Publications

2006
Dov Steiner, Assaf Aharoni, Uri Banin, and Oded Millo. 2006. “Level structure of InAs quantum dots in two-dimensional assemblies.” Nano letters, 6, 10, Pp. 2201-2205. Publisher's Version Abstract

The electronic level structure of colloidal InAs quantum dots (QDs) in two-dimensional arrays, forming a QD-solid system, was probed using scanning tunneling spectroscopy. The band gap is found to reduce compared to that of the corresponding isolated QDs. Typically, the electron (conduction-band) ground state red shifts more than the hole (valence-band) ground state. This is assigned to the much smaller effective mass of the electrons, resulting in stronger electron delocalization and larger coupling between electron states of neighboring QDs compared to the holes. This is corroborated by comparing these results with those for InAs and CdSe nanorod assemblies, manifesting the effects of the electron effective mass and arrangement of nearest neighbors on the band gap reduction. In addition, in InAs QD arrays, the levels are broadened, and in some cases their discrete level structure was nearly washed out completely and the tunneling spectra exhibited a signature of two-dimensional density of states.

Dan Oron, Miri Kazes, Itzik Shweky, and Uri Banin. 2006. “Multiexciton spectroscopy of semiconductor nanocrystals under quasi-continuous-wave optical pumping.” Physical Review B, 74, 11, Pp. 115333. Publisher's Version Abstract

We present a method for performing multiexciton spectroscopy in colloidal semiconductor nanocrystals. In this regime multiexcitonic states are generated sequentially via a “ladder-climbing” mechanism. The distribution of multiexcitonic states, determined by a steady-state rate equation, adiabatically follows the illumination intensity, allowing for “slow” detection of multiexcitonic spectra. Emission spectra of multiexcitonic states with a small number of excitons are obtained without passing through states with a large number of excitons. This is in contrast with impulsive excitation schemes utilizing picosecond pulses, where in order to significantly populate a given multiexcitonic state, many of the dots necessarily pass through states with a larger number of excitons due to the Poissonian distribution of the number of absorbed photons. In particular, we observe directly the order of appearance of the various multiexcitonic peaks. This enables us to determine the threshold conditions for Auger ionization, shedding more light on the nature of this process. We are also able to observe the short-lived excitations at higher energies than triexcitons in CdSe quantum dots. Finally, we demonstrate bi- and triexcitonic optical gain in a close-packed film under quasi-continuous-wave pumping.

Ron Gill, Ronit Freeman, Jian-Ping Xu, Itamar Willner, Shira Winograd, Itzik Shweky, and Uri Banin. 2006. “Probing Biocatalytic Transformations with CdSe− ZnS QDs.” Journal of the American Chemical Society, 128, 48, Pp. 15376-15377. Publisher's Version Abstract

CdSe/ZnS QDs enable the optical probing of the biocatalytic oxidation of tyrosine derivatives and of the scission of peptides by thrombin. CdSe/ZnS QDs were modified with tyrosine methyl ester or with a tyrosine-containing peptide. The tyrosine units were reacted with tyrosinase/O2 to yield the respective l-DOPA and quinone derivatives. The luminescence of QDs modified by the enzyme-generated quinone units is quenched. The quinone-functionalized peptide associated with the QDs was cleaved by thrombin, a process that restored the luminescence of the QDs.

Dong Hee Son, Joshua S Wittenberg, Uri Banin, and Paul A Alivisatos. 2006. “Second harmonic generation and confined acoustic phonons in highly excited semiconductor nanocrystals.” The Journal of Physical Chemistry B, 110, 40, Pp. 19884-19890. Publisher's Version Abstract

The photo-induced enhancement of second harmonic generation and the effect of nanocrystal shape and pump intensity on confined acoustic phonons in semiconductor nanocrystals have been investigated with time-resolved scattering and absorption measurements. The second harmonic signal showed a sublinear increase of the second-order susceptibility with respect to the pump pulse energy, indicating a reduction of the effective one-electron second-order nonlinearity with increasing electron−hole density in the nanocrystals. The coherent acoustic phonons in spherical and rod-shaped semiconductor nanocrystals were detected in a time-resolved absorption measurement. Both nanocrystal morphologies exhibited oscillatory modulation of the absorption cross section, the frequency of which corresponded to their coherent radial breathing modes. The amplitude of the oscillation also increased with the level of photoexcitation, suggesting an increase in the amplitude of the lattice displacement as well.

