Symposium CP02—Design and In Situ TEM Characterization of Self-Assembling Colloidal Nanosystems

Growth of single crystals via assembly of colloidal nanocrystals is widespread phenomenon in both synthetic and natural environments. In mineral systems, this process exhibits a range of styles including face-specific oriented attachment of like phases, attachment of a nanoscopic phase to a bulk phase followed by transformation, and mis-oriented aggregation followed by coarsening to a single crystal. While descriptions of assembly in these systems must share a commonality with continuum-based DLVO-type theories for simple colloids, mineral systems present additional complexities, including face-specificity of dielectric properties, the presence of structured nanoscale interfaces, and the consequent solvent-responses that are expected to be of a comparable length scale. To understand the relationship between nanocrystal structure, interparticle forces and consequent motion leading to assembly, we are investigating colloidal assembly in the titanium, iron and zinc oxide systems. We use in situ TEM to observe assembly dynamics, force spectroscopy with custom-made single crystal tips to measure face-specific interparticle forces, AFM to probe interfacial structure, and molecular simulations to understand the origins of these features. For both ZnO and the iron oxyhydroxide ferrihydrite, TEM demonstrates that direction-specific forces align randomly diffusing particles prior to contact and that attachment occurs on lattice matched faces via a sudden jump to contact followed by coarsening to produce compact single crystal structures. Direct measurements of the forces between both TiO₂ and ZnO surfaces reveal orientation dependent attractive interactions that exhibit the symmetry of the underlying lattice. A combination of molecular dynamics and classical DFT simulations indicate a combination of hydration, ion correlation and dispersion forces are responsible for orienting the particles and that the barriers caused by hydration layers, which scale with surface hydrophobicity, cannot be overcome by simple Brownian motion in the absence of fluctuations in those layers. Addition of organic ligands, like oxalate, to the ferrihydrite system produces a dramatically different pathway and outcome: Nanoparticles of hematite nucleate sporadically from the ferrihydrite and, once formed are covered with oxalate and drive all new hematite particles to nucleate within about 1 nm of the surface, after which they jump to contact. The oxalate inhibits growth and coarsening of the particles, leading to spindle-shaped aggregates of coaligned particles with a fixed and constant aspect ratio. Thus the random assembly process seen in the pure ferrihydrite system is replaced with one that is deterministic. Classical DFT calculations show that gradients in Fe³⁺ chemical potential created by the ligands drive this interfacially driven bias towards growth by oriented attachment.

We use colloidal nanoparticle superlattice formation as a model system to study the fundamental laws of order emergence from a dispersion of anisotropic nanoparticles. Specifically, the single-particle level imaging enabled by liquid-phase transmission electron microscopy allows us to map out, quantitatively, strain propagation pathways as the nanoparticles vibrate collectively to anneal defects as well as to sample energetically degenerated lattice states. Such strain originates from highly-directional interactions between nanoparticles, which also drives the oriented attachment of different nanoparticle lattice domains as they approach and form into larger crystals. Such growth mode is distinct from crystallization behavior of micron-sized colloids, and resembles more the faceted crystals formed from atoms. Our systematic study provides detailed dynamic information over the superlattice development and relaxation at single nanoparticle resolution, which can serve as a general guideline for materials design and engineering from the bottom-up.

The directed assembly of nanoscale building blocks into complex superstructures is of widespread scientific and technological interest. Scientists and engineers have been intrigued by the prospects of tailoring self-assembly processes to create materials whose properties and function can be tuned through the interaction between constituent particles. In particular, recent reports of epitaxially connected colloidal quantum dot superlattices with long-range atomic coherence have generated significant interest as a platform for novel, quasi 2D ‘designer materials’. Experimental protocols for the formation of high quality superlattices in which constituent quantum dots are registered to within a single atomic bond length have been established; however, significant gaps persist in our fundamental understanding of several aspects of the underlying mechanism by which these structures form.
Our approach to gain mechanistic insights into the assembly and subsequent attachment leverages unique in-situ multiprobe characterization techniques including TEM and X-ray scattering. The combination of in-situ wide- and small-angle X-ray scattering informs the temporal evolution of nanoparticle orientation and assembly at the fluid interface. We will present recent advances with in-situ analysis TEM analysis of quanutum dot solids deposited on a heating chip to gain atomic-level insights into the formation of the interdot bond. In-situ TEM experiments at varying temperatures and ramp rates hold significant potential to provide new mechanistic insights into the temporal evolution of interdot bridge width and structure.

In the past decades, rare-earth nickelate perovskite RNiO₃ (R=rare earth) have been demonstrated to exhibit rich phase diagram concomitant with complicate magnetic property. It has been found that these nickelates can undergo a structure transition from Orthorhombic to Monoclinic (Pnma to P2₁/c) accompanied with metal-insulator transition (MIT) upon cooling process. Currently, atomic-resolution scanning transmission electron microscopy (STEM) equipped with a cooling holder can enable imaging with picometer resolution at cryogenic temperature, providing an intuitive method to analyze the change of atomic structure at low temperature. In this work, we will use aberration-corrected STEM combined with in-situ cooling to analyze the phase transition found in LaNiO_3-δ. During in-situ cooling experiment, we have found the reversible disappearance/appearance of superlattice peak from diffraction pattern of LaNiO_2.75 as a function of temperature. Electron energy-loss spectroscopy (EELS) will be used to examine the change in local electronic structure as a function of temperature.

Colloidal nanocrystals offer a route toward engineering new classes of materials by acting as discrete units that can be assembled to construct composite solids. The self-assembly of two sizes of spherical nanocrystals has revealed a surprisingly diverse library of structures. To date, at least fifteen distinct binary nanocrystal superlattice (BNSL) structures have been documented. The stability of the observed binary phases cannot be fully explained using the traditional conceptual framework treating the assembly process as entropy-driven crystallization of rigid spherical particles. We evaluate new theoretical models treating the co-crystallization of deformable spheres and to formulate new hypotheses about the factors affecting the nucleation and growth of the binary superlattices. The deviation from hard sphere behavior can be explained by specific topological textures developed within deformable layers of surface ligands. Our results also suggest that the relative abundance of BNSL phases is determined not only by their thermodynamic phase stability but also by a postulated pre-ordering of the binary fluid into local structures with icosahedral or polytetrahedral structures prior to nucleation.
Strong electronic coupling between individual nanocrystals within a superlattice is an important prerequisite for the emergence of non-additive physical properties. However, a simultaneous realization of strong electronic coupling and dense ordered packing of nanocrystal solids has remained elusive. We report a method for growing all-inorganic highly ordered solids of electrostatically-stabilized nanocrystals with the interstitial space filled with a glassy metal chalcogenide matrix which, combined with the short separation between particles leads to very strong electronic coupling. Temperature-dependent conductivity measurements show metallic transport across our supercrystals. The formation of strongly-coupled all-inorganic nanocrystal assemblies represents an important step toward the bottom-up design of functional nanostructured composites.

Monodisperse shape-controlled nanocrystals (NCs) are ideal building blocks for the assembly of new thin films and devices. These NCs can be thought of as "artificial atoms" with tunable electronic, optical, magnetic properties that are allowing the development of a new periodic table for design at the mesoscale. In this talk, I will briefly outline the current state of the art in synthesis, purification, and integration of single phase NCs and core-shell (heterostructures) NCs emphasizing the design of semiconductor building blocks with tunable shapes (spheres, roads, cubes, discs, octahedra, etc... I will then share how these tailored NCs can be directed to assemble into single-component, binary, ternary NC superlattices providing a scalable route to the production of multi-functional thin films. The in situ studies of NC organization and transformation will be carried out using a combination of x-ray scattering and electron microscopy/tomography. The modular assembly of these NCs allows the desirable features of the underlying quantum phenomena to be retained and enhanced even as the interactions between the NCs allow new delocalized properties to emerge. Synergies in electronic, optical coupling between NCs will be emphasized as we push toward the realization of artificial solids with a new 3D and structure and high mobility device integration. I will share specific case studies in thin film transistors, thermo-electric materials and solution-processable photovoltaic devices build with these strongly coupled nanocrystal solids highlighting the recent developments in wafer scale NC superlattice deposition and patterning may provide a path to scalable fabrication. In a final example of hetero-integration, I will present our progress in the co-assembly of plasmonic resonators together with nanoscale emitters as a route to the scalable self-assembled MetaMaterials with novel linear and non-linear optical properties. Progress toward the design of micron-sized 3D superparticle composed NC building blocks will also be shared.

