Ionic Crystal Surfaces Research Articles

Atomic reconstructions and dynamic-evolutionary pathways on a polar surface of a wurtzite ionic crystal: Ga-doped ZnO(0001)

Preparing atomic reconstructions and tracking their evolutionary pathways on polar surfaces of wurtzite ionic crystals remain a big challenge in surface science. Here, we addressed this challenge by introducing Ga surface segregation through Ar+ bombardment and subsequent annealing treatment. By employing ZnO(0001) surface as a model system for wurtzite ionic crystals, we generated various atomic reconstructions and observed three distinct evolutionary pathways. These pathways involved complex dynamic processes including atom desorption and movement, and each pathway started with a disordered surface and ended with a (2 × 8) reconstruction. Tracking all the pathways, the overall reconstruction route can be put in order as “disorder → (√3 × √3)R30° → (2 × 2) → (2 × 4) → (2 × 8)”, as the surface Ga concentration is increased. These atomic reconstructions are proposed to originate from the alternations in the valence state of cation and the influence of surface segregation on the vacant states of dangling bonds. Our discoveries present a comprehensive framework for generating a multitude of atomic reconstructions and elucidating their evolutionary pathway on intricate polar surfaces of wurtzite ionic crystals.

Ultra-high vacuum cleaver for the preparation of ionic crystal surfaces.

Cleaving single crystals in situ under ultra-high vacuum conditions provides a reliable and straightforward approach to prepare clean and atomically well-defined surfaces. Here, we present a versatile sample cleaver to efficiently prepare ionic crystal surfaces under ultra-high vacuum conditions, which is suitable for preparation of softer materials, such as alkali halides, and harder materials, such as metal oxides. One of the advantages of the presented cleaver design is that the cleaving blade and anvil to support the crystal are incorporated into the device. Therefore, no particularly strong mechanical manipulator is needed, and it is compatible with existing vacuum chambers equipped with an xyz-manipulator. We demonstrate atomically flat terraces and the atomic structure of NaCl(001), KBr(001), NiO(001), and MgO(001) cleavage planes prepared in situ under ultra-high vacuum conditions and imaged by low-temperature non-contact atomic force microscopy.

Stability of Two-Dimensional Ionic Clusters at Solid-Liquid Interfaces.

The stability of two-dimensional clusters (2DCs) at the interface between ionic crystals and their solutions was investigated by molecular dynamics simulations. We found that 2DCs show a remarkable feature of odd-even alternation in stability. In NaCl and NaBr systems, the clusters containing an odd number of ions are more stable than those with an even number of ions, while in KCl systems, it is the other way round. Accordingly, the stability of water molecules in the first hydration shell of 2DCs also shows an odd-even alternation, which is consistent with the associated 2DCs. The odd-even alternation is discussed based on a competition mechanism between two factors: the Coulomb repulsion in charged 2DCs and the interaction between charges and water dipoles. Our discussion indicates that this odd-even alternation should be a universal feature in similar systems and would be important for understanding the nucleation and crystallization of solutions on ionic crystal surfaces.

Structure Analysis of Water Ice Crystallites on NaCl(001), KCl(001), and CaF2(111) by Reflection High-Energy Electron Diffraction

Interactions of water with ionic-crystal surfaces, such as NaCl(001), KCl(001), and CaF2(111), were investigated in terms of epitaxial ice growth and hydration based on diffraction patterns of tran...

Dominant Polar Surfaces of Colloidal II–VI Wurtzite Semiconductor Nanocrystals Enabled by Cation Exchange

Polar surfaces of ionic crystals are of growing technological importance, with implications for the efficiency of photocatalysts, gas sensors and electronic devices. Creation of ionic nanocrystals with large percentages of polar surfaces is an option to improve their efficiency in aforementioned applications but is hard to be accomplished because they are less thermodynamically stable and prone to vanish during the growth process. Herein we developed a strategy that is capable of producing polar surface dominated II-VI semiconductor nanocrystals including ZnS and CdS, from copper sulfide hexagonal nanoplates through cation exchange reactions. The obtained hexagonal prism-shaped wurtzite ZnS hexagonal nanoplates have dominant {002} polar surfaces, occupying up to 97.8% of all surfaces. Density functional calculations reveal the polar surfaces can be stabilized by a charge transfer of 0.25 eV/formula from the anion-terminated surface to the cation-terminated surface, which also explains the presence of polar surfaces in the initial Cu1.75S hexagonal nanoplates with cation deficiency prior to cation exchange reactions. Experimental results showed that the HER activity could be boosted by the surface polarization of polar surface dominated ZnS hexagonal nanoplates. We anticipate this strategy is general and could be used to other systems to prepare nanocrystals with dominant polar surfaces. Furthermore, the availability of colloidal semiconductor nanocrystals with dominant polar surfaces produced through this strategy open a new avenue for improving their efficiency in catalysis, photocatalysis, gas sensing and other applications.

