Crystallization-Induced Flower-like Superstructures via Peptoid Helix Assembly.

We report a unique method to construct hierarchical superstructures based on molecular programming of peptidomimetics. Chiral steric hindrance in the polymer backbone stabilizes peptoid helices that crystallize into nanosheets during solvent evaporation. The stacking of nanosheets results in flower-like superstructures. The helical peptoid, nucleated from chiral monomers, is characterized as locally stiffer and more extended than the unstructured peptoid. Molecular dynamics (MD) simulations further suggest a constraint on the dihedral angles and a preference toward the trans configuration, resulting in an extended chain structure. The nanosheet assemblies at various length scales indicate an extent of intermolecular ordering amplified by chiral steric hindrance. Such molecular programming and processing protocols will benefit the future design and controlled assembly of hierarchical peptidomimetics.

Ultrafast deposition of faceted lithium polyhedra by outpacing SEI formation.

Electrodeposition of lithium (Li) metal is critical for high-energy batteries1. However, the simultaneous formation of a surface corrosion film termed the solid electrolyte interphase (SEI)2 complicates the deposition process, which underpins our poor understanding of Li metal electrodeposition. Here we decouple these two intertwined processes by outpacing SEI formation at ultrafast deposition current densities3 while also avoiding mass transport limitations. By using cryogenic electron microscopy4-7, we discover the intrinsic deposition morphology of metallic Li to be that of a rhombic dodecahedron, which is surprisingly independent of electrolyte chemistry or current collector substrate. In a coin cell architecture, these rhombic dodecahedra exhibit near point-contact connectivity with the current collector, which can accelerate inactive Li formation8. We propose a pulse-current protocol that overcomes this failure mode by leveraging Li rhombic dodecahedra as nucleation seeds, enabling the subsequent growth of dense Li that improves battery performance compared with a baseline. While Li deposition and SEI formation have always been tightly linked in past studies, our experimental approach enables new opportunities to fundamentally understand these processes decoupled from each other and bring about new insights to engineer better batteries.

Operando spectral imaging of the lithium ion battery's solid-electrolyte interphase.

The lithium-ion battery is currently the preferred power source for applications ranging from smart phones to electric vehicles. Imaging the chemical reactions governing its function as they happen, with nanoscale spatial resolution and chemical specificity, is a long-standing open problem. Here, we demonstrate operando spectrum imaging of a Li-ion battery anode over multiple charge-discharge cycles using electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Using ultrathin Li-ion cells, we acquire reference EELS spectra for the various constituents of the solid-electrolyte interphase (SEI) layer and then apply these "chemical fingerprints" to high-resolution, real-space mapping of the corresponding physical structures. We observe the growth of Li and LiH dendrites in the SEI and fingerprint the SEI itself. High spatial- and spectral-resolution operando imaging of the air-sensitive liquid chemistries of the Li-ion cell opens a direct route to understanding the complex, dynamic mechanisms that affect battery safety, capacity, and lifetime.

Direct In Situ Measurement of Quantum Efficiencies of Charge Separation and Proton Reduction at TiO2-Protected GaP Photocathodes.

Photoelectrochemical solar fuel generation at the semiconductor/liquid interface consists of multiple elementary steps, including charge separation, recombination, and catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Here, we report simultaneous in situ transient photocurrent and transient reflectance spectroscopy (TRS) measurements of titanium dioxide-protected gallium phosphide photocathodes for water reduction in photoelectrochemical cells. Transient reflectance spectroscopy enables the direct probe of the separated charge carriers responsible for water reduction to follow their kinetics. Comparison with transient photocurrent measurement allows the direct probe of the initial charge separation quantum efficiency (ϕCS) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (ϕabs), charge separation (ϕCS), and photoreduction (ϕred), i.e., IPCE = ϕabsϕCSϕred. Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces ϕCS and interfacial recombination at the junction that decreases ϕred. Although both loss pathways can be reduced at a more negative applied bias, for GaP/TiO2, the initial charge separation loss is the key efficiency limiting factor. Our combined transient reflectance and photocurrent study provides a time-resolved view of microscopic steps involved in the overall light-to-current conversion process and provides detailed insights into the main loss pathways of the photoelectrochemical system.

