Since the first discovery of graphene in 2004, 1 2D materials have intrigued the research community due to their rich physical properties. Graphene has a unique band structure, 2 exceptional mechanical strength, 3 ultrahigh electrical and thermal conductivity, 4 as well as robust chemical stability. 5 Accordingly, graphene has been applied in transparent electrodes, 6,7 flexible electronics, 8,9 and electrochemical systems. 10 Semiconducting transition metal dichalcogenides (TMDCs) with a direct bandgap at the monolayer limit 11 are particularly promising for optoelectronic applications. Insulating hexagonal boron nitride (hBN) can also provide excellent encapsulation for 2D material systems. 12 With tunable bandgaps ranging from the ultraviolet to the infrared range, 2D materials exhibit distinct light-matter interactions across a broad spectrum, 13 including enhanced photoluminescence (PL), 14,15 thickness-dependent polaritons, [16][17][18] and ultrafast carrier transport in 2D heterostructures. 19,20 Thus, 2D materials have been introduced in many applications, including high-efficiency photovoltaics, [21][22][23] broadband photodetectors, 24,25 single-photon quantum emitters, 26,27 valleytronics, [28][29][30] and fast-switching transistors. 31,32 The current methods to fabricate 2D materials-based devices generally involve complex procedures. First, pristine 2D flakes are prepared by either exfoliation or chemical synthesis 33,34 and then transferred onto a target substrate. Next, these 2D materials are processed in multiple steps that may involve patterning, thickness sculpting, carrier density control, defect engineering, phase transition, and alloying to achieve desired geometrical configurations and properties. Elaborate methods, such as electron beam lithography, are typically used for device fabrication. 35 Reactive ion etching and plasma doping are widely used to modify the physical properties of 2D materials. 36 However, these processes are prone to introducing undesired defects and structural changes to 2D materials. 37 The aforementioned 2D material device fabrication processes involve capital instrumentation, high fabrication cost, long processing time, and may incur sample damage and contamination.
Laser-aided technologies present promising alternatives to lithographical and chemical modification methods for the processing and functionalization of 2D materials. By controlling highly localized laser-matter interactions across a wide range of temporal scales and at different irradiated energy densities (fluences), 38 laser processing provides a versatile platform to selectively pattern and modify 2D materials without the need for masks or photoresists, 39 thereby effectively avoiding contamination and chemical damage. Continuous-wave Applied Physics Letters PERSPECTIVE pubs.aip.org/aip/apl (CW) lasers and/or pulsed lasers can be used, depending on the materials processing and manufacturing objectives. CW lasers can be employed to drive processes evolving over relatively long timescales and typically induce controllable, mild temperature fields. On the other hand, pulsed lasers are suitable for fast photo-physical removal or breakdown of materials or molecules and can achieve high resolution patterning with reduced residual thermal damage. Due to their structure, 2D materials are highly sensitive to external stimuli, including light, heat, and electric fields. This sensitivity allows the development of laser processing methods for controlling the properties of monolayer flakes and defining the number of layers in multilayer stacks. 40 In this Perspective, we summarize advances in laser processing and functionalization of 2D materials on site-selective laser thinning, directlaser-writing and photothermal patterning, laser-induced doping, laser-enabled 2D material device engineering, and discuss challenges and potential opportunities of laser-aided 2D materials processing. Detailed accounts on laser-aided 2D materials device integration and related applications can be found in recent comprehensive reviews. 11,[41][42][43] In this Perspective, we focus on the laser materials modification mechanisms as well as on the developments of laser processing technologies (Fig. 1).
