Photoelectron Spectroscopic Investigations of Chemical Bonding in

Aug 30, 2005 - 85, D-22607 Hamburg, Germany, Institute for Atomic Physics and Teacher Training, Technical University of Berlin, Hardenbergstr. 36, D-1...
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J. Phys. Chem. B 2005, 109, 17422-17428

Photoelectron Spectroscopic Investigations of Chemical Bonding in Organically Stabilized PbS Nanocrystals A. Lobo,*,† T. Mo1 ller,†,‡ M. Nagel,§ H. Borchert,§ S. G. Hickey,§ and H. Weller§ HASYLAB/DESY, Notkestr. 85, D-22607 Hamburg, Germany, Institute for Atomic Physics and Teacher Training, Technical UniVersity of Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany, and Institute for Physical Chemistry, UniVersity of Hamburg, Grindelallee 117, D-20146 Hamburg, Germany ReceiVed: May 17, 2005; In Final Form: July 18, 2005

The surface structure of organically capped PbS nanocrystals using synchrotron radiation excited core-level photoelectron spectroscopy has been studied. The nanocrystallites prepared by methods of colloidal chemistry have average diameters of 3.1, 3.9, 4.6, and 7.6 nm with narrow size distributions and are stabilized either with oleic acid only or with a combination of trioctylphosphine and oleic acid as ligands. High resolution photoelectron spectroscopy measurements allowed the surface structure to be studied and in particular how the organic ligands bind to the surface of the PbS nanocrystals to be elucidated. The results indicate that the trioctylphosphine ligands passivate only the surface S sites while oleic acid ligands appear to bind mainly to Pb sites.

1. Introduction The IV-VI materials (e.g., PbS, PbSe, and PbTe) exhibit considerable quantum size effects, and the regime of strong confinement of both electron and hole can be easily accessed in these materials.1,2 Hence, the nanocrystals of these materials possess interesting properties relevant to fundamental explorations as well as technological applications. For example, quantum dots of these materials provide unique opportunities for a strongly size-quantized electronic system.2 Technologically, IV-VI materials have potential advantages over other materials with regard to their nonlinear optical response due to the possibilities of achieving strong confinement of the excitons.3,4 Also, due to their narrow band gap, quantum dots of these materials have their lowest energy optical transitions in the technologically important near-infrared wavelengths.5,6 However, all semiconductors often possess unsatisfied dangling bonds at the surfaces, which give rise to surface states in the band gap. Due to the large surface-to-volume ratio in low dimensional systems, the properties are strongly influenced by their surface structure. The preparation of semiconductor nanocrystals by the method of colloidal chemistry usually involves capping of the nanocrystal surface with stabilizing ligands which fulfill various functions, including the passivation of dangling bonds at the surfaces. Therefore, the capping of the nanocrystals is important for the synthesis of high quality nanocrystals, and hence, a detailed understanding of the ligandsurface interactions is necessary in order to fully understand the properties of these nanocrystals and to improve them in order to take full advantage of their many technological applications. Over the past decade, such work has mainly concentrated on II-VI and III-V semiconductor nanocrystals,7-10 and little experimental information is available on IV-VI semiconductor nanocrystal surfaces. * Corresponding author. E-mail: [email protected]. Fax: +49 40 8998 4475. † HASYLAB/DESY. ‡ Technical University of Berlin. § University of Hamburg.

