Symposium S.CT08—Crystallization via Nonclassical Pathways in Synthetic, Biogenic and Geologic Environments

Crystallization by assembly of primary nanocrystals is a widespread phenomenon in synthetic and natural environments. In mineral systems, this process exhibits diverse styles ranging from oriented attachment of like phases to mis-oriented aggregation of disparate phases followed by coarsening to single crystals. While descriptions of 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, inherent diopolar interactions, structured nanoscale interfaces, and solvent-responses of a length scale comparable to particle size. To understand the relationship between nanocrystal structure, interparticle forces and assembly dynamics, we are investigating colloidal assembly of titanium, iron and zinc oxides. For ZnO, in situ TEM demonstrates that direction-specific forces align diffusing particles at distances far in excess of what is expected based on DLVO theory. Particles approach one another in a diffusion limited regime and then attach lattice matched via a sudden jump to contact unopposed by any significant energy barrier. Direct AFM-based measurements of the forces between both TiO₂ and ZnO surfaces reveal orientation dependent attractive interactions exhibiting the symmetry of the underlying lattice. MD simulations for both systems predict that solvation forces are responsible for the orientation dependence, creating solvent separated minima and barriers to contact. However, classical density functional theory applied to ZnO predicts that ion correlation forces lead to generally attractive interactions with little or no barrier, enabling random attachment, which is not observed. The long range nature of the forces and torques in the ZnO system are shown to be a consequence of the inherent dipole moments of ZnO particles and it is these dipole-dipole interactions that enforce alignment even in the absence of solvation-defined orientation dependent forces. Addition of organic ligands, like oxalate, to the FeOOH system produces a dramatically different pathway and outcome: Nanoparticles of Fe₂O₃ nucleate sporadically from the FeOOH and, once formed, are covered with oxalate, which drives all new Fe₂O₃ particles to nucleate within about 1 nm of the interface, 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 FeOOH system is replaced with one that is deterministic.

We use liquid-phase transmission electron microscopy to directly image a series of crystallization pathways of colloidal nanoparticles into superlattices. We find that there exist similarities to the prevalent model system of micron-sized colloids, such as a non-classical two-step crystallization pathway, where amorphous intermediates in the form of nanoparticle clusters exist preceding the nucleation. Capillary wave theory can be used to describe the fluctuation and energy of the surfaces. But there are also differences, in particular, a universal layer-by-layer growth mode that we observe consistently for diverse nanoparticle shapes. Single particle tracking, trajectory analysis, and simulations combined unravel the energetic and kinetic features rendering this crystal growth mode possible and universal at the unexplored nanoscale, enabling advanced crystal engineering.

We utilize nanoparticle superlattice as a model system to study the fundamental laws of order emergence and evolution from a dispersion of nanosized entities. Liquid-phase TEM makes it possible to directly conduct single-particle level imaging in real time and real space, to reveal pathways of nanoparticle crystallization. To be specific, we observe how different types of diffusion events (e.g., surface and volume diffusion) involved in the layer-by-layer growth of a nanoparticle superlattice, where the high spatiotemporal resolution enables us to identify single particles throughout the whole process and to map out their trajectories sampling the energy landscape. Furthermore, oriented attachment of different nanoparticle lattice domains is observed, where fast translational and rotational diffusions of the domains drive the neck formation. The single-particle analysis aids correlating order evolution to differently ranged interactions. Such crystallization behavior is distinct from that of micron-sized colloids, resembling more the faceted crystals formed from atoms. Our study provides new dynamic information on the superlattice development at high spatial and temporal resolution, which can serve as a general guideline for bottom-up material design.

Nucleation and growth are universally important in systems from the atomic to the micrometer scale as they dictate structural and functional attributes of crystals. However, at the nanoscale, the pathways toward crystallization have been largely unexplored owing to the challenge of resolving the motion of individual building blocks in a liquid medium. Here we address this gap by directly imaging the full transition of dispersed nanoparticles to a superlattice at the single-particle level. We utilize liquid-phase transmission electron microscopy at low dose rates to control nanoparticle interactions without affecting their motions. Combining particle tracking with Monte Carlo simulations, we reveal that positional ordering of the superlattice emerges from orientational disorder. Otherwise elusive parameters including line tension and phase coordinates are measured, charting the nonclassical nucleation pathway involving a dense, amorphous intermediate. We elucidate the origin of this two-step crystallization by experimentally measuring the energy barrier for nucleation, which is lower in the presence of the amorphous precursor. We demonstrate the versatility of our approach via crystallization of different nanoparticles, pointing the way to more general applications. Our real space nanoscopic imaging platform not only offers new understanding on the crystallization kinetics, but also opens opportunities to other nanoscale fluctuating systems, from biofouling, lubrication, melting, phonon transport, to dendrite formation.

The self-assembly of quantum dots (QDs) into epitaxially connected superlattices called quantum dot solids (QDSs), made possible by a growing library of synthesis methods for precisely controllable QD building blocks, represents exciting opportunities for advancing optoelectronic devices and realizing exotic electronic phenomena. However, the myriad complex interactions at play during the assembly and oriented attachment processes lead to imperfections in epitaxial connections between QDs. Such imperfections affect electronic coupling between QDs and lead to local translational disorder, which propagates through the film and hinders tunable miniband formation. Heating QDSs is a promising method for annealing out defects, thus improving electronic properties. Understanding the nuanced effects on epitaxial connections, however, requires visualization of atomic-scale structure over large enough fields of view to access a statistically meaningful number of QDs.

Here, we use in situ MEMS heating in combination with aberration-corrected HAADF-STEM and image analysis to assess the effects of heating on epitaxial connections in QDSs over extended fields of view. We analyze PbS QDs self-assembled into square SLs and epitaxially connected along the <100> directions. Localized heating provided by the MEMS device maintains stage stability such that a region of interest can be tracked throughout a heating cycle and images can be acquired at atomic-resolution over ~100 nm. We identify four distinct strain states in epitaxial connections: tension, shear, bending and torsion, noting that they appear concurrently in some cases. Using a local sine wave fitting technique similar to the phase lock-in method, we visualize the tension, shear and bending strain states. Given the limitation of STEM to projection imaging, torsion is investigated indirectly by identifying the out-of-plane orientation of each QD, nearest neighbor mismatches in which indicate torsion and out-of-plane bending strain states between connected QDs. The orientation data, in combination with wave fitting analysis, reveal that mild annealing between 150 and 200°C causes dramatic relaxation of tensile, shear and torsional strains, while bending strain is more persistent. These results give insight into the energetics of the types of defects present in epitaxial connections and represent a path toward improved carrier delocalization and miniband formation.

There is an increasing need for the development of multifunctional lightweight materials with high strength and toughness. Natural systems have evolved efficient strategies, exemplified in the biological tissues of numerous animal and plant species, to synthesize and construct composites from a limited selection of available starting materials that often exhibit exceptional mechanical properties that are similar, and frequently superior to, mechanical properties exhibited by many engineering materials. These biological systems have accomplished this feat by establishing controlled synthesis and hierarchical assembly of nano- to micro-scaled building blocks. This controlled synthesis and assembly require organic that is used to transport mineral precursors to organic scaffolds, which not only precisely guide the formation and phase development of minerals, but also significantly improve the mechanical performance of otherwise brittle materials.

Here, we investigate an organism that have taken advantage of hundreds of millions of years of evolutionary changes to derive structures, which are not only strong and tough, but also demonstrate abrasion resistance. All of this is controlled by the underlying organic-inorganic components. Specifically, we discuss the formation of heavily crystallized radular teeth the chitons, a group of elongated mollusks that graze on hard substrates for algae.

Our investigation of the formation of a fully mature radular tooth from Cryptochiton stelleri found in the eastern Pacific, occurs over a series of more than 40 teeth. The initial stage of tooth formation begins with the synthesis of a three-dimensional fibrous α-chitin organic matrix, prior to the onset of crystallization. Microscope analysis of early stage teeth shows ferrihydrite mineral particles growing on α-chitin fibers. Synchrotron x-ray, combined with TEM analyses show that ferrihydrite exists as randomly oriented aggregates, but undergoes a phase transformation to magnetite within a few rows of teeth. We discuss potential mechanisms of nucleation of ferrihydrite, its transformation to magnetite as well as its subsequent mesoscale ordering, crystal growth and the resulting mechanical properties. From the investigation of synthesis-structure-property relationships in these unique organisms, we are now developing and fabricating multifunctional engineering materials for energy and water purification based applications.

A novel method for the solidification of metallic alloys involving spiral self-organization is presented as a new strategy for producing large-area chiral patterns with emergent structural and optical properties, with attention to the underlying mechanism and dynamics. This study reports the discovery of a new growth mode for metastable, two-phase spiral patterns from a liquid metal. Crystallization proceeds via a non-classical, two-step pathway consisting of the initial formation of a polytetrahedral seed crystal, followed by ordering of two solid phases that nucleate heterogeneously on the seed and grow in a strongly coupled fashion. Crystallographic defects within the seed provide a template for spiral self-organization. These observations demonstrate the ubiquity of defect-mediated growth in multi-phase materials and establish a pathway towards bottom-up synthesis of chiral materials with an inter-phase spacing comparable to the wavelength of infrared light. Given that liquids often possess polytetrahedral short-range order, our results are applicable to many systems undergoing multi-step crystallization.

Crystallization induced microstructure plays a central role in tailoring materials
properties, yet our understanding of its formation process is limited due to its complexity.
Classical crystallization theories assume that nucleation and growth are separate processes and
growth ought to be temperature-dependent only. Local thermodynamically unstable clusters are
considered to fluctuate in the nucleation stage but do not participate in the later growth. Here, we
apply in-situ transmission electron microscope (TEM) to nanostructured metallic glasses (MGs)
and directly probe crystallization kinetics at the unprecedented time and spatial resolution^{1, 2,
3,4}. In particular, the growth kinetics of single crystalline Pt_57.5Cu_14.6Ni_5.3P_22.5 MG nanorods
were studied in situ by observing isothermal crystallization at the atomic level. We found that
the crystal growth rates of heated metallic glass are much higher than those of cooled metallic
liquid despite the same isothermal crystallization temperature. The observation is in direct
contrast to one would expect from classical growth models. We, therefore, propose a
supercluster-coupled growth model and hypothesize that structural ordering can strongly
enhance the crystal growth. The asymmetric growth upon heating is thus attributed to the higher
degree of structural ordering and a greater number of clusters in the MGs nanorods. The
structure-coupled growth is further supported by manipulating the density of the small clusters
through nanoscale confinement and thermal treatment of the MG nanorods. In addition,
molecular dynamics simulations of binary Lennard-Jones glasses quenched at different cooling
rates show a growth rate trend that agrees with experimental observations. The present results
demonstrate the inadequacy of the classical crystallization model on modern complex materials
systems.

