Graphene-based Electronic Devices Research Articles

Graphene thermocouple fabricated on a flexible and transparent substrate

We demonstrate the realization of reliable, high-quality, micro-sized graphene-based field-effect devices on a flexible and transparent substrate, ethylene vinyl acetate (EVA)/polyethylene terephthalate (PET), using a convenient hot-press lamination transfer and employing parylene-N as a dielectric material for gating. Using this technique, we fabricate a graphene thermocouple on the EVA/PET substrate. Specifically, the graphene is patterned in a U-shape, and its legs are equipped with two independent top gates. Full control of the carrier density and type by electrostatic gating in the two graphene regions allow the formation of a thermocouple layout, exhibiting an enlarged thermovoltage signal when the two regions are doped with opposite types of carriers and leading to a maximum sensitivity with a thermopower of ∼73 µV/K. This agrees well with the working principle of thermocouple, and it proves the good compatibility and functionality of the graphene thermocouple on the EVA/PET substrate. Our findings suggest possible applications for producing scalable and reliable graphene-based electronic devices on flexible and transparent substrates in a simple way.

Effects of the in-plane uniaxial and biaxial strains on the electronic and optical properties of the graphene/β-Si3N4 heterostructure

van der Waals heterostructures can enable adjustment of graphene's electronic properties, which is important for the application of graphene-based electronic devices. The structural, electronic and optical properties of heterostructures composed of graphene and β-Si3N4 under different strain conditions are studied in this paper using first-principle calculation. The result shows that the heterojunction constituted with β-Si3N4 opens a small bandgap at the Brillouin zone Γ, which is about 0.099 eV, and the bandgap is highly sensitive to the direction and magnitude of strain. Under uniaxial compressive (tensile) strain, the heterojunction bandgap increases linearly, reaching 1.901 eV and 1.614 eV at −11 % and 11 %, respectively; under biaxial strain, the heterojunction bandgap decreases linearly with the compressive strain, decreasing to 0.087 eV at −11 %, and increases linearly with the tensile strain, reaching 0.113 eV at 11 %. And the rate of the bandgap under uniaxial strain is larger than that under biaxial strain, which suggests that the uniaxial strain regulation is more effective. Optical property study shows that uniaxial (biaxial) compressive strain improves the light absorption of the heterojunction in the blue-ultraviolet region band, especially when the uniaxial compressive strain reaches 9 %, the light absorption of the heterojunction increases significantly in the whole spectral interval. These research results will provide a theoretical basis for the practical applications of graphene and β-Si3N4 heterostructures in the fields of pressure sensors and optical modulators.

Application of digital twins for simulation based tailoring of laser induced graphene

In the era of man–machine interfaces, digital twins stand as a key technology, offering virtual representations of real-world objects, processes, and systems through computational models. They enable novel ways of interacting with, comprehending, and manipulating real-world entities within a virtual realm. The real implementation of graphene-based sensors and electronic devices remains challenging due to the integration complexities of high-quality graphene materials with existing manufacturing processes. To address this, scalable techniques for the in-situ fabrication of graphene-like materials are essential. One promising method involves using a CO2 laser to convert polyimide into graphene. Optimizing this graphitization process is hindered by complex parameter interactions and nonlinear terms. This article explores how these digital replicas can enhance the fabrication of laser-induced graphene (LIG) through laser simulation and machine learning methods to enable rapid single-step LIG patterning. This approach aims to create a universal simulation for all CO2 lasers, calculating optical energy flux and utilizing machine learning to control and predict LIG conductivity (ability to conduct current), morphology, and electrical resistance. The proposed procedure, integrating digital twins in the LIG production process, will avoid or reduce the preliminary tests required to determine the proper laser parameters to reach the desired LIG characteristics. Accordingly, this approach will reduce the time and costs associated with these tests and thus increase the efficiency and optimize the procedure.

Sensitive Characterization of the Graphene Transferred onto Varied Si Wafer Surfaces via Terahertz Emission Spectroscopy and Microscopy (TES/LTEM).

Graphene shows great potential in developing the next generation of electronic devices. However, the real implementation of graphene-based electronic devices needs to be compatible with existing silicon-based nanofabrication processes. Characterizing the properties of the graphene/silicon interface rapidly and non-invasively is crucial for this endeavor. In this study, we employ terahertz emission spectroscopy and microscopy (TES/LTEM) to evaluate large-scale chemical vapor deposition (CVD) monolayer graphene transferred onto silicon wafers, aiming to assess the dynamic electronic properties of graphene and perform large-scale graphene mapping. By comparing THz emission properties from monolayer graphene on different types of silicon substrates, including those treated with buffered oxide etches, we discern the influence of native oxide layers and surface dipoles on graphene. Finally, the mechanism of THz emission from the graphene/silicon heterojunction is discussed, and the large-scale mapping of monolayer graphene on silicon is achieved successfully. These results demonstrate the efficacy of TES/LTEM for graphene characterization in the modern graphene-based semiconductor industry.

