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Oxide Nanoparticle Thin Films Created Using Molecular Templates J. A. Gardener,†,O I. Liaw,‡,[ G. Aeppli,† I. W. Boyd,‡,z S. Fiddy,§ G. Hyett,^ T. S. Jones,|| S. Lauzurica,‡ R. G. Palgrave,^ I. P. Parkin,^ G. Sankar,^ M. Sikora,#,þ A. M. Stoneham,†,& G. Thornton,^ and S. Heutz*,4 †
Department of Physics and London Centre for Nanotechnology, University College London, London WC1E 6BT, U.K. Department of Electronic and Electrical Engineering and London Centre for Nanotechnology, University College London, London WC1E 6BT, U.K. § Daresbury Laboratory, Daresbury Science and Innovation Campus, Warrington WA4 4AD, U.K. ^ Department of Chemistry and London Centre for Nanotechnology, University College London, London WC1E 6BT, U.K. Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K. # ESRF, 6 rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex, France 4 Department of Materials and London Centre for Nanotechnology, Imperial College London, London SW7 2AZ, U.K.
)
‡
ABSTRACT: We present a new generic method to synthesize nanostructured metal(II) oxide films on a substrate such as silicon. Vacuum ultraviolet (VUV) excimer lamps rupture metalorganic precursors, creating volatile organic fragments, while the metal species at the surface form oxides. X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) confirm that 172 nm VUV irradiation of manganese and copper phthalocyanines yields manganese(II) and copper(II) oxides, respectively. The morphology of the precursor film provides a template from which metal oxide nanoparticles are formed, which we demonstrate for particles with dimensions 40 40 10 nm3. Our procedure is both simple and flexible, with low thermal budget and potential for patterning.
’ INTRODUCTION Metal oxide nanoparticles display a wealth of applications, encompassing medicine,13 optics,46 catalysis,79 magnetism,10,11 photovoltaics,12,13 and microelectronic devices,14 with still further potential if organized arrays of nanosized metal oxides could be created on surfaces, as demonstrated for the optical properties of ZnO nanorods.6 In particular, thin films of manganese oxides have been investigated for a wide range of applications such as ultracapacitors15 or electron injection layers in organic light emitting devices.16 MnO deposited onto Si has a high dielectric constant that could be used in field-effect transistors.17 MnO is antiferromagnetic below the Neel temperature of ∼120 K,18 but the magnetic and electronic properties of the thin films depend on the film thickness and type of substrate;19,20 indeed, recent studies have highlighted the presence of a ferromagnetic phase in MnO nanoparticles.21 The ferromagnetism is mediated by surface states, which are a wellknown driver of unexpected magnetic properties even in the simplest oxides such as MgO.22 Therefore, anticipated magnetic properties can occur in either continuous or nanostructured thin films of MnO. Yet, there are limitations to the many current approaches to create designed metal oxide nanostructure morphologies and compositions, often including added nanostructural features. The so-called bottom-up approaches23 capitalize on the varieties of nanoparticle shapes that can be synthesized24,25 and then r 2011 American Chemical Society
arranged on surfaces through self-assembly,26 while techniques such as stamping and dip-pen lithography27 create patterned nanoparticle films. Top-down approaches include atomic layer deposition,28 chemical vapor deposition,29 or wet chemical methods,30 followed by techniques such as electron beam lithography.31 These approaches are labor and/or cost intensive, especially if a wide variety of nanostructured assemblies are sought. Current film growth methods, mainly pulsed laser deposition and atomic layer deposition, suffer from a number of limitations. Indeed, they rely on high substrate temperatures and crystallinities, yield polycrystalline films with little flexibility regarding morphology, and are susceptible to producing phases with higher oxidation states. Our approach addresses the need for a generic method to produce nanostructured, single valence, metal oxide films at low temperature and with low thermal budget. Here, we exploit the nanosized topographical features of metal-containing thin molecular films to produce metal(II) oxide particulates. This approach is motivated by the diverse range of nanostructures that can be constructed from molecular building blocks32 and the low cost and temperature associated with each step of the synthesis. Briefly, we grow a uniform film comprising nanocrystalline domains on Si(100) substrates. This film is then irradiated Received: January 19, 2011 Revised: May 5, 2011 Published: June 17, 2011 13151
