One of the specialties of the Rosenthal group is ultrafast fluorescence upconversion spectroscopy. Our lab pioneered the use of this technique to study the carrier dynamics of CdSe nanocrystals on the ultrafast time scale. This allows us the ability to study the photogenerated charges virtually in real time.
Our ultrafast titanium:sapphire laser system is a commercial system from Coherent, Inc. It consists of a Verdi V-18 laser which pumps a mode-locked Ti:sapphire laser (Mira 900 Basic), Regenerative Amplifier (RegA 9000) and Optical Parametric Amplifier (OPA 9400). The OPA gives us the ability to produce wavelengths from 400 nm to ~2000 nm by combining the signal, idler, and residual second harmonic pump source.
The experiment itself is probably the most intuitive of the ultrafast laser spectroscopic techniques. The data comes in the form of fluorescence with respect to time, which is analogous to what most commercial fluorimeters would call a kinetics script for observing fluorescence intensity with time. The major difference between the two is that instead of time of observation being counted in minutes, seconds or even milliseconds, we observe fluorescence on the femtosecond (10-15 seconds) time scale.
Ultrafast fluorescence upconversion is a technique that requires two ultra-short laser pulses: an excitation pulse and a gate pulse. The wavelength of the excitation pulse is tuned to the sample’s absorption spectrum. The gate pulse acts much like a camera shutter, in that only when the gate pulse is mixed temporally and spatially with the sample fluorescence is a signal observed, much like an image being collected only while the shutter exposes the film to light. Time resolution is achieved by delaying one pulse relative to the other by changing the distance the excitation pulse must travel.
Typical fluorescence upconversion data appears as a sum of decaying exponentials, with the caveat that the shape of the laser pulse must be taken into account. This is necessary because the laser pulse duration is nearly on the order of the time constants for the decay of the fluorescence intensity. The data fitting function is therefore a convolution of a function representing the laser pulse (in our case a Gaussian) and a sum of decaying exponentials.
In quantum dots, high surface-to-volume ratios coupled with the strong geometric confinement of the exciton lead to significant interaction of excited charge carriers with the surface of the nanocrystal. These surface interactions occur on the femto- and picosecond time scale directly after excitation and dictate the ultimate fate of the excited state. Any charge carriers that become trapped at surface or internal defect states become localized and cannot radiatively recombine to emit a photon. Since the experiment monitors fluorescence from the nanocrystals, charge carrier trapping is realized as a decrease in fluorescence intensity as a function of time. Utilizing fluorescence upconversion spectroscopy, we are able to monitor the fluorescence intensity from the nanocrystals with <250 fs temporal resolution. Additionally, we can monitor specific wavelengths in the emission spectrum from the nanocrystals which allows us to preferentially probe the excited state at the band edge.
The Rosenthal lab conducted some of the first ultrafast dynamics studies on CdSe nanocrystals and helped to establish the foundation for the rich field of carrier dynamics in nanostructures. Some of our first work was in establishing a model based on the competition between recombination from band edge states and surface Se dangling-bond electrons that can act as states to localize charges. The decay pathway from trap states can be either radiative (resulting in deep trap emission) or non-radiative (resulting in quenching of the fluorescence) recombination. Both the band edge and trap state dynamics are directly dependent on the size of the nanocrystal, a direct result of quantum confinement. Since then, our lab has investigated the ultrafast carrier dynamics for a wide array of different nanocrystal structures and compositions including: ultrasmall <2 nm white light emitting CdSe nanocrystals, CdSe nanocrystals with varying organic ligands and surface chemistry, core/shell nanocrystals of different semiconductor materials, and both homogeneous and heterogenous (graded) alloy nanocrystals.
Keene, J. D.; McBride, J. R.; Orfield, N. J.; Rosenthal, S. J., Elimination of Hole-Surface Overlap in Graded CdSxSe1-x Nanocrystals Revealed By Ultrafast Fluorescence Upconversion Spectroscopy. ACS Nano 2014, 8 (10), 10665-10673.
Bowers, M. J., II; McBride, J. R.; Garrett, M. D.; Sammons, J. A.; Dukes, A. D., III; Schreuder, M. A.; Watt, T. L.; Lupini, A. R.; Pennycook, S. J.; Rosenthal, S. J., Structure and Ultrafast Dynamics of White-Light-Emitting CdSe Nanocrystals. J. Am. Chem. Soc. 2009, 131 (16), 5730-5731.
Garrett, M. D.; Bowers, M. J., II; McBride, J. R.; Orndorff, R. L.; Pennycook, S. J.; Rosenthal, S. J., Band Edge Dynamics in CdSe Nanocrystals Observed by Ultrafast Fluorescence Upconversion. J. Phys. Chem. C 2008, 112 (2), 436-442.
Garrett, M. D.; Dukes, A. D., III; McBride, J. R.; Smith, N. J.; Pennycook, S. J.; Rosenthal, S. J., Band Edge Recombination in CdSe, CdS and CdSxSe1-x Alloy Nanocrystals Observed by Ultrafast Fluorescence Upconversion: The Effect of Surface Trap States. J. Phys. Chem. C 2008, 112 (33), 12736-12746.
Kippeny, T. C.; Bowers, M. J., II; Dukes, A. D., III; McBride, J. R.; Orndorff, R. L.; Garrett, M. D.; Rosenthal, S. J., Effects of surface passivation on the exciton dynamics of CdSe nanocrystals as observed by ultrafast fluorescence upconversion spectroscopy. J. Chem. Phys. 2008, 128 (8), 084713/1-084713/7.
Underwood, D. F.; Kippeny, T.; Rosenthal, S. J., Ultrafast Carrier Dynamics in CdSe Nanocrystals Determined by Femtosecond Fluorescence Upconversion Spectroscopy. J. Phys. Chem. B 2001, 105 (2), 436-443.