Laser sintering of silver nanoparticle thin films: microstructure and optical properties

We present a method for the sintering of silver (Ag) nanoparticle thin films by millisecond pulsed laser irradiation. The microstructure of sintered thin films and sintering behaviors of nanoparticles were systematically investigated in this paper. Absorption spectra of sintered thin films showed blue-shifted surface plasmon resonances (SPR) from 500 nm to 480 nm and red-shifted from 480 nm to 550 nm when laser power was varied from 100 W to 140 W and from 140 W to 200 W, respectively. This indicates a new technique to control light absorption through joining nanoparticles with laser sintering. According to theoretical calculations based on a heat diffusion model, the melting temperature of these Ag nanoparticles was estimated to be 440 °C during laser irradiation.     

Control of plasmon resonance of gold nanoparticles via excimer laser irradiation

A significant shift of the surface plasmon resonance absorption spectrum of gold nanoparticles was obtained by the oxidation of the nanoparticle surface via pulsed excimer laser irradiation. The high UV-light absorption of gold nanoparticles chemically produced by citrate reduction led to the important surface oxidation up to 26%. As a result of laser irradiation, the gold/gold oxide core-shell nanoparticles with little variation of the nanoparticle size were produced. After only 5 min of laser irradiation, a 12-nm blue shift in surface plasmon resonance was obtained. The possible mechanisms governing the modification in surface plasmon resonance by laser irradiation of gold nanoparticles were discussed.     

Surface plasmons in porous gold films

The surface plasmon resonance effects in porous gold (por-Au) films—nanocomposite porous films containing an ensemble of disordered gold nanoparticles—have been investigated by modulation-polarization spectroscopy. Por-Au films have been obtained by pulsed laser deposition (using a direct particle flow from an erosion torch formed by a YAG:Nd³⁺ laser in argon). The spectral and angular dependences of the polarization difference ρ(λ, θ) of internal-reflection coefficients of s- and p-polarized radiation in the Kretschmann geometry and the spectral dependences of isotropic reflection angles at ρ(θ) = 0 are measured. Two types of surface plasmon resonance are found: one occurs on isolated nanoparticles (dipole and multipole modes), and the other is due to the dipole–dipole interaction of neighboring nanoparticles. The frequency of electron plasma oscillations for the nanoparticle ensemble and the frequencies and decay parameters of resonances are determined. Dispersion relations for the radiative and nonradiative modes are presented. The negative sign of the dispersion branch of nonradiative modes of dipole–dipole interaction is explained by the spatial dispersion of permittivity. The relationships between the formation conditions of the films, their structure, and established resonance parameters (determining the resonant-optical properties of films) are discussed.     

Heat-generating property of a local plasmon resonator under illumination

We have investigated the heat generation from gold nanoparticles resulting from their local plasma resonance. We have demonstrated the self-assembly of Au nanoparticle arrays/dielectric layer/Ag mirror sandwiches, i.e., a local plasmon resonator, using a dynamic oblique deposition technique. The thicknesses of the Au and dielectric layers were changed combinatorially on a single substrate. As a result, local plasmon resonator chips were successfully fabricated. Because of strong interference, their optical absorption can be controlled between 0.0% and 97% in the near-IR region, depending on the thickness of the dielectric layer. We evaluated the heat generation from Au nanoparticles by measuring the temperature of water with which a cell prepared on a chip is filled under laser illumination. The change in the water temperature is proportional to the optical absorption of the local plasmon resonator chips. This suggests that the photothermal conversion efficiency can be controlled by interference. These features make the application of the local plasmon resonator to nanoheaters, which can spatiotemporally control heat generation, suitable.     

Effect of deposition parameters on morphology and size of FeCo nanoparticles synthesized by pulsed laser ablation deposition

The experimental parameters that control the surface morphology and size of iron cobalt nanoparticles synthesized at room temperature by pulsed laser ablation deposition (PLAD) technique have been systematically investigated. The nanoparticle synthesis has been achieved at higher operating gas pressures of argon. It was found that nanoparticles upon deposition formed small clusters, the size of which increases with decreasing pressure, increasing laser-energy density, and decreasing target-to-substrate distance. This trend could be attributed to change in the kinetic energy of deposited nanoparticles with varying argon pressure, laser-energy, and target-to-substrate distance. The nanoparticles size and size distribution showed strong dependence on argon pressure and weak dependence on laser-energy density and target-to-substrate distance.     

