Laser-induced Reduction Research Articles

Ultrasensitive Detection and In Situ Imaging of Analytes on Graphene Oxide Analogues Using Enhanced Raman Spectroscopy.

We demonstrate how algorithm-improved confocal Raman microscopy (ai-CRM), in combination with chemical enhancement by two-dimensional substrates, can be used as an ultrasensitive detection method for rhodamine (R6G) molecules adsorbed from aqueous solutions. After developing a protocol for laser-induced reduction of graphene oxide, followed by noninvasive Raman imaging, a limit of detection (LOD) of 5 × 10–10 M R6G was achieved using ai-CRM. An equivalent subnanomolar LOD was also achieved on another graphene oxide analogue −UV/ozone-oxidized graphene. These record-breaking detection capabilities also enabled us to study the adsorption kinetics and image the spatial distribution of the adsorbed R6G. These findings indicate a strong potential for algorithm-improved graphene-enhanced Raman spectroscopy as a facile method for detecting, imaging, and quantifying trace amounts of adsorbing molecules on a variety of 2D substrates.

Copper circuits fabricated on flexible polymer substrates by a high repetition rate femtosecond laser-induced selective local reduction of copper oxide nanoparticles.

In this work, copper circuits were fabricated on flexible polyimide (PI) substrates by high repetition rate femtosecond laser-induced selective local reduction of copper oxide nanoparticles (CuO NPs). The effects of laser pulse energy and laser scanning velocity on the quality of the copper circuit were studied. By optimizing laser processing parameters, we prepared a Cu circuit of a line width of 5.5 µm and an electrical resistivity of 130.9 µΩ·cm. The Cu/O atomic ratio of the Cu circuit reaches ∼10.6 and the proportion of Cu is 91.42%. We then studied the formation mechanism of the copper circuit by simulating the temperature field under the irradiation of high repetition rate femtosecond laser pulses. The results show that the thermochemical reduction reaction induced by the high repetition rate femtosecond laser reduces CuO NPs into Cu NPs. Under the thermal effect of the high repetition rate femtosecond laser, Cu NPs agglomerate and grow to form a uniform and continuous Cu circuit.

Penetration-depth control in a remote laser-welding system based on an optical triangulation loop

Remote laser-welding systems are being used more frequently because of their larger working areas, shorter downtimes and ability to weld different seam types with high accuracy at greater speeds in comparison to conventional welding. Therefore, precise process-monitoring methods are needed in order to achieve weld traceability and good process and quality control to accompany different welding situations. This paper proposes the use of optical triangulation feedback on a remote laser-welding system that makes it possible to monitor a larger working area. This configuration means we can monitor the interaction zone itself, analyze the 3D position of the laser beam and key process estimators as a result of laser welding. AISI steel plates were welded in a lap configuration to show that stable partial penetration can be achieved. First, a dependency matrix was constructed for the different welding parameters (material thicknesses, welding speeds and laser powers) in order to describe the change of the weld's penetration depth with respect to the key estimators. An approximation was used to characterize the change of the weld's depth according to the change of the estimators. The experimental results demonstrate that the interaction zone's area can be used to successfully control the laser's power output in order to achieve a stable partial penetration with an error of less than 7 % of the desired target weld depth. Longitudinal macrographies show a significantly more constant weld penetration depth and laser-induced plume reduction during welding.

Synthesis of manganese (IV) oxide at activated carbon on reduced graphene oxide sheets via laser irradiation technique for organic binder-free electrodes in flexible supercapacitors

