Thermal Chemical Vapor Deposition Research Articles

Order-of-Magnitude Increase in Carbon Nanotube Yield Based on Modeling Transient Diffusion and Outgassing of Water From Reactor Walls

Abstract While reactor wall preconditioning was previously shown to influence the yield in chemical vapor deposition (CVD), especially for the growth of carbon nanotubes (CNTs), it was limited to studying accumulating carbonaceous deposits over a number of runs. However, the effects of temperature and duration as the reactor walls are exposed to hot humidity for extended periods between growth runs were not previously studied systematically. Here, we combine experimental measurements with a mathematical model to elucidate how the thermochemical history of reactor walls impacts growth yield, especially knowing that only a specific range of humidity promotes growth. Importantly, we demonstrate a one-order-of-magnitude higher CNT yield by increasing the interim, i.e., the time between runs. We explain the results based on previously unexplored process sensitivity to trace amounts of oxygen-containing species in the reactor. In particular, we model the effect of small amounts of water vapor being desorbed from reactor walls during growth. Our results reveal the outgassing dynamics and show the underlying mechanism of generating growth-promoting molecules. By installing a humidity sensor in our custom-designed multizone rapid thermal CVD reactor, we are able to uniquely correlate the amount of moisture within the reactor to real-time measurements of growth kinetics, as well as ex situ characterization of CNT alignment and atomic defects. Our findings enable a scientifically grounded approach toward both boosting growth yield and improving its consistency by reducing run-to-run variations. Accordingly, engineered dynamics recipes with added preprocessing steps can be envisioned to leverage this phenomenon for improving manufacturing process scalability and robustness.

Morphology-Controlled ZnO@ZnWO4 Hetero-Nanostructures for Efficient Photooxidation of Water in Near-Neutral pH.

One-dimensional ZnO nanorods (NRs) have been extensively studied as photoanodes because of their unique optical properties, high electron mobility, and suitable band positions for water oxidation. However, their practical efficiency is often compromised by chemical instability during water oxidation and high carrier recombination rates. To overcome this issue, precise morphological control of ZnO@ZnWO4 core-shell structured photoanodes, featuring a ZnO core and a ZnWO4 shell was used. This was accomplished by depositing WO3 onto hydrothermally grown ZnO NRs using the thermal chemical vapor deposition process. The photoelectrochemical performance of ZnO@ZnWO4 with an optimized morphology outperforms that of pristine ZnO NRs. Systematic optical and electrochemical analyses of ZnO@ZnWO4 demonstrated that the enhancement is attributed to the enhanced charge transfer efficiency facilitated by the optimized ZnWO4 shells.

Graphene nanowalls formation investigated by Electron Energy Loss Spectroscopy

The properties of layered materials are significantly dependent on their lattice orientations. Thus, the growth of graphene nanowalls (GNWs) on Cu through PECVD has been increasingly studied, yet the underlying mechanisms remain unclear. In this study, we examined the GNWs/Cu interface and investigated the evolution of their microstructure using advanced Scanning transmission electron microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). GNWs interface and initial root layers of comprise graphitic carbon with horizontal basal graphene (BG) planes that conform well to the catalyst surface. In the vertical section, the walls show a mix of graphitic and turbostratic carbon, while the latter becomes more noticeable close to the top edges of the GMWs film. Importantly, we identified growth process began with catalysis at Cu interface forming BG, followed by defect induction and bending at ‘coalescence points’ of neighboring BG, which act as nucleation sites for vertical growth. We reported that although classical thermal CVD mechanism initially dominates, growth of graphene later deviates a few nanometers from the interface to form GNWs. Nascent walls are no longer subjected to the catalytic action of Cu, and their development is dominated by the stitching of charged carbon species originating in the plasma with basal plane edges.

Highly enhanced and stable field emission performance of CNT – Dielectric (Si3N4) hybrids

In the present study, vertically aligned carbon nanotubes (CNTs) were grown onto Si substrates using a simple thermal chemical vapour deposition (TCVD) technique. Subsequently, the dielectric layer of Si3N4 was coated over the pristine CNTs using the Plasma Enhanced CVD (PECVD) method. Various characterizations were carried out on the as-deposited samples to study the structural properties, surface morphology, elemental composition, and electronic structure. The field emission measurements executed on CNT – Si3N4 samples showed an enhanced current density of 8.28 mA/cm2 compared to the 2.85 mA/cm2 current density of the pristine CNTs. A detailed study reveals that, apart from the basic conventional field emission parameters, the electronic structure of the CNTs, along with the presence of disordered carbon atoms and the presence of the dielectric material, are responsible for enhancing the current density and temporal stability of the CNT – Si3N4 hybrids. The study aids in exploring CNT-dielectric hybrids for better field emission properties with improved stability for field emission applications.

Evaluation of vertical alignment in carbon nanotubes: A quantitative approach

An automated quantitative study of the alignment of vertically aligned carbon nanotubes (VA-CNTs) from scanning electron microscopy (SEM) images has been demonstrated. It is based on the fact that the image gradient (directional change in intensity or color) at every pixel in the image contains the information about the anisotropy at that particular pixel i.e., the local alignment is maintained in the orthogonal direction to the gradient. Structure tensor metrics are formulated demonstrating to be able to summarize the distribution of gradient directions within the neighborhood of any pixel within a two-dimensional domain (surface). An image analysis is presented that evaluates the alignment in desired regions of interest (ROIs) in SEM images of VA-CNTs. This method has been exploited to study the alignment of two different kinds of VA-CNTs: one grown via the thermal chemical vapor deposition (T-CVD) method and the second synthesized via the plasma enhanced chemical vapor deposition (PE-CVD) method.

