Haimei Zheng - Academia.edu (original) (raw)

Papers by Haimei Zheng

Nature Materials, 2020

Electron microscopy touches on nearly every aspect of modern life, underpinning materials develop... more Electron microscopy touches on nearly every aspect of modern life, underpinning materials development for quantum computing, energy and medicine. We discuss the open, highly integrated and data-driven microscopy architecture needed to realize transformative discoveries in the coming decade.

Nano letters, Jan 20, 2016

Nanoparticle self-assembly has been well studied theoretically, but it remains challenging to dir... more Nanoparticle self-assembly has been well studied theoretically, but it remains challenging to directly observe and quantify individual nanoparticle interactions. With our custom image analysis method, we track the trajectories of nanoparticle movement with high precision from a stack of relatively noisy images obtained using liquid cell transmission electron microscopy. In a time frame of minutes, Pt-Fe nanoparticles self-assembled into a loosely packed hcp lattice. The energetics and stability of the dynamic assembly were studied quantitatively. From velocity and diffusion measurements, we experimentally determined the magnitude of forces between single particles and the related physical properties. The results illustrate that long-range anisotropic forces drive the formation of chains, which then clump and fold to maximize close range van der Waals interactions.

Small (Weinheim an der Bergstrasse, Germany), Jan 24, 2016

The high-yield and scalable production of single-layer ternary transition metal dichalcogenide na... more The high-yield and scalable production of single-layer ternary transition metal dichalcogenide nanosheets with ≈66% of metallic 1T phase, including MoS2x Se2(1-x) and Mox W1-x S2 is achieved via electrochemical Li-intercalation and the exfoliation method. Thin film MoS2 x Se2(1- x ) nanosheets drop-cast on a fluorine-doped tin oxide substrate are used as an efficient electrocatalyst on the counter electrode for the tri-iodide reduction in a dye-sensitized solar cell.

Nano letters, Jan 6, 2015

We report the observation of lithiation/delithiation of MoS2 nanosheets in a LiPF6/EC/DEC commerc... more We report the observation of lithiation/delithiation of MoS2 nanosheets in a LiPF6/EC/DEC commercial electrolyte for the application of lithium ion batteries using electrochemical liquid cell transmission electron microscopy (TEM). Upon discharge in a voltage range of 1.8 ~ 1.2 V, MoS2 on the Ti electrode underwent irreversible decomposition resulting in shattering of the MoS2 nanosheets into 5 ~ 10 nm MoS2 nanoparticles. Repeated experiments also show that some MoS2 nanosheets do not decompose upon lithiation. Instead, lithiation induced structural expansion and deformation has been observed. A solid electrolyte interface (SEI) was formed on the anode side of the Ti electrode in contact with Li metal. The SEI layer is composed of LiF nanocrystals distributed within the entire layer with the constituent elements C, O and F. However, no passivation film was observed on the cathode side of the Ti electrode with MoS2 nanosheets on it. Such in situ electrochemical liquid cell TEM study ...

Journal of visualized experiments : JoVE, Jan 20, 2012

The recent development for in situ transmission electron microscopy, which allows imaging through... more The recent development for in situ transmission electron microscopy, which allows imaging through liquids with high spatial resolution, has attracted significant interests across the research fields of materials science, physics, chemistry and biology. The key enabling technology is a liquid cell. We fabricate liquid cells with thin viewing windows through a sequential microfabrication process, including silicon nitride membrane deposition, photolithographic patterning, wafer etching, cell bonding, etc. A liquid cell with the dimensions of a regular TEM grid can fit in any standard TEM sample holder. About 100 nanoliters reaction solution is loaded into the reservoirs and about 30 picoliters liquid is drawn into the viewing windows by capillary force. Subsequently, the cell is sealed and loaded into a microscope for in situ imaging. Inside the TEM, the electron beam goes through the thin liquid layer sandwiched between two silicon nitride membranes. Dynamic processes of nanoparticle...

MRS Communications, 2014

Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fasc... more Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fascinated scientists for decades because they provide degrees of freedom to explore in condensed matter physics and design-coupled multifunctionlities. Recently, of particular interest is the perovskite-spinel-based VHN, covering a wide spectrum of promising applications. In this review, fabrication of VHN, their growth mechanism, control, and resulting novel multifunctionalities are discussed thoroughly, providing researchers a comprehensive blueprint to construct promising VHN. Following the fabrication section, the state-of-the-art design concepts for multifunctionalities are proposed and reviewed by suitable examples. By summarizing the outlook of this field, we are excitedly expecting this field to rise with significant contributions ranging from scientific value to practical applications in the foreseeable future.

