Wet Transfer Process Research Articles

Efficient high-throughput method utilizing neural network potentials to calculate interaction energies, validated by clean transfer experiment of CVD graphene with polymer mixtures

In this study, we designed a computational model involving energy decomposition using ground state energy minimized geometries resulting from a general-purpose neural network potential (ANI-1ccx). The numerical simulations show a distribution of energies, which indicate a two-fold reduction in interaction energy and polarity shift in electrostatic interaction, highlighting the computational novelty in exploring over a million metastable configurations. Experimentally, we validate our model by observing that using a mixture of two distinct polymers in the wet transfer process reduces transfer-induced doping and strain on transferred CVD graphene compared to the conventional single polymer wet transfer method, primarily due to decreased polymer contamination from the transfer process. This reduction is linked to the decreased interaction energy in the mixture of polymethyl methacrylate and angelica lactone polymer on graphene.

The effect of photodissociation of confined water on photoemission behaviors of monolayer MoS2

The effects of confined water on the photoluminescence of 2D MoS2 depending on the type of isotope, substrate, and surface termination were investigated. The monolayer MoS2 film showed quenched photoluminescence intensity when exposed to water, whereas it was not observed upon exposure to heavy water. This result is attributed to the overtone vibrational energy transfer between 2D MoS2 and water. Furthermore, the investigation of the photoluminescence in 2D MoS2 depending on the surface characteristics indicated that it can be controlled by the adsorbed water on the substrate. Real-time photoluminescence measurements revealed that the two types of adsorbed water affected the variation in the optical properties and that the orientation of the adsorbed water was also significant for reverting to the intrinsic 2D MoS2 by catalytic photodissociation process through exciton-to-vibronic transfer, facilitated in a confined space. Our results indicate that the substrate surface is crucial for controlling the adsorbed water formed during wet-transfer process. Therefore, this study is effective for applications that utilize large-scale two-dimensional materials.

Controllable Fabrication of Sub-10 nm Graphene Nanopores via Helium Ion Microscopy and DNA Detection.

Solid-state nanopores have become a prominent tool in the field of single-molecule detection. Conventional solid-state nanopores are thick, which affects the spatial resolution of the detection results. Graphene is the thinnest 2D material and has the highest spatial detection resolution. In this study, a graphene membrane chip was fabricated by combining a MEMS process with a 2D material wet transfer process. Raman spectroscopy was used to assess the quality of graphene after the transfer. The mechanism behind the influence of the processing dose and residence time of the helium ion beam on the processed pore size was investigated. Subsequently, graphene nanopores with diameters less than 10 nm were fabricated via helium ion microscopy. DNA was detected using a 5.8 nm graphene nanopore chip, and the appearance of double-peak signals on the surface of 20 mer DNA was successfully detected. These results serve as a valuable reference for nanopore fabrication using 2D material for DNA analysis.

Molecular dynamics simulation-based study to analyse the properties of entrapped water between gold and graphene 2D interfaces.

Heterostructures based on graphene and other 2D materials have received significant attention in recent years. However, it is challenging to fabricate them with an ultra-clean interface due to unwanted foreign molecules, which usually get introduced during their transfer to a desired substrate. Clean nanofabrication is critical for the utilization of these materials in 2D nanoelectronics devices and circuits, and therefore, it is important to understand the influence of the "non-ideal" interface. Inspired by the wet-transfer process of the CVD-grown graphene, herein, we present an atomistic simulation of the graphene-Au interface, where water molecules often get trapped during the transfer process. By using molecular dynamics (MD) simulations, we investigated the structural variations of the trapped water and the traction-separation curve derived from the graphene-Au interface at 300 K. We observed the formation of an ice-like structure with square-ice patterns when the thickness of the water film was <5 Å. This could cause undesirable strain in the graphene layer and hence affect the performance of devices developed from it. We also observed that at higher thicknesses the water film is predominantly present in the liquid state. The traction separation curve showed that the adhesion of graphene is better in the presence of an ice-like structure. This study explains the behaviour of water confined at the nanoscale region and advances our understanding of the graphene-Au interface in 2D nanoelectronics devices and circuits.

