arXiv subjects

Haimei Zheng

Publications and source records attributed to Haimei Zheng.

11 recordsLinked to original sources

arXiv2026

Visualizing Crystallization Dynamics and Transformation Pathways of Disordered Rocksalt Oxides During Thermally Activated Sol-Gel Synthesis

Sol-gel synthesis is a wet-chemical processing route for the fabrication of functional materials offering control over composition, morphology, and microstructure at relatively low processing temperatures compared to conventional solid-state synthesis methods. While the sol-gel process initiates with intermixed molecular precursors, the transformation pathways at the early nucleation stage are insufficiently understood. Here, the chemical and structural transformation of disordered rocksalt (DRX) Li1.2Mn0.4Ti0.4O2 (LMTO), a promising cathode material for lithium batteries, is studied by multiscale characterization tools. In situ heating transmission electron microscopy (TEM) using a liquid cell visualizes and identifies crystallization pathways at nanoscale. While some regions follow a classical multi-step transition through thermodynamically stable intermediates, others exhibit a kinetic shortcut in which intermediate nanocrystals dissolve into a localized amorphous matrix that directly precipitates the DRX structure. Macroscale FTIR corroborates the findings to be related to chemically distinct microenvironments in the gel precursor, with transition metal ions more strongly incorporated into the acetate-coordinated network than lithium. Although in situ heating TEM captures diverse local transformation pathways, in situ SXRD indicates that the macroscopic transformation proceeds predominantly through spinel LMTO and lithium titanite intermediates toward DRX-LMTO. The findings shed light on the spatiotemporal chemical and structural transformations in sol-gel derived DRX-LMTO materials, and call for fine tuning of such sol-gel chemistries to manipulate the crystallization pathways and achieve target material homogeneity more efficiently.

cond-mat.mtrl-sci↗

arXiv2026

Nanocrystal Geometry Governs Phase Transformation Pathways in Palladium Hydride

Pathways and structural dynamics of phase transformations impact performance of materials in energy and information storage technologies. Palladium hydride ($\mathrm{PdH}_x$) nanocrystals are an ideal model system for studying solute-induced phase transformations, where elastic energy from lattice mismatch between $α$-$\mathrm{PdH}_x$ and $β$-$\mathrm{PdH}_x$ phases is often considered a key to determining the transformation pathways. $α/β$-$\mathrm{PdH}_x$ interfacial elastic energy is affected by the confined geometry of a nanocrystal. However, how nanocrystal geometry influences phase transformation pathways is largely unknown. Using in situ liquid phase transmission electron microscopy, we directly visualize hydrogenation in Pd nanocrystals with two geometries -- a nanocube and a hexagonal nanoplate. Both follow similar sequences of an initially curved nucleus, interface flattening, and reverse-stage nucleation; however, their evolving $α/β$-$\mathrm{PdH}_x$ interfaces exhibit geometry-dependent crystallographic alignments. In nanocubes, $\{100\}$-aligned configurations conform to static elastic energy ordering, representing a pathway that maintains a local mechanical equilibrium, whereas nanoplates display both $\{110\}$- and $\{211\}$-aligned interfaces. Theoretical simulations show that geometry determines the accessibility of alternative phase transformation pathways as the system is driven far from equilibrium during hydrogenation. These findings identify geometry as a fundamental parameter for directing phase transformation pathways, offering design principles for accessing atypical configurations and improving properties of intercalation-based devices.

cond-mat.mes-hall↗

arXiv2024

Atomic evolution of hydrogen intercalation wave dynamics in palladium nanocrystals

Solute-intercalation-induced phase separation creates spatial heterogeneities in host materials, a phenomenon ubiquitous in batteries, hydrogen storage, and other energy devices. Despite many efforts, probing intercalation processes at the atomic scale has been a significant challenge. We study hydrogen (de)intercalation in palladium nanocrystals as a model system and achieve atomic-resolution imaging of hydrogen intercalation wave dynamics by utilizing liquid-phase transmission electron microscopy. Our observations reveal that intercalation wave mechanisms, instead of shrinking-core mechanisms, prevail at ambient temperature for palladium nanocubes ranging from ~60 nm down to ~10 nm. We uncover the atomic evolution of hydrogen intercalation wave transitioning from non-planar and inclined boundaries to those closely aligned with {100} planes. Our kinetic Monte Carlo simulations demonstrate the observed intercalation wave dynamics correspond to sorption pathways minimizing the lattice mismatch strain at the phase boundary. Unveiling the atomic intercalation pathways holds profound implications for engineering intercalation-mediated devices and advancements in energy sciences.

