Charge and Heat Transport in Polycrystalline Metallic Nanostructures

Metals are typically good conductors in which the abilities to transport charge and to transport heat can be related through the Wiedemann-Franz law. Here we report on an abnormal charge and heat transport in polycrystalline metallic nanostructures in which the ability to transport charge is weakened more obviously than that to transport heat. We attribute it to the influence of the internal grain boundaries and have formulated a novel relation to predict the thermal conductivity. The Wiedemann-Franz law is then modified to account for the influence of the grain boundaries on the charge and heat transport with the predictions now agreeing well with the measured results.     

Phonon spectroscopy measurements at amorphous films

Phonon spectroscopy measurements were used to examine the scattering of high frequency phonons (300 GHz-1 THz) in amorphous materials. The experiments were done with the use of time and frequency resolved measurements of the phonon transmission behaviour through amorphous single films of different thicknesses. The typical film thicknesses were of the order of 10 nm. In contrast to the pure amorphous semiconductors Si and Ge our experiments show inelastic phonon scattering processes in the case of SiO₂ and SiH. This inelastic phonon scattering also occurs when the pure semiconductors Si and Ge are prepared in an O₂ or H₂ atmosphere, but is missing when the preparation process is done in an N₂ atmosphere. In films of the pure semiconductors a-Si and a-Ge we only found evidence to elastic scattering processes. In further experiments at heated a-SiH samples we could examine the atomical bonded hydrogen to be the center of the inelastic phonon scattering.The measurements and investigations described in this work were done in time of preparing a thesis at: Physikalisches Institut Teil 1, Universität Stuttgart, Pfaffenwaldring 57, D-70569 Stuttgart, Germany     

The Casimir atomic pendulum

We want to introduce an atomic pendulum whose driving force (torque) is due to the quantum vacuum fluctuations. Applying the well-known Casimir-Polder effect to a special configuration (a combined structure of an atomic nanostring and a conducting plate), an atomic pendulum (Casimir atomic pendulum) is designed. Using practically acceptable data corresponding to the already known world of nanotechnology and based on reasonable/reliable numerical estimates, the period of oscillation for the pendulum is computed. This pendulum can be considered as both a new micro(nano)-electromechanical system and a new simple vacuum machine. Its design may be considered as a first step towards realizing the visualized vacuum (Casimir) clock!     

Etching of buried photoresist layers and its application to the formation of three-dimensional layered structures

The fabrication of three-dimensional layered structures with 180-nm-thick TaO_x top layers supported by 1.5-μm-thick Mo pillars formed on a glass substrate is presented. The photoresist used for planarization was successfully removed through the TaO_x layers using heat treatment at 270 °C with mixed vapors of ethyl alcohol and pure water at high pressure for 3 h. Vacancies underlying the TaO_x layers were consequently formed. The possibility of rapid and lateral crystallization of amorphous silicon films was demonstrated when the silicon films formed on the TaO_x overlaying the vacancy regions were irradiated using a frequency-doubled YAG laser at 250 mJ/cm². Energy sensors using Cr/Al metal wires, with a high sensitivity of 0.07 mW/cm², were also demonstrated using the present structure with vacancy regions for reduction of heat diffusion. Received: 22 January 2001 / Accepted: 24 January 2001 / Published online: 27 June 2001     

Cantilevered actuator using magnetostrictive thin film

This paper describes a cantilevered magnetic actuator driven by magnetostriction in a low magnetic field. The dimensions of the two layers actuator were 1×5 mm and amorphous FeSiB was used as the magnetostrictive material. Since the FeSiB has excellent soft magnetic characteristics, the actuator with FeSiB was able to work in magnetic field strength of less than 10 kA/m. The theoretical formulas for the amount of the displacement and the force of the actuator were obtained. The theoretical results agreed with the experimental one. According to the theoretical formula, the displacement was calculated with the parameter of the mechanical properties of the substrate. To obtain the large displacement, the actuator with Co substrate was designed based on the theoretical formula. The displacement of 153 μm was obtained using Cu substrate of 1.1 μm thickness in the magnetic field of 10 kA/m. © 2008 Elsevier B.V. All rights reserved     

