Nanoscale reduction of graphene fluoride via thermochemical nanolithography

Recent Advances in Fluorinated Graphene from Synthesis to Applications: Critical Review on Functional Chemistry and Structure Engineering

Fluorinated graphene (FG), as an emerging member of the graphene derivatives family, has attracted wide attention on account of its excellent performances and underlying applications. The introduction of a fluorine atom, with the strongest electronegativity (3.98), greatly changes the electron distribution of graphene, resulting in a series of unique variations in optical, electronic, magnetic, interfacial properties and so on. Herein, recent advances in the study of FG from synthesis to applications are introduced, and the relationship between its structure and properties is summarized in detail. Especially, the functional chemistry of FG has been thoroughly analyzed in recent years, which has opened a universal route for the functionalization and even multifunctionalization of FG toward various graphene derivatives, which further broadens its applications. Moreover, from a particular angle, the structure engineering of FG such as the distribution pattern of fluorine atoms and the regulation of interlayer structure when advanced nanotechnology gets involved is summarized. Notably, the elaborated structure engineering of FG is the key factor to optimize the corresponding properties for potential applications, and is also an up-to-date research hotspot and future development direction. Finally, perspectives and prospects for the problems and challenges in the study of FG are put forward.

Preparation and Applications of Fluorinated Graphenes

The present review focuses on the numerous routes for the preparation of fluorinated graphene (FG) according to the starting materials. Two strategies are considered: (i) addition of fluorine atoms on graphenes of various nature and quality and (ii) exfoliation of graphite fluoride. Chemical bonding in fluorinated graphene, related properties and a selection of applications for lubrication, energy storage, and gas sensing will then be discussed.

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.

Complex three-dimensional graphene structures driven by surface functionalization

The origami technique can provide inspiration for fabrication of novel three-dimensional (3D) structures with unique material properties from two-dimensional sheets. In particular, transformation of graphene sheets into complex 3D graphene structures is promising for functional nano-devices. However, practical realization of such structures is a great challenge. Here, we introduce a self-folding approach inspired by the origami technique to form complex 3D structures from graphene sheets using surface functionalization. A broad set of examples (Miura-ori, water-bomb, helix, flapping bird, dachshund dog, and saddle structure) is achieved via molecular dynamics simulations and density functional theory calculations. To illustrate the potential of the origami approach, we show that the graphene Miura-ori structure combines super-compliance, super-flexibility (both in tension and compression), and negative Poisson's ratio behavior.

Thermal scanning probe lithography-a review

Fundamental aspects and state-of-the-art results of thermal scanning probe lithography (t-SPL) are reviewed here. t-SPL is an emerging direct-write nanolithography method with many unique properties which enable original or improved nano-patterning in application fields ranging from quantum technologies to material science. In particular, ultrafast and highly localized thermal processing of surfaces can be achieved through the sharp heated tip in t-SPL to generate high-resolution patterns. We investigate t-SPL as a means of generating three types of material interaction: removal, conversion, and addition. Each of these categories is illustrated with process parameters and application examples, as well as their respective opportunities and challenges. Our intention is to provide a knowledge base of t-SPL capabilities and current limitations and to guide nanoengineers to the best-fitting approach of t-SPL for their challenges in nanofabrication or material science. Many potential applications of nanoscale modifications with thermal probes still wait to be explored, in particular when one can utilize the inherently ultrahigh heating and cooling rates.

Fluorinated Graphene: A Promising Macroscale Solid Lubricant under Various Environments

Graphene-related materials are promising solid lubricants owing to their easy shear between lattice layers. However, the coefficient of friction (COF) of graphene is not sufficiently low at the macroscale, and the lubrication performance is largely restricted by the external environment. In this study, we fabricated a fluorinated graphene (FG) coating on a stainless-steel substrate by a simple electrophoretic deposition in ethanol. The FG coating exhibited an excellent lubrication performance, which reduced the COF by 54.0 and 66.2% compared to those of pristine graphene and graphene oxide coatings, respectively. The lubrication enhancement of FG coating is attributed to its extremely low surface energy and interlaminar shear strength. The formation of ionic metal-fluorine chemical bonds provided a robust solid tribofilm and transfer layer on the friction pairs, which further increased the lubrication performance of the FG coating. The limited influence of the humidity on the lubrication performance of the FG coating is attributed to the hydrophobicity of the FG nanoflakes, which could prevent the influence of water molecules on the sliding interface. The excellent lubrication performance and better environmental adaptability of the FG make it a promising solid lubricant for mechanical engineering applications.

