1:30pm - 2:00pmInvited
Construction of Novel 2D Atomic Crystals on Transition Metal Surfaces and Physical Properties: Graphene, Silicene, Germanene, Hafnene, PtSe2 and HfTen
Institute of Physics, Chinese Academy of Sciences, China
The novel properties of graphene-like honeycomb structure have spurred tremendous interest in investigating other two-dimensional (2D) layered structures beyond graphene. In this lecture, I will present construction of graphene, silicene, germanene, hafnium honeycomb lattice, monolayer PtSe2 as well as HfTe3/HfTe5, a superconductor-topological insulator layered heterostructure, on transition metal surfaces (TMS) (for example, Ru(0001), Pt(111), Hf(0001) and Ir(111)). Molecular beam epitaxial growth technique is used to form the large scale 2D atomic crystals on TMS. Low electron energy diffraction (LEED) and scanning tunneling microscopy/spectroscopy (STM/S) together with density functional theory (DFT) calculations are employed to confirm the formed structures on the TMS. In addition, we have successfully intercalated Si-layer at the interface between the formed graphene and the Ru(0001). The intercalation mechanism has been clarified with STM observations at an atomic level and the DFT calculations. We expect that these new 2D crystals materials will show very interesting physical property and its promising potential applications in nanoscale devices.
In collaboration with Y.L. Wang, S.X. Du, H.M. Guo, L. Huang, H.T. Yang, J.T. Sun, Y. Pan, L. Meng, L.F. Li, G. Li, Y.Q. Wang, X. Wu, L.Z. Zhang, S.R. Song, J.B. Pan et al. from Institute of Physics, CAS; Z.H. Qin from Wuhan Institute of Physics and Mathematics, CAS; S.Y. Zhou from Tsinghua University; S. Pantelides from Vanderbilt University, US; A. Ferrari from University of Cambridge, UK; M. Ouyang from Maryland University, US; W.A. Hofer from the University of Liverpool, F. Liu from University of Utah, US.
 Y. Pan et al., Adv. Mater. 21 (2009) 2777.
 L. Meng et al., Nano Lett. 13 (2013) 685.
 L.F. Li et al., Nano Lett. 13 (2013). 4671.
 L.F. Li et al., Adv. Mater. 26 (2014) 4820.
 Y.L. Wang et al., Nano Lett. 15 (2015) 4013.
 G. Li et al., J. Am. Chem. Soc. 137 (2015) 7099.
 Y.Q. Wang et al., Adv. Mater. DOI: 10.1002/adma.201600575 (2016).
2:00pm - 2:30pmInvited
Growing New Types of 2D Transition Metal Dichalcogenide Monolayer
King Abdullah University of Science and Technology, Saudi Arabia
Our recent demonstration in vapor phase growth of TMD monolayer such as MoS2 and WSe2 has stimulated the research in growth and applications. However, this commonly used CVD approach only allows to grow single material in one synthetic process. Two or more growing processes are required to grow second material or to form heterojunctions . So far there is no efficient way to control the location of growth for each TMDs. In this work, we developed a novel synthesis method to grow more than one TMDs in single process on the same substrate, where the growth location of each material can be precisely controlled for circuit fabrication. Specifically, in single CVD process, we grow WSe2p-channel and MoSe2n-channel all together in the newly developed growth process. As a result, first bottom-up inverter has been demonstrated.
In addition to the symmetry 2D materials, we have also developed a method that can precisely manipulate arrangement of chalcogenide atoms (S and Se) along the vertical direction of TMD by applying plasma thinning along with selenization (or sulfurization). This new strategy is not only to control the composition of upmost chalcogenide layer but also able to fabricate MoSSe Janus structure with 1:1 Se to S ratio, a material never exists in nature. In this Janus structure, the transition metals are sandwiched by selenium at upmost and sulfur at bottom. Such a Janus 2D monolayer exhibits piezoelectric responses and optical dipole along out-of-plane direction. The breakthrough here opens a new area in 2D materials research.
 Y.-H. Lee et al. Adv. Mater. 24, 2320 (2012)
 M.-Y. Li et al. Science 349, 524 (2015)
2:30pm - 2:45pmOral
Controllable Growth and Characterization of Monolayer MoS2 and its Heterostructures
Nanyang Technological University, Singapore
Monolayer MoS2 has been demonstrated to be good candidate for advanced electronics, optoelectronics, and other applications. As newly explored material, Controllable production of large-scale atomically thin MoS2 with high-quality is essential for translating their unique properties into industrial applications. In this talk, we report scalable growth of monolayer continuous MoS2 films over entire SiO2 substrates by home-designed CVD, which provides a route towards scaled-up synthesis of MoS2 films for multi-functional devices. Furthermore, we found the growth of MoS2 domains can be greatly improved by creatively introducing small amount of etching agent oxygen into growth environment. The role of oxygen is not only to effectively prevent the poisoning of precursors but also to eliminate defects during the growth. The single-crystal monolayer MoS2 domains can be achieved with sizes up to ∼350 μm and room-temperature mobility up to ∼90 cm2/(V·s) on SiO2.
