Laser scribed graphene for supercapacitors Opto-Electronic

Supercapacitors, with the merits of both capacitors for safe and fast charge and batteries for high energy storage have drawn tremendous attention. Recently, laser scribed graphene has been increasingly studied for supercapacitor applications due to its unique properties, such as flexible fabrication, large surface area and high electrical conductivity. With the laser direct writing process, graphene can be directly fabricated and patterned as the supercapacitor electrodes. In this review, facile laser direct writing methods for graphene were firstly summarized. Various precursors, mainly graphene oxide and polyimide were employed for laser scribed graphene and the modifications of graphene properties were also dis-cussed. This laser scribed graphene was applied for electrochemical double-layer capacitors, pseudo-capacitors and hybrid supercapacitors. Diverse strategies including doping, composite materials and pattern design were utilized to enhance the electrochemical performances of supercapacitors. Featured supercapacitors with excellent flexible, ultrafine-structured and integrated functions were also reviewed.


Introduction
Supercapacitors, also called electrochemical capacitors or ultracapacitors, have been intensively studied over the past few years to meet the rapidly growing demand for highly efficient energy storage devices 1−3 . Owing to their unique advantages including high power density (10 kW/kg), short charge/discharge duration (in seconds), and long cycle life (over 1 million cycles), supercapacitors were considered to be promising candidates in the applications of consumer electronics, hybrid electric vehicles and industrial power management 4,5 . Supercapacitors with properties such as high energy and power densities, small sizes, light weights and mechanical flexibility have been highly demanded 6−8 . Graphene, a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, has become a hot research topic since its discovery 9,10 . It shows great potential for supercapacitor applications due to its exceptional high theoretical surface area (2630 m 2 ·g −1 ) and electrical conductivity 11 . Many approaches such as mechanical exfoliation 12 , chemical vapor deposition (CVD) 13 and the reduction of graphene oxide 14,15 have been developed to produce graphene materials. Recently, a laser direct writing method for graphene has drawn tremendous attention, because of its unique advantages including selective and localized reduction, flexible patterning and no requirement for additional chemicals 16,17 . Based on the laser scribed graphene (LSG), various applications have been demonstrated, including hologram 18 , energy storage 19−21 , strain sensor 22,23 , biosensor 24,25 and antennas 26 . Unlike conventional microfabrication methods such as lithography, this laser technique does not require the utilization of masks, expensive materials, post-processing and cleanroom operations 27−32 . The LSG can be directly prepared by laser irradiation and simultaneously patterned for the electrodes of supercapacitors. This facile and cost-effective method of graphene fabrication has demonstrated great potential for commercial supercapacitor applications.
In this review, the recent developments of the LSG supercapacitors were summarized. Firstly, we concluded the fabrication and modification of LSG. Particular attention is paid to the application of electrochemical double-layer capacitors (EDLCs), pseudo-capacitors and hybrid supercapacitors, and the diverse strategies to achieve high-performance, flexible, ultrafine-structured and integrated supercapacitors. Current challenges and future advancements of LSG based supercapacitors were also discussed.
Different precursors: graphene oxide, commercial polyimides and other carbon resources were employed for laser scribed graphene production in the literature. The resulting graphene materials are referred differently in scientific reports as laser reduced graphene oxide 33 , laser-induced graphene 34 , laser-scribed graphene 35 , or laser carbonized nanomaterials 36 . The term 'laser scribed graphene (LSG)' is adopted throughout this paper.