Comprehensive Study and Design of Graphene Transistor.

Graphene, renowned for its exceptional electrical, optical, and mechanical properties, takes center stage in the realm of next-generation electronics. In this paper, we provide a thorough investigation into the comprehensive fabrication process of graphene field-effect transistors. Recognizing the pivotal role graphene quality plays in determining device performance, we explore many techniques and metrological methods to assess and ensure the superior quality of graphene layers. In addition, we delve into the intricate nuances of doping graphene and examine its effects on electronic properties. We uncover the transformative impact these dopants have on the charge carrier concentration, bandgap, and overall device performance. By amalgamating these critical facets of graphene field-effect transistors fabrication and analysis, this study offers a holistic understanding for researchers and engineers aiming to optimize the performance of graphene-based electronic devices.

Transparent Patternable Large-Area Graphene p-n Junctions by Photoinduced Electron Doping.

We present a novel approach to achieve n-type doping in graphene and create graphene p-n junctions through a photoinduced electron doping method using photobase generators (PBGs). The unique properties of PBGs allow us to spatially and temporally control the doping process via light activation. The selective irradiation of specific regions on the graphene film enables switching their doping from p- to n-type, as confirmed by changes in the electromotive force and Seebeck and Hall coefficients. We demonstrate a stable (over 2 months) high electron mobility exceeding 1000 cm2 V-1s-1 using Hall effect measurements. The precise control of doping and the creation of p-n junctions in graphene offer exciting possibilities for various electronic, optoelectronic, and thermoelectric applications. Furthermore, we fabricate transparent graphene thermocouples with a high electromotive force of approximately ca. 80 μV/K, which validates the reliability and effectiveness of our approach for temperature sensing applications. This work paves the way for high-performance graphene-based electronic devices via controlled doping and patterning techniques. These findings provide valuable insights for the practical implementation of graphene in various fields.

Characterizing and controlling infrared phonon anomaly of bilayer graphene in optical-electrical force nanoscopy

We, for the first time, report the nanoscopic imaging study of anomalous infrared (IR) phonon enhancement of bilayer graphene, originated from the charge imbalance between the top and bottom layers, resulting in the enhancement of E1u mode of bilayer graphene near 0.2 eV. We modified the multifrequency atomic force microscope platform to combine photo-induced force microscope with electrostatic/Kelvin probe force microscope constituting a novel hybrid nanoscale optical-electrical force imaging system. This enables to observe a correlation between the IR response, doping level, and topographic information of the graphene layers. Through the nanoscale spectroscopic image measurements, we demonstrate that the charge imbalance at the graphene interface can be controlled by chemical (doping effect via Redox mechanism) and mechanical (triboelectric effect by the doped cantilever) approaches. Moreover, we can also diagnosis the subsurface cracks on the stacked few-layer graphene at nanoscale, by monitoring the strain-induced IR phonon shift. Our approach provides new insights into the development of graphene-based electronic and photonic devices and their potential applications.

Laser-Induced Graphene-based Flexible Substrate with Photothermal Conversion and Photoresponse Performance on Polyimide Film.

Graphene-based flexible electronic devices are widely used in photoelectric components and photodetectors. However, it remains a huge challenge to fabricate graphene-based flexible devices efficiently and economically. Compared with the flexible electronic devices made by combining the flexible film with metal and semiconductor materials, the graphene-based flexible substrate (GFS) can be efficiently and conveniently induced by laser direct writing on the flexible film. In this paper, the GFS with a resistance of as low as 15 Ω was successfully induced by CO2 laser on a polyimide (PI) film in one step, and the GFS surface covered with carbon nanoparticles (GFSC) with a resistance of 25 Ω was further induced by femtosecond (fs) laser reprocessing. Benefiting from the laser-induced porous graphene structure, the absorptivity of GFS is up to 90% in the wavelength range of 200-2000 nm. The formation of carbon nanoparticles on the GFSC surface further improves the absorptivity to 97.5% in a wide spectral range. Under white light irradiation of 1 sun, the surface temperature of GFS reaches 65.7 °C and that of GFSC is up to 70.8 °C within 2 min. Under the irradiation of a light-emitting diode (LED) with a central wavelength of 365 nm, the highest photoresponsivity of GFS and GFSC was 8.8 and 1.3 mA/W, respectively. The response time and recovery time of GFS are 8 and 7.3 s, and those of GFSC are 8.3 and 6.7 s, respectively. Importantly, GFSC has a more stable photoresponse performance due to the better electron capture and transfer capability of carbon nanoparticles. It is believed that GFS and GFSC have great application potential in flexible photodetectors and sensors.

