Learning Complexity to Guide Light-Induced Self-Organized Nanopatterns.

Ultrafast laser irradiation can induce spontaneous self-organization of surfaces into dissipative structures with nanoscale reliefs. These surface patterns emerge from symmetry-breaking dynamical processes that occur in Rayleigh-Bénard-like instabilities. In this study, we demonstrate that the coexistence and competition between surface patterns of different symmetries in two dimensions can be numerically unraveled using the stochastic generalized Swift-Hohenberg model. We originally propose a deep convolutional network to identify and learn the dominant modes that stabilize for a given bifurcation and quadratic model coefficients. The model is scale-invariant and has been calibrated on microscopy measurements using a physics-guided machine learning strategy. Our approach enables the identification of experimental irradiation conditions for a desired self-organization pattern. It can be generally applied to predict structure formation in situations where the underlying physics can be approximately described by a self-organization process and data is sparse and nontime series. Our Letter paves the way for supervised local manipulation of matter using timely controlled optical fields in laser manufacturing.

Towards Room Temperature Phase Transition of W-Doped VO2 Thin Films Deposited by Pulsed Laser Deposition: Thermochromic, Surface, and Structural Analysis.

Vanadium dioxide (VO2) with an insulator-to-metal (IMT) transition (∼68 °C) is considered a very attractive thermochromic material for smart window applications. Indeed, tailoring and understanding the thermochromic and surface properties at lower temperatures can enable room-temperature applications. The effect of W doping on the thermochromic, surface, and nanostructure properties of VO2 thin film was investigated in the present proof. W-doped VO2 thin films with different W contents were deposited by pulsed laser deposition (PLD) using V/W (+O2) and V2O5/W multilayers. Rapid thermal annealing at 400-450 °C under oxygen flow was performed to crystallize the as-deposited films. The thermochromic, surface chemistry, structural, and morphological properties of the thin films obtained were investigated. The results showed that the V5+ was more surface sensitive and W distribution was homogeneous in all samples. Moreover, the V2O5 acted as a W diffusion barrier during the annealing stage, whereas the V+O2 environment favored W surface diffusion. The phase transition temperature gradually decreased with increasing W content with a high efficiency of -26 °C per at. % W. For the highest doping concentration of 1.7 at. %, VO2 showed room-temperature transition (26 °C) with high luminous transmittance (62%), indicating great potential for optical applications.

Learning PDE to Model Self-Organization of Matter.

A self-organization hydrodynamic process has recently been proposed to partially explain the formation of femtosecond laser-induced nanopatterns on Nickel, which have important applications in optics, microbiology, medicine, etc. Exploring laser pattern space is difficult, however, which simultaneously (i) motivates using machine learning (ML) to search for novel patterns and (ii) hinders it, because of the few data available from costly and time-consuming experiments. In this paper, we use ML to predict novel patterns by integrating partial physical knowledge in the form of the Swift-Hohenberg (SH) partial differential equation (PDE). To do so, we propose a framework to learn with few data, in the absence of initial conditions, by benefiting from background knowledge in the form of a PDE solver. We show that in the case of a self-organization process, a feature mapping exists in which initial conditions can safely be ignored and patterns can be described in terms of PDE parameters alone, which drastically simplifies the problem. In order to apply this framework, we develop a second-order pseudospectral solver of the SH equation which offers a good compromise between accuracy and speed. Our method allows us to predict new nanopatterns in good agreement with experimental data. Moreover, we show that pattern features are related, which imposes constraints on novel pattern design, and suggest an efficient procedure of acquiring experimental data iteratively to improve the generalization of the learned model. It also allows us to identify the limitations of the SH equation as a partial model and suggests an improvement to the physical model itself.

Boosted Spontaneous Formation of High-Aspect Ratio Nanopeaks on Ultrafast Laser-Irradiated Ni Surface.

The capacity to synthesize and design highly intricated nanoscale objects of different sizes, surfaces, and shapes dramatically conditions the development of multifunctional nanomaterials. Ultrafast laser technology holds great promise as a contactless process able to rationally and rapidly manufacture complex nanostructures bringing innovative surface functions. The most critical challenge in controlling the growth of laser-induced structures below the light diffraction limit is the absence of external order associated to the inherent local interaction due to the self-organizing nature of the phenomenon. Here high aspect-ratio nanopatterns driven by near-field surface coupling and architectured by timely-controlled polarization pulse shaping are reported. Electromagnetic coupled with hydrodynamic simulations reveal why this unique optical manipulation allows peaks generation by inhomogeneous local absorption sustained by nanoscale convection. The obtained high aspect-ratio surface nanotopography is expected to prevent bacterial proliferation, and have great potential for catalysis, vacuum to deep UV photonics and sensing.

