Thermal Chemical Vapor Deposition Research Articles

Asymmetric contact-induced selective doping of CVD-grown bilayer WS2 and its application in high-performance photodetection with an ultralow dark current.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are excellent candidates for high-performance optoelectronics due to their high carrier mobility, air stability and strong optical absorption. However, photodetectors made with monolayer TMDs often exhibit a high dark current, and thus, there is a scope for further improvement. Herein, we developed a 2D bilayer tungsten disulfide (WS2) based photodetector (PD) with asymmetric contacts that exhibits an exceptionally low dark current and high specific detectivity. High-quality and large-area monolayer and bilayer WS2 flakes were synthesized using a thermal chemical vapor deposition system. Compared to conventional symmetric contact electrodes, utilizing metal electrodes with higher and lower work functions relative to bilayer WS2 aids in achieving asymmetric lateral doping in the WS2 flakes. This doping asymmetry was confirmed through the photoluminescence spectral profile and Raman mapping analysis. With the asymmetric contacts on bilayer WS2, we find evidence of selective doping of electrons and holes near the Ti and Au contacts, respectively, while the WS2 region away from the contacts remains intrinsic. When compared with the symmetric contact case, the dark current in the WS2 PD with asymmetric (Au, Ti) contact decreases by an order of magnitude under reverse bias with a concomitant increase in the photocurrent, resulting in an improved on/off ratio of ∼105 and overall improved device performance under identical illumination conditions. We explained this improved performance based on the energy band alignment showing a unidirectional charge flow under light illumination. Our results indicate that the planar device structure and compatibility with current nanofabrication technologies can facilitate its integration into advanced chips for futuristic low-power optoelectronic and nanophotonic applications.

Growth of Vertical Graphene Sheets on Silicon Nanoparticles Well-Dispersed on Graphite Particles for High-Performance Lithium-Ion Battery Anode.

With rapidly increasing demand for high energy density, silicon (Si) is greatly expected to play an important role as the anode material of lithium-ion batteries (LIBs) due to its high specific capacity. However, large volume expansion for silicon during the charging process is still a serious problem influencing its cycling stability. Here, a Si/C composite of vertical graphene sheets/silicon/carbon/graphite (VGSs@Si/C/G) is reported to address the electrochemical stability issues of Si/graphite anodes, which is prepared by adhering Si nanoparticles on graphite particles with chitosan and then in situ growing VGSs by thermal chemical vapor deposition. As a promising anode material, due to the buffering effect of VGSs and tight bonding between Si and graphite particles, the composite delivers a high reversible capacity of 782.2mAhg-1 after 1000 cycles with an initial coulombic efficiency of 87.2%. Furthermore, the VGSs@Si/C/G shows a diffusion coefficient of two orders higher than that without growing the VGSs. The full battery using VGSs@Si/C/G anode and LiNi0.8Co0.1Mn0.1O2 cathode achieves a high gravimetric energy density of 343.6Whkg-1, a high capacity retention of 91.5% after 500 cycles and an excellent average CE of 99.9%.

Low-k SiOx/AlOx Nanolaminate Dielectric on Dielectric Achieved by Hybrid Pulsed Chemical Vapor Deposition.

Selective and smooth low-k SiOx/AlOx nanolaminate dielectric on dielectric (DOD) was achieved by a hybrid water-free pulsed CVD process consisting of 50 pulses of ATSB (tris(2-butoxy)aluminum) at 330 °C and a 60 s TBS (tris(tert-butoxy)silanol) exposure at 200 °C. Aniline selective passivation was demonstrated on W surfaces in preference to Si3N4 and SiO2 at 300 °C. At 200 °C, TBS pulsed CVD exhibited no growth on W or SiO2, but its growth was catalyzed by AlOx. Using a two-temperature pulsed CVD process, ∼2.7 nm selective SiOx/AlOx nanolaminate was deposited on Si3N4 in preference to aniline passivated W. Nanoselectivity was confirmed and demonstrated on nanoscale W/SiO2 patterned samples by TEM analysis. For a 1:1 Si:Al ratio, a dielectric constant (k) value of 3.3 was measured. For a 2:1 Si:Al ratio, a dielectric constant (k) value of 2.5 was measured. The k value well below that of Al2O3 and SiO2 is consistent with the formation of a low-density, low-k SiO2/Al2O3 nanolaminate in a purely thermal process. This is the first report of a further thermal CVD process for deposition of a low-k dielectric and the first report for a selective low-k process on the nanoscale.