Unveiling the mechanism of surface corrugation formation on a quasi free-standing bi-layer graphene via experimental and modeling investigations

Extensive studies have been conducted to accomplish the controls over the formation of surface corrugations on chemical vapor deposited (CVD) graphene. However, the underlying mechanism of the surface corrugation formation on graphene remains elusive, due to the difficulty delineating the growth-induced surface corrugations from wrinkles formed during graphene transfer with the use of a supporting layer casted on top of the sample. In this study, “quasi free-standing” graphene was realized using the weakly-interacting h-BN layer during a wet transfer process to study the surface corrugation formation on graphene. The weak interaction of graphene with the supporting h-BN layer allows for the growth-induced surface corrugations to be released, enabling the investigation of the primary mechanism for the graphene corrugation behavior during the transfer process. The experimental measurements of surface corrugations were conducted using atomic force microscopy (AFM) to reveal that surface corrugations are formed following the graphene’s crystallographic directions. Density functional theory calculations using the projector augmented-wave (PAW) method and Perdew-Burke-Ernzerhof (PBE) exchange correlation functional were also conducted for two models of “atomic-scale ripples” and “nanoscale wrinkles” and showed that all of them are formed along the crystallographic axis of graphene, which is in good agreement with the experimental observations.

A surface plasmon resonance haemoglobin concentration sensor based on D‐type optical fibre with a graphene‐gold surface architecture

AbstractIn this study, a surface plasmon resonance sensor for detecting haemoglobin concentration based on a D‐type optical fibre with a graphene‐gold surface architecture is proposed. The graphene‐gold surface architecture included a 50 nm thick gold film and monolayer graphene film, which were decorated on a 10 mm long D‐type sensing region. The proposed sensor worked in wavelength interrogation mode, with the light wavelength ranging from 400 to 1100 nm. The authors realised the D‐type optical fibre surface polishing process, the gold film vacuum coating process, and the chemical‐vapour‐deposited graphene's wet transfer process. Furthermore, the fabricated sensors were used to detect the refractive index (RI) of haemoglobin samples, which varied from 1.331 to 1.346. Experiment results show that the fitted RI sensitivity of the sensor decorated with gold and graphene reaches 1874.41 nm/RIU, 4.995% higher than that of the sensor decorated only with gold. The concentration sensitivity of the sensor coated with gold and graphene film is 4.96 nm/(g/dL), and the proposed sensor can provide a resolution of 20.2 mg/dL for haemoglobin concentration detection.

Electrical performance of monolayer MoS2 transistor with MoS2 nanobelt metallic edges as electrodes

The contact electrodes have great influence on the performance of monolayer MoS2 devices. In this paper, monolayer MoS2 and MoS2 nanobelts were synthesized on SiO2/Si substrates via the chemical vapor deposition method. By using wet and dry transfer process, MoS2 nanobelt metallic edges were designed as the source/drain contact electrodes of monolayer MoS2 field effect transistor. The ‘nanobelt metallic edges’ refers to the top surface of the nanobelt being metallic. Because the base planes of MoS2 nanobelt vertically stand on the substrate, which makes the layer edges form the top surface of the nanobelt. The nonlinear I ds–V ds characteristics of the device indicates that the contact between the monolayer MoS2 and MoS2 metallic edges displays a Schottky-like behavior. The back-gated transfer characteristics indicate that monolayer MoS2 device with MoS2 nanobelt metallic edges as electrodes shows an n-type behavior with a mobility of ∼0.44 cm2 V−1·s−1, a carrier concentration of ∼7.31 × 1011 cm−2, and an on/off ratio of ∼103. The results enrich the electrode materials of two-dimensional material devices and exhibit potential for future application of MoS2 metallic edges in electronic devices.

