CVD growth of large area smooth-edged graphene nanomesh by nanosphere lithography

Custom-made holey graphene via scanning probe block co-polymer lithography

Oxidative chemical etching of metal nanoparticles (NPs) to produce holey graphene (hG) suffers from the presence of aggregated NPs on the graphene surface triggering heterogeneous etching rates and thereby producing irregular sized holes. To encounter such a challenge, we investigated the use of scanning probe block co-polymer lithography (SPBCL) to fabricate precisely positioned silver nanoparticles (AgNPs) on graphene surfaces with exquisite control over the NP size to prevent their aggregation and consequently produce uniformly distributed holes after oxidative chemical etching. SPBCL experiments were carried out via printing an ink suspension consisting of poly(ethylene oxide-b-2-vinylpyridine) and silver nitrate on a graphene surface in a selected pattern under controlled environmental and instrumental parameters followed by thermal annealing in a gaseous environment to fabricate AgNPs. Scanning electron microscopy revealed the uniform size distribution of AgNPs on the graphene surface with minimal to no aggregation. Four main sizes of AgNPs were obtained (37 ± 3, 45 ± 3, 54 ± 2, and 64 ± 3 nm) via controlling the printing force, z-piezo extension, and dwell time. Energy dispersive X-ray spectroscopy analysis validated the existence of elemental Ag on the graphene surface. Subsequent chemical etching of AgNPs using nitric acid (HNO₃) with the aid of sonication and mechanical agitation produced holes of uniform size distribution generating hG. The obtained I _D/I _G ratios ≤ 0.96 measured by Raman spectroscopy were lower than those commonly reported for GO (I _D/I _G > 1), indicating the removal of more defective C atoms during the etching process to produce hG while preserving the remaining C atoms in ordered or crystalline structures. Indeed, SPBCL could be utilized to fabricate uniformly distributed AgNPs of controlled sizes on graphene surfaces to ultimately produce hG of uniform hole size distribution.

Playing with sizes and shapes of colloidal particles via dry etching methods

Monolayers of self-assembled quasi-spherical colloidal particles are essential building blocks in the field of materials science and engineering. More typically, they are used as a template for the fabrication of nanostructures if they serve, for instance, as a mask for deposition of new material on the surface on which particles are assembled or for etching of the material underneath; in this case, they are removed afterwards. This is what occurs in colloidal or nanosphere lithography. In some other cases, they are not used as a sacrificial material but they are incorporated in the final structure because they are inherently interesting for their properties. Independently of their specific use and application, different strategies have been devised in order to modify size and shape of colloidal particles, so as to enrich the variety of attainable patterns and to tailor the properties of the final structures and materials. In this review, we will focus on one of the most widespread methods to shape spherical colloidal particles, i.e. dry etching techniques. We will follow the development of such approaches until recent days, so as to trace an extensive panorama of the diverse parameters that can be harnessed to achieve specific morphological changes and highlight the characteristic features of the variants of this method. We will finally discuss how particles modified via dry etching can be used for patterning or can be resuspended in solvents for very diverse applications.

A graphene-printed paper electrode for determination of H2O2 in municipal wastewater during the COVID-19 pandemic

Graphene prepared through exfoliation process was printed on paper substrate using inkjet-printer and then printed paper electrode was used as an electrochemical sensor for analysis of H2O2 in cyclic voltammetry.

Recent Progress in the Transfer of Graphene Films and Nanostructures

The one-atom-thick graphene has excellent electronic, optical, thermal, and mechanical properties. Currently, chemical vapor deposition (CVD) graphene has received a great deal of attention because it provides access to large-area and uniform films with high-quality. This allows the fabrication of graphene based-electronics, sensors, photonics, and optoelectronics for practical applications. Zero bandgap, however, limits the application of a graphene film as electronic transistor. The most commonly used bottom-up approaches have achieved efficient tuning of the electronic bandgap by customizing well-defined graphene nanostructures. The postgrowth transfer of graphene films/nanostructures to a certain substrate is crucial in utilizing graphene in applicable devices. In this review, the basic growth mechanism of CVD graphene is first introduced. Then, recent advances in various transfer methods of as-grown graphene to target substrates are presented. The fabrication and transfer methods of graphene nanostructures are also provided, and then the transfer-related applications are summarized. At last, the challenging issues and the potential transfer-free approaches are discussed.

Synthesis of holey graphene for advanced nanotechnological applications

Holey or porous graphene, a structural derivative of graphene, has attracted immense attention due to its unique properties and potential applications in different branches of science and technology. In this review, the synthesis methods of holey or porous graphene/graphene oxide are systematically summarized and their potential applications in different areas are discussed. The process-structure-applications are explained, which helps relate the synthesis approaches to their corresponding key applications. The review paper is anticipated to benefit the readers in understanding the different synthesis methods of holey graphene, their key parameters to control the pore size distribution, advantages and limitations, and their potential applications in various fields.

