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Carbon nanofibers are high-aspect ratio graphitic materials that have been investigated for numerous applications due to their unique physical properties such as high strength, low density, metallic conductivity, tunable morphology, chemical and environmental stabilities, as well as compatibility with organochemical modification. Surface studies are extremely important for nanomaterials because not only is the surface structurally and chemically quite different from the bulk, but its properties tend to dominate at the nanoscale due to the drastically increased surface-to-volume ratio. This review surveys recent developments in surface analysis techniques used to characterize the surface structure and chemistry of carbon nanofibers and related carbon materials. These techniques include scanning probe microscopy, infrared and electron spectroscopies, electron microscopy, ion spectrometry, temperature-programed desorption, and atom probe analysis. In addition, this article evaluates the methods used to modify the surface of carbon nanofibers in order to enhance their functionality to perform across an exceedingly diverse application space. ©2008 American Institute of Physics

Contents

• INTRODUCTION
• SURFACE CHARACTERIZATION TECHNIQUES
  • A.Scanning probe microscopy
  • B.Infrared spectroscopy
  • C.Electron spectroscopy
  • D.Electron microscopy
  • E.Secondary ion mass spectrometry
  • F.Temperature-programed desorption
  • G.Atom probe
• SURFACE MODIFICATION TECHNIQUES
  • A.Chemical vapor deposition of thin film coatings
  • B.Electro- or electroless plating
  • C.Chemical functionalization
  • D.Biochemical functionalization
• SUMMARY AND OUTLOOK
• ACKNOWLEDGMENTS
• REFERENCES
• FIGURES
• FOOTNOTES

The rapid surge in new synthetic nanomaterials demands complementary advancement in surface characterization techniques and modification methods in order to understand and utilize these materials. It is well known that surface properties often deviate substantially from the bulk material properties due to a difference in physical structure and chemistry. Moreover, the surface is a dynamic system that interacts with the environment, a characteristic which is exploited for many applications. The surfaces of nanostructured materials are of special significance because of their enhanced role in determining functional properties—a phenomenon which becomes more pronounced as the surface-to-volume ratio increases. At the same time the complex morphology of nanostructured surfaces also creates new challenges for their characterization. To a large extent, the development of surface science of the last century has been based on the assumption that the sample presents a flat, well-defined surface, which is examined under ultrahigh vacuum. The translation of traditional surface characterization techniques to the study of complex three-dimensional functional surfaces is nontrivial and the methods employed are often specific to each particular family of nanostructured materials.

Among the multitude of nanomaterials, carbon nanostructures hold a special place due to their mechanical strength and chemical stability. In addition, the covalent chemistry of carbon with oxygen, hydrogen and nitrogen provides facile routes for functionalization of carbon surfaces with organic or biological molecules. In elemental form, carbon constructs allotropes with different kinds of carbon-carbon bonds, such as sp²-based graphite and sp³-based diamond, resulting from the variety of covalent bonding arrangements provided by orbital hybridization.¹ Fullerenes are a class of sp²-based nanostructured materials that can be seen as close-cage derivatives of a hexagonal network of carbon atoms.² In its simplest form, such a hexagonal network terminated by hydrogen atoms is represented by a graphene sheet, Fig. 1(a). A carbon nanotube (CNT), more specifically a single-walled carbon nanotube (SWCNT), can then be considered as a graphene sheet rolled into a cylinder, where multiple concentric sheets create a multiwalled carbon nanotube [MWCNT, Fig. 1(b)]. The introduction of five and seven member rings into the graphene allows the formation of curved structures such as “buckyballs”³ and nanocones.⁴ Carbon nanofibers (CNFs) are a class of these materials that have curved graphene layers or nanocones stacked to form a quasi-one-dimensional filament, whose internal structure can be characterized by the angle between the graphene layers and the fiber axis [Fig. 1(c)].⁵ One common distinction noted in the literature is between the two main fiber types: “herringbone” [Fig. 1(d)], with dense conical graphene layers and large , and “bamboo” [Fig. 1(e)], with cylindrical cuplike graphene layers and small , which is more similar to a MWCNT. However, in the case of a true CNT is zero.

Figure 1.

Despite distinct differences in their internal structures, nanofibers are often called nanotubes as they can display similar morphology to MWCNTs; however, their physical and chemical properties are quite different. While nanotubes are reported to display ballistic electron transport⁶ and diamondlike tensile strength along their axis,⁷ nanofibers have proven their robustness as individual, freestanding structures with higher chemical reactivity and electron transport across their sidewalls, important for functionalization^8,9,10,11 and electrochemical applications,^8,12,13 respectively. In fact, early studies of highly oriented pyrolytic graphite (HOPG) and glassy carbon have shown that the edge planes of graphite have electron transfer rates on the order of 10⁵ times higher than basal planes.¹⁴ Only recently has there been demonstrated control over the modulation of the internal graphitic structure of CNFs, in turn modulating the density of edge plane termination on the nanofiber surface.¹⁵

Vertically aligned carbon nanofibers [VACNFs, Fig. 1(f)], synthesized by plasma enhanced chemical vapor deposition (PECVD),⁵ are highly compatible with microfabrication, thereby facilitating their incorporation as functional nanostructured components of a large variety of devices. Demonstrated CNF applications include electron field emitters,^16,17,18,19 charge and hydrogen storage media,^20,21 composite materials,^22,23 biosensors,^8,9,24,25 gene delivery arrays,^26,27,28,29 synthetic membrane structures,^30,31 electrochemical probes,^13,32,33 electrodes for neuronal interface,^34,35 and scanning probe microscopy (SPM) tips.^36,37,38 The structure and surface chemistry of the nanofibers play a crucial role in the performance characteristics of these nanofiber-based devices. For many applications it is necessary to modify the surface in order to change its properties and induce additional functionality.³⁹ The surfaces of CNFs can vary substantially depending on synthesis and postsynthesis processing conditions such as those encountered during microfabrication, and subsequent operations such as heat treatment or oxidation.⁴⁰ For each of the many possible applications of interest, but specifically for biological and composite applications, the goal is often to manipulate the surface chemistry in order to amplify the number of potential attachment sites, maximize the specificity and selectivity of adsorption processes, or just to maintain the stability of the surface. In order to understand the benefits that surface modifications provide and to quantify their effectiveness, it is necessary to characterize the physical and chemical changes that they cause.

Biological applications have been one of the most significant examples of the successful implementation of carbon nanostructures, generating a swiftly growing appreciation of the surface functionality of these materials. Synthetic nanoscale structures offer a particularly suitable means of interfacing with biological systems because they intervene at the scale where life processes proceed—the molecular level. CNFs are especially appropriate for biological interfacing because of their high surface area coupled with an abundance of dangling bonds terminated in hydrogen or other functional groups. Consequently, CNFs have exhibited excellent specificity and reversibility in binding DNA probes⁹ as well as superior long term chemical stability even at elevated temperatures. The success of nanomaterials-enabled biology has led to a renewed interest in surface characterization, as exemplified by an ample number of journal publications on the analysis of CNF surfaces employing a variety of techniques.

This review focuses on the surface characterization and modification of CNFs. Surface reviews on CNTs published thus far have been limited to methods of functionalization only, in which case the chemistry for graphite basal surfaces differs from nanofiber sidewall chemistry. Furthermore, this review examines the advantages of surface analysis by various techniques including scanning probe and electron microscopies, infrared and electron spectroscopies, ion spectrometry, temperature-programed desorption, and atom probe analysis. In addition, substantial emphasis is placed on exploring the recent methods of CNF surface modification for achieving a wide range of functionality.

This section organizes the experimental surface characterization work by technique, presenting a brief explanation of the method with its strengths and limitations, and illustrating with several interesting examples of how the technique has been applied to CNF surface studies. While we cannot cover all surface techniques, we attempt to review the most common, relevant, and novel techniques from the literature.

SPM surveys small areas of surface with high lateral resolution, as opposed to techniques such as x-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy, which sample relatively large surface areas and yield mean values for surface properties. SPM techniques, including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), are capable of providing high surface sensitivity with atomic-level resolution; however, the yield of acceptable images is low and image interpretation can be difficult. When performing high-resolution scans, large numbers of images must be taken to ensure the data is representative.

STM can map the surfaces of electrically conductive materials with sub-Angstrom vertical resolution. Paredes et al. used STM observations to address the atomic-scale reorganization of the CNF surface after oxidation.^41,42 Figure 2 shows STM images of a nanofiber surface before and after a 5  min oxygen plasma treatment, from which they drew two main conclusions about the surface structure. First, the untreated CNF surfaces, Fig. 2(a), displayed basal graphite characteristics on a local scale of several atom lengths, but lacked long range atomic order. Second, oxygen plasma treatment resulted in the annihilation of the short-range basal graphitic order [Fig. 2(b)], leading to a significant and uniform increase in the atomic-scale disorder of the CNF surface. The results agree with previous studies of oxygen plasma etching of HOPG surfaces resulting in atomic-scale defects.⁴³ These atomic defects introduced in the plasma environment ultimately led to the addition of functional oxygen groups to the CNF surface and may be resolved by atomic force microscopy, XPS, IR, or other methods.

Figure 2.

Surface graphitization can be improved by postgrowth heat treatment. Paredes et al. monitored the graphitization process of CNF surfaces at various annealing temperatures by using STM imaging.⁴⁴ These images provided clear, atomic-sale evidence for the evolution of the CNF surface consistent with spatially averaged characterization results obtained by x-ray diffraction (XRD) and Raman spectroscopy. In particular, STM revealed a nanoscale surface transformation from platelet morphology to larger ordered areas and finally to atomically flat terraces on faceted nanofibers at 2800  °C. This corresponded in the atomic-scale images to a conversion from small, highly defective crystallites coalescing to form larger, defective crystallites and finally to true graphitic domains in many areas. Endo et al. also observed similar graphitization results using STM imaging.⁴⁵ Another STM study by Yoon et al. found that graphitized platelet CNFs are actually assembled of carbon nano-rod units, where the edges of the graphene layers formed closed caps.⁴⁶

The AFM has the inherent advantage in that it can generate high lateral resolution images with superb z-height discrimination from all types of surfaces—even those that are wet or insulating. Furthermore, the AFM exhibits sensitivity to chemical changes via molecular recognition and friction imaging, otherwise known as chemical force microscopy. Recently, an AFM-based approach was developed to resolve hydrophilic oxygen group regions on carbon surfaces.⁴³ This method is based on the detection of phase changes in the noncontact tapping mode of the AFM. Paredes et al. applied this approach to detect and map the oxygen functionalized content of CNF surfaces.⁴² However, phase image contrast is a relative measure within a given image, which limits sample-to-sample comparisons. Paredes et al. avoided this issue by providing a common reference for evaluation of two nanofiber samples by placing them on a HOPG substrate. Shown in Fig. 3 are line profiles from the noncontact tapping mode phase images of a CNF before and after oxygen plasma treatment.

Figure 3.