A Simple Method to Produce an Aluminum Oxide-Passivated Tungsten Diselenide/n-Type Si Heterojunction Solar Cell with High Power Conversion Efficiency

Transition metal dichalcogenide (TMDC) materials are attractive candidates for 2D solar cell devices thanks to their straightforward integration with various substrates and traditional semiconductor technologies, wide band gap ranges over the visible light spectrum, and high absorption coefficient values. Although there are several previous reports on the fabrication of 2D material-based solar cells, difficult and complex processes in the fabrication are highly required to be modified for wider use in daily life applications. Photolithography, the most commonly used manufacturing process for TMDC-based solar cells, is complicated. In this study, we demonstrate that the fabrication of 2D tungsten diselenide (WSe2) by adopting a wet transfer process with thermal release tape simplifies the manufacturing steps for TMDC-based solar cell devices. This simplification not only reduces the production cost by excluding several factors such as transmittance, thermal expansion, surface flatness, and durability but also improves the yield. Furthermore, a proof-of-concept demonstration of creating a WSe2/Si junction with an aluminum oxide (Al2O3) antireflective coating provided a power conversion efficiency of 6.39%, which is a significant improvement over that of a WSe2/Si solar cell without the antireflective coating layer (1.08%).

Effects of post-transfer annealing and substrate interactions on the photoluminescence of 2D/3D monolayer WS2/Ge heterostructures.

The ultraflat and dangling bond-free features of two-dimensional (2D) transition metal dichalcogenides (TMDs) endow them with great potential to be integrated with arbitrary three-dimensional (3D) substrates, forming mixed-dimensional 2D/3D heterostructures. As examples, 2D/3D heterostructures based on monolayer TMDs (e.g., WS2) and bulk germanium (Ge) have become emerging candidates for optoelectronic applications, such as ultrasensitive photodetectors that are capable of detecting broadband light from the mid-infrared (IR) to visible range. Currently, the study of WS2/Ge(100) heterostructures is in its infancy and it remains largely unexplored how sample preparation conditions and different substrates affect their photoluminescence (PL) and other optoelectronic properties. In this report, we investigated the PL quenching effect in monolayer WS2/Ge heterostructures prepared via a wet transfer process, and employed PL spectroscopy and atomic force microscopy (AFM) to demonstrate that post-transfer low-pressure annealing improves the interface quality and homogenizes the PL signal. We further studied and compared the temperature-dependent PL emissions of WS2/Ge with those of as-grown WS2 and WS2/graphene/Ge heterostructures. The results demonstrate that the integration of WS2 on Ge significantly quenches the PL intensity (from room temperature down to 80 K), and the PL quenching effect becomes even more prominent in WS2/graphene/Ge heterostructures, which is likely due to synergistic PL quenching effects induced by graphene and Ge. Density functional theory (DFT) and Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations show that the interaction of WS2 and Ge is stronger than in adjacent layers of bulk WS2, thus changing the electronic band structure and making the direct band gap of monolayer WS2 less accessible. By understanding the impact of post-transfer annealing and substrate interactions on the optical properties of monolayer TMD/Ge heterostructures, this study contributes to the exploration of the processing-properties relationship and may guide the future design and fabrication of optoelectronic devices based on 2D/3D heterostructures of TMDs/Ge.

Origin of Tearing Paths in Transferred Graphene by H2 Bubbling Process and Improved Transfer of T ear-Free Graphene Films U sing a H eat Press

Among efforts to improve techniques for the chemical vapor deposition of large-area and high-quality graphene films on transition metal substrates, being able to reliably transfer these atomistic membranes onto the desired substrate is a critical step for various practical uses, such as graphene-based electronic and photonic devices. However, the most used approach, the wet etching transfer process based on the complete etching of metal substrates, remains a great challenge. This is mainly due to the inevitable damage to the graphene, unintentional contamination of the graphene layer, and increased production cost and time. Here, we report the systematic study of an H2 bubbling-assisted transfer technique for graphene films grown on Cu foils, which is nondestructive not only to the graphene film but also to the Cu substrate. Also, we demonstrate the origin of the graphene film tearing phenomenon induced by this H2 bubbling-assisted transfer process. This study reveals that inherent features are produced by rolling Cu foil, which cause a saw-like corrugation in the poly(methyl methacrylate) (PMMA)/graphene stack when it is transferred onto the target substrate after the Cu foil is dissolved. During the PMMA removal stage, the graphene tearing mainly appears at the apexes of the corrugated PMMA/graphene stack, due to weak adhesion to the target substrate. To address this, we have developed a modified heat-press-assisted transfer technique that has much better control of both tearing and the formation of residues in the transferred graphene films.

