Publications

2017
Ido Hadar, John P Philbin, Yossef E Panfil, Shany Neyshtadt, Itai Lieberman, Hagai Eshet, Sorin Lazar, Eran Rabani, and Uri Banin. 2017. “Semiconductor seeded nanorods with graded composition exhibiting high quantum-yield, high polarization, and minimal blinking.” Nano letters, 17, 4, Pp. 2524-2531. Publisher's Version Abstract

Abstract Image

Seeded semiconductor nanorods represent a unique family of quantum confined materials that manifest characteristics of mixed dimensionality. They show polarized emission with high quantum yield and fluorescence switching under an electric field, features that are desirable for use in display technologies and other optical applications. So far, their robust synthesis has been limited mainly to CdSe/CdS heterostructures, thereby constraining the spectral tunability to the red region of the visible spectrum. Herein we present a novel synthesis of CdSe/Cd1–xZnxS seeded nanorods with a radially graded composition that show bright and highly polarized green emission with minimal intermittency, as confirmed by ensemble and single nanorods optical measurements. Atomistic pseudopotential simulations elucidate the importance of the Zn atoms within the nanorod structure, in particular the effect of the graded composition. Thus, the controlled addition of Zn influences and improves the nanorods’ optoelectronic performance by providing an additional handle to manipulate the degree confinement beyond the common size control approach. These nanorods may be utilized in applications that require the generation of a full, rich spectrum such as energy-efficient displays and lighting.

Jiajia Ning and Uri Banin. 2017. “Magic size InP and InAs clusters: synthesis, characterization and shell growth.” Chemical Communications, 53, 17, Pp. 2626-2629. Publisher's Version Abstract

 c6cc09778b-f2.tif

Magic size III–V semiconductor nanoclusters were synthesized. Non-continuous transition of the absorption spectra upon mild heating of the reaction solution is observed, indicating transformation between differently sized clusters. Further manipulation of the clusters is demonstrated for an InP MSC by growing a ZnS and ZnSe shell aiming at blue fluorescence. The quantum yield is limited due to formation of interfacial defects for thick ZnS shells.

Orian Elimelech, Jing Liu, Anna M Plonka, Anatoly I Frenkel, and Uri Banin. 2017. “Size Dependence of Doping by a Vacancy Formation Reaction in Copper Sulfide Nanocrystals.” Angewandte Chemie, 129, 35, Pp. 10471-10476. Publisher's Version Abstract

image

Doping of nanocrystals (NCs) is a key, yet underexplored, approach for tuning of the electronic properties of semiconductors. An important route for doping of NCs is by vacancy formation. The size and concentration dependence of doping was studied in copper(I) sulfide (Cu2S) NCs through a redox reaction with iodine molecules (I2), which formed vacancies accompanied by a localized surface plasmon response. X‐ray spectroscopy and diffraction reveal transformation from Cu2S to Cu‐depleted phases, along with CuI formation. Greater reaction efficiency was observed for larger NCs. This behavior is attributed to interplay of the vacancy formation energy, which decreases for smaller sized NCs, and the growth of CuI on the NC surface, which is favored on well‐defined facets of larger NCs. This doping process allows tuning of the plasmonic properties of a semiconductor across a wide range of plasmonic frequencies by varying the size of NCs and the concentration of iodine. Controlled vacancy doping of NCs may be used to tune and tailor semiconductors for use in optoelectronic applications.

