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.
The resistance switching current-voltage (I-V) characteristics in polycrystalline NiO films were investigated in the temperature range of 10K<T<300K. Very clear reversible resistive switching phenomena were observed in the entire temperature range. An analysis of the temperature dependence of the resistance switching transport revealed additional features, not reported in previous studies, that weak metallic conduction and correlated barrier polaron hopping coexist in the high-resistance off state and that relative dominance depends on the temperature and defect configuration. In addition, the authors propose that metallic Ni defects, existing near polycrystalline (or granular) boundaries, play a key role in the formation of a metallic channel.
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