Publications

2017
Yuval Ben-Shahar and Uri Banin. 2017. “Hybrid semiconductor–metal nanorods as photocatalysts.” In Photoactive Semiconductor Nanocrystal Quantum Dots, Pp. 149-174. Springer. Publisher's Version Abstract

Hybrid nanoparticles combine two or more disparate materials on the same nanosystem and represent a powerful approach for achieving advanced materials with multiple functionalities stemming from the unusual materials combinations. This review focuses on recent advances in the area of semiconductor–metal hybrid nanoparticles. Synthesis approaches offering high degree of control over the number of components, their compositions, shapes, and interfacial characteristics are discussed, including examples of advanced architectures. Progress in hybrid nanoscale inorganic cage structures prepared by a selective edge growth mechanism of the metal onto the semiconductor nanocrystal is also presented. The combined and often synergistic properties of the hybrid nanoparticles are described with emphasis on optical properties, electronic structure, electrical characteristics, and light induced charge separation effects. Progress toward the application of hybrid nanoparticles in photocatalysis is overviewed. We conclude with a summary and point out some challenges for further development and understanding of semiconductor–metal hybrid nanoparticles. This progress shows promise for application of hybrid nanoparticles in photocatalysis, catalysis, optical components, and electronic devices.

Uri Banin and Oded Millo. 2017. “Optical and Tunneling Spectroscopy of Semiconductor Nanocrystal Quantum Dots.” In Nanocrystal Quantum Dots, Pp. 281-310. CRC Press. Publisher's Version
Adam Faust, Yorai Amit, and Uri Banin. 2017. “Phonon–Plasmon Coupling and Active Cu Dopants in Indium Arsenide Nanocrystals Studied by Resonance Raman Spectroscopy.” The journal of physical chemistry letters, 8, 11, Pp. 2519-2525. Publisher's Version Abstract

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Doping of semiconductor nanocrystals is an emerging tool to control their properties and has recently received increased interest as the means to characterize the impurities and their effect on the electronic characteristics of the nanocrystal evolve. We present a temperature-dependent Raman scattering study of Cu-doped InAs nanocrystals observing changes in the relative scattering intensities of the different modes upon increased dopant concentrations. First, the longitudinal optical (LO) phonon overtone mode is suppressed, indicating weakening of the coupling strength related to the effect of screening by the free electrons. Second, the transverse optical (TO) mode is relatively enhanced compared to the LO mode, which is attributed to the appearance of a coupled phonon–plasmon mode analogous to observations for n-type doped bulk InAs. These signatures indicate that the Cu impurities serve as active dopants and occupy an impurity-related pseudo sub-band akin to the heavy doping limit.

Yuval Ben-Shahar, Kathy Vinokurov, Héloïse de Paz-Simon, Yosef Gofer, Matan Leiter, Uri Banin, and Yaron S Cohen. 2017. “Photoelectrochemistry of colloidal Cu 2 O nanocrystal layers: the role of interfacial chemistry.” Journal of Materials Chemistry A, 5, 42, Pp. 22255-22264. Publisher's Version Abstract

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Colloidal Cu2O nanocrystal layers on Au substrates are studied as photocathodes in the context of solar electrochemical water-splitting applications. The photoelectrochemical response of the nanocrystal layers in aqueous solutions under simulated solar light conditions depends strongly on the interfacial chemistry and its impact on the transport of the charge carriers across the Au/nanocrystals/liquid interfaces. The Cu2O nanocrystals are originally stabilized with octadecylamine ligands. While octadecylamine is an efficient capping ligand for the colloidal synthesis of highly uniform nanocrystals, its low conductivity impedes the charge transport across the Au/nanocrystals/liquid interfaces. The photoresponse of the nanocrystals can be enhanced by the replacement of the octadecylamine ligands with more conductive and hydrophilic molecules, such as 1,2-ethanedithiol and benzene-1,4-dithiol. The conductivity and hydrophilicity of the ligands were investigated and found to be important for the photo-induced charge separation and transport across the Au/nanocrystals/liquid interfaces and transfer to the liquid. Furthermore, the interfacial energetics of the Au/nanocrystals/liquid junction and the resulting photoresponse of the Cu2O nanocrystal photocathode can be optimized by rational design of the exchanging ligands with desired functionalities and dipoles at the specific interfaces. A comparison of the photoresponse of Cu2O nanocrystal layers to that of electrodeposited Cu2O layers shows that the former is, yet, lower, due to the apparent low conductivity of the ligands. However, the nanocrystal organic ligands impart high hydrophobicity, which prevents the contact of the aqueous solution with the nanocrystals and improves their stability against photocorrosion and reduction to Cu0, as confirmed by X-ray diffraction measurements.