In the present work, we have studied the ligand-surface interactions in colloidally prepared PbS nanocrystals and have determined the nature of the particle-ligand interface using high resolution photoelectron spectroscopy (PES). PES can provide direct information about the chemical bonding at the surfaces. The use of synchrotron radiation allows tuning of the kinetic energy of the photoelectrons. Because of the dependence of the mean free path on the photoelectron kinetic energy, the sampling depth can be varied. This allows one to distinguish between the atoms on the surface and those in the volume of the nanocrystal. To the authors’ knowledge, this is the first such detailed analysis available on colloidally prepared organically capped PbS nanocrystals. The PbS nanocrystallites investigated in this study have average diameters (L) of 3.1, 3.9, 4.6, and 7.6 nm with a narrow size distribution and with oleic acid (OA) and trioctylphosphine (TOP) as ligands. The use of synchrotron radiation has allowed Pb 4f and S 2p core levels to be measured at surface- and volume-sensitive photoelectron kinetic energies, which has enabled the identification of different Pb and S species present in the nanocrystal. The Pb 4f and S 2p core-level spectra have been discussed in detail, which provides important information about the chemical bonds that exist between the nanocrystal surface and the capping OA/TOP molecules. 2. Experimental Section The PbS nanocrystals are synthesized by methods of colloidal chemistry in a solvent medium under water and oxygen free conditions.5,11,12 Diphenyl ether was used as the solvent, and the capping ligands were oleic acid and tri-n-octylphosphine (TOP). As Pb2+ sources, lead(II) acetate or lead(II) oxide were employed, and as the sulfur precursor, bis(trimethylsilyl)sulfide was used. With a rapid injection of the sulfur precursor, size control could be gained through an excess of lead, a lower injection temperature, and also a shorter growth time, all of which resulted in smaller nanocrystals. The coordinating tri-noctylphosphine and oleic acid mixture plays an essential role in the control of the growth, size distribution, colloidal stability,

10.1021/jp0525888 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/30/2005

Organically Stabilized PbS Nanocrystals

Figure 1. TEM images of PbS nanocrystals with average particle diameters (L) of (a) 3.1 nm, (b) 3.9 nm, and (c) 7.6 nm. The 10 nm size bar applies to all inset images.

crystallinity, and shape of the resulting nanocrystals. A systematic combination of the reaction and growth conditions allows the possibility to produce high quality PbS nanocrystals over a broad range of diameters with narrow size distributions without any size selective precipitation techniques being applied. The smallest nanoparticles with a diameter of 3.1 nm (Figure 1a) were synthesized using an excess of lead. The amount of lead in the reaction mixture was 3 times higher than that of sulfur. Furthermore, only oleic acid was employed as the capping ligand and mixed with the solvent in a volume ratio of 60% oleic acid to 40% solvent. The injection temperature

J. Phys. Chem. B, Vol. 109, No. 37, 2005 17423 employed was 40 °C. For the syntheses of the particles with a diameter of 3.9 nm (Figure 1b), an amount of oleic acid was replaced with tri-n-octylphosphine which resulted in a solution consisting of 41% solvent, 6% oleic acid, and 53% tri-noctylphosphine (vol/vol), while the molar ratio of lead to sulfur remained 3:1. The sulfur precursor was injected at a temperature of 150 °C. The ratio of lead to sulfur for the synthesis resulting in nanoparticles with a diameter of 7.6 nm (Figure 1c) was 1:1 which necessitated a solution of 57% solvent, 23% oleic acid, and 20% tri-n-octylphosphine (vol/vol) when an injection temperature of 150 °C was used. A short chained alcohol was used to precipitate the nanocrystals out of the reaction mixture which were subsequently washed with butan-1-ol to remove any excess of ligands. Four samples with average particle diameters of 3.1, 3.9, 4.6, and 7.6 nm have been synthesized and analyzed in the present study. The smallest nanocrystals (3.1 nm) were capped with OA only, while all other samples were stabilized with mixtures of OA and TOP. Nanoparticle shapes, sizes, and size distributions were evaluated by transmission electron microscopy (TEM) using a Philips CM-300UT microscope. The samples for TEM analysis were prepared by drying the colloidal solution containing PbS nanocrystals on an amorphous carbon coated copper grid. Figure 1 shows the TEM images of PbS nanocrystals from different syntheses. The TEM image (Figure 1a) of PbS nanocrystals capped with only OA ligands shows some amount of coalescence or interconnection between particles. However, the TEM images (Figure 1b and c) of PbS nanocrystals capped with a combination of OA and TOP show that the samples are homogeneous with respect to the size and shape of the particles. The average size of the particles shown in Figure 1 was found to be 3.1, 3.9, and 7.6 nm, respectively. The TEM images show faceted nanocrystals with regular interparticle spacing determined by the surrounding capping ligand shells, and long range hexagonal ordering is observed. The particles were also characterized using X-ray diffraction (XRD), absorbance, and photoluminescent emission. Thin film X-ray diffraction on a silicon wafer reveals the crystalline, cubic rock-salt structure (Fm3m) of bulk PbS. The nanocrystals exhibit sharp features and rich substructure in the absorption spectra as well as size dependent luminescence spectra which is also evidence of the narrow size distribution. The photoelectron spectroscopy measurements were performed at the undulator beamline BW3 of the DORIS III storage ring at HASYLAB/DESY in Hamburg, Germany. Spectra were recorded using an Omicron EA 125 hemispherical electron analyzer in normal emission mode. The combined experimental resolution of photon source and spectrometer was better than 300 meV for all photon energies below 600 eV. The samples were prepared by depositing a drop of the colloidal nanocrystal solution on sputter cleaned Au substrates. Within approximately 1 min after applying the drop on the substrate, the sample was transferred into the experimental chamber through a fast-entry load lock. By this procedure, it was possible to prepare a thin coating layer of nanocrystals, which yielded relatively intense photoemission signals from the constituents along with Au lines from the substrate. The Au 4f7/2 line was used as a reference to calibrate binding energies. No charging effect was observed on these samples, which was cross-checked by measuring both the peak line widths and positions and comparing spectra recorded at both an earlier stage and at the end of the measurements. The spectra were also carefully checked to detect any beam induced modifications in

