Vapor Phase Technique Research Articles

Growth Processing and Strategies: A Way to Improve the Gas Sensing Performance of Nickel Oxide-Based Devices

The review paper provides a comprehensive analysis of nickel oxide (NiO) as an emerging material in environmental monitoring by surveying recent developments primarily within the last three years and reports the growth processing and strategies employed to enhance NiO sensing performance. It covers synthesis methods for pristine NiO, including vapor-phase, liquid-phase, and solution-processing techniques, highlighting advantages and limitations. The growth mechanisms of NiO nanostructures are explored, with a focus on the most recent research studies. Additionally, different strategies to improve the gas sensing performance of NiO are discussed (i.e., surface functionalization by metallic nanoparticles, heterostructure formation, carbon-based nanomaterials, and conducting polymers). The influence of these strategies on selectivity, sensitivity, response time, and stability of NiO-based sensors is thoroughly examined. Finally, the challenges and future directions that may lead to the successful development of highly efficient NiO-based gas sensors for environmental monitoring are introduced in this review.

Low-Temperature Vapor-Phase Growth of 2D Metal Chalcogenides.

2D metal chalcogenides (MCs) have garnered significant attention from both scientific and industrial communities due to their potential in developing next-generation functional devices. Vapor-phase deposition methods have proven highly effective in fabricating high-quality 2D MCs. Nevertheless, the conventionally high thermal budgets required for synthesizing 2D MCs pose limitations, particularly in the integration of multiple components and in specialized applications (such as flexible electronics). To overcome these challenges, it is desirable to reduce the thermal energy requirements, thus facilitating the growth of various 2D MCs at lower temperatures. Numerous endeavors have been undertaken to develop low-temperature vapor-phase growth techniques for 2D MCs, and this review aims to provide an overview of the latest advances in low-temperature vapor-phase growth of 2D MCs. Initially, the review highlights the latest progress in achieving high-quality 2D MCs through various low-temperature vapor-phase techniques, including chemical vapor deposition (CVD), metal-organic CVD, plasma-enhanced CVD, atomic layer deposition (ALD), etc. The strengths and current limitations of these methods are also evaluated. Subsequently, the review consolidates the diverse applications of 2D MCs grown at low temperatures, covering fields such as electronics, optoelectronics, flexible devices, and catalysis. Finally, current challenges and future research directions are briefly discussed, considering the most recent progress in the field.

(Invited) Electrodeposition of Manganese Arsenides (MnxAs) as Promising Candidates in Spintronics

Spintronics is a new branch of electronics in which the spin of the electrons as well as their charge is manipulated to produce a desired outcome. Spintronic devices are particularly attractive for memory storage and magnetic sensor applications, and potentially for quantum computing where the electron spin would represent a bit of information. Nowadays many efforts are made to increase the efficiency and lower the production costs of these materials, among others the use of molecular inorganic compounds and cheap preparation techniques [1].Most of these layers are synthesized with physical and/or vapor phase techniques [2] which allow to obtain materials with a very few defects, but which are complex to manage and typically very expensive. For this reason, in this study we have searched for a way to electrochemically deposit a compound with interesting spintronic properties in the aqueous medium. In fact, electrodeposition is notoriously a very cheap, versatile and easily scalable technique.With this purpose Mn-As system was selected thanks to its good magnetic and transport properties to be used as building blocks for spin valve [3]. MnAs has potential applications in spintronics, for electrical spin injection into GaAs and Si based devices [4] and DFT simulations predict that Mn2As and Mn3As have a low magnetization saturation and a high spin polarization at the Fermi-level [5]. Starting from MnSO4 and NaAsO2 precursors solutions with various concentrations, supporting electrolytes and pH were tested.The electrochemical behaviour of the precursors was investigated with cyclic voltammetries and both potentiostatic as well as galvanostatic deposition were carried out. The deposits obtained were morphologically and compositional characterised with scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray fluorescence spectroscopy (XRF), the chemical states were analysed with X-ray photoelectron spectroscopy (XPS), the crystalline structure was investigated with X-ray diffraction measurements (XRD). The preliminary magnetic and transport measurements evidence the promising properties of this deposition process to provide spinfiltering materials.The authors acknowledge Regione Toscana POR CreO FESR 2014-2020 – azione 1.1.5 sub-azione A1 – Bando 1 “Progetti Strategici di ricerca e sviluppo” which made possible the project “RAM-PVD” (CUP 3647.04032020.157000057_1225).