Surface Properties of CVD-Grown Graphene Transferred by Wet and Dry Transfer Processes.

Graphene, an atomically thin material, has unique electrical, mechanical, and optical properties that can enhance the performance of thin film-based flexible and transparent devices, including gas sensors. Graphene synthesized on a metallic catalyst must first be transferred onto a target substrate using wet or dry transfer processes; however, the graphene surface is susceptible to chemical modification and mechanical damage during the transfer. Defects on the graphene surface deteriorate its excellent intrinsic properties, thus reducing device performance. In this study, the surface properties of transferred graphene were investigated according to the transfer method (wet vs. dry) and characterized using atomic force microscopy, Raman spectroscopy, and contact angle measurements. After the wet transfer process, the surface properties of graphene exhibited tendencies similar to the poly(methyl methacrylate) residue remaining after solvent etching. The dry-transferred graphene revealed a surface closer to that of pristine graphene, regardless of substrates. These results provide insight into the utilization of wet and dry transfer processes for various graphene applications.

Wrinkle-Free Graphene Films on Fluorinated Self-Assembled Monolayer-Modified Substrates for Enhancing the Electrical Performance of Transistors

Self-assembled monolayer (SAM)-functionalized substrates are widely used for tailoring the electronic properties (i.e., p- and n-type doping) of two-dimensional materials, which might suppress the charge scattering, impurities, and wrinkles that can severely decline electrical properties. The fluorinated self-assembled monolayer (FSAM) is also used for promoting electrical properties by eliminating the small number of water molecules/residues between the substrate and graphene during the typical wet transfer process. However, taking graphene as an example, most of the graphene/SAM or graphene/FSAM studies are focused on the enhancement of their electrical properties and lack investigation on their surface morphology after transfer and the correlation between them. Herein, a strategy is discovered in which FSAM-modified substrates help construct a wrinkle-free graphene film with the dry transfer process due to the low surface energy between the graphene and FSAM. Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FDTS) was selected as a precursor toward the formation of the monolayer and highly uniform FSAM using a facile dip-coating route, which is feasible on versatile substrates such as Si, SiO2/Si, and poly(ethylene terephthalate) (PET). The surface roughness of the FSAM-modified SiO2 substrate reduces from 2.98 to 0.13 nm. Therefore, the lower surface energy (from 60.80 to 8.12 mN/m) was found to enhance the carrier mobility of the transferred graphene by about 1.77 times, increasing from 894.6 to 1588 cm2/(V·s). Furthermore, a top-gated field-effect transistor based on graphene/FSAM was fabricated and characterized in which the FSAM decouples from the strong substrate interference and significantly improved the field-effect mobility by up to 17 times compared to that of graphene without substrate modification. Thus, this work provides a facile strategy to avoid wrinkle defects and suppress charge scattering through the decoupling from the substrate, which could be applied to other two-dimensional (2D) materials for improving their electrical performance on transistors and flexible electronics.

Scalable and Versatile Transfer of Sensitive Two-dimensional Materials.