2016
Defect luminescence from wurtzite CuInS2 nanocrystals: combined experimental and theoretical analysis
Alice DP Leach, Xiao Shen, Adam Faust, Matthew C Cleveland, Andrew D La Croix, Uri Banin, Sokrates T Pantelides, and Janet E Macdonald. 2016. “Defect luminescence from wurtzite CuInS2 nanocrystals: combined experimental and theoretical analysis.” The Journal of Physical Chemistry C, 120, 9, Pp. 5207-5212. Publisher's Version Abstract
jpcc2016CuInS2 nanocrystals with the wurtzite structure show promise for applications requiring efficient energy transport due to their anisotropic crystal structure. We investigate the source of photoluminescence in the near-infrared spectral region recently observed from these nanocrystals. Spectroscopic studies of both wurtzite CuInS2 itself and samples alloyed with Cd or Zn allow the assignment of this emission to a radiative point defect within the nanocrystal structure. Further, by varying the organic passivation layer on the material, we are able to determine that the atomic species responsible for nonradiative decay paths on the nanocrystal surface are Cu- or S-based. Density functional theory calculations of defect states within the material allow identification of the likely radiative species. Understanding both the electronic structure and optical properties of wurtzite CuInS2 nanocrystals is necessary for their efficient integration into potential biological, photovoltaic, and photocatalytic applications.
Image icon 2016_jpcc_02.gif
Chunfan Yang, Adam Faust, Yorai Amit, Itay Gdor, Uri Banin, and Sanford Ruhman. 2016. “Impurity sub-band in heavily Cu-doped InAs nanocrystal quantum dots detected by ultrafast transient absorption.” The Journal of Physical Chemistry A, 120, 19, Pp. 3088-3097. Publisher's Version Abstract

The effect of Cu impurities on the absorption cross section, the rate of hot exction thermalization, and on exciton recombination processes in InAs quantum dots was studied by femtosecond transient absorption. Our findings reveal dynamic spectral effects of an emergent impurity sub-band near the bottom of the conduction band. Previously hypothesized to explain static photophysical properties of this system, its presence is shown to shorten hot carrier relaxation. Partial redistribution of interband oscillator strength to sub-band levels reduces the band edge bleach per exciton progressively with the degree of doping, even though the total linear absorption cross section at the band edge remains unchanged. In contrast, no doping effects were detected on absorption cross sections high in the conduction band, as expected due to the relatively high density of sates of the undoped QDs.

Nir Waiskopf, Yuval Ben-Shahar, Michael Galchenko, Inbal Carmel, Gilli Moshitzky, Hermona Soreq, and Uri Banin. 2016. “Photocatalytic reactive oxygen species formation by semiconductor–metal hybrid nanoparticles. Toward light-induced modulation of biological processes.” Nano letters, 16, 7, Pp. 4266-4273. Publisher's Version Abstract

nirSemiconductor–metal hybrid nanoparticles manifest efficient light-induced spatial charge separation at the semiconductor–metal interface, as demonstrated by their use for hydrogen generation via water splitting. Here, we pioneer a study of their functionality as efficient photocatalysts for the formation of reactive oxygen species. We observed enhanced photocatalytic activity forming hydrogen peroxide, superoxide, and hydroxyl radicals upon light excitation, which was significantly larger than that of the semiconductor nanocrystals, attributed to the charge separation and the catalytic function of the metal tip. We used this photocatalytic functionality for modulating the enzymatic activity of horseradish peroxidase as a model system, demonstrating the potential use of hybrid nanoparticles as active agents for controlling biological processes through illumination. The capability to produce reactive oxygen species by illumination on-demand enhances the available peroxidase-based tools for research and opens the path for studying biological processes at high spatiotemporal resolution, laying the foundation for developing novel therapeutic approaches.

 

Jing Liu, Yorai Amit, Yuanyuan Li, Anna M Plonka, Sanjit Ghose, Lihua Zhang, Eric A Stach, Uri Banin, and Anatoly I Frenkel. 2016. “Reversed nanoscale Kirkendall effect in Au–InAs hybrid nanoparticles.” Chemistry of Materials, 28, 21, Pp. 8032-8043. Publisher's Version Abstract

