Volume 106, Issue 9, 02 March 2015
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- superconductivity and superconducting electronics
- dielectrics, ferroelectrics, and multiferroics
- nanoscale science and technology
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
Using e-beam nanolithography, the current injection/transport area in organic light-emitting diodes (OLEDs) was confined into a narrow linear structure with a minimum width of 50 nm. This caused suppression of Joule heating and partial separation of polarons and excitons, so the charge density where the electroluminescent efficiency decays to the half of the initial value (J0 ) was significantly improved. A device with a narrow current injection width of 50 nm exhibited a J0 that was almost two orders of magnitude higher compared with that of the unpatterned OLED.
- PHOTONICS AND OPTOELECTRONICS
106(2015); http://dx.doi.org/10.1063/1.4913533View Description Hide Description
For hybrid light emitting devices (LEDs) consisting of GaN quantum wells and colloidal quantum dots, it is necessary to explore the physical mechanisms causing decreases in the quantum efficiencies and the energy transfer efficiency between a GaN quantum well and CdSe quantum dots. This study investigated the electro-luminescence for a hybrid LED consisting of colloidal quantum dots and a GaN quantum well patterned with photonic crystals. It was found that both the quantum efficiency of colloidal quantum dots on a GaN quantum well and the energy transfer efficiency between the patterned GaN quantum well and the colloidal quantum dots decreased with increases in the driving voltage or the driving time. Under high driving voltages, the decreases in the quantum efficiency of the colloidal quantum dots and the energy transfer efficiency can be attributed to Auger recombination, while those decreases under long driving time are due to photo-bleaching and Auger recombination.
106(2015); http://dx.doi.org/10.1063/1.4913607View Description Hide Description
Using an optical parametric oscillation laser as the excitation source, the dependence of the saturable absorption of multiple-layer graphene upon photon energy is investigated and, in all cases, the saturation intensity is lower for lower excitation photon energy. This result experimentally proves the hourglass shape of the energy band of graphene, which is a well-known theoretical deduction from first principles. The modulation depth increases from 1.5% to 5.1% when the layer number increases from a monolayer to 5–7 layers and, at the same time, the saturation intensity decreases with increasing number of layers. The results demonstrate that, as a saturable absorber of a pulsed laser, graphene can more easily achieve optical modulation in the low energy region, i.e., the infrared waveband.
106(2015); http://dx.doi.org/10.1063/1.4913688View Description Hide Description
The emission properties of GeSn heterostructure pin diodes have been investigated. The devices contain thick (400–600 nm) Ge 1− y Sn y i-layers spanning a broad compositional range below and above the crossover Sn concentration y c where the Ge 1− y Sn y alloy becomes a direct-gap material. These results are made possible by an optimized device architecture containing a single defected interface thereby mitigating the deleterious effects of mismatch-induced defects. The observed emission intensities as a function of composition show the contributions from two separate trends: an increase in direct gap emission as the Sn concentration is increased, as expected from the reduction and eventual reversal of the separation between the direct and indirect edges, and a parallel increase in non-radiative recombination when the mismatch strains between the structure components is partially relaxed by the generation of misfit dislocations. An estimation of recombination times based on the observed electroluminescence intensities is found to be strongly correlated with the reverse-bias dark current measured in the same devices.
Manipulation of nanoscale V-pits to optimize internal quantum efficiency of InGaN multiple quantum wells106(2015); http://dx.doi.org/10.1063/1.4914116View Description Hide Description
We systematically investigated the influence of nanoscale V-pits on the internal quantum efficiency (IQE) of InGaN multiple quantum wells (MQWs) by adjusting the underlying superlattices (SLS). The analysis indicated that high barrier energy of sidewall MQWs on V-pits and long diffusion distance between the threading dislocation (TD) center and V-pit boundary were crucial to effectively passivate the non-radiative centers of TDs. For a larger V-pit, the thicker sidewall MQW on V-pit would decrease the barrier energy. On the contrary, a shorter distance between the TD center and V-pit boundary would be observed in a smaller V-pit, which could increase the carrier capturing capability of TDs. An optimized V-pit size of approximately 200–250 nm in our experiment could be concluded for MQWs with 15 pairs SLS, which exhibited an IQE value of 70%.