High-Density Nanowells Formation in Ultrafast Laser-Irradiated Thin Film Metallic Glass.

We present an effective approach for fabricating nanowell arrays in a one-step laser process with promising applications for the storage and detection of chemical or biological elements. Biocompatible thin films of metallic glasses are manufactured with a selected composition of Zr65Cu35, known to exhibit remarkable mechanical properties and glass forming ability. Dense nanowell arrays spontaneously form in the ultrafast laser irradiation spot with dimensions down to 20 nm. The flared shape observed by transmission electron microscopy is ideal to ensure chemical or biological material immobilization into the nanowells. This also indicates that the localization of the cavitation-induced nanopores can be tuned by the density and size of the initial nanometric interstice from the columnar structure of films deposited by magnetron sputtering. In addition to the topographic functionalization, the laser-irradiated amorphous material exhibits structural changes analyzed by spectroscopic techniques at the nanoscale such as energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. Results reveal structural changes consisting of nanocrystals of monoclinic zirconia that grow within the amorphous matrix. The mechanism is driven by local oxidation process catalyzed by extreme temperature and pressure conditions estimated by an atomistic simulation of the laser-induced nanowell formation.

On the Insignificant Role of the Oxidation Process on Ultrafast High-Spatial-Frequency LIPSS Formation on Tungsten.

The presence of surface oxides on the formation of laser-induced periodic surface structures (LIPSS) is regularly advocated to favor or even trigger the formation of high-spatial-frequency LIPSS (HSFL) during ultrafast laser-induced nano-structuring. This paper reports the effect of the laser texturing environment on the resulting surface oxides and its consequence for HSFLs formation. Nanoripples are produced on tungsten samples using a Ti:sapphire femtosecond laser under atmospheres with varying oxygen contents. Specifically, ambient, 10 mbar pressure of air, nitrogen and argon, and 10-7 mbar vacuum pressure are used. In addition, removal of any native oxide layer is achieved using plasma sputtering prior to laser irradiation. The resulting HSFLs have a sub-100 nm periodicity and sub 20 nm amplitude. The experiments reveal the negligible role of oxygen during the HSFL formation and clarifies the significant role of ambient pressure in the resulting HSFLs period.

Initial Morphology and Feedback Effects on Laser-Induced Periodic Nanostructuring of Thin-Film Metallic Glasses.

Surface nanostructuring by femtosecond laser is an efficient way to manipulate surface topography, creating advanced functionalities of irradiated materials. Thin-film metallic glasses obtained by physical vapor deposition exhibit microstructures free from grain boundaries, crystallites and dislocations but also characterized by a nanometric surface roughness. These singular properties make them more resilient to other metals to form laser-induced nanopatterns. Here we investigate the morphological response of Zr65Cu35 alloys under ultrafast irradiation with multipulse feedback. We experimentally demonstrate that the initial columnar microstructure affects the surface topography evolution and conditions the required energy dose to reach desired structures in the nanoscale domain. Double pulses femtosecond laser irradiation is also shown to be an efficient strategy to force materials to form uniform nanostructures even when their thermomechanical properties have a poor predisposition to generate them.

Self-Organization Regimes Induced by Ultrafast Laser on Surfaces in the Tens of Nanometer Scales.

A laser-irradiated surface is the paradigm of a self-organizing system, as coherent, aligned, chaotic, and complex patterns emerge at the microscale and even the nanoscale. A spectacular manifestation of dissipative structures consists of different types of randomly and periodically distributed nanostructures that arise from a homogeneous metal surface. The noninstantaneous response of the material reorganizes local surface topography down to tens of nanometers scale modifying long-range surface morphology on the impact scale. Under ultrafast laser irradiation with a regulated energy dose, the formation of nanopeaks, nanobumps, nanohumps and nanocavities patterns with 20-80 nm transverse size unit and up to 100 nm height are reported. We show that the use of crossed-polarized double laser pulse adds an extra dimension to the nanostructuring process as laser energy dose and multi-pulse feedback tune the energy gradient distribution, crossing critical values for surface self-organization regimes. The tiny dimensions of complex patterns are defined by the competition between the evolution of transient liquid structures generated in a cavitation process and the rapid resolidification of the surface region. Strongly influencing the light coupling, we reveal that initial surface roughness and type of roughness both play a crucial role in controlling the transient emergence of nanostructures during laser irradiation.