Yuval Ebenstein, Eyal Yoskovitz, Ronny Costi, Assaf Aharoni, and Uri Banin. 2006. “Interaction of scanning probes with semiconductor nanocrystals; physical mechanism and basis for near-field optical imaging.” The Journal of Physical Chemistry A, 110, 27, Pp. 8297-8303. Publisher's Version Abstract

We investigate the modification of photoluminescence (PL) from single semiconductor nanocrystal quantum dots (NCs) in the proximity of metal and semiconducting atomic force microscope (AFM) tips. The presence of the tip alters the radiative decay rate of an emitter via interference and opens efficient nonradiative decay channels via energy transfer to the tip material. These effects cause quenching (or enhancement) of the emitter's PL intensity as a function of its distance from the interacting tip. We take advantage of this highly distance-dependent effect to realize a contrast mechanism for high-resolution optical imaging. AFM tips are optimized as energy acceptors by chemical functionalization with InAs NCs to achieve optical resolution down to 30 nm. The presented experimental scheme offers high-resolution optical information while maintaining the benefits of traditional AFM imaging. We directly measure the PL intensity of single NCs as a function of the tip distance. Our results are in good agreement with calculations made by a classical theoretical model describing an oscillating dipole interacting with a planar mirror.

Itzhak Shweky, Assaf Aharoni, Taleb Mokari, Eli Rothenberg, Moshe Nadler, Inna Popov, and Uri Banin. 2006. “Seeded growth of InP and InAs quantum rods using indium acetate and myristic acid.” Materials Science and Engineering: C, 26, 5-7, Pp. 788-794. Publisher's Version Abstract

A synthesis of soluble III–V semiconductor quantum rods using gold nanoparticles to direct and catalyze one-dimensional growth isdeveloped. The growth takes place via the solution–liquid–solid (SLS) mechanism where proper precursors are injected into a coordinatingsolvent. We report the synthesis of InP nanorods using indium acetate and myristic acid with gold nanoparticles as the catalysts in the SLS growthmode. A similar route was successfully developed for the growth of InAs nanorods. We find that the amount of Au catalyst in the reaction is animportant parameter to achieve shape control. Transmission electron microscope (TEM) images of InP and InAs nanocrystals revealed that thecrystals are mostly rod-shaped, and provide strong evidence for Au presence in one edge. The rods were characterized structurally using X-raydiffraction and high-resolution TEM and optically by absorption and photoluminescence.D2005 Elsevier B.V. All rights reserved.

Aaron E Saunders, Inna Popov, and Uri Banin. 2006. “Synthesis of hybrid CdS− Au colloidal nanostructures.” The Journal of Physical Chemistry B, 110, 50, Pp. 25421-25429. Publisher's Version Abstract

We explore the growth mechanism of gold nanocrystals onto preformed cadmium sulfide nanorods to form hybrid metal nanocrystal/semiconductor nanorod colloids. By manipulating the growth conditions, it is possible to obtain nanostructures exhibiting Au nanocrystal growth at only one nanorod tip, at both tips, or at multiple locations along the nanorod surface. Under anaerobic conditions, Au growth occurs only at one tip of the nanorods, producing asymmetric structures. In contrast, the presence of oxygen and trace amounts of water during the reaction promotes etching of the nanorod surface, providing additional sites for metal deposition. Three growth stages are observed when Au growth is performed under air:  (1) Au nanocrystal formation at both nanorod tips, (2) growth onto defect sites on the nanorod surface, and finally (3) a ripening process in which one nanocrystal tip grows at the expense of the other particles present on the nanorod. Analysis of the hybrid nanostructures by high-resolution TEM shows that there is no preferred orientation between the Au nanocrystal and the CdS nanorod, indicating that growth is nonepitaxial. The optical signatures of the nanocrystals and the nanorods (i.e., the surface plasmon and first exciton transition peaks, respectively) are spectrally distinct, allowing the different stages of the growth process to be easily monitored. The initial CdS nanorods exhibit band gap and trap state emission, both of which are quenched during Au growth.

