Germanene is a material made up of a single layer of germanium atoms. [2] The material is created in a process similar to that of silicene and graphene, in which high vacuum and high temperature are used to deposit a layer of germanium atoms on a substrate. [2] [3] [4] High-quality thin films of germanene have revealed unusual two-dimensional structures with novel electronic properties suitable for semiconductor device applications and materials science research.
In September 2014, G. Le Lay and others reported the deposition of a single atom thickness, ordered and two-dimensional multi-phase film by molecular beam epitaxy upon a gold surface in a crystal lattice with Miller indices (111). The structure was confirmed with scanning tunneling microscopy (STM) revealing a nearly flat honeycomb structure. [5]
We have provided compelling evidence of the birth of nearly flat germanene—a novel, synthetic germanium allotrope which does not exist in nature. It is a new cousin of graphene.
— Guy Le Lay from Aix-Marseille University, New Journal of Physics
Additional confirmation was obtained by spectroscopic measurement and density functional theory calculations. The development of high quality and nearly flat single atom films created speculation that germanene may replace graphene if not merely add an alternative to the novel properties of related nanomaterials. [2] [5] [6] [7] [8] [9]
Bampoulis and others [10] have reported the formation of germanene on the outermost layer of Ge2Pt nanocrystals. Atomically resolved STM images of germanene on Ge2Pt nanocrystals reveal a buckled honeycomb structure. This honeycomb lattice is composed of two hexagonal sublattices displaced by 0.2 Å in the vertical direction with respect to each other. The nearest-neighbor distance was found to be 2.5±0.1 Å, in close agreement with the Ge-Ge distance in germanene.
Based on STM observations and density functional theory calculations, formation of an apparently more distorted form of germanene has been reported on platinum. [5] [11] Epitaxial growth of germanene crystals on GaAs(0001) has also been demonstrated, and calculations suggest that the minimal interactions should allow germanene to be readily removed from this substrate. [12]
Germanene's structure is described as "a group-IV graphene-like two-dimensional buckled nanosheet". [13] Adsorption of additional germanium onto the graphene-like sheet leads to formation of "dumbbell" units, each with two out-of-plane atoms of germanium, one on either side of the plane. Dumbbells attract each other. Periodically repeating arrangements of dumbbell structures may lead to additional stable phases of germanene, with altered electronic and magnetic properties. [14]
In October 2018, Junji Yuhara and others reported that germanene is easily prepared by a segregation method, using a bare Ag thin film on a Ge substrate and achieved in situ its epitaxial growth. [15] The growth of germanene, akin to graphene and silicene, by a segregation method, is considered to be technically very important for the easy synthesis and transfer of this highly promising 2D electronic material.
Germanene's electronic and optical properties have been determined from ab initio calculations, [16] and structural and electronic properties from first principles. [17] [18] These properties make the material suitable for use in the channel of a high-performance field-effect transistor [19] and have generated discussion regarding the use of elemental monolayers in other electronic devices. [20] The electronic properties of germanene are unusual, and provide a rare opportunity to test the properties of Dirac fermions. [21] [22] Germanene has no band gap, but attaching a hydrogen atom to each germanium atom creates one. [23] These unusual properties are generally shared by graphene, silicene, germanene, stanene, and plumbene. [22] [24] [25] [26]
Graphene is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure. The name is derived from "graphite" and the suffix -ene, reflecting the fact that the graphite allotrope of carbon contains numerous double bonds.
Phosphorene is a two-dimensional material consisting of phosphorus. It consists of a single layer of black phosphorus, the most stable allotrope of phosphorus. Phosphorene is analogous to graphene. Among two-dimensional materials, phosphorene is a competitor to graphene because it has a nonzero fundamental band gap that can be modulated by strain and the number of layers in a stack. Phosphorene was first isolated in 2014 by mechanical exfoliation. Liquid exfoliation is a promising method for scalable phosphorene production.
Graphene nanoribbons are strips of graphene with width less than 100 nm. Graphene ribbons were introduced as a theoretical model by Mitsutaka Fujita and coauthors to examine the edge and nanoscale size effect in graphene.
Walter Alexander "Walt" de Heer is a Dutch physicist and nanoscience researcher known for discoveries in the electronic shell structure of metal clusters, magnetism in transition metal clusters, field emission and ballistic conduction in carbon nanotubes, and graphene-based electronics.
Silicene is a two-dimensional allotrope of silicon, with a hexagonal honeycomb structure similar to that of graphene. Contrary to graphene, silicene is not flat, but has a periodically buckled topology; the coupling between layers in silicene is much stronger than in multilayered graphene; and the oxidized form of silicene, 2D silica, has a very different chemical structure from graphene oxide.
Graphyne is an allotrope of carbon. Its structure is one-atom-thick planar sheets of sp and sp2-bonded carbon atoms arranged in crystal lattice. It can be seen as a lattice of benzene rings connected by acetylene bonds. The material is called graphyne-n when benzene rings are connected by n sequential acetylene molecules, and graphdiyne for a particular case of n = 2.