Sub-100 nm 2D nanopatterning on a large scale by ultrafast laser energy regulation.

Coupling ultrafast light irradiation to surface nanoreliefs leads to periodic patterns, achieving record processing scales down to tens of nanometers. Driven by near-field interactions, the promising potential of the spontaneous pattern formation relies on the scaling up of one-step manufacturing processes. Here, we report the self-assembly of unconventional arrays of nanocavities of 20 nm diameter with a periodicity down to 60 nm upon ultrafast laser irradiation of a nickel surface. In stark contrast to laser-induced surface ripples, which are stochastic and suffer from a lack of regularity, the 2D patterns present an unprecedented uniformity on extreme scales. The onset of nanocavity arrays ordered in a honeycomb lattice is achieved by overcoming the anisotropic polarization response of the surface by a delayed action of cross-polarized laser pulses. The origin of this self-arrangement is identified as a manifestation of Marangoni convection instability in a nanoscale melt layer, destabilized by the laser-induced rarefaction wave.

Electroanalytical Performance of Nitrogen-Doped Graphene Films Processed in One Step by Pulsed Laser Deposition Directly Coupled with Thermal Annealing.

Graphene-based materials are widely studied to enable significant improvements in electroanalytical devices requiring new generations of robust, sensitive and low-cost electrodes. In this paper, we present a direct one-step route to synthetize a functional nitrogen-doped graphene film onto a Ni-covered silicon electrode substrate heated at high temperature, by pulsed laser deposition of carbon in the presence of a surrounding nitrogen atmosphere, with no post-deposition transfer of the film. With the ferrocene methanol system, the functionalized electrode exhibits excellent reversibility, close to the theoretical value of 59 mV, and very high sensitivity to hydrogen peroxide oxidation. Our electroanalytical results were correlated with the composition and nanoarchitecture of the N-doped graphene film containing 1.75 at % of nitrogen and identified as a few-layer defected and textured graphene film containing a balanced mixture of graphitic-N and pyrrolic-N chemical functions. The absence of nitrogen dopant in the graphene film considerably degraded some electroanalytical performances. Heat treatment extended beyond the high temperature graphene synthesis did not significantly improve any of the performances. This work contributes to a better understanding of the electrochemical mechanisms of doped graphene-based electrodes obtained by a direct and controlled synthesis process.

Review of Graphene Growth From a Solid Carbon Source by Pulsed Laser Deposition (PLD).

Graphene is a remarkable two-dimensional (2D) material that is of great interest to both academia and industry. It has outstanding electrical and thermal conductivity and good mechanical behavior with promising applications in electronic devices, supercapacitors, batteries, composite materials, flexible transparent displays, solar cells, and sensors. Several methods have been used to produce either pristine graphene or doped graphene. These include chemical vapor deposition (CVD), mechanical exfoliation, decomposition of SiC, liquid-phase exfoliation, pulsed laser deposition (PLD). Among these methods, PLD, which is routinely used for growing complex oxide thin films has proved to be an alternative to the more widely reported CVD method for producing graphene thin films, because of its advantages. Here we review the synthesis of graphene using PLD. We describe recent progress in preparing pristine graphene and doped graphene by PLD, including deposition processes and characterization. The goal of this complete survey is to describe the advantages of using the technique for graphene growth. The review will also help researchers to better understand graphene synthesis using the PLD technique.

Nano-Architecture of nitrogen-doped graphene films synthesized from a solid CN source.

