Electrical Transport Through a Single Nanoscale SemiconductorBranch Point

Semiconductor tetrapods are three dimensional branched nanostructures, representing a new class of materials for electrical conduction. We employ the single electron transistor approach to investigate how charge carriers migrate through single nanoscale branch points of tetrapods. We find that carriers can delocalize across the branches or localize and hop between arms depending on their coupling strength. In addition, we demonstrate a new single-electron transistor operation scheme enabled by the multiple branched arms of a tetrapod: one arm can be used as a sensitive arm-gate to control the electrical transport through the whole system. Electrical transport through nanocrystals, molecules, nanowires and nanotubes display novel quantum phenomena. These can be studied using the single electron transistor approach to successively change the charge state by one, to reveal charging energies, electronic level spacings, and coupling between electronic, vibrational, and spin degrees of freedom. The advent of colloidal synthesis methods that produce branched nanostructures provides a new class of material which can act as conduits for electrical transport in hybrid organic-inorganic electrical devices such as light emitting diodes and solar cells. Already, the incorporation of branched nanostructures has yielded significant improvements in nanorod/polymer solar cells, where the specific pathways for charge migration can have a significant impact on device performance. Progress in this area requires an understanding of how electrons and holes migrate through individual branch points, for instance do charges delocalize across the branches or do they localize and hop between arms. Here we employ the single electron transistor approach to investigate the simplest three dimensional branched nanostructure, the semiconductor tetrapod, which consists of a pyramidal shaped zinc blende-structured ''core'' with four wurzite-structured arms projecting out at the tetrahedral angle. Monodisperse CdTe tetrapods with arms 8 nm in diameter and 150 nm in length were synthesized as previously reported. The tetrapods dispersed in toluene were deposited onto {approx}10 nm thick Si{sub 3}N{sub 4} dielectrics with alignment markers and a back gate (see Supporting Information). A tetrapod spontaneously orients with one arm pointing perpendicularly away from the substrate and three arms projecting down towards the surface. Individual 60 nm-thick Pd electrodes were placed by EBL onto each of the three arms downwards so that there are four terminals (three arms and a back gate) as shown schematically in Fig. 1 top inset. Figure 1 bottom inset shows a typical scanning electron micrograph (SEM) of the devices. The center brighter spot is due to the fourth arm pointing up away from the substrate although its controlled breaking is possible. The separation between the metal electrodes and the tetrapod branch point ranges from 30 to 80 nm in our devices. The devices were loaded into a He{sup 4}-flow cryostat for low-temperature ({approx}5K) electrical measurements