Search

Newtonian fluid mechanics, in which the shear stress is proportional to the strain rate, is synonymous with the flow of simple liquids such as water. We report the measurement and theoretical verification of non-Newtonian, viscoelastic flow phenomena produced by the high-frequency (20 GHz) vibration of gold nanoparticles immersed in water-glycerol mixtures. The observed viscoelasticity is not due to molecular confinement, but is a bulk continuum effect arising from the short time scale of vibration. This represents the first direct mechanical measurement of the intrinsic viscoelastic properties of simple bulk liquids, and opens a new paradigm for understanding extremely high frequency fluid mechanics, nanoscale sensing technologies, and biophysical processes.

International audience; Colloidal quantum dots (CQDs) with mid-infrared (IR) tunable bandgaps present a new paradigm for mid-IR materials, but advances in charge transport and collection are needed for practical use in electronic applications. By replacing the organic ligands in films of HgTe CQDs with As 2 S 3 , a problem of oxidation in air is avoided and charge carrier mobilities are improved 100 fold compared to the standard organic ligand exchange. The composite inorganic HgTe/As 2 S 3 material allows for ambipolar field effect transistors with on-off ratios up to 10 7 , and photodetectors with high sensitivity, reaching 3x10 10 Jones at 230 K with a 3.5 micron cutoff wavelength.

The vibrational dynamics of CO at two electrochemical interfaces is studied as a function of the electrode potential, for COPt(100) in 0.1M aqueous H2SO4 and COPt(110) in 0.05M C16H36ClNO4 acetonitrile electrolyte. The measured lifetime of ∼1.7 ps is in line with those previously determined for dry CO-metal interfaces. The lifetime appears to be independent of the electrolyte composition, and unaffected by an electrode-potential variation as large as 2 V achieved for the non-aqueous electrolyte experiment. These measurements suggest that the 2π∗ CO acceptor orbital involved in the substrate/adsorbate charge transfer process is much broader than 0.8 eV.

We review our recent nonlinear spectroscopies of nanocrystals and synthetic efforts to improve their luminescence properties. A two-photon spectroscopic study of CdSe nanocrystals as a function of size is presented and compared with predictions from the effective mass model with spherical confinement. We also detail our efforts at improving the luminescence properties of nanocrystals which have culminated in a 50% fluorescence quantum yield for inorganic capping of some sizes of CdSe nanocrystals. Finally, we present the application of two-photon microscopy to resolve fluorescence from single nanocrystals at room temperature and cryogenic temperatures.

A rigid scanning tunneling microscope design with an easily implemented two‐dimensional approach is presented. Also described is a simple current–voltage converter optimized for low‐current (pA) applications. Capabilities of the microscope include atomic resolution on Au(111) and molecular resolution of poorly conducting self‐assembled monolayers.

We describe the synthesis of ZnS-capped CdSe semiconductor nanocrystals using organometallic reagents by a two-step single-flask method. X-ray photoelectron spectroscopy, transmission electron microscopy and optical absorption are consistent with nanocrystals containing a core of nearly monodisperse CdSe of 27−30 A diameter with a ZnS capping 6 ± 3 A thick. The ZnS capping with a higher bandgap than CdSe passivates the core crystallite removing the surface traps. The nanocrystals exhibit strong and stable band-edge luminescence with a 50% quantum yield at room temperature.

Fluctuations in the fluorescence from macroscopic ensembles of colloidal semiconductor quantum dots have the spectral form of 1/f noise. The measured power spectral density reflects the fluorescence intermittency of individual dots with power-law distributions of "on" and "off" times, and can thus serve as a simple method for characterizing such blinking behavior.

The vibrational relaxation of the CO stretching vibration when the molecule is adsorbed at a (100) platinum single crystal electrode in aqueous electrolyte is investigated by sum-frequency generation. The measured vibration lifetime of 1.5 ± 0.5 ps is in line with those previously observed for dry CO/metal interfaces. No lifetime modification upon electrode potential shift of 0.5 V could be detected. Consequently, the electrolyte or the interfacial electric field has little effect on the fast vibrational dynamics resulting from the non-adiabatic coupling of the vibration to the substrate electrons. It is speculated that the observation of a significant lifetime modification upon electrode potential shift would require non-aqueous electrolytes where the CO/Pt interface can be stable over a larger potential range.

