J. Phys. Chem. B 2000, 104, 9981-9986
9981
Role of SidO in the Photoluminescence of Porous Silicon Fulin Zhou and John D. Head* Department of Chemistry, UniVersity of Hawaii, Honolulu, Hawaii 96822 ReceiVed: May 2, 2000; In Final Form: August 8, 2000
It has been proposed recently that localized excitations and recombinations in the silanone SidO bond is the source for the red photoluminescence (PL) observed for oxygen-exposed porous silicon (PS). One concern with this proposal is the expected low chemical stability of the SidO bond. By using quantum chemistry calculations and molecular clusters to simulate the PS surface, we show that the hydrated silanone bond is less stable than the hydroxylated Si(OH)2 surface species. However, we also determine there is a low barrier for the conversion of the hydroxylated Si to a hydrated silanone surface species. Our calculations suggest that the origin of the red PL for oxygen-exposed PS first involves the conversion of hydroxylated Si species, presumably by energy provided by the incident photon source, to a surface with a larger concentration of silanone groups. These metastable ground-state silanone groups would then be excited by additional photons to produce the observed PL.
Introduction The room temperature photoluminescence (PL) of electrochemically etched porous silicon (PS) wafers has been the subject of an intense research effort primarily because of its potential applications as a Si-based device in optoelectronics, displays, and sensors. The structure of PS is composed of crystalline Si spheres and columnar structures with dimensions of a few nanometers covered with various surface species.1 Although several models have been proposed for the origin of the PL, its source still remains controversial. The existence of nanometer-sized Si crystallites in PS has given rise to the popular hypothesis that the luminescence results from the radiative recombination of quantum-confined electrons and holes as in a quantum dot.2-5 A second explanation has focused on the importance of surface-localized states, formed on the irregularly shaped small crystallites that are not perfectly passivated, wherein the elementary excitations are first trapped prior to recombination.6-8 But it seems there is not enough evidence to fully support either of these two explanations. A third explanation for the visible emission from porous silicon contends that the luminescence results from the presence of surface-confined molecular emitters, such as siloxenes.9,10 More recently two separate groups have proposed that silanone groups, SidO, on the PS surface are involved with the photoluminescence. Gole, Dixon, and co-workers (GD) in an extensive series of investigations using ab initio molecular-orbital theory and density-functional theory (DFT) and a wide range of molecular Si clusters which model possible surface species on PS have suggested that the SidO group is the main origin of the PL, with a -(RO)SidO fluorophor being responsible for the orangered emission,11-15 whereas the Fauchet, Allan, and Delerue group (FAD) in a different study have proposed that both the quantum confinement model and surface passivation by silanone groups play a role in determining the PL of PS.16 FAD investigated the electronic structure of the silanone groups by using the self-consistent tight-binding model and DFT calculations. While there are several appealing features to the proposal that the silanone group may be responsible for the PL of PS, one purpose of this paper is to comment on the expected low
stability of a silanone group relative to some other potential species on the PS surface. GD in their investigation of the origin of the photoluminescence of PS essentially computed the ground- and excited-state energies and geometries for various molecular clusters, such as H2SidO and (H3Si)2SidO, to model the local excitations taking place in the SidO chromophore bonded to a more extensive Si substrate of the PS surface. The H atoms in these clusters complete the tetravalency of the Si atom to which they are bonded and in many cases effectively serve as pseudo-Si atoms approximating the extended substrate. GD also examined a number of molecular clusters containing an O atom which bridges the silanone Si atom to either a H or another Si atom, such as X(HO)SidO and X(H3SiO)SidO, where X simulates