The challenges of creating designed materials via self-assembly of nanoscale blocks require establishing robust yet highly tailorable methods for directing nanoscale interactions. While it has been recognized that anisotropic bonds allow for the coordination control, and, consequently, for the regulation of formed structures, the development of practical realizations was notoriously difficult. We have developed an assembly platform that through a regulation of nano-object valence allows to design a coordination symmetry. That approach permits an assembly of prescribed nanoscale architectures and complex lattices. Moreover, a variety of nano-object types can be integrated into these self-assembled hetero-architectures. This new assembly methodology requires direct 3D nanoscale visualizations, both in-situ and ex-situ, in order to probe the imperfections, a structure formation and assembly pathways. Our advances in the development of suitable x-ray- and electron- based methods for 3D characterizations will be also discussed.

Polyelemental nanoparticles have novel chemical and physical properties, which make them promising for a wide range of fields spanning catalysis, plasmonics, and electronics. However, the combinations of elements that have thus far been studied in these systems are limited by a lack of methods for preparing and analyzing multiplexed nanoparticle systems. To explore the vast number of elemental and structural combinations possible, we developed scanning probe block copolymer lithography (SPBCL), a method for synthesizing polyelemental nanoparticle arrays from polymer nanoreactors that contain metal precursors in defined amounts and ratios. Through heat treatments or electron irradiation, small nanoparticles nucleate, grow, and coarsen inside each nanoreactor eventually resulting in a single nanoparticle. The structure and formation of these spatially encoded nanoparticles can be studied using electron microscopy, both in situ and ex situ, as a function of their composition and thermal treatment. Although metal species aggregate at different rates, the final structures of the nanoparticles attain thermodynamic equilibrium. Additionally, by utilizing the fluidic polymer nanoreactors as windowless liquid “cells,” the motion and interaction of ~1 nm nanoparticles during electron beam-induced solvent evaporation can be tracked in real time using an electron microscope. Taken together, SPBCL serves as a novel method for synthesizing polyelemental nanoparticles in a combinatorial manner, and the polymer nanoreactors provide a versatile platform for studying the structural evolution and final structure of complex, polyelemental systems.

Self-assembly of synthetic nanoparticles enables the positioning of nanoparticles into one to three dimensional ordered arrays, facilitating integration of nanoparticle lattices into nanophotonic and nanoelectronic architectures. The functional properties of these particle materials are expected to be highly sensitive to structural factors such as coordination number, degree of long-range order, or interparticle separation distance, requiring the development of robust self- and directed-assembly pathways forprecise control of structural parameters to improve optical and electronic properties of functional nanoparticles. In this presentation, I will review our past efforts in development of self-assembled nanoparticles thin film arrays and in-situ structural evolution at ambient condition. I will then extend my presentation to our recent progress in development of a new Stress-Induced Fabrication method in which we applied high pressure or stress to nanoparticle arrays to induce structural phase transition and to consolidate new nanomaterials with precisely controlled structures and tunable properties. By manipulating nanoparticle coupling through external pressure, a reversible change in their assemblies and properties can be achieved and demonstrated. In addition, over a certain threshold, the external pressure will force these nanoparticles into contact, thereby allowing the formation and consolidation of one- to three-dimensional nanostructures. Through stress induced nanoparticle assembly, materials engineering and synthesis become remarkably flexible without relying on traditional crystallization process where atoms/ions are locked in a specific crystal structure. Therefore, morphology or architecture can be readily tuned to produce desirable properties for practical applications.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Supramolecular chemistry is an ideal means to dynamically program structural hierarchy in both polymer and composite materials, as it uses reversible interactions between atomically precise building blocks to create well-defined nano- to macroscopic structures. Simple synthetic adjustments in molecular composition can alter how polymers bind to one another in three-dimensional space, and this in turn can modulate the nanoscale morphology of the assembled material. Conversely, once assembled, these larger-scale structures can act as scaffolds that allow multiple supramolecular binders to behave as a collective unit. Because the shape of these larger length scale motifs can affect the ability of individual supramolecular motifs or polymer chains to coordinate their binding, material form at the nanoscale could potentially be used as a design handle to tune the supramolecular groups’ collective behavior. Understanding how to holistically control structural hierarchy in this manner therefore necessitates the development of a synthetic building block whose nanoscale morphology can be tuned without significantly altering the chemical structure of the individual supramolecular binding groups themselves.
We have recently demonstrated a new polymer brush-particle building block capable of directed assembly via complementary hydrogen bonding motifs, called the Nanocomposite Tecton (NCT). NCTs consist of an inorganic nanoparticle core coated with a dense polymer brush, where each polymer chain terminates in a supramolecular bonding group; interparticle interactions drive particle assembly by creating dynamic nanoscopic bonds consisting of multiple supramolecular linkages.
NCT assembly is distinct from other materials synthesis methods reliant on paired hydrogen bonding motifs, as each NCT-NCT connection consists of multiple supramolecular interactions that behave collectively not because individual groups are closely linked via covalent bonds, but rather because they are geometrically confined to a localized volume at the NCT surface. An NCT’s nanoscale geometry (as dictated by the particle core size, polymer length, and polymer morphology) therefore has a strong influence on its assembly behavior, and can be used to tune NCT-NCT bond strength without changing molecule-molecule bond strength. Here we show how nanoscale structure can be used to manipulate material morphology in a holistic manner, where structural features at the molecular, nano, and mesoscopic length scales influence one another in a programmable manner, and each can be used to dictate material behavior.

Owing to their unique collective properties, colloidal crystals are attracting more and more attention. They offer us a novel method to fabricate functional materials. Nucleation and growth processes are two key factors for us to obtain colloidal crystals. Here, by changing the reaction temperature, we realized precise control of nucleation and growth rates of colloidal crystals. For the saturated solution of Cs-Pb-Br nanoparticles, a relevantly fast cooling led to nucleation of colloidal crystals, and then a very slow cooling was applied to grow colloidal crystals. Through this method, we acquired ordered cubic colloidal crystals, which was made up of 10 nm cubic Cs-Pb-Br nanoparticles. The size of those uniform colloidal crystals was about 2 μm. We found out that precursors concentration, solvents, which would influence the nucleation and growth processes, had a great influence on the final product. Through time sequence experiment, we briefly described the growth dynamics of colloidal crystal, and built the relationship between quality of colloidal crystals and nucleation/growth rates.

Self-assembling polymer-grafted nanoparticles (PGNs) are advancing roll-to-roll manufacturing of photovoltaics, microsensors, opto-electronics, memory, and multifunctional coatings. Architectural features of the polymeric canopy (e.g. grafting density, chain length) not only determine processibility and assembly, but also robustness, such as mechanical toughness and dielectric breakdown strength. To control PGN assembly, and therefore tailor properties, a detailed understanding of solution phase behavior is imperative. However, while phase properties of macromolecules have been extensively explored, comparable knowledge of PGN solution phase behavior is lacking and fails to elucidate the role of the PGN constituents. Here we map the phase space of polystyrene-grafted gold-nanoparticles in cyclohexane by establishing the upper critical solution coexistence curve via UV-vis spectroscopy, dynamic light scattering, and small angle X-ray scattering. Increasing the molecular weight of the polystyrene graft (20 → 50 kDa) advances the coexistence curve to higher temperatures (T_c ~ 12 → 20 ^oC @ 10 nM (φ ~3e-5)) and increases the estimated theta temperature (θ ~ 25 → 27 ^oC). Considering the well-established phase behaviors of structured macromolecules, the coexistence curves and theta temperatures of PGNs are less than those of polystyrene stars (θ ~ 30 ^oC) and linear chains (θ ~ 33 ^oC). Using a TEM solution cell the impact of the proximity to the coexistence curve (e.g. spinodal decomposition v. nucleation and growth) on the assembly process is demonstrated. Such insight refines processing protocols for slot-die coating techniques, allowing for rapid self-assembly of large-area ordered PGN monolayers directly on solid substrates.

