Bond-order-length-strength Research Articles

Size- and composition-induced band-gap change of nanostructured compound of II–VI semiconductors

We have investigated the joint effect of size- and composition-induced band-gap change of semiconductive nanocompounds from the recently developed bond-order-length-strength (BOLS) correlation mechanism using the approach of local bond average (LBA). An analytical solution has been developed to connect the band-gap energy with the bonding identities of the nanocompounds. Agreement between the model predictions with the available experimental measurements of band-gap change of II–VI semiconductor nanocompounds showed that both the particle size and the composition with alloying effect can be used as factors tuning the band-gap energy, suggesting an effective way to realize the desirable properties of semiconductive nanocompounds.

FACTORS CONTROLLING THE STRONGEST SIZES IN THE INVERSE HALL–PETCH RELATIONSHIP

Incorporating the bond–order–length–strength correlation mechanism [C. Q. Sun, Prog. Solid State Chem.35, 1 (2007)] and Born's criterion for melting [J. Chem. Phys.7, 591 (1939)] into the conventional Hall–Petch relationship has turned out an analytical expression for the size and temperature dependence of the mechanical strength of nanograins, known as the inverse Hall–Petch relationship (IHPR). Reproduction of the measured IHPR of Ni , NiP , and TiO 2 nanocrystals revealed that: (i) the competition between the size-induced energy–density gain and atomic cohesive energy loss in the surface skins of nanograins originate from the IHPR; (ii) the competition between the activation and inhibition of atomic dislocations motion activate the entire IHPR behavior; (iii) the bond nature involved and the T/Tm ratio between the temperature of operating and the temperature of melting dictate the measured strongest sizes of a given specimen; (iv) a quasimolten phase present before melting determines the size-induced softening and the superplasticity of nanostructures.

Size Dependence of the Pressure-Induced Phase Transition in Nanocrystals

Extending the recently developed bond-order−length-strength (BOLS) correlation mechanism [Sun, C. Q. Prog. Solid-State Chem. 2007, 35, 1−159] to the pressure domain has led to atomistic insight into the phase stability of nanostructures under the varied stimuli of pressure and solid size. It turns out that the competition between the pressure-induced overheating (TC elevation) and the size-induced undercooling (TC depression) dominates the measured size trends of the pressure-induced phase transition. Reproduction of the measured size and pressure dependence of the phase stability for CdSe, Fe2O3, and SnO2 nanocrystals evidence the validity of the solution derived from the perspective of atomic cohesive energy and its response to the external stimulus.

Atomistic origin and temperature dependence of Raman optical redshift in nanostructures: a broken bond rule

AbstractConsistent insight is presented into the atomistic origin and temperature (T) dependence of the redshift of Raman optical modes in nanostructures from the perspective of surface bond unsaturation and the bond‐order‐length‐strength (BOLS) correlation [Sun et al., Phys Rev B 72, 134301 (2005)]. It turns out that: (1) the size‐induced redshift of Raman optical modes results from the effect of bond order deficiency of atoms on the surface and its consequences on the force constants of bonds between the under‐coordinated surface atoms; (2) the temperature‐induced redshift arises from thermally induced bond expansion and bond weakening and (3) the magnitude of vibration of atoms in the surface or surrounding defects is always greater than the fully coordinated bulk atoms, while the frequency of such under‐coordinated atoms are relatively lower. It is demonstrated that a combination of the freedoms of solid size and temperature of testing with Raman spectroscopy could lead to information about the vibrational frequency of a monatomic chain and single‐bond energy, which is beyond the scope of the traditional size‐ and temperature‐independent approaches. Copyright © 2007 John Wiley & Sons, Ltd.

Change in thermal conductivity of cylindrical silicon nanowires induced by surface bonding modification

The impact of bond order loss of surface atoms on thermal conductivity of cylindrical silicon nanowires has been examined using the isotropic elastic continuum model. A core-shell structure with a modified Young’s modulus in the surface skin of the wire has been used. Thermal conductivity is calculated from modified phonon dispersion relations based on recent developed bond-order-length-strength (BOLS) correlation. This work extends the previous linear approximation [Yeung et al. Phys. Rev. B 74, 155317 (2006)] to calculate the phonon dispersion relation of torsional mode and longitudinal mode using the nonlinear approach. A significant increase in thermal conductivity is found compared to the case without using BOLS correlation.

