CdSe Nanorods Functionalized with Thiol-Anchored Oligothiophenes

Oct 25, 2007 - Bryan C. Sih, and Michael O. Wolf* ..... Hinz , Marius Schmid , Denis V. Seletskiy , Helmut Cölfen , Alfred Leitenstorfer , Stefan Mec...
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J. Phys. Chem. C 2007, 111, 17184-17192

ARTICLES CdSe Nanorods Functionalized with Thiol-Anchored Oligothiophenes Bryan C. Sih and Michael O. Wolf* Department of Chemistry, UniVersity of British Columbia, VancouVer, British Columbia, Canada, V6T 1Z1 ReceiVed: May 31, 2007; In Final Form: August 2, 2007

A series of thiol-substituted oligothiophenes were synthesized and used to functionalize the surface of CdSe nanorods, via exchange with tetradecylphosphonic acid-capped particles. Attachment of the oligothiophenes to the CdSe nanorods results in little change to the electronic structure of the oligothiophenes as shown by the similarities in the absorption spectra to the unbound oligothiophenes. The emission spectra of the oligothiophene-capped nanorods show that oligothiophene emission is quenched after attachment to the CdSe surface due to either an electron or energy transfer mechanism. The intensity of emission from the CdSe nanorods differs depending on the number of oligothiophenes attached to the CdSe surface. A larger number of bound thiols act as hole traps leading to quenching of the nanoparticle emission.

Introduction Semiconductor nanoparticles (NPs) have attracted significant research attention due to size-tunable optical and electronic properties1 that are promising for various applications such as biological labeling,2,3 photovoltaics,4,5 and light-emitting diodes (LEDs).6,7 It is of interest to modify the surface of semiconductor NPs with functional organic groups. This may be achieved by replacement of tri-n-octylphosphine oxide (TOPO) or phosphonic acids, which are typically used to cap the particles during synthesis, with functional molecules. For applications in photovoltaics and LEDs, the TOPO or phosphonic acid capping groups act as an insulating layer that prevents efficient charge transfer into or out of the NP,4 and it has been shown that conducting polymers directly attached to the semiconductor NP surface results in more efficient charge transfer.4,8,9 To understand this charge-transfer mechanism, oligomers such as oligoaniline,10 oligo-(p-phenylenevinylenes),9,11 and oligothiophenes12,13 have been attached to semiconductor NPs such as CdSe. In the case of oligothiophenes, the focus has been largely on longer oligomers (g3 thiophene rings) because these have a sufficiently high-energy highest occupied molecular orbital (HOMO) to quench the fluorescence of CdSe NPs via electron transfer from the oligothiophene. In photovoltaic devices where separation of electron and holes is required for improved device efficiencies, this is useful. Shorter oligothiophenes, which have HOMO levels below the CdSe valence band, do not quench NP emission but can still act as a hole injection layer, and this may be useful in LEDs based on CdSe NPs. Oligothiophene-capped CdSe NPs with shorter oligothiophenes (1-3 rings) are therefore of interest. Oligothiophenes have been previously attached to CdSe NPs with phosphonic acids as the anchoring groups.12,13 The phosphonic acid group attached to the oligothiophene has a similar affinity for the CdSe surface as TOPO14 or hexylphosphonic * To whom correspondence should be addressed. E-mail: mwolf@ chem.ubc.ca.

acid12 attached to the particle surface during NP preparation. Thus, the passivating groups (TOPO or hexylphosphonic acid) must first be displaced by heating the CdSe particles in neat pyridine, and then the phosphonic acid group attached to the oligothiophene can be used to displace the more weakly bound pyridine groups.12,14 The extra pyridine substitution step may be eliminated by using thiol-anchoring groups that are known to bind more strongly to the CdSe surface and thus may be used in a direct displacement process.15 Rod-shaped CdSe NPs show anisotropic charge-transport properties5 and polarized emission.16,17 Various methods have been developed to control the growth mechanism of semiconductor NPs to induce different shapes,18 and it has been shown that by using CdO as a precursor, rod-shaped NPs can be prepared.19 Oligothiophenes have not been previously attached to rod-shaped CdSe NPs. We report here the preparation of oligothiophenes with 1-3 thiophene rings containing a thiolanchoring group. These functionalized oligothiophenes are attached to the surface of rod-shaped CdSe NPs, and the properties of the oligothiophene-capped CdSe NPs are studied. Experimental Section General. Chemicals were purchased from Aldrich except for CdO, selenium, trioctylphosphine (TOP), and trioctylphosphine oxide (TOPO), which were obtained from Alfa Aesar, and tetradecyl phosphonic acid (TDPA) obtained from Poly Carbon Industries. 2-Bromo-3-hexylthiophene,20 5-(2-(3-hexyl)thienyl)3-hexyl-2-thiophenecarboxylic acid and benzyl 5-(3-hexyl-5(2-(3-hexyl)thienyl)thienyl)-3-hexyl-2-thiophenecarboxylate21 were all prepared according to literature procedures. Electrochemical measurements were conducted using a Pine AFCBP1 bipotentiostat. A Pt disk was used as the working electrode, the counter electrode was a Pt coil wire, and the reference electrode a silver wire. An internal reference (decamethylferrocene) was added to correct the measured potentials with respect to saturated calomel electrode (SCE). [(n-Bu)4N]PF6 was used as the supporting electrolyte and was purified by triple crystallization

10.1021/jp074193p CCC: $37.00 © 2007 American Chemical Society Published on Web 10/25/2007

