New Opportunities in Two-dimensional Materials

New Opportunities in Two-dimensional Materials

Yuanbo Zhang （张远波）

Dept. of Physics, Fudan University, China

A Brief History of Materials

The Stone Age The Bronze Age

The Iron Age

The Silicon Age?

“Interface is the Device”

The first transistor, Bell Lab, 1947

Graphene: The Beginning of 2D Material Research

Geim group (2004)

Graphene : Dirac Fermions in 2D

pc E

Band Structure of Graphene

pc E  ^F

kv E 

k_x k_y

Energy

Momentum, hk

Pseudo-spin

P. R. Wallace, Phys. Rev. 71, 622 (1947).

T. Ando et al, J. Phys. Soc. Jpn 67, 2857 (1998).

Less is different

Graphite

c

High Tc Materials Such as

Bi₂Sr₂CaCu₂O_8-x

Zr N Cl

b- ZrNCl Metal

Chalcogenides (M= Nb, Ta, Va, …

X= S, Se, Te ) M X

X

Families of New Materials in 2D

Hundreds of 2D crystals waiting to be explored

Opportunities to Tune the Material Properties in 2D

New device paradigm?

 Black phosphorus (semiconductor)

 1T-TaS₂ (metal) Outline

 Gate-controlled intercalation

 Tunable Phase in 1T-TaS₂

White Phosphorus Red Phosphorus

Allotropes of Phosphorus

Black Phosphorus

Allotropes of Phosphorus: Black Phosphorus

P. W. Bridgman, JACS 36,1344 (1914)

Layered crystal structure

Review:

Morita, Applied Physics A 39, 227–242 (1986).

Phosphorene v.s. Graphene

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Phosphorene

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Graphene

Y. Takao, et al., J. Phys. Soc. Jpn.

50, 3362 (1981)

Band gap ~ 2 eV

Phosphorene

pc E

k_x k_y

Energy

Graphene

P. R. Wallace, Phys. Rev. 71, 622 (1947).

Phosphorene v.s. Graphene

Direct band gap ~ 0.3 eV Band structure of the bulk

Thickness-dependent Bandgap in few-layer Phosphorene

Thickness-dependent bandgap

Direct bandgap tunable by varying thickness

Thickness-dependent Band Gap

Bridging the gap

Churchill and Jarillo-Herrero, Nature Nano. (2014)

Si

Black Phosphorus

Black Phosphorus Field-effect Transistor

Likai Li et al. Nature Nano. 9, 372 (2014).

Likai Li Fangyuan Yang

See also:

Liu, H. et al. ACS Nano 8, 4033 (2014).

Koenig, S. P. et al., APL 104, 103106 (2014).

Xia, F. et al., Nature Comm. 5, 4458 (2014).

Highest on-off ratio ~ 10⁵

Black Phosphorus FET

5 nm sample

Room temperature

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Limiting Factors of Carrier Mobility

Before After

Sample left in air for 3 days

Optic Image of

Black Phosphorus on BN Cross-sectional View

Black Phosphorus on Hexagonal Boron Nitride

Protecting the bottom surface with hBN

Quantum Oscillations in Black Phosphorus on hBN

B = 31T, T = 0.3K

2D Electron and Hole Gases in Black Phosphorus

2D instead of 3D Fermi surface

2D Electron and Hole Gases in Black Phosphorus

2D confinement at the surfave Charge distribution

2D electron and hole gases are confined to ~ 2 atomic layers

2D Electron and Hole Gases in Black Phosphorus

Crucial information obtained from the quantum oscillations

Likai Li et al. Nature Nano., Advance Online Publication (arXiv:1411.6572).

See also:

Tayari, V. et al., arXiv:1412.0259 (2014).

Chen, X. et al., arXiv:1412.1357 (2014).

Gillgren, N. et al., 2D Mater. 2, 011001 (2015).

Even Higher Mobility?

