J. Phys. Chem. B 2006, 110, 20895-20900
20895
Magnetic and Electronic Properties of Palladium Nanoparticles Coated with π-Conjugated Tetrathiafulvalenes Derivative Weixia Tu,†,‡ Ken-ichi Fukui,† Akira Miyazaki,† and Toshiaki Enoki*,† Department of Chemistry, Tokyo Institute of Technology, Tokyo 152-8551, Japan, and Key Lab for Nanomaterials, Ministry of Education, College of Chemical Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China ReceiVed: June 5, 2006; In Final Form: August 10, 2006
Magnetic and electronic properties are investigated for Pd nanoparticles coated with a TTF-derivative (EDTTTF(SCH3)(SC10H20SH)) and octadecanethiol organic mixed monolayer. The temperature-independent spin susceptibility and spin concentration of Pd decrease upon the increasing proportion of the TTF-derivative in the organic layer. With the introduction of the derivative, the ESR broad signal originating from the interior of the Pd nanoparticles tends to vanish following the appearance of sharp signals due to the TTF radical. The presence of the TTF molecules enhances the charge transfer from the core Pd nanoparticles. The electronic state of the Pd nanoparticles changes as a consequence of the contributions of both the quantum-size effect and the charge-transfer effect between the Pd core, TTF-derivative, and alkanethiol.
Introduction Recently, metal nanoparticles have attracted considerable interest in supramolecular chemistry and nanoscopic physics as they have unconventional properties and functionalities, which are different from those of the bulk metal and a single metal atom.1 Among the interesting issues, the quantum-size effect has been one of the important targets in condensed-matter physics, where the energy level discreteness is responsible for the unconventional features of the particles. Recent development of research works on self-assembled monolayers (SAMs) has added a different aspect to the chemistry and physics of metal nanoparticles. Actually, works on nanoparticles with their surfaces covered with alkanethiol SAMs, which are easily prepared by the colloidal solution technique, have triggered the development of functionalized metal nanoparticles, leading to one of the important issues in supramolecular chemistry. Using SAMs of molecules intentionally designed for special purposes, functionalized metal nanoparticles can be obtained which work with functions such as magnetic, electron reservoir, optical, and chemical functions. In functionalizing metal nanoparticles, the interface between the core nanoparticle and the covering SAM can cooperate uniquely. Indeed, recent work on Pt nanoparticles with their surface covered with alkanethiol SAMs has revealed the electron confinement around the interface area, which can work as an electron reservoir.2 Charge transfer taking place from the core metal nanoparticle to the sulfur atoms of thiol molecules makes the interface electron-enriched with the core of the metal nanoparticle electron-deficient. The electrons are considered to be weakly bound around the interface area. In the present work, we focus on the electron confinement at the interface and investigate its electronic properties on the basis of the results obtained in the previous paper.2 Here, we employ the tetrathiafulvalene (TTF)-derivative of an alkanethiol for the * Corresponding author. Tel/Fax: 81-(0)3-5734-2242. E-mail: tenoki@ chem.titech.ac.jp. † Tokyo Institute of Technology. ‡ Beijing University of Chemical Technology.
