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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.