**4. Nano-probe electrochemistry on amino-terminated self-assembled monolayers toward nano memory**

In this chapter, we investigated that an amino-terminated SAM was electrochemically converted into an oxidized SAM by SPL at positive bias voltages. Moreover, this oxidized SAM was then reconverted into an amino-terminated SAM by SPL at negative bias voltages. The chemical conversions of amino groups were confirmed by Kelvin probe force microscopy (KPFM), atomic force microscopy (AFM) and the site-selective adsorption of carboxyl-modified fluorescent spheres. We examined the chemical changes undergone in the scanned area from the viewpoint of surface-potential reversibility. Fig. 7 schematically illustrates of the experimental procedure. An amino-terminated SAM was prepared by chemical vapor deposition (CVD) from *p*-aminophenyltrimethoxysilane (APhS) on n-type silicon (111) wafers with electrical resistivity of 4-6 Ω/cm. First, the silicon substrate was cleaned in acetone, ethanol, and deionized water, in that order. After cleaning, the silicon substrate was irradiated by 172 nm vacuum ultraviolet light in air for 20 min. This removed organic contaminants and introduced silanol groups onto the substrate surface. Next, each cleaned silicon substrate was placed together with a glass vessel filled with APhS liquid in a Teflon container. The Teflon container was sealed and placed in an oven with the temperature kept at 100 °C. The reaction time was 1 h. The heated APhS liquid vaporized and hydrolyzed. The hydrolyzed APhS reacted with the silanol groups on the silicon substrate resulting in the fabrication of an amino-terminated SAM.

Fig. 7. Preparation and electrochemical scanning probe lithography of APhS-SAM [N. Saito, SH. Lee, T. Ishizaki, J. Hieda, H. Sugimura, O. Takai, Surface potential reversibility of an amino-terminated self-assembled monolayer based on nanoprobe chemistry, J. Phys. Chem. B, 109, 11602-11605 (2005) . Copyright@American Chemical Society (2005)]

We fabricated the APhS-SAM through the CVD method. The formation of APhS SAM was confirmed by the ellipsometry, water contact angle and X-ray photoelectron spectroscopy (XPS) measurement. In our SPL system, electrons were transferred between a gold-coated

Scanning Probe Lithography on Organic Monolayers 485

Fig. 9. Topographic and surface-potential images scanned at +2 and +5 V in air and vacuum. [N. Saito, SH. Lee, T. Ishizaki, J. Hieda, H. Sugimura, O. Takai, Surface potential reversibility of an amino-terminated self-assembled monolayer based on nanoprobe chemistry, J. Phys.

Chem. B, 109, 11602-11605 (2005) . Copyright@American Chemical Society (2005)]

probe and the silicon substrate through the APhS SAM and an adsorbed water layer. The adsorbed water played the same role as water in a typical, macroscopic electrochemical cell system. The electrochemical conversion was conducted with a gold-coated silicon nanoprobe with a force constant of 1.8 N/m and a resonance frequency of 23.2 kHz. The probe was scanned in air at a relative humidity of 35% to 40%.

Firstly, the formation of APhS SAM was confirmed by the ellipsometry, water contact angle and XPS measurement. The static water contact angle of the sample surface after preparation was about 60°. Its film thickness obtained by ellipsometry was ca. 0.58 nm. This thickness was considered reasonable since the chain length of the APhS molecule was estimated to be 0.6 nm by semiempirical molecular orbital calculation using the AM1 Hamiltonian. The XPS N1s spectrum of the sample seen in Fig. 8 shows that the N1s binding energy is 399.6 eV. This binding energy agrees approximately with a previously reported value (400.0 eV). These results indicated the formation of APhS SAM on the SiO2 substrate.

To show the effect of adsorbed water, the nano-probe was scanned at the pressure of 10-6 Pa. Fig. 9 shows topographic and surface-potential images scanned at 2 and 5 V in both air and vacuum. On the basis of these images, it is obvious that the chemical reaction does not proceed in vacuum. Thus, adsorbed water is considered necessary for the chemical conversion.