the extended substrate. From the computed adiabatic excitation energies coupled with changes in the excited-state geometry relative to the ground-state geometry for these different molecular clusters, GD proposed that the green PL is due to the -RSidO species, whereas the more stable orange-red emission is due to the -(RO)SidO surface species, where R is either a H atom or a SiH3-based group.11-15 Although the GD adiabatic excitation energies provide a reasonable correlation with the different experimentally observed PL emission energies, we demonstrate that an alternative interpretation for the origin of the PL becomes necessary when the vertical excitation energies are used to estimate the initial absorption energy needed to excite the silanone group. Our calculations find the vertical excitation energies computed at the ground-state geometry of the molecular clusters containing the -(RO)SidO species are large enough to prevent it from readily being photoexcited, to suggest that the excited state for these species may not be populated and may not participate in the PL emission. We find even higher vertical excitation energies when we extend the calculations to treat the molecular clusters (HO)2SidO and (H3SiO)2SidO having a silanone group bridged with two oxygen atoms. Rather than model surface species on PS, the (RO)2SidO-type clusters would serve as a first approximation to a silanone group being present on a SiO2 surface. Thus, by the explicit consideration of vertical excitation energies for various molecular Si clusters,
10.1021/jp001650s CCC: $19.00 © 2000 American Chemical Society Published on Web 10/04/2000
9982 J. Phys. Chem. B, Vol. 104, No. 43, 2000 we are able to explain why PL occurs for a silanone group on PS but not on a SiO2 surface. A much broader issue with the proposal that SidO groups play an important role in the origin of the photoluminescence in PS is the expected low chemical stability of the SidO double bond. One would expect hydroxylated Si(OH)x groups to be the more stable species and more likely to be present on the surface of PS. We find confirmation for this in the present work; for example, we find that the optimized structure for H2Si(OH)2 is 2.93 eV more stable than the separately optimized H2SidO and H2O molecules combined using a Lanl2DZ basis set and MP2 level calculations. While it might be argued that not many SidO groups are needed to generate the PL intensity observed for PS, their low energetic stability makes it surprising that any should be formed at all. Therefore, in the present work we have investigated the stability of the SidO group further and conclude that the silanone is probably a short-lived intermediate formed after the phototransformation of hydroxylated Si(OH)x groups to a hydrated silanone group. Once a sufficient number of these surface silanone groups are formed, the PS should produce the observed PL. This paper is organized as follows. In the next section we give details of the quantum chemical methods and the molecular clusters used to model the local geometries and excitation energies for some of the surface silicon oxyhydrides present on PS. Then in the Results and Discussion we first present ground- and excited-state geometries and the corresponding excitation and emission energies for various Si compounds containing silanone and hydroxyl groups. In the second part of the Results and Discussion we discuss the conversion of hydroxylated Si to a hydrated silanone and argue how the silanone is a short-lived intermediate responsible for the PL in oxidized PS. Concluding remarks are made in the final section of the paper. Method Similar to the work of GD,11-15 quantum chemistry calculations were performed on a number of different molecular clusters which simulate different silicon oxyhydride species likely to be present on the PS surface. For example, the simplest model of a surface silanone group is provided by the H2SidO molecule, where the H atoms complete the tetravalency of the silanone Si and act as pseudo-Si atoms which crudely model the more extensive Si substrate of PS. A better approximation to the effects of the Si substrate should be provided with the larger molecules such as (H3Si)2SidO. GD have restricted most of their attention to clusters of the types XRSidO and X(RO)SidO, where X simulates the extended substrate. We have also