Chemical Deposition Research Articles

High mobility p-channel GaN heterostructures grown by MOCVD through impurity engineering

The p-GaN/AlGaN/GaN heterostructures with integrated n-channel and p-channel have been extensively applied in p-channel field effect transistor (p-FET) devices and complementary (CMOS) logic circuits. However, the hole mobility of the p-channel is still low, especially in the heterostructures grown by metalorganic chemical vapor deposition (MOCVD). In this work, an impurity engineering was designed by introducing Ga vacancies in the p-channel, so that the diffused Mg impurity could substitute the Ga site and form MgGa−1 rather than Mginter+2. The charged impurity scattering was hence suppressed due to the reduction of the impurity charge. As a result, the GaN heterostructure with a hole mobility of 21.8 cm2/V·s with a sheet hole density of 1.02 × 1013/cm2 was realized at room temperature by MOCVD. This work paves a way for improving the transport properties of GaN heterostructures and lays a foundation of high performance GaN-based p-FET devices and CMOS logic circuits on Si substrates.

A Coplanar Crystalline InGaO Thin Film Transistor with SiO2 Gate Insulator on ZrO2 Ferroelectric Layer: A New Ferroelectric TFT Structure

AbstractFerroelectric transistors with a large memory window (MW) and operational stability have been of increasing interest recently. In this study, a ferroelectric thin‐film transistor (FE‐TFT) with a novel metal‐insulator‐semiconductor‐ferroelectric (MISF) structure is proposed. With the ferroelectric layer located under the semiconductor, the TFT process can be similar to a conventional coplanar structure with a SiO2 gate insulator (GI). In this work, both FE and active semiconductors are deposited by spray pyrolysis which is beneficial for large‐area and low‐cost manufacturing. The FE ZrO2 by spray pyrolysis has a nanocrystalline phase, and the semiconductor InGaO shows a polycrystalline structure. The TFT exhibits a MW of 5.6 V with an operating voltage range of −10–10 V. The device shows a low leakage current of 10−12 A, and thus the on/off ratio is >107 at VDS = 1.0 V. The device shows stable performance with increasing temperatures up to 80 °C. The endurance of the device is 5000 cycles at 0.1 Hz pulse with a negligible variation of MW less than 0.1 V, indicating excellent operational stability.

Defect-Mediated Exciton Localization and Relaxation in Monolayer MoS2.

Defects in chemical vapor deposition (CVD)-grown monolayer MoS2 are unavoidable and provide a powerful approach to creating single-photon emitters and quantum information systems through localizing excitons. However, insight into the A- trion and B/C exciton localization in monolayer MoS2 remains elusive. Here, we investigate defect-mediated A- trion and B/C exciton localization and relaxation in CVD-grown monolayer MoS2 samples via transient absorption spectroscopy. The localization rate of A- trions is five times faster than B excitons, which is attributed to the distinctions in the Bohr radius, diffusion rate, and multiphonon emission. Furthermore, we obtain unambiguous experimental evidence for the direct excitation of localized C excitons. Varying gap energy at the band-nesting region revealed by first-principles calculations explains the anomalous dependence of localized C exciton energy on delay time. We also find that the rapid dissociation of localized C excitons features a short characteristic time of ∼0.14 ps, while the measured relaxation time is much longer. Our results provide a comprehensive picture of the defect-mediated excitonic relaxation and localization dynamics in monolayer MoS2.

Sustainable Synthesis of Diamond-like Carbon and Giant Carbon Allotropes from Hyperbaric Methanol–Water Mixtures Through the Critical Point

Freeform carbon fibres were 3D-printed from CH3OH:H2O mixtures using hyperbaric-pressure laser chemical vapour deposition (HP-LCVD). The experiment overlapped a region of known diamond growth, with the objective of depositing diamond-like carbon without the use of plasmas or hot filaments. A high-pressure regime was investigated for the first time through the precursor’s critical point. Seventy-two C-fibres were grown from 13 different CH3OH:H2O mixtures at total pressures between 7.8 and 180 bar. Maximum steady-state axial growth rates of 14 µm/s were observed. Growth near the critical point was suppressed, ostensibly due to thermal diffusion and selective etching. In addition to nanostructured graphite, various carbon allotropes were synthesised at/within the outer surface of the fibres, including diamond-like carbon, graphite polyhedral crystal, and tubular graphite cones. Several allotropes were oversized compared to structures previously reported. Raman spectral pressure–temperature (P-T) maps and a pictorial P-T phase diagram were compiled over a broad range of process conditions. Trends in the Raman ID/IG and I2D/IG intensity ratios were observed and regions of optimal growth for specific allotropes were identified. It is intended that this work provide a basis for others in optimising the growth of specific carbon allotropes from methanol using HP-LCVD and similar CVD processes.