Amol Ashok Pawar, Shira Halivni, Nir Waiskopf, Yuval Ben-Shahar, Michal Soreni-Harari, Sarah Bergbreiter, Uri Banin, and Shlomo Magdassi. 2017. “Rapid three-dimensional printing in water using semiconductor–metal hybrid nanoparticles as photoinitiators.” Nano letters, 17, 7, Pp. 4497-4501. Publisher's Version Abstract

Additive manufacturing processes enable fabrication of complex and functional three-dimensional (3D) objects ranging from engine parts to artificial organs. Photopolymerization, which is the most versatile technology enabling such processes through 3D printing, utilizes photoinitiators that break into radicals upon light absorption. We report on a new family of photoinitiators for 3D printing based on hybrid semiconductor–metal nanoparticles. Unlike conventional photoinitiators that are consumed upon irradiation, these particles form radicals through a photocatalytic process. Light absorption by the semiconductor nanorod is followed by charge separation and electron transfer to the metal tip, enabling redox reactions to form radicals in aerobic conditions. In particular, we demonstrate their use in 3D printing in water, where they simultaneously form hydroxyl radicals for the polymerization and consume dissolved oxygen that is a known inhibitor. We also demonstrate their potential for two-photon polymerization due to their giant two-photon absorption cross section.

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

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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.

Yuval Ben-Shahar, Ilka Kriegel, Francesco Scotognella, Nir Waiskopf, Stefano Dal Conte, Luca Moretti, Giulio Cerullo, Eran Rabani, and Uri Banin. 2017. “Ultrafast carrier dynamics unravel role of surface ligands and metal domain size on the photocatalytic hydrogen evolution efficiency of Au-tipped CdS nanorods: an ultrafast transient absorption spectroscopy study”. Publisher's Version Abstract

Semiconductor-metal hybrid nanostructures are interesting materials for photocatalysis. Their tunable properties offer a highly controllable platform to design light-induced charge separation, a key to their function in photocatalytic water splitting. Hydrogen evolution quantum yields are influenced by factors as size, shape, material and morphology of the system, additionally the surface coating or the metal domain size play a dominant role.

 

In this paper we present a study on a well-defined model system of Au-tipped CdS nanorods. We use transient absorption spectroscopy to get insights into the charge carrier dynamics after photoexcitation of the bandgap of CdS nanorods. The study of charge transfer processes combined with the hydrogen evolution efficiency unravels the effects of surface coating and the gold tip size on the photocatalytic efficiency. Differences in efficiency with various surface ligands are primarily ascribed to the effects of surface passivation. Surface trapping of charge carriers is competing with effective charge separation, a prerequisite for photocatalysis, leading to the observed lower hydrogen production quantum yields. Interestingly, non-monotonic hydrogen evolution efficiency with size of the gold tip is observed, resulting in an optimal metal domain size for the most efficient photocatalysis. These results are explained by the sizedependent interplay of the metal domain charging and the relative band-alignments. Taken together our findings are of major importance for the potential application of hybrid nanoparticles as photocatalysts.

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

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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

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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.
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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

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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

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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

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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.

Uri Banin and Hagai Arbell. 2016. “Polarizing lighting systems.” United States of America.
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

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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.

Chunfan Yang, Itay Gdor, Yorai Amit, Adam Faust, Uri Banin, and Sanford Ruhman. 2015. “Exciton dynamics in Cu-doped InAs colloidal quantum dots.” In Ultrafast Phenomena XIX, Pp. 267-270. Springer.
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

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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.