Vertically Aligned One-Dimensional Crystal-Structured SbSe for High-Efficiency Flexible Solar Cells Regulating Selenization Kinetics.

Recently, antimony selenide (SbSe) has exhibited an exciting potential for flexible photoelectric applications due to its unique one-dimensional (1D) chain-type crystal structure, low-cost constituents, and superior optoelectronic properties. The 1D structure endows SbSe with a strong anisotropy in carrier transport and a lasting mechanical deformation tolerance. The control of the crystalline orientation of the SbSe film is an essential requirement for its device performance optimization.

Liquid-Phase van der Waals Epitaxy of a Few-Layer and Unit-Cell Thick Ruddlesden-Popper Halide Perovskite.

2D Ruddlesden-Popper (RP) halide perovskites with natural multiple quantum well structures are an ideal platform to integrate into vertical heterostructures, which may introduce plentiful intriguing optoelectronic properties that are not accessible in a single bulk crystal. Here, we report liquid-phase van der Waals epitaxy of a 2D RP hybrid perovskite (4,4-DFPD)PbI (4,4-DFPD is 4,4-difluoropiperidinium) on muscovite mica and fabricate a series of perovskite-perovskite vertical heterostructures by integrating it with a second 2D RP perovskite R-NPB [NPB = 1-(1-naphthyl)ethylammonium lead bromide] sheets. The grown (4,4-DFPD)PbI nanobelt array can be multiple layers to unit-cell thin and are crystallographically aligned on the mica substrate.

Giant pyroelectricity in nanomembranes.

Pyroelectricity describes the generation of electricity by temporal temperature change in polar materials. When free-standing pyroelectric materials approach the 2D crystalline limit, how pyroelectricity behaves remained largely unknown. Here, using three model pyroelectric materials whose bonding characters along the out-of-plane direction vary from van der Waals (InSe), quasi-van der Waals (CsBiNbO) to ionic/covalent (ZnO), we experimentally show the dimensionality effect on pyroelectricity and the relation between lattice dynamics and pyroelectricity.

Tunable 2D Group-III Metal Alloys.

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.

Flexo-photovoltaic effect in MoS.

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, MoS, by using a strain-gradient engineering approach based on the structural inhomogeneity and phase transition of a hybrid system consisting of MoS and VO.

High-Crystallinity Epitaxial SbSe Thin Films on Mica for Flexible Near-Infrared Photodetectors.

The V-VI binary chalcogenide, SbSe, has attracted considerable attention for its applications in thin film optoelectronic devices because of its unique 1D structure and remarkable optoelectronic properties. Herein, we report an SbSe thin film epitaxially grown on a flexible mica substrate through a relatively weak (van der Waals) interaction by vapor transport deposition. The epitaxial SbSe thin films exhibit a single (120) out-of-plane orientation and a 0.

Chemical reaction induced carrier localization in nanometer-thin Al/Ru, Al/Co, and Al/Mo superlattices.

It is well-known that the electrical conductivity of a metallic film reduces dramatically when the film becomes very thin. This effect is mainly attributed to surface scattering of the conducting carriers. In a multilayer structure, interface scattering also reduces the conductance, but chemical reactions at the interfaces can have equal or bigger effects.

Vanadium disulfide flakes with nanolayered titanium disulfide coating as cathode materials in lithium-ion batteries.

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.

Remote Phononic Effects in Epitaxial Ruddlesden-Popper Halide Perovskites.

Despite their weak nature, van der Waals (vdW) interactions have been shown to effectively control the optoelectronic and vibrational properties of layered materials. However, how vdW effects exist in Ruddlesden-Popper layered halide perovskites remains unclear. Here we reveal the role of interlayer vdW force in Ruddlesden-Popper perovskite in regulating phase-transition kinetics and carrier dynamics based on high-quality epitaxial single-crystalline (CHNH)PbI flakes with controlled dimensions.

Remote epitaxy of copper on sapphire through monolayer graphene buffer.

In this work, we show that remote heteroepitaxy can be achieved when Cu thin film is grown on single crystal, monolayer graphene buffered sapphire(0001) substrate via a thermal evaporation process. X-ray diffraction and electron backscatter diffraction data show that the epitaxy process forms a prevailing Cu crystal domain, which is remotely registered in-plane to the sapphire crystal lattice below the monolayer graphene, with the (111) out-of-plane orientation. As a poor metal with zero density of states at its Fermi level, monolayer graphene cannot totally screen out the stronger charge transfer/metallic interactions between Cu and substrate atoms.

van der Waals Epitaxy of Antimony Islands, Sheets, and Thin Films on Single-Crystalline Graphene.

Antimony (Sb) nanostructures, including islands, sheets, and thin films, of high crystallinity were epitaxially grown on single-crystalline graphene through van der Waals interactions. Two types of graphene substrates grown by chemical vapor deposition were used, the as-grown graphene on Cu(111)/ c-sapphire and the transferred graphene on SiO/Si. On the as-grown graphene, deposition of ultrathin Sb resulted in two growth modes and associated morphologies of Sb.

Diffusion-Controlled Epitaxy of Large Area Coalesced WSe Monolayers on Sapphire.

A multistep diffusion-mediated process was developed to control the nucleation density, size, and lateral growth rate of WSe 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 HSe to promote surface diffusion of tungsten-containing species to form oriented WSe islands with uniform size and controlled density. The growth conditions were then adjusted to suppress further nucleation and laterally grow the WSe islands to form a fully coalesced monolayer film in less than 1 h.

Revealing the Crystalline Integrity of Wafer-Scale Graphene on SiO/Si: An Azimuthal RHEED Approach.

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 SiO/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.

Photon Transport in One-Dimensional Incommensurately Epitaxial CsPbX Arrays.

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 CsPbX (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.

Effects of nanoscale surface roughness on the resistivity of ultrathin epitaxial copper films.

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.

Air stability of low-temperature dehydrogenation of Pd-decorated Mg blades.

We demonstrated that Pd-decorated Mg blades are air-stable for hydrogen storage with a low desorption temperature of 373 K. Pd-catalyst-decorated Mg blades were prepared by 64° oblique incident angle thermal deposition on a rotatable substrate with the rotation axis perpendicular to the substrate. The hydrogen desorption from Pd-decorated Mg blades was performed and recorded by temperature-programmed desorption (TPD) for repeated hydrogenation–dehydrogenation cycles.

Biaxially textured Al film growth on CaF2 nanostructures toward a method of preparing single-crystalline Si film on glass substrates.

We report the room temperature growth of biaxially textured Al films and further demonstrate the use of these Al films in preparing single-crystalline Si layers on glass substrates. The formation of the biaxial texture in Al film relies on the existence of the CaF(2) buffer layer prepared using oblique angle physical vapor deposition, which consists of single-crystalline nanorods with caps that are in the form of inverted nanopyramids. The single-crystalline Si film was obtained upon crystallization of the amorphous Si film deposited through physical evaporation on the biaxially textured Al film.