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 SiO₂. 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 WSe₂ on an insulator <i>via</i> laser irradiation assisted selenization (LIAS) process directly, for which the amorphous WO₃ film undergoes a reduction process in the presence of selenium gaseous vapors to form WSe₂, 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 WSe₂ concept was demonstrated by patterning the WO₃ film followed by the laser irradiation. By combining the patternable process, the transfer-free WSe₂ back gate field effect transistor (FET) devices are realized on 300 nm-thick SiO₂/P⁺Si substrate with extracted field effect mobility of ∼0.2 cm² V^–1 s^–1. Similarly, the reduction process by the laser irradiation can be also applied for the synthesis of other TMDs such as MoSe₂ from other metal oxides such as MO₃ 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 (MS₂) 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 (MoS₂) and tungsten disulfide (WS₂), are investigated in this work. Material characterization reveals the diffusion of sulfur into the oxide layer prior to the formation of the MS₂. By controlling the sulfur diffusion, we are able to synthesize continuous MS₂ 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 WS₂ 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

Additional file 1: of Few-Layer Graphene Sheet-Passivated Porous Silicon Toward Excellent Electrochemical Double-Layer Supercapacitor Electrode

Figure S1. (a)~(c) Top-view SEM images of PSi structures after annealing at 1000, 1050, and 1100 °C. (DOCX 243 kb

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 H₂ 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_0.2Ti_0.8O₃ Nanowire Arrays: From Controlled Growth by Pulsed Laser Deposition to Piezopotential Measurements

Single crystalline PbZr_0.2Ti_0.8 (PZT) nanowires arrays (NWAs) with taper morphology were epitaxially grown on SrTiO₃ (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 MoS₂/WSe₂ 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 MoS₂/WSe₂ van der Waals (vdW) heterostructures using a dual-gate device architecture. The electric potential and carrier concentration of MoS₂ and WSe₂ 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

Enhancing Quantum Yield in Strained MoS₂ Bilayers by Morphology-Controlled Plasmonic Nanostructures toward Superior Photodetectors

Recently, extracting hot electrons from plasmonic nanostructures and utilizing them to enhance the optical quantum yield of two-dimensional transition-metal dichalcogenides (TMDs) have been topics of interest in the field of optoelectronic device applications, such as solar cells, light-emitting diodes, photodetectors, and so on. The coupling of plasmonic nanostructures with nanolayers of TMDs depends on the optical properties of the plasmonic materials, including radiation pattern, resonance strength, and hot electron injection efficiency. Herein, we demonstrate the augmented photodetection of a large-scale, transfer-free bilayer MoS2 by decorating this TMD with four different morphology-controlled plasmonic nanoparticles. This approach allows engineering the band gap of the bilayer MoS2 due to localized strain that stems up from plasmonic nanoparticles. In particular, the plasmonic strain blue shifts the band gap of bilayer MoS2 with 32 times enhanced photoresponse demonstrating immense hot electron injection. Besides, we observed the varied photoresponse of MoS2 bilayer hybridized with different morphology-controlled plasmonic nanostructures. Although hot electron injection was a substantial factor for photocurrent enhancement in hybrid plasmonic semiconductor devices, our investigations further show that other key factors such as highly directional plasmonic modes, high-aspect-ratio plasmonic nanostructures, and plasmonic strain-induced beneficial band structure modifications were crucial parameters for effective coupling of plasmons with excitons. As a result, our study sheds light on designing highly tailorable plasmonic nanoparticle-integrated transition-metal dichalcogenide-based optoelectronic devices