Benjamin H. Savitzky

Charge transport and localization in atomically coherent quantum dot solids

Epitaxial attachment of quantum dots into ordered superlattices enables the synthesis of quasi-two-dimensional materials that theoretically exhibit features such as Dirac cones and topological states, and have major potential for unprecedented optoelectronic devices. Initial studies found that disorder in these structures causes localization of electrons within a few lattice constants, and highlight the critical need for precise structural characterization and systematic assessment of the effects of disorder on transport. Here we fabricated superlattices with the quantum dots registered to within a single atomic bond length (limited by the polydispersity of the quantum dot building blocks), but missing a fraction (20%) of the epitaxial connections. Calculations of the electronic structure including the measured disorder account for the electron localization inferred from transport measurements. The calculations also show that improvement of the epitaxial connections will lead to completely delocalized electrons and may enable the observation of the remarkable properties predicted for these materials.

Atomic lattice disorder in charge-density-wave phases of exfoliated dichalcogenides (1T-TaS2)

Significance Low-dimensional materials, such as 1T-TaS2, permit unique phases that arise through electronic and structural reshaping known, respectively, as charge-density waves and periodic lattice distortions (PLDs). Determining the atomic structure of PLDs is critical toward understanding the origin of these charge-ordered phases and their effect on electronic properties. Here we reveal the microscopic nature of PLDs at cryogenic and room temperature in thin flakes of 1T-TaS2 using atomic resolution scanning transmission electron microscopy. Real-space characterization of the local PLD structure across the phase diagram will enable harnessing of emergent properties of thin transition-metal dichalcogenides. Charge-density waves (CDWs) and their concomitant periodic lattice distortions (PLDs) govern the electronic properties in several layered transition-metal dichalcogenides. In particular, 1T-TaS2 undergoes a metal-to-insulator phase transition as the PLD becomes commensurate with the crystal lattice. Here we directly image PLDs of the nearly commensurate (NC) and commensurate (C) phases in thin, exfoliated 1T-TaS2 using atomic resolution scanning transmission electron microscopy at room and cryogenic temperature. At low temperatures, we observe commensurate PLD superstructures, suggesting ordering of the CDWs both in- and out-of-plane. In addition, we discover stacking transitions in the atomic lattice that occur via one-bond-length shifts. Interestingly, the NC PLDs exist inside both the stacking domains and their boundaries. Transitions in stacking order are expected to create fractional shifts in the CDW between layers and may be another route to manipulate electronic phases in layered dichalcogenides.

Propagation of Structural Disorder in Epitaxially Connected Quantum Dot Solids from Atomic to Micron Scale

Epitaxially connected superlattices of self-assembled colloidal quantum dots present a promising route toward exquisite control of electronic structure through precise hierarchical structuring across multiple length scales. Here, we uncover propagation of disorder as an essential feature in these systems, which intimately connects order at the atomic, superlattice, and grain scales. Accessing theoretically predicted exotic electronic states and highly tunable minibands will therefore require detailed understanding of the subtle interplay between local and long-range structure. To that end, we developed analytical methods to quantitatively characterize the propagating disorder in terms of a real paracrystal model and directly observe the dramatic impact of angstrom scale translational disorder on structural correlations at hundreds of nanometers. Using this framework, we discover improved order accompanies increasing sample thickness and identify the substantial effect of small fractions of missing epitaxial bonds on statistical disorder. These results have significant experimental and theoretical implications for the elusive goals of long-range carrier delocalization and true miniband formation.

Bending and breaking of stripes in a charge ordered manganite

In charge-ordered phases, broken translational symmetry emerges from couplings between charge, spin, lattice, or orbital degrees of freedom, giving rise to remarkable phenomena such as colossal magnetoresistance and metal–insulator transitions. The role of the lattice in charge-ordered states remains particularly enigmatic, soliciting characterization of the microscopic lattice behavior. Here we directly map picometer scale periodic lattice displacements at individual atomic columns in the room temperature charge-ordered manganite Bi0.35Sr0.18Ca0.47MnO3 using aberration-corrected scanning transmission electron microscopy. We measure transverse, displacive lattice modulations of the cations, distinct from existing manganite charge-order models. We reveal locally unidirectional striped domains as small as ~5 nm, despite apparent bidirectionality over larger length scales. Further, we observe a direct link between disorder in one lattice modulation, in the form of dislocations and shear deformations, and nascent order in the perpendicular modulation. By examining the defects and symmetries of periodic lattice displacements near the charge ordering phase transition, we directly visualize the local competition underpinning spatial heterogeneity in a complex oxide.Charge-lattice coupling plays a central role in the exotic behaviors of multiferroic complex oxides, such as manganites, however, obtaining a microscopic picture is challenging. Here, Savitzky et al. map periodic lattice displacement fields at the picometer scale to study local order-disorder competition.

