NEWS

The Externship Program is a research exchange program that allows New Mexico graduate students (with an existing assistantship) to spend a semester or summer doing research at a partnering New Mexico university or research facility. This report is from NMSU student Tom Nakotte about his time as an extern at Los Alamos National Labs.

I have recently started working with colloidal quantum dots (QD) as the focus of my PhD research. With the help of the NM EPSCoR externship program I was fortunate enough to work with a group in Los Alamos National Laboratory, headed by Jeff Pietryga and Victor Klimov, two well-known figures in the field. The group is made up of two separate teams; the chemistry team as well as the physics (or spectroscopy) team, these two teams work together to synthesize QD’s, make devices (such as FET’s and solar cells), and study their physical, electrical and optical properties.

I joined the group the as a student chemist and was taught how to synthesize quantum dots by one of the post-docs in the group, almost immediately after I completed the required trainings to begin work in the chemistry lab. The first project that I was tasked to work on was the radiation detection project, in which the group was attempting to make a detector for special nuclear materials using lead selenide (PbSe) QDs. For this project I was asked to make PbSe QDs about 6 nm in diameter and perform a ligand exchange to a much smaller charged ligand, this ligand exchange process would allow the quantum dots to pack much closer together when they are made into a film. The closer packing of QDs within the film would lead to higher conductivity because it would be easier for the electrons or holes to “jump” from dot to dot. After preparing the quantum dots the next task was to prepare a thick film (on the order of microns). The film processing was done in two ways; spin coating and Dr. Blading, the doctor blading method produced thicker films but the spin coating method was more advantageous for our purposes because there was less possibility of defects (such as cracks or chips) in the film when spin coating was used.

Another project that I am still currently working on, is a new structure, which is a PbTe/CdTe/CdSe (core/shell/shell) quantum dot. This structure looks like it could be a promising candidate for carrier multiplication (creating multiple excitons from one photon) which is a major factor for creating high efficiency quantum dot solar cells. The project is still in the works, however we have found a good low temperature method for performing cation exchange of Cd for Pb which has allowed us to make PbTe/CdTe (core/shell) dots with good reproducibility and size distribution. The next step is to grow a CdSe layer onto these dots, we are still exploring options but one of the more promising techniques would be a layer by layer growth method because the PbTe cores cannot handle the high temperatures typically needed for CdSe shell growth.

Overall this NM EPSCoR externship has been a valuable experience for me, not only in learning the fundamentals of the field but also seeing how a research group operates and understanding what is expected of each individual. The group works together well and everyone was nice and willing to teach new things so I learned a lot about the field of quantum dots as well as proper use of instruments and how research group works.