Fig. 8. XPS *N1s* spectrum of the Si surface covered with APhS-SAM.

probe and the silicon substrate through the APhS SAM and an adsorbed water layer. The adsorbed water played the same role as water in a typical, macroscopic electrochemical cell system. The electrochemical conversion was conducted with a gold-coated silicon nanoprobe with a force constant of 1.8 N/m and a resonance frequency of 23.2 kHz. The

Firstly, the formation of APhS SAM was confirmed by the ellipsometry, water contact angle and XPS measurement. The static water contact angle of the sample surface after preparation was about 60°. Its film thickness obtained by ellipsometry was ca. 0.58 nm. This thickness was considered reasonable since the chain length of the APhS molecule was estimated to be 0.6 nm by semiempirical molecular orbital calculation using the AM1 Hamiltonian. The XPS N1s spectrum of the sample seen in Fig. 8 shows that the N1s binding energy is 399.6 eV. This binding energy agrees approximately with a previously reported value (400.0 eV). These results indicated the formation of APhS SAM on the SiO2 substrate. To show the effect of adsorbed water, the nano-probe was scanned at the pressure of 10-6 Pa. Fig. 9 shows topographic and surface-potential images scanned at 2 and 5 V in both air and vacuum. On the basis of these images, it is obvious that the chemical reaction does not proceed in vacuum. Thus, adsorbed water is considered necessary for the chemical

probe was scanned in air at a relative humidity of 35% to 40%.

Fig. 8. XPS *N1s* spectrum of the Si surface covered with APhS-SAM.

conversion.

Fig. 9. Topographic and surface-potential images scanned at +2 and +5 V in air and vacuum. [N. Saito, SH. Lee, T. Ishizaki, J. Hieda, H. Sugimura, O. Takai, Surface potential reversibility of an amino-terminated self-assembled monolayer based on nanoprobe chemistry, J. Phys. Chem. B, 109, 11602-11605 (2005) . Copyright@American Chemical Society (2005)]

Scanning Probe Lithography on Organic Monolayers 487

Fig. 11. (a) Surface potential images obtained by KFM and (b) the change in surface potential

The nanoprobe was scanned across the sample surface over an area of 20m X 20m at bias voltages of 0.5-6 V. Fig. 10 shows both AFM topographic images and the height difference against the nonlithographic area after scanning. A slight protuberance can be observed in the topographic images of samples scanned at the bias voltages of 4 to 6 V. This protuberance became much higher at the bias voltages of 4 to 6V. These protuberances are due to the production of SiO2, which resulted from the decomposition of the as-deposited APhS SAM and the oxidation of silicon. This demonstrates that there is no possibility of chemical reversibility at these bias voltages since the SAM has been damaged. Specifically, there is no possibility of surface-potential reversibility. On the other hand, no change can be observed in the topographic images of samples scanned at the bias voltages of 0.5-3 V. At these voltages, it is possible that the framework of the SAM molecules remained intact. Fig. 11 shows both surface-potential images obtained by KFM and the change in surface potential on the scanned region. The surface potential shifted negatively, which can be roughly explained by the apparent dipole moment of the SAM. The apparent dipole

on the area scanned at positive bias voltages.

Fig. 10. (a) AFM topographic images and (b) the height difference against the non-scanned area after scanning at positive bias voltages.

Fig. 10. (a) AFM topographic images and (b) the height difference against the non-scanned

area after scanning at positive bias voltages.

Fig. 11. (a) Surface potential images obtained by KFM and (b) the change in surface potential on the area scanned at positive bias voltages.

