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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)

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.

Electron transport through iron porphyrin (FeP) molecules self-assembled on a gold (Au) substrate was investigated using conductive atomic force microscopy (AFM) to measure current–voltage (I–V) ch...

The V–VI binary chalcogenide, Sb2Se3, 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 Sb2Se3 thin film epitaxially grown on a flexible mica substrate through a relatively weak (van der Waals) interaction by vapor transport deposition. The epitaxial Sb2Se3 thin films exhibit a single (120) out-of-plane orientation and a 0.25° full width at half-maximum of (120) rocking curve in X-ray diffraction, confirming the high crystallinity of the epitaxial films. The Sb2Se3(120) plane is epitaxially aligned on mica(001) surface with the in-plane relationship of Sb2Se3[210]//mica­[010] and Sb2Se3[001]//mica­[100]. Compared to the photodetector made of a nonepitaxial Sb2Se3 film, the photocurrent of the epitaxial Sb2Se3 film photodetector is almost doubled. Furthermore, because of the flexibility and high sensitivity of the epitaxial Sb2Se3 film photodetector on mica, it has been successfully employed to detect the heart rate of a person. These encouraging results will facilitate the development of epitaxial Sb2Se3 film-based devices and potential applications in wearable electronics.

Most high-quality CdTe thin films are epitaxially grown on single-crystalline substrates through chemical bondings by either molecular beam epitaxy or metalorganic chemical vapor deposition. The ep...

Single crystal graphite is an extremely useful substrate to grow functional single crystalline films for future electronic and optoelectronic device applications. Due to weak van der Waals interaction, it allows one to grow high quality epitaxial films without the restriction of lattice matching and the films are relaxed at the interface without generating a high density of misfit induced defects even with very large lattice mismatch systems. However, the sizes of single crystalline graphite substrates are typically very small when cleaved from the natural graphite or exfoliated from the commercial highly oriented graphite. In this study we grew large scale single crystalline AB-stacking graphite films by chemical vapor deposition of graphite on wafer size, single crystalline Ni(111) films that were epitaxially grown by magnetron sputtering on spinel (MgAl2O4(111)) substrates. Our results show that smooth, single crystalline graphite films can be achieved at temperatures below 925 °C. Growth temperatures higher than 1000 °C promotes much rougher and thicker graphite films resulted from the inhomogeneous graphite segregation and precipitation processes in the Ni film. These large single crystalline graphite films may serve as substrates to grow functional semiconductor films for electronic and optoelectronic device applications.

We present the magnetic properties of three-bilayer superlattices [Co(0.8 nm)/Cu(1.1 nm)]3 and Co(0.8 nm)/Al(2.2 nm)]3 grown by magnetron sputter deposition on SiO2 substrates. Using the surface magneto-optical Kerr effect we observed longitudinal magnetic hysteresis loops with low coercivity values of

Atmospheric pressure chemical vapor deposition (APCVD) is employed for the synthesis of layered vanadium disulfide. By tuning several critical growth parameters, we achieve VS2 flakes with lateral ...

Traditionally, a high-quality CdTe film can only be grown on a single crystal substrate with a small lattice mismatch. Herein, we report the epitaxy of CdTe films on monolayer single crystal graphene buffered amorphous SiO2/Si(100) substrates, despite a 86% lattice mismatch between CdTe(111) and graphene. X-ray pole figure, electron backscatter diffraction mapping and transmission electron microscopy all confirm that the epitaxial CdTe films are composed of two domains: the primary and the Σ3 twin. The crystal quality of films is shown to improve as the post-deposition annealing temperature increases. However, the rotational misalignment in CdTe remains large even after annealing. Through density functional theory calculations on the charge transfer distribution at the interface of CdTe and graphene, it is found that the interface is dominated by the weak van der Waals interaction, which explains the large spread of in-plane orientation in CdTe films. Furthermore, the rotational misalignment in graphene itself is also confirmed to produce the large in-plane orientation spread in CdTe films. Although imperfect in epitaxy quality, this work demonstrates that monolayer single crystal graphene can buffer amorphous substrates for growing epitaxial films, and hence hints an opportunity for developing advanced thin film devices using graphene as a template.

