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The atomistic origin of the atomic vacancy or nanocavity induced hardening in hollow nanotubes and nanoporous structures has long been a puzzle. An analysis from the perspective of bond-order–length-strength correlation mechanism [C. Q. Sun, Prog. Solid State Chem. 35, 1 (2007)] has led to solutions that show that the shortened and strengthened bonds between the undercoordinated atoms in the negatively curved surface skins dominate the observed nanocavity strengthening and thermal instability of the porous structures. It is suggested that the broken bond derived local strain and quantum trapping and the associated energy densification provide pinning centers for inhibiting atomic dislocations and that the broken bond induced cohesive energy dropping dominate the thermal instability. On the other hand, nanocavities also provide sites that initiate the structure failure under plastic deformation. The agreement between predictions and the experimentally observed size dependence of mechanical strength of some nanoporous materials and the well-known phenomenon of hollow tube strengthening evidences for the proposed mechanism. [ABSTRACT FROM AUTHOR]

With the miniaturization of a solid device, quantum and interface effects become dominant not only in the static properties but also in the transport dynamics in the solid. Here we examine the dc conductance of a nanosolid by introducing the depression of the intra-atomic trapping potential in the surface skin [Sun, Phys. Rev. B 69, 045105 (2004)] as a perturbation to the conventional “quantum well” for tunneling electrons. It is derived that downshifts of the dc conductance peaks (that is, the incident energies for resonant tunneling) happens in an oscillatory way, depending on the strength of the perturbation. Besides, the downshifts of the peak positions due to the trapping potential well depression decrease as the size of the nanosolid is increased. [ABSTRACT FROM AUTHOR]

CF4 plasma-passivation enhanced size dependence of the blueshift in photoemission and photoabsorption, E2p-level shift, and band-gap expansion of porous silicon has been measured and analyzed numerically based on the recent “bond order-length-strength” correlation [ C. Q. Sun, Phys. Rev. B 69, 045105 (2004) ]. Matching predictions to the measurements conducted before and after fluorination reveals that fluorination further enhances both the crystal binding intensity that determines the band gap and core level shift and the electron-phonon coupling that contributes to the energies of photoemission and photoabsorption. This approach enables us to discriminate the effect of surface-bond contraction from the effect of surface-bond nature alteration on the unusual behavior of photons, phonons, and electrons in nanosolid Si. [ABSTRACT FROM AUTHOR]

An approach improving diamond–metal adhesion has been developed based on modeling predictions and experimental verifications on the interfacial stresses modified by catalytic reaction. It is found that N-plasma irradiating onto Ti and tungsten-carbide substrates generates tensile surface stresses while C-plasma irradiation creates strongly compressive stress at the surfaces, both of which deteriorate the diamond–metal adhesion. It is also found that surface oxidation prevents diamond nucleation. Therefore, we applied a graded TiCN interlayer with carefully adjusting the ratio of C and N in the gas mixture to neutralize the interfacial stress and, hence, we have improved the diamond–metal adhesion substantially. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Correlation between the size-enhanced Stokes shift and the size-enlarged band gap expansion of porous silicon has been derived, which allows us to discriminate the effect of electron-phonon coupling from the effect of crystal binding on the blueshift in photoemission and photoabsorption of nanosolid silicon. Matching predictions to the measured peak shift of both photoemission and absorption and to the measured band gap expansion evidences the essentiality and validity of the recent bond order-length-strength correlation mechanism which suggests that the atomic coordination imperfection dictates the shape and size dependency of a nanosolid. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of surface bond-order loss on the electronic thermal conductivity of metallic polycrystalline films is examined using Boltzmann transport theory. A modification of the grain boundary potential barrier has been made by adding depressed potential wells of intra-atomic trapping [C. Q. Sun, Phys. Rev. B 69, 045105 (2004)] to both sides of the grain boundaries. Electron scattering by film surfaces is also considered to follow the line of Fuchs’ convention. Results show that the thermal conductivity of the films is sensitive to the film thickness, mean grain size, and Fermi energy. In particular, thermal conductivity increases significantly with the depth of the atomic trapping due to the bond-order loss induced surface bond contraction and associated bond strength gain. [ABSTRACT FROM AUTHOR]

Deep level transient spectroscopy and optical absorption spectroscopy measurement revealed three outstanding features of density-of-states (DOS) appeared above the valence band edge (Ev) of Al oxide thin films. The broad peak located at 0.39 eV above Ev disappears while the other two located at 1.0 and 1.3 eV shift in position and attenuate in intensity upon annealing at 200 °C. The latter two peaks are removed by annealing at temperature up to 400 °C. The observed midgap DOS feature dynamics is in accordance with the oxygen lone pair features as confirmed earlier with Raman spectroscopy in the low-frequency ranges [C. Q. Sun et al., J. Appl. Phys. 90, 2615 (2001)] and thermal desorption measurements of other oxide surfaces. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Dielectric impedance measurements of porous silicon within the frequency range of 50 Hz-1.0 MHz and temperature range of 298-798 K revealed three semicircles in a Cole-Cole plot when the temperature is raised to 773 K; they are thought to correspond to contributions from the grain interior, grain boundary, and electrode/film interface, respectively. The enhancement in conductivity by heating follows an Arrhenius law with an activation energy transition from 0.07 to 0.79 eV at ∼565 K, which originates from band tail hopping that occurs around the Fermi edge. At a critical temperature, a high degree of dispersion in the real and imaginary parts of the permittivity also occurs at low frequencies. This dispersion behavior is interpreted as a combination of electron-lattice polarization associated to the band tail hopping and the crystal field weakening due to thermal expansion. [ABSTRACT FROM AUTHOR]