Effects of A Magnetic Field on the Transport and Noise Properties of a Graphene Ribbon with Antidots

We perform a numerical simulation of the effects of an orthogonal magnetic field on charge transport and shot noise in an armchair graphene ribbon with a lattice of antidots. This study relies on our envelope-function based code, in which the presence of antidots is simulated through a nonzero mass term and the magnetic field is introduced with a proper choice of gauge for the vector potential. We observe that by increasing the magnetic field, the energy gap present with no magnetic field progressively disappears, together with features related to commensurability and quantum effects. In particular, we focus on the behavior for high values of the magnetic field: we notice that when it is sufficiently large, the effect of the antidots vanishes and shot noise disappears, as a consequence of the formation of edge states crawling along the boundaries of the structure without experiencing any interaction with the antidots.

Recent Advances in Graphene Patterning

As an emerging field of research, graphene patterning has received considerable attention because of its ability to tailor the structure of graphene and the respective properties, aiming at practical applications such as electronic devices, catalysts, and sensors. Recent efforts in this field have led to the development of a variety of different approaches to pattern graphene sheets, providing a multitude of graphene patterns with different shapes and sizes. These established patterning techniques in combination with graphene chemistry have paved the road towards highly attractive chemical patterning approaches, establishing a very promising and vigorously developing research topic. In this review, an overview of commonly used strategies is presented that are categorized into top-down and bottom-up routes for graphene patterning, focusing mainly on new advances. Other than the introduction of basic concepts of each method, the advantages/disadvantages are compared as well. In addition, for the first time, an overview of chemical patterning techniques is outlined. At the end, the challenges and future perspectives in the field are envisioned.

Polymer-Assisted Self-Assembly of Multicolor Carbon Dots as Solid-State Phosphors for Fabrication of Warm, High-Quality, and Temperature-Responsive White-Light-Emitting Devices

White-light-emitting devices (WLEDs) are considered to be a promising illumination source; especially, the WLEDs based on carbon dots (CDs) with white fluorescence have attracted extensive research interest. Herein, we report the design and implementation of solid white-light-emitting phosphors (WCDs@PS), which combine blue and orange emissive CDs (BCDs and OCDs) assisted by polystyrene (PS) through a self-assembly technique. Based on these phosphors (OCDs/BCDs = 1.2:1), the obtained WLEDs display a warm white light with International Commission on Illumination (CIE) coordinates of (0.35, 0.36), a high color rendering index of 93.2, a low correlated color temperature of 4075 K, and a luminous efficiency of up to 14.8 lm·W^-1. Interestingly, these WLEDs exhibit temperature-dependent emission performance, whose light-emission spectrum can be adjusted in situ from white (λ ∼ 400-730 nm) to blue (λ ∼ 440 nm) in the range of 20-80 °C. A change in CIE coordinates from (0.35, 0.36) to (0.32, 0.23) was also observed. The temperature-driven tunable LEDs as a thermochromism device could broaden the application of CDs-based lighting systems in special displays.

Lithographic band structure engineering of graphene

Two-dimensional materials such as graphene allow direct access to the entirety of atoms constituting the crystal. While this makes shaping by lithography particularly attractive as a tool for band structure engineering through quantum confinement effects, edge disorder and contamination have so far limited progress towards experimental realization. Here, we define a superlattice in graphene encapsulated in hexagonal boron nitride, by etching an array of holes through the heterostructure with minimum feature sizes of 12-15 nm. We observe a magnetotransport regime that is distinctly different from the characteristic Landau fan of graphene, with a sizeable bandgap that can be tuned by a magnetic field. The measurements are accurately described by transport simulations and analytical calculations. Finally, we observe strong indications that the lithographically engineered band structure at the main Dirac point is cloned to a satellite peak that appears due to moiré interactions between the graphene and the encapsulating material.

Ink-jet patterning of graphene by cap assisted barrier-guided CVD

Barrier-guided CVD growth could provide a new route to printed electronics by combining high quality 2D materials synthesis with scalable and cost-effective deposition methods. Unfortunately, we observe the limited stability of the barrier at growth conditions which results in its removal within minutes due to hydrogen etching. This work describes a route towards enhancing the stability of an ink-jet deposited barrier for high resolution patterning of high quality graphene. By modifying the etching kinetics under confinement, the barrier film could be stabilized and high resolution barriers could be retained even after 6 hours of graphene growth. Thus produced microscopic graphene devices exhibited an increase in conductivity by 6 orders of magnitude and a decrease in defectiveness by 48 times yielding performances that are superior to devices produced by traditional lithographical patterning which indicates the potential of our approach for future electronic applications.

High-resolution graphene patterning through ink-jet deposition of a barrier and subsequent CVD is achieved by a confinement-assisted growth process.

Modulating capacitive response of MoS2 flake by controlled nanostructuring through focused laser irradiation

Unlike graphene nanostructures, various physical properties of nanostructured MoS₂ have remained unexplored due to the lack of established fabrication routes. Herein, we have reported unique electrostatic properties of MoS₂ nanostructures, fabricated in a controlled manner of different geometries on 2D flake by using focused laser irradiation technique. Electrostatic force microscopy has been carried out on MoS₂ nanostructures by varying tip bias voltage and lift height. The analysis depicts no contrast flip in phase image of the patterned nanostructure due to the absence of free surface charges. However, prominent change in phase shift at the patterned area is observed. Such contrast changes signify the capacitive interaction between tip and nanostructures at varying tip bias voltage and lift height, irrespective of their shape and size. Such unperturbed capacitive behavior of the MoS₂ nanostructures offer modulation of capacitance in periodic array on 2D MoS₂ flake for potential application in capacitive devices.