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−1 @200 mA g−1 current density) and rate capability (~70 mAh g−1 @1000 mA g−1), while achieving capacity retention close to 100% after 400 charge−discharge steps., VS2 is a promising cathode material for lithium-ion batteries, but is susceptible to Peierls distortion during (de)lithiation. Here the authors show that VS2 cathodes can be stabilized by conformally coating them with a nanoscale TiS2 protective layer, leading to impressive electrochemical performance.

We present a simple method of variable temperature process that can potentially enhance the hydrogen storage properties of a large variety of solid state materials. In this approach, hydrogen gas is first introduced at about room temperature, which is followed by a gradual increase to a preset maximum temperature value, Tmax. Using this approach, we investigated hydrogen absorption properties of vertically aligned arrays of magnesium nanotrees and nanoblades fabricated by glancing angle deposition (GLAD) technique, and conventional Mg thin film. Weight percentage (wt%) storage values were measured by quartz crystal microbalance (QCM). After exposing Mg samples to H2 at 30 bar and 30 °C, dynamic absorption measurements were conducted as the temperature was increased from 30 °C to maximum values of Tmax = 100, 200, and 300 °C all within 150 min. QCM measurements revealed that variable temperature method results in significant improvements in hydrogen storage values over the ones obtained by conventional constant temperature process. At a low effective temperature Teff = 165 °C (Tmax = 300 °C), we achieved storage values of 6.19, 4.76, and 2.79 wt% for Mg nanotrees, nanoblades, and thin film, respectively.

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

The V-VI binary chalcogenide, Sb

Pyroelectricity describes the generation of electricity by temporal temperature change in polar materials

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 (C4H9NH3)2PbI4 flakes with controlled dimensions. Both substrate–perovskite epitaxial interaction and interlayer vdW interaction play significant roles in suppressing the structural phase transition. With reducing flake thickness from ∼100 to ∼20 nm, electron–phonon coupling strength decreases by ∼30%, suggesting the ineffectiveness of phonon confinement of the natural quantum wells. Therefore, the conventional understanding that vdW perovskite is equivalent to a multiple quantum well has to be substantially amended due to significant nonlocal phononic effect...

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 SiO2/Si. On the as-grown graphene, deposition of ultrathin Sb resulted in two growth modes and associated morphologies of Sb. One was Sb islands grown in Volmer–Weber (VW) mode, and the other was Sb sheets grown in Frank–van der Merve (FM) mode. In contrast, only Sb islands grown in VW mode were found in a parallel growth experiment on the transferred graphene. The existence of Sb sheets on the as-grown graphene was attributed to the remote epitaxy between Sb and Cu underneath the graphene. In addition, Sb thin films were grown on both the as-grown and transferred graphene substrates. Both films indicated high quality, and no significant difference can be found between th...

Conventional hetero-epitaxial films are typically grown on lattice and symmetry matched single crystal substrates. We demonstrated the epitaxial growth of orthorhombic SnS film (∼500 nm thick) on single crystal, monolayer graphene that was transferred on the amorphous SiO2/Si substrate. Using X-ray pole figure analysis we examined the structure, quality and epitaxy relationship of the SnS film grown on the single crystal graphene and compared it with the SnS film grown on commercial polycrystalline graphene. We showed that the SnS films grown on both single crystal and polycrystalline graphene have two sets of orientation domains. However, the crystallinity and grain size of the SnS film improve when grown on the single crystal graphene. Reflection high-energy electron diffraction measurements show that the near surface texture has more phases as compared with that of the entire film. The surface texture of a film will influence the growth and quality of film grown on top of it as well as the interface formed. Our result offers an alternative approach to grow a hetero-epitaxial film on an amorphous substrate through a single crystal graphene buffer layer. This strategy of growing high quality epitaxial thin film has potential applications in optoelectronics.

