Publications

2004
Taleb Mokari, Eli Rothenberg, Inna Popov, Ronny Costi, and Uri Banin. 2004. “Selective growth of metal tips onto semiconductor quantum rods and tetrapods.” Science, 304, 5678, Pp. 1787-1790. Publisher's Version Abstract

We show the anisotropic selective growth of gold tips onto semiconductor (cadmium selenide) nanorods and tetrapods by a simple reaction. The size of the gold tips can be controlled by the concentration of the starting materials. The new nanostructures display modified optical properties caused by the strong coupling between the gold and semiconductor parts. The gold tips show increased conductivity as well as selective chemical affinity for forming self-assembled chains of rods. Such gold-tipped nanostructures provide natural contact points for self-assembly and for electrical devices and can solve the difficult problem of contacting colloidal nanorods and tetrapods to the external world.

2003
Oded Millo, David Katz, Dov Steiner, Eli Rothenberg, Taleb Mokari, Miri Kazes, and Uri Banin. 2003. “Charging and quantum size effects in tunnelling and optical spectroscopy of CdSe nanorods.” Nanotechnology, 15, 1, Pp. R1. Publisher's Version Abstract

We summarize our correlated scanning tunnelling microscopy and optical spectroscopy investigations of the electronic level structure and single-electron charging effects in CdSe quantum rods. Both optical and tunnelling spectra show that the level structure depends primarily on rod diameter and not on length. With increasing diameter, the bandgap and the excited state level spacings shift to the red. The level structure is assigned using a multi-band effective-mass model. The tunnelling spectra also exhibit, depending on the tunnel-junction parameters, single-electron charging effects that yield information on the degeneracy of the electronic states.

Fernando Patolsky, Ron Gill, Yossi Weizmann, Taleb Mokari, Uri Banin, and Itamar Willner. 2003. “Lighting-up the dynamics of telomerization and DNA replication by CdSe− ZnS quantum dots.” Journal of the American Chemical Society, 125, 46, Pp. 13918-13919. Publisher's Version Abstract

Abstract Image

CdSe−ZnS core−shell quantum dots (QDs) act as photochemical centers for lighting-up the dynamics of telomerization or DNA replication.

Taleb Mokari and Uri Banin. 2003. “Synthesis and properties of CdSe/ZnS core/shell nanorods.” Chemistry of materials, 15, 20, Pp. 3955-3960. Publisher's Version Abstract

Abstract Image

A method for the synthesis of CdSe/ZnS core/shell nanorods is reported. In the first step rods are grown, and in a second step a shell of ZnS is overgrown at moderate temperatures in a mixture of trioctylphosphine-oxide and hexadecylamine. Structural and chemical characterization using transmission electron microscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy were performed providing direct evidence for shell growth. The emission quantum yield significantly increases by over 1 order of magnitude for the core/shell nanorods compared to the original rods because of the improved surface passivation. Rods with lengths up to ∼30 nm were investigated, and in this size regime the maximal achievable QY showed little dependence on length and strong dependence on rod diameter, with increased QY in smaller diameters. Color tunability is available via tuning of the rod diameter. The stability against photooxidation was significantly improved in core/shell nanorods compared with rods coated by organic ligands.

ShiHai Kan, Taleb Mokari, Eli Rothenberg, and Uri Banin. 2003. “Synthesis and size-dependent properties of zinc-blende semiconductor quantum rods.” Nature materials, 2, 3, Pp. 155. Publisher's Version Abstract

Dimensionality and size are two factors that govern the properties of semiconductor nanostructures1,2. In nanocrystals, dimensionality is manifested by the control of shape, which presents a key challenge for synthesis3,4,5. So far, the growth of rod-shaped nanocrystals using a surfactant-controlled growth mode, has been limited to semiconductors with wurtzite crystal structures, such as CdSe (ref. 3). Here, we report on a general method for the growth of soluble nanorods applied to semiconductors with the zinc-blende cubic lattice structure. InAs quantum rods with controlled lengths and diameters were synthesized using the solution–liquid–solid mechanism6 with gold nanocrystals as catalysts7. This provides an unexpected link between two successful strategies for growing high-quality nanomaterials, the vapour–liquid–solid approach for growing nanowires8,9,10,11,12, and the colloidal approach for synthesizing soluble nanocrystals13,14,15. The rods exhibit both length- and shape-dependent optical properties, manifested in a red-shift of the bandgap with increased length, and in the observation of polarized emission covering the near-infrared spectral range relevant for telecommunications devices16,17.

