LiF Thin Films Research Articles

Effect of Annealing Temperature on Thermally Evaporated Lithium Fluoride Thin Films

Abstract Lithium fluoride thin films were successfully deposited on glass substrates using a well known vacuum evaporation method by means of resistive heating at (10-3Torr). These thin films were then annealed for 350°C, 400°C and 450°C respectively. The annealed thin films were characterized for structural surface morphological and optical properties; the annealing temperature strongly affects the properties of LiF thin films the crystallite size was found to increase with respect to increase in annealing temperature. The increase in annealing temperature also affects the optical properties of the LiF thin films show improved transmittance with respect to increase in annealing temperature, the refractive index increases whereas optical band decreases with increase in annealing temperature.

Visible photoluminescence of color centers in lithium fluoride detectors for low-energy proton beam Bragg curve imaging and dose mapping

In the last few years, the visible photoluminescence (PL) of radiation-induced laser-active F2 and F3+ aggregate color centers (CCs) in lithium fluoride, LiF, has been successfully used for advanced diagnostics of low-energy proton beams. A systematic study of the optical properties of LiF crystals and thin films exposed in different irradiation geometries to a proton beam of nominal energy 7 MeV is under way. The ionization induces the stable formation of primary and aggregate CCs in the crystalline lattice, among which the F2 and F3+ defects possess broad emission bands in the red and green spectral ranges, respectively, when optically pumped at wavelengths close to 450 nm. Their PL intensity has been found to be linearly proportional to the dose absorbed by the host material over at least three orders of magnitude, so that LiF solid-state dosimeters based on spectrally-integrated PL reading can be envisaged. A simple defect formation model that takes into account the energy released in the material, crystals and films, together with the saturation of CC concentration at high doses, allows obtaining both the full Bragg curve reconstruction with its dose distribution with depth and the accumulated bi-dimensional transversal dose-mapping from the visible fluorescence images latently stored in LiF crystals and thin films, respectively. Results obtained by using a fluorescence microscope are presented and discussed with the aim of highlighting some quantitative aspects of the PL behavior of CCs both in LiF crystals and films, which are relevant for low-energy proton-beam Bragg curve imaging and dose mapping in a wide interval of doses up to the ones at which strong CC saturation occurs.

Refractive Index Sensor Using Long-Range Surface Plasmon Resonance with Prism Coupler

Long-range surface plasmon resonance (LRSPR)-based sensors exhibit high sensitivity as compared to the conventional SPR sensors due to low losses. A high refractive index prism and low refractive index dielectrics are required to excite long-range surface plasmon (LRSP) mode. In this work, a symmetrical structure consisting of metal (Au) layer sandwiched between the two similar refractive index dielectrics has been utilized to excite LRSPR modes. Two dielectrics, i.e., LiF and MgF2 thin films have been chosen for glucose sensing. Theoretical simulations for optimizing the thickness of two dielectrics have been carried out. Highly sensitive refractive index sensors based on LRSPR have been developed using the two dielectric thin films and their sensitivities have been compared.

Atomic Layer Deposition of Lithium Fluoride Optical Coatings for the Ultraviolet

Lithium fluoride is an important material for ultraviolet optical systems, possessing among the largest optical bandgaps of dielectric materials. We report on the development of an atomic layer deposition (ALD) process for lithium fluoride that is capable of depositing thin films in a self-limiting manner, with an approximate deposition rate of approximately 0.15 Å per ALD cycle at a substrate temperature of 150 °C. Films are characterized by spectroscopic ellipsometry, atomic force microscopy, X-ray photoelectron spectroscopy, and far ultraviolet reflectometry. For substrate temperatures of 150 °C and greater, films showed significant microroughness with a correlated reduction in effective refractive index. This behavior was mitigated by a reduction in substrate temperature to as low as 100 °C. Films deposited on silicon substrates were subjected to long-term storage testing to evaluate the environmental sensitivity of the deposited layers. Protected aluminum mirrors were also fabricated with ALD LiF overcoats, yielding a reflectance of 84% at a wavelength of 125 nm. The performance relative to state-of-the-art LiF thin films deposited by physical vapor deposition methods is discussed, along with the prospects for future optimization.

