An understanding of how facets of a nanocrystal develop is critical for controlling nanocrystal shape and designing novel functional materials. However, the atomic pathways of nanocrystal facet development are mostly unknown because of the lack of direct observation. We report the imaging of platinum nanocube growth in a liquid cell using transmission electron microscopy with high spatial and temporal resolution. The growth rates of all low index facets are similar until the {100} facets stop growth. The continuous growth of the rest facets leads to a nanocube. Our calculation shows that the much lower ligand mobility on the {100} facets is responsible for the arresting of {100} growing facets. These findings shed light on nanocrystal shape-control mechanisms and future design of nanomaterials.

Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure-property relationships at the fundamental level.

Watching it all fall apart The control of the shape and size of metal nanoparticles can be very sensitive to the growth conditions of the particles. Ye et al. studied the reverse process: They tracked the dissolution of gold nanoparticles in a redox environment inside a liquid cell within an electron microscope, controlling the particle dissolution with the electron beam. Tracking short-lived particle shapes revealed structures of greater or lesser stability. The findings suggest kinetic routes to particle sizes and shapes that would otherwise be difficult to generate. Science , this issue p. 874

Abstract:Understanding structural details of colloidal nanoparticles is required to bridge our knowledge about their synthesis, growth mechanisms, and physical properties. We introduce a method for determining 3D structures of individual nanoparticles in solution.We combine a graphene liquid cell, high-resolution transmission electron microscopy, a direct electron detector, and an algorithm for single-particle 3D reconstruction originally developed for analysis of biological molecules to produce two near-atomic resolution 3D structures of individual Pt nanocrystals. Since our method derives the 3D structure from images of individual nanoparticles rotating freely in solution, it enables the analysis of heterogeneous populations of potentially unordered nanoparticles that are synthesized in solution, thereby providing a means to understand the structure and stability of defects at the nanoscale. Main Text:Colloidal nanoparticles are clusters of hundreds to thousands of inorganic atoms typically surrounded by organic ligands that stabilize them in solution. The atomic arrangement of colloidal nanoparticles determines their chemical and physical properties, which are distinct from bulk materials and can be exploited for many applications in biological imaging, renewable energy, catalysis, and more. The 3D atomic arrangement on the surface and in the core of a nanocrystal influences the electronic structure, which affects how the nanocrystal functions in catalysis or how it interacts with other components at the atomic scale (1). Introduction of atomic dopants, surface adatoms, defects, and grain boundaries alters the chemical properties of nanocrystals (2). Ensembles of synthesized nanocrystals in solution are structurally inhomogeneous due to the stochastic nature of nanocrystal nucleation and growth (3,4). Therefore, a method for determination of the 3D atomic arrangement of individual unique nanoparticles in solution is needed. 3Electron tomography is routinely used for 3D analysis of materials (5-9). This method cannot be applied to individual particles in a liquid because it relies on acquisition of images of a single object at many different tilt angles over 2 to 5 hours, assuming the object is static during the entire acquisition. Single particle cryo-electron microscopy (cryo-EM) is a common method for the determination of 3D structures in biological sciences. The average 3D Coulomb potential map (density) of a protein is reconstructed from tens of thousands of TEM images of randomly oriented copies of the same protein embedded in vitreous ice (10). The unknown 3D projection angles of the images are determined by computational methods (11). Single-particle cryo-EM has succeeded in reconstructing biological molecules with nearly 3 Å resolution (10, 12). A similar approach was recently applied to reconstruct the atomic structure of homogeneous ultrasmall gold clusters (13). However, the single-particle method is not readily applicable to 3D reconstruction of colloidal nanoparticles due to their intrinsic struc...

In 1959, Richard Feynman challenged the electron microscopy community to locate the positions of individual atoms in substances 3 . Over the last 55 years, significant advances have been made in electron microscopy. With the development of aberration-corrected electron optics 4,5 , scanning transmission electron microscopy (STEM) has reached sub-0.5 Å resolution in two dimensions 6 . In a combination of STEM 7-9 and a 3D image reconstruction method known as equal slope tomography (EST) 10,11 , electron tomography has achieved 2.4 Å resolution and was applied to image the 3D core structure of edge and screw dislocations at atomic resolution 12,13 . More recently, transmission electron microscopy (TEM) has been used to determine the 3D atomic structure of gold nanoparticles by averaging 939 particles 14 . Notwithstanding these important developments, Feynman's 1959 challenge  3D localization of the coordinates of individual atoms in a substance without using averaging or a priori knowledge of sample crystallinity  remains elusive. Here, we determine the 3D coordinates of 3,769 individual atoms in a tungsten needle sample with a precision of ~19 picometers and identify a point defect inside the sample in three dimensions. The acquisition of a high-quality tilt series with an aberration-corrected STEM and 3D EST reconstruction allow us to trace individual atomic coordinates from the reconstructed intensity and refine the 3D atomic model. direction from 0 to 180, a tilt series of 62 angles was acquired with equal slope increments ( Supplementary Fig. 1). The 0 (Fig. 1 inset) and 180 images of the tilt series are compared in Supplementary Fig. 2, indicating minimal change of the sample structure throughout the experiment. After correcting sample drift, scan distortion, and performing background subtraction on each image (Methods), the tilt series was aligned to a common rotation axis by a centre of mass method 12 . Only the apex of the needle ( Fig. 1 inset and Supplementary Fig. 1) was used for the EST reconstruction due to the 4 STEM depth of focus and to minimize dynamical scattering. Three different schemes were implemented to reconstruct our experimental data. First, a direct EST reconstruction was performed on the tilt series (termed the raw reconstruction). Second, 3DWiener filtering was applied to the raw reconstruction to reduce the noise 22 . Third, the tilt series images were denoised by a sparsity based algorithm 23 ( Supplementary Fig. 3) and then reconstructed by EST (Methods).The EST reconstruction provides an estimate of the intensity distribution inside the tungsten tip, and further analysis known as atom tracing is needed to determine atomic coordinates. We traced and verified the 3D positions of individual atoms using two independent reconstructions: one using Wiener filtering and the other using sparsity denoising (Methods). During atom tracing, a 3D Gaussian function was fit to each local intensity maximum in both reconstructions. Then, we screened each of these plausible atoms by its fi...