2D Nanoflakes of MoS2 Towards Efficient Photoluminescence Sensing and Hydrogen Evolution Reaction
Central University of Kerala, India
The demand for protection of biosphere and maintaining the ecological balance are important governing factors for a sustainable future. Among these, the fabrication of sensors that can detect various analytes are of considerable technological importance in environmental monitoring, medical diagnosis and in industrial workplace. As an emerging family of materials to replace silicon, layered transition metal dichalcogenides (TMDs) have aroused great interest due to their unique chemical, mechanical, electronic and optoelectronic properties. MoS2, having an indirect to direct band-gap transition, is a well-known TMDs that exhibits novel electronic and optoelectronic properties in the atomic scale as well as its applications such as transistors, catalysts, LEDs, sensors etc. In this work, we report the excitation wavelength dependent down-conversion and up-conversion photoluminescent behavior of hydrothermally grown MoS2 nanostructures and a viable designing of MoS2-based optical photoluminescence sensors towards serious pollutants, some of which also serves as a metabolic input in biosphere. Increased sensitivity towards certain analytes demonstrates MoS2 sensors can help in the diagnosis of diabetics and kidney dysfunctions such as liver cirrhosis. In addition, this work also elucidates the catalytic property of MoS2 towards hydrogen production which is comparable to that of noble metals. The distinctive optical properties and HER activity originate from the structural and electronic modulations in MoS2 acquired through quantum confinement effects.
A Chemical Method for MoS2 Film Synthesis
1University of Southampton, United Kingdom; 2Institute of Materials Research & Engineering, Agency for Science, Technology and Research (A*STAR), Singapore
Since the discovery of graphene in 2004, the nanoscience world has sought 2D materials with bandgaps suitable for nanoscale optoelectronics. Transition metal dichalcogenides (TMDCs) are one class of 2D materials possessing a direct bandgap in the monolayer limit. Exfoliation of TMDCs has been used in producing crystalline monolayers for fundamental materials studies and proof-of-concept device applications. Scalable techniques for synthesising will have a significant impact on research and practical applications. The challenges in synthesis have been in obtaining defect-free crystals on a large scale. Physical (PVD) and chemical vapour deposition (CVD) have been extensively studied for the synthesis of graphene, h-BN, HfO2 and MoS2 with some success. Liquid-based chemical techniques have the advantages of low cost and large volume. They have been used for the synthesis of metal—organic films and metal oxides such as TiO2. Here we report a surface-limited chemical method for the synthesis of MoS2 films based on modification of existing deposition techniques, and the detailed characterisation of the films by Raman spectroscopy, photoluminescence, scanning electron microscopy and transmission electron microscopy.
Anticorrosion and Antibiofouling Graphene-Based Coating for Marine Industry
1Nanyang Technological University, Singapore; 2Lomonosov Moscow State University, Russian Federation
Corrosion and biofouling are serious global problems that cost hundreds of billion dollars annually. Current coating technologies suffer from poor performance, environmental concerns, and high cost. The protective coating is the most widespread technology among various methods used in anti-corrosion and anti-fouling. Traditional coating materials (e.g. Cr-based, Zn-based or biocide-containing anti-corrosive/fouling coatings) suffer from poor performance, environmental hazard and/or high cost. Present work deals with developing coating materials with graphene oxide additives that provide triple protection (anticorrosion, antifouling and anti-decay) and are also environmentally friendly, non-cytotoxic, low cost and long lasting.
Graphene consists of one atomic layer of sp2-hybridized carbon atoms in the hexagonal lattice. The graphene layer is impermeable to all atoms and molecules at room temperature, which is the most promising barrier material for protective coating. Graphene also possesses other outstanding properties that ideally suited for coating applications: extremely high mechanical strength, exceptional chemical inertness, non-cytotoxicity, outstanding thermal and electrical conductivity, and very strong absorption of UV and visible light to protect the paint from degradation.
The composite coating with epoxy resin was prepared in the wide range of graphene oxide concentration from 0,00075% to 0,4%, applied to stainless steel 316L and corrosion behavior was investigated by electrochemical measurement. According to the Tafel plot experiment, the potential and current of the corrosion were obtained and corrosion rate was calculated. The investigation of the graphene oxide – epoxy polymer coating showed good corrosion protection properties and corrosion rate had not linear dependence on concentration due to changing the stages of corrosion protection mechanism which were described. It was detected that the concentration behavior of corrosion rate had two minimum and addition 0,003% or 0,047% of graphene oxide decreased corrosion rate more than three times.
Aptamer-based Sensor with AIEgen for Detection of Cocaine
Department of Chemistry, Hong Kong University of Science and Technology (HKUST), Hong Kong S.A.R. (China)
Cocaine is a kind of strong central nervous system stimulant and also one of the most frequently used illegal drug globally. Aptamers are single-stranded RNA or DNA molecules that can specifically bind to targets with high affinity. With aggregation-induced emission luminogens (AIEgens), aptamer can be used as biosensor to detect specific target. Here, we report a simple and label-free method of aptamer-based AIEgen sensor allowing for cocaine detection. In the presence of cocaine, the sensor would associate with cocaine to cause conformational change and form a tripartite complex being prevented from absorbing to graphene oxide (GO). A turn-on signal can be achieved. Without cocaine, aptamers and AIEgens would be absorbed by GO resulting in fluorescence being quenched. The proposed method has advantages of selectivity, cost-effectiveness, and rapidness, which holds great potential for in-situ drug detection and analysis.
Control the Electronic Structure in Semimetal MoTe2 at the Atomic Scale
1Department of Chemistry, National University of Singapore, Singapore; 2Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore
The emergence of layered transition metal dichalcogenides (TMDs) enables intense studies down to atomic scale, where new phenomena and properties have been continuously revealed. The semiconducting hexagonal (2H) phase TMDs have been involved in most studies because the 2H phase are the most thermodynamically stable phase for most TMDs. One exception is MoTe2, which can form stable structure both in 2H semiconducting and 1T’ (monoclinic) metallic phase. In the bulk form, 1T’ MoTe2 has been observed to show the signature of type II Weyl semimetal. Here, we show that the electronic structure of 1T’ MoTe2 can be well tuned by its dimension. By thinning down the crystal from bulk to atomic scale, we observed metal-insulator transition and different gate tunable magneto-transport behavior. Both the localization and antilocalization effect affect the evolution of electronic structure with layer number and we will discuss in detail through the combination of transport measurements and calculations.
