Surfactant-Induced Modulation of Light Emission in Porous Silicon

Jul 26, 2006 - (1) Uhlir, A., Jr. Bell System Technical Journal 1956, 35, 333-347. (2) Turner ..... first monolayer of oxide requires breakup of the S...
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Anal. Chem. 2006, 78, 6058-6064

Surfactant-Induced Modulation of Light Emission in Porous Silicon Produced by Metal-Assisted Electroless Etching Soma Chattopadhyay and Paul W. Bohn*

Department of Chemistry, Beckman Institute for Advanced Sciences and Technology, and the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801

Photoluminescent porous silicon (PSi) was produced by Pt-assisted electroless chemical etching of p--Si in a 1:1:2 (v/v/v) solution of HF, methanol, and H2O2. Upon irradiation with ultraviolet light PSi produced under these conditions luminesces with a peak emission near 590 nm that is sufficiently intense to be visible by eye. Because PSi light emission is an attractive modality for chemical sensing, the effect of charged surfactant adsorbates on the photoluminescence (PL) intensity was investigated. PSi was exposed to aqueous solutions of cationic, cetyltrimethylammonium bromide (CTAB), and anionic, sodium dodecyl sulfate (SDS), surfactants as a function of solution concentration and pH. Adsorption produces both chemical and physical changes at the PSi-solution interface, which were followed by a combination of PL and infrared absorption spectroscopy. Luminescence is quenched in the presence of CTAB and enhanced in the presence of SDS, both in a pH-dependent manner, the behavior being explained by a depletion layer model. PSi crystallites generated from p-Si exhibit a hole-depletion layer at the Si-solution interface, and the depletion layer expands in the presence of cationic surfactant and contracts in the presence of anionic surfactant. Because the surface depletion region is nonemissive (dead layer), surfactant adsorbate-induced modulation of the depletion layer width determines the luminescence intensity of PSi. At very basic pH, PL quenching was observed independent of surfactant identity or concentration, an observation likely tied to the dissolution of the PSi nanocrystallites in strong base. Porous silicon (PSi) was first reported by Ulhir,1 followed by Turner,2 during electropolishing studies of silicon, but it received widespread attention only after Canham reported visible room temperature luminescence from PSi in 1990.3 Light emission from PSi is particularly intriguing, because bulk crystalline Si, being an indirect band gap semiconductor, is an inefficient emitter. In addition to obvious applications involving optoelectronics, significant effort has been expended in developing applications that * To whom correspondence should be addressed. E-mail: [email protected]. (1) Uhlir, A., Jr. Bell System Technical Journal 1956, 35, 333-347. (2) Turner, D. R. J. Electrochem. Soc. 1958, 105, 402-408. (3) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046-1048.