Reference
1. Xie, Yujun, et al. "Supercluster-coupled crystal growth in metallic glass forming liquids."
Nature communications 10 (2019).
2. Xie, Yujun, et al. "Direct observation through in situ transmission electron microscope of
early states of crystallization in nanoscale metallic glasses." JOM 69.11 (2017): 2187-2191.
3. Sohn, Sungwoo*, Xie, Yujun*, et al. "Tailoring crystallization phases in metallic glass
nanorods via nucleus starvation." Nature communications 8.1 (2017): 1980. (*co-first author)
4. Sohn, Sungwoo, Xie, Yujun, et al. "Nanoscale size effects in crystallization of metallic glass
nanorods." Nature communications 6 (2015): 8157.

Crystallization from solution follows either a single or a multistep pathway. The latter case involves the formation, aggregation and transformation of precursor/intermediate particles to the final crystalline phase. This has been observed for several important minerals, such as CaCO₃, CaPO₄, CaSO₄ or CSH [e.g. 1-3], but also for macromolecular crystals [e.g. 4,5]. Moreover, this particle-mediated crystallization pathway can be “fossilized” in the structure of the final crystals [e.g. 6].
In the idealized laboratory environment pure solutions are often studied, but in real-world natural and engineered crystallization processes soluble additives, e.g. inorganic or organic molecules, are omnipresent. In fact, they frequently play a key role in the formation of crystalline phases and are a central part of biomineralization, functional material synthesis and anti-scaling strategies [e.g. 7]. Traditionally, models of additive-controlled crystallization have considered a single step nucleation with additives adsorbing on the surface of nuclei, impeding their further growth. Hence, to advance our understanding and gain a better control of multistep crystallization processes, the interaction of additives with the precursors and/or intermediate particles needs to be unraveled.
To achieve this, we constructed a holistic nanoscopic view of a particle-mediated crystallization pathway in an additive free system [e.g. 5-8], based on in situ nanoscale observations. Subsequently, we conducted macroscopic and microscopic observations of additive-controlled crystal formation. Opposing the classical view, we found that the inhibition of nucleation can be ascribed to additives that hinder the formation of precursor species and/or their aggregation/transformation by prevailing in their vicinity and thereby reducing their interaction probability [e.g. 7, 10-11].

[1] De Yoreo et al., Science 349, 489, 2015. [2] Van Driessche et al., New perspectives on mineral nucleation and growth. Springer-Verlag, 2017. [3] Krautwurst et al., Chem. Mater. 30, 9, 2018. [4] Van Driessche et al., Nature 556, 89-94, 2018. [5] Sleutel and Van Driessche Nanoscale 10, 12256-12267, 2018. [6] Stawski et al., Cryst. Growth Des. 19, 3714-3721, 2019. [7] Van Driessche et al., Chem. Geol. 119274, 2019. [8] Stawski et al., Nat. Commun. 7, 11177, 2016. [9] Stawski et al., J. Phys. Chem. C. 123, 23151-23158, 2019. [10] Nicoleau et al., Cement Concrete Res. 105837, 2019. [11] Ossorio et al., Minerals 7, 140, 2017.

The progress of energy and electronic industry relies on the design and engineering of new nanostructures with fine control in the atomic scale. Examples include the use of twisted two-dimensional materials for the tuning of electronic structure, the surface engineering to create surface steps in metal nanowires that show enhanced catalytic activity. Manipulation of the nanostructures with more precise control requires the understanding of how atoms interplay with various stimuli, which must be characterized by advanced imaging tools that can not only reveal the materials behavior during growth and use, but also provides insights on the change of composition and bonding. Recent development of aberration-corrected transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM) allows the combination of low-dose imaging with in situ methods to study the materials dynamics at high spatial resolution, when coupled with high-speed direct electron detectors, the electric potential and field that can influence the materials interaction can also be imaged. Here we first demonstrate the atomic resolution imaging of a Pt precursor compound and the dynamics of its transformation to Pt nanoclusters.[1] Specifically, under an ultra-low electron dose, the atomic structure of pristine K₂PtCl₄ crystallite is resolved in the image captured by the CMOS-based direct electron detector. With further electron beam irradiation, the dynamics of staged reduction of Pt compound and nucleation of Pt cluster are captured and identified. Results show that the ionic bond between K⁺ and PtCl₄^2- is first broken, which is followed by PtCl₄^2- forming PtCl₂; reduction of the dual-valent Pt then leads to the formation of Pt clusters. The solid-state transition is deciphered in real time and real space. Later, using a gas cell, we show that the sintering process of Pt clusters can be modified by gas adsorbates:[2] in H₂, the Pt nuclei approach and attach to each other only along specific orientations, and gradually form short nanowires. In ex situ experiment, we successfully synthesized extended long Pt and PtNi nanowires in solid state assisted by H₂. Compared to the Pt-based NWs grown by other methods, our catalysts showed both enhanced mass specific activity and stability in ORR. We will also show our four-dimensional STEM study at the interface of complex oxides that reveals the penetration of electric field from the ferroelectric BiFeO₃ into the insulating SrTiO₃,[3] and discuss how advanced electron microscopy can further our study of nanostructures.

[1] Wenpei Gao, Peter Tieu, Christopher Addiego, Yanling Ma, Jianbo Wu, Xiaoqing Pan, Science Advances, 5(1), eaau9590, 2019
[2] Yanling Ma, Wenpei Gao, Hao Shan, Wenlong Chen, Wen Shang, Peng Tao, Chengyi Song, Chris Addiego, Tao Deng, Xiaoqing Pan, Jianbo Wu, Advanced Materials, 1703460, 2017
[3] Wenpei Gao, Christopher Addiego, Hui Wang, Xingxu Yan, Yusheng Hou, Dianxiang Ji, Colin Heikes, Yi Zhang, Linze Li, Huaixun Huyan, Thomas Blum, Toshihiro Aoki, Yuefeng Nie, Darrell Schlom, Ruqian Wu, Xiaoqing Pan, Nature, in press, 2019

Biosynthesis of NPs is a striking phenomenon in nature. Inspiring examples include storage and transport of iron atoms in a non-toxic form of a ferric oxyhydroxide cluster in the ferritin protein cage, using metal salts as terminal sinks in electron transfer pathways and formation of metal particles by bacteria, and biomineralization of magnetite nanocrystals in specific organelle magnetosome that enables coordinated movement of magnetotactic bacteria. Under certain experimental conditions, metal complexes and nanoclusters can be obtained in mammalian cells or in a whole animal, mainly for use in imaging. However, the mechanisms of this biosynthesis are often spontaneous, poorly understood and, therefore, difficult to control. In our approach, a pathological process, namely hypoxia of the brain accompanied by changes in the NADH/NAD+ co-enzymes ratio, initiates and controls the biosynthesis of biocompatible Au nanoparticles from precursor salts in the immediate vicinity of the hypoxia site, thereby restoring the redox status of the brain.

Nanoscale DOI: 10.1039/c9nr05794c (2019)

The early observation of strong circular dichroism for individual nanoparticles and their assemblies have developed into a rapidly expanding field of chiral inorganic nanostructures. The chiral inorganic nanostructures encompass sophisticated constructs from metals, semiconductors, ceramics, and nanocarbons with multiple chiral geometries with characteristic scales from Ångströms to microns. Such versatility enables their functional engineering over a broad range of physical and chemical properties but the relationship between the chirality of surface ligands, NP cores, and resulting assemblies remains obscure. Similarly, the relationship between their chiral geometries and their chiroptical properties. This talk will address (1) the origin of the uniquely high values of optical anisotropy; (2) the mechanisms of chirality transfer in inorganic materials; and (3) differences/similarities with chiral supramolecular, liquid crystal, and biological assemblies. Long-range correlations and critical phenomena in chiral nanoassemblies leading to the emergence of geometrically complex structures will be demonstrated. The novel venues for practical realizations of chiral nanoassemblies included in the talk will be photocatalytic C-C coupling in biomimetic chiral supraparticles and polarization spectroscopy in terahertz (THz) spectral window with chiroplasmonic kirigami composites.

Exerting control over non-classical pathways to crystallization to direct the growth, polymorphism, and self-assembly of inorganic nanoparticles into higher-order structures shapes the future of materials science ^[1]. To achieve this, the dominant strategy currently under development is directed towards our ability to govern the nucleation stage in a multitude of precipitation reactions occurring far from thermodynamic equilibrium ^[2]. These reactions share the common purpose of converting solution precursors into solid mineral materials, and, hence, are based on a number of different approaches that overcome the free-energy barrier to nucleation. We have examined the feasibility of an alternative approach that could not only overcome the limitations associated with the dominant strategy mentioned above, but also shape our way of manufacturing novel materials suitable for modern needs. We believe that future approaches will deliberately skip over the nucleation stage on which little control can be exercised, and will focus on the conversion of solid, metastable amorphous nanoparticles into their crystalline counterparts. Such metastable amorphous nanoparticles that latter transform into higher-order, hierarchical mineral structures have been observed across various biomineralizing taxa such as stony corals ^[3], but their use in synthetic systems remains extremely limited. Here we used Amorphous Calcium Magnesium Carbonate (ACMC) nanoparticles as models, and we monitored their pathway to crystallization in aqueous medium to show their potential usefulness in the development of novel materials. To this end, we applied a combination of ultrahigh-resolution three-dimensional imaging, and in situ, solid-state nuclear magnetic resonance (ssNMR) spectroscopy.

Our results reveal the underlying mechanism of the phase transformation of these Amorphous Calcium Magnesium Carbonate (ACMC) nanoparticles into crystals in wet conditions. First, they suggest that these amorphous nanoparticles are coated by a rigid hydration shell of bound H₂O molecules when soaked in water. Our results also show that fast chemical exchanges occur between hydrogens from the nanoparticles and those from the free H₂O molecules of the surrounding aqueous medium. Crystallization spontaneously occurs after the amorphous nanoparticles were soaked in water. Using two-dimensional ssNMR spectroscopy, we especially show that the nascent crystalline environments, together with the starting amorphous materials, belong to the same particles. This is a clear spectroscopic evidence of a solid-state phase transformation of the starting amorphous nanoparticles into crystals. In addition, we reveal that the starting amorphous nanoparticles remain hydrated while crystallization progresses. There is also evidence of fast chemical exchanges between hydrogens from the amorphous nanoparticles partially converted into crystals, and those from the free H₂O molecules of the surrounding aqueous medium. Hence, our results question the role played by these unceasing chemical exchanges towards crystallization. We imagine that they are likely to be associated with an enhanced mobility of the ions/molecules that compose the amorphous nanoparticles which, in turn, allows for rearrangement of these ions/molecules into crystalline domains. Lastly, using ultrahigh-resolution three-dimensional imaging, we show that these amorphous nanoparticles aggregate to form ordered crystalline structures through crystal growth by particle attachment; and this conclusion is further supported by our ssNMR results. As such, these metastable amorphous nanoparticles open new avenues to develop novel ceramic or inorganic-organic hybrid materials through strategies that are no longer limited by the nucleation stage.