investigated the molecular clusters (HO)2SidO and (H3SiO)2Sid O, which can be regarded as being surface models for a silanone group on SiO2, rather than PS, although it is well-known that Madelung corrections are needed to treat such oxide surfaces accurately.17 The ground- and lowest triplet excited-state geometries and energies for the different molecular clusters were computed with the GAUSSIAN 94 program.18 For Si we used the Hay-Wadt effective core potential and basis set (Lanl2DZ)19 augmented with the d (exponent 0.45) polarization function suggested by Rohlfing and Raghavachari.20 The Dunning-Hay basis was used for H and O, with the O basis augmented with diffuse p (0.059) and d (0.85) polarization functions.21 Our Lanl2DZ calculations on the small molecular clusters give results very consistent with those of the Si all-electron Dunning-Hay basis set.21 In this paper we only report the optimized geometries and energies computed at the MP2 level, with the
Zhou and Head TABLE 1: Summary of the Optimized Geometries for the Ground and Lowest Triplet Excited States of Various Silanones and Silicon Hydroxylated Species at the MP2/ Lanl2dz Levela molecule
r(SisO) GS singlet
r(SisO) ES triplet
∆r(SisO)
∆r(SisX)
H2SidO H2SidO‚OH2 H2SidO‚2OH2 (H3Si)2SidO (H3Si)2SidO‚OH2 H(HO)SidO H(HO)SidO‚OH2 (H3Si)(HO)SidO (HO)2SidO (HO)2SidO‚OH2 (H3SiO)2SidO H2Si(OH)2 (H3Si)2Si(OH)2 Si(OH)4
1.524 1.535 1.558 1.541 1.546 1.518 1.530 1.525 1.518 1.529 1.520 1.652 1.665 1.637
1.680 1.689b 1.694 1.663 1.670c 1.692 1.689 1.695 1.691 1.685c 1.691 1.672 1.661 1.671
0.156 0.154 0.136 0.122 0.124 0.174 0.159 0.170 0.173 0.156 0.171 0.020 0.004 0.034
0.004 0.001 0.006 0.003 0.008 0.023 0.021 0.032 0.023 0.022 0.012 0.005 2.330 0.012
a r(Si-O) refers to either the silanone bond length or the Si-O distance in the hydroxylated species without silanone bonds present, whereas ∆r(Si-X) gives the average bond length change of other H, Si, or O atoms bound to the central Si atom. All distances are in angstroms. GS ) ground state and ES ) excited state. b Both excited structures have the same SisO and SisH distances. c The excitedstate structure has water coordinating the silanone Si atom.
open-shell systems being treated using the unrestricted formalism. For comparison between singlet and triplet energy separations we also report some vertical excitation energies computed by the singly excited configuration interaction (CIS) method.22 Results and Discussion Silanone and Hydroxylated Si Ground- and Excited-State Geometries and Energies. The optimized bond lengths calculated at the MP2/Lanl2DZ level for the singlet ground states and the lowest-lying triplet excited states of the model silanone and hydroxylated Si compounds are summarized in Table 1. Our results for H2SidO, (SiH3)2SidO, and (HO)2SidO are very consistent with those given previously by GD.11-15 In the ground state, the XYSidO framework is planar and we also find a substantial ∼0.15 Å lengthening of the SidO bond to form a nonplanar XYSidO species when the silanone chromophore is excited. Table 1 shows the bond distances between the silanone Si and the H, SiH3, and OH groups to be relatively unaffected by the silanone excitation. In the optimized hydroxylated species XYSi(OH)2, the four atoms coordinating the central Si atom have the expected tetrahedral arrangement and the lengths of the SisO bonds are not appreciably modified by the excitation. In exactly the same way as previously noted by GD, our results in Table 1 demonstrate that significant geometric changes do accompany the silanone transitions to their excited triplet states. As we discuss in more detail below, GD have correlated these changes with the large Stokes shift observed for the photoluminescence from porous silicon. In contrast, the much smaller changes in the HsSi, SisSi, and SisO bond distances should be expected to produce much smaller shifts in the observed emission spectrum. In Table 2 we summarize the vertical excitation, vertical emission, and relaxed emission energies calculated at the MP2/ Lanl2DZ level for the silicon oxide compounds that we have considered. The vertical excitation energy is computed as the difference between the energies of the ground-state optimized geometry and the lowest excited state at the same geometry. The emission energies are computed from the energy of the optimized geometry at the UMP2/Lanl2DZ level for the lowest
