RESEARCH - National University of · PDF fileTable of Contents Faculty of Science Research ... BioBoss tiles used in performing large-scale field experiments at Pulau Hantu. ... film

RESEARCH Volume 21 Number 2 December 2016

FEATURESOxide interfaces for future electronics beyond MooreA chemist’s approach to two-dimensional materials researchStrange optics of ultra-thin semiconductorsOxides for magnetic refrigerationExcitons in correlated electron systemsTwo-dimensional materials for wearable electronics

A special issue on Advanced Materials:Oxides and Two-Dimensional Materials

Table of Contents

Faculty of Science ResearchThe Faculty of Science conducts basic and applied experimental, theoretical and simulation research over a broad spectrum of science, mathematics and technology domains. We cover most of the key fields in biological sciences, chemistry, physics, pharmacy, mathematics and statistics.

Faculty of Science Research is published online twice a year. It is written for a broad scientific audience interested to keep up with some of the key areas of science pioneered by researchers at the Faculty of Science.

This publication may be reproduced in its original form for personal use only. Modification or commercial use without prior permission from the copyright holder is prohibited.

For further information on the research in this newsletter, please contact:

Editor: SOH Kok Hoe ([email protected])Deputy Editor: Janice QUAH ([email protected])Consultant: Thorsten WOHLAND ([email protected])

Dean’s Office, Faculty of ScienceNational University of SingaporeBlk S16, Level 5, Science Drive 2Singapore 117546

For the latest research news, please refer to:URL: www.science.nus.edu.sg/research/research-news

NEWS ROUNDUP2 Medicinal value from tropical plants2 Biodiversity and biological interactions in Singapore’s reservoirs and waterways3 Boosting our marine sustainability efforts

RESEARCH FEATURES4 Oxide interfaces for future electronics beyond Moore6 A chemist’s approach to two-dimensional materials research8 Strange optics of ultra-thin semiconductors10 Oxides for magnetic refrigeration12 Excitons in correlated electron systems14 Two-dimensional materials for wearable electronics

FOS RESEARCH | VOL. 21 | NUMBER 2 | DECEMBER 2016 1

On the cover: Illustration of energy transfer between two semiconductor layers. The process is triggered by excitation with a green laser and results in emission of red light.


Singapore’s abundant botanical resources, with many different types of tropical fruits and vegetables and its multi-cultural society, make it an ideal place to discover plants with medicinal value. Many of the herbs which are used in traditional remedies are grown in the Healing Gardens located in the Singapore Botanic Gardens, the only tropical garden honoured as a UNESCO World Heritage Site. These traditional herbs, together with diverse tropical fruits, vegetables, and other flora available locally represent an untapped resource for new bioactive compounds that maintain good health and provide treatments for chronic diseases. Despite the huge potential, there is little systematic research on this among the tropical botanicals. Prof HUANG Dejian from the Food Science and Technology Programme at the Department of Chemistry, NUS has partnered Kikkoman Singapore R&D Laboratory to discover novel bioactive

compounds from tropical botanicals with potential use in food products. In the present phase, a high throughput screening assay developed in-house will be used to screen and identify suitable active compounds from medicinal plant samples which have health benefits. These active compounds will be characterised and assessed for use in new food

products.

Prof Huang says, “This research collaboration synergises the research capability of our lab on bioactive natural products with the commercialisation capability of Kikkoman Corporation to accelerate the translation of our scientific discovery to commercial products.”

Medicinal value from tropical plants

NEWS ROUNDUP

Building on earlier work which first documented the biodiversity in Singapore’s reservoirs and a recent pilot study on quantitative biodiversity sampling and biomanipulation at six local reservoirs, Prof Darren YEO from the Department of Biological Sciences, NUS is expanding his collaboration with PUB, Singapore’s National Water Agency, to study the aquatic biodiversity and ecology in the 11 remaining reservoirs. This is expected to give a more complete biodiversity picture for Singapore’s reservoirs.

“This study not only addresses fundamental gaps in our knowledge of tropical fresh waters, but also has translational implications,” said Prof YEO. “Understanding food web

and trophic models in our reservoirs can inform adaptive management approaches, and potentially bring about savings by reducing trial and error.” He added that these studies also advance the longer-term goal of having environmentally-friendly, biomanipulation approaches for sustainable water quality management.

This study also assesses for the first time, the biodiversity and food webs in Kallang River @ Bishan-Ang Mo Kio Park (KRBAP), an iconic and popular “naturalised” waterway jointly developed by PUB and the National Parks Board (NParks). It will provide a broad comparison between waterways and reservoirs, and also benefit PUB and NParks in terms of their resource

management, public education and outreach activities at KRBAP.

Biodiversity and biological interactions in Singapore’s reservoirs and waterways

Tropical plants contain many undiscovered bioactive compounds with potential medicinal value.

Obtaining size and population data from a captured fish specimen before releasing it back into the reservoir.

Phot

o cr

edit:

Fre

shw

ater

and

Inva

sion

Biol

ogy

Labo

rato

ry


NUS marine researchers are boosting local marine research and sustainability efforts to cope with emerging challenges brought about by climate change and urbanisation.

Prof Peter TODD and Prof HUANG Danwei both from the Department of Biological Sciences, NUS are strengthening our marine science research capabilities as part of the Marine Science Research and Development Programme funded by the Singapore National Research Foundation. In this programme, Prof Todd is partnering the National Parks Board and the Housing & Development Board to create “green” seawalls on Singapore’s coastlines to enhance native marine biodiversity and improve ecosystem resilience, while Prof Huang is partnering the National Parks Board to integrate historical and modern-day approaches to uncover the mechanisms coral reefs use to cope with urbanisation and deploy them as real-time biological monitors of ecosystem change.

Ecologically engineered seawalls to enhance biodiversity

Coastal urban development and climate change-associated threats have resulted in an increase in the construction of hard defences such as seawalls. These are generally rock or concrete structures emplaced along the shoreline to guard against coastal erosion and flooding. They are usually steep and do not function as surrogates for the natural habitats which they replace, resulting in the loss of ecosystem services and resilience.

Prof Todd is enhancing the native biodiversity on coastal defences in Singapore using ecological engineering principles. The research will develop future-ready “green” seawalls that can host a diverse array of native species with positive effects on neighbouring habitats, contributing to an overall increase in ecosystem resilience.

Prof Todd commented, “Over 60% of Singapore’s coastline is seawall. It is imperative that we work towards making these structures more habitable for marine life.”

Adaptation and resilience of coral reefs to environmental change

Coral reefs around the world are facing dramatic declines due to many environmental stressors. Despite these impacts, Singapore’s reefs have managed to persist, with over 200 species of reef-building corals and about 200 species of fish documented on local reefs.

