Graphene Patterns Research Articles

Clean-Room Lithographical Processes for the Fabrication of Graphene Biosensors.

This work is on developing clean-room processes for the fabrication of electrolyte-gate graphene field-effect transistors at the wafer scale for biosensing applications. Our fabrication process overcomes two main issues: removing surface residues after graphene patterning and the dielectric passivation of metallic contacts. A graphene residue-free transfer process is achieved by using a pre-transfer, sacrificial metallic mask that protects the entire wafer except the areas around the channel, source, and drain, onto which the graphene film is transferred and later patterned. After the dissolution of the mask, clean gate electrodes are obtained. The multilayer SiO2/SiNx dielectric passivation takes advantage of the excellent adhesion of SiO2 to graphene and the substrate materials and the superior impermeability of SiNx. It hinders native nucleation centers and breaks the propagation of defects through the layers, protecting from prolonged exposition to all common solvents found in biochemistry work, contrary to commonly used polymeric passivation. Since wet etch does not allow the required level of control over the lithographic process, a reactive ion etching process using a sacrificial metallic stopping layer is developed and used for patterning the passivation layer. The process achieves devices with high reproducibility at the wafer scale.

Size-dependent mechanical properties of twin graphene

Twin Graphene, a novel 2D planar semiconducting carbon allotrope predicted recently, has attracted tremendous attention due to its potential applications in nano electromechanical systems (NEMS). In this paper, we use Molecular Dynamics simulations to investigate the mechanical properties of twin graphene. By performing uniaxial tensile loading, we find that the Young’s modulus, failure stress and fracture strain of a twin graphene sheet are strongly dependent on its size. Rectangular sheets show more apparent anisotropy of mechanical properties than square sheets. Our results also demonstrate that the fracture pattern of twin graphene is dependent on its geometry, as a result of its diverse bond types and orientations. These findings present an in-depth understanding of size dependent mechanical properties of twin graphene, and may benefit its future applications as building blocks of NEMS devices.

Patterning Graphene Films by H2O-Based Magnetic-Assisted UV Photolysis.

Properly cutting graphene into certain high-quality micro-/nanoscale structures in a cost-effective way has a critical role. Here, we report a novel approach to pattern graphene films by H2O-based magnetic-assisted ultraviolet (UV) photolysis under irradiation at 184.9 nm. By virtue of the paramagnetic characteristic, the photo-dissociated hydroxyl [OH(X2Π)] radicals are magnetized and have their oxidation capability highly enhanced through converting into an accelerated directional motion. Meanwhile, the precursor of H2O(X̃1A1) molecules distributes uniformly thanks to its weak diamagnetic characteristic, and there exists no instable diamagnetic intermediate to cause lateral oxidation. Possessing these unique traits, the H2O-based magnetic-assisted UV photolysis has the capability of making graphene microscale patterns with the linewidth down to 8.5 μm under a copper grid shadow mask. Furthermore, it is feasible to pattern graphene films into 40 nm-wide ribbons under ZnO nanowires and realize hybrid graphene/ZnO nanoribbon field-effect transistors with a hole mobility up to 7200 cm2·V-1·s-1. The X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses reveal that OH(X2Π) radicals act as a strong oxidant and that another product of H(12S) adsorbs weakly on graphene.

A graphene-based dual-band THz absorber design exploiting the impedance-matching concept

A THz wave absorber is designed by exploiting a unique concept and method. Three layers including dielectric and graphene patterns are incorporated on top of a thick gold surface to ensure zero transmission of THz waves through the structure. A favorable dual-bias scheme is used for the periodic arrays of graphene ribbons in the first layer, while the second layer consists of periodic arrays of graphene disks. Also, a top layer consisting of a continuous graphene sheet is used to enhance the overall absorption. All the components are modeled via a circuit model approach developed based on passive circuit elements. This leads to an explicit impedance calculation for the structure, paving the way to fully predict the device behavior. By applying impedance-matching theory, the structural parameters are tuned to obtain perfect dual-band absorption at frequencies of 5.5 and 8.5 THz. Extensive simulations verify the robustness of the structure versus variations in its geometrical parameters, while a great ability to tune the absorption peaks is achieved via variation of the chemical potentials. Such robust and tunable absorbers are attractive for the design of optical sensors, detectors, and modulators.