High-Flux ZIF-8 Membranes on ZnO-Coated Supports for Propane/Propylene Separation

Zeolitic-imidazolate framework ZIF-8, a subclass of metal–organic frameworks (MOFs) with sodalite topology, has been a promising candidate for gas separation applications. Nevertheless, fabricating a high-flux propylene-selective ZIF-8 membrane is still challenging owing to the intrinsic crystalline nature of ZIF-8. In this study, an effective vapor-phase technique has been used to synthesize thin ZIF-8 membranes. ZnO was coated on anodic alumina oxide (AAO) supports via atomic layer deposition (ALD). The ZnO layer was vapor-phase treated with 2-methyl imidazole, which converted the ZnO layer to ZIF-8. The vapor-phase synthesized ZIF-8 membranes were evaluated for propylene/propane separation. With a ZIF-8 layer of ∼400 nm thickness, a high propylene/propane separation factor of 58 was obtained, concomitant with a high propylene permeance of 3.6 × 10–7 mol m–2 s–1 Pa–1. It was found that the vapor-phase solvent-free synthesis minimized the number of chemicals needed to fabricate a thin, highly permeable, and propylene-selective ZIF-8 membrane.

Development of AZO TCOs with ALD for HEMT and HJSC Solar Cell Applications

Transparent Conductive Oxide (TCO) films are widely used in optoelectronic devices, such as solar cells, LEDs, and Lasers. Utilization of these contacts directly affects the device efficiencies. Purpose of this study is to produce and optimize properties of Aluminum doped Zinc Oxide (AZO) using a vapor phase technique, Atomic Layer Deposition (ALD) for (n+) a-Si:H surface of silicon Heterojunction Solar Cells (HJSCs) and High Electron Mobility Transistor (HEMT) applications. This study is focused on the effect of the deposition temperature and aluminum atomic concentration on structural, electrical and optical properties of ALD grown AZO ohmic contact films. The results show that as-deposited films have 80-90% transmittance in the visible spectra, low resistance (2.04x10-3 ohm.cm) and mobility value of 5.25 cm2/V.s.

Self-powered photodetector based on liquid phase exfoliated Cu–WSe2 nanosheets

According to its tuneable optoelectronic capabilities that enhanced the application of the photo-sensing device, the 2D-TMDC (transition metal dichalcogenides) deposited thin films have been the subject of additional research. Herein, we describe the high-quality crystalline behaviour of 10% incorporation of Cu in the pristine WSe2 nanosheets using two step liquid phase exfoliation process and its use as a broad range photodetector. The vapour phase technique is used to grown compounds. The elemental mapping and purity were analysed by EDAX. Powder Xray diffraction shows that all exfoliated nanosheets have a hexagonal lattice structure. The structural morphology was investigated by SEM and HR-TEM. The Raman Spectra reveal that the exfoliated nanosheets have in plane E2g vibration mode. The effect of alloy engineering on photo responsiveness has been investigated. The maximum photodetector parameters are examined for the ITO based Cu (10%) WSe2 photodetector under various wavelength dependent monochromatic light sources for 40 mW/cm2 power intensity.

Gd-Encapsulated Carbon Nanotubes as Dual-Modal Probes for Magnetic Resonance and Second Near-Infrared Emission

We report a second near-infrared (NIR-II) fluorescence and magnetic resonance (MR) dual-modal probe composed of fluorescent carbon nanotubes (CNTs) encapsulated with paramagnetic Gd compounds. The Gd-filled CNTs are synthesized via a vapor-phase technique using gadolinium halides as precursors, and the CNTs exhibit the strong photoluminescence even after Gd filling. We have investigated NIR-II fluorescence and MR imaging in vivo as well as in vitro, demonstrating that the Gd-filled CNTs can be used for dual-modal imaging of tissues and vessels in living mice, creating avenues for disease diagnosis and prognosis.