The nanoprobe was scanned across the sample surface over an area of 20m X 20m at bias voltages of 0.5-6 V. Fig. 10 shows both AFM topographic images and the height difference against the nonlithographic area after scanning. A slight protuberance can be observed in the topographic images of samples scanned at the bias voltages of 4 to 6 V. This protuberance became much higher at the bias voltages of 4 to 6V. These protuberances are due to the production of SiO2, which resulted from the decomposition of the as-deposited APhS SAM and the oxidation of silicon. This demonstrates that there is no possibility of chemical reversibility at these bias voltages since the SAM has been damaged. Specifically, there is no possibility of surface-potential reversibility. On the other hand, no change can be observed in the topographic images of samples scanned at the bias voltages of 0.5-3 V. At these voltages, it is possible that the framework of the SAM molecules remained intact. Fig. 11 shows both surface-potential images obtained by KFM and the change in surface potential on the scanned region. The surface potential shifted negatively, which can be roughly explained by the apparent dipole moment of the SAM. The apparent dipole

Scanning Probe Lithography on Organic Monolayers 489

Fig. 13. (a) Schematic illustration of selective adsorption of carboxylate-modified polystyrene spheres after AFM lithography and (b) dark field image acquired by optical microscope after immersion in a pH 4 solution containing carboxylate-modified polystyrene

spheres, followed by successive scanning probe lithography.

moment of the untreated APhS SAM is in the direction from the sample surface to the substrate. This can explain the negative shifts of the surface potential in the scanned area. In addition, amino surfaces were converted into nitroso surface at the bias voltages of 1 V to 3 V because surface potential contrast was nearly constant. In surface potential image, the surface potential reversed with the applied bias voltage.

To confirm surface-potential reversibility, a nanoprobe scanning series was performed as follows. At first, (a) a 60 μm × 60 μm square region was oxidized, and (b) a 20 μm × 20 μm square region in the 60 μm × 60 μm square region was reduced. The scan rates were 0.5 and 1.0 Hz, respectively. Fig. 12 shows illustrates of these (a) experimental processes and (b) surface potential image of scanned area. In surface potential image, the surface potential reversed with the applied bias voltage. Fig. 13 shows (a) schematic illustration of selective adsorption of carboxyl- modified fluorescent spheres after AFM lithography and (b) dark field image acquired by optical microscope after immersion of the sample in Fig. 12 in a pH 4 solution containing carboxyl-modified fluorescent spheres. The -COOH and -NH2 groups in the pH 4 solution were converted into -COO– and -NH3+ ion groups. Thus, the selective adsorption of fluorescent spheres onto the NH2 regions proceeded due to attractive electrostatic interaction. In the pH 4 solution, amino-modified fluorescent spheres were repulsed in regions with –NO terminated groups. Therefore, the bright and dark areas correspond to the NH2 and NO surface, respectively. These indicated that the nitroso terminated surfaces were reconverted into amino terminated surfaces with a negative bias voltage.

Fig. 12. (a) Schematic illustrations for a series of the lithography and (b) the obtained surface potential image.

moment of the untreated APhS SAM is in the direction from the sample surface to the substrate. This can explain the negative shifts of the surface potential in the scanned area. In addition, amino surfaces were converted into nitroso surface at the bias voltages of 1 V to 3 V because surface potential contrast was nearly constant. In surface potential image, the

To confirm surface-potential reversibility, a nanoprobe scanning series was performed as follows. At first, (a) a 60 μm × 60 μm square region was oxidized, and (b) a 20 μm × 20 μm square region in the 60 μm × 60 μm square region was reduced. The scan rates were 0.5 and 1.0 Hz, respectively. Fig. 12 shows illustrates of these (a) experimental processes and (b) surface potential image of scanned area. In surface potential image, the surface potential reversed with the applied bias voltage. Fig. 13 shows (a) schematic illustration of selective adsorption of carboxyl- modified fluorescent spheres after AFM lithography and (b) dark field image acquired by optical microscope after immersion of the sample in Fig. 12 in a pH 4 solution containing carboxyl-modified fluorescent spheres. The -COOH and -NH2 groups in the pH 4 solution were converted into -COO– and -NH3+ ion groups. Thus, the selective adsorption of fluorescent spheres onto the NH2 regions proceeded due to attractive electrostatic interaction. In the pH 4 solution, amino-modified fluorescent spheres were repulsed in regions with –NO terminated groups. Therefore, the bright and dark areas correspond to the NH2 and NO surface, respectively. These indicated that the nitroso terminated surfaces were reconverted into amino terminated surfaces with a negative bias

Fig. 12. (a) Schematic illustrations for a series of the lithography and (b) the obtained surface

surface potential reversed with the applied bias voltage.

voltage.

potential image.