Hierarchical Nickel Cobalt Phosphide @ Carbon Nanofibers Composite Microspheres: Ultrahigh Energy Densities of Electrodes for Supercapacitors.

Supercapacitors (SCs) are widely used in energy storage devices due to their superior power density and long cycle lifetime. However, the limited energy densities of SCs hinder their industrial application to a great extent. In this study, we present a new combination of metallic phosphide-carbon composites, synthesized by directly carbonizing (Ni1-xCox)5TiO7 nanowires via thermal chemical vapor deposition (TCVD) technology. The new method uses one-dimensional (1D) (Ni1-xCox)TiO7 nanowires as precursors and supporters for the in situ growth of intertwined porous CNF microspheres. These 1D nanowires undergo microstructure transformation, resulting in the formation of CoNiP nanoparticles, which act as excellent interconnected catalytic nanoparticles for the growth of porous 3D CNF microspheres. Benefiting from the synergistic effect of a unique 1D/3D structure, the agglomeration of nanoparticles can effectively be prevented. The resulting CNF microspheres exhibit an interconnected conductive matrix and provide a large specific surface area with abundant ion/charge transport channels. Consequently, at a scanning rate of 10 mV s-1, its specific capacitance in 1.0 M Na2SO4 + 0.05 M Fe(CN)63-/4- aqueous solution is as high as 311.7 mF cm-2. Furthermore, the CoNiP@CNFs composite film-based symmetrical SCs show an ultrahigh energy density of 20.08 Wh kg-1 at a power density of 7.20 kW kg-1, along with outstanding cycling stability, with 87.2% capacity retention after 10,000 cycles in soluble redox electrolytes. This work provides a new strategy for designing and applying high-performance binary transition metal phosphide/carbon composites for next-generation energy storage devices.

One-step facile growth of nitrogen-doped graphene nanowalls by catalyst-free thermal chemical vapor deposition

We report an effective technique for the growth of graphene nanowalls (GNWs) and nitrogen-doped graphene nanowalls (N-GNWs) using a thermal chemical vapor deposition (TCVD) on the catalyst-free substrate. Ethanol and mixed ethanol-imidazole were used as carbon and carbon-nitrogen sources for the growth of GNWs and N-GNWs, respectively. The effect of imidazole concentration on the morphology, nanostructure, crystal structure, crystallinity, chemical bonding and chemical structure of GNWs and N-GNWs were investigated by scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy. Focused ion beam combined with TEM was utilized to analyze the growth mechanism. The results show that GNWs and N-GNWs are vertically oriented graphene nanosheets on in-plane-oriented graphitic layer/carbon nanolayer. An increase in imidazole concentration from 2.5 to 5.0 wt% increases the nitrogen content in the N-GNWs from 1.24 to 2.61 at%. The incorporated nitrogen atoms are mainly graphitic-N. One-step facile growth of N-GNWs by TCVD method without the need for a metal catalyst and plasma system was achieved.

