Scanning probe microscopy and spectroscopy of colloidal semiconductor nanocrystals and assembled structures

Colloidal semiconductor nanocrystals become increasingly important in materials science and technology, due to their optoelectronic properties that are tunable by size. The measurement and understanding of their energy levels is key to scientific and technological progress. Here we review how the confined electronic orbitals and related energy levels of individual semiconductor quantum dots have been measured by means of scanning tunneling microscopy and spectroscopy. These techniques were originally developed for flat conducting surfaces, but they have been adapted to investigate the atomic and electronic structure of semiconductor quantum dots. We compare the results obtained on colloidal quantum dots with those on comparable solid-state ones. We also compare the results obtained with scanning tunneling spectroscopy with those of optical spectroscopy. The first three sections provide an introduction to colloidal quantum dots, and a theoretical basis to be able to understand tunneling spectroscopy on dots attached to a conducting surface. In sections 4 and 5 , we review the work performed on lead-chalcogenide nanocrystals and on colloidal quantum dots and rods of II-VI compounds, respectively. In section 6 , we deal with colloidal III-V nanocrystals and compare the results with their self-assembled counter parts. In section 7 , we review the work on other types of semiconductor quantum dots, especially on Si and Ge nanocrystals

Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics

Quantised charging of monolayer-protected nanoparticles

Metal nanoparticles coated with an organic monolayer, so-called monolayer protected clusters (MPCs), can show quantised charging at room temperature due to their sub-attofarad capacitance arising from the core size and the nature of the protecting monolayer. In this tutorial review, we examine the factors affecting the energetics of MPC charging. In the first section, the underlying physics of quantised charging is outlined and we give an overview of the various methods that can be used to measure single electron transfer to nanoparticles. In the subsequent sections, we discuss how electrochemical measurements can be used to give information on the quantised charging of freely diffusing and films of immobilised MPCs. The predictions of models used to determine MPC capacitance are compared with experimental data from the literature

Ion limited charging of nanoparticle thin films

This article reports on the charging behavior of thin films of alkanethiol protected gold nanoparticles, socalled monolayer protected clusters (MPCs), immersed in aqueous solution. We demonstrate that the oxidation of the MPCs is controlled by the transfer of counter-ions across the aqueous/MPC film interface. A model is developed to describe this and its predictions compared with experiment. Langmuir-Schaefer MPC thin films were transferred to a glassy carbon electrode and film charging behavior investigated in a series of aqueous solutions comprised of different base electrolyte anions. The dependence of peak current on film thickness and peak position on anion lipophilicity for MPC oxidative charging can all be accounted for with the ion-transfer limited model. The impact of ion partitioning into the film at equilibrium is also discussed and the effect of the aqueous phase cation is theoretically considered. In addition, the effect of co-transported water with hydrophilic ions transferring into the film on film charging is rationalized

Synthesis and stability of monolayer-protected Au38 clusters

A synthesis strategy to obtain monodisperse hexanethiolate-protected Au38 clusters based on their resistance to etching upon exposure to a hyperexcess of thiol is reported. The reduction time in the standard Brust−Schiffrin two-phase synthesis was optimized such that Au38 were the only clusters that were fully passivated by the thiol monolayer which leaves larger particles vulnerable to etching by excess thiol. The isolated Au38 was characterized by mass spectrometry, thermogravimetric analysis, optical spectroscopy, and electrochemical techniques giving Au38(SC6)22 as the molecular formula for the cluster. These ultrasmall Au clusters behave analogously to molecules with a wide energy gap between occupied (HOMO) and unoccupied levels (LUMO) and undergo single-electron charging at room temperature in electrochemical experiments. Electrochemistry provides an elegant means to study the electronic structure and the chemical stability of the clusters at different charge states. We used cyclic voltammetry and scanning electrochemical microscopy to unequivocally demonstrate that Au38 can be reversibly oxidized to charge states z = +1 or +2; however, reduction to z = −1 leads to desorption of the protecting thiolate monolayer. Although this reductive desorption of thiol from the cluster surface is superficially analogous to electrochemical desorption of planar self-assembled monolayers (SAMs) from macroscopic electrodes, the molecular details of the process are likely to be complicated based on the current view that the thiolate monolayer in clusters is in fact composed of polymeric Au−S complexes