Consistency between x-ray photoelectron spectroscopy measurements and density-function theory calculations confirms our bond order-length-strength notation-incorporated tight-binding theory predictions on the quantum entrapment of Si solid skin and atomic clusters. It has been revealed that bond-order deficiency shortens and strengthens the Si-Si bond, which results in the local densification and quantum entrapment of the core and valence electrons. Unifying Si clusters and Si(001) and (111) skins, this mechanism has led to quantification of the 2p binding energy of 96.089 eV for an isolated Si atom, and their bulk shifts of 2.461 eV. Findings evidence the significance of atomic undercoordination that is of great importance to device performance. [ABSTRACT FROM AUTHOR]

We have formulated the size and temperature dependence of the phonon relaxation dynamics for CdS, CdSe, Bi2Se3, and Sb2Te3 nanostructures based on the framework of bond order-length-strength correlation, core-shell configuration, and local bond averaging approach. The Raman shifts are correlated directly to the identities (nature, order, length, and energy) of the representative bond of the specimen without needing involvement of the Grüneisen mode parameters or considering the processes of phonon decay or multi-phonon resonant scattering. Quantitative information of the Debye temperature, the atomic cohesive energy, the reference frequencies from which the Raman shifts proceed, and the effective coordination numbers of the randomly sized particles, as well as the length and energy of the representative bond, has been obtained. It is clarified that the size-induced phonon softening arises intrinsically from the cohesive weakening of the undercoordinated atoms in the skin up to three atomic layers and the thermally derived phonon softening results from the thermally lengthening and weakening of bonds. Developed approach empowers the Raman technique in deriving quantitative and direct information regarding bond stiffness relaxation with applied stimuli such as coordination, mechanical, thermal, and chemical environment, which are crucial to practical applications. [ABSTRACT FROM AUTHOR]

Correlation between the elastic modulus (B) and the Raman shift (Δω) of TiO2 and their responses to the variation of crystal size, applied pressure, and measuring temperature have been established as a function depending on the order, length, and energy of a representative bond for the entire specimen. In addition to the derived fundamental information of the atomic cohesive energy, binding energy density, Debye temperature and nonlinear compressibility, theoretical reproduction of the observations clarified that (i) the size effect arises from the under-coordination induced cohesive energy loss and the energy density gain in the surface up to skin depth; (ii) the thermally softened B and Δω results from bond expansion and bond weakening due to vibration; and, (iii) the mechanically stiffened B and Δω results from bond compression and bond strengthening due to mechanical work hardening. With the developed premise, one can predict the changing trends of the concerned properties with derivatives of quantitative information as such from any single measurement alone. [ABSTRACT FROM AUTHOR]

Although the physics behind the bulk modulus, B(T,P), as a function of temperature (T) and pressure (P), has been intensively investigated, an atomic scale understanding of this attribute remains a high challenge. Here, we show that the B(T,P) for BaXO3 (X=Ti,Zr,Nb) can be established by connecting the B directly to the bond length and bond energy and their response to the applied T and P in the form of binding energy density, B[E/d3(T,P)]. Besides an estimation of the Debye temperature and single bond energy, outcomes clarified that the thermally softened B arises from bond expansion and bond weakening due to lattice vibration and the mechanically stiffened B results from bond compression and bond strengthening due to mechanical work hardening. [ABSTRACT FROM AUTHOR]

The functional dependence of stress, elastic modulus, melting point, and their interdependence on the identities (bond order, nature, length, and strength) of a representative bond of the specimen has been established for deeper insight into the transition from the conventional Hall-Petch relationship (HPR) to the inverse HPR (IHPR) for nanostructured TiO2. Theoretical reproduction of the observed inverse HPR suggests that the intrinsic competition between the energy-density gain (elastic modulus enhancement) and the cohesive-energy remnant (melting point depression) in the grain boundaries originates and the extrinsic competition between the activation and the inhibition of atomic dislocations activates the IHPR. [ABSTRACT FROM AUTHOR]

In this work, ballistic phonon transport in atomic chain nanostructures is investigated by atomic nonequilibrium Green's functions and embedded atom method. Bond length and strength modification in atomic chain (low-dimensional structure) was taken into consideration by using bond-order-length-strength correlation premise. We especially focus on the contact interface effects on phonon transmission and thermal conductance. It is found that the contact interfaces between an atomic chain and contact reservoir, i.e., neck region or buffer layers, play an important role in phonon transport. The more buffer layers the less thermal conductance. [ABSTRACT FROM AUTHOR]