Surface plasmon enhanced quantum transport in a hybrid metal nanoparticle array

Hybrid Pd–Ag nanoparticle arrays composed of randomly distributed Pd nanoparticles in dense packing and a small number of dispersed Ag nanoparticles were fabricated with controlled coverage. Photo-enhanced conductance was observed in the nanoparticle arrays. Largest enhancement, which can be higher than 20 folds, was obtained with 450 nm light illumination. This wavelength was found to correlate with the surface plasmon resonance of the Ag nanoparticles. Electron transport measurements showed there were significant Coulomb blockade in the nanoparticle arrays and the blockade could be overcome with the surface plasmon enhanced local field of Ag nanoparticles induced by light illumination.     

Pulsed laser deposition of nanostructured Ag films

Ultra-thin (0.5-5 nm) films of Ag have been prepared by pulsed laser deposition in vacuum using a 26 ns KrF excimer laser at 1 J cm⁻². The deposition was controlled using a Langmuir ion probe and a quartz crystal thickness monitor. Transmission electron microscopy showed that the films are not continuous, but are structured on nanometer size scales. Optical absorption spectra showed the expected surface plasmon resonance feature, which shifted to longer wavelength and increased in strength as the equivalent film thickness was increased. It is shown that Maxwell Garnett effective medium theory can be used to calculate the main features of optical absorption spectra.     

银纳米颗粒阵列的表面增强拉曼散射效应研究

利用具有高密度拉曼热点的金属纳米结构作为表面增强拉曼散射(SERS)基底,可以显著增强吸附分子的拉曼信号.本文通过阳极氧化铝模板辅助电化学法沉积制备了高密度银(Ag)纳米颗粒阵列;利用扫描电子显微镜和反射谱表征了样品的结构形貌和表面等离激元特性;用1, 4-苯二硫醇(1, 4-BDT)为拉曼探针分子,研究了Ag纳米颗粒阵列的SERS效应.通过优化沉积时间,制备出高SERS探测灵敏度的Ag纳米颗粒阵列,检测极限可达10~(-13)mol/L;时域有限差分法模拟结果证实了纳米颗粒间存在强的等离激元耦合作用,且发现纳米颗粒底端的局域场增强更大.研究结果表明Ag纳米颗粒阵列可作为高效的SERS基底.     

Pulsed-laser generation of gold nanoparticles with on-line surface plasmon resonance detection

Size of nanoparticles is an important parameter for their applications. The real-time monitoring is required for reliable and reproducible production of nanoparticles with controllable size. We present results of our research on development of the system for the online nanoparticle characterization during their production by a laser. The laser ablation chamber which allows measurements of surface plasmon resonance spectra during the nanoparticle generation process has been designed and fabricated. The online characterization system was tested by producing and modification of gold nanoparticles. Nanoparticles were generated by nanosecond-laser (wavelength 1064 nm) ablation of gold target in deionized water, and optimal conditions for the highest nanoparticle productivity were estimated. The mean diameter of nanoparticles was determined using their absorption spectra measured in the real-time during the ablation experiments and from the TEM images analysis, and it varied from 20 to 45 nm. The mismatch between nanoparticle diameters, estimated using these two methods, is due to the polydispersity of the generated nanoparticles. The further experiments of laser-induced modification of colloidal gold nanoparticles were carried out using second harmonic (wavelength 532 nm) of nanosecond Nd:YAG laser and alteration in nanoparticle size were acquired by the online measurement system.     

XRD study on the effect of the deposition condition on pulsed laser deposition of ZnO films

Using a pulsed laser deposition (PLD) process on a ZnO target in an oxygen atmosphere, thin films of this material have been deposited on Si(111) substrates. An Nd: YAG pulsed laser with a wavelength of 1064 nm was used as the laser source. The influences of the deposition temperature, laser energy, annealing temperature and focus lens position on the crystallinity of ZnO films were analyzed by X-ray diffraction. The results show that the ZnO thin films obtained at the deposition temperature of 400°C and the laser energy of 250 mJ have the best crystalline quality in our experimental conditions. The ZnO thin films fabricated at substrate temperature 400°C were annealed at the temperatures from 400°C to 800°C in an atmosphere of N₂. The results show that crystalline quality has been improved by annealing, the optimum temperature being 600°C. The position of the focusing lens has a strong influence on pulsed laser deposition of the ZnO thin films and the optimum position is 59.5 cm from the target surface for optics with a focal length of 70 cm.      

