Laser-Induced Galfenol Embedded Multi-Layer Graphene-Oxide in Laser-Induced Galfenol Embedded Multi-Layer Graphene-Oxide in Solution Solution

The proposed work demonstrates the direct synthesis of nanomaterial-embedded laser-induced few-layer graphene-oxide by directly ablating galfenol in a water-based solution for the first time. Laser-induced multilayer graphene-oxide (GO) embedded with galfenol (gallium–iron alloy) nanoparticles (NPs) is created through a method of direct laser inscription of bulk galfenol in deionized (DI) water with femtosecond laser ablation. The NP-embedded GO is achieved by irradiating a near-infrared (near-IR) femtosecond laser at 1040 nm on a bulk galfenol material submerged in a solution comprising DI water and a small concentration (5%/wt.) of polyvinylpyrrolidone followed by a second ablation in pure DI water. Results show nanoparticles with a mean diameter of ∼ 30 nm embedded in GO sheets with visible folds spaced at ∼ 0.63 nm. The composition of iron and gallium shifts by less than 2% during the laser ablation process, and the few-layer GO sheets exhibit similar Raman peaks to bulk graphite


INTRODUCTION
Recently, laser-induced graphene (LIG) from polyimide films gained prominence due to its simplistic synthesis method of direct carbonization of polyamide into graphene.Graphene has multiple applications, with some of the most popular being humidity sensing, [1][2][3][4] gas sensing, [5][6][7] liquid sensing, 8 strain sensing, [9][10][11][12][13] biosensing, [14][15][16][17] and energy storage. 18,19LIG is a relatively inexpensive and straightforward process when compared to chemical vapor deposition (CVD) 20,21 and other traditional synthesis methods. 22,23IG has been shown to coat components intended for use in harsh conditions such as deep water or space. 24These components can self-monitor mechanical wear or deformation. 18,19IG created from polyimide (PI) films has shown graphene production with prevalent porosity. 258][29] However, porous graphene has been shown to possess a relatively large bandgap, resulting in lowered electronic conductivity than pristine graphene. 26,30][33][34] Creation of nanomaterial embedded LIG can be structurally advantageous as it has been shown to increase the hardness, 35 resilience, 31 and diffusion. 34In addition, the embedded nanomaterial provides additional unique properties.For example, MoS 2 embedded graphene created by spin-coating MoS 2 sheets on polyamide films prior to laser inscription 26,27 exhibits stable and durable electrical properties that can be used in flexible electronics 31 and electrodes, 30 for supercapacitors, 32 and in multilevel resistive switching memory. 36The embedded MoS 2 enhanced the conductivity, capacitance, energy density, and cycle stability. 31,32Other examples of nanoparticle embedded graphene include Pt nanoparticle embedded graphene 25 and metal-oxide nanoparticle embedded LIG with Co 3 O 4 , MoO 2 , and Fe 3 O 4 nanoparticles. 37These embedded graphene sheets were used to create highly sensitive (gauge factor of ∼1,242) and reliable (hysteresis of ∼2.75) strain sensors with fast relaxation time (∼0.17 s). 31 Recently, researchers 25 demonstrated that NP-embedded LIG could be achieved, wherein platinum (Pt) was embedded into graphene by drop-casting Pt nanoparticles onto a thin solution film made of Pt(acac) 2 and illuminating the sample with a picosecond laser.This ablation induced the formation of graphene while simultaneously reducing Pt.The resulting strain sensors demonstrated high sensitivity (gauge factor ∼489.3) and a stable response at over 5000 cycles. 25However, even in this process, the nanoparticles must be synthesized separately, and an additional step of producing and curing the film is required.Our method of ablating a bulk material in a solution containing polyvinylpyrrolidone (PVP) creates a few-layer graphene-oxide (GO) and nanoparticles simultaneously.The generated NP-embedded GO dispersed in a deionized (DI) water solution is an ideal method to create a water-based ink and apply it to the printing technology for flexible hybrid electronics.
In the proposed work, we demonstrate for the first time the direct synthesis of nanomaterial-embedded laser-induced few-layer GO by directly ablating a galfenol in a water-based solution.This synthesis method will be the first time galfenol NPs and grapheneoxide are directly created and embedded through femtosecond laser ablation.Our production method is different from other standard methods 25,31,32,35,37 as it does not require the production of a polyimide film prior to ablation.
Galfenol was first discovered by the U.S. Navy and studied for its magnetostrictive properties, which cause a change in the shape of the structure in response to a magnetic field.Interest and demand for wearable, high-performance, and non-restricting flexible electronics rapidly increase.The magnetostrictive properties of galfenol make it an excellent material for micro-electromechanical systems (MEMS).9][40][41][42] We believe that due to the high energy density, high power density, and a long cycle time over a wide voltage range exhibited by galfenol, 43 graphene embedded with galfenol NPs has a huge potential for MEMS.
Our method of embedding nanoparticles is more environmentally conscious as it does not require the synthesis and ablation of a film, chemical contamination, high processing temperatures, or a harsh chemical etching agent.Our method produces GO and embedded NPs using the same femtosecond laser without the need for a separate synthesis process or method.The following sections describe the synthesis parameters used to create galfenol embedded laser-induced GO verified by analysis of transmission electron microscope (TEM) images, electron diffraction patterns, XRD, and the Raman spectrum.