Stanene is a topological insulator, which may display dissipationless currents at its edges near room temperature. It is composed of tin atoms arranged in a single layer, in a manner similar to graphene. Stanene got its name by combining stannum with the suffix -ene used by graphene. Research is ongoing in Germany and China, as well as at laboratories at Stanford and UCLA.
Borophene is a crystalline atomic monolayer of boron, i.e., it is a two-dimensional allotrope of boron and also known as boron sheet. First predicted by theory in the mid-1990s, different borophene structures were experimentally confirmed in 2015.
Potential graphene applications include lightweight, thin, and flexible electric/photonics circuits, solar cells, and various medical, chemical and industrial processes enhanced or enabled by the use of new graphene materials.
Transition-metal dichalcogenide (TMD or TMDC) monolayers are atomically thin semiconductors of the type MX2, with M a transition-metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms. They are part of the large family of so-called 2D materials, named so to emphasize their extraordinary thinness. For example, a MoS2 monolayer is only 6.5 Å thick. The key feature of these materials is the interaction of large atoms in the 2D structure as compared with first-row transition-metal dichalcogenides, e.g., WTe2 exhibits anomalous giant magnetoresistance and superconductivity.
In materials science, the term single-layer materials or 2D materials refers to crystalline solids consisting of a single layer of atoms. These materials are promising for some applications but remain the focus of research. Single-layer materials derived from single elements generally carry the -ene suffix in their names, e.g. graphene. Single-layer materials that are compounds of two or more elements have -ane or -ide suffixes. 2D materials can generally be categorized as either 2D allotropes of various elements or as compounds.
A two-dimensional semiconductor is a type of natural semiconductor with thicknesses on the atomic scale. Geim and Novoselov et al. initiated the field in 2004 when they reported a new semiconducting material graphene, a flat monolayer of carbon atoms arranged in a 2D honeycomb lattice. A 2D monolayer semiconductor is significant because it exhibits stronger piezoelectric coupling than traditionally employed bulk forms. This coupling could enable applications. One research focus is on designing nanoelectronic components by the use of graphene as electrical conductor, hexagonal boron nitride as electrical insulator, and a transition metal dichalcogenide as semiconductor.
A rapidly increasing list of graphene production techniques have been developed to enable graphene's use in commercial applications.
Graphene is a 2D nanosheet with atomic thin thickness in terms of 0.34 nm. Due to the ultrathin thickness, graphene showed many properties that are quite different from their bulk graphite counterparts. The most prominent advantages are known to be their high electron mobility and high mechanical strengths. Thus, it exhibits potential for applications in optics and electronics especially for the development of wearable devices as flexible substrates. More importantly, the optical absorption rate of graphene is 2.3% in the visible and near-infrared region. This broadband absorption characteristic also attracted great attention of the research community to exploit the graphene-based photodetectors/modulators.
Graphene is a semimetal whose conduction and valence bands meet at the Dirac points, which are six locations in momentum space, the vertices of its hexagonal Brillouin zone, divided into two non-equivalent sets of three points. The two sets are labeled K and K'. The sets give graphene a valley degeneracy of gv = 2. By contrast, for traditional semiconductors the primary point of interest is generally Γ, where momentum is zero. Four electronic properties separate it from other condensed matter systems.
Single-layer graphene was first unambiguously produced and identified in 2004, by the group of Andre Geim and Konstantin Novoselov, though they credit Hanns-Peter Boehm and his co-workers for the experimental discovery of graphene in 1962; while it had been explored theoretically by P. R. Wallace in 1947. Boehm et al. introduced the term graphene in 1986.
Epitaxial graphene growth on silicon carbide (SiC) by thermal decomposition is a method to produce large-scale few-layer graphene (FLG). Graphene is one of the most promising nanomaterials for the future because of its various characteristics, like strong stiffness and high electric and thermal conductivity. Still, reproducible production of Graphene is difficult, thus many different techniques have been developed. The main advantage of epitaxial graphene growth on silicon carbide over other techniques is to obtain graphene layers directly on a semiconducting or semi-insulating substrate which is commercially available.
Two dimensional hexagonal boron nitride is a material of comparable structure to graphene with potential applications in e.g. photonics., fuel cells and as a substrate for two-dimensional heterostructures. 2D h-BN is isostructural to graphene, but where graphene is conductive, 2D h-BN is a wide-gap insulator.
Plumbene is a material made up of a single layer of lead atoms. The material is created in a process similar to that of graphene, silicene, germanene, and stanene, in which high vacuum and high temperature are used to deposit a layer of lead atoms on a substrate. High-quality thin films of plumbene have revealed two-dimensional honeycomb structures. First researched by Indian scientists, further investigations are being done around the world.
Maria C. Asensio is a Spanish-Argentinian physical chemist, academic, researcher, and author. She is a Full Research Professor at the Materials Science Institute of Madrid (ICMM) of the Spanish National Research Council (CSIC) and Chair of the CSIC Research Associated Unit-MATINÉE created between the ICMM and the Institute of Materials Science (ICMUV) of the Valencia University.
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