CaF2 Surface Research Articles

Interpretation of linear dichroism at S L2,3 x-ray absorption edges of small organic molecules at surfaces

Resonant soft x-ray absorption was applied at the S L2,3 (2p) edges of thin films of 1,4-benzenedimethanethiol (BDMT) on gold and of sexithiophene (6 T) on flat and patterned CaF2 surfaces. Linear dichroism effects were clearly observed depending on the orientation of the electric field vector of the incoming radiation. They were related to the organization of the molecules. For the interpretation of the angular dependencies of the spectral features, density functional theory (DFT) simulations of the absorption cross sections were calculated. It was possible to confirm the generally accepted chemisorption geometry of BDMT on Au(111), composed of molecules nearly vertical at the surface with one of the S atoms participating in the bonding with Au and the other left pending at the outermost surface and contributing mostly to the measured x-ray absorption. For 6 T on CaF2 a new chemisorption phase was identified on ridge-patterned CaF2(110), coexisting with a previously known phase given by 6 T crystallites highly oriented along the ridges. The new phase being composed of molecules oriented perpendicular to the ridges and lying on their edges.

Physical properties of fluorides barium and calcium nanopowders produced by the pulsed electron beam evaporation method

The mesoporous nanocrystal powders BaF2 and CaF2 with a specific surface up to 34.8 and 88.7 sq.m/g, respectively, are produced by the pulsed electron beam evaporation method in vacuum. The influence of thermal annealing of nanoparticles on air in the range of temperature from 200 to 900°C on the size, morphology of particles and change of their magnetic and luminescent properties investigated have been investigared. The essential stoichiometric impurity (overage of metals) and significant growth in a specific surface of nanopowders (NP) BaF2 and CaF2 after annealing at the temperature of 200°C have been detected. It is established that the synthesized NP BaF2 is a paramagnetic while initial material in the bulk state is diamagnetic. After annealing at 900°C appears the small ferromagnetic contribution at NP BaF2. Produced NP CaF2 showed ferromagnetic behavior. In literature there is no information about the ferromagnetism of CaF2. Appearance of the ferromagnetic response can be explained with formation of structural and radiation defects (F-centers, etc.). The analysis of PCL and magnetization curves of samples BaF2 and CaF2 allows drawing conclusions about their connection.

Electronic Structure of Single-Crystal CaF<sub>2</sub>(111) with Nanoscale Phases of Ca and Si

The impact of Ca and Si nano-scale structures on parameters and density of states of single-crystalline CaF2(111) was studied. It was shown that at low concentration of ions of Ar+ (D ≤ 5 × 1015 cm-2) one witnesses formation of nanoscale phases on CaF2 surface. It was revealed that these phases lead to narrowing of the forbidden band Еg between the phases by 4 - 4.5 eV. At higher concentrations (D ≈ 6 × 1016 cm-2) the surface completely is covered by Ca atoms. It was shown that deposition of θ = 10 thick Si single layer on CaF2 surface manifests island picture. The concentration of Ca and Si nano-scale phases on the surface of CaF2 and the band gap of the phases were investigated as a function of (hν) of passing light. Nano-scale phases and nano-scale films of Ca were obtained by using the technique of bombardment with ions of Ar+ of CaF2 surface. Formation of nano-scale phases were accompanied by change in the composition and structure of CaF2 zones located between the phases. These changes led to narrowing of the forbidden band of CaF2 down to 7.5 - 8 eV. The concentration of Ca and Si nano-scale phases on the surface of CaF2 and the band gap of the phases were investigated as a function of (hν) of passing light.

Epitaxial Ni nanoparticles on CaF2(001), (110) and (111) surfaces studied by three-dimensional RHEED, GIXD and GISAXS reciprocal-space mapping techniques