Correlated Fluorescence Blinking in 2D Semiconductor Heterostructures
Nanyang Technological University, Singapore
Two-dimensional (2D) van der Waals heterostructures that are stacked vertically in Lego-like fashion provide additional knobs by which to manipulate the electron motions, leading to unprecedented functionalities that are beyond the reach of any single component. Pioneering progresses with this concept have demonstrated bright future in design and fabrication of new materials, both for exploration of new fundamental physics and real-world device applications. Here, by the construction of loosely stacked 2D semiconductor heterostructures with a type-II band alignment, we uncover a correlated fluorescence blinking effect. As an interesting and puzzling phenomenon, fluorescence blinking, i.e., random switching between ‘ON’ (bright) and ‘OFF’ (dark) states of an emitter, has been widely studied in zero-dimensional (0D) quantum dots and molecules, and scarcely in one-dimensional (1D) systems. A generally accepted mechanism for blinking in quantum dots is related with random switching between distinct charge state (or accompanied by fluctuations in charge-carrier traps). Unlike single electron charging and discharging events in quantum dots, exciton fluctuations due to intermittent interlayer carrier transfer (IICT) process is traced to be the origin of blinking in 2D heterostructures, which is a collective effect and the two monolayer components are always negatively correlated. For instance, whereby a bright emission state occurs in one monolayer while a dark state occurs in the other, and vice versa. Steady-state and transient fluorescence cross-correlation spectroscopy experiments have been carried out to investigate the details of this new type of blinking. Our results provide a unique platform for the study of charge-transfer dynamics and non-equilibrium-state physics. Rational tailoring of multi-component heterostructures may lead to delicate optoelectronic devices with valley functionalities, or even correlated quantum emitters.
 W. G. Xu et al., “Correlated fluorescence blinking in two-dimensional semiconductor heterostructures”, Nature, 541, 62-67 (2017).
Correlating Transport and Raman Signatures of Uniaxial Strain in Suspended Graphene
1Department of Physics, University of Basel, Switzerland; 2Swiss Nanoscience Institute, University of Basel, Switzerland; 3Empa - Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland
Since its discovery, graphene has not only attracted a lot of interest for its electronic, but also for its mechanical properties. Understanding and controlling strain in graphene is a prerequisite for predicting the behavior of devices and can be used as a tool for the investigation of fundamental properties. Raman spectroscopy has successfully been used as a tool to identify the magnitude of strain and distinguish crystallographic orientations. While most results have been obtained on a polymer substrate, we present results from our recent experiments on freely suspended graphene with uniaxial strain applied by tuning the distance between metallic contacts in a three-point bending geometry. We correlate the observed shifts in Raman modes with the measured electronic transport properties as a function of gate voltage.
Efficient n-type Doping in Epitaxial Graphene through Strong Lateral Orbital Coupling of Ti Adsorbate
1Department of Physics, National Cheng Kung University, Taiwan; 2Center for Nanotechnology Innovation, National Enterprise for nanoScience and nanoTechnology (NEST), Istituto Italiano di Tecnologia, Italy; 3National Enterprise for nanoScience and nanoTechnology, Istituto Nanoscienze-CNR and Scuola Normale Superiore, Italy; 4National Synchrotron Radiation Research Center, Taiwan
Recently, many different types of doping methods for epitaxial graphene have been demonstrated through atom substitution and adsorption. Then, here we observe by angle-resolved photoemission spectroscopy (ARPES) a coupling-induced Dirac cone renormalization when depositing small amounts of Ti onto epitaxial graphene on SiC. We obtain a remarkably high doping efficiency and a readily tunable carrier velocity simply by changing the amount of deposited Ti. First-principles theoretical calculations show that a strong lateral (non-vertical) orbital coupling leads to an efficient doping of graphene by hybridizing the 2pz orbital of graphene and the 3d orbitals of the Ti adsorbate, which attached on graphene without creating any trap/scattering states. This Ti-induced hybridization is adsorbate-specific and has major consequences for efficient doping as well as applications towards adsorbate-induced modification of carrier transport in graphene.
Electrical Studies on MoS2 Schottky Diode with Asymmetric Metal Contacts Fabricated Using Electron Beam Lithography
Department of Physics, Indian Institute of Technology Delhi, India
Two-dimensional (2D) material based devices have been investigated with great interest in recent past for their potential application in electronic and opto-electronic devices. Molybdenum disulfide (MoS2) has been considered as a promising candidate due to its unique properties such as presence of both indirect and direct bandgap and high ON/OFF ratio. All devices rely on metal contacts, therefore understanding of metal/MoS2 interface is crucial for device application.
In our work, MoS2 was transferred from bulk crystal on SiO2/Si substrate using mechanical exfoliation method. Metal electrodes on MoS2 were patterned using electron beam lithography and deposited using RF sputtering system followed by lift off in acetone. Various metals were studied such as Cr, Ni and Pt depending on the electron affinity of MoS2 and work function of metals. Symmetric contacts such as Cr/MoS2/Cr, Ni/MoS2/Ni showed nearly ohmic behavior. On the contrary, asymmetric contacts such as Ni/MoS2/Pt showed Schottky behavior. Work function of MoS2 was investigated using KPFM. Surface potential was measured to be -350 mV and work function was calculated as 5.1 eV. Schottky barrier height and ideality factor in case of asymmetric contacts were calculated to be 0.56 eV and 2.04, respectively at room temperature using thermionic emission (TE) current equation.
Electrically and Optically Tunable Properties of Two Dimensional Heterostructures
Institute of Materials Research and Engineering (IMRE), Agency for Science Technology and Research (A*STAR), Singapore
Two dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted tremendous attention for optoelectronic applications due to their suitable bandgap that covers wide energy range. It is often required to form low resistance contact in order to have efficient optoelectronic devices and graphene is always used to form Ohmic and transparent contact with TMDCs.
However, how the graphene electrode affects the optoelectronic properties of semiconducting TMDCs is still not clear and there are contradicting results that awaits further clarification. Here, we fabricated TMDC/graphene heterostructures with both p-type and n-type monolayer TMDCs. We achieved reversible and dynamic control of heterostructure properties through both electrostatic gating and photo irradiation. Using the heterostrucutres, we were able to clarify the controversy regarding to the interface interactions. Our results provide general guidelines to fabricate efficient and controllable optoelectronic devices with 2D TMDCs.
