Controlled Number Of Layers Research Articles

WS2 with Controllable Layer Number Grown Directly on W Film.

As a layered material with single/multi-atom thickness, two-dimensional transition metal sulfide WS2 has attracted extensive attention in the field of science for its excellent physical, chemical, optical, and electrical properties. The photoelectric properties of WS2 are even more promising than graphene. However, there are many existing preparation methods for WS2, but few reports on its direct growth on tungsten films. Therefore, this paper studies its preparation method and proposes an innovative two-dimensional material preparation method to grow large-sized WS2 with higher quality on metal film. In this experiment, it was found that the reaction temperature could regulate the growth direction of WS2. When the temperature was below 950 °C, the film showed horizontal growth, while when the temperature was above 1000 °C, the film showed vertical growth. At the same time, through Raman and band gap measurements, it is found that the different thicknesses of precursor film will lead to a difference in the number of layers of WS2. The number of layers of WS2 can be controlled by adjusting the thickness of the precursor.

The effect of annealing and multilayer structure on promoting the electrochemical performance of Al/Si thin film anodes

C/(Al/Si)n/TiO2 multilayer thin films with the controllable number of (Al/Si) layers (n = 1, 2, 3, 4) were prepared by magnetron sputtering. The effect of the inserted Al layers and annealing process on the electrochemical performance of the anodes have been discussed. Among the samples before annealing, C/(Al/Si)4/TiO2 thin film anode exhibits a better capacity retention rate after 500 cycles. The annealing process was performed to facilitate the diffusion of Al and Si layers in the film, improving the conductivity of the anode. The result is a decrease in the resistance and an increase in the Li+ diffusion rate of the anode. The annealing process also forms the Al-Si phase. During the cycling process, the Al-Si phase reacts with the electrolyte to form a Li-Al-Si ternary phase, which can improve the stability of the solid electrolyte interphase (SEI) layer. C/(Al/Si)4/TiO2 thin film anode annealed at 300 °C for 30 min (C/(Al/Si)4/TiO2-30 min) forms a stable block structure during cycling, which can alleviate volume expansion, enhance Li+ diffusion rate and exhibit adaptability to high current densities of the thin film anode. It is shown that C/(Al/Si)4/TiO2-30 min thin film anode obtains an initial discharge capacity of 2419 mA h g−1 and a capacity retention of 80.5 % after 500 cycles. In addition, this thin film anode exhibits superior rate performance, maintaining a capacity of 1453 mA h g−1 after 500 cycles at 5 A/g and a capacity above 1463 mAhg−1 even at 30 A/g.

Investigating the Fundamental Conditions for Quantitative Growth to Obtain High-Quality WS2 Using a Process of Physical Vapor Deposition

Two-dimensional layered transition-metal dichalcogenides (2D-TMDs) have garnered significant attention due to their layer number-dependent electronic properties, making them promising candidates for atomically thin electronics and optoelectronics. However, current research has primarily focused on exfoliated TMD materials, which have limitations in size, layer number control, and yield. Therefore, a crucial challenge remains in producing large single TMD crystals with precise control over the layer number. A comprehensive understanding and precise control of the growth conditions are imperative to address this challenge. This study systematically investigated key growth conditions, including temperature, precursor flow, growth duration, material quantity, gas flow, and slide position. By optimizing these parameters, we successfully synthesized TMD materials with an impressive size of 850 µm. Notably, we achieved the preparation of monolayer WS2 single crystals on a large scale within a remarkably short duration of 10 min, exhibiting a lateral growth rate of up to 1.4 μm/s, which is comparable to the best-exfoliated monolayers. The findings from our study provide a robust pathway for the rapid growth of high-quality TMD single crystals, facilitating further advancements in this field.

Trapping Hydrogen Molecules between Perfect Graphene.

There is a lack of deep understanding of hydrogen intercalation into graphite due to many challenges faced during characterization of the systems. Therefore, a suitable route to trap isolated hydrogen molecules (H2) between the perfect graphite lattices needs to be found. Here we realize the formation of hydrogen bubbles in graphite with controllable density, size, and layer number. We find that the molecular H2 cannot be diffused between nor escape from the defect-free graphene lattices, and it remains stable in the pressurized bubbles up to 400 °C. The internal pressure of H2 inside the bubbles is strongly temperature dependent, and it decreases as the temperature rises. The proton permeation rate can be estimated at a specific plasma power. The producing method of H2 bubbles offers a useful way for storing hydrogen in layered materials, and these materials provide a prospective research platform for studying nontrivial quantum effects in confined H2.

