Surface Structural Transition Research Articles

Surface roughening and filling of nanoholes in Au surfaces under electron-beam irradiation

Electron-beam irradiation has been used to sinter nanocrystalline metals. Since the surface structure and its transitions control the migration behavior of surfaces, understanding surface structural transitions under electron-beam irradiation is an essential factor in the development of nanoscale sintering, such a study that, regrettably, has been missing. To elucidate the possibility of surface structural transitions, thin Au (1 0 0) and (1 1 0) surface specimens were irradiated at various current densities at an acceleration voltage of 300 keV by transmission electron microscopy. At a low current density, nanometer polygonal holes, which were composed of atomically sharp facets, were formed in both surfaces. At a higher current density, the holes became filled and, simultaneously, some of the facet surfaces of the holes were roughened. This surface structural transition with varying current density acted as the origin of the variation in hole behavior.

Surface-dependent stress-corrosion cracking in Ni-rich layered oxide cathodes

Structural degradation is the principal driving force for rapid voltage decay and capacity fading of Ni-rich layered oxide (NLO) cathode materials upon cycling, but its working mechanism is not yet fully elucidated. In this work, multi-scale electron microscopy/spectroscopy techniques and theoretical calculations are applied on both polycrystalline and single-crystal NLOs. We discover that both the intergranular and intragranular cracks initiate along polar (001) basal plane, while surface structure evolution and transition metal dissolution occur on nonpolar (104) fresh surface. A new chain stress corrosion mechanism from anisotropic elastic (001) tensile deformation, microcrack generation, nonpolar surface reconstruction, HF attack to metal dissolution is proposed to paint the full picture of the structural degradation of NLOs. This surface-dependent stress-corrosion coupling effect indicates that severe intergranular cracking that accumulates within the polycrystalline NLO aggregates accounts mostly for the fast voltage decay and capacity fading, whereas minor intragranular cracking and less surface damage lead to substantial improvements on cyclability and reversible capacity of single-crystal NLOs. The surface-dependent stress-corrosion cracking in both polycrystalline and single-crystal NLOs provides grain-boundary engineering clues on designing new cathode materials with high energy density and long cycle life.

Antibiotic Zwitterionic Nanogel Membrane: from Molecular Dynamics Simulation to Structure Manipulation.

Membrane separation has been considered as one of the most revolutionary technologies for the removal of oils, dyes, or other pollutants from wastewater. However, most membranes still face great challenges in water permeability, antifouling property, and even antibiotic ability. Possessing a pathogen-repellent surface is of great significance as it can enable membranes to minimize the presence of active viral pathogens. Herein, we demonstrate a distinct design with a molecular dynamics simulation-guided experiment for the surface domination of antibiotic zwitterionic nanogel membranes. The zwitterionic nanoparticle gel (ZNG)/Cu2+/glutaraldehyde (GA) synergy system is first simulated by introducing a ZNG into a preset CuCl2 brine solution and into a GA ethanol solution, in which the nanogel is observed to initially swell and subsequently shrink with the increase of GA concentration, leading to the membrane surface structure transition. Then, the corresponding experiments are performed under strict conditions, and the results suggest the surface structure transition from nanoparticles to network nanoflowers, which are consistent with the simulated results. The obtained network structure membrane with superhydrophilic and underwater superoleophobic abilities can significantly enhance the water permeability as high as almost 40% with its original rejection rate in comparison with unoptimizable ZNG-PVDF (polyvinylidene difluoride) membranes. Moreover, the obtained membrane achieves additional excellent antibiofouling capacity with the antibiotic efficiency exceeding 99.3%, manifesting remarkable potential for disinfection applications. By comparison, the conventional antibiotic methods generally improve the membrane's antibiotic property solely but can hardly improve the other properties of the membrane. That is to say, our simulation combined with the experimental strategy significantly improved the zwitterionic membrane property in this work, which provides a new perspective on the design of high-performance functional materials.

