Patterning, characterization, and chemical sensing applications of graphene nanoribbon arrays down to 5 nm using helium ion beam lithography

Electronic, magnetic, and optical properties of Np and Pu decorated armchair graphene nanoribbons: a DFT study

We employed Density Functional Theory (DFT) to investigate the electronic, magnetic, and optical characteristics of armchair graphene nanoribbons (AGNRs) decorated with neptunium (Np) and plutonium (Pu). Our analysis delves deeply into the intricate orbital hybridizations associated with C-Np, C-Pu, C-C, Np-Np, and Pu-Pu chemical bonds. Through this approach, we explore the electronic band structure, band-decomposed charge densities, spin-charge distributions, and Van Hove singularities in the density of states. Furthermore, our examination successfully correlates optical excitation with electronic band energy. Our results indicated that these rare-earth atoms are strongly bound to the edge structure of AGNRs, significantly altering their electronic, magnetic, and optical properties. Theoretical exploration not only reveals the intriguing physical and chemical properties of rare-earth (Np/Pu) decorated AGNRs but also presents a practical pathway for synthesizing novel materials.

Cross-Scale Topography Achieved by MOPL with Positive Photoresist to Regulate the Cell Behavior

Cross-scale micro-nano structures play an important role in semiconductors, MEMS, chemistry, and cell biology. Positive photoresist is widely used in lithography due to the advantages of high resolution and environmental friendliness. However, cross-scale micro-nano structures of positive photoresist are difficult to flexibly pattern, and the feature resolution is limited by the optical diffraction. Here, cross-scale patterned micro-nano structures are achieved using the positive photoresist based on the femtosecond laser maskless optical projection lithography (MOPL) technique. The dependence between exposure dose and groove width is comprehensively analyzed, and a feature size of 112 nm is obtained at 110 µW. Furthermore, large-area topography considering cell size is efficiently fabricated by the MOPL technique, which enables the regulation of cell behavior. The proposed protocol of achieving cross-scale structures with the exact size by MOPL of positive photoresist would provide new avenues for potential applications in nanoelectronics and tissue engineering.

Thermal Scanning-Probe Lithography for Broad-Band On-Demand Plasmonic Nanostructures on Transparent Substrates

Thermal scanning-probe lithography (t-SPL) is a high-resolution nanolithography technique that enables the nanopatterning of thermosensitive materials by means of a heated silicon tip. It does not require alignment markers and gives the possibility to assess the morphology of the sample in a noninvasive way before, during, and after the patterning. In order to exploit t-SPL at its peak performances, the writing process requires applying an electric bias between the scanning hot tip and the sample, thereby restricting its application to conductive, optically opaque, substrates. In this work, we show a t-SPL-based method, enabling the noninvasive high-resolution nanolithography of photonic nanostructures onto optically transparent substrates across a broad-band visible and near-infrared spectral range. This was possible by intercalating an ultrathin transparent conductive oxide film between the dielectric substrate and the sacrificial patterning layer. This way, nanolithography performances comparable with those typically observed on conventional semiconductor substrates are achieved without significant changes of the optical response of the final sample. We validated this innovative nanolithography approach by engineering periodic arrays of plasmonic nanoantennas and showing the capability to tune their plasmonic response over a broad-band visible and near-infrared spectral range. The optical properties of the obtained systems make them promising candidates for the fabrication of hybrid plasmonic metasurfaces supported onto fragile low-dimensional materials, thus enabling a variety of applications in nanophotonics, sensing, and thermoplasmonics.

Electron-phonon interaction toward engineering carrier mobility of periodic edge structured graphene nanoribbons

Graphene nanoribbons have many extraordinary electrical properties and are the candidates for semiconductor industry. In this research, we propose a design of Coved GNRs with periodic structure ranged from 4 to 8 nm or more, of which the size is within practical feature sizes by advanced lithography tools. The carrier transport properties of Coved GNRs with the periodic coved shape are designed to break the localized electronic state and reducing electron-phonon scattering. In this way, the mobility of Coved GNRs can be enhanced by orders compared with the zigzag GNRs in same width. Moreover, in contrast to occasional zero bandgap transition of armchair and zigzag GNRs without precision control in atomic level, the Coved GNRs with periodic edge structures can exclude the zero bandgap conditions, which makes practical the mass production process. The designed Coved-GNRs is fabricated over the Germanium (110) substrate where the graphene can be prepared in the single-crystalline and single-oriented formants and the edge of GNRs is later repaired under "balanced condition growth" and we demonstrate that the propose coved structures are compatible to current fabrication facility.

Graphene: Preparation, tailoring, and modification

Graphene is a 2D material with fruitful electrical properties, which can be efficiently prepared, tailored, and modified for a variety of applications, particularly in the field of optoelectronic devices thanks to its planar hexagonal lattice structure. To date, graphene has been prepared using a variety of bottom-up growth and top-down exfoliation techniques. To prepare high-quality graphene with high yield, a variety of physical exfoliation methods, such as mechanical exfoliation, anode bonding exfoliation, and metal-assisted exfoliation, have been developed. To adjust the properties of graphene, different tailoring processes have been emerged to precisely pattern graphene, such as gas etching and electron beam lithography. Due to the differences in reactivity and thermal stability of different regions, anisotropic tailoring of graphene can be achieved by using gases as the etchant. To meet practical requirements, further chemical functionalization at the edge and basal plane of graphene has been extensively utilized to modify its properties. The integration and application of graphene devices is facilitated by the combination of graphene preparation, tailoring, and modification. This review focuses on several important strategies for graphene preparation, tailoring, and modification that have recently been developed, providing a foundation for its potential applications.