Atomic-scale tailoring of chemisorbed atomic oxygen on epitaxial graphene for graphene-based electronic devices

Graphene, with its unique band structure, mechanical stability, and high charge mobility, holds great promise for next-generation electronics. Nevertheless, its zero bandgap challenges the control of current flow through electrical gating, consequently limiting its practical applications. Recent research indicates that atomic oxygen can oxidize epitaxial graphene in a vacuum without causing unwanted damage. In this study, we have investigated the effects of chemisorbed atomic oxygen on the electronic properties of epitaxial graphene using scanning tunneling microscopy (STM). Our findings reveal that oxygen atoms effectively modify the electronic states of graphene, resulting in a bandgap at its Dirac point. Furthermore, we demonstrate that it is possible to selectively induce desorption or hopping of oxygen atoms with atomic precision by applying appropriate bias sweeps with an STM tip. These results suggest the potential for atomic-scale tailoring of graphene oxide, enabling the development of graphene-based atomic-scale electronic devices.

Stone-Wales Defect in Graphene.

2D graphene the most investigated structures from nanocarbon family studied in the last three decades. It is projected as an excellent material useful for quantum computing, artificial intelligence, and next generation advanced technologies. Graphene exists in several forms and its extraordinary thermal, mechanical, and electronic properties, principally depend on the kind of perfection of the hexagonal atomic lattice. Defects are always considered as undesired components but certain defects in graphene could be an asset for electrochemistry and quantum electronics due to the engineered electronclouds and quantum tunnelling. The authors carefully discuss the Stone-Wales imperfections in graphene and its derivatives comprehensively. A specific emphasis is focused on the experimental and theoretical aspects of the Stone-Wales defects in graphene with respect to structure-property relationships. The corroboration of extrinsic defects like external atomic doping, functionalization, edge distortion in the graphene consisting of Stone-Wales imperfections, which are very significant in designing graphene-based electronic devices, are summarized.

Bottom-up synthesis of mesoscale nanomeshes of graphene nanoribbons on germanium

The synthesis of functional graphene nanostructures on Ge(001) provides an attractive route toward integrating graphene-based electronic devices onto complementary metal oxide semiconductor-compatible platforms. In this study, we leverage the phenomenon of the anisotropic growth of graphene nanoribbons from rationally placed graphene nanoseeds and their rotational self-alignment during chemical vapor deposition to synthesize mesoscale graphene nanomeshes over areas spanning several hundred square micrometers. Lithographically patterned nanoseeds are defined on a Ge(001) surface at pitches ranging from 50 to 100 nm, which serve as starting sites for subsequent nanoribbon growth. Rotational self-alignment of the nanoseeds followed by anisotropic growth kinetics causes the resulting nanoribbons to be oriented along each of the equivalent, orthogonal Ge⟨110⟩ directions with equal probability. As the nanoribbons grow, they fuse, creating a continuous nanomesh. In contrast to nanomesh synthesis via top-down approaches, this technique yields nanomeshes with atomically faceted edges and covalently bonded junctions, which are important for maximizing charge transport properties. Additionally, we simulate the electrical characteristics of nanomeshes synthesized from different initial nanoseed-sizes, size-polydispersities, pitches, and device channel lengths to identify a parameter-space for acceptable on/off ratios and on-conductance in semiconductor electronics. The simulations show that decreasing seed diameter and pitch are critical to increasing nanomesh on/off ratio and on-conductance, respectively. With further refinements in lithography, nanomeshes obtained via seeded synthesis and anisotropic growth are likely to have superior electronic properties with tremendous potential in a multitude of applications, such as radio frequency communications, sensing, thin-film electronics, and plasmonics.

Graphene Strain-Effect Transistor with Colossal ON/OFF Current Ratio Enabled by Reversible Nanocrack Formation in Metal Electrodes on Piezoelectric Substrates

Extraordinarily high carrier mobility in graphene has led to many remarkable discoveries in physics and at the same time invoked great interest in graphene-based electronic devices and sensors. However, the poor ON/OFF current ratio observed in graphene field-effect transistors has stymied its use in many applications. Here, we introduce a graphene strain-effect transistor (GSET) with a colossal ON/OFF current ratio in excess of 107 by exploiting strain-induced reversible nanocrack formation in the source/drain metal contacts with the help of a piezoelectric gate stack. GSETs also exhibit steep switching with a subthreshold swing (SS) < 1 mV/decade averaged over ∼6 orders of magnitude change in the source-to-drain current for both electron and hole branch amidst a finite hysteresis window. We also demonstrate high device yield and strain endurance for GSETs. We believe that GSETs can significantly expand the application space for graphene-based technologies beyond what is currently envisioned.

