18178
J. Phys. Chem. C 2008, 112, 18178–18182
Structural, Optical, and Electronic Properties of Self-Assembled Di-(3-diaminopropyl)-viologen on Indium Tin Oxide Electrode Surfaces Kyung-Hee Hyung,†,‡ Jaegeun Noh,*,† Wonjoo Lee,† and Sung-Hwan Han*,† Department of Chemistry, Hanyang UniVersity, 17 Haengdang-dong, Sungdong-ku, Seoul, Korea 133-791, and Electronic Chemical Materials R&D Center, Cheil Industries Inc., 332-2, Gocheon-dong, Uiwang-si, Kyoungki-do, Korea 437-711 ReceiVed: June 2, 2008; ReVised Manuscript ReceiVed: September 8, 2008
Di-(3-aminopropyl)-viologen (DAPV) on indium tin oxide (ITO) films were prepared as self-assembled monolayers (SAMs), and were well characterized using scanning tunneling microscopy (STM), UV-vis absorption spectroscopy, and electrochemistry. Furthermore, DAPV on ITO SAMs film was applied to a photoinduced charge transfer system. The STM images of the ordered SAMs on ITO showed round-shaped small domains with an average size of 15 nm, and the periodicity of the molecular rows in the domains was approximately 2.1 ( 0.2 nm. The energy band gap, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) of DAPV on ITO were measured using a UV-vis absorption spectrum and a cyclic voltammogram. The energy band gap, HOMO, and LUMO of DAPV on ITO were 1.6, -5.7, and -4.1 eV, respectively. Under illumination, the DAPV on ITO films showed a current change of 28 nA/cm2 under an air mass (AM) 1.5 condition (I ) 89 mW/cm2) in a vacuum (10-6 torr). Furthermore, SAM films on ITO showed a photocurrent of 52.5 µA/cm2 under an AM 1.5 condition (λ ) 530 nm, 3.3 mW/cm2) in the presence of I-/I3-/CH3CN. Introduction Self-assembled monoayers (SAMs) have attracted great attention due to their prospects for applications as well as fundamental research.1-9 SAMs have many advantages such as exceptional stability, a highly packed and ordered nature, good insulating power, and other unique properties, which use perspective scaffolds for a wide range of applications.6-9 SAMs can be used in sensors, solar cells, patterned devices, light-emitting devices, and other nanoscale devices.5-7 To prepare SAMs with useful properties, efficient characterization methods should be considered that provide precise information of the structural, optical, and electronic properties of SAMs films. Not surprisingly, many studies have used the precise information of SAMs in versatile characterization methods such as electrochemistry, UV-vis absorption spectroscopy, scanning tunneling microscopy (STM), atomic force microscope (AFM), and scanning electron microscopy (SEM).3,10-21 Cho and co-workers reported the formation of (60) fullerene metal clusterporphyrin dyad SAMs on indium tin oxide (ITO) for photocurrent generation.6 They also provided direct spectroscopic evidence for the formation of [diazabicyclooctaneZn porphyrin] · + in the photoinduced electron transfer process of Zn porphyrin-C60/ITO. Recently, we reported the formation of diamine SAMs films on electrodes for a photoinduced charge transfer system at the molecular level.3,4,17-22 Formation of diamine on ITO was quantitatively characterized by electrochemical techniques and Rutherford back scattering spectroscopy (RBS). The acceptor-sensitizer dyad SAMs were constructed on the ITO surface, showing good photosensing * Corresponding authors. Phone: +822-2220-0934. Fax: +822-22990762. E-mail:
[email protected] (S.-H.H.);
[email protected] (J.N.). † Hanyang University. ‡ Cheil Industries Inc.
