Two-Dimensional Visible Spectroscopy For Studying Colloidal Semiconductor Nanocrystals Elsa Cassette, Jacob C. Dean, and Gregory D. Scholes*
Possibilities offered by 2D visible spectroscopy for the investigation of the properties of excitons in colloidal semiconductor nanocrystals are overviewed, with a particular focus on their ultrafast dynamics. The technique of 2D electronic spectroscopy is illustrated with several examples showing its advantages compared to 1D ultrafast spectroscopic techniques (transient absorption and time-resolved photoluminescence).
1. Introduction Semiconductor nanocrystals have attracted vast interest given their large absorption cross section and size-dependent optical and electronic properties. Since the early 1990s, colloidal synthesis has offered extraordinary possibilities for tuning the nanocrystals optical and electronic properties via the control of size, shape and crystallographic structure, as well as with the development of a large variety of hetero- and hybrid nanostructures.[1–3] The applications of these colloidal nanostructures range from optoelectronic, including photovoltaics, diodes, and photodetectors, to bioimaging,[5,6] and, more recently, photocatalysis.[7,8] Despite the fact that their fundamental properties have been extensively scrutinised, numerous questions remain with regards to the ultrafast photophysics occurring in these nanostructures following excitation, i.e., on timescales below 100 fs. The development of pulsed laser sources and spectroscopic techniques has opened new possibilities to retrieve information that is lost in steady state measurements while simultaneously capturing ultrafast dynamics on the femtosecond time scale. Coherent multidimensional spectroscopies provide high temporal and energy resolution, lending access to these femtosecond dynamics. 2D spectroscopy[9–11] in the visible range has unveiled couplings and coherences in molecular systems.[12–16] In the near-infrared, 2D spectroscopy has been E. Cassette, J. C. Dean, G. D. Scholes Department of Chemistry Princeton University Princeton, NJ 08544, USA E-mail: [email protected]
used to investigate spectral linewidths and manybody interactions in epitaxial quantum wells.[17–19] It is only recently that visible 2D spectroscopy has been applied to colloidal nanocrystals.
1.1. Transient Absorption and Time-Resolved Photoluminescence Time-resolved spectroscopic methods have allowed the study of dynamical processes occurring in semiconductor nanostructures over a large set of time scales and wavelengths, and thus have yielded insights into optical and electronic properties of these systems. The two predominant timeresolved methods used to study colloidal nanocrystals and their photophysical properties are transient absorption (TA) and time-resolved photoluminescence (TR-PL) spectroscopy. Photoluminescence gives information on the nature of the photoexcited species (bound exciton, free carriers, localized exciton on defect/doping state, etc.) and their lifetime. Measuring the luminescence kinetic decay is typically realized with time-correlated single-photon counting techniques. In these experiments, exciton lifetime with radiative recombination, as well as nonradiative decays, can be investigated with a temporal resolution of approximately 10–100 ps, generally limited by the detector (timing resolution of the electronics). Photoluminescence reveals information about the population of the lowest energy state, i.e., at the band edge, since exciton relaxation (internal conversion) occurs at faster time scales (hundreds of femtoseconds to several picoseconds) than the instrument response function. High-quality nanocrystals with a narrow size distribution can have a photoluminescence quantum yield close to 1 and almost monoexponential PL decay at room temperature.
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Thus the radiative decay time can be determined (generally tens to hundreds of nanoseconds), thereby allowing estimation of electron and hole wavefunction overlap. In most cases however, the nanocrystal surface is imperfectly passivated and the PL decay curves are moderately to strongly non-monoexponential. The early-time part of the PL decay indicates exciton dissociation by surface traps, while delayed PL reveals recombination of trapped carriers. Exciton–exciton and carrier–carrier interactions lead to specific power-dependent behavior and are used to identify the nature of the species generated after photoexcitation as well as their fate. Temperature-dependent TR-PL measurements have revealed information about the fine structure of the lowest energy exciton in CdSe nanocrystals (also known as colloidal quantum dots, Qds) with noticeable biexponential behavior at low temperature due to the presence of a low-emitting exciton state (called a “dark state”) just below the lowest bright exciton state.[23,24] The sensitivity of fluorescence detection has enabled the fluorescence lifetime of single nanocrystals to be recorded, leading to intrinsic decay rates sometimes being concealed in ensemble measurements. These measurements have been extended to different shapes and materials.[26–28] The use of streak cameras and optical-gated fluorescence detection techniques combined with ultrashort laser pulses (typically from femtosecond Ti:Sapphire lasers) are used to improve the temporal resolution of the detection to picoseconds and hundreds of femtoseconds, respectively. With streak cameras, researchers can directly record a map of the emission spectrum over time. Multi-exciton spectra can be measured before Auger recombination occurs, and then compared with single exciton spectra to study spectral shifts and biexciton binding energies. At shorter times, emission signal from higher energy states within the first exciton fine structure can be detected[30–33] and exciton cooling from higher electronic states can be measured directly.[34,35] Like other time-resolved measurements, particular care should be taken with the choice of laser repetition rate to ensure that the system has returned to the ground state before the following excitation pulse arrives. So far, very few research groups have used fluorescence up-conversion to access the dynamics of colloidal semiconductor nanocrystals at shorter time scales (