CdSe Nanorods with Thiol-Anchored Oligothiophenes from ethanol and dried at 90 °C under vacuum for 3 days. Methylene chloride used for cyclic voltammetry (CV) was purified by passage through an activated alumina tower. Solution electronic absorption spectra were obtained on a Varian Cary 5000 UV-vis/NIR spectrometer in CHCl3, and solid state absorption spectra were acquired from films deposited on glass. Solution emission and excitation spectra were obtained using a Photon Technology International Quantamaster fluorimeter, and solid-state emission spectra were acquired on films deposited on glass. 1H and 31P NMR spectra were acquired on a Bruker AV-300 spectrometer, and 13C NMR spectra were obtained on a Bruker AV-400 spectrometer. 1H and 13C NMR spectra were referenced to residual solvent, and 31P NMR spectra were referenced to external 85% H3PO4. Transmission electron microscopy (TEM) images were taken using a Hitachi H7600 electron microscope operating at 80 kV. NPs were dropcast from CHCl3 solutions onto carbon-coated 300-mesh copper grids. The particle dimensions were measured using the image processing program Quartz PCI 5. A total of 150 particles were counted resulting in mean dimensions. Density functional theory (DFT) calculations were performed using a B3LYP/6-31G*(d,p) basis set implemented using the Gaussian 03 program.22 Synthesis. All reactions were performed using standard Schlenk techniques with dry solvents under a nitrogen atmosphere, and unless otherwise noted all reagents were used without further purification. 3-Hexyl-2-thiophenecarboxylic acid (4). A solution of 2-bromo3-hexylthiophene20 (2.5 g, 10 mmol) in 50 mL of dry tetrahydrofuran (THF) was cooled to -78 °C and n-BuLi (6.3 mL, 1.6 M, 10 mmol) was added dropwise while stirring. The mixture was stirred for 1.5 h, and then excess dry ice was added. The mixture was stirred for an additional 2 h and allowed to warm up to room temperature. The mixture was quenched with 1 M HCl and extracted three times with 50 mL of diethyl ether. The combined organic layers were washed with 1 M HCl and deionized water followed by drying over MgSO4. After evaporation of the solvent, the residue was purified with column chromatography over silica gel (CH2Cl2-ethyl acetate [80:20]) yielding the product (1.97 g, 92%) as a white solid. 1H NMR (300 MHz, CDCl3): δ 7.49 (d, J ) 5.0 Hz, 1H, 5-H), 6.99 (d, J ) 5.0 Hz, 1H, 4-H), 3.02 (t, J ) 7.6 Hz, 2H, -CH2), 1.701.57 (m, 2H, -CH2), 1.42-1.29 (m, 6H, -CH2-), 0.90 (t, J ) 6.9 Hz, 3H, -CH3). MS (EI): m/z [M+] ) 268. Anal. Calcd. for C11H16O2S: C, 62.23; H, 7.60. Found: C, 62.63; H, 7.95. 2,5-Dioxo-pyrrolidin-1-yl 3-hexyl-2-thiophenecarboxylate (7). N-hydroxysuccinimide (0.298 g, 2.59 mmol) and N-(3dimethylamino-propyl)-N′-ethylcarbodiimide hydrochloride (0.50 g, 2.4 mmol) were added to a solution of 4 (0.5 g, 2.4 mmol) in 30 mL dry CH2Cl2. The mixture was stirred at room temperature overnight. The solvent was removed, and the residue was purified by column chromatography over silica gel (CH2Cl2) yielding 7 (0.60 g, 82%) as a clear oil. 1H NMR (300 MHz, CDCl3): δ 7.60 (d, J ) 5.0 Hz, 1H, 5-H), 7.05 (d, J ) 5.0 Hz, 1H, 4-H), 2.99 (t, J ) 7.8 Hz, 2H, -CH2-), 2.89 (s, 4H, OCCH2CH2CO), 1.69-1.56 (m, 2H, -CH2-), 1.39-1.23 (m, 6H, -CH2-), 0.87 (t, J ) 6.7 Hz, 3H, -CH3). MS (EI): m/z [M+] ) 309. Anal. Calcd. for C15H19NO4S: C, 58.23; N, 4.53; H, 6.19. Found: C, 58.36; N, 4.80; H, 6.04. N-(2-Mercaptoethyl)-3-hexyl-2-thiophenecarboxamide (1). Triethylamine (5 mL) and cysteamine (0.47 g, 6.1 mmol) were added to a solution of 7 (1.7 g, 5.5 mmol) in 15 mL dry CH2Cl2. The mixture was stirred for 24 h at room temperature. The solvent was removed, and the residue was purified by column chromatography over silica gel (CH2Cl2-ethylacetate [20:1]),

