Ultrahigh infrared normal spectral emissivity of microstructured silicon coating Au film

We studied infrared normal spectral emissivity on quasi-periodic microstructured silicon, which was prepared by femtosecond laser irradiation in SF6 ambient gas, coated with 100 nm thick Au thin film. The observed emissivity is higher than any reported previously for a flat material with a thickness of less than 0.5 mm, at a temperature range of 200 °C to 400 °C. The emissivity over the measured wavelength region increases with temperature and the spike height. These results show the potential to be used as a flat blackbody source or for applications in infrared thermal sensor, detector, and stealth military technology.     

表面微构造的硅材料--一种新型的光电功能材料

在SF6气体氛围内用飞秒激光照射硅表面,可在硅表面产生准规则排列的微米量级尖峰结构,形成“黑硅”新材料．初步研究表明,这种“黑硅”新材料对波长为250-2500nm的光波有大于90％的吸收,同时它还具有相当好的场致发射特性．由于具有这些奇特的光电性质,这种表面微构造的硅材料在光电探测器、太阳能电池、平板显示器等器件制造领域有着重要的潜在应用价值．     

紫外光辐照对TiO2薄膜光学性能的影响

 采用电子束热蒸发技术在不同工艺下制备了TiO2薄膜,利用椭偏仪和分光光度计研究了紫外光辐照后薄膜光学特性的变化。实验结果表明：不同工艺下制备的TiO2薄膜经相同条件的紫外光辐照后,其折射率均有所下降,折射率的变化量随着沉积速率上升、基底温度上升、工作真空度下降分别有增大的趋势。薄膜的透射率在紫外光辐照后有一定下降。相同工艺条件下制备的TiO2薄膜,其折射率随着辐照时间的增加,先迅速降低,随后又有所增加,但均低于辐照前薄膜的折射率。     

紫外光下纳米TiO2薄膜亲水性机理的电化学研究

利用溶胶 凝胶方法在透明导电玻璃ITO (SnO2 ∶In)表面制备纳米TiO2 薄膜 ,XRD谱图表明TiO2 是锐钛矿晶型 ,AFM (Atomic Force Microscope)测得薄膜表面粒子约为 10 0nm .研究了ITO表面纳米TiO2 薄膜的光致亲水性变化 .通过循环伏安技术测定TiO2 薄膜电极在 2 5 3.7nm的紫外光照射后的电化学行为推测光致亲水性机理 .发现在紫外光照射一定时间后 ,TiO2 薄膜电极的循环伏安图在 +0 .0 35V处出现新的氧化峰 ;且随光照时间的增加 ,氧化峰的峰电流增大 ,溶液中的溶解氧对峰电流的大小有明显影响 .实验表明 ,在紫外光照下电极表面有Ti3 + 产生 ,证实了TiO2 薄膜的光致亲水性转变过程与Ti3 + 的生成导致的表面结构变化有关     

The formation of different structures in the interaction between a single femtosecond laser pulse and a thin Au film

The interactions between femtosecond (fs) laser pulses and a thin Au film deposited on a silica glass substrate were systematically investigated based on experimental data. Different structures, including microholes, nanoholes, and nanobumps, are obtained when pulses with different energies are incident on the surface of a gold film. The experimental results are discussed according to specific experimental parameters. Two physical models were constructed in order to explain the experimental results. The formation of nanoholes in a silica substrate is attributed to etching by higher order harmonic generations (HHG) when the femtosecond laser pulse interacts with the generated plasma layer, while the formation of nanobumps on the surface of an Au film is attributed to the elastic and plastic characteristics of the metal film under laser pulse irradiation.     

Stimulation of the local growth of aligned carbon nanotubes by pulse laser exposure of the substrate

The local stimulation of carbon nanotubes (CNT) growth at the laser-modified sites that have been obtained by excimer laser irradiation at 248 nm causing a local surface modification has been investigated by two different processing methods. The influence of the laser processing parameters on the CNT growth is compared for the irradiation of thin spin-coated iron nitrate films on silicon substrates and the backside irradiation of a fused silica substrate being in contact with an iron nitrate solution. Both techniques cause the formation of catalytic surface sites either by decomposition of the film or by deposition from the solution. For both laser modification approaches the local growth of vertical aligned nanotubes has been observed. In the case of spin-coated film the laser irradiation conditions have only a small influence on the CNT growth whereas at backside modification by means of a solution a strong dependence on the laser processing parameters has been found.     

