Surface Debye Temperature Research Articles

Surface Debye temperature determination from LEED: correlation to defects in epitaxial films

Low energy electron diffraction (LEED) peak intensities were measured to estimate surface Debye temperature for epitaxially-grown silicon thin films and for bulk Si (001). Rutherford backscattering spectroscopy (RBS, random and channeling modes) and positron annihilation spectroscopy (PAS) were used to quantify defect density and distribution in the near-surface layers. The surface Debye temperature of bulk Si (001), and 1.0 μm, and 0.6 μm Si on sapphire were measured to be 333 K, 299 K, and 260 K, respectively. RBS and PAS showed that the defect concentration was highest near the film/substrate interface, presumably due to the lattice mismatch, and decreased toward the top surface. The thicker film presented fewer defects in the surface and near-surface layers. We showed that a larger concentration of defects Nd in Si epitaxial films correlates with a lower surface Debye temperature following the empirical relation θD=(365±14)−(8.1±1.5)×10−13Nd for our set of samples. Our study suggests the further development of LEED to estimate near-surface defect concentrations.

Electron\u2013phonon coupling in superconducting 1T-PdTe2

We have determined the electron–phonon interaction in type II Dirac semimetallic 1T-PdTe2 by means of helium atom scattering. While 1T-PdTe2 is isostructural with 1T-PtTe2, only the former is superconductor. The difference can be traced to the substantially larger value of the electron–phonon coupling in 1T-PdTe2, λ = 0.58, obtained from the Debye-Waller attenuation of the He specular peak. With this value and the surface Debye temperature, ΘD = 106.2 K, we have figured out the superconducting critical temperature, Tc = 1.83 K given by the BCS theory, which is in good agreement with Tc = (1.95 ± 0.03) K obtained with low-temperature scanning tunneling microscopy. The value of the effective mass related to ΘD indicates that the large electron–phonon coupling in 1T-PdTe2 is due to coupling, not only with the zone-center optical mode O2 at 9.2 meV, as proposed in a recent theoretical study, but also with the zone-boundary acoustic mode LA. Our results suggest that the topological states of a Dirac cone play a negligible role on the onset of superconductivity.

Analytical study of thermodynamic quantities with surface tension and Debye temperature variations in Quinoline & p-Xylene at T = 303.15 K to 318.15 K

Abstract Thermo dynamic physical quantities viz., compressibility (β), molecular free length (Lf) and impedance (Z) are calculated in the binary organic liquid mixture having Quinoline + p-Xylene at temperatures T = 303.15 K to 318.15 K with 5 K variation using experimentally measured ultrasonic velocities, densities and viscosities with the molar concentration of Quinoline. Theoretical estimations are used to calculate Debye temperature and Surface tension in the above liquid mixture at different temperatures. A correlation of Surface Tension and Debye Temperature with the thermodynamic physical quantities are examined to bring out the inclusion of structural changes occurred due to the temperature variations in the aforesaid binary mixture. The output data is analysed the molecular association and to discuss component molecules structural behaviour with the temperature in the binary organic liquid mixture.

Structural, magnetic, vibrational and optical studies of structure transformed spinel Fe2+-Cr nano-ferrites by sintering process

This investigation presents the optical, structural, vibrational and magnetic studies of the cubic spinel Fe2+CrFeO4 nano-ferrites, which transformed to maghemite structure by annealed at 250 °C for 3 h and to hexagonal hematite at temperatures T ≥ 500 °C in air. These nanomaterials were synthesized by the co-precipitation route and sintered at different temperatures. A split of the lattice constant a to a and c occurred for T ≥ 500 °C due to the transformation to hexagonal hematite, where c/a ≈ 2.7. The density and trend of crystallite size (R) values showed increase against T, whereas the calculated and experimental specific surface area, threshold energy, Debye temperature and elastic wave velocities showed decrease. The trend of dislocation density, inter-chain distance, distortion parameter, saturation magnetization (Ms), squareness, remnant magnetization, magnetic moment and anisotropy constant was decreased with T. The main IR absorption bands; ν1 and ν2 of the nanomaterials were observed in the IR spectra where their positions proved dependence on T. The porosity (P), strain (ɛ), force constants, elastic and stiffness constants and coercivity revealed dependence on T. Variation of all deduced parameters with T revealed the structure transformation from cubic to hexagonal hematite. The optical band gab energy (Eg) showed increase as T increases. Eg, S and ɛ proved dependence on R, whereas Ms proved dependence on P. So, Dx, Ms and Hc proved effect on the elastic wave velocities.