Abstract Image

Metal–semiconductor hybrid nanoparticles (NPs) offer interesting synergistic properties, leading to unique behaviors that have already been exploited in photocatalysis, electrical, and optoelectronic applications. A fundamental aspect in the synthesis of metal–semiconductor hybrid NPs is the possible diffusion of the metal species through the semiconductor lattice. The importance of understanding and controlling the co-diffusion of different constituents is demonstrated in the synthesis of various hollow-structured NPs via the Kirkendall effect. Here, we used a postsynthesis room-temperature reaction between AuCl3 and InAs nanocrystals (NCs) to form metal–semiconductor core–shell hybrid NPs through the “reversed Kirkendall effect”. In the presented system, the diffusion rate of the inward diffusing species (Au) is faster than that of the outward diffusing species (InAs), which results in the formation of a crystalline metallic Au core surrounded by an amorphous, oxidized InAs shell containing nanoscale voids. We used time-resolved X-ray absorption fine-structure (XAFS) spectroscopy to monitor the diffusion process and found that both the size of the Au core and the extent of the disorder of the InAs shell depend strongly on the Au-to-NC ratio. We have determined, based on multielement fit analysis, that Au diffuses into the NC via the kick-out mechanism, substituting for In host atoms; this compromises the structural stability of the lattice and triggers the formation of In–O bonds. These bonds were used as markers to follow the diffusion process and indicate the extent of degradation of the NC lattice. Time-resolved X-ray diffraction (XRD) was used to measure the changes in the crystal structures of InAs and the nanoscale Au phases. By combining the results of XAFS, XRD, and electron microscopy, we correlated the changes in the local structure around Au, As, and In atoms and the changes in the overall InAs crystal structure. This correlative analysis revealed a co-dependence of different structural consequences when introducing Au into the InAs NCs. Therefore, this study of diffusion effects in nanocrystals has relevance to powerful concepts in solid-state nanochemistry related to processes of cation exchange, doping reactions, and diffusion mechanisms.

Kathy Vinokurov, Orian Elimelech, Oded Millo, and Uri Banin. 2016. “Copper Sulfide Nanocrystal Level Structure and Electrochemical Functionality towards Sensing Applications.” ChemPhysChem, 17, 5, Pp. 675-680. Publisher's Version Abstract

image

The level structure of copper sulfide nanocrystals of different sizes was investigated by correlating scanning tunneling spectroscopy and cyclic voltammetry data in relation to sensing applications. Upon oxidation of Cu2S nanocrystals in the low‐chalcocite phase, correlated changes are detected by both methods. The cyclic voltammetry oxidation peak of Cu(1+) down shifts, while in‐gap states, adjacent to the valence‐band edge, appeared in the tunneling spectra. These changes are attributed to Cu vacancy formation leading to a Cu depleted phase of the nanocrystals. The relevance of the oxidation to the use of copper sulfide nanocrystals in hydrogen peroxide sensing was also addressed, showing that upon oxidation the sensitivity vanishes. These findings bare significance to the use of copper sulfide nanocrystals in glucose sensing applications.

Yuval Ben-Shahar, Francesco Scotognella, Ilka Kriegel, Luca Moretti, Giulio Cerullo, Eran Rabani, and Uri Banin. 2016. “Optimal metal domain size for photocatalysis with hybrid semiconductor-metal nanorods.” Nature communications, 7, Pp. 10413. Publisher's Version Abstract

figure1

Semiconductor-metal hybrid nanostructures offer a highly controllable platform for light-induced charge separation, with direct relevance for their implementation in photocatalysis. Advances in the synthesis allow for control over the size, shape and morphology, providing tunability of the optical and electronic properties. A critical determining factor of the photocatalytic cycle is the metal domain characteristics and in particular its size, a subject that lacks deep understanding. Here, using a well-defined model system of cadmium sulfide-gold nanorods, we address the effect of the gold tip size on the photocatalytic function, including the charge transfer dynamics and hydrogen production efficiency. A combination of transient absorption, hydrogen evolution kinetics and theoretical modelling reveal a non-monotonic behaviour with size of the gold tip, leading to an optimal metal domain size for the most efficient photocatalysis. We show that this results from the size-dependent interplay of the metal domain charging, the relative band-alignments, and the resulting kinetics.