Preparation and characterization of laser-induced graphene derived from polyimide for dye-sensitized solar cell applications

The synthesis of laser-induced graphene (LIG) from polyimide (PI) sheets provides a cost-effective and direct patterning approach for graphene-based electronic devices. However, the fabrication of LIG-based electrodes toward photovoltaic applications urgently needs to be explored. Herein, facile high-yield production of the porous LIG counter electrodes for DSSC applications was demonstrated by laser induction of PI film. The electrical conductivity and electrocatalytic activity of the LIG electrodes increased with the applied laser power due to the formation of a unique hierarchical porous graphene structure with abundant edges and defects, which resulted in an evident improvement in power conversion efficiency (PCE) of 3.56%. In addition, a one-step in situ synthesis of Pt/LIG composite was further demonstrated by laser irradiation on the PI sheet with low loadings of Pt precursor to obtain a Pt-less counter electrode. It is found that the Pt/LIG composite electrode-based DSSC achieved a higher PCE of 3.80%, which is comparable to that of pure Pt-based DSSC. This study demonstrates that the porous LIG from PI films is a promising candidate as a carbonaceous counter electrode for DSSCs.

Highly stable self-passivated MoO3-doped graphene film with nonvolatile MoO layer

The realization of high-performance graphene-based electronics, including transparent electrodes, flexible devices, and energy storage, is often hindered by the lack of adequate doping, which provides a stable and low sheet resistance. In this study, we demonstrate a highly stable MoO 3 -doped graphene obtained simply through a self-passivation. Graphene deposited with a 5-nm-thick MoO 3 exhibited a significant decrease in sheet resistance upon annealing at 400 °C under a hydrogen atmosphere. Surface and structural analyses confirmed that MoO 3 was converted to MoO x by thermal annealing, which consisted of mainly crystalline MoO 3 and Mo 4 O 11 with coexisting MoO 2 . A field-effect transistor fabricated using the MoO x -doped graphene exhibited a p-type characteristic similar to that of the MoO 3 -doped graphene. However, unlike the MoO 3 -doped graphene severely degraded by environment, the MoO x -doped graphene exhibited stable electrical properties after air exposure and chemical immersion owing to the chemically inert Mo 4 O 11 and MoO 2 acting as passivation layers while maintaining the p-type doping by MoO 3 . Thus, we expect that the highly stable MoO x -doped graphene obtained via the simple method will facilitate the fabrication and contribute to the performance reliability of various graphene-based electronic devices. • Highly stable MoO 3 -doped graphene through self-passivation by oxygen deficient MoO x • Simple way of thermal annealing for the synthesis of oxygen deficient MoO x -doped graphene • MoO x -doped graphene with p-type semiconducting characteristics • Excellent stability and chemical durability of MoO x -doped graphene

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.

Tafel-Kinetics-Controlled High-Speed Switching in a Electrochemical Graphene Field-Effect Transistor.

Graphene field-effect transistors (FETs) have attracted tremendous attention owing to the single-atomic-layer thickness and high electron mobility for potential applications in next-generation electronics. With regards to switching methodology, the electric-field-induced metal-insulator transition offers a new strategy to produce a large on/off current ratio through reversible electrochemical hydrogenation of the graphene channels. Therefore, the performance of such electrochemical graphene FETs greatly relies on the kinetics of hydrogenation reaction. Here, we show that the switching time can be systemically controlled by the applied gate voltages and geometries of graphene channels. The turn-on and turn-off time display an exponential dependence on the gate voltages, manifesting the dominated Tafel-form kinetics of hydrogenation reaction in a two-dimensional limit. Moreover, the turn-off time is inversely proportional to the channel width but independent of the length, while the turn-on time relies on both the width and length, as well as the off-state gate voltage and duration. Our work improves the response time to the magnitude of tens of microseconds and advances the application of graphene-based electronic devices.

Topological Stone-Wales Defects Enhance Bonding and Electronic Coupling at the Graphene/Metal Interface.