Atomistic Simulation of Ice Nucleation on Silver Iodide (0001) Surfaces with Defects

Small particles of silver iodide (AgI) are known to have excellent ice nucleating capabilities and have been used in rain seeding applications. It is widely believed that the silver-terminated (0001) surface of β-AgI acts as a template for the basal plane of hexagonal ice. However, the (0001) surface of ionic crystals with the wurtzite structure is polar and will therefore exhibit reconstructions and defects. Here, we use atomistic molecular dynamics simulations to study how the presence of defects on AgI(0001) affects the rates and mechanism of heterogeneous ice nucleation at moderate supercooling at −10 °C. We consider AgI(0001) surfaces exhibiting vacancies, step edges, terraces, and pits and compare them to simulations of the corresponding ideal surface. We find that, while point defects have no significant effect on ice nucleation rates, step edges, terraces, and pits reduce both the nucleation and growth rates by up to an order of magnitude. The reduction of the ice nucleation rate correlates well with the fraction of the surface area around the defects where perturbations of the hydration layer hinder the formation of a critical ice nucleus.

Formation of Thin Films and Fine Single Crystals from Gas Phase on Ionic Crystal Surfaces under Heating and an Electric Field

The behavior of surfaces of ionic crystals of various crystallographic orientations and surfaces of cleavage under heating in an electric field was studied. On the surfaces, viscous liquid drops or fine single crystals were observed. The charge appearing on crystal surfaces resulted from depletion or saturation of the near-surface layers with positive ions that led to a stoichiometric change in the crystal substance. Single crystals were formed from ionic gas, which appeared due to the sublimation of matrix ions from cleaved surface.

Образование тонких пленок и малоразмерных монокристаллов из газовой фазы на поверхностях ионных кристаллов в условиях нагрева и электрического поля

The work investigated the behavior of surfaces of ionic crystals of different crystallographic orientation and surface of the chip in terms of heat and electric field. Changes on the surfaces of the samples are manifested in the form of formation of a viscous liquid or small-sized single crystals. On the surfaces of crystals, a charge is formed as a result of depletion or saturation of the near-surface layers of the crystal with positive ions, which leads to a stoichiometric change in the substance of the crystal. Single crystals are formed from ion gas, the appearance of which is initiated by sublimation of matrix ions from the surface of the chip.

Study of the potassium bromide surface after laser effects using x-ray photoelectron spectroscopy

The surface of KBr crystals outside and in the crater was examined using an x-ray photoelectron spectrometer (XPS). The comparative analysis of the XPS spectra showed that high-intensity laser irradiation has a significant impact on state and composition of the ionic crystal surface.

Compensating a polar surface

Surface Science An ionic crystal surface can be electrostatically unstable, and the surface must reconstruct in some way to avoid this “polar catastrophe.” Setvin et al. used scanning probe microscopies and density functional theory to study the changes in the polar surface of the perovskite KTaO3. They observed several structural reconstructions as the surface cleaved in vacuum was heated to higher temperatures. These ranged from surface distortions to the formation of oxygen vacancies to the development of KO and TaO2 stripes. Hydroxylation after exposure to water vapor also stabilized the surface. Science , this issue p. [572][1] [1]: /lookup/volpage/359/572?iss=6375

Polarity compensation mechanisms on the perovskite surface KTaO3(001).

The stacking of alternating charged planes in ionic crystals creates a diverging electrostatic energy-a "polar catastrophe"-that must be compensated at the surface. We used scanning probe microscopies and density functional theory to study compensation mechanisms at the perovskite potassium tantalate (KTaO3) (001) surface as increasing degrees of freedom were enabled. The as-cleaved surface in vacuum is frozen in place but immediately responds with an insulator-to-metal transition and possibly ferroelectric lattice distortions. Annealing in vacuum allows the formation of isolated oxygen vacancies, followed by a complete rearrangement of the top layers into an ordered pattern of KO and TaO2 stripes. The optimal solution is found after exposure to water vapor through the formation of a hydroxylated overlayer with ideal geometry and charge.

