To tailor the performance to suit particular applications, the electron transport into the QDs and the conduction mechanism in the QD-assembled films should be explored. Two-probe electrical characterization of the electron-beam lithographically fabricated QD device is one feasible approach. Scanning tunnelling microscopy (STM) of QDs dispersed on metallic surfaces offers another handy probe.

In a recent study on semiconductor QDs, researchers in Taiwan have adopted STM to look into single-electron charging and interdot coupling effects. The conditions for the self-assembly of colloidal QD islands are controlled to form QD arrays of different sizes, widths and lengths. The size dependence on the tunnelling spectra between the STM tip, the array and the metal substrate can then be unearthed.

The tunnelling spectrum through a single QD demonstrates single-electron tunnelling and agrees well with classical single-electron charging, orthodox theory. The team has noticed, however, that the tunnelling spectra through a QD array cannot be described by the classical theory. Neither a single, large QD nor a number of individual QDs provide an adequate fit to the data.

Instead, the tunnelling spectra through a QD array, displaying a threshold voltage and a power law dependence on I-V with a scaling exponent, can be described by collective transport (collective Coulomb blockade) theory. The group from National Chiao Tung University has observed that as the array size increases and the interdot coupling engages, the threshold voltage decreases and the scaling component increases to reveal more open channels for electron tunnelling between the STM tip and the metal substrate. The variation will finally come close to a saturation value, implying a finite screening length in the QD arrays.

To investigate the memory effect, the researchers plan to vary the separation distance between the QDs. Tuning the environmental conditions for the QD assembly and the post thermal treatment, allows the team to prepare samples having different QD separation distances. Additional tunnelling spectra measurements will then be performed to study the conduction mechanism and electrical bistability effect in more detail to create real applications.

The researchers presented their work in Nanotechnology.