Monochalcogenides enrich library of 2D crystals

The field of two-dimensional materials is possibly one of the fastest expanding fields in material science and condensed matter research worldwide. The interest on this class of materials was boosted by the fast development of ever more efficient methods to synthesize them at atomically thin level.

Within the ever-growing library of 2D crystals, layered group-IV monochalcogenides (MC) have become an increasingly important group of materials. In particular, the binary IV-VI compounds SnS, SnSe, GeS, and GeSe, which form a subgroup with orthorhombic structure. SnS can be found in nature: its orthorhombic α phase, also known as herzenbergite, is a naturally occurring (nontoxic) mineral with an optical band gap of ≈ 1.3 eV, in the range of optimal values for solar cells (1.1 to 1.5 eV). Such properties boosted experimental and theoretical research on SnS in recent years.

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Gate-Tunable Giant Stark Effect in Few-Layer Black Phosphorus

Two-dimensional black phosphorus has sparked enormous research interest due to its high carrier mobility, layer-dependent direct bandgap and outstanding in-plane anisotropic property. It is one of the few 2D materials where it is possible to tune the bandgap over a wide energy range from the visible to the IR spectrum.

When a few atomic layers of BP are exposed to an electric field, a physical phenomenon known as the Stark effect is observed. In atomic spectra, the Stark effect is the shifting and splitting of atomic energy levels under the influence of an externally applied electric field. Similarly, this Stark effect causes the conduction and valence band to shift towards each other, resulting in the reduction of the bandgap of few-layer BP.

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CA2DM-NUS team pioneers two-dimensional polymer breakthrough that could revolutionise energy storage

The novel ultra-thin two-dimensional polymer sheet, which is the organic analogue of graphene, heralds new opportunities for long lasting sodium rechargeable batteries

Polymers, such as plastic and synthetic textiles, are very useful technological commodities that have revolutionised daily life and industries. A research team from the National University of Singapore (NUS) has successfully pushed the frontier of polymer technology further by creating novel two-dimensional (2D) graphene-like polymer sheets.

In the last century, scientists have successfully developed molecules which can be crosslinked to form one-dimensional and three-dimensional polymers. These are used to produce a wide range of technological products. However, making 2D polymers has met with little success, as most molecules are not flat and they tend to rotate in solution, making it difficult to control their linking to a 2D plane,” said Professor Loh Kian Ping, Head of 2D Materials Research in the Centre for Advanced 2D Materials at NUS. He also holds an appointment with the Department of Chemistry at the NUS Faculty of Science.

 

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2D materials for all-optical photonic devices

There is an ultimate limit for electronic miniaturization imposed by electron-electron interactions and by the Pauli exclusion principle that forbids two electrons to occupy the same quantum state. This will eventually prevent the development of denser circuitry, as well as of multiplexed or parallel schemes using electrons as information carrier units. Photons, however, can share a same logical gate, without interacting among one another, unless mediated by the supporting material and its nonlinear optical properties. This suggests that, in the near future, technology based on all-optical photonic devices will partially take over electronics and, possibly, extend classical computing in use today to include new quantum protocols and techniques. In this photon controlling photon type of devices, materials that provide efficient nonlinear optical interaction will indubitably play a central role.

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Science review on 2D materials and van der Waals heterostructures

Writing in Science, leading 2D materials researchers estimate that research on combining materials of just a few atomic layers in stacks called heterostructures is at the same stage that graphene was 10 years ago, and can expect the same rapid progress graphene has experienced.

Graphene was the first 2D material, isolated at The University of Manchester in 2004. Its range of superlative properties, including fantastic strength, conductivity, flexibility and transparency, has paved the way for applications ranging from water filtration to bendable smartphones; from rust-proof coatings to anti-cancer drug delivery systems.

Combining graphene with other materials, which individually have excellent characteristics complimentary to the extraordinary properties of graphene, has resulted in exciting scientific developments and could produce applications as yet beyond our imagination.     

The authors of the review article, from The University of Manchester and National University of Singapore, state that early applications could be high-mobility transistors for superfast electronics and LED devices using graphene as a transparent electrode.

However, such in the range of possible combinations of materials, researchers believe that heterostructures could deliver designer materials, made to order to meet the demands of industry.

