11 research outputs found
The Essential Role of Cu Vapor for the Self-Limit Graphene via the Cu Catalytic CVD Method
Because of the inconsistent observations,
the Cu catalytic decomposition
of methane for graphene synthesis is reexamined, i.e., via the surface
absorption, decomposition to atomic carbon, and segregation. Here,
we experimentally show the quantity of ambient Cu vapor is the key
factor in graphene synthesis, which influences the dropwise condensations
for airborne Cu clusters during growth. The massive carburization
in Cu clusters and the calculation of carbon solubility in nanosized
clusters are performed, experimented, and further examined from the
growth of diamond-like-carbon films and ball-like diamonds via Cu
vapor assisted growth on SiO<sub>2</sub>. The affinitive interactions
between Cu vapor, ambient gases, and solid surface are embodied. By
combining the molecular dynamics for the redeposited Cu clusters to
surface, the vehicle theory of Cu clusters, which transports the atomic
carbon to the surface and completes the graphene growth, is thus proposed
as the essential puzzle we considered
Ultrafast and Low Temperature Synthesis of Highly Crystalline and Patternable Few-Layers Tungsten Diselenide by Laser Irradiation Assisted Selenization Process
Recently, a few attempts to synthesize monolayers of transition metal dichalcogenides (TMDs) using the chemical vapor deposition (CVD) process had been demonstrated. However, the development of alternative processes to synthesize TMDs is an important step because of the time-consuming, required transfer and low thermal efficiency of the CVD process. Here, we demonstrate a method to achieve few-layers WSe<sub>2</sub> on an insulator <i>via</i> laser irradiation assisted selenization (LIAS) process directly, for which the amorphous WO<sub>3</sub> film undergoes a reduction process in the presence of selenium gaseous vapors to form WSe<sub>2</sub>, utilizing laser annealing as a heating source. Detailed growth parameters such as laser power and laser irradiation time were investigated. In addition, microstructures, optical and electrical properties were investigated. Furthermore, a patternable WSe<sub>2</sub> concept was demonstrated by patterning the WO<sub>3</sub> film followed by the laser irradiation. By combining the patternable process, the transfer-free WSe<sub>2</sub> back gate field effect transistor (FET) devices are realized on 300 nm-thick SiO<sub>2</sub>/P<sup>+</sup>Si substrate with extracted field effect mobility of ∼0.2 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. Similarly, the reduction process by the laser irradiation can be also applied for the synthesis of other TMDs such as MoSe<sub>2</sub> from other metal oxides such as MO<sub>3</sub> film, suggesting that the process can be further extended to other TMDs. The method ensures one-step process to fabricate patternable TMDs, highlighting the uniqueness of the laser irradiation for the synthesis of different TMDs
Low Temperature Growth of Graphene on Glass by Carbon-Enclosed Chemical Vapor Deposition Process and Its Application as Transparent Electrode
A novel carbon-enclosed chemical
vapor deposition (CE-CVD) to grow
high quality monolayer graphene on Cu substrate at a low temperature
of 500 °C was demonstrated. The quality of the grown graphene
was investigated by Raman spectra, and the detailed growth mechanism
of high quality graphene by the CE-CVD process was investigated in
detail. In addition to growth of high quality monolayer graphene,
a transparent hybrid few-layer graphene/CuNi mesh electrode directly
synthesized by the CE-CVD process on a conventional glass substrate
at the temperature of 500 °C was demonstrated, showing excellent
electrical properties (∼5 Ω/□ @ 93.5% transparency)
and ready to be used for optical applications without further transfer
process. The few-layer graphene/CuNi mesh electrode shows no electrical
degradation even after 2 h annealing in pure oxygen at an elevated
temperature of ∼300 °C. Furthermore, the few-layer graphene/CuNi
mesh electrode delivers an excellent corrosion resistance in highly
corrosive solutions such as electroplating process and achieves a
good nucleation rate for the deposited film. Findings suggest that
the low temperature few-layer graphene/CuNi mesh electrode synthesized
by the CE-CVD process is an excellent candidate to replace indium