ShiHai Kan, Taleb Mokari, Eli Rothenberg, and Uri Banin. 2003. “Synthesis and size-dependent properties of zinc-blende semiconductor quantum rods.” Nature materials, 2, 3, Pp. 155-158. Publisher's Version Abstract

Dimensionality and size are two factors that govern the properties of semiconductor nanostructures1,2. In nanocrystals, dimensionality is manifested by the control of shape, which presents a key challenge for synthesis3,4,5. So far, the growth of rod-shaped nanocrystals using a surfactant-controlled growth mode, has been limited to semiconductors with wurtzite crystal structures, such as CdSe (ref. 3). Here, we report on a general method for the growth of soluble nanorods applied to semiconductors with the zinc-blende cubic lattice structure. InAs quantum rods with controlled lengths and diameters were synthesized using the solution–liquid–solid mechanism6 with gold nanocrystals as catalysts7. This provides an unexpected link between two successful strategies for growing high-quality nanomaterials, the vapour–liquid–solid approach for growing nanowires8,9,10,11,12, and the colloidal approach for synthesizing soluble nanocrystals13,14,15. The rods exhibit both length- and shape-dependent optical properties, manifested in a red-shift of the bandgap with increased length, and in the observation of polarized emission covering the near-infrared spectral range relevant for telecommunications devices16,17.

2002
Miri Kazes, David Y Lewis, Yuval Ebenstein, Taleb Mokari, and Uri Banin. 2002. “Lasing from semiconductor quantum rods in a cylindrical microcavity.” Advanced Materials, 14, 4, Pp. 317-321. Publisher's Version Abstract

Lasers based on colloidal semiconductor nanostructures can benefit from the remarkable spectral coverage afforded through the quantum confinement effect. The first observation of lasing for colloidal CdSe/ZnS quantum rods in solution using a cylindrical microcavity is reported here (see also inside front cover). Lasing in the same configuration was also observed for spherical nanocrystal quantum dots. For the quantum dots lasing is not polarized, but in quantum rods the laser emission is highly linearly polarized, a desirable feature for laser and photonic applications.

David Katz, Tommer Wizansky, Oded Millo, Eli Rothenberg, Taleb Mokari, and Uri Banin. 2002. “Size-dependent tunneling and optical spectroscopy of CdSe quantum rods.” Physical review letters, 89, 8, Pp. 086801. Publisher's Version Abstract

Figure

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.

Andreas Eichhöfer, Assaf Aharoni, and Uri Banin. 2002. “Synthesis, structure, and optical properties of new cadmium chalcogenide clusters of the type [Cd10E4 (E'Ph) 12 (PR3) 4],(E, E'= Te, Se, S).” Zeitschrift für anorganische und allgemeine Chemie, 628, 11, Pp. 2415-2421. Publisher's Version Abstract

New cadmium chalcogenide cluster molecules [Cd10E4(E'Ph)12(Pn Pr3)4], E = Te, E' = Te () and [Cd10E4(E'Ph)12 (Pn Pr2Ph)4] E = Te, E' = Se (); E = Te E' = S (); E = Se, E' = S () have been synthesized and structurally characterized by single crystal X‐ray structure analysis. The influence of the variation of the chalcogen atom is investigated by structural means and by optical spectroscopy. All cluster‐molecules have a broad emission in the blue‐visible range at low temperature as indicated by photo luminescence (PL) measurements. A clear classification of the emission peak position can be made based on the E' species suggesting that the emission is assigned to transitions associated with the cluster surface ligands. Photoluminescence excitation and absorption measurements display a systematic shift of the band gap to the higher energies with the variation of E and E' from Te to Se to S, as also occurs in the respective series of the bulk semiconductors.

Nir Tessler, Vlad Medvedev, Miri Kazes, ShiHai Kan, and Uri Banin. 2002. “Efficient near-infrared polymer nanocrystal light-emitting diodes.” Science, 295, 5559, Pp. 1506-1508. Publisher's Version Abstract

Conjugated polymers and indium arsenide–based nanocrystals were used to create near-infrared plastic light-emitting diodes. Emission was tunable from 1 to 1.3 micrometers—a range that effectively covers the short-wavelength telecommunications band—by means of the quantum confinement effects in the nanocrystals. The external efficiency value (photons out divided by electrons in) is ∼0.5% (that is, >1% internal) and is mainly limited by device architecture. The near-infrared emission did not overlap the charge-induced absorption bands of the polymer.