IR femtochemistry on the surface of wide-gap ionic crystals

We have found and studied a phenomenon of the growth of films resulting from decomposition of some organic and silicon-containing molecules adsorbed on the surface of ionic crystals under the action of IR (1.4–5.4 µm) femtosecond radiation of a moderate intensity, ~1011 W cm−2. In the gas phase, these molecules do not decompose. Microstructured films consisting of amorphous carbon, graphite oxide, and silicon dioxide have been obtained. The formation of carbon films was accompanied by the appearance of different hydrocarbons in the gas phase. The extensive films of graphite oxide have been obtained. The decomposition of molecules on the surface is apparently caused by non-resonant ionization and subsequent deep fragmentation. The mechanisms of ionization at relatively low intensities of the femtosecond IR radiation have been discussed.

Lattice-constant and electron-affinity effects on negative-ion conversion in atom–ionic-crystal-surface grazing scattering

The effects of the lattice constant and electron affinity on the negative-ion conversion of a neutral atom undergoing grazing scattering on an ionic-crystal surface over the complete velocity range were investigated. Here, a comparison of negative-ion conversion of neutral ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100),{\mathrm{F}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100)$, and ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KI}(100)$ surface systems shows that the pronounced difference in the efficiency of negative-ion formation between ${\mathrm{F}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100)$ and ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100)$ is caused by the large difference in their projectile electron affinities, whereas the difference between ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KI}(100)$ and ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100)$ is caused by the difference in their lattice constants.

Local Structure and Composition Variation at Surface of Lithium-Ion Conducting Solid Electrolyte

Recently, lithium ion conducting solid electrolytes attract many interests because of their potential application to all-solid-state batteries (ASSBs), which are supposed to be one of candidates of next-generation energy storage devices with high safety and reliability [1-3]. With great efforts of development of solid electrolytes, several materials with high lithium ionic conductivity have been reported, which facilitated hope of realization of ASSBs [2,4-8]. Now, researchers face several problems on application of solid electrolytes to ASSBs. Most of those problems are related to interfaces of between solid electrolytes and active materials, for example, contact areas, interdiffusion of elements on assembly and/or electrochemical processes of ASSBs. To solve these issues, much attention has been paid to design and fabrication techniques. In addition to the problems on the fabrication, several reports point out intrinsic phenomena related change in ion concentration at interfaces of solid electrolytes [1, 9, 10]. We have so far studied the local structure at the interfacial region by fabricating nano-composites of active materials and solid electrolytes, and confirmed that the interfacial region with distorted lattice ranges for at least 20-50 nm in thickness [9, 10]. Although more researches with variety of experimental analysis are required for further comprehension of the interfacial phenomena, difficulty in the analysis of solid/solid interface limits experimental tools. It is known that surface of solid electrolytes is also related to interfacial resistance. Kliewer proposed cation depletion at surface of ionic crystals when free energy of defect formation is lower for cation than for anion [11]. In addition, we observed that ball-milling of a poor ionic conductor (Li2SiO3, LSO) resulted in increase in ionic conductivity without change in activation energy [9, 12]. In this study, three different surface-sensitive techniques, grazing incidence X-ray diffraction (GIXD), attenuated total reflectance Fourier transform infrared (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS), were employed to analyze the local structure and composition around the surface region of a lithium ion conducting solid electrolyte sheet (NASICON-type in the system of Li-Al-Ti-Ge-Si-P-O, Ohara Inc. abbreviated as LICGC). It was revealed that the local structure and composition changed depending on depth from the top surface of the sheet. At the very surface, there was a layer with expanded lattice (Figure 1) and high Li composition (Figure 2). And with increasing the depth from the surface, lattice shrank sharply, and then expanded again gradually. The change in the lattice seemed to be accompanied by Li and Al composition. It was supposed that the change in the Li composition and structure is induced by combination of chemical reaction, segregation, and defects distribution [13].

The formation of carbonic and silicon dioxide structured films through the decomposition of molecules on the surface of ionic crystals under the action of IR femtosecond laser radiation

This study relates to the formation of carbon and silicon dioxide films that occurs as a result of the decomposition of organic and silicon-containing molecules on the surface of ionic crystals under IR femtosecond laser radiation of moderate intensity (~1011 W cm−2) without molecular decomposition in the gas phase. We found that transparent graphite oxide films formed in the case of CO2 molecule decomposition.

Surface energetics of the hydroxyapatite nanocrystal-water interface: a molecular dynamics study.