Laser scribed graphene
Preparation of laser scribed graphene

Graphene oxide
Graphene oxide (GO), which has the skeleton of graphene decorated with oxygen components, is considered as an important precursor of LSG 15 . It can be produced in large scale by cost-effective chemical methods, forming stable aqueous colloids that are favored by industrial processes 37 . GO films were prepared by spincoating, drop-casting, blade, or freezing-drying method. Graphene devices on diverse substrates, including leaf, lens, fabrics etc. can be fabricated by laser technology 38 . Xiao et al., fabricated LSG microcircuits with the line widths of 500 nm by the laser irradiation on GO films 33 . Figure 1(a-c) shows the fabrication of GO films, the LSG microstructures, and the scanning electron microscopy (SEM) images of the microcircuits. After the laser writing, the thicknesses of film decrease and the color of film turns to black. An obvious removal of oxygen components can be observed as indicated in AFM and XPS results in Fig. 1(d-f). The mechanism of laser reduction of GO was strongly related to the photochemical and photothermal effect of laser 15 . The threshold of GO photoreduction was 3.2 eV (390 nm) 39 . For the laser with wavelength < 390 nm, the photochemical effect of laser can trigger the C−O bonding weakening and the oxygen removal. Meanwhile, it was reported that the exothermic reduction of GO occurs at a temperature between 200 -230 °C 40 . The high temperature induced by laser could easily break the C−O and C=O bonds, leading to the reduction of GO. In this laser reduction process, two sub-processes, namely the direct conversion from sp 3 carbon to sp 2 carbon and removal of oxygen functional groups can occur, resulting in the reduced graphene oxide (rGO) 41 . The ultrafast thermal transferred process triggered by the laser spot can also induce the simultaneous exfoliation and reduction of GO, and thereby enhance its specific surface area 42 . Beside GO film, GO in solution, GO fiber and GO aerogel can also be reduced to rGO with laser treatment. Figure 1(g) shows the dramatic color change of the GO solution with ammonia before and after the pulsed laser irradiation 43 . Upon pulsed laser irradiation, the yellow-brown color instantaneously turned black, indicating the effective reduction of GO in solution. GO fiber were also region-specifically reduced by laser irradiation to fabricate a flexible fiber supercapacitor with reduced GO layers as electrodes and GO as the separator 44 . Figure 1(h) shows that a precursor of GO aerogel, after being exposed to a laser spot, was reduced in only tens of milliseconds and converted to graphene bulks 45 . In another report, the self-assembled GO liquid crystals on the surface of GO solutions can also be reduced by laser 46 . Ibrahim et al. introduced the production of reduced GO gels by focusing a femtosecond laser on air/GO solution interfaces 47 .

Polymer and biomass
In 2014, Tour's group reported the fabrication of porous LSG films from commercial polymer films using a CO 2 laser 48 . The polyimide (PI) and ployetherimide out of 15 different polymers were successfully converted to LSG under laser irradiation. The LSG with excellent electrical conductivity (5 -25 S·cm −1 ) can be readily patterned to interdigitated electrodes for in-plane supercapacitors with specific capacitances of < 44 mF·cm −2 and power densities of ～9 mW·cm −2 . Figure 2(a-c) shows the laser pattern on PI and the SEM images of LSG, exhibiting high surface area (～340 m 2 ·g −1 ) with pore sizes < 9 nm. The Raman spectrum of LSG ( Fig. 2(d)) demonstrates a 2D Raman band (centered at 2700 cm −1 ), typically found in that of 2D graphite. The XRD pattern of LSG ( Fig.  2(e)) exhibits an intense peak centered at 2θ=25.9°, indicating the high degree of graphitization. This LSG formation is attributed to the extremely high localized temperature (>2500 °C) triggered by the CO 2 laser beam, which can break C−O, C=O and C−N bonds and rearrange the aromatic compounds to graphene structures. In a similar process, Zhang et al., converted phenolic resin into LSG with 3D porous structures with low resistance (~44 Ω·sq −1 ) and good mechanical properties in large scale by a laser scribing 49 . The polybenzoxazine resin poly (Ph-ddm) with good flexibility, high thermal stability and superior chemical resistance was also employed for the fabrication of LSG by straightforward CO 2 lasing 50 . The graphitization of sulfonated poly (ether ether ketone) (SPEEK) film was obtained for an all-SPEEK flexible supercapacitor using a pulsed CO 2 laser 51 . The resulting LSG can act as the binder-free electrode. The current collector and the SPEEK is employed as both separator and polymeric electrolyte. With laser treatment, natural precursors such as wood 52 , cloth, paper, potato skins, coconut shells, cork 53 and lignin 54 , which are inexpensive, abundant, and biodegradable, can also be transformed into graphene, as shown in Fig. 2(f-j). Kaner et al. converted carbon nanodots (CNDs) into high-surface-area 3D graphene networks with excellent electrochemical properties by an irradiation with an infrared laser 55 . The fabricated 3D LSG electrodes show high specific volumetric capacitance of 27.5 mF·cm −3 and extremely fast charging rates with a relaxation time of 3.44 ms.

