Combined Chemical Vapor Deposition Research Articles

Influence of platinum content (X) in the LaNi5PtX catalyst and reaction conditions on the properties of ferromagnetic Ni-filled CNTs

This research highlights the synthesis of carbon nanostructures using the catalytic chemical vapor deposition (CCVD) method, a hybrid process combining chemical vapor deposition (CVD) and Gas-Solid Heterogeneous Catalysis (GSHC). Intermetallic catalysts, LaNi5Ptx (x = 0, 0.05, 0.5, 1.0), were fabricated through an arc melting procedure to serve as templates for the catalytic conversion of carbon precursors into solid materials. These catalysts, featuring the ferromagnetic transition metal Ni, tend to aggregate into larger particles, reducing the rate of hydrocarbon cracking at the catalyst surface. Rare earth metals like Lanthanum were introduced to form an alloy with a higher melting point to prevent thermal aggregation. Additionally, Pt was incorporated to modify the catalytic properties of the alloy. The study primarily explores the impact of catalyst composition on carbon nanotube (CNT) production. The introduction of platinum content to the catalyst influences the rate at which Ni is filled. The research aimed to alter the catalyst's composition by varying platinum doping in the Pauli paramagnetic lanthanum nickel alloy (LaNi5) to understand the growth mechanism and morphological variations of Ni-filled CNTs. Platinum and lanthanum oxides significantly influence nickel filling in CNTs, resulting in an increased filling rate as the catalyst's Pt content varies from 0 to 1.0. However, higher Pt doping content disrupts the CNT structure with spontaneous nickel encapsulation.

Electrochemical polishing of chemical vapor deposited niobium thin films

Combining chemical vapor deposition (CVD) with electrochemical polish (EP) operation is a promising route to producing performance-capable superconducting films for use in the fabrication of cost-effective components for superconducting radiofrequency (SRF) particle accelerators and superconducting quantum computers. The post-deposition EP process enables a critically necessary reduction in surface roughness of niobium (Nb) thin films to promote optimal superconducting surface conditions. This work investigated surface morphology, roughness, and crystal orientation of the CVD-grown and EP-polished Nb films. The CVD films were found to comprise steps and pyramidal features, resulting in undesirable large peak-to-valley distances. EP was demonstrated to significantly diminish the height of pyramids and effectively minimize the overall surface roughness. EP results showed a probable dependence on the crystal orientation. These understandings identify the EP principles tied to CVD-grown Nb films that allow further refinement of surface profiles for film-based SRF applications.

Enhanced Electrical Conductivity and Tensile Strength of Cu/Single-Layer Graphene/Cu Nanomaterials

Copper (Cu) is widely used for electrical conduction but the inherent resistive heating not only wastes significant amounts of energy but also creates severe reliability issues; therefore, how to increase copper’s electrical conductivity has been a goal with a long history. Although nanometer-thin graphene has been regarded with great potential to increase copper’s conductive performance, published results are far from expectations and the quest remains about the actual enhancement efficiency of graphene. Here, we demonstrated that single-layer graphene in a Cu-sandwiched Cu/graphene/Cu composite structure fabricated by combining chemical vapor deposition (CVD) of graphene with magnetron sputtering deposition of copper can significantly increase annealed Cu’s electrical conductivity and strength at the same time. With a graphene content of less than 0.0008 vol % (a single-layer graphene sandwiched by 260 nm sputtering copper and 45 μm copper foil), the resultant electrical conductivity is about 110% of the annealed copper foil. In addition, the tensile strength is about 187% of the annealed copper. The converted enhancement efficiency per unit volume fraction of graphene is about a factor of 125 for the electrical conductivity and about a factor of 1096 for the tensile strength, respectively. The increased electrical conductivity and strength of Cu/graphene/Cu-sandwiched composite is attributed to the soundness of single-layer graphene obtained from the CVD growth, the graphene quality retention through the sputtering deposition process, and the good interfacial bonding formed between graphene and copper nanofilms that materializes band gap tuning by doping.

