UC Berkeley

Strategies for the device integration and electrical characterization of individual colloidal semiconductor nanocrystals are presented. Results emphasize the insight gained by studying the electronic structure of individual nanocrystals as opposed to measurements of ensembles of particles. An analysis of a variety device geometries, material systems, and nanocrystal morphology and functionality shows that the method of electrical contact has a dominating role in the electrical behavior of the samples. Further, interactions with the electrode contact reflect the unique electronic and surface structure of the individual nanocrystals.

In studies utilizing nanoscale lithography to directly deposit metal electrodes onto nanocrystals under vacuum, samples behave as single electron transistors (SET). Devices made from CdTe nanorods contacted by Pd display strong electron-electron correlations, which limit the flow of current to one electron at a time across the nanocrystal. Measurements also indicate that chemical reactions induced by the electrode metal cause diffusion of interface species and compositional modification of the nanoparticle. Interface chemical reactions may completely transform the nanocrystal under study, also altering the nanocrystal electronic structure.

To avoid these complications, alternative strategies for device fabrication take advantage of the self-assembly of heterostructure nanoparticles. Synthetic methods for the direct solution-phase growth of Au electrodes on CdSe nanorod tips provide a 100,000-fold increase in the conductivity of single particles. Device response indicates ensemble electron physics and a Schottky barrier at the electrode contact, allowing quantitative determination of interface electronic structure.

The methods of self-assembly are extended to a variety of heterostructure nanoparticles optimized for electronic and optoelectronic functionality. This work demonstrates the increasing sophistication of high-quality electrical devices achievable via self-assembly and verifies the process as an excellent route to the next generation of electronic and optoelectronic devices utilizing colloidal semiconductor nanocrystals.