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

Graphene • 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 synthesis • Graphene first deposited on copper substrates using chemical vapour deposition method (CVD method) • Then transferred onto 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 CH 4 and 2 sccm for H 2. Sample Temperature Duration of Power of plasma grown/ ̊ C growth/ min used / w A 1015 15 none B 800 15 100

Results from Raman Spectroscopy • Sample A shows the presence 8500 D peak G peak 2D peak 8000 of “2D” band, which is the 7500 Sample A 7000 characteristic peak of 6500 Intensity (a.u.) 6000 graphene. 5500 5000  successfully grown graphene 4500 4000 with plasma at 800  C 3500 3000 Sample B 2500 2000 1500 1000 1500 2000 2500 3000 -1 ) Raman Shift (cm

Results from IV IV measurements • Resistance of plasma 20 assisted CVD graphene = 22kΩ 10 Voltage (V) • Resistance of normal 0 CVD graphene -10 = 10kΩ A -20 B -30 -0.0010 -0.0005 0.0000 0.0005 0.0010 Current (mA)

Results from IV IV measurements • Could be because plasma assisted CVD Conduction band process caused the fermi level to shift  Band gap to form  display characteristics of Valence band doped graphene, and be more insulating

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 sp 3 configuration, as opposed to • graphene's sp 2 configuration  less delocalized electrons to conduct electricity  Thus graphene became more insulating Legend: After exposure to Carbon atom H 2 plasma 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. 25 20 • This could be because between graphene 15 and silicon there is a potential gradient 10 • With enough energy, electrons will spill from Voltage (V) 5 the graphene in to silicon. 0 • The transferred electron cannot move back to -5 the graphene due to the electron not having -10 enough energy to cross the Schottky barrier. -15 • This causes the graphene to exhibit p type -0.0010 -0.0005 0.0000 0.0005 0.0010 doping, be more insulating as well as exhibit Current (mA) properties of a diode.

Graphene on sil ilicon • The photo current increase when light -5 3.0x10 is shone Without exposure to hydrogen plasma -5 2.5x10 After 5 min of exposure • This could be because when light is Photocurrent (A) After 10 min of exposure -5 2.0x10 shining on the graphene/silicon interface, the electrons absorb energy -5 1.5x10 from light, -5 1.0x10 • Causing them to dislodge from the -6 5.0x10 graphene and become free electrons • Improving the conductivity of 0.0 graphene. 20 40 60 80 100 120 Time (s)

Graphene on sil ilicon • After exposure to hydrogen plasma  photocurrent increase increases. -5 3.0x10 Without exposure to hydrogen plasma • This could be due to pronounced -5 2.5x10 After 5 min of exposure electron-hole separation efficiency and Photocurrent (A) After 10 min of exposure -5 2.0x10 low electron hole recombination. -5 1.5x10  Graphene/ silicon interface can be -5 1.0x10 used as photodetector and exposure to hydrogen plasma increases its sensitivity -6 5.0x10 to light 0.0 20 40 60 80 100 120 Time (s)

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.

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