Synthesis and characterization of magnetically-active nickel-yttrium oxide (Ni-Y2O3) nanocomposite particles prepared with modified ultrasound spray pyrolysis device

AbstractThe synthesis of magnetically-active nickel-yttrium oxide (Ni-Y2O3) nanocomposite particles is described in this work. The investigated material is produced with a modified ultrasound spray pyrolysis (USP) device which differs from a common USP setup in terms of use of three independently heating zones. They provide a direct feed of H2 to the second reaction zone and allow controlling the formation of the nanocomposite particles and facilitating their post-reaction stabilization with polyvinylpyrrolidone (PVP). According to the morphological and structural studies, the Ni-Y2O3 material takes a form of nanoparticles whose sizes are not homogeneously distributed as well as shapes are not smooth due to the successful formation of composite material with two interpenetrating phases. Moreover, the organic layer is detected on the surface of the nanoparticles which confirms the presence of PVP stabilizer. The magnetic investigations confirm that the Ni-Y2O3 nanocomposite reveals a spin glass-like behavior in which a collective freezing of magnetic moments might occur due to the interparticle interactions between Ni nanocrystallites presented in the sample.

Functionalized Dual/Multiligand Metal-Organic Frameworks for Efficient CO2 Capture under Flue Gas Conditions.

Reducing carbon dioxide (CO2) emissions has become increasingly urgent for China, particularly in the industrial sector. Striking a balance between a high CO2 adsorption capacity and long-term stability under practical conditions is crucial for effectively capturing CO2 from flue gas. In this study, a series of functionalized MFM-136 adsorbents were synthesized in which -NO2 and -NH2 groups were grafted onto the kagome lattice of MFM-136. Modifications with -NH2 groups were found to be highly effective for CO2 adsorption, specifically, the CO2 adsorption capacity peaked at 4.35 mmol/g for NH2(0.6)-MFM-136, representing a 55% enhancement more than MFM-136. Concurrently, the CO2/N2 selectivity for NH2(0.6)-MFM-136 was increased 1.57 times. Verification of novel adsorption sites introduced by NH2-H2L4 was conducted by using in situ DRIFT analysis and DFT calculations. It turns out that NH2-H2L4 modification can effectively mitigate the chemical deposition from the impurity gases and significantly improve the adsorbent's hydrophobicity and its tolerance to impurity gases. Remarkably, the reduction in the CO2 absorption capacity for NH2(0.6)-MFM-136 was 34% less than that for MFM-136 after 24 h of exposure to simulated flue gas, making NH2(0.6)-MFM-136 a promising candidate for the potential application of stable and selective CO2 capture under industrial flue gas conditions.

Controlled Chemical Vapor Deposition and Modification of Carbon Layers inside Quartz Nanopipettes.

Carbon nanopipettes (CNPs) have attracted much attention in nanoscale electrochemical applications recently, while the carbon structure and surface oxygen-containing groups limit its applications. Herein, we grow the carbon nanotubes (CNTs) inside the quartz nanopipet via the chemical vapor deposition method, and the fabricated carbon nanotube nanopipettes (CNT-NPs) exhibit better electrochemical responses toward biomolecules such as glutathione and ascorbic acid, compared to the conventional CNPs. In addition, the carbon nanopipette can also be easily doped by a chemical reaction with urea, to display positive surface charges and high electrochemical activity for H2O2 oxidation/reduction. This work provides an easy way to tailor the surface structure and charges of the deposited carbon inside the pipettes and thus would further promote its broader usage in electrochemical sensing applications in biological fields.

