Fabrication, functionalisation and characterisation of epitaxial graphene devices

PhD ThesisGraphene has attracted considerable attention in recent years due to its remarkable material properties and its potential for applications in next-generation nanoelectronics. In particular, the high specific surface area, extremely high electrical conductivity and exceptionally low electrical noise make graphene an ideal material for surface-sensitive applications such as chemical sensing, biological sensing and DNA sequencing. The surface cleanliness of graphene devices is critical for these applications, along with low contact resistance at metal/graphene interfaces. In addition, having pristine surface is also essential to carry out controlled functionalisation of graphene to target its chemical reactions with designated analyte species. However, it was found that conventional lithography processing techniques used for graphene device fabrication significantly contaminates the graphene surface with resist residues, which cannot be removed by any known organic solvents. The presence of such chemical contamination degrades the intrinsic properties of graphene and also significantly affects the performance of graphene based electronic devices. In this thesis, two methods were developed to address this issue, where, for the first method, rapid thermal annealing of graphene devices was performed in N2/H2 atmosphere, whilst for the second method, a metal sacrificial layer was used to prevent graphene from coming into direct with photoresist during the lithography process. Chemical, electrical, structural and surface morphological analysis showed that clean graphene surfaces can be achieved by both these methods, which allowed the intrinsic properties of graphene to be measured. In addition to surface contamination, the performance of graphene devices is also limited by contact resistance associated with the metal-graphene interface, where an unique challenge arise as charge carriers are injected from a three-dimensional metal film into a two-dimensional graphene sheet. The quantitative analysis of the data demonstrates that highly reactive metals such as Ti destroys the graphene lattice and results in high contact resistance, whereas metals with higher work functions and lower lattice mismatch to graphene (such as Ni) was found to result in significantly lower contact resistance. The work function, binding energy, diffusion energy and the lattice mismatch of the deposited metals were used to explain the electrical and structural characteristics of different types of metal/graphene interfaces. ABSTRACT vi In order to enhance the chemical reactivity of graphene surfaces, controlled functionalisation of epitaxial graphene films using electron-beam generated oxygen plasma has been demonstrated at room temperature. It was found that oxygen functionalisation not only introduces different oxygen functional groups onto the graphene surface, but also results in strain relaxation, in which the intrinsic compressive strain present in epitaxial graphene film decreased progressively with the increasing plasma pressure. A detailed study on the effect of e-beam plasma treatment on the chemical, electrical, structural and morphological characteristics of epitaxial graphene films have been investigated from initial to advanced oxidation stages. Finally, the effectiveness of oxygen functionalised graphene as a chemical sensor for detecting a wide range of polar chemical vapours in the ambient atmosphere has been demonstrated. The sensing characteristics of oxygen functionalised graphene devices showed ultra-fast response (10 s) and recovery times (100 s) to different chemical vapours, whilst unfunctionalised graphene sensors showed considerably weaker sensitivity and extremely slow recovery time in the range of ∼1.5 to 2 hours. A strong correlation between the dipole moment of the chemical and the magnitude of the response was observed, in which oxygen functionalised sensors displayed a twofold increase in the sensitivity over un-functionalised sensors with the increasing dipole moment from 2.0 D to 4.1 D. The sensing properties of graphene and the effect of oxygen functionalization on sensor responses were critically examined in an effort to provide a detailed understanding on the graphene sensing mechanism and provide a pathway for future advancements in the graphene sensor research.EPSRC and BAE system

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

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.We report a novel approach for the fabrication of micro- and nano-scale graphene devices via the in-situ plasma functionalization and in-situ lithographic patterning of large-area graphene directly on CVD catalytic metal (Cu) substrates. This enables us to create graphene-based devices in their entirety prior to any transfer processes, simplifying very significantly the device fabrication process and potentially opening up the route to the use of a wider range of target substrates. We demonstrate the capabilities of our technique via the fabrication of a flexible, transparent, graphene/graphene oxide humidity sensor that outperforms a conventional commercial sensor.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 result

Fast High-Responsivity Few-Layer MoTe2 Photodetectors

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.The Transition Metal Dichalcogenide MoTe2 is fabricated via mechanical exfoliation into few-layer Field Effect Transistors (FETs) having a hole mobility of 2.04 V/cm2/s. Four-layer MoTe2 FETs show a high photoresponsivity of 6 A/W and a response time, at around 160 μs, over 100 times faster than previously reported for MoTe2. Few-layer MoTe2 thus appears as a strong candidate for high speed and high sensitivity photodetection applications.CDW would like to acknowledge funding via EPSRC grants EP/M015173/1 and EP/M015130/1. TJO acknowledges funding from the EPSRC Centre for Doctoral Training in Metamaterials, grant number EP/L015331/

Humidity‐Controlled Ultralow Power Layer‐by‐Layer Thinning, Nanopatterning and Bandgap Engineering of MoTe2

