GRAPHENE/METAL INTERFACES FOR LARGE SPIN-ORBIT INTERACTIONS (NIE jr - - PowerPoint PPT Presentation

DEVELOPMENT OF GRAPHENE/METAL INTERFACES FOR LARGE SPIN-ORBIT INTERACTIONS (NIEjr06)

NATHANAEL TONG HU DING GUAN

Presentation Flow

2

• Background
• Aims & Objectives
• Methodology
• Results & Discussion
• Conclusions & Further Research

BACKGROUND

3

Graphene

• Monolayer sheet of sp2 bonded carbon atoms.
• σ bonds between neighbouring carbon atoms

due to overlapping sp2 orbitals

• Bonding energies of neighbouring Cpz orbitals of

each carbon atom form the π and π* states.

4

C2

σ

C1

π π

• Solid state objects follow band theory

 conductivity is based on its electrons’ quantum states

• Free movement of electrons = conduction

5

Band theory

Conduction band

Valence band

Conduction band Conduction band

Valence band Valence band

Band gap

Conductor Semiconductor Insulator

• Linear distribution of band structure

 valence & conduction bands meet

• Formation of Dirac cones
• Little energy is required for electrons

to move freely between the 2 bands

6

Band theory

7

RESEARCH GAP

• The absence of a band gap in graphene makes it

difficult to show contrast between the graphene transistors on/off states

Band bending

• At the junction of an interface, the phenomenon of

band bending at the Fermi Level is observed.

• Difference in work functions of metal (ϕM) and

graphene (ϕG)

• Induces a local barrier height (ϕBH) = band gap
• f the graphene

8

AIMS & OBJECTIVES

9

Create

a substrate-induced band gap in graphene and

Compare

induced band gaps across different metal interfaces

METHODOLOGY

11

12

CVD graphene growth PMMA-assisted wet transfer Obtain Gr/Metal interface

METHODOLOGY

CHEMICAL VAPOUR DEPOSITION

13

(2 sccm) (10 sccm)

60 minutes 1100°C

SOLVENT ETCHING AND TRANSFER

14

• Spin coat

Poly(Methyl- methacrylate) layer onto Cu/Gr

• Etch Cu

substrate with FeCl3 solution

• Transfer

graphene layer

• nto substrate

manually

RESULTS AND ANALYSIS

15

16

RESULTS & ANALYSIS

Scanning Electron Microscopy Raman Spectroscopy I-V Measurements I-Z Spectra

Using Scanning Electron Microscope (SEM) imaging, we took high magnification images of

• ur graphene samples.

Using the images taken, we were able to identify any visual defects on the graphene surface.

17

SCANNING ELECTRON MICROSCOPY

18

SCANNING ELECTRON MICROSCOPY

SEM image of Cu/Gr (x600) SEM image of ITO/Gr (x600) SEM image of Pt/Gr (x600) SEM image of W/Gr (x600)

19

SCANNING ELECTRON MICROSCOPY

SEM image of Cu/Gr (x2,000) SEM image of ITO/Gr (x2,000) SEM image of Pt/Gr (x2,000) SEM image of W/Gr (x2,000)

Using Raman Spectroscopy, we characterized the G and 2D peaks of graphene in our samples. These principle peaks tell us more about the quality

• f our graphene samples,

and helps us confirm our samples are actually graphene.

20

RAMAN SPECTROSCOPY

21

RAMAN SPECTROSCOPY

SiO2 ITO Si W Pt

Using a 2 probe method, we measured the I-V characteristics of the graphene samples. We plotted the voltage (V) as a function of current (A) to obtain a linear graph.

22

I-V CHARACTERISTICS

23

I-V CHARACTERISTICS

Si W Cu ITO Pt SiO2

24

I-V CHARACTERISTICS

Substrate Resistance/Ω Pt 4012 W 2919 Cu 4746 ITO 3413 Si 5041 SiO2 6655

• Taking the relationship

𝑆 =

𝑊 𝐽 , we can deduce

the resistance of the graphene layer on the substrate

Using a Scanning Tunneling Microscope (STM), we obtained the I-Z spectra of the graphene samples. The I-Z spectras yield information on the local barrier height (band gap)

• f the graphene samples

25

I-Z SPECTRA

26

I-Z SPECTRA

• From the quantum tunneling principle, the relationship

between the magnitude of the tunneling current ( I(Z) ) and the tip sample distance ( 𝑨 ) can be expressed as I(z) ∝ 𝑓−2𝜆𝑨, where κ (decay constant), dependent on the barrier height (ϕBH), is given as κ = 5.1√ϕBH(eV )nm−1

27

Substrate Barrier height/eV Pt 0.4 W 0.7 Undoped Graphene 0.1

I-Z SPECTRA

• By taking 𝑨 as a function
• f ln[I(Z)], we obtain a

linear graph with a gradient of -2𝜆

• The barrier height (ϕBH)

can hence be calculated.

CONCLUSIONS & FUTURE RESEARCH

28

CONCLUSIONS

29

• Graphene  quality consistent with literature
• Successfully induced band gap in interface

 Band bending at Fermi level  Possible utility in spintronic devices

CONCLUSIONS

30

• Defects caused during manual transfer

 a relatively small band gap induced in pure, un-doped graphene

• Contact resistance between probe & graphene

 Uncertainty of recorded resistance

FURTHER RESEARCH

31

• Measurement of electron mobility & quantum spin-

hall effect

• Ultraviolet Photoelectron Spectroscopy (UPS)
• X-ray Photoelectron Spectroscopy (XPS)

ACKNOWLEDGEMENTS

32

Our utmost appreciation to our supervisor, Dr. Rohit Medwal, for his unwavering guidance, mentorship and encouragement during this past year. Thanks to Prof. R. S. Rawat, Head/NSSE, for allowing us access to the lab and facilities for us to carry out our research. We would like to express our sincerest gratitude to our mentor, Ms Loh Yuhui, for her support throughout this project; Dr Joseph Vas for offering his knowledge and expertise; Mr Pae Jian Yi for his patience and advice; Mr Mishra Mayank for his time and patience; Mr Avinash for his kindness and support; our fellow NRP participants Charmaine and Jin Feng for lending us their time and help. Thank you to Nanyang Technological University for offering us this invaluable opportunity to participate in NRPjr 2018.

33

THANK YOU!

34