Taleb Mokari

Lasing from Semiconductor Quantum Rods in a Cylindrical Microcavity

For transient ellipsometry, the sample is placed between crossed polarizers at an angle of 54 between the sample normal and the laser beam. A laser diode at a wavelength k = 905 nm is employed and the residual birefringence of the sam- ple is compensated by a crystal compensator. The transmission through the el- lipsometry setup is measured in response to a step function of the electric field applied to the sample with a rise time of ~100 ls. The two-wave mixing is measured in a standard geometry (29) with the two beams having external angles of 30 and 60 to the sample normal and beam ra- tio 1:1. The laser source is a Kr ion laser at a wavelength of 647 nm. The time transients are measured by opening a shutter for both beams with a switching time of about 150 ls. After the measurements, the gratings are erased by a larg- er, non-Bragg matched erasing beam. In the beam fanning experiments, the same setup as in the two-wave mixing is used except only one beam is present. The angle of the beam to the sample normal is 60 and the applied electric field is inverted compared to two-wave mixing to enhance the fanning. Additionally, an aperture is placed in the beam path 50 cm behind the sample, which clips only 5 % of the unperturbed beam.

Selective Growth of Metal and Binary Metal Tips on CdS Nanorods

Here, we demonstrate an approach for the selective growth of Pt, PtNi, and PtCo on CdS nanorods. The hybrid nanostructures prepared via an organometallic synthesis have promise for photocatalytic and magnetic applications.

Fluorescence quantum yield of CdSe/ZnS nanocrystals investigated by correlated atomic-force and single-particle fluorescence microscopy

Correlated atomic-force and fluorescence microscopy are used to study single-particle versus ensemble fluorescence quantum yields (QY) of semiconductor nanocrystals by measuring a simultaneous map of the topography and the single-particle fluorescence. CdSe/ZnS nanocrystal quantum dots and quantum rods with high QY were investigated. A significant portion of dark particles is detected. Comparison with the ensemble solution QY shows that samples with higher QY have a larger fraction of bright particles accompanied by an increased single-particle QY. Saturated emission from single nanocrystals could not be detected because of particle darkening under high-power excitation.

Size-dependent tunneling and optical spectroscopy of CdSe quantum rods

Photoluminescence excitation spectroscopy and scanning-tunneling spectroscopy are used to study the electronic states in CdSe quantum rods that manifest a transition from a zero-dimensional to a one-dimensional quantum-confined structure. Both optical and tunneling spectra show that the level structure depends primarily on the diameter of the rod and not its length. With increasing diameter, the band gap and the excited state level spacings shift to the red. The level structure was assigned using a multiband effective-mass model, showing a similar dependence on rod dimensions.

Enhanced Semiconductor Nanocrystal Conductance via Solution Grown Contacts

We report a 100000-fold increase in the conductance of individual CdSe nanorods when they are electrically contacted via direct solution phase growth of Au tips on the nanorod ends. Ensemble UV-vis and X-ray photoelectron spectroscopies indicate this enhancement does not result from alloying of the nanorod. Rather, low temperature tunneling and high temperature (250-400 K) thermionic emission across the junction at the Au contact reveal a 75% lower interface barrier to conduction compared to a control sample. We correlate this barrier lowering with the electronic structure at the Au-CdSe interface. Our results emphasize the importance of a nanocrystal surface structure for robust device performance and the advantage of this contact method.

Efficient Multiple Exciton Generation Observed in Colloidal PbSe Quantum Dots with Temporally and Spectrally Resolved Intraband Excitation

We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs.

Synthesis of metal sulfide nanomaterials via thermal decomposition of single-source precursors

In this report, we present a synthetic method for the formation of cuprous sulfide (Cu2S) and lead sulfide (PbS) nanomaterials directly on substrates from the thermolysis of single-source precursors. We find that the final morphology and arrangement of the nanomaterials may be controlled through the concentration of the dissolved precursors and choice of solvent. One-dimensional (1-D) morphologies may also be grown onto substrates with the addition of a metal catalyst layer through solution-liquid-solid (SLS) growth. These synthetic techniques may be expanded to other metal sulfide materials.

Studying the chemical, optical and catalytic properties of noble metal (Pt, Pd, Ag, Au)–Cu2O core–shell nanostructures grown via a general approach

We studied the chemical, optical and catalytic properties of metal (Pt, Pd, Ag, Au)–Cu2O core–shell nanoparticles grown via a simple and reproducible approach which utilizes aqueous-phase reactions at room temperature. We were able to control the thickness of the Cu2O shell and examine the effect of the cores shape and size on the structure and properties of the hybrid nanocrystals. We also studied the optical properties of the hybrid nanocrystals, in particular the effect of the Cu2O shell thickness on the frequency of the plasmon of gold nanorods. In addition, the catalytic activity of the hybrid nanostructures was examined by testing the reduction reaction of 4-nitrophenol with NaBH4. Finally, the hybrid metal–Cu2O nanostructures were used as templates to form the yolk–shell of metal–Cu2S materials. The interface and the crystalline structures of the four hybrid nanostructures were extensively characterized by high resolution transmission electron microscopy (HRTEM), energy-filtered TEM (EFTRM) and X-ray diffraction (XRD).

