Atomic structure of graphene on SiO2

Manipulating 2D Materials through Strain Engineering

This review explores the growing interest in 2D layered materials, such as graphene, h-BN, transition metal dichalcogenides (TMDs), and black phosphorus (BP), with a specific focus on recent advances in strain engineering. Both experimental and theoretical results are delved into, highlighting the potential of strain to modulate physical properties, thereby enhancing device performance. Various strain engineering methods are summarized, and the impact of strain on the electrical, optical, magnetic, thermal, and valleytronic properties of 2D materials is thoroughly examined. Finally, the review concludes by addressing potential applications and challenges in utilizing strain engineering for functional devices, offering valuable insights for further research and applications in optoelectronics, thermionics, and spintronics.

Clean Transfer of Two-Dimensional Materials: A Comprehensive Review

The growth of two-dimensional (2D) materials through chemical vapor deposition (CVD) has sparked a growing interest among both the industrial and academic communities. The interest stems from several key advantages associated with CVD, including high yield, high quality, and high tunability. In order to harness the application potentials of 2D materials, it is often necessary to transfer them from their growth substrates to their desired target substrates. However, conventional transfer methods introduce contamination that can adversely affect the quality and properties of the transferred 2D materials, thus limiting their overall application performance. This review presents a comprehensive summary of the current clean transfer methods for 2D materials with a specific focus on the understanding of interaction between supporting layers and 2D materials. The review encompasses various aspects, including clean transfer methods, post-transfer cleaning techniques, and cleanliness assessment. Furthermore, it analyzes and compares the advances and limitations of these clean transfer techniques. Finally, the review highlights the primary challenges associated with current clean transfer methods and provides an outlook on future prospects.

Dielectric engineered graphene transistors for high-performance near-infrared photodetection

Graphene, known for its ultrahigh carrier mobility and broadband optical absorption, holds significant potential in optoelectronics. However, the carrier mobility of graphene on silicon substrates experienced a marked decrease due to surface roughness, phonon scattering affects. Here we report carrier mobility enhancement of graphene dielectric engineering. Through the fabrication of devices utilizing Si/SiO2/Al2O3/graphene layers and subsequent electrical characterization, our findings illustrate the navigable nature of the Al2O3 dielectric layer is navigable for reducing the SiO2 phonon scattering and increasing graphene's carrier mobility by up to ∼8000 cm2V-1s-1. Furthermore, the improvement in carrier mobility of graphene has been utilized in the hybrid near-infrared photodetector, resulting in outstanding responsivity of ∼400 AW-1, detectivity of ∼2.2 ✕ 1011 Jones in the graphene/Ag2Te detector. Our study establishes pathways for the seamless integration of graphene or other 2D materials within the standard CMOS processes, thereby facilitating the fabrication of advanced optoelectronic devices.

Electrochemical and Electrical Biosensors for Wearable and Implantable Electronics Based on Conducting Polymers and Carbon-Based Materials

Bioelectronic devices are designed to translate biological information into electrical signals and vice versa, thereby bridging the gap between the living biological world and electronic systems. Among different types of bioelectronics devices, wearable and implantable biosensors are particularly important as they offer access to the physiological and biochemical activities of tissues and organs, which is significant in diagnosing and researching various medical conditions. Organic conducting and semiconducting materials, including conducting polymers (CPs) and graphene and carbon nanotubes (CNTs), are some of the most promising candidates for wearable and implantable biosensors. Their unique electrical, electrochemical, and mechanical properties bring new possibilities to bioelectronics that could not be realized by utilizing metals- or silicon-based analogues. The use of organic- and carbon-based conductors in the development of wearable and implantable biosensors has emerged as a rapidly growing research field, with remarkable progress being made in recent years. The use of such materials addresses the issue of mismatched properties between biological tissues and electronic devices, as well as the improvement in the accuracy and fidelity of the transferred information. In this review, we highlight the most recent advances in this field and provide insights into organic and carbon-based (semi)conducting materials' properties and relate these to their applications in wearable/implantable biosensors. We also provide a perspective on the promising potential and exciting future developments of wearable/implantable biosensors.

First-principles study of highly sensitive graphene/hexagonal boron nitride heterostructures for application in toxic gas-sensing devices

Graphene-based sensors exhibit high sensitivity, fast response, and good selectivity towards toxic gases but have low mechanical stability. The combination of graphene and two-dimensional hexagonal boron nitride (h-BN) is expected to increase the mechanical stability and enhance the adsorption performance of these gas sensors. Using first-principles calculations, we demonstrate that two-dimensional graphene/h-BN double layers can be used as good substrates for gas sensors with a small lattice mismatch of only 1.78%. Moreover, the presence of a h-BN layer widens the band gap by about 38 meV and considerably increases the work function, thus positively affecting the gas adsorption performance. Although these graphene/h-BN heterostructures do not change the physical adsorption mechanism of these sensors concerning the graphene-based materials, these bilayers significantly enhance the sensitivity of these sensors for detecting CO2, CO, NO, and NO2 toxic gases. Particularly, compared to the pristine graphene-based materials, the gas adsorption energies of graphene/h-BN increased by up to 13.78% for the adsorption of NO, and the shortest distances between the graphene/h-BN substrates and adsorbed gas molecules decreased. We also show that the graphene/h-BN heterostructure is more selective towards NOx gases while more inert towards COx gases, based on the different amounts of charge transferred from the substrate to the adsorbed gas molecules. Using the non-equilibrium Green functions in the context of density functional theory, we quantitatively associated these charge transfers with the reduction of the current passing through these scattering regions. These results demonstrate that graphene/h-BN heterostructures can be exploited as highly sensitive and selective room-temperature gas sensors for detecting toxic gases.

Two-Legged Molecular Walker and Curvature: Mechanochemical Ring Migration on Graphene

Attaining controllable molecular motion at the nanoscale can be beneficial for multiple reasons, spanning from optoelectronics to catalysis. Here we study the movement of a two-legged molecular walker by modeling the migration of a phenyl aziridine ring on curved graphene. We find that directional ring migration can be attained on graphene in the cases of both 1D (wrinkled/rippled) and 2D (bubble-shaped) curvature. Using a descriptor approach based on graphene's frontier orbital orientation, we can understand the changes in binding energy of the ring as it translates across different sites with variable curvature and the kinetic barriers associated with ring migration. Additionally, we show that the extent of covalent bonding between graphene and the molecule at different sites directly controls the binding energy gradient, propelling molecular migration. Importantly, one can envision such walkers as carriers of charge and disruptors of local bonding. This study enables a new way to tune the electronic structure of two-dimensional materials for a range of applications.

