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The design of antenna–reactor photocatalysts has become a powerful strategy to covert transition metal reactors from traditional thermocatalysts to novel photocatalysts. Plasmonic metals are often used as the optical antenna. Here, we demonstrate that conventional dielectric supports with high refractive index are able to achieve comparable performance as the plasmonic antennas, giving rise to a huge enhancement of the visible light absorption in the small Pt nanoparticles (NPs) of the core–satellite antenna–reactor photocatalysts through resonance energy transfer. The absorption enhancement can be mediated not only by the electric resonances of the plasmonic antenna but also by the magnetic resonances of the dielectric antenna. A large enough dielectric antenna or a small plasmonic antenna is desired for the generation of strong optical resonances. Judged by those Pt NPs with strong visible light absorption enhancement, the potential catalytically active sites are mainly distributed at the back side of the dielectric antenna, while they may be widely distributed over the surface of the plasmonic antenna with probably lower activities. The expanding scope of antenna–reactor photocatalysts offers new opportunities for solar to chemical energy conversion using nonplasmonic catalytic transition metals.

As a promising plasmonic photocatalyst, an Ag-Pt core-shell nanostructure is able to convert sunlight into chemical energy. To fully exploit each function of the constituent materials, a numerical study on the optimal design of the hybrid nanostructures is presented in this work. Ag-Pt is demonstrated to be a good material configuration for the core-shell nanostructure because Ag has strong intrinsic plasmonic responses and a low imaginary dielectric function in the visible region, while Pt is catalytically active and has a large imaginary dielectric function. Considering the hot carrier generation and transfer processes in both plasmonic photocatalysis and photothermal catalysis, the catalytically active sites at the Pt shell can be revealed by high local heating power densities. For the dipole resonance, these sites distribute alternately with the spots where local electric fields are greatly enhanced. The former are along the “equatorial belt” of the nanoparticle, while the latter are in the two polar regions. It is then found that the high-efficiency hot carrier generation is related to multiple factors, including at least an ultrathin shell and a core of high aspect ratio with sharp tips. The physics behind these factors is further addressed.As a promising plasmonic photocatalyst, an Ag-Pt core-shell nanostructure is able to convert sunlight into chemical energy. To fully exploit each function of the constituent materials, a numerical study on the optimal design of the hybrid nanostructures is presented in this work. Ag-Pt is demonstrated to be a good material configuration for the core-shell nanostructure because Ag has strong intrinsic plasmonic responses and a low imaginary dielectric function in the visible region, while Pt is catalytically active and has a large imaginary dielectric function. Considering the hot carrier generation and transfer processes in both plasmonic photocatalysis and photothermal catalysis, the catalytically active sites at the Pt shell can be revealed by high local heating power densities. For the dipole resonance, these sites distribute alternately with the spots where local electric fields are greatly enhanced. The former are along the “equatorial belt” of the nanoparticle, while the latter are in the two polar ...

Depending on the experimental conditions and plasmonic systems, the correlations between near-field surface enhanced Raman scattering (SERS) behaviors and far-field optical responses have sometimes been accepted directly, or argued, or explored. In this work, we have numerically demonstrated the anomalous spectral correlations between the near- and far-field properties for roughened Au mesoparticles. As a counterexample, it is witnessed that the dipole extinction peak of the mesoparticles may mislead us in seeking favorable SERS performance. The simple Rayleigh scattering spectra can also be misguided in the presence of dark modes. For roughened mesoparticles with a moderate size here, the huge near-field enhancement is a synergistic result of the overall dark quadrupole mode and the substructural bonding dipole coupling. The conclusions demonstrated here would be of general interest to the field of plasmonics, especially the optimization of single-particle SERS substrates.

This letter investigates repeated uniaxial mechanical stress-induced degradation behavior in flexible amorphous In-Ga-Zn-O thin-film transistors (TFTs) of different geometric structures. Two types of via-contact structure TFTs are investigated: symmetrical and UI structure (TFTs with I- and U-shaped asymmetric electrodes). After repeated mechanical stress, I-V curves for the symmetrical structure show a significant negative threshold voltage (VT) shift, due to mechanical stress-induced oxygen vacancy generation. However, degradation in the UI structure TFTs after stress is a negative VT shift along with the parasitic transistor characteristic in the forward-operation mode, with this hump not evident in the reverse-operation mode. This asymmetrical degradation is clarified by the mechanical strain simulation of the UI TFTs.

