Molecular Vapor Deposition Research Articles

Novel fabrication of mixed wettability micromodels for pore-scale studies of fluid-rock interactions.

Wettability plays a crucial role in multiphase fluid flow in porous media, impacting various geological applications such as hydrocarbon extraction, aquifer remediation, and carbon dioxide sequestration. Microfluidic methods have attracted interest for their capacity to explore and visualize essential multiphase flow dynamics at the pore level, mimicking actual rock pore structures. However, creating micromodels with representative mixed wettability is currently a challenge. Existing technology is limited to producing micromodels with a singular wettability, either water-wet or oil-wet, leaving a gap in representing mixed-wet scenarios. In this study, we introduce a novel method to fabricate microfluidic devices with controlled spatial distribution of wettability at the micro-scale, mimicking actual configurations of mixed-wet rocks arising from varied mineralogy and pore structures. The proposed method combines the soft lithography process with thin film deposition techniques. The micromodels were designed to mimic the pore network of actual reservoir rocks, and a silicon substrate served as the foundation for the photolithography process optimization and wettability alteration methodology. Perfluorodecyltrichlorosilane coating was applied using molecular vapor deposition technology for surface wettability modification. The coated parts of the microdevice substrate altered the localized wetting state of the silicon towards hydrophobic, while the wettability remained unchanged in the non-coated areas. We utilized surface measurements, including contact angle, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, and atomic force microscopy, to assess the wettability, composition, thickness, shape, roughness, and overall quality of the coating. Our fabrication process successfully produced a microfluidics device with tailored mixed-wet attributes at the micro-scale, which is, to our best knowledge, the first achievement in the field. This method enables the replication of mixed-wet characteristics commonly seen in various applications, such as carbonates and shales within underground rocks, providing a more accurate examination of fundamental multiphase fluid dynamics and rock interactions at the pore level.

Production Methods and Energy Applications of Borophene

Two dimensional materials have unique physical, chemical and electrical properties. Many theoretical studies on borophene revealed important possible properties, such as metallicity, transparency, conductivity and chemical activity; due to vacancies present in its structure. It was first synthesized in 2015 and experimental studies on this new exciting 2D material is few in number. It was found that many of these studies still focus on expensive and sophisticated production methods such as Molecular Beam Epitaxy (MBE) and Chemical Vapor Deposition (CVD). Although graphene, first of these 2D materials, is being studied more than two decades, it could not do the expected impact on energy industry due to lack of production methods in industrial scale. In this review, it was intended to build a narrative on the experimental production methods and resulting structures of borophene rather than theoretical background. It was found that liquid phase exfoliation method to synthesize borophene sheets might be the most promising method to upscale borophene production. Theoretical, computational and experimental studies suggested that β12 and χ3 borophene structures were stable and could be produced by sonochemical exfoliation method. In addition, possible uses in energy applications and some future prospects were also discussed. Borophene produced this way, might be used in batteries, supercapacitors hydrogen evolution (HER) and oxygen evolution (OER) reactions.

Fabrication of tunable bandgap epitaxial β‐(AlxGa1−x)2O3 films using a spin‐coating method

Abstractβ‐(AlxGa1−x)2O3 films have several critical properties of interest to the research community, including a wide bandgap that may be used in the development of new electronic, optoelectronic, and photonic devices. Here we demonstrate the first time fabricated metal‐alkoxide‐based spin‐coated single‐phase epitaxial β‐(AlxGa1−x)2O3 films on c‐sapphire substrates with orientation and good crystallinity that is comparable to the films fabricated using other film deposition techniques, such as molecular beam epitaxy and chemical vapor deposition. Using this technique, we generated films with broad Al compositions (x) of 0.3, 0.5, and 0.7 with bandgap energies of 5.15, 5.56, and 6.16 eV, respectively, estimated from the X‐ray photoelectron spectroscopy inelastic energy‐loss spectra. Photoluminescence emission spectra in the ultraviolet and visible (blue) wavelength range highlighted several intrinsic defects in the film structure that functioned as luminescence centers, including self‐trapped exciton and recombining donor‐to‐acceptor band. Detailed analysis of the structural and optical properties of β‐(AlxGa1−x)2O3 epitaxial films revealed that this low‐cost and scalable solution‐deposition approach coupled with a spin‐coating technique could be used to fabricate β‐(AlxGa1−x)2O3 films with tunable properties.

