Matthew D. Barnes

Properties and applications of chemically functionalized graphene

The vast and yet largely unexplored family of graphene materials has great potential for future electronic devices with novel functionalities. The ability to engineer the electrical and optical properties in graphene by chemically functionalizing it with a molecule or adatom is widening considerably the potential applications targeted by graphene. Indeed, functionalized graphene has been found to be the best known transparent conductor or a wide gap semiconductor. At the same time, understanding the mechanisms driving the functionalization of graphene with hydrogen is proving to be of fundamental interest for energy storage devices. Here we discuss recent advances on the properties and applications of chemically functionalized graphene.

High Quality Monolayer Graphene Synthesized by Resistive Heating Cold Wall Chemical Vapor Deposition.

The growth of graphene using resistive‐heating cold‐wall chemical vapor deposition (CVD) is demonstrated. This technique is 100 times faster and 99% lower cost than standard CVD. A study of Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and electrical magneto‐transport measurements shows that cold‐wall CVD graphene is of comparable quality to natural graphene. Finally, the first transparent flexible graphene capacitive touch‐sensor is demonstrated.

Multilevel Ultrafast Flexible Nanoscale Nonvolatile Hybrid Graphene Oxide–Titanium Oxide Memories

Graphene oxide (GO) resistive memories offer the promise of low-cost environmentally sustainable fabrication, high mechanical flexibility and high optical transparency, making them ideally suited to future flexible and transparent electronics applications. However, the dimensional and temporal scalability of GO memories, i.e., how small they can be made and how fast they can be switched, is an area that has received scant attention. Moreover, a plethora of GO resistive switching characteristics and mechanisms has been reported in the literature, sometimes leading to a confusing and conflicting picture. Consequently, the potential for graphene oxide to deliver high-performance memories operating on nanometer length and nanosecond time scales is currently unknown. Here we address such shortcomings, presenting not only the smallest (50 nm), fastest (sub-5 ns), thinnest (8 nm) GO-based memory devices produced to date, but also demonstrate that our approach provides easily accessible multilevel (4-level, 2-bit per cell) storage capabilities along with excellent endurance and retention performance-all on both rigid and flexible substrates. Via comprehensive experimental characterizations backed-up by detailed atomistic simulations, we also show that the resistive switching mechanism in our Pt/GO/Ti/Pt devices is driven by redox reactions in the interfacial region between the top (Ti) electrode and the GO layer.

Functionalised hexagonal-domain graphene for position-sensitive photodetectors

Graphenes unique photoresponse has been largely used in a multitude of optoelectronics applications ranging from broadband photodetectors to wave-guide modulators. In this work we extend the range of applications to position-sensitive photodetectors (PSDs) using FeCl3-intercalated hexagonal domains of graphene grown by atmospheric pressure chemical vapour deposition (APCVD). The FeCl3-based chemical functionalisation of APCVD graphene crystals is affected by the presence of wrinkles and results in a non-uniform doping of the graphene layers. This doping profile creates multiple p-p+ photoactive junctions which show a linear and bipolar photoresponse with respect to the position of a focused light spot, which is ideal for the realization of a PSD. Our study paves the way towards the fabrication of flexible and transparent PSDs that could be embedded in smart textile and wearable electronics.

A simple process for the fabrication of large-area CVD graphene based devices via selective in situ functionalization and patterning

This work was carried out under the auspices of the EU FP7 project CareRAMM. This project received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no 309980. The authors are grateful for helpful discussions with all CareRAMM partners, particularly Prof A. Ferrari and Ms A. Ott at the University of Cambridge, and Dr A. Sebastian and Dr W. Koelmans at IBM Research Zurich. We also gratefully acknowledge the assistance of the National EPSRC XPS User’s Service (NEXUS) at Newcastle University, UK (an EPSRC Mid-Range Facility) in carrying out the XPS measurements and the assistance of Prof S. Russo at the University of Exeter in carrying out humidity sensing measurements. A.M.A. would also like to thank Dr E. Alexeev for useful ideas for this Letter and pleasurable discussions of the results