2001
David Katz, Oded Millo, Shi-Hai Kan, and Uri Banin. 2001. “Control of charging in resonant tunneling through InAs nanocrystal quantum dots.” Applied Physics Letters, 79, 1, Pp. 117-119. Publisher's Version Abstract

Tunneling spectroscopy of InAs nanocrystals deposited on graphite was measured using scanning tunneling microscopy, in a double-barrier tunnel-junction configuration. The effect of the junction symmetry on the tunneling spectra is studied experimentally and modeled theoretically. When the tip is retracted, we observe resonant tunneling through the nanocrystal states without charging. Charging is regained upon reducing the tip–nanocrystal distance, making the junction more symmetric. The effect of voltage distribution between the junctions on the measured spectra is also discussed.

Michal Jacobsohn Eilon, Taleb Mokari, and Uri Banin. 2001. “Surface exchange effect on hyper Rayleigh scattering in CdSe nanocrystals.” The Journal of Physical Chemistry B, 105, 51, Pp. 12726-12731. Publisher's Version Abstract

The surface and bulk contributions to the second-order nonlinear optical response of CdSe nanocrystals is studied. The first hyperpolarizability, βn, was measured for the nanocrystals in solution using the hyper-Rayleigh scattering method. Tri-n-octylphosphineoxide-capped nanocrystals show an enhancement in the value of the second hyperpolarizibility per unit cell, β, with reduced size. The two-state model can explain the enhancement for nanocrystals with radius down to about 1.7 nm, related with the concentration of oscillator strength, but for smaller particles the enhancement is larger than the prediction. This additional enhancement is assigned to a surface response. The contribution of surface ligands to the second harmonic signal for the nanocrystals was investigated by exchanging the tri-n-octylphosphineoxide ligands with the nonlinear chromophore nitrothiophenol. Surface exchange was evidenced through the change in particle solubility, by a substantial reduction of the fluorescence intensity, and by the vibrational spectra. The substitution to nitrothiophenol ligands leads to a size-dependent enhancement of βn compared to the original particles. The difference in βn between the surface-substituted and nonsubstituted particles scales with the number of surface sites for nanocrystals of different sizes. Surface exchange also leads to an enhancement in βn for a rod-shaped sample (aspect ratio 3.3:1). The contributions to βn for such nanocrystals can therefore be attributed to a combination of a bulk-like part arising from the nonlinear electronic response of Cd−Se bonds and a surface part effected by the nature of the ligands.

ShiHai Kan, Israel Felner, and Uri Banin. 2001. “Synthesis, characterization, and magnetic properties of α‐MnS nanocrystals.” Israel Journal of Chemistry, 41, 1, Pp. 55-62. Publisher's Version Abstract

MnS nanocrystals have been prepared by a colloidal synthesis route through the reaction of MnCl2 and S[Si(CH3)3]2 in trioctylphosphineoxide. The nanocrystals were characterized using X‐ray diffraction and transmission electron microscopy. The magnetic properties were studied with SQUID magnetometry. X‐ray diffraction shows that the nanocrystals are of the thermodynamically stable α‐MnS (alabandite) structure. Size control was achieved by changing the concentration of the precursors. Nanocrystal sizes were measured by transmission electron microscopy, and three samples of average diameters 20, 40, and 80 nm were obtained, with narrow size distribution (σ˜9%). The zero field cooled magnetization curves for the 80‐, 40‐, and 20‐nm samples showed a cusp at 116 K, 97 K, and 50 K respectively, all smaller than the antiferromagnetic transition temperature, N = 130 K, of bulk α‐MnS. Below N the magnetization exhibits a paramagnetic behavior unlike typical antiferromagnetic materials. These results indicate that there is a mixture of paramagnetic and antiferromagnetic phases in the nanocrystals. The size dependence shows a general trend of decrease of N with reduced particle size, indicating size dependent magnetic ordering.