Sb2Se3 thin film is an emerging photon absorber used in solar cells. We report the study of surface and interface structures of Sb2Se3(1 2 0) film grown on mica substrate by a high-rate vapor transport method. The interface epitaxial relationship between Sb2Se3 and mica examined by the cross- sectional TEM images and diffraction patterns along the [0 0 1] and [10] 2 ¯ directions of Sb2Se3 reveal a rectangular structure with lengths of 4.03 ± 0.1 A and 5.29 ± 0.1 A, consistent with the [1 2 0] out-of-plane direction of Sb2Se3 bulk lattice parameters. In contrast, the two-dimensional reciprocal space map (2D map) constructed from azimuthal reflection high-energy electron diffraction (ARHEED) patterns from the surface exhibits a decorated hexagonal structure. This surface structure emerges from six epitaxial orientation domains/rods and each domain has a rectangular unit mesh of 3.94 ± 0.09 A and 26.95 ± 1.16 A along the [0 0 1] and [10] 2 ¯ directions. The 26.95 A is consistent with the unit mesh of the outermost layer of the Sb2Se3(1 2 0) domains/rods. Our 2D map reveals surface information that are not easily observed by other diffraction techniques.

Monolayer WSe2, a 2D transition metal dichalcogenide (TMDCs), has been demonstrated as a good candidate for potential applications in optoelectronics. It is imperative to know the crystalline quality of WSe2 over the wafer scale prior to its applications. Azimuthal reflection high-energy electron diffraction (ARHEED) is demonstrated to be a powerful technique to measure the symmetry, lattice constants, and in-plane orientation domain dispersion in wafer-scale, continuous monolayer WSe2 epitaxially grown by metal organic chemical vapor deposition on c-plane sapphire substrate. The constructed 2D reciprocal map from ARHEED reveals few degrees’ dispersion in WSe2 orientation domains due to the step meandering/bunching/mosaic of sapphire substrate. Minor 30° orientation domains are also observed. The methodology can be applied to study other TMDCs epitaxial monolayers, graphene, and confined atomically thin hetero-epitaxial metals.

There has been increasing interest in the fabrication of thin film materials with mixed dimensions, in particular, 2D to 3D and 3D to 2D heterostructures. Often, if the interface interaction is weak, the lattice matching criterion between the substrate and overlayer can be lifted. If the overlayer lattice is completely relaxed, it can form an incommensurate film on the mismatched substrate. In this work, we show that domain boundaries are inherent in the incommensurate epitaxial films due to random nucleation sites of domains in an overlayer. The nature and origin of the incommensurate domain boundaries are different from the conventional dislocation boundaries that come from the relaxation of strain due to film–substrate lattice mismatch. We propose that the formation of such domain boundaries can be studied through Voronoi tessellation. Using a case study of monolayer WS2 on sapphire (2D on 3D), we show the formation of domain boundaries that compared well with a recent experiment reported in the literature. In the Voronoi tessellation, we also show quantitatively that the average domain size depends on the density of nucleation sites. The conclusion of this case study may be generalized to any incommensurate epitaxial films when the interface interaction is weak.

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.

Vanadium dioxide (VO2) and vanadium pentoxide (V2O5) thin films change their properties in response to external stimuli such as photons, temperature, electric field and magnetic field and have applications in electronics, optical devices, and sensors. Due to the multiple valence states of V and non-stoichiometry in thin films, it is challenging to grow epitaxial, single-phase V-oxide on a substrate, or a heterostructure of two epitaxial V-oxides. We report the formation of a heterostructure consisting of a few nm thick ultrathin V2O5 epitaxial layer on pulsed laser deposited tens of nm thick epitaxial VO2 thin films grown on single crystal Al2O3(001) substrates without post annealing of the VO2 film. The simultaneous observation of the ultrathin epitaxial V2O5 layer and VO2 epitaxial film is only possible by our unique reflection high energy electron diffraction pole figure analysis. The out-of-plane and in-plane epitaxial relationships are V2O5[100]||VO2[010]||Al2O3[001] and V2O5[03 2 ¯ ]||VO2[100]||Al2O3[1 1 ¯ 0], respectively. The existence of the V2O5 layer on the surface of the VO2 film is also supported by X-ray photoelectron spectroscopy and Raman spectroscopy.

