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 (JP), and 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.

Here, the team 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 growth process is characterized in detail by complementary experimental techniques such as Raman, TEM, and EDX, and the growth mechanisms are rationalized by first-principles DFT calculations. The remarkably high optical quality of the synthesized Janus monolayers is demonstrated by optical and magneto-optical measurements, which reveal a high polarization of up to 25% and an enhanced exciton G-factor of -3.3.

The physical properties of single layer (ML) transition metal dichalcogenides (TMDs) can be tailored for specific applications by stacking them in heterostructures,[1,2] alloying them,[3] or more recently by fabricating ML Janus TMDs.[4] 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., [8]). 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 [4, 12b]. Therefore, to realize the potential of Janus TMDs for science and applications, technologically relevant methods for their fabrication still need to be established.

The synthesis method of ML Janus SeMoS is schematically shown in Figure 1. Our one-pot synthesis method consists of two steps. i) Our growth process starts with CVD growth of ML-MoSe2 single crystals.[13b] Then, the sample is cooled to room temperature. 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 plays an important role in the conversion, leading to dissociation of the S molecules forming the vapor phase,[14] (Sn with n = 2, 3, 4, 6, 7 and 8) and chemisorption of the S atoms on the gold substrate.[15] 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 S1.

Figure 1B 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 1C shows the Janus MLs after their transfer to a SiO2/Si wafer by electrochemical delamination transfer technique[15] The atomic force microscopy (AFM) topography image of Janus SeMoS on SiO2/Si is shown in Figure 1D. The AFM height profile (inset in Figure 1D) shows an average thickness of 0.8 ± 0.2 nm, as expected for ML Janus SeMoS.

To better understand and optimize the transformation process, we performed a series of 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 1E). The black spectrum shows the Raman signal of grown ML-MoSe2 with the characteristic A'1 peak at 240 cm-1.[17] When the temperature is further increased (green and purple spectra), significant changes in the Raman spectra are observed: decreasing peak intensity, peak broadening, and the appearance of additional peaks characteristic of MoS2. This behavior is consistent with the formation of a random MoS2(1-x)Se2x alloy,[17, 20] 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), see the full article by Gan et al. in Adv. Mater. 2022, 34, 2205226.

Aberration-corrected transmission electron microscopy (TEM) along with spatially resolved energy dispersive X-ray spectroscopy (EDX) was used to further verify the Janus-SeMoS structure. The sample grown on the Au substrate was transferred to a Si wafer with a native SiO2 layer that served as an atomically flat substrate for the cross-sectional study. Figure 2A 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 S16 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 2B. 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.[23,2b] We have applied this approach to our Janus ML SeMoS as outlined in Figure 3A, where the bottom hBN layer is 50 nm thick and the top layer is 10 nm thick. In Figure 3B, we see the PL emission is both equipolarized and counterpolarized with respect to the polarization of the excitation laser. We extract the circular degree of polarization Pc = (I?+ - I?-)/(I?+ + I?-) of the X emission. For an excitation photon energy ≈180 meV above the X emission energy, we measure a high polarization of Pc = 25% for temperatures up to 100 K (Figure 3D), since chiral selection rules apply to the direct transition at the nonequivalent K points at the edge of the Brillouin zone.[4, 27] We find a drastic drop in Pc for temperatures T > 100 K and at 200 K we find Pc ≈ 0. For comparison, the PL emission from localized states L is unpolarized over the temperature range studied in Figure 3B. Strikingly, the sudden decrease in Pc for the X peak occurs in the same temperature range where drastic changes in peak position and linewidth are also observed, shown in Figure 3C as the emission energy of the X peak (exciton) as a function of temperature. Fitting the characteristic redshift of the bandgap allows us to extract an average phonon energy ???? = (38.9 ± 0.7) meV, which we use in fitting the transition linewidth as a function of temperature,[24] ?=?(0)+a?T(?/(e^(??/(k_B T))-1), here ?(0) = (18.5 ± 0.1) meV is the linewidth of X at T = 0 K, a = (24 ± 5) ?eV K-1 is the linear broadening due to acoustic phonons, and ? = (124 ± 2) meV is the strength of phonon coupling. The value of ? found here is considerably larger than the values previously reported for MoSe2 and MoS2 monolayers.[25b]

In Figure 4A we show power-dependent PL spectra using a HeNe laser (? = 633 nm), and in Figure 4B we plot 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[29]. In addition to our data on the energy shift and broadening with temperature, this behavior indicates that the X emission is indeed excitonic in origin. In the reflectivity measurements shown in the same panel, we distinguish the A and B exciton resonances, labeled X and XB, respectively. The difference ∆_(X_B-X)≈177 meV lies between the values reported for MoS2 and MoSe2 monolayers.[30]

Figure 4C 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 has already been reported in the literature.[17, 20] Interestingly, the average phonon energy ???? = (38.9 ± 0.7) meV (corresponding to ≈313 cm-1) extracted in the temperature-dependent measurements in Figure 3C,D falls within the range of the main phonon energies we find in Raman spectroscopy. To provide further information on the electronic structure, we show PL measurements as a function of excitation laser energy (PLE) in Figure 4D,E. The data show a strong resonance in the absorption at about 1.965 eV. By comparison with reflectivity measurements (Figure 4A), we attribute this resonance to the B exciton of the Janus SMoS monolayer. Our experiments show that the HeNe laser excitation (633 nm; 1.96 eV) is nearly in resonance with the B exciton at T = 4K, 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 4D, see Ref. [31]). 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.

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, 2205226.