We report a novel, green, scalable technique to synthesize binder-free, high-purity conductive composite comprising activated carbon (AC), manganese dioxide nanorods (MnO2), and reduced graphene oxide sheets (rGO) for flexible supercapacitors with outstanding electrochemical performance. UV pulsed laser irradiation of GO-based composite dispersion (AC/GO or MnO2@AC/GO) in ethanol aqueous medium was used to induce a photocatalytic reduction of GO and simultaneous anchor AC particles or AC loaded MnO2 nanorods (MnO2@AC) on the reduced GO sheets (rGO) at room temperature and atmospheric pressure. rGO sheets serve as a large surface area, conductive binder to enhance the ion adsorption, electrical conductivity, and mechanical flexibility of supercapacitor electrodes. This laser-induced photocatalytic reduction method was used to prepare two different rGO-based colloidal composites AC/rGO (CG) and MnO2@AC/rGO (MCG). The prepared rGO-based colloidal composites were used to fabricate symmetric supercapacitors (CG//CG and MCG//MCG) and asymmetric supercapacitors (MCG//CG) in which MCG is the positive electrode and CG is the negative one. All prepared rGO-based supercapacitors demonstrated significant improvement in their electrochemical performance compared with rGO-free AC based supercapacitors. The enhancement in the electrochemical properties of rGO-based supercapacitors could be attributed to the intrinsic characteristics of rGO, such as high surface area, excellent electrical conductivity, and super mechanical flexibility. Our approach is a one-step, scalable, cost-effective synthesis technique to produce all binder-free AC/rGO based composites for flexible energy-storage devices.

Laser-Induced Magnetization Precession in Individual Magnetoelastic Domains of a Multiferroic Co40Fe40B20/BaTiO3 Composite

Using a magneto-optical pump-probe technique with micrometer spatial resolution we show that magnetization precession can be launched in individual magnetic domains imprinted in a Co$_{40}$Fe$_{40}$B$_{20}$ (CoFeB) layer by elastic coupling to ferroelectric domains in a BaTiO$_{3}$ substrate. The dependence of the precession parameters on external magnetic field strength and orientation reveal that by laser-induced ultrafast partial quenching of the magnetoelastic coupling parameter of CoFeB by $\approx$27% along with 10% ultrafast demagnetization trigger the magnetization precession. The relation between the laser-induced reduction of the magnetoelastic coupling and the demagnetization is approximated by the $n(n+1)/2$-law with n$\approx$2. This correspondence confirms the thermal origin of the laser-induced anisotropy change. Based on the analysis and modeling of the excited precession we find signatures of laser-induced precessional switching, which occurs when the magnetic field is applied along the hard magnetization axis and its value is close to the effective magnetoelastic anisotropy field. The precession excitation process in an individual magnetoelastic domain is found to be unaffected by neighboring domains. This makes laser-induced changes of magnetoelastic anisotropy a promising tool for driving magnetization dynamics and switching in composite multiferroics with spatial selectivity.

Laser-induced photocatalytic reduction of CO2 into methanol over perovskite LaMnO3

Perovskite oxides offer efficient photocatalytic properties for CO2 reduction to methanol. In this study, LaMnO3+δ nanocrystals were fabricated by the solgel combustion method. The structural properties including lattice constants, occupancies, Mn–O lengths, and Mn–O–Mn angle were studied by Rietveld refinement. Jahn–Teller distortion and O-2p and Mn-3d hybridization have been investigated using the structural parameters and lattice dynamics characterized by Fourier-transform far-infrared spectroscopy. The electron transitions near the lower edge of conduction band were studied by diffuse reflectance spectroscopy. LaMnO3+δ exhibited laser-induced photocatalytic activity for reductive conversion of CO2 to methanol under both visible (532 nm) and infrared (1064 nm) irradiation. The methanol yield as a function of irradiation time was investigated by the gas chromatographic analysis. After 20 min of irradiation at 532 nm, the methanol yield showed the highest value (970 μmol), while the methanol started to degrade with further irradiation time. The recycling stability test from the repeatedly used LaMnO3+δ sample showed the high stability of the catalyst. The authors demonstrated that in addition to the appropriate bandgap energy, the length of eg↑ in the conduction band plays a significant role in the photocatalytic properties of the perovskites with Jahn–Teller distortion under visible and infrared irradiations.

Fs-laser based hybrid micromachining for polymer micro-opto electrical systems

We report on femtosecond laser direct writing of electrically conductive copper structures on transparent cyclic olefin copolymer based planar optical chips. The process is based on a laser-induced reduction of CuO nanoparticles dissolved in a water-soluble resin. Thus generated conductive copper structures are characterized with respect to their chemical composition and electrical resistivity. In addition, the application of ultrashort laser pulses enables a hybrid micromachining approach comprising ablation of polymer substrates and the fabrication of copper patterns in a single laser processing setup. A possible electro-optical application of this approach is demonstrated by employing the copper structures as an electro-thermal microheater in combination with a polymer planar optical Bragg grating sensor. This, in turn, highlights the potential of generating copious micro-opto-electro-mechanical polymer structures for numerous sensing applications ranging from Lab-on-Chip to environmental applications using the presented hybrid micromachining approach.

