Uri Banin - Academia.edu (original) (raw)

Papers by Uri Banin

Chemistry of Materials, Oct 17, 2016

Metal-semiconductor hybrid nanoparticles (NPs) offer interesting synergistic properties, leading ... more 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 post-synthesis, room-temperature reaction between AuCl 3 and InAs nanocrystals (NCs) to form metal-semiconductor core-shell hybrid NPs, through the "reverse Kirkendall effect". In the presented system the diffusion rate of the inward diffusing specie (Au) is faster than that of the outward diffusion specie (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 (TR) 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 Auto -NC ratio. We have determined, based on multi-element 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 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 co-dependency of different structural consequences when introducing Au into the InAs NCs. This study of diffusion effects in nanocrystals therefore has relevance to powerful concepts in solid-state nanochemistry related to processes of cation exchange, doping reactions, and diffusion mechanisms.

Coupled colloidal quantum dot (CQD) dimers represent a new class of artificial molecules composed... more Coupled colloidal quantum dot (CQD) dimers represent a new class of artificial molecules composed of fused core/shell semiconductor nanocrystals. The electronic coupling and wavefunction hybridization is enabled by the formation of an epitaxial connection with a coherent lattice between the shells of the two neighboring quantum dots where the shell material and its dimensions dictate the quantum barrier characteristics for the charge carriers. Herein we introduce a colloidal approach to control the neck formation at the interface between the two CQDs in such artificial molecular constructs. This allows the tailoring of the neck barrier in pre-linked homodimers formed via fusion of multifaceted wurtzite CdSe/CdS CQDs. The effects of reaction time, temperature and excess ligands is studied. The neck filling process follows an intraparticle ripening mechanism at relatively mild reaction conditions while avoiding inter-particle ripening. The degree of surface ligand passivation plays a key role in activating the surface atom diffusion to the neck region. The degree of neck filling strongly depends also on the initial relative orientation of the two CQDs, where homonymous plane attachment allows for facile neck growth, unlike the case of heteronymous plane attachment. Upon neck-filling, the observed redshift of the absorption and fluorescence measured both for ensemble and single dimers, is assigned to enhanced hybridization of the confined wavefunction in CQD dimer molecules, as supported by quantum calculations. The fine tuning of the particle interface introduced herein provides therefore a powerful tool to further control the extent of hybridization and coupling in CQD molecules.

Angewandte Chemie International Edition, 2021

Supporting information for this article is given via a link at the end of the document.

Nanotechnology, 2020

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the applicati... more This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: ‘high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing’ to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as descri...

Nanotechnology, 2020

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the applicati... more This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: ‘high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing’ to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as descri...

Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the ... more Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the key strategy for achieving high fluorescence quantum efficiency and essential stability for optoelectronic applications and biotagging with emissive QDs. Herein we investigate the effect of shell growth rate on the structure and optical properties in blue-emitting ZnSe/ ZnS QDs with narrow emission line width. Tuning the precursor reactivity modifies the growth mode of ZnS shells on ZnSe cores transforming from kinetic (fast) to thermodynamic (slow) growth regimes. In the thermodynamic growth regime, enhanced fluorescence quantum yields and reduced on−off blinking are achieved. This high performance is ascribed to the effective avoidance of traps at the interface between the core and the shell, which are detrimental to the emission properties. Our study points to a general strategy to obtain high-quality core/shell QDs with enhanced optical properties through controlled reactivity yielding shell growth in the thermodynamic limit.

ECS Meeting Abstracts, 2013

The Journal of Physical Chemistry C, 2019

The electrical functionality of an array of semiconductor nanocrystals depends critically on the ... more The electrical functionality of an array of semiconductor nanocrystals depends critically on the free carriers that may arise from impurity or surface doping. Herein, we used InAs nanocrystals thin films as a model system to address the relative contributions of these doping mechanisms by comparative analysis of as-synthesized and Cu-doped nanocrystal based field effect transistor (FET) characteristics. By applying FET simulation methods used in conventional semiconductor FETs, we elucidate surface and impurity-doping contributions to the overall performance of InAs NCs based FETs. As-synthesized InAs nanocrystal based FETs show n-type characteristics assigned to the contribution of surface electrons accumulation layer that can be considered as an actual electron donating doping level with specific doping density and is energetically located just below the conduction band. The Cu-doped InAs NCs FETs show enhanced n-type conduction as expected from the Cu impurities location as an interstitial n-dopant in InAs nanocrystals. The simulated curves reveal the additional contribution from electrons within an impurity sub band close to the conduction band onset of the InAs NCs. The work therefore demonstrates the utility of the bulk FET simulation methodology also to NC based FETs. It provides guidelines for control of doping of nanocrystal arrays separately from surface contributions and impurity doping in colloidal semiconductor NCs towards their future utilization as building blocks in bottom-up prepared optoelectronic devices.

