UROS 2017 Project: Phonons in Nanomaterials – Introducing Copper deficiency states in CulnS2 Nanocrystals
By Niall Garry
Nanocrystals are tiny light emitting particles on the nanometre scale. They have typical dimensions of between 1-100nm which means they bridge the gap between small molecules and large crystals. The production of nanocrystals is low-cost, relatively easy and allows a lot of control over their specific properties. This coupled with their fluorescence makes them useful for numerous applications. Our nanocrystals were categorised as ‘quantum dots’. This refers to very small semiconductor nanocrystals where the band gap of the material corresponds to the emitted wavelength of light. These properties make the nanocrystals useful for applications such as; solar energy, bio-labelling and display technologies.
We were interested in working with copper indium sulphide (CuInS2) crystals. For our research, we were interested in reducing the copper to indium ratio in the CuInS2 nanocrystals with the hope that a copper deficiency would introduce a defect state between the valence and conduction band. This would hopefully improve their efficiency in applications such as solar energy since a defect state would mean less energy is ‘lost’ as emitted light. From background research we made the assumption that a copper to indium ratio of around 0.7:1 would yield the optimum fluorescence.
Copper iodide was used as the source of copper and Indium acetate was used as the source of indium. Dodecanethiol was also used as the source of Sulphur as well as the solvent of the reaction. The ratio of copper to indium was then altered by reducing the mass of Copper iodide. The nanocrystals were synthesised in a high-heat reaction in a nitrogen-flushed flask.
Once the nanocrystals had been synthesised and cleaned we were able to do a number of tests on the samples. Firstly X-ray diffraction (XRD) was completed to confirm all the samples were in fact CuInS2 and that reducing the copper had not caused the synthesis of another compound. The peaks of each XRD curve corresponded to an element causing X-rays to diffract at a particular angle. The 3 peaks all lining up confirmed that all the samples were in fact CuInS2 nanocrystals. Next was X-ray spectroscopy (XPS) which was completed at Liverpool University. XPS uses the photoelectric effect to gain information on the energy levels within the compound. When the three curves are compared a clear ‘shoulder’ on the main copper peak is seen as the sample reaches 0.7:1 ratio. This confirmed not only that a copper deficiency band was present but that 0.7:1 seemed to be the optimum ratio to introduce it. A simple test that was completed on the samples is the one that produced the most visible results, observing the samples under a UV lamp. This allows us to view the fluorescence of the samples and made it clear that reducing the ratio of copper to indium produced a clear increase in fluorescence.
The results also showed a ‘blue shift’ in the emitted light as it appeared to get ‘more red’, something we were not able to find described in literature.
*To view Aaron’s project poster, please click on the thumbnail below