Author Archives: Maria Maragkou

2D materials devices: Challenges in device fabrication

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Semiconducting 2D materials such as transition metal dichalcogenides are beginning to generate a lot of interest as a candidate for ultra thin body electronics and optoelectronics. There have been significant advances in the last 2 to 3 years and a shift from papers reporting fundamental properties of these materials towards proof of concept devices and high quality, larger scale synthesis. In technology speak one could say the field has graduated into a Technology Readiness Level of 3-4 where scientists all over the world are conducting research to understand feasibility of using these materials in electronics and optoelectronic components.

This is rapid progress indeed but the jump to TRL 5-6 will require clear understanding of all facets of the device fabrication and integration in order to preserve the figures of merit such as the mobility, subthreshold swing, responsivity and luminescence quantum yield.  R&D would therefore need to focus on optimizing each step of device fabrication :

  1. Growth/deposition of 2D materials

This is the first and main step of fabrication and has therefore received the most attention. There are numerous papers reporting the large area deposition of MoS2 and other 2D materials using powder precursors in a furnace.  While this is a great first step in producing high quality crystals, there are several unanswered questions regarding the continuity, grain boundary structure and scalability of these films. Metal organic precursor based techniques such as MOCVD and ALD present an attractive alternative to this technique and optimization of film properties via this method would be critical to the progress of this technology. An aspect in this realm that has not received much attention is the importance of substrates and the influence of stresses in the resulting films on the aforementioned figures of merit.

  1. Contacts

Scalable deposition of high quality Ohmic contacts is still an unsolved problem that needs careful study to minimize parasitic effects.  Successful strategies in the lab involve the deposition pure metal contacts such as Au and Pd under ultra high vacuum or patterning/surface treatment of the area under the contact to create stronger interaction between the inert 2D surface and the metal. This in turn has implications for high frequency operation (fmax) etc.  In order to set the challenge, best values of contact resistance on MoS2 are on the order of ~700-900 Ω-μm while state-of-the-art Si-MOSFETs with values as low 80 Ω-μm have been reported. An improvement in this figure will certainly go a long way in affecting the next TRL jump.

  1. Gate dielectric and passivation layers.

One of the reasons for the rise of silicon as ubiquitous material in everyday electronics is the stability of its oxide this is not true of Germanium which was the material used for Shockley’s transistor!  More recently the ability to deposit thin, high-k dielectrics by techniques such as ALD with extremely low interface density states and high breakdown voltages not only extended the scaling limits for Si based devices but also enabled the commercialization of III-V based devices such as HEMTs.  The inert nature of Van der Waals layers endow them with properties conducive to these applications but this by it’s very nature causes homogeneity and quality problems for the deposition of high-k dielectrics. Several workarounds have been reported ranging from the deposition of a metallic seed layer to functionalization of the 2D layers which show great promise.

  1. Etching of 2D materials

While it seems trivial due to its atomically thin structure controlled etch of 2D materials is critical to achieve sophisticated device structures such as LEDs, tunnel transistors or for side contacts to 2D materials. The relatively mature etch technology can therefore be easily tuned to etch stacks of these layers with profiles dictated by the device architecture. At the limit of controlled etching is the emerging field of Atomic Layer Etching (ALE).  Significant advances in ALE methods of both conventional and 2D materials would enable further advances in atomic scale devices.

  1. Metrology and in-line quality control

Important advances in the solutions of the above challenges requires non destructive characterization techniques both in-line and post fabrication. Techniques such as Raman spectroscopy and ThZ spectroscopy show great promise in providing significant insights into the structural and electronic properties.  Correlation of material quality by evaluating device performance with spectral features obtained using these techniques would aid in standardization. This definition would enable scientists across the world to work towards the same goal and thereby further accelerate development of this exciting new technology.

