Center for Integrated Nanostructure Physics (CINAP)

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2 Center for Integrated Nanostructure Physics (CINAP) - Institute for Basic Science (IBS) was launched in 2012 by the Korean government to promote basic science in Korea - Our Center was established in 2012 at SKKU - Focus on growth of 2D materials (Gr, BN, TMD, and others) and eplore unprecedented physical and chemical properties including carrier dynamics and thermoelectrics - Seven tenure tracks, 30 postdocs & research professors - About 100 graduate students - New building (~9000 m 2 ) in July, 2015

3 Periodic table: 2D layered materials CINAP What could we do with layered structures of materials with just the right layers? -Feynman - Metal, insulator, semiconductors (small or large E g ) - Vertical stacking - New tunneling vertical devices - New optoelectronic/magnetic properties - Fleible, transparent sp 2 sp 2 New material and physics world in 2D!!!!

4 Atomic layered materials Nat. Photon. 8, (2014)

5 Vertical structure New device structures by van der Waals stacking A. K. Geim et al. Nature 499, 419 (2013)

6 What is new in 2D? Strong Coulomb interaction or less charge screening Coulomb interaction

7 PL (a.u.) Ecitons at room temperature Large eciton binding energy H. S. Lee et al. PRL, ASAP 0.5 W A XX A T A X 50 W V G (V) Emergence of ecitons at room temperature : E b ~ 1 ev - Emergence of multiecitons at room temperature : trions, E b tri ~ E b + 30 ~ 40 mev : biecitons, E b bi ~ E b tri + 30 mev 500 W Photon energy (ev)

8 Layer dependence Band structures: Indirect bandgap (ML) => direct bandgap (1L)

9 Optical properties CINAP a b c d Ecitation with 514 nm laser (a,b) Li, H. et al. Adv. Funct. Mater. 2012, 22, (c,d) Eda, G. et al. Nano Lett. 2011, 11,

10 I sd (A) R s ( ) Al contact for n-type Tr 10-5 t o = 300 nm SS ~ 9 V/dec K T = 300K Layer dependence Transport properties Al V sd = 0.2 V 1 V V BG (V) FE (cm 2 /Vs) D. Perrelo et al., Nature Comm. 6, 1-8 (2015) K Al t ~ 3.5 nm t ~ 8 nm t ~ 13 nm V BG (V) Type conversion with flake thickness: n-type to ambipolar Graphene-like electron and hole mobilities Mobility increases in proportion to film thickness Bandgap shrinkage and surface scattering reduction FE (cm 2 /Vs)

11 Layer dependence Structural phase transition

12 Why MoS 2? CINAP Google.com - Reduced charge screening - High mobility ~200 cm 2 V -1 s -1 (MoS 2 ) - Optical properties: Indirect bandgap (bulk) => direct bandgap (1L) Wide range of bandgap (1.0 ~ 2.5 ev) H. Wang et al., PNAS 11, (2013) - Various phases eist - Tailoring such phases is a big challenge - MoS 2 phase transition: 2H (semicond) => 1T (metal) by Li intercalation - Difficult to realize - Severe lattice distortion - Local phenomena

13 Why MoTe 2? CINAP Similar to MoS 2 but.. - Bandgap : ~ 1 ev, similar to Si - good for energy harvesting - good for TFET - Cohesive energy difference between 2H and 1T is smaller than that of MoS 2 - Rich physics: CDW, superconductivity

14 Polymorph engineering of MoTe 2 CINAP Phase stability Nature Comm. 5, 4214 (2014) Cohesive energy of MOTe 2 0 ΔE per unit cell 1.0 (ev) 0.5 1T 1T' 2H +0.2 Neutral -0.2 Doping per unit cell (e) -> Polymorph engineering is easier in MoTe 2

15 Single crystal MoTe 2 by flu method Single crystal growth (flu method): mi Mo and Te powder with sodium flu & sintering 1T 2H TEM comparison (100) (100) (010) 5 1/nm 3 nm -1 (010) 3 nm -1 1 nm 1 nm 2H-MoTe 2 1T -MoTe 2

16 Structures of MoTe 2 D. H. Keum & S. Y. Cho et al., Nature Phys. 11, 482 (2015) Semiconductor 2H Metal 1T Small bandgap metal 1T Heagonal Octahedral Distorted octahedral Peierls distortion ->Band inversion 1T phase is more stable than 1T phase!

