Two-step Chemical Vapor Deposition Process Research Articles

High-Performance WS2 MOSFETs with Bilayer WS2 Contacts.

WS2 is a promising transition-metal dichalcogenide (TMDC) for use as a channel material in extreme-scaled metal-oxide-semiconductor field-effect transistors (MOSFETs) due to its monolayer thickness, high carrier mobility, and its potential for symmetric n-type and p-type MOSFET performance. However, the formation of stable, low-barrier-height contacts to monolayer TMDCs continues to be a challenge. This study introduces an innovative approach to realize high-performance WS2 MOSFETs by utilizing bilayer WS2 (2L-WS2) in the contact region grown through a two-step chemical vapor deposition process. The 2L-WS2 devices demonstrate a high I ON/I OFF ratio of 108 and a saturated drain current, I D(SAT), of 280 μA/μm (386 μA/μm) at room temperature (78 K), even while still using conventional metal (Pd or Ni) contacts. Devices featuring a 1L-WS2 channel and 2L-WS2 in the contact regions were also fabricated, and they exhibited performance comparable to that of 2L-WS2 devices. The devices also exhibit good stability with nearly identical performance after storage over a 13 month period. The study highlights the benefits of a hybrid channel thickness approach for TMDC transistors.

Direct Selective Epitaxy of 2D Sb2Te3 onto Monolayer WS2 for Vertical p-n Heterojunction Photodetectors.

Two-dimensional transition metal dichalcogenides (2D-TMDs) possess appropriate bandgaps and interact via van der Waals (vdW) forces between layers, effectively overcoming lattice compatibility challenges inherent in traditional heterojunctions. This property facilitates the creation of heterojunctions with customizable bandgap alignments. However, the prevailing method for creating heterojunctions with 2D-TMDs relies on the low-efficiency technique of mechanical exfoliation. Sb2Te3, recognized as a notable p-type semiconductor, emerges as a versatile component for constructing diverse vertical p-n heterostructures with 2D-TMDs. This study presents the successful large-scale deposition of 2D Sb2Te3 onto inert mica substrates, providing valuable insights into the integration of Sb2Te3 with 2D-TMDs to form heterostructures. Building upon this initial advancement, a precise epitaxial growth method for Sb2Te3 on pre-existing WS2 surfaces on SiO2/Si substrates is achieved through a two-step chemical vapor deposition process, resulting in the formation of Sb2Te3/WS2 heterojunctions. Finally, the development of 2D Sb2Te3/WS2 optoelectronic devices is accomplished, showing rapid response times, with a rise/decay time of 305 μs/503 μs, respectively.

Growth of Single-Walled Carbon Nanotubes from Solid Carbon Nanoparticle Seeds via Cap Formation Engineering with a Two-Step Growth Process and Water Vapor Supply.

Solid carbon nanoparticles are promising growth seeds to prepare single-walled carbon nanotubes (SWCNTs) at high temperatures, at which the SWCNT crystallinity should be improved significantly but conventional metal catalyst nanoparticles are unstable and suffer from aggregation. The noncatalytic nature of solid carbon nanoparticles, however, makes SWCNT growth inefficient, resulting in a limited growth yield. In this study, we develop a two-step chemical vapor deposition process to efficiently synthesize high-crystallinity SWCNTs at high temperatures from solid carbon nanoparticles obtained from nanodiamond. Based on thermodynamic considerations, the growth conditions are separately adjusted to supply different growth driving forces which are suitable for the formation of the initial cap structures and for the stationary elongation of SWCNTs. This process, called cap formation engineering, improves the nucleation density of the cap structures. We examined the changes in crystallinity, amorphous carbon deposition, diameter, and yield of SWCNTs with respect to the synthesis conditions. By controlling the initial growth conditions, high-quality SWCNTs are grown with improved yield. With the addition of water vapor as the etchant, deposition of amorphous carbon at high temperatures was further prevented. The results provide a pathway for precise growth control of SWCNTs from unconventional solid growth seeds.