Ultra-high-rate deposition of diamond-like carbon films using Ar/C2H2 plasma jet CVD in combination with substrate-stage discharge

Diamond-like carbon (DLC) films with excellent mechanical properties are used as functional surface protective films for cutting tools. The deposition rate of DLC films using conventional plasma chemical vapor deposition (CVD) methods is several hundred nm min−1. This study applied a negative DC pulsed voltage to a substrate stage irradiated with an Ar/C2H2 mixed plasma jet. The effect of the voltage applied to the stage on the fabricated DLC films was investigated based on the thicknesses and characteristics of the films. DLC films were formed on Si substrates using an Ar/C2H2 plasma jet CVD system. Ar plasma was ejected from a circular nozzle, and C2H2 gas was supplied as a carbon source gas from the center of the circular nozzle. DLC films with a nanoindentation hardness of 17 GPa were obtained by applying −500 V to the stage with a deposition rate of 2140 nm min−1.

A hierarchically ordered porous Fe, N, S tri-doped carbon electrocatalyst with densely accessible Fe-Nx active sites and uniform sulfur-doping for efficient oxygen reduction reaction

Doping FeNC catalysts with heteroatoms has been widely recognized as a most promising way to improve the electrocatalytic activity to achieve the replacement of Pt-based electrocatalysts for efficient oxygen reduction reaction (ORR). However, it is still a challenge to combine delicate control of the catalyst structure with heteroatom-doping to achieve optimal electrocatalytic efficiency. Herein, a strategy of combining chemical vapor deposition (CVD) with dual-template cooperative pyrolysis was rationally designed to synthesize the novel electrocatalyst with densely isolated single Fe atoms scattered on hierarchically ordered porous N and S codoped carbon (ISA-Fe/HOPNSC). The three-dimensional hierarchically ordered interconnected macro/mesoporous structure and uniform sulfur-doping make ISA-Fe/HOPNSC exhibit superb ORR performance in both alkaline and acidic electrolytes with a positive half-wave potential (0.920 V in 0.1 M KOH and 0.795 V in 0.5 M H2SO4) as well as outstanding methanol tolerance and long-time stability. As cathode catalyst in Zn-air battery, ISA-Fe/HOPNSC also provides much better maximum power density (210.7 mW cm−2) and specific capacity (796.7 mAh gZn−1) than those of Pt/C (112.6 mW cm−2 and 712.6 mAh gZn−1). Remarkably, the synthesis strategy in this work proposes myriad opportunities for combining delicate control of the catalyst structure with heteroatom doping to synthesize high-performance ORR catalysts.

A low-temperature preparation strategy of SiC/ZrB2-CrSi2-Si/SiC multilayer oxidation-resistant coating for C/C composites: Process, kinetics and mechanism research

In this work, a strategy of preparing multilayer SiC/ZrB2-CrSi2-Si/SiC coating at low-temperature by combining chemical vapor deposition (CVD), brush-sintering and preoxidation was proposed for C/C composites. The coated C/C composites could maintain stable and long-term oxidation resistance at 1473–1673 K, and the specific mass loss was only 8.05 mg∙cm−2 after 1673 K/340 h oxidation. The coupling mechanism of the preparation process on the microstructure and performance, the kinetics process of oxidation behavior and the protection/failure mechanism of the coating were analyzed. Furthermore, the importance of organosilicon (SiCxOy) in the process of self-healing and oxidation resistance was enlightened.

