Denis Mongin, Ehud Shaviv, Paolo Maioli, Aurélien Crut, Uri Banin, Natalia Del Fatti, and Fabrice Vallée. 2012. “Ultrafast photoinduced charge separation in metal–semiconductor nanohybrids.” ACS nano, 6, 8, Pp. 7034-7043.
Yorai Amit, Adam Faust, Itai Lieberman, Lior Yedidya, and Uri Banin. 2012. “Semiconductor nanorod layers aligned through mechanical rubbing.” physica status solidi (a), 209, 2, Pp. 235-242. Publisher's Version Abstract


Large‐scale lateral alignment of nanorods (NRs) is of interest for manifestation of their anistropic properties including polarized emission and directional electrical transport. This study investigates the utility of mechanical rubbing for macroscopic scale alignment of colloidal semiconductor NRs. CdSe/CdS seeded‐rods, exhibiting linearly polarized emission, are aligned by mechanical rubbing of a spin‐coated glass substrate. The dragging force exerted by the rubbing fibers results in deflection and reorientation of the NRs along the rubbing direction. The rubbed samples were characterized by various methods including absorption, polarized emission, optical fluorescence microscopy, atomic force microscopy, and ultra‐high resolution scanning electron microscopy. The emission polarization contrast ratio (CR), defined as the ratio between emission intensities parallel and perpendicular to the rubbing direction, was used to characterize the rods alignment. The effects of substrate treatments on the CR were studied, showing that partially hydrophobic surface provides optimal conditions for alignment. Excess organic ligands added to the deposited NR solution strongly affect the extent of alignment. This was studied for a series of NR samples of different dimensions and an optimal additive ratio of ∼3 ligand molecules per 1 nm2 NR surface area was found to yield the highest CR. Average CR values of 3.5 were detected over the entire 6 cm2 substrate area, with local values exceeding 4.5. While samples of rubbed spherical quantum dots and spin‐coated films of NRs show no emission polarization, the emission intensity from rubbed NR samples is polarized obeying Malus' law (wherein, the intensity is proportional to cos2(θ)). Mechanical rubbing, well known for its use in LC devices, may be considered as a method for large‐scale alignment of NRs on substrates.

Ehud Shaviv, Olaf Schubert, Marcelo Alves-Santos, Guido Goldoni, Rosa Di Felice, Fabrice Vallee, Natalia Del Fatti, Uri Banin, and Carsten Sönnichsen. 2011. “Absorption properties of metal–semiconductor hybrid nanoparticles.” ACS nano, 5, 6, Pp. 4712-4719.
Moshe G Harats, Ilai Schwarz, Adiel Zimran, Uri Banin, Gang Chen, and Ronen Rapaport. 2011. “Enhancement of two photon processes in quantum dots embedded in subwavelength metallic gratings.” Optics express, 19, 2, Pp. 1617-1625.
Nitzan Livneh, Ayelet Strauss, Ilai Schwarz, Itamar Rosenberg, Adiel Zimran, Shira Yochelis, Gang Chen, Uri Banin, Yossi Paltiel, and Ronen Rapaport. 2011. “Highly directional emission and photon beaming from nanocrystal quantum dots embedded in metallic nanoslit arrays.” Nano letters, 11, 4, Pp. 1630-1635. Publisher's Version Abstract

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We demonstrate a directional beaming of photons emitted from nanocrystal quantum dots that are embedded in a subwavelength metallic nanoslit array with a divergence angle of less than 4°. We show that the eigenmodes of the structure result in localized electromagnetic field enhancements at the Bragg cavity resonances, which could be controlled and engineered in both real and momentum space. The photon beaming is achieved using the enhanced resonant coupling of the quantum dots to these Bragg cavity modes, which dominates the emission properties of the quantum dots. We show that the emission probability of a quantum dot into the narrow angular mode is 20 times larger than the emission probability to all other modes. Engineering nanocrystal quantum dots with subwavelength metallic nanostructures is a promising way for a range of new types of active optical devices, where spatial control of the optical properties of nanoemitters is essential, on both the single and many photons level.