Electron-phonon coupling and surface Debye temperature of Bi2Te3 (111) from helium atom scattering

We have studied the topological insulator Bi$_2$Te$_3$(111) by means of helium atom scattering. The average electron-phonon coupling $\lambda$ of Bi$_2$Te$_3$(111) is determined by adapting a recently developed quantum-theoretical derivation of the helium scattering probabilities to the case of degenerate semiconductors. Based on the Debye-Waller attenuation of the elastic diffraction peaks of Bi$_2$Te$_3$(111), measured at surface temperatures between $110~\mbox{K}$ and $355~\mbox{K}$, we find $\lambda$ to be in the range of $0.04-0.11$. This method allows to extract a correctly averaged $\lambda$ and to address the discrepancy between previous studies. The relatively modest value of $\lambda$ is not surprising even though some individual phonons may provide a larger electron-phonon interaction. Furthermore, the surface Debye temperature of Bi$_2$Te$_3$(111) is determined as ${\rm \Theta}_D = (81\pm6)~\mbox{K}$. The electronic surface corrugation was analysed based on close-coupling calculations. By using a corrugated Morse potential a peak-to-peak corrugation of 9% of the lattice constant is obtained.

Structural, elastic and magnetic studies of the as-synthesized Co1−xSrxFe2O4 nanoparticles

A series of as-synthesized Co1−xSrxFe2O4 nano-ferrites, 0.0 ≤ x ≤ 0.6, were prepared by the chemical co-precipitation method. The techniques used in this investigation were x-ray diffraction, transmission electron microscope, infrared spectrometer and vibrating sample magnetometer. The results proved that the samples have single phase cubic spinel structure. This study revealed that the crystallite size, strain and elastic wave velocities were decreased by increasing strontium content x, whereas the density, theoretical density, specific surface area, force constants, Debye temperature and elastic moduli were increased. The true lattice constant and porosity proved dependence on Sr2+ion content x. TEM images showed agglomeration of the nanoparticles, where the average particle size values ranged 10.56–23.66 nm which agree with R values. Six absorption bands were observed in the IR spectra and assigned to the corresponding site bonds. The saturation magnetization (Ms), coercivity, magnetic moment, anisotropy constant and remnant magnetization were affected by Sr2+ ion content x. The vibrational frequencies; ν1 and ν2 showed decrease with increasing Ms. On the other hand, Ms proved dependence on the crystallite size R, whereas the elastic wave velocities were affected by the sample density.

Detection of the adsorption of water monolayers through the ion oscillation frequency in the magnesium oxide lattice by means of low energy electron diffraction

We investigate the variation of the oscillation frequency of the Mg2+ and O2− ions in the magnesium oxide lattice due to the interactions of the surface with water monolayers by means of Low Energy Electron Diffraction. Our key result is a new technique to determine the adsorbate vibrations produced by the water monolayers on the surface lattice as a consequence of their change in the surface Debye temperature and its chemical shift. The latter was systematically investigated for different annealing times and for a constant external thermal perturbation in the range of 110–300 K in order to accomplish adsorption or desorption of water monolayers in the surface lattice.

Graphene on Ni(111): Electronic Corrugation and Dynamics from Helium Atom Scattering.

