13776
J. Phys. Chem. 1996, 100, 13776-13780
A Comparison of Porous Silicon and Silicon Nanocrystallite Photoluminescence Quenching with Amines Beata Sweryda-Krawiec, Robin R. Chandler-Henderson, and Jeffery L. Coffer* Department of Chemistry, Texas Christian UniVersity, Fort Worth, Texas 76129
Young Gyu Rho and Russell F. Pinizzotto* Department of Materials Science, UniVersity of North Texas, Denton, Texas 76203 ReceiVed: March 15, 1996; In Final Form: May 21, 1996X
We report here a comparison of the ability of the monodentate Lewis base n-propylamine and the bidentate molecule ethylenediamine to quench the photoluminescence of light-emitting silicon in three different structural environments. These include porous silicon fabricated from p-type substrates; porous silicon from n-type substrates, and Si nanocrystallites derived from porous silicon. Both types of porous Si substrates were characterized by conventional transmission electron microscopy, while the Si nanocrystallites were characterized by high-resolution transmission electron microscopy. The fractional changes in photoluminescence as a consequence of amine addition were measured and fitted to a static equilibrium model from which adduct formation constants were calculated. It is found that the Si lumophores embedded in the porous oxides on n- and p-type substrates interact similarly with n-propylamine and ethylenediamine; however, ultrasonic extraction of the nanocrystalline Si fragments from the porous oxide layers and subsequent exposure to these amines reveals a near order of magnitude difference between the equilibrium constants for n-propylamine (log K ∼ 4.9) and ethylenediamine (log K ∼ 5.8) addition.
Introduction The discovery of visible light emission from porous Si (PS)1 has stimulated an extensive number of studies designed to understand the fundamental mechanism of its luminescence and exploit it in selected optoelectronic applications.2 Kinetic quantum confinement appears to be the fundamental photoluminescence (PL) mechanism operative in porous Si. Radiationless Auger-type processes are suppressed in Si nanocrystals,3 although the precise role of recombination from trapped carriers at the surface remains a point of some debate.4 One specific area of expanding interest involves probing the effects of exposure of the surface of porous silicon to small molecules and the resultant effects on the photoluminescence. Such experiments are relevant in providing indirect information not only on the accessibility of surface states in this semiconductor but also on the feasibility of its use as a chemical sensor. One of the first studies in this area was reported by Sailor and co-workers, who found that a number of solvent molecules reversibly quenched the luminescence of porous Si;5 the extent of the PL quenching was found to correlate with the dipole moment of the solvent. In a subsequent report, mild oxidation of the PS surface was found to increase the ability of water vapor to quench the PL.6 Bocarsly and co-workers have noted the quenching of porous Si PL upon exposure to a number of Brønsted bases, with such quenching reversible with exposure to a weak acid such as trifluoroacetic acid.7 Other reports have described the quenching of porous Si PL by metal ions8 and polycyclic organic species.9 A recent report by Rehm et al. describes the differences in red- and blue-emitting porous Si, each with a different surface composition and morphology, exposed to methanol.10 In our laboratories, we have focused on studies probing both steric and electronic effects in the quenching of porous Si light * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00806-4 CCC: $12.00
emission by using Lewis bases such as amines. Initially, porous Si prepared by stain-etch techniques was employed as a substrate, but exposure of this type of PS to an amine was found to irreversibly deplete the concentration of silicon hydride surface species upon quenching.11 A more stable porous Si stain film has been recently developed, however.12 Subsequent measurements on relatively more robust anodized porous Si samples have demonstrated that while comparable percentages of porous Si PL can be quenched for a particular p-type substrate with equivalent amounts of n-propyl-, n-butyl-, or n-pentylamine, the restoration of PL over time shows that the PS substrate discriminates between n-pentylamine and n-propyl/nbutylamine(s).13 These results suggested that the ability of small amines to quench the PL of porous Si and subsequent restoration of light emission are a function of the steric bulk of the amine. Complementary to these previous studies is a comparison of the PL quenching ability of two or more organoamines of comparable steric volume, but which differ by the number of reactive functional groups per molecule. By examining different types of porous Si samples