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17185 yielding 1 (0.91 g, 60%) as a clear oil. 1H NMR (300 MHz, CDCl3): δ 7.27 (d, J ) 5.0 Hz, 1H, Th-H), 6.94 (d, J ) 5.0 Hz, 1H, Th-H), 6.25 (br, 1H, -NH), 3.60 (dt, J ) 6.2 Hz, 2H, -NCH2), 2.93 (t, J ) 7.9 Hz, 2H, -CH2-), 2.77 (m, 2H, -CH2S), 1.70-1.57 (m, 2H, -CH2-), 1.37-1.22 (m, 6H, -CH2-), 0.88 (t, J ) 7.0 Hz, 3H, -CH3). 13C NMR (400 MHz, CDCl3): δ 164.12, 147.8, 131.8, 131.35, 127.48, 43.75, 32.67, 31.68, 30.58, 30.21, 25.70, 23.60, 15.09. MS (EI): m/z [M+] ) 271. Anal. Calcd. for C13H21NOS2: C, 57.52; H, 7.80. Found: C, 57.90; H, 7.90. 2,5-Dioxo-pyrrolidin-1-yl 5-(2-(3-hexyl)thienyl)-3-hexyl-2thiophenecarboxylate (8). Following the synthetic method used to prepare 7, 5-(2-(3-hexyl)thienyl)-3-hexyl-2-thiophenecarboxylic acid21 (5) (1.26 g, 3.35 mmol) was reacted with N-hydroxysuccinimide (0.424 g, 3.69 mmol) and N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (0.706 g, 3.69 mmol). After chromatography (silica gel, CH2Cl2), 8 was obtained (1.35 g, 85%) as a yellow oil. 1H NMR (300 MHz, CDCl3): δ 7.26 (d, J ) 5.0 Hz, 1H, Th-H), 7.05 (s, 1H, Th-H), 6.96 (d, J ) 5.0 Hz, 1H, Th-H), 2.98 (t, J ) 7.7 Hz, 2H, ThCH2-), 2.89 (s, 4H, OCCH2CH2CO), 2.79 (t, J ) 7.9 Hz, 2H, -CH2-) 1.71-1.58 (m, 4H, -CH2-), 1.42-1.24 (m, 12H, -CH2), 0.88 (t, J ) 6.4 Hz, 6H, -CH3). MS (ESI): m/z [M + Na+] ) 498. Anal. Calcd. for C25H33NO4S2: C, 63.13; N, 2.95; H, 6.99. Found: C, 63.10; N, 3.20; H, 7.13. N-(2-Mercaptoethyl)-5-(2-(3-hexyl)thienyl)-3-hexyl-2-thiophenecarboxamide (2). Following the synthetic method used to prepare 1, compound 8 (1.1 g, 2.3 mmol) was reacted with 2.4 mL of triethylamine and cysteamine (0.20 g, 2.6 mmol). After chromatographic workup (silica gel, CH2Cl2-hexanes [80: 20]), 2 was obtained (0.43 g, 43%) as a yellow oil. 1H NMR (300 MHz, CDCl3): δ 7.20 (d, J ) 5.1 Hz, 1H, Th-H), 6.94 (s, 1H, 3-H), 6.94 (d, J ) 5.1 Hz, 1H, Th-H), 6.23 (br, 1H, -NH), 3.61 (dt, J ) 6.1 Hz, 2H, -NCH2-), 2.93 (t, J ) 7.7 Hz, 2H, -CH2-), 2.83-2.71 (m, 4H, -CH2- -CH2S-), 1.72-1.57 (m, 4H, -CH2-), 1.38-1.21 (m, 12H, -CH2-), 0.88 (t, J ) 6.3 Hz, 6H, -CH3). 13C NMR (400 MHz, CDCl3): δ 163.88, 148.11, 141.66, 138.42, 131.19, 130.67, 130.50, 130.14, 125.48, 43.82, 32.71, 32.66, 31.62, 31.55, 30.74, 30.29, 30.25, 30.19, 25.72, 23.63, 15.11. MS (ESI): m/z [M + H+] ) 438. Anal. Calcd. for C23H35NOS3: C, 63.11; N, 3.20; H, 8.06. Found: C, 63.46; N, 3.39; H, 8.11. 5-(3-Hexyl-5-(2-(3-hexyl)thienyl)thienyl)-3-hexyl-2-thiophenecarboxylic acid (6). Benzyl 5-(3-hexyl-5-(2-(3-hexyl)thienyl)thienyl)-3-hexyl-2-thiophenecarboxylate21 (1.01 g, 1.60 mmol) and [Bu4N]OH‚30 H2O (2.60 g, 1.19 mmol) were dissolved in dry THF (250 mL) and heated to reflux for 3 h. The mixture was cooled to room temperature and acidified with 1 M HCl. Diethylether (300 mL) was added to the mixture then washed (three times) with 1 M HCl (50 mL) and finally with water. The organic phase was dried over MgSO4, and the solvent was removed. The residue was purified using column chromatography on silica gel (CH2Cl2-ethylacetate [80:20]) yielding 6 (0.55 g, 63%) as a yellow-orange solid. 1H NMR (300 MHz, CDCl3): δ 7.18 (d, J ) 5.1 Hz, 1H, Th-H), 7.01 (s, 1H, Th-H), 6.96 (s, 1H, Th-H), 6.94 (d, J ) 5.1 Hz, 1H, Th-H), 3.01 (t, 7.8 Hz, 2H, -CH2-), 2.83-2.75 (m, 4H, -CH2-), 1.72-1.59 (m, 6H, -CH2-), 1.47-1.27 (m, 18H, -CH2-), 0.94-0.83 (m, 9H, -CH3). MS (ESI): m/z [M + Na+] ) 567.5. Anal. Calcd. for C31H44O2S3: C, 68.33; H, 8.14. Found: C, 68.53; H, 8.31. 2,5-Dioxo-pyrrolidin-1-yl 5-(3-hexyl-5-(2-(3-hexyl)thienyl)thienyl)-3-hexyl-2-thiophenecarboxylate (9). Following the synthetic method used to prepare 7, compound 6 (0.59 g, 1.08 mmol) was reacted with N-hydroxysuccinimide (0.137 g, 1.19