Prof Huang will be integrating historical and modern-day approaches to uncover the mechanisms reefs employ to cope with urbanisation. This is the

first for a reef system. The research will reconstruct the genealogical and environmental history of Singapore’s reefs over the last few centuries by profiling coral communities (living and fossil) and their connections between populations. It will also cover the response of corals to various contemporary stress factors, focusing on the changes due to land reclamation and seabed dredging, which have led to losses of more than 60% of the original reef area and up to 37% of coral species.

Prof Huang commented, “Singapore’s coral reefs have been impacted by multiple stresses. To better manage them, we need to understand the genetic mechanisms underlying their survival, historically and contemporarily. ”

Boosting our marine sustainability efforts

NEWS ROUNDUP

BioBoss tiles used in performing large-scale field experiments at Pulau Hantu.

Coral reef at Raffles Lighthouse showing high biodiversity.

Oxide interfaces for future electronics beyond MooreDiscoveries in complex oxides can provide new functionalities for electronic devicesIntroduction

Silicon-based microelectronics is the engine that enables information processing, storage and transmission in modern society. Moore’s law has until now focused on scaling down and squeezing more silicon transistors (building blocks of silicon-based microelectronics) into tiny chips to provide increased computational power. With the transistor size fast approaching the limit of nature (the size of a single atom) and issues with high heat generation at the chip level, it is becoming increasingly clear that we have to “go beyond Moore” to meet industry demand. As our society adopts the “internet of things” with physical devices become more interconnected and wearable personalised devices becoming a way of life, electronic devices need to provide more functionalities. Many electronic companies are now starting to look at developing and integrating more functional materials such as oxides and two-dimensional (2D) materials with silicon processing technology to meet these needs.

Oxide Research

Oxides have unique properties that are not exhibited by silicon and these can significantly increase device functionality. For example, by using ferroelectrics (a material that has its own electric field), large amounts of electric charges can be switched on rapidly in a device to provide new functionalities, such as physical motions or changes in magnetic states. With advanced material growth techniques, layers of different oxides can be combined in a heterostructure with an atomically-sharp interface and a high degree of thickness control. This is similar to stacking blocks of LEGO but in this case, each unit block has a thickness of 0.00000004 centimetres

(see Figure 1). The sensitivity of physical parameters (such as charge, spin and orbital) that determine the property of an oxide to its atomic structure makes oxide heterostructures and interfaces a playground for exploring unique phenomena which are absent in bulk constituents. Many scientists are amazed by the unexpected phenomena emerging at the oxide heterostructures and interfaces, which may lead to new device functionalities. Our group at the Department of Physics and the NUS Nanoscience and Nanotechnology Initiative (NUSNNI)-Nanocore at NUS is one of the leading experts in this field.

Electrical Conductivity at Oxide Interface

The most widely used material for creating oxide heterostructures and interfaces is a crystal frequently used as artificial diamonds, called Strontium-Titanate (SrTiO3). This material may look simple but it carries a strong appeal for material scientists. In 1969, Marvin COHEN, a well-known materials scientist, said, “If SrTiO3 had magnetic properties, a complete study of this material would require a thorough knowledge of all of solid state physics.” Two other well-known scientisits, J. Georg BEDNORZ and K. Alex MULLER included two electrically insulating crystals SrTiO3 and LaAlO3 as key materials in their study that led to the discovery of high-temperature superconductivity in cuprate perovskites (which was awarded the 1987 Nobel prize in Physics). Following this, A. OHTOMO and H.Y. HWANG showed in 2004 for the first time that the contact area (interface) between a LaAlO3 film and a SrTiO3 substrate which are insulators is able to conduct electricity (see Figure 1). Notably, these two materials marked the beginning of a new chapter in the research on oxides interfaces. In 2011, my research team [1] at NUS showed

that an oxide interface can exhibit metallic and magnetic properties in the form of a phase separation that has not been observed before in either oxide component. These novel properties emerge at their interface when the non-magnetic insulating oxide crystal, LaAlO3 is grown perfectly aligned on a layer of the other non-magnetic insulting oxide, SrTiO3 (see Figure 1) using a technique called pulsed laser deposition. The interface becomes metallic in a form of a two-dimensional electron gas and it also acts like a magnet. This discovery was also observed subsequently by other research groups (including groups from Stanford University and Massachusetts Institute of Technology) where they showed the coexistence of magnetism and superconductivity at the very same interface. The electronic and magnetic properties of this interface are confined locally to only a few atomic layers and found to be very sensitive to external stimuli such as magnetic fields. This may be used in novel nano-sized sensors to measure external disturbances influencing the interface, such as the local oxygen concentration or external magnetic fields. These developments open up exciting possibilities for oxide-based nanoelectronics; one can imagine a completely transparent electronic circuit in an automobile windshield providing a variety of guidance functions, or a room-temperature superconductor allowing electrical current to flow without energy loss.

Switching Magnetism in Oxides

In a recent collaboration between NUS, Trinity College Dublin, and University of Twente, we have also discovered unique magnetic properties from another oxide heterostructure based on lanthanum-manganese-oxide (LaMnO3) [2,3]. By growing a thin stack of LaMnO3 on a perfectly flat crystal of


RESEARCH FEATURES

Ariando is an Associate Professor in the Department of Physics and a Deputy Director at NUSNNI -NanoCore, NUS. He received his Ph.D. degree from the University of Twente, the Netherlands. His research interest is in strongly correlated electron systems and multifunctional devices involving oxide interfaces, heterostructures and superlattices.

References[1] Ariando, et al., “Electronic phase separation at the LaAlO3/SrTiO3 interface” NATURE COMMUNICATIONS Volume: 2 Article Number: 188 DOI: 10.1038/ncomms1192 Published: 2011.[2] Wang XR, et al., “Imaging and control of ferromagnetism in LaMnO3/SrTiO3 heterostructures” SCIENCE Volume: 349 Issue: 6249 Pages: 716-719 DOI: 10.1126/science.aaa5198 Published: 2015.[3] Anahory Y, et al., “Emergent nanoscale superparamagnetism at oxide interfaces” NATURE COMMUNICATIONS Volume: 7 Article Number: 12566 DOI: 10.1038/ncomms12566 Published: 2016.[4] Huang Z, et al., “The Effect of Polar Fluctuation and Lattice Mismatch on Carrier Mobility at Oxide Interfaces” NANO LETTERS Volume: 16 Issue: 4 Pages: 2307-2313 DOI: 10.1021/acs.nanolett.5b04814 Published: 2016.

nonmagnetic SrTiO3, magnetism in the LaMnO3 layer is switched on abruptly when the number of manganese atomic layers changes from five to six. Adding the sixth atomic layer switches the magnetism in LaMnO3 from antiferromagnetic (antiferromagnets produce no magnetic field) to ferromagnetic. Such an abrupt transition has never been seen before. Using SQUID, a scanning microscope that employs superconducting electronics to measure magnetic fields with high sensitivity (one hundred thousand times smaller than the earth’s field), an image of the change in magnetic properties was obtained experimentally. The abrupt switch from antiferromagnetism to ferromagnetism which, is due to an extra atomic layer, can be explained by an avalanche of electronic charge inside the LaMnO3 travelling from the top surface of the film to the bottom.

