Showing papers in "Applied Physics Letters in 2013"
TL;DR: In this paper, 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.
Abstract: 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.
1,422 citations
TL;DR: In this paper, the authors fabricate MoS2 field effect transistors on both SiO2 and polymethyl methacrylate (PMMA) dielectrics and measure charge carrier mobility in a four-probe configuration.
Abstract: We fabricate MoS2 field effect transistors on both SiO2 and polymethyl methacrylate (PMMA) dielectrics and measure charge carrier mobility in a four-probe configuration. For multilayer MoS2 on SiO2, the mobility is 30–60 cm2/Vs, relatively independent of thickness (15–90 nm), and most devices exhibit unipolar n-type behavior. In contrast, multilayer MoS2 on PMMA shows mobility increasing with thickness, up to 470 cm2/Vs (electrons) and 480 cm2/Vs (holes) at thickness ∼50 nm. The dependence of the mobility on thickness points to a long-range dielectric effect of the bulk MoS2 in increasing mobility.
640 citations
TL;DR: In this article, the authors proposed two couples composed of monolayer transition metal dichalcogenides (TMDs) with sizable band gaps for low-power TFET applications.
Abstract: Tunnel field effect transistors (TFETs) based on vertical stacking of two dimensional materials are of interest for low-power logic devices. The monolayer transition metal dichalcogenides (TMDs) with sizable band gaps show promise in building p-n junctions (couples) for TFET applications. Band alignment information is essential for realizing broken gap junctions with excellent electron tunneling efficiencies. Promising couples composed of monolayer TMDs are suggested to be VIB-MeX2 (Me = W, Mo; X = Te, Se) as the n-type source and IVB-MeX2 (Me = Zr, Hf; X = S, Se) as the p-type drain by density functional theory calculations.
631 citations
TL;DR: In this paper, a 3D printed active composite material is realized by directly printing glassy shape memory polymer fibers in an elastomeric matrix, and the initial configuration is created by 3D printing, and then the programmed action of the shape memory fibers creates time dependence of the configuration.
Abstract: We advance a paradigm of printed active composite materials realized by directly printing glassy shape memory polymer fibers in an elastomeric matrix. We imbue the active composites with intelligence via a programmed lamina and laminate architecture and a subsequent thermomechanical training process. The initial configuration is created by three-dimension (3D) printing, and then the programmed action of the shape memory fibers creates time dependence of the configuration—the four-dimension (4D) aspect. We design and print laminates in thin plate form that can be thermomechanically programmed to assume complex three-dimensional configurations including bent, coiled, and twisted strips, folded shapes, and complex contoured shapes with nonuniform, spatially varying curvature. The original flat plate shape can be recovered by heating the material again. We also show how the printed active composites can be directly integrated with other printed functionalities to create devices; here we demonstrate this by creating a structure that can assemble itself.
579 citations
TL;DR: In this article, the authors demonstrate that a fundamental performance bottleneck for hydrazine processed kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with efficiencies reaching above 11% can be the formation of band-edge tail states.
Abstract: We demonstrate that a fundamental performance bottleneck for hydrazine processed kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with efficiencies reaching above 11% can be the formation of band-edge tail states, which quantum efficiency and photoluminescence data indicate is roughly twice as severe as in higher-performing Cu(In,Ga)(S,Se)2 devices. Low temperature time-resolved photoluminescence data suggest that the enhanced tailing arises primarily from electrostatic potential fluctuations induced by strong compensation and facilitated by a lower CZTSSe dielectric constant. We discuss the implications of the band tails for the voltage deficit in these devices.
577 citations
TL;DR: In this article, single-crystal gallium oxide (Ga2O3) metal-oxide-semiconductor field effect transistors were fabricated on a semi-insulating β-Ga 2O3 (010) substrate.