 

 

Dr Ravi S Sundaram

Market Manager: Research and Emerging Technologies

Oxford Instruments Plasma Technology

North End, Yatton,

BS49 4AP

https://bit.ly/2rtQ4kt

https://www.linkedin.com/in/ravi-shankar-sundaram-86241727

Nature Materials: Focus on 2D materials beyond graphene

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One of the many things that were revealed with the isolation of graphene, was the pursuit of atomically thin forms of other materials: semiconductors, boron nitride and, more recently, Xenes, are offering endless possibilities not only to explore fundamental physics, but also to demonstrate improved or even entirely novel applications.

2D materials have a lot to offer in terms of optoelectronics applications and in a wide range of wavelengths (microwave to the visible),  as they exhibit something that graphene does’t have: a bandgap. Tony Low, Frank Koppens and colleagues review the physics and applications of different kinds of polaritons (exciton-, plasmon- and phonon) in layered 2D materials.

Alessandro Molle, Deji Akinwande and collaborators offer a critical overview of the issues that remain open when it comes to Xenes, where atoms from the group IVA elements are organized into a single layered, honeycomb-like lattice. Several issues remain still under debate, such as their stability, while various theoretical studies are predicting a plethora of interesting topological properties.

A unique characteristic of 2D materials, however, is the possibility to easily form (horizontal or vertical) van der Waals heterostructures, following the stacking of layers of different materials and thicknesses. Such heterostructures however are not only limited to combinations of 2D materials; Deep Jariwala, Tobin Marks and Mark Hersam explain in their Review that 2D materials can be combined with non-2D materials, such as organic molecules and quantum dots (0D), carbon nanotubes (1D) and bulk Si, Ge, III–V and II–VI semiconductors (3D), that adhere primarily through non-covalent interactions.

 

Maria Maragkou (Nature Materials)

 

Nature Reviews Materials: Focus on 2D materials

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The applications of 2D materials are numerous and diverse, ranging from electronics to catalysis, and from information storage to medicine.

A Focus Issue just published in Nature Reviews Materials covers the synthesis and fundamental properties of sFocus on 2D materialseveral 2D materials, as well as the devices they enable, combining Reviews, Comments and Research Highlights. In particular, a Review by Manish Chhowalla explores the use
of graphene, hexagonal boron nitride, transition metal dichalcogenides (TMDCs), phosphorene and silicene as channels in field-effect transistors. The emerging field of valleytronics and its implementation in graphene and TMDCs is the topic of a Review by John Schaibley, Xiaodong Xu and colleagues. Beyond electronics, graphene (and, more in general, carbon-based materials) is attracting growing interest as a low-cost catalyst for renewable energy production and storage: the use of heteroatom-doped graphene as a metal-free catalyst is the topic of a Review by Xien Liu and Liming Dai. In another Review, by Castro Neto and colleagues, the synthesis, properties and applications of phosphorene are discussed.

There is clearly a lot of fundamental research being carried out on 2D materials; however, it is also necessary to address their translation into commercial or medical devices. This problem is analyzed in two Comments, one by Seongjun Park and one by Kostas Kostarelos. A common theme emerging from these opinion pieces is the need for stronger collaboration between academia and industry or medical professionals.

As it was argued in other posts in this blog, the road to the commercialization of 2D-materials-based products is still long, and many challenges lie ahead. But numerous exciting developments in the field of 2D materials are keeping researchers busy, and we hope that the overview provided in this Focus Issue will be a useful tool for the community to explore them.

 

Giulia Pacchioni (Nature Reviews Materials)

 

Graphene commercialization: a voice from industry

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The commercialization of graphene-based products is a recurring theme in this blog. Why after much talking about graphene being a wonder material the most high-tech graphene-based product we can buy is still a tennis racket? Nature Reviews Materials asked this question to Seongjun Park, an engineer working in Samsung and studying graphene. In a Comment piece, he reminded the readers that the commercialization of new materials and technologies always takes time, often decades — optical memory devices and phase-change memories are good examples, as it took more than 30 years to take them to the market. Compared to them, graphene is still a young technology: it is only 12 years that scientists and engineers are playing with it and tweaking its properties.