Controlling SERS intensity by tuning the size and height of a silver nanoparticle array

It is demonstrated that the surface-enhanced Raman scattering (SERS) intensity of R6G molecules adsorbed on a Ag nanoparticle array can be controlled by tuning the size and height of the nanoparticles. A firm Ag nanoparticle array was fabricated on glass substrate by using nanosphere lithography (NSL) combined with reactive ion etching (RIE). Different sizes of Ag nanoparticles were fabricated with seed polystyrene nanospheres ranging from 430 nm to 820 nm in diameter. By depositing different thicknesses of Ag film and lifting off nanospheres from the surface of the substrate, the height of the Ag nanoparticles can be tuned. It is observed that the SERS enhancement factor will increase when the size of the Ag nanoparticles decreases and the deposition thickness of the Ag film increases. An enhancement factor as high as 2×10⁶ can be achieved when the size of the polystyrene nanospheres is 430 nm in diameter and the height of the Ag nanoparticles is 96 nm. By using a confocal Raman mapping technique, we also demonstrate that the intensity of Raman scattering is enhanced due to the local surface plasmon resonance (LSPR) occurring in the Ag nanoparticle array.     

Vapor phase synthesis and characterization of bimetallic alloy and supported nanoparticle catalysts

The laser vaporization controlled condensation (LVCC) technique coupled with a differential mobility analyzer (DMA) is used to synthesize size-selected alloy nanoparticles and nanoparticle catalyst systems. The formation of Au–Ag alloy nanoparticles is concluded from the observation of only one plasmon band. The maximum of the plasmon absorption is found to vary linearly with the gold mole fraction. For the Au–Pd system, the XRD data confirms the formation of the alloy nanoparticles with no evidence of any of the pure components. The Au/CeO₂ nanoparticle catalyst prepared by the LVCC method is a promising catalyst for low temperature CO oxidation due to its high activity and stability.     

Cerium oxide nanoparticles protect primary mouse bone marrow stromal cells from apoptosis induced by oxidative stress

The sintering of a silver (Ag) nanoparticle film by laser beam irradiation was studied using a CW DPSS laser. The laser sintering of the Ag nanoparticle thin film gave a transparent conductive film with a thickness of ca. 10 nm, whereas a thin film sintered by conventional heat treatment using an electronic furnace was an insulator because of the formation of isolated silver grains during the slow heating process. The laser sintering of the Ag nanoparticle thin film gave a unique conductive network structure due to the rapid heating and quenching process caused by laser beam scanning. The influences of the laser sintering conditions such as laser scan speed on the conductivity and the transparency were studied. With the increase of scan speed from 0.50 to 5.00 mm/s, the surface resistivity remarkably decreased from 4.45 × 10⁸ to 6.30 Ω/sq. The addition of copper (Cu) nanoparticles to silver thin film was also studied to improve the homogeneity of the film and the conductivity due to the interaction between the oxidized surface of Cu nanoparticle and a glass substrate. By adding 5 wt% Cu nanoparticles to the Ag thin film, the surface resistivity improved to 2.40 Ω/sq.     

Modification of structure, morphology and physical properties of tin sulfide thin films by pulsed laser irradiation

This paper presents modification of tin sulfide (SnS) thin films by pulsed laser irradiation. Tin sulfide films of 1 μm thickness were prepared using chemical bath deposition (CBD) technique. The chemical bath contained 5 ml acetone, 12 ml of triethanolamine, 8 ml of 1 M thioacetamide, 10 ml of 4 M ammonium hydroxide and 65 ml of distilled water. The chemical bath was kept at a constant temperature of 60 °C for 6 h which resulted in SnS films with 500 nm thickness. By double deposition, the final thickness of SnS thin films obtained was 1 μm. Laser processing was conducted to modify the structure, morphology and physical properties of the SnS thin films. The laser specifications were pulsed Nd:YAG laser with 532 nm wavelength, 300 mJ pulse energy and 10 ns pulse width. Properties of the laser-irradiated SnS thin films were compared with the as-prepared SnS thin films. The changes in structure, morphology, optical and electrical properties of the laser-irradiated SnS thin films were described.     

