Analysis of pulsed laser plasmon-assisted photothermal heating and bubble generation at the nanoscale

A study is presented of photothermal effects associated with nanosecond-pulsed laser-illuminated subwavelength metallic nanoparticles in aqueous solutions. Computational electromagnetic and fluid analysis are used to model fundamental aspects of the photothermal process taking into account energy conversion within the nanoparticle at plasmon resonance, heat transfer to the fluid, homogeneous bubble nucleation, and the dynamic behaviour of the bubble and surrounding fluid. Various nanoparticle geometries are modelled including spheres, nanorods and tori. The analysis demonstrates that the laser intensity and pulse duration can be tuned to achieve controllable bubble generation without exceeding the melting temperature of the particle. The analysis also shows that the particle geometry can be tuned to optimize photothermal energy conversion for bubble generation at wavelengths that span the UV to NIR spectrum. Multiparticle systems are studied and a cooperative heating effect is demonstrated for particles that are within a few radii of each other. This provides more robust bubble generation using substantially reduced laser energy as compared to single-particle systems. The modelling approach is discussed in detail and should be of considerable use in the development of new photothermal applications.     

Fabrication of crystalline silicon spheres by selective laser heating in liquid medium

Micrometer and submicrometer crystalline silicon spheres were fabricated by selective laser heating of irregular silicon particles in liquid medium. TEM, SEM, XRD, and XPS characterized the structure and morphology of the prepared silicon spheres. The results suggested that they were spherical with a single crystalline structure. In this study, the formation mechanism of the spheres is analyzed, and the process parameters are optimized to obtain high-quality silicon spheres. A theoretical deduction regarding the relationship between critical laser energy density and particle size is also discussed, by which we can predict that larger spheres can be obtained at higher laser energy densities.     

Radiation dynamics of post-breakdown laser induced plasma

A radiation dynamic model is developed for a post-breakdown stage of a laser induced plasma expanding into vacuum. The model describes the plasma formed on a small solid particle, which is completely vaporized by a laser. The symmetry of the expanding plasma is spherical. The time frame for the applicability of the model is somewhat between a hundreds of nanoseconds, after the laser pulse is terminated, and a few microseconds, when the plasma ceases to emit. The model is based on a system of gas dynamic equations coupled with the equation of radiative transfer. Local thermodynamic equilibrium is assumed, allowing the application of the collision-dominated plasma model as well as standard statistical distributions. Calculations are performed for a dual SiC system, although calculations for any arbitrary number of system's components are permitted. The model has two implications. First, an analytical expression for the plasma radiation dynamics is obtained by artificially setting the initial conditions. Second, from experimentally measured plasma parameters, information is deduced about the initial state of the plasma. The main model input parameters are the total number and distribution of plasma species and the initial distribution of temperature. Some of the other model inputs, such as the speed of the plasma front and the temperature profile across the plasma can be directly measured, thus providing valuable experimental feedback to the model. The model outputs are the evolution of plasma temperature, the spatial and temporal distributions of atoms, ions and electron number densities and the evolution of the plasma spectrum in a desirable spectral window (e.g. 280–290 nm for the chosen in this work SiC system).     

Unsteady heating of metallic particles in a rarefied plasma

Analytical results are presented concerning the unsteady heating of a metallic spherical particle innnersed in a rarefied plasma. The results show that the tinte periods required for the solid-phase heating, melting, liquid-phase heating, and evaporation are all proportional to the particle radius. For estimating the time needed for the solid-phase heating and that for the melting, the additional heat transfer rmechanism due to the thermionic emission front the particle surface is usually negligible since the surface temperatures of the particle heated in the plasma are, in general, compartively low during those heating steps. Thermionic emission assumes its effect only as the higher surface temperatures of the heated particle are involved (e.g., higher than 4000 K), while radiation loss shows its effects at much lower wall temperatures. As the plasma temperature is comparatively low, radiation heat loss may restrict the surface temperature of a particle to such a low value that the effect of thermionic emission on the overall heating time can he neglected and complete evaporation of refractor y metallic particles becomes impossible. The uncertainty in the calculation of the effect of thermionic emission is associated with the choice of the value of the effective work function for the particle material.     