Role of SidO in the Photoluminescence of PS
J. Phys. Chem. B, Vol. 104, No. 43, 2000 9983
TABLE 2: Excitation and Emission Energies (eV) for Various SiO Molecular Clusters vertical excitation molecule H2SidO H2SidO‚OH2 H2SidO‚2OH2 (H3Si)2SidO (H3Si)2SidO‚OH2 (H3Si)2SidO‚2OH2 H(HO)SidO H(HO)SidO‚OH2 (H3Si)(HO)SidO (HO)2SidO (HO)2SidO‚OH2 (H3SiO)2SidO H2Si(OH)2 (H3Si)2Si(OH)2 Si(OH)4
UMP2 CIS(T) CIS(S) 4.33 5.94 6.77 3.52 4.78 5.37 5.88 7.03 5.32 7.15 7.55 6.91 8.61 5.95 8.94
3.99 6.32 7.79 3.16 4.64 5.32 5.45 7.30 5.06 6.73 7.89 6.53 9.05 6.04 9.44
4.98 6.87 8.20 3.78 5.32 6.07 6.56 8.08 5.96 7.85 8.76 7.67 9.79 7.16 10.2
relaxed vertical emission emission UMP2
UMP2
2.97 3.53a 3.56 2.76 3.10b
1.49 1.52 1.38 1.74 1.79
3.52 4.00 3.59 3.59 3.89b 3.33 5.07 3.17 5.20
1.58 1.53 1.93 0.72 0.83 1.02 0.10 1.07 0.04
a
The two different H2SidO‚OH2 optimized excited-state structures have the same relaxed emission energy. b The excited-state structure has water coordinating the silanone Si atom.
Figure 1. Schematic diagram illustrating the different types of excitation and emission processes for the silanones.
triplet excited state. In Table 2 we list both the relaxed emission energy computed from the differences between the energies of the optimized triplet excited- and singlet ground-state structures, and the vertical emission energy computed using the excitedstate optimized geometry. We have diagrammed these energies schematically in Figure 1 and note the initial absorption or excitation process of a surface species is usually associated with the vertical excitation energy to a singlet state, while the energy of the PL is usually correlated with the relaxed emission energy. Optical detection of magnetic resonance experiments by Stutzman et al. have established the PS red emission is from a triplet state.9 Our relaxed emission energies calculated for H2SidO (2.97 eV), (SiH3)2SidO (2.76 eV), and (HO)2SidO (3.59 eV) agree very closely with the uncorrected ground-state singlet-excitedstate triplet adiabatic energy separations, ∆E(S-T), given by GD.11-15 GD report corrected MP2/DZP ∆E(S-T) values obtained by reducing the actual energy separation by 0.21 eV, where their correction is based on the results of coupled cluster and large basis set (CCSD(T)/TZ2PF) calculations on H2Sid O. Instead of calculating explicit vertical excitation and emission energies, GD make reasonable, yet qualitative, assignments based on geometry differences between the optimized lowest excited triplet state and the ground state of the molecular clusters. Due to the large increase in the SidO bond length on excitation, GD argue, using the equivalent of our Figure 1, that
the absorption will occur at much higher energy than their ∆E(S-T) values, whereas the emission is red shifted relative to the ∆E(S-T) values. By further observing that the change in the silanone bond length, ∆r(SisO), is consistently on the order of 0.17 Å whenever an OH or OSiH3 group is bound to the Si in the silanone bond while the ∆r(SisO) is 0.015-0.05 Å shorter in the absence of such O-containing groups, coupled with the low dependence of the ∆E(S-T) values on the different silanone substituents, GD postulated that the green PL correlates with a -RSidO fluorophor on PS, whereas the more oxidized silanone bonding in the -(RO)SidO fluorophor produces the more stable orange-red PL.11-15 Even though the GD work does provide a chemically reasonable origin for the different colors of emission observed for PS, we do not understand why these authors did not directly compute vertical excitation and emission energies for the different molecular clusters they investigated. In Table 2 we list three types of vertical absorption energies: the lowest singlet CIS(S) and triplet CIS(T) excited-state separation relative to the singlet ground state using the singles only configuration interaction (CIS) method,22 and the lowest triplet to singlet ground-state energy separation using UMP2 theory. The CIS calculations, which are performed at the MP2 ground-state geometry, demonstrate that the singlet excited state is generally 0.5-1.0 eV higher in energy than the triplet state. The UMP2 vertical absorption energies for the lowest triplet excitation include more correlation effects than the CIS calculations and yet essentially reproduce the