Doped Quantum Dots

When preparing a doped QD material, guest ions are generally added in during the preparation and are incorporated into the growing inorganic lattice. However, the number of dopants per QD cannot be controlled. Simply put, some QDs won’t be doped, some will have 1 dopant, some 2, etc. As shown by Mocatta et al. (Science, 2011, 332, 77-81), QDs with different levels of dopants have different properties. Thus, we can prepare doped quantum dots, but each quantum dot is different from the others and our measurements on these materials are thus inexact. Realizing this back in 2007, our group has worked to develop a method to address this issue using the cluster seed method developed by Nanoco, Inc. (Manchester, GB).

Cluster-Seeding QDs

Nanoco developed a method to use clusters of Cd₁₀Se₄(SPh)₁₆ to seed the growth of CdSe QDs; essentially, the number of QDs produced in a batch process is the same as the number of clusters added (Pickett, N. U.S. Patent 7,867,556). Our group did some follow up characterization, and found that the use of a large number of clusters results in the formation of small QDs (due to competition of larger number of dots for less precursors) and that no quantum dots are synthesized if the clusters are absent. Our group then examined if clusters that do not contain cadmium, rather, dopant ions, could act as seeds. If so, then we could possibly beat Poisson statistics and make QDs with an exact number of dopant ions.

 Copper Cluster Seeds Create CdSe:Cu₄ Quantum Dots

A breakthrough with exact-doping using the cluster seed method occurred in 2010 when Dr. Ali Jawaid was able to synthesize a homogeneous copper₄ cluster, [Cu₄(SPh)₆], as verified with X-ray crystallography as shown on the left. Dr. Jawaid then synthesized CdSe nanocrystals in the presence of an increasing level of copper clusters and found that the number of QDs synthesized was linearly correlated to the number of clusters used in the synthesis. We also did a large number of other characterizations, such as elemental analysis, X-ray Photoelectron Spectroscopy, X-ray Near-Edge, and X-ray Extended Fine Structure Absorption Spectroscopy at Argonne National lab.

Since this discovery, our group has used exactly-doped QDs to interrogate the fundamental redox behavior of dopants in semiconductors using time-resolved EXAFS spectroscopy at Argonne National Lab with our collaborator Dr. Xiaoyi Zhang. We are now synthesizing new materials and investigating dopants other than copper. Overall, these results demonstrate that it is possible to dope quantum dots with an exact, stoichiometric number of dopants and that such control of the chemical structure is necessary to develop a fundamental understanding of dopant photophysics.

Core/Shell Reducing Dots

Our group recently reported in Nanoscale that core/shell quantum dots can be efficient reducing agents. This idea is antithetical to the general concept of core/shell particles having reduced efficacy as redox agents due to a reduction in wavefunction overlap with the substrate. However, several properties of a nanomaterial are altered via inorganic surface passivation, and these can offset the detrimental redox barriers created by a shell. For example, core/shell QDs have longer lifetimes afforded by mollification of surface trap states. More importantly, we found that using materials with a hole degeneracy allows for ion paring between the oxidized QD and reduced substrate. This lowers the energy of interaction which favors substrate reduction. The fact that core/shell QDs may be good reducing agents is of value due to their substantially enhanced stability, robustness, and resilience after processing into various devices or cap exchange for phase transfer.