Phase engineering of seamless heterophase homojunctions with co-existing 3R and 2H phases in WS₂ monolayers

Abstract

Self-organized semiconductor-semiconductor heterostructures (3R-2H) that coexist in atomically thin 2D monolayers forming homojunctions are of great importance for next-generation nanoelectronics and optoelectronics applications. Herein, we investigated the defect controlled growth of heterogeneous electronic structure within a single domain of monolayer WS to enable in-plane homojunctions consisting of alternate 2H semiconducting and 3R semiconducting phases of WS. X-ray photoelectron, Raman, and photoluminescence spectroscopy along with fluorescence and Kelvin probe force microscopy imaging confirm the formation of homojunctions, enabling a direct correlation between chemical heterogeneity and electronic heterostructure in the atomically thin WS monolayer. Quantitative analysis of phase fractions shows 59% stable 2H phase and 41% metastable 3R phase estimated over WS flakes of different sizes. Time-resolved fluorescence lifetime imaging confirms distinct contrast between 2H and 3R phases with two distinct lifetimes of 3.2 ns and 1.1 ns, respectively. Kelvin probe force microscopy imaging revealed an abrupt change in the contact potential difference with a depletion width of ∼2.5 μm, capturing a difference in work function of ∼40 meV across the homojunction. Further, the thermal stability of coexisting phases and their temperature dependent optical behavior show a distinct difference among 2H and 3R phases. The investigated aspects of the controlled in plane growth of coexisting phases with seamless homojunctions, their properties, and their thermal stability will enable the development of nanoscale devices that are free from issues of lattice mismatch and grain boundaries.

Mass Production of High‐Quality Transition Metal Dichalcogenides Nanosheets via a Molten Salt Method

“…The peaks of W 4f 5/2 (35.2 eV) and W 4f 7/2 (33.0 eV) agree well with previous studies of WS 2 (Figure d). The small peak at around 38.8 eV is assigned to W 4f 5/2 from WO 3 due to the oxidation during the sample preparation, and the W 4f 7/2 from WO 3 is buried by W 4f 5/2 from WS 2 . As for MoSe 2 and WSe 2 , Se 3d 3/2 and Se 3d 5/2 located at 55.0 and 54.2 eV exhibit around the same peak positions for MoSe 2 and WSe 2 (Figure h,k).…”

Growth Mechanism of Alternating Defect Domains in Hexagonal WS2 via Inhomogeneous W‐Precursor Accumulation

“…The radiohazard pattern with alternating WV and SV domains in h-WS 2 was observed regardless of the strains (confirmed by transferred sample in Figure S5, Supporting Information), [28] contamination by impurities or precursors ( Figures S6 and S7, Supporting Information), [24] and phase transition (neglected by Raman mapping in Figure S8, Supporting Information). [29][30][31] To investigate the accumulation process of the W-precursors at the facet edges, the time evolution of WS 2 growth is carried out and analyzed accordingly with PL ( Figure 3a) and Raman spectra ( Figure S9, Supporting Information) as well as with AFM topography (Figure 3b) at each growth time step. In the initial growth stage (12 min), a small flake of triangular (t-) WS 2 starts to grow with weak PL intensity (multiplied by 10 times due to its weak intensity) with a large number of the W-precursors remaining on the entire substrate.…”

Nearly 90% Circularly Polarized Emission in Monolayer WS2 Single Crystals by Chemical Vapor Deposition

“…For instance, monolayer 5 hexagonal-shape WS 2 (h-WS 2 ) flakes with triangular heterogeneous defect domains have been synthesized by CVD under hydrogen-rich growth conditions, as reported by several research groups. [23][24][25][26][27] Curiously, the optical emission in the h-WS 2 flakes exhibits alternating areas of bright and dark photoluminescence (PL) emission within each h-WS 2 flake, and the resulting PL image looks similar to the radioactive hazard symbol, as exemplified by the left panel of Figure 1a. According to previous studies, [23][24][25][26][27] the domains with a stronger PL intensity and higher mobility are associated with W-edges and S-vacancies (SVs), and the -domains with a significantly quenched PL intensity and lower electron mobility exhibit a blue-shifted PL peak position, and are associated with Sedges and W-vacancies (WVs).…”

“…[23][24][25][26][27] Curiously, the optical emission in the h-WS 2 flakes exhibits alternating areas of bright and dark photoluminescence (PL) emission within each h-WS 2 flake, and the resulting PL image looks similar to the radioactive hazard symbol, as exemplified by the left panel of Figure 1a. According to previous studies, [23][24][25][26][27] the domains with a stronger PL intensity and higher mobility are associated with W-edges and S-vacancies (SVs), and the -domains with a significantly quenched PL intensity and lower electron mobility exhibit a blue-shifted PL peak position, and are associated with Sedges and W-vacancies (WVs). A schematic illustration of the heterogeneous defect domains for WVs and SVs in a single crystalline h-WS 2 is shown in the right panel of Similar to other monolayer TMDCs in the 2H-phase, monolayer WS 2 is an ideal candidate for valleytronic applications due to its inequivalent K and K valleys at the edge of the Brillouin zone.…”