Spatial Heterogeneities and Onset of Passivation Breakdown at Lithium Anode Interfaces

Effective passivation of lithium metal surfaces, and prevention of battery-shorting lithium dendrite growth, are critical for implementing lithium-metal-anodes for batteries with increased power densities. Nanoscale surface heterogeneities can be "hot spots" where anode passivation breaks down. Motivated by the observation of lithium dendrites in pores and grain boundaries in all-solid batteries, we examine lithium metal surfaces covered with Li(2)O and/or LiF thin films with grain boundaries in them. Electronic structure calculations show that, at >0.25 V computed equilibrium overpotential, L(2)O grain boundaries with sufficiently large pores can accommodate Li(0) atoms which aid electron leakage and passivation breakdown. Strain often accompanies Li-insertion, applying a ~1.7% strain already lowers the computed overpotential to 0.1 V. Lithium metal nanostructures as thin as 12 Angstroms are thermodynamically favored inside cracks in Li(2)O films, becoming "incipient lithium filaments." LiF films are more resistant to lithium metal growth. The models used herein should in turn inform passivating strategies in all-solid-state batteries.

Lithium fluoride thin film detectors for low-energy proton beam diagnostics by photoluminescence of colour centres

Optically transparent LiF thin films thermally evaporated on glass and Si(100) substrates were used for advanced diagnostics of proton beams of energies from 1.4 to 7 MeV produced by a linear accelerator for protontheraphy under development at ENEA C.R. Frascati. The proton irradiation induces the formation of stable colour centres, among them the aggregate F2 and F3+ optically active defects. After exposure of LiF films grown on glass perpendicularly to the proton beams, their accumulated transversal spatial distributions were carefully measured by reading the latent two-dimensional (2-D) fluorescence images stored in the LiF thin layers by local formation of these broad-band visible light-emitting defects with an optical microscope under blue lamp excitation. Taking advantage from the low thickness of LiF thin films and from the linear behaviour of the integrated F2 and F3+ photoluminescence intensities up to the irradiation fluence of ∼5x1015 p/cm2, placing a cleaved LiF film grown on Si substrate with the cutted edge perpendicular to the proton beam, the 2-D fluorescence image of the film surface could allow to obtain the depth profile of the energy released by protons, which mainly lose their energy at the end of the path.

Swift heavy ions induced nano-grain fragmentation in fluoride thin films

Influence of the swift heavy ions (SHIs) at various ion fluences in nanodimensional polycrystalline fluoride thin films (LiF, CaF2 and BaF2) were investigated by glancing angle X-ray diffraction (GAXRD), complemented by atomic force microscopy (AFM). Nano-grain fragmentation is observed with increasing ion fluences as evident from the increase of the peak width in GAXRD pattern. It is also observed that after certain fluence SHIs irradiation, there is a saturation in the nano-grain fragmentation. The track radius and damage cross section are deduced from peak width and area of the dominant diffraction. The track radii are 5.5 and 14.5 nm, 5.0 and 9.7 nm and 5.5 and 6.5 nm for BaF2, LiF, CaF2, deduced from width and area, respectively. It is noteworthy that the track radius is found to be higher than the previously estimated from sputtering studies of LiF thin films.

Lithium fluoride colour centres-based imaging detectors for proton beam characterization at high doses

Proton beams of 7 MeV energy, produced by a linear accelerator, were used to irradiate LiF crystals and thin films thermally evaporated on glass substrates in the dose range from 103 to 4 × 106 Gy, inducing the formation of stable photoluminescent colour centres (mainly F2 and F3+), emitting in the visible spectral range. Using a conventional fluorescence microscope, the transversal proton beam intensity was mapped by acquiring the photoluminescence image of the irradiated spots. Image analysis allowed measuring the integrated photoluminescence intensity as a function of the irradiation dose: a linear optical response was obtained up to different maximum dose values, after which a quenching was observed, depending on the nature of the samples (crystals or films). The colour centres formation was investigated by optical absorption spectroscopy at room temperature and the Principal Component Analysis was applied to the absorption spectra of irradiated LiF crystals. In samples irradiated at highest doses, it allowed clearly identifying the formation of more complex aggregate defects, which appears strictly related to the observed photoluminescence quenching effect.