Monolayer MoS2 Nanoribbon Transistors Fabricated by Scanning Probe Lithography

Monolayer MoS₂ is a promising material for nanoelectronics; however, the lack of nanofabrication tools and processes has made it very challenging to realize nanometer-scale electronic devices from monolayer MoS₂. Here, we demonstrate the fabrication of monolayer MoS₂ nanoribbon field-effect transistors as narrow as 30 nm using scanning probe lithography (SPL). The SPL process uses a heated nanometer-scale tip to deposit narrow nanoribbon polymer structures onto monolayer MoS₂. The polymer serves as an etch mask during a XeF₂ vapor etch, which defines the channel of a field-effect transistor (FET). We fabricated seven devices with a channel width ranging from 30 to 370 nm, and the fabrication process was carefully studied by electronic measurements made at each process step. The nanoribbon devices have a current on/off ratio > 10⁴ and an extrinsic field-effect mobility up to 8.53 cm²/(V s). By comparing a 30 nm wide device with a 60 nm wide device that was fabricated on the same MoS₂ flake, we found the narrower device had a smaller mobility, a lower on/off ratio, and a larger subthreshold swing. To our knowledge, this is the first published work that describes a working transistor device from monolayer MoS₂ with a channel width smaller than 100 nm.

Reductive defluorination of graphite monofluoride by weak, non-nucleophilic reductants reveals low-lying electron-accepting sites

Graphite monofluoride (GF) can undergo reductive defluorination in the presence of weak, non-nucleophilic reductants. This leads to a new approach to GF-polyaniline composites as cathode materials for significantly improving the discharge capacity of primary lithium batteries. We postulate that the reduction is mediated by residual π-bonds in GF.

Parallelized biocatalytic scanning probe lithography for the additive fabrication of conjugated polymer structures

Scanning probe lithography (SPL) offers a more accessible alternative to conventional photolithography as a route to surface nanofabrication. In principle, the synthetic scope of SPL could be greatly enhanced by combining the precision of scanning probe systems with the chemoselectivity offered by biocatalysis. This report describes the development of multiplexed SPL employing probes functionalized with horseradish peroxidase, and its subsequent use for the constructive fabrication of polyaniline features on both silicon oxide and gold substrates. This polymer is of particular interest due to its potential applications in organic electronics, but its use is hindered by its poor processability, which could be circumvented by the direct in situ synthesis at the desired locations. Using parallelized arrays of probes, the lithography of polymer features over 1 cm2 areas was achieved with individual feature widths as small as 162 ± 24 nm. The nature of the deposited materials was confirmed by Raman spectroscopy, and it was further shown that the features could be chemically derivatized postlithographically by Huisgen [2 + 3] "click" chemistry, when 2-propargyloxyaniline was used as the monomer in the initial lithography step.

Nanopatterning of GeTe phase change films via heated-probe lithography

The crystallization of amorphous germanium telluride (GeTe) thin films is controlled with nanoscale resolution using the heat from a thermal AFM probe. The dramatic differences between the amorphous and crystalline GeTe phases yield embedded nanoscale features with strong topographic, electronic, and optical contrast. The flexibility of scanning probe lithography enables the width and depth of the features, as well as the extent of their crystallization, to be controlled by varying probe temperature and write speed. Together, these technologies suggest a new approach to nanoelectronic and opto-electronic device fabrication.

Friction and conductance imaging of sp(2)- and sp(3)-hybridized subdomains on single-layer graphene oxide

We investigated the subdomain structures of single-layer graphene oxide (GO) by characterizing local friction and conductance using conductive atomic force microscopy. Friction and conductance mapping showed that a single-layer GO flake has subdomains several tens to a few hundreds of nanometers in lateral size. The GO subdomains exhibited low friction (high conductance) in the sp(2)-rich phase and high friction (low conductance) in the sp(3)-rich phase. Current-voltage spectroscopy revealed that the local current flow in single-layer GO depends on the quantity of hydroxyl and carboxyl groups, and epoxy bridges within the 2-dimensional carbon layer. The presence of subdomains with different sp(2)/sp(3) carbon ratios on a GO flake was also confirmed by chemical mapping using scanning transmission X-ray microscopy. These results suggest that spatial mapping of the friction and conductance can be used to rapidly identify the composition of heterogeneous single-layer GO at nanometer scale, which is essential for understanding charge transport in nanoelectronic devices.