Graphene-metasurface for wide-incident-angle terahertz absorption

We demonstrate a graphene-metasurface structure for tunable wide-incident-angle terahertz wave absorption, which involves depositing planar arrays of Omega-shaped graphene patterns on a silicon dioxide substrate. We also discuss how the graphene Fermi-level layer and various substrates affect the absorption characteristics. The absorption of the proposed terahertz absorber is above 80% at an incident angle of 0°–60° in frequencies ranging from 0.82 to 2.0 THz. Our results will be very beneficial in the application of terahertz wave communications and biomedical imaging/sensing systems.

Integrated Sensing and Warning Multifunctional Devices Based on the Combined Mechanical and Thermal Effect of Porous Graphene.

Wearable devices with integrated alarm functions play a vital role in daily life and can help people prevent potential hazards. Although many wearable sensors have been extensively studied and proposed to monitor various physiological signals, most of them are needed to integrate with the external alarm elements to realize warning, such as light-emitting diodes and buzzers, resulting in the system complexity and poor flexibility. In this paper, an integrated sensing and warning multifunctional device based on the mechanical and thermal effect of porous graphene is proposed on a bilayer asymmetrical pattern of laser-induced graphene (LIG). Compared with the strain sensor with nonpatterned LIG, the mechanical performance is greatly improved with the highest gauge factor value of up to 950 for the strain sensor with mesh-patterned LIG. On the contrary, the heating performance of the heater with nonpatterned LIG is better than that with mesh-patterned LIG. Combining the performance differences of different LIG patterns, the integrated wearable device with a bilayer asymmetrical LIG pattern is demonstrated. It can generate enough heating energy to warn the person when the detected signal meets the threshold condition measured in real time by the ultrasensitive strain sensor. This work will provide a new way to construct an integrated wearable device for realizing multifunctional applications. This integrated multifunctional device shows great potential toward the applications in healthcare monitoring and timely warning.

Patterning of graphene for highly sensitive strain sensing on various curved surfaces

AbstractIn recent years, graphene based flexible strain sensors have attracted great attention. Although various designed micro/nanostructure morphologies have been introduced to optimize the performance parameters such as sensitivity and working range, there are relatively limited studies on the effective patterning of graphene on various substrates on curved surfaces to manufacture functional devices. In this work, we report a general route to realize the patterning of graphene into predesigned structure on a variety of complex curved substrates. The methodology involves a combination of polydopamine surface modification of the substrate and the spraying process of graphene. The polydopamine modification method preserves the intrinsic electrical properties and stability of graphene, and the modification process by self‐polymerization of dopamine is available on many types of materials of complex shape. This general approach of interface modification to achieve free patterning has capability of large‐scale fabrication. By this interface‐enhancement patterning method, sensing structure of cut‐through channel crack has been obtained, resulting in flexible strain sensors with adjusted high sensitivity (gauge factor of 100–2000 at 1% strain), little hysteresis and conformality of complex surfaces. In addition, the strain sensor array has also been fabricated to detect the deformation spatial distribution of complex curved structural components, thereby expanding their potential applications in wearable electronics.

Centimeter-Scale Ge-Assisted Grown Graphene Directly on SiO2/Si for NO2 Gas Sensors

For the manufacture of graphene based electronic devices, graphene growth directly on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates is a prerequisite. However, the problem of metal contamination is a key issue that cannot be avoided when synthesizing graphene on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates. Additionally, the direct fabrication of patterned graphene still suffers from inefficient and low repeatable problems. In this article, an approach based on a germanium (Ge)-assisted chemical vapor deposition (CVD) is proposed. It can produce graphene directly on any dielectric substrates. Graphene is grown directly on Ge-covered SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates due to the catalysis of Ge. By depositing the second Ge layer with a pattern by a designed hard mask onto the as-grown centimeter-scale and continuous graphene on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates followed by etching the graphene and removing the Ge, the monolayer graphene pattern was acquired. This process has the advantages of no metal pollution and high repeatability. Subsequently, the patterned and highly conductive graphene prepared on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates was used for gas sensor. The gas sensor exhibited well NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas sensing properties because of large charge transferred from graphene to NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> molecules.