Electrically Contacting Self-Assembled PbS Nanocrystals using Graphene
1Cavendish Laboratory, University of Cambridge, United Kingdom; 2Cambridge Graphene Centre, University of Cambridge, United Kingdom; 3Department of Physics, University of Lancaster, United Kingdom; 4Center for Nanophotonics, AMOLF, Amsterdam, The Netherlands
Using molecular junctions as electrical components often implies low scalability and complex fabrication: horizontal architectures generally require costly and sequential processes such as electron-beam lithography , whilst vertical stacking arrangements using metal evaporation can damage the molecules or cause short circuits . Recent architectures with molecular self-assembled monolayers (SAMs) and graphene have enabled molecular tunnel junctions to be built with a yield of 90% . Here, we use graphene to make arrays of ~1µm2 junctions contacting SAMs of PbS nanocrystals (5nm diameter) as quantum dots to obtain films with more complex low-dimensional transport characteristics. Our junctions exhibit Coulomb blockade [1,5] in the nanocrystals with a yield above 40% before optimisation, thus demonstrating single-electron effects in a robust and scalable architecture. The design is adapted for electron-beam lithography to contact areas down to nanometre sizes. This enables a comparison of transport over a large range of nanocrystal numbers, from single digits up to tens of thousands. Statistical analysis and topographical imaging allow us to investigate the conduction parameters in these complex films.
 D.L. Klein et al., Appl. Phys. Lett. 68 (1996), 2574.
 H. Haick et al., J. Phys. Chem. C, 111 (2007) 2318.
 G. Wang et al., Adv. Mater, 23 (2011), 755.
 H. Jeong et al., Nanotechnology 26 (2015), 025601.
 U. Meirav and E. B. Foxman, Sem. Sci. Tech. 11 (1996) 255.
Electronic and magnetic properties of single-layer MPX3 metal phosphorous trichalcogenides
1Sungkyunkwan University, South Korea; 2University of Seoul, South Korea; 3The University of Texas at Austin, United States
We systematically investigate the electronic structure and magnetic properties of two dimensional (2D) MPX3 (M= V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and X = S, Se, Te) transition metal chalcogenophosphates to examine their potential role as single-layer van der Waals materials that possess magnetic order. Our ab initio calculations predict that most of these single-layer materials are antiferromagnetic semiconductors. The band gaps of the antiferromagnetic states decrease as the atomic number of the chalcogen atom increases (from S to Se to Te), leading in some cases to half-metallic ferromagnetic states or to non-magnetic metallic states. We find that the phase transition boundary from antiferromagnetic semiconductor to ferromagnetic half-metal can be substantially influenced by gating or by strain engineering. The sensitive interdependence we find between magnetic, structural, and electronic properties establishes the potential of this 2D materials class for applications in spintronics.
Exfoliated 2-D MoS2 Layer Control by Shear Stress with the Solution Process
Ajou University, South Korea
Moly disulfide (MoS2), which is one of the typically two-dimensional materials, has a great potential in the next generation electronic applications because the material has unique electrical, mechanical, and chemical properties. MoS2 has been especially reported as having good flexible transistor characteristics. In an effort to apply MoS2 to the practical use for flexible transistors, fabrication process compatible to the large area manufacturing with a low temperature should be guaranteed. Solution based process can meet these conditions. For the mass production of MoS2 with a solution process, it is essential to develop inks that is well dispersed and results in few layers MoS2 nanosheet.
In this study, we propose a method for separating few layer (below ~ 10 nm) MoS2 nanosheet layers from MoS2 powder by adding shear stress and centrifugation speed. After repeating the centrifuging and sonication processes, the MoS2 nanosheet becomes more fine and uniform. The main goal of this study is to obtain the optimal ink which can be dissolved by the water or ethanol via control the mixing time and sonication time. This has some advantages that it is economical and can be volatilized at a low temperature with a good dispersion. In order to characterize the MoS2 properties, the various spectroscopic analyzing method was performed such as UV/VIS spectrophotometer, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and tunneling electron microscope (TEM).
Finally, we fabricated the MoS2 based 2-D transistor with a low temperature process, which is applied to flexible transistors production and possible applications as the gas sensor detecting CO, CO2 and NOx.
Experimental and Theoretical Investigation on the Substrate Effects of Monolayer Molybdenum Disulfide
Department of Physics, Xiamen University, China
Monolayer molybdenum disulfide, a semiconducting two-dimensional material, has received lots of attention in recent years due to its unique properties and potential applications in high performance electronic and optoelectronic devices. The preparation of uniform MoS2 with high quality is a critical issue for fundamental researches and practical applications. Other than the mechanical transfer technique, the methods of direct growth have been attempted to synthetize MoS2 layer on substrates recently. The growth mechanism and structures of the MoS2 sheets would be affected by the growth conditions and substrate structures, and hence influence the intrinsic properties of the prepared 2D heterostructures. Here we present our investigation on the epitaxial growth of MoS2 on single layer graphene as well as graphite substrate in a home-built MBE system. The epitaxial behaviors and structural properties of the grown MoS2 are studied by utilizing scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning tunneling microscopy. Models for the corresponding heterostructures are built based on the experimental observations, and used to investigate the atomic structures, electronic properties, interfacial interaction as well as the substrate effects. Our findings demonstrate that one can tune the band gap of MoS2 sheet and yet retain the main characteristic of band structures by appropriate substrates.
Hydrogenated Defected Graphene as an Anode Material for Sodium and Calcium Ion Batteries
1Centre for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Australia; 2School of Chemistry and Molecular Biosciences, The University of Queensland, Australia
Selection of a suitable anode material plays an important role in achieving good performance and high electrical capacity of rechargeable batteries. Although sodium (Na) and calcium (Ca) offer some advantages over lithium (Li), they have larger atomic radii and therefore are not able to be used as a replacement for Li in graphite electrodes because of their inability to intercalate. We have recently carried out quantum chemical calculations to consider the performance of layered bulk graphdyine (GDY) an as an anode material for sodium ion batteries . It was found that GDY can accept a reasonable amount of Na with a capacity of NaC2.57, equivalent to electrical capacity of 497 mAhg-1 for single layer and also bulk layers with 28% expansion rate. Without expansion, the capacity will be NaC5.14 equivalent to 316 mAhg-1. Moreover, we have considered hydrogenated defected graphene materials to see if the hydrogenation can increase the loading and interlayer spacing of graphene. A material with one hydrogen atom on each dangling sp2/sp3 carbon atom in a mono-vacancy has been studied computationally. This material can be synthesized by passing controlled hydrogen gas stream over the defective graphene, which is assumed to contain uniformly distributed mono-vacancies. According to our computational results, a supercell of 6×6 hexagonal rings with four carbon vacancies (5.55%) and four hydrogen atoms (C68H4) can accommodate up to 16 Na and Calcium (Ca) atoms, equivalent to 173.2 and 231.4 mAhg-1, respectively. Moreover, C68H4 has a greater interlayer distance than the non-hydrogenated defected graphene (C68), by about 11% for AA stacking type. In addition, results for Na and Ca loading on the bulk layers of C68H4 with optimal stacking will be presented.
