All That Matters

It’s been only a week ago that I wrote about the increasing competition for graphene. But as I said then, there are still some exciting advances based on graphene. An example is photonics, which is an area where traditionally graphene perhaps has not been as strong as in electronics. A reason for this is that being only a single atomic layer thin, graphene initially wasn’t expected to show much interaction with light. One of the more intriguing historic results in this area has been the fact that the absorption of light in graphene is determined by one of nature’s most fundamental numbers, the fine structure constant.

Plasmons in graphene can be created by illuminating the tip of an atomic force microscope (grey) with an infrared laser beam (red). Reprinted by permission from Macmillan Publishers Ltd. Fei Z. et al. Nature 487, 82–85 (2012). doi:10.1038/nature11253

But absorption of light is not where the true potential of graphene lies, namely on the nanoscale. On the same scale as electronic applications, because ultimately the aim is to achieve photonic functionality on a chip.

However, the control of light on the nanoscale typically requires surface plasmons. These are collective movements of electrons at the surface of metals. So in a sense surface plasmons function a bit like antenna that can focus light into tiny spots. [...]

Move over graphene, there is competition in town. A new type of two-dimensional materials – with the far less appealing family name, transition metal dichalcogenides –  are increasingly gaining attention. Well, at least they’re giving it a shot. Graphene, a sheet of carbon atoms only one atomic layer thick, still has plenty going for itself in terms of electronic, optical and mechanical properties. There seems nothing that graphene can’t do.

On the other hand, there are also limits. When it comes to its electronic properties graphene is not a semiconductor in the same was as silicon is. It is lacking a bandgap, a gap in its electronic states that is important for light emitters and for some electronic devices.

Schematic model of transition metal dichalcogenide atomic layers. The yellow balls represent the chalcogenide atoms, the blue ones the transition metals. Reprinted by permission from Macmillan Publishers Ltd. Nature Nanotechnology (2012). doi:10.1038/nnano.2012.193

Transition metal dichalcogenides offer an advantage there. They are semiconductors, and they can have a bandgap. And as their name says, they are formed by a combination of chalcogens such as sulphur or selenium and transition metals such as molybdenum or tungsten. Typical examples are MoS₂ or MoSe₂. These materials have become such hot stuff now that their properties have been reviewed in this month’s issue of Nature Nanotechnology. And even though the field is still young, there is plenty to review. [...]

The study of materials is one of the major areas of science, with legions of researchers in physics, chemistry and materials science working on this topic. Condensed matter physics is one of the largest research areas in physics. Yet, it makes me often uneasy how the benefits of materials science are promoted. It is all too often about applications, and not about fundamental physics. How materials such as graphene might revolutionize electronics. And how new physical concepts could be used to develop materials for energy applications: solar cells, batteries and so on. In classical materials science it’s often about tougher materials, such as enhanced steels, and less about the fundamental insights they are based on. Of course, applications are an important aspect in the study of materials. But does this mean that too often fundamental insights are neglected in favour of a material’s commercial potential?

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Intel's 3D tri-gate transistors have a feature size of only 22 nm. The thin structures are the silicon channels, the thicker ones are the gates and the contacts. Several gates can be used next to each other to enhance the efficiency of the switching. (c) Intel

Finally I am getting around to blog about the latest generation of transistors that Intel presented earlier this months. These transistors reach feature sizes of only 22 nanometres, down from 32 nm. To give you some perspective what this amazingly high integration means: 4,000 of those 22 nm structures fit across the width of a human hair, or similarly, 100 million of these transistors fit on the head of a pin.

Now how did they reduce transistor length scales down by almost a third? Well, even though Intel (and others) is in the business of shrinking transistor for more than 40 years, this time it’s a bit more than a mere scaling exercise. For the first time we have a commercial 3D transistor design on such a scale. In a typical ‘field-effect’ transistor, two electrical contacts are used to run an electric current through a silicon layer. The transistor is switched between an electrically conducting and an insulating state by a gate on top of the silicon. The voltage applied to that gate determines whether current can flow or not. Thereby the gate is able to set the digital ’1′ and ’0′ in a transistor.

