Quantification of Unsupported Thin-Film X-ray Spectra Using Bulk Standards.

A quantification model which uses standard X-ray spectra collected from bulk materials to determine the composition and mass thickness of single-layer and multilayer unsupported thin films is presented. The multivariate model can be iteratively solved for single layers in which each element produces at least one visible characteristic X-ray line. The model can be extended to multilayer thin films in which each element is associated with only one layer. The model may sometimes be solved when an element is present in multiple layers if additional information is added in the form of independent k-ratios or model assumptions. While the algorithm is suitable for any measured k-ratios, it is particularly well suited to energy-dispersive X-ray spectrometry where the bulk standard spectra can be used to deconvolve peak interferences in the thin-film spectra. The algorithm has been implemented and made available in the Open Source application National Institute of Standards and Technology DTSA-II. We present experimental data and Monte Carlo simulations supporting the quantification model.

Carbon-Nanotube-Encapsulated-Sulfur Cathodes for Lithium-Sulfur Batteries: Integrated Computational Design and Experimental Validation.

To mitigate lithium-polysulfides (Li-PSs) shuttle in lithium-sulfur batteries (LiSBs), a unique carbon-nanotube-encapsulated-sulfur (S@CNT) cathode material with optimum open-ring sizes (ORSs) on the CNT walls were designed using an integrated computational approach followed by experimental validation. By calculating the transport barrier of Li+ ion through ORSs on the CNT walls and comparing the molecular size of solvents and Li-PSs with ORSs, optimum open-rings with 16-30 surrounding carbon atoms were predicted to selectively allow transportation of Li+ ion and evaporated sulfur while blocking both Li-PS and solvent molecules. A CNT oxidation process was proposed and simulated to generate these ORSs, and the results indicated that the optimum ORSs can be achieved by narrowly controlling the oxidation parameters. Subsequently, S@CNT cathodes were experimentally synthesized, confirming that optimum ORSs were generated in CNT oxidized at 475 K and exhibited more stable cycling behavior.

Thickness-Dependent Permeance of Molecular Layer-By-Layer Polyamide Membranes.

We present the thickness-dependent permeance of highly cross-linked polyamide (PA) membranes formed by a molecular layer-by-layer (mLbL) deposition process. The deposition allows for the synthesis of extremely smooth, uniform PA films of tunable thickness, which is counter to the less controlled interfacial polymerization process used commercially. The ability to control and measure the membrane thickness allows us to elucidate the relationships among network structure, transport properties, and separation performance. In this work, a series of large-area mLbL PA membranes is prepared with thickness ranging from less than 5 nm to greater than 100 nm, which can be transferred defect-free via a film floating technique onto a macroporous support layer and challenged with salt solutions. A critical thickness of 15 nm is identified for efficient desalination, and water permeance is described using a multi-layer solution diffusion model that allows for the extraction of material properties relevant to transport. Finally, the model demonstrates the existence of two distinct layers in the mLbL films, one layer comprised of a (5 to 10) nm graded or less cross-linked layer at the surface and a more densely cross-linked layer in the interior of the film. This graded layer appears inherent to the mLbL deposition process and is observed at all film thicknesses.

From Microparticles to Nanowires and Back: Radical Transformations in Plated Li Metal Morphology Revealed via in Situ Scanning Electron Microscopy.

Li metal is the preferred anode material for all-solid-state Li batteries. However, a stable plating and stripping of Li metal at the anode-solid electrolyte interface remains a significant challenge particularly at practically feasible current densities. This problem usually relates to high and/or inhomogeneous Li-electrode-electrolyte interfacial impedance and formation and growth of high-aspect-ratio dendritic Li deposits at the electrode-electrolyte interface, which eventually shunt the battery. To better understand details of Li metal plating, we use operando electron microscopy and Auger spectroscopy to probe nucleation, growth, and stripping of Li metal during cycling of a model solid-state Li battery as a function of current density and oxygen pressure. We find a linear correlation between the nucleation density of Li clusters and the charging rate in an ultrahigh vacuum, which agrees with a classical nucleation and growth model. Moreover, the trace amount of oxidizing gas (≈10-6 Pa of O2) promotes the Li growth in a form of nanowires due to a fine balance between the ion current density and a growth rate of a thin lithium-oxide shell on the surface of the metallic Li. Interestingly, increasing the partial pressure of O2 to 10-5 Pa resumes Li plating in a form of 3D particles. Our results demonstrate the importance of trace amounts of preexisting or ambient oxidizing species on lithiation processes in solid-state batteries.