 Farokh Niaei, A.H., et al., Sodium-intercalated bulk graphdiyne as an anode material for rechargeable batteries. Journal of Power Sources, 2017. 343: p. 354-363.
In-line Printing of Graphene-Based Barrier Film on Flexible Lighting Device
Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research (A*STAR), Singapore
Quick drop of performance and short lifetime of novel electronic device induced by fast degradation of device materials, is one of the critical issues that limits the translation of research investigation to products level. Novel 2D nanomaterials, such as graphene, have been studied as new barrier materials to stretch the limit of current barrier film. For the first time, transparent and roll-to-roll in-line printable graphene-based barrier layer has been fabricated on the flexible electroluminescence (EL) device. Based on the finding from defect analysis of EL device, the barrier layer was printed on top of the EL back electrode under atmosphere. The enhancement of barrier performance was reflected by the 31.6 % improvement in WVTR compared with bare PET, inhibition of corrosion and oxidation on copper surface, as well as high transmittance of 63% at 550 nm. With further optimization in printing process and barrier performance, the in-line printable graphene-based barrier layer will greatly simplify the fabrication process of electronic devices. Noteworthy, it opens new opportunities for flexible and stretchable hybrid electronic devices in 2D or even 3D fabrication.
Integrating Al2O3 on BP Crystalline using Plasma Enhanced Atomic Layer Deposition
Fudan University, China
It is quite challenging to grow high quality and uniform dielectric on black phosphorus (BP) because of easy reactions between BP and O2 or H2O in ambient. In this work, we have directly grown Al2O3 on BP using plasma enhanced atomic layer deposition (PEALD). The surface roughness of BP with covered Al2O3 film can reduce significantly. It was also found there is an interfacial layer of POx in between amorphous Al2O3 film and crystallized BP, evidenced by both XPS and TEM measurements. The POx can be converted into fully oxidized P2O5 as the deposition temperature increases to 350oC. Futhermore, Al2O3 as a passivation layer can effectively protect BP from the ambient degration. These findings provide insightful information on passivation and top-gate dielectric integration for future applications in BP devices.
Investigating Metal/2D Semiconductor Contacts through BEEM
1Department of Physics, National University of Singapore, Singapore; 2NUS Graduate School of Integrated Sciences and Engineering, National University of Singapore, SIngapore; 3Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore
Two dimensional (2D) semiconductors based on transition metal dichalcogenides (TMDs) such as MoS2, MoSe2, WS2 and WSe2 are promising materials for use in next generation electronics due the presence of a bandgap in these materials and their tunable electronic and optical properties. One of the major challenges in current research is to understand the nature of the metal/2D semiconductor contacts as the charge transport into and through the little understood interfaces are limiting the performance of these devices. [1, 2] In this poster, we present our strategy for investigating the Au/MoS2 Schottky interface using scanning tunneling microscopy (STM) and ballistic electron emission microscopy (BEEM) in the Omicron UHV-Nanoprobe and explain its merit as a model system for understanding the metal/2D semiconductor interface.
 Liu, H.; Si, M.; Deng, Y.; Neal, A. T.; Du, Y.; Najmaei, S.; Ajayan, P. M.; Lou, J.; Ye, P. D. ACS Nano 2014, 8, 1031
 Zhang, W.; Chiu, M. -H.; Chen, C. -H.; Chen, W.; Li, L. -J.; Wee, A. T. S. ACS Nano 2014, 8, 8653
Low-temperature MOCVD of Wafer-scale Two-dimensional Molybdenum Disulfide
1Center for Vacuum Technology, Korea Research Institute of Standards and Science, South Korea; 2Center for Nanometrology, Korea Research Institute of Standards and Science, South Korea; 3Department of Advanced Device Technology, University of Science and Technology, South Korea
Chemical vapor deposition (CVD) using molybdenum trioxide (MoO3) and sulfur powder is a highly effective method for growing molybdenum disulfide (MoS2) atomic layers on a dielectric substrate. Studies with a similar method have demonstrated the effective growth of large-area, high-quality MoS2 with a larger grain size. However, to the best of our knowledge, a feasible method for growing a MoS2 at low-temperatures of below 400 °C has not yet been reported, as it still requires the sulfurization of MoO3−x at high temperatures ranging from 650 to 850 °C. Herein, we report direct one-step low-temperature CVD process for the growth of high-quality layered MoS2 with control of the cluster size and the nucleation sites using Mo(CO)6 and H2S as the precursor and reaction gas, respectively. In addition, the electrical and gas sensing properties of MoS2 was examined.
At a lower PSR/PMoP, irregular 3D islands with small grain sizes were grown. As PSR/PMoP increases, the morphology was changed to a mixed structure which was consisted of irregular 3D islands and 2D triangular islands. At a much higher PSR/PMoP, the structure completely changed to 2D triangular islands with larger grain sizes. The grown MoS2 using our method exhibit the characteristic of layer-by-layer growth without changing other parameters. Different surface colors were observed for different numbers of layers in which highly uniform large-area MoS2 were grown on SiO2 substrates and grown at the wafer scale up to 3” in size, as confirmed by an ellipsometry mapping analysis. The low-temperature grown monolayer MoS2 was used to fabricate a back-gate FET to examine the electrical performance. The FET device exhibits conventional n-type semiconductor behavior with a mobility of 0.15 cm2V−1s−1. Furthermore, gas sensing property of grown MoS2 was examined, and showed significant sensitivity up to parts per million detection level for O2 and CO2.
MoS2 Thin Film Transistors for Flexible Memory Device
National Institute for Materials Science, Japan
A main purpose of this study is to improve the device performance of MoS2TFTs and to investigate the effects of organic ferroelectric materials on resulting MoS2 TFTs for application in TFTs-based flexible memory.
The flexible memory devices based on ferroelectric TFTs have been widely studied owing to their simple and low-cost device fabrication, non-volatile and non-destructive data. Consequently, the judicious choice of the materials and elaborate manufacturing condition are essential by influencing on the electrical properties of devices. A MoS2 satisfies the various requirements as the channel layer for application in memory devices by exhibiting excellent electrical properties. On the other hand, organic ferroelectric materials have many attractive properties for the flexible memory. A Gd@C82, consisted of encapsulation of metallic species inside fullerene cages, is promising material for ferroelectric memory. This exhibits unique properties due to their electric dipole moment which leads to possibilities where the molecular orientation or rotation is controlled by external electric fields. A P(VDF−TrFE) also has been widely studied as a ferroelectric materials because of its advantages such as large spontaneous polarization, short switching times, low leakage and solution processing at low temperature.