Defects play a critical role for the functionality and performance of materials, but the understanding of the related effects is often lacking, because the typically low concentrations of defects make them difficult to study. A prominent case is the topological defects in two-dimensional materials such as graphene. The performance of graphene-based (opto-)electronic devices depends critically on the properties of the graphene/metal interfaces at the contacting electrodes. The question of how these interface properties depend on the ubiquitous topological defects in graphene is of high practical relevance, but could not be answered so far. Here, we focus on the prototypical Stone-Wales (S-W) topological defect and combine theoretical analysis with experimental investigations of molecular model systems. We show that the embedded defects undergo enhanced bonding and electron transfer with a copper surface, compared to regular graphene. These findings are experimentally corroborated using molecular models, where azupyrene mimics the S-W defect, while its isomer pyrene represents the ideal graphene structure. Experimental interaction energies, electronic-structure analysis, and adsorption distance differences confirm the defect-controlled bonding quantitatively. Our study reveals the important role of defects for the electronic coupling at graphene/metal interfaces and suggests that topological defect engineering can be used for performance control.

Interfacial characterization in defective graphene/PET substrate structure through traction separation models: A molecular dynamics study

Among various graphene-based flexible electronic devices, the graphene/polymer substrate is one of the most common microstructures. Its interfacial mechanical properties directly determine the performance and reliability of these devices. Accordingly, the interaction between single-layer two-dimensional material such as graphene and polymer substrate has become an urgent research field in the manufacture and application of flexible electronic devices. In this study, traction-separation (T–S) models are established to study the interfacial mechanical behaviors of graphene/polyethylene terephthalate (PET) substrate structures using molecular dynamics (MD) simulations. The calculated interface parameters are verified by simulating the blister test with MD simulation and finite element analysis (FEA). Two common types of defects (Stone-Wales (S–W) and single vacancy (S–V)) are considered. By monitoring the aromatic ring distribution (order parameter and concentration) in PET substrate, the results reveal that under the normal loading, the S-W defect in graphene enhances the interfacial strength, while the separation energy is not sensitive to the existence of the S–W defect. The S–V defect in graphene degrades both the normal interfacial strength and separation energy owing to the loss of carbon atoms. Under the shear loading, it is found that the surface roughness of graphene caused by defects is an essential factor affecting the interfacial shear properties at the nanoscale. In addition, the results indicate that a low-concentration S–V defect can increase the surface roughness of graphene to obtain stronger mechanical interlocking, thereby enhancing the interfacial shear properties.

Symmetry-enforced nodal lines in the band structures of vacancy-engineered graphene

We elaborate that single-layer graphene with periodic vacancies can have a band structure containing nodal lines or nodal loops, opening the possibility of graphene-based electronic or spintronic devices with novel functionalities. The principle is that by removing carbon atoms such that the lattice becomes nonsymmorphic, every two sublattices in the unit cell will map to each other under glide plane operation. This mapping yields degenerate eigenvalues for the glide plane operation, which guarantees that the energy bands must stick together pairwise at a boundary of the Brillouin zone. Moving away from the Brillouin zone boundary causes the symmetry-enforced nodal lines to split, resulting in accidental nodal lines caused by the crossings of the split bands. Moreover, the density of states at the Fermi level may be dramatically enhanced if the nodal lines crosses the Fermi level. The nodal lines occur a variety of vacancy configurations even in the presence of Rashba spin-orbit coupling. Finally, our theory also explains the nodal loops surrounding the entire Brillouin zone of a chevron-type nanoporous graphene fabricated in a recent experiment.

A novel pre-deposition assisted strategy for inkjet printing graphene-based flexible pressure sensor with enhanced performance

Inkjet printing of graphene-based materials represents a highly promising deposition method, as benefits of its flexibility for graphical printing and simple operation process. However, the huge challenge associated with inkjet printing of graphene-based inks is to overcome the coffee ring effect, which results in ununiformity of the printed pattern. In this paper, we innovatively demonstrate a novel and facile pre-deposition assisted strategy for inkjet printing of graphene ink. By simply pre-depositing an ethanol layer, a series of procedures including homogenization, solvent exchange, post-stretching, and air drying would take place, leading to uniform deposition of graphene nanosheets with a denser structure. The as-printed pattern was proved to have a flatter surface. Subsequently, the approach was applied to construct an interdigital capacitance pressure sensor with a polyethylene terephthalate (PET) substrate. Such sensors showed high sensitivity (the relative capacitance change reached 33 under 10000 Pa), low detection limits (clearly perceive the weight of 0.1 g), and excellent stability. Notably, it exhibited negligible thermal hysteresis and decreased performance compared with the sensor fabricated by direct printing, which stems from its compact structure and lower air content. The method we proposed opens up opportunities for practical applications of inkjet printing low-cost graphene-based electronic devices with desirable performance.