10 key papers on: Graphene

1 Electric field effect in atomically thin sheets by Novoselov et al.

This was the paper that started the graphene field. It details how the now-famous sticky-tape method was used to isolate carbon films from graphite that had thicknesses down to a few atomic layers. Layers this thin were not thought to be thermodynamically stable, and showing that these films existed was a revelation that kick-started the field of graphene research. Notice how the title uses the term atomically thin carbon films, as the word graphene was not yet as famous as it would become.

Further to isolating the films, the authors also performed electrical measurements on the films. These showed that the films had very high charge-carrier concentrations (there are lots of electrons (holes) available to move charge) and very high charge-carrier mobilities (and they can move very fast). These measurements were the first inkling of graphene’s superb electrical performance. The authors could not confidently say they had isolated single-atom-thin sheets explicitly, but they did note that as the carbon films got thinner, their electrical performance got closer to what would be expected for a single layer of graphene. In an endnote, however, they did say that they thought the thinnest layers were probably monolayers, but impurities on the samples disturbed the measurements. Again this was the first suggestion that graphene’s exciting electronic structure was measurable.

Read more: https://arxiv.org/pdf/cond-mat/0410550.pdf

2 Two-dimensional atomic crystals by Novoselov et al.

This paper followed from Manchester a year later, and within, the power of mechanical exfoliation (the sticky-tape method) was revealed. Here they didn’t just look at carbon layers from graphite, but they managed to repeat the procedure with other layered materials like MoS2 and NbSe2. They also showed how transmission electron microscopy could be used to count the edges of the films, using an example of a double layer of MoS2. Further, atomic force microscopy showed steps on the surface of the material that are the same as the distance between layers in a thick crystal. Overall, this paper showed directly how mechanical exfoliation could be used to separate films of layered materials and how direct imaging tools could show the number of layers in the films.

Read more: https://arxiv.org/pdf/cond-mat/0503533.pdf

3 Two-dimensional gas of massless Dirac fermions in graphene by Novoselov et al.

While the first paper on this list suggested some of graphene’s exciting electrical possibilities, it was this paper that outlined them in detail. The key part of this is to do with what equations from quantum mechanics can be used to describe the behaviour of charge carriers in a material. This understanding is important because the application of quantum mechanics to charge carriers in materials is what led to the use of semiconductors in transistors, which then caused the staggering rise in technology over recent decades. The exciting thing about graphene is that the equations we use to describe its charge carriers are different from those normally used. This paper shows how they are best described as relativistic particles with zero rest mass, which is extremely unusual for materials. It is this property that makes graphene such an excellent conductor. This result is not just exciting for graphene’s prospects as a new material, but also because it demonstrated the possibility of examining quantum physics on a small-laboratory scale.

Read more: self-archived version, Nature

4 The rise of graphene by Novoselov and Geim

This is a brief review of the progress in graphene research in the three years since it had been first isolated. It is from the perspective of Kostya Novoselov and Andre Geim (also authors of the previous three papers), who would go on to win the Nobel Prize in Physics in 2010 for the isolation of graphene. This gives this paper a unique perspective on where they think the field of graphene research is headed. The understanding of graphene’s properties had been developed and some of the possible application routes had been explored. Despite the progress, as they say, experimental work was yet to catch up with theory as new techniques and handling processes for the new material were still in their early days. This caused them to dampen some of the hype that was surrounding graphene at the time, reminding us that electronic devices were still 20 years away. The final paragraph claims we have only uncovered “the very tip of the iceberg” and that graphene is not a fleeting fad, a view that has been confirmed over the last decade.

Read more: Nature Materials

5 Fine structure constant defines visual transparency of graphene by Nair et al.

One of graphene’s exciting properties is its transparency, because materials that are strong, conductive, and transparent are rare. They are, however, important, as they are essential in technologies like touch-screens and solar cells. This paper published how much light a layer of graphene absorbed (2.3%), and how each extra layer absorbed a further 2.3%. They also explained the origin of this absorption, which again lies in graphene’s unique electronic structure.

Read more: self-archived version, Science

6 Measurement of the elastic properties and intrinsic strength of monolayer graphene by Lee et al.

This paper was the first to measure the mechanical strength of a monolayer of graphene, and they found that it did have the extremely high strength that was theoretically predicted. To do this, they started by suspending monolayer graphene sheets over holes cut in silicon. Then they used an atomic force microscope to press a sharp tip on to the sheet. They used this to measure how the sheet bends while being pressed, and also at what point the sheet broke. Graphene turned out to be the strongest material ever measured, which is outstanding considering it is only a single atom thick.