Modification of laser scribed graphene
The chemical component, structure and morphology of LSG is strongly affected by the laser scribing process and can be modified by adjusting the laser parameters, laser process, laser system, precursors and environment. Laser systems with varied wavelengths, including CO 2 laser (10.6 μm) 36 , near-infrared (NIR) laser (1064 nm) 56 , 780 nm femtosecond (fs) laser 41   reduction process is attributed to the photochemical effect and photothermal effect induced by laser 15 . The surface modification of LSG, including modulating the surface morphologies, carbonization and wettability was demonstrated by adjusting laser powers, scanning speeds and pulse repetition frequencies 36,58,59 . Different morphologies, namely "sheet ", "needle " and "porous ", can be achieved with the optimized laser-writing recipes 60 . With repeated laser irradiations, the structure of PI-derived LSG can be transformed from its original macroporous foam to an intermediate concave corrugated tile structure, and finally a carbon nanotube structure 61 . Instead of a focused round beam, a wide line beam was employed to transform GO into LSG, improving the efficiency of large area laser reduction 62 . The resulting LSG electrode exhibits a high specific capacitance (~130 F·g −1 ) at a current density of 1 A·g −1 . Sun et al., reported a hierarchicalstructuring and synchronous GO photoreduction using a nanosecond laser holography technology 63 . The laser beam was split into two with an equal intensity, and then interfered on the surface of a GO film. The periodic light field patterns generated by the interfered laser beams resulted in the periodic micro-nano structures of LSG. It was found that the laser reduction of PI resulted in selfnitrogen-doped porous LSG (sourcing from the N element of PI precursor), improving its conductivity and electrochemical performance 24

Laser scribed graphene based supercapacitor
Based on the different energy storage mechanisms, supercapacitors (SCs) can be divided into two types: electrochemical double-layer capacitors (EDLCs) and pseudo-capacitors 66 . EDLCs, which are non-Faradaic ca-pacitors, store energy by building up charges in the layers of the electrical double-layer formed at the interface of electrode/electrolyte 67 . Owing to the fast physical charging and discharging process, EDLCs show great advantages of short charging time, high power densities and long lifespans. However, the capacitance of EDLCs is relatively low due to the limited effective surfaces of electrodes. In this way, LSG electrodes with excellent conductivities and high surface areas are very promising to improve the energy density of EDLCs. Different from EDLCs, the capacitance of pseudo-capacitors is acquired from the storage of charge in the bulk of a redox material following a redox reaction 66 . This fast redox reaction  acts like capacitance (hence the name pseudo-capacitance) and contributes to an enhanced capacitance 68 . The poor conductivity of pseudocapacitive materials limits the power density. Moreover, the material deterioration during the redox process shortens the cycle lives of devices. The porous LSG with good conductivity and chemical stability was considered as a good framework of pseudo-capacitors. The LSG has been widely studied to fabricate both EDLCs and pseudo-capacitors. The electrochemical performances of diverse LSG-based SCs in literature were listed in Table 1.