In-situ deposition of diamond on functionally graded copper scaffold for improved thermal conductivity and mechanical properties

A novel functionally graded structure of diamond was developed with significantly enhanced thermal conductivity and compressive strength, based on synergistic effects combining chemical vapor deposition (CVD) and selective laser melting (SLM). In-situ diamond growth was achieved without conventionally complex pretreatment steps thanks to the high surface roughness of as-printed graded copper scaffold. The continuous diamond film coated on graded copper scaffold forms a monolith with interconnected network, acting as an effective layer for heat conduction with an exceptional increase (459%) in thermal conductivity compared with copper counterpart. The functionally graded structure also demonstrates a progressive mechanical response to loading compared with the uniform structure. This work opens a new avenue for realizing three-dimensional diamond structure and proves its multifunctionalities, especially for high-power electronics applications.

Improving Transport Properties of GaN-Based HEMT on Si (111) by Controlling SiH4 Flow Rate of the SiNx Nano-Mask

The AlGaN/AlN/GaN high electron mobility transistor structures were grown on a Si (111) substrate by metalorganic chemical vapor deposition in combination with the insertion of a SiNx nano-mask into the low-temperature GaN buffer layer. Herein, the impact of SiH4 flow rate on two-dimensional electron gas (2DEG) properties was comprehensively investigated, where an increase in SiH4 flow rate resulted in a decrease in edge-type threading dislocation density during coalescence process and an improvement of 2DEG electronic properties. The study also reveals that controlling the SiH4 flow rate of the SiNx nano-mask grown at low temperatures in a short time is an effective strategy to overcome the surface desorption issue that causes surface roughness degradation. The highest electron mobility of 1970 cm2/V·s and sheet carrier concentration of 6.42 × 1012 cm−2 can be achieved via an optimized SiH4 flow rate of 50 sccm.

Nanoparticle and Solution Doping for Efficient Holmium Fiber Lasers

Efficient holmium fiber lasers have been studied as attractive laser sources operating around 2.1 μm. We report on holmium-doped silica fibers prepared by the modified chemical vapor deposition in combination with either a solution-doping method or a nanoparticle-doping method. A set of 15 fibers with various compositions was characterized and compared with respect to their fluorescence lifetime, laser slope efficiency and laser threshold. This set of fibers in wide concentration ranges allowed us to assess reliably the influence of material composition and the influence of doping method. The best-performance fibers exhibited slope efficiency 83.1%, laser threshold 155 mW and a record value of upper laser level lifetime of 1.35 ms. These results were achieved in fibers with holmium concentration lower than 800 molar ppm and Al/Ho molar ratio greater than 70. Significant differences between fibers prepared by solution doping and nanoparticle doping were not observed. The behavior of Al2O3 nanoparticles during fiber preparation is discussed in details.

Surface Functionalization of Grown-on-Tip ZnO Nanopyramids: From Fabrication to Light-Triggered Applications

We report on a combined chemical vapor deposition (CVD)/radio frequency (RF) sputtering synthetic strategy for the controlled surface modification of ZnO nanostructures by Ti-containing species. Specifically, the proposed approach consists in the CVD of grown-on-tip ZnO nanopyramids, followed by titanium RF sputtering under mild conditions. The results obtained by a thorough characterization demonstrate the successful ZnO surface functionalization with dispersed Ti-containing species in low amounts. This phenomenon, in turn, yields a remarkable enhancement of photoactivated superhydrophilic behavior, self-cleaning ability, and photocatalytic performances in comparison to bare ZnO. The reasons accounting for such an improvement are unravelled by a multitechnique analysis, elucidating the interplay between material chemico-physical properties and the corresponding functional behavior. Overall, the proposed strategy stands as an amenable tool for the mastering of semiconductor-based functional nanoarchitectures through ad hoc engineering of the system surface.

Performance-Enhanced Activated Carbon Electrodes for Supercapacitors Combining Both Graphene-Modified Current Collectors and Graphene Conductive Additive.

