Large Area High Quality Research Articles

Synthesis of large-area high quality 2D BiOI for highly sensitive ultraviolet photodetection

High-quality 2D layered BiOI crystals have been synthesized by a novel space-confined molten salt method.

Advancements in the Preparation and Application Research of TMDCs

Two-dimensional transition metal dichalcogenides(TMDCs)have been widely applied in the fields of light, electricity, magnetism, and heat due to their excellent physicochemical properties, including high photoluminescence quantum yield, outstanding thermal conductivity, good mechanical flexibility, and high charge carrier mobility. Tungsten disulfide (WS2) is a typical TMDCs, and optimizing its crystal structure and achieving high-quality large-area synthesis are of great significance for realizing its large-scale applications. This paper introduces the structure and electronic properties of tungsten disulfide, provides a comprehensive review of its main preparation methods, and summarizes its research progress in various application fields. Finally, the future development directions in the field of two-dimensional materials, including tungsten disulfide, are discussed, emphasizing the importance of further research and development to promote their widespread applications in various fields.

Omnidirectional diffusion of organic amine salts assisted by ordered arrays in porous lead iodide for two-step deposited large-area perovskite solar cells

The porous lead iodide layer with ordered arrays structure is firstly fabricated to facilitate the omnidirectional diffusion of organic amine salts and realize high quality large-area perovskite film in two-step sequential deposition.

Industrial-scale production of high-quality graphene sheets by millstone grinders

Graphene exhibits a variety of unprecedented innate properties and has sparked great interest in both fundamental science and regarding prospective commercial applications. To meet the ever-increasing demand for high-quality graphene sheets, an industrial-scale, reliable, environmental-friendly, low-cost production process is required. However, large-scale production high quality graphene remains elusive. Here we demonstrate a scalable mechanical cleavage method for large-quantity production of high quality large-area and few-layer graphene sheets by introducing a millstone grinding process. The average thickness of the graphene sheets is around 5 nm. This procedure is simpler than the state-of-the-art methods that allows for scalable preparation of graphene dispersion in hundreds of litres by mechanical cleavage of graphite, and the yield is 30%–40%. The size of the prepared graphene sheets can be tuneable from few micrometers to tens of micrometers by varying the dimension of raw graphite, which is larger than that produced by the state-of-the-art methods. Moreover, comparing to conductive agents, the conductivity of wafers containing graphene can be increased by one order of magnitude, suggesting a high potential of the prepared graphene sheets for the application as conductive agent in lithium battery cathodes. This allows the requirements of different sizes graphene sheets for industry applications in different fields.

Spontaneous Relaxation of Heteroepitaxial Thin Films by van der Waals-Like Bonding on Te-Terminated Sapphire Substrates.

Here the novel direct heteroepitaxial growth method of a 3D heteroepitaxial system is demonstrated on a 3D substrate, CdTe (111)/Al2 O3 (0001), which forms a spontaneous vdW-like bond at the interface, instead of the two 3D crystals being strongly bound. Despite a large lattice mismatch, the thin films are single crystals and maintain high quality due the compliance of the interface which accommodates strain. This weak bonding interface is accomplished by the self-assembly of a pseudomorphic chalcogenide layer on the sapphire surface during growth. Since the vdW-like interface forms spontaneously in situ during growth, it is easily scalable to large wafer sizes, without the need to layer transfer 2D materials onto the growth substrate for remote epitaxy. Further, the weak adhesion of the films on the substrates allow for epitaxial film transfer to a variety of other substrates, leaving the original growth substrate for future reuse. This type of 3D/3D vdW-like interface is exploitable as a compliant interlayer for additional epitaxy, and may even be observable directly in other material systems grown on complex oxides, allowing for the production of large area high quality freestanding and layer transferred epitaxial devices for material systems not currently possible by conventional processing techniques.

Protective Layers Based on Carbon Paint To Yield High-Quality Large-Area Molecular Junctions with Low Contact Resistance.

A major obstacle for transforming large-area molecular junctions into a viable technology is the deposition of a top, metallic contact over the self-assembled monolayer (SAM) without chemically damaging the molecules and preventing an interface-limited charge transport. Often a thin conducting layer is softly deposited over the SAM to protect it during the deposition of the metal electrode which requires conditions under which organic molecules are not stable. We report a new protective layer based on carbon paint which is highly conductive and has metallic-like behavior. Junctions made of SAMs of n-alkanethiolates supported by Au were characterized with both dc and ac techniques, revealing that carbon paint protective layers provide a solution to three well-known challenges in molecular junctions: series resistance of the leads, poor interface conductance, and low effective contact area related to the roughness of the interfaces. Transport is constant with coherent tunneling down to 10 K, indicating the carbon paint does not add spurious thermally activated components. The junctions have both high reproducibility and good stability against bias stressing. Finally, normalized differential conductance analysis of the tunneling characteristics of the junctions as a function of molecular length reveals that the scaling voltage changes with molecular length, indicating a significant voltage drop on the molecules rather than on the molecule–electrode interface. There is a clear inverse dependence of the scaling voltage on length, which we deduced has a tunneling barrier height of close to 2 eV. The paper establishes the reliability of carbon paint protective layers and provides a procedure for discriminating genuine molecular effects from interfacial contributions.