Expanding the cryogenic electron microscopy toolbox to reveal diverse classes of battery solid electrolyte interphase.

Advancements in energy storage technologies such as Li-based batteries depend on a deep understanding of the chemical and structural aspects of critical interfaces. Among these, the solid electrolyte interphase (SEI) governs how batteries operate yet remains one of the most elusive to characterize due to its rapid degradation under an electron beam and sensitivity to ambient conditions. In recent years, cryogenic electron microscopy (cryo-EM) has emerged as a promising technique to provide atomic-resolution imaging of beam-sensitive battery materials. Distinct SEIs have been discovered with unique chemical compositions and structural features. In this perspective, the role of cryo-EM in uncovering the physicochemical properties of three classes of SEIs (i.e., compact, extended, and indirect SEI) will be defined and discussed. Furthermore, an in-depth analysis of new cryo-EM imaging modalities will be provided to highlight directions for the future development of cryo-EM.

Visualizing the Electron Wind Force in the Elastic Regime.

With continued scaling toward higher component densities, integrated circuits (ICs) contain ever greater lengths of nanowire that are vulnerable to failure via electromigration. Previously, plastic electromigration driven by the "electron wind" has been observed, but not the elastic response to the wind force itself. Here we describe mapping, via electron energy-loss spectroscopy, the density of a lithographically defined aluminum nanowire with sufficient precision to determine both its temperature and its internal pressure. An electrical current density of 108 A/cm2 produces Joule heating, tension upwind, and compression downwind. Surprisingly, the pressure returns to its ambient value well inside the wire, where the current density is still high. This spatial discrepancy points to physics that are not captured by a classical "wind force" model and to new opportunities for optimizing electromigration-resistant IC design.

Vibrational Sum Frequency Generation Spectroscopy of Surface Hydroxyls on Nickel Phyllosilicate Nanoscrolls.

Phyllosilicate clays are layered structures with diverse nanoscale morphology depending on the composition. Size mismatch between the sheets can cause them to form nanoscrolls, a spiral structure with different inner and outer surface charges. The hydroxyls on the exposed surface of the nanoscrolls determine the adsorption properties and hydrophilicity of the surface. Vibrational sum frequency generation (VSFG) spectroscopy was applied to study the surface hydroxyls of nickel phyllosilicate (Ni3Si2O5(OH)4), revealing three distinct in-phase OH-stretch modes: 3642, 3645, and 3653 cm-1. The relative signs of the peaks allow their assignment as "outward" and "inward" pointing hydroxyls on the opposite sides of the scrolled sheet, consistent with the crystal structure. Orientational analysis of polarization-selected VSFG spectra is consistent with a broad (140-164°) step-function distribution of the OH-stretch tilt angles, which we attribute to the uncompensated portion of the scroll rolled more than a whole number of full turns.

Nanoscale TiO2 Protection Layer Enhances the Built-In Field and Charge Separation Performance of GaP Photoelectrodes.

Nanoscale oxide layer protected semiconductor photoelectrodes show enhanced stability and performance for solar fuels generation, although the mechanism for the performance enhancement remains unclear due to a lack of understanding of the microscopic interfacial field and its effects. Here, we directly probe the interfacial fields at p-GaP electrodes protected by n-TiO2 and its effect on charge carriers by transient reflectance spectroscopy. Increasing the TiO2 layer thickness from 0 to 35 nm increases the field in the GaP depletion region, enhancing the rate and efficiency of interfacial electron transfer from the GaP to TiO2 on the ps time scale as well as retarding interfacial recombination on the microsecond time scale. This study demonstrates a general method for providing a microscopic view of the photogenerated charge carrier's pathway and loss mechanisms from the bulk of the electrode to the long-lived separated charge at the interface that ultimately drives the photoelectrochemical reactions.

Discovery of a Wurtzite-like Cu2FeSnSe4 Semiconductor Nanocrystal Polymorph and Implications for Related CuFeSe2 Materials.

