A multistep diffusion-mediated process was developed to control the nucleation density, size, and lateral growth rate of WSe2 domains on c-plane sapphire for the epitaxial growth of large area ...monolayer films by gas source chemical vapor deposition (CVD). The process consists of an initial nucleation step followed by an annealing period in H2Se to promote surface diffusion of tungsten-containing species to form oriented WSe2 islands with uniform size and controlled density. The growth conditions were then adjusted to suppress further nucleation and laterally grow the WSe2 islands to form a fully coalesced monolayer film in less than 1 h. Postgrowth structural characterization demonstrates that the WSe2 monolayers are single crystal and epitaxially oriented with respect to the sapphire and contain antiphase grain boundaries due to coalescence of 0° and 60° oriented WSe2 domains. The process also provides fundamental insights into the two-dimensional (2D) growth mechanism. For example, the evolution of domain size and cluster density with annealing time follows a 2D ripening process, enabling an estimate of the tungsten-species surface diffusivity. The lateral growth rate of domains was found to be relatively independent of substrate temperature over the range of 700–900 °C suggesting a mass transport limited process, however, the domain shape (triangular versus truncated triangular) varied with temperature over this same range due to local variations in the Se/W adatom ratio. The results provide an important step toward atomic level control of the epitaxial growth of WSe2 monolayers in a scalable process that is suitable for large area device fabrication.
One-dimensional nanoscale epitaxial arrays serve as a great model in studying fundamental physics and for emerging applications. With an increasing focus laid on the Cs-based inorganic halide ...perovskite out of its outstanding material stability, we have applied vapor phase epitaxy to grow well aligned horizontal CsPbX3 (X: Cl, Br, or I or their mixed) nanowire arrays in large scale on mica substrate. The as-grown nanowire features a triangular prism morphology with typical length ranging from a few tens of micrometers to a few millimeters. Structural analysis reveals that the wire arrays follow the symmetry of mica substrate through incommensurate epitaxy, paving a way for a universally applicable method to grow a broad family of halide perovskite materials. The unique photon transport in the one-dimensional structure has been studied in the all-inorganic Cs-based perovskite wires via temperature dependent and spatially resolved photoluminescence. Epitaxy of well oriented wire arrays in halide perovskite would be a promising direction for enabling the circuit-level applications of halide perovskite in high-performance electro-optics and optoelectronics.
Unlike the vast majority of transition metal dichalcogenides which are semiconductors, vanadium disulfide is metallic and conductive. This makes it particularly promising as an electrode material in ...lithium-ion batteries. However, vanadium disulfide exhibits poor stability due to large Peierls distortion during cycling. Here we report that vanadium disulfide flakes can be rendered stable in the electrochemical environment of a lithium-ion battery by conformally coating them with a ~2.5 nm thick titanium disulfide layer. Density functional theory calculations indicate that the titanium disulfide coating is far less susceptible to Peierls distortion during the lithiation-delithiation process, enabling it to stabilize the underlying vanadium disulfide material. The titanium disulfide coated vanadium disulfide cathode exhibits an operating voltage of ~2 V, high specific capacity (~180 mAh g
@200 mA g
current density) and rate capability (~70 mAh g
@1000 mA g
), while achieving capacity retention close to 100% after 400 charge-discharge steps.
The ability to reconfigure spin structure and spin‐photon interactions by an external electric field is a prerequisite for seamless integration of opto‐spintronics into modern electronics. In this ...study, the use of electric field on the tuning of circular photo galvanic effect in a quasi‐2D oxyhalide perovskite Bi4NbO8Cl is reported. The electrical transport measurements are applied to study the switching characteristics of the microsheet devices. The electric field is used to tune the nanoscale devices and an optical orientation approach is applied to understand the field‐tuned spin‐polarized band structures. It is found that the circular photogalvanic current can be erased and re‐created by poling, indicating the electric‐field‐based control over spin structure. The study enriches the basic understanding of the symmetry‐regulated optoelectronic response in ferroelectrics with spin‐orbit coupling.
An oxyhalide semiconductor Bi4NbO8Cl with strong spin‐orbit coupling, unique symmetry, and reasonable chemical stability is proposed as a potential candidate for hosting a switchable and nonvolatile Dresselhaus effect. Experimentally, it is found that the circular photo galvanic current can be erased and re‐created by electric voltage, indicating the electric‐field‐based control over spin structure.
Chemically stable quantum‐confined 2D metals are of interest in next‐generation nanoscale quantum devices. Bottom‐up design and synthesis of such metals could enable the creation of materials with ...tailored, on‐demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon‐near‐zero behavior, or wavelength‐specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air‐stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large‐area 2D‐InxGa1−x alloys are synthesized byConfinement Heteroepitaxy (CHet). Near‐complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.
Air‐stable large‐area 2D‐InxGa1−x alloys with tunable composition and no evidence of phase segregation are realized by confinement heteroepitaxy. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.
