Probing the Dependence of Long-Range, Four-Atom Interactions on Intermolecular Orientation. 4. The Dissociation Dynamics of H₂/D₂···ICl(B,<i>v</i>′=3) and the Observation of Efficient Vibrational–Rotational Energy Transfer

The vibrational predissociation dynamics of H2/D2···I35Cl(B,v′=3) complexes containing both para- and ortho-hydrogen prepared in different intermolecular vibrational levels were investigated. The Δv = −1 I35Cl(B,v = 2,j) rotational product-state distributions measured for excitation to the lowest-energy T-shaped levels of these complexes are mostly bimodal. The rotational distributions measured for excitation of the H2···I35Cl(B,v′=3) complexes are colder than those of the D2···I35Cl(B,v′=3) complexes, and there are only slight differences between those measured for the para- and ortho-hydrogen containing complexes. Excitation of the delocalized bending levels results in slightly colder rotational product-state distributions. The distributions suggest the dynamics result from more than impulsive dissociation off of the inner repulsive wall of the lower-energy H2/D2 + I35Cl(B,v = 2) potential surfaces of the products. The depths of these potentials and the energies available to these products also contribute to the dynamics. The formation of the Δv = −2, I35Cl(B,v = 1) product channel was only identified for excitation of levels within the ortho(j = 0)-D2 + I35Cl(B,v′=3) potential. The formation of this channel occurs via I35Cl(B,v′=3) vibrational to D2 rotational energy transfer forming the ortho(j = 2)-D2 + I35Cl(B,v = 1,j) products

Dynamic Quantum-State Renormalization and Effects of Competing Pathways on Carrier Relaxation in Semiconductor Nanoparticles

The magnitude and temporal evolution of the quantum-state renormalization (QSR), or the energetic shifting of the quantum-confinement states caused by photoexcitation and changes in electron screening, were probed in transient absorption (TA) spectroscopy measurements of colloidal semiconductor nanoparticles. Experiments were performed on high- and lower-quality wurtzite CdTe quantum wires (QWs) with photoluminescence quantum yields of 8.8% and ∼0.2% using low-excitation fluences. The QSR shifts the spectral features to lower energies in both samples, with larger shifts measured in the high-quality QWs. The TA spectral features measured for both samples shift uniquely with time after photoexcitation, illustrating dynamic QSR that depends on the quantum-confinement states and on the states occupied by carriers. The higher fraction of carriers that reach the band-edge states in the high-quality QWs results in larger renormalization, with the energies of the band-edge states approaching the Stokes shift of the steady-state photoluminescence feature below the band-edge absorption energy. The intraband relaxation dynamics of charge carriers photoexcited in semiconductor nanoparticles was also characterized after accounting for contributions from QSR in the TA data. The intraband relaxation to the band-edge states was slower in the high-quality QWs than in the lower-quality QWs, likely due to the reduced number of trap states accessible. The contrasting relaxation time scales provide definitive evidence for a dependence of the photoluminescence efficiency on excitation energy. These studies reveal the complicated interplay between the energetics and relaxation mechanisms of carriers within semiconductor nanoparticles, even those with the same dimensionality

Spectroscopic Properties of Phase-Pure and Polytypic Colloidal Semiconductor Quantum Wires

We report ensemble extinction and photoluminesence spectra for colloidal CdTe quantum wires (QWs) with nearly phase-pure, defect-free wurtzite (WZ) structure, having spectral line widths comparable to the best ensemble or single quantum-dot values, to the single polytypic (having WZ and zinc blende (ZB) alternations) QW values, and to those of two-dimensional quantum belts or platelets. The electronic structures determined from the multifeatured extinction spectra are in excellent agreement with the theoretical results of WZ QWs having the same crystallographic orientation. Optical properties of polytypic QWs of like diameter and diameter distribution are provided for comparison, which exhibit smaller bandgaps and broader spectral line widths. The nonperiodic WZ–ZB alternations are found to generate non-negligible shifts of the bandgap to intermediate energies between the quantum-confined WZ and ZB energies. The alternations and variations in the domain sizes result in inhomogeneous spectral line width broadening that may be more significant than that arising from the 12–13% diameter distributions within the QW ensembles

Synchronous Photoluminescence Intermittency (Blinking) along Whole Semiconductor Quantum Wires