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Figure 2. XPS survey spectra of PbS nanocrystals capped with TOP and OA ligands.

the surface chemical bonding, by comparing the spectra measured within a minute of exposure to the synchrotron radiation and at later stages. All photoelectron spectra were measured at room temperature. 3. Results and Discussion Photoelectron spectroscopy is very extensively used due to its capability of providing information on the chemical environment of an atom and the electronic structure of a material. The binding energy of a core-level electron of an atom in the solid depends on the potential energy at that position, which in turn depends on the chemical environment of the atom. The difference in the oxidation state, molecular environment, and lattice site can result in the nonequivalence of the atoms. These atoms in the solid can give rise to core-level peaks with measurably different binding energies in the X-ray photoelectron spectra. Figure 2 shows typical photoemission survey spectra of differently sized PbS nanocrystals, obtained using 620 eV photons and a 50 eV spectrometer pass energy. They show Pb and S photoelectron lines from the nanocrystals and Au photoelectron lines from the substrate. The spectra also show P, C, and O peaks from the ligands. There can be additional contributions to the C and O line intensities from adsorbed gaseous molecules such as CO and/or H2O on the nanocrystals and on the substrate. High resolution scans of the Pb and S peaks were performed to investigate the surface chemical bonding in these nanocrystals. Figure 3a represents S 2p photoelectron spectra comprised of 2p3/2 and 2p1/2 lines measured at bulk and surface-sensitive electron kinetic energies for the 3.9 nm PbS nanocrystals. Using a simplex fitting routine (Unifit), the peaks were decomposed into multiple components of spin-orbit doublets with a polynomial background. The smallest possible number of peaks was taken and fitted with Voigt line shapes. During the fitting procedure, the Lorentzian width was restricted to being between 0.2 and 0.25 eV and the Gaussian width was allowed to vary. The spin-orbit splitting for all components was fixed at 1.2 eV, and an intensity ratio of 2:1 was used. Numerous spectra have been carefully evaluated by fitting routines. Table 1 summarizes the fitting parameters used for the S 2p core level for differently sized PbS nanocrystals. A good fit to the experimental data was obtained with four sets of Voigt functions in the binding energy range from 160 to 168 eV, which are

Figure 3. High resolution S 2p core-level spectra of (a) L ) 3.9 nm, (b) L ) 7.6 nm, and (c) L ) 4.6 nm PbS nanocrystals measured at different excitation energies and fitted with spin-orbit split doublets of Voigt functions. The nanocrystals in all three samples are stabilized with a combination of TOP and OA ligand molecules.

