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2007, 111, 14049-14054 Published on Web 09/05/2007

Synthesis of Colloidal PbSe/PbS Core-Shell Nanowires and PbS/Au Nanowire-Nanocrystal Heterostructures Dmitri V. Talapin,*,†,‡,| Heng Yu,† Elena V. Shevchenko,†,‡ Arun Lobo,§ and Christopher B. Murray† IBM Research DiVision, T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and DESY/HASYLAB, Notkestrasse 85, 22603 Hamburg, Germany ReceiVed: June 4, 2007; In Final Form: July 11, 2007

The oriented attachment of PbSe nanocrystals results in single-crystalline colloidal PbSe nanowires. The addition of different surfactants allows tailoring nanowire morphology, choosing between straight, zigzag, helical, and branched nanowires. In this work, we studied the formation of coaxial PbSe/PbS core-shell heterostructures and observed Stranski-Krastanov growth regime of PbS shell on PbSe nanowires at low reaction temperature (150 °C) that switched to layer-by-layer epitaxial growth above 180 °C. The protection of PbSe nanowires with a PbS shell substantially improves the nanowire stability against oxidation. We also developed a technique for controllable decoration of colloidal PbSe nanowires with Au nanoparticles and found that the morphology of nanowire template had a strong effect on nucleation and growth of gold nanoparticles.

1. Introduction Semiconductor nanowires stay among the hottest research topics of the past decade owing to their potential in electronics,1-3 sensors,4 thermoelectric devices,5 lasers,6 photodetectors and solar cells.7,8 The development of cost-effective techniques for synthesis of high-quality nanowires with desired properties is an important milestone on the way toward broad commercial application of these materials. Vapor-liquid-solid (VLS) synthesis is generally used in preparation of semiconductor nanowires.9,10 However, the high cost of VLS-grown nanowires looks like a road blocker for their applications in photovoltaics, thermoelectrics, and other areas requiring large quantities of materials. Solution-phase (colloidal) nanowire syntheses might provide low-cost alternatives to chemical vapor deposition (CVD)-based techniques.11,12 Moreover, colloidal syntheses allow obtaining narrow, sub-10 nm diameter semiconductor nanowires, which are difficult to synthesize by gas phase approaches.11,12 We recently demonstrated the possibility to synthesize single-crystalline PbSe nanowires in a colloidal solution by the oriented attachment and fusion of PbSe nanocrystals.13 The synthesis by oriented attachment allows tailoring the morphology of single-crystalline PbSe nanowires from straight to zigzag, helical, or branched. The presence of dangling bonds at the nanowire surface can dramatically change recombination kinetics and carrier mobility in the nanowires. In many cases, these surface dangling bonds * Corresponding author. E-mail: [email protected]. † IBM T. J. Watson Research Center. ‡ The Molecular Foundry. § DESY/HASYLAB. | Current address: Chemistry Department, The University of Chicago, Chicago, IL 60637.

10.1021/jp074319i CCC: $37.00

introduce midgap states and behave as oxidation sites. The passivation of surface-dangling bonds can be achieved by growing epitaxial core-shell structures, as it was demonstrated for colloidal nanocrystals14,15 and CVD-grown semiconductor nanowires.16 At the same time, little is known about solutionphase synthesis of nanowire heterostructures.17 Colloidal semiconductor nanowires could also be used as templates for designing various multifunctional materials. Combining several materials with different functionalities at the nanoscale is a powerful approach for development of novel materials and “metamaterials”.18 For example, placing magnetic or plasmonic nanomaterial in a close proximity to a semiconducting nanowire might create novel properties originating from a cross-talk between the components with different functionalities.19 The development of nanowire heterostructures and composite materials using colloidal chemistry techniques is expected to pave the way to the creation of novel pathways for designing materials and devices with diverse functions. 2. Experimental Section Nanowire Synthesis. Synthesis of PbSe nanowires with different morphology has been described in detail in ref 13. Briefly, PbSe nanowires have been synthesized by reacting lead oleate with trioctylphosphine selenide at 250 °C in phenyl ether solution in the presence of oleic acid as a capping ligand. The addition of cosurfactants (alkylphosphonic acids, alkylamines, etc.) allowed tailoring the morphology of PbSe nanowires. Thus, the presence of n-tetradecylphosphonic acid resulted in straight nanowires while the addition of hexadecylamine as a cosurfactant allowed us obtaining single-crystalline zigzag and helical nanowires. Branched PbSe nanowires were synthesized by using dodecylamine as a cosurfactant and precisely controlling the © 2007 American Chemical Society