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exploit the ease of fabrication, high surface area, biocompatibility,4 and the options for facile chemical derivatization5,6 to produce tailored surfaces characteristic of PSi. The special material properties of PSi have been exploited for applications in chemical7-12 and biochemical sensing,13-16 as a sacrificial layer in micromachining applications,17 matrix-free mass spectrometry of biological molecules,18,19 and as compliant substrates in epitaxial growth of semiconductors.20 PSi has been prepared by a variety of approaches, but it is most commonly prepared by anodic (electrochemical) etching in HF-based solutions.21 Alternatively, PSi can be produced without any electrochemical bias by stain etching,22,23 in which oxidative dissolution takes place in the presence of HNO3 or other strong oxidizing agents. Distinct from these two approaches, a new fabrication technique for PSi, metal-assisted electroless etching, (4) Canham, L. T.; Stewart, M. P.; Buriak, J. M.; Reeves, C. L.; Anderson, M.; Squire, E. K.; Allcock, P.; Snow, P. A. Phys. Status Solidi A 2000, 182, 521525. (5) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. 1998, 37, 3257-3260. (6) Buriak, J. M.; Allen, M. J. J. Lumin. 1999, 80, 29-35. (7) Le´tant, S. E.; Sailor, M. J. Adv. Mater. 2000, 12, 355-359. (8) Zangooie, S.; Bjorklund, R.; Arwin, H. Sens. Actuators, B 1997, 43, 168174. (9) Taliercio, T.; Dilhan, M.; Massone, E.; Foucaran, A.; Gue´, A. M.; Bretagnon, T.; Fraisse, B.; Monte`s, L. Sens. Actuators, A 1995, 46, 43-46. (10) Le´tant, S. E.; Content, S.; Tan, T. T.; Zenhausen, F.; Sailor, M. J. Sens. Actuators, B 2000, 69, 193-198. (11) Archer, M.; Christopherson, M.; Fauchet, P. M. Sens. Actuators, B 2005, 106, 347-357. (12) Mulloni, V.; Pavesi, L. Appl. Phys. Lett. 2000, 76, 2523-2525. (13) Lin, V. S.-Y.; Motesharei, K.; Dancil, K.-P. S.; Sailor, M. J.; Ghadiri, M. R. Science 1997, 278, 840-843. (14) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925-7930. (15) Starodub, V. M.; Fedorenko, L. L.; Sisetskiy, A. P.; Starodub, N. F. Sens. Actuators, B 1999, 58, 409-414. (16) Thust, M.; Scho ¨ning, M. J.; Frohnhoff, S.; Arens-Fischer, R.; Kordos, P.; Lu ¨ th, H. Meas. Sci. Technol. 1996, 7, 26. (17) Hedrich, F.; Billat, S.; Lang, W. Sens. Actuators, A 2000, 84, 315-323. (18) Wei, J.; Buriak, J. M.; Siuzdak, G. Nature 1999, 399, 243-246. (19) Kruse, R. A.; Li, X.; Bohn, P. W.; Sweedler, J. V. Anal. Chem. 2001, 73, 3639-3645. (20) Soldatenkov, F. Y.; Ulin, V. P.; Yakovenko, A. A.; Fedorova, O. M.; Konnikov, S. G.; Korol’kov, V. I. Technol. Phys. Lett. 1999, 25, 852-854. (21) Cullis, A. G.; Canham, L. T.; Calcott, P. D. J. J. Appl. Phys. 1997, 82, 909965. (22) Fathauer, R. W.; George, T.; Ksendzov, A.; Vasquez, R. P. Appl. Phys. Lett. 1992, 60, 995-997. (23) Sarathy, J.; Shih, S.; Jung, K.; Tsai, C.; Li, K.-H.; Kwong, D.-L.; Campbell, J. C. Appl. Phys. Lett. 1992, 60, 1532-1534. 10.1021/ac060411j CCC: $33.50

© 2006 American Chemical Society Published on Web 07/26/2006

has been developed24 in which a thin film of metal, typically Pt, 3 nm e d e 20 nm, is deposited onto crystalline Si prior to immersion in an etchant composed of HF and H2O2 in methanol. PSi is produced from crystalline Si in seconds, in the dark without any external circuit and in principle is much simpler than anodization. Upon irradiation with ultraviolet light, PSi emits in the visible spectral range, and the peak emission wavelength can be tuned simply by varying the time of etching.25 A key characteristic of the metal-assisted electroless etching approach is the manner in which the deposited Pt directs the spatial distribution of etched material, making it possible to pattern PSi reproducibly. Pt may be patterned in a direct-write process by focused ion beam-assisted degradation of an organometallic Pt precursor and combined with electroless etching to pattern PSi submicrometer characteristic dimensions.26 The simplicity of fabrication and ability to pattern PSi produced by metal-assisted electroless etching opens up the possibility of creating sensing devices, such as addressable in-plane arrays. However, to realize the full potential of PSi luminescence in chemical sensing, a comprehensive understanding of luminescence efficiency in different media is necessary. Pristine PSi exhibits a hydrogen-terminated surface that slowly oxidizes upon prolonged exposure to the atmosphere, meaning that the initially prepared hydrophobic surface slowly acquires a hydrophilic character. Such chemical lability may be detrimental to the application of PSi in chemical sensing, and efforts to counter it have focused on methods to stabilize the surface chemically.27 However, many successful surface stabilization methods for PSi adversely affect the luminescence properties. For the implementation of PSi as a sensor material, a thorough understanding of how its luminescence properties evolve over time and how that temporal behavior depends on the nature of the PSi surface is crucial. Furthermore, since the light emission properties of PSi are highly modulated by the preparation conditions, understanding how the surface affects the luminescence behavior of PSi prepared by metal-assisted electroless etching is critical to realize its potential applications in chemical and biochemical sensing. An additional concern with PSi in contact with liquids is the ability of the solvent to infiltrate the pores, especially for high surface energy solvents, such as water. The study of surfactant effects on PSi luminescence intensity28-30 is motivated by the fact that surfactants might be necessary in chemical sensing applications to fully exploit the high surface area of PSi and that surfactant adsorption as a function of concentration, chain length, pH, and ionic strength has been often used to probe the surface properties of oxide materials.31 Moreover, the presence of adsorbed electrostatic charge, such as that present on cationic and anionic surfactants, may influence the light emission characteristics of (24) Li, X.; Bohn, P. W. Appl. Phys. Lett. 2000, 77, 2572-2574. (25) Chattopadhyay, S.; Li, X.; Bohn, P. W. J. Appl. Phys. 2002, 91, 6134-6140. (26) Chattopadhyay, S.; Bohn, P. W. J. Appl. Phys. 2004, 96, 6888-6894. (27) Stewart, M. P.; Buriak, J. M. Adv. Mater. 2000, 12, 859-869. (28) Shane, M. J.; Heinrich, J. L.; Smith, R. C.; Sailor, M. J. Proc. Electrochem. Soc. 1996, 95-25, 278-285. (29) Canaria, C. A.; Huang, M.; Cho, Y.; Heinrich, J. L.; Lee, L. I.; Shane, M. J.; Smith, R. C.; Sailor, M. J.; Miskelly, G. M. Adv. Funct. Mater. 2002, 12, 495-500. (30) Bjorklund, R. B.; Zangooie, S.; Arwin, H. Langmuir 1997, 13, 1440-1445. (31) Hough, D. B.; Rendall, H. M. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983.