tin oxide (ITO) as transparent conductive material (TCM) in the next
generation
Transfer-Free Growth of Atomically Thin Transition Metal Disulfides Using a Solution Precursor by a Laser Irradiation Process and Their Application in Low-Power Photodetectors
Although chemical vapor deposition is the most common
method to synthesize transition metal dichalcogenides (TMDs), several
obstacles, such as the high annealing temperature restricting the
substrates used in the process and the required transfer causing the
formation of wrinkles and defects, must be resolved. Here, we present
a novel method to grow patternable two-dimensional (2D) transition
metal disulfides (MS<sub>2</sub>) directly underneath a protective
coating layer by spin-coating a liquid chalcogen precursor onto the
transition metal oxide layer, followed by a laser irradiation annealing
process. Two metal sulfides, molybdenum disulfide (MoS<sub>2</sub>) and tungsten disulfide (WS<sub>2</sub>), are investigated in this
work. Material characterization reveals the diffusion of sulfur into
the oxide layer prior to the formation of the MS<sub>2</sub>. By controlling
the sulfur diffusion, we are able to synthesize continuous MS<sub>2</sub> layers beneath the top oxide layer, creating a protective
coating layer for the newly formed TMD. Air-stable and low-power photosensing
devices fabricated on the synthesized 2D WS<sub>2</sub> without the
need for a further transfer process demonstrate the potential applicability
of TMDs generated via a laser irradiation process
Low Vacuum Annealing of Cellulose Acetate on Nickel Towards Transparent Conductive CNT–Graphene Hybrid Films
We
report a versatile method based on low vacuum annealing of cellulose
acetate on nickel (Ni) surface for rapid fabrication of graphene and
carbon nanotube (CNT)–graphene hybrid films with tunable properties.
Uniform films mainly composed of tri-layer graphene can be achieved
via a surface precipitation of dissociated carbon at 800 °C for
30 seconds under vacuum conditions of ∼0.6 Pa. The surface
precipitation process is further found to be efficient for joining
the precipitated graphene with pre-coated CNTs on the Ni surface,
consequently, generating the hybrid films. As expected, the hybrid
films exhibit substantial opto-electrical and field electron emission
properties superior to their individual counterparts. The finding
suggests a promising route to hybridize the graphene with diverse
nanomaterials for constructing novel hybrid materials with improved
performances
Quantum Size Effects on the Chemical Sensing Performance of Two-Dimensional Semiconductors
We investigate the role of quantum confinement on the
performance
of gas sensors based on two-dimensional InAs membranes. Pd-decorated
InAs membranes configured as H<sub>2</sub> sensors are shown to exhibit
strong thickness dependence, with ∼100× enhancement in
the sensor response as the thickness is reduced from 48 to 8 nm. Through
detailed experiments and modeling, the thickness scaling trend is
attributed to the quantization of electrons which favorably alters
both the position and the transport properties of charge carriers;
thus making them more susceptible to surface phenomena
Taper PbZr<sub>0.2</sub>Ti<sub>0.8</sub>O<sub>3</sub> Nanowire Arrays: From Controlled Growth by Pulsed Laser Deposition to Piezopotential Measurements
Single crystalline PbZr<sub>0.2</sub>Ti<sub>0.8</sub> (PZT) nanowires arrays (NWAs) with taper morphology were epitaxially grown on SrTiO<sub>3</sub> (STO) substrate using pulse laser deposition. The taper morphology was attributed to the overcoating of PZT layer <i>via</i> a lateral growth of PZT clusters/adatoms during PZT NW growth. The growth window for PZT film or nanowire was systematically studied at varied temperatures and pressures. The proposed growth mechanism of the taper PZT NWAs was investigated from a layer by layer growth <i>via</i> Frank–Van Der Merwe growth, followed by a formation of three-dimensional islands <i>via</i> Stranski–Krastanow growth, and then axial growth on the lowest energy (001) plane with growth direction of [001] <i>via</i> vapor–solid growth mechanism. However, under certain conditions such as at higher or lower pressure (>400 or <200 mTorr) or substrate temperatures (>850 °C and <725 °C), formation of the PZT NWs is suppressed while the epitaxial PZT thin film <i>via</i> the layer-by-layer growth remains. The controllable growth directions of the PZT NWAs on (001), (110), and (111) STO substrates were demonstrated. The piezopotential of the taper PZT NWAs using a conducting atomic force microscope with the average voltage output of ∼18 mV was measured. The theoretical piezopotential of a PZT NW was calculated to compare with the measured outputs, providing a comprehensively experimental and theoretical understanding of the piezoelectricity for the PZT NW
