Direct laser writing of graphene electrodes

Lasers are important in many aspects of modern technologies, from fundamental science to industrial production.^1–3 When a laser beam is scanned using a galvo system, or the laser head is mounted on a moving platform, the controlled laser beam can artificially create patterns according to computer-aided designs.⁴ The high scalability of laser writing has significantly shortened the time scale from fundamental material research and development to industrial production.

Graphene is an emerging carbon allotrope with unique two-dimensional nanostructures and a Dirac core shape semiconducting band structure.⁵ Graphene family materials have outstanding optical, electrical, and chemical properties for wide application areas.^6–8 With the laser meeting graphene, direct laser writing of graphene electrode paved a novel research field with various emerging applications.⁹ 

Direct laser writing of graphene can be categorized into three groups, according to the physical nature of the reactions.

The photothermal effect on the direct laser writing of graphene electrodes can be categorized into two types: the photothermal reduction of graphene oxide and direct synthesis through laser induced thermal heating for the thermal reorganization of the carbon atoms into the graphene structures.¹⁰ For the first category, graphene oxide has rich chemical bonding on the graphene skeleton, which can be removed through thermal treatment. Due to the high energy density of the laser, the absorption of laser energy can be significantly converted to heat, as illustrated in Fig. 1(a). As a result, the focus area can have a significantly high temperature (typically over 2000°C), which can effectively break the weak chemical bonding, such as –COOH and –OH, as shown in the inset of Fig. 1(a).¹¹ Therefore, this photothermal effect has been widely studied for converting graphene oxide into reduced graphene oxide.¹² However, the relative low-power laser treatment on graphene might result in oxidizing graphene, especially in ambient or oxygen rich environments.¹³ For the second category, graphene can be synthesized directly from the carbon source under laser heating. With the presence of the metal catalyst materials, the solute atoms on the metallic catalyst surfaces can be directly grown to the graphene structure under a suitable thermal environment due to laser heating. ¹⁴ 

The large photon energy of the laser is also an important factor for the direct writing of graphene electrodes, in addition to the thermal effect. When the laser wavelength is small, the energy of each photon is increased. When the wavelength of external light is approaching the UV region (typically smaller than 400 nm), the photochemical effects will significantly influence the synthesis of graphene, usually by removing the oxygen residuals on the graphene skeleton.¹⁵ Differing from the photothermal effect due to local heating, the absorbed photon energy can directly break the chemical bonding, especially when the photon energy is larger than the dissociation energy, as illustrated in Fig. 1(b) and the inset.⁴ So the photochemical effect of the laser reduction of graphene can be applied for direct writing of graphene electrodes without significantly heating the substrate.¹⁶ So far, the raw materials for the photochemical synthesized graphene are limited only with graphene oxide, because significant heating is needed for converting the nonaromatic structure to graphene skeletons.

Additive manufacturing has become a trending technology in recent years, due to the rapid development of material sciences and robotic manufacturing. Within the direct laser writing process, synthesized graphene can be simultaneously deposited onto other substrates due to the kinetic momentum induced by laser heating, which is generated due to the volume expansion of the synthesized graphene from the polyimide precursor and the gaseous by-products, such as CO₂ and NO_x.¹⁷ By utilizing the layer by layer approach, graphene desired in 3D structures can be artificially fabricated as a computer-aided design, as illustrated in Fig. 1(c). Laser induced forward transfer,¹⁸ laminated object manufacturing,¹⁹ and direct laser synthesized on 3D structures have all been developed for the successful 3D printing of graphene using laser.²⁰ 

The recent development of direct laser synthesis of graphene can be categorized according to the different types of raw materials.

Graphene oxide is the earliest and most widely studied raw material for the direct laser synthesis of graphene.²¹ There are three main advantages of the reduction of graphene oxide. Firstly, the weak –OH and –COOH bonding on the edge of the graphene oxide can be easily removed due to the external heating effects (Fig. 2).^22,23 The heating during laser writing can quickly overcome the thermal barrier for the breaking of the C–O bonds. Secondly, the absorption of photons in graphene oxide is significantly large, due to the narrow band structures of graphene oxide. Lastly, the graphene oxide flakes can be easily deployed on various substrates, due to the ease of dispersion of the graphene oxide within common solvents. It can be quickly applied using spin coating, spray coating, or inkjet printing methods. Thus, graphene oxide can be easily written to reduced graphene oxide with even weak photon laser, such as CO₂ laser,²⁴ near IR laser,²⁵ or even laser diode for DVD texture marking, as shown in Fig. 3.²⁶ 

