Post on 08-Mar-2021
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
S. Cruces Research Laboratory Course 3
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
S. Cruces Research Laboratory Course 4
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/
S. Cruces Research Laboratory Course 5
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
S. Cruces Research Laboratory Course 6
Tape residue removal
100 µm
Before acetone After acetone
100 µm
▪ 10 min in acetone▪ Blow dried with N2
→ Cleaner surface & flakes remain
S. Cruces Research Laboratory Course 7
Exfoliation of large area graphene flakes
20 μm
▪ Less number of subsequent exfoliations▪ 75 °C for 3 min→ Clean, thin & bigger flakes
20 μm
S. Cruces Research Laboratory Course 8
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
S. Cruces Research Laboratory Course 9
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
S. Cruces Research Laboratory Course 10
▪ 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
S. Cruces Research Laboratory Course 11
All-dry transfer method: Transfer tool
PDMS
PC
Glass slide
flake
S. Cruces Research Laboratory Course 12
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
S. Cruces Research Laboratory Course 13
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
S. Cruces Research Laboratory Course 14
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
S. Cruces Research Laboratory Course 15
▪ 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)
S. Cruces Research Laboratory Course 16
▪ 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
S. Cruces Research Laboratory Course 17
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