In this letter, the effects of hot carriers on amorphous In-Ga-Zn-O thin film transistors (TFTs) of different geometric structures were investigated. Three types of via-contact structure TFTs are used in this experiment, defined as source-drain large (SD large), source-drain normal (SD normal), and fork-shaped. After hot-carrier stress, the I-V curves of both SD normal and fork-shaped TFTs with U-shaped drains show a threshold voltage shift along with the parasitic transistor characteristic in the reverse-operation mode. Asymmetrical degradation is exhibited in an ISE-TCAD simulation of the electric field, which shows the distribution of hot electrons injected into the etch-stop layer below the redundant drain electrode.

This paper investigates the degradation behavior of InGaZnO thin film transistors (TFTs) under negative bias illumination stress (NBIS). TFT devices with two different source and drain layouts were exanimated: one having a parallel format electrode and the other with UI format electrode. UI means that source/drain electrodes shapes is defined as a forked-shaped structure. The I-V curve of the parallel electrode exhibited a symmetric degradation under forward and reverse sweeping in the saturation region after 1000 s NBIS. In contrast, the I-V curve of the UI electrode structure under similar conditions was asymmetric. The UI electrode structure also shows a stretch-out phenomenon in its C-V measurement. Finally, this work utilizes the ISE-Technology Computer Aided Design (ISE-TCAD) system simulations, which simulate the electron field and IV curves, to analyze the mechanisms dominating the parallel and UI device degradation behaviors.

A voltage-controllable multiguide directional coupler is designed with a nematic liquid crystal cell, comprising a grating-like indium-tin-oxide electrode. An incident polarized laser beam can couple into adjacent channels by periodic distribution of the refractive index. This capability is attributed to the reorientation of the periodic liquid crystal caused by the application of a voltage. The coupling variation of the optical power in the transverse direction parallel to the waveguide surface is investigated. This work presents a promising coupling function for potential applications of optical communication devices.

Field emission from carbon nanotubes grown on carbon cloth has been studied. An extremely low electric field of less than 0.4V∕μm is required to reach an emission current density of 1mA∕cm2. This ultralow operating electric field of carbon nanotubes grown on carbon cloth is mainly due to a very high field enhancement factor of 1.882×104, which is the result of geometrical configuration of the carbon nanotubes and the substrate. In addition to the field enhancement, the highly disordered microstructure of carbon nanotubes grown on carbon cloth plays an important role to field emission. This unexpected result indicates that the roughness of the substrates on which carbon nanotubes grow is very important. This result also brings us significantly closer to practical applications such as highly efficient lamps, field emission displays, micro vacuum electron sources, etc.

The effects of TaON/LaON dual passivation interlayer on the interfacial and electrical properties of Ge metal-oxide-semiconductor (MOS) capacitor with HfO2 gate dielectric are investigated. As compared to its counterpart with only LaON as passivation interlayer, the formation of HfGeOx and LaHfOx, which would degrade the interfacial quality, is effectively suppressed due to the strong blocking role of the TaON barrier layer against Hf diffusion. As a result, excellent interfacial and electrical properties are achieved for the Ge MOS device with the TaON/LaON dual passivation interlayer: high k value (20.9), low interface-state density (5.32 × 1011 cm−2 eV−1) and oxide-charge density (−3.90 × 1012 cm−2), low gate leakage current density (1.77 × 10−4 A/cm2 at Vg = Vfb + 1 V), and high reliability under high-field stress.

Large-quantity (grams) one-dimensional ZnO nanowires of different sizes have been synthesized by a simple thermal evaporation of ZnO powder in a tube furnace at a temperature controlled to 1000–1200 °C and pressure to 1–2 Torr air. A mixture of ZnO and graphite powder was used as the source. Fine graphite flakes were used to promote the growth. The graphite flakes are the key for large-quantity yield and were easily removed by oxidation in flowing O2 at about 700 °C for 1–3 h. The scanning- and transmission-electron-microscopic studies show that the diameter and length of the nanowires vary from 20 to 100 nm and 0.5 to 10 μm, respectively. Room temperature photoluminescence studies found that the luminescent intensity depends on the processing conditions. A reduced band edge ultraviolet (380 nm) and deep-band green (520 nm) emission have been observed for these nanowires. Most importantly, the method can be extended to any other oxide nanowires that will be the building block of future nanoscale devices.

