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Effects of Processing Conditions on the Work Function and Energy-Level Alignment of NiO Thin Films Mark T. Greiner,*,† Michael G. Helander,† Zhi-Bin Wang,† Wing-Man Tang,† and Zheng-Hong Lu*,†,‡ Department of Materials Science and Engineering, UniVersity of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4, and Department of Physics, Yunnan UniVersity, 2 Cuihu Beilu, Yunnan, Kunming 650091, People’s Republic of China ReceiVed: August 31, 2010; ReVised Manuscript ReceiVed: October 12, 2010
We have investigated NiO thin films prepared by in situ and ex situ ozone oxidation, as well as air-exposed and vacuum-annealed NiO films. The core-level and valence-level photoemission spectra, as well as the work function and energy-level alignment with a common hole-injection material, have been measured using X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. We found that in situ oxidation results in the formation of a purely NiO film, while ex situ oxidation and air exposure result in a hydroxide-terminated NiO film. Work functions as high as 6.7 eV can be achieved for in situ-oxidized NiO; however, the work function decreases rapidly with time due to adsorption of residual gases in vacuum. The work functions of ex situ and air-exposed NiO were significantly lower, between 5.2 and 5.6 eV, due to hydroxylation of the oxide surface. We have examined the rate at which the work function decreases with air exposure and found there to be a very rapid initial decrease in work function, followed by a much slower continual decline. Despite the decrease in work function, energy-level alignment of R-NPD is not affected until the work function drops below a threshold value. We have also examined the effect of vacuum annealing of NiO and found that it becomes highly defective with oxygen vacancies, causing the Fermi level position of the oxide to move away from the valence band maximum and decreasing the work function. Introduction Transition metal oxide (TMO) thin films are now commonly used in organic electronic applications,1 such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic thin-film transistors (OTFTs). Nickel monoxide (NiO) has recently drawn attention for these applications because it has very favorable energy-level alignment with several important hole-transport organic semiconductors,2 due to its high work function, and it is a p-type transparent conductive oxide.3 It has been used as a hole-injection buffer layer in OLEDs2,4 and OPVs;5 it has been used in several dye-sensitized solar cell designs6 and as an electrode buffer layer in OTFTs.7 All of these applications involve NiO/organic interfaces. In order to keep fabrication costs of such devices low, it is necessary to fabricate them under low-temperature conditions and using procedures that are simple to carry out. For example, NiO films are often formed by oxidation of nickel in ultraviolet (UV)-generated ozone in ambient atmosphere,8 by O2-plasma oxidation,9 or by mild annealing of NiO precursors.5b Organic layers are to be solution processable, which often requires that the NiO be exposed to atmospheric conditions. NiO is known to be catalytic and to bond to many types of atmospheric molecules,10 as well as to form several hydroxide structures.11 Such changes to the NiO surface can alter its properties, which is especially a concern if it worsens the energy-level alignment with organic layers, thereby worsening device performance. It is important to know how the surface properties of NiO can * To whom correspondence should be addressed. E-mail: mark.greiner@ utoronto.ca (M.T.G.);
[email protected] (Z.-H.L.). † University of Toronto. ‡ Yunnan University.
change with treatment procedures and how these changes will affect organic energy-level alignment. In the present work we compare the properties of in situwith ex situ-prepared NiO films. Most reports of ex situ-prepared NiO films quote a work function of between 5.0 and 5.6 eV,4c,5b,9,12 while we have found that in situ preparation can yield a work function as high as 6.7 eV. The work function, however, is very sensitive to adsorbate exposure. It drops very rapidly as water and other atmospheric gases, such as O2 and CO2, adsorb to the surface. We have examined the effect of atmospheric adsorbates on the work function, X-ray photoemission spectra (XPS), and ultraviolet photoemission spectra (UPS) of NiO films, as well as the effect on energy-level alignment of NiO with the common hole-transport material N,N′-diphenyl-N,N′bis-(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (R-NPD). We find that in situ ozone oxidation forms NiO, and during exposure to air, a surface hydroxide is formed. We also find that ex situ ozone treatment results in a fully hydroxylated surface, resulting in a lower work function than in situ-prepared NiO. Prolonged exposure to air results in further decrease of the work function, due to adsorption of CO2, O2, and H2O. We find, however, that even though the work function rapidly decreases with air exposure, the energy-level alignment with organic semiconductors does not worsen until the work function of the NiO becomes smaller than the ionization energy of the organic semiconductor. Experimental Section Nickel films were prepared by magnetron sputtering of 99.999% pure nickel targets onto polished Si wafers. All nickel substrates were sputter cleaned with argon ions prior to oxidation. Ex situ oxidation was carried out in an ambient
10.1021/jp108281m 2010 American Chemical Society Published on Web 11/02/2010
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Figure 1. Photoemission spectra of ex situ-oxidized NiO (solid gray line), in situ-oxidized NiO (solid black line), and air-exposed NiO (dotted line). (a) Ni 2p3/2 XPS spectra, (b) O 1s XPS spectra, (c) C 1s XPS spectra, (d) secondary electron cutoff, (e) He IR UPS valence spectra of the substrates, and (f) He IR UPS valence spectra after 1 nm R-NPD was deposited on each substrate.