Ultrathin NbS2 films were synthesized from sputter-deposited ultrathin Nb films on SiO2/Si and quartz substrates at 850 °C under sulfur vapor pressure. The structure and surface composition of the ...synthesized films were characterized by grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy. The films have rhombohedral 3R-NbS2 structure and are nearly stoichiometric. The optical bandgaps of ultrathin NbS2 samples were determined from ultraviolet–visible-near–infrared spectrometry to be in the range of ∼0.43 to ∼0.90 eV and indirect. This implies that the ultrathin NbS2 film is semiconducting and differs from the metallic nature of bulk NbS2. The Raman shifts show distinct Raman active modes that depend on film thickness. The simple growth method developed can be applied to other TMDCs in which the metal has a high oxide heat of formation.
The symmetry of graphene is usually determined by a low-energy electron diffraction (LEED) method when the graphene is on the conductive substrates, but LEED cannot handle graphene transferred to ...SiO2/Si substrates due to the charging effect. While transmission electron microscopy can generate electron diffraction on post-transferred graphene, this method is too localized. Herein, we employed an azimuthal reflection high-energy electron diffraction (RHEED) method to construct the reciprocal space mapping and determine the symmetry of wafer-size graphene both pre- and post-transfer. In this work, single-crystalline Cu(111) films were prepared on sapphire(0001) and spinel(111) substrates with sputtering. Then the graphene was epitaxially grown on single-crystalline Cu(111) films with a low pressure chemical vapor deposition. The reciprocal space mapping using azimuthal RHEED confirmed that the graphene grown on Cu(111) films was single-crystalline, no matter the form of the monolayer or multilayer structure. While the Cu(111) film grown on sapphire(0001) may occasionally consist of 60° in-plane rotational twinning, the reciprocal space mapping revealed that the in-plane orientation of graphene grown atop was not affected. The proposed method for checking the crystalline integrity of the post-transferred graphene sheets is an important step in the realization of the graphene as a platform to fabricate electronic and optoelectronic devices.
The knowledge on the influence of surface roughness and the electron-phonon (el-ph) interaction on electrical transport properties of nanoscale metal films is important from both fundamental and ...technological points of view. Here we report a study of the temperature dependent electron transport properties of nanoscale copper films by measuring temperature dependent electrical resistivity with thickness ranging from 4 to 500 nm. We show that the residual resistivity, which is temperature independent, can be described quantitatively using both measured vertical surface root-mean-square roughness and lateral correlation length in the nanoscale, with no adjustable parameter, by a recent quasi-classical model developed by Chatterjee and Meyerovich (2010 Phys. Rev. B 81 245409-10). We also demonstrate that the temperature dependent component of the resistivity can be described using the Bloch-Grüneisen equation with a thickness dependent el-ph coupling constant and a thickness dependent Debye temperature. We show that the increase of the el-ph coupling constant with the decrease of film thickness gives rise to an enhancement of the temperature dependent component of the resistivity.
Flexo-photovoltaic effect in MoS2 Jiang, Jie; Chen, Zhizhong; Hu, Yang ...
Nature nanotechnology,
08/2021, Letnik:
16, Številka:
8
Journal Article
Recenzirano
The theoretical Shockley–Queisser limit of photon–electricity conversion in a conventional p–n junction could be potentially overcome by the bulk photovoltaic effect that uniquely occurs in ...non-centrosymmetric materials. Using strain-gradient engineering, the flexo-photovoltaic effect, that is, the strain-gradient-induced bulk photovoltaic effect, can be activated in centrosymmetric semiconductors, considerably expanding material choices for future sensing and energy applications. Here we report an experimental demonstration of the flexo-photovoltaic effect in an archetypal two-dimensional material, MoS2, by using a strain-gradient engineering approach based on the structural inhomogeneity and phase transition of a hybrid system consisting of MoS2 and VO2. The experimental bulk photovoltaic coefficient in MoS2 is orders of magnitude higher than that in most non-centrosymmetric materials. Our findings unveil the fundamental relation between the flexo-photovoltaic effect and a strain gradient in low-dimensional materials, which could potentially inspire the exploration of new optoelectronic phenomena in strain-gradient-engineered materials.A strain-gradient approach induced by the phase-change transition enables the observation of the flexo-photovoltaic effect in MoS2.
Recently, Sb2Se3, an Earth-abundant constituents compound, has emerged as a promising low-cost thin film photovoltaic material. The Sb2Se3 solar cells with a polycrystalline CdS buffer layer suffer ...from unsatisfactory photoexcitation stability. Herein, we fabricate flexible Sb2Se3 thin film solar cells by utilizing the transferable van der Waals epitaxial CdS on mica as a buffer layer. Compared to the nonepitaxial CdS buffer layer, the epitaxial CdS can significantly reduce the interface defects and interfacial diffusion. With the improved Sb2Se3/CdS interface, we obtained a power conversion efficiency of 7.15%, and the flexible Sb2Se3 solar cell exhibits an excellent stability under the continuous illumination. In addition, after 1000 bending cycles with a 50° bending angle, the flexible device maintained over 90% of the initial efficiency. These encouraging results demonstrate a potential application of Sb2Se3 solar cells in flexible, lightweight, and wearable power supply.
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•High-quality van der Waals epitaxial CdS was used as a buffer layer.•Epitaxial CdS layer reduced the interface defects and interfacial diffusions.•Flexible Sb2Se3 solar cells with an efficiency of 7.15% was obtained.•Epitaxial CdS buffer layer enabled no VOC degradation.
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