Accurate detection of subsurface microcavity by bimodal atomic force microscopy.

Subsurface detection capability of bimodal atomic force microscopy (AFM) was investigated using the buried microcavity as a reference sample, prepared by partially covering a piece of highly oriented pyrolytic graphite (HOPG) flake with different thickness on a piece of a cleaned CD-R disk substrate. This capability can be manifested as the image contrast between the locations with and without the buried microcavities. The theoretical and experimental results demonstrated that the image contrast is significantly affected by the critical parameters, including the second eigenmode amplitude and frequency as well as local structural and mechanical properties of the sample itself. Specifically, improper parameter settings generally lead to incorrect identification of the buried microcavity due to the contrast reduction, contrast reversal and even disappearance. For accurate detection, the second eigenmode amplitude should be as small as possible on the premise of satisfying the signal-to-noise ratio and second eigenmode frequency should be close to the resonance frequency of the cantilever. In addition, the detectable depth is closely related to microcavity dimension (thickness and width) of the HOPG flake and local stiffness of the sample. These results would be helpful for further understanding of the detection mechanism of bimodal AFM and facilitating its application in nano-characterization of subsurface structures, such as the micro-/nano- channels to direct the flow of liquids in lab-on-a-chip devices.

Interlayer Structure Manipulation of FeOCl/MXene with Soft/Hard Interface Design for Safe Water Production Using Dechlorination Battery Deionization.

Suffering from the susceptibility to decomposition, the potential electrochemical application of FeOCl has greatly been hindered. The rational design of the soft-hard material interface can effectively address the challenge of stress concentration and thus decomposition that may occur in the electrodes during charging and discharging. Herein, interlayer structure manipulation of FeOCl/MXene using soft-hard interface design method were conducted for electrochemical dechlorination. FeOCl was encapsulated in Ti3C2Tx MXene nanosheets by electrostatic self-assembly layer by layer to form a soft-hard mechanical hierarchical structure, in which Ti3C2Tx was used as flexible buffer layers to relieve the huge volume change of FeOCl during Cl- intercalation/deintercalation and constructed a conductive network for fast charge transfer. The CDI dechlorination system of FeOCl/Ti3C2Tx delivered outstanding Cl- adsorption capacity (158.47 ± 6.98 mg g-1), rate (6.07 ± 0.35 mg g-1 min-1), and stability (over 94.49 % in 30 cycles), and achieved considerable energy recovery (21.14 ± 0.25 %). The superior dechlorination performance was proved to originate from the Fe2+/Fe3+ topochemical transformation and the deformation constraint effect of Ti3C2Tx on FeOCl. Our interfacial design strategy enables a hard-to-soft integration capacity, which can serve as a universal technology for solving the traditional problem of electrode volume expansion.

Operando Imaging of Over-Discharge-Induced Surface Morphology Evolutions of LiMn2O4 Submicron-Sized Particles by Electrochemical High-Speed Atomic Force Microscopy.

Spinel LiMn2O4 is a promising cathode material but suffers from severe capacity fading during battery operation. One of capacity fade mechanisms results from changes in its morphology and structure due to over-discharge. In this work, for the first time, we successfully tracked the morphologic evolution of LiMn2O4 submicron-sized particles during over-discharging by our home-made electrochemical high-speed atomic force microscopy (EC-HS-AFM). Seven hundred and sixty successive EC-HS-AFM images were stably captured at an imaging speed of ∼0.85 fps at corresponding potentials during over-discharging in ∼15 min, from which evolutions of nanoscale wrinkle-like and step-like structures on the particle surface were clearly observed. The phenomena could be resulted from the complex stresses due to structural distortion during the phase transformation from cubic (LiMn2O4) to tetragonal (Li2Mn2O4), and the formation of the Li2Mn2O4 phase was confirmed by ex situ XRD. Moreover, the particle surface area as a function of the potential was quantitatively extracted from the EC-HS-AFM images, revealing the irreversible expansion/contraction of the particles, and this finding obtained at the nanoscale was consistent with the macroscopic results tested by cyclic voltammetry and galvanostatic charge/discharge methods. These results demonstrate that the EC-HS-AFM is a powerful tool to establish the correlation between the over-discharge-induced surface morphology changes and irreversibility of the Li-ion insertion/extraction as well as capacity fading.

