Chemical Vapor Deposition Step Research Articles

Inkjet Printing-Manufactured Boron-Doped Diamond Chip Electrodes for Electrochemical Sensing Purposes.

Fabrication of patterned boron-doped diamond (BDD) in an inexpensive and straightforward way is required for a variety of practical applications, including the development of BDD-based electrochemical sensors. This work describes a simplified and novel bottom-up fabrication approach for BDD-based three-electrode sensor chips utilizing direct inkjet printing of diamond nanoparticles on silicon-based substrates. The whole seeding process, accomplished by a commercial research inkjet printer with piezo-driven drop-on-demand printheads, was systematically examined. Optimized and continuous inkjet-printed features were obtained with glycerol-based diamond ink (0.4% vol/wt), silicon substrates pretreated by exposure to oxygen plasma and subsequently to air, and applying a dot density of 750 drops (volume 9 pL) per inch. Next, the dried micropatterned substrate was subjected to a chemical vapor deposition step to grow uniform thin-film BDD, which satisfied the function of both working and counter electrodes. Silver was inkjet-printed to complete the sensor chip with a reference electrode. Scanning electron micrographs showed a closed BDD layer with a typical polycrystalline structure and sharp and well-defined edges. Very good homogeneity in diamond layer composition and a high boron content (∼2 × 1021 atoms cm-3) was confirmed by Raman spectroscopy. Important electrochemical characteristics, including the width of the potential window (2.5 V) and double-layer capacitance (27 μF cm-2), were evaluated by cyclic voltammetry. Fast electron transfer kinetics was recognized for the [Ru(NH3)6]3+/2+ redox marker due to the high doping level, while somewhat hindered kinetics was observed for the surface-sensitive [Fe(CN)6]3-/4- probe. Furthermore, the ability to electrochemically detect organic compounds of different structural motifs, such as glucose, ascorbic acid, uric acid, tyrosine, and dopamine, was successfully verified and compared with commercially available screen-printed BDD electrodes. The newly developed chip-based manufacture method enables the rapid prototyping of different small-scale electrode designs and BDD microstructures, which can lead to enhanced sensor performance with capability of repeated use.

Annealing Enhanced Phase Transition in VO2 Thin Films Deposited on Glass Substrates via Chemical Vapor Deposition

In this work, VO2 is deposited on borosilicate glass substrates via a single step Chemical Vapor Deposition (CVD). Though the structural characterization revealed the presence of phase pure polycrystalline VO2 in simple monoclinic crystal structure (P21/c), electrical measurements did not show a sharp transition. To understand the ambiguity, surface characterization was performed with X-ray Photoelectron Spectroscopy (XPS) on the as-deposited VO2 thin films. Vanadium was present in V3+, V4+ and V5+ states. Following these measurements, annealing was performed in ambience at 200 °C for 10 minutes. Subsequent electrical measurements showed an improved phase transition, enhanced by an order of magnitude. We attribute the enhancement in transition to annealing of oxygen vacancies present in VO2 thin films because of the deposition conditions. Decrease in oxygen vacancies was correlated with a decrease in the fraction of V3+ from XPS measurements. The sample annealed at 200°C for 10 minutes exhibited a higher resistance ratio (ΔA = 305) with an optimum hysteresis (4°C) and transition width(12°C), whereas the as-deposited sample exhibited a resistance ratio of ΔA = 32. The thermal treatment affected the characteristics of the transition. A narrow hysteresis of 4°C and a sharp transition width with a reasonable resistance ratio were obtained for the sample annealed at 200°C for 10 minutes. From this study, we understand that our synthesis of VO2 thin films via CVD is prone to be off-stoichiometric and therefore, contains defects such as oxygen vacancies. We could mitigate the effect of these vacancies and engineer the stoichiometry by performing a simple annealing which in turn improved the strength of the phase transition. Drawbacks such as a poor-quality transition, resulting from synthesis parameters can be mitigated if we understand the effect of vanadium and oxygen vacancies.