EXPERIMENT Laser-induced graphene synthesis
The schematic in Fig. 2 shows the experimental setup to create the galfenol-oxide NP embedded LIG in a standard atmospheric environment (20 ○ C and normal pressure).A Spirit-One 1040 nm, 400 femtoseconds (fs), 100 MHz laser was focused on the sample through a waveplate for precise power control.It utilizes a Thorlabs uncoated plano-convex lens with a focal length of 7.5 cm.The sample is a 2 g bulk of galfenol submerged in a 25 ml deionized (DI) water solution and 1.25 g (5%/wt.) of polyvinylpyrrolidone.We confirmed that the source of graphene is PVP By ablating the bulk galfenol sample in DI water alone.We did not observe characteristic graphene sheets in the TEM image, as shown in Fig. 1.The galfenol sample rests on a motorized linear stage obtained from Newport to move at 2.6 mm/s.The laser light is focused directly on the bulk galfenol at the point where a bright white light reflects off the surface of the bulk material.At this focus, visible clouds of nanoparticles appear in the solution.The laser power is 3 W with a repetition of 1 MHz, and ablation of the sample occurs for 10 minutes.The sample moves in a back-and-forth motion, scanning the laser light over the material's surface with assistance from a linear stage to avoid producing craters on the surface and disrupting the incident light's focus.After this initial ablation, the sample is removed from the solution and left to dry.The sample is then transferred to a glass Petri dish with a 25 ml solution of pure DI water and ablated a second time for 10 minutes with identical parameters.Finally, the bulk is removed from the Petri dish, and the remaining colloidal solution of NPs and DI water is transferred to a sealed glass vessel.
The distance from the focusing lens to the material being lased is necessary to ensure consistent NP and graphene production.If the focused light is not meeting the material at the focus plane, the spot size on the sample will be much larger.Therefore, the laser power incident on the sample will be less intense as it is being spread over a larger surface area.The spot size for a gaussian beam in the TEM 00 mode using 2ω 0 = 4 M 2 λf/πD gives a beam diameter when focused on the surface of the material of ∼49.7 μm, 44 where M 2 is the beam divergence, lambda is the wavelength of the laser light, f is the focal length of the lens, and D is the diameter of the beam entering the lens.As a result of the galfenol sample being submerged in a DI water solution, the sample cannot simply be positioned the exact distance away from the sample as the lens focal distance (f = 7.5 cm).As light travels from air to water, it diffracts as a result of entering material with a higher refractive index.This causes the beam of light to travel a further distance. 44Using this method of ablation in water requires awareness of the level of water above the sample and the distance of the lens from the target.In our experiment, the depth of the water over the sample is ∼1 cm.Assuming a refractive index of 1.00 for air and 1.33 for deionized water with 1,040 nm light, 45 using Snell's law, concludes that the actual distance traveled in the 1 cm of water is ∼1.513 cm.Therefore, if the focal distance of the lens is 7.5 cm, the distance between the lens and the target must be 6.99 cm.

Material characterization
We utilized several methods to characterize and verify the synthesized GO layers.First, a dynamic light scattering (DLS) measurement was taken to estimate the average hydrodynamic diameters of the embedded nanoparticles, depicted in Fig. 3 I).Finally, an energy-dispersive x-ray spectroscopy (EDS) system on the TEM was used to verify the composition of the NPs, the GO sheets, and the oxide layer present on many of the nanoparticles [Fig.4(d)]. 3