in this type of experiment, it is hoped that additional information regarding the interrelationship between size and Lewis base character of a quencher molecule and the porous Si substrate structure can be ascertained. Thus, we report here results for the following luminescent Si systems: (a) porous silicon fabricated from p-type substrates, (b) porous silicon fabricated from n-type substrates, and (c) Si nanocrystallites derived from p-type porous silicon. These three materials are probed with respect to PL quenching by both the monodentate amine n-propylamine and the bidentate amine ethylenediamine. The choice of these particular Lewis bases is motivated in part by previous work by Ellis and co-workers, who demonstrated that diamines such as ethylenediamine are capable of chelation to the II-VI single-crystal semiconductor surfaces of CdS(e).14 For our particular studies with luminescent silicon, the percentage of PL quenched by a given organoamine at saturation for each substrate is one criterion of these matrix © 1996 American Chemical Society
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J. Phys. Chem., Vol. 100, No. 32, 1996 13777
effects. For each substrate the incremental changes in PL observed during titration experiments were measured and fit to an equilibrium model from which adduct formation constants were calculated. Experimental Section Porous Si Preparation. Rectangular pieces (5 × 15 mm) of p-type, , boron-doped CZ Si (6-8 Ω‚cm) and n-type, , phosphorus-doped CZ Si (2-4 Ω‚cm) were employed in a lateral anodic etch process. The samples were etched galvanostatically at 7 mA/cm2 in a 1:1 solution of 48% HF: 95% ethanol for 5 min. After etching, the wafers were rinsed with ethanol and dried in a stream of nitrogen. Crystallites Derived from Porous Si. Colloidal solutions of crystallites derived from anodically etched porous Si wafers from p-type substrates were prepared according to the method of Sailor and co-workers.15 Porous Si wafers were anodically etched as described above, then the wafers were suspended in degassed toluene (under N2) and sonicated for 90 min. During sonication, crystallites leached out from the porous Si network, and the suspension became pale yellow. Titrant Solutions. Solutions (0.01 M) of each titrant (npropylamine (98%) and ethylenediamine (98%) (Mallinckrodt, Organic Reagents)) were prepared in heptane for wafer titration experiments and in toluene for crystallite titration experiments. All amines were distilled prior to use. Instrumentation. A Branson 5200 Ultrasonic cleaning apparatus was used to prepare the crystallite suspensions. Steady-state PL measurements were recorded using a Spex Fluorolog-2 0.22 m double spectrometer at an excitation wavelength of 375 nm. All emission spectra were corrected for fluctuations in photomultiplier tube response. All Si samples were illuminated with a Xe lamp until steady-state PL intensity was reached. Infrared spectra were recorded on a Midac systems FT IR with DTGS detector at a resolution of 4 cm-1. Electron microscopy was performed using a JEOL 200CX operating at 200 kV and a top-entry stage Hitachi H-9000 highresolution electron microscope operating at 300 kV. Samples were analyzed using a combination of bright field, dark field, lattice imaging, and electron diffraction. Amine Titration Experiments. PL measurements were performed on Si crystallite solutions in a 1 cm fluorescence cuvette with the emission monitored at a right angle to the excitation beam. PL measurements were performed on anodically etched porous Si wafers by cutting the wafers into 5 × 5 mm pieces and placing them in a tapered Teflon holder with a small window at the end; the samples were secured with a small nylon screw. The holder was designed to fit snugly into a 1 cm fluorescence cuvette. Mounted samples were immersed in heptane under UV irradiation for about 40 min until a steadystate condition was achieved. The holder possesses a small (1 mm) orifice by which dilute heptane solutions of the organoamines can be added during titration. Great care was taken not to disturb the wafer position or to introduce additional oxygen into the cell at any time during a titration. Results reported are the average of three or more runs. Results and Discussion The relatively high internal surface area of porous silicon and the large number of accessible surface states make luminescence quenching processes quite feasible in this nanophase material, through either static or dynamic mechanism(s) (interfacial energy or electron transfer). It is anticipated that the morphology of porous Si (i.e., the distribution of pore sizes within the network structure) should have a profound impact
Figure 1. PL spectra of anodically etched porous Si, exposed to (i) 0.0, (ii) 1.3 × 10-6, (iii) 1.3 × 10-5, and (iv) 2.5 × 10-4 M solutions of ethylenediamine in heptane.