Apart from the powder form graphene oxide, commercial polymers can also work as precursors for direct laser synthesis of graphene. Polyimide is an emerging raw material for direct laser writing of graphene electrodes, as illustrated in Fig. 3(a), because of three advantages. Firstly, there is abundant hexagonal crystalline carbon within the imide structures, which can serve as a precursor for the synthesis of graphene. In the meantime, polyimide also has a large absorption at the IR region and near UV region, so many types of laser can directly carbonize the polyimide film to graphene structures.²⁷ In addition, the film form of polyimide can be easily integrated into roll-to-roll production, which can be straightforwardly scaled up for mass production.

With the extensive study of laser synthesis for graphene, it has recently been found that almost any carbon source can be transformed into graphene under optimized conditions.²⁸ The carbon atoms within the wood, toast, and sugar can all be converted into graphene during the high temperature laser process, especially with the aid of metallic catalysts and under the illumination of a high energy laser, such as the CO₂ 10.6 μm laser, as illustrated in Fig. 3(b).^20,28,29 In addition, similar to the catalyst effect of nickel during the chemical vapor deposition (CVD) synthesis of monolayer graphene within a furnace tube, the existing transition metal can prompt the production of graphene during laser synthesis.

Direct laser writing of graphene electrodes can also be categorized according to the types of lasers, from excimer laser to CO₂ laser, as shown in Fig. 4.

The excimer laser possesses great photon energy and a uniform photon spatial distribution, and it can effectively generate graphene using direct laser writing techniques. The strong cleavage capability of the high energy photon can directly break the chemical bond within the carbon-containing precursors. A nanosecond excimer laser at a wavelength from 248 nm^16,30 to 308 nm³¹ has been successfully studied for fabricating supercapacitors using graphene oxide as the raw material. Due to the short wavelength of such lasers, however, the poor penetration effects of the excimer laser can only carbonize the top surface area.³² So the graphene electrode can be directly written by a UV laser, without significantly heating the substrate through the photothermal effect. As a result, the areal capacitance of the graphene supercapacitor fabricated using excimer laser is usually lower, compared to long wavelength lasers.

The carbon dioxide laser is one of the earliest lasers ever developed. The gas discharge of CO₂ molecules can effectively produce large laser energy at IR wavelength, such as 10.6 μm and 9.4 μm.³³ Due to the low cost and easy maintenance, it is still one of the most economic lasers for industrial production. According to the literature, the CO₂ laser is also the most widely used laser for direct laser writing of graphene electrodes.³⁴ 

Common precursor materials for the synthesis of graphene, such as graphene oxide,³⁵ polyimide,³⁶ and other natural carbon sources, such as bread and wood,³⁷ all possess large absorption at the IR region. So the CO₂ laser has been widely studied for the fabrication of supercapacitors,³⁶ electrochemical sensors,³⁶ dew point sensors,³⁸ multiflavor detecting sensors,³⁹ and other important applications of various types of raw materials. However, due to the large absorption of photon energy, the carbonization process starts as soon as the photon enters the raw material.¹⁹ So only the top surface of the precursor, especially for polyimide, is carbonized to graphene.⁴⁰ For the application of such graphene on different substrates, additional steps are needed for transferring the graphene, such as by peeling off the precast polydimethylsiloxane (PDMS).⁴¹ 

Near IR lasers, such as Nd:YAG laser at 1064 nm, fiber laser at 1070 nm, or similar wavelength lasers, show different absorptions for different raw materials. These solid-state lasers can be integrated into an environmentally controlled chamber and can be equipped with a fast galvo scanning system, usually with a nanosecond pulse, for rapid prototyping and large scale production.

The dominating photothermal effects of near IR lasers demonstrated the successful synthesis of graphene electrode, which is similar to the traditional thermal reduction of graphene oxide using hydrothermal protocols.⁴² Various applications have been successfully demonstrated using near IR lasers, such as supercapacitors,⁴³ lithium batteries,⁴⁴ and conducting leads for interconnecting circuits.²⁵ 