Using helium atom scattering, we have studied the structure and dynamics of a graphene layer prepared in situ on a Ni(111) surface. Graphene/Ni(111) exhibits a helium reflectivity of ∼20% for a thermal helium atom beam and a particularly small surface electron density corrugation ((0.06 ± 0.02) Å peak to peak height). The Debye–Waller attenuation of the elastic diffraction peaks of graphene/Ni(111) and Ni(111) was measured at surface temperatures between 150 and 740 K. A surface Debye temperature of θD = (784 ± 14) K is determined for the graphene/Ni(111) system and θD = (388 ± 7) K for Ni(111), suggesting that the interlayer interaction between graphene and the Ni substrate is intermediary between those for strongly interacting systems like graphene/Ru(0001) and weakly interacting systems like graphene/Pt(111). In addition we present measurements of low frequency surface phonon modes on graphene/Ni(111) where the phonon modes of the Ni(111) substrate can be clearly observed. The similarity of these findings with the graphene/Ru(0001) system indicates that the bonding of graphene to a metal substrate alters the dynamic properties of the graphene surface strongly and is responsible for the high helium reflectivity of these systems.

Structural properties and magnetic interactions in Sr-doped Mg–Mn nanoparticle ferrites

As-prepared Mg0.2−xSrxMn0.8Fe2O4 nanoferrites, 0 ≤ x ≤ 0.2, were synthesized through the chemical co-precipitation route. The true lattice constant, relative atomic ratio of elements, mean site ionic radii, hopping and bond lengths, site edges, oxygen positional parameter u, specific surface area, strain, Debye temperature and threshold frequency were determined and discussed as functions of Sr2+ content x. Mössbauer spectra showed coexistence of magnetic and paramagnetic phases in all the samples which proved that they have superparamagnetic property. The spectra were analyzed and attributed to two magnetic subpatterns and two quadrupole doublets due to Fe3+ ions at the A- and B-sites. This study proved that the quadrupole and isomer shifts, hyperfine magnetic fields, HA and HB, quadrupole doublet splitting, ΔEA and ΔEB, and outermost line width at the A- and B-sites, respectively, were x dependent, where the cation distribution was estimated. HA and HB proved dependence on the bond length at the corresponding site, whereas ΔEA and ΔEB showed dependence on the crystallite size R and u. FMR spectra indicated a ferrimagnetic nature of all the samples, where the Lande factor, line width and resonance field showed nonlinear dependence on x. TEM images showed that the average particle size ranges 22–35 nm and ferromagnetically ordered nano clusters may be existed in the samples.

The surface stability of Cr2O3(0 0 0 1)

The surface of chromia (Cr2O3) has a surface electronic structure distinct from the bulk and a packing density distinct from the bulk. More than a demarcation between the solid and the vacuum, the surface differs from the bulk of chromia, not just because of a partial occupancy of chromium sites, but also because of an increased number of unoccupied surface oxygen sites (vacancy sites), evident in angle-resolved core level photoemission. In spite of the structural differences that exist at the surface, there is, as yet, no evidence that these complications affect the surface Debye temperature beyond the most simple of assumptions regarding the lower coordination of the surface. Using low-energy electron diffraction (LEED), the effective surface Debye temperature (∼490 K) is found to be lower than the bulk (∼645 K) Debye temperature of Cr2O3(0 0 0 1). This surface effective Debye temperature, indicative of vibrations along the surface normal, uncorrected for anharmonic effects, has a value reduced from the effective bulk Debye temperature yet close to the value √2 expected from a simple mean field argument.

Surface Debye temperature and vibrational dynamics of Antimony(111) from helium atom scattering measurements

The surface Debye temperature and the vibrational dynamics of Sb(111) were studied using helium atom scattering. The Debye–Waller attenuation of the elastic diffraction peaks was measured at surface temperatures between 98K and 447K. A surface Debye temperature of (155±3)K is obtained within the description originally derived for electron diffraction. The attractive well depth for the He–Sb(111) interaction is determined to be (4.5±0.5) meV. The perpendicular mean-square displacement for the surface at room temperature is estimated to be (1.8±0.4)×10−2Å2.