Abrupt changes in surface topography also lead to phase contrast. In the aforementioned case, evidence of this effect was manifest as a large change in phase at the edge of the nanofiber. Since such effects were only of topographical origin and were not related to the surface properties, they were disregarded. Thus, in this case, comparison was only made between the top of the nanofiber (where it is locally atomically flat) and the HOPG surface, indicated by arrows in Fig. 3. Typical phase profiles implied that the phase of the untreated CNF was very similar (within 1°) to that of the HOPG substrate, while the phase of the plasma treated CNF was consistently 3° higher than the HOPG substrate. This 3° shift had been previously reported for oxygenated planar HOPG surfaces as well.⁴³

IR spectroscopy, with the addition of the interferometer, is a robust and easy method for characterization of organic surfaces. Surface sensitive IR techniques can probe depths of a few centimeters to as shallow as 100  nm below the surface. Since CNFs are generally on the order of 20–200  nm in diameter, the spectra in this case convey “bulk” chemical information. However, this can be useful to distinguish which surface groups are present. For example, Ros et al. compared the surface structure of untreated bamboo-type to herringbone-type CNFs using transmission IR.⁴⁷ They hypothesized that herringbone fibers, in which the sidewalls terminate with a greater number of reactive graphitic edges, would be more susceptible to surface oxidation than bamboo CNFs. By keeping the transmission levels from each sample the same, the two fiber types are compared in Fig. 4. Overall, they concluded that the CNFs have a defect-rich structure and that carbon-hydrogen bonds are present on herringbone as well as bamboo fibers (stretching at 3012, 2947, 2917, and 2846  cm⁻¹). However, only herringbone fibers showed evidence of carboxyl groups (1717-1712 stretch) and a loss of mass as CO₂ when annealed.

Figure 4.

Additionally, by comparing spectra from before and after processing treatments the change in surface structure can be evaluated. In many cases the surface of the fiber is oxidized to create reactive carboxylic acid sites useful for further functionalization. Ros et al. surveyed several methods of surface oxidation and found that a mixture of concentrated nitric and sulfuric acids proved to be the most effective method for creating oxygen-containing surface groups.⁴⁸ Furthermore, the IR spectra revealed that these surface groups occurred at defect sites on the fiber surface and that oxidation proceeds first from carbonyl groups and other oxides to carboxy and carboxylic anhydride groups.

Ros et al. also investigated the immobilization (by covalent bonding) of metal complexes onto CNFs by first treating the oxidized fibers in anthranilic acid.⁴⁹ The success of this treatment is demonstrated by the IR spectra in Fig. 5. After oxidation, an additional peak appears at 1724  cm⁻¹. This band originates from the CO stretching vibration of carboxyl and/or carbonyl groups. In the spectrum of CNF-AA, the carboxyl CO stretching vibration at 1724  cm⁻¹ of CNF-OX all but disappeared, indicating that an amide bond between the carboxyl groups on the CNFs and the amine groups of anthranilic acid was formed. Additionally, the aromatic CH bending mode at 746  cm⁻¹ (signifying four adjacent H atoms) occurring in the spectrum of CNF-AA is a good indication for the presence of anthranilic acid on the surface of the CNFs.

Figure 5.

Li et al. derivatized CNFs with concentrated nitric acid at 140  °C for 4  h and then acylated the surface-oxidized fibers using thionyl chloride for 24  h.⁵⁰ Following acylation various amine compounds were then surface bound to the fibers. IR results showed that the graphitic backbone CC stretching peaks remain unaffected by the nitric acid treatment. IR also verified that a new CO stretching band appeared following the acid oxidation, which was then reduced by the acylation and amine treatments. Unfortunately this technique was not useful for verifying the expected carbonyl stretching band at 1616  cm⁻¹ for amide functional groups since it overlaps with the intense graphene CC peak at 1578  cm⁻¹.

Han et al. used Fourier transform IR (FTIR) to verify that oxidized multiwalled CNTs made a covalent bond (CH₃ to CH₂ transition) with alkanethiolate-coated gold nanoparticles,⁵¹ showing the importance of linker molecules in creating composite materials. Additionally, they have shown via FTIR that the organic shells can later be removed from the gold nanoparticles on the surfaces of the MWCNTs by a thermal anneal at 300  °C. Efforts by Oh et al. utilized FTIR to demonstrate the covalent grafting of polyetherketones onto the surfaces of both MWCNTs and CNFs treated with phosphoric acid.⁵²

There are several ways of obtaining atomic composition and chemical bonding information from the surface. When x-rays of sufficient frequency (energy) interact with an atom, inner shell electrons in the atom are excited to outer, empty orbitals, or they may be ejected from the atom completely, so ionizing the atom. These electrons ejected by incident x-rays are called photoelectrons and can be detected by XPS. Likewise, an ultraviolet excitation source can also be used and the generated photoelectrons are measured by ultraviolet photoelectron spectroscopy (UPS). Once a surface atom loses a photoelectron the atom desires to return to a relaxed state; thus the inner shell “hole” left by the photoelectron will then be filled by electrons from outer orbitals and their excess energy must be given off in the form of either x-ray fluorescence (XRF) or an ejected Auger electron. The ejected Auger electrons can be analyzed via Auger electron spectroscopy (AES).

Thus there are two competing atomic relaxation mechanisms. For lighter elements, the probability of Auger electron emission is significantly higher than emission of an x-ray photon. However, collection of characteristic x-ray photons is more efficient for measuring the composition of heavier elements. XRF from photon excitation generally is not very surface sensitive with contributions from a few microns to millimeters deep within the sample, and is therefore beyond the scope of this review. X-ray emission as a result of electron induced fluorescence, however, will be discussed further in Sec. II D 2.

The major advantages of photoelectron and Auger spectroscopy techniques are the high surface sensitivity and the ease of depth profiling. By sputtering the sample surface with an ion beam between successive spectra collections, depth-resolved chemical information can be obtained.

XPS is considered the “workhorse” of surface analysis because of its robustness and versatility. Using an x-ray source with energy in the keV range, the escape depth of core-level photoelectrons is only several atom layers deep with a lateral spatial resolution from ~30  µm to a few millimeters. The data produced permit the detection of all elements except hydrogen and helium with a sensitivity of better than 1  at.  %. One drawback of this technique is that it gives spatially averaged information across a large sampling area, as opposed to AES which can probe areas down to 10  nm with a focused scanning electron beam. Overall, there is a plethora of literature characterizing carbon nanostructures by XPS. Thus the scope of the examples presented in this section will be limited to selected articles that focus on CNF studies, mainly surface oxidation treatment and the surface adsorption of metals.

Hoshi et al. utilized XPS to verify the purity of untreated CNFs with a C  1s peak of 284.7  eV.⁵³ Since pure CC XPS peaks of graphite reside at ~284.5  eV and C—O bonding is the vicinity of 288 eV, they inferred that there was less than 5% oxygen binding on the surface.

According to Toebes et al., the most dominant effect of acid oxidation is a surface area increase upon opening of an inner tubular channel along the length of the fibers.⁵⁴ XPS was again used to establish the amount of oxygen on the 2-3 nm sub-surface of the CNF. They demonstrated that total oxygen content, as well as the number of acidic groups, was a function of the type of oxidizing agent and the treatment time.

Lakshminarayanan et al. used XPS to take an in-depth look at what occurs during the HNO₃ oxidation process.⁵⁵ They found that maximum levels of ~20% oxygen were reached after only a 10  min treatment. This level remained fairly constant until oxidation times longer than a few hours lead to carboxyl group removal and replacement by ester, anhydride, quinoid, and phenolic hydroxyl groups.

Bubert et al. investigated the formation of functional surface groups on CNFs from plasma treatment using various gases, pressures, and powers.⁵⁶ Figure 6 displays an XPS sputter depth profile of a CNF showing most of the oxygen is in the outer 2  nm of the sample. Additionally, from the tabulated results, they concluded that the highest oxygen concentration (NF-6) is achieved at low gas pressures (1.5  mbars). The kind of oxygen source gas, whether O₂ or CO₂, did not appear to make a significant difference.

Figure 6.

Winter et al. used a combination of XPS and transmission electron microscopy (TEM) to determine the percentage of Co and Pd nanoparticles that had adsorbed to the inner core of the nanofiber rather than the external surface following an oxidative HNO₃ treatment.⁵⁷ TEM elucidated the average particle size and the weight percent loading of particles to fiber, but it could not distinguish which particles were located in the central cavity of the fiber as opposed to those adsorbed to the surface. However, metal particles located on the interior of the CNFs gave no contribution in XPS signal due to the short mean free path of photoelectrons in graphite (~2.6  nm). From these two sets of data they were able to calculate the weight percent of metal particles located in the interior of the nanofiber to be 10% for Pd and 28% for Co.

Xia et al. studied the process of depositing iron oxide particles on CNFs for additional growth of secondary fibers using XPS.⁵⁸ The specific surface area of the nanocomposite was enhanced by the growth of secondary nanofibers, and it was possible to tune the morphology of the nanofiber-nanofiber composites by the process parameters. First, CNFs were exposed to oxygen plasma to introduce oxygen-containing functional groups. Next, the chemical vapor deposition (CVD) of ferrocene was carried out under oxidizing conditions, yielding nanofiber-supported iron oxide nanoparticles. Secondary CNFs with diameters in the range from 10  to  20  nm were subsequently catalyzed by the sintered metallic iron nanoparticles. XPS was applied to monitor the chemical changes of the surface composition and the sintering of the metallic iron particles. The XPS results shown in Fig. 7 were used to derive the surface composition of both the as-synthesized iron oxide coated fibers and the heat treated (sintered) sample. The surface concentrations of both Fe and O decreased after heat treatment, which indicated severe sintering and a loss of oxygen-containing surface groups on the CNF.

Figure 7.

UPS is similar to XPS but uses ultraviolet radiation (10–50  eV) to excite valence-level photoelectrons of much lower kinetic energies. While the penetration depth and lateral spatial resolution are similar to XPS, the type of information obtained is quite different because UPS gives molecular orbital bonding and electronic structure information. Additionally, the spectral resolution of UPS is greater than XPS because the photon linewidth for ultraviolet is less than the linewidth for x rays. The UPS technique works best with a synchrotron source, which generates a beam of high intensity with a narrow spectral excitation width.

In the literature there is an absence of work specifically on UPS characterization of CNFs; however, there are a few articles characterizing MWCNTs by UPS. Most studies focused on comparing CNTs to graphite electronic configuration or looking at the effects of oxidation. The early UPS work done by Chen et al. characterized MWCNTs produced by catalytic CO disproportionation.⁵⁹ They found that the CNT valence band is basically the same as graphite with minor variations. The main differences in the spectra, shown in Fig. 8, are that the intensity for the 2p- binding energy (2.0–7.6  eV) is lower and the 2p- (~11.5  eV) is slightly higher compared to graphite, an effect explained by the curvature of the nanotubes and - hybridization. Analogous research by Umishita et al. compared MWCNT samples produced by arc discharge before and after purification.⁶⁰ UPS and XPS showed many similarities between the MWCNTs and graphite; however, they attributed the variations between the spectra to the nonalignment of the MWCNT samples.

Figure 8.

Ago et al. studied the work function and density of states of MWCNTs affected by oxidation of the surface.⁶¹ Pure MWCNTs were found to have a slightly lower work function of 4.3  eV than graphite (4.4  eV) but after oxygen plasma treatment the density of states was affected and the work function increased up to 4.8  eV as shown in Fig. 9. The investigations of Lim et al. on MWCNT oxidation effects using UPS and TEM led to similar results.⁶² They found that with increasing oxygen exposure the carbon 2p- states at 3  eV below the Fermi level diminished while the 2p- states around 6  eV significantly increased. Furthermore, annealing appeared to increase the density of states around the Fermi level.

Figure 9.

