The effect of size and shape of gold nanoparticles on thin film properties

This work describes the properties of gold nanoparticles (AuNPs) thin film on Si substrate. AuNPs can be divided into two major shapes: gold nanospheres (AuNSs) and gold nanorods (AuNRs). Two sizes of AuNSs (15 nm and 30 nm) and three aspect ratios of AuNRs (3.12, 3.39, 3.60 aspect ratio) were synthesized and determined by transmission electron microscopy (TEM). As shown in Figure 1, the monodispersed spherical shape of 30 nm AuNSs was obtained by growing of 15 nm AuNSs added with hydroxylamine. This approach helps to grow AuNSs into larger particles and improve the monodispersity [1]. From Figure 2, the AuNRs were obtained by the seed-mediated growth approach. The presence of cetyltrimentylammonium bromide (CTAB) is crucial for structure directing [2]. AuNPs produced were deposited on silicon (Si) substrate by using a spin coating method followed by a heat treatment process. Deposition layers of AuNPs were varied; 1, 3 and 5 layers, respectively. From the field emission scanning electron microscopy (FESEM), the AuNPs were uniformly distributed on the Si substrate surface. The AuNPs distribution increased with increasing number of AuNPs deposition layers. As shown in Figure 3, well distributed of AuNSs thin film was obtained. X-ray diffraction (XRD) spectra showed that AuNPs thin film was in face centered cubic gold structure. Besides that, the Raman and photoluminescence (PL) scattering proved the presence of gold in the samples. PL result showed a strong and a weak peak. The strong peak was attributed to the band emission of Au whereas the weak peak could be attributed to defects in the samples [3]. Various deposited layers of different shapes and sizes of AuNPs were analyzed using current-voltage (I-V) measurement. The results showed that the resistivity of samples became lower in light condition as the AuNPs deposition layers increased on Si substrate due to large amount of AuNPs particles which had better properties in absorbing and scattering the light intens- ty. Among these samples, 15 nm of AuNSs and 3.12 aspect ratio of AuNRs in 5 layers thin film gave the best response of resistivity in light-dark condition which is shown in Tables 1 and 2. In conclusion, the higher response implied better sensing and recovery properties since it can amplify small signal from the same light source power/intensity with more free electrons generated.