CIS(T) results. The agreement between the CIS(T) and UMP2 results provides some justification for modeling the triplet excited states for these systems with a single determinant Hartree-Fock calculation followed by correlation at the MP2 level. If one uses the vertical UMP2 excitation energy as an estimate for the absorption energy to reach the lowest triplet state, then Table 2 shows the molecules (SiH3)2SidO, (SiH3)2SidO‚OH2, and (SiH3)2SidO‚2OH2 have absorption energies in the range 3.5-5.5 eV. The CIS(S) results for these molecules show that the absorption energy is less than 1 eV higher to reach a singlet excited state. Thus, our computed vertical excitation energies show that these silanone species do have absorption energies very close to the energies provided by the typical experimental excitation sources such as the KrF laser (5.01 eV) or the N2 laser (3.69 eV). In contrast Table 2 shows the UMP2 initial vertical excitation energies for the clusters with two additional O atoms bonded to the silanone, (HO)2SidO (7.15 eV), (HO)2SidO‚OH2 (7.55 eV), and (H3SiO)2SidO (6.91 eV), are much higher than for the species where H or Si atoms are directly bonded to the silanone Si atom. That is, the (HO)2Sid O, (HO)2SidO‚OH2, and (H3SiO)2SidO computed vertical excitation energies are much larger than the energies provided by the typical laser excitation sources. Since these O-bridged silanone molecular clusters could serve as models for surface species on the SiO2 oxide surface, these larger excitation energies may explain why SiO2 materials do not exhibit the same type of PL as observed for PS. Finally, the UMP2 initial vertical excitation energies for compounds with only one OH group bound to the silanone Si are somewhat intermediate between the above two sets of energies: H(HO)SidO (5.88 eV), H(HO)SidO‚OH2 (7.03 eV), and (H3Si)(HO)SidO (5.32 eV). Thus, our calculations suggest that the oxyhydride compounds where the silanone Si atom is bonded to an OR group may not be as readily excited as the R2SidO species by the commonly available laser sources into an initial excited state which could then eventually decay by PL emission. Table 2 also shows the
9984 J. Phys. Chem. B, Vol. 104, No. 43, 2000 H2Si(OH)2, (SiH3)2Si(OH)2, and Si(OH)4 compounds, where the silanone group is absent, to have high vertical absorption energies and suggests the lowest-lying excitation energies for these surface species will lie far into the ultraviolet region. These results are also consistent with higher silanone vertical excitation energies when there is an OR group bonded to a silanone Si atom. The relatively low vertical absorption energy for (SiH3)2Si(OH)2 is because it is the SisSi bond, and not the SisO bond, which becomes excited. The triplet UMP2 level calculations on (SiH3)2Si(OH)2 produce an excited-state geometry where the SisSi bond is effectively broken, and consequently these molecules still show large Stokes shifts in the computed relaxed emission energies. In H2Si(OH)2 and Si(OH)4, our calculations find the lowest excited triplet state corresponds to one of the OsH bonds being excited. For completeness we have included the adiabatic relaxed emission and vertical emission energies for the H2Si(OH)2, (SiH3)2Si(OH)2, and Si(OH)4 compounds; although these energies have no physical significance since the high excitation energies for these compounds should prevent their excited states from actually being populated on PS, the negative vertical emission energies are indicative of state crossings and bond breaking which would take place if these excited states were populated. Our computed relaxed emission energies are listed in Table 2 and are in close agreement with the ∆E(S-T) values previously given by GD.11-15 Although these relaxed emission energies for the molecular clusters containing a silanone group show substantial Stokes shifts from the vertical excitation energies, both our and the GD ∆E(S-T) values are at least 1 eV higher than the energy associated with the red PL. GD have argued this additional red shift might be achieved when one considers the vertical emission energy. We have performed these calculations and in Table 2 show that the UMP2 vertical emission energies for the silanone groups are indeed in much better agreement with the observed red PL for PS. The actual energy of the peak intensity for the