The discovery of a critical thickness to control the ferromagnetism makes it possible to tailor nanoscale magnetic structures by controlling the layers of oxide materials. We expect that such phenomena could possibly also result from the application of electric fields or through absorbing specific molecules. Magnetism in nanoscale layers only a few tens of atoms thick is one of the foundations of the big data revolution—most of the information available on the internet is stored magnetically on hard disks in server farms located across the world. With magnetic memory elements approaching nano dimensions, this discovery promises new approaches for magnetic recording and computing. The field of oxide interface research, which has expanded to include other oxide interface combinations with many new exciting functional

properties [4], brings great potential for advanced electronics beyond Moore.


Figure 2: An image of the magnetic field recorded by scanning the surface of a LaMnO3 film grown on a substrate crystal with a tiny superconducting coil placed over it. The magnetic left-hand side is seven LaMnO3 blocks thick (3 nm) while the nonmagnetic right-hand side is five blocks thick (2 nm).

Figure 1: (Left) A LEGO model of an oxide heterostructure and interface between two different oxides. In this example, the two oxides are LaAlO3 (the building block is shown at the bottom left) and SrTiO3 (the building block is shown at the bottom right). Both of these oxides are insulating and non-magnetic in their bulk form. When the LaAlO3 is placed over SrTiO3, two-dimensional electron gas emerges spontaneously at their interface. The interface can become magnetic and superconducting (conduct electricity without energy loss) in a correct environment. (Right) Atomically resolved image of an interface between two oxides showing individual atoms of each oxide building block (1 nm = 0.0000001 cm).

A chemist’s approach to two-dimensional materials researchTwo-dimensional (2D) materials can provide new functionalities to enable innovationsIntroduction

My research focuses on the growth and processing of two dimensional (2D) materials and discovering their unique properties suited for technological applications. Post-graphene discovery, my research interests have shifted to alternative forms of 2D materials which may have thickness-dependent bandgaps. These include black phosphorus, molybdenum disulfide, etc. Research on 2D transition metal dichalcogenides (TMD), a class of 2D materials, is still in its infancy and the following challenges have to be addressed for long-term sustainability in this area: (i) new applications utilising the unique optical and electronic properties provided by 2D materials, and (ii) scalable methods for the synthesis of 2D materials in sufficiently large quantities at an acceptable cost to consumers.

Many 2D materials available today are produced in a non-scalable way and this restricts their usage. Certain 2D materials are also highly unstable in air, meaning that they can be used only with surface protection. Using a range of techniques ranging from chemical vapour deposition [1] to molecular beam epitaxy, we have developed strategies which allow the growth of single crystals of TMD over macroscopically large areas. This is supplemented by chemical strategies to passivate and protect their metastable phases. We are now working on the use of these materials as potential electrode materials for energy conversion and storage applications.

Electric Field Effect

At the fundamental level, 2D materials are interesting because of their thickness-dependent optical and electronic properties. The screening of

electric fields in 2D materials becomes less effective when the material is reduced to a few layers thick. This means that the charge carrier density in these materials, and even its band gap, can be tuned by applying an electric field. In collaboration with NUS colleagues, this concept was applied to study how electric fields can be used to tune many body interactions in TiSe2, a 2D material. It also allowed us to study the details of the phase transitions between many-body states [1].

High Performance Battery Electrode

Recently, we discovered that quasi-2D materials can be made from their three-dimensional (3D) counterparts by chemical intercalation. This is a reversible process where a guest molecule or ion is inserted into compounds with layered structures. An example is LixMoS2, shown in Figure 1. It is well-known that the intercalation of Li in MoS2 converts the semiconducting 2H MoS2 to a metallic 1Tʹ phase. The transition to the 1T’ phase is caused by electron transfer from intercalated Li, which destabilises the original trigonal-prismatic 2H-MoS2 structure and drives it to an octahedrally coordinated phase. We observed that in the 2H-to-1T’ phase conversion, there is a concomitant crystal domain size reduction in LixMoS2, converting the bulk material to a nano-crystalline state. This is because the low-symmetry 1Tʹ

phase has three orientation variants, resulting from the three equivalent directions of Peierls distortion, which pre-empts the material to form nano-domains during the 2H-to-1T’ phase conversion. When this material is used as an electrode in an alkali ion battery, the nano-structuring effect which converts the 2H-MoS2 into a bulk nanophase material allows for a more homogeneous phase conversion and uniform charge distribution in the material. As a result, the 1Tʹ metallic Li1.0MoS2 exhibits very impressive cycle stability (tested to 3000 cycles at 5 A g-1 with a capacity of 450 mAh g-1) and rate capability when tested in reversible ion batteries [2].

2D Polymers

Following the discovery of graphene, there is renewed interest in the rational organic synthesis of π-conjugated 2D polymers. These polymers offer greater flexibility in terms of composition, topology and other physical properties compared to graphene. The combination of sp2 and sp3 type bondings in 2D organic sheets enhances spin-orbit coupling and this is expected to create new emergent phenomena. Although there are infinite methods of polymerising molecules to produce linear, cross-linked or branched polymers with a diverse range of functional groups and properties, the synthesis of crystalline 2D polymers comprising repeating


RESEARCH FEATURES

Figure 1: Phase restructuring of bulk 2H-MoS2 into a 1T’-nanophase material with the chemical intercalation of lithium naphthelenide [2].

LOH Kian Ping is Provost’s Chair Professor in the Department of Chemistry, NUS. He also heads the 2D materials group in the Centre for Advanced 2D Materials and is a director with the Shenzhen-NUS Joint Collaborative Innovation Centre for Optoelectronic Science & Technology. Since 2008, he has established a strong research programme in graphene and 2D materials at NUS. He was awarded the American Chemical Society Nano Lectureship award in 2013 and the President’s Science Award in 2014.

