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Title: Optical properties of luminescent alkylated-silicon quantum dots
Author: Rostron, Rebecca Joy
ISNI:       0000 0004 2719 5979
Awarding Body: University of Newcastle Upon Tyne
Current Institution: University of Newcastle upon Tyne
Date of Award: 2011
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Under excitation by visible light, alkylated-silicon quantum dots emit an orange- coloured luminescence, peaking at around 650 nm. Following continuous illumination, a decay of the luminescence over a time-scale of 5-10 minutes was monitored concur- rently with a photo current generated by ejection of electrons from the dots. The photoluminescence and current both decayed to non-zero, steady-state values during irradiation by visible laser light at incident intensities in the range 0.25-0.3 ± 0.01 kW /cm2; on cessation, the non-conducting photoluminescent state was substantially regained. These observations are consistent with a model in which the decay is as- cribed to autoionization of the alkylated-silicon quantum dots with a mean lifetime {Ta), depending on particle size, and recovery of luminescence to electron-hole re- combination characterized by a mean lifetime {Teh). Values of {Ta) = 1.08 ± 0.03 sand {Teh) = 770 ± 300 s were extracted from nonlinear least squares fitting to the time dependence of the photoluminescence intensity. The temporal behaviour of the transient photocurrent was found to be quantitatively consistent with a one- dimensional model of diffusion of charge carriers between quantum dots. Integration of the time-dependence of the photo current response coupled with an estimate of the volume irradiated by the laser light suggests ionization of one electron per quantum dot during photon irradiation. Measurements of the time-resolved decay of orange-band emission over a time scale of tens of microseconds and of the dependence on applied intensity of luminescence from the quantum dots were performed using pulsed laser sources. The dependence of luminescence on time was found to be strongly non-exponential and was optimally ac- counted for by a probability density function which describes a continuous distribution of two decay times: the temporal behaviour is characteristic of a pair of elementary steps connected with light emission within a distribution of local environments, or a single rate process supported by two environments. Non-linear least-squares fits to the time dependent luminescence formulated on this basis with a Gaussian, Lorentzian or log-normal distribution of rates returned most probable lifetimes T1 = 21 ± 1 μS and T2 = 3.7 ± 0.8 μS. The widths of the distributions vary between σ1 = 0.01-D.03μs-1 and az = 0.14-1.1 μs-1 associated with 1/T1 and 1/T2 respectively. The intensity of luminescence displays a linear power dependence on the intensity of the applied field, from which an exponent n = 0.94±0.02 commensurate with single-photon absorption was derived.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available