Optical Recrystallization of Nanocrystalline Silicon Ribbons

The Silicon on Dust Substrate (SDS) is a gas-to-wafer process that produces multicrystalline silicon ribbons directly from gaseous feedstock (silane), avoiding the standard industry steps of polysilicon deposition, crystal growth, and wafering. The SDS technique consists of three main steps: (i) micrometric-sized silicon powder production by grinding silicon chunks; (ii) chemical vapor deposition (CVD) of silicon over this silicon powder substrate; and (iii) zone-melting recrystallization (ZMR) of the nanocrystalline pre-ribbon obtained in the CVD step. Several samples were produced by this technique. During CVD, mechanically self-sustained nanocrystalline pre-ribbons were grown over silicon powder substrates, with growth rates in the order of 50 µm/min. The ZMR process performance is substantially impacted by the pre-ribbon physical characteristics. The best and largest recrystallizations were achieved on pre-ribbons grown over powder substrates with smaller particle sizes, which also have lower substrate powder incorporation ratios. Multicrystalline silicon ribbons with crystalline areas as large as 2 × 4 cm2 were successfully produced. These areas have visible columnar crystal growth with crystal lengths up to 1 cm. The SDS ribbons’ measured resistivity confirmed the high powder content of the resulting material. The ability to produce solar cells on SDS multicrystalline silicon ribbons was demonstrated.

Atomistic reaction mechanism of CVD grown MoS2 through MoO3 and H2S precursors

Chemical vapor deposition (CVD) through sulfidation of MoO3 is one of the most important synthesis techniques to obtain large-scale and high-quality two-dimensional (2D) MoS2. Recently, H2S precursor is being used in the CVD technique to synthesize 2D MoS2. Although several studies have been carried out to examine the mechanism of MoS2 growth in the presence of sulfur and MoO3 precursors, the growth of MoS2 in the presence of H2S precursor has largely remained unknown. In this study, we present a Reactive molecular dynamics (RMD) simulation to investigate the reaction mechanism of MoS2 from MoO3 and H2S precursors. The intermediate molecules formation, the reason behind those formations, and the surface compositions of MoOxSyHz during the initial steps of CVD have all been quantified. Surprisingly, a sudden separation of sulfur atoms from the surface was observed in the H2S precursor system due to the substantial oxygen evolution after 1660 K. The sulfur detachments and oxygen evolution from the surface were found to have a linear relationship. In addition, the intermediate molecules and surface bonds of MoS2 synthesized by MoO3 and H2S precursors were compared to those of a system using S2 and MoO3 precursors. The most stable subsidiary formation from the H2S precursor was found to be H2O, whereas in case of S2 precursor it was SO. These results provide a valuable insight in the formation of large-scale and high-quality 2D MoS2 by the CVD technique.

Patterned Growth of Transition Metal Dichalcogenide Monolayers and Multilayers for Electronic and Optoelectronic Device Applications.

A simple, large area, and cost-effective soft lithographic method is presented for the patterned growth of high-quality 2D transition metal dichalcogenides (TMDs). Initially, a liquid precursor (Na2 MoO4 in an aqueous solution) is patterned on the growth substrate using the micromolding in capillaries technique. Subsequently, a chemical vapor deposition step is employed to convert the precursor patterns to monolayer, few layers, or bulk TMDs, depending on the precursor concentration. The grown patterns are characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and photoluminescence spectroscopy to reveal their morphological, chemical, and optical characteristics. Additionally, electronic and optoelectronic devices are realized using the patterned TMDs and tested for their applicability in field effect transistors and photodetectors. The photodetectors made of MoS2 line patterns show a very high responsivity of 7674 A W-1 and external quantum efficiency of 1.49 × 106 %. Furthermore, the multiple grain boundaries present in patterned TMDs enable the fabrication of memtransistor devices. The patterning technique presented here may be applied to many other TMDs and related heterostructures, potentially advancing the fabrication of TMDs-based device arrays.

Nanofabrication of Isoporous Membranes for Cell Fractionation

Cell fractionations and other biological separations frequently require several steps. They could be much more effectively done by filtration, if isoporous membranes would be available with high pore density, and sharp pore size distribution in the micro- and nanoscale. We propose a combination of two scalable methods, photolithography and dry reactive ion etching, to fabricate a series of polyester membranes with isopores of size 0.7 to 50 μm and high pore density with a demonstrated total area of 38.5 cm2. The membranes have pore sizes in the micro- and submicro-range, and pore density 10-fold higher than track-etched analogues, which are the only commercially available isoporous polymeric films. Permeances of 220,000 L m−2 h−1bar−1 were measured with pore size 787 nm. The method does not require organic solvents and can be applied to many homopolymeric materials. The pore reduction from 2 to 0.7 μm was obtained by adding a step of chemical vapor deposition. The isoporous system was successfully demonstrated for the organelle fractionation of Arabidopsis homogenates and could be potentially extended to other biological fractionations.

AACVD Synthesis and Characterization of Iron and Copper Oxides Modified ZnO Structured Films.

