Artificial atoms on semiconductor surfaces

Abstract

Semiconductor nanocrystals are called artificial atoms because of their atom-like discrete electronic structure resulting from quantum confinement. Artificial atoms can also be assembled into artificial molecules or solids, thus, extending the toolbox for material design. We address the interaction of artificial atoms with bulk semiconductor surfaces. These interfaces are model systems for understanding the coupling between localized and delocalized electronic structures. In many perceived applications, such as nanoelectronics, optoelectronics, and solar energy conversion, interfacing semiconductor nanocrystals to bulk materials is a key ingredient. Here, we apply the well established theories of chemisorption and interfacial electron transfer as conceptual frameworks for understanding the adsorption of semiconductor nanocrystals on surfaces, paying particular attention to instances when the nonadiabatic Marcus picture breaks down. We illustrate these issues using recent examples from our laboratory.

Fig. 1.

Scanning electron microscope images of submonolayers of PbSe QDs (diameter = 5.5 ± 0.4 nm) adsorbates on a Si surface. The QDs are capped with (A) oleic acid, or (B) ethanedithiol. Image courtesy of A. Wolcott.

Fig. 2.

Radial probability density distributions ρ(r) = 4πr²|ψ|² for (A) the two lowest-energy S (ℓ = 0) eigenstates in an idealized 3 nm radius spherical CdSe nanocrystal and (B) the lowest-energy eigenstates in 3 nm radius CdSe and PbSe nanocrystals, calculated according to the method described by Brus (17). The discontinuity in the first derivative of the probability density at the nanocrystal surface arises from different effective masses of the electron inside and outside the nanocrystal core. The wave functions of higher-energy eigenstates extend farther beyond the nanocrystal surface, facilitating strong electronic coupling to neighboring nanocrystals or bulk semiconductor substrates.

Fig. 3.

Optical absorption spectra for one monolayer of 5.4 nm diameter PbSe QDs adsorbed on the native oxide terminated silicon surface. The QDs are capped with the long oleic acid molecules (blue) or shorter EDT.

Fig. 4.

UPS of one monolayer PbSe QDs (3.4 nm diameter) assembled on the TiO₂(110) surface. The oleic acid capping molecules have been removed by reaction with hydrazine (red) or replaced with EDT (blue). Also shown is a spectrum for clean TiO₂(110). Energy scale is referenced to the substrate Fermi level. Experimental details can be found in refs.  and .

Fig. 5.

Time-resolved SHG (dots) of the TiO₂ surface coated with 1.5 monolayers of EDT-treated 6.7 nm PbSe nanocrystals probed with p-polarized light with the electric field of the optical wave in the plane of incidence. The sample temperature was 12 K. Both pump and probe were 50 fs pulses of 810 nm light. The intensity of reflected second harmonic light at 405 nm was recorded as a function of time delay between the pump and probe pulses. The blue curve shows a least-squares fit incorporating electron injection and recombination (red) and three coherent phonon modes. Adapted from Tisdale et al., ref. .

Fig. 6.

Free energy as a function of configuration coordinate (Q) for electron transfer from one of the QD exited states (red) to a delocalized conduction band (blue). The dotted (crossing) curves are parabolic diabatic free energy surfaces in the weak-coupling limit. The solid anticrossing curves (purple) are adiabatic free energy surfaces of the coupled donor-acceptor system. Δ is electronic coupling strength and λ the reorganization energy.