Laser fluence dependence of the elastic properties of diamond-like carbon films prepared by pulsed-laser deposition

We studied the temperature dependence of internal friction of variety of amorphous diamond-like carbon films prepared by pulsed-laser deposition. Like the most of amorphous solids, the internal friction below 10 K exhibits a temperature independent plateau, which is caused by the atomic tunnelling states—a measure of structure disorder. In this work, we have varied the concentration of sp³ versus sp² carbon atoms by increasing laser fluence from 1.5 to 30 J/cm². Our results show that the internal friction has a nonmonotonic dependence on sp³/sp² ratio with the values of the internal friction plateaus varying between 6×10⁻⁵ and 1.1×10⁻⁴. We explain our findings as a result of a possible competition between the increase of atomic bonding and the increase of internal strain in the films, both of which are important in determining the tunneling states in amorphous solids. The importance of the internal strain in diamond-like carbon films is consistent with our previous study on laser fluence, doping, and annealing, which we will review as well. In contrast, no significant dependence of laser fluence is found in shear moduli of the films, which vary between 220 and 250 GPa.     

Low temperature acoustic properties in YBa2Cu3O7−δ

Temperature dependencies of acoustic lossesQ ^–1 and of relative sound velocity change v/v in YBa₂Cu₃O_7– up to 60 K are calculated by the tunneling model theory. The tunneling systems are related to the off-centered positions of the apical oxygen atoms O(A) and are described through the pseudo-Jahn-Teller effect. Tunneling systems' parameters are distributed in narrow range of values and are in correspondence with the experimentally observed infrared phonon spectra and thermal ellipsoids of O(A). Respective relaxation times are calculated by the adapted reaction rate method. The calculatedQ ^–1(T) and v(T)/v dependencies are in good agreement with the experimental data, which is an additional support to the conclusion about the existence of tunneling systems in YBa₂Cu₃O_7– due to the pseudo Jahn-Teller effect.     

Size-Dependent Melting Behaviour of Nanometre-Sized Pb Particles Studied by Dynamic Mechanical Analysis

Nanometre-sized （hereafter nano-） Pb particles embedded in an Al matrix are prepared by ball milling. It is found that the size of nano-Pb particles was decreased with increasing milling time. The melting behaviour of nano-Pb particles embedded in the Al matrix is studied by means of dynamic mechanical analysis, and a single internal friction peak in the vicinity of Pb melting temperature is observed. The onset temperature of the peak moves to lower temperature with the decrease of particles size and the internal friction peak height is increased, which indicates a size-dependent melting behaviour of nano-Pb particles. It is suggested that the size-dependent melting behaviour is associated with surface melting.     

Mechanical spectrum study of glass transition by a composite method

Normalized mechanical spectra of glycerol, 1,2-propanediol carbonate and poly(vinyl chloride)/di(2-ethyl-hexyl) phthalate (PVC/DOP) blends were studied in the temperature range from 100 to 300 K by a composite method. The dynamic glass transition was observed, which exhibits a peak of temperature-dependent loss modulus. The peak moves toward higher temperature with higher measuring frequency, which accords with the relaxation feature of the dynamic glass transition. Another characteristic temperature can be marked in the mechanical spectrum by the onset of storage modulus change, which is labeled as T_gm. T_gm is found to be nearly equal to the calorimetric glass transition temperature in glycerol, 1,2-propanediol carbonate and di(2-ethyl-hexyl) phthalate. As we expected, this onset temperature in the mechanical spectrum has an intimate relation with the calorimetric glass transition of materials, and it can be regarded as a representative when the calorimetric glass transition temperature is not available. Finally, normalized mechanical spectra of PVC/DOP blends with different PVC content were obtained and mechanical glass transition temperatures T_gm were determined.     