Although barium titanate (BaTiO3) ferroelectrics at the nanoscale has been extensively investigated, the physical origin of their unusual performance, in particular, the suppressed thermal stability remains yet unclear despite existing models from various perspectives. Based on the consideration of the short range bond order-length-strength correlation and the long range dipole-dipole interaction, we have developed a dual-shell model for the size and shape induced suppression of the Curie temperature for BaTiO3 nanocrystals. One surface shell of three atomic layers represents the short range interaction due to the shorter and stronger bonds between under-coordinated atoms, which intrinsically lowers the mean atomic cohesive energy of the crystal. The other shell with thickness of Kc (critical number of atomic layers) characterizes the long rang dipole-dipole interaction. If one moves a BaTiO3 unit cell from the center of the nanocrystal outwards, the unit cell will lose its ferroelectrics gradually upon reaching the Kc point. The modeling predictions have been verified by the presented experimental observations and results documented in the open literature. The least Kc has been optimized to be 9 for BaTiO3 spherical particles. [ABSTRACT FROM AUTHOR]

The effect of surface bond reconstruction (SBR) of Si contact surfaces on phonon transport in Si atomic wire is investigated. Green’s function method is applied to calculate the thermal conductance and local heat currents. Results show that the phonon transport in atomic wires is enhanced significantly by SBR at the thermal contact surface. A blue shift for phonon transmission function is induced by the SBR. [ABSTRACT FROM AUTHOR]

Phonon transport in atomic wire coupled by two semi-infinite thermal contacts is investigated by atomic nonequilibrium Green’s functions. The effect on phonon transport due to anharmonicity in atomic wire and surface bond reconstruction of thermal contact is analyzed. It is found that surface bond reconstruction affects phonon transport significantly in the presence of anharmonicity. [ABSTRACT FROM AUTHOR]

The effect of substrate temperature (TS) on the behavior of field emission, microstructure, optical band gap, and the surface energy of N-doped SrTiO3 thin films coated on silicon tip arrays has been examined in detail. Results indicate that the TS dominates the chemical states of nitrogen added to the sputtered SrTiO3 films and hence the observations. At the critical temperature of 600 °C, nitrogen atoms incorporate into the oxide film with sp-hybridization features. The generation of the nonbonding lone pair states narrows the optical band gap and the lone pair induced antibonding dipoles lower the threshold field for electron emission substantially. At lowered TS, molecular adsorption of nitrogen dominates. Contact angle measurements further evidence for the presence of antibonding dipole states at the surfaces which is responsible for the adsorbate-induced surface stress. [ABSTRACT FROM AUTHOR]

The effect of nitrogen (N) doping on the behavior of field emission, surface energy and the band structure of strontium titanate (SrTiO3) thin films coated on silicon tip arrays has been examined in detail. Measurements using x-ray photoelectron spectroscopy, ellipsometry, water contact angle and field emission testing revealed that the optimal 50%-nitrogen partial pressure (PN) could improve substantially the threshold field of electron emission of the SrTiO3 films accompanied with narrowed band gap, lowered surface energy and work function and a negative energy shift of the N 1s level from 404 to 396 eV. Results evidence consistently the presence of the nonbonding lone pairs and the lone pair induced antibonding dipoles upon tetrahedron formation which is responsible for the observations. At PN below and above the optimal value physisorption and hydrogen bond likes formation like to occur. [ABSTRACT FROM AUTHOR]

The thermally induced softening of the elastic and vibronic identities in crystals and their correlations have long been a puzzle. Analytical solutions have been developed, showing that the detectable elastic and vibronic properties could be related directly to the bonding parameters, such as bond length and strength, and their response to the temperature change. Reproduction of measured T-dependent Young’s modulus and Raman shift of Si, Ge, and diamond reveals that the thermally driven softening of the elasticity and the optical Raman frequency arises from bond expansion and vibration, with derived information about the atomic cohesive energy and clarification of their interdependence. [ABSTRACT FROM AUTHOR]

Carbon nitride films have been deposited by rf reactive magnetron sputtered graphite carbon in an N[SUB2] discharge. The process parameters, viz., nitrogen partial pressure (P[SUBN[SUB2]]), substrate temperature (T[SUBs]), and substrate bias (V[SUBb]) were varied in order to investigate their influence on the field emission properties. The effective work function for carbon nitride films determined using the Fowler-Nordheim equation is in the range of 0.01-0.1 eV. Insight is presented into the nitrogen-lowered threshold of cold cathode electron emission of carbon from the perspective of nitride tetrahedron bond formation. The involvement of nonbonding (lone pair) and lone-pair-induced antibonding (dipole) states is suggested to be responsible for lowering the work function and hence the electron emission threshold. It is found that the substrate temperature of 200 °C, floating potential at the substrate, and nitrogen partial pressure of 0.3 Pa are favorable to promote the reaction that lowers the work function. [ABSTRACT FROM AUTHOR]