Image (a) is a TEM micrograph of a cadmium-selenide nanocrystal before gold tip growth in solution and image; (b) is after tips have been added. Image (c) is a SEM micrograph of a single nanocrystal two-terminal device. (Credit: LBL)
Researchers at Berkeley Lab reported a technique by which the electrical conductivity of nanorod crystals of cadmium-selenide was increased 100,000 times.
“The key to our success is the fabrication of gold electrical contacts on the ends of cadmium-selenide rods via direct solution phase-growth of the gold tips,” says Paul Alivisatos, interim-director of Berkeley Lab, who led this research. “Solution-grown contacts provide an intimate, abrupt nanocrystal-metal contact free of surfactant, which means that unlike previous techniques for adding metal contacts, ours preserves the intrinsic semiconductor character of the starting nanocrystal.”
Demand for energy is expected to triple by the end of the century, and nanocrystals could be a sustainable and carbon-neutral means to meet global demands, particularly in solar electricity and solar fuel technologies.
“Standard contacting procedures that deposit metal onto semiconductor nanocrystals directly, such as those used in commercial wafer-scale chip fabrication, cause alloying and chemical reactions at the metal-semiconductor interface,” says Matthew Sheldon, lead author of a paper published in NanoLetters. “This means that the finished electrical device is actually made of a different material than the starting nanocrystal.”
Matthew Sheldon, a member of the Paul Alivisatos research group, was part of a BNL team that developed a technique to boost the electrical conductivity of nanorod crystals of the semiconductor cadmium-selenide by a factor of 105. (Credit: Roy Kaltschmidt, BNL)
In this new study, Sheldon, Alivisatos and their co-authors used single nanostructure electrical measurements to make comparisons between cadmium-selenide nanorods with and without gold tips. The solution-grown tipping process started with the addition of gold salt to a solution of toluene and cadmium-selenide nanorods, which resulted in gold metal being selectively deposited on the nanorod tips. A silicon wafer test chip was then dipped in this nanorod solution. After submersion, the evaporation of the toulene solvent oriented individual cadmium-selenide nanorods across pre-defined gold electrodes, which were fabricated through electron beam lithography. The results were gold-tipped cadmium-selenide heterostructure devices whose electrical conductance was characterized in a two-terminal geometry as a function of source-drain voltage and temperature.
Says Alivisatos, “Our work demonstrates the increasing sophistication of high-quality electrical devices that can be achieved through self-assembly and verifies this process as an excellent route to the next generation of electronic and optoelectronic devices utilizing colloidal nanocrystals.”
Adds Sheldon, “We believe our approach is an ideal strategy for making future devices from nanocrystals because it preserves the semiconductor character of the nanocrystal as synthesized with the precise control of their synthesis developed over the past decades.”