Growth behavior and substrate selective deposition of polypyrrole, polythiophene, and polyaniline by oxidative chemical vapor deposition and molecular layer deposition

Bottom-up self-aligned area-selective deposition (ASD) plays an important role in patterning of advanced electronic devices. Specifically, ASD of organic materials can be utilized for nucleation inhibitors, sacrificial layers, and air-gap materials for next-generation nanoscale processing. This work introduces fundamental growth behavior of various conjugated polymers including polypyrrole, polythiophene, and polyaniline via oxidative molecular layer deposition and chemical vapor deposition. Effects of process parameters on film properties are described, and ASD behavior of different polymers are quantitatively characterized. These findings expand fundamental understanding of conjugated polymer deposition and provide new perspectives for ASD of organic thin films.

Synthesis of monolayer 2D MoS2 quantum dots and nanomesh films by inorganic molecular chemical vapor deposition for quantum confinement effect control

AbstractBandgap engineering is an important prerequisite for various applications of two‐dimensional (2D) transition metal dichalcogenides (TMD). A reduction in the dimension from 2D has been one of the methods to control the electronic structure, by which a quantum confinement effect in the additional axis(es) can result in the widening of band structures. A vapor‐phase process for synthesizing monolayer MoS2 nanomesh film or MoS2 quantum dot is developed based on the inorganic molecular chemical vapor deposition. The vapor‐phase process can control MoS2 coverage by adjusting growth times. Therefore, the formation of nanostructures can be confirmed based on the growth time. The quantum confinement effect in monolayer MoS2 nanomesh films and MoS2 quantum dots is also confirmed via spectroscopic investigations, which induce a blue shift, indicating bandgap widening. Consequently, this approach can be used to synthesize lower‐dimensional TMD materials for bandgap engineering, which is an essential process in optical or optoelectrical applications.

Electron radiation effects on carrier relaxation in molecular beam and vapor deposition grown GaAs test structures

We exposed a series of AlGaAs/GaAs double heterostructure test articles grown by molecular beam epitaxy and metalorganic chemical vapor deposition to electron radiation. The active regions of the test articles were doped p-type, n-type, or unintentionally. Above bandgap steady state and time-resolved photoluminescence spectroscopy were used to characterize radiation-induced changes in the band structure and carrier dynamics of the test articles. The effects of 1 MeV electron radiation on the low temperature steady state photoluminescence and room temperature carrier lifetime varied with dopant type and concentration. The doping dependence of the lifetime damage coefficient indicates that dopants can mitigate the impact of radiation-induced non-radiative carrier recombination processes.

Wafer‐Scale InN/In2S3 Core–Shell Nanorod Array for Ultrafast Self‐Powered Photodetection

AbstractSelf‐powered photodetectors have paved the way for electronic applications in fields such as civilian communication, infrared mapping, and industrial automatic control. However, most self‐powered photodetectors have faced photoresponse‐speed and device‐scale bottlenecks. Herein, a novel, self‐powered detector with an ultrafast response speed based on a core–shell InN/In2S3 nanorod array is proposed. A wafer‐scale InN/In2S3 nanorod array with good homogeneity is synthesized on Si substrates via a simple two‐step method involving molecular beam epitaxy and chemical vapor deposition. The photodetector device exhibits excellent self‐powered properties and a high current on/off ratio of 5 × 103. Further analyses determined that the device have an excellent photovoltaic responsivity and detectivity of 140 mA·W−1 and 4.0 × 1010 Jones, respectively (0 V). Impressively, the device exhibits an ultrafast photoresponse with a rise/fall time of 22/32 µs. The self‐powered InN/In2S3 photodetector with an ultrafast response speed shows superior potential for electronic applications. The core–shell nanostructure hybrid heterojunction introduces a novel idea for wafer‐scale nano‐photodetectors.

Application of Molecular Vapour Deposited Al2O3 for Graphene-Based Biosensor Passivation and Improvements in Graphene Device Homogeneity.

Graphene-based point-of-care (PoC) and chemical sensors can be fabricated using photolithographic processes at wafer-scale. However, these approaches are known to leave polymer residues on the graphene surface, which are difficult to remove completely. In addition, graphene growth and transfer processes can introduce defects into the graphene layer. Both defects and resist contamination can affect the homogeneity of graphene-based PoC sensors, leading to inconsistent device performance and unreliable sensing. Sensor reliability is also affected by the harsh chemical environments used for chemical functionalisation of graphene PoC sensors, which can degrade parts of the sensor device. Therefore, a reliable, wafer-scale method of passivation, which isolates the graphene from the rest of the device, protecting the less robust device features from any aggressive chemicals, must be devised. This work covers the application of molecular vapour deposition technology to create a dielectric passivation film that protects graphene-based biosensing devices from harsh chemicals. We utilise a previously reported “healing effect” of Al2O3 on graphene to reduce photoresist residue from the graphene surface and reduce the prevalence of graphene defects to improve graphene device homogeneity. The improvement in device consistency allows for more reliable, homogeneous graphene devices, that can be fabricated at wafer-scale for sensing and biosensing applications.

