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Growing Monodispersed PbS Nanoparticles on Self-Assembled Monolayers of 11-Mercaptoundecanoic Acid on Au(111) Substrate Peng Jiang,* Zhong-Fan Liu, and Sheng-Min Cai Center for Nanoscale Science & Technology (CNST) and College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China Received December 21, 2001. In Final Form: March 8, 2002 Lead sulfide, PbS, nanoparticles with the average size of 3.2 ( 0.4 nm have been synthesized by exposing self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid adsorbed Pb2+ ions on Au(111) substrate into a H2S atmosphere. The preparation process of the PbS nanoparticles consists of the formation of the SAMs, deprotonation, and Pb2+ ion adsorption of outer surface of the SAMs, followed by a gas/solid chemical reaction. The whole process was traced by infrared reflectance spectra with grazing incidence reflection mode and X-ray photoelectron spectroscopy. In addition, scanning tunneling microscopy and high-resolution transmission electron microscopy images provided the direct proof of formation of the small-sized PbS nanoparticles. A possible nucleation model of PbS growth on the SAM has been suggested.
Introduction The design of novel nanoscaled inorganic/organic composite materials has been a continuing hot point in modern materials science due to their potential application in nonlinear optics, light energy conversion, photocatalysis, and microelectronics. A particular emphasis in the latest research1,2 has been focused on developing new methods to organize metal or semiconductor nanoparticles with defined size, morphology, and orientation into two- or three-dimensional ordered architecture. Among them, the most common route employed involves synthesizing the nanoparticles by a colloidal wetting chemical method and then organizing them on a suitable substrate.3-6 The method usually comprises two separate steps. In particular, coupling them onto a solid substrate to construct the desired devices needs special consideration regarding interaction between the nanoparticles and the substrate. As an alternative, using an organic matrix with a defined structure as template, we can combine two steps into one by directly nucleating a nanoparticle array on various interfaces. Ideally, the nanoparticle film growth is expected to occur only by heterogeneous chemical reaction. Furthermore, nanoparticle crystal structure, orientation, * To whom correspondence should be addressed. Present address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Physical Chemistry, Faradayweg 4-6, D-14195 Berlin, Germany. Tel: (+49)-30-84135133. E-mail: peng@ fhi-berlin.mpg.de. (1) (a) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (b) Alivisatos, A. P. Science 2001, 289, 736. (c) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (2) (a) Wang, Z. L.; Dai, Z.; Sun, S. Adv. Mater. 2000, 12, 1944. (b) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (c) Wang, Z. L. Adv. Mater. 1998, 10, 13. (3) (a) Kiely, C. J.; Fink, J.; Zheng, J. G.; Brust, M.; Bethell, D.; Schiffrin, D. J. Adv. Mater. 2000, 12, 640. (b) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (4) (a) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358. (b) Pileni, M. P. Langmuir 1997, 13, 3266. (5) (a) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545. (b) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335 (6) (a) Jiang, P.; Xie, S. S.; Yao, J. N.; Pang, S. J.; Gao, H. J. J. Phys. D: Appl. Phys. 2001, 34, 2255. (b) Jiang, P.; Xie, S. S.; Yao, J. N.; He, S. T.; Zhang, H. X.; Shi, D. X.; Pang, S. J.; Gao, H. J. Chin. Sci. Bull. 2001, 46, 996.