Self-assembly of decanethiol on Ag(111) is studied using ultrahigh impedance scanning tunneling microscopy. The surface frequently shows monatomic silver islands covering about 20% of the surface. Molecular domains with an average size of ∼70 A as well as the molecules themselves are also imaged. From these images, three important quantities regarding the molecular structure are determined : (1) the lattice constant is 4.61 ± 0.15 A ; (2) two sets of 6-fold lattice orientations separated by 20.7 ± 2.3° are observed ; (3) far from domain boundaries, height variations of nearest neighbors are less than 0.1 A. A model is proposed to explain how such a structure could have arisen.

Electrochemical effects on self-assembled monolayers ofalkanethiols, C n H 2n+1 SH (n = 9, 10, 18), on Au, Ag(111), and Pt(111) are investigated using sum-frequency generation. Layers on Au and Ag(111) show no noticeable effects. Layers on Pt(111) show gauche transformations that are reversibly eliminated at negative or positive potentials.

International audience; Thermal deflection spectroscopy allows to measure very small absorption and uncovers absorption tails extending well below the bulk bandgap energy for CdSe quantum dots films after ligand exchange by sulfide. In this monodispersed system, the redshift, the broadening, and the absorption tails cannot be solely attributed to electronic coupling between the dots. Instead, mixing of hole states from the quantum dot and surface is proposed to dominate the changes of the interband spectra at the absorption edge.

International audience; The use of colloidal material offers an interesting alternative to top down approaches for the realization of low cost infrared detectors. We demonstrate photoconduction in thin films of a colloidal material in the mid-infrared (up to 7 μm), using HgTe colloidal quantum dots. Thin films of the colloidal quantum dots have a large absorption coefficient (>104 cm−1), and the photoconductive response is dramatically improved by encapsulating the nanoparticle into an inorganic matrix of As2S3. Such devices show fast response and large detectivity (>1010 jones) at temperatures above 200 K.

The transport and thermal properties of HgTe colloidal quantum dot films with cut-off wavelengths in the mid-IR are investigated. The cut-off wavelength of this material can be tuned over the 3-5 \mu m range, which makes it a promising alternative to existing high cost detectors. Post deposition processes such as ligand exchange and atomic layer deposition are investigated as a way to increase the carrier mobility., Comment: 7 pages, 7 figures

Precise knowledge of the properties of semiconductor nanocrystallites (NCs) may determine their use in future optical devices. However, since synthesis methods are not able to yield perfectly homogeneous NCs, spectral and dynamic responses are usually averaged properties. We examine single NCs using two-photon microscopy and obtain narrow luminescence linewidths (15 meV in 55 A CdS). This technique should be broadly applicable to single molecule spectroscopy and local probing of buried interfaces, such as quantum wells.

Electron injection into nanocrystal thin films is reversibly controlled by applying an electrochemical potential. Complete bleach of the visible interband transition and strong midinfrared intraband absorption are observed upon electron injection into the nanocrystal films. Quenching of the photoluminescence of the nanocrystal film is also observed.

HgTe colloidal quantum dots are prepared via a simple two-step injection method. Absorption and photodetection with sharp edges, as well as narrow photoluminescence, are tunable across the near and mid-IR between 1.3 and 5 μm.

International audience; We report on HgTe colloidal-based quantum-dot mid-infrared (IR) photode-tectors. We demonstrate tuning the cutoff wavelength of this detector from the near IR up to 7 lm. Responsivity values of films of HgTe nanoparticles a few hundred nanometers thick are in the 100 mA W À1 range even at room temperature. The detectivity of this film is limited by large 1/f noise. Time responses as short as 200 ns are achieved.

Researchers show that thin films containing HgTe quantum dots with diameters of around 10 nm exhibit a photoresponse in the mid-infrared that extends to wavelengths as long as 5 µm. Such films could become the basis of a new form of low-cost mid-infrared photodetector.