effects by changing the current density of ITO upon illumination.3,18,21,22 Furthermore, a molecular photocurrent generation system was applied with a good quantum yield. However, the characterization method for SAMs films has not yet been fully understood. In particular, none of the reports in the literature have disclosed the structural characteristics of SAMs on ITO using STM techniques. STM enables us not only to take images of individual atoms and molecules, but also to manipulate them for bottom-up structuring. Furthermore, researchers have been able to use a variety of scanning probe techniques to characterize the electrical, magnetic, and optical properties of the localized surface.23-25 In continuation of our research on SAMs,3,11,16-22 our purpose is to well characterize self-assembled DAPV on ITO using STM and electrochemistry. To the best of our knowledge, this is the first STM image result showing SAM formation by organic molecules on an ITO surface. Furthermore, the SAMs films were applied to a photoinduced charge transfer system. STM images of DAPV on polished ITO showed round-shaped small domains with an average size of 15 nm. The periodicity of molecular rows in the domains is approximately 2.1 ( 0.2 nm. The UV-vis absorption spectrum of DAPV on ITO showed maximum absorption at 349 and 530 nm, which indicated excellent formation of the DAPV dimer [(DAPV•+)2] on the ITO surface. The energy band gap, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) of the DAPV dimers [(DAPV•+)2] on ITO were quantitatively measured by a UV-vis absorption spectrum and cyclic voltammogram. The energy band gap, HOMO, and LUMO of the DAPV dimers on ITO were 1.6, -5.7, and -4.1 eV, respectively. Under illumination, DAPV on ITO films effectively showed a photoinduced charge transfer and changed current density of the ITO. The DAPV on ITO films showed a current change of 28 nA/cm2 under an air mass (AM) 1.5
10.1021/jp807232e CCC: $40.75 2008 American Chemical Society Published on Web 10/29/2008
DAPV on ITO Electrode Surfaces
J. Phys. Chem. C, Vol. 112, No. 46, 2008 18179
condition (I ) 89 mW/cm2) in a vacuum (10-6 torr). Furthermore, SAMs films on ITO showed a photocurrent of 52.5 µA/ cm2 under AM 1.5 condition (λ ) 530 nm, 3.3 mW/cm2) in the presence of I-/I3-/CH3CN. Experimental Section Materials. The DAPV was prepared as reported.3,18,20 ITO (10 Ω/0 and 1 kΩ/0) glass was obtained from Samsung Corning Co. (Korea) and Hoya Co. (Japan). The thickness of the ITO on glass was about 100 nm. The ITO substrates were washed by acetone, ethanol, and triply distilled water in an ultrasonication bath for 15 min and finally washed with 2-propanol. High-purity water (Milli-Q, Millipore) was used for all experiments. Preparation and Characterization of DAPV/ITO. The formation of DAPV on ITO was carried out similar to the reported procedure.3,20 The SAMs of DAPV on ITO were prepared by immersing the ITO in 2 mM DAPV/0.1 M phosphate buffer (pH 7.0) at 40-45 °C for 1 min followed by washing with water. Electrochemical measurements (BAS 100B, Bioanalytical Systems, Inc.) were used with a standard single compartment three-electrode glass cell. Ag/AgCl was used for the reference electrode, and Pt wire was used for the counter electrode. The area of the ITO electrode exposed to the solution was maintained at a constant 0.5 cm2. The aqueous phase measurements were carried out under nitrogen conditions. A two-probe method was used to measure the current density change of the ITO. The SAMs on the ITO (1 kΩ/0, 1.5 × 2.5 cm2) were illuminated with 89 mW/cm2 white light with AM 0 and 1.5 filters as a solar simulator in the presence of a water filter (450 W xenon lamp, Oriel Instruments) in a vacuum (10-6 torr). The current was measured at 0.6 mV by a Kiethley 2400 source meter. A sandwich-type photocurrent generation cell was assembled with DAPV/ITO and Pt-sputtered ITO glass [electrolyte: 0.3 M LiI and 30 mM I2 in (Bu)4NBF4/CH3CN]. The photocurrent was measured under irradiation with a single wavelength (Photon counting spectrometer, ISS Inc.). The light intensity was monitored by an optical power meter (Newport Co.). The light intensity was 3.3 mW/cm2 at 530 nm. The currents were measured by a Kiethley 2400 source meter. STM measurements were performed using a NanoScope E (Veeco, Santa Barbara, CA) with a commercial Pt/Ir (80: 20) tip. All STM images were obtained in air using the constant current mode with a bias voltage (Vb) ranging from 400 to 700 mV and a tunneling current (It) ranging from 0.25 to 0.60 nA between the tip and the sample surface. Results and Discussion Structural Properties. Figure 1 shows the STM images of bare polished ITO (10 Ω/0) and DAPV on polished ITO. The STM image of Figure 1a shows the surface morphology of the bare polished ITO, which has poor roughness (rootmean-square-roughness: 1 nm) as compared to a flat gold electrode surface (root-mean-square-roughness: >1 nm).11,25 The surface of the polished ITO has no structural ordering (Figure 1a). However, the high-resolution STM images in Figure 1b-d are quite different from the STM image of the bare polished ITO. From a nanoscopic viewpoint, we found that DAPV SAMs on ITO have round-shaped small domains with an average size of 15 nm. As compared to the formation of large domains of 30-50 nm for alkanethiol SAMs on Au (111),23,24 the domain size of DAPV SAMs on ITO is
Figure 1. (a) STM image of a bare polished ITO surface (500 nm × 500 nm, Vb ) 0.53 V, and It ) 0.25 nA). (b), (c), and (d) STM images of DAPV SAMs on a polished ITO surface: (b) 500 nm × 500 nm, Vb ) 0.53 V, and It ) 0.22 nA; (c) 170 nm × 170 nm, Vb ) 0.60 V, and It ) 0.25 nA; and (d) 100 nm × 100 nm, Vb ) 0.60 V, and It ) 0.25 nA.
