Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe/WSe vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe and WSe layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe and WSe.
van
der Waals heterostructures composed of two different monolayer
crystals have recently attracted attention as a powerful and versatile
platform for studying fundamental physics, as well as having great
potential in future functional devices because of the diversity in
the band alignments and the unique interlayer coupling that occurs
at the heterojunction interface. However, despite these attractive
features, a fundamental understanding of the underlying physics accounting
for the effect of interlayer coupling on the interactions between
electrons, photons, and phonons in the stacked heterobilayer is still
lacking. Here, we demonstrate a detailed analysis of the strain-dependent
excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive
strain that enables the interlayer interactions to be modulated along
with the electronic band structure. We find that the strain-modulated
interlayer coupling directly affects the characteristic combined vibrational
and excitonic properties of each monolayer in the heterobilayer. It
is further revealed that the relative photoluminescence intensity
ratio of WS2 to MoS2 in our heterobilayer increases
monotonically with tensile strain and decreases with compressive strain.
We attribute the strain-dependent emission behavior of the heterobilayer
to the modulation of the band structure for each monolayer, which
is dictated by the alterations in the band gap transitions. These
findings present an important pathway toward designing heterostructures
and flexible devices.
We report on an insulating two-dimensional material, hexagonal boron nitride (h-BN), which can be used as an effective wrapping layer for surface-enhanced Raman spectroscopy (SERS) substrates. This material exhibits outstanding characteristics such as its crystallinity, impermeability, and thermal conductance. Improved SERS sensitivity is confirmed for Au substrates wrapped with h-BN, the mechanism of which is investigated via h-BN thickness-dependent experiments combined with theoretical simulations. The investigations reveal that a stronger electromagnetic field can be generated at the narrowed gap of the h-BN surface, which results in higher Raman sensitivity. Moreover, the h-BN-wrapped Au substrate shows extraordinary stability against photothermal and oxidative damages. We also describe its capability to detect specific chemicals that are difficult to analyze using conventional SERS substrates. We believe that this concept of using an h-BN insulating layer to protect metallic or plasmonic materials will be widely used not only in the field of SERS but also in the broader study of plasmonic and optoelectronic devices.
Heterostructures of hexagonal boron nitride (h-BN) and graphene have attracted a great deal of attention for potential applications in 2D materials. Although several methods have been developed to produce this material through the partial substitution reaction of graphene, the reverse reaction has not been reported. Though the endothermic nature of this reaction might account for the difficulty and previous absence of such a process, we report herein a new chemical route in which the Pt substrate plays a catalytic role. We propose that this reaction proceeds through h-BN hydrogenation; subsequent graphene growth quickly replaces the initially etched region. Importantly, this conversion reaction enables the controlled formation of patterned in-plane graphene/h-BN heterostructures, without needing the commonly employed protecting mask, simply by using a patterned Pt substrate.
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