Post-CMOS processing challenges and design developments of CMOS-MEMS microheaters for local CNT synthesis

Carbon nanotubes (CNTs) can be locally grown on custom-designed CMOS microheaters by a thermal chemical vapour deposition (CVD) process to utilize the sensing capabilities of CNTs in emerging micro- and nanotechnology applications. For such a direct CMOS-CNT integration, a key requirement is the development of necessary post-processing steps on CMOS chips for fabricating CMOS-MEMS polysilicon heaters that can locally generate the required CNT synthesis temperatures (~650–900 °C). In our post-CMOS processing, a subtractive fabrication technique is used for micromachining the polysilicon heaters, where the passivation layers in CMOS are used as masks to protect the electronics. For dielectric etching, it is necessary to achieve high selectivity, uniform etching and a good etch rate to fully expose the polysilicon layers without causing damage. We achieved successful post-CMOS processing by developing two-step reactive ion etching (RIE) of the SiO2 dielectric layer and making design improvements to a second-generation CMOS chip. After the dry etching process, CMOS-MEMS microheaters are partially suspended by SiO2 wet etching with minimum damage to the exposed aluminium layers, to obtain high thermal isolation. The fabricated microheaters are then successfully utilized for synthesizing CNTs by a local thermal CVD process. The CMOS post-processing challenges and design aspects to fabricate CMOS-MEMS polysilicon microheaters for such high-temperature applications are detailed in this article. Our developed process for heterogeneous monolithic integration of CMOS-CNT shows promise for wafer-level manufacturing of CNT-based sensors by incorporating additional steps in an already existing foundry CMOS process.

Hydrophobic sponge derived from natural loofah for efficient oil/water separation

Polymer sponge and carbon aerogel have received widespread attention because of their high specific surface area and controllable wettability. However, the complex manufacturing processes and non-renewable raw materials of these porous materials have limited their practical applications. Herein, we conducted the thermal chemical vapor deposition (CVD) of hydrophobic methyltrichlorosilane (MTCS) on loofah sponge (LS), which led to the formation of a novel oil-adsorbing hydrophobic MTCS-coated loofah sponge (MTCS-LS). The obtained MTCS-LS retained the interconnected 3D porous structure and robust structure stability of natural LS, while exhibited dramatically improved hydrophobicity with contact angle of 137° in the pH range of 2–12. This unique characteristic conferred MTCS-LS selective oil removal ability from water, and MTCS-LS possessed oil sorption capacity of 13 times of its dry weight and high separation efficiency (>92%). Results of the adsorption-squeezing tests proved that the excellent reversible compressibility of natural LS was maintained after the silanization modification. Compared to other known oil-adsorbing materials, the MTCS-LS displayed apparent advantage in adsorbing oils with high viscosity (e.g. crude oil). Our study provides a simple approach to exploitation of biomass in pollutants removal.

열화학 기상 증착법을 이용한 베타갈륨옥사이드 나노구조 증착

열화학 기상 증착법을 이용한 베타갈륨옥사이드 나노구조 증착

Cooking oil waste from AYAMAS as a carbon source in forming multilayer graphene films

ABSTRACT Industrial oil waste has become a novel carbon source for graphene due to its carbon-rich and renewable nature. A new approach was proposed to produce multilayer graphene from waste industrial cooking oil (WICO) sourced from project collaborators, the AYAMAS Food Corporation Sdn Bhd. Graphene was synthesized from WICO through a double thermal chemical vapor deposition (DT-CVD) method. Nickel foil was used as a substrate to initiate the process. The amount of WICO added was varied by 10 µL increments, ranging from 10 to 50 µL. The precursor and deposition temperatures were fixed at 350°C and 1000°C, respectively. Eventually, precipitation and segregation resulted in the formation of graphene. Raman spectroscopy analysis of the graphene obtained revealed an I2D/IG ratio of less than 0.5, indicating its multilayered nature. Besides, it has 4 graphene layers by calculating the number of atomic layers. This finding was further supported by outcomes shown through Ultraviolet – visible (UV-Vis) spectroscopy, where sharp peaks of 30 µL and 40 µL were observed at 250 nm. Finally, Atomic Force Microscopy (AFM) representative images demonstrated an inverse relationship between layer numbers and surface roughness in high-WICO graphene, where a gain in layers smoothened rather than roughened its appearance. Multilayer graphene has high conductivity and a wide bandgap. These properties are applicable in electrical and thermal applications. Through a method known as DT-CVD, it was achievable to successfully produce multilayer graphene by using the usage of AYAMAS, which is usage from the frying oil industry, as a carbon source.

