Carbon nanotubes and quantum dots, the subjects of the first Kavli Prize in Nanoscience ten years ago, are among the best examples of what nanoscience and nanotechnology are all about
By Fabio Pulizzi - Chief Editor, Nature Nanotechnology
It has already been ten years. The first Kavli Prizes, in astrophysics, nanoscience and neuroscience were awarded in 2008. Sumio Iijima and Louis Brus shared the award in nanoscience for their work on carbon nanotubes and semiconductor nanocrystals, also known as colloidal quantum dots, respectively. The choice could not have been more appropriate, especially considering how those two materials systems had dominated research and indeed scientific literature from the 1990s.
But it’s more than that. Reflecting on the award after a decade, one aspect that comes to mind is that these two types of structure represent clearly the essential aspects of nano – a word that I use here to refer to both nanoscience and nanotechnology. First of all, the properties of a material change when the dimensions are reduced to the nanoscale, for example because quantum physics effects become obvious or because the properties of the material are dominated by the surface rather than by bulk.
Iijima reported the observation of carbon nanotubes in a paper in 1991 [ 1 ]. Inspired by the previous discovery of fullerenes, he was exploring the fabrication of carbon nanostructures through a technique known as arc discharge, which consists in the application of high voltage between two graphite electrodes and the consequent release of small structures. He discovered these very small needles that once examined at the electron microscope revealed a tubular shape and a graphitic structure: carbon nanotubes.
The 1991 paper stimulated intense activity. Nanotubes were demonstrated to exhibit excellent electronic transport and mechanical properties. During the years they have been used as a platform to study quantum transport phenonema and to realize proof of concept devices like single nanotube transistors and electromechanical resonators, or even for potential applications in biomedicine.
CdSe quantum dots of different sizes, illuminated by UV light. In courtesy of Khosro Zangeneh Kamali / Alamy Stock Photo
Brus’s work preceded that by Iijima by a few years. In 1984 he published a paper on the light emission of small colloidal suspensions of semiconductors [ 2 ]. More precisely, the colour of the emission under UV illumination varied with the size of the semiconductor nanoparticles. The report was very soon confirmed by research performed by Alexei Ekimov and Alexander Efros who were working on small semiconductor particles in glasses and also observed a variation of emission colour with size [ 3 ]. Such effect was due to a change in the forbidden energy gap of the material – arguably the most defining property of a semiconductor- and was a direct manifestation of quantum mechanics.
Quantum mechanics does not just determine the size dependence. In a quantum dot the energy levels of electrons and holes - the charged particles that dominate the optical properties of semiconductors - are quantized, and can assume only discrete values, a bit like in atoms. A single nanocrystal can therefore be seen as an ideal platform to study the interaction among a small number of electrons, in terms of their charge and of their spins. Plus, due to the controllable and stable fluorescence, it could be used in bioimaging.
Thus, both nanotubes and quantum dots are model systems to study properties that emerge at the nanoscale and are different from the bulk form of their respective materials. But there is another essential aspect of nano that both these materials system fulfil. That is, the potential of creating large scale materials systems with new functionalities starting from small, nanoscopic entities. Let’s start with nanotubes again. The fact that these graphitic cylinders are excellent conductors of electricity has stimulated a great deal of research to fabricate electronic components to replace those based on silicon. A great obstacle to achieving this goal has always been the fact that according to their geometry, nanotubes can be semiconducting or metallic. Only semiconducting ones can be used in electronic components and typically ensembles of as-grown nanotubes contain elements of both types. Scientists in both academia and industry – including research groups at IBM – have worked on separating nanotubes of different classes, on fabricating arrays of aligned nanotubes and eventually on realizing efficient devices. The progress has led, in 2013, to the realization of a computer fully based on carbon nanotubes by a group at Stanford University [ 4 ]. A slow computer perhaps, but still an impressive proof of concept. In 2016 a group at University of Wisconsin-Madison realized a transistor with performance matching or bettering silicon, yet another important milestone [ 5 ].
Scanning electron microscopy image of the full carbon nanotube computer. From ref. 4, Macmillan Publishers Ltd
Beyond electronics, extensive interest has been given to the combination of high conductivity and strength. Carbon nanotubes have been investigated as additives in matrices of other materials like polymers to create strong, flexible, conductive membranes for a variety of applications, including flexible and transparent displays, energy storage, water treatment or construction. Indeed, the first commercial applications that have emerged use carbon nanotubes as additives in battery electrodes and in structural materials.
Researchers working on quantum dots have developed easy techniques to synthesise nanocrystals with uniform size, shape and performance, with the idea of using them as tuneable active ingredients in a range of optoelectronic devices including solar cells, lasers and optical displays. The underlying concept is that, because of the colloidal nature of nanocrystals, all such applications could be produced through cost-effective and scalable solution based procedures. Thanks to the quantum confinement effect, it should be possible to largely overcome the performance and design flexibility of more traditional bulk materials. Progress has been made in many fields, with the realization of solar cells with efficiency around 10% and with recent progress towards very low threshold lasers by the group of Victor Klimov at Los Alamos [ 6 ]. Perhaps the most tangible demonstration of the potential of quantum dots is their incorporation in displays for enhanced colour definition. The idea was pioneered by small spin offs like Nanosys who presented a so called quantum dot enhancement layer in 2011, and it was successfully implemented by large companies like Sony, Samsung LG and TCL in television displays.
One of the most intriguing lines of research for quantum dots has been the idea of building artificial crystals, or superlattices, in which the dots replace the atoms. As already mentioned, the electronic properties in quantum dots resemble that of atoms, with the advantage that they can be easily controlled by changing the size, the composition and even the shape. The number of possibilities is infinite and we can imagine creating materials with properties tailored to whatever need we have. In practice, the interaction between adjacent quantum dots is not as strong as that among adjacent atoms, so the properties of any artificial solid will be dominated by those of the single nanocrystals, rather than by the ensemble. Still, dedicated efforts have led to achieve impressive properties, like electron motilities high enough to indicate a collective behaviour [ 7 ].
Let us now play devil’s advocate for a moment. All the progress and the proof-of-concept devices may be impressive from an academic perspective, but the truth is that after 34 years of quantum dots and 27 of carbon nanotubes, the real world applications are limited to some use in structural materials and colour enhancement in TV displays. Perfectly respectable applications of course, but not exactly the sort of material revolution from the bottom up that has often been claimed nano should bring. There are two ways to answer this point. The first, perhaps more obvious, is that it is a question of time and eventually both carbon nanotubes and quantum dots will prompt more futuristic technologies with disrupting positive effects on our life. The second, a bit more profound, is that it is not a question of whether all nanomaterials, even the most studied, will be directly employed in real life applications. Let us consider for example that fundamental and engineering studies of carbon nanotubes have created many of the premises for the work on graphene and other 2D materials that dominate the scientific literature these days. Also, current work on perovskites, the focus of substantial research in optoelectronics and energy-related topics is relying heavily on our knowledge of materials previously studied, primarily quantum dots. Generalizing the concept, studies on nano-objects will help us understand and control matter at a microscopic level, which is the first step to synthesise nanostructured materials with improved properties respect to those of materials existing in nature. Isn’t that, perhaps, the real essence of nano?
Acknowledgements: I would like to thank Pulickel Ajayan, Alexander Efros and Sergio Brovelli for fruitful discussions during the writing of this article