Centimeter-level MoS2 films with controllable number of layers by face-to-face chemical vapor deposition strategy

Two-dimensional layered transition metal dichalcogenides (TMDCs) are highly promising in field-effect-transistors (EFTs), photodetectors, flexible devices, but the limitation of uncontrollable size and number of layers is a huge challenge for the development of these devices. Although some breakthroughs like wafer-level MoS2 have been achieved on specific substrates such as copper foil or sapphire using advanced chemical vapor deposition, electrical and optoelectronic testing requires the transfer of material from these substrates to SiO2/Si substrates. In this paper, we developed a face-to-face confined space CVD scheme for the preparation of centimeter-level MoS2 thin films on amorphous SiO2/Si substrate. In contrast to traditional confined-space CVD scheme where the MoO3 powder is often piled like a small hill in the small quartz boat, The MoO3 powder is evenly spread in the small quartz boat, facing directly up to the growing substrate with a “face-to-face” growth mode. Therefore, the nucleation centers are uniformly distributed on the growing substrate, finally resulting in a uniform and clean continuous film originating from the coalescence of numerous MoS2 domains grown up from these nucleation centers. The layer number can be well controlled by adjusting growing parameters like growing temperature, gas flow rate and growth time. Our facile but robust face-to-face growth strategy may be a great supplementary to the wafer-level growth of 2D TMDCs.

An aptamer-functionalized photonic crystal sensor for ultrasensitive and label-free detection of aflatoxin B1

As a novel optical responsive material, photonic crystal is a promising sensing material in the recognition and detection of small molecules. Herein, a label-free composite sensor for aflatoxin B1 (AFB1) based on aptamer-functionalized photonic crystal arrays was successfully developed. Three-dimensional photonic crystals (3D PhCs) with a controllable number of layers were produced by a layer-by-layer (LBL) approach, and the introduction of gold nanoparticles (AuNPs) facilitated the immobilization procedure of recognition element aptamers, thus creating the AFB1 sensing detection system (AFB1-Apt 3D PhCs). The sensing system AFB1-Apt 3D PhCs exhibited a good linearity in the wide range of 1 pg mL−1-100 ng mL−1 AFB1 with a limit of detection (LOD) of 0.28 pg mL−1. Furthermore AFB1-Apt 3D PhC was successfully applied in the determination of AFB1 in the millet and beer samples with good recovery. The sensing system performed ultrasensitive and label-free detection to the target, which could be further applied in the fields of food safety, clinical diagnosis or environmental monitoring, establishing an efficient and rapid universal detection platform.

Conversion of Carbon Dioxide into Chemical Vapor Deposited Graphene with Controllable Number of Layers via Hydrogen Plasma Pre-Treatment.

In this work, we report the conversion of carbon dioxide (CO2) gas into graphene on copper foil by using a thermal chemical vapor deposition (CVD) method assisted by hydrogen (H2) plasma pre-treatment. The synthesized graphene has been characterized by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results show the controllable number of layers (two to six layers) of high-quality graphene by adjusting H2 plasma pre-treatment powers (100–400 W). The number of layers is reduced with increasing H2 plasma pre-treatment powers due to the direct modification of metal catalyst surfaces. Bilayer graphene can be well grown with H2 plasma pre-treatment powers of 400 W while few-layer graphene has been successfully formed under H2 plasma pre-treatment powers ranging from 100 to 300 W. The formation mechanism is highlighted.

Highly optically transparent graphene mesh for electromagnetic interference shielding

The update of optoelectronic technology requires that the optical window has the dual functions of optical transparency and electromagnetic shielding. A transparent electromagnetic shielding method based on graphene mesh structure is proposed. The optical transmittance of the graphene mesh was theoretically analyzed. Graphene mesh is fabricated by ordinary mask photolithography technology to arbitrarily adjust the structure and number of layers. It retains the same properties as the unpatterned graphene film. Through mesh structure design and layer number control, graphene mesh can be used to replace graphene film with uniform thickness. The average transmittance of the four-layer graphene mesh in the 3–5 μm band is 95.26%, and the average shielding efficiency in the 12–18 GHz band is 3.86 dB. It shows that the graphene mesh maintains high optical transmittance and has good electromagnetic shielding effectiveness.