Investigation and simulation of the structural and electronic properties of LiF:Mg, LiF:Cu, LiF:P nano-layer dosimeters

In this study, electronic structure of lithium fluoride thin films in pure state and doped with magnesium (Mg), copper (Cu) and phosphorus (P) impurities was studied using WIEN2K Code. The structural and electronic properties of two LiF thin films with 1.61 and 4.05 nm thicknesses were studied and compared. Results show that the distance of atoms in the surface and central layers of pure LiF are 1.975 and 2.03 nm, respectively. Electronic density of the valence band around the surface atoms is greater than that around middle atoms of the supercell. The band gap of bulk LiF is 9 eV. But, in the case of thin films, it is reduced to 2 eV. Electronic and hole-traps were not observed in composition of LiF thin films doped with Mg and P with 1.61 and 4.05 nm thickness and in fact, metallic properties were observed. When Cu atoms were doped in composition of an LiF thin film, the thin film was converted to semiconductor.

Photoluminescence of colour centres in lithium fluoride thin films: From solid-state miniaturised light sources to novel radiation imaging detectors

Luminescence properties of point defects in insulating materials are successfully used for solid state light sources and radiation detectors. Among them, colour centres in lithium fluoride, LiF, are well known for their application in tuneable lasers and dosimeters. Broad-band light-emitting F2 and F3+ electronic defects, stable at room temperature, are produced in LiF crystals and films by different kinds of radiation. Under blue optical pumping in their overlapping absorption bands, the efficient photoluminescence spans over the green–red visible spectral range. Novel LiF thin-film radiation imaging detectors based on the exploitation of the peculiar spectral characteristics of F2 and F3+ defects and of the optical properties and radiation sensitivity of the LiF material have been proposed and successfully tested for extreme ultraviolet radiation, soft and hard X-ray imaging, including micro-radiography of biological objects, even for in vivo samples. After exposure to X-rays, the latent images stored in the LiF thin layers by local formation of active defects are read with conventional and advanced optical fluorescence microscopes. Among the main advantages of the LiF thin-film imaging detectors, there are intrinsic very high spatial resolution, large field of view and wide dynamic range. Moreover, these solid state radiation detectors are easy to handle, as insensitive to ambient light, and no development process is needed. Recently their use has been extended to proton beam advanced diagnostics. In this paper, a short review of their properties, results and applications in X-ray biological imaging and proton dose-mapping is presented, underlying the great versatility and potentialities offered by LiF thin films, whose photoluminescence response can be improved through the choice of suitable substrates and growth conditions.

Photoluminescence of radiation-induced color centers in lithium fluoride thin films for advanced diagnostics of proton beams

Systematic irradiation of thermally evaporated 0.8 μm thick polycrystalline lithium fluoride films on glass was performed by proton beams of 3 and 7 MeV energies, produced by a linear accelerator, in a fluence range from 1011 to 1015 protons/cm2. The visible photoluminescence spectra of radiation-induced F2 and F3+ laser active color centers, which possess almost overlapping absorption bands at about 450 nm, were measured under laser pumping at 458 nm. On the basis of simulations of the linear energy transfer with proton penetration depth in LiF, it was possible to obtain the behavior of the measured integrated photoluminescence intensity of proton irradiated LiF films as a function of the deposited dose. The photoluminescence signal is linearly dependent on the deposited dose in the interval from 103 to about 106 Gy, independently from the used proton energies. This behavior is very encouraging for the development of advanced solid state radiation detectors based on optically transparent LiF thin films for proton beam diagnostics and two-dimensional dose mapping.

Primary and aggregate color centers in proton irradiated LiF crystals and thin films for luminescent solid state detectors

Proton beams of 3 MeV energy, produced by the injector of a linear accelerator for proton therapy, were used to irradiate at room temperature lithium fluoride crystals and polycrystalline thin films grown by thermal evaporation. The irradiation fluence range was 1011-1015 protons/cm2. The proton irradiation induced the stable formation of primary and aggregate color centers. Their formation was investigated by optical absorption and photoluminescence spectroscopy. The F2 and F3+ photoluminescence intensities, carefully measured in LiF crystals and thin films, show linear behaviours up to different maximum values of the irradiation fluence, after which a quenching is observed, depending on the nature of the samples (crystals and films). The Principal Component Analysis, applied to the absorption spectra of colored crystals, allowed to clearly identify the formation of more complex aggregate defects in samples irradiated at highest fluences.

