New Opportunities in Two-dimensional Materials Yuanbo Zhang Dept. - - PowerPoint PPT Presentation

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New Opportunities in Two-dimensional Materials Yuanbo Zhang Dept. - - PowerPoint PPT Presentation

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New Opportunities in Two-dimensional Materials Yuanbo Zhang 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

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New Opportunities in Two-dimensional Materials

Yuanbo Zhang （张远波）

  • Dept. of Physics, Fudan University, China
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A Brief History of Materials

The Stone Age The Bronze Age The Iron Age

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The Silicon Age?

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“Interface is the Device”

The first transistor, Bell Lab, 1947

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Graphene: The Beginning of 2D Material Research

Geim group (2004)

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Graphene : Dirac Fermions in 2D

pc E 

Band Structure of Graphene

pc E 

F

kv E  

kx ky

Energy Momentum, hk Pseudo-spin

  • P. R. Wallace, Phys. Rev. 71, 622 (1947).
  • T. Ando et al, J. Phys. Soc. Jpn 67, 2857 (1998).
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Less is different

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Graphite

c

High Tc Materials Such as Bi2Sr2CaCu2O8-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

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Opportunities to Tune the Material Properties in 2D

New device paradigm?

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 Black phosphorus (semiconductor)  1T-TaS2 (metal) Outline

 Gate-controlled intercalation  Tunable Phase in 1T-TaS2

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White Phosphorus Red Phosphorus

Allotropes of Phosphorus

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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).

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Phosphorene v.s. Graphene

  • Puckered honey-comb lattice
  • 5 valence electrons
  • Fully filled valence band
  • Gapped semiconductor

Phosphorene

  • Planar honey-comb lattice
  • 4 valence electrons
  • Half-filled conduction band
  • Zero-gap semiconductor

Graphene

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

50, 3362 (1981)

Band gap ~ 2 eV

Phosphorene

pc E 

kx ky

Energy

Graphene

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

Phosphorene v.s. Graphene

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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

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Thickness-dependent Band Gap

Bridging the gap

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

Si

Black Phosphorus

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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).

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Highest on-off ratio ~ 105

Black Phosphorus FET

5 nm sample Room temperature  High mobility up to

1000 cm2/Vs

  • Saturation in I-V

Characteristics

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Limiting Factors of Carrier Mobility

Before After

Sample left in air for 3 days

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Optic Image of Black Phosphorus on BN Cross-sectional View

Black Phosphorus on Hexagonal Boron Nitride

Protecting the bottom surface with hBN

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Quantum Oscillations in Black Phosphorus on hBN

B = 31T, T = 0.3K

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2D Electron and Hole Gases in Black Phosphorus

2D instead of 3D Fermi surface

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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

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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).

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Even Higher Mobility?

Top View Side View

Graphite local gate screens impurity potential, leads to high mobility

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High Mobility Black Phosphorus 2DEG

Factor of 3 increase in mobility

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Quantum Hall Effect in Black Phosphorus 2DEG

Likai Li et al. arXiv:1504.07155 (2015)

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Holes Electrons

Landau Level Energy Landscape

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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?

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1T-TaS2 : a Strongly Correlated 2D Material

Yijun Yu

Crystal structure of 1T-TaS2

1T

6Å

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Nearly Commensurate

NCCDW

Incommensurate

ICCDW

Commensurate

CCDW & Mott

100 200 300 400 500 10

  • 1

10 10

1

b u l k

R() T (K)

Various CDW Phases in 1T-TaS2

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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).

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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)

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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

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Gate-controlled Doping by Intercalation

Device Structure

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

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1 2 3 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Normalized R Vg(V)

1T-TaS2 Ionic Field-effect Transistors (iFET) iFET operates through ion diffusion

NCCDW ICCDW

1 2 3 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Vg(V)

B

1 2 3 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Vg(V)

C

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Gate-controlled Doping by Intercalation

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Gate-controlled Doping by Intercalation

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Gate-controlled Doping by Intercalation

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Gate-controlled Doping by Intercalation

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Gate-controlled Doping by Intercalation

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Gate-controlled Doping by Intercalation

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Gate-controlled Doping by Intercalation

14 nm sample

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Electron Doping from Charge Transfer

~ 20% electron doping from charge transfer from Li

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Mott

Tunable Phases in 1T-TaS2 iFET

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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)
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Summary

  • Black Phosphorus Transistor
  • Tunable Phases in 1T-TaS2
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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