Quantum Transport Chun Ning Lau in (Jeanie) 2D Atomic...

Chun Ning Lau (Jeanie)

Quantum Transport !in!

2D Atomic Membranes!

April 2015 NSF US EU Workshop on 2D Layered Materials & Devices

2D Materials and Heterostructures!

•  Conductors, e.g. graphene, few-layer graphene •  Semiconductors, e.g MoS2, WS2, •  Superconductors, Nb2Se3

•  Insulators, e.g. hBN •  Charge density waves, e.g. NbSe •  Ferromagnets, e.g. VSe2

Geim, Nature 2013.

PERSPECTIVEdoi:10.1038/nature12385

Van der Waals heterostructuresA. K. Geim1,2 & I. V. Grigorieva1

Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leadingtopics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomicplanes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. Thefirst, already remarkably complex, such heterostructures (often referred to as ‘van der Waals’) have recently beenfabricated and investigated, revealing unusual properties and new phenomena. Here we review this emergingresearch area and identify possible future directions. With steady improvement in fabrication techniques and usinggraphene’s springboard, van der Waals heterostructures should develop into a large field of their own.

G raphene research has evolved into a vast field with approxi-mately ten thousand papers now being published every yearon a wide range of graphene-related topics. Each topic is covered

by many reviews. It is probably fair to say that research on ‘simplegraphene’ has already passed its zenith. Indeed, the focus has shiftedfrom studying graphene itself to the use of the material in applications1

and as a versatile platform for investigation of various phenomena.Nonetheless, the fundamental science of graphene remains far frombeing exhausted (especially in terms of many-body physics) and, asthe quality of graphene devices continues to improve2–5, more break-throughs are expected, although at a slower pace.

Because most of the ‘low-hanging graphene fruits’ have already beenharvested, researchers have now started paying more attention to othertwo-dimensional (2D) atomic crystals6 such as isolated monolayers andfew-layer crystals of hexagonal boron nitride (hBN), molybdenumdisulphide (MoS2), other dichalcogenides and layered oxides. Duringthe first five years of the graphene boom, there appeared only a few

experimental papers on 2D crystals other than graphene, whereas thelast two years have already seen many reviews (for example, refs 7–11).This research promises to reach the same intensity as that on graphene,especially if the electronic quality of 2D crystals such as MoS2 (refs 12, 13)can be improved by a factor of ten to a hundred.

In parallel with the efforts on graphene-like materials, anotherresearch field has recently emerged and has been gaining strength overthe past two years. It deals with heterostructures and devices made bystacking different 2D crystals on top of each other. The basic principle issimple: take, for example, a monolayer, put it on top of another mono-layer or few-layer crystal, add another 2D crystal and so on. The resultingstack represents an artificial material assembled in a chosen sequence—asin building with Lego—with blocks defined with one-atomic-plane pre-cision (Fig. 1). Strong covalent bonds provide in-plane stability of 2Dcrystals, whereas relatively weak, van-der-Waals-like forces are sufficientto keep the stack together. The possibility of making multilayer vander Waals heterostructures has been demonstrated experimentally only

1School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK. 2Centre for Mesoscience and Nanotechnology, University of Manchester, Manchester M13 9PL, UK.

Graphene

hBN

MoS2

WSe2

Fluorographene

Figure 1 | Building van der Waalsheterostructures. If one considers2D crystals to be analogous to Legoblocks (right panel), the constructionof a huge variety of layered structuresbecomes possible. Conceptually, thisatomic-scale Lego resemblesmolecular beam epitaxy but employsdifferent ‘construction’ rules and adistinct set of materials.

2 5 J U L Y 2 0 1 3 | V O L 4 9 9 | N A T U R E | 4 1 9

Macmillan Publishers Limited. All rights reserved©2013


Outline!

•  There is still life in graphene….