A novel design for the combination of electrochemical atomic force microscopy and Raman spectroscopy in reflection mode for in situ study of battery materials.

The emergence of functional materials, especially energy materials made up of various structures with different properties, requires the development of complementary or integrated characterization technologies. The combination of atomic force microscopy and Raman spectroscopy (AFM-Raman) offers a powerful technique for the in situ characterization of physical properties (AFM) and chemical composition (Raman) of materials simultaneously. To further extend the potential application in the battery's field, we here present an electrochemical AFM-Raman (EC-AFM-Raman) in the reflection mode, developed by designing a novel structure including water-immersion objective lens-based optics for high-sensitivity Raman excitation/collection, optical level detection for AFM imaging in the solution, and a dual-cell for electrochemical reaction. EC-AFM imaging and Raman measurement can be realized simultaneously. Dynamic morphologic evolution and phase transition of the LiMn2O4 particles during cyclic voltammetry measurement were successfully observed. This technique will provide the possibility of probing physicochemical phenomena of the battery materials and other surface/interface processes such as the formation of the solid electrolyte interphase layer.

Tracking Electrochemical-Cycle-Induced Surface Structure Evolutions of Cathode Material LiMn2O4 with Improved Operando Raman Spectroscopy.

Linking surface structure evolution to the capacity fading of cathode materials has been a problem in lithium ion batteries. Most of the strategies used to solve this problem are focused on the differences between the unaged and aged materials, leading to the loss of intermediate dynamic change information during cycling. Raman spectroscopy is a convenient, nondestructive, and highly sensitive tool for characterizing the surface/near-surface region structure. In this work, we improved an operando Raman system, which is able to record in situ and in real time a series of Raman spectra during charging/discharging cycles and is even able to record very weak Raman peaks without the use of SRES enhancement, which facilitates sample preparation. These series of Raman spectra revealed an inherent correlation between the electrode potential/Li content and the surface structure changes of the as-prepared pure LiMn2O4 film, including the biphase reaction, the evolution of the peroxo O-O bond, and the formation of the Mn3O4 surface phase. They were the first to show that the number of peroxo O-O bonds was decreased with an increasing number of cycles and that this decrease was accompanied by an increase in the Mn3O4 phase. With the help of the data measured by XPS, c-AFM, electrochemical testing equipment, and the calculation based on density functional theory, the causes of the capacity fading of the material are discussed. This work not only showed a direct correlation between the surface structure evolution and the capacity fading of the LiMn2O4 but also could provide an alternative operando Raman system that could be widely used for the in situ characterization of battery electrode materials.

Bimodal AFM-Based Nanocharacterization of Cycling-Induced Topographic and Mechanical Evolutions of LiMn2O4 Cathode Films.

Evolution of LiMn2O4 mechanical property during charge/discharge cycles is a critical issue because it is closely related to the performance of lithium-ion batteries. Extensive studies have been conducted by first-principles calculations/molecular dynamics simulation at the atomic level and by the nanoindentation technique at the micron scale. In this study, cycling-induced topographic and mechanical evolutions of the LiMn2O4 films are investigated at the nanoscale using the bimodal atomic force microscopy (AFM), which provides a complementary approach to bridge the gap between atomic-level calculation and micron-scale measurement. The topographic change and elastic modulus degradation of the LiMn2O4 films during the charge/discharge cycles are found to occur simultaneously and irreversibly. Moreover, a dramatic decrease in the elastic modulus of the films takes place at the first 10 cycles, which is consistent with the significant loss of the capacity and the change of the Coulombic efficiency measured by the galvanostatic method. By considering the nanoscale phenomena and the macroscopic measurement results, the reasons for the elastic modulus degradation are discussed. This study would be a valuable addition to a better understanding of the degradation mechanisms of this cathode material.

Development of electrochemical high-speed atomic force microscopy for visualizing dynamic processes of battery electrode materials.

