Optoelectronic devices based on van der Waals heterostructures

In this thesis we investigate the use of van der Waals heterostructures in optoelec- tronic devices. An improvement in the optical and electronic performance of specific devices can be made by combining two or more atomically thin materials in layered structures. We demonstrate a heterostructure photodetector formed by combining graphene with tungsten disulphide. These photodetectors were found to be highly sensitive to light due to a gain mechanism that produced over a million electrons per photon. This arises from the favourable electrical properties of graphene and the strong light-matter interaction in WS2 . An analysis of the photodetector per- formance shows that these devices are capable of detecting light under moonlight illuminations levels at video-frame-rate speeds with applications in night vision ima- ging envisaged. We also report a novel method for the direct laser writing of a high-k dielectric embedded inside a van der Waals heterostructure. Such structures were shown to be capable of both light-detection and light-emission within the same de- vice architecture, paving the way for future multifunctional optoelectronic devices. Finally we address a more fundamental problem in the properties of aligned grap- hene/hBN heterostructures. Strain distributions are shown to modify the electronic properties of graphene due to a change in the interlayer interaction. We demon- strates a method to engineer these strain patterns by contact geometry design and thermal annealing strategies.Engineering and Physical Sciences Research Council (EPSRC

Graphene-based light sensing: fabrication, characterisation, physical properties and performance

This is the final version. Available from MDPI via the DOI in this record.Graphene and graphene-based materials exhibit exceptional optical and electrical properties with great promise for novel applications in light detection. However, several challenges prevent the full exploitation of these properties in commercial devices. Such challenges include the limited linear dynamic range (LDR) of graphene-based photodetectors, the lack of efficient generation and extraction of photoexcited charges, the smearing of photoactive junctions due to hot-carriers effects, large-scale fabrication and ultimately the environmental stability of the constituent materials. In order to overcome the aforementioned limits, different approaches to tune the properties of graphene have been explored. A new class of graphene-based devices has emerged where chemical functionalisation, hybridisation with light-sensitising materials and the formation of heterostructures with other 2D materials have led to improved performance, stability or versatility. For example, intercalation of graphene with FeCl3 is highly stable in ambient conditions and can be used to define photo-active junctions characterized by an unprecedented LDR while graphene oxide (GO) is a very scalable and versatile material which supports the photodetection from UV to THz frequencies. Nanoparticles and quantum dots have been used to enhance the absorption of pristine graphene and to enable high gain thanks to the photogating effect. In the same way, hybrid detectors made from stacked sequences of graphene and layered transition-metal dichalcogenides enabled a class of detectors with high gain and responsivity. In this work we will review the performance and advances in functionalised graphene and hybrid photodetectors, with particular focus on the physical mechanisms governing the photoresponse in these materials, their performance and possible future paths of investigation.Funding: M.F.C. and S.R. acknowledge financial support from: Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, projects EP/M002438/1, EP/M001024/1, EPK017160/1, EP/K031538/1, EP/J000396/1; the Royal Society, grant title "Room temperature quantum technologies" and "Wearable graphene photovolotaic"; Newton fund, Uk-Brazil exchange grant title "Chronographene" and the Leverhulme Trust, research grants "Quantum drums" and "Quantum revolution". J.D.M. acknowledges financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials, Grant No. EP/L015331/1

Laser writable high-K dielectric for van der Waals nano-electronics

This is the author accepted manuscript. The final version is available from American Association for the Advancement of Science via the DOI in this record.Like silicon-based semiconductor devices, van der Waals heterostructures will require integration with high-K oxides. This is needed to achieve suitable voltage scaling, improved performance as well as allowing for added functionalities. Unfortunately, commonly used high-k oxide deposition methods are not directly compatible with 2D materials. Here we demonstrate a method to embed a multi-functional few nm thick high-k oxide within van der Waals devices without degrading the properties of the neighbouring 2D materials. This is achieved by in-situ laser oxidation of embedded few layer HfS2 crystals. The resultant oxide is found to be in the amorphous phase with a dielectric constant of k~15 and break-down electric fields in the range of 0.5-0.6 V/nm. This transformation allows for the creation of a variety of fundamental nano-electronic and opto-electronic devices including, flexible Schottky barrier field effect transistors, dual gated graphene transistors as well as vertical light emitting and detecting tunnelling transistors. Furthermore, upon dielectric break-down, electrically conductive filaments are formed. This filamentation process can be used to electrically contact encapsulated conductive materials. Careful control of the filamentation process also allows for reversible switching between two resistance states. This allows for the creation of resistive switching random access memories (ReRAMs). We believe that this method of embedding a high-k oxide within complex van der Waals heterostructures could play an important role in future flexible multi-functional van der Waals devices.F.W acknowledges support from the Royal Academy of Engineering. J.D.M. acknowledges financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1). S.R. and M.F.C. acknowledge financial support from EPSRC (Grant no. EP/K010050/1, EP/M001024/1, EP/M002438/1), from Royal Society international Exchanges Scheme 2016/R1, from The Leverhulme trust (grant title “Quantum Revolution” and "Quantum Drums"). A.P Rooney and S.J Haigh acknowledge support from the EPSRC postdoctoral fellowship and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement ERC-2016-STG-EvoluTEM-715502) and the Defence Threat Reduction Agency (HDTRA1-12-1-0013). I.A. acknowledges financial support from The European Commission Marie Curie Individual Fellowships (Grant number 701704)