•  Beyond graphene

•  Few Layer MoS2

•  Few-layer Phosphorene


•  Metal – insulator transition, Tc ~ 35K •  Thermal activation measurement yields

Δ ~ 41 meV •  G(Vbias) curves at E⊥=n=0 yield Δ ∼ 42 meV

Dual-Gated Suspended ABC Trilayer Graphene

42 mV

400

200

0

G (µ

S)

80400

T (K)

2

4

102

4

1002

4

G ( µ

S)

0.050.001/T (1/K)

2x104

0dI

/dV

(µS

)

-40 40Vbias (V)Vbias (mV)

mobility 20,000 – 90,000 cm2/Vs


Effect of electric and magnetic fields Differential conductance G vs source drain bias V at n=0"

•  gap educed symmetrically by |E⊥|!à not layer polarized; arises from electronic interactions"

•  gap reduced by parallel magnetic field at 30T"

Y. Lee, D. Tran, K. Myhro, J. V. Jr., N. Gillgren, C. N. Lau, Y. Barlas, J. M. Poumirol, D. Smirnov, and F. Guinea, Nature Communications, 5, 5656 (2014)

40

0

-40

V (m

V)

300 B|| (T)

10x103

50

G(µS)


Proposed Phase Diagram!U⊥

B||

Canted Anti- Ferromagnet

Quantum Valley Hall

Layer Anti- Ferromagnet

Ferromagnet

Y. Lee, D. Tran, K. Myhro, J. V. Jr., N. Gillgren, C. N. Lau, Y. Barlas, J. M. Poumirol, D. Smirnov, and F. Guinea, Nature Communications, 5, 5656 (2014)

Current EU collaboration: Paco Guinea (CSIC, Spain; Machester) Frank Koppens (ICFO; Spain)


MoS2

•  gapped, On/Off ratio >106 •  direct-to-indirect band gap

transition as function of thickness

•  valley physics

But Mobility <~ 200 – 500 cm2/Vs

Wu et al, Nat. Phys. 2013.

Radisavljevic et al, Nat. Nanoetchnol. 2011.

and many others

What is the mobility bottlenck?


Suspending MoS2

F. Wang, M. Gray, P. Stepanov and C.N. Lau, Nanotechnology, in press (2015)

•  the mobility is even lower, 0.1 -50 cm2/Vs

•  gas annealing à 200 cm2/Vs •  Removing substrates does not

significantly improve mobility •  Other mobility bottlenecks:

•  Schottky barriers at contact •  impurity scattering •  defects


Ionic liquid gating of MoS2

F. Wang, M. Gray, P. Stepanov and C.N. Lau, in preparation (2015)

•  Ionic liquids are molten salts with low melting point •  can induce high carrier density (up to 1014 cm-2)

S" D"

IL"gate"

IL"

Si"SiO2"

VILg"

In collaboration with Robert Haddon at UCR

•  To date all IL gating are performed on substrate-supported devices •  Suspended devices – enable gating from both surfaces


Comparing Suspended and non-suspended devices


•  use DEME-TFSI •  all suspended devices are more

conductive by at least 1-2 orders of magnitude

à IL gating is more effective in free-standing devices

Performed IL gating of 9 suspended and 9 substrate-supported samples

Mechanism: 1.  Higher charge density

2.  Better screening


Transport Mechanism

(e)

(a)

VIlg=0

I ∝ exp a V −ΦB

kBT

$

%&

'

()

Schottky emission at MoS2-electrode interfaces

a = e e4πε0εrd

slope yields εr ~ 11 à dielectric constant of DEME-TFSI ~ 14.5 à agrees with literature values -5

5

I (µΑ)

-1 1Vds (V)

-16

8

I (µΑ)

-1 1Vds (V)

Fujimoto, T.; Awaga, K. Phys Chem Chem Phys 15, 8983 (2013).