Multimodal Characterization of the Morphology and Functional Interfaces in Composite Electrodes for Li-S Batteries by Li Ion and Electron Beams.

We report the characterization of multiscale 3D structural architectures of novel poly[sulfur-random-(1,3-diisopropenylbenzene)] copolymer-based cathodes for high-energy-density Li-S batteries capable of realizing discharge capacities >1000 mAh/g and long cycling lifetimes >500 cycles. Hierarchical morphologies and interfacial structures have been investigated by a combination of focused Li ion beam (LiFIB) and analytical electron microscopy in relation to the electrochemical performance and physicomechanical stability of the cathodes. Charge-free surface topography and composition-sensitive imaging of the electrodes was performed using recently introduced low-energy scanning LiFIB with Li+ probe sizes of a few tens of nanometers at 5 keV energy and 1 pA probe current. Furthermore, we demonstrate that LiFIB has the ability to inject a certain number of Li cations into the material with nanoscale precision, potentially enabling control of the state of discharge in the selected area. We show that chemical modification of the cathodes by replacing the elemental sulfur with organosulfur copolymers significantly improves its structural integrity and compositional homogeneity down to the sub-5-nm length scale, resulting in the creation of (a) robust functional interfaces and percolated conductive pathways involving graphitic-like outer shells of aggregated nanocarbons and (b) extended micro- and mesoscale porosities required for effective ion transport.

Comparison of Nanoscale Focused Ion Beam and Electrochemical Lithiation in ß-Sn Microspheres.

The development of Li focused ion beams (Li-FIB) enables controlled Li ion insertion into materials with nanoscale resolution. We take the first step toward establishing the relevance of the Li-FIB for studies of ion dynamics in electrochemically active materials by comparing FIB lithiation with conventional electrochemical lithiation of isolated ß-Sn microspheres. Samples are characterized by cross-sectioning with Ga FIB and imaging via electron microscopy. The Li-FIB and electrochemical lithiated Sn exhibit similarities that suggest that the Li-FIB can be a powerful tool for exploring dynamical Li ion-material interactions at the nanoscale in a range of battery materials.

Analytical Multimode Scanning and Transmission Electron Imaging and Tomography of Multiscale Structural Architectures of Sulfur Copolymer-Based Composite Cathodes for Next-Generation High-Energy Density Li-S Batteries.

Poly[sulfur-random-(1,3-diisopropenylbenzene)] copolymers synthesized via inverse vulcanization represent an emerging class of electrochemically active polymers recently used in cathodes for Li-S batteries, capable of realizing enhanced capacity retention (1,005 mAh/g at 100 cycles) and lifetimes of over 500 cycles. The composite cathodes are organized in complex hierarchical three-dimensional (3D) architectures, which contain several components and are challenging to understand and characterize using any single technique. Here, multimode analytical scanning and transmission electron microscopies and energy-dispersive X-ray/electron energy-loss spectroscopies coupled with multivariate statistical analysis and tomography were applied to explore origins of the cathode-enhanced capacity retention. The surface topography, morphology, bonding, and compositions of the cathodes created by combining sulfur copolymers with varying 1,3-diisopropenylbenzene content and conductive carbons have been investigated at multiple scales in relation to the electrochemical performance and physico-mechanical stability. We demonstrate that replacing the elemental sulfur with organosulfur copolymers improves the compositional homogeneity and compatibility between carbons and sulfur-containing domains down to sub-5 nm length scales resulting in (a) intimate wetting of nanocarbons by the copolymers at interfaces; (b) the creation of 3D percolation networks of conductive pathways involving graphitic-like outer shells of aggregated carbons;

Electrolyte stability determines scaling limits for solid-state 3D Li ion batteries.