[1] J. Lee et al., Nat. Rev. Mater. 2016, 1, 16034.
[2] E. Nakouzi, O. Steinbock, Sci. Adv. 2016, 2, e1601144.
[3] S. Von Euw et al., Science 2017, 356, 933.

In recent years, significant achievements have been made in the synthesis of various colloidal nanostructures. The anisotropic nanostructures, such as nanowires or two-dimensional (2D) materials have attracted a lot of attention due to their unique properties. However, their formation mechanisms have not been fully understood. They seem to be contradictory to the predictions based on the conventional crystal growth theories. And, it adds to the complexity that nanocrystals often change structure and morphology during growth. We explore the growth mechanisms of anisotropic nanocrystals using in situ liquid cell transmission electron microscopy (TEM). In this talk, I will show the growth of 2D transition metal oxide nanosheets, e.g. cobalt oxide and cobalt nickel oxide. The 2D nanosheets have been achieved in a liquid cell, which resemble those synthesized in flasks. Our direct observations reveal that 3D nanoparticles are initially formed from the molecular precursor solution and then transform into 2D nanosheets. Ab initio calculations show that a small nanocrystal is dominated by positive edge energy, but when it grows to a certain size, the negative surface energy becomes dominant, driving the transformation of the 3D nanocrystal into a 2D structure. Uncovering these growth pathways, including the 3D-to-2D transition, provides opportunities for the future design and solution synthesis of novel materials.

References:
J. Yang, Z. Zeng, J. Kang, C. Czarnik, X. Zhang, C. Ophus, C. Yu, K. Bustillo, M. Pan, J. Qiu, L. W. Wang, H. Zheng, “Formation of two-dimensional transition metal oxide nanosheets with nanoparticles as intermediates.” Nature Materials 18, 970-976 (2019).
H. Zheng et al., “In situ electron microscopy imaging of nanocrystal growth and transformations”, Trends in Chemistry invited review (2020).

Acknowledgements:
The project was funded by U.S. 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 in situ TEM program (KC22ZH). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Microorganisms have been found living in one of the driest and oldest deserts on the earth, the Atacama Desert in Chile. In this hyper-arid environment, microbial communities colonize the interior of rocks and obtaining water is critical for their survival. Gypsum (CaSO₄●2H₂O) is one of the main components in rocks from the Atacama Desert and is a main source of water for these communities, which are mainly constituted of cyanobacteria and heterotrophic bacteria. We report that these microorganisms extract crystalline water from the gypsum rock, inducing a phase transformation from gypsum (CaSO₄●2H₂O) to anhydrite (CaSO₄). Cyanobacteria cultures were inoculated on gypsum rock samples to investigate and validate the water extraction and phase transformation mechanisms found in the geological environment. It is shown that cyanobacteria attach on high surface energy crystal planes and generate a thin biofilm on the gypsum samples. Due to the acidic nature of the biofilm, gypsum dissolution is observed, accompanied with water extraction and subsequent transformation to anhydrite via reprecipitation and oriented attachment of mesocrystals. This is the first time showing gypsum – anhydrite phase transformation and non-classical crystal growth induced by microorganisms. The results in the present work not only shed light on how these organisms survive in such extreme environments, but also can provide insight into potential life in xeric environments such as Mars as well as offer strategies for advanced water storage methods.

Metal nanocrystals have gained tremendous attention due to their superior performance in various applications, ranging from selective catalysis to electronic devices to plasmonic applications, such as photovoltaics and sensing. The properties of nanocrystals are highly sensitive to their size and shape. To this end, solution-phase synthetic protocols have been highly successful at producing a variety of nanocrystal structures. However, great challenges remain in achieving high selectivity to particular nanostructures. A significant difficulty lies in understanding and controlling shape evolution in these systems.

A common adage is that solution-phase additives, such as capping molecules, can direct the shapes of colloidal nanocrystals by adsorbing selectively to certain facets. Though our first-principles-based studies indicate that capping molecules alone can direct the formation of nanocrystal shapes in certain systems, experiments demonstrate that more robust syntheses can be achieved in the co-presence of halide additives. We use DFT-based ab initio thermodynamics calculations to probe the influence of chloride on Ag nanoshapes. Consistent with experiment, these calculations indicate that chloride adsorption alone can lead to truncated Ag cubes. We find that adsorbed chlorine works synergistically with PVP, a common capping agent for Ag, to enhance the tendency for Ag cubes. We also demonstrate that adsorbed Cl works synergistically with HDA capping molecules to facilitate the growth of Cu nanowires via a kinetic pathway - consistent with experiment. These studies indicate that halides and capping molecules can be a powerful combination to create selective nanoshapes in solution-phase syntheses.

In this study, electrospinning was successfully used to fabricate hybrid Carbon-Silicon Carbide (C-SiC) fibers using polyacrylonitrile (PAN) and Silicon (Si) nanoparticles as precursors. Silicon-to-carbon precursor ratios and heat treatment were optimized to fabricate hybrid nano-fibers with high oxidation resistance. After heat treatment, the nano-fibers showed two-dimensional ordered carbons and SiC nano-phases formation. As a result, a noticeable improvement in mechanical and thermal properties was achieved. Samples with 90wt% PAN/10wt% Si showed about four-time improvements in char yield as compared with 100 wt % PAN. In addition, after the heat treatment of the fiber at 1250 ^oC, SiC was formed. The carbon and SiC crystallinities had a great impact on the oxidation and mechanical property improvement of hybrid nanofibers. The determination of crystal size was carried out using X-ray results obtained at different temperatures and holding times. The SiC grain growth was predicted using kinetic models and its exponent was found to be around n = 4 with activation energy around 35 KJ/mol.K. For such growth, the dominant grain growth mechanism was grain boundary diffusion.

The unique properties of nanoporous aluminosilicates (zeolites) are exploited in a variety of applications spanning from ion exchange and separations to catalysis and drug delivery. The ability to selectively control zeolite synthesis to achieve desired physicochemical properties relies upon detailed understandings of the thermodynamic and kinetic factors regulating crystal nucleation and growth, which are generally lacking. Designing innovative approaches to tailor zeolite crystallization and elucidate structure-performance relationships has the potential to produce materials with superior properties beyond what is achievable by conventional routes. In this talk, I will discuss efforts to characterize the complex mechanisms of zeolite crystallization, which occur by nonclassical pathways^1-2 involving the self-assembly and structural evolution of amorphous precursors.^3-5 There is still much that we do not understood regarding the role of precursors in nucleation⁶ and the influence of growth conditions on the selection of crystal topology,⁷ which underscores the need for molecular-level studies of zeolite crystallization. Our group addresses these challenges using a broad range of techniques that include the use solvothermal atomic force microscopy to capture time-resolved images of growing zeolite surfaces in real time.⁸ This technique has led to the first in situ characterization of zeolite growth with the capability of resolving surface dynamics at the spatiotemporal scales necessary to elucidate mechanistic pathways of crystallization. Based on our findings, we observe that growth occurs via multiple (cooperative) pathways that differ from one material to the next. In this talk, we will summarize our findings for several zeolite structures.


References
1. De Yoreo, J. J.; Gilbert, P.; Sommerdijk, N.; Penn, R. L.; Whitelam, S.; Joester, D.; Zhang, H. Z.; Rimer, J. D.; Navrotsky, A.; Banfield, J. F.; Wallace, A. F.; Michel, F. M.; Meldrum, F. C.; Colfen, H.; Dove, P. M., Crystallization by particle attachment in synthetic, biogenic, and geologic environments. Science 2015, 349 (6247), 498-+.
2. Kumar, M.; Choudhary, M. K.; Rimer, J. D., Transient modes of zeolite surface growth from 3D gel-like islands to 2D single layers. Nat. Commun. 2018, 9, 2129 (1-9).
3. Kumar, M.; Li, R.; Rimer, J. D., Assembly and Evolution of Amorphous Precursors in Zeolite L Crystallization. Chem. Mat. 2016, 28 (6), 1714-1727.
4. Oleksiak, M. D.; Soltis, J. A.; Conato, M. T.; Penn, R. L.; Rimer, J. D., Nucleation of FAU and LTA Zeolites from Heterogeneous Aluminosilicate Precursors. Chem. Mat. 2016, 28 (14), 4906-4916.
5. Kumar, M.; Luo, H.; Roman-Leshkov, Y.; Rimer, J. D., SSZ-13 Crystallization by Particle Attachment and Deterministic Pathways to Crystal Size Control. J. Am. Chem. Soc. 2015, 137 (40), 13007-13017.
6. Rimer, J. D.; Tsapatsis, M., Nucleation of open framework materials: Navigating the voids. MRS Bull. 2016, 41 (5), 393-398.
7. Maldonado, M.; Oleksiak, M. D.; Chinta, S.; Rimer, J. D., Controlling Crystal Polymorphism in Organic-Free Synthesis of Na-Zeolites. J. Am. Chem. Soc. 2013, 135 (7), 2641-2652.
8. Lupulescu, A. I.; Rimer, J. D., In Situ Imaging of Silicalite-1 Surface Growth Reveals the Mechanism of Crystallization. Science 2014, 344 (6185), 729-732.

A scarcely investigated class of compounds, tetrel-pnictides of rare-earth and transition metals, R(M)-T-Pn (M = transition metal; T = Si, Ge; Pn = P, As), possess complex structural chemistry and potentially interesting properties. By combining rare-earth and transition metals, new quaternary tetrel-pnictide phases (R-M-T-Pn), both doping phases and new structural types, can be formed. The synthesis of these compounds is a challenge and it prompted us to look for new approach. Due to the volatility of phosphorus and arsenic, it is not possible to directly melt the reactants without strong depleting of sample with pnictogen due to evaporation. Target phases decompose peritectically below 1000^oC, below this temperature Si and Ge are inert. Multicomponent nature of the target phase prevents effective use of molten flux reactions due to different solubility and dissolution rates of constituent components, Pn>R>M>>T. Gas transport reactions also appeared to be ineffective for quaternary systems. As such, a synthesis using pre-reacted {R, M, and T} quasi- or true ternary compositions with phosphorus or arsenic becomes very appealing. Although R-M-T binaries and ternaries typically have melting points above 1000^oC, our in-situ X-ray diffraction studies show that they are reactive in the presence of a pnictogen-rich atmosphere at temperatures as low as 650^oC, thereby forming desired tetrel-pnictide compounds. Therefore, careful optimization of the starting reactant ratios can yield phase-pure single-phase compounds, where excess P and As can be sublimed away from the phase of interest. Furthermore, this method can generate single crystals for structural characterization due to the P and As acting as transport agents in addition to being reactants. The structural chemistry, polymorphism, and properties of the family of {R,T}-Si-Pn compounds will be discussed.