Consistent pattern printing of the gap structure in femtosecond laser DMD projection lithography

Maskless lithography technologies have been developed and played an important role in the fabrication of functional micronano devices for microelectronics, biochips and photonics. Optical projection lithography based on digital micromirror device (DMD) is an efficient maskless lithography technology that can rapidly fabricate complex structures. The precise modulation of gap width by DMD maskless optical projection lithography (MOPL) using femtosecond laser becomes important for achieving micronano structures. Herein, we have investigated the relationship between the structure morphology and the light intensity distribution at the image plane by multi-slit diffraction model and Abbe imaging principle, and optimized the gap width more accurately by modulating exposure energy. The aperture diameter of the objective lens has a substantial effect on the pattern consistency. The continuously adjustable structural gap widths of 2144 nm, 2158 nm and 1703 nm corresponding to 6, 12, 24 pixels are obtained by varying the exposure energy in the home-built MOPL system. However, the ideal gap structure cannot be obtained only by adjusting the exposure energy when the gap width is small, such as 1 or 2 pixels. Furthermore, we have proposed an alternative way to achieve fine gap structures through the structural decomposition design and precise control of exposure energy in different regions without changing the MOPL optical system. This study would provide a promising protocol for fabricating gap microstructures with controllable configuration using MOPL technique.

Versatile Approach of Silicon Nanofabrication without Resists: Helium Ion-Bombardment Enhanced Etching

Herein, we report a helium ion-bombardment enhanced etching method for silicon nanofabrication without the use of resists; furthermore, we demonstrate its unique advantages for straightforward fabrication on irregular surfaces and prototyping nano-electro-mechanical system devices, such as self-enclosed Si nanofluidic channels and mechanical nano-resonators. This method employs focused helium ions to selectively irradiate single-crystal Si to disrupt the crystal lattice and transform it into an amorphous phase that can be etched at a rate 200 times higher than that of the non-irradiated Si. Due to the unique raindrop shape of the interaction volumes between helium ions and Si, buried Si nanofluidic channels can be constructed using only one dosing step, followed by one step of conventional chemical etching. Moreover, suspended Si nanobeams can be fabricated without an additional undercut step for release owing to the unique raindrop shape. In addition, we demonstrate nanofabrication directly on 3D micro/nano surfaces, such as an atomic force microscopic probe, which is challenging for conventional nanofabrication due to the requirement of photoresist spin coating. Finally, this approach can also be extended to assist in the etching of other materials that are difficult to etch, such as silicon carbide (SiC).

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Achieving high aspect ratio in plasmonic lithography for practical applications with sub-20 nm half pitch

Plasmonic lithography, which exploits a bowtie nanoaperture (BNA) for the purpose of subwavelength near-field focusing, has the capability of high-resolution patterning. However, the ultra-small feature size is achieved at the price of sharply decay of the surface plasmon waves (SPWs) in the photoresist (PR) layer, which directly leads to some unfavorable patterning issues, such as non-uniformity and shallow pattern depth even over small exposure areas. In this work, a special hybrid plasmonic waveguide (HPW) patterning system, which is composed of the plasmonic BNA-PR layer-silver reflector, is designed to facilitate high spatial frequency selection and amplify the evanescent field in the PR layer. Theoretical calculations indicate that the antisymmetric coupled SPWs and plasmonic waveguide modes excited by the HPW structure can remove the exponential decay and ensure uniform exposure over the entire depth of the PR layer. Importantly, the hyperbolic decaying characteristic of the SPWs in the PR layer plays a noticeable role in the improvement of achievable resolution, depth-of-field, and line array pattern profile. It is worth to note that the uniform periodic patterns in sub-20 nm feature can be achieved with high aspect ratio. Additionally, further numerical simulation results are presented to demonstrate the achievement of spatial frequency selection of high-k mode in HPW structure by controlling the PR thickness and gap size. Our findings may provide a new perspective on the manufacture of surface nanostructures and broaden the potential promising applications of plasmonic lithography in nanoscale patterning.

Advanced wearable biosensors for the detection of body fluids and exhaled breath by graphene

Given the huge economic burden caused by chronic and acute diseases on human beings, it is an urgent requirement of a cost-effective diagnosis and monitoring process to treat and cure the disease in their preliminary stage to avoid severe complications. Wearable biosensors have been developed by using numerous materials for non-invasive, wireless, and consistent human health monitoring. Graphene, a 2D nanomaterial, has received considerable attention for the development of wearable biosensors due to its outstanding physical, chemical, and structural properties. Moreover, the extremely flexible, foldable, and biocompatible nature of graphene provide a wide scope for developing wearable biosensor devices. Therefore, graphene and its derivatives could be trending materials to fabricate wearable biosensor devices for remote human health management in the near future. Various biofluids and exhaled breath contain many relevant biomarkers which can be exploited by wearable biosensors non-invasively to identify diseases. In this article, we have discussed various methodologies and strategies for synthesizing and pattering graphene. Furthermore, general sensing mechanism of biosensors, and graphene-based biosensing devices for tear, sweat, interstitial fluid (ISF), saliva, and exhaled breath have also been explored and discussed thoroughly. Finally, current challenges and future prospective of graphene-based wearable biosensors have been evaluated with conclusion. Graphene is a promising 2D material for the development of wearable sensors. Various biofluids (sweat, tears, saliva and ISF) and exhaled breath contains many relevant biomarkers which facilitate in identify diseases. Biosensor is made up of biological recognition element such as enzyme, antibody, nucleic acid, hormone, organelle, or complete cell and physical (transducer, amplifier), provide fast response without causing organ harm.

Highly enhanced ferroelectricity in HfO2-based ferroelectric thin film by light ion bombardment

Continuous advancement in nonvolatile and morphotropic beyond-Moore electronic devices requires integration of ferroelectric and semiconductor materials. The emergence of hafnium oxide (HfO₂)-based ferroelectrics that are compatible with atomic-layer deposition has opened interesting and promising avenues of research. However, the origins of ferroelectricity and pathways to controlling it in HfO₂ are still mysterious. We demonstrate that local helium (He) implantation can activate ferroelectricity in these materials. The possible competing mechanisms, including He ion-induced molar volume changes, vacancy redistribution, vacancy generation, and activation of vacancy mobility, are analyzed. These findings both reveal the origins of ferroelectricity in this system and open pathways for nanoengineered binary ferroelectrics.

