Ablative patterning and pattern transfer of graphene by ultrafast lasers

In graphene device fabrication, typically a synthesized graphene layer on a metal foil is transferred to a target substrate, followed by patterning. However, these processes involve wet chemical steps and can unintentionally modify the graphene properties. ^{Reference Pirkle, Chan, Venugopal, Hinojos, Magnuson, McDonnell, Colombo, Vogel, Ruoff and Wallace14,Reference Cheng, Zhou, Wang, Li, Wang and Fang15} Furthermore, most of the transferred graphene area is etched away during the patterning step, so little area is used. Hence there is room for improvement in current graphene fabrication methods.

Femtosecond laser-ablative patterning of suspended and supported graphene under ambient conditions is summarized in Figure 1 . ^{Reference Yoo, In, Park, Jeon and Grigoropoulos16} Direct ablation was observed upon single pulse irradiation of suspended graphene (Figure 1a), wherein graphene folds were formed in the periphery of ablated supported graphene spots (Figure 1b). Multiscale graphene patterns have been also transferred to arbitrary substrates by applying femtosecond laser pulses on a graphene/metal foil prior to the transfer step (Figure 1c–d). The diminished thermal transport accompanying femtosecond laser processing enables formation of high-resolution graphene patterns, even on thermally conductive metal foil. ^{Reference Park, Yoo and Grigoropoulos17} Furthermore, a laser-induced pattern transfer (LIPT) method was developed. Femtosecond laser patterning is performed on a graphene/poly(methyl methacrylate) (PMMA) donor layer, and the patterned graphene is simultaneously transferred to a target substrate. This results in the simultaneous patterning and transfer of graphene in a single processing step. The LIPT method simplifies the complex graphene fabrication process while augmenting flexibility in graphene device design. Graphene/PMMA microscale patterns on a flexible substrate (Figure 1e) and suspended graphene cantilever structures were fabricated via the developed single-step method (Figure 1f). ^{Reference Yoo, Park, Ahn and Grigoropoulos18}

Figure 1.

Direct laser patterning of graphene. (a) Scanning electron microscope (SEM) image of a single pulse irradiated suspended single-layer graphene on a transmission electron microscope grid. Arrows indicate cracks developed by ablation. The inset shows a pristine suspended graphene. The scale bar indicates 200 nm. (b) SEM image of folded graphene structures on a SiO₂/Si substrate. ^{Reference Yoo, In, Park, Jeon and Grigoropoulos16} The scale bar indicates 2 μm. Optical micrographs of (c) various graphene patterns on a graphene/copper foil after femtosecond laser patterning and (d) transferred graphene patterns on a SiO₂/Si substrate. Scale bars are 100 μm. (e) Optical microscope image of transferred graphene/poly(methyl methacrylate) (PMMA) patterns on a flexible polyimide film over a curved surface. The entire view is shown in the upper inset. The bottom inset shows a transferred pattern with higher magnification. (f) Tilted SEM image of graphene/PMMA cantilevers over a trench in a silicon substrate. The inset shows a SEM top view image of the cantilever pattern. ^{Reference Yoo, Park, Ahn and Grigoropoulos18} Scale bars in (e–f) are 200 μm.

Direct nanoscale patterning of suspended and supported graphene has been demonstrated, where a vortex phase plate beam shaping technique was applied to achieve resolution down to 20 nm. ^{Reference Stohr, Kolesov, Xia and Wrachtrup9} When graphene is patterned into nanoribbon shapes, an energy gap is created due to quasi-one-dimensional carrier confinement. ^{Reference Han, Ozyilmaz, Zhang and Kim19} By employing high repetition rate ultrafast lasers, precise graphene patterning was achieved due to local oxidation and conversion into CO₂. Based on Raman spectroscopy, the defect-induced D-line was not detected after the patterning, indicating that high-atomic-structure quality was maintained in the atomic scale even at the cut edges. Hence direct patterning of graphene using ultrafast pulse lasers is a promising technique for nanoscale graphene fabrication, enabling defect engineering at the nanoribbon edges, which could be free from unintended contamination.