I2-II-IV-VI4 and I-III-VI2 semiconductor nanocrystals have found applications in photovoltaics and other optoelectronic technologies because of their low toxicity and efficient light absorption into the near-infrared. Herein, we report the discovery of a metastable wurtzite-like polymorph of Cu2FeSnSe4, a member of the I2-II-IV-VI4 family of semiconductors containing only earth-abundant metals. Density functional theory calculations on this metastable polymorph of Cu2FeSnSe4 indicate that it may be a superior semiconductor for solar energy and optoelectronics applications compared to the thermodynamically preferred stannite polymorph, since the former displays a sharper dispersion of energy levels near the conduction band minimum that can enhance electron mobility and suppress hot electron cooling. The experimental optical band gap was measured by the inverse logarithmic derivative method to be direct, in agreement with theory, and in the range of 1.48-1.59 eV. Mechanistic studies reveal that this metastable phase derives from intermediate Cu3Se2 nanocrystals that serve as a structural template for the final hexagonal wurtzite-like product. We compare the chemistry of wurtzite-like Cu2FeSnSe4 to the related CuFeSe2 material system. Our experimental and computational comparisons between Cu2FeSnSe4 and CuFeSe2 help explain both the crystal chemistry of CuFeSe2 that prevents it from forming wurtzite-like polymorphs and the essential role of Sn in stabilizing the metastable structure of Cu2FeSnSe4. This work provides insight into the importance of elemental composition when designing syntheses for metastable materials.

Crystal Structure of Colloidally Prepared Metastable Ag2Se Nanocrystals.

Structural polymorphism is known for many bulk materials; however, on the nanoscale metastable polymorphs tend to form more readily than in the bulk, and with more structural variety. One such metastable polymorph observed for colloidal Ag2Se nanocrystals has traditionally been referred to as the "tetragonal" phase. While there are reports on the chemistry and properties of this metastable polymorph, its crystal structure, and therefore electronic structure, has yet to be determined. We report that an anti-PbCl2-like structure type (space group P21/n) more accurately describes the powder X-ray diffraction and X-ray total scattering patterns of colloidal Ag2Se nanocrystals prepared by several different methods. Density functional theory (DFT) calculations indicate that this anti-PbCl2-like Ag2Se polymorph is a dynamically stable, narrow-band-gap semiconductor. The anti-PbCl2-like structure of Ag2Se is a low-lying metastable polymorph at 5-25 meV/atom above the ground state, depending on the exchange-correlation functional used.

High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals.

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS3, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures.

Electron-Transparent Thermoelectric Coolers Demonstrated with Nanoparticle and Condensation Thermometry.

More efficient thermoelectric devices would revolutionize refrigeration and energy production, and low-dimensional thermoelectric materials are predicted to be more efficient than their bulk counterparts. But nanoscale thermoelectric devices generate thermal gradients on length scales that are too small to resolve with traditional thermometry methods. Here we fabricate, using single-crystal bismuth telluride (Bi2Te3) and antimony/bismuth telluride (Sb2-xBixTe3) flakes exfoliated from commercially available bulk materials, functional thermoelectric coolers (TECs) that are only 100 nm thick. These devices are the smallest TECs ever demonstrated by a factor of 104. After depositing indium nanoparticles to serve as nanothermometers, we measure the heating and cooling produced by the devices with plasmon energy expansion thermometry (PEET), a high-spatial-resolution, transmission electron microscopy (TEM)-based thermometry technique, demonstrating a ΔT = -21 ± 4 K from room temperature. We also establish proof-of-concept for condensation thermometry, a quantitative temperature-change mapping technique with a spatial precision of ≲300 nm.

Electron beam-induced current imaging with two-angstrom resolution.

An electron microscope's primary beam simultaneously ejects secondary electrons (SEs) from the sample and generates electron beam-induced currents (EBICs) in the sample. Both signals can be captured and digitized to produce images. The off-sample Everhart-Thornley detectors that are common in scanning electron microscopes (SEMs) can detect SEs with low noise and high bandwidth. However, the transimpedance amplifiers appropriate for detecting EBICs do not have such good performance, which makes accessing the benefits of EBIC imaging at high-resolution relatively more challenging. Here we report lattice-resolution imaging via detection of the EBIC produced by SE emission (SEEBIC). We use an aberration-corrected scanning transmission electron microscope (STEM), and image both microfabricated devices and standard calibration grids.

