When the dimensions of conventional solid shrink down, there is so-called high surface-to-volume ratio that can roughly portray the properties' change. From a physical point of view, as dimensionality changing, the solid will experience some 'critical transitions' corresponding to a variety of phenomenon-dependent characteristic lengths. Confessedly, nanowires, nanotubes and nanoparticles, along with porous or dendrite materials hold this line.

This surface-to-volume ratio can be infinite. Three typical structures possess this limit. One such entity is carbon-60 molecular (C60). The other one is single-wall carbon nanotube (CNT). Obviously, the last expected is single sheet of the atomic lattice. Thereby, stimulated by the huge potentials C60 and CNT, and considering that natural graphite has a layered structure like book where the carbon atoms all lie in the sheets stacked via Van der Waals forces, we really hope to extract a single piece — Graphene, for all expectations. However, it's somewhat of a surprise that we just, strictly saying, almost got it last year, because what we had is a corrugated sheet but not a flat one, reported Jannik Meyer, Andre Geim & the colleagues in 1 March 2007 issue of Nature [ 1 ].

Andre Geim
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Graphene 2007 

Bright-field TEM image of a suspended graphene membrane. Its central part (bluish) is monolayer graphene. Electron diffraction images from different areas of the flake show that it is a single crystal without domains. The typical distance between the golden bars is 500 nm.

Graphene 2007

Credit: Lin PU, Scidea Art 2007. Source: ScideaNews.com

At the first glance, it seems easy to get graphene. Unfortunately, long before it was made, two of our physics teachers, Peierls and Landau had pointed out that this kind of sheet, strictly two-dimensional crystal, could not exist naturally. It's because the thermal fluctuations should shock the lattice with amplitude as comparable as the interatomic distance, which unavoidably destroy long-range order, resulting melting of the lattice into clusters at any finite temperature [ 2 ]. But it's fortunate that, so often the laboratory fellows are incorrigible like Don Quixote, and finally, they bypassed, but not tunneled through, the Peierls-Landau barrier with an astounding free-standing shown by an individual graphene sheet [ 1 ].

Generally, enlightening with stable solid surfaces, one conceptual fabrication of graphene comes as a film-on-substrate geometry via standard heteroepitaxy in chemical vapor deposition (CVD) or molecular-beam epitaxy (MBE), and then the thing to do is grow film as thin as possible. But it's well known that the film from this technique prefers a thick growth condition rather than a very thin one. Because thin film will collapse and condense to particles ascribed to surface tension, thermal and lattice mismatch in heteroepitaxy, and without question, the above-mentioned Peierls-Landau barrier. So it's not surprise why we had to fight on paper for so many years.

Kostya Novoselov 
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Graphene 2004 

(a) Scanning electron micrograph of a fallen mesa of graphite. This is the way graphene molecules were "extracted" from bulk graphite. To be reasonably visible in SEM, the authors show a 10 nm carbon flake (30 layer thick). 

(b) Graphene nanofabric. SEM micrograph of a strongly crumpled graphene sheet on a Si wafer. Note that it looks just like silk thrown over a surface. Lateral size of the image is 20 microns. Si wafer is at the bottom-right corner.

Until 2004, the dawn broke with a pioneer fabrication of the graphene-on-SiO2, by Novoselov, Geim and colleagues from Russia and United Kingdom, reported in the 22 October 2004 issue of Science [ 3 ]. By repeated peeling of small mesas of highly oriented pyrolytic graphite, they can obtain the few-, double- or single-layer graphene that supported by the substrate, a 300 nm SiO2 film on n⁺-doped Si wafer. The method was found to be highly reliable to prepare few-layer graphene up to 10 micrometers in size.

The work was continuing. Using a standard microelectronics procedure, they deposited a ladder-like metal grid (3 nm Cr and 100 nm Au) on top of the few- or single-layer graphene-on-SiO2, and then carefully removed of the substrate via chemical etching. As expected, they got it [ 1 ]. The existence of the monolayer graphene was proved through the diffraction analysis (using a transmission electron microscopy, TEM). Because, "the key for the identification of monolayer graphene is that its reciprocal space has only the zero-order Laue zone and, therefore, no dimming of the diffraction peaks should occur at any angle, in contrast to the behaviour of crystal lattices extended in the third dimension. This is exactly the behaviour we observed experimentally", said the authors.

However, the monolayer graphene suspended on the metal grid is not flat but corrugated. These out-of-plane corrugations in the third dimension (observed on a lateral scale of ≈10 nm), which involving a significant elastic strain, can intrinsically provide the thermodynamic stability of the film without the aid of the bond defects suggested by previous theory, the authors deduced. Although further experimental and theoretical studies are deserved to figure out the detailed mechanism of the stability, I intuitively believe that the authors' explanation should be just correct. There are some daily-life samples with similar stability, for example, in corrugated abatis or copper net, or a piece of paper, where the "frozen" stain field across the whole surface is the major contribution for stably "standing" (Figure).

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Hold soft paper with strain

Credit: Scidea Art 2007. Source: ScideaNews.com

What does the monolayer graphene mean? Just how stirring is its prospects? In fact, it's beyond expression. Graphene nanostructures are stable down to true nanometer sizes, and possibly even down to a single benzene ring. This allows the exploration of  a region somewhere in between single-electron-transistor and molecular electronics [ 4 ]. The physics and possible applications are surely exciting, and sometimes, so unexpected.

For example, it has been confirmed that this zero-gap semiconductor can allow ballistic transport on a submicrometer scale at 300 K, namely a SET-like transistor operational at room temperature (SET: single-electron-transistor), whereas resistive (rather than traditional tunnel) barriers can be used to induce Coulomb blockade [ 4 ]. 

Another brilliant work is the graphene lens for electrons where a p-n junction in graphene can focus electrons with high precision (negative refraction) [ 5 ], just as light is focused in Veselago's lens that is capable of sub-wavelength resolution for photons [ 6 ]. But then, the present graphene lens has no complete analogy with Veselago's lens, because it is perfectly transmitting only when electrons strike the surface exactly at a perpendicular angle, but otherwise not so. However, because electron wavelengths are generally very small, sub-wavelength resolution is less of an issue than it is for photons, comments J. B. Pendry [ 7 ].

The topic has recently become so hot. Someone may think that the graphene, probably allowing all the promised applications to reach an industrial stage, should compete with CNTs in the myriad of potentials suggested, and could make nanotubes obsolete. But it should be noted that the "dimensionality is one of the most defining material parameters", so two-dimensional graphene or one-dimensional carbon nanotube possesses their own unique fingerprints, that is, "graphene's prospects can be sometimes superior, sometimes inferior, and most often completely different from those of carbon nanotubes", Geim and Novoselov fair pointed out [ 4 ].

Beyond all doubt, the shining stones of graphene-based electronics will soon come one after another. It's so lucky that we can sometimes dance with some simple systems like graphene. Yes, it's simple but with so so wonderful physics. It indeed deserves the gold medal.□  

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Graphene from Research group of Prof. Andre Geim at The University of Manchester, UK. 

References    

Citation  

L. Pu 

Lin PU, Graphene stands up with frozen waves. Scidea Sketch 1(3), ss20070301a1 (2007). 

□ doi:: 10.3128/ ss20070301a1 | Scidea ::  Abs . Full | CrossRef
□ Scidea Sketch:: ISSN: 1992 - 8548