In this paper, we report an evolution of surface morphology of silver film irradiated by a 1 kHz femtosecond laser. By SEM
observations, it is noted that different nanostructures with respective surface features depend highly on the number of pulses
and the laser fluence. Especially when the laser fluence is below the threshold fluence of film breakdown, a textured nanostructure
including many nanobumps and nanocavities will appear on the surface of silver film. In order to determine an optimal regime
for nanostructuring silver film and to further study the underlying mechanism, we perform a quantitative analysis of laser
fluence and pulse number. The results show that this nanostructure formation should be due to a sequential process of laser
melting, vapor bubbles bursting, heat stress confinement, and subsequent material redistribution. As a potential application,
we find this nanostructured silver film can be used as the active substrate for surface enhanced Raman scattering effect. 相似文献
The physical mechanisms responsible for the formation of nanobump structures on a surface of a thin metal film irradiated
by a tightly focused femtosecond laser pulse are investigated in a large-scale molecular dynamics simulation. The simulation
is performed with a combined atomistic-continuum model adapted for an adequate representation of laser-induced processes at
the length-scale of the entire laser spot. The relaxation of the compressive stresses generated by the fast laser heating
is identified as the main driving force responsible for the separation of the metal film from the substrate and formation
of the nanobump. The kinetics of the transient melting and resolidification, occurring under conditions of the fast cooling
due to the two-dimensional electron heat conduction, defines the shape of the nanobump. The predictions of the simulation
are related to the surface structures observed in femtosecond laser nanostructuring. 相似文献
We propose a physics method to study the effect of laser field and mechanical force on the melting process of double-stranded deoxyribonucleic acid (DNA). A two-dimensional lattice model is established for DNA molecules stuck on the surface, and the stretching energy of the hydrogen bond and stacking energy for each DNA molecule are calculated by using a nonlinear potential. A real-time algorithm is employed to deal with the dynamics process of DNA melting. Numerical results explain the experimental observations. The spatial distribution of the laser field determines the sequences of DNA melting. The simulation has shown the dependence of the final number of melted DNA on the laser field and mechanical force. 相似文献
This paper reports tribological properties of diamond-like carbon (DLC) films nanostructured by femtosecond (fs) laser ablation. The nanostructure was formed in an area of more than 15 mm × 15 mm on the DLC surface, using a precise target-scan system developed for the fs-laser processing. The frictional properties of the DLC film are greatly improved by coating a MoS2 layer on the nanostructured surface, while the friction coefficient can be increased by surface texturing of the nanostructured zone in a net-like patterning. The results demonstrate that the tribological properties of a DLC surface can be controlled using fs-laser-induced nanostructuring. 相似文献
Heating and cooling rates as well as melting and solidification velocity of surface layers of irradiated samples by laser pulses are semi-quantitatively described in terms of heat flow concepts based on the assumption that laser light is directly converted in lattice heating. The range of validity of this approach compared with a more complete scheme taking into account the free carrier plasma evolution is sketched and the importance of Auger effect in the plasma-lattice coupling mechanism is detailed. The most important consequences of the quencing rates achievable by short laser pulse irradiation on the structure modification of semiconductor surface layers are reviewed with more details on the liquid to amorphous silicon transition. This is in fact the more new and less understood fast solidification process induced by pulsed laser irradiation. 相似文献
Numerical simulation of melting and solidification processes induced in CdTe by nanosecond radiation of ruby laser (λ = 694 nm, τ = 20 and 80 ns) and KrF excimer laser (λ = 248 nm, τ = 20 ns) taking into account components diffusion in melt and their evaporation from the surface has been carried out. Cd atoms evaporation has shown to essentially affect the dynamics of phase transitions in the near-surface region. Thus, in the case of the influence of ruby laser irradiation intensive surface cooling results in the formation of nonmonotone temperature profile with maximum temperature in semiconductor volume at the distance of ∼20 nm from the surface. The melt formed under the surface extends both to the surface and to the semiconductor volume as well. As a result of cadmium telluride components evaporation and diffusion in the melt the near-surface region is enriched with tellurium. The obtained melting threshold value of irradiation energy density is in a reasonable agreement with experimental data. 相似文献
Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied
numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid
interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering
the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law
to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of
surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process
are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature,
deeper melting and ablation depths. 相似文献
A temperature field on a moving thin sheet heated by a high-quality laser beam (TEM00) is calculated. Effects of melting and solidification of sheet material is taken into account. Convective and radiation cooling is included into a mathematical model. Nonlinear effects connected with temperature dependencies of material properties are considered. An explicit finite-difference numerical method was used to calculate the non-steady-state 2D and 3D Stefan problems. A surface, on which the melting occur, is replaced by a zone of finite thickness. All physical properties in this zone are smoothed and considered as continuous values. Evaporation of liquid metal begins when temperature reaches some critical value. This effect is also included in the developed model. As a result, the geometry of the melting zone can be derived and is compared with a cross section of real laser welding seem. 相似文献
This study describes a self-consistent theoretical model of simulating diffusion-controlled kinetics on the liquid–solid phase boundary during high-speed solidification in the melt pool after the selective laser melting (SLM) process for titanium matrix composite based on Ti–TiC system. The model includes the heat transfer equation to estimate the temperature distribution in the melt pool and during crystallization process for some deposited layers. The temperature field is used in a micro region next to solid–liquid boundary, where solute micro segregation and dendrite growth are calculated by special approach based on transient liquid phase bonding. The effect of the SLM process parameters (laser power, scanning velocity, layer thickness and substrate size) on the microstructure solidification is being discussed. 相似文献
The threshold character of ultra-short-pulse laser ablation allows the formation of sub-diffraction-limited structures. In order to achieve nanostructuring of an extended area in reasonable production times, parallel production is highly desirable. In this paper we analyze the results obtained by nanostructuring using a self-assembled microlens array formed by deposition of quartz spheres directly on a noble-metal surface or on a quartz spacer layer. The quartz spheres are removed by a single laser pulse, so the structures formed are the result of single-shot ablation. The size of the holes formed depends on the laser fluence and the thickness of the transparent spacer layer. The hole depths are significantly larger than the optical penetration depth, indicating that heat diffusion plays an important role. The results are analyzed by solving the two-temperature diffusion model numerically in one dimension. The results from the numerical simulation lead to the formulation of a simple analytical model for the ablation at high fluence, which reproduces the results of the simulation quite well and is in qualitative agreement with the experimental data. PACS 61.80.Ba; 78.47.+p; 81.16.Rf; 81.65.Cf 相似文献
Molecular dynamics (MD) simulations were performed to investigate the role of core volume fraction and number of fusing nanoparticles (NPs) on the melting and solidification of Cu/Al and Ti/Al bimetallic core/shell NPs during a superfast heating and slow cooling process, roughly mimicking the conditions of selective laser melting (SLM). One recent trend in the SLM process is the rapid prototyping of nanoscopically heterogeneous alloys, wherein the precious core metal maintains its particulate nature in the final manufactured part. With this potential application in focus, the current work reveals the fundamental role of the interface in the two-stage melting of the core/shell alloy NPs. For a two-NP system, the melting zone gets broader as the core volume fraction increases. This effect is more pronounced for the Ti/Al system than the Cu/Al system because of a larger difference between the melting temperatures of the shell and core metals in the former than the latter. In a larger six-NP system (more nanoscopically heterogeneous), the melting and solidification temperatures of the shell Al roughly coincide, irrespective of the heating or cooling rate, implying that in the SLM process, the part manufacturing time can be reduced due to solidification taking place at higher temperatures. The nanostructure evolution during the cooling of six-NP systems is further investigated.
