Neil Richard Wilson

There is rising interest in two-dimensional (2D) ferroelectrics, particularly as functional components in 2D heterostructures using their reconfigurable polarisation to create low-power memory devices. Understanding and optimising their application will require investigations that can unravel, at the nanoscale, the effect of ferroelectric switching on the local electronic structure. Using contact mode scanning tunnelling spectroscopy (CM-STS), we observe polarisation-dependent band-edge shifts in an all 2D heterostructure that combines the ferroelectric CuInP ₂ S ₆ (CIPS) with the semiconductor WSe ₂ . We employ scanning probe microscopy to characterise the polarisation and conductivity in the CIPS layer, and correlate these measurements with CM-STS spectra, revealing changes from p-type to n-type doping in the WSe ₂ layer as the out-of-plane polarisation of the underlying CIPS is switched by the external electric field. The data demonstrate the importance of considering the channel and contact resistances, along with tip-induced-band-bending, when using the tunnelling current to probe the local density of states (LDOS) in 2D semiconductor heterostructures. The significant band-edge shift of 0.8 eV demonstrates efficient coupling of the semiconductor to the polarisation field in the ferroelectric, consistent with clean interfaces in the heterostructure without dead layers. The results show that probing the LDOS by CM-STS gives valuable insight into 2D ferroelectric based electronics and enables direct correlation between electronic structure and functional properties.

Among the ever-growing family of two-dimensional (2D) materials, in recent years alpha phase indium selenide ( α -In 2 Se 3 ), has attracted interest for its 2D ferroelectricity down to the monolayer limit, high photoresponsivity and thickness-dependent band-gap. However, the environmental instability of α -In 2 Se 3 presents a significant obstacle to harnessing these properties in devices. Understanding the underlying mechanism and practical limits this degradation imposes is necessary. In this work, we employ various characterization techniques in order to probe how the structure, surface morphology, composition and ferroelectric properties of α -In 2 Se 3 change over a period of up to 14 days. We resolve some of the previous contradictions in the interpretation of Raman spectra for these and similar air-sensitive 2D materials, particularly accounting for the potentially confounding role of beam damage in elemental Se spectral features. We find that the bulk structural properties of α -In 2 Se 3 remain largely unchanged owing to the formation of a uniform surface oxide layer preventing further oxidation below the surface. In addition, piezoresponse force microscopy studies show that ferroelectric polarization in α -In 2 Se 3 remains stable for up to 2 h but decays rapidly within 22–24 h, providing a quantitative time frame for the handling of α -In 2 Se 3 in air before its ferroelectric properties are compromised. As well as these findings aiding in the implementation of α -In 2 Se 3 in devices, they can also be used to inform the characterization of air-degradation in other related 2D transition metal chalcogenides.

We report the electronic structure of monolayer CrSBr exfoliated onto mica template-stripped gold substrates. Angle-resolved photoemission spectroscopy reveals charge transfer from the substrate, populating the conduction band of monolayer CrSBr, accompanied by a pronounced reduction in the quasiparticle band gap. Furthermore, we observe two separate conduction bands that exhibit a splitting at the X point. This indicates a breaking of glide-mirror symmetry, which in the bulk or in a free-standing monolayer protects the band degeneracies at the Brillouin zone boundary. Our results demonstrate that ultraflat gold substrates do more than modify carrier densities and screening: they can lift symmetry-protected degeneracies and thus fundamentally reshape the band topology of 2D materials.

We report the electronic structure of monolayer CrSBr exfoliated onto mica template-stripped gold substrates. Angle-resolved photoemission spectroscopy reveals charge transfer from the substrate, populating the conduction band of monolayer CrSBr, accompanied by a pronounced reduction in the quasiparticle band gap. Furthermore, we observe two separate conduction bands that exhibit a splitting at the X point. This indicates a breaking of glide-mirror symmetry, which in the bulk or in a free-standing monolayer protects the band degeneracies at the Brillouin zone boundary. Our results demonstrate that ultraflat gold substrates do more than modify carrier densities and screening: they can lift symmetry-protected degeneracies and thus fundamentally reshape the band topology of 2D materials.

Understanding the interface between metals and two-dimensional materials is critical for their application in electronics and for the development of metal-mediated exfoliation of large area monolayers. Studying the intricate interactions at the interface requires model systems that enable control of the roughness, purity, and crystallinity of the metal surface. Here, we use angle-resolved photoemission spectroscopy to investigate the layer-dependent electronic structure of WSe₂ on template-stripped gold (TSG) substrates fabricated using both silicon and mica templates, giving crystallographically disordered and Au(111) ordered surfaces, respectively, and contrast these findings with ab initio predictions. We observe strong hybridisation around the Brillouin zone centre at Γ ― , indicating a covalent admixture in the gold-WSe₂ interaction, and band shifts that suggest charge rearrangement at the Au(111)/WSe₂ interface. Core-level spectroscopy shows a single chemical environment for the interfacial WSe₂ layer on the TSG, distinct from the subsequent layers. These results reveal a mixture of van der Waals and covalent interactions, best described as a covalent-like quasi-bonding with intermediate interaction strength.

Stacking two semiconducting transition metal dichalcogenide (MX₂) monolayers to form a heterobilayer creates a new variety of semiconductor junction with unique optoelectronic features, such as hosting long-lived dipolar interlayer excitons. Despite many optical, transport, and theoretical studies, there have been few direct electronic structure measurements of these junctions. Here, we apply angle-resolved photoemission spectroscopy with micron-scale spatial resolution (µARPES) to determine the band alignments in MoSe₂/WSe₂ heterobilayers, using in-situ electrostatic gating to electron-dope and thus probe the conduction band edges. By comparing spectra from heterobilayers with opposite stacking orders, that is, with either MoSe₂ or WSe₂ on top, we confirm that the band alignment is type II, with the valence band maximum in the WSe₂ and the conduction band minimum in the MoSe₂. The overall band gap is E _G = 1.43 ± 0.03 eV, and to within experimental uncertainty it is unaffected by electron doping. However, the offset between the WSe₂ and MoSe₂ valence bands clearly decreases with increasing electron doping, implying band renormalisation only in the MoSe₂, the layer in which the electrons accumulate. In contrast, µARPES spectra from a WS₂/MoSe₂ heterobilayer indicate type I band alignment, with both band edges in the MoSe₂. These insights into the doping-dependent band alignments and gaps of MX₂ heterobilayers will be useful for properly understanding and ultimately utilizing their optoelectronic properties.

Photoluminescence has widely been used to study excitons in semiconducting transition metal dichalcogenide (MX₂) monolayers, demonstrating strong light-matter interactions and locked spin and valley degrees of freedom. In heterobilayers composed of overlapping monolayers of two different MX₂, an interlayer exciton can form, with the hole localised in one layer and the electron in the other. These interlayer excitons are long-lived, field-tunable, and can be trapped by moiré patterns formed at small twist angles between the layers. Here we demonstrate that emission from radiative recombination of interlayer excitons can be observed by cathodoluminescence from a WSe₂/MoSe₂ heterobilayer encapsulated in hexagonal boron nitride. The higher spatial resolution of cathodoluminescence, compared to photoluminescence, allows detailed analysis of sample heterogeneity at the 100 s of nm lengthscales over which twist angles tend to vary in dry-transfer fabricated heterostructures.

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.