As mentioned before, a fundamental advantage of AES is that the yield of Auger electrons is highest for the lighter elements such as C, Si, N, and O. In addition, the incident electron beam can be focused to a fine spot giving excellent lateral spatial resolution on the order of a few tens of nanometers. Furthermore, since Auger electrons have lower energies, their escape depth is even shallower than that of x-ray induced photoelectrons and so they describe a region just a few atoms deep. Auger spectroscopy must therefore be performed in ultrahigh vacuum to maintain a clean surface, and in addition, the sample must be a good electrical conductor or else charging induced by the incident beam will result in a shift of the energies of the characteristic emission edges.

Throughout the literature AES has been used to characterize the surface of CNFs for many types of studies including irradiation changes and the composition of surface coatings. One of the most fundamental studies, by Dementjev et al., looked at the relationship between the x-ray excited C KVV Auger line shape and the layered structure of graphite.⁶³ A plot of the angular dependence of the Auger lineshape for several types of carbon is displayed in Fig. 10, where the Auger spectra can be divided into two groups: first the HOPG at grazing emission, fullerene, quarterphenyl and SWCNT, and second, HOPGn at normal emission and MWCNT. The MWCNT sample has a regular interwall spacing of ~3.4  Å so its spectrum is very similar to HOPGn. Dementjev et al. concluded that the C KVV Auger spectra of single-layer (i.e., CNTs, fullerenes) and multilayer structures (i.e., MWCNTs, the most similar to CNFs) with sp² bonds are significantly different and may be used as a fingerprint for the single-layer and multilayer growth modes.

Figure 10.

Zhu et al. investigated the interaction of MWCNTs with Ar⁺ beam irradiation using in situ AES and XPS characterization techniques.⁶⁴ Following irradiation with the Ar⁺ beam, the kinetic energy of the C KLL lines in x-ray induced AES decreased 1.6  eV in the MWCNT samples, becoming more similar to graphite, as seen in Fig. 11. They hypothesized that the conjugated -bonds in the tube structure of the MWCNTs were destroyed by irradiation and that the nanotube was converted into an amorphous carbon rod containing many dangling bonds but no conjugated -bonds even though the carbon is sp² hybridized.

Figure 11.

The Auger technique can also be useful for characterizing species deposited on the surface of carbon nanostructures during growth or following postgrowth treatments. Wang et al. grew conical CNFs with Co catalysts by microwave PECVD with a high substrate bias.⁶⁵ Similarly, Cui et al. grew conical CNFs from Fe-Pt catalysts using direct current PECVD.⁶⁶ Both authors used focusable, electron excited AES to sample the chemical composition at the base and the tip of the cone and found significant levels of C, N, O, and Si. They attributed the high levels of silicon to sputtering of the substrate and redeposition on the sidewalls of the CNFs during the growth process. The oxygen and nitrogen incorporation onto the surface may have resulted from the sample's exposure to air.

An amorphous carbon layer can also be deposited under certain growth conditions (discussed further in Sec. III C 1). The work of Teo et al. looked at the effect of the gas ratio used for CNF growth on the deposition of noncatalytic carbon on the substrate⁶⁷ and fiber sidewalls.⁶⁸ They found that the CNFs contain well-ordered graphitic carbon in contrast to the amorphous carbon by-product on the substrate. By adjusting gas ratios during PECVD, the fibers range from cylindrical to conical depending on the amount of carbon sidewall deposition, as conveyed in Figs. 12(a)12(b). Auger analysis in Fig. 12(c) of the tips and bodies of cylindrical and conical CNFs demonstrates two things: first, that the body of the CNF does not contain detectable amounts of Ni and second, that only the cylindrical CNF contains a significant level of N, O, and Si. Additionally, AES depth profiling in Fig. 12(d) reveals that gas ratios greater than 30% C₂H₂:NH₃ resulted in an amorphous carbon-nitrogen film detected on the substrate. In all cases, a 5–10  nm amorphous interfacial layer was detected on the surface of the silicon substrate.

Figure 12.

Auger spectroscopy can also be useful in observing postgrowth coatings deposited on the CNFs. Wang et al. employed AES and electron microscopy to characterize each step of a process whereby the catalyst tips of the CNFs were removed and then the fibers were coated with a phase-change alloy GeSbTeSn.⁶⁹ The AES results showed that following heat treatment of the phase-change alloy coated fibers, the composition of the alloy changed, presumably due to the faster evaporation rates of Sb and Te than Ge. To obtain the desired end-point alloy composition, the initial deposition ratio must be modified.

Secondary electron microscopy (SEM) is perhaps the most frequently used method of characterizing the morphological structure and topography of a sample. TEM and scanning transmission electron microscopy (STEM) convey information about the nanostructure as a whole, but can also show the presence of surface layers and reveal the atomic structure of the surface interface by high-resolution TEM (HRTEM). A common companion tool to both the SEM and the TEM is an energy dispersive x-ray (EDX) spectrometer, which readily gives the elemental composition of the sample and can also be useful in generating elemental maps. Electron energy loss spectroscopy (EELS) is a valuable analytical tool both for determining the composition of TEM specimens and for providing information about the valence state and electronic structure of the material under examination. The atom's response to electron bombardment is similar to its relaxation from UV or x-ray stimulation, discussed in Sec. II C. There are still two main competing relaxation mechanisms: characteristic x-ray emission and Auger electron ejection, although for higher atomic number materials relaxation can also efficiently occur by means of nonradiative mechanisms. Backscattered electron imaging (BSE) is generally not very useful for surface analysis of CNFs because the escape depths of backscattered electrons are on the order of 1  µm or greater, and is therefore not discussed here.

Although scanning electron microscopy is the most widely used surface imaging technique, the depth from which the relevant secondary electrons typically escape (usually ranging from 5  to  50  nm deep depending on the material) results in the image containing both surface and bulk information. Image contrast and brightness can also be ambiguous and not quantitatively topographical; edges are often highlighted and surface charging can result in large fluctuations in signal level as well as in distortions of the scan raster. Nevertheless, there are numerous examples of the use of SEM for studying surface morphology and process control studies related to CNFs. For the sake of brevity, only two studies are described.

For example, the work of Xia et al., mentioned earlier in Sec. II C 1, shows a morphological change in the fiber surface, depicted in Fig. 13.⁵⁸ This morphological transition from a smooth fiber in Fig. 13(a) to one with protrusions from the surface in Fig. 13(b), agrees with the processing steps taken to grow secondary fibers from iron oxide particles deposited on the oxidized CNFs. As a result the surface area of the fibers was significantly increased.

Figure 13.

Similarly, Klein et al. used SEM to document a change in surface morphology of VACNFs with respect to catalyst composition.⁷⁰ Small levels of Cu in Ni-rich alloy catalysts resulted in split particles and branching nanofibers [Fig. 14(a)]; however, as the alloy composition became Cu-rich, the nanofibers became more conical as seen in Figs. 14(b)14(c).

Figure 14.

EDX measures the energies of the characteristic x-rays generated from ionizations induced within the specimen in an electron microscope. Each element emits a unique fingerprint of x-ray energies related to the difference in binding energies of the electron shells involved in the relaxation process. A semiconductor diode detects each incoming photon and also measures its energy to provide a histogram display depicting the emitted x-ray emission spectrum from the irradiated area of the sample. This spectrum can then be analyzed to identify the elements present (“qualitative analysis”) and to determine the chemical composition of the material (“quantitative analysis”). Therefore, EDX is a tool often used along with electron microscopy imaging to give complementary chemical information. Depending on the energy of the electron beam and the density and chemistry of the specimen the lateral spatial resolution can range from micrometers to tens of nanometers, while the depth resolution is a fraction of the incident beam range and so is on the scale of hundreds of nanometers or less.

For example, the study by Klein et al., mentioned in the previous section, investigated catalyst effects on CNF morphology using EDX to determine the composition of the fibers and location of catalyst material.⁷⁰ From the EDX information, shown in Fig. 15, it was noted that the copper from the Cu-Ni catalyst particles segregated out of alloy phase to reside at the base of the nanofiber. Furthermore, they found that the fiber itself was encapsulated in a thick in situ deposited silicon-rich coating. These data lead the authors to conclude that Cu-rich alloys were not highly active catalysts for carbon growth, but rather the Ni portion segregated to catalyze a very thin CNF, which served as a scaffold for the condensation of predominantly silicon species from the plasma.

Figure 15.

In a second example, Weng et al. reported significantly enhanced field emission (FE) properties from VACNFs on a Si substrate treated briefly with an argon plasma.⁷¹ Analysis by electron microscopy and EDX, shown in Fig. 16, suggests that a structural transformation of the fibers is a result of a cosputtering/deposition process by energetic plasma ions, revealed by the Si counts increasing with longer Ar plasma treatment. They attributed the field emission enhancement to the combined effect of additional Si/C layer coverage, catalytic nanoparticle removal, and the physical sharpening of the CNFs tips. However, with longer treatment times (>5  min), structural damage effects were seen to decrease the effectiveness of the plasma treatment as conveyed by the FE measurements in Fig. 16(c).

Figure 16.

Hang et al. found EDX mapping useful in their search for the best nanocarbon material for Fe/C composite air battery anodes.²¹ Hollow- and herringbone-type CNFs exhibited favorable results, including improved conductivity and higher redox currents of the Fe/C electrode. EDX revealed that on the Fe/nanofiber surface, iron was more dispersed than on Fe/graphite after cycling. They concluded that such a high dispersion of iron on nanofiber surfaces may improve the electrochemical behavior of the iron redox species.

The surface as well as internal structure of CNFs can be analyzed using STEM or TEM. Both real space (“image”) and reciprocal space (“diffraction”) data, together with chemical and electronic analytical information (derived from EDX or EELS), can be obtained from the same nanoscale area. The TEM and STEM therefore provide a wide and deep range of data about the specimen of interest.

For instance, a study on the nanofiber structural effects of different catalysts (Ni versus Pd) was performed by Ominami et al.⁷² It can be seen from the STEM images in Fig. 17 that the Ni-catalyzed fibers have graphitic planes ending on sidewalls of the fiber in a herringbone fashion, whereas the Pd-catalyzed fibers have multiwalled nanotubelike layers on the outer walls of the fiber. Ominami et al. attributed the increased electrical conductivity along the length of the Pd-catalyzed CNFs to this improvement in the graphitic structure and lower interface resistance with the substrate.

Figure 17.

Similarly, Naguib et al. used HRTEM to analyze the nanofiber wall structure in order to compare processing methods for biological applications.⁴⁰ They reported that heat treatment of CNFs at 3000  °C formed graphitic loops on their surface [Fig. 18(a)], eliminating chemically active dangling bonds and leading to hydrophobic behavior. However, CNFs that were pyrolytically stripped in carbon dioxide to remove polyaromatic hydrocarbon layers from their outer surface had significantly more disordered surfaces [Fig. 18(b)], and exhibited hydrophilic behavior. As a result of improved wetting and increased protein binding capacity of the nanofiber surface, Naguib et al. concluded that antibody coatings were more successful on the pyrolytically stripped CNFs.

Figure 18.

The work by Han et al., previously described in the IR spectroscopy section,⁵¹ is also good example of the utility of TEM for surface characterization. TEM imaging, such as in Fig. 19(a), can give an idea of the degree of functional sites available by the dispersion of gold nanoparticles on the surface of the fibers. Additionally, the HRTEM image in Fig. 19(b) shows a closeup of how the graphitic planes are terminated by a gold nanoparticle.