Single crystal CdTe films are desirable for optoelectronic device applications. An important strategy of creating films with high crystallinity is through epitaxial growth on a proper single crystal substrate. We report the metalorganic chemical vapor deposition of epitaxial CdTe films on the CdS/mica substrate. The epitaxial CdS film was grown on a mica surface by thermal evaporation. Due to the weak van der Waals forces, epitaxy is achieved despite the very large interface lattice mismatch between CdS and mica (∼21–55%). The surface morphology of mica, CdS and CdTe were quantified by atomic force microscopy. The near surface structures, orientations and texture of CdTe and CdS films were characterized by the unique reflection high-energy electron diffraction surface pole figure technique. The interfaces of CdTe and CdS films and mica were characterized by X-ray pole figure technique and transmission electron microscopy. The out-of-plane and in-plane epitaxy of the heteroepitaxial films stack are determined to be CdTe(111)//CdS(0001)//mica(001) and [ 1 ¯ 2 1 ¯ ]CdTe//[ 1 ¯ 100]CdS//[010]mica, respectively. The measured photoluminescence (PL), time resolved PL, photoresponse, and Hall mobility of the CdTe/CdS/mica indicate quality films. The use of van der Waals surface to grow epitaxial CdTe/CdS films offers an alternative strategy towards infrared imaging and solar cell applications.

A two-step process for synthesizing stable Cs2SnI6 perovskite thin films is reported in this letter. The two-step process includes the co-evaporation of two precursors SnI2 and CsI onto a glass substrate, followed by a post thermal annealing process in iodine vapor. Using this technique, pure Cs2SnI6 perovskite thin films were successfully synthesized without any wet process. These perovskite thin films are found to be stable under ambient conditions. They also show an electron mobility up to 509 cm2 V−1 s−1, which is higher than the mobilities of films prepared by solution processes reported in the literature.

Semiconductor thin films grown on weakly interacting substrates have recently attracted much attention due to the fact that one can peel/release the film from its substrate for flexible electronic and optoelectronic applications. In this work we report separable CdTe(1 1 1) epitaxial films grown on mica substrate via vapor transport deposition. The CdTe films were separated from the mica substrate using double-sided Cu tape, exposing the CdTe interface-surface. Azimuthal reflection high-energy electron diffraction (ARHEED) was used to characterize the fresh CdTe interface-surface and the original CdTe surface. Dramatic differences between these two surfaces were observed. (1) Diffraction streaks were observed from the interface-surface, indicating a smooth surface. Diffraction spots were observed from the surface, indicating a rough surface. (2) The rough CdTe surface has twin and secondary twin structures, which are difficult to be observed with conventional X-ray diffraction. (3) The full-width-at-half maximum (FWHM) analysis of the streaks and spots in RHEED patterns reveals that despite the smoothness of the interface-surface, there is a lower degree of lateral long-range order as compared with the rough surface. This etchant-free method to create smooth interface-surface and ARHEED characterization may be applicable to other films that are detachable from weakly interacting substrates.

Molybdenum disulfide (MoS2) on the c-plane sapphire has been a very popular system to study in the two-dimensional (2D) materials community. Bottom-up synthesis of monolayer (ML) MoS2 with excellent electrical properties has been achieved on sapphire by various methods, making it a very promising candidate to be used in the next generation nano-electronic devices. However, large-area ML MoS2 with comparable quality as the relatively small size exfoliated ML remains quite a challenge. To overcome this bottle neck, a comprehensive understanding of the structure of the as-grown ML material is an essential first step. Here, we report a detailed structural characterization of wafer-scale continuous epitaxial ML MoS2 grown by metalorganic chemical vapor deposition on sapphire using an azimuthal reflection high-energy electron diffraction (ARHEED) technique. With ARHEED we can map not only 2D but also 3D reciprocal space structure of the ML statistically. From the oscillation in the ARHEED intensity profile along the vertical direction of the ML, we derived a real space distance of ~3 Å at the interface of ML and sapphire. Quantitative diffraction spot broadening analyses of the 3D reciprocal space map reveals low density defects and a small angular misalignment of orientation domains in ML MoS2. Based on atomic force microscopy height distribution analysis, cross-section scanning transmission electron microscopy, and density functional theory calculations, we suggest that there exists a passivation layer between MoS2 ML and sapphire substrate. This ARHEED methodology also has been applied to ML WS2 and is expected to be applicable to other ML transition metal dichalcogenides on arbitrary crystalline or non-crystalline substrates.

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 ...