References[1] Li LJ, et al., “Controlling many-body states by the electric-field effect in a two-dimensional material”, NATURE Volume: 529 Issue: 7585 Pages: 185-U129 DOI: 10.1038/nature16175 Published: 2016.[2] Leng K, et al., “Phase Restructuring in Transition Metal Dichalcogenides for Highly Stable Energy Storage” ACS NANO Volume: 10 Issue: 10 Pages: 9208-9215 DOI: 10.1021/acsnano.6b05746 Published: 2016.[3] Wei Liu, et al., “A Two-dimensional Conjugated Aromatic Polymer via C-C Coupling Reaction” Nature Chemistry (Accepted).[4] Xu H, et al., “Oscillating edge states in one-dimensional MoS2 nanowires” NATURE COMMUNICATIONS Volume: 7 Article Number: 12904 DOI: 10.1038/ncomms12904 Published: 2016.

units that can create topologically planar macromolecules (rather than linear) is rarely reported. This is the holy grail of polymer synthesis, the construction of 2D conjugated polymer crystals with strong and stable linkages, such as C-C bonds. We recently discovered that a viable strategy may lie with the endogenous solid-state polymerisation (without the presence of solvents, initiators or catalysts) of structurally pre-organised monomers in their crystalline state. This takes place via thermal-initiated dehalogenation and concomitant C-C coupling where the building blocks themselves could act as templates to kinetically confine the propagation of the sheet to the 2D space (see Figure 2) [3]. These 2D polymers have very well-defined pore sizes which are suitable for ultrafiltration and molecular separation membranes in water treatment. They are also suitable for ion storage in batteries and supercapacitors. Two examples,

2D-CAP-1 and 2D-CAP-2 shown in Figure 2, have highly homogeneous pore sizes of six angstroms and five angstroms, respectively.

1D Nanoribbons

Besides 2D TMDs, we have also built the smallest or narrowest one-dimensional (1D) TMD ribbon, constructed entirely of basic molecular scale building blocks [4]. By making use of surface-templated assembly, atomic units of Mo and S, or Mo and Se, were assembled to create the narrowest possible nanoribbons (see Figure 3). In these 1D nanoribbons, the edge atoms can dominate the properties of the system relative to the basal plane atoms, leading to exotic electronic superlattice and magnetic ordering. Our studies show that the pliability of the single MoS2 nanowire allows it to form strong coupling with the substrate. This gives rise to a substrate-modulated superlattice potential along the edges of the

nanoribbon. The existence of these oscillating edge states generates well-defined periodic transmission channels for studying coherent transport and quantum interference in low-dimensional nanodevices.

Going forward, I believe that chemists can contribute greatly to the success of 2D materials research by developing cost-effective methods for scaling up the production of these materials and applying them in a wide range of applications. Other than 2D inorganic materials, 2D organic materials such as covalent organic frameworks and 2D hybrid organic-inorganic perovskites offer great flexibility in the design and composition for new technological applications.


Figure 2: Endogenous polymerisation of 2D polymers from pre-packed crystalline monomers. 2D polymers synthesised by these methods have highly homogeneous pores [3].

Figure 3: Bottom-up synthesis of MoS2 nanowire array on the vicinal Au surface. (a) Schematic illustration of the step-templated growth of MoS2 nanowires on the Au(755) surface. (b) Large-area STM image of MoS2 triangular nanoclusters and MoS2 nanowires grown on an Au(111) surface. (c) Large-area STM image of MoS2 nanowires aligned along the steps on the Au(755) surface. (d) Magnified view of the rectangle-enclosed region in (c) shows the atomic-resolved lattice of a single MoS2 nanowire (S atom positions are indicated by yellow circles). Scale bars are 10 nm in (b) and (c), and 1 nm in (d).

Strange optics of ultra-thin semiconductorsA new approach to engineering light-matter interaction

Let There be Light

Our interpretation of the world largely relies on the visual information acquired through the process of light entering and stimulating our eyes. What we “see” is light after it has gone through some form of interaction with our surroundings. We can identify one substance from another simply by looking at them because light interacts with different substances in specific ways. A piece of glass appears transparent because light pass through the material without being absorbed. A piece of metal is shiny and reflective because light tends to bounce off its surface before entering our eyes. The black color of charcoal is a result of efficient absorption and attenuation of light. We can enjoy the richness of the hue and texture of the objects around us with our eyes thanks to the physics that governs the interaction between light and matter.

Light-Matter Interaction

This interaction between light and matter is closely related to the basic properties of matter at a fundamental level. Let’s take solid materials for the sake of discussion. For example, materials that conduct electricity well like silver and copper reflect light efficiently. This is because the electrons (carriers of electricity) in the material respond to light such that they prevent light from propagating below the surface. On the other hand, materials that are optically transparent tend to be electrically insulating. For this reason, materials that are both electrically conducting and optically transparent are rarely found in nature. However, materials with both of these characteristics are essential for many technological applications such as solar cells, displays, and touch screens. A conventional approach to preparing such a material involves introducing

impurities into a transparent electrical insulator until it becomes semi-transparent and moderately conducting. This approach is widely used in industry but suffers from the inevitable trade-off between electrical conductivity and transparency. The big challenge is finding an alternative approach to realising materials with these nominally incompatible characteristics.

Transparent Metal

My research team recently discovered a unique approach to making metals semi-transparent to light. This unusual effect was realised by hybridising metal particles with atomically thin semiconductor layer. We found that metal particles placed on top of an ultrathin semiconductor layer are more transparent to light than the metal particles alone (see Figure 1). This observation is counterintuitive because attenuation of light is an additive process. That is, the more material light interacts with, the more attenuated its intensity, and therefore less light transmitted. In this case, the metal-semiconductor hybrid should appear more opaque compared to metal particles alone. Surprisingly, our observation was just the opposite of the general expectations.

How does a thin veil of semiconductor

make metal particles “disappear”? In a joint effort with international collaborators, we found that this peculiar effect is not simply a result of light interacting with metal and semiconductor independently, but is related to a cooperative interaction between light and the excited states of the two materials. Upon illumination, metal particles and the semiconductor layer can independently capture the energy of light and become excited for a short period of time. When the two materials are in close proximity with each other, however, they are not only excited by light but also by each other. Once excited by light, the metal particles can transfer its energy to the semiconductor layer, and the reverse route is also possible. We attribute the enhanced light transmission to the complex exchange of energy between light, the metal particles, and the semiconductor layer. The exact nature of this phenomenon is still open to discussion but it is evident that the interaction between the two materials plays a fundamental role.

Resonating Strings

The observed energy transfer is a resonance process, which is similar to resonance between guitar strings. If you place two guitars side by side and pluck a string on one of them, the


RESEARCH FEATURES

Figure 1: A beam of light transmitted through silver particles placed on top of an ultrathin semiconductor layer.

EDA Goki is an Assistant Professor in both the Departments of Physics and Chemistry, NUS. He also holds joint appointments in the Centre for Advanced 2D Materials and the Solar Energy Research Institute of Singapore, NUS. He has done extensive research work on a variety of two-dimensional (2D) materials such as graphene oxide and molybdenum disulfide. He was awarded the President’s Science and Technology Young Scientist Award 2015 for his research in the fundamental properties of 2D materials. He is also a recipient of the IPS Omicron Nanotechnology Award and the NUS Young Scientist Award.

