Temperature-Dependent Transient Absorption Spectroscopy Elucidates Trapped-Hole Dynamics in CdS and CdSe Nanorods

Abstract

Charge-carrier traps play a central role in the excited-state dynamics of semiconductor nanocrystals, but their influence is often difficult to measure directly. In CdS and CdSe nanorods of nonuniform width, spatially separated electrons and trapped holes display relaxation dynamics that follow a power-law function in time that is consistent with a recombination process limited by trapped-hole diffusion. However, power-law relaxation can originate from mechanisms other than diffusion. Here we report transient absorption spectroscopy measurements on CdS and CdSe nanorods recorded at temperatures ranging from 160 to 294 K. We find that the exponent of the power law is temperature-independent, which rules out several models based on stochastic activated processes and provides insights into the mechanism of diffusion-limited recombination in these structures. The data point to weak electronic coupling between trap states and suggest that surface-localized trapped holes couple strongly to phonons, leading to slow diffusion. Trap-to-trap hole hopping behaves classically near room temperature, while quantum aspects of phonon-assisted tunneling become observable at low temperatures.

Influence of Surface Ligands on Charge-Carrier Trapping and Relaxation in Water-Soluble CdSe@CdS Nanorods

“…Additionally, a broad, minute positive amplitude was recorded for probe photon energies lower than 2.1 eV (600-750 nm, Figure S23), which builds up within the first ps. Previous studies attributed this spectral feature to the dissociation of excitons into electrons and holes with subsequent trapping of holes on the surface [46,68,69]. The subsequent behavior is characterized by the decay of the CdS bleach feature, accompanied by a small red-shift of the spectral position of the minimum of circa 50 meV, and the decay of the CdSe bleach feature, accompanied by a small blue-shift of the minimum of circa 30 meV.…”

“…For the range 530-630 nm (indicating changes in excited CdSe), two time constants and an offset were needed for DHLA-and DHLA-PEG-NR, and three and an offset for TOPO-NR, MUA-NR, and HS-PEG-NR, and PEI-NR. Parallel to the initial CdS and CdSe bleach formation processes formation of a broad positive amplitude for probe photon energies lower than 2.1 eV ( Figure S23), indicative of surface trapping of holes [46,68,69], is observed. This process exhibits a minor ligand-dependence: for all thiol-based ligands, the risetime is in the order of 0.3 ps and below, while TOPO-and PEI-NR exhibit slightly higher risetimes (0.8 and 0.5 ps, respectively).…”

“…Corresponding DAS are spectrally very similar with negative amplitudes at 2.71 and 2.70 eV for all NR except PEI-NR, for which these features are blue-shifted by circa 30 meV in agreement with the positions in the ground state absorption spectra. These decays describe intrinsic decay processes such as recombination processes between holes and electrons involving holes trapped on the surface or the CdS rod [46,68,74,75]. Further, processes involving trapping of electrons at the surface opening additional decay pathways can contribute to this signal decay, if respective trap sites are available in the structures [76].…”

The Motion of Trapped Holes on Nanocrystal Surfaces

“…We found that trapped holes on the surfaces of CdS nanorods are not stationary on the time scale of electron–hole recombination but instead undergo a diffusive random walk on the particle surface . In this Perspective, we summarize our experimental and theoretical efforts to understand the motion of surface-trapped holes in CdS and CdSe nanocrystalline materials. − We first describe how transient absorption (TA) spectroscopy experiments and theoretical modeling of NCs with a unique morphology led to the observation of trapped-hole diffusion. We then summarize insights into the mechanism of hole hopping from theory, computations, and TA experiments in multiple materials recorded over a range of temperatures and time scales.…”

Sub-Bandgap Optical Modulation of Quantum Dot Blinking Statistics

“…Our dominant observation is that the modulation beam affects the slope of the ON probability distribution, leaving the OFF-state behavior unaltered. We considered the possibility that this effect arose from additional excitations into the absorption tail, either phonon-assisted band-to-band transitions or to surface-directed shallow defect states that may interconvert with the relaxed exciton. − However, a slope change has not been reported for intensity-dependent studies with conventional supra-gap excitation, , which disfavors a straightforward effect from additional ground-state photoexcitations. Moreover, we expect that this additional flux of excitations from the modulation laser is small relative to the excitation rate from the λ: 470 nm laser.…”