Figure 19.

EELS measures the energy spectrum of the electron beam transmitted through the sample in the TEM or STEM. The spectrum contains chemical data, which are complementary to those derived from EDX, but with much higher sensitivity for the lower atomic number elements (Z<20). In addition, other phenomena such as plasmon excitations, ionization edge fine structure, and extended fine structure effects provide a detailed look at valence, bond structures, radial distribution functions, and other descriptors of microstructure on the atomic scale.

Hollow CNF wall structure investigations by Ye et al. revealed that the exterior surface was covered by a 5 Å hairlike layer in the TEM micorgraphs.⁷³ Further investigation by EELS suggested that the “hairs” were functional groups containing oxygen and carbon, thus corroborating the hydrophilic behavior of the as-synthesized CNFs.

Chen et al. developed a method for synthesizing boron nitride (BN) coatings on the surface of CNFs without damaging the graphitic walls of the fiber.⁷⁴ A reaction between boric acid and ammonia was initiated on the surface of the fibers to form the coatings. HRTEM and EELS were utilized to characterize the BN coating quality, thickness, and interface with the underlying CNF. An EELS chemical line scan is shown in Fig. 20(a), along with a TEM image [Fig. 20(b)] illustrating there was about ~20  nm thick BN layer covering the surface. In addition, they found that the surface structure of the CNF significantly influenced the morphology of the BN coating. If the surface of the hollow CNFs was relatively free of defects (i.e., dangling bonds), then highly crystallized BN sheaths would encapsulate the CNFs. However, if the surface of the CNF was disordered due to pyrolytic stripping, for instance, then a thinner polycrystalline BN sheath was produced.

Figure 20.

Secondary ion mass spectrometry (SIMS) is a powerful characterization tool with outstanding surface sensitivity on the scale of a few atomic layers. SIMS also provides detection limits surpassing those attainable with XPS. The drawbacks of SIMS include sample charging and the complexity of the instrumentation and data analysis. In SIMS, energetic, heavy ions such as Ar⁺ are used to decompose the surface into atoms or molecules. A small percentage (<10%) of the surface fragments are ejected as charged particles or “secondary ions,” which can be sorted on the basis of their charge to mass ratio or their velocity. There are two types of SIMS experiments, dynamic SIMS (D-SIMS) and static SIMS (S-SIMS).

In dynamic SIMS, a continuous, high-flux stream of primary ions having an energy of 1–20  keV bombards the surface and fragments it as much as possible to maximize the generation of charged atomic species. Additionally, this intense bombardment serves a second purpose of eroding (sputtering) the sample's surface for elemental depth profile information. D-SIMS has ppt to ppm sensitivity with about a 2  nm depth resolution and 50  nm lateral resolution. For this reason, SIMS is often used for doping experiments or to detect trace level elements. Uniquely, all elements, including hydrogen, can be detected with this method and isotopes can also be differentiated.

The work of Khare et al. appears to be the only application of D-SIMS for the surface analysis of carbon nanomaterials.⁷⁵ In this work SWCNTs were irradiated by hydrogen ions and the effects evaluated by SIMS and FTIR, as shown in Fig. 21. The SIMS results showed that the hydrogen concentration was much higher for the irradiated sample than for the control. This was in agreement with the FTIR spectra, which confirmed the presence of CH stretching modes only in the proton treated sample.

Figure 21.

In contrast to dynamic SIMS, the goal in static SIMS is to maintain the molecular integrity of the surface fragments as much as possible. Therefore, a pulsed source is often used, the primary beam flux is reduced, and the beam is rastered to generate larger charged sample fragments. Because many of the detected species are not distinctly identifiable, S-SIMS is only qualitative. However, with this method isomers can usually be distinguished because each fragment of the isomer should be measured in predictable levels in the mass spectrum relative to a base peak.

Static SIMS has been utilized most often for characterizing linker molecules and polymer films bound to the nanofibers. For instance, Okpalugo et al. reported on surface-to-depth analysis of functionalized MWCNTs using SIMS and XPS.⁷⁶ The CNTs in this work were first treated with nitric acid followed by dimethylformamide and further modified by covalently bound 1-ethyl-3-(3-dimethylaminopropyl carbodiimide). The functionalized nanotubes possessed several surface species with levels of OH and CN steady throughout the depth profile, indicating a deeply penetrating chemical modification had occurred.

Chen et al. have described success with preparing polyaniline films on MWCNTs.⁷⁷ Using time-of-flight S-SIMS, they determined that the primary functional group on the surface of the modified CNTs was C₆H₆N. Similarly, He et al. applied S-SIMS to confirm the deposition of a carbon-fluorine polymer film onto CNFs by comparing the before and after spectra.⁷⁸ As a final example, Shi et al. utilized time-of-flight S-SIMS to characterize a plasma-polymerized pyrrole film deposited on the surface of hollow CNFs.⁷⁹ A small amount of C₆F₁₄ was copolymerized to distinguish the polymer coating from the hydrocarbon fiber. The results of the TEM and S-SIMS analysis are shown in Fig. 22. The TEM image in Fig. 22(b) illustrates a thin pyrrole film (~7  nm) on the outer surface of the fiber and an ultrathin film (~1–3  nm) on the inner surface of the fiber. Since the fibers were open ended and hollow, some of the precursor gas was able to polymerize on the interior of the fiber. The mass spectra in Figs. 22(c)22(d) confirm that large carbon-fluorine molecules were incorporated into the film. Judging from the size of the ionized polymer pieces, Shi et al. deduced that the polymer coating was highly branched and cross-linked.

Figure 22.

Temperature-programed desorption (TPD), also called thermal desorption spectroscopy (TDS), is a method of characterizing adsorbed surface species by heating the sample while under vacuum and simultaneously detecting the residual gas by means of a mass spectrometer. As the temperature rises, certain absorbed species will have enough energy to escape or desorb from the surface and will be detected as a rise in pressure for a particular mass component. A continued rise in temperature will lead to a reduction in the amount of the species on the surface causing the pressure to drop again, which manifests in a peak in the pressure versus time plot. The temperature at the pressure peak maximum provides information on the binding energy of the bound species, while the identity of the bound species is deduced from its atomic mass. Additional information can be obtained if sample weight is measured simultaneously with the desorption spectra in a technique called thermogravimetric analysis-mass spectrometry (TGA-MS).

TPD is a commonly used technique for characterization of surface oxygen complexes.⁸⁰ These species are often created by reaction with oxidizing gases (O₂, O₃, and CO₂) or by treatment with aqueous solutions of HNO₃, H₂SO₄, or H₂O₂. Such treatments afford CNFs with surface functionality that can be utilized in biofunctionalization, catalyst support, or composite applications. Upon heating the oxidized carbon at a constant rate, the evolved gases of H₂O, CO₂, and CO are determined using a mass spectrometer. With an appropriate deconvolution procedure, TPD can be used to characterize both the type and amount of surface oxygen complexes on CNFs. In general it is assumed that carboxylic acids and lactones result in CO₂ peak only, while carboxylic anhydrides produce both CO and CO₂ peaks.

Zhou et al. combined XPS and FTIR for disambiguation of TPD spectra obtained from gas and liquid-phase oxidized CNFs.⁸¹ The TPD spectra, shown in Fig. 23, have overlapping peaks of desorbed carbon oxides decomposed from different oxygen complexes. In their work they offer a deconvolution method that uses peak assignment (for similarly treated nanofibers based on peak temperature, T_m): including 280  °C for carboxyl, 460  °C for carboxylic anhydride, 520  °C for peroxide, 570  °C for hydroxyl, 620 and 720  °C for lactones, 660  °C for ether or carbonyl, 790  °C for carbonyl, and 930  °C for pryrone-type structures. Other example work, by Toebes et al., used TGA-MS to demonstrate that liquid oxidation in mixtures of HNO₃ and H₂SO₄ changes the surface by opening the inner tubelike cavities of the fibers.⁵⁴ They also concluded that the total oxygen content of the oxidized nanofibers is much higher than what can be attributed to the surface alone, suggesting that some oxygen is bound 2–3  nm deep within the subsurface.

Figure 23.

Another common application of TPD is the characterization of carbon nanostructures for hydrogen storage.^20,82,83,84 Dillon et al. used TPD to show that hydrogen condensed inside SWCNTs under conditions that do not induce adsorption within a standard mesoporous activated carbon, which shows the potential of CNTs as effective media for hydrogen storage.⁸⁴ Chambers et al. applied TPD to confirm the presence of chemisorbed hydrogen in CNFs after exposure to hydrogen at room temperature and high pressure.²⁰ In this hydrogen storage study they found an observable difference between stored and recoverable volumes of hydrogen at room temperature.

The atom probe is an instrument that combines both a probe-aperture field ion microscope (with atomically high resolution) and a mass spectrometer (with single particle sensitivity). Over the years, the atom probe has evolved to include the unique capability of atomic layer-by-layer depth profiling using time-of-flight mass analysis. The scanning atom probe (SAP) works by field evaporating surface atoms from the sample for mass analysis; thus for field enhancement purposes, protruding, high aspect ratio structures are ideal. Only recently has there been atom probe study of carbon nanostructures.^85,86 Nishikawa et al. have found that carbon specimens adsorb a large amount of hydrogen and may serve as highly efficient reservoirs. In addition, they also have found that MWCNT and SWCNT samples exhibit a significant difference in the adsorption and desorption characteristics of hydrogen.

Mass peaks from MWCNTs are fairly broad with a tail to the right, as seen in Fig. 24(a), indicating that the field evaporated CH cluster ions were dissociated while the ions travel from the specimen surface to the ion detector, losing neutral hydrogen atoms along the way.⁸⁶ The width of the peaks indicates the abundance of hydrogen. Clusters with odd numbers of carbon atoms are more common than even, reflecting the binding state in the specimen. After annealing the MWCNT sample in vacuum for 10  min at 1000  °C [Fig. 24(b)], the peaks become slightly sharper but the sample still contains large amounts of hydrogen and contaminants.

Figure 24.

Mass peaks from a rod made of SWCNTs, measured immediately after introducing the sample into the vacuum [Fig. 24(c)], exhibit a wide tail to the right side of the peaks, again implying that the dissociation of CH clusters occurred prior to entering the mass spectrometer and an abundance of hydrogen.⁸⁶ However, after holding the sample in the vacuum over a 40  h period [Fig. 24(d)], the initially high hydrogen level was significantly reduced, implying that the SWCNT specimen has considerably lower activation energy for hydrogen adsorption and desorption as compared to the MWCNT sample. In fact, even though CVD diamond has a higher hydrogen to carbon ratio, the CH binding in SWCNTs proved weaker than that in diamond. Furthermore, Watanabe et al. coupled the mass spectra with FE data from SWCNT samples to conclude that the work function of the SWCNTs is lower than that of other carbon materials and increases two- to threefold with the removal of adsorbed hydrogen.⁸⁵

Improvement of the SAP technique in the future may open up new approaches to analyzing the properties of carbon nanomaterials at the atomic scale, which cannot be explored by conventional techniques such as AES and SIMS.