RESULTS
The DLS data in Fig. 3(a) illustrates the hydrodynamic diameter distribution among the galfenol-oxide nanoparticles.The distribution shows results for 100 independent trials of the material at the same laser fluence.We observed that average NP sizes were consistent across all 100 samples and resulted in mean effective diameter of 30.83 nm with a variance of 29.90 nm.The majority of the galfenol-oxide nanoparticles present in the sample were 50 nm in diameter or less.Figure 3(b) shows the Raman spectrum observed for the few-layer GO.Raman measurements were carried out using  a Horiba LabRAM HR Evolution at room temperature at 633 nm.A 100× objective was used to locate the few-layer GO sheets, and care was taken to avoid damaging the sample during analysis.Three characteristic peaks exist in the Raman spectrum of graphitic materials.They are known commonly as the G-peak, D-peak, and 2D-peak. 46Bulk graphite exhibits a G-peak at 1580 cm −1 and a 2D peak at 2700 cm −1 .The galfenol embedded few-layer GO exhibited a D-peak at ∼1380 cm −1 , G-peak at ∼1680 cm −1 , and a 2D-peak at ∼2800 cm −1 .TEM images of galfenol-oxide nanoparticles embedded in laser-induced GO are shown in Figs.4(d Many few-layer GO sheets with galfenol-oxide embedded NPs are depicted in Fig. 5(a).We computed the TEM lattice dimension and the number of folds present in the GO.Figures 5(b) and 5(c) reveal few-layer GO from 3 to 9 layers.A profile measurement was taken using the TEM to compute the distance between the edges in the multilayer GO.The distance between the edges in the observed GO sheet shown in Fig. 5(c) is ∼0.63 nm.Measurements were taken for five multilayer GO sheets found in different locations on the TEM grid with an average interlayer spacing of ∼0.598 nm.This distance is a crucial characteristic to observe as thinner graphene sheets are expected to have finer wrinkle-wavelengths, or distances from peak to peak of periodical ripples. 47The distance between layers is much greater than the average lattice spacing for graphite, reported as 0.34 nm. 48However, research has commonly shown larger interlayer spacings in graphene, such as 0.45 nm, 22 and graphene-oxide, about ∼0.80 nm. 49he average interlayer spacing observed indicates few to multilayer graphene-oxide present in the produced sample.
An electron diffraction pattern can be seen in Fig. 5(d), showing the diffraction rings exhibited by the galfenol-oxide nanoparticles.The clear diffraction spots, or bright white spots in the image, indicate that the material possesses a crystalline structure characteristic of a gallium-iron alloy.The inverse of the radius is proportional to the d-spacing, or space between planes of atoms.From the XRD database, it can be seen that bulk graphite has a d-spacing of 2.110 Å in the (100) orientation.The electron diffraction pattern of the synthesized few-layer graphene shown in Fig. 5(d) resulted in a d-spacing within 10% error of values for bulk graphite for all orientations observed in the XRD image, with a corresponding d-spacing of 2.270 Å at an orientation of (100), as shown in Table I.

CONCLUSION
In conclusion, we have reported a unique and simple process of creating nanomaterial embedded few-layer grapheneoxide by ablation with a femtosecond laser directly into the waterbased polyamide solution, followed by a second ablation submerged in only DI water.Embedding NPs in GO is desired as it can enhance the physical and chemical properties of graphene.Typical methods include laser-induced graphene in polyamide film and embedding NPs using chemical vapor deposition or electrochemical exfoliation.Even in the recent work by Dr. Liu 25 that shows laser-induced graphene in a solvent, the nanoparticles had to be drop-casted in a solvent, making it a complex process.Our method is cost-effective and greener as it does not require chemical synthesis, contaminants, a controlled environment, or post-processing.

FIG. 1 .
FIG. 1. TEM image of galfenol nanoparticles synthesized through laser ablation in DI water.

FIG. 2 .
FIG. 2. Lab setup for laser induced graphene-oxide production.The red line depicts the path of the near-IR laser light.
(a).Then a Raman spectrum was gathered on the few-layer GO to verify the composition [Fig.3b].A transmission electron microscope (TEM) was used to generate high-resolution images of the size and presence of galfenol nanoparticles and GO sheets [Figs.3(d)-3(f)].XRD was also carried out to compare interplanar spacing in the few-layer GO with the TEM database (Table
)-4(f).The EDS data in Figs.4(a)-4(c) correspond with the TEM images directly below.The folded edges of the graphene sheets are easily identifiable and are a characteristic trait of this material.Figure4(a) shows EDS data from the center of the particle depicted in Fig.4(d), demonstrating a large quantity of iron (Fe) and gallium (Ga).Large amounts of copper (Cu) and Oxygen (O) are present in the sample due to the TEM grid being made of copper and oxidation of the sample occurring during laser ablation.The large intensities in the EDS data shown in Fig.4(b) indicate that the oxide layer around the galfenol NPs is rich in iron and oxygen.The EDS composition data for the large region in Fig.4(f) show an area including GO and NPs, proving that polymer exists in the solution.The PVP polymer contains carbon, nitrogen, and oxide elements, as confirmed by the EDS peaks.The atomic composition of bulk galfenol at the time of purchase was Fe 81.6 Ga 18.4.The atomic composition of the galfenol-oxide embedded NPs after ablation was found to be Fe 83.55 Ga 16.45 through analysis by EDS.Therefore, a shift of 1.95% in favor of iron occurred during the laser ablation process.

FIG. 5 .
FIG. 5. TEM images of (a) grapheneoxide folds and galfenol-oxide NPs, (b) GO folds, (c) lattice dimensions of folds, and (d) the electron diffraction pattern of a galfenol NP.The scale bar for (d) reads 10 1/nm, meaning the scale bar is 10 nm in length if the inverse of the measured diameter of rings is taken.

TABLE I .
Interplanar spacing of few-layer graphene compared to the database.