relatively small. This result can be attributed to the differences in the roughness of ITO, the lattice structures of the substrates, or the interactions between the molecules and the substrates. These differences strongly affect the diffusion of adsorbed molecules on a surface in the processes of selfassembly. In addition, van der Waals interactions between π-systems in DAPV molecules are considered to play a major role in the formation of two-dimensionally ordered SAMs. Interestingly, we observed the ordered molecular rows with different directions, as indicated by the white arrows in Figure 1c and d. In particular, two different molecular rows were observed in the STM image of Figure 1d. From this result, we can confirm that these molecular rows are not the result of an imaging artifact because such distortion usually appears from only one direction in an STM image. The periodicity of the molecular rows was approximately 2.1 ( 0.2 nm. We assume that there are several additional molecular rows between the two observed rows. However, even if we try to observe a molecularly resolved image, it is hard to observe the molecular image of DAPV SAMs on an ITO surface. This is probably due to the roughness of the ITO surface. However, from our STM study, we realized that adsorption of DAPV molecules on an ITO surface leads to ordered SAMs having round-shaped small domains. To the best of our knowledge, this is the first STM image result showing SAM formation by organic molecules on an ITO surface. Optical Properties. Figure 2 shows the UV-vis absorption spectrum of DAPV/ITO. The adsorption of viologen moiety has a maximum absorption at 349 and 530 nm. The strong UV-vis absorption indicated excellent formation of DAPV layers on the ITO surface.3,20,21 The absorption bands in the visible region strongly indicate the formation of the π complex. It has already been reported that the two absorption bands are associated with the formation of the dimers, (DAPV•+)2 (Scheme 1).22 The two viologen functional groups form cation radicals with a face-to-face configuration. The
18180 J. Phys. Chem. C, Vol. 112, No. 46, 2008
Figure 2. UV-vis absorption spectrum of the self-assembled DAPV on ITO.
Figure 3. Cyclic voltammogram of DAPV/ITO in 0.1 M TBATFB [(Bu)4NBF4]/CH3CN as the electrolyte at a scan rate of 50 mV/s.
SCHEME 1: Schematic Diagram of the Self-Assembled Monolayer of the DAPV
viologen dimers can be formed only in a confined environment such as in cyclodextrin or in zeolite.26-29 In the process of SAMs formation, the ITO surface provides a confined environment that facilitates the formation of viologen dimers. However, the DAPV can exist as a mixture of monomers and dimers on the ITO surface. Electronic Studies. The SAMs of DAPV on ITO (10 Ω/0) were also investigated by cyclic voltammetry. Figure 3 shows the cyclic voltammogram of DAPV on ITO, which is monitoring the reduction of viologen moiety. Viologen is well-known to give two characteristic redox peaks in cyclic voltammetry in solution (EC1 ) -586 mV and EC2 ) -916 mV vs Ag/AgCl).21 However, the cyclic voltammogram of DAPV on ITO shows three redox waves at EC1 ) -450 mV, EC2 ) -563 mV, and EC3 ) -900 mV. These are different from those of the free DAPV in an aqueous solution. The
Hyung et al.