and size can also be easily controlled by choosing suitable organic matrix and experimental conditions. Many important results have been reported by utilizing the method.7,8 For example, Fendler and co-workers9 have employed ordered Langmuir monolayers at the solution/ air interface to systematically investigate the growing process of a variety of semiconductor nanocrystals. An epitaxial relationship between crystal and monolayer lattices has been deduced for different systems. On the other hand, lead sulfide (PbS) is an important semiconductor material. Bulk PbS belongs to a direct band gap semiconductor with a rather narrow band gap of 0.41 eV and a large exciton Bohr radius of 18 nm. The characteristic implies that one can conveniently tailor dimension of the material to study the effect of size confinement. It has been reported that the band gap of PbS can be widen to a few electronvolts from the bulk value when particle size falls in the nanometer regime.10,11 Thus, a great deal of research effort has been devoted to the method development for the synthesis of PbS particles with various sizes in a controllable manner. In this paper, we explore a new synthetic strategy to prepare nearly monodispersed PbS nanoparticles with the average size of 3.2 ( 0.4 nm by exposing self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid salts modified on Au(111) substrate in an H2S atmosphere. Compared with other methods,8,9 an advantage of the method is that the formation process of the nanoparticles is confined to the initial stage of nucleation over a short period of time due to the limited Pb2+ ions on the outer SAMs surface. No further growth of the nanoparticles takes place over an extended period of time due to the depletion of Pb2+ ions. The process is very facilitated to produce monodispersed PbS nanoparticles in the range of (7) Guo, S. W.; Konopny, L.; Popovitz-Biro, R.; Cohen, H.; Porteanu, H.; Lifshitz, E.; Lahav, M. J. Am. Chem. Soc. 1999, 121, 9589. (8) Onuma, K.; Oyane, A.; Kokubo, T.; Treboux, G.; Kanzaki, N.; Ito, A. J. Phys. Chem. B 2000, 104, 11950. (9) (a) Bekele, H.; Fendler, J. H.; Kelly, J. W. J. Am. Chem. Soc. 1999, 121, 7266. (b) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (10) Kane, S. R.; Cohen, R. E.; Silbey, R. J. Phys. Chem. 1996, 100, 7928. (11) Wang, Y. Acc. Chem. Res. 1991, 24, 133.
10.1021/la015757y CCC: $22.00 © 2002 American Chemical Society Published on Web 04/26/2002
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Figure 1. Schematic representation of nucleation process of PbS nanoparticles.
several nanometers. In particular, it also enables one to investigate initial nucleation kinetics of the nanoparticles. The route consists of three major steps: adsorption of an 11-mercaptoundecanoic acid monolayer on Au(111) substrate, deprotonation of carboxylic groups at the outside surface, and subsequent adsorption of Pb2+ ions, followed by a reaction with H2S gas. The general process of the formation of PbS nanoparticles, as illustrated in Figure 1, has been traced by infrared reflectance spectra (IRS) and X-ray photoelectron spectroscopy (XPS) detection techniques. Furthermore, scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) have also been used to verify the formation of the PbS nanoparticles. Experimental Section Gold Substrates. Au(111) substrate served as STM measurement was prepared as follows.12 A section of 0.5 mm diameter polycrystalline Au wire (∼99.999% Nilaco Co., Tokyo, Japan) was first immersed in “piranha solution” (3:1 concentrated H2SO4/30% H2O2) for cleaning and subsequently rinsed with pure water. After that, the wire was then put into hydrogen-oxygen flame and allowed to melt until a ∼2 mm diameter bead had been formed at the end of the wire, followed rapidly by quenching the bead into ultrapure water. The obtained Au ball provided us with several large flat Au(111) facets on its surface. Gold thin films evaporated on Cr-primed glass slides were used for IRS and XPS characterizations. The substrates were slightly annealed in a hydrogen flame, subsequently precleaned