In 2004, first experimental synthesis of graphene as the first 2D material gave the go-ahead for the search to counterparts with z-numbers above carbon. Stepping down the carbon group 2D counterparts of numerous elements were realized in recent years. Two-dimensional materials often form a honeycomb structure, which means their atoms assemble hexagonally. According to theoretical predictions and experimental evidence, these materials have extraordinary electronic properties. Most of these characteristics differ significantly from their 3D relatives. For example, so-called two-dimensional Dirac materials obtain an outstanding high mobility of charge carriers (Dirac fermions) resulting from their Dirac-like dispersion. Elements with increasing atomic number Z show an increasing spin-orbit coupling, as well as an increasing buckling. This leads to the applicability of remarkable properties such as quantum spin hall effect or topological insulators.
Since the structure of 2D materials has major influence on the electronic properties of the material, we focus on structural analysis. Furthermore, the interface structure is of utmost importance for substrate grown 2D-materials, because the substrate induces structural changes, which again lead to new electrical properties. For this reason we concentrate on the analysis of chemical and atomic structures at the interfaces, as well as on revealing of electronic properties of 2D materials in dependency on its structure.
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Quasi-particle zoo (A) Charge carrier with effective mass m* as typically described by the Schrödinger equation in condensed matter physics. (B) Relativistic particles in the limit of zero rest mass following the Dirac equation. (C) So-called massless Dirac fermions in graphene, described by a 2D analog of the Dirac equation and (D) the more massive particles in bilayer graphene. A. K. Geim, Science 324, 1530 (2009). |
Buckling of 2D materials P. Vogt, Beilstein J. Nanotechnol. 9, 2665 (2018). |
Since the first preparation of graphene in 2004 by Novoselov and Geim by exfoliating a graphene sheet from a graphite crystal with scotch tape, various preparation methods have been invented. The most promising techniques are chemical vapor deposition (CVD), reduction of graphene oxides and sublimation graphene from silicon carbide (SiC) crystals. We use the sublimation method, since it excels both methods in homogeneity of the graphene film.
This method yields the potential preparation of whole SiC-wafer-scale graphene with outstanding homogeneity and electronic properties on an insulating substrate.
We use the confinement controlled sublimation (CCS) method in an argon atmosphere. When the first bilayer of SiC lacks all of its silicon the so-called buffer layer is formed. Decoupled from the substrate the buffer layer converts to epitaxial graphene. Decoupling from the substrate can also be archived by intercalation of the buffer layer by metals or gases. This enables us to produce multi-layer systems and add new interesting properties induced by the intercalated materials. Also, multi-layer graphene can be established. Furthermore, graphene can be used as substrate for other 2D-materials.
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CCS-method For the CCS-method a graphite crucible is heated by an induction coil. The sublimation is controlled with a leak at the top. W. A. de Heer et al., PNAS 108, 16900 (2011). |
The discovery of graphene paved the way for a new field of two-dimensional materials in solid state physics. One novel material beyond graphene, silicene, means the 2D version of silicon. It forms a slightly buckled honeycomb structure hence astonishing electronic properties like dirac characteristic and an opening band gap can be observed. Structural properties of silicene such as buckling and interaction with the growing substrate have a direct impact on the electronic behaviour. Therefore, we focus on the chemically sensitive structural determination of silicene phases on different substrates, like Ag(111) and Au(111).
In 2015 the first realization of a silicene based field effect transistor, operating at room temperature succeeded. The synthesis is performed by creating a multi-layer system, containing silicene as a buried interface. For that reason we are using XPS- and XPD-measurements to reveal the structure of buried silicene.
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From planar to buckled silicene. The evolution from sp2 to sp3 hybridization of silicenes chemical bonds induces a structural change from a planar phase to a buckled phase. Often further electronic properties, like a band gap develop as well. A. Molle et al., Chem. Soc. Rev. 47, 6370 (2018). |
Silicene field-effect transistors operating at room temperature. A schematic of the fabrication process for silicene field-effect transistors. L. Tao et al., Nature Nanotech. 10, 227 (2015). |
Other representatives of the 2D materials are systems like self-assembled nanotubes, nanowires or nanoribbons. Those narrow strips of silicon were grown on different substrates like Ag(110) or Au(110). For application on other materials the nano-ribbons can be removed from the surface via STM tip.
Although silicon is a semiconductor, it reveals metallic behaviour in the one-dimensional limit. Silicon nano-ribbon systems have great potential for application in spintronic devices. As XPS and XPD methods are extremely surface sensitive, a detailed structural and chemical analysis is possible. Using these methods we investigate silicon nano-ribbons on Ag(110) [1] and on Au(110) [2].