Synthesis of FeCo nanoparticles by pulsed laser deposition in a diffusion cloud chamber

Magnetic FeCo nanoparticles were successfully synthesized in a diffusion cloud chamber setup within pulsed laser deposition (PLD) equipment. The variation of morphology and size of FeCo nanoparticles with the number of laser pulses, ambient gas pressure and temperature gradient was studied. It was observed that the morphology of the nanoparticles changes from “cloud-like” fractal clusters to particle chains; average particle size increased at higher argon gas pressure. Increasing the temperature gradient considerably reduced the agglomeration of the nanoparticles. Nanoparticles deposited using the diffusion cloud chamber are found to be crystalline.     

Size controlled synthesis of semiconductor nanocrystals in a continuous-flow mode microcapillary reactor

A microcapillary reactor with 320 μm inner diameter was utilized for CdSe nanoparticle synthesis. The influence of the reaction temperature and flow rate of precursors on the size and size distribution of prepared CdSe nanoparticles was systematically studied. The as-prepared nanoparticles exhibit sharp excitonic absorption and photoluminescence peak (FWHM 30 nm) with a quantum-yield around 10–40%. The microcapillary reactor was also used for CdSe/ZnS core-shell nanoparticle synthesis in continuous-flow mode. The quantum yield of the core-shell nanoparticles was found to be considerably influenced by the reactor temperature and have a close correlation with the thickness of ZnS shell under growth. An optimized quantum yield up to 70% was obtained for the CdSe/ZnS core-shell nanoparticles.     

Plasmon enhanced light trapping in thin film GaAs solar cells by Al nanoparticle array

Light trapping is a crucial factor to enhance the performance of thin film solar cells. For effective light trapping, we introduced Al nanoparticle array on the top and rear surface of thin film GaAs solar cells. The effect of both array on the optical absorption and current density of solar cells is investigated by using finite difference time domain (FDTD) method. The optimization process of top and rear array in solar cells is done systematically. The results indicate that by plasmonic action of arrays, the optical absorption is significantly enhanced and optimized structure yields a current density of 25.77 mA/cm². These enhancements are mainly attributed to surface plasmon effects induced by Al nanoparticles and the light grating properties of the arrays.     

激光诱导玻璃内部金纳米颗粒的析出及光谱

使用钛宝石飞秒激光引发和热处理相结合，实现了在含有金离子的硅酸盐玻璃内部，有空间选择性地析出金纳米颗粒。对吸收光谱的研究表明，随着热处理温度的升高，吸收峰强度增大且红移；随着激光功率密度的增大，金纳米颗粒也增大。在特定的激光和热处理条件作用下，可以在玻璃内部有空间选择性的使金离子还原后聚集，形成金纳米颗粒，具有量子尺寸效应。改变激光功率和热处理温度可以控制所析出的金属纳米粒子的尺寸，从而实现多色显示，飞秒激光诱导金纳米颗粒具有稳定性，颜色具有持久性。     

Nanoparticle formation during laser ablation of metals at different pressures of surrounding noble gases

We demonstrate that the nanoparticle formation during laser ablation of metals by short (of a few tens of ps) laser pulses strongly depends on the concentration of surrounding gas. While, at vacuum conditions, nanoparticle formation shows very “sharp” atomic force microscope images of aggregated clusters, following with clear appearance of plasmon resonance on the absorption spectra of deposited films, an addition of gas particles starts to decrease the probability of cluster formation. This process shows a threshold for both helium (33 torr) and xenon (12 torr) above which no surface plasmon resonance and correspondingly no observable nanoparticles on the deposited surfaces were detected. The destruction of nanoparticle formation was attributed to the negative influence of surrounding gas particles on ablated particles aggregation.     

Preparation of super-hard coatings by pulsed laser deposition

Amorphous diamond-like carbon (DLC) films and nanocrystalline cubic boron nitride (c-BN) films were prepared by pulsed laser deposition. DLC films with 80 to 85% sp³ bonds prepared at a laser fluence above 6 J/cm² and a substrate temperature below 100 °C show high compressive stresses in the range of 8 to 10 GPa. Those stresses can be completely removed by means of pulsed laser annealing, allowing the preparation of DLC films with several-micrometre thickness. c-BN films were prepared with additional ion-beam bombardment at a substrate temperature of 250 °C. The properties of DLC and c-BN films deposited at high growth rates up to 100 nm/min are presented . PACS 81.15.Fg; 68.60.Bs: 62.40.+i