Electric Field Effects on Curved Graphene Quantum Dots

The recent and continuous research on graphene-based systems has opened their usage to a wide range of applications due to their exotic properties. In this paper, we have studied the effects of an electric field on curved graphene nanoflakes, employing the Density Functional Theory. Both mechanical and electronic analyses of the system have been made through its curvature energy, dipolar moment, and quantum regeneration times, with the intensity and direction of a perpendicular electric field and flake curvature as parameters. A stabilisation of non-planar geometries has been observed, as well as opposite behaviours for both classical and revival times with respect to the direction of the external field. Our results show that it is possible to modify regeneration times using curvature and electric fields at the same time. This fine control in regeneration times could allow for the study of new phenomena on graphene.

Superlubricity and Stress-Shielding of Graphene Enables Ultra Scratch-Resistant Glasses

Glasses, when subjected to scratch loading, incur damages affecting their optical and mechanical integrity. Here, it is demonstrated that silica glasses protected with mechanically exfoliated few-layer graphene sheets can exhibit remarkable improvement in scratch resistance. To this extent, the friction and wear characteristics of silica glasses with exfoliated graphene using atomic force microscopy (AFM) are explored. The friction forces recorded during AFM scratch tests of the graphene-glass surfaces at multiple loads exhibit ∼98% reduction compared to that of the bare silica glass, with the friction coefficient falling in the superlubricity regime. This dramatic reduction in friction achieved by the graphene sheets results in significantly lower wear of the graphene-glass surfaces postscratching. Further investigations employing atomistic simulations reveal that the stress-shielding mechanism is due to the reduced deformation of graphene-glass surfaces, thereby curtailing the overall damage. Altogether, the present work provides a new fillip toward the development of glasses with enhanced scratch resistance exploiting two-dimensional coatings.

Critical Point Drying of Graphene Field-Effect Transistors Improves Their Electric Transport Characteristics

A critical point drying (CPD) technique is reported with supercritical CO2 as a cleaning step for graphene field-effect transistors (GFETs) microfabricated on oxidized Si wafers, which results in an increase of the field-effect mobility and a decrease of the impurity doping. It is shown that the polymeric residues remaining on graphene after the transfer process and device microfabrication are significantly reduced after the CPD treatment. Moreover, the CPD effectively removes ambient adsorbates such as water therewith reducing the undesirable p-type doping of the GFETs. It is proposed that CPD of electronic, optoelectronic, and photonic devices based on 2D materials as a promising technique to recover their intrinsic properties after the microfabrication in a cleanroom and after storage at ambient conditions.

Nanoprocessing of Self-Suspended Monolayer Graphene and Defect Formation by Femtosecond-Laser Irradiation

We demonstrate the femtosecond-laser processing of self-suspended monolayer graphene grown by chemical vapor deposition, resulting in multipoint drilling with holes having a diameter of <100 nm. Scanning transmission electron microscopy revealed the formation of many nanopores on the laser-irradiated graphene. Furthermore, atomic-level defects as well as nanopores were found in the graphene membrane by high-resolution transmission electron microscopy, while the overall crystal structure remained intact. Raman spectroscopy showed an increase in the defect density with an increase in the number of laser shots, suggesting that the nanopore formation triggered the creation of the <100 nm holes. The approach presented herein can offer an experimental insight into the simulation of atomic dynamics in graphene under femtosecond-laser irradiation. The thorough examination of the atomic defect formation and secondary effect of surface cleaning observed in this study would help develop engineering methods for graphene and other two-dimensional materials in the future.

Identification of Ubiquitously Present Polymeric Adlayers on 2D Transition Metal Dichalcogenides

The interest in 2D materials continues to grow across numerous scientific disciplines as compounds with unique electrical, optical, chemical, and thermal characteristics are being discovered. All these properties are governed by an all-surface nature and nanoscale confinement, which can easily be altered by extrinsic influences, such as defects, dopants or strain, adsorbed molecules, and contaminants. Here, we report on the ubiquitous presence of polymeric adlayers on top of layered transition metal dichalcogenides (TMDs). The atomically thin layers, not evident from common analytic methods, such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), or scanning electron microscopy (SEM), could be identified with highly resolved time-of-flight secondary ion mass spectrometry (TOF-SIMS). The layers consist of hydrocarbons, which preferentially adsorb to the hydrophobic van der Waals surfaces of TMDs, derived from the most common methods. Fingerprint fragmentation patterns enable us to identify certain polymers and link them to those used during preparation and storage of the TMDs. The ubiquitous presence of polymeric films on 2D materials has wide reaching implications for their investigation, processing, and applications. In this regard, we reveal the nature of polymeric residues after commonly used transfer procedures on MoS₂ films and investigate several annealing procedures for their removal.

Theoretical studies on optical properties of Beltrami-shaped curved graphene

We compute the optical conductivity and polarization for an out-of-plane deformation in graphene nanostructure using a theoretical approach based on Dirac equation solutions on curved2+1dimensional space-time, where the space part is considered to correspond to the Beltrami pseudosphere which belongs to the family of surfaces with negative constant Gaussian curvature. We found that different parameters of deformation along one direction translate into an enhancement of the optical conductivity peaks and polarization magnitude in the far-infrared frequencies. This allows for a very high degree of polarization with a single layer graphene and opens up a potential prospect of employing graphene layer as efficient polarizers. Therefore the experimental predictions related to the electronic configuration of the corresponding graphene-like sample may be explicitly worked out.

Thickness measurements of graphene oxide flakes using atomic force microscopy: results of an international interlaboratory comparison

Flake thickness is one of the defining properties of graphene-related 2D materials (GR2Ms), and therefore requires reliable, accurate, and reproducible measurements with well-understood uncertainties. This is needed regardless of the production method or manufacturer because it is important for all GR2M products to be globally comparable. An international interlaboratory comparison on thickness measurements of graphene oxide flakes using atomic force microscopy has been completed in technical working area 41 of versailles project on advanced materials and standards. Twelve laboratories participated in the comparison project, led by NIM, China, to improve the equivalence of thickness measurement for two-dimensional flakes. The measurement methods, uncertainty evaluation and a comparison of the results and analysis are reported in this manuscript. The data and results of this project will be directly used to support the development of an ISO standard.