We present the design and development of a MEMS (Micro ElectroMechanical System) -based liquid cell that enables in-situ liquid electrochemistry inside the TEM (Transmission Electron Microscope). The presented device, referred to as the Nano-Cell, enables the user to work in liquid conditions with a three-electrode configuration (i.e. reference, working and counter electrodes) in order to trigger and measure chemical reactions as a function of an electrical stimulus. The design of the Nano-Cell has been done in such a way that a thin liquid layer (to minimize electron scattering and maximize resolution) is guaranteed. Furthermore, the patented on-chip inlet and outlet combined with the design of the fluidic channel offer unrivalled control of the liquid flow. As a result, the complete system gives new insights on materials characterization at the nanoscale with applications in corrosion, Lithium batteries, electroplating and materials science.
Here, we present results showing good agreement between experimental cyclic voltammetry of Ferrocenemethanol, a one -electron redox molecule in an aqueous solution and finite element modeling, allowing to characterize mass transport to the electrode and to extract reaction rates and other material properties. We demonstrate the superior fluid flow control through the MEMS based microfluidic channel. Using a pressure-based pump, both the pressure and the flow can be accurately controlled in the sample area.
The resulting liquid layer thickness, in hundreds of nanometers, allows to maximize spatial resolution in TEM: while diffusion is impeded by this thin liquid layer, it mimics diffusion-limited mass transport observed in classical macroscopic electrochemistry. With additional controlled fluid flow, faster mass transport can be achieved in a similar way to other classical approaches using a rotating disc electrode or a flow cell, showing the relevance of this approach.
Finally, we demonstrate experimentally controlled flow and increased mass transport to the electrode and its influence in tuning electrodeposition of copper crystals.

Nanoparticles have been used in a variety of applications such as catalysts, pharmaceuticals and sensors. Controlling their growth and assembling them into a hierarchical structure is critical for the application. Current understanding of the nanoparticle growth and its assembly is mainly derived from their post-synthesis characterization. An atomic level understanding of the mechanism of nanoparticle formation and assembly during the synthesis process will be interesting and important. Tandem in-situ small angle X-ray scattering (SAXS)-X-ray Absorption Spectroscopy (XAS) will be a unique way to achieve this goal. In this presentation, I will discuss the growth of nanoparticles using SAXS-XAS.

The self-assembly of nanoparticles (NPs) into superlattice structures attracts great attention due to their unique optical, thermoelectric, magnetic, energy storage and catalytic properties. Understanding of and mechanisms of particle assembly and interaction between active species can help to establish conditions to control the assembly process and supperlattice structure, which are closely tied to the physical and chemical properties. However, little is known about the driving forces and controlling factors during the process of particle self-assembly. In this work, we analyzed dynamic information of the process and investigated the Van der Waals force, Brownian force, and the hydrodynamic force, based on measured particle positions and velocities by directly observing the process of 2-dimensional self-assembly of nanoparticles and tracking individual particles as a function of time via in situ TEM. In addition, we studied the steric hindrance factor between particles due to the polymer attached on particle surface. We estimated the most stable conditions based on our calculations and understanding of all process during the nanoparticle self-assembly process.

Gibbsite (α-Al(OH)₃) is important hydroxide of aluminum in nature that also play diverse roles across a plethora of industrial applications. In addition, it’s prominent component in high-level nuclear waste stored in large quantities at the Hanford Site, Washington and at the Savannah River Site, South Carolina. Future processing of these waste materials depends on an understanding of the behavior of gibbsite crystallization, dissolution, and transformation. As a consequence, precise synthesis of the gibbsite nanocrystals with controlled particle size, shape and properties is important. Mechanisms of crystallization of gibbsite nanocrystals still remain poorly understood, particularly in complex industrial systems, which can include concentrated sodium hydroxide at low water activity. In this work, in situ magic angle spinning nuclear magnetic resonance (MAS-NMR) combine with high resolution atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, and X-ray Pair Distribution Function (PDF) techniques were conducted to investigate the crystallization of gibbsite nanocrystals from amorphous aluminum hydroxide gel precursors and also aluminum (III) based cluster solutions. By focusing on the dynamics of aluminum coordination change from tetrahedral in solution or clusters to octahedral in solids, and the intermediate pentacoordinate state, some unifying principles governing these transformation emerge, which are important for controlling morphology of gibbsite, and management of nuclear waste.

Solution-phase self-assembly of nanocrystals into mesoscale structures is a promising strategy for creating functional materials from nanoscale building blocks. Liquid environments are key to self-assembly since they allow suspended nanocrystals to diffuse and interact freely, but they also complicate experiments. Existing approaches for studying self-assembly have until recently been limited to assessing the final product ex-situ, or tracking long-range order in-situ in solution by reciprocal space methods. In-situ liquid cell electron microscopy is the only technique that provides access to real-space dynamics, especially at small scale, and is capable of probing self-assembly processes in real space in the native liquid environment.

Here I will illustrate the power of in-situ liquid-cell electron microscopy to probe self-assembly of nanocrystals in solution from monomers to extended assemblies with nanometer scale resolution, and extracting fundamental information on assembly pathways and forces. Examples include self-assembly of polyhedral nanoparticles and highly anisotropic nanocrystals controlled by non-specific interactions, and programmable crystallization of DNA-nanoparticle conjugates and their real-time reconfiguration in response to external stimuli.

Our results demonstrate that real-time electron microscopy can advance our understanding of solution phase self-assembly processes as a basis for designing materials with tailored functionality.

A diverse class of materials exhibit characteristics of mesocrystals: single crystals composed of distinct nano-sized domains that are atomically aligned. The formation of such structures is often attributed to crystallization through oriented attachment (OA). However, many unanswered questions about the fundamental drivers and dynamic progression of this phenomenon remain. Here we focus on the crystallization of hematite (hm, Fe₂O₃) mesocrystals from ferrihydrite (fh) nanoparticles. In pure solution the resulting hm crystals are well faceted cuboctahedra, but in the presence of oxalate hm forms a nanoporous spindle-shaped single crystal elongated along [001]. The spindles are hierarchically organized across two length scales. At the shortest length scale, they consist of atomically aligned nanometer size domains. These then form a second order structure consisting of chains of the hm domains. To investigate the formation process, we applied in situ liquid phase TEM with heating on solutions of fh held at temperatures of 70 °C into which hm seeds were added. We directly observed both the dissolution of fh and the nucleation of new hm particles, which all formed within close proximity (~ 1 nm) of the hm/solution interface. Immediately after nucleation, the hm particles attached to the nearby seed to form a hm mesocrystal. Post growth analyses using EDX mapping and electron diffraction after disassembling the liquid cell confirmed the growth of spindle-shaped hematite during the liquid phase TEM experiments. In addition, we utilized a freeze-and-look approach in which indexed TEM grids were used to cycle samples between the growth reactor and the TEM in order to track the pathway of crystallization over long periods of time. The results were consistent with those of the in situ experiments and confirmed that the fh serves as a buffer that sets the Fe³⁺ concentration and the spindles grow by creation of new particles in the solution near the hm interface. Based on ATR FTIR measurements of the relative binding strength of oxalate to the (001) and (012) faces and calculations of chemical potential gradients near the interface, we propose that oxalate plays the role of inhibiting classical monomer-by-monomer growth of the hematite particles while promoting the nucleation of new hm particles at the hm/solution interface by creating a gradient in Fe³⁺ chemical potential. In this way, the oxalate ligands bias the growth process away from classical mechanisms and towards oriented attachment.