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Silicon nano-ribbons on a Ag(110) surface form a regular buckled pentagonal structure. P. Espeter et al., Nanotechnology 28, 455701 (2017). |
On the way of preparing 2D materials an important role in surface science play surface alloy systems [3]. As in between states of clean surfaces and a well preparated 2D system, surface alloys likely reveal new periodic structures with different electronic properties.
[1] P. Espeter, Dissertation (2017)
[2] P. Roese, Dissertation (2017)
[3] N. Si et al., Nano Today 30, 100805 (2020).
Germanene is one of graphene's latest cousins and represents the next two-dimensional material in the carbon group of the periodic table. Because of its electronic structure germanene also belongs to the dirac materials where charge carriers behave like massless dirac fermions. Due to its stronger buckling and higher atomic number the material boasts a larger band gap and stronger spin-orbit coupling than silicene. This enables the applicability in spintronic technology, such as spin transistors, using the quantum spin hall effect.
One big issue is the preparation of free-standing germanene, chemically isolated from the substrate. We are using high energy resolution XPS to resolve interactions of germanene to the substrate as well as XPD to determine the surface and interface structure.
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Germanene on Au(111). Structure model of epitaxially grown germanium on Au(111). M. E. Dávila and G. Le Lay, Sci. Rep. 6, 20714 (2016). |
Dirac characteristic of germanene on Au(111). N. B. M. Schröter et al., 2D Mater. 4, 031005 (2017). |
As in the case of silicon and germanium, a two-dimensional graphene analogue can be found for tin, the next heaviest element of the carbon group, the so-called stanene.
Stanene was successfully prepared for the first time in 2015 by Zhu et al.
In contrast to atomically flat graphene, the heavier elements of the fourth main group of the periodic table show an increasingly larger buckling with increasing nuclear charge number Z [1].
Furthermore, the influence of spin-orbit coupling increases with Z. Thus, for stanene, non-trivial topologically protected states are expected even at room temperature.
Theoretical calculations also expect a band gap of 0.13 eV for stanene, making stanene a promising material for applications in spintronics [2].
The outstanding electronic properties of stanene vary significantly with its structure.
By varying the substrate as well as by chemical functionalization, different structured stanene phases of different electronic properties can be found.
To further understand stanenes structural properties, we are currently investigating tin on the Au(111) surface, resulting in numerous different stable reconstructions, depending on the heating temperature of the system after the epitaxial growth of the tin layer [3].
[1]: A. Molle et al., Nat. Mater. 16, 163 (2017).
[2]: C. C. Liu et al. Phys. Rev. B 84, 195430 (2011).
[3]: M. Maniraj et al., Commun. Phys. 2 (2019).
Many theoretical predictions were followed by the first experimental synthesis of single-layer (two-dimensional) lead, as the last group 14 element. In 2019, the so-called plumbene was realized and characterized as a surface of an Pd-Pb alloy by Yuhara et al. [2]. They discovered this unique lead structure epitaxially grown by segregation.
Like every two-dimensional counterpart of its bulk material, plumbene can offer many interesting and improved characteristics as well. It is predicted that plumbene has the largest band gap with 0.4 eV and the highest spin-orbit coupling from all group 14 elements due to its high mass [2]. In addition, its Quantum Hall effect at room temperature qualifies plumbene in general, like every element in this main group, to be an ideal candidate for topological insulators and future electronic devices.
[1]: J. Yuhara et al., Adv. Mater. 31 (2019)
[2]: D. K. Das et al., Comput. Mater. Sci. 151 (2018)
In addition to the carbon group, many elements of the 5th main group such as phosphorus, arsenic and antimony are also interesting substances for 2D materials.
Similar to the previously mentioned 2D materials, antimony also reconstructs in a buckled honeycomb structure, the so-called antimonene. However, the synthesis of flat antimonene is also possible as shown by Y. Shao et. al. in 2018 [1].
While bulk antimony belongs to the semi-metal, a single-layer of antimony shows characteristic properties of an indirect semiconductor with a band gap of 2.28eV. Due to the fact that indirect band gaps are less efficient, it is of special interest that antimonene changes into a direct semiconductor under biaxial strain. In addition, the band gap can be varied under strain, which makes antimonene a very interesting material for future optoelectronic applications [2].
[1]: Y. Shao et al., Nano Lett. 18, 2133-2139 (2018)
[2]: S. Zhang et al., Angew. Chem. 127, 3155-3158