Bio-inspired dual surface modification to improve tribological properties at small-scale

In miniaturized devices like micro/nano-electro-mechanical systems (MEMS/NEMS), the critical forces, namely adhesion and friction restrict the smooth operation of the elements that are in relative motion. MEMS/NEMS are traditionally made of silicon, whose tribological properties are not good. In this paper, we present an investigation on the approach of dual surface modification of silicon surfaces and their tribological properties at micro-scale. The dual surface modification is a combination of topographical and chemical modifications. As the topographical modification, micro-patterns with varying shapes of pillars and channels were fabricated on Si(1 0 0) wafer surfaces using photolithography method. Chemical modification included the coating of micro-patterns with diamond-like carbon (DLC) and Z-DOL (perfluoropolyether, PFPE) thin films. The surfaces with combined modification were evaluated for their micro-friction behavior in comparison with those of bare Si(1 0 0) flat surfaces and the topographically/chemically modified silicon surfaces. Results showed that the surfaces with dual modification exhibited superior tribological properties. These results indicate that a combination of topographical and chemical modification is very effective in enhancing tribological properties at small-scale. The combined surface treatments such as the ones investigated in the current work could be useful for tribological applications in small-scale devices such as MEMS/NEMS. The motivation for undertaking the dual modification approach comes from an earlier observation made on the significant influence of the surface characteristics of lotus leaf on its micro-friction behavior.     

Squeezing Effect of a Nanomechanical Resonator Coupled to a Two-Level System: an Equilibrium Approach

The squeezing effect of a nanomechanical resonator coupled to a two-level system is studied by variational calculations based on both the displaced-squeezed-state （DSS） and the displaced-oscillator-state （DOS）. The stable region of the DSS ground state at both T = 0 and T ≠ 0 and the corresponding squeezing factor are calculated. It is found that when the resonator frequency lies in （kBT,△）, where A is the tunnelling splitting of the two-level-system in the presence of dissipation, tunnelling splitting of a DSS ground state decreases with the temperature, while tunnelling splittihg of a DOS ground state increases with the temperature in low temperature region. This opposite temperature dependence can help to distinguish between the DSS and DOS ground state in the experiment.     

Resin-bonded permanent magnetic films with out-of-plane magnetization for MEMS applications

Resin-bonded permanent magnets with out-of-plain direction of magnetization and improved magnetic properties for magnetic MEMS actuator have been created. The material investigated consists of magnetically anisotropic strontium ferrite particles embedded into epoxy resin matrix upto a volume loading of 80%. Intrinsic coercivity H_ci of 6000 Oe (480 kA/m), residual magnetic flux density B_r up to 4000 G (0.4 T) and maximum energy product (BH)_max of 3.0 MG Oe (23.6 kJ/m³) have been attained due to magnetic-field-induced alignment of the ferrite particles during curing process.     

An Array of One-Dimensional Porous Silicon Photonic Crystal Reflector Islands for a Far-Infrared Image Detector

With the aid of photolithography, an array of one-dimensional porous silicon photonic crystai reflector islands for a far infrared image detector ranging from 10 μm to 14 μm is successfully fabricated. Silicon nitride formed by low pressure chemical vapor deposition （LPCVD） was used as the masking layer for the island array formation. After etching, the microstructures were examined by a scanning electron microscope and the optical properties were studied by Fourier transform infrared spectroscopy, the result indicates that the multilayer structure could be obtained in the perpendicular direction via periodically alternative etching current in each pre-pattern. At the same time, the island array has a well-proportioned lateral etching effect, which is very useful for the thermal isolation in lateral orientation of the application in devices. It is concluded that regardless of the absorption of the deposition layer on the substrate, the localized photonic crystalline islands have higher reflectivity. The designed islands structure not only prevents the cracking of the porous silicon layers but is also useful for the application in the cold part for the sensor devices and the iliterconnection of each pixel.     

Improvement of the outcoupling efficiency of an organic light-emitting device by attaching microstructured films

In this paper, we present and analyse the optical characteristics, such as spectral shift, CIE coordinates, viewing angle dependence, luminous current efficiency and luminous power efficiency, of an organic light-emitting device (OLED) with a commercial diffuser film or a brightness-enhancement film (BEF) attached. Compared to a planar green OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 29% and 23%, respectively. The overall luminous power efficiencies are enhanced by 28% and 7%. Compared to the planar green device, we observe blue shifts at different viewing angles when microstructured films are attached, which is the evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifts and the full width at the half maximum (FWHM) decrease with increasing viewing angles due to the microcavity effect. When the diffuser is attached, the spectral peak has a constant blue shift (6 nm) compared to that of the planar OLED. On the other hand, in the BEF case, the spectral shift depends on the viewing angle (2-12 nm blue shifts from 0 to 80°). This is due to the different operating principles (scattering and redirected light) of the diffuser and BEF. Since the transmittance spectra of both the diffuser film and the BEF are flat over the visible range, it is suitable for lighting applications by using white OLED. When attaching the films on a commercial white OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 34% and 31%, respectively. The overall luminous power efficiencies are enhanced by 42% and 8%.     

