Effects of laser pulse wavelength and laser fluence on the characteristics of silver nanoparticle generated by laser ablation

Electrically pumped ultraviolet random lasing was achieved in metal-insulator-semiconductor (MIS) diodes based on ZnO films at room temperature. The ZnO films were grown by plasma assisted molecular beam epitaxy. Two different kinds of insulator layers, SiO _x (0<x≤2) and AlO _x (0<x≤1.5) were deposited by electron beam evaporation. X-ray diffraction experiments found these oxide layers were amorphous (or microcrystals), and X-ray photoelectron spectroscopy confirmed the Si and Al were fully oxidized. Compared with devices using SiO _x as the insulator layer, diodes with evaporated AlO _x layers showed a lower working threshold forward current (～20 mA to ～26 mA) and higher emission intensity. Periodic features indicating formation of closed-loop paths were deduced by the power Fourier transform of electroluminescence spectra. The cavity length of both devices increased as forward currents increased, while a larger cavity length was always obtained in the AlO _x -involved device under the same working current. The improved performance was attributed to larger hole amount in AlO _x layers. These results revealed that evaporated AlO _x can serve as good electron blocking and hole supplying layers for hetero-structures.     

Gold nanoparticle production by laser ablation in liquid nitrogen medium followed by cryogenic medium substitution with ethanol

The use of liquid nitrogen as a medium for laser ablation made it possible to obtain Au particles shaped as cores, cores/hollow shells, and hollow shells by radiation of a picosecond Nd:YAG laser. In this case, the substitution of the cryogenic liquid medium of the colloid by evaporating on the surface of a room-temperature liquid causes the shift and broadening of the plasmon resonance peak of Au nanoparticles, which results from the formation of fractal nanoparticle aggregates.     

Ultra fine carbon nitride nanocrystals synthesized by laser ablation in liquid solution

Crystalline carbon nitride nanopowders and nanorods have been successfully synthesized at room temperature and pressure using the novel technique of pulsed laser ablation of a graphite target in liquid ammonia solution. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and Fourier transform infrared spectroscopy (FTIR) were used to systematically study the morphology, nanostructure and chemical bonding. The experimental composition and structure of the nanoparticles are consistent with the theoretical calculations for α-C₃N₄. After 2 h ablation the particles had a size distribution ∼8–12 nm, whereas after 5 h ablation the particles had grown into nanorod-like structures with a crystalline C₃N₄ tip. A formation mechanism for these nanorods is proposed whereby nanoparticles are first synthesized via rapid formation of an embryonic particle, followed by a slow growth, eventually leading to a one-dimensional nanorod structure.     

Laser ablation of liquid surface in air induced by laser irradiation through liquid medium

The pulse laser ablation of a liquid surface in air when induced by laser irradiation through a liquid medium has been experimentally investigated. A supersonic liquid jet is observed at the liquid–air interface. The liquid surface layer is driven by a plasma plume that is produced by laser ablation at the layer, resulting in a liquid jet. This phenomenon occurs only when an Nd:YAG laser pulse (wavelength: 1064 nm) is focused from the liquid onto air at a low fluence of 20 J/cm². In this case, as Fresnel’s law shows, the incident and reflected electric fields near the liquid surface layer are superposed constructively. In contrast, when the incident laser is focused from air onto the liquid, a liquid jet is produced only at an extremely high fluence, several times larger than that in the former case. The similarities and differences in the liquid jets and atomization processes are studied for several liquid samples, including water, ethanol, and vacuum oil. The laser ablation of the liquid surface is found to depend on the incident laser energy and laser fluence. A pulse laser light source and high-resolution film are required to observe the detailed structure of a liquid jet.     

