Article pubs.acs.org/JPCC
Thermal Transformation of 4,4″-Diamino‑p‑terphenyl on a Si(111)‑7 × 7 Surface Analyzed by X‑ray Photoemission Spectroscopy and Scanning Tunneling Microscopy Takashi Nishimura,† Akira Sasahara,† Hideyuki Murata,† Toyoko Arai,‡ and Masahiko Tomitori*,† †
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan
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ABSTRACT: Analysis using X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM) was carried out for 4,4″-diamino-pterphenyl (DAT) deposited on a Si(111)-7 × 7 surface with a low coverage at room temperature and subsequently heated at temperatures from 300 to 800 °C. The XPS analysis showed that nitrogen atoms in amino groups of DAT partly reacted with the Si at 350 °C and were transformed to silicon nitride at 600 °C, while carbon atoms in phenyl rings of DAT partly reacted with the Si at 600 °C. The aggregations were found after being heated at 800 °C by STM observation and were attributed to silicon carbide and silicon nitride with carbon atoms in the phenyl rings out of DAT by XPS analysis. The XPS and STM combined analysis revealed the critical temperatures of decomposition and transformation of molecules on substrates with their arrangements and chemical states at a molecular level.
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INTRODUCTION
A great variety of organic thin films have been extensively studied in regard to their applications of electronic devices since organic thin films have unique functionalized properties different from those of inorganic semiconductors.1 Hybridization of organic thin films with inorganic semiconducting materials is expected to make enormous progress in the next generation of electronic devices. When electronic devices are fabricated with an organic thin film on a solid substrate, the electronic properties of the interface between them should be well controlled.2,3 For example, it is required that the contact resistance at the interface be low for carrier injection with high efficiency under a forward bias voltage and high for rectification with low leakage current under a reversed bias voltage. For the high electronic performance of molecular devices, it is indispensable to analyze and to control the arrangements and chemical states of molecules on the substrates at a molecular level. To date, the states of a large number of organic molecules onto silicon (Si) substrates as a standard semiconducting material have been intensively examined on a nanoscale.4,5 As a basic study of an organic thin film on a silicon substrate, we reported the bonding state of a π-conjugated organic molecule, 4,4″-diamino-p-terphenyl (DAT), vapor deposited on a Si(111)-7 × 7 surface at room temperature in ultrahigh vacuum (UHV), which was analyzed by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM).6 DAT consists of a linear but twisted three-phenyl ring chain (π-conjugated), i.e., terphenyl, to each end of which an amino group is attached (Figure 1(a)). DAT has been used for a monomer of a conjugated polymer, polyazomethine, which was employed for organic electroluminescent (EL) © 2014 American Chemical Society
Figure 1. (a) Simplified chemical structure of 4,4″-diamino-pterphenyl (DAT). (b) Intuitive model for the initial adsorption of DAT bonded with a Si adatom on Si(111)-7 × 7.
devices prepared by vapor deposition polymerization.7 Once DAT is attached to the silicon surface, the extension of conjugation length can be controlled by deposition of aromatic dialdehydes and further deposition of DAT. Our XPS and STM Received: August 27, 2014 Revised: October 2, 2014 Published: October 6, 2014 25104
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evaluated from XPS peak intensities for various coverages was approximately two times the surface density of the protrusion, we concluded that the protrusion corresponded to one DAT molecule, having two N atoms, on Si(111)-7 × 7. The typical flux was calculated to be 2 molecules/7 × 7 unit cell/s at the position of the substrate (the area of a 7 × 7 unit cell is 6.27 nm2). In this study we prepared samples covered with DAT at one molecule per unit cell by controlling the opening time of the shutter. STM images of Si(111)-7 × 7 surfaces covered with DAT were acquired at a tip bias voltage of −1.5 V and a constant current of 0.05 nA with electrochemically etched W tips cleaned in UHV by heating before STM scanning.11−13 An Xray photoelectron spectrometer (PHI 5600, ULVAC PHI) was used to characterize the samples, which were transferred from the UHV STM system to the XPS chamber using a transfer vessel filled with dry N2 gas without exposing the samples to air. XPS spectra were obtained using a monochromated Al Kα source (1486.6 eV) with a pass energy of 11.75 eV, the energy peak positions of which were referred to a binding energy of 99.4 eV for Si(2p3/2). The background of the spectra was subtracted using the Shirley method, followed by a nonlinear least-squares fitting to a mixed Gaussian−Lorentzian peak shape.14 The XPS spectra shown were normalized with respect to the peak of Si(2p) for each sample.