17 Temperature-dependent XRD Intensity (a. u.) (0 0 4) (0 0 4) 1T 720 C 680 C 640 C 600 C 500 C 400 C θ (degree) 2θ (degree) (0 0 2) (0 0 4) 2H Structural phase transition by temperature -> absence of Te deficiency -> reversible phase transition (0 0 2) (0 0 4) (0 0 6) (0 0 6) (0 0 8) (0 0 8) 1T -MoTe 2 2H-MoTe θ (degree)

18 Intensity (a. u.) Raman & absorption spectroscopy B g (cm -1 ) Absorbance (a.u.) A g (cm -1 ) Intensity (a. u.) A 1g (cm -1 ) Absorbance (a.u.) E 2g (cm -1 ) D. H. Keum & S. Y. Cho et al., Nature Phys. 11, 482 (2015) 1T -MoTe 2 2H-MoTe 2 B g Peak position A 1g 172 Peak position 234 A g B g A g 11L 9L 4L 3L Raman shift (cm -1 ) Layer number Energy (mev) Few-layer MoTe E 2g 230 A 1g E 2g 4L 3L 1.0 2L 0.5 1L Layer number Raman shift (cm -1 ) Energy (ev) -> Sign of small bandgap in monoclinic TMDs -> New results on Raman and absorption spectroscopy in 1T -MoTe 2

19 Energy (ev) Large spin-orbit coupling band splitting in VBM at K - SOC effect 2H 1T Γ M K Γ 2H-MoTe 2

20 Mobility (cm 2 V -1 s -1 ) Electrical properties S (μv/k) Carrier mobility of 1T -MoTe 2 Seebeck constants of MoTe cm 2 V -1 S 1.8 K 300 2H-MoTe T -MoTe T -MoTe Temperature (K) T (K) -> High carrier mobility at low temperature -> High power factor : 0.04 ~ 11 mw/mk 2 (4 mw/mk 2 : state-of-the-art thermoelectric materials)

21 Temperature ( C) mied ln σ (a. u.) Electronic phase transition in MoTe 2 ASM Database 1180 D. H. Keum & S. Y. Cho et al., Nature Phys. 11, 482 (2015) Our work T -MoTe 2 Arrhenius plot Bulk 1T -MoTe α-mote 2 β-mote H-MoTe 2 1T -MoTe H-MoTe /T (K -1 ) 2H-MoTe 2 Few-layered 1T -MoTe ecessive Te (%) deficient

22 Ohmic contact in 2D?? CINAP - Ohmic contact in Si: Ion implantation - Ohmic contact in 2D? - ion implantation damages 2D layer - metal/2d: weak van der Waals interface => side contact to provoke covalent bonds? - phase transition from 2H to 1T by Li intercalation in MoS 2 - phase transition by light irradiation - BN to modulate Schottky barrier?? H. Wang et al., PNAS 11, (2013)

23 Intensity (a. u.) Light-induced phase transition in MoTe 2 S. Cho et al., Science 349, 625 (2015) Light illumination!! Laser irradiation at a chosen local area => Phase control with 1 μm spatial resolution 1T 2H 2μm 2H 2μm 1T 1T 2H 2H-MoTe 2 Laser 1T Laser 1T A g (1T ) A g (1T ) A g (1T ) A g (2H) A g (2H) A g (1T ) E 2g (2H) E 2g (2H) (4) (3) (2) SiO 2 SiO 2 SiO 2 SiO 2 (1) (2) (3) (4) A g (2H) E 2g (2H) (1) - Laser thinning - Phase conversion to 1T Raman shift (cm -1 )