Unsteady heating and radiation effects of small particles in a thermal plasma

Based on exact solutions for the heat flux to a particle exposed to a thermal plasma given in a previous paper, initial unsteady heating (including heating of the solid phase, melting of the solid phase, heating of the liquid phase, and evaporation) and radiation effects are considered. Closed-form solutions can be obtained for particles with infinite thermal conductivities. The results show that the time periods required for the various steps are all proportional to the square of the particle radius, suggesting that reduced time periods which are independent of the particle radius are appropriate bases for comparison. Results are presented for three materials (alumina, tungsten, and graphite) and three types of plasmas (argon, argon-hydrogen mixture, and nitrogen). It is shown that evaporation (or sublimation) is by the slowest step among all processes in a plasma reactor if complete evaporation (or sublimation) of the particles is desired. Studies of the temperature history of particles with finite thermal conductivities show that temperature gradients within the particles depend on the ratio of the particles' thermal resistance to that of the plasma. In spite of the difference in initial heating, the analytical expressions based on infinite thermal conductivities predict the correct total time spent for both heating and evaporation even for low-conductivity materials such as alumina. The effect of radiation losses from a particle during heating becomes important for large particles, for high-boiling-point materials, and for low enthalpy differences between the plasma and the particle surface.On leave from the Department of Engineering Mechanics, Tsinghua University, Beijing, P.R.C.     

Analysis of blackbody-like radiation from laser-heated gas-phase tungsten nanoparticles

Thermal (blackbody-like) radiation that originated from laser-heated tungsten nanoparticles was measured using optical emission spectroscopy. The nanoparticles were generated via ArF excimer laser-assisted photolytic decomposition of WF6/H2/Ar gas mixtures, and the laser heating was applied parallel to the deposition. The temperature of the nanoparticles was determined, and its dependence on time, with respect to the 15-ns laser pulse (full width at half-maximum, fwhm) and laser fluence (phi), has been presented. At phi > 90 mJ/cm2, the particles reached the melting point (shortly after the laser pulse). Dominant cooling mechanisms, such as evaporation (above approximately 3000 K) and a combination of heat transfer by the ambient gas and radiative cooling (below approximately 3000 K), were observed for the nanoparticles, which were approximately 10 nm in diameter. The degree of inelasticity for the (predominantly) argon-gas collisions and the total emissivity of the particles (in the 2500-3000 K temperature region) could also be derived. The measured cooling rate and temperature data indicate that, depending on experimental parameters, evaporation and surface reactions can have a definite effect on the growth of particles.     

Mid-infrared refractive indices of the nematic mixture E7

E7 is a room temperature nematic liquid crystal mixture with a high positive dielectric anisotropy and a high chemical stability. Because of its relatively high nematic-to-isotropic transition temperature, it is suitable for mid-infrared laser applications, where the absorption of laser light is not negligible and gives rise to a certain heating of the medium. In this paper we give a measurement of the refractive indices of the liquid crystal E7 at a wavelength of 10.6 μm as a function of temperature. An empirical formula to represent the experimental data is also provided.     

Mid-infrared refractive indices of the nematic mixture E7

E7 is a room temperature nematic liquid crystal mixture with a high positive dielectric anisotropy and a high chemical stability. Because of its relatively high nematic-to-isotropic transition temperature, it is suitable for mid-infrared laser applications, where the absorption of laser light is not negligible and gives rise to a certain heating of the medium. In this paper we give a measurement of the refractive indices of the liquid crystal E7 at a wavelength of 10.6 µm as a function of temperature. An empirical formula to represent the experimental data is also provided.     