Ultramicroscopy, 2013

The atomic structure and interfaces of CdS/Cu 2 S heterostructured nanorods are investigated with... more The atomic structure and interfaces of CdS/Cu 2 S heterostructured nanorods are investigated with the aberration-corrected TEAM 0.5 electron microscope operated at 80 kV and 300 kV applying in-line holography and complementary techniques. Cu 2 S exhibits a low-chalcocite structure in pristine CdS/Cu 2 S nanorods. Under electron beam irradiation the Cu 2 S phase transforms into a high-chalcocite phase while the CdS phase maintains its wurtzite structure. Time-resolved experiments reveal that Cu +-Cd 2+ cation exchange at the CdS/Cu 2 S interfaces is stimulated by the electron beam and proceeds within an undisturbed and coherent sulfur sub-lattice. A variation of the electron beam current provides an efficient way to control and exploit such irreversible solid-state chemical processes that provide unique information about system dynamics at the atomic scale. Specifically, we show that the electron beaminduced copper-cadmium exchange is site specific and anisotropic. A resulting displacement of the CdS/ Cu 2 S interfaces caused by beam-induced cation interdiffusion equals within a factor of 3-10 previously reported Cu diffusion length measurements in heterostructured CdS/Cu 2 S thin film solar cells with an activation energy of 0.96 eV.

Science, 2009

Mergers and Acquisitions The crystallization of small molecules or polymers is often described in... more Mergers and Acquisitions The crystallization of small molecules or polymers is often described in terms of a nucleation stage, where initial clusters form, followed by a distinct growth stage. Growth can come from the addition of unbound molecules, or through “Ostwald ripening” where larger crystals grow at the expense of smaller ones due to thermodynamic effects. Zheng et al. (p. 1309 ) studied the growth of platinum nanocrystals inside a transmission electron microscope using a special liquid cell, allowing observation of crystal growth in situ. Both monomer addition to growing particles and the coalescence of two particles were observed. The specific growth mechanism appeared to be governed by the size of each of the particles. The combination of growth processes makes it possible for an initially broad distribution of particles to narrow into an almost uniform one.

Physical Chemistry Chemical Physics, 2012

SnS 2 nanoparticle-loaded graphene nanocomposites were synthesized via one-step hydrothermal reac... more SnS 2 nanoparticle-loaded graphene nanocomposites were synthesized via one-step hydrothermal reaction. Their electrochemical performance was evaluated as the anode for rechargeable lithium-ion batteries after thermal treatment in an Ar environment. The electrochemical testing results show a high reversible capacity of more than 800 mA h g À1 at 0.1 C rate and 200 mA h g À1 for up to 5 C rate. The cells also exhibit excellent capacity retention for up to 90 cycles even at a high rate of 2 C. This electrochemical behavior can be attributed to the well-defined morphology and nanostructures of these as-synthesized nanocomposites, which is characterized by high-resolution transmission electron microscopy and electron energy-loss spectroscopy.

Nano Letters, 2009

We report solar cells based on highly confined nanocrystals of the ternary compound PbS x Se 1-x.... more We report solar cells based on highly confined nanocrystals of the ternary compound PbS x Se 1-x. Crystalline, monodisperse alloyed nanocrystals are obtained using a one-pot, hot injection reaction. Rutherford back scattering and energy filtered transmission electron microscopy suggest that the S and Se anions are uniformly distributed in the alloy nanoparticles. Photovoltaic devices made using ternary nanoparticles are more efficient than either pure PbS or pure PbSe based nanocrystal devices. Colloidal semiconductor nanocrystals display a wealth of size-dependent physical and chemical properties, including quantum confinement effects, shape dependent electronic structure, 1, 2 and control over assembly through modification of surface functionalization. 3, 4 Photovoltaic devices are an easily recognized potential application for nanocrystals due, in part, to their high photoactivity, solution processability and low cost of production. Several schemes for using nanocrystals in solar cells are under active consideration, including nanocrystal-polymer composites, 5 nanoparticle array solar cells, 6 films of partially sintered nanoparticles, 7 and nanocrystal analogues to dye-sensitized solar cells. 8 A persistent challenge for any nanoparticle-based solar cell is to take advantage of quantum confinement effects to improve the optical absorption process without overly hindering the subsequent transport of charge to the electrodes. Various