2015
Yehonadav Bekenstein, Orian Elimelech, Kathy Vinokurov, Oded Millo, and Uri Banin. 2015. “Charge Transport in Cu2S Nanocrystals Arrays: Effects of Crystallite Size and Ligand Length.” Zeitschrift für Physikalische Chemie, 229, 1-2, Pp. 179-190.
Ido Hadar, Shira Halivni, Na’ama Even-Dar, Adam Faust, and Uri Banin. 2015. “Dimensionality effects on fluorescence resonance energy transfer between single semiconductor nanocrystals and multiple dye acceptors.” The Journal of Physical Chemistry C, 119, 7, Pp. 3849-3856. Publisher's Version Abstract

Abstract Image

Colloidal semiconductor nanocrystals are outstanding donors in energy transfer processes due to their unique size and shape dependent optical properties, their exceptional photostability, and chemical processability. We examine the dimensionality effect in energy transfer between single heterostructure nanocrystals of spherical and rod shape, serving as donors, and multiple dye molecules attached to their surface acting as acceptors. Förster resonant energy transfer (FRET) to individual dyes attached to the surface of a single nanocrystal is identified via step-like changes in both acceptor and donor emission, enabling to calculate the efficiency of energy transfer and distance of each acceptor individually. This offers a unique tool to study the surface chemistry of various nanocrystals. The dimensionality of the nanocrystals is reflected by the acceptors distribution, which enables to study the inner geometry of these heterostructures, such as the location of the seed and shell thickness. Additionally, the nanocrystals serve as an optical antenna that enhances the excitation and emission of the dye molecules through the FRET interaction. These measurements enable to gain deeper understanding of the energy transfer process between semiconductor nanocrystals of various geometries and dye molecules and promote its utilization for extremely sensitive sensing applications at the single molecule level.

Reut Timor, Hana Weitman, Nir Waiskopf, Uri Banin, and Benjamin Ehrenberg. 2015. “PEG-phospholipids coated quantum rods as amplifiers of the photosensitization process by FRET.” ACS applied materials & interfaces, 7, 38, Pp. 21107-21114. Publisher's Version Abstract

Abstract Image

Singlet oxygen (1O2) generated upon photostimulation of photosensitizer molecules is a highly reactive specie which is utilized in photodynamic therapy. Recent studies have shown that semiconductor nanoparticles can be used as donors in fluorescence resonance energy transfer (FRET) process to excite attached photosensitizer molecules. In these studies, their unique properties, such as low nanoscale size, long-term photostability, wide broad absorbance band, large absorption cross section, and narrow and tunable emission bands were used to provide advantages over the traditional methods to produce singlet oxygen. Previous studies that achieved this goal, however, showed some limitations, such as low FRET efficiency, poor colloidal stability, nonspecific interactions, and/or complex preparation procedure. In this work, we developed and characterized a novel system of semiconductor quantum rods (QRs) and the photosensitizer meso-tetra(hydroxyphenyl) chlorin (mTHPC), as a model system that produces singlet oxygen without these limitations. A simple two-step preparation method is shown; Hydrophobic CdSe/CdS QRs are solubilized in aqueous solutions by encapsulation with lecithin and PEGylated phospholipid (PEG–PL) of two lipid lengths: PEG350 or PEG2000. Then, the hydrophobic photosensitizer mTHPC, was intercalated into the new amphiphilic PEG–PL coating of the QR, providing a strong attachment to the nanoparticle without covalent linkage. These PEGylated QR (eQR)–mTHPC nanocomposites show efficient FRET processes upon light stimulation of the QR component which results in efficient production of singlet oxygen. The results demonstrate the potential for future use of this concept in photodynamic therapy schemes.