Development of lithium ion batteries with ultrafast charging rate as well as high energy/power densities and long cycle-life is one of the imperative works in the field of batteries. To achieve this goal, it requires not only to develop new electrode materials but also to develop nano-characterization techniques that are capable of investigating the dynamic evolution of the surface/interface morphology and property of fast charging electrode materials during battery operation. Although electrochemical atomic force microscopy (EC-AFM) holds high spatial resolution, its imaging speed is too slow to visualize dynamics occurring on the timescale of minutes. In this article, we present an electrochemical high-speed AFM (EC-HS-AFM), developed by addressing key technologies involving optical detection of small cantilever deflection, dual scanner capable of high-speed and wide-range imaging, and electrochemical cell with three electrodes. EC-HS-AFM imaging from 1 fpm to ∼1 fps with a maximum scan range of 40 × 40 µm2 has been stably and reliably realized. Dynamic morphological changes in the LiMn2O4 nanoparticles during cyclic voltammetry measurements in the 0.5 mol/l Li2SO4 solution were successfully visualized. This technique will provide the possibility of tracking dynamic processes of fast charging battery materials and other surface/interface processes such as the formation of the solid electrolyte interphase layer.

Constructing an Adaptive Heterojunction as a Highly Active Catalyst for the Oxygen Evolution Reaction.

Electrochemical water splitting is of prime importance to green energy technology. Particularly, the reaction at the anode side, namely the oxygen evolution reaction (OER), requires a high overpotential associated with OO bond formation, which dominates the energy-efficiency of the whole process. Activating the anionic redox chemistry of oxygen in metal oxides, which involves the formation of superoxo/peroxo-like (O2 )n - , commonly occurs in most highly active catalysts during the OER process. In this study, a highly active catalyst is designed: electrochemically delithiated LiNiO2 , which facilitates the formation of superoxo/peroxo-like (O2 )n - species, i.e., NiOO*, for enhancing OER activity. The OER-induced surface reconstruction builds an adaptive heterojunction, where NiOOH grows on delithiated LiNiO2 (delithiated-LiNiO2 /NiOOH). At this junction, the lithium vacancies within the delithiated LiNiO2 optimize the electronic structure of the surface NiOOH to form stable NiOO* species, which enables better OER activity. This finding provides new insight for designing highly active catalysts with stable superoxo-like/peroxo-like (O2 )n - for water oxidation.

Visualization of Electrochemical Cycling-Induced Dimension Change in LiMn2O4 Nanoparticles by High-Speed Atomic Force Microscopy.

Exploring dynamic dimension change and lithium-ion diffusion kinetics of active nanoparticles is important to further improve the qualities of lithium-ion batteries (LIBs), such as the cycle life and charge rate. For advancing such research, an imaging technique that is capable of operating in an electrochemical environment with high spatial and temporal resolutions is really needed. In this work, we successfully developed electrochemical high-speed atomic force microscopy (EC-HS-AFM), which enabled nanoscale imaging at the rate of ∼1 frame/s during electrochemical cycling. The dimensional evolutions of LiMn2O4 single nanoparticles accompanying an insertion/extraction reaction of lithium ions were visualized. The surface area-potential hysteresis loops of the single nanoparticles at different sweep rates were quantitatively extracted from the successive HS-AFM images or video. The first-order derivative of the hysteresis loop was interestingly similar to the cyclic voltammetry (CV). Moreover, the EC-HS-AFM experiments confirmed that the utilization of the nanoparticles in the cathode can indeed improve the rate performance of the LIBs. These results demonstrated that EC-HS-AFM would be a promising tool to study dimensional evolutions and lithium-ion diffusion kinetics at a nanoscale.

In situ probing behaviors of single LiNiO2 nanoparticles by merging CAFM and AM-FM techniques.

Probing single active nanoparticles of Li-ion battery electrodes is challenging but important to reveal their behaviors including morphology, mechanical properties and electrochemical reactions with an electrolyte. In this work, we in situ investigated voltage-induced behaviors of single LiNiO2 nanoparticles by merging conductive atomic force microscopy (CAFM) and amplitude modulation-frequency modulation (AM-FM) techniques. The former was used to apply a voltage between a selected single nanoparticle and a substrate through its tip, while the latter was done for imaging. Evolution in the morphology and stiffness of the nanoparticles induced by different voltages under air and dried argon atmospheres was tracked, respectively. The evolution mechanisms related to electrochemical reactions were discussed in detail. These results suggest that the merged techniques would provide an indirect and effective approach to study the behaviors and electrochemical reactions of electrode materials on the nanometer scale and even single nanoparticles.