The development of growth techniques aimed at the fabrication of nanoscale heterostructures with layers of ferroic 3d metals on semiconductor substrates is very important for their potential usage in magnetic media recording applications. A structural study is presented of single-crystal nickel island ensembles grown epitaxially on top of CaF2/Si insulator-on-semiconductor heteroepitaxial substrates with (111), (110) and (001) fluorite surface orientations. The CaF2 buffer layer in the studied multilayer system prevents the formation of nickel silicide, guides the nucleation of nickel islands and serves as an insulating layer in a potential tunneling spin injection device. The present study, employing both direct-space and reciprocal-space techniques, is a continuation of earlier research on ferromagnetic 3d transition metals grown epitaxially on non-magnetic and magnetically ordered fluorides. It is demonstrated that arrays of stand-alone faceted nickel islands with a face-centered cubic lattice can be grown controllably on CaF2 surfaces of (111), (110) and (001) orientations. The proposed two-stage nickel growth technique employs deposition of a thin seeding layer at low temperature followed by formation of the islands at high temperature. The application of an advanced three-dimensional mapping technique exploiting reflection high-energy electron diffraction (RHEED) has proved that the nickel islands tend to inherit the lattice orientation of the underlying fluorite layer, though they exhibit a certain amount of {111} twinning. As shown by scanning electron microscopy, grazing-incidence X-ray diffraction (GIXD) and grazing-incidence small-angle X-ray scattering (GISAXS), the islands are of similar shape, being faceted with {111} and {100} planes. The results obtained are compared with those from earlier studies of Co/CaF2 epitaxial nanoparticles, with special attention paid to the peculiarities related to the differences in lattice structure of the deposited metals: the dual-phase hexagonal close-packed/face-centered cubic lattice structure of cobalt as opposed to the single-phase face-centered cubic lattice structure of nickel.

An attempt to apply the inelastic thermal spike model to surface modifications of CaF2 induced by highly charged ions: comparison to swift heavy ions effects and extension to some others material

Surface damage appears on materials irradiated by highly charged ions (HCI). Since a direct link has been found between surface damage created by HCI with the one created by swift heavy ions (SHI), the inelastic thermal spike model (i-TS model) developed to explain track creation resulting from the electron excitation induced by SHI can also be applied to describe the response of materials under HCI which transfers its potential energy to electrons of the target. An experimental description of the appearance of the hillock-like nanoscale protrusions induced by SHI at the surface of CaF2 is presented in comparison with track formation in bulk which shows that the only parameter on which we can be confident is the electronic energy loss threshold. Track size and electronic energy loss threshold resulting from SHI irradiation of CaF2 is described by the i-TS model in a 2D geometry. Based on this description the i-TS model is extended to three dimensions to describe the potential threshold of appearance of protrusions by HCI in CaF2 and to other crystalline materials (LiF, crystalline SiO2, mica, LiNbO3, SrTiO3, ZnO, TiO2, HOPG). The strength of the electron–phonon coupling and the depth in which the potential energy is deposited near the surface combined with the energy necessary to melt the material defines the classification of the material sensitivity. As done for SHI, the band gap of the material may play an important role in the determination of the depth in which the potential energy is deposited. Moreover larger is the initial potential energy and larger is the depth in which it is deposited.

Swift heavy ion irradiation of CaF2 – from grooves to hillocks in a single ion track

A novel form of ion-tracks, namely nanogrooves and hillocks, are observed on CaF2 after irradiation with xenon and lead ions of about 100 MeV kinetic energy. The irradiation is performed under grazing incidence (0.3°–3°) which forces the track to a region in close vicinity to the surface. Atomic force microscopy imaging of the impact sites with high spatial resolution reveals that the surface track consists in fact of three distinct parts: each swift heavy ion impacting on the CaF2 surface first opens a several 100 nm long groove bordered by a series of nanohillocks on both sides. The end of the groove is marked by a huge single hillock and the further penetration of the swift projectile into deeper layers of the target is accompanied by a single protrusion of several 100 nm in length slowly fading until the track vanishes.By comparing experimental data for various impact angles with results of a simulation, based on a three-dimensional version of the two-temperature-model (TTM), we are able to link the crater and hillock formation to sublimation and melting processes of CaF2 due to the local energy deposition by swift heavy ions.

Self-organisation processes and phase-structural states in hybrid metal-mesogenic nanosystems

ABSTRACTHybrid nanosystems including silver and copper and long-chain mesogenic derivatives of alkylcyanobiphenyls have been obtained by low temperature vacuum co-condensation of reagent vapors on cooled surfaces of quartz, KBr, CaF2 or polished copper under molecular beam conditions. Controlled thermal treatment of the samples allowed the directed formation of metal nanoparticles of definite size from 2 up to 100 nm. It was shown that manner of temperature treatment and molecular self-organisation of different liquid crystalline phases controlling of the size and morphology of nanoparticles and their aggregates formed in the system. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) data of the samples show the formation of orientationally ordered structures in nematic mesophases. Formation of flat 2D-aggregates has been found in layered smectic mesophases Optical absorbance spectra of silver/4-pentyl-4-cyanobiphenyl (Ag/5СВ) and copper/4-pentyl-4-cyanobiphenyl (Cu/5CB) samples encapsulated in polymer poly-para-xylylene at 300 K contain characteristic bands of plasmonic absorbance of metal nanoparticles at 420–440 nm and 560–600 nm. Rising of metal concentration in the sample led to the performable growth of rod-like metal particles with anisometric ratio l/d>20 and intensive absorbance at higher wavelengths (λ ≥ 650 nm)