SAM molecule and a Pd nanoparticle for the metal core particle. TTF acting as an electron donor is expected to participate in the electron-transferring process at the interface.3,4 Meanwhile, palladium is known to have anomalous electronic features associated with the presence of a large electron density of states around the Fermi energy,5 which brings about a strong paramagnetic feature. The electron transfer from two constituents, TTF and Pd nanoparticle, is expected to give interesting electron-reserving features at the interface. Palladium nanoparticles covered with mixtures of TTFderivative (EDT-TTF(SCH3)(SC10H20SH)) and octadecanethiol (ODT) are synthesized and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, electron spin resonance, and magnetic susceptibility in order to clarify the structure and the electronic properties generated at the interface between the metal core particle and the coating layer. The varied properties caused by the different coverage of the TTF-derivative are discussed. Experimental Section Synthesis of EDT-TTF(SCH3)(SC10H20SH) was taken from starting materials of carbon disulfide and sodium following the methods reviewed in refs 6-8. Its constitutional formula is shown below with n ) 10:
Pd nanoparticles covered with octadecanethiol (ODT) were synthesized according to the literature method.9 The typical process is as follows: 0.5 mmol of palladium acetate ((CH3COO)2Pd) and 0.5 mmol of octadecanethiol were dissolved in 10 mL of THF freshly distilled from sodium/benzophenone ketyl prior to use, and then a solution of lithium triethylborohydride in THF (1.0 M, 5 mL) was added. The color of the mixture
10.1021/jp063463l CCC: $33.50 © 2006 American Chemical Society Published on Web 09/21/2006
20896 J. Phys. Chem. B, Vol. 110, No. 42, 2006 turned immediately from bright yellow to dark brown. After stirring for 2 h, the resulting solution gave colloidal Pd nanoparticles covered by ODT. The nanoparticles were precipitated by adding anhydrous ethanol and were washed several times to remove all starting materials. Finally, the nanoparticles were dried under vacuum condition (symbolized as Pd(ODT)). The Pd(ODT) nanoparticle powder was dispersed in dichloroform with the addition of different amounts of EDT-TTF(SCH3)(SC10H20SH). The mixture was stirred for 24 h to exchange ODT by EDT-TTF(SCH3)(SC10H20SH). Then, the nanoparticles can be precipitated under ethanol and washed by ethanol and acetone. The as-obtained nanoparticles were dried under vacuum (symbolized as Pd(ODT)(TTF). The composition ratios of the obtained nanoparticles were determined from C, H, N, and S elemental analysis on the basis of molecular formulas of octadecanethiol and EDT-TTF(SCH3)(SC10H20SH). Transmission electron microscopy (TEM) photographs were taken by using a JEOL FE-TEM JEM-2010F instrument with an acceleration voltage of 200 kV. Specimens were prepared by dropping the redissolved nanoparticle solution in dichloroform upon a copper grid covered with a perforated carbon film and then evaporating the solvent at room temperature. The average diameter of the particles was determined from diameters of 300 nanoparticles found in an arbitrarily chosen area in the TEM photographs. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an X-ray photoelectron spectrometer Phi5500 MT (ESCA/MC/SAM) with a Mg KR X-ray source. The Pd nanoparticle powder was put on an indium foil. All binding energy values were referred to carbon (C1s ) 284.6 eV). The magnetic susceptibility and magnetization of the Pd nanoparticle powder were measured with Quantum Design MPMS-5 DC-SQUID magnetometer in the temperature range 2-300 K under magnetic fields up to 5.5 T. Electron spin resonance (ESR) spectra were measured at room temperature with an X-band spectrometer (JEOL JES-TE200), where the magnetic field and microwave frequency were calibrated using a NMR field meter and a frequency counter, respectively. The calculation of spin concentration was obtained by using an ESR maker (MnO/MgO) as an inner standard, whose intensity was calibrated by DPPH (1,1-diphenyl-2-picrylhydrazyl) with a known spin concentration.10
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Figure 1. TEM photographs and size distribution histograms of Pd(ODT)0.75 (a) and Pd(ODT)0.17(TTF)0.5 (b) nanoparticles.
Results Palladium nanoparticles protected by octadecanethiol were synthesized by a chemical reduction method.2 An exchange reaction was carried out to make EDT-TTF(SCH3)(SC10H20SH) cover the surface of the Pd nanoparticles accompanied with the desorption of the ODT from the particle surface. Conventional C-H-N-S elemental analysis confirmed the obtained Pd nanoparticles with different compositions. They are denoted as Pd(ODT)0.75, Pd(ODT)0.35(TTF)0.05, Pd(ODT)0.45(TTF)0.25, and Pd(ODT)0.17(TTF)0.5, where the subscripts indicate the molar ratio of the organic molecule to Pd. In these Pd particles, the molar proportion of the TTF-derivative in the organic layer of the nanoparticles is 0, 12.5%, 36%, and 75%, respectively. Figure 1 shows TEM photographs and size distribution histograms of Pd(ODT)0.75 and Pd(ODT)0.17(TTF)0.5. It can be seen that Pd(ODT)0.75 has an average diameter () 2.2 nm and a standard derivation () 0.35 nm with spherical shape. After the exchange reaction by the TTF-derivative, the mean size of the Pd nanoparticles is not obviously changed ( 2.2 nm and 0.36 nm). This means that the core of the Pd particle has no change in shape and size, and the change occurs only on the component of the coating organic layer.
