BaO Thin Films Research Articles

The Impact of Lattice Distortions on the Magnetic Stability of Single Atoms: Dy and Ho on BaO(100)

AbstractX‐ray magnetic circular dichroism, atomic multiplet simulations, and density functional theory calculations are employed to identify criteria for the optimum combination of supporting alkaline earth oxide and adsorption site maximizing the spin lifetimes of lanthanide single‐atom magnets. Dy and Ho atoms adsorbed on BaO(100) thin films on Pt(100) are characterized and compared with previous results for the same two elements on MgO/Ag(100). Dy shows hysteresis in magnetic fields up to ≈3.5 T and long spin lifetime, exceeding 300 s at 2.5 K and 0.5 T. Dy displays superior magnetic stability on the bridge site than on the top‐O site. Surprisingly, Ho shows paramagnetism, as opposed to its long spin lifetime on MgO. These differences originate from the local surface distortions induced by the adatoms. On MgO, minimal distortions involve only the closest O atoms, while, on BaO, they affect both the closest anions and cations. This trend reflects the decrease of the lattice energy along the series of the alkaline earth oxides, going from MgO to BaO. This study represents a step ahead in the understanding of the factors determining the spin dynamics of surface‐adsorbed single‐atom magnets in order to achieve their operation as qubits and memories.

Investigation of polymer-based BaO and rGO nanocomposites for application in low energy X ray attenuation

Polymeric materials can serve as a matrix for the dispersion of nanomaterials with good attenuation features, resulting in lightweight, conformable, flexible, lead-free and easy-to-process materials. Thus, some well-known radiation shielding materials could be used in low proportion as a filler, for the formation of new materials. On the other hand, nanostructured carbon materials, such as graphene oxide (GO) have been reported recently to show enhanced attenuation properties. For the present work, poly(vinylidene fluoride) [PVDF] homopolymers and its fluorinated copolymers were filled with metallic oxides and nanosized reduced graphene oxides (rGO) in order to produce nanocomposites with increased low energy X ray attenuation efficiency. We objective is to investigate the X ray shielding features of multilayered PVDF/rGO and P(VDF-TrFE)/BaO composites. PVDF/rGO overlapped with P(VDF-TrFE)/BaO thin films were sandwiched between two layers of kapton films of different thickness. The linear attenuation coefficients were measured for monochromatic X ray photons with energy of 8.1 keV. The samples were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Ultraviolet–visible (UV-vis) and Fourier-Transform Infrared (FTIR) Spectroscopy. The linear attenuation coefficient of the multilayered sample was evaluated and compared with the linear attenuation of the individual constituents. It was observed an increase in the attenuation coefficient of the overlapping samples. It is demonstrated that thin films of rGO nanocomposite with thickness of only 0.32 mm can attenuate up to 50% of X ray beams with energy of 8.1 keV, justifying further investigation of these nanocomposites as X ray or gamma radiation attenuators

Work function and quantum efficiency study of metal oxide thin films on Ag(100)

Increasing the quantum efficiency (QE) of metal photocathodes is in the design and development of photocathodes for free-electron laser applications. The growth of metal oxide thin films on certain metal surfaces has previously been shown to reduce the work function (WF). Using a photoemission model B. Camino et al. [Comput. Mater. Sci. 122, 331 (2016)] based on the three-step model combined with density functional theory calculations we predict that the growth of a finite number of MgO(100) or BaO(100) layers on the Ag(100) surface increases significantly the QE compared with the clean Ag(100) surface for a photon energy of 4.7 eV. Different mechanisms for affecting the QE are identified for the different metal oxide thin films. The addition of MgO(100) increases the QE due to the reduction of the WF and the direct excitation of electrons from the Ag surface to the MgO conduction band. For BaO(100) thin films, an additional mechanism is in operation as the oxide film also photoemits at this energy. We also note that a significant increase in the QE for photons with an energy of a few eV above the WF is achieved due to an increase in the inelastic mean-free path of the electrons.

