Synthesis of Gold Nanodroplets with Field Enhancement of 105 at their Tips using a Simple Wet-Chemical Method

Summary form only given. Strong local field enhancement by plasmonic nanostructures has made them potential candidates in sensing, surface-enhanced Raman spectroscopy, and nanoscale microscopy. Anisotropic nanostructures with sharp tips lead to strong field enhancements. In this paper, we report the wet-chemical synthesis of nanodroplets (AuND), a new nanostructure that has a sharp tip at one end and a spherical base at the other. The AuNDs have two plasmon resonances corresponding to plasmon oscillations along the transverse (base diameter) and longitudinal (height) directions. The longitudinal plasmon resonance is tunable over 800-900 nm by changing the silver salt concentration during synthesis. Numerical simulations indicate that strong field enhancement of 10
⁵
can be achieved at the tip of the AuND when excited at the longitudinal plasmon resonance. The AuNDs were synthesized by following a seed-mediated growth method. The seeds were prepared by rapid reduction of chloroauric acid (HAuC1
₄
.3H
₂
0) with sodium borohydride (NaBI-1 4 ) in 100 mM cetyl trimethylammonium bromide (CTAB) solution. The seeds were aged in the dark for seven days before being used in the growth solution. A volume of 50 L of seed (-10 nm diameter) was added to the growth solution containing 2.5 mL of 1 mM HAuC1
₄
.3H
₂
0, 2.5 mL of 0.2M CTAB, 25 1 of 10 mM silver nitrate (AgNO
₃
), and 1.25 ml of 60 mM hydroquinone. The seeds were allowed to grow for 12 hours in the dark at 40°C. The formation of AuNDs was confirmed from the SEM images. The length and base diameter, averaged over 100 AuNDs, were 106 ± 7nm and 33 ± 5nm respectively. The extinction spectrum of the AuNDs has two prominent peaks at 570 nm and 804 nm. Numerical simulations indicate the peak at 570 nm corresponds to the plasmon resonance along the transverse direction (blue curve) while the peak at 804 nm corresponds to the resonance along the longitudinal direction (magenta curve). The longitudinal plasmon resonance of the AuNDs can be tuned over -100 nm by varying the AgNO
₃
concentration of the growth solution. The experimental longitudinal plasmon resonance (red curve) at -800 nm is broader than the simulated spectrum (magenta curve) because the dimension of the AuNDs in the solution has some statistical variation. The fi eld enhancement was calculated to be more than 10
⁵
at the tips. The high fi eld enhancement makes these AuNDs promising candidates for surface -enhanced Raman spectroscopy (SERS) and highly sensitive sensing application. Future work will focus on increasing the yield and exploring sensing applications.