Nano Letters, 2009

We have directly observed motion of inorganic nanoparticles during fluid evaporation using a Tran... more We have directly observed motion of inorganic nanoparticles during fluid evaporation using a Transmission Electron Microscope. Tracking real-time diffusion of both spherical (5-15 nm) and rod-shaped (5x10 nm) gold nanocrystals in a thin-film of water-15%glycerol reveals complex movements, such as rolling motions coupled to large-step movements and macroscopic violations of the Stokes-Einstein relation for diffusion. As drying patches form during the final stages of evaporation, particle motion is dominated by the nearby retracting liquid front.

Microscopy and Microanalysis, 2012

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, ... more Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Journal of the American Chemical Society, 2009

We show that nanocrystals (NCs) with well-established synthetic protocols for high shape and size... more We show that nanocrystals (NCs) with well-established synthetic protocols for high shape and size monodispersity can be used as templates to independently control the NC composition through successive cation exchange reactions. Chemical transformations like cation exchange reactions overcome a limitation in traditional colloidal synthesis, where the NC shape often reflects the inherent symmetry of the underlying lattice. Specifically we show that full or partial interconversion between wurtzite CdS, chalcocite Cu 2 S, and rock salt PbS NCs can occur while preserving anisotropic shapes unique to the assynthesized materials. The exchange reactions are driven by disparate solubilites between the two cations by using ligands that preferentially coordinate to either monovalent or divalent transition metals. Starting with CdS, highly anisotropic PbS nanorods are created, which serve as an important material for studying strong two-dimensional quantum confinement, as well as for optoelectronic applications. In NC heterostructures containing segments of different materials, the exchange reaction can be made highly selective for just one of the components of the heterostructure. Thus, through precise control over ion insertion and removal, we can obtain interesting CdS|PbS heterostructure nanorods, where the spatial arrangement of materials is controlled through an intermediate exchange reaction.

Applied Physics Letters, 2004

For ͑001͒ c oriented BaTiO 3 thin films, it has been found that epitaxial constraint can result i... more For ͑001͒ c oriented BaTiO 3 thin films, it has been found that epitaxial constraint can result in a dramatic increase in the temperature of a tetragonal ͑T͒ structural phase transition. For 2000-Å-thick films grown directly on SrTiO 3 substrates, a T → cubic ͑C͒ phase transition was found on heating at Ͼ950 K, where the lattice constant changed smoothly with temperature. It was also found for films of the same thickness that the T → C transition is nearly restored to that of bulk crystals by the use of a buffer layer, which relaxes the epitaxial constraint. These results provide evidence of an epitaxially induced high temperature structural phase transition in BaTiO 3 thin films, where the ferroelectric (internal) and structural (external) aspects of the phase transition are decoupled.

Angewandte Chemie International Edition, 2012

ACS Nano, 2012

Direct imaging of nanoparticle solutions by liquid phase transmission electron microscopy has ena... more Direct imaging of nanoparticle solutions by liquid phase transmission electron microscopy has enabled unique in situ studies of nanoparticle motion and growth. In the present work, we report on real-time formation of two-dimensional nanoparticle arrays in the very low diffusive limit, where nanoparticles are mainly driven by capillary forces and solvent fluctuations. We find that superlattice formation appears to be segregated into multiple regimes. Initially, the solvent front drags the nanoparticles, condensing them into an amorphous agglomerate. Subsequently, the nanoparticle crystallization into an array is driven by local fluctuations. Following the crystallization event, superlattice growth can also occur via the addition of individual nanoparticles drawn from outlying regions by different solvent fronts. The dragging mechanism is consistent with simulations based on a coarse-grained lattice gas model at the same limit.