Effects of ambient gas and laser fluence on the compositional changes in iron oxide particle aggregated films prepared by laser ablation

Iron oxide nanoparticle aggregated films were prepared using the excimer laser ablation technique by adopting an off-axis configuration and a gas condensation process. Sintered iron oxide targets (Fe₂O₃) were ablated in Ar, O₂, He, N₂, Ne and Xe by an ArF excimer (193 nm) under various pressures. It was found that both the ambient gas and its pressure strongly affected the composition of the deposition product. Depending on the ablation parameters, the product of ablation was comprised of Fe₂O₃ or a mixture of Fe₂O₃ and FeO. Ablation in argon at the lowest fluence of 4.9 J/cm² led to deposition of two oxides, FeO and Fe₂O₃ in the entire pressure range tested. The most pronounced change in the relative amounts of these phases was observed in the 13.3–350 Pa pressure range. Changing the Ar pressure from the lower to the upper limit of this range varied the product chemistry from being close to a single (Fe₂O₃) phase to being two phase (Fe₂O₃+FeO). Ablation experiments in all the background gases used showed that when the irradiation fluence was sufficiently high the deposition became stoichiometric (Fe/O ratio the same as that of the target) within a pressure range specific to each gas. With further increase in fluence, the pressure ranges within which the deposition was stoichiometric became broadened. Changes in the product chemistry were analyzed in terms of the energetics of the ablated species. For monoatomic gases, the maximum pressure (P_S(X)) at which the product was still a single Fe₂O₃ phase decreased monotonically with increasing atomic mass of the gas at lower fluence, and at higher fluence this dependence became non-monotonic. Diatomic gases, such as N₂ and O₂, were more efficient in cooling down the evaporated species, leading to non-stoichiometric deposition even at lower pressures. PACS 81.05.Ys; 81.15.Fg; 61.10.-i     

Impurity doping in silicon nanowires synthesized by laser ablation

Boron (B) or phosphorus (P) doped silicon nanowires (SiNWs) were synthesized by laser ablation. Local vibrational modes of B were observed in B-doped SiNWs by micro-Raman scattering measurements at room temperature. Fano broadening due to a coupling between the discrete optical phonon and a continuum of interband hole excitations was also observed in the Si optical phonon peak for B-doped SiNWs. An electron spin resonance signal due to conduction electrons was observed only for P-doped SiNWs. These results prove that B and P atoms were doped in substitutional sites of the crystalline Si core of SiNWs during laser ablation and electrically activated in the sites.     

Effect of the laser fluence on structural and optical characterization of thin CdS films synthesized by femtosecond pulsed laser deposition

CdS thin films have been grown on Si(1 1 1) and quartz substrates using femtosecond pulsed laser deposition. X-ray diffraction, atomic force microscopy, photoluminescence measurement, and optical transmission spectroscopy were used to characterize the structure and optical properties of the deposited CdS thin films. The influence of the laser fluence (laser incident energy in the range 0.5–1.5 mJ/pulse) on the structural and optical characterizations of CdS thin films has been studied. The results indicate that the structure and optical properties of the CdS thin films can be improved as increasing the per pulse output energy of the femtosecond laser to 1.2 mJ. But when the per pulse output energy of the femtosecond laser is further increased to 1.5 mJ, which leads to the degradation of the structure and optical properties of the CdS thin films.     

Effects of laser fluence on near-field surface nanostructuring

In this work, molecular dynamics simulation is performed to explore the long-time (up to 5 ns) behavior of argon crystal in surface nanostructuring with an extremely localized near-field laser beam. The surface nanostructuring region is limited to tens of nanometers in diameter, although the simulated systems are much larger (comprised of more than 770,000 atoms). This study focuses on the long-time solidification and crystallization procedure, which is driven by the heat conduction in the material. The effect of the computational domain on the final nanostructure is studied in detail. Different laser fluences are used in the simulation to explore how and to what extent the energy input affects the dynamic melting behavior and the final dimension and profile of the surface nanostructure. In-depth theoretical investigation gives satisfactory explanation of the effect of the laser fluence on the melting depth. Spot-like structural defects in the sub-surface region are observed and investigated until full solidification.     