Fe Doping in LiMn1.5Ni0.5O4 by Atomic Layer Deposition Followed by Annealing: Depths and Occupation Sites

The development of high-voltage cathodes is very important to meet the pursuit of high energy and power density for next-generation lithium-ion batteries. LiMn1.5Ni0.5O4 (LMNO), as one of the most promising high-voltage cathodes, is still challenged by unstable surface chemistry, metal dissolution, and structure transition during charge/discharge cycling. In this work, a porous structure combined with Fe doping in LMNO was achieved by iron oxide (FeOx) atomic layer deposition (ALD) and postannealing. The formation of the porous structure was closely related to FeOx ALD, and the sites of Fe doping in LMNO spinel were also controlled by annealing temperature. After this modification, Li+ transport and cycling stability were both significantly improved.

Surface-Dependent Stress-Corrosion Cracking in Ni-Rich Layered Oxide Cathodes

Structural degradation is the principal driving force for rapid voltage decay and capacity fading of Ni-rich layered oxide (NLO) cathode materials upon cycling, but its working mechanism is not yet fully elucidated. Here we apply multi-scale electron microscopy/spectroscopy techniques and theoretical calculations on both polycrystalline and single-crystal NLOs, and describe their structural evolution upon cycling. We discover that both the intergranular and intragranular cracks initiate along polar (001) basal plane due to its large elastic anisotropy upon cycling and surface structure evolution and transition metal dissolution occur on nonpolar (104) fresh surface. With this surface-dependent stress-corrosion coupling effect, severe intergranular cracking that accumulates within the polycrystalline NLO aggregates accounts mostly for the fast voltage decay and capacity fading, whereas minor intragranular cracking and less surface damage lead to substantial improvements on cyclability and reversible capacity of single-crystal NLOs. A universal understanding of the surface-dependent degradation in both polycrystalline and single-crystal NLOs provides clues on designing new cathode materials with high energy density and long cycle life through grain-boundary engineering.

Coherent control of a surface structural phase transition.

Active optical control over matter is desirable in many scientific disciplines, with prominent examples in all-optical magnetic switching1,2, light-induced metastable or exotic phases of solids3-8 and the coherent control of chemical reactions9,10. Typically, these approaches dynamically steer a system towards states or reaction products far from equilibrium. In solids, metal-to-insulator transitions are an important target for optical manipulation, offering ultrafast changes of the electronic4 and lattice11-16 properties. The impact of coherences on the efficiencies and thresholds of such transitions, however, remains a largely open subject. Here, we demonstrate coherent control over a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system. A femtosecond double-pulse excitation scheme17-20 is used to switch the system from the insulating to a metastable metallic state, and the corresponding structural changes are monitored by ultrafast low-energy electron diffraction21,22. To govern the transition, we harness vibrational coherence in key structural modes connecting both phases, and observe delay-dependent oscillations in the double-pulse switching efficiency. Mode-selective coherent control of solids and surfaces could open new routes to switching chemical and physical functionalities, enabled by metastable and non-equilibrium states.

Molecular dynamics study of the removal mechanism of SiC in a fixed abrasive polishing in water lubrication

The mechanical removal mechanism of SiC substrates in the water-lubricated environment is investigated by molecular dynamics (MD) simulation. The surface quality, subsurface damage, removal efficiency and structural phase transition of SiC substrates were analyzed in a fixed abrasive polishing (FAP) under various water film thickness and external load. In water-lubricated environment, a small number of water molecules are pressed on the interface between the SiC substrates and the diamond abrasive. The results show that the largest pressing depth decreases with the increase of the water film thickness, contrary to the effect of the external load. The phase transition and the frictional heat both decrease as the pressing depth decreases, which further reduces the stress and temperature of the substrate in the FAP. The water layer has positive impact on the surface roughness and subsurface damage depth. Furthermore, the removal efficiency in the water-lubricated nano-abrading is lower than that in the dry process. Our researches benefit to understand the mechanical removal mechanism of SiC substrates under a water-lubricated environment and give theoretical guide for improving the machining technology of FAP.