Rechargeable, all-solid-state Li ion batteries (LIBs) with high specific capacity and small footprint are highly desirable to power an emerging class of miniature, autonomous microsystems that operate without a hardwire for power or communications. A variety of three-dimensional (3D) LIB architectures that maximize areal energy density has been proposed to address this need. The success of all of these designs depends on an ultrathin, conformal electrolyte layer to electrically isolate the anode and cathode while allowing Li ions to pass through. However, we find that a substantial reduction in the electrolyte thickness, into the nanometer regime, can lead to rapid self-discharge of the battery even when the electrolyte layer is conformal and pinhole free. We demonstrate this by fabricating individual, solid-state nanowire core-multishell LIBs (NWLIBs) and cycling these inside a transmission electron microscope. For nanobatteries with the thinnest electrolyte, ≈110 nm, we observe rapid self-discharge, along with void formation at the electrode/electrolyte interface, indicating electrical and chemical breakdown. With electrolyte thickness increased to 180 nm, the self-discharge rate is reduced substantially, and the NWLIBs maintain a potential above 2 V for over 2 h. Analysis of the nanobatteries' electrical characteristics reveals space-charge limited electronic conduction, which effectively shorts the anode and cathode electrodes directly through the electrolyte. Our study illustrates that, at these nanoscale dimensions, the increased electric field can lead to large electronic current in the electrolyte, effectively shorting the battery. The scaling of this phenomenon provides useful guidelines for the future design of 3D LIBs.

Methanol, ethanol and hydrogen sensing using metal oxide and metal (TiO(2)-Pt) composite nanoclusters on GaN nanowires: a new route towards tailoring the selectivity of nanowire/nanocluster chemical sensors.

We demonstrate a new method for tailoring the selectivity of chemical sensors using semiconductor nanowires (NWs) decorated with metal and metal oxide multicomponent nanoclusters (NCs). Here we present the change of selectivity of titanium dioxide (TiO(2)) nanocluster-coated gallium nitride (GaN) nanowire sensor devices on the addition of platinum (Pt) nanoclusters. The hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN NWs followed by the deposition of TiO(2) and/or Pt nanoclusters (NCs) using the sputtering technique. This paper present the sensing characteristics of GaN/(TiO(2)-Pt) nanowire-nanocluster (NWNC) hybrids and GaN/(Pt) NWNC hybrids, and compare their selectivity with that of the previously reported GaN/TiO(2) sensors. The GaN/TiO(2) NWNC hybrids showed remarkable selectivity to benzene and related aromatic compounds, with no measurable response for other analytes. Addition of Pt NCs to GaN/TiO(2) sensors dramatically altered their sensing behavior, making them sensitive only to methanol, ethanol and hydrogen, but not to any other chemicals we tested. The GaN/(TiO(2)-Pt) hybrids were able to detect ethanol and methanol concentrations as low as 100 nmol mol(-1) (ppb) in air in approximately 100 s, and hydrogen concentrations from 1 µmol mol(-1) (ppm) to 1% in nitrogen in less than 60 s. However, GaN/Pt NWNC hybrids showed limited sensitivity only towards hydrogen and not towards any alcohols. All these hybrid sensors worked at room temperature and are photomodulated, i.e. they responded to analytes only in the presence of ultraviolet (UV) light. We propose a qualitative explanation based on the heat of adsorption, ionization energy and solvent polarity to explain the observed selectivity of the different hybrids. These results are significant from the standpoint of applications requiring room-temperature hydrogen sensing and sensitive alcohol monitoring. These results demonstrate the tremendous potential for tailoring the selectivity of the hybrid nanosensors for a multitude of environmental and industrial sensing applications.

The use of plasmon spectroscopy and imaging in a transmission electron microscope to probe physical properties at the nanoscale.