Epitaxial rutile-TiO2 and anatase-TiO2 films were grown at 800 °C on Al2O3(1102) and LaAlO3(001), respectively, using pulsed laser deposition. Both films showed high crystalline quality, evidenced by x-ray diffraction and high-resolution electron microscopy. The formation of different phases on different substrates could be qualitatively explained by the atomic arrangements at the interfaces. We also deposited epitaxial rutile-TiO2 and anatase-TiO2 films on conductive RuO2 and La0.5Sr0.5CoO3 electrodes, respectively. Using a Kelvin probe, we measured the photovoltaic properties of these multilayer structures. A rutile-TiO2 film grown on RuO2 showed a very broad peak in the visible light region. An epitaxial anatase-TiO2 film grown on La0.5Sr0.5CoO3 showed a strong peak with a threshold energy of 3.05 eV.

Both the linear and nonlinear optical properties of Bi3.15Nd0.85Ti3O12 (BNT0.85) ferroelectric thin films deposited on quartz substrates were investigated. The fundamental optical constants were determined as a function of light wavelength by optical transmittance measurements. By performing single-beam Z-scan experiments with femtosecond laser pulses at a wavelength of 800 nm, the two-photon absorption (TPA) coefficient β and third-order nonlinear refraction index γ were measured to be 1.15 × 102 cm/GW and −8.15 × 10−3 cm2/GW, respectively. The large TPA is attributed to an indirect transition process via the intermediate energy levels and the large refractive nonlinearity is the result of the electronic polarization and ferroelectric polarization arisen from the femtosecond midinfrared radiation. The results indicate that the BNT0.85 thin film is a promising candidate for applications in nonlinear photonic devices.

We have found that phase transformation can occur in cobalt when subjected to ball milling. The two modifications of cobalt, i.e., face‐center‐cubic (fcc) and hexagonal close‐packed (hcp) phases, which usually coexist at room temperature and are often difficult to be separated from each other, can now be easily separated by using the simple ball milling technique. The phase formation of cobalt was found to depend on the mill intensity. Under different mill intensity or different milling time, the phase transformations follow the routes of hcp+fcc→hcp, hcp+fcc→hcp→fcc+hcp, and hcp+fcc→hcp→fcc+hcp→fcc, respectively. Our results indicate that the phase formation of cobalt induced by ball milling was determined by the accumulation of structure defects. Different mill intensity may adjust the rate and level of the accumulation of defects.

Anomalous output characteristic shift of the n-type lateral diffused metal-oxide-semiconductor transistor with floating P-top layer is investigated. It shows that the linear drain current has obvious decrease when the output characteristic of fresh device is measured for two consecutive times. The charge pumping experiments demonstrate that the decrease is not from hot-carrier degradation. The reduction of cross section area for the current flowing, which results from the squeezing of the depletion region surrounding the P-top layer, is responsible for the shift. Consequently, the current capability of this special device should be evaluated by the second measured output characteristic.

In this Letter, the mechanism of double forming process phenomenon revealing in ZrO2/HfO2 bilayer resistive random access memory structure is investigated. This phenomenon caused by the formation of TiON interfacial layer can be well explained by using the energy band diagram. The TiON interfacial layer will be a tunneling barrier during the first forming process when a negative voltage applied on the device, while it will breakdown when applying a positive voltage. Besides, due to the double forming process, an asymmetric conductive filament with narrower size at ZrO2/HfO2 interface is formed in the device. The point for formation and rupture of the conductive filament can be confined at the ZrO2/HfO2 interface, and it will suppress the consumption of oxygen ions during endurance test. Therefore, high speed (40 ns) and large endurance (107 cycles) characteristics are achieved in this device structure.

Resonant tunneling by fast neutron irradiation with doses from 1×1012 to 1×1016 n/cm2 was studied. We observed that peak and valley positions shifted to higher voltages, and peak‐to‐valley ratios decreased with higher doses in static current‐voltage characteristics. Several models which take into account ionized impurities were used to give consistent explanation to such phenomena.

Luminescence in the near band-edge spectral region of Mg-doped GaN films grown by metalorganic chemical vapor deposition has been studied at liquid-helium temperatures. Radiative transitions at 3.37 and 3.416 eV were observed to evolve in cathodoluminescence spectra during electron-beam irradiation at 5 kV. The intensity of the 3.37 eV peak correlates monotonically with the resistivity of the films. By annealing the films in N2 and N2/H2 atmospheres, the 3.37 and 3.416 eV transitions are shown to be related to hydrogen.