Non-modified (ZnO) and modified (Fe2O3@ZnO and CuO@ZnO) structured films are deposited via aerosol assisted chemical vapor deposition. The surface modification of ZnO with iron or copper oxides is achieved in a second aerosol assisted chemical vapor deposition step and the characterization of morphology, structure, and surface of these new structured films is discussed. X-ray photoelectron spectrometry and X-ray diffraction corroborate the formation of ZnO, Fe2O3, and CuO and the electron microscopy images show the morphological and crystalline characteristics of these structured films. Static water contact angle measurements for these structured films indicate hydrophobic behavior with the modified structures showing higher contact angles compared to the non-modified films. Overall, results show that the modification of ZnO with iron or copper oxides enhances the hydrophobic behavior of the surface, increasing the contact angle of the water drops at the non-modified ZnO structures from 122° to 135° and 145° for Fe2O3@ZnO and CuO@ZnO, respectively. This is attributed to the different surface properties of the films including the morphology and chemical composition.

Hierarchically structured 3D carbon nanotube electrodes for electrocatalytic applications.

Hierarchically structured 3-dimensional electrodes based on branched carbon nanotubes (CNTs) are prepared on a glassy carbon (GC) substrate in a sequence of electrodeposition and chemical vapor deposition (CVD) steps as follows: Primary CNTs are grown over electrodeposited iron by CVD followed by a second Fe deposition and finally the CVD growth of secondary CNTs. The prepared 3-dimensional CNT structures (CNT/CNT/GC) exhibit enhanced double-layer capacitance and thus larger surface area compared to CNT/GC. Pt electrodeposition onto both types of electrodes yields a uniform and homogeneous Pt nanoparticle distribution. Each preparation step is followed by scanning electron microscopy, while the CNTs were additionally characterized by Raman spectroscopy. In this way it is demonstrated that by varying the parameters during the electrodeposition and CVD steps, a tuning of the structural parameters of the hierarchical electrodes is possible. The suitability of the hierarchical electrodes for electrocatalytic applications is demonstrated using the methanol electro-oxidation as a test reaction. The Pt mass specific activity towards methanol oxidation of Pt-CNT/CNT/GC is approximately 2.5 times higher than that of Pt-CNT/GC, and the hierarchical electrode exhibits a more negative onset potential. Both structures demonstrate an exceptionally high poisoning tolerance.

Influence of HfC nanowires on the growth behavior, microstructure and ablation resistance of CVD-HfC coating

HfC nanowire-toughened HfC ablation resistant coating was prepared on carbon/carbon composites by two steps of chemical vapor deposition. Effects of HfC nanowires on the growth behavior, microstructure and ablation resistance of the HfC coating were researched. Due to the incorporation of HfC nanowires, the deposition rate of the HfC coating was improved, the HfC coating was composed of particle-stacked crystals. After incorporating HfC nanowires, the bonding strength and fracture toughness of the HfC coating increased. HfC nanowires could restrain the crack propagation of HfC coating during ablation, contributing to improving the ablation resistance of HfC coating. After ablation for 60 s, the mass ablation rate of the HfC-coated C/C sample reduced from 0.44 to 0.26 mg/s because of the incorporation of HfC nanowires.

Maskless direct growth of carbon nanotube micropatterns on metallic substrates

Herein, we report on a simple process, which is suitable for producing carbon nanotube micropatterns on steel alloys without the use of catalyst deposition. The method is based on a pulsed laser beam induced surface microstructuring process, followed by a thermal chemical vapor deposition growth of carbon nanotubes that emerge exclusively from the surface areas of the alloys previously exposed to the laser pulses. As concluded from X-ray diffraction, electron microscopy, Raman and X-ray photoelectron spectroscopy analyses of pristine and laser treated surfaces, the area-selective growth is caused by a formation of a thin and nanostructured metal oxide layer during the laser treatment, which is then partially reduced during the subsequent nanotube growth steps. The partially reduced oxide nanoparticles provide catalytic sites for nanotube growth and are also believed to disable diffusion of catalytic metal atoms to the subsurface regions, giving rise to stable catalyst nanoparticle formation and nanotube growth in the chemical vapor deposition step. The proposed method is fast, scalable and has potential for use in multiple applications that require close metal to nanotube contacts with arbitrary microscopic pattern definition on two and three-dimensional surfaces such as those used in electromechanical contacts, electrochemical electrodes, field emitters, and thermal interfaces.

