Phase Change and Piezoelectric Properties of Two-Dimensional Materials

Transcription

1 Phase Change and Piezoelectric Properties of Two-Dimensional Materials April 22, 2015 Karel-Alexander Duerloo, Yao Li, Yao Zhou, Evan Reed Department of Materials Science and Engineering Stanford University

2 CONCLUSIONS o Many single layer materials exhibit emergent piezoelectricity, e.g. MoS 2 o Duerloo, Ong, Reed, J. Phys. Chem. Lett. 3, 2871 (2012). o Some single layer materials (e.g. MoTe 2 ) exhibit structural metal-insulator phase transitions under tensile strains. o Duerloo, Li, Reed, Nat. Comm. 5, 4214 (2014);

3 Integrating PIEZOELECTRIC Piezoelectric Dynamic Control Applications MATERIAL in Nanodevices APPLICATIONS Stress Sensors Acoustic transducers for signal processing Scanning Tunneling Microscope (STM) Injet Printing

4 Integrating Piezoelectric Dynamic PIEZOELECTRICITY Control Applications in Nanodevices Piezoelectric Effect P i =e ijk ε jk Converse Piezoelectric Effect ε ij =d ijk E k Polarization Electric Field Piezoelectric materials must exhibit: 1. An electronic bandgap 2. A lack of centrosymmetry Inversion symmetry of a crystal => d ijk = ( 1) 3 d ijk = d ijk So d ijk =0 for crystals with inversion symmetry!

5 GRAPHENE IS NOT PIEZOELECTRIC graphene Inversion symmetry => non-piezoelectric Semi-metallic character => nonpiezoelectric

6 Transition Metal Dichalcogenides: MoS 2, MoSe 2. MoTe 2, WS 2, WSe 2 Trigonal prismatic structure:! Semiconducting (E gap ~ 1-2 ev)! Not centrosymmetric TMD MONOLAYERS HAVE POTENTIAL FOR PIEZOELECTRICITY! 3m point group leads to non-zero d 11 and e 11 coefficients

7 WE DISCOVER THAT A VARIETY OF TMDS HAVE SIGNIFICANT PIEZOELECTRIC EFFECTS Our HPC-enabled approach enables identification of promising materials Piezo-coefficients of trigonal prismatic TMD structures are comparable to bulk wurtzite structures K.-A. Duerloo, M. T. Ong, E. J. Reed., J. Phys. Chem. Lett. 3, 2871 (2012).

8 PIEZOELECTRICITY IN 2D MATERIALS HAS RECENTLY BEEN OBSERVED Observation of Piezoelectricity in monolayer MoS 2, Zhu, Wang, Xiao, Liu, Xiong, Wong, Ye, Yin, Zhang, Nature Nanotechnology (2014). Reported e 11 within 20% of predicted. Piezoelectricity of single atomic layer MoS 2 Wu, Wang, Hone, Wang et al., Nature (2014). Piezoelectricity in C 3 N 4 few layers: Zelisko, et al, Nature Communications (2014).

9 BILAYER Conclusions EMERGENT ELECTROMECHANICAL CURVATURE We find that TMDC monolayers are piezoelectric while their bulk host crystals exhibit an inversion center and are therefore not piezoelectric! We predict bilayer BN exhibits an electromechanical curvature effect. K.-A. Duerloo, E. J. Reed., Nano Letters (2013).

10 A TMD FRONTIER: MONOLAYERS SURFACE CAN CHEMISTRY EXIST IN MULTIPLE BEYOND CRYSTAL PLANE-WAVE STRUCTURES DFT Semiconducting (1-2 ev) Metallic Semi-metallic

11 A TMD FRONTIER: MONOLAYERS SURFACE MIGHT CHEMISTRY EXIST IN MULTIPLE BEYOND CRYSTAL PLANE-WAVE STRUCTURES DFT Can the phases of monolayers be engineered and employed in devices? Semiconducting Metallic Metallic Two phases have been observed in chemically exfoliated MoS 2 and WS 2. Eda et al, ACS Nano 6, 7311 (2012); Voiry et al, Nat. Mat. (2013).

12 A FRONTIER: CAN TENSILE SURFACE STRAIN CHEMISTRY CAUSE A STRUCTURAL BEYOND PLANE-WAVE PHASE TRANSITION? DFT Duerloo will be visiting ARL in July for a week to participate in this ARL experiment with Madan Dubey et al.

13 A FRONTIER: WE COMPUTE SURFACE STRAIN CHEMISTRY PHASE BOUNDARIES BEYOND PLANE-WAVE FOR MONOLAYERS DFT Our DFT/PBE calculations of TMD monolayer phase boundaries in strain. Energy calculations on a 5x5 grid in (a,b) lattice constants. Lagrange interpolation for phase boundaries. 2H 5 10 WTe 2 2 2H 1T Tensile strain of 6% along b axis croses phase boundary in MoTe Duerloo, Li, Reed, Nature Communications, (2014).

14 HYBRID A FRONTIER: EXCHANGE SURFACE AND CHEMISTRY ENTROPIC EFFECTS BEYOND IMPACT PLANE-WAVE MoTe 2 BOUNDARY DFT BOUNDARY Hybrid exchange (HSE06) moves phase boundary closer to ambient conditions Incorporation of entropic effects (F=E-TS) accomplished using quasiharmonic approximation with computed phonon bands!(!,!)!!"#$%&' (!,!) +! 1 2 ħ!!(!,!) +!!! ln 1!!ħ!!(!,!)/!!! HSE06 with room T vibrations predict phase boundary within 3% tensile strain in MoTe 2.

15 A FRONTIER: CONTEXT: SURFACE PHASE CHANGE CHEMISTRY MATERIALS BEYOND PLANE-WAVE FOR ELECTRONICS DFT New materials are needed to mitigate escalating power requirements for electronic devices. Metal-oxides: VO Chalcogenide alloys: 2 Ge 2 Sb 2 Te 5 (GST) Sharoni et al. Phys. Rev. Lett. 101, (2008) Suitable phase change materials are rare animals. We discover a promising new material. Xu et al., PNAS. 109, 1055 (2012) Application spaces: Information and energy storage Low power electronics Infrared technologies

16 ACKNOWLEDGEMENTS Acknowledgements Conclusions Karel-Alexander Duerloo Mitchell Ong Yao Li Yao Zhou This work is supported by: Army High Performance Computing Research Center (AHPCRC) DARPA N NSF EECS and DMR National Energy Research Science Computing Center (NERSC)

17 CONCLUSIONS o Many single layer materials exhibit emergent piezoelectricity, e.g. MoS 2 o Duerloo, Ong, Reed, J. Phys. Chem. Lett. 3, 2871 (2012). o Some single layer materials (e.g. MoTe 2 ) exhibit structural metal-insulator phase transitions under tensile strains. o Duerloo, Li, Reed, Nat. Comm. 5, 4214 (2014).