We verified that the heat dissipation of surface plasmon resonance can be measured using photothermal deflection method. The maximum value of photothermal deflection corresponded to the occurrence of surface plasmon resonance. The detection of anti-integrin and integrin biomolecules that exhibit specific binding was demonstrated using this technique. From our experimental results it shows that the photothermal deflection method can be used to obtain the surface plasmon resonance angles and angle shift.

We report the thermoelectric characteristics of core-shell p-type germanium nanowires (GeNWs) (lightly doped core, heavily doped shell). Overall, the thermoelectric characteristics are dominated by the heavily doped shell. Experimental data indicate that surface states produce dopant deactivation when the heavily doped shell is removed. Under this situation, the thermoelectric figure of merit is degraded. Etching the heavily doped shell resulted in a rough germanium nanowire with a thermal conductivity close to 1.1 W/m-K at 300 K, which is one of the smallest k measured for nanowires and comparable to the thermal conductivity of bulk SiO2.

The electronic and optical properties of visible InGaN quantum-well (QW) structures grown on In0.03Ga0.97N underlayers have been investigated. A significant improvement of the QW emission is observed as a result of the insertion of the underlayers, which is associated with blueshift in the emission energy, reduced recombination lifetime, increased spatial homogeneity in the QW luminescence, and weaker internal fields inside the QWs. These are explained by partial strain relaxation evidenced by reciprocal space mapping of the X-ray diffraction intensity. Electrostatic potential profiles obtained by electron holography provide evidence for enhanced carrier injection by tunneling from the underlayer into the first QW.

The unipolar resistive switching behavior of Pt/LixZn1−xO/Pt structures fabricated via radio-frequency magnetron sputtering is investigated. Various Li doping concentrations influence the defect types (i.e., VO, Lii, and LiZn) in LixZn1−xO films for adjustable resistance ratio. The resistance ratio reaches 108 due to the minimized leakage current in the high-resistance state (HRS) at 6 at. % Li dopants. The dominant conduction mechanisms are explained in terms of Ohmic behavior and Poole-Frenkel (PF) emission. The coefficient r of the PF emission in the HRS is evaluated to confirm that the total number of defects in LixZn1−xO films decreases with increasing Li content.

We report the observation of a different defect generation phenomenon in layer-by-layer crystal growth. Steps at a nanowire liquid-solid growth interface, resulting from edge nucleated defects, are found to cause a gradual multiplication of stacking faults in the regions bounded by two edge defects. In the presence of a twin boundary, these generated defects continue to propagate along the entire nanowire length. This rocking chair generation mechanism is a unique feature of nanoscale layer-by-layer growth and is significantly different from well-known defect multiplication mechanisms in bulk materials. Published version

In this work, hybrid heterojunction solar cells are demonstrated based on a conjugate polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) directly spun-cast on micro-textured n-type crystalline silicon wafers. The fabrication conditions suggest that the organic coverage on the micro-textured surface is excellent and key to achieve high efficiency, leading to an average power conversion efficiency of 9.84%. A one-dimensional drift-diffusion model is then developed based on fitting the device characteristics with experimentally determined PEDOT:PSS parameters and projects an ultimate efficiency above 20% for organic/inorganic hybrid photovoltaics. The simulation results reveal the impacts of defect densities, back surface recombination, doping concentration, and band alignment.

Folding straight Ag nanorods into zig-zag structures could generate corners or bends that become potential hot spots for surface-enhanced Raman scattering (SERS). Using a dynamic shadowing growth method, zig-zag silver nanorod arrays of different bending number N with a fixed total rod length are fabricated, and their SERS performance are measured and compared using the Raman probe Rhodamine 6G. The SERS intensity increases with N when N 4. The results suggest that folding silver nanorods into three-dimensional structures is a promising way to design highly sensitive SERS substrates.

The In xAl 1-xN/GaN system is found to show compositional instability at the lattice-matched composition (x = 0.18) in epitaxial layers grown by metal organic chemical vapor deposition. The breakdown in compositional homogeneity is triggered by threading dislocations with a screw component propagating from the GaN underlayer, which tend to open up into V-grooves at a certain thickness of the In xAl 1-xN layer. The V-grooves coalesce at ?200 nm and are filled with material that exhibits a significant drop in indium content and a broad luminescence peak. Transmission electron microscopy suggests that the structural breakdown is due to heterogeneous nucleation and growth at the facets of the V-grooves.