Carbon fibers have broad applications in aerospace, automobiles, wind turbines, medical devices, and implants, among many others. Carbon fibers with high-end applications have previously been manufactured from precursor fibers of rayon, pitch, and polyacrylonitrile (PAN) fibers. New precursor fibers have been conventionally fabricated by homogenously mixing the reinforcement/nanofiller and polymer matrix mechanically and spinning the dope into a coagulation bath, such as PAN processing in the case of dry-jet wet spinning followed by the drawing stage and heat treatment at different temperatures. Mechanical properties of this fiber are dependent on the continuity, density and interfacial action of the hierarchical graphitic structures. The presence of voids formed during crystallization due to the random dispersion of reinforcement/nanofiller in pre-carbon fibers (pCFs) deteriorates these properties.
In this project, a specialized templating graphene channel was developed via an in-house designed spinneret. This channel containing graphene and varied PAN solutions running along the axial direction of the fiber was parallel to the outer and inner PAN channels. On drawing the as-spun fibers the GNP was aligned in the axial direction. This channel then allowed for crystallization along the axial direction and minimizes the formation of voids leading to enhanced mechanical properties. The mechanical test showed the composite as-spun fiber with GNP templating channel has 18.5% and 51.4% increased Young’s modulus and tensile strength, respectively, as compared to the pure PAN fiber. The thermodynamics and kinetics of the heat-treatment procedures were analyzed to correlate the microstructure features and mechanical/thermal properties of obtained carbon fibers.

Although non-classical crystallization has been studied extensively in the solution growth, not much attention has been paid to the non-classical crystallization in the gas phase synthesis of thin films and nanostructures by chemical vapor deposition (CVD). Extensive studies were made in CVD processes and the non-classical crystallization was shown to be a general growth mechanism of thin films and nanostructures [1], where the charged nanoparticles become the building block. Generation of charged nanoparticles was confirmed in most CVD processes, including the growth of films and nanostructures of diamond, carbon nanotubes, ZrO₂, Si, ZnO and GaN. Small charged nanoparticles are favorable for epitaxial growth whereas large ones are favorable for nanostructure growth.
Understanding made from the non-classical crystallization behaviour in the CVD process can provide the valuable information as to the non-classical crystallization in solution. For example, building block of nanoparticles are electrically charged in the CVD process and the presence of charge plays a critical role in the growth of thin films and nanostructures. The charge is responsible for not only self-assembly but also liquid-like coalescence, which result in the evolution of dense films and nanostructures. The role of charge could be clearly revealed by comparing the deposition behavior of silicon charged nanoparticles between electrically floating and grounded silicon substrates. Single crystalline silicon nanowires grew on the floating substrate whereas nanoparticles grew on the grounded substrate. In other processing conditions, a dense silicon film grew on the floating substrate whereas a porous silicon film grew on the grounded substrate.
The fact that charged nanoparticles can be a building block of films and nanowires without leaving any void behind indicates that charged nanoparticles are quasi-solid, having a liquid-like property in diffusion. This means that the charge enhances the atomic diffusion, which is a newly discovered physical phenomenon. The charge-enhanced kinetics could be analyzed by comparing the behavior of Au nanoparticles on conducting and insulating membranes during transmission electron microscopy (TEM) observation. Au nanoparticles on insulating membranes were actively changing their orientation during TEM observation whereas those on conducting membranes were not. Also plasma dewetting behavior of sputtered Sn films was compared between grounded and floating conditions. The dewetting kinetics of Sn films in the floating condition was much higher than that in the grounded condition. Both experimental results support charge-enhanced kinetics.
In this presentation, some important aspects of the non-classical crystallization in the CVD process would be introduced, emphasizing that the non-classical crystallization occurs not only in solution but also in the gas phase synthesis.

[1] N.M. Hwang, Non-Classical Crystallization of Thin Films and Nanostructures in CVD and PVD Processes, vol. 60, Springer Series in Surface Sciences (Springer, 2016).

Two-dimensional (2D) materials have attracted significant attention due to their unique structure and fascinating properties. However, our understanding of their growth mechanisms in solution is still limited. Here, using indium chloride (InCl₃) as a model system, we performed liquid phase transmission electron microscopy (TEM) study of the 2D growth in situ. We combined the low temperature techniques with liquid phase TEM. The nucleation and growth of 2D nanosheets in solution via both classical and non-classical pathways were observed with the atomic resolution in real time at room temperature as well as low temperature (e.g. 1-2 °C). We found the drastically decreased growth kinetics at the low temperature compared to that at the room temperature. Observation also showed that the fast growth induced the high density of crystal defects (e.g., nanotwins). Comparatively, perfect 2D nanocrystals were obtained through the low temperature growth. In addition, we did the atomic structural analysis of the nanosheets and chemical mapping of the reaction front at the Cryogenic temperature using the same liquid cell sample. The results revealed the growth mechanisms of InCl₃ 2D nanosheets and the formation of nanotwins in solution. And, the liquid phase TEM with variable temperature control opens many opportunities for future depth study of nanoscale materials growth and transformations.

Synthesis design remains a paramount problem in materials science. Significant research has been dedicated to advancing tools to predict and aid in synthesis development. Here we describe an ab-initio approach to evaluate water-based synthesis processes such as hydrothermal synthesis. We show how to use electrochemical Pourbaix diagram, constructed from first principles density functional theory (DFT) calculations, to understand thermodynamic forces of complex Cu oxides and Cu oxychalcogenides in aqueous environments. We discuss how to construct these complex diagrams consisting of many degrees-of-freedom, including multiple elemental species, compounds, and complexes as well as nonstandard state conditions. We describe how the driving forces for the products evolve with changes in pH, oxidation state, and the activity of reactants; this understanding is then used to justify successful experimental synthesis of the targeted phase. Scale dependent nucleation is also examined. Last, we propose Pourbaix diagrams are a useful synthesis-processing design tool for accelerated discovery.

Common growth models assume a structure-less liquid composed of atomic flow units and describe crystal growth as a phenomenon of transferring atoms at the interface between a growing solid and a structure-less liquid. However, for liquid metals and polymers, structural ordering has been shown, raising the possibility of structure-coupled crystal growth. By performing in situ transmission electron microscopy (TEM) crystallization experiments on Pt-based metallic glass nanorods, we show that local structural order of the glass strongly affects crystal growth, in contrast to assumptions from conventional growth models [1].

Metallic glasses serve as a good model system to study crystallization due to their non-directional metallic bonds, relatively slow crystallization kinetics, and moderate crystallization temperatures, which readily allow direct visualization of crystallization events at the atomic scale via in situ TEM experiments. At the nanoscale, we have observed crystallization phenomena that deviate greatly from the macroscopic behaviors due to the geometrical confinement effects. These include suppression of crystallization for very small metallic glass nanorods [2], emergence of a metastable, single crystalline phase out of multi-component metals [3], and the structure-coupled crystallization [1]. The structure-coupled crystallization was inferred from the asymmetric growth rates of a heated metallic glass versus a cooled metallic liquid. We demonstrate that thermodynamically unstable structural ordering in the glass state can be kinetically stable enough to enhance crystal growth during crystallization. Thus our works highlight crystallization via non-classical pathways.


References:
[1] Y. Xie*, S. W. Sohn*, M. Wang, et. al., Nat. Commun. 10 (2019) 915.
[2] S. W. Sohn*, Y. Jung*, Y. Xie, et. al., Nat. Commun. 6 (2015) 8157.
[3] S. W. Sohn*, Y. Xie*, Y. Jung, et. al., Nat. Commun. 8 (2017) 1980.
(* indicates equal contributions)

We describe a theoretical and computational framework for adiabatic shear banding (ASB) and dynamic recrystallization (DRX) in polycrystalline materials. The work is based on the thermodynamic dislocation theory (TDT) by Langer et al., to which we add the grain boundary density as an internal state variable. TDT provides a convenient and thermodynamically consistent way to represent the viscoplastic and thermal behavior of the material, with minimal ad-hoc assumptions regarding the initiation of yielding or onset of shear banding. Within the TDT framework, dynamic recrystallization emerges as an entropic effect arising from the competition between dislocation creation and grain boundary formation, and contributes to material weakening and failure. We implement the TDT-DRX theory in conjunction with a finite-element computational framework. Favorable comparison to experimental measurements on a top-hat AISI 316L stainless steel sample compressed with a split-Hopkinson pressure bar suggests the accuracy and usefulness of the TDT-DRX framework, and demonstrates the crucial role of DRX in strain localization.

The color purity of indium phosphide (InP) quantum dots (QDs) lags behind that of cadmium selenide (CdSe) QDs, especially at their largest sizes and reddest wavelengths. In order to improve the quality of InP QDs, we have developed a new synthetic method that features a slow precursor injection rate in contrast to the rapid injection rates that have worked so well for CdSe QD syntheses and that have traditionally been applied to InP. The absorption features of the InP QD batches produced by our method are redder than those of batches of comparably-disperse InP QDs and narrower than those of batches of comparably-sized InP QDs that are produced by other methods. We prepared InP QDs with first excitonic absorption peaks that are redder than 630 nm and have half widths at half maximum less than 85 meV. After adding a zinc selenide shell, we achieved core/shell QDs that are redshifted slightly with respect to the core and have photoluminescence full widths at half maximum under 55 nm. Our method also allows us to synthesize even larger InP cores with defined first excitonic absorption peaks that are redder than 660 nm. We used MALDI mass spectrometry to characterize the size and size dispersity of the QDs and to investigate the presence of low molecular weight clusters and secondary populations. Lastly, we find that the concentration of QDs increases throughout the entire course of the reaction, and we propose a nucleation and growth model to explain our observations.

The self-assembly of colloidal nanoparticles into ordered superlattices provides significant expansions for the capability of functional materials design. As the new type of building blocks, the geometric shapes of nanocrystals, such as sphere, cube and octahedron, play critical roles in determining the structure of assembled supercrystal. In the nanocube system, the observed superlattices were dominated by the three phases of simple cubic (SC), rhombohedral (Rh), and face-centered cubic (FCC), but how they nucleate and grow and transform from one to another remain largely unknown. In this work, we applied highly monodispersive magnetite nanocubes with an average side length of 12.5 nm, and controlled various assembly environments, and revealed the formation and growth of three major phases (SC, acute Rh and FCC). Taking advantage of synchrotron-based SAXS techniques, we also explored the structural correlations between three superlattices phases. With additional simulations and computations, we further discussed the driving forces and constructed a reasonable superlattice phase diagram of nanocube-based assembly. Our results not only dramatically expand our understanding on the shape-dependent phase behaviors of nanocrystal, but also provide insights for design and fabrication of novel functional materials.

