modulating inter-layer coupling in layered semiconductors · 2017. 9. 5. · 2 the wu group:...

Modulating inter-layer coupling

in layered semiconductors

Junqiao Wu

Department of Materials Science and Engineering, University of California, Berkeley

and

Division of Materials Sciences, Lawrence Berkeley National Laboratory

http://mse.berkeley.edu/~jwu

E3S, 2/25/2014February 25, 2015 For Internal E3S Use Only. These Slides May Contain Prepublication Data and/or Confidential Information.

2

the Wu Group: Electronic Materials Research

Thermal physics and

thermoelectric materials

Phase transitions in

quantum materials

Low-dimensional

materials and

interfaces

February 25, 2015 For Internal E3S Use Only. These Slides May Contain Prepublication Data and/or Confidential Information.

3

Outline

• Preparation of monolayers and hetero-

bilayers of layered semiconductors

• Ultra-fast charge transfer between layers

• Mechanical properties of 2D semiconductors

• Interlayer de-coupling: 3D material behaves

as 2D

• Enforced interlayer coupling: vibrational

spectrum renormalization


4

12

3 eV

GaSe

MoO3

MoSe2

MoS2

GaS

MoTe2

VS2, graphene

metallic

1.55 m

WS2

WSe2

BN, Mg(OH)2

widegap

bulk 4-layer 2-layer 1-layer

Kis Group, Nature Nano. 2011; Heinz Group, PRL, 2010;

Wang Group, Nano Lett. 2010; etc.

Monolayer Semiconductors as 2D Materials

Layered

materials:

inter-layer

coupling defines

the “2Dness”


5

Interlayer coupling defines the “2Dness”

MoS2/WS2

bilayer


6

2D materials: van der Waals (vdW) epitaxy

Geim, et al, Nature, 2013; Wang et al, Nature Nano, 2012; Polman et al, Nature Mater, 2012; etc.


7

Large-area growth of monolayers

CVD growth of monolayer semiconductors

Nano Lett., 14, 3185 (2014)

Isolated islands

Continuous layer


8

Preparation of hetero-layers

Nano Lett., 14, 3185 (2014)

Transfer and stamping to form heterostructures

MoS2 MoS2/WS2


9

Band offset enables novel 2D charge-splitting devices

2D heterostructures MoS2/WS2

February 25, 2015

For Internal E3S Use Only. These Slides May Contain

Prepublication Data and/or Confidential Information.

10

Anneal to enhance MoS2/WS2 coupling

Nano Lett., 14, 3185 (2014)

Annealing leads to renormalized PL spectrumFebruary 25, 2015



11

How fast is the inter-layer charge transfer?

Wang et al, Nature Nanotech. 9, 682 (2014)

Transient absorption by pump-probe optics

h+h+

e-e-

h+

e-

h+

e-

MoS2 WS2

pump

probe

A

B

WS2

A exciton

MoS2

B exciton


12

Ultra-fast charge transfer across the vdW gap

Fitting shows < 50fs hole transfer from MoS2 to WS2

WS2

A exciton

in MoS2/WS2

MoS2

B exciton

in MoS2


13

Mechanical properties of 2D materials

32 2 3

0 2( ) ( )D D q

F Er

q=1/(1.05 – 0.15n – 0.16n2)

Nano Lett., 14, 5097(2014)


14

Inter-layer sliding in bilayers?


15

Young’s modulus of mono and bilayers

𝐸3𝐷 =𝐸2𝐷

𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠

Nano Lett., 14, 5097(2014)February 25, 2015 For Internal E3S Use Only. These Slides May Contain Prepublication Data and/or Confidential Information.

16

Interlayer-decoupled layered material

2D behavior in 3D (bulk) materials?

Nature Commun., 5, 3252 (2014)


17

ReS2: interlayer-decoupled layered material

A 3D (bulk) material acts as 2D!February 25, 2015

For Internal E3S Use Only. These Slides May Contain Prepublication Data and/or Confidential Information.

18

ReS2: interlayer-decoupled layered material

diamond anvil cell

Nature Commun., 5, 3252 (2014)

• Hydrostatic pressure is isotropic,

thus avoids cracking

• Pressure up to 50 GPa = 500k atm

• Raman, PL, XRD, TDTR and

transport under DAC

• Maximally modulate interlayer

coupling


High hydrostatic pressure on 2D Materials

Unpublished data (2014)

MoS2 monolayer

February 25, 2015


20

Nayak et al, Nature Commun., 5, 3731 (2014)

High hydrostatic pressure on multilayers

ambient

pressured

February 25, 2015

For Internal E3S Use Only.