17186 J. Phys. Chem. C, Vol. 111, No. 46, 2007 mmol) and N-(3-dimethylamino-propyl)-N′-ethylcarbodiimide hydrochloride (0.229 g, 1.19 mmol). After chromatographic workup (silica gel, CH2Cl2), 9 was obtained (0.67 g, 88%) as an orange oil. 1H NMR (300 MHz, CDCl3): 7.20 (d, J ) 5.2 Hz, 1H, Th-H), 7.06 (s, 1H, Th-H), 6.97 (s, 1H, Th-H), 6.94 (d, J ) 5.2 Hz, 1H, Th-H), 2.99 (t, J ) 7.7 Hz, 2H, -CH2-), 2.90 (s, 4H, OCCH2CH2CO), 2.83-2.74 (m, 4H, -CH2-), 1.731.59 (m, 6H, -CH2-), 1.44-1.25 (m, 18H, -CH2-), 0.93-0.83 (m, 9H, -CH3). MS (ESI): m/z [M + Na+] ) 664.4. Anal. Calcd. for C35H47NO4S3: C, 65.48; H, 7.38. Found: C, 65.56; H, 7.40. N-(2-Mercaptoethyl)-5-(2-(3-hexyl)thienyl)thienyl)-3-hexyl-2thiophenecarboxamide (3). Following the synthetic method used to prepare 1, compound 9 (0.63 g, 0.98 mmol) was reacted with triethylamine (1 mL) and cysteamine (0.085 g, 1.1 mmol). After chromatography workup (silica gel, CH2Cl2), compound 3 was obtained (0.252 g, 43%) as an orange solid. 1H NMR (300 MHz, CDCl3): 7.17 (d, J ) 5.1 Hz, 1H, Th-H), 6.96 (s, 1H, Th-H), 6.94 (s, 1H, Th-H), 6.93 (d, J ) 5.2 Hz, 1H, Th-H), 6.24 (br, 1H, -NH), 3.61 (dt, J ) 6.1 Hz, 2H, -NCH2-), 2.93 (t, J ) 7.7 Hz, 2H, -CH2-), 2.83-2.71 (m, 6H, -CH2-,-CH2S-), 1.74-1.58 (m, 6H, -CH2-), 1.46-1.23 (m, 19H, -CH2-,-SH), 0.93-0.82 (m, 9H, -CH3). 13C NMR (400 MHz, CDCl3): δ 163.79, 148.28, 141.96, 140.94, 138.11, 136.15, 131.16, 131.14, 130.41, 130.33, 130.01, 129.80, 124.89, 43.74, 32.67, 31.62, 31.42, 30.78, 30.44, 30.29, 30.26, 30.21, 25.81, 23.62, 15.09. MS (ESI): m/z [M + Na+] ) 626.5. Anal. Calcd. for C23H35NOS3: C, 65.62; N, 2.32; H, 8.18. Found: C, 65.51; N, 2.41; H, 8.45. TDPA-Capped CdSe Nanorods (TDPA-CdSe). The synthesis of the TDPA-capped CdSe nanorods followed procedures established by Peng and co-workers.23 CdO (0.13 g, 1.0 mmol) was dissolved in a solution of TOPO (9.442 g, 24.42 mmol) and tetradecylphosphonic acid (TDPA) (0.56 g, 2.1 mmol) and heated to 300-320 °C. The temperature was then reduced to 270 °C, and a solution of Se/TOP (0.2 M, 6.3 mL, 1.25 mmol) was injected quickly. After injection, the nanorods were allowed to grow for 4 min, and then the heat source was removed, stopping the growth process. The nanorod mixture was cooled to 60 °C, and then degassed CHCl3 (10 mL) was added. The mixture was centrifuged to remove unreacted CdO and Se that was left after decanting the solution. The nanorods were precipitated out of solution by adding degassed methanol and centrifuged. The filtrate was discarded and the reprecipitations repeated until no more trioctylphosphine oxide was detected in the washings by 1H NMR spectroscopy. Oligothiophene-Capped CdSe Nanorods (1-CdSe, 2-CdSe, 3-CdSe). A solution of TDPA-CdSe nanorods (50 mg) was dissolved in degassed CHCl3 (2 mL). Thiol 1 (0.4 mmol) dissolved in degassed CHCl3 was added to this solution. The mixture was allowed to stir for 24 h at room temperature. The 1-CdSe particles were precipitated by the addition of degassed methanol, and the precipitate was collected by centrifugation. The 1-CdSe particles were redissolved in CHCl3, precipitated with methanol, and centrifuged to remove excess unbound 1. The reprecipitations were repeated until 1 was no longer detectable in the washings by thin layer chromatography (TLC). The same procedure was used to obtain 2-CdSe and 3-CdSe. These particles were reprecipitated with degassed methanol from hexanes to remove any excess unbound capping groups. Reprecipitations were repeated 20 times in both cases. However, even after 20 reprecipitations, there were still some unbound capping groups detectable in the TLC of the washings, sug-