Triethylamine borane thermal decomposition for BN low pressure chemical vapour deposition

Triethylamine borane (TEAB) complex was studied as a potential precursor for the thermal low pressure chemical vapour deposition (LPCVD) of pure crystalline sp2-hybridised BN. In particular, its thermal decomposition with or without ammonia was characterised from thermodynamic and experimental points of view in the temperature ranges 400–2000 °C and 300–1300 °C, respectively. NH3 plays a role in the gas phase equilibrium by providing a nitriding source that promotes HCN formation at high temperatures. NH3 is also a source of hydrogen for light hydrocarbons formation as it decomposes. It is thus possible to promote gaseous carbon species formation at high temperatures and limit carbon introduction into coatings by adding ammonia. A turbostratic BN coating could be obtained by CVD from a TEAB/NH3/N2 mixture and conditions chosen thanks to the gas phase decomposition study.

Low energy Si+, SiCH5+, or C+ beam injections to silicon substrates during chemical vapor deposition with dimethylsilane

We found that the atomic-concentration-ratio of carbon to silicon (C/Si ratio) in silicon carbide (SiC) films formed by thermal chemical vapor deposition (CVD) was much greater than 1 when the source gas for CVD was dimethylsilane (DMS). Thus, we tried to change carbon-inclusion levels in the film by injecting some ion beams into a depositing SiC film during the CVD process with DMS. Three ion beams, i.e., Si+, SiCH5+, or C+ ions were injected to depositing SiC films. The energy of Si+, SiCH5+, and C+ ions was 110 eV. The temperature of the substrate was 800 °C. X-ray diffraction of the deposited films showed that 3C–SiC was included in all three samples. X-ray photoelectron spectroscopy (XPS) showed that the C/Si ratio of the obtained SiC film increased significantly following the Si+ or C+ ion beam irradiations. The XPS measurements also showed that the C/Si ratio of the SiC film obtained by injecting SiCH5+ beam during thermal CVD with DMS was lower than that of the SiC film formed by thermal CVD with DMS alone.

Flexible and lightweight graphene grown by rapid thermal processing chemical vapor deposition for thermal management in consumer electronics

Flexible and lightweight graphene grown by rapid thermal processing chemical vapor deposition for thermal management in consumer electronics

Synthesis of SnO2 nanowires using thermal chemical vapor deposition with SnO powder and their application as self-powered ultraviolet photodetectors

We report on the performance of self-powered ultraviolet (UV) photodetectors composed of SnO2 nanowire (NW) networks. SnO2 NWs with a length of several hundred micrometers can be synthesized by thermal chemical vapor deposition (CVD) using SnO powder as the raw material, which has the advantages of a low process temperature and no requirement for a reducing agent. Based on the characterization results obtained through synchrotron X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy (TEM), we determined the growth behavior of SnO2 NWs via a vapor-liquid-solid mechanism with Au nanoparticles. The NW growth switched from an initial in-plane growth to subsequent vertical growth, forming NW cotton. This was started at a process temperature of 600 ℃ and optimized at 800 ℃. Moreover, the XRD and TEM results indicate that the NWs mainly grew in the form of SnO2, although the formation of SnO NWs was also possible. Metal−SnO2 NW−metal type photodetectors were fabricated, and their photoresponsivity to UV light in the range from 200 to 400 nm was investigated. The device exhibited a photo-to-dark current ratio of ∼2.17 × 106 at an applied bias of 10 V and 254 nm UV exposure. A maximum responsivity of about 1100 A/W was estimated at a wavelength of 270 nm, and the cutoff edge wavelength appeared around 350 nm. In particular, the self-powered photoresponse at nominal zero bias was ∼1.23 nA. The results of this study support the idea that SnO2 NWs are promising candidates for self-powered deep-UV photodetectors and that thermal CVD using SnO powder is suitable for synthesizing SnO2 NWs at temperatures as low as 700 °C.

