Blueshift of the $A$-exciton peak in folded monolayer $1H$-MoS${}_2$

The large family of layered transition-metal dichalcogenides is widely believed to constitute a second family of two-dimensional (2D) semiconducting materials that can be used to create novel devices that complement those based on graphene. In many cases these materials have shown a transition from an indirect band gap in the bulk to a direct band gap in monolayer systems. In this work we experimentally show that folding a 1$H$ molybdenum disulfide (MoS${}_2$) layer results in a turbostratic stack with enhanced photoluminescence quantum yield and a significant shift to the blue by ∼90 meV. This is in contrast to the expected 2$H$-MoS${}_2$ band-structure characteristics, which include an indirect gap and quenched photoluminescence. We present a theoretical explanation for the origin of this behavior in terms of exciton screening.

Figure 1

Schematics of a single 1$H$-MoS${}_2$ layer (a), single 2$H$-MoS${}_2$ monolayer (b), and folded 1$H$-MoS${}_2$ layer (c). Note the structural similarity of the latter to a 2$H$-MoS${}_2$ monolayer far from the connection region.

Figure 2

Examination of the circled regions in these before (a) and after (b) photographs reveals that processing can sometimes remove some triangles completely, while in other cases folds are generated in those triangles that have undergone partial detachment from the substrate. A backscatter SEM micrograph shown in the lower right of (b) reveals a large 1$H$-MoS${}_2$ folded region.

Figure 3

Optical (a), secondary electron (b), backscatter (c), and AFM (d) images of folded MoS${}_2$, as well as corresponding scans from 1$H$ layer (e), folded 1$H$ layer (f), and 2$H$ monolayer (g) regions.

Figure 4

High-resolution maps of Raman peak intensity (a),(c) and position (b),(d) of the in-plane (a),(b) and out-of-plane (c),(d) modes in a folded 1$H$-MoS${}_2$ layer.

Figure 5

High-resolution maps showing the PL intensity (a) and peak position (b) in folded 1$H$-MoS${}_2$.

Figure 6

Raman (a) and PL (b) spectra for a 1$H$-MoS${}_2$ monolayer (blue), folded 1$H$-MoS${}_2$ (red), and a 2$H$-MoS${}_2$ monolayer (green). Inset: Expanded view of curves revealing the peak position of the $B$ exciton.

Figure 7

Optical micrographs depicting the fabrication flow of a back-gated FET on folded 1$H$-MoS${}_2$ (a)–(c): application of photoresist over the folded region (a), PMMA patterned to deposit Ohmic contacts on the etched region (b), and the finished device (c). $I_{\mathrm{DS}}$-$V_{\mathrm{BG}}$ characteristics for both 1$H$-MoS${}_2$ (red) and folded 1$H$-MoS${}_2$ (blue) are plotted in both logarithmic and linear scales for an $L=800$ nm, $W$ $=$ 1 μm FET measured using the two- point-probe technique (d).

Figure 8

Feynman diagram for intersheet exciton-exciton collision. Exciton-$eh$ vertices are denoted by squares, the diamond shows an $eh$-photon/phonon vertex, and the triangle shows intersheet h-h elastic scattering due to the fold edge. Red lines delineate the time ordering of this particular process. The symbol × denotes the vertex for converting hole $h_1$ into hole $h_2^{\prime }$, i.e., transferring a hole from one sheet to another, via an infinitely heavy scatterer (see Ref. 29). The most likely agent for this process is nontunneling injection through the fold connection region.