atmosphere UV-ozone treatment system (SEN Lights Corp., model PL16-110). In situ oxidation was carried out by backfilling an oxidation chamber (base pressure of 3 × 10-9 torr) of 99.999% pure O2 and generating ozone by shining UV through a quartz window. The UV light source was a 110 W, low-pressure mercury lamp (SEN Lights Corp. SUV110GS36) powered by a SEN Lights Corp. model UV-110-1H power source. Samples were annealed in vacuum by heating them with a 300 W halogen light bulb, positioned 3 cm away from the sample. All photoemission spectra were measured using a PHI 5500 MultiTechnique XPS system. The analysis chamber had a base pressure of 3 × 10-10 torr. The X-ray photon source was monochromatic Al KR (hν ) 1486.7 eV) and the ultraviolet (UV) photon source was nonmonochromated He IR, generated by a helium plasma lamp. Work function measurements were taken with the sample biased at -15 V and the sample tilted with its surface perpendicular to the XPS electrostatic lens system (i.e., 90° photoelectron takeoff angle). Discussion Nickel oxide films formed by ex situ oxidation, or having been exposed to air, have been quoted as being composed of Ni2O3, based on the Ni 2p3/2 binding energy from XPS spectra.13 However, Ni(OH)2 has nearly the same Ni 2p3/2 binding energy as Ni2O3. We have found that NiO formed by in situ UV-ozone oxidation of nickel using dry oxygen forms only NiO. Furthermore, air exposure of in situ-prepared NiO results in the appearance of the same peaks in the XPS spectra, which are commonly attributed to Ni2O3. Figure 1a shows the Ni 2p3/2 XPS spectra of ex situ-oxidized nickel, in situ-oxidized nickel, and in situ-oxidized nickel after exposure to air for 30 min. The ex situ-prepared sample contains a peak at ∼855.3 eV, which is either Ni2O3 or Ni(OH)2. The spectrum of the in situ sample is characteristic of pure NiO. It has a peak at ∼853.9 eV and a lower intensity shoulder at ∼855.6 eV. The spectrum of the air-exposed sample contains both the NiO peak at 853.9 eV and the Ni2O3/Ni(OH)2 peak at 855.3 eV. It is a convolution of the in situ and ex situ spectra.
The O 1s spectra of the ex situ, in situ, and air-exposed samples are shown in Figure 1b. The in situ sample consists of a main peak at ∼529.4 eV, which originates from the lattice O atoms of NiO, as well as multiple higher binding energy shoulders. These shoulders have been discussed extensively in the literature and are believed to arise from defects and physisorbed oxygen.14 The ex situ sample also contains the peaks of NiO, but the most intense peak is at ∼530.6 eV. Again, this peak can be attributed to either Ni2O3 or Ni(OH)2. The air-exposed sample has the same set of peaks, but with the Ni2O3/Ni(OH)2 peak being lower in intensity than the NiO peak. Based on these observations, we conclude that in situ ozone treatment results in only NiO, while exposure of clean NiO to air results in chemisorption of water and the formation of Ni(OH)2 at the surface. We also conclude that ex situ UV-ozone oxidation of nickel forms Ni(OH)2-terminated NiO, not Ni2O3. In fact, the existence of Ni2O3 has been a point of contention since the 1930s.15 There is still no conclusive evidence in the literature to show that Ni2O3 is a thermodynamically stable compound. The only unambiguous evidence of the existence of an Ni2O3-like compound comes from XPS results of NiO samples which were bombarded with O2- ions in vacuum.16 In this case, it appears that an oxide containing Ni3+ was made, but whether a stable stoichiometry of Ni2O3 was achieved is uncertain. If Ni2O3 was formed it is clearly nonequilibrium, as it was made only by ion bombardment, and likely not crystalline, nor stable under normal atmospheric conditions. In addition to forming surface nickel hydroxide, ex situprepared NiO also adsorbs carbonaceous species, such as CO2. The C 1s spectra of in situ- and ex situ-prepared NiO are shown in Figure 1c. Note that even though the ex situ-prepared sample was loaded into the XPS vacuum chamber within 10 min of being oxidized, it still has a significant amount of carbonaceous species adsorbed. The surface nickel hydroxide and adsorbed carbonaceous species affect the work function of the oxide. Figure 1d shows the secondary electron cutoff (used to determine work function) for the ex situ, in situ, and air-exposed samples. The work functions are 6.43, 5.50, and 5.23 eV, respectively. From these
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TABLE 1: Summary of Substrate Work Functions and r-NPD HOMO Binding Energies for in Situ-Oxidized, ex Situ-Oxidized, and Air-Exposed NiO Films sample