PSi. For example, chemically induced changes in the depletion or accumulation conditions in nanostructured materials exhibit increased sensitivity when compared to planar surfaces. Nanotube FET gas sensors have been demonstrated that employ the binding of electron-withdrawing NO2 or electron-donating NH3 to gate electronic conduction,32 and the binding of negatively charged streptavidin has been detected by biotin-modified p-type Si nanowires, by modulating the width of the depletion region.33 In other words, electron donors or acceptors or charges bound to semiconductor nanostructures modify the depletion or accumulation condition at the interface, which is then manifested in altered electrical properties. This work focuses on how the photoluminescence of PSi is altered by exposure to anionic and cationic surfactants as a function of pH and surfactant concentration. The observed changes in emission intensity are interpreted within a depletion layer model. The surface of PSi crystallites produced in these studies exhibits a p-type hole-depletion layer, which is nonemissive. Adsorbed charge may alter the electronic characteristics of the near-surface region by increasing or decreasing the width of the surface depletion region, thereby affecting the efficiency of radiative recombination and, hence, light emission intensity. EXPERIMENTAL SECTION Materials. Wafers of p--Si (100) were cut into small, 1 cm × 1 cm, square pieces and sputter-coated with 60 Å of Pt using a Desk II TSC Turbo Sputter Coater from Denton Vacuum. A mask, with rows of 1.5-mm-diameter holes spaced by 2.5 mm (edge to edge), was used to pattern the metal deposition on the sample wafer. The etchant mixture was prepared by mixing methanol (MeOH) (Aldrich), HF 49% semiconductor grade (Transene Inc.), and H2O2 30% (Fisher) in a 1:1:2 (v/v/v) ratio. The Pt-patterned Si sample was immersed in the etchant for 5 min, after which it was rinsed with copious amounts of MeOH and dried in a stream of N2. Upon irradiation with ultraviolet light, the sample emits in the visible spectral range and is sufficiently intense to be visible to the eye. Detailed investigations regarding luminescence and morphology of electrolessly etched porous silicon has been reported elsewhere.24,25 Prior to surfactant adsorption studies, the samples were stored overnight in a N2-purged container. Cationic surfactant, cetyltrimethylammonium bromide (CTAB) (Fisher), and anionic surfactant, sodium dodecyl sulfate (SDS) (Fisher), were chosen for surfactant adsorption studies on PSi. The adsorption studies were performed at surfactant concentrations both around and below the critical micelle concentrations (CMC), which for CTAB and SDS are 1 and ∼10 mM, respectively, at 25 °C. Surfactant solutions were prepared using deionized (DI) water (F ∼ 18 MΩ cm) and divided into three parts. The pH was adjusted to pH 3, 7, and 10 by addition of H2SO4 (SDS), HCl (CTAB), or NaOH as appropriate. The PSi sample was immersed into the surfactant solution in a quartz fluorometer cuvette (Starna). Photoluminescence (PL) measurements of the immersed PSi sample were conducted in situ by irradiating the sample surface off the initial Pt deposit, with 360-nm excitation in a right-angle configuration on a commercial dual monochromator fluorescence spectrometer (Spex, (32) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622-625. (33) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289-1292.

