Vapor Phase Growth Research Articles

Effective shape-controlled synthesis of gallium selenide nanosheets by vapor phase deposition

The controlled synthesis of large and uniform gallium selenide (GaSe) crystals is crucial for its various applications based on the attractive properties of this emerging material. In this work, vapor phase growth of high-quality monolayer GaSe nanosheets with multiple shape and size is achieved by tuning the Ga/GaSe ratio in the precursor. A theoretical model based on density functional theory calculations and kinetic Wulff construction theory describe the observed shape evolution of the GaSe nanosheets. Results show that the Ga/Se ratio plays a critical role in the evolution of the domain shape and size. Moreover, the as-grown GaSe nanosheets show improved performance with photoresponse time less than 0.7 ms and responsibility up to 3,000 A/W. This study presents a previously unexplored strategy for the controlled growth of two-dimensional (2D) GaSe nanosheets for promising applications in next-generation optoelectronics.

Large-Scale Production of Nanocrystalline Black Phosphorus Ceramics.

Black phosphorus is currently among the most explored two-dimensional (2D) materials. Currently, the synthesis methods are dominantly based on vapor-phase growth of black phosphorus. In this manuscript, we demonstrate large-scale synthesis of black phosphorus by rapid high-pressure transition of red phosphorus. The high-pressure conversion of red phosphorus led to high-density nanocrystalline black phosphorus ceramics. The resulting material was explored in detail including structural and morphological characterization in addition to thermal and electrical transport and basic thermophysical properties. The nanocrystalline black phosphorus can be employed for large-scale production of stable few/single-layer black phosphorus colloidal solutions in various solvents.

Synthesis and Characterization of Monoclinic Gallium Oxide Nanomaterials for High-Concentration Ethanol Vapor Detection

This study was conducted to synthesize Gallium Oxide nanomaterials from its bulk form using a low-cost and repeatable method and investigate on its potential as sensing element for Ethanol (EtOH). Horizontal Vapor Phase Growth (HVPG) Technique was utilized to synthesize monoclinic Gallium Oxide (β-Ga2O3) nanowires. Images obtained from the Scanning Electron Microscope showed high density nanowires specifically in the area of highest temperature gradient. SEM also showed that the grown structures using HVPG Technique were at the nanoscale with diameters ranging from 51.60 to 908.38 nm. Higher surface-to-volume ratio was also noted in the area of highest temperature gradient which was subjected to an applied magnetics field. The Gallium to Oxygen ratio was verified via EDX to be approximately 2:3 which agrees with the atomic ratio of Ga2O3. The monoclinic structure of the grown nanomaterials was investigated using Raman Spectroscopy. Raman peaks of the samples were at 199 and 486 cm−1 which was accounted to the presence of two Raman-active modes of the b-polymorph of Ga2O3. The Raman spectrum of the grown nanowires confirmed that the material is monoclinic in structure and belongs to C2/m space group. The I-V Curve of the grown nanowires were also determined using two-point probe which illustrates a non-linear curve similar to that of a semiconductor material. Furthermore, additional fundamental properties such as resistivity and specific conductance of the materials were also determined via van der Pauw Technique. Results showed that the material has high specific conductance and low resistivity. The synthesized nanomaterial was responsive to Ethanol vapor. Exposure to the said compound increased its resistance. Graphs showed that there is a significant increase in the resistance after exposure to ethanol vapor.

One-step vapour phase growth of two-dimensional formamidinium-based perovskite and its hot carrier dynamics.

Formamidinum lead iodide perovskite is one of the most promising materials for application in solar cells due to its narrow band gap and higher thermal stability. In this work, we demonstrate the facile synthesis of square-shaped formamidinium lead iodide single crystals on indium tin oxide (ITO) substrates using a one-step vapour phase deposition method. Formamidinium lead iodide-based two-dimensional layered perovskite crystals were successfully synthesized by controlling the deposition conditions. These crystals exhibited a blue-shifted photoluminescence (PL) compared to the conventional formaminium lead iodide perovskite crystals. Power law fittings of the excitation power dependent PL spectra revealed that Auger heating becomes dominant at high excitation densities. In addition, we observed an asymmetric broadening of the PL peak tail at the high energy side, indicating light emission from hot carriers even under steady-state illumination conditions. Phonon-bottleneck effect and Auger heating were considered as the main mechanisms for retardation of hot carrier cooling. Further analysis of the high energy tails using Maxwell-Boltzmann fitting revealed hot-carrier temperatures as high as 690 K. Our findings provide an important aspect of the synthetic approach of perovskites for their potential application in hot carrier solar cells.

