Publication date: 27th June 2014
Doping semiconductor nanocrystals (NCs) is a highly pursued challenge, providing another means, along with size and shape, for controlling their electronic properties. We present a new impurity free method for NC doping by thermal treatment at moderate temperatures (T below 400K), thus creating vacancies leading to free charge carriers. This method is applied here for Cu2S NCs, where Cu vacancies easily form, resulting in p-type doping. This thermal doping procedure is used here to controllably increase the conductance of Cu2S-NC arrays, achieving up to 6 orders of magnitude enhancement, for which we extract the activation energy for Cu vacancy formation, ~1.6eV. The thermal doping effect manifests itself also in tunneling spectra by the emergence of in-gap states and ~280meV shift of the Fermi level towards the valence band, signifying p-type doping. The effect of vacancy formation was further verified via powder X-ray diffraction depicting a gradual irreversible switch between the Cu rich and Cu poor states of Cu2S upon the thermal process. In addition, we demonstrated local thermal doping of the NC array via a focused laser beam serving as the heating source. This method yielded a similar increase in device conductance and further characterization obtained via Kelvin probe microscopy portrayed ~80meV increase in the potential of the NC array in the laser heated areas verifying that p-doping also takes place in the laser induced thermal process. This method opens an innovative route for low temperatures patterned doping of NC arrays.(1)
(1) Bekenstein, Y.; Vinokurov, K.; Keren-Zur, S.; Hadar, I.; Schilt, Y.; Raviv, U.; Millo, O.; Banin, U. Nano Lett. 2014.
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