The ability to dynamically organize functional nanoparticles (NPs) via the use of environmental triggers (temperature, pH, light, or solvent polarity) opens up important perspectives for rapid and convenient construction of a rich variety of complex assemblies and materials with new structures and functionalities. Here we report an unconventional strategy for crafting stable hairy NPs with light- as well as thermal-enabled reversible and reliable self-assembly and tunable optical and catalytic properties. Central to our strategy is to judiciously design amphiphilic star-like diblock copolymers comprising inner hydrophilic blocks and outer hydrophobic stimuli-responsive blocks as nanoreactors to direct the synthesis of monodisperse plasmonic NPs intimately and permanently capped with stimuli-responsive polymers. The size and shape of hairy NPs can be precisely tailored by modulating the length of inner hydrophilic block of star-like diblock copolymers. The perpetual anchoring of stimuli-responsive polymers on the NP surface renders the attractive feature of self-assembly and disassembly of NPs on demand using stimuli of different wavelengths or temperature, as revealed by tunable surface-plasmon resonance absorption of NPs and the reversible transformation of NPs between their dispersed and aggregated states. By extension, the star-like block copolymer strategy enables the crafting of a family of stable stimuli-responsive NPs (e.g., pH-sensitive polymer-capped magnetic, ferroelectric, upconversion, or semiconducting NPs) and their assemblies for fundamental research in self-assembly and crystallization kinetics of NPs as well as potential applications in optics, optoelectronics, magnetic technologies, sensory materials and devices, catalysis, nanotechnology, and biotechnology.

We study the crystallization front evolution in a growing nanoparticle superlattice using real-time low-dose liquid-phase transmission electron microscopy. This interface between the co-existing crystal and dispersed nanoparticles fluctuates due to thermal agitations, which defines the final shape of the superlattice. The tracked interface profile at the single particle resolution allows us to decompose an otherwise complicated interface into a series of Fourier components based on the capillary wave theory. Characteristics such as the interfacial stiffness at different lattice orientations and the lateral fluctuation correlation lengths are measured for the first time from real-space imaging. Our experiments can provide insights into the engineering the crystallization pathways and growing defect-free superlattices from solution.

Self-assembly of colloidal nanocrystals is a promising approach for integrating nanocrystals into larger macroscopic materials with collective and emergent optical, electronic, and mechanical properties. Conventionally, nanocrystal self-assembly is carried out slowly and with weak attractive interparticle interactions to avoid kinetic traps. For instance, evaporative self-assembly of nanocrystal superlattices requires up to a week under carefully controlled environmental conditions. Recent experiments have demonstrated rapid (minutes) crystallization of colloidal nanocrystals into 3D superlattices during high temperature nanocrystal synthesis or under electric field stimulus. The mechanism and underlying kinetic processes enabling formation of nearly perfect self-assembled structures in such short times remains unclear. Here we show direct liquid cell electron microscopy of the formation of 3D platinum supraparticles by concurrent nanocrystal synthesis and self-assembly. Electron beam induced reduction of an aqueous platinum salt formed monodisperse ~3 nm spherical platinum nanocrystals in solution, which rapidly self-assembled into platinum supraparticles with a variety of structures, including densely packed spherical caps, faceted plates, and core-shell structures. We investigate the effects of nucleation and assembly kinetics and addition of small molecule and polymer capping ligands on the kinetics of supraparticle formation and the final supraparticle structure.

Despite splendorous synthetic achievements in anisotropic nanoparticles, spatial control of their surface chemistry remains primitive. Here we demonstrate a single-pot site-specific polymer coating on anisotropic nanoparticles due to the local nanoparticle surface curvature difference. No post-coating manipulation is needed to induce the spatial patterning. Surface characterization and three-dimensional morphology of these patchy nanoparticles under scanning transmission electron microscopy and atomic force microscopy reveal the full geometry of regiospecific functionalization. Mechanistic insights gained in this study via quantitative analysis on thickness, local surface curvature, and confined area on surface of patchy nanoparticles may be applicable to design hybrid anisotropic nanoparticles with directional and “soft-interaction” between themselves, which can be further expanded to bottom-up reconfigurable assembly.

Nanocrystal superlattices experience structural transformations due to ligand exchange, where each individual nanocrystal acts as an artificial atom. Drastic changes of the electronic properties have been observed resulting from the superlattice transformations. For example, the electron transfer characteristics for PbSe nanocrystal superlattices are drastically different before and after PbCl₂ treatments. We report our study of PbSe superlattice structural transformations due to ligand exchange with in situ liquid cell TEM. Direct observations reveal the rare behavior of individual nanocrystals and intermediates during superlattice structural transformations. Assisted with theoretical calculations/computation, we are able to develop an understanding of the particle-particle interactions modified by ligand displacement from nanoparticle surfaces. This work also demonstrates that in situ liquid cell TEM is an indispensable approach for revealing of the pathways of superlattice transformations at the single nanoparticle level.

Acknowledgements: This work was funded by Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-05-CH11231 within the KC22ZH program. The collaboration and program support through the UC Lab Fees project are also acknowledged. Dr. Yu Wang and Xinxing Peng performed the in situ TEM experiments.

Self-assembly of nanoparticles has been used to fabricate structurally diverse colloidal crystals, including binary, ternary, and quasicrystalline superlattices, many of which were found to exhibit unanticipated optical, electronic, and catalytic properties. In this talk, I will describe how these nanoparticle superlattices can be further (post-assembly) subjected to chemical reactions and transformed into novel classes of materials. I will focus on non–close-packed nanoparticle arrays, which we created through the selective removal of one of two components comprising binary nanoparticle superlattices. I will also discuss the importance of the liquid on the structure of binary superlattices assembled at the liquid-air interface and the choice of the liquid can lead to the fabrication of superlattices featuring previously unknown types of packing of the constituent nanoparticles.

The assembly process of nanoparticles from individual atoms, and nanostructures from nanoparticles in solution is fundamental for materials engineering and “bottom-up” fabrication of functional nanodevices.
Using dynamic in situ TEM imaging [1-3] in liquids, I will describe how inorganic and organic nanoparticles form in solution and how these nanoparticles interact with each other. First, I will discuss how phase separation of a solution containing Au ions into solute-rich and solute-poor phases leads to the formation of Au nanocrystal through a pathway that does not follow classical nucleation theory (CNT). Namely, I will show that there are multiple steps that lead to the formation of nuclei from which nanocrystals grow [4]. These steps are: 1) phase separation of the liquid solution into solute-poor and solute-rich phases, from which 2) amorphous nanoparticles which serve as a precursor for nuclei emerges. This is followed by 3) crystallization of amorphous nanoparticles into crystalline nuclei. I will show that similar is true for more complex structures such as MOFs
Next, I will highlight the role of intermolecular forces between nanoparticles in solution and describe their role in the assembly of nanostructures from individual nanoparticle building blocks (bottom-up approach) [5]. Specifically, I will show how the balance between repulsive hydration force and attractive van der Waals (vdW) force results in a metastable nanoparticle-pair which promotes their subsequent attachment to each other [6]. I will also briefly discuss the effect of other interactions such as electrostatic, H-bonding [7-8], and capillary forces[9] and hydrophobic interactions [10] in the context of their effect on the nanoparticle self-assembly dynamics.
These findings highlight the role of solvent-mediated physical and chemical forces in material synthesis and self-assembly of nanoparticles. Our observations also emphasize the importance of direct nanoscale observation in uncovering previously unknown intermediate states that are pivotal for synthesis and self-assembly.
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[10] S. F. Tan, S. Raj, G. Bisht, H. Annadata, C. A Nijhuis, P. Kral, U. Mirsaidov, Advanced Materials 30 (2018), p. 1707077.