analysis revealed that DAT was chemically bonded to the silicon surface even at room temperature through one amino group at one end of the DAT, and the other amino group at the other end was free from the silicon surface (Figure 1(b)). Through the chemical bond between the amino group and the silicon surface, the interface between them would be stable and controllable. In order to realize further vapor deposition polymerization on the DAT layer, the DAT layer should be heated around 300 °C.7 Thus, it is significant to evaluate the state of the DAT layer on the Si(111) surface at high temperatures. In this paper, we carry out XPS and STM analysis of the DAT layer on a Si(111)-7 × 7 surface heated at temperatures from 300 to 800 °C in order to disclose the change in the chemical states and topography of the surface. It is interesting to examine the transformation of DAT to silicon compounds compared with that to silicon nitride from NH3 and N2 molecules adsorbed on Si surfaces at high temperatures.8,9 Understanding of the thermal reaction of molecules on Si surfaces is of great importance in terms of establishing the processes of molecular device fabrication and in terms of extending the potential of their applications.
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EXPERIMENTAL METHODS We used a home-built UHV STM system with a base pressure of 2 × 10−11 Torr operated at room temperature. The system has a sample preparation chamber with a base pressure of a low of 10−10 Torr connected to the STM main chamber through gate valves. A preparation chamber was equipped with a molecule deposition apparatus consisting of a tungsten carbide crucible wound with a sheath heater with a thermocouple, a water-cooling jacket, a quartz thickness monitor (CRTM-9000, ULVAC), and a mechanical shutter to control the deposition amount. The STM main chamber was equipped with an apparatus to clean a silicon substrate by resistive heating. The temperature of the substrate was monitored using an infrared pyrometer above 600 °C. A characteristic curve of the temperature above 600 °C versus heating current into the substrate was used to extrapolate and to estimate the substrate temperature below 600 °C. We confirmed that the estimated temperature approximately corresponded to the substrate temperature measured with a thermocouple in the preparation chamber. A rectangular cut (3 × 12 × 0.35 mm3) from a P-doped Si(111) wafer with a resistivity of 2−4 Ω cm was used as a substrate and introduced into the preparation chamber after being cleaned with an ozone cleaner (NL-UV253, Nippon Laser & Electronics Lab) for 1.5 h, followed by resistive heating at 600 °C for 10 h for degassing. Subsequently, the cut was flashed at 1200 °C, leading to the surface reconstruction of 7 × 7, which was confirmed by the STM. The DAT was purchased from Lancaster Synthesis Ltd. and used after having been purified three times by the train sublimation method.10 DAT molecules were evaporated by heating the crucible at 180 °C below the DAT melting temperature of about 240 °C, resulting in a pressure rise in the order of 10−9 Torr, while the substrate was at room temperature. The deposited amount of DAT was monitored by the quartz thickness monitor. The reading of the thickness monitor was calibrated from the number of protrusions depicted in STM images at low coverages;6 since the number of protrusions increased linearly with an increase of the amount of deposition and since the nitrogen (N) atom surface density