Size dependence of nanostructures: Impact of bond order deficiency

This report deals with the mechanism behind the unusual behavior of nanostructures in mechanical strength, thermal stability, acoustics (lattice dynamics), photonics, electronics, magnetism, dielectrics, and chemical reactivity and its indication for designing and fabricating nanostructured materials with desired functions. A bond-order-length-strength (BOLS) correlation mechanism has been initiated and intensively verified, which has enabled the tunability of a variety of properties of a nanosolid to be universally reconciled to the effect of bond order deficiency of atoms at sites surrounding defects or near the surface edges of the nanosolid. The BOLS correlation indicates that atomic coordination imperfection causes the remaining bonds of the under-coordinated atom to contract spontaneously associated with bond strength gain and the intraatomic trapping potential well depression. Consequently, localized densification of charge, energy and mass occurs to the surface skin, which modify the atomic coherency (the product of bond number and the single bond energy), electroaffinity (separation between the vacuum level and the conduction band edge), work function, and the Hamiltonian of the nanosolid. Therefore, any detectable quantity can be functionalized depending on the atomic coherency, electroaffinity, work function, Hamiltonian or their combinations. For instances, the perturbed Hamiltonian determines the entire band structure such as the band-gap expansion, core-level shift, Stokes shift (electron-phonon interaction), and dielectric suppression (electron polarization); The modified atomic coherency dictates the thermodynamic process of the solid such as self-assembly growth, atomic vibration, phase transition, diffusitivity, sinterbility, chemical reactivity, and thermal stability. The joint effect of atomic coherency and energy density dictates the mechanical strength (surface stress, surface energy, Young's modulus), and compressibility (extensibility, or ductility) of a nanosolid. Most strikingly, a combination of the new freedom of size and the original BOLS correlation has allowed us to gain quantitative information about the single energy levels of an isolated atom and the vibration frequency of an isolated dimer, and the bonding identities in the metallic monatomic chains and in the carbon nanotubes. A survey and analysis of the theoretical and experimental observations available to date demonstrated that the under-coordinated atoms in the surface skin of 2-3 atomic layers dictate the performance of nanostructures yet atoms of the interior remain as they are in the bulk counterpart. Further extension of the BOLS correlation and the associated approaches to atomic defects, impurities, liquid surfaces, junction interfaces, and amorphous states and to the temperature domain would be more challenging, fascinating, and rewarding.

Size and Confinement Effect on Nanostructures

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Size-induced acoustic hardening and optic softening of phonons in InP,CeO2,SnO2, CdS, Ag, and Si nanostructures

It has long been a puzzle that the Raman optical modes shift to lower frequency (or termed optical mode softening or redshift) associated with the creation of Raman acoustic modes that shift to higher energy (or called acoustic hardening or blueshift) upon a nanosolid being formed and its size being reduced. Understandings of the mechanism behind the size-induced Raman shifts have been quite controversial. On the basis of the bond-order--length--strength (BOLS) correlation mechanism [Phys. Rev. B 69, 045105 (2004)], we show that the optical softening arises from atomic cohesive energy weakening of atoms in the surface skins, whereas the acoustic mode hardening is predominated by intergrain interactions. Agreement between predictions and observations has been realized for Ag, Si, CdS, InP, ${\mathrm{TiO}}_{2}$, ${\mathrm{CeO}}_{2}$, and ${\mathrm{SnO}}_{2}$ nanostructures with elucidation of vibration frequencies of the corresponding isolated dimers from fitting the optical softening.

Distinguishing the effect of crystal-field screening from the effect of valence recharging on the 2p 3/2 and 3d 5/2 level energies of nanostructured copper

Incorporating the recent bond order–length–strength correlation mechanism [C.Q. Sun, Phys. Rev. B 69 (2004) 045105] to the size dependence of Auger photoelectron coincidence spectra of Cu nanoparticles with and without being passivated has enabled us to gain quantitative information about the 2p and 3d level energies of an isolated Cu atom and their shift upon bulk formation. The developed approach also enabled us to discriminate the effect of crystal-field screening from the effect of valence recharging (due to surface passivation and substrate–particle interaction) on the binding energies to the electrons at different energy levels of a specimen.

ATOMIC-COORDINATION-IMPERFECTION-ENHANCED Pd-3d5/2 CRYSTAL BINDING ENERGY

Crystal binding energy of a core electron at the 3d 5/2 level of a Pd atom has been estimated to be -4.0±0.02 eV by decoding the X-ray photoelectron spectra obtained from both the Pd surfaces and the Pd nanoparticles. Findings indicate that the increase in the binding energy originates from the effect of coordination number (CN) imperfection of atoms at a flat surface or at the curved surface of a nanosolid. The CN imperfection shortens the remaining bonds of the lower-coordinated atoms spontaneously associated with bond energy increase, which perturbs the Hamiltonian of an extended solid and hence shifts the core level to higher binding energy. Therefore, the significance of atomic CN imperfection cannot be overlooked in dealing with a low-dimensional system, and the recent bond-order–length–strength (BOLS) correlation mechanism is essentially adequate, for the significance of atomic CN imperfection.

Oxidation Electronics: Bond—Band—Barrier Correlation and Its Applications

AbstractFor Abstract see ChemInform Abstract in Full Text.

Size-effect on the electronic structure and the thermal stability of a gold nanosolid

Consistent insight into the size-enhanced E4f-level shift and the size-suppressed melting point of Au nanosolids has been obtained based on the bond order-length-strength (BOLS) correlation mechanism [Sun et al., Acta Mater. 51 (2003) 4631]. Consistency between theory predictions and observations reveals that the atomic-coordination number (CN)-imperfection induced bond contraction and the associated bond energy increase dictate these changes. The increase of binding energy density per unit volume in the relaxed surface region perturbs the Hamiltonian that determines the core-level shift; the decrease of atomic cohesive energy (a product of atomic CN and the single bond energy) determines the thermal energy required for melting. Extending the knowledge to the lower end of the size limit suggests that the metallic bond in the gold monatomic-chain contracts by ∼30% associated with ∼43% magnitude rise of the bond energy. The crystal binding intensity contributing from all the atoms in the solid to the Au-E4f electrons in the bulk is determined to be −2.87 eV.