Figure 4. (a) Schematic diagram of self-assembled DAPV on ITO for electron accumulation on ITO and (b) current density change of bare ITO (O) and self-assembled DAPV on ITO (b).
second reduction peak (EC2 ) -568 mV) is due to (DAPV2+/ DAPV•+), and the third reduction peak (EC3 ) -900 mV) is due to (DAPV•+/DAPV°). These peaks are the same as free viologen molecules in solution. It is important to note that an irreversible reduction peak appeared at -465 mV. This peak can be attributed to the formation of π-complex dimers of cation radicals (DAPV•+)2.20 However, the self-assembled DAPV can exist as a mixture of monomers and dimers on the ITO surface. The monomer-dimer equilibrium was observed by spectroscopy and electrochemistry, and a faceto-face configuration between the viologen functional groups was reported in the dimers formation.20,22 An oxidation peak corresponding to the dimers was not observed. It can be suggested that electrons were easily and quickly transferred from dimers (DAPV•+)2 (-4.3 eV) to ITO (-4.7 eV). The surface concentration of DAPV on ITO was calculated by the integration of the reduction peak current. The surface concentration of DAPV on ITO was measured to give 5.6 × 10-10 mol/cm2 by integrating the reduction peak in the cyclic voltammogram. The surface concentrations of the DAPV film were independently measured by Rutherford backscattering spectroscopy to give 8.7 × 10-10 mol/cm2.18,20 The UV-vis absorption spectrum and the cyclic voltammogram were used to calculate the energy band gap, HOMO, and LUMO of the DAPV/ITO. The energy band gap of viologen dimers, (DAPV•+)2, was measured to be 1.6 eV from the UV-vis absorption band edge of (DAPV•+)2/ITO (λ ) 775 nm, Figure 2). The on-set potential of (DAPV•+)2 was used to calculate the LUMO level (Figure 3). The irreversible nature of the first peak indicates that the electrons in the LUMO of the DAPV dimers on ITO (-4.1 eV) are easily transferred to the ITO side (-4.7 eV). The HOMO of DAPV dimer on ITO is -5.7 eV, which was calculated from the energy band gap and LUMO of the DAPV dimer. Photoinduced Electron Transfer. The energy band gap of 1.6 eV is within the visible region, and the visible light can excite electrons in DAPV dimers from the ground state to the excited state. The LUMO of the DAPV dimers was -4.1 eV, relatively higher than the ITO of -4.7 eV (Figure 4a). The excited electrons in the DAPV dimers, then, have a strong chance of moving to the ITO side. The energy-level alignment with a proper HOMO/LUMO level facilitates a photoinduced charge transfer from the HOMO to the LUMO of the DAPV dimers and up to the ITO side. Therefore, the (DAPV•+)2/ITO system can perform a photoinduced charge transfer in the visible region. The irradiation of the (DAPV•+)2/ ITO system brought about a photoinduced electron transfer and caused a change in the electron density of the ITO. Consequently, the ITO current density changed. Excellent photoresponses in current density upon irradiation are shown in Figure 4b. A two-probe method was used
DAPV on ITO Electrode Surfaces
Figure 5. (a) Schematic diagram of photocurrent generation of DAPV on ITO and (b) photocurrent of DAPV on the ITO device under monochromic irradiation (λ ) 530 nm, 3.3 mW/cm2) in the presence of I-/I3-/CH3CN.