2000
Yunwei Cao and Uri Banin. 2000. “Growth and properties of semiconductor core/shell nanocrystals with InAs cores.” Journal of the American Chemical Society, 122, 40, Pp. 9692-9702. Publisher's Version Abstract

Core/shell semiconductor nanocrystals with InAs cores were synthesized and characterized. III−V semiconductor shells (InP and GaAs), and II−VI semiconductor shells (CdSe, ZnSe, and ZnS) were overgrown on InAs cores with various radii using a two step synthesis. In the first step cores were prepared, and in the second step the shells were grown using high-temperature pyrolysis of organometallic precursors in a coordinating solvent. Core/shell growth was monitored by absorption and photoluminescence spectroscopy. The band gap shifts to the red upon growth of InP or CdSe shells, while for ZnSe and ZnS shells that have larger band offsets with respect to InAs, the band gap energy is maintained. This behavior is reproduced by band gap energy calculations using a particle within a spherical box model. The photoluminescence quantum yield is quenched in InAs/InP core/shells but increases substantially up to 20% for InAs/CdSe and InAs/ZnSe core/shells. For InAs/ZnS core/shells the enhancement of the photoluminescence quantum yields is smaller, up to 8%. The core/shell nanocrystals were characterized using transmission electron microscopy, X-ray photoelectron spectroscopy, and powder X-ray diffraction. X-ray photoelectron spectroscopy provides evidence for shell growth. The X-ray diffraction peaks shift and narrow upon shell growth, providing evidence for an epitaxial growth mode. Simulations of the X-ray diffraction patterns reproduce both effects, and show that there is one stacking fault present for every four to five layers in the core and core/shell nanocrystals. The stability of InAs/CdSe and InAs/ZnSe core/shells against oxidation is substantially improved compared with the cores, and the photostability is significantly better compared with a typical near-IR laser dye IR140. Core/shell nanocrystals with InAs cores are suggested as a novel type of fluorophores covering the near-IR spectral range, with high emission quantum yields and improved stability compared with traditional near-IR laser dyes.

Chieh-Ju Lee, Ari Mizel, Uri Banin, Marvin L Cohen, and Paul A Alivisatos. 2000. “Observation of pressure-induced direct-to-indirect band gap transition in InP nanocrystals.” The Journal of Chemical Physics, 113, 5, Pp. 2016-2020. Publisher's Version Abstract

We investigate the quantum size effects in the pressure-induced direct-to-indirect band gap transition in InP nanocrystals. Hydrostatic pressures of up to 13 GPa are applied to two different sizes of InP nanocrystal samples in a diamond anvil cell. The band gap pressure dependence and the nature of the emitting states are studied by photoluminescence (PL) and fluorescence line narrowing (FLN) techniques at 10 K. Pressure-dependent FLN spectra show that the nature of the emitting states at pressures up to 9 GPa is similar to that at ambient pressure, suggesting that no direct-to-indirect transition happens below 9 GPa. For both sizes, the PL peak energy exhibits a strong blueshift with rising pressure until approximately 9 to 10 GPa. Above this pressure, the PL peak position slightly shifts red. Beyond 12 GPa, the band gap emission intensity becomes extremely weak and trap emission dominates the PL spectra. As the pressure is released, both the luminescence intensity and the peak position recover in a fully reversible manner. The change in the sign of the band gap energy pressure dependence and the disappearance of the band edge luminescence indicate the pressure-induced direct-to-indirect band gap transition. Contrary to theoretical calculations, no substantial reduction of the transition pressure is observed in the nanocrystal cases compared to the bulk transition pressure.

1999
Yun-Wei Cao, Julia Aksenton, Victor Soloviev, and Uri Banin. 1999. “Colloidal synthesis and properties of InAs/InP and InAs/CdSe core/shell nanocrystals.” MRS Online Proceedings Library Archive, 571. Publisher's Version Abstract

High-temperature colloidal synthesis of InAs/InP and InAs/CdSe core/shell nanocrystal quantum dots is reported. InP and CdSe shells with several thicknesses were grown on InAs cores ranging in diameter between 20 to 50 Å. Optical spectra, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to analyze the core/shell quantum dots and determine their chemical composition, average size, size distributions, and structures. The experimental results indicate that shell growth is uniform, expitaxial, and controllable. For both InP and CdSe shells, growth is accompanied by a red shift of the band gap energy as a result of the extension of the electron wavefunction into the shell region. An increase of the room temperature photoluminescence quantum yield by a factor of∼4 is observed with CdSe shell growth on InAs Cores. The growth of InP shells, however, quenches the photoluminescence quantum yield. The difference is assigned to outer surface effects in core/shell nanocrystals.