φ (eV)
HOMO (eV)
in situ air-exposed ex situ
6.43 5.50 5.23
0.31 0.32 0.48
measurements, we see that the in situ-prepared oxide can have a very high work function. When the samples were prepared in a well-baked vacuum system, we have observed values as high as 6.73 eV. However, adsorbates stick very rapidly to NiO, and even trace amounts of background gases will cause the oxide work function to drop rapidly. After the samples were exposed to air for only 10 min, the work function decreased by almost 1 eV. The valence band UPS spectra of the in situ, air-exposed, and ex situ samples are shown in Figure 1e. All three samples have similar features, arising from the NiO valence band. There is a peak at ∼1.5 eV and one at ∼4.5 eV. The air-exposed and ex situ samples, however, have additional features, with peaks growing in at ∼7 and 9 eV. These additional peaks likely result from oxygen states of the hydroxide. In spite of the different work functions and slight differences in valence bands of the three substrates, the energy-level alignment of R-NPD was not severely affected. In fact, the HOMO binding energy of R-NPD is the same on in situ and air-exposed samples, despite the 0.93 eV difference in work function. The HOMO binding energy on the ex situ NiO, however, is slightly higher, about 0.15 eV further away from the Fermi level. The HOMO binding energies of R-NPD and work functions of the three substrates are shown in Table 1. More discussion of the relationship between work function and HOMO binding energy will be presented below. In this study, the work functions of in situ-prepared NiO samples generally ranged from 6.2 to 6.7 eV; however, the work function is extremely sensitive to adsorbate coverage and it rapidly decreases even in a vacuum with ∼10-10 torr background pressure of water (other background gases such as He and H2 were present in negligible quantities, as measured using a residual gas analyzer). In device production of solution-
Figure 2. Plot of NiO work function versus background gas exposure in langmuirs. The expanded window shows data points taken in vacuum of 3 × 10-10 torr. The three data points with the highest exposures represent samples exposed to atmospheric pressure of ambient air for 10, 30, and 70 min, respectively.
Figure 3. Plot of R-NPD HOMO binding energy versus NiO work function. The vertical dashed line represents the ionization energy of R-NPD (IE ) 5.3 eV).
processed devices, NiO electrodes would need to be carried through the air for a short duration before additional organic layers can be deposited. It is important to know what kind of time budget one has before the work function of a freshly prepared NiO electrode drops below an unacceptable level. Figure 2 shows a plot of NiO work function versus air exposure in langmuirs (1 langmuir is 1 s of exposure at 10-6 torr). The data points in the blow-up panel were taken from the sample while kept in vacuum, while the other points were taken after the sample was exposed briefly to air. The last three data points represent air exposure for 10, 30, and 70 min. One can see that the most rapid drop in the work function of NiO occurs during the first few langmuirs of exposure. The work function drops by 0.75 eV during the first ∼200 langmuirs of exposure and then drops only an additional 0.36 eV with 10 orders of magnitude more exposure. It may appear from this figure that the work function of NiO levels off at ∼5.6 eV; however, this in not the case. The work function of a sample left for 3 days in atmosphere had dropped to 4.7 eV (data point not shown). The continual drop in work function arises because the NiO surface continues to adsorb carbonaceous species and other atmospheric contaminants even after it has been fully hydroxylated. It was seen in the comparison of in situ and air-exposed NiO shown above that the energy-level alignment is not always affected by a reduction in work function. We have found, however, that if the work function drops below a certain value, then HOMO binding energy begins to increase, moving away from the Fermi level. Figure 3 shows a plot of R-NPD HOMO binding energy versus NiO work function. The work function of NiO can be altered by leaving the sample in vacuum, exposing to air, and annealing to induce defects and remove excess oxygen. Here we see that when the substrate work function is between 5.5 and 6.5 eV the HOMO binding energy is invariable at ∼0.3 eV. For substrate work functions below about 5.3 eV, the HOMO binding energy increases with decreasing work function. We propose that the cutoff point, below which decreasing the substrate work function increases the HOMO binding energy, is the ionization energy of the organic adsorbate. The vertical dashed line in Figure 3 is the ionization energy of R-NPD (IE ) 5.3 eV). When the work function of the substrate is greater than this value, the HOMO binding energy does not move any closer to the Fermi level. However, when the work function of the substrate is less than the ionization energy of the organic, the HOMO binding energy