Here, the processing optimization of multi-step CVD method for a MoS2 thin film is reported. This facilitated the fabrication of patterned MoS2 thin films with good crystallinity, excellent uniformity, thickness controllability and large-area processing. A MoS2 TFTs was subsequently fabricated that possesses a carrier mobility of 0.8 cm2/Vs, an on/off ratio of ~ 103 and a threshold voltage of -3.0 V with negligible hysteresis. An initial investigation of organic ferroelectric materials, e.g. Gd@C82 andP(VDF−TrFE), on surface and electrical properties is also undertaken. In a further investigation, organic ferroelectric materials for application in the MoS2 TFTs-based ferroelectric memory will be used and the resulting changes in electrical performance will be reported.
Optoelectronic Properties of Twisted Multilayer MoS2
1NUS Nanoscience and Nanotechnology Institute, National University of Singapore, Singapore; 2NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore; 3University of California, Berkeley, United States
Large exciton binding energy in MoS2 enables a quasi-resonant excitation of excitonic resonances below the single-particle band gap. For a monolayer, this creates a non-equilibrium distribution of carriers predominantly in the K-valleys leading to strong excitonic emission at 1.9 eV (direct band-gap) and a weak peak at 1.2 eV (indirect band-gap). With increase in the number of layers, the indirect band gap increases monotonically resulting in an increased intra-band relaxation rate from the excitonic state. As a result, the PL-intensity drastically decreases. Notably here the interlayer coupling plays a very important role. In this context, our CVD grown large area MoS2 on transition metal oxide substrates (such as SrTiO3) exhibit an anomalous behavior where even multilayered (more than 4-5 layers) samples show a very strong direct band-gap luminescence and a much less layer dependence. This arises from mutual rotated layers of MoS2 stacked on each other which we can grow by this protocol. We have characterized our samples with HRSTEM where the planar view images and their corresponding FFT suggest that each layer gives a slightly shifted set of Bragg peaks which implies that the layers are relatively rotated. This reduces intra-band relaxation of the excitonic states resulting into a strong PL emission at the direct gap for thicker samples. In context of the current surge of interest on twisted 2D materials this a very interesting observation where the twisting is inherently a result of our growth protocol. We have also conducted vertical transport measurements to investigate the effect of this rotation of each layer on bulk electronic transport. In general, multilayers of TMDCs have never been of much interest but our results do indicate a number of interesting possibilities emerging out of this phenomenon of interlayer rotation.
Oxygen Passivation Mediated Tunability of Trion and Excitons in Molybdenum Disulfide
1Department of Physics, Faculty of Science, National University of Singapore, Singapore; 2Singapore Synchrotron Light Source, National University of Singapore, Singapore; 3Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore; 4Department of Electrophysics, National Chiao Tung University, Taiwan; 5Physical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Saudi Arabia; 6NUS Nanoscience and Nanotechnology Initiative (NUSNNI)-NanoCore, National University of Singapore, Singapore
The current vigorous research efforts in the field of ultrathin two dimensional transition metal dichalcogenides (TMDCs) have paid rich dividends with results showing fundamental physical implications as well as potential for applications. One of the most prominent aspects of the electronic structure and optical properties of these materials is the presence of strong many-body interactions. These are manifested as strongly bound excitons and other types of exciton complexes such as trions and biexcitons. In this work, using wide spectral range in situ spectroscopic ellipsometry, we demonstrate the tunability of exciton (and trion) peaks in MoS2 via high temperature annealing at ultrahigh vacuum conditions and in situ exposure to oxygen. The spectral weights of the A (1.92 eV) and B (2.02 eV) exciton peaks in the dielectric function reduce significantly upon annealing. Interestingly, the original spectral weights are recovered upon oxygen exposure. This tunability of the excitonic effects is likely due to passivation and re-emergence of the gap states in the bandstructure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. The adsorbed oxygen also causes variation to the local potential enhancing the inhomogeneous broadening of the exciton (and trion) lineshapes. This work unveils the important role of adsorbed oxygen in the optical spectra and many-body interactions of MoS2.
Photocatalytic Hydrogen Transfer from Water to Halides by Porous CdSe Nanosheets
National University of Singapore, Singapore
Photochemical generated hydrogen from water is a safe and green reductant in organic synthesis. However, the considerable energy cost needed for water splitting as well as sluggish H transfer reactions restrict its application. Herein, we demonstrate that by using porous two-dimensional CdSe nanosheets as the photocatalyst, highly efficient hydrodehalogenation of halides can be achieved by photochemical generated hydrogen from water. This strategy works well on various classes of halogenated molecules to give the hydrogenated products in good to excellent yields with good selectivity and functional groups tolerance. Most importantly, this approach is powerful enough to hydrogenate less reductively labile C-Br, C-Cl and C-F bonds. Mechanistic studies reveal a tandem radical coupling pathway on the surface of porous CdSe upon photo-excitation. A series of deuterium-labelled molecules can be readily prepared by employing D2O as D donor.
Photoluminescence Imaging of a Few Layer MoS2
1Chonbuk National University, South Korea; 2Korea Institute of Science and Technology, South Korea
Ultrathin layered materials have attracted a great deal of attention due to their potential applications for nanoscale optoelectronic devices. In the case of two-dimensional transition metal dichalcogenide crystals, which have a layered structure through van der Waals interlayer interactions, the electronic structure is significantly changed according to the number of layers. In this study, we report spatially resolved photoluminescence (PL) study of a few layer MoS2, the imaging area of which consists of monolayer, bilayer, trilayer, and quadrilayer. A PL spectrum revealed the A and B excitonic transitions at ~1.82 and ~1.96 eV, respectively. Interestingly, the A and B excitons varied in intensity as a function of the number of layers. The B exciton intensity increased when the layer number increased from 1 to 2. With a further increase of the number of layers, the B exciton intensity decreased and finally quenched for thick MoS2. The intensity variation of the A exciton was somewhat different from that of the B exciton. The A exciton intensity did not seem to vary when the layer number increased from 1 to 2. Furthermore, the A exciton intensity in the monolayer exhibited a significant enhancement adjacent to the bilayer, which indicated electronic coupling between the monolayer and bilayer. The energy and spectral width changes of the A exciton as a function of the number of layers were also somewhat different from those of the B exciton.
* This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) (Grant No. 2016R1D1A1B03935270) funded by the Ministry of Education.
