Higher Ultrasonic Frequency Liquid Phase Exfoliation as a More Efficient Method to Deposit Monolayer to Few-Layer Flakes of 2D Layered Materials

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Abstract Body: Dimensionality is the determining critical parameter in any material. Graphene and transition metal chalcogenides are two-dimensional (2D) layered nanomaterials which demonstrate wide array of physical and chemical properties when approaching few-layer thicknesses [1-2]. Large lateral size of these van der Waals nanomaterials with thinner mono- or few-layered flakes possess higher aspect ratios, which makes them suitable for electronic and optoelectronic applications. Among the most reliable techniques for exfoliation, sonication-assisted liquid-phase exfoliation (LPE) is considered to be a simple and cost-effective method for preparing graphene and its 2D inorganic counterparts at reasonable sizes and acceptable levels of defects. Although there were rapid advances in this field, improper sonication may result in unstable dispersions with wide flake sizes and thicknesses distributions, impurities and defects. Previous studies were mainly focused on the effects of solvent selection (predominantly, using high-boiling point solvents or surfactants), sonication time, and power input (usually, high power from probe/tip sonication can cause significant damage to the material). However, the effect of the sonication frequency is poorly understood and often ignored resulting in a low exfoliation efficiency. Here, we demonstrate that simple mild bath sonication at high frequency at relatively low powers contributes to the thickness, size and quality of the final exfoliated products. We show that monolayer graphene flakes can be directly exfoliated from graphite using only ethanol as a solvent at a reasonable sonication time by increasing the frequency of the bath sonicator from 37 kHz (which is usually used) to 80 kHz. The key mechanism involved is the generation of small-sized bubbles resulting from higher frequencies, which can easily penetrate between the interlayer spacing of the 2D materials to facilitate the delamination process with reduced damage to the material [3]. Detailed morphology studies by atomic force microscopy (AFM) indicate that the obtained product is dominated by large lateral size (10-15 μm) monolayer graphene flakes. High resolution TEM (HRTEM) show that the graphene flakes are of high-quality without any significant structural defects, although X-ray photoelectron spectroscopy (XPS) shows evidence of a small oxide population. Additionally, we show that this facile technique can be applied to other inorganic 2D nanomaterials such as digenite (Cu9S5) and silver sulfide (Ag2S). This simple and effective method facilitates graphene and transition metal chalcogenides’ processing for a wide range of applications.