Fabrication of Magnetic α-Fe2O3/Fe3O4 Composite Particles by Nanosecond Laser Irradiation of α-Fe2O3 Powder in Water

Magnetic α-Fe2O3/Fe3O4 composite spherical particles were synthesized by nanosecond pulsed laser irradiation to α-Fe2O3 nanoparticle powder suspended in pure water and acetone. It was shown that the laser-induced reduction efficiencies in two solvents were comparable. The obtained submicrometer spheres exhibited a ferromagnetic property having a higher coercivity than that of commercial Fe3O4.

Direct Ultraviolet Laser-Induced Reduction of Disulfide Bonds in Insulin and Vasopressin.

Ultraviolet (UV) light has been shown to induce reduction of disulfide bonds in proteins in solution. The photoreduction is proposed to be a result of electron donation from excited Tyr or Trp residues. In this work, a powerful UV femtosecond laser was used to generate photoreduced products, while the hypothesis of Tyr/Trp mediation was studied with spectroscopy and mass spectrometry. With limited irradiation times of 3 min or less at 280 nm, the laser-induced reduction in arginine vasopressin and human insulin led to significant yields of ∼3% stable reduced product. The photogenerated thiols required acidic pH for stabilization, while neutral pH primarily caused scrambling and trisulfide formation. Interestingly, there was no direct evidence that Tyr/Trp mediation was a required criterion for the photoreduction of disulfide bonds. Intermolecular electron transfer remained a possibility for insulin but was ruled out for vasopressin. We propose that an additional mechanism should be increasingly considered in UV light-induced reduction of disulfide bonds in solution, in which a single UV photon is directly absorbed by the disulfide bond.

Photochemical Printing of Plasmonically Active Silver Nanostructures.

In this paper, we demonstrate plasmonic substrates prepared on demand, using a straightforward technique, based on laser-induced photochemical reduction of silver compounds on a glass substrate. Importantly, the presented technique does not impose any restrictions regarding the shape and length of the metallic pattern. Plasmonic interactions have been probed using both Stokes and anti-Stokes types of emitters that served as photoluminescence probes. For both cases, we observed a pronounced increase of the photoluminescence intensity for emitters deposited on silver patterns. By studying the absorption and emission dynamics, we identified the mechanisms responsible for emission enhancement and the position of the plasmonic resonance.

Photo-induced electrodeposition of metallic nanostructures on graphene.

Graphene, a single atomic layer of sp2 hybridized carbon, is a promising material for future devices due to its excellent optical and electrical properties. Nevertheless, for practical applications, it is essential to deposit patterned metals on graphene in the micro and nano-meter scale in order to inject electrodes or modify the 2D film electrical properties. However, conventional methods for depositing patterned metals such as lift-off or etching leave behind contamination. This contamination has been demonstrated to deteriorate the interesting properties of graphene such as its carrier mobility. Therefore, to fully exploit the unique properties of graphene, the controlled and nano-patterned deposition of metals on graphene films without the use of a sacrificial resist is of significant importance for graphene film functionalization and contact deposition. In this work, we demonstrate a practical and low-cost optical technique of direct deposition of metal nano-patterned structures without the need for a sacrificial lift-off resist. The technique relies on the laser induced reduction of metal ions on a graphene film. We demonstrate that this deposition is optically driven, and the resolution is limited only by the diffraction limit of the light source being used. Patterned metal features as small as 270 nm in diameter are deposited using light with a wavelength of 532 nm and a numerical aperture of 1.25. Deposition of different metals such as Au, Ag, Pd, Pb and Pt is shown. Additionally, change in the Fermi level of the graphene film through the nano-patterned metal is demonstrated through the electrical characterization of four probe field effect transistors.