The effect of CdCl2 heat treatment on the ZnTe layer used as the back electron reflector layer in polycrystalline CdTe solar cells was investigated. A thin film of ZnTe was grown on top of polycrystalline CdTe layer using metalorganic vapor phase epitaxy, which was then subjected to CdCl2 heat treatment. The effect was examined using various characterization techniques such as X-ray diffraction, energy dispersive X-ray spectroscopy and scanning electron microscope images and it was found that the ZnTe layer transformed into CdTe and ZnO due to the treatment. This was verified by performing CdCl2 heat treatment on pure ZnTe thin layer directly grown on glass which also showed the conversion of ZnTe to CdTe and ZnO. To study the effect of this transformation on final device performance, thin film CdTe solar cell structures were fabricated with ZnTe electron reflector layer with and without the CdCl2 heat treatment. The device without the ZnTe layer seemed to perform better than the devices with ZnTe layer and the CdCl2 heat treatment.

Epitaxial Ge films are useful as a substrate for high-efficiency solar cell applications. It is possible to grow epitaxial Ge films on low cost, cube textured Ni(001) sheets using CaF2(001) as a buffer layer. Transmission electron microscopy (TEM) analysis indicates that the CaF2(001) lattice has a 45o in-plane rotation relative to the Ni(001) lattice. The in-plane epitaxy relationships are CaF2[110]//Ni[100] and CaF2[110]//Ni[010]. Energy dispersive spectroscopy (EDS) shows a sharp interface between Ge/CaF2 as well as between CaF2/Ni. Electron backscatter diffraction (EBSD) shows that the Ge(001) film has a large grain size (~50 µm) with small angle grain boundaries (< 8o). The epitaxial Ge thin film has the potential to be used as a substrate to grow high quality III-V and II-VI semiconductors for optoelectronic applications.

Novel materials suitable for optoelectronics are of great interest due to limited and diminishing energy resources and the movement toward a green earth. We report a simple film growth method to tune the S composition, x from 1 to 2 in semiconductor ultrathin SnSx films on quartz substrates, that is, single phase SnS, single phase SnS2, and mixed phases of both SnS and SnS2 by varying the sulfurization temperature from 150 to 500 °C. Due to the ultrathin nature of the SnSx films, their structural and optical properties are characterized and cross-checked by multiple surface-sensitive techniques. The grazing incidence X-ray diffraction (GIXRD) shows that the single phase SnS forms at 150 °C, single phase SnS2 forms at 350 °C and higher, and mixed phases of SnS and SnS2 form at temperature between. GIXRD shows structures of SnS film and SnS2 film are orthorhombic and 2H hexagonal, respectively. To complement the GIXRD, the reflection high energy electron diffraction pattern analysis shows that both pure pha...

Realizing high-quality heteroepitaxy of a wide variety of films of very large lattice misfit, f ≥ 10%, with the substrate is a great challenge, but also a potential advancement, because the films may be made threading-dislocation-free as all the dislocations will be confined at the interface. In spite of the numerous experimental findings in the literature, first-principles theory for such systems is virtually nonexistent due to their intrinsic heterogeneity; namely, away from the interface, the film is strain free, but at the interface, not only strain but also misfit dislocation develop. Here, a modular approach is proposed to study such heterogeneous films by a combined first-principles and elasticity theory method to predict, for example, their epitaxial relationship. Four representative metal–semiconductor interfaces, Al(111)/Si(111), Cu(111)/Si(111), Cu(001)/Si(001), and CaF2(111)/Ni(001), are considered. By taking into account the chemical bonding information at the interface by first-principles th...

Cube-textured Ni(001) foils have been considered as a viable alternative substrate to grow high quality functional films for large area optoelectronic devices. In this work, we report the heteroepitaxial growth of CaF2(001) films on cube-textured Ni(001) foils at 350–600 °C with in-plane orientation of CaF2[110]//Ni[100] and CaF2[ 1 - 10 ]//Ni[010] with 45° rotation respect to the Ni(001) substrate. Unlike CaF2(111)/Ni(001) films where there exist four independent rotational domains with rotational domain boundaries, CaF2(001)/Ni(001) contains no rotational domains or rotational domain boundaries. This makes CaF2(001)/Ni(001) films better candidates as templates for the growth of high quality functional semiconductors. We also demonstrate that Ge(001) film with no rotational domains and with a grain size of ~ 50 μm similar to that of the Ni substrate can be grown on the CaF2(001) buffered Ni substrate.