Face-specific interfacial energies and structures of water at ionic crystal surfaces play a dominant role in a wide range of biological, environmental, technological, and industrial processes. Nanosized, plate-shaped crystals of calcium phosphate (CaP) with nonideal stoichiometry of hydroxyapatite (HAP, ideal stoichiometry Ca10(PO4)6(OH)2) comprise the inorganic component of bone and dentin. The crystal shape and size contribute significantly to these tissues' biomechanical properties. Plate-shaped HAP can be grown in the presence of biomolecules, whereas inorganically grown HAP crystals have a needlelike shape. Crystal morphology reflects the relative surface areas of the faces and, for an ideal inorganically grown crystal, should be governed by the surface energies of the faces with water. Interfacial energies and dynamics also affect biomolecule adsorption. Obtaining face-specific surface energies remains experimentally challenging because of the difficulty in growing large HAP single crystals. Here we employed molecular dynamics (MD) simulations to determine nanocrystalline HAP-water interfacial energies. The (100) face was found to be the most favorable energetically, and (110) and (004) were less hydrophilic. The trend in increasing interfacial energy was accompanied by a decrease in the average coordination number of water oxygen to surface calcium ions. The atomic-level geometry of the faces influenced interfacial energy by limiting lateral diffusion of water and by interrupting the hydrogen bond network. Such unfavorable interactions were limited on (100) compared to the other faces. These results provide a thermodynamic basis for the empirically observed trends in relative surface areas of HAP faces. The penetration of charged biomolecules through the interfacial water to form direct interactions with HAP faces, thus potentially influencing morphology, can also be rationalized.

Dynamics of point defect formation, clustering and pit initiation on the pyrite surface

The collective behavior of point defects formed on the free surfaces of ionic crystals under redox conditions can lead to initiation of local breakdown by pitting. Here, we controllably generated sulfur vacancies on single crystal FeS2(100) through in vacuo annealing, and investigated the resulting evolution of surface chemistry using synchrotron x-ray photoelectron spectroscopy (XPS). By measuring the S 2p photoemission signal intensity arising from sulfur defects as a function of temperature, the enthalpy of formation of sulfur vacancies was found to be 0.1±0.03eV, significantly lower than the reduction enthalpy of bulk FeS2. Above 200°C, the created sulfur vacancies together with preexisting iron vacancies condensed into nm-scale defect clusters, or pits, at the surface, as evidenced by scanning tunneling microscopy (STM). We provide a mechanistic description for the initiation of pits that requires the concerted behavior of both the sulfur and iron vacancies, and validate this model with kinetic Monte Carlo (kMC) simulations. The model probes realistic length and time scales, providing good agreement with the experimental results from XPS and STM measurements. Our results mechanistically and quantitatively describe the atomic scale processes occurring at pyrite surfaces under chemically reducing environments, important in many natural and technological settings, ranging from its role as a passivating film in corrosion to its potential use as a photovoltaic absorber in solar energy conversion.

Controlled surface modification of various substrates with SnO2nanoparticles

We describe a simple, cost-effective approach for surface modification of various substrates with SnO2 nanoparticles by controlled hydrolysis, which is not confined on specific chemical states of the surface. Specifically, these SnO2 nanoparticles were loaded onto the surfaces of carbon-based materials (graphene), ionic crystals (tin sulfide), polymer materials (silk fiber) and biological materials (yeast) with uniform distribution, despite the great differences in surface chemistries. Moreover, the formation mechanism of the SnO2 nanoparticles has been discussed, confirming that the process described can be easily implemented and adapted to other systems. These as-synthesized nanocomposites are expected to have wide applications in the fields of gas sensing, photocatalysis and biomaterials.

Measuring Electric Field Induced Subpicometer Displacement of Step Edge Ions

We provide unambiguous evidence that the applied electrostatic field displaces step atoms of ionic crystal surfaces by subpicometers in different directions via the measurement of the lateral force interactions by bimodal dynamic force microscopy combined with multiscale theoretical simulations. Such a small imbalance in the electrostatic interaction of the shifted anion-cation ions leads to an extraordinary long-range feature potential variation and is now detectable with the extreme sensitivity of the bimodal detection.

Dipole-driven self-organization of zwitterionic molecules on alkali halide surfaces.

We investigated the adsorption of 4-methoxy-4′-(3-sulfonatopropyl)stilbazolium (MSPS) on different ionic (001) crystal surfaces by means of noncontact atomic force microscopy. MSPS is a zwitterionic molecule with a strong electric dipole moment. When deposited onto the substrates at room temperature, MSPS diffuses to step edges and defect sites and forms disordered assemblies of molecules. Subsequent annealing induces two different processes: First, at high coverage, the molecules assemble into a well-organized quadratic lattice, which is perfectly aligned with the <110> directions of the substrate surface (i.e., rows of equal charges) and which produces a Moiré pattern due to coincidences with the substrate lattice constant. Second, at low coverage, we observe step edges decorated with MSPS molecules that run along the <110> direction. These polar steps most probably minimize the surface energy as they counterbalance the molecular dipole by presenting oppositely charged ions on the rearranged step edge.