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Figure 1. PbSe/PbS core-shell nanowires. (a) PbSe/PbS nanoparticles synthesized by growing PbS shells around spherical PbSe nanocrystals using different sulfur precursors. (b) PbSe nanowires used as a template for growing coaxial PbSe/PbS nanoheterostructures. (c,d) Different stages of shell growth around the PbSe nanowire at 150 °C. (e) HRTEM image demonstrating nucleation of PbS pyramids at PbSe nanowire surface. (g) PbSe/PbS nanowire with uniform PbS shell grown at 180 °C. Corresponding overview TEM images are shown in Supporting Information, Figures S2-S4. All scale bars are 5 nm.

temperature profile during the reaction.13 Figure S1 from Supporting information shows representative transmission electron microscopy (TEM) images of PbSe nanowires with different morphologies. As-synthesized PbSe nanowires form stable colloidal solutions in chloroform. Synthesis of Coaxial PbSe/PbS Nanowire Heterostructures. Coaxial core-shell PbSe/PbS nanowires have been synthesized by adding precursors of the shell material (lead oleate and sulfur dissolved in 1-octadecene) to the colloidal solution of PbSe nanowires. In a typical synthesis, straight PbSe nanowires capped with n-tetradecylphosphonic acid and oleic acid were first synthesized as described in ref 13. The nanowires were isolated from crude solutions, redispersed in chloroform, and stored under inert atmosphere. Pb/S stock solution was prepared by dissolving 0.755 g of lead acetate trihydrate in 2 mL of oleic acid and 10 mL of octyl ether. The lead stock solution was heated to 150 °C and dried by flowing nitrogen above the solution for 30 min. Two milliliters of dried lead oleate stock solution was mixed with 1.2 mL of 0.5 M solution of sulfur in 1-octadecene. About 20 mg of PbSe nanowires dispersed in 1.2 mL of chloroform was added to Pb:S stock solution and injected into 12 mL of dried trioctylamine at 110 °C. The temperature of reaction mixture was increased up to 180 °C over 3 min followed by fast cooling with a water bath. The PbSe/PbS core-shell nanowires were collected by centrifugation and redispersed in chloroform. Photoelectron Spectroscopy. Photoelectron spectroscopy was performed at beamline BW3 at HASYSLAB/DESY in Hamburg, Germany. The synchrotron radiation has been tuned in the energy range from 110 to 720 eV, and an Omicron EA 125 hemispherical energy analyzer has been used for acquiring the photoemission spectra. PbSe and PbSe/PbS nanowires properly washed from an excess of stabilizing agents and

dissolved in chloroform were deposited on a Pt foil by dropping a small amount of solution on the substrate and allowing the solvent to evaporate. Decoration of PbSe Nanowires with Au Nanoparticles. In a typical synthesis, 8-10 mg of PbSe nanowires (straight, zigzag, or branched) was dispersed in a solution containing 3 mL of hexane, 0.36 mL of oleylamine, and 0.11 mL of oleic acid. The mixture was stirred at room temperature for 10 min, and a dispersion of 10-25 mg of HAuCl4‚3H2O in 6 mL of hexane (obtained by sonication of HAuCl4‚3H2O in hexane for ∼10 min) was added to the nanowire solution. The reaction mixture was left stirring at 35 °C for 1 h under nitrogen atmosphere. The size of Au nanoparticles formed at the nanowire surface can be tuned by varying the treatment time from 30 min up to 3 h. PbSe nanowires decorated with Au nanoparticles have been precipitated from the crude solution by adding hexane/ethanol (1:2 by volume) mixture and collected by centrifugation. The PbSe-Au composite nanowires can be redispersed in chloroform. 3. Results and Discussion Synthesis of Coaxial PbSe/PbS Core-Shell Nanowires. Oxidation of selenium surface atoms can cause slow degradation of lead selenide nanomaterials.20 To improve the stability and processability of colloidal PbSe nanowires, we studied the possibility of growing PbS shell around PbSe wires. The protection of nanomaterials against oxidation with a thin inorganic epitaxial shell is widely used in highly luminescent II-VI and III-V nanocrystals14,15,21 and nanorods.22 It has been demonstrated that nanoscale materials can tolerate large mismatch of lattice parameters due to the relaxation of interfacial strain through coherent expansion or shrinkage of crystalline