New routes to the functionalization patterning and manufacture of graphene-based materials for biomedical applications

Graphene-based materials are being widely explored for a range of biomedical applications, from targeted drug delivery to biosensing, bioimaging and use for antibacterial treatments, to name but a few. In many such applications, it is not graphene itself that is used as the active agent, but one of its chemically functionalized forms. The type of chemical species used for functionalization will play a key role in determining the utility of any graphene-based device in any particular biomedical application, because this determines to a large part its physical, chemical, electrical and optical interactions. However, other factors will also be important in determining the eventual uptake of graphene-based biomedical technologies, in particular the ease and cost of manufacture of proposed device and system designs. In this work, we describe three novel routes for the chemical functionalization of graphene using oxygen, iron chloride and fluorine. We also introduce novel in situ methods for controlling and patterning such functionalization on the micro- and nanoscales. Our approaches are readily transferable to large-scale manufacturing, potentially paving the way for the eventual cost-effective production of functionalized graphene-based materials, devices and systems for a range of important biomedical applications.

Wafer scale FeCl3 intercalated graphene electrodes for photovoltaic applications

The alteration of graphenes electrical properties through chemical functionalization is a necessary process in order for graphene to fulfill its potential as a transparent conducting electrode. In this work, we present a method for the transfer and intercalation of large area (wafer scale) graphene samples to produce highly doped FeCl3 intercalated Few Layer Graphene (FeCl3-FLG). Given its excellent flexibility, transmission, and a sheet resistance, comparable to that of Indium Tin Oxide, FeCl3-FLG has potential to replace alternative flexible transparent electrodes as well as compete with rigid transparent electrodes. We assess the effect of functionalization temperature on the degree of intercalation in the large area samples and comparing results to that of 1 cm2 FeCl3-FLG samples. Raman spectroscopy is then used to characterize samples, where we introduce a new figure of merit ({PosG}) by which to assess the degree of intercalation in a sample. This is an average G peak position, weighted by the areas of the constituent peaks, which can then be used to map the charge carrier concentration of the sample. The inhomogeneity of the graphene grown by chemical vapor deposition is found to be one of the limiting factors in producing large area, high quality FeCl3-FLG.

A New Facile Route to Flexible and Semi-Transparent Electrodes Based on Water Exfoliated Graphene and their Single-Electrode Triboelectric Nanogenerator

Wearable technologies are driving current research efforts to self-powered electronics, for which novel high-performance materials such as graphene and low-cost fabrication processes are highly sought.The integration of high-quality graphene films obtained from scalable water processing approaches in emerging applications for flexible and wearable electronics is demonstrated. A novel method for the assembly of shear exfoliated graphene in water, comprising a direct transfer process assisted by evaporation of isopropyl alcohol is developed. It is shown that graphene films can be easily transferred to any target substrate such as paper, flexible polymeric sheets and fibers, glass, and Si substrates. By combining graphene as the electrode and poly(dimethylsiloxane) as the active layer, a flexible and semi-transparent triboelectric nanogenerator (TENG) is demonstrated for harvesting energy. The results constitute a new step toward the realization of energy harvesting devices that could be integrated with a wide range of wearable and flexible technologies, and opens new possibilities for the use of TENGs in many applications such as electronic skin and wearable electronics.

Role of defect states in functionalized graphene photodetectors

The functionalization of graphene can enhance the optoelectronic properties of graphene, allowing the creation of highly sensitive broadband photodetectors. Presently, the role played by defects, induced by the functionalization of graphene, on the performance of graphene photodetectors is not well understood. Here, we investigate the optoelectronic properties of van der Waals heterostructures comprising of graphene and a functionalized partner, formed by pristine and fluorinated graphene. We find that the electrical properties of graphene are preserved upon formation of the heterostructure. A negligible charge transfer is observed across the interface between the two materials which limits the performance of the photodetector due to the vertical separation of the two materials.