Thin-film solar cells based on Methylammonium triiodideplumbate (CH3NH3PbI3) halide perovskites have recently shown remarkable performance. First-principle calculations show that CH3NH3PbI3 has unusual defect physics: (i) Different from common p-type thin-film solar cell absorbers, it exhibits flexible conductivity from good p-type, intrinsic to good n-type depending on the growth conditions; (ii) Dominant intrinsic defects create only shallow levels, which partially explain the long electron-hole diffusion length and high open-circuit voltage in solar cell. The unusual defect properties can be attributed to the strong Pb lone-pair s orbital and I p orbital antibonding coupling and the high ionicity of CH3NH3PbI3. (C) 2014 AIP Publishing LLC.
Black phosphorus exhibits a layered structure similar to graphene, allowing mechanical exfoliation of ultrathin single crystals. Here, we demonstrate few-layer black phosphorus field effect devices on Si/SiO2 and measure charge carrier mobility in a four-probe configuration as well as drain current modulation in a two-point configuration. We find room-temperature mobilities of up to 300 cm(2)/Vs and drain current modulation of over 10(3). At low temperatures, the on-off ratio exceeds 10(5), and the device exhibits both electron and hole conduction. Using atomic force microscopy, we observe significant surface roughening of thin black phosphorus crystals over the course of 1 h after exfoliation. (C) 2014 AIP Publishing LLC.
Recently, fabricated two dimensional (2D) phosphorene crystal structures have demonstrated great potential in applications of electronics. Mechanical strain was demonstrated to be able to significantly modify the electronic properties of phosphorene and few-layer black phosphorus. In this work, we employed first principles density functional theory calculations to explore the mechanical properties of phosphorene, including ideal tensile strength and critical strain. It was found that a monolayer phosphorene can sustain tensile strain up to 27% and 30% in the zigzag and armchair directions, respectively. This enormous strain limit of phosphorene results from its unique puckered crystal structure. We found that the tensile strain applied in the armchair direction stretches the pucker of phosphorene, rather than significantly extending the P-P bond lengths. The compromised dihedral angles dramatically reduce the required strain energy. Compared to other 2D materials, such as graphene, phosphorene demonstrates superior flexibility with an order of magnitude smaller Young's modulus. This is especially useful in practical large-magnitude-strain engineering. Furthermore, the anisotropic nature of phosphorene was also explored. We derived a general model to calculate the Young's modulus along different directions for a 2D system. (C) 2014 AIP Publishing LLC.
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. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4774090
Moisture is assumed to be detrimental to organometal trihalide perovskite, as excess water can damage the crystallinity of the perovskite structure. Here, we report a growth mode for via thermal annealing of the perovskite precursor film in a humid environment (e. g., ambient air) to greatly improve the film quality, grain size, carrier mobility, and lifetime. Our method produces devices with maximum power conversion efficiency of 17.1% and a fill factor of 80%, revealing a promising route to achieve high quality perovskite polycrystalline films with superior optoelectronic properties that can pave the way towards efficient photovoltaic conversion. (C) 2014 AIP Publishing LLC.
Using atomic resolved scanning tunneling microscopy, we present here the experimental evidence of a silicene sheet (graphenelike structure) epitaxially grown on a close-packed silver surface [Ag(111)]. This has been achieved via direct condensation of a silicon atomic flux onto the single-crystal substrate in ultrahigh vacuum conditions. A highly ordered silicon structure, arranged within a honeycomb lattice, is synthesized and present two silicon sublattices occupying positions at different heights (0.02 nm) indicating possible sp 2 -sp 3 hybridizations.
Efficient hole-conductor-free organic lead iodide thin film solar cells have been fabricated with a sequential deposition method, and a highest efficiency of 10.49% has been achieved. Meanwhile, the ideal current-voltage model for a single heterojunction solar cell is applied to clarify the junction property of the cell. The model confirms that the TiO2/CH3NH3PbI3/Au cell is a typical heterojunction cell and the intrinsic parameters of the cell are comparable to that of the high-efficiency thin-film solar cells. (C) 2014 AIP Publishing LLC.
We report a giant spin Hall effect in beta-W thin films. Using spin torque induced ferromagnetic resonance with a beta-W/CoFeB bilayer microstrip, we determine the spin Hall angle to be vertical bar theta(beta-W)(SH)vertical bar = 0.30 +/- 0.02, large enough for an in-plane current to efficiently reverse the orientation of an in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction adjacent to a thin beta-W layer. From switching data obtained with such 3-terminal devices, we independently determine vertical bar theta(beta-W)(SH)vertical bar = 0.33 +/- 0.06. We also report variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable (alpha and beta) phase composition. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4753947
Cu2O is one of the attractive photovoltaic materials for solar cells because of its low cost, nontoxicity, and good mobility. In this paper, an obvious enhancement of power conversion efficiency (PCE) for ZnO/Cu2O solar cells with perfectly oriented and micrometer grain sized Cu2O films was experimentally demonstrated. Cu2O was fabricated using radical oxidation of Cu foils at a low temperature of 500 °C. When followed by a rapid quenching and post annealing treatment, the perfectly oriented and micrometer sized Cu2O crystals (3∼4 μm) could be obtained. The crystal structure and optical properties of Cu2O were investigated in detail. Compared to conventional solar cells without any treatment, the PCE of the solar cells based on Cu2O with treatment was 3.18%, corresponding to a significant PCE improvement of 60.6%.
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. (C) 2013 AIP Publishing LLC.