Antimonene: a tuneable post-graphene material for advanced applications in optoelectronics, catalysis, energy and biomedicine

The post-graphene era is undoubtedly marked by two-dimensional (2D) materials such as quasi-van der Waals antimonene. This emerging material has a fascinating structure, exhibits a pronounced chemical reactivity (in contrast to graphene), possesses outstanding electronic properties and has been postulated for a plethora of applications. However, chemistry and physics of antimonene remain in their infancy, but fortunately recent discoveries have shed light on its unmatched allotropy and rich chemical reactivity offering a myriad of unprecedented possibilities in terms of fundamental studies and applications. Indeed, antimonene can be considered as one of the most appealing post-graphene 2D materials reported to date, since its structure, properties and applications can be chemically engineered from the ground up (both using top-down and bottom-up approaches), offering an unprecedented level of control in the realm of 2D materials. In this review, we provide an in-depth analysis of the recent advances in the synthesis, characterization and applications of antimonene. First, we start with a general introduction to antimonene, and then we focus on its general chemistry, physical properties, characterization and synthetic strategies. We then perform a comprehensive study on the allotropy, the phase transition mechanisms, the oxidation behaviour and chemical functionalization. From a technological point of view, we further discuss the applications recently reported for antimonene in the fields of optoelectronics, catalysis, energy storage, cancer therapy and sensing. Finally, important aspects such as new scalable methodologies or the promising perspectives in biomedicine are discussed, pinpointing antimonene as a cutting-edge material of broad interest for researchers working in chemistry, physics, materials science and biomedicine.

Carbon Nanostructure Embedded Novel Sensor Implementation for Detection of Aromatic Volatile Organic Compounds: An Organized Review

For field-like environmental gas monitoring and noninvasive illness diagnostics, effective sensing materials with exceptional sensing capabilities of sensitive, quick detection of volatile organic compounds (VOCs) are required. Carbon-based nanomaterials (CNMs), like CNTs, graphene, carbon dots (Cdots), and others, have recently drawn a lot of interest for their future application as an elevated-performance sensor for the detection of VOCs. CNMs have a greater potential for developing selective sensors that target VOCs due to their tunable chemical and surface properties. Additionally, the mechanical versatility of CNMs enables the development of novel gas sensors and places them ahead of other sensing materials for wearable applications. An overview of the latest advancements in the study of CNM-based sensors is given in this comprehensive organized review.

Gaussian Curvature Effects on Graphene Quantum Dots

In the last few years, much attention has been paid to the exotic properties that graphene nanostructures exhibit, especially those emerging upon deforming the material. Here we present a study of the mechanical and electronic properties of bent hexagonal graphene quantum dots employing density functional theory. We explore three different kinds of surfaces with Gaussian curvature exhibiting different shapes-spherical, cylindrical, and one-sheet hyperboloid-used to bend the material, and several boundary conditions regarding what atoms are forced to lay on the chosen surface. In each case, we study the curvature energy and two quantum regeneration times (classic and revival) for different values of the curvature radius. A strong correlation between Gaussian curvature and these regeneration times is found, and a special divergence is observed for the revival time for the hyperboloid case, probably related to the pseudo-magnetic field generated by this curvature being capable of causing a phase transition.

Multiscale compression-induced restructuring of stacked lipid bilayers: From buckling delamination to molecular packing

Lipid membranes in nature adapt and reconfigure to changes in composition, temperature, humidity, and mechanics. For instance, the oscillating mechanical forces on lung cells and alveoli influence membrane synthesis and structure during breathing. However, despite advances in the understanding of lipid membrane phase behavior and mechanics of tissue, there is a critical knowledge gap regarding the response of lipid membranes to micromechanical forces. Most studies of lipid membrane mechanics use supported lipid bilayer systems missing the structural complexity of pulmonary lipids in alveolar membranes comprising multi-bilayer interconnected stacks. Here, we elucidate the collective response of the major component of pulmonary lipids to strain in the form of multi-bilayer stacks supported on flexible elastomer substrates. We utilize X-ray diffraction, scanning probe microscopy, confocal microscopy, and molecular dynamics simulation to show that lipid multilayered films both in gel and fluid states evolve structurally and mechanically in response to compression at multiple length scales. Specifically, compression leads to increased disorder of lipid alkyl chains comparable to the effect of cholesterol on gel phases as a direct result of the formation of nanoscale undulations in the lipid multilayers, also inducing buckling delamination and enhancing multi-bilayer alignment. We propose this cooperative short- and long-range reconfiguration of lipid multilayered films under compression constitutes a mechanism to accommodate stress and substrate topography. Our work raises fundamental insights regarding the adaptability of complex lipid membranes to mechanical stimuli. This is critical to several technologies requiring mechanically reconfigurable surfaces such as the development of electronic devices interfacing biological materials.

Graphene delamination from chemical vapor deposited turbostratic multilayer graphene for TEM analysis

Turbostratic multilayer graphene (MLG) is of great interest due to its unique electronic properties resulting from a linear band dispersion at the K point, which is similar to that of single-layer graphene. The band structure is derived from the stacking structure of turbostratic MLG where graphene layers have random in-plane rotations with respect to each other. Although wafer-scale growth of turbostratic MLG has been demonstrated, the crystallinity of individual graphene layers is still challenging to investigate. In this study, we present a new approach to reveal the grain structure of turbostratic MLG by transmission electron microscopy (TEM) observation. Mechanical delamination is demonstrated for the chemical vapor deposited MLG to peel off the topmost graphene layers by using a polydimethylsiloxane sheet. Micrometer-scale patterning of the MLG prior to the delamination is found to be effective to obtain graphene films with the designed shape and arrangement. Furthermore, the delaminated graphene films are successfully transferred onto a TEM grid, enabling us to estimate the grain size of the turbostratic MLG. This method is potentially applicable for not only preparing samples but also fabricating vertically stacked heterostructure devices using 2D materials.