TABLE 1: Formation Constants (Kf) and Percent Quenching (%Q)a for the Three Types of Substrates with Respect to Two Types of Titrant Solutions (Standard Deviations are Listed in Parentheses) PS from p-type wafers titrant type
log Kf
%Q
PS from n-type wafers log Kf
%Q
Si crystallites log Kf
%Q
n-propylamine 4.97(10) 77(7) 5.01(7) 84(2) 4.92(10) 79(12) ethylenediamine 4.97(3) 82(8) 4.82(7) 70(18) 5.83(2) 90(4) a
%Q ) [ PL1 - PLsat/PL1] × 100%.
on the ability of small molecules to affect the PL. Even prior to the discovery of its ability to efficiently emit visible light, porous silicon was known to possess a complex morphology. This morphology is typically viewed in terms of two extremes, either a “wire” or a “sponge” network. 16 The precise type of pore morphology obtained generally depends on the type and concentration of dopant in the starting silicon substrate and the anodization conditions. According to Smith,17 porous silicon derived from p-type substrates possesses extremely small pore diameters and interpore spacings (1-5 nm), characterized as a randomly directed pore network. For porous Si from n-type substrates the pore diameters are considerably larger than those for p-type silicon and tend to form straight channels or colums, especially at low dopant concentrations. The differences between these two extremes can be blurred, however, by moderate dopant levels or extended anodization times. A radically different structural host for the Si nanoparticles can be achieved by totally extracting the Si nanocrystallite lumophore from its porous oxide matrix, leaving behind a transparent colloidal solution of the Si nanocrystals in a solvent. The first series of experiments in this study involved a comparison of PS from p-type substrates with a porosity of 65% exposed to n-propylamine and ethylenediamine. As with other amines, the addition of a dilute solution of either reagent results in a substantial reduction of the visible PL intensity of porous Si near 620 nm; a gradual increase of amine concentration results in further quenching of PL until a steady-state level is reached (Figure 1). The maximum percentage of integrated PS PL which may be quenched by these two amines is approximately 80%, with the bidentate amine quenching a small and statistically insignificant larger amount of light emission (Table I). Unlike small aromatic hydrocarbons, plots of amine quencher concentration do not satisfactorily fit a simple Stern-Volmer model, as some curvature is observed in plots of I0/I versus amine concentration. This curvature is not attributed to irreversible surface chemical processes in the porous Si matrices, however, since the porous Si PL is almost completely reversible and can be restored with time.13 Another possible criterion for a quantitative evaluation of the differences between these particular amines is an estimation of
13778 J. Phys. Chem., Vol. 100, No. 32, 1996
Sweryda-Krawiec et al.
Figure 2. Θ (fractional decrease in PL using eq 4) as a function of ethylenediamine concentration. Inset shows the corresponding doublereciprocal plot (1/Θ vs 1/C); adduct formation constants are calculated from the slope of the double-reciprocal plot using eq 6.