106(2015); http://dx.doi.org/10.1063/1.4914477View Description Hide Description
A quantum cascade laser (QCL) with an output power of 203 W is demonstrated in pulsed mode at 283 K with an angled cavity. The device has a ridge width of 300 μm, a cavity length of 5.8 mm, and a tilt angle of 12°. The back facet is high reflection coated, and the front facet is anti-reflection coated. The emitting wavelength is around 4.8 μm. In distinct contrast to a straight cavity broad area QCL, the lateral far field is single lobed with a divergence angle of only 3°. An ultrahigh brightness value of 156 MW cm−2 sr−1 is obtained, which marks the brightest QCL to date.
106(2015); http://dx.doi.org/10.1063/1.4914479View Description Hide Description
In periodic systems of low-symmetry, the Bragg condition for the complete interference of waves along the contour of the Brillouin zone (BZ) boundary is not generally satisfied. As a result, band-gaps do not necessarily occur at this boundary. This letter demonstrates this experimentally by recording the iso-frequency contours for surface plasmon polaritons (SPPs) supported on a diffraction grating with an underlying 2D oblique Bravias lattice. It is shown that these contours do not intersect the BZ boundary perpendicularly, as the symmetry operations of the lattice place no conditions on the surface wave interference at this boundary.
106(2015); http://dx.doi.org/10.1063/1.4914003View Description Hide Description
Light reflected off a material or absorbed within it exerts radiation pressure through the transfer of momentum. Micro/nano-mechanical transducers have become sensitive enough that radiation pressure can influence these systems. However, photothermal effects often accompany and overwhelm the radiation pressure, complicating its measurement. In this letter, we investigate the radiation force on an uncoated silicon nitride microcantilever in ambient conditions. We identify and separate the radiation pressure and photothermal forces through an analysis of the cantilever's frequency response. Further, by working in a regime where radiation pressure is dominant, we are able to accurately measure the radiation pressure. Experimental results are compared to theory and found to agree within the measured and calculated uncertainties.
Towards an optical far-field measurement of higher-order multipole contributions to the scattering response of nanoparticles106(2015); http://dx.doi.org/10.1063/1.4914117View Description Hide Description
We experimentally show an all-optical multipolar decomposition of the lowest-order eigenmodes of a single gold nanoprism using azimuthally and radially polarized cylindrical vector beams. By scanning the particle through these tailored field distributions, the multipolar character of the eigenmodes gets encoded into 2D-scanning intensity maps even for higher-order contributions to the eigenmode that are too weak to be discerned in the direct far-field scattering response. This method enables a detailed optical mode analysis of individual nanoparticles.
106(2015); http://dx.doi.org/10.1063/1.4914343View Description Hide Description
An all-optical tunabe chirality is realized in a photonic metamaterial, the metamolecule of which consists of a nonlinear nano-Au:polycrystalline indium-tin oxide layer sandwiched between two L-shaped gold nano-antennas twisted 90° with each other. The maximum circular dichroism reached 30%. Under excitation of a 40 kW/cm2 weak pump light, the peak in the circular dichroism shifts 45 nm in the short-wavelength direction. An ultrafast response time of 35 ps is maintained. This work not only opens up the possibility for the realization of ultralow-power and ultrafast all-optical tunable chirality but also offers a way to construct ultrahigh-speed on-chip biochemical sensors.
106(2015); http://dx.doi.org/10.1063/1.4914344View Description Hide Description
We report the generation of 10 mJ, 5-cycle pulses at 1.8 μm (30 fs) at 100 Hz repetition rate using an optical parametric amplifier pumped by a high energy Titanium-Sapphire laser system (total energy of 23 mJ for Signal and Idler). This is the highest reported peak power (0.33 TW) in the infrared spectral range. This high-energy long wavelength laser source is well suited for driving various nonlinear optical phenomena such as high harmonic generation for high flux ultrafast soft X-ray pulses.