ACS Nano, 2012

Simultaneously probing the electronic structure and morphology of materials at the nanometer or a... more Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope (ETEM) equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy (EELS) in situ for cobalt catalysts under reaction conditions. Analysis revealed quantitative correlation of the cobalt valence states to the nanoporous structures. The in situ experiments were performed on nanoporous cobalt particles coated with silica while a 15 mTorr hydrogen environment was maintained at various temperatures (300-600 ºC). When the

ACS Nano, 2008

S ince the first report in 2004, 1 the formation of hollow nanoparticles via the nanoscale Kirken... more S ince the first report in 2004, 1 the formation of hollow nanoparticles via the nanoscale Kirkendall effect has been observed in dozens of materials. 2-5 In the simplest scenario, an elemental metallic nanocrystal reacts to form a compound nanoparticle; hollow, polycrystalline nanoparticles result when the diffusion of the cationic species outward is faster than the inward diffusion of the anionic species. Some examples of hollow particles formed in this way include oxides, sulfides, selenides, and phosphides of cobalt, iron, molybdenum, and nickel (e.g.

ACS Nano, 2011

Nanorod Synthesis & Assembly Details of the synthetic and assembly procedures for the nanorods ca... more Nanorod Synthesis & Assembly Details of the synthetic and assembly procedures for the nanorods can be found in the supporting information of our previous work 1. One important point to note about repeatability of this assembly is that relatively fresh nanorods (synthesized within ~1 month of assembly attempt) had higher rates of successful assembly. Also, while monodispersity was important (which requires cleaning), over-cleaning was found to result in rods that laid parallel to the substrate. Finding an appropriate balance of monodispersity and cleanliness was difficult and sometimes took a few synthetic attempts before successful alignment. The required cleaning procedure for each batch of rods was slightly different, despite the fact that the syntheses themselves were identical. Assembly substrates (Si3N4 for RBS measurements; 150nm 20Ω/square ITO on glass for electrical measurements) were cleaned by sequential sonication in water, toluene, isopropanol and acetone, followed by an oxygen plasma etch. Rutherford Backscattering Spectroscopy (RBS) Rutherford backscattering data was taken with a 2.5 MeV Van de Graaff accelerator. The spectra were taken using a 2 MeV He + ion beam with a Si surface barrier detector at 165 o with respect to the incident beam. The samples were tilted by 50 deg in order to improve depth resolution. The spectra were analyzed using the RUMP software package. `

Nature Materials, 2020

Electron microscopy touches on nearly every aspect of modern life, underpinning materials develop... more Electron microscopy touches on nearly every aspect of modern life, underpinning materials development for quantum computing, energy and medicine. We discuss the open, highly integrated and data-driven microscopy architecture needed to realize transformative discoveries in the coming decade.

Nano letters, Jan 20, 2016

Nanoparticle self-assembly has been well studied theoretically, but it remains challenging to dir... more Nanoparticle self-assembly has been well studied theoretically, but it remains challenging to directly observe and quantify individual nanoparticle interactions. With our custom image analysis method, we track the trajectories of nanoparticle movement with high precision from a stack of relatively noisy images obtained using liquid cell transmission electron microscopy. In a time frame of minutes, Pt-Fe nanoparticles self-assembled into a loosely packed hcp lattice. The energetics and stability of the dynamic assembly were studied quantitatively. From velocity and diffusion measurements, we experimentally determined the magnitude of forces between single particles and the related physical properties. The results illustrate that long-range anisotropic forces drive the formation of chains, which then clump and fold to maximize close range van der Waals interactions.

Small (Weinheim an der Bergstrasse, Germany), Jan 24, 2016

The high-yield and scalable production of single-layer ternary transition metal dichalcogenide na... more The high-yield and scalable production of single-layer ternary transition metal dichalcogenide nanosheets with ≈66% of metallic 1T phase, including MoS2x Se2(1-x) and Mox W1-x S2 is achieved via electrochemical Li-intercalation and the exfoliation method. Thin film MoS2 x Se2(1- x ) nanosheets drop-cast on a fluorine-doped tin oxide substrate are used as an efficient electrocatalyst on the counter electrode for the tri-iodide reduction in a dye-sensitized solar cell.