Caged Quantum Dots

Photoactivatable organic fluorophores and fluorescent proteins have been widely adopted for cellular imaging and have been critical for increasing temporal and spatial resolution, as well as for the development of superresolution microscopy techniques. At the same time, semiconducting nanocrystal quantum dots (QDs) have shown superior brightness and photostability compared to both organic fluorophores and proteins. As part of our efforts to develop nanoparticles with novel optical properties, we have synthesized caged quantum dots, which are nonluminescent under typical microscopic illumination but can be activated with stronger pulses of UV light. We show that ortho-nitrobenzyl groups efficiently quench QDs of different compositions and emissions and can be released from the nanoparticle surface with UV light, both in solution and in live cells. This caging is dependent on the emission of the QD, but it is effective through the visible spectrum into the nIR, offering a large array of new colors for photoactivatable probes. Like organic and protein-based photoactivatable probes, caged QDs can confer increased spatial and temporal resolution, with the added brightness and photostability of QDs.

Synthesis of Lead Chalcogenide Alloy and Core–Shell Nanowires†

Control over the dimensions and shape of nanostructures represents one of the main challenges in modern materials science. Morphology control of a variety of materials can be achieved using vapor–liquid–solid or solution–liquid–solid techniques to obtain one-dimensional (1D) systems. The unique optical and electrical properties of 1D nanostructures make them one of most important building blocks for nanoscience and nanotechnology applications, and provide the opportunity for their integration in electronic, photonic, thermoelectric, and sensor-based devices. Size control has been traditionally important and necessary to tune the optical and electrical properties of nanomaterials by changing the band gap. This is particularly important in the strong confinement region, where one of the dimensions is smaller than the corresponding excitonic Bohr diameter. Semiconductor alloy and core–shell nanowire systems represent another interesting direction towards functional nanostructures with enhanced structural and property tunability. Herein, we focus on preparing novel 1D heterostructures of IV–VI semiconductor nanomaterials. Lead chalcogenides are known to be good materials for thermoelectrics due to their low thermoconductivity. Pseudobinary (e.g. PbSeTe) and pseudoternary alloys (e.g. PbSnSeTe) have even lower lattice thermal conductivities than the binary compounds due to disorder-induced phonon scattering processes. Lead chalcogenide materials are also good candidates for multiexciton-generation (MEG) solar cells. For example, previous reports showed quantum efficiencies as high as 300% and 700% for PbSe nanoparticles. Heterostructured alloy and core–shell nanomaterials have previously been shown for various materials, mainly II–VI semiconductor nanocrystals. For example, a quasi 1D system of CdSe–ZnS has been reported, other systems include PbSe–PbS core–shell and alloy spherical nanoparticles developed by Lifshitz and co-workers. In addition, Talapin et al. have demonstrated the growth of PbS and Au onto PbSe nanowires. The physical properties of these heterostructured nanosystems are of interest for various applications as shown by the electronic structure calculations carried out by different groups. Here we demonstrate the formation of lead chalcogenide heterostructure nanowires by a solution-phase synthesis at moderate temperatures (see the Experimental Section). Two types of heterostructures (alloy and core–shell) were prepared by changing the concentration and temperature of the reaction. We were able to control the composition of the alloy and the thickness of the shell by changing the growth parameters. Three different systems, PbSexS1 x alloys, and PbSe–PbS and PbSe–PbTe core–shell nanowires were prepared. Achieving these three targeted structures is nontrivial due to various competitive processes such as ripening and formation of pure PbS (PbTe) nanoparticles. The synthesis of PbSe nanowires is based on a previous report by Murray and co-workers. The same procedure was used to prepare the PbSe nanowires used here as templates for further growth to give the alloy and core–shell nanostructures. The diameter of the core nanowires could be controlled and varied from 4 nm up to 100 nm, with a length of a few tens of micrometers. The PbSe nanowires (Figure 1A) were used as templates to form PbSexS1 x alloy wires. Figure 1B shows PbSe0.4S0.6 alloy nanowires that were prepared by the slow addition of Pb and S precursors to a hot solution containing PbSe nanowires. (a detailed description of the synthesis can be found in the Experimental Section). The diameter of the alloy nanowires increased from 6 nm (pure PbSe nanowires) to ca. 10 nm, indicating the incorporation of additional material into the nanowires. Structural characterization of the alloy system was carried out using various methods as shown in Figure 1. Figure 1D shows a high-resolution transmission electron microscopy (HRTEM) image of the PbSe0.4S0.6 nanowires. The latticeresolved image indicates that the nanowires are growing along the h100i direction. X-ray diffraction (XRD) measurements of the alloy nanowires are shown in Figure 1C. The pattern can be indexed to a structure intermediate between the cubic PbSe and cubic PbS bulk phases, which strongly supports the formation of an alloyed structure. An energydispersive X-ray (EDX) spectrum (Figure 1E) taken on a small area of the alloy nanowire, shown in Figure 1D, indicates the presence of Se from the original PbSe nanowires, Pb from the original and added materials, and Cu from the TEM grid. However, due to overlap between the Pb and S peaks, electron energy loss spectroscopy (EELS) was necessary to detect the incorporation of S. The energy loss peak for S was observed at 165 eV (Figure 1F), providing clear evidence for the existence of S in the alloy nanowires. The EDX and EELS spectra were taken from the same area of the nanowire shown in Figure 1D. Tuning the alloy composition can be achieved by simply controlling the reaction conditions. For example, altering the S concentration will act to tune the alloy composition. The actual composition was determined by [*] Dr. T. Mokari, S. E. Habas, Prof. P. Yang Department of Chemistry, University of California Berkeley, CA 94720 (USA) Fax: (+1)510-642-7301 E-mail: p_yang@berkeley.edu