Influence of the long-range ordering of gold-coated Si nanowires on SERS

Controlling the location and the distribution of hot spots is a crucial aspect in the fabrication of surface-enhanced Raman spectroscopy (SERS) substrates for bio-analytical applications. The choice of a suitable method to tailor the dimensions and the position of plasmonic nanostructures becomes fundamental to provide SERS substrates with significant signal enhancement, homogeneity and reproducibility. In the present work, we studied the influence of the long-range ordering of different flexible gold-coated Si nanowires arrays on the SERS activity. The substrates are made by nanosphere lithography and metal-assisted chemical etching. The degree of order is quantitatively evaluated through the correlation length (ξ) as a function of the nanosphere spin-coating speed. Our findings showed a linear increase of the SERS signal for increasing values of ξ, coherently with a more ordered and dense distribution of hot spots on the surface. The substrate with the largest ξ of 1100 nm showed an enhancement factor of 2.6 · 10³ and remarkable homogeneity over square-millimetres area. The variability of the signal across the substrate was also investigated by means of a 2D chemical imaging approach and a standard methodology for its practical calculation is proposed for a coherent comparison among the data reported in literature.

Structurally Controlled Large-Area 10 nm Pitch Graphene Nanomesh by Focused Helium Ion Beam Milling

Graphene nanomesh (GNM) is formed by patterning graphene with nanometer-scale pores separated by narrow necks. GNMs are of interest due to their potential semiconducting characteristics when quantum confinement in the necks leads to an energy gap opening. GNMs also have potential for use in phonon control and water filtration. Furthermore, physical phenomena, such as spin qubit, are predicted at pitches below 10 nm fabricated with precise structural control. Current GNM patterning techniques suffer from either large dimensions or a lack of structural control. This work establishes reliable GNM patterning with a sub-10 nm pitch and an < 4 nm pore diameter by the direct helium ion beam milling of suspended monolayer graphene. Due to the simplicity of the method, no postpatterning processing is required. Electrical transport measurements reveal an effective energy gap opening of up to ∼450 meV. The reported technique combines the highest resolution with structural control and opens a path toward GNM-based, room-temperature semiconducting applications.

Approaches to self-assembly of colloidal monolayers: A guide for nanotechnologists

Self-assembly of quasi-spherical colloidal particles in two-dimensional (2D) arrangements is essential for a wide range of applications from optoelectronics to surface engineering, from chemical and biological sensing to light harvesting and environmental remediation. Several self-assembly approaches have flourished throughout the years, with specific features in terms of complexity of the implementation, sensitivity to process parameters, characteristics of the final colloidal assembly. Selecting the proper method for a given application amidst the vast literature in this field can be a challenging task. In this review, we present an extensive classification and comparison of the different techniques adopted for 2D self-assembly in order to provide useful guidelines for scientists approaching this field. After an overview of the main applications of 2D colloidal assemblies, we describe the main mechanisms underlying their formation and introduce the mathematical tools commonly used to analyse their final morphology. Subsequently, we examine in detail each class of self-assembly techniques, with an explanation of the physical processes intervening in crystallization and a thorough investigation of the technical peculiarities of the different practical implementations. We point out the specific characteristics of the set-ups and apparatuses developed for self-assembly in terms of complexity, requirements, reproducibility, robustness, sensitivity to process parameters and morphology of the final colloidal pattern. Such an analysis will help the reader to individuate more easily the approach more suitable for a given application and will draw the attention towards the importance of the details of each implementation for the final results.

Directed growth of graphene nanomesh in purified argon via chemical vapor deposition

Graphene nanomeshes (GNMs), new graphene nanostructures with tunable bandgaps, are potential building blocks for future electronic or photonic devices, and energy storage and conversion materials. In previous works, GNMs have been successfully prepared on Cu foils by the H₂ etching effect. In this paper, we investigated the effect of Ar on the preparation of GNMs, and how the mean density and shape of them vary with growth time. In addition, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (TEM) revealed the typical hexagonal structure of GNM. Atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) indicated that large copper oxide nanoparticles produced by oxidization in purified Ar can play an essential catalytic role in preparing GNMs. Then, we exhibited the key reaction details for each growth process and proposed a growth mechanism of GNMs in purified Ar.

Epitaxial Growth of Large-Grain NiSe Films by Solid-State Reaction for High-Responsivity Photodetector Arrays

Film-based photodetectors have shown superiority for the fabrication of photodetector arrays, which are desired for integrating photodetectors into sensing and imaging systems, such as image sensors. But they usually possess a low responsivity due to low carrier mobility of the film consisting of nanocrystals. Large-grain semiconductor films are expected to fabricate superior-responsivity photodetector arrays. However, the growth of large-grain semiconductor films, normally with a nonlayer structure, is still challenging. Herein, this study introduces a solid-state reaction method, in which the growth rate is supposed to be limited by diffusion and reaction rate, for interface-confined epitaxial growth of nonlayer structured NiSe films with grain size up to micrometer scale on Ni foil. Meanwhile, patterned growth of NiSe films allows the fabrication of NiSe film based photodetector arrays. More importantly, the fabricated photodetector based on as-grown high-quality NiSe films shows a responsivity of 150 A W^-1 in contrast to the value of 0.009 A W^-1 from the photodetector based on as-deposited NiSe film consisting of nanocrystals, indicating a huge responsivity-enhancement up to four orders of magnitude. It is ascribed to the enhanced charge carrier mobility in as-grown NiSe films by dramatically decreasing the amount of grain boundary leading to scattering of charge carrier.