Enhanced electrical properties of amorphous In-Sn-Zn oxides through heterostructuring with Bi2Se3 topological insulators

Amorphous indium tin zinc oxide (a-ITZO)/Bi2Se3 nanoplatelets (NPs) were fabricated using a two-step procedure. First, Bi2Se3 NPs were synthesized through thermal chemical vapor deposition at 600 °C on a glass substrate, and then a-ITZO was deposited on the surface of the Bi2Se3 NPs via magnetron sputtering at room-temperature. The crystal structures of the a-ITZO/Bi2Se3 NPs were determined via X-ray diffraction spectroscopy and high-resolution transmission electron microscopy. The elemental vibration modes and binding energies were measured using Raman spectroscopy and X-ray photoelectron spectroscopy. The morphologies were examined using field-emission scanning electron microscopy. The electrical properties of the a-ITZO/Bi2Se3 NPs were evaluated using Hall effect measurements. The bulk carrier concentration of a-ITZO was not affected by the heterostructure with Bi2Se3. In the case of the Bi2Se3 heterostructure, the carrier mobility and conductivity of a-ITZO were increased by 263.6% and 281.4%, respectively, whereas the resistivity of a-ITZO was reduced by 73.57%. This indicates that Bi2Se3 significantly improves the electrical properties of a-ITZO through its heterostructure, expanding its potential applications in electronic and thermoelectric devices.

Modified 3D Graphene for Sensing and Electrochemical Capacitor Applications.

Less defective, nitrogen-doped 3-dimensional graphene (N3DG) and defect-rich, nitrogen-doped 3-dimensional graphene (N3DG-D) were made by the thermal CVD (Chemical Vapor Deposition) process via varying the carbon precursors and synthesis temperature. These modified 3D graphene materials were compared with pristine 3-dimensional graphene (P3DG), which has fewer defects and no nitrogen in its structure. The different types of graphene obtained were characterized for morphological, structural, and compositional assessment through Scanning Electron Microscopy (SEM), Raman Spectroscopy, and X-ray Photoelectron Spectroscopy (XPS) techniques. Electrodes were fabricated, and electrochemical characterizations were conducted to evaluate the suitability of the three types of graphene for heavy metal sensing (lead) and Electric Double-Layer Capacitor (EDLC) applications. Initially, the various electrodes were treated with a mixture of 2.5 mM Ruhex (Ru (NH3)6Cl3 and 25 mM KCl to confirm that all the electrodes underwent a reversible and diffusion-controlled electrochemical process. Defect-rich graphene (N3DG-D) revealed the highest current density, followed by pristine (P3DG) and less-defect graphene (N3DG). Further, the three types of graphene were subjected to a sensing test by square wave anodic stripping voltammetry (SWASV) for lead detection. The obtained preliminary results showed that the N3DG material provided a great lead-sensing capability, detecting as little as 1 µM of lead in a water solution. The suitability of the electrodes to be employed in an Electric Double-Layer Capacitor (EDLC) was also comparatively assessed. Electrochemical characterization using 1 M sodium sulfate electrolyte was conducted through cyclic voltammetry and galvanostatic charge-discharge studies. The voltammogram and the galvanostatic charge-discharge (GCD) curves of the three types of graphene confirmed their suitability to be used as EDLC. The N3DG electrode proved superior with a gravimetric capacitance of 6.1 mF/g, followed by P3DG and N3DG, exhibiting 1.74 mF/g and 0.32 mF/g, respectively, at a current density of 2 A/g.

Preparation of vertically oriented aromatic polyester thin films by thermal chemical vapor deposition

Aromatic polyesters exhibit high thermal conductivity and large nonlinear optical effects by controlling the orientation of their main chains. Especially in recent years, with the development of flexible optical and electronic devices utilizing polymer thin films, out-of-plane orientation control in thin films on the order of several hundred nm is required. However, due to the rigidity of aromatic polyesters, it is difficult to control vertical orientation in thin film growth from melt or solution. In this study, we attempted to control the vertical orientation of aromatic polyester thin films from the vapor phase using a thermal CVD. From a single head-to-tail type monomer, aromatic polyester thin films with the most primitive structure, whose mp exceeds 500 °C, were successfully grown. Furthermore, it was found that the vertical orientation of the main chains was enhanced by substrate surface treatment.

Interaction of graphene and WS2 with neutrophils and mesenchymal stem cells: implications for peripheral nerve regeneration.

Graphene and bidimensional (2D) materials have been widely used in nerve conduits to boost peripheral nerve regeneration. Nevertheless, the experimental and commercial variability in graphene-based materials generates graphene forms with different structures and properties that can trigger entirely diverse biological responses from all the players involved in nerve repair. Herein, we focus on the graphene and tungsten disulfide (WS2) interaction with non-neuronal cell types involved in nerve tissue regeneration. We synthesize highly crystalline graphene and WS2 with scalable techniques such as thermal decomposition and chemical vapor deposition. The materials were able to trigger the activation of a neutrophil human model promoting Neutrophil Extracellular Traps (NETs) production, particularly under basal conditions, although neutrophils were not able to degrade graphene. Of note is that pristine graphene acts as a repellent for the NET adhesion, a beneficial property for nerve conduit long-term applications. Mesenchymal stem cells (MSCs) have been proposed as a promising strategy for nerve regeneration in combination with a conduit. Thus, the interaction of graphene with MSCs was also investigated, and reduced viability was observed only on specific graphene substrates. Overall, the results confirm the possibility of regulating the cell response by varying graphene properties and selecting the most suitable graphene forms.

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.