Please visit http://www.physics.nus.edu.sg/~phyeda/index.html for more information about the research work of his Nanomaterials & Devices Group.

same string on the other guitar will start vibrating because the two strings are in resonance with each other. We believe that a similar resonance process takes place over a nanoscopic distance in our hybrid system and with high efficiency. According to theory, this resonant energy transfer process must be very fast in order for the enhanced transparency effect to be observable. But how fast is fast? Typical time scale of ordinary energy transfer processes is known to be around one billionth of a second. This may sound like a fast process but our estimate for our hybrid system is even faster by thousand times. That is, energy is exchanged in the hybrid within one trillionth of a second!

In this study, the transparency effect was limited to red and near-infrared light and the degree of transmission was well below 100 %. But we believe that it will be possible to significantly enhance the transparency window by more precisely engineering the structure of the metal at the nanometer-scale and preparing a high quality semiconductor layer. This unconventional and unique approach to engineering optical response of materials could open up avenues to many technological applications from displays to optical communication.

Close but not Too Close

In what other systems do we expect to find such fast energy exchange and interesting optical phenomena? The rate of resonant energy transfer is known to increase quickly for decreasing separation between the donor and acceptor of energy. So the excited states of the resonating systems need to be brought as close as possible

to achieve the ultimate energy transfer rates. But the two excited states need to be sufficiently separated that they maintain their unique identity. Based on these considerations, we investigated another kind of a nano-hybrid system consisting of two different ultrathin semiconductor layers stacked together. Each layer is only three-atom thick and the two layers are naturally separated by a tiny gap that can barely fit a small atom.

We used a technique called photoluminescence excitation spectroscopy to study this new hybrid. Here we excite the system with a laser beam and detect the light that is emitted as a result of de-excitation. For carefully prepared hybrid stacks, we found, under specific conditions, that excitation of one layer results in light emission from the other layer (see Figure 2). This is an indication that the energy of the laser beam that was first captured by one layer was transferred to the other layer before it was released

as light. We estimated the time scale of this process and found it to be within one trillionth of a second, similar to the case of the metal-semiconductor hybrid. Such a fast energy transfer rate is among the fastest reported to date for nano-hybrid systems.

There are many implications to such ultrafast energy transfer dynamics. From a technological point of view, fast and efficient capture and delivery of light energy to a desired location is important for realising efficient solar energy harvesting. For this reason, ultrathin semiconductors that we used in these studies are attractive building blocks for solar cells. From a fundamental point of view, ultrafast energy transfer is a key ingredient in realising novel quasi-particles with unique hybrid characteristics of the interacting excited states. It is an exciting scientific and technological challenge to engineer nano-hybrid systems to realise such new states of matter and unconventional optical phenomena.


Figure 2: Illustration of energy transfer between two semiconductor layers. The process is triggered by excitation with a green laser (green arrow) and results in emission of red light (red arrow).

Oxides for magnetic refrigerationMagnetic oxides can be used for solid state cooling over a wide temperature range

Introduction

Magnets have numerous applications ranging from a simple button in a cell phone pouch to powerful electrical motors in hydroelectric power stations, aeroplanes and windmills. Magnetic materials are also used in non-conventional applications such as magnetic refrigeration. Magnetic refrigeration is a novel cooling/ heating method in which a magnetic material exposed to a changing magnetic field cools down or heats up depending on its magnetic state.

Refrigerators and air conditioners have become indispensable in daily life. A refrigerator removes heat using a gas compression/ expansion cycle. Most of them use Hydrochlorofluorocarbon (HCFC), which is harmful to the ozone layer. The energy efficiency of a refrigerator is also low, at only 40% of the ideal Carnot engine. Magnetic refrigeration (MREF) is a viable technology to replace gas compression/ expansion-based refrigerators. It uses only solid (magnetic) materials and a water-ethanol mixture as heat exchange fluid instead of HCFC and it does not need a bulky compressor to work. Another advantage of MREF is the possibility of cooling over a wide temperature range, from room temperature to micro kelvin.

Physics behind Magnetic Refrigeration

The physics behind MREF is similar to that of gas compression/ expansion cycle for refrigeration. In the absence of a magnetic field, the spins of electrons in a solid are disordered (point randomly), leading to high magnetic entropy (Sm). Under the influence of a magnetic field, they become ordered (aligned along the magnetic field direction) and the magnetic entropy (Sm) decreases. Apart from electrons’ spin entropy,

the entropy of lattice vibrations (Sl) and the electron occupancy (Sel) also contribute to the total entropy of the solid (Stotal = Sm+Sl+Sel). In an adiabatic process, there is no flow of heat (DQ = 0) from the sample to the surrounding and the total entropy change is zero (Stotal remains a constant). Sel is usually independent of the influence by a magnetic field.

The process starts with the adiabatic application of an external magnetic field to a magnetic solid. This reduces its spin entropy (Sm reduces) and increases the lattice entropy (Sl increases), causing the solid to heat up when magnetised. This excess heat is removed by a coolant (usually water) and then the magnetic solid

is isolated from the surrounding and demagnetised. This causes the magnetic entropy (Sm) to increase. However, as there is no heat exchange with the surroundings, the lattice entropy (Sl) decreases to conserve the total entropy, causing the temperature of the solid to go below its starting value. This phenomenon is known as the magnetocaloric effect (MCE) (see Figure 1).

Although paramagnetic salts have been used for the past 60 years to reach 0.1 K, the general scientific community, with the exception of a few specialists, paid little attention to the discovery of new magnetic materials which can be used for solid state cooling over a wide temperature range, from room


Figure 1: Illustration of the magnetocaloric effect used in the magnetic refrigeration cycle. (1) Antiferromagnetic EuTiO3 is adiabatically magnetised by applying external magnetic fields. Arrows indicate antiferromagnetic spin alignment of Eu2+:4f7 spins in zero field; (2) Antiferromagnetic spin configuration changes to ferromagnetic and the sample “heats up” by DTad due to an increase in lattice entropy; (3) Heat is removed and the temperature of the sample goes back to the original state as in (1) but with ferromagnetic spin alignment; (4) Magnetic field is removed adiabatically, i.e. the sample is demagnetised. Temperature of the sample decreases below the starting temperature in (1). The above cycle then repeats itself. Inset: Temperature dependence of magnetic entropy (Sm) without (H = 0) and with a magnetic field H. DSm is the isothermal magnetic entropy change and DTad is the adiabatic temperature change.