For metallic electrocatalysts for energy applications, understanding of structure-performance relationship can only be built on the precise characterization on the phase, interfacial and surface structures. Advanced (scanning) transmission electron microscopy ((S)TEM) techniques have been widely applied to investigate the structure of catalyts in atomic scale. With the help of multimodal characterization techniques, it is possible to establish a direct link from the composition and surface structure to the performance of electrocatalysis. In this talk, I will discuss the recent progresses in the Pt-based catalysis for proton exchange membrane fuel cells. The first case is on how the strain and ligand effects affect the surface electronic structure and then the overall performance. In case of PtPb/Pt(110) faceted core-shell nanoplates, the biaxial strain from PtPb to Pt may help to optimize the Pt-oxygen bond strength and therefore boost their activity and stability for oxygen reduction reaction (ORR). Another case is on using in-situ TEM to study the synthesis process of Pt-Co and Pt-Ni alloys. L₁₂ ordering are found in these alloys which is supposed to enhance the electrochemical performances. This talk highlights the importance of multimodal study for developing electrocatalysts, in combination of advanced TEM techniques, electrochemical characterization and first principles calculations.

The shape details of nanoparticle building blocks are critical to the dynamics and packing symmetry of their self-assembly process. Using liquid-phase transmission electron microscopy, we observed dynamic assembly and transformation of arrow-shaped gold nanoparticles in multiple geometric configurations. In-situ monitoring of the individual nanoparticles shows that they follow side-to-side, face-to-face or closed packing units directed by the convex or concave shape details with different relative orientations. The aspect ratio of these nanoparticles can be precisely tuned to achieve structural complexity and tunability, which can also be illustrated by the solvent evaporation-driven self-assembly. Our study provides guidance for future works of anisotropic particle self-assembly with complex structural details to have good controls in shape-oriented interaction and to enrich the library of self-assembly structures and properties.

Liquid-liquid phase separation (LLPS) of proteins into concentrated microdroplets (also called coacervation) is a phenomenon that is increasingly recognized to occur in many biological processes, both inside and outside the cell [1]. While it has been established that LLPS can be described as a spinodal decomposition leading to demixing of an initially homogenous protein solution, little is known about the assembly pathways by which soluble proteins aggregate into dense microdroplets. Using a recently developed technique enabling the observation of matter suspended in liquid by transmission electron microscopy (TEM), we observed how a model intrinsically disordered protein (IDP) phase-separates in liquid environment. The model protein used in these experiments is Histidine-rich Beak Proteins 2 (HBP-2), a protein present in the beak of squids that can be recombinantly obtained [2]. Our observations reveal for the first time dynamic mechanisms by which soluble proteins self-organize into condensed microdroplets, with nano-scale and milli-second space and time resolution, respectively. With this method, the nucleation and initial growth steps of the LLPS could be captured, opening the door for a deeper understanding of other biomacromolecular complexes exhibiting LLPS ability.

[1] Y. Shin and C.P. Brangwynne, Liquid phase condensation in cell physiology and disease, Science, 357, 1253 (2017).

[2] H. Cai, B. Gabryelczyk, M.S.S. Manimekelai, G. Gruber, S. Salentinig, A. Miserez, Self-coacervation of modular squid beak proteins – a comparative study, Soft Matter, 13, 7740 (2017).

Despite the study of BaTiO₃ for more than a half of a century, the physical and functional properties of this material at the nanoscale are still poorly known. This is due to the presence of multiple concomitant factors that are difficult to disentangle from macroscopic experiments. One of the distinctive characteristics of BaTiO₃ is the ferroelectricity, which is associated to the coherent and collective displacement of Ti ions that give origin to an electrical charge separation at the surface. However, at reduced crystals size (tens or couple hundreds of nanometers) the polar ordering of this material changes significantly from the bulk. From commonly employed experimental techniques used to study the crystal structure and phase transitions, such as XRD, Raman, DSC, and dielectric spectroscopy; the reduction of the crystal size results in peaks broadening and higher signal to noise ratio. Therefore, the crystal structure, phase transition temperature, and polar ordering of perovskite nanocrystals at the nanoscale is still unclear. To overcome these issues, PFM and HR-STEM have been employed to study the ferroelectricity and the atomic columns displacement of single nanocrystals of BaTiO₃, with sizes of 15 nm, 30-50 nm, and ~100 nm. The results demonstrated that the 100 nm nanocrystals show behavior similar to bulk BaTiO₃, with a sharp ferroelectric to paraelectric phase transition around 110 °C. By decreasing the size of the nanocrystals at around 40 nm, it has been observed from HR-STEM images that the polarization was not anymore along the c axis and could be either oriented along the [011] or [111] directiion. In support to the HR-STEM data, the PFM hysteresis loops have shown a shape variation while increasing the temperature that has been associated to an orthorhombic/rombohedral to tetragonal phase transition. Moreover, the tetragonal to cubic phase transition has been found to be around ~170 °C, which is well above the bulk temperature of 130°C. The smaller nanocrystals, showed by PFM measurements, an elongated hysteresis loop with smaller coercive field, which is in good agreement with our HR-STEM investigation that shows a randomized dipole pattern in a single nanocrystal. This study, clearly shows that in a small size range, of about 30 nm to 50 nm, the nanocrystals are stable at room temperature with a displacement of the Ti ions along the [011] or [111] vector, in contrast of the common [001] of the tetragonal system. This can open the door to the design of optimized electronic devices since the coercive field at room temperature decreases, allowing a more energy efficient polarization switching, and also the polar state is stable up to 170 °C that will allow higher thermal stability.

Eutectic Gallium-Indium alloy (EGaIn), a room temperature liquid metal, is of interest for applications which leverage its liquid state and rapid alloying behavior for application such as reconfigurable and self-healing electronics. Additionally, its tendency to rapidly self-passivate by production of native gallium oxide allows for simple production of EGaIn colloids which resist spontaneous recoalescence. Although EGaIn has been used in applications such as these, to date, most approaches in this area have an ad hoc character due to a lack of fundamental knowledge on the chemical and mechanical nature of the native gallium oxide and the process by which it develops. To provide insight in these areas, we demonstrate the production and functionalization of EGaIn colloids with the aim of carefully tuning the gallium oxide shell thickness and thus the derivative chemical and mechanical properties of the particles. To accomplish this, orthogonal ligand chemistries consisting of thiols or phosphonic acids are chosen to selectively bond to either elemental gallium, native gallium oxide, or both. Characterization with XPS and nanoindentation of the resultant particles shows that not only can the gallium oxide shell thickness be tuned over nearly a full order of magnitude, but the particles also show good adherence to Reissner’s scaling theory which is commonly applied to other core-shell particles. We anticipate approaches such as these to find application in devices such as self-healing electronics where fine control over particle mechanical properties is desirable to ensure a robust and reliable response to an initiating stimulus.

State-of-the-art in situand operandotransmission electron microscopy (TEM) applications have skyrocketed in recent years, permitting scientists to observe dynamic processes at the nanoscale. As the techniques evolved, so have the designs for liquid cell specimen holders. New advanced holders are capable of controlling liquid environments while simultaneously delivering external stimuli [1]. Samples encapsulated in liquid retain their freedom to move as real-time analysis provides new insights for protein aggregation, diffusion, particle interactions, and mechanistic changes [1]. Thus, researchers can gain a more complete understanding of individual systems beyond static snapshots acquired by traditional TEM methodologies.

Combining liquid-phase TEM (LP-TEM) with low-dose imaging and down-stream computational procedures provides unique quantifiable data of biological entities. Recent work to determine aggregation properties of a protein-based therapeutic reagent, PEGylated Interferon α2a, revealed diffusion and growth behaviours as a function of time [3]. Understanding these aggregation steps is beneficial for developing effective drugs with low side-affects. Such studies represent a new means to improve our understanding of aggregation dynamics under physiologically relevant conditions.

Using the same techniques, we were able to correlate the nanoscale motion of actively transcribing double-layered rotavirus particles (DLPs) with cryo-EM reconstructions. A side-by-side comparison of the two techniques showed increased internal disorder in actively transcribing DLPs versus those in a resting state (not actively transcribing). These correlative applications advance our ability to quantify molecular dynamics in the context of three-dimensional structural information.

Finally, a new exciting path for LC-TEM involves tomographic data collection of samples in liquid. In closed cell specimen holders, measurements are limited by a reduced ability to tilt the holder to higher angles. To address this issue, we developed a sandwich approach that utilizes a conventional carbon TEM grid in combination with a silicon nitride microchip having integrated microwells to enclose liquid specimens. Tomographic data from this work proved useful for microbiology studies and the technique is currently in development to monitor host-cell pathogen interactions in real-time.