One-Step Synthesis of NbSe2/Nb-Doped-WSe2 Metal/Doped-Semiconductor van der Waals Heterostructures for Doping Controlled Ohmic Contact.

van der Waals heterostructures (vdWHs) of metallic (m-) and semiconducting (s-) transition-metal dichalcogenides (TMDs) exhibit an ideal metal/semiconductor (M/S) contact in a field-effect transistor. However, in the current two-step chemical vapor deposition process, the synthesis of m-TMD on pregrown s-TMD contaminates the van der Waals (vdW) interface and hinders the doping of s-TMD. Here, NbSe2/Nb-doped-WSe2 metal-doped-semiconductor (M/d-S) vdWHs are created via a one-step synthesis approach using a niobium molar ratio-controlled solution-phase precursor. The one-step growth approach synthesizes Nb-doped WSe2 with a controllable doping concentration and metal/doped-semiconductor vdWHs. The hole carrier concentration can be precisely controlled by controlling the Nb/(W + Nb) molar ratio in the precursor solution from ∼3 × 1011/cm2 at Nb-0% to ∼1.38 × 1012/cm2 at Nb-60%; correspondingly, the contact resistance RC value decreases from 10 888.78 at Nb-0% to 70.60 kΩ.μm at Nb-60%. The Schottky barrier height measurement in the Arrhenius plots of ln(Isat/T2) versus q/KBT demonstrated an ohmic contact in the NbSe2/WxNb1-xSe2 vdWHs. Combining p-doping in WSe2 and M/d-S vdWHs, the mobility (27.24 cm2 V-1 s-1) and on/off ratio (2.2 × 107) are increased 1238 and 4400 times, respectively, compared to that using the Cr/pure-WSe2 contact (0.022 cm2 V-1 s-1 and 5 × 103, respectively). Together, the RC value using the NbSe2 contact shows 2.46 kΩ.μm, which is ∼29 times lower than that of using a metal contact. This method is expected to guide the synthesis of various M/d-S vdWHs and applications in future high-performance integrated circuits.

Crystalline Nanodiamond-Induced Formation of Carbon Nanotubes for Stable Hydrogen Sensing

Herein, multiwalled carbon nanotubes (MWCNTs) were synthesized through a two-step chemical vapor deposition process via an ultra-nanocrystalline diamond (ND) as a carbon source for the first time, ...

A two-step chemical vapor deposition process for the growth of continuous vertical heterostructure WSe2/h-BN and its optical properties.

The expansion of two-dimensional (2D) van der Waals heterostructure materials growth and synthesis leads to impressive results in the development and improvement of electronic and optoelectronic applications. Herein, a vertical WSe2/hBN heterostructure was obtained via a dual CVD system, in which prior to the WSe2 growth a continuous monolayer hBN was obtained on a SiO2/Si substrate. Comparing growth on SiO2/Si and quartz substrates, we found that the underlayer of hBN leads to a desorption/diffusion process of tungsten (W) and selenium (Se) producing high-quality and large-area WSe2 growth. In contrast with WSe2/SiO2 and WSe2/quartz heterostructures, the photoluminescence properties of WSe2/hBN exhibit a sharp intense WSe2 peak at 790 nm with a narrow full width at half-maximum (80 meV) due to no dangling bonds and dielectric effect of the hBN interface. The photoluminescence results suggest that the WSe2/hBN heterostructure has high crystallinity with a defect-free interface.

Probing photoresponse of aligned single-walled carbon nanotube doped ultrathin MoS2

We report a facile method to produce ultrathin molybdenum disulfide (MoS2) hybrids with polarized near-infrared (NIR) photoresponses, in which horizontally-aligned single-walled carbon nanotubes (SWNTs) are integrated with single- and few-layer MoS2 through a two-step chemical vapor deposition process. The photocurrent generation mechanisms in SWNT-MoS2 hybrids are systematically investigated through wavelength- and polarization-dependent scanning photocurrent measurements. When the incident photon energy is above the direct bandgap of MoS2, isotropic photocurrent signals are observed, which can be primarily attributed to the direct bandgap transition in MoS2. In contrast, if the incident photon energy in the NIR region is below the direct bandgap of MoS2, the maximum photocurrent response occurs when the incident light is polarized in the direction along the SWNTs, indicating that photocurrent signals mainly result from the anisotropic absorption of SWNTs. More importantly, these two-dimensional (2D) hybrid structures inherit the electrical transport properties from MoS2, displaying n-type characteristics at a zero gate voltage. These fundamental studies provide a new way to produce ultrathin MoS2 hybrids with inherited electrical properties and polarized NIR photoresponses, opening doors for engineering various 2D hybrid materials for future broadband optoelectronic applications.