Customizing 3D Structures of Vertically Aligned Carbon Nanotubes to Direct Neural Stem Cell Differentiation.

Neural tissue-related illnesses have a high incidence and prevalence in society. Despite intensive research efforts to enhance the regeneration of neural cells into functional tissue, effective treatments are still unavailable. Here, a novel therapeutic approach based on vertically aligned carbon nanotube forests (VA-CNT forests) and periodic VA-CNT micropillars produced by thermal chemical vapor deposition is explored. In addition, honeycomb-like and flower-like morphologies are created. Initial viability testing reveals that NE-4C neural stem cells seeded on all morphologies survive and proliferate. In addition, free-standing VA-CNT forests and capillary-driven VA-CNT forests are created, with the latter demonstrating enhanced capacity to stimulate neuritogenesis and network formation under minimal differentiation medium conditions. This is attributed to the interaction between surface roughness and 3D-like morphology that mimics the native extracellular matrix, thus enhancing cellular attachment and communication. These findings provide a new avenue for the construction of electroresponsive scaffolds based on CNTs for neural tissue engineering.

Synthesis of Manganese Silicide Nanowires by Thermal Chemical Vapor Deposition for the Hydrogen Evolution Reaction

Synthesis of Manganese Silicide Nanowires by Thermal Chemical Vapor Deposition for the Hydrogen Evolution Reaction

Structural, electronic, and magnetic properties of MnSi and Mn4Si7 nanowires

The importance of nanoscale magnetism under tailored phase evolution has been gaining considerable interest in the past few decades, attributable to the surging demand for memory devices and emerging electromagnetism. Herein, we present experimental demonstrations and theoretical studies on the structural, electronic, and magnetic properties of MnSi and Mn4Si7 nanowires grown by thermal chemical vapor deposition. The crystallinity of the nanowires was confirmed by a high-resolution transmission electron microscope and elemental distribution. The ratio of Mn to Si (∼0.96–1.04) indicates uniformity in MnSi composition. Additionally, the Mn4Si7 nanowire has an Mn/Si ratio of ∼0.78–0.87, which deviates from the homogenous Mn4Si7 ratio of ∼ 0.6 due to a higher Mn content at the nucleation sites. We found that the MnSi nanowires remained ferromagnetic (FM), similar to bulk MnSi, whereas the Mn4Si7 nanowires exhibited an FM order in contrast to the non-magnetic nature of bulk Mn4Si7. Field-cooled and zero-field-cooled magnetization curves reveal that the Mn4Si7 nanowires exhibited an FM order with a transition temperature (Tc) above room temperature. In contrast, the MnSi nanowires have a lower Tc of ∼ 30 K. It is noting that the First-principles density functional theory calculations demonstrated that both MnSi and Mn4Si7 nanowires possess a metallic characteristic. The spin-charge density of Mn4Si7 nanowires was found to be localized near the surface, implying a surface-induced magnetism. These findings suggest the possibility of tunning the phase evolution of Si-based nanowires by simply changing the diffusion mechanism of the Mn precursor. These metallic nanowires may be used as nanoscale interconnects and gate electrodes in nanoscale technologies.

Synthesis of ZnO nanowires by thermal chemical vapor deposition technique: Role of oxygen flow rate

