Sahu et al. Nanoscale Research Letters 2011, 6:178 http://www.nanoscalereslett.com/content/6/1/178 to ≡Si0 defect sites rather to than nitrogen-dangling bonds. From the above discussions, it can be ascertained that the predominant mechanism responsible for the PL behavior can be due to the recombination of defect states. Figure 5a shows the evolution of PL of sample S3 con- taining 29 at.% of excess Si with thermal annealing treatment of up to 1100°C. This evolution is similar for all other samples and is depicted in Figure 5b,c. As evi- dent from Figure 5b, PL intensity increases and reaches a maximum at an annealing temperature of 700°C. A further increase in annealing temperature leads to a sig- nificant reduction of the PL intensity. The PL intensity drops to approximately 24% and approximately 10% of its peak value at 900 and 1100°C, respectively. The decrease in PL peak can be attributed to introduction of considerable amount of non-radiative defects after high- temperature annealing. It is reseaonable to expect more disordered structure, which is caused by the breaking of Page 6 of 10 hydrogen bonds and subsequent effusion of hydrogen during high temperature thermal treatment. This phe- nomenon induces an increase in the number of dangling bonds, giving rise to an enhanced number of non-radia- tive recombination centers. Thus, the PL observed in the as-deposited sample quenches after high-tempera- ture annealing above 700°C. In addition, a phase separa- tion generally occurs when silicon-rich silicon nitride samples are annealed at high temperature (≥ 950°C), resulting in the formation of Si-nps embedded in a nearly stoichiometric silicon nitride matrix. Although, Si3N4 matrix itself could passivate some dangling bonds, causing non-radiative quenching, there are still a large number of dangling bonds existing in the film, especially at the interface region between Si-nps and Si3N4 matrix. Previously, it has been shown that one dangling bond is sufficient to quench the luminescence of a Si-np [42]. In this respect, the passivation of silicon- and nitrogen- dangling bonds acting as non-radiative recombination Figure 5 Room temperature PL spectra. (a) (Color online) Room temperature PL spectra of the sample S3 (29 at.% of silicon excess) subjected to thermal annealing within the temperature range of 500-1100°C, evolution of PL intensities (b), and energies (c) as a function of annealing temperature, (d) evolution of PL intensity of the sample S2, which has been subjected to annealing at 1100°C in N2, and subsequently in forming gas (10% H2 + 90% N2.