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Laser-Triggered Bottom-Up Transcription of Chemical Information: Toward Patterned Graphene/MoS2 Heterostructures

Efficiently assembling heterostructures with desired interface properties, stability, and facile patternability is challenging yet crucial to modern device fabrication. Here, we demonstrate an interface coupling concept to bottom-up construct covalently linked graphene/MoS2 heterostructures in a spatially defined manner. The covalent heterostructure domains are selectively created in analogy to the traditional printmaking technique, enabling graphic patterns at the bottom MoS2 layer to be precisely transferred to the top graphene layer. This bottom-up connection and transcription of chemical information is achieved simply via laser beam irradiation. Our approach opens up a new paradigm for heterostructure construction and integration. It enables the efficient generation and real-time visualization of spatially well-resolved covalent graphene/MoS2 heterostructures, facilitating further design and integration of patterned heterostructures into new generations of high-performance devices.


Preparation of patterned MoS2 surface
The patterned MoS2 was prepared according to the protocol we have developed previously. 1 Briefly, CVD-MoS2 was firstly covered by a single PMMA layer. Then it was accelerated to 3500 rpm for 45 s by a Convac ST 145 spin-coater, and heated at 180 °C for 90 s. The hardened resist (thickness 200 nm) was patterned in a subsequent e-beam lithography (EBL) step, using a SUPRA Zeiss SEM (10 keV). Irradiated areas of the resist were removed by immersion in isopropanol/ isobutyl methyl ketone solution. As a result, the open-lying areas of CVD-MoS2 can be locally functionalized. In an argon filled glovebox (˂0.1 ppm O2 and H2O), the CVD-MoS2 with above prepared guiding pattern was firstly treated with Na/K (molar ratio 1:3) alloy for 2 h. After the alloy layer was removed, the film was immediately soaked in 4-bromobenzenediazonium tetrafluoroborate/ anhydrous methanol (0.1 mmol/mL, 3 mL) solution in darkness for 0.5 h. The resulting film was thoroughly washed with methanol and de-ionized water to remove all the physisorbed residues. Finally, the PMMA mask was removed by soaking the sample in an acetone bath for three times (30 min for each). The sample was then dried under ambient conditions.

Assembly of patterned G/MoS2 heterostructures
The patterned G/MoS2 heterostructures were fabricated by placing the single layer graphene directly on the top of the patterned MoS2 surface followed by the laser irradiation. Specifically, the single layer graphene with PMMA mask was placed on the top of the patterned MoS2 surface followed by drying in the fumehood at room temperature overnight. Subsequently the sample was heated at 130 °C for 0.5 h. The vertical stack was then subjected to the laser irradiation, resulting in the formation of patterned G/MoS2 heterostructures. In the end, the PMMA mask was removed by soaking the sample in an acetone bath for three times (30 min for each). The sample was then dried under ambient conditions.

Raman spectroscopy
Raman spectroscopic measurements were performed on a confocal Raman microscope (WITec, alpha 300RA) with a 532 nm excitation laser, which was focused with a 100× objective (NA = 0.90, 0.26 mm working distance). The Raman emission was dispersed by 600 grooves/mm. The spectrometer was calibrated in frequency using a standard silicon wafer. For the single irradiation process, the laser energy of 7 mW with the integration time 10 s was applied. The Raman maps with the better contrast were obtained by employing a writing-reading cycle. For the laser writing process, the covalent domains were irradiated with a green laser at 17 mW for 1s. For the reading process, the laser energy of 5 mW with the exposure time of 1 s was applied to the entire patterned surface.

Photoluminescence (PL)
PL measurements were carried out on a confocal Raman microscope (WITec, alpha 300RA) with a 532 nm excitation laser, which was focused with a 100× objective (NA = 0.90, 0.26 mm working distance). The PL spectra were recorded with a lens-based spectrometer with a CCD camera (1024×128 pixels, cooled to -65 °C) using a grating of 600 lines/mm. The diffraction-limited focus resulted in a lateral resolution of about 200 nm, and a focal depth of about 1 μm. For the PL spatial mapping, the laser energy of 3 mW with the integration time 0.1-0.5 s was applied.