PL will be determined by the Franck-Condon overlap between the vibrational states belonging to the excited and ground states and should lie somewhere between the computed relaxed and vertical emission energies. We have already described how GD have used the slightly different geometry changes ∆r(SisO) they computed when exciting the SidO species with different substituents to explain how the PL transforms from a green to a final orangered emission. They attribute the green luminescence to -RSid O species and correlate the larger ∆r(SisO) found for -(RO)SidO with the orange-red PL. Some support for the GD proposal is provided by the much smaller vertical emission energies, given in Table 2, of the (HO)2SidO, (HO)2SidO‚ OH2, and (H3SiO)2SidO molecular clusters. However, as we have already noted, the large vertical excitation energy values computed by us for these (RO)2SidO species suggest the initial excitation does not readily take place and hence these species should not contribute to PL emission on PS. Interestingly Table 2 shows the H(HO)SidO, H(HO)SidO‚OH2, and (H3Si)(HO)SidO clusters, with only one OH group bonding directly to the silanone Si, have much larger vertical emission energies than the (RO)2SidO species. This latter result suggests that the -(RO)SidO fluorophor may not be as drastically red shifted from that of the -RSidO emission as previously postulated by GD. Alternatively, FAD have attributed the PL transformation from a green to a final orange-red emission as being due to the oxidation of an initially O-free Si surface on PS to presumably produce a surface silanone.16 FAD argue the green PL is due
Zhou and Head
Figure 2. Computed energies along the reaction coordinate for the transformation from H2Si(OH)2 to H2SidO‚OH2. The values in parentheses are for the reaction of (SiH3)2Si(OH)2 to (SiH3)2SidO‚ OH2.
to the recombination of quantum-confined electron/hole pairs in approximately 2 nm sized O-free Si crystallites. Computational limitations prevent both GD and us from exploring the properties of Si nanoparticles in the 2 nm size range with the present cluster calculations, and we cannot exclude the possibility of quantum confinement effects in the O-free PS. The present calculations support the idea that local excitations of the SidO bond result in the orange-red PL on PS, but using the calculated geometry change ∆r(SisO) taking place in the excitation to predict the extent of the red shift, as performed by GD, may not be reliable enough to distinguish between the different possible fluorophors producing green or orange-red PL emission. Formation and Electronic Properties of the Hydrated Silanones. Although the calculations by GD, FAD, and us all provide support for the proposal that the SidO bond has an emission energy which can be correlated with the orange-red photoluminescence observed for PS, the strong preference by Si to be tetrahedrally coordinated suggests a low stability for the silanone bond. For example, we find at the MP2 level, using the Lanl2DZ basis set, the optimized H2Si(OH)2 structure to be 2.93 eV more stable than the separately optimized H2SidO and H2O molecules combined. We also find a water molecule readily hydrates the silanone Si to give the structure shown in Figure 3, where a 0.80 eV hydration energy is released by forming H2SidO‚OH2 from the H2SidO and H2O molecules. The water O atom is 2.05 Å from the Si atom, suggesting that the water may readily react with SidO to produce H2Si(OH)2, or more significantly, as we describe in more detail below, the reverse reaction of dehydration of hydroxylated silicon to form silanone may readily take place. While it might be argued that not many SidO groups are needed to generate the PL intensity observed, our computed low silanone stability makes it surprising that any should be formed at all. An alternative possibility is that the SidO group is a short-lived oxide intermediate formed along the pathway from the initially hydroxyl-passivated PS to the photoluminescently active surface and that the PL excitation mechanism for PS involves a two-step excitation process. The
Role of SidO in the Photoluminescence of PS
J. Phys. Chem. B, Vol. 104, No. 43, 2000 9985
Figure 3. Optimized ground-state and two lowest triplet excited-state structures for H2SidO‚OH2 obtained at the MP2/Lanl2DZ level. All distances are in angstroms.