Nature Communications, 2019

Semiconductor heterostructure nanocrystals, especially with core/shell architectures, are importa... more Semiconductor heterostructure nanocrystals, especially with core/shell architectures, are important for numerous applications. Here we show that by decreasing the shell growth rate the morphology of ZnS shells on ZnSe quantum rods can be tuned from flat to islands-like, which decreases the interfacial strain energy. Further reduced growth speed, approaching the thermodynamic limit, leads to coherent shell growth forming unique helical-shell morphology. This reveals a template-free mechanism for induced chirality at the nanoscale. The helical morphology minimizes the sum of the strain and surface energy and maintains band gap emission due to its coherent core/shell interface without traps, unlike the other morphologies. Reaching the thermodynamic controlled growth regime for colloidal semiconductor core/shell nanocrystals thus offers morphologies with clear impact on their applicative potential.

Journal of the American Chemical Society, Jan 12, 2018

Semiconductor nanocrystals serve as outstanding model systems for studying quantum confined size ... more Semiconductor nanocrystals serve as outstanding model systems for studying quantum confined size and shape effects. Shape control is an important knob for controlling their properties but so far it has been well developed mainly for heavy-metal containing semiconductor nanocrystals, limiting their further widespread utilization. Herein, we report a synthesis of heavy-metal free ZnSe nanocrystals with shape and size control through utilization of well-defined molecular clusters. In this approach, ZnSe nanowires are synthesized and their length and shape control is achieved by introduction of controlled amounts of molecular clusters. As a result of [Zn(SPh)](MeN) clusters (Zn clusters) addition, short ZnSe nanorods or ZnSe nanodots can be obtained through tuning the ratio of Zn clusters to ZnSe. A study using transmission electron microscopy revealed the formation of a hybrid inorganic-organic nanowire, whereby the ligands form a template for self-assembly of ZnSe magic size clusters....

Physical Review B, 1999

Confined acoustic phonons in InAs nanocrystals are observed in the time domain by femtosecond pum... more Confined acoustic phonons in InAs nanocrystals are observed in the time domain by femtosecond pumpprobe spectroscopy. The size dependence of both frequency and damping time is investigated for InAs. The frequency of the discrete modes varies between 18 and 30 cm Ϫ1 for a nanocrystal radius ranging from 28 to 12 Å, and the dependence deviates from the expected 1/R behavior at small sizes. The damping rate is found to vary linearly with 1/R, suggesting coupling to the matrix through the particle surface as a main damping route. This is corroborated by observing faster damping for the acoustic mode in solution compared to a polymer environment. The coupling strength is found to depend on material parameters: in CdSe nanocrystals, weaker coupling is observed. The role of acoustic phonon coupling in the dephasing dynamics of semiconductor nanocrystals is discussed. ͓S0163-1829͑99͒01627-6͔

Chemistry of Materials, 2016

Metal−semiconductor hybrid nanoparticles (NPs) offer interesting synergistic properties, leading ... more 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 AuCl 3 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 finestructure (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 Auto -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.

ACS nano, Jan 21, 2015

Tuning of the electronic properties of pre-synthesized colloidal semiconductor nanocrystals (NCs)... more Tuning of the electronic properties of pre-synthesized 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 post-synthesis diffusion based doping reaction to introduce Ag impurities into InAs NCs. Optical absorption spectroscopy along with 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 concentration of the impurities in the reaction vessel. X-ray absorption fine-structure (XAFS) spectroscopy was employed to determine the impu...

Small, 2014

This is the peer reviewed version of the following article: "Effect of surface coating on the pho... more This is the peer reviewed version of the following article: "Effect of surface coating on the photocatalytic function of hybrid CdS-Au nanorods" (Small, 2015.