Controlling the surface chemistry of CNFs is critical to defining their functionality. Whether being used for microfluidic or intracellular devices, the surface charge, hydrophobicity, and chemical reactivity of CNFs can be altered through both physical and chemical modifications, as detailed in this chapter. Contamination, poor solubility, and chemical inertness of carbon nanostructures have been obstacles to their use in applications ranging from biosensors to composite materials. Surface modification and functionalization techniques are necessary for resolving these issues. In the literature it can be seen that surface coatings (Sec. III A s3B) not only improve the mechanical strength and chemical stability of CNFs but also add functionality such as variable conductivity/electrical isolation or the ability to selectively activate certain regions on the surface through microfabrication routes. A second method, covalent attachment of functional groups (Sec. III C), is commonly used to increase wettability, dispersiblility, and surface reactivity of CNFs, enabling further biochemical functionalization (Sec. III D).

Deposition of a thin film coating on the external surface of a CNF provides a means to impart additional properties and functionality to the nanofiber. These properties may be far beyond the scope of those attainable with the carbon nanostructure alone, thereby making coatings an extremely useful component. A diverse set of material coatings has been deposited successfully on CNF substrates including dielectrics, metals, and polymers. Because of the high aspect ratio and the limited line-of-site access to the CNF bases, the most common approaches to thin film coatings for nanofibers are CVD routes. This section discusses both in situ and postsynthesis deposition of inorganic and polymer coatings as they are applied to CNF surfaces.

The processes involved in the catalytic synthesis of carbon nanostructures not only control the internal graphitic structure,¹⁵ but also significantly affect the condition of the nanofiber surface. This is especially pronounced in the PECVD nanofiber growth process, as plasma activation of gas species is frequently used for noncatalytic thin film deposition. Some degree of sidewall deposition is practically unavoidable in PECVD processes involving high aspect ratio structures, thus minimizing sidewall deposition requires significant effort if clean, graphitic surfaces are desired. There are essentially two types of thin films that get deposited on carbon nanostructure sidewalls: (1) carbon films from the source gas,^67,68,87,88 as mentioned in Sec. II C 3 and (2) compounds of redeposited substrate material, for instance silicon, which can react with the nitrogen from the ammonia etchant gas to form Si_xN_y coatings on the surface.^66,70,89

The sidewall deposition of carbon films was first reported by Chen et al. in a high temperature, high pressure, hot filament PECVD process.⁸⁷ They observed formation of conical structures, shown in Fig. 25, which had graphitic carbon branches due to sidewall deposition during growth. Amorphous carbon coatings have also been realized on the nanofiber sidewalls and substrate.^67,68,88 If the carbon source gas ratio is increased relative to the etchant gas, then carbon condenses on the walls of the fibers and substrate simultaneously along with the catalytic vertical growth of the CNF as in Fig. 26. By changing the source/etchant gas ratio, the cone angle can be controlled.⁸⁸ This effect is most pronounced for sparse arrays compared to dense forests of nanofibers where geometric shielding becomes a factor.⁹⁰

Figure 25.

Figure 26.

The second type of sidewall deposition occurs in a more etching regime (higher NH₃ flow), in which amorphous carbon is prevented from condensing. In this regime the substrate, unprotected by carbon film, is etched by the plasma species and the etch products redeposit on the sidewalls of growing CNFs. In the case of CNF growth on silicon substrates using a C₂H₂/NH₃ mixture it was determined that Si_xN_y deposits with x and y around 0.5.^18,89 Thus there is a delicate balance to the gas ratios used in regard the desired surface condition; for CNFs without an amorphous carbon coating a C₂H₂:NH₃ ratio of 20% or below must be used;⁶⁸ however for ratios lower than this, sidewall deposition of Si_xN_y material becomes more favorable.

Other types of in situ surface modification include the control of graphene edge termination groups or the doping of the graphene structure itself. In situ nitrogen doping has been recently reviewed by Ewels and Glerup.⁹¹ The doping can be achieved in a catalytic CVD process by incorporating a nitrogen-containing gas, for example, Villalpando-Paez et al. used benzylamine (C₇H₉N) as part of ferrocene/ethanol/benzylamine mixture.⁹²

One of the most common coatings deposited on both CNFs and CNTs via CVD methods is silicon oxide.^{24,29,32,81,93,94,95,96,97} Silicon oxide films deposited on CNFs will be commonly referred to here as SiO_x since the film stoichiometry following deposition on CNF surfaces has not been specifically quantified. Indeed, the literature regarding the deposition of silicon dioxide on CNFs is limited, although SiO₂ has been deposited on SWCNTs by the decomposition of silicon tetra-acetate using a PECVD technique.⁹⁶ The surface coating composition was stoichiometric SiO₂ as determined by XPS analysis of the deposited layer 2–8  nm in annular thickness.

Silicon oxide coatings deposited on CNF surfaces by PECVD have exhibited several advantageous physical properties. Moon et al. coated MWCNTs with SiO_x and found that the thin layer prevents high temperature oxidation and thermal degradation as well as improves the field emission properties.⁹⁷ FTIR analysis of the SiO_x-MWCNT interface detected intercalated SiO, SiC, and SiOC characteristic bonding.

Thicker SiO_x coatings have also been shown to improve mechanical strength and electrically insulate CNFs from the surrounding environment for electrochemical probe or biosensing applications.^24,32 Freestanding, hollow SiO_x nanopipes have been produced by either partially or entirely removing the interior CNF scaffold by excavating the top of the oxide coating followed by reactive ion etching of the CNF in an O₂ plasma.^29,95 An example recipe for SiO_x film deposition by PECVD includes growth at a total pressure of 1  Torr under a dual gas flow of 125  cm³/min SiH₄ and 125  cm³/min NO₂ at a substrate temperature of 400  °C.⁹³ The resulting deposition rate under these conditions is ~45  nm/min. Generally, the coating deposits in a conformal fashion on CNF surfaces over short PECVD growth times of 1–10  min. Conformal deposition creates a smoother surface and retains the overall CNF geometry. In addition, the deposition rate is linear, making PECVD a simple and reproducible process. However, for a longer silicon dioxide deposition time, the final shape of the coated CNF diverges from the template CNF. Fowlkes et al. attributed this shape change to the mobility of deposited species.⁹³ The high growth temperature (T~400  °C), coupled with plasma excitation, imparts energy to deposited species, which drives their subsequent thermal migration on the evolving coating surface; the surface free energy is minimized as a result. Thick 0.5–1  µm columnar SiO_x coatings form on the exterior of the tapered, underlying CNF. In some instances, to minimize surface free energy, clustered CNFs in close proximity ultimately merge into a single, vertical SiO_x pillar for excessively long film growth times (t<30  min, thickness >1000  nm).⁹³

In this way, predominantly hydrophobic CNFs are converted to hydrophilic structures by coating the CNF surfaces with SiO_x.⁹⁴ Moreover, the deposition of silicon oxide on CNF surfaces enables silane-based chemical functionalization strategies, providing a link to biorelated applications such as turning the CNFs into active arrays for delivering biomolecules or into passive arrays where biological species are delivered to the CNFs.

One of the early reports of successful thermal CVD coating of carbon nanostructures was by Chen et al. who coated hollow CNFs with boron nitride.⁷⁴ They used a two-step process in which they initially infiltrated the CNFs with a saturated boric acid solution and subsequently nitridized the fibers in an ammonia ambient at 1100–1200  °C. Furthermore, they correlated the surface structure of the starting nanofibers that were either pyrolytically stripped versus annealed at ~3000  °C to the resultant BN coatings. They determined that the annealed fibers had a more ordered surface and resulted in BN coatings with better crystallinity.

Xia et al. introduced a process in which a homogenous distribution of secondary CNFs could be grown on CNFs via a totally gas-phase process.⁵⁸ The initial step of the process was an oxygen plasma treatment, which oxygen-functionalized the primary CNFs. Subsequently, a ferrocene-oxygen CVD process was performed and the resultant film was sintered to form FeO_x nanoparticles on the primary CNF surface. Finally, a uniform distribution of secondary “nanobranches” of CNFs was catalytically grown by thermal CVD from the FeO_x nanoparticles in a reducing H₂/cylcohexane atmosphere.

As a final example, Abdi et al. recently coated CNFs with TiO₂ using an atmospheric pressure CVD process.⁹⁸ While the particular CVD processing details were not divulged, the specific application they were developing was a FE lithography source in which they used an elaborate process: (1) initially CNFs were grown using Ni catalysts, (2) they subsequently coated the CNFs with a conformal CVD TiO₂ film, (3) then they polished the tips of the TiO₂-coated CNFs, and finally (4) etched the emancipated tips in an oxygen plasma. The coated and etched nanofibers were successfully used to expose a line in an electron beam resist.

Several groups have also coated CNTs and CNFs with polymer thin films using CVD routes. Shi et al. has used a plasma-induced polymerization process coupled with a fluidized bed reactor to coat CNTs with ultrathin polymer films.⁷⁹ Their initial work demonstrated pyrrole coatings on hollow CNFs, where careful control of the plasma parameters enabled them to deposit coating thicknesses in the range of 2–7  nm. Later using a similar plasma polymerization process they coated VACNFs with a carbon-flourine polymer.⁷⁸ Dhindsa et al. uniformly coated patterned VACNFs with parylene and compared their electrowetting characteristics to aluminum oxide coated nanofibers described in Sec. III A 4.⁹⁹ Figure 27(a) shows a SEM image of a patterned CNF template which has been uniformly coated with parylene.

Figure 27.

Atomic layer deposition (ALD) occurs similarly to standard CVD, except that in an ALD process the CVD reaction is broken into two half-reactions, keeping the precursor materials separate throughout the process. ALD has been used by a few research groups to coat CNTs and CNFs.^99,100,101 ALD is particularly suited for coating large area and high aspect ratio structures because it allows atomic layer thickness control over an entire surface by a sequence of self-limiting reactions. Lee et al. was first to report coating CNFs via ALD as they successfully coated bamboo-type nanofibers with Al₂O₃ using trimethylaluminum and distilled water.¹⁰¹ They demonstrated uniform and controlled growth of amorphous Al₂O₃ on the outside of the fibers and also demonstrated that the interior cavities of the fibers could also be coated to varying degrees. Later, Herrmann et al. also coated CNTs with Al₂O₃ and Al₂O₃/W bilayer via ALD.¹⁰⁰ The Al₂O₃ was deposited using a similar process to that of Lee et al. and the tungsten ALD layer was accomplished by alternating cycles of WF₆ and Si₂H₆. In both cases, exceptional thickness control and uniformity were demonstrated and applications of the coaxial CNT/Al₂O₃/W/Al₂O₃ structure, such as a miniature patch clamp device, were suggested. Furthermore, the Al₂O₃ coating was functionalized with a hydrophobic monolayer to demonstrate the utility of the ALD coating for subsequent surface functionalization. These different functionalization approaches are schematically illustrated in Fig. 28 along with a TEM image of a CNT/Al₂O₃/W/Al₂O₃ composite structure. As a last example, Dhindsa et al. also coated arrays of VACNFs with an ALD Al₂O₃ layer and demonstrated controlled and reversible electrowetting on the coated CNF scaffold, shown in Fig. 27(b).⁹⁹ Contact angle measurements and capacitance measurements were performed to illustrate the electrowetting behavior.

Figure 28.