LSG based electrochemical double-layer capacitors
In 2012, EI-Kady et al., fabricated the LSG based EDLCs by employing a standard LightScribe DVD optical drive, as shown in Fig. 3(a) 69 . The GO films were deposited on the disk and then treated with laser. The resulting LSG shows a high electrical conductivity (1738 S·m −1 ) and a specific surface area (1520 m 2 ·g −1 ). An ion porous separator was sandwiched between two identical LSG electrodes for SCs (total thickness < 100 μm). The fabricated LSG SCs exhibit a energy density of 1.36 mWh·cm −3 and a power density of ~20 W·cm −3 . With the similar laser direct writing technology, this research group demonstrated a scalable fabrication of LSG in-plane supercapa-citors, Fig. 3(b) 70 . The GO irradiated by laser was converted into graphene and applied as the SC electrodes. The unirradiated GO served as the separator between the positive and negative interdigitated electrodes. More than 100 LSG SCs can be produced on a single disc in 30 minutes or less. These efficiently fabricated LSG supercapacitors exhibit an ultrahigh power of 200 W·cm −3 and an excellent cycling stability retaining 96% of the initial performance after 10000 charge-discharge cycles. Gao et al. demonstrated laser reduction and patterning of hydrated GO films for all-carbon monolithic supercapacitors 20 . The LSG electrodes were fabricated for both inplane and conventional sandwiched supercapacitor. The hydrated GO, which contains substantial amounts of trapped water serves as both the electrolyte and the electrode separator. The resulting LSG SCs show a good cyclic stability (30% drop in the capacitance after 10000 cycles) and a high areal capacitance (0.51 mF·cm −2 ). By laser scribing, the amorphous carbon nanospheres (CNS) precursors are transformed into highly turbostratic graphitic carbon (CNS-LSG) 82 . The sandwiched supercapacitors based on the CNS-LSG electrodes exhibit a high volumetric power density of 28 W·cm −3 . For high quality LSG SCs, a CO 2 laser beam was employed to fabricate LSG with high crystallinity and a low degree of defects.  Then a successive ultraviolet (UV) pulsed laser direct carving was performed for high resolution electrode pattern 83 . The fabricated SCs with an electrode width of 50 μm exhibit a areal capacitance of 43.7 mF·cm −2 and a 90% capacitance retention after 3000 cycles. In addition to the laser irradiation method, a successive electrochemical reduction was combined to fabricate highly conductive graphene networks for current collectors of supercapacitors 84 . The developed LSG supercapacitor shows notable improvement of the stability performances (100000 cycles). A large cell voltage of 10.8 V was realized by modularizing nine devices in series, exhibiting rectangular shapes of the cyclic voltammetry curves at high scan rates of 100 V·s −1 .

Doped LSG for supercapacitors
Doping with heteroatoms (such as boron, nitrogen, phosphorus, and sulfur) has been regarded as an effective way to tailor the electrochemical properties of graphene-derived materials and to enhance their capacitive performances 85−87 . Heteroatom-doped graphene materials were intensely studied as active electrodes in energy storage devices. Tour et al., demonstrated that boron-doped porous graphene can be prepared from boric acid containing polyimide sheets using a facile laser induction process in ambient air, as shown in Fig. 4(a) 72 . At the same time, the LSG was patterned for electrodes of flexible supercapacitors. Fig. 4 , which is about 10 times higher than that of the bare LSG.  16 20

Intercalated LSG for supercapacitors
Owing to the intensive pi -pi interaction of graphene, graphene sheets exhibit the strong tendency to restack together. The restacking issue leads to a significant decrease of ion-accessible surface area and thus a low capacitance of the graphene-based SC 91 . To prevent the restacking of LSG layers, carbon nanotubes (CNTs) with a smaller diameter (1−2 nm) were employed to insert between GO sheets before laser treatment, as shown in Fig. 5(a) 73 . The GO/CNTs hybrid material was patterned into LSG/CNTs supercapacitors, yielding increased ionaccessible surface area. Figure 5( 93 . Attributed to a larger specific surface area and lots of mesopores, the LSG/Zn SC exhibited a nearly 4 times increase in the energy density. Besides intercalated graphene, components with advanced properties were also combined with LSG for enhanced performances. Li et al., reduced the mixture GO and chloroauric acid (HAuCl 4 ) nanocomposite with a laser irradiation, patterning of LSG electrodes and producing Au nanoparticles in a one-step process, as shown in Fig. 5(f-i) 71 .
The porous LSG/Au electrode demonstrates a high conductivity of 1.   polyimide onto 3D nickel foam, as a porous electrode with laser processing 95 . The LSG/Ni electrode shows a high electrical conductivity (359712 S·m −1 ) and the fabricated LSG/Ni supercapacitor demonstrated a large areal specific capacitance (995 mF·cm −2 ), a power densitiy (9.39 mW·cm −2 ) and over 98% capacitance retention after 10000 cycles.