Antimycobacterial and antibiofilm activity of garlic essential oil using vapor phase techniques

Antimycobacterial and antibiofilm activity of garlic essential oil using vapor phase techniques

Atomic Layer Deposition of Li4Ti5O12: Enabling Ultrafast Charging 3D Thin-Film Batteries

Atomic layer deposition (ALD) of lithium (Li)-compound thin-films has aroused significant interest in recent years. Promising applications are Li-ion thin-film batteries (TFB), protective particle coatings, interface model systems, and neuromorphic computing [1,2]. Here, we demonstrate the ultrafast charging with long-cycle life of Li4Ti5O12 (LTO) fabricated by ALD. The “zero-strain” anode will enable on-chip three-dimensional (3D) solid-state thin-film batteries to power upcoming autonomous sensor systems.The simultaneous increase of power and energy density of full cell 3D TFBs by coating the battery layer stack over microstructured substrates was recently demonstrated [3]. The required conformal, pinhole-free deposition and stoichiometric control of nanometer-thin films on highly structured surfaces are only accessible via ALD. The vapor-phase technique based on sequential, self-limiting surface reactions is well understood. However, the direct deposition of Li-compound anodes remains challenging [1].In this work, ALD LTO is electrochemically evaluated for the first time. The study is based on a thermal three-step ALD process for Li-containing mixed oxides on 200 mm silicon wafers [4]. This process suppresses the undesired hygroscopic nature of lithium hydroxide during deposition with water as a co-reactant. Lithium-tert-butoxide (LTB) and lithium hexamethyldisilazide (LiHMDS) are both proven as suitable precursors forming high-quality spinel LTO with low impurities after rapid thermal processing. The substrate has a strong influence on the crystallization and the film texture, determining the electrochemical behavior. Planar 30 nm films reveal capacities close to the theoretical value of 600 mAh cm-3 at 1 C. Excellent fast dis-/charging capability is achieved by optimized film texture obtaining around 50 % of the maximum capacity at extreme rates of 200 C. Outstanding cycle performance with high Coulomb efficiency is demonstrated, exhibiting only 2% capacity loss after 1000 cycles at 100 C. Excellent conformality, adhesion, and recent electrochemical results of ALD LTO in a 3D hole structure with high aspect ratios of up to 20:1 will be discussed. The superior performance of ALD LTO, compared to other deposition techniques, is a key enabler for scalable high-volume production of high-power 3D TFBs.

(Invited) Unidirectional Epitaxy of TMD Monolayers on Sapphire

Wafer-scale synthesis of semiconducting transition metal dichalcogenide (TMDs) monolayers is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Promising results have been demonstrated for epitaxial films deposited by vapor phase techniques such as CVD and MOCVD. However, the three-fold symmetry of TMDs such as MoS2 and WSe2, results in two energetically equivalent domain alignments, often referred to as 0o and 60o domains, when grown on substrates such as c-plane sapphire and graphene. The oppositely oriented domains give rise to inversion domain boundaries (IDBs) upon coalescence which exhibit a metallic character and are generally undesirable. In this study, we demonstrate the epitaxial growth of unidirectional TMD monolayers on 2” diameter c-plane sapphire substrates with a significantly reduced density of inversion domains. Steps on the sapphire surface are shown to break the surface symmetry giving rise to a preferred domain orientation.Metalorganic chemical vapor deposition (MOCVD) was used for the epitaxial growth of WSe2 and WS2 monolayers on c-plane sapphire in a cold-wall horizontal quartz-tube reactor. The as-received sapphire substrates, which are miscut ~0.2o toward <112 ̅0>, consist of steps with sub-1 nm step height separated by 50-70 nm wide terraces. A three-step nucleation-ripening-lateral growth process, carried out at temperatures ranging from 850oC to 1000oC, was used to achieve epitaxial films using W(CO)6, H2Se and H2S as precursors in a H2 carrier gas. Nucleation was observed to occur at the terrace edge and the growing domains align epitaxially with the underlying (0001) sapphire lattice. As a result of the nucleation process, the domains grow with a zig-zag edge facing the terrace edge which imparts a preferential direction to the domains. The percentage of domains with a preferred direction ranges from 75%-86% depending on MOCVD growth conditions. Continued lateral growth for times ranging from 10-30 minutes results in fully coalesced TMD monolayers that are epitaxially oriented on the sapphire, as assessed by in-plane x-ray diffraction, with a reduced density of inversion domain boundaries. The results demonstrate the important role of surface structure in nucleation and epitaxial growth of TMD monolayers.