Current and future directions in electron transfer chemistry of graphene

The participation of graphene in electron transfer chemistry, where an electron is transferred between graphene and other species, encompasses many important processes that have shown versatility and potential for use in important applications.

Band-tunable photodetectors based on graphene/alloyed ZnxCd1−xS film hybrids

We have achieved band-tunable photodetectors based on graphene/alloyed ZnxCd_1–xS film hybrids with cut-off edge in spectra response gradually changed from 410 nm to 580 nm.

Bilayered graphene/h-BN with folded holes as new nanoelectronic materials: modeling of structures and electronic properties

The latest achievements in 2-dimensional (2D) material research have shown the perspective use of sandwich structures in nanodevices. We demonstrate the following generation of bilayer materials for electronics and optoelectronics. The atomic structures, the stability and electronic properties of Moiré graphene (G)/h-BN bilayers with folded nanoholes have been investigated theoretically by ab-initio DFT method. These perforated bilayers with folded hole edges may present electronic properties different from the properties of both graphene and monolayer nanomesh structures. The closing of the edges is realized by C-B(N) bonds that form after folding the borders of the holes. Stable ≪round≫ and ≪triangle≫ holes organization are studied and compared with similar hole forms in single layer graphene. The electronic band structures of the considered G/BN nanomeshes reveal semiconducting or metallic characteristics depending on the sizes and edge terminations of the created holes. This investigation of the new types of G/BN nanostructures with folded edges might provide a directional guide for the future of this emerging area.

Enhancing electric-field control of ferromagnetism through nanoscale engineering of high-Tc MnxGe1-x nanomesh

Voltage control of magnetism in ferromagnetic semiconductor has emerged as an appealing solution to significantly reduce the power dissipation and variability beyond current CMOS technology. However, it has been proven to be very challenging to achieve a candidate with high Curie temperature (T_c), controllable ferromagnetism and easy integration with current Si technology. Here we report the effective electric-field control of both ferromagnetism and magnetoresistance in unique Mn_xGe_1−x nanomeshes fabricated by nanosphere lithography, in which a T_c above 400 K is demonstrated as a result of size/quantum confinement. Furthermore, by adjusting Mn doping concentration, extremely giant magnetoresistance is realized from ∼8,000% at 30 K to 75% at 300 K at 4 T, which arises from a geometrically enhanced magnetoresistance effect of the unique mesh structure. Our results may provide a paradigm for fundamentally understanding the high T_c in ferromagnetic semiconductor nanostructure and realizing electric-field control of magnetoresistance for future spintronic applications.

Voltage control of magnetism in ferromagnetic semiconductor is appealing for spintronic applications, which is yet hindered by compound formation and low Curie temperature. Here, Nie et al. report electric-field control of ferromagnetism in Mn_xGe_1−x nanomeshes with a Curie temperature above 400 K and controllable giant magnetoresistance.

Nanopore Creation in Graphene by Ion Beam Irradiation: Geometry, Quality, and Efficiency

Ion beam irradiation is a promising approach to fabricate nanoporous graphene for various applications, including DNA sequencing, water desalination, and phase separation. Further advancement of this approach and rational design of experiments all require improved mechanistic understanding of the physical drilling process. Here, we demonstrate that, by using oblique ion beam irradiation, the nanopore family is significantly expanded to include more types of nanopores of tunable geometries. With the hopping, sweeping, and shoving mechanisms, ions sputter carbon atoms even outside the ion impact zone, leading to extended damage particularly at smaller incident angles. Moreover, with lower energies, ions may be absorbed to form complex ion-carbon structures, making the graphene warped or curly at pore edges. Considering both efficiency and quality, the optimal ion energy is identified to be 1000 eV at an incident angle of 30° with respect to the graphene sheet and 400-500 eV at higher incident angles. All of these results suggest the use of oblique ion beam and moderate energy levels to efficiently fabricate high-quality nanopores of tunable geometries in graphene for a wide range of applications.

Design methodology for a confocal imaging system using an objective microlens array with an increased working distance

In this study, a design methodology for a multi-optical probe confocal imaging system was developed. To develop an imaging system that has the required resolving power and imaging area, this study focused on a design methodology to create a scalable and easy-to-implement confocal imaging system. This system overcomes the limitations of the optical complexities of conventional multi-optical probe confocal imaging systems and the short working distance using a micro-objective lens module composed of two microlens arrays and a telecentric relay optical system. The micro-objective lens module was fabricated on a glass substrate using backside alignment photolithography and thermal reflow processes. To test the feasibility of the developed methodology, an optical system with a resolution of 1 μm/pixel using multi-optical probes with an array size of 10 × 10 was designed and constructed. The developed system provides a 1 mm × 1 mm field of view and a sample scanning range of 100 μm. The optical resolution was evaluated by conducting sample tests using a knife-edge detecting method. The measured lateral resolution of the system was 0.98 μm.