Valence EELS and energy-filtering TEM appear to be powerful tools to explore diverse nanoscale phenomena. The techniques enable real-time information on the band structure, bonding, dielectric and optical response and phase compositions of nanostructured materials. Furthermore, electron beam-induced excitations in the 0 to 50 eV energy loss range dominated by plasmons are sensitive to valence electron states primarily responsible for intrinsic materials properties. We used universality and scaling in relationships between the volume plasmon energy and cohesive energy, elastic moduli and hardness to derive analytical expressions for quantitative determination of the properties. Based on this approach, cohesive and elastic properties of metastable nanoprecipitates in structural alloys and hardness of diesel engine soot nanoparticles have been evaluated. Spatially-resolved plasmon spectroscopic imaging techniques offer possibilities to determine and image in situ multiple physical properties of nanoscale materials and to monitor their changes during dynamic transformations, thus establishing new capabilities for material research.

Investigation of the excitations of plasmons and surface exciton polaritons in monoclinic gadolinium sesquioxide by electron energy-loss spectroscopy and plasmon spectroscopic imaging.

The monoclinic gadolinium sesquioxide (denoted as m-Gd2O3) with its lower crystal symmetry exhibits larger dielectric permittivity (κ) than the cubic Gd2O3 (denoted as c-Gd2O3). Recently, a few nanometers thick m-Gd2O3 thin film has been successfully epitaxially grown on a GaN substrate as a promising candidate gate oxide in metal-oxide-semiconductor field-effect transistors (MOSFETs). Thus, it is important to understand the electronic excitations in m-Gd2O3 and investigate them by electron energy loss spectroscopy (EELS) performed with aloof electron beams and electron diffraction to gain the spatial and momentum resolutions. In this study, using scanning transmission electron microscopy combined with EELS (STEM-EELS) in the aloof electron beam setup, we observed low-loss spectral features at 13 eV and 14.5 eV at the specimen edge in a grazing incidence and the material interior, which can be interpreted as a surface plasmon (SP) and a volume plasmon (VP), respectively. Surface exciton polaritons (SEPs), which represents surface resonances associated with excitonic onsets above the bandgap, were also observed at about 7, 10.2, and 36 eV energy loss. Their surface excitation character was confirmed by energy-filtered transmission electron microscopy spectrum imaging (EFTEM-SI) and using relativistic energy versus-momentum (E-k) map calculations. The momentum (q)-dependent EELS indicates that the SEP features near the bandgap represented a function of q and revealed a nondispersive behavior for VP and SEP at 36 eV. The oscillator strengths for VP and SEP at 36 eV dropped at different q values along with different q directions, revealing the anisotropic electronic structures of m-Gd2O3.

Emergent Moiré Phonons Due to Zone Folding in WSe2-WS2 Van der Waals Heterostructures.

Bilayers of 2D materials offer opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs) where the constituent monolayers have different lattice constants, a moiré superlattice forms with a length scale larger than the lattice constant of either constituent material regardless of twist angle. Here, we report the appearance of moiré Raman modes from nearly aligned WSe2-WS2 vdWHs in the range of 240-260 cm-1, which are absent in both monolayers and homobilayers of WSe2 and WS2 and in largely misaligned WSe2-WS2 vdWHs. Using first-principles calculations and geometric arguments, we show that these moiré Raman modes are a consequence of the large moiré length scale, which results in zone-folded phonon modes that are Raman active. These modes are sensitive to changes in twist angle, but notably, they occur at identical frequencies for a given small twist angle away from either the 0-degree or 60-degree aligned heterostructure. Our measurements also show a strong Raman intensity modulation in the frequency range of interest, with near 0 and near 60-degree vdWHs exhibiting a markedly different dependence on excitation energy. In near 0-degree aligned WSe2-WS2 vdWHs, a nearly complete suppression of both the moiré Raman modes and the WSe2 A1g Raman mode (∼250 cm-1) is observed when exciting with a 532 nm CW laser at room temperature. Temperature-dependent reflectance contrast measurements demonstrate the significant Raman intensity modulation arises from resonant Raman effects.

