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Creation of micro/nano surface structures on silver using collinear double femtosecond laser pulses with different pulse separation

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Abstract and Figures

Self-organized mound-like micro/nanoscale structures are reported for the first time on silver using a dual-pulse femtosecond laser surface processing technique. The dual-pulse laser processing technique reported in this paper uses femtosecond laser pulse pairs with a controlled temporal delay between the leading and trailing pulses. Using dual pulses at higher fluence values, mound-like micro/nanostructures have been created on silver samples for the first time. Formation of the self-organized microstructures is shown to be dependent on the time delay between the leading and trailing pulses. Mound-like microstructures do not develop on silver for overlapped pulses or using single-pulse femtosecond laser surface processing for the parameter space studied. Subsurface microstructure characterization of a single mound-like surface structure is analyzed by cross-sectional analysis using focused ion beam milling followed by scanning electron microscopy and energy dispersive X-ray spectroscopy.
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Multiscale and Multidisciplinary Modeling, Experiments and Design (2018) 1:145–153
Creation of micro/nano surface structures on silver using collinear
double femtosecond laser pulses with different pulse separation
Nicholas Roth1·Craig Zuhlke1·Edwin Peng2·Scott Hansen3·Jeffrey E. Shield2·Dennis Alexander1
Received: 27 March 2018 / Accepted: 9 April 2018 / Published online: 23 April 2018
© Springer International Publishing AG, part of Springer Nature 2018
Self-organized mound-like micro/nanoscale structures are reported for the first time on silver using a dual-pulse femtosecond
laser surface processing technique. The dual-pulse laser processing technique reported in this paper uses femtosecond laser
pulse pairs with a controlled temporal delay between the leading and trailing pulses. Using dual pulses at higher fluence
values, mound-like micro/nanostructures have been created on silver samples for the first time. Formation of the self-organized
microstructures is shown to be dependent on the time delay between the leading and trailing pulses. Mound-like microstructures
do not develop on silver for overlapped pulses or using single-pulse femtosecond laser surface processing for the parameter
space studied. Subsurface microstructure characterization of a single mound-like surface structure is analyzed by cross-
sectional analysis using focused ion beam milling followed by scanning electron microscopy and energy dispersive X-ray
Keywords Femtosecond ·Laser ·Silver ·Materials ·Nanotechnology ·Functionalization
1 Introduction
Femtosecond laser surface processing (FLSP) is a develop-
ing technique for creating micro/nanoscale surfaces with a
wide range of applications including improved heat transfer
(Kruse et al. 2013,2015,2016), medical implants (Vorobyev
and Guo 2007), improving efficiency of electrolysis cells
(Anderson et al. 2015), and controlling material wetting prop-
erties (Zuhlke et al. 2013). In most of these applications, the
greatest enhancements result from a surface with a combi-
nation of micro and nanoscale surface features. Creation of
self-organized micro/nanoscale structures has been demon-
strated using FLSP for a range of metals including stainless
steel, titanium, nickel, and many other transition metals but
BNicholas Roth
1Department of Electrical and Computer Engineering,
University of Nebraska–Lincoln, 209N Scott Engineering
Center, Lincoln, NE 68588, USA
2Department of Mechanical and Materials Engineering,
University of Nebraska–Lincoln, W342 Nebraska Hall,
Lincoln, NE 68588, USA
3EC6: Thermal Systems Branch, NASA Johnson Space
Center, 2101 NASA Parkway, Houston, TX 77058, USA
has been challenging in general for noble metals (Vorobyev
and Guo 2013). The difficulty in forming microscale surface
structures and the difference in structure formation on noble
metals versus other metals has not been adequately explained
and this paper explores the phenomena.
In this paper, a dual-pulse FLSP technique is used to
produce self-organized microscale mound-like structures on
silver with controlled temporal delay between the pulse
pairs. The mound-like structures are similar to the struc-
tures reported in the literature for other metals (Vorobyev
and Guo 2007; Kietzig et al. 2009; Kruse et al. 2013; Zuhlke
et al. 2013; Peng et al. 2017a,b). The dual-pulse technique
reported in this paper uses a similar experimental setup for
controlled temporal delay between pulse pairs as pump/probe
experiments used in time-resolved spectroscopy. The char-
acteristics of dual-pulse laser interactions have been studied
in the past in the context of laser-induced breakdown spec-
troscopy (LIBS) (Schiffern et al. 2007). Past experiments
show that overlapped double pulses result in an increased
ablation depth compared to double pulses with a delay greater
than 10 ps. Therefore, the energy does not go into removal of
material when a delay between pulses is introduced (Semerok
and Dutouquet 2004). The current consensus, in the literature
review, indicates this change in energy absorption for differ-
ent delay times is a result of plasma shielding from the first
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... The chaotic nature of fluid mechanics and redeposition creates structure interiors with voids. Figure 4.14 is an image adopted from a paper published covering this research [29]. This image compares an unprocessed silver surface shown in ...