cond-mat.stat-mech↗

arXiv2024

Operando probing of nanocracking in CuO-derived Cu during CO$_2$ electroreduction

Identifying and controlling active sites in electrocatalysis remains a grand challenge due to restructuring of catalysts in the complex chemical environments during operation. Inactive precatalysts can transform into active catalysts under reaction conditions, such as oxide-derived Cu (OD-Cu) for CO$_2$ electroreduction displaying improved production of multicarbon (C$_{2+}$) chemicals. Revealing the mechanism of active site origin in OD-Cu catalysts requires in situ/operando characterizations of structure, morphology, and valence state evolution with high spatial and temporal resolution. Applying newly developed electrochemical liquid cell transmission electron microscopy combined with X-ray absorption spectroscopy, our multimodal operando techniques unveil the formation pathways of OD-Cu active sites from CuO bicrystal nanowire precatalysts. Rapid reduction of CuO directly to Cu within 60 seconds generates a nanocrack network throughout the nanowire, via formation of "boundary nanocracks" along the twin boundary and "transverse nanocracks" propagating from the surface to the center of the nanowire. The nanocrack network further reconstructs, leading to a highly porous structure rich in Cu nanograins, with a boosted specific surface area and density of active sites for C$_{2+}$ products. These findings suggest a means to optimize active OD-Cu nanostructures through nanocracking by tailoring grain boundaries in CuO precatalysts. More generally, our advanced operando approach opens new opportunities for mechanistic insights to enable improved control of catalyst structure and performance.

cond-mat.mtrl-sci↗

arXiv2024

Engineering liquid-liquid interfaces for high-entropy alloy synthesis

High-entropy alloys (HEAs) with various promising applications have attracted significant interest. However, alloying immiscible metal elements while controlling the morphology and crystallinity remains extremely challenging. We report a general route, by engineering liquid-liquid interfaces, for the synthesis of HEAs with controlled crystallinity (single crystal, mesocrystal, polycrystal, amorphous), morphology (0-dimension, 2-dimension, 3-dimension), and high composition diversity (20 elements) under mild conditions (room temperature to 80 degrees celsius). As reactions can be spatially confined at the liquid-liquid interface, it provides an opportunity to control the kinetics and reduce the alloying temperature. Our real-time observation of the alloying of HEAs reveals hydrogen-enhanced mixing and isothermal solidification that kinetically traps the mixing states due to composition changes. The concept of liquid-liquid interface engineering can be applied to obtain other high-entropy compounds.

cond-mat.mtrl-sci↗

arXiv2022

Strong structural and electronic coupling in metavalent PbS moire superlattices

Moire superlattices are twisted bilayer materials, in which the tunable interlayer quantum confinement offers access to new physics and novel device functionalities. Previously, moire superlattices were built exclusively using materials with weak van der Waals interactions and synthesizing moire superlattices with strong interlayer chemical bonding was considered to be impractical. Here using lead sulfide (PbS) as an example, we report a strategy for synthesizing of moire superlattices coupled by strong chemical bonding. We use water-soluble ligands as a removable template to obtain free-standing ultra-thin PbS nanosheets and assemble them into direct-contact bilayers with various twist angles. Atomic-resolution imaging shows the moire periodic structural reconstruction at superlattice interface, due to the strong metavalent coupling. Electron energy loss spectroscopy and theoretical calculations collectively reveal the twist angle26 dependent electronic structure, especially the emergent separation of flat bands at small twist angles. The localized states of flat bands are similar to well-arranged quantum dots, promising an application in devices. This study opens a new door to the exploration of deep energy modulations within moire superlattices alternative to van der Waals twistronics.

cond-mat.mtrl-sci↗

arXiv2016

The Nature of Electron Transport and visible light absorption in Strontium Niobate -- A Plasmonic Water Splitter

Semiconductor compounds are widely used for water splitting applications, where photo-generated electron-hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr$_{1-x}$NbO$_3$, 0.03 < x < 0.20) have shown competitive photocatalytic efficiency, opening up the material space available for finding optimizing performance in water-splitting applications. The origin of the visible light absorption in these powders was reported to be due to an interband transition and the charge carrier separation was proposed to be due to the high carrier mobility of this material. In the current work we have prepared epitaxial thin films of Sr$_{0.94}$NbO$_{3+δ}$ and found that the bandgap of this material is ~4.1 eV, which is very large. Surprisingly the carrier density of the conducting phase reaches 10$^{22}$ cm$^{-3}$, which is only one order smaller than that of elemental metals and the carrier mobility is only 2.47 cm$^2$/(V$\cdot$s). Contrary to earlier reports, the visible light absorption at 1.8 eV (~688 nm) is due to the bulk plasmon resonance, arising from the large carrier density, instead of an interband transition. Excitation of the plasmonic resonance results in a multifold enhancement of the lifetime of charge carriers. Thus we propose that the hot charge carriers generated from decay of plasmons produced by optical absorption is responsible for the water splitting efficiency of this material under visible light irradiation.