by “piranha solution”, and rinsed with ultrapure water and ethanol before used. Preparation of SAM and PbS Nanoparticles. The fresh gold substrate mentioned above was dipped in ∼1 mM ethanolic solution of 11-mercaptoundecanoic acid (MUA) (synthesized in our laboratory) for 12 h after successive rinsing with deionized H2O and ethanol. The substrate modified by a MUA monolayer was then taken out of this solution, rinsed in pure ethanol, and placed into a 0.1 M NaOH solution to deprotonate for 10 min, followed by rinsing with a great deal of ultrapure water. After that, the substrate was deposited in 10-4 M Pb(NO3)2 (Aldrich) solution to adsorb Pb2+ ions for 6 h via electrostatic interaction. The Pb2+-ion-adsorbed substrate was further washed with ultrapure water, dried in N2, and then exposed into H2S gas for 15 min to form PbS nanoclusters on them. Infrared Reflectance Spectra (IRS). IR spectra of the samples were recorded using a Perkin-Elmer system 2000 FTIR in the grazing incidence reflection mode. The spectrometer was purged by dry nitrogen delivered from a gas source to eliminate the interference of water vapor in air. The incident angle of the p-polarized light was set to 86° relative to the surface normal. The reflected light was detected by a liquid nitrogen cooled MCT detector. Three kinds of Au substrate samples modified by MUA, MUA-Pb2+, and MUA-PbS were selected to perform the experiments. All of measurements were done after 10 min when mounting a new sample. For all measured IRS data, the resolution was set to 4 cm-1 and 1000 scans were accumulated to average to obtain an acceptable signal-to-noise ratio. X-ray Photoelectron Spectroscopy (XPS). XPS (VG ESCALAB 220 i-XL photoelectron spectroscopy) was employed for analyzing the change of chemical composition of the layers grown on the carboxylic thiol SAMs. The samples used for XPS analysis were prepared just like as those used in IRS research. The samples (12) Demir, U.; Shannon, C. Langmuir 1994, 10, 2794.
Figure 2. IRS spectra of (a) MUA, (b) MUA-Pb2+, and (c) MUA-PbS on Au substrates. Table 1. Mode Assignments for the Diagnostic IR Bands (cm-1) in the High- and Low-Frequency Regions for MUA, MUA-Pb2+, and MUA-PbS Monolayers mode assignment
MUA
MUA-Pb2+
MUA-PbS
νa(CH2) νs(CH2) νCdO(CO2H) νa(CO2-)
2920.0 2851.8 1741
2920.1 2851.9
2920.2 2852.0 1718
δ(CH2) νs(CO2-)
1472
1542 1523 1512 1472 1423
1472
purged into a box with N2 gas protection before XPS experiments. XPS measurements were performed using a monochromatized Al (KR) source. The backbone carbon peak at 284.6 eV serves as a reference for final calibration of the energy scale. Scanning Tunneling Microscopy (STM). A commercial NanoScope IIIa (Digital Instruments, DI Inc.) was used to image Au(111) substrate and PbS nanoparticles formed on the Au(111) modified by the MUA monolayer in ambient. All of the STM images shown here were collected in the constant-current mode using an etched Au tip with a typical bias voltage of 130 mV and a tunneling current of 20 pA.
Results and Discussion Infrared Spectra. Grazing angle reflectance infrared spectroscopy is one of a very few methods which can provide information about orientation of a long-chain organic molecule adsorbed on Au substrate.13 In the experiments, the whole process of PbS nanoparticle formation was traced by the technique. Figure 2 shows the IRS spectra of MUA, MUA-Pb2+, and MUA-PbS on Au(111) substrates at high- and low-frequency regions, respectively. The corresponding possible assignments and peak positions have been summarized in Table 1. In the low-frequency region of Figure 2, the most obvious characteristic is that the two peaks at 1741 and 1718 cm-1, assigned to ν(CdO) bands for -COOH groups of free or non-hydrogen-bonded and side-by-side dimeric hydrogenbonded modes,14 respectively, for the MUA monolayer disappear when the SAM is deprotonated and adsorbed by Pb2+ ions (see Figure 2b). At the same time, new bands (13) McKelvy M. L.; Britt T. R.; Davis B. L.; Gillie J. K.; Lentz L. A.; Leugers A.; Nyquist R. A.; Putzig C. L. Anal. Chem. 1996, 68, 93R.