2005
Eli Rothenberg, Miri Kazes, Ehud Shaviv, and Uri Banin. 2005. “Electric field induced switching of the fluorescence of single semiconductor quantum rods.” Nano letters, 5, 8, Pp. 1581-1586. Publisher's Version Abstract

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The exceptional fluorescence properties of single CdSe quantum rods (QRs) arising from internal and external electric fields are studied. Reversible external field induced switching of the emission in single QRs is reported for the first time. This effect was correlated with local field induced emission intensity reduction and newly observed darkening mechanism. Bimodal spectral jumps under a zero field were also observed and assigned to charged exciton emission, a phenomenon that was likewise directly controlled through an external field. These phenomena point to the use of single QRs as spectrally tunable charge sensitive fluorophores with polarized emission in fluorescence tagging and optical switching applications.

Taleb Mokari, Claudia G Sztrum, Asaf Salant, Eran Rabani, and Uri Banin. 2005. “Formation of asymmetric one-sided metal-tipped semiconductor nanocrystal dots and rods.” Nature Materials, 4, 11, Pp. 855-863. Publisher's Version Abstract

Multicomponent nanoparticles represent a new approach for creating smart materials, requiring the development of the growth of different material types on one particle. Here, we report the synthesis of asymmetric metal–semiconductor heterostructures where gold is grown on one side of CdSe nanocrystal quantum rods and dots, creating nanostructures offering intrinsic asymmetry for diverse device functionalities such as diode elements, along with one-sided chemical accessibility through the gold tips. Surprisingly, one-sided growth is preceded by two-sided growth and is generally observed in different particle shapes. Theoretical modelling in a lattice-gas model and experimental analysis show that a ripening process drives gold from one end to the other, transforming two-sided growth to one-sided growth. Ripening is therefore occurring on the nanostructure itself, leading to a phase-segregated structure. This thereby extends the realm of ripening phenomena and their significance in nanostructure synthesis, in particular for nanocrystals composed of different materials.

Dan T Fuchs, Ronen Rapaport, Gang Chen, Ylva K Olsson, Vikram C Sundar, L Lucas, S Vilan, Assaf Aharoni, Uri Banin, and Jonathan S Steckel. 2005. “Making waveguides containing nanocrystalline quantum dots”. Publisher's Version Abstract

A new material platform is described that enables inclusion of nanocrystalline quantum dots into a polymer. This technology is compatible with semiconductor processing and may enable integration of active materials into current waveguide technologies. We describe the steps preformed to fabricate a waveguide chip that contains IR-emitting quantum dots. Optical tests demonstrate guiding and preservation of the quantum dots optical properties through the processing steps. Time resolved optical measurements indicate presence of gain in the InAs quantum dot impregnated polymer.

Gang Chen, Ronen Rapaport, Dan T Fuchs, Leah Lucas, Andrew J Lovinger, Sahar Vilan, Assaf Aharoni, and Uri Banin. 2005. “Optical gain from InAs nanocrystal quantum dots in a polymer matrix.” Applied Physics Letters, 87, 25, Pp. 251108. Publisher's Version Abstract

We report on the observation of optical gain from InAs nanocrystal quantum dots which emit at 1.55microns1.55microns and are imbedded in a novel polymer platform. The measurements are based on a three-beam time resolved pump-probe technique, which enables extracting the intrinsic gain cross section, lifetime, and recovery time. These experiments are another step toward the realization of active optical devices based on InAs nanocrystals.

Olga Solomeshch, Ariel Kigel, Aldona Saschiuk, Vlad Medvedev, Assaf Aharoni, Alexey Razin, Yoav Eichen, Uri Banin, Efrat Lifshitz, and Nir Tessler. 2005. “Optoelectronic properties of polymer-nanocrystal composites active at near-infrared wavelengths.” Journal of Applied Physics, 98, 7, Pp. 074310. Publisher's Version Abstract

We report a systematic study of the optoelectronic processes occurring in composites made of near-infrared (IR) emitting nanocrystals and conjugated polymers. We focus on PbSe and InAs/ZnSe blended with polyphenylenevinylene-type polymers. We find that the process responsible for quenching the visible luminescence of the polymer by the nanocrystal varies depending on the nanocrystal composite. Moreover, the high (66%) energy-transfer efficiency from the polymer to the PbSe nanocrystal does result in significant emission at the near IR. Our measurements suggest that the host may be doping the PbSe nanocrystal, thus making the nonradiative Auger process favorable. For InAs we find the energy levels well aligned inside the polymer band gap, making it an efficient charge trap which acts as a luminescence center. Through two-dimensional numerical modeling of the charge transport in such composite films we highlight the importance of morphology (nanocrystal distribution) control.