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to H-termination passivating the surface, since formation of the first monolayer of oxide requires breakup of the Si-H bonds. Graf et al.37 have postulated a mechanism for oxidation of Si in an aqueous environment. Freshly prepared PSi produced from an HF-based solution may contain some Si-F termination in addition to the more prevalent Si-H termination. SiOH groups initially appear on the surface due to fast exchange of Si-F.

Si-F + H2O f Si-OH + HF

(1)

The presence of the OH groups weakens the Si-SiOH back-bond, making it prone to attack by water.

Si-SiOH + H2O f Si-H + OH-Si-OH Figure 1. FT-IR absorption spectra of p-type PSi (a) immediately after metal-assisted electroless etching and (b) after aging for 1 day in the laboratory ambient.

The Si-H bond formed slowly reacts with water according to

Si-H + H2O f Si-OH + H2 model DM1B) with thermoelectrically cooled photomultiplier tube. The peak emission wavelength of a PSi sample etched for 5 min by metal-assisted electroless etching is typically near 590 nm. Integrated PL spectra were obtained at regular intervals after surfactant exposure and plotted after normalizing to unit intensity at time t ) 0. Fourier transform infrared (FT-IR) absorption spectra were acquired at normal incidence using a Digilab FTS-60A spectrometer (Bio-Rad, Cambridge, MA) equipped with liquid-nitrogencooled HgxCd1-xTe detector. The FT-IR chamber was purged with N2 during acquisition of spectra. Typically, spectra were acquired by co-adding 512 scans at a resolution of 4 cm-1. All spectra were referenced to a scan in N2 and baseline-corrected for accurate comparison of peak intensities. FT-IR spectra were obtained from all PSi samples both before and after immersion into the surfactant solution for comparison of effects of surfactant adsorption. RESULTS AND DISCUSSION Evolution of the PSi Surface. Lightly p-doped Si is transparent throughout much of the infrared region, so transmission FTIR taken through the Si sample can reveal the presence of chemical species on the PSi sample surface. Freshly prepared PSi samples immediately after metal-assisted electroless etching are predominantly hydrogen-terminated, but this surface oxidizes with prolonged storage at atmospheric pressure. Figure 1a reveals the baseline-corrected FT-IR spectrum of a freshly prepared PSi sample produced by metal-assisted electroless etching. The spectrum consists of ν(Si-Hx)29,34 stretching modes in the vicinity of 2100 cm-1, a δ(Si-H2) scissor mode around 908 cm-1, and features associated with Si lattice modes34 at 667 and 628 cm-1. Upon aging the sample for a day under ambient laboratory conditions, Figure 1b, some oxidation takes place, as indicated by the growth of the ν(Si-O)35 band centered at 1064 cm-1. The formation of oxide on a Si surface treated in HF is much slower than the untreated surface,36 the kinetic barrier being attributed (34) Gupta, P.; Colvin, V. L.; George, S. M. Phys. Rev. B 1988, 37, 8234-8243. (35) Shih, S.; Jung, K. H.; Kwong, D. L.; Kovar, M.; White, J. M. Appl. Phys. Lett. 1993, 62, 1780-1782. (36) Zhang, X. G. Electrochemistry of Silicon and its Oxide; Kluwer Academic/ Plenum Publishers: Norwell, MA, 2001.

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(2)

(3)

Finally, condensation of the Si-OH groups yields the oxide.

Si-OH + Si-OH f Si-O-Si + H2O

(4)

The PSi produced by metal-assisted electroless etching, being produced in an HF-containing reaction environment, is susceptible to this type of oxidation process, as evidenced by the growth of the ν(Si-O) band in Figure 1. Adsorbate Induced Modulation of PSi Photoluminescence. The influence of adsorbed surfactants on the light emission of PSi can be interpreted in light of the space charge region near the semiconductor surface.38 In general, adsorbed molecules modulate the band bending of semiconductors and change the width of the depletion region. Because the width of the space charge region figures prominently in determining carrier recombination kinetics, any perturbation of the depletion region, for example, by adsorbed species, affects the photoluminescence efficiency. Moreover, in lightly doped Si, small changes of the surface charge can produce large effects in the depletion layer width.11 Immersion of p--Si in solution results in two electrically segregated layers: the interfacial (silicon/solution) ionic double layer and the space charge layer.11,39 The solution double layer mediates the adsorption of ions, to which the semiconductor space charge region responds by compensating the effect of the charge until equilibrium is reestablished. The p--PSi crystallites obtained by metal-assisted electroless etching exhibit a p-type hole depletion layer. In the presence of adsorbed positive charges, the width of the depletion layer increases, whereas the opposite is expected in the case of adsorbing negative charges (cf. Figure 2). Analytical models relating changes in the depletion width to photoluminescence intensity have been proposed by Hollingsworth and Sites40 and by Mettler.41 In both models, the surface (37) Gra¨f, D.; Grundner, M.; Schulz, R. J. Vac. Sci. Technol., A 1989, 7, 808813. (38) Seker, F.; Meeker, K.; Kuech, T. F.; Ellis, A. B. Chem. Rev. 2000, 100, 2505-2536. (39) Bott, A. W. Curr. Sep. 1998, 17, 87-91. (40) Hollingsworth, R. E.; Sites, J. R. J. Appl. Phys. 1982, 53, 5357-5358.