Highly selective GaN-nanowire/TiO2-nanocluster hybrid sensors for detection of benzene and related environment pollutants.

Nanowire-nanocluster hybrid chemical sensors were realized by functionalizing gallium nitride (GaN) nanowires (NWs) with titanium dioxide (TiO(2)) nanoclusters for selectively sensing benzene and other related aromatic compounds. Hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN NWs followed by the deposition of TiO(2) nanoclusters using RF magnetron sputtering. The sensor fabrication process employed standard microfabrication techniques. X-ray diffraction and high-resolution analytical transmission electron microscopy using energy-dispersive x-ray and electron energy-loss spectroscopies confirmed the presence of the anatase phase in TiO(2) clusters after post-deposition anneal at 700 °C. A change of current was observed for these hybrid sensors when exposed to the vapors of aromatic compounds (benzene, toluene, ethylbenzene, xylene and chlorobenzene mixed with air) under UV excitation, while they had no response to non-aromatic organic compounds such as methanol, ethanol, isopropanol, chloroform, acetone and 1,3-hexadiene. The sensitivity range for the noted aromatic compounds except chlorobenzene were from 1% down to 50 parts per billion (ppb) at room temperature. By combining the enhanced catalytic properties of the TiO(2) nanoclusters with the sensitive transduction capability of the nanowires, an ultra-sensitive and selective chemical sensing architecture is demonstrated. We have proposed a mechanism that could qualitatively explain the observed sensing behavior.

Twist Angle-Dependent Atomic Reconstruction and Moiré Patterns in Transition Metal Dichalcogenide Heterostructures.

Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of materials with weak interlayer bonding, which enables one to create so-called van der Waals heterostructures (vdWH). One promising attribute of vdWH is the ability to rotate the layers at arbitrary azimuthal angles relative to one another. Recent work has shown that control of the twist angle between layers can have a dramatic effect on TMD vdWH properties, but the twist angle has been treated solely through the use of rigid-lattice moiré patterns. No atomic reconstruction, that is, any rearrangement of atoms within the individual layers, has been reported experimentally to date. Here, we demonstrate that vdWH of MoSe2/WSe2 and MoS2/WS2 at twist angles ≤1° undergo significant atomic level reconstruction leading to discrete commensurate domains divided by narrow domain walls, rather than a smoothly varying rigid-lattice moiré pattern as has been assumed in prior experimental work. Using conductive atomic force microscopy (CAFM), we show that TMD vdWH at small twist angles exhibit large domains of constant conductivity. The domains in samples with R-type stacking are triangular, whereas the domains in samples with H-type stacking are hexagonal. Transmission electron microscopy provides additional evidence of atomic reconstruction in MoSe2/WSe2 structures and demonstrates the transition between a rigid-lattice moiré pattern for large angles and atomic reconstruction for small angles. We use density functional theory to calculate the band structures of the commensurate reconstructed domains and find that the modulation of the relative electronic band edges is consistent with the CAFM results and photoluminescence spectra. The presence of atomic reconstruction in TMD heterostructures and the observed impact on nanometer-scale electronic properties provide fundamental insight into the behavior of this important class of heterostructures.

Synthesis of High-Quality Monolayer MoS2 by Direct Liquid Injection.

We report the synthesis of high-quality single monolayer MoS2 samples using a novel technique that utilizes direct liquid injection (DLI) for the delivery of precursors. The DLI system vaporizes a liquid consisting of a selected precursor dissolved in a solvent into small, micron-sized droplets in an expansion chamber maintained at a selected temperature and pressure, before delivery to the deposition chamber. We demonstrate the synthesis of monolayer MoS2 on SiO2/Si substrates using the DLI technique with film quality superior to exfoliated samples or those grown by traditional tube furnace chemical vapor deposition (CVD) methods. Photoluminescence measurements of DLI monolayers exhibit consistently brighter emission, narrower line width, and higher emission energy than their exfoliated and CVD counterparts. Conductive atomic force microscopy identifies a defect density of 8.3 × 1011/cm2 in DLI MoS2, lower than the measured density in CVD material and nearly an order of magnitude improvement over the exfoliated MoS2 investigated under the same conditions. The DLI method is directly applicable to many other van der Waals materials, which require the use of challenging low vapor pressure precursors, to the growth of alloys, and sequential growths of dissimilar materials leading to van der Waals heterostructures.