Sih and Wolf gesting that these capping groups dissociate to some extent during the reprecipitation process. Results Synthesis. The oligothiophenes 1-3 with a thiol attached at one of the R-positions through an amide linker were prepared via the synthetic route summarized in Schemes 1 and 2. Thiols have been shown to bind strongly to the surface of CdSe nanoparticles.15,24 Hexyl substituents are attached to the 3-position of the thiophene rings to aid in the solubility of the nanoparticles. The synthetic pathway involves first the synthesis of the carboxylic acid derivatives 4-6. Compound 4 was prepared by lithiating 2-bromo-3-hexylthiophene20 followed by quenching with solid CO2 and protonation with HCl. The bithiophene (5) and terthiophene (6) analogs were prepared using sequential Suzuki-coupling reactions.21 The thiol groups were then attached to the oligothiophenes via the amine-reactive N-hydroxy succinimide (NHS) esters. This approach has been previously shown to give good overall yields of the desired amide products.25 The carboxylic acids 4-6 were reacted with N-(3-dimethylamino-propyl)-N′-ethylcarbodiimide hydrochloride to give the amine-reactive Oacylisourea intermediates, which in the presence of NHS undergo substitution to yield the NHS esters (7-9). The subsequent substitution reaction of cysteamine with NHSleaving groups yields the desired thiol functionalized oligothiophenes 1-3, which were purified using column chromatography. The UV-vis spectra of the capping groups (1-3) are shown in Figure 1 and the spectral data tabulated in Table 1. The capping groups have one major absorption band that bathochromically shifts as the number of linked thiophene rings in the oligothiophene increases from 1 to 3, resulting from an increase in the conjugation length and decrease in the HOMOlowest unoccupied molecular orbital (LUMO) gap. DFT calculations were carried out in Gaussian 0322 on 1-3 using a B3LYP/6-31G*(d,p) basis set. The frontier orbitals and energies are shown in Figure 2. The calculated HOMO-LUMO differences for 1-3 are close in energy to the experimentally measured UV-vis absorption maxima. The calculations indicate that the LUMO is localized on the thiophene π-system in each case, and the thiol contributes less orbital density to the HOMO as the length of the oligothiophene increases. In the monothiophene (1), the HOMO has significant orbital density localized on the thiol, and the absorption band in the UV-vis spectrum is therefore assigned as an nfπ* transition. For the longer oligothiophenes (2 and 3), the HOMO has larger thiophene π character and the absorption band is therefore assigned as a πfπ* transition. With an increase in the number of thiophene rings, the relative change in the HOMO-LUMO energy difference decreases with each additional ring. This is typical of conjugated oligomers where the red-shift decreases with increasing chain length and has been previously observed for other oligothiophenes.26,27 The HOMO energies of the oligothiophene-capping groups were determined experimentally by CV, and the voltammograms are shown in Figure 3. The shortest oligomer 1 shows no oxidation below the upper potential limit of CH2Cl2 (2.0 V versus SCE). The higher oxidation potential of 1 compared to 2 and 3 is due to a lower HOMO energy as a consequence of its shorter π-conjugated system. With an increase in the conjugation length by an additional thiophene ring, the oxidation potential decreases (1.48 V for 2) due to an increase in the HOMO energy. There is however no clear reduction peak

CdSe Nanorods with Thiol-Anchored Oligothiophenes

J. Phys. Chem. C, Vol. 111, No. 46, 2007 17187

SCHEME 1

SCHEME 2

associated with the oxidation wave indicating that the oxidation is irreversible. Repeated potential cycling of 2 from 0 to 1.7 V does not show any new peaks or peak growth indicating that no material is electrodeposited onto the working electrode. It is possible that dimerization does occur after oxidation of 2, but the dimerized form is sufficiently soluble in CH2Cl2 that it does not deposit onto the electrode. Extending the π-conjugated system with a third thiophene ring decreases the oxidation potential even more (1.09 V for 3) with two reduction peaks observed at 0.99 and 0.76 V. The wave at 0.99 V is due to the reduction of oxidized 3 while the wave at lower potential (0.76 V) is assigned to reduction of surface bound material formed upon oxidation, possibly dimerized 3. With successive cycling of the potential from 0 to 1.3 V, the peak at 0.76 V grows in intensity with each successive scan (Supporting Information, Figure S1), consistent with electrodeposition of conductive material, possibly dimerized 3. The shortest capping group (1) is nonemissive at room temperature similar to unsubstituted thiophene.28 The excitation

and emission spectra of the longer oligothiophene-capping groups (2 and 3) in CHCl3 are shown in Supporting Information, Figure S2, with the data tabulated in Table 2. The emission maxima bathochromically shift as the number of repeat units increases from 2 to 3 due to increased conjugation and a decrease in the HOMO-LUMO gap. Solid-state emission spectra for the capping groups show a bathochromic shift compared to solution spectra (Table 2). A similar red shift has been observed in the solid-state emission of bithiophene and terthiophene compared to solution;29 this was attributed to increased planarity in the solid state compared to free rotation in solution.30 TDPA-capped CdSe NPs were prepared via the method developed by Peng et al.23 using nonpyrophoric CdO instead of Cd(CH3)2.31 It has been shown that CdSe NPs prepared using this method results in nanorods instead of spherical particles, and the nanorods are surface capped with TDPA.19 To prepare oligothiophene-capped CdSe nanoparticles, an exchange reaction is carried out on the TDPA-CdSe with the oligothiophene-

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Figure 1. UV-vis absorption spectra of capping groups (1-3) in CHCl3 (obtained with an approximate maximum absorbance of 0.1).

TABLE 1: UV-vis Absorption Maxima of Capping Groups (1-3), TDPA-CdSe and Oligothiophene-Capped CdSe NPs in CHCl3 sample

UV-vis absorption λmax (nm)