Figure 3. Integrated photoluminescence intensity from PSi samples exposed to pH 3, 7, and 10 solutions of 8.5 µM CTAB as a function of time. All the spectra are normalized to unit intensity at time t ) 0. Solid lines are included as guides to the eye. (inset) Normalized integrated PL intensities from a PSi sample exposed to pH 7 DI H2O. Note the longer time axis.

Figure 2. (a) Schematic illustration of the spatial dependence of the semiconductor energy bands (EV ) valence band edge, EC ) conduction band edge, EF ) Fermi level) showing a hole depletion layer in untreated p-type PSi. Before adsorption from solution, the positively charged surface states are just balanced by the accumulated negative charge in the depletion region of width, wd. (b) Increase in wd upon adsorption of a positively charged adsorbate. (c) Decrease in wd upon adsorption of a negatively charged adsorbate.

depletion region is nonemissive upon photon excitation and is, thus, a dead layer. Electron-hole pairs generated by photoexcitation in the depletion region have a reduced density of carriers available for radiative recombination due to the depletion of majority carriers. This results in increased carrier lifetime, which in turn enhances the probability of nonradiative recombination. Furthermore, the photogenerated electron-hole pairs in the depletion region are swept apart by the near-surface electric field, with minority carriers being driven toward the surface by the field, where nonradiative recombination processes likely dominate. Therefore, the PL arises from radiative recombination in the charge-neutral region in the interior away from the dead layer. By affecting the magnitude of the surface charge density, adsorbates modify the dead layer thickness, wd, which in turn modulates the PL intensity. Ellis42 has proposed a quantitative measure relating the adsorbate-induced PL behavior with the thickness of the dead layer according to

I0 ) exp(-R∆w) Ix

(5)

where I0 and Ix are the PL intensities before and after exposure to the adsorbate, respectively; R is the semiconductor absorptivity; and ∆w ) (w0 - wx) is the change in the dead layer thickness (41) Mettler, K. App. Phys. (Berlin) 1977, 12, 75-82. (42) Geisz, J. F.; Kuech, T. F.; Ellis, A. B. J. Appl. Phys. 1995, 77, 1233-1240.

induced by the adsorbate, where w0 and wx are depletion widths in the untreated and treated samples, respectively. In the presence of adsorbed positive charges, the width of the depletion layer increases, that is wx > w0 making ∆w negative, and the PL intensity, Ix, decreases, whereas for adsorption of negative charges, the opposite holds true. In PSi, the morphology is dominated by nanocrystallites, and as the dimension of the crystallite approaches the surface depletion width, distinctive changes in PL efficiency can be observed. Adsorption of Cationic Surfactants. Solutions of the cationic surfactant CTAB were prepared in 18 MΩ cm DI water at two different concentrations: 1 mM and 8.5 × 10-6 M, at and below the CMC, respectively, and the solution pH was adjusted to the desired value. Further increases in the surfactant concentration invariably increase the concentration of micelles, which are not generally surface-active.31 Changes in the normalized integrated photoluminescence intensities for the PSi sample in [CTAB] ) 8.5 × 10-6 M are plotted as a function of time in Figure 3. There is a gradual loss in the photoluminescence intensity for samples immersed in pH 3 and 7 solutions; however the pH 10 surfactant solution quenches the luminescence almost completely within the first 10 min. Exposure of identically prepared PSi samples to DI water does not give rise to any measurable quenching under identical measurement conditions (cf. Figure 3, inset). Quenching of the luminescence of PSi upon exposure to 1 mM CTAB at all pH values (not shown) is complete and immediate, that is, within the time required to acquire fluorescence data. FT-IR spectra obtained upon emersing the samples from 1 mM solutions after 30 min are shown in Figure 4. Increasing the surfactant concentration from ∼10-5 M to 10-3 M lowers surface tension such that CTAB infiltrates the pores better, and the presence of adsorbed CTAB is signaled by the appearance of bands assigned to aliphatic νC-H stretching modes of the surfactant at 2923 and 2858 cm-1. The intensities of the νSi-H stretching vibrations in the region 2080-2140 cm-1 decrease upon exposure to the surfactant (cf. Figure 1), whereas the νSi-O vibrational mode at 1075 cm-1 is much larger. A new band arises at ∼2250 cm-1, which is ascribed to the hydride of silicon bonded to oxygen Analytical Chemistry, Vol. 78, No. 17, September 1, 2006