However, for directly writing on polyimide, these near IR lasers show quite different optical absorbing phenomena, compared to other laser types. Because polyimide films show high transmission (over 80%) at the near IR region, the absorption of laser energy is quite low compared to CO₂ lasers. So no carbonization is observed when the laser energy is low or under defocusing conditions on polyimide films. However, by fine-tuning the laser writing parameters, my research team created graphene with different properties on polyimide compared to CO₂ lasers. By focusing the laser beam of a 1064 nm laser at polyimide, we carbonized the polyimide film with a superhydrophobic bottom and superhydrophilic top layers, simultaneously. So we can use these porous and through membranes for desalination application under solar illumination.⁴⁵ In addition, we can create patterns on polyimide films with different superwetting statuses using two state laser writing, which can be used for simultaneous droplet manipulation and electrochemical sensing.⁴⁶ Furthermore, by optimizing the laser power, the synthesized graphene can be simultaneously deposited onto receiving substrates, using the continuous wave laser induced forward transfer strategy. The resulting 3D printed graphene supercapacitor had almost 1 F/cm² areal specific capacitance with 100 layers of printing.⁴⁰ 

Direct laser writing of graphene, based on a laser diode, is one of the lowest cost methods. A commercial LightScribe laser with a 780 nm laser diode and open-source UV laser diode based laser engraving system is mainly used for direct laser writing of graphene research.⁴⁷ 

With an open-source microcontroller and step motors, the total cost of the laser diode-based system can be lower than 200 USD.⁴ Both graphene oxide and polyimide show large absorption at the near UV region, so these 405 nm or 450 nm laser diodes with output power ranging from 15 to 1000 mW can easily carbonize the raw materials.^48–50 By combining computer-aided designs, artificial patterns can be created for various applications, such as supercapacitors,⁵¹ strain sensors,⁵² dielectric actuators,⁵³ and electrochemical sensors.⁵⁴ 

LightScribe was initially developed for patterning text and figures on surfaces of optical DVDs.⁵⁵ A laser diode with a power of 5 mW and a wavelength around 780 nm was mounted within the DVD writing chamber.⁵⁶ Graphene oxide can be directly applied on the commercial DVD surfaces as a raw material, as shown in Fig. 5.⁴⁷ With the computer-aided design, artificial structures can be directly created on the DVD substrate. Due to the ease of processing, a wide range of applications have been developed with the DVD laser, such as supercapacitors,⁵⁷ flexible circuits,⁵⁸ electrochemical sensors,⁵⁹ and resistive random access memory units.⁶⁰ 

Furthermore, other types of lasers have also been studied for carbonizing the graphene as electrodes directly. Femtosecond lasers with wavelengths ranging from 780 to 800 nm have been used for synthesizing graphene for applications such as electrocatalysts,⁶¹ electrochemical sensing,⁶² supercapacitors,⁶³ and OLEDs.⁶⁴ Other pulse lasers, such as second harmonic generated 532 nm⁶⁵ and third harmonic generated 355 nm laser,⁶⁶ with 1064 nm pumping at nanosecond and picosecond pulse widths have also been studied for graphene-based supercapacitors⁶⁷ and electrochemical sensing applications.⁶⁸ However, the expensive cost of these lasers limits the development of such graphene applications, mostly at the in-lab level research, and still lacks the scalability for mass production.

Although differing from monolayer graphene generated from CVD, this type of graphene synthesized by direct laser writing also shows a similar structural (usually characterized by Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy) and physical properties (electrical, optical, and mechanical), which are useful for various applications.

The supercapacitor is an electrical energy storage device based on the mechanism of electrical double layer capacitance.⁶⁹ Graphene is one of the most widely used materials for supercapacitor research, due to its high conductivity, large surface area, and low cost.⁶ Direct laser writing of graphene as a supercapacitor electrode is also popular among the scientific community, because of the ease of fabrication and characterization.³⁴ They can be easily fabricated to flexible, stretchable, and transparent supercapacitors, with emerging applications in wide areas.⁷⁰ For planar supercapacitors, the most important factor of performance is the specific capacitance.

The supercapacitor electrode based on direct laser writing on precoated graphene oxide was developed with various types of lasers, including CO₂,^35,71 excimer,^30,31 femtosecond,⁷² DVD LightScribe,^26,73 and Nd:YAG 1064 nm lasers.⁴³ Additionally, the interdigital configuration is the most widely adopted structure, where the two electrodes can be directly connected to external circuits.⁷⁴ The areal specific capacitance, ranging from 2.4 μF/cm² to 32.6 mF/cm², has been achieved using graphene oxide as a precursor.⁷⁵ 

Although graphene oxide can be fabricated as a supercapacitor electrode with excellent performance, the extra steps for deploying the graphene oxide increase the fabrication duration. In contrast, polyimide can be directly carbonized to graphene under laser writing, as illustrated in Fig. 6. Furthermore, the absorption of 10 600 nm photon energy is considerably large for polyimide, so the CO₂ laser can directly convert the aromatic rings within the polyimide into defect rich graphene.³⁶ The published areal specific capacitances ranged from 4 to 995 mF/cm², which are significantly larger than the ones achieved using pure graphene oxide.⁴⁰ The introduction of 3D porous structures with enlarged surface areas would increase the specific capacitance due to the electrical double layer capacitance, thanks to the optimized laser parameters for direct writing of the graphene electrode.