1 2 3 TDPA-CdSe 1-CdSe 2-CdSe 3-CdSe

258 326 363 547 266, 555 319, 544 362, 544

capping groups. Thiols have been shown to have a high affinity for CdSe surfaces, and TDPA may be displaced with thiols without the need to prepare a pyridine-capped CdSe intermediate.15,24 Scheme 3 outlines the procedure used for attaching thiolfunctionalized oligothiophenes 1-3 to the CdSe NPs. The oligothiophenes 1-3 were dissolved in CHCl3 with TDPACdSe and stirred at room temperature overnight. 1-CdSe particles were purified by repeated precipitation with methanol from CHCl3 solutions. The precipitations were halted when 1 was no longer observed via TLC in the washings. 2-CdSe and 3-CdSe are both more soluble than 1-CdSe and remained dissolved in CHCl3 even when excess methanol was added. 2-CdSe and 3-CdSe were purified by repeated precipitations with methanol from hexane solutions. The washings were examined by TLC, but even after 20 precipitations trace free thiol was still detected. Previous studies have shown that thiolates attached to the surface of CdSe NPs can readily dissociate,32 and an equilibrium between surface bound and solution species is established.33 When 2-CdSe and 3-CdSe are dissolved in hexanes or CH2Cl2, the capping groups appear to dissociate from the surface; this is observed as an increase in emission from unbound capping group as a function of time (Supporting Information, Figure S3). Thiols 2 and 3 are strongly emissive, and even a very small amount of dissociation is evident via TLC. 1H and 31P NMR spectra of the washed thiol-capped NPs 1-CdSe still show TDPA present (Supporting Information, Figure S4a,d, respectively) indicating the exchange reaction is incomplete. 31P NMR spectra of 2-CdSe and 3-CdSe on the other hand show no observable signal due to TDPA (Supporting Information, Figure S4e,f); however, the 1H NMR spectra of 2-CdSe and 3-CdSe show TDPA still present (Supporting Information, Figure S4b,c) also indicating incomplete substitution. The lack of signal in the 31P NMR spectra of 2-CdSe and

Sih and Wolf 3-CdSe could be due to longer correlation times resulting in a broadened signal. The region of the 1H NMR spectra of 1-CdSe, 2-CdSe, and 3-CdSe showing the thiophene proton signals (Supporting Information, Figure S4a-c, inset) demonstrates that the oligothiophenes are present; however, the signals are broader when compared to unbound capping groups, possibly due to restricted rotation. Some of the proton signals have also shifted downfield after attachment to the NP surface. Such a downfield shift has been observed previously in related systems34 and has been attributed to π-electron density being inductively drawn toward the CdSe surface causing deshielding of the oligothiophene protons. No sharp peaks due to unbound capping groups are observed in the 1H NMR spectra of the oligothiophenecapped CdSe NPs. TEM images of TDPA-CdSe dropcast onto carbon-coated TEM grids are shown in Figure 4a. The CdSe nanoparticles are rod shaped with dimensions of ∼3 × 15 nm, similar to those prepared by Peng.19 After the exchange with 3, no change in the size and shape of the particles is observed for 3-CdSe (Figure 4b). The UV-vis absorption spectra of the TDPA-CdSe NPs are shown in Figure 5a. The band edge energy of TDPA-CdSe is 547 nm (2.27 eV). After exchange with the oligothiophenes 1-3, the absorption spectra of the oligothiophene-capped CdSe NPs show two peaks (Figure 5b-d). The lower energy peak corresponds to the band edge of the NPs while the peak at higher energy corresponds to the nfπ* and πfπ* absorptions of the oligothiophene-capping groups. The UV-vis absorption spectrum of TDPA-CdSe subtracted from the absorption spectra of each of the oligothiophene-capped CdSe NPs (1-CdSe, 2-CdSe, and 3-CdSe) confirms this assignment (insets, Figure 5b-d) as the difference spectra have identical features to the absorption spectra of the corresponding capping group used (13). The average number of capping groups on the surfaces of 1-CdSe, 2-CdSe, and 3-CdSe can be calculated from the UVvis absorption data. The molar extinction coefficients for the capping groups are  ) 8190, 14 860, and 20 200 L mol-1 cm-1 for 1, 2, and 3, respectively. The extinction coefficient for the CdSe NPs is  ) 94 850 L mol-1 cm-1 estimated from the first excitonic peak position.30 Deconvoluting the absorption spectra (Figure 5) for the contribution of the capping group and CdSe NP gives an average of 69, 29, and 47 capping groups present in 1-CdSe, 2-CdSe, and 3-CdSe, respectively. The smaller number of capping groups for 2-CdSe and 3-CdSe compared to 1-CdSe are possibly due to the large steric bulk of the longer oligothiophene-capping groups and poorer affinity for the CdSe surface compared to TDPA. The emission spectra of the CdSe NPs in CHCl3 excited at 350 nm are plotted in Figure 6. The TDPA-CdSe NPs have an emission maximum at 562 nm exhibiting a large shift from the excitation wavelength. This large shift is due to the large number of excitonic transitions that nonradiatively decay to the LUMO, emitting at the energy of the first excitonic peak (HOMO-LUMO energy gap).35 When oligothiophenes are exchanged for the TDPA on the CdSe NP surface, interesting results are observed. Figure 6b shows the emission spectra of 1-CdSe and TDPA-CdSe at the same optical density. The presence of 1 on the CdSe surface quenches the NP fluorescence. This is in contrast to the results for 2-CdSe and 3-CdSe where no significant quenching of the NP fluorescence is observed (Figure 6c,d). When 2-CdSe and 3-CdSe are left in solution, the intensity of the higher energy emission (from oligothiophene) increases with time (Figure S3). After allowing the solution to equilibrate

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Figure 2. Calculated frontier orbitals of 1-3 with energy level diagram depicting the HOMO and LUMO levels.