Synthesis of germanium/silicon (Ge/Si) core/shell nanowire heterostructures is typically accompanied by unwanted gold (Au) diffusion on the Ge nanowire sidewalls, resulting in rough surface morphology, undesired whisker growth, and detrimental performance of electronic devices. Here, we advance understanding of this Au diffusion on nanowires, its diameter dependence and its kinetic origin. We devise a growth procedure to form a blocking layer between the Au seed and Ge nanowire sidewalls leading to elimination the Au diffusion for in situ synthesis of high quality Ge/Si core/shell heterostructures.

With the availability of Landau wave functions in real space, we investigate the magneto-optical spectra of ordinary bulk graphite within a tight-binding model. The interlayer interactions significantly affect the structure, frequency and intensity of absorption peaks, as well as their field evolution and the double-peak absorptions. Those unique features of the absorption constitute important characterization of graphite and serve to differentiate the bulk spectra from those of true monolayer and bilayer graphenes.

A correlation has been established between the optical and structural properties of blue-light-emitting diode structures grown on free-standing m-plane GaN. A double-peak InGaN quantum well emission at room temperature has been observed with a strong inhomogeneous spatial distribution, which can be associated with the presence of dislocations originating at the InGaN layers. It is argued that the difference in peak energy positions is due to in-plane piezoelectric fields within the dislocated regions.

The transmission spectra of bowtie aperture consisting of opposing triangular holes arranged in rectangular array on silver/silicon structure were investigated. It is found that the second order surface plasmon polaritons mode is much stronger than the fundamental one. In addition, by increasing the separation and angles of the tips or silver film thickness, the second order modes decrease gradually. These phenomena suggest magnetic field coupling between metal tips which results in higher order mode enhancement.

We present the analysis of linewidth roughness (LWR) in nanowire-mask-based graphene nanoribbons (GNRs) and evaluate its impact on the device performance. The data show that the LWR amplitude decreases with the GNR width, possibly due to the etching undercut near the edge of a nanowire-mask. We further discuss the large variation in GNR devices in the presence of LWR by analyzing the measured transport properties and on/off ratios.

Lithium (Li) dendrite formation has been recognized as one of the major safety concerns for Li metal batteries but not for conventional Li ion batteries (LIBs) where Li metal is not used. With the advanced in situ transmission electron microscopy enabling direct observation of battery operation, we found that Li fibers with length up to 35 μm grew on nanowire tip after charging. The Li fibers growth were highly directional, i.e., nucleating from the nanowire tip, and extending along the nanowire axis, which was attributed to the strong electric field enhancement effect induced by the sharp nanowire tip. This study reveals a potential safety concern of short-circuit failure for LIBs using nanowire anodes.

Unusually large and compressively strained Si in nanoheterostructures of Ni silicide/Si/Ni silicide, in which the strain of the Si region can be achieved up to 10%, has been produced with point contact reactions between Si and Ni nanowires in an ultrahigh vacuum transmission electron microscope. The growth rate and relationships between the strain and the spacing of the Si region have been measured. Based on the rate and relationships, we can control the Si dimension and, in turn, the strain of remaining Si can be tuned with appropriate spacing. Since one-dimensional nanoheterostructures may have potential applications in nanoelectronic devices, the existent strain will further affect carrier mobility and piezoresistance coefficients in the Si region. Electrical measurements on the nanodevices from such nanoheterostructures show that the current output closely correlates with the Si channel length and compressive strain.

In this study, we have investigated the effect of solution viscosity on generating the aligned and long DNA nanostrands array via modified molecular combing/soft lithography technique (or patterned molecular combing). By using the solution with higher viscosity, a low peeling-off speed can be applied to generate the long DNA nanostrands and the thornlike structures can be eliminated effectively. The DNA nanostrands array can also be generated by utilizing the polydimethylsiloxane stamp having microwells with larger diameter; however, variation of the lengths of the DNA nanostrands becomes greater as the solution viscosity and the size of microwells increase.

We report all-polymer solar cells (PSCs) based on a polymer blend of poly[3-(10-n-octyl-3-phenothiazine-vinylene)thiophene-co-2,5-thiophene] as donor and poly[1,4-dioctyloxyl-p-2,5-dicyanophenylenevinylene] as acceptor. Strong photoluminescence quenching is observed in the polymer blend indicating the photoinduced charge transfer occurs between the two polymers. The power conversion efficiency of the PSC was improved from 0.41% to 0.8% after 120 °C postannealing, under the illumination of AM 1.5, 100 mW/cm2. We attribute the improved efficiency to better morphological features of the photoactive layer after thermal annealing.