Besides, other carbon precursors have also been studied for the fabrication of graphene-based supercapacitors using direct laser writing. Lignin,⁷⁶ sulfonated poly(ether ether ketone),⁷⁷ bakelite,⁷⁸ polyvinyl alcohol (PVA),⁷⁹ polyether ether ketone,⁸⁰ and some composite materials with polyimide or graphene oxide components have all be reported for direct laser writing for supercapacitor applications.⁸¹ So far, almost all types of lasers and many kinds of precursors have been studied for direct writing of the graphene electrode for supercapacitor applications, and their typical values have been shown in Table I. Since the surface area of the laser written graphene electrode will be the key characteristic for the electrical double layer capacitance, increased surface area sizes will be the main factors for achieving higher specific capacitance.

Due to the larger surface area and the ease of losing edge electrons, graphene has been widely studied for electrochemical sensing.⁸⁸ Graphene electrodes fabricated by direct laser writing of graphene oxide or polyimide raw materials, as illustrated in Fig. 7, also demonstrated an extraordinary performance toward the detection of various materials.

As an important indicator for diabetic disease, the glucose level within human fluid is the most crucial and widely studied composition. For glucose sensing applications, important factors are the detection limit and linear range. Using polyimide as a precursor, glucose levels between 0.025 and 4.5 mM can be detected using a low-power laser diode.⁵⁴ Due to the flexibility of polyimide films, the graphene electrode fabricated on a polyimide film can also be integrated as a wearable glucose sensor.⁴⁶ When copper nanoparticles are included as the detecting elements, the linear range of glucose sensing can be further enhanced from 1 μM to 4.54 mM, using a LightScribe laser diode.⁸⁹ 

Besides glucose, various types of chemicals can also be detected accurately using laser written graphene. Polylactic acid (PLA) and polyimide have been carbonized by using lasers for the detection of NaCl and HCl, as electrical tongue sensors.⁹⁰ Polyimide derived graphene sensor after CO₂ or 405 nm laser diode writing can realize 1–150 μM⁹¹ or 300 nM–5 μM linear range for dopamine sensing applications.⁴⁹ The graphene electrode after laser writing on polyimide also shows an excellent performance toward the sensing of heavy metals such as Cd (5–380 μg/l) and Pb (0.5–380 μg/l) elements.^92,93 Other chemicals, such as serum,⁹⁴ bisphenol A,⁹⁵ hydroquinone and catechol,⁹⁶ potassium ferricyanide,⁵⁹ and ascorbic acid,⁹⁷ have all been successfully detected using laser written graphene as the electrochemical electrode. However, there is still much room for further study of electrochemical sensors with laser written graphene electrodes. Active sites of sensing elements are crucial for high accuracy sensing. By fine-tuning the laser writing parameters and choosing optimized precursors, more applications will be developed for advanced sensing technologies. Higher sensitivity electrochemical sensors based on the laser written graphene electrode can be realized with enhanced surface areas, better electrical conductivity between the graphene and active metallic sites, and novel composite materials.

Humidity sensors are usually fabricated in the configuration of interdigital electrodes. Laser writing is a convenient approach to easily draw such patterns as graphene, as shown in Fig. 8. Furthermore, laser reduced graphene oxide can be used directly as a humidity sensor, with the detection limit at 5000 ppm water vapor.⁹⁸ In comparison with graphene oxide, the polyimide generated a graphene sensor performed with superior thermal stability of 3.26 fF/°C at 10 kHz after 405 nm laser treatment.⁹⁹ With the rapid development of wearable electronics and the Internet of things (IoTs), more humidity sensors fabricated by laser direct writing will emerge, in more compact sizes and higher accuracy.