Enhanced current rectification in graphene nanoribbons: effects of geometries and orientations of nanopores

We discuss the possibility of getting rectification operation in graphene nanoribbon (GNR). For a system to be a rectifier, it must be physically asymmetric and we induce the asymmetry in GNR by introducing nanopores. The rectification properties are discussed for differently structured nanopores. We find that shape and orientation of the nanopores are critical and sensitive to the degree of current rectification. As the choice of Fermi energy is crucial for obtaining significant current rectification, explicit dependence of Fermi energy on the degree of current rectification is also studied for a particular shape of the nanopore. Finally, the role of nanopore size and different spatial distributions of the electrostatic potential profile across the GNR are explored. The stability of the nanopores is also discussed with a possible solution. Given the simplicity of the proposed method and promising results, the present proposition may lead to a new route of getting current rectification in different kinds of materials where nanopores can be formed selectively.

A Fullerene-Platinum Complex for Direct Functional Patterning of Single Metal Atom-Embedded Carbon Nanostructures

The development of patterning materials ("resists") at the nanoscale involves two distinct trends: one is toward high sensitivity and resolution for miniaturization, the other aims at functionalization of the resists to realize bottom-up construction of distinct nanoarchitectures. Patterning of carbon nanostructures, a seemingly ideal application for organic functional resists, has been highly reliant on complicated pattern transfer processes because of a lack of patternable precursors. Herein, we present a fullerene-metal coordination complex as a fabrication material for direct functional patterning of sub-10 nm metal-containing carbon structures. The attachment of one platinum atom per fullerene molecule not only leads to significant improvement of sensitivity and resolution but also enables stable atomic dispersion of the platinum ions within the carbon matrix, which may gain fundamentally new interest in functional patterning of hierarchical carbon nanostructures.

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.

Evolution of Graphene Patterning: From Dimension Regulation to Molecular Engineering

The realization that nanostructured graphene featuring nanoscale width can confine electrons to open its bandgap has aroused scientists' attention to the regulation of graphene structures, where the concept of graphene patterns emerged. Exploring various effective methods for creating graphene patterns has led to the birth of a new field termed graphene patterning, which has evolved into the most vigorous and intriguing branch of graphene research during the past decade. The efforts in this field have resulted in the development of numerous strategies to structure graphene, affording a variety of graphene patterns with tailored shapes and sizes. The established patterning approaches combined with graphene chemistry yields a novel chemical patterning route via molecular engineering, which opens up a new era in graphene research. In this review, the currently developed graphene patterning strategies is systematically outlined, with emphasis on the chemical patterning. In addition to introducing the basic concepts and the important progress of traditional methods, which are generally categorized into top-down, bottom-up technologies, an exhaustive review of established protocols for emerging chemical patterning is presented. At the end, an outlook for future development and challenges is proposed.

Super-Resolution Nanolithography of Two-Dimensional Materials by Anisotropic Etching

Nanostructuring allows altering of the electronic and photonic properties of two-dimensional (2D) materials. The efficiency, flexibility, and convenience of top-down lithography processes are, however, compromised by nanometer-scale edge roughness and resolution variability issues, which especially affect the performance of 2D materials. Here, we study how dry anisotropic etching of multilayer 2D materials with sulfur hexafluoride (SF₆) may overcome some of these issues, showing results for hexagonal boron nitride (hBN), tungsten disulfide (WS₂), tungsten diselenide (WSe₂), molybdenum disulfide (MoS₂), and molybdenum ditelluride (MoTe₂). Scanning electron microscopy and transmission electron microscopy reveal that etching leads to anisotropic hexagonal features in the studied transition metal dichalcogenides, with the relative degree of anisotropy ranked as: WS₂ > WSe₂ > MoTe₂ ∼ MoS₂. Etched holes are terminated by zigzag edges while etched dots (protrusions) are terminated by armchair edges. This can be explained by Wulff constructions, taking the relative stabilities of the edges and the AA' stacking order into account. Patterns in WS₂ are transferred to an underlying graphite layer, demonstrating a possible use for creating sub-10 nm features. In contrast, multilayer hBN exhibits no lateral anisotropy but shows consistent vertical etch angles, independent of crystal orientation. Using an hBN crystal as the base, ultrasharp corners can be created in lithographic patterns, which are then transferred to a graphite crystal underneath. We find that the anisotropic SF₆ reactive ion etching process makes it possible to downsize nanostructures and obtain smooth edges, sharp corners, and feature sizes significantly below the resolution limit of electron beam lithography. The nanostructured 2D materials can be used themselves or as etch masks to pattern other nanomaterials.

Broadband and switchable terahertz polarization converter based on graphene metasurfaces

In this work, we propose broadband and switchable terahertz (THz) polarization converters based on either graphene patch metasurface (GPMS) or its complementary structure (graphene hole metasurface, GHMS). The patch and hole are simply cross-shaped, composed of two orthogonal arms, along which plasmonic resonances mediated by Fabry-Perot cavity play a key role in polarization conversion (PC). An incidence of linear polarization will be converted to its cross-polarization (LTL) or circular polarization (LTC), as the reflected wave in the direction of two arms owning the same amplitude and π phase difference (LTL), or ±π/2 phase difference (LTC). Such requirements can be met by optimizing the width and length of two arms, thickness of dielectric layer, and Fermi level E_F of graphene. By using GPMS, LTL PC of polarization conversion ratio (PCR) over 90% is achieved in the frequency range of 2.92 THz to 6.26 THz, and by using GHMS, LTC PC of ellipticity χ ≤ -0.9 at the frequencies from 4.45 THz to 6.47 THz. By varying the Fermi level, the operating frequency can be actively tuned, and the functionality can be switched without structural modulation; for instance, GPMS supports LTL PC as E_F = 0.6 eV and LTC PC of χ ≥ 0.9 as E_F = 1.0 eV, in the frequency range of 2.69 THz to 4.19 THz. Moreover, GHMS can be optimized to sustain LTL PC and LTC PC of |χ| ≥ 0.9, in the frequency range of 4.96 THz to 6.52 THz, which indicates that the handedness of circular polarization can be further specified. The proposed polarization converters of broad bandwidth, active tunability, and switchable functionality will essentially make a significant progress in THz technology and device applications, and can be widely utilized in THz communications, sensing and spectroscopy.