... 21: Schematic of short pulse heating of solid-density plasma. Adopted from[29]. ...... 39 ...
... 14: SEM images adopted from[29] which show a) the grain boundary in unprocessed silver b) the grain boundary in dual pulse FLSP structures created in silver c) grain structure within bulk silver d) small and large pores within the silver structures. ...
A new technique has been developed for creating multiscale micro/nanoscale surfaces on metals. This technique is based on past work in femtosecond laser surface processing (FLSP) and dual pulse laser induced breakdown spectroscopy (LIBS). Using a femtosecond laser, pulse pairs were created with varying pulse separations between 0 ps to 500 ps. The creation of surface structures with dual pulse FLSP was studied on Ag, Cu, Ti, Al, Ni, and 304 stainless steel in relation to pulse separation. Using dual pulse FLSP micro/nano structures have been created for the first time on silver. The silver structures are multiscale in nature with an average height of 32 μm and a nanoparticle aggregate covering them. This dual pulse method also reduces the needed fluence to create micro/nano structures on copper when compared to FLSP performed with a single pulse train. It was found that the formation of these surface structures is dependent on the pulse separation between pulse pairs. The initial pulse temporarily decreases the material reflectivity which causes increased absorption of the second pulse. The reduction in reflectivity increases the laser energy being coupled into the material. Advisor: Dennis Alexander
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В данной работе методом молекулярной динамики с использованием потенциала сильной связи проведено моделирование процесса молекулярно-лучевой эпитаксии с целью определения закономерностей при формировании фрактальных металлических пленок платины на поверхности родия. Установлена возможность формирования фрактальных структур как в островковых пленках платины на поверхности родия, так и в сплошной пленке. Установлены параметры компьютерного эксперимента, определяющие переход от отдельных островковых пленок к сплошной пленке в указанной системе. С использованием различных программных продуктов Gwyddion и Image Analysis, а также собственной разработки FractalSurface проанализирован диапазон изменения фрактальной размерности при различных условиях молекулярно-динамического эксперимента методом подсчета кубов. Полученные значения фрактальной размерности в целом находятся в приемлемом согласии между собой, однако существует ряд исключений, которые обсуждаются более подробно. Сравнительный анализ получаемых результатов позволяет формулировать рекомендации для методики создания, корректировки и прецизионного контроля при «выращивании» структур с заданной морфологией поверхности. In this work, the molecular dynamics method and the tight-binding potential are used to simulate the process of molecular beam epitaxy in order to determine the regularities in the formation of fractal platinum metal films on the rhodium surface. The possibility of formation of fractal structures both in island platinum films on the rhodium surface and in a continuous film has been established. The parameters of the computer experiment, which determine the transition from individual island films to a continuous film in the indicated system, have been established. Using various software products Gwyddion and Image Analysis, as well as our own software FractalSurface, the range of changes in the fractal dimension has been analyzed under various conditions of a molecular dynamics experiment by the method of cube counting. The obtained values of the fractal dimension are generally in acceptable agreement with each other; however, there is a number of exceptions, which are discussed in more detail. A comparative analysis of the results obtained allows one to formulate recommendations for the methodology for creating, adjusting and precision control when «growing» structures with a given surface morphology.
Improving the heat transfer characteristics of materials can be accomplished by increasing the surface area. This surface roughening can be accomplished efficiently and directly using pulsed-laser processing. Here, laser processing of copper using a picosecond laser pulse technique produced mound-like structures, with surface morphologies and subsurface microstructures dependent on laser pulse count, with higher pulse counts producing a unique layering of Cu and Cu2O in the mounds. Processing in ambient air resulted in the formation of surface oxides. The presence of oxides deleteriously influenced the heat transfer characteristics, with a lower heat transfer coefficient compared to unprocessed Cu. To remove the oxidation, the laser-processed copper was subjected to different acid treatments. It was found that treatment using citric acid resulted in efficient and effective removal of oxides both surface and subsurface. For low pulse count samples, an improvement in the heat transfer characteristics was observed after oxide removal, and outperformed the polished reference sample at heat fluxes above approximately 90 W/cm². However, the removal of oxides using citric acid was found to actually decrease heat transfer characteristics for high pulse count samples with the onion-like subsurface layers, as oxide removal left behind voids in the structure which were detrimental to heat transfer. Thus, optimized laser surface processing which avoids subsurface onion-like layer formation enhances heat transfer performance.
