Search

A series of morphologically distinct cadmium halide complexes were obtained from potentially pentadentate, serpentine ligands 2,6-bis([(2-pyridylmethyl)oxy]methyl)-pyridine (L) and 2,6-bis([(2-pyridylmethyl)thio]methyl)pyridine (L*). Methods were developed to isolate crystalline materials ranging from mononuclear [CdLCl2] (1) to zig-zag one-dimensional polymer {[CdL(μ-Cl)]2(CdCl4)}n·n(CH3CN) (2) for L and from one-dimensional helical polymers [CdL*Cl2]n (3) and [CdL*Br2]n (4) to dinuclear [CdL*(μ-Br)(CdBr3)]2 (toluene solvate 5a, ether solvate 5b) or three-dimensional polymer [((Cd(μ-Cl)2)2L*)2]n (6) for L*. Compounds 1 and 2 had pentagonal bipyramidal metal centers with L in a pseudo-planar spiral, pentadentate mode. In contrast, the complexes of L* had octahedral metal centers with a bis-N,S metal-bridging ligand binding mode linking Cd(halogen)2 units in polymeric complexes 3, 4, and 6, but a pentadentate Λ2/Δ2 ligand conformation in dinuclear complexes 5a and 5b. Interactions involved in supramolecular aggregation included offset π-π stacking interactions and edge-to-face σ-π interactions, as well as anion-π(pyridine) interactions, chalcogen–chalcogen interactions, halogen-chalcogen interactions and C H⋯X–Cd halide hydrogen bonds. Additional characterizations of the complexes included elemental analysis, solution 1H NMR spectroscopy, FTIR, TGA and PXRD.

Transient absorption (TA) and photolumines- cence excitation (PLE) anisotropy measurements were used to investigate the polarization of band-edge and above-band-edge excitonic states in ensembles of CdSe nanocrystals with aspect ratios of 1:1, 3:1, and 10:1, dispersed in hexanes. The 1:1 nanocrystals (quantum dots) are isotropic absorbers and emitters. The 10:1 nanorods have a nonzero but featureless anisotropy spectrum above the band edge due to heterogeneity in the crystal structure and, therefore, electronic structure within single nanorods. The nanocrystals with an aspect ratio of 3:1, which are largely single crystals, have PLE and TA anisotropy spectra with features that correspond to those in the absorption spectrum. Direct measurement of the TA anisotropy spectrum of the nanorods and comparison with the PLE anisotropy spectrum reveal that the band-edge absorptive and emissive transitions contain both linear (z) and planar (xy) character. The degree of planar character at the band-edge states, modulated by classical local field effects arising from the dielectric contrast between the nanorod and the solvent, limits the degree of photoselection at this wavelength. The variation in the magnitude of the xy projection of the absorptive transitions within states above the band edge is responsible for the wavelength dependence of the absorption and emission anisotropies.

An unbranched N3O2 ligand 2,6-bis[((2-pyridinylmethyl)oxy)methyl]pyridine (L1) was used to prepare new mononuclear heteroleptic Group 12 perchlorate complexes characterized by IR, (1)H NMR and X-ray crystallography. Racemic complexes with pentadentate L1 and one to four oxygens from either water or perchlorate bound to a metal ion were structurally characterized. Octahedral [Zn(L1)(OH2)](ClO4)2 (1) and pentagonal bipyramidal [Cd(L1)(OH2)(OClO3)]ClO4 (2) structures were found with lighter congeners. The polymorphic forms of [Hg(L1)(ClO4)2] characterized (3 in P1[combining macron] and 4 in P21/c) had a mix of monodentate, anisobidentate and bidentate perchlorates, providing the first examples of a tricapped trigonal prismatic Hg(ii) coordination geometry, as well as additional examples of a rare square antiprismatic Hg(ii) coordination geometry. Solution state (1)H NMR characterization of the Group 12 complexes in CD3CN indicated intramolecular reorganization remained rapid under conditions where intermolecular M-L1 exchange was slow on the chemical shift time scale for Zn(ii) and on the J(M(1)H) time scale for Cd(ii) and Hg(ii). Solution studies with more than one equivalent of ligand also suggested that a complex with a 1 : 2 ratio of M : L1 contributed significantly to solution equilibria with Hg(ii) but not the other metal ions. The behavior of related linear pentadentate ligands with Group 12 perchlorate salts is discussed.