This is the final version. Available on open access from Wiley via the DOI in this recordA highly effective laser thinning method is demonstrated to accurately control the thickness of MoTe2 layers. By utilizing the humidity present in the ambient atmosphere, multilayered MoTe2 films can be uniformly thinned all the way down to monolayer with layer-by-layer precision using an ultralow laser power density of 0.2 mW µm−2. Localized bandgap engineering is also performed in MoTe2, by creating regions with different bandgaps on the same film, enabling the formation of lateral homojunctions with sub-200 nm spatial resolution. Field-effect transistors fabricated from these thinned layers exhibit significantly improved electrical properties with an order of magnitude increase in on/off current ratios, along with enhancements in on-current and field-effect mobility values. Thinned devices also exhibit the fastest photoresponse (45 µs) for an MoTe2-based visible photodetector reported to date, along with a high photoresponsivity. A highly sensitive monolayer MoTe2 photodetector is also reported. These results demonstrate the efficiency of the presented thinning approach in producing high-quality MoTe2 films for electronic and optoelectronic applications.Office of Naval Research GlobalEngineering and Physical Sciences Research Council (EPSRC)Defence Science and Technology Laborator

Multilevel ultrafast flexible nanoscale nonvolatile hybrid graphene oxide-titanium oxide memories

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.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.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. Andrea Ferrari and Ms. Anna Ott at the University of Cambridge, and Drs. Abu Sebastian and Wabe Koelmans at IBM Research Zurich. We also gratefully acknowledge the assistance of the National EPSRC XPS User’s Service (NEXUS) at Newcastle University, U.K. (an EPSRC Mid-Range Facility) in carrying out the XPS measurement

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

This is the author accepted manuscript. The final version is available from Royal Society via the DOI in this record.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-functionalised forms. The type of chemical species used for functionalisation will play a key role in determining the utility of any graphene-based device in any particular biomedical application, since 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 functionalisation of graphene using oxygen, iron chloride and fluorine. We also introduce novel in-situ methods for controlling and patterning such functionalisation on the micro- and nano-scales. Our approaches are readily transferable to large-scale manufacturing, potentially paving the way for the eventual cost-effective production of functionalised graphene-based materials, devices and systems for a range of important biomedical applications.AA, VKN, MFC and CDW acknowledge funding via the EU FP7 project CareRAMM (grant no. 309980). SR and MFC. acknowledge financial support from the Engineering and Physical Sciences Research Council (grant nos. EP/J000396/1, EP/K017160/1, EP/K010050/1, EP/G036101/1, EP/M001024/1, and EP/M002438/1)

Photoconductivity of Few-Layer MoTe2

This is the final version of the paper. Available from metaconferences.org via the URL in this record.A photoconductivity study of few-layer MoTe2 in a field effect transistor (FET) configuration was performed to find the photoresponsivity and photocurrent response of the material. The mechanisms for MoTe2 with no applied gate voltage were found to be dominated by the photovoltaic effect, showing its potential for use in solar cells. Due to the band gap of MoTe2 being 1.1 eV, MoTe2 is a suitable photodetector for optical wavelengths and potentially the near infrared

A nonvolatile phase-change metamaterial color display

This is the final version. Available from Wiley via the DOI in this record.Chalcogenide phase-change materials, which exhibit a marked difference in their electrical and optical properties when in their amorphous and crystalline phases and can be switched between these phases quickly and repeatedly, are traditionally exploited to deliver nonvolatile data storage in the form of rewritable optical disks and electrical phase-change memories. However, exciting new potential applications are now emerging in areas such as integrated phase-change photonics, phase-change optical metamaterials/metasurfaces, and optoelectronic displays. Here, ideas from these last two fields are fused together to deliver a novel concept, namely a switchable phase-change metamaterial/metasurface resonant absorber having nonvolatile color generating capabilities. With the phase-change layer, here GeTe, in the crystalline phase, the resonant absorber can be tuned to selectively absorb the red, green, and blue spectral bands of the visible spectrum, so generating vivid cyan, magenta, and yellow pixels. When the phase-change layer is switched into the amorphous phase, the resonant absorption is suppressed and a flat, pseudowhite reflectance results. Thus, a route to the potential development is opened-up of nonvolatile, phase-change metamaterial color displays and color electronic signage.Engineering and Physical Sciences Research Council (EPSRC

STUDY, BEHAVIOUR AND CLASSIFICATION OF FIBER REINFORCED CONCRETE

This can be a preliminary study from the fundamental conduct of fiber strengthened concrete. The best goal with this research study would be to develop an analytical model to simulate the conduct of fiber strengthened concrete structures under dynamic loading in addition to help in creating fiber strengthened concrete structure. This project examined fiber strengthened concrete through several experiments. To higher understand fiber strengthened concrete qualities, test examples were statically examined. Exterior fiber strengthened concrete sections be utilized for that purpose of blast and impact protection, with the additional advantage of reducing construction cost and time by decreasing the needed quantity of conventional steel. Adding unconventional reinforcement to concrete, particularly fiber reinforcement, continues to be proven to achieve the preferred qualities for blast and impact resistance including elevated sturdiness, toughness and energy absorption. This structure serves multiple functions. It's been recommended that for compressive strength, spitting tensile strength, and more importantly average residual strength, all using ASTM standards when relevant. Ale fiber strengthened concrete to hold load past initial cracking is shown by average residual strength