labeled as VS, Surf1, Surf2, and Surf3. The individual doublet Voigt components are shown by the solid lines with the overall fit superimposed on the experimental data points. The decrease in the surface sensitivity by increasing the kinetic energy of ejected photoelectrons through increasing the photon energy from 220 to 620 eV leads to changes in the spectrum caused by the reduction in the intensity of the Surf1, Surf2, and Surf3 components relative to the increase in component VS. From this change, it can be inferred that components Surf1, Surf2, and Surf3 represent sulfur species located at the particle surface, while component VS originates from sulfur atoms in the interior of the nanocrystal. Therefore, we identify component VS as the sulfur atoms of PbS present in the volume of the nanocrystals. The binding energy value of this component is 160.97 ( 0.1 eV which is close to the S 2p binding energy in a PbS bulk crystal (160.7 eV) observed in other studies.13,14

Organically Stabilized PbS Nanocrystals

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TABLE 1: Core-Level Spectra Fitting Parameters for S 2pa average diameter, ligand

binding energy (VS) Γ VS Lorentzian S.O split SCLS, Surf Γ Surf SCLS, Surf1 Γ Surf1 SCLS, Surf2 Γ Surf2 SCLS, Surf3 Γ Surf3 a

L ) 3.1 nm, OA

L ) 3.9 nm, OA/TOP

L ) 4.6 nm, OA/TOP

L ) 7.6 nm, OA/TOP

161.23 0.60 0.25 1.2 +0.40 0.8

160.97 0.60 0.25 1.2

161.25 0.58 0.25 1.2

161.05 0.59 0.25 1.2

+1.12 (0.05) 0.97 +3.08 (0.1) 1.0 +7.07 (0.05) 1.1

+1.15 (0.05) 0.74 +2.73 (0.1) 0.98 +5.60 (0.05) 0.99

+1.03 (0.05) 0.80 +2.67 (0.1) 0.82 +6.82 (0.05) 0.80

+2.265 (0.1) 0.72 +5.57 (0.05) 0.85

All parameters are in electronvolts. Γ, Gaussian broadening; S.O split, spin-orbit split; SCLS, surface core-level shift.

Note that here it was not possible to measure the absolute peak positions referenced to the valence band edge of the PbS nanocrystal, due to the interference of the Au valence band of the substrate. The other three components identified as originating from the surface sulfur atoms have their 2p3/2 signals centered at 162.09, 164.05, and 168.04 eV respectively. The physical origin of these surface components will be discussed later. The S 2p core-level spectra of PbS nanocrystals with an average size of 7.6 nm are presented in Figure 3b. These spectra also exhibit evidence of multiple components similar to Figure 3a. The spectra are once again well described in terms of one volume component and three surface components. The energy separation between the different components and their widths are very similar (a few millieletronvolts less) to those of the 3.9 nm PbS nanocrystals. These values are presented in Table 1. One difference is in the relative intensity of the Surf1 component to the VS component, which is reduced when compared to the 3.9 nm PbS nanocrystals, which is reasonable given that the larger particles have a reduced ratio of surface atoms to volume atoms. Additionally, we have synthesized and analyzed 4.6 nm, TOP/ OA capped PbS nanocrystals. The photoelectron spectra of these particles are similar to 3.9 nm sized PbS nanocrystals but with slightly different intensity ratios. The surface-sensitive scan of the S 2p core level is presented in Figure 3c. Further, to check the possible binding sites of ligand molecules, an additional sample has been synthesized using only OA as a stabilizing agent. The average particle size measured using TEM and XRD analysis was found to be 3.1 nm. Figure 4 represents the Voigt function fitted volume- and surfacesensitive S 2p photoemission spectra from such PbS nanocrystals. In these nanocrystals, the Surf1 component, which was observed in 3.9 and 7.6 nm nanocrystals, is absent. However, we observed a Surf component at the high binding energy side with a chemical shift of 0.4 eV. This component must also be on the nanocrystal surface, as its intensity is larger in the surfacesensitive spectrum. Component Surf is real, since attempts to fit the spectrum without this component show larger residuum (Figure 4(i)). It is also reproducible, as similar spectra have been measured and analyzed on different samples. In these nanocrystals, the Surf2 component is present with very low intensity and the intensity of the Surf3 component is very weak. From the energy dependence of the various photoelectron lines, it can be concluded that in total four different surface sites related to sulfur are observed. Whether the states are populated depends on the composition of the nanocrystal surface.