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corresponding to asymmetric (1542, 1523, 1512 cm-1) and symmetric (1423 cm-1) stretching vibrations of -CO2emerge, indicating the group transformation from -COOH to -CO2- at the outer surface of MUA SAMs. However, after the MUA-Pb2+ SAM reacts with H2S gas, the new bands disappear and ν(CdO) bands for -COOH groups occur again, but the intensity of absorbance peak obviously decreases in comparison with that of original MUA SAMs, demonstrating a change of the SAM structure after reaction. In the high-frequency region, two important features can be observed in the spectrum region of the methylene. One is gradual decrease in the absorbance of νa(CH2) (2920 cm-1) and νs(CH2) (2851.8 cm-1) modes (see Figure 2), and the other is slight shift in their frequencies toward higher frequencies (see Table 1), with the change of the monolayer film composition from MUA to MUAPb2+ to MUA-PbS. In general, when ordered SAMs are formed on Au(111) substrate, the transition dipole moment of νs(CH2) is expected to locate in the methylene plane and is orthogonal to the backbone axis of the alkyl chain. In this situation, the alkyl chains will adopt an all-trans conformation.15 According to the selection rule of grazing angle reflectance FTIR on a metal surface: only the transition having a nonzero projection of its dipole moment in the direction of the surface normal can bring about absorption, the decrease of the peak intensities indicates that projection of the dipole moment in the direction perpendicular to the surface gradually reduces, implying that a tilting of the alkyl chain away from the surface normal occurs. In addition to the orientation of the alkyl chains, the frequencies of the C-H stretching vibration of the methylene group can provide information on the packing status of the alkyl chains. The slight increase in the frequencies of these two methylene modes indicates a change in the alkyl chain conformation away from the normal all-trans one, reflecting that the packing status among the hydrocarbon chains is changing toward a looser packed and less ordered style with the process of the reaction. X-ray Photoelectron Spectroscopy Characterization. XPS measurements were performed to gain much more insight into the formation process of PbS nanoparticles by the suggested route. Binding energies and the lines of the various elements such as C(1s), O(1s), Pb(4f), and S(2p) can provide considerable rich information on the headgroup structures of the films. The XPS assignments are summarized in Table 2 and Figure 3. In Figure 3, high-resolution regions of XPS spectra of the C(1s), O(1s), and S(2p) were fitted with multiple Gaussians. For MUA SAMs in the C(1s) region (see Figure 3a), it appears that there are two obvious distinct components at 284.6 and 289.1 eV, which were assigned to the methylene carbon and the carboxyl carbon, respectively. Curve fitting in the O(1s) region resulted in two peaks at 532.1 eV for carboxyl (CdO) oxygen and 533.0 eV for the hydroxyl (-C(O)OH) oxygen. In the S(2p) region, the peaks at 162.1 and 163.4 eV are assigned to the S(2p3/2) and S(2p1/2) photoelectrons.16 Once Pb2+ ions are adsorbed on MUA, it can be seen that the C(1s) signal for the carboxyl carbon in XPS spectrum (see Figure 3b) exhibits a small shift to 288.3 eV and the O(1s) line becomes much more symmetrical and demonstrates a dominant peak at 531.7 eV, (14) (a) Tao, Y.-T.; Lin, W.-L.; Hietpas, G. D.; Allara, D. L. J. Phys. Chem. B 1997, 101, 9732. (b) Kepley, L. J.; Crooks, R. M. Anal. Chem. 1992, 64, 3191. (c) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (15) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (16) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723.