Eu3+-doped CaF2 nanoparticles functionalized by salicylic acid: synthesis, structural, optical and morphological studies

Luminescent CaF2:Eu3+ nanoparticles sensitized by organic ligands have shown a great potential in the areas of optoelectronics. The present study reports the synthesis of salicylic acid functionalized CaF2:Eu3+ nanoparticles and their structural, optical and morphological properties have been discussed. The effect on photoluminescent properties with different doping concentrations of salicylic acid was explained by the energy transfer from salicylic acid to Eu3+ ion which resulted in the sensitization of luminescence with increase in the amount of salicylic acid up to at 2.5 mmol and then decreases with the addition of more salicylic acid due to concentration quenching. The change in the PL intensity has also been studied at different excitation wavelengths 248, 298 and 350 nm. The UV–Vis absorption spectra of the synthesized nanoparticles also show a slight red shift relative to the salicylic acid, suggests the coordination of salicylic acid to the surface of CaF2:Eu3+ nanoparticles. The FTIR spectra indicates the coordination of salicylic acid to the CaF2:Eu3+ nanoparticles and the transmission electron microscopy (TEM) depicts the morphology of the synthesized nanoparticles, indicating that the Eu3+ ion is surrounded by the dispersed salicylic acid functionalized CaF2:Eu3+ nanoparticles. The results indicate that salicylic acid can efficiently sensitize the luminescence of Eu3+ ion embedded CaF2 nanoparticles and these photoluminescent properties can be efficiently utilized for use in optoelectronic devices.

Band structure and decay channels of thorium‐229 low‐lying isomeric state for ensemble of thorium atoms adsorbed on calcium fluoride

AbstractThe results are presented on the study of the electronic structure of thorium atoms adsorbed by the liquid atomic layer deposition from aqueous solution of thorium nitrate on the surface of CaF2. The chemical state of the atoms and the change of the band structure in the surface layers of Th/CaF2 system on CaF2 substrate were investigated by XPS and REELS techniques. It was found that REELS spectra for Th/CaF2 system include peaks in the region of low energy losses (3‐7 eV) which are missing in the similar spectra for pure CaF2. It is concluded that the presence of the observed features in the REELS spectra is associated with the chemical state of thorium atoms and is caused by the presence of uncompensated chemical bonds at the Th/CaF2 interface, and, therefore, by the presence of unbound 6d‐ and 7s‐electrons of thorium atoms. Assuming the equivalence of the electronic configuration of thorium‐229 and thorium‐232 atoms, an estimate was made on the time decay of the excited state of thorium‐229 nuclei through the channel of the electron conversion. It was found that the relaxation time is about 40 μs for 6d‐electrons, and about 1 μs for 7s‐electrons. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hillock formation on CaF2, A12O3, c-SiO2 and MgO single crystal surfaces by ion impact - From potential energy deposition to electronic energy loss

SynopsisFor impact of very slow highly charged ions on single crystal surfaces the appearance of hillocks can be linked to a threshold in potential energy while for swift heavy ions a minimum electronic energy loss per unit length is necessary. Recent investigations on CaF2 in the medium energy range bridge the gap between these two extreme cases. These investigations are now extended to other target materials.

Experimental search for the low-energy nuclear transition in 229Th with undulator radiation

To search for the lowest energy nuclear isomeric transition in 229Th in solid samples, a novel adsorption technique which prepares 229Th atoms on a surface of CaF2 is developed. Adsorbed 229Th is exposed to highly intensive undulator radiation in the wavelength range between 130 and 320 nm, which includes the indirectly measured nuclear resonance wavelength 160(10) nm. After the excitation, fluorescence from the sample is detected with a VUV sensitive photomultiplier tube. No clear signal relating to the nuclear transition is observed and possible reasons are discussed.

