## 2 Experimental Section
### Synthesis and Fabrication of Monolayer Triangular Silver Nanoplate Thin Film  
The triangular silver nanoplate solution was prepared via a direct chemical reduction approach at ambient temperature in air. A typical synthesis process is described in the following: 0.2 mL of 0.05 M silver nitrate (AgNOs) was mixed into 96.56 mL of deionized water. Then, the solution was vigorously stirred by a magnetic stirrer at 900 rpm. After that, 2.0 mL of 75 mM trisodium citrate was mixed into the solution followed by 0.24 mL of 30% hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)). To start a reduction process of Ag ions to Ag metal, 0.1 M sodium borohydride (NaBH4) was injected quickly into the stirred solution. The ultimate stable formation of triangular silver nanoplates in solution can be roughly seen through the color change of the solution. A clear, colorless solution changed to a dark blue solution after vigorously stirring for 5 min. This solution was then subjected to ultrasonic treatment for 1 h at 27 \({}^{\circ}\)C. The output power of ultrasonic bath is about 160 Watts. Afterward, the solution was centrifuged at 6000 rpm for 30 min. A pellet eventually formed at the bottom of centrifuge tube and the pellet was extracted to make the solutions used in the deposition of triangular nanoplate films on the silicon surface.  
The monolayer triangular silver nanoplate film was grown on the silicon surface by a self-assembly technique. This approach was typically started by cleaning a silicon substrate via immersing in piranha solution for 1 h to remove any organic waste on the silicon surface. Next, the substrate was cleaned in acetone and then in ethanol with ultrasonic treatment for 15 min. After that, the clean silicon was immersed in 5% 3-aminopropyltrimethoxysilane (APTMS) in ethanol for 1 h at room temperature to functionalize the silicon surface. The APTMS film on silicon substrate was then rinsed with copious amounts of ethanol and the surface was dried. Finally, this APTMS film was heated on a hot plate for 2 h at 100 \({}^{\circ}\)C. The deposition of the triangular nanoplates on the silicon surface was done by immersing the APTMS-covered silicon into a silver nanoplate solution formed by redissolving the centrifuged pellet in water.  
To study changes in surface coverage of the triangular nanoplates on the surface, the exposure time of the APTMS-covered silicon to the nanoplate suspensions was varied from 15 min to 22 h. The films were then characterized using atomic force microscopy, AFM (Veeco, Riverside, CA, USA), and field emission scanning electron microscopy, FESEM (Hitachi, Kuala Lumpur, Malaysia), to determine the morphology and coverage of the monolayer triangular silver nanoplate thin films. Meanwhile, transmission electron microscopy, TEM (Philips, New South Wales, Australia), optical absorbance (Cary 5G spectrophotometer; Varian, Melbourne, Australia), and dynamic light scattering, DLS (HPLS Maples, Adelaide, Australia), were recorded for the silver nanoplate solution to study the shape andsize of the triangular nanoplates. The SERS measurement was recorded using a Raman spectrometer (Horiba Scientific, New South Wales, Australia) with a 532 nm excitation laser wavelength with 10 seconds integration time.