Oxidative molecular layer deposition of PEDOT using volatile antimony(V) chloride oxidant

Molecular layer deposition and chemical vapor deposition are emerging and promising techniques for the incorporation of high-performance conductive polymers into high surface area devices, such as sintered tantalum anodes for electrolytic capacitors. Until recently, vapor-phase synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) has relied on solid reactants which require relatively high temperatures and complex dosing schemes for sequential layer-by-layer processes. This work introduces a facile and high-performing layer-by-layer oxidative molecular layer deposition (oMLD) scheme using the volatile liquid oxidant antimony(V) chloride (SbCl5) to deposit PEDOT thin films. Effects of reactor parameters on PEDOT film characteristics are described, and the necessary foundation for future studies aiming to understand the nucleation and growth of layer-by-layer oMLD PEDOT is detailed.

Magnetic and electronic properties of Fe3O4/PtSe2/Fe3O4 junctions

Two-dimensional (2D) materials with single or a few-atomic layers have attracted significant attention from the scientific community due to their transport properties and potential for technological applications. A variety of 2D materials beyond graphene with different bandgaps have been synthesized in recent years. One of them is platinum diselenide (PtSe2) with the bandgap energy of 1.2 eV at one monolayer. However, the low throughput synthesis of high quality 2D thin films has thus far hindered the development of PtSe2 devices. In this work methods of molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) have been used to fabricate PtSe2 thin films. We have fabricated a prototypical PtSe2 based magnetic tunnel junction (MTJ) with Fe3O4 electrodes. For this purpose, Fe3O4-PtSe2-Fe3O4 trilayers were grown on MgO(100) substrate. The magnetoresistance of the trilayer at room temperature indicates that PtSe2 is a potential candidate as a barrier material for Fe3O4 based magnetic tunnel junctions.

Methods of transferring two-dimensional materials

The advent of two-dimensional (2D) materials, a family of materials with atomic thickness and van der Waals (vdWs) interlayer interactions, offers a new opportunity for developing electronics and optoelectronics. For example, semiconducting 2D materials are promising candidates for extending the Moore's Law. Typical 2D materials, such as graphene, hexagonal boron nitride (h-BN), black phosphorus (BP), transition metal dichalcogenides (TMDs), and their heterostrcutures present unique properties, arousing worldwide interest. In this review the current progress of the state-of-the-art transfer methods for 2D materials and their heterostructures is summarized. The reported dry and wet transfer methods, with hydrophilic or hydrophobic polymer film assistance, are commonly used for physical stacking to prepare atomically sharp vdWs heterostructure with clear interfaces. Compared with the bottom-up synthesis of 2D heterostructures using molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), the construction of 2D heterostructures by transfer methods can be implemented into a curved or uneven substrate which is suitable for pressure sensing, piezoelectric conversion as well as other physical properties’ research. Moreover, the transfer of 2D materials with inert gas protected or in vacuum operation can protect moisture-sensitive and oxygen-sensitive 2D materials from degerating and also yield interfaces with no impurities. The efficient and non-destructive large-area transfer technology provides a powerful technical guarantee for constructing the 2D heterostructures and exploring the intrinsic physical and chemical characteristics of materials. Further development of transfer technology can greatly facilitate the applications of 2D materials in high-temperature superconductors, topological insulators, low-energy devices, spin-valley polarization, twistronics, memristors, and other fields.

Patterned GeSn Strip Growth on Si By Sn Self-Catalyzed Method

GeSn has been attracted much attention in recent years due to its direct band characteristic. However, due to the large lattice mismatch and the low solid solubility of Sn in Ge, the growth of GeSn on Si is still a challenge. Thick-layer GeSn alloy have been grown by molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) by applying the low-temperature-grown Ge buffer layer, but there is still high the dislocation density(more than 107cm-2) and it limits its application in optoelectronics. We have reported a simple way to grow the defect-free GeSn alloy strips on Si substrates with the assistance of Sn metal. It can be lateral grown on Si(111) by Sn self-catalyzed method. The GeSn materials are grown in a MBE system with a base pressure of 5 × 10−8 Pa.Here we will report our recent progress of the growth of GeSn wires and GeSn pattern on pattered substrate. It is found that GeSn materials can be selected grown in the patterned window. The size of wires can be down to 100 nm. We can grow 10x10 GeSn pattern with size of 2um x 2 um and 5 um x 5um. Here we will also discuss the growth mechanism and the impact factors on the growth of GeSn materials. It is found that Sn has an important impact on morphology of the GeSn wires and different depositions and annealing times of Sn will impact their morphology. GeSn alloy strips show a good quality by the TEM micrographs and the SAED patterns. It has an important application in Si based laser and photodetectors.