Top View Side View

Graphite local gate screens impurity potential, leads to high mobility

High Mobility Black Phosphorus 2DEG

Factor of 3 increase in mobility

Quantum Hall Effect in Black Phosphorus 2DEG

Likai Li et al. arXiv:1504.07155 (2015)

Holes Electrons

Landau Level Energy Landscape

Even-denominator fractional quantum Hall states in ZnO

Falson, J. et al. Nature Physics 11, 347 (2015)

Black phosphorus potentially harbors similar FQH states

Anyons in Black Phosphorus 2DEG?

1T-TaS₂ : a Strongly Correlated 2D Material

Yijun Yu

Crystal structure of 1T-TaS₂

1T

6Å

Nearly Commensurate

NCCDW Incommensurate

ICCDW

Commensurate

CCDW & Mott

0 100 200 300 400 500

10^-1 10⁰ 10¹

R() bulk

T (K)

Various CDW Phases in 1T-TaS₂

Various CDW Phases in 1T-TaS₂

Commensurate CDW and Mott Insulator State

Fazekas, P. & Tosatti, E. Philos. Mag. B (1979) Wilson et al., Adv. Phys. (1975)

Sipos, et al. Nat. Mater. (2008).

Gate Doping Limits

Electrolyte SiO₂

Conventional Dielectric Gating Ion Liquid Gating

Maximum n ~ 10¹³ cm^-2 Maximum n ~ 10¹⁴-10¹⁵ cm^-2 Only top atomic layer

K.Ueno,Nat.Mater.(2008); D.K.Efetov,Phys.Rev.Lett.(2010); J.T.Ye,Nat.Mater.(2010);

J.G.Checkelsky,Nat.Phys.(2012); Nakano,Nature(2012); J.T.Ye,Science(2013)

Tuning TaS₂ through Gate-controlled intercalation

Ion Gel (LiClO₄/PEO)

TaS₂ Sample

Gate Electrode

Gate-controlled intercalation in TaS₂

n ~ 10¹⁵ cm^-2 for EACH atomic layer

Gate-controlled Doping by Intercalation

Device Structure

Yijun Yu et al. Nature Nano., 10, 270 (2015).

0 1 2 3 0.5

0.6 0.7 0.8 0.9 1.0 1.1

Normalized R

V_g(V)

1T-TaS₂ Ionic Field-effect Transistors (iFET)

iFET operates through ion diffusion

NCCDW

ICCDW

0 1 2 3

0.5 0.6 0.7 0.8 0.9 1.0 1.1

V_g(V)

B

0 1 2 3

0.5 0.6 0.7 0.8 0.9 1.0 1.1

V_g(V)

C

Gate-controlled Doping by Intercalation

Gate-controlled Doping by Intercalation

Gate-controlled Doping by Intercalation

Gate-controlled Doping by Intercalation

Gate-controlled Doping by Intercalation

Gate-controlled Doping by Intercalation

Gate-controlled Doping by Intercalation

14 nm sample

Electron Doping from Charge Transfer

~ 20% electron doping from charge transfer from Li

Mott

Tunable Phases in 1T-TaS₂ iFET

Mott

Temperature

pressure,

isovalent substitution ICCDW

NCCDW

CCDW

&Mott

SC

Intercalation Compared with Pressure and Isovalent Substitution

Connection btw intercalation and pressure/isovalent substitution??

Sipos, et al. Nat. Mater. (2008).

L. J. Li et al. EPL (2012) R. Ang et al. PRL (2012)

Summary

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Acknowledgement

Fangyuan Yang Yijun Yu

Likai Li Liguo Ma

Fudan Univ.

Prof. Xianhui Chen Guo Jun Ye

Xiu Fang Lu Ya Jun Yan

USTC

Prof. Sang-Woo Cheong

Rutgers Univ.

Prof. Donglai Feng

Prof. Hua Wu Xuedong Ou Qinqin Ge

Y. H. Cho Prof. Young Woo Son

KIAS, Korea

NIMS, Japan

Dr. Takashi Taniguchi Dr. Kenji Watanabe

Prof. Li Yang

Univ. of Washington

Vy Tran

Ruixiang Fei

Institute of Metal Research

Prof. Wencai Ren