Low-temperature and scalable CVD route to WS2 monolayers on SiO2/Si substrates

Tungsten disulfide (WS2) monolayers are promising for next-generation flat electronics, but few scalable deposition methods are currently available. Here, the authors report the fabrication of tungsten disulfide monolayers through a novel two-step chemical vapor deposition process involving the deposition of an amorphous tungsten sulfide layer at a relatively mild temperature from the W(CO)6 and 1,2-ethanedithiol precursors, followed by a short annealing at 800 °C under an inert atmosphere. This two-step process allows the fabrication of a crystalline WS2 deposit with a low thermal budget. Raman, x-ray photoelectron, and wavelength dispersive x-ray fluorescence spectroscopic studies performed before and after annealing confirmed the deposition of a sulfur-rich amorphous intermediate, and further confirmed its conversion upon annealing toward oriented 2D WS2 crystals in the 1–2 monolayer range, as corroborated by high-resolution transmission electron microscopy.

Thermal Shock Resistance of SiCNW-SiC Coating for Carbon/Carbon Composites Fabricated by Successive Two-Step CVD Process

The oxidation protective SiC coating toughened by SiC nanowires was fabricated by two-step chemical vapor deposition (CVD) process using MTS as precursor. First, a random-orientation SiC nanowires network was synthesized by atmospheric pressure chemical vapor deposition (APCVD) on carbon/carbon (C/C) composites with Ni assistant. Second, the SiC nanowires network was infiltrated and enwrapped by SiC coating during low pressure chemical vapor deposition (LPCVD) process. The nanowires could effectively toughen the SiC coating by nanowire pullout, nanowire bridging, micro-crack deflection and good interaction between nanowire and matrix interface. The weight loss of SiCNW-SiC coated C/C composites was 2.38% after 25 times thermal cycling between 1773 K and room temperature, while SiC coated C/C composites was 9.50%. The result of the thermal shock showed that the SiCNW-SiC coating had better thermal shock resistance than SiC coating.

Some Like It Flat: Decoupled h-BN Monolayer Substrates for Aligned Graphene Growth.

On the path to functional graphene electronics, suitable templates for chemical vapor deposition (CVD) growth of high-mobility graphene are of great interest. Among various substrates, hexagonal boron nitride (h-BN) has established itself as one of the most promising candidates. The nanomesh, a h-BN monolayer grown on the Rh(111) surface where the lattice mismatch of h-BN and rhodium leads to a characteristic corrugation of h-BN, offers an interesting graphene/h-BN interface, different from flat graphene/h-BN systems hitherto studied. In this report, we describe a two-step CVD process for graphene formation on h-BN/Rh(111) at millibar pressures and describe the influence of the surface texture on the CVD process. During a first exposure to the 3-pentanone precursor, carbon atoms are incorporated in the rhodium subsurface, which leads to decoupling of the h-BN layer from the Rh(111) surface. This is reflected in the electronic band structure, where the corrugation-induced splitting of the h-BN bands vanishes. In a second 3-pentanone exposure, a graphene layer is formed on the flat h-BN layer, evidenced by the appearance of the characteristic linear dispersion of its π band. The graphene layer grows incommensurate and highly oriented. The formation of graphene/h-BN on rhodium opens the door to scalable production of well-aligned heterostacks since single-crystalline thin-film Rh substrates are available in large dimensions.

Synthesis of three-dimensional carbon nanotube/graphene hybrid materials by a two-step chemical vapor deposition process

A three-dimensional carbon nanotube (CNT)/graphene hybrid material was synthesized by a two-step chemical vapor deposition (CVD) process. Due to the separated CVD processes for graphene and CNTs, the structures of the hybrid materials could be easily controlled. It is revealed that graphene film was tightly connected with one end of the CNT arrays, forming “jellyfish” structures. Moreover, our results indicate that the presence of graphene influenced the precipitation and growth rate of CNTs. The precipitation of CNTs was postponed due to the existence of graphene. However, the average growth rate of CNTs in the graphene region for the whole process was faster than that in the region without graphene.