Abstract: Single-crystal gallium oxide (Ga2O3) metal-oxide-semiconductor field-effect transistors were fabricated on a semi-insulating β-Ga2O3 (010) substrate. A Sn-doped n-Ga2O3 channel layer was grown by molecular-beam epitaxy. Si-ion implantation doping was performed to source and drain electrode regions for obtaining low-resistance ohmic contacts. An Al2O3 gate dielectric film formed by atomic layer deposition passivated the device surface and significantly reduced gate leakage. The device with a gate length of 2 μm showed effective gate modulation of the drain current with an extremely low off-state drain leakage of less than a few pA/mm, leading to a high drain current on/off ratio of over ten orders of magnitude. A three-terminal off-state breakdown voltage of 370 V was achieved. Stable transistor operation was sustained at temperatures up to 250 °C.
544 citations
TL;DR: In this paper, the effects of annealing temperature (Tanneal) and film thickness (tf) on the crystal structure and ferroelectric properties of Hf 0.5Zr0.5O2 films were examined.
Abstract: The effects of annealing temperature (Tanneal) and film thickness (tf) on the crystal structure and ferroelectric properties of Hf0.5Zr0.5O2 films were examined. The Hf0.5Zr0.5O2 films consist of tetragonal, orthorhombic, and monoclinic phases. The orthorhombic phase content, which is responsible for the ferroelectricity in this material, is almost independent of Tanneal, but decreases with increasing tf. In contrast, increasing Tanneal and tf monotonically increases (decreases) the amount of monoclinic (tetragonal) phase, which coincides with the variations in the dielectric constant. The remanant polarization was determined by the content of orthorhombic phase as well as the spatial distribution of other phases.
506 citations
TL;DR: In this paper, a high-quality graphene transparent conductive film was fabricated by roll-to-roll chemical vapor deposition (CVD) synthesis on a suspended copper foil and subsequent transfer.
Abstract: A high-quality graphene transparent conductive film was fabricated by roll-to-roll chemical vapor deposition (CVD) synthesis on a suspended copper foil and subsequent transfer. While the high temperature required for the CVD synthesis of high-quality graphene has prevented efficient roll-to-roll production thus far, we used selective Joule heating of the copper foil to achieve this. Low pressure thermal CVD synthesis and a direct roll-to-roll transfer process using photocurable epoxy resin allowed us to fabricate a 100-m-long graphene transparent conductive film with a sheet resistance as low as 150 Ω/sq, which is comparable to that of state-of-the-art CVD-grown graphene films.
452 citations
TL;DR: In this article, the authors report high mobility (>60 cm2/Vs at room temperature) field-effect transistors that employ unencapsulated single-layer MoS2 on oxidized Si wafers with a low level of extrinsic contamination.
Abstract: Ultra-thin MoS2 has recently emerged as a promising two-dimensional semiconductor for electronic and optoelectronic applications. Here, we report high mobility (>60 cm2/Vs at room temperature) field-effect transistors that employ unencapsulated single-layer MoS2 on oxidized Si wafers with a low level of extrinsic contamination. While charge transport in the sub-threshold regime is consistent with a variable range hopping model, monotonically decreasing field-effect mobility with increasing temperature suggests band-like transport in the linear regime. At temperatures below 100 K, temperature-independent mobility is limited by Coulomb scattering, whereas, at temperatures above 100 K, phonon-limited mobility decreases as a power law with increasing temperature.
399 citations
TL;DR: In this paper, the authors demonstrate pronounced ambipolar device characteristics of multilayer WSe2 FETs using different contact electrodes and reveal that nickel electrodes facilitate electron injection while palladium electrodes are more efficient for hole injection.
Abstract: One of the most relevant features that a semiconducting channel material can offer when used in a field-effect transistor (FET) layout is its capability to enable both electron transport in the conduction band and hole transport in the valence band. In this way, complementary metal-oxide-semiconductor type applications become feasible once similar electron and hole drive current densities are achieved, and the threshold voltages are properly adjusted. In this article, we demonstrate pronounced ambipolar device characteristics of multilayer WSe2 FETs using different contact electrodes. Our study reveals that nickel electrodes facilitate electron injection while palladium electrodes are more efficient for hole injection. We also show, as an interesting demonstration, that by using nickel as the source contact electrode and palladium as the drain contact electrode, ambipolar device characteristics with similar on-state performance for both the electron and the hole branch can be achieved in WSe2 FETs. Finally, we discuss a unique technique based on the asymmetry in the ambipolar device characteristics to extract the Schottky barrier heights for such metal to WSe2 contacts.
