Compositional tuning of Zn- based semiconducting nanoparticles by Atmospheric Pressure Plasmas and study of their optoelectronic properties

Abstract

The main objective of this work is to study the impact of compositional variation on the material property of zinc-based semiconductors. Non-thermal atmospheric pressure plasma systems are used to synthesise these nanosized semiconductors. Relevant applications are explored, specifically photovoltaics and supercapacitors.

In this context, porous zinc oxide (ZnO) film, ZnO quantum dots (QDs) and zinc nitride (Zn2N3) nanoparticles (NPs) are synthesised with gas-phase atmospheric pressure plasma. Further, doped ZnO and zinc peroxide (ZnO2) NPs are synthesised with liquid-phase non-thermal atmospheric pressure plasma. This work also includes extensive material characterization of synthesised nanomaterials to analyse their properties.

Firstly, porous ZnO is synthesised and directly deposited from the gas phase plasma system. Detailed material characterization has validated the purity of the synthesised material. Further, analysis of band alignment has shown the intactness of electronic properties of ZnO. This synthesised porous ZnO is used as an electron transport layer in photovoltaics. Initial results have shown that porous ZnO is not an appropriate choice for perovskite based solar cells. More specifically, control over the device fabrication is difficult task in ZnO/ perovskite solar cells. In another solar cell configuration, N doped carbon quantum dots are used as an active material. Preliminary results have shown promising results in using porous ZnO as electron transport layer with quantum dots active solar cells.

The second important finding of this work is the formation of defect-free selfhydroxyl passivated ultra-small ZnO QDs (~1.9 nm) from gas phase plasma system. This passivation happens because of the high surface energy of the small size QDs. Hydroxyl passivation of the ZnO QDs has shown the surface defect termination. This defect-termination has led to the very stable ultra-violet emission of ZnO QDs without any visible emission.

Later, ZnO2 is produced with a hybrid liquid-plasma system. Impact of synthesis current is analysed on intrinsic crystal defects. The results have shown that intrinsic defects in the lattice has reduced with increasing synthesis current. Analysis of ZnO2 formation mechanism has revealed that ionized oxygen species plays a major role in X the final synthesis of ZnO2.

Further, doped ZnO is also synthesised successfully with a hybrid liquid-plasma system. Detailed characterization has revealed many interesting outcomes of doping. With doping, the absorption edge is extended in the visible region and bandgap is reduced to 2.98 eV. The dopant position in the lattice was analysed with the help of many characterization techniques. Results have indicated that at a lower dopant concentration, substitutional doping plays a major role. Later, an in-built constraint towards the substitutional occupation site is observed and interstitial dopant concentration dominates over substitutional with increase in doping concentration. Also, initial experiments have shown pseudocapacitive nature in N-doped and C-N doped ZnO for the first time. Highest areal capacitance (1.6 F/ cm2 ) was observed for C-N doped ZnO. Collectively, these results have also indicated the positive impact of substitution doping on the visible absorption and storage capacity of the ZnO.

Finally, for very first time, the synthesis of Zn2N3 nanoparticle in gas-phase atmospheric pressure plasma is studied. The complexity of producing Zn2N3 at atmospheric pressure is demonstrated with the help of different nitrogen precursor. At first, pure nitrogen gas was used as a nitrogen precursor where zinc, zinc oxide and zinc nitride mixed phase were synthesised. However, a great degree of difficulty was observed in identifying the chemical composition of synthesised material. In another attempt, ammonia gas was used as a nitrogen precursor. Synthesis of pure a phase Zn2N3 was observed with ammonia. Also, surface oxidation was observed after atmospheric exposure of synthesised Zn2N3.

Date of Award	Jan 2019
Original language	English
Sponsors	Engineering and Physical Sciences Research Council
Supervisor	Davide Mariotti (Supervisor)