Germanene: the germanium analogue of graphene

TLDR

Germanene, a buckled germanium analogue of graphene, has been grown by several groups and exhibits Dirac‑fermion behavior with a large spin‑orbit gap (~24 meV) that enables the quantum spin Hall effect at accessible temperatures, making it a promising 2D material. This review seeks to survey synthesis routes, summarize key DFT findings, and evaluate germanene’s potential for field‑effect devices and other applications. The authors discuss band‑gap opening strategies—including external electric fields, foreign‑atom adsorption, and substrate coupling—alongside synthesis methods and DFT analyses. DFT calculations reveal that free‑standing germanene is a 2D Dirac‑fermion system with massless relativistic electrons.

Abstract

Recently, several research groups have reported the growth of germanene, a new member of the graphene family. Germanene is in many aspects very similar to graphene, but in contrast to the planar graphene lattice, the germanene honeycomb lattice is buckled and composed of two vertically displaced sub-lattices. Density functional theory calculations have revealed that free-standing germanene is a 2D Dirac fermion system, i.e. the electrons behave as massless relativistic particles that are described by the Dirac equation, which is the relativistic variant of the Schrödinger equation. Germanene is a very appealing 2D material. The spin-orbit gap in germanene (~24 meV) is much larger than in graphene (<0.05 meV), which makes germanene the ideal candidate to exhibit the quantum spin Hall effect at experimentally accessible temperatures. Additionally, the germanene lattice offers the possibility to open a band gap via for instance an externally applied electrical field, adsorption of foreign atoms or coupling with a substrate. This opening of the band gap paves the way to the realization of germanene based field-effect devices. In this topical review we will (1) address the various methods to synthesize germanene (2) provide a brief overview of the key results that have been obtained by density functional theory calculations and (3) discuss the potential of germanene for future applications as well for fundamentally oriented studies.