Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities

Abstract

We present a comprehensive, ground-state density functional theory study of the size dependence of the optical and electronic properties and the stability of spherical silicon nanocrystals (NCs) with different impurities on the surface. We vary the size of the NCs from 1.0 to 3.5 nm, considering single-bonded (CH 3 , F, Cl, OH) and double-bonded (O, S) impurities and bridged oxygen. We show that the density of states (DOS) and absorption indices of the NCs with single-bonded impurities are very similar to each other and the fully hydrogenated NCs, except for the 1.0-nm NCs, where a slight difference is present. In the case of the NCs with double-bonded impurities, the DOS and absorption indices exhibit a significant difference, compared to the fully hydrogenated NCs, for sizes up to 2.5 nm. We argue that this difference arises from the difference in the contribution from the impurity to the states around the gap, which can considerably change the character of the states. We demonstrate that the double-bonded impurities contribute significantly to the states around the gap, compared to the single-bonded impurities, causing changes in the symmetry of these states. This observation was further supported by analyzing the changes of the Fourier transform of the charge densities of the highest occupied and lowest unoccupied eigenstate. We also show that the formation energies of NCs with bridged oxygen and fluorine are the lowest, regardless of the size. Furthermore, we show that high hydrogen concentration can be used to suppress the addition of oxygen and fluorine on the surface of the Si NCs.

Mapping of Defects in Individual Silicon Nanocrystals Using Real-Space Spectroscopy

“…Comparison of a 1-D LDOS map of this NC (Figure S7b) to those of its defect-free variant (Figure S8b) shows that the bandgap of the defective NC is reduced at the defect location by ∼300 meV; however, the energy differences between the delocalized peaks H 1 and E 1 are nearly identical for both NCs, suggesting that the defect-induced peaks H 1 * and E 1 * effectively correspond to trap states within the defect-free bandgap. In contrast, bandgaps obtained from theoretical LDOS of both Si–O–Si and Si–OH defects (Figure b and Figure S9b, respectively) are quite similar to that of the defect-free SiNC (Figure S8b), consistent with previous theoretical results. ,,, The impact of these defects is primarily in the redistribution of the SiNC LDOS, resulting in enhanced LDOS at the location of the corresponding defect (compare peaks H 1 with maxima H 1 * in Figure e–g and Figures S9e–g and S8e–g). The varied impacts of the different types of defects on the SiNC LDOS are directly related to the specific spatial distributions of the corresponding electronic states (Figures S10–S15).…”

“…The presence of oxidative defects on the SiNCs surface has been shown to produce red-shifted photoluminescence (PL), with photon energies being considerably less sensitive to the SiNC size than predicted for emission from quantum-confined states. − The exact origin of this emission remains poorly understood, which is due, in part, to the diversity of chemical defects resulting from surface oxidation of silicon , and the lack of experimental techniques capable of directly identifying the chemical structures of defects responsible for the red-shifted PL. Theoretical studies suggest that Si–O–Si bridged oxygen, − as well as sufficient coverage of Si–OH surface groups may result in red-shifted PL. , Indeed, it would be natural to expect these defects to play a role in PL of SiNCs because both defects appear prominently in Fourier transform infrared spectroscopy (FTIR) of porous hydrogen-passivated SiNCs within minutes of exposure to ambient air . Alternatively, a red shift consistent with the experimental data may be attributed to emission from individual defects associated with surface silicon–oxygen double (SiO) bonds, ,− ,− which have not, however, been observed in FTIR spectra of emissive SiNCs.…”

“…Theoretical studies suggest that Si–O–Si bridged oxygen, − as well as sufficient coverage of Si–OH surface groups may result in red-shifted PL. , Indeed, it would be natural to expect these defects to play a role in PL of SiNCs because both defects appear prominently in Fourier transform infrared spectroscopy (FTIR) of porous hydrogen-passivated SiNCs within minutes of exposure to ambient air . Alternatively, a red shift consistent with the experimental data may be attributed to emission from individual defects associated with surface silicon–oxygen double (SiO) bonds, ,− ,− which have not, however, been observed in FTIR spectra of emissive SiNCs. Finally, completely oxidized SiNCs have also been shown to possess delocalized electronic states with calculated energies consistent with the experimentally observed PL .…”

“…In contrast, bandgaps obtained from theoretical LDOS of both Si−O−Si and Si−OH defects (Figure 4b and Figure S9b, respectively) are quite similar to that of the defectfree SiNC (Figure S8b), consistent with previous theoretical results. 23,31,33,35 The impact of these defects is primarily in the redistribution of the SiNC LDOS, resulting in enhanced LDOS at the location of the corresponding defect (compare peaks H 1 with maxima H 1 * in Figure 4e−g and Figures S9e−g and S8e− g). The varied impacts of the different types of defects on the SiNC LDOS are directly related to the specific spatial distributions of the corresponding electronic states (Figures S10−S15).…”

“…To investigate the nature of observed locally enhanced peaks H 1 * and H 1 **, we carried out DFT calculations of the electronic structures of SiNCs containing individual chemical defects . Because a substantial fraction (∼22%) of studied SiNCs showed defects of the type shown in Figure ; these are likely to be of the same chemical nature and must be very common for the hydrogen-passivated SiNCs used in our study.…”