Facile synthesis of diverse graphene nanomeshes based on simultaneous regulation of pore size and surface structure

Recently, graphene nanomesh (GNM) has attracted great attentions due to its unique porous structure, abundant active sites, finite band gap and possesses potential applications in the fields of electronics, gas sensor/storage, catalysis, etc. Therefore, diverse GNMs with different physical and chemical properties are required urgently to meet different applications. Herein we demonstrate a facile synthetic method based on the famous Fenton reaction to prepare GNM, by using economically fabricated graphene oxide (GO) as a starting material. By precisely controlling the reaction time, simultaneous regulation of pore size from 2.9 to 11.1 nm and surface structure can be realized. Ultimately, diverse GNMs with tunable band gap and work function can be obtained. Specially, the band gap decreases from 4.5-2.3 eV for GO, which is an insulator, to 3.9-1.24 eV for GNM-5 h, which approaches to a semiconductor. The dual nature of electrophilic addition and oxidizability of HO(•) is responsible for this controllable synthesis. This efficient, low-cost, inherently scalable synthetic method is suitable for provide diverse and optional GNMs, and may be generalized to a universal technique.

Self-organized growth of graphene nanomesh with increased gas sensitivity

A bottom-up chemical vapor deposition (CVD) process for the growth of graphene nanomesh films is demonstrated. The process relies on silicon nanospheres to block nucleation sites for graphene CVD on copper substrates. These spheres are formed in a self-organized way through silicon diffusion through a 5 μm copper layer on a silicon wafer coated with 400 nm of silicon nitride. The temperature during the growth process disintegrates the Si3N4 layer and silicon atoms diffuse to the copper surface, where they form the nanospheres. After graphene nanomesh growth, the Si nanospheres can be removed by a simple hydrofluoric acid etch, leaving holes in the graphene film. The nanomesh films have been successfully transferred to different substrates, including gas sensor test structures, and verified and characterized by Auger, TEM and SEM measurements. Electrical/gas-exposure measurements show a 2-fold increase in ammonia sensitivity compared to plain graphene sensors. This improvement can be explained by a higher adsorption site density (edge sites). This new method for nanopatterned graphene is scalable, inexpensive and can be carried out in standard semiconductor industry equipment. Furthermore, the substrates are reusable.

High- and Reproducible-Performance Graphene/II-VI Semiconductor Film Hybrid Photodetectors

High- and reproducible-performance photodetectors are critical to the development of many technologies, which mainly include one-dimensional (1D) nanostructure based and film based photodetectors. The former suffer from a huge performance variation because the performance is quite sensitive to the synthesis microenvironment of 1D nanostructure. Herein, we show that the graphene/semiconductor film hybrid photodetectors not only possess a high performance but also have a reproducible performance. As a demo, the as-produced graphene/ZnS film hybrid photodetector shows a high responsivity of 1.7 × 10⁷ A/W and a fast response speed of 50 ms, and shows a highly reproducible performance, in terms of narrow distribution of photocurrent (38–65 μA) and response speed (40–60 ms) for 20 devices. Graphene/ZnSe film and graphene/CdSe film hybrid photodetectors fabricated by this method also show a high and reproducible performance. The general method is compatible with the conventional planar process, and would be easily standardized and thus pay a way for the photodetector applications.

Rapid Stencil Mask Fabrication Enabled One-Step Polymer-Free Graphene Patterning and Direct Transfer for Flexible Graphene Devices

We report a one-step polymer-free approach to patterning graphene using a stencil mask and oxygen plasma reactive-ion etching, with a subsequent polymer-free direct transfer for flexible graphene devices. Our stencil mask is fabricated via a subtractive, laser cutting manufacturing technique, followed by lamination of stencil mask onto graphene grown on Cu foil for patterning. Subsequently, micro-sized graphene features of various shapes are patterned via reactive-ion etching. The integrity of our graphene after patterning is confirmed by Raman spectroscopy. We further demonstrate the rapid prototyping capability of a stretchable, crumpled graphene strain sensor and patterned graphene condensation channels for potential applications in sensing and heat transfer, respectively. We further demonstrate that the polymer-free approach for both patterning and transfer to flexible substrates allows the realization of cleaner graphene features as confirmed by water contact angle measurements. We believe that our new method promotes rapid, facile fabrication of cleaner graphene devices, and can be extended to other two dimensional materials in the future.

Reduced graphene oxide nanoshells for flexible and stretchable conductors

Graphene has been extensively investigated for its use in flexible electronics, especially graphene synthesized by chemical vapor deposition (CVD). To enhance the flexibility of CVD graphene, wrinkles are often introduced. However, reports on the flexibility of reduced graphene oxide (RGO) films are few, because of their weak conductivity and, in particular, poor flexibility. To improve the flexibility of RGO, reduced graphene oxide nanoshells are fabricated, which combine self-assembled polystyrene nanosphere arrays and high-temperature thermal annealing processes. The resulting RGO films with nanoshells present a better resistance stabilization after stretching and bending the devices than RGO without nanoshells. The sustainability and performance advances demonstrated here are promising for the adoption of flexible electronics in a wide variety of future applications.