Pattern and structure of LSG based supercapacitors
Besides the electrode materials, the structure design of SCs has been also intensively studied to further enhance the performance of devices. Li et al., fabricated a flexible high-voltage LSG-SCs ranging from a few to thousands of volts with a planar in-series architecture, shown in Fig.  6(a) 74 . 210 isolated porous LIG squares were firstly pat-terned on PI by a programmable CO 2 laser system. The electrolyte was then added by brush coating process. The 209 V SC could achieve a high capacitance of 0.43 μF at a low applied current of 0.2 μA, and a capacitance of 0.18 μF at a high applied current of 5.0 μA. Figure 6(b) shows that a new structural design inspired by the traditional Japanese paper-cutting craftwork (known as "Kirigami"), has been employed to manufacture highly deformable SCs by laser-assisted graphitic conversion and cutting 96 . There is less than 2% shift in the LSG capacitance when the device is elongated to 382.5% of its initial length 97 . Figure 6(c) shows that vertically stacked LSG supercapacitors were assemblyed to multiply its electrochemical performance by laser induction on both sides of PI sheets 34 Fig. 6(e−f). Compared to the conventional planar supercapacitors, the Hilbert fractal designed SC increased the ratio of active surface area to volume of the electrodes and reduced the electrolyte ionic path. The energy density is thus significantly increased to ~10 −1 Wh·cm −3 , more than 30 times higher than that achievable by the planar interdigital electrodes.

LSG based pseudo-capacitors
Compared to the EDLCs, pseudo-capacitors can achieve much higher capacitances since they store energy through a Faradic process, involving fast and reversible redox reactions between electrolytes and electro-active materials on electrode surfaces 68 . Transition metal oxides, hydroxides and conducting polymers are usually used as the electrodes for pseudo-capacitors 99 . However, owing to the poor electrical conductivity and the unstable structure of materials during the redox process, pseudo-capacitors demonstrate relatively low power densities and cycling unstabilities, which hinder their practical applications 66 . To overcome these drawbacks, graphene materials with high electrical conductivity and large specific surface area are merged with these active   Fig.  7(a-c) 35 . A CO 2 laser beam was firstly used to convert the PI into porous LIG with an interdigitated architecture. Then, the conductive Ni-catecholate-based metal-organic frameworks (Ni-CAT MOF) were selectively grown on 3D LSG with a low-temperature solvothermal method. Figure 7(d) depicts that the tiny uniform nanorods are anchored on the LSG sheets and the black-grey pattern turns into dark blue (inset of Fig. 7(d)). Figure 7(e-f) demonstrates the cyclic voltammetry (CV) curves and the galvanostatic charge/discharge (GCD) curve of the bare LSG electrode and that of the LSG/Ni-CAT composite.

LSG based hybrid supercapacitor
To maximize the benefits of existing supercapacitors (high power density and stable cycle performance) and lithium-ion batteries (high energy density), hybrid supercapacitors were proposed by Naoi in 2009 by using an asymmetric electrode 106,107 . Asymmetric supercapacitors have been extensively explored by combining Faradic electrodes and capacitive electrodes to enhance energy density of high-power SCs 108 . Liu et al., reported a facile fabrication of an in-plane hybrid supercapacitor with the Fe 3 O 4 nanoparticle-anchored LSG (LSG/Fe 3 O 4 ) as the anode and LSG as the cathode 77 . Figure 8(a) demonstrates the preparation of LSG/Fe 3 O 4 with laser irradiation on FeCl 3 crystal-coated PI film and subsequent laser annealing. Figure 8(b-c) shows the 3D porous LSG was well wrapped by the Fe 3 O 4 nanoparticles. Figure  8( 78 . The LSG supercapacitors exhibit high areal specific capacitance up to 34.7 mF·cm −2 , while that with the acid gel electrolyte is 8 mF·cm −2 . This substantial enhancement is considered to be due to the combination of Faradaic intercalation and non-Faradaic absorption of the Li-ions at the LSG electrodes.