The ablation processes of Cu film are investigated using temporal shaped femtosecond pulse trains. The depth is modulated by changing the number and interval of the sub-pulses. The underlying ultrafast dynamic processes are discussed based on plasma shielding and electron multiple heating mechanisms. When the sub-pulse interval is less than 0.4 ps electron multiple heating is the dominant mechanism, while the plasma shielding dominates the subsequent ablation processes when the sub-pulse interval is larger than 0.4 ps. The curve of depth obtained by three pulse trains shows more significant oscillation as the function of sub-pulse interval under the low-fluence. We propose that the oscillation of depth is due to the coherent phonon oscillation excited by the pulse train. The study provides a basis for giving insight into the ultrafast dynamics for improving micromachining and nano-fabrications using shaped femtosecond pulse trains.
A comparison is made between the self-organized surface features that form on copper using femtosecond versus picosecond laser pulses. Modification of material surfaces (like metals and dielectrics) have shown to affect the properties for applications including enhancing heat transfer and producing anti-corrosive or antibacterial surfaces. Femtosecond laser surface processing is a precise and repeatable technique to produce uniform self-organized quasi-periodic structures on the surface for most metals. However, it is difficult to produce uniform quasi-periodic structures on noble metals (e.g. copper). Using femtosecond pulses in the processing of copper results in non-uniform structure formation with areas that have little to no mound development. Lengthening the pulses from femtoseconds to picoseconds is shown to result in uniform quasi-periodic structure formation with high repeatability. The subsurface microstructure formed using femtosecond versus picosecond pulses while keeping other processing parameters the same was investigated, providing insight into mound formation processes. Picosecond pulses resulted in finer-grained regions within the mounds, and after a critical pulse count threshold was reached, onion-like layers with alternating high-density copper and highly porous copper oxide regions formed near the surface and blanketed the mounds. Multiple formation mechanisms are proposed to help explain the microstructures that formed using picosecond pulses.
Laser processing of metal surfaces by ultrafast pulse lasers is a developing technology with many potential uses including applications in heat transfer, medical implants, and tribology. Laser processed silver surfaces has several potential applications such as biomedical devices, antibacterial surfaces, and chemical sensors. However, there is a lack of previous research on laser processing of silver is more difficult to process compared with other metals. A newly investigated dual-pulse femtosecond laser surface processing technique was capable of producing self-organized, micro/nanoscale surface features on silver where single-pulse techniques had previously failed. Three-dimensional (3D) scanning electron microscopy (SEM) was used to examine mound-like structures produced by this new method to determine their composition and formation processes. The interior microstructure revealed that the mounds were comprised mostly of resolidified Ag grains with approximately 1% porosity. Hydrodynamically-driven fluid flow was the primary process that forms these surface structures without significant oxidation.
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Electron density dynamics in air, nitrogen and argon femtosecond filament plasma channel is studied under different pressure up to hundreds of picoseconds after ionization by transverse interferometry method. It is revealed that initial electron density in nitrogen plasma rises significantly under the pressure over 4 atmospheres. Optical refractive index anisotropy, which precedes and accompanies ionization, related with intense pulse propagation is revealed.
In the recent decades, there has been much interest in functionalized surfaces produced by ultrafast laser processing. Using pulse lasers with nanosecond to femtosecond time scale, a wide range of micro/nanoscale structures can be produced on virtually all metal surfaces. These surface structures create special optoelectronic, wetting, and tribological properties with a diverse range of potential applications.^ The formation mechanisms of these surface structures, especially microscale, mound-like structures, are not fully understood. There has been wide study of ultrafast laser processing of metals. Yet, the proposed formation models present in current literature often lack sufficient experimental verification. Specifically, many studies are limited to surface characterization, e.g. scanning electron microscopy of the surfaces of these micro/nanoscale structures. Valuable insight into the physical processes responsible for formation can be obtained if standard material science characterization methods are performed across the entire mound.^ In our study, we examined mound-like structures formed on three metal alloys. Using cross section and 3D slice and view operations by a dual beam scanning electron microscope-focused ion beam, the interior microstructures of these mounds are revealed. Taking advantage of amorphous phase formation during laser processing of Ni60Nb40, we verified the fluence-dependent formation model: mounds formed at low fluence are primarily the result of ablation while mounds formed at high fluence are formed by both ablation and rapid resolidification by hydrodynamical fluid flow. For the first time, we revealed the cross section of a wide variety of mound-like structures on titanium surfaces. The increased contribution to mound formation by fluid flow with increasing fluence was observed. Finally, a 3D scanning electron microscopy technique was applied for mounds produced on silver surface by delayed-pulse laser processing. The interior microstructure demonstrated that most of the volume comprised of resolidified silver grains with 1% porosity.