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The Journal of Physical Chemistry C with 172 nm vacuum ultraviolet (VUV) photons produced by excimer lamps, which represent a high power, uniform light source that is clean, cheap, and readily scalable for mass production technologies.33,34 The VUV light photodecomposes the molecular film, releasing the metal and producing volatile organic fragments. We have previously shown that this procedure can lead to the diffusion of some transition metals into the substrate and have used it to produce interstitially doped Mn in Si(100).35 Here we focus instead on the film which is formed on top of the substrate and find that it is a metal oxide film which retains the morphology and metal oxidation state of the precursor film. We demonstrate our procedure using metal phthalocyanine (MPc, M = metal) thin films. These molecules are ideal candidates, given the wide variety of metal variants readily and cheaply available.36 In addition, they are thermally robust, and a wide range of nanostructures can be easily formed via the choice of deposition technique and experimental parameters used. For example, nanowires, -ribbons, and -brushes can be obtained via organic vapor phase deposition,37,38 while thin films,39 nanofibers,40 or even nanoflowers41 can be grown by organic molecular beam deposition. These nanostructures are routinely created via sublimation onto a wide range of metallic, semiconducting, and insulating substrates. Deposition can also be achieved from the solution phase,42 either as chemisorption of single layers opening up new possibilities for coating pores and other structure geometries43 or as thicker LangmuirBlodgett films,36 thus further increasing the potential applications of our technique. High surface areas are indeed critical for applications in catalysis or electrode materials in batteries, while flat precursor films would be chosen if the oxide is to be used in a multilayer magnetoresistive structure or as a transparent conducting oxide. One further advantage of MPcs is that they are isostructural for small transition metal centers and that homogeneous mixtures can therefore easily be formed by coevaporation. This would offer an additional route to create more complex oxides, for example, the dilute magnetic semiconductor Zn1xMnxO, without any risk for phase segregation. Here, we have deposited thin MPc films using organic molecular beam deposition. The choice of substrate temperature44 and film thickness45 enables us to form different shapes and sizes of domains. In particular, the formation of CuO and MnO is demonstrated from CuPc and MnPc thin films, respectively, and examples were chosen to highlight the versatility and wide range of applications of this new procedure. Nanosized CuO is of interest as a catalyst,46 while the size-dependent magnetism in MnO could be exploited for magnetic data storage applications.10 Further, multiple stable oxidation states exist for both oxide species, and current methods to produce MnO often yield higher oxide contaminants. Therefore, our studies will also highlight the selectivity of the preparation method for obtaining a single oxidation state in the metal.
’ EXPERIMENTAL METHODS Si(100) substrates were prepared by immersing in HF (VWR, 50% VLSI, selectipure grade) for 30 s and then rinsed in DI water and dried in a nitrogen stream immediately before loading into a Kurt J. Lesker organic molecular beam deposition chamber (base pressure of 5 108 mbar). Manganese and copper phthalocyanine were purchased from Sigma Aldrich (97% pure) and further purified by two cycles of gradient sublimation, prior to loading in a ceramic crucible. Thin films of each MPc (5 nm thick
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film, as monitored by an in situ quartz crystal microbalance) were deposited onto the Si(100) substrates held at room temperature. During deposition, the purified MPc material was heated to typically 330400 C, resulting in typical growth rates of 0.51 nm/min. The MPc films were then transferred to an ex situ second vacuum chamber wherein 172 nm of VUV light was generated by a home-built array of four parallel Xe 750 W excimer lamps. The output power was 3 W as stated by Boyd et al.33 This corresponds to a homogeneous illumination density of ∼7 mW/cm2 for samples placed 8 cm below the lamp array. Prior to irradiation, this chamber was evacuated to typically 0.1 mbar and backfilled with nitrogen gas to a pressure of 5.0 mbar; therefore, experiments were performed with an oxygen partial pressure of ∼0.02 mbar. Ex situ characterization of the films was performed using a Renishaw Raman spectrometer, with a continuous wave green laser (λexc = 514.5 nm) at a power of 0.97 mW (a low power was chosen to avoid laser-induced degradation). Topographic characterization was achieved using a Veeco Dimension 3100 AFM in tapping mode, and the images were analyzed using WSxM software.47 X-ray photoelectron spectroscopy (XPS) was performed by an Escalab 220i-XL instrument using Al KR radiation (typical base pressure of 2 109 mbar). We calibrated the XPS spectra using the known binding energy of benzene, the most abundant form of carbon in the MPc compounds.48,49 Gaussian fitting was applied to the high-resolution XPS spectra after performing a Shirley background subtraction. X-ray absorption spectroscopy (XAS) measurements were performed at two synchrotron facilities. Those performed at the Mn K-edge were obtained from the European Synchrotron Radiation Facility (Grenoble, France), Station ID26, using a 13 element Ge detector for KR fluorescence, at an energy resolution of 0.5 eV. The Cu K-edge data were collected at SRS, Daresbury Laboratory, Station 7.2, again in fluorescence mode. The XAS spectra were analyzed using Athena and ExCurve98 software (SRS, Daresbury Laboratory).