Figure 4. High resolution S 2p core-level spectra of L ) 3.1 nm PbS nanocrystals measured at surface- and volume-sensitive photon energies. In this sample, the nanocrystals are stabilized only with oleic acid molecules.

The different states can be assigned to various sites on the basis of the surface core-level shift. As will be shown below, the following sites can be identified. The component Surf, which is shifted by 0.4 eV toward higher binding energy, can be assigned to unpassivated S atoms on the nanocrystal surface, although in studies of bulk PbS slightly negative surface core-level shifts were observed.13 A similar situation was observed for the S 2p photoelectron spectra of CdS. Wiklund et al. found a surface component shifted by 0.4 eV to a lower binding energy for CdS(100).15 However, in the case of CdS nanocrystals, Winkler et al.8 and Nanda et al.9 observed a surface component shifted to the high binding energy side of the bulk component and could assign it to unpassivated S atoms at the surface. Furthermore, a small positive surface core-level shift has also been predicted by theoretical calculations for PbS.16 On the basis of the photoemission results of the 3.1 nm nanocrystals, it can be concluded that the Surf1 component observed only in PbS nanocrystals stabilized additionally with TOP originates from the binding of TOP molecules to the surface sulfur atoms. The increase in the binding energy of this component is due to the covalent coupling of the P atom of TOP. This assignment is consistent with the attribution of the

17426 J. Phys. Chem. B, Vol. 109, No. 37, 2005 component Surf to unpassivated sulfur atoms in the nanocrystals stabilized with OA only. If the Surf1 component, observed in TOP and OA stabilized nanocrystals, was related to unpassivated surface sulfur atoms, then the component should have been present also in the 3.1 nm particles stabilized only with OA, and due to the increased surface-to-volume ratio of these nanocrystals, its relative intensity should even have been enlarged. In conclusion, the results provide clear evidence that TOP ligands bond to S atoms at the surface of PbS nanocrystals, and the absence of the component Surf in the case of a TOP/ OA stabilizing mixture shows that the passivation of the surface sulfur sites is more or less complete. The origin of component Surf2 requires further discussion. With three different nanocrystal sizes, we observed that the intensity of component Surf1 increases relative to component VS with decreasing nanocrystal size. This is due to the increase in the surface-to-volume ratio with decreasing size for an approximately spherical particle. However, such a trend was not observed in the intensity of component Surf2. Furthermore, in all of the nanocrystal samples that were measured, the Surf2 component with an average binding energy value of 163.9 ( 0.2 eV has been observed. This value is close to the S 2p3/2 binding energy for sulfur in either an elemental form or in the form of polysulfides. Hence, the component Surf2 can be attributed to the contribution of the S-S type of bonding present on the nanocrystal surface. Such a bonding type can be expected involving the corner atoms or at the defect sites. Winkler et al. observed a component with a similar binding energy in a study of CdS nanocrystals and also attributed it to S-S bonds.8,17 A similar chemical shift can also be related to the S atoms being located closer to anions and polarized by their electric field. In a theoretical study, Becker and Hochella suggested that a chemical shift of ∼3 eV from the monosulfide peak can be expected for a S atom bonded to oxygen or near two oxygen atoms.18 The presence of excess sulfur at the surface can in fact be checked by calculating the intensity ratios from the measured photoelectron spectra. To check the composition of Pb and S in our samples, S 2p-to-Pb 4f intensity ratios have been calculated after normalizing the intensities as outlined in ref 19. The normalized intensity ratios Inorm(S 2p)/Inorm(Pb 4f) were found to be ∼1.1 for these nanocrystal samples. This provides us with evidence that a slight excess of sulfur was indeed present on the surface of these PbS nanocrystals. It has been demonstrated previously that the binding energy 163.8 ( 0.3 eV is the same as the binding energies for the S not bonded to the metals in Cu, Pt, and Rh polysulfides.20-22 The binding energies for sulfur atoms not bonded to the metal are at the same position in all metal complexes and on all of the sulfide surfaces studied (PbS, (Zn, Fe)S, FeS2). The Surf3 component was observed at a high binding energy of 167-168 eV. This doublet can be attributed to the strongly oxidized surface sulfur atoms in the form of S2O32-, SO32-, or SO42- with S in the 2+, 4+, or 6+ oxidation state, respectively. Such types of oxidation products have been observed on bulk PbS.23,24 The Pb 4f core-level spectra, recorded with surface- and volume-sensitive photon energies, reveal the existence of two components located at 138.02 and 138.70 eV for 3.9 nm particles and 137.88 and 138.42 eV for 7.6 nm particles, as shown in Figure 5. Table 2 summarizes the fitting parameters used for the Pb 4f core level for differently sized PbS nanocrystals. By increasing the surface sensitivity, through decreasing the energy of excitation, the intensity of component CPb increases with respect to component VPb. Hence, we attribute the component

