Marek Szablewski

A novel liquid–liquid interface method for producing thin films and van der Waals heterostructures of two-dimensional (2D) materials is presented, offering broad applicability to 2D materials held in surfactant-stabilised aqueous suspensions.

Cu ₂ ZnSnS ₄ (CZTS) is a p-type absorber layer semiconductor, which is a promising candidate material for advanced solar cell devices. CZTS nanoparticle thin films were synthesized via the hot-injection spin-coating method. Nanoparticle inks were deposited on soda lime glass substrates. Different reaction temperatures during synthesis were investigated by using X-ray diffraction (XRD), focused ion beam microscopy (FIB-SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and Ultraviolet-visible spectroscopy to study the effects of the synthesis reaction temperature on CZTS thin films. The energy band-gap decreased with increasing reaction temperatures from 1.55 to 1.34 eV for the temperature range 225 °C–300 °C. These energy bandgaps show values close to the optimum for a CZTS solar cell device. XRD confirmed the kesterite structure, which is the preferred structure in CZTS for high efficiency PV device performance. In addition, a clear increasing trend of the scattering domain size with the reaction temperature is found to be in good agreement with the FIB-SEM images. Raman spectra showed no secondary phases. EDS of thin films demonstrated compositions consistent with start-up precursor ratios (Cu-poor Zn-rich composition). And 250 °C was identified as the most suitable reaction temperature for CZTS devices.

CZTS nanoparticle inks are synthesised and deposited as thin films by spin coating. We report variations in the chemical composition of films synthesised under nominally identical conditions, and the impact of annealing in a sulphur environment.

Bournonite (CuPbSbS3) is an emerging absorber material for solar cells. There is little literature on some of its fundamental structural and optical properties. We report the first complete Raman spectra of bournonite as well as the first luminescence signal. A chosen area has been mapped with both Raman spectra and cathodoluminescence (CL) to investigate the structural and optical uniformity. Raman mapping reveals a new secondary phase, tetrahedrite, but otherwise a uniform compositional thin-film. CL mapping reveals slight redshift at grain boundaries (GBs). Preliminary devices made with a bournonite absorber layer show rectification under standard test conditions.

Novel fluorescent Langmuir-Blodgett (LB) films have been constructed from three different amphiphilic dicynaoquinodimethanes (DADQs). The DADQs varied in functional group structure, which had an impact on the LB film structure and the fluorescence properties. As the fluorescence of DADQs competes with non-radiative decay (conformational change), the packing and/or free volume in the LB film will influence the average fluorescence lifetime and integrated intensity. The pristine (blank) LB films were then exposed to a selection of non-fluorescent target analytes (some with environmental relevance) and the fluorescence was measured and analyzed relative to the pristine LB film. Exposure of the LB films to selected target analytes results in a modulation of the fluorescence, both with respect to average fluorescence lifetime and integrated intensity. The modulation of the fluorescence is different for different DADQ LB films and can be attributed to restricted non-radiative decays or charge transfer reactions between target analyte and DADQ LB film. The response from the DADQ LB films shows that these systems can be developed into sensing surfaces based on fluorescence measurements.

The high temperature dielectric transition within the organic functional material, TEA(TCNQ) ₂ , is studied. Both the structure and dynamics of this transition has been mapped out.

The metal-organic charge transfer compound NiTCNQ₂ has been subjected to pressure where both magnetisation and muon experiments show a suppression of T _C with an applied pressure. At 12 kbar, it appears that the sample is paramagnetic and this gives an estimate of the pressure required for switching the transition on/off. The present work also provides some insight into the molecular structure since this behaviour can be explained if the structure is 3D. It is proposed that the TCNQ interactions increase along a stack with increasing pressure which causes spins on the TCNQ to couple into a singlet state with just isolated Ni ions left, which behave paramagnetically.

With interest in charge transfer compounds growing steadily, it is important to understand all aspects of the underlying physics of these systems, including the properties of the defects and interfaces that are universally present in actual experimental systems. For the study of these defects and their interactions a spin-Peierls (SP) system provides a useful testing ground. This work presents an investigation within the SP phase of potassium TCNQF₄ where anomalous features are observed in both the magnetic susceptibility and ESR spectra for temperatures between 60 K and 100 K. Muon spin spectroscopy measurements confirm the presence of these anomalous magnetic features, with low temperature zero-field data exhibiting the damped oscillatory form that is a characteristic signature of static magnetic order. This ordering is most likely due to the interaction between structurally correlated magnetic defects in the system. The critical behaviour of the temperature dependent muon spin rotation frequency indicates that a 2D Ising model is applicable to the magnetic ordering of these defects. We show that these observations can be explained by a simple model in which the magnetic defects are located at stacking faults, which provide them with a 2D structural framework to constrain their interactions.