The family of 2D crystals is expanding all the time, meaning that new possibilities for combining them in stacks can be explored.

The next challenge is to work out how to mass produce 2D materials; a similar problem that faced graphene in the early years after it was isolated.

Sir Kostya Novoselov, who together with Professor Sir Andre Geim won the Nobel prize for Physics in 2010 for demonstrating the remarkable properties of graphene, believes 2D materials are one of the most exciting and promising areas of research.

He said: “With 2D materials, we are currently where we were about 10 years ago with graphene – plenty of interesting science and unclear prospects for mass production.

“Given the fast progress of graphene technology over the past few years, we can expect similar advances in the production of heterostructures, making the science and applications more achievable.”

Co-author Professor Antonio Castro Neto, Director of the Centre for Advanced 2D Materials at the National University of Singapore, added: “In the search for revolutionary and disruptive new technologies, van der Waals heterostructures and devices based on two dimensional materials emerge as major players.

“This review covers the latest developments in one of the fastest growing fields that bridges science, materials science, and engineering.”

Source: The University of Manchester

CA2DM crosses h-index = 50 in less than 5 years

In a brief span of 5 years, the Centre for Advanced Two-Dimensional Materials (CA2DM), and its predecessor, Graphene Research Centre, has just quietly passed a milestone in academic research. This is the h-index = 50 mark which signifies that 50 of its papers have at least 50 citations by peer publications. Putting things into its proper context, what this really means is that the work of CA2DM’s scientists is being acknowledged by their peers.

Drawn from over 400 papers published to date, CA2DM’s 50 most cited articles (attached) show a breadth of diversity and strength across many topics in 2D materials like graphene oxide, transition metal dichalcogenides, biomaterials, topological semimetals, organic catalysts, and energy storage materials.

As of this writing, 16% of the 50 papers have garnered more than 300 citations each with the highest just tipping over the 1000 mark. These papers are published in a broad range of well-known academic journals such as Nature (2), Science (6), Nature’s sister journals, e.g. Nature Nanotechnology (2) and Nature Communications (8), ACS Nano (6), Nano Letters (6) and others like Advanced Materials (2).

Papers in two specific areas of research stand out amongst the 50 in being classified by the Web of ScienceTM as both Hot Papers and Highly Cited Papers. Generally, they have had the most citations within the shortest space of time since publication when benchmarked against peer papers. The two areas are black phosphorus (3) and Weyl semimetals (3). The key scientists that carry out seminal work on black phosphorus are Professor Antonio H Castro Neto and Dr Alexandra Carvalho. CA2DM’s leading researcher on Weyl semimetals and the related topological insulators is Assistant Prof Hsin Lin.

As an indicator of their profundity and prolificacy, Professors Castro Neto and Loh Kian Ping have the honour of the most number of highly cited papers in a diverse array of topics. Two fast-rising researchers with highly cited papers in this group are Assistant Professor Goki Eda (transition metal dichalcogenides) and Associate Professor Christian Nijhuis (plasmonics).

In conclusion, as CA2DM crosses over this significant landmark, it is poised to take a firm grasp on its leading position in 2D materials research, moving towards even more groundbreaking work.

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Choreographing the dance of electrons

NUS scientists have discovered how to manipulate many body states in thin semiconductors by encapsulating them in atomically thin materials and changing the electric field

Scientists at the National University of Singapore (NUS) have demonstrated a new way of controlling many body states in correlated electron systems by confining them in a device made out of atomically thin materials, and applying external electric and magnetic fields. This research, published on 23 December 2015 in the prestigious scientific journal Nature, was led by Professor Antonio Castro Neto and his research team at the Centre for Advanced 2D Materials (CA2DM) of the NUS Faculty of Science.

Almost all modern technology like motors, light bulbs and semiconductor chips runs on electricity, harnessing the flow of electrons through devices. Explained Prof Castro Neto, “Not only are electrons small and fast, they naturally repel each other due to their electric charge. They obey the strange laws of quantum physics, making it difficult to control their motion directly.”

To control electron behaviour, many semi-conductor materials require chemical doping, where small amounts of a foreign material are embedded in the material to either release or absorb electrons, creating a change in the electron concentration that can in turn be used to drive currents.