Knowledge of the kinetics of oxide formation on an ultrathin copper (Cu) film is important both for fundamental understanding and for application, such as the use of copper as interconnects in semiconductor integrated circuits. We used angle resolved X-ray photoelectron spectroscopy to study the growth mechanism of Cu oxides at ambient conditions over times ranging from 1 h to 120 days after the preparation of a 50 nm-thick epitaxial Cu(1 0 0) film. We analyzed high resolution spectra near O 1s, Cu 2p and Cu LMM peaks to understand the kinetics of oxidation and to estimate overall oxide thickness as a function of time. We demonstrate that the oxide thickness of the epitaxial Cu film follows approximately an inverse logarithmic growth rate law, and that the rate of oxidation is substantially slower in epitaxial films than in polycrystalline films reported in the literature.

Remarkable properties of layered metal dichalcogenides and their potential applications in various fields have raised intense interest worldwide. We report tens of microns-sized ultrathin single crystal SnS2 flakes grown on amorphous substrates using a simple one-step thermal coevaporation process. X-ray pole figure analysis reveals that a majority of flakes are oriented with the (0001) plane parallel to the substrate and a preferred fiber texture. For few-layer-thick SnS2, Moire patterns of 6-fold and 12-fold symmetries are observed by transmission electron microscopy imaging and diffraction. These patterns result from the relative rotation between SnS2 layers in the ultrathin flake. The 12-fold symmetry is consistent with a known quasicrystal pattern. The photoluminescence spectrum supports that these ultrathin flakes possess a direct bandgap. Carrier lifetime measured by time-resolved photoluminescence of a single flake is a few nanoseconds. These results improve our understanding of the formation and ...

Ge1−xSnx films were grown via physical vapor deposition below the crystallization temperature of Ge on single crystal (111) and (100) CaF2 substrates to assess the role of Sn alloying in Ge crystallization. By studying samples grown at several growth temperatures ranging from 250 °C to 400 °C we report temperature-dependent trends in several of the films' properties. X-ray diffraction theta vs. two-theta (θ/2θ) scans indicate single orientation Ge1−xSnx(111) films are grown on CaF2(111) substrates at each temperature, while a temperature-dependent superposition of (111) and (100) orientations are exhibited in films grown on CaF2(100) above 250 °C. This is the first report of (111) oriented Ge1−xSnx grown on a (100) oriented CaF2 substrate, which is successfully predicted by a superlattice area matching model. These results are confirmed by X-ray diffraction pole figure analysis. θ/2θ results indicate substitutional Sn alloying in each film of about 5%, corroborated by energy dispersive spectroscopy. Additionally, morphological and electrical properties are measured by scanning electron microscopy, atomic force microscopy and Hall mobility measurements and are also shown to be dependent upon growth temperature.

High quality heteroepitaxy of CdTe is challenging due to lattice mismatches of CdTe with many substrates. Herein, we demonstrate the epitaxial growth of single crystalline CdTe films, in multilevel island format, on mica using metalorganic chemical vapor deposition, regardless of large in-plane lattice mismatch (\ensuremath{\sim}13%) between CdTe(111) and mica(001). X-ray and electron diffractions suggest that CdTe is epitaxially aligned with mica: out-of-plane $\mathrm{CdTe}(111)\text{//mica}(001)$ and in-plane CdTe $[\overline{1}2\overline{1}]$//mica $[010]$. Full-width-at-half-maximum (FWHM) of x-ray rocking curve and FWHM of x-ray azimuthal in-plane angular dispersion of CdTe are shown to be $0.{11}^{\ensuremath{\circ}}$ and $0.{38}^{\ensuremath{\circ}}$, respectively, better than most CdTe films reported. Electron backscattering diffraction shows that CdTe grains are tens of \ensuremath{\mu}m and, if twin boundaries are excluded, in excess of 250 \ensuremath{\mu}m in size. In contrast to the belief that overlayer growth on mica is purely through van der Waals interaction, our first-principle calculations uncover that van der Waals interaction only contributes to 20% of the total interfacial energy, and 80% of the interfacial energy comes from chemical interaction. We believe such a strong chemical interaction is accountable for the high-quality epitaxy. The demonstration of epitaxial growth of high-quality semiconductor on mica with a large lattice mismatch creates opportunities for flexible optoelectronic devices.