Laser thinning of 2D layered materials

TMDCs are attractive 2D layered semiconductor materials that have unique mechanical, electrical, and optical properties. ^{Reference Wang, Kalantar-Zadeh, Kis, Coleman and Strano20} For example, monolayer MoS₂ has a direct bandgap of ∼1.85 eV instead of the indirect bandgap of 1.2 eV for the bulk counterpart. A tightly focused continuous-wave laser beam was utilized to thin MoS₂ down to a single layer. ^{Reference Castellanos-Gomez, Barkelid, Goossens, Calado, van der Zant and Steele4} A similar approach was also applied to process multilayer graphene for obtaining single-layer graphene. ^{Reference Han, Chae, Kim, Gunes, Lee, Lee, Lee, Lim, Jeong, Jeong and Lee21} Although laser light may be absorbed in both mono- and multilayered materials, the thinning process is self-regulated due to the variation of the thermal conductivity with the number of layers in conjunction with the heatsink effect of the substrate. The MoS₂ thinning process can be further controlled by choosing the appropriate laser wavelength and by using ultrafast pulses, shorter than ∼1 ps. Multilayer MoS₂ is opaque to near-infrared Ti:Sapphire femtosecond laser radiation. In contrast, single-layer MoS₂ is transparent, implying potential for selective-layer processing. A two-photon absorption process was experimentally confirmed in the course of the ultrafast laser-based MoS₂ thinning process. ^{Reference Paradisanos, Kymakis, Fotakis, Kioseoglou and Stratakis10}

Ultrafast laser direct writing of graphene patterns

Although pattern-transfer approaches have demonstrated high-quality graphene patterning capabilities, these methods involve multiple processing steps and may cause structural damage during graphene transfer. To resolve these issues, researchers must address these key challenges with the development of direct and transfer-free writing of graphene on desired substrates. To this end, laser direct-writing methods such as laser-induced thermal reduction of graphene oxide (GO) films, ^{Reference Zhou, Bao, Varghese, Tang, Tan, Sow and Loh22,Reference Zhang, Guo, Wei, He, Xia, Chen, Sun and Xiao23} or laser-assisted chemical vapor deposition (CVD), ^{Reference Park, Xiong, Gao, Qian, Xie, Mitchell, Zhou, Han, Jiang and Lu24,Reference Park, Xiong, Xie, Gao, Qian, Mitchell, Mahjouri-Samani, Zhou, Jiang and Lu25} have been examined. However, the laser-induced thermal reduction of GO films yields a substantial number of defects, significantly deteriorating electrical conductivity. The quality of the graphene pattern by laser-assisted CVD on Ni foils was significantly improved over that of thermally reduced GO, ^{Reference Park, Xiong, Xie, Gao, Qian, Mitchell, Mahjouri-Samani, Zhou, Jiang and Lu25} yet a postgrowth graphene-transfer process (i.e., etched removal from Ni foil, then transferred to the target substrate) was necessary.

Ultrafast laser direct writing has been employed for directly growing graphene patterns on various insulating substrates under ambient conditions through rapid heating of cosputtered Ni/C thin films (the detailed process sequence is depicted in Figure 2a). ^{Reference Xiong, Zhou, Hou, Jiang, Gao, Fan, Jiang, Silvain and Lu26} The sheet resistance of the resulting graphene patterns is approximately two times lower than the bilayer graphene fabricated by the laser-assisted CVD and one order of magnitude lower than that for the reduced GOs, possibly enabled by the preservation of a high-quality graphene lattice by the transfer-free growth process, and a Ni doping effect. ^{Reference Giovannetti, Khomyakov, Brocks, Karpan, van den Brink and Kelly27,Reference Rigo, Martins, da Silva, Fazzio and Miwa28} The as-measured optical transmittance was 94.3% at 550 nm, supporting potential use as a transparent electrode.

Figure 2.

