Amine-ligated {CdSe­[n-octylamine]0.53±0.06} quantum belts undergo instantaneous, reversible exchange of the L-type amine ligation for Z-type ligation provided by the neutral metal halides MX2 (M = Cd, Zn; X = Cl, Br, I). The CdX2-ligated quantum belts are determined to have the compositions {CdSe­[CdCl2]1.23}, {CdSe­[CdBr2]1.03}, and {CdSe­[CdI2]0.42}, corresponding to two, two, and one CdX2 monolayer shells, respectively. Exchange from L-type to Z-type MX2 ligation results in large shifts of the quantum-belt absorption (extinction) features to lower energy, which are 252, 151, and 157 meV for CdCl2, CdBr2, and CdI2 ligation, respectively. These spectral shifts, which are due to changes in the strain states of the quantum belts (QBs), and extension of the CdSe crystal lattice by surface coordination of the ligand Cd atoms, are rapidly and completely reversed by back exchange to n-octylamine ligation. Similar, reversible spectral shifts are observed upon L-type to Z-type exchange with ZnX2 ligation, although the shifts are smaller in magnitude. The results demonstrate the facility, reversibility, and generality of L-type for Z-type surface exchanges in wurtzite CdSe quantum belts, and that neutral metal halides may function as Z-type ligands in semiconductor nanocrystals.

The excitation energy dependence (EED) of the photoluminescence quantum yield (Φ PL ) of semiconductor nanoparticles with varying dimensionalities is reported. Specifically, the EEDs of CdSe quantum dots, CdSe quantum platelets, CdSe quantum belts, and CdTe quantum wires were determined via measurements of individual Φ PL values and photoluminescence efficiency (PL Eff (E)) spectra. There is a general trend of overall decreasing efficiency for radiative recombination with increasing excitation energy. In addition, there are often local minima in the PL Eff (E) spectra that are most often at energies between quantumconfinement transitions. The average PL lifetimes of the samples do not depend on the excitation energy, suggesting that the EED of Φ PL arises from charge carrier trapping that competes efficiently with intraband carrier relaxation to the band edge. The local minima in the PL Eff (E) spectra are attributed to excitation into optically coupled states that results in the loss of carriers in the semiconductor. The EED data suggest that the PL Eff (E) spectra depend on the sample synthesis, preparation, surface passivation, and environment.

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.

We report on the time-dependent, energetic shifting of quantum-confinement states within photoexcited CdTe quantum wires that were identified by using transient absorption (TA) spectroscopy. The initial photoexcitation promotes an electron to states in the conduction band (CB) and generates a hole in the valence band (VB), and the changes in carrier densities give rise to quantum-state renormalization (QSR). This mechanism is akin to the generation of electron and hole polarons that alter the effective masses of the states in the CB and VB that are probed in the TA experiments. Since the energies of the quantum-confinement states are inversely proportional to the effective mass of the carrier, the states undergo QSR that evolves as the carriers relax to the band edge. A simple model is presented which separates the bleach and induced-absorption signals associated with QSR in the TA data from additional bleach signals attributed to the occupancy of carriers in the shifted states. The states are observed to shift independently with time as the carriers relax. The lowest-energy occupancy feature for the CdTe quantum wires shifts from −25 to +3 meV within 600 fs and then to −17 meV on longer time scales after excitation. The Stokes shift of the photoluminescence of the CdTe quantum wires is dominated by the long-time scale QSR of the band-edge states. QSR is likely prominent in other direct-gap semiconductor nanoparticles, especially those with a significant Stokes shift of the photoluminescence from the absorption band-gap transitions.

Time-resolved photoluminescence (PL) intensity decay profiles were recorded for room-temperature ensemble samples of CdTe quantum wires (QWs) with varying PL quantum yields (Φ PL ). The PL lifetimes for samples with Φ PL > 4% are nearly an order of magnitude greater than the radiative lifetime of CdTe QDs, ≥200 versus ∼25 ns. The photogenerated electron−hole pairs relax to the lowest exciton state, correlating with the 1Σ e and 1Σ 3/2h quantum-confinement states, and are bound together as onedimensional (1D) excitons. These 1D excitons have a thermal distribution of translational kinetic energy along the long, unconfined dimension of the QWs. The extended lifetimes are justified via the constraints imposed by the conservation of wave vector (or momentum) and the large mismatch between the wave vector of the moving 1D excitons and of the photons emitted during radiative relaxation. The long charge-carrier lifetimes and the dimensionality of these high-quality semiconductor QWs offer distinct advantages for use in photovoltaics.