Enhancing interface doping in graphene-metal hybrid devices using H2 plasma clean

Synthetic graphene suffers from surface contamination after the removal of transfer polymer. This impacts the chemical and electronic properties at the interface of graphene-metal structures. Hydrogen plasma treatment is promising to effectively clean, dope and pattern graphene. However, it also causes irreversible damage to the graphene lattice or reversible graphene hydrogenation which primarily depends on the flux and energy of plasma radicals/ions. Here, we optimize a downstream Ar/H2 plasma with moderate flux and low energy thermal H radicals to eliminate polymer residues and enhance the charge transfer at the graphene-metal interface. A systematic study is carried out to evaluate the plasma-induced doping and defectivity in ruthenium capped graphene devices. Single layer graphene is found to be n-doped after plasma treatment accompanied by an increase in defect density. Similarly, bilayer graphene (BLG) also undergoes n-doping but does not incur significant plasma-induced damage. Sheet resistance of Ru capped plasma cleaned BLG is found to decrease by 18%. After device fabrication, a final plasma clean reduces the surface roughness of the capping metal, and further improves the electrical conductivity. This study provides a viable basis to effectively clean and dope graphene-metal interface with a fab-compliant plasma technique while preserving the high quality of graphene.

Organic Thin Film Transistors Fabricated by a Solution Process Using Direct Patterned Single-Layer Graphene Electrodes.

We propose the direct transfer method of single-layer graphene (SLG) from metal catalyst Cu-foil to a polymeric insulator and the direct patterning method of the SLG for electrodes of organic thin-film transistors (OTFTs) without contamination using soft-lithography. Through soft-lithography, SLG can be formed in various patterns relatively easily in comparison with the conventional photolithography method that has multiple complex process steps to make graphene patterns. Furthermore, the 6,13-bis(triisopropylsilylethynyl) pentacene OTFTs are fabricated in solution with SLG source and drain electrodes. As a result, the field-effect mobility of OTFTs based on SLG electrodes was enhanced about 4 times in comparison with that of OTFTs using typical metal electrodes due to the decrease in contact resistance.

Laser‐Induced Graphene Tapes as Origami and Stick‐On Labels for Photothermal Manipulation via Marangoni Effect

AbstractDirect light‐to‐work conversion enables remote actuation through a non‐contact manner, among which the photothermal Marangoni effect is significant for developing light‐driven robots because of the diversity of applicable photothermal materials and light sources, as well as the high energy conversion efficiency. However, the lack of nanotechnologies that enable flexible integration of advanced photothermal materials with actuators of complex configurations significantly restricts their practical applications. In this paper, laser‐induced graphene (LIG) tape is reported as stick‐on photothermal labels for developing light‐driven actuators based on the Marangoni effect. With the help of direct laser writing technology, graphene patterns with superior photothermal properties are prepared on the PI tape. The patterned LIG tape can be stuck on any desired objects and generates an asymmetric photothermal field under light irradiation, forming a photothermal Marangoni actuator. Additionally, the PI tape with LIG patterns can be folded into 3D origami actuators that permit photothermal Marangoni actuation including both translation and rotation. The graphene‐based photothermal Marangoni actuators feature biocompatibility, which is confirmed by MDA‐MB‐231 cells proliferation experiments. Owing to the excellent photothermal property of LIG patterns, the as‐produced photothermal actuators can be manipulated by a variety of light sources, holding great promise for developing light‐driven soft robots.

Chemical Patterning of Graphene via Metal-Assisted Highly Energetic Electron Irradiation for Graphene Homojunction-Based Gas Sensors.