Because macroscopic clusters of elemental carbon at low (ambient) pressure sublimate to vapor near 4000K, liquid carbon is seldom considered to play a role in nature even though quenched carbon droplets have been reported in laboratory laser ablation studies [1,2]. However, a subset of meteoritic carbon particles from red giant atmospheres show a “graphene-core”/graphite-rim structure [3,4], likely following condensation of super-cooled carbon droplets from the vapor phase, which nucleated graphene sheets on randomly-oriented 5-member rings before solidification and subsequent coating with a graphite rim. Similar core-rim particles can be formed by slow cooling of carbon vapor in the lab [5].
It remains a challenge to study the properties of liquid carbon in a lab due to the difficulty of experimentation under extreme conditions. Therefore, we investigated nucleation and growth of carbon rings and graphene sheets using density functional theory (DFT). We assume low pressure, with an experimental 1.8 g/cc density estimate, and relax randomized positions of liquid-like 13/20-carbon atom clusters in a cubic supercell. Inter-atom distances characteristic of covalent vs. metallic interactions (with a gap in 1.7-2.0 Å range) allow us to identify covalent “bonds” with small separation. Local energy minima at T = 0K show a mix of sp² and sp, plus few sp³ coordination numbers as in the literature which reports that liquid carbon favors sp coordination at lower densities, sp³ coordination at higher densities, and a mix of sp and sp² coordination at intermediate densities [6-10]. Ring sizes vary from triangle to heptagon, but pentagons are more abundant than hexagons, also consistent with previous reports [9,10]. Further work remains to see if such ab initio studies can confirm that abundant pentagonal rings at this density, during slow cooling of the liquid, can nucleate the growth of faceted pentacones as suggested by electron phase-contrast imaging of the presolar cores [11].
In summary DFT relaxation of random configurations of 13/20-atom clusters in a supercell, along with literature reports of the same, suggest that: (a) covalent bonds can be identified by interatom spacings below 1.85Å, (b) sp² coordination is expected at that density, and (c) pentagonal rings are more abundant than hexagonal rings as possible nucleation seeds. The properties of unlayered graphene sheets in a frozen liquid matrix, e.g. as a diffusion barrier, should be interesting to explore further both experimentally and computationally.
References
[1] D. Kasuya et al., (2002) J. Phys. Chem B 106, 4947.
[2] W.A. De Heer et al., (2005) Science 307:5711, pp. 907–910.
[3] P. Fraundorf and M. Wackenhut (2002) Ap. J. Lett. 578(2):L153-156
[4] T. Kevin Croat et al., (2014) Elements 10, 441-446.
[5] P. Fraundorf et al., (2019) HAL-02238804.
[6] Christopher B. Cannella and Nir Goldman (2015), J.Phys.Chem 119, 29-31.
[7] J. R. Morris et al., (1995) Phys. Rev. B 52.
[8] Christine J. Wu et al., (2002), Phys.Rev.Lett 89,135701
[9] N. A. Marks et al.,(2002) Phys. Rev. B 65, 075411.
[10] Volker L. Deringer and Gábor Csányi (2017) Phys. Rev. B 95, 094203.
[11] Eric Mandell, "Electron Beam Characterization of Carbon Nanostructures" (2007) Ph.D. Dissertation at UM-St Louis/Rolla.

Core/shell fibers have many broad applications in the biomedical field, such as drug delivery and tissue engineering, and are also used in environmental-related applications like water treatment and CO₂ absorption. This research demonstrated an alternative technique of gel-spinning for fabricating core/shell structures other than the traditionally used melt spinning or electrospinning technique. These fibers were porous in the shell and had capabilities to load particles in the core. Polystyrene (PS) and polyethylene glycol (PEG) were used as an example for demonstrating the feasibility of the core/shell fiber fabrication. A two-phase liquid-state microstructure was first achieved via the co-axial spinneret with the exterior layer composed of PS pellets (PSp)/PEG/organic solvent (e.g., xylene or dimethylformamide) solutions and the interior layer containing dispersed PS spheres (PSs)/PEG/water solutions. Procedures of coagulation, heat-treatment, and phase-separation-based selective dissolution were used to generate the porous-shell/particle-core fibers. The pore sizes and density were controlled by the porogen (i.e., PEG) concentrations and examined using scanning electron microscopy (SEM). The fiber formation dynamics and microstructural features were studied via rheology tests and gelation measurements, tensile tests, thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC).

A diverse class of materials exhibit characteristics of mesocrystals: single crystals constituting distinct nano-sized domains that are atomically aligned. The formation of such structures is often attributed to crystallization through oriented attachment. However, many questions about the fundamental drivers and dynamic progression of this phenomenon remain. And the process become more complexed when coupled with phase transformation. Here we focus on the crystallization of hematite (hm, Fe₂O₃) mesocrystals from ferrihydrite (fh) nanoparticles. In the oxalate-free solution, the resulting hm crystals are well faceted rhombohedron, while in the presence of oxalate, hm forms nanoporous spindle-shaped mesocrystals. We applied in situ liquid phase TEM with heating temperatures of 80 °C to investigate the formation. We directly observed both the dissolution of fh and the nucleation of new hm particles, which 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 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 quasi in situ approach of using indexed TEM grids to cycle samples between the growth reactor and the TEM in order to track the pathway of crystallization. The results were consistent with those of the in situ experiments and confirmed that the fh serves as a buffer and the Hm grow by creation of new particles in the solution near the hm interface. Based on ATR FTIR measurements, ion chromatography and classic DFT calculation 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 modifying the interfacial energy of hm close to the hm/solution interface. In this way, the oxalate ligands bias the growth process away from classical mechanisms and towards oriented attachment.

We use liquid-phase transmission electron microscopy to directly image a series of crystallization pathways of colloidal nanoparticles into superlattices. We find that there exist similarities to the prevalent model system of micron-sized colloids, such as a non-classical two-step crystallization pathway, where amorphous intermediates in the form of nanoparticle clusters exist preceding the nucleation. Capillary wave theory can be used to describe the fluctuation and energy of the surfaces. But there are also differences, in particular, a universal layer-by-layer growth mode that we observe consistently for diverse nanoparticle shapes. Single particle tracking, trajectory analysis, and simulations combined unravel the energetic and kinetic features rendering this crystal growth mode possible and universal at the unexplored nanoscale, enabling advanced crystal engineering.

Crystallization from solution follows either a single or a multistep pathway. The latter case involves the formation, aggregation and transformation of precursor/intermediate particles to the final crystalline phase. This has been observed for several important minerals, such as CaCO₃, CaPO₄, CaSO₄ or CSH [e.g. 1-3], but also for macromolecular crystals [e.g. 4,5]. Moreover, this particle-mediated crystallization pathway can be “fossilized” in the structure of the final crystals [e.g. 6].
In the idealized laboratory environment pure solutions are often studied, but in real-world natural and engineered crystallization processes soluble additives, e.g. inorganic or organic molecules, are omnipresent. In fact, they frequently play a key role in the formation of crystalline phases and are a central part of biomineralization, functional material synthesis and anti-scaling strategies [e.g. 7]. Traditionally, models of additive-controlled crystallization have considered a single step nucleation with additives adsorbing on the surface of nuclei, impeding their further growth. Hence, to advance our understanding and gain a better control of multistep crystallization processes, the interaction of additives with the precursors and/or intermediate particles needs to be unraveled.
To achieve this, we constructed a holistic nanoscopic view of a particle-mediated crystallization pathway in an additive free system [e.g. 5-8], based on in situ nanoscale observations. Subsequently, we conducted macroscopic and microscopic observations of additive-controlled crystal formation. Opposing the classical view, we found that the inhibition of nucleation can be ascribed to additives that hinder the formation of precursor species and/or their aggregation/transformation by prevailing in their vicinity and thereby reducing their interaction probability [e.g. 7, 10-11].

[1] De Yoreo et al., Science 349, 489, 2015. [2] Van Driessche et al., New perspectives on mineral nucleation and growth. Springer-Verlag, 2017. [3] Krautwurst et al., Chem. Mater. 30, 9, 2018. [4] Van Driessche et al., Nature 556, 89-94, 2018. [5] Sleutel and Van Driessche Nanoscale 10, 12256-12267, 2018. [6] Stawski et al., Cryst. Growth Des. 19, 3714-3721, 2019. [7] Van Driessche et al., Chem. Geol. 119274, 2019. [8] Stawski et al., Nat. Commun. 7, 11177, 2016. [9] Stawski et al., J. Phys. Chem. C. 123, 23151-23158, 2019. [10] Nicoleau et al., Cement Concrete Res. 105837, 2019. [11] Ossorio et al., Minerals 7, 140, 2017.

There is an increasing need for the development of multifunctional lightweight materials with high strength and toughness. Natural systems have evolved efficient strategies, exemplified in the biological tissues of numerous animal and plant species, to synthesize and construct composites from a limited selection of available starting materials that often exhibit exceptional mechanical properties that are similar, and frequently superior to, mechanical properties exhibited by many engineering materials. These biological systems have accomplished this feat by establishing controlled synthesis and hierarchical assembly of nano- to micro-scaled building blocks. This controlled synthesis and assembly require organic that is used to transport mineral precursors to organic scaffolds, which not only precisely guide the formation and phase development of minerals, but also significantly improve the mechanical performance of otherwise brittle materials.

Here, we investigate an organism that have taken advantage of hundreds of millions of years of evolutionary changes to derive structures, which are not only strong and tough, but also demonstrate abrasion resistance. All of this is controlled by the underlying organic-inorganic components. Specifically, we discuss the formation of heavily crystallized radular teeth the chitons, a group of elongated mollusks that graze on hard substrates for algae.

Our investigation of the formation of a fully mature radular tooth from Cryptochiton stelleri found in the eastern Pacific, occurs over a series of more than 40 teeth. The initial stage of tooth formation begins with the synthesis of a three-dimensional fibrous α-chitin organic matrix, prior to the onset of crystallization. Microscope analysis of early stage teeth shows ferrihydrite mineral particles growing on α-chitin fibers. Synchrotron x-ray, combined with TEM analyses show that ferrihydrite exists as randomly oriented aggregates, but undergoes a phase transformation to magnetite within a few rows of teeth. We discuss potential mechanisms of nucleation of ferrihydrite, its transformation to magnetite as well as its subsequent mesoscale ordering, crystal growth and the resulting mechanical properties. From the investigation of synthesis-structure-property relationships in these unique organisms, we are now developing and fabricating multifunctional engineering materials for energy and water purification based applications.

We utilize nanoparticle superlattice as a model system to study the fundamental laws of order emergence and evolution from a dispersion of nanosized entities. Liquid-phase TEM makes it possible to directly conduct single-particle level imaging in real time and real space, to reveal pathways of nanoparticle crystallization. To be specific, we observe how different types of diffusion events (e.g., surface and volume diffusion) involved in the layer-by-layer growth of a nanoparticle superlattice, where the high spatiotemporal resolution enables us to identify single particles throughout the whole process and to map out their trajectories sampling the energy landscape. Furthermore, oriented attachment of different nanoparticle lattice domains is observed, where fast translational and rotational diffusions of the domains drive the neck formation. The single-particle analysis aids correlating order evolution to differently ranged interactions. Such crystallization behavior is distinct from that of micron-sized colloids, resembling more the faceted crystals formed from atoms. Our study provides new dynamic information on the superlattice development at high spatial and temporal resolution, which can serve as a general guideline for bottom-up material design.