New synthesis routes to tailor graphene properties by controlling the concentration and chemical configuration of dopants show great promise. Herein we report the direct reproducible synthesis of 2-3% nitrogen-doped 'few-layer' graphene from a solid state nitrogen carbide a-C:N source synthesized by femtosecond pulsed laser ablation. Analytical investigations, including synchrotron facilities, made it possible to identify the configuration and chemistry of the nitrogen-doped graphene films. Auger mapping successfully quantified the 2D distribution of the number of graphene layers over the surface, and hence offers a new original way to probe the architecture of graphene sheets. The films mainly consist in a Bernal ABA stacking three-layer architecture, with a layer number distribution ranging from 2 to 6. Nitrogen doping affects the charge carrier distribution but has no significant effects on the number of lattice defects or disorders, compared to undoped graphene synthetized in similar conditions. Pyridinic, quaternary and pyrrolic nitrogen are the dominant chemical configurations, pyridinic N being preponderant at the scale of the film architecture. This work opens highly promising perspectives for the development of self-organized nitrogen-doped graphene materials, as synthetized from solid carbon nitride, with various functionalities, and for the characterization of 2D materials using a significant new methodology.

Growth Twinning and Generation of High-Frequency Surface Nanostructures in Ultrafast Laser-Induced Transient Melting and Resolidification.

The structural changes generated in surface regions of single crystal Ni targets by femtosecond laser irradiation are investigated experimentally and computationally for laser fluences that, in the multipulse irradiation regime, produce sub-100 nm high spatial frequency surface structures. Detailed experimental characterization of the irradiated targets combining electron back scattered diffraction analysis with high-resolution transmission electron microscopy reveals the presence of multiple nanoscale twinned domains in the irradiated surface regions of single crystal targets with (111) surface orientation. Atomistic- and continuum-level simulations performed for experimental irradiation conditions reproduce the generation of twinned domains and establish the conditions leading to the formation of growth twin boundaries in the course of the fast transient melting and epitaxial regrowth of the surface regions of the irradiated targets. The observation of growth twins in the irradiated Ni(111) targets provides strong evidence of the role of surface melting and resolidification in the formation of high spatial frequency surface structures. This also suggests that the formation of twinned domains can be used as a sensitive measure of the levels of liquid undercooling achieved in short pulse laser processing of metals.

Robust Electrografting on Self-Organized 3D Graphene Electrodes.

Improving graphene-based electrode fabrication processes and developing robust methods for its functionalization are two key research routes to develop new high-performance electrodes for electrochemical applications. Here, a self-organized three-dimensional (3D) graphene electrode processed by pulsed laser deposition with thermal annealing is reported. This substrate shows great performance in electron transfer kinetics regarding ferrocene redox probes in solution. A robust electrografting strategy for covalently attaching a redox probe onto these graphene electrodes is also reported. The modification protocol consists of a combination of diazonium salt electrografting and click chemistry. An alkyne-terminated phenyl ring is first electrografted onto the self-organized 3D graphene electrode by in situ electrochemical reduction of 4-ethynylphenyl diazonium. Then the ethynylphenyl-modified surface efficiently reacts with the redox probe bearing a terminal azide moiety (2-azidoethyl ferrocene) by means of Cu(I)-catalyzed alkyne-azide cycloaddition. Our modification strategy applied to 3D graphene electrodes was analyzed by means of atomic force microscopy, scanning electron microscopy, Raman spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy (XPS). For XPS chemical surface analysis, special attention was paid to the distribution and chemical state of iron and nitrogen in order to highlight the functionalization of the graphene-based substrate by electrochemically grafting a ferrocene derivative. Dense grafting was observed, offering 4.9 × 10(-10) mol cm(-2) surface coverage and showing a stable signal over 22 days. The electrografting was performed in the form of multilayers, which offers higher ferrocene loading than a dense monolayer on a flat surface. This work opens highly promising perspectives for the development of self-organized 3D graphene electrodes with various sensing functionalities.

Electrochemical boron-doped diamond film microcells micromachined with femtosecond laser: application to the determination of water framework directive metals.

Planar electrochemical microcells were micromachined in a microcrystalline boron-doped diamond (BDD) thin layer using a femtosecond laser. The electrochemical performances of the new laser-machined BDD microcell were assessed by differential pulse anodic stripping voltammetry (DPASV) determinations, at the nanomolar level, of the four heavy metal ions of the European Water Framework Directive (WFD): Cd(II), Ni(II), Pb(II), Hg(II). The results are compared with those of previously published BDD electrodes. The calculated detection limits are 0.4, 6.8, 5.5, and 2.3 nM, and the linearities go up to 35, 97, 48, and 5 nM for, respectively, Cd(II), Ni(II) Pb(II), and Hg(II). The detection limits meet with the environmental quality standard of the WFD for three of the four metals. It was shown that the four heavy metals could be detected simultaneously in the concentration ratio usually measured in sewage or runoff waters.