RESEARCH FEATURES

Ramanathan MAHENDIRAN joined the Department of Physics, NUS in 2005 and has been an Associate Professor since 2012. He obtained his Ph.D. in Condensed Matter Physics from the Indian Institute of Science, Bangalore, India in 1997. His current research focuses on spintronics; thermoelectric, magneto and electrocaloric effects; magnetostrictive, multiferroic properties and high frequency magneto-transport in transition metals oxides; Heusler alloys and other novel materials.

References[1] Midya A, et al., “Large adiabatic temperature and magnetic entropy changes in EuTiO3” PHYSICAL REVIEW B Volume: 93 Issue: 9 Article Number: 094422 DOI: 10.1103/PhysRevB.93.094422 Published: 2016.

[2] Rubi K, et al., “Giant magnetocaloric effect in magnetoelectric Eu1-xBaxTiO3” APPLIED PHYSICS LETTERS Volume: 104 Issue: 3 Article Number: 032407 DOI: 10.1063/1.4862981 Published: 2014.

[3] Km Rubi and R. Mahendiran, to be submitted to Nature series journal.

[4] Repaka DVM, et al., “Magnetocaloric effect and magnetothermopower in the room temperature ferromagnet Pr0.6Sr0.4MnO3” JOURNAL OF APPLIED PHYSICS Volume: 112 Issue: 12 Article Number: 123915 DOI: 10.1063/1.4769876 Published: 2012.

temperature to a few kelvin below the boiling temperature of liquid helium (4.2 K). Following global warming in recent years, magnetic refrigeration is now receiving renewed attention.

Transition Metal Oxides

Over the last few years, we have been researching the electrical and thermophysical responses of transition metal oxides to external electrical and magnetic fields. Our research focus include:

(i) generation of electrical voltage due to imposed temperature gradient under magnetic field (magneto-Seebeck effect or magneto-thermopower);

(ii) magnetic and electrical field modulation of resistance

(magnetoresistance and electroresistance);

(iii) magnetic field induced temperature and entropy changes (magnetocaloric effect); and

(iv) magnetic field induced shape change (magnetostriction).

We are particularly interested in EuTiO3 oxide, which belongs to the perovskite titanate family (RTiO3 where R is a rare earth ion). EuTiO3 is antiferromagnetic insulator at liquid helium temperature. It has magnetically active Eu2+ ion with spin polarised 4f-band (4f7) and ferroelectric active Ti4+ ion with empty d-shell (d0). This is unique among RTiO3 oxides because most other rare earth ions (e.g., R = Gd, Y, Pr) adopt 3+ valance states along with Ti in trivalent state as Ti3+ (d1). We found that by applying a magnetic field of 7 T at 5 K, the temperature of the sample increases by DTad = 22 K [1] (see Figure 2) to 27 K. This means that if the sample temperature is 27 K initially and the magnetic field is adiabatically reduced from 7 T to 0 T, the temperature of the sample will decrease to 5 K. This is the highest adiabatic temperature change found in transition metal oxides. This effect could potentially be used to liquefy hydrogen gas, a clean energy source. Prior to this work, we have also shown that the oxide series Eu1-

xBaxTiO3, which changes behaviour from being an antiferromagnetic insulator (x = 0) to a paramagnetic ferroelectric (x = 0.9), shows large changes in its magnetic entropy, more than any other perovskite oxides [2]. In addition, we found very large

magnetoresistance effects in EuTiO3 [3]. The magnetoresistance could change up to 98% in a field of 0.5 T and it shows a marginal increase to 99.99% when the field is further increased from 0.5 T to 7 T at 2 K (see Figure 3). This is the first time that such a large magnetoresistance effect has been observed in a rare earth titanate family.

EuTiO3 has many exotic properties which could be developed for new applications. We intend to investigate the impact of charge carrier doping in EuTiO3 on its electrical and thermophysical properties in the coming months. While EuTiO3 could be an excellent magnetic material to reach temperatures below 25 K by adiabatic demagnetisation, we are also exploring other magnetic oxides for magnetic cooling applications at room temperature. The manganite series Pr1-xSrxMnO3 (x = 0.4 - 0.6) could be a potential candidate [4].


Figure 2: A three-dimensional contour plot of adiabatic temperature change (DTad along z- axis) upon magnetisation as a function of the starting temperature (x-axis) and increasing magnetic field (y-axis). Adiabatic application of 7 T magnetic field at 5 K causes temperature of the sample to increase to 27 K. This implies that if a magnetic field of 7 T is applied at 27 K and removed adiabatically, the temperature of the sample will reduce to 5 K. This could have potential applications for liquefying hydrogen gas.

Figure 3: Resistivity of EuTiO3 as a function of temperature under different magnetic fields. Note that the resistivity decreases by more than 4 orders of magnitude under 7 T. Inset: Magnetic field dependence of magnetoresistance. It indicates major change in the magnetoresistance at 2.5 K, occurring below 0.5 T.

Excitons in correlated electron systemsNewly discovered resonant excitons may open up new optical applications

Introduction

Electron-electron (e-e) and electron-hole (e-h) interactions are often associated with many exotic phenomena in correlated electron systems. These phenomena result in transition metal oxides having unique physical properties, which include high-temperature superconductivity, unusual insulator-metal transition, half-metallic ferromagnetic effects, and colossal magneto resistance. These properties, which are interesting for fundamental science, also open up new technological applications.

One outcome of the e-h interaction in conventional semiconductors is the formation of an exciton. An exciton, which is an elementary excitation of condensed matter, results from the e-h pair bound by the Coulomb potential. It is an electrically neutral quasiparticle of coherent light–matter interaction which is important for optical processes such as photosynthesis. In conventional semiconductors, excitons typically appear below their optical band gap, from far infra-red to the visible spectrum.

Recently, our research team discovered a new type of excitons in transition metal oxides, known as resonant excitons. These resonant excitons appear well above its optical band gap. This new observation not only introduces a new physical concept but may open up new optical applications in the ultraviolet to deep ultraviolet spectrum. Below, we discuss briefly our observations of resonant excitons in Ta1-xTixO2 and graphene.

Emerging Giant Resonant Exciton Induced by Ta Substitution in Anatase TiO2: A Tunable Correlation Effect [1]

Doped or defective titanium dioxide

(TiO2) exhibits unique electronic transport and optical properties. TiO2 is opaque in visible sunlight and highly efficient at absorbing ultraviolet (UV) light. This makes it particularly useful for photocatalysis applications. The first step in photoexcitation is the formation of e-h pair quasiparticles (excitons), which may either recombine or decay into free charges. These free charges react with molecules on the material surface, enhancing the photocatalytic effects by forming reactive free radicals. Although excitons and their spatial behaviour play a key role, the precise nature and behaviour of excitons in TiO2-based materials remain unclear in some aspects.