The advent of electrically addressable VACNF material has enabled a multitude of electrochemical modification strategies. Guillorn et al. utilized the electrodeposition of gold to decorate and thereby validate that only the extreme tips of sidewall-passivated nanofibers were electroactive, and that the sidewalls and underlying interconnect structures were adequately passivated (by methods mentioned in Sec. III A 2).³² Melechko et al. demonstrated the electrodeposition of gold upon VACNF material that was deeply recessed into a nanopipe structure.⁹⁵ In this process, arrays of high aspect ratio tubular cavities were produced by coating VACNFs with silicon oxide and subsequent etching the carbon-VACNF material from within the pipe. Electrodeposition of gold within the pipe demonstrated that the interior of the nanostructure was wetted and viable for electrochemical methods.

Electrochemical deposition of Ni has been demonstrated on the surface of herringbone-type carbon material to produce a vertical nanotube heterojunction.¹⁰² Ni electrodeposition was conducted in 0.10M sulfuric acid containing 0.10M Ni(NO₃)₂ at −0.7  V versus a silver quasireference electrode. Ye et al. have employed the electrodeposition of molybdenum oxide onto aligned MWCNTs for the electrocatalytic reduction of bromate.¹⁰³ Ngo et al. demonstrated that copper could be preferentially electrodeposited in the interstitial spaces between vertically aligned MWCNTs on an addressed substrate, thereby providing mechanical support and improved heat dissipation for a MWCNT heat transfer material.¹⁰⁴ “Gap filling” of the Cu was achieved by passivating the CNF material with polyethyleneglycol (PEG) during the electrodeposition of the metal.

Extremely high surface area metallic electrodes have been demonstrated by Metz et al. using electroless deposition of gold onto herringbone-type VACNFs as shown in Fig. 29.¹⁰⁵ Exposed graphite planes of the vertical carbon material were first functionalized with carboxylic acid sites using photochemical capture and deprotect of the methyl ester of undecylinic acid under nitrogen-purged 254  nm irradiation. The resultant COOH sites were then decorated with electrolessly deposited gold by tin sensitization, silver activation, and gold deposition. Gold coverage on the nanofibers ranged from sparse, <20  nm nanoparticles to dense spikelike nucleations based on immersion time in the gold electroless bath (1–22  h).

Figure 29.

Electropolymerization of pyrrole has been utilized by several groups for dramatically different applications. Chen et al. demonstrated that polypyrrole (PPy) could be electropolymerized from an acidic monomer solution as a conformal, electronically conductive film over the entire length of vertically aligned carbon material for potential use as high performance electrode in rechargeable batteries.¹⁰⁶ Nguyen-Vu et al. employed this same modification on VACNF surfaces to both increase the mechanical stability of herringbone nanofiber electrodes as well as to improve biocompatibility with neuronal cells.³⁴ Fletcher et al. have also polymerized PPy on VACNF electrodes as a means of controlling the spacing and pore structure of nanofiber-based microfluidic structures.¹⁰⁷ Lastly, McKnight et al. have demonstrated photoresist-based templating strategies for achieving localization of polypyrrole along the vertical length of nanofiber electrodes, ultimately for application to their nanofiber-based gene delivery methodologies.^11,26

Graphitic materials, when they are in their pristine state and free of surface oxygen groups, have a hydrophobic nature; however, following routine activation procedures these materials can exhibit some degree of hydrophilic character.¹⁰⁸ The chemical stability of CNFs allow their surface chemistry to be controlled with variable wettability; both hydrophobic^40,109,110 and hydrophilic^40,48,73,111 surfaces have been demonstrated. Chemical, biochemical, and electrochemical functionalizations of VACNF material are often preceded by an oxidation step in which amorphous material is removed and oxygen-containing moieties are generated on the surface; most commonly noted are carboxyl surface terminus species.^{10,26,28,48,49,51,111,112}

Based on the recalcitrance of low defect-harboring CNT material, early attempts with carbon nanostructure functionalization employed aggressive acidic pretreatment methods. For instance, Ros et al. surveyed several surface oxidation methods and concluded that a mixture of concentrated nitric and sulfuric acids was the most effective method for attaching oxygen-containing surface groups to defect sites on the nanofiber surface.⁴⁸ IR study has shown that the creation of oxygen-containing surface groups occurs at graphitic defect sites and that oxidation process proceeds via carbonyls and other oxides to carboxy and carboxylic anhydride groups.

Sato et al. used an aggressive purification/functionalization procedure to investigate the cytotoxic factors affecting cell activation on functionalized herringbone CNFs.¹¹³ First the CNFs were pyrolyzed in air then introduced to 6M HCl to dissolve the Ni catalyst material, followed by an anneal in vacuum and finished with ultrasonication in a 3:1 mixture of H₂SO₄ and HNO₃. This lengthy treatment effectively removed all Ni and generated carboxyl surface groups, which were then functionalized with 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS). While the water-soluble CNFs did not affect cell viability, they found that the functional group on the surface of the nanofibers was a significant cytotoxic factor affecting cell activation.

Lim et al. noted that HNO₃ treatment helped to recover the free edges previously closed by annealing platelet-type, graphitized CNFs.¹¹⁴ Similarly, Li et al. derivatized CNFs by treatment in concentrated nitric acid at 140  °C for 4  h.^22,50 However, it must be conveyed that exfoliation under such harsh conditions not only leads to oxidation but can also degrade the electrical and mechanical properties of CNTs. Even for herringbone/bamboo-oriented carbon, Nguyen et al. noted that acidic pretreatments often compromise the mechanical integrity of the vertically aligned material.¹¹⁵ Therefore they employed spin-on glass to enhance the mechanical stability of their VACNF samples. Subsequent work by this group with electrically addressable carbon arrays has alternatively employed an electrochemical etch in 1M NaOH (1.5  V, 1–2  min) to provide both VACNF sidewall etching and carboxylic acid sites.^25,116

For MWCNT material with a low density of sidewall defects, acidic pretreatment can be used to selectively generate COOH and OH groups at nanotube tips, due to a higher defect density and reactivity at these sites. Taft et al. employed this selectivity to site specifically functionalize MWCNT material with different oligonucleotides at the tips versus on the sidewalls.¹¹⁰

Early stuides by Esumi et al. on surface treatments of mesocarbon microbeads found that oxygen plasma oxidized the carbon surface, resulting in the formation of acidic groups; in contrast, nitrogen and ammonia plasma treatments produced basic functionalities.¹¹⁷ Plasma etching has commonly been used to provide COOH sites for both changing the hydrophobicity of nanofiber samples and for subsequent functionalization. He and Dai reported the use of an acetic acid plasma to provide COOH functionality,¹¹⁸ while and Fletcher et al. and McKnight et al., among others, reported the use of an oxygen plasma etch prior to further functionalization steps.^10,26 Paredes et al. observed reorganization of the CNF surface after oxidation treatment and concluded that exposure to oxygen plasma leads to a significant and uniform increase in the atomic-scale disorder of the nanofiber surface.⁴¹

Bubert et al. conducted a comparative study investigating the formation of functional surface groups on CNFs from low pressure plasma treatment using various gases, flow rates, pressures and powers.⁵⁶ From XPS analysis of the nanofiber surface they inferred that there was an ~1  nm functionalized layer composed of mainly hydroxyl, carbonyl, and carboxyl groups. Bubert et al. also noted that after the initial functionalization of this layer, further plasma treatment did not enhance the oxygen content, but rather changed slightly the proportions of the functional groups. Additionally, they concluded that the kind of oxygen source gas, whether O₂ or CO₂, did not appear to make a considerable difference in the resultant surface composition.

Chen et al. have developed a novel approach for chemical modification of aligned CNTs by carrying out radio frequency glow-discharge plasma treatment, and subsequent reactions characteristic of the plasma-induced surface groups.¹¹⁹ In the first of two reaction schemes, they reported the surface immobilization of polysaccharide (amino-dextran) chains onto acetaldehyde plasma-activated MWCNTs through the formation of Schiff-base linkages, which were further stabilized by reduction with sodium cyanoborohydride. In a second approach, the nanofiber surfaces were first aminated using ethylenediamine plasma, followed by Schiff-base formation with aldehyde groups of periodate oxidized Dextran-FITC. Again, the immobilized material was reductively stabilized with sodium cyanoborohydride.

A technique that allows for the UV-induced reaction of H groups on the nanofiber sidewalls with alkene terminated molecules has been developed by Baker et al.⁹ In their work CNFs having both COO and NH functional groups were produced using protected forms of acid and amine terminated molecules (methyl ester protected undecylenic acid and tBOC-10-aminodec-1-ene, where tBOC=tert-butyloxycarbamate). Solutions of the protected molecules were incubated with freshly grown CNFs and illuminated under nitrogen-purged 254  nm light for 16  h. Deprotection of the tBOC protected surfaces was carried out using a treatment of 1:1 trifluoroacetic acid: methylene chloride for 1  h, immersed in 10% ammonium hydroxide, and rinsed with de-ionized (DI) water. Deprotection of the methyl ester protected surfaces was carried out by immersion in a slurry of 250  mM potassium t-butoxide in dimethyl sulfoxide for 6  min. This treatment was followed by a rinsing in 0.1M hydrochloric acid and DI water. In these reactions it has been reported that the use of fibers immediately following growth provides optimal results. Baker et al. attribute this to the “highly reducing, hydrogen-rich” environment of the CNF growth chamber.

Metz et al. have employed this same photochemical functionalization technique to achieve extremely high coverage of COOH upon VACNF material as demonstrated in Fig. 29.¹⁰⁵ Herringbone nanofibers with high hydrogenation were reacted with the methyl ester of undecylenic acid using 254  nm irradiation for 16–18  h. Then they converted the methyl ester groups to carboxylic acid, yielding CNFs functionalized with carboxylic acid groups. The resultant COOH sites were electrolessly plated with gold as shown in the last step of Figs. 29(a)29(b). They concluded that this method combining photochemical oxidation of the fibers with electroless gold deposition increased tenfold the electrically active surface area of the CNF template, amounting to a hundred times higher than the planar substrate.

Annealing at high temperatures in inert environments has traditionally been used to remove oxygen complexes and defects to increase the graphitic order of CNFs.^{44,81,114,120} However, thermal treatment in oxidizing atmospheres has also been reported for removal of amorphous material and oxidation of graphitic material. Combustion of thermal CVD carbon nanofiber products has been used by Sato et al. as a purification step,¹¹³ while Wang et al. heated a MWCNT array to 400  °C in air in order to increase hydrophilicity via generation of surface oxides and defect sites on the sterically aligned material.¹²¹ Chen et al. and Naguib et al. compared pyrolytically stripped CNFs to those that had been annealed at 3000  °C.^40,74 The pyrolytically stripped CNFs possessed a disordered, turbostratic surface due to the thermal etching, whereas the high temperature annealed fibers formed energetically and chemically stable closed graphitic edges on their surface.⁷⁴ Furthermore, CNFs that were pyrolytically stripped in carbon dioxide exhibited hydrophilic behavior while the elimination of chemically active dangling bonds by vacuum annealing led to hydrophobic behavior.⁴⁰

The covalent addition of linker molecules and polymers is another way to enhance functionality. Li et al. derivatized CNFs with concentrated nitric acid at 140  °C for 4  h and then acylated the surface-oxidized fibers using thionyl chloride for 24  h.⁵⁰ After acylation, various amine compounds were then surface bound to the fibers. In subsequent work, CNFs were functionalized to render hydrophobic or hydrophilic graphitic CNF polymer brushes.²² Following oxidation in nitric acid at 400  °C, thionyl chloride was used to acylchlorinate the surface, which was in turn reacted with either (4-hydroxymethyl)benzyl-2-bromopropionate (HBBP) or 2-hydroxyethyl-2-bromoprionate (HEBP) to provide surface bound radical initiators. Atom transfer radical polymerization (ATRP) was then employed with acrylate and methacrylate esters to generate hydrophobic composite nanofiber-polymer brush materials. Subsequent ester hydrolysis of the latter could also be used to hydrophilize the material, demonstrating tailored control of the material hydrophobicity without significant alteration of the structural integrity of the underlying nanofiber material.