Room-temperature gas sensing of laser-modified anatase TiO2 decorated with Au nanoparticles

This article reports on fabrication of activated TiO2 and gold decorated TiO2 prepared by irradiation with millisecond laser and on their gas sensing properties. Commercially available anatase TiO2 was used as starting material whose nanoparticles were modified and decorated upon irradiating its water suspension by pulsed laser. Formation of Au nanoparticles was achieved by laser-induced reduction of Au(III) ions, followed by their deposition onto TiO2 nanoparticle surface. In order to evaluate the effect of gold concentration in Au@TiO2 nanocomposites on their sensing properties, two gold-containing samples with 1 and 5 wt% of Au were prepared. All nanomaterials were characterized by XRD, XPS and SEM methods and then tested as gas sensing devices towards a number of volatile compounds. XPS analysis revealed formation of oxygen vacancies and Ti3+ ions on the TiO2 surface upon laser irradiation, while the latter defects disappeared after decoration with gold. Depending on the degree of Au loading, the samples demonstrated selectivity towards ammonia, acetaldehyde or benzene.

Laser patterning and induced reduction of graphene oxide functionalized silk fibroin

This paper presents a one-step method for patterning and reducing graphene oxide functionalized in silk fibroin using femtosecond laser induced forward transfer (LIFT). Such approach renders fibroin, a natural biopolymer with excellent biocompatibility, chemical stability, and mechanical properties, improved electrical conductivity in a controlled and localized way. Composite films based on silk fibroin and graphene oxide/reduced graphene oxide were fabricated by spin coating at room temperature. The films composition was characterized by micro-Raman spectroscopy demonstrating the presence of graphene oxide in the silk fibroin matrix. The influence of pulse energy on the fs laser transfer process was investigated, yielding a threshold energy of 24.0 nJ. Additionally, the laser-induced reduction of graphene oxide attained by LIFT was determined by Raman spectroscopy and further confirmed by an increase in the electrical conductivity. The approach presented here proved to be an efficient route to simultaneously produce micrometric patterns of graphene oxide/silk fibroin composite (with line widths on the order of 1 μm), and reduction of graphene oxide, opening new opportunities for the development of green composites for electronics devices, including for instance microfluidic sensors, capacitors and LEDs.

Uniform graphene oxide films fabrication via spray-coating for sensing application

A spray-coating technique for deposition of thin uniform graphene oxide films with a thickness of several tens of nanometers was developed. Among distinctive features of the technique is the low substrate heating temperature, which allows preserving most oxygen-containing functional groups and coat a wide range of substrates. The morphology of the spray-coated graphene oxide films with flakes of different lateral sizes was investigated. The local laser-induced photo-thermal reduction of thin graphene oxide thin films was demonstrated. The sensitivity of reduced graphene oxide to water vapor was also measured. Spray-coating and laser reduction techniques can be further applied for humidity sensor fabrication.

Controlling the Morphologies of Silver Aggregates by Laser-Induced Synthesis for Optimal SERS Detection.

Controlling the synthesis of metallic nanostructures for high quality surface-enhanced Raman scattering (SERS) materials has long been a central task of nanoscience and nanotechnology. In this work, silver aggregates with different surface morphologies were controllably synthesized on a glass–solution interface via a facile laser-induced reduction method. By correlating the surface morphologies with their SERS abilities, optimal parameters (laser power and irradiation time) for SERS aggregates synthesis were obtained. Importantly, the characteristics for largest near-field enhancement were identified, which are closely packed nanorice and flake structures with abundant surface roughness. These can generate numerous hot spots with huge enhancement in nanogaps and rough surface. These results provide an understanding of the correlation between morphologies and SERS performance, and could be helpful for developing optimal and applicable SERS materials.

Raman spectroscopy coupled with AFM scan head: A versatile combination for tailoring graphene oxide/reduced graphene oxide hybrid materials

Raman spectroscopy is a typical technique used for quality control and identification of graphene oxide, able to generate a laser-induced reduction process, even at low power, during spectroscopic characterization of such systems. This phenomenon, far from being a problem, brought up novel routes to obtain hybrid carbonaceous materials for diverse applications. Motivated by this feature, the present study provides new insights into controllable patterning of graphene oxide by introducing the combined use of Raman spectroscopy with synchronized X-Y scanning using an AFM head, as lithographical approach for tailoring conducting/insulating reduced graphene oxide/graphene oxide channels pattern. Thus, a systematic study of spectral, topographical and chemical characterizations performed on laser-induced hybrid structures is presented, showing the dependence on a proper selection of laser wavelength, power, and irradiation times as well as scanning parameters to achieve the effective fabrication of microchanneled patterns with nanometric depths of interest in nanotechnology applications.