- SURFACES AND INTERFACES
106(2015); http://dx.doi.org/10.1063/1.4914016View Description Hide Description
Significant advances in atomically resolved imaging of crystals and surfaces have occurred in the last decade allowing unprecedented insight into local crystal structures and periodicity. Yet, the analysis of the long-range periodicity from the local imaging data, critical to correlation of functional properties and chemistry to the local crystallography, remains a challenge. Here, we introduce a Sliding Fast Fourier Transform (FFT) filter to analyze atomically resolved images of in-situ grown La5/8Ca3/8MnO3 (LCMO) films. We demonstrate the ability of sliding FFT algorithm to differentiate two sub-lattices, resulting from a mixed-terminated surface. Principal Component Analysis and Independent Component Analysis of the Sliding FFT dataset reveal the distinct changes in crystallography, step edges, and boundaries between the multiple sub-lattices. The implications for the LCMO system are discussed. The method is universal for images with any periodicity, and is especially amenable to atomically resolved probe and electron-microscopy data for rapid identification of the sub-lattices present.
Improved carrier mobility of chemical vapor deposition-graphene by counter-doping with hydrazine hydrate106(2015); http://dx.doi.org/10.1063/1.4913702View Description Hide Description
We developed a counter-doping method to tune the electronic properties of chemical vapor deposition (CVD)-grown graphene by varying the concentration and time of graphene exposure to hydrazine hydrate (N2H4·H2O). The shift of G and 2D peaks of Raman spectroscopy is analyzed as a function of N2H4·H2O concentration. The result revealed that N2H4·H2O realized n-type doping on CVD grown graphene. X-ray photoelectron spectroscopy measurement proved the existence of nitrogen, which indicated the adsorption of N2H4 on the surface of graphene. After counter-doping, carrier mobility, which was measured by Hall measurements, increased three fold.
Impact of post-deposition annealing on interfacial chemical bonding states between AlGaN and ZrO2 grown by atomic layer deposition106(2015); http://dx.doi.org/10.1063/1.4914351View Description Hide Description
The effect of post-deposition annealing on chemical bonding states at interface between Al0.5Ga0.5N and ZrO2 grown by atomic layer deposition (ALD) is studied by angle-resolved x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. It has been found that both of Al-O/Al 2p and Ga-O/Ga 3d area ratio decrease at annealing temperatures lower than 500 °C, which could be attributed to “clean up” effect of ALD-ZrO2 on AlGaN. Compared to Ga spectra, a much larger decrease in Al-O/Al 2p ratio at a smaller take-off angle θ is observed, which indicates higher effectiveness of the passivation of Al-O bond than Ga-O bond through “clean up” effect near the interface. However, degradation of ZrO2/AlGaN interface quality due to re-oxidation at higher annealing temperature (>500 °C) is also found. The XPS spectra clearly reveal that Al atoms at ZrO2/AlGaN interface are easier to get oxidized as compared with Ga atoms.
Excellent wear life of silicon nitride/tetrahedral amorphous carbon bilayer overcoat on functional tape heads106(2015); http://dx.doi.org/10.1063/1.4914353View Description Hide Description
Developing ultrathin and highly wear-resistant overcoats for magnetic tape heads is one of the current research areas of interest, because of its potential to delay pole tip recession and increase the operational lifetime of high areal density tape drives. Using optimized process conditions and an appropriate overcoat design, we report on the development of a ∼20 nm thick silicon nitride/tetrahedral amorphous carbon (Si/SiNx/ta-C) bilayer overcoat, where the ta-C film was deposited by a filtered cathodic vacuum arc process. The bilayer overcoat deposited on a functional tape head survived 40–50 × 106 m of testing with commercial tape media under standard industrial testing conditions. The excellent wear resistance of the overcoat was attributed to the generation of high (∼72%) sp3 carbon content and the formation of strong interfacial bonds, such as Si-C, C=N, nitrile, and (Al, Ti)N at the interfaces, as confirmed by various spectroscopic techniques. This study demonstrates the pivotal role of high sp3 carbon bonding combined with enhanced interfacial bonding in developing an ultrathin yet durable protective overcoat for magnetic tape heads.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
Hopping conduction and piezoelectricity in Fe-doped GaN studied by non-contacting resonant ultrasound spectroscopy106(2015); http://dx.doi.org/10.1063/1.4913973View Description Hide Description
Using the antenna-transmission acoustic-resonance technique, we measured temperature dependencies of mechanical resonance frequencies and attenuation of an Fe-doped GaN. A strong internal-friction peak appears during temperature change, at which reduction in frequency occurs. The peak temperature rises as frequency increases, indicating the phonon-assisted hopping conduction of carriers between Fe centers. The Arrhenius plot yields the activation energy of the hopping conduction to be 0.23 ± 0.05 eV. The frequency reduction of a quasi-plane-shear resonance mode yields the piezoelectric coefficient e 15 = 0.332 ± 0.03 C/m2.