Dual-Gated MoS<sub>2</sub>/WSe<sub>2</sub> van der Waals Tunnel Diodes and Transistors
Two-dimensional layered semiconductors present a promising material platform for band-to-band-tunneling devices given their homogeneous band edge steepness due to their atomically flat thickness. Here, we experimentally demonstrate interlayer band-to-band tunneling in vertical MoS<sub>2</sub>/WSe<sub>2</sub> van der Waals (vdW) heterostructures using a dual-gate device architecture. The electric potential and carrier concentration of MoS<sub>2</sub> and WSe<sub>2</sub> layers are independently controlled by the two symmetric gates. The same device can be gate modulated to behave as either an Esaki diode with negative differential resistance, a backward diode with large reverse bias tunneling current, or a forward rectifying diode with low reverse bias current. Notably, a high gate coupling efficiency of ∼80% is obtained for tuning the interlayer band alignments, arising from weak electrostatic screening by the atomically thin layers. This work presents an advance in the fundamental understanding of the interlayer coupling and electron tunneling in semiconductor vdW heterostructures with important implications toward the design of atomically thin tunnel transistors
Large Scale and Orientation-Controllable Nanotip Structures on CuInS<sub>2</sub>, Cu(In,Ga)S<sub>2</sub>, CuInSe<sub>2</sub>, and Cu(In,Ga)Se<sub>2</sub> by Low Energy Ion Beam Bombardment Process: Growth and Characterization
One-step facile methodology to create
nanotip arrays on chalcopyrite materials (such as CuInS<sub>2</sub>, CuÂ(In,Ga)ÂS<sub>2</sub>, CuInSe<sub>2</sub>, and CuÂ(In,Ga)ÂSe<sub>2</sub>) via a low energy ion beam bombardment process has been demonstrated.
The mechanism of formation for nanotip arrays has been proposed by
sputtering yields of metals and reduction of metals induced by the
ion beam bombardment process. The optical reflectance of these chalcopyrite
nanotip arrays has been characterized by UV–vis spectrophotometer
and the efficient light-trapping effect has been observed. Large scale
(∼4′′) and high density (10<sup>10</sup> tips/cm<sup>2</sup>) of chalcopyrite nanotip arrays have been obtained by using
low ion energy (< 1 kV), short processing duration (< 30 min),
and template-free. Besides, orientation and length of these chalcopyrite
nanotip arrays are controllable. Our results can be the guide for
other nanostructured materials fabrication by ion sputtering and are
available for industrial production as well
Crystalline Engineering Toward Large-Scale High-Efficiency Printable Cu(In,Ga)Se<sub>2</sub> Thin Film Solar Cells on Flexible Substrate by Femtosecond Laser Annealing Process
Ink-printing
method emerges as a viable way for manufacturing large-scale
flexible CuÂ(In,Ga)ÂSe<sub>2</sub> (CIGS) thin film photovoltaic (TFPV)
devices owing to its potential for the rapid process, mass production,
and low-cost nonvacuum device fabrication. Here, we brought the femtosecond
laser annealing (fs-LA) process into the ink-printing CIGS thin film
preparation. The effects of fs-LA treatment on the structural and
optoelectronic properties of the ink-printing CIGS thin films were
systematically investigated. It was observed that, while the film
surface morphology remained essentially unchanged under superheating,
the quality of crystallinity was significantly enhanced after the
fs-LA treatment. Moreover, a better stoichiometric composition was
achieved with an optimized laser scanning rate of the laser beam,
presumably due to the much reduced indium segregation phenomena, which
is believed to be beneficial in decreasing the defect states of In<sub>Se</sub>, V<sub>Se</sub>, and In<sub>Cu</sub>. Consequently, the
shunt leakage current and recombination centers were both greatly
decreased, resulting in a near 20% enhancement in photovoltaic conversion
efficiency