An Investigation into Solution Processing and Assembly Routes for SnS Nanosheet Devices

Abstract

In this work tin(II) sulfide and germanium(II) sulfide were exfoliated via ultrasonication in NMP followed by cascade centrifugation to obtain nanosheets of varying thicknesses in order to create a range of photodetectors with varying band gaps via the use of three assembly methods: Langmuir-Blodgett deposition, liquid-liquid deposition and inkjet printing. Ways to improve these devices via doping and a change in architecture were then investigated. In chapter 1 an introduction to nanotechnology was given with a literature review detailing key publications on the synthesis methods, properties and applications of bulk and 2D SnS in particular, literature was also provided on GeS as well as the assembly methods and techniques used within this work including Langmuir-Blodgett assembly, liquid-liquid assembly, inkjet printing and doping of nanomaterials. Chapter 2 discussed the operational mechanisms of the characterisation equipment used as well as the liquid phase exfoliation and cascade centrifugation methods used to prepare the nanomaterials, the assembly of the nanomaterials via Langmuir-Blodgett, liquid-liquid and inkjet printing methods as well as discussing the methods utilised in the production and optoelectronic testing of the devices and finally a list of terms used throughout the work was provided. Chapter 3 discussed the liquid phase exfoliation of both SnS and GeS followed by the cascade centrifugation and analysis of the nanomaterial properties via multiple methods in order to determine properties such as the band gap and nanosheet thickness. Smaller nanosheets with an increasing band gap (0.9 eV – 1.58 eV) were produced with increasing centrifugation speeds (180 g – 11620 g) for both materials. A brief investigation into other surfactants and solvents which may have been capable of exfoliating SnS was also performed, CTAB was considered as a promising surfactant for further analysis due to an increased length: thickness ratio (6.61 : 1 vs 4.04 : 1 for NMP). In chapter 4 the nanosheets produced were assembled into films via Langmuir deposition both with and without barrier compression as well as by liquid-liquid assembly and inkjet printing. These films were characterised via AFM, SEM and UV-vis spectroscopy with the creation of progressively thinner, higher band gap films obtained from nanosheets separated at increased centrifugation speed. Band gaps of 0.72 eV and 1.4 eV for 180 g and 11620 g were obtained respectively for LL films. A significant change in film roughness and surface coverage was observed between equivalent thickness liquid-liquid and inkjet printed films with 71.9 nm Ra and 40.1 % coverage for SnS 180 - 3040 g inkjet printed film vs 31.1 nm Ra and 80.1 % coverage for a 180 - 3040 g liquid-liquid film. A p-n junction ZnO/ consisting of multiple liquid-liquid SnS nanosheet layers formed into a ZnO/SnS heterostructure was created with the properties of each layer characterised. Chapter 5 investigated the properties of the devices produced via all methods. A photoresponse under bias was observed with an increase in response for films prepared with more highly centrifuged, thicker SnS nanosheets (4.24 × 10-9 A W-1 and 2.39 × 10-9 A W-1 average for 180 g – 3040 g PLB and LL devices) compared to 8.56 × 10-10 A W-1 and 3.17 × 10-11 A W-1 for 3040 g – 5900 g LB and LL devices). Printed devices with top printed Ag contacts exhibited a more gradual time dependent photoresponse without an initial sharp rise observed in Langmuir and liquid-liquid devices. Layered ZnO/ SnS devices exhibited a far greater responsivity (0.0124 – 0.49 A W-1) compared to lateral photoconductor devices, however these devices were damaged during testing, making further characterisation difficult. In chapter 6; a means to improve the band gap properties of the nanosheets including a transition from indirect to direct gap via doping to improve solar absorption was discussed. SnS was doped with Pb to form via a dithiocarba

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Brian Derby (Supervisor) & David Lewis (Supervisor)