Solid state detectors based on point defects in lithium fluoride for advanced proton beam diagnostics

Proton beams of 3 and 7MeV energies, produced by a linear accelerator, were used to irradiate lithium fluoride crystals and thermally evaporated LiF thin films in the fluence range of 1011–1015protons/cm2. The irradiation induces the formation of stable colour centres, mainly the primary F centre and the aggregate F2 and F3+ defects. By optical pumping in the blue spectral region, the F2 and F3+ centres emit broad photoluminescence bands in the visible spectral range. By conventional fluorescence microscopy, the integrated photoluminescence intensity was carefully measured in LiF crystals and thin films as a function of the irradiation fluence: a linear optical response was obtained in a large range of fluence, which is dependent on the used LiF samples and the selected beam energy. It was possible to record the transversal proton beam intensity profile by acquiring the photoluminescence image of the irradiated spots on LiF films by a standard optical microscope. Using LiF films grown on silicon substrates irradiated in a particular geometry, the same optical reading microscopy technique allowed one to measure the distribution of colour centres photoluminescence along the depth and direct imaging the Bragg peak position, which gives a rough estimation of the initial proton beam energy.

Analyzing the LiF thin films deposited at different substrate temperatures using multifractal technique

The Atomic Force Microscopy technique is used to characterize the surface morphology of LiF thin films deposited at substrate temperatures 77K, 300K and 500K, respectively. It is found that the surface roughness of thin film increases with substrate temperature. The multifractal nature of the LiF thin film at each substrate temperature is investigated using the backward two-dimensional multifractal detrended moving average analysis. The strength of multifractility and the non-uniformity of the height probabilities of the thin films increase as the substrate temperature increases. Both the width of the multifractal spectrum and the difference of fractal dimensions of the thin films increase sharply as the temperature reaches 500K, indicating that the multifractility of the thin films becomes more pronounced at the higher substrate temperatures with greater cluster size.

Optical spectroscopy and imaging of colour centres in lithium fluoride crystals and thin films irradiated by 3 MeV proton beams

Lithium fluoride is a well-known dosimeter material and it is currently under investigation also for high-resolution radiation imaging detectors based on colour centre photoluminescence. In order to extend their applications, proton beams of 3MeV energy, produced by a linear accelerator, were used to irradiate LiF crystals and thin films in the fluence range of 1010–1015protons/cm2. The irradiation induces the formation of colour centres, mainly the primary F centre and the aggregate F2 and F3+ defects, which are stable at room temperature. By optical pumping in the blue spectral region, the F2 and F3+ centres emit broad photoluminescence bands in the visible spectral range. By conventional fluorescence microscopy, the integrated photoluminescence intensity was carefully measured in LiF crystals and thin films as a function of the irradiation fluence: a linear optical response was obtained in a large range of fluence, which is dependent on the used LiF samples. Colour centres concentrations were estimated in LiF crystals by optical absorption spectroscopy. It was possible to record the transversal proton beam intensity profile by acquiring the photoluminescence image of the irradiated spots on LiF films.

Stopping powers of LiF thin films deposited onto self-supporting Al foils for swift protons

