孩子胆小>大年三十是几号
The ri of graphene
A. K. Geim1 & K. S. Novolov1
网上社保Abstract
Graphene is a rapidly rising star on the horizon of materials science and condend-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discusd here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condend-matter physics, where quantum relativistic phenomena, some of which are unobrvable in high-energy physics, can now be mimicked and tested in table-t
op experiments. More generally, graphene reprents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never cead to surpri and continues to provide a fertile ground for applications.
形意拳五行拳
Introduction
Graphene is the name given to a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities (Fig. 1). It can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite. Theoretically, graphene (or '2D graphite') has been studied for sixty years1,2,3, and is widely ud for describing properties of various carbon-bad materials. Forty years later, it was realized that graphene also provides an excellent condend-matter analogue of (2+1)-dimensional quantum electrodynamics
4,5,6, which propelled graphene into a thriving theoretical toy model. On the other hand, although known as an integral part of 3D materials, graphene was presumed not to exist in the free state, being described as an 'academic' material5 and was believed to be unstable with respect to the formation of curved structures such as soot, fullerenes and nanotubes. Suddenly, the vintage model turned into reality, when free-standing graphene was unexpectedly found three years ago7,8 — and especially when the follow-up experiments9,10 confirmed that its charge carriers were indeed massless Dirac fermions. So, the graphene 'gold rush' has begun.
Figure 1: Mother of all graphitic forms.
职教论文Graphene is a 2D building material for carbon materials of all other dimensionalities. It ca
n be wrapped up into 0D buckyballs, rolled into 1D nanotubes or stacked into 3D graphite.
老师介绍
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幼教中心
Materials that should not exist
More than 70 years ago, Landau and Peierls argued that strictly 2D crystals were thermodynamically unstable and could not exist11,12. Their theory pointed out that a divergent contribution of thermal fluctuations in low-dimensional crystal lattices should lead to such displacements of atoms that they become comparable to interatomic distances at any finite temperature13. The argument was later extended by Mermin14 and is strongly supported by an omnibus of experimental obrvations. Indeed, the meltin
g temperature of thin films rapidly decreas with decreasing thickness, and the films become unstable (gregate into islands or decompo) at a thickness of, typically, dozens of atomic layers15,16. For this reason, atomic monolayers have so far been known only as an integral part of larger 3D structures, usually grown epitaxially on top of monocrystals with matching crystal lattices15,16. Without such a 3D ba, 2D materials were presumed not to exist, until 2004, when the common wisdom was flaunted by the experimental discovery of graphene7 and other free-standing 2D atomic crystals (for example, single-layer boron nitride and half-layer BSCCO)8. The crystals could be obtained on top of non-crystalline substrates8,9,10, in liquid suspension7,17 and as suspended membranes18.
Importantly, the 2D crystals were found not only to be continuous but to exhibit high crystal quality7,8,9,10,17,18. The latter is most obvious for the ca of graphene, in which charge carriers can travel thousands of interatomic distances without scattering7,8,9,10. With the benefit of hindsight, the existence of such one-atom-thick crystals can be reconciled with theory. Indeed, it can be argued that the obtained 2D crystallites are quenched in a metastable state becau they are extracted from 3D materials, whereas their small size (<<1 mm) and strong interatomic bonds ensure that thermal fluctuations cannot lead to the generation of dislocations or other crystal defects even at elevated temperature13,14. A complementary viewpoint is that the extracted 2D crystals become intrinsically stable by gentle crumpling in the third dimension18,19 (for an artist's impression of the crumpling, e the cover of this issue). Such 3D warping (obrved on a lateral scale of 10 nm)18 leads to a gain in elastic energy but suppress thermal vibrations (anomalously large in 2D), which above a certain temperature can minimize the total free energy19.
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Brief history of graphene
Before reviewing the earlier work on graphene, it is uful to define what 2D crystals are. Obviously, a single atomic plane is a 2D crystal, whereas 100 layers should be considered as a thin film of a 3D material. But how many layers are needed before the structure is regarded as 3D? For the ca of graphene, the situation has recently become reasonably clear. It was shown that the electronic structure rapidly evolves with the number of layers, approaching the 3D limit of graphite at 10 layers20. Moreover, only graphene and, to a good approximation, its bilayer has simple electronic spectra: they are both zero-gap miconductors (they can also be referred to as zero-overlap mimetals) with one type of electron and one type of hole. For three or more layers, the spectra become increasingly complicated: Several charge carriers appear7,21, and the conduction and valence bands start notably overlapping7,20. This allows single-, double- and few- (3 to <10) layer graphene to be distinguished as three different types of 2D crystals ('graphenes'). Thicker structures should be considered, to all intents and purpos, as thin films of graphite. From the experimental point of view, such a definition is also nsible. The screening length in graphite is only 5 Å (that is, less than two layer
s in thickness)21k线图咋看 and, hence, one must differentiate between the surface and the bulk even for films as thin as five layers21,22.
