We demonstrate quantitative determination of surface Fermi level pinning position in InN nanowires using polymer electrolyte gating and three-dimensional (3D) electrostatic modeling of charge distribution. We find pinning of the Fermi level 0.6–0.7 eV above the conduction band minimum at the surface of the nanowires. After taking into account the Fermi level pinning, doping concentration and carrier mobilities are also evaluated and compared with InN thin films. This general approach of combining electrolyte gating experiments with 3D numerical modeling can be applied to nanowires of other materials to determine their surface Fermi level pinning position.
The band offsets and heterostructures of monolayer and few-layer transition-metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) are investigated from first principles calculations. The band alignments between different MX2 monolayers are calculated using the vacuum level as reference, and a simple model is proposed to explain the observed chemical trends. Some of the monolayers and their heterostructures show band alignments suitable for potential applications in spontaneous water splitting, photovoltaics, and optoelectronics. The strong dependence of the band offset on the number of layers also implicates a possible way of patterning quantum structures with thickness engineering.
Interactions between metal and atomically thin two-dimensional (2D) materials can exhibit interesting physical behaviors that are of both fundamental interests and technological importance. In addition to forming a metal–semiconductor Schottky junction that is critical for electrical transport, metal deposited on 2D layered materials can also generate a local mechanical strain. We investigate the local strain at the boundaries between metal (Ag, Au) nanoparticles and MX2 (M = Mo, W; X = S) layers by exploiting the strong local field enhancement at the boundary in surface plasmon-enhanced Raman scattering (SERS). We show that the local mechanical strain splits both the in-plane vibration mode E2g(1) and the out-of-plane vibration mode A1g in monolayer MoS2, and activates the in-plane mode E1g that is normally forbidden in backscattering Raman process. In comparison, the effects of mechanical strain in thicker MoS2 layers are significantly weaker. We also observe that photoluminescence from the indirect bandgap transition (when the number of layers is ≥2) is quenched with the metal deposition, while a softened and broadened shoulder peak emerges close to the original direct-bandgap transition because of the mechanical strain. The strain at metal–MX2 boundaries, which locally modifies the electronic and phonon structures of MX2, can have important effects on electrical transport through the metal–MX2 contact.
We report on tuning the electronic and magnetic properties of metallic transition metal dichalcogenides (mTMDCs) by 2D to 1D size confinement. The stability of the mTMDC monolayers and nanoribbons is demonstrated by the larger binding energy compared to the experimentally available semiconducting TMDCs. The 2D MX2 (M = Nb, Ta; X = S, Se) monolayers are non-ferromagnetic metals and mechanically softer compared to their semiconducting TMDCs counterparts. Interestingly, mTMDCs undergo metal-to-semiconductor transition when the ribbon width approaches to ∼13 Å and ∼7 Å for zigzag and armchair edge terminations, respectively; then these ribbons convert back to metal when the ribbon widths further decrease. Zigzag terminated nanoribbons are ferromagnetic semiconductors, and their magnetic properties can also be tuned by hydrogen edge passivation, whereas the armchair nanoribbons are non-ferromagnetic semiconductors. Our results display that the mTMDCs offer a broad range of physical properties spanning from metallic to semiconducting and non-ferromagnetic to ferromagnetic that is ideal for applications where stable narrow bandgap semiconductors with different magnetic properties are desired.
Read moreAbstract Using low temperature molecular beam epitaxy (LT‐MBE) technique we have overcome the miscibility gap of GaAs and GaN alloys and successfully synthesized GaN 1‐x As x alloys in the whole composition range on crystalline (sapphire and silicon) and amorphous (Pyrex glass) substrates. On the N‐rich side we found an increased incorporation of As with decreasing growth temperature. At high enough As content the films lose their crystallinity and become amorphous. On sapphire substrate, the alloys are amorphous in the composition range of 0.17<x<0.75. For the films grown on glass substrates, the composition range for amorphous alloys extends to x∼0.1. However, films grown on silicon shows small fraction of As‐rich and N‐rich GaNAs nanocrystals for films with As content up to 36%. These amorphous GaNAs films have smooth morphology, homogeneous composition and sharp, well defined optical absorption edges. The bandgap energy of the GaN 1‐x As x alloys covers a broad energy range from ∼3.4 eV in GaN to∼0.8 eV at x∼0.85. This provides an almost perfect fit to the solar spectrum offering the opportunity to design high efficiency multijunction solar cells using a single ternary alloy system. The amorphous nature of this alloy over a wide alloy range can also be advantageous since they can be deposited on low‐cost glass substrate. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Read moreViews Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation A. X. Levander, K. M. Yu, S. V. Novikov, A. Tseng, C. T. Foxon, O. D. Dubon, J. Wu, W. Walukiewicz; GaN1−xBix: Extremely mismatched semiconductor alloys. Appl. Phys. Lett. 4 October 2010; 97 (14): 141919. https://doi.org/10.1063/1.3499753 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioApplied Physics Letters Search Advanced Search |Citation Search
Read moreWe study field-effect transistors realized from VO2 nanobeams with HfO2 as the gate dielectric. When heated up from low to high temperatures, VO2 undergoes an insulator-to-metal transition. We observe a change in conductance (~ 6 percent) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system and we discuss its possible origins.
Read moreElectronic structure engineering is essential for producing materials suited for efficient solid-state devices. Mismatched semiconductors offer wide tunability of electronic structure with only a small change in composition. Here, we report a combined compound-elemental source vapor transport method for synthesis of mismatched alloy nanowires (NWs) of ZnSe1−xTex across the composition range. The alloy composition can be continuously tuned by varying the growth temperature from ZnSe (x = 0) at higher temperature to ZnTe (x = 1) at lower temperature. The nanowires have structure and bandgaps consistent with their compositions, with lattice parameters varying with Vegard’s law and emissions following predicted extreme bandgap bowing.