Picosecond laser ablation of nano-sized WTi thin film

Interaction of an Nd:YAG laser, operating at 532 nm wavelength and pulse duration of 40 ps, with tungsten-titanium (WTi) thin film (thickness, 190 nm) deposited on single silicon (100) substrate was studied. Laser fluences of 10.5 and 13.4 J/cm² were found to be sufficient for modification of the WTi/silicon target system. The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following WTi/silicon surface morphological changes were observed: (i) ablation of the thin film during the first laser pulse. The boundary of damage area was relatively sharp after action of one pulse whereas it was quite diffuse after irradiation with more than 10 pulses; (ii) appearance of some nano-structures (e.g., nano-ripples) in the irradiated region; (iii) appearance of the micro-cracking. The process of the laser interaction with WTi/silicon target was accompanied by formation of plasma.     

Topological evolution of self-induced silicon nanogratings during prolonged femtosecond laser irradiation

Gradual evolution of self-induced silicon surface topology from one-dimensional ridge-like to two-dimensional spike-like nanogratings and then to isotropic sets of micro-columns was observed by evenly increasing IR and UV femtosecond laser irradiation dose. This topological evolution exhibits clear indications of consequent melting and vaporization processes being set up during the prolonged laser irradiation. Monotonously decreasing cumulative IR and UV femtosecond laser-nanostructuring thresholds may indicate an increase of optical absorbance of the laser-nanostructured silicon surfaces versus the increasing laser dose, consistent with the consequent onset of the abovementioned thermal modification processes.     

Partial crystallization of silicon by high intensity laser irradiation

Commercial single crystal silicon wafers and amorphous silicon films piled on single crystal silicon wafers were irradiated with a femtosecond pulsed laser and a nanosecond pulsed laser at irradiation intensities between 10¹⁷ W/cm² and 10⁹ W/cm². In the single crystal silicon substrate, the irradiated area was changed to polycrystalline silicon and the piled silicon around the irradiated area has spindly column structures constructed of polycrystalline and amorphous silicon. In particular, in the case of the higher irradiation intensity of 10¹⁶ W/cm², the irradiated area was oriented to the same crystal direction as the substrate. In the case of the lower irradiation intensity of 10⁸ W/cm², only amorphous silicon was observed around the irradiated area, even when the target was single crystal silicon. In contrast, only amorphous silicon particles were found to be piled on the amorphous silicon film, irrespective of the intensity and pulse duration.Three-dimensional thermal diffusion equation for the piled particles on the substrate was solved by using the finite difference methods. The results of our heat-flow simulation of the piled particles almost agree with the experimental results.     

The effects of a thin film dopant precursor on the structure and properties of femtosecond-laser irradiated silicon

Femtosecond (fs) laser irradiation of a silicon substrate coated with a thin film is a flexible approach to producing metastable alloys with unique properties, including near-unity sub-band gap absorptance extending into the infrared. However, dopant incorporation from a thin film during fs-laser irradiation is not well understood. We study the thin film femtosecond-laser doping process through optical and structural characterization of silicon fs-laser doped using a selenium thin film, and compare the resulting microstructure and dopant distribution to fs-laser doping with sulfur from a gaseous precursor. We show that a thin film dopant precursor significantly changes the laser-material interactions, modifying both the surface structuring and dopant incorporation processes and in turn affecting p–n diode behavior.     

Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas

We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H₂ or H₂S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c     

TiO2膜光催化剂的改进及表征

提出用粉末一溶胶法制备TiO2薄膜型光催化剂,介绍了粉溶法的制备工艺及改变pH值对催化活性的影响。研究结果表明,利用粉溶法或添加浓HNO3后制备的TiO2薄膜光催化剂表现出了较高的光催化活性,结合结构表征详细分析了其中的原因,认为粉溶法制备的催化剂表面更粗糙,比表面积增大,膜厚度减小,这些都可能是其催化活性提高的原因,而加酸后膜催化活性提高主要是因为酸性条件下表面几乎没有Ti^3+表面态,利于光生电子与光生穴穴的分离。     

Analysis of sunlight loss for femtosecond laser microstructed silicon and its solar cell efficiency

Black silicon, which is obtained by irradiating the surface of a Si wafer with femtosecond laser pulses in the presence of a sulfur-bearing gas, holds great promise in the preparation of high-performance intermediate band silicon solar cells. Using a three-level model, the enhanced usefulness of sunlight of the microstructured silicon was firstly analyzed. A detailed study on the relationship between the light loss, the ionization energy of doped impurities in silicon and the impurity band width were given. Then the effect of the position of intermediate band within the forbidden gap of silicon on the theoretical conversion efficiency for the corresponding solar cell is discussed using the Detailed Balance Theory. Finally problems need to be resolved in making intermediate band solar cells based on femtosecond laser microstructured silicon are pointed out with great emphasis.     

基于相似测度算法的激光诱导薄膜损伤表面特征研究

鉴于薄膜激光损伤性能评价是增强抗激光红外观察窗口性能的重要保证,给出薄膜在脉冲激光诱导作用下的损伤表面特性及其机理。实验采用YAG脉冲激光器对TiO2薄膜样片进行1-on-1方式的激光诱导。通过CCD采集TiO2薄膜激光辐照前后2幅图像,将这2幅图像进行匹配,建立差异图像测度算法;实验得出TiO2薄膜样片的差异能量测度可判别出损伤情况,即测度值M＜0.1为未发生损伤,0.1＜M＜0.2为轻度损伤,0.2＜M＜0.5为中度损伤,M＞0.5为严重损伤。薄膜样片经过能量密度为0.5 J/cm2的激光辐照后粗糙度明显增大。研究结果表明,采用激光辐照前后图像匹配的测试方法可实现薄膜激光损伤与否的判别。     

Structure and microtribological characteristics of laser-arc deposited TiB2 thin film

TiB2 thin film was deposited by laser-arc deposition method on the surface of single crystalline silicon. The morphology, composition, structure and microtribological properties of the film were studied by using XPS, XRD and atomic force/friction force microscope (AFM/FFM). The results show that TiB2 (100) preferred growth on the Si(100) substrate, TiB2(001) preferred growth on the Si(111) substrate. The TiB2 thin film was composed of TiB2 and a small amount of TiO2. The friction coefficient of TiB2 film on substrates Si (100) and Si(111) in microtribological process were 0.087 and 0.073, respectively. TiB2 thin film displayed distinct ability of anti-scratch and wear-resistance.     

Structure and microtribological characteristics of laser-arc deposited TiB2 thin film

TiB2 thin film was deposited by laser-arc deposition method on the surface of single crystalline silicon. The morphology, composition, structure and microtribological properties of the film were studied by using XPS, XRD and atomic force/friction force microscope (AFM/FFM). The results show that TiB2 (100) preferred growth on the Si(100)substrate, TiB2(001) preferred growth on the Si(111) substrate. The TiB2 thin film was composed of TiB2 and a small amount of TiO2. The friction coefficient of TiB2 film on substrates Si (100) and Si(111) in microtribological process were 0.087 and 0.073,respectively. TiB2 thin film displayed distinct ability of anti-scratch and wear-resistance.     

Role of thermal stresses on pulsed laser irradiation of thin films under conditions of microbump formation and nonvaporization forward transfer

This paper presents a theoretical analysis of the processes in thin solid films irradiated by short and ultrashort laser pulses in the regimes of film structuring and laser-induced forward transfer. The regimes are considered at which vaporization of the film materials is insignificant and film dynamics is governed mainly by mechanical processes. Thermoelastoplastic modeling has been performed for a model film in one- and two-dimensional geometries. A method has been proposed to estimate the height of microbumps produced by nanosecond laser irradiation of solid films. Contrary to femtosecond laser pulses, in nanosecond pulse regimes, stress waves across the film are weak and cannot induce film damage. The main role in laser-induced dynamics of irradiated films is played by radial thermal stresses which lead to the formation of a bending wave propagating along the film and drawing the film matter to the center of the irradiation spot. The bending wave dynamics depends on the hardness of the substrate underlying the film. The causes of the receiver substrate damage sometimes observed upon laser-induced forward transfer in the scheme of the direct contact between the film and the receiver are discussed.     