This review highlights the developments in synthetic methods for colloidal quantum dots that have expanded the range of achievable sizes, shapes, materials, and surface chemistries over the past 30 years, and how these methods have enabled optimization of properties like photoluminescence quantum yield, monodisperse size distributions, and conductivity in the solid state.

This paper describes the use of the spectral signatures of near-infrared (NIR) absorbing cyanine dyes to quantitatively analyze their intermolecular interactions upon adsorption to colloidal CdSe quantum dots (QDs) with diameters of 2–3 nm. Spectroscopic characterization of the disaggregation of two types of sulfonate-functionalized cyanine molecules, IR783 and IR820, from H-aggregate dimers to monomers upon addition of methanol yields spectral signatures of aggregation used to analyze the response of the dyes to exposure to CdSe QDs. The spectrally distinct absorbances of the cyanines and QDs enable a factor analysis procedure that decomposes the absorbance spectrum of the QD/cyanine mixture into three distinct components—solution-phase cyanine molecules (in monomer and H-aggregate form), QD-bound cyanine monomers, and disordered, QD-bound cyanine aggregates—as a function of the molar ratio of cyanine to QD. The presence of these three distinct components strongly suggests that cyanines initially bind to...

This Perspective discusses recent work on mechanisms by which organic ligands affect the electronic structure and exciton dynamics of colloidal quantum dots (QDs). Much of the work described here uses some combination of steady-state absorption, transient absorption, steady-state photoluminescence, and transient photoluminescence spectroscopies to characterize QD–ligand complexes. Ligands affect the ground-state electronic structure of QDs via mixing of the frontier orbitals at the QD–ligand interface and influence the dynamics of excitonic decay by mediating charge trapping or by participating in charge transfer. This Perspective highlights strategies to address the various forms of structural and chemical heterogeneity of QD ensembles in identifying the mechanisms of these ligand-mediated processes. Finally, four-wave mixing techniques are discussed as promising methods for direct measurement of ligand-mediated nonradiative dissipation of the QD exciton.

This paper describes a study of the mechanism of photoinduced photoluminescence enhancement, termed “photobrightening” (PB), of ∼150-nm-thick films of CdSe quantum dots (QDs) in a dry N2 (g) atmosphere. Steady-state photoluminescence (PL) and ultrafast transient absorption measurements of films photoexcited continuously and shot-wise revealed that: (i) PL enhancement occurred in all of the close-packed films during periods of continuous photoexcitation and continued after the excitation source was turned off; (ii) the time-dependence of PB (both during excitation and in the dark) was initially linear and became exponential as the PB reached saturation; (iii) the rate of PB and the maximum PB achieved by the film depended on the degree of surface passivation of the QDs; (iv) the PL peak shifted to lower energy and broadened during PB; and (v) rates of nonradiative trapping of excitonic electrons decreased during PB. These data were utilized to construct a model for PB based on migration of photoexcited ele...

A major focus within the field of magnonics involves the manipulation and control of spin-wave modes. This is usually done by patterning continuous soft magnetic films. Here, we report on work in which we use topographic modifications of a continuous magnetic thin film, rather than lithographic patterning techniques, to modify the ferromagnetic resonance spectrum. To demonstrate this technique we have performed in-plane, broadband, ferromagnetic resonance studies on a 100-nm-thick permalloy film sputtered onto a colloidal crystal with individual sphere diameters of 200 nm. Effects resulting from the, ideally, sixfold-symmetric underlying colloidal crystal were studied as a function of the in-plane field angle through experiment and micromagnetic modeling. Experimentally, we find two primary modes; the ratio of the intensities of these two modes exhibits a sixfold dependence. Detailed micromagnetic modeling shows that both modes are quasiuniform and nodeless in the unit cell but that they reside in different demagnetized regions of the unit cell. Our results demonstrate that topographic modification of magnetic thin films opens additional directions for manipulating ferromagnetic resonant excitations.