Mesophase Formation Stabilizes High-Purity Magic-Sized Clusters

Magic-sized clusters (MSCs) are renowned for their identical size and closed-shell stability that inhibit conventional nanoparticle (NP) growth processes. Though MSCs have been of increasing interest, understanding the reaction pathways toward their nucleation and stabilization is an outstanding issue. In this work, we demonstrate that high concentration synthesis (1000 mM) promotes a well-defined reaction pathway to form high-purity MSCs (>99.9%). The MSCs are resistant to typical growth and dissolution processes. On the basis of insights from in situ X-ray scattering analysis, we attribute this stability to the accompanying production of a large (>100 nm grain size), hexagonal organic-inorganic mesophase that arrests growth of the MSCs and prevents NP growth. At intermediate concentrations (500 mM), the MSC mesophase forms, but is unstable, resulting in NP growth at the expense of the assemblies. These results provide an alternate explanation for the high stability of MSCs. Whereas the conventional mantra has been that the stability of MSCs derives from the precise arrangement of the inorganic structures (i.e., closed-shell atomic packing), we demonstrate that anisotropic clusters can also be stabilized by self-forming fibrous mesophase assemblies. At lower concentration (<200 mM or >16 acid-to-metal), MSCs are further destabilized and NPs formation dominates that of MSCs. Overall, the high concentration approach intensifies and showcases inherent concentration-dependent surfactant phase behavior that is not accessible in conventional (i.e., dilute) conditions. This work provides not only a robust method to synthesize, stabilize, and study identical MSC products but also uncovers an underappreciated stabilizing interaction between surfactants and clusters.

Strain Accommodation and Coherency in Laterally-Stitched WSe 2 /WS 2 Junctions

Monolayer transition metal dichalcogenides (TMDs), which are flexible and stretchable semiconductor films with direct band gaps, have attracted much interest for their applications in electronics and optoelectronics. We have used MOCVD [1] process for the epitaxial growth of defect-free lateral stitched heterojunctions between TMDs. These include WSe2/WS2 junctions, also previously reported by others [2,3], which should have a ~4.4% lattice mismatch based on their isolated lattice constants, while the reported absence of defects would imply large lattice strains at the interface, and these have been predicted to influence electronic and optical properties [3]. Here we examined this interface using dark field TEM (DF-TEM) and atomic-resolution annular dark field scanning transmission electron microscopy (ADF-STEM), where the contrast is proportional to ~Z. We mapped the lattice mismatch and strain using geometric phase analysis (GPA) and bond-length calculations. We observed that WSe2 and WS2 exert uniaxial lattice strain parallel to the junction, resulting in largely coherent, latticematched structures, while the lattice perpendicular to the junction is distorted consistent with a Poisson dilation/contraction.

Successive Ionic Layer Absorption and Reaction for Postassembly Control over Inorganic Interdot Bonds in Long-Range Ordered Nanocrystal Films

Epitaxially connected assemblies of nanocrystals (NCs) present an interesting new class of nanomaterial in which confinement of charge carriers is intermediate between that of a quantum dot and a quantum well. Despite impressive advances in the formation of high-fidelity assemblies, predicted collective properties have not yet emerged. A critical knowledge gap toward realizing these properties is the current lack of understanding of and control over the formation of epitaxial interdot bonds connecting the NCs within the assemblies. In this work we demonstrate successive ionic layer absorption and reaction (SILAR) to enhance the interdot bonding within the NC assembly. SILAR treatment improved the fraction of interdot bonds from 82% to 91% and increased their width from 3.1 to 4.0 nm. Absorption spectra and charge transport measurements indicate that the effect of postassembly growth on quantum confinement in this system depends on the composition of the SILAR shell material. Increased NC film conductance following SILAR processing indicates that building and strengthening interdot bonds lead to increased electronic coupling and doping in the assemblies. The postassembly film growth detailed here presents an opportunity to repair structural defects and to tailor the balance of quantum confinement and interdot coupling in epitaxially connected NC assemblies.