Kinetic Control of [AuCl4]- Photochemical Reduction and Gold Nanoparticle Size with Hydroxyl Radical Scavengers.

Laser-induced photochemical reduction of aqueous [AuCl4]- is a green synthesis approach requiring no chemical reducing agents or stabilizers; but size control over the resulting gold nanoparticles remains a challenge. Under optical breakdown conditions producing hydrated electrons (eaq-) and hydroxyl radicals (OH•) through decomposition of water, [AuCl4]- reduction kinetics follow an autocatalytic rate law, which is governed by rate constants: nucleation rate k1, dependent on eaq-; and growth rate k2, dependent on the OH• recombination product, H2O2. In this work, we add the hydroxyl radical scavengers isopropyl alcohol and sodium acetate to limit H2O2 formation. Higher scavenger concentrations both lowered k2 values and produced smaller gold nanoparticles with Gaussian size distributions and remarkably narrow mass-weighted size distributions. With sufficiently high scavenger concentrations, the mean nanoparticle size could be tuned from 3.8 to 6.1 nm with polydispersity indices below 0.08. Both the higher surface area-normalized catalytic activity of the gold nanoparticles synthesized in the presence of scavengers, and FTIR measurements, indicate no capping ligands on the nanoparticle surfaces. These results demonstrate that the size distributions of "naked" gold nanoparticles produced by photochemical [AuCl4]- reduction can be effectively tuned by controlling the reaction kinetics.

High-power laser-patterning graphene oxide: A new approach to making arbitrarily-shaped self-aligned electrodes

We demonstrate the fabrication of self-aligned laser-reduced graphene oxide patterns with a spatial resolution/laser spot size ratio of 1/10, lower than anything reported before using laser-reduction. Laser light modifies graphene oxide (GO) by removing the oxygen-containing groups turning GO into a more graphene-like nanomaterial. Our method is based on high laser power density used for the reduction of GO that results in ablation of the GO film. This enabled us to remove the laser spot illuminated area while inducing the selective graphene oxide reduction at the periphery of the laser spot achieving resistivity of 1.6·10−5 Ω m, as low as values previously reported for other rGO. Therefore, we can exploit laser-induced reduction at high laser power density to pattern GO films with conductive dimensions that are a fraction of the laser spot size. This innovative method is scalable, inexpensive, and straightforward, allowing conductive circuits on arbitrary, flexible, and transparent substrates for applications in lightweight electronics and wearables.

Laser-Induced Reduction of Graphene Oxide by Intensity-Modulated Line Beam for Supercapacitor Applications.

Supercapacitors are irreplaceable energy-storage devices for high power output and rapid charge/discharge of electrical energy. In this study, the laser-based fabrication of reduced graphene oxide (rGO) electrodes for supercapacitors is demonstrated with several new features of laser irradiation. A conventional CO2 laser irradiation system is equipped with (1) a nitrogen blower to avoid combustion of the GO paper, (2) a cylindrical lens for producing a wide line beam, and (3) an optical chopper system for generating an intensity-modulated laser beam. Scanning of the intensity-modulated line beam transforms an extended area of GO into chemically reduced and physically porous graphene. The effects of the laser beam modifications and scanning parameters on the electrochemical performance of the rGO electrode are investigated. The rGO electrode exhibits a high specific capacitance (up to ∼130 F/g) at a current density of 1 A/g. This work can serve as a reference for the process optimization of laser-induced GO reduction.

Rapid in situ synthesis of polymer-metal nanocomposite films in several seconds using a CO2 laser

We demonstrate the rapid in situ synthesis of polymer-metal nanocomposite films using a CO2 laser at 10.6 μm. The mechanism of our method is that the precursor of the metal nanoparticles, i.e., the metallic ions, is very rapidly reduced in the laser-heated polymer matrix without any reducing agent. Unlike other known laser-induced reduction methods using UV lasers, which produce radicals to promote reduction, the CO2 laser energy is mainly absorbed by the glass substrate, and the laser-heated substrate heats the polymer matrix through heat diffusion to promote reduction. The superiority of the use of CO2 lasers over nanosecond visible~UV lasers is also demonstrated in terms of the damage to the film. The developed method can be a new alternative to quickly synthesize a variety of polymer-metal nanocomposite films.