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 cm(2)/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 cm(2)/Vs (electrons) and 480 cm(2)/Vs (holes) at thickness similar to 50 nm. The dependence of the mobility on thickness points to a long-range dielectric effect of the bulk MoS2 in increasing mobility. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789365
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. (C) 2013 AIP Publishing LLC.
The effects of residues introduced during the transfer of chemical vapor deposited graphene from a Cu substrate to an insulating (SiO2) substrate on the physical and electrical of the transferred graphene are studied. X-ray photoelectron spectroscopy and atomic force microscopy show that this residue can be substantially reduced by annealing in vacuum. The impact of the removal of poly(methyl methacrylate) residue on the electrical properties of graphene field effect devices is demonstrated, including a nearly 2 x increase in average mobility from 1400 to 2700 cm(2)/Vs. The electrical results are compared with graphene doping measurements by Raman spectroscopy. (C) 2011 American Institute of Physics. [doi: 0.1063/1.3643444
We report a demonstration of single-crystal gallium oxide (Ga2O3) metal-semiconductor field-effect transistors (MESFETs). A Sn-doped Ga2O3 layer was grown on a semi-insulating beta-Ga2O3 (010) substrate by molecular-beam epitaxy. We fabricated a circular MESFET with a gate length of 4 mu m and a source-drain spacing of 20 mu m. The device showed an ideal transistor action represented by the drain current modulation due to the gate voltage (V-GS) swing. A complete drain current pinch-off characteristic was also obtained for VGS < -20 V, and the three-terminal off-state breakdown voltage was over 250 V. A low drain leakage current of 3 mu A at the off-state led to a high on/off drain current ratio of about 10000. These device characteristics obtained at the early stage indicate the great potential of Ga2O3-based electrical devices for future power device applications. (C) 2012 American Institute of Physics. [doi:10.1063/1.3674287
With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm(2) 2-terminal monolithic perovskite/silicon multijunction solar cell with a V-OC as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%. (C) 2015 AIP Publishing LLC.
We predict enormous, anisotropic piezoelectric effects in intrinsic monolayer group IV monochalcogenides (MX, M=Sn or Ge, X=Se or S), including SnSe, SnS, GeSe, and GeS. Using first-principle simulations based on the modern theory of polarization, we find that their piezoelectric coefficients are about one to two orders of magnitude larger than those of other 2D materials, such as MoS2 and GaSe, and bulk quartz and AlN which are widely used in industry. This enhancement is a result of the unique "puckered" C-2v symmetry and electronic structure of monolayer group IV monochalcogenides. Given the achieved experimental advances in the fabrication of monolayers, their flexible character, and ability to withstand enormous strain, these 2D structures with giant piezoelectric effects may be promising for a broad range of applications such as nano-sized sensors, piezotronics, and energy harvesting in portable electronic devices. (C) 2015 AIP Publishing LLC.
Under strong laser radiation, a Dirac material, the topological insulator (TI) Bi2Te3, exhibits an optical transmittance increase as a result of saturable absorption. Based on an open-aperture Z-scan measurement at 1550 nm, we clearly show that the TI, Bi2Te3 under our investigation, is indeed a very-high-modulation-depth (up to 95%) saturable absorber. Furthermore, a TI based saturable absorber device was fabricated and used as a passive mode locker for ultrafast pulse formation at the telecommunication band. This contribution unambiguously shows that apart from its fantastic electronic property, a TI (Bi2Te3) may also possess attractive optoelectronic property for ultrafast photonics. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767919
High quality factor resonances are extremely promising for designing ultra-sensitive refractive index label-free sensors, since it allows intense interaction between electromagnetic waves and the analyte material. Metamaterial and plasmonic sensing have recently attracted a lot of attention due to subwavelength confinement of electromagnetic fields in the resonant structures. However, the excitation of high quality factor resonances in these systems has been a challenge. We excite an order of magnitude higher quality factor resonances in planar terahertz metamaterials that we exploit for ultrasensitive sensing. The low-loss quadrupole and Fano resonances with extremely narrow linewidths enable us to measure the minute spectral shift caused due to the smallest change in the refractive index of the surrounding media. We achieve sensitivity levels of 7.75 x 10(3) nm/refractive index unit (RIU) with quadrupole and 5.7 x 10(4) nm/RIU with the Fano resonances which could be further enhanced by using thinner substrates. These findings would facilitate the design of ultrasensitive real time chemical and biomolecular sensors in the fingerprint region of the terahertz regime. (C) 2014 AIP Publishing LLC.
The band structure of MoS2 strongly depends on the number of layers, and a transition from indirect to direct-gap semiconductor has been observed recently for a single layer of MoS2. Single-layer MoS2 therefore becomes an efficient emitter of photoluminescence even at room temperature. Here, we report on scanning Raman and on temperature-dependent, as well as time-resolved photoluminescence measurements on single-layer MoS2 flakes prepared by exfoliation. We observe the emergence of two distinct photoluminescence peaks at low temperatures. The photocarrier recombination at low temperatures occurs on the few-picosecond timescale, but with increasing temperatures, a biexponential photoluminescence decay with a longer-lived component is observed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3636402
High absorption efficiency is particularly desirable at present for various microtechnological applications including microbolometers, photodectors, coherent thermal emitters, and solar cells. Here we report the design, characterization, and experimental demonstration of an ultrathin, wide-angle, subwavelength high performance metamaterial absorber for optical frequencies. Experimental results show that an absorption peak of 88% is achieved at the wavelength of ∼ 1.58 μ m , though theoretical results give near perfect absorption.