Collectively, these results demonstrate the wide variety of fundamental processes that benefit from in situand in situ-adjacent imaging strategies to visualizeminute biological systems that inhabit the nanoworld around us.

References:
[1] Demmert et al., in (2016) ‘Visualizing Macromolecules in Liquid at the Nanoscale’, in Ross, F.M. (ed.) Liquid Cell Electron Microscopy. Cambridge: Cambridge University Press, pp. 334–355.
[2] Varano et al., Chem. Commun., 2015, 51, 16176-16179.
[3] DiMemmo et al., Lab Chip, 17 (2017) p. 315

Galvanic replacement is a useful method for synthesizing architecturally complex bimetallic nanostructures from monometallic templates. The oxidation of a monometallic template by ions of a more noble metal is well studied; however, chemically heterogeneous templates offer more than one type of reaction site and potentially structurally more complex materials. Yet, mechanistic studies with such templates are limited. Here, the reactivities of Ag and Pd in Janus-style AgPd dimers are compared when Au ions capable of oxidizing both metals are introduced, revealing (1) the selectivity of an oxidant in galvanic replacement for oxidizing one of two available reductants, (2) the interaction between galvanic replacement and solid-state diffusion, and (3) the similar replacement mechanisms for different redox pairs. Specifically, when in the presence of Ag and Pd nanocrystals, Au ions with various reduction potentials and oxidation states almost exclusively replace Ag first and Pd second, and when Ag and Pd are physically joined in dimers, Ag oxidation facilitates diffusion of the remaining Ag toward the Pd domain. These results provide mechanistic insight into the kinetically linked processes involved in the galvanic replacement of complex multimetallic templates, and demonstrate the importance of understanding these interactions in order to achieve structural and compositional control over the resulting nanostructures.

Calcium carbonate is a ubiquitous material, which has a high commercial importance and a variety of important applications. For example, calcium carbonate has traditionally found uses in the construction industry in the production of lime mortar and cement while its whiteness and high refractive index leads to its use in the manufacture of high-gloss paper and as a white pigment in the paint industry. In addition, CaCO₃ demonstrates excellent biocompatibility, enabling its biomedical applications such as in antacid tablets, bone regeneration and drug delivery systems.

The main goal of our work is the development of new CaCO₃ based nano- and micro-structural materials using various synthetic techniques. Here, we present our work on CO₂ bubbling, precipitation-based syntheses, and a novel dry ice carbonation method for the preparation of nano- and micro-particles of CaCO₃. CO₂ bubbling, which is the preferred method for industrial carbonation, was studied and the parameters affecting its synthesis were subsequently refined. Precipitation reactions were used to finely control the synthesis of unique CaCO₃ materials, and subsequently their chiral and luminescent properties. Finally, a unique dry ice carbonation method was developed enabling us to produce CaCO₃ morphologies which are not typically attainable using standard preparation approaches. We believe that this work will contribute to further development of CaCO₃ based materials with a range of potential applications.

1. Y. Boyjoo, V. K. Pareek and J. Liu; J. Mater. Chem. A, 2, 14270–14288, (2014).
2. A. Som, R. Raliya, L. M. Tian, W. Akers, J. E. Ippolito, S. Singamaneni, P. Biswas and S. Achilefu; Nanoscale, 8, 12639–12647, (2016).
3. F.C. Donnelly, F. Purcell-Milton, V. Framont, O. Cleary, P.W. Dunne, Y.K. Gun’ko; Chem. Commun., 53, 6657–6660, (2017).

Poly(N-vinylpyrrolidone) (PVP) is ubiquitously used in shape-controlled polyol synthesis of metal nanoparticles (NPs). Among the various systems using PVP, Ag nanocubes (NCs) synthesis has emerged as one of the most robust systems, where PVP is considered as the structure-directing agent and ethylene glycol (EG) as the reducing agent for Ag⁺ at elevated temperature. While several reports indicate molecular weight (M_w) and monomer concentration (C_m) of PVP impact the final shape of NPs, a general consensus on the role of PVP during the synthesis of Ag NCs using polyol method is thus far lacking. Recent experimental studies from our group indicate the differential heat of adsorption of PVP to (100) versus (111) facets is too small to predict a cube over an octahedron via a thermodynamic Wulff construction. We further demonstrate that chloride (Cl^–) added as HCl is the structure-directing agent in synthesis of Ag NCs. However, it has been observed that PVP can impact the final shape as its C_m or M_w is changed, suggesting PVP may influence the rate of reduction, leading to kinetically-preferred shapes even in the presence of Cl^–. We demonstrate PVP acts both as stabilizer and the dominant reducing agent in the synthesis of Ag NCs using polyol method. Introduction of small molecules such as 2-pyrrolidone (2P) into the reaction mixture of Ag NC synthesis drastically reduces the yield of Ag NCs owing to displacement of the capped PVP molecules, thereby demonstrating the stabilizing role of PVP. Optical studies of Ag⁺ reduction at different PVP C_m and M_w shows faster reduction rate at higher C_m for the same M_w of PVP and at lower M_w for the same C_m, suggesting PVP rather than EG plays a dominant role in the reduction of Ag⁺ and the reducing effect originates from the end groups of PVP. We constructed an experimental phase diagram for the formation of Ag NCs by varying the PVP C_m and M_w at constant temperature. Between the high and the low boundaries of the phase diagram, any combination of PVP C_m and M_w, can yield uniform Ag NCs. The lower boundary is related to a required minimum amount of PVP for stabilization, while the upper boundary is related to the role of PVP in Ag⁺ reduction. Finally, in order to bridge the gap over existing arguments regarding the identity of end functional groups in commercial PVP (–OH or –CHO) and the functional group in PVP molecule responsible for Ag⁺ reduction, we synthesized PVP with modifiable end-groups and a precise control over the average M_w for a systematic comparative study of Ag NC synthesis. Experiments suggest end groups are indeed responsible for Ag⁺ reduction kinetics; PVP with –OH end groups are more potent reducing agent than those with –CHO end groups. This work provides key insights into understanding the role of PVP in controlling reduction kinetics of Ag⁺ and thereby influencing the shape of Ag NPs during polyol synthesis.

Self-assembly of nanoparticles into mesoscopic structures is a proofed concept in material science and chemistry for the fabrication of hybrid systems with collective properties for different purpose. Liquid cell TEM provides a direct imaging approach that enables to watch the movement of nanoparticles in real time. Here, it was employed to study the self-assembly process of nanoparticles with different charges to unravel the charge effect on self-assembly. It was found that the positively charged nanoparticles can assemble to quasi one dimensional structures when the electron beam intensity exceeds a threshold, while negatively charged particles do not move under the e-beam. We also found size select assembly. The high energy electron beam effect is the driven factor on the self-assembly process. The individual particles show a preference to attach to the ends of existing dimers, trimers to form trimer and tetramer. The long distance dipolar interaction and short distance Van der Waals are the dominant control interaction in the formation of anisotropic one dimensional structure. We further investigate the self-assembly of Au nanoparticles (positively charged) under biased conditions in liquid environment. The different biased voltage was applied to nanoparticle suspension. It was found that the nanoparticles start moving at certain biasing condition. By statistics, the nanoparticle displacement is roughly the same under different biased voltage. But the number of moving nanoparticles is increasing along with the higher biased voltage. We also studied the self-assembly of non-charged nanoparticles. The fractal structure was formed and the hierarchical assembly was observed.
This work was performed at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357.

