Large area Janus monolayer transition metal dichalcogenides with high optical quality

July 30, 2022 - Due to the different electronegativity of the upper and lower chalcogen layers, Janus TMDs have a built-in electric dipole that breaks the symmetry of the out-of-plane mirror plane and leads to a variety of new physical phenomena. Scientists from the Universities of Jena and Ulm (Germany), the University of Toulouse (France), the Helmholtz-Zentrum Dresden-Rossendorf, the National Institute for Materials Science in Tsukuba (Japan), and the Aalto University in Finland have demonstrated the growth process of 2D SMoSe with high optical quality, characterized them by complementary experimental techniques, and rationalized the growth mechanism by first-principle calculations.

The physical properties of single layer (ML) transition metal dichalcogenides (TMDs) can be tailored for specific applications by stacking them in heterostructures,[1-3] alloying them,[4] or more recently by fabricating ML Janus TMDs.[5] Due to the different electronegativity of the upper and lower chalcogen layers, Janus TMDs have a built-in electrical dipole that breaks the out-of-plane symmetry of the mirror plane, leading to a variety of novel physical phenomena (see, e.g., [6]). However, the existing synthetic routes for ML-Janus TMDs based on plasma-induced exchange of the top chalcogen layer of a parent TMD are quite complex, and obtaining large-scale samples with high quality remains a challenge [5, 7]. Therefore, to realize the potential of Janus TMDs for science and applications, technologically relevant methods for their fabrication still needed to be established.

The team now reports the one-pot growth of large-area Janus-SeMoS monolayers from chemical vapor deposition (CVD) with asymmetric upper (Se) and lower (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. These 2D semiconductor monolayers are formed by the thermodynamic equilibrium exchange of the lower Se atoms of the MoSe₂ single crystals originally grown on gold foils with S atoms. The team now characterized the growth process by complementary experimental techniques including Raman spectroscopy and transmission electron microscopy (TEM), and rationalized the growth mechanisms by first-principles DFT calculations. They could demonstrate a remarkably high optical quality of the synthesized Janus monolayers.

The synthesis method of ML Janus SeMoS is schematically shown in Figure 1. The scientists one-pot synthesis method consists of two steps. i) CVD growth of ML-MoSe₂ single crystals.[8, 9] and ii) The conversion of MoSe₂ to Janus-SeMoS occurs when the temperature is raised and annealed at 700 °C in the presence of S-vapor. The gold substrate played an important role in the conversion, leading to dissociation of the S molecules forming the vapor phase,[10] (Sn with n = 2, 3, 4, 6, 7 and 8) and chemisorption of the S atoms on the gold substrate.[11] After their diffusion under the MoSe₂ crystals, they replace the lower Se layer, forming a ML Janus-SeMoS. A detailed description of the experimental procedures and a schematic representation of the CVD process can be found in Figure 2.

To better understand and optimize the transformation process, the scientists performed a series of DFT calculations [12] , see for example Figure 3,. They then performed their experiments at different sulfurization temperatures ranging from 650 °C to 800 °C, with a sulfurization time of 10 min. Subsequently, the samples were analyzed by Raman spectroscopy directly on Au foils (Figure 4E). The black spectrum shows the Raman signal of grown ML-MoSe₂ with the characteristic A'₁ peak at 240 cm^-1.[13] When the temperature is further increased (green and purple spectra), the Raman spectra changed significantly and the peak intensity decreased, the peak broadened, and the appearance of additional peaks characteristic of MoS₂ was obersevd. This behavior is consistent with the formation of a random MoS_2(1-x)Se_2x alloy,[13, 14] as expected for the replacement of the lower and upper Se layers by S. The chemical composition and asymmetric structure of synthesized ML Janus SeMoS were determined by angle-resolved X-ray photoelectron spectroscopy (ARXPS).

Figure 4 shows a light microscopy image of the grown ML Janus SeMoS crystals. A high density and homogeneous distribution of the triangular crystals can be clearly seen. Figure 4C shows the Janus MLs after their transfer to a SiO₂/Si wafer by electrochemical delamination transfer technique[11] The atomic force microscopy (AFM) topography image of Janus SeMoS on SiO₂/Si is shown in Figure 4D. The AFM height profile (inset in Figure 4D) shows an average thickness of 0.8 ± 0.2 nm, as expected for ML Janus SeMoS.

