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While metal oxides and metal sulfides have been extensively studied for gas sensing applications, there are no extensive reports on gas sensing properties of metal diborides. Here, for the first time, we have investigated the conductometric gas sensing behavior of HfB2 nanoparticles. The HfB2 nanoparticles is synthesized via a sol-gel method and characterized using X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The gas sensor is fabricated by drop casting the HfB2 nanoparticles on interdigitated Ag/Pd electrodes. The sensor exhibited a promising NO2 sensing performance at the operating temperature of 200 °C. Interestingly, it is found that resistance of the sensor decreases upon exposure to both oxidizing and reducing gases, which does not follow the gas sensing behaviors of the widely investigated metal oxides. In addition, the sensor response is minimally influenced by oxygen background concentration. These results highlight that HfB2 nanoparticles exhibit very unique sensing characteristics.

This paper reports on the fabrication of marking units of controllable size ranging from 300 to 500 μm equipped with anti-reflective micro-structures using defocused femtosecond laser on titanium alloy TC4. The results show that the range of diameters of marking units (laser ablation-based craters) goes through three stages with the increase of the pulse number. The craters are too shallow for the first stage and too deep for the third stage to meet the criterion of depth, so the second stage turns out to be the optimal stage of parameter selection, where the diameter and depth of marking units are within a desired range with modification of laser energy and defocusing amount. Besides meeting the marking requirements of the size and morphology, the anti-reflective micro-structures with great color difference are formed on the surface of marking units, which contributes to the high recognition rate. Compared with the conventional marking methods, this method has a great recognition rate without recast layer and micro-cracks. Therefore, the femtosecond laser-based processing would provide a new marking technology with high efficiency and quality.

Carbon nanowires have many interesting properties in electronic, opto-electronic and nanoelectromechanical devices due to their high aspect ratios. Carbon nanotubes are ideal templates for the synthesis of one-dimensional carbon nanowires. T-carbon nanowires have been recently produced by the pseudo-topotactic conversion of multi-walled carbon nanotubes (MWCNTs) under picosecond laser irradiation. There is no data about the relationship between the carbon product morphology and laser pulse duration reported so far. Pulsed laser induced reaction is a fast and far from equilibrium process. Carbon nanostructures with different morphology have been produced from carbon nanotube suspension in methanol under laser irradiation with different pulse duration. Carbon nanoparticles, porous carbon nanorods, and even diamond nanobundles have been obtained under nanosecond laser irradiation. The one-dimensional template growth from MWCNTs to carbon nanowires is not successful under nanosecond laser irradiation. However, different carbon nanowires with diameters similar to shortened carbon nanotubes have been easily produced from carbon nanotube suspension under the irradiation of picosecond and femtosecond pulsed laser. The morphology-directing transformations of carbon nanotubes have also explained using reported mechanisms.

This study reported on the high capacity color code obtained by femtosecond laser-induced surface oxidation on titanium alloy. The color code with quaternary numbers was designed. Various colors were obtained by changing the pulse energy and scanning speed. Moreover, the XPS survey spectra showed the surface chemical composition of different colored areas was TiO2. Experimental reflectance data and simulations showed that the increase of oxide layer thickness led to peak shift and color change. Taking into account the effect of color code unit size on coloring, the processing areas with an area of 1 × 1 mm2 and 6 × 6 mm2 were prepared. Since the color difference showed that the colors obtained under different unit sizes were different, the effect of size should be considered in the actual processing. By changing the scanning speed, different color areas with obvious contrast could be obtained to meet the conditions of color code realization. Therefore, this research further realized the encryption of information on the basis of increasing the information capacity of the marking.

The bonding strength between environmental barrier coatings (EBCs) and silicon carbide fiber reinforced silicon carbide (SiC/SiC) was improved by preparing microstructure on SiC/SiC surface with femtosecond laser. The processing morphologies with different parameters were compared. There was oxidation phenomenon at the edge of laser processing, and the degree of graphitization was increased. Without damage the internal fiber structure of SiC/SiC, reasonable processing parameters were selected, and microgroove structure was processed on the SiC/SiC surface using the femtosecond laser. The tension test results showed that, after the femtosecond laser machining, the bonding strength between EBCs and SiC/SiC could be improved by about 5.5%, and the time spent to reach the critical load was extended by 11.2%. The EBCs regularly adheres to the microgrooves formed by the femtosecond laser processing, while EBCs in the area without the laser processing was completely detached.

