Germanene: a novel two-dimensional germanium allotrope akin to graphene and silicene
TL;DR: In this article, an atom-thin, ordered, two-dimensional multi-phase film was grown in situ through germanium molecular beam epitaxy using a gold surface as a substrate.
Abstract: We have grown an atom-thin, ordered, two-dimensional multi-phase film in situ through germanium molecular beam epitaxy using a gold (111) surface as a substrate. Its growth is similar to the formation of silicene layers on silver (111) templates. One of the phases, forming large domains, as observed in scanning tunneling microscopy, shows a clear, nearly flat, honeycomb structure. Thanks to thorough synchrotron radiation core-level spectroscopy measurements and advanced density functional theory calculations we can identify it as a ?3????3 R(30?) germanene layer in conjunction with a ?7????7 R(19.1?) Au(111) supercell, presenting compelling evidence of the synthesis of the germanium-based cousin of graphene on gold.
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TL;DR: More than twenty 2D carbides, nitrides and carbonitrides of transition metals (MXenes) have been synthesized and studied, and dozens more predicted to exist.
Abstract: The family of 2D transition metal carbides, carbonitrides and nitrides (collectively referred to as MXenes) has expanded rapidly since the discovery of Ti3C2 in 2011. The materials reported so far always have surface terminations, such as hydroxyl, oxygen or fluorine, which impart hydrophilicity to their surfaces. About 20 different MXenes have been synthesized, and the structures and properties of dozens more have been theoretically predicted. The availability of solid solutions, the control of surface terminations and a recent discovery of multi-transition-metal layered MXenes offer the potential for synthesis of many new structures. The versatile chemistry of MXenes allows the tuning of properties for applications including energy storage, electromagnetic interference shielding, reinforcement for composites, water purification, gas- and biosensors, lubrication, and photo-, electro- and chemical catalysis. Attractive electronic, optical, plasmonic and thermoelectric properties have also been shown. In this Review, we present the synthesis, structure and properties of MXenes, as well as their energy storage and related applications, and an outlook for future research. More than twenty 2D carbides, nitrides and carbonitrides of transition metals (MXenes) have been synthesized and studied, and dozens more predicted to exist. Highly electrically conductive MXenes show promise in electrical energy storage, electromagnetic interference shielding, electrocatalysis, plasmonics and other applications.
4,745 citations
University of Cambridge1, Istituto Italiano di Tecnologia2, Lancaster University3, University of Manchester4, Catalan Institution for Research and Advanced Studies5, Technical University of Denmark6, Nokia7, fondazione bruno kessler8, Queen Mary University of London9, University of Trento10, Technische Universität München11, Polytechnic University of Milan12, Centre national de la recherche scientifique13, University of Trieste14, University of Ioannina15, University of Geneva16, Trinity College, Dublin17, Texas Instruments18, University of Paris19, Spanish National Research Council20, Leiden University21, Delft University of Technology22, University of Patras23, École Normale Supérieure24, Radboud University Nijmegen25, Nest Labs26, Airbus UK27, Seoul National University28, Yonsei University29, University of Oxford30, Chalmers University of Technology31, University of Groningen32, STMicroelectronics33, Chemnitz University of Technology34, Max Planck Society35, Aalto University36
TL;DR: An overview of the key aspects of graphene and related materials, ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries are provided.
Abstract: We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.
2,560 citations
Pennsylvania State University1, Rensselaer Polytechnic Institute2, Oak Ridge National Laboratory3, University of Central Florida4, Yale University5, Purdue University6, Carnegie Mellon University7, Massachusetts Institute of Technology8, University of California, Berkeley9, Lawrence Berkeley National Laboratory10, Rice University11, Chinese Academy of Sciences12, University of Texas at Austin13, University of California, Santa Barbara14
TL;DR: Insight is provided into the theoretical modeling and understanding of the van der Waals forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies.