Damage-free transfer of large-area two-dimensional (2D) materials is indispensable to unleash their full potentials in a wide range of electronic, photonic, and biochemical applications. However, the all-surface nature of 2D materials renders many of them vulnerable to surrounding environments, especially etchants and water involved during wet transfer process. Up to now, a scalable and damage-free transfer method for sensitive 2D materials is still lacking. Here, we report a general damage-free transfer method for sensitive 2D materials. The as-transferred 2D materials exhibit well-preserved structural integrity and unaltered physical properties. We further develop a facile TEM sample preparation technique that allows direct recycling of materials on TEM grids with high fidelity. This recycling technique provides an unprecedented opportunity to precisely relate structural characterization with physical/chemical/electrical probing for the same samples. This method can be readily generalized to diverse nanomaterials for large-area damage-free transfer and enables in-depth investigation of structure-property relationship.

Defects Produced during Wet Transfer Affect the Electrical Properties of Graphene.

Graphene has been widely used due to its excellent electrical, mechanical and chemical properties. Defects produced during its transfer process will seriously affect the performance of graphene devices. In this paper, single-layer graphene was transferred onto glass and silicon dioxide (SiO2) substrates by wet transfer technology, and the square resistances thereof were tested. Due to the different binding forces of the transferred graphene surfaces, there may have been pollutants present. PMMA residues, graphene laminations and other defects that occurred in the wet transfer process were analyzed by X-ray photoelectron spectroscopy and Raman spectroscopy. These defects influenced the square resistance of the produced graphene films, and of these defects, PMMA residue was the most influential; square resistance increased with increasing PMMA residue.

Piezo-resistive effects of Area-arrayed Dumbbell-Shape Graphene Nano-ribbon Structure for Highly Sensitive Micro-Scale Tactile Sensor

In order to develop a highly-sensitive small-scale tactile sensor, piezoresistive effect of graphene nanoribbon was applied to the sensor. It was confirmed that the graphene nanoribbon narrower than about 70 nm showed semiconductive properties and thus, large piezo-resistive constant. The dumbbell shape was applied to the stable junction between metallic graphene and semiconductive graphene. It was easy to form stable ohmic contact between the metallic graphene and metallic electrode. An area-arrayed nanoscale GNR structure with varying width was fabricated for a highly sensitive tactile sensor. Graphene was synthesized inside a copper pocket using a low-pressure chemical vapour deposition (LP-CVD) method. PMMA- assisted wet transfer process was used to transfer graphene grown on the copper to the target substrate. Then, the electrodes were deposited using electron-beam deposition. A four-point bending test was carried out on the sensor to determine the piezo-resistive effect of the DS-GNR. Finally, the gauge factor was determined under the application of various amounts of strain.

New leak element based on transfer-free single-layer graphene membrane

In this work, a new leak element based on transfer-free single-layer graphene was developed. The single-layer graphene was synthesized on Cu foil and small holes of about 10 μm were drilled on Cu foil to reveal graphene membrane. The conductance of He, N2 and Ar through the graphene leak element was measured based on the difference method. The results imply that various gases exhibit molecular flow regime under pressures ranging from vacuum up to 105 Pa, and the conductance of the transfer-free graphene is about two orders of magnitude smaller than that of the wet-transferred graphene. The difference of conductance may result from the generation of micron-sized cracks and tears on graphene surface during wet-transfer process. The transfer-free single-layer graphene developed in this work is an excellent material for low-permeation leak element.

Residue-free photolithographic patterning of graphene

Photolithography of graphene using an organic photoresist (PR), whereby the graphene film is patterned into a desired geometry, is required for the fabrication of graphene-based sensors and electronic devices. However, because the PR hardens during the O2 plasma etching process, its complete removal is difficult. Herein, we present a new route to obtain residue-free patterned graphene by introducing a buffer layer between graphene and the PR. The modified photolithography process allows complete removal of the hardened PR and eliminates poly(methyl methacrylate) residues on the graphene surface during the wet transfer process. Density functional theory calculations revealed that the binding strength between the buffer layer and graphene is weaker than that between the PR and graphene. This led to a clean graphene surface without PR and other organic residues. The graphene films patterned using the modified photolithography process are of superior quality; they have the lowest surface roughness, natural hydrophobicity, and the highest transmittance of 94.1% at the wavelength of 550 nm. In addition, graphene-based field-effect transistors fabricated using the new process present the electron mobility of 800 cm2 V−1 s−1, and their charge neutrality point is located near zero bias, indicating that the unintended doping effect was effectively eliminated. This work demonstrates a new process that is compatible with silicon-based nanosystems and can be readily applied in the mass production of residue-free graphene electronics.