Large-Area Exfoliated Lead-Free Perovskite-Derivative Single-Crystalline Membrane for Flexible Low-Defect Photodetectors.

Wide applications of personal consumer electronics have tended to cause a huge demand for smart and portable electronics, featuring mechanical flexibility, lightweight, and environmental friendliness. However, most of the recently reported flexible photodetectors based on microcrystalline and amorphous components commonly suffer from severe drawbacks, including plenty of grains, boundaries, and surface defects. Here, we present a new lead-free chiral perovskite-derivative light absorber of (aminoguanidinium)3Bi2I9 (AG3Bi2I9), which displays a narrow direct band gap of about 1.89 eV. High-quality bulk single crystals were successfully grown with dimensions up to 24 × 12 × 5 mm3. Emphatically, as-grown bulk single crystals are easy to be exfoliated for large-area ultrathin wafers with an exfoliated area up to 0.6 cm2, showing promise for low-defect flexible optoelectronic applications. The remarkable surface smoothness and crystalline quality of single-crystalline thin layers were further confirmed by TEM, HRTEM, AFM, single-crystalline X-ray diffraction, and space-charge limited current (SCLC) measurements. As expected, the planar photodetectors based on flexible exfoliated wafers are first fabricated and exhibit notable photoelectric performance. This work represents an important step forward as it offers an effective strategy for the fabrication of high-quality large-area flexible exfoliated wafer devices.

Controlled Graphene Synthesis from Solid Carbon Sources

Here the authors present the investigation of controllable growth of large area high quality graphene from different solid carbon sources. A home‐made cold‐wall chamber is used for chemical vapor deposition realization to grow graphene on copper foils. It is supplied with tools for in situ monitoring the temperature along the entire sample. Monolayer graphene films are synthesized using solid poly(methyl methacrylate) (PMMA) and paraffin precursors at a growth temperature of about 850 °C and under low pressure conditions. The area of 1 × 1 cm2 appears to be covered by graphene. Raman spectroscopy and scanning electron microscopy are used to estimate the quality of graphene films fabricated and the graphene layer number.

Thickness controlled nanostructure formation in RF sputtered WS2 thin film

Large area high quality WS2 nanostructured thin films are grown on silicon at 350 °C substrate temperature using RF sputtered technique. XRD results show that the films are highly crystalline in nature with the observation of significance of 002 peak located ∼13°. FESEM images confirm that, the surface morphology can be modified easily by controlling the thickness and growth temperature of the films. Interestingly, the planar films slowly transform to nanostructure with increase of the film thickness which is also observed by AFM results. The observation of two strong Raman peak further conform the formation of WS2 film on Si substrate. Temperature dependent I–V curve behaves the films are pure semiconducting in nature and resistivity of the sample is decreased by increasing temperature. This exciting finding encourages WS2 based metal-chalcogenide is a potential candidate for its use in several semiconductor and other electronic devices.

Free-standing Three Dimensional Graphene Incorporated with Gold Nanoparticles as Novel Binder-free Electrochemical Sensor for Enhanced Glucose Detection

The electrochemical sensing performance of metal-graphene hybrid based sensor may be significantly decreased due to the dissolution and aggregation of metal catalyst during operation. For the first time, we developed a novel large-area high quality three dimensional graphene foam-incorporated gold nanoparticles (3D-GF@Au) via chemical vapor deposition method and employed as free-standing electrocatalysis for non-enzymatic electrochemical glucose detection. 3D-GF@Au based sensor is capable to detect glucose with a wide linear detection range of 2.5 μM to 11.6 mM, remarkable low detection limit of 1 μM, high selectivity, and good stability. This was resulted from enhanced electrochemical active sites and charge transfer possibility due to the stable and uniform distribution of Au NPs along with the enhanced interactions between Au and GF. The obtained results indicated that 3D-GF@Au hybrid can be expected as a high quality candidate for non-enzymatic glucose sensor application. Keywords: Gold nanoparticles, Graphene foam, Nanohybrid, Catalyst, Glucose sensor