CO2/N2 injection into CH4 + C3H8 hydrates for gas recovery and CO2 sequestration

CO2 injection into CH4 hydrate (sI) is a promising method for simultaneous energy production and global warming mitigation. In fact, the natural abundance of sII hydrates have been also confirmed. The gas exchange kinetics in sII hydrates were significantly different from those in CH4 hydrate. In this work, the injection of CO2 with or without N2 into CH4-C3H8 hydrate-bearing sediments was experimentally investigated to explore gas recovery and CO2 sequestration during the replacement. The results indicated that the CH4-CO2 exchange in large cages was always controlled by gas diffusion through newly-formed mixed hydrate layer. Differently, the CH4-N2 exchange in small cages changed from gas exchange-controlled to gas diffusion-controlled with the increase of N2 partial pressure. It was found that the recovery of CH4 was much more continuous than that of C3H8 in gaseous CO2/N2 replacement while CH4 and C3H8 were released synchronously in liquid CO2 replacement. CH4-C3H8 hydrate particles soaked in liquid CO2 may undergo almost complete structural transitions from sII to sI hydrate rich in CO2. The analysis of heterogeneous compositions of the replaced hydrates indicated that the presence of N2 may have inhibition effects on the structural transition of CH4-C3H8 hydrates. With the injection of gaseous CO2/N2, there was only a surface structure-transition followed by an iso-structural conversion in the deeper layers of hydrate particles. The structural transition played an important role in the replacement of sII hydrate, which made sII gas hydrate reservoirs to be promising target regions for efficient gas production and CO2 sequestration.

Ultrasound driven mound to self-affine surface structure transition in nanostructured films: one-pot route to cubic CuInS2 quantum dots with controlled structure and tunable band gap energy

In the current study, the first method for synthesis of 3D assemblies composed by close-packed QDs of the cubic modification of CuInS2in thin film form is presented. The developed one-pot synthetic approach enables high tunability of materials’ properties by combining ultrasonic irradiation of the reactor with the one-step colloidal chemical approach or by controlled post-deposition annealing of the deposited samples. The average crystal size of the material can be tuned from ~ 1 up to ~ 36 nm. In-depth studies of the structure, surface morphology and optical properties were carried out. Manifestations of size-quantization effects in the optical properties were analyzed in details. Analysis of the second-order statistical properties of thin films’ surfaces has shown that ultrasonically-supported synthesis enables generation of self-affine surfaces, while the colloidal chemical one leads to mounded surfaces. Such change of the surfaces’ character was attributed to heterogeneous sonochemical effects, leading ultimately to more efficient transport of the clusters formed in the bulk liquid and their improved energizing, enabling them to overcome various diffusion barriers on the growing surface. Sonochemically deposited films are characterized by notably larger values of the Hurst exponent than the chemically deposited ones, and an order of magnitude smaller correlation length. Smaller fractal dimensions in sonochemical films are in line with the smoother surfaces generate by ultrasonically-assisted route.

Surface structural phase transition induced by the formation of metal-organic networks on the Si(111)--In surface.

We studied the adsorption of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on the Si(111)- -In surface, a known surface superconductor. Scanning tunneling microscopy shows the development of a surface-confined metal-organic network (SMON) where TCNQ molecules coordinate with indium atoms from the underlying reconstruction. The formation of the SMON causes a surface structural phase transition from the reconstruction to a previously unknown 5 × 5 reconstruction of the Si(111)-In surface. Scanning tunneling spectroscopy measurements indicate that the 5 × 5 reconstruction has a stronger insulating character than the reconstruction. Density-functional-theory calculations are used to evaluate the atomic arrangement and stability of the 5 × 5 and reconstructions as a function of In coverage, and suggest that the structural phase transition is driven by a slight reduction of the In coverage, caused by the incorporation of indium atoms into the SMON.