Structure and field-emission properties of W/WO2.72 heterostructures fabricated by vapor deposition

One dimensional W/WO2.72 heterostructures were successfully synthesized using WO3 as the raw material by a simple two-step chemical vapor deposition process. The morphology and microstructure of the W/WO2.72 heterostuctures were characterized using scanning electron microscopy and transmission electron microscopy. The results indicate that the long and straight central axial W whiskers grow along [110] direction, while the branched WO2.72 nanowires grow on the side surface of the W whiskers along the radial direction, with [010] as the growth direction. The as-synthesized heterostuctures exhibit enhanced field emission property over the single W whiskers, and could be used as a candidate for field-emission devices and ultrahigh sensitivity sensors due to their unique composition and structure.

Rational and scalable fabrication of high-quality WO3/CdS core/shell nanowire arrays for photoanodes toward enhanced charge separation and transport under visible light

High-quality one-dimensional WO3/CdS core/shell nanowire arrays used as photoanodes in photoelectrochemical (PEC) cells were for the first time prepared via a rational, two-step chemical vapor deposition process. The narrow band-gap CdS shell was homogeneously coated on the entire surface of as-grown WO3 core nanowire arrays, forming coaxial heterostructures. The one-dimensional core/shell heterostructure facilitates the photogenerated electron-hole pair separation and the electron transfer from CdS to WO3 nanowires under visible light illumination. Moreover, the core nanowire arrays provide a direct pathway for the electron transport. The present results imply that the WO3/CdS core/shell heterostructure nanowire arrays may be useful in the design of nanostructure photoanodes toward highly efficient PEC cells.

Synthesis of few-to-monolayer graphene on rutile titanium dioxide

We demonstrate a chemical-vapor-deposition (CVD)-based approach for the direct synthesis of graphene on insulator with high-dielectric-constant (high-κ). Rutile titanium dioxide (TiO2), an insulator with reported k value of 80–125, is selected as the growth-initiating layer for graphene. A two-step CVD process is shown to grow graphene directly on TiO2 crystals or exfoliated ultrathin TiO2 nanosheets without using any metal catalyst. Various material characterization techniques confirm the growth of few-to-monolayer of graphene. Annealing of the growth substrate at 1100°C under atmospheric pressure, prior to the low-pressure CVD process, is needed for activating nucleation sites in subsequent graphene synthesis. Electrical behavior of a field-effect transistor fabricated on the graphene/TiO2 heterostructure shows p-type doping in the CVD-synthesized graphene.

High-Yield Chemical Vapor Deposition Growth of High-Quality Large-Area AB-Stacked Bilayer Graphene

Bernal-stacked (AB-stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electric field. Mechanical exfoliation can be used to produce AB-stacked bilayer graphene flakes but typically with the sizes limited to a few micrometers. Chemical vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixture of AB- and randomly stacked structures. Herein we report a rational approach to produce large-area high-quality AB-stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H(2)/CH(4) ratio in a low-pressure CVD process to enable the continued growth of bilayer graphene. A high-temperature and low-pressure nucleation step is found to be critical for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90%) and high coverage (up to 99%). The electrical transport studies demonstrate that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB-stacked bilayer graphene with the highest carrier mobility exceeding 4000 cm(2)/V · s at room temperature, comparable to that of the exfoliated bilayer graphene.

Correction to “Switchable Wettability in SnO2 Nanowires and SnO2@SnO2 Heterostructures”

■ ABSTRACT Tin oxide nanowires and heterostructures thereof were synthesized by sequential aplication of thermal and plasmaassisted chemical vapor deposition (CVD) methods. The wetting properties of surfaces based on individual nanowires, hyperbranched SnO2@SnO2 structures, and SnO2@SnO2@ SiOx core−shell heterostructures could be modulated from superhydrophilic to superhydrophobic by changing both chemical composition and the geometrical architecture of nanoheterostructures. Whereas the randomly grown SnO2 nanowires with a contact angle of 3° exhibited superhydrophilicity, the contact angle of SnO2@SnO2 heterostructures synthesized by a two-step CVD process increased to 133°. The corresponding contact angle of SnO2@SnO2@SiOx heterostructures was enhanced to 155.8° by a hydrophobic SiOx coating. Switchable surface wettability in SnO2@SnO2@ SiOx heterostructures was observed by alternation of UV irradiation, storage in dark, and O2 annealing, indicating that geometric microstructure was the major determinant in the switchable wettability. The significant change in the surface wettability upon changing the geometrical features is of potential interest in functional coatings such as antifinger print and self-cleaning surfaces.

