Operando Spatial and Temporal Tracking of Axial Stresses and Interfaces in Solid-state Batteries.

Solid-state batteries have the potential to replace the current generation of liquid electrolyte batteries. However, the major limitation resulting from their solid-state architecture is the gradual loss of ionic conductivity due to the loss of physical contact between the individual battery components during charging/discharging. This is mainly due to mechanical stresses caused by volume changes in the cathode and anode during lithiation and delithiation. To date, limited research has been devoted to understanding the spatio-temporal distribution of stresses during battery operation. Here, operando scanning high-energy X-ray diffraction to quantify cross-sectional axial stresses with a spatial resolution of 10 µm is used. It is shown how a non-monotonous stress distribution evolves over time during the cycling of the solid-state battery. In addition, degradation of the solid-state electrolyte in the vicinity of the lithium anode is observed and tracked periodic changes in the unit cell volume in the cathode. The presented methodology of tracking the chemo-mechanically induced stresses and interface morphology in real time in correlation with other battery parameters is believed, can provide a valuable platform for the future optimization of solid-state batteries.

Green Colloidal Synthesis of MoS2 Nanoflakes.

Currently, two approaches dominate the large-scale production of MoS2: liquid-phase exfoliation, referred to as the top-down approach, and bottom-up colloidal synthesis from molecular precursors. Known colloidal synthesis approaches utilize toxic precursors. Here, an alternative green route for the bottom-up synthesis of MoS2 nanoflakes (NFs) is described. The NFs were synthesized by colloidal synthesis using [Mo(CH3COO)2]2 and a series of sulfur (S)-precursors including thioacetamide (TAA), 3-mercaptopropionic acid (3-MPA), l-cysteine (L-CYS), mercaptosuccinic acid (MSA), 11-mercaptoundecanoic acid (MUA), 1-dodecanethiol (DDTH), and di-tert-butyl disulfide (DTBD). While TAA, an S-precursor most commonly used for MoS2 NF preparation, is a known carcinogen, the other investigated S-precursors have low or no known toxicity. High-resolution scanning transmission electron microscopy (HR-STEM) and grazing incidence wide-angle X-ray scattering (GIWAXS) confirmed that in all cases, the syntheses yielded single-layer MoS2 NFs with lateral sizes smaller than 15 nm and a well-defined crystal structure. Electronic absorption and Raman spectra showed characteristic features associated with the MoS2 monolayers. The evolution of the absorption spectra of the growth solution during the syntheses reveals how the kinetics of the NF formation is affected by the S-precursor as well as the nature of the coordinating ligands.

Tailored Langmuir-Schaefer Deposition of Few-Layer MoS2 Nanosheet Films for Electronic Applications.

Few-layer MoS2 films stay at the forefront of current research of two-dimensional materials. At present, continuous MoS2 films are prepared by chemical vapor deposition (CVD) techniques. Herein, we present a cost-effective fabrication of the large-area spatially uniform films of few-layer MoS2 flakes using a modified Langmuir-Schaefer technique. The compression of the liquid-phase exfoliated MoS2 flakes on the water subphase was used to form a continuous layer, which was subsequently transferred onto a submerged substrate by removing the subphase. After vacuum annealing, the electrical sheet resistance dropped to a level of 10 kΩ/sq, being highly competitive with that of CVD-deposited MoS2 nanosheet films. In addition, a consistent fabrication protocol of the large-area conductive MoS2 films was established. The morphology and electrical properties predetermine these films to advanced detecting, sensing, and catalytic applications. A large number of experimental techniques were used to characterize the exfoliated few-layer MoS2 flakes and to elucidate the formation of the few-layer MoS2 Langmuir film.

Evolution of Defects, Morphology, and Strain during FAMAPbI3 Perovskite Vacuum Deposition: Insights from In Situ Photoluminescence and X-ray Scattering.

At present, the power conversion efficiency of single-junction perovskite-based solar cells reaches over 26%. The further efficiency increase of perovskite-based optoelectronic devices is limited mainly by defects, causing the nonradiative recombination of charge carriers. To improve efficiency and ensure reproducible fabrication of high-quality layers, it is crucial to understand the perovskite nucleation and growth mechanism along with associated process control to reduce the defect density. In this study, we investigate the growth kinetics of a promising narrow bandgap perovskite, formamidinium methylammonium lead iodide (FAMAPbI3), for high-performance single-junction solar cells. The temporal evolution of structural and optoelectronic properties during FAMAPbI3 vacuum codeposition was inspected in real time by grazing-incidence wide-angle X-ray scattering and photoluminescence. Such a combination of analytical techniques unravels the evolution of intrinsic defect density and layer morphology correlated with lattice strain from the early stages of the perovskite deposition.

