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 explore unprecedented physical and chemical

properties including carrier dynamics and thermoelectrics

- Seven tenure tracks, 30 postdocs & research professors

- About 100 graduate students

- New building (~9000 m2) in July, 2015

CINAP Periodic table: 2D layered materials

sp2

sp2

- Metal, insulator, semiconductors (small or large Eg)

- Vertical stacking

- New tunneling vertical devices

- New optoelectronic/magnetic properties

- Flexible, transparent

New material and physics world in 2D !!!!

What could we do with layered structures of materials with just the right layers? -Feynman

(SKKU) (SKKU) Nat. Photon. 8, 899-907 (2014)

Atomic layered materials

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Vertical structure

A. K. Geim et al. Nature 499, 419 (2013)

New device structures by van der Waals stacking

(SKKU) (SKKU)

What is new in 2D?

Strong Coulomb interaction or less charge screening

Coulomb interaction

(SKKU) (SKKU)

Excitons at room temperature

Large exciton binding energy

- Emergence of excitons at room temperature

: Ebx ~ 1 eV

- Emergence of multiexcitons at room temperature

: trions, Ebtri ~ Eb

x + 30 ~ 40 meV

: biexcitons, Ebbi ~ Eb

tri + 30 meV 1.8 1.9 2.0 2.1

VG (V)

-70

-50

-30

-10

10

30

50

70

0.5 W

AX

ATA

XX

PL (

a.u

.)

50 W

Photon energy (eV)

500 W

H. S. Lee et al. PRL, ASAP

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Layer dependence

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

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CINAP

(a,b) Li, H. et al. Adv. Funct. Mater. 2012, 22, 1385–1390

(c,d) Eda, G. et al. Nano Lett. 2011, 11, 5111–5116

a

d c

b

Excitation with 514 nm laser

Optical properties

(SKKU) (SKKU)

Layer dependence Transport properties

-60 -40 -20 0 20 40 60

10-1

100

101

102

103

104

t ~ 3.5 nm

t ~ 8 nm

t ~ 13 nm

F

E (

cm

2/V

s)

Rs (

)V

BG(V)

300K

103

104

105

106

107

108

109

Al

• 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

D. Perrelo et al., Nature Comm. 6, 1-8 (2015) Al contact for n-type Tr

Vsd = 0.2 V

→ 1 V

Al

-20 0 20 40 6010

-10

10-9

10-8

10-7

10-6

10-5

tox

= 300 nm

SS ~ 9 V/dec

I sd (

A)

VBG

(V)

300K

0

300

600

900

1200

F

E (

cm

2/V

s)

T = 300K

(SKKU) (SKKU)

Layer dependence

Structural phase transition

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CINAP Why MoS2?

- Reduced charge screening

- High mobility ~200 cm2 V-1s-1 (MoS2)

- Optical properties:

Indirect bandgap (bulk) => direct bandgap (1L)

Wide range of bandgap (1.0 ~ 2.5 eV)

- Various phases exist

- Tailoring such phases is a big challenge

- MoS2 phase transition:

2H (semicond) => 1T (metal)

by Li intercalation

- Difficult to realize

- Severe lattice distortion

- Local phenomena

H. Wang et al., PNAS 11, 19701 (2013)

Google.com

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CINAP Why MoTe2?

Similar to MoS2 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 MoS2

- Rich physics: CDW, superconductivity…

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CINAP

Nature Comm. 5, 4214 (2014)

Polymorph engineering of MoTe2

Phase stability

-> Polymorph engineering is easier in MoTe2

Cohesive energy of MOTe2

1T

1T'

2H

Neutral -0.2 +0.2

Doping per unit cell (e)

0

1.0

ΔE p

er

unit c

ell

(eV)

0.5

(SKKU)

Single crystal MoTe2 by flux method

1T’

2H

Single crystal growth (flux method): mix Mo and Te powder with sodium flux & sintering

5 1/nm

1 nm

3 nm-1

(100)

(010)

3 nm-1

(100)

(010)

1 nm

TEM comparison

2H-MoTe2 1T’-MoTe2

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Peierls distortion ->Band inversion

Hexagonal Octahedral Distorted octahedral

Structures of MoTe2

Semiconductor Metal Small bandgap metal

1T’ phase is more stable than 1T phase!

2H 1T 1T’

D. H. Keum & S. Y. Cho et al., Nature Phys. 11, 482 (2015)

(SKKU) (SKKU)

Inte

nsi

ty (

a. u.)

