Article pubs.acs.org/JPCC
Fluorine-Passivated Silicon Nanocrystals: Surface Chemistry versus Quantum Confinement Yeshi Ma, Xiaodong Pi,* and Deren Yang State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
ABSTRACT: The technological importance of silicon nanocrystals (Si NCs) largely originates from their remarkable optical properties. However, the detailed mechanisms for the optical behavior of Si NCs remain controversial. The controversy mainly centers on the interplay between quantum confinement and surface chemistry. By using a model system of fluorine (F)passivated Si NCs in the framework of density functional theory, we demonstrate how both quantum confinement and surface chemistry impact the optical properties of Si NCs with sizes (diameters) < 2 nm. It is found that the effect of surface chemistry on both the excitation energy and emission energy of a Si NC becomes more significant as F coverage at the NC surface increases. For a Si NC whose size changes from 1.7 to 1.4 nm at both ground state and excited state, high F coverage (>50%) increasingly enables the effect of surface chemistry to prevail over that of quantum confinement. For a very small Si NC (40% after atmospheric oxidation. This indicates the beneficial role of F in the enhancement of light emission efficiency of Si NCs, but further experiments in which only F and
Figure 5. (a) Excitation energy and (b) emission energy of a Si NC with respect to the number of Si atoms in the NC. The numbers of Si atoms 30, 47, 71, 99, and 135 correspond to the sizes of Si NCs 0.8, 1.2, 1.4, 1.6, and 1.7 nm, respectively. F coverage at the NC surface varies from 0% to 100%.
Figure 6. Distribution of electron wave functions for the HOMO and LUMO of a 0.8 nm Si NC at the ground state and the excited state. F coverage at the NC surface changes from 0% to 100%. The blue/yellow lobes represent the isosurfaces of the positive/negative values of the square moduli of wave functions. 5405
dx.doi.org/10.1021/jp211177d | J. Phys. Chem. C 2012, 116, 5401−5406
The Journal of Physical Chemistry C
Article
(12) Degoli, E.; Cantele, G.; Luppi, E.; Magri, R.; Ninno, D.; Bisi, O.; Ossicini, S. Phys. Rev. B 2004, 69, 155411. (13) Ramos, L. E.; Degoli, E.; Cantele, G.; Ossicini, S.; Ninno, D.; Furthmüller, J.; Bechstedt, F. Phys. Rev. B 2008, 78, 235310. (14) Delerue, C.; Allan, G.; Lannoo, M. Phys. Rev. B 1993, 48, 11024−11036. (15) Watanabe, M.; Matsunuma, T.; Maruyama, T.; Maeda, Y. Jpn. J. Appl. Phys. 1998, 37, L591−L593. (16) Liptak, R. W.; Devetter, B.; Thomas, J. H. III; Kortshagen, U.; Campbell, S. A. Nanotechnology 2009, 20, 035603. (17) Pi, X. D.; Liptak, R. W.; Deneen Nowak, J.; Wells, N. P.; Carter, C. B.; Campbell, S. A.; Kortshagen, U. Nanotechnology 2008, 19, 245603. (18) Ioannou-Sougleridis, V.; Nassiopoulou, A. G.; Ouisse, T.; Bassani, F.; d’Avitaya, F. A. Appl. Phys. Lett. 2001, 79, 2076−2078. (19) Pi, X. D.; Chen, X. B.; Yang, D. J.Phys. Chem. C 2011, 115, 9838−9843. (20) Pi, X. D.; Mangolini, L.; Campbell, S. A.; Kortshagen, U. Phys. Rev. B 2007, 75, 085423. (21) Delerue, C.; Lannoo, M.; Allan, G. Phys. Rev. Lett. 2000, 84, 2457. (22) Kovalev, D.; Heckler, H.; Ben-Chorin, M.; Polisski, G.; Schwartzkopff, M.; Koch, F. Phys. Rev. Lett. 1998, 81, 2803−2806.
H are at the NC surface need to be carried out to directly test the theoretical results presented in the current work.
4. CONCLUSION We have shown that the effect of surface chemistry on both the excitation energy and emission energy of a Si NC becomes more significant as F coverage increases. For a Si NC whose size changes from 1.7 to 1.4 nm at both the ground state and the excited state, high F coverage (>50%) increasingly enables the effect of surface chemistry to prevail over that of quantum confinement. This signifies that the rule of the blueshift of optical absorption/emission with the decrease of size for Si NCs can not always be applied. For a very small Si NC (