Synthesis of Few Layer Amorphous 1T/2H MoS2 by a One-Step Ethanol/Water Solvothermal Method and Its Hydrodesulfurization Performance

Few layer (< 5 layers) MoS2 mixed with a 1T phase has the characteristics of a large specific surface area and an improved active site distribution. Therefore, it has been increasingly study for the catalytic hydrodesulfurization of polycyclic compounds. In this paper, few-layer MoS2 with a high proportion of the 1T phase is synthesized by a one-step solvothermal method using a green non-toxic ethanol/water solvent. Results show that few-layer MoS2 (3–5 layers) with a 1T phase up to 48% can be synthesized in a 50% ethanol solvent. This MoS2 is used to catalyse the hydrodesulfurization reaction of dibenzothiophene, and the target product conversion rate reaches 84.2%. Among them, biphenyl accounts for 70.2%, phenylcyclohexane accounts for 13.9%, and bicyclohexane accounts for 0.4%. According to the Hansen solubility parameter (HSP) distance Ra, the effects of solvents on the structure and morphology of MoS2 and the conversion of dibenzothiophene hydrodesulfurization are determined. This work is significant for the synchronous synthetic exfoliation of MoS2 with a controllable number of layers, a higher ratio of the 1T phase and good catalytic HDS of polycyclic compounds. Few layers and high proportion of 1T phase MoS2 by solvothermal synthesis for catalytic hydrodesulfurization of dibenzothiophene.

Patterning of transition metal dichalcogenides catalyzed by surface plasmons with atomic precision

Summary Plasmon-induced charge transfer has drawn considerable interests and spurred rapid developments of photovoltaics and photocatalysis. Various plasmonic metal/transition metal dichalcogenide (TMD) heterostructures have been developed to promote charge separation. For plasmon-induced catalysis, the transformation of TMDs is rare, and the erosion of TMDs has not yet been reported. Here, we report the etching of two-dimensional TMDs by planar surface plasmon polaritons (SPPs). We found that SPPs can etch various TMDs into desired layers and lateral size through controlling the light power and incident direction. In aqueous media, the strong plasmonic coupling at Au/MoS2 interface generates holes in the valence band of MoS2 that can weaken its interlayer interaction. As a synergistic effect, the plasmonic hot electrons produce oxidizing H2O2 to dissolve MoS2 from the top layers. Our results provide a new perspective for understanding the plasmonic coupling at metal-TMD interfaces and a reliable route toward fabricating well-defined TMDs.

Chemical Vapor Deposition Growth of Two-Dimensional Compound Materials: Controllability, Material Quality, and Growth Mechanism

CONSPECTUS: Two-dimensional (2D) compound materials are promising materials for use in electronics, optoelectronics, flexible devices, etc. because they are ultrathin and cover a wide range of properties. Among all methods to prepare 2D materials, chemical vapor deposition (CVD) is promising because it produces materials with a high quality and reasonable cost. So far, much efforts have been made to produce 2D compound materials with large domain size, controllable number of layers, fast-growth rate, and high quality features, etc. However, due to the complicated growth mechanism like sublimation and diffusion processes of multiple precursors, maintaining the controllability, repeatability, and high quality of CVD grown 2D binary and ternary materials is still a big challenge, which prevents their widespread use. Here, taking 2D transition metal dichalcogenides (TMDCs) as examples, we review current progress and highlight some promising growth strategies for the growth of 2D compound materials. The key technology issues which affect the CVD process, including non-metal precursor, metal precursor, substrate engineering, temperature, and gas flow, are discussed. Also, methods in improving the quality of CVD-grown 2D materials and current understanding on their growth mechanism are highlighted. Finally, challenges and opportunities in this field are proposed. We believe this review will guide the future design of controllable CVD systems for the growth of 2D compound materials with good controllability and high quality, laying the foundations for their potential applications.