High-Speed and Large-Scale Electromagnetically Actuated Resonant MEMS Optical Scanner

A high speed electromagnetically actuated resonant micro-electromechanical systems (MEMS) optical scanner with large mirror area of 6×4mm² has beendeveloped. The MEMS optical scanner chip is fabricated using bulk silicon micromachining process and electroplating technique, and can generate the maximum optical deflection angle of ±6.8°circ at the 2.95kHz resonant frequency with a quality factor of 197 in air under the low power consumption of 50mW, when it is immersed in a constant 510.2mT magnetic field parallel to the coil plane. In addition, the surface roughness of less than 20nm for scanning mirror has been measured and the optical reflectivity of mirror atwavelength of 1550nm is high up to 85%. The results show that the device can satisfy the demands of mm-sized scanning systems in optical communications.     

Electromechanical Properties of Microcantilever Actuated by Enhanced Piezoelectric PZT Thick Film

Pb(Zr_0.53,Ti_0.47)O₃ (PZT) films with thicknesses of 0.8μm, 2μm and 4μm are prepared by a sol-gel method and their longitudinal piezoelectric coefficients are analysed. The results show that the PZT thick films, whose density is closer to bulk PZT, has the better crystallization, with d₃₃ and density much larger than those of PZT thin films. A piezoelectric microcantilever actuated by a 4-μm-thick PZT film is fabricated and its displacement is measured in different frequencies and voltages. The displacement increases linearly with the increasing bias, and the maximum displacement of 0.544μm is observed at 30kHz for 5V bias. The resonant frequency obtained in the experiment matches quite well with the theoretical result, and it is shown that the resonant frequency of PZT microcantilever could be controlled and predicated.     

Efficiency improvement and spectral shift of an organic light-emitting device with a square-based microlens array

In this paper, we present and analyze the influences of the fill factor and the height ratio of square-based microlens arrays on the optical characteristics of an organic light-emitting device (OLED). These properties include spectral shift, CIE coordinates, viewing-angle dependence, luminous current efficiency and luminous power efficiency. Both the luminous current efficiency and luminous power efficiency of the OLED were found to increase linearly with increasing the fill factor of microlenses. It is also found that the full width at half maximum (FWHM) of the OLED spectra and CIE coordinate decreased linearly with increasing the fill factor of the microlenses. Besides, the efficiency improvement of the OLED increased with the height ratio of attached microlenses. Compared to the OLED, the luminous current efficiency and luminous power efficiency of the device can be enhanced by 42% and 47%, respectively, by attaching the microlens array having a fill factor of 90% and a height ratio of 0.57. We also observed blue shifts at different viewing angles when microlens arrays were attached to the OLED, which is the evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifted and the FWHM decreased with increasing the viewing angles due to the microcavity effect.     

Critical current density of a YBa2Cu3O7 film: Comparison between experiment and collective pinning theory

With the vibrating reed technique we have measured the critical current densityJ _c of a Y 123 200 nm thin film as a function of magnetic field perpendicular to the CuO₂ planes (O T<B8 T) and temperature (20KT60 K). At fieldsB<0.1 TJ _c is magnetic field independent and decreases at higher fields. A comparison with theory indicates that a crossover from a single pinning to a small bundle collective pinning regime may explain the observed behaviour. According to our estimate the main pinning centers are weak point pins due to oxygen vacancies. From the temperature dependence ofJ _c atB0 we obtain a temperature dependence of the thermodynamical critical fieldB _c (1–T/T _c )² forT20 K which agrees with the anomalous temperature dependence ofB _c2 observed recently in highly anisotropic high temperature superconductors.