Read more: self-archived version, Science

7 The electronic properties of graphene by Castro Neto et al.

This review article lays out the theory behind the electronic structure of graphene, which had now been carefully explored by both theorists and experimentalists. The first part gives a concise review on the work leading up to this paper, with a focus on the electronic properties. It is the second part that introduces the basics of the theory of graphene’s electronic properties, including the effect of reducing the size of the graphene sheet (like in nanoribbons) and the effect of magnetism. Later parts look at deviations from the pristine graphene system, with the introduction of disorder.

Read more: https://arxiv.org/pdf/0709.1163v2.pdf

8 Large-area synthesis of high-quality and uniform graphene films on copper foils by Li et al.

One significant challenge with graphene technology is its production. The original exfoliation method can yield perfect graphene, but only in tiny sheets; another route, chemical production methods, can yield substantial volumes of large sheets, but the graphene is often defective. This paper introduced the chemical vapour deposition (CVD) method, which is currently the most technologically viable route to large-area, defect-free graphene. Since this paper, the method has been extended to making sheets of graphene 30-inches across, and even to producing them in a continuous roll-to-roll process.

Read more: https://arxiv.org/pdf/0905.1712v2.pdf

9 Graphene: status and prospects by Geim

Since the rush of results over the 5 years after its first isolation, graphene has attracted significant attention. This is mainly motivated by its single atom thickness, and its long list of superlative properties. It is these properties that are discussed in this accessible review, written by Andre Geim. An update on production, applications and basic science is presented. However, it is the focus on the prospects, especially from this perspective, that makes this article the most interesting. Speculating about all the possible applications and unexplored research areas makes this article an exciting read.

Read more: https://arxiv.org/pdf/0906.3799v1.pdf

10 Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems by Ferrari et al.

If you are interested to find out how far away graphene technologies are, this review will help. It has been compiled by over 60 authors from the Graphene Flagship – the 1 billion euro research effort, coordinated across Europe, into graphene’s (and related materials) properties and applications. The review aims to give an update on the progress in each of the Flagship’s areas, which include production, electronics, sensors, and biomedical applications. There are outlines of the basic science underlying each application, as well as examples of the technologies that are available. With over 2300 references cited, it is an extremely useful roadmap for the progress in graphene research.

Read more: Open Access from Nanoscale

VOPc on Graphene

The majority of the electronics that we use are made from inorganic materials that are often hard to recycle, include dangerous elements like lead, and sometimes include rare elements that are expensive. This combines to increase both the financial and environmental cost of the vast electronics industry.

One alternative is to use organic molecules. These are molecules that are mostly made from carbon, and, in contrast to inorganics, they are often cheap, disposable, and easy to process. Organic molecules, like one named vanadyl phthalocyanine (or VOPc), could be used to make cheaper, more environmentally friendly solar cells and transistors. But the best attempts of using organic materials have yet to surpass their inorganic counterparts. The performance of organics lies in how the molecules stick together to form semiconducting crystals.

In this paper, we looked at the growth of VOPc crystals on graphene. Graphene is chosen as a growth substrate for two key reasons. One, is that it is an excellent conductor that is almost completely transparent to visible light, and so could be used in solar cells and LEDs. The second reason is that it is also virtually transparent to electrons. This allows us to deposit VOPc onto freely suspended graphene, and use the latest transmission electron microscopes to study how the VOPc crystals grow. This is something you cannot do with traditional substrates like glass.

diagram

We made three key findings. The first is that heating the graphene substrate during the VOPc deposition causes large crystals to grow. This is likely because molecules move around faster on a hot substrate and so are more likely to find and stick to an already-existing VOPc island than to make their own. This allows crystals to grow larger, and larger crystals generally perform better than smaller ones in devices.

VOPc_AFM

Atomic force microscopy (AFM) from graphene on copper after VOPc deposition at a) room temperature, b) 75 °C, c) 125 °C, d) 155 °C, and e) 175 °C. The graph in f) shows how the islands get bigger but the coverage drops as the graphene gets hotter during deposition.

The second finding is that the VOPc crystals do not necessarily align with the graphene crystal structure underneath. This lack of epitaxy (a term used to describe how one layer can influence the orientation of the layer above) is quite important, as epitaxy is often used to increase crystal size. These results show, then, that it is not essential to rely on epitaxy to grow large crystals.

The third result is that the large-crystal behaviour seen for graphene is not seen in graphene oxide (GO). GO is a much cheaper, solution-based form of graphene that is easier to process, and so is also being carefully considered for large-scale applications. Its structure is a graphene sheet with oxygen groups attached, and it is these oxygen groups that disrupt the large crystal growth: the VOPc molecules are more likely to nucleate at these groups than reach the already-existing islands.

Overall, the results show that graphene could be an ideal substrate for organic molecular crystal growth because it enables large crystal growth, without the need to rely on epitaxy. However, the graphene should be clean and flat; any disruption (like the oxygen groups in graphene oxide) can hinder the growth.

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