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

Abstract

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.

Transport in a Single Self-Doped Nanocrystal

“…6 However, obtaining reliable and stable tunnel junctions with single QDots remains a difficult challenge. 7 So far, most of the efforts regarding tunnel spectroscopy of colloidal QDots have been focused on wide band gap materials, [8][9][10] where the introduction of quantum confinement marginally affects the electronic structure. On the other hand, in narrow band gap semiconductors or semimetals, the quantum confinement leads to a drastic modification of the electronic spectrum.…”

Real Space Observation of Electronic Coupling between Self-Assembled Quantum Dots

“…Rectangular pieces of 3 mm x 8 mm were cut from a thinned p-type GaAs((001) substrate after MBE growth of the In(Ga)As/GaAs structure and marked by a notch to guide the in situ cleavage performed under UHV and perpendicular to the long [1][2][3][4][5][6][7][8][9][10] axis. Cleaved samples exposing a cross section (110) surface through the QDs structure were transferred within 5 min after cleavage in the precooled low temperature STM head.…”

“…Self-assembled In(Ga)As/GaAs QDs grown by molecular beam epitaxy have appeared as interesting objects to implement these blocks. 1,[3][4][5][6] Indeed, stacks of vertically coupled QDs can be grown thanks to the strain field which drives the growth of the upper dot on the top of the lower QDs. 7 The electronic coupling has already been demonstrated by optical means and by several teams 4,[8][9][10] in case of double dots.…”

Quantum state readout of individual quantum dots by electrostatic force detection

“…The described technique, termed single-electron electrostatic force microscopy (e-EFM), was first demonstrated on QDs formed in carbon nanotube SETs [20,21,22]. As the present technique requires no patterned electrode to be defined around the QDs, it can open up the spectroscopy on individual QDs of various kinds that have been very difficult to investigate, such as colloidal nanoparticle dots and those having complex structures [7,9,23].…”