Publications

2009
Miri Kazes, Tsiala Saraidarov, Renata Reisfeld, and Uri Banin. 2009. “Organic–inorganic sol–gel composites incorporating semiconductor nanocrystals for optical gain applications.” Advanced Materials, 21, 17, Pp. 1716-1720. Publisher's Version Abstract

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Organic‐inorganic sol‐gel composite incorporating highly emitting CdSe/ZnS quantum dots and CdSe/CdS/ZnS quantum rods is reported. Optical gain measurements conducted on a quantum rod/sol‐gel composite film show stable room temperature amplified spontaneous emission under ambient conditions.

Ehud Shaviv, Asaf Salant, and Uri Banin. 2009. “Size dependence of molar absorption coefficients of CdSe semiconductor quantum rods.” ChemPhysChem, 10, 7, Pp. 1028-1031. Publisher's Version Abstract

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Fundamental properties: The molar absorption coefficients of CdSe quantum rods are determined experimentally as a function of their dimensions (see figure). Far above the band gap a simple dependence on volume is seen. The behavior at the band gap manifests a concentration of oscillator strength with decreased diameter in agreement with strong quantum confinement behavior.

Meike L Schipper, Gopal Iyer, Ai Leen Koh, Zhen Cheng, Yuval Ebenstein, Assaf Aharoni, Shay Keren, Laurent A Bentolila, Jianquing Li, and Jianghong Rao. 2009. “Particle size, surface coating, and PEGylation influence the biodistribution of quantum dots in living mice.” Small, 5, 1, Pp. 126-134. Publisher's Version Abstract

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This study evaluates the influence of particle size, PEGylation, and surface coating on the quantitative biodistribution of near-infrared-emitting quantum dots (QDs) in mice. Polymer- or peptide-coated 64Cu-labeled QDs 2 or 12 nm in diameter, with or without polyethylene glycol (PEG) of molecular weight 2000, are studied by serial micropositron emission tomography imaging and region-of-interest analysis, as well as transmission electron microscopy and inductively coupled plasma mass spectrometry. PEGylation and peptide coating slow QD uptake into the organs of the reticuloendothelial system (RES), liver and spleen, by a factor of 6-9 and 2-3, respectively. Small particles are in part renally excreted. Peptide-coated particles are cleared from liver faster than physical decay alone would suggest. Renal excretion of small QDs and slowing of RES clearance by PEGylation or peptide surface coating are encouraging steps toward the use of modified QDs for imaging living subjects.

2008
Dov Steiner, Dirk Dorfs, Uri Banin, Fabio Della Sala, Liberato Manna, and Oded Millo. 2008. “Determination of band offsets in heterostructured colloidal nanorods using scanning tunneling spectroscopy.” Nano letters, 8, 9, Pp. 2954-2958. Publisher's Version Abstract

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The ability to tailor the properties of semiconductor nanocrystals through creating core/shell heterostructures is the cornerstone for their diverse application in nanotechnology. The band-offsets between the heterostructure components are determining parameters for their optoelectronic properties, dictating for example the degree of charge-carrier separation and localization. So far, however, no method was reported for direct measurement of these factors in colloidal nanocrystals and only indirect information could be derived from optical measurements. Here we demonstrate that scanning tunneling spectroscopy along with theoretical modeling can be used to determine band-offsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type I band structure where both the hole and electron ground state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization. The generality of the approach is further demonstrated in ZnSe/CdS nanocrystals where their type II band alignment, leading to electron−hole separation, is manifested.

Einat Elmalem, Aaron E Saunders, Ronny Costi, Asaf Salant, and Uri Banin. 2008. “Growth of photocatalytic CdSe–Pt nanorods and nanonets.” Advanced Materials, 20, 22, Pp. 4312-4317. Publisher's Version Abstract

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A hybrid Pt‐CdSe nanoparticle system is prepared in aqueous media, displaying rod or nanonet structures depending on the pH. The new hybrids display morphology‐dependent photocatalytic activity under visible light irradiation. Separation between photo‐generated charge carriers occurs at the metal/semiconductor interface (see figure), allowing redox reactions to take place at the nanoparticle surface.

Uri Banin. 2008. “Light-emitting diodes: Bright and stable.” Nature Photonics, 2, 4, Pp. 209. Publisher's Version Abstract

The use of inorganic charge transport layers has enabled the fabrication of bright, environmentally stable LEDs that are based on electrically pumped colloidal solutions of quantum dots.

Assaf Aharoni, Dan Oron, Uri Banin, Eran Rabani, and Joshua Jortner. 2008. “Long-range electronic-to-vibrational energy transfer from nanocrystals to their surrounding matrix environment.” Physical review letters, 100, 5, Pp. 057404. Publisher's Version Abstract

A radiationless transition process of long-range, resonance interconversion of electronic-to-vibrational energy transfer (EVET) is discovered between the band-gap excitation of nanocrystal quantum dots to matrix vibrational overtone modes using fluorescence lifetime measurements. A theoretical analysis based on long-range dipole-dipole nonadiabatic couplings, being distinct from the traditional adiabatic or "static-coupling" pictures, is given and is in qualitative agreement with experiments. EVET should be considered in matrix choices for near-infrared optoelectronic applications of nanocrystals.