Femtosecond laser surface processing (FLSP) can be used to functionalize many surfaces, imparting specialized properties such as increased broadband optical absorption or super-hydrophobicity/-hydrophilicity. In this study, the subsurface microstructure of a series of mound-like FLSP structures formed on commercially pure titanium using five combinations of laser fluence and cumulative pulse counts was studied. Using a dual beam Scanning Electron Microscope with a Focused Ion Beam, the subsurface microstructure for each FLSP structure type was revealed by cross-sectioning. The microstructure of the mounds formed using the lowest fluence value consists of the original Ti grains. This is evidence that preferential laser ablation is the primary formation mechanism. However, the underlying microstructure of mounds produced using higher fluence values was composed of a distinct smaller-grained α-Ti region adjacent to the original larger Ti grains remaining deeper beneath the surface. This layer was attributed to resolidification of molten Ti from the hydrodynamic Marangoni effect driven fluid flow of molten Ti, which is the result of the femtosecond pulse interaction with the material.
A detailed structural and chemical analysis of a class of self-organized surface structures, termed aggregated nanoparticle spheres (AN-spheres), created using femtosecond laser surface processing (FLSP) on silicon, silicon carbide, and aluminum is reported in this paper. AN-spheres are spherical microstructures that are 20–100 μm in diameter and are composed entirely of nanoparticles produced during femtosecond laser ablation of material. AN-spheres have an onion-like layered morphology resulting from the build-up of nanoparticle layers over multiple passes of the laser beam. The material properties and chemical composition of the AN-spheres are presented in this paper based on scanning electron microscopy (SEM), focused ion beam (FIB) milling, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX) analysis. There is a distinct difference in the density of nanoparticles between concentric rings of the onion-like morphology of the AN-sphere. Layers of high-density form when the laser sinters nanoparticles together and low-density layers form when nanoparticles redeposit while the laser ablates areas surrounding the AN-sphere. The dynamic nature of femtosecond laser ablation creates a variety of nanoparticles that make-up the AN-spheres including Si/C core-shell, nanoparticles that directly fragmented from the base material, nanoparticles with carbon shells that retarded oxidation, and amorphous, fully oxidized nanoparticles.
The use of a plasma mirror to improve the temporal contrast of few cycle laser pulses has been considered. Pre-plasma features, prior to the main pulse, have been evaluated using an analytical model that has been verified using hydrodynamic code. The temporal/spectral profile, reflectivity, and broadening of the reflected pulse have been parametrically analysed using an analytical formulation that describes the reflection of broadband ultra-short pulses from the plasma gradient. The analytical estimate for the pulse reflectivity is in good agreement with experimental measurements. The consistency of the analytical expressions for the collisionless case has been validated via comparison with a 1D particle in cell simulations.
Femtosecond laser surface processing is a technology that can be used to functionalize many surfaces, imparting specialized properties such as increased broadband optical absorption or superhydrophilicity/superhydrophobicity. In this study, two unique classes of surface structures, below surface growth (BSG) and above surface growth (ASG) mounds, were formed by femtosecond laser surface processing on amorphous and polycrystalline Ni60Nb40 with two different grain sizes. Cross sectional imaging of these mounds revealed thermal evidence of the unique formation processes for each class of surface structure. BSG mounds formed on all three substrates using the same laser parameters had similar surface morphology. The microstructures in the mounds were unaltered compared with the substrate before laser processing, suggesting their formation was dominated by preferential valley ablation. ASG mounds had similar morphology when formed on the polycrystalline Ni60Nb40 substrates with 100 nm and 2 μm grain size. However, the ASG mounds had significantly wider diameter and higher peak-to-valley heights when the substrate was amorphous Ni60Nb40. Hydrodynamic melting was primarily responsible for ASG mound formation. On amorphous Ni60Nb40 substrates, the ASG mounds are most likely larger due to lower thermal diffusivity. There was clear difference in growth mechanism of femtosecond laser processed BSG and ASG mounds, and grain size does not appear to be a factor.