Study on the friction and wear performance of graphene/porous bronze composites

Abstract The porous surface of air bearings may suffer from surface scratching and wear issues during the start-stop processes or under extreme working conditions. To enhance the lubrication properties and improve the service life of the porous restrictors, the graphene/porous bronze composites were prepared by combining powder metallurgy (PM) and chemical vapor deposition (CVD) in this study. On this basis, studies on the friction and wear performance of porous composites were carried out. The prepared porous composites and the friction surfaces were described by various means such as friction coefficient, morphology scanning, surface roughness, SEM, and EDS, which revealed the friction and wear process of the composites. Moreover, during the composite generation, the flow rates of carbon sources were set to be 3, 6, 9, and 12 sccm, respectively, to investigate the effect of graphene content on the lubrication properties of the original porous matrix. The experimental results showed that the lubricating performance of the composite was optimized at a flow rate of 12 sccm. Compared to the original porous bronze, the friction coefficient was decreased by 8.4%, and the wear rate was reduced by 10.5%. Furthermore, the research findings indicated that the grown graphene also affects the sintering process of the porous matrix, leading to weak connections between powders and differences in the final pore structure. Thus, the final pore structure, along with the presence of graphene, jointly influences the friction and wear performance of the porous composites.

Effect of annealing temperature onto physical properties of Cu doped ZnO thin films prepared using spray pyrolysis

Abstract Herein we report the effect of annealing on spray-pyrolysis-deposited Cu-doped zinc oxide thin films, with a fixed 3 wt% copper concentration and annealing temperatures of 450 and 500 °C. Various analytical techniques were employed to evaluate the effect of annealed films, which exhibited high stability in physical properties and minimal influence from the annealing process. XRD analysis confirmed that all films maintained a hexagonal ZnO structure without any additional phases, indicating the high purity of the films, with the (002) peak serving as the main diffraction peak for both as-deposited and annealed films. Crystallite size, calculated using the Halder-Wagner equation, revealing an increase from 13.96 nm for the as-deposited film to 14.26 nm for film annealed at 450 °C and 14.65 nm for film annealed at 500 °C. Microstrain values were measured at 2.3 × 10−3, 2.5 × 10−3, and 1.3 × 10−3 for the as-deposited and annealed films. Surface imaging with FE-SEM revealed average grain sizes of 57.25 nm, 68 nm, and 67.8 nm for the as-deposited film and those annealed at 450 °C and 500 °C, respectively. The estimated band gap values were 3.14 eV for the as-deposited films, 3.15 eV for those annealed at 450 °C, and 3.16 eV for films annealed at 500 °C. According to the Spitzer-Fan model, both the density of states and plasma frequency remained constant across the films, while the relaxation time and optical mobility were lowest at 450 °C, where the high-frequency dielectric constant reaches its peak.

High-temperature growth of Cu2O crystals via mist chemical vapor deposition on Si (111) substrates

Abstract Cuprous oxide (Cu2O) crystals were grown on Si(111) substrates at a high temperature of approximately 960 °C using mist chemical vapor deposition (mist-CVD) by varying the mist carrier gas parameters. The mist-CVD process has a potentially low environmental impact and low cost due to the absence of a vacuum system and the use of low-toxic raw materials. The as-grown crystals were evaluated using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Based on these results, the mixture ratio of the mist carrier gas is significant for realizing Cu2O crystal growth. 


Synthesis of diamond films with reduced roughness in microwave plasma

The work studies the effect of nitrogen additions on the secondary nucleation (nucleation) of diamond during its synthesis by chemical vapor deposition (CVD). A series of polycrystalline diamond (PCD) films 2 μm thick were grown on silicon substrates in methane-hydrogen-nitrogen gas mixtures with different nitrogen concentrations (0–1%). The structure and roughness of the grown films were studied using scanning electron microscopy (SEM) and optical profilometry. It has been shown that small additions of nitrogen play a key role in the processes of secondary nucleation of diamond, having a significant impact on the morphology of films. The comparison of the characteristics of grown PCD allowed us to find the optimal nitrogen concentration [N2] ≈ 0.2% for the formation of nanocrystalline diamond (NCD) films with low surface roughness and increased growth rate. The results obtained are expected to be used to optimize the parameters of CVD synthesis of PCD films for use as protective or friction-reducing layers, as well as for the manufacture of superhard cutting tools.