Figure 2. TTF-derivative ratio dependence of the temperatureindependent susceptibility (χ0Pd) (a), and the total spin concentration (Ns) (b) of the palladium nanoparticles (b) and the spin concentration of the TTF π-electron origin (O). χ0Pd of the bulk Pd is also shown.
The magnetic susceptibility χ measured under 1 T for all the samples obeys the Curie law in the whole temperature range investigated, as expressed by χ ) C/T + χ0, where C and χ0 are the Curie constant and the temperature-independent term of the susceptibility, respectively. Due to the core diamagnetic susceptibility of the coating organic compound, the constant susceptibility χ0 is negative for all the particles. By subtracting the contribution of the organic component, the temperatureindependent susceptibility for Pd nanoparticles (χ0Pd) can be calculated. Figure 2, parts a and b, give the TTF-derivative composition ratio dependence of localized spin concentration (Ns) and χ0Pd of palladium, respectively. The χ0Pd values for all the nanoparticles are much smaller than that of bulk Pd (5.4796 × 10-4 emu/mol), which indicates the change in the electronic structure of the core of the Pt nanoparticles due to the quantumsize effect and interface effect between the core and the coating
Pd Nanoparticles Coated with TTF-Derivative
Figure 3. The magnetization curves of the Pd nanoparticles at 2 K accompanying the Brillouin curves of s ) 1/2 and s ) 1.
Figure 4. ESR signals of the Pd nanoparticle samples: Pd(ODT)0.75 (a), Pd(ODT)0.35(TTF)0.05 (b), Pd(ODT)0.45(TTF)0.25 (c), and Pd(ODT)0.17(TTF)0.5 (d).
organic layer.2 It can be seen that χ0Pd decreases as the amount of TTF-derivative in the particles increases. For Pd(ODT)0.17(TTF)0.5 with 75% TTF-derivative in the organic layer, χ0Pd shows obviously a negative value (-6.708 × 10-5 emu/mol). The spin concentrations (Ns) given in Figure 2b are estimated from the Curie constant C with an assumption the spin value of s ) 1/2 and the g-value of g ∼ 2. Ns is estimated as 1.5 spin per particle for the Pd(ODT)0.75 particles. With the increasing amount of the TTF-derivative in the particles, Ns decreases gradually. (The average spins per Pd atom for Pd(ODT)0.75, Pd(ODT)0.35(TTF)0.05, Pd(ODT)0.45(TTF)0.25, and Pd(ODT)0.17(TTF)0.5 are 0.0039, 0.0030, 0.0025, and 0.0018, respectively.) For Pd(ODT)0.17(TTF)0.5, Ns is 0.70 spin per particle. These results reveal that the TTF-derivative affects the spin states of the nanoparticles. Figure 3 shows the magnetization curves of the Pd nanoparticles at 2 K accompanying Brillouin curves of s ) 1/2 and 1. For all the samples, the observed magnetization curves show smaller magnetization than that expected in comparison with the Brillouin curves with s ) 1/2 and 1. This indicates the presence of interaction between the spins, although the data analysis can be also partly responsible for the result. Figure 4 shows the ESR signals of the Pd nanoparticles. Pd(ODT)0.75 nanoparticles have a broad signal with a g-value of 2.0748 and a line width of 20 mT. The large line width and the g-value deviation from the free electron spin g-value (2.0023) indicate that the Pd electrons subjected to the large spin-orbit interaction are responsible for the broad ESR signal. When 12.5% TTF-derivative is introduced into the Pd particles (Pd(ODT)0.35(TTF)0.05), the broad signal weakens and a new
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Figure 5. Sharp components of the ESR signals of the samples Pd(ODT)0.35(TTF)0.05 (a), Pd(ODT)0.45(TTF)0.25 (b), and Pd(ODT)0.17(TTF)0.5 (c) in the magnetic field range of 318-328 mT.