The Journal of Physical Chemistry B, 2004

Semiconductor nanocrystals are used to functionalize atomic force microscope (AFM) tips. Such tip... more Semiconductor nanocrystals are used to functionalize atomic force microscope (AFM) tips. Such tips may be useful for fluorescence resonance energy transfer (FRET) microscopy. CdSe/ZnS nanocrystals are chemically bound to the surface of an AFM tip by three coating methods utilizing organosilane linker molecules. Binding of nanocrystals by these methods was characterized on silicon and glass surfaces by AFM, scanning electron microscopy (SEM), and optical measurements. The use of mercaptopropyltrimethoxysilane as the linker molecule is found to provide the optimal linking scheme. Via the linking of the nanocrystals to the tip, the unique photophysical properties of the quantum dots and their tunability via chemical synthesis are exploited to create light-emitting scanning probes with controlled emission color, using a single excitation source. The functionalized probes retain their sharpness for high-resolution AFM topography acquisition. These properties are desirable for FRET schemes where the nanocrystals on the tip serve as either FRET donors or acceptors interacting with chromophores on the scanned sample. This interaction provides a contrast mechanism for high-resolution optical imaging in the near field. The potential of these coated tips for FRET-based imaging is demonstrated by localized binding of acceptor dye molecules to the functionalized tips resulting in distinctive FRET signals.

The Journal of Physical Chemistry A, 2006

We investigate the modification of photoluminescence (PL) from single semiconductor nanocrystal q... more We investigate the modification of photoluminescence (PL) from single semiconductor nanocrystal quantum dots (NCs) in proximity of metal and semiconducting Atomic Force Microscope (AFM) tips. The presence of the tip alters the radiative decay rate of an emitter via interference and opens efficient non radiative decay channels via energy transfer to the tip material. These effects cause quenching (or enhancement) of the emitter's PL intensity, as a function of its distance from the interacting tip. We take advantage of this highly distance dependent effect to realize a contrast mechanism for high resolution optical imaging. AFM tips are optimized as energy acceptors by chemical functionalization with InAs NCs to achieve optical resolution down to 30 nm. The presented experimental scheme offers high resolution optical information while maintaining the benefits of traditional AFM imaging. We directly measure the PL intensity of single NCs as a function of the tip distance. Our results are in good agreement to calculation made by a classical theoretical model describing an oscillating dipole interacting with a planar mirror.

Superlattices and Microstructures, 1997

The near band-gap level structure in high-quality colloidal InAs nanocrystal quantum dots within ... more The near band-gap level structure in high-quality colloidal InAs nanocrystal quantum dots within the very strong confinement regime is investigated. Size-selective photoluminescence excitation and fluorescence line narrowing measurements reveal a size-dependent splitting between the absorbing and the emitting states. The splitting is assigned to the confinementenhanced electron-hole exchange interaction. The size dependence of the splitting significantly deviates from the idealized 1/r 3 scaling law for the exchange splitting. A model incorporating a finite barrier which allows for wavefunction leakage is introduced. The model reproduces the observed 1/r 2 dependence of the splitting and good agreement with the experimental data is obtained. The smaller barriers for embedded InAs dots grown by molecular-beam epitaxy, are predicted to result in smaller exchange splitting as compared with colloidal dots with a similar number of atoms.

Small, 2009

This study evaluates the influence of particle size, PEGylation, and surface coating on the quant... more This study evaluates the influence of particle size, PEGylation, and surface coating on the quantitative biodistribution of near‐infrared‐emitting quantum dots (QDs) in mice. Polymer‐ or peptide‐coated 64Cu‐labeled QDs 2 or 12 nm in diameter, with or without polyethylene glycol (PEG) of molecular weight 2000, are studied by serial micropositron emission tomography imaging and region‐of‐interest analysis, as well as transmission electron microscopy and inductively coupled plasma mass spectrometry. PEGylation and peptide coating slow QD uptake into the organs of the reticuloendothelial system (RES), liver and spleen, by a factor of 6–9 and 2–3, respectively. Small particles are in part renally excreted. Peptide‐coated particles are cleared from liver faster than physical decay alone would suggest. Renal excretion of small QDs and slowing of RES clearance by PEGylation or peptide surface coating are encouraging steps toward the use of modified QDs for imaging living subjects.

Physical Review Letters, 2002

Photoluminescence excitation spectroscopy and scanning tunneling spectroscopy are used to study t... more 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 rod diameter and not on length. With increasing diameter, the band-gap and the excited state level spacings shift to the red. The level structure was assigned using a multi-band effective-mass model, showing a similar dependence on rod dimensions.

