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"Wilson, Neil"
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Visualizing electrostatic gating effects in two-dimensional heterostructures
The ability to directly monitor the states of electrons in modern field-effect devices—for example, imaging local changes in the electrical potential, Fermi level and band structure as a gate voltage is applied—could transform our understanding of the physics and function of a device. Here we show that micrometre-scale, angle-resolved photoemission spectroscopy
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(microARPES) applied to two-dimensional van der Waals heterostructures
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affords this ability. In two-terminal graphene devices, we observe a shift of the Fermi level across the Dirac point, with no detectable change in the dispersion, as a gate voltage is applied. In two-dimensional semiconductor devices, we see the conduction-band edge appear as electrons accumulate, thereby firmly establishing the energy and momentum of the edge. In the case of monolayer tungsten diselenide, we observe that the bandgap is renormalized downwards by several hundreds of millielectronvolts—approaching the exciton energy—as the electrostatic doping increases. Both optical spectroscopy and microARPES can be carried out on a single device, allowing definitive studies of the relationship between gate-controlled electronic and optical properties. The technique provides a powerful way to study not only fundamental semiconductor physics, but also intriguing phenomena such as topological transitions
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and many-body spectral reconstructions under electrical control.
Changes in the electronic states of two-dimensional semiconductor devices resulting from electrical gating can be monitored directly using micrometre-scale angle-resolved photoemission spectroscopy.
Journal Article
Atomic reconstruction in twisted bilayers of transition metal dichalcogenides
Van der Waals heterostructures form a unique class of layered artificial solids in which physical properties can be manipulated through controlled composition, order and relative rotation of adjacent atomic planes. Here we use atomic-resolution transmission electron microscopy to reveal the lattice reconstruction in twisted bilayers of the transition metal dichalcogenides, MoS2 and WS2. For twisted 3R bilayers, a tessellated pattern of mirror-reflected triangular 3R domains emerges, separated by a network of partial dislocations for twist angles θ < 2°. The electronic properties of these 3R domains, featuring layer-polarized conduction-band states caused by lack of both inversion and mirror symmetry, appear to be qualitatively different from those of 2H transition metal dichalcogenides. For twisted 2H bilayers, stable 2H domains dominate, with nuclei of a second metastable phase. This appears as a kagome-like pattern at θ ≈ 2°, transitioning at θ → 0 to a hexagonal array of screw dislocations separating large-area 2H domains. Tunnelling measurements show that such reconstruction creates strong piezoelectric textures, opening a new avenue for engineering of 2D material properties.Lattice reconstruction in twisted transition metal dichalcogenides manifest in intrinsic asymmetry of electronic wavefunctions for 3R homo-bilayers and strong piezoelectric textures in 2H homo-bilayers.
Journal Article
Tunnel junctions based on interfacial two dimensional ferroelectrics
Van der Waals heterostructures have opened new opportunities to develop atomically thin (opto)electronic devices with a wide range of functionalities. The recent focus on manipulating the interlayer twist angle has led to the observation of out-of-plane room temperature ferroelectricity in twisted rhombohedral bilayers of transition metal dichalcogenides. Here we explore the switching behaviour of sliding ferroelectricity using scanning probe microscopy domain mapping and tunnelling transport measurements. We observe well-pronounced ambipolar switching behaviour in ferroelectric tunnelling junctions with composite ferroelectric/non-polar insulator barriers and support our experimental results with complementary theoretical modelling. Furthermore, we show that the switching behaviour is strongly influenced by the underlying domain structure, allowing the fabrication of diverse ferroelectric tunnelling junction devices with various functionalities. We show that to observe the polarisation reversal, at least one partial dislocation must be present in the device area. This behaviour is drastically different from that of conventional ferroelectric materials, and its understanding is an important milestone for the future development of optoelectronic devices based on sliding ferroelectricity.
The authors study tunneling junctions in rhombohedral MoS
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bilayers and correlate their performance with the local domain layout. They show that the switching behavior in sliding ferroelectrics is strongly dependent on the pre-existing domain structure.
Journal Article
Adiabatic versus non-adiabatic electron transfer at 2D electrode materials
2D electrode materials are often deployed on conductive supports for electrochemistry and there is a great need to understand fundamental electrochemical processes in this electrode configuration. Here, an integrated experimental-theoretical approach is used to resolve the key electronic interactions in outer-sphere electron transfer (OS-ET), a cornerstone elementary electrochemical reaction, at graphene as-grown on a copper electrode. Using scanning electrochemical cell microscopy, and co-located structural microscopy, the classical hexaamineruthenium (III/II) couple shows the ET kinetics trend: monolayer > bilayer > multilayer graphene. This trend is rationalized quantitatively through the development of rate theory, using the Schmickler-Newns-Anderson model Hamiltonian for ET, with the explicit incorporation of electrostatic interactions in the double layer, and parameterized using constant potential density functional theory calculations. The ET mechanism is predominantly adiabatic; the addition of subsequent graphene layers increases the contact potential, producing an increase in the effective barrier to ET at the electrode/electrolyte interface.
This combined experimental and theoretical study reveals the nature of electron transfer at graphene as grown on copper. The authors find that outer-sphere electron transfer occurs adiabatically with slower kinetics for multi- than for monolayer graphene.
Journal Article
Eastern Europe
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Book
Ultra-thin van der Waals crystals as semiconductor quantum wells
Control over the quantization of electrons in quantum wells is at the heart of the functioning of modern advanced electronics; high electron mobility transistors, semiconductor and Capasso terahertz lasers, and many others. However, this avenue has not been explored in the case of 2D materials. Here we apply this concept to van der Waals heterostructures using the thickness of exfoliated crystals to control the quantum well dimensions in few-layer semiconductor InSe. This approach realizes precise control over the energy of the subbands and their uniformity guarantees extremely high quality electronic transport in these systems. Using tunnelling and light emitting devices, we reveal the full subband structure by studying resonance features in the tunnelling current, photoabsorption and light emission spectra. In the future, these systems could enable development of elementary blocks for atomically thin infrared and THz light sources based on intersubband optical transitions in few-layer van der Waals materials.
A plethora of solid-state nanodevices rely on engineering the quantization of electrons in quantum wells. Here, the authors leverage the thickness of exfoliated 2D crystals to control the quantum well dimensions in few-layer semiconductor InSe and investigate the resonance features in the tunnelling current, photoabsorption and light emission spectra.
Journal Article