sharp signal appears. With the larger TTF proportions (Pd(ODT)0.45(TTF)0.25 and Pd(ODT)0.17(TTF)0.5), the broad signal becomes invisible and the sharp signal is pronounced. Figure 5 reveals the behavior of the sharp ESR signal of Pd(ODT)0.35(TTF)0.05, Pd(ODT)0.45(TTF)0.25, and Pd(ODT)0.17(TTF)0.5 in the magnetic field range of 318-328 mT. The sharp signal having multiple peaks can be explained in terms of randomly oriented anisotropic molecules having three different g-values along the principal axes. From the analysis of the signal, the g-values along the principal axes are estimated at g1 ) 2.0069, g2 ) 2.0063, and g3 ) 2.0048, while the line widthes are 0.7 mT, 0.8 mT, and 0.8 mT, respectively. These observed g-values demonstrate that the TTF+ radical is responsible for the sharp ESR signal, where g1, g2, and g3 can be assigned to the g-value along the long molecular axis and the short molecular axis, and perpendicular to the molecular plane of the TTF molecule.11 The presence of the TTF+ radical spin signal proves that charge transfer is taking place from the TTF moiety to the surrounding. Taking into account that TTF-substituted alkanethiol molecules are oriented roughly perpendicular to the surface of the Pd nanoparticles, TTF molecules bound to the spherical-shaped nanoparticle can give the ESR signal of randomly oriented molecules as observed. Table 1 gives the calculation results of the spin concentration for the whole Pd nanoparticle sample (NsTotal) and the contribution of the TTF-derivative in the organic layer (NsTTF). The results reveal that the spin contribution from the TTF-derivative is pronounced as the amount of the TTFderivative increases in the organic layer. XPS gives an important clue in characterizing the electronic state of the Pd nanoparticles. As shown in Figure 6a, the binding energies (BEs) of Pd3d5/2 and Pd3d3/2 in Pd(ODT)0.75 are 335.5 and 340.7 eV, respectively, which shift to higher binding energies by 0.4 eV compared with those of bulk Pd (Pd3d5/2, 335.1 eV; Pd3d3/2, 340.3 eV).12 In contrast, the S2p BE for the Pd(ODT)0.75 particles is 162.8 eV (Figure 6c), which shifts to the lower binding energy by 1.0 eV than the S2p BE of the pure octadecanethiol molecules (163.8 eV).13 These results indicate the electron transfer from Pd to S in the ODT molecules, which is the consequence of the thiol-bonding effect.14 The BEs of Pd3d5/2 and Pd3d3/2 for the Pd(ODT)0.45(TTF)0.25 nanoparticles are 336.4 and 341.7 eV (Figure 6b), respectively, with a positive energy shift of another 0.9 eV from those of Pd(ODT)0.75. The size of the Pd core had no change by exchange reaction by the TTF derivative as noted above. Thus, the charging energy of the Pd nanoparticles (e2/2C, where C is the capacitance between the Pd core and the indium substrate) associated with the electron emission is not affected or slightly decreased by an increase of the capacitance (due to the increase of the dielectric constant
20898 J. Phys. Chem. B, Vol. 110, No. 42, 2006
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TABLE 1: Spin Concentration of the Pd Nanoparticle Samples molar ratio of the TTF-derivative in the organic layer (%)
NsTTF from ESR (1020 per mole Pd sample)
NsTTF from ESR (1020 per mole TTF)
NsTotal from Curie constant (1021 per mole Pd sample)
NsTTF/ NsTotal (%)
12.5 36 75
0.21 0.95 2.9
4.3 3.8 5.8
1.8 1.5 1.1
1.2 6.3 26
by introduction of TTF derivative), which should have resulted in the shift of Pd3d to a lower BE from that for Pd(ODT)0.75. Therefore, the shift of Pd3d to a higher BE should be attributed to the more positive valence of the Pd in Pd(ODT)0.45(TTF)0.25 than that in Pd(ODT)0.75 nanoparticles. The BE of the S2p peak (Figure 6d) of the Pd(ODT)0.45(TTF)0.25 nanoparticles is 163.3 eV, which should consist of contributions from the TTF moiety and S atoms bound to the Pd particle. Note that the contribution of the TTF moiety to the spectrum is larger from the content of S atoms in the TTF moiety and those bound to the Pd core (ca. 2.9:1). Meanwhile, the pristine TTF-derivative in Figure 6e has a peak with a shoulder corresponding to S2p3/2 and S2p1/2, respectively, where the overall full width at half-maximum (fwhm) is 2.002 eV and the maximum peak position of S2p BE is 163.8 eV. These results