Lead toxicity has sparked interest into alternative halide nanomaterials with properties similar to CsPbX₃ perovskites. A promising alternative suggested from bulk studies is the family of double perovskites of the form Cs₂AgMX₆. Here, we report the robust and tunable synthesis of colloidal nanocrystals of Cs₂AgInCl₆ and Cs₂AgSbCl₆ via injection of acyl halides into a metal acetate solution under atmospheric conditions and relatively mild temperatures. We demonstrate the synthesis of single crystalline cubic nanocrystals of about 10 nm side length, and showcase their morphological similarities to other double perovskite structures in terms of their [200] facet termination and decoration with Ag⁽⁰⁾ spots. To compare the stabilities of the synthesized materials, we developed a titration assay based on the degradation of nanocrystals with amines as a proxy for degradation by humidity, which provides a quantifiable stability metric. This measurement showed that Cs₂AgSbCl₆ releases more than twice the decomposition energy compared to Cs₂AgInCl₆ or CsPbCl₃ and degrades in the presence of ca. one molar equivalent of amine, whereas the other two materials require more than a hundredfold excess. Using facile chemical titration to quantitatively determine chemical stability deepens the basic understanding of what makes materials environmentally stable.

Over the past five years, we have witnessed a rapid growth in liquid and gas flow holders for TEM and X-ray microscopes. These holders have enabled direct imaging of material transformations in liquid and gaseous environments with submicron-scale to atomic-scale spatial resolution. In particular, research regarding electrode materials in lithium ion batteries and nanocatalysts in heterogeneous catalysis has greatly benefited from the emergence of these capabilities. Despite their initial success in in-situ battery studies, similar class of problems in electrocatalysis has been rarely addressed using existing liquid electrochemical holders. In this talk, I will showcase the capability of direct nanoscale visualization of electrochemical formation and degradation of electrocatalysts utilizing an operando TEM liquid holder and I will also discuss existing challenges that the in-situ EM field need to address.

Additive manufacturing (AM) techniques have undergone enormous growth in the past couple of decades and have already revolutionised the process of rapid prototyping and manufacture of multifunctional, complex architectures. Due their interesting structure and surface chemistry, 2D colloids can be used in additive manufacturing to create complex structures with many unique characterises, or can be used in composite formulations for a wide range of applications including; photoelectronics, flexible transparent electrodes for energy storage to water purification.
Extrusion-based 3D printing, also known as Direct Ink Writing (DIW) or robocasting, provides a unique approach to introduce advanced and high-added-value materials with limited availability into lab-scale in manufacturing. Robocasting involves continuous extrusion of colloidal pastes or gels through a fine nozzle to create 3D structures layer-by-layer. Formulation design aims to ensure “printability” through careful control of rheology. The inks or pastes must be shear thinning to easily flow through the nozzle at modest shear rates and then immediately set into a non-flowing structure once printed. They must also be able to support multiple layers on top and retain the shape across spans without deformation. “Printability” however is still a broad concept due to the diverse range of applications within the field. This work compiles different rheology methodologies combining shear, extensional and oscillatory rheology that will enable quantifying the printability of soft materials.
2D colloids of graphene oxide (GO) exhibit a fascinating rheology and can aid the processing of different materials to develop ‘printable’ formulations. GO colloids in water form printable networks at relatively low concentrations and can behave as multifunctional additives. Due to unique surface chemistry GO, can be used to act as a rheology modifier that imparts a strongly shear-thinning behaviour and yield stress (σy) to suspensions of other materials even when is used in small concentrations. This work provides an in-depth rheological study of GO suspensions with a wide range of behaviours from Newtonian-like to viscoelastic ‘printable’ soft solids. The combination of extensional and shear rheology reveals the network formation process as GO concentration increases from <0.1 vol% to 3 vol%. Results from the extensional tests showed how the GO transitions from a Newtonian-like liquid (<0.1vol%) to weakly (<0.8vol%) and then strongly-established structural network as concentration is increased. Our results also demonstrate that the quantification of ‘printability’ can be based on three rheology parameters: the stiffness of the network via the storage modulus (G’), the solid-to-liquid transition or flow stress (σf), and the flow transition index, which relates the flow and yield stresses (FTI=σf/σy).

Assemblies of colloidal particles exhibit unique collective behaviors based on particle geometry, composition and arrangement, which enables tailored design of novel materials for diverse applications. Methods to deposit and self-assemble ordered particle solids from suspension are typically limited to fabrication of films and patterns, and commonly utilize surface tension to confine particles against substrates. In contrast, direct-write methods to build 3D structures rely on cohesion between particles in high-density suspensions to achieve structural rigidity, but this precludes particle ordering. Here, we present a method to build freestanding structures from self-assembled colloidal particles. The structures have mm-cm scale dimensions and can be built in freeform with aspect ratios greater than 10, yet retain polycrystalline order. We derive a scaling law that governs the rate of assembly, show how macroscale structural color can be tailored via the size and crystalline ordering of polystyrene particles, and build exemplary freestanding functional structures. Owing to the diversity of colloidal building blocks and means to control their interactions, direct-write assembly could therefore enable novel composites, photonics, electronics, and other materials and devices.

DNA-mediated programmable assembly is a promising route for synthesizing novel materials. This strategy has been successfully used to synthesize crystalline structures with more than 35 different symmetries and over 500 structures. However, most of the properties of these colloidal crystals are dictated by the identity of the building blocks, instead of their structural arrangements. In contrast, crystal structure plays a crucial role in determining the properties of several traditional molecular/atomic crystals such as porous (e.g. MOFs, clathrates, zeolites) and anisotropic crystals (e.g. uniaxial crystals, biaxial crystals). Analogous porous or anisotropic colloidal crystals are difficult to fabricate given that the most commonly used building blocks are highly symmetric spherical particles. Unlike spherical particles, anisotropic particles can directionally guide the formation of DNA bonds with specific angles, which affects the lattice symmetry and crystal habit of the resulting colloidal crystals. In this work, we show two examples of how low symmetry nanoparticles assemble into either porous or anisotropic crystals. <!--![endif]---->
<!--[endif]---->Natural clathrates are mainly hydrate frameworks with a variety of host molecules such as methane. The cages in natural clathrates are formed when molecular or atomic nodes adopt discrete bond angles between 100 and 125°. To assemble the nanoparticle analog, oblate trigonal bipyramids were identified as ideal building blocks as they can promote the formation of DNA bonds with ~110° bond angle. DNA-mediated assembly of triangular bipyramids generated three different clathrate architectures, which, to date, are the most sophisticated architectures made by programmable assembly. The cavities in the clathrates may have potential for host-guest recognition applications (such as proteins or virus) as well as catalysis.
Natural anisotropic crystals have different refractive indices along different crystallographic axes which allows them to modulate the phase or path of light. Using low symmetry nanoparticles such as nanoscale pentabipyramids and nanorods, anisotropic colloidal crystals such as rhombohedra, hexagonal prisms, and rhombic prisms were assembled. In particular, colloidal rhombic prisms which have face-centered-orthorhombic symmetry, the lowest symmetry achieved by programmable assembly to date, were prepared. Importantly, these represent the first examples of biaxial colloidal crystals. Anisotropic colloidal crystals such as these have potential as important optical components in micro-optical systems. <!--![endif]---->

In last few years piezoelectric materials have found its position as one of the leading stress sensing element in real life applications. Hence the demand for developing highly effective and sensitive piezoelectric devices is the all-time high. We present an unpoled high performance flexible piezoelectric nanogenerator comprising of three-dimensional(3D) surface modified Lithium doped Zinc Oxide (Li-ZnO) with Multiwalled carbon nanotube (MWCNT) in bulk matrix of Polyvinylidene fluoride (PVDF). 3D Li-ZnO was synthesized hydrothermally followed by surface modification by polyethylene glycol (PEG-20000). The polyethylene glycol coating served as an effective solution for avoiding electrical poling and enhanced the proportion of the PVDF ß-phase while MWCNTs acted to increase conductivity and to reinforce the composite during mechanical stressing. This also acted as a cost-effective multifunctional piezoelectric device. The piezoelectric nanocomposite was tested with different body motions to assess this piezoelectric device’s efficiency in real-world application. The piezoelectric device was found with promising a linear response to a gradual increase in normal stresses. This device also shows a promising use for biomedicine as PVDF, ZnO is FDA approved.

