Nanoscale Electronic Inhomogeneity in In₂Se₃ Nanoribbons Revealed by Microwave Impedance Microscopy

Abstract

Driven by interactions due to the charge, spin, orbital, and lattice degrees of freedom, nanoscale inhomogeneity has emerged as a new theme for materials with novel properties near multiphase boundaries. As vividly demonstrated in complex metal oxides 1-5 and chalcogenides 6,7 , these microscopic phases are of great scientific and technological importance for research in hightemperature superconductors 1,2 , colossal magnetoresistance effect 4 , phase-change memories 5,6 , and domain switching operations [7][8][9] . Direct imaging on dielectric properties of these local phases, however, presents a big challenge for existing scanning probe techniques. Here, we report the observation of electronic inhomogeneity in indium selenide (In 2 Se 3 ) nanoribbons 10 by near-field scanning microwave impedance microscopy [11][12][13] . Multiple phases with local resistivity spanning six orders of magnitude are identified as the coexistence of superlattice, simple hexagonal lattice and amorphous structures with ~100nm inhomogeneous length scale, consistent with high-resolution transmission electron microscope studies. The atomic-force-microscope-compatible microwave probe is able to perform quantitative sub-surface electronic study in a noninvasive manner. Finally, the phase change memory function in In 2 Se 3 nanoribbon devices can be locally recorded with big signal of opposite signs. 2While the conventional wisdom on solids largely results from the real-space periodic structures and k-space band theories 14 , recent advances in physics have shown clear evidence that microscopic inhomogeneity, manifested as sub-micron spatial variations of the material properties, could indeed occur under certain conditions. Utilizing various probe-sample coupling mechanisms 15 , spatial inhomogeneity has been observed as nanometer gap variations in high-Tc superconductors 1,2 , coexisting electronic states in VO 2 near the metal-insulator transition 3 , ferromagnetic domains in manganites showing colossal magnetoresistance effect 4 , and in an ever-growing list. Probing these non-uniform phases provides not only much knowledge of the underlying interactions, but also valuable information for applications of the domain structures. In particular, spatially resolved properties are of significant interest for phase change and other switching materials to be on board the nanoelectric era [5][6][7][8][9] .While a number of contrast mechanisms 15 have been employed to visualize the electronic inhomogeneity, established scanning probe techniques do not directly access the low-frequency (f) complex permittivity ε(ω) = ε′ + iσ/ω, where ε′ is the dielectric constant and σ the conductivity, which holds a special position to study the ground state properties of materials. For local electrodynamic response, near-field technique is imperative to resolve spatial variations at length scales well below the radiation wavelength 16 . For this study, the working frequency is set at ~1GHz, i.e., in the microwave regime, to stay below resonant excitation...

Controlling Growth High Uniformity Indium Selenide (In2Se3) Nanowires via the Rapid Thermal Annealing Process at Low Temperature

“…The strong intralayer bonding and weak interlayer Van der Waals interaction lead to highly anisotropic structural, electrical, optical, and mechanical properties [16,17]. Layer-structure In 2 Se 3 nanowires and nanoribbons have been synthesized by using metal nanoparticles as the catalyst via the vapor-liquid-solid (VLS) process [2,[18][19][20]. The properties of NWs depend not only on their shape anisotropy but also on their crystallographic anisotropy [21].…”

Thickness-Dependent Dielectric Constant of Few-Layer In2Se3 Nanoflakes

“…[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][13][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 topological insulator nature. 19,20 The chemical and structural compatibility between the two chalcogenides enables the growth of In2Se3/Bi2Se3 heterostructures.…”

“…When the sample was rapidly cooled down (> 100C/min) from the deposition temperature, a superlattice phase can be observed by transmission electron microscopy (TEM), 12,13,23 [120] directions give rise to specific electron diffraction or FFT patterns, suggesting that In2Se3 has the R3m structure. Although multiple phases were observed in previous work, 13,19 extensive TEM images reveal that the as-grown samples in this study are predominantly -phase with R3m space group, which is also confirmed by the Raman spectra in Fig. 1 (d).…”

“…A microwave impedance microscope (MIM) based on the atomic-force microscopy (AFM) platform 13,[26][27][28] was employed to measure the thickness and dielectric response of fewlayer In2Se3 flakes, as illustrated in Fig. 2(a).…”

Nanoscale Conductivity Contrast by Scattering-Type Near-Field Optical Microscopy in the Visible, Infrared and THz Domains

“…Common to all microwave SNOMs is that no free-space beam is involved in the coupling to the tip, rather, coaxial or coplanar transmission lines are used for this purpose [15]. Microwave SNOMs have the demonstrated capability of quantifying the local conductivity of lowly doped semiconductors over at least two orders of magnitude, at a spatial resolution of 100 nm [16], and better [17].…”