A review of defect engineering, ion implantation, and nanofabrication using the helium ion microscope

The helium ion microscope has emerged as a multifaceted instrument enabling a broad range of applications beyond imaging in which the finely focused helium ion beam is used for a variety of defect engineering, ion implantation, and nanofabrication tasks. Operation of the ion source with neon has extended the reach of this technology even further. This paper reviews the materials modification research that has been enabled by the helium ion microscope since its commercialization in 2007, ranging from fundamental studies of beam-sample effects, to the prototyping of new devices with features in the sub-10 nm domain.

Tunable reflective dual-band line-to-circular polarization convertor with opposite handedness based on graphene metasurfaces

In this letter, we propose a dual-band tunable reflective linear-to-circular (LTC) polarization converter, which is composed of a graphene sheet etched with an I-shaped carved-hollow array. In the mid-infrared region, two LTC bands with opposite handedness are simultaneously realized due to the excitation of the three graphene surface plasmon (GSP) modes. The band of line-to-right-circular-polarization (LTRCP) ranges from 9.87 to 11.03THz with ellipticity χ <-0.95, and from 9.69 to 11.36 THz with an axial ratio of less than 3 dB; the band of line-to-left-circular-polarization (LTLCP) ranges from 13.16 to 14.43THz with χ >0.95, and from 12.79 to 14.61 THz with an axial ratio of less than 3 dB. The tunable responses of the reflective polarizer with Fermi energy (Ef) and electron scattering time (τ) are discussed, and especially the perfect LTLCP can be changed to LTRCP with increasing Ef. Also, the influences of geometric parameters, incident angle, and polarization angle on the performances of the dual-band LTC are also investigated, and it is found that our polarizer converter shows angle insensitivity. All simulation results are conducted by the finite element method. Our design enriches the research of tunable LTC polarizers and has potential applications in integrated terahertz systems.

Nanopatterning of monolayer graphene by quantum optical lithography

Quantum optical lithography, a diffraction-unlimited method, was applied to pattern monolayer graphene at 10 nm resolution. In our tests with chemical vapor deposition monolayer graphene samples, we have succeeded in producing flat surfaces of a sandwich of monolayer graphene-resist on Si, Si₃N₄, or glass substrates. Complex patterns have been written on monolayer graphene samples by a nanoablation process. The method could be used to realize monolayer graphene nanodevices.

Molecular embroidering of graphene

Structured covalent two-dimensional patterning of graphene with different chemical functionalities constitutes a major challenge in nanotechnology. At the same time, it opens enormous opportunities towards tailoring of physical and chemical properties with limitless combinations of spatially defined surface functionalities. However, such highly integrated carbon-based architectures (graphene embroidery) are so far elusive. Here, we report a practical realization of molecular graphene embroidery by generating regular multiply functionalized patterns consisting of concentric regions of covalent addend binding. These spatially resolved hetero-architectures are generated by repetitive electron-beam lithography/reduction/covalent-binding sequences starting with polymethyl methacrylate covered graphene deposited on a Si/SiO₂ substrate. The corresponding functionalization zones carry bromobenzene-, deutero-, and chloro-addends. We employ statistical Raman spectroscopy together with scanning electron microscopy/energy dispersive X-ray spectroscopy for an unambiguous characterization. The exquisitely ordered nanoarchitectures of these covalently multi-patterned graphene sheets are clearly visualized.

Covalently 2D-patterning graphene with different chemical functionalities is an attractive way to tailor its physical and chemical properties. Here, the authors realize spatially defined 2D-hetereoarchitectures of graphene via a strategy of molecular embroidering.

Multi-functional polarization conversion manipulation via graphene-based metasurface reflectors

In this work, we present an efficient polarization conversion device via using a hollow graphene metasurface. The platform can simultaneously realize a series of excellent performances, including the broadband x-to-y cross polarization conversion (CPC) function with near unity polarization conversion ratio (PCR), dual-frequency linear-to-circular polarization conversion (LTC-PC) function, and highly sensitive polarization conversion function manipulation under wide oblique incidence angle range. For instance, the proposed device obtains an x-to-y CPC function with the bandwidth up to 1.83 THz (χPCR ≥98.8%). Moreover, the x-to-y CPC function can be switched to LTC-PC function via artificially tuning the Fermi energy of graphene. The maximal frequency shift sensitivity (S) of polarization conversion function reaches 23.09 THz/eV, suggesting a frequency shift of 2.309 THz for the LTC-PC function when the chemical potential is changed by 0.1 eV. Based on these superior performances, the polarization converter can hold potential applications in integrated and compact devices, such as polarization sensor, switches and other optical polarization control components.

Focused Ion Beam Processing for 3D Chiral Photonics Nanostructures

The focused ion beam (FIB) is a powerful piece of technology which has enabled scientific and technological advances in the realization and study of micro- and nano-systems in many research areas, such as nanotechnology, material science, and the microelectronic industry. Recently, its applications have been extended to the photonics field, owing to the possibility of developing systems with complex shapes, including 3D chiral shapes. Indeed, micro-/nano-structured elements with precise geometrical features at the nanoscale can be realized by FIB processing, with sizes that can be tailored in order to tune optical responses over a broad spectral region. In this review, we give an overview of recent efforts in this field which have involved FIB processing as a nanofabrication tool for photonics applications. In particular, we focus on FIB-induced deposition and FIB milling, employed to build 3D nanostructures and metasurfaces exhibiting intrinsic chirality. We describe the fabrication strategies present in the literature and the chiro-optical behavior of the developed structures. The achieved results pave the way for the creation of novel and advanced nanophotonic devices for many fields of application, ranging from polarization control to integration in photonic circuits to subwavelength imaging.