Manipulating the chiroptical at the nanoscale is of great importance in stereoselective reactions, enantioseparation, self assembly and biological phenoma. In recent years, carbon dots have garnered great attention due to their favorable properties such as tunable fluorescence, high biocompatibility, and facile, scalable synthetic procedures. Herein, we report for the first time the unusual behavior of cyclic amino acids on the surface of carbon dots prepared via microwave-based carbonization. Various amino acids were introduced on the surface of carbon dots via EDC: NHS conjugation at room temperature. Circular dichroism results revealed that although most of the surface conjugated amino acids can preserve their chirality on negatively charged, ‘bare’ carbon dots, the ‘handedness’ of cyclic α-amino acids can be flipped when covalently attached on carbon dots. Moreover, these chiroptical carbon dots were found to interact with the cellular membrane or its mimic in a highly selective manner due to their acquired asymmetric selectivity. Comprehensive inhibitor study was conducted to investigate the pathway of cellular trafficking of these carbon dots. Overall, it was concluded that chirality of amino acid on the surface of carbon dots could regulate many of the cellular processes.

Due to their amazing optoelectronic properties, colloidal lead halide perovskite nanocrystals (NCs) are receiving increasing attention in recent times.^1,2 Perovskite NCs possess properties that are not accessible in their bulk or thin-film counterparts. For example, perovskite phase of CsPbI₃ is unstable in ambient condition in bulk or thin-film, but they are phase stable in their quantum confined form. This particular material has shown to have record efficiency for quantum dot (QD) solar cells.^3,4 Another interesting advantage of these QD materials is that their compositions can be tuned without changing the crystal framework either by direct synthesis or by post-synthetic ion exchanges. Particularly, X-site ion exchange in the perovskite QDs with general formula ABX₃ (where A= Cesium-Cs, methylammonium-MA, formamidinium-FA etc.; B= Pb or Sn; X= Cl, Br, I) has shown to be very facile.⁵ On the other hand, A-site composition tunability is very limited in these materials, or even in the corresponding thin films. For example, it is known that any arbitrary composition in Cs_1-xFA_xPbI₃ cannot be achieved both in QDs and thin films, and it has been shown that only compositions with 1-x>0.4 can be realized in the pure usable perovskite phase. This is due to thermal instability of FAPbI₃ (crystallizes at around 130 ^oC) at temperatures required to crystallize CsPbI₃ (above 300 ^oC). Here, we present a simple post synthetic cross-cation exchange reaction between colloidal solutions of CsPbI₃ and FAPbI₃ nanocrystals just by mixing them at temperatures slightly above the room temperature that enables us to achieve compositions in the whole range of 0<x<1. This helps us to realize compositions that were not known previously. The photoluminescence (PL) kinetics studies reveal that the activation energy required to inter-exchange the Cs⁺ and FA⁺ ions is around 0.65 eV, higher than that for X-site exchange in lead halide perovskites. We use these alloyed colloidal perovskite quantum dots to fabricate photovoltaic devices. We applied these alloyed NC ink in solar cell and demonstrated that they exhibited high open circuit voltage (V_OC) of ~ 90% of their Shockley-Quiesser maximium and have lower losses than the thin film perovskite devices of similar compositions.⁶

References
1. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V., Nano Lett. 2015, 15 (6), 3692.
2. Akkerman, Q. A.; Rainò, G.; Kovalenko, M. V.; Manna, L., Nat. Mater. 2018, 17 (5), 394.
3. Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M., Science 2016, 354 (6308), 92.
4. Sanehira, E. M.; Marshall, A. R.; Christians, J. A.; Harvey, S. P.; Ciesielski, P. N.; Wheeler, L. M.; Schulz, P.; Lin, L. Y.; Beard, M. C.; Luther, J. M., Sci. Adv. 2017, 3 (10).
5. Akkerman, Q. A.; D’Innocenzo, V.; Accornero, S.; Scarpellini, A.; Petrozza, A.; Prato, M.; Manna, L., J. Am. Chem. Soc. 2015, 137 (32), 10276.
6. Abhijit Hazarika, Qian Zhao, E. Ashley Gauldin, Jeffrey A. Christians, Benjia Dou, Ashley R. Marshall, Taylor Moot, Joseph J. Berry, Justin C. Johnson, and Joseph M. Luther, ACS Nano, 2018, 12 (10), 10327.

The graphene quantum dots (GQDs) have attracted the attention of researchers due to their excellent properties and potential applications in biomedicines, energy storage devices and photovoltaics. The photoluminescence is one of the most important characteristics of GQDs. The doping of GQDs with Nitrogen atoms is one of the most effective ways to tune their photoluminescence emission and to increase quantum yield. In this work, high-quality Nitrogen-doped graphene quantum dots (N-GQDs) were synthesized by using pulsed laser synthesis method at various irradiation powers of pulsed laser and changing the concentration of nitrogen doping to effectively tune the photoluminescence emission and improve the quantum yield (QY) of as synthesized N-GQDs. The TEM, HRTEM, XPS, XRD, Raman spectroscopy and FTIR were carried out to observe the morphology, size distribution, crystalline structure and to prove successful doping of GQDs with nitrogen atoms. To observe optical properties of as synthesized N-GQDs, the UV-vis and Photoluminescence measurements were carried out. The as-synthesized NGQDs exhibit high quality crystalline structure of graphene. A high quantum yield was exhibited by the obtained N-GQDs as compare to the pristine GQDs. The obtained N-GQDs with oxygen-rich functional groups exhibit a strong emission. This work may be helpful to expand the scope of GQDs especially in biomedical applications.

In this presentation, we will describe the colloidal synthesis of phase-pure nanocrystals (NCs) of a highly abundant mineral, chalcopyrite (CuFeS₂). The steady state optical extinction spectrum of these NCs offers few surprises due to Fe which introduces deep energy levels in the electronic band structure, as supported by our density functional theory (DFT) calculations. The spectrum is characterized by absorption bands centered at around 480 and 950 nm, spanning almost the entire visible and near infrared regions. These are ascribable to electronic transitions from the valence band (VB) to the empty intermediate band (IB), located in the so-called fundamental band-gap and composed predominantly of Fe 3d orbitals. We further demonstrate, through spectroscopic measurements and ab initio calculations that these NCs actually sustain metal-like quasi-static optical resonances despite the absence of free carriers in the NC ground state owing to this unique band structure. Laser-irradiation (at 808 nm) of an aqueous suspension of these CuFeS₂ NCs exhibited significant heating, with a photothermal conversion efficiency of 49%. Such efficient heating is most likely due to the carrier relaxation within the broad IB band, as corroborated by transient absorption measurements and further described by DFT calculations. The intense absorption and high photothermal transduction efficiency (PTE) of these NCs in the so-called biological window (650−900 nm) make them suitable for photothermal therapy as demonstrated by tumor cell annihilation upon laser irradiation.
We intend to put forward the chemistry behind the synthesis of these low band-gap ternary semiconductor NCs and describe the optoelectronic properties using experimental and theoretical points of view, through this presentation. A suite of spectroscopic techniques, which include visible-infrared transient absorption, x-ray absorption and emission, resonant inelastic x-ray scattering (RIXS) were used in our analyses. The subsequent use as a photothermal agent is an offshoot of this understanding.The presence of the deep Fe levels constituting the IB is the origin of such enhanced PTE, which can be used to design other high performing NC photothermal agents.

References:
1. Colloidal CuFeS₂ Nanocrystals: Intermediate Fe d-Band Leads to High Photothermal Conversion Efficiency. Sandeep Ghosh, Tommaso Avellini, Alessia Petrelli, Ilka Kreigel, Roberto Gaspari, Guilherme Almeida, Giovanni Bertoni, Andrea Cavalli, Francesco Scotognella, Teresa Pellegrino, and Liberato Manna Chem. Mater. 28(13), 4848- 4858 (2016).

2. Quasi-Static Resonances in the Visible Spectrum from All-Dielectric Intermediate Band Semiconductor Nanocrystals. Roberto Gaspari, Giuseppe Della Valle, Sandeep Ghosh, Ilka Kriegel, Francesco Scotognella, Andrea Cavalli, and Liberato Manna Nano Lett. 17(12), 7691–7695 (2017).