Scanning tunneling spectroscopy of individual PbSe quantum dots and molecular aggregates stabilized in an inert nanocrystal matrix

The electronic local density of states (LDOS) of single PbSe quantum dots (QDs) and QD molecules is explored using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Both individual PbSe QDs and molecular aggregates of PbSe QDs (dimers, trimers, etc.) are mechanically stabilized in a twodimensional superlattice of wide band gap CdSe QDs acting as an inert matrix. The LDOS measured at individual QDs dispersed in the matrix is identical to that of single isolated QDs chemically linked to a substrate. We investigate the degree of quantum mechanical coupling between the PbSe QDs in molecular aggregates by comparing the LDOS measured at each site in the aggregates to that of an individual PbSe QD. We observe a variable broadening of the resonances indicating a spatially dependent degree of electron delocalization in the molecular aggregates

Scanning tunneling spectroscopy of individual PbSe quantum dots and molecular aggregates stabilized in an inert nanocrystal matrix

The electronic local density of states (LDOS) of single PbSe quantum dots (QDs) and QD molecules is explored using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Both individual PbSe QDs and molecular aggregates of PbSe QDs (dimers, trimers, etc.) are mechanically stabilized in a twodimensional superlattice of wide band gap CdSe QDs acting as an inert matrix. The LDOS measured at individual QDs dispersed in the matrix is identical to that of single isolated QDs chemically linked to a substrate. We investigate the degree of quantum mechanical coupling between the PbSe QDs in molecular aggregates by comparing the LDOS measured at each site in the aggregates to that of an individual PbSe QD. We observe a variable broadening of the resonances indicating a spatially dependent degree of electron delocalization in the molecular aggregates

Tunneling spectroscopy of semiconductor nanocrystals in superlattices

Colloidal semiconductor nanocrystals (NCs) are quantum-size-effect tunable and processible from organic or aqueous solution onto rigid or flexible substrates, thus making them quite appealing for the fabrication of low-cost electronic devices. While these devices are expected to consist of NC solids, with the expectation that the properties of such solids will be much improved over the combined properties of the individual NCs, key questions exist regarding the conduction properties of such thin films. Scanning tunnelling microscopy is an ideal tool to characterize individual NCs. Generally the NCs have to be attached to a conducting substrate to get stable substrate/NC/tip junction, but here we will show that NCs are better stabilized in the tunnelling junction between a tip and a conductive substrate, when they self-assemble to form a monolayer of nanocrystals. By using scanning tunnelling spectroscopy at low temperatures, we will investigate the transport through individual nanocrystals in the monolayer and show that tunnelling currents with higher intensities are driven through the NCs in a monolayer with respect to the tunnelling currents used on isolated NCs. From theoretical calculations, we will discuss the origin of the linewidth for the resonances observed in the differential conductance spectra. Depending on the chemical nature of the NCs, different mechanisms, including a strong electron-phonon coupling, a large intervalley coupling or a variable degree of electronic coupling between the NCs, are found to be involved in the broad resonances with widths in the range of tens of meV

Can scanning tunnelling spectroscopy measure the density of states of semiconductor quantum dots?

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Amine-terminated silicon nanoparticles: synthesis, optical properties and their use in bioimaging

Very stable and bright emitting amine-terminated Si nanoparticles (NPs) with different alkyl chain lengths between the Si core and amine end-group are synthesized. The obtained NPs have a spherical shape and homogeneous size distribution (1.57 ± 0.24 nm). Their emission can be tuned from the UV to the blue spectral region, in a controllable fashion, by only changing the alkyl spacer length. The emission quantum yields are 12% for all synthesized Si NPs. Excited state lifetimes are in the ns range and point to a direct band gap excitation. NH2-terminated Si NPs exhibit an exceptional stability over a wide pH range (1–13) and high temperatures (120 °C). The diffusion coefficient of prepared Si NPs is determined by fluorescence correlation spectroscopy (FCS) to be 3.3 × 10-10 m2 s-1. The derived size of Si NPs from mobility corresponds to 1.4 nm which is in a good agreement with the size obtained by transmission electron microscopy (TEM). Prepared Si NPs are shown to be highly suitable for bioimaging studies as they are readily taken up by BV2 cells. Si NPs are located in the cells cytosol. Proliferation of stained BV2 cells is observed and showed that newly formed cells are also stained with Si NPs, indicating their minimal toxicity. By using Si NPs it is possible to stain multiple cell generations by only staining the mother cells

Binary superlattices of PbSe and CdSe nanocrystals

In this paper we show that self-organization of colloidal PbSe and CdSe semiconductor nanocrystals with a size ratio of 0.57 leads to binary structures with a AB2 or a cuboctahedral AB13 lattice. The type of superlattice formed can be regulated by the relative concentration of both nanocrystals in the suspension