Nano letters, Jan 6, 2015

We report the observation of lithiation/delithiation of MoS2 nanosheets in a LiPF6/EC/DEC commerc... more We report the observation of lithiation/delithiation of MoS2 nanosheets in a LiPF6/EC/DEC commercial electrolyte for the application of lithium ion batteries using electrochemical liquid cell transmission electron microscopy (TEM). Upon discharge in a voltage range of 1.8 ~ 1.2 V, MoS2 on the Ti electrode underwent irreversible decomposition resulting in shattering of the MoS2 nanosheets into 5 ~ 10 nm MoS2 nanoparticles. Repeated experiments also show that some MoS2 nanosheets do not decompose upon lithiation. Instead, lithiation induced structural expansion and deformation has been observed. A solid electrolyte interface (SEI) was formed on the anode side of the Ti electrode in contact with Li metal. The SEI layer is composed of LiF nanocrystals distributed within the entire layer with the constituent elements C, O and F. However, no passivation film was observed on the cathode side of the Ti electrode with MoS2 nanosheets on it. Such in situ electrochemical liquid cell TEM study ...

Journal of visualized experiments : JoVE, Jan 20, 2012

The recent development for in situ transmission electron microscopy, which allows imaging through... more The recent development for in situ transmission electron microscopy, which allows imaging through liquids with high spatial resolution, has attracted significant interests across the research fields of materials science, physics, chemistry and biology. The key enabling technology is a liquid cell. We fabricate liquid cells with thin viewing windows through a sequential microfabrication process, including silicon nitride membrane deposition, photolithographic patterning, wafer etching, cell bonding, etc. A liquid cell with the dimensions of a regular TEM grid can fit in any standard TEM sample holder. About 100 nanoliters reaction solution is loaded into the reservoirs and about 30 picoliters liquid is drawn into the viewing windows by capillary force. Subsequently, the cell is sealed and loaded into a microscope for in situ imaging. Inside the TEM, the electron beam goes through the thin liquid layer sandwiched between two silicon nitride membranes. Dynamic processes of nanoparticle...

MRS Communications, 2014

Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fasc... more Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fascinated scientists for decades because they provide degrees of freedom to explore in condensed matter physics and design-coupled multifunctionlities. Recently, of particular interest is the perovskite-spinel-based VHN, covering a wide spectrum of promising applications. In this review, fabrication of VHN, their growth mechanism, control, and resulting novel multifunctionalities are discussed thoroughly, providing researchers a comprehensive blueprint to construct promising VHN. Following the fabrication section, the state-of-the-art design concepts for multifunctionalities are proposed and reviewed by suitable examples. By summarizing the outlook of this field, we are excitedly expecting this field to rise with significant contributions ranging from scientific value to practical applications in the foreseeable future.

Ultramicroscopy, 2013

The atomic structure and interfaces of CdS/Cu 2 S heterostructured nanorods are investigated with... more The atomic structure and interfaces of CdS/Cu 2 S heterostructured nanorods are investigated with the aberration-corrected TEAM 0.5 electron microscope operated at 80 kV and 300 kV applying in-line holography and complementary techniques. Cu 2 S exhibits a low-chalcocite structure in pristine CdS/Cu 2 S nanorods. Under electron beam irradiation the Cu 2 S phase transforms into a high-chalcocite phase while the CdS phase maintains its wurtzite structure. Time-resolved experiments reveal that Cu +-Cd 2+ cation exchange at the CdS/Cu 2 S interfaces is stimulated by the electron beam and proceeds within an undisturbed and coherent sulfur sub-lattice. A variation of the electron beam current provides an efficient way to control and exploit such irreversible solid-state chemical processes that provide unique information about system dynamics at the atomic scale. Specifically, we show that the electron beaminduced copper-cadmium exchange is site specific and anisotropic. A resulting displacement of the CdS/ Cu 2 S interfaces caused by beam-induced cation interdiffusion equals within a factor of 3-10 previously reported Cu diffusion length measurements in heterostructured CdS/Cu 2 S thin film solar cells with an activation energy of 0.96 eV.

Science, 2009

Mergers and Acquisitions The crystallization of small molecules or polymers is often described in... more Mergers and Acquisitions The crystallization of small molecules or polymers is often described in terms of a nucleation stage, where initial clusters form, followed by a distinct growth stage. Growth can come from the addition of unbound molecules, or through “Ostwald ripening” where larger crystals grow at the expense of smaller ones due to thermodynamic effects. Zheng et al. (p. 1309 ) studied the growth of platinum nanocrystals inside a transmission electron microscope using a special liquid cell, allowing observation of crystal growth in situ. Both monomer addition to growing particles and the coalescence of two particles were observed. The specific growth mechanism appeared to be governed by the size of each of the particles. The combination of growth processes makes it possible for an initially broad distribution of particles to narrow into an almost uniform one.