The Mechanism of Impedance Increase in Nickel-Rich NMC Cathodes

The electrochemical performance of nickel-rich NMC cathode materials in Li-ion batteries depends on the kinetic control of side reactions at NMC-electrolyte/interface such as electrolyte oxidation, surface film formation, surface structure transformation and transition metal dissolution. However, the exact mechanism i.e. interdependence, sequence and impact of these interfacial phenomena is still poorly understood. The effect of surface reconstruction from layered- to spinel- or rock salt-phase(1, 2) in NMC materials during cycling on the kinetics of charge/discharge process remains unclear. As much as the exact chemical and physical properties of such surface reconstruction layer are still largely unknown, it is likely that it occurs simultaneously with other surface reactions and phenomena, which all together are difficult to deconvolute and evaluate. To understand the mechanism of the surface reconstruction layer formation in Ni-rich NMC material during battery storage and its possible effect on the electrode and cell impedance a surface modification was deliberately induced in NMC particles artificially a priori via exposure to the electrolyte at elevated temperatures. The reaction of the electrolyte with NMC powders and the phase reconstruction of NMC surface were probed by surface sensitive techniques, including Raman, IR, SEM and X-ray absorption spectroscopy in total electron yield mode (XAS-TEY). Accompanying electrochemical testing including electrochemical impedance spectroscopy (EIS) was performed with half-cell configuration coin cells. Through comparison of the surface chemical properties and electrochemical performance between pristine NMC and treated samples, we were able to demonstrate that this artificial reconstruction layer on NMC does not cause a critical resistance in the battery. The treatment was shown to demonstrate some positive effects on the performance: i) higher capacity retention and ii)less resistance growth during electrochemical cycling. Surface chemical analysis shows that treated electrodes had reduced amount of organic film formation and less subsequent growth of reconstruction layers during electrochemical cycling compared with pristine NMC, indicating that an artificial reconstruction layer might suppress other degradation process of the battery, such as film formation and metal dissolution. Acknowledgement This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Reference 1. F. Lin, I. M. Markus, D. Nordlund, T. C. Weng, M. D. Asta, H. L. L. Xin and M. M. Doeff, Nat Commun, 5(2014). 2. S. K. Jung, H. Gwon, J. Hong, K. Y. Park, D. H. Seo, H. Kim, J. Hyun, W. Yang and K. Kang, Adv Energy Mater, 4 (2014).

The Evidence of Giant Surface Flexoelectric Field in (111) Oriented BiFeO3 Thin Film.

In this work, the surface structure of a single-domain epitaxial BiFeO3 film with (111) orientation was investigated by in situ grazing incidence X-ray diffraction and X-ray reflectivity. We found that a large strain gradient exists in the surface region (2-3 nm) of the BiFeO3 film. The strain gradient is approximately 107 m-1, which is 2 or 3 orders of magnitude larger than the value inside the film. Moreover, we found that a surface layer with a lower electron density compared with the underlying BiFeO3 layer exists on the surface of BiFeO3 film, and this layer exhibits an irreversible surface structure transition occurs at 500 K, which should be associated with the surface flexoelectric field. We considered that this large strain gradient is originated from the surface depolarization field of ferroelectrics. Our results suggest a coupling between the surface structure and the flexoelectricity and imply that the surface layer and properties would be controlled by the strain gradient in ferroelectric films.

Top surface modification of carbon film on its structure, morphology and electrical resistivity using electron-ion hybrid irradiation in ECR plasma

A two-step electron-ion hybrid irradiation process in electron cyclotron resonance (ECR) plasma was proposed as a flexible method to obtain carbon film with low roughness and electrical conductive surface. By the combination of either electron irradiation or ion irradiation during and after film deposition, two modes of hybrid irradiation were realized: In i-e hybrid mode, smooth amorphous carbon film was firstly deposited under ion irradiation, and its electrical resistivity was reduced from 1.4 to 0.12Ω·cm−1 after 5-min electron irradiation. In e-i hybrid mode, conductive graphene nanocrystallited film was firstly deposited, and its surface Ra roughness was decreased from 15.7 to 0.063nm after 5-min ion irradiation. The top surface structural transition under electron irradiation in the i-e mode were demonstrated by transmission electron microscopy (TEM), Raman spectra, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), and the effective depth of electron irradiation on carbon film surface were discussed. This study provided a practical route to the massive production of ultra-smooth carbon films with novel nanostructure and outstanding physical properties for potential applications in nano-machinery and nano-devices. The hybrid irradiation method also expanded the way of plasma utilization in the fields of film deposition and surface modification, and will inspire various new films and coatings with improved top surfaces.