Revealing Water Films Structure from Force Reconstruction in Dynamic AFM

The structure of water films in contact with surfaces has direct implications in many important interfacial processes, from biology to climatology, as well as in ice nucleation. Here we report on the detection of individual ice-like water layers adsorbed on surfaces in ambient conditions. Reconstructed force profiles obtained in amplitude modulation atomic force microscopy (AM-AFM) on top of (111) BaF2 surfaces, with a lattice constant close to the distance of facing water molecules in hexagonal ice (Ih), showed characteristic oscillations in the attractive regime with a periodicity of 3.7 A. This distance matches the thickness of a bilayer of Ih ice and is absent in force profiles on (111) CaF2 surfaces, which show a different lattice parameter. A thickness of 2.6 A is measured for the first water layer in contact with the surface, corresponding to a high-density liquid film structure predicted from calculations in the literature. Our results indicate that, although epitaxial Ih growth of the first water...

Capillary and van der Waals interactions on CaF2 crystals from amplitude modulation AFM force reconstruction profiles under ambient conditions.

There has been much interest in the past two decades to produce experimental force profiles characteristic of the interaction between nanoscale objects or a nanoscale object and a plane. Arguably, the advent of the atomic force microscope AFM was instrumental in driving such efforts because, in principle, force profiles could be recovered directly. Nevertheless, it has taken years before techniques have developed enough as to recover the attractive part of the force with relatively low noise and without missing information on critical ranges, particularly under ambient conditions where capillary interactions are believed to dominate. Thus a systematic study of the different profiles that may arise in such situations is still lacking. Here we employ the surfaces of CaF2, on which nanoscale water films form, to report on the range and force profiles that might originate by dynamic capillary interactions occurring between an AFM tip and nanoscale water patches. Three types of force profiles were observed under ambient conditions. One in which the force decay resembles the well-known inverse-square law typical of van der Waals interactions during the first 0.5–1 nm of decay, a second one in which the force decays almost linearly, in relatively good agreement with capillary force predicted by the constant chemical potential approximation, and a third one in which the attractive force is almost constant, i.e., forms a plateau, up to 3–4 nm above the surface when the formation of a capillary neck dominates the tip–sample interaction.

Extraction of pure spectral signatures and corresponding chemical maps from EPR imaging data sets: identifying defects on a CaF2 surface due to a laser beam exposure.

A calcium fluoride (CaF2) plate was exposed to pulsed laser irradiations inducing surface morphological and ionization changes on its surface. More precisely surface damages mainly correspond to intrinsic defects. Electron paramagnetic resonance (EPR) hyperspectral imaging is a powerful technique able to characterize the defects formed on the CaF2 surface. Indeed, EPR hyperspectral images provide spatial and spectral information about the sample studied. In fact, these images possess a great potential to obtain accurate and reliable knowledge about the chemical composition and the distribution of the component due to the presence of the spatial aspect. However, the complexity of such hyperspectral data sets imposes the use of advanced chemometric tools to extract valuable information on the considered physicochemical system. Therefore, Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) is proposed to identify and locate the different constituents in the images. The originality of this work is that it reports on the application of MCR-ALS, for the first time, on electron paramagnetic resonance (EPR) imaging data sets that will furnish the distribution maps and the spectral signatures of all components present in the sample. The results show the identification of different intrinsic defects on a CaF2 sample from the sole information in the raw image measurements and, therefore, confirm the potential of this methodology and the important role of spatial information contained in the image.

Wetting of selected fluorite surfaces by water

The interfacial chemistry features, particularly the wetting characteristics of selected fluorite (CaF2) surfaces (111, 110 and 100 crystal surfaces), were examined under nonequilibrium conditions (unsaturated solution), in the absence of reagents, including water contact angle measurements and interfacial water structure. Of special interest was the examination of the effect of crystal surface structure on the wetting characteristics of interfacial water molecules at CaF2 surfaces. Different contact angles were found for the three CaF2 surfaces, which have been attributed to the differences in water interaction at these crystallographic surfaces. Specifically, the low-energy (111) surface of CaF2 (the cleavage plane) has a contact angle of ~20°, whereas the (100) and (110) surfaces are completely hydrophilic, with a zero contact angle in each case. Detailed analysis of interfacial water structure was conducted under nonequilibrium conditions in order to understand the wetting characteristics of water molecules at the molecular level using molecular dynamics simulation as well as sum-frequency vibrational spectroscopy. For equilibrium conditions, when water is saturated with calcium fluoride, it appears that recrystallization occurs and the low-energy (111) surface state exists at all surfaces as revealed from contact angle measurements.