Incorporating nanocrystals into future electronic or optoelectronic devices will require a means of controlling charge-injection processes and an understanding of how the injected charges affect the properties of nanocrystals. We show that the optical properties of colloidal semiconductor nanocrystal quantum dots can be tuned by an electrochemical potential. The injection of electrons into the quantum-confined states of the nanocrystal leads to an electrochromic response, including a strong, size-tunable, midinfrared absorption corresponding to an intraband transition, a bleach of the visible interband exciton transitions, and a quench of the narrow band-edge photoluminescence.

Relatively monodisperse, highly luminescent zinc blende ZnSe nanocrystals are synthesized in a hexadecylamine/trioctylphosphine coordination solvent. The controlled growth process affords tunable sample sizes. Pure band-edge fluorescence size-tunable between 2.8 and 3.4 eV is obtained at room temperature with quantum yields between 20% and 50% relative to Stilbene 420.

Incorporating nanocrystals into future electronic or optoelectronic devices will require a means of controlling charge-injection processes and an understanding of how the injected charges affect the properties of nanocrystals. We show that the optical properties of colloidal semiconductor nanocrystal quantum dots can be tuned by an electrochemical potential. The injection of electrons into the quantum-confined states of the nanocrystal leads to an electrochromic response, including a strong, size-tunable, midinfrared absorption corresponding to an intraband transition, a bleach of the visible interband exciton transitions, and a quench of the narrow band-edge photoluminescence.

Visible light-induced IR absorption in colloidal CdSe quantum dots is observed by step-scan FTIR spectroscopy. Size dependence and the effect of surface modification are investigated. The introduct...

Starting with 11-ps pulses of 8-mJ energy at 1.064 microm and using a sequence of a KTP crystal and a CdSe crystal, we generate infrared pulses from 10 to 20 microm with energies ranging from 40 to 5 microJ. The quantum efficiency of the last stage is higher than 50% from 10 to 18 microm.

Using charged films of colloidal CdSe/CdS core/shell quantum dots of approximately 3.5 to 4.5 nm core diameters and 0.6 to 1.2 nm thick CdS shells, the radiative and nonradiative decay of the negatively charged exciton, the trion T-, are measured. The T- radiative rate is faster than the exciton by a factor of 2.2 +/- 0.4 and estimated at approximately 10 ns. The T- lifetime is approximately 0.7-1.5 ns for the samples measured and is longer than the biexciton lifetime by a factor or 7.5 +/- 1.7.

Quantum dots are nanometre-sized semiconductor particles exhibiting unique size-dependent electronic properties. In order to passivate the nanocrystals surface and to protect them from oxidation, we grow a shell composed of a second semiconductor w ith a larger bandgap on the core (for example a core / shell CdS / ZnS). However, the lattice mismatch between the two materials (typically 7% between ZnS and CdS) induces mechanical stress which can lead to dislocations. To be tter understand these mechanisms, it is important to be able to measure the pressure induced on the semiconductor core. We used a nanocrystal doped with manganese ions Mn 2+ , which provide a phosphorescence signal depending on the local pressure. A few dopant atoms per nanoparticle were placed at controlled radial positions in a ZnS shell formed layer by layer. The experimental pressure measurements are in very good agreement with a simple spherically symmetric elastic continuum model[1]. Using manganese as a pressure gauge could be used to better understand some structural phenomena observed in these nanocrystals, such as crys talline phases transition, or shell cracking. Keywords: Doped Quantum dots, strain and stress

We investigated the effectiveness of low temperatures in reducing plasmon damping by measuring the homogeneous linewidth of plasmon resonances $(\ensuremath{\sim}1.5\text{ }\text{eV})$ in nanoscale gold bipyramids at temperatures from 293 to 6 K. The linewidth drops linearly with temperature and approaches a constant value at approximately 50 K. Measurements were performed on monodisperse ensembles as well as on single particles. The 30% decrease in the homogeneous linewidth with decreasing temperature is well accounted for by the reduced electron-phonon scattering. The other relaxation mechanisms---electron-electron scattering, electron-surface scattering, and radiative relaxation---do not change significantly with temperature.

Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of ∼0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.

We study theoretically a dipole emitter placed near a metal nanoparticle and near small chains of two and three nanoparticles. The emitter can efficiently excite dark, or nonradiative, surface-plasmon modes in the nanostructures, in addition to the well-known bright modes. In the case of coupled nanoparticles, the origins of the bright and dark modes can be understood in the context of plasmon hybridization. Excitation of dark modes in nanoparticle chains allows for subwavelength guiding of optical energy with no radiative losses and thus with improved propagation lengths.

The surface plasmon resonance shift of gold bipyramids deposited on glass slides is investigated as a function of dielectric environement. Due to their narrow ensemble resonance, the bipyramids prove to be highly sensitive near-infrared sensors.

The fluorescence of Mn2+ doped in the shell of CdS/ZnS core/shell nanocrystals at radially controlled position is used as a local probe of pressure in the nanocrystal shell. The redshift of the fluorescence with increasing shell thickness indicates a pressure of more than 4 GPa for 7.5 monolayers of ZnS. The radial dependence and magnitude of the pressure derived from the Mn2+ fluorescence shift are in good agreement with the spherically symmetric elastic continuum model and the 7% misfit between the core and the epitaxial shell.

Near- and far-field optical properties of several types of gold nanoparticle are investigated with rigorous three-dimensional computational electrodynamics. The primary focus is on the results obtained with the finite-difference time-domain method, although some results obtained with the discrete dipole approximation are also given. We first consider spheres and prolate spheroids, where analytical solutions are available for comparison. The spectra of gold nanorods and pentagonal bipyramids are then investigated and excellent agreement with recent experimental optical spectra is found. The local field enhancement $(\ensuremath{\mid}\mathbf{E}\ensuremath{\mid}∕\ensuremath{\mid}{\mathbf{E}}_{0}\ensuremath{\mid})$ is studied at the longitudinal plasmon resonance. Sharper structural features produce more significant enhancement and the largest enhancement of more than a factor of 200 is seen around the poles of the bipyramid. The fields within the nanoparticles are also studied. Whereas the field magnitude is nearly uniform within small spheres and spheroids, it can be nonuniform for nanorods and bipyramids. The field magnitude decreases from the center toward the poles in the case of nanorods, but increases rapidly in the case of bipyramids. A large internal field enhancement by more than a factor of 30 is seen for the bipyramids, which suggests that these particles will exhibit strong optical nonlinearities.

The interband and intraband spectra of colloidal II-VI CdS and CdSe quantum dot cores and CdS∕ZnSe, CdS∕CdSe, CdSe∕CdS, and CdSe∕ZnSe core/shell systems are reported. Infrared absorption peaks between 0.5 and 0.2eV are observed. The slope of the intraband energy versus the first interband absorption feature is characteristic of the relative band alignments of the materials constituting the core and the shell and it is analyzed within an effective mass model. The analysis provides a new estimate of the band gap of zinc blende CdSe as well as the band offsets in zinc blende and wurtzite CdSe, CdS, and ZnSe.

We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods, Au/Ag core/shell nanorods, and Au bipyramids in solution, using the longitudinal surface-plasmon resonance to enhance optical forces. Laser light that is detuned slightly to the long-wavelength side of the resonance traps individual and multiple particles for up to 20 minutes; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under stable-trapping conditions, the trapping time of individual particles depends exponentially on laser power, in agreement with a Kramers escape process. Trapped particles have their long axes aligned with the trapping-laser polarization, as evidenced by a suppression of rotational diffusion about the short axis. When multiple particles are trapped simultaneously, evidence of interparticle interactions is observed, including a nonlinearly increasing two-photon fluorescence intensity, increasing fluorescence fluctuations, and changing fluorescence profiles as the trapped particle number increases.