Surface Energy, Surface Debye Temperature and Specific Heat of Nanocrystals

The present study provides insight into the role of surface free energy and surface Debye temperatures in the specific heat of nanocrystals. In the case of small phases, consisted of comparable number of bulk and surface atoms, the variation of heat capacity with respect to that of infinitely large phase is significant. This effect, not accounted for in the Debye model is due to the substantial impact of interface energy of clusters. Here, in the framework of classical Gibbs thermodynamics, we introduce surface heat capacity that accounts for surface free energy contribution. Defined as a difference, between surface and bulk terms, the excess heat capacity reveals temperature gap and size limits of nanoclusters where the contribution of surface energy is essential. Considering the role of surface Debye temperatures, we evaluate the deviation of specific heat for nanoclusters having different surface atomic density and surface orientation. Being in agreement with recent experimental findings, the present model opens up a way to predict basic, size-dependent thermodynamic properties of nanocomposite systems.

Vibrational dynamics and surface structure of Bi(111) from helium atom scattering measurements

The Bi(111) surface was studied by elastic scattering of helium atoms at temperatures between 118 and 423 K. The observed diffraction patterns with clear peaks up to third order were used to model the surface corrugation using the eikonal approximation as well as the GR method. Best fit results were obtained with a rather large corrugation height compared to other surfaces with metallic character. The corrugation shows a slight enhancement of the surface electron density in between the positions of the surface atoms. The vibrational dynamics of Bi(111) were investigated by measurements of the Debye–Waller attenuation of the elastic diffraction peaks and a surface Debye temperature of (84 ± 8) K was determined. A decrease of the surface Debye temperature at higher temperatures that was recently observed on Bi nanofilms could not be confirmed in the case of our single-crystal measurements.

The effective surface Debye temperature of Yb:GaN

The effective Debye temperature of ytterbium and gallium in Yb:GaN thin films has been obtained using X-ray photoemission spectroscopy. The vibrational motion normal to the surface results in a dimunition of photoemission intensities from which we have estimated the effective Debye temperatures of 221 ± 30 K and 308 ± 30 K for Yb and Ga, respectively. The difference between the measured values for Yb and Ga suggests that the Debye temperatures are influenced by the local environment. The smaller effective surface Debye temperature for Yb correlates to a soft, strained surface, possibly due to an increased Yb―N bond length as compared to the Ga―N bond length.

Helium atom diffraction measurements of the surface structure and vibrational dynamics of CH3–Si(111) and CD3–Si(111) surfaces

The surface structure and vibrational dynamics of CH(3)-Si(111) and CD(3)-Si(111) surfaces were measured using helium atom scattering. The elastic diffraction patterns exhibited a lattice constant of 3.82 Å, in accordance with the spacing of the silicon underlayer. The excellent quality of the observed diffraction patterns, along with minimal diffuse background, indicated a high degree of long-range ordering and a low defect density for this interface. The vibrational dynamics were investigated by measurement of the Debye-Waller attenuation of the elastic diffraction peaks as the surface temperature was increased. The angular dependence of the specular (θ(i)=θ(f)) decay revealed perpendicular mean-square displacements of 1.0×10(-5) Å(2) K(-1) for the CH(3)-Si(111) surface and 1.2×10(-5) Å(2) K(-1) for the CD(3)-Si(111) surface, and a He-surface attractive well depth of ∼7 meV. The effective surface Debye temperatures were calculated to be 983 K for the CH(3)-Si(111) surface and 824 K for the CD(3)-Si(111) surface. These relatively large Debye temperatures suggest that collisional energy accommodation at the surface occurs primarily through the Si-C local molecular modes. The parallel mean-square displacements were 7.1×10(-4) and 7.2×10(-4) Å(2) K(-1) for the CH(3)-Si(111) and CD(3)-Si(111) surfaces, respectively. The observed increase in thermal motion is consistent with the interaction between the helium atoms and Si-CH(3) bending modes. These experiments have thus yielded detailed information on the dynamical properties of these robust and technologically interesting semiconductor interfaces.

Softening versus hardening transition in surface bilayer bonding of bismuth nanofilm

We have found that the surface bilayer of Bi(001) undergoes an unusual reversible transition at 350 K, well below the bulk melting temperature. Surface Debye temperature abruptly decreases without an anharmonic signature and the vertical-to-lateral ratio of surface thermal expansion increases in a critical manner, as revealed by high-resolution diffraction measurement. Because of the metallic-covalent bilayer structure of semimetal Bi, the most feasible explanation for the result is that bond softening occurs between the first and second bilayers at 350 K. This softening transition leads to the simultaneous hardening of the topmost intrabilayer bonds.

