Dependence of silicon nanoparticles luminescence with size and superficial oxidation

MANUEL JOSÉ LLANSOLA PORTOLÉS; PIS DIEZ, REINALDO; DELLARCIPRETE, M. L.; DANIEL MARTIRE; GONZALEZ, MÓNICA C.

Congreso; 21st I-APS Conference; 2011

Inter American Photochemical Society

Silicon nanoparticles (NP) with sizes in the range of 1 to 10 nm exhibit photoluminescence in the UV-Vis upon excitation with light of 280-400 nm. The excitation-emission spectra depends on different factors such as size [1], superficial coating [2] and environment. However, the trends and causes of these effects still remain controversial. In order to better understand the effect of size on the emission of NP, the emission of two sizes NP of 1.5 nm (NP1.5syn) and 3.0 nm (NP3syn) were compared and contrasted with the theoretical emission spectra of NP of different sizes. The geometry of NP with sizes of 0.42 nm, 0.64 nm, 0.86 nm, 1.12 nm, and 1.22 nm (NP1.22calc) was optimized using the semiempirical Density Functional Tight Binding (DFTB) method. The emission spectra were calculated at the B3LYP/LANL2DZ level of theory within the framework of the Time Depended Density Functional Theory as implemented in the FIREFLY program. Calculations for larger particles where avoided because of computational limitations. The maximum emission and excitation wavelengths (emmax, exmax) measured for NP1.5syn and NP3syn are (360 nm, 300 nm) and (430 nm, 370 nm), respectively. The observed size-dependent spectrum is in line with that expected for a quantum confinement-controlled Si bandstructure [1]. The theoretical calculations show that emmax goes from 280 to 400 nm when the size increases from 0.42 to 1.22 nm. The experimental spectra of NP1.5syn acceptably agree with that theoretically calculated for NP1.22calc. To study the effect of superficial oxidation on the excitation-emission spectra, theoretical calculations were performed oxidising the NP1.22calc particles with an oxygen monolayer. The oxidized particle structure was optimized to place the oxygen atoms in the most stable configuration. The emission spectrum of the thus oxidized particle is displaced towards higher energies with respect to that of NP1.2calc. A small displacement towards higher energy is also observed for emmax. In fact, the spectrum structure resembles that of NP0.86calc. Oxidation experiments of NP1.5syn with H2O2 results in an oxidized particle also showing emission in the 500-700 nm range [3] but with emmax shifted to longer wavelengths. The obtained results indicate that the combination of DFTB and B3LYP/LANL2DZ methods adequately reproduces the emission spectrum of < 1.22 nm silicon-based nanoparticles. Theoretical and experimental results on the effect of superficial oxidation are not consistent and need further investigation. Generation of surface traps may be the cause of the red shift observed in the emission of surface oxidized synthetic particles [4]. Further experiments will involve the detection of possible surface radicals by esr spectroscopy. (1) Belomoin, G.; Therrien, J.; Smith, A.; Rao, S.; Twesten, R.; Chaieb, S.; Nayfeh, M. H.; Wagner, L.; Mitas, L. Appl. Phys. Lett. 2002, 80, 841-843. (2) Rogozhina, E.; Belomoin, G.; Smith, A.; Abuhassan, L.; Barry, N.; Akcakir, O.; Braun, P. V.; Nayfeh, M. H. Appl. Phys. Lett. 2001, 78, 3711-3713. (3) Belomoin, G.; Therrien, J.; Nayfeh, M. Appl. Phys. Lett. 2000, 77, 779-781. (4) Kuntermann, V.; Cimpean, C.; Brehm, G.; Sauer, G.; Kryschi, C.; Wiggers, H. Physical Review B 2008, 77, 115343.