Nucleation and growth are universally important in systems from the atomic to the micrometer scale as they dictate structural and functional attributes of crystals. However, at the nanoscale, the pathways toward crystallization have been largely unexplored owing to the challenge of resolving the motion of individual building blocks in a liquid medium. Here we address this gap by directly imaging the full transition of dispersed nanoparticles to a superlattice at the single-particle level. We utilize liquid-phase transmission electron microscopy at low dose rates to control nanoparticle interactions without affecting their motions. Combining particle tracking with Monte Carlo simulations, we reveal that positional ordering of the superlattice emerges from orientational disorder. Otherwise elusive parameters including line tension and phase coordinates are measured, charting the nonclassical nucleation pathway involving a dense, amorphous intermediate. We elucidate the origin of this two-step crystallization by experimentally measuring the energy barrier for nucleation, which is lower in the presence of the amorphous precursor. We demonstrate the versatility of our approach via crystallization of different nanoparticles, pointing the way to more general applications. Our real space nanoscopic imaging platform not only offers new understanding on the crystallization kinetics, but also opens opportunities to other nanoscale fluctuating systems, from biofouling, lubrication, melting, phonon transport, to dendrite formation.

Biosynthesis of NPs is a striking phenomenon in nature. Inspiring examples include storage and transport of iron atoms in a non-toxic form of a ferric oxyhydroxide cluster in the ferritin protein cage, using metal salts as terminal sinks in electron transfer pathways and formation of metal particles by bacteria, and biomineralization of magnetite nanocrystals in specific organelle magnetosome that enables coordinated movement of magnetotactic bacteria. Under certain experimental conditions, metal complexes and nanoclusters can be obtained in mammalian cells or in a whole animal, mainly for use in imaging. However, the mechanisms of this biosynthesis are often spontaneous, poorly understood and, therefore, difficult to control. In our approach, a pathological process, namely hypoxia of the brain accompanied by changes in the NADH/NAD+ co-enzymes ratio, initiates and controls the biosynthesis of biocompatible Au nanoparticles from precursor salts in the immediate vicinity of the hypoxia site, thereby restoring the redox status of the brain.

Nanoscale DOI: 10.1039/c9nr05794c (2019)

Crystallization induced microstructure plays a central role in tailoring materials
properties, yet our understanding of its formation process is limited due to its complexity.
Classical crystallization theories assume that nucleation and growth are separate processes and
growth ought to be temperature-dependent only. Local thermodynamically unstable clusters are
considered to fluctuate in the nucleation stage but do not participate in the later growth. Here, we
apply in-situ transmission electron microscope (TEM) to nanostructured metallic glasses (MGs)
and directly probe crystallization kinetics at the unprecedented time and spatial resolution^{1, 2,
3,4}. In particular, the growth kinetics of single crystalline Pt_57.5Cu_14.6Ni_5.3P_22.5 MG nanorods
were studied in situ by observing isothermal crystallization at the atomic level. We found that
the crystal growth rates of heated metallic glass are much higher than those of cooled metallic
liquid despite the same isothermal crystallization temperature. The observation is in direct
contrast to one would expect from classical growth models. We, therefore, propose a
supercluster-coupled growth model and hypothesize that structural ordering can strongly
enhance the crystal growth. The asymmetric growth upon heating is thus attributed to the higher
degree of structural ordering and a greater number of clusters in the MGs nanorods. The
structure-coupled growth is further supported by manipulating the density of the small clusters
through nanoscale confinement and thermal treatment of the MG nanorods. In addition,
molecular dynamics simulations of binary Lennard-Jones glasses quenched at different cooling
rates show a growth rate trend that agrees with experimental observations. The present results
demonstrate the inadequacy of the classical crystallization model on modern complex materials
systems.

Reference
1. Xie, Yujun, et al. "Supercluster-coupled crystal growth in metallic glass forming liquids."
Nature communications 10 (2019).
2. Xie, Yujun, et al. "Direct observation through in situ transmission electron microscope of
early states of crystallization in nanoscale metallic glasses." JOM 69.11 (2017): 2187-2191.
3. Sohn, Sungwoo*, Xie, Yujun*, et al. "Tailoring crystallization phases in metallic glass
nanorods via nucleus starvation." Nature communications 8.1 (2017): 1980. (*co-first author)
4. Sohn, Sungwoo, Xie, Yujun, et al. "Nanoscale size effects in crystallization of metallic glass
nanorods." Nature communications 6 (2015): 8157.

Crystallization by assembly of primary nanocrystals is a widespread phenomenon in synthetic and natural environments. In mineral systems, this process exhibits diverse styles ranging from oriented attachment of like phases to mis-oriented aggregation of disparate phases followed by coarsening to single crystals. While descriptions of 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, inherent diopolar interactions, structured nanoscale interfaces, and solvent-responses of a length scale comparable to particle size. To understand the relationship between nanocrystal structure, interparticle forces and assembly dynamics, we are investigating colloidal assembly of titanium, iron and zinc oxides. For ZnO, in situ TEM demonstrates that direction-specific forces align diffusing particles at distances far in excess of what is expected based on DLVO theory. Particles approach one another in a diffusion limited regime and then attach lattice matched via a sudden jump to contact unopposed by any significant energy barrier. Direct AFM-based measurements of the forces between both TiO₂ and ZnO surfaces reveal orientation dependent attractive interactions exhibiting the symmetry of the underlying lattice. MD simulations for both systems predict that solvation forces are responsible for the orientation dependence, creating solvent separated minima and barriers to contact. However, classical density functional theory applied to ZnO predicts that ion correlation forces lead to generally attractive interactions with little or no barrier, enabling random attachment, which is not observed. The long range nature of the forces and torques in the ZnO system are shown to be a consequence of the inherent dipole moments of ZnO particles and it is these dipole-dipole interactions that enforce alignment even in the absence of solvation-defined orientation dependent forces. Addition of organic ligands, like oxalate, to the FeOOH system produces a dramatically different pathway and outcome: Nanoparticles of Fe₂O₃ nucleate sporadically from the FeOOH and, once formed, are covered with oxalate, which drives all new Fe₂O₃ particles to nucleate within about 1 nm of the interface, 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 FeOOH system is replaced with one that is deterministic.

The progress of energy and electronic industry relies on the design and engineering of new nanostructures with fine control in the atomic scale. Examples include the use of twisted two-dimensional materials for the tuning of electronic structure, the surface engineering to create surface steps in metal nanowires that show enhanced catalytic activity. Manipulation of the nanostructures with more precise control requires the understanding of how atoms interplay with various stimuli, which must be characterized by advanced imaging tools that can not only reveal the materials behavior during growth and use, but also provides insights on the change of composition and bonding. Recent development of aberration-corrected transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM) allows the combination of low-dose imaging with in situ methods to study the materials dynamics at high spatial resolution, when coupled with high-speed direct electron detectors, the electric potential and field that can influence the materials interaction can also be imaged. Here we first demonstrate the atomic resolution imaging of a Pt precursor compound and the dynamics of its transformation to Pt nanoclusters.[1] Specifically, under an ultra-low electron dose, the atomic structure of pristine K₂PtCl₄ crystallite is resolved in the image captured by the CMOS-based direct electron detector. With further electron beam irradiation, the dynamics of staged reduction of Pt compound and nucleation of Pt cluster are captured and identified. Results show that the ionic bond between K⁺ and PtCl₄^2- is first broken, which is followed by PtCl₄^2- forming PtCl₂; reduction of the dual-valent Pt then leads to the formation of Pt clusters. The solid-state transition is deciphered in real time and real space. Later, using a gas cell, we show that the sintering process of Pt clusters can be modified by gas adsorbates:[2] in H₂, the Pt nuclei approach and attach to each other only along specific orientations, and gradually form short nanowires. In ex situ experiment, we successfully synthesized extended long Pt and PtNi nanowires in solid state assisted by H₂. Compared to the Pt-based NWs grown by other methods, our catalysts showed both enhanced mass specific activity and stability in ORR. We will also show our four-dimensional STEM study at the interface of complex oxides that reveals the penetration of electric field from the ferroelectric BiFeO₃ into the insulating SrTiO₃,[3] and discuss how advanced electron microscopy can further our study of nanostructures.

[1] Wenpei Gao, Peter Tieu, Christopher Addiego, Yanling Ma, Jianbo Wu, Xiaoqing Pan, Science Advances, 5(1), eaau9590, 2019
[2] Yanling Ma, Wenpei Gao, Hao Shan, Wenlong Chen, Wen Shang, Peng Tao, Chengyi Song, Chris Addiego, Tao Deng, Xiaoqing Pan, Jianbo Wu, Advanced Materials, 1703460, 2017
[3] Wenpei Gao, Christopher Addiego, Hui Wang, Xingxu Yan, Yusheng Hou, Dianxiang Ji, Colin Heikes, Yi Zhang, Linze Li, Huaixun Huyan, Thomas Blum, Toshihiro Aoki, Yuefeng Nie, Darrell Schlom, Ruqian Wu, Xiaoqing Pan, Nature, in press, 2019

A novel method for the solidification of metallic alloys involving spiral self-organization is presented as a new strategy for producing large-area chiral patterns with emergent structural and optical properties, with attention to the underlying mechanism and dynamics. This study reports the discovery of a new growth mode for metastable, two-phase spiral patterns from a liquid metal. Crystallization proceeds via a non-classical, two-step pathway consisting of the initial formation of a polytetrahedral seed crystal, followed by ordering of two solid phases that nucleate heterogeneously on the seed and grow in a strongly coupled fashion. Crystallographic defects within the seed provide a template for spiral self-organization. These observations demonstrate the ubiquity of defect-mediated growth in multi-phase materials and establish a pathway towards bottom-up synthesis of chiral materials with an inter-phase spacing comparable to the wavelength of infrared light. Given that liquids often possess polytetrahedral short-range order, our results are applicable to many systems undergoing multi-step crystallization.

The self-assembly of quantum dots (QDs) into epitaxially connected superlattices called quantum dot solids (QDSs), made possible by a growing library of synthesis methods for precisely controllable QD building blocks, represents exciting opportunities for advancing optoelectronic devices and realizing exotic electronic phenomena. However, the myriad complex interactions at play during the assembly and oriented attachment processes lead to imperfections in epitaxial connections between QDs. Such imperfections affect electronic coupling between QDs and lead to local translational disorder, which propagates through the film and hinders tunable miniband formation. Heating QDSs is a promising method for annealing out defects, thus improving electronic properties. Understanding the nuanced effects on epitaxial connections, however, requires visualization of atomic-scale structure over large enough fields of view to access a statistically meaningful number of QDs.