Au3-Decorated graphene as a sensing platform for O2 adsorption and desorption kinetics

The adsorption and desorption kinetics of molecules is of significant fundamental and applied interest. In this paper, we present a new method to quantify the energy barriers for the adsorption and desorption of gas molecules on few-atom clusters, by exploiting reaction induced changes of the doping level of a graphene substrate. The method is illustrated for oxygen adsorption on Au₃ clusters. The gold clusters were deposited on a graphene field effect transistor and exposed to O₂. From the change in graphene's electronic properties during adsorption, the energy barrier for the adsorption of O₂ on Au₃ is estimated to be 0.45 eV. Electric current pulses increase the temperature of the graphene strip in a controlled way and provide the required thermal energy for oxygen desorption. The oxygen binding energy on Au₃/graphene is found to be 1.03 eV and the activation entropy is 1.4 meV K^-1. The experimental values are compared and interpreted on the basis of density functional theory calculations of the adsorption barrier, the binding energy and the activation entropy. The large value of the activation entropy is explained by the hindering effect that the adsorbed O₂ has on the fluxional motion of the Au₃ cluster.

Simulation of Figures of Merit for Barristor Based on Graphene/Insulator Junction

We investigated the tunneling of graphene/insulator/metal heterojunctions by revising the Tsu-Esaki model of Fowler-Nordheim tunneling and direct tunneling current. Notably, the revised equations for both tunneling currents are proportional to V³, which originates from the linear dispersion of graphene. We developed a simulation tool by adopting revised tunneling equations using MATLAB. Thereafter, we optimized the device performance of the field-emission barristor by engineering the barrier height and thickness to improve the delay time, cut-off frequency, and power-delay product.

Fabrication of Graphene-Based TiO2@CeO2 and CeO2@TiO2 Core-Shell Heterostructures for Enhanced Photocatalytic Activity and Cytotoxicity

Development of light-harvesting properties and inhibition of photogenerated charge carrier recombination are of paramount significance in the photocatalytic process. In the present work, we described the synthesis of core-shell heterostructures, which are composed of titanium oxide (TiO₂) and cerium oxide (CeO₂) deposited on a reduced graphene oxide (rGO) surface as a conductive substrate. Following the synthesis of ternary rGO-CeO₂@TiO₂ and rGO-TiO₂@CeO₂ nanostructures, their photocatalytic activity was investigated toward the degradation of rhodamine B dye as an organic pollutant under UV light irradiation. The obtained structures were characterized with high-resolution transmission electron microscopy, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy surface analysis, and UV-Vis spectroscopy. Various parameters including pH, catalyst dosage, temperature, and contact time were studied for photocatalysis optimization. Heterostructures showed considerable advantages because of their high surface area and superior photocatalytic performance. In contrast, rGO-CeO₂@TiO₂ showed the highest photocatalytic activity, which is attributed to the more effective electron-hole separation and quick suppression of charge recombination at core-shell phases. A biological assay of the prepared heterostructure was performed to determine the cytotoxicity against breast cancer cells (MCF-7) and demonstrated a very low survival rate at 7.65% of cells at the 17.5 mg mL^-1 concentration of applied photocatalyst.

Advances in Two-Dimensional Materials for Optoelectronics Applications

The past one and a half decades have witnessed the tremendous progress of two-dimensional (2D) crystals, including graphene, transition-metal dichalcogenides, black phosphorus, MXenes, hexagonal boron nitride, etc., in a variety of fields. The key to their success is their unique structural, electrical, mechanical and optical properties. Herein, this paper gives a comprehensive summary on the recent advances in 2D materials for optoelectronic approaches with the emphasis on the morphology and structure, optical properties, synthesis methods, as well as detailed optoelectronic applications. Additionally, the challenges and perspectives in the current development of 2D materials are also summarized and indicated. Therefore, this review can provide a reference for further explorations and innovations of 2D material-based optoelectronics devices.

Role of Surface Adsorbates on the Photoresponse of (MO)CVD-Grown Graphene-MoS2 Heterostructure Photodetectors

A promising strategy toward ultrathin, sensitive photodetectors is the combination of a photoactive semiconducting transition-metal dichalcogenide (TMDC) monolayer like MoS₂ with highly conductive graphene. Such devices often exhibit a complex and contradictory photoresponse as incident light can trigger both photoconductivity and photoinduced desorption of molecules from the surface. Here, we use metal-organic chemical vapor deposition (MOCVD) to directly grow MoS₂ on top of graphene that is deposited on a sapphire wafer via chemical vapor deposition (CVD) for realizing graphene-MoS₂ photodetectors. Two-color optical pump-electrical probe experiments allow for separation of light-induced carrier transfer across the graphene-MoS₂ heterointerface from adsorbate-induced effects. We demonstrate that adsorbates strongly modify both magnitude and sign of the photoconductivity. This is attributed to a change of the graphene doping from p- to n-type in case adsorbates are being desorbed, while in either case, photogenerated electrons are transferred from MoS₂ to graphene. This nondestructive probing method sheds light on the charge carrier transfer mechanisms and the role of adsorbates in two-dimensional (2D) heterostructure photodetectors.

Suspended Graphene Membranes to Control Au Nucleation and Growth

Control of nucleation sites is an important goal in materials growth: nuclei in regular arrays may show emergent photonic or electronic behavior, and once the nuclei coalesce into thin films, the nucleation density influences parameters such as surface roughness, stress, and grain boundary structure. Tailoring substrate properties to control nucleation is therefore a powerful tool for designing functional thin films and nanomaterials. Here, we examine nucleation control for metals deposited on two-dimensional materials in a situation where substrate effects are absent and heterogeneous nucleation sites are minimized. Through quantification of faceted, epitaxial Au island nucleation on graphene, we show that ultralow nucleation densities with nuclei several micrometers apart can be achieved on suspended graphene under conditions where we measure 2-3 orders of magnitude higher nucleation density on the adjacent supported substrate. We estimate diffusion distances using nucleation theory and find a strong sensitivity of nucleation and diffusion to suspended graphene thickness. Finally, we discuss the role of surface roughness as the main factor determining nucleation density on clean free-standing graphene.