Electrochemically induced high ion and electron conductive interlayer in porous multilayer Si film anode with enhanced lithium storage properties

Si has been regarded as one of the most promising anodes for next-generation lithium ion batteries for its high theoretical capacity (3579 mA h g−1) and low operating potential of 0.4 V vs Li/Li+. However, the huge volume change during (de)lithiation and low electronic conductivity impede its further commercial applications. In this work, Zn/Si porous multi-layer films with controllable layer number (from 1 to 6 layers) are constructed firstly, and then a ZnO thin layer is sputtered on the top surface of the Zn/Si films to fabricate the Zn/Si/ZnO porous multi-layer films. When used as binder-free anodes, the designed 4 layers Zn/Si/ZnO thin film anode delivers the highest specific capacity, best cycling and rate performance than other multi-layer film anodes and the bulk Zn/Si/ZnO film anode with the same overall thickness. The excellent electrochemical properties of the prepared 4 layers Zn/Si/ZnO porous composite film anode can be attributed to its porous and multilayer structure, which can moderate the volume change, decrease the internal stress and benefit to the penetration of electrolyte. Moreover, the 3D electron and Li+ ion conductive channels resulted from the formed Li–Zn alloy during cycling can help to increase the electrochemical reaction kinetics.

Controllable synthesis of graphene by CVD method

<p indent=0mm>In 2004, graphene was successfully prepared by micromechanical exfoliation of graphite, which attracts wide attention in the scientific research community. After that, related research of two-dimensional (2D) materials has received continuous exploration. In particular, academia and industry have paid tremendous attention and high expectations on graphene, owing to its remarkable and tunable physical, electrical, optical, magnetic properties and its potential applications in various fields. These practical applications include high performance electronics and optoelectronics, such as field effect transistors, transparent and flexible electrodes, spintronic devices, solar cells and so on. However, as is well-known, the practical commercial applications are based on mature and repeatable preparation of materials. Great achievements have been made in developing synthesis techniques, including micromechanical exfoliation, epitaxial growth, chemical vapor deposition (CVD), chemical exfoliation, arc discharge, segregation growth and bottom-up synthesis and so on. Among these methods, CVD method is regarded as the most versatile platform to obtain high quality, large area, and controllable number of layers with good repeatability and low cost for real commercial applications. Up to now, the past decade has witnessed significant developments in the field of CVD graphene synthesis. Herein, we present the development of this field by firstly introducing the origin of graphene and the background of CVD method. Then, the main achievements in the development of graphene since 2008 are presented from the point of view of controllable preparation. The CVD synthesis of graphene is focused mainly on four aspects: the selection and modification of catalytic substrates, the manipulation of reaction conditions, bandgap engineering and clean transfer. The selection of catalytic substrate plays a critical role in the controllable preparation of graphene by CVD method. The representative catalytic mechanisms of different substrates for growing graphene are introduced, followed by various modifications of catalytic metal substrates. Besides, surface morphology and microstructure of substrates have a great impact on the quality and uniformity of as-grown graphene. Thus, due to the disadvantages of non-uniform surface energy, defects, wrinkles and impurities on the surface of solid metal, the introduction of liquid copper catalytic system helps to better control the nucleation uniformity and optimize the graphene growth. Moreover, the preparation of large-area monocrystalline metal foils for obtaining large-scale single-crystal graphene was also reviewed. In order to avoid the disadvantages such as the damage, wrinkle and polymer pollution of graphene associated with transfer process occurred in the cases of using metal substrates to grow graphene, direct synthesis of graphene on dielectric substrates is presented with focuses on two basic controls of nucleation density and growth rate. A series of techniques such as reaction medium regulation, substrate surface modification, and plasma-assisted growth, the introduction of gaseous metals and direct preparation of graphene on liquid glass are introduced. The development of graphene vertical and planar heterostructures with hexagonal boron nitride is also presented. Moreover, the disadvantage of graphene limits its application in logic circuits due to its zero bandgap without an acceptable on/off ratio, and three main ways to open graphene bandgap are discussed including doping, the preparation of a large area of AB stacking bilayer graphene and engineering graphene nanoribbons. After that, a brief introduction of transfer method used in graphene post-processing is given. Finally, we summarize the problems existing in the field, and the future opportunities and challenges are discussed.