366 citations
TL;DR: In this article, the use of reactive electropositive metal contacts is proposed to lower contact resistance in MoS2 devices, based on calculations of the sulfur vacancy in the screened exchange (sX) hybrid functional.
Abstract: The use of reactive electropositive metal contacts is proposed to lower contact resistance in MoS2 devices, based on calculations of the sulfur vacancy in MoS2 by the screened exchange (sX) hybrid functional. sX gives band gaps of 1.88 eV and 1.34 eV for monolayer and bulk MoS2. The S vacancy has a formation energy of 2.35 eV in S-rich conditions, while the Mo vacancy has a large formation energy of 8.02 eV in Mo-rich conditions. The S vacancy introduces defect levels 0/−1 at 1.23 eV and −1/−2 at 1.28 eV in the upper gap. Its formation energy becomes small or negative for EF near the conduction band edge, leading to EF pinning near the conduction band for reactive metal contacts and lower contact resistances.
Journal Article•
TL;DR: In this article, the authors presented a work on capacitance characterization of organic photovoltaic cells and rectified a mistake which appeared in Carr and Chaudhary's work.
Abstract: In the 100th volume of Applied Physics Letters Carr and Chaudhary have presented a work on capacitance characterization of organic photovoltaic cells. The work concerns small signal measurements of various organic photovoltaic structures. The authors however limit their considerations to one part of small signal response, namely to capacitance measured either in parallel mode or in series mode. This attitude generally does not lead to accurate capacitance in organic devices and only for certain cases can result in appropriate values. Moreover, confusing C’’ with C’ they classify our work into “improper” models. In this comments, we go into details of capacitance characterization of organic photovoltaic cells and we rectify a mistake which appear in Carr and Chaudhary’s work.
TL;DR: Magnetic field-induced strain (MFIS) of 12% was reported in ferromagnetic Ni 46Mn24Ga22Co4 Cu 4 martensite exhibiting non-modulated (NM) tetragonal crystal structure with lattice parameter ratio c / a > 1.
Abstract: Magnetic field-induced strain (MFIS) of 12% is reported in ferromagnetic Ni 46Mn24Ga22Co4 Cu 4 martensite exhibiting non-modulated (NM) tetragonal crystal structure with lattice parameter ratio c / a > 1 . The strain was measured at ambient temperature in a magnetic field of the order of 1 T. The twinning stress σ T W and the magnetic stress σ M A G were also measured and the condition for a giant MFIS observation σ T W < σ M A G was confirmed. The MFIS was achieved in NM Ni 46Mn24Ga22Co4 Cu 4 martensite by considerable lowering of the σ T W value as compared to the values for NM martensites in ternary Ni-Mn-Ga system.
TL;DR: In this article, a metasurface lens that focuses light and controls its polarization at a wavelength of 2'μm is presented, which demonstrates high transmission and complete phase control within a subwavelength thickness at near-infrared frequencies.
Abstract: A metasurface lens that focuses light and controls its polarization at a wavelength of 2 μm is presented. This lens demonstrates high transmission and complete phase control within a subwavelength thickness at near-infrared frequencies. By cascading four patterned sheets, the efficiency is dramatically improved over more common single sheet designs. In addition, by utilizing anisotropic sheets, arbitrary birefringence can be achieved. A planar lens that both focuses light and converts its polarization from linear to circular is analyzed.
TL;DR: In this paper, a magnetically coupled nonlinear piezoelectric energy harvester was investigated by altering the angular orientation of its external magnets for enhanced broadband frequency response.