suggest that the TTF moiety remains almost neutral. This is in good agreement with the observed spin concentration of TTF+ radicals, which is estimated as about 0.1% of the total number of TTF molecules as shown in Table 1. Compared with Figure 6c, S atoms bound to the Pd core (1/3.9 contribution in the S2p signal) should have a peak around 162.8 eV or at a smaller binding energy if a larger charge transfer from the Pd core than that for Pd(ODT)0.75 is taken into account. This point will be discussed later. Discussion The previous paper on alkanethiol-SAM-coated Pt nanoparticles2 demonstrates two important issues in discussing the electronic structure of metal nanoparticles with their surface covered with SAMs. The first one is the interface effect, where the alkanethiol SAMs present around the surface of the nanoparticles induces charge transfer from the interior of the
Figure 6. XPS spectra of Pd and S atoms for Pd(ODT)0.75 (a,c), Pd(ODT)0.45(TTF)0.25 (b,d) nanoparticles, and EDT-TTF(SCH3)(SC10H20SH) (e). The dotted lines are guides for comparison.
nanoparticle to the sulfur atoms at the interface with the core Pt nanoparticles being electron-deficient. In general, the Pt nanoparticles having the even number of electrons tend to have a decrease of the spin susceptibility as the temperature is lowered enough compared with the energy discreteness δ. However, the electron-deficient feature of Pt nanoparticles makes the Pt nanoparticles deviate from the even-electron system, resulting in the large increase in the localized spin concentration. The second issue is the quantum-size effect, for which the energy level discreteness is responsible. Indeed, the alkanethiol-SAM-coated Pt nanoparticles show that the Pauli susceptibility at temperatures high enough compared to the energy discreteness δ decreases upon the decrease of the particle size, as explained by the relations χ ) µB2/δ and δ ∼ EF/N with the Fermi energy EF and the particle number included N. The experimental findings obtained in the present results also suggest the importance of these two kinds of effect, although the participation of TTFderivative SAM in the charge transfer makes the electronic structure more complicated in addition to the change in the electronic structure by substituting Pt atoms with Pd atoms. Here, the electronic structures of the present nanoparticles are discussed on the basis of the two effects. We begin the discussion with the susceptibility, which reflects sensitively the change in the electronic structures. As we explained in the results, the susceptibility observed consists of the temperature-independent term and the Curie’s term of localized spins, the former of which involves the Pauli paramagnetic susceptibility as the major contribution. The localized spin concentration observed in the ODT-SAM-coated Pd nanoparticles (1.5 spins/particle) is in the range similar to that observed in SAM-coated Pt nanoparticles,2 as shown in Figure 2b. Taking into account that the localized spins observed are assigned to the Pd electrons having the large spin-orbit interaction from the large ESR line width (∆H ) 20 mT) and the large g-value (g ) 2.0748), this suggests that the electron deficiency produced by charge transfer from the core Pd nanoparticle to the interface enhances the localized spin concentration observed at low temperatures. The electron deficiency of the core Pd nanoparticles is confirmed by the high energy shift of BE of the Pd3d XPS spectra. The spin concentration of the Pd nanoparticles was larger than that of Pt nanoparticles. Actually, in the case of Pt nanoparticles, the spin concentration observed suggests that each Pt nanoparticle has 0 or 1 spin. In contrast, each Pd nanoparticle can have more than 1 spin. The larger spin concentration is considered to be related to the strong paramagnetic features of Pd, which originates from the large density of states around the Fermi energy, as will be discussed later with the temperature-independent susceptibility. The participation of TTF-derivative molecules in the SAM tends to reduce the localized spin concentration, where the spin concentration becomes 0.7 spin/ particle at the TTF composition ratio of 75%. Here, it should be noted that part of the TTFalkanethiol molecules are also in the paramagnetic radical state TTF+, as evidenced by the presence of localized spins assigned to the TTF moieties. Therefore, the spin concentration of the Pd spins has to be discussed by subtracting the contribution of