Preparation of Ultra-small Nanodots with Two-dimensional Structure
Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, Singapore
Engineering the size and dimension of nanomaterials is promising to endow them with novel properties and broaden their applications. For example, ultra-small nanodots have exhibited unique electronic and optical properties due to the quantum confinements and edge effects. Recently, semiconductors with two-dimensional structures, such as transition metal dichalcogenides (TMDs) and black phosphorus (BP), have drawn great attention in electronics, catalysis, biomedicine, sensing and energy storage. However,high-yield production of TMD and BP nanodots still remain a challenge and their applications need to be explored. Herein, we report a facile method for the preparation of a number of layered TMD nanodots (including MoS2, WS2, ReS2, TaS2, MoSe2, WSe2 and NbSe2) and BP nanodots in high yield.[2,3] All the prepared nanodots have small size and good dispersity. As a proof-of-concept application, the synthesized nanodots mixed with polyvinylpyrrolidone (PVP) are used as active layers for fabrication of memory devices.
 X. Zhang, Z. C. Lai, C. L. Tan, H. Zhang*, Angew. Chem. Int. Ed., 55 (2016), pp. 8816-8838.
 X. Zhang, Z. C. Lai, Z. D. Liu, C. L. Tan, Y. Huang, B. Li, M. T. Zhao, L. H. Xie, W. Huang, H. Zhang*, Angew. Chem. Int. Ed., 18 (2015), pp. 5425–5428.
 X. Zhang, H. M. Xie, Z. D. Liu, C. L. Tan, Z. M. Luo, H. Li, J. D. Lin, L. Q. Sun, W. Chen, Z. C. Xu, L. H. Xie, W Huang, H. Zhang*, Angew. Chem. Int. Ed., 54 (2015), pp. 3653–3657.
Raman and Photoluminescence Fingerprints of Interlayer Charge Transfer in Twisted WS2/MoS2 Heterostructures
1Nanyang Technological University, Singapore; 2Fudan University, China; 3Nanjing Tech University, China
The advanced assembly technique enables the formation of diverse van der Waals heterostructures with novel properties and functionalities. The two-dimensional (2D) transition metal dichalcogenide (TMD) heterostructures exhibit the characteristics of type II interlayer excitons and charge transfer, which possess promising properties for novel excitonic devices. Although significant progress has been made in explaining the interlayer exciton dynamics in TMD heterostructures through femtosecond transient absorption spectroscopy, photoluminescence excitation spectroscopy and scanning tunneling microscopy and spectroscopy, probing the interlayer charge transfer directly through Raman spectroscopy is technically much more facile and thus highly demanded so far.
In this work, we will present systematic Raman and photoluminescence (PL) measurements of the interlayer charge transfer in WS2/MoS2 heterostructures with variable stacking. Experimentally, WS2 monolayers were first grown on sapphire by chemical vapor deposition (CVD), and then transferred onto triangle MoS2 to form twisted heterobilayers using developed dry transfer method. The Raman and PL measurements were then conducted on twisted WS2/MoS2 heterostructures, pristine WS2 and MoS2, respectively. Compared with the pristine positions of A1g mode, A1g (WS2) obviously blueshifts, while A1g (MoS2) predominantly redshifts, indicating the charge transfer between WS2 and MoS2. Additionally, the E2g (WS2) and E2g (MoS2) both display the identical shift, which probably manifests the presence of lattice mismatch-induced strain. Simultaneously, with twisted angle varying from 0° to 60°, the shifts of A1g and E2g modes vary and show a relatively clear trend. Furthermore, the PL of the heterostructures quenches a lot and the PL peaks display a redshift for WS2, while a blueshift for MoS2, which also indicates the same charge transfer between two layers. According to the Density of Function Theory calculations, the band structure of WS2/MoS2 heterostructures demonstrates a type-II band alignment, which is consistent with our experimental results, as well.
Solid Argon as a Possible Substrate for Quasi-freestanding Silicene
1King Abdullah University of Science and Technology, Saudi Arabia; 2Department of Chemistry and Chemical Biology, Cornell University, USA
We study the structural and electronic properties of silicene on solid Ar(111) substrate using ab initio calculations. We demonstrate that due to weak interaction, quasi-freestanding silicene is realized in this system. The small binding energy of only -32 meV per Si atom also indicates the possibility to separate silicene from the solid Ar(111) substrate. In addition, a band gap of 11 meV and a significant splitting of the energy levels due to spin-orbit coupling are observed.
Spatially-resolved Raman Study of Electrically Biased Graphene/h-BN Field-effect Transistor
1Chonbuk National University, South Korea; 2Korea Research Institute of Standards and Science, South Korea
Understanding the role of power dissipation in graphene-based heterostructures is important for thermal management of future graphene devices. In this study, we report in situ Raman imaging of a hexagonal boron nitride (h-BN) layer underneath an electrically biased single-layer graphene field-effect transistor (FET). To study thermal transport properties of the graphene/h-BN FET, the E2g phonon energy in the h-BN layer was mapped over the entire channel of the biased graphene. When a bias voltage was applied to the graphene channel, the E2g phonon energy of the h-BN shifted downward, indicative of an increase in temperature under current. For example, we found that an average temperature through the entire h-BN area underneath the graphene channel increased with increasing Vsd, i.e., ~ 43 °C and ~ 76 °C at Vsd = 10 and 15 V, respectively. The increase of the h-BN temperature along the channel showed excellent agreement with an analytical calculation considering the interfacial thermal resistance between the graphene and h-BN. Our result demonstrates that the Raman thermography provides useful means to probe the heat dissipation in the graphene/h-BN FET.
* This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) (Grant No. 2016R1D1A1B03935270) funded by the Ministry of Education.
Structural Evolution and Electronic Effects of Flower Defects in Epitaxial Graphene on SiC(0001)
State Key Laboratory of Surface Physic & Department of Physics, Fudan University, China
The topological defects in graphene can modify its electronic structures even for rather small concentration, and therefore will influence its chemical, mechanical, transport and magnetic properties. In this work, the epitaxial graphene with flower-like defects was prepared on SiC(0001) substrates via thermal decomposition. The structural evolution processes of flower defects at high temperature annealing were investigated by using scanning tunneling microscopy, Raman spectroscopy and the density functional theory calculations. The experiments were carried out mainly in an Omicron ultrahigh-vacuum STM system. The STM measurements were performed in situ at room temperature in the constant current mode. Raman spectra were measured ex situ, with a laser of 632.8nm as excitation source. The DFT calculations were performed using the Vienna ab initio simulation package code to investigate the electronic structures and densities of states of graphene with flower defects, as well as the energies of the flower defect formation. The experiment results reveal that flower defects emerging mainly in bilayer graphene can increase in amount with the increasing of annealing time till they nucleate into the complex defect structures. As a new topological defect and a direct experimental evidence of the structural evolution, the conjoined-twins defect was detected in epitaxial graphene with two of 2/3 flower defects joined together. The theoretical calculations show that the conjoined-twins defect has a calculated energy 2.7eV smaller than that of two isolated flower defects. The flower defects in monolayer graphene can bring about a bandgap of 34meV. The calculation results also demonstrate that new van Hove singularity states will be generated in the vicinity of +0.7eV and -0.2eV in the density of states for the dislocated carbon rings of the conjoined-twins defects, which will probably improve the electronic transport properties of graphene-based devices.