Dirac cone move and bandgap on/off switching of graphene superlattice

Using the density functional theory with generalized gradient approximation, we have studied in detail the cooperative effects of degenerate perturbation and uniaxial strain on bandgap opening in graphene. The uniaxial strain could split π bands into π_a and π_z bands with an energy interval E_s to move the Dirac cone. The inversion symmetry preserved antidot would then further split the π_a (π_z) bands into π_a1 (π_z1) and π_a2 (π_z2) bands with an energy interval E_d, which accounts for the bandgap opening in a kind of superlattices with Dirac cone being folded to Γ point. However, such antidot would not affect the semimetal nature of the other superlattices, showing a novel mechanism for bandstructure engineering as compared to the sublattice-equivalence breaking. For a superlattice with bandgap of ~E_d opened at Γ point, the E_s could be increased by strengthening strain to close the bandgap, suggesting a reversible switch between the high velocity properties of massless Fermions attributed to the linear dispersion relation around Dirac cone and the high on/off ratio properties associated with the sizable bandgap. Moreover, the gap width actually could be continuously tuned by controlling the strain, showing attractive application potentials.

Large and pristine films of reduced graphene oxide

A new self-assembly concept is introduced to form large and pristine films (15 cm in diameter) of reduced graphene oxide (RGO). The resulting film has different degrees of polarity on its two different sides due to the characteristic nature of the self-assembly process. The RGO film can be easily transferred from a glass substrate onto water and a polymer substrate after injection of water molecules between the RGO film and glass substrate using an electric steamer. The RGO film can also be easily patterned into various shapes with a resolution of around ± 10 μm by a simple taping method, which is suitable for mass production of printed electronics at low cost.

Non-templated ambient nanoperforation of graphene: a novel scalable process and its exploitation for energy and environmental applications

Nano-perforation of 2D graphene sheets is a recent and strategically significant means to exploit such materials in modern applications such as energy production and storage. However, current options for the synthesis of holey graphene (hG) through nano-perforation of graphene involve industrially undesirable steps viz., usage of expensive/noble metal or silica nanoparticle templates and/or hazardous chemicals. This severely hampers its scope for large scale production and further exploitation. Herein, we report for the first time a scalable non-templated route to produce hG at ambient conditions. Nano-perforation is achieved with tunable pore size via the simple few layer co-assembly of silicate-surfactant admicelles along the surface of graphene oxide. A gentle alkali treatment and a reduction at optimized conditions readily yielded holey graphene with a remarkable capacitance (∼250 F g(-1)) and interesting adsorption abilities for pollutants. Density functional theory based computational studies reveal interesting insights on the template free nano-perforation at a molecular level. This simple rapid process not only excludes the need for expensive templates and harmful chemicals to yield hG at attractively ambient, chemically placid and industrially safer conditions, but also creates no hurdles in terms of scaling up.

Programmable Extreme Pseudomagnetic Fields in Graphene by a Uniaxial Stretch

Many of the properties of graphene are tied to its lattice structure, allowing for tuning of charge carrier dynamics through mechanical strain. The graphene electromechanical coupling yields very large pseudomagnetic fields for small strain fields, up to hundreds of Tesla, which offer new scientific opportunities unattainable with ordinary laboratory magnets. Significant challenges exist in investigation of pseudomagnetic fields, limited by the nonplanar graphene geometries in existing demonstrations and the lack of a viable approach to controlling the distribution and intensity of the pseudomagnetic field. Here we reveal a facile and effective mechanism to achieve programmable extreme pseudomagnetic fields with uniform distributions in a planar graphene sheet over a large area by a simple uniaxial stretch. We achieve this by patterning the planar graphene geometry and graphene-based heterostructures with a shape function to engineer a desired strain gradient. Our method is geometrical, opening up new fertile opportunities of strain engineering of electronic properties of 2D materials in general.

Photocurrent generation in lateral graphene p-n junction created by electron-beam irradiation

Graphene has been considered as an attractive material for optoelectronic applications such as photodetectors owing to its extraordinary properties, e.g. broadband absorption and ultrahigh mobility. However, challenges still remain in fundamental and practical aspects of the conventional graphene photodetectors which normally rely on the photoconductive mode of operation which has the drawback of e.g. high dark current. Here, we demonstrated the photovoltaic mode operation in graphene p-n junctions fabricated by a simple but effective electron irradiation method that induces n-type doping in intrinsic p-type graphene. The physical mechanism of the junction formation is owing to the substrate gating effect caused by electron irradiation. Photoresponse was obtained for this type of photodetector because the photoexcited electron-hole pairs can be separated in the graphene p-n junction by the built-in potential. The fabricated graphene p-n junction photodetectors exhibit a high detectivity up to ~3 × 10¹⁰ Jones (cm Hz^1/2 W⁻¹) at room temperature, which is on a par with that of the traditional III–V photodetectors. The demonstrated novel and simple scheme for obtaining graphene p-n junctions can be used for other optoelectronic devices such as solar cells and be applied to other two dimensional materials based devices.