This work presents the fabrication of convex structures by laser irradiation on dual-layer polymethylmethacrylate (PMMA). The surface PMMA layer can prevent gaseous product from escaping and the high absorption of underlying black PMMA layer can ensure enough gas products produced. It is shown that convex structures can only be formed in a particular focus range. The focus position range for appearance of convex structures is determined in our experiments to be 180–400 μm. And then the dependences of height and diameter of convex structures on pulse energy, pulse number and film thickness of surface layer have been investigated. The result demonstrates that the size (both diameter and height) of convex structures could be tuned by pulse energy; compared with the diameter, the height of convex structures is more sensitive to pulse number and film thickness of surface transparent layer. The formation of convex structures is attributed to the sensitively balanced combination effect between the softening of surface material and expansion of underlying material. Finally, large-area-arrays of convex structures with high consistency and variable tunable sizes were generated. The diameter and height of convex structures were measured to be 149 μm and 43 μm, respectively.

Ultrafast laser-induced surface modification of materials has attracted significant attention in recent years. The mechanism of formation of ultrafast laser-induced periodic surface structures (LIPSS), which were evenly distributed over the large area of indium tin oxide, was studied both theoretically and experimentally. Films were deposited using 10-ps 1064-nm laser. Resistance and transmission characteristics of LIPSSs on ITO film were analyzed. The resistance of LIPSSs increases with the increasing of transmittance in infrared band within a certain range. The paper reports the optimal processing parameters for the realization of LIPSSs on ITO films established based on the resistance and transmission properties of the film. The film with the optimized structure can significantly improve the infrared transmittance function of ITO while ensuring the low resistance, which can greatly improve the power generation efficiency of the thin film solar cells.

This study reported the fabrication of a large area of micro/nano structures with different morphologies and sizes by the deposition of ablated material and melting of material on silicon through a line-shaped femtosecond laser beam irradiation. The evolution of micro/nano structures on the silicon surface was demonstrated with the laser fluence of 0.64 J/cm2. It was found that the melting of material was responsible for the formation of the micro-protrusions from laser-induced periodic surface structures (LIPSSs). Additionally, the deposition fell on the surface of the micro-protrusions in oblique incidence way, causing LIPSSs obscure and even invisible. As a consequence, those micro-protrusions gradually evolved into the micro-spikes with the ladder-like surface. Then, various laser fluences were applied to regulate the deposition and melting behaviors of silicon, to obtain the micro/nano structures with different morphologies and sizes. The formation mechanism of these micro/nano structures was analyzed. On this basis, the optical properties test showed that best anti-reflectivity was referred to the sample full of micro-spikes with the ladder-like surface, and the average reflectance has decreased from ∼38.17% of the planar silicon to∼4.75% in the waveband between 300 and 1000 nm.

Low-power microheater platforms are promising in lowering power consumption during gas sensing processes. However, the small amount of activated-material and poor electrical contact greatly affect the sensitivity. Here, via in-situ annealing of a porous metal organic framework (MOF), ZIF-8, using a miniature heater electrode with a fast ramp rate (ca. 60 °C/s), we demonstrate the formation of a well-connected nanoparticle network with high porosity. Nanoparticle networks prepared in-situ exhibit significantly enhanced response to ethanol, defined as the ratio of sensor’s resistance before and after gas exposure, compared to ex-situ annealed counterparts (>10 times larger response) and to commercially available nanoparticles (∼4 times larger response) at a sensing temperature of 250 °C. The mechanism of the enhanced performance is studied using AC impedance spectroscopy. The results indicate that the large number of highly accessible and effective adsorption sites on the in-situ annealed material are responsible for the enhancement.