Surface Structural Transition Induced by Gradient Polyanion‐Doping in Li‐Rich Layered Oxides: Implications for Enhanced Electrochemical Performance

Lithium‐rich layered oxides (LLOs) exhibit great potential as high‐capacity cathode materials for lithium‐ion batteries, but usually suffer from capacity/voltage fade during electrochemical cycling. Herein, a gradient polyanion‐doping strategy is developed to initiate surface structural transition to form a spinel‐like surface nanolayer and a polyanion‐doped layered core material in LLOs simultaneously. This strategy integrates the advantages of both bulk doping and surface modification as the oxygen close‐packed structure of LLOs is stabilized by polyanion doping, and the LLO cathodes are protected from steady corrosion induced by electrolytes. A LLO material modified with 5 at% phosphate (5%P@LLO) shows a high reversible discharge capacity of ≈300 mAh g−1 at 0.1 C, excellent cycling stability with a capacity retention of 95% after 100 cycles, and enhanced electrode kinetics. This gradient doping strategy can be further extended to other polyanion‐doped LLO materials, such as borate and silicate polyanions.

Effect of Defects on Decay of Voltage and Capacity for Li[Li0.15Ni0.2Mn0.6]O2 Cathode Material.

Lithium-rich manganese metal layered oxides are very promising cathode materials for high-energy-density lithium-ion batteries, but improvement in voltage decay and capacity fade is a great challenge, which is mainly related to the structural instability or reconstruction of material's surface. Defects, such as part lattice distortions, local cation disordering and atomic ununiformity, often aggravate the further structural changes upon cycling. In this paper, we found that PEG contributed to form better layered structure, well crystallinity, uniform composition and polyhedral nanoparticles for Li[Li0.15Ni0.2Mn0.6]O2 (LNMO). On the basis of the comparative trial, a mechanism of electronegativity difference is proposed to elucidate cation nonuniform distribution. Higher electronegativity of Ni (1.91) than Mn (1.55) show a stronger ability of attraction between Ni and O atoms, and then led to Ni atoms show stronger diffusion driving force toward particle surface to contact the rich O atoms during sintering in air. However, PEG polymer can form a better barrier for more O atoms to attract Ni and Mn atoms on particle surface so that facilitated a uniform distribution. The electrochemical test indicated that the decay of discharge capacity and working voltage was mitigated, which was identified by the result of HRTEM analysis that the initial less defect structure obviously retarded the phase transformation from the layered to spinel after 50 cycles. Therefore, defects are crucial for understanding the voltage fade and capacity decay, and the improvement of performance also demonstrates that designing optimum compositions and ordering atomic arrangements will contribute to stabilize the surface structure and restrain inherent phase transitions.

Enhancement on the Cycling Stability of the Layered Ni-Rich Oxide Cathode by In-Situ Fabricating Nano-Thickness Cation-Mixing Layers

The layered Ni-rich oxide cathode (LiNi0.8Co0.1Mn0.1O2) has been significantly attractive due to its large reversible capacity (200 mAh g−1). However, the layered Ni-rich oxide cathode suffers from a tremendous structural degradation during long-term cycling, causing the drastic rise of electrode impedance and deterioration of the capacity retention. In this paper, we develop a novel and facile approach to stabilize its structure by per-cycling of the layered Ni-rich oxide cathode at high voltage of 4.5 V (vs Li/Li+). The nano-thickness cation-mixing layers with Fm-3m structure were in-situ formed on the layered Ni-rich oxide LiNi0.8Co0.1Mn0.1O2 cathode by the surface structural transition from rhombohedral phase to cubic phase. The as-prepared LiNi0.8Co0.1Mn0.1O2 cathode coated with the nano-thickness cation-mixing layers shows superior cycling life at a cutoff voltage of 4.3 V, and it keeps a capacity retention of 83% after 500 cycles, which could be attributed to that the nano-thickness cation-mixing layers stabilize the structure of LiNi0.8Co0.1Mn0.1O2 during long-term cycling.

