Surface chemistry altering electronic behaviour of liquid metal-derived tin oxide nanosheets.

Possessing excellent electronic properties and high chemical stability, semiconducting n-type two-dimensional (2D) tin dioxide (SnO2) nanosheets have been featured in sensing and electrocatalysis applications recently. Derived from non-layered crystal structures, 2D SnO2 has abundant unsaturated dangling bonds existing at the surface, providing interfacial activity. How the surface chemistry alters the electronic properties of 2D SnO2 nanomaterials remains unexplored. In this study, we synthesised ultra-thin 2D SnO2 nanosheets using a liquid metal (LM) touch printing technique and investigated experimentally and theoretically how the interactions of organic solvents composed of alkyl and hydroxyl groups with the surface of LM-derived 2D SnO2 modulate the electronic properties. It was found that alkane solvents can physically absorb onto the SnO2 surface with no impact on the material conductivity. Alcohol-based solvents on the other hand interact with the SnO2 surface via chemical absorptions primarily, in which oxygen atoms of hydroxyl groups in the alcohols form bonds with the surface atoms of SnO2. The binding stability is determined by the length and configuration of the hydrocarbon chain in alcohols. As representative long-chain alcohols, 1-octanol and 1-pentanol attach onto the SnO2 surface strongly, lowering the binding energy of Sn4+ and reducing the electron transfer ability of SnO2 nanosheets. Consequently, the electronic properties, i.e. conductivity and electronic mobility of SnO2 nanosheet-based electronic devices are decreased significantly.

Broken-gap energy alignment in two-dimensional van der Waals heterostructures for multifunctional tunnel diodes.

Two-dimensional (2D) materials are promising platforms for future nanoelectronic technologies as they provide the building blocks for atomically thin devices, including switches, amplifiers, and oscillators. When 2D materials are layered on top of each other, forming van der Waals heterostructures (vdWHs), they can provide unique properties not possessed by the individual layers. Here we consider the vdWHs HfS2/MoTe2, HfS2/WTe2, 1T-HfS2/WTe2, TiS2/WSe2, TiS2/ZnO, and TiSe2/WTe2 as potential Esaki (or tunnel) diodes that can be incorporated into electronic devices. In this work, the strongly constrained and appropriately normed (SCAN) meta-generalised-gradient approximation (meta-GGA) functional is employed for the structural properties, whereas the Heyd-Scuseria-Ernzerhof (HSE) functional is used for the electronic properties. We establish that the band alignments in these systems form broken-band heterojunctions. We show that the electronic properties of the systems can be effectively modulated by applying lateral strain or an external electric field. Importantly, we demonstrate that the band gap of the vdWHs can be widened by up to 0.65 eV by applying an electric force field of -1 to +1 eV Å-1. This work demonstrates a set of 6 vdWHs with properties suitable for application as 2D Esaki tunnel diodes, 4 of which could be applied as multifunctional devices. These materials not only offer new device properties, but their small dimensions allow for the creation of ultrathin devices.

Repairing and Preventing Photooxidation of Few-Layer Black Phosphorus with ß-Carotene.

Few-layer black phosphorus (FLBP), a technologically important 2D material, faces a major hurdle to consumer applications: spontaneous degradation under ambient conditions. Blocking the direct exposure of FLBP to the environment has remained the key strategy to enhance its stability, but this can also limit its utility. In this paper, a more ambitious approach to handling FLBP is reported where not only is FLBP oxidation blocked, but it is also repaired postoxidation. Our approach, inspired by nature, employs the antioxidant molecule ß-carotene that protects plants against photooxidative damages to act as a protecting and repairing agent for FLBP. The mechanistic role of ß-carotene is established by a suite of spectro-microscopy techniques, in combination with computational studies and biochemical assays. Transconductance studies on FLBP-based field effect transistor (FET) devices further affirm the protective and reparative effects of ß-carotene. The outcomes indicate the potential for deploying a plethora of natural antioxidant molecules to enhance the stability of other environmentally sensitive inorganic nanomaterials and expedite their translation for technological and consumer applications.

Ferroelectric van der Waals heterostructures of CuInP2S6for non-volatile memory device applications.