Amit Sitt, Asaf Salant, Gabi Menagen, and Uri Banin. 2011. “Highly emissive nano rod-in-rod heterostructures with strong linear polarization.” Nano letters, 11, 5, Pp. 2054-2060.
Eyal Yoskovitz, Ido Hadar, Amit Sitt, Itai Lieberman, and Uri Banin. 2011. “Interplay of quenching and enhancement effects in apertureless near-field fluorescence imaging of single nanoparticles.” The Journal of Physical Chemistry C, 115, 32, Pp. 15834-15844.
Nir Waiskopf, Itzhak Shweky, Itai Lieberman, Uri Banin, and Hermona Soreq. 2011. “Quantum dot labeling of butyrylcholinesterase maintains substrate and inhibitor interactions and cell adherence features.” ACS chemical neuroscience, 2, 3, Pp. 141-150. Publisher's Version Abstract

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Butyrylcholinesterase (BChE) is the major acetylcholine hydrolyzing enzyme in peripheral mammalian systems. It can either reside in the circulation or adhere to cells and tissues and protect them from anticholinesterases, including insecticides and poisonous nerve gases. In humans, impaired cholinesterase functioning is causally involved in many pathologies, including Alzheimer’s and Parkinson’s diseases, trait anxiety, and post stroke conditions. Recombinant cholinesterases have been developed for therapeutic use; therefore, it is important to follow their in vivo path, location, and interactions. Traditional labeling methods, such as fluorescent dyes and proteins, generally suffer from sensitivity to environmental conditions, from proximity to different molecules or special enzymes which can alter them, and from relatively fast photobleaching. In contrast, emerging development in synthesis and surface engineering of semiconductor nanocrystals enable their use to detect and follow molecules in biological milieus at high sensitivity and in real time. Therefore, we developed a platform for conjugating highly purified recombinant human BChE dimers (rhBChE) to CdSe/CdZnS quantum dots (QDs). We report the development and characterization of highly fluorescent aqueous soluble QD-rhBChE conjugates, present maintenance of hydrolytic activity, inhibitor sensitivity, and adherence to the membrane of cultured live cells of these conjugates, and outline their advantageous features for diverse biological applications.

Yossi Shemesh, Janet E Macdonald, Gabi Menagen, and Uri Banin. 2011. “Synthesis and Photocatalytic Properties of a Family of CdS‐PdX Hybrid Nanoparticles.” Angewandte Chemie, 123, 5, Pp. 1217-1221.
David Mocatta, Guy Cohen, Jonathan Schattner, Oded Millo, Eran Rabani, and Uri Banin. 2011. “Heavily doped semiconductor nanocrystal quantum dots.” Science, 332, 6025, Pp. 77-81. Publisher's Version Abstract

Doping of semiconductors by impurity atoms enabled their widespread technological application in microelectronics and optoelectronics. However, doping has proven elusive for strongly confined colloidal semiconductor nanocrystals because of the synthetic challenge of how to introduce single impurities, as well as a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We developed a method to dope semiconductor nanocrystals with metal impurities, enabling control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing. Our method yields n- and p-doped semiconductor nanocrystals, which have potential applications in solar cells, thin-film transistors, and optoelectronic devices.

Ronny Costi, Aaron E Saunders, and Uri Banin. 2010. “Colloidal hybrid nanostructures: a new type of functional materials.” Angewandte Chemie International Edition, 49, 29, Pp. 4878-4897. Publisher's Version Abstract

One key goal of nanocrystal research is the development of experimental methods to selectively control the composition and shape of nanocrystals over a wide range of material combinations. The ability to selectively arrange nanosized domains of metallic, semiconducting, and magnetic materials into a single hybrid nanoparticle offers an intriguing route to engineer nanomaterials with multiple functionalities or the enhanced properties of one domain. In this Review, we focus on recent strategies used to create semiconductor–metal hybrid nanoparticles, present the emergent properties of these multicomponent materials, and discuss their potential applicability in different technologies.

Hadar Steinberg, Omri Wolf, Adam Faust, Asaf Salant, Yigal Lilach, Oded Millo, and Uri Banin. 2010. “Electrical current switching in single CdSe nanorods.” Nano letters, 10, 7, Pp. 2416-2420. Publisher's Version Abstract

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Electrical current measurements through individually wired colloidal CdSe nanorods exhibit pronounced multistability. This current switching is analogous to the widely observed fluorescence intermittency in similar systems and may be associated with surface charge dynamics. Such association is quantitatively established for the case when the current is bistable, where the probability of the sojourn time t at the high or low current state follows an exponential dependence. Remarkably, this behavior can be modeled by charging dynamics of a single surface trap, whose position could be estimated from the intermittent current−voltage characteristics. The methodology presented here provides a unique route for charge dynamic sensing at the nanoscale, where the nanorod senses its own surface charge.