Tunable Thermal Energy Transport across Diamond Membranes and Diamond-Si Interfaces by Nanoscale Graphoepitaxy.

The development of electronic devices, especially those that involve heterogeneous integration of materials, has led to increased challenges in addressing their thermal operational temperature demands. The heat flow in these systems is significantly influenced or even dominated by thermal boundary resistance at the interface between dissimilar materials. However, controlling and tuning heat transport across an interface and in the adjacent materials has so far drawn limited attention. In this work, we grow chemical vapor-deposited diamond on silicon substrates by graphoepitaxy and experimentally demonstrate tunable thermal transport across diamond membranes and diamond-silicon interfaces. We observed the highest diamond-silicon thermal boundary conductance (TBC) measured to date and increased diamond thermal conductivity due to strong grain texturing in the diamond near the interface. Additionally, nonequilibrium molecular dynamics simulations and a Landauer approach are used to understand the diamond-silicon TBC. These findings pave the way for tuning or increasing thermal conductance in heterogeneously integrated electronics that involve polycrystalline materials and will impact applications including electronics thermal management and diamond growth.

Low Thermal Boundary Resistance Interfaces for GaN-on-Diamond Devices.

The development of GaN-on-diamond devices holds much promise for the creation of high-power density electronics. Inherent to the growth of these devices, a dielectric layer is placed between the GaN and diamond, which can contribute significantly to the overall thermal resistance of the structure. In this work, we explore the role of different interfaces in contributing to the thermal resistance of the interface of GaN/diamond layers, specifically using 5 nm layers of AlN, SiN, or no interlayer at all. Using time-domain thermoreflectance along with electron energy loss spectroscopy, we were able to determine that a SiN interfacial layer provided the lowest thermal boundary resistance (<10 m2K/GW) because of the formation of an Si-C-N layer at the interface. The AlN and no interlayer samples were observed to have TBRs greater than 20 m2K/GW as a result of a harsh growth environment that roughened the interface (enhancing phonon scattering) when the GaN was not properly protected.

Hierarchical Carbon-Coated Ball-Milled Silicon: Synthesis and Applications in Free-Standing Electrodes and High-Voltage Full Lithium-Ion Batteries.

Lithium-ion batteries have been regarded as one of the most promising energy storage devices, and development of low-cost batteries with high energy density is highly desired so that the cost per watt-hour ($/Wh) can be minimized. In this work, we report using ball-milled low-cost silicon (Si) as the starting material and subsequent carbon coating to produce low-cost hierarchical carbon-coated (HCC) Si. The obtained particles prepared from different Si sources all show excellent cycling performance of over 1000 mAh/g after 1000 cycles. Interestingly, we observed in situ formation of porous Si, and it is well confined in the carbon shell based on postcycling characterization of the hierarchical carbon-coated metallurgical Si (HCC-M-Si) particles. In addition, lightweight and free-standing electrodes consisting of the HCC-M-Si particles and carbon nanofibers were fabricated, which achieved 1015 mAh/g after 100 cycles based on the total mass of the electrodes. Compared with conventional electrodes, the lightweight and free-standing electrodes significantly improve the energy density by 745%. Furthermore, LiCoO2 and LiNi0.5Mn1.5O4 cathodes were used to pair up with the HCC-M-Si anode to fabricate full cells. With LiNi0.5Mn1.5O4 as cathode, an energy density up to 547 Wh/kg was achieved by the high-voltage full cell. After 100 cycles, the full cell with a LiNi0.5Mn1.5O4 cathode delivers 46% more energy density than that of the full cell with a LiCoO2 cathode. The systematic investigation on low-cost Si anodes together with their applications in lightweight free-standing electrodes and high-voltage full cells will shed light on the development of high-energy Si-based lithium-ion batteries for real applications.