As is known, van der Waals heterostructures stacked from various 2D atomic sheets are emerging materials for new type of functional devices. The characteristics of these heterostructures are usually dominated by the interface and layer stacking configurations. Here, we also report the epitaxial growth of both AA and AB stacking MoS2 on WS2 via two-step CVD growth method. Compared with transferred heterostructures, our epitaxial samples show atomic clean interface and strong interlayer coupling. Low frequency Raman interlayer breathing mode and shear mode can be observed in our AA and AB stacked bilayers for the first time, which can serve as persuasive fingerprints for interfacial contact quality and stacking configurations. Based on the platform of as-grown MoS2/WS2 system, using energy-state-resolved ultrafast visible/infrared microspectroscopy, we obtain unambiguous experimental evidence of the charge transfer intermediate state. The observations explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the momentum mismatch and excitonic localization in 2D heterostructures and devices.
2:45pm - 3:00pmOral
Tunable Layer-dependent Electro and Optical Properties in 2D Materials
Nanyang Technological University, Singapore
The optical and electronic structures of two dimensional transition metal dichalcogenide (2D TMD) materials and perovskite often show very strong layer-dependent properties. Detailed understanding of the inter-layer interaction will help greatly in tailoring the properties of 2D TMD materials for applications. Raman/Photoluminescence (PL) spectroscopy and imaging have been extensively used in the study of nano-materials and nano-devices. They provide critical information for the characterization of the materials such as electronic structure, optical property, phonon structure, defects, doping and stacking sequence.
In this talk, we use Raman and PL techniques to study few-layer 2D TMD samples and perovskites under low temperature and high pressure. The pressure clearly tune the interactions between the layers, which are clearly shown in both the PL and ultra-low wavenumber Raman. The Raman and PL spectra also show clear correlation with layer-thickness and stacking sequence. Our ab initio calculations reveal that difference in the electronic structures mainly arises from competition between spin-orbit coupling and interlayer coupling in different structural configurations.
3:00pm - 3:15pmOral
Preparation and Application of Transition Metal Dichalcogenides Ultra-small Nanodots with Layered Structure
Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, Singapore
Recently, two-dimensional transition metal dichalcogenides (TMDs) nanosheets have exhibited great potential in many applications such as electronics, catalysis, biomedicine, sensing and energy storage. Bear this in mind, further engineering the size and dimension of TMD nanomaterials is promising to endow them with novel properties and broaden their applications. As a typical example, it has been reported that ultra-small MoS2 nanodots exhibited great catalytic activity toward hydrogen evolution reaction and unique optical properties due to the quantum confinements and edge effects.[2, 3] However, high-yield production of TMD nanodots is still under investigation and their applications remain to be explored. Herein, we report the preparation of a number of layered TMD nanodots (including MoS2, WS2, ReS2, TaS2, MoSe2, WSe2 and NbSe2) in high yield. 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.
 D. Gopalakrishnan, D. Damien and M. M. Shaijumon, ACS Nano, 8 (2014), pp. 5297–5303.
 S. J. Xu, D. Li and P. Y. Wu, Adv. Funct. Mater. 25 (2015), pp. 1127-1136.
 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., 54 (2015), pp. 5425–5428.
3:15pm - 3:30pmOral
Tuning the Quasiparticles in MoS2 by Direct Growth on Transition Metal Oxide Substrates
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; 4Rice University, United States
2D MoS2 is a non-centrosymmetric material with a direct energy gap of 1.9 eV causing a strong photoluminescence with efficient valley and spin control. Also, the reduced dielectric screening in 2 dimensions along with the heavy effective mass of Mo-centered d-electrons contribute to the formation of the quasiparticles viz. excitons along with trions. These quasiparticles are, in particular, important since they open up promises like many-body interactions which are not just fundamentally significant but also enable optoelectronic tunability. So far, the routes to tune the MoS2 quasiparticles have been via conventional methods like application of electric field or by changing the dielectric environment. Tuning these quasiparticles via many body interactions with the substrate itself has not been investigated. This is mainly due to weak interactions of MoS2 with conventional substrates (like SiO2). In this work, we have grown MoS2 directly on SrTiO3, a transition metal oxide well known in ‘oxide electronics’ for its tunable temperature dependent dielectric constant and rich phonon diagram. Direct CVD growth on these substrates provides us with a strongly coupled system as compared to exfoliation or transfer. Temperature dependent PL measurements show that while the charge neutral exciton does not interact much with SrTiO3, it is possible to selectively tune the lifetime and binding energy of the negatively charged trion by modulating the substrate environment. Electric field dependent luminescence studies on MoS2/SrTiO3 devices which help us manipulate the phonons in SrTiO3, support our hypothesis. Our experiments on various substrates and calculations help us clearly establish the effect of dielectric, temperature and even phonon interactions of SrTiO3 on MoS2. Our results open an unprecedented route to tune quasiparticles in a 2D TMDC through interactions with transition metal oxides for high efficiency valleytronics.