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Figure 2. High-resolution photoemission spectra of Se 3d core-level of PbSe and PbSe/PbS nanowires. (a) Se 3d core-level XPS spectrum of PbSe nanowires prepared under nitrogen atmosphere and (b) same sample but after exposure to air for several days. (c,d) Se 3d corelevel photoemission spectra of PbSe/PbS core-shell nanowires stored in air and measured at different photon energies.

lattices.23 The passivation of surface-dangling bonds substantially reduces the concentration of midgap states serving as carrier traps, significantly enhancing luminescence efficiency and carrier mobility.14,15,21-24 First, we studied the growth of PbS shells around spherical PbSe nanocrystals. The formation of PbSe/PbS core-shell nanoparticles has been extensively studied during the last few years.25-30 Bulk PbSe and PbS crystallize in the rock salt structure (SG 225); the mismatch between PbSe and PbS lattice constants is 3.1%. This value is low enough to tolerate coherent shell growth in spherical PbSe/PbS nanocrystals as was confirmed by high-resolution TEM (HRTEM) investigation (Figure 1a and Supporting Information, Figure S2). When trioctylphosphine sulfide was used as the sulfur precursor and the reaction was performed at 220 °C, uniform shell growth resulted in nearly spherical PbSe/PbS core-shell nanocrystals. On the other hand, cubic PbSe/PbS core-shell nanocrystals can be synthesized by using more reactive elemental sulfur dissolved in octadecene and carrying out the process of shell growth at 150 °C (Figure 1a). We found that PbS shell grows differently on PbSe nanowires and nanocrystals. At 150 °C, the growth of PbS shell around 7 nm diameter PbSe nanocrystals occurs mostly via layer-bylayer epitaxy, whereas under the same experimental conditions 7 nm diameter PbSe nanowires show the behavior similar to the Stranski-Krastanov growth (Figure 1b,c,d and Supporting Information, Figure S3). The interfacial strain prevents nucleated PbS islands from lateral growth, resulting in PbS pyramids grown along the PbSe nanowire as shown in Figure 1e. Upon growth, the PbS pyramids merge forming a complete but undulated PbS shell (Figure 1d and Supporting Information, Figure S3). The difference in behavior of PbSe nanocrystals and nanowires might originate from easier strain relaxation in spherical nanocrystals compared to cylindrical nanowires.22 The fast growth of the PbS shell at 180 °C allows uniform coating of PbSe nanowires with ∼3 nm thick PbS shell (Figure 1g and Supporting Information, Figure S4). Suppression of the (111), (311), and (222) reflections in X-ray diffraction

Figure 3. PbSe/PbS core-shell nanowires formed by self-assembly and partial oriented attachments of cubic PbS nanoparticles around PbSe nanowires. (a) Decoration of PbSe nanowires with cubic PbS nanoparticles. (b) TEM images of three-dimensional superlattices of cubic PbS nanoparticles grown around PbSe nanowires. (c) HRTEM image showing oriented attachment between cubic PbS nanoparticles and PbSe nanowire. (d) Selected area electron diffraction pattern taken from the nanoparticle-nanowire interface.