Application of templated vapor-liquid-solid growth to heteroepitaxy of InP on Si

Direct growth of III–V semiconductors on Si promises to combine the superior optoelectronic properties of III–Vs with the existing large-scale fabrication capabilities for Si. Vapor-liquid-solid-based growth techniques have previously been used to grow optoelectronic-quality III–Vs in polycrystalline films and various photolithography-defined features. We show that templated vapor-liquid-solid growth can produce epitaxial material when performed on crystalline substrates. In templated vapor-liquid-solid growth, the metal group-III precursor is evaporated along with a capping SiO2 layer on the crystalline substrate, then melted and converted with exposure to a vapor-phase group-V precursor. We demonstrate homoepitaxial growth of InP on InP wafers using two forms of the SiOx capping layer to confine the liquid metal: evaporated SiO2 and solgel SiOx, the latter of which is necessary for growth on Si. We then demonstrate heteroepitaxial growth of InP islands on Si substrates from both evaporated and electroplated In metals. The templated vapor-liquid-solid process provides better material utilization and growth rates than common vapor-phase techniques, with similar control and convenience, providing a path toward the large-scale fabrication of integrated optoelectronic components.

Structural reversibility of Cu doped NU-1000 MOFs under hydrogenation conditions.

The metal-organic framework (MOF), NU-1000, and its metalated counterparts have found proof-of-concept application in heterogeneous catalysis and hydrogen storage among others. A vapor-phase technique, akin to atomic layer deposition (ALD), is used to selectively deposit divalent Cu ions on oxo, hydroxo-bridged hexa-zirconium(IV) nodes capped with terminal -OH and -OH2 ligands. The subsequent reaction with steam yields node-anchored, CuII-oxo, hydroxo clusters. We find that cluster installation via AIM (ALD in MOFs) is accompanied by an expansion of the MOF mesopore (channel) diameter. We investigated the behavior of the cluster-modified material, termed Cu-AIM-NU-1000, to heat treatment up to 325 °C at atmospheric pressure with a low flow of H2 into the reaction cell. The response under these conditions revealed two important results: (1) Above 200 °C, the initially installed few-metal-ion clusters reduce to neutral Cu atoms. The neutral atoms migrate from the nodes and aggregate into Cu nanoparticles. While the size of particles formed in the MOF interior is constrained by the width of mesopores (∼3 nm), the size of those formed on the exterior surface of the MOF can grow as large as ∼8 nm. (2) Reduction and release of Cu atoms from the MOFs nodes is accompanied by the dynamic structural transformation of NU-1000 as it reverts back to its original dimension following the release. These results show that while the MOF framework itself remains intact at 325 °C in an H2 atmosphere, the small, AIM-installed CuII-oxo, hydroxo clusters are stable with respect to reduction and conversion to metallic nanoparticles only up to ∼200 °C.

Predicting the Conditions for the Vapor-Phase Epitaxy of the III–V Compounds

Chloride-hydride epitaxy is the main vapor-phase technique for forming layers of functional homostructures and heterostructures for microelectronics and optoelectronics. At present, nanoheterostructures are obtained by MOS-hydride and molecular epitaxy; the molecular-layering technique is currently being perfected. The occurrence of new materials requires long-term development of the optimal technological conditions for their fabrication and the mathematical, physical, and other principles for modeling these processes. The chloride-hydride method continues to be perfected for forming relatively thick layers of functional heterostructures. The work outlines the fundamentals of physicochemical modeling by the example of chloride-hydride epitaxy. A physicochemical model of the variation in the technological modes of the vapor-phase epitaxy of different compounds under the corresponding conditions, which facilitate the formation of compounds with the same degree of disorder, is discussed. Equations for predicting the conditions for the epitaxy of other materials of the same group by the well-developed technology of a material have been derived. The obtained regularities can be used to optimize the chloride-hydride epitaxy of gallium phosphide and solid solutions based on it. The calculated conditions for gallium-nitride epitaxy are shown to be in good agreement with the conditions of real technological developments made by other authors.