Geometrically Structured Nanomaterials for Nanosensors, NEMS, and Nanosieves

Recently, geometrically structured nanomaterials have received great attention due to their unique physical and chemical properties, which originate from the geometric variation in such materials. Indeed, the use of various geometrically structured nanomaterials has been actively reported in enhanced-performance devices in a wide range of applications. Recent significant progress in the development of geometrically structured nanomaterials and associated devices is summarized. First, a brief introduction of advanced nanofabrication methods that enable the fabrication of various geometrically structured nanomaterials is given, and then the performance enhancements achieved in devices utilizing these nanomaterials, namely, i) physical and gas nanosensors, ii) nanoelectromechanical devices, and iii) nanosieves are described. For the device applications, a systematic summary of their structures, working mechanisms, fabrication methods, and output performance is provided. Particular focus is given to how device performance can be enhanced through the geometric structures of the nanomaterials. Finally, perspectives on the development of novel nanomaterial structures and associated devices are presented.

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.

Laser-Induced Graphene Electrochemical Immunosensors for Rapid and Label-Free Monitoring of Salmonella enterica in Chicken Broth

Food-borne illnesses are a growing concern for the food industry and consumers, with millions of cases reported every year. Consequently, there is a critical need to develop rapid, sensitive, and inexpensive techniques for pathogen detection in order to mitigate this problem. However, current pathogen detection strategies mainly include time-consuming laboratory methods and highly trained personnel. Electrochemical in-field biosensors offer a rapid, low-cost alternative to laboratory techniques, but the electrodes used in these biosensors require expensive nanomaterials to increase their sensitivity, such as noble metals (e.g., platinum, gold) or carbon nanomaterials (e.g., carbon nanotubes, or graphene). Herein, we report the fabrication of a highly sensitive and label-free laser-induced graphene (LIG) electrode that is subsequently functionalized with antibodies to electrochemically quantify the food-borne pathogen Salmonella enterica serovar Typhimurium. The LIG electrodes were produced by laser induction on the polyimide film in ambient conditions and, hence, circumvent the need for high-temperature, vacuum environment, and metal seed catalysts commonly associated with graphene-based electrodes fabricated via chemical vapor deposition processes. After functionalization with Salmonella antibodies, the LIG biosensors were able to detect live Salmonella in chicken broth across a wide linear range (25 to 10⁵ CFU mL^-1) and with a low detection limit (13 ± 7 CFU mL^-1; n = 3, mean ± standard deviation). These results were acquired with an average response time of 22 min without the need for sample preconcentration or redox labeling techniques. Moreover, these LIG immunosensors displayed high selectivity as demonstrated by nonsignificant response to other bacteria strains. These results demonstrate how LIG-based electrodes can be used for electrochemical immunosensing in general and, more specifically, could be used as a viable option for rapid and low-cost pathogen detection in food processing facilities before contaminated foods reach the consumer.

High sensitive gas sensor based on vertical graphene field effect transistor

A gas sensor made from graphene vertical field effect transistor (VGr-FET) has been fabricated using graphene as the source electrode, C₆₀ thin film as the semiconductor layer and aluminum thin film as the drain electrode. The on/off ratio of transistor gated by bottom electrode with ionic liquid gel as dielectric layer is derived to be 10³ from measured source-drain current I _ds. The apparent energy barrier height between the graphene and polycrystalline fullerene was calculated from the model of heterojunction diode I-V response curves. The barrier height φ _BH was altered by the gating potential vertically applied on graphene sheet, resulting the large on/off ratio of the transistor. The effect of surface adsorption of water vapor, oxygen, ammonia and isoprene gas phase molecules on the I _ds was measured. The lower limit of detection (LOD) for ammonia (86 ppb) than that of isoprene (420 ppb) is attributed to the donor nature of ammonia contact with p-type graphene, and the adsorbed donor leads to a corresponding positive gating effect to the VGr-FET. This facile, low cost and quick responsive device shows promise for early diagnose of severe human respiratory diseases.

Laser Patterning a Graphene Layer on a Ceramic Substrate for Sensor Applications

This paper describes a method for patterning the graphene layer and gold electrodes on a ceramic substrate using a Nd:YAG nanosecond fiber laser. The technique enables the processing of both layers and trimming of the sensor parameters. The main aim was to develop a technique for the effective and efficient shaping of both the sensory layer and the metallic electrodes. The laser shaping method is characterized by high speed and very good shape mapping, regardless of the complexity of the processing. Importantly, the technique enables the simultaneous shaping of both the graphene layer and Au electrodes in a direct process that does not require a complex and expensive masking process, and without damaging the ceramic substrate. Our results confirmed the effectiveness of the developed laser technology for shaping a graphene layer and Au electrodes. The ceramic substrate can be used in the construction of various types of sensors operating in a wide temperature range, especially the cryogenic range.

Spatially Controlled Fabrication and Mechanisms of Atomically Thin Nanowell Patterns in Bilayer WS2 Using in Situ High Temperature Electron Microscopy

We show controlled production of atomically thin nanowells in bilayer WS₂ using an in situ heating holder combined with a focused electron beam in a scanning transmission electron microscope (STEM). We systematically study the formation and evolvement mechanism involved in removing a single layer of WS₂ within a bilayer region with 2 nm accuracy in location and without punching through to the other layer to create a hole. Best results are found when using a high temperature of 800 °C, because it enables thermally activated atomic migration and eliminates the interference from surface carbon contamination. We demonstrate precise control over spatial distributions with 5 nm accuracy of patterning and the width of nanowells adjustable by dose-dependent parameters. The mechanism of removing a monolayer of WS₂ within a bilayer region is different than removing equivalent sections in a monolayer film due to the van der Waals interaction of the underlying remaining layer in the bilayer system that stabilizes the excess W atom stoichiometry within the edges of the nanowell structure and facilitates expansion. This study offers insights for the nanoengineering of nanowells in two-dimensional (2D) transitional metal dichalcogenides (TMDs), which could hold potential as selective traps to localize 2D reactions in molecules and ions, underpinning the broader utilization of 2D material membranes.