Ronny Costi, Aaron E Saunders, Einat Elmalem, Asaf Salant, and Uri Banin. 2008. “Visible light-induced charge retention and photocatalysis with hybrid CdSe− Au nanodumbbells.” Nano Letters, 8, 2, Pp. 637-641. Publisher's Version Abstract

Visible light photocatalysis is a promising route for harnessing of solar energy to perform useful chemical reactions and to convert light to chemical energy. Nanoscale photocatalytic systems used to date were based mostly on oxide semiconductors aided by metal deposition and were operational only under UV illumination. Additionally, the degree of control over particle size and shape was limited. We report visible light photocatalysis using highly controlled hybrid gold-tipped CdSe nanorods (nanodumbbells). Under visible light irradiation, charge separation takes place between the semiconductor and metal parts of the hybrid particles. The charge-separated state was then utilized for direct photoreduction of a model acceptor molecule, methylene blue, or alternatively, retained for later use to perform the reduction reaction in the dark.

Eyal Yoskovitz, Dan Oron, Itzhak Shweky, and Uri Banin. 2008. “Apertureless near-field distance-dependent lifetime imaging and spectroscopy of semiconductor nanocrystals.” The Journal of Physical Chemistry C, 112, 42, Pp. 16306-16311. Publisher's Version Abstract

Apertureless near-field scanning optical microscopy, along with time-resolving capabilities, is used to produce optical imaging and spectroscopy measurements of single-semiconductor nanocrystals, in correlation with the AFM topography scan. The strongly distance-dependent energy transfer between the excited particle and silicon or metallic-coated AFM tips provides a contrast mechanism for subdiffraction-limited optical imaging. Fluorescence lifetime optical images show excellent contrast, sharpness, sensitivity, and resolution equivalent to that of the AFM topography images (sub 20 nm) and significantly improved over fluorescence intensity images. The sharper resolution of lifetime images is consistent with model predictions of energy transfer between an emitting dipole and a dielectric surface. Lifetime images also enable resolving multiple emitters located in the excitation spot. The comprehensive time and distance dependent data is used to study the imaging mechanism and the properties of silicon tips and platinum-coated tips as energy acceptors and quenchers. The findings provide a basis for use of lifetime imaging, in conjunction with apertureless near field microscopy, for simultaneous high-resolution topography and optical imaging.

Orly Kletenik-Edelman, Elina Ploshnik, Asaf Salant, Roy Shenhar, Uri Banin, and Eran Rabani. 2008. “Drying-mediated hierarchical self-assembly of nanoparticles: A dynamical coarse-grained approach.” The Journal of Physical Chemistry C, 112, 12, Pp. 4498-4506. Publisher's Version Abstract

16. Rabani JPCC 2008

A coarse-grained lattice gas model is developed to study the drying-mediated self-assembly of nanoparticles on diblock copolymer substrates. The model describes the nanoparticles, the solvent and the diblock copolymer on length scales that are typical to the solvent bulk correlation length. Monte Carlo simulation techniques are used to delineate the various mechanisms of this out-of-equilibrium hierarchical self-assembly. Several different assembly scenarios corresponding to different selectivity of the nanoparticles/liquid/substrate are discussed. The role of surface tension, evaporation rate, diffusion rate, nanoparticle coverage, and diblock copolymer periodicity is explored. Optimal conditions to form a stripe phase of nanoparticles along with predictions of novel 3D structures resulting from high nanoparticle and solvent selectivity are described.

Dov Steiner, Doron Azulay, Assaf Aharoni, Assaf Salant, Uri Banin, and Oded Millo. 2008. “Electronic structure and self-assembly of cross-linked semiconductor nanocrystal arrays.” Nanotechnology, 19, 6, Pp. 065201. Publisher's Version Abstract

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We studied the electronic level structure of assemblies of InAs quantum dots and CdSe nanorods cross-linked by 1,4-phenylenediamine molecules using scanning tunneling spectroscopy. We found that the bandgap in these arrays is reduced with respect to the corresponding ligand-capped nanocrystal arrays. In addition, a pronounced sub-gap spectral structure commonly appeared which can be attributed to unpassivated nanocrystal surface states or associated with linker-molecule-related levels. The exchange of the ligands by the linker molecules also affected the structural array properties. Most significantly, clusters of close-packed standing CdSe nanorods were formed.