Electrocatalysts are a multidiscipline research field for catalytic chemical reactions.¹⁰⁰ The porous structure of laser induced graphene enables it to be an efficient catalyst for various chemical reactions. The laser can directly write graphene and the composite electrode as catalytic working electrodes, as illustrated in Fig. 9. The graphene electrode after CO₂ treatment of polyimide,¹⁰¹ poly(acrylic acid) (PAA) precursor,¹⁰² pine, birch, and oak have been studied for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) reaction as water splitting catalysts.²⁸ When nickel and platinum were introduced, the laser induced graphene and metal composite electrode can work as electrocatalysts for methanol oxidation.^61,103,104 A better understanding of the electrical contact between synthesized graphene and active metal sites are still challenges in electrocatalyst research. The fundamental studies of the graphene structures on the chemical reaction within the electrocatalytic reactions can further advance the performance of electrocatalysts.

The battery electrode is another important application for graphene materials.¹⁰⁵ Lasers can write graphene on copper foils, which can work directly as anodes for battery applications, as shown in Fig. 10. By generating porous electrodes for storing lithium ions, direct laser writing of graphene electrodes has been intensively studied in lithium ion batteries.^106,107 Discharge capacity ranging from 156 to 830 mAh/g has been achieved with IR laser writing on graphene oxide raw materials.^11,108 Beside lithium ion batteries, the laser induced graphene electrode was also used in sodium ion batteries, with 425 mAh/g capacity at a 0.1 A/g discharging rate.¹⁰⁹ A wide range of materials will be developed in the near future, for emerging types of batteries. Higher mechanical stabilities within the battery electrode can facilitate the laser written graphene electrodes as robust electrode materials for metal ion batteries.

Because of the fast scanning speed of galvo scanning systems, graphene conducting leads for circuit patterns can be rapidly fabricated using laser writing on various substrates, as illustrated in Fig. 11. Highly conducting leads were fabricated using a 240 nm excimer laser,¹⁶ an 800 nm femtosecond laser,¹¹⁰ a 1064 nm nanosecond laser,²⁵ and a 1070 nm fiber laser at a continuous wave mode on graphene oxide,¹¹¹ with resistance down to 1.07 × 10⁻⁴ Ω m. The graphene-based flexible substrate can significantly boost the future development of wearable electronics.¹¹² In combination with other advanced manufacturing methods, such as inkjet printing, more advanced wearable sensors can be created for versatile applications.

As an important optoelectronic application, the photodetector can also be prepared by using laser written electrodes.¹¹³ The graphene electrode generated by direct laser writing can be used as the sensing element in photodetectors, as shown in Fig. 12. By carbonizing the polyimide using the CO₂ laser, a bias voltage of 10.0 V was achieved when illuminated with 380 nm light at a light power of 17.88 mW.¹¹⁴ When ZnO was introduced into graphene oxide, the power density for the photodetector could be in 0.66–20.03 mW/cm².¹¹⁵ With the emerging development of halide perovskite materials, the combination of laser synthesized graphene, especially with a cold laser (due to the poor thermal stability of halide perovskite), will find vast applications as composite electrodes for photodetecting applications.

The laser written graphene electrodes can also work as electrical actuator electrodes, as shown in Fig. 13. After the graphene oxide is treated with a 650 nm laser, a bending angle ranging from 10° to 270° can be realized, with a fast response of 8 s and a recovery time of 19 s.^116,117 When combined with an ionic polymer metal composite, the actuator is stable at a 4.5 V input voltage for over 6 h.¹¹⁸ In addition, laser induced graphene from polyimide can also be used as an artificial throat with sound sensing ability.¹¹⁹ When combined with the functional materials, more advanced actuators will be developed with laser written electrodes.

The laser synthesized graphene electrode is also a potential candidate for many different applications, such as electrically conductive water treatment, solar cells, OLEDs, hydrodynamic pumps, and triboelectric nanogenerators. By carbonizing the polyethersulfone using a 0.05 W CO₂ laser, 63% increased bovine serum albumin rejection was experimentally measured.¹²⁰ It can also be used in dye sensitized solar cells with a power conversion efficiency of 5.1%.¹²¹ Field emission transistors (FETs) were successfully fabricated with laser reduced graphene having a mobility of 0.27 cm²/V s.¹²² The heating chips can also be made with laser written graphene, and a heating rate of 6 °C/s was established.¹²³ The patterned electrodes can also work as electrodes in OLED⁶⁴ and RC circuits.¹²⁴ Active devices such as electrodes for magnetic composite hydrodynamic pumps were fabricated, with a flow velocity of 3.4 mm/s and a current density of 30 mA/cm².¹²⁵ Triboelectric nanogenerators were also fabricated using laser induced graphene with an open circuit voltage over 3500 V.¹²⁶ With the rapid development of materials science, many more applications are anticipated with electrodes fabricated by laser writing.