Here, we use in situ MEMS heating in combination with aberration-corrected HAADF-STEM and image analysis to assess the effects of heating on epitaxial connections in QDSs over extended fields of view. We analyze PbS QDs self-assembled into square SLs and epitaxially connected along the <100> directions. Localized heating provided by the MEMS device maintains stage stability such that a region of interest can be tracked throughout a heating cycle and images can be acquired at atomic-resolution over ~100 nm. We identify four distinct strain states in epitaxial connections: tension, shear, bending and torsion, noting that they appear concurrently in some cases. Using a local sine wave fitting technique similar to the phase lock-in method, we visualize the tension, shear and bending strain states. Given the limitation of STEM to projection imaging, torsion is investigated indirectly by identifying the out-of-plane orientation of each QD, nearest neighbor mismatches in which indicate torsion and out-of-plane bending strain states between connected QDs. The orientation data, in combination with wave fitting analysis, reveal that mild annealing between 150 and 200°C causes dramatic relaxation of tensile, shear and torsional strains, while bending strain is more persistent. These results give insight into the energetics of the types of defects present in epitaxial connections and represent a path toward improved carrier delocalization and miniband formation.

In recent years, significant achievements have been made in the synthesis of various colloidal nanostructures. The anisotropic nanostructures, such as nanowires or two-dimensional (2D) materials have attracted a lot of attention due to their unique properties. However, their formation mechanisms have not been fully understood. They seem to be contradictory to the predictions based on the conventional crystal growth theories. And, it adds to the complexity that nanocrystals often change structure and morphology during growth. We explore the growth mechanisms of anisotropic nanocrystals using in situ liquid cell transmission electron microscopy (TEM). In this talk, I will show the growth of 2D transition metal oxide nanosheets, e.g. cobalt oxide and cobalt nickel oxide. The 2D nanosheets have been achieved in a liquid cell, which resemble those synthesized in flasks. Our direct observations reveal that 3D nanoparticles are initially formed from the molecular precursor solution and then transform into 2D nanosheets. Ab initio calculations show that a small nanocrystal is dominated by positive edge energy, but when it grows to a certain size, the negative surface energy becomes dominant, driving the transformation of the 3D nanocrystal into a 2D structure. Uncovering these growth pathways, including the 3D-to-2D transition, provides opportunities for the future design and solution synthesis of novel materials.

References:
J. Yang, Z. Zeng, J. Kang, C. Czarnik, X. Zhang, C. Ophus, C. Yu, K. Bustillo, M. Pan, J. Qiu, L. W. Wang, H. Zheng, “Formation of two-dimensional transition metal oxide nanosheets with nanoparticles as intermediates.” Nature Materials 18, 970-976 (2019).
H. Zheng et al., “In situ electron microscopy imaging of nanocrystal growth and transformations”, Trends in Chemistry invited review (2020).

Acknowledgements:
The project was funded by U.S. 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 in situ TEM program (KC22ZH). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Microorganisms have been found living in one of the driest and oldest deserts on the earth, the Atacama Desert in Chile. In this hyper-arid environment, microbial communities colonize the interior of rocks and obtaining water is critical for their survival. Gypsum (CaSO₄●2H₂O) is one of the main components in rocks from the Atacama Desert and is a main source of water for these communities, which are mainly constituted of cyanobacteria and heterotrophic bacteria. We report that these microorganisms extract crystalline water from the gypsum rock, inducing a phase transformation from gypsum (CaSO₄●2H₂O) to anhydrite (CaSO₄). Cyanobacteria cultures were inoculated on gypsum rock samples to investigate and validate the water extraction and phase transformation mechanisms found in the geological environment. It is shown that cyanobacteria attach on high surface energy crystal planes and generate a thin biofilm on the gypsum samples. Due to the acidic nature of the biofilm, gypsum dissolution is observed, accompanied with water extraction and subsequent transformation to anhydrite via reprecipitation and oriented attachment of mesocrystals. This is the first time showing gypsum – anhydrite phase transformation and non-classical crystal growth induced by microorganisms. The results in the present work not only shed light on how these organisms survive in such extreme environments, but also can provide insight into potential life in xeric environments such as Mars as well as offer strategies for advanced water storage methods.

The early observation of strong circular dichroism for individual nanoparticles and their assemblies have developed into a rapidly expanding field of chiral inorganic nanostructures. The chiral inorganic nanostructures encompass sophisticated constructs from metals, semiconductors, ceramics, and nanocarbons with multiple chiral geometries with characteristic scales from Ångströms to microns. Such versatility enables their functional engineering over a broad range of physical and chemical properties but the relationship between the chirality of surface ligands, NP cores, and resulting assemblies remains obscure. Similarly, the relationship between their chiral geometries and their chiroptical properties. This talk will address (1) the origin of the uniquely high values of optical anisotropy; (2) the mechanisms of chirality transfer in inorganic materials; and (3) differences/similarities with chiral supramolecular, liquid crystal, and biological assemblies. Long-range correlations and critical phenomena in chiral nanoassemblies leading to the emergence of geometrically complex structures will be demonstrated. The novel venues for practical realizations of chiral nanoassemblies included in the talk will be photocatalytic C-C coupling in biomimetic chiral supraparticles and polarization spectroscopy in terahertz (THz) spectral window with chiroplasmonic kirigami composites.

Common growth models assume a structure-less liquid composed of atomic flow units and describe crystal growth as a phenomenon of transferring atoms at the interface between a growing solid and a structure-less liquid. However, for liquid metals and polymers, structural ordering has been shown, raising the possibility of structure-coupled crystal growth. By performing in situ transmission electron microscopy (TEM) crystallization experiments on Pt-based metallic glass nanorods, we show that local structural order of the glass strongly affects crystal growth, in contrast to assumptions from conventional growth models [1].

Metallic glasses serve as a good model system to study crystallization due to their non-directional metallic bonds, relatively slow crystallization kinetics, and moderate crystallization temperatures, which readily allow direct visualization of crystallization events at the atomic scale via in situ TEM experiments. At the nanoscale, we have observed crystallization phenomena that deviate greatly from the macroscopic behaviors due to the geometrical confinement effects. These include suppression of crystallization for very small metallic glass nanorods [2], emergence of a metastable, single crystalline phase out of multi-component metals [3], and the structure-coupled crystallization [1]. The structure-coupled crystallization was inferred from the asymmetric growth rates of a heated metallic glass versus a cooled metallic liquid. We demonstrate that thermodynamically unstable structural ordering in the glass state can be kinetically stable enough to enhance crystal growth during crystallization. Thus our works highlight crystallization via non-classical pathways.


References:
[1] Y. Xie*, S. W. Sohn*, M. Wang, et. al., Nat. Commun. 10 (2019) 915.
[2] S. W. Sohn*, Y. Jung*, Y. Xie, et. al., Nat. Commun. 6 (2015) 8157.
[3] S. W. Sohn*, Y. Xie*, Y. Jung, et. al., Nat. Commun. 8 (2017) 1980.
(* indicates equal contributions)

The unique properties of nanoporous aluminosilicates (zeolites) are exploited in a variety of applications spanning from ion exchange and separations to catalysis and drug delivery. The ability to selectively control zeolite synthesis to achieve desired physicochemical properties relies upon detailed understandings of the thermodynamic and kinetic factors regulating crystal nucleation and growth, which are generally lacking. Designing innovative approaches to tailor zeolite crystallization and elucidate structure-performance relationships has the potential to produce materials with superior properties beyond what is achievable by conventional routes. In this talk, I will discuss efforts to characterize the complex mechanisms of zeolite crystallization, which occur by nonclassical pathways^1-2 involving the self-assembly and structural evolution of amorphous precursors.^3-5 There is still much that we do not understood regarding the role of precursors in nucleation⁶ and the influence of growth conditions on the selection of crystal topology,⁷ which underscores the need for molecular-level studies of zeolite crystallization. Our group addresses these challenges using a broad range of techniques that include the use solvothermal atomic force microscopy to capture time-resolved images of growing zeolite surfaces in real time.⁸ This technique has led to the first in situ characterization of zeolite growth with the capability of resolving surface dynamics at the spatiotemporal scales necessary to elucidate mechanistic pathways of crystallization. Based on our findings, we observe that growth occurs via multiple (cooperative) pathways that differ from one material to the next. In this talk, we will summarize our findings for several zeolite structures.


References
1. De Yoreo, J. J.; Gilbert, P.; Sommerdijk, N.; Penn, R. L.; Whitelam, S.; Joester, D.; Zhang, H. Z.; Rimer, J. D.; Navrotsky, A.; Banfield, J. F.; Wallace, A. F.; Michel, F. M.; Meldrum, F. C.; Colfen, H.; Dove, P. M., Crystallization by particle attachment in synthetic, biogenic, and geologic environments. Science 2015, 349 (6247), 498-+.
2. Kumar, M.; Choudhary, M. K.; Rimer, J. D., Transient modes of zeolite surface growth from 3D gel-like islands to 2D single layers. Nat. Commun. 2018, 9, 2129 (1-9).
3. Kumar, M.; Li, R.; Rimer, J. D., Assembly and Evolution of Amorphous Precursors in Zeolite L Crystallization. Chem. Mat. 2016, 28 (6), 1714-1727.
4. Oleksiak, M. D.; Soltis, J. A.; Conato, M. T.; Penn, R. L.; Rimer, J. D., Nucleation of FAU and LTA Zeolites from Heterogeneous Aluminosilicate Precursors. Chem. Mat. 2016, 28 (14), 4906-4916.
5. Kumar, M.; Luo, H.; Roman-Leshkov, Y.; Rimer, J. D., SSZ-13 Crystallization by Particle Attachment and Deterministic Pathways to Crystal Size Control. J. Am. Chem. Soc. 2015, 137 (40), 13007-13017.
6. Rimer, J. D.; Tsapatsis, M., Nucleation of open framework materials: Navigating the voids. MRS Bull. 2016, 41 (5), 393-398.
7. Maldonado, M.; Oleksiak, M. D.; Chinta, S.; Rimer, J. D., Controlling Crystal Polymorphism in Organic-Free Synthesis of Na-Zeolites. J. Am. Chem. Soc. 2013, 135 (7), 2641-2652.
8. Lupulescu, A. I.; Rimer, J. D., In Situ Imaging of Silicalite-1 Surface Growth Reveals the Mechanism of Crystallization. Science 2014, 344 (6185), 729-732.