Figure 3. Cyclic voltammetry of (a) 2 and (b) 3 in CH2Cl2 containing 0.1 M [(n-Bu)4N]PF6.

for 4 h, the growth of this emission halts. The emission at shorter wavelength is at least partially attributable to unbound 2 and 3 present in solution. The solid-state emission spectra of 2-CdSe and 3-CdSe (Supporting Information, Figure S5) support this deduction where there is a much weaker high-energy band than in solution due to the absence of dissociated oligothiophenecapping groups. This data suggests that emission from the capping groups 2 and 3 attached to the CdSe surface in 2-CdSe and 3-CdSe is partially or completely quenched. The emission

from 2 or 3 in solution is strong, so even a small number of molecules dissociating from the NP surface appears to dwarf the NP emission (Figure 6). The excitation spectrum of 1-CdSe at an emission wavelength of 562 nm is identical to that of TDPA-CdSe demonstrating that no charge or energy is transferred from 1 to the NP. On the other hand, excitation spectra of 2-CdSe and 3-CdSe are quite different from that of TDPA-CdSe. Figure 7 shows the excitation spectrum of TDPA-CdSe subtracted from the excitation spectra of 2-CdSe and 3-CdSe. The difference spectra for 2-CdSe and 3-CdSe have maxima at 355 and 383 nm, respectively. These difference excitation spectra have maxima that are red-shifted from the excitation maxima of 1 and 2 by 20 nm in 2-CdSe and 16 nm in 3-CdSe. These red shifts in the excitation spectra may be due to the increased planarity in the oligothiophene backbone that is expected to result from close packing between adjacent molecules at the NP surface. A similar red shift was observed in the solid-state absorbance spectra of bithiophene and terthiophene compared to solution29 due to increased planarity in the solid state.30 The solid-state absorbance spectra of 2 and 3 both display a 15 nm red shift compared to the absorption in solution. Therefore, excitation of the oligothiophene-capping groups bound to the 2-CdSe and 3-CdSe NP surface results in emission from the NPs at 562 nm.

17190 J. Phys. Chem. C, Vol. 111, No. 46, 2007

Sih and Wolf

TABLE 2: Excitation and Emission Maxima of Capping Groups (1-3), TDPA-CdSe, and Oligothiophene-Capped CdSe NPs in CHCl3 and in the Solid State

sample

λmax in CHCl3 [emission wavelength] (nm)

λmax in CHCl3 [excitation wavelength] (nm)

solid-state emission maxima [excitation wavelength] (nm)

1 2 3 TDPA-CdSe 1-CdSe 2-CdSe 3-CdSe

no emission 335 [414] 367 [468] broad broad broad broad

no emission [258] 414 [326] 468 [363] 562 [350] 560 [350] 415, 562 [350] 468, 565 [350]

no emission 448 [399] 485 [450] 569 [350] 566 [350] 407, 566 [350] 465, 567 [350]

Electrodeposition of the oligothiophene-capped CdSe NPs by oxidative coupling was attempted both by potential cycling and electrolysis. However, no electrodeposited material was detected for any of the samples. 1-CdSe and 2-CdSe showed no oxidation peaks in the accessible potential window for CH2Cl2. Electrochemical potential cycling of 3-CdSe did show a small oxidation at ∼1.1 V (Supporting Information, Figure S1b) with a small broad reduction peak associated with it on the reverse scan at ∼0.7 V. There was a slight increase in current on the second scan compared to the first but subsequent scans after that showed no further increase. This suggests that some oxidative coupling may have occurred on the first scan but potential cycling after that results in no further crosslinking. This is in contrast to terthiophene-capped gold NPs, which can readily be electrochemically polymerized giving films up to 1 micron thick.36 This difference could be related to the higher inherent conductivity of the Au NP core relative to the CdSe NPs.37,38 When 3-CdSe is electrochemically deposited after the first scan, the particles may act as an insulating layer between the electrode and other 3-CdSe particles in solution thus preventing further oxidative crosslinking from occurring. Discussion The observation that excitation of the oligothiophene-capping groups in 2-CdSe and 3-CdSe results in emission from the CdSe NPs can be rationalized via the energy diagram shown in Figure 8. The band edges of a CdSe nanoparticle39 and the HOMO and LUMO energies of 2 and 3 determined from UV-vis absorption and CV are shown in this Figure. Because the LUMOs of 2 and 3 are higher in energy than the lower edge of the NPs conduction band, electron transfer is possible from excited 2 or 3 into the CdSe NP. The transferred electron can then radiatively relax to the CdSe valence band where the SCHEME 3

emitted photon’s energy equals the CdSe band edge energy. A similar electron-transfer mechanism has been observed in a blend of polyvinylpyrrolidone/CdSe nanoparticles where the energy levels are similar.40,41 Another possible mechanism is Fo¨rster resonance energy transfer (FRET) where the excited oligothiophene may undergo nonradiative transfer of energy to the CdSe NP giving radiative emission from the CdSe NP. Examples of FRET from a conjugated polymer to CdSe NPs have also been previously observed.42,43 According to the CV measurements, the HOMOs of 1-3 are all lower in energy relative to the valence band edge of the CdSe NP and therefore cannot act as hole traps. This explains why no significant quenching of the CdSe NP luminescence is observed in 2-CdSe or 3-CdSe, but does not explain the quenching of the CdSe NP luminescence observed in 1-CdSe because the HOMO of 1 is also lower than the HOMOs of both 2 and 3. A possible explanation for the anomalous behavior of 1-CdSe is based on consideration of the number of thiolate groups on the NP surface. The choice of functional group used to attach to the surface has been shown to greatly affect CdSe NP luminescence.44 Thiolates in particular have been shown to quench CdSe NP fluorescence. It has been theorized that quenching occurs due to thiolates acting as hole traps (electron transfer from the HOMO of the thiolate into the CdSe particle).45 However, not all thiolate-capped NPs show luminescence quenching and some thiolate concentrations result in slightly enhanced NP fluorescence.46 CdSe NP surfaces have intrinsic electron trap states that are deactivated by electron-donating thiolates at low concentrations. As the number of thiolates on the CdSe NP surface exceeds the number of electron-trapping sites, the excess thiolates becomes hole-trapping sites that quench fluorescence.