A problem in shrinking transistors has been the fact that those three electric contacts need a certain minimum space of their own. Furthermore, as the gate has become smaller and smaller, it has been increasingly inefficient to switch the electric current in the silicon layer underneath. For smaller gates the electric fields from the gate just don’t reach that far down into the silicon layer. [...]

The past year has been a great year for science with major advances in several areas. Too many exciting results to mention here. Instead, to reflect about the past year I have chosen a representative paper for each month of the year that I hope can serve as an example of the great science going on in a number of research fields. Of course, this is a highly subjective and personal collection, and indeed there might be others worth mentioning. But the aim was also to provide a balanced overview of the year that covers a variety of topics.

Of course, if you have an exciting paper to add, please feel free to use the comments section below to let us know!

Anyway, enough said, here are some of my highlights from the past year:

Simulations of electronic excitations in an iron-based superconductor. Image by Oak Ridge National Laboratory via flickr.

JANUARY – iron-based superconductors

Since they were discovered in 2008, iron-based superconductors, the pnictides, have been one of the hottest topics in condensed matter physics. Part of their appeal stems from the fact that they are based on iron, which is a magnetic element. Normally, magnets and superconductivity exclude each other.

The iron-based compounds have a similar crystal structure as the so-called cuprates, which are the materials with the highest superconducting temperatures known. The mechanism for these high-temperature superconductors is unknown, and studying the iron-based superconductors may also be relevant to the understanding of the cuprates.

This paper published in Science shows for the first time that the electrons in the iron-based superconductors show a periodic arrangement that is different to the periodicity of the atoms in the crystal. Similar observations have been made in the cuprates, and their understanding is considered important to the mechanism of high-temperature superconductivity.

Chuang, T., Allan, M., Lee, J., Xie, Y., Ni, N., Bud’ko, S., Boebinger, G., Canfield, P., & Davis, J. (2010). Nematic Electronic Structure in the “Parent” State of the Iron-Based Superconductor Ca(Fe1-xCox)2As2 Science, 327 (5962), 181-184 DOI: 10.1126/science.1181083

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Following this year’s Nobel prize in physics to Andre Geim and Konstantin Novoselov, the relevance of graphene hardly needs to be stated. Graphene-based devices have a real potential owing to the material’s unique electronic properties. If graphene, which is metallic, is cut into small pieces it becomes semiconducting and could be used as a transistor. The problem is however the edges of such small graphene devices. These perturb the operation of graphene transistors, and this is the reason one has to be cautious when it comes to immediate relevance for applications.

To figure out what exactly happens with atoms at the edges of graphene, Kazu Suenaga and Masanori Koshino from Japan’s National Institute of Advanced Industrial Science and Technology imaged and characterized the electronic properties of single atoms at the edge of graphene with a high-resolution scanning electron microscope. Their findings on how these atoms bond with each other are published this week in Nature.

When it comes to transistors that are smaller than anything that could be done with silicon, graphene is one of the materials of choice. As transistors shrink so much they consist of only a few atoms the electric currents that atomic bonds have to carry can become huge. Only a few materials can sustain this and graphene would be perfect for it. “The bonds between the carbon atoms in graphene are very strong and can carry exceptionally high currents,” Andre Geim told me once when researching a feature on future computing technologies. Moreover, electrons can travel through graphene for long distances, easily comparable to the distance between the source and drain electrical contacts of a transistor. “Your electrons would move between source and drain without scattering,” says Geim.

Characterising individual atoms of graphene. The atoms depicted in red, blue and green colours represent atoms at different positions on a graphene sheet. Their energy loss spectra mirror their different properties. Reprinted by permission from Nature doi: 10.1038/nature09664 (2010).

While all this is true for the centre of graphene sheets, the edges are a different matter. There, electrons scatter and all these nice benefits of graphene are diminished. And the smaller the transistors get the more edges there are in relation to the rest of the surface.

The atoms at the edges of graphene have of course been imaged many times before. What the researchers have now achieved is that they are also able to measure their energy absorption. This leaves a spectral fingerprint on how these atoms are bonded to their neighbours, depending on their position in the atomic structure. The identification of novel electronic states is one of the key findings of their study according to Suenaga. “No one else has ever seen the peaks we report in this work.”

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