Carbon dioxide (CO2) has been shown to contribute to human health consequences indoors, such as shortness of breath, nasal and optic irritation, dizziness, and nausea. In this work, we explore the potential of metal–organic frameworks (MOFs) as highly-porous, crystalline sorbents for sensitive colorimetric CO2 detection. In particular, the zeolitic imidazolate framework (ZIF-8) is chosen as the sorptive material due to its chemical stability and tunable CO2 affinity. The colorimetric gas sensor is developed in methanol with three components: (i) MOF ZIF-8 as a high surface area adsorbent; (ii) ethylenediamine (ED) as the CO2-affinitive basic function; and (iii) phenolsulfonpthalein (PSP) as the pH indicator. Colorimetric assays and ratiometric analysis confirm a colorimetric response to variable CO2 concentrations of relevance to indoor air quality. The color response is attributed to a zwitterion mechanism whereby ED reacts with CO2 to form a zwitterionic intermediate. This intermediate is then deprotonated by the pH indicator, shifting the pH and inducing a color change. Given its simple fabrication, rapid and obvious response, and stability in ambient environment, the ZIF-8-based colorimetric sensor provides a promising route for an improved indoor air quality monitoring.

In this study, we report on the synthesis of the NiO/ZnO heterostructured nanowires by a facile two-step liquid phase route and their gas sensing characteristics employing Au interdigitated electrodes integrated on a miniature ceramic heater. Microstructural characterizations indicate that flocculent NiO particles are uniformly assembled on the outer surfaces of the single-crystalline ZnO nanowires, with diameters around 50 nm and lengths ranging from 500 nm to several μm. The gas sensing investigation indicates that the sensors based on NiO/ZnO heterostructured nanowires exhibit high sensitivity towards ethanol, good reversibility, reproducibility, stability, robustness towards humidity, and fast response/recovery rates at the determined optimum operating temperature of 300 °C. Interestingly, the sensor shows higher ethanol response but longer recovery time in N2 compared with those in air. An ethanol sensing mechanism is proposed to explain the experimental results.

A theoretical study, based on the split as well as experiments, was conducted to investigate the formation of high-spatial-frequency laser-induced periodic surface structures (HSFLs) on rough indium-tin-oxide (ITO) films under 10-ps 532-nm-wavelength laser irradiation. At a peak laser fluence of 0.472 J/cm2, the theoretical periods of HSFLs (130–190 nm) matched the experimental values (128–200 nm). Both the theoretical and experimental results demonstrated that the transformation mechanism of laser-induced periodic surface structures (LIPSSs) from low-spatial-frequency LIPSSs (LSFLs) to HSFLs was attributed to split and the irregular period difference of HSFLs and LSFLs was attributed to the surface roughness. Deeper ablation occurred for LIPSSs with a larger period, and the difference at the ablated depth increased with increasing spot number. Therefore, the LIPSSs with the larger period were clearer demarcated and the initial pits in the convex portion of LIPSSs disappeared due to the laser-induced melting. Consequently, sub-100-nm-perioded HSFLs were invisible in spite of the theoretical minimum period of ~88.5 nm. Then, for pits of different depths, the difference of the ablated depth induced by a subsequent pulse can be narrowed by reducing the laser fluence. On this method, 83-nm-perioded HSFLs were obtained by reducing the peak laser fluence to 0.432 J/cm2. Keywords: Ripples, Split, Indium-tin-oxide film, Picosecond laser, Surface roughness, Drude-Lorentz model

This study reported on the formation of sub-5-μm microstructures covered on titanium by cracks growth under 10-ns laser radiation at the wavelength of 532 nm and its induced light modification for production of nanostructures. The electric field intensity and laser power density absorbed by commercial pure titanium were computed to investigate the self-trapping introduced by cracks and the effect of surface morphology on laser propagation characteristics. It is found that nanostructures can form at the surface with the curvature radius below 20 μm. Meanwhile, variable laser fluences were applied to explore the evolution of cracks on commercial pure titanium with or without melt as spot overlap number increased. Experimental study was first performed at the peak laser fluence of 1.063 J/cm 2 to investigate the microstructures induced only by cracks growth. The results demonstrated that angular microstructures with size between 1.68 μm and 4.74 μm was obtained and no nanostructure covered. Then, at the peak laser fluence of 2.126 J/cm 2 , there were some nanostructures covered on the melt-induced curved microstructured surface. However, surface molten material submerged in the most of cracks at the spot overlap number of 744, where the old cracks disappeared. The results indicated that there was too much molten material and melting time at the peak laser fluence of 2.126 J/cm 2 , which was not suitable for obtainment of perfect micro-nano structures. On this basis, peak laser fluence was reduced down to 1.595 J/cm 2 and the sharp sub–5 μm microstructures with nanostructures covered was obtained at spot overlap number of 3720.