to measure the conductivity change of ITO. The SAMs on ITO (1 kΩ/0, 1.5 × 2.5 cm2) were illuminated with 89 mW/ cm2 white light with AM 0 and 1.5 filters as a solar simulator in the presence of a water filter (450 W xenon lamp, Oriel Instruments) in a vacuum (10-6 torr). The current density was measured at 0.6 mV by a Kiethley 2400 source meter. The current density increased when the light was switched on and returned to its original position when the light was off. This result shows that the electron density of the ITO changed upon irradiation due to the electron injection from the LUMO of the DAPV dimers. The current was changed 28 nA/cm2 and demonstrated the photosensing effects of the system. Photocurrent Generation. The formation of DAPV dimers on ITO was further applied to construct photocurrent generation devices (Figure 5a). A sandwich-type photocurrent generation cell was prepared with the (DAPV•+)2 on ITO (10 Ω/0) as well as a platinum counter electrode having an electrolyte of 0.3 M LiI and 30 mM I2 in acetonitrile. The light intensity was 3.3 mW/cm2 at 530 nm. Upon irradiation, the electrons in the HOMO of the DAPV dimers were excited to the LUMO and transferred to the ITO. The DAPV dimers were oxidized by the irradiation. The electrolyte with I-/ I3-, a redox mediator, provides electrons to the oxidized dimers, which return to their original ground state. It is remarkable to note the large photocurrent generation, 52.5 µA/cm2, with clear on/off behaviors (Figure 5b). The strong response to irradiation clearly demonstrates that the DAPV dimer is an excellent photosensitizer in the visible region. The SAMs formation of DAPV provides a convenient route to prepare a photocurrent generation system on the ITO surface. The photocurrent can be converted into quantum yields. The quantum yields were determined in wavelength terms using photocurrent density, absorbance on the electrode, and input power at the applied potential:
φ ) (i/e)/I(1 - 10-A),I ) Wλ/hc where i is the photocurrent density, e is the elementary charge, I is the number of photons per unit area and unit time, λ is the wavelength of light irradiation, A is the absorbance of the adsorbed dyes at λ nm, W is the light power irradiated at nm, c is the light velocity, and h is Planck’s constant.21 As a result, the quantum efficiency of the DAPV dimer was 0.11%. This value is similar to the quantum efficiency of photoelectrochemical cells using porphyrin SAMs (0.1%).30 Conclusions The formation of the DAPV on ITO was prepared using SAMs. To precisely understand the structural, optical, and electronic properties of SAMs films, the DAPV on ITO was
J. Phys. Chem. C, Vol. 112, No. 46, 2008 18181 characterized by STM, UV-vis absorption spectroscopy, and electrochemistry. The STM images of the ordered SAMs on ITO showed round-shaped small domains with an average size of 15 nm, and the periodicity of molecular rows in the domains was approximately 2.1 ( 0.2 nm. The UV-vis absorption spectrum of DAPV on ITO showed maximum absorption at 349 and 530 nm. This indicates excellent formation of the DAPV dimer on the ITO surface. The energy band gap, HOMO, and LUMO of the DAPV dimers on ITO were quantitatively measured by UV-vis absorption spectroscopy and electrochemistry. The energy band gap, HOMO, and LUMO of the DAPV dimers on ITO were 1.6, -5.7, and -4.1 eV, respectively. Under illumination, the DAPV dimer on ITO films showed an effective photoinduced charge transfer and changed the current density of the ITO. The DAPV on ITO films showed the current change of 