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RESULTS AND DISCUSSION XPS analysis was carried out for samples covered with DAT of one molecule per 7 × 7 unit cell deposited at a substrate temperature of room temperature in UHV and subsequently heated at a temperature of 300, 350, 600, or 800 °C for 3 min by passing current into the Si substrates. Typical XPS spectra around N(1s) at a binding energy of about 400 eV are shown in Figure 2. For a nonheated sample without DAT (not shown here), no XPS peaks were found around 400 eV. For a nonheated but DAT covered sample, a broad XPS peak of N(1s) was found, which was decomposed into two peaks at 399.9 and 398.8 eV, labeled as N1 and N2, respectively (Figure 2(a)). The ratio of the intensity area of N1 to that of N2 was 1.2. Peak N1 was assigned to a N atom in an amino group of DAT free from the Si surface, and peak N2 was assigned to a N atom in an amino group chemically bonded to a Si surface atom, corresponding to Si−NH−terphenyl.6 As mentioned in the Introduction (Figure 1(b)), this indicates that DAT stands at a slant, which is chemically bonded to the Si surface through one amino group at one end of the DAT and has the other amino group intact at the other end. For the sample covered with DAT heated at 300 °C, the XPS peak of N(1s) was also decomposed into two peaks at 399.9 and 398.8 eV (Figure 2(b)), regarded as N1 and N2, respectively. The ratio of the intensity area of N1 to that of N2 was 1.1. Thus, it was concluded that the states of the two amino groups of DAT on the Si(111)-7 × 7 surface did not change upon heating at 300 °C. For the samples heated above 300 °C, the peak around N(1s) noticeably changed. As shown in Figure 2(c) for heating at 350 °C, the peak was decomposed into three peaks at 399.9, 398.8, and 398.1 eV (labeled as N3). The peaks at 399.9 and 398.8 eV were regarded as N1 and N2, respectively, and the ratios of the intensity area of N1 and N3 to that of N2 were 1.1 and 0.9, respectively. The total peak intensity area of N(1s) decreased to be about 80% of that for the nonheated sample and the sample heated at 300 °C, as shown in Figure 3, which 25105
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chemisorbed on Si(111)-7 × 7 as Si−N(CH3)2.15 This peak appearance is discussed later in detail with the change in XPS peaks of C(1s). The ratio of the intensity area of N4 to that of N4 + N5 was about 0.9, while the total intensity area decreased to be about 80% (Figure 3). The above results show that DAT on the Si surface started to be decomposed by heating at 350 °C; some amino groups of the DATs (about 24% = N3/(N1 + N2 + N3) × 0.8) reacted with Si atoms and resulted in peak N3, while 20% of N atoms in amino groups disappeared from the Si surface. Peak N3 probably corresponded to transformation to silicon nitride. For the sample heated at 600 °C almost all amino groups of DAT transformed to silicon nitride, while 20% of N atoms in the amino groups disappeared from the Si surface. For the sample heated at 800 °C, the peak of N(1s) was decomposed into two peaks at 398.0 and 399.5 eV, labeled as N6 and N7, respectively (Figure 2(e)). The peak energy of N6 seemed to shift higher by 0.4 eV from that of N4 for heating at 600 °C (Figure 2(d)). The similar peak shifts of N(1s) were also reported;16,17 when Si(111)-7 × 7 surfaces covered with NH3 were heated at 300−500 °C, the XPS peak shifted by 0.5 eV for N(1s) from 397.6 to 398.1 eV.16 This was explained by a decrease in relaxation energy when the matrix surrounding (Si)3N sites changes from a semiconducting state of initial nitridation stages on the Si(111)-7 × 7 surface to an insulating one of bulk nitride, i.e., Si3N4 crystal. It is likely that similar changes took place for the DAT on the Si(111)-7 × 7 surfaces, as nitridation was promoted by heating at 600−800 °C. Figure 4 shows the XPS spectra around the peak of C(1s) at a binding energy of about 285 eV. A reference of an XPS spectrum for a nonheated surface without DAT deposition exhibited two peaks at 284.8 eV as major and 286.2 eV as minor after XPS peak