Graphene has been widely used in the active material, conductive agent, binder or current collector for supercapacitors, due to its large specific surface area, high conductivity, and electron mobility. However, works simultaneously employing graphene as conductive agent and current collector were rarely reported. Here, we report improved activated carbon (AC) electrodes (AC@G@NiF/G) simultaneously combining chemical vapor deposition (CVD) graphene-modified nickel foams (NiF/Gs) current collectors and high quality few-layer graphene conductive additive instead of carbon black (CB). The synergistic effect of NiF/Gs and graphene additive makes the performances of AC@G@NiF/G electrodes superior to those of electrodes with CB or with nickel foam current collectors. The performances of AC@G@NiF/G electrodes show that for the few-layer graphene addition exists an optimum value around 5 wt %, rather than a larger addition of graphene, works out better. A symmetric supercapacitor assembled by AC@G@NiF/G electrodes exhibits excellent cycling stability. We attribute improved performances to graphene-enhanced conductivity of electrode materials and NiF/Gs with 3D graphene conductive network and lower oxidation, largely improving the electrical contact between active materials and current collectors.

Hybridizing graphene aerogel into three-dimensional graphene foam for high-performance composite phase change materials

Three-dimensional (3D) graphene foam (GF) produced by template-directed chemical vapour deposition (CVD) has shown great potential for applications in composite phase change materials (PCMs) with enhanced thermal conductivity and graphene aerogel (GA) has been proved to be effective supporting scaffold to improve the shape-stability of organic PCMs. Here, hybrid graphene aerogel (HGA) is encapsulated in GF framework to obtain a 3D GF/HGA (GH) hybrid microstructure via combined self-assembly and CVD techniques. The thermal conductivity of paraffin wax (PW)/GH composite PCMs increases by 574% and 98% compared with pure PW and PW/GF composite PCMs, respectively. Meanwhile, PW/GH composite PCM exhibits better shape stability than PW/GF composite PCM, high thermal energy storage density, good thermal reliability and chemical stability. PW/GH composite PCMs also realize efficient light-to-thermal energy conversion and storage owing to the excellent photoabsorption. This study sheds light on the development of the composite PCMs with good comprehensive properties, which are potentially to be used widely in energy storage systems.

All-fiber linear polarization and orbital angular momentum modes amplifier based on few-mode erbium-doped fiber and long period fiber grating

A few-mode erbium-doped fiber (FM-EDF) is fabricated using modified chemical vapor deposition in combination with liquid solution. The core and cladding diameters of the fiber are approximately 19.44 and 124.12 μm, respectively. The refractive index difference is 0.98%, numerical aperture (NA) is 0.17, and normalized cut-off frequency at 1550 nm is 6.81. Therefore, it is a five-mode fiber, and can be used as a higher-order mode gain medium. Furthermore, a long period fiber grating (LPFG) is fabricated, which can convert LP01 mode to LP11 mode, and its conversion efficiency is up to 99%. The first-order orbital angular momentum (OAM) is also generated by combining the LPFG and polarization controller (PC). Then, an all-fiber amplification system based on the FM-EDF and LPFG, for LP11 mode and first-order OAM beams, is built up. Its on-off gain of the LP11 mode beam is 37.2 dB at 1521.2 nm. The variation, whose transverse mode field intensity of first-order OAM is increased with the increase of pumping power, is obvious. These show that both the LP11 mode and first-order OAM beams are amplified in the all-fiber amplification system. This is a novel all-fiber amplification scheme, which can be used in the optical communication fields.

Scalable van der Waals Heterojunctions for High-Performance Photodetectors.

Atomically thin two-dimensional (2D) materials have attracted increasing attention for optoelectronic applications in view of their compact, ultrathin, flexible, and superior photosensing characteristics. Yet, scalable growth of 2D heterostructures and the fabrication of integrable optoelectronic devices remain unaddressed. Here, we show a scalable formation of 2D stacks and the fabrication of phototransistor arrays, with each photosensing element made of a graphene-WS2 vertical heterojunction and individually addressable by a local top gate. The constituent layers in the heterojunction are grown using chemical vapor deposition in combination with sulfurization, providing a clean junction interface and processing scalability. The aluminum top gate possesses a self-limiting oxide around the gate structure, allowing for a self-aligned deposition of drain/source contacts to reduce the access (ungated) channel regions and to boost the device performance. The generated photocurrent, inherently restricted by the limited optical absorption cross section of 2D materials, can be enhanced by 2 orders of magnitude by top gating. The resulting photoresponsivity can reach 4.0 A/W under an illumination power density of 0.5 mW/cm2, and the dark current can be minimized to few picoamperes, yielding a low noise-equivalent power of 2.5 × 10-16 W/Hz1/2. Tailoring 2D heterostacks as well as the device architecture moves the applications of 2D-based optoelectronic devices one big step forward.