Synthesis and Characterization of Gallium Oxide/Tin Oxide Nanostructures via Horizontal Vapor Phase Growth Technique for Potential Power Electronics Application

The monoclinic β‐gallium oxide (Ga2O3) was viewed as a potential candidate for power electronics due to its excellent material properties. However, its undoped form makes it highly resistive. The Ga2O3/SnO2 nanostructures were synthesized effectively via the horizontal vapor phase growth (HVPG) technique without the use of a magnetic field. Different concentrations of Ga2O3 and SnO2 were varied to analyze and describe the surface morphology and elemental composition of the samples using the scanning electron microscopy (SEM) and energy‐dispersive X‐ray (EDX) spectroscopy, respectively. Meanwhile, the polytype of the Ga2O3 was confirmed through the Fourier transform infrared (FTIR) spectroscopy. The current‐voltage (I–V) characteristics were established using a Keithley 2450 source meter. The resistivity was determined using the van der Pauw technique. The mobility and carrier concentration was done through the Hall effect measurements at room temperature using a 0.30‐Tesla magnet. It was observed that there was an increase in the size of the nanostructures, and more globules appeared after the concentration of SnO2 was increased. It was proven that the drop in the resistivity of Ga2O3 was due to the presence of SnO2. The data gathered were supported by the Raman peak located at 662 cm−1, attributed to the high conductivity of β‐Ga2O3. However, the ε‐polytype was verified to appear as a result of adding SnO2. All the samples were considered as n‐type semiconductors. High mobility, low power loss, and low specific on‐resistance were attained by the highest concentration of SnO2. Hence, it was clinched as the optimal n‐type Ga2O3/SnO2 concentration and recommended to be a potential substrate for power electronics application.

Sensitivity Improvement of Urchin-Like ZnO Nanostructures Using Two-Dimensional Electron Gas in MgZnO/ZnO

This paper introduces a strategy for improving the sensitivity of a gas sensor to NO2 gas. The gas sensor was fabricated using urchin-like ZnO nanostructures grown on MgO particles via vapor-phase growth and decorated with MgZnO nanoparticles via a sol-gel process. The urchin-like ZnO gas sensor decorated with MgZnO showed higher sensitivity to NO2 gas than a pristine urchin-like ZnO gas sensor. When ZnO and MgZnO form a heterojunction, a two-dimensional electron gas is generated. This improves the performance of the fabricated gas sensor. The growth morphology, atomic composition, and phase structure were confirmed through field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction, respectively.

Vapor Phase Selective Growth of Two-Dimensional Perovskite/WS2 Heterostructures for Optoelectronic Applications.

Organic-inorganic hybrid perovskites have attracted increased interest owing to their exceptional optoelectronic properties and promising applications. Monolayers of transition metal dichalcogenides (TMDCs), such as tungsten disulfide (WS2), are also intriguing because of their unique optoelectronic properties and their atomically thin and flexible structures. Therefore, the combination of these different types of materials is very attractive in terms of fundamental science of interface interaction, as well as for the realization of ultrathin optoelectronic devices with high performance. Here, we demonstrate the controlled synthesis of two-dimensional (2D) perovskite/WS2 heterostructures by an all vapor-phase growth approach. This involves the chemical vapor deposition (CVD) growth of monolayer WS2, followed by the vapor-phase selective deposition of 2D PbI2 onto the WS2 with the successive conversion of PbI2 to organic-inorganic perovskite (CH3NH3PbI3). Moreover, the selective growth of the perovskite on prepatterned WS2 enables the direct synthesis of patterned heterostructures, avoiding any damage to the perovskite. The photodetectors utilizing the perovskite/WS2 heterostructure show increased responsivities compared with isolated thin perovskite obtained by conventional solution methods. The integration of 2D perovskite with TMDCs opens a new avenue to fabricate advanced devices by combining their unique properties and overcoming current processing difficulties of perovskites.