decomposition, in Figure 4(a). These peaks were attributed to carbon contamination adsorbed on the Si(111)-7 × 7 surface during transfer of the sample from the UHV STM chamber to the XPS chamber using the vessel. The XPS peak around C(1s) for the nonheated sample covered with DAT at one molecule per 7 × 7 unit cell was decomposed into two peaks at 284.7 eV (labeled as C1) and 285.8 eV (labeled as C2), in Figure 4(c). Since the peak intensity in Figure 4(c) was prominently larger than that in Figure 4(a), peaks C1 and C2 were attributed to carbon atoms in the phenyl rings of DAT. To confirm the origins of peaks C1 and C2, we deposited DAT at 100 molecules/unit cell (for 50 s at a flux of 2 molecules/7 × 7 unit cell/s) in Figure 4(b); the DAT molecules were thought to be mostly physisorbed over the first layer of DAT chemically bonded to the Si(111)-7 × 7 surface. Peaks C1 and C2 clearly increased at the same binding energies, respectively. This ensured the above assignment of the peak origin. In addition, although C(1s) peaks in terphenyl-derived molecules on noble metal surfaces were found at 284.1−284.2 eV,18 the C(1s) peak of terphenyl on a hydrogenated Si(111) surface was found at 284.5−284.7 eV,19 which coincided with that in this study. The ratio of peak intensity area of C1 to C2 was 7.4:1.0. Peaks C1 and C2 also appeared for the samples with DAT heated at 300 and 350 °C in Figure 4(d) and (e), similarly to the nonheated samples (Figures 4(b) and (c)). Thus, it is deduced that the state of carbon atoms in DAT did not change very much when heated to 350 °C. The peak energy value of C1 (284.7 eV) coincided with that of the XPS peak found for the aniline multilayer on Ni(100),20 which originated from carbon atoms in the phenyl ring and, furthermore, with that of carbon atoms C2 and C3 in 4-
Figure 2. Normalized XPS spectra around N(1s) of Si(111)-7 × 7 surfaces covered with DAT at one molecule per 7 × 7 unit cell. (a) For a nonheated sample (room temperature (RT)). (b)−(e) Each sample was heated at 300, 350, 600, and 800 °C for 3 min, respectively. The XPS peak of Si(2p) for each sample was used as an internal standard for intensity normalization.
displays the plots of the intensity area change of peaks around N(1s) versus heating temperature.
Figure 3. Plots of the normalized intensity areas of XPS peaks around N(1s) on Si(111)-7 × 7 surfaces versus heating temperature. The surfaces were covered with DAT at one molecule per 7 × 7 unit cell. The XPS peak of Si(2p) for each sample was used as an internal standard for intensity normalization.
For the sample heated at 600 °C, the peak was decomposed into two peaks at 397.6 eV (labeled as N4) and 399.1 eV (labeled as N5). The peak energy of N4 closely coincided with that at 397.4 eV for triple coordinated silicon nitride ((Si)3 N) for NH3-adsorbed Si heated at about 600 °C,8 where N atoms were not bonded to H atoms. The peak energy of N5 coincided with that at 399.1 eV for dimethylamine (DMA) 25106
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4(f)). Peak C3 possibly corresponded to C atoms in an intermediate chemical state between the phenyl rings of DAT and silicon carbide. The ratio of the intensity area of C1, C3, and C4 to that of the total peak of C(1s) was 0.66, 0.26, and 0.08, respectively, indicating that about 30% of C atoms in phenyl rings of DAT reacted with the Si surfaces at 600 °C, while in the XPS spectra the total number of carbon atoms decreased to be about 90% compared to that for the nonheated sample, as shown in Figure 5.
Figure 5. Normalized intensity area of C(1s) XPS spectra of Si(111)-7 × 7 surfaces covered with DAT at one molecule per 7 × 7 unit cell versus heating temperature. The XPS peak of Si(2p) for each sample was used as an internal standard for intensity normalization.
