UROS 2017 Project: Synthesis of Iron Doped Chalcopyrite (CulnFeS2) Quantum Dots (Semiconductor Nanocrystals)
By Robert Sharp
The purpose of this project was to synthesize iron-doped chalcopyrite (CuInFeS2) quantum dots to investigate the predicted intermediate energy level band gap due to the presence of iron. Quantum dots are semiconductor nanostructures that govern the motion of conduction band electrons, valance band holes and excitons in all three spatial directions. In other words, the energy gap between the valence band (the highest filled energy state) and the conduction band (the lowest filled empty state) is tuneable, depending on the properties of the quantum dots, e.g. size or doping (in this case iron).
The colloidal synthesis method was used to grow the nanocrystals, which primarily consisted of thermally decomposing a mixture of three precursors (copper iodide, indium acetate and iron acetylacetonate) in a solvent (dodecanethiol) with a high boiling point under N2 gas flow. Due to the heat, the precursors are chemically altered and nucleate into nanocrystals, whose properties can be altered by variables such as temperature. Therefore, the procedure for synthesis was repeated for each different sample to grow crystals of the same size for each sample. The only difference was the mass of iron acetylacetonate used as this meant different concentrations of samples were synthesized; ranging from 1% to 30%.
The samples were then washed by adding them into new specimen containers and topping up with hexane. These were then placed in a centrifuge (after ensuring the masses were equal) for ten minutes and spun at around 7000RPM. Once complete, any unreacted precursors or ligands (and hexane) in the samples could then be poured away. This process was repeated three times.
Once washed, the samples were then ready to be analysed using X-ray photo-electron spectroscopy (XPS) and X-ray diffraction (XRD). XPS was used to distinguish the intermediate band gap and achieved this by utilising the photoelectric interaction between monoenergetic x-rays and the nanocrystals, whose electrons were ejected (photoelectrons) with discrete energy levels. The kinetic energy distribution of the emitted photoelectrons was measured using an electron energy analyser and a photoelectron spectrum was produced, with the peaks characterising the binding energy of each element present in the quantum dots. Upon reviewing the XPS spectra, the 690-730eV region was inspected as this corresponded to iron (Fe) 2p states (and indium 3p states) for copper iron disulphide. The NIST XPS database was then referred to in order to verify that the peaks in the spectra were representative of iron states. Indeed, all peaks (5%, 10% and 20%) contained a peak at approximately 722eV, corresponding to an Fe 2p (1/2) energy level, as well as the 20% core level sample containing a peak at 708eV which corresponded to Fe 2p (3/2). This also indicated that the 20% sample did in fact contain more iron than the other samples, since the spectrum contained two peaks for iron energy levels as opposed to just the one. Finally, the XRD spectra showed that all samples comprised of similar crystalline structures, with the sizes of the crystals also similar, with the peaks having similar widths.
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