Atomic Layer Deposition on Dispersed Materials in Liquid Phase by Stoichiometrically Limited Injections.

Atomic layer deposition (ALD) is a well-established vapor-phase technique for depositing thin films with high conformality and atomically precise control over thickness. Its industrial development has been largely confined to wafers and low-surface-area materials because deposition on high-surface-area materials and powders remains extremely challenging. Challenges with such materials include long deposition times, extensive purging cycles, and requirements for large excesses of precursors and expensive low-pressure equipment. Here, a simple solution-phase deposition process based on subsequent injections of stoichiometric quantities of precursor is performed using common laboratory synthesis equipment. Precisely measured precursor stoichiometries avoid any unwanted reactions in solution and ensure layer-by-layer growth with the same precision as gas-phase ALD, without any excess precursor or purging required. Identical coating qualities are achieved when comparing this technique to Al2 O3 deposition by fluidized-bed reactor ALD (FBR-ALD). The process is easily scaled up to coat >150 g of material using the same inexpensive laboratory glassware without any loss in coating quality. This technique is extended to sulfides and phosphates and can achieve coatings that are not possible using classic gas-phase ALD, including the deposition of phosphates with inexpensive but nonvolatile phosphoric acid.

Corrosion protection of Cu by atomic layer deposition

Atomic layer deposition (ALD) is a vapor phase technique that is able to deposit uniform, conformal thin films with an excellent thickness control at the atomic scale. 18 nm thick Al2O3 and TiO2 coatings were deposited conformaly and pinhole-free onto micrometer-sized Cu powder, using trimethylaluminum and tetrakis(dimethylamido)titanium(IV), respectively, as a precursor and de-ionized water as a reactant. The capability of the ALD coating to protect the Cu powder against corrosion was investigated. Therefore, the stability of the coatings was studied in solutions with different pH in the range of 0–14, and in situ raman spectroscopy was used to detect the emergence of corrosion products of Cu as an indication that the protective coating starts to fail. Both ALD coatings provide good protection at standard pH values in the range of 5–7. In general, the TiO2 coating shows a better barrier protection against corrosion than the Al2O3 coating. However, for the most extreme pH conditions, pH 0 and pH 14, the TiO2 coating starts also to degrade.

ZnO coated fiber optic microsphere sensor for the enhanced refractive index sensing

Optical fiber-based sensors are expected to become key components in the control of industrial processes, and the tuning and the enhancement of their sensing properties are crucial for the further development of this technology. Atomic Layer Deposition (ALD), a vapor phase technique allowing for the deposition of conformal thin films, is particularly suited for the deposition of controllable thin films on challenging substrates. In this work, we report the tuning of fiber-optic microsphere sensors using an ALD process of zinc oxide (ZnO) based on diethylzinc (DEZ) and H2O at 90 °C. Nanolayers of 50 and 100 nm were deposited on the fiber-optic microspheres sensors, using 250 and 500 ALD cycles respectively. The fabricated samples were characterized using Scanning Electron Microscopy (SEM), and the spectral responses of the devices were investigated theoretically and experimentally, by measuring the refractive index of different oils. It has been found that the biocompatible ZnO functional nanocoatings of the fiber-optic microsphere sensors allowed for a wider measurement range of refractive indexes, opening up new prospects for fiber-based sensing devices.

Effects of NaCl solution on the swelling and shrinkage behavior of compacted bentonite under one-dimensional conditions

Because of its low hydraulic conductivity, high swelling capacity, and good adsorption properties, Gaomiaozi (GMZ) bentonite has been proposed as a suitable buffer/backfill material for construction of artificial barriers in deep geological repositories for high-level nuclear waste (HLW) disposal in China. By varying the concentration of NaCl solution and total suction, the swelling and shrinkage characteristics of compacted GMZ bentonite were studied under one-dimensional conditions. Results show that the swelling strain of compacted GMZ bentonite decreases as the concentration of NaCl solution increases under one-dimensional conditions. During drying with suction control by vapor phase technique, the density increases with suction. Simultaneously, the sample saturated with salt solution has a larger void ratio than that of the sample saturated with distilled water under the same suction. The void ratio and water content of the samples decrease with increasing suction. For a given suction, the sample saturated with salt solution has a higher degree of saturation than the sample saturated with distilled water. With the obtained water content (w), void ratio (e), and the degree of saturation (Sr), soil-water retention curves (SWRCs) were calculated for samples initially hydrated with different concentrations of NaCl solution. The SWRCs reveal that the sample with salt solution has higher water retention capacity than the sample with distilled water. A modified SWRC equation was proposed to account for the effect of NaCl solutions on the drying process of compacted GMZ bentonite under one-dimensional conditions. The results confirm that the modified SWRC equation can effectively describe the SWRCs of compacted GMZ bentonite considering the effects of NaCl solutions.