the strength of surface adduct formation. Fractional changes in PL intensity have been previously employed to monitor adduct formation of organoamines/ phosphines/arsines and related derivatives with single-crystal bulk CdS(e)14,18 and quantum-confined CdS nanoparticles.19 Assuming a uniform surface binding site and a static quenching mechanism, we can calculate adduct formation constants (Kf) for the following equilibrium:
PSsurface site + base f PS-base adduct Kf )
[PS-base adduct] [PSsurface sites][base]
(1) (2)
The concentration of adducts formed is assumed to be proportional to the change in PL peak area observed after base addition, and the total number of available surface sites is proportional to the maximum change in PL intensity. According to the model, the equilibrium constant for adduct formation can be expressed as
Kf )
∆PL [(PL1 - PLsat) - ∆PL]C
(3)
where ∆PL ) PL intensity (initial) - PL intensity (at concentration C), PLsat ) PL intensity at saturation, PL1 ) PL intensity of the parent sample, and C ) concentration of base. The fractional change in PL peak area (Θ) after each titrant addition is calculated from the following:
Θ)
∆PL PL1 - PLsat
(4)
Substitution of Θ into the Kf expression yields
Θ (1 - Θ)C
(5)
KfC 1 1 or )1+ 1 + KfC Θ KfC
(6)
Kf ) which can be rewritten as
Θ)
A plot of fractional change in PL intensity (Θ) versus C yields a plot which follows that of a Langmuir-type adsorption isotherm. The value of Kf is determined from the slope of a double-reciprocal plot of 1/Θ versus 1/C (see inset, Figure 2). Figure 2 illustrates typical results for porous Si derived from p-type substrates of 65% porosity and quenched with ethylene-
Figure 3. Cross-sectional transmission electron micrographs of (a) porous Si derived from p-type substrates by 5 min anodization of CZ Si and (b) porous Si derived from n-type substrates, also by a 5 min anodization process.
diamine. Note that the intercept of the double-reciprocal plot is near unity, as required by the model. There is some slight curvature observed in the double-reciprocal plot of this figure, which occurs only in the case of the bidentate ethylenediamine. It is believed that such curvature arises in part because of competitive multisite binding processes which are possible with this adsorbate only at relatively high concentrations in a restricted porous environment. The log Kf values for these two amines with regard to this particular type of porous Si surface are identical (4.92); thus, it is concluded that the second amino terminus in ethylenediamine does not play a measurable role in the observed PL quenching processes, which is most likely a reflection of the steric hindrance of the porous matrix. In order to probe possible morphological differences due to dopant type, the results for p-type porous Si of 65% porosity were compared with those for n-type porous Si of comparable porosity (52%). As shown in Table 1, the maximum percentage of integrated PS PL quenched by n-propylamine is slightly larger than that of ethylenediamine (84% versus 70%), but the standard deviation for ethylenediamine is larger than this difference. This observation is mirrored in the values of Kf for these two amines with the n-type PS substrate, with the bidentate amine value of 4.82(7) statistically equivalent to the mondentate amine value of 5.01(7) (Table 1). These results are consistent with crosssectional analyses of these two substrates by transmission electron microscopy (TEM). Typical micrographs are shown in Figure 3. To a first approximation, there are no appreciable differences in morphology between the two substrates at the nanometer scale. This is consistent with the virtually identical quenching behavior demonstrated by these two PS substrates; i.e., there is no evidence for significant interaction of two amino groups of ethylenediamine simultaneously interacting with the porous Si surface. As a control experiment, one can also examine the effects of exposure of n-propylamine and ethylenediamine to a colloidal suspension of Si nanocrystallites which have been ultrasonically extracted from anodically etched porous Si layers.15 These nanocrystallites have PL properties essentially the same as the parent PS layers supported by the crystalline substrate, with the exception that the luminescent Si centers are no longer sterically confined in a PS network. A typical high-resolution electron micrograph (HREM) of the Si nanoparticles extracted from
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J. Phys. Chem., Vol. 100, No. 32, 1996 13779
Figure 4. Typical high-resolution electron micrograph of Si nanoparticles extracted from porous Si.
Figure 6. Infrared spectra of a typical anodized porous silicon substrate and silicon crystallites employed in this work in the region of 4000 to 400 cm-1: (i) freshly prepared sample after UV illumination to steadystate photoluminescence intensity; (ii) silicon crystallites in KBr.