2θ (degree) 20.0 40.0 60.0

(0 0 2) (0 0 4)

(0 0 6) (0 0 8)

(0 0 2)

(0 0 4)

(0 0 6)

(0 0 8)

2H-MoTe2

1T’-MoTe2

25 26 27

10 20 30 40 50 60

2θ (degree) 2θ (degree)

400 °C

500 °C

600 °C

640 °C 680 °C

720 °C

(0 0

4)

(0 0

4)

2H

1T’

Temperature-dependent XRD

Structural phase transition by temperature -> absence of Te deficiency -> reversible phase transition

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0 10 20 30 40

162

164

166

258

260

262

264

Inte

nsi

ty (

a. u.)

150 300 200 250

Bg (cm

-1)

Ag (cm

-1)

Abso

rbance

(a.u

.)

50 60 70 80 90

3L

4L

9L

Energy (meV)

Peak position

Ag Bg

11L

Ag

Bg

Layer number

0

0.5

1.0

Raman shift (cm-1)

0 5 10 15 20 25

170

172

230

232

234

Inte

nsi

ty (

a. u.)

150 300 200 250

A1g (c

m-1)

E2g (c

m-1)

Abso

rbance

(a.u

.)

0.6 0.8 1.0 1.2 1.4

1L

2L

3L

Energy (eV)

Peak position

E2g A1g

4L Layer number

0

0.5

1.0

Raman shift (cm-1)

E2g

A1g

Raman & absorption spectroscopy

2H-MoTe2 1T’-MoTe2

-> Sign of small bandgap in monoclinic TMDs -> New results on Raman and absorption spectroscopy in 1T’-MoTe2

D. H. Keum & S. Y. Cho et al., Nature Phys. 11, 482 (2015)

Few-layer MoTe2

Large spin-orbit coupling

2H-MoTe2

band splitting in VBM at K

- SOC effect

Γ M K Γ

En

erg

y (

eV

)

2H 1T’

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Electrical properties

0 100 200 300

Temperature (K)

100

102

Mobili

ty (

cm2V

-1s-

1)

1T’-MoTe2

104

Carrier mobility of 1T’-MoTe2

4000 cm2V-1S-1 @ 1.8 K

S (

μV/K

)

100

200

300

300 340 380 T (K)

2H-MoTe2

1T’-MoTe2

Seebeck constants of MoTe2

-> High carrier mobility at low temperature -> High power factor : 0.04 ~ 11 mW/mK2

(4 mW/mK2 : state-of-the-art thermoelectric materials)

(SKKU) (SKKU)

α-M

oTe

2

β-M

oTe

2

2H

-MoTe

2

1T’-M

oTe

2

mix

ed

750

900

1000

1100

988

820

650

500

880

Tem

pera

ture

(°C

)

1180

ASM Database Our work

66 68

Te (%)

65 67 66.7

x

x

x

x

x x

x

x

x

x x

x x x

x

x

x

x x x

x

x

excessive deficient

Bulk 1T’-MoTe2

Few-layered 1T’-MoTe2

2H-MoTe2

1T’-MoTe2

2H-MoTe2

ln σ

(a. u.)

10000/T (K-1)

Arrhenius plot

Electronic phase transition in MoTe2

D. H. Keum & S. Y. Cho et al., Nature Phys. 11, 482 (2015)

(SKKU) (SKKU)

CINAP Ohmic contact in 2D??

H. Wang et al., PNAS 11, 19701 (2013)

- 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 MoS2

- phase transition by light irradiation

- BN to modulate Schottky barrier??

(SKKU) (SKKU)

250 300 200 150 100

Raman shift (cm-1)

Inte

nsi

ty (

a. u.)

Ag(1T’)

Ag(1T’)

(1)

(2)

(4) Ag (1T’)

E2g(2H) Ag(2H)

E2g(2H) Ag(2H)

E2g(2H) Ag(2H)

Ag(1T’)

(3)

SiO2 SiO2 SiO2

Laser

SiO2

Laser 1T’

1T’

2H

-MoTe

2

(1) (2) (3) (4)

Light-induced phase transition in MoTe2

S. Cho et al., Science 349, 625 (2015)

Laser irradiation at a chosen local area

=> Phase control with 1 μm spatial resolution

- Laser thinning

- Phase conversion to 1T’

2H 1T’

1T’

2μm 2μm 2H

1T’

2H

Light illumination!!

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2H contact Φb~200 meV

1T’ contact Φb~10 meV

10000/T (K-1)

ln σ

(a. u.)