Direct writing of graphene patterns by a femtosecond laser. (a) Process sequence of femtosecond laser direct-writing process to fabricate graphene patterns under ambient conditions. (b) Dependence of the graphene quality (Raman D/G and 2D/G ratios; G band induced by in-plane vibrational mode, D band by defect or disorder, and 2D band by the second-order defect) on laser average powers. At an optimal laser power of 24 mW, a minimal D/G ratio indicates high-quality crystallinity, and a 2D/G ratio close to 1 confirms formation of bilayer graphene. (c) Optical micrograph of large-scale graphene patterns on a glass substrate fabricated by femtosecond laser direct writing, for integrated-circuit applications. (d) Electrical characterization of graphene devices fabricated by femtosecond laser direct writing, current–voltage curve of the four-terminal electrical device with eight graphene straight line channels. The linear trend and high electrical conductance of the current–voltage curve show metallic characteristics of the fabricated graphene patterns, indicating its potential as an electrode. Reproduced with permission from Reference Reference Xiong, Zhou, Hou, Jiang, Gao, Fan, Jiang, Silvain and Lu26. © 2014 Nature Publishing Group.

The linewidth of the as-fabricated graphene patterns was close to the focal spot size of the femtosecond laser direct-writing system (submicrometer for the tested setup). By heating the cosputtered Ni/C thin film using the femtosecond laser, graphene growth was completed within an order of ms, enabled by the highly efficient energy delivery to the substrate maintained at room temperature. In the controlled comparison experiments, the conventional rapid thermal annealing process ^{Reference Xiong, Zhou, Jiang, Sarkar, Mahjouri-Samani, Xie, Gao, Ianno, Jiang and Lu29} required exposure for a few minutes at 1100°C under vacuum. As shown in Figure 2b, a distinct femtosecond laser processing window exists to achieve optimum bilayer graphene formation at ∼24 mW laser average power (confirmed by the 2D/G band ratio in the Raman spectrum being close to 1; the G band indicates in-plane vibrational mode induced peak intensity, and the 2D band is induced by the second-order defect or disorder band). Lower laser power yields multiple graphene layers with defects, while higher laser power forms additional defects through lattice damage.

Under the optimized condition for high-quality graphene growth, large-scale graphene-based integrated-circuit patterns have been fabricated in a programmable manner (Figure 2c) with high-quality electrical performance (measured I–V curve shown in Figure 2d). A similar approach was recently applied to fabricate a flexible solid-state microsupercapacitor based on porous carbon formation in polyimide. ^{Reference Bin In, Hsia, Yoo, Hyun, Carraro, Maboudian and Grigoropoulos30} Considering the high degree of simplicity, flexibility, and reliability in producing graphene patterns, the ultrafast laser direct-writing method could open the door for fabricating a wide range of advanced graphene-based devices.

Laser doping of 2D layered materials

Laser-assisted doping has reportedly modified the semiconductor device characteristics in many 2D materials. ^{Reference Choi, Jeong, Jung, Byun, Choi, Hong, Choi and Lee31–Reference Kim, Ko, Kim, Chen, Suh, Ryu, Wu, Meng, Suslu, Tongay, Wu and Grigoropoulos34} Since doping is facilitated by defect sites, GO was typically subjected to laser illumination under a controlled dopant precursor gas environment for the simultaneous reduction of GO into doped graphene. When the doping process is carried out using nanosecond lasers, ultraviolet wavelengths are used since they are absorbed by the GO material. ^{Reference Savva, Lin, Petridis, Kymakis, Anthopoulos and Stratakis32} A similar process using a femtosecond laser (800 nm wavelength) in a controlled ammonia environment showed efficient reduction into n-doped graphene. ^{Reference Guo, Zhang, Han, Jiang, Wang, Li, Xia, Feng, Chen and Sun33}

Two-dimensional TMDC layered materials with the formula MX₂ (M = Mo, W; X = S, Se, Te) have direct bandgaps ranging from 1.5 to 2.0 eV, therefore, a visible range laser is an effective choice for processing. Laser-assisted phosphorus doping in ultrathin (monolayer and bilayer) TMDCs, including n-type MoS₂ and p-type WSe₂, was recently demonstrated using a 532-nm-wavelength laser, which is a versatile, air-stable, and tunable method, carried out by varying the duration and intensity of a focused laser irradiating in a phosphine environment to accomplish site-specific doping ( Figure 3 ). ^{Reference Kim, Ko, Kim, Chen, Suh, Ryu, Wu, Meng, Suslu, Tongay, Wu and Grigoropoulos34} Extending the laser-doping method to emerging 2D layered materials will not only allow site-specific selectivity, but also prevent the redistribution of the impurity profile via a pulsed laser with short irradiation duration. As mentioned earlier, ultrafast lasers may provide a new pathway to TMDC layer doping through atomic-scale control of vacancy generation and the subsequent refilling with impurity atoms via non-thermal lattice disturbance processes. ^{Reference Komsa, Kotakoski, Kurasch, Lehtinen, Kaiser and Krasheninnikov11}

Figure 3.