Single Crystal Growth and Crystal Structure Analysis of Novel Orange-Red Emission Pure Nitride CaAl2Si4N8:Eu2+ Phosphor.

Single crystal of novel orange-red emission CaAl2Si4N8:Eu2+ phosphor was grown by our original vapor phase technique, and the precise crystal structure was investigated for the first time by single crystal X-ray diffraction analysis. The crystal structure of CaAl2Si4N8:Eu2+ single crystal was verified to be an α-Si3N4-type trigonal structure (space group P31c) with a = 0.79525(9) nm, c = 0.57712(8) nm, and z = 2. The Ca atoms were coordinated to seven nitrogen atoms and located in the three-dimensional framework formed by (Al, Si)N4 tetrahedra. The single-crystal CaAl2Si4N8:Eu2+ phosphors exhibited a broad reddish-yellow emission with a peak at 600 nm under near-UV and blue light excitation. When increasing the temperature up to 150 °C, the emission intensity of the CaAl2Si4N8:Eu2+ phosphor decreased to 88% of the initial value at 25 °C.

Experimental and Modeling Study of Soil Water Retention Curves of Compacted Bentonite considering Salt Solution Effects

Due to its low hydraulic conductivity, high swelling capacity, and good adsorption properties, the Gaomiaozi (GMZ) bentonite has been selected as potential buffer/backfill materials for construction of engineered barriers in the deep geological repository for disposal of high-level nuclear waste (HLW) in China. Investigation of salt solution effects on the water retention properties of compacted bentonite is of great importance in the context of geological disposal of HLW based on the multibarrier concept. In this study, amended specimens were obtained through a spray of different concentrations of salt solutions to reach target salt contents, respectively. With employment of the vapor phase technique for suction control, water retention tests were conducted on densely compacted GMZ bentonite (1.7 Mg/m3) with different salt contents under confined conditions. Corresponding soil water retention curves (SWRCs) were obtained. Analysis indicates that, for a given suction, the measured water content of GMZ bentonite specimen increases as the salt content (or pore fluid concentration) increases. The influencing rate depends on suction. For lower suctions (lower than 38 MPa), the water retention capacity increases as the salt content increases, while for higher suctions (higher than 38 MPa), the influence can be negligible. Based on the Fredlund and Xing (1994) equation, a soil water retention model was proposed for simulation of the SWRCs of compacted GMZ bentonite with consideration of pore fluid chemistry. Parameters were analyzed and determined with consideration of influences of the pore fluid concentration. Verification indicates that the SWRCs simulated by the proposed model are well agreed with the measured ones.

Highly efficient thulium-doped high-power laser fibers fabricated by MCVD.

We report a hybrid process by combining both vapor-phase and solution-doping techniques of rare-earth doped preform fabrication in conjunction with the MCVD technique, in order to fabricate highly efficient Tm-doped laser fibers. The proposed fabrication route takes advantage of co-doping silica with high alumina content through the vapor-phase doping process, which is otherwise difficult to achieve using conventional solution doping technique. In addition, by employing the solution doping method, high-purity thulium halide precursors that have low vapor pressures up to several hundred degree Celsius. These high-purity thulium halide precursors can be used to dope the fiber core region with a high thulium concentration that is optimized for an efficient two-for-one cross-relaxation process for 79xnm diode pumped thulium-doped fiber laser. Fibers fabricated using the hybrid approach show more homogeneous and flat-top dopant profiles, compared with the conventional approach, where both aluminum and thulium are incorporated in the core through solution doping. This will ensure that more doped region will take part in the cross-relaxation process. Superior laser performance with a slope efficiency of >70% in the two-micron band has been demonstrated when diode pumped at ~790nm.