Paraffin-Enabled Compressive Folding of Two-Dimensional Materials with Controllable Broadening of the Electronic Band Gap

Abstract

The capability to manipulate the size of the electronic band gap is of importance to semiconductor technology. Among these, a wide direct band gap is particularly helpful in optoelectronic devices due to the efficient utilization of blue and ultraviolet light. Here, we reported a paraffin-enabled compressive folding (PCF) strategy to widen the band gap of two-dimensional (2D) materials. Due to the large thermal expansion coefficient of paraffin, folded 2D materials can be achieved via thermal engineering of the paraffin-assisted transfer process. It can controllably introduce 0.2–1.3% compressive strain onto folded structures depending on the temperature differences and transfer the folding product to both rigid and soft substrates. Exemplified by MoS2, its folded multilayers demonstrated blue-shifts at direct gap transition peaks, six times stronger photoluminescence intensity, almost double mobility, and 20 times higher photoresponsivity over unfolded MoS2. This PCF strategy can attain controllable widening band gap of 2D materials, which will open up novel applications in optoelectronics.

Tuning photoluminescence behaviors in strained monolayer belt-like MoS2 crystals confined on TiO2(001) surface

“…This phenomenon can be understood that the compressive stain may induce an up-shift in the conduction band minima (CBM) at the K-point and a down-shift in the local valence band maxima (VBM) at the Γ-point. Accordingly, the optical band gap ban be broadened, which has been predicted in the former theoretical results [33,37,51,56,57]. Under the influence of the strain, the PL intensity for the belt-like and triangular samples on the TiO 2 surface exhibits an overt dissimilarity.…”

One‐Interlayer‐Twisted Multilayer MoS2 Moiré Superlattices

“…[ 37 ] For folded monolayer with ≈0° twist angle, the A exciton peak intensity is 3.5 times larger than 3R‐stacked bilayer and 2 times larger than unfolded monolayer, which is in good agreement with previous works. [ 31,35 ] The A exciton peak intensities for≈0° folded bilayer and trilayer are respectively 3 and 2 times stronger than corresponding untwisted MoS 2 (4L and 6L, respectively), with almost the same intensities as their untwisted building blocks (2L and 3L, respectively). The above A exciton peak intensity enhancement can be understood by indirect bandgap broadening induced by larger interlayer distance (see detailed discussion in Note S4, Supporting Information), which clearly reflects the weak interlayer coupling of FIALs.…”

“…Multilayer MoS 2 Moiré superlattices with one twisted interface are fabricated by applying a paraffin‐assisted folding strategy [ 31 ] (see Experimental Section) on the CVD‐grown untwisted MoS 2 ( Figure a). High crystallinity and good quality of our CVD‐grown samples is confirmed by microscopic and spectroscopic characterizations (Note S1 and Figure S1, Supporting Information).…”

“…Another approach to fabricate a twisted multilayer structure is by artificial stacking, but the continuous transfer of one layer after another with a wide range of twist angle can be very tedious, which is not suitable for systematic study of twist-angle dependent properties of the material. [17,18,[28][29][30] In this work, we demonstrate the fabrication of multilayer Moiré superlattices with only one twisted lattice interface via a simple paraffin-assisted folding strategy [31] on non-twisted stacked multilayer MoS 2 . The stacking and twisting structure of one-interface-twisted multilayer Moiré superlattices are confirmed by selected-area electron diffraction (SAED) and transmission electron microscopy (TEM).…”

“…In this work, we demonstrate the fabrication of multilayer Moiré superlattices with only one twisted lattice interface via a simple paraffin‐assisted folding strategy [ 31 ] on non‐twisted stacked multilayer MoS 2 . The stacking and twisting structure of one‐interface‐twisted multilayer Moiré superlattices are confirmed by selected‐area electron diffraction (SAED) and transmission electron microscopy (TEM).…”