Abstract: The isolation of graphene in 2004 from graphite was a defining moment for the “birth” of a field: two-dimensional (2D) materials In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement Here, we review significant recent advances and important new developments in 2D materials “beyond graphene” We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (ie, silicene, phosphorene, etc) and transition metal carbide- and carbon nitride-based MXenes We then discuss the doping and functionalization of 2
2,036 citations
TL;DR: In this paper, the authors reported the successful fabrication of 2D stanene by molecular beam epitaxy, confirmed by atomic and electronic characterization using scanning tunnelling microscopy and angle-resolved photoemission spectroscopy, in combination with first-principles calculations.
Abstract: Following the first experimental realization of graphene, other ultrathin materials with unprecedented electronic properties have been explored, with particular attention given to the heavy group-IV elements Si, Ge and Sn. Two-dimensional buckled Si-based silicene has been recently realized by molecular beam epitaxy growth, whereas Ge-based germanene was obtained by molecular beam epitaxy and mechanical exfoliation. However, the synthesis of Sn-based stanene has proved challenging so far. Here, we report the successful fabrication of 2D stanene by molecular beam epitaxy, confirmed by atomic and electronic characterization using scanning tunnelling microscopy and angle-resolved photoemission spectroscopy, in combination with first-principles calculations. The synthesis of stanene and its derivatives will stimulate further experimental investigation of their theoretically predicted properties, such as a 2D topological insulating behaviour with a very large bandgap, and the capability to support enhanced thermoelectric performance, topological superconductivity and the near-room-temperature quantum anomalous Hall effect.
1,187 citations
Iran University of Medical Sciences1, Sharif University of Technology2, Research Institute of Petroleum Industry3, Shahid Beheshti University4, University of Kerala5, Kharazmi University6, Islamic Azad University7, Tehran University of Medical Sciences8, Boston University9, Harvard University10, University of Tehran11, Massachusetts Institute of Technology12
TL;DR: This review highlights the recent advances of smart MNPs categorized according to their activation stimulus (physical, chemical, or biological) and looks forward to future pharmaceutical applications.
Abstract: New achievements in the realm of nanoscience and innovative techniques of nanomedicine have moved micro/nanoparticles (MNPs) to the point of becoming actually useful for practical applications in the near future. Various differences between the extracellular and intracellular environments of cancerous and normal cells and the particular characteristics of tumors such as physicochemical properties, neovasculature, elasticity, surface electrical charge, and pH have motivated the design and fabrication of inventive “smart” MNPs for stimulus-responsive controlled drug release. These novel MNPs can be tailored to be responsive to pH variations, redox potential, enzymatic activation, thermal gradients, magnetic fields, light, and ultrasound (US), or can even be responsive to dual or multi-combinations of different stimuli. This unparalleled capability has increased their importance as site-specific controlled drug delivery systems (DDSs) and has encouraged their rapid development in recent years. An in-depth understanding of the underlying mechanisms of these DDS approaches is expected to further contribute to this groundbreaking field of nanomedicine. Smart nanocarriers in the form of MNPs that can be triggered by internal or external stimulus are summarized and discussed in the present review, including pH-sensitive peptides and polymers, redox-responsive micelles and nanogels, thermo- or magnetic-responsive nanoparticles (NPs), mechanical- or electrical-responsive MNPs, light or ultrasound-sensitive particles, and multi-responsive MNPs including dual stimuli-sensitive nanosheets of graphene. This review highlights the recent advances of smart MNPs categorized according to their activation stimulus (physical, chemical, or biological) and looks forward to future pharmaceutical applications.
1,072 citations
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TL;DR: In this paper, first-principles calculations of structure optimization, phonon modes, and finite temperature molecular dynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, honeycomb structures.
Abstract: First-principles calculations of structure optimization, phonon modes, and finite temperature molecular dynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, these puckered structures are ambipolar and their charge carriers can behave like a massless Dirac fermion due to their pi and pi(*) bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.
2,624 citations
Journal Article•
TL;DR: First-principles calculations of structure optimization, phonon modes, and finite temperature molecular dynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, honeycomb structures, which show remarkable electronic and magnetic properties, which are size and orientation dependent.
