Patterned arrays of lateral heterojunctions within monolayer two-dimensional semiconductors

The formation of semiconductor heterojunctions and their high-density integration are foundations of modern electronics and optoelectronics. To enable two-dimensional crystalline semiconductors as building blocks in next-generation electronics, developing methods to deterministically form lateral heterojunctions is crucial. Here we demonstrate an approach for the formation of lithographically patterned arrays of lateral semiconducting heterojunctions within a single two-dimensional crystal. Electron beam lithography is used to pattern MoSe2 monolayer crystals with SiO2, and the exposed locations are selectively and totally converted to MoS2 using pulsed laser vaporization of sulfur to form MoSe2/MoS2 heterojunctions in predefined patterns. The junctions and conversion process are studied by Raman and photoluminescence spectroscopy, atomically resolved scanning transmission electron microscopy and device characterization. This demonstration of lateral heterojunction arrays within a monolayer crystal is an essential step for the integration of two-dimensional semiconductor building blocks with different electronic and optoelectronic properties for high-density, ultrathin devices.

pulses, target to substrate distance of 11 cm, and laser fluence of 0.5 J/cm 2 ). At 400-500°C, the original Raman peak of the MoSe 2 nanosheet at 238 cm -1 disappears and MoS 2 peaks at 387 and 403 cm -1 appear. Also, the corresponding PL peak at 805 nm shifts to lower wavelengths. Disorder peaks at ~220 and 260 cm -1 indicate incomplete conversion and formation of intermediate alloys. Complete conversion is observed for temperatures above 600°C. c, d, Raman and PL spectra showing conversion evolution as a function of sulfur pulse number at a constant temperature (700°C) and the same distance (11 cm) and laser fluence (0.5 J/cm 2 ). The Raman and PL peaks of MoSe 2 slowly shift toward those associated with MoS 2 as the number of sulfur pulses increase, thereby resulting in a tunable bandgap. showing symmetrical curves with n-type transport characteristics.

Supplementary Note 1. Synthesis of MoSe 2 monolayer crystals
The MoSe 2 nanosheets were synthesized by PLD-assisted and conventional vapor phase transport (VTG) techniques. Conventional VTG involved vaporization of MoO 3 (99%, Sigma Aldrich) powder and selenium shots (99.99999%, Sigma Aldrich) in a tube furnace under argon (100 sccm) and hydrogen (10 sccm) flow and background pressure of 30 Torr at 800 o C for 30 min. PLD-assisted VTG, on the other hand, employed PLD to first deposit a uniform and precise amount of stoichiometric precursor nanoparticles onto a source substrate at room temperature that was then covered by a receiver substrate which was placed in contact and on top of the source substrate to form a confined VTG system. Both methods provided isolated and continuous monolayer MoSe 2 nanosheets. The assynthesized MoSe 2 nanosheets were first identified by optical and atomic force microscopy (AFM) imaging and further studied under PL and Raman spectroscopy before patterning and conversion processes. As shown by optical and AFM images in Supplementary Figure 1a

Supplementary Note 2. Conversion Process
Prior to the patterning process and formation of heterojunction arrays, the conversion of 2D monolayers was studied at various temperatures and laser pulse numbers to understand and optimize the process. PL and Raman spectroscopies were utilized to investigate the extent of conversion and to reveal their optical properties.
To investigate the effect of substrate temperature on the conversion process, a large number of laser pulses (400) were delivered to provide sufficient sulfur for reaction at different substrate temperatures (400-800°C). As shown by the Raman and PL spectra in Supplementary Figure 2a, b, MoSe 2 nanosheets were totally converted to MoS 2 for temperatures above 600°C. Below 600°C, however, intermediate alloys where formed. At 400°C, for example, the E 1 2g mode of MoSe 2 at 238 cm -1 disappeared while the A 1 g and E 1 2g modes of MoS 2 at 383 and 403 cm -1 were observed. However, as can be seen from the spectra, the Raman peak at 250 cm -1 changes significantly, possibly due to the crystal lattice distortion by partial alloying. This peak slowly weakened as the temperature increased and completely disappeared at 700 and 800°C. The corresponding PL spectra also show the optical bandgap evolution at these temperatures. The effect of various laser pulse numbers was also investigated at a fixed substrate temperature. Supplementary Figure 2c, d shows the Raman and PL evolution as a function of laser pulse number at a constant substrate temperature of 700°C. Here, different laser pulse numbers ranging from 5 to 400 provided precise amounts of sulfur that resulted in a tunable bandgap that ranged from MoSe 2 at 800 nm to MoS 2 at 660 nm.

Supplementary Note 3. Conversion and formation of WSe 2 /WS 2 heterojunctions
To show the broad and compelling technological advantages of our approach, we have also demonstrated the conversion of WSe 2 to WS 2 similar to MoSe 2 / MoS 2 process. We found that WSe 2 nanosheets are totally converted to WS 2 for substrate temperatures of 700°C and 300 laser-vaporized Raman peaks similar to those reported in the literatures.

Supplementary Note 4. Sample transfer to TEM grids
The transfer of nanosheets onto TEM grids was performed by mechanical transfer using a PMMAassisted method (PMMA=poly (methyl methacrylate) provided by Micro Chem, product number 950 PMMA A4). After spin-coating the sample with PMMA, the SiO 2 layer between the monolayer flakes 8 and the Si substrate was etched by a 2M KOH solution, thereby promoting lifted-off of the PMMA/flakes ensemble, which was then transferred onto TEM grids and allowed to air dry. The PMMA was then removed by soaking in acetone followed by a final rinse in isopropanol.
Supplementary Figure 4a, b shows the optical and SEM images of a monolayer crystal transferred to a TEM grid.

Supplementary Note 5. First-principles calculations
First-principles calculations, based on density functional theory, were performed using the Vienna Ab initio Simulation Package. The projector augmented wave method was used to mimic the ionic cores, while the LDA considering the Ceperly-Alder-Perdew and Zunger (CA-PZ) functional was employed for the XC functional. Atomic positions, as well as lattice parameters, were optimized using a conjugate gradient algorithm. The ionic and electronic relaxations were performed by applying a convergence criterion of 1×10 -2 eV/Å per ion and 10 -5 eV per electronic step, respectively. The rectangular MoS 2 -MoSe 2 hybrid structure, 44.1×3.2 Å, was used for the LDOS calculations, and a vacuum of 20 Å between the hybrids was considered. Also, 1×10×1 Monkhorst-Pack meshes were used to perform the integration over the Brillouin zone.