Plasma Assisted Low Temperature Synthesized Graphene and its use as Hydrogen Sensor and Photodetector

Charmine Tay

• An allotrope of carbon

• Consist of a single layer ofcarbon atoms arranged ina hexagonal lattice

• Exceptional conductivity,mechanical strength, andthermal stability

Graphene

Graphene synthesis• Graphene first deposited on

copper substrates using chemicalvapour deposition method (CVDmethod)

• Then transferred onto desiredsubstrates

Copper substrates

Limitations of Graphene synthesis1. Transferring process

Degradation of the transferred graphene

Direct growth on desired substrate much preferable

Limitations of Graphene synthesis2. 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 beforedeposition.

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 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

of “2D” band, which is the characteristic peak of graphene.

successfully grown graphene with plasma at 800 C

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Sample B

D peak 2D peak

Inte

nsity (

a.u

.)

Raman Shift (cm-1)

G peak

Sample A

Results from 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

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Vo

lta

ge

(V

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Current (mA)

A

B

Results from 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 plasma• Resistance of graphene increased

• After 20 mins, resistance saturates

Exposure to hydrogen plasma reduces the conductivity of the graphene.

Results after exposure to hydrogen plasma• 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 atomElectronHydrogen atomBond

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 silicon• 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

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25

Vo

lta

ge

(V

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Current (mA)

Graphene on silicon• 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

Pho

tocurr

ent

(A)

Time (s)

Without exposure to hydrogen plasma

After 5 min of exposure

After 10 min of exposure

Graphene on silicon• 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

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1.0x10-5

1.5x10-5

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Pho

tocurr

ent

(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