STM Characterization of MoS 2 films

Transcription

1 Center for the Computational Design of Functional Layered Materials STM Characterization of MoS 2 films Maria Iavarone Physics Department, Temple University EFRC Annual Meeting May 12-13,2016

2 STM -Iavarone TEM -Zhu Thin film- Xi Theoretical Modeling Bansil Forum A EFRC Annual Meeting May 12-13,2016 2

3 New properties in old materials: the layered dichalcogenides New avenues: dimensionality, strain and bending play an important role in determining the physical properties. Possibility to tune the band gap for electronic applications, thermoelectric applications and catalysis 3

4 Motivation and Approach Need to understand the coupling between defects, grain boundaries, strain, dimensionality and electronic properties Combine different capabilities in coherent effort study key systems of scientific interest: MoS 2 : layer dependence of electronic properties Future STM local electronic structure STEM/EELS Local structure chemical XPEEM Valence band Electronic structure understanding electronic properties in 2D materials 4

5 MoS 2 : The Prototypical TMDC Layered indirect band gap (1.2eV) semiconductor in bulk that transitions to direct gap* (1.8eV) at one monolayer. *Visible range favorable for optoelectronics Osgood Jr., M., Phys. Rev. Lett. 111: (2013) Chhowalla, Nature Chem 5(4): (2013) Atomically thin nature invites the potential of transparent and flexile electronics. - Monolayer transistor fabricated with mobility greater than 200 cm 2 (V s) -1 and with high on-off ratios. Radisavljevic, Nature Nanotech 6: (2011). Strong spin orbit coupling and inversion asymmetry in single layer spintronic and valleytronic applications. - Spin polarization present only on odd layer numbers Bansil, A. et al Scientific Reports 4, 6270 (2014). 5

6 MoS 2 : Layer Dependence Electronic Properties (STM) 3 ML 4 ML 5 ML 39Å 1 ML HOPG 2 ML 0Å 5 ML di/dv (a. u.) 4 ML 3 ML 2 ML 1 ML Films grown by CVD (Trainer, Wolak, Xi)-Poster V (V) Trainer, Putilov, Iavarone Manuscript in preparation 6

7 E b = 8.4 ev 10x10 μm 2 Valence band location resolved : Photoemission Electron Spectroscopy E b = 2.7 ev 10x10 μm 2 E b = 1.8 ev 10x10 μm 2 A C B At energy E b = 1.8 ev a clear photoemission intensity contrast is observed between the mono- and bilayer MoS 2 due to the difference in the valence band edge. Shift in the Valence Band Maxima from Mono- to Bilayer MoS 2 reflect the trend seen in the STS. PES (a. u.) hν = 100 ev A HOPG B 1ML C 2ML hν = 100 ev Valence Band Maxima 2ML Binding Energy (ev) PES (a. u.) 1ML HOPG Gray-Chandrasena (Temple)-Kronast (Helmholtz-Zentrum Berlin ) Binding Energy (ev) Manuscript in preparation 7

8 Layer evolution of the tight-binding band structure in MoS 2 Experimental tunneling spectra di/dv (a. u.) 5 ML 4 ML 3 ML 2 ML 1 ML 1 ML V (V) Calculated tunneling spectra 2 ML 3 ML 8

9 Band Decomposition for three ML of MoS 2 Bansil, Nieminen, Saari, Wang manuscript in preparation 9

10 Local Tunneling Spectroscopy across a grain boundary Left Grain 12x12 nm 2 Right Grain 12x12 nm 2 20x20nm 2 0Å 5.9 Å Atomic resolution on opposite sides of grain boundary show 6º lattice rotation. Strain induced reduction of band-gap over grain boundary as Valence Band Edge shifts toward E F. Trainer, Putilov, Iavarone 10

11 Low Temperature experiments Unpublished data not shown 11

12 Low Temperature experiments: Moire patterns HOPG Area #4: 10.3Å MoS2 > HOPG: 7 o MoS2 > Moire: 26.3 o STEP SIZE: 0.13nm 1ML 13 x 13 nm 2 2ML 19º C atom S atom of MoS 2 Numerical didv (a.u.) Area #3: 6.8Å MoS2 > HOPG: 26 o MoS2 > Moire: o 1456x500 nm 2 Moiré pattern MoS V (V) Angle between MoS 2 and HOPG lattices: 11º 12

13 325 x 325 nm 2 HOPG Spectroscopy at Defects sites Parallel Grain Edges Defect #1 Defect #2 Defect #3 V = +1.5V I = 10pA 1ML MoS 2 V = +1.0V I = 30pA 23.4 x 23.4 nm 2 Unpublished data not shown 13

14 Summary of findings Layer dependence of the gap has been observed in MoS 2 few layers films. Band structure calculations suggest that interfacial Sulphur orbitals have a strong contribution to the shift of the valence band edge. Ongoing experimental work Strong effect on local density of states has been observed by STM at defects and grain boundaries Need to correlate different kind of grain boundaries by STM/STS/TEM and identify local Sulphur structures. Ongoing theoretical work Band structure calculations, electric and chemical behavior of MoS 2 grain boundaries, defects, effect of substrate 14

15 Collaborations and Acknowledgements D. Trainer, A. Putilov, C. Di Giorgio, M. Iavarone STM/STS characterization Department of Physics, Temple University, Philadelphia PA Xiaoxing Xi Fabrication of MoS 2 thin films Department of Physics, Temple University, Philadelphia PA Yimei Zhu, Qiao Qiao TEM characterization of Mos 2 Brookhaven National Laboratory, Department of Condensed Matter Physics and Materials Science Alexander Gray, Ravini Chandrasena XPEEM Department of Physics, Temple University, Philadelphia, PA Florian Fronast XPEEM Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein- Straße 15, Berlin, Germany Arun Bansil, Baokai Wang, Christopher Lane Theoretical Modeling Department of Physics, Northeastern University, Boston, MA Jouko Nieminen and Timo Saari Theoretical Modeling Department of Physics, Tampere University of Technology, Tampere, Finland Tay-Rong Chang Theoretical Modeling Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan Horng-Tay Jeng, Theoretical Modeling Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan Institute of Physics, Academia Sinica, Taipei 11529, Taiwan Hsin Lin Theoretical Modeling Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore This work was supported as part of the CCDM, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under Award #DE-SC

16 300 mk STM lab UHV Torr minimum temp. 300 mk magnetic field up to 9 T Cold cleavage stage Tip prep. in-situ (e-beam) LEED-Auger E-beam evaporator Substrate e-beam heater 1500 C Substrate cooling stage -130 C Scanning Probe Microscopy group at Temple Iavarone 16