Study of Optical Properties from Exfoliated and Grown Molybdenum Disulphide (MoS2)
1Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore; 2Department of Electrical and Computer Engineering, National University of Singapore, Singapore
Molybdenum Disulphide (MoS2), one of the transition metal dichalcogenides (TMDCs), attracts research interest in optoelectronics due to its unique properties such as indirect-to-direct bandgap transformation when it changes from multi-layer to monolayer. To achieve monolayer and few layers MoS2, several techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), sulfurization of molybdenum oxides and exfoliation have been used. In this paper, we report the optical properties such as reflection and absorption of different PVD grown and tape-based mechanical exfoliated MoS2 samples. In additional to optical properties of exfoliated MoS2, a few black areas were observed on the exfoliated MoS2 sample during exfoliation process and investigation reveals that they are kind of folded MoS2.
Surface Energy and Wettability of van der Waals Structures
1National University of Singapore, Singapore; 2Nanoelectronic Science and Engineering Laboratory, Department of Advanced Materials Science & Engineering, School of Engineering, Sungkyunkwan University, South Korea; 3SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), Sungkyunkwan University, South Korea
Wetting behaviour of surfaces is believed to be affected by van der Waals (vdW) forces; however, there is no clear demonstration of this. With the isolation of two-dimensional vdW layered materials it is possible to test this hypothesis. In this paper, we report the wetting behaviour of vdW heterostructures which include, chemical vapor deposition (CVD) grown graphene, molybdenum disulfide (MoS2) and tungsten disulfide (WS2) on few layers of hexagon boron nitride (h-BN) and SiO2/Si. Our study clearly shows that while this class of two-dimensional materials are not completely wetting transparent, there seems to be a significant amount of influence on their wetting properties by the underlying substrate due to dominant vdW forces. Contact angle measurements indicate that graphene and graphene-like layered transitional metal dichalcogenides invariably have intrinsically dispersive surfaces with a dominating London-vdW force-mediated wettability.
Synthesis and Characterization of WS2 Nanotubes Grown by Sulfurization of Tungsten Thin Films
Tata Institute of Fundamental Research, India
Among the layered transition metal dichalcogenide (TMDC) materials WS2 is one of the most interesting material because of its large valence band splitting, direct band gap in monolayer form, low effective mass and a theoretically predicted high mobility. Most of work on the growth and characterization of WS2 has been done on exfoliated flakes or thin films. There is a relatively less reported on the study on WS2 in nanotube form. Semiconducting WS2 nanotubes would be potential candidate for photocatalysis and other device applications. There are only a few reports on the growth of WS2 nanotubes by direct sulfurization of amorphous tungsten thin films. In our study we discuss the formation of WS2 nanotube by sulfurization of tungsten in detail. WS2 nanotubes were synthesized by sulfurization of tungsten films deposited by e-beam evaporation in the three-zone furnace at atmospheric pressure. The tungsten films were heated from room temperature (25 oC) to ~900-1000 oC under a continuous nitrogen flow, which was used to transport sulfur vapor carried to the sample from a boat kept upstream at a lower temperature. Depending on the growth conditions and tungsten film thickness different morphologies can be obtained. Under appropriate conditions WS2 flakes of dimension ~100nm are initially formed which then wrap around themselves to form nanotubes of outer diameter ~20nm and length ~100nm. We will present XRD, SEM and TEM characterization of the surface morphology and structure of these WS2 nanotubes.
Synthesis of Calcium Carbonate Molecular Sheets
New York University Abu Dhabi, United Arab Emirates
Biological systems produce polymer-inorganic hybrids in the form of self-organized, hierarchical and ordered structures with excellent mechanical properties. The development of new biomimetic materials via synthetic and bio-inspired methods is an important field of research. Herein, we describe a scheme to create nacre like lamellar structures of molecular sheets of CaCO3, analogous to graphene sheets, interleaved with amorphous carbon. A formulation of poly(acrylic) acid (PAA) and calcium acetate were used to create these lamellar stacks of single crystal sheets of CaCO3. Computational modeling, ab initio molecular orbital calculations via “oligomer approach”, was used to confirm a net-like polymer structure bridged by calcium ion as the mechanism for the formation of the molecular sheets. The process reported here could be readily applied for the creation molecular sheets of other important inorganic materials.
Temperature Dependent Electrical Properties of Graphene / n-Si Schottky Diode
Department of Physics, Indian Institute of Technology Delhi, India
The present work reports the fabrication and detailed macroscopic characteristics of CVD grown graphene/n-Si Schottky diodes. Temperature dependent electrical transport characteristics of CVD grown graphene/n-Si Schottky diodes are investigated. Surface potential of graphene is found to be 417 mV by Kelvin probe force microscopy, which gives the work function of graphene 4.54 eV. Ideality factor and barrier height are found in the range of 5.1 to 1.9 and 0.23 to 0.79 eV, respectively. It is found that barrier height increases and ideality factor decreases with increasing temperature and the graphene/n-Si Schottky diode is found to have a lower level of barrier inhomogeneity.
The Shottky Barrier Tuning Effect of Post Rapid Thermal Process on Al2O3 Passivated BP FETs
Fudan University, China
It is widely acknowledged that black phosphorus(BP) was unstable in ambient, hence typical methods such as PMMA and insulation capping are provided to protect the BP FETs from degradation. Here we fabricated few-layer BP transistors with the capping layer of Al2O3, which was grown by atomic layer deposition (ALD) using H2O and TMA precursor. The BP devices can be well protected for weeks. It was also observed that the Shottky barrier of BP FETs can be effectively tuned by the Al2O3 passivation, leading to the raise of electron current in transfer characteristics. Moreover, the Shottky barrier can be tuned reversely in O2 atmosphere (RTP) under the temperature of 200˚C.
