Conference Programme

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Overview
Session
H-04: Properties: characterization and modeling
Time:
Tuesday, 20/Jun/2017:
1:30pm - 3:30pm

Session Chair: Kian Ping Loh, National University of Singapore
Session Chair: Kai Xiao, Oak Ridge National Laboratory
Location: Rm 323

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Presentations
1:30pm - 2:00pm
Invited

Atomic Level Characterization of Dopants, Defects and Grain Boundaries in Monolayer Transition Metal Dichalcogenides

Jamie WARNER

Oxford University, United Kingdom

Monolayer transition metal dichalcogendies such as MoS2 and WS2 are direct band gap semiconductors in the red spectral region and offer exciting prospects for future ultrathin 2D semiconductor electronics. I will present recent results on the atomic level characterization of monolayer TMDs using aberration corrected transmission electron microscopy. Defects, such as vacancies, will be discussed, along with dislocations in grain boundaries and the impact of mirror twin boundaries on the bilayer stacking. I will present results on detecting single atom Cr and V dopants in TMDs using annular dark field scanning transmission electron microscopy and electron energy loss spectroscopy. The ability to manipulate single metal atoms in MoS2 will be demonstrated. Finally I will discuss the atomic level interactions between single Pt atoms and the surface of MoS2.


2:00pm - 2:30pm
Invited

Structure, Defect, and Growth of Phosphorene: A First-principles Exploration

Yong-Wei ZHANG1, Gang ZHANG1, Yongqing CAI1, Junfeng GAO1, Andrey KISTANOV1,2, Kun ZHOU2

1Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR), Singapore; 2School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore

Phosphorene has attracted increasing attention recently due to its fascinating properties, such as finite, direct band characteristic and high carrier mobility [1]. Currently, conditions to grow phosphorene, behavior of its atomic vacancies, and origin for its notoriously low structural stability remain poorly understood. Clearly, understanding its growth conditions and defect behavior and uncovering the mechanism for its instability are crucial to enhance its structural stability and promote its applications.

In this talk, we will discuss several interesting phoenmena associated with phosphorene. First, we will disucss the itinerant behavior of atomic vacancies due to the unusual energtics and kinetics of its atomic vacacies. This highly itinerant behavior, which can occur at room temperature, may explain the difficulty in imaging its atomic vacancies by electronic spectroscopy at room temperature, and its low intrinsic structural stability [3]. Second, we examine the role of water and oxygen molecules played in the structural instability of phosphorene. We find that oxygen meolcules plays a critical role in breaking down the structure of phosphorene, but water molecules are not the origin for the chemical instability [3]. Finally, we will discuss the interactions of a phosphorene nanoflake with different subsutrates, focusing on its structural stability and choice of substrate for growing phosphoroene [4].

References:

[1] Sorkin et al. “Recent advances in the study of phosphorene and its nanostructures”. Crit. Rev. Solid State & Mater. Sci. 42, 1-82 (2016)

[2] Cai et al. “Highly itinerant atomic vacancies in phosphorene”. J. Am. Chem. Soc. 138, 10199 (2016).

[3] Kistanov et al. “The role of water and oxygen molecules and vacancies in the structure instability of phosphorene”. 2D Mater. 4, 015010 (2016).

[4] Gao et al. “The critical role of substrate in stabilizing phosphorene nanoflake” A theoretic exploration”. J. Am. Chem. Soc. 138, 4763 (2016).


2:30pm - 2:45pm
Oral

Direct Measurement of the Layer-dependent Quasiparticle Band Gap of Two-dimensional Gallium Selenide on Epitaxial Graphene

Zeineb BEN AZIZA, Abdelkarim OUERGHI

Centre de Nanosciences et de Nanotechnologies, Centre Nationnal de la Recherche Scientifique (CNRS), University of Paris-Sud, University of Paris-Saclay, C2N – Marcoussis, France

Two dimensional (2D) materials have recently been the focus of extensive research. Stacking these 2D materials in so-called van der Waals (vdW) heterostructures provides the ability to obtain hybrid systems which are suitable to design optoelectronic devices[1]. Here[2,3], we report the structural and electronic properties of gallium selenide (GaSe) directly grown by Molecular beam Epitaxy (MBE) on bilayer graphene substrate. Micro-Raman spectroscopy showed that, after the vdW hetero-interface formation, the Raman signature of pristine graphene is preserved. The uniformity of GaSe and the sharp interface were examined by scanning transmission electron microscopy (STEM). Using scanning tunneling microscopy and spectroscopy (STM/STS), we demonstrate the tunability of the quasiparticle energy gap of few layered GaSe as varying its thickness. Our results show that the band gap is about 3.50 ± 0.05 eV for single-tetralayer (1TL), 3.00 ± 0.05 eV for bi-tetralayer (2TL) and 2.30 ± 0.05 eV for tri-tetralayer (3TL). This band gap evolution of GaSe, in particularly the shift of the valence band with respect to the Fermi level, was confirmed by angle-resolved photoemission spectroscopy (ARPES) measurements and our theoretical calculations. Given the position of the Fermi level, the few- layered GaSe was shown to be n doped contrarily to the p type character observed for bulk GaSe. This n type doping is the result of the important charge transfer at the interface of this vdW heterostructure which was evidenced by the 100 meV shift of the graphene Dirac point toward lower binding energies compared to the pristine graphene. These findings allow deeper understanding of the electronic structure and the interfacial properties of GaSe/graphene vdW heterostructure.

References:

[1] Geim, A. K. et al. Nature 499, 419–425 (2013).