Aerobic and anaerobic H2 sensing sensors fabricated by diffusion membranes depositing on Pt-ZnO film

Utilizing sensor to detect H2 under different oxygen concentrations is essential in monitoring status of nuclear weapon. A membrane covering on a sensing film could influence the diffusion of oxygen, thus changing the sensing performance of a metal oxide (MOX) gas sensor. In this research, silica and alumina (SiO2/Al2O3) composite membranes were deposited in-situ on Pt-modified ZnO by combining chemical vapor deposition (CVD) and ultrasonic spray pyrolysis (USP) methods Through adjusting the ratio of silicon to aluminum (Si/Al), the composition of membranes, the diffusion rate of oxygen and H2 sensing property under different oxygen concentrations were regulated. With the decrease of the Si/Al ratio, the H2 responses of the sensors increased first and decreased then under high (10,000ppm) oxygen concentration. While under low (50ppm) oxygen concentration, the H2 responses of the sensors went down first and rose then. The sensors covered with membranes of high Si/Al ratio exhibited aerobic H2 sensing property. Meanwhile, sensors deposited with membranes of low Si/Al ratio or without membrane showed anaerobic H2 sensing property. This study contributes to further understanding of the influence of oxygen diffusion on MOX sensing properties.

Low-temperature formation of crystalline Si:H/Ge:H heterostructures by plasma-enhanced CVD in combination with Ni-nanodots seeding nucleation

Hydrogenated microcrystalline (µc) Si/Ge heterostructures were prepared on quartz substrates by plasma-enhanced chemical vapor deposition (CVD) from VHF inductively coupled plasma of SiH4 just after GeH4 employing Ni nanodots (NDs) as seeds for crystalline nucleation. The crystallinity of the films and the progress of grain growth were characterized by Raman scattering spectroscopy and atomic force microscopy (AFM), respectively. When the Ge films were grown on Ni-NDs at 250 °C, the growth of µc-Ge films with crystallinity as high as 80% was realized without an amorphous phase near the Ge film/quartz substrate interface. After the subsequent Si film deposition at 250 °C, fine grains were formed in the early stages of film growth on µc-Ge films with compositional mixing (µc-Si0.85Ge0.15:H) caused by the release of large lattice mismatch between c-Si and c-Ge. With further increase in Si:H film thickness, the formation of large grain structures accompanied by fine grains was promoted. These results suggest that crystalline Si/Ge heterojunctions can be used for efficient carrier collection in solar cell application.

Novel synthesis of Ag3PO4/CNFs/silica-fiber hybrid composite as an efficient photocatalyst

Novel Ag3PO4/CNFs/silica-fiber composite photocatalyst was successfully synthesized via a facile combined chemical vapor deposition (CVD) technique and hydrothermal process. The as-prepared Ag3PO4/CNFs/silica-fiber was characterized by X-Ray diffraction, field emission scanning electron microscopy, ultraviolet–visible diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The photocatalytic activity of Ag3PO4/CNFs/silica-fiber was evaluated by the degradation of rhodamine B (RhB) under visible-light irradiation. Note that the existence of unique featured CNFs not only has good adsorption ability for RhB molecules, but also benefits to the effective separation of photogenerated electron–hole pairs and thus improves the photocatalytic performance of the catalyst. Specially, the as-prepared Ag3PO4/CNFs/silica-fiber photocatalyst can easily reduce the cost for the practical application of Ag3PO4 photocatalyst. The new synthetic procedure is simple and could be applicable to a large number of efficient and inexpensive visible-light-driven photocatalytic materials.