The atomic layer thickness causes quantum confinement and interlayer coupling. Consequently, the electrical and optical properties of 2D materials strongly depend on the number of layers. 49 For example, TMDCs transition from indirect to direct bandgap semiconductors in monolayer form, leading to strong PL emission. 50 Black phosphorus' bandgap changes from 0.3 eV in the bulk form to 1.5-2.0 eV in monolayers. 51 Furthermore, many 2D materials exhibit intriguing thermal and mechanical properties as the number of layers is reduced. For instance, the elastic modulus increases from the gigapascal range in graphite to 1 TPa in monolayer graphene. 3 The inplane thermal conductivity of hBN increases from around 300 W/mK in the bulk to 751 W/mK in monolayers. 52 The reduced dimensionalities in 2D monolayers confine the electron profiles, producing nonlinear optical effects and unique exciton dynamics. 17,53,54 Therefore, controlling the thickness of 2D materials is crucial for tailoring their properties and performance in various applications. While exfoliation and plasma etching have been employed for this purpose, they often introduce defects and are prone to low yield. Consequently, there is a need for developing more precise thinning methods. 55,56 Laser thinning has emerged as a versatile technique, enabling control over the layer thickness of 2D flakes. Upon focused laser irradiation, layered 2D materials experience localized hightemperature hotspots. The unusually low (<$20 MW m À2 K À1 ) thermal boundary conductance between 2D materials and the underlying bulk substrates effectively confines heat within the ultrathin films whose thermal mass is extremely small. 57 Therefore, the localized heat confinement produces an essentially uniform temperature across the 2D flake and facilitates layer removal through oxidation or sublimation. Moreover, the interlayer bonding energy of 2D materials can be tuned by exploiting optoelectronic or photochemical interactions, allowing for precise thinning under gas and liquid ambient conditions.
The initial work was conducted on thinning multilayer graphene flakes. Han et al. developed a self-limited approach to thin few-layer graphene into monolayer on a Si/SiO 2 substrate upon CW laser irradiation. 58 As the local temperature rose to $1450 C, the top layers oxidized and burnt. The weak absorption of the laser power into the remnant monolayer induced lower temperature, hence preventing
FIG. 1. Schematic showing laser-aided processing of 2D materials. Laser thinning figure adapted with permission from Castellanos-Gomez et al., Nano Lett. 12, 3187-3192 (2012). Copyright 2012 American Chemical Society. 44 Laser patterning figure adapted with permission from Park et al., Appl. Phys. Lett. 101, 043110 (2012). Copyright 2012 AIP
oxidation. Materials, such as MoS 2 , begin to sublime at relatively low temperatures, and therefore, laser thinning can be achieved thermally. Castellanos-Gomez et al. reported a CW laser-based method for thinning multilayer MoS 2 into a single-layer 2D flake [Fig. 2(a)]. 44 Under CW laser illumination, multilayer MoS 2 absorbed incident optical power to reach sublimation temperature ($450 C). The bottom layer stayed intact [Fig. 2(b)]. Substantially higher laser fluence was required for its removal because of the weaker optical absorption. Due to the transition from indirect to direct bandgap, the laser-thinned monolayer MoS 2 exhibited a 4-5 times stronger PL emission compared to the few-layer flake [Fig. 2(c)].
Aside from self-limited thinning to monolayers, obtaining a specified number of layers is desired for applications involving bandgap engineering. Rho et al. demonstrated layer-by-layer precision thinning by initiating the film removal from edge defects via oxygen mediated anisotropic etching. 59 The low energy barrier for the reaction of oxygen molecules at edge defects (0.31 eV for the Mo zigzag edge, whereas 0.8 eV for bulk MoS 2 ) significantly reduced the required laser fluence, removing the topmost layer while keeping the underlying film intact. Time-resolved reflectance of the processed area was acquired with nanosecond resolution using a dual-color pump-probe configuration shown in Fig. 2(d) [Fig. 2(e)]. The transient signal could reveal the exact layer number during the thinning process. This in situ optical probing unraveled differences between the sublimation process that is signified by the rapid drop in the reflectance at high laser flux and the oxygen mediated etching that is represented by the gradual decrease in reflectance at a lower laser flux. Despite the precise thickness control afforded by laser thinning, the lateral resolution of this technique is typically limited by the optical diffraction limit. To overcome this constraint, Rho et al. demonstrated atomic force microscopy (AFM) tipbased thermomechanical laser thinning of MoTe 2 , achieving nanoscale lateral resolution. 60 In this work, a silicon AFM tip was heated to reach approximately 1000 K at its apex by focusing a 800 nm femtosecond laser of 80 MHz repetition rate onto the tip body. Upon contact with the MoTe 2 surface, the heated tip induced a high-temperature domain of nanometric dimensions, facilitating local oxidation of the topmost layer [Fig. 2(f)]. The oxidized layer was then continuously removed by tip scribing upon the exerted mechanical indentation force [Figs. 2(g) and 2(h)]. This scanning probe mediated thinning method has been extended to other 2D materials of higher oxidation temperatures, including black phosphorus. 61