Novel circuit design for high-impedance and non-local electrical measurements of two-dimensional materials

This is the author accepted manuscript. The final version is available from AIP Publishing via the DOI in this recordTwo-dimensional materials offer a novel platform for the development of future quantum technologies. However, the electrical characterisation of topological insulating states, non-local resistance and bandgap tuning in atomically thin materials, can be strongly affected by spurious signals arising from the measuring electronics. Common-mode voltages, dielectric leakage in the coaxial cables and the limited input impedance of alternate-current amplifiers can mask the true nature of such high-impedance states. Here, we present an optical isolator circuit which grants access to such states by electrically decoupling the current-injection from the voltagesensing circuitry. We benchmark our apparatus against two state-of-the-art measurements: the non-local resistance of a graphene Hall bar and the transfer characteristic of a WS2 field-effect transistor. Our system allows the quick characterisation of novel insulating states in two-dimensional materials with potential applications in future quantum technologies.J.D.M. acknowledges financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1). S.F.R acknowledges financial support from the Higher Committee for Education Development in Iraq (HCED). S.R. and M.F.C. 13 acknowledge financial support from EPSRC (Grant no. EP/K010050/1, EP/M001024/1, EP/M002438/1), from Royal Society international Exchanges Scheme 2016/R1, from The Leverhulme trust (grant title ”Quantum Revolution”). A.D.S, S.R. and M.F.C. acknowledge financial support from Royal Society international Exchanges Grant ”Energy Harvesting Fabric”

Hot-Carrier Cooling in High-Quality Graphene is Intrinsically Limited by Optical Phonons

Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand, and eventually control, the cooling dynamics of the photoinduced hot-carrier distribution. There is, however, still an active debate on the different mechanisms that contribute to hot-carrier cooling. In particular, the intrinsic cooling mechanism that ultimately limits the cooling dynamics remains an open question. Here, we address this question by studying two technologically relevant systems, consisting of high-quality graphene with a mobility >10,000 cm$^2$V$^{-1}$s$^{-1}$ and environments that do not efficiently take up electronic heat from graphene: WSe$_2$-encapsulated graphene and suspended graphene. We study the cooling dynamics of these two high-quality graphene systems using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to the environment is relatively inefficient in these systems, predicting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a timescale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the hot-carrier distribution with enough kinetic energy emit optical phonons. During phonon emission, the electronic system continuously re-thermalizes, re-creating carriers with enough energy to emit optical phonons. We develop an analytical model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights into the intrinsic cooling mechanism of hot carriers in graphene will play a key role in guiding the development of graphene-based optoelectronic devices

A pre-time-zero spatiotemporal microscopy technique for the ultrasensitive determination of the thermal diffusivity of thin films

Diffusion is one of the most ubiquitous transport phenomena in nature. Experimentally, it can be tracked by following point spreading in space and time. Here, we introduce a spatiotemporal pump-probe microscopy technique that exploits the residual spatial temperature profile obtained through the transient reflectivity when probe pulses arrive before pump pulses. This corresponds to an effective pump-probe time delay of 13 ns, determined by the repetition rate of our laser system (76 MHz). This pre-time-zero technique enables probing the diffusion of long-lived excitations created by previous pump pulses with nanometer accuracy and is particularly powerful for following in-plane heat diffusion in thin films. The particular advantage of this technique is that it enables quantifying thermal transport without requiring any material input parameters or strong heating. We demonstrate the direct determination of the thermal diffusivities of films with a thickness of around 15 nm, consisting of the layered materials MoSe2 (0.18 cm2/s), WSe2 (0.20 cm2/s), MoS2 (0.35 cm2/s), and WS2 (0.59 cm2/s). This technique paves the way for observing nanoscale thermal transport phenomena and tracking diffusion of a broad range of species

Unraveling Heat Transport and Dissipation in Suspended MoSe 2 from Bulk to Monolayer

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental-theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe. Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in-plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat-carrying modes around ≈0.1 THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out-of-plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe by an order of magnitude. These results are crucial for the design of (flexible) TMD-based (opto-)electronic applications