The color purity of indium phosphide (InP) quantum dots (QDs) lags behind that of cadmium selenide (CdSe) QDs, especially at their largest sizes and reddest wavelengths. In order to improve the quality of InP QDs, we have developed a new synthetic method that features a slow precursor injection rate in contrast to the rapid injection rates that have worked so well for CdSe QD syntheses and that have traditionally been applied to InP. The absorption features of the InP QD batches produced by our method are redder than those of batches of comparably-disperse InP QDs and narrower than those of batches of comparably-sized InP QDs that are produced by other methods. We prepared InP QDs with first excitonic absorption peaks that are redder than 630 nm and have half widths at half maximum less than 85 meV. After adding a zinc selenide shell, we achieved core/shell QDs that are redshifted slightly with respect to the core and have photoluminescence full widths at half maximum under 55 nm. Our method also allows us to synthesize even larger InP cores with defined first excitonic absorption peaks that are redder than 660 nm. We used MALDI mass spectrometry to characterize the size and size dispersity of the QDs and to investigate the presence of low molecular weight clusters and secondary populations. Lastly, we find that the concentration of QDs increases throughout the entire course of the reaction, and we propose a nucleation and growth model to explain our observations.

A scarcely investigated class of compounds, tetrel-pnictides of rare-earth and transition metals, R(M)-T-Pn (M = transition metal; T = Si, Ge; Pn = P, As), possess complex structural chemistry and potentially interesting properties. By combining rare-earth and transition metals, new quaternary tetrel-pnictide phases (R-M-T-Pn), both doping phases and new structural types, can be formed. The synthesis of these compounds is a challenge and it prompted us to look for new approach. Due to the volatility of phosphorus and arsenic, it is not possible to directly melt the reactants without strong depleting of sample with pnictogen due to evaporation. Target phases decompose peritectically below 1000^oC, below this temperature Si and Ge are inert. Multicomponent nature of the target phase prevents effective use of molten flux reactions due to different solubility and dissolution rates of constituent components, Pn>R>M>>T. Gas transport reactions also appeared to be ineffective for quaternary systems. As such, a synthesis using pre-reacted {R, M, and T} quasi- or true ternary compositions with phosphorus or arsenic becomes very appealing. Although R-M-T binaries and ternaries typically have melting points above 1000^oC, our in-situ X-ray diffraction studies show that they are reactive in the presence of a pnictogen-rich atmosphere at temperatures as low as 650^oC, thereby forming desired tetrel-pnictide compounds. Therefore, careful optimization of the starting reactant ratios can yield phase-pure single-phase compounds, where excess P and As can be sublimed away from the phase of interest. Furthermore, this method can generate single crystals for structural characterization due to the P and As acting as transport agents in addition to being reactants. The structural chemistry, polymorphism, and properties of the family of {R,T}-Si-Pn compounds will be discussed.

Although non-classical crystallization has been studied extensively in the solution growth, not much attention has been paid to the non-classical crystallization in the gas phase synthesis of thin films and nanostructures by chemical vapor deposition (CVD). Extensive studies were made in CVD processes and the non-classical crystallization was shown to be a general growth mechanism of thin films and nanostructures [1], where the charged nanoparticles become the building block. Generation of charged nanoparticles was confirmed in most CVD processes, including the growth of films and nanostructures of diamond, carbon nanotubes, ZrO₂, Si, ZnO and GaN. Small charged nanoparticles are favorable for epitaxial growth whereas large ones are favorable for nanostructure growth.
Understanding made from the non-classical crystallization behaviour in the CVD process can provide the valuable information as to the non-classical crystallization in solution. For example, building block of nanoparticles are electrically charged in the CVD process and the presence of charge plays a critical role in the growth of thin films and nanostructures. The charge is responsible for not only self-assembly but also liquid-like coalescence, which result in the evolution of dense films and nanostructures. The role of charge could be clearly revealed by comparing the deposition behavior of silicon charged nanoparticles between electrically floating and grounded silicon substrates. Single crystalline silicon nanowires grew on the floating substrate whereas nanoparticles grew on the grounded substrate. In other processing conditions, a dense silicon film grew on the floating substrate whereas a porous silicon film grew on the grounded substrate.
The fact that charged nanoparticles can be a building block of films and nanowires without leaving any void behind indicates that charged nanoparticles are quasi-solid, having a liquid-like property in diffusion. This means that the charge enhances the atomic diffusion, which is a newly discovered physical phenomenon. The charge-enhanced kinetics could be analyzed by comparing the behavior of Au nanoparticles on conducting and insulating membranes during transmission electron microscopy (TEM) observation. Au nanoparticles on insulating membranes were actively changing their orientation during TEM observation whereas those on conducting membranes were not. Also plasma dewetting behavior of sputtered Sn films was compared between grounded and floating conditions. The dewetting kinetics of Sn films in the floating condition was much higher than that in the grounded condition. Both experimental results support charge-enhanced kinetics.
In this presentation, some important aspects of the non-classical crystallization in the CVD process would be introduced, emphasizing that the non-classical crystallization occurs not only in solution but also in the gas phase synthesis.

[1] N.M. Hwang, Non-Classical Crystallization of Thin Films and Nanostructures in CVD and PVD Processes, vol. 60, Springer Series in Surface Sciences (Springer, 2016).

Metal nanocrystals have gained tremendous attention due to their superior performance in various applications, ranging from selective catalysis to electronic devices to plasmonic applications, such as photovoltaics and sensing. The properties of nanocrystals are highly sensitive to their size and shape. To this end, solution-phase synthetic protocols have been highly successful at producing a variety of nanocrystal structures. However, great challenges remain in achieving high selectivity to particular nanostructures. A significant difficulty lies in understanding and controlling shape evolution in these systems.

A common adage is that solution-phase additives, such as capping molecules, can direct the shapes of colloidal nanocrystals by adsorbing selectively to certain facets. Though our first-principles-based studies indicate that capping molecules alone can direct the formation of nanocrystal shapes in certain systems, experiments demonstrate that more robust syntheses can be achieved in the co-presence of halide additives. We use DFT-based ab initio thermodynamics calculations to probe the influence of chloride on Ag nanoshapes. Consistent with experiment, these calculations indicate that chloride adsorption alone can lead to truncated Ag cubes. We find that adsorbed chlorine works synergistically with PVP, a common capping agent for Ag, to enhance the tendency for Ag cubes. We also demonstrate that adsorbed Cl works synergistically with HDA capping molecules to facilitate the growth of Cu nanowires via a kinetic pathway - consistent with experiment. These studies indicate that halides and capping molecules can be a powerful combination to create selective nanoshapes in solution-phase syntheses.

In this study, electrospinning was successfully used to fabricate hybrid Carbon-Silicon Carbide (C-SiC) fibers using polyacrylonitrile (PAN) and Silicon (Si) nanoparticles as precursors. Silicon-to-carbon precursor ratios and heat treatment were optimized to fabricate hybrid nano-fibers with high oxidation resistance. After heat treatment, the nano-fibers showed two-dimensional ordered carbons and SiC nano-phases formation. As a result, a noticeable improvement in mechanical and thermal properties was achieved. Samples with 90wt% PAN/10wt% Si showed about four-time improvements in char yield as compared with 100 wt % PAN. In addition, after the heat treatment of the fiber at 1250 ^oC, SiC was formed. The carbon and SiC crystallinities had a great impact on the oxidation and mechanical property improvement of hybrid nanofibers. The determination of crystal size was carried out using X-ray results obtained at different temperatures and holding times. The SiC grain growth was predicted using kinetic models and its exponent was found to be around n = 4 with activation energy around 35 KJ/mol.K. For such growth, the dominant grain growth mechanism was grain boundary diffusion.

Carbon fibers have broad applications in aerospace, automobiles, wind turbines, medical devices, and implants, among many others. Carbon fibers with high-end applications have previously been manufactured from precursor fibers of rayon, pitch, and polyacrylonitrile (PAN) fibers. New precursor fibers have been conventionally fabricated by homogenously mixing the reinforcement/nanofiller and polymer matrix mechanically and spinning the dope into a coagulation bath, such as PAN processing in the case of dry-jet wet spinning followed by the drawing stage and heat treatment at different temperatures. Mechanical properties of this fiber are dependent on the continuity, density and interfacial action of the hierarchical graphitic structures. The presence of voids formed during crystallization due to the random dispersion of reinforcement/nanofiller in pre-carbon fibers (pCFs) deteriorates these properties.
In this project, a specialized templating graphene channel was developed via an in-house designed spinneret. This channel containing graphene and varied PAN solutions running along the axial direction of the fiber was parallel to the outer and inner PAN channels. On drawing the as-spun fibers the GNP was aligned in the axial direction. This channel then allowed for crystallization along the axial direction and minimizes the formation of voids leading to enhanced mechanical properties. The mechanical test showed the composite as-spun fiber with GNP templating channel has 18.5% and 51.4% increased Young’s modulus and tensile strength, respectively, as compared to the pure PAN fiber. The thermodynamics and kinetics of the heat-treatment procedures were analyzed to correlate the microstructure features and mechanical/thermal properties of obtained carbon fibers.

Two-dimensional (2D) materials have attracted significant attention due to their unique structure and fascinating properties. However, our understanding of their growth mechanisms in solution is still limited. Here, using indium chloride (InCl₃) as a model system, we performed liquid phase transmission electron microscopy (TEM) study of the 2D growth in situ. We combined the low temperature techniques with liquid phase TEM. The nucleation and growth of 2D nanosheets in solution via both classical and non-classical pathways were observed with the atomic resolution in real time at room temperature as well as low temperature (e.g. 1-2 °C). We found the drastically decreased growth kinetics at the low temperature compared to that at the room temperature. Observation also showed that the fast growth induced the high density of crystal defects (e.g., nanotwins). Comparatively, perfect 2D nanocrystals were obtained through the low temperature growth. In addition, we did the atomic structural analysis of the nanosheets and chemical mapping of the reaction front at the Cryogenic temperature using the same liquid cell sample. The results revealed the growth mechanisms of InCl₃ 2D nanosheets and the formation of nanotwins in solution. And, the liquid phase TEM with variable temperature control opens many opportunities for future depth study of nanoscale materials growth and transformations.

Synthesis design remains a paramount problem in materials science. Significant research has been dedicated to advancing tools to predict and aid in synthesis development. Here we describe an ab-initio approach to evaluate water-based synthesis processes such as hydrothermal synthesis. We show how to use electrochemical Pourbaix diagram, constructed from first principles density functional theory (DFT) calculations, to understand thermodynamic forces of complex Cu oxides and Cu oxychalcogenides in aqueous environments. We discuss how to construct these complex diagrams consisting of many degrees-of-freedom, including multiple elemental species, compounds, and complexes as well as nonstandard state conditions. We describe how the driving forces for the products evolve with changes in pH, oxidation state, and the activity of reactants; this understanding is then used to justify successful experimental synthesis of the targeted phase. Scale dependent nucleation is also examined. Last, we propose Pourbaix diagrams are a useful synthesis-processing design tool for accelerated discovery.