Switchable Wettability in SnO2 Nanowires and SnO2@SnO2 Heterostructures

We present here the synthesis of SnO2 nanowires SnO2@SnO2 and SnO2@SnO2@SiOx heterostructures via the chemical vapor deposition (CVD) method. Surface wettability could be modulated from superhydrophilic to superhydrophobic by changing both chemical composition and the geometrical architecture of nanoheterostructures. The random SnO2 nanowires with a contact angle of 3° presented a superhydrophilic property. The contact angle of SnO2@SnO2 heterostructures synthesized by a two-step CVD process increased to 133°. The corresponding contact angle of SnO2@SnO2@SiOx heterostructures was 155.8°. Switchable surface wettability of the SnO2@SnO2@SiOx heterostructure was observed by alternation of UV irradiation, dark storage, and O2 annealing, indicating that geometric microstructure was the major determinant in the switchable wettability from superhydrophilic to superhydrophobic. The switchable surface wettability of SnO2 nanostructures may develop the significant potential for industrial coating and self-cleaning ...

Direct Growth of Graphene/Hexagonal Boron Nitride Stacked Layers

Graphene (G) and atomic layers of hexagonal boron nitride (h-BN) are complementary two-dimensional materials, structurally very similar but with vastly different electronic properties. Recent studies indicate that h-BN atomic layers would be excellent dielectric layers to complement graphene electronics. Graphene on h-BN has been realized via peeling of layers from bulk material to create G/h-BN stacks. Considering that both these layers can be independently grown via chemical vapor deposition (CVD) of their precursors on metal substrates, it is feasible that these can be sequentially grown on substrates to create the G/h-BN stacked layers useful for applications. Here we demonstrate the direct CVD growth of h-BN on highly oriented pyrolytic graphite and on mechanically exfoliated graphene, as well as the large area growth of G/h-BN stacks, consisting of few layers of graphene and h-BN, via a two-step CVD process. The G/h-BN film is uniform and continuous and could be transferred onto different substrates for further characterization and device fabrication.

Graphene Films with Large Domain Size by a Two-Step Chemical Vapor Deposition Process

The fundamental properties of graphene are making it an attractive material for a wide variety of applications. Various techniques have been developed to produce graphene and recently we discovered the synthesis of large area graphene by chemical vapor deposition (CVD) of methane on Cu foils. We also showed that graphene growth on Cu is a surface-mediated process and the films were polycrystalline with domains having an area of tens of square micrometers. In this paper, we report on the effect of growth parameters such as temperature, and methane flow rate and partial pressure on the growth rate, domain size, and surface coverage of graphene as determined by Raman spectroscopy, and transmission and scanning electron microscopy. On the basis of the results, we developed a two-step CVD process to synthesize graphene films with domains having an area of hundreds of square micrometers. Scanning electron microscopy and Raman spectroscopy clearly show an increase in domain size by changing the growth parameters. Transmission electron microscopy further shows that the domains are crystallographically rotated with respect to each other with a range of angles from about 13 to nearly 30°. Electrical transport measurements performed on back-gated FETs show that overall films with larger domains tend to have higher carrier mobility up to about 16,000 cm(2) V(-1) s(-1) at room temperature.

Flexible orientation control of ultralong single-walled carbon nanotubes by gasflow

Gas flow is used to guide the growth of ultralong single-walled carbon nanotubes(SWCNTs) with complex shapes. Large-area cross-shaped networks of ultralong SWCNTsare obtained by a two-step chemical vapor deposition process. Also non-straight SWCNTsof different shapes are prepared by modifying the gas flow with tiny barriers.The shapes of the SWCNTs replicate the streamline patterns accordingly. TheSWCNTs with designed patterns and shapes may be used in high performancenanoelectronics.