Chemical Vapor Deposition Synthesis of Large-Area Graphene Films

Graphene films were synthesized with the method of chemical vapor deposition using gaseous methane as the source of carbon and copper foil as the substrate for the deposition. The following conditions were found optimal to grow large-area high quality graphene films: preliminary annealing of the foil in the argon/hydrogen mixture at 970–990 °С for 30–40 min; simultaneous supply of the argon/hydrogen mixture (100 cm3/min) and methane (10 cm3/min) for 5–10 minutes, and subsequent cooling in an inert atmosphere. As a result, 1–10 layered graphene films were obtained to fully coat the copper foil over the area up to 50 cm2. Several methods have been developed to transfer graphene to dielectric substrates such as silicon oxide and flexible polymer films. The obtained graphene films were used to create a flexible transparent conductive touch panel and a highly sensitive resistive humidity sensor exhibiting fast response-recovery time.

Role of Carbon Interstitials in Transition Metal Substrates on Controllable Synthesis of High-Quality Large-Area Two-Dimensional Hexagonal Boron Nitride Layers

Reliable and controllable synthesis of two-dimensional (2D) hexagonal boron nitride (h-BN) layers is highly desirable for their applications as 2D dielectric and wide bandgap semiconductors. In this work, we demonstrate that the dissolution of carbon into cobalt (Co) and nickel (Ni) substrates can facilitate the growth of h-BN and attain large-area 2D homogeneity. The morphology of the h-BN film can be controlled from 2D layer-plus-3D islands to homogeneous 2D few-layers by tuning the carbon interstitial concentration in the Co substrate through a carburization process prior to the h-BN growth step. Comprehensive characterizations were performed to evaluate structural, electrical, optical, and dielectric properties of these samples. Single-crystal h-BN flakes with an edge length of ∼600 μm were demonstrated on carburized Ni. An average breakdown electric field of 9 MV/cm was achieved for an as-grown continuous 3-layer h-BN on carburized Co. Density functional theory calculations reveal that the interstitial carbon atoms can increase the adsorption energy of B and N atoms on the Co(111) surface and decrease the diffusion activation energy and, in turn, promote the nucleation and growth of 2D h-BN.

Long-term electrical conductivity stability of graphene under uncontrolled ambient conditions

The stability of graphene under oxidative conditions is crucial to both its applications as an electrical material and the lifetime of graphene-based devices. In this work, large-area high quality graphene film was synthesized through chemical vapor deposition (CVD-G) and transferred onto SiO2/Si substrate. Based on the electrical resistance monitoring, its conductivity stability in air was excellent under uncontrolled ambient conditions within 500 days. After kept in air over years, only tiny resistance oscillation and small increase of Raman D peak intensity of CVD-G have been observed. During a year, CVD-G had the highest resistance in July with high temperature and high relative humidity (the relative resistance changes were no more than 10%). The conductivity of CVD-G returned to the normal value again in the subsequent autumn (October) due to its reversible absorptions of oxidative gas species in air. The morphology characterizations of CVD-G showed it had a well-maintained continuous structure, therefore its electrical resistance was stable.

A comparative study on macroscopic and nanoscale polarization mapping on large area PLD grown PZT thin films

By optimizing laser raster scanning conditions and controlling PbO evaporation, we have grown high quality large area PbZr.52Ti.48O3 (PZT) thin films using custom-built off-axis pulsed laser deposition (PLD) technique. The realization of high quality PZT thin films over a large area with uniform polarization is a challenging task due to the formation of non-ferroelectric or pyrochlore like PbTi3O7 phases at the processing temperature (>600°C). Our results show the polycrystalline nature of the sample with a uniform chemical surface composition. The macroscopic level of polarization mapping shows excellent saturated P-E loops with a narrow variation (6%) of the remnant polarization (2Pr) across the large area thin films. A comparative study of macroscopic and nanoscale level of mapping of polarization switching are performed, to demonstrate the existence of stoichiometry ferroelectric PZT thin films over a large area. The achievement of high quality large area PZT thin films with uniform polarization might be used for under-water SONAR devices and in power harvesting.

Large-area SnSe2/GaN heterojunction diodes grown by molecular beam epitaxy

We report on the synthesis and properties of wafer-scale two-dimensional/three-dimensional (2D/3D) n-SnSe2/n-GaN(0001) heterojunctions. The hexagonal crystal structure of crystalline SnSe2 grown by molecular beam epitaxy was confirmed via in-situ reflection high-energy electron diffraction and off-axis X-ray diffraction. Current-voltage (I-V) measurements of SnSe2/GaN diodes exhibited 9 orders of magnitude rectification, and the SnSe2/GaN heterojunction barrier height was estimated to be 1 eV using capacitance-voltage measurements and internal photoemission measurements. Vertical electronic transport analyzed using temperature-dependent I-V measurements indicates thermionic field emission transport across the junction. This work demonstrates the potential of epitaxial growth of large area high quality 2D crystals on 3D bulk semiconductors for device applications involving carrier injection across 2D/3D heterojunctions.

