LiF Single Crystals Research Articles

Low-temperature excitonic, electron–hole and interstitial-vacancy processes in LiF singlecrystals

The emission spectra and the excitation spectra of various emissions have been measured inLiF crystals at 9 K using VUV radiation of 10–33 eV. In contrast to the luminescence ofself-trapped excitons (3.4 eV), the efficiency of several extrinsic emissions (4.2, 4.6 and5.8 eV) is very low in the region of an exciton absorption (12.4–14.2 eV). A single excitingphoton of 28–33 eV is able to create a primary electron–hole (e–h) pair and a secondaryexciton. The tunnel phosphorescence has been detected after the irradiation of LiF byan electron beam or x-rays at 6 K, and several peaks of thermally stimulatedluminescence (TSL) at 12–170 K appeared at the heating of the sample. It wasconfirmed that the TSL at 130–150 K is related to the diffusion of self-trapped holes(VK centres). TheTSL peak at ∼160 K is ascribed to the thermal ionization ofF′ centres. The TSL at 20–30 K and 50–65 K is caused by the diffusion of interstitialfluorine ions (I centres) or H interstitials, respectively. The TSL peak at∼13 K, the most intense after electron or x-irradiation, cannot be detected after LiFirradiation by VUV radiation, selectively forming excitons or e–h pairs. Thecreation of a spatially correlated anion exciton and an e–h pair is needed forthe appearance of this peak: an exciton decays into an F–H pair, a hole forms aVK and an electron transformsH into I (an F–I–VK group is formed) or an F centre into a two-electronF′ centre(an F′-H -VK group). The analysis of the elementary components of the 9–16 K TSLshowed that a phonon-induced radiative tunnel recombination ofF′–VK (5.6 eV),F–H (∼3 eV)and F–VK (3.4 eV) occurs within these groups.

Silicon strip detector applied to X-ray diffractometer: Angular resolution and counting rate

The silicon strip detector (128 strips with 100 μm strip pitch) was mounted at the arm of laboratory X-ray diffractometer with 230 mm instrument radius. With the use of a LaB 6 standard sample we demonstrate the angular resolution of 0.05° at low angles for classical Bragg–Brentano configuration. Experiments with a large LiF single crystal makes it possible to experimentally determine the detector limit range of linearity (30 000 cps/strip) and maximum counting range (100 000 cps/strip).

Selective creation of colour centres and peaks of thermally stimulated luminescence by VUV photons in LiF single crystals

Processes of radiation creation and annealing of Frenkel defects as well as electron-hole processes have been studied in LiF single crystals with a various content of impurity ions by means of highly sensitive method of thermally stimulated luminescence (TSL) and other optical methods. In highly pure LiF crystals, X-irradiated at 4.2 K, the TSL peaks connected with the annealing of interstitial fluorine ions (25–40 K) or atoms, i.e. H centres (50–65 K) and self-trapped holes (120–140 K) have been separated. For the first time, the creation spectra of the TSL peaks at 480 and 550 K by 10–33 eV-photon irradiation at 295 K have been measured. The anomalously high creation efficiency of the TSL peak at 480 K by 11.7–12.3 eV and 26–27 eV photons is interpreted as the creation of near-impurity electronic excitations both, directly by photons and by hot conduction electrons. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Instability of Plastic Flow in the Microstructure of Alkali Halide Crystals

Spatial localization of deformation bands in LiF and KCl single crystals caused by instability of plastic flow in the strain rate range from 5 × 10−6 to 2 × 10−4 s−1 was studied experimentally. The geometrical parameters of localized shift bands (LSB) were studied as a function of strain rate and temperature. To study the LSB relief, a surface profilometry technique was used for the first time, which made it possible to determine the LSB parameters at the early stages of plastic flow (for strains in the range from 0.5 to 2%). The formation and branching of LSB steps on the surface of a deformed crystal due to the generation and motion of dislocations were found to be scaled. It was shown experimentally that the LSB formation is a thermally activated process that occurs through dislocation glide and is limited by dislocation creep.

X-ray Diffraction Study of Low- and High-Angle Misorientations in LiF Crystals during Creep

Low-and high-angle lattice misorientations in LiF single crystals subjected to tensile deformation in the temperature range 573–973 K were studied using the following x-ray diffraction techniques: the ω-scanning method, the Laue method, and the Kossel and Fujiwara widely divergent beam (WDB) methods. The fraction of the high-angle misorientation is shown to be low at low temperature, where a block structure is absent, and at high temperature, where blocks are large (∼100 μm). The high-angle misorientation increases with strain faster than the low-angle misorientation. The misorientations measured by different techniques can differ because of the different areas which are illuminated by an x-ray beam and over which the misorientations are averaged. The results obtained are compared with available data on the structure of deformed alkali-halide crystals.