Novel catalyst-free synthesis of hierarchical zinc oxide (ZnO) nanowires (NWs) by atmospheric pressure thermal chemical vapor deposition (TCVD) technique and the impact of different oxygen (O2) flow rates (25, 50, 75, and 100 mL/min) on the optical behavior, crystalline structure, emission response, and surface morphology of the prepared samples were studied in detail. The hierarchical ZnO NWs were characterized by ultraviolet–visible (UV–Vis) spectrophotometer, X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, and field emission scanning electron microscope (FESEM) measurements. The physical characteristics of the samples were found to be intensely influenced by the O2 flow rate. UV–Vis spectrophotometer measurements indicated that the optical energy gap (Eg) of the samples varies from 3.24 eV to 3.10 eV with regulating O2 flow rates from 25 to 100 mL/min. XRD analyses confirmed that all the prepared samples were polycrystalline in nature with hexagonal wurtzite-type crystal structure and a strong preferential direction along the (002) plane. Photoluminescence results revealed that oxygen vacancies were the major defects causing the blue emissions. Besides, the higher flow rate of O2 (100 mL/min) was efficient in decreasing the number of oxygen vacancies accountable for wide visible emission band intensity. FESEM images demonstrated a high density of hierarchical ZnO NWs with diameters ranging from 33 ± 2 nm to 88 ± 5 nm. Based on the FESEM observations, the potential growth mechanism of the hierarchical ZnO NWs was a vapor-solid (VS) process. Our simple systematic process for growth and characterization may provide toward the development of the hierarchical ZnO NWs-based gas sensors.

Field emission characteristics of MWCNT- bimetal (Cu and In) hybrids: Correlation with morphology, microstructure and electronic structure

In the current study, vertically aligned multiwalled carbon nanotubes (MWCNTs) and bi-metal (Cu and In) decorated CNTs were synthesized over Si substrates using Thermal Chemical Vapour Deposition technique and the thermal evaporation method. Different characterization techniques studied the surface morphology, structural properties, and elemental composition of the prepared samples. Field emission measurements were executed on the as-prepared samples. In-Cu-MWCNTs exhibit an enhanced current density of 7.46 mA/cm2 as compared to 2.83 mA/cm2 of pristine CNTs. A meticulous study of the electronic structure along with the basic field emission parameters reveals that the graphitic content of carbon (C) as well as the density of state and electronic structure variation are responsible for the improved field emission properties of the CNT-bimetal hybrids. The study opens a new area of research in CNT-bi-metal decorated hybrids for various field emission applications.

Flexible and lightweight graphene grown by rapid thermal processing chemical vapor deposition for thermal management in consumer electronics

Next-generation consumer electronics require excellent thermal management. Graphene is a good choice because its thermal conductivity is 13 times that of copper. Single-, bi- and few-layer graphene (SLG, BLG, FLG) with large sp2 domains were grown by rapid thermal processing chemical vapor deposition (RTP-CVD) from CH4 and H2 using Ar as the diluting gas. The quality of graphene was investigated by Raman spectroscopy and TEM. To demonstrate the heat dissipation capability of RTP-CVD-grown graphene, a 2 TB solid state drive was used and the temperature was measured by a FLIR thermal camera. Results indicate that high thermal conductivity graphene was prepared by diluting the precursor gas with Ar. SLG was prepared at a growth temperature of 1 000 °C and a time of 25 min. A transition from FLG to high-quality BLG was observed at low H2 concentrations. Using SLG, there was a 5 °C lower temperature rise than using a commercial copper heat dissipator. The heat dissipation ability of SLG was approximately 200 times that of commercial copper heat dissipators.

Restriction of residual defects of indium phosphide nanowires: Integration into single nanowire electronic device and gas sensing

Indium phosphide nanowires (InPNWs) with an average diameter size of 87 nm were successfully synthesised through thermal chemical vapour deposition (CVD) method. The InPNWs were synthesised by reacting indium metalloid and red phosphorus at 600 °C. They were synthesised possessing residual stress defects such as stacking faults. However, in this study we demonstrated how to control the generation and distribution of these defects by using indium metalloid catalyst and a modified synthesis technique. The synthesis technique was optimised to control the crystal structure, morphology, and size distribution. A single indium phosphide nanowire back-gated electronic device was fabricated using electron beam lithography. The device had resistivity of 50 Ω.cm, which confirmed that the electrical properties of indium phosphide nanowire was intact despite the presence of residual defects. The InPNWs were tested for gas sensing, and they were found to have response time of 320 s for nitric oxide gas (NO2)