57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

The main goal of our PRF grant is to explore and develop fluorescent probes of the local electrochemical potential.  The system we chose is semiconductor nanocrystals because of the demonstrated sensitivity of nanocrystals to injected charges, their robust fluorescence and the possibility of reversibly going over an indefinite number of redox cycles, which are altogether a unique combination of properties.  Following recent suggestions in the literature, we expect that nanocrystals with smaller electron kinetic energies, either because of reduced confinement or alloying, will slow down Auger processes and lengthen the Trion lifetimes, allowing to retain luminescence in charged states and the sensitivity of the PL to the presence of extra charges.

The grant supported Wei Qin, who is now starting his third year as a Physical Chemistry PhD student.  There are now two papers published in ACS Nano that result from his work and acknowledge the PRF grant.  The first one is “CdSeS/ZnS Alloyed Nanocrystal Lifetime and Blinking Studies under Electrochemical Control”.  In this paper, Wei synthesized alloyed CdSeS nanocrystals.  This allowed tuning between the CdSe and CdS band edges while retaining relatively small dots, of the order of 5 nm diameter. The PRF grant also allowed for the upgrade of the microscope system, in particular with a 405 nm pulsed diode laser and optimized filters and microscope objectives for improved confocal detection and lifetime measurements.  Wei was then able to raise the photocounts about an order of magnitude compared to our earlier work.  With ensemble intraband spectroscopy, we verified that the CdSeS cores of ~ 5 nm diameter lead to a reduced electron confinement energy and with time resolved bleach measurements we found that this correlated with a slower biexciton decay rate compared to similar sizes CdSe cores. Then, a ZnS shell synthesis procedure was developed to allow bright stable CdSeS/ZnS colloidal quantum dots (QDs) suitable for single dot imaging.  Remarkably, the conditions for shell growth on these alloyed nanocrystals had to be different than on the CdSe system.  These CdSeS/ZnS nanocrystals of total size of 6.5 nm were then compared to CdSe/ZnS of similar core size and we found that blinking behavior was essentially similar between the two systems.  However, using electrochemistry with ensemble and single dot measurements, we did find a significant lengthening of the Trion lifetime of CdSeS/ZnS dot,  resolved to be ~ 1 ns,  while it was about 0.3 ns for CdSe/ZnS.  Since we were able to monitor the PL of the charged dots, we were also able to confirm that the blinking behavior of single CdSeS/ZnS QDs was almost completely suppressed when in the Trion state.  In his second paper, Wei Qin studied the CdSe/CdS dots which had been previously introduced by others as systems exhibiting strongly suppressed blinking in the limit of very thick CdS shells.  The paper, “Evidence for the Role of Holes in the Blinking: Negative and Oxidized CdSe/CdS Dots” has just been published in ACS Nano.  We found that thin shell CdSe/CdS colloidal quantum dots, with a small 3 nm core diameter and a 1 nm shell for a total diameter of ~ 5 nm, exhibit typical blinking and a binary PL intensity distribution.  However, since the Trion has a long enough PL lifetime, it can be monitored and we found that electrochemical charging with one electron suppresses the blinking, as for the previous CdSeS/ZnS. With a larger core of 5 nm and a 1 nm shell thickness for a total dot diameter of ~7 nm, the blinking power law statistics of on and off states is similar to that of a smaller core but the dots also display a state of intermediate brightness, called the grey state.  We found that that state had a finite duration time (~6 ms) on glass and not a power law statistics. However, the grey state disappeared when the nanocrystals where placed on an electron-accepting ZnO nanocrystals film. In addition, the grey state PL lifetime on glass was found to be similar to the Trion lifetime measured from electrochemically charged dots. Therefore, the grey state was assigned to negative dots arising from a process of photocharging. It was then concluded that the grey state, or negative photocharged dot, may always be present as the dots get irradiated even though it might not be observed due to the brightness of the Trion and/or the stability of the negative charge. We also investigated thick shell CdSe/CdS dots of about 10 nm total diameter.  For these systems, the intraband spectroscopy indicated a small confinement energy, consistent with the reduced kinetic energy of the electron.  Under electrochemical control, the charged dots remain quite bright, and we were able to observe PL of single dot with multiple charging, up to 4 electrons per dot.  This was monitored as sequential changes in the photoluminescence lifetime which could be described by the Nernst equation. The small potential increment confirmed the weak electron confinement with the thick CdS shell. Finally, we proposed that for the CdSe based system, a mechanism of hole-trapping and surface oxidation by the hole could account for the grey state and off state in the blinking.

In the coming year, the photoluminescence of other systems, alloys and core shell, will be studied further.  On the fundamental aspects of this study, we would like to answer whether gradient core/shell or reduced kinetic energy can eliminate the quenching of Trions at small nanocrystal sizes.  We are also developing a system to control the electron transfer rate, aiming to bring it to a low enough rate that we can isolate, in time, the individual charged states of the nanocrystals.  The successful development of bright charged dots might then lead to local probes of electrochemical potential which will further impact the study of nanostructured materials, with potential applications for corrosion studies, electrocatalysis or fuel cells.