Frank G. Shi

SIZE DEPENDENT THERMAL VIBRATIONS AND MELTING IN NANOCRYSTALS

A simple model for the size-dependent amplitude of the atomic thermal vibrations of a nanocrystal is presented which leads to the development of a model for the size dependent melting temperature in nanocrystals on the basis of Lindemanns criterion. The two models are in terms of a directly measurable parameter for the corresponding bulk crystal, i.e., the ratio between the amplitude of thermal vibrations for surface atoms and that for interior ones. It is shown that the present model for the melting temperature offers not only a qualitative but even an excellent quantitative agreement with the experimentally observed size-dependent superheating, as well as melting point suppression in both the supported and embedded metallic and semiconductor nanocrystals.

Towards model-based engineering of optoelectronic packaging materials: dielectric constant modeling

Abstract Increases in data transmission speeds of optoelectronic devices have consequently increased high-frequency requirements for optoelectronic packaging materials including substrate, EMC/EMI shielding, adhesive and encapsulant (molding and underfill) materials. Most of those materials are polymer/filler composites, and critical materials properties for the device design and packaging include the effective dielectric constant, dielectric loss and their frequency and filler concentration dependence. This work presents a systematic theoretical investigation of the effective dielectric constant of polymer/filler composite materials, and its dependence on the filler concentration, the filler/polymer interaction, and the size of fillers. Our results demonstrate that, in contrary to the prevailing views, the filler concentration dependence of the effective dielectric constant is non-monotonic. Depending on the dielectric constant ratio between filler and polymer matrix, and the degree of interaction between filler and matrix, the effective dielectric constant exhibits an extreme as a function of filler concentration. In addition, our model is demonstrated to contain the Maxwell–Wagner formulation as an asymptotic limit. The present results have significant implications to the targeted formulation of optoelectronic packaging materials.

Characterizing the interphase dielectric constant of polymer composite materials: Effect of chemical coupling agents

Recent research into the dielectric characteristics of polymer–ceramic composites has shown that the interphase region of the composite can have a dielectric constant significantly different from that of the polymer phase due to covalent bonding of the polymer molecules to the surface of the filler particles. Chemical coupling agents and surfactants such as functional silanes, organotitanates, organometallic chelating agents, phosphate esters, and various ionic and nonionic organic esters are commonly employed to enhance the compatibility between the polymer phase and dispersed filler phase of composite systems. Using experimental data and molecular dipole polarization calculations, we determine the effect of such coupling agents on the interphase dielectric constant. Our results show that the addition of functional silane coupling agents or nonionic surfactants at concentrations of 0.5 wt % or less of the total organics of a polymer–ceramic composite system has significant effects on the dielectric const...

Studies of Phosphor Concentration and Thickness for Phosphor-Based White Light-Emitting-Diodes

The dependence of luminous efficacy on phosphor concentration and thickness for high-power white light-emitting-diode (WLED) lamps is investigated by employing three-dimensional ray-tracing simulations. The simulations show that the brightness or luminous efficacy of WLED lamps highly depends on the combination of phosphor concentration and phosphor thickness (or phosphor-matrix composite volume). The package with lower concentration and higher phosphor thickness has higher luminous efficacy because the light trapping efficiency is lower with the low phosphor concentration. At the correlated color temperature (CCT) value of around 4000 K, ray-tracing simulation and experimental results show 20% and 23% improvement in lumen, respectively, with a 1.8-mm-phosphor package over a 0.8-mm-phosphor package. A package with convex lens can improve the lumen output over flat lens, but this improvement is small, and it requires higher amount of phosphor, up to 25%, to achieve same CCT value.

Complex permittivity of composite systems: a comprehensive interphase approach

The present paper reports the development of a unique model to provide physical insights to the complex permittivity of composite systems. The model takes into account interactions between the components of the composite system in the form of interphase regions. The resultant model, termed the interphase power-law (IPL) model, relies on the permittivities of the filler component, the matrix component and the interphase region as well as the volume fractions of each. The model is applicable to any uniform composite system of discrete particles dispersed within a matrix. Trends in the composite materials effective permittivity as a function of filler volume fraction, interphase permittivity, filler surface area and filler particle shapes are explored.

Thermal stability of crystalline thin films

Abstract A simple model, free of any adjustable parameter, is introduced for the size-dependent melting of crystalline thin films. It predicts that the melting temperature of a crystalline thin film decreases as its thickness decreases. The prediction is supported by the available experimental results on Pb, In, Sn and Fe thin films.

Effect of Phosphor Particle Size on Luminous Efficacy of Phosphor-Converted White LED

In this paper, the influence of YAG:Ce phosphor particle sizes on the lumen output and the conversion efficiency of both in-cup phosphor and top remote phosphor LED packages are investigated with 3-D ray-tracing simulations. The lumen output and the conversion efficiency of both types of phosphor-converted (pc) white LED packages are dependent on the size of YAG:Ce particles. The lumen output and conversion efficiency of both types of pc-white LED packages are minimal at the phosphor particle size with the size parameter of around one and are highest at the particle size in micron size. The simulation results show that both in-cup and top remote phosphor packages have the highest lumen output and the highest conversion efficiency at the particle size of around 20 mum.

Stable photoluminescence of zinc oxide quantum dots in silica nanoparticles matrix prepared by the combined sol–gel and spray drying method

A sol–gel method was employed to produce a zinc oxide (ZnO) colloid consisting of ZnO nanocrystalline particles with an average diameter of ∼3 nm, and subsequently mixed with a silica (SiO2) colloid. The mixture was finally spray dried to form a powder nanocomposite. It was found that the green photoluminescence (PL) exhibited by the composite was very stable: the intensity, position, and shape do not change even after being aged over 30 days. Thus, the ZnO/SiO2 nanocomposite has a much improved PL stability over ZnO colloids, which is often found to undergo a significant redshift even after aging over a few days. Our results are expected to have significant technological implications.

Thickness dependent dielectric strength of a low-permittivity dielectric film

The dielectric strength of a promising interlevel low relative permittivity dielectric is investigated for various film thicknesses and temperatures by using I-V measurements with metal-insulator-semiconductor (MIS) structures. It is found that the dielectric breakdown mechanism also depends on thickness. For relatively thick films (thickness >500 nm), the dielectric breakdown is electromechanical in origin, i.e. the dielectric strength is proportional to the square root of Youngs modulus of the films. By scanning electron microscopy (SEM) observation, a microcrack in thicker films may contribute to a lower value of Youngs modulus, which may confirm that the electromechanical breakdown is the dominant mechanism for dielectric breakdown of thicker films. In addition, the thickness dependent dielectric strength can be described by the well-known inverse power-law relation by using different exponents to describe different thickness ranges, However for thinner films, i.e., <500 nm, the experimentally observed relationships among the dielectric strength, Youngs modulus, and film thickness cannot be explained by the existing models.

Size-dependent electrical constriction resistance for contacts of arbitrary size: from Sharvin to Holm limits

Abstract A general analytical expression for the size-dependent constriction (contact) resistance is obtained for non-quantum contacts of arbitrary size as a solution of Laplace’s equation with appropriate boundary conditions. The new analytical expression contains both the Holm resistance and the Sharvin resistance as the asymptotic limits for the respective large and small contact sizes relative to the mean free path of electrons. This first general theoretical result is fully supported by available experimental data on the pressure dependence of contact resistance.