Perovskite Single-Crystal Thin Film Devices Using Lithography Assisted Epitaxy

Strain engineering is an emerging technique to physically manipulate and enhance semiconductor device performance. Chen et al. recently combined strain engineering with halide perovskites, demonstrating a novel approach toward improvement in optoelectronic properties.

Characterization and performance of graphene–PbSe thin film heterojunction

An infrared detector with high responsivity based on graphene–PbSe thin film heterojunction was reported. High-quality PbSe thin film and graphene were prepared by molecular beam epitaxy and chemical vapor deposition, respectively. The physical characteristics of PbSe thin film and graphene were performed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman measurement. The phototransistor using PbSe thin film as a sensitizer and graphene as a channel to transport excitons exhibits peak responsivity and detectivity up to ~ 420 A·W−1 and 5.9 × 1011 Jones (radiation intensity: 0.75 mW·cm−2) at room temperature in the near-infrared (NIR) region, respectively. The high optical response is attributed to the photo-excited holes transferring from PbSe film to graphene under irradiation. Moreover, it is revealed that the responsivity of graphene–PbSe phototransistor is gate-tunable which is important in photodetectors.

Atomic simulation of homoepitaxial AlN on non-polar (11-20) plane

ABSTRACTLarge-scale molecular dynamics vapour deposition simulations have been performed to study the homoepitaxial growth of AlN films on non-polar (11-20) AlN surfaces. The effect of growth temperature and injected N: Al flux ratio on the crystallinity and surface morphology of deposited AlN film are investigated. We find that as the growth temperature increases from 1000 K to 3000 K at injected N:Al flux ratio of 2.0, the crystallinity and surface morphology of deposited AlN film both reach the optimum at 3000 K. Increasing the growth temperature makes the adatoms more mobile to move to their ideal lattice sites. Thus, the amorphousness and defects can be effectively reduced and good crystallinity and surface morphology could be obtained. Moreover, when the N:Al flux ratio increases from 1.2 to 2.4 under the growth temperature of 2600 K, the crystallinity and surface morphology of deposited AlN film both reach the optimum at injected N: Al flux ratio of 1.6. The improved crystallinity and surface morphology can be reached by using the optimised N: Al flux ratio. Furthermore, our simulation deepens the understanding and provides guidance for the growth of AlN on the non-polar (11-20) surface in the future.

Molecular dynamics study of tilt grain boundary evolution during the growth of beryllium thin films

• Grain boundaries can maintain its type as the epitaxial layer growing. • Large incident angle results in the formation of more defects at the grain boundary. • Incident energy about 5 eV is desired to form improved Beryllium film. Beryllium films fabricated by magnetron sputtering deposition unavoidably contain grain boundaries (GBs), which have a substantial effect on the resulting material properties. The growth dynamics and evolution of various tilt GBs were explored using a direct molecular dynamics vapor deposition simulation. The effects of the incident energy, incident angle and substrate temperature on the film morphologies were also considered. The results show that GBs and dislocations were extended to the epitaxial layer during the growth of the beryllium thin films. The types of GBs were maintained and were closely related to the surface topography of the substrate. An increased incident energy and substrate temperature enabled the formation of good-quality beryllium films. However, large incident angles resulted in the formation of defect structures in the interior of the beryllium films. The low-angle GBs were more stable than the high-angle GBs. Large incident energies (above 10 eV) with large incident angles (above 38°) always accompanied increased numbers of reflected atoms. In brief, during the growth of the beryllium thin films, the GBs were extended to the epitaxial layer, and the types of GBs were maintained. In addition, incident energies about 5 eV with incident angles less than 30° are desired to form improved Be films.