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Figure 3. XPS spectra of C(1s), O(1s) of carbonyl, and S(2p), Pb(4f5/2, 4f7/2) in the MUA (a), MUA-Pb2+ (b), and MUA-PbS (c). Table 2. Binding Energies (eV) and Composition Assignments for XPS Spectra of MUA, MUA-Pb2+, and MUA-PbS assignment
MUA
MUAb-Pb2+
MUA-PbS
C(1s)
284.6 289.1 532.1 533.0 162.1 163.4
288.3
284.6 289.0 532.1 533.0 160.7 161.5 162.1 163.4 138.0 142.9
O(1s) S(2p)
Pb(4f)
531.7 162.1 163.4 138.8 143.6
reflecting the equal chemical environment of the two oxygen atoms after MUA was modified by Pb2+ ions. In comparison, S(2p) peaks seem not to be influenced by the adsorption of Pb2+ ions. In addition, the occurrence of new lines at 138.8 and 143.6 eV, originating from Pb(4f7/2) and Pb(4f5/2), respectively, verifies the existence of Pb2+ ions on the outer surface of the modified MUA. After reaction with H2S gas, O(1s) peaks obtained by multiple Gaussian fitting occur again at 532.1 and 533.0 eV (see Figure 3c). Interestingly, in the S(2p) region, curve fitting produces two new peaks at 160.7 and 161.5 eV. The two peaks can be attributed to the S(2p3/2) and S(2p1/2) due to the product of PbS naoparticles.17 More notably, the peaks corresponding to Pb(4f7/2) and Pb(4f5/2) shift to lower binding energies (138.0 and 142.9 eV, respectively). The shift of the Pb lines reflects the change of chemical environment near Pb2+ ions,18 further accounting for the formation of PbS nanoparticles. STM and TEM Characterizations. STM is a powerful tool for investigating surface microstructure due to its high atomic scale resolution, which is being widely used to image various surfaces. Figure 4a shows a typical STM image of Au(111) facet obtained by melting Au wire in a hydrogen-oxygen flame, on which the monatomic step (17) Takahashi M.; Ohshima Y.; Nagata K.; Furuta S. J. Electroanal. Chem. 1993, 359, 281. (18) Reiche, R.; Thielsch, R.; Oswald, S.; Wetzig, K. J. Electron Spectrosc. Phenom. 1999, 104, 161.
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Figure 4. STM images of a typical of Au(111) substrate (an insert shows section analysis along the white line) (a) and PbS nanoparticles nucleated on MUA SAMs on the Au(111) (an insert represents a distribution of the PbS nanoparticles) (b). Figure 6. Hexagonal coverage scheme for MUA and possible adsoprtion positions for Pb2+ on deprotonated MUA on Au(111) substrate. The open circles represent Au atoms, the gray circles are -COOH groups, and the dark circles are Pb2+ ions.
Figure 5. A typical STM image of an Au tip prepared by electrochemical etching (a) and HRTEM micrograph of a PbS nanoparticle on the apex of the tip (b).
lines are found to intersect at either 60° or 120°. The height of adjacent terraces is about 0.26 nm (see insert Figure 4a), being in agreement with the monatomic step height of Au(111). When the Au(111) substrate was modified by PbS nanoparticles through chemical reaction, a great number of protrusions are observed in the STM image, as shown in Figure 4b. The dots have a representative average size of 3.2 ( 0.4 nm, obtained by STM section analysis from the 100 PbS nanoparticles (see insert in Figure 4b). Interestingly, it can be seen in Figure 4b that almost all of the PbS nanoparticles seem to orient roughly in the same direction. The observation probably suggests that the growth of the PbS nanoparticles is high related to the long-range order of the MUA SAMs on Au(111). To further verify the formation of PbS nanoparticles on the substrate, we also employed TEM to observe the PbS nanoparticles. Due to the limitation of the support being transparent to the electron beam, an Au tip prepared by electrochemical etching had to be used to fit the TEM measurement. The obtained tip usually has an apex with a curvature radius of 10-20 nm, as evidenced by the scanning electron micrograph image (SEM Amray 1919 field emission microscope (USA)) (see Figure 5a). Figure 5b demonstrates a representative TEM image of the tip apex region of the SAM/PbS-modified gold tip, on which a PbS nanoparticle with the size of about 4 nm is clearly found. The distance between the planes was measured to be 0.34 nm, very consistent with that of PbS (111) (d ) 0.3429 nm). On the basis of the characterizations and discussions mentioned above, we propose a possible growth model of PbS nanoparticle on the carboxylic thiol SAMs (see Figure 6). Fendler and co-workers19 have investigated epitaxial growth of PbS microcrystals under arachidic acid (AA) monolayers fabricated at the air-water interface by the Langmuir-Blodgett (LB) technique. Their research re(19) Yang, J. P.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5505.