Source identification and method for drastic reduction of Fe contamination on wet transferred graphene

Graphene is a 2D material with extraordinary electronic and mechanical properties and has a wide range of applications. Among existing synthesis methods, chemical vapor deposition (CVD) onto Cu foil is the most prominent because it allows large area high quality graphene to be prepared at relatively low costs and then subsequently wet transferred to arbitrary substrates. Typically, PMMA and FeCl3 are used as a polymeric support layer and a metal etchant respectively during wet transfer. Unfortunately, this method leaves graphene susceptible to contamination, degrading transferred graphene's extraordinary properties. In this paper, contamination in the form of black particulates is identified to be Fe wrapped in graphene and a model of how it originates was developed. Then, a procedure to reduce this contamination during wet transfer was developed and its effectiveness was systematically studied. Transfers were characterized using Raman spectroscopy and optical microscopy with image segmentation. Experimental data obtained indicates that this simple and economic method is very effective in reducing Fe contamination on transferred CVD graphene, offering a way to improve the wet transfer process.

Consequence of oxidation method on graphene oxide produced with different size graphite precursors

Exfoliation method is a most cost effective method for producing chemically derived-graphene (CDG) yet electronic performance of CDG strongly depends on the lateral and vertical dimensions of graphene especially in regards to the development of macroscopic scale films. This work demonstrates the variation in functionality of Graphene oxide (GO) prepared via Marcano-Tour’s and modified Hummer’s method using two different sized graphite precursors. Reduced GO papers were fabricated using vacuum filtration and examined by Hall probe and Raman spectroscopy to ensure their quality. A correlation between oxidation-controlled GO size with their overall electrical conductivity is explored by Fourier Transform Infrared spectroscopy (FTIR), Thermo Gravimetric Analysis (TGA) and X-ray Diffraction (XRD) techniques. This study reveals that the modified Hummer’s method suitably preserves basal planes of GO produced from larger GO flakes with less inter-sheet resistance and illustrates the importance of comparing synthetic methods for producing large area high quality graphene related nanostructures.

The Preparation of Large-Area Single Crystal MoS2 on Quartz Substrate for Photodetector

Controllable synthesis of high quality large area monolayer MoS2 is still a challenge for its practice application. In this work, we report an Au foil assistant synthesis large size (400 μm) single crystals MoS2 on quartz substrate via chemical vapor deposition (CVD). The Raman and photoluminescence spectroscopy studies indicate the synthesized monolayer MoS2 has a high homogenous optical characteristic. The optoelectronic performance of MoS2 photodetector were also fabricated which shows a high photo-response of 550mA/W and a fast photo-response time of 230 ms. The growth techniques described here will beneficial for other atomically thin TMD materials growth.

High quality uniform YBCO film growth by the metalorganic deposition using trifluoroacetates

A need exists for the large-area superconducting YBa2Cu3O7-x (YBCO) films with high critical current density for microwave communication and/or electric power applications. Trifluoroacetic metalorganic (TFA-MOD) method is a promising low cost technique for large-scale production of YBCO films, because it does not need high vacuum device and is easily applicable to substrates of various shape and size. In this paper, double-sided YBCO films with maximum 2in diameter were prepared on LaAlO3 substrates by TFA-MOD method. Inductive critical current densitiy Jc, microwave surface resistance Rs, as well as the microstructure were characterized.A newly homemade furnace system was used to epitaxially grown YBCO films, which can improve the uniformity of YBCO film significantly by gas supply and temperature distribution proper design. Results showed that the large area YBCO films were very uniform in microstructure and thickness distribution, an average inductive Jc in excess of 6MA/cm2 with uniform distribution, and low Rs (10GHz) below 0.3mΩ at 77K were obtained. Andthe film filter may be prepared to work at temperatures lower than 74K. These results are very close to the highest value of YBCO films made by conventional vacuum method, so we show a very promising route for large-scale production of high quality large-area YBCO superconducting films at a lower cost.

Toward fast growth of large area high quality graphene using a cold-wall CVD reactor

In this work we provide a detailed analysis on graphene synthesis by Chemical Vapor Deposition (CVD) using a cold wall CVD reactor to achieve fast production of large area high quality graphene.