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Figure 4. FT-IR absorption spectra of p-type PSi after exposure to (a) pH 3, (b) pH 7, and (c) pH 10 solutions of 1 mM CTAB.

Figure 5. Evolution of the integrated PL intensity from PSi exposed to 20 µM SDS at pH 3, 7, and 10. All the spectra are normalized to unit intensity at time t ) 0. Solid lines are included as guides to the eye.

atoms. The νC-H stretching modes disappear after rinsing the sample with EtOH, indicating that the cationic surfactant CTAB is physisorbed to the surface, as would be expected for this quaternary ammonium surfactant. However, rinsing does not remove any of the oxide bands, indicating that the surface termination of the PSi is altered upon interaction with the surfactant. Close association of water with the PSi surface leads to oxidation of the surface, modifying the Si-H termination. Not surprisingly, oxidation is more aggressive for samples immersed at pH 7 and 10, whereas the intensities of νSi-H and νSi-O indicate that the PSi sample in 1 mM CTAB at pH 3 suffers comparatively less oxidation, which is expected, because the high H+ ion concentration suppresses oxidation. In the depletion layer model, adsorption of the cationic surfactant CTAB should increase wd, thereby increasing the likelihood of nonradiative recombination and quenching the luminescence. For 8.5 × 10-6 M solution, which is relatively dilute, there is moderate quenching at pH 3 and 7, but significant quenching at pH 10. Adsorption from relatively dilute concentration of 8.5 × 10-6 M solution results in a modest, but temporally increasing, number of adsorbates on the surface, thus leading to significant quenching in the 0-30-min time window. Longer time experiments (not shown) give a significantly quenched but nonzero steady-state luminescence at pH 3, whereas the data for pH 7 are already close to their ultimate steady state at t ) 30 min. The differences in the temporal behavior between pH 3 and 7 evident in Figure 3 can be ascribed to the slower kinetics of adsorption at lower pH; however at pH 10, the PL is rapidly and completely quenched, even at low surfactant concentration. This behavior cannot be attributed to the surfactant alone. Comparison of the ratios of the intensities of the νSi-H and νSi-O stretching modes indicates that PSi samples in pH 7 and 10 solutions are substantially oxidized. Not surprisingly, PSi samples immersed in pH 3 surfactant solution are only modestly oxidized, because oxidation is suppressed with increasing H+ ion concentration, following the mechanism of Graf37 et al. Furthermore, increased surfactant concentration lowers the surface tension and facilitates increased wetting of the pores, thereby further aiding the oxidation of the nanocrystallites. Thus, although oxidation clearly dominates at pH 10, it is not possible to apportion the

quenching independently between oxidation and depletion layer effects at pH 3 or 7 and [CTAB] ) 1 mM, since both oxidation and cation adsorption give rise to luminescence quenching. Adsorption of Anionic Surfactant. SDS solutions were prepared in 18 MΩ cm DI water at 20 µM and 10 mM, below and near the CMC, respectively, and the solution pH was adjusted to the desired value. Normalized integrated photoluminescence intensities for the PSi sample in 20 µM SDS solution are plotted in Figure 5. Unlike the cationic surfactant, exposure to anionic SDS solutions does not quench the luminescence of PSi at pH 3 or 7. The luminescence intensity remains largely unchanged for PSi samples in pH 3 and 7 solutions but quenches within 10 min for the PSi sample in pH 10 solution. In the depletion layer, model anionic SDS is not expected to cause quenching of luminescence from PSi samples. Rather, adsorption of negative charges on PSi is expected to decrease the depletion layer width, increasing the photoluminescence intensity. The experiments at 20 µM represent dilute surfactant solutions, and FT-IR spectra (not shown) confirm that minimal SDS adsorption occurs at this concentration. In the absence of appreciable surfactant adsorption, the luminescence of PSi remains largely unperturbed; however, the luminescence from PSi samples immersed in pH 10 solution is gradually quenched, even in the dilute anionic surfactant solution. Photoluminescence measurements from PSi samples immersed in 10 mM SDS solution at pH 3, 7, and pH 10 are shown in Figure 6. Luminescence is enhanced by SDS adsorption from pH 3 and 7 solutions but quenched by immersion in pH 10 solution. FTIR spectra obtained after surfactant adsorption from each of these solutions are shown in Figure 7. Upon exposure to anionic SDS, new bands appear in the FT-IR spectrum; the bands at 2850 and 2920 cm-1 are assigned to symmetric and asymmetric νC-H stretching, respectively. In addition, νSi-H bands can be seen in the vicinity of 2100 cm-1, and an additional νSi-O vibration is seen around 1077 cm-1, although their intensities change following surfactant exposure. Comparison of the intensities of the νSi-H and νSi-O bands shows that there are no appreciable changes in the surface composition at pH 3 or 7. Peaks associated with δC-H bending vibrations around 1467 cm-1 and the bands appearing in the region of 1230 cm-1,