Park likens the process of commercialization to a jigsaw puzzle, in which many pieces need to fit together in order to produce a recognizable image. Many studies are carried out on graphene, but they often focus on one specific property, whereas for creating a graphene-based device multiple properties have to be optimized at the same time, and multiple engineering challenges have to be addressed. Currently, two types of applications are under development: those that are easy to develop and promise low reward, and those that are very challenging but are potential game-changers. The way to go, Park reckons, is to develop the first kind of products while we wait for the second kind to mature. One factor that is slowing down the process of graphene commercialization is the fact that academics tend to focus on research lines that are likely to lead to high-profile publications, expecting engineers in industry to develop commercial products on their own; papers reporting results from industry are often judged incremental and of little interest. A stronger link between academia and industry is thus needed to speed up graphene commercialization.

It is often said that in the Gartner hype cycle graphene has passed the peak of inflated expectations. Now is time to take a more realistic approach to what researchers and engineers can develop in the short term. It is very early to lose faith in the potential of graphene to enable new, revolutionary products.

 

Giulia Pacchioni (Nature Reviews Materials)

 

Celebrating Nature Nanotechnology

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Nature Nanotechnology has recently turned 10. To celebrate this milestone, a number of experts from different areas of nanotechnology have been invited to describe how the field has evolved in the last ten years. The ever growing demand for improved functionalities and nanoscale miniaturisation of electronic devices, in addition to the approaching limits of current silicon-based technology, has driven the quest for materials enabling alternative technological solutions. In this context, two-dimensional materials have significantly shaped the nanotechnology landscape over this decade. In the feature entitled “Nano on reflection”, Dr Silvia Milana, Associate editor at Nature Communications, outlines the development of two-dimensional materials, highlighting both promising achievements and associated challenges.

2D goes 3D

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Van der Waals heterostructures

Two-dimensional layered materials and van der Waals heterostructures. From Nature Reviews Materials 16042 (2016) “Van der Waals heterostructures and devices“

If you are reading this blog, you probably already think that 2D materials are awesome. However, stacks combining several 2D materials could be even better — they open almost endless possibilities for new properties and devices, as they draw from a wide library of 2D materials with different electronic properties, ranging from insulating to metallic, conductive and superconductive, which can be mixed and matched to create hybrid structures with unique functionalities. Xiangfeng Duan and colleagues bring us on an inspiring journey to discover van der Waals heterostructures in a newly published Review in Nature Reviews Materials. Flexible and transparent electronic and optoelectronic devices based on van der Waals stacks have already been demonstrated, including tunneling transistors, vertical field-effect transistors, wearable electronics and innovative solar cells.
The ‘ingredients’ for the heterostructures feature graphene as the most common component, but they can also include boron nitride, transition metal dichalcogenides, phosphorene and other materials. Thanks to the fact that the interlayer interactions are van der Waals in nature, highly disparate materials can be integrated without limitations imposed, for example, by lattice mismatches. Because all the components of a device can be integrated in a single membrane without needing to incorporate a substrate, flexible and adaptable devices can be obtained.
There are still important challenges that lie ahead; namely, the difficulty of developing fabrication methods that are both scalable and precise, and the need to produce reliable contacts. But van der Waals heterostructures promise to enable amazing functionalities in electronic and optoelectronic devices — maybe it is this growth in the third dimension that will realize the full potential of 2D materials.

 

Giulia Pacchioni (Nature Reviews Materials)

 

Graphene Week 2016, Warsaw

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The third week of June found many academics and industry representatives in Poland for the annual Graphene Week conference. Editors from various Nature journals were also there, to hear some excellent talks on the most recent developments with graphene and 2D materials, roam around the poster sessions and appreciate (for a little bit) the beauty of Warsaw.

The program was dense, covering of course both fundamental issues and applications. No doubt there was something for each one of the nearly 800 attendees. Most of the plenary talks and sessions were devoted to fundamental physics. Of course, this can be justified given the plenty remaining unanswered questions. There is nevertheless a clear boost towards applications and more synergy between academia and industry. To this end, there were 3 sessions as part of the so called Innovation Forum, dealing with standardisation, commercialisation and the road ahead. They brought together the industry perspective (from industry giant BOSCH to RD Support Limited, from small established companies like Graphenea, to larger enterprises seeking to enter in the graphene world, such as Airbus), academics with an eye to applications (like Professors Kostya Novoselov and Ian Kinloch from Manchester), and spin-off companies (like BeDimensional, from the Italian Institute of Technology.