Abstract: We investigate a magnetically coupled nonlinear piezoelectric energy harvester by altering the angular orientation of its external magnets for enhanced broadband frequency response. Electromechanical equations describing the nonlinear dynamic behavior include an experimentally identified polynomial for the transverse magnetic force that depends on magnet angle. Up- and down-sweep harmonic excitation tests are performed at constant acceleration over the range of 0–25 Hz. Very good agreement is observed between the numerical and experimental open-circuit voltage output frequency response curves. The nonlinear energy harvester proposed in this work can cover the broad low-frequency range of 4–22 Hz by changing the magnet orientation.
TL;DR: In this paper, a unique approach to efficiently rotate the linear polarization of terahertz wave in a broadband configuration with tri-layer metasurfaces was proposed, where the Fabry-Perot cavity effect was attributed to the underlying mechanism of high transmittance and polarization rotation.
Abstract: Polarization conveys valuable information for electromagnetic signal processing exhibiting tremendous potential in developing application driven photonic devices. Manipulation of polarization state of an electromagnetic wave has drawn a lot of research interests in many different fields, especially in the terahertz regime. Here, we propose a unique approach to efficiently rotate the linear polarization of terahertz wave in a broadband configuration with tri-layer metasurfaces. We experimentally observe a nearly perfect orthogonal polarization conversion with an ultrahigh efficiency, demonstrating a ultrathin terahetz rotator. The Fabry-Perot cavity effect in the tri-layer metasurfaces is attributed to the underlying mechanism of high transmittance and polarization rotation.
TL;DR: In this paper, the authors theoretically investigated the switching of a perpendicular magnetic layer by in-plane charge current due to the spin Hall effect, and they found that the threshold switching current is independent of the damping constant and is almost linearly proportional to both effective perpendicular magnetic anisotropy field and external inplane field applied along the current direction.
Abstract: We theoretically investigate the switching of a perpendicular magnetic layer by in-plane charge current due to the spin Hall effect. We find that in the high damping regime, the threshold switching current is independent of the damping constant and is almost linearly proportional to both effective perpendicular magnetic anisotropy field and external in-plane field applied along the current direction. We obtain an analytic expression of the threshold current, in excellent agreement with numerical results. Based on the expression, we find that magnetization switching induced by the spin Hall effect can be practically useful when it is combined with voltage-controlled anisotropy change.
TL;DR: In this paper, a record efficiency was achieved for SnS-based thin-film solar cells by varying the oxygen-to-sulfur ratio in Zn(O,S).
Abstract: SnS is a promising earth-abundant material for photovoltaic applications. Heterojuction solar cells were made by vapor deposition of p-type tin(II) sulfide, SnS, and n-type zinc oxysulfide, Zn(O,S), using a device structure of soda-lime glass/Mo/SnS/Zn(O,S)/ZnO/ITO. A record efficiency was achieved for SnS-based thin-film solar cells by varying the oxygen-to-sulfur ratio in Zn(O,S). Increasing the sulfur content in Zn(O,S) raises the conduction band offset between Zn(O,S) and SnS to an optimum slightly positive value. A record SnS/Zn(O,S) solar cell with a S/Zn ratio of 0.37 exhibits short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) of 19.4 mA/cm2, 0.244 V, and 42.97%, respectively, as well as an NREL-certified total-area power-conversion efficiency of 2.04% and an uncertified active-area efficiency of 2.46%.
TL;DR: In this article, the switching stability of a Si-doped HfO2 film under bipolar pulsed-field operation was investigated and a de-pinning of domains due to reduction of the defect concentration at bottom electrode interface was suggested as the origin of the wake-up.
Abstract: Hafnium oxide based ferroelectric thin films have shown potential as a promising alternative material for non-volatile memory applications This work reports the switching stability of a Si-doped HfO2 film under bipolar pulsed-field operation High field cycling causes a “wake-up” in virgin “pinched” polarization hysteresis loops, demonstrated by an enhancement in remanent polarization and a shift of negative coercive voltage The rate of wake-up is accelerated by either reducing the frequency or increasing the amplitude of the cycling field We suggest de-pinning of domains due to reduction of the defect concentration at bottom electrode interface as origin of the wake-up
TL;DR: In this article, a pair of compliant electrodes comprising silver nanowire networks embedded in the surface layer of polyurethane matrix, and a highly compliant dielectric spacer sandwiched between the electrodes are demonstrated for the detection of deformation and pressure.