Physical Chemistry Chemical Physics, 2012

SnS 2 nanoparticle-loaded graphene nanocomposites were synthesized via one-step hydrothermal reac... more SnS 2 nanoparticle-loaded graphene nanocomposites were synthesized via one-step hydrothermal reaction. Their electrochemical performance was evaluated as the anode for rechargeable lithium-ion batteries after thermal treatment in an Ar environment. The electrochemical testing results show a high reversible capacity of more than 800 mA h g À1 at 0.1 C rate and 200 mA h g À1 for up to 5 C rate. The cells also exhibit excellent capacity retention for up to 90 cycles even at a high rate of 2 C. This electrochemical behavior can be attributed to the well-defined morphology and nanostructures of these as-synthesized nanocomposites, which is characterized by high-resolution transmission electron microscopy and electron energy-loss spectroscopy.

Nano Letters, 2009

We report solar cells based on highly confined nanocrystals of the ternary compound PbS x Se 1-x.... more We report solar cells based on highly confined nanocrystals of the ternary compound PbS x Se 1-x. Crystalline, monodisperse alloyed nanocrystals are obtained using a one-pot, hot injection reaction. Rutherford back scattering and energy filtered transmission electron microscopy suggest that the S and Se anions are uniformly distributed in the alloy nanoparticles. Photovoltaic devices made using ternary nanoparticles are more efficient than either pure PbS or pure PbSe based nanocrystal devices. Colloidal semiconductor nanocrystals display a wealth of size-dependent physical and chemical properties, including quantum confinement effects, shape dependent electronic structure, 1, 2 and control over assembly through modification of surface functionalization. 3, 4 Photovoltaic devices are an easily recognized potential application for nanocrystals due, in part, to their high photoactivity, solution processability and low cost of production. Several schemes for using nanocrystals in solar cells are under active consideration, including nanocrystal-polymer composites, 5 nanoparticle array solar cells, 6 films of partially sintered nanoparticles, 7 and nanocrystal analogues to dye-sensitized solar cells. 8 A persistent challenge for any nanoparticle-based solar cell is to take advantage of quantum confinement effects to improve the optical absorption process without overly hindering the subsequent transport of charge to the electrodes. Various

Nano Letters, 2009

We have directly observed motion of inorganic nanoparticles during fluid evaporation using a Tran... more We have directly observed motion of inorganic nanoparticles during fluid evaporation using a Transmission Electron Microscope. Tracking real-time diffusion of both spherical (5-15 nm) and rod-shaped (5x10 nm) gold nanocrystals in a thin-film of water-15%glycerol reveals complex movements, such as rolling motions coupled to large-step movements and macroscopic violations of the Stokes-Einstein relation for diffusion. As drying patches form during the final stages of evaporation, particle motion is dominated by the nearby retracting liquid front.

Microscopy and Microanalysis, 2012

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, ... more Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Journal of the American Chemical Society, 2009

We show that nanocrystals (NCs) with well-established synthetic protocols for high shape and size... more We show that nanocrystals (NCs) with well-established synthetic protocols for high shape and size monodispersity can be used as templates to independently control the NC composition through successive cation exchange reactions. Chemical transformations like cation exchange reactions overcome a limitation in traditional colloidal synthesis, where the NC shape often reflects the inherent symmetry of the underlying lattice. Specifically we show that full or partial interconversion between wurtzite CdS, chalcocite Cu 2 S, and rock salt PbS NCs can occur while preserving anisotropic shapes unique to the assynthesized materials. The exchange reactions are driven by disparate solubilites between the two cations by using ligands that preferentially coordinate to either monovalent or divalent transition metals. Starting with CdS, highly anisotropic PbS nanorods are created, which serve as an important material for studying strong two-dimensional quantum confinement, as well as for optoelectronic applications. In NC heterostructures containing segments of different materials, the exchange reaction can be made highly selective for just one of the components of the heterostructure. Thus, through precise control over ion insertion and removal, we can obtain interesting CdS|PbS heterostructure nanorods, where the spatial arrangement of materials is controlled through an intermediate exchange reaction.