Light-matter interaction at the nanoscale is of particular interest for future photonic integrated circuits and devices with applications ranging from communication to sensing and imaging. In this Letter a combination of transient absorption (TA) and the use of third harmonic generation as a probe (THG-probe) has been adopted to investigate the response of the localized surface plasmon resonances (LSPRs) of vertically aligned indium tin oxide rods (ITORs) upon ultraviolet light (UV) excitation. TA experiments, which are sensitive to the extinction of the LSPR, show a fluence-dependent increase in the frequency and intensity of the LSPR. The THG-probe experiments show a fluence-dependent decrease of the LSPR-enhanced local electric field intensity within the rod, consistent with a shift of the LSPR to higher frequency. The kinetics from both TA and THG-probe experiments are found to be independent of the fluence of the pump. These results indicate that UV excitation modulates the plasma frequency of ITO on the ultrafast time scale by the injection of electrons into, and their subsequent decay from, the conduction band of the rods. Increases to the electron concentration in the conduction band of ∼13% were achieved in these experiments. Computer simulation and modeling have been used throughout the investigation to guide the design of the experiments and to map the electric field distribution around the rods for interpreting far-field measurement results.

This paper describes a study of the rates of photoinduced electron transfer (PET) from CdSe quantum dots (QDs) to poly(viologen) within thin films, as a function of the length of the ligands passivating the QDs. Ultrafast (10 ps), quantitative PET occurs from CdSe QDs coated with HS-(CH(2))(n)-COOH for n = 1, 2, 5, and 7 to viologen units. The observed decrease in the magnitude of the PET rate constant with n is weaker than that expected from the decay of the electron tunneling probability across extended all-trans mercaptocarboxylic acids but well-described by electron tunneling across a collapsed ligand shell. The PET rate constants for films with n = 10 and 15 are much slower than those expected based on the trend for n = 1-7; this deviation is ascribed to the formation of bundles of ligands on the surface of the QD that make the tunneling process prohibitively slow by limiting access of the viologen units to the surfaces of the QDs. This study highlights the importance of molecular-level morphology of donor and acceptor materials in determining the rate and yield of interfacial photoinduced electron transfer in thin films.

Replacement of the native (as-synthesized) ligands of colloidal CdSe QDs with varying concentrations of a series of para-substituted anilines (R-An), where R ranges from strongly electron-withdrawing to strongly electron-donating, decreases the PL of the QDs. The molar ratio of R-An to QD ([R-An]:[QD]) at which the PL decreases by 50% shifts by 4 orders of magnitude over the series R-An. The model employed to describe the data combines a Freundlich binding isotherm (which reflects the dependence of the binding affinity of the amine headgroups of R-An on the substituent R) with a function that describes the response of the PL to R-An ligands once they are bound at their equilibrium surface coverage. The latter function includes as a parameter the rate constant, k(nr), for nonradiative decay of the exciton at a site to which an R-An ligand is coordinated. The value of this parameter reveals that the predominant mechanism of QD-ligand interaction is passivation of Cd(2+) surface sites through sigma-donation for R-An ligands with R = H, Br, OCF(3), and reductive quenching through photoinduced hole transfer for R = MeO, (Me)(2)N.

We describe surface-enhanced Raman scattering measurements from a benzenethiol monolayer adsorbed on a silver-coated film that is, in turn, deposited on an artificial opal, where the latter is a close-packed three-dimensional dielectric lattice formed from polystyrene spheres. Data for a range of sphere sizes, silver film thicknesses, and laser excitation wavelengths are obtained. Enhancement factors can be in the range of 10(7). To partially explain these large enhancements, we have performed model finite-difference time domain simulations of the position-dependent electric fields generated at the opal surfaces for several experimentally studied laser wavelengths and sphere diameters.