Improved Gas Sensing Capabilities of MoS2/Diamond Heterostructures at Room Temperature.

Molybdenum disulfide (MoS2) and nanocrystalline diamond (NCD) have attracted considerable attention due to their unique electronic structure and extraordinary physical and chemical properties in many applications, including sensor devices in gas sensing applications. Combining MoS2 and H-terminated NCD (H-NCD) in a heterostructure design can improve the sensing performance due to their mutual advantages. In this study, the synthesis of MoS2 and H-NCD thin films using appropriate physical/chemical deposition methods and their analysis in terms of gas sensing properties in their individual and combined forms are demonstrated. The sensitivity and time domain characteristics of the sensors were investigated for three gases: oxidizing NO2, reducing NH3, and neutral synthetic air. It was observed that the MoS2/H-NCD heterostructure-based gas sensor exhibits improved sensitivity to oxidizing NO2 (0.157%·ppm-1) and reducing NH3 (0.188%·ppm-1) gases compared to pure active materials (pure MoS2 achieves responses of 0.018%·ppm-1 for NO2 and -0.0072%·ppm-1 for NH3, respectively, and almost no response for pure H-NCD at room temperature). Different gas interaction model pathways were developed to describe the current flow mechanism through the sensing area with/without the heterostructure. The gas interaction model independently considers the influence of each material (chemisorption for MoS2 and surface doping mechanism for H-NCD) as well as the current flow mechanism through the formed P-N heterojunction.

Tunable Subnanometer Gaps in Self-Assembled Monolayer Gold Nanoparticle Superlattices Enabling Strong Plasmonic Field Confinement.

Nanoparticle superlattices produced with controllable interparticle gap distances down to the subnanometer range are of superior significance for applications in electronic and plasmonic devices as well as in optical metasurfaces. In this work, a method to fabricate large-area (∼1 cm2) gold nanoparticle (GNP) superlattices with a typical size of single domains at several micrometers and high-density nanogaps of tunable distances (from 2.3 to 0.1 nm) as well as variable constituents (from organothiols to inorganic S2-) is demonstrated. Our approach is based on the combination of interfacial nanoparticle self-assembly, subphase exchange, and free-floating ligand exchange. Electrical transport measurements on our GNP superlattices reveal variations in the nanogap conductance of more than 6 orders of magnitude. Meanwhile, nanoscopic modifications in the surface potential landscape of active GNP devices have been observed following engineered nanogaps. In situ optical reflectance measurements during free-floating ligand exchange show a gradual enhancement of plasmonic capacitive coupling with a diminishing average interparticle gap distance down to 0.1 nm, as continuously red-shifted localized surface plasmon resonances with increasing intensity have been observed. Optical metasurfaces consisting of such GNP superlattices exhibit tunable effective refractive index over a broad wavelength range. Maximal real part of the effective refractive index, nmax, reaching 5.4 is obtained as a result of the extreme field confinement in the high-density subnanometer plasmonic gaps.

Lithium-Induced Reorientation of Few-Layer MoS2 Films.

Molybdenum disulfide (MoS2) few-layer films have gained considerable attention for their possible applications in electronics and optics and also as a promising material for energy conversion and storage. Intercalating alkali metals, such as lithium, offers the opportunity to engineer the electronic properties of MoS2. However, the influence of lithium on the growth of MoS2 layers has not been fully explored. Here, we have studied how lithium affects the structural and optical properties of the MoS2 few-layer films prepared using a new method based on one-zone sulfurization with Li2S as a source of lithium. This method enables incorporation of Li into octahedral and tetrahedral sites of the already prepared MoS2 films or during MoS2 formation. Our results discover an important effect of lithium promoting the epitaxial growth and horizontal alignment of the films. Moreover, we have observed a vertical-to-horizontal reorientation in vertically aligned MoS2 films upon lithiation. The measurements show long-term stability and preserved chemical composition of the horizontally aligned Li-doped MoS2.

Nonequilibrium phases of nanoparticle Langmuir films.

We report on an in-situ observation of the colloidal silver nanoparticle self-assembly into a close-packed monolayer at the air/water interface followed by a 2D to 3D transition. Using the fast tracking GISAXS technique, we were able to observe the immediate response to the compression of the self-assembled nanoparticle layer at the air/water interface and to identify all relevant intermediate stages including those far from the equilibrium. In particular, a new nonequilibrium phase before the monolayer collapse via the 2D to 3D transition was found that is inaccessible by the competing direct space imaging techniques such as the scanning and transmission electron microscopies due to the high water vapor pressure and surface tension.

Thickness Dependence of Electronic Structure and Optical Properties of F8BT Thin Films.

