Synthesized Graphene and it its use as Hydrogen Sensor and - - PowerPoint PPT Presentation

Pla lasma Assisted Low Temperature Synthesized Graphene and it its use as Hydrogen Sensor and Photodetector

Charmine Tay

• An allotrope of carbon
• Consist of a single layer of

carbon atoms arranged in a hexagonal lattice

• Exceptional

conductivity, mechanical strength, and thermal stability

Graphene

Graphene synthesis

• Graphene

first deposited

• n

copper substrates using chemical vapour deposition method (CVD method)

• Then

transferred

• nto

desired substrates

Copper substrates

Limitations of Graphene synthesis

1. Transferring process Degradation of the transferred graphene Direct growth on desired substrate much preferable

Limitations of Graphene synthesis

2. Requires high temperatures  >1000 ̊C  Does not allow graphene to be grown on the substrates required in wearable and flexible electronics  As they are damaged at high temperatures.

Plasma assisted CVD

• The carbon precursor will be

exposed to radio frequency (RF) plasma before deposition. Dissociate carbon precursor and promote the graphene growth even at very low temperature

Gas sensing

• High surface to volume ratio
• Remarkable conductivity

 Promising for gas molecule sensing

Photo detection - Graphene/Silicon interface

• The silicon opens a band gap in graphene
• Enables it to detect light
• Effect of hydrogen functionalisation on the

graphene/silicon interface was also explored

Graphene Silicon

Aim: To synthesize graphene at low temperatures with the help of plasma and investigates its use as a hydrogen gas sensor and photodetector.

CVD method

 used to grow graphene

Transferring

• Used the PMMA assisted wet transfer method
• Transferred onto on to PET and silicon

Raman spectroscopy

• Confirm the existence and

quality of the graphene grown

• shows the presence of

impurities (if there are any)

Photocurrent measurements

• Taken by measuring

the current following through the sample when a bias voltage is applied to it

IV IV measurements

• A current was allowed to flow through the

graphene and the voltage through the graphene was measured.

• IV graph was obtained.
• By Ohm’s law, the resistance of the

graphene can be calculated from the gradient of that graph.

Results

Flow rate was kept at 10 sccm for CH4 and 2 sccm for H2. Sample Temperature grown/ ̊C Duration of growth/ min Power of plasma used / w A 1015 15 none B 800 15 100

Results from Raman Spectroscopy

• Sample A shows the presence
• f “2D” band, which is the

characteristic peak of graphene.  successfully grown graphene with plasma at 800 C

1000 1500 2000 2500 3000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500

Sample B

D peak 2D peak

Intensity (a.u.) Raman Shift (cm

• 1)

G peak

Sample A

Results from IV IV measurements

• Resistance of plasma

assisted CVD graphene = 22kΩ

• Resistance of normal

CVD graphene = 10kΩ

• 0.0010
• 0.0005

0.0000 0.0005 0.0010

• 30
• 20
• 10

10 20

Voltage (V) Current (mA) A B

Results from IV IV measurements

• Could be because

plasma assisted CVD process caused the fermi level to shift Band gap to form display characteristics of doped graphene, and be more insulating

Conduction band Valence band

Results after exposure to hydrogen pla lasma

• Resistance of graphene increased
• After 20 mins, resistance

saturates  Exposure to hydrogen plasma reduces the conductivity of the graphene.

Results after exposure to hydrogen pla lasma

• As the hydrogen plasma reacts with the graphene
• Hydrogenated graphene is formed
• Which means that the carbon bonds are in a sp3 configuration, as opposed to

graphene's sp2 configuration  less delocalized electrons to conduct electricity  Thus graphene became more insulating

After exposure to H2 plasma

Legend: Carbon atom Electron Hydrogen atom Bond

Results after heating

• Resistance of the graphene went

nearly back to the original resistance before hydrogenation.

• This could be because when

heated, the bound hydrogen atoms thermally desorbs,

• Restoring the graphene to its

pristine stage  This shows that plasma assisted CVD graphene is suitable to be used as a gas sensor for hydrogen gas.

Graphene on sil ilicon

• This graph shows telling characteristics of a

diode.

• This could be because between graphene

and silicon there is a potential gradient

• With enough energy, electrons will spill from

the graphene in to silicon.

• The transferred electron cannot move back to

the graphene due to the electron not having enough energy to cross the Schottky barrier.

• This causes the graphene to exhibit p type

doping, be more insulating as well as exhibit properties of a diode.

• 0.0010
• 0.0005

0.0000 0.0005 0.0010

• 15
• 10
• 5

5 10 15 20 25

Voltage (V) Current (mA)

Graphene on sil ilicon

• The photo current increase when light

is shone

• This could be because when light is

shining on the graphene/silicon interface, the electrons absorb energy from light,

• Causing them to dislodge from the

graphene and become free electrons

• Improving the conductivity of

graphene.

20 40 60 80 100 120 0.0 5.0x10

• 6

1.0x10

• 5

1.5x10

• 5

2.0x10

• 5

2.5x10

• 5

3.0x10

• 5

Photocurrent (A) Time (s) Without exposure to hydrogen plasma After 5 min of exposure After 10 min of exposure

Graphene on sil ilicon

• After exposure to hydrogen plasma

 photocurrent increase increases.

• This could be due to pronounced

electron-hole separation efficiency and low electron hole recombination. Graphene/ silicon interface can be used as photodetector and exposure to hydrogen plasma increases its sensitivity to light

20 40 60 80 100 120 0.0 5.0x10

• 6

1.0x10

• 5

1.5x10

• 5

2.0x10

• 5

2.5x10

• 5

3.0x10

• 5

Photocurrent (A) Time (s) Without exposure to hydrogen plasma After 5 min of exposure After 10 min of exposure

Conclusions

• Graphene was successfully grown with plasma at 800 C.
• Plasma assisted CVD graphene is shown to be suitable

to be used as a gas sensor for hydrogen gas.

• A graphene/silicon-based photodiode was also

successfully demonstrated

• The sensitivity of the graphene/silicon photodiode

improves with hydrogenation of graphene.

Thank you