Laser-assisted doping of MoS₂. (a) Schematic diagram of laser-assisted doping. Focused laser locally illuminated onto the MoS₂ flakes (blue atoms, Mo; yellow atoms, S) located on a SiO₂/Si substrate under diluted phosphine (PH₃) dopant gas environment. The function of laser illumination is to create sulfur vacancies and simultaneously dissociate the dopant molecules in preparation for the subsequent doping process. (b) Optical image of as-prepared monolayer (1L) and five-layer (5L) MoS₂ on a SiO₂/Si substrate (blue color represents MoS₂). (c) Atomic force microscope image of the (b) zoomed area. Its thickness is approximately 0.7 nm, in good agreement with the thickness of monolayer MoS₂. The circle in (c) is the laser spot area in the laser doping. (d) Photoluminescence (PL) mapping of the zoomed area in (c) that clearly shows the PL intensity enhancement of the laser-assisted doped area. ^{Reference Kim, Ko, Kim, Chen, Suh, Ryu, Wu, Meng, Suslu, Tongay, Wu and Grigoropoulos34}

Further ultrafast laser processing of 2D layered materials and in situ diagnostics

Considering that engineering of imperfections or defects in 2D layered materials (e.g., vacancies, edges, dopants, adsorbates, or substrates) can play a critical role on the properties (e.g., electrical, optical, thermal, or magnetic) of 2D layered materials owing to their low dimensionality, atomic-scale subtractive and additive control over the 2D layered materials can be a powerful processing tool. To this end, optical near-field technology can be used to improve lateral processing resolution, as demonstrated at the nanoscale by various processing examples, ^{Reference Ryu, Kim, Hwang and Grigoropoulos35–Reference Hwang, Ryu, Misra, Jeon and Grigoropoulos37} or laser-assisted atomic layer deposition or etching to achieve atomic thickness level control toward 2D layered heterostructure fabrication. ^{Reference Grigoropoulos, Hwang and Chimmalgi36,Reference Ohtsu, Kobayashi, Ito and Lee38}

For achieving high throughput and precision, various schemes have been proposed for scalable and programmable processing on the basis of multiple laser light confining elements in conjunction with in situ diagnostics and metrology. ^{Reference Pan, Hwang, Ko, Clem, Frechet, Bauerle and Grigoropoulos39} Integrating in situ process monitoring tools, such as both optical far- and near-field laser illumination arrangements, into scanning and transmission electron microscopes will enable investigation of the fundamental interaction mechanisms of ultrafast lasers with 2D layered materials. ^{Reference In, Ryu, Lee, Ahn, Zheng, Hwang and Grigoropoulos40–Reference Hwang, Xiang, Ryu, Dubon, Minor and Grigoropoulos42} Further, the structural modification signature measured by the imaging and analytical diagnostic functions of electron microscopes can be correlated with in situ measured optical spectroscopic signals (e.g., in situ Raman spectroscopy ^{Reference Allen, Kim, Ryu, Ozdol, Grigoropoulos and Minor43,Reference Allen, Kim, Andresen, Grigoropoulos and Minor44} ).

Conclusion and outlook

Ultrafast lasers can drive a wide range of subtractive and additive processes for the patterning and functionalization of atomic layered materials. Mechanisms such as multiphoton absorption, minimized thermal effects, non-thermal interactions, as well as compatibility and integration with advanced diagnostics can be beneficial in this effort. Research at the fundamental level with the aid of in situ diagnostics tools is necessary to achieve atomic-scale manipulation. Furthermore, ultrafast lasers can be configured to scalable processing tools and therefore realize the next-generation high-performance portable, flexible, and wearable devices based on 2D materials.