Ultrafast Exciton Dynamics in Scalable Monolayer MoS2 Synthesized by Metal Sulfurization.

Excitons in monolayer transition metal dichalcogenides (TMDs) have exceptionally large binding energies and dominate the optical properties of materials. Exploring the relaxation behavior of excitons is crucial for understanding the fundamental physics as well as the performance of TMD-based optoelectronic devices. However, ultrafast carrier dynamics is sensitive to the structural defects and surface conditions of TMDs, depending on the growth or transfer process. Here, we utilized pump-probe transient absorption (TA) spectroscopy with a white-light probe to investigate the dynamics of excitons in monolayer MoS2 synthesized by the metal sulfurization method. The sulfurization method was used for the fabrication of large-scale, continuous, and uniform thin films with a controllable number of layers. The excitation dynamics of the wafer-size monolayer MoS2 is found to be comparable to that of monolayer MoS2 flakes grown by chemical vapor deposition (CVD). The dominant processes of carrier relaxation in the monolayer MoS2 are exciton–exciton annihilation (hundreds of femtoseconds), the trapping of the excitons by surface states (a few picoseconds), and interband carrier-phonon scattering (tens of picoseconds). Moreover, the induced absorption due to mid-gap defects, which is often observed for samples fabricated by growth methods, such as CVD, is not observed for our continuous and uniform monolayer films. Understanding the charge carrier dynamics of the exciton in the scalable and uniform monolayer MoS2 can provide physical insights that are valuable in the design and development of complex 2D devices.

Dosed carbon precipitation and graphene layer number control on nickel micro-electromechanical systems surfaces

In this paper, we report on the in situ synthesis of graphene layers by means of chemical vapor deposition (CVD), directly on nickel micro-electromechanical systems (MEMS) surfaces. We have developed MEMS structures of which the temperature can be increased locally by Joule heating while in a methane environment. For our MEMS structures, the thermal time constant is 28 μs. As a result, we have control over the carbon precipitation time, thereby governing how many graphene layers are formed. Bi-layer to multi-layer graphene was observed using micro-Raman spectroscopy, but not single-layer graphene, as it gives no Raman signal when coupled on a nickel surface. The corresponding precipitation control theory is also presented in this paper, in which we relate the out-diffusion of carbon atoms from the grains of the nickel structure to the resulting number of graphene layers. Our method provides regulated carbon segregation from nickel and allows a prescribed number of graphene layers to form by tuning the precipitation time. In this way, we enable the direct in situ synthesis of graphene locally on the top and sidewalls of nickel MEMS structures, so that e.g. such graphene-coated MEMS surfaces can contribute towards a promising solution against friction and wear for MEMS devices with sliding components.

Supercritical hydrothermal synthesis of MoS2 nanosheets with controllable layer number and phase structure.

Molybdenum disulfide (MoS2), an attractive material for energy conversion devices, is known to exhibit varying properties depending on the number of layers and the phase structure. In this study, we developed a supercritical hydrothermal process that allows the controllable synthesis of MoS2 nanosheets with various structural and morphological characteristics. Detailed characterization of the synthesized materials confirmed that the number of layers in the MoS2 nanosheets could be controlled by varying the organic reducing agent under supercritical hydrothermal conditions. In addition, the MoS2 phase could be controlled kinetically by varying the reaction time with ascorbic acid as the reducing agent. Because water and elemental sulfur were respectively used as the solvent and sulfur source and as the reaction time was minimal, the developed hydrothermal process represents a facile processing method for different types of MoS2 nanosheets.