Charge Density Induced in Suspended MoS2


Compare ΔVbg and ΔVIL needed to induce the same change in conductance ratio of ionic liquid gate to back gate: up to 450 à  α up to 4.6x1013 cm-2 V-1 > 2-4x previous values


IL-tuned Metal Insulator Transition

•  metal insulator transition observed as VILg is tuned

•  At small VILg, transport via thermal activation

obtained from I-V curves

0.1

1

10

100

σs

(µS)

200120T (K)

VILg =3V2V

1.5V

1V

0V

-0.5V0.1

1

10

100

σ s (µS)

200120T (K)

0.0080.0041/T (1/K)

VILg =3V

2V

1.5V

1V

0V

-0.5V

I ∝ exp a V −ΦB

kBT

$

%&

'

() a = e e

4πε0εrd


Conclusion


•  Mobility not limited by substrate in current generation of devices

•  Bottleneck: Schottky barrier at MoS2-electrode interface à critical: contact engineering

•  Ionic liquid gating of suspended devices à  ion accumulation on both surfaces à  higher charge density, enhanced screening

•  Further optimization à Ultra-high density regime for new phases •  p-doping à spin/valley transport

see Cui et al, arXiv 1412.5977 (2014)


Outline!

•  Few Layer Graphene

•  Few Layer MoS2

•  Fabrication and annealing of suspended MoS2

•  Ionic liquid gating

•  Few-layer Phosphorene

•  Fabrication of air-stable, high mobility devices

•  Observation of quantum oscillation


“Curse of 2D Materials”

Graphene •  Mobility ~ 105 – 106 cm2/Vs •  Gapless

MoS2,WS2, MoSe2, WSe2, etc •  Mobility ~ 100 cm2/Vs •  Gapped

Black Phosphorus •  most stable form of phosphorus •  layered structure •  bulk mobility up to 60,000 cm2/Vs

peroidictable.com


Black Phosphorus

Tran et al, PRB 2014

•  only other layered element •  Puckered atoms within layers •  Anisotropic •  Thickness dependent band

gap, 0.3 - 2 eV •  Direct band gap for all

thickness Asahina & Morita, J. Phys. C, 1986


Few-Layer Black Phosphorus Transistors

Li et al, Nature Nanotechnol 2014

•  ambipolar transport •  gapped, on/off ration ~105 •  Anisotropic Transport •  Mobility ~100-1000 cm2/Vs

for thickness ~2 – 20 nm

Liu et al, ACS Nano 2014 Xia et al, Nature Comm. 2014

•  Best of both worlds!


Challenges

Kroenig et al, APL 2014

Instability in air •  react with water and O2 to form phosphoric acid •  reaction accelerated by light

Island et al, 2D Materials 2014

Favor et al, arxiv 2014


Encapsulation for stable, high mobility Devices hexagonal boron nitride (hBN)

from wikipedia

•  atomically flat •  no dangling bonds à little trapped charges •  high mobility graphene/hBN devices demonstrated

Columbia group, Nature Nanotechnol. 2012

Encapsulate few-layer phosphorene with hBN?


Device Fabrication

•  Dry transfer to form hBN/few-layer phosphorene/hBN heterostructure sandwiches

•  etch to expose edges of phosphorene •  1D metallic contact to 2D layers

Wang et al, Science 2013

hBN SiO2

PDMS

phosphorene hBN

electrode

top gate

Si/SiO2


Device Stability

•  Device left in air for 2 weeks •  Slight shift in charge neutrality point •  Only slight decrease in conductance & mobility

16

Figure 5. Time dependence of few-layer BP FET device characteristics. (a) Transfer curves for an unen-capsulated BP FET with Ti/Au contacts, measured as a function of ambient exposure time. (b) Transfer curves for a BP FET measured immediately before and after encapsulation. (c) Transfer curves for a ~30 nm thick ALD AlOx encapsulated BP FET with Ti/Au contacts, measured as a function of ambient exposure time. (d) Transfer curves for a ~30 nm thick ALD AlOx encapsulated BP FET with Ni/Au contacts, meas-ured against ambient exposure time. Comparison of the (e) ION/IOFF ratio and (f) hole mobility for encapsu-lated and unencapsulated BP FETs versus ambient exposure time.