Electronic devices based on polymer thin films have experienced a tremendous increase in their efficiency in the last two decades. One of the critical factors that affects the efficiency of polymer solar cells or light emitting devices is the presence of structural defects that controls non-radiative recombination. The purpose of this report is to demonstrate a non-trivial thickness dependence of optoelectronic properties and structure (dis)order in thin conductive poly(9,9-dioctyfluorene-alt-benzothiadiazole), F8BT, polymer films. The UV-Vis absorption spectra exhibited blue shift and peak broadening; significant changes in 0-0 and 0-1 radiative transition intensity was found in photoluminescence emission spectra. The density of state (DOS) was directly mapped by energy resolved-electrochemical impedance spectroscopy (ER-EIS). Satellite states 0.5 eV below the lowest unoccupied molecular orbital (LUMO) band were revealed for the thinner polymer films. Moreover, the decreasing of the deep states density in the band gap manifested an increment in the material structural ordering with increasing thickness. Changes in the ratio between crystalline phases with face-on and edge-on orientation of F8BT chains were identified in the films by grazing-incidence wide angle X-ray scattering technique. A thickness threshold in all investigated aspects of the films at a thickness of about 100 nm was observed that can be attributed to the development of J-H aggregation in the film structure and mutual interplay between these two modes. Although a specific structure-property relationship thickness threshold value may be expected for thin films prepared from various polymers, solvents and under different process conditions, the value of about 100 nm can be generally considered as the characteristic length scale of this phenomenon.

Evolution of Structure and Optoelectronic Properties During Halide Perovskite Vapor Deposition.

The efficiency of perovskite-based solar cells has increased dramatically over the past decade to as high as 25%, making them very attractive for commercial use. Vapor deposition is a promising technique that potentially enables fabrication of perovskite solar cells on large areas. However, to implement a large-scale deposition method, understanding and controlling the specific growth mechanisms are essential for the reproducible fabrication of high-quality layers. Here, we study the structural and optoelectronic kinetics of MAPbI3, employing in-situ photoluminescence (PL) spectroscopy and grazing-incidence small/wide-angle X-ray scattering (GI-SAXS/WAXS) simultaneously during perovskite vapor deposition. Such a unique combination of techniques reveals MAPbI3 formation from the early stages and uncovers the morphology, crystallographic structure, and defect density evolution. Furthermore, we show that the nonmonotonous character of PL intensity contrasts with the increasing volume of the perovskite phase during the growth, although bringing valuable information about the presence of defect states.

A wide-angle X-ray scattering laboratory setup for tracking phase changes of thin films in a chemical vapor deposition chamber.

The few-layer transition metal dichalcogenides (TMD) are an attractive class of materials due to their unique and tunable electronic, optical, and chemical properties, controlled by the layer number, crystal orientation, grain size, and morphology. One of the most commonly used methods for synthesizing the few-layer TMD materials is the chemical vapor deposition (CVD) technique. Therefore, it is crucial to develop in situ inspection techniques to observe the growth of the few-layer TMD materials directly in the CVD chamber environment. We demonstrate such an in situ observation on the growth of the vertically aligned few-layer MoS2 in a one-zone CVD chamber using a laboratory table-top grazing-incidence wide-angle X-ray scattering (GIWAXS) setup. The advantages of using a microfocus X-ray source with focusing Montel optics and a single-photon counting 2D X-ray detector are discussed. Due to the position-sensitive 2D X-ray detector, the orientation of MoS2 layers can be easily distinguished. The performance of the GIWAXS setup is further improved by suppressing the background scattering using a guarding slit, an appropriately placed beamstop, and He gas in the CVD reactor. The layer growth can be monitored by tracking the width of the MoS2 diffraction peak in real time. The temporal evolution of the crystallization kinetics can be satisfactorily described by the Avrami model, employing the normalized diffraction peak area. In this way, the activation energy of the particular chemical reaction occurring in the CVD chamber can be determined.

Optical Characterization of Few-Layer PtSe2 Nanosheet Films.

Thin films of transition-metal dichalcogenides are potential materials for optoelectronic applications. However, the application of these materials in practice requires knowledge of their fundamental optical properties. Many existing methods determine optical constants using predefined models. Here, a different approach was used. We determine the sheet conductance and absorption coefficient of few-layer PtSe2 in the infrared and UV-vis ranges without recourse to any particular model for the optical constants. PtSe2 samples with a thickness of about 3-4 layers were prepared by selenization of 0.5 nm thick platinum films on sapphire substrates at different temperatures. Differential reflectance was extracted from transmittance and reflectance measurements from the front and back of the sample. The film thickness, limited to a few atomic layers, allowed a thin-film approximation to calculate the optical conductance and absorption coefficient. The former has a very different energy dependence in the infrared, near-infrared, and visible ranges. The absorption coefficient exhibits a strong power-law dependence on energy with an exponent larger than three in the mid-infrared and near-infrared regions. We have not observed any evidence for a band gap in PtSe2 thin layers down to an energy of 0.4 eV from our optical measurements.