Chemistry of Materials, Oct 17, 2016

Metal-semiconductor hybrid nanoparticles (NPs) offer interesting synergistic properties, leading ... more 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 post-synthesis, room-temperature reaction between AuCl 3 and InAs nanocrystals (NCs) to form metal-semiconductor core-shell hybrid NPs, through the "reverse Kirkendall effect". In the presented system the diffusion rate of the inward diffusing specie (Au) is faster than that of the outward diffusion specie (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 (TR) 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 Auto -NC ratio. We have determined, based on multi-element 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 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 co-dependency of different structural consequences when introducing Au into the InAs NCs. This study of diffusion effects in nanocrystals therefore has relevance to powerful concepts in solid-state nanochemistry related to processes of cation exchange, doping reactions, and diffusion mechanisms.

Coupled colloidal quantum dot (CQD) dimers represent a new class of artificial molecules composed... more Coupled colloidal quantum dot (CQD) dimers represent a new class of artificial molecules composed of fused core/shell semiconductor nanocrystals. The electronic coupling and wavefunction hybridization is enabled by the formation of an epitaxial connection with a coherent lattice between the shells of the two neighboring quantum dots where the shell material and its dimensions dictate the quantum barrier characteristics for the charge carriers. Herein we introduce a colloidal approach to control the neck formation at the interface between the two CQDs in such artificial molecular constructs. This allows the tailoring of the neck barrier in pre-linked homodimers formed via fusion of multifaceted wurtzite CdSe/CdS CQDs. The effects of reaction time, temperature and excess ligands is studied. The neck filling process follows an intraparticle ripening mechanism at relatively mild reaction conditions while avoiding inter-particle ripening. The degree of surface ligand passivation plays a key role in activating the surface atom diffusion to the neck region. The degree of neck filling strongly depends also on the initial relative orientation of the two CQDs, where homonymous plane attachment allows for facile neck growth, unlike the case of heteronymous plane attachment. Upon neck-filling, the observed redshift of the absorption and fluorescence measured both for ensemble and single dimers, is assigned to enhanced hybridization of the confined wavefunction in CQD dimer molecules, as supported by quantum calculations. The fine tuning of the particle interface introduced herein provides therefore a powerful tool to further control the extent of hybridization and coupling in CQD molecules.

Angewandte Chemie International Edition, 2021

Supporting information for this article is given via a link at the end of the document.

Nanotechnology, 2020

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the applicati... more This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: ‘high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing’ to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as descri...

Nanotechnology, 2020

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the applicati... more This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: ‘high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing’ to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as descri...

Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the ... more Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the key strategy for achieving high fluorescence quantum efficiency and essential stability for optoelectronic applications and biotagging with emissive QDs. Herein we investigate the effect of shell growth rate on the structure and optical properties in blue-emitting ZnSe/ ZnS QDs with narrow emission line width. Tuning the precursor reactivity modifies the growth mode of ZnS shells on ZnSe cores transforming from kinetic (fast) to thermodynamic (slow) growth regimes. In the thermodynamic growth regime, enhanced fluorescence quantum yields and reduced on−off blinking are achieved. This high performance is ascribed to the effective avoidance of traps at the interface between the core and the shell, which are detrimental to the emission properties. Our study points to a general strategy to obtain high-quality core/shell QDs with enhanced optical properties through controlled reactivity yielding shell growth in the thermodynamic limit.

ECS Meeting Abstracts, 2013

The Journal of Physical Chemistry C, 2019

The electrical functionality of an array of semiconductor nanocrystals depends critically on the ... more The electrical functionality of an array of semiconductor nanocrystals depends critically on the free carriers that may arise from impurity or surface doping. Herein, we used InAs nanocrystals thin films as a model system to address the relative contributions of these doping mechanisms by comparative analysis of as-synthesized and Cu-doped nanocrystal based field effect transistor (FET) characteristics. By applying FET simulation methods used in conventional semiconductor FETs, we elucidate surface and impurity-doping contributions to the overall performance of InAs NCs based FETs. As-synthesized InAs nanocrystal based FETs show n-type characteristics assigned to the contribution of surface electrons accumulation layer that can be considered as an actual electron donating doping level with specific doping density and is energetically located just below the conduction band. The Cu-doped InAs NCs FETs show enhanced n-type conduction as expected from the Cu impurities location as an interstitial n-dopant in InAs nanocrystals. The simulated curves reveal the additional contribution from electrons within an impurity sub band close to the conduction band onset of the InAs NCs. The work therefore demonstrates the utility of the bulk FET simulation methodology also to NC based FETs. It provides guidelines for control of doping of nanocrystal arrays separately from surface contributions and impurity doping in colloidal semiconductor NCs towards their future utilization as building blocks in bottom-up prepared optoelectronic devices.

