Synthesis and characterization of transparent conducting oxides for Stark e ect applications in single molecule spectroscopy

Abstract

The desire to study the characteristics of transparent conducting oxides (TCOs) for Stark application in single molecule spectroscopy (SMS) stemmed from our interest in the dark states of light harvesting complex II (LHCII) of plants. Investigation of this mechanism requires a Stark cell. The high optical quality required by the SMS procedure demands a very speci c thickness microscope slide at the objective to sample interface. Since no commercial TCOs of this standard were available, we had to synthesize our own and spincoat our SMS setup's stock microscope slides. As a rst attempt, uorine doped tin oxide (FTO) thin lms were synthesized at 0 to 3 percent atomic doping levels. The optical quality of the lms were low, as they appeared almost burned (blackish). X-ray di raction (XRD) con rmed the lms to be FTO by the FTO powder di raction le (PDF 00-041-1445). The average optical band gap achieved was 3.860 eV, in good agreement to literature. A minimum resistivity of 0.37

:cm􀀀�1 was obtained for the 2% sample. Scanning electron microscopy revealed that the poor optical quality of the lms was due to agglomeration of tin, most likely due to a too high tin concentration in the solution. Due to availability and experience in the department with zinc oxide thin lms, the following study was on the synthesis of aluminium doped zinc oxide (AZO). Thin lms of atomic doping percentages ranging from 0 to 5 percent in steps of 1 percent were fabricated. A new deposition of layer procedure was also used, described in the next paragraph. AZO was successfully synthesized, con rmed by XRD correspondence to zinc oxide's PDF (00- 036-1451) and the average optical band gap of 3.2873 eV agreement to literature. SEM revealed polycrystalline morphology in all lms. All lms had high transparency in the visible, as showcased by ultraviolet to visible (UV-Vis) spectroscopy. A minimum resistivity of 8.61 :cm􀀀�1 was obtained for the 2% sample. From experience gained in the FTO study, we hypothesized that a di erent Deposition of thin lm layer technique could have a signi cant impact on the quality of TCO lms. Simply drying a layer before addition of a new layer would not solidify the layer in a signi cant way. Addition of new solution would then solubilize a good amount of dried particles of the previous layers. This would result in a new pseudo-solution of much higher concentration, resulting in aggregation and low reproducibility. Our results indicated that the Deposition of thin lm layer technique had little impact on the optical quality of the lms, however, the crystalline and electrical qualities were both signi cantly better for TCO application. This procedure was used for the AZO study.