Layer-modulated synthesis of uniform tungsten disulfide nanosheet using gas-phase

precursors. Jusang Park*

School of Electrical and Electronics Engineering,

Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul, Korea

2D material family

New materials for advanced application : Beyond graphene Transition metal dichalcogenides (TMDC)

We can choose all kinds of properties for everything : semiconductor, ferromagnetic, or nonmagnetic metals…

New era for atomic nanosheet technology !

Thermal decomposition of thiosalts

Transition metal oxides with chalcogen

Transport and recrystallization from TMD

Vapor Phase Deposition

→The precursor(NH4)2MoS4 was dipcoated on SiO2/Si →Two-step annealing process

→The facilitated nucleation by seeding the substrate with graphene-like species

S. Wu, C. Huang et al,ACS Nano, 7, 2768–2772,(2013)

Y.-H. Lee, L. Yu er al,Nano Lett.13,1852–1857,(2013)

Liu K-K et al, Nano Lett. 12, 1538–1544, (2012)

→Vapour–solid (VS) growth method for synthesizing a high optical quality MoS2 monolayer.

Growth of CVD WS2

• Initial growth of CVD WS2 Time dependent lateral growth of WS2

Precursor

H2S Gas

Vacuum furnace

Pump out

<SEM>

<Previous CVD method>

Lee Y-H et al, Adv. Mater, 24, 2320–2325, (2012)

<Gas Phase CVD WS2>

WCl6 Tube furnace

Growth temperature : 700 °C

Time dependent Layer Control <Optical Microscopy>

<AFM>

Number of layer dependent on the cycle number of ALD WO3

Coalescence of CVD WS2

Coalescence of two or more domains Seed for the growth of second WS2 layer

Sina Najmaei et al, Nat Mat, 12, 754–759, (2013)

<AFM> CVD 10 min Transferred WS2

200 nm

Seed for second layer of WS2

200 nm

First WS2

SiO2

CVD 10 min Transferred WS2

Seed for second layer of WS2

Raman of CVD WS2 <Raman, laser excitation λ = 633 nm>

Raman spectra for the 1L, 2L and 4L WS2 nanosheets Relative Raman peak intensities and peak distances of the E1

2g and A1g modes Dependent on the number of layer

Optical Property of CVD WS2 <PL>

PL spectra for the 1L, 2L and 4L WS2 nanosheets

Indirect to direct band gap transition with reducing number of layer

I peak from PL spectrum of 2L and 4L WS2

<Electronic structure of WS2>

Zeng, H. et al, Sci, Rep. 3, 1608, (2013)

Chemical Composition of CVD WS2

CVD WS2 show good stoicheometry with no Cl contamination (from WCl6)

1L WS2

W 33.6

S 66.4

Cl <1%

4L WS2

W 33.1

S 67

Cl <1%

CVD grown 1L WS2

CVD grown 4L WS2

<XPS>

Large Area Uniformity of CVD WS2

Color dependency on the number of layers

Large-area uniformity on 1 cm X 7 cm SiO2

<Raman analysis>

Atomic Arrangement of CVD WS2

<HRTEM> <Inversed FFT> <TEM>

Low-magnification TEM image for a 1L WS2 nanosheet on a TEM grid HRTEM image of 1L WS2 nanosheet at a selected region and (inset) the SAED pattern Inverse FFT by applying a mask

(100) and (110) crystal directions Lattice spacing: 0.26 nm and 0.16 nm for the (100) and (110) planes

Graphene/WS2 Photo-Detector

• △Id/Id @ Vg=0 V -> 4% with monochromatic green light (~550 nm) @ 1 W/m2

• Lower than exfoliated few-layer MoS2 with CVD graphene photo-detector (~ 7% @ 0.6 W/m2)

<Transfer curve> <Output curve> <OM image>

Si++/SiO2

Unpublished Data

Transferred CVD Graphene on WS2

20 µm

1L WS2

Kallol Roy et al, Nat nanotech, 8, 826–830, (2013)

Graphene/WS2 Photo-Detector

• Photo-excited electron injection to hole doped graphene resulting in reduced hole channel current when Vg « VT

• CVD WS2 based photo-detector can be fabricated large area

Kallol Roy et al, Nat nanotech, 8, 826–830, (2013)

Vg « VT Vg > VT

EF

WS2

SiO2

P++ Si

(back-gate)

EF

WS2

SiO2

P++ Si

(back-gate)

Summary CVD WS2 nanosheets are

synthesized using gas phase S

reactant

Lateral growth and coalescence of

two or more domains are observed

Number of layer can be controlled

by reaction time

Graphene/WS2 hetero-structure

shows properties of photo

detecting