Colloidal growth of plasmonic nanostructures may present some advantages such as shape control at the nm scale with atomic smoothness of the surfaces and possibly reduced damping. We show that the seed-mediated growth of gold nanostructures is strongly dependent on the gold seed nanocrystal structure. Starting with gold seed solutions prepared such that they are either single crystalline or multiply twinned, growth yields either nanorods with good control over the aspect ratio (~10%) or elongated bipyramidal nanoparticles. The nanorods are single crystalline while the gold bipyramids are penta-fold-twinned. The gold bipyramids are also strikingly monodisperse in shape with the sharpest ensemble surface plasmon resonance reported so far. Silver can be coated onto the gold nanostructures leading to a large blue-shift of the longitudinal plasmon resonance. Surprisingly, even a thin silver layer introduces much additional damping explained as scattering at the Au/Ag interface. Silver can be converted to silver sulphide yielding a large red-shift. The metal-semiconductor composite materials may present interesting nonlinear optical properties which are being currently investigated. Finally, the nonlinear optical scattering from individual Au nanorods was measured under excitation by ultrafast laser pulses on resonance with their longitudinal plasmon mode. Surprisingly, the ultrafast nonlinearity can be attributed entirely to the heating of conduction electrons and does not exhibit any response associated with coherent plasmon oscillation. This indicates an unanticipated damping of strongly driven plasmons.

In colloidal quantum dots, Energy dissipation by intraband, interband, and 'Auger' relaxation have very different mechanisms and rates which are being probed by transient optical measurements. Surface vibrations, phonons and spectator carrier play important roles.

We have measured nonlinear scattering from plasmons in individual Au nanorods and have correlated second-harmonic activity of Ag nanoparticles and clusters to morphology. The measurements reveal novel ultrafast nonlinear phenomena related to electron confinement and enhanced plasmon dephasing.

The photovoltaic response of thin films of HgTe colloidal quantum dots in the 3–5 μm range is observed. With no applied bias, internal quantum efficiency exceeding 40%, specific detectivity above 1010 Jones and microseconds response times are obtained at 140 K. The cooled devices detect the ambient thermal radiation. A detector with 5.25 μm cut-off achieves Background Limited Infrared Photodetection at 90 K.

Relatively monodisperse, highly luminescent zinc blende ZnSe nanocrystals are synthesized in a hexadecylamine/trioctylphosphine coordination solvent. The controlled growth process affords tunable sample sizes. Pure band-edge fluorescence size-tunable between 2.8 and 3.4 eV is obtained at room temperature with quantum yields between 20% and 50% relative to Stilbene 420.

Intraband spectra of CdSe nanocrystal colloids are observed. The photoexcited nanocrystals, of average diameters 31.5, 38, and 43 A, exhibit an isolated and strong infrared-induced absorption with peak energy between 0.5 and 0.3 eV. The energies and cross sections of the resonance are consistent with a one-electron transition between the delocalized 1Se and 1Pe states of the strongly confined quantum dots.

We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods in solution, taking advantage of the longitudinal surface-plasmon resonance to enhance optical forces. Stable trapping is achieved using laser light that is detuned slightly to the long-wavelength side of the resonance; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under stable-trapping conditions, the trapping time depends exponentially on laser power, in agreement with a Kramers escape process. Trapped rods have their long axes aligned with the trapping-laser polarization, as evidenced by a suppression of rotational diffusion about the short axis. The ability to trap and orient individual metal nanoparticles may find important application in assembly of functional structures, sorting of nanoparticles according to their shape, and development of novel microscopy techniques.

We have measured nonlinear scattering from plasmons in individual Au nanorods and have correlated second-harmonic activity of Ag nanoparticles and clusters to morphology. The measurements reveal novel ultrafast nonlinear phenomena related to electron confinement. Surprisingly, the coherent plasmon response is suppressed relative to the hot electron response indicating enhanced plasmon dephasing. In a parallel set of studies we demonstrate nanometer scale localization of the nonlinear optical response of single nanoparticles and aggregates and correlate this with their morphology. Position markers are fabricated on an optical and electron-transparent substrate (Si3N4 thin film) that allows optical measurements and transmission electron microscopy (TEM) imaging of the identical nanoparticles or aggregates. The second harmonic (SH) activity optical image of individual Ag nanostructures is registered with the TEM image. Centroid localization of the optical signals allows correlation with better than 25 nm precision. This is sufficient to determine the origin of optical "hot spots" within multi-particle aggregates.