Ron Gill, Itamar Willner, Itzhak Shweky, and Uri Banin. 2005. “Fluorescence resonance energy transfer in CdSe/ZnS− DNA conjugates: probing hybridization and DNA cleavage.” The Journal of Physical Chemistry B, 109, 49, Pp. 23715-23719. Publisher's Version Abstract

Nucleic-acid-functionalized CdSe/ZnS quantum dots (QDs) were hybridized with the complementary Texas-Red-functionalized nucleic acid. The hybridization was monitored by following the fluorescence resonance energy transfer from the QDs to the dye units. Treatment of the QD/dye DNA duplex structure with DNase I resulted in the cleavage of the DNA and the recovery of the fluorescence properties of the CdSe/ZnS QDs. The luminescence properties of the QDs were, however, only partially recovered due to the nonspecific adsorption of the dye onto the QDs. Similarly, nucleic-acid-functionalized Au nanoparticles (Au NPs) were hybridized with the complementary Texas-Red-labeled nucleic acid. The hybridization was followed by the fluorescence quenching of the dye by the Au NPs. Treatment of the Au NP/dye DNA duplex with DNase I resulted in the cleavage of the DNA and the partial recovery of the dye fluorescence. The incomplete recovery of the dye fluorescence originated from the nonspecific binding of the dye units to the Au NPs. The nonspecific binding of the dye to the CdSe/ZnS QDs and the Au NPs is attributed to nonprotected surface vacancies in the two systems.

Taleb Mokari, Claudia G Sztrum, Asaf Salant, Eran Rabani, and Uri Banin. 2005. “Formation of asymmetric one-sided metal-tipped semiconductor nanocrystal dots and rods.” Nature Materials, 4, 11, Pp. 855. Publisher's Version Abstract

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Multicomponent nanoparticles represent a new approach for creating smart materials, requiring the development of the growth of different material types on one particle. Here, we report the synthesis of asymmetric metal–semiconductor heterostructures where gold is grown on one side of CdSe nanocrystal quantum rods and dots, creating nanostructures offering intrinsic asymmetry for diverse device functionalities such as diode elements, along with one-sided chemical accessibility through the gold tips. Surprisingly, one-sided growth is preceded by two-sided growth and is generally observed in different particle shapes. Theoretical modelling in a lattice-gas model and experimental analysis show that a ripening process drives gold from one end to the other, transforming two-sided growth to one-sided growth. Ripening is therefore occurring on the nanostructure itself, leading to a phase-segregated structure. This thereby extends the realm of ripening phenomena and their significance in nanostructure synthesis, in particular for nanocrystals composed of different materials.

Oded Millo, Dov Steiner, David Katz, Assaf Aharoni, ShiHai Kan, Taleb Mokari, and Uri Banin. 2005. “Transition from zero-dimensional to one-dimensional behavior in InAs and CdSe nanorods.” Physica E: Low-dimensional Systems and Nanostructures, 26, 1-4, Pp. 1-8. Publisher's Version Abstract

Tunneling and optical spectroscopy performed on InAs nanorods 7–25 nm long, reveal a clear dependence of the band gap on length. This (zero-dimension like) behavior is different from that of CdSe rods, where the band gap is nearly independent of length, a signature of quasi-one-dimensionality. The transition between these two regimes is governed by the ratio between the Bohr radius and the nanorods length. The gaps measured by tunneling spectroscopy are larger than the optical gaps by a factor that depends on the tunneling configuration. This is attributed to a combination of the Coulomb interaction and the voltage division between the two tunnel junctions in the STM experiment. However, the tunneling gaps were found to reduce in dense aggregates of rods.