Influence of graphene oxide content on the morphology and properties of carbon fiber/epoxy composites

AbstractIn this article, the influence of graphene oxide (GO) content on the morphology and properties of carbon fiber/epoxy (CF/EP) composites was investigated. To obtain CF/EP composites with uniform dispersion of GO, electromagnetic stirring and ultrasonic dispersion techniques were employed in the wet transfer process. Combined with a new molding process, CF/EP composites with different GO content were prepared. The test results showed that the bending strength of CF/EP composites could be effectively improved by adding GO, among which 0.5 wt% GO–CF/EP composite exhibited the best bending strength and modulus increased by 47.20% and 21.24%, up to 1158.9 MPa and 115.3 GPa. Likewise, the mode I interlaminar fracture toughness (GIC) of the composite at the beginning of crack and the maximum during the propagation increased by 28.13% and 21.57%, reached 327.5 and 656.6 J/m2, respectively. The distribution of GO in the composites and the fracture morphology of the composites were observed. Finally, according to the test results and microscopic morphology analysis, the enhancement and failure mechanism of GO in CF/EP composites were clarified. It was found that GO could improve the properties of CF/EP composites by enhancing the interface properties of CF/matrix, enhancing the strength of the matrix and forming a net‐like structure inside the matrix.

Solubility of Monolayer MoS2 and Expected Bioabsorbable LED

Transient electronic technology is a new technology, which is characterized by the ability to dissolve, decompose in a controlled way. Expected adhibitions include bioabsorbable or biodegradable medical implants, hardware-safe storage devices and biodegradable sensors.2D materials may play a vital role in these systems due to their unique electrical, thermal, mechanical and optical properties. Monolayer MoS2 is a recently discovered biosoluble two-dimensional material. Monolayer MoS2 has unique semiconductor characteristics and biological absorption properties. Single-layer MoS2 is a direct band gap semiconductor material, and it has a high electron mobility, good current switching ratio, very low power consumption. These characteristics of single-layer MoS2 make it have great potential in electronic/optoelectronic devices, sensors, photothermal therapeutics, and biomedical applications. In this aspect, the ability to dissolve single-layer MoS2 in the biological liquid can be used to regulate the performance and life of the relevant bio-absorbable devices and systems. Based on monolayer MoS2, we designed a bio-absorbable LED. Furthermore, the wet transfer process of single layer MoS2 was optimized to some extent.

Monolayer Graphene Transfer onto Hydrophilic Substrates: A New Protocol Using Electrostatic Charging.

In the present work, we developed a novel method for transferring monolayer graphene onto four different commercial hydrophilic micro/ultra-filtration substrates. The developed method used electrostatic charging to maintain the contact between the graphene and the target substrate intact during the etching step through the wet transfer process. Several measurement/analysis techniques were used in order to evaluate the properties of the surfaces and to assess the quality of the transferred graphene. The techniques included water contact angle (CA), atomic force microscopy (AFM), and field emission scanning electron microscopy (FESEM). Potassium chloride (KCl) ions were used for the transport study through the developed graphene-based membranes. The results revealed that 70% rejection of KCI ions was recorded for the graphene/polyvinylidene difluoride (PVDF1) membrane, followed by 67% rejection for the graphene/polyethersulfone (PES) membrane, and 65% rejection for graphene/PVDF3 membrane. It was revealed that the smoothest substrate was the most effective in rejecting the ions. Although defects such as tears and cracks within the graphene layer were still evolving in this new transfer method, however, the use of Nylon 6,6 interfacial polymerization allowed sealing the tears and cracks within the graphene monolayer. This enhanced the KCl ions rejection of up to 85% through the defect-sealed graphene/polymer composite membranes.