Numerical simulation of nanoparticle melting inside a matrix

A theoretical model is proposed for describing the melting of a metal nanoparticle embedded into a solid matrix. The model is based on a thermodynamic approach that takes into account matrix elasticity. The melting process is described for gold nanoparticles embedded in a solid matrix whose elastic modulus is varied in a wide range. Both spherical and ellipsoidal particles are considered. It is shown that particle melting temperature can be both higher and lower than the melting point of a bulk sample depending on the interaction intensity of the solid and liquid particle surfaces with the matrix. An increase in the shear modulus of the matrix causes a rise in the nanoparticle melting temperature, with the effect of the matrix elasticity becoming noticeable at some critical shear modulus. The conditions are revealed at which only a surface layer of a nanoparticle, the thickness of which depends on the particle radius and temperature, is melted.     

Reduction Mechanism of Transition Metal Oxide Particles in Thermally Induced Nanobubbles during Pulsed Laser Melting in Ethanol

Pulsed laser melting in liquid (PLML) is a technique to fabricate spherical submicrometer particles (SMPs) wherein nanosecond pulsed laser (several tens to several hundreds of mJ pulse⁻¹ cm⁻²) irradiates raw particles dispersed in liquid. Raw particles are transiently heated above the melting point to form spherical particles, which enables pulsed heating of surrounding liquid to form thermally induced bubbles by liquid vaporization. These transient bubbles play an important role as a thermal barrier to rapidly heat the particle. Reduced SMPs are generated from raw metal-oxide nanoparticles by PLML process in ethanol. This reduction cannot be explained by high-temperature thermal decomposition, but by mediation of molecules decomposed from ethanol. Computational simulations of ethanol decomposition by pulsed heating for 100 ns at the temperature 1000–4000 K revealed that ethylene is generated as the main product. Gibbs free energies of oxide reduction reactions mediated by ethylene greatly decreased compared to those without ethylene mediation. This explanation can be applied to reductive SMP formation from various transition metal oxides by PLML.     

Molecular kinetic aspects of vaporization of volatile coordination compounds with organic ligands

We present results of the experimental study and numerical simulation of radiation-convective heat and mass transfer during the sublimation of spherical particles of metal β-diketonates in a high-temperature inert gas flow (argon or helium). The sublimation process is visualized, and experimental data on the temperature variation dynamics and particle size are obtained. It is shown that at stable transfer of the compound from the particle surface the sublimation proceeds with the formation of large pores in its structure. The effect of inert gas properties on the kinetics of the vaporization process of precursor particles with various initial diameters is analyzed in the temperature range from 200 °C to 330 °C. Due to a higher thermal conductivity and heat capacity of helium as compared with argon, the choice of helium as carrier gas causes an increase in the sublimation intensity.     

Photoluminescence of ZnO nanoparticles prepared by laser ablation in different surfactant solutions

ZnO nanoparticles were prepared by laser ablation of a zinc metal plate in a liquid environment using different surfactant (cationic, anionic, amphoteric, and nonionic) solutions. The nanoparticles were obtained in deionized water and in all surfactant solutions except the anionic surfactant solution. The average particle size and the standard deviation of particle size decreased with increasing amphoteric and nonionic surfactant concentrations. With the increase of the amphoteric surfactant concentration, the intensity of the defect emission caused by oxygen vacancies of ZnO rapidly decreased, while the exciton emission intensity increased. This indicates that anionic oxygen in the amphoteric surfactant molecules effectively occupied the oxygen vacancy sites at the ZnO nanoparticle surface due to charge matching with the positively charged ZnO nanoparticles.     

纳米2,4-二羟基苯甲酸铅(Ⅱ)配合物的合成及其燃烧催化性能研究

采用液相分散沉淀法制备了纳米2,4-二羟基苯甲酸-Pb(Ⅱ)配合物粉体,并用TG,XRD,TEM,IR和元素分析对产物进行了表征.研究了反应条件对产物粒径和粒子形貌的影响.用DSC考察了产物对吸收药热分解的催化作用.结果表明,在水溶液中得到的产物为15nm×40nm的棒状粒子,而在乙醇溶液中得到的是粒径约为50nm的球形粒子.产物对吸收药热分解有显著的催化效果,使吸收药的分解峰温降低5.6℃,分解热增加918J/g.     