Lithographically defined synthesis of transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) promise to revolutionize optoelectronic applications. While monolayer exfoliation and vapor phase growth produce extremely high quality 2D materials, direct fabrication at wafer scale remains a significant challenge. Here, we present a method that we call ‘lateral conversion’, which enables the synthesis of patterned TMD structures, with control over the thickness down to a few layers, at lithographically predefined locations. In this method, chemical conversion of a metal-oxide film to TMD layers proceeds by diffusion of precursor propagating laterally between silica layers, resulting in structures where delicate chalcogenide films are protected from contamination or oxidation. Lithographically patterned WS2 structures were synthesized by lateral conversion and analyzed in detail by hyperspectral Raman imaging, scanning electron microscopy and transmission electron microscopy. The rate of conversion was investigated as a function of time, temperature, and thickness of the converted film. In addition, the process was extended to grow patterned MoS2, WSe2, MoSe2 structures, and to demonstrate unique WS2/SiO2 multilayer structures. We believe this method will be applicable to a variety of additional chalcogenide materials, and enable their incorporation into novel architectures and devices.

Integration of VLS-Grown WO3 Nanowires into Sensing Devices for the Detection of H2S and O3.

The inspiration behind this research is the development of tungsten oxide (WO3) nanowires based, highly sensitive and selective sensing devices directly on the active sensing platform. WO3 one-dimensional nanowires were synthesized via the vapour-phase growth technique. This approach allows the production of well-aligned and uniform nanowires on alumina substrates with their diameter and length in the nanometer range. The morphological and structural properties of nanowires have been investigated by means of the field effect electron microscopy, grazing incidence X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Finally, the fabricated WO3 nanowire sensing devices and their gas sensing performance were investigated in the presence of different oxidizing and reducing gases (especially environmental gases) at different temperatures. The WO3 sensors demonstrate high performance toward H2S and O3 at the optimal working temperatures of 400 and 200 °C, respectively, with the detection limit in the ppb level.

Limitations during Vapor Phase Growth of Bulk (100) 3C-SiC Using 3C-SiC-on-SiC Seeding Stacks.

The growth of 3C-SiC shows technological challenges, such as high supersaturation, a silicon-rich gas phase and a high vertical temperature gradient. We have developed a transfer method creating high-quality 3C-SiC-on-SiC (100) seeding stacks, suitable for use in sublimation “sandwich” epitaxy (SE). This work presents simulation data on the change of supersaturation and the temperature gradient between source and seed for the bulk growth. A series of growth runs on increased source to seed distances was characterized by XRD and Raman spectroscopy. Results show a decrease in quality in terms of single-crystallinity with a decrease in supersaturation. Morphology analysis of as-grown material indicates an increasing protrusion dimension with increasing thickness. This effect limits the achievable maximal thickness. Additional polytype inclusions were observed, which began to occur with low supersaturation (S ≤ 0.06) and prolonged growth (increase of carbon gas-species).

Ultrahigh-Performance Optoelectronics Demonstrated in Ultrathin Perovskite-Based Vertical Semiconductor Heterostructures.