Growth and lattice dynamics of ultrathin BaO films on Pt(0 0 1)

We report on the surface phonons of long-range ordered BaO thin films grown on Pt(0 0 1). In the thickness range between 4 and 28 ML, we find unstrained BaO(0 0 1)-() bulk-like terminated films which coincide with a Pt(0 0 1)-c() lattice periodicity. The dipole-active lattice vibrations were determined using high-resolution electron energy loss spectroscopy. For all BaO film thicknesses, a single Fuchs–Kliewer phonon-polariton is observed. Its intensity increases and its frequency softens with increasing film thickness. These thickness-dependent properties and the spectral shape are quantitatively discussed on the basis of dielectric theory, where a proper modeling requires three components: the dielectric response of the BaO film itself, the plasmonic response of the metallic substrate, and a weak damping due to a defect-induced doping within the oxide film. For a full description, also the quantization of the phonon wavevector due to the confinement within the film of finite thickness has to be taken into account.

Initial growth stages of titanium and barium oxide films on SrTiO3(001)

Ultrathin titanium and barium oxide films were grown on SrTiO3(001) substrate surfaces via Ti/Ba deposition and oxidation. When Ti is deposited onto the SrTiO3(001) surface, it forms a series of SrTiO3 surface reconstructions such as c(4×2), 5×5−R26.6∘, (6×2) and (9×2), which can be controlled by varying the Ti concentration. Epitaxial anatase TiO2(001) islands with (1×3) and (1×5) reconstructions are formed by further increasing the Ti deposition amount and post-annealing. The initial growth of BaO thin films depends on the surface condition of the substrate. BaO nanoparticles form on reconstructed SrTiO3(001), while a locally ordered c(4×4) BaO structure is observed on the disordered sample surface. Co-deposition of Ba and Ti on SrTiO3(001) results in phase separation with the deposited Ba forming BaO nanoparticles and the Ti contributing to Ti-rich SrTiO3 surface reconstructions. No formation of BaTiO3 is observed at the initial growth stages between 0.3 monolayer (ML) and 1.8MLs.

Quantum Yield Investigation of Ag−BaO Thin Film

Ag−BaO thin films have been fabricated by evaporating silver into BaO semiconductor thin film in ultrahigh vacuum and the quantum yield distribution in the visible region between 1.5 and 3.7 eV has been measured. The quantum yield spectra have a peak when the energy of the incident light is 2.5 eV. The direct transitions and surface plasmon resonance of silver nanoparticles appear to be very important in determining the shape of quantum yield spectra. The explanation for the small discrepancy between the experimental result and the theoretical data is given.

Ultrafast optical Kerr effect of Ag–BaO composite thin films

We report the ultrafast optical Kerr effect of Ag–BaO composite thin films by the femtosecond time-resolved pump-probe technique. The Ag–BaO thin films with Ag nanoparticles embedded into the BaO semiconductor matrix were prepared using a vacuum evaporation-deposition multimetallic layer method. The third-order nonlinear optical susceptibility of the thin films with the thickness of approximately 300 nm and the volume fraction of Ag nanoparticles in the thin films of about 25% was estimated to be 4.8×10−10 esu at the incident laser wavelength of 820 nm. The response time, i.e., the full width at half maximum of the Kerr signal, was as fast as 210 fs. The intrinsic third-order optical nonlinearity, or the optical Kerr effect of the thin films, can be attributed to the change of refractive index due to the intraband transition of electrons from the occupied state near the Fermi level to the unoccupied state in the Ag nanoparticles. Such nonlinearity is further enhanced by the local field effect that is present when the metallic nanoparticles are embedded into the semiconductor matrix.

A method of preparing thin‐film micro‐PTC thermistors based on BaTiO3 using YAG‐laser irradiation

AbstractA technique is proposed for obtaining thin‐film positive temperature coefficient (PTC) microthermistors made of BaTiO3‐based material. In this technique, stacked layers of Ti and BaO thin films are irradiated in air by a YAG laser beam, which creates BaTiO3 semiconductor microregions with PTC characteristics. The experiments indicate that by suitably choosing the optical power of the laser beam and its scanning rate over the stacked layers, it is possible, and in fact easy, to control the resistance of the resulting BaTiO3 semiconductor microregions. We found that the resistance of the BaTiO3 semiconductor material created in this way must be sufficiently low in order for the material to exhibit PTC characteristics. In this case, the material becomes a thermistor, that is, its resistance increases rapidly with increasing temperature. As the temperature increases, the resistance suddenly begins to increase starting at a certain threshold temperature. In an attempt to control this threshold temperature, we modified the technique described in this research by replacing some of the Ti sites with Zr. This modified technique led us to conclude that, just as in bulk BaTiO3, such control was indeed possible. When our technique is used, high‐temperature heating of the entire substrate is not necessary, either during the deposition of the thin film or after its deposition. In this way we confirm that it is possible to create compound device modules by fabricating multiple thin‐film microdevices on a single substrate, including PTC microthermistors. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 85(11): 25–31, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.1113