Abstract: Department of Physics, Bilkent University, Ankara 06800, Turkey(Received 27 November 2008; published 12 June 2009)First-principles calculations of structure optimization, phonon modes, and finite temperature moleculardynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, honeycombstructures. Similar to graphene, these puckered structures are ambipolar and their charge carriers canbehave like a massless Dirac fermion due to their and bands which are crossed linearly at the Fermilevel. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Geshow remarkable electronic and magnetic properties, which are size and orientation dependent. Theseproperties offer interesting alternatives for the engineering of diverse nanodevices.
2,002 citations
TL;DR: It is demonstrated that silicene with topologically nontrivial electronic structures can realize the quantum spin Hall effect (QSHE) by exploiting adiabatic continuity and the direct calculation of the Z(2) topological invariant.
Abstract: We investigate the spin-orbit opened energy gap and the band topology in recently synthesized silicene as well as two-dimensional low-buckled honeycomb structures of germanium using first-principles calculations. We demonstrate that silicene with topologically nontrivial electronic structures can realize the quantum spin Hall effect (QSHE) by exploiting adiabatic continuity and the direct calculation of the Z(2) topological invariant. We predict that the QSHE can be observed in an experimentally accessible low temperature regime in silicene with the spin-orbit band gap of 1.55 meV, much higher than that of graphene. Furthermore, we find that the gap will increase to 2.9 meV under certain pressure strain. Finally, we also study germanium with a similar low-buckled stable structure, and predict that spin-orbit coupling opens a band gap of 23.9 meV, much higher than the liquid nitrogen temperature.
1,940 citations
TL;DR: It is shown that two-dimensional, epitaxial silicene forms through surface segregation on zirconium diboride thin films grown on Si wafers and that the buckling and thus the electronic properties of silicenes are modified by epitaxials strain.
Abstract: As the Si counterpart of graphene, silicene may be defined as an at least partially $s{p}^{2}$-hybridized, atom-thick honeycomb layer of Si that possesses $\ensuremath{\pi}$-electronic bands. Here we show that two-dimensional, epitaxial silicene forms through surface segregation on zirconium diboride thin films grown on Si wafers. A particular buckling of silicene induced by the epitaxial relationship with the diboride surface leads to a direct $\ensuremath{\pi}$-electronic band gap at the $\ensuremath{\Gamma}$ point. These results demonstrate that the buckling and thus the electronic properties of silicene are modified by epitaxial strain.
1,459 citations
TL;DR: In this article, the authors derived the low energy effective Hamiltonian involving spin-orbit coupling (SOC) for silicene, which is the analog to the graphene quantum spin Hall effect (QSHE) Hamiltonian.
Abstract: Starting from symmetry considerations and the tight-binding method in combination with first-principles calculation, we systematically derive the low-energy effective Hamiltonian involving spin-orbit coupling (SOC) for silicene. This Hamiltonian is very general because it applies not only to silicene itself but also to the low-buckled counterparts of graphene for the other group-IVA elements Ge and Sn, as well as to graphene when the structure returns to the planar geometry. The effective Hamitonian is the analog to the graphene quantum spin Hall effect (QSHE) Hamiltonian. As in the graphene model, the effective SOC in low-buckled geometry opens a gap at the Dirac points and establishes the QSHE. The effective SOC actually contains the first order in the atomic intrinsic SOC strength ${\ensuremath{\xi}}_{0}$, while this leading-order contribution of SOC vanishes in the planar structure. Therefore, silicene, as well as the low-buckled counterparts of graphene for the other group-IVA elements Ge and Sn, has a much larger gap opened by the effective SOC at the Dirac points than graphene, due to the low-buckled geometry and larger atomic intrinsic SOC strength. Further, the more buckled is the structure, the greater is the gap. Therefore, the QSHE can be observed in low-buckled Si, Ge, and Sn systems in an experimentally accessible temperature regime. In addition, the Rashba SOC in silicene is intrinsic due to its own low-buckled geometry, which vanishes at the Dirac point $K$, while it has a nonzero value with deviation of $\stackrel{P\vec}{k}$ from the $K$ point. Therefore, the QSHE in silicene is robust against the intrinsic Rashba SOC.
1,107 citations