Figure 5. Histogram illustrating the distribution of particle sizes for the Si nanoparticles extracted from a porous Si substrate.
porous Si is shown in Figure 4. The nanocrystalline Si remnants in these images appear as small appendages of nanometer size emanating from larger crystalline Si particles, which are very irregularly shaped. Selected area electron diffraction patterns (SADPs) of these crystalline particles confirm the expected lattice spacings for nanocrystalline Si in the diamond cubic phase. A size distribution analysis (Figure 5) of over 300 particles shows an average particle size of 4.6 nm, with a relatively broad size distribution. If the above observations are indeed a consequence of steric constriction of the luminescent Si nanoparticles in the porous oxide matrix, then, in principle, extraction of the Si lumophores into an organic solvent should yield quite different results for the bidentate amine, ethylenediamine, as compared to n-propylamine. As with the other substrates, the incremental PL changes in Si lumophore intensity of the nanocrystals as a consequence of n-propylamine or ethylenediamine addition were also fitted to the Langmuir model, and formation constants Kf were calculated. The average log Kf values and percent quenching values of luminescent Si PL at saturation (%Q) for these crystallites with respect to both amines are presented in Table I. The Kf values for the interaction of n-propylamine with these nanocrystals are comparable to the Kf’s for n- and p-type porous Si. However, there is a clear difference between the log Kf values for the Si nanocrystals exposed to n-propylamine and ethylenediamine: log Kf ) 4.92 and 5.83, respectively. The percent quenching values of luminescent Si PL at saturation are also somewhat different, with a slightly larger value for ethylenediamine (90%) than for n-propylamine (79%); once again large standard deviations mask these differences.
Given comparable surface chemical composition(s), one anticipates a priori that the Si crystallites suspended in an organic solvent are more accessible to quenching by Lewis bases such as amines. An examination of the relevant IR spectra for a typical porous Si film along with that of Si nanocrystallites extracted from PS (and recorded as a KBr film) is shown in Figure 6. A mixture of surface oxide and hydride moieties is found in each case; hence, our assumptions with regard to surface composition are assumed to be valid. It should be noted that the oxide surrounding the Si nanoparticles apparently is partially comprised of Si hydroxide moieties; however, this is not considered to be a dominant factor in the differences between log Kf values for ethylenediamine with regard to the different Si host environments, since comparable log Kf values for n-propylamine exposure are obtained for both PS substrates and Si nanoparticles derived from porous Si. The larger formation constant value for silicon nanocrystallites suspended in solution and exposed to ethylenediamine relative to the corresponding value for n-propylamine can be interpreted in terms of the ability of a bidentate Lewis base to have additional interactions on a nanocrystallite surface. There are two possibilities: first, a true “chelate” effect when both amino groups interact with a single Si atom (unlikely given the local Si coordination environment), or second, both amino groups of a single ethylenediamine molecule interact with the surface and bridge surface acidic sites located on different surface atoms. In either case a larger Kf value is expected compared to that of a monodentate amine. Given our observation of an order of magnitude difference