Quantum yield of photoemission of Ag–BaO thin films for detecting ultrashort infrared laser pulses

The Ag–BaO thin films are a new kind of photoemissive thin films, which have transient optical and ultrafast photoelectronic response. The properties of the Ag–BaO thin films establish the application foundation of detecting ultrashort infrared laser pulses. The measurement of the quantum yield of photoemission for ultrashort laser pulses is different from that for normal visible light. The experimental results present that the quantum yield of Ag–BaO thin films is of the order of magnitude of 10 −6 under the action of a single laser pulse when the incident infrared laser wavelength is 1.06 μm. It is two orders of magnitude higher than that of pure metals at the same conditions. The quantum yield under the action of a single-chain laser pulse, which comprises 11 single pulses of duration 40 ps separated by 10 ns, increases rapidly following increasing incident energy of the laser. In this case, the Ag–BaO thin films present multiphoton emission of 2-photon process and higher quantum yield of 10 −5.

STUDY OF ENHANCED PHOTOABSORPTION OF BaO THIN FILMS IN THE NEAR-ULTRAVIOLET BAND WITH APPLIED VERTICAL ELECTRIC FIELD ON THE SURFACE

The optical absorption properties of BaO semiconductor thin films deposited by v acuum evaporation were measured when a vertical electric field was applied on th e surface of the thin films.Enhanced absorption in the near-ultraviolet band was observed.The absorption was increased with the increased intensity of applied e lectric field.Theoretical analysis indicates that the energy-band edges of BaO s emiconductor become bent on the application of electric field,and there is an in creased probability for electrons in the valence band to tunnel through the forb idden gap according to the quantum-mechanics.As a result,it becomes possible for the photons with energy less than Eg to be absorbed by the electrons ,and which leads to the enhanced photoabsorption of BaO thin films in the near-u ltraviolet band.Franz-Keldysh effect and Stark effect were used to explain the e lectroabsorption spectrum when metal oxide semiconductor was excited by the phot ons with energy less or more than Eg.

Observation of coherent phonons in silver nanoparticles embedded in BaO thin films

Three kinds of coherent phonons (0.30, 1.90, and 3.75 THz) have been observed in silver nanoparticles embedded in BaO thin films by means of the femtosecond time-resolved pump-probe technique. The generation of 0.3 THz coherent phonons is attributed to the resonant excitation of localized surface plasmon of silver nanopaticles, and the resonant impulsive stimulated Raman scattering in silver aggregates is responsible for the generation of 1.90 and 3.75 THz coherent phonons.

Atomic-scale Analysis of Laser MBE Growth of Oxide Thin Films by in situ Rheed and Caiciss

ABSTRACTAtomic-scale growth analysis of oxide thin films was performed by in situ reflection high energy electron diffraction (RHEED) and coaxial impact collision ion scattering spectroscopy (CAICISS) combined with Laser MBE. On single crystal substrates with atomically flat terrace and step structures, the two-dimensional nucleation followed by molecular layer-by-layer growth was verified by in situ monitoring of RHEED intensity oscillations, as well as ex situ atomic force microscopy (AFM) observation, for the growth of BaTiO3, Al2O3 and BaO thin films. The epitaxial BaTiO3 films grown on SrTiO3(100) and c-axis oriented Bi2Sr2CaCu2Ox (Bi2212) superconducting films were subjected to in situ CAICISS measurements in order to examine the topmost surface structure. The key factors to develop oxide lattice engineering are discussed with respect to not only in situ monitoring of the growth process using RHEED but also the atomic regulation of the substrate surface by AFM and ion scattering spectroscopy. The present work also demonstrates the advanced oxide thin film processing based on the laser MBE to control the growth and surface of films on an atomic scale.