Two-dimensional (2D) ferroelectric materials are providing promising platforms for creating future nano- and opto-electronics. Here we propose new hybrid van der Waals heterostructures, in which the 2D ferroelectric material CuInP2S6(CIPS) is layered on a 2D semiconductor for near-infrared (NIR) memory device applications. Using density functional theory, we show that the band gap of the hybrid bilayers formed with CIPS can be tuned and that the optical and electronic properties can be successfully modulated via ferroelectric switching. Of the 3712 heterostructures considered, we identified 19 structures that have a type II band alignment and commensurate lattice matches. Of this set, both the CuInP2S6/PbSe and CuInP2S6/Ge2H2heterostructures possess absorption peaks in the NIR region that change position and intensity with switching polarisation, making them suitable for NIR memory devices. The CuInP2S6/ISSb, CuInP2S6/ISbSe, CuInP2S6/ClSbSe and CuInP2S6/ZnI2heterostructures had band gaps which can be switched from direct to indirect with changing the polarisation of CIPS making them suitable for optoelectronics and sensors. The heterostructures formed with CIPS are exciting candidates for stable ferroelectric devices, opening a pathway for tuning the band alignment of van der Waal heterostructures and the creation of modern memory applications that use less energy.

Tuning the Schottky barrier height in a multiferroic In2Se3/Fe3GeTe2 van der Waals heterojunction.

The ferroelectric material In2Se3 is currently of significant interest due to its built-in polarisation characteristics that can significantly modulate its electronic properties. Here we employ density functional theory to determine the transport characteristics at the metal-semiconductor interface of the two-dimensional multiferroic In2Se3/Fe3GeTe2 heterojunction. We show a significant tuning of the Schottky barrier height as a result of the change in the intrinsic polarisation state of In2Se3: the switching in the electric polarisation of In2Se3 results in the switching of the nature of the Schottky barrier, from being n-type to p-type, and is accompanied by a change in the spin polarisation of the electrons. This switchable Schottky barrier structure can form an essential component in a two-dimensional field effect transistor that can be operated by switching the ferroelectric polarisation, rather than by the application of strain or electric field. The band structure and density of state calculations show that Fe3GeTe2 lends its magnetic and metallic characteristics to the In2Se3 layer, making the In2Se3/Fe3GeTe2 heterojunction a potentially viable multiferroic candidate in nanoelectronic devices like field-effect transistors. Moreover, our findings reveal a transfer of charge carriers from the In2Se3 layer to the Fe3GeTe2 layer, resulting in the formation of an in-built electric field at the metal-semiconductor interface. Our work can substantially broaden the device potential of the In2Se3/Fe3GeTe2 heterojunction in future low-energy electronic devices.

Broad-Spectrum Solvent-free Layered Black Phosphorus as a Rapid Action Antimicrobial.

Antimicrobial resistance has rendered many conventional therapeutic measures, such as antibiotics, ineffective. This makes the treatment of infections from pathogenic micro-organisms a major growing health, social, and economic challenge. Recently, nanomaterials, including two-dimensional (2D) materials, have attracted scientific interest as potential antimicrobial agents. Many of these studies, however, rely on the input of activation energy and lack real-world utility. In this work, we present the broad-spectrum antimicrobial activity of few-layered black phosphorus (BP) at nanogram concentrations. This property arises from the unique ability of layered BP to produce reactive oxygen species, which we harness to create this unique functionality. BP is shown to be highly antimicrobial toward susceptible and resistant bacteria and fungal species. To establish cytotoxicity with mammalian cells, we showed that both L929 mouse and BJ-5TA human fibroblasts were metabolically unaffected by the presence of BP. Finally, we demonstrate the practical utility of this approach, whereby medically relevant surfaces are imparted with antimicrobial properties via functionalization with few-layer BP. Given the self-degrading properties of BP, this study demonstrates a viable and practical pathway for the deployment of novel low-dimensional materials as antimicrobial agents without compromising the composition or nature of the coated substrate.

Tuning the work function of the silicene/4 × 4 Ag(111) surface.

Silicene, the silicon analog of graphene, is an atomically thin two-dimensional material with promising applications in gas sensing, storage and as components in modern electronic devices. Silicene epitaxially grown on the Ag(111) surface can expand the utility of the silver surface by enabling the tuning of its work function through the functionalisation of silicene. Here we examine the electronic and structural properties and the thermodynamic stability of functionalised silicene/4 × 4 Ag(111) using density functional theory calculations coupled with ab initio molecular dynamics (AIMD) simulations. We focus on 11 functional groups, namely phenyl, methyl, hydroxyl, cyano, methoxyl, amino and ethylmethylamine, in addition to 4 halogen atoms. These functional groups are shown to endow the Si/Ag(111) surface with a large variation in the work function. Our AIMD simulations confirm the thermodynamic stability of these 11 functionalised structures. This work shows the possibility of tuning the electronic structure of silicene by functionalisation, which could then be utilized in polymer solar cells and nanoelectronic circuit components.