24 ln σ (a. u.) Mobility (cm 2 V -1 s -1 ) H (nm) Laser-patterned Ohmic junction in MoTe 2 Laser patterning on transistor (only at source/drain area) 1T 2H Cr/Au (source) Cr/Au (drain) S. Cho et al., Science 349, 625 (2015) Laser thinning Local 1T at S & D Cr/Au (source) Cr/Au (drain) 4 2H -23 V SD =1 V, V G =0 V Distance (μm) -25 1T contact Φ b ~10 mev 10 1T contact μ~50 cm 2 V -1 s H contact Φ b ~200 mev 1 2H contact μ~1 cm 2 V -1 s /T (K -1 ) Temperature (K) -> New ohmic contact formed at the junction

25 Resistance (mohm) Summary & Future works CINAP Key findings - Reversible Structural Phase Transition - Bandgap Opening in 1T -MoTe 2 by Spin-Orbit Coupling - Local Phase Transition -> Ohmic contact in 2D - Room-Temperature Phase Transition by Small Strain Future projects - Superconductivity in MoTe 2 - Bandgap ~ 1 ev => Photovoltaic, Optoelectronic devices - Vertical tunneling device T 5mT 10mT 20mT 100mT 300mT 500mT Temperature (K)

26 Environmental susceptability Physical and chemical properties can be easily modulated by environment

27 Strain effect B. G. Shin et al., submitted! Bandgap is strongly modulated with local strain in MoS 2! SiO 2 Roughness ~ 1 nm HOPG

28 B. G. Shin et al., submitted! 80% area is converted to indirect bandgap from direct bandgap!

29 Room-temperature phase transition CINAP High phase transition T to low phase transition T??? S. H. Song et al., submitted Tensile strain Fully reversible upon release 1100 K at zero strain => 300 K at 0.2% tensile strain First-order M-I transition at RT can be induced by small tensile strain (0.2%)

30 Dielectric constant modulation Elias et al., Nature Physics 2011 From the renormalization group theory: Lin et al., NanoLett. 14, 5569 (2014)

31 CPD (V) Work function modulation CPD (V) Work function of MoS 2 is modulated by O 2 adsorption/desorption! : 0.4 ev, similar to graphene S. Y. Lee et al., submitted a KFM Tip b High density monolayer MoS 2 on SiO 2 e MoS 2 + _ V DC (Si) ~ V AC (Tip) + _ V DC (Tip) c In air (4.36 ev) Pumping out UHV (4.04 ev) Time (min) d UHV (4.05 ev) + O 2 With O 2 (4.47 ev) Time (min)

32 van der Waals stacking Quantum mechanical phenomena

33 IQE (%) Normalized ecited carrier density ( n / n 0 ) 1L-MoS 2 vs 6L-MoS 2 Multi-layer charge transport Continuity equation d dt d n D d 2 2 d n n n g d W. J. Yu et al., submitted 1L-MoS 2? 6L-MoS 2 Monolayer charge transport Tunneling probability A Ae Moving distance (nm) 1L-MoS 2 6L-MoS 2 40 hv Photon energy (ev)

34 Thermoelectric Properties (Bi 0.5 Sb 1.5 Te 3 ) Dimensionless Figure of Merit, zt S. I. Kim et al., Science 348, (2015) World-best record for RT TE! Poudel, Science (2008) Nanocomposite zt = S 2 / T

35 Synthesis of 2D materials - Large-area, monolayer monocrystalline graphene (Adv. Mat. 27, 1376 (2015)) - Large-area, AB stacking bilayer graphene (unpublished) - Large-area, monolayer MoS 2, seed growth by CVD (Nature Comm. (2015)) -Large-area monolayer WSe 2 on Au substrate by CVD (ACS Nano 9, 5510 (2015) - Seed growth for MX 2 (M: Mo, W, X: S, Se) (unpublished) - Thin MoTe 2 film from Mo metal (ACS Nano 9, 6548 (2015)) - Multilayer hbn is also available => on SiO 2 /Si, unpublished => on Fe : Nature Comm., ASAP - on Pt: Large area monolayer hbn, ACS NANO 8, 8520 (2014) - on Au: mm-size monolayer singlecrystalline hbn, unpublished

36 Summary van der Waals engineering! There is a plenty of room for new phenomena in 2D materials!