Zhong et al. similarly activated nitric acid-etched nanofibers with thionyl chloride for reaction with 3–4-oxydianiline (ODA) to provide pendant free amino groups.²³ Extension of the ODA linker molecule with a reactive diluent, butyl glycidyl ether, and incorporation into epoxy resin provided nanofiber/epoxy nanocomposites with enhanced flexural strengths, exemplifying the utility of linker molecules in creating composite materials. Wang et al. also employed thionyl chloride with acid oxidized nanofibers to generate hydroxylated nanofiber feedstock, by reaction of the acylchloride functionalized nanofibers with glycol at 120  °C for 48  h.¹²² Biodegradable poly(caprolactone) (PCL) was then covalently grafted onto the CNF-OH feedstock using in situ ring opening polymerization (ROP). Oh et al. also utilized a covalent grafting technique to attach polyetherketones to the surfaces of both MWCNTs and CNFs treated with phosphoric acid.⁵² Additional work, mentioned in Sec. II B, demonstrated the covalently bonding of oxidized MWCNTs to alkanethiolate-coated gold nanoparticles, where the organic coating was later removed from the gold nanoparticle-MWCNT composites by a thermal anneal at 300  °C.⁵¹

Preparation of VACNFs for bioapplications requires a high level of control over their surface chemistry. Whether being used to modify molecular transport or act as a platform for biomolecule immobilization, the surface charge, hydrophobicity, and chemical reactivity of CNFs can be tailored through both chemical and physical modifications, as detailed in Sec. III A s3B s3C. Most initial strategies for biomolecule immobilization focus on the decoration of surfaces with prescribed densities of COO or NH functional groups. This provides researchers with reasonable control over surface charge and chemical reactivity, allowing either passive adsorption or covalent cross-linking strategies to be used for biomolecule immobilization.

In addition to changing the morphology of nanofibers and nanofiber assemblies, physical modification of the CNF surfaces via coating with gold (Sec. III B) or silicon dioxide (Sec. III A 2) can be an effective strategy for making fibers amenable to specific chemical modification strategies using well-established thiol and silane chemistries. However, some sensing applications benefit from using native CNF graphitic surfaces for biomolecular immobilization. In these cases, control of surface chemistry can be achieved during growth (Sec. III A 1) or through the use of postgrowth modification techniques (Sec. III C).

Treatment of CNFs with an oxygen plasma or wet etch produces COO groups on the surface, which can then be modified using conventional carbodiimide strategies and commercial reagents. As an alternative to oxygen plasma treatment, the native H groups that terminate graphene edges on the fiber surface have been modified using photochemical and electrochemical reactions to yield COO and amine terminated surfaces that can be modified using conventional cross-linking and adsorption strategies.^8,9,24,105 Such techniques have been used for both DNA and protein immobilization. However, contaminates and materials that redeposit on the nanofiber sidewalls can limit the degree to which functional groups are distributed over the surfaces.

Protein attachment can provide an efficient way to evaluate the biologically accessible area of a sample. Baker et al. took advantage of avidin's high affinity for the small molecule biotin to quantify VACNF surface activity.⁹ By binding a large molecule in solution (avidin, ~50  Å diameter), to a smaller molecule (biotin, ~10  Å), which is covalently tethered at an excess of binding sites on nanofiber surface, the quantity of large molecules binding to the surface from solution becomes limited by their packing density. Initially the biotin was attached to the VACNFs using a linker containing a disulfide bond, thus after the biotin groups had bound one monolayer of avidin molecules, the disulfide bond was cleaved, releasing the avidin into solution (where it can be quantitatively measured). Although the exact size of the surface-bound avidin molecule and the surface area of the VACNFs is not known precisely, this method gives a good measure of the relative surface areas of different samples and yields an absolute measure of protein molecules per planar area of the surface. Baker et al. stated that this method (for measuring the active surface area using the self-terminating adsorption of avidin onto biotin-modified VACNFs) agreed with their DNA hybridization experiments, demonstrating an eightfold higher biologically accessible surface area as compared to planar substrates.

Another study by Baker et al. involved the adsorption of cytochrome C, a redox-active protein, to treated VACNF surfaces.⁸ It was found that COO terminated surfaces were the most favorable for protein adsorption as demonstrated by significant reduction peaks in cyclic voltammograms taken from the addressable “treated” CNF electrodes. It is also conceivable that photolithographic masking could be used to carry out selective VACNF functionalization allowing for the functionalization of individual CNF chips with a collection of different biofunctional molecules.

Naguib et al. have demonstrated how differences in the surface structure of heat treated versus pyrolytically stripped CNFs significantly affect the adsorption of phycoerythrin–conjugated anti-CD3 antibody.⁴⁰ The pyrolytically stripped CNFs with disordered surfaces exhibited hydrophilic behavior. Thus as a result of improved wetting and increased protein binding capacity, Naguib et al. concluded that antibody coatings were more prolific on the pyrolytically stripped nanofibers. They also established that protein adsorption could be appreciably enhanced by pretreating dispersions of CNFs with poly-L-lysine.

Protein attachment to CNFs has also been utilized for in vivo work. Nguyen-Vu et al. combined the electropolymerization of pyrrole films with the incorporation of extracellular matrix proteins, to promote the attachment of mammalian cells.³⁴ They noted that neuronal cells only proliferated on the VACNF arrays after coating them with collagen proteins.

DNA modification of VACNFs has been implemented for both sensing and gene delivery applications. A fluorescence-based nucleic acid sensor employing bamboo VACNFs was first described by Nguyen et al. in 2002.¹¹⁵ The vertically aligned CNF material was embedded within spin-on glass to provide mechanical stability to subsequent oxidative etching of the carbon material. This etching comprised thermal treatment at 430  °C in air for 1  h followed by acidic treatment. Resultant COOH groups on the exposed CNFs were then used to capture both dye-labeled amine-terminated 21-mer oligonucleotides and unlabelled 11-mer peptide nucleic acid (PNA) oligos using the water soluble coupling reagents 1-ethyl-3(3-dimethyl aminopropyl) carbodiimide hydrochloride (EDC) and N-hydroxysulfosuccinimide (sulfo-NHS). Hybridization of complementary dye-labeled oligos to the bound PNA was verified using laser fluorescence microscopy. Li et al. advanced this effort using electrically addressable VACNF arrays to transduce DNA complementation.^25,116 A less harsh oxidative treatment was employed to generate carboxylic acid binding sites for subsequent EDC chemistry wherein oxide sidewalls insulating the CNF electrodes were electrochemically etched in 1M NaOH at 1.5  V for 1–2  min. This treatment both tailored the emergent length of the exposed carbon electrode material and introduced COOH and OH groups to the surface for subsequent DNA capture. Subattomole DNA detection limits were observed for complementary unlabeled targets using [Ru(bpy)₃]²⁺ mediated guanine oxidation.

Parallel work by He and Dai demonstrated DNA hybridization detection using the oxidation signal of ferrocene (FCA) on FCA-labeled target strands.¹¹⁸ The approach taken was similar; however, as opposed to electrochemical oxidation of the electrode in NaOH, an acetic acid plasma treatment was used to generate COOH binding sites.

Taft et al. investigated the immobilization of amine-terminated oligos onto the tips and sidewalls of large-diameter CNTs templated using an aluminum oxide nanoporous membrane.¹¹⁰ A two part functionalization strategy was employed to provide tip-specific and sidewall-specific alteration of the vertically aligned material. Acid pretreatment (50% sulfuric/nitric acid, 3:1 in H₂O, 1  h) introduced COOH groups to defect sites predominantly located at the open tips. Amine-terminated DNA was condensed to these sites using an organic phase cross-linking reaction (TFTU/DEAI in DMF). Subsequently, the less-reactive sidewalls of the MWCNTs were functionalized by absorption of pyrene-terminated oligos. SEM imaging of these structures, following incubation with complementary oligos labeled with different sized gold nanoparticles, indicated that there was little crosstalk between the two functionalization schemes, attributed again to the presence of defect sites at the open tips of the CNT material.

In pursuit of orthogonal or sequential functionalization of spatially separated nanofiber bundles Lee et al. used diazonium chemistry, individually addressable electrodes, and selective reduction of nitro to amine groups in order to demonstrate the functionalization of particular nanofiber bundles with different DNA sequences.¹²³ The graphene sidewalls of addressed nanotube/nanofiber arrays were first modified with aromatic nitro groups by 24  h immersion of the array in 4-nitrobenzenediazonium tetrafluoroborate and sodium dodecyl sulfate. Electrochemical reduction was then employed at discrete electrodes to reduce nitro groups to primary amines. Thiolized oligonucleotides were subsequently captured at aminated electrodes using the heterobifunctional cross-linker, sulfosuccinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate (SSMCC). Successful attachment of distinct oligo probes at various electrodes was validated by capture of fluorescently tagged complimentary oligonucleotides. In additional work by the same group, Baker et al. carried out the sequential, electrochemically addressed functionalization of CNFs by reducing nitrophenyl groups on different electrodes and exposing the amines to sulfo-SMCC and thiol modified DNA strands.⁹ Throughout their work the group relied both on IR spectroscopy techniques and quantitative fluorescence measurements to confirm fiber modification and deprotection. Their measurements show that both photochemical and diazonium functionalization routes yielded DNA-modified CNFs exhibiting excellent specificity and reversibility in binding to DNA probes. In addition, the eightfold greater hybridization of DNA to the nanofiber samples as compared to planar substrates demonstrated that most of the DNA was bound to the sidewalls of the nanofibers.

Using dense arrays of VACNFs, Fletcher et al. illustrated relatively homogenous functionalization of the tips and sidewalls of oxygen plasma etched samples following EDC-condensed capture of amine terminated oligos, as shown in Fig. 30.¹⁰ Confocal microscopy following incubation with complementary dye-labeled oligos presented fluorescent response along the entire length of 4  µm tall nanofibers, putatively due to the presence of COOH and oligo capture along the entire length of the herringbone-structured fibers.

Figure 30.

McKnight et al. demonstrated the capture and transcriptional activity of larger (5081  bp) double stranded DNA sequences on VACNF arrays.^26,28 Periodic arrays of VACNFs at 5  µm pitch were oxygen plasma etched and functionalized using an overnight incubation of plasmid DNA and EDC in acidic buffer (MES, 2-morpholinoethanesulfonic acid, pH 4.5–5.0). Following extensive rinsing, nanofibers remained functionalized with covalently bound, active full-length promoter/gene sequences, as evidenced by expression of fluorescent proteins encoded by these genes, following penetration of the nanostructured arrays into mammalian cells. Quantitative analysis of the amount and transcriptional activity of tethered DNA was subsequently documented by Mann et al. using quantitative polymerase chain reaction (PCR) and in vitro transcription bioassay.²⁷

The control of material properties at the nanoscale lies at the very crux of nanotechnology, and for the many applications involving interfaces, the critical behaviors are driven by surfaces. Detailed characterization of surface structure and composition are an integral part of understanding the properties of CNFs and how they can be modified to enable diverse applications. This literature review examined carbon nanostructure characterization via various microscopy, spectroscopy, and spectrometry techniques. Examples from the current literature pertaining to CNF surface studies were discussed in depth. It has been shown that each surface characterization technique offers unique advantages and disadvantages and we emphasize the importance of coupling complementary information from both spatially averaged and high-resolution techniques whenever possible to get a clear representation of the surface structure and properties. Additionally, in situ and postsynthesis CNF functionalization methods were reviewed, from CVD thin film coatings to biomolecule attachment. This control over the surface chemistry of CNFs has significantly broadened the application space.

