Reaching the Excitonic Limit in 2D Janus Monolayers by In Situ Deterministic Growth

Abstract

Named after the two‐faced Roman god of transitions, transition metal dichalcogenide (TMD) Janus monolayers have two different chalcogen surfaces, inherently breaking the out‐of‐plane mirror symmetry. The broken mirror symmetry and the resulting potential gradient lead to the emergence of quantum properties such as the Rashba effect and the formation of dipolar excitons. Experimental access to these quantum properties, however, hinges on the ability to produce high‐quality 2D Janus monolayers. Here, these results introduce a holistic 2D Janus synthesis technique that allows real‐time monitoring of the growth process. This prototype chamber integrates in situ spectroscopy, offering fundamental insights into the structural evolution and growth kinetics, that allow the evaluation and optimization of the quality of Janus monolayers. The versatility of this method is demonstrated by synthesizing and monitoring the conversion of SWSe, SNbSe, and SMoSe Janus monolayers. Deterministic conversion and real‐time data collection further aid in conversion of exfoliated TMDs to Janus monolayers and unparalleled exciton linewidth values are reached, compared to the current best standard. The results offer an insight into the process kinetics and aid in the development of new Janus monolayers with high optical quality, which is much needed to access their exotic properties.

Angular Dependence of the Second-Order Nonlinear Optical Response in Janus Transition Metal Dichalcogenide Monolayers

“…In comparing the in-plane lattice parameters and thicknesses for Janus TMD monolayers (Figure 1S in the Supporting Information) and the corresponding values for TMD monolayers, 29 we note that the overall relationship is similar, as expected, and any deviations due to the two different chalcogen Low strain commensurate heterostructures of Janus and TMD monolayers may result due to the similarity of the in-plane lattice parameters and growth methods. 15,19 For example, the WSSe lattice parameter (a = 3.231 Å) is similar to the WSe 2 lattice parameter (a = 3.284 Å) which, if grown, would require the two layers to compensate for a strain of approximately 1.6%.…”

“…Calculated electronic band structures for the Janus TMDs (Figure 2S and Table 2S) indicate that the PBE+SOC bandgaps result in errors of 10% and 15% for MoSSe and WSSe, respectively, compared to experiment. 15,18,19,52 Such errors in the bandgap are expected when using a generalized gradient approximation (GGA) functional and comparing it to experimental measurement. However, the errors are larger, about 20%, compared to other calculated values using PBE, 13,56 PBE+SOC, 49,50,53 HSE, 11,54 and HSE+SOC.…”

“…For the direct bandgap MoSSe and WSSe Janus TMDs, we find good agreement with 3.8% and 3.6% average error, respectively, for the A, B, C, and D peak energies when compared to experimental measurements. 15,19,61 The A and B peaks in an indirect bandgap Janus monolayer (Figure 2S(b−f)) are unresolved because the calculated absorption spectra only consider momentum-conserving transitions.…”

Chemical Vapor Deposition of High‐Optical‐Quality Large‐Area Monolayer Janus Transition Metal Dichalcogenides

“…In Figure 3C, we plot the emission energy of the X peak (exciton) as a function of temperature. Fitting the characteristic redshift of the bandgap (see Note S8, Supporting Information) allows us to extract an average phonon energy 〈ℏω〉 = (38.9 ± 0.7) meV that we use in the fit of the transition linewidth as a function of temperature, [24] 0…”

Pentagonal 2D Transition Metal Dichalcogenides: PdSe2 and Beyond

“…The isolation of atomically thin single-crystalline graphene by mechanical cleavage in 2004 [1] ignited much interest in 2D materials. [2][3][4][5][6] Being only one monolayer thick, 2D materials exhibit unconventional properties including optical, [7][8][9][10] electrical, [11][12][13][14][15] chemical, [16][17][18][19] and magnetic properties. [20][21][22][23][24] 2D materials promise novel properties such as superconductivity, [25][26][27] magnetism, [28][29][30] as well as applications in electronics, [31][32][33] optics, [34][35][36] energy storage, [37][38][39] sensors, [40][41][42][43] and biomedicine.…”