Engineering excitons in semiconductor nanocrystals

Semiconductor nanocrystals are a building block of modern nanotechnology. By controlled synthesis of nanocrystals using solution chemistry, materials can be made with highly engineered optical and electrical properties that are not possible by other routes. The nanocrystals can then be incorporated into devices such as solar cells for renewable energy or light emitting diodes for energy efficient lighting, or they can be attached to other molecules for use as highly fluorescent sensors in the life sciences.

Our research focuses on the engineering of optoelectronic properties by controlling the geometry and dimensions of excited states (excitons) generated in the nanocrystals. Nanocrystals are grown and characterised by electron microscopy and optical spectroscopy in-house, including state of the art single nanocrystal spectroscopy to understand the carrier dynamics at the single electron level. Modelling of the nanocrystal behaviour is carried out using a variety of methods including effective mass theory, Monte Carlo, and tight binding theory calculations.

Experimental setup
Fig. 1 Apparatus for single nanocrystal characterisation experiments.
10K spectrum
Fig.2 (left) Fluorescence blinking from single nanocrystals, (middle) fluorescence spectrum from two nanocrystals at T=10K, (right) Monte carlo simulation of fluorescence decay.

Recent publications and presentations

K. D. Wegner, P. T. Lanh, T. Jennings, E. Oh, V. Jain, S. M. Fairclough, J. M. Smith, E. Giovanelli, N. Lequeux, T. Pons, and N. Hildebrandt, Influence of luminescence quantum yield, surface coating, and functionalisation of quantum dots on the sensitivity of time-resolved FRET bioassays, ACS Applied Materials and Interfaces 5, 2881 (2013)

C. A. Cattley, C. Cheng, S. M. Fairclough, L. M. Droessler, N. P. Young, J. H. Warner, J. M. Smith, H. E. Assender, and A. A. R. Watt, Low temperature phase selective synthesis of Cu2ZnSSnS4 quantum dots, Chem. Comm. 49, 3745 (2013).

S. M. Fairclough, E. J. Tyrrell, D. M. Graham, P. J. B. Lunt, S. J. O. Hardman, A. Pietzsch, F. Hennies, J. Moghal, W. R. Flavell, A. A. R. Watt, and J. M. Smith, Growth and characterization of strained and alloyed type-II ZnTe/ZnSe coreshell nanocrystals, J. Phys. Chem. C 116, 26898 (2012).

Z. Y. Di, H. V. Jones, P. R. Dolan, S. M. Fairclough, M. B. Wincott, J. Fill, G. M. Hughes, and J. M. Smith, Controlling the emission from semiconductor quantum dots using ultra-small tunable optical microcavities, New J. Phys. 14, 103048 (2012).

M. Cadirci, S. K. Stubbs, S. M. Fairclough, E. J. Tyrrell, A. A. R. Watt, J. M. Smith, and D. J. Binks, Ultrafast dynamics in type II ZnTe-ZnSe colloidal quantum dots, Phys. Chem. Chem. Phys. 14 13638 (2012).

E. J. Tyrrell and J. M. Smith, Effective mass modeling of excitons in type-II quantum dot heterostructures, Phys. Rev. B 84, 165328 (2011).

S. M. Fairclough, E. J. Tyrrell, J. Moghal, A. A. R. Watt and J. M. Smith, Excitonic transitions in type-II heterostructured ZnTe/ZnSe core-shell nanocrystals, Symposium on Quantum Dots, University of Bristol, January 10, (2011).

S. M. Fairclough, E. J. Tyrrell, A. A. R. Watt, and J. M. Smith, Synthesis and optical characterisation of colloidal Type-II heterostructured ZnTe/ZnSe and ZnTe/ZnSe/ZnS nanoparticles, Quantum Dot 2010, University of Nottingham, UK, April 26-30, (2010).

A. Stavrinadis, J. M. Smith, C. A. Cattley, A. G. Cook, P. S. Grant, and A. A. R. Watt, SnS/PbS nanocrystal heterojunction photovoltaics, Nanotechnology 21, 185202 (2010).

A. Stavrinadis, S. Xu, J. H. Warner, J. L. Hutchison, J. M. Smith and A. A. R. Watt, Superstructures of PbS nanocrystals in a conjugated polymer and the aligning role of oxidation, Nanotechnology 20, #445608 (2009).

D. J. Keeble, E. A. Thomsen, A. Stavrinadis, I. D.W. Samuel, J. M. Smith, and A. A. R. Watt, Paramagnetic point defects and charge carriers in PbS and CdS nanocrystal polymer composites, J. Phys. Chem. C 113, p.17306 (2009).

P. K. Santra, R Viswanatha, S Daniels, N. L. Pickett, J. M Smith, P. O'Brien and D. D. Sarma, Investigation of the internal heterostructure of highly luminescent quantum dot - quantum well nanocrystals, J. Am. Chem. Soc. 131, p470 (2009).

A. Stavrinadis, R. Beal, J. M. Smith, H. E. Assender, and A. A. R. Watt, Direct formation of PbS nanorods in a conjugated polymer, Advanced Materials 20, p3105 (2008).

S. H. Kim, P. H. Sher, Y. B. Hahn, and J. M. Smith, Luminescence from single CdSe nanocrystals embedded in ZnO thin films using atomic layer deposition, Nanotechnology 19, #365202 (2008).

P. H. Sher, J. M. Smith, P. A. Dalgarno, R. J. Warburton, X. Chen, P. J. Dobson, S. M. Daniels, N. L. Pickett, and P. O'Brien, Power law carrier dynamics in semiconductor nanocrystals at nanosecond time scales, Appl. Phys. Lett. 92, #101111 (2008).

P. H. Sher, X. Chen, P. J. Dobson, S. M. Daniels, N. L. Pickett, P. O'Brien, and J. M. Smith, Measuring carrier trapping dynamics in single semiconductor nanocrystals over ten decades in time, 5th International Symposium on Surface Science (ISSS-5), Tokyo, Japan, November 9-13, (2008).

P. H. Sher, J. M. Smith, S. M. Daniels, N. L. Pickett, P. O'Brien, P. A. Dalgarno, and R. J. Warburton, Power law dynamics in the fluorescence from semiconductor nanocrystals, Condensed Matter and Materials Physics, University of Leicester, April 11-13, (2007).