The role of ligands in determining the exciton relaxation dynamics in semiconductor quantum dots.

This article reviews the mechanisms through which molecules adsorbed to the surfaces of semiconductor nanocrystals, quantum dots (QDs), influence the pathways for and dynamics of intra- and interband exciton relaxation in these nanostructures. In many cases, the surface chemistry of the QDs determines the competition between Auger relaxation and electronic-to-vibrational energy transfer in the intraband cooling of hot carriers, and between electron or hole-trapping processes and radiative recombination in relaxation of band-edge excitons. The latter competition determines the photoluminescence quantum yield of the nanocrystals, which is predictable through a set of mostly phenomenological models that link the surface coverage of ligands with specific chemical properties to the rate constants for nonradiative exciton decay.

The chemical environments of oleate species within samples of oleate-coated PbS quantum dots.

A combination of FT-IR, (1)H NMR, nuclear Overhauser effect (NOESY), and diffusion-ordered (DOSY) NMR spectroscopies shows that samples of oleate-coated PbS quantum dots (QDs) with core radii ranging from 1.6 to 2.4 nm, and purified by washing with acetone, contain two species of oleate characterized by the stretching frequencies of their carboxylate groups, the chemical shifts of their protons, and their diffusion coefficients. One of these oleate species exists primarily on the surfaces of the QDs and either chelates a Pb(2+) ion or bridges two Pb(2+) ions. The ratio of bridging oleates to chelating oleates on the surfaces of the QDs is approximately 1:1 for all sizes of the QDs we studied. The second oleate species in these samples bridges two Pb(2+) ions within clusters or oligomers of lead oleate (with a hydrodynamic radius of ~1.4 nm), which are byproducts of the QD synthesis. The concentration of these clusters increases with increasing size of the QDs because larger QDs are produced by increasing the concentration of the oleic acid ligand in the reaction mixture. The oleate molecules on the surfaces of the QDs and within the lead oleate clusters are in rapid exchange with each other. Additional washes with methanol progressively eliminate the contaminating clusters from the PbS QD samples. This work quantitatively characterizes the distribution of binding geometries at the inorganic/organic interface of the nanocrystals and demonstrates the utility of using organic ligands as probes for the composition of a colloidal QD sample as a function of the preparation procedure.

Spontaneous multielectron transfer from the surfaces of PbS quantum dots to tetracyanoquinodimethane.

This paper describes an investigation of the interfacial chemistry that enables formation of a multielectron ground-state charge-transfer (CT) complex of oleate-coated PbS quantum dots (QDs) and tetracyanoquinodimethane (TCNQ) in CHCl3 dispersions. Thermodynamically spontaneous electron transfer occurs from sulfur ions on the surfaces of the QDs (radius = 1.6 nm) to adsorbed TCNQ molecules and creates indefinitely stable ion pairs that are characterized by steady-state visible and mid-infrared absorption spectroscopy of reduced TCNQ and by NMR spectroscopy of the protons of oleate ligands that coat the QDs. The combination of these techniques shows that (i) each QD reduces an average of 4.5 TCNQ molecules, (ii) every electron transfer event between the QD and TCNQ occurs at the QD surface, (iii) sulfur ions on the surfaces of the QDs (and not delocalized states within the QDs) are the electron donors, and (iv) some TCNQ molecules adsorb directly to the surface of the QDs while others adsorb upon displacement of oleate ligands.

A molecule to detect and perturb the confinement of charge carriers in quantum dots.

This paper describes unprecedented bathochromic shifts (up to 970 meV) of the optical band gaps of CdS, CdSe, and PbS quantum dots (QDs) upon adsorption of an organic ligand, phenyldithiocarbamate (PTC), and the use of PTC to map the quantum confinement of specific charge carriers within the QDs as a function of their radius. For a given QD material and physical radius, R, the magnitude of the increase in apparent excitonic radius (ΔR) upon delocalization by PTC directly reflects the degree of quantum confinement of one or both charge carriers. The plots of ΔR vs R for CdSe and CdS show that exciton delocalization by PTC occurs specifically through the excitonic hole. Furthermore, the plot for CdSe, which spans a range of R over multiple confinement regimes for the hole, identifies the radius (R∼1.9 nm) at which the hole transitions between regimes of strong and intermediate confinement. This demonstration of ligand-induced delocalization of a specific charge carrier is a first step toward eliminating current-limiting resistive interfaces at organic-inorganic junctions within solid-state hybrid devices. Facilitating carrier-specific electronic coupling across heterogeneous interfaces is especially important for nanostructured devices, which comprise a high density of such interfaces.