Erratum to: Effect of CaF2 Surface Layers on the Friction Behavior of Copper-Based Composite

Erratum to: Effect of CaF2 Surface Layers on the Friction Behavior of Copper-Based Composite

Structural plasticity of calmodulin on the surface of CaF2 nanoparticles preserves its biological function.

Nanoparticles are increasingly used in biomedical applications and are especially attractive as biocompatible and biodegradable protein delivery systems. Herein, the interaction between biocompatible 25 nm CaF2 nanoparticles and the ubiquitous calcium sensor calmodulin has been investigated in order to assess the potential of these particles to serve as suitable surface protein carriers. Calmodulin is a multifunctional messenger protein that activates a wide variety of signaling pathways in eukaryotic cells by changing its conformation in a calcium-dependent manner. Isothermal titration calorimetry and circular dichroism studies have shown that the interaction between calmodulin and CaF2 nanoparticles occurs with physiologically relevant affinity and that the binding process is fully reversible, occurring without significant alterations in protein secondary and tertiary structures. Experiments performed with a mutant form of calmodulin having an impaired Ca(2+)-binding ability in the C-terminal lobe suggest that the EF-hand Ca(2+)-binding motifs are directly involved in the binding of calmodulin to the CaF2 matrix. The residual capability of nanoparticle-bound calmodulin to function as a calcium sensor protein, binding to and altering the activity of a target protein, was successfully probed by biochemical assays. Even if efficiently carried by CaF2 nanoparticles, calmodulin may dissociate, thus retaining the ability to bind the peptide encompassing the putative C-terminal calmodulin-binding domain of glutamate decarboxylase and activate the enzyme. We conclude that the high flexibility and structural plasticity of calmodulin are responsible for the preservation of its function when bound in high amounts to a nanoparticle surface.

Experimental observations of temperature effects in the near-field regime of the Casimir–Polder interaction

We investigate the temperature dependence of the Casimir–Polder interaction on the electrostatic limit. This unusual phenomenon relies on the coupling between a virtual atomic transition and a thermal excitation of surface polariton modes. We first focus on the scenario in which a Cs(8P3/2) atom is next to a CaF2 or BaF2 surface. Our theoretical predictions show a strong temperature dependence of the van der Waals coefficient at experimentally accessible conditions. A series of spectroscopic measurements performed in a specially designed Cs vapor cell containing a CaF2 tube is presented. Our results illustrate the sensitivity of atom–surface interaction experiments to the quality and chemical stability of the surface material and emphasize the need for using more durable materials, such as sapphire. Finally, we discuss selective reflection experiments with Cs(7D3/2) in an all-sapphire cell that clearly demonstrate a temperature-dependent van der Waals coefficient.

First-principles simulations of H centers in CaF2

H center, a hole trapped at an interstitial anion site, placed in the bulk and the (111) surface of calcium fluoride CaF2, has been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. The H center orients the (111) direction for the bulk case and the (100) direction for the surface case, and the hole is mainly localized on the interstitial fluorine. The surface H center leads to a remarkable XY-translation of the surface atoms. Spin and hyperfine coupling calculations show a considerable interaction between the unpaired spin and the spin of neighboring nuclei, and the surface effect strengthens the spin polarization and hyperfine structure. The hole induces an empty level in the β-spin band gap, located 2.88eV above the valence band (VB) top, corresponding to the first optical absorption band, and the surface effect heightens the hole level considerably. Density of states (DOS) calculations reveal that the hole band mainly consists of the H-center p-orbitals, and the interstitial fluorine does the major contribution. Further study regarding the electron–hole pair, named FH pair in this paper, shows that the geometrical structure is similar to an F center and an H center paired together, whereas the hole localized on the H center in the isolated H-center case, moves to the fluorine vacancy (VF) site. The electron–hole pair induces seven defect levels in the VB–CB (conduction band) gap, three of them located near the Fermi energy, being occupied, and four of them located above the Fermi level, forming the hole bands. The p-orbitals of the interstitial fluorine form the three electron bands and the four hole bands are composed of the s- and p-orbitals of the VF.

Nonmetallic substrates for growth of silicene: an ab initio prediction

By means of first-principles calculations we predict the stability of silicene layers as buckled honeycomb lattices on Cl-passivated Si(1 1 1) and clean CaF2(1 1 1) surfaces. The van der Waals interaction between silicene and the inert substrate stabilizes the adsorbate system while not destroying the Si pz-derived linear bands forming Dirac cones at the Brillouin zone corners. Only small gaps of about 3 and 52 meV are opened.