Aerosol Assisted Chemical Vapour Deposition (AACVD) of Zinc dichalcogenoimidodiphosphinate Complexes for the Deposition of Zinc Selenide Thin Films.

Dichalcogenoimidodiphosphinate complexes of zinc [Zn{(EPiPr2)2N}2], [E=Se,Se; S,Se] were synthesized through metathetical reactions from the dichalcogenoimidodiphosphinate ligands [(EE'PiPr2NH)] (E, E'=Se, Se; S, Se). These complexes were characterized and used as single-source precursors through Aerosol-Assisted Chemical Vapour Deposition (AACVD) for the deposition of cubic zinc selenide (ZnSe) films on glass substrates. The deposition temperature occurred at 500 and 525 °C, while the flow rates of the carrier gas was 160 and 240 standard cubic centimetre (sccm). The morphology of the deposited films ranged between dense fibrous network and woven fibres. X-ray photoelectron spectroscopy (XPS) confirmed the presence of the constituent elements in zinc selenide.

Synthesis and Characterization of Optically Transparent and Electrically Conductive Mo-Doped ZnO, F-Doped ZnO, and Mo/F-Codoped ZnO Thin Films via Aerosol-Assisted Chemical Vapor Deposition

Synthesis and Characterization of Optically Transparent and Electrically Conductive Mo-Doped ZnO, F-Doped ZnO, and Mo/F-Codoped ZnO Thin Films via Aerosol-Assisted Chemical Vapor Deposition

Spray Black Coating for High‐Efficiency Light Absorption

AbstractBlack coatings have emerged as a research focus due to their excellent light absorption performance over a wide wavelength range. They play a crucial role in precision optical devices and solar thermal applications. Among various preparation methods, spray coating has attracted great attention due to its simple preparation process, low cost, scalability, and applicability to complex structures. Herein, the recent progress in spray black coatings is comprehensively presented. Various spray coating methods employed in the preparation of black coatings, including air spraying, ultrasonic spraying, electrostatic spraying, spray pyrolysis, and thermal spraying are summarized and compared. Black spray coatings based on metal sulfide, metal oxide, cermet, polymer, and carbon are then reviewed. In addition to the intrinsic absorption properties of the black coatings, light‐trapping structures are key to achieving high‐efficiency light absorption. Typical structural design strategies for enhancing absorption are highlighted. Moreover, the trade‐off between absorptance and adhesion in the design of robust spray black coatings is indicated. The remaining challenges and outlook for the spray black coatings are discussed. This review is expected to provide valuable guidelines for the future development of spray black coatings.

Nano-Sized Graphene Cages Realize Highly Loaded “Solid-Solid” Process in Cathode Materials of Lithium Sulfur Batteries

Abstract The effects of polysulfide shuttle and volume expansion in lithium sulfur batteries limit their practical applications. In this work, a two-dimensional fabric with graphene cage interweaving was designed by the chemical vapor deposition method to resolve these issues. It has a large number of micropores, mesopores, and macropores. Sulfur was deposited in these pores by melting method to form sulfur/graphene complex. Polysulfides were confined within these pores, making them less likely to diffuse and shuttle. Graphene walls have strong flexibility and can alleviate the volume expansion effect during charge and discharge process. Sulfur/graphene complex as active materials were directly coated on aluminum foil by graphene oxide to build lithium-sulfur battery, where graphene oxide served not only as an adhesive but also as a conductive agent to improve the electrochemical performances of batteries. The lithium-sulfur battery had an initial capacity of 1400 mAh g-1 and high capacity retention (500 mAh g-1 after 200 cycles at 0.2 C rate). These results suggest that the reasonable design of material morphology for sulfur loading will effectively improve the performance of lithium sulfur batteries.