vealed the growth of PbS from its {111} plane and the formation of oriented equilateral triangular crystallites, which were attributed to the organized LB template and an excellent match between the {111} plane of PbS and the headgroup distance of AA monolayers. In our case, MUA monolayers are expected to form (x3 × x3)R30° structure SAMs on Au(111) substrate. Theoretically, the closely packed hexagonal structure has an expected lattice parameter a ) 5.00 Å.8,20 PbS crystals are known to possess a typical NaCl cubic structure with a lattice parameter of a ) 5.95 Å. The Pb-Pb or S-S spacing (4.20 Å) of the PbS (111) face almost perfectly match the lattice line distance (d ) 4.33 Å) for MUA, as shown in Figure 6. The spatial mismatch between them is only on the order of 3%. Obviously, the growth of PbS nanoparticles on MUA SAMs seems also to obey the epitaxial model suggested by Fendler. Meldrum et al.21 reported a similar synthesis system of PbS nanoparticles, in which thiourea was used as a reactant instead of H2S gas. Surface plasmon spectroscopy, scanning electron microscopy, and TEM have been employed to investigate the nucleation and growth process of PbS nanoparticle film. They did not find an epitaxial relationship between the PbS crystallites and the 16-mercaptohexadecanoic acid self-assembled film substrate and attributed it to the intrinsic limited lateral order of the SAMs. We think that Meldrum and co-workers investigated the initial stage of PbS nanoparticle nucleation by TEM, but some details with high resolution in TEM images were not demonstrated. In our case, the ordered arrangement of the small sized PbS nanoparticles has been observed by STM. TEM measurement also provided the evidence of formation of a PbS nanocrystal. Furthermore, lateral order of the SAMs is available over a larger area, based on the quality of single-crystal Au(111). Onuma and co-workers8 investigated the nucleation of calcium phosphate on 11-marcaptoundecanoic acid SAMs by atomic force microscopy, in which the ordered molecular arrangement of the carboxylic thiol SAMs was shown. A two-dimensional hexagonal closely packed structure of calcium phosphate nanodots with the size of 5-10 nm has been confirmed in the initial stage of nucleation. After this stage, random nucleated nanodots with a size of 20-30 nm will occur. Thus, the results from Meldrum et al. probably reflect the disordered state in (20) Chidsey, C. E. D.; Loiacono, D. N. Langmiur 1990, 6, 682. (21) Meldrum, F. C.; Flath, J.; Knoll, W. Langmiur 1997, 13, 2033.
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the later stage of growth. So, as far as nucleation mechanism, an epitaxial relationship still probably exists between the PbS crystallites and the carboxyl-terminated alkanethiol SAMs. Conclusions Orderly arranged monodispersed PbS nanoparticles with the average size of 3.2 ( 0.4 nm have been synthesized by the nucleation on MUA SAMs assembled on Au(111) substrate through gas-solid chemical reaction. IRS and XPS techniques have been employed to trace the formation
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process of the PbS nanoparticles. STM and TEM results have also verified the formation of the almost monodispersed PbS nanoparticles. Moreover, a growth model has been suggested. We believe that the arrangement of the PbS nanoparticles reflects the structure of the thiol outer surface and the nucleation of them on MUA SAMs obeys the epitaxial model at least in the range of several nanometers. LA015757Y