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increases with decreasing work function. This conclusion predicts that even though the work function of NiO will decrease as it is exposed to air, energy-level alignment will not be affected until the work function falls below the ionization energy of the organic. In previous works on metal/organic interfaces, it has been proposed that the charge-neutrality level (CNL) of organic molecules is an important parameter for energy-level alignment. Said theory states that the CNL determines the HOMO level offset with respect to the metal Fermi level (less some screening factor). In the present work, we claim that the HOMO binding energy reaches a minimum when the work function of the substrate is above a threshold a value, which is close to the ionization energy of the organic semiconductor. Due to the error in these measurements, and the fact that CNLs of many organic molecules are often very close in magnitude to their ionization energies, we cannot eliminate the possibility that the threshold work function value is the CNL of the organic. Perhaps the CNL of the organic only determines the minimum binding energy that can be achieved when using a substrate of a high work function. Further investigations are necessary to determine if this is the case. The other property of NiO we will discuss is its ability to become reduced. Most transition metals can form more than one stable oxide, each one differing in terms of the oxidation state of the metal cation. The cation oxidation state often determines the electronic properties of the oxide. For example, an oxide may be an insulator in its high oxidation state and a conductor in its low oxidation state. The cation oxidation state also changes an oxide’s work function, which can alter the energy-level alignment. It is important for an oxide’s properties to remain constant during device operation. Transition metals having many stable oxides may be prone to oxidation or reduction during operation, which could change the conductivity or energy-level alignment within the device. For example, CuO can be reduced to Cu2O, which has a lower work function and worse energy-level alignment.1d Unlike many other TMOs, NiO cannot be reduced to a lower-oxidation-state oxide. Annealing a transition metal oxide in vacuum generally reduces it to a lower oxidation state. However, since there is no stable nickel oxide with a lower oxidation state than NiO, the oxide becomes highly defective and eventually reduces to metallic nickel. Figure 4a shows the Ni 2p3/2 XPS spectrum of NiO and vacuum-annealed NiO (annealed for 4 h at 180 °C in 1 × 10-9 torr). One can see that the XPS peak of metallic nickel (binding energy ) 852.7 eV) appears in the annealed NiO spectrum. Furthermore, the oxidized nickel peak of NiO shifts to higher binding energy by ∼0.7 eV. The O 1s spectra of the annealed and nonannealed NiO are shown in Figure 4b. One can see that the main O 1s peak is also shifted to higher binding energy, but by slightly less (∼0.6 eV). Also, the higher-bindingenergy shoulder (at about 531.5 eV) decreases in intensity relative to the main oxide peak. This higher-binding-energy peak has been attributed previously to interstitial Ni3+ cations.14,17 Its decrease in intensity is expected when the oxide becomes reduced; however, even though some parts of the oxide had reduced all the way to nickel metal, it is difficult to remove all the Ni3+ defects. The secondary electron cutoff and valence band UPS spectra of the annealed and nonannealed NiO samples are shown in Figure 4c and d, respectively. The secondary electron cutoff shows that the work function decreases by 1.4 eV after annealing. The valence band spectrum of annealed NiO has also shifted to higher binding energy by ∼0.7 eV, similar to the Ni
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Figure 4. Photoemission spectra of in situ-oxidized NiO (solid black line) and vacuum-annealed NiO (dotted line). (a) Ni 2p3/2 XPS spectra, (b) O 1s XPS spectra, (c) secondary electron cutoff, (d) He IR UPS valence spectra of the substrates. The inset in (d) shows an expanded view of the Fermi level.