The scientists then used aberration-corrected transmission electron microscopy (TEM) along with spatially resolved energy dispersive X-ray spectroscopy (EDX) to further verify the Janus-SeMoS structure. To do so, they transferred the sample grown on the Au substrate to a Si wafer with a native SiO₂ layer that served as an atomically flat substrate for the cross-sectional study. Figure 5A shows a TEM image of the SeMoS/SiO₂/Si interfaces in high angle annular dark field (HAADF). The image is dominated by Z contrast, with the intensity scaling approximately by a power of two, with heavier elements exhibiting much brighter contrast than lighter elements. Two brighter layers are visible along with the lower intensity layer (similar contrast to the atomic cleavages of the Si substrate), indicating the presence of two layers of heavier elements (Se and Mo) and a lighter element (S) with a similar Z number to Si. The SeMoS structure is visible as three contiguous layers rather than atomic columns corresponding to the individual atomic layers because there is no epitaxial relationship with the SiO₂/Si substrate. The three individual atomic layers of the Janus-SeMoS structure are detected; however, due to the different contrast mechanism of HRTEM (interference contrast), the S, Mo, and Se layers appear with very little contrast difference. Figure 6 shows an atomically resolved, aberration-corrected HRTEM image showing the typical honeycomb lattice of a TMD monolayer, demonstrating the high crystallinity of the Janus SMoSe. EDX elemental maps of the interface regions are shown in Figure 5B. Together with the corresponding HAADF reference map (top), three individual layers of S, Mo, and Se can be clearly identified. The RGB color map and the corresponding line profiles of the individual EDX signals show that the three layers are slightly separated from each other (elemental arrangement from substrate to surface as S/Mo/Se).

Janus TMD MLs are expected to exhibit pulse-dependent spin splitting and obey chiral optical selection rules. The optical transition widths of CVD-grown TMD monolayers (typically 50-100 meV) can be significantly reduced by removing the samples from the growth substrate and encapsulating them in high-quality hBN.[3, 15, 16] The authors have then applied this approach to their Janus ML SeMoS as outlined in Figure 7A, where the bottom hBN layer is 50 nm thick and the top layer is 10 nm thick. In Figure 7B, we see the PL emission is both equipolarized and counterpolarized with respect to the polarization of the excitation laser. For an excitation photon energy ≈180 meV above the X emission energy, the measured polarization was P_c = 25% for temperatures up to 100 K (Figure 7D), since chiral selection rules apply to the direct transition at the nonequivalent K points at the edge of the Brillouin zone.[5, 19] For comparison, the PL emission from localized states L is unpolarized over the temperature range studied in Figure 7B. By fitting the characteristic redshift of the bandgap they could extract an average phonon energy (38.9 ± 0.7) meV, which they used for fitting the transition linewidth as a function of temperature,[17] The linear broadening due to acoustic phonons found now in the new study was considerably larger than the values previously reported for MoSe₂ and MoS₂ monolayers.[18]

To further confirm the successful synthesis, the authors studied power-dependent PL spectra using a HeNe laser (λ = 633 nm). Figure 8B showsd the integrated PL intensity IPL as a function of laser power P. The intensity of the X peak increases linearly with power, which is the signature of a free exciton peak, while at the L transition the contributing states saturate[20]. In addition the new data on the energy shift and broadening with temperature indicated that the X emission is indeed excitonic in origin. In the reflectivity measurements shown in the same panel, the A and B exciton resonances, can be clearly distinhuished. Their difference, Δ_XB-X≈177 meV lies between the values reported for MoS₂ and MoSe₂ monolayers.[21] Figure 8C also shows the measurements on a MoSSe monolayer of random alloy (with hBN capsule) measured under the same conditions. The Raman spectra of the random alloy and the ordered Janus layer are clearly different. For the alloy, we find the MoS₂-like doublet at 400 cm^-1, which had already been reported in the literature.[13, 14] Interestingly, the average phonon energy (38.9 ± 0.7) meV (corresponding to ≈313 cm^-1) extracted in the temperature-dependent measurements in Figure 7C,D falls within the range of the main phonon energies.

"We have developed a simple, reproducible one-pot CVD synthesis of large-area Janus SeMoS MLs on Au foils.", the authors said. Their experiments show that the HeNe laser excitation (633 nm; 1.96 eV) is nearly in resonance with the B exciton at T = 4 K, which explains the comparatively high PL signal and the very rich structure in the Raman data compared to the 532 nm excitation. The PL data also show underlying periodic features that are at a fixed energy with respect to the laser energy and are likely related to phonon-assisted absorption and/or emission (Figure 8, see Ref. [22]). Note that these periodic features are not present in the random alloy monolayer sample. For comparison, we show the PLE data for the alloy monolayer, for which we find a resonance at a very similar energy position, but the PLE spectrum at higher energies (>1.97 eV) is very different compared to the Janus monolayer due to the different band structure. "The remarkably high optical quality of the synthesized Janus monolayers demonstrated by optical and magneto-optical measurements reveal their potential use for science and applications."

Resource: Gan, Z., Paradisanos, I., Estrada‐Real, A., Picker, J., Najafidehaghani, E., Davies, F., Neuman, C., Robert, C., Wiecha, P., Watanabe, K., Taniguchi, T., Marie, X., Biskupek, J., Mundszinger, M., Leiter, R., Kaiser, U., Krasheninnikov, A. V., Urbaszek, B., George, A., & Turchanin, A. (2022). Chemical vapor deposition of high‐optical‐quality large‐area monolayer Janus transition metal dichalcogenides. Advanced Materials, 34(38), 2205226.