In this study, a two-dimensional thermo-chemical reaction model with temperature-dependent thermo-physical parameters on Si3N4 with 10 ns laser was developed to investigate the ablated size, volume and surface morphology after single pulse. For model parameters, thermal conductivity and heat capacity of β-Si3N4 were obtained from first-principles calculations. Thermal-chemical reaction rate was fitted by collision theory, and then, reaction element length was deduced using the relationship between reaction rate and temperature distribution. Furthermore, plasma absorption related to energy loss was approximated as a function of electron concentration in Si3N4. It turned out that theoretical ablated volume and radius increased and then remained constant with increasing laser energy, and the maximum ablated depth was not in the center of the ablated zone. Moreover, the surface maximum temperature of Si3N4 was verified to be above 3000 K within pulse duration, and it was much higher than its thermal decomposition temperature of 1800 K, which indicated that Si3N4 was not ablated directly above the thermal decomposition temperature. Meanwhile, the single pulse ablation of Si3N4 was performed at different powers using a TEM00 10 ns pulse Nd:YAG laser to validate the model. The model showed a satisfactory consistence between the experimental data and numerical predictions, presenting a new modeling technology that may significantly increase the accuracy of the predicated results for laser ablation of materials undergoing thermo-chemical reactions.

This study reported on the fabrication of nanoripples via femtosecond laser-induced periodic surface oxidation on titanium film, which can work both for the structural colors in reflection mode and transmission mode. The periods and heights of nanoripples were obtained under different pulse energies and scanning speeds. To obtain the structural colors in the transmission mode, the height of the nanoripples was above 175 nm. The experimental results showed that the spectral changes in the reflection mode and transmission mode were consistent, which conformed to the principle of grating diffraction. Moreover, it could be seen that the colors were very sensitive to the angles. The ripples orientation affected the intensity of the color, while the viewing angle and the incident angle affected the type of the color. According to the spectral characteristics of structural colors, the display and encryption of information were realized. On this basis, we used the method of pattern segmentation to achieve the encryption and decryption of information on the condition that the overall pattern was not missing. This study provided a new idea to realize the color display, information security, and anti-counterfeiting on both sides of the sample.

The fabrication of uniform and intact nanostructures on semiconductor thin films is an essential process to alleviate the many electronic defects that are usually present. In this study, nanostructures with good photoelectric properties were fabricated on ITO films by direct laser irradiation and secondary sputtering. A Nd:YLF femtosecond laser system, with a wavelength of 800 nm and a repetition rate of 1 kHz based on pulse regenerative amplification, was used. Laser-induced periodic surface structures with different fluences and scan speeds were studied. With a fluence of 1.86 J cm−2, a scan speed of 1.1 mm s−1, a scan spacing of 4.5 μm, and a focused laser spot of 18.5 μm, a large area of uniform nanostructures with little damage are obtained under optimized parameters. The transmittance increases by up to 21%, and the sheet resistance increases more than 10 times. A definite thickness of the ITO thin film was sputtered by secondary sputtering to improve the electrical conductivity, and the heights of nanostructures can be exactly controlled. Compared with the untreated surface of ITO films, the transmittance increases by 19% and the sheet resistance decreases to less than 1.6 times with the optimal nanostructure height of 90 nm. This method to fabricate nanostructures can greatly improve the photoelectric properties without any complex processing and expensive cost, which can be greatly used in a wide area of the industry application.