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Figure 4. TEM images of (a) TDPA-CdSe and (b) 3-CdSe drop-cast onto a carbon-coated TEM grid.

Figure 7. Excitation spectrum of TDPA-CdSe subtracted from excitation spectra of (a) 2-CdSe and (b) 3-CdSe (emission wavelength ) 562 nm).

Figure 5. UV-vis absorption spectra of (a) TDPA-CdSe, (b) 1-CdSe, (c) 2-CdSe, and (d) 3-CdSe in CHCl3. Insets: difference spectra between the respective oligothiophene-capped CdSe NPs and TDPACdSe.

Figure 8. HOMO and LUMO energy potential diagram for 1, 2, and 3. Band edge values for CdSe are based on ionization potentials from reference 41 and the band gap is from UV-vis absorption of TDPACdSe.

Figure 6. Emission spectra of (a) TDPA-CdSe, (b) 1-CdSe, (c) 2-CdSe, and (d) 3-CdSe excited at 350 nm in CHCl3. For 1-CdSe, 2-CdSe, and 3-CdSe, the emission spectrum of TDPA-CdSe at the same optical density is plotted on the same graph for comparison.

The average number of capping groups on the surfaces of 1-CdSe, 2-CdSe, and 3-CdSe calculated from the UV-vis absorption data correlates with the NP fluorescence. 1-CdSe has the largest average number of capping groups on the surface

and shows quenching of the NP fluorescence. In this case, a large number of thiolates relative to electron trap sites may result in 1 acting as hole-trapping states that quench fluorescence. On the other hand, 2-CdSe and 3-CdSe with fewer capping groups present do not have excess thiolate acting as hole-trapping states, and fluorescence is not quenched. To test this hypothesis, an excess amount of thiol was added to 1-CdSe and 2-CdSe in solution. Dodecanethiol was used instead of 1 or 2 because strong emission from 2 completely swamps out any CdSe NP emission. Furthermore, dodecanethiol is less bulky than 1 and 2 and may be able to insert between molecules already tethered to the surface of 1-CdSe and 2-CdSe. This results in the immediate reduction of fluorescence in both 1-CdSe and 2-CdSe by half. After 30 min, complete quenching is observed in 2-CdSe, while for 1-CdSe the fluorescence decreases steadily up to 1 h after which time there remains a weak fluorescence. Dodecanethiol added to 1-CdSe and 2-CdSe in solution presumably binds to the NP surface and quenches the fluores-

17192 J. Phys. Chem. C, Vol. 111, No. 46, 2007 cence from the CdSe NP. With time, more dodecanethiol molecules bind to the CdSe surface and eventually no more emission from the NP is observed. This observation is consistent with the proposed explanation that the presence of excess thiolate on the NP surface acts as hole traps and quenches the NP fluorescence. Conclusions A series of thiol-substituted oligothiophenes were used to functionalize the surface of CdSe nanoparticles. Attachment of the oligothiophenes to the CdSe nanoparticles results in little change to the electronic structure of the oligothiophenes as shown by the similarities in the absorption spectra to the unbound oligothiophenes. However, the oligothiophene emission is quenched after attachment to the CdSe surface due to either an electron or energy transfer mechanism. Depending on the number of oligothiophenes attached to the CdSe surface, the emission from the CdSe nanoparticles differs. A larger number of bound thiols act as hole traps leading to quenching of the NP emission. Attempts to electrochemically crosslink the oligothiophene-capped CdSe nanoparticles were unsuccessful possibly due to the intrinsic resistivity in the particles. Acknowledgment. We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding. Supporting Information Available: Cyclic voltammograms, fluorescence spectra, and NMR spectra (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Gaponenko, S. V. Optical Properties of Semiconductor Nanocrystals; Cambridge University Press: Cambridge, 1998. (2) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (3) Chan, W. C. W.; Nile, S. Science 1998, 281, 2016. (4) Greenham, N. C.; Peng, X.; Alivisatos, A. P. Phys. ReV. B. 1996, 54, 17628. (5) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (6) Coe, S.; Woo, W.-K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800. (7) Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. AdV. Mater. 2000, 12, 1102. (8) Liu, J.; Tanaka, T.; Sivula, K.; Alivisatos, A. P.; Frechet, J. M. J. J. Am. Chem. Soc. 2004, 126, 6550. (9) Odoi, M. Y.; Hammer, N. I.; Sill, K.; Emrick, T.; Barnes, M. D. J. Am. Chem. Soc. 2006, 128, 3506. (10) Querner, C.; Reiss, P.; Bleuse, J.; Pron, A. J. Am. Chem. Soc. 2004, 126, 11574. (11) Skaff, H.; Sill, K.; Emrick, T. J. Am. Chem. Soc. 2004, 126, 11322. (12) Locklin, J.; Patton, D.; Deng, S.; Baba, A.; Millan, M.; Advincula, R. C. Chem. Mater. 2004, 16, 5187. (13) Milliron, D. J.; Alivisatos, A. P.; Pitois, C.; Edder, C.; Frechet, J. M. J. AdV. Mater. 2003, 15, 58. (14) Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G. J. Chem. Phys. 1997, 106, 9869. (15) Wisher, A. C.; Bronstein, I.; Chechik, V. Chem. Commun. 2006, 1637. (16) Hikmet, R. A. M.; Chin, P. T. K.; Talapin, D. V.; Weller, H. AdV. Mater. 2005, 17, 1436. (17) Hu, J.; Li, L.-s.; Yang, W.; Manna, L.; Wang, L.-w.; Alivisatos, A. P. Science 2001, 292, 2060.

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