Dopant-enhanced ablation of nitrocellulose by a nitrogen laser

The photoetching behavior of pure nitrocellulose and of nitrocellulose dyed with stilbene-420, coumarin-120 and rhodamine 6G by 337 nm nitrogen laser pulses has been studied. Ablation with a low power nitrogen laser is hereby reported for the first time. A two step photochemical mechanism is proposed to account for the ablation of the pure material. With the addition of dyes strongly absorbing at 337 nm the photoetching rate of nitrocellulose can be increased significantly. This increase is proportional to the molar extinction coefficient of the dye at 337 nm and its concentration in the polymer. The photoetching mechanism and the energy transfer processes from the dye to the polymer are discussed in detail.     

Difference in formation of carbon cluster cations by laser ablation of graphene and graphene oxide

The distributions of positive carbon cluster ions produced by laser ablation of graphene (G) and graphene oxide (GO) are found to be quite different. Under a typical experimental condition, narrow distributions of even-numbered clusters from C60+ to C162+ were observed for G, and broad distributions including even-numbered clusters from C100+ to C400+ and odd-numbered clusters from C189+ to C395+ were observed for GO. The threshold of laser energy for G is lower than that of GO. Further results of collision-activated dissociation mass spectrometry indicate that the cluster ions generated from G are structurally similar but are different with those generated from GO or nanodiamonds. It is proposed that the experimentally observed difference can be attributed to the different mechanisms behind the process. A top-down mechanism including both direct transformation of G to fullerene and fragmentation of large-sized fullerenes is suggested for the generation of carbon cluster cations in the process of laser ablation of G. For GO, the experimental results are close to those of nanodiamonds and other materials reported previously and can be explained by the generally accepted bottom-up mechanism.     

A novel method of nanocrystal fabrication based on laser ablation in liquid environment

Metal nanoparticles can be prepared by a novel technique that consists of the laser ablation of a solid target immersed in a water solution of a metal salt. Silicon was chosen as the most adequate target to synthesize silver and gold nanoparticles from a water solution of either AgNO₃ or HAuCl₄. The influence of both the silver nitrate concentrations and the irradiation time of the Si target on the optical properties of the Au and Ag nanoparticles have been investigated. The crystalline nature of the metal nanoparticles has been determined by X-ray diffraction (XRD). Average size and particle size distribution have been measured by means of TEM. The absorbance spectra show the characteristic band of the surface resonant plasmon of silver and gold nanoparticles.     

Quantifying the quality of femtosecond laser ablation of graphene

The influence of beam intensity on laser ablation quality and ablation size is experimentally studied on graphene-coated silicon/silicon dioxide substrates. With an amplified femtosecond-pulsed laser system, by systematically decreasing the average power, periodic stripes with decreasing widths are ablated. Histogram analyses of the untouched and ablated regions of scanning electron microscope images of the fabricated structures make it possible to quantify the ablation quality. These analyses reveal that submicron ablation can be achieved while maintaining 75 % ablation accuracy by adjusting the beam intensity around the ablation threshold.     

Antibacterial and surface-enhanced Raman scattering (SERS) activities of AgCl cubes synthesized by pulsed laser ablation in liquid

We used pulsed laser ablation in liquid to fabricate silver chloride (AgCl) nanocubes directly from a bulk Ag target in sodium chloride (NaCl) solution. We optimized particle size and investigated the surface properties of the cubes for their Surface Enhanced Raman Scattering (SERS) behavior relative to Rhodamine 6G (R6G). The SERS behavior was related to the surface properties, clearness, and morphology, i.e., varied atomic arrangements and surface energies of different facets of the cubes. In addition, we have demonstrated that our easily synthesized AgCl cubes were antibacterial with a high efficiency to decontaminate Escherichia coli upon contact. Our results can be extended to generate particle-based coatings with antibacterial properties.