2004
Miri Kazes, David Y Lewis, and Uri Banin. 2004. “Method for Preparation of Semiconductor Quantum‐Rod Lasers in a Cylindrical Microcavity.” Advanced Functional Materials, 14, 10, Pp. 957-962. Publisher's Version Abstract

An efficient method for preparation of semiconductor quantum rod films for robust lasing in a cylindrical microcavity is reported. A capillary tube, serving as the laser cavity, is filled with a solution of nanocrystals and irradiated with a series of intense nanosecond laser pulses to produce a nanocrystal film on the capillary surface. The films exhibit intense room‐temperature lasing in whispering‐gallery modes that develop at the film–capillary interface as corroborated from the spacing detected for the lasing modes. Good lasing stability is observed at moderate pump powers. The method was applied successfully to several quantum‐rod samples of various sizes.

Eyal Nahum, Yuval Ebenstein, Assaf Aharoni, Taleb Mokari, Uri Banin, Nira Shimoni, and Oded Millo. 2004. “Transport and charging in single semiconductor nanocrystals studied by conductance atomic force microscopy.” Nano Letters, 4, 1, Pp. 103-108. Publisher's Version Abstract

Electrical transport measurements through single InAs and CdSe semiconductor nanocrystals embedded in a thin polymer film were performed using conductance atomic force microscopy. The current and topography images showed excellent correlation, where current was detected only over the nanocrystals. A rapid current decay in consecutive scans was observed for positive sample bias, while remaining intact at negative bias. This current decay was accompanied by bias-dependent changes in the height of the nanocrystals. These phenomena, which were not observed for gold nanocrystals, are attributed to long-sustained charging of the nanocrystals.

Eli Rothenberg, Yuval Ebenstein, Miri Kazes, and Uri Banin. 2004. “Two-photon fluorescence microscopy of single semiconductor quantum rods: Direct observation of highly polarized nonlinear absorption dipole.” The Journal of Physical Chemistry B, 108, 9, Pp. 2797-2800. Publisher's Version Abstract

Two-photon polarization fluorescence microscopy is used to study the nature of the emission and nonlinear absorption dipole of single CdSe/ZnS quantum rods. Rods showed strongly polarized nonlinear excitation with sharp angular dependence, following a cos4(φ) functional form, in agreement with the predicted two-photon absorption process. The two-photon absorption is parallel to the emission polarization and allows high orientation selectivity in excitation to be achieved. This further demonstrates the role of single molecule measurements in unraveling basic principles of light−matter interactions otherwise masked by ensemble averaging.

Dov Steiner, David Katz, Oded Millo, Assaf Aharoni, ShiHai Kan, Taleb Mokari, and Uri Banin. 2004. “Zero-dimensional and quasi one-dimensional effects in semiconductor nanorods.” Nano Letters, 4, 6, Pp. 1073-1077. Publisher's Version Abstract

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Scanning tunneling microscopy (STM) and optical spectroscopy measurements were performed on InAs nanorods 7 to 25 nm long. Both methods reveal a clear dependence of the band-gap on length, with a red shift for longer rods. This (zero-dimension like) behavior is different than that of CdSe rods, where the band-gap is nearly insensitive to length, a signature of quasi one-dimensionality. The transition between these two regimes is governed by the ratio between the Bohr radius and the nanorod length. The gaps measured by tunneling spectroscopy are larger than the optical gaps by a factor that depends on the tunneling configuration. This is attributed to a combination of the Coulomb interaction and the voltage division between the two tunnel junctions in the STM experiment.

The present invention provides a new method for the production of inorganic semiconductor nanocrystals having a rod-like shape. More specifically the present invention provides a method of synthesizing rod shaped Group III-V semiconductor nanocrystals. The method comprises: reacting, in a high-boiling point organic solvent, a two-source precursor solution comprising at least one metal source and at least one nonmetal source, or a single-source precursor solution, with a metal catalyst or an agent capable of producing said metal catalyst, said high-boiling point organic solvent having a temperature above 200° C., thereby forming a reaction product comprising semiconductor nanocrystals of various shape; cooling the reaction product, and subsequently exposing said cooled reaction product to at least one centrifugal step so as to obtain semiconductor nanocrystals having substantially rod-like shape.

The rod-shaped nanocrystals obtained by the method of the invention usually have organic ligands as a coating on their outer surfaces. Such organic ligands affect the solubility of the particles and may be substituted or removed, according to the application intended for said particles after the reaction is completed.