Lobo et al.

Figure 5. High resolution Pb 4f core-level spectra of (a) L ) 3.9 nm and (b) L ) 7.6 nm PbS nanocrystals measured at different excitation energies and fitted with spin-orbit split doublets of Voigt functions. The nanocrystals in both samples are stabilized with a combination of TOP and OA ligand molecules. At surface-sensitive energies, the P 2p spectrum is also present.

TABLE 2: Core-Level Spectra Fitting Parameters for Pb 4fa average diameter, ligand

binding energy (VPb) Γ VPb Lorentzian S.O split SCLS, CPb Γ CPb

L ) 3.9 nm, OA/TOP

L ) 7.6 nm, OA/TOP

138.02 0.84 0.20 4.86 +0.68 (0.05) 1.04

137.88 0.76 0.20 4.86 +0.54 (0.05) 0.97

a All parameters are in electronvolts. Γ, Gaussian broadening; S.O split, spin-orbit split; SCLS, surface core-level shift.

VPb to Pb in the volume of the nanocrystal and the component CPb to the Pb atoms on the surface of the nanocrystal passivated by the ligand molecules. The observed chemical shift is close to the value of Pb-O bonding of 138.4 eV.25 The shift to higher binding energy can be explained taking into account the fact that these atoms are bound to the more electronegative oxygen of OA molecules. The different components observed for Pb and S in the photoelectron spectra and their assignment are summarized in Table 3.

Organically Stabilized PbS Nanocrystals

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TABLE 3: Summary of Sulfur and Lead Components Observed in the Photoelectron Spectra of PbS Nanocrystals and Their Assignments component

assignment

VS Surf Surf1 Surf2 Surf3 VPb CPb

sulfur atoms in the volume of the nanocrystal unpassivated surface sulfur atoms surface sulfur atoms passivated by TOP ligands surface sulfur atoms with S-S bonding strongly oxidized surface sulfur atoms lead atoms in the volume of the nanocrystal surface lead atoms passivated with oleic acid ligands