When the sample was heated at 800 °C, the XPS peak was decomposed into three peaks at 283.2 eV (labeled as C5), 284.9 eV (labeled as C6), and 286.2 eV (labeled as C7) as a minor peak (Figure 4(g)). The ratio of the intensity area of C5, C6, and C7 to that of the total of C(1s) was 0.32, 0.59, and 0.09, respectively. The peak energy of 283.2 eV (C5) closely coincided with that of silicon carbide,15 and the peak energy of 284.9 eV (C6) closely coincided with that of C1 (284.7 eV), attributed to carbon atoms in the phenyl rings of DAT. This means that about 30% of carbon atoms in phenyl rings of DAT reacted with the Si surfaces, resulting in transformation to silicon carbide, and about 60% remained in the phenyl ring(s) even at 800 °C, although it is unclear whether they were in a phenyl ring, biphenyl, terphenyl, or other related configurations. When the samples were heated at 600 and 800 °C, the minor XPS peaks of C4 and C7 appeared. These peak energies closely coincided with 286.4 eV of the XPS peak of C(1s) of dimethylamine (DMA) chemisorbed on Si(111)-7 × 7 at room temperature as Si−N(CH3)2.15 Meanwhile, the peak energy around N(1s) for DMA on Si(111)-7 × 7 was also identical to the binding energy of N5 for DAT with a ratio of N5 to the total intensity of 10%. Thus, the bonding state of 10% amino groups in DAT heated at 600 and 800 °C was likely similar to that of DMA. However, the respective ratios of C4 and C7 to the total intensity of C(1s) of 8% and 9% seemed high compared with that of N5 because the original atomic number ratio of C to N is 18:2. When 10% of amino groups in DAT are adsorbed as the configuration of DMA absorbed on the Si(111)-7 × 7, the necessary ratio of C atoms to total C atoms in DAT is only 2%, which is less than the summed ratio (8% +
Figure 4. Normalized XPS spectra around C(1s) of Si(111)-7 × 7 surfaces. (a) The surface was not covered with DAT nor heated (room temperature (RT)). (b) For a sample covered with DAT at 100 molecules per 7 × 7 unit cell without heating. (c) For a sample covered with DAT at one molecule per 7 × 7 unit cell without heating. (d)−(g) For samples with DAT at one molecule per 7 × 7 unit cell heated at 300, 350, 600, and 800 °C for 3 min, respectively. The XPS peak of Si(2p) for each sample was used as an internal standard for intensity normalization.
chloroaniline (H 2 NC 1 (C 2 HC 3 H) 2 C 4 Cl) physisorbed on Si(111)-7 × 7,21 in which the carbon atoms of C2 and C3 do not bond to Cl nor to NH2. The peak energy of C2 (285.8 eV) closely coincided with that at 285.7 eV for carbon atom C1 bonded to NH2 in 4-chloroaniline physisorbed on Si(111)-7 × 7.21 The ratio of the peak intensity area of C1 to C2 of the DAT, which was 7.4:1.0 at 100 DAT molecules/unit cell, almost coincided with that of the number ratio of carbon atoms between ones not bonded to NH2 and ones bonded to NH2 in the three phenyl rings of DAT (8:1). Thus, C1 and C2 were assigned to the carbon atoms not bonded to NH2 and to the carbon atoms bonded to NH2 in DAT, respectively. For the sample heated at 600 °C, the XPS peak of C(1s) was decomposed into three peaks at 284.7 (C1), 283.6 eV (labeled as C3), and 286.4 eV(labeled as C4) as a minor peak (Figure 25107
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9% = 17%) of peaks C4 and C7. At present this is unsettled, but it is likely that some fragmented derivatives with more C atoms out of DAT were adsorbed and exhibited the same XPS peak as that of DMA. From the above XPS analysis, the following statements are noted: the N atoms and C atoms of DAT remained intact on the Si surfaces up to 300 °C. The amino groups in DAT started to be partly decomposed at 350 °C, while the terphenyl in DAT did not. At 600 °C about 30% of the carbon atoms in the terphenyl transformed to silicon carbide, and most amino groups of DAT transformed to silicon nitride. It is possible that the C atoms in the phenyl rings remained at 800 °C. The N atoms and C atoms started to leave the surfaces by