Abstract: Highly flexible transparent capacitive sensors have been demonstrated for the detection of deformation and pressure. The elastomeric sensors employ a pair of compliant electrodes comprising silver nanowire networks embedded in the surface layer of polyurethane matrix, and a highly compliant dielectric spacer sandwiched between the electrodes. The capacitance of the sensor sheets increases linearly with strains up to 60% during uniaxial stretching, and linearly with externally applied transverse pressure from 1 MPa down to 1 kPa. Stretchable sensor arrays consisting of 10 × 10 pixels have also been fabricated by patterning the composite electrodes into X-Y addressable passive matrix.
TL;DR: In this article, a pressure sensor based on the piezoresistive effect of graphene is presented, which is a 100'nm thick, 280'μm wide square silicon nitride membrane with graphene meander patterns located on the maximum strain area.
Abstract: We present a pressure sensor based on the piezoresistive effect of graphene. The sensor is a 100 nm thick, 280 μm wide square silicon nitride membrane with graphene meander patterns located on the maximum strain area. The multilayer, polycrystalline graphene was obtained by chemical vapor deposition. Strain in graphene was generated by applying differential pressure across the membrane. Finite element simulation was used to analyze the strain distribution. By performing electromechanical measurements, we obtained a gauge factor of ∼1.6 for graphene and a dynamic range from 0 mbar to 700 mbar for the pressure sensor.
TL;DR: In this article, the phonon linewidth and thermal conductivity of monolayer MoS2 were investigated using ab initio calculations, and it was shown that the thermal conductivities can be further increased by 30% in 10'μm sized samples.
Abstract: Using ab initio calculations, we have investigated the phonon linewidths and the thermal conductivity (κ) of monolayer MoS2. κ for a typical sample size of 1 μm is 83 W/m K at room temperature in the completely rough edge limit, suggesting κ is not a limiting factor for the electronic application of monolayer MoS2. κ can be further increased by 30% in 10 μm sized samples. Due to strong anharmonicity, isotope enhancement of room temperature κ is only 10% for 1 μm sized samples. However, linewidths can be significantly reduced, for instance, for Raman active modes A1g and E2g1, in isotopically pure samples.
TL;DR: In this article, the effects of surface wettability, porosity, and roughness on the critical heat flux (CHF) of water were examined using engineered surfaces and the results showed that porosity had little effect on the smooth non-porous surface CHF.
Abstract: The separate effects of surface wettability, porosity, and roughness on the critical heat flux (CHF) of water were examined using engineered surfaces. Values explored were 0, 5, 10, and 15 μm for Rz (roughness), 110° for static contact angle (wettability), and 0 and 50% for pore volume fraction. The porous hydrophilic surface enhanced CHF by 50%–60%, while the porous hydrophobic surface resulted in a reduction of CHF by 97%. Wettability had little effect on the smooth non-porous surface CHF. Surface roughness (Ra, Rq, Rz) had no effect on CHF within the limit of this database.
TL;DR: In this paper, a back metal reflector was used to enhance voltage through photon recycling that improves light extraction in one-sun GaInP solar cells, achieving 20.8% solar conversion efficiency, 8% external radiative efficiency, and 80-90% internal radiative capacity.
Abstract: We demonstrate 1.81 eV GaInP solar cells approaching the Shockley-Queisser limit with 20.8% solar conversion efficiency, 8% external radiative efficiency, and 80–90% internal radiative efficiency at one-sun AM1.5 global conditions. Optically enhanced voltage through photon recycling that improves light extraction was achieved using a back metal reflector. This optical enhancement was realized at one-sun currents when the non-radiative Sah-Noyce-Shockley junction recombination current was reduced by placing the junction at the back of the cell in a higher band gap AlGaInP layer. Electroluminescence and dark current-voltage measurements show the separate effects of optical management and non-radiative dark current reduction.
TL;DR: In this article, a modulated surface photovoltage spectroscopy (SPV) was used to investigate CH3NH3PbI3 layers during heating in vacuum.