This study developed an optically controllable transflective spatial filter in a dye-doped cholesteric liquid crystal (DDCLC) film. The mechanism to induce the spatial filter is attributable to the photoisomerization-induced controllability of the redshift of the reflection band in the DDCLC cell. At various pumped intensities, different spatial distributions of the diffraction pattern of the object can be selected to be filtered, such that high- and low-pass or notch- and band-pass transmitted and reflected images can be simultaneously obtained, respectively.

Electronic structures of the nanorods of RuO2 and IrO2 metallic oxides were investigated by x-ray absorption near-edge structure (XANES) and scanning photoelectron microscopy (SPEM). O K-, Ru, and Ir L3-edge XANES results reveal that hybridization between O 2p and metal t2g orbitals is weaker in IrO2 than in RuO2. The enhancement of the tip-region SPEM intensities relative to those in the sidewall regions for both RuO2 and IrO2 nanorods is found to extend over a large energy range in contrast to those of carbon nanotubes and ZnO nanorods, which are confined to deep below and near the Fermi level, respectively.

The dynamic behavior of surface relief grating of dye-doped polymer film has been under investigation. The enhancement of the first-order diffraction efficiency for dye-doped polymer film can be demonstrated through the use of nematic liquid crystals as the interface. The larger surface modulation of relief grating measured by the atomic force microscopy is consistent with the enhancement of the first-order diffraction efficiency. The time evolution of diffraction efficiency for dye-doped polymer film in contact with nematic liquid crystals shows the pattern of rise-decay-rise to a stable level. The decay of diffraction efficiency is temperature dependent. The pump-probe transmission experiment confirms that the decay of diffraction efficiency is attributed to the inhomogeneous orientation of nematic liquid crystals due to photothermal fluctuation.

Atomic-scale microstructure changes of carbon nanotubes under high bias∕high current conditions were studied by in situ high-resolution transmission electron microscopy. We found that high bias voltage caused significant structure changes, such as crystallization and elimination of amorphous coating on the surface of nanotube walls, removal of nanotube walls, and formation of atomic-scale kinks. These structural changes are attributed to high temperatures induced by high bias resistive heating on the nanotubes. These structural changes cause dramatic electronic property changes of the nanotubes correspondingly. The results provide an efficient route to tailor the electronic properties of nanotubes by appropriate structural modifications.

Recently, we have reported an extremely strong field emission from carbon nanotubes grown on carbon cloth, with the field-enhancement factor of up to 18 800. In this paper, we study the origins of this effect, by investigating field emission from individual carbon nanotubes, in a transmission electron microscope equipped with a piezo manipulator. Microscopic analysis reveals a multistage structure of some of the nanotubes, characterized by an order of magnitude smaller nanotubes branching off the tips of bigger nanotubes (or carbon fibers). The multistage structure causes a macroscopic enhancement of the electric field, which can match that of a single macroscopically long nanotube with length equal to the combined length of all stages, and the tip radius equal to that of the thinnest nanotube in the structure. This not only explains the observed giant field enhancement, but also provides important clues for the design of nanotube emitters for electronic applications.

The specific heat of multiwalled titanium dioxide (anatase phase) nanotubes has been measured between 1.5 and 95K. Bulk anatase and rutile were also measured. The nanotube specific heat approaches that of bulk anatase at high temperatures. Below about 50K the nanotube specific heat begins to show large enhancements compared to bulk. Using an isotropic elastic continuum model, this can be understood qualitatively as a transition to low-dimensional behavior. Below about 3K there is a second transition and the nanotube specific heat becomes nearly constant, exceeding bulk anatase by an order of magnitude or more at 1.5K.

Low-temperature magnetotransport measurements were performed on AlxGa1−xN∕GaN two-dimensional electron systems. By studying the beating pattern in the Shubnikov–de Haas oscillations in a perpendicular magnetic field, we are able to measure the zero-field spin-splitting energies in our systems. Our experimental results demonstrate that the Rashba term due to structural inversion asymmetry is the dominant mechanism which gives rise to the measured zero-field spin splitting in our wurzite AlGaN∕GaN structures. By utilizing the persistent photoconductivity (PPC) effect, we are able to increase the carrier density n in our AlGaN∕GaN two-dimensional electron system. It is found that the Rashba spin-orbit splitting parameter α decreases with increasing n. We suggest that the formation of long-lived electron-hole pairs induced by the PPC effect decreases the large electric field near the AlGaN∕GaN interface, causing α to decrease with increasing n.