’ RESULTS AND DISCUSSION The MPc film is an ideal precursor since it is of high purity and homogeneous over an area of several centimeters. Analysis of the vibrational modes of the MPc films via Raman spectroscopy provides a quantitative analysis of the amount of MPc present as a function of VUV exposure. Figure 1(a) shows a series of Raman spectra of a 5 nm CuPc film. Here, the black spectrum (most intense signal) is of the as-deposited CuPc layer on Si. A series of well-defined peaks are observed due to the main breathing modes of this molecule, the most intense occurring at 1530 cm1 which is attributed to a vibration of B1 symmetry.50 Upon increasing UV exposure, these peaks are seen to sequentially decrease in intensity, implying a systematic reduction in CuPc content. We note that an analogous behavior has been observed for 5 nm MnPc films35 and 600 nm thick CoPc films.51 No CuPc signature can be detected after 20 min of irradiation, suggesting that at this stage the Pc ligand has been fully removed and/or destroyed. Thermal desorption of the entire molecules can be excluded, as even after 40 min of continuous irradiation the sample temperature remains less than 60 C, which is comfortably below the sublimation temperature of MPc’s (typically above 300 C). Although structural changes can occur as a result of annealing, such phase transitions are induced at much higher temperatures and do not lead to a loss of molecular material.44,52 Instead, we 13152
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Figure 1. (a) Raman spectra (λexc = 514.5 nm) of 5 nm thick CuPc films on Si(100) as a function of UV exposure time (durations stated in the legend). The most intense peaks are observed for the as-deposited film, with the intensities of each peak systematically decreasing as a function of increasing irradiation time. The molecular structure of metal phthalocyanine is shown in the inset. Tapping mode AFM images (1 1 μm2) of a 5 nm thick CuPc film (b) before and (c) after 40 min UV exposure show that morphological features are retained after degradation (note that a and b are not from identical areas but are representative of the whole film which is homogeneous over 1 cm2). The profiles (obtained from the AFM images above along the marked black line) are shown for the 5 nm CuPc film before (d) and after (e) 40 nm UV exposure.
attribute the systematic reduction in MPc signal of Figure 1(a) to photoassisted decomposition by the 7.2 eV photons emitted by the UV excimer.33 On the basis of the dissociation energies of the bonds present in the phthalocyanine ligand,53 it is likely that the CN bonds are the most susceptible to rupture.44,52 Inspection of the molecular structure (inset of Figure 1(a)) suggests that benzene and pyrrole-containing complexes are formed. Such species are volatile and will have desorbed prior to our Raman measurements. The MPc binding energies are of the same order of magnitude as the UV photons.54,55 Even a partial decomposition of the MPc molecule via rupturing the CN bonds will destabilize the MN bonds, further reducing the MPc binding energy. It is therefore feasible that the metal may be left behind and form part of the degradation product, as will be demonstrated shortly. Tapping mode AFM has been used to study the topography of the MPc films. As seen in Figure 1(b), 5 nm thick MPc films on Si comprise many small protrusions, resulting in a root-meansquare film roughness of 2.2 nm. These are characteristic of
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Figure 2. XPS analyses of a 5 nm MnPc film on Si(100) before and after 40 min UV irradiation. (a) Comparison of the spectra across a large binding energy range (offset for clarity). The identity of the most intense peaks is labeled accordingly. A significant reduction in C and N, coupled with an increase in O, is seen from the as-deposited film (top, black) and UV exposed film (bottom, red). High-resolution XPS spectra of the (b) C 1s and (c) Mn 2p peaks. The raw data are plotted as a thick black line, while Gaussian peaks, along with the calculated Shirley background and summed fitted data (red), are also shown. Dramatic UV-induced changes are observed for the C 1s peaks, demonstrating that the organic framework is photodecomposed, while more subtle variations are seen for the two Mn 2p spectra.