Direct-Write Lithiation of Silicon Using a Focused Ion Beam of Li.

Electrochemical processes that govern the performance of lithium ion batteries involve numerous parallel reactions and interfacial phenomena that complicate the microscopic understanding of these systems. To study the behavior of ion transport and reaction in these applications, we report the use of a focused ion beam of Li+ to locally insert controlled quantities of lithium with high spatial resolution into electrochemically relevant materials in vacuo. To benchmark the technique, we present results on direct-write lithiation of 35 nm thick crystalline silicon membranes using a 2 keV beam of Li+ at doses up to 1018 cm-2 (104 nm-2). We confirm quantitative sub-µm control of lithium insertion and characterize the concomitant morphological, structural, and functional changes of the system using a combination of electron and scanning probe microscopy. We observe saturation of interstitial lithium in the silicon membrane at ≈10% dopant number density and spillover of excess lithium onto the membrane's surface. The implanted Li+ is demonstrated to remain electrochemically active. This technique will enable controlled studies and improve understanding of Li+ ion interaction with local defect structures and interfaces in electrode and solid-electrolyte materials.

Metal particle growth during glucose hydrogenation over Ru/SiO2 evaluated by X-ray absorption spectroscopy and electron microscopy.

Biorenewable resources such as carbohydrates are considered alternative feedstocks for oxygenated chemicals. This work investigates the stability of silica-supported Ru catalysts in the aqueous phase conversion of glucose to sorbitol. In situ X-ray absorption spectroscopy at the Ru K edge revealed that air-exposed silica-supported Ru was in an oxidized state but was subsequently reduced in aqueous solutions saturated with 40 bar H(2) at 373 K. Furthermore, exposure to aqueous phase conditions resulted in the sintering of Ru particles on the silica surface. However, the presence of glucose in the aqueous phase stabilized the growth of the Ru particles. Batchwise hydrogenation of glucose at 373 K and 80 bar H(2) over a Ru/SiO(2) (2.67 wt %) catalyst is nearly 100% selective to sugar alcohol with an average turnover frequency of 0.21 +/- 0.04 s(-1). The hydrogenation reaction was not mass transfer limited according to the Madon-Boudart criterion.

Elemental Sulfur and Molybdenum Disulfide Composites for Li-S Batteries with Long Cycle Life and High-Rate Capability.

The practical implementation of Li-S technology has been hindered by short cycle life and poor rate capability owing to deleterious effects resulting from the varied solubilities of different Li polysulfide redox products. Here, we report the preparation and utilization of composites with a sulfur-rich matrix and molybdenum disulfide (MoS2) particulate inclusions as Li-S cathode materials with the capability to mitigate the dissolution of the Li polysulfide redox products via the MoS2 inclusions acting as "polysulfide anchors". In situ composite formation was completed via a facile, one-pot method with commercially available starting materials. The composites were afforded by first dispersing MoS2 directly in liquid elemental sulfur (S8) with sequential polymerization of the sulfur phase via thermal ring opening polymerization or copolymerization via inverse vulcanization. For the practical utility of this system to be highlighted, it was demonstrated that the composite formation methodology was amenable to larger scale processes with composites easily prepared in 100 g batches. Cathodes fabricated with the high sulfur content composites as the active material afforded Li-S cells that exhibited extended cycle lifetimes of up to 1000 cycles with low capacity decay (0.07% per cycle) and demonstrated exceptional rate capability with the delivery of reversible capacity up to 500 mAh/g at 5 C.