鼓泡浆态反应器中浸没表面的传热研究

在内径98mm的鼓泡浆态反应器内，考察了工艺参数对浸没表面与浆液间的传热系数的影响。浆态反应器轴向装有一个外径20mm，长120mm的测量传热膜系数用的铜制元件。为了模拟浆态FT合成反应系统，三相系统由N2、液体石蜡和石英砂（平均粒径53μm、110μm、180μm）或63μm以下的Fe2O3组成。工艺参数变化范围如下：表观气速0.005m/s～0.08m/s, 温度353K～453K, 压力0.1MPa～0.8MPa，固体的质量分数0～20%,初始液位高度625mm～1240mm。本研究使用单孔板、多孔板、烧结金属板三种气体分布器类型。结合实验数据，应用最小二乘法求得各个参数值，得到的无因次传热系数关联式为St=0.179(ReFr)－0.25Pr－0.66，相关指数0.98，最大偏差18％。该关联式可应用于气－液和粒径小于100μm的气－液－固体系。     

Explosive cavitation in superheated liquid argon

The kinetics of explosive boiling-up of liquid argon has been investigated at negative pressures created by the reflection of a compression pulse 3-5 mus long from the free surface of a liquid by the method of liquid pulse heating on a thin platinum wire (with a rate of temperature increase of about 1 Kmus). The limiting superheats T(*) (stretches p(*)), the effective nucleation rate J(*), and the derivative G(T)=(d ln JdT)(T=T(*) ) have been determined by experimental data on the thermal perturbation of a wire probe and the results of solution of the problem on the initial stage of explosive boiling-up of a liquid. The experimental data are compared with homogeneous nucleation theory.     

The stage of nonisothermal nucleation of supercritical particles of a new phase under nonstationary conditions of particle diffusion growth and heat transfer to a medium

An analytical theory has been formulated for the stage of nonisothermal nucleation of supercritical particles in a metastable medium with instantaneously generated initial supersaturation. The theory takes into account the nonuniformities of metastable substance concentration and temperature, which result from the nonstationary diffusion of the substance to growing particles and the nonstationary transfer of the heat of the phase transition from the particles to the medium. The formulated theory extends the approach based on the concept of excluded volume that has recently been used in the theory of the stage of nucleation under isothermal conditions. This approach implies that the nucleation intensity of new particles is suppressed in spherical diffusion regions with certain sizes that surround previously nucleated supercritical particles and remaining unchanged in the rest of the medium. It has been shown that, when self-similar solutions are used for nonstationary equations of substance diffusion to particles and heat transfer from the particles, the ratio between the excluded volume and the particle volume is independent of particle size, thereby enabling one to analytically solve the integral equation for the excluded volume throughout a system as a time function at the stage of nucleation. The main characteristics of the phase transition have been found for the end of the stage of nucleation. Comparison has been carried out with the characteristics obtained in terms of the isothermal and nonisothermal nucleation theory upon uniform vapor consumption and heat dissipation (the mean-field approximation of vapor supersaturation and temperature).     

Equilibrium adsorption data from temperature-programmed desorption experiments

This work describes a novel method that enables the calculation of a series of adsorption isotherms basically from a single Temperature-Programmed Desorption (TPD) experiment. The basic idea is to saturate an adsorbent packed in a fixed bed at a certain feed concentration and temperature and to subsequently increase its temperature linearly with time, while maintaining a constant feed concentration.We measured TPD response curves for carbon dioxide on activated carbon at different heating rates for various combinations of feed concentration, molar flow rate and particle size. Response curves from an axially dispersed plug flow model were fitted to experimental data by adjustment of the Langmuir parameters. Adsorption isotherms calculated with these fitted parameters are in good agreement with adsorption data obtained by other methods over the full temperature range.The influence of heating rate on intraparticle mass transfer resistance is discussed.     

