A Study of Synthesis and Optoelectronics of Copper Iron Chalcogenide Nanocrystals
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Copper iron chalcogenides constitute a promising class of optoelectronic materials courtesy of their narrow bandgaps and earth abundant constitution. However, they are yet to receive the attention they deserve due to the lack of easy synthetic protocols and poorly understood material properties. Discordant narratives in the literature regarding their optoelectronic properties has also prevented them from being used for device-based applications. This thesis is aimed at rectifying a few of these issues. The objective of this thesis is to synthesize and study the properties of copper iron chalcogenide nanocrystals viz., CuFeS2 and CuFeSe2, and to explore their utility in the context of optoelectronic devices. Chapter 1 provides a brief introduction to the fundamental concepts related to the work described in this thesis. The chapter further discusses the scope and motivation behind the work carried out in this thesis. Chapter 2 describes our efforts to assign the nature of a feature in the optical absorption spectrum of CuFeS2 nanocrystals occurring at ~500 nm. Using a combination of steady-state and time-resolved optical spectroscopy as well as transport measurements we assign the feature to be a localized surface plasmon resonance and attribute the peculiar properties exhibited by CuFeS2 nanocrystals to this feature. Further, the transport measurements revealed that films of these nanocrystals can support a photoresponse. Chapter 3 describes the fabrication and characterization of a broadband photodetector based on CuFeS2 nanocrystals. Briefly, we fabricated heterojunctions of CuFeS2 nanocrystals with bulk n type silicon and demonstrated a broadband photoresponse from 460 nm-2200 nm with response time of the order of microseconds. The photodetector was further found to possess a photothermal response that is bolometric in nature, which allows the device to sense hot objects at room temperature. Chapter 4 describes our efforts to synthesize and study the optoelectronic properties of CuFeSe2 and CuFeSe2-CdS core-shell nanocrystals. We synthesized CuFeSe2 nanocrystals and studied their properties using structural, optical and electrical characterization techniques. The nanocrystals were found to have a very narrow bandgap of 0.11 eV and were also found to exhibit a plasmon resonance at ~410 nm. We further found that the films of these nanocrystals exhibited a photoresponse in the MIR, thus making them a promising candidate for infrared photodetection. We further synthesized highly luminescent CuFeSe2-CdS core-shell nanocrystals and found that the energetic position of their emission is greatly dependent on the sequence in which the shell growth precursors are added to the reaction mixture. Using optical and structural characterization techniques, we find that there are two different core-shell variants that result from the synthesis and their formation is determined by which one of the shell growth precursors is added to the reaction mixture first. The key difference between the two variants were found to be the presence of an interfacial CdSe layer which occurs whenever the cation precursor is added to the reaction mixture first. Chapter 5 describes the synthesis of CuFexGa1-xS2 nanocrystals, a hitherto unknown composition of nanocrystals. Using alloying as a strategy, we synthesized CuFexGa1-xS2 nanocrystals corresponding to different Fe:Ga ratios. The properties of the resulting nanocrystals were found to be greatly dependent on their composition.