These Slides May Contain Prepublication Data and/or Confidential Information.

21

Monolayer versus multilayers

Distinct

pressure

behavior of

Raman

peaks

in

monolayer

versus

multilayer

Unpublished data (2015) February 25, 2015


22

From isolated monolayers to a coupled bilayer


23

High hydrostatic pressure on vdW junctions

0GPa

40GPa

MoS2 / WS2 bilayer

Raman shift (1/cm)

In preparation (2015)

350

400

450

500

550

0 5 10 15 20 25 30 35 40

Ra

ma

n p

eak (

1/c

m)

Pressure (GPa)

E (MoS2)mono, hetero

E (WS2)mono, hetero

A (WS2)hetero

A (MoS2)hetero

A (MoS2)mono

A (WS2)mono

Out-of-plane

In-plane

Pressure behavior of MoS2/WS2 versus monolayers:

- In-plane modes: no difference

- Out-of-plane modes: repel each other in MoS2/WS2

hetero-bilayersFebruary 25, 2015 For Internal E3S Use Only. These Slides May Contain Prepublication Data and/or Confidential Information.

24

Renormalization of vibration spectrum by

enforced interlayer coupling

In preparation (2015)February 25, 2015 For Internal E3S Use Only. These Slides May Contain Prepublication Data and/or Confidential Information.

25

𝜔1 = 𝑘1/𝑀, 𝜔2 = 𝑘2/𝑀

Renormalization of vibration spectrum by

enforced interlayer coupling

Initial eigen-frequencies:

After k12 coupling,

𝜔𝑛𝑒𝑤 = 𝑘1+𝑘

2+2𝑘

12

2𝑀±

1

2𝑀𝑘1− 𝑘2

2+ 4𝑘122,

𝜔1new> 𝜔1 > 𝜔2>𝜔2

new

In preparation (2015)February 25, 2015 For Internal E3S Use Only. These Slides May Contain Prepublication Data and/or Confidential Information.

26

• 2D semiconductors: probe and modulate inter-layer

coupling

• Growth and preparation of 2D monolayers and bilayers

• 3D material behaves as 2D due to interlayer decoupling

• Vibrational spectrum renormalization by enforced coupling

• Acknowledgements

– Collaborators (J. C. Grossman,

R. Sinclair, F. Wang, B. Li,

Y. S. Huang, B. Chen,

K. Hippalgaonkar, et al)

– Wu Group members (Sef Tongay,

Joonki Suh, Kai Liu, Wen Fan,

Yabin Chen, et al)

– Funding from U.S. DOE, U.S. NSF and SinBeRISE

Summary


Thank you!

27


28


29

Story I: Thermally driven tendency of inter-

layer decoupling in multi-layer MoSe2

• Ordinary multi-layer materials (e.g., MoS2): T rises hot PL weakens

• Unusual multi-layer MoSe2: T rises hot PL intensifies

• Reason: direct/indirect gaps nearly degenerate T rises neighboring

layers tend to decouple more toward direct-gap PL intensifies

2-layer 1-layer

hot

emissiondirect

emission

Tongay et al, Nano Lett. 2012


30


DFT: easier inter-layer decoupling in MoSe2B

and

ga

p (

eV

)

February 25, 2015

For Internal E3S Use Only. These Slides May Contain Prepublication

Data and/or Confidential Information.

31

High pressure behavior of PL from

heterolayers

Unpublished (2015)February 25, 2015 For Internal E3S Use Only. These Slides May Contain Prepublication Data and/or Confidential Information.

32


33

2D Heterostructures: Tune Interlayer Coupling

Fan et al, Nano Lett. 2014


34

Band alignment of 2D semiconductors

DFT calculated

February 25, 2015



35

2D Semiconductors: van der Waals Epitaxy

Geim, et al, Nature, 2013

Wang et al, Nature Nano, 2012

Polman et al, Nature Mater, 2012

Multi-junction solar cell with

Van der Waals epitaxy


36

Drastic enhancement in room-temperature PL of monolayers by

gentle thermal annealing

Defects Mediated Optical Emission



37

Exposure to different gas species further enhances PL

intensity of annealed monolayers: “molecular gating”

Defects-Molecules Interaction



38pristine: n-type

trion emission

Defects Modulated Excitonic Effects

O2

MoS2

S double vacancy

trion X- exciton X0

defects+molecule: n-depleted

exciton emission

EtrionEexciton


modulating inter-layer coupling in layered semiconductors · 2017. 9. 5. · 2 the wu group:...

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