pattern of PbSe/PbS nanowires points to the epitaxial relation between central wire and PbS phase for all nanowires in the sample (Supporting Information, Figure S5). The lattice parameter of PbSe/PbS nanowires is intermediate to PbSe and PbS phases, characteristic of either PbSe/PbS core-shell or alloyed PbSe1-xSx nanowires. The latter could, in principle, form due to partial alloying of core and shell materials during shell growth at 180 °C. To investigate the internal structure of PbSe/PbS nanowires, we compared Se 3d level components in the highresolution X-ray photoemission (XPS) spectra of PbSe and PbSe/PbS nanowires. The photoemission spectra of Se 3d corelevel have been recorded at a series of excitation energies from 160 to 650 eV. Tuning the photon energy allows varying the sampling depth because attenuation of the emitted photoelectrons is strongly dependent on the photoelectron kinetic energy.31 Assynthesized PbSe nanowires showed a single component with the binding energy of about 54 eV and the spin-orbit splitting of 0.84 eV (Figure 2a), in agreement with literature values for bulk PbSe. After long-term exposure of PbSe nanowires to air,

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Figure 4. Decoration of PbSe nanowires with Au nanoparticles. (a-c) PbSe nanowires with straight, zigzag, and branched morphologies, respectively, used as templates. (d-f) PbSe-Au nanocomposites synthesized using PbSe nanowires with different morphologies. (g) Overview TEM image of Au-decorated PbSe nanowires.

the second component with binding energy of about 58.5 eV appeared in the Se 3d core-level spectrum of PbSe nanowires (Figure 2b). The chemical shift between the components in the Se 3d core level spectrum suggests the formation of Se bonds with oxygen at the nanowire surface.32,33 At the same time, no Se-O bonds were detected in as-prepared PbSe nanowires, showing that both oleic and tetradecylphosphonic acids selectively bind to lead surface sites. The Se component is strongly suppressed in the photoelectron spectra of PbSe/PbS nanowires shown in the Figure 2c,d. We could not detect any signal from Se 3d core level at the excitation energy of 160 eV. This photon energy corresponds to the surface sensitive regime, that is, most of the photoelectrons come from the nanowire surface.31 When the energy is increased, the attenuation is reduced and the interior of the sample contributes more and more to the observed photoemission intensity. Increasing the excitation energy to 350 and 650 eV resulted in significant increase of intensity of Se 3d core level component (Figure 2c,d). Such behavior strongly supports the core-shell morphology of our PbSe/PbS nanowires. The coreshell structure is also supported by the absence of the component corresponding to the Se-O bonds in the XPS spectrum of airexposed PbSe/PbS nanowires (Figure 2d). The XPS data confirm that the PbS shell successfully protects PbSe nanowire surface from oxidation. PbSe nanowires uniformly passivated with PbS shells could find use as photo- and electroluminescent materials for mid-IR spectral range. In a separate series of experiments, we explored the possibility of forming PbSe/PbS core-shell nanowires by self-assembly

and oriented attachment of PbS nanoparticles around PbSe nanowires. Addition of cubic PbS nanoparticles to a colloidal solution of PbSe nanowires in trioctylamine at 120 °C results in a uniform decoration of the nanowires with PbS nanoparticles (Figure 3a). This process can be driven by the van der Waals interactions between nanowires and nanocrystals. Nanoparticle self-assembly can progress toward the formation of thick layers of nanocubes assembled around PbSe nanowires (Figure 3b). This self-assembly approach can generate fairly thick PbS “shells” around PbSe nanowires, whereas the shells grown from the molecular precursors are usually only several nanometer thick. Annealing the nanostructures at 180 °C for 15 min promoted at least partial oriented attachment between nanoparticles and nanowires as show in Figure 3c. The matching of the lattice fringes in PbSe nanowire and adjacent PbS nanoparticles as well as single-crystal like selected area electron diffraction pattern (Figure 4d) confirm fusion of PbS nanoparticles with PbSe nanowire. Decoration of PbSe nanowires with PbS nanoparticles provides a promising architecture for photovoltaic cells with carrier multiplication.34 Indeed, attaching PbS nanoparticles to a PbSe nanowire seems to be the most promising way to separate multiple excitons generated inside PbS nanocrystals34 on a sub-100 ps time scale, before their Auger recombination. In bulk, PbSe and PbS form a type-II heterostructure with the positive offsets for conduction and valence bands at the PbSe/PbS interface.35 This type of band alignment is favorable for efficient separation of photogenerated carriers, because it allows collecting electrons in central PbSe

