Publications

2010
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

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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

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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.