between adduct formation constants for ethylenediamine and n-propylamine for Si nanocrystallites, we believe that the data most likely imply some type of bridging interaction. As observed in the interaction of these two amines with singlecrystal CdS(e) surfaces, a true “chelate” effect would produce an adduct formation constant at least 2 orders of magnitude larger in the case of a bidentate amine.14 Conclusions The results reported here complement earlier studies of the quenching of porous Si visible PL by amines by examining the effects of the PS substrate on PL quenching behavior for amine molecules of comparable steric volume. For PS substrates prepared at short anodization times and at relatively low current
13780 J. Phys. Chem., Vol. 100, No. 32, 1996 densities, one cannot distinguish between the monodentate n-propylamine and bidentate ethylenediamine and their ability to quench the visible PL of porous Si. However, for Si nanoparticles ultrasonically liberated from the porous oxide matrix, there is an appreciable difference in quenching behavior with respect to adduct formation constants for these monodentate and bidentate amines that may be related to a bridging interaction. Acknowledgment. The authors gratefully acknowledge financial support of this research by the Robert A. Welch Foundation and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. Support by the Texas Advanced Technology Program is also gratefully acknowledged. References and Notes (1) Canham, L. T. Appl. Phys.Lett. 1990, 57, 1046. (2) (a) Iyer, S.; Collins, R.; Canham, L. Mater. Res. Soc. Symp. Proc. 1992, 256, 2. (b) Fauchet, P.; Tsai, C. C.; Canham, L.; Shimizu, I.; Aoyagi, Y. Mater. Res. Soc. Symp. Proc. 1993, 283, 3. (c) Tischler, M., Collins, R.; Thewalt, M.; Abstreiter, G. Mater. Res. Soc. Symp. Proc. 1993, 298, 3. (3) Brus, L. E.; Szajowski, P. F.; Wilson, W. L.; Harris, T. D.; Schupler, S.; Citrin, P. H. J. Am. Chem. Soc. 1995, 117, 2915. (4) (a) Kanemitsu, Y. Phys. ReV. 1993, B48, 12357. (b) Kanemitsu, Y.; Ogawa, T.; Shiraishi, K.; Takeda, K. Phys. ReV. 1993, B48, 4883. (c) Kanemitsu, Y. Phys. ReV. 1994, B49, 16845.
Sweryda-Krawiec et al. (5) Lauerhaas, J. M.; Credo, G.; Heinrich, J.; Sailor, M. J. J. Am. Chem. Soc. 1992, 114, 1911. (6) Lauerhaas, J. M.; Sailor, M. J. Science 1993, 261, 1567. (7) Chun, J.; Bocarsly, A.; Cottrell, T.; Benzinger, J.; Lee, J. J. Am. Chem. Soc. 1993, 115, 3024. (8) Andsager, D.; Hilliard, J.; Hetrick, J. M.; AbuHassan, L. H.; Plisch, M.; Nayfeh, M. H. J. Appl. Phys. 1993, 74, 4783. (9) Ko, M. C.; Meyer, G. J. Chem. Mater. 1995, 7, 12. (10) Rehm, J. M.; McLendon, G. L.; Tsybescov, L.; Fauchet, P. M. Appl. Phys. Lett. 1995, 66, 3669. (11) Coffer, J. L.; Lilley, S. C.; Martin, R. A.; Files-Sesler, L. J. Appl. Phys. 1993, 74, 2094. (12) Chandler-Henderson, R. R.; Coffer, J. L. J. Electrochem. Soc. 1994, 141, L166. (13) Chandler, R. R.; Sweryda-Krawiec, B.; Coffer, J. L. J. Phys. Chem. 1995, 99, 8851. (14) Lisensky, G. C.; Penn, R. L.; Murphy, C. J.; Ellis, A. B. Science 1990, 248, 840. (15) Heinrich, J. L.; Curtis, C. L.; Credo, G. M.; Kavanagh, K. L.; Sailor, M. J. Science 1992, 255, 66. (16) Lehman, V.; Gosele, V. AdV. Mater. 1992, 4, 114. (17) Smith, R. L.; Collins, S. D. J. Appl. Phys. 1992, 71, 8 (18) Meyer, G. J.; Lisensky, G. C.; Ellis, A. B. J. Am. Chem. Soc. 1988, 110, 4914. (19) (a) Dannhauser, T.; O’Neil, M.; Johansson, K.; Whitten, O.; McLendon, G. J. Phys. Chem. 1986, 90, 6074. (b) Chandler, R. R.; Coffer, J. L.; Atherton, S. J.; Snowden, P. T. J. Phys. Chem. 1992, 96, 2713, (c) Chandler, R.; Coffer J. L. J. Phys. Chem. 1991, 95, 4.
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