Two-dimensional (2D) atomic layered semiconductor (e.g., transition metal dichalcogenides, TMDCs) heterostructures display diverse novel interfacial carrier properties and have potential applications in constructing next generation highly compact electronics and optoelectronics devices. However, the optoelectronic performance of this kind of semiconductor heterostructures has difficulty reaching the expectations of practical applications, due to the intrinsic weak optical absorption of the atomic-thick component layers. Here, combining the extraordinary optoelectronic properties of quantum-confined organic-inorganic hybrid perovskite (PVK), we design an ultrathin PVK/TMDC vertical semiconductor heterostructure configuration and realize the controlled vapor-phase growth of highly crystalline few-nanometer-thick PVK layers on TMDCs monolayers. The achieved ultrathin PVKs show strong thickness-induced quantum confinement effect, and simultaneously form band alignment-engineered heterointerfaces with the underlying TMDCs, resulting in highly efficient interfacial charge separation and transport. Electrical devices constructed with the as-grown ultrathin PVK/WS2 heterostructures show ambipolar transport originating from p-type PVK and n-type WS2, and exhibit outstanding optoelectronic characteristics, with the optimized response time and photoresponsivity reaching 64 μs and 11174.2 A/W, respectively, both of which are 4 orders of magnitude better than the heterostructures with a thick PVK layer, and also represent the best among all previously reported 2D layered semiconductor heterostructures. This work provides opportunities for 2D vertical semiconductor heterostructures via incorporating ultrathin PVK layers in high-performance integrated optoelectronics.

Synthesis and magnetoresistance of (Cd1−x Znx)3As2 (x = 0,007) crystals

The vapor phase growth of Cd3As2—Zn3As2 (in the following (Cd1−x Znx)3As2 solid solutions process is described. The (Cd0,993 Zn0,007)3As2 solid solution single crystals were synthesized. Scanning electron microscopy and electron diffraction data suggest high crystalline quality of studied sample. Its structure and surface morphology, indicating the presence of growth nuclei and cleavage planes, were investigated. Giant anisotropic magnetoresistance and Shubnikov — de Haas oscillations were observed at low temperatures. Obtained results suggests that peculiarities of Dirac semimetal phase persist in (Cd1−x Znx)3As2 solid solution at low zinc content. At the same time, there are indications of some differences with initial Cd3As2 properties.

Growth and Properties of Mn-Doped PbI2 Crystals

This paper presents an experimental study of the effect of doping with manganese on the vapor phase growth of lead(II) iodide crystals in a closed system and their properties. The manganese concentration in the grown crystals and the mass transport rate in the PbI2–I2 system are determined by the Mn concentration in the growth charge and the temperature of the source zone. We have obtained single crystals of the following morphological types: elongated ribbons, ribbons with sharp ends, plates, and their combinations and intergrowths. Doping with manganese has been shown to influence low-temperature (T = 5 K) photoluminescence spectra of PbI2.

Preferred orientations and microstructures of lanthanum phosphate films prepared via laser chemical vapor deposition

In this study, the vapor-phase growth of lanthanum phosphate (LaPO4 monazite) films having preferred orientations on polycrystalline alumina substrates was demonstrated via laser chemical vapor deposition using metal-organic precursors. The effects of the laser power densities and oxygen (O2) partial pressure on the crystal orientations, microstructures, and textures of the LaPO4 films were investigated. Monoclinic LaPO4 monazite was grown at laser power densities of 62–96 W cm−2 (deposition temperatures of 1075–1120 K) and an O2 partial pressure of 257 Pa. The (111¯)-oriented LaPO4 films with small pyramidal facets of less than 1 μm in sizes and large platelets were grown at laser power densities lower than 85 W cm−2. The preferred orientation of the monoclinic LaPO4 changed from (111¯) to (101¯) as the laser power density was increased to 95 W cm−2, at which large platelets inclined from the substrate surface were formed. Deposition with a reduced O2 partial pressure of 200 Pa induced the growth of highly (111¯)-oriented LaPO4 films with small pyramidal facets, which comprised (001), (100), and (010) faceted planes. At a low O2 partial pressure of 133 Pa, (100)-oriented LaPO4 films with platelet grains vertically grown from the substrate were formed. The deposition rates of the LaPO4 films were as high as 58.6 μm h−1.