Abstract: Layers of CH3NH3PbI3 are investigated by modulated surface photovoltage spectroscopy (SPV) during heating in vacuum. As prepared CH3NH3PbI3 layers behave as a p-type doped semiconductor in depletion with a band gap of 1.5 eV. After heating to 140 °C the sign of the SPV signals of CH3NH3PbI3 changed concomitant with the appearance of a second band gap at 2.36 eV ascribed to PbI2, and SPV signals related to charge separation from defect states were reduced after moderate heating.
TL;DR: In this paper, temperature-dependent cathodoluminescence spectra were measured from (001) unintentionally doped, (100) Si-doped, and (010) Mgdoped β-Ga2O3 substrates prepared by either the floating zone growth or edge-defined film-fed growth methods.
Abstract: Temperature-dependent cathodoluminescence spectra were measured from (001) unintentionally doped, (100) Si-doped, and (010) Mg-doped β-Ga2O3 substrates prepared by either the floating zone growth or edge-defined film-fed growth methods. Although β-Ga2O3 is expected to be an indirect bandgap material, direct Γ-Γ transitions were found to be dominant in the optical transmittance spectra. The substrates exhibited no near-band-edge emission and instead exhibited ultraviolet luminescence, blue luminescence (BL), and green luminescence bands. Since the BL intensity strongly depended on the resistivity in the crystals, there was evidence of a correlation between the BL intensity and formation energy of oxygen vacancies.
TL;DR: In this article, a comparative study of different tip cross-sections for small scale wind energy harvesting based on galloping phenomenon is presented, and it is recommended that the square section should be used for small wind galloping energy harvesters.
Abstract: This letter presents a comparative study of different tip cross-sections for small scale wind energy harvesting based on galloping phenomenon. A prototype device is fabricated with a piezoelectric cantilever and a tip body with various cross-section profiles (square, rectangle, triangle, and D-shape) and tested in a wind tunnel. Experimental results demonstrate the superiority of the square-sectioned tip for the low cut-in wind speed of 2.5 m/s and the high peak power of 8.4 mW. An analytical model is established and verified by the experimental results. It is recommended that the square section should be used for small wind galloping energy harvesters.
TL;DR: In this article, a planar plasmonic metamaterial on thin corrugated metal films in microwave and terahertz frequencies was proposed, which can sustain highly localized SPPs along two orthogonal directions.
Abstract: We propose a planar plasmonic metamaterial on thin corrugated metal films in microwave and terahertz frequencies. From theoretical simulation and experiment, we show that spoof surface plasmon polaritons (SPPs) can propagate along a thin metal film by corrugating its edge with periodic array of grooves. We demonstrate that such a planar plasmonic metamaterial can sustain highly localized SPPs along two orthogonal directions. We have designed and simulated a planar ring resonator in the terahertz frequency, and made experiment in the microwave frequency, both of which exhibit excellent performance. The proposed planar plasmonic metamaterials can play important roles in integrated plasmonic circuits and systems.
TL;DR: In this paper, the electrical characteristics of field effect transistors (FETs) with single-crystal molybdenum disulfide (MoS2) channels synthesized by chemical vapor deposition (CVD) were reported.
Abstract: We report the electrical characteristics of field-effect transistors (FETs) with single-crystal molybdenum disulfide (MoS2) channels synthesized by chemical vapor deposition (CVD). For a bilayer MoS2 FET, the field-effect mobility is ∼17 cm2 V−1 s−1 and the on/off current ratio is ∼108, which are much higher than those of FETs based on CVD polycrystalline MoS2 films. By avoiding the detrimental effects of the grain boundaries and the contamination introduced by the transfer process, the quality of the CVD MoS2 atomic layers deposited directly on SiO2 is comparable to or better than the exfoliated MoS2 flakes. The result shows that CVD is a viable method to synthesize high quality MoS2 atomic layers.
TL;DR: In this paper, a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN was shown to have a reduced energy bandgap for hexagonal AlN.
Abstract: Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced energy bandgap as expected for hexagonal AlN.