the so-called “R-phase” polymorph, wherein densely packed cylindrical columns extend outward from the substrate surface.45 The domains shown in Figure 1(b) are all of similar size, typically 3040 nm in diameter and ∼5 nm in height, as seen from the linescan in Figure 1(d). After 40 min of irradiation, corresponding to double the time required to fully degrade the molecular film (as determined by Raman and UVvis spectroscopy), the general topography of the precursor films is retained (Figure 1(c)). The number density of the domains is identical to the one in the starting film, although the height has decreased to about 2.5 nm (Figure 1(e)) due to the loss of organic fragments. The lateral sizes are however similar to the precursor film and range between 30 and 40 nm. Therefore, the retention of the two-dimensional morphology of the MPc film after its photodecomposition demonstrates the ability to use the initial structure as a template. Having established that the UV light degrades the MPc molecules while leaving nanosized features on the surface, we now focus on the chemical composition of the residue formed. Figure 2(a) compares X-ray photoelectron spectroscopy (XPS) survey spectra of a MnPc film before and after 40 min of UV exposure (i.e., after all MPc molecules have been photodecomposed). 13153
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The Journal of Physical Chemistry C For the as-deposited films, we observe an intense C 1s presence, in addition to N 1s and Mn 2p peaks (Figure 2(a), top trace)). After correcting for the relative sensitivity factors,56 we deduce a C:N:Mn ratio of 32.0 ( 2.4:7.4 ( 1.4:1.2 ( 0.3. These values are in excellent agreement with the MPc empirical formula (32:8:1), confirming the integrity of the initial film. Photoelectrons arising from absorption events in the substrate (the Si 2p peaks) are also detected, verifying that the entire film thickness is probed by this technique. The XPS analyses were performed ex-situ; between sample preparation and analysis each sample was exposed to air, resulting in the partial oxidation of the Si(100) surface and leading to the detection of small O 1s peaks. After exposure to the 172 nm irradiation (Figure 2(a), bottom trace) the relative intensities of the main spectral lines are dramatically altered. In particular, the intensities of the C 1s and N 1s peaks are reduced, which we attribute to photodecomposition of the organic ligand and subsequent sublimation of any volatile fragments formed. Most significantly, the ratio of C to Mn decreases to 6.3 ( 1.4:1.0 ( 0.1, a factor of ∼4 increase in relative Mn content from the initial film. Clearly, some Mn remains at or near the surface after the irradiation procedure, with some also diffused into the bulk Si,35 while the majority of the organic matter is removed. Further changes in the local chemical environment induced by the UV treatment were analyzed using high-resolution XPS spectra (Figure 2(b) and (c)). For the case of MnPc, peaks are observed in the C 1s spectrum at 284.8, 286.3, and 288.0 eV. The first of these arises from the presence of benzene, while the second is from the CN bonds. The relative intensities of these peaks are 74.8:25.2, in excellent agreement with that of 75:25 obtained from the molecular structure. The low intensity higher binding energy peak is a C 1s satellite.49 As seen from Figure 2(b), UV exposure drastically changes the line shape of the C 1s profile, an unambiguous demonstration of photodegradation. A new peak is observed at 289.2 eV; this originates from carbonyl groups57 that are either formed during the processing or arise from posttreatment surface contamination. The Mn 2p spectra before and after UV exposure are compared in Figure 2(c). In the as-deposited films, features corresponding to the spinorbit split 2p1/2 (653.9 eV) and 2p3/2 (642.6 eV) peaks are observed, along with their lower intensity satellites.48 The close agreement between the fitted and experimental data, combined with relatively narrow linewidths, confirms that only one oxidation state of Mn is present after irradiation. Subtle variations in the line shape of the Mn profile are observed after UV exposure, accompanied by a drastic increase in the relative oxygen content after UV processing (Figure 2(a)). This implies that metal oxides have been formed. After UV exposure, the metal 2p