Kelvin probe force microscopy (KPFM)
The KPFM measurement was performed under ambient conditions using a Bruker Dimension Icon microscope with non-contact mode (AFM tip: SCM-PIT-v2, platinum-iridium coated). Amplitudemodulated KPFM in a dual pass configuration was used. A modulation bias of 1.5 V was applied to the cantilever. In the first scan the topography is received and in the second pass the potential profile is measured while keeping the tip at a constant distance (50 nm) from the sample surface. Table S1. Summary of Raman characteristics of graphene related peaks. The band position (Pos), full width at half maximum (FWHM), and peak intensity ratios are extracted from the Lorentzian function fitted Raman spectra.

A. Attempts to generate covalent G/MoS2 heterostructures using MoS2 modified by other light inert groups as the bottom layer.
In an argon filled glovebox (˂0.1 ppm O2 and H2O), the CVD-MoS2 was firstly treated with Na/K (molar ratio 1:3) alloy for 2 h. After the alloy layer was removed, the film was immediately soaked in diazonium salts (4-nitrobenzenediazonium tetrafluoroborate or 4-methoxybenzenediazonium tetrafluoroborate)/ anhydrous methanol (0.1 mmol/mL, 3 mL) solution in darkness for 0.5 h. The resulting film was thoroughly washed with methanol and de-ionized water to remove all the physisorbed residues, yielding the 4-nitrophenyl group functionalized MoS2 (denoted as NO2-MoS2) or 4-methoxylphenyl group functionalized MoS2 (denoted as OMe-MoS2). The monolayer graphene with PMMA mask was placed on the top of the functionalized MoS2 surface followed by drying in the fumehood at room temperature overnight. Then the sample was heated at 130 °C for 0.5 h. The vertical stack of graphene and functionalized MoS2 was subjected to the Raman laser irradiation. None of these stacked structures show an increased intensity of D band in Raman spectra (Figure S9), suggesting that graphene was not functionalized in these cases and the covalent heterostructures cannot form by using these light inert linkers nor without linkers.

B1. Functionalization of graphene with bromobenzene.
One drop of bromobenzene (BB)/ ethanol (80 mM) solution was drop-cast on a clean surface of a silicon wafer followed by evaporating off the solvent in the fumehood at room temperature overnight, yielding the BB@Si/SiO2. Then the monolayer graphene with PMMA mask was placed on the top of the BB@Si/SiO2. After drying in the fumehood at room temperature overnight, the sample was heated at 130 °C for 0.5 h. The vertical stack of G/BB was then subjected to the laser irradiation ( Figure   S10A). With increasing laser intensity, the Raman spectra ( Figure S10B) of G/BB display significantly increased D band and reduced 2D band, suggesting the graphene layer in G/BB was successfully functionalized by laser irradiation.

B2. Fabrication of covalently linked graphene and SiO2 heterostructures.
The generation of a self-assembled (SAM) layer on a silicon substrate was firstly performed according to the literature reported procedure. 2 Specifically, the freshly cleaned silicon wafer was immersed into a freshly prepared 4-bromobenzylphosphonic acid/ THF solution (80 mM) for 24 h followed by drying with N2 flow. The sample was then annealed at 100 º C under Ar for 1 h. After cooling to room temperature, the sample was washed with THF and ethanol, giving the SAM modified silicon surface (SAM@Si/SiO2). Subsequently, the single layer graphene with PMMA mask was placed on the top of SAM@Si/SiO2. After drying out in the fumehood at room temperature overnight, the sample was heated at 130 °C for 0.5 h. The vertical stack was then subjected to the laser irradiation ( Figure S11A).
The covalently linked graphene and SiO2 heterostructures (G/SAM@Si/SiO2) formed along the laserscanned path, as evidenced by the increased D band and decreased 2D band in the Raman spectra of irradiated regions (Figure S11B).