In addition to heat mediated thinning, other laser thinning techniques leveraging chemical reactions of 2D materials in a controlled Copyright 2012 American Chemical Society. 44 (d) Schematic of the in situ reflectance probing during laser thinning of MoS 2 . (e) Reflectance of the probe beam from the onset of the thinning process with different laser powers. Adapted with permission from Rho et al., ACS Appl. Mater. Interfaces 11, 39385-39393 (2019). Copyright 2019 American Chemical Society. 59 (f) Schematic showing femtosecond laser heated tip-based oxidative thinning of MoTe 2 . (g) AFM topographies of a MoTe 2 flake after initial contact with the tip (top) and after scribing of hot tip (bottom). Both scale bars are 100 nm. (h) The height profile of dashed lines A and B in (g). Adapted with permission from Rho et al., Adv. Mater. Interfaces 9, 2200634 (2022). Copyright 2022 Wiley-VCH. 60 environment have been demonstrated. 62 For instance, Huang et al. immersed MoS 2 in electrically biased de-ionized water and applied laser beams of different wavelengths to perform thinning. 63 As electrons escaped into the liquid, excess holes at defect sites in MoS 2 could be electrochemically oxidized in the presence of hydroxide ions [Fig. 3(a)]. Self-limited laser thinning was achieved by optical excitation at different laser wavelengths based on thickness-dependent bandgaps of MoS 2 [Fig. 3(b)]. The PL mapping in Fig. 3(c) demonstrated the uniformity of the thinning process, revealing four different regions, including a completely etched area (532 nm laser), thinned monolayer (785 nm laser), thinned bilayer (850 nm laser), and intact flake (980 nm laser). The laser-directed electrochemical process could on-demand yield specific number of layers and lower the laser power threshold for thinning, hence reducing thermal damage.
Laser thinning of 2D materials has also been demonstrated in liquid water and vapor. Zhai et al. reported the light-induced exfoliation of MoS 2 in water by vaporizing water molecules under laser illumination. 64 In that experiment, the exfoliation process intermittently progressed from the center to the edge of the flake. Nagareddy et al. demonstrated humidity-controlled layer-by-layer thinning of MoTe 2 . 65 Cyclic etching of MoTe 2 flakes was achieved by laser irradiation under controlled relative humidity in a vacuum chamber [Fig. 3(d)]. This thinning process showed reduced power density threshold with increasing humidity [Fig. 3(e)] while preserving the crystallinity and uniformity of underlying layers.
In addition to thinning of 2D materials, complete film removal is needed for applications such as channel shaping in transistors, 66,67 structuring metamaterials, 68,69 as well as defining optical waveguides and resonators. [70][71][72][73] Direct laser patterning using femtosecond laser 74 and photothermal patterning with CW laser have produced fine resolution features in 2D materials.
Direct patterning of graphene is considered challenging due to its high oxidation and sublimation temperature. Various patterning approaches involving chemical oxidation were proposed, such as goldassisted anodic oxidation 75 and hydroxyl radical oxidation. 35 However, femtosecond laser irradiation can raise the temperature within tens of picoseconds, 76 Applied Physics Letters PERSPECTIVE pubs.aip.org/aip/apl
Compared to femtosecond laser, CW laser irradiation generates a much broader heat affected zone and yields slower temperature rise, which is a serious impediment to efficient and high-resolution laser processing. To overcome these limitations, Lin et al. developed an optothermoplasmonic nanolithography technique that utilized plasmon-enhanced localized heating to achieve patterning of graphene and MoS 2 with a CW laser. 78 Graphene and MoS 2 were transferred onto a thermoplasmonic substrate coated by a layer of quasicontinuous Au nanoparticles. Under low-power CW laser illumination, localized temperature higher than $500 K was obtained to remove graphene by oxidation or ablate MoS 2 through sublimation [Fig. 4(d)]. Figures 4(e) and 4(f) showed tunable feature size via this method by varying the exposure time and optical power, where a minimum linewidth down to 300 nm was demonstrated. By scanning the laser or moving the sample stage, large, high-resolution 2D patterns can be created on-demand and transferred onto receiving substrates.
In addition to the thinning and patterning of 2D materials, laser processing can offer site-specific modification of their optoelectronic properties, including bandgaps, carrier distributions, and mobilities. Doping is the backbone for creating 2D material-based p-n junction devices such as photodiodes 79 and electronic switches. 80 Various doping strategies have been attempted using CVD methods to substitutionally diffuse gas-phase dopants into 2D materials. [81][82][83] Despite its easy integration with 2D material synthesis, the CVD method cannot offer spatial selectivity, hence hindering the development of nanoscale 2D heterojunctions. For this reason, laser-induced doping presents a promising alternative to traditional methods. 84,85 Surface functionalization and substitutional doping are both encountered in the fabrication of 2D material-based devices. The former involves adsorption of dopants on the surface, while the latter refers to replacement of atoms in the lattice.