Tailored Nanopatterning by Controlled Continuous Nanoinscribing with Tunable Shape, Depth, and Dimension

We present that the tailored nanopatterning with tunable shape, depth, and dimension for diverse application-specific designs can be realized by utilizing controlled dynamic nanoinscribing (DNI), which can generate bur-free plastic deformation on various flexible substrates via continuous mechanical inscription of a small sliced edge of a nanopatterned mold in a compact and vacuum-free system. Systematic controlling of prime DNI processing parameters including inscribing force, temperature, and substrate feed rate can determine the nanopattern depths and their specific profiles from rounded to angular shapes as a summation of the force-driven plastic deformation and heat-driven thermal deformation. More complex nanopatterns with gradient depths and/or multidimensional profiles can also be readily created by modulating the horizontal mold edge alignment and/or combining sequential DNI strokes, which otherwise demand laborious and costly procedures. Many practical user-specific applications may benefit from this study by tailor-making the desired nanopattern structures within desired areas, including precision machine and optics components, transparent electronics and photonics, flexible sensors, and reattachable and wearable devices. We demonstrate one vivid example in which the light diffusion direction of a light-emitting diode can be tuned by application of specifically designed DNI nanopatterns.

Intact Crystalline Semiconducting Graphene Nanoribbons from Unzipping Nitrogen-Doped Carbon Nanotubes

Unzipping carbon nanotubes (CNTs) may offer a valuable route to synthesize graphene nanoribbon (GNR) structures with semiconducting properties. Unfortunately, currently available unzipping methods commonly rely on a random harsh chemical reaction and thereby cause significant degradation of the crystalline structure and electrical properties of GNRs. Herein, crystalline semiconducting GNRs are achieved by a synergistic, judiciously designed two-step unzipping method for N-doped CNTs (NCNTs). NCNTs are effectively unzipped by damage-minimized, dopant-specific electrochemical unzipping and subsequent sonochemical treatment into long ribbon-like nanostructures with crystalline basal planes. Owing to the nanoscale dimension originating from the dense nucleation of the unzipping reaction at highly NCNTs, the resultant GNRs demonstrate semiconducting properties, which can be exploited for chemiresistor-type gas-sensing devices and many other applications.

A Review of Recent Applications of Ion Beam Techniques on Nanomaterial Surface Modification: Design of Nanostructures and Energy Harvesting

Nanomaterials have gained plenty of research interest because of their excellent performance, which is derived from their small size and special structure. In practical applications, to acquire nanomaterials with high performance, many methods have been used to modulate the structure and components of materials. To date, ion beam techniques have extensively been applied for modulating the performance of various nanomaterials. Energetic ion beams can modulate the surface morphology and chemical components of nanomaterials. In addition, ion beam techniques have also been used to fabricate nanomaterials, including 2D materials, nanoparticles, and nanowires. Compared with conventional methods, ion beam techniques, including ion implantation, ion irradiation, and focused ion beam, are all pure physical processes; these processes do not introduce any impurities into the target materials. In addition, ion beam techniques exhibit high controllability and repeatability. Here, recent progress in ion beam techniques for nanomaterial surface modification is systematically summarized and existing challenges and potential solutions are presented.

Time-effective strategies for the fabrication of poly- and single-crystalline gold nano-structures by focused helium ion beam milling

Milling with the focused helium ion beam of a helium ion microscope is one of the most accurate ways to produce nano-structures such as plasmonic nanoantennas. In addition to good and immediate control of the dimensions, features in the sub-10 nm regime are achievable. Especially small gaps and sharp tips in this regime may lead to very high field enhancement under excitation. However, the milling rate of 30 keV helium ions is rather low, making it time-consuming to cut nano-structures out of a gold film. We present two processes to work around the low milling rate to obtain arrays of nano-structures with maximum precision within a reasonable time. These strategies can both be adapted to either poly-crystalline gold films or single-crystalline gold flakes. Using single crystals from a fabrication point of view enables even higher precision due to constant etch rates over the whole crystal as well as straight edges and vertical side-walls due to the uniform crystalline structure.

Fabrication of Uniaxially Aligned Silica Nanogrooves with Sub-5 nm Periodicity on Centimeter-Scale Si Substrate Using Poly(dimethylsiloxane) Stamps

The large-area fabrication of aligned nanopatterns with sub-5 nm feature size is crucial for developing nanodevices. Highly ordered nanostructures fabricated through molecular self-assembly exhibit substantial potential for sub-5 nm patterning techniques. Previously, we had reported the fabrication of silica nanogrooves with sub-5 nm periodicity on a Si substrate by using the outermost surfaces of cylindrical surfactant micelles as a template. However, uniaxial alignment of nanogrooves on the entire substrate surface has not yet been achieved. In this study, uniaxially aligned silica nanogrooves were prepared on the entire surface of a Si substrate (2 cm × 2 cm) by utilizing a poly(dimethylsiloxane) (PDMS) stamp with a striped pattern. The PDMS stamp was placed on the surface of a surfactant thin film precoated on the substrate, although the stamp was not in direct contact with the substrate as in the case of the soft nanoimprint technique. The substrate was then exposed to water vapor, during which cylindrical micelles were aligned in the direction of the guide. Subsequently, by exposing the substrate to an NH₃-water vapor mixture, the outermost surfaces of the aligned micelles facing the substrate were replicated with soluble silicate species. The formation of uniaxially aligned nanogrooves on the entire surface of the centimeter-scale substrate was verified by scanning electron microscopy observations and grazing-incidence small-angle X-ray scattering analysis. Thus, herein we provide a simple large-area fabrication method for uniaxially aligned nanopatterns with ultrafine pitch.