Read moreFinite element simulations of novel InGaN solar cells, requiring no p-type InGaN, were carried out using the commercial software package APSYS. Simulations show that efficient, compositionally graded p-GaN/n-In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N solar cells can be achieved, provided the graded layer is confined within the depletion region. These compositionally graded solar cells can be used as the top cell in an InGaN/Si double-junction cell to achieve AM 1.5 efficiencies over 27% using realistic material parameters.
Read moreWe investigated external-stress-induced metal-insulator phase transitions in cantilevered single-crystal VO[subscript 2] nanobeams at variable temperatures using a combined theoretical and experimental approach. An atomic force microscope was used to measure the force-displacement curve of the nanobeams, which showed nonlinearity that signifies activation and expansion of domains of a new phase out of the old one. Superelasticity of the VO[subscript 2] nanobeam and supersaturation of the phase transition were clearly observed and quantified within the general theory of first-order phase transitions. Phase field modeling was employed to understand the energetics of the domain formation.
Read moreWe demonstrate the isolation of two free carrier scattering mechanisms as a function of radial band bending in InN nanowires via universal mobility analysis, where effective carrier mobility is measured as a function of effective electric field in a nanowire field-effect transistor. Our results show that Coulomb scattering limits effective mobility at most effective fields, while surface roughness scattering only limits mobility under very high internal electric fields. High-energy α particle irradiation is used to vary the ionized donor concentration, and the observed decrease in mobility and increase in donor concentration are compared to Hall effect results of high-quality InN thin films. Our results show that for nanowires with relatively high doping and large diameters, controlling Coulomb scattering from ionized dopants should be given precedence over surface engineering when seeking to maximize nanowire mobility.
Read moreIn article number 1500388, J. Wu, J. Y. Park, and co-workers present a new route to control friction in micro- and nanomechanical applications. The combined effect of phonons and electrons in friction is investigated by probing the nanotribological and electrical properties of VO2 across a thermally-driven metal–insulator transition temperature. Depending on whether it is insulating or metallic, the friction of VO2 behaves quite differently with temperature.
Read moreWe report the results of pressure-induced phase transitions and metallization in VO2 based on synchrotron x-ray diffraction, electrical resistivity, and Raman spectroscopy. Our isothermal compression experiments at room temperature and 383 K show that the room temperature monoclinic phase (M1,P21/c) and the high-temperature rutile phase (R,P42/mnm) of VO2 undergo phase transitions to a distorted M1 monoclinic phase (M1′,P21/c) above 13.0 GPa and to an orthorhombic phase (CaCl2-like, Pnnm) above 13.7 GPa, respectively. Upon further compression, both high-pressure phases transform into a new phase (phase X) above 34.3 and 38.3 GPa at room temperature and 383 K, respectively. The room temperature M1−M1′ phase transition structurally resembles the R−CaCl2 phase transition at 383 K, suggesting a second-order displacive type of transition. Contrary to previous studies, our electrical resistivity results, Raman measurements, as well as ab initio calculations indicate that the new phase X, rather than the M1′ phase, is responsible for the metallization under pressure. The metallization mechanism is discussed based on the proposed crystal structure.
Read moreThe remarkable turnover of friction on a vanadium dioxide (VO 2 ) surface driven by the metal–insulator transition is revealed using temperature‐variable atomic force microscopy in ultrahigh vacuum. Phononic and electronic contributions are known as two major components in mediating friction energy dissipation. Here, a VO 2 thin film is prepared on a silicon wafer with preferential orientations of (100) and (120) in the monoclinic phase using pulsed laser deposition. Corresponding friction and conductivity images show that friction decreases below the critical temperature, above which two trends are seen as the temperature increases: less friction on the insulating domains and higher friction on the metallic domains. This distinct temperature dependence of friction is attributed to the combined effects of thermal lubricity and electronic contributions. This study indicates the promising potential for vanadium oxide to tune friction in the electric regime as well as with temperature.
Read moreAbstract The relation between structural perfection and optical properties of InGaN with 10% In are discussed. Transmission Electron Microscopy, X‐ray diffraction and Rutherford backscattering spectrometry measurements show that only strained layers with a thickness not exceeding 100 nm are defect free and In concentration is lower than the nominal value. Extension of layer thickness leads to layer sequestration into sublayers with different In contents and the formation of planar defects as a result of layer relaxation. In concentration in such sublayers reach and in some cases exceed the nominal concentration. A single band edge photoluminescence peak is observed only for the thinnest layer. Samples with larger film thickness showed multiple PL peaks corresponding to layers with different In content. Much higher In content would be required to explain the presence of some PL peaks, suggesting that some PL peaks originate from the defective areas of the film. This was confirmed by cathodoluminescence studies performed on the same samples used earlier for TEM studies. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Read moreAbstract We review recent progress towards increasing solar cell efficiencies beyond the Shockley‐Queisser efficiency limit. Four main approaches are highlighted: multi‐junction cells, intermediate‐band cells, hot carrier cells and spectrum conversion. Multi‐junction cells use multiple solar cells that selectively absorb different regions of the solar spectrum. Intermediate‐band cells use one junction with multiple bandgaps to increase efficiencies. Hot‐carrier cells convert the excess energy of above‐bandgap photons into electrical energy. Spectrum conversion solar cells convert the incoming polychromatic sunlight into a narrower distribution of photons suited to the bandgap of the solar cell.
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