Controlling the size of colloidal nanocrystals is essential to optimizing their performance in optoelectronic devices, catalysis, and imaging applications. Traditional synthetic methods control size by terminating the growth, an approach that limits the reaction yield and causes batch-to-batch variability. Herein we report a library of thioureas whose substitution pattern tunes their conversion reactivity over more than five orders of magnitude and demonstrate that faster thiourea conversion kinetics increases the extent of crystal nucleation. Tunable kinetics thereby allows the nanocrystal concentration to be adjusted and a desired crystal size to be prepared at full conversion. Controlled precursor reactivity and quantitative conversion improve the batch-to-batch consistency of the final nanocrystal size at industrially relevant reaction scales.

We study the impact of surface chemistry on the photoluminescence (PL) quantum yield of CdSe core, CdSe/CdS core/shell, and CdSe/CdZnS/ZnS core/shell/shell nanocrystals prepared by multiple synthetic routes. We expose as-synthesized particles to varying concentrations of n-alkylamine and n-alkanethiol ligands and verify that the addition of n-alkanethiols to CdSe and CdSe/CdS nanocrystal solutions quenches their PL. We also show that the addition of n-alkylamines to nanocrystal solutions can increase or decrease nanocrystal PL, an effect that depends on the concentration of both nanocrystals and ligands. We demonstrate the importance of considering the nanocrystal concentration when fitting ligand binding curves, and show that common solvent impurities can affect the PL and ligand binding data. While alkanethiols quench CdSe nanocrystals prepared using multiple synthetic procedures, we find the exact shape of the quenching curve depends on the synthetic route chosen. We emphasize that the ligand binding data extracted from PL quenching curves are contingent on the assumptions made during fitting. By fitting our PL quenching curves to a Langmuir isotherm and accounting for the particle surface sites, we estimate a lower limit for the equilibrium CdSe−alkanethiol binding constant on the order of 109 M-1 with different numbers of thiol binding sites depending on the method of nanocrystal synthesis.

The near-field effects of plasmonic optical antennas are being explored in applications ranging from biosensors to solar cells. We demonstrate that photoluminescence emission enhancement from CdSe quantum dots (QDs) can be obtained in the absence of any excitation enhancement near single silver nanoprisms. The spectral dependence of the radiative and nonradiative decay rate of the QDs closely follows the silver nanoparticle plasmon scattering spectrum. Using both experiment and theory we show that, in the absence of excitation enhancement, the ratio of radiative to nonradiative decay rate enhancement is proportional to the silver nanoparticle scattering efficiency. These results provide guidelines both for separating excitation and emission enhancement effects in sensing and device applications and for tailoring emission enhancement effects using plasmonic nanostructures.

We study plasmon-enhanced fluorescence from CdSe∕CdS∕CdZnS∕ZnS core/shell quantum dots near a variety of Ag and Au nanoparticles. The photoluminescence excitation (PLE) spectrum of quantum dots closely follows the localized surface plasmon resonance (LSPR) scattering spectrum of the nanoparticles. We measure excitation enhancement factors of ∼3 to 10 for different shapes of single metal nanoparticles.

Colloidal nanocrystal quantum dots (QDs) are solution-processable chromophores with size-tunable bandgaps, high photoluminescence (PL) quantum efficiency (QE), excellent photostability, narrow emission line widths (< 30 nm), and large spin-orbit coupling. These factors make them good candidates for use in next-generation thin-film optoelectronic devices. Indeed, colloidal QDs are currently being explored for use in photovoltaics, [1][2][3][4] photodetectors, [5,6] and light emitting diodes, [7][8][9][10][11][12][13][14][15][16][17][18] often in hybrid structures that incorporate both QDs and conjugated polymers or small-molecule organic semiconductors. Despite the potential advantages of using QDs as emitters, early QD light-emitting diodes (QD-LEDs) exhibited low efficiencies, and often produced broad voltage-dependent emission with spectral contributions from both the QDs and the organic host materials. However, drawing from lessons learned from the field of all-organic LEDs, the MIT group reported a multilayer LED structure incorporating a monolayer of CdSe/ZnS core/shell QDs sandwiched between small molecule hole and electron transport layers. These devices exhibited a maximum external quantum efficiency (Q ext ) of ∼ 0.5 % and a luminous efficiency (LE) of 1.9 cd/ A at a brightness of 100 cd/ m 2 , although pure emission spectra at high brightness were not achieved in the initial report. [8,16] With subsequent refinements, the same authors have achieved maximum Q ext of > 2 % and luminous power efficiency (LPE) > 1 lm/W.[17]Recently, we reported an alternative strategy for QD-LED fabrication that allows for independent control of the QD and hole-transport layer (HTL) thicknesses by spin-coating the QD layer onto a thermally cross-linked HTL.[18] Using this flexible fabrication strategy, we demonstrated that graded structures comprising multiple hole-transport and injection layers could be used to further improve Q ext of the devices. The best devices exhibited good efficiency (Q ext > 0.8 % at 100 cd/ m 2 ), narrow EL spectra (∼ 30 nm FWHM) and maximum brightness in excess of 1000 cd/ m 2 . However, because of the high turn-on voltage for our first QD-LEDs, the LPE was not high.Herein, we describe how a substantial improvement in QD-LED performance, especially the LPE, can be obtained both by using an improved polymer hole-injection layer (HIL)/ HTL structure and by performing a thermal annealing of the QD layer prior to the final deposition of the organic electrontransport layer. In particular, the annealing step results in a significant performance improvement with these devices. In order to lay the scientific groundwork for future improvements in QD-LED performance, we characterize the changes in the chemical, photophysical, and electronic properties of the structures that occur due to the annealing process.