The future challenge of surface characterization lies in understanding the functionality of nanomaterials. This requires tools that not only probe nonflat and nonperiodic structures, but also probe the operational environment in which the nanomaterial functions. More specifically, for a better understanding of CNFs and their interactions with biological systems (such as proteins and nucleic acids), there is still a need for high-resolution imaging at the nanoscale using analysis tools that allow for straightforward sample preparation and in situ observation.

A central issue relating to CNF research involves understanding how changes in the atomic-level structure of nanofibers affect their surface chemistry and their electrochemical properties. In particular, for CNFs, the ratio of edge planes versus basal planes and the degree of graphitic order is important to obtaining a better understanding of the physical and chemical properties of this diverse group of nanomaterials. One important challenge that remains is the implementation of controlled synthesis methods that allow for the selection of appropriate graphitic structure specific for the desired function. In addition, ongoing development of new chemical functionalization methods will continue to advance the integration of nanofibers with a variety of molecular, biomolecular, and electrocatalytic materials.

M.L.S. and A.V.M. acknowledge support from the Division Material Sciences and Engineering of the DOE Office of Science. K.L.K., J.D.F., S.T.R., and P.D.R. acknowledge support from the Center for Nanophase Materials Sciences. T.E.M. was supported in part by the National Institute for Biomedical Imaging and Bioengineering Grant No. R01EB006316. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities.

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Fig. 1. (Color online) Schematic of carbon nanostructures: (a) single sheet of graphite, (b) CNT consisting of concentric graphene sheets, and (c) CNF composed of stacked graphene cones at an angle alpha with respect to the axis of the fiber. The two primary CNF structures: (d) herringbone-type CNF and (e) bamboo-type CNF. (f) Representative VACNF composed of a Ni catalyst nanoparticle at the tip and a graphitic fiber body. [(a)–(e)] Adapted with permission from Ref. 5. First citation in article

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Fig. 2. Atomic-scale STM images of a CNF surface (a) before and (b) after oxygen plasma treatment. Reprinted with permission from Ref. 42, Copyright 2003 American Chemical Society. First citation in article

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Fig. 3. Typical noncontact tapping mode line profile of a phase image taken in the direction perpendicular to the axis of (a) an untreated “fresh” fiber on HOPG and (b) a fiber exposed to 5  min of oxygen plasma on HOPG. Reprinted in part with permission from Ref. 42, Copyright 2003 American Chemical Society. First citation in article

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Fig. 4. TEM of untreated (a) fishbone (herringbone) and (b) parallel (bamboo) CNFs with the corresponding IR spectroscopy results below. Adopted with permission from Ref. 47, Copyright 2002 Blackwell Publishing. First citation in article

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Fig. 5. IR spectra of untreated (CNF-U), oxidized (CNF-OX), anthranilic-acid-treated (CNF-AA) CNFs, and of a physical mixture of anthranilic acid and CNFs (AA-phys). Adopted with permission from Ref. 49, Copyright 2002 Blackwell Publishing. First citation in article

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Fig. 6. XPS sputter depth profile of a CNF treated in Ar/O plasma for 10  min. The table lists XPS atomic composition results for six nanofiber samples treated with different plasma conditions. The values in parentheses at the top of the table are the mean binding energies of the fit lines, given in eV. Reprinted with permission from Ref. 56, Copyright 2002 Elsevier. First citation in article

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Fig. 7. XPS results: (a) Fe 2p and (b) O 1s spectra of iron oxide coated CNFs, with (1) as-synthesized and (2) heated to 700  °C in hydrogen and helium (followed by exposure to air). Summary of surface compositions is summarized in the table. Reprinted in part with permission from Ref. 58, Copyright 2003 American Chemical Society. First citation in article

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Fig. 8. UPS He  II valence band spectra of the CNTs (solid line) and graphite (dotted line). Reprinted with permission from Ref. 59, Copyright 1999 American Physical Society. First citation in article

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Fig. 9. UPS spectra of (a) HOPG, (b) purified MWCNT film, (c) air oxidized MWCNT film and (d) plasma oxidized MWCNT film, with He  II 40.8  eV. Arrows indicate pi-derived density of states. Adopted with permission from Ref. 61, Copyright 1999 Elsevier. First citation in article

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Fig. 10. C KVV Auger spectra of HOPGn at normal emission and MWCNT, SWCNT, fullerene, quarterphenyl, and HOPG at an emission angle of 5°. Reprinted with permission from Ref. 63, Copyright 2005 Elsevier. First citation in article

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Fig. 11. TEM images of MWCNT samples (a) before and (b) after irradiation. (c) AES carbon KLL spectra from the nanotube samples is shown (A) before irradiation, (B) after 30  min sputter time, (C) after 210  min sputter time, and (D) graphite carbon. Adopted with permission from Ref. 64, Copyright 1999 Elsevier. First citation in article

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Fig. 12. SEM images of cylindrical CNFs deposited using (a) 20% C₂H₂:NH₃ gas ratio and (b) conical CNFs deposited using 75% C₂H₂:NH₃ gas ratio. Auger chemical composition analysis is presented in (c), where (1) is from the head of the cylindrical CNF in (a), (2) is from the body of the conical CNF in (a), (3) is from the head of the conical CNF in (b), and (4) is from the body of the conical CNF in (b). (d) Summary of Auger depth profiles of the substrate surface at various C₂H₂:NH₃ ratios. Adopted with permission from Ref. 68. First citation in article

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Fig. 13. SEM images of plasma-treated CNFs (a) before and (b) after a 5  min growth of secondary nanofibers. Reprinted in part with permission from Ref. 58, Copyright 2003 American Chemical Society. First citation in article

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Fig. 14. SEM images of CNFs grown from Cu-Ni alloys with (a) 81% Ni, (b) 39% Ni, and (c) 20% Ni. Adopted with permission from Ref. 70, Copyright 2005 Elsevier. First citation in article

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Fig. 15. (Color online) EDX line scan showing the elemental composition along the length of a conical nanofiber (grown from Cu₈₀Ni₂₀ catalyst) shown in the SEM image on the right. Reprinted with permission from Ref. 70, Copyright 1999 Elsevier. First citation in article

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Fig. 16. (Color online) EDX spectra (a) from the body of an argon treated fiber (b). FE measurements (c) of typical J-E curves for VACNFs treated with Ar plasma for various times. Adopted with permission from Ref. 71. First citation in article

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Fig. 17. STEM images and cartoons of the nanofiber outer wall structure: [(a) and (b)] using a Ni catalyst and [(c) and (d)] using a Pd catalyst. The white arrows in the STEM images point towards the catalyst particle at the tip of the fiber. Adopted with permission from Ref. 72. First citation in article

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Fig. 18. HRTEM images showing the surface morphology differences between (a) heat treated and (b) pyrolytically stripped, hollow CNFs. Adopted with permission from Ref. 40, Copyright 2005 Institute of Physics. First citation in article

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Fig. 19. (a) TEM image of Au-nanoparticle CNF composites after thermal activation at 300  °C. (b) HRTEM image showing a single nanoparticle on the CNF surface. Reprinted with permission from Ref. 51, Copyright 2004 American Chemical Society. First citation in article

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Fig. 20. (a) EELS analysis of a BN-coated pyrolytically stripped hollow CNF and (b) HRTEM image of a heat treated BN-coated hollow CNF. Adopted with permission from Ref. 74, Copyright 2004 Blackwell Publishing. First citation in article

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Fig. 21. (a) Surface hydrogen concentration as measured by D-SIMS and (b) FTIR spectra of a 1  MeV proton bombarded SWCNT film. Reprinted with permission from Ref. 75, Copyright 2003 American Chemical Society. First citation in article

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Fig. 22. TEM images of hollow CNFs (a) before and (b) after pyrrole deposition. Time-of-flight SIMS spectra of (c) untreated and (d) polymer treated CNFs. Adopted with permission from Ref. 79. First citation in article

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Fig. 23. (Color online) Temperature-programed desorption spectra of from CNFs treated with HNO₃ for 12  h at room temperature for (a) CO and (b) CO₂. The peak temperature (T_m) of the different types of surface oxygen complexes used in fitting were 280  °C for carboxyl, 460  °C for carboxylic anhydride, 520  °C for peroxide, 570  °C for hydroxyl, 620  °C and 720  °C for lactones, 660  °C for ether or carbonyl, 790  °C for carbonyl, and 930  °C for pyrone-type structures. Reprinted with permission from Ref. 81, Copyright 2007 Elsevier. First citation in article

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Fig. 24. Mass spectrum of (a) MWCNT initially, (b) MWCNT after heating in vacuum to 1000  °C for 10  min, (c) SWCNT rod initially, and (d) SWCNT rod after 40  h in vacuum. Na⁺, K⁺, and NaOH⁺ are adsorbed contaminants. Adopted with permission from Ref. 86, Copyright 2003 Japanese Journal of Applied Physics. First citation in article

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Fig. 25. (a) Low magnification TEM image of a conical CNT showing numerous graphite branches grown around the main tube; (b) a closer view of the tip where the hollow tube and catalyst particle decorating the tip are clearly visible. Adopted with permission from Ref. 87, Copyright 2000 Springer Science and Business Media. First citation in article

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Fig. 26. Schematic representation of the growth of (a) a CNF using conventional thermal CVD, (b) a vertically aligned carbon nanostructure using PECVD, and (c) a carbon nanocone formed due to additional precipitation of C on the outer walls during PECVD. Reprinted with permission from Ref. 88. First citation in article

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Fig. 27. Scanning electron images of (a) parylene-coated and (b) Al₂O₃ VACNFs. Reprinted in part with permission from Ref. 99, Copyright 2006 American Chemical Society. First citation in article

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Fig. 28. Schematic cross-section images of functionalized nanotubes coated via ALD: (a) Al₂O₃ ALD film, (b) multilayered Al₂O₃/W/Al₂O₃ ALD film, and (c) functionalized monolayer on an Al₂O₃ ALD seed layer. (d) TEM image of a multiwalled nanotube coated via ALD with a multilayer as shown in (b). Reprinted with permission from Ref. 100. First citation in article

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Fig. 29. (Color online) (a) Schematic illustration of the steps involved in the functionalization of CNFs and subsequent procedure for electroless deposition. (b) Chemical transformations involved in the nanofiber modification. Reprinted with permission from Ref. 105, Copyright 2006 American Chemical Society. First citation in article

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Fig. 30. (Color online) Illustration of biomolecular functionalization of a CNF with an amine-terminated oligonucleotide, first four bases shown of 5-amino-c6-G-GGG… (courtesy of M. Fuentes-Cabrera). Attachment upon the nanofiber is provided by an amide bond, such as that resulting from an EDC condensation reaction, at a putative nanofiber-COOH site. First citation in article

^*Electronic mail: SimpsonML1@ornl.gov.

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