APPLICATIONS OF PLASMONIC AND METAL-SEMICONDUCTOR COMPOSITE NANOPARTICLES

A. SERS substrates for multiple excitation wavelengths

• Trace detection of different dyes in nano-molar concentration range (about 500 nM) using silver-gold alloy nanoparticle films as SERS substrates suitable for multiple excitation wavelengths.
• Preparation method: Sequential pulsed laser deposition:

A novel single step growth method for composition controlled Ag-Au alloy nanoparticle films of tunable LSPR response (Fig. 1)


Fig. 1 AFM image (left) of nanoparticle film and (right) LSPR tuning by varying alloy composition and film thickness for SERS based trace detection of 500 nM Rhodamine 6G dye.



• Silver and silver rich films have relatively high SERS intensity at lower excitation wavelength.
• By controlling nanoparticle size and alloy composition, SERS substrates suitable in wavelength range of 450-900 nm were produced (Fig.2).
• Shelf life of SERS substrates: ~2 months for silver and silver-rich alloy films and > 6 months for Au films.

Fig. 2 Trace level detection of different dye molecules using SERS silver-gold alloy based substrates of varied Ag concentrations, suitable for different excitation wavelengths: 633, 473 and 785 nm.

Publications: Shweta Verma et al., Journal of Alloys and Compounds, 753, 2018, 395-406.

• Producing SERS substrates based on densely packed gold nanoparticle films with shelf life of more than 6 months by controlling growth temperature and film thickness.
• Preparation method: Pulsed laser deposition / thermal evaporation / sputtering
• For same mass thickness of gold films, SERS intensity was significant when produced at optimum growth temperature.
• Films with high LSPR strength near the excitation wavelength showed the highest SERS enhancement (Fig. 3).



Fig. 3 SERS intensity of Rh6G dye [500 nM] in presence of gold films grown at different substrate temperatures.

Publications:

1. Shweta Verma et al., Appl. Surf. Sci., 346, 2015, 379-387.
2. Shweta Verma, B. Tirumala Rao and L. M. Kukreja, Proc. DAE BRNS 6th National Symposium on Pulsed Laser Deposition of Thin Films and Nanostructured Materials (PLD), Nov. 2011, Bengaluru.

• Efficient detection of dyes and pesticides upto 200 nanomolar concentrations using SERS substrates based on Ag/Au nanoparticle deposited Si nanowires.
• Preparation method: Plasmonic metal assisted chemical etching for nano-structuring of Si wafers followed by deposition of Ag/Au nanoparticles.
• Nanostructuring of Si substrates in form of nanoporus and nanowire (diameter:100-200 nm, length: few microns) morphologies (Fig.4) controlled by growth temperature varied thin Ag/Au films.



Fig. 4 Scanning electron microscope image of Si nanowires (a) top and (b) cross-section view and (c) silver nanoparticle coated



• Super-hydrophilicity of these nanostructured Si substrates (contact angle < 10o) allowed uniform dispersion of anlaytes for efficient and reproducible SERS substrates (Fig.5).



Fig. 5 SERS spectra of pesticide thiram using Si nanowire based SERS substrates

Publications: J. Jenitta, B. Tirumala Rao et al, Proc. DAE-BRNS National Laser Symposium-NLS 28, 2019, VIT Chennai.

Salient features:

• Cost-effective fabrication of large area, uniform films
• Suitable for different metal nanoparticles of varied shapes
• Films can be supported on various types of substrates like glass, silicon etc.



Fig. SEM image of uniform films of gold nanoparticles of spherical and rod shape