Letters wire while the holes can be extracted into a matrix of p-conducting polymer, such as MEH-PPV.36 Decoration of PbSe Nanowires with Au Nanoparticles. To further explore the routes toward multifunctional nanowire-based materials, we investigated decoration of colloidal PbSe nanowires with Au nanoparticles. Addition of HAuCl4 to a solution of PbSe nanowires in the presence of oleic acid and oleylamine resulted in nucleation and growth of Au nanoparticles along PbSe nanowires. It has been shown by Mokari et al. that Au nanoparticles can be selectively nucleated at the tips of CdSe nanorods and nanotetrapods.37 Long straight nanowires (Figure 4a) do not have topological features that could serve as nucleation centers for gold nanoparticles. However, we observed nearly equidistant distribution of Au nanoparticles along PbSe nanowires with the mean interparticle spacing of ∼40-60 nm (Figure 4d). The equidistant distribution of gold nanoparticles along the nanowire could be caused by strain generated in PbSe lattice in the proximity of attached gold nanoparticles. Lattice strain is known to result in the formation of ordered arrays of quantum dots during MBE growth.38 If the growth of Au nanoparticles is limited by diffusion of molecular species from the bulk of solution, the equidistant distribution of Au nanoparticles along PbSe straight nanowires could also originate from local depletion of the solution in the vicinity of growing gold nanoparticles. The decoration density of PbSe nanowires with Au nanoparticles can be controllably varied by changing the morphology of the nanowire template. When we used PbSe nanowires with zigzag morphology (Figure 4b), Au nanoparticles nucleated mostly at zigzag edges, and the spacing between adjacent nanoparticles was on the order of 10 nm (Figure 4e). The decoration density can be further increased by using PbSe nanowires with side branches as the starting material (Figure 4c). In that case, the nanowire surface has been almost densely covered with 5 nm Au nanoparticles (Figure 4f and Supporting Information, Figure S6). Gold nanoparticles selectively nucleated at nanowire edges and branches. The average number of dangling bonds per surface atom at the nanowire surface depends on the surface curvature and should be significantly larger at sharp edges and tips. These sites should be more reactive because of the increased interfacial energy,37,39 favoring the nucleation of Au nanoparticles. The formation of gold nanoparticles is accompanied with a partial dissolution of PbSe lattice, especially in the vicinity of the nanoparticles. Thus, Au nanoparticles replaced side branches at the nanowire surface (Figure 1f and Supporting Information, Figure S5). Probably, the reduction of [AuCl4]- to Au0 concurred with dissolution of the PbSe phase. Gold atoms can form relatively strong bonds with selenium,40 anchoring Au nanoparticles to the surface of PbSe nanowires. An overview TEM image in Figure 1g shows that this simple technique allows obtaining large quantities of gold-decorated PbSe nanowires. The combination of a semiconductor nanowire and metal nanoparticles can provide novel functionalities to the nanostructure. Under illumination, surface plasmon resonances generated by Au nanoparticles can significantly enhance the nanowire absorption cross-section.41 Metal nanowire nanostructures might find use in photo- and electrocatalysis. Finally, gold nanoparticles can serve as the anchor points and recognition elements for directed self-assembly of nanowire networks and devices.37,42 Acknowledgment. We want to thank A. P. Alivisatos (University of California at Berkeley) for stimulating discussions and Robert L. Sandstrom (IBM T. J. Watson Research Center, IBM) for technical support. Work at the Molecular Foundry