Nature Communications, 2019

Semiconductor heterostructure nanocrystals, especially with core/shell architectures, are importa... more Semiconductor heterostructure nanocrystals, especially with core/shell architectures, are important for numerous applications. Here we show that by decreasing the shell growth rate the morphology of ZnS shells on ZnSe quantum rods can be tuned from flat to islands-like, which decreases the interfacial strain energy. Further reduced growth speed, approaching the thermodynamic limit, leads to coherent shell growth forming unique helical-shell morphology. This reveals a template-free mechanism for induced chirality at the nanoscale. The helical morphology minimizes the sum of the strain and surface energy and maintains band gap emission due to its coherent core/shell interface without traps, unlike the other morphologies. Reaching the thermodynamic controlled growth regime for colloidal semiconductor core/shell nanocrystals thus offers morphologies with clear impact on their applicative potential.

Journal of the American Chemical Society, Jan 12, 2018

Semiconductor nanocrystals serve as outstanding model systems for studying quantum confined size ... more Semiconductor nanocrystals serve as outstanding model systems for studying quantum confined size and shape effects. Shape control is an important knob for controlling their properties but so far it has been well developed mainly for heavy-metal containing semiconductor nanocrystals, limiting their further widespread utilization. Herein, we report a synthesis of heavy-metal free ZnSe nanocrystals with shape and size control through utilization of well-defined molecular clusters. In this approach, ZnSe nanowires are synthesized and their length and shape control is achieved by introduction of controlled amounts of molecular clusters. As a result of [Zn(SPh)](MeN) clusters (Zn clusters) addition, short ZnSe nanorods or ZnSe nanodots can be obtained through tuning the ratio of Zn clusters to ZnSe. A study using transmission electron microscopy revealed the formation of a hybrid inorganic-organic nanowire, whereby the ligands form a template for self-assembly of ZnSe magic size clusters....

Physical Review B, 1999

Confined acoustic phonons in InAs nanocrystals are observed in the time domain by femtosecond pum... more Confined acoustic phonons in InAs nanocrystals are observed in the time domain by femtosecond pumpprobe spectroscopy. The size dependence of both frequency and damping time is investigated for InAs. The frequency of the discrete modes varies between 18 and 30 cm Ϫ1 for a nanocrystal radius ranging from 28 to 12 Å, and the dependence deviates from the expected 1/R behavior at small sizes. The damping rate is found to vary linearly with 1/R, suggesting coupling to the matrix through the particle surface as a main damping route. This is corroborated by observing faster damping for the acoustic mode in solution compared to a polymer environment. The coupling strength is found to depend on material parameters: in CdSe nanocrystals, weaker coupling is observed. The role of acoustic phonon coupling in the dephasing dynamics of semiconductor nanocrystals is discussed. ͓S0163-1829͑99͒01627-6͔

Chemistry of Materials, 2016

Metal−semiconductor hybrid nanoparticles (NPs) offer interesting synergistic properties, leading ... more 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 AuCl 3 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 finestructure (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 Auto -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.

ACS nano, Jan 21, 2015

Tuning of the electronic properties of pre-synthesized colloidal semiconductor nanocrystals (NCs)... more Tuning of the electronic properties of pre-synthesized 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 post-synthesis diffusion based doping reaction to introduce Ag impurities into InAs NCs. Optical absorption spectroscopy along with 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 concentration of the impurities in the reaction vessel. X-ray absorption fine-structure (XAFS) spectroscopy was employed to determine the impu...

Small, 2014

This is the peer reviewed version of the following article: "Effect of surface coating on the pho... more This is the peer reviewed version of the following article: "Effect of surface coating on the photocatalytic function of hybrid CdS-Au nanorods" (Small, 2015.