The comparison between CdS thin films grown on Si(111) substrate and quartz substrate by femtosecond pulsed laser deposition

Two kinds of cadmium sulfate (CdS) thin films have been grown at 600 °C onto Si(111) and quartz substrates using femtosecond pulsed laser deposition (PLD). The influence of substrates on the structural and optical properties of the CdS thin films grown by femtosecond pulsed laser deposition have been studied. The CdS thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL) and Raman spectroscopy. Although CdS thin films deposited both on Si(111) and quartz substrates were polycrystalline and hexagonal as shown by the XRD , SEM and AFM results, the crystalline quality and optical properties were found to be different. The size of the grains for the CdS thin film grown on Si(111) substrate were observed to be larger than that of the CdS thin film grown on quartz substrate, and there is more microcrystalline perpendicularity of c-axis for the film deposited on the quartz substrate than that for the films deposited on the Si substrate. In addition, in the PL spectra, the excitonic peak is more intense and resolved for CdS film deposited on quartz than that for the CdS film deposited on Si(111) substrate. The LO and TO Raman peaks in the CdS films grown on Si(111) substrate and quartz substrate are different, which is due to higher stress and bigger grain size in the CdS film grown on Si(111) substrate, than that of the CdS film grown on the amorphous quartz substrate. All this suggests that the substrates have a significant effect on the structural and optical properties of thin CdS films. PACS 81.15.Fg; 81.05.Ea; 78.20.-e; 78.67.-n; 42.62.-b     

Sub-micron scale patterning using femtosecond laser and self-assembled monolayers interaction

Standard positive photoresist techniques were adapted to generate sub-micron scale patterns of gold substrate using self-assembled monolayers (SAMs) and femtosecond laser. Self-assembled monolayers formed by the adsorption of alkanethiols onto gold substrate are employed as very thin photoresists. The process underlying photopatterning of SAMs on gold is well-known at the phenomenological level. Alkanethiolates formed by the adsorption of alkanethiols are oxidized on exposure to UV light in the presence of air to alkylsulfonates. Specifically, it is known that deep UV light of wavelength less than 200 nm is necessary for oxidation to occur. In this study, solid state femtosecond laser of wavelength 800 nm is applied for photolithography. The results show that ultrafast laser of near infrared (NIR) range wavelength can replace deep UV laser source for photopatterning using thin organic films. The essential basis of our approach is the photochemical excitation of specific reactions in a particular functional group (in this case a thiolate sulfur atom) distributed with monolayer coverage on a solid surface. Femtosecond laser photolithography could be applied to fabricate the patterning of surface chemical structure and the creation of three-dimensional nanostructures by combination with suitable etching methods.     

Ultra fast melting process in femtosecond laser crystallization of thin a-Si layer

In this paper, we investigated the mechanism of crystallization induced by femtosecond laser irradiation for an amorphous Si (a-Si) thin layer on a crystalline Si (c-Si) substrate. The fundamental, SHG, THG wavelength of a Ti:Sapphire laser was used for the crystallization process. To investigate the processed areas we performed Laser Scanning Microscopy (LSM), Transmission Electron Microscopy (TEM) and Imaging Pump-Probe measurements. Except for 267 nm femtosecond laser irradiation, the crystallization occurred well. The threshold fluences for the crystallization using 800 nm and 400 nm femtosecond laser irradiations were 100 mJ/cm² and 30 mJ/cm², respectively. TEM observation revealed that the crystallization occurred by epitaxial growth from the boundary surface between the a-Si layer and c-Si substrate. The melting depths estimated by Imaging Pump-Probe measurements became shallower when the shorter wavelength was used.