Elina Ploshnik, Asaf Salant, Uri Banin, and Roy Shenhar. 2010. “Hierarchical Surface Patterns of Nanorods Obtained by Co‐Assembly with Block Copolymers in Ultrathin Films.” Advanced Materials, 22, 25, Pp. 2774-2779. Publisher's Version Abstract


Co‐assembly of cadmium selenide nanorods in block copolymer films gives rise to anisotropic, hierarchical nanorod superstructures at the film surface. Unlike their observed behavior in the bulk composite, the nanorods preferentially orient perpendicular to the direction of the block copolymer domain, and the number of nanorods assembled across the domain is controlled by the ratio between the nanorod length and the domain width.

Eyal Yoskovitz, Gabi Menagen, Amit Sitt, Ella Lachman, and Uri Banin. 2010. “Nanoscale near-field imaging of excitons in single heterostructured nanorods.” Nano letters, 10, 8, Pp. 3068-3072. Publisher's Version Abstract

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The mixed 0D-1D dimensionality of heterostructured semiconductor nanorods, resulting from the dot-in-rod architecture, raises intriguing questions concerning the location and confinement of the exciton and the origin of the fluorescence in such structures. Using apertureless near-field distance-dependent lifetime imaging together with AFM topography, we directly map the emission and determine its location with high precision along different types of nanorods. We find that the fluorescence is emanating from a sub-20 nm region, correlated to the seed location, clearly indicating exciton localization.

Asaf Salant, Menny Shalom, Idan Hod, Adam Faust, Arie Zaban, and Uri Banin. 2010. “Quantum dot sensitized solar cells with improved efficiency prepared using electrophoretic deposition.” ACS nano, 4, 10, Pp. 5962-5968. Publisher's Version Abstract

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Quantum dot sensitized solar cells (QDSSC) may benefit from the ability to tune the quantum dot optical properties and band gap through the manipulation of their size and composition. Moreover, the inorganic nanocrystals may provide increased stability compared to organic sensitizers. We report the facile fabrication of QDSSC by electrophoretic deposition of CdSe QDs onto conducting electrodes coated with mesoporous TiO2. Unlike prior chemical linker-based methods, no pretreatment of the TiO2 was needed, and deposition times as short as 2 h were sufficient for effective coating. Cross-sectional chemical analysis shows that the Cd content is nearly constant across the entire TiO2 layer. The dependence of the deposition on size was studied and successfully applied to CdSe dots with diameters between 2.5 and 5.5 nm as well as larger CdSe quantum rods. The photovoltaic characteristics of the devices are greatly improved compared with those achieved for cells prepared with a linker approach, reaching efficiencies as high as 1.7%, under 1 sun illumination conditions, after treating the coated electrodes with ZnS. Notably, the absorbed photon to electron conversion efficiencies did not show a clear size-dependence indicating efficient electron injection even for the larger QD sizes. The electrophoretic deposition method can be easily expanded and applied for preparations of QDSSCs using diverse colloidal quantum dot and quantum rod materials for sensitization.

Hybrid semiconductor−metal nanoparticles exhibit a combination of properties from the disparate components or even, more interestingly, synergetic properties which arise from the coupling between the two materials. In this work, we study the second harmonic generation (SHG) in CdSe−Au hybrid nanoparticles in comparison with their components, using the Hyper-Rayleigh scattering (HRS) method. Possible contribution of symmetry effects was studied by comparing symmetric two-sided gold-tipped CdSe nanodumbbells with asymmetric one-sided quantum dot−Au (QD−Au) hybrids. A simplistic view of a symmetry effect is disproved in this case by the experimental data, which shows an unexpected reduction in the SHG response in both cases, compared to the respective mixtures of Au and CdSe nanoparticles. For CdSe−Au hybrids with a long semiconductor rod segment, we find that the SHG response corresponds to a sum of the contributions from the semiconductor and the metal components. However, for QD−Au and smaller dumbbells, the SHG response is smaller than expected from a simple sum of the contribution from both components. This reduction is assigned to the effects related to the CdSe−Au interfacial region within these hybrids. A first plausible contribution to the reduction is the dephasing induced by the gold, leading to diminished SHG from the CdSe component. This reduced response of the semiconductor component is accompanied by reduced SHG from the gold component which is assigned to a partial change of the surface of the gold once an interface with CdSe is formed. These observations regarding the SHG response manifest the unique properties that arise from the combination of a semiconductor and a metal within one hybrid nanoparticle.

Elina Ploshnik, Asaf Salant, Uri Banin, and Roy Shenhar. 2010. “Co-assembly of block copolymers and nanorods in ultrathin films: effects of copolymer size and nanorod filling fraction.” Physical Chemistry Chemical Physics, 12, 38, Pp. 11885-11893. Publisher's Version Abstract

Schematic representation of the interaction counting model comparing the perpendicular and parallel nanorod orientations for two NR lengths. Blue colored nanorods denote square fragments of the assembly used for interaction counting, with peripheral attractive and repulsive interactions depicted as green and red stripes, respectively. PMMA domains are colored orange, and PS domains underneath the NRs are colored yellow. In the short NR case the lower enthalpic driving force for the perpendicular orientation can be offset by entropic effects, leading to increased morphological defects.