To gain the target functionality of graphene for gas detection, nonfocused and large-scale compatible MeV electron beam irradiation on graphene with Ag patterns is innovatively adopted in air for chemical patterning of graphene. This strategy allows the metal-assisted site-specific oxidation of graphene to realize monolithically integrated graphene-chemically patterned graphene (CPG)-graphene homojunction-based gas sensors. The size-tunable CPG patterns can be mediated by regulating the size of Ag prepatterns. The impacts of highly energetic electron irradiation (HEEI) on graphene are summarized as follows: (i) the selective p-type doping and the defect generation of graphene by the HEEI-induced oxidation, (ii) the resistance of the homojunction devices manipulated by the HEEI dose, (iii) the band gap opening of graphene as well as the lowering of the Fermi level, (iv) the work function values for pristine graphene and CPG corresponding to 4.14 and 4.88 eV, respectively, and (v) graphene-CPG-graphene homojunction for NO2 gas, revealing an 839% enhanced gas response compared with that of the pristine graphene-based gas sensor.

Tunable broadband terahertz absorber based on a single-layer graphene metasurface.

In this paper, a broadband and tunable terahertz absorber based on a graphene metasurface in a sandwiched structure is introduced. A single-layered graphene patterned with hollow-out squares is applied in this design, which is continuously connected to provide convenience for electrical tuning and fabrication. Plasmonic coupling and hybridization inside the graphene pattern can significantly enhance the absorption bandwidth. Moreover, polarization-insensitive and omnidirectional performances are also guaranteed by the symmetrical design. Full wave simulations demonstrate that the absorber exhibits over 90% absorbance within 1.14∼3.31 THz with a fractional bandwidth up to 97.5%. The device reveals tunable absorbance from 14% to almost 100% by manipulating the graphene chemical potential from 0 to 0.9 eV. When the incident angle sweeps up to 55°, the absorbance remains more than 90% from 1.77 to 3.42 THz for TE polarization, while over 90% absorbance maintains around 3.3 THz for TM polarization. These superior abilities guarantee the applicability of the presented absorber in THz cloaking, tunable sensor and photovoltaic devices.

Graphene-based multi-layers THz absorber: Circuit model representation

Leveraging both concept and method, a unique THz wave absorber is presented. Three layers include dielectric and graphene patterns that are exploited on top of a thick golden surface to ensure zero THz waves transmitting via the structure. Auspicious dual bias scheme is used for periodic arrays of graphene ribbons in the first layer while the second layer consists of periodic arrays of graphene disk. Also, the top layer is a continuous graphene sheet to enhance the whole absorption. All involved components are modeled via the developed circuit model approach as passive circuit elements. This leads to an exclusive impedance calculation of the structure, paves the way to the fully predictable behavior of the device. Thanks to impedance matching theory, the structure parameters are tuned to obtain perfect dual-band absorption at 5.5THz and 8.5THz frequencies. Ample simulations verify the robustness of the structure versus geometrical parameters while a great capability of tuning absorption peaks is achieved via chemical potentials variation. Such a robust and tunable absorber is attractive for designing optical sensors, detectors, and modulators.

Graphene based metasurface with near unity broadband absorption in the terahertz gap

This paper reports a novel graphene metasurface exhibiting near-unity absorption over a wide bandwidth of 9.74 THz ranging from 2.06 to 11.80 THz with a fractional bandwidth of 140.86% with respect to the center frequency of 6.93 THz within the terahertz gap of the electromagnetic spectrum. The proposed structure consists of two layers of graphene separated by silicon dioxide (SiO2). On the top of SiO2, a modified fractal-shaped 1 nm thick graphene pattern is deposited while a square-shaped graphene pattern of identical thickness completely covers the backside of the SiO2 substrate. The absorber was found to be polarization independent under the normal incidence of the approaching electromagnetic wave (EM) toward the top layer of the structure due to its 4-fold structural symmetry. The designed structure offers stable absorption up to 60° incident angle under TE polarization and 40° incident angle under TM polarization. The structure is ~λ/55.9 thin and maintains periodicity of ~λ/21.7; thereby validating the effective homogeneity condition in the sub-wavelength domain. This graphene-based absorber can find several applications including, sensors, detectors, spatial light modulators, and in spectroscopic detectors, and so forth, in the “Terahertz Gap” for next-generation 5G communication system and beyond.

