2, University of California, Irvine, Irvine, California, United States
Lead-Chalcogenide quantum dots (QDs) have attracted attention due to their tunable electrical and optical properties. In the QD solids, bulk-like electronic bands with bandwidth of 100~200 meV are expected to form which yield much higher carrier mobility and diffusion length compared to weakly-coupled QDs. However, the electronic properties (the local density of states, electron delocalization) of highly ordered QD arrays are not fully understood. In this work, the local density of state of a highly ordered monolayer PdSe superlattice was studied by low temperature scanning tunneling microscopy (STM).
A monolayer of PbSe QDs was prepared using the Langmuir Schaefer deposition technique. First, oleate-capped PbSe QDs dispersed in hexane were drop casted onto diethylene glycol surface. After the hexane was evaporated, a (111) in-plane oriented polycrystalline FCC superlattice was formed on the diethylene glycol surface. NH4SCN solution was applied onto the oleate-capped PbSe superlattice film. The injection of NH4SCN initiates the ligand exchange and phase transformation from an FCC to a simple cubic structure superlattice. A HF cleaned (001) Si substrate was stamped on the PbSe-SCN layer for superlattice film transfer to observe the cross section of the QDs by TEM. 200 cycle of Al2O3 ALD was employed at 75C to yield complete infilling of the QD layers. For STM measurements, HOPG (highly ordered pyrolytic graphite) substrates were cleaned by mechanical exfoliation using an adhesive tape. A monolayer QD superlattice was prepared on a HOPG substrate using same technique. Afterward, the HOPG sample was loaded into a commercial UHV scanning tunneling microscopy chamber (Omicron Technology) with a base pressure of 1x10-10 torr. The sample was annealed at 75C for 3hr to remove the hydrocarbon and ligands from the surface. The topography of the QDs was observed with a tungsten tip prepared by electrochemical etching of a tungsten wire. The STM images were acquired in constant current mode (I = 0.03 nA) with a tip bias of +2V.
STM imaging showed that the PbSe QD monolayer had 4 fold symmetry with an average inter QD spacing of 7nm. It is also found the height fluctuation of the QDs was 1nm indicating size variation of the QDs and imperfect crystal structure of the superlattice. Scanning tunneling spectroscopy (STS) was performed to investigate the electronic structure of the PdSe QDs using a constant z-mode with an external lock-in amplifier in the bias range of -2 to 2V. The single point STS showed the band gap measured by STS varied from 0.8 eV to 1.2 eV indicating dependence of the bandgap on the size of the QDs. The STS measurements also showed the local variation on the electronic structure of the QDs within a same QD which can be attributed to the defect site. This result provides important information in the design of extended array of QDs with controlled properties.