A systematic study of the growth and characterization of few-layer graphene on Ni foils

Graphene, a two-dimensional material, has gained ample interest in today’s society owing to its remarkable properties such as high conductivity, thermal stability, mechanical strength. Up to date, various graphene synthesis methods are available and out of them, atmospheric pressure chemical vapor deposition (APCVD) is one of the best synthesis due to a very low diffusivity coefficient and a critical step for graphene-based device fabrication. In this work, we have optimized the growth conditions for the synthesis of graphene and its layers on Ni foil using APCVD. It is observed that the temperature i.e., heating and cooling rate, plays an essential role in the growth process. It is also found that annealing of substrate also plays a vital role in the growth of graphene and its layers and the annealing time has been varied from 30 to 120 min the substrate. From XRD and surface roughness analysis, the optimized time for annealing is optimized at 60 min. This led to continuous graphene layers comparable to those achieved with high-temperature CVD with full surface coverage and excellent quality. Our APCVD method is expected to allow the direct growth of graphene in device manufacturing processes for practical applications while keeping underlying devices intact. From TEM analysis, the layers can be seen in the size range from 50 to 100 nm. The presented method in this work provides an integrated device technology for wafer scale deposition with a controllable number of graphene layers.

Precise Vapor-Phase Synthesis of Two-Dimensional Atomic Single Crystals.

Two-dimensional atomic single crystals (2DASCs) have drawn immense attention because of their potential for fundamental research and new technologies. Novel properties of 2DASCs are closely related to their atomic structures, and effective modulation of the structures allows for exploring various practical applications. Precise vapor-phase synthesis of 2DASCs with tunable thickness, selectable phase, and controllable chemical composition can be realized to adjust their band structures and electronic properties. This review highlights the latest advances in the precise vapor-phase synthesis of 2DASCs. We thoroughly elaborate on strategies toward the accurate control of layer number, phase, chemical composition of layered 2DASCs, and thickness of non-layered 2DASCs. Finally, we suggest forward-looking solutions to the challenges and directions of future developments in this emerging field.

Realizing Room-Temperature Resonant Tunnel Magnetoresistance in Cr/Fe/MgAl2O4 Quasi-Quantum Well Structures.

The quantum well (QW) realizes new functionalities due to the discrete electronic energy levels formed in the well‐shaped potential. Magnetic tunnel junctions (MTJs) combined with a quasi‐QW structure of Cr/ultrathin‐Fe/MgAl2O4(001)/Fe, in which the Cr quasi‐barrier layer confines Δ 1 up‐spin electrons to the Fe well, are prepared with perfectly lattice‐matched interfaces and atomic layer number control. Resonant peaks are clearly observed in the differential conductance of the MTJs due to the formation of QWs. Furthermore, enhanced tunnel magnetoresistance (TMR) peaks at the resonant bias voltages are realized for the MTJs at room temperature, i.e., it is observed that TMR ratios at specific and even high bias‐voltages (V bias) are larger than zero‐bias TMR ratios for the MTJs with odd Fe atomic layers, in contrast to the earlier experimental studies. In addition, a new finding in this study is unique sign changes in the temperature coefficient of resistance (TCR) depending on the Fe thickness and V bias, which is interpreted as a signature of the QW formation of Δ1 symmetry electronic states. The present study suggests that the spin‐dependent resonant tunneling via the QWs formed in Cr/ultrathin‐Fe/MgAl2O4/Fe structures should open a new pathway to achieve a large TMR at practically high V bias.

Adjusting acetylene gas flow to grow a spheroidal graphene film with controllable layer number and lattice defects

Spheroidal graphene film (SGF) is of importance for the spatial sensor matrix applications due to its large specific surface area and more chemical modification sites. It is difficult to in-situ grow the SGF on the desired substrate, specifically, the control of the number of layers and lattice defects of SGF is difficult. In this work, the SGF is grown in-situ on a cemented carbide surface with controllable layer number and lattice defects. The Co, a binder phase in cemented carbide, was used as a catalytic metal; amorphous SiC (a-SiC) film was used as a carbon source on the surface of the cemented carbide and catalytic reaction occurred under the annealing condition. The controllable preparation of a number of layers and lattice defects of graphene was achieved by controlling the flow rate of acetylene gas during the deposition of the a-SiC film. It was found that the acetylene gas flow rate can directly control the C atom content of the interfacial layer. With the increase of the content of C atoms, the lattice defects of the resulting graphene tend to decrease, while the number of layers of SGF tends to increase. The reaction mechanism of growth of SGF is discussed. The content of C atom may induce the change of the phase composition of the interfacial layer, which acts as a constituting element of the graphene film. As a result, the content of C atoms is in close relationship with the number of layers and lattice defects of the SGF.