Nature and evolution of incommensurate charge order in manganites visualized with cryogenic scanning transmission electron microscopy

Significance Charge order is a modulation of the electron density and is associated with unconventional phenomena, including colossal magnetoresistance and metal–insulator transitions. Determining how the lattice responds provides insights into the nature and symmetry of the ordered state. Scanning transmission electron microscopy can measure lattice displacements with picometer precision, but its use has been limited to room-temperature phases only. Here, we demonstrate high-resolution imaging at cryogenic temperature and map the nature and evolution of charge order in a manganite. We uncover picometer-scale displacive modulations whose periodicity is strongly locked to the lattice and visualize temperature-dependent phase inhomogeneity in the modulations. These results pave the way to understanding the underlying structure of charge-ordered states and other complex phenomena. Incommensurate charge order in hole-doped oxides is intertwined with exotic phenomena such as colossal magnetoresistance, high-temperature superconductivity, and electronic nematicity. Here, we map, at atomic resolution, the nature of incommensurate charge–lattice order in a manganite using scanning transmission electron microscopy at room temperature and cryogenic temperature (∼93 K). In diffraction, the ordering wave vector changes upon cooling, a behavior typically associated with incommensurate order. However, using real space measurements, we discover that the ordered state forms lattice-locked regions over a few wavelengths interspersed with phase defects and changing periodicity. The cations undergo picometer-scale (∼6 pm to 11 pm) transverse displacements, suggesting that charge–lattice coupling is strong. We further unearth phase inhomogeneity in the periodic lattice displacements at room temperature, and emergent phase coherence at 93 K. Such local phase variations govern the long-range correlations of the charge-ordered state and locally change the periodicity of the modulations, resulting in wave vector shifts in reciprocal space. These atomically resolved observations underscore the importance of lattice coupling and phase inhomogeneity, and provide a microscopic explanation for putative “incommensurate” order in hole-doped oxides.Incommensurate charge order in hole-doped oxides is intertwined with exotic phenomena such as colossal magnetoresistance, high-temperature superconductivity, and electronic nematicity. Here, we map at atomic resolution the nature of incommensurate order in a manganite using scanning transmission electron microscopy at room temperature and cryogenic temperature (∼ 93K). In diffraction, the ordering wavevector changes upon cooling, a behavior typically associated with incommensurate order. However, using real space measurements, we discover that the underlying ordered state is lattice-commensurate at both temperatures. The cations undergo picometer-scale (∼6-11 pm) transverse displacements, which suggests that charge-lattice coupling is strong and hence favors lattice-locked modulations. We further unearth phase inhomogeneity in the periodic lattice displacements at room temperature, and emergent phase coherence at 93K. Such local phase variations not only govern the long range correlations of the charge-ordered state, but also results in apparent shifts in the ordering wavevector. These atomically-resolved observations underscore the importance of lattice coupling and provide a microscopic explanation for putative ”incommensurate” order in hole-doped oxides.

Connectivity of centermost chromatophores in Rhodobacter sphaeroides bacteria: Connectivity of centermost chromatophores

The size of whole Rhodobacter sphaeroides prevents 3D visualization of centermost chromatophores in their native environment. This study combines cryo‐focused ion beam milling with cryo‐electron tomography to probe vesicle architecture both in situ and in 3D. Developing chromatophores are membrane‐bound buds that remain in topological continuity with the cytoplasmic membrane and detach into vesicles when mature. Mature chromatophores closest to the cell wall are typically isolated vesicles, whereas centermost chromatophores are either linked to neighboring chromatophores or contain smaller, budding structures. Isolated chromatophores comprised a minority of centermost chromatophores. Connections between vesicles in growing bacteria are through ~10 nm‐long, ~5 nm‐wide linkers, and are thus physical rather than functional in terms of converting photons to ATP. In cells in the stationary phase, chromatophores fuse with neighboring vesicles, lose their spherical structure, and greatly increase in volume. The fusion and morphological changes seen in older bacteria are likely a consequence of the aging process, and are not representative of connectivity in healthy R. sphaeroides. Our results suggest that chromatophores can adopt either isolated or connected morphologies within a single bacterium. Revealing the organization of chromatophore vesicles throughout the cell is an important step in understanding the photosynthetic mechanisms in R. sphaeroides.

Aberration-Corrected STEM/EELS at Cryogenic Temperatures

Today’s aberration-corrected scanning transmission electron microscopes (STEM) routinely focus highenergy electrons down to a spot smaller than 1Å in diameter to perform scattering experiments that allow us to study the atomic-scale structure of materials and devices. When combined with electron energy loss spectroscopy analysis of the inelastically scattered electrons, these narrow probes can also provide atomic-scale information about the composition and local electronic structure of bulk materials, defects and interfaces [1, 2].