The Journal of Physical Chemistry B, 2004

Semiconductor nanocrystals are used to functionalize atomic force microscope (AFM) tips. Such tip... more Semiconductor nanocrystals are used to functionalize atomic force microscope (AFM) tips. Such tips may be useful for fluorescence resonance energy transfer (FRET) microscopy. CdSe/ZnS nanocrystals are chemically bound to the surface of an AFM tip by three coating methods utilizing organosilane linker molecules. Binding of nanocrystals by these methods was characterized on silicon and glass surfaces by AFM, scanning electron microscopy (SEM), and optical measurements. The use of mercaptopropyltrimethoxysilane as the linker molecule is found to provide the optimal linking scheme. Via the linking of the nanocrystals to the tip, the unique photophysical properties of the quantum dots and their tunability via chemical synthesis are exploited to create light-emitting scanning probes with controlled emission color, using a single excitation source. The functionalized probes retain their sharpness for high-resolution AFM topography acquisition. These properties are desirable for FRET schemes where the nanocrystals on the tip serve as either FRET donors or acceptors interacting with chromophores on the scanned sample. This interaction provides a contrast mechanism for high-resolution optical imaging in the near field. The potential of these coated tips for FRET-based imaging is demonstrated by localized binding of acceptor dye molecules to the functionalized tips resulting in distinctive FRET signals.

The Journal of Physical Chemistry A, 2006

We investigate the modification of photoluminescence (PL) from single semiconductor nanocrystal q... more We investigate the modification of photoluminescence (PL) from single semiconductor nanocrystal quantum dots (NCs) in proximity of metal and semiconducting Atomic Force Microscope (AFM) tips. The presence of the tip alters the radiative decay rate of an emitter via interference and opens efficient non radiative decay channels via energy transfer to the tip material. These effects cause quenching (or enhancement) of the emitter's PL intensity, as a function of its distance from the interacting tip. We take advantage of this highly distance dependent effect to realize a contrast mechanism for high resolution optical imaging. AFM tips are optimized as energy acceptors by chemical functionalization with InAs NCs to achieve optical resolution down to 30 nm. The presented experimental scheme offers high resolution optical information while maintaining the benefits of traditional AFM imaging. We directly measure the PL intensity of single NCs as a function of the tip distance. Our results are in good agreement to calculation made by a classical theoretical model describing an oscillating dipole interacting with a planar mirror.

Superlattices and Microstructures, 1997

The near band-gap level structure in high-quality colloidal InAs nanocrystal quantum dots within ... more The near band-gap level structure in high-quality colloidal InAs nanocrystal quantum dots within the very strong confinement regime is investigated. Size-selective photoluminescence excitation and fluorescence line narrowing measurements reveal a size-dependent splitting between the absorbing and the emitting states. The splitting is assigned to the confinementenhanced electron-hole exchange interaction. The size dependence of the splitting significantly deviates from the idealized 1/r 3 scaling law for the exchange splitting. A model incorporating a finite barrier which allows for wavefunction leakage is introduced. The model reproduces the observed 1/r 2 dependence of the splitting and good agreement with the experimental data is obtained. The smaller barriers for embedded InAs dots grown by molecular-beam epitaxy, are predicted to result in smaller exchange splitting as compared with colloidal dots with a similar number of atoms.

Small, 2009

This study evaluates the influence of particle size, PEGylation, and surface coating on the quant... more This study evaluates the influence of particle size, PEGylation, and surface coating on the quantitative biodistribution of near‐infrared‐emitting quantum dots (QDs) in mice. Polymer‐ or peptide‐coated 64Cu‐labeled QDs 2 or 12 nm in diameter, with or without polyethylene glycol (PEG) of molecular weight 2000, are studied by serial micropositron emission tomography imaging and region‐of‐interest analysis, as well as transmission electron microscopy and inductively coupled plasma mass spectrometry. PEGylation and peptide coating slow QD uptake into the organs of the reticuloendothelial system (RES), liver and spleen, by a factor of 6–9 and 2–3, respectively. Small particles are in part renally excreted. Peptide‐coated particles are cleared from liver faster than physical decay alone would suggest. Renal excretion of small QDs and slowing of RES clearance by PEGylation or peptide surface coating are encouraging steps toward the use of modified QDs for imaging living subjects.

Physical Review Letters, 2002

Photoluminescence excitation spectroscopy and scanning tunneling spectroscopy are used to study t... more 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 rod diameter and not on length. With increasing diameter, the band-gap and the excited state level spacings shift to the red. The level structure was assigned using a multi-band effective-mass model, showing a similar dependence on rod dimensions.