Pd Nanoparticles Coated with TTF-Derivative the TTF spins from the total spin concentration. Figure 2b shows the spin concentrations of the Pd electrons and the TTF π-electrons as a function of TTF composition ratio. The Pd spin concentration decreases from 1.5 to 0.5 spin/particle as the TTF composition ratio increases from 0 to 75%, whereas the TTF spin concentration is elevated from 0 to 0.2 spin/particle. The monotonical decrease in the Pd spin concentration is apparently inconsistent with the XPS results. Indeed, the increase in the TTF composition ratio makes the BE of the Pd3d shift to higher energy, suggesting that TTF contributes to making Pd nanoparticles more electron-deficient. Consistency can be obtained by taking the experimental finding that each Pd nanoparticle can have more than 1 spin. In the case in which each nanoparticle has less than 1 spin, the spin can behave as an isolated localized spin, whose concentration monotonically increases upon the increase in the charge-transfer rate. In contrast, when the spin concentration exceeds 1 spin/ particle, the localized spins present in the same nanoparticle interact with each other antiferromagnetically, resulting in the reduction of the effective spin concentration, as can be seen in Figure 2b. Therefore, the reduction in the Pd spin concentration gives evidence that the presence of TTF enhances the charge transfer from the core Pd nanoparticles. Next, we approach the change in the electronic structure from the behavior of the Pauli susceptibility. The value of the temperature-independent susceptibility χ0, which the Pauli susceptibility governs, is reduced considerably from the bulk to nanoparticle shown in Figure 2a, where χ0 drops to 12% of the bulk value at the TTF composition ratio of 0%. The reduction in χ0 can be explained partly in terms of the quantumsize effect, as discussed for the Pt nanoparticles in the previous paper.2 Indeed, the naked Pt nanoparticles having a mean size of 2.2 nm show a 28% drop of χ0 from that in the bulk Pt. In addition to this, the ODT-coated Pd nanoparticles are subjected to the charge-transfer effect at the interface. This large change in the value of χ0 cannot be explained simply by the contribution of the quantum-size effect. According to Chen et al.,5 bulk Pd has a sharp density-of-states peak around the Fermi level associated with the contribution of the d-electron levels. This contributes to an increase the Pauli paramagnetic susceptibility that is exceptional for bulk pristine Pd. The charge transfer taking place from the core Pd nanoparticles induces the shift of the Fermi energy to the low-energy side. The lowering in the Fermi level works to reduce the density of states around EF, resulting in the reduction of the Pauli paramagnetic susceptibility. Eventually, the observed large reduction of the Pauli paramagnetic susceptibility in the Pd nanoparticles is the consequence of the contributions of both the quantum-size effect and the charge-transfer effect. The introduction of TTF tends to reduce the value of χ0 more and make it negative above the TTF composition ratio of 36%. This is reasonably explained by the participation of TTF molecules in the charge-transfer process. The core Pd nanoparticles become more positive with the introduction of the larger portion of TTF, as evidenced from the BE of the Pd3d. The Fermi energy shift induced by the charge transfer reduces again the density of states at the Fermi energy, resulting in the additional reduction in the Pauli susceptibility upon the increase of the TTF content. Finally, the role of the TTF molecules in the charge transfer is addressed. A previous report15 suggested that PdS is formed on the surface of polycrystalline Pd thin films by partial decomposition of alkanethiol groups during the SAM formation process. From our XPS results, the increase in the S2p signal vs