Multi-scale electronics transport properties in non-ideal CVD graphene sheet

In this work, we benchmark non-idealities and variations in the two-dimensional graphene sheet. We have simulated more than two hundred graphene-based devices structure. We have simulated distorted graphene sheets and have included random, inhomogeneous, asymmetric out-of-plane surface corrugation and in-plane deformation corrugation in the sheet through autocorrelation function in the non-equilibrium Green's function (NEGF) framework to introduce random distortion in flat graphene. These corrugation effects inevitably appear in the graphene sheet due to background substrate roughness or the passivation encapsulation material morphology in the transfer step. We have examined the variation in density of state, propagating density of transmission modes, electronic band structure, electronic density, and hole density in those device structures. We have observed that the surface corrugation increases the electronic and hole density distribution variation across the device and creates electron-hole charge puddles in the sheet. This redistribution of microscopic charge in the sheet is due to the lattice fields' quantum fluctuation and symmetry breaking. Furthermore, to understand the impact of scattered charge distribution on the sheet, we simulated various impurity effects within the NEGF framework. The study's objective is to numerically simulate and benchmark numerous device design morphology with different background materials compositions to elucidate the electrical property of the sheet device.

Grafting Ink for Direct Writing: Solvation Activated Covalent Functionalization of Graphene

Covalent functionalization of graphene (CFG) has shown attractive advantages in tuning the electronic, mechanical, optical, and thermal properties of graphene. However, facile, large-scale, controllable, and highly efficient CFG remains challenging and often involves highly reactive and volatile compounds, requiring complex control of the reaction conditions. Here, a diazonium-based grafting ink consisting of only two components, i.e., an aryl diazonium salt and the solvent dimethyl sulfoxide (DMSO) is presented. The efficient functionalization is attributed to the combination of the solvation of the diazonium cations by DMSO and n-doping of graphene by DMSO, thereby promoting electron transfer (ET) from graphene to the diazonium cations, resulting in the generation of aryl radicals which subsequently react with the graphene. The grafting density of CFG is controlled by the reaction time and very high levels of functionalization, up to the failing of the Tuinstra-Koenig (T-K) relation, while the functionalization layer remains at monolayer height. The grafting ink, effective for days at room temperature, can be used at ambient conditions and renders the patterning CFG by direct writing as easy as writing on paper. In combination with thermal sample treatment, reversible functionalization is possible by subsequent writing/erasing cycles.

Two-Dimensional Ultrathin Silica Films

Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO₂ stoichiometry. It consists of precisely two layers of tetrahedral [SiO₄] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials.

Quantum Revivals in Curved Graphene Nanoflakes

Graphene nanostructures have attracted a lot of attention in recent years due to their unconventional properties. We have employed Density Functional Theory to study the mechanical and electronic properties of curved graphene nanoflakes. We explore hexagonal flakes relaxed with different boundary conditions: (i) all atoms on a perfect spherical sector, (ii) only border atoms forced to be on the spherical sector, and (iii) only vertex atoms forced to be on the spherical sector. For each case, we have analysed the behaviour of curvature energy and of quantum regeneration times (classical and revival) as the spherical sector radius changes. Revival time presents in one case a divergence usually associated with a phase transition, probably caused by the pseudomagnetic field created by the curvature. This could be the first case of a phase transition in graphene nanostructures without the presence of external electric or magnetic fields.

Mechanochemical Molecular Migration on Graphene

In this study, we propose that the curvature of graphene can be exploited to perform directional molecular motion and provide atomistic insights into the curvature-dependent molecular migration through density functional theory calculations. We first reveal the origin of the different migration trends observed experimentally for aromatic molecules with electron-donating and -withdrawing groups on p-doped functionalized graphene. Next, we show that the kinetic barrier for migration depends on the amount and nature of the curvature, that is, positive versus negative curvature. We find that the molecular migration on a wrinkled/rippled graphene sheet preferentially happens from the valley (positive curvature) to the mountain (negative curvature) regions. To understand the origin of such curvature-dependent molecular motion and migrational kinetic barrier trends, we develop a descriptor based on the frontier orbital orientation of graphene. Finally, based on these findings, we predict that time- and space-varying curvature can drive directional molecular motion on graphene and thus further propose that efforts should focus on exploring other two-dimensional materials as active platforms for performing controlled molecular motion.

Nanoscale Self-Wetting Driven Monatomization of Ag Nanoparticle for Excellent Photocatalytic Hydrogen Evolution

Metal nanoparticles (NPs) with <10 nm have demonstrated many novel applications including surprisingly low melting point, astonishing liquid-like pseudoelasticity, and outstanding hydrogen evolution performance. Here, a nanoscale self-wetting driven monatomization of Ag NPs with <5 nm on carbon nitride (CN) to fabricate Ag single-atom catalyst (Ag₁ /CN SAC) is demonstrated, and a thermodynamic approach to elucidate Ag NPs decomposing into single atoms is established. Dynamic dispersion process of Ag NPs into atoms on CN is recorded using in situ AC-HADDF-TEM techniques. Density functional theory calculations and molecular dynamics simulations suggest that the spontaneous dispersion origins from the nanoscale self-wetting effect in thermodynamics. In atomic scale, the driving force of self-wetting derived from the balance between cohesive energy of Ag NPs and excess energy of Ag atoms in CN vacations. The fabricated Ag₁ /CN SAC proved a higher efficiency for photocatalytic hydrogen evolution activity (3690 μmmol g^-1 h^-1 ) than Pt nanoparticles on CN (3192 μmmol g^-1 h^-1 ). This spontaneous monatomization resulting from the interaction between metal NPs and substrate provides a simple method to prepare SACs with a high active photocatalytic hydrogen evolution.

Electrical and Low Frequency Noise Characterization of Graphene Chemical Sensor Devices Having Different Geometries

Chemiresistive graphene sensors are promising for chemical sensing applications due to their simple device structure, high sensitivity, potential for miniaturization, low-cost, and fast response. In this work, we investigate the effect of (1) ZnO nanoparticle functionalization and (2) engineered defects onto graphene sensing channel on device resistance and low frequency electrical noise. The engineered defects of interest include 2D patterns of squares, stars, and circles and 1D patterns of slots parallel and transverse to the applied electric potential. The goal of this work is to determine which devices are best suited for chemical sensing applications. We find that, relative to pristine graphene devices, nanoparticle functionalization leads to reduced contact resistance but increased sheet resistance. In addition, functionalization lowers 1/f current noise on all but the uniform mesa device and the two devices with graphene strips parallel to carrier transport. The strongest correlations between noise and engineering defects, where normalized noise amplitude as a function of frequency f is described by a model of A_N/f^γ, are that γ increases with graphene area and contact area but decreases with device total perimeter, including internal features. We did not find evidence of a correlation between the scalar amplitude, A_N, and the device channel geometries. In general, for a given device area, the least noise was observed on the least-etched device. These results will lead to an understanding of what features are needed to obtain the optimal device resistance and how to reduce the 1/f noise which will lead to improved sensor performance.

