C. H. Kam

Yellow-to-violet upconversion in neodymium oxide nanocrystal/titania/ormosil composite sol–gel thin films derived at low temperature

Neodymium (III) oxide nanocrystals synthesized by the inverse microemulsion technique were dispersed in sol–gel titania/organically modified silane (ormosil) composite thin films for photonic applications. X-ray diffraction analysis and transmission electron microscopy observation show that the neodymium oxide nanoparticles have a nanocrystalline structure and that the size of the nanoparticles is in the range of 5–60 nm. An intense room-temperature yellow-to-violet upconversion emission at 402 nm (4D3/2→4I13/2) was measured from the composite thin film heated at 100 °C upon excitation with a yellow light at 587 nm. In addition to this violet emission, ultraviolet emission at 372 nm and blue emission at 468 nm were also observed. The lifetime of the violet emission was measured and the mechanism of the upconversion emission is proposed, and they indicate excited-state absorption originating from the long-lived 4F3/2 excited state should be responsible for the yellow-to-violet upconversion.

Phonon transport in nanowire with contacts: Size and doping

The phonon transport in Lennard-Jones silicon wire with contacts is investigated using non-equilibrium Green’s function. With the size decreasing, the significant reduction in the number of phonon modes leads to a smaller thermal conductance density. The dopant (Ge) atoms are used to substitute the atom in the wire to study the doping effect. For thin wire, its thermal conductance is very sensitive to the location of dopants. It is also found that the interior atom substitution has more impact on the thermal conductance over surface atom; substitution near contact surface reduces thermal conductance significantly; thermal conductance is suffering a 10%–20% variation due the random distribution of dopants; 17% of Ge content is sufficient to reduce thermal conductance by 80%.

Influence of electron scatterings on thermoelectric effect

In this work, we employed non-equilibrium Green’s function to investigate the electron transport properties in a nanowire in the presence of scatterings. The scattering mechanism is modelled by the Buttiker probe. The effect of electron scattering is analyzed under three conditions: absence of external field; with a bias voltage; and with a finite temperature difference. It is found that weak and strong scattering strengths affect the electron transport in different ways. In the case of weak scattering strength, electron trapping increases the electron density, thereby boosting the conductance significantly. Although the increment in conductance would reduce the Seebeck coefficient slightly, the power factor still increases. In the case of strong scattering strength, electron diffraction causes the redistribution of electrons; accumulation of electrons at the ends of the wire blocks current flow; hence the conductance is reduced significantly. Although the Seebeck coefficient increases slightly, the power...

Phonon transport in atomic chains coupled by thermal contacts: The role of buffer layer

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.

The upper limit of thermoelectric figure of merit : importance of electronic thermoelectric efficiency

To improve thermoelectric (TE) efficiency, the physical phenomenon of TE effect is revisited. The important TE figure of merit (FOM) is expressed in terms of powers, and it is mapped by two fundamental quantities. One is the electronic TE efficiency, which is purely determined by a probability distribution function of electron transport. Furthermore, electronic TE efficiency plays an important role in the upper limit of TE FOM, which is an important index to judge the quality of a TE device. For any TE device with FOM more than one, its electronic TE efficiency must be greater than 0.5. For demonstration purpose, the TE properties of silicon nanowire are investigated.

The effect of surface bond reconstruction of thermal contact surfaces on phonon transport in atomic wire

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.

Photoluminescence of erbium oxide nanocrystals/TiO2/γ-glycidoxypropyltrimethoxysilane (GLYMO) composite sol–gel thin films derived at low temperature

Erbium (III) oxide nanocrystals prepared using the inverse microemulsion technique were dispersed in sol–gel TiO2/γ-glycidoxypropyltrimethoxysilane (GLYMO) composite thin films at low temperature. X-ray diffraction analysis and transmission electron microscopy observation show that the erbium (III) oxide nanoparticles have a nanocrystalline structure and their size is in the range from 5 to 30 nm. A relatively strong room-temperature photoluminescence (PL) was observed at 1.531 μm with a full width at half-maximum (FWHM) of 22 nm. The shape, peak position, and FWHM of the PL curve from the composite thin films are quite comparable to those prepared by other methods. The PL peak of the composite thin film reached a maximum intensity after a heat treatment at 300 °C. Fourier transform infrared results show that water and hydroxyl groups are present in the films and are responsible for the luminescence peak intensity reduction of the films heated at higher temperature.

Effect of surface bond-order loss on the dc conductance of a metallic nanosolid

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.

Effect of surface bond-order loss on the electronic thermal conductivity of metallic polycrystalline films

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.