TABLE 3: Geometries and Lowest Triplet Excitation Energies in the Transformation from H2Si(OH)2 to H2SidO‚OH2a Optimized GS H2Si(OH)2 r(SisOs) r(SisOw) r(SisH1) r(SisH2) ∠(HsSisH) ∠(HsSisO)
1.652 1.652 1.462 1.462 113.2 103.6 111.7
TS
H2SidO‚OH2
1.573 1.870 1.464 1.470 109.8 122.5 122.4
1.535 2.033 1.468 1.474 108.7 123.9 123.9
Optimized ES H2SidO‚OH2 H2Si(OH)2 r(SisOs) r(SisOw) r(SisH1) r(SisH2) ∠(HsSisH) ∠(HsSisO) vertical excitation relaxed emission vertical emission
1.672 1.641 1.467 1.458 115 101 109 8.61 5.07 0.10
TS
6.46
ESA
ESB
1.692 2.831 1.470 1.470 113.8 103.7 103.8 5.94 3.53 1.51
1.689 4.272 1.470 1.470 112.9 105.0 105.0 5.94 3.53 1.52
a H and H label the H atom bonded to Si. O and O distinguish 1 2 s w between the O atoms in the silanone bond and water molecule in H2SidO‚OH2. Os and Ow are symmetrically equivalent in the H2Si(OH)2 ground state. There are two ES structures optimized for H2SidO‚OH2: (A) the water O coordinating with Si and (B) one of the water H atoms forming a H bond with the silanone O atom. All distances are in anstroms, and the energies are in electronvolts.
surface of the porous silicon after electrochemical etching in the presence of air is most likely made up of silicon hydroxyl, Si(OH)x, species, while the lower stability of the silanone Sid O suggests that not many of these groups should be present on the surface. In the initial photoexcitation of PS the Si(OH)x species may be converted to a silanone-based oxyhydride coordinated with water. Then these silanone-based oxyhydrides absorb the additional photons to generate excited-state silanones with the elongated SidO bond. The de-excitation of the silanones then gives rise to the subsequent visible PL. To explore the energetics of the hydroxy to silanone interconversion, we have computed the optimized ground-state singlet structures for the molecules H2Si(OH)2 and H2SidO‚OH2 and determined the optimized transition-state (TS) structure connecting these two molecules. The geometries for the first excited triplet states of H2Si(OH)2 and H2SidO‚OH2 have also been optimized. All of the relevant results are summarized in Table 3, and a summary of the energetics for the transformation from H2Si(OH)2 to H2SidO‚OH2 is given in Figure 2. The vertical excitation energy
Figure 4. Optimized ground-state and lowest triplet excited-state structures for H2SidO‚2OH2 obtained at the MP2/Lanl2DZ level. All distances are in angstroms.
