Effect of nanoparticles on liquid state dewetting of

bismuth thin filmKrishna Kumar, Mrudula Kavuri, and P. Swaminathan

Department of Metallurgical and Materials Engineering,Indian Institute of Technology Madras, Chennai – 600036, India

Email : krishnaiitm14@gmail.com

❑ Thin films are thermally unstable and

agglomerate into islands on annealing, known

as dewetting

❑ Dewetting can be in solid or liquid state

❑ Detrimental for high temperature operation

❑ We investigate role of nanoparticles on liquid

state dewetting

DewettingObjective

• Thin metal films on oxide substrates are metastable

• Dewetting is driven by surface energy minimization

• Final morphology depends on temperature and thickness

Different types of dewetting

Experimental protocolSubstrate –

a-C gridCustom built grid

holder

Clamping on substrate holder

Sequential deposition of

different metals

Thickness monitored by quartz crystal

oscillator

Thermal evaporationBase Pressure –

3 x 10-6 mbar

Deposition of metals

• Bismuth is evaporated on a-C grid

Thickness – 10 nm

• Gold, Silver and Copper deposited

for different time intervals – 5 ,

10, and 20 s

• Annealing – 400 °C – 5h – air

in air

Solid state Liquid state

• Occurs below m.p.• Furnace annealing• Surface diffusion • Generally for metals and

alloys

• Occurs above m.p.• For metallic films - laser, ion or

electron beam induced annealing• Hydrodynamic flow of liquid• Generally observed in polymer

films

Effect of nanoparticles on solid state dewetting

• Added silver hole size

• Ag pins down the grain boundary

preventing growth of holes

• Ag suppresses solid state dewetting of

Cu thin film

• Dewetting combined with reactive ion

etching is an easy route to fabricate

silica nanopillars for antireflection and

self cleaning properties

Effect of nanoparticles on liquid state dewetting

Morphology of as deposited filmsMetal

nanoparticles

Substrate exposure (%) after annealing for 400 °C – 5h

5 s 10 s 20 s

Au 16.3 ± 1.9 18.9 ± 0.8 20.8 ± 3.8

Ag 11.8 ± 3.1 15.9 ± 1.7 20.1 ± 1.4

Cu 1.4 ± 0.4 1.7 ± 0.37 1.8 ± 0.4

Morphology of annealed films

Effect of nanoparticles

• Deposition time nanoparticle size

• Films with Au and Ag nanoparticles have moresubstrate exposure as compared to pure Bi

• Au nanoparticles dissolve in Bi matrix

• Au and Ag destabilizes Bi film

• Cu stabilizes the Bi film

References1. C.V. Thomson, Annu. Rev. Mater. Res., 42, 399 (2012)

2. Mukherjee et al., Soft Matter., 4, 2086 (2008)

3. Swaminathan et al., Phys. Rev. B, 78, 115416 (2008)

4. K.Kumar and P. Swaminathan, Thin Solid Films, 642, 364 (2017)

5. K.Kumar and P. Swaminathan, Appl. Surf. Sci.,456, 915 (2018)

Conclusions• Annealing leads to melting of film and exposure of substrate

• Ag and Au nanoparticles lowers the melting point of the film

• Ag and Au nanoparticles destabilizes Bi thin films

• Cu nanoparticles stabilizes Bi thin film

Acknowledgements

• Research supported by NRB : Project No: NRB/4003/PG/354.

• TEM was done in Department of Metallurgical and Materials Engineering, IIT Madras.

Scale bar – 500 nm

Scale bar – 50 nm

12

3

4

5

50 µm

Stages of dewettingHoleformation Hole

growth

Formation of ribbons

Droplets

Metal

nanoparticles

Size of nanoparticles (nm) for deposition time

5 s 10 s 20 s

Au 4.4 ± 1.5 6.1± 2.0 8.1 ± 2.1

Ag 4.3 ± 0.7 5.4 ± 1.1 7.9 ± 1.7

Cu 5.6 ± 3.1 7.3 ± 1.8 12.3 ± 4.7

Average grain size, Bi - 83.1 ± 39.4 nm

Pure Cu

Pure Bi

As

de

po

site

dA

nn

eale

d

Substrate exposure %

C.A – 135 °

4

3t= 5 s t= 10 s t= 20 s

• Substrate exposure of pure Bi (%) - 10.1 ± 1.4