Ultrafast cooling of photoexcited electrons in gold nanoparticle-thiolated DNA conjugates involves the dissociation of the gold-thiol bond

Using UV-visible extinction spectroscopy and femtosecond pump-probe transient absorption spectroscopy, we have studied the effect of femtosecond laser heating on gold nanoparticles attached to DNA ligands via thiol groups. It is found that femtosecond pulse excitation of the DNA-modified nanoparticles at a wavelength of 400 nm leads to desorption of the thiolated DNA strands from the nanoparticle surface by the dissociation of the gold-sulfur bond. The laser-initiated gold-sulfur bond-breaking process is a new pathway for nonradiative relaxation of the optically excited electrons within the DNA-modified gold nanoparticles, as manifested by a faster decay rate of the excited electronic distribution at progressively higher laser pulse energies. The experimental results favor a bond dissociation mechanism involving the coupling between the photoexcited electrons of the nanoparticles and the gold-sulfur bond vibrations over one involving the conventional phonon-phonon thermal heating processes. The latter processes have been observed previously by our group to be effective in the selective photothermal destruction of cancer cells bound to anti-epidermal growth factor receptor-conjugated gold nanoparticles.     

A model of nanofluids effective thermal conductivity based on dimensionless groups

Thermal conductivity is an important parameter in the field of nanofluid heat transfer. This article presents a novel model for the prediction of the effective thermal conductivity of nanofluids based on dimensionless groups. The model expresses the thermal conductivity of a nanofluid as a function of the thermal conductivity of the solid and liquid, their volume fractions, particle size and interfacial shell properties. According to this model, thermal conductivity changes nonlinearly with nanoparticle loading. The results are in good agreement with the experimental data of alumina-water and alumina-ethylene glycol based nanofluids.     

Phenomenological studies on structure and elemental composition of nanosecond and femtosecond laser-generated aerosols with implications on laser ablation inductively coupled plasma mass spectrometry

In laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), the properties of laser-generated aerosols, such as size and composition, are crucial for matrix-independent quantification. In this study, the aerosol particle morphology and elemental composition generated by two state-of-the-art laser systems (ArF excimer nanosecond-UV laser and Ti:sapphire femtosecond-IR laser) were investigated by electron microscopic techniques. Electrostatic sampling of the aerosols directly onto transmission electron microscopy (TEM) grids allowed us to study the morphology and elemental composition of the aerosols using TEM and TEM–EDX (energy dispersive X-ray spectroscopy) analyses, respectively. The results of the electron microscopic studies were finally compared to the LA-ICPMS signals of the main matrix components. The investigations were carried out for non-conducting materials (glass and zircon), metallic samples (steel and brass) and semiconductors (sulfides). The studies confirm that ns-LA-generated aerosols dominantly consist of nanoparticle agglomerates while conducting samples additionally contain larger spherical particles (diameter typically 50 to 500 nm). In contrast to ns-laser ablation, fs-LA-generated aerosols consist of a mixture of spherical particles and nanoparticle agglomerates for all investigated samples. Surprisingly, the differences in elemental composition between nanoparticle agglomerates and spherical particles produced with fs-LA were much more pronounced than in the case of ns-LA, especially for zircon (Si/Zr fractionation) and brass (Cu/Zn fractionation). These observations indicate different ablation and particle formation mechanisms for ns- and fs-LA. The particle growth mechanism for ns-LA is most likely a gas-to-particle conversion followed by agglomeration and additional hydrodynamic sputtering for conducting samples. On the other hand, phase explosion is assumed to be responsible for the mixture of large spherical particles and nanoparticle agglomerates as found for fs-LA-generated aerosols. Based on these mechanisms, the overall temporal elemental fractionation effects in ns-LA-ICPMS seem to occur mainly during the ablation. This effect was not observed for fs-LA-ICPMS despite the element separation into different particle fractions, which, on the other hand, could induce severe ICP-induced fractionation.