Lateral buckling and mechanical stretchability of fractal interconnects partially bonded onto an elastomeric substrate106(2015); http://dx.doi.org/10.1063/1.4913848View Description Hide Description
Fractal-inspired designs for interconnects that join rigid, functional devices can ensure mechanical integrity in stretchable electronic systems under extreme deformations. The bonding configuration of such interconnects with the elastomer substrate is crucial to the resulting deformation modes, and therefore the stretchability of the entire system. In this study, both theoretical and experimental analyses are performed for postbuckling of fractal serpentine interconnects partially bonded to the substrate. The deformation behaviors and the elastic stretchability of such systems are systematically explored, and compared to counterparts that are not bonded at all to the substrate.
Realization of a reversible switching in TaO2 polymorphs via Peierls distortion for resistance random access memory106(2015); http://dx.doi.org/10.1063/1.4913904View Description Hide Description
Transition-metal-oxide based resistance random access memory (RRAM) is a promising candidate for next-generation universal non-volatile memories. Searching and designing appropriate materials used in the memories becomes an urgent task. Here, a structure with the TaO2 formula was predicted using evolutionary algorithms in combination with first-principles calculations. This triclinic structure (T-TaO2) is both energetically and dynamically more favorable than the commonly believed rutile structure (R-TaO2). The metal-insulator transition (MIT) between metallic R-TaO2 and T-TaO2 (band gap: 1.0 eV) is via a Peierls distortion, which makes TaO2 a potential candidate for RRAM. The energy barrier for the reversible phase transition is 0.19 eV/atom and 0.23 eV/atom, respectively, suggesting low power consumption for the resistance switch. The present findings about the MIT as the resistance-switch mechanism in Ta-O system will stimulate experimental work to fabricate tantalum oxides based RRAM.
106(2015); http://dx.doi.org/10.1063/1.4913999View Description Hide Description
By employing a metal-coated central platelet and a rigid mesh electrode which is transparent to acoustic wave, we show that the membrane-type acoustic metamaterials (MAMs) can be easily tuned by applying an external voltage. With static voltage, the MAM's eigenfrequencies and therefore the phase of the transmitted wave are tunable up to 70 Hz. The MAM's vibration can be significantly suppressed or enhanced by using phase-matched AC voltage. Functionalities such as phase modulation and acoustic switch with on/off ratio up to 21.3 dB are demonstrated.
106(2015); http://dx.doi.org/10.1063/1.4914011View Description Hide Description
We introduce a strategy to attain reconfigurable, highly focused, subwavelength wave patterns in cellular metamaterials via electromechanical tuning of their microstructures. The metamaterial cells feature a population of auxiliary microstructural elements instrumented with piezoelectric patches connected to negative capacitance shunting circuits. By tuning the circuital characteristics of selected subsets of shunts, we relax the symmetry of the cell material property landscape, thus affecting the global directivity and enabling a plethora of wave manipulation capabilities. The acoustic reconfiguration is decoupled from other mechanical functions and is carried out without affecting the shape or the static properties of the host cellular structure.
Exploring electrical conductivity anomalies across the martensite transition in Fe7Pd3 ferromagnetic shape memory alloys: Experiments and ab-initio calculations106(2015); http://dx.doi.org/10.1063/1.4914004View Description Hide Description
Conductivity in Fe7Pd3 is characterized by an anomalous increase when traversing the face–centered–cubic (fcc) austenite to face–centered–tetragonal (fct) martensite transition, contrary to most other conventional and ferromagnetic shape memory alloys. Experiments on molecular– beam–epitaxy–grown single crystals indicate a resistivity change of ≈20% during the transformation on top of a quadratic temperature dependence reaching up to room temperature. The physical foundations of residual resistivity changes along the full Bain path are addressed by a Kubo– Greenwood approach within the framework of density functional theory. To do so, a concept to reliably extract the DC conductivities is proposed that yields reproducible results consistent with experiments. Finding that conductivity peaks in the fct phase, we identify a large density of states paired with high velocities at the Fermi level in the majority spin sub–bands in presence of minimum s–d electron scattering as underlying physical origin.