Although many applications have been successfully developed for direct laser writing of graphene electrodes, there is still plenty of space for further exploration because of its outstanding properties and potentials.

For supercapacitor applications, although electrical double layer capacitors have been well established, there are still only a few reports on developing supercapacitors with larger energy densities in terms of using pseudocapacitance.¹²⁷ Future work might involve one-step laser synthesis of composite electrodes containing pseudocapacitive precursors, for generating in situ redox sites, such as MnO₂ and RuO₂.¹²⁸ Advanced electrolytes are also promising for higher performance supercapacitors.^129,130

For battery applications, the current development of laser induced graphene is still at the initial stage in this emerging research field. More efforts will be made to address the poor mechanical stability of existing porous graphene structures.¹³¹ Beyond the reported lithium and sodium ion batteries with laser synthesized graphene, there are still many different types of battery electrodes to be studied, such as metal-air electrodes and metal-sulfur electrodes.^132,133 Other functional materials can also be combined as composite electrodes for achieving better performance.¹⁰⁵ 

For sensing applications, there is nevertheless plenty of room for wearable sensing development with laser induced graphene electrodes. The flexible and even stretchable electrodes from direct laser writing can be widely used in wearable sensing, not only for glucose but also other chemicals, for real-time detection.¹³⁴ With the low cost of raw materials and the superwetting surfaces, more functional sensing platforms will be developed very soon. In addition, more accurate sensors can also be developed when combining other sensing elements such as composite electrodes. It is envisioned that composite electrodes with nanoparticles, such as Pt, Cu, Ni, and Ru, will be synthesized within the graphene matrix for more accurate monitoring of physiological medical conditions, as shown in Fig. 14(a).^135,136

At the same time, there are emerging applications for laser written graphene in nonelectrode applications. Laser induced graphene after 1064 nm laser treatment can be used as a floating membrane, for the evaporation of seawater to water vapor under solar steam generation, as illustrated in Fig. 14(b).⁴⁵ Superhydrophobic graphene after laser treatment can also be used for recycling oil floating on ocean surfaces. Many other functional applications for laser synthesized graphene will be explored in the near future.¹³⁷ 

Furthermore, we can foresee that with the recent development of artificial intelligence and robotic techniques, more research outputs based on the automatic nature of the laser writing of graphene electrodes will be generated.¹³⁸ Taking advantage of the deep learning algorithms of machine learning, the development of laser induced graphene will develop at unprecedented speeds.¹³⁹ 

Although direct laser writing of graphene electrodes might not possess the same physical properties as monolayer graphene synthesized using CVD or mechanical exfoliation methods, the excellent scalability, low cost, and ease of synthesis facilitate these types of laser synthesized graphene with more advantages for real applications.¹⁴⁰ Although conductive, the mobility of the laser written graphene is still poor compared to monolayer graphene. The removal of the side residual from the GO bond not only reduces the precursor, some conductive paths among the sp² bonding are also broken, which is evinced by the strong defect peaks from the Raman characterization. It is still challenging to maintain the high conductivity of graphene when laser treating the raw precursor materials. The optical transparency of laser induced graphene is still poor compared to the monolayer or few layer graphene. The poor mechanical stability of porous graphene written by laser is also challenging for many applications. In addition, some fundamental understanding of laser synthesized graphene is still worthy of study. For example, how the individual photons interact with the oxidized residual on the precursor is still poorly understood. There are still no reports on purely UV cleavage of chemical bonding between the carbon ring and the –COOH/–OH bonds. The further developments of such fields are important for wider applications of the direct laser writing of graphene electrodes.

This perspective summarizes the cutting-edge status of the direct laser writing of graphene electrodes, with wide applications in electrochemistry, electrical sensing, and electrical transducers. The potential future applications in these field areas are also discussed, especially for the future development of battery electrodes and nonelectrode applications. We can foresee that this research field will be further developed with the emerging artificial intelligence and robotic technologies, as illustrated in Fig. 15.

G. Li acknowledges the funding support of State Key Laboratories in Hong Kong from the Innovation and Technology Commission (ITF) (No. 1-BBX9) of the Government of the Hong Kong Special administrative Region (HKSAR), China.

The author declares no conflict of interest.