J. Phys. Chem. C, Vol. 111, No. 38, 2007 14053 was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract DEAC02-05CH11231. Supporting Information Available: TEM images of PbSe nanowires with different morphologies (Figure S1). TEM images of PbSe/PbS nanocrystals (Figure S2) and PbSe/PbS nanowires at different stages of the shell growth (Figures S3 and S4). XRD patterns of PbSe and PbSe/PbS nanowires (Figure S5). TEM images of Au-decorated PbSe nanowires (Figure S5). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Cui, Y.; Zhong, Z.; Wang, D.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 149. (2) Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K.-H.; Lieber, C. M. Science 2001, 294, 1313. (3) Duan, X.; Huang, Y.; Leiber, C. M. Nano Lett. 2002, 2, 487. (4) Patolsky, F.; Zheng, G.; Hayden, O.; Lakadamyali, M.; Zhuang, X.; Lieber, C. M. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 14017. (5) Lin, Y.-M.; Dresselhaus, M. S. Phys. ReV. B: Condens. Matter 2003, 68, 075304. (6) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (7) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (8) Hayden, O.; Agarwal, R.; Lieber, C. M. Nat. Mater. 2006, 5, 352. (9) Gudiksen, M. S.; Wang, J.; Lieber, C. M. J. Phys. Chem. B 2001, 105, 4062. (10) Law, M.; Goldberg, J.; Yang, P. Annu. ReV. Mater. Res. 2004, 34, 83. (11) Wang, F.; Dong, A.; Sun, J.; Tang, R.; Yu, H.; Buhro, W. E. Inorg. Chem. 2006, 45, 7511. (12) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237240. (13) Cho, K.-S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (14) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468. (15) Talapin, D. V.; Mekis, I.; Gotzinger, S.; Kornowski, A.; Benson, O.; Weller, H. J. Phys. Chem. B 2004, 108, 18826. (16) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M. Nature 2002, 420, 57. (17) Dong, A.; Wang, F.; Daulton, T. L.; Buhro, W. E. Nano Lett. 2007, 7, 1308. (18) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55. (19) Lee, J.; Hernandez, P.; Lee, J.; Govorov, A. O.; Kotov, N. A. Nat. Mater. 2007, 6, 291. (20) Steckel, J. S.; Coe-Sullivan, S.; Bulovic´, V.; Bawendi, M. B. AdV. Mater. 2003, 15, 1862. (21) Cao, Y. W.; Banin, U. J. Am. Chem. Soc. 2000, 122, 9692. (22) Manna, L.; Scher, E. C.; Li, L.-S.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7136. (23) Baranov, A. V.; Rakovich, Yu. P.; Donegan, J. F.; Perova, T. S.; Moore, R. A.; Talapin, D. V.; Rogach, A. L.; Masumoto, Y.; Nabiev, I. Phys. ReV. B: Condens. Matter 2003, 68, 165306. (24) Xiang, J.; Lu, W.; Hu, Y.; Wu, Y.; Yan, H.; Lieber, C. M. Nature 2006, 441, 489. (25) Sashchiuk, A.; Langof, L.; Chaim, R.; Lifshitz, E. J. Cryst. Growth 2002, 240, 431. (26) Brumer, M.; Kigel, A.; Amirav, L.; Sashchiuk, A.; Solomesch, O.; Tessler, N.; Lifshitz, E. AdV. Funct. Mater. 2005, 15, 1111. (27) Xu, J.; Cui, D.; Zhu, T.; Paradee, G.; Liang, Z.; Wang, Q.; Xu, S.; Wang, A. Y. Nanotechnology 2006, 17, 5428. (28) Xu, J.; Ge, J.-P.; Li, Y.-D. J. Phys. Chem. B 2006, 110, 2497. (29) Lifshitz, E.; Brumer, M.; Kigel, A.; Sashchiuk, A.; Bashouti, M.; Sirota, M.; Galun, E.; Burshtein, Z.; Le Quang, A. Q.; Ledoux-Rak, I.; Zyss, J. J. Phys. Chem. B 2006, 10, 25356. (30) Stouwdam, J. W.; Shan, J.; Van Veggel, Frank, C. J. M.; PattantyusAbraham, A. G.; Young, J. F.; Raudsepp, M. J. Phys. Chem. C 2007, 111, 1086. (31) Hu¨fner, S. Photoelectron spectroscopy, Springer-Verlag, Berlin, (1995). (32) Borchert, H.; Talapin, D. V.; McGinley, C.; Adam, S.; de Castro, A. R. B.; Mo¨ller, T.; Weller, H. J. Chem. Phys. 2003, 119, 1800. (33) Lobo, A.; Moller, T.; Nagel, M.; Borchert, H.; Hickey, S. G.; Weller, H. J. Phys. Chem. B 2005, 109, 17422.

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