The other peak with strong intensity observed in Figure 5 for the lower photon energy (hν ) 194.7 eV) spectrum corresponds to the P 2p core level. Because of the different photoelectric cross sections for the Pb 4f and P 2p levels and also due to variations in the probing depth, at higher photon energies the P 2p peak has much less intensity with respect to the Pb 4f peak. The P 2p3/2 core level has a binding energy of 132.8 eV. The spectra were fitted using the Voigt function with a spin-orbit splitting of 0.88 eV adapted from an InP bulk study.26 The strong broadening of the P 2p level is typical for ligands attached to the nanocrystal at different surface sites. Since the binding energy values of P in P-O and P-S bonds are very close, a component corresponding to P in oxidized TOP could not be resolved if it were present in our samples.27,28 In the smaller sized nanocrystal sample, a higher intensity of the P 2p signal with respect to Pb 4f has been observed. Correspondingly, in the S 2p and Pb 4f levels, the intensity of the surface components reduces with respect to that of the volume components with increasing nanocrystal size. This is simply due to the decrease of the surface-to-volume ratio with increasing size for an approximately spherical particle. Similar results have been reported for other nanocrystals such as CdS, ZnS, and InAs.8,9,19 Since synchrotron radiation can easily lead to radiation damage, we have also made stability tests. Parts a and c of Figure 6 show the S 2p and Pb 4f spectra, respectively, measured at an early stage and parts b and d after about an hour of irradiation under identical conditions. One can notice a certain amount of radiation induced modifications of the surface sites. The intensity of all surface components has decreased. Possibly, some of the ligands are desorbing from the surface either by a local heating effect or due to electron stimulated desorption. Winkler et al. observed similar effects upon heating CdS nanocrystals stabilized by thiol ligands.29 A detailed understanding of the radiation induced reactions is rather difficult to ascertain and requires more investigations. However, in the present study, the attribution of component Surf1 to sulfur atoms bound to ligands holds. If the origin of the Surf1 component was unpassivated surface sulfur atoms, one would not expect the intensity of this component to decrease. 4. Summary Samples of differently sized colloidally prepared PbS nanocrystals stabilized with only OA and OA/TOP have been studied by high resolution photoelectron spectroscopy. The analysis allowed the existence of various chemical species present at the nanocrystal surface and the chemical bonds between surface atoms and the organic capping molecules to be elucidated. Pb 4f spectra provided evidence for the bonding of OA ligands to Pb atoms at the nanocrystal surface. Since no indications were found for OA bound also to surface S atoms, it seems likely that these ligands bind to Pb surface sites only. Consistent with this result, nanocrystals stabilized with OA only present

Figure 6. High resolution (a) S 2p and (c) Pb 4f core-level spectra of L ) 3.9 nm PbS nanocrystals at an early stage of the experiment. The spectra in parts b and d were taken at a late stage of the measurement and reveal slight radiation induced modifications of the nanocrystal surface sites.

unpassivated S atoms at the surface as revealed by photoelectron S 2p spectra. In contrast to this, S 2p spectra of PbS nanocrystals stabilized with OA/TOP mixtures did not show the component associated with unpassivated S atoms at the surface. Instead, a new component is observed which can be associated with bonds between TOP molecules and surface S atoms. Thus, there is clear evidence that TOP efficiently passivates the S surface sites. Apart from bonds between the organic stabilizers and the nanocrystal surface, evidence for S-S bonds as well as for some strongly oxidized S species at the particle surfaces was also found. In conclusion, the analysis demonstrates that the surfaces of PbS nanocrystals can be efficiently passivated using a mixture of OA and TOP where each of the two types of ligands attaches to one kind of surface site. Until this investigation, there were no data available to demonstrate to which site each respective ligand was attached. The present study clearly demonstrates the existence of such interactions between ligands and the nanocrystal surface sites and has helped to clear up ongoing discussions on this topic. Since low photoluminescence efficiency in semiconductor nanocrystals is usually a result of the existence of traps for electrons or holes due to inadequate passivation of the nanocrystal surface, information as provided by the present work is highly important for the design of new materials in nanosciences. Acknowledgment. The authors thank R. Do¨hrmann for technical support at the XPS experimental station and the BW3 beamline. S.G.H. wishes to acknowledge the support of the European community’s Human Potential Programme (contract HPRN-CT-2002-00320). This work was supported by the German Science Foundation (DFG) within the framework of the SFB 508.