Noble metal nanoparticles attached to the surface of 2D materials can produce doping effects. Kim et al. proposed a laser-induced reduction method to deposit silver nanoparticles (AgNPs) on the MoS 2 surface to achieve a stable p-type doping. 86 The irradiation of a 532 nm CW laser on MoS 2 sample immersed in AgNO 3 solution could selectively synthesize AgNPs on the MoS 2 surface through recombination of optically induced free electrons in MoS 2 and positively charged Ag ions. Compared to the pristine sample, the Ag-decorated MoS 2 exhibited enhancement in excitonic PL emission and shifted the threshold voltage in field effect transistor (FET) devices.
The surface adsorption of dopants provides a viable way to shift the Fermi level of graphene and tune its electrical properties. While physisorbed dopants, such as ammonia, can be easily desorbed by pumping in vacuum, 87 chlorine ionically bonds to graphene carbon atoms with good chemical stability. However, plasma-doped graphene showed limited carrier mobility after chlorine coverage. 88,89 Rho et al. developed a dual laser-assisted reversible doping approach without compromising the carrier mobility of graphene. 46 A UV nanosecond laser was directed parallel to the graphene sample to dissociate flowing Cl 2 gas into Cl radicals that diffused and bonded to the underlying graphene [Fig. 5(a)]. Subsequently, the chlorine dopant could be photothermally desorbed using a focused CW laser beam, enabling rewritable doping patterns. The ratio of I 2D and I G peaks in the Raman spectra decreased when graphene was doped with chlorine and increased after the dopant removal process [Fig. 5(b)]. The adsorption of chlorine on graphene was further evidenced through X-ray photoelectron spectroscopy, where the existence of C-Cl and C-Cl 2 bonds was clearly observed [Fig. 5(c)]. Due to the mild processing condition, defects were not introduced into the underlying graphene monolayer, making this noninvasive method advantageous in retaining high carrier mobility in a repeatable manner.
Compared to surface functionalization of graphene, substitutional doping is difficult due to strong sp 2 carbon bonding and low controllability in carbon vacancy generation for dopant incorporation. 91 Early efforts on laser-assisted graphene substitutional doping exploited high defect densities at edge sites 92,93 or incorporated plasma ions to induce defects. 94 Guo et al. provided an alternative solution starting with graphene oxide and irradiating the sample with a femtosecond laser in NH 3 atmosphere. 95 Owing to the weaker bonding of carbon and oxygen, nitrogen atoms could be doped into the oxygen vacancies as a ntype dopant.
The laser doping method in gaseous atmosphere was also applied to TMDC materials. Kim et al. demonstrated site-selective substitutional p-type doping for MoS 2 and WSe 2 monolayers in a PH 3 environment. 90 A CW laser was used to heat the sample, generate sulfur/selenide vacancies, and at the same time dissociate PH 3 molecules due to the heat transfer into the ambient gas. Phosphorous then diffused to occupy these vacancies [Fig. 5(d)]. A drastic difference in the emission spectrum before and after doping could be observed comparing MoS 2 and WSe 2 . MoS 2 is intrinsically n-type with excessive electrons, giving rise to a negative trion (X À ) emission in the PL spectrum. After laser doping, a population of holes was introduced into monolayer MoS 2 , suppressing the trion emission and enhancing the neutral exciton emission at 1.88 eV [Fig. 5(e)]. 96,97 In contrast, WSe 2 , an intrinsically p-type semiconductor, became more p-type after the phosphorus doping, favoring the formation of more positive trions (X þ ) over neutral excitons [Fig. 5(f)]. This substitutional phosphorous doping remained stable after the sample was placed in ambient conditions for weeks. A similar laser-assisted doping experiment was performed by Copyright 2016 Wiley-VCH. 90 Applied Physics Letters PERSPECTIVE pubs.aip.org/aip/apl
One important aspect of 2D material-based electronic devices is to control the 2D semiconductor-to-metal interface characteristics. 99 In this regard, several strategies have been explored, such as pure edge contact between metal and 2D materials to avoid the "tunnel barrier" 100 and covalent bonding between specific metals and 2D materials to reduce the metal-semiconductor gap. 101,102 Aside from the geometric contact limitations, the Schottky barrier or nonlinear contact formed between 2D materials and metals is also a concern in many applications. 103