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Figure 6. Evolution of the integrated PL intensity from PSi exposed to 10 mM SDS at pH 3, 7, and 10. All the spectra are normalized to unit intensity at time t ) 0. Solid lines are included as guides to the eye.

Figure 7. FT-IR absorption spectra of p-type PSi after exposure to (a) pH 3, (b) pH 7, and (c) pH 10 solutions of 10 mM SDS.

attributed to the νS-O of the SDS surfactant, are also clearly visible. Comparison of the intensities of the νSi-H and νSi-O reveal that not much oxidation has taken place for the samples exposed to 10 mM SDS at pH 3 and 7. Even for PSi samples exposed to 10 mM SDS at pH 10, the oxidation is not as extensive as that on PSi samples in CTAB solutions, indicating that PSi is less extensively oxidized in SDS than in CTAB. In the presence of adsorbed negative charges arising from adsorption of anionic SDS, the width of the hole depletion layer diminishes in the crystallites of PSi which, at high SDS loadings, enhances PSi luminescence. At pH 3, the PSi surface in surfactant solution is less susceptible to oxidation than at pH 7. Moreover, the negatively charged Si-O- sites are largely protonated at pH 3, and the sulfate moiety is likely partially protonated as well. Upon SDS adsorption, both of these factors lead to greater negative charge buildup in PSi immersed in pH 7 surfactant relative to pH 3. The cumulative negative charge on the PSi surface affects the depletion layer width accordingly, shrinking the depletion width region (cf. Figure 2c) and enhancing the photoluminescence. However, these arguments clearly do not hold at pH 10, in which the luminescence is quenched quickly upon surfactant exposure.

In fact, the results of luminescence and FT-IR studies demand that the behavior of PSi samples in pH 10 solutions be discussed separately. Effect of Residual Pt. Pt plays a crucial role in the metalassisted electroless etching process used here. A very thin (∼3-6 nm) film of Pt is deposited, typically in circular patterns, onto the Si surface prior to etching. As shown previously, electroless etching produces PSi exhibiting an in-plane distribution of morphology and light emission dependent on the location relative to where the Pt was initially deposited.25,26 Naturally, because trace amounts of transition metals are known to quench luminescence, it is important to understand whether residual Pt is playing a role in modulating the luminescence intensities with or without surfactant adsorption. The first piece of evidence is derived from Figure 2 of reference 25, which shows a series of luminescence images of PSi prepared by Pt-assisted electroless etching with varying etch times. These images, which show both the area originally deposited with Pt and the area adjacent to the Pt pads, demonstrate that although the luminescence characteristics change with etch time, i.e., λem shifts to the blue, luminescence is observed both from the area originally deposited with Pt and the area adjacent to the Pt pad at all etch times.25 The images suggest that Pt is removed completely from these large area structures, consistent with XPS experiments (data not shown) that showed that any residual Pt in the original Pt-coated areas was below the ∼1% detection limit for Pt by XPS. Furthermore, in ref 26, small (200 nm < l < 10 µm) PSi pixel areas were prepared using focused ion beam dissociation of trimethyl(methylcyclopentadienyl)Pt to deposit Pt.26 Comparison of the luminescence and Pt auger images from these pixels, namely, Figure S1 (Supporting Information), illustrate that areas containing residual Pt remain dark in the luminescence image, as would be expected if Pt were an effective quencher of PSi luminescence. Finally one might be concerned that the increase in luminescence intensity observed with the anionic surfactant might result from selective complexation of residual Pt by the sulfonate moieties. Control experiments were carried out with the zwitterionic surfactant, 3-(N,N-dimethylmyristylammonio)propanesulfonate, which contains both the sulfonate and quaternary ammonium moieties characteristic of the SDS and CTAB surfactants, respectively. If the sulfonate were complexing residual Pt, one would expect to see the luminescence intensity increase as it does with SDS; however, the luminescence intensity decreases in the presence of the zwitterionic surfactant (cf. Figure S2). Collectively, these data strongly indicate that the level of residual Pt remaining after etching in these large area structures is sufficiently small that it has a negligible effect on the luminescence intensities observed. Luminescence Quenching at pH 10. Irrespective of the concentration or the nature of the surfactant, that is, CTAB vs SDS, PSi luminescence is always quenched in pH 10 solution, which is consistent with the prior work of Chun et al.43 In all likelihood, surfactant adsorption and surface passivation by suppressing oxidation do not influence the luminescence behavior of PSi samples in pH 10 solutions. In these strongly basic solutions, PSi obviously undergoes fundamental changes, causing detrimen(43) Chun, J. K. M.; Bocarsly, A. B.; Cottrell, T. R.; Benziger, J. B.; Yee, J. C. J. Am. Chem. Soc. 1993, 115, 3024.