2p3/2 and O 1s main peaks. The valence features of the oxide are otherwise the same, indicating that the oxide did not change its electronic structure significantly upon annealing. The decrease in work function and shifting of the main Ni 2p3/2, O 1s, and valence peaks is likely a result of a change in the position of the Fermi level of the oxide due to the presence of oxygen vacancies or removal of interstitials. While the electronic structure of the oxide does not appear very different in the annealed oxide, it is likely that its conductivity has changed. When sufficient oxygen vacancies have been formed, nickel cations become reduced to metallic nickel. The valence spectrum of the annealed oxide shows evidence of metallic nickel. The expanded region of the spectrum, near the Fermi level, is shown the panel at the top right of Figure 4d. Here the Fermi edge of nickel metal can be seen. The reduction of NiO directly to Ni metal, rather than forming an n-type defective oxide, is a manifestation of the doping asymmetry of transition metal oxides.18 NiO is known to be a p-type transparent conductive oxide when oxygen interstitial or Ni2+ vacancies are present.3 NiO cannot be easily made (if at all) n-type.18 The n-type TMOs, such as the d0 oxides (WO3, MoO3, TiO2, V2O5, CrO3), can easily form oxygen vacancies, which give rise to conductivity via n-type states. By trying to induce oxygen vacancies in NiO, however, the oxide is reduced directly to Ni metal. This reduction is likely a result of the instability of a Ni+ oxide. All the d0 oxides, on the other hand, have stable lower-oxidationstate forms. These considerations must be kept in mind when one is attempting to induce oxygen vacancies to improve the conductivity of NiO. Oxygen vacancies can lead to reduction to metallic nickel, which would decrease the transparency of the material. The defective oxide, with its much decreased work function, also gives rise to poor energy-level alignment with R-NPD. The defective oxide has a work function of 4.26 eV, which is well below the ionization energy of R-NPD, and results in a HOMO binding energy of ∼1.15 eV.
NiO Thin Films Conclusion We have compared nickel oxides prepared by in situ and ex situ UV-ozone oxidation of nickel metal, as well as in situprepared NiO which has been briefly exposed to air. We have found that ex situ oxidation and air exposure result in the formation of a surface terminated by Ni(OH)2. Clean NiO surfaces were found to have work functions as much as 1.5 eV higher than hydroxylated surfaces. Furthermore, we have found that the peak commonly attributed to Ni2O3 in ex situ-prepared samples is actually from Ni(OH)2. In addition to the surface hydroxide, atmospheric exposure of NiO results in adsorption of carbonaceous species, which also decreases the work function of NiO. Exposure of clean NiO to air results in an initial, rapid decline in the work function, followed by a continual slow decrease. We have found, however, that a decrease in work function of NiO does not affect energylevel alignment until the work function falls below a threshold value, close to the ionization energy of the organic semiconductor with which it forms an interface. We have also examined the effect of annealing NiO in vacuum. Annealing NiO in vacuum causes oxygen vacancies to form, which shifts the position of the Fermi level and causes valence- and core-level binding energies to increase. Furthermore, since there is no lower-oxidation-state nickel oxide than NiO, it reduces directly to metallic nickel. Acknowledgment. We gratefully acknowledge the financial support of the National Science and Engineering Research Council of Canada. References and Notes (1) (a) You, H.; Dai, Y. F.; Zhang, Z. Q.; Ma, D. G. J. Appl. Phys. 2007, 101 (2), 026105. (b) Zhang, D. D.; Feng, J.; Liu, Y. F.; Zhong, Y. Q.; Bai, Y.; Jin, Y.; Xie, G. H.; Xue, Q.; Zhao, Y.; Liu, S. Y.; Sun, H. B. Appl. Phys. Lett. 2009, 94 (22), 223306. (c) Wang, F. X.; Qiao, X. F.; Xiong, T.; Ma, D. G. Org. Electron. 2008, 9 (6), 985–993. (d) Murdoch, G. B.; Greiner, M.; Helander, M. G.; Wang, Z. B.; Lu, Z. H. Appl. Phys. Lett. 2008, 93 (8), 083309. (e) Meyer, J.; Winkler, T.; Hamwi, S.; Schmale, S.; Johannes, H. H.; Weimann, T.; Hinze, P.; Kowlasky, W.; Riedl, T. AdV. Mater. 2008, 20 (20), 3839. (f) Matsushima, T.; Kinoshita, Y.; Murata, H. Appl. Phys. Lett. 2007, 91 (25), 253504. (g) Lee, H.; Cho, S. W.; Han, K.; Jeon, P. E.; Whang, C. N.; Jeong, K.; Cho, K.; Yi, Y. Appl. Phys. Lett. 2008, 93 (4), 043308. (h) Matsushima, T.; Jin, G. H.; Murata, H. J. Appl. Phys. 2008, 104 (5), 054501. (i) Qiu, C. F.; Xie, Z. L.; Chen, H. Y.; Wong, M.; Kwok, H. S. J. Appl. Phys. 2003, 93 (6), 3253–3258. (j) Helander, M. G.; Wang, Z. B.; Greiner, M. T.; Qiu, J.; Lu, Z. H. Appl. Phys. Lett. 2009, 95 (8), 083301.
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