In an attempt to solve the nonlinear current-voltage issue rising from the Schottky contact, Cho et al. demonstrated a laser-induced 2H-to-1T 0 phase transition in MoTe 2 to create a heterophase homojunction with Ohmic behavior. 48 The MoTe 2 beneath the metal contacts was phase-patterned by a focused laser [Fig. 6 46 Applied Physics Letters PERSPECTIVE pubs.aip.org/aip/apl picosecond laser of 80 MHz repetition rate was scanned over the Au electrode to induce a transient temperature profile at the metal-semiconductor interface [Fig. 6(d)]. Due to the short time interval (12.5 ns) between two consecutive pulses, heat accumulates forming an almost uniform temperature profile that differs by $10 K across the electrode thickness [Fig. 6(e)]. Previous studies had demonstrated the tenfold reduction in contact resistance by annealing the metal-MoS 2 contact in furnace at 473 K, 104 as the thermal process is beneficial for stress relief and improving conformity on the interface. However, laser annealing offers spatial selectivity and a higher effective temperature. The improved field-effect mobilities for asymmetric contact and symmetric contact indicate that laser annealing reduces Coulomb scattering through a deduction in interfacial traps between metal and MoS 2 [Fig. 6(f)].
Doping is another alternative for achieving low contact resistance at the metal-2D material heterojunctions. Yang et al. demonstrated that heavy doping of TMDCs in contact with an electrode could enhance the source/drain current through electron tunneling. 105 Similarly, 2D devices with doped materials exhibited an increased photocurrent. Rho et al. demonstrated the tunable photoelectric effect by localized and reversible chlorine doping in graphene [Fig. 6(g)]. 46 Three photocurrent maps of the pristine, chlorinated, and chlorineremoved device showed distinct features where the channel doping concentration was modified [Fig. 6(h)]. This laser functionalization strategy presents a promising technique for incorporating graphene into future active optoelectronic devices.
Laser-based techniques offer substantial advantages for engineering 2D materials and devices. However, challenges and opportunities remain for further expanding the versatility and refining the processing resolution. For instance, 2D heterostructures have gained growing interest due to their intriguing exciton and carrier characteristics. Complete control over the geometry and composition of 2D heterostructures is desired for their implementation in device applications. While previous efforts incorporated lithographically defined regions for chemical modification and synthesis, 106 laser-based processing can be utilized to fabricate heterojunctions at specified locations in a single processing step. Current optical patterning of 2D material heterojunctions still yields relatively rough polycrystalline interfaces. 107 However, sophisticated processing protocols utilizing specific laser-material interaction mechanisms can be developed by selecting the laser wavelength and temporal profile. Other potential applications of hybrid 2D materials include their incorporation into nanocomposites to alter their mechanical and optical properties, paving their way for the development of materials with tailored properties 108 and sensing functionalities. 109 2D materials are also highly sensitive to defects, which can significantly alter their electrical, optical, and magnetic properties. 110 Mildtemperature thermal treatment by CW lasers, such as those described in Sec. II, can generate chalcogen vacancies in TMDCs and alter their PL emission. 111,112 Although thermal annealing in chalcogen gas environments could repair these vacancies, the prevention of defect introduction during laser processing remains a challenge. The defect atomistic configurations may differ depending on the lattice structures and bonding energies of specific 2D materials, where defects can serve as carrier donors, scattering sites, Coulomb traps, or recombination centers. 113 Ultrafast lasers have been explored to create spin defects in hBN 114 and anneal vacancies in TMDCs. 115 Future advances in local defect control can leverage methods such as tip-based scanning probe techniques to bypass the optical diffraction limit. In addition to materials processing, laser instrumentation is the core of powerful optical diagnostics to understand nanoscale local properties in 2D materials, including defect location, 116 carrier density, 117,118 exciton dynamics, 119,120 interfaces, 121 and strains. 122 In summary, laser-assisted processing and functionalization techniques enable precise shape patterning and compositional structuring of 2D materials by dialing various process parameters, chiefly the incident spatial optical intensity distribution, laser wavelength, and temporal irradiation profile. Such methods offer distinct advantages, in terms of the excellent versatility, high throughput, and nanoscale resolution. The progress accomplished on the development of laser processing techniques highlights the significant potential of these emerging materials to push the frontiers of both scientific discoveries and practical applications.