Applied Physics Letters, 2004

For ͑001͒ c oriented BaTiO 3 thin films, it has been found that epitaxial constraint can result i... more For ͑001͒ c oriented BaTiO 3 thin films, it has been found that epitaxial constraint can result in a dramatic increase in the temperature of a tetragonal ͑T͒ structural phase transition. For 2000-Å-thick films grown directly on SrTiO 3 substrates, a T → cubic ͑C͒ phase transition was found on heating at Ͼ950 K, where the lattice constant changed smoothly with temperature. It was also found for films of the same thickness that the T → C transition is nearly restored to that of bulk crystals by the use of a buffer layer, which relaxes the epitaxial constraint. These results provide evidence of an epitaxially induced high temperature structural phase transition in BaTiO 3 thin films, where the ferroelectric (internal) and structural (external) aspects of the phase transition are decoupled.

Angewandte Chemie International Edition, 2012

ACS Nano, 2012

Direct imaging of nanoparticle solutions by liquid phase transmission electron microscopy has ena... more Direct imaging of nanoparticle solutions by liquid phase transmission electron microscopy has enabled unique in situ studies of nanoparticle motion and growth. In the present work, we report on real-time formation of two-dimensional nanoparticle arrays in the very low diffusive limit, where nanoparticles are mainly driven by capillary forces and solvent fluctuations. We find that superlattice formation appears to be segregated into multiple regimes. Initially, the solvent front drags the nanoparticles, condensing them into an amorphous agglomerate. Subsequently, the nanoparticle crystallization into an array is driven by local fluctuations. Following the crystallization event, superlattice growth can also occur via the addition of individual nanoparticles drawn from outlying regions by different solvent fronts. The dragging mechanism is consistent with simulations based on a coarse-grained lattice gas model at the same limit.

ACS Nano, 2012

Simultaneously probing the electronic structure and morphology of materials at the nanometer or a... more Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope (ETEM) equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy (EELS) in situ for cobalt catalysts under reaction conditions. Analysis revealed quantitative correlation of the cobalt valence states to the nanoporous structures. The in situ experiments were performed on nanoporous cobalt particles coated with silica while a 15 mTorr hydrogen environment was maintained at various temperatures (300-600 ºC). When the

ACS Nano, 2008

S ince the first report in 2004, 1 the formation of hollow nanoparticles via the nanoscale Kirken... more S ince the first report in 2004, 1 the formation of hollow nanoparticles via the nanoscale Kirkendall effect has been observed in dozens of materials. 2-5 In the simplest scenario, an elemental metallic nanocrystal reacts to form a compound nanoparticle; hollow, polycrystalline nanoparticles result when the diffusion of the cationic species outward is faster than the inward diffusion of the anionic species. Some examples of hollow particles formed in this way include oxides, sulfides, selenides, and phosphides of cobalt, iron, molybdenum, and nickel (e.g.

ACS Nano, 2011

Nanorod Synthesis & Assembly Details of the synthetic and assembly procedures for the nanorods ca... more Nanorod Synthesis & Assembly Details of the synthetic and assembly procedures for the nanorods can be found in the supporting information of our previous work 1. One important point to note about repeatability of this assembly is that relatively fresh nanorods (synthesized within ~1 month of assembly attempt) had higher rates of successful assembly. Also, while monodispersity was important (which requires cleaning), over-cleaning was found to result in rods that laid parallel to the substrate. Finding an appropriate balance of monodispersity and cleanliness was difficult and sometimes took a few synthetic attempts before successful alignment. The required cleaning procedure for each batch of rods was slightly different, despite the fact that the syntheses themselves were identical. Assembly substrates (Si3N4 for RBS measurements; 150nm 20Ω/square ITO on glass for electrical measurements) were cleaned by sequential sonication in water, toluene, isopropanol and acetone, followed by an oxygen plasma etch. Rutherford Backscattering Spectroscopy (RBS) Rutherford backscattering data was taken with a 2.5 MeV Van de Graaff accelerator. The spectra were taken using a 2 MeV He + ion beam with a Si surface barrier detector at 165 o with respect to the incident beam. The samples were tilted by 50 deg in order to improve depth resolution. The spectra were analyzed using the RUMP software package. `