Simultaneous determination of the adsorption constant and the photoinduced electron transfer rate for a CdS quantum dot-viologen complex.

Transient absorption (TA) spectroscopy of solution-phase mixtures of colloidal CdS quantum dots (QDs) with acid-derivatized viologen molecules, N-[1-heptyl],N'-[3-carboxypropyl]-4,4'-bipyridinium dihexafluorophosphate (V(2+)), indicates electron transfer occurs from the conduction band of the QD to the LUMO of V(2+) after photoexcitation of a band-edge exciton in the QD. Analysis of the magnitude of the ground state bleach of the QD as a function of the molar ratio QD:V(2+) yields the QD-ligand adsorption constant, K(a) (4.4 × 10(4) M(-1)) for V(2+) ligands adsorbed in geometries conducive to electron transfer. The value of K(a), together with the measured rates of (i) formation of the V(+•) electron transfer product and (ii) recovery of the ground state bleach of the QD, enables determination of the intrinsic rate constant for charge separation, k(CS,int) ~ 1.7 × 10(10) s(-1), the rate for a single QD-V(2+) donor-acceptor pair. This analysis confirms previous reports that the number of ligands adsorbed to each QD is well-described by a Poisson distribution. This is the first report where the QD-ligand charge transfer and binding equilibria are quantitatively investigated simultaneously with a single technique.

Probing silver nanoparticles during catalytic H2 evolution.

Employing silver nanoparticles from a recently developed synthesis [Evanoff, D. D.; Chumanov, G. J. Phys. Chem. B 2004, 108, 13948] and a well-studied probe molecule, p-aminothiophenol, we follow changes at the surface of the particles during the conditioning and eventually the catalytic production of hydrogen from water using strongly reducing radicals. Injection of electrons into the particles causes pronounced variations in the intensity of the surface enhanced Raman scattering (SERS) spectrum of the probe molecule. These spectral changes are caused by changes in the Fermi-level energy that are in turn caused by changes in the silver ion concentrations or in the pH, or by changes in electron density in the particle. This correlation highlights the effect of the chemical potential on the SERS enhancement at the end of the particles synthesis. The intensity of the SERS spectra increases in the presence of the silver ions when excitation at 514 nm is utilized. When the Ag(+) ions in the colloidal suspension are completely reduced by the radicals and the particles operate in the catalytic mode, the SERS spectrum is too weak to record, but it can reversibly be recovered upon the addition of Ag(+). The effect of pH on the SERS intensity is similar in nature to that of the silver ions but is complicated by the pKa of the aminothiol and the point of zero charge (pzc) of the particles. It is hypothesized that as the particles cross the pzc (around neutral pH) the electrostatic interaction between the protonated amine headgroup of the probe and the positively charged surface increases the density of probe molecules in the perpendicular orientation at the expense of a competing species. This conversion results in enhanced SERS signals and is observable during the preconditioning stage of the particles. Indeed, adsorption isotherms of the probe indicate the presence of two species. In analogous previous observations these two species have been attributed to perpendicular and flat adsorption orientations of the deprotonated probe molecule relative to the particle surface. However, preliminary density functional calculations on relevant prototypes raise the possibility that the two species may be the probe molecule and a cationic form produced by charge transfer in the ground state from the chemisorbed probe to the metal. These two forms of the probe have differing electronic structures and vibrational frequencies, with perhaps differing orientations relative to the surface. Whichever is the correct interpretation, a neutral molecule in a flat orientation or a radical cation, this species is easier to replace than the other in competitive adsorption by ethanethiol.