Conduction in thin films of PbSe nanocrystals doped by electrochemical gating has been studied. Charging the film, with either electrons or holes, increases the conductance by orders of magnitude. The electrons in the 1S(e) state of nanocrystals in these films have a mobility as high as 5.0 x 10(-3) cm(2) V(-1) s(-1). Electrons in the 1P(e) state were found to have a differential mobility up to 3-5 times greater than the electrons in the 1S(e) state, and a mobility minima was found corresponding to the complete filling of the 1S(e) state. The temperature and electric field dependence of conductance in the film, measured between 4.3 and 135 K, were both well described by a variable range hopping model.

The seed-mediated growth of gold nanostructures is shown to be strongly dependent on the gold seed nanocrystal structure. The gold seed solutions can be prepared such that the seeds are either single crystalline or multiply twinned. With added silver(I) in the cetyltrimethylammonium bromide (CTAB) aqueous growth solutions, the two types of seeds yield either nanorods or elongated bipyramidal nanoparticles, in good yields. The gold nanorods are single crystalline, with a structure similar to those synthesized electrochemically (Yu, Y. Y. et al. J. Phys. Chem. B 1997, 101, 6661). In contrast, the gold bipyramids are pentatwinned. These bipyramids are strikingly monodisperse in shape. This leads to the sharpest ensemble longitudinal plasmon resonance reported so far for metal colloid solutions, with an inhomogeneous width as narrow as 0.13 eV for a resonance at approximately 1.5 eV. Ag(I) plays an essential role in the growth mechanism. Ag(I) slows down the growth of the gold nanostructures. Ag(I) also leads to high-energy side facets that are {110} for the single crystalline gold nanorods and unusually highly stepped {11n} (n approximately 7) for the bipyramid. To rationalize these observations, it is proposed that it is the underpotential deposition of Ag(I) that leads to the dominance of the facets with the more open surface structures. This forms the basis for the one-dimensional growth mechanism of single crystal nanorods, while it affects the shape of the nanostructures growing along a single twinning axis.

Charging of semiconductor nanoparticles is a basic issue that may be behind many unexplained effects including blinking, Stark effects and dipole moments. Spectroelectrochemistry has provided a deep insight into the possibility of redox processes involving the quantum states of nanocrystals. It is now possible to inject electrons in the conduction band of several quantum dot materials leading to switchable and reversible electrochromic changes in the visible and in the mid-IR spectral ranges. There remains still to explain the mechanism underlying the fluorescence quenching observed during charging.

We have measured nonlinear scattering from plasmons in individual Au nanorods and have correlated second-harmonic activity of Ag nanoparticles and clusters to morphology. The measurements reveal novel ultrafast nonlinear phenomena related to electron confinement.

We measure third-order optical nonlinearities of plasmons in single chemically-synthesized gold nanorods using a novel interferometric scattering technique. We observe ultrafast, coherent material nonlinearities that are enhanced by localization of light in the rods.

The intraband relaxation between the 1Pe and 1Se state of CdSe colloidal quantum dots is studied by pump-probe time-resolved spectroscopy. Infrared pump-probe measurements with approximately 6-ps pulses show identical relaxation whether the electron has been placed in the 1Se state by above band-gap photoexcitation or by electrochemical charging. This indicates that the intraband relaxation of the electrons is not affected by the photogenerated holes which have been trapped. However, the surface ligands are found to strongly affect the rate of relaxation in colloid solutions. Faster relaxation (8 ps) is obtained with phosphonic acid and oleic acid ligands. Alkylamines lead to longer relaxation times of approximately 10 ps and the slowest relaxation is observed for dodecanethiol ligands with relaxation times approximately 30 ps. It is concluded that, in the absence of holes or when the holes are trapped, the intraband relaxation is dominated by the surface and faster relaxation correlates with larger interfacial polarity. Energy transfer to the ligand vibrations may be sufficiently effective to account for the intraband relaxation rate.