Healing of cracks in alkali halide crystals

The healing of micro-and macrocracks under local heating and x-ray irradiation of LiF single crystals is studied. The main features revealed in crack healing due to local actions are described. The contribution of the plastic zone formed upon the arrest and healing of a crack to the strength of the crystal is estimated.

Color center production by femtosecond pulse laser irradiation in LiF crystals

We report the production of color centers in LiF single crystals by ultrashort high intensity laser pulses (60fs, 10 GW). An intensity threshold for color centers creation of 2 TW/cm(2) was determined, which is slightly smaller than the continuum generation threshold. We could identify a large amount of F centers that gave rise to aggregates such as F(2), F(2) (+) and F(3) (+). The proposed mechanism of formation is based on multiphoton excitation that also produce short lived F(2) (+) centers. It is also shown that it is possible to write tracks in the LiF crystals with dimensional control.

Quantification of Fluorine Density in the Outermost Atomic Layer

The outermost atomic layer of perfluorinated thiol monolayers on gold and poly(tetrafluoroethylene) (PTFE) is analyzed by low-energy ion scattering. Absolute quantification of fluorine density in this layer was achieved after calibrating the fluorine signal with a freshly cleaved LiF(100) single crystal. The fluorine density of monolayers of a C8F17-thiol on gold was 1.48 x 10(15) F atoms/cm2, whereas for PTFE a value of 1.24 x 1015 F atoms/cm2 was observed. This difference was explained by the different tilt angles of the thiol on gold and PTFE chains with respect to the surface normal. Both a configurational and a molecular interpretation on the perfluorinated thiol monolayer on gold are given.

Quantum properties of nanoscale metallic Li colloids formed by electron irradiation in LiF

We present experimental results on the nucleation of nanoscale metallic lithium colloids, of well-controlled diameter (2–5 nm), in MeV electron-irradiated LiF single crystals. Conduction electron spin resonance experiments show a clear-cut quantum effect in these colloids: the spin susceptibility follows a mixed Curie–Pauli law, characteristic for tiny metallic particles in which the mean level spacing is comparable to kT. The behavior of the spin relaxation times ( T 1 and T 2) is presented and a discussion of quantum size effect in small metallic particles is proposed.

Track formation on LiF single crystal surface induced by high-energy Xe ions

Energetic-ion-induced surface and island tracks (i.e. tracks in island gold film) on the surface of LiF single crystal were studied by electron microscopy. Irradiations were carried out by 4.17 MeV/nucleon Xe ions at incidence angles of 1°, 2°, 3°, 6°, 20°, 50° and 75° relative to the crystal surface. The size and shape of the observed surface and island tracks were compared. At the three most grazing irradiations we applied upper and lateral collectors in order to compare the total amount of removed islands with the collected ones. Collectors were studied by both electron microscopy and atomic force microscopy. We discuss our observations in the framework of some of the existing sputtering mechanisms.

The role of vacuum ultraviolet in H 2 plasma treatment on SiO 2 aerogel film

SiO 2 aerogel film has been drawn an attention as excellent dielectric material due to its ultra low dielectric constant. SiO 2 aerogel film could exhibit the improved electrical property after plasma treatment due to the removal of organic group. However, the role of vacuum ultraviolet (VUV) was not investigated in spite of high energy of VUV radiated during plasma processing. In this work, the radiation effect of VUV on SiO 2 aerogel film was investigated during H 2 plasma treatment. H 2 gas was selected from its lightest atomic mass and strong photon emission in the VUV region. LiF single crystal was used as a VUV filter to permit the transmittance of the VUV radiated from H 2 plasma but not with chemical species and reactive ions. It was found that VUV radiation during H 2 plasma treatment plays a role of removal of organic group and rearrangement of siloxane bond.

Time-resolved x-ray diffraction measurements and analysis to investigate shocked lithium fluoride crystals

Experimental methods to permit continuous, time-resolved x-ray diffraction measurements in plate impact experiments were developed and used to examine lattice deformation in shock compressed LiF single crystals. Using an x-ray streak camera diffraction data with 2–4 ns resolution were obtained from crystals subjected to both shock and ramp wave loading along the [111] direction. Because of the penetration depth of x rays into the sample, interpretation of the ramp wave loading data required an analytic model to simulate the results. The penetration depth used in the model was determined experimentally from the time-resolved shock wave loading data. Good agreement between the simulations and experimental data was found for both loading conditions, suggesting that the analytic model has broad applicability.

SFM study of ion-induced hillocks on LiF exposed to thermal and optical annealing

Single crystals of LiF were irradiated at 10 different temperatures from room temperature to 780 K with Pb ions of 4.1 MeV/u. The irradiated surfaces were analyzed with scanning force microscopy, which revealed ion-induced hillocks with diameters of ∼20 nm and with heights of a few nm. Above 450 K, the number of hillocks strongly decreased with irradiation temperature. No hillocks were created under irradiation at 780 K. In addition, LiF samples irradiated at room temperature with Ni (2.5 MeV/u) and U ions (11.1 MeV/u) were bleached with UV-light on part of the crystal surface. In the bleached area, the characteristic F- and F 2-centers disappeared, whereas the mean diameter and height of the hillocks did not show any significant change.

