Massimo Perucca, PhD - Diad Group: Surface assembly and positioning of individual quantum dots with sub-100nm accuracy
Many groups have explored theoretical systems in which a quantum dot is coupled to a nanowire or nanocavity. However, placing an emission source within the influence of such structures requires accuracy approaching molecular-length scales. The capability to position individual quantum dots near metallic nanostructures is highly desirable for constructing active optical devices that can manipulate light at the single photon level. Researchers Nitipat Pholchai, Volker Sorger, Dr Tae-Jin Yim, Dr Rupert Oulton and Prof. Xiang Zhang at the University of California Berkeley, US and Rebecca K Kramer, now at Harvard University in Cambridge, MA, US, have recently shown that by changing the chemistry of a surface, a high level of control over the position of individual quantum dots is achievable. The team used a chemical fabrication process to place quantum dots in predetermined patterns on metallic substrates. The predetermined patterns consisted of squares of different sizes, the smallest of which was 100 nm2, to test the feasibility of positioning quantum dots in increasingly small areas. Patterns of functionalized quantum dots were defined by electron-beam lithography, followed by a series of surface chemical functionalization processes that allowed for the programmed assembly of DNA-linked colloidal quantum dots. More explicitly, streptavidin-coated quantum dots were bonded to DNA strands with a biotin group at one end, exploiting the streptavidin–biotin protein interaction. The opposite end of DNA strands consisted of a thiol group that bonded to either gold or silver after surface preparation. The end result demonstrates the ability to reliably position quantum dots with sub-100 nm accuracy, which is several times smaller than the diffraction limit of a quantum dot's emission light.