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. The primary Cu domain thus has good crystal quality as manifested by a narrow crystal misorientation distribution. On the other hand, we show that graphene interface imperfections, such as bilayers/multilayers, wrinkles and interface contaminations, can effectively weaken the atomic interactions between Cu and sapphire. This results in a second Cu domain, which directly grows on and follows the graphene hexagonal lattice symmetry and orientation. Because of the weak van der Waals interaction between Cu and graphene, this domain has inferior crystal quality. The results are further confirmed using graphene buffered spinel(111) substrate, which indicates that this remote epitaxial behavior is not unique to the Cu/sapphire system.

The van der Waals (vdW) bond is traditionally believed to be orders of magnitude lower than the typical chemical bond (i.e., ionic, metallic, or covalent), and hence the effects caused by a vdW interface are thought to be trivial. In this paper, by investigating the vdW epitaxial growth of a mechanically soft perovskite on the vdW substrate, we obtained the solid proof of strong non-negligible vdW interaction. The experimental results illustrate the formation of cracks and holes for the relaxation of the vdW strain as well as a lattice-constant-dependent epitaxial angle evolution and a pronounced band-structure change. The first-principles calculations indicate that the contribution of the vdW interaction energy at the epitaxial interface reaches up to more than a quarter of the overall interaction energy far exceeding the traditionally recognized vdW bonding strength. Both the experimental and the theoretical work show that given the appropriate material system, the vdW interaction could be strong enough to significantly manipulate material properties.

A multistep diffusion-mediated process was developed to control the nucleation density, size, and lateral growth rate of WSe

We report two different CdTe homojunction solar cell structures. Single-crystal CdTe homojunction solar cells were grown on GaAs single-crystal substrates by metalorganic chemical vapor deposition. Arsenic and iodine were used as dopants for p-type and n-type CdTe, respectively. Another homojunction solar cell structure was fabricated by growing n-type CdTe directly on bulk p-type CdTe single-crystal substrates. The electrical properties of the different layers were characterized by Hall measurements. When arsine was used as arsenic source, the highest hole concentration was ~6 × 1016 cm–3 and the activation efficiency was ~3%. Very abrupt arsenic doping profiles were observed by secondary ion mass spectrometry. For n-type CdTe with a growth temperature of 250°C and a high Cd/Te ratio the electron concentration was ~4.5 × 1016 cm–3. Because of the 300 nm thick n-type CdTe layer, the short circuit current of the solar cell grown on the bulk CdTe substrate was less than 10 mA/cm2. The open circuit voltage of the device was 0.86 V. According to a prediction based on measurement of short circuit current density (J sc) as a function of open circuit voltage (V oc), an open circuit voltage of 0.92 V could be achieved by growing CdTe solar cells on bulk CdTe substrates.

A simple method for the synthesis of Nb2O5 films of thicknesses ranging from tens to several hundreds of nanometers on amorphous silicon dioxide or quartz substrates is presented. Nb2O5 films were formed by annealing the sputter deposited Nb films under an Ar flow and without oxygen plasma in a quartz tube within a furnace at 850 °C. The structural, compositional, optical, and vibrational properties were characterized by grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet visible spectroscopy, and Raman scattering. Each of the Nb2O5 films is polycrystalline with an orthorhombic crystal structure. We observed vibrational modes including longitudinal optical, transverse optical, and triply degenerate modes, and measured the indirect optical band gap to be ∼3.65 eV. The transmittance spectrum of the ∼20 nm thick Nb2O5 film shows over 90% transmittance below the band gap energy in the visible wavelength range and decreases to less than 20% in the ultraviolet regime. The optical properties of the films in the UV-vis range show potential applications as UV detectors.

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

The texture orientation of 2.5, 5, and 100 nm thick Mo films grown by sputter deposition on SiO2 membranes at room temperature has been studied quantitatively using transmission electron microscopy. The sample tilt angle dependent diffraction patterns of these films revealed a fiber texture with the [110] out-of-plane direction for all film thicknesses. Films grown at different sputtering power and substrate–target distance showed a similar texture orientation. These results would imply that the growth was in zone II, according to the microstructure classification of the structural zone model. This is in contrast to the conventional assignment of zone I for Mo deposition at room temperature based on the low Mo homologous temperature of ∼0.1. It was found that texture dispersion was reduced as the thickness of the film increased. Possible mechanisms, including energetic particles bombardment during growth, which could affect the surface mobility and texture of the films, were discussed.