[2] Ben Aziza, Z. et al. ACS Nano 10, 9679−9686 (2016).

[3] Ben Aziza, Z. et al. Submitted (2017).


2:45pm - 3:00pm
Oral

In Operando Transmission Electron Microscopy of 2D-based Devices

Martial DUCHAMP1, Massimo SPINA2, Tao LIU3, Slaven GARAJ2,3

1School of Materials Science and Engineering, Nanyang Technological University, Singapore; 2Centre for Advanced 2D Materials, National University of Singapore, Singapore; 3Department of Physics, National University of Singapore, Singapore

Two-dimensional (2D) materials offer a rich opportunity to explore fundamental physical phenomena that could reveal new functionalities for device applications. Charge transport, which is very sensitive to defects, dislocations, interfaces, edges and adatom adsorption in this reduced dimension play a central role in applications. The understanding of the relation between electrical transport properties and the local atomic and structural environment is of fundamental significance for identifying optimal device architectures and futures applications.

Atomic resolution capabilities of the modern transmission electron microscopes (TEMs) have mainly be used to monitor the sample quality and in some rare cases to directly observe 2D-based devices. Nevertheless, the correlation between the electronic properties of a 2D-based device and the structural and chemical composition can be established by performing in operando experiments inside a TEM, i.e., by observing a 2D device under operation inside a TEM at atomic resolution.

Free-standing two terminals devices based on MoS2, TaS2 and WSe2 2D materials are fabricated on top of MEMS chips compatible with in situ TEM holder using clean-room processes. The electrical properties of the devices are accessed in operando the TEM. Different device configurations, 2D material layer thicknesses, defects types and edge terminations are observed and put in relation with the electrical properties of the devices. This approach provides a new inside on the influence of the local atomic configurations on the electrical properties of 2D devices and allows a finer analysis of the device properties.


3:00pm - 3:15pm
Oral

Theoretical Study of Quantum Anomalous Hall Related States in Two-dimensional Materials

Tong ZHOU, Xinyuan WEI, Zhongqin YANG

State Key Laboratory of Surface Physics and Key Laboratory for Computational Physical Sciences (MOE) & Department of Physics, Fudan University, China

In this talk, I will primarily introduce two types of quantum anomalous Hall (QAH) related states found in two-dimensional materials: quantum spin-quantum anomalous Hall (QSQAH) and non-Dirac quantum anomalous Hall (NDQAH) states, based on density-functional theory and Wannier function methods. One new category of valley polarized topological insulators, QSQAH, where the QAH effect occurs at one valley and the quantum spin Hall (QSH) effect occurs at the other valley, is predicted in the decorated Sb(111) monolayers. The time reversal symmetry is broken by doping certain magnetic atoms in the sheet or the magnetic substrate. The induced drastic exchange field, together with strong spin-orbit coupling (SOC) of Sb atoms, coming from Sb px and py orbitals, generates the novel QSQAH state. Dissipationless chiral charge edge states related to one valley are found to emerge along the both sides of the sample, while low-dissipation spin edge states related to the other valley flow only along one side of the sample. The NDQAH state is predicted in MoS2 monolayers with magnetic atoms doped, which is formed by quadratic bands of dx2−y2 and dxy orbitals of the adatoms. The mechanism is very different from the traditional Dirac-type QAH effect. The special two-meron pseudospin texture is found to contribute to the topology.


3:15pm - 3:30pm
Oral

Cathodoluminescence in Monolayer Transitional Metal Dichalcogenides

Shoujun ZHENG1,2, Jinkyu SO1, Fucai LIU3, Zheng LIU3, Hong Jin FAN1,2

1Centre for Disruptive Photonic Technologies, Nanyang Technological University, Singapore; 2Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore; 3School of Materials Science and Engineering, Nanyang Technological University, Singapore

Cathodoluminescence (CL), excited by an electron beam, is widely applied in cathode-ray tubes, oscilloscopes, fluorescent screens, minerals analysis and so on. However, it is unable to directly study monolayers of 2-dimensional (2D) TMDs by CL technique because of the low excitation volume. Here, we realized the observation of the CL in monolayer TMDs for the first time, including WSe2, MoS2 and WS2. Two peaks were observed in the low temperature CL spectra of WSe2, which correspond to exciton and trion of WSe2. Our method opens a door to study optical properties of 2D materials in nanoscale by CL technique and might be applied to 2D emitters powered by an electron beam.


3:30pm - 3:45pm
Oral

Linear and Nonlinear Magneto-optical Absorption Coefficients and Refractive Index Changes in Phosphorene

Dinh LE1, Van-Tung LUONG2, Vinh-Phuc HUYNH2

1Hue University, Vietnam; 2Dong Thap University, Vietnam

In this work, we theoretically study the magneto-optical absorption coefficients and refractive index changes in monolayer phosphorene under a perpendicular magnetic field. We evaluate the linear, the third-order nonlinear, and the total absorption coefficients and relative refractive index changes as functions of the incident photon energy and the magnetic field. The magneto-optical absorption coefficients and relative refractive index changes appear in two different regimes: the microwave to THz and the visible frequency. The amplitude of intra-band transition peaks is larger than that of the inter-band transitions. The resonant peaks shift towards higher photon energies and decrease with the magnetic field. Our results demonstrate the potential of monolayer phosphorene as a new two-dimensional material for applications in nano-electronic and optical devices as a promising alternative to graphene.

References:

[1] C.V. Nguyen, N.N. Hieu, C.A. Duque, D.Q. Khoa, N.V. Hieu, L.V. Tung, and H.V. Phuc, J. Appl. Phys. 121 (2017) 045107



 
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