Silicon on Dust Substrate: The Effect of Powder Size on Ribbon Production

The silicon on dust substrate process is a two‐step technique to produce multicrystalline silicon (mc‐Si) ribbons directly from gaseous feedstock. Silicon pre‐ribbons of very small grain size (ranging from nano to microcrystalline), with a porous structure and dimensions up to 25 × 100 mm2, are obtained using an inline optical chemical vapor deposition (CVD) system operating at atmospheric pressure and at low temperatures (<600 °C). Using silane as the gaseous precursor, nano or microcrystalline silicon layers can be grown, on top of silicon powder substrates, moving at constant speed, and crossing three hot deposition regions several times. The growth rates (GR) vary from 13.4 to 73.2 μm min−1, depending on the grain size of silicon powder. The last step is a floating zone (FZ) recrystallization technique, where the silicon pre‐ribbons obtained in the CVD step, become solid mc‐Si ribbons. The success of recrystallization depends on the grain size of silicon powder used as substrate on the CVD step, with lower grain sizes powders delivering better results. For the lower size powder substrates, multi‐crystalline areas of 5–20 mm2 were obtained, with an average crystal size in the (0.1; 1) mm range.

Single step synthesis of Schottky-like hybrid graphene - titania interfaces for efficient photocatalysis

The development of 2D nanomaterial coatings across metal surfaces is a challenge due to the mismatch between the metal microstructure and the nanoscale materials. The naturally occurring thin oxidative layer present across all metal surfaces, may lead to low adherence and connectivity. In this paper, graphene/titania/Titanium hybrid films were for the first time fabricated by a single step chemical vapour deposition process across Titanium foils. The presence of graphene as a dopant was found to enhance the photocatalytic performance of the final products, applied to the degradation of organic molecules and to lead to Schottky-like junction formation at the metal/oxide interface. These Schottky junctions, where vacancies are present across the titania material due to the graphene doping and where Ti3+ ions are predominantly located, yield enhanced catalytic performance. The highest degradation rate was found to be 9.66 × 10−6 min−1, achieved by the sample grown at 700 °C for 5 min, which was 62% higher than the sample just treated at that temperature without graphene growth. This work provides evidence that graphene may be grown across pure Titanium metal and opens new avenues in biomedical devices design, tribological or separation applications.

Stacking-mode confined growth of 2H-MoTe2/MoS2 bilayer heterostructures for UV–vis–IR photodetectors

The atomic thin, vertically-stacked 2H-MoTe2/MoS2 heterostructures are successfully synthesized using the single step chemical vapor deposition (CVD) method and a magnet-assisted secondary precursor delivery tool. The second material (MoTe2) was grown in a well-controlled, unique and epitaxial 2H-stacking mode atop the first material (MoS2), starting from the edges. This led to the construction of a vertical p-n junction with a broadband photoresponse from the ultraviolet (UV, 200 nm) to the near-infrared (IR, 1100 nm) regions. The high crystallinity of MoTe2/MoS2 heterostructures with a modulation of sulfur and tellurium distribution is corroborated by multiple characterization methods, including Raman spectroscopy, photoluminescence (PL) spectroscopy and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Furthermore, the photoelectrical measurements exhibit a tremendous photoresponsivity with an external quantum efficiency (EQE) as high as 4.71 A/W and 532% at 1100 nm, while as 4.67 A/W and 1935% at 300 nm, one to two orders of magnitude higher than other exfoliated MoTe2 heterostructure devices have been reported so far. This synthetic method is a controllable stacking mode confined synthesis approach for 2D heterostructures, and paves the way for the fabrication of high-performance functional telluride-based broadband photodetectors.

Direct Synthesizing In‐Plane Heterostructures of Graphene and Hexagonal Boron Nitride in Designed Pattern

AbstractIn‐plane heterostructure of graphene (G) and hexagonal boron nitride (h‐BN) has attracted increasing attentions owing to its exceptional properties and potentials in atomically thin circuitry. However, direct growth of patterned in‐plane G/h‐BN heterostructures (G‐BNs) by chemical vapor deposition (CVD) method in a controllable way remains a great challenge. Here, a reliable way to directly synthesize G‐BN in designed pattern by one CVD step is demonstrated. First, ultraviolet lithography and magnetron sputtering are utilized to deposit nickel nanofilm in designed pattern onto copper foil surface. Then the copper foil with prepatterned nickel nanofilm is used as the synthesis substrate. Benefiting from different mechanisms of graphene growth on Cu and Cu–Ni alloy surfaces, graphene grows solely on the Ni‐free surface of the substrate, followed by h‐BN growth on the exposed Ni‐pattern surface, realizing the direct growth of patterned G‐BN. The method presented here paves a viable road for controllable growth of 2D in‐plane heterostructures for variable applications.

Single step route to highly transparent, conductive and hazy aluminium doped zinc oxide films.