heating at 350 °C and above, possibly owing to their diffusion into the Si bulk or to their evaporation from the Si surface. Figure 6 shows STM images of the DAT at an amount of one molecule per 7 × 7 unit cell. For the nonheated sample, spherical protrusions were observed, corresponding to individual DAT molecules (the STM image is not shown here; see Figure 2 in ref 6). The similar protrusions were also observed for the sample heated at 300 °C for 3 min, in Figure 6(a). Subsequently, the sample was heated at 600 °C for 3 min; aggregated protrusions as denoted by arrowhead A and deep darker regions as denoted by arrowhead B were observed, while the number of spherical protrusions decreased (Figure 6(b)). It is noted that the 7 × 7 reconstruction remained in the other areas. From the above-mentioned XPS analysis, the protrusions are ascribed to the formation of silicon nitride, to the initial formation of silicon carbide out of DAT reacted with Si atoms, and to the phenyl rings out of DAT. The darker areas were possibly induced by adsorption of amino related atoms and groups like N, H, NH, or hydrocarbons decomposed from DAT. Furthermore, the sample was heated at 800 °C for 3 min; evolving aggregated features as denoted by arrow C were observed (Figure 6(c)). The features possibly corresponded to the origins for the energy shifts of XPS peaks for N(1s) from 397.6 eV (N4) to 398.0 eV (N6) and from 399.1 eV (N5) to 399.5 eV (N7), and C(1s) from 284.7 (C1) to 284.9 (C6). The features were probably composed of N atoms and C atoms out of DAT, including the chemical configuration of Si3N4. For nitridation of Si(111)-7 × 7 in N2 gas above 870 °C, Tabe et al. observed the growth of aggregations on the surface by STM and suggested that nitrogen atoms on the surface were certainly segregated by surface migration.17 The migration possibly induced the aggregations of silicon nitride and carbide. It is noteworthy that the 7 × 7 reconstruction was clearly observed owing to the desorption of N, H, and so on, except the large aggregates. By combining the STM observation and XPS analysis, it is concluded that the DAT on Si(111)-7 × 7 did not change at 300 °C, started to be decomposed above 350 °C and to aggregate at 600 °C, and finally transformed into larger clusters of silicon carbide, nitride, and aggregate at 800 °C, while some phenyl rings of DAT remained.
Figure 6. STM images of a Si(111)-7 × 7 surface covered with DAT at one molecule/unit cell. (a) After being heated at 300 °C for 3 min, (b) subsequently heated at 600 °C for 3 min, and (c) further heated at 800 °C for 3 min. Scanning area: about 23 nm × 23 nm. STM imaging conditions: Vtip = −1.5 V, Itunnel = 0.05 nA.
CONCLUSION DAT molecules deposited on Si(111)-7 × 7 surfaces at a low coverage and heated at temperatures from 300 to 800 °C were analyzed using XPS and STM. The adsorption state of DAT heated at 300 °C did not change; however, at 350 °C amino groups in DAT partly reacted with Si atoms, while no change in the carbon state in the phenyl rings of DAT was detected. Meanwhile, the numbers of carbon atoms and of nitrogen atoms tended to decrease above 350 °C. At 600 °C some
phenyl rings of DAT reacted with the Si atom; the intermediate chemical state of carbon atoms in between the phenyl rings of DAT and silicon carbide was formed, indicating that the terphenyl was tough enough for the heating. At 800 °C, aggregates possibly consisting of silicon carbide, nitride, and phenyl rings out of DAT were found by STM. XPS- and STMcombined analysis enables us to elucidate the critical temperature of decomposition and transformation of the molecules on substrates.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: +81 761 51 1501. Fax: +81 761 51 1149. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research from the Japanese Society for the Promotion of Science (JSPS) KAKENHI Grant Number 24246014, 24340068, 26600024, 26600098, and 26630330.
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