NSRRC Activity Report 2022

020 NSRRC ACTIVITY REPORT 2022 which generates or “opens” Se vacancies (V Se ) in the monolayer. In Phase II, chromium(III) oxide is placed near the sample and heated up to the reaction temperature and it undergoes defect-assisted interdiffusion to serve as a source Cr dopant. Chromium oxide undergoes reduction at elevated temperatures according to the chemical equation Cr 2 O 3 + 3H 2 → 2Cr + 3H 2 O. The Cr species reaching the WSe 2 surface form low-energy Cr–V Se complexes, which become embedded into the lattice and can undergo interdiffusion with W atoms. This is the “replace” phase as Cr atoms replace W atoms in the lattice. Finally, in Phase III, chalcogen vapor is introduced, which repairs the lattice and “closes” the reaction. Figure 1(a) (right panel) shows a schematic of the area-selective doping of Cr in a monolayer WSe 2 film using graphene as a mask. Figure 1(b) shows a scanning electron microscopy (SEM) image of the pristine WSe 2 (light grey) film covered by a pre-patterned graphene mask made by e-beam lithography. Figure 1(c) shows the SEM image after the ORC process is carried out. Figure 1(d) shows the atomic-resolution STEM annular dark field (ADF) image of WSe 2 exposed to H 2 flow before Cr doping, which corresponds to Phase I, in which a high number of Se vacancies are observed (highlighted by yellow dotted circles). Finally, to confirm the successful doping of Cr in the unmasked WS 2 region, authors performed STEM characterization of the ORC-reacted WS 2 /WSe 2 interface. Figure 1(e) shows the ADF image of the interface between Cr:WS 2 and Gr/WSe 2 . It can be seen that the Cr dopants are substituted at the W sites and exist only in the WS 2 domain with a darker contrast (highlighted by purple dotted circles). Fig. 1 : Schematic of the three phases (I–II–III) of the ORC reaction for the area-selective substitution of the W sublattice in WSe 2 . (a) Schematic of the area-selective doping of Cr in a monolayer WSe 2 film using graphene as the mask. (b) SEM image of the pristine WSe 2 (light grey) film covered by a pre-patterned graphene layer. (c) SEM image of the Cr-doped WSe 2 film in the same area as shown in (b). (d) An ADF image of WSe 2 after heating at 700 °C under hydrogen flow. The Se vacancies are denoted by yellow dotted circles. (e) An ADF image of the interface of Cr:WS 2 and Gr/WSe 2 . Cr dopants and Se vacancies are represented by purple dotted circles and yellow dotted circles, respectively. [Reproduced from Ref. 4] Fig. 2 : SHG mapping comparison of pristine and Cr-doped WS 2 . (a) SEM image of the three selected WS 2 single crystals for SHG spatial mapping with the top two covered by graphene and the bottom one exposed for Cr doping. (b) Corresponding SHG images recorded at three excitation wavelengths. For Gr/WS 2 , the SHG image under excitation at 1260 nm exhibits a uniform intensity with no trion contribution. For Cr:WS 2 under excitation at 1260 nm, SHG is clearly enhanced in the triangular region, which is attributed to the trion resonance. Weak exciton resonances are observed at 1220 nm, a spatial distribution contrary or opposite to the trion distribution in Cr:WS 2 , but the exciton resonance is relatively uniform in Gr/WS 2 . At 840 nm, the SHG intensity is enhanced by the band-nesting energy resonance, but exhibits a uniform intensity in both Cr:WS 2 and Gr/WS 2 . At all excitation wavelengths, Gr also produces very weak SHG. Intensity is indicated by a color scale. [Reproduced from Ref. 4] (a) (b) Next, the authors carried out a standard characterization of the WSe 2 and Cr:WS 2 films using Raman spectroscopy and photoluminescence to confirm the systematic increase of the Cr doping content. After confirming the systematic increase in the Cr content, the authors used multiphoton laser scanning microscopy to perform rapid imaging of second-harmonic-generation (SHG) resonances via the graphene-masked ORC reaction on individual triangular WS 2 single crystals. SHG excitations were measured at 1220 and 1260 nm, corresponding to half the excitation energies of 2.03 and 1.97 eV in the SHG process and similar to the energies of the excitons and trions of WS 2 , respectively. 5