Structures, Morphological Control, and Antibacterial Performance of Ag/TiO2 Micro-Nanocomposite Materials

Structures, morphological control, and antibacterial activity of silver-titanium dioxide (Ag/TiO2) micro-nanocomposite materials against Staphylococcus aureus are investigated in this study. Horizontal vapor phase growth (HVPG) technique was used to synthesize the Ag/TiO2 micro-nanomaterials, with parameters of growth temperature and baking time. The materials were characterized by using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, and atomic force microscope (AFM). The result indicated that the HVPG technique is able to synthesize Ag/TiO2 with many shapes in micro- and nanoscale such as nanoparticles, nanorods, triangular nanomaterials, and nanotubes. The results showed that the shape of micro- and nanocomposites material could be arranged by adjusting the parameters. The results revealed that the nanorods structure were obtained at 1000°C growth temperature and that 8 hours of baking time was ideal for antibacterial application. Treating the S. aureus stock with Ag/TiO2 nanocomposites is able to reduce bacterial growth with a significant result.

Atomic-Scale Rational Designs of Superionic Sulfide-Based Solid-State Electrolytes By Atomic Layer Deposition

Liquid electrolytes currently are widely utilized in lithium batteries, serving as a lithium-ion conductor but an electrical insulator. However, their flammable nature poses serious safety concerns and their reactivity to electrodes underlies performance-fading of lithium batteries. In this context, solid-state electrolytes are highly regarded as a safe and reliable alternative and undergoing intensive investigation. Traditional processes (e.g., solid-state reaction, mechanochemical method, and melt-quenching method) have enabled various superionic solid-state electrolytes, but there have been challenges to achieve intimate assembling contacts between solid-state electrolytes and electrodes. To this end, an in-situ method will be the most desirable for growing solid-state electrolytes directly on electrodes with minimal interfacial resistance. In this regard, atomic layer deposition (ALD) recently has raised an increasing interest for solid-state electrolytes, featuring its defect-free uniformity, unrivaled conformal deposition, low temperature, and rational tunability. To date, ALD has mainly reported for synthesizing oxide-based solid-state electrolytes, realizing an ionic conductivity of 2.2 x 10-6 S/cm [1] at best. Recently we conducted the first study on sulfide-based solid-state electrolytes using ALD and brought a big breakthrough for achieving superionic solid-state electrolytes. Through rationally combining two ALD processes for binary Li-S and Al-S compounds [2, 3], in the study we synthesized a series of ternary compounds of LixAlyS. Our impedance measurements revealed that the resultants LixAlyS films are promising solid-state electrolytes with tunable ionic conductivities up to over 10-3 S/cm, at least three orders of magnitude higher than those of previous oxide-based counterparts by ALD. In the study, we also characterized the composition of the resultant LixAlyS films using quartz crystal microbalance (QCM), inductively coupled plasma (ICP) mass spectrometry, and X-ray photoelectron spectroscopy (XPS). In addition, we investigated the growth of the LixAlyS films using in-situ Fourier transform infrared spectroscopy (FTIR) and QCM. Very interestingly, we demonstrated that the resultant LixAlyS films have exceptional properties in inhibiting the growth of lithium dendrite structures in lithium batteries. Thus, this study is significant for developing all-solid-state batteries via the in-situ ALD growth of superionic solid-state electrolytes. Kazyak, E., et al., Atomic layer deposition and first principles modeling of glassy Li3BO3–Li2CO3 electrolytes for solid-state Li metal batteries. Journal of Materials Chemistry A, 2018. 6(40): p. 19425-19437.Meng, X., et al., Vapor-phase atomic-controllable growth of amorphous Li2S for high-performance lithium–sulfur batteries. ACS Nano, 2014. 8(10): p. 10963-10972.Meng, X., et al., Atomic layer deposition of aluminum sulfide: growth mechanism and electrochemical evaluation in lithium-ion batteries. Chemistry of Materials, 2017. 29(21): p. 9043-9052.

Vapor Phase Growth of Semiconductor Nanowires: Key Developments and Open Questions.

Nanowires are filamentary crystals with a tailored diameter that can be obtained using a plethora of different synthesis techniques. In this review, we focus on the vapor phase, highlighting the most influential achievements along with a historical perspective. Starting with the discovery of VLS, we feature the variety of structures and materials that can be synthesized in the nanowire form. We then move on to establish distinct features such as the three-dimensional heterostructure/doping design and polytypism. We summarize the status quo of the growth mechanisms, recently confirmed by in situ electron microscopy experiments and defining common ground between the different synthesis techniques. We then propose a selection of remaining defects, starting from what we know and going toward what is still to be learned. We believe this review will serve as a reference for neophytes but also as an insight for experts in an effort to bring open questions under a new light.