Gap-mode excitation, manipulation, and refractive-index sensing application by gold nanocube arrays

The challenges in fabricating two-dimensional metallic nanostructures over large areas, which normally involve expensive and time-consuming nanofabrication techniques, have severely limited the exploration of the related applications based on plasmon-induced effects. Here, we cost-efficiently prepared large-area Au nanocube arrays (NCAs) using only the electrostatic forces between colloidal Au nanocubes and polyelectrolyte layers. This method provides a flexible way for obtaining controlled Au NCAs with various fill fractions and single-cube sizes. When the Au NCAs were arranged to be coupled with a continuous Au film, the plasmonic gap mode could be excited and manipulated, leading to significant and tunable light absorbance from the visible to the near-infrared parts of the spectrum. Besides, the as-prepared Au NCAs were used to construct a prototype refractive-index (RI) sensor, which exhibited excellent stability and sensitivity over 560 nm per RI unit.

Defect Localization and Nanofabrication for Conductive Structures with Voltage Contrast in Helium Ion Microscopy

As the dimensions of feature sizes in electronic devices decrease to nanoscale, an easy method for failure analysis and evaluation of processing steps is required. Gallium-focused ion beam (Ga-FIB) or scanning electron microscope is an efficient approach to detect voltage contrast for addressing failure analysis in semiconductor devices and processing. However, Ga-FIB may cause damage or implantation to the surface of the analyzed area, and its resolution is low. Helium ion microscopy (HIM) uses a light ion beam (helium or neon) for imaging and fabrication at nanoscale. With passive voltage contrast (PVC) in HIM images, the defect localization for failure of conductive structures can be rapidly and easily detected with a sufficient voltage contrast. Furthermore, a defect gap as narrow as sub-10 nm can be investigated with HIM imaging. PVC with HIM is an efficient method for defect localization at nanoscale with a minimal damage to the analyzed area. For circuit edit and failure analysis, it may be necessary to intentionally cut the conductive connection. In this circumstance, final results can be easily verified using PVC imaging with HIM. With XeF₂ gas assistance, both helium and neon ion beams can be used to perform nanofabrication for metal disconnection. XeF₂ gas plays an important role in preventing deposition of conductive materials on etching region and enhancing material removal rates to achieve electrically isolated structures. The etching rate with a neon ion beam is much faster than that of a helium ion beam. PVC in HIM images with controllable operation and dimensions using a helium ion beam with XeF₂ gas assistance could also be used to localize a hidden defect for a single-location-defect situation. With neon ion beam irradiation on a defective location, PVC can be used to find the defect locations in the case of a series of defects.

Solution and on-surface synthesis of structurally defined graphene nanoribbons as a new family of semiconductors

Graphene nanoribbons (GNRs) are quasi-one-dimensional subunits of graphene and have open bandgaps in contrast to the zero-bandgap graphene. The high potential of GNRs as a new family of carbon-based semiconductors, e.g. for nanoelectronic and optoelectronic applications, has boosted the research attempts towards fabrication of GNRs. The predominant top-down methods such as lithographical patterning of graphene and unzipping of carbon nanotubes cannot prevent defect formation. In contrast, bottom-up chemical synthesis, starting from tailor-made molecular precursors, can achieve atomically precise GNRs. In this account, we summarize our recent research progress in the bottom-up synthesis of GNRs through three different methods, namely (1) in solution, (2) on-surface under ultrahigh vacuum (UHV) conditions, and (3) on-surface through chemical vapour deposition (CVD). The solution synthesis allows fabrication of long (>600 nm) and liquid-phase-processable GNRs that can also be functionalized at the edges. On the other hand, the on-surface synthesis under UHV enables formation of zigzag GNRs and in situ visualization of their chemical structures by atomic-resolution scanning probe microscopy. While the on-surface synthesis under UHV is typically costly and has limited scalability, the industrially viable CVD method can allow lower-cost production of large GNR films. We compare the three methods in terms of the affordable GNR structures and the resulting control of their electronic and optical properties together with post-processing for device integration. Further, we provide our views on future perspectives in the field of bottom-up GNRs.

Gallium, neon and helium focused ion beam milling of thin films demonstrated for polymeric materials: study of implantation artifacts

Focused ion beam milling of ∼200 nm polymer thin films is investigated using a multibeam ion microscope equipped with a gallium liquid metal ion source and a helium/neon gas field-ionization source. The quality of gallium, neon, and helium ion milled edges in terms of ion implantation artifacts is analyzed using a combination of helium ion microscopy, transmission electron microscopy and light microscopy. Results for a synthetic polymer thin film, in the form of cryo-ultramicrotomed sections from a co-extruded polymer multilayer, and a biological polymer thin film, in the form of the base layer of a butterfly wing scale, are presented. While gallium and neon ion milling result in the implantation of ions up to tens of nanometers from the milled edge and local thinning near the edge, helium ion milling produces much sharper edges with dramatically reduced implantation. These effects can be understood in terms of the minimal lateral scatter and larger stopping distance of helium compared with the heavier ions, whereby due to the thin film geometry, most of the incident helium ions will pass straight through the material. The basic result demonstrated here for polymer thin films is also expected for thin films of hard materials such as metals and ceramics.

Edge-Functionalized Graphene Nanoribbon Chemical Sensor: Comparison with Carbon Nanotube and Graphene

With growing focus on the use of carbon nanomaterials in chemical sensors, one-dimensional graphene nanoribbon (GNR) has become one of the most attractive channel materials, owing to its enhanced conductance fluctuation by quantum confinement effects and dense, abundant edge sites. Due to the narrow width of a basal plane with one-dimensional morphology, chemical modification of edge sites would greatly affect the electrical channel properties of a GNR. Here, we demonstrate for the first time that chemically functionalizing the edge sites with aminopropylsilane (APS) molecules can significantly enhance the sensing performance of the GNR sensor. The resulting APS-functionalized GNR has a sensitivity ((Δ R/ R_b)_max) of ∼30% at 0.125 ppm nitrogen dioxide (NO₂) and an ultrafast response time (∼6 s), which are, respectively, 7- and 15-fold enhancements compared to a pristine GNR sensor. This is the fastest and most sensitive gas-sensing performance of all GNR sensors reported. To demonstrate the superiority of the GNR-APS sensor, we compare its sensing performance with that of APS-functionalized carbon nanotube (CNT) and reduced graphene oxide (rGO) sensors prepared in identical synthesis conditions. Very interestingly, the GNR-APS sensor exhibited 30- and 93-fold enhanced sensitivity compared to the CNT-APS and rGO-APS sensors. This might be attributed to highly active edge sites with superior chemical reactivity, which are not present in CNT and rGO materials. Density functional theory clearly shows that the greatly enhanced gas response of GNR with edge functionalization can be attributed to the higher electron densities in the highest occupied molecular orbital levels of GNR-APS and incorporation of additional adsorption sites. This finding is the first demonstration of the importance of edge functionalization of GNR for chemical sensors.