Serrated plastic deformation in LiF single crystals at high temperatures

The serrated plastic flow in LiF single crystals has been studied in the mode of active deformation at high temperatures (T = 573–1093 K). The parameters of the jumps in the deforming stresses (normalized amplitude and relaxation time of stress oscillations) were determined at the stage of strain softening under conditions of uniaxial compression and tension. It was shown that the jump parameters are essentially dependent on the type of the stressed state and the deformation temperature. The activation energy of serrated deformation in shear bands was established to be close to the migration energy for cation vacancies.

Nanoindentation size effect in alkali-halide single crystals

Nanoindentation size effect has been investigated at very low loads in (001) LiF and KCl single crystals. All the measurements were carried out with DI Dimension 3100 equipped with a diamond tip. Simultaneously, the topography of the sample surface were done by using tapping mode atomic force microscopy (AFM). For both crystals, the indentation size effect (for loads from 5 to 400 μN) was found. The results obtained are discussed in the terms of the model of geometrically necessary dislocations proposed to interpretation of indentation size effect. The experiments show that the square of the nanohardness H 2 vs. the inverse of indentation depth, 1/ h, is a linear dependence, that is in a good qualitative agreement with the predictions of the model. However, as follows from the experiments presented, for quantitative estimates, it is necessary to take into account a microscopic mechanisms of the indent formation.

VACUUM ULTRAVIOLET ABSORPTION SPECTRA OF SOLID HYDROGEN HALIDES

Absorption spectra of solid HCl, HBr and HI films deposited on LiF single crystals cooled at 103–104 K and those annealed have been obtained in the 4–11.5 eV region. The first peaks found around the absorption edges correspond to the first bands (dissociative) in isolated molecules, so that they are regarded as Frenkel exciton peaks. The spectral feature of as-deposited films changed after annealing irreversibly, which suggests the structural transition from an amorphous phase to a crystalline phase in HCl and HBr. In HI, it was revealed that the first band in the gaseous phase consists of three components. The broad structures found above the first peaks do not resemble the structures of the isolated molecules in the same energy region which consist of many lines. The band structures of solid hydrogen halides seem to resemble those of sodium halide crystals and solid rare gases.

Dislocation glide and dynamic recrystallization in LiF single crystals

Single crystals of LiF with 〈111〉 orientation have been dynamically recrystallized during steady state of plastic deformation at 673 K. It is shown that localization of single dislocation glide plays an important role in the initiation of continuous dynamic recrystallization. Bands of elongated subgrains of rectangular shape are formed in areas of localized deformation at small strains. Upon subsequent deformation uniform multiple slip occurs and leads to the formation of equiaxed subgrains between bands of elongated subgrains. Further plastic deformation leads to a continuous increase in misorientation of subgrain boundaries and their eventual conversion into high angle boundaries. The mechanisms of the formation of recrystallized grains with different grain boundary configurations and the relationship between grain boundary configuration and character of dislocation slip are discussed.

Jetlike component in sputtering of LiF induced by swift heavy ions.

Angular distributions of sputtered atoms from SiO2 and LiF single crystals were measured under the irradiation of 1 MeV/u swift heavy ions. In contrast to the almost isotropic distribution of SiO2, an additional jetlike component was observed for LiF. The total sputtering yield of SiO2 ( approximately 10(2) atoms/ion) can be reproduced by an extended inelastic thermal spike model, whereas the huge yield of LiF ( approximately 10(4) atoms/ion) needs a substantial decrease of the sublimation energy to be described by the model.

Novel ultra-fast luminescence from incipient ion tracks of insulator crystals: electron–hole plasma formation in the track core

Measurements of fast luminescence decay and time-resolved spectra revealed novel ultra-fast luminescence with the lifetime of several tens ps in heavy-ion-irradiated single crystals of LiF, NaF, NaCl, KCl, KBr, KI, RbI, CsCl, CsBr, CsI, α-alumina, and MgO. The luminescence is furthermore characterized by a super-linear increase in the efficiency with increasing excitation density, non-tailed decay curve, and temperature-insensitive decay-rate and yield. The results mean that the luminescence neither originates from localized excited states such as self-trapped excitons, free excitons, excited defects, and excited impurity centers nor their interaction. A process which does not contradict the experimental results is the formation of the e–h plasmas and the luminescence from them.

Interaction of high-energy Pb ions with an island metal film on the surface of a LiF single crystal

Track formation in an island gold film on the surface of a LiF single crystal was studied by means of transmission electron microscopy. Irradiation by Pb ions with an energy of 4.48 MeV/nucleon was carried out at the angles of 0.5° and 2° with respect to the crystal surface. The quantity and shape of the islands removed from the surface during the track formation process was investigated with the help of collectors. Possible track formation mechanisms are discussed.