Fabrication of UV Laser-Induced Porous Graphene Patterns with Nanospheres and Their Optical and Electrical Characteristics.

Many studies have been conducted to fabricate unique structures on flexible substrates and to apply such structures to a variety of fields. However, it is difficult to produce unique structures such as multilayer, nanospheres and porous patterns on a flexible substrate. We present a facile method of nanospheres based on laser-induced porous graphene (LIPG), by using laser-induced plasma (LIP). We fabricated these patterns from commercial polyimide (PI) film, with a 355 nm pulsed laser. For a simple one-step process, we used laser direct writing (LDW), under ambient conditions. We irradiated the PI film at a defocused plane −4 mm away from the focal plane, for high pulse overlap rate. The effect of the laser scanning speed was investigated by FE-SEM, to observe morphological characterization. Moreover, we confirmed the pattern characteristics by optical microscope, Raman spectroscopy and electrical experiments. The results suggested that we could modulate the conductivity and structural color by controlling the laser scanning speed. In this work, when the speed of the laser is 20 mm/s and the fluence is 5.28 mJ/cm2, the structural color is most outstanding. Furthermore, we applied these unique characteristics to various colorful patterns by controlling focal plane.

Graphene gets bent

Graphene gets bent

Suspended graphene sensor with controllable width and electrical tunability via direct-write functional fibers

Abstract In our study, we report Near-field electrospinning as a novel method of patterning and tuning suspended graphene channel. As a proof of concept, a resistive sensor has been fabricated for the detection of air carrying NH3 gas vapors. The suspended graphene channel, performing as the primary sensing element, changes its resistivity when NH3 molecules are absorbed on the surface of the graphene, effectively functioning as scattering centers. By utilizing the Near-field electrospinning for the deposition of the functional fibers, not only the width of the bridge but also the chemical doping effect on graphene can be readily tuned through the adjustment of process parameters and implementation of a wide range of polymers. About 78 % boost in sensitivity is confirmed when the width of suspended graphene channel is decreased from 15 μm to 5 μm. The presented process is expected to provide a simple, cost-effective and one-step precise manufacturing method by easily creating suspended graphene channels for a wide range of sensing applications not just limited to gas detection.

Multifunctional and tunable trigate graphene metamaterial with “Lakes of Wada” topology

Many plasmon-induced transparency (PIT) metamaterials previously reported had limited functions. Their tunabilities were realized by complex discrete structures, which greatly increased the difficulty and cost of device fabrication and adversely affected their resonance characteristics. It is an open question to adjust the Fermi levels of many graphene patterns with only a few in-plane electrodes. We propose and numerically study a novel electrically tunable and multifunctional trigate graphene metamaterial (TGGM) based on the concept of "Lakes of Wada". Benefiting from the trigate regulation, our proposed TGGM turns out to exhibit excellent characteristics, that can not only be used for terahertz band-stop filter, terahertz refractive index sensor, near-field optical switch, slow-light device, but also for double PIT window metamaterial with broad transparency windows and large tunable frequency range.

Optical contrast calculations to quatify modifications induced on trilayer graphene by Ti:Sapphire laser thinning process

A laser thinning process was developed for producing desired uniform patterns onto trilayer graphene. In contrast of exfoliated graphene which contains small domains with different layer numbers, we used large-size CVD trilayer graphene. This made possible designing of desired graphene patterns on the substrate surface with various laser fluences and scanning speeds. Such control are required for electronic and optical applications. Raman spectroscopy was used to compare produced patterns that were obtained using different laser parameters. We demonstrate optical contrast calculations as an efficient technique to distinguish between produced patterns on trilayer graphene. Comparison of optical contrasts of individually produced patterns under various conditions was performed using optical images taken by optical microscope with different objective lenses. This method can also be applied for graphene with more layers for rapid evaluation of patterns, especially in applications which need fast economic processing.