However, chemical doping has limitations as a research technique, since it causes irreversible chemical change in the material being studied. The foreign atoms embedded into the material also disrupt its natural ordering, often masking important electronic states of the pure material.

The NUS research team was able to replicate the effects of chemical doping in this study by using only external electric and magnetic fields applied to an atomically thin material, titanium diselenide (TiSe2), encapsulated with boron-nitride (hBN). The researchers were able to control the behaviour of the electrons accurately and reversibly, making measurements that had been theoretical up to now. The thinness of the two materials was crucial, confining the electrons within the material to a two-dimensional layer, over which the electric and magnetic fields had a strong, uniform effect.

“In particular, we could also drive the material into a state called superconductivity, in which electrons move throughout the material without any heat or energy loss,” Prof Castro Neto said.

Because they are atomically thin, two-dimensional superconducting materials would have advantages over traditional superconductors, in applications such as smaller, portable magnetic resonance imaging (MRI) machines.

One specific goal of the NUS research team is to develop high-temperature two-dimensional superconducting materials. Current materials require an extremely cold temperature of -270°C to function, ruling out exciting applications such as lossless electrical lines, levitating trains and MRI machines.

The technique, which took the researchers two years to develop, will enable new experiments that shine light on high-temperature superconductivity and other solid-state phenomena of interest. With a wide range of materials awaiting testing, electric field doping greatly widens the possibilities of solid-state science.


Original Publication: L. J. Li et al. Controlling many-body states by the electric-field effect in a two-dimensional material, Nature (2015). DOI: 10.1038/nature16175


Commentary in Nature Nanotechnology: Peter Abbamonte, 2D superconductivity: Electric tuning of many-body states, Nature Nanotechnology (2016). DOI:10.1038/nnano.2016.7


Media coverage as of December 27th

 

NUS Researchers set New Benchmarks for Magnetic Field Sensors

It is not often that one piece of research achieves multiple ground-breaking firsts and garners invaluable scientific insights as well. A team of NUS researchers comprising Dr. Kalon Gopinadhan, Dr. Shin Youngjun, Prof. Antonio H. Castro Neto, Prof. T. Venky Venkatesan, and Prof. Yang Hyunsoo, from the Centre for Advanced Two-Dimensional Materials, the Department of Electrical and Computer Engineering, and NUS Nanoscience and Nanotechnology Institute, has done exactly that with their latest invention in sensor technology.

Prof. Yang and his colleagues, together with Prof. Andre Geim from the University of Manchester, have developed a new type of sensor that leaves those in the market, and laboratories, literally, in the dust. They have also carried out a definitive study of graphene-based MR sensors that hints at their immense promise in developing into the next generation of extremely sensitive sensors.

Their invention utilizes a characteristic property of many materials, i.e. magnetoresistance (MR), in which electrical resistance is changed by an external magnetic field. This very exciting piece of research, spearheaded by Prof. Yang, has just been published (in Sep 2015) in the prestigious journal, Nature Communications, highlighting graphene’s crucial role in making the accomplishment possible.

The significant benchmarks attained by the sensor include extremely high sensitivity to low and high magnetic fields, tunable MR effects that expands its potential scope of applications, very small resistance variations due to temperature, and the ability to act as thermal switches due to heat-related MR effects. In particular, the 2000% MR measured at 400 K (the practical sensor temperature) is a gain of more than 8 times on previously reported laboratory results and easily over 200 times that of most Hall sensors in the market.

When compared to existing silicon-based Hall sensors, the team’s graphene- boron nitride (BN) MR sensor has a much higher sensitivity due to its higher mobility. It is also cheaper to produce since raw material costs for graphene are very low. An added bonus is its very small change in resistance with temperature unlike other sensors in the market.

Another breakthrough came with the finding that the mobility of the graphene multilayers can be partially adjusted by tuning the voltage across the sensor. This is a huge advantage in terms of possible applications over other sensors in the market. The discovery of heat-related MR effects of nearly 90,000% also suggests that graphene-based thermal switches are additional applications to add to an already long list.

This invention is broadly flexible in that other 2D materials such as transition metal heterostructures or other graphene derivatives may work as well including varying the substrates that have been used with graphene in other research.