Many-body e-e and e-h interactions determine the physical properties of excitons. Excitons usually occur below the direct band gap in semiconductor and insulator materials, but they may involve higher energy bands when there is strong electronic correlation. Unlike excitons in conventional semiconductors, the resonant excitons can occur at energy bands well above the optical band gap of the material. This effect can be probed directly using high-energy optical conductivity [2]. A detailed understanding of the role of e-e and e-h interactions in TiO2-based materials remains elusive and resonant excitons have not yet been observed in the material. The main research focus is on tuning its optical properties, which is usually done via doping and/ or nano-engineering. However, understanding the role of d-electrons in materials and the possible functionalisation of d-electron properties are still major challenges.

Using a combination of an innovative experimental technique, high-energy optical conductivity and state-of-the-art ab initio electronic structure calculations, we recently reported an emerging, novel resonant exciton

in the deep ultraviolet region of the optical response (see Figure 1). The resonant exciton develops from the low-concentration Ta substitution in anatase TiO2 films. It is surprisingly robust and is related to strong e-e and e-h interactions.

We argue that these experimental findings show that tunable e-e and e-h correlations play a key role in the observed resonant excitons in the TaxTi1−xO2 system and can be used in a model for resonant excitonic effects.

Observation of Room Temperature High-Energy Resonant Excitonic Effects in Graphene and Conductivity Renormalisation of Graphene on SrTiO3 Due to Resonant Excitonic Effects Mediated by Ti 3d Orbitals [3,4]

Graphene, the thinnest material to be successfully isolated, shows prominent many-body effects of


RESEARCH FEATURES

Figure 1: Room temperature measurements of the real part of the optical conductivity for pure TiO2 (solid black line), 1.8% (short blue dotted line), and 3.8% Ta-substituted TiO2 (red dashed-dotted line). The polarisation vector is perpendicular to the [001] direction. Inset: Details of the real part of the optical conductivity in the visible and low-ultraviolet regions.

Andrivo RUSYDI is an Assistant Professor at the Department of Physics, NUS. He received his Ph.D. in Physics from University of Groningen, the Netherlands in 2006. Prior to joining NUS, he was a postdoctoral fellow at University of Hamburg, Germany, where he has been part of a team to develop VUV-Raman Spectroscopy at the Centre for Free-Electron Laser Science.

References[1] Yong Z, et al., “Emerging giant resonant exciton induced by Ta substitution in anatase TiO2: A tunable correlation effect” PHYSICAL REVIEW B Volume: 93 Issue: 20 Article Number: 205118 DOI: 10.1103/PhysRevB.93.205118 Published: 2016.[2] Rusydi A, et al., “Metal-insulator transition in manganites: Changes in optical conductivity up to 22 eV” PHYSICAL REVIEW B Volume: 78 Issue: 12 Article Number: 125110 DOI: 10.1103/PhysRevB.78.125110 Published: 2008.[3] Santoso I, et al., “Observation of room-temperature high-energy resonant excitonic effects in graphene” PHYSICAL REVIEW B Volume: 84 Issue: 8 Article Number: 081403 DOI: 10.1103/PhysRevB.84.081403 Published: 2011.[4] Gogoi PK, et al., “Optical conductivity study of screening of many-body effects in graphene interfaces” EPL (Europhysics Letters) Volume: 99 Issue: 6 Article Number: 67009 DOI: 10.1209/0295-5075/99/67009 Published: 2012.

e-e and e-h interactions, which can be manipulated by using substrate materials. The role of interacting quasiparticles in graphene, particularly in the form of considerable e-e and e-h interactions, has been reported. These include the renormalisation of the Fermi velocity with distortion of the Dirac cone, the fractional quantum Hall effect and prominent excitonic effects and interactions, which

occur at high energy in the optical absorbance spectra (see Figure 2). In the context of many-body effects, optical conductivity measurements of graphene allow one to reveal the roles of both e-e and particularly e-h interactions as optical transitions involve the creation of concomitant e-h states.

Recently, we presented evidence of a drastic renormalisation of the

optical conductivity of graphene on SrTiO3 resulting in almost full transparency in the ultraviolet region (see Figure 3). These findings are attributed to resonant excitonic effects and verified by ab initio Bethe-Salpeter equation and density functional theory calculations. The (π,π*) orbitals of graphene and Ti-3d t2g orbitals of SrTiO3 are strongly hybridised and the interactions of e-h states residing in those orbitals play a dominant role in graphene optical conductivity. These results open a possibility to manipulate interaction strengths in graphene via d orbitals, which could be crucial for optical applications.


Figure 3: Real part of the optical conductivity [σ1(ω)]. (a) The axis in left is for sheet conductivity, σ1(ω) of GSiO2/Si (in brown) while the right axis is for the bulk conductivity of SiO2 (in indigo). (b) The axis on the left is for sheet conductivity, σ1(ω) of GSrTiO3 (in blue) while the right axis is for the bulk conductivity of SrTiO3 (in orange).

Figure 2: Optical conductivity (σ1) of (a) substrate and graphene (N = 1,2,8) and (b) graphene (N = 22,75) and graphite shows three peaks at 5.4 eV (label A), 6.3 eV (high-energy resonant excitons, label B), and 14.1 eV (label C). Inset 1 shows the comparison of σ1 between experimental data and theoretical calculations. Insets 2 and 3 show the σ1 on an expanded scale at various energy ranges.

Copy

right

201

1 by

the

Amer

ican

Phy

sical

Soc

iety

.Fi

gure

cre

dit:

Adap

ted

with

per

miss

ion

from

[3].

Two-dimensional materials for wearable electronicsHigh crystallinity combined with atomic thinness unlock some unique applications for two-dimensional (2D) materials in the field of human-machine interfaces and flexible electronics

Introduction

Communication between humans and machines has historically relied on a multitude of mechanical devices, including switches, levers, sliders, knobs and push buttons. It is only recently that electronic methods of interfacing have emerged, such as capacitive touch sensing, which are driven by the rise of display-based consumer electronic devices.

Today, touch-based interfaces can be found in almost all consumer devices, ranging from mobile phones, tablets and other portable devices, to household appliances, automotive and industrial applications. Among all the sensing methods, capacitive touch sensors have found favour over competing methods such as resistive or infra-red diode-based touch sensing, primarily due to their compatibility with multi-touch technology, fast response times and high level of accuracy.

Capacitive touch sensing relies on measuring small changes in capacitance, which occur whenever a human finger is introduced in the vicinity of the sensor. The material used to detect these capacitance changes is usually a transparent conducting oxide, such as Indium Tin Oxide (ITO), whose low sheet resistance allows it to read small changes in capacitance with a high signal-to-noise ratio.