The energy losses of ∼(0.273–3.334)MeV protons in LiF thin films deposited by vacuum evaporation onto self-supporting Al foils have been measured using the transmission method. The thicknesses of selected and used LiF/Al target samples were accurately determined via systematic energy loss measurements for alpha particles from a very thin mixed 241Am/239Pu/233U radioactive source. The samples were investigated in detail for their stoichiometry and their impurity contents by backscattering Rutherford spectrometry and nuclear reaction analysis. Then, LiF stopping powers have been determined with overall relative uncertainty of less than 2.7% arising mainly from errors in the determination of target sample thicknesses. These S(E) data are reported and discussed in comparison to previous experimental data sets from the literature and to values calculated by the Sigmund–Schinner binary collision stopping theory both for molecular LiF, and for the LiF compound assuming Bragg–Kleeman’s additivity rule. Our S(E) data show to be in excellent agreement with the latter theory for molecular LiF over the whole proton energy range explored, which supports the use of modified electronic hydrogen wave functions for evaluating atomic shell corrections in the case of low-Z2 target materials. In contrast, they exhibit a slightly increasing deviation from theoretical values derived for the LiF compound with assuming stopping force additivity as the proton energy decreases from E≈400keV towards lower proton velocities. This deviation in excess relative to experimental data, amounting only up to (at most) ∼2.5%, can be ascribed to strong effects of 2s-state valence electrons of Li atoms within the LiF compound. Besides, the comparison to values calculated by the SRIM-2008 computer code indicates that this program satisfactorily accounts for our S(E) data above E≈1.30MeV but underestimates them with substantially increasing deviations (up to ∼11%) towards lower proton velocities where the Bragg–Kleeman additivity rule therefore appears to be inapplicable.

Atomic Layer Deposition of LiF Thin Films from Lithd and TiF4 Precursors

AbstractLithium fluoride (LiF) is an important optical material with a low refractive index and a large band gap. In this study, thin films of LiF are deposited using atomic layer deposition (ALD). Lithd and TiF4 are used as precursors, and they produce crystalline LiF in the temperature range 250–350 °C. The films are studied with UV‐Vis spectrometry, field emission scanning electron microscopy (FESEM), X‐ray diffraction (XRD), atomic force microscopy (AFM) and elastic recoil detection analysis (ERDA). Adhesion of the films is tested by a Scotch tape test. This ALD process results in LiF films with a growth rate of approximately 1 Å per cycle at 325 °C. According to ERDA measurements, the films are pure LiF with only small O, C, and H impurities.

Atomic Layer Deposition of LiF Thin Films from Lithd, Mg(thd)2, and TiF4 Precursors

Lithium fluoride is an interesting material because of its low refractive index and large band gap. Previously LiF thin films have been deposited mostly by physical methods. In this study a new way of depositing thin films of LiF using atomic layer deposition (ALD) is presented. Mg(thd)2, TiF4 and Lithd were used as precursors, and they produced crystalline LiF at a temperature range of 300–350 °C. The films were studied by UV–vis spectrometry, field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), atomic force microscopy (AFM), time-of-flight elastic recoil detection analysis (ToF-ERDA), and energy dispersive X-ray spectroscopy (EDX). In addition, film adhesion was tested by a Scotch tape test. This method results in LiF films with a growth rate of approximately 1.4 Å per cycle. According to the ToF-ERDA measurements, the films are pure LiF with very small Mg and Ti impurities, the largest impurity being hydrogen with contents below 1 atom %.

Thickness analysis of LiF thin films by using 7Li(p,n) and 19F(p,αγ) nuclear reactions

A neutron time of flight (n-TOF) technique is applied for analyzing the thickness of a LiF thin film used as a neutron target. Measuring the flight time of the neutron generated from the neutron target gives information on the proton attenuation energy corresponding to the film’s thickness. Three thin-film targets were fabricated at an applied electron voltage of 4.3 kV and an electron current of about 20 mA by using an evaporating system with an electron gun. Two methods, weight on a micro-balance and the excitation function for the narrow resonance band of 483.8 keV on the 19F(p,αγ) reaction, were used to confirm the target thickness. The thicknesses of the three targets were found to be 95.5, 109.7, and 206.8 µg/cm2 by treating statistically the results of the three methods. The results for the three methods agreed within 7%.

Fractal and multifractal analysis of LiF thin film surface

Fractal and multifractal analysis is performed on the atomic force microscopy (AFM) images of the surface morphologies of the LiF thin films of thickness 10nm, 20nm, and 40nm, respectively. Autocorrelation function, height–height correlation function, and two-dimensional multifractal detrended fluctuation analysis (MFDFA) are used for characterizing the surface. It is found that the interface width, average roughness, lateral correlation length, and fractal dimension of the LiF thin film increase with the thickness of the film, whereas the roughness exponent decreases with thickness. Thus, the complexity and roughness of the LiF thin films increases as thickness increases. It is also demonstrated that the LiF thin films are multifractal in nature. Strength of the multifractality increases with thickness of the film.