Comparative Optical Studies of Chemically Synthesized Silicon Nanocrystals
- PDF / 494,383 Bytes
- 6 Pages / 412.92 x 646.2 pts Page_size
- 89 Downloads / 193 Views
The PSi technique gives a surface largely passivated by an oxide layer, while the synthetic route provides an organic (methyl and methoxy groups) surface passivation layer. We performed various optical studies on these nanocrystalline Si systems, which include absorption, photoluminescence (PL), and photoluminescence excitation (PLE) spectroscopy. The experimental results are compared to theoretical models of nanocrystalline Si. Our analysis provides compelling evidence for quantum confinement in these two systems. Finally, our results indicate that the blue emission from these Si nanocrystals corresponds to the bandedge emission, while the red emission results from trapping and radiative relaxation of the initial excitation generated by bandedge absorption [14]. EXPERIMENT Porous silicon is made by anodic electrochemical etching of silicon wafers in hydrofluoric acid solutions [1-3]. The nanostructured surface of the PSi was mechanically scraped off the wafer and place in appropriate solvents. A colloidal suspension of silicon nanocrystals in various solvents was prepared by ultrasonicating the PSi remnants for several days. The final colloidal suspension was centrifuged and filtered through 0.45 gtm filters. Colloidal suspensions of Si nanocrystals derived via a chemical synthetic route were prepared through a controlled low temperature phase chemical reaction. This technique is described in greater detail in Ref.13 and will not be considered here. Several major advantages were gain through this synthetic procedure. Smaller sized Si nanocrystals were achieved and surface termination with different organic groups was possible. Colloidal suspensions of both nanocrystalline systems in various solvents, such as hexane, methanol and toluene, all gave identical behavior thus indicating that the solvent does not interfere with the mechanism for light emission. RESULTS AND DISCUSSION PL spectra from PSi gave a broad red peak centered at 680 nm. The PL from the PSi is consistent with other optical studies on PSi. However, no blue emission is observed even after prolonged periods (5 months) of exposure to ambient conditions. After processing the PSi to give monodispersed nanocrystals in colloidal suspensions, the broad red peak disappeared and a different broad red peak at lower energies appeared along with an intense blue emission [14]. Similar spectral features are observed for the synthetic nanocrystals. 3.0
1.4 1.2
o
1.0
2.5 • •
2 2.0
""•:•
0
0Cn
0.6 0.4 0.2 0.0o 2.0
1.5
1.0 0.5 0.0o 2.5
3.0
3.5
4.0
2
Energy (eV)
3
4
5
6
Energy (eV)
Figure 1. Absorption spectrum of Si nanocrystals derived from PSi.
Figure 2. Absorption spectrum of chemically synthesized Si nanocrystals.
Absorption spectra for both types of Si nanocrystals are smooth and nearly structureless and are shown in figures 1 and 2. Theoretical models by Delerue et al. [15] and by Wang and 178
Zunger [16] calculate absorption spectra for nanocrystals of a specific size. The calculated absorption edge shifts to higher energies due spec
Data Loading...