peaks do not show any significant shift in binding energy compared to the MPc profile, suggesting that the Mn remains in a 2þ oxidation state. Further, the Mn 2p3/2 peak appears truncated with a line width of ∼1.0 eV; such a profile is characteristic of MnO.58 The peak positions are also consistent with literature values for Mn oxides, although it is difficult to unambiguously attribute the oxidation state from the Mn 2p lines. For example, Tan et al. report a difference in Mn 2p3/2 positions of only 0.5 eV when oxidation states vary from (II) to (IV).59 These differences are smaller than the range of peak position that has been reported for MnO, and therefore the attributions from XPS can be inconclusive.60,61 We have also performed an analogous XPS study of UV-irradiated CuPc films (not shown here), the results of which imply the formation of
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CuO, but similar reservations about the accuracy of the method have to be made. XAS allows us to resolve these issues. Our XAS measurements were performed using fluorescencemode detection, to enhance the surface sensitivity. X-ray absorption near edge structures (XANES) of the MnPc and CuPc films before and after UV exposure are compared in Figures 3(a) and (b). Mn is in the neutral oxidation state in pure foil form and so provides a reliable reference for energy calibration. We also compare to MnO powder previously published62 and taken in conditions identical to our data, as verified by the correspondence of the Mn foil data in both experiments. Comparisons of the absorption edge positions (defined as the maxima of the first derivative of the absorption spectra) with respect to the Mn foil can give an indication of the oxidation state of each sample. We observe shifts with respect to the Mn foil of þ9.4, þ11.9, and þ9.4 eV for the MnPc powder (not shown) film and UV-exposed film, respectively. These are all consistent with Mn2þ, in line with the inference of MnO formation from our XPS studies. Subtle variations in the line shape of the pre- and post-UV treated films are observed. The UV treated MnPc film shows a small pre-edge at ∼6540 eV along with maxima at 6553, 6567, and 6597 eV, in good agreement with the known spectral features of bulk MnO58,63,64 and similar to the reference MnO spectrum. Slight discrepancies between MnO and the degraded film might be due to differences in crystallinity and size effects in the nanoparticulate films. The observations for CuPc are similar, as shown in Figure 2(b). In particular, the absorption edge profiles of the CuPc precursor, degraded CuPc film, and CuO all coincide, demonstrating that Cu2þ is retained upon UV irradiation. Further, the line shape of the degraded film is in good agreement with that of CuO, although subtle differences exist due to the nanoscale morphology of our samples compared to bulk CuO.65 We have also performed analysis of the extended X-ray absorption fine structure (EXAFS) oscillations of the post-UV MnPc and CuPc films to further support our claim of forming generic MO (M = metal) nanoparticles. Owing to the low signalto-noise ratio obtained from such thin films and the anticipated signal from any metal incorporated into the bulk substrate,35 only the first shell has been fitted over the first two oscillations in each case. The Fourier transforms of the experimental and simulated data are compared in Figure 3(c) and (d). The best fit in Figure 3(c) yields a MnO bond distance of 2.23 Å and coordination number of ∼6.0, in excellent agreement with the known bond length of 2.22 Å for bulk MnO.66 In a similar manner, a bond length of 1.93 Å and coordination number of ∼4.6 were found from Figure 3(d), in agreement with the known 1.93 Å CuO bond distance of CuO clusters.67 The combination of our XPS, XAS, and EXAFS analyses therefore confirms that MnO and CuO are the most abundant species produced. Following the elucidation of the chemical composition of the particulates formed by photodegratation, we can attempt to rationalize the morphological features observed in Figure 1b,c, namely, the retention of lateral feature sizes and decrease in particle height. The mechanism of degradation is likely to proceed with minimal lateral diffusion of the metal atoms upon removal of the organic ligands, and the position of the metallic centers in the CuPc structure can serve as a starting point for understanding the CuO feature sizes. It has been shown before that R-CuPc deposited on silicon and glass crystallizes with the (100) plane nearly parallel to the substrate, using the indexation proposed by Hoshino et al.68 Since CuPc molecules are separated by 12.9 Å along the direction normal to the substrate, a 5 nm 13154