Unfortunately, oxide-based materials such as ITO suffer from an inherent brittleness that prevents them from achieving the same level of capacitive sensing performance for emerging applications such as flexible electronics and wearable systems. Next generation flexible and wearable devices, in the era of internet of things, require a completely new set of electronic materials.

Two-Dimensional Materials

One of the most important developments in materials science from the past decade was the discovery of a new class of nano-materials, called two-dimensional (2D) crystals. The first such material to be isolated and characterised was graphene, which in 2010 was recognised with a Nobel Prize in Physics to Sir Andre GEIM and Sir Konstantin NOVOSELOV. Graphene is a single sheet of perfectly-ordered carbon atoms and can be viewed as a monolayer of graphite. Graphene has the thickness of a single atom, which is more than a million times thinner than a strand of hair, and yet possesses considerable mechanical strength and good electrical conductance. Since this discovery, research has expanded towards a multitude of atomically-thin 2D materials which researchers hope could play a pivotal role in the development of next generation technology.

What makes 2D materials special is the marriage of high crystallinity with atomic thinness. The near-perfect lattice structure allows these materials to exhibit excellent transport of electric charge, spin polarisation, and heat waves, as well as high mechanical tensile strength, and near-perfect gas impermeability. The atomic thinness provides an additional set of desirable physical properties, such as mechanical flexibility, optical transparency and 2D electron confinement, which are not normally associated with crystalline solids. The combination of these properties in a single material makes 2D crystals one of the most exciting platforms for fundamental and applied research.

Beside some common defining properties, each 2D material comes with its own characteristic, expressed by its electronic band structure.

Depending on the constituent atomic elements, 2D crystals can be insulators, conductors, semiconductors, or could even exhibit complex behaviour like superconductivity. Most of the research has been devoted to semiconducting 2D crystals, because they are expected to be technologically important for next generation electronic devices (for example, transistors, light detectors, diodes, and solar cells). A semiconductor material is able to operate both as a good electrical conductor and an insulator. Despite its excellent conduction properties, graphene cannot operate as an insulator, because it does not have energy states with no free electrons (or a band gap). Therefore, most research focused on semiconducting 2D crystals with the chemical formula MX2 where M is a transition metal atom (usually Mo or W) and X is a chalcogen (S, Se, or Te).

A New Two-Dimensional Material: Phosphorene

In 2014, our team at the Department of Physics, NUS was among the first to experimentally isolate and study a new 2D crystal, called phosphorene [1]. Similar to graphene, it consists of a single sheet of ordered atoms, but instead of carbon, phosphorene is built from phosphorus (arranged in a slightly more complicated 2D lattice). Most importantly, phosphorene was found to be an excellent semiconductor that exhibits both an insulating state which is better than in graphene, and electrical conduction which is higher than the MX2 materials.

As with any new material, phosphorene also comes with a set of specific challenges related to its isolation, purification, and practical device fabrication. The product of intense research, the solutions to these material-specific problems and


RESEARCH FEATURES

Barbaros ÖZYILMAZ is a Professor in the Department of Physics, NUS. He is also the Head of Graphene Research and Deputy Director (Translation) in the Centre for Advanced 2D Materials, NUS. He obtained his Ph.D. from New York University and did his postdoctoral research at Columbia University, joining NUS in 2007. He received the NRF Fellowship Award in 2008 and was awarded the NUS Young Scientist Award in 2013.

References[1] Koenig SP, et al., “Electric field effect in ultrathin black phosphorus” APPLIED PHYSICS LETTERS Volume: 104 Issue: 10 Article Number: 103106 DOI: 10.1063/1.4868132 Published: 2014.[2] Doganov RA, O’Farrell ECT, Koenig CP, et al., “Transport properties of pristine few-layer black phosphorus by van der Waals passivation in an inert atmosphere” NATURE COMMUNICATIONS Volume: 6 Article Number: 6647 DOI: 10.1038/ncomms7647 Published: 2015.[3] Avsar A, et al., “Air-Stable Transport in Graphene-Contacted, Fully Encapsulated Ultrathin Black Phosphorus-Based Field-Effect Transistors” ACS NANO Volume: 9 Issue: 4 Pages: 4138-4145 DOI: 10.1021/acsnano.5b00289 Published: 2015.[4] Koenig SP, Doganov RA, et al., “Electron Doping of Ultrathin Black Phosphorus with Cu Adatoms” NANO LETTERS Volume: 16 Issue: 4 Pages: 2145-2151 DOI: 10.1021/acs.nanolett.5b03278 Published: 2016.

their overall practicality ultimately determine the faith of any new material. In the case of phosphorene, the major initial challenge was the gradual reaction of the phosphorus atoms with ambient air and the resulting degradation of the 2D crystal. We have developed a method to protect phosphorene by covering it with another impermeable 2D crystal in an oxygen- and moisture-free environment [2,3]. This allowed a better understanding of how to control the amount of electrons in phosphorene and enabled the fabrication of a state-of-the-art electronic device, which can serve as a zero and one logic gate, similar to those found in computer processors [4].

Research on 2D materials is still expanding and the field offers plenty of opportunities for scientific and technological breakthroughs. Graphene and 2D MX2 crystals are all likely to find their own places. Phosphorene, a new material, has become one of the most exciting 2D crystals in a span of less than three years. The challenges are aplenty, but the future of 2D crystals holds great promise.


Figure 1: Visuals and schematics of next generation transparent and flexible electronics.

Figure 2: (a) The lattice structure of two layers of phosphorene. (b) Atomic force microscopy image of phosphorene partially covered with graphene. The white dashed lines indicate the regions protected from degradation by graphene [2]. (c) A schematic of a phosphorene-based logical inverter device. (d) Electrical characterisation of a 0 - 2V phosphorene logical inverter.

Phot

o cr

edits

: (m

iddl

e im

age)

wiki

med

ia.o

rg, ht p

s://c

omm

ons.

wiki

med

ia.o

rg/w

iki/F

ile:To

uchS

cree

n_ca

pacit

ve.

svg,

Mer

cury

13,

Figu

re c

redi

t: Ad

apte

d w

ith p

erm

issio

n fr

om [4

].Co

pyrig

ht 2

016

Amer

ican

Che

mic

al S

ocie

ty.

CC-B

Y-SA

3.0

(rig

ht im

age)

f ick

r.com

, ht p

://w

ww

.f ick

r.com

/ph

otos

/rde

com

/414

6880

795/

, U.S.

Arm

y RED

COM

, CC-

BY 2

.0.

Documents

RESEARCH - National University of · PDF fileTable of Contents Faculty of Science Research ... BioBoss tiles used in performing large-scale field experiments at Pulau Hantu. ... film