Structures, mechanical properties and antibacterial activity of Ag/TiO2 nanocomposite materials synthesized via HVPG technique for coating application

In this study, the structures and mechanical properties of the silver-titanium dioxide nanocomposite material were investigated using Atomic Force Microscopy (AFM). These properties include surface roughness, hardness, and reduced Young's modulus. The nanocomposite material was successfully synthesized using the Horizontal Vapor Phase Growth (HVPG) technique which yielded shapes such as nanoparticles, nanospheres, nanorods, triangular nanocomposites, and nanocrystals. Characterization of nanocomposite materials was done through Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) spectroscopy to elucidate material shape, diameter, and composition. The pour plate technique combined with McFarland standards was used to evaluate the antibacterial activity of the nanocomposite material against Staphylococcus aureus. The nanocomposite material was able to eradicate bacteria and was suitable for coating applications effectively.

Increasing Photoluminescence Quantum Yield by Nanophotonic Design of Quantum-Confined Halide Perovskite Nanowire Arrays.

High-photoluminescence quantum yield (PLQY) is required to reach optimal performance in solar cells, lasers, and light-emitting diodes (LEDs). Typically, PLQY can be increased by improving the material quality to reduce the nonradiative recombination rate. It is in principle equally effective to improve the optical design by nanostructuring a material to increase light out-coupling efficiency (OCE) and introduce quantum confinement, both of which can increase the radiative recombination rate. However, increased surface recombination typically minimizes nanostructure gains in PLQY. Here a template-guided vapor phase growth of CH3NH3PbI3 (MAPbI3) nanowire (NW) arrays with unprecedented control of NW diameter from the bulk (250 nm) to the quantum confined regime (5.7 nm) is demonstrated, while simultaneously providing a low surface recombination velocity of 18 cm s-1. This enables a 56-fold increase in the internal PLQY, from 0.81% to 45.1%, and a 2.3-fold increase in OCEy to increase the external PLQY by a factor of 130, from 0.33% up to 42.6%, exclusively using nanophotonic design.

Direct van der Waals epitaxial growth of 1D/2D Sb2Se3/WS2 mixed-dimensional p-n heterojunctions

The mixed-dimensional integration of two-dimensional (2D) materials with non-2D materials can give rise to prominent advances in performance or function. To date, the mixed-dimensional one-dimensional (1D)/2D heterostructures have been fabricated using various physical assembly approaches. However, direct epitaxial growth method which has notable advantages in preparing large-scale products and obtaining perfect interfaces is rarely investigated. Herein, we demonstrate for the first time the direct synthesis of the 1D/2D mixed-dimensional heterostructures by sequential vapor-phase growth of Sb2Se3 nanowires on WS2 monolayers. X-ray diffraction (XRD) pattern and Raman spectrum confirm the composition of the Sb2Se3/WS2 heterostructures. Transmission electron microscope (TEM) measurement demonstrates high quality of the heterostructures. Electrical transport characterization reveals that Sb2Se3 nanowire exhibits p-type characteristic and that WS2 monolayer exhibits n-type behavior, and that the p-n diode from 1D/2D mixed-dimensional Sb2Se3/WS2 heterostructure possesses obvious current rectification behavior. Optoelectronic measurements of the heterostructures show apparent photovoltaic response with an open-circuit voltage of 0.19 V, photoresponsivity of 1.51 A/W (Vds = 5 V) and fast response time of less than 8 ms. The van der Waals epitaxial growth mode of Sb2Se3 nanowires on WS2 monolayers is verified by stripping the Sb2Se3 nanowire from the heterostructures using tape. Together, the direct van der Waals epitaxy opens a facile pathway to 1D/2D mixed-dimensional heterostructures for functional electronic and optoelectronic devices.