Alvin T Yeh, Giulio Cerullo, Uri Banin, Alf Mews, Paul A Alivisatos, and Charles V Shank. 1999. “Dynamics of exciton localization in CdS/HgS quantum-dot quantum wells.” Physical Review B, 59, 7, Pp. 4973. Publisher's Version Abstract

Localization of carrier wave functions to the quantum-well portion of the CdS/HgS quantum-dot quantum well (QDQW) is investigated. Nanosecond hole-burning (HB) spectra measure the photoinduced exciton coupling to a 250−cm−1 HgS phonon mode indicative of localization. Femtosecond pump-probe spectroscopy of these QDQW, however, show the photoinduced exciton couples to coherent 300−cm−1 CdS longitudinal optical-phonon modes, which is indicative of delocalization throughout the QDQW.  Femtosecond HB and three pulse pump-dump experiments reveal these results are dependent on the time scale of the experiment. These experiments indicate that the initially photoexcited electron and hole wave functions are weakly confined to the HgS monolayer. Only after long times (∼400 fs) will the exciton localize to the HgS well. These results indicate that the primary optical interaction excites electrons from a delocalized QDQW ground state and not from a localized HgS well state.

Uri Banin, Yunwei Cao, David Katz, and Oded Millo. 1999. “Identification of atomic-like electronic states in indium arsenide nanocrystal quantum dots.” Nature, 400, 6744, Pp. 542. Publisher's Version Abstract

Semiconductor quantum dots, due to their small size, mark the transition between molecular and solid-state regimes, and are often described as ‘artificial atoms’ (13). This analogy originates from the early work on quantum confinement effects in semiconductor nanocrystals, where the electronic wavefunctions are predicted4 to exhibit atomic-like symmetries, for example ‘s ’ and ‘p ’. Spectroscopic studies of quantum dots have demonstrated discrete energy level structures and narrow transition linewidths5,6,7,8,9, but the symmetry of the discrete states could be inferred only indirectly. Here we use cryogenic scanning tunnelling spectroscopy to identify directly atomic-like electronic states with s and p character in a series of indium arsenide nanocrystals. These states are manifest in tunnelling current–voltage measurements as two- and six-fold single-electron-charging multiplets respectively, and they follow an atom-like Aufbau principle of sequential energy level occupation10.

Yun‐Wei Cao and Uri Banin. 1999. “Synthesis and characterization of InAs/InP and InAs/CdSe core/shell nanocrystals.” Angewandte Chemie International Edition, 38, 24, Pp. 3692-3694. Publisher's Version Abstract

The effect of the outer surface of core/shell nanocrystals on the fluorescence quantum yield was observed for InAs/InP and InAs/CdSe core/shells (see picture). For InAs/CdSe we observed substantial enhancement of the fluorescence quantum yield compared to the InAs core, and up to two times larger than the laser dye IR‐140. Such core/shell nanocrystals have potential use as biological fluorescent markers in the near IR spectral range.

1998
Erez Gershgoren, Uri Banin, and Sanford Ruhman. 1998. “Caging and geminate recombination following photolysis of triiodide in solution.” The Journal of Physical Chemistry A, 102, 1, Pp. 9-16. Publisher's Version Abstract

A survey of caging and geminate recombination dynamics following the UV photolysis of I3- in a series of polar solvents is presented. Transient absorption in both the near-IR and UV was measured out to delays of 0.4 ns, probing evolution of the nascent product and recombined reactants, respectively. The fate of photolysis fragments is suggested to be determined shortly after the act of bond fission. Kinetic analysis shows caged fragments either recombine directly and vibrationally relax within a few picoseconds or produce long-lived complexes of unknown structure that decay exponentially in ∼40 ps, and both routes lead to ground-state I3-. The persistent complex exhibits a near-IR absorption spectrum that is broadened and red-shifted relative to free I2-. A very shallow and slow residual component of recombination may be associated with encounters of geminate pairs that initially escape the solvent cage. The choice of solvent strongly effects the probability and dynamics of caging, but not the decay rate of complex caged pairs. This is not altered by varying the temperature of an isobutyl alcohol solution from 5 to 45 °C. The results are discussed in an effort to illuminate the role played by the solvent in triiodide recombination in solution.