 

Ido Hadar, Tsafrir Abir, Shira Halivni, Adam Faust, and Uri Banin. 2015. “Size‐Dependent Ligand Layer Dynamics in Semiconductor Nanocrystals Probed by Anisotropy Measurements.” Angewandte Chemie, 127, 42, Pp. 12640-12644.
Yuval Ben‐Shahar, Francesco Scotognella, Nir Waiskopf, Ilka Kriegel, Stefano Dal Conte, Giulio Cerullo, and Uri Banin. 2015. “Effect of surface coating on the photocatalytic function of hybrid CdS–Au nanorods.” Small, 11, 4, Pp. 462-471. Publisher's Version Abstract

image
Hybrid semiconductor–metal nanoparticles are interesting materials for use as photocatalysts due to their tunable properties and chemical processibility. Their function in the evolution of hydrogen in photocatalytic water splitting is the subject of intense current investigation. Here, the effects of the surface coatings on the photocatalytic function are studied, with Au‐tipped CdS nanorods as a model hybrid nanoparticle system. Kinetic measurements of the hydrogen evolution rate following photocatalytic water reduction are performed on similar nanoparticles but with different surface coatings, including various types of thiolated alkyl ligands and different polymer coatings. The apparent hydrogen evolution quantum yields are found to strongly depend on the surface coating. The lowest yields are observed for thiolated alkyl ligands. Intermediate values are obtained with L‐glutathione and poly(styrene‐co ‐maleic anhydride) polymer coatings. The highest efficiency is obtained for polyethylenimine (PEI) polymer coating. These pronounced differences in the photocatalytic efficiencies are correlated with ultrafast transient absorption spectroscopy measurements, which show a faster bleach recovery for the PEI‐coated hybrid nanoparticles, consistent with faster and more efficient charge separation. These differences are primarily attributed to the effects of surface passivation by the different coatings affecting the surface trapping of charge carriers that compete with effective charge separation required for the photocatalysis. Further support of this assignment is provided from steady‐state emission and time‐resolved spectral measurements, performed on related strongly fluorescing CdSe/CdS nanorods. The control and understanding of the effect of the surface coating of the hybrid nanosystems on the photocatalytic processes is of importance for the potential application of hybrid nanoparticles as photocatalysts.

Yorai Amit, Yuanyuan Li, Anatoly I Frenkel, and Uri Banin. 2015. “From Impurity Doping to Metallic Growth in Diffusion Doping: Properties and Structure of Silver-Doped InAs Nanocrystals.” ACS nano, 9, 11, Pp. 10790-10800. Publisher's Version Abstract

Abstract Image

Tuning of the electronic properties of presynthesized colloidal semiconductor nanocrystals (NCs) by doping plays a key role in the prospect of implementing them in printed electronics devices such as transistors and photodetectors. While such impurity doping reactions have already been introduced, the understanding of the doping process, the nature of interaction between the impurity and host atoms, and the conditions affecting the solubility limit of impurities in nanocrystals are still unclear. Here, we used a postsynthesis diffusion-based doping reaction to introduce Ag impurities into InAs NCs. Optical absorption spectroscopy and analytical inductively coupled plasma mass spectroscopy (ICP-MS) were used to present a two-stage doping model consisting of a “doping region” and a “growth region”, depending on the impurity to NC ratio in the reaction vessel. X-ray absorption fine-structure (XAFS) spectroscopy was employed to determine the impurity location and correlate between the structural and electronic properties for different sizes of InAs NCs and dopant concentrations. The resulting structural model describes a heterogeneous system where the impurities initially dope the NC, by substituting for In atoms near the surface of the NC, until the “solubility limit” is reached, after which the rapid growth and formation of metallic structures are identified.

 

Shira Halivni, Shay Shemesh, Nir Waiskopf, Yelena Vinetsky, Shlomo Magdassi, and Uri Banin. 2015. “Inkjet printed fluorescent nanorod layers exhibit superior optical performance over quantum dots.” Nanoscale, 7, 45, Pp. 19193-19200. Publisher's Version Abstract