Band-gap tunability and dynamical instability in strained monolayer and bilayer phosphorenes

Very recently, field-effect transistors based on few-layer phosphorene crystals with a thickness of down to a few nanometres were successfully fabricated, triggering interest in this new functional two-dimensional material. In this work, we apply first-principles calculations to studying the evolution of electronic and phononic structures with out-of-plane strain for monolayer and bilayer phosphorenes. It is found that the vertical stress can be used to tune the band gap of a semiconducting phosphorene in a wide range. On the other hand, the vertical stress can make the phosphorene lattice become dynamically unstable and surface reconstruction or structural phase transition may occur. Due to the interlayer van der Waals coupling, the dynamically stable range of bilayer phosphorene under vertical stress is wider than that of monolayer phosphorene. It is proposed whether or not a semiconductor-semimetal transition occurring in a strained phosphorene is determined not only by its band gap closing, but also by its lattice stability against strain. This information is essential for the strain engineering of phosphorene and future device fabrication.

Temperature Dependence of Surface Polar State of SrTiO3 Ceramics Obtained by Piezoresponse Force Microscopy

The existence of polar state on the surface of strontium titanate ceramics was obtained by piezoresponse force microscopy (PFM) in temperature range from 8 to 295 K. This polar state is nonswitchable and relaxes fast at room temperature. This fact is attributed to charge injection from tip, which provoked the formation of oxygen vacancies in subsurface layer. Low temperature dynamics of surface piezoresponse reveals an increasing of PFM contrast at 110–130 K, associated with surface structural phase transition. Temperature dependence of the average level of piezoresponse signal exhibits two peaks due to bulk structural phase transition and coherent quantum state.

Deposition of Fluorine-Containing Thin Film by Atmospheric Pressure Plasma Jet and Film Surface Structural Transition

Plasma polymerized fluorine-containing (SiOxCyFz) thin films were deposited with an atmospheric pressure plasma jet, a useful technique to form thin films of fluorocarbon/silica nanohybrids. This atmospheric pressure plasma polymerization was fed with argon/1H,1H,2H,2H-perfluorooctyltriethoxysilane mixtures. AFM analysis found the nano-textured SiOxCyFz thin films obtained a high degree of surface roughness with an average size of 30 nm. FTIR and XPS found varying working distances in their chemical structure transitions. As the working distance increased, the fluorine compositions of the SiOxCyFz films increased from 12 to 50%, and at a greater working distance, the chemical structure of the atmospheric pressure plasma polymerized thin films was converted to the fluorine-containing dominion. The dielectric constant property of the SiOxCyFz thin film was examined by ellipsometry. From experimental results indicated, the decrease in the dielectric constant of the SiOxCyFz thin films can be attributed a decreased electronic polarization. This investigation demonstrates that the working distance of atmospheric pressure plasma polymerization affords good control of fluorine-containing film properties.

AFM analysis of the surface morphology, structure, and mechanical properties of methylcellulose mixtures with colloidal silver dispersions

The surface morphology, structure, mechanical properties, and physical transition temperatures of film compositions between methylcellulose (MC) and colloidal silver dispersions (CSDs) stabilized by poly-N-vinylpyrrolidone (PVP) are studied via atomic-force microscopy (AFM), X-ray diffraction and thermomechanical analyses, and mechanical tests. It is established that composite films exhibit the characteristic granular (nanodomain) surface morphology. When the CSD content in the compositions increases, the degree of ordering of MC macromolecules enhances, grain sizes decrease from ∼75 to ∼60 nm (at 3 and 20 wt % of the CSD, respectively), and the film surface relief becomes smoother. For example, the surface roughness of composite films is three times less than that of the initial MC film. In other words, owing to CSD introduction, film compositions are structurized at the nanolevel. In this case, Young’s modulus and the yield stress of composite films increase successively. High-temperature heating increases the average nanodomain size to 120–180 nm (for mixtures with 20 wt % of the CSD) and stimulates the local ordering of MC macromolecular fragments with sizes of ∼3–5 nm.