Multi-characterization of LiCoO2 cathode films using advanced AFM-based techniques with high resolution.

ABSTARCT: The thin film Li-ion batteries have been extensively used in micro-electronic devices due to their miniaturization, high capacity density and environmental friendliness, etc. In order to further prolong the lifetime of the film batteries, one of important tasks is to explore the aging mechanisms of the cathode films. In this paper, we especially focused on the multi-characterization of the LiCoO2 film in nanoscale, which is carried out by combining advanced AFM-based techniques with capacity measurement. The surface morphology, contact stiffness as well as surface potential were measured by amplitude modulation-frequency modulation (AM-FM) and kelvin probe force microscope (KPFM), respectively. Remarkable changes after different numbers of charge/discharge cycling were observed and the intrinsic reasons of them were discussed in detail. To acknowledge the relationship with these microscopic changes, the macro-capacity of the thin films was also measured by the galvanostatic charge/discharge method. These comprehensive results would provide a deep insight into the fading mechanism of the cathode film, being helpful for the design and selection of the cathode film materials for high performance batteries.

On-chip modulation for rotating sensing of gyroscope based on ring resonator coupled with Mach-Zehnder interferometer.

An improving structure for resonance optical gyro inserting a Mach-Zehnder Interferomete (MZI) into coupler region between ring resonator and straight waveguide was proposed. The different reference phase shift parameters in the MZI arms are tunable by thermo-optic effect and can be optimized at every rotation angular rate point without additional phase bias. Four optimum paths are formed to make the gyroscope to work always at the highest sensitivity.

Optical waveguides in TiO2 formed by He ion implantation.

We report on the formation and the optical properties of the planar and ridge optical waveguides in rutile TiO2 crystal by He+ ion implantation combined with micro-fabrication technologies. Planar optical waveguides in TiO2 are fabricated by high-energy (2.8 MeV) He+-ion implantation with a dose of 3 × 10¹6 ions/cm² and triple low energies (450, 500, 550) keV He+-ion implantation with all fluences of 2 × 10¹6 ions/cm² at room temperature. The guided modes were measured by a modal 2010 prism coupler at wavelength of 1539 nm. There are damage profiles in ion-implanted waveguides by Rutherford backscattering (RBS)/channeling measurements. The refractive-index profile of the 2.8 MeV He+-implanted waveguide was analyzed based on RCM (Reflected Calculation Method). Also ridge waveguides were fabricated by femtosecond laser ablation on 2.8 MeV ion implanted planar waveguide and Ar ion beam etching on the basis of triple keV ion implanted planar waveguide, separately. The loss of the ridge waveguide was estimated. The measured near-field intensity distributions of the planar and ridge modes are all shown.

High-efficiency second-harmonic generation in doubly-resonant χ(²) microring resonators.

By directly simulating Maxwell's equations via the finite-difference time-domain (FDTD) method, we numerically demonstrate the possibility of achieving high-efficiency second harmonic generation (SHG) in a structure consisting of a microscale doubly-resonant ring resonator side-coupled to two adjacent waveguides. We find that ≳ 94% conversion efficiency can be attained at telecom wavelengths, for incident powers in the milliwatts, and for reasonably large bandwidths (Q ∼ 1000s). We demonstrate that in this high efficiency regime, the system also exhibits limit-cycle or bistable behavior for light incident above a threshold power. Our numerical results agree to within a few percent with the predictions of a simple but rigorous coupled-mode theory framework.

Planar and ridge waveguides formed in LiNbO3 by proton exchange combined with oxygen ion implantation.

We report on the fabrication of planar and ridge waveguides in lithium niobate by proton exchange combined with oxygen ion implantation. The implanted energy ranges from 600 to 1400 keV with a dose of 1 x 10(15) ions/cm(2). The modes in proton exchanged waveguide can be modulated by O ion implantation. There are different damage profiles in proton-exchanged and ion-implanted waveguides in Rutherford backscattering/channeling spectra. The refractive index profile in single-mode waveguide in lithium niobate has been obtained based on Intensity Calculation Method. Also ridge waveguide was fabricated on the basis of planar waveguide by Ar ion beam etching. The measured near-field intensity distributions of the ridge waveguide modes show a reasonable agreement with the simulated ones. The estimated propagation loss was approximately 2.2 dB/cm.