Ultra-broadband detection by photovoltaic and thermoelectric coupling based on large-scale single-crystalline SnSe thin film

Recently, the layered tin selenide (SnSe) has attracted intense attention from the researchers due to its distinguished thermoelectric properties, thus giving this compound quite a promising potential application for photothermoelectric detectors. However, the low-cost epitaxial growth method toward a millimeter scale single phase SnSe thin film is still rarely reported, thus limiting its fabrications in arraying photoelectric sensors. Here, we synthesized a large-scale SnSe thin film on the SrTiO3 substrate by using the crack of PbS thin film-assisted nucleation in the chemical vapor deposition, achieving a homogeneous single-crystal SnSe thin film with a centimeter scale, as revealed by the x-ray diffraction and scanning electron microscope measurement. In addition, a two-terminal device is fabricated to study the photoelectric properties of this film. Surprisingly, this SnSe detector shows a synergetic photovoltaic and thermoelectric effect, achieving an ultrabroad band detection ranging from visible (405 nm) to mid-infrared (10.0 μm) at room temperature. Significantly, this detector also shows an impressive performance with an optimized response time of 2.81 ms (at 4.0 μm), a responsivity of 290.9 V W−1 (at 4.0 μm), and a detectivity of 5.5×108 Jones (at 4.0 μm). The above results addressed the bottleneck in SnSe film synthesis, and accelerated its applications in future high-performance photoelectronic devices.

Chemical Vapor Deposition of Pyridinic-Nitrogen-Dominated Graphene Model Electrocatalysts and in Situ Raman Spectroscopic Evidence on Their Active Sites for Oxygen Reduction Reaction

Chemical Vapor Deposition of Pyridinic-Nitrogen-Dominated Graphene Model Electrocatalysts and in Situ Raman Spectroscopic Evidence on Their Active Sites for Oxygen Reduction Reaction

A simple route to synthesize cubic gauche polymeric nitrogen

Abstract The long sought cubic gauche polymeric nitrogen (cg-N) consisting of N-N single bonds has been synthesized by a simple route using sodium azide as a precursor at ambient conditions. The recrystallization process was designed to expose crystal faces with low activation energy that facilitates initiating the polymeric reaction at ambient conditions. The azide was considered as a precursor due to the low energy barrier in transforming double bonded N=N to single bonded cg-N. Raman spectrum measurements detected the emerging vibron peaks at 635 cm-1 for the polymerized sodium azide samples, demonstrating the formation of cg-N with N-N single bonds. Different from traditional high pressure technique and recently developed plasma enhanced chemical vapor deposition method, the route achieves the quantitative synthesis of cg-N at ambient conditions. The simple method to synthesize cg-N offers potential for further scale up production as well as practical applications of polymeric nitrogen based materials as high energy density materials.

Zn concentration’s effects on the structure, optical, and photoconductivity properties of ZnxCd1–xS thin films synthesized by chemical spray pyrolysis

Zn concentration’s effects on the structure, optical, and photoconductivity properties of ZnxCd1–xS thin films synthesized by chemical spray pyrolysis

Tuning Flame Spray Pyrolysis for Variation of the Crystallite Size in Cu/ZnO/ZrO2 and its Influence on the Performance in CO2‐to‐Methanol Synthesis

AbstractCO2 hydrogenation to methanol (MeOH) is a key transformation in the Power‐to‐liquid concept, which aims to store energy in chemical energy carriers and chemicals. Cu/ZnO/ZrO2 (CZZ) shows great promise due to its enhanced stability in the presence of water, a critical by‐product when utilizing CO2‐based feedstocks. The structure‐sensitivity of this reaction, especially for particle sizes below 10 nm and in three‐component systems, remains highly debated. Herein, we systematically prepared a series of CZZ catalysts by flame spray pyrolysis (FSP) to vary the crystallite size and to study its effect on methanol synthesis in this three‐component system. FSP enabled us to maintain a fixed Cu/Zn/Zr ratio close to the commercial composition (61/29/10 atomic ratio), while varying the precursor feed rate. This resulted in variation in the crystallinity. The characterization by X‐ray diffraction and electron microscopy revealed an increase in crystallite size with rising feed rate for Cu and t‐ZrO2, whereas ZnO remained mostly unaffected. The testing of the materials in methanol synthesis uncovered an increase in performance, higher space time yield and MeOH selectivity, with decreasing crystallite size for two (Cu, t‐ZrO2) of its three components. The increased selectivity with smaller sizes might be attributed to an increase in interfacial sites.