Surface Debye Temperatures and Specific Heat of Nanocrystals

The present study deals with the contribution of the surface atomic layer to the specific heat of nanocrystals. In the case of small phases with comparable number of bulk and surface atoms, the variation of heat capacity with respect to that of infinitely large phase is significant. To evaluate this variation, in the framework of the classical Debye model, we introduce surface excess heat capacity that accounts for the surface layer contribution. On that physical background, we draw a functional dependence of the specific heat of nano clusters on the number of their bulk and surface atoms and corresponding bulk, , and surface, , Debye temperatures. Since depends on the crystallographic orientation of the surface, the presented model also accounts for the symmetry and atomic density of the respective crystal faces of the nanocrystal.

Elastic and inelastic scattering components in the angular intensity distribution of He scattered from graphite

The angular intensity distribution of thermal energy He beam scattered from highly oriented pyrolytic graphite (HOPG) surface has been measured by means of supersonic molecular beam scattering technique in the wide surface temperature range. To separate the elastic and inelastic scattering components, simple analysis method has been developed by applying the classical binary collision theory of the hard cube model (HCM). From the extracted elastic scattering component in the scattering distribution, the Debye temperature of the HOPG surface has been derived as 590 ± 30 K. On the basis of the HCM analysis for the extracted inelastic scattering components of He beam, the effective mass for the HOPG surface has been found to be 72 u (six carbon atoms).

Phase Transition of In/Si(111) Surface Studied by Reflection High-Energy Positron Diffraction

We determined the structures of Si(111)-4×1-In and Si(111)-8×2-In surfaces that are formed at 293 K and 60 K, respectively, through the rocking curve analyses of reflection high-energy positron diffraction (RHEPD). The structure of Si(111)-4×1-In surface is in good agreement with the zigzag chain structure determined by the surface X-ray diffraction [O. Bunk et al., Phys. Rev. B 59, 12228 (1999)]. In the Si(111)-8×2-In surface, In atoms are displaced in two dimensional directions from the positions of zigzag chain structure. The structure of Si(111)-8×2-In surface determined here is compatible to the hexagon structure predicted by the first principles study [C. Gonzalez, F. Flores, and J. Ortega, Phys. Rev. Lett. 96, 136101 (2006)]. We determined the surface Debye-temperatures of 4×1 and 8×2 phases to be 80 K and 130 K, respectively. [DOI: 10.1380/ejssnt.2009.436]

Surface dynamics measurements of silica glass

We present helium atom scattering experiments from the vitreous silica surface. Time-of-flight data of scattered He-atoms have been recorded at varying temperature and as a function of parallel momentum transfer. A dominant elastic peak and two broad features at $\ensuremath{\approx}4\text{ }\text{meV}$, one each on the phonon annihilation and creation side, are observed in all spectra. The thermal measurements between 127.0 and 368.5 K unveil clear Debye-Waller attenuation behavior of the elastic peak and indicate that scattering occurs in the single-phonon regime below 200 K. The root-mean-square vibration amplitude at room temperature is estimated to be 0.014 nm with a corresponding surface Debye temperature of $340\ifmmode\pm\else\textpm\fi{}20\text{ }\text{K}$ and an effective mass of 18 a.u. The obtained values agree well with bulk values under the assumption of force constants at the surface reduced by one-half. The wave-vector dependence of the Debye-Waller exponent indicates a well depth of the atom-surface interaction potential of $10\ifmmode\pm\else\textpm\fi{}2\text{ }\text{meV}$. The broad inelastic features on the phonon creation and annihilation sides are identified as the surface manifestation of the boson peak phenomenon in amorphous materials. An unusual shifting of the boson peak modes to higher energies with increasing temperature is observed. Measurements at different parallel momentum transfers unveil a nondispersive behavior of the boson peak mode.