A

B D

C

F

E

Wood et al, Nano Letters 2014

Encapsulated in hBN (our data)

N. Gillgren, D. Wickramaratne, Y. Shi, T. Espiritu, J.Yang, J. Hu, J. Wei, X. Liu, Z. Mao, K. Watanabe, T. Taniguchi, Marc Bockrath, Yafis Barlas, R. K. Lake, C.N. Lau, 2D Materials, 2, 011001 (2014)


Device mobility

•  Ambipolar transport •  On/off ratio ~ 105

•  linear I-V à ohmic contact

16

Figure 5. Time dependence of few-layer BP FET device characteristics. (a) Transfer curves for an unen-capsulated BP FET with Ti/Au contacts, measured as a function of ambient exposure time. (b) Transfer curves for a BP FET measured immediately before and after encapsulation. (c) Transfer curves for a ~30 nm thick ALD AlOx encapsulated BP FET with Ti/Au contacts, measured as a function of ambient exposure time. (d) Transfer curves for a ~30 nm thick ALD AlOx encapsulated BP FET with Ni/Au contacts, meas-ured against ambient exposure time. Comparison of the (e) ION/IOFF ratio and (f) hole mobility for encapsu-lated and unencapsulated BP FETs versus ambient exposure time.

A

B D

C

F

E

Rxx

(Ω)

Rxx

(Ω)

•  Metal-insulator transition •  highly hole-doped: metallic, µ up to 4000

•  towards band edge: insulating, µ ê with T


Quantum Oscillations

Rxx with smooth background subtracted •  oscillations periodic in 1/B •  oscillations periodic in Vg ~n •  doubling frequency in for B>8T à

Zeeman splitting

ΔRxx (Ω)

a

c d

N. Gillgren, D. Wickramaratne, Y. Shi, T. Espiritu, J.Yang, J. Hu, J. Wei, X. Liu, Z. Mao, K. Watanabe, T. Taniguchi, Marc Bockrath, Yafis Barlas, R. K. Lake, C.N. Lau, 2D Materials, 011001 (2015)


Temperature Dependence Quantum Oscillations

•  effective mass of charge carriers ~0.25 to 0.31 me as Fermi energy increases towards band edge

•  agree with DFT calculations within 50%

b

Oscillations’ amplitude dependence on T

N. Gillgren, D. Wickramaratne, Y. Shi, T. Espiritu, J.Yang, J. Hu, J. Wei, X. Liu, Z. Mao, K. Watanabe, T. Taniguchi, Marc Bockrath, Yafis Barlas, R. K. Lake, C.N. Lau, 2D Materials, 011001 (2014)


Conclusion

b

20

0N

umbe

r121 2014 month

•  Few layer phosphorene has both high mobility and band gap •  Stable via hBN encapsulation

Outlook •  Physics

•  strain-dependent band gap •  large anisotropy (up to factor of 60, electrical and thermal

transport, thermopower) •  electric field effect •  quantum Hall effect

•  Electronics and optoelectronics •  hBN encapsulation of reactive 2D materials

see Cao et al, arXiv: 1502.03755


Acknowledgments!

Undergraduate Students Tim Espiritu Kevin Thilahar Mason Gray Ziqi Pi

Nathaniel Gillgren"

UCOP

Graduate Students

Yongjin Lee " Jhao-wun Huang "

Kevin Myhro "

Fenglin Wang"

Yanmeng Shi"

Petr Stepanov " Son Tran "


Collaborators!

Florida Mag Lab Dmitry Smirnov Jean-Marie Poumirol

UCR Physics Marc Bockrath

UCR EE Roger Lake

CSIC Paco Guinea

UCR Physics Yafis Barlas

Tulane Zhiqiang Mao

Tulane Jiang Wei

UCR Chem. & CEE Robert Haddon

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