Flexible LSG supercapacitor
With the increasing development of wearable devices, flexible energy storage units are highly demanded. Gu et al., reported large-scale flexible LSG supercapacitors with dimension 100 cm 2 fabricated on textiles in 3 minutes as shown in Fig. 9(a) 112 . The fabrics were paint-coated with the GO/Matte binder solution to form thin films with thicknesses of 3 μm, which were then treated with a CO 2 laser. The fabricated SCs demonstrate an excellent water stability, an areal capacitance of 49 mF·cm −2 , an energy density of 6.73 mWh·cm −2 and a power density of 2.5 mW·cm −2 . The LSG SCs show stable CV results for a maximum of 200% stretchability and a high capacitance retention of 88% under 500 cycles of 200% stretching condition, as shown in Fig. 9(b-c). Xie et al., presented LSG SCs on the flexible substrate of Poly(ethylene terephthalate) (PET, 6 μm-thick) 79 . With electrostatic spray deposition method, the uniform GO film was deposited on a PET covered with Ni film (500 nm). By adjusting the laser power, the reduction and patterning of LSG electrode arrays can be fabricated in just one batch. These ultrathin ( stability, retaining 84% of their initial capacitance after 1000 cycles in stretching condition and almost 90% in the bending condition. Furthermore, a PDMS/PI powder composite was directly treated by a CO 2 laser, resulting in the graphitization of polyimide for the application of flexible strain gauge and supercapacitor 114 . The laserwritten PDMS/PI substrates are sufficiently electrically conductive and mechanically stable for flexible electronics. A flexible melamine foam was employed as the skeleton to attach the GO sheets 115 . After laser irradiation, the sandwiched LSG -GO -LSG foam supercapacitor shows a high capacitance performance which can be easily regulated by adjusting the compressive state of electrodes. The supercapacitors show a volumetric energy density of 0.04 mWh·cm −3 and 1 mWh·cm −3 under 0% and 90% strain, respectively.

Miniaturized LSG supercapacitor
The large dimension of LSG significantly limits the density of graphene electrodes of the supercapacitor, decreases the effective surface area and thus severely deteriorates the energy densities of supercapacitors 116 . Consequently, the fabrication of LSG with a high spatial resolution is a promising approach to enhance energy densities of supercapacitors. Due to the diffraction limit and the heat diffusion generated during the laser reduction, the linewidth of LSG electrodes in a supercapacitor is strongly related to the laser direct writing system 117 . The fs laser fabrication is mainly considered as a non-thermal fabrication process, which involves the multiphoton ab-sorption within the time scales of less than a picosecond 18

Integrated LSG supercapacitor
With the recent rapid growth of portable and multifunctional electronic devices, the studies of integrated energy devices have attracted enormous attention. Gu et al. integrated the in-plane supercapacitors with commercial c-Si solar cells by using a CO 2 laser to scribe the GO film on the reverse side of solar cells, as shown in Fig. 11(a) 122 . Under light illumination, electron-hole pairs are generated in the silicon solar cell and eventually collected by   Fig. 11(b-d) 124 . The LSG SC and a ZnO nanoparticles-based photodetector were prepared by a one-step laser direct writing process and were integrated with commercial solar panels. It was demonstrated that the SC can be easily charged within 1 minute by the solar panel and remain above 0.55 V after 2 h, which is sufficient to drive the UV photodetector. Based on the laser direct writing on a PI film, the same research group also combined micro-circuits for wireless charging with LSG SC for energy storage, Fig. 11(e-f). This integrated device can be wirelessly charged by a commercial wireless charger 57 . Kim et al., reported a novel thermally chargeable supercapacitor that can convert thermal energy to electricity and then store charge simultaneously, Fig. 11(g-h) 125 . These devices were fabricated with laser irradiation on GO films intercalated by sulfate ions. With a temperature gradient of 10.5 K, a thermally charged voltage of 58 mV can be generated. This supercapacitor can perform as long as a temperature gradient exist. Therefore, any heat dissipating objects including the human body and power-consuming devices can be utilized as power source for charging. This laser direct writing for graphene shows a facile and versatile process, can be compatible with various devices and indicates great potential for integrated multifunctional units.