Thin-Film Formation of 2D MoS2 and its Use as a Hole-Transporting Layer in Planar Perovskite Solar Cells
Indian Association for the Cultivation of Science, India
Out of all layered transitional metal chalcogenides (TMDs), MoS2 has attracted most attention of the global scientific community due to its unique physical properties. However in contrast to various fabrication procedures of TMD nanosheets,cost effective film formation techniques are still not up to the mark. Solution based approaches, namely spin-coating and drop-casting are low cost but without ligand treatment they have serious issues related to the quality of the films. In this report, the method of centrifugal casting has been used to fabricate large area and uniform few layered MoS2 nanosheet thin film. We have characterized the film with SEM, cross sectional SEM and AFM. Compared to the previously reported film morphologies the film appeared much robust with a pin hole free, uniform and considerably smooth morphology over a large area. Beside the green color the few layered characteristic is also verified by shift in Raman spectra of the film compared to bulk MoS2. The XPS spectra also verified the absence of any residual NMP. It is apparent that the film could be prepared uniformly on any surface with area only limited by the dimension of the centrifuge tube. In order to verify the functional effectiveness of the as fabricated film we implemented them as hole transporting layer in solution processed planar perovskite solar cells. The Fermi level of MoS2 nanosheet was engineered by plasma treatment to fabricate a type-II band alignment with methyl ammonium lead iodide (MAPbI3) perovskite. The charge transfer from absorber to MoS2 was verified by PL quenching of the perovskite layer. All photovoltaic devices (ITO/MoS2/MAPbI3/PCBM/Al) made with different MOS2 layer thicknesses were responded under 1 sun illumination. Although our work is entirely based on MoS2, it is obvious that the procedure could be used to fabricate any kind of TMD nanosheet film with satisfactory quality.
Transport of Gases and Molecules through Ultra-Smooth Nanocapillaries of 2D-Material Heterostructures
The University of Manchester, United Kingdom
Atomically smooth nanocapillaries have thus far not been explored in a systematic way as there are no easy, reliable and reproducible routes to making them. Nanoscale capillaries have been studied because of their significance in many natural phenomena and use in numerous applications such as nanofiltration, nanopore analytics, models for biological channels etc. Nanocapillaries can be made by two principle routes; top-down and bottom-up. In the top-down approach, micro and nanofabrication techniques are employed and channels down to 2 nm in average height have been demonstrated. As an alternative bottom-up approach, chemical synthesis is used. Despite its many advantages for scalable manufacturing, the latter approach provides limited flexibility, especially for making capillaries with dimensions larger than several Å. Nevertheless, it has proved extremely difficult to control the capillary sizes and maintain smooth walls at the nanometer scale using conventional materials and techniques. Through the advent of mechanical exfoliation techniques for bulk crystal graphene and other 2D-atomic crystals, such as hBN, MoS2, and WS2, it has been possible to yield layers with desired thickness and atomically smooth surfaces. This proposed project exploits both the flatness of 2D-atomic crystals and their atomic thinness in combination with much of the flexibility offered by microfabrication techniques to create ultra-smooth nanocapillaries. These state-of-the art nanocapillaries will be used for separation and transport of gases and molecules.
 B. Radha, A. Esfandiar, F. C. Wang, A. P. Rooney, K. Gopinadhan, A. Keerthi, A. Mishchenko, A. Janardanan, P. Blake, L. Fumagalli, M. Lozada-Hidalgo, S. Garaj, S. J. Haigh, I. V. Grigorieva, H. A. Wu & A. K. Geim, Molecular transport through capillaries made with atomic-scale precision. Nature 538, 222-225 (2016).
Tunable Half-metallic Magnetism in Atom-thin Holey Two-dimensional C2N Monolayer
Singapore University of Technology and Design (SUTD), Singapore
Exploring two-dimensional (2D) materials with magnetic ordering is a focus of current research. It remains a challenge to achieve tunable magnetism in a material of one-atom-thickness without introducing extrinsic magnetic atoms or defects. Here, based on first-principles calculations, we propose that tunable ferromagnetism can be realized in the recently synthesized holey 2D C2N (h2D-C2N) monolayer via purely electron doping that can be readily achieved by gating. We show that owing to the prominent van Hove singularity in the band structure, the material exhibits ferromagnetism even at a small doping level. Remarkably, over a wide doping range of 4×1013/cm2 to 8×1013/cm2 , the system becomes half-metallic, with carriers fully spin-polarized. The estimated Curie temperature can be up to 320 K. Besides gating, we find that the magnetism can also be effectively tuned by lattice strain. Our result identifies h2D-C2N as the first material with single-atom-thickness that can host gate-tunable room-temperature half-metallic magnetism, suggesting it as a promising platform to explore nanoscale magnetism and flexible spintronic devices.
Wafer-Scale Epitaxial Growth of Highly-Crystalline Molybdenum Disulfide Films on Hexagonal Boron Nitride
1Department of Chemistry, National University of Singapore, Singapore; 2SinBeRISE CREATE, National Research Foundation, Singapore; 3Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore; 4Department of Chemistry, Department of Energy Engineering, Low Dimensional Carbon Materials Center, UNIST, South Korea; 5UNIST Central Research Facilities, Ulsan National Institute of Science and Technology, South Korea; 6School of Physical Sciences, CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, China; 7Materials Science and Technology Division, Oak Ridge National Laboratory, United States
Atomically thin molybdenum disulphide (MoS2), a direct-band-gap semiconductor, is promising for applications in electronics and optoelectronics and has attracted increased research interest following the discovery of graphene. Large single-crystalline MoS2 flakes with sub-millimetre size have been grown, but controlling the lattice orientation of these flakes to form continuous single-crystalline films over macroscopic length scales remains challenging. An atomically flat epitaxial substrate such as hexagonal boron nitride (h-BN) can be a natural choice for such heteroepitaxy. Here we report the successful epitaxial growth of a continuous, uniform, highly-crystalline monolayer MoS2 film on h-BN by molecular beam epitaxy. Atomic force microscopy and electron microscopy studies reveal that two primary types of MoS2 grains, i.e. zero-degree aligned and sixty-degree anti-aligned domains, merge seamlessly into a highly-crystalline film with nearly no grain boundaries at elevated growth temperatures. The epitaxially grown MoS2 films exhibit excellent electrical properties that are comparable to those of exfoliated MoS2 monolayers. Large-scale monolayer MoS2 film can be grown on a 2-inch h-BN/sapphire wafer for which surface morphology and Raman mapping confirm good spatial uniformity. Our study represents a significant step in the scalable synthesis of highly-crystalline MoS2 films on atomically flat surfaces and paves the way to large scale applications.