Fabrication of Metallic Nanoparticle Array Using Nanosphere Lithography Michael Hoha, S K Sahoob, P S V R V Tejasvib, A Preetib, R Vaibhavb and Dr. V H S Moorthy* a Kaisersslautern University of Technology, Germany. bDepartment of Electronics and Communication, Manipal Institute of Technology, Manipal University, Manipal- 576104, India.
Abstract: Nanosphere lithography1 (NSL) – liquid surface self assembly technique- is an inexpensive, table top and scalable technique for the fabrication of periodic nanostructures of varying size, shape and periodicity. The self assembled mono-dispersed 2D colloidal crystals (polystyrene nanospheres of 450 nm size) on pre-treated Si (100) substrate is used as a mask to create periodic nanostructures of Ag. The polystyrene nanospheres are mono-dispersed on air-DI water interface by using custom-built pipette. A surfactant (Triton X-100/Sodium dodecyl sulphate) is added to the water surface to reduce the surface tension and condense the particles into a closely packed monolayer. Then the hydrophilic Si (100) substrates are introduced beneath the water surface and the water is carefully drained such that the polystyrene film is descended onto the substrate and subsequently left for drying in a clean environment. The resultant self-assembled polystyrene templates are characterized for different defect structures- by optical microscopy and scanning electron microscopy (SEM –JEOL8610) and modifications in the deposition technique are effected to obtain larger domains of hexagonally close packed (hcp) structures of polystyrene nanospheres. On this polystyrene hcp template, an ultrathin film (320 Å) of Ag is deposited by vacuum evaporation technique. The periodic metallic (Ag) nanostructures are obtained by subsequent removal of the under layer hcp structure of polystyrene nanospheres. The periodic nanostructures are characterized by scanning electron microscopy. The periodic structure is characterized by triangular shaped nanoparticles with average size of 120nm and periodicity 450nm.
Different NSL methods
Rod Coating Spin coating Drop coating Dip coating Lift up method
Characteristic/ Surfactant used
Sodium Dodecyl Sulphate 20% by weight
Sodium Dodecyl Sulphate 20% by weight
Triton X100 1:100 diluted
Triton X100 1:100 diluted
Triton X100 1:100 diluted (Sample 1)
Density of PS nanospheres on airwater interface
~70%
~70%
~70%
~70%
~100%
Sample orientation
Inclined 0 (2.97 )
NonInclined
Inclined 0 (7.758 )
NonInclined
NonInclined
Domain size
8-10 µm
6-7 µm
6-8 µm
8-9 µm
3-4 µm
Packing Density
5.252 5.45/µm
4.42 4.76/µm
5.122 5.31/µm
4.762 5.76/µm
4.5/µm
Line defects
Less
Moderate
Moderate
Less
Moderate
Double layers
Nil
Nil
Very less
Very less
Nil
Triplets/particulates
Very Less
Very Less
Less
Less
Less
2
One of the significant challenges in our experiment was determining the optimum condition for the fabrication of large area, defect-free, periodic nanospheres monolayer template, with repeatability. Parameters like concentration of different surfactants in different solutions, concentration of nanospheres spread, evaporation time, duration of vesseltilting and angle of elevation of the substrate were monitored.
Importance of NSL-a Bottom-Up approach
Results
Cost- effective Flexible Simple High-throughput Attractive in the present research scenario
Applications of Nanoparticles
Photonic Crystals Catalysis Plasmonic biosensors
Principle of NSL: COLLOIDAL
Fig 2: SEM image of a large uniform hcp layer (nanosphere mask- before etching) and Silver nanoparticles post-etching
SELF-ASSEMBLY
Spontaneous organization of colloidal particles into a relatively stable structure through non-covalent interactions Self-assembly at the liquid-gas interface
References: Fig 1: Custom-built micropipette used in Nanosphere Lithography and the procedure of NSL (schematic and experiment)
[1] J. C. Hulteen and R. P. Van Duyne: Nanosphere lithography, J. Vac. Sci. Technol. A 13(3), May/Jun 1995