Exactly Doped Quantum Dots

Poisson Statistics

When preparing a doped QD material, guest ions are generally added in during the preparation and are incorporated (sometimes) into the growing inorganic lattice. However, the number of dopants per QD cannot be controlled with any finesse- while one can add a certain number of dopants into a known number of QDs, the dopant levels will follow Poissonian statistics. 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 over 5 years. Fortunately, in that same year I went to an industry conference where a speaker from Nanoco, Inc. (Manchester, GB) presented a method developed by the company’s founder (Nigel Pickett) to produce large quantities of QDs in a single batch- a procedure they termed the cluster-seed method.

Cluster-Seeding QDs

Although the company never published the cluster-seed method in the peer-reviewed literature, Nanoco has stipulated that clusters of Cd10Se4(SPh)16 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). We verified the efficacy of the cluster seed method recently (Jawaid et al. ACS Nano asap). Basically, we synthesized CdSe nanocrystals in the presence of an increasing number of clusters and then determined the number of QDs synthesized; it was found that the number of clusters and nanocrystals synthesized are linearly correlated as show here. Additionally, 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. Here is where the story gets interesting- my first thought back in 2007 was whether 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

image004Our group has worked since 2008 to determine whether the cluster seed method could be altered to synthesize exactly-doped quantum dots. A breakthrough occurred in 2010 with the lead author, Dr. Ali Jawaid, was able to synthesize a copper4 cluster, [Cu4(SPh)6], as verified with X-ray crystallography by our collaborator, Prof. Donald J. Wink of UIC. This novel compound is 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 as shown here on the right. 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 with our collaborator Dr. Soma Chattopadhyay in the summer of 2012. Dr. Chattopadhyay’s work demonstrated that copper dopants are in the +1 state in the CdSe QD matrix, and that they become oxidized and translocate within the QD matrix after exposure to UV irradiation. It was also found that studying Poissonian-doped nanocrystals did not reveal the same level of chemical dynamics; as such, the use of exactly-doped QDs is necessary to fully understand dopant photophysics.

Last, we found that the dopants were located within the core of the quantum dots. Etching the surface did not change the doped QD’s photophysical properties, and we found that the coupling of the nanocrystal’s electronic structure with dopants was stronger than that observed with Poissonian-doped QDs that were loaded with significantly higher levels of 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.

Future Studies

There exist an infinite number of doped semiconductor QD systems that can be created using the cluster-seed method; we may be able to produce and study QDs that are totally non-toxic and can be used in cancer diagnostics as well as efficiently produce hydrogen from sunlight. We have also shown that the examination of exactly-doped nanomaterials is necessary to fundamentally understand their basic photophysical behavior. In the long term, we hope that we can couple the development of exactly-doped QDs with our other research endeavors, such as the creation of cancer diagnostic fluorophores and semiconductor catalysts that are efficient producers of hydrogen for renewable energy generation.