A Comprehensive Review on Supercapacitor Applications and Developments

The storage of enormous energies is a significant challenge for electrical generation. Researchers have studied energy storage methods and increased efficiency for many years. In recent years, researchers have been exploring new materials and techniques to store more significant amounts of energy more efficiently. In particular, renewable energy sources and electric vehicle technologies are triggering these scientific studies. Scientists and manufacturers recently proposed the supercapacitor (SC) as an alternating or hybrid storage device. This paper aims to provide a comprehensive review of SC applications and their developments. Accordingly, a detailed literature review was first carried out. The historical results of SCs are revealed in this paper. The structure, working principle, and materials of SC are given in detail to be analysed more effectively. The advantages and disadvantages, market profile, and new technologies with manufacturer corporations are investigated to produce a techno-economic analysis of SCs. The electric vehicle, power systems, hybrid energy storage systems with integration of renewable energy sources, and other applications of SCs are investigated in this paper. Additionally, SC modelling design principles with charge and discharge tests are explored. Other components and their price to produce a compact module for high power density are also investigated.

Polymer grafted magnetic graphene oxide as a potential nanocarrier for pH-responsive delivery of sparingly soluble quercetin against breast cancer cells

In this work, polymer grafted magnetic graphene oxide (GO–PVP–Fe₃O₄) was successfully synthesized for efficient delivery of anticancer drug. Firstly, GO was functionalized with the hydrophilic and biocompatible polymer polyvinylpyrrolidone (PVP) and then grafted with magnetic nanoparticles (Fe₃O₄) through an easy and effective chemical co-precipitation method. Quercetin (QSR) as an anticancer drug was loaded onto the surface of GO–PVP–Fe₃O₄via non-covalent interactions. The drug loading capacity was as high as 1.69 mg mg⁻¹ and the synthesized magnetic nanocarrier shows pH-responsive controlled release of QSR. The cellular cytotoxicity of the synthesized nanocarrier with and without drugs was investigated in human breast cancer MDA MB 231 cells and their effects compared on non-tumorigenic epithelial HEK 293T cells. These results reveal that the drug loaded GO–PVP–Fe₃O₄ nanohybrid was found to be more toxic than the free drug towards MDA MB 231 cells and exhibits biocompatibility towards HEK 293T cells. Overall, a smart drug delivery system including polymer grafted magnetic graphene oxide as a pH-responsive potential nanocarrier could be beneficial for targeted drug delivery, controlled by an external magnetic field as an advancement in chemotherapy against cancer.

Polymer grafted magnetic graphene oxide (GO–PVP–Fe₃O₄) as an efficient nanocarrier for the delivery of anticancer drug quercetin.

Soft Bioelectronics Based on Nanomaterials

Recent advances in nanostructured materials and unconventional device designs have transformed the bioelectronics from a rigid and bulky form into a soft and ultrathin form and brought enormous advantages to the bioelectronics. For example, mechanical deformability of the soft bioelectronics and thus its conformal contact onto soft curved organs such as brain, heart, and skin have allowed researchers to measure high-quality biosignals, deliver real-time feedback treatments, and lower long-term side-effects in vivo. Here, we review various materials, fabrication methods, and device strategies for flexible and stretchable electronics, especially focusing on soft biointegrated electronics using nanomaterials and their composites. First, we summarize top-down material processing and bottom-up synthesis methods of various nanomaterials. Next, we discuss state-of-the-art technologies for intrinsically stretchable nanocomposites composed of nanostructured materials incorporated in elastomers or hydrogels. We also briefly discuss unconventional device design strategies for soft bioelectronics. Then individual device components for soft bioelectronics, such as biosensing, data storage, display, therapeutic stimulation, and power supply devices, are introduced. Afterward, representative application examples of the soft bioelectronics are described. A brief summary with a discussion on remaining challenges concludes the review.

Exponentially selective molecular sieving through angstrom pores

Two-dimensional crystals with angstrom-scale pores are widely considered as candidates for a next generation of molecular separation technologies aiming to provide extreme, exponentially large selectivity combined with high flow rates. No such pores have been demonstrated experimentally. Here we study gas transport through individual graphene pores created by low intensity exposure to low kV electrons. Helium and hydrogen permeate easily through these pores whereas larger species such as xenon and methane are practically blocked. Permeating gases experience activation barriers that increase quadratically with molecules' kinetic diameter, and the effective diameter of the created pores is estimated as ∼2 angstroms, about one missing carbon ring. Our work reveals stringent conditions for achieving the long sought-after exponential selectivity using porous two-dimensional membranes and suggests limits on their possible performance.

Adiabatic versus non-adiabatic electron transfer at 2D electrode materials

2D electrode materials are often deployed on conductive supports for electrochemistry and there is a great need to understand fundamental electrochemical processes in this electrode configuration. Here, an integrated experimental-theoretical approach is used to resolve the key electronic interactions in outer-sphere electron transfer (OS-ET), a cornerstone elementary electrochemical reaction, at graphene as-grown on a copper electrode. Using scanning electrochemical cell microscopy, and co-located structural microscopy, the classical hexaamineruthenium (III/II) couple shows the ET kinetics trend: monolayer > bilayer > multilayer graphene. This trend is rationalized quantitatively through the development of rate theory, using the Schmickler-Newns-Anderson model Hamiltonian for ET, with the explicit incorporation of electrostatic interactions in the double layer, and parameterized using constant potential density functional theory calculations. The ET mechanism is predominantly adiabatic; the addition of subsequent graphene layers increases the contact potential, producing an increase in the effective barrier to ET at the electrode/electrolyte interface.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Structure, Properties and Applications of Two-Dimensional Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) has emerged as a strong candidate for two-dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. Super-thin h-BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti-corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h-BN followed by a comprehensive account of state-of-the-art synthesis strategies for 2D h-BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal-mechanical-chemical stability of 2D h-BN, various potential applications of these structures are described.