17428 J. Phys. Chem. B, Vol. 109, No. 37, 2005 References and Notes (1) Kang, I.; Wise, F. W. J. Opt. Soc. Am. 1997, 14, 1632. (2) Wise F. W. Acc. Chem. Res. 2000, 33, 773. (3) Guerreris, P. T.; Ten, S.; Borrelli, N. F.; Butty, J.; Jabbour, G. E.; Peyghambarian, N. Appl. Phys. Lett. 1997, 71, 1595. (4) Wundke, K.; Poetting, S.; Auxier, J.; Schuelzgen, A.; Peyghambarian, N. Appl. Phys. Lett. 2000, 76, 10. (5) Bakueva, L.; Musikhin, S.; Hines, M. A.; Chang, T. W. F.; Tzolov, M.; Scholes, G. D.; Sargent, E. H. Appl. Phys. Lett. 2003, 82, 2895. (6) Bakueva, L.; Konstantatos, G.; Levian, L.; Musikhin, S.; Sargent, E. H. Appl. Phys. Lett. 2004, 84, 3459. (7) Alivisatoes, A. P. J. Phys. Chem. 1996, 100, 13226. (8) Winkler, U.; Eich, D.; Chen, Z. H.; Fink, R.; Kulkarni, S. K.; Umbach, E. Chem. Phys. Lett. 1999, 306, 95. (9) Nanda, J.; Sarma, D. D. J. Appl. Phys. 2001, 90, 2504. (10) McGinley, C.; Riedler, M.; Moeller, T.; Borchert, H.; Haubold, S.; Haase, M.; Weller, H. Phys. ReV. B 2002, 65, 245308. (11) Murray, C.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. DeV. 2001, 45, 47. (12) Wehrenberg, B. L.; Wang, C.; Guyot-Sionnest, P. J. Phys. Chem. B 2002, 106, 10634. (13) Leiro, J. A.; Laajalehto, K.; Kartio, I.; Heinonen, M. H. Surf. Sci. 1998, 412/413, L918. (14) Smart, R. St. C.; Skinner, W. M.; Gerson, A. R. Surf. Interface Anal. 1999, 28, 101. (15) Wiklund, S.; Magnusson, K. O.; Flodstroem, S. A. Surf. Sci. 1990, 238, 187.

Lobo et al. (16) Allan, G. Phys. ReV. B 1991, 43, 9594. (17) Dollefeld, H.; McGinley, C.; Almousalami, S.; Moeller, T.; Weller, H.; Eychmueller, A. J. Chem. Phys. 2002, 117, 8953. (18) Becker, U.; Hochella, M. F., Jr. Geochim. Cosmochim. Acta 1996, 60, 2413. (19) Borchert, H.; Haubold, S.; Haase, M.; Weller, H.; McGinley, C.; Riedler, M.; Moeller, T. Nano Lett. 2002, 2, 151. (20) Scaini, M. J.; Bancroft, G. M.; Lorimer, J. W.; Maddox, L. M. Geochim. Cosmochim. Acta 1995, 59, 2733. (21) Termes, S. C.; Buckley, A. N.; Gillard, R. D. Inorg. Chim. Acta 1987, 126, 79. (22) Kartio, I.; Laajalehto, K.; Kaurila, T.; Suoninen, E. Appl. Surf. Sci. 1996, 93, 167. (23) Godocikova, E.; Balaz, P.; Bastl, Z.; Brabec, L. Appl. Surf. Sci. 2002, 200, 36. (24) Szargan, R.; Schaufuss, A.; Rossbach, P. J. Electron Spectrosc. Relat. Phenom. 1999, 100, 357. (25) Laajalehto, K.; Smart, R. St. C.; Ralston, J.; Suoninen, E. Appl. Surf. Sci. 1993, 64, 29. (26) Kendelewicz, T.; Mahowald, P. H.; Bertness, K. A.; McCants, C. E.; Lindau, I.; Spicer, W. E. Phys. ReV. B 1987, 36, 6543. (27) Lau, W. M.; Jin, S.; Wu, X.-W.; Ingrey, S. J. Vac. Sci. Technol., A 1991, 9, 994. (28) Yuan, Z. L.; Ding, X. M.; Lai, B.; Hou, X. Y.; Lu, E. D.; Xu, P. S.; Zhang, X. Y. Appl. Phys. Lett. 1998, 73, 2977. (29) Winkler, U.; Eich, D.; Chen, Z. H.; Fink, R.; Kulkarni, S. K.; Umbach, E. Phys. Status Solidi A 1999, 173, 253.