Thomas J. Whittles

Electronic and optical properties of single crystal SnS2: an earth-abundant disulfide photocatalyst

Tin disulfide is attractive as a potential visible-light photocatalyst because its elemental components are cheap, abundant and environmentally benign. As a 2-dimensional semiconductor, SnS2 can undergo exfoliation to form atomic layer sheets that provide high surface areas of photoactive material. In order to facilitate the deployment of this exciting material in industrial processes and electrolytic cells, single crystals of phase pure SnS2 are synthesised and analysed with modern spectroscopic techniques to ascertain the values of relevant semiconductor properties. An electron affinity of 4.16 eV, ionisation potential of 6.44 eV and work function of 4.81 eV are found. The temperature dependent band gap is also reported for this material for the first time. We confirm the valence band is formed predominately by a mixture S 3p and Sn 5s, while the conduction band consists of a mixture of Sn 5s and 5p orbitals and comment on the agreement between experiment and theory for values of band gaps.

Direct Measurements of Fermi Level Pinning at the Surface of Intrinsically n-Type InGaAs Nanowires

Surface effects strongly dominate the intrinsic properties of semiconductor nanowires (NWs), an observation that is commonly attributed to the presence of surface states and their modification of the electronic band structure. Although the effects of the exposed, bare NW surface have been widely studied with respect to charge carrier transport and optical properties, the underlying electronic band structure, Fermi level pinning, and surface band bending profiles are not well explored. Here, we directly and quantitatively assess the Fermi level pinning at the surfaces of composition-tunable, intrinsically n-type InGaAs NWs, as one of the prominent, technologically most relevant NW systems, by using correlated photoluminescence (PL) and X-ray photoemission spectroscopy (XPS). From the PL spectral response, we reveal two dominant radiative recombination pathways, that is, direct near-band edge transitions and red-shifted, spatially indirect transitions induced by surface band bending. The separation of their relative transition energies changes with alloy composition by up to more than ∼40 meV and represent a direct measure for the amount of surface band bending. We further extract quantitatively the Fermi level to surface valence band maximum separation using XPS, and directly verify a composition-dependent transition from downward to upward band bending (surface electron accumulation to depletion) with increasing Ga-content x(Ga) at a crossover near x(Ga) ∼ 0.2. Core level spectra further demonstrate the nature of extrinsic surface states being caused by In-rich suboxides arising from the native oxide layer at the InGaAs NW surface.

Photochemical CO2 reduction in water using a co-immobilised nickel catalyst and a visible light sensitiser

A dye-sensitised CO2 reduction photocatalyst that operates in water is reported. Transient spectroscopy demonstrates that the facile co-immobilisation of a Ru dye and a Ni CO2 reduction electrocatalyst enables efficient on-particle electron transfer leading to photocatalytic activity that greatly exceeds the equivalent solution based system.

Core Levels, Band Alignments, and Valence-Band States in CuSbS2 for Solar Cell Applications

The earth-abundant material CuSbS2 (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core levels of the constituent elements, surface contaminants, ionization potential, and valence-band spectra are studied by X-ray photoemission spectroscopy. The ionization potential and electron affinity for this material (4.98 and 3.43 eV) are lower than those for other common absorbers, including CuInxGa(1-x)Se2 (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the valence band maximum is raised by the lone pair electrons from the antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies.

Cu(I)Cu(II)BTC, a microporous mixed-valence MOF via reduction of HKUST-1

Metal–organic frameworks (MOFs) are typically crystalline microporous materials with metal ions of usually one oxidation state (although mixed-valence MOFs have been reported) and one type of co-ordination structure. Here, a microporous mixed-valence Cu(I,II)-MOF featuring a unique network of distinct Cu(I) and Cu(II) coordination sites is prepared by solvothermal reduction of HKUST-1. Like HKUST-1, this compound is assembled solely from Cu ions and benzene 1,3,5-tricarboxylate (BTC) ligands. The new MOF exhibits dual micropore size distribution and shows superior water stability compared to HKUST-1.

Characterization of sulfurized CuSbS 2 thin films for PV applications

CuSbS₂ is emerging as a novel absorber for sustainable photovoltaics. We fabricated CuSbS₂ thin films by the sulfurization of sputtered metallic layers. Characterization of the morphological, optical, electrical and surface properties is presented. Overpressure of inert gas during sulfurization reduced antimony loss. The largely single-phase films exhibit optimal characteristics for PV applications, including a band gap of about 1.5 eV and p-type conductivity. The carrier concentrations were ~10¹⁷ cm^-3 and the mobility was generally about 10 cm² V^-1 s^-1. Surface property measurements are dominated by Sb₂O₃ and a secondary phase may be responsible for PL emission above the optical gap.

Atomic layer deposition of Nb-doped ZnO for thin film transistors.

We present physical and electrical characterization of niobium-doped zinc oxide (NbZnO) for thin film transistor (TFT) applications. The NbZnO films were deposited using atomic layer deposition. X-ray diffraction measurements indicate that the crystallinity of the NbZnO films reduces with an increase in the Nb content and lower deposition temperature. It was confirmed using X-ray photoelectron spectroscopy that Nb5+ is present within the NbZnO matrix. Furthermore, photoluminescence indicates that the band gap of the ZnO increases with a higher Nb content, which is explained by the Burstein-Moss effect. For TFT applications, a growth temperature of 175 °C for 3.8% NbZnO provided the best TFT characteristics with a saturation mobility of 7.9 cm2/Vs, the current On/Off ratio of 1 × 108, and the subthreshold swing of 0.34 V/decade. The transport is seen to follow a multiple-trap and release mechanism at lower gate voltages and percolation thereafter.

Physical and electrical characterization of Mg-doped ZnO thin-film transistors

The effect of Mg-doping on the valence and conduction bands of ZnO grown at 200 °C using atomic layer deposition has been investigated using a range of physical characterization techniques: X-ray photoemission spectroscopy, inverse photoemission spectroscopy and spectrocopic ellipsometry. The conduction band minimum is seen to increase with Mg content hence confirming the increased band gap. The physical characterization has been linked with modeling of thin-film transistor structures whereby a defect state based model has been employed to explain the transport mechanisms within the film.

The Electronic Structures of SnS, SnS 2 , and Sn 2 S 3 for Use in PV

This chapter involves the electronic characterisation of the single crystal tin sulphides: SnS, SnS2, and Sn2S3, for PV devices and other uses.

The Electronic Structure of Cu 3 BiS 3 for Use as a PV Absorber

This chapter involves measurements of thin-film samples of CuSbS2 (CAS), and the electronic characterisation of this material for use in PV devices.