Revisited Cassie's law to incorporate microstructural capillary effects

The equilibrium contact angle and the receding contact angle of water droplets suspended on surfaces comprising arrays of equidistant, uniformly sized square micropillars has been measured with goniometry. Surfaces with distinct pillar size and spacing were fabricated via photolithography and deep reactive-ion etching prior to hydrophobization via the molecular vapor deposition of perflourodecyltrichlorosilane (FDTS). The surfaces exhibited superhydrophobic properties and the measured equilibrium contact angle was compared with the prediction of the Cassie equation based on the measured contact angle on a flat FDTS surface and the surface morphology. A poor agreement between the experimental data and the data predicted by the Cassie equation was found when the spacing between structures was less than the width of the pillars. For more closely spaced structures, the deviation between the measured and predicted values increased. In this roughness region, the measured angle is unchanged by the spacing but the receding angle continues to be dependent on the surface structure. A microscopic examination of the interface between the surface and the droplet revealed that the liquid-gas portion of the contact line was distorted at the pillar edges. The extent of the distortion could not be accurately quantified but it was shown that if the capillary region was assumed to be semicircular and extending half of the width of a pillar in to the liquid-gas region of the contact line, that the contact angle could be predicted well. Moreover, a good prediction of the experimental data of a prior study of droplets on closely spaced circular polydimethylsiloxane micropillar arrays is presented.Received 20 March 2019DOI:https://doi.org/10.1103/PhysRevFluids.4.081601©2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasContact line dynamicsDrops & bubblesLiquid bridgesSurface tension effectsWettingFluid Dynamics

Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have been attributed to in-gap states associated with chalcogen vacancies, even in the absence of direct experimental evidence. Here, we combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe2 and WS2 monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively. Surprisingly, we observe no in-gap states. Our results strongly suggest that the common chalcogen defects in the described 2D-TMD semiconductors, measured in vacuum environment after gentle annealing, are oxygen substitutional defects, rather than vacancies.

Efectos Isotópicos del Agua Ligera (H2O) y del Agua Pesada (D2O) en las Interacciones Hidrofóbicas

Las propiedades fisicoquímicas del agua ligera (H2O) y del agua pesada (D2O) difieren en cierta medida debido a las diferencias que surgen de los efectos de los isótopos. Para comprender mejor los efectos de los isótopos en las interacciones hidrófobas, informamos la comparación experimental directa de las interacciones hidrófobas en agua ligera versus agua pesada entre superficies de mica hidrofobizadas molecularmente extendidas y lisas utilizando un Aparato de Fuerzas de Superficie (SFA). En este estudio sintetizamos superficies hidrófobas estables, lisas y fácilmente reproducibles mediante la deposición de monocapas de perfluorodeciltriclorosilano (FDTS) sobre superficies de mica activadas por plasma utilizando la técnica de deposición de vapor molecular (MVD). La humectabilidad y la morfología de la superficie de las muestras de mica recubiertas con FDTS se controlaron directamente utilizando una célula de ángulo de contacto y un microscopio de fuerza atómica (AFM). La inestabilidad en la curva de fuerza se observa durante el acercamiento y la retracción de las superficies en los experimentos SFA y nuestros experimentos preliminares de espectroscopia de fuerza demostraron que la magnitud de la interacción hidrofóbica entre las superficies de mica recubiertas con FDTS es 20% más fuerte en agua pesada que en agua ligera. Nuestros resultados indican que los perfiles de fuerza-distancia obtenidos para los casos tanto de H2O como de D2O no pudieron ser descritos razonablemente por la teoría clásica de Derjaguin − Landau − Verwey − Overbeek (DLVO) y la fuerte adhesión medida entre las capas FDTS-FDTS que interactúan es dominado por las interacciones hidrófobas.

Differentiating Polymorphs in Molybdenum Disulfide via Electron Microscopy.

The presence of rich polymorphs and stacking polytypes in molybdenum disulfide (MoS2 ) endows it with a diverse range of electrical, catalytic, optical, and magnetic properties. This has stimulated a lot of interest in the unique properties associated with each polymorph. Most techniques used for polymorph identification in MoS2 are macroscopic techniques that sample averaged properties due to their limited spatial resolution. A reliable way of differentiating the atomic structure of different polymorphs is needed in order to understand their growth dynamics and establish the correlation between structure and properties. Herein, the use of electron microscopy for identifying the atomic structures of several important polymorphs in MoS2 , some of which are the subjects of mistaken assignment in the literature, is discussed. In particular, scanning transmission electron microscopy-annular dark field imaging has emerged as the most effective and reliable approach for identifying the different phases in MoS2 and other 2D materials because its images can be directly correlated to the atomic structures. Examples of the identification of polymorphs grown under different conditions in molecular beam epitaxy or chemical vapor deposition, for example, 3R, 1T, 1T'-phases, and 1T'-edges, are presented, including their atomic structures, fascinating properties, growth methods, and corresponding thermodynamic stabilities.