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Figure 3. X-ray absorption analyses of MnPc and CuPc films after being subjected to 40 min UV irradiation. (a) XANES spectra performed at the Mn KR edge of Mn metal foil, MnO powder (from ref 62), and a 5 nm thick MnPc film before UV exposure. The onset of X-ray absorption occurs at similar energies for the MnPc and MnO spectra, which are all shifted to a significantly higher energy than that of the pure Mn metal. (b) XANES spectra obtained at the Cu KR edge of CuPc powder, CuO powder, and a 5 nm CuPc film after UV exposure. The onset of X-ray absorption occurs at similar energies for all three spectra, confirming that they are of the same oxidation state. Fourier transforms of the EXAFS oscillations measured for 5 nm thick (c) MnPc and (d) CuPc films after 40 min of UV exposure (black lines). These are compared to theoretically simulated fits (red), for which the position of the first peaks is seen to coincide with the experimental data in both cases.
thick film corresponds to an average of four copper atoms perpendicular to the substrate. Nearest-neighbor distances between Cu centers are 3.86 Å along the CuOCu bond, as observed in our studies and in the literature.67 Therefore, if arranged in CuOCu chains, four Cu atoms would lead to a thickness of 1.5 nm, which is close to the values observed. Within the plane of the substrate, the M atoms in the MPc crystal form chains separated by 3.7 Å along the [010] direction and by 12.0 Å along the [001] direction. The former dimension is close to the MM or MOM distances in the MO particle, and therefore the crystal size is not expected to change upon degradation. The latter is indeed much larger in the MPc precursor; however, we expect that the gaps generated between the MO particles are where the carbon residue, observed by XPS, is most likely to be accommodated, and therefore morphological change is minimized.
with residual oxygen from the chamber (pO2 ∼ 2 102 mbar) and adsorbed on the film. After UV exposure, the nanostructure of the MPc film is retained, and nanoparticles with a height of 2 nm and a diameter of about 3040 nm are formed. The process that we have proposed could be readily extended to manufacture a range of oxide nanoparticles, including ternary oxides, since there is considerable control over the choice of precursor film composition and morphology, with potential for patterning. A wide range of phthalocyanine polymorphs exist, each with different grain shapes and sizes, while a diverse range of metal phthalocyanine species are commercially available. However, our approach should not be limited to MPc thin films. The conservation of the oxidation states, coupled with the controllable formation of homogeneous nanostructured films, suggests that our approach could be extended to other metalorganic precursors with different oxidation states and morphologies.
’ CONCLUSIONS We have shown that UV excimer irradiation of metal phthalocyanines photodecomposes the precursor films, forming metal(II) oxides. This appears to be a novel, highly reproducible, and versatile method to generate MnO and CuO nanoparticles on surfaces, with very low thermal budget. Given the many distinct forms of oxides of these metals (seven stable forms of manganese, two of copper), it is therefore very significant that we have formed metal oxides in the 2þ oxidation states. We believe that the formation of MnO and CuO in this manner is a direct result of the presence of M2þ within the precursor molecule combined
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses O
Department of Physics, Harvard University, Cambridge, MA 02138, USA. [ School of Chemistry, University of Melbourne, Victoria 3010, Australia. z Melbourne Centre for Nanofabrication, Monash University, VIC 3800, Australia. 13155
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Department of Solid State Physics, AGH University of Science and Technology, 30-059 Krakow, Poland.
Notes &
A.M. Stoneham passed away on February 18, 2011.