Significantly enhanced photothermal catalytic hydrogen evolution over Cu2O-rGO/TiO2 composite with full spectrum solar light

Reduced graphene oxide (rGO) has conspicuous photothermal characteristics in photothermal applications. Thus in our previous work, we used reduced graphene oxide (rGO) supported titanium dioxide (TiO₂) nanocomposite (rGO/TiO₂) to absorb the ultraviolet and infrared light in the photothermal hydrogen evolution process. In order to make use of the full spectrum solar energy into other clear energy, the visible light should be also considered in following research. Herein, we report a cuprous oxide (Cu₂O) decorated reduced graphene oxide (rGO) supported titanium dioxide (TiO₂) (Cu₂O-rGO/TiO₂) catalysts, which can absorb full spectrum solar light in an innovative way. The Cu₂O-rGO/TiO₂ catalyst is synthesized through a one-step hydrothermal method. The rates of hydrogen evolution are 17800 μmol·g^-1h^-1 under photothermal condition (90°C), 3800 μmol·g^-1h^-1 under photocatalysis condition only (25°C) and 0 μmol·g^-1h^-1 under thermal catalysis condition only. The result of photothermal catalytic hydrogen evolution rate is about 4.7 times that of the sum of the photocatalytic and thermal reactions. The photothermal synergetic effect promotes the photo-generated electron-holes separation through the rGO due to the temperature rising, and accelerates the reaction rates on the catalyst surface in hydrogen evolution process simultaneously. This work could provide us a new promising way for the conversion of full spectrum solar energy to hydrogen energy.

Graphene Transfer: A Physical Perspective

Graphene, synthesized either epitaxially on silicon carbide or via chemical vapor deposition (CVD) on a transition metal, is gathering an increasing amount of interest from industrial and commercial ventures due to its remarkable electronic, mechanical, and thermal properties, as well as the ease with which it can be incorporated into devices. To exploit these superlative properties, it is generally necessary to transfer graphene from its conductive growth substrate to a more appropriate target substrate. In this review, we analyze the literature describing graphene transfer methods developed over the last decade. We present a simple physical model of the adhesion of graphene to its substrate, and we use this model to organize the various graphene transfer techniques by how they tackle the problem of modulating the adhesion energy between graphene and its substrate. We consider the challenges inherent in both delamination of graphene from its original substrate as well as relamination of graphene onto its target substrate, and we show how our simple model can rationalize various transfer strategies to mitigate these challenges and overcome the introduction of impurities and defects into the graphene. Our analysis of graphene transfer strategies concludes with a suggestion of possible future directions for the field.

Low defect and high electrical conductivity of graphene through plasma graphene healing treatment monitored with in situ optical emission spectroscopy

Fundamental studies on graphene (Gr) and its real device applications have been affected by unavoidable defects and impurities which are usually present in synthesized Gr. Therefore, post treatment methods on Gr have been an important subject of research followed by the community. Here, we demonstrate a post-treatment of cm-sized CVD-grown graphene in a Radio Frequency-generated low-pressure plasma of methane and hydrogen to remove oxygen functional groups and heal the structural defects. The optimum plasma treatment parameters, such as pressure, plasma power, and the ratio of the gases, are optimized using in-situ optical emission spectroscopy. This way we present an optimal healing condition monitored with in situ OES. A twofold increase in the conductivity of plasma-treated Gr samples was obtained. Plasma treatment conditions give insights into the possible underlying mechanisms, and the method presents an effective way to obtain improved Gr quality.

Measurements of the Electrical Conductivity of Monolayer Graphene Flakes Using Conductive Atomic Force Microscopy

The intrinsic electrical conductivity of graphene is one of the key factors affecting the electrical conductance of its assemblies, such as papers, films, powders, and composites. Here, the local electrical conductivity of the individual graphene flakes was investigated using conductive atomic force microscopy (C-AFM). An isolated graphene flake connected to a pre-fabricated electrode was scanned using an electrically biased tip, which generated a current map over the flake area. The current change as a function of the distance between the tip and the electrode was analyzed analytically to estimate the contact resistance as well as the local conductivity of the flake. This method was applied to characterize graphene materials obtained using two representative large-scale synthesis methods. Monolayer graphene flakes synthesized by chemical vapor deposition on copper exhibited an electrical conductivity of 1.46 ± 0.82 × 10⁶ S/m. Reduced graphene oxide (rGO) flakes obtained by thermal annealing of graphene oxide at 300 and 600 °C exhibited electrical conductivities of 2.3 ± 1.0 and 14.6 ± 5.5 S/m, respectively, showing the effect of thermal reduction on the electrical conductivity of rGO flakes. This study demonstrates an alternative method to characterizing the intrinsic electrical conductivity of graphene-based materials, which affords a clear understanding of the local properties of individual graphene flakes.

Effects of temperature and repeat layer spacing on mechanical properties of graphene/polycrystalline copper nanolaminated composites under shear loading

In the present study, the characteristics of graphene/polycrystalline copper nanolaminated (GPCuNL) composites under shear loading are investigated by molecular dynamics simulations. The effects of different temperatures, graphene chirality, repeat layer spacing, and grain size on the mechanical properties, such as failure mechanism, dislocation, and shear modulus, are observed. The results indicate that as the temperature increases, the content of Shockley dislocations will increase and the maximum shear stress of the zigzag and armchair directions also decreases. The mechanical strength of the zigzag direction is more dependent on the temperature than that of the armchair direction. Moreover, self-healing occurs in the armchair direction, which causes the shear stress to increase after failure. Furthermore, the maximum shear stress and the shear strength of the composites decrease with an increase of the repeat layer spacing. Also, the shear modulus increases by increasing the grain size of copper.