Kinetics of nanoparticle reassembly mediated by UV-photolysis of surfactant.

Real-time reassembly of an ordered nanoparticle monolayer due to UV-photolysis of the surfactant shell of nanoparticles was observed. The technique of grazing-incidence small-angle X-ray scattering provided the possibility to track in situ the nanoparticle pair correlation function of the sample processed in a UV-ozone reactor. The analysis revealed a total shift of approximately 1 nm of the nanoparticle nearest-neighbor distance. The temporal evolution of the interparticle distance proved to be the first-order process governed by the UV-photolysis and described by a single-exponential decay function. The nanoparticles tend to agglomerate into a labyrinth-like structure with a typical length scale of some 30 nm.

Development of pink-beam 4D phase CT for in-situ observation of polymers under infrared laser irradiation.

Four-dimensional phase computed tomography (4D phase CT) by an X-ray Talbot interferometer (XTI) with white synchrotron radiation has ever been demonstrated at a temporal resolution of about 1 s for soft-matter samples. However, the radiation damage to samples caused by white synchrotron radiation occasionally hampers our understanding of the sample dynamical properties. Based on the fact that XTI functions with X-rays of a bandwidth up to ca. 10% with performance comparable to that by monochromatic X-rays, filtering white synchrotron radiation to generate a 'pink-beam' of a 10% bandwidth is effective to reduce radiation damage without degrading the image quality and temporal resolution. We have therefore developed pink-beam 4D phase CT at SPring-8, Japan by installing a multilayer mirror with a 10% bandwidth and a 25 keV central photon energy. XTI optimal at this photon energy was built downstream, and a CMOS-based X-ray detector was used to achieve fast image acquisitions with an exposure time of 1 ms (or 0.5 ms) per moiré image. The resultant temporal resolution of pink-beam 4D phase CT was 2 s (1 s). We applied the pink-beam 4D phase CT to in-situ observation of polypropylene, poly(methyl methacrylate), and polycarbonate under infrared laser irradiation (1064 nm). The dynamics of melting, bubbling, and ashing were successfully visualized in 3D movies without problematic radiation damage by synchrotron radiation.

Tuning the orientation of few-layer MoS2 films using one-zone sulfurization.

Few-layer MoS2 films are promising candidates for applications in numerous areas, such as photovoltaics, photocatalysis, nanotribology, lithium batteries, hydro-desulfurization catalysis and dry lubricants, especially due to their distinctive electronic, optical, and catalytic properties. In general, two alignments of MoS2 layers are possible - the horizontal and the vertical one, having different physicochemical properties. Layers of both orientations are conventionally fabricated by a sulfurization of pre-deposited Mo films. So far, the Mo thickness was considered as a critical parameter influencing the final orientation of MoS2 layers with horizontally and vertically aligned MoS2 grown from thin (1 nm) and thick (3 nm) Mo films, respectively. Here, we present a fabrication protocol enabling the growth of horizontally or vertically aligned few-layer MoS2 films utilizing the same Mo thickness of 3 nm. We show that the sulfur vapor is another parameter influencing the growth mechanism, where a sulfurization with higher sulfur vapor pressure leads to vertical MoS2 layers and slow sulfur evaporation results in horizontally aligned layers for a thicker Mo starting layer.

Kinetics of Polymer-Fullerene Phase Separation during Solvent Annealing Studied by Table-Top X-ray Scattering.

Solvent annealing is an efficient way of phase separation in polymer-fullerene blends to optimize bulk heterojunction morphology of active layer in polymer solar cells. To track the process in real time across all relevant stages of solvent evaporation, laboratory-based in situ small- and wide-angle X-ray scattering measurements were applied simultaneously to a model P3HT:PCBM blend dissolved in dichlorobenzene. The PCBM molecule agglomeration starts at ∼7 wt % concentration of solid content of the blend in solvent. Although PCBM agglomeration is slowed-down at ∼10 wt % of solid content, the rate constant of phase separation is not changed, suggesting agglomeration and reordering of P3HT molecular chains. Having the longest duration, this stage most affects BHJ morphology. Phase separation is accelerated rapidly at concentration of ∼25 wt %, having the same rate constant as the growth of P3HT crystals. P3HT crystallization is driving force for phase separation at final stages before a complete solvent evaporation, having no visible temporal overlap with PCBM agglomeration. For the first time, such a study was done in laboratory demonstrating potential of the latest generation table-top high-brilliance X-ray source as a viable alternative before more sophisticated X-ray scattering experiments at synchrotron facilities are performed.