Photoluminescence microscopy studies have detected synchronous-photoluminescence-intensity fluctuations along entire cadmium selenide quantum wires under continuous illumination. While similar photoluminescence blinking has been reported previously for semiconductor quantum dots and rods, the observation of synchronous blinking spanning the entire length of quantum wires, with diameters ≈9 nm and lengths >5 μm, is remarkable. We propose a mechanism to account for the synchronous blinking that is based on a dynamic, photolytic filling of surface-trap sites

Facet-Specific Electron Transfer in Pseudo-Two-Dimensional Wurtzite Cadmium Selenide Nanocrystals

Ligand-exchange reactions of wurtzite CdSe quantum platelets (QPs) and quantum belts (QBs) with methyl viologen (MV2+) and the derivative ligands MV2+(CH2)nNH2 (n = 2, 4, or 6) are investigated. The QP and QB photoluminescence is quenched after partial ligand exchange. Spectroscopic and compositional data establish that this initial ligand substitution occurs on the thin QP and QB edges. The MV2+(CH2)nNH2 ligands are shown to be more-efficient photoluminescence quenchers than the parent MV2+ ion. The ligands on the thin, nonpolar, long-edge facets quench the photoluminescence via the trapping of excitons. Transient absorption experiments indicate the excitons dissociate, and electron transfer to the MV2+(CH2)nNH2 ligands only occurs at the polar, short-edge facets of the wurtzite CdSe QPs and QBs. Electron transfer to the MV2+(CH2)nNH2 ligands occurs within 100 fs when exciting at the band edge and on longer time scales, due to intraband relaxation, when exciting at higher energies

Synchronous Photoluminescence Intermittency (Blinking) along Whole Semiconductor Quantum Wires

Photoluminescence microscopy studies have detected synchronous-photoluminescence-intensity fluctuations along entire cadmium selenide quantum wires under continuous illumination. While similar photoluminescence blinking has been reported previously for semiconductor quantum dots and rods, the observation of synchronous blinking spanning the entire length of quantum wires, with diameters ≈9 nm and lengths >5 μm, is remarkable. We propose a mechanism to account for the synchronous blinking that is based on a dynamic, photolytic filling of surface-trap sites

Solution–Liquid–Solid Growth of Semiconductor Quantum-Wire Films

We report the growth of cadmium-selenide (CdSe) quantum-wire (QW) films on a variety of substrates by the solution–liquid–solid (SLS) method. Our SLS syntheses employ size-controlled, near-monodisperse bismuth (Bi) nanoparticles (NPs) as the catalysts for QW growth, which offers several advantages over Bi NPs thermally generated from thin Bi films, including mean QW diameter control, narrow diameter distributions, small diameters in the quantum-confinement regime, and control of the QW density on the substrates. The Bi NPs are deposited on the substrates via drop casting of a Bi-NP solution and subsequently annealed in a reducing atmosphere, a key step to ensure firm attachment of the Bi NPs onto the substrates and maintenance of their catalytic activity for the QW-film growth. The QW growth density is proportional to the Bi-NP coating density, which is determined by the concentration of the Bi-NP deposition solution. Lower concentrations are used for small Bi NPs to reduce their high tendency for agglomeration and to achieve control over mean QW diameter and to produce narrow diameter distributions. Spectroscopic evidence of quantum confinement is provided. Related films of InP, InAs, and PbSe QWs are also described

Synchronous Photoluminescence Intermittency (Blinking) along Whole Semiconductor Quantum Wires

Photoluminescence microscopy studies have detected synchronous-photoluminescence-intensity fluctuations along entire cadmium selenide quantum wires under continuous illumination. While similar photoluminescence blinking has been reported previously for semiconductor quantum dots and rods, the observation of synchronous blinking spanning the entire length of quantum wires, with diameters ≈9 nm and lengths >5 μm, is remarkable. We propose a mechanism to account for the synchronous blinking that is based on a dynamic, photolytic filling of surface-trap sites

Methods for the ICP-OES Analysis of Semiconductor Materials

The techniques employed in the compositional analysis of semiconductor materials by inductively coupled plasma optical emission spectroscopy (ICP-OES) dramatically influence the accuracy and reproducibility of the results. We describe methods for sample preparation, calibration, standard selection, and data collection. Specific protocols are suggested for the analysis of II–VI compounds and nanocrystals containing the elements Zn, Cd, S, Se, and Te. We expect the methods provided will apply more generally to semiconductor materials from other families, such as to III–V and IV–VI nanocrystals