Large-Area Semiconducting Graphene Nanomesh Tailored by Interferometric Lithography

Graphene nanostructures are attracting a great deal of interest because of newly emerging properties originating from quantum confinement effects. We report on using interferometric lithography to fabricate uniform, chip-scale, semiconducting graphene nanomesh (GNM) with sub-10 nm neck widths (smallest edge-to-edge distance between two nanoholes). This approach is based on fast, low-cost, and high-yield lithographic technologies and demonstrates the feasibility of cost-effective development of large-scale semiconducting graphene sheets and devices. The GNM is estimated to have a room temperature energy bandgap of ~30 meV. Raman studies showed that the G band of the GNM experiences a blue shift and broadening compared to pristine graphene, a change which was attributed to quantum confinement and localization effects. A single-layer GNM field effect transistor exhibited promising drive current of ~3.9 μA/μm and ON/OFF current ratios of ~35 at room temperature. The ON/OFF current ratio of the GNM-device displayed distinct temperature dependence with about 24-fold enhancement at 77 K.

Gold nanodome-patterned microchips for intracellular surface-enhanced Raman spectroscopy

Top-down patterned gold nanodome microchips are taken up by living cells and serve as a uniform and reproducible sensor for intracellular surface-enhanced Raman scattering.

Functionalization of graphene at the organic/water interface

A simple method for the deposition of noble metal (Pd, Au) nanoparticles on a free-standing chemical vapour deposited graphene monolayer is reported. Metal deposition can proceed using either spontaneous or electrochemically-controlled processes. The resultant nanoclusters are characterized using atomic force and electron microscopy techniques, and mapping mode Raman spectroscopy.

A simple method for the deposition of noble metal (Pd, Au) nanoparticles on a free-standing chemical vapour deposited graphene (CVD GR) monolayer is reported. The method consists of assembling the high purity CVD GR, by transfer from poly (methyl methacrylate) (PMMA), at the organic/water interface. Metal deposition can then proceed using either spontaneous or electrochemically-controlled processes. The resultant graphene-based metal nanoclusters are characterized using atomic force and electron microscopy techniques, and the location of the nanostructures underneath the graphene layer is determined from the position and the intensity changes of the Raman bands (D, G, 2D). This novel process for decoration of a single-layer graphene sheet with metal nanoparticles using liquid/liquid interfaces opens an alternative and useful way to prepare low dimensional carbon-based nanocomposites and electrode materials.

Graphene nanomesh: new versatile materials

Graphene, an atomic-scale honeycomb crystal lattice, is increasingly becoming popular because of its excellent mechanical, electrical, chemical, and physical properties. However, its zero bandgap places restrictions on its applications in field-effect transistors (FETs). Graphene nanomesh (GNM), a new graphene nanostructure with a tunable bandgap, shows more excellent performance. It can be widely applied in electronic or photonic devices such as highly sensitive biosensors, new generation of spintronics and energy materials. These illustrate significant opportunities for the industrial use of GNM, and hence they push nanoscience and nanotechnology one step toward practical applications. This review briefly describes the current status of the design, synthesis, and potential applications of GNM. Finally, the perspectives and challenges of GNM development are presented and some suggestions are made for its further development and exploration.

Exceptionally strong and robust millimeter-scale graphene-alumina composite membranes

Graphene has attracted attention as a potential strengthening material and functional component in suspended membranes as utilized in micro and nanosystems. Development of a practical and scalable fabrication process is a necessary step to allow the exceptional material properties of graphene to be fully exploited in composite structures. Using standard and scalable microfabrication processes, we fabricated free-standing chemical vapor deposition monolayer graphene-reinforced Al2O3 composite membranes, 0.5 mm in diameter, that are strong and robust. Bulge tests revealed that the graphene reinforcement increased the membrane fracture strength by a factor of at least three and maximum sustainable strain from 0.28% to at least 0.69%. We show that the graphene-reinforced membranes are even tolerant to significant cracking without loss of membrane integrity. The graphene composite membranes' freestanding area of ∼ 200 000 μm(2) is almost a thousand times larger than suspended graphene membranes reported elsewhere. The presented graphene composite membranes may be seen as representing an interesting new class of durable composite materials warranting further study and having potential for broad applicability in a variety of fields.

Scalable holey graphene synthesis and dense electrode fabrication toward high-performance ultracapacitors

Graphene has attracted a lot of attention for ultracapacitor electrodes because of its high electrical conductivity, high surface area, and superb chemical stability. However, poor volumetric capacitive performance of typical graphene-based electrodes has hindered their practical applications because of the extremely low density. Herein we report a scalable synthesis method of holey graphene (h-Graphene) in a single step without using any catalysts or special chemicals. The film made of the as-synthesized h-Graphene exhibited relatively strong mechanical strength, 2D hole morphology, high density, and facile processability. This scalable one-step synthesis method for h-Graphene is time-efficient, cost-efficient, environmentally friendly, and generally applicable to other two-dimensional materials. The ultracapacitor electrodes based on the h-Graphene show a remarkably improved volumetric capacitance with about 700% increase compared to that of regular graphene electrodes. Modeling on individual h-Graphene was carried out to understand the excellent processability and improved ultracapacitor performance.