Two-dimensional, hierarchical assemblies of nanorods were obtained by exploiting the structures afforded by block copolymers in ultrathin films. Under the appropriate conditions, the nanorods segregate to the film surface already upon casting the composite film, and organize with the block copolymer through phase separation. In this paper we compare the structures formed by CdSe nanorods of three different lengths and two polystyrene-block-poly(methyl methacrylate) copolymers with different nanorods/copolymer ratios, and study the temporal evolution of the structure in each case. It is found that the initial morphology of the film largely dictates the resulting structure. The combination of short nanorods and/or short copolymers is shown to be more prone to morphological defects, while assembling long nanorods with long copolymers leads to highly organized nanorod morphologies. These phenomena are explained by a combination of kinetic and thermodynamic factors.

Michal Soreni‐Harari, David Mocatta, Marina Zimin, Yair Gannot, Uri Banin, and Nir Tessler. 2010. “Interface Modifications of InAs Quantum‐Dots Solids and their Effects on FET Performance.” Advanced Functional Materials, 20, 6, Pp. 1005-1010. Publisher's Version Abstract


InAs nanocrystals field‐effect transistors with an ON/OFF ratio of 105 are reported. By tailoring the interface regions in the active layer step‐by‐step, the evolution of the ON/OFF ratio can be followed from approximately 5 all the way to around 105. The formation of a semiconducting solid from colloidal nanocrystals is achieved through targeted design of the nanocrystal–nanocrystal interaction. The manipulation characteristics of the nanocrystal interfaces include the matrix surrounding the inorganic core, the interparticle distance, and the order of nanocrystals in the 3D array. Through careful analysis of device characteristics following each treatment, the effect of each on the physical properties of the films are able to be verified. The enhanced performance is related to interparticle spacing, reduction in sub‐gap states, and better electronic order (lower σ parameter). Films with enhanced charge transport qualities retain their quantum‐confined characteristics throughout the procedure, thus making them useful for optoelectronic applications.

Hadar Steinberg, Yigal Lilach, Asaf Salant, Omri Wolf, Adam Faust, Oded Millo, and Uri Banin. 2009. “Anomalous temperature dependent transport through single colloidal nanorods strongly coupled to metallic leads.” Nano letters, 9, 11, Pp. 3671-3675. Publisher's Version Abstract

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We report wiring of individual colloidal nanorods (NRs), 30−60 nm long by 3.5−5 nm diameter. Strong electrical coupling is achieved by electron beam induced deposition (EBID) of metallic lines targeting NR tips with nanometric precision. At T = 4 K many devices exhibit smooth I(V) curves with no sharp onset features, which remarkably fit a Fowler−Nordheim tunneling model. All devices exhibit an anomalous exponential temperature dependence of the form I ∼ exp(T/T0). This irregular behavior cannot be explained by any hopping or activation model and is interpreted by accounting for the lowering of the NR conduction band due to lattice dilation and phonon coupling.

Gabi Menagen, Janet E Macdonald, Yossi Shemesh, Inna Popov, and Uri Banin. 2009. “Au growth on semiconductor nanorods: photoinduced versus thermal growth mechanisms.” Journal of the American Chemical Society, 131, 47, Pp. 17406-17411. Publisher's Version Abstract

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Gold growth on CdS nanorods and on seeded CdSe/CdS nanorods with and without illumination at different temperatures was studied. Two competing mechanisms were identified: thermal and light-induced growth. The thermal mechanism leads to growth of small gold particles at defects along the rod body and can be suppressed at lower temperatures. This control is attributed to a phase transition of the alkyl chains of the surface amine ligands to a static phase at lower temperatures, blocking the Au precursor’s access to the nanorod surfaces. While a long-chain (C18) amine shows effective blocking at 293 K, a shorter chain (C12) amine shows the same result only at 273 K; however, in the case of a bulky trialkylamine, defect growth was observed even at 273 K. Light-induced growth leads to selective deposition of gold on one end of the rods. The tip was shown to grow on sulfur-rich facets of the nanorod, producing end-on and angled tip orientations. Growth under illumination with decreased temperature provides a highly selective synthesis of hybrid semiconductor nanorods with a single gold tip. Such anisotropic semiconductor−metal hybrids are of interest for self-assembly and photocatalysis and as building blocks in optoelectronic devices.