A pool boiling phenomenon referred to as secondary boiling effects is discussed. Based on the experimental trends, a mechanism is proposed that identifies the parameters that lead to this phenomenon. Secondary boiling effects refer to a distinct decrease in the wall superheat temperature near the critical heat flux due to a significant increase in the heat transfer coefficient. Recent pool boiling heat transfer experiments using femtosecond laser processed Inconel, stainless steel, and copper multiscale surfaces consistently displayed secondary boiling effects, which were found to be a result of both temperature drop along the microstructures and nucleationcharacteristic length scales. The temperature drop is a function of microstructure height and thermal conductivity. An increased microstructure height and a decreased thermal conductivity result in a significant temperature drop along the microstructures. This temperature drop becomes more pronounced at higher heat fluxes and along with the right nucleationcharacteristic length scales results in a change of the boiling dynamics. Nucleation spreads from the bottom of the microstructure valleys to the top of the microstructures, resulting in a decreased surface superheat with an increasing heat flux. This decrease in the wall superheat at higher heat fluxes is reflected by a “hook back” of the traditional boiling curve and is thus referred to as secondary boiling effects. In addition, a boiling hysteresis during increasing and decreasing heat flux develops due to the secondary boiling effects. This hysteresis further validates the existence of secondary boiling effects.
Femtosecond laser surface processing (FLSP) is an emerging technique for creating functionalized surfaces with specialized properties, such as broadband optical absorption or superhydrophobicity/ superhydrophilicity. It has been demonstrated in the past that FLSP can be used to form two distinct classes of mound-like, self-organized micro/nanostructures on the surfaces of various metals. Here, the formation mechanisms of below surface growth (BSG) and above surface growth (ASG) mounds on polycrystalline Ni60Nb40 are studied. Cross-sectional imaging of these mounds by focused ion beam milling and subsequent scanning electron microscopy revealed evidence of the unique formation processes for each class of microstructure. BSG-mound formation during FLSP did not alter the microstructure of the base material, indicating preferential valley ablation as the primary formation mechanism. For ASG-mounds, the microstructure at the peaks of the mounds was clearly different from the base material. Transmission electron microscopy revealed that hydrodynamic melting of the surface occurred during FLSP under ASG-mound forming conditions. Thus, there is a clear difference in the formation mechanisms of ASG- and BSG-mounds during FLSP.
Femtosecond Laser Surface Processing (FLSP) is a versatile technique for the fabrication of a wide variety of micro/nanostructured surfaces with tailored physical and chemical properties. Through control over processing conditions such as laser fluence, incident pulse count, polarization, and incident angle, the size and density of both micrometer and nanometer-scale surface features can be tailored. Furthermore, the composition and pressure of the environment both during and after laser processing have a substantial impact on the final surface chemistry of the target material. FLSP is therefore a powerful tool for optimizing interfacial phenomena such as wetting, wicking, and phasetransitions associated with a vapor/liquid/solid interface. In the present study, we utilize a series of multiscale FLSPgenerated surfaces to improve the efficiency of vapor generation on a structured surface. Specifically, we demonstrate that FLSP of stainless steel 316 electrode surfaces in an alkaline electrolysis cell results in increased efficiency of the water-splitting reaction used to generate hydrogen. The electrodes are fabricated to be superhydrophilic (the contact angle of a water droplet on the surface is less than 5 degrees). The overpotential of the hydrogen evolution reaction (HER) is measured using a 3-electrode configuration with a structured electrode as the working electrode. The enhancement is attributed to several factors including increased surface area, increased wettability, and the impact of micro/nanostructures on the bubble formation and release. Special emphasis is placed on identifying and isolating the relative impacts of the various contributions.
A theoretical approach using ab initio calculations was applied to study the interaction of an ultrashort laser pulse with the metal alloy Fe0.72Cr0.18Ni0.1 (AISI 304). The electronic structure was simulated by taking into account the chemical and magnetic disorder of the alloy by the coherent potential approximation implemented in a fully relativistic Korringa-Kohn-Rostoker formalism in the framework of spin density functional theory. A self-consistent calculation of the electronic structure using the Matsubara technique in the paramagnetic state of Fe0.72Cr0.18Ni0.1 for finite temperatures was applied. Utilizing these predictions we determined the electron heat capacity and the electron-phonon coupling factor of Fe0.72Cr0.18Ni0.1 in dependence on the electron temperature for two-temperature model applications. Compared with pure Fe a maximum deviation of 5% for the electron heat capacity and 25% for the electron-phonon coupling factor was found.