Infra-Red Active Dirac Plasmon Serie in Potassium Doped-Graphene (KC8) Nanoribbons Array on Al2O3 Substrate

A theoretical formulation of the electromagnetic response in graphene ribbons on dielectric substrate is derived in the framework of the ab initio method. The formulation is applied to calculate the electromagnetic energy absorption in an array of potassium-doped graphene nanoribbons (KC8-NR) deposited on a dielectric Al2O3 substrate. It is demonstrated that the replacement of the flat KC8 by an array of KC8-NR transforms the Drude tail in the absorption spectra into a series of infrared-active Dirac plasmon resonances. It is also shown that the series of Dirac plasmon resonances, when unfolded across the extended Brillouin zones, resembles the Dirac plasmon. The Dirac plasmon resonances' band structure, within the first Brillouin zone, is calculated. Finally, an excellent agreement between the theoretical absorption and recent experimental results for differential transmission through graphene on an SiO2/Si surface is presented. The theoretically predicted micrometer graphene nanoribbons intercalation compound (GNRIC) in a stage-I-like KC8 is confirmed to be synthesized for Dirac plasmon resonances.

Mechanisms of Spontaneous Curvature Inversion in Compressed Graphene Ripples for Energy Harvesting Applications via Molecular Dynamics Simulations

Electrically conductive, highly flexible graphene membranes hold great promise for harvesting energy from ambient vibrations. For this study, we built numerous three-dimensional graphene ripples, with each featuring a different amount of compression, and performed molecular dynamics simulations at elevated temperatures. These ripples have a convex cosine shape, then spontaneously invert their curvature to concave. The average time between inversion events increases with compression. We use this to determine how the energy barrier height depends on strain. A typical convex-to-concave curvature inversion process begins when the ripple's maximum shifts sideways from the normal central position toward the fixed outer edge. The ripple's maximum does not simply move downward toward its concave position. When the ripple's maximum moves toward the outer edge, the opposite side of the ripple is pulled inward and downward, and it passes through the fixed outer edge first. The ripple's maximum then quickly flips to the opposite side via snap-through buckling. This trajectory, along with local bond flexing, significantly lowers the energy barrier for inversion. The large-scale coherent movement of ripple atoms during curvature inversion is unique to two-dimensional materials. We demonstrate how this motion can induce an electrical current in a nearby circuit.

Efficient ReSe2 Photodetectors with CVD Single-Crystal Graphene Contacts

Rhenium-based 2D transition metal dichalcogenides such as ReSe₂ are suitable candidates as photoactive materials for optoelectronic devices. Here, photodetectors based on mechanically exfoliated ReSe₂ crystals were fabricated using chemical vapor deposited (CVD) graphene single-crystal (GSC) as lateral contacts. A “pick & place” method was adopted to transfer the desired crystals to the intended position, easing the device fabrication while reducing potential contaminations. A similar device with Au was fabricated to compare contacts’ performance. Lastly, a CVD hexagonal boron nitride (hBN) substrate passivation layer was designed and introduced in the device architecture. Raman spectroscopy was carried out to evaluate the device materials’ structural and electronic properties. Kelvin probe force measurements were done to calculate the materials’ work function, measuring a minimal Schottky barrier height for the GSC/ReSe₂ contact (0.06 eV). Regarding the electrical performance, I-V curves showed sizable currents in the GSC/ReSe₂ devices in the dark and under illumination. The devices presented high photocurrent and responsivity, along with an external quantum efficiency greatly exceeding 100%, confirming the non-blocking nature of the GSC contacts at high bias voltage (above 2 V). When introducing the hBN passivation layer, the device under white light reached a photo-to-dark current ratio up to 10⁶.

Towards field-effect controlled graphene-enhanced Raman spectroscopy of cobalt octaethylporphyrin molecules

During the last decade graphene-enhanced Raman spectroscopy has proven to be a powerful tool to detect and analyze minute amounts of molecules adsorbed on graphene. By using a graphene-based field-effect device the unique opportunity arises to gain a deeper insight into the coupling of molecules and graphene as graphene's Fermi level can be controlled by the transistor`s gate voltage. However, the fabrication of such a device comes with great challenges because of contaminations stemming from processing the device inevitably prevent direct adsorption of the molecules onto graphene rendering it unsuitable for field-effect controlled graphene-enhanced Raman spectroscopy measurements/experiments. In this work, we solve this problem by establishing two different fabrication procedures for such devices, both of which are in addition compatible with large area and scalable production requirements. As a first solution, selective argon cluster irradiation is shown to be an efficient way to remove resist residues after processing. We provide evidence that after the irradiation the enhancement of the molecular Raman signal can indeed be measured, demonstrating that this procedure cleans graphene's surface sufficiently enough for direct molecular adsorption. As a second solution, we have developed a novel stacking method to encapsulate the molecules in between two graphene layers to protect the underlying graphene and molecular layer from the harsh conditions during the photolithography process. This method combines the advantages of dry stacking, which leads to a perfectly clean interface, and wet stacking processes, which can easily be scaled up for large area processing. Both approaches yield working graphene transistors with strong molecular Raman signals stemming from cobalt octaehtylporphyrin, a promising and prototypical candidate for spintronic applications, and are therefore suitable for graphene based molecular sensing applications.

A comparative study of electrical and opto-electrical properties of a few-layer p-WSe2/n-WS2 heterojunction diode on SiO2 and h-BN substrates

Since the innovation of van der Waals heterostructures of 2D materials, the p–n junction diode, a building block of electronics and opto-electronics has been studied in various ways. To date most of them have been studied on SiO₂ or other oxide substrates, although the oxide substrates cause significant degradation of the 2D material's intrinsic properties and device performances. Whereas using hexagonal boron nitride (h-BN) as an underlying layer to the 2D materials is known to preserve their properties. Here we have carefully analyzed the electrical and opto-electrical properties of a p-WSe₂/n-WS₂ van der Waals heterojunction diode on SiO₂ and the h-BN substrates. Besides the usual enhancement of the field-effect mobility of WSe₂ and WS₂, we have achieved a significant enhancement of the diode rectification ratio and excellent photovoltaic characteristics on the h-BN substrate. We have obtained more than an order-of-magnitude enhancement of the diode rectification ratio and about two-fold increments in the overall opto-electronics behavior on the h-BN substrate compared with those on the SiO₂ substrate. The values of self-powered photo responsivity and external quantum efficiency are 3 A/W and 588% respectively on the h-BN substrate at 10 mW cm⁻² photo-power density and 633 nm wavelength, whereas they reduce to about one-half on the SiO₂ substrate.

A few-layer WSe₂/WS₂ heterojunction diode on an h-BN substrate shows improved electronic and optoelectronic characteristics with a robust diode rectification ratio and photo responsivity compared to that on a SiO₂ substrate.