’ ACKNOWLEDGMENT We thank Professor Andrew Evans, Dr. Mary Ryan, and Mr. David Gonzalez Arellano for useful discussions. We are grateful to Dr. Steve Firth for assistance with the Raman measurements. Financial support from the Research Council UK and the Engineering and Physical Sciences Research Council (EPSRC) Basic Technology grants “Putting the Quantum into Information Technology” and “Molecular Spintronics” is gratefully acknowledged. We thank the Royal Society for a Dorothy Hodgkin Research Fellowship (SH) and a Royal Society Wolfson Research Merit Award (GA). We acknowledge the ESRF for provision of their facilities and P. Gl€atzel and T.-C. Weng for assistance. We thank CLRC for facilities and technical support at the SRS in Daresbury. ’ REFERENCES (1) Jaffer, F. A.; Libby, P.; Weissleder, R. Circulation 2007, 116, 1052. (2) Stoimenov, P. K.; Klinger, R. L.; Marchin, G. L.; Klabunde, K. J. Langmuir 2002, 18, 6679. (3) Lee, J. H.; Huh, Y.-M.; Jun, Y.-W.; Seo, J.-W.; Jang, J.-T.; Song, H.-T.; Kim, S.; Cho, E.-J.; Yoon, H.-G.; Suh, J.-S.; Cheon, J. Nat. Med. 2007, 13, 95. (4) Li, L.; Zhou, Z.; Wang, X.; Huang, W.; He, Y.; Yang, M. Phys. Chem. Chem. Phys. 2008, 10, 6829. (5) Wu, Y. L.; Lim, C. S.; Fu, S.; Tok, A. I. Y.; Lau, H. M.; Boey, F. Y. C.; Zeng, X. T. Nanotechnology 2007, 18, 215604. (6) Zhang, X.; Lin, D.; Zhang, L.; Li, W.; Gao, M.; Ma, W.; Ren, Y.; Zeng, Q.; Niu, Z.; Zhou, W.; Xie, S. J. Mater. Chem. 2009, 19, 962. (7) Li, L.; Sun, X.; Qiu, X.; Xu, J.; Li, G. Inorg. Chem. 2008, 47, 8839. (8) Vajda, S.; Pellin, M. J.; Greeley, J. P.; Marshall, C. L.; Curtiss, L. A.; Ballentine, G. A.; Elam, J. W.; Catillon-Mucherie, S.; Redfern, P. C.; Mehmood, F.; Zapol, P. Nat. Mater. 2009, 8, 213. (9) Shi, F.; Tse, M. K.; Pohl, M.-M.; Radnik, J.; Bruckner, A.; Zhang, S.; Beller, M. J. Mol. Catal. A: Chem. 2008, 292, 28. (10) Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J. T. Angew. Chem., Int. Ed. 2004, 43, 1115. (11) Jun, Y.-W.; Seo, J. W.; Cheon, J. Acc. Chem. Res. 2008, 41, 179. (12) Yang, Z.; Chiang, C.-K.; Chang, H.-T. Nanotechnology 2008, 19, 025604. (13) Boucle, J.; Ravirajan, P.; Nelson, J. J. Mater. Chem. 2007, 17, 3141. (14) Nag, J.; Haglund, R. F. J. Phys.: Condens. Matter. 2008, 20, 264016. (15) Pang, S.-C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444. (16) Luo, J.; Xiao, L.; Chen, Z.; Gong, Q. Appl. Phys. Lett. 2008, 93, 133301. (17) Dakhel, A. A. Thin Solid Films 2006, 496, 353. (18) Shull, C. G.; Strauser, W. A.; Wollan, E. O. Phys. Rev. 1951, 83, 333. (19) Neubeck, W.; Ranno, L.; Hunt, M. B.; Vettier, C.; Givord, D. Appl. Surf. Sci. 1999, 138, 195. (20) Nagel, M.; Biswas, I.; Nagel, P.; Pellegrin, E.; Schuppler, S.; Peisert, H.; Chasse, T. Phys. Rev. B 2007, 75, 195426. (21) Lee, Y.-C.; Pakhomov, A. B.; Krishnan, K. M. J. Appl. Phys. 2010, 107, 09E124. (22) Stoneham, M. J. Phys.: Condens. Matter 2010, 22, 074211. (23) Lu, W.; Lieber, C. M. Nat. Mater. 2007, 6, 841.
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