Vertical stacking of monolayers via van der Waals assembly is an emerging field that opens promising routes toward engineering physical properties of two-dimensional (2D) materials. Industrial exploitation of these engineering heterostructures as robust functional materials still requires bounding their measured properties so to enhance theoretical tractability and assist in experimental designs. Specifically, the shortrange attractive van der Waals forces are responsible for the adhesion of chemically inert components and are recognized to play a dominant role in the functionality of these structures. Here we reliably quantify the the strength of van der Waals forces in terms of an effective Hamaker parameter for CVD-grown graphene and show how it scales by a factor of two or three from single to multiple layers on standard supporting surfaces such as copper or silicon oxide. Furthermore, direct measurements on freestanding graphene provide the means to discern the interplay between the van der Waals potential of graphene and its supporting substrate. Our results demonstrated that the underlying substrates could enhance or reduce the van der Waals force of graphene surfaces, and its consequences are explained in terms of a Lifshitz theorybased analytical model.
Atomically
thin, two-dimensional (2D) indium selenide (InSe) has attracted considerable
attention because of the dependence of its bandgap on sample thickness,
making it suitable for small-scale optoelectronic device applications.
In this work, by the use of Raman spectroscopy with three different
laser wavelengths, including 488, 532, and 633 nm, representing resonant,
near-resonant, and conventional nonresonant conditions, a conclusive
understanding of the thickness dependence of lattice vibrations and
electronic band structure of InSe and InSe/graphene heterostructures
is presented. Combining our experimental measurements with first-principles
quantum mechanical modeling of the InSe systems, we identified the
crystal structure as ε-phase InSe and demonstrated that its
measured intensity ratio of Raman peaks in the resonant Raman spectrum
evolves with the number of layers. Moreover, graphene coating enhances
Raman scattering of few-layered InSe and also makes its photoluminescence
stable under higher intensity laser illumination. The optically induced
charge transfer between van der Waals graphene/InSe heterostructures
is observed under excitation of the E′ transition in InSe,
where the observed mechanism may potentially be a route for future
integrated electronic and optoelectronic devices.
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