2007
Uri Banin. 2007. “Nanocrystals: Tiny seeds make a big difference.” Nature materials, 6, 9, Pp. 625. Publisher's Version Abstract

A study was to demonstrate a seeded growth approach that provides shape-controlled bimetallic nanocrystals, generating new ways for selecting new nanoscale building blocks. It was demonstrated that semiconductor and oxide nanowires were grown, using metal seeding approach that acted as catalysts. Gold rod growth in solution was also carried out by seeding with small gold particles, while semiconductor rods were grown using semiconductor nanoparticles as seeds. The new approach also used faceted metal nanocrystals as nucleation centers for the overgrowth of a secondary metal. It was also observed that the ability of the new approach to synthesize binary-metal nanoparticles has significant implications for diverse applications in memory devices, electrical contacts, biological applications, and catalysis.

Miri Kazes, Dan Oron, Itzhak Shweky, and Uri Banin. 2007. “Temperature dependence of optical gain in CdSe/ZnS quantum rods.” The Journal of Physical Chemistry C, 111, 22, Pp. 7898-7905. Publisher's Version Abstract

We studied the optical gain characteristics of CdSe/ZnS core/shell colloidal quantum rods, investigated their temperature dependence, and compared the gain properties with quantum dots (QD). The gain was measured systematically for close-packed films of rods and dots under quasi-CW nanosecond optical pumping, using the variable stripe length method measuring the amplified spontaneous emission (ASE). Tunable ASE can be achieved by changing the rod diameter. Optical gain factors of up to 350 cm-1 at a temperature range of 10−120 K were measured for quantum rods. Above 120 K, the gain decreased sharply, but by increasing the pump power, ASE was easily achieved also at room temperature. The temperature dependence was assigned to the Auger heating process and phonon assisted thermal relaxation. QD of similar diameters as the rods showed much smaller gain values (∼50 cm-1) and a sharp decrease in gain at lowered temperatures (∼50 K), and ASE could not be detected at room temperature even at high pump powers. The significantly improved gain values in quantum rods as compared with dots were attributed to the slower Auger relaxation rates, the higher absorption cross-section, and the reduced self-absorption due to the larger Stokes shift. The temperature dependence of the threshold power for the quantum rods, used to characterize the thermal insensitivity of the system, showed two distinct temperature regions. In the low-temperature region, a very high T0 value of 3500 K was measured, as predicted for a low-dimensional quantum confined system.

Dan Oron, Miri Kazes, and Uri Banin. 2007. “Multiexcitons in type-II colloidal semiconductor quantum dots.” Physical Review B, 75, 3, Pp. 035330. Publisher's Version Abstract

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The spectroscopy and dynamics of multiple excitations on colloidal type-II CdTe∕CdSe core-shell quantum dots (QDs) are explored via quasi-cw multiexciton spectroscopy. The charge separation induced by the band offset redshifts the exciton emission and increases the radiative lifetime. In addition, we observe a significant modification of multiexciton properties compared with core-only or type-I QDs. In particular, the Auger recombination lifetimes are significantly increased, up to a nanosecond time scale. While in type-I QDs the Auger lifetime scales with the volume, we find for type-II QDs a scaling law that introduces a linear dependence also on the radiative lifetime. We observe a blueshift of the biexciton emission and extract biexciton repulsion of up to 30meV in type-II QDs. This is assigned to the dominance of the Coulomb repulsion as the positive and negative charges become spatially separated, which overwhelms the correlation binding term. Higher electronic excited states can remain type I even when the lowest transition is already type II, resulting in a different size dependence of the triexciton emission. Finally, we discuss the possibilities of “multiexciton band gap engineering” using colloidal type-II QDs.

2006
Tsiala Saraidarov, Renata Reisfeld, Miri Kazes, and Uri Banin. 2006. “Blue laser dye spectroscopic properties in solgel inorganic-organic hybrid films.” Optics letters, 31, 3, Pp. 356-358. Publisher's Version Abstract

A blue solid-state laser material based on 4,4′4′ dibenzyl carbamido stilbene-2,2′2′ disulfonic acid incorporated into solgel zirconia and inorganic–organic hybrid matrices is presented. The absorption maxima of the dye in various matrices are around 339–361 nm, and the broad fluorescence peaks are at 411–413 nm. Optical gain measurements using the variable stripe method show amplified spontaneous emission peaking at 437 nm.

Asaf Salant, Ella Amitay-Sadovsky, and Uri Banin. 2006. “Directed self-assembly of gold-tipped CdSe nanorods.” Journal of the American Chemical Society, 128, 31, Pp. 10006-10007. Publisher's Version Abstract

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Gold-tipped CdSe rods (nanodumbbells) were solubilized in an aqueous phase and self-assembled in a head-to-tail manner using biotin disulfide and avidin. The disulfide end of the biotin molecule attaches to the gold tip of the nanodumbbell, and the biotin end of the molecule is able to conjugate to an avidin protein. The avidin can strongly conjugate up to four biotin molecules. Changing the ratios of biotin to nanodumbbells leads to the formation of dimers, trimers, and flowerlike structures. To further improve the distribution of chain lengths, a separation method based upon weight was applied using a concentration gradient. The gold tips provide effective anchor points for constructing complex nanorod structures by self-assembly.

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.