The MR sensor developed in this research is perfectly poised to pose a serious challenge in a market estimated at USD1.8 billion in 2014 and expected to grow to USD2.9 billion by the year 2020. With sterling credentials matched by its capacity to fill the performance gaps of existing sensors, the potential of this novel device for making an impact is probably very substantial, to say the least.

Some closing words from Prof. Yang sum up this research as: “… an opportunity to understand magnetic and transport properties of few-layer graphene at practical device temperatures of 300–400 K, which has not been reported previously. As we have demonstrated that the field sensitivity and magnetoresistance can be engineered in graphene/boron-nitride heterostructures, our results indicate a promising avenue for magnetic field sensing applications.”

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Yale-NUS, NUS and UT Austin researchers establish theoretical framework for graphene physics

Making strides towards using graphene to create new electronic devices

Since the discovery of graphene about a decade ago, scientists have been studying ways to engineer electronic band gaps in the material to produce semiconductors which can create new electronic devices. A team of researchers from Yale-NUS College, the Center for Advanced 2D Materials and Department of Physics at the National University of Singapore (NUS) and the University of Texas at Austin, USA (UT Austin) have established a theoretical framework to understand the elastic and electronic properties of graphene. The findings were published in February 2015 in Nature Communications, one of the most prestigious research journals in the world.

Graphene, a single-atom-thick sheet of carbon atoms arranged in a honeycomb-like lattice, is one of the simplest materials with unrivalled mechanical and electronic properties. The material has been hailed by scientists as an extremely good conductor of electrons due to its strength and its light weight. In 2013, researchers from the Massachusetts Institute of Technology (MIT) discovered that placing graphene on top of hexagonal boron nitride, another one-atom-thick material with similar properties will create a hybrid material that shares graphene’s amazing ability to conduct electrons, while adding the band gap necessary to form transistors and other semiconductor devices. Semiconductors, which can switch between conducting and insulating states, are the basis for modern electronics. The reasons behind why the hybrid material performed as such were unexplained until this new theoretical framework was created by researchers from Yale-NUS, NUS and UT Austin.

To fully harness the hybrid material’s properties for the creation of viable semiconductors, a robust band gap without any degradation in the electronic properties is a necessary requirement. The researchers concluded that it is necessary to use a theoretical framework that treats electronic and mechanical properties equally in order to make reliable predictions for these new hybrid materials.

Shaffique Adam, Assistant Professor at Yale-NUS College and NUS Department of Physics, said,” This theoretical framework is the first of its kind and can be generally applied to various two dimensional materials. Prior to our work, it was commonly assumed that when one 2D material is placed on top of another, they each remain planar and rigid.  Our work showed that their electronic coupling induces significant mechanical strain, stretching and shrinking bonds in three dimensions, and that these distortions change the electronic properties. We find that the band gap depends on several factors including the angle between the two sheets and their mechanical stiffness. Going forward, we will continue to theoretically explore the optimal parameters to create larger bandgaps that can be used for a wide range of technologies. ”

Pablo Jarillo-Herrero, the Mitsui Career Development Associate Professor of Physics at MIT, whose research team first reported band gaps in this new graphene hybrid material said, “This theory work has increased the accuracy and predictability of calculating the induced band gap in graphene, which may enable applications of graphene in digital electronics and optoelectronics. If we are able to increase the magnitude of the band gap through new research, this could pave the way to novel flexible and wearable nanoelectronic and optoelectronic devices.”

The research work in Singapore was funded by the National Research Foundation and the Ministry of Education.

CA2DM paper featured in cover and editor’s choice of Physical Review Letters

A recent paper by Prof. Antonio Castro Neto and collaborators was featured in the cover of the March 27th edition of Physical Review Letters.

The paper demonstrated for the first time the site-dependent g factor of a single magnetic molecule, with intramolecular resolution, using low-temperature, high-magnetic-field scanning tunneling microscopy of dehydrogenated Mn-phthalocyanine molecules on Au(111).

This was achieved by exploring the magnetic-field dependence of the extended Kondo effect at different atomic sites of the molecule. Importantly, an inhomogeneous distribution of the g factor inside a single molecule is revealed.

The results open up a new route to access local spin properties within a single molecule.