24 32 40

-27

-25

-23 VSD=1 V, VG =0 V

Temperature (K) 200 300 400 500

10

1

Mobili

ty (

cm2V

-1s-

1)

1T’ contact μ~50 cm2V-1s-1

2H contact μ~1 cm2V-1s-1

H (

nm

)

0

4

Distance (μm) 0 4 2

2

102

1T’

2H

Cr/Au (source)

2H

Cr/Au (source)

Cr/Au (drain)

Cr/Au (drain)

Laser-patterned Ohmic junction in MoTe2

-> New ohmic contact formed at the junction

Laser patterning on transistor

(only at source/drain area) Local 1T’ at S & D

Laser thinning

S. Cho et al., Science 349, 625 (2015)

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CINAP

Key findings

- Reversible Structural Phase Transition

- Bandgap Opening in 1T’-MoTe2 by ‘Spin-Orbit Coupling’

- Local Phase Transition -> Ohmic contact in 2D

- Room-Temperature Phase Transition by Small Strain

Summary & Future works

Future projects

- Superconductivity in MoTe2

- Bandgap ~ 1 eV => Photovoltaic, Optoelectronic devices

- Vertical tunneling device

0 1 2 3

0

1

2

Resis

tance (

mO

hm

)

Temperature (K)

0T

5mT

10mT

20mT

100mT

300mT

500mT

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Environmental susceptability

Physical and chemical properties can be

easily modulated by environment

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Bandgap is strongly modulated with local strain in MoS2 !

SiO2

HOPG

Roughness ~ 1 nm

B. G. Shin et al., submitted!

Strain effect

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80% area is converted to indirect bandgap from direct bandgap!

B. G. Shin et al., submitted!

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CINAP Room-temperature phase transition

S. H. Song et al., submitted

First-order M-I transition at RT can be induced by small tensile strain (0.2%)

Tensile strain

Fully reversible upon release

1100 K at zero strain

=> 300 K at 0.2% tensile strain

High phase transition T to low phase transition T???

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Dielectric constant modulation

Lin et al., NanoLett. 14, 5569 (2014)

From the renormalization group theory:

Elias et al., Nature Physics 2011

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Work function of MoS2 is modulated by O2 adsorption/desorption!

0 5 10 15 30 35 40 45

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

With O2

(4.47 eV)

CP

D (

V)

Time (min)

UHV

(4.05 eV)

+ O2

e

0 10 20 80 90 100 110 120 130

0.0

0.1

0.2

0.3

0.4

UHV

(4.04 eV)

CP

D (

V)

Time (min)

In air

(4.36 eV)

Pumping out

a

c d

High density monolayer MoS2

on SiO2

MoS2

KFM Tip

+ _

~ VAC(Tip)

VDC(Tip)

+

_

VDC(Si)

b

S. Y. Lee et al., submitted

: 0.4 eV, similar to graphene

Work function modulation

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van der Waals stacking

Quantum mechanical phenomena

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gn

ndx

dn

dx

dDn

dt

d

2

2Continuity

equation

1L-MoS2 vs 6L-MoS2

Multi-layer charge transport

0 1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d e

xc

ite

d

ca

rrie

r d

en

sit

y (n

x/

n0)

Moving distance (nm)

6L-MoS2

1L-MoS2?

Tunneling probability

Monolayer charge transport

hv

1.8 2.0 2.2 2.4 2.6 2.8

0

20

40

60

80

100

IQE

(%

)

Photon energy (eV)

1L-MoS2

6L-MoS2xAeA 2

W. J. Yu et al., submitted

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Thermoelectric Properties (Bi0.5Sb1.5Te3)

Poudel, Science (2008) Nanocomposite

Dimensionless Figure of Merit, zT

S. I. Kim et al., Science 348, 109-114 (2015)

zT = S2 / T

World-best record

for RT TE!

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Synthesis of 2D materials

- Multilayer hBN is also available

=> on SiO2/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

- Large-area, monolayer monocrystalline graphene

(Adv. Mat. 27, 1376 (2015))

- Large-area, AB stacking bilayer graphene (unpublished)

- Large-area, monolayer MoS2, seed growth by CVD

(Nature Comm. (2015))

-Large-area monolayer WSe2 on Au substrate by CVD

(ACS Nano 9, 5510 (2015)

- Seed growth for MX2 (M: Mo, W, X: S, Se) (unpublished)

- Thin MoTe2 film from Mo metal

(ACS Nano 9, 6548 (2015))

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Summary

There is a plenty of room

for new phenomena in

2D materials!

van der Waals engineering!