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tal effects on the luminescence. This is supported by observation that anodic PSi luminescence quenches in aqueous bases and is consistent with a mechanism in which Si is attacked in alkaline solution, forming soluble silicate Si(OH)2O22- in solution36 leading to the overall reaction

Si + 2H2O + 2OH- f Si(OH)2O22- + 2H2

(6)

Thus, in alkaline solution, PSi crystallites may dissolve, forming soluble silicates. Luminescence quenching in alkaline surfactant solutions can then be attributed to loss of the porous nanocrystallite structure. As a result, in alkaline solutions, leaching of the silicon is the dominant effect, overshadowing effects arising from surfactant adsorption and consequent modulation of the depletion layer. CONCLUSIONS Metal-assisted electroless etching produces PSi marked by nanocrystallites of silicon in a porous network that exhibit photoluminescence in the visible spectral range. The ease of fabrication, biocompatibility, and large internal surface area make PSi attractive for developing sensing applications; however, these applications can only be optimally exploited through a comprehensive understanding of the material properties. Freshly prepared PSi is hydrophobic as a result of its H-termination, but it develops progressively more hydrophilic character as the material gradually oxidizes when exposed to the laboratory ambient conditions. Exposure of PSi to solutions of surfactant molecules produces a complex set of chemical and physical changes that are determined by the nature of the surfactant, its concentration, and the pH. Surfactant molecules naturally facilitate increased wetting of the PSi surface and consequent changes to the surface termination and photoluminescence upon adsorption of cationic, CTAB, and anionic, SDS, surfactants. FT-IR spectra reveal that oxidation of PSi in aqueous solutions is suppressed at low pH and favored at high pH, whereas the visible luminescence of PSi is quenched

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by adsorption of cationic CTAB and enhanced by the adsorption of anionic SDS. Such light emission behavior of PSi can be explained in the context of a space charge model, which recognizes that the surface of p-type PSi is characterized by hole depletion. The space charge region responds to the charge felt by the crystallites of PSi, such that adsorbed negative charges shrink the depletion layer, while adsorbed positive charges increase it. Increased depletion width increases the relative efficiency of nonradiative recombination, resulting in decreased photoluminescence intensity in the presence of cationic surfactants, whereas the opposite is true for anionic surfactants. The luminescence behavior of PSi also clearly tracks the surfactant concentration below the CMC, with more dramatic quenching (CTAB) and enhancement (SDS) observed at higher surfactant concentrations. Irrespective of the concentration or whether the surfactant is cationic or anionic, the luminescence from PSi is always quenched upon exposure to pH 10 solutions. This likely indicates dissolution of the PSi nanocrystallites at high pH, an effect that dominates the adsorbed electrostatic charges from cationic or anionic surfactants. ACKNOWLEDGMENT This work was supported by the Air Force Office of Scientific Research under Grant F49620-02-1-0381 and by the Department of Energy under Grant DE FG02 91ER45439. SEM imaging was carried out at the Center for Microanalysis of Materials, supported via the Department of Energy under Grant DE FG02 91ER45439. SUPPORTING INFORMATION AVAILABLE Data relevant to the presence of residual Pt under various experimental conditions is given. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review March 6, 2006. Accepted June 22, 2006. AC060411J