Basal Plane Fluorination of Graphene by XeF2 via a Radical Cation Mechanism

Graphene fluorination with XeF2 is an attractive method to introduce a nonzero bandgap to graphene under mild conditions for potential electro-optical applications. Herein, we use well-defined graphene nanostructures as a model system to study the reaction mechanism of graphene fluorination by XeF2. Our combined experimental and theoretical studies show that the reaction can proceed through a radical cation mechanism, leading to fluorination and sp(3)-hybridized carbon in the basal plane.

Patterning magnetic regions in hydrogenated graphene via e-beam irradiation

Partially hydrogenated graphene is ferromagnetic and may be patterned by electron-beam irradiation. Sequential patterning produces a patterned magnetic array. Removal of the hydrogen atoms also can convert electrically insulating fully hydrogenated graphene back into conductive graphene, enabling the writing of chemically isolated, dehydrogenated graphene nanoribbons as narrow as 100 nm.

Fluorination of graphene enhances friction due to increased corrugation

The addition of a single sheet of carbon atoms in the form of graphene can drastically alter friction between a nanoscale probe tip and a surface. Here, for the first time we show that friction can be altered over a wide range by fluorination. Specifically, the friction force between silicon atomic force microscopy tips and monolayer fluorinated graphene can range from 5-9 times higher than for graphene. While consistent with previous reports, the combined interpretation from our experiments and molecular dynamics simulations allows us to propose a novel mechanism: that the dramatic friction enhancement results from increased corrugation of the interfacial potential due to the strong local charge concentrated at fluorine sites, consistent with the Prandtl-Tomlinson model. The monotonic increase of friction with fluorination in experiments also demonstrates that friction force measurements provide a sensitive local probe of the degree of fluorination. Additionally, we found a transition from ordered to disordered atomic stick-slip upon fluorination, suggesting that fluorination proceeds in a spatially random manner.

Advanced scanning probe lithography

The nanoscale control afforded by scanning probe microscopes has prompted the development of a wide variety of scanning-probe-based patterning methods. Some of these methods have demonstrated a high degree of robustness and patterning capabilities that are unmatched by other lithographic techniques. However, the limited throughput of scanning probe lithography has prevented its exploitation in technological applications. Here, we review the fundamentals of scanning probe lithography and its use in materials science and nanotechnology. We focus on robust methods, such as those based on thermal effects, chemical reactions and voltage-induced processes, that demonstrate a potential for applications.

Harnessing catalysis to enhance scanning probe nanolithography

The use of scanning probes bearing catalysts to perform surface nanolithography combines the exquisite spatial precision of scanning probe microscopy with the synthetic capabilities of (bio)chemical catalysis. The ability to use these probes to direct a variety of localised chemical reactions enables the generation of nanoscale features with a high degree of chemical complexity in a "direct-write" manner. This article surveys the range of reactions that have been employed and the key factors necessary for the successful use of such catalytic scanning probes. These factors include the experimental parameters such as write speed, force applied to the probes and temperature; as well as the processes involved in the preparation of the catalysts on the probes and the surface that is to be fabricated. Where possible, the various reactions are also compared and contrasted; and future perspectives are discussed.

Parallelization of thermochemical nanolithography

One of the most pressing technological challenges in the development of next generation nanoscale devices is the rapid, parallel, precise and robust fabrication of nanostructures. Here, we demonstrate the possibility to parallelize thermochemical nanolithography (TCNL) by employing five nano-tips for the fabrication of conjugated polymer nanostructures and graphene-based nanoribbons.

Chemical stability of graphene fluoride produced by exposure to XeF2

Fluorination can alter the electronic properties of graphene and activate sites for subsequent chemistry. Here, we show that graphene fluorination depends on several variables, including XeF2 exposure and the choice of substrate. After fluorination, fluorine content declines by 50-80% over several days before stabilizing. While highly fluorinated samples remain insulating, mildly fluorinated samples regain some conductivity over this period. Finally, this loss does not reduce reactivity with alkylamines, suggesting that only nonvolatile fluorine participates in these reactions.