A novel approach to obtain in-situ growth carbon nanotube reinforced aluminum foams with enhanced properties

In-situ growth carbon nanotube (CNT) reinforced Al foams have been fabricated successfully through a method combined chemical vapor deposition (CVD), ball milling and space-holder technique. Experimental results showed that the CNTs with integrated structure were well dispersed in Al powders, and the CNTs and Al matrix were well-bonded. The morphology and size of pores could be well controlled by the space-holder technique. The compressive yield strength of ball-milled 2wt%-CNT/Al composite foams was nearly 25% and 67% higher than the pure Al foams and non-ball-milled 2wt%-CNT/Al composite foams respectively, and the energy absorption capacity was also significantly enhanced.

Combinatorial Characterization of TiO2 Chemical Vapor Deposition Utilizing Titanium Isopropoxide.

The combinatorial characterization of the growth kinetics in chemical vapor deposition processes is challenging because precise information about the local precursor flow is usually difficult to access. In consequence, combinatorial chemical vapor deposition techniques are utilized more to study functional properties of thin films as a function of chemical composition, growth rate or crystallinity than to study the growth process itself. We present an experimental procedure which allows the combinatorial study of precursor surface kinetics during the film growth using high vacuum chemical vapor deposition. As consequence of the high vacuum environment, the precursor transport takes place in the molecular flow regime, which allows predicting and modifying precursor impinging rates on the substrate with comparatively little experimental effort. In this contribution, we study the surface kinetics of titanium dioxide formation using titanium tetraisopropoxide as precursor molecule over a large parameter range. We discuss precursor flux and temperature dependent morphology, crystallinity, growth rates, and precursor deposition efficiency. We conclude that the surface reaction of the adsorbed precursor molecules comprises a higher order reaction component with respect to precursor surface coverage.

Evaluation of niobium dimethylamino-ethoxide for chemical vapour deposition of niobium oxide thin films

Chemical vapour deposition (CVD) processes depend on the availability of suitable precursors. Precursors that deliver a stable vapour pressure are favourable in classical CVD processes, as they ensure process reproducibility. In high vacuum CVD (HV-CVD) process vapour pressure stability of the precursor is of particular importance, since no carrier gas assisted transport can be used. The dimeric Nb2(OEt)10 does not fulfil this requirement since it partially dissociates upon heating. Dimethylamino functionalization of an ethoxy ligand of Nb(OEt)5 acts as an octahedral field completing entity and leads to Nb(OEt)4(dmae). We show that Nb(OEt)4(dmae) evaporates as monomeric molecule and ensures a stable vapour pressure and, consequently, stable flow. A set of HV-CVD experiments were conducted using this precursor by projecting a graded molecular beam of the precursor onto the substrate at deposition temperatures from 320°C to 650°C. Film growth rates ranging from 8nm·h−1 to values larger than 400nm·h−1 can be obtained in this system illustrating the high level of control available over the film growth process. Classical CVD limiting conditions along with the recently reported adsorption–reaction limited conditions are observed and the chemical composition, and microstructural and optical properties of the films are related to the corresponding growth regime. Nb(OEt)4(dmae) provides a large process window of deposition temperatures and precursor fluxes over which carbon-free and polycrystalline niobium oxide films with growth rates proportional to precursor flux are obtained. This feature makes Nb(OEt)4(dmae) an attractive precursor for combinatorial CVD of niobium containing complex oxide films that are finding an increasing interest in photonics and photoelectrochemical water splitting applications. The adsorption–reaction limited conditions provide extremely small growth rates comparable to an atomic layer deposition (ALD) process indicating that HV-CVD has the potential to be an alternative to ALD for growth of ultrathin films on low aspect ratio substrates.