Dirac model of electronic transport in graphene antidot barriers

In order to use graphene for semiconductor applications, such as transistors with high on/off ratios, a band gap must be introduced into this otherwise semimetallic material. A promising method of achieving a band gap is by introducing nanoscale perforations (antidots) in a periodic pattern, known as a graphene antidot lattice (GAL). A graphene antidot barrier (GAB) can be made by introducing a 1D GAL strip in an otherwise pristine sheet of graphene. In this paper, we will use the Dirac equation (DE) with a spatially varying mass term to calculate the electronic transport through such structures. Our approach is much more general than previous attempts to use the Dirac equation to calculate scattering of Dirac electrons on antidots. The advantage of using the DE is that the computational time is scale invariant and our method may therefore be used to calculate properties of arbitrarily large structures. We show that the results of our Dirac model are in quantitative agreement with tight-binding for hexagonal antidots with armchair edges. Furthermore, for a wide range of structures, we verify that a relatively narrow GAB, with only a few antidots in the unit cell, is sufficient to give rise to a transport gap.

The selective formation of graphene ranging from two-dimensional sheets to three-dimensional mesoporous nanospheres

This research presents a template-free solvothermal method which offers selective preparation of graphene ranging from two-dimensional sheets to 3-dimensional nanospheres. The thus prepared nanospheres have size-defined mesopores with a huge surface area and, after doping with nitrogen, exhibited stronger electrocatalytic activity toward oxygen reduction than commercial Pt/C catalysts.

Fabrication of a graphene nanomesh using a platinum nano-network as a pattern mask

Here, we report a facile method to fabricate a graphene nanomesh (GNM) by using a platinum (Pt) metal nano-network as a pattern mask. A hexagonally ordered Pt nano-network (i.e. nanomesh) with high-density arrays of periodic nano-holes was synthesized using an anodized alumina template, which served perfectly as a pattern mask for generating GNMs with tunable pore neck widths. Altering the neck width of the pores allows the modulation of the electrical conductivity of the GNMs. Resultant GNMs were further characterized using Raman spectroscopy and their electrical properties as conducting channels in field-effect transistors (FETs) were evaluated as a function of neck width. This synthetic route for producing GNMs provides a low-cost and simple way to fabricate GNMs for use in future fundamental studies related to graphene.

Porous graphene materials for advanced electrochemical energy storage and conversion devices

Combining the advantages from both porous materials and graphene, porous graphene materials have attracted vast interests due to their large surface areas, unique porous structures, diversified compositions and excellent electronic conductivity. These unordinary features enable porous graphene materials to serve as key components in high-performance electrochemical energy storage and conversion devices such as lithium ion batteries, supercapacitors, and fuel cells. This progress report summarizes the typical fabrication methods for porous graphene materials with micro-, meso-, and macro-porous structures. The structure-property relationships of these materials and their application in advanced electrochemical devices are also discussed.

Scalable, flexible and high resolution patterning of CVD graphene

The unique properties of graphene make it a promising material for interconnects in flexible and transparent electronics. To increase the commercial impact of graphene in those applications, a scalable and economical method for producing graphene patterns is required. The direct synthesis of graphene from an area-selectively passivated catalyst substrate can generate patterned graphene of high quality. We here present a solution-based method for producing patterned passivation layers. Various deposition methods such as ink-jet deposition and microcontact printing were explored, that can satisfy application demands for low cost, high resolution and scalable production of patterned graphene. The demonstrated high quality and nanometer precision of grown graphene establishes the potential of this synthesis approach for future commercial applications of graphene. Finally, the ability to transfer high resolution graphene patterns onto complex three-dimensional surfaces affords the vision of graphene-based interconnects in novel electronics.

Bulk preparation of holey graphene via controlled catalytic oxidation

Structural manipulation of the two dimensional graphene surface has been of significant interest as a means of tuning the properties of the nanosheets for enhanced performance in various applications. In this report, a straightforward and highly scalable method is presented to prepare bulk quantities of "holey graphenes", which are graphene sheets with holes ranging from a few to tens of nm in average diameter. The approach to their preparation takes advantage of the catalytic properties of silver (Ag) nanoparticles toward the air oxidation of graphitic carbon. In the procedure, Ag nanoparticles were first deposited onto the graphene sheet surface in a facile, controllable, and solvent-free process. The catalyst-loaded graphene samples were then subjected to thermal treatment in air. The graphitic carbons in contact with the Ag nanoparticles were selectively oxidized into gaseous byproducts, such as CO or CO2, leaving holes in the graphene surface. The Ag was then removed by refluxing in diluted nitric acid to obtain the final holey graphene products. The average size of the holes on the graphene was found to correlate with the size of the Ag nanoparticles, which could be controlled by adjusting the silver precursor concentration. In addition, the temperature and time of the air oxidation step, and the catalyst removal treatment conditions were found to strongly affect the morphology of the holes. Characterization results of the holey graphene products suggested that the hole generation might have started from defect-rich regions present on the starting graphene sheets. As a result, the remaining graphitic carbon structures on the holey graphene sheets were highly crystalline, with no significant increase of the overall defect density despite the presence of structural holes. Preliminary experiments are also presented on the use of holey graphene sheets as fillers for polymeric composites. The results indicated that these sheets might be better reinforcing fillers than the starting graphene sheets due to their perforated structure. Other unique potentials of these materials, such as for energy storage applications, are also discussed.