The message from the companies was clear. Adopting graphene and embracing such a tremendous shift in industry would require a reduction in cost or an increase in performance, both of the order of 10%; and we are not there yet. The latter has been in fact pinned down to one thing: the quality of hexagonal boron nitride, the “ideal” substrate for graphene that seems to bring out its best qualities (in terms of mobility for example).

The moment for this clear shift of interest towards applications is not surprising. The Flagship has approached the end of the Ramp-up period, and has entered the Horizon 2020 phase (Core 1, 2014-2020) with over 150 partners in 23 countries. The specific goals are very clearly laid out: managing knowledge and IP and for exploitation of results, benchmarking (both with other graphene-based approaches and with competing technologies) and further strengthening of the activities among the work packages of the Flagship.

Indeed the pressure is building up; because so did expectations during the last decade. It has become clear that the commercialisation of graphene is not easy; recent discussions over the National Graphene Institute at the University of Manchester have shown as much. Graphene, much like other advanced materials trying to make their way to the market, face similar problems, like high capital costs and technology uncertainty. Is it fair though to expect the universities to bring the product that close to market, given their limited, public resources? Probably not, but the continuing reduction of industrial research and development spending during the last years is certainly not helping.

While nobody can say with certainty yet what the outcome of this multi-million effort will be, there is no doubt that the field of graphene and 2D materials has gone a long way rather quickly, in only 12 years since the isolation of graphene in fact. Such impressive progress should keep us optimistic about the future.

 

Maria Maragkou (Nature Materials)

Silvia Milana (Nature Communications)

 

npj 2D materials and applications

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The field of 2D materials, based on ultrathin sheets that otherwise form layered materials was started and dominated for a long time by graphene. While graphene can be described by a lot of superlatives, just like everything else in life, it can also have limitations when it comes to certain applications. One example is the absence of a band gap which is an important drawback for applications that require semiconductors, for example in electronics where a band gap is needed to make a transistor switch that can be turned off or for solar cells that efficiently converts light into electricity. Graphite is not the only layered material though, so the realisation that experimental approaches originally developed for graphene can be extended to other layered materials such as transition metal dichalcogenides (TMDCs) with semiconducting molybdenum disulphide (MoS2) as one of the best known examples, has fuelled the rapid growth of interest in 2D materials beyond graphene. We now know of over 500 such materials to choose from, all with interesting properties. Examples include NbSe2 which is a superconductor, VS2 predicted to be a ferromagnet or CrSiTe3 predicted to be a semiconducting ferromagnet.

People very often ask me “which 2D material is best?” Our civilisations are based on using a multitude of materials and the choice of the best material is determined by its application. It is time to adopt a similar approach in nanoscience and stop expecting that one single wonder material can solve all our problems. Given the history of the 2D materials field, some materials such as TMDCs are better explored with some key applications emerging such as in valleytronics, while others such as PdSe2 have yet to be explored. Yet they all have something new to offer; we only have to try hard enough to find what it is. As the Editor-in-chief of npj 2D materials and applications I will strive to keep a balance and select the best papers on “more established” and “new” 2D materials, being reported on for the first time. This new journal is part of the nature research portfolio and of the nature partner journal series. Open access and online only, I hope that it will serve the 2D materials’ community by widely disseminating the most exciting and impactful new findings and stimulating discussion and further research.

 

Prof. Andras Kis

Laboratory of Nanoscale Electronics and Structures,

EPFL, Lausanne, Switzerland

andras.kis@epfl.ch
https://lanes.epfl.ch

 

Graphene in biomedicine

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The May editorial of Nature Materials discusses the recent launch of a Work Package on Biomedical Technologies by the Graphene Flagship, and highlighted the questions that researchers in the field will try to answer in the next few years.