cond-mat.mtrl-sci↗

arXiv2012

Revealing Correlation of Valence State with Nanoporous Structure in Cobalt Catalyst Nanoparticles by in Situ Environmental TEM

Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope (ETEM) equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy (EELS) in situ for cobalt catalysts under reaction conditions. Analysis revealed quantitative correlation of the cobalt valence states to the particles' nanoporous structures. The in situ experiments were performed on nanoporous cobalt particles coated with silica while a 15 mTorr hydrogen environment was maintained at various temperatures (300-600\degreeC). When the nanoporous particles were reduced, the valence state changed from cobalt oxide to metallic cobalt and concurrent structural coarsening was observed. In situ mapping of the valence state and the corresponding nanoporous structures allows quantitatively analysis necessary for understanding and improving the mass activity and lifetime of cobalt-based catalysts, i.e., for Fischer-Tropsch synthesis that converts carbon monoxide and hydrogen into fuels, and uncovering the catalyst optimization mechanisms.

cond-mat.mtrl-sci↗

arXiv2012

On-column 2p bound state with topological charge \pm1 excited by an atomic-size vortex beam in an aberration-corrected scanning transmission electron microscope

Atomic-size vortex beams have great potential in probing materials' magnetic moment at atomic scales. However, the limited depth of field of vortex beams constrains the probing depth in which the helical phase front is preserved. On the other hand, electron channeling in crystals can counteract beam divergence and extend the vortex beam without disrupting its topological charge. Specifically, in this paper, we report atomic vortex beams with topological charge \pm1 can be coupled to the 2p columnar bound states and propagate for more 50 nm without being dispersed and losing its helical phase front. We gave numerical solutions to the 2p columnar orbitals and tabulated the characteristic size of the 2p states of two typical elements, Co and Dy, for various incident beam energies and various atomic densities. The tabulated numbers allow estimates of the optimal convergence angle for maximal coupling to 2p columnar orbital. We also have developed analytic formulae for beam energy, convergence-angle, and hologram dependent scaling for various characteristic sizes. These length scales are useful for the design of pitch-fork apertures and operations of microscopes in the vortex-beam imaging mode.

cond-mat.mes-hall↗

arXiv2004

Evidence for power-law frequency dependence of intrinsic dielectric response in the CaCu$_{3}$Ti$_{4}$O$_{12}$

We investigated the dielectric response of CaCu$_3$Ti$_4$O$_{12}$ (CCTO) thin films grown epitaxially on LaAlO$_3$ (001) substrates by Pulsed Laser Deposition (PLD). The dielectric response of the films was found to be strongly dominated by a power-law in frequency, typical of materials with localized hopping charge carriers, in contrast to the Debye-like response of the bulk material. The film conductivity decreases with annealing in oxygen, and it suggests that oxygen deficit is a cause of the relatively high film conductivity. With increase of the oxygen content, the room temperature frequency response of the CCTO thin films changes from the response indicating the presence of some relatively low conducting capacitive layers to purely power law, and then towards frequency independent response with a relative dielectric constant $ε'\sim10^2$. The film conductance and dielectric response decrease upon decrease of the temperature with dielectric response being dominated by the power law frequency dependence. Below $\sim$80 K, the dielectric response of the films is frequency independent with $ε'$ close to $10^2$. The results provide another piece of evidence for an extrinsic, Maxwell-Wagner type, origin of the colossal dielectric response of the bulk CCTO material, connected with electrical inhomogeneity of the bulk material.

cond-mat.mtrl-sci↗

arXiv2004

High-Performance Carbon Nanotube Transistors on SrTiO3/Si Substrates

Single-walled carbon nanotubes (SWNTs) have been grown via chemical vapor deposition on high-kappa dielectric SrTiO3/Si substrates, and high-performance semiconducting SWNT field-effect transistors have been fabricated using the thin SrTiO3 as gate dielectric and Si as gate electrode. The transconductance per channel width is 8900 S/m. The high transconductance cannot be explained by the increased gate capacitance; it is proposed that the increased electric field at the nanotube-electrode interface due to the high-kappa SrTiO3 decreases or eliminates the nanotube-electrode Schottky barrier.

cond-mat.mes-hall↗