28 nA/ cm2 under AM 1.5 conditions (I ) 89 mW/cm2) in a vacuum (10-6 torr). Furthermore, SAMs films on ITO showed a photocurrent of 52.5 µA/cm2 under AM 1.5 condition (λ ) 530 nm, 3.3 mW/cm2) in the presence of I-/I3-/CH3CN. Investigations into the application of the DAPV/ITO to extremely thin absorber solar cells are currently underway. Acknowledgment. This research was supported by the Nano R&D program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science, and Technology (M1080300131008M 030031010). We wish to thank Brain Korea 21 for financial support. References and Notes (1) Sheeney-Haj-Ichia, L.; Wasserman, J. AdV. Mater. 2002, 14, 1323– 1326. (2) Soto, E.; MacDonald, J. C.; Cooper, C. G. F.; McGimpsey, W. G. J. Am. Chem. Soc. 2003, 125, 2838–2839. (3) Oh, S.-Y.; Han, S.-H. Langmuir 2000, 16, 6777–6779. (4) Lee, W.; Lee, J.; Lee, S.-H.; Chang, J.; Yi, W.; Han, S.-H. J. Phys. Chem. C 2007, 111, 9110–9115. (5) Kondo, M.; Nakamura, Y.; Fujii, K.; Nagata, M.; Suemori, Y.; Dewa, T.; Iida, K.; Gardiner, A. T.; Cogdell, R. J.; Nango, M. Biomacromolecules 2007, 8, 2457–2463. (6) Cho, Y.-J.; Ahn, T. K.; Song, H.; Kim, K. S.; Lee, C. Y.; Seo, W. S.; Lee, K.; Kim, S. K.; Kim, D. H.; Park, J. T. J. Am. Chem. Soc. 2005, 127, 2380–2381. (7) Chukharev, V.; Vuorinen, T.; Efimov, A.; Tkachenko, N. V.; Kimura, M.; Fukuzumi, S.; Imahori, H.; Lemmetyinen, H. Langmuir 2005, 21, 6385–6391. (8) Ma, H.; Kang, M.-S.; Xu, Q.-M.; Kim, J.-S.; Jen, A. K.-Y. Chem. Mater. 2005, 17, 2896–2903. (9) Kim, K.-S.; Kang, M.-S.; Ma, H.; Jen, A. K.-Y. Chem. Mater. 2004, 16, 5058–5062. (10) Sheeney-Haj-Ichia, L.; Wasserman, J.; Willner, I. AdV. Mater. 2002, 14, 1323–1326. (11) Lee, W.; Min, S.-K.; Shin, S.; Han, S.-H.; Lee, S.-H. Appl. Phys. Lett. 2008, 92, 023507/1-3. (12) Min, S.-K.; Joo, O.-S.; Jung, K.-D.; Mane, R. S.; Han, S.-H. Electrochem. Commun. 2006, 8, 223–226. (13) Choi, Y.; Jeong, Y.; Chung, H.; Ito, E.; Hara, M.; Noh, J. Langmuir 2008, 24, 91–96. (14) Lee, S. W.; Pyo, E.; Kim, J. O.; Noh, J. G.; Lee, H. J. Appl. Phys. 2007, 101, 044905/1-5. (15) Lee, W.; Mane, R. S.; Min, S.-K.; Yoon, T. H.; Han, S.-H.; Lee, S.-H. Appl. Phys. Lett. 2007, 90, 263503/1-3. (16) Kim, Y.-H.; Lee, S.-H.; Noh, H.; Han, S.-H. Thin Solid Films 2006, 510, 305–310. (17) Oh, S.-Y.; Yun, Y.-J.; Kim, D.-Y.; Han, S.-H. Langmuir 1999, 15, 4690–4692. (18) Hyung, K.-H.; Han, S.-H. Synth. Met. 2003, 137, 1441–1442. (19) Hyung, K. H.; Kim, D.-Y.; Han, S.-H. New J. Chem. 2005, 29, 1022–1026. (20) Lee, W.; Hyung, K.-H.; Kim, Y.-H.; Cai, G.; Han, S.-H. Electrochem. Commun. 2007, 9, 729–734. (21) Lee, W.; Mane, R. S.; Lee, S.-H.; Han, S.-H. Electrochem. Commun. 2007, 9, 1502–1507.
18182 J. Phys. Chem. C, Vol. 112, No. 46, 2008 (22) Lee, W.; Chang, G. K.; Mane, R. S.; Min, S. K.; Cai, G.; Ganesh, T.; Koo, G.; Chang, J.; Cho, B. W.; Kim, S.-K.; Han, S.-H. Mater. Chem. Phys. 2008, 112, 208–212. (23) Noh, J.; Hara, M. Langmuir 2001, 17, 7280–7285. (24) Noh, J.; Hara, M. Langmuir 2002, 18, 1953–1956. (25) Martin, D. S.; Blanchard, N. P.; Weightman, P. Surf. Sci. 2003, 1, 532–535. (26) Park, Y. S.; Lee, K.; Lee, C.; Yoon, K. B. Langmuir 2000, 16, 4470–4477.
Hyung et al. (27) John, S. A.; Kitamura, F.; Tokuda, K.; Ohsaka, T. J. Electroanal. Chem. 2000, 492, 137–144. (28) John, S. A.; Kitamura, F.; Tokuda, K.; Ohsaka, T. Electrochim. Acta 2000, 45, 4041–4048. (29) John, S. A.; Okajima, T.; Ohsaka, T. J. Electroanal. Chem. 1999, 466, 67–74. (30) Imahori, H.; Norieda, H.; Ozawa, S.; Ushida, K.; Yamada, H.; Azuma, T.; Tamaki, K.; Sakata, Y. Langmuir 1998, 14, 5335–5338.
JP807232E