Conclusions and outlook
As concluded in this review, great progress has been made in the research field of LSG SCs. LSG can be fabricated from various precursors including GO, polymer and biomass with different laser systems. And its modifications were achieved by adjusting the laser parameters, fabrication processing and environment. The laser direct writing technology can simply induce graphene and simultaneously pattern the graphene electrodes for supercapacitors. Based on LSG, the fabrication of EDLCs, pseudo-capacitors and hybrid supercapacitors and their performances were discussed. Numerous studies have devoted to developing LSG SCs with enhanced performances by element doping, intercalation and pattern optimization. Diverse supercapacitors with advanced fea-tures such as being flexible, high power density, miniature, high voltage and new pattern design were also illustrated. The LSG SCs hold great potential for energy storage in the future. However, there is still room for further improvements. Due to the increasing demand for the portable devices, the miniaturization of energy units is highly expected 126 . Currently, the resolution of laser direct writing, which is typically several micrometres or even millimetres has severally restricted the size of LSG SCs. Meanwhile, the number of LSG electrodes on devices was decreased due to the low resolution, which hampers the energy density of LSG SCs. It was highly expected to further improve the resolution of LSG for fabricating the nano-supercapacitors and enhancing their performances. For the fabrication of asymmetric SCs, the laser direct writing process is always followed with electrodeposition process to deposit the pseudo-capacitive materials on one of the electrodes of the in-plane supercapacitors. However, this process may not be suitable for the fabrication of asymmetric MSCs considering the size. Other leading methods or technologies, such as printing or lithography, should be explored and combined with laser direct writing method.
Moreover, a broad range of precursors should be explored for LSG, considering not only the properties of LSG but also their impacts on environment and potential for massive production. Various carbon sources, which were traditionally considered as waste, could be reused as precursors of LSG fabrication. The development of potential precursors can effectively utilize resource, reduce pollution and promote the massive production of LSG. In addition, the integration of LSG SCs with energy-generating and energy-consuming components should be further developed. Various energy devices, including thermal or mechanical energy generator, can be investigated as future energy suppliers of LSG SCs. The energy-consuming devices including diverse LSG sensors and LSG transistors can also be integrated. Importantly, these integrated all-LSG devices entirely engraved by laser will facilitate scalable manufacture and the industrialization of LSG SCs. The integration of supercapacitors, solar cells and radiative coolers can also be very attractive. Radiative cooling is a promising cooling method without external energy consumption. The radiative coolers can simultaneously possess a high solar reflection up to 97% and strong infrared emission, cooling the object below ambient temperature 127,128 . The reflected solar can be redirected to solar cell for higher absorption efficiency. The decreased temperature can improve the efficiency of the integrated devices since the rising operation temperature deteriorates the performance and reliability of solar cells 129,130 .
Furthermore, the laser technology should be combined with other advanced technologies. As a significant technique, artificial intelligence (AI) is emerging as an effective approach to solving complicated problems in various fields and is becoming more and more important nowadays 131,132 . AI system can adapt its parameters and generate desired outputs from given inputs. It has been applied to emulate the human cognitions, including autonomous decision, deduction, adaption and interpretation 133 . In the research of LSG SCs, AI can be very favorable in several key areas, including pattern designs, fabrications and applications. The AI-driven inverse design has demonstrated great potential in the demanded design of structures and devices 134,135 . Giving certain conditions and constraints, a sequence of patterns can be inversed designed by AI for modelling of the LSG SCs and prediction of their performances. Through the "training" phase, the unseen internal nonlinear relationships between the layout of electrodes and the performances of the corresponding LSG SCs can be statistically acquired. To meet the requirements of various applications, the LSG SCs can be custom-designed to achieve different energy densities, power densities and reliabilities. Avoiding conventional regulatory and constrains, AI offers tremendous potential for the novel patterning designs of LSG SCs. Meanwhile, the self-learning ability of AI gives it great advantage of biomimetic design. The bioinspired structures can largely enhance the variation of the electrode layout. With the assistance of the AI, diverse biomimetic designs for LSG SCs can be further optimized to improve the accessible active area of the electrodes and reduce the electrolyte ionic path. AI can also be utilized for accurate fabrication of LSG SCs. In the LSG production, laser spot sizes, laser powers and laser scan speeds strongly affect the reduction degree and the morphology of LSG, which largely determine the performance of fabricated LSG SCs. A variety of algorithms can be developed, combining AI with domain knowledge in laser parameters and properties of the resulting LSG. AI facilitates the production of LSG with targeted properties. Ultra-narrow LSG electrodes with accurately controlled gaps can be realized to reduce the size of devices and increase the density of electrodes. With the assistance of AI, 3D LSG SCs with complex configurations will also be precisely manufactured. AI can play a significant and decisive role to improve the fabrication capability of LSG SCs and achieve their desired performances. For the applications of LSG SCs, the implement of AI is very promising for developing intelligent energy management systems. With the properties of LSG SCs, energy production units and energy consuming components in the system, AI can forecast the power demanding, tune the output of individual device and balance the power supplies within the energy system. Especially for the highly integrated SCs systems, this overall predication and evaluation of the device performances can effectively enhance the efficiencies and reliabilities of the whole energy systems.