deterministic transfer of h-bn/ graphene heterostructures ... · deterministic transfer of h-bn/...

Deterministic transfer of h-BN/ graphene heterostructures for THz antennas

Sofía Cruces27.01.2021

fs laser pulse

Surface plasmon polaritons

THz emission

Research Laboratory Course

SPPs: jacopobertolotti.com/

Transfer tool at LMN

S. Cruces Research Laboratory Course 2

Graphene and its conductivity

Anwar, et al. Digital Communications and Networks, 2018. 4(4), 244-257.

▪ Resonance condition: L=λSPP

2=

𝜋

𝑘𝑆𝑃𝑃

▪ Plasmon dispersion relation: 1

𝑘𝑆𝑃𝑃2−

𝜔2

𝑐2

+𝜀

𝑘𝑆𝑃𝑃2−𝜀

𝜔2

𝑐2

= −𝑖𝜎 ωμ𝑐τ

ωε0

→Challenge: high-quality graphene!

Chemical potential (𝛍𝐜)▪ Tuned by bias voltage or doping▪ Determines number of n & p in antennaRelaxation time (τ)▪ Average life time of charges▪ Determined by graphene quality

For functional antenna → large μ𝑐 & τ


Structure of THz Antenna

+ -

50 μm

5 μm

200 μm

200 μm

Dipole antenna ▪ GaAs substrate (Eg = 1.42 eV)▪ Conduction lines & contact: Cr / Au▪ Two graphene patches

▪ Supports SPPs in THz band (0.1 – 10 THz)▪ Graphene SPP waves tuned by doping▪ Design/shape of graphene antennas


Mechanical exfoliation

Freire Soler, Doctoral dissertation, 2014.

Advantages▪ Monocrystalline Graphene▪ Defect-free→ Cleanliness, high conductivity

Challenges▪ Low yield▪ Non-deterministic▪ Small and irregular flakes

Parameters:▪ Graphite source▪ Tape (Nitto, Tesa)▪ Force, speed, direction applied▪ Number of subsequent exfoliations

https://lmn.mb.uni-siegen.de/


Challenges of mechanical exfoliation

50 μm

Nitto tape Tesa tape

Challenge → Residue removal

Residue

▪ Multiple subsequent exfoliations▪ 100 °C for 1 min→ Small, irregular & thick flakes

50 μm


Tape residue removal

100 µm

Before acetone After acetone

100 µm

▪ 10 min in acetone▪ Blow dried with N2

→ Cleaner surface & flakes remain


Exfoliation of large area graphene flakes

20 μm

▪ Less number of subsequent exfoliations▪ 75 °C for 3 min→ Clean, thin & bigger flakes

20 μm


Thickness determination using contrast spectra

Novoselov, et al. Applied physics letters, 2007, 91(6), 063124.

▪ Model based on the Fresnel law▪ White light source and no filter

SiO2: 𝑛2(λ)= 1.47

Graphene: 𝑛1 = 2.6 – 1.3𝑖

Air: 𝑛0=1

𝑑1=0.34 nm

𝑑2=285 nm

Contrast equation

𝐶 =𝐼 𝑛1 = 1 − 𝐼(𝑛1)

𝐼(𝑛1 = 1)

Graphene

Substrate

Si: 𝑛3(λ)= 5.6 – 0.4i

Contrast of graphene on SiO2/Si


Thickness determination using contrast spectra

Ni, et al. Nano letters, 2007, 7(9), 2758-2763.

𝐶 =𝐼 𝑛1 = 1 − 𝐼(𝑛1)

𝐼(𝑛1 = 1)

C = 0.0046 + 0.0925N − 0.00255𝑁2

20 μm

MLGTLG

4LG

▪ Intensities with ImageJ

Contrast equation

Number of layers


▪ No D – peak (1350 cm-1) ▪ G´ band – single Lorentzian profile▪ FWHM 22.3 cm-1

▪ IG/IG´ (0.37)→ Typical values for defect-free, ME graphene

20 μm

▪ 532 nm laser▪ 1200 l/mm grating▪ 600 nm center wavelength

1400 1600 1800 2000 2200 2400 2600 2800

Inte

nsity (

arb

. units)

Raman shifts (cm-1)

GaAs-Gr-02

Fit peak

1589.92

2685.26

2466.65

G

G*

G´

Raman spectroscopy


All-dry transfer method: Transfer tool

PDMS

PC

Glass slide

flake


Stamping Scheme

Preparation of graphene/hBN flakes

x/y

β

α

Visual control

Substrate temperature[40 °C – 200 °C ]

1

2

3

4

5

Quality control of flakes / flake search


Pick up technique of flakes

PDMS

PC 5wt %

Glass slide

hBN

SiO240°C

PDMS

PC 5wt %

Glass slide

hBN

SiO2

Selection of h-BN flake on SiO2/Si substrate h-BN flake on stamp

200 μm 100 μm


Challenges▪ PC coating would break▪ PDMS sticked to the substrate→Parameter optimization

PDMS

PC 5wt %

Glass slide

hBN

SiO2100°C

Graphene

PDMS

PC 5wt %

Glass slide

SiO2100°C

PDMS

PC 5wt %

Glass slide

SiO240°C

i. ii.

iii.

Drop-down

Pick-up20 μm

Pizzocchero, et al. Nature communications, 2016, 7(1), 1-10.

Selection of graphene on SiO2/Si substrate

Pick up technique of flakes


▪ Extremely low contrast on GaAs

▪ Raman measurements required

▪ High contrast at contact with Au 1400 1600 1800 2000 2200 2400 2600 2800

Inte

nsity/ a.u

Raman shifts/ cm-1

Sample Si-GR01

Fitted plot

1601.66

2740.88

2459.17

Flake on antenna (Transfer OM)

PDMS

Glass slide

Flake

SiO270°C

200 μm

50 μm

100 μm

50 μm

Thin flake on PDMS

Visibility of graphene/graphite on GaAs substrate

Flake on antenna (Transfer OM)


▪ Raman spectroscopy mapping

▪ Rapid optical thickness using contrast

▪ Correlation of contrast and thickness on GaAs

▪ Complimentary techniques (AFM)

▪ Correlation of contrast and thickness on PDMS

Outlook


Prof. Benjamin ButzM.Sc. Charles OgollaLMN group members

Prof. Peter Haring BolívarInstitute of High Frequency and Quantum Electronics

Prof. Mario AgioLaboratory of Nano-Optics

Acknowledgements

deterministic transfer of h-bn/ graphene heterostructures ... · deterministic transfer of h-bn/...

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