Among the different applications being currently explored for graphene and other 2D materials, diagnostic and therapeutic medical tools certainly have huge commercial potential. However, will the performance and fabrication costs of graphene-based devices ever be able to compete with those of Silicon lab-on-chips, metallic electrodes for brain stimulation and flexible neural recording sensors based on organic semiconductors or other materials?

A sensible approach to answer this riddle is trying to understand which physical and chemical properties of graphene can make the difference in such applications. For instance, membranes of monolayer graphene are ion-impermeable, yet they can be patterned to create nanopores that allow the controlled transport of ionic currents, polymer chains and DNA molecules. These architectures have been studied for DNA sequencing applications, although several practical challenges remain to be solved.  Graphene can also be easily functionalized , which may prove advantageous for the fabrication of efficient drug delivery systems. Single or few layers of graphene or graphene oxide could be used as carriers that immobilize drugs on their surface and release them only in proximity to a specific target in the body.  First, however, it’s essential to understand how cells and organs react to graphene, and it seems now clear that systematic studies on how  the biocompatibility of 2D materials is affected by their size, structural integrity and chemical functionalization will be required to address this point.

As a final example, it is worth mentioning that also the electronic properties of graphene may be leveraged to realize advanced biomedical devices. A high charge mobility improves the sensing performance of transistors used to record the electrical activity of neurons, and indeed  flexible graphene transistors that detect the action potential generated by cell cultures with sensitivity comparable with Silicon transistors have been recently reported. Scaling up this technology and translating it into in vivo applications — for example to realize devices able to record the brain activity from more than 1,000,000 neurons simultaneously, as required by a recent call for proposals from DARPA — is a massive engineering challenge that will certainly need much more work, and it is not clear at this stage if graphene will be the winning solution in bioelectronics on the long term.

For those interested in learning more about the potential of graphene in biomedicine, Nature Materials, Nature Physics and the Graphene Flagship are organising a Nature Symposium on Biomedical Applications during the Graphene Week conference coming next June. Stay tuned.

 

Luigi Martiradonna (Nature Materials)

Into the fold

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Graphene boasts a number of exceptional properties, but arguably its best feature may be its form as a strong and flexible sheet of material. After all, the ancient art of origami teaches us that an infinite range of shapes can be created by folding up paper-like sheets like graphene. 3D is the new 2D: folding techniques may be employed for bottom-up fabrication of intricate 3D nanostructures or for assembling flat-packed devices that can unfold where or when needed.

A square twist mechanical switch

A square twist mechanical switch: recently shown possible due to hidden degrees of freedom.
From Nature Materials 14, 389–393 (2015) “Origami structures with a critical transition to bistability arising from hidden degrees of freedom”.

Origami-led concepts have already inspired the design of numerous soft devices made from 2D polymer sheets such as miniature robots that self-assemble and become mobile by folding and unfolding. Recently, mechanical metamaterials were constructed, using a paper folding technique called snapology, that can smoothly change shape from a flat sheet to a voluminous block of material, and back.

In recent years, the bug of origami-inspired design has also caught among graphene researchers as the possibilities seem endless. Controlled crumpling and folding of graphene can produce new types of 3D nanostructures of interest for flexible electrodes in batteries and supercapacitors. In another strand of research, scientists have unleashed the power of kirigami – origami with cutting – to make super-stretchable, ultrathin graphene electrodes that could be used in biomedical applications. In a more classic origami demonstration, graphene oxide paper sheets can self-fold into boxes that could be used as miniature containers that open and close on demand. Proposals also exist for folding single layer graphene into thin but robust nanocages for high-density molecular storage.

Astoundingly, new discoveries are still made in origami research as the mechanics of folding materials yields new surprises and previously unknown geometries (see figure). Graphene researchers may want to pay attention as the inspiration for the next breakthrough in graphene materials may be found in novel geometries enabled by the art of paper folding.

 

Liesbeth Venema (Senior Editor, Nature)