J. Phys. Chem. B, Vol. 110, No. 42, 2006 20899 Pd3d signal by exchange of some ODT to the TTF derivative can be reasonably explained by the contribution of the amount of introduced TTF moiety. Therefore, the formation of PdS is not a major process in our case. However, it may be possible to form a very small amount of PdS at the interface during an exchange reaction, which contributes to the slight increase of the charge transfer from Pd to the interfacial S that forms the sulfide. It is difficult to determine the origin of enhanced charge transfer from the Pd core in our experiments. In the meantime, according to the ESR experimental results, the TTF radical spin concentration is only 0.1% even in Pd(ODT)0.17(TTF)0.5 that has the highest TTF concentration. It is considered that the ODT and TTF-derivative molecules are randomly distributed on the SAM surface with some structural irregularity and defects of remaining sulfur atoms. On the surface, most of these longshaped molecules are aligned perpendicular to the surface. However, a small fraction of the molecules around the defect points can be laid down flat on the surface of Pd nanoparticles, since the molecules having long alkyl chain groups are easily bent due to their flexibility. In this situation, TTF-derivative molecules that have four sulfur atoms in the TTF moiety are expected to have interaction with negatively charged surface sulfur atoms through the sulfur-sulfur atomic contacts, resulting in the charge transfer from TTF to the interface area. This can create TTF-radical spin species as observed in the present experiment. Summary Pd nanoparticles coated with the TTF-derivative (EDT-TTF(SCH3)(SC10H20SH)) and an octadecanethiol organic monolayer are synthesized through an exchange process from octadecanethiol-coated Pd nanoparticles. The core size of the particles has no change with the average diameter of 2.2 nm after the introducion of different amounts of TTF-derivative into the SAM. The low-temperature susceptibility shows a Curie-type divergence, which is due to the charge transfer from the Pd to the organic layer. The charge transfer results in the deviation from the even-electron system of Pd. The introduction of the TTFderivative affects the magnetic and electronic properties of the Pd nanoparticles. The Pauli susceptibility and the spin concentration of Pd decrease upon the increasing proportion of the TTF-derivative in the organic layer. The ESR broad signal originating from the interior Pd nanoparticles decreases with the appearance of the sharp signals from the TTF radical. The existence of the TTF-derivative causes the change in the electronic structure at the interface between Pd and the organic layer. Acknowledgment. We thank Prof. Toshiharu Teranishi for his valuable discussion. Thanks to Y. Yokota and M. Shibasaki are also expressed for their help and discussion on the synthesis. The present work was partly supported by a Grant-in-Aid for Scientific Research on Priority Area (No. 12046231). W.T. was supported by the Japan Science Promotion Society Postdoctoral Fellowship for Foreign Researchers and Natural Science Foundation of Young Teachers in Beijing University of Chemical Technology (No. QN0401). References and Notes (1) Schmid, G. Clusters and Colloids; VCH: Weinheim, 1994. (2) Tu, W.; Takai, K.; Fukui, K.; Miyazaki, A.; Enoki, T. J. Phys. Chem. B 2003, 107, 10134. (3) Bryce, M. R. Chem. Soc. ReV. 1991, 20, 335. (4) Roncali, J. J. Mater. Chem. 1997, 7, 2307. (5) Chen, H.; Nrener, N. E.; Callaway, J. Phys. ReV. B 1989, 40, 1443. (6) Svenstrup, N.; Becher, J. Synthesis 1995, 215.
20900 J. Phys. Chem. B, Vol. 110, No. 42, 2006 (7) Simonsen, K. B.; Svenstrup, N.; Lau, J.; Simonsen, O.; Mørk, P.; Kristensen, G. J.; Becher, J. Synthesis 1996, 407. (8) Binet, L.; Fabre, J. M. Synthesis 1997, 1179. (9) Yee, C. K.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3486. (10) Fujito, T.; Enoki, T.; Ohya-Nishiguchi, H.; Deguchi, Y. Chem. Lett. 1972, 557-560. (11) Sugano, T.; Saito, G.; Kinoshita, M. Phys. ReV. B 1986, 34, 117.
Tu et al. (12) Practical Surface Analysis Vol. 1, Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, 1990. (13) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (14) Fu, X.; Wang, Y.; Wu, N.; Gui, L.; Tang, Y. J. Colloid Interface Sci. 2001, 243, 326. (15) Love, J. C.; Wolfe, D. B.; Haasch, R.; Chabinyu, M. L.; Paul, K. E.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 2597.