Light scattering yet transparent electrodes are important for photovoltaics as they increase device efficiency by prolonging light path lengths. Here, we present a novel single step route to highly textured Al doped ZnO thin films on glass substrates that show a minimum resistivity of ∼3 × 10−3 Ω cm and high visible light transmittance of 83% while still maintaining high haze factor of 63%. Roughness was imparted into the ZnO films during the synthetic process using acetylacetone and deionized water as additives. The highly hazy yet visible and near infrared transparent nature of the conductive ZnO:Al films allow it to be potentially used as an electrode material in amorphous and microcrystalline silicon solar cells.

A scalable and facile synthesis of carbon nanospheres as a metal free electrocatalyst for oxidation of l-ascorbic acid: Alternate fuel for direct oxidation fuel cells

Large-scale, highly crystalline, high surface area carbon nanospheres (CNSs; ~250nm) were synthesized on Cu substrate by optimized two step chemical vapor deposition (CVD) approach from a mixture of camphor and naphthalene as a source of carbon at 950°C using Ar as a carrier gas. On the basis of host of characterization techniques the mechanistic pathway proposed for the formation of CNSs having ordered graphite crystal structure. The present approach is capable of producing monodispersed, high yield and ultra-high purity (no other carbon impurities) nanospheres. This work further report on the electrocatalytic performance of as-synthesized CNSs and acid treated CNSs for the ascorbic acid (AA) oxidation reaction as a model reaction for direct oxidation fuel cells. These CNSs based electrocatalytic systems exhibits enhanced current densities and lower oxidation overvoltages response, demonstrating excellent catalytic activities towards AA oxidation could be due to their advantageous structural features.

Preparation of silica-graphene nanohybrid as a stabilizer of emulsions

In the present study, a novel silica-graphene nanohybrid was synthesized through a facile and single step chemical vapor deposition of acetylene on silica aerogel in the presence of hydrogen as a reducing agent at atmospheric pressure and 600°C. The physiochemical properties of the nanohybrid were thoroughly characterized by XRD, TGA, Raman, BET, DLS, FT-IR, SEM, and TEM. The amount of graphene in the silica-graphene nanohybrid was found to be 16wt% while a remarkable specific surface area about 843.64m2/g has been accomplished. The as-prepared nanohybrid was employed for stabilization of decalin/water emulsion. In addition, the effective factors on the droplets size and condition of stabilization for emulsion preparation were investigated through Taguchi L16 statistical design. Ultimately, the Wilhemy plate method shows that by dispersion of nanohybrid in water/decalin emulsion the interfacial tension (IFT) can be considerably declined from 55 to 30.12mN/m.

One‐Step In‐Situ Growth of Core–Shell SiC@Graphene Nanoparticles/Graphene Hybrids by Chemical Vapor Deposition

A one‐step in‐situ route to free standing core–shell silicon carbide in graphene nanoparticles on monolayer graphene is presented. The core–shell SiC@Graphene nanoparticle growth is realized by a simple chemical vapor deposition (CVD) process where carbon and silica precursors are simultaneously introduced into the growth chamber. This process permits the synthesis of a monolayer graphene sheet dressed with silicon carbide nanoparticles in a single CVD step, with the product controlled by growth temperature and the carbon/SiO2 exposure time. Growth of a high density SiC@Graphene distribution on a continuous graphene layer requires long exposure times (>1 h) and high temperature (1000 °C). The growth process proceeds by a carbothermal mechanism. The simultaneous growth of graphene and SiC nanoparticles enables uniform core–shell SiC@Graphene nanoparticle formation rather than SiC/carbon nanofiber growth. As a proof of concept, the functionalization of preformed nanoparticle graphene surface with a diazonium salt is studied, demonstrating an increase in grafting rate with increasing nanoparticle population. This work provides a general procedure for one‐step synthesis, with further investigation required to develop precursors for hybrid core–shell CVD material growth.

Large area chemical vapor deposition growth of monolayer MoSe2 and its controlled sulfurization to MoS2

Abstract

Direct CVD Growth of Multi-Layered Graphene Closed Shells around Alumina Nanofibers

In this work, a catalyst-free direct deposition of multi-layered graphene closed shells around highly aligned alumina nanofibers with aspect ratio of 107 is demonstrated for the first time. A single – step chemical vapor deposition process of specified parameters was used for development of hybrid structures of carbon shells around the core alumina nanofibers. Transmission electron microscopy and Raman spectroscopy were used to confirm formation of graphene layers and to understand the morphology of the various structures. The developed routine for growth of peculiar carbon nanostructures opens new opportunities for deposition of the tailored carbon structures on dielectric substrates.