Natalie A. Gibson
University of California, Berkeley
Natalie Gibson is a Ph.D. Candidate at the University of California, Berkeley in the research group of Professor Stephen Leone. She uses single molecule spectroscopy to study carrier dynamics in semiconductor nanostructures.
Semiconductor nanocrystals are attractive materials for a range of optoelectronic devices, due in large to their tunable optical and electronic properties. However, the localization of excited charges at defect states impedes both charge mobility and radiative recombination and limits the efficiency of any nanocrystal-based device. Since this leads to intermittencies in the emitted photoluminescence (PL) of a single nanocrystal, it can be directly probed by analyzing PL trajectories of individual nanocrystals.
While such experiments have contributed to an understanding of light-matter interactions in type II-VI, III-V and IV-VI semiconductors,1 these interactions in an emerging class of semiconductor nanocrystals, lead halide perovskites, are largely unknown.
Time-correlated single photon counting is used to record PL trajectories of single CsPbBr3 nanocrystals. An embedding polymer used to prevent aggregation also decreases the photostability and quantum yield of the nanocrystals. To overcome these issues, a low repetition rate laser is used to avoid rapid photobleaching, and a bin-free changepoint analysis (CPA) method2,3 is used to analyze trajectories that lack a clear separation of intensity levels.
Figure 1 shows a PL trajectory of a single CsPbBr3 nanocrystal, obtained by binning photon counts (black) and using CPA (red). Once a trajectory is separated into distinct ‘on’ and ‘off’ levels via CPA, the kinetics of each level can be investigated by calculating a probability distribution. This is found to follow an exponentially-truncated power law, P(τ)=τ-mexp(-τ/τc), where P(τ) is the probability that a duration of length τ occurs, m is the power-law exponent, and τc is the time at which the power law diverges into an exponential.
The intensity dependence of the on-state τc is investigated in over 130 nanocrystals, and appears to decrease with increasing excitation intensity (Figure 2). A superlinear dependence at low exciton formation is observed, followed by a rapid saturation (Figure 3).
These results suggest that a single-exciton mechanism, or a combination of single and multi-exciton mechanisms, is responsible for the intermittent PL behavior in CsPbBr3 nanocrystals.4 Future studies will focus on isolating the nanocrystals in a polymer-free environment to understand the role of the surrounding polymer.
[1] 10.1021/jp0467548
[2] 10.1021/acs.jpcc.6b09780
[3] 10.1039/C2CS35452G
[4] 10.1063/1.1993567
Optical properties of nanostructures , Quantum dots and colour centres