for H2Si(OH)2 is computed to be 8.61 eV and is therefore not very likely to occur with the laser excitation sources typically used with PS. However, the TS shown in Figure 2 connecting H2Si(OH)2 to H2SidO‚OH2 is computed to be only 2.38 eV higher in energy than H2Si(OH)2. This suggests that H2Si(OH)2 could be converted by energy from the initial photon flux to the hydrated silanone H2SidO‚OH2. The relatively low barrier from hydroxy to silanone conversion suggests a relatively large concentration of the hydrated silanone groups could develop on the PS surface during the initial exposure to radiation. After conversion the hydrated silanone surface species can be readily excited by the laser sources to give the observed PS visible PL. We have computed a similar magnitude for the TS energy barrier to the (SiH3)2Si(OH)2 and (SiH3)2SidO‚OH2 interconversion; we summarize these energy values in parentheses in Figure 2. Molecular water adsorption and dissociation have also been observed to readily occur on clean Si surfaces.23 We also find the hydrated silanone H2SidO‚OH2 is readily hydrated by a second water to produce the H2SidO‚2OH2 structure shown in Figure 4. The addition of the second water has a 0.55 eV stability to generate an overall hydration energy of 1.35 eV. Substantial changes in the water coordination to the H2SidO take place upon excitation. In Figure 4 we show the lowest UMP2 triplet-state geometry computed for H2SidO‚2OH2. Like the unhydrated H2SidO, the excited-state geometry of the silanone framework is nonplanar, and has only one water molecule still coordinated to the Si atom although this Si to water O distance increases to 2.76 Å. The other water is no longer coordinated to the Si atom, and instead H bonds to both the O atom in the SisO bond and the Si-coordinated water molecule. In this H2SidO‚2OH2 excited-state structure the total hydration energy is reduced to 0.76 eV. In Figure 3 we show the two UMP2 triplet optimized structures which we find for H2SidO‚OH2. Essentially, these two structures correspond to the two types of waters discussed for the H2SidO‚2OH2 excited-state structure. Structure A models water coordinating the Si atom with the Si to water O distance increased to 2.83 Å, and structure B corresponds to a H-bond interaction between water and the O atom in the SisO bond. It is difficult to distinguish the two structures energetically as both have hydration energies of 0.24 eV. Thus, in the excitation and relaxation process to the final excited state prior to PL, there will be significant rearrangement of any water or other molecules coordinating with the silanone, and this is a possible mechanism broadening the width of the PL emission energies. Finally, the low stability for the silanone could catalyze the rapid oxidation of H-passivated PS surfaces and explain the
9986 J. Phys. Chem. B, Vol. 104, No. 43, 2000 fairly rapid transformation of the green PL to the longer lived orange-red emission observed when H-passivated PS is exposed to air.16 Conclusions Similar to the work by GD11-15 and FAD,16 our calculations suggest that localized excitations and recombinations within the silanone SidO bond is responsible for orange-red photoluminescence observed for porous silicon. From consideration of our computed vertical excitation, relaxed emission, and vertical emission energies, we conclude the orange-red PL is from silanone groups which are directly bonded to other Si atoms present in the underlying substrate. The higher vertical excitation energies computed for the (RO)2SidO clusters suggest that silanone groups linked to the Si substrate via bridging O atoms would not be as readily excited, thereby reducing their ability to produce a PL emission. Finally we also find relatively high excitation energies for the R(HO)SidO clusters where the silanone Si atom is bound to an OH group. In any event, even if the silanone bond is excited in the R(HO)SidO cluster, in contrast to the earlier GD work our computed vertical emission energies provide little support for the GD assignment of green PL to the -RSidO fluorophor and orange-red PL to the -(RO)SidO fluorophor. At present we feel the green PL is probably due to the quantum confinement effects discussed by FAD. Unfortunately our cluster calculations are not large enough to provide any additional verification for the origin of the green PL. The present work also confirms that the silanone bond is not very stable. However, we compute a relatively low barrier for the conversion of hydroxylated Si, XYSi(OH)2, to a hydrated silanone species. The surface of the oxygen-exposed PS should initially consist of hydroxylated Si, which would be first converted presumably by energy from the initial photon flux to produce a surface with a higher concentration of silanone groups. These metastable ground-state silanone groups would then be able to absorb an additional photon to produce an excited-state species, which can then produce the orange-red PL observed for PS. Acknowledgment. We thank Professors David Harwell and Randy Larsen for their many helpful discussions on this work.
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