An attempt has been made to achieve the crystallization of silicon thin film on metallic foils by long pulse duration excimer laser processing. Amorphous silicon thin films (100 nm) were deposited by radiofrequency magnetron sputtering on a commercial metallic alloy (N42-FeNi made of 41 % of Ni) coated by a tantalum nitride (TaN) layer. The TaN coating acts as a barrier layer, preventing the diffusion of metallic impurities in the silicon thin film during the laser annealing. An energy density threshold of 0.3 J?cm?2, necessary for surface melting and crystallization of the amorphous silicon, was predicted by a numerical simulation of laser-induced phase transitions and witnessed by Raman analysis. Beyond this fluence, the melt depth increases with the intensification of energy density. A complete crystallization of the layer is achieved for an energy density of 0.9 J?cm?2. Scanning electron microscopy unveils the nanostructuring of the silicon after laser irradiation, while cross-sectional transmission electron microscopy reveals the crystallites’ columnar growth. 相似文献
The solidification behavior of liquid gold nanowires with about 1.84 nm in diameter has been studied by using molecular dynamics simulation with an embedded atom potential. It is found the cooling rate has great effect on the final structure of the gold nanowires during solidification from liquid. With the decrease of cooling rates, the final structure of the gold nanowires varies from amorphous to crystalline via helical multi-shelled structure. The face-centered cubic structure of the gold nanowires is proven energetically the most stable form. 相似文献
We find the femtosecond laser induced microripple beside the focused femtosecond laser spot and along the movement direction of the laser spot on polydimethylsiloxane (PDMS) surface. The microripple may be due to the melting of PDMS induced by femtosecond laser pulses and the subsequent cool-down solidification of the melting PDMS along with the movement of the femtosecond laser spot. This result will be helpful to understand the interaction between the femtosecond laser and the polymer. 相似文献
Excimer laser annealing (ELA) is frequently employed to fabricate low-temperature polycrystalline silicon films on glass substrate. The grain size and crystallinity of polycrystalline silicon films are significantly affected by the resolidification behavior during ELA. A real-time in situ time-resolved optical measurement system is developed to record the rapid phase transformation process during ELA. The average solidification velocity of liquid-Si is calculated from these optical spectra using MATLAB and Excel softwares. Field emission scanning electron microscopy images reveal maximum grain size of poly-Si films with a diameter of 1 μm, which is obtained in the complete melting regime of both frontside ELA and backside ELA. Recrystallization mechanisms of complete melting of Si thin films in frontside ELA and backside ELA are demonstrated. Resolidification scenarios of partial melting, near-complete melting and complete melting in frontside ELA and backside ELA are proposed. 相似文献
We present an extensive but concise review of our present understanding, largely based on theory and simulation work from our group, on the equilibrium behavior of solid surfaces and nanosystems close to the bulk melting point. In the first part we define phenomena, in particular surface melting and nonmelting, and review some related theoretical approaches, from heuristic theories to computer simulation. In the second part we describe the surface melting/nonmelting behavior of several different classes of solids, ranging from van der Waals crystals, to valence semiconductors, to ionic crystals and metals. In the third part, we address special cases such as strained solids, the defreezing of glass surfaces, and rotational surface melting. Next, we digress briefly to surface layering of a liquid metal, possibly leading to solid-like or hexatic two-dimensional phases floating on the liquid. In the final part, the relationship of surface melting to the premelting of nanoclusters and nanowires is reviewed. 相似文献
Laser‐supported processes can be used to modify the electrical and thermal properties of ceramic substrates locally. These processes are characterized by a strong thermal interaction between the laser beam and the ceramic surface that leads to localized melting. During the dynamic melting process an additive material is injected into the melt pool in order to modify the physical properties. The heat and mass transfer during this dynamic melting and solidification process has been studied numerically in order to identify the dominating process parameters. Simulation tools based on a finite‐volume method have been developed to describe the heat transfer, fluid flow and the phase change during the melting and solidification of the ceramic. The results of the calculation have been validated against experimental results. 相似文献