William E. Buhro

Oleylamine-ligated wurtzite CdSe quantum platelets (QPs) and quantum belts (QBs) are exchanged with carboxylate-terminated, methyl-viologen-containing MV2+(CH2)nCO2– ligands (where n = 2, 4, or 6), resulting in the progressive quenching of the QP and QB photoluminescence (PL). Spectroscopic and compositional analyses indicate that the initial ligand exchange responsible for PL quenching occurs on the nonpolar, thin, long-edge nanocrystal facets. The X-type MV2+(CH2)nCO2– ligands are shown to be more-effective PL quenchers than the corresponding L-type MV2+(CH2)nNH2 ligands studied previously [ Meyer, H. M.; et al.J. Phys. Chem. C2023, 127, 18506–18517.]. Transient-absorption spectroscopy measurements reveal electron transfer to the ligands occurs for fully quenched QPs, but not for the fully quenched QBs, establishing that electron transfer occurs selectively at the polar, thin, short-edge facets of the nanocrystals. The electron transfer occurs in <50 fs for the X-type ligation and within 100 fs for the L-type ligation. This difference is ascribed to the greater preference for the binding of X-type over L-type ligands on polar facets, and this results in a significant contribution of electron transfer to the overall PL quenching.

Wurtzite CdSe quantum belts with L-type n-octylamine, L-type ammonia, or Z-type Cd(oleate)₂ ligands are exchanged for several metal-dithiocarbamate ligands [M(S₂CNR₁R₂)₂]: Cd(S₂CNPhMe)₂, Cd(S₂CNEt₂)₂, Zn(S₂CNPhMe)₂, and Zn(S₂CNEt₂)₂. Successful ligand exchange with all M(S₂CNR₁R₂)₂ compounds occurs from {CdSe[Cd(oleate)₂]_0.19} quantum belts (QBs), which induce similar spectral shifts in the absorption spectra of the ligand-exchanged QBs. Spectroscopic data, experimentally determined lattice strains, and ligand exchanges with [Na][Et₂NCS₂] and [NH₄][MePhNCS₂] establish that the [M(S₂CNR₁R₂)₂] ligands bind as bound-ion-paired X-type ligands with (S₂CNR₁R₂)⁻ groups ligated directly to the QB surfaces and [M(S₂CNR₁R₂)]⁺ groups serving as the charge-balancing ion-paired countercations. The X-type dithiocarbamate ligands do not impart any special electronic effects to the CdSe QBs.

The state-to-state intraband relaxation dynamics of charge carriers photogenerated within CdTe quantum wires (QWs) are characterized via transient absorption spectroscopy. Overlapping signals from the energetic-shifting of the quantum-confinement features and the occupancy of carriers in the states associated with these features are separated using the quantum-state renormalization model. Holes generated with an excitation energy of 2.75 eV reach the band edge within the instrument response of the measurement, ~200 fs. This extremely short relaxation time is consistent with the low photoluminescence quantum yield of the QWs, ~0.2%, and the presence of alternative relaxation pathways for the holes. The electrons relax through the different energetically available quantum-confinement states, likely via phonon coupling, with an overall rate of ~0.6 eV ps^–1.