Exfoliation of Alpha‐Germanium: A Covalent Diamond‐Like Structure

Abstract

2D materials have opened a new field in materials science with outstanding scientific and technological impact. A largely explored route for the preparation of 2D materials is the exfoliation of layered crystals with weak forces between their layers. However, its application to covalent crystals remains elusive. Herein, a further step is taken by introducing the exfoliation of germanium, a narrow‐bandgap semiconductor presenting a 3D diamond‐like structure with strong covalent bonds. Pure α‐germanium is exfoliated following a simple one‐step procedure assisted by wet ball‐milling, allowing gram‐scale fabrication of high‐quality layers with large lateral dimensions and nanometer thicknesses. The generated flakes are thoroughly characterized by different techniques, giving evidence that the new 2D material exhibits bandgaps that depend on both the crystallographic direction and the number of layers. Besides potential technological applications, this work is also of interest for the search of 2D materials with new properties.

Solvent‐Assisted Anisotropic Cleavage of Transition Metal Carbide into 2D Nanoflakes

“…Among the different crystallographic planes, the one with minimum bond strength is the least stable one under the shock wave and will induce the cleavage. [12] With the continuous sonication treatment, the cleavage tendency of the plane with lowest energy will be enhanced. (110), one of TiC crystallographic planes, is intersected.…”

“…Ajayan's group demonstrated that nonlayered hematite can be exfoliated into 2D material "hematene." They attributed such exfoliation to the facet that direction perpendicular to {001} [12] has the highest broken bond density, so it is less stable, which results in preferable cleavage along [001] and [010] directions when hydrodynamic forces are applied. [13] More recently, 2D magnesium was fabricated after the crystals were treated in liquid nitrogen, which induced Mechanical exfoliation of bulk layered materials into atomic-layer thin nanosheets has become a general strategy to obtain 2D nanomaterials, which seems intuitively unsuitable for non-van der Waals crystals.…”

“…specific inactivated slip systems, so that cleavage occurred along the inactive slip plane once mechanical stress was applied. [14] Nonlayered iron pyrite [15] and α-germanium [12] can also be exfoliated due to the presence of relatively weak covalent bonds. Fabrication of 2D selenium [16] and tellurium [17] was reported by Zhang's group, due to their chain-like structure crystals.…”

Self‐Limiting Synthesis of Ultrathin Ge(110) Single Crystal via Liquid Metal

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Given these novel physical properties of germanium, uncovering the nanostructure and characteristics of the 2D Ge systems are timely.Unfortunately, the 3D diamond-like structure with strong covalent bonds impedes the layer-by-layer exfoliation, making it hard to generate ultrathin germanium with high crystallization quality by exfoliation. [8] Furthermore, the strong covalent bonds greatly hinder 2D anisotropic growth. Only some examples of nanoparticles [9][10][11] and ultrathin Ge(111) flakes were investigated.

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Liquid Phase Exfoliation of Rubrene Single Crystals into Nanorods and Nanobelts

“…e ., that high-aspect-ratio sheets can be prepared when the in-plane bonds are much stronger than those out-of-plane. This implies that both scission and delamination occur, and as such, the layered nature of the crystal is not a prerequisite and LPE can also be used to produce nanomaterials from nonlayered materials such as pyrite and germanium . However, in all these cases, the individual layers of the final nanomaterials are held together by strong in-plane covalent or coordination bonds.…”