Thickness-Dependent Dielectric Constant of Few-Layer In₂Se₃ Nanoflakes

Abstract

ABSTRACT:The dielectric constant or relative permittivity (r) of a dielectric material, which describes how the net electric field in the medium is reduced with respect to the external field, is a parameter of critical importance for charging and screening in electronic devices. Such a fundamental material property is intimately related to not only the polarizability of individual atoms, but also the specific atomic arrangement in the crystal lattice. In this letter, we present both experimental and theoretical investigations on the dielectric constant of few-layer In2Se3 nano-flakes grown on mica substrates by van der Waals epitaxy. A nondestructive microwave impedance microscope is employed to simultaneously quantify the number of layers and local electrical properties. The measured r increases monotonically as a function of the thickness and saturates to the bulk value at around 6 ~ 8 quintuple layers. The same trend of layer-dependent dielectric constant is also revealed by first-principle calculations. Our results of the dielectric response, being ubiquitously applicable to layered 2D semiconductors, are expected to be significant for this vibrant research field.KEYWORDS: microwave impedance microscopy, layer-dependent dielectric constant, In2Se3 nano-flakes, layered materials, polarization, first-principle calculations 2The rapid rise of graphene in the past decade has led to an active research field on twodimensional (2D) layered materials.1, 2 Of particular interest here are layered semiconductors, such as many metal chalcogenides, for their roles as gate dielectrics or channel materials in nextgeneration electronics. 3 Due to the strong intralayer covalent bonding and weak van der Waals (vdW) interactions, most physical properties are already anisotropic in the bulk form, with a clear 3D-2D crossover when approaching the monolayer thickness. In particular, the number of layers (n) in a thin-film 2D system is expected to strongly influence its dielectric constant, a fundamental electrical property that determines the capacitance and charge screening in electronic devices. 4-6The 2D material in this study is the layered semiconducting chalcogenide In2Se3, a technologically important system for phase-change memory, thermoelectric, and photoelectric applications 7 . The In-Se phase diagram is among the most complex ones in binary compounds.Even at the exact stoichiometry of In:Se = 2:3, multiple phases can occur under different temperatures and pressures. [8][9][10][11] By controlling the synthesis parameters or thermal/electrical pretreatment processes, several phases (superlattice, simple hexagonal -phase, simple hexagonal -phase, and amorphous state) with vastly different electrical conductivity can coexist at the ambient condition, 12-14 which explains the research interest of In2Se3 as a prototypical phasechange material. 7,[12][13][14][15][16][17][18] In addition, the lattice constant of In2Se3 matches well with Bi2Se3, which is heavily investigated for its high thermoelectric figure-of-merit and ...

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