 c5nr06248a-f1.tif

Semiconductor nanocrystals exhibit unique fluorescence properties which are tunable in size, shape and composition. The high quantum yield and enhanced stability have led to their use in biomedical imaging and flat panel displays. Here, semiconductor nanorod based inkjet inks are presented, overcoming limitations of the commonly reported quantum dots in printing applications. Fluorescent seeded nanorods were found to be outstanding candidates for fluorescent inks, due to their low particle–particle interactions and negligible self-absorption. This is manifested by insignificant emission shifts upon printing, even in highly concentrated printed layers and by maintenance of a high fluorescence quantum yield, unlike quantum dots which exhibit fluorescence wavelength shifts and quenching effects. This behavior results from the reduced absorption/emission overlap, accompanied by low energy transfer efficiencies between the nanorods as supported by steady state and time resolved fluorescence measurements. The new seeded nanorod inks enable patterning of thin fluorescent layers, for demanding light emission applications such as signage and displays.

2014
Amit Halevi, Shira Halivni, Meirav Oded, Axel HE Müller, Uri Banin, and Roy Shenhar. 2014. “Co-assembly of A–B diblock copolymers with B′-type nanoparticles in thin films: effect of copolymer composition and nanoparticle shape.” Macromolecules, 47, 9, Pp. 3022-3032. Publisher's Version Abstract

 

41. co-assembly of B'-type nanorods in A-B block copolymers

The coassembly of A B diblock copolymers with B'-type nanoparticles (i.e., nanoparticles that are slightly incompatible with the B domain) leads to hierarchical structures, where the block copolymer phase separates first and the nanoparticles create close-packed arrays within the B domains due to a slower, secondary phase separation process. Here we report the results of a comprehensive study, which focused on two aspects: the influence of the nanoparticle shape (spherical vs rod-like) and the effect of the volume composition of the blocks. Three polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymers featuring similar molecular weights but differing in PS volume fraction were mixed with spherical and rod-shaped poly(ethylene oxide)- (PEO-) capped CdS nanoparticles at different filling fractions and cast as thin films. Our results highlight the mutual influence between the block copolymer and the nanoparticles on the resulting morphology, demonstrating the ability to control the film morphology by the filling fraction of the nanoparticles and their tendency to localize at the film surface, and by confinement-induced nanoparticle aggregation. Most importantly, the results reveal the influence of the nanoparticle shape on the structure of the film.

Guohua Jia and Uri Banin. 2014. “A general strategy for synthesizing colloidal semiconductor zinc chalcogenide quantum rods.” Journal of the American Chemical Society, 136, 31, Pp. 11121-11127. Publisher's Version Abstract

Abstract Image

Quasi-one-dimensional (1D) semiconductor nanocrystals manifest linearly polarized emission, reduced lasing threshold, and improved charge transport compared with their counterparts such as spherical quantum dots. Present investigations of colloidal semiconductor quantum rods are mainly based on cadmium chalcogenide systems because of their facile synthetic accessibility. However, it is still a big challenge to fabricate quasi-1D zinc chalcogenide nanocrystals with controlled aspect ratios. Here we report a general strategy for synthesizing zinc chalcogenide quantum rods via a colloidal chemical synthetic approach. Unlike the most common growth mechanisms of quasi-1D colloidal nanocrystals such as monomer attachment and particle coalescence, the synthesis of zinc chalcogenide quantum rods is performed by a ripening process starting from their respective ultrathin nanowires through thermodynamically driven material diffusion. We anticipate that this strategy is general and could be applied to other systems to construct quasi-1D nanostructures. Moreover, the presence of cadmium-free (or “green”) zinc chalcogenide quantum rods synthesized through this strategy provides a desirable platform for eco-friendly photocatalysis, optoelectronic devices, biolabeling, and other applications.

Uri Banin, Yuval Ben-Shahar, and Kathy Vinokurov. 2014. “Hybrid semiconductor–metal nanoparticles: from architecture to function.” Chemistry of Materials, 26, 1, Pp. 97-110.
Shlomit Kraus‐Ophir, Yuval Ben‐Shahar, Uri Banin, and Daniel Mandler. 2014. “Perpendicular Orientation of Anisotropic Au‐Tipped CdS Nanorods at the Air/Water Interface.” Advanced Materials Interfaces, 1, 1, Pp. 1300030.