Graphene nanogap electrodes in electrical biosensing

Graphene nanogap electrodes are reported here for the first time in an electrical biosensor for the detection of biomolecular interactions. Streptavidin-biotin was chosen as a model system for evaluating the sensor's performance. High-affinity interactions of streptavidin-gold nanoparticles (strep-AuNPs) to the biotin-functionalized nanogap localizes AuNPs, thereby bridging the gap and resulting in changes in device conductance. Biosensing performance was optimized by varying the gap size, AuNP diameter, and streptavidin coverage on AuNPs. The sensitivity and limit of detection (LOD) of streptavidin detection with the optimized parameters were determined to be 0.3 µA/nM and 0.25 pM, respectively. The proposed platform suggests high potential as a portable point-of-use biosensor for the detection of other affinity-based biomolecular interactions, such as antigen-antibody, nucleic acid, or chemo-selective interactions.

Near-field imaging of graphene triangles patterned by helium ion lithography

Plasmon nanoresonators in graphene have many applications in biosensing, photodetectors and modulators. As a result, an efficient and precise patterning technique for graphene is required. Helium ion lithography (HIL) emerges as a promising tool for direct writing fabrication because it owns improved fabrication precision compared to electron beam lithography and conventional gallium focused ion beam technique. In this paper, utilizing HIL, a set of graphene triangles are patterned and excellent plasmon response is detected. Particularly, the evolution of breathing mode in these structures is unveiled by scattering-type scanning near-field optical microscopy. Besides, the plasmon response of graphene structures can be efficiently tuned by adjusting the irradiated ion dose during the etching process, which can be explained by the phenomenal simulation model. Our work demonstrates that HIL is a feasible way for precise plasmonic nanostructure fabrication, and can be applied to graphene plasmon control at the nanoscale as well.

Multi-layer nanoarrays sandwiched by anodized aluminium oxide membranes: an approach to an inexpensive, reproducible, highly sensitive SERS substrate

A large-scale sub-5 nm nanofabrication technique is developed based on double layer anodized aluminium oxide (AAO) porous membrane masking. This technique also provides a facile route to form multilayer nano-arrays (metal nanoarrays sandwiched by AAO membranes), which is very challenging for other techniques. Normally the AAO mask has to be sacrificed, yet in this work it is preserved as a part of the nanostructure. The preserved AAO layers as the support for the second/third layer of the metal arrays provide a high-refractive index background for the multilayer metal arrays. This background concentrates the local E-field more significantly and results in a much higher Surface-Enhanced Raman Spectroscopy (SERS) signal than single layer metal arrays. This technique may lead to the advent of an inexpensive, reproducible, highly sensitive SERS substrate. Moreover, the physical essence of the plasmonic enhancement is unveiled by finite element method based numerical simulations. Enhancements from the gaps and the multilayer nanostructure agree very well with the experiments. The calculated layer-by-layer electric field distribution determines the contribution from different layers and provides more insights into the 3D textured structure.

Scanning Nanowelding Lithography for Rewritable One-Step Patterning of Sub-50 nm High-Aspect-Ratio Metal Nanostructures

The development of a new nanolithographic strategy, named scanning nanowelding lithography (SNWL), for the one-step fabrication of arbitrary high-aspect-ratio nanostructures of metal is reported in this study. Different from conventional pattern transfer and additive printing strategies which require subtraction or addition of materials, SNWL makes use of a sharp scanning tip to reshape metal thin films or existing nanostructures into desirable high-aspect-ratio patterns, through a cold-welding effect of metal at the nanoscale. As a consequence, SNWL can easily fabricate, in one step and at ambient conditions, sub-50 nm metal nanowalls with remarkable aspect ratio >5, which are found to be strong waveguide of light. More importantly, SNWL outweighs the existing strategies in terms of the unique ability to erase the as-made nanostructures and rewrite them into other shapes and orientations on-demand. Taking advantages of the serial and rewriting capabilities of SNWL, the smart information storage-erasure of Morse codes is demonstrated. SNWL is a promising method to construct arbitrary high-aspect-ratio nanostructure arrays that are highly desirable for biological, medical, optical, electronic, and information applications.

Exploring proximity effects and large depth of field in helium ion beam lithography: large-area dense patterns and tilted surface exposure

Helium ion beam lithography (HIL) is an emerging nanofabrication technique. It benefits from a reduced interaction volume compared to that of an electron beam of similar energy, and hence reduced long-range scattering (proximity effect), higher resist sensitivity and potentially higher resolution. Furthermore, the small angular spread of the helium ion beam gives rise to a large depth of field. This should enable patterning on tilted and curved surfaces without the need of any additional adjustments, such as laser-auto focus. So far, most work on HIL has been focused on exploiting the reduced proximity effect to reach single-digit nanometer resolution, and has thus been concentrated on single-pixel exposures over small areas. Here we explore two new areas of application. Firstly, we investigate the proximity effect in large-area exposures and demonstrate HIL's capabilities in fabricating precise high-density gratings on large planar surfaces (100 μm × 100 μm, with pitch down to 35 nm) using an area dose for exposure. Secondly, we exploit the large depth of field by making the first HIL patterns on tilted surfaces (sample stage tilted 45°). We demonstrate a depth of field greater than 100 μm for a resolution of about 20 nm.