In this study, a method for dot-matrix marking with femtosecond laser on nickel-based alloy using diaphragm modulation was proposed. The simulation results showed that the edge quality and recognition uniformity of the crater were improved because of the diffraction effect of the diaphragm. Moreover, the confinement of the diaphragm could ensure the position accuracy. First, the reasonable size of diaphragm turned out to be from 8 to 10 mm. With the addition of the diaphragms, the regular variation of the diameters of the craters under different parameters was beneficial to control the size of the marking units. Thanks to the edge diffraction effect, the melt phenomenon on the edge of the craters almost disappeared, so the boundary of the crater was clear. In addition, the roundness of the crater increased by a maximum of 6%. Because the difference of gray value of different areas within the crater decreased by 60%, the recognition uniformity and the recognition rate of the marking were improved. Moreover, it was found that the contaminated area did not affect the identification of the marking. Therefore, with spatial shaping of laser beam with diaphragm, the femtosecond laser marking technology can have the advantage of high position precision, dimensional accuracy, processing quality and recognition rate.

To meet the ultra-broadband perfect absorption of visible-infrared light on silicon surfaces, a green, efficient and economical method for fabricating multi-scale micro-nano composite structures in ambient air is proposed. We experimentally demonstrate laser cleaning assisted femtosecond laser ablation for fabricating anti-reflection structures. Laser cleaning technology not only effectively eliminates oxide deposition on the laser textured surfaces, but also manufactures the small scale fine-microstructures and nanostructures. A focused ellipse laser spot is innovatively applied to realize large area and energy decays continuously multiple laser cleaning of laser-treated surfaces, and solve the problem that new oxide deposition is generated in the cleaning process. The processing efficiency is also increased by 4.8 times. The average reflectance of 2.06% is reached from 300 to 2500 nm. Great enhancement of infrared light absorption of silicon from 2.5 to 16 μm is realized experimentally. The average reflectance is reduced to 4.98% with a broadband reflectance below 6.6%. Especially, a reflectance below 5.0% from 2.5 to 10 μm and an average reflectance of 4.3% is achieved, which is the least reported to date by laser processing techniques as far as we know. This strategy for anti-reflection structures is excellent candidate for future optoelectronic devices.

Microlens arrays were fabricated on polymethyl methacrylate (PMMA) substrate using a microlens array (MLA) with rear-side ps laser swelling in this study. The modified beam energy can ensure the fabricated microlenses are uniform. Using the foci of a microlens array (MLA), approximately 630 microlenses were fabricated with a single series of pulses, which provides greater processing throughput compared to point-to-point direct laser writing. The formation of smooth microlenses is attributed to photolysis of methyl red (MR) and molecular relaxation. The dependence of height and diameter of the microlenses on laser fluence and irradiation time were investigated. The results show that the size of microlenses could be controlled using the appropriate fluence and irradiation time. Finally, an array of mirolenses with high uniformity was fabricated over a large area. The diameter and height of the microlenses were measured to be 55 and 12 µm, respectively.

Nanosecond laser ablation of metal is a complicated process, which consists of many strongly coupled physical phenomena, including material heating, melting, evaporation, vapour dynamics, and plasma shield. In this work, the nanosecond laser ablation process of titanium is investigated at 1064 nm wavelength. A multi-physics axisymmetric two-dimensional (2D) model is presented. The evolution and the distribution of titanium target’s temperature were solved using governing equations and the vapour dynamics was determined using the Knudsen relations. The maximum temperature of titanium grown slower with the increase in laser fluence and the maximum flow velocity of liquid materials reached 121 m/s with the laser fluence of 12 J/cm2. In addition, the plasma shield effect was taken into account to correct the energy distribution of the incident laser. As the laser fluence increases, the energy efficiency decreases. At the laser fluence of 12 J/cm only 55.9% of the energy was absorbed at the centre of titanium. Furthermore, the surface morphology profiles were analysed after the laser ablation on different laser fluences lying within the range of 2 - 12 J/cm2. The results showed that the surface morphology after ablation has a crater-like form and the increment of laser fluence leads to a slower non-linear increment in ablation depth and diameter of melt zone. The calculated results are in good agreement with the experimental results. The study provides useful information for nanosecond laser precision fabrication.