Dendritic spines are the primary site of excitatory synaptic input onto neurons, and are biochemically isolated from the parent dendritic shaft by their thin neck. However, due to the lack of direct electrical recordings from spines, the influence that the neck resistance has on synaptic transmission, and the extent to which spines compartmentalize voltage, specifically excitatory postsynaptic potentials, albeit critical, remains controversial. Here, we use quantum-dot-coated nanopipette electrodes (tip diameters ~15–30 nm) to establish the first intracellular recordings from targeted spine heads under two-photon visualization. Using simultaneous somato-spine electrical recordings, we find that back propagating action potentials fully invade spines, that excitatory postsynaptic potentials are large in the spine head (mean 26 mV) but are strongly attenuated at the soma (0.5–1 mV) and that the estimated neck resistance (mean 420 MΩ) is large enough to generate significant voltage compartmentalization. Nanopipettes can thus be used to electrically probe biological nanostructures.

Coupled plasmonic/chromophore systems are of interest in applications ranging from fluorescent biosensors to solar photovoltaics and photoelectrochemical cells because near-field coupling to metal nanostructures can dramatically alter the optical performance of nearby materials. We show that CdSe quantum dots (QDs) near single silver nanoprisms can exhibit photoluminescence lifetimes and quantum yields that depend on the excitation wavelength, in apparent violation of the Kasha-Vavilov rule. We attribute the variation in QD lifetime with excitation wavelength to the wavelength-dependent coupling of higher-order plasmon modes to different spatial subpopulations of nearby QDs. At the QD emission wavelength, these subpopulations are coupled to far-field radiation with varying efficiency by the nanoprism dipolar resonance. These results offer an easily accessible new route to design metachromophores with tailored optical properties.

We report a family of substituted thiocarbonates, thiocarbamates, and thioureas and their reaction with cadmium oleate at 180-240 °C to form zincblende CdS nanocrystals (d = 2.2-5.9 nm). To monitor the kinetics of CdS formation with UV-vis spectroscopy, the size dependence of the extinction coefficient for λ max (1S e-1S 1/2h) is determined. The precursor conversion kinetics span five orders of magnitude depending on the precursor structure (2˚-thioureas > 3˚-thioureas ≥ 2˚-thiocarbamates > 2˚-thiocarbonates > 4˚-thioureas ≥ 3˚-thiocarbamates). The concentration of nanocrystals formed by the nucleation reaction increases with increasing precursor conversion reactivity, allowing the final size to be controlled by the precursor structure. 1 H NMR spectroscopy is used to monitor the reaction of dip -tolyl thiocarbonate and cadmium oleate where dip -tolyl carbonate and oleic anhydride coproducts can be identified. These coproducts further decompose into p-tolyl oleate and p-cresol. The spectral features of CdS nanocrystals produced from thiocarbonates are exceptionally narrow (95-161 meV FWHM) compared to those made from thioureas (137-174 meV FWHM) under otherwise identical conditions, indicating that particular precursors nucleate narrower size distributions than others. Additional nanocrystal synthesis and precursor coproduct identification Figures S1-S23 and 1 H, 13 C{ 1 H}, and 19 F{ 1 H} NMR characterization of molecules. (PDF)