Fig. 13. (a) Schematic illustration of selective adsorption of carboxylate-modified polystyrene spheres after AFM lithography and (b) dark field image acquired by optical microscope after immersion in a pH 4 solution containing carboxylate-modified polystyrene spheres, followed by successive scanning probe lithography.

Scanning Probe Lithography on Organic Monolayers 491

potential reversibility on the amino-terminated SAM by controlling the applied bias voltage.

 -NH2 + H2O ↔ -NO + 4H+ + 4e- (3) Finally, by making use of this phenomenon of surface-potential reversibility, we demonstrated surface-potential memory. First, a square region 10 μm × 10 μm was oxidized at the bias voltage of 2 V. Next, dotted areas in the oxidized region were selectively reduced at the bias voltage of -2 V. Finally, the 10 μm × 10 μm square region was again oxidized at the bias voltage of 2 V. Fig. 15 shows the surface potential changes corresponding to (a) "writing" and (b) "erasing" with the experimental process. In Fig. 15 (a), sixteen bright areas corresponding to the chemically converted region can be observed. These sixteen areas disappeared after "erasing," as shown in Fig. 15 (b). Although this "surface potential memory" has not yet been highly integrated, it has the potential to perform as ultra-

Fig. 15. Surface-potential memory: (a) "writing" state, (b) "erasing" state (c) and the

This reaction formula is as follows .

integrated memory.

experimental process.

Fig. 14. Illustrations of reversibility processes ((a)-(d)) and the surface potential image of multu-scanned area (e).

In addition, we investigated the multi-reversible conversion of the APhS surface. Firstly, (a) a 80 μm × 80 μm square region was oxidized, and (b) a 60 μm × 60 μm square region in the 80 μm × 80 μm square region was reduced. Moreover, (c) the 40 μm × 40 μm square region in the 60 μm × 60 μm square region was oxidized, and then (d) the 20 μm × 20 μm square region the 40 μm × 40 μm square region was reduced. The scan rates were 0.5, 0.7, 1.0, and 2.0 Hz, respectively. Fig. 14 (a) and (b) show illustrations of these experimental processes and the surface potential image of the scanned area, respectively. In the surface potential image, the surface potential reversed with the applied bias voltage. These indicated the multi-reversible conversion of amino terminated surfaces. Thus, we can control the surface-

Fig. 14. Illustrations of reversibility processes ((a)-(d)) and the surface potential image of

In addition, we investigated the multi-reversible conversion of the APhS surface. Firstly, (a) a 80 μm × 80 μm square region was oxidized, and (b) a 60 μm × 60 μm square region in the 80 μm × 80 μm square region was reduced. Moreover, (c) the 40 μm × 40 μm square region in the 60 μm × 60 μm square region was oxidized, and then (d) the 20 μm × 20 μm square region the 40 μm × 40 μm square region was reduced. The scan rates were 0.5, 0.7, 1.0, and 2.0 Hz, respectively. Fig. 14 (a) and (b) show illustrations of these experimental processes and the surface potential image of the scanned area, respectively. In the surface potential image, the surface potential reversed with the applied bias voltage. These indicated the multi-reversible conversion of amino terminated surfaces. Thus, we can control the surface-

multu-scanned area (e).

potential reversibility on the amino-terminated SAM by controlling the applied bias voltage. This reaction formula is as follows .

$$\text{-NH}\_2 + \text{H}\_2\text{O} \leftrightarrow \text{-NO} + 4\text{H}^\* + 4\text{e}^\* \tag{3}$$

Finally, by making use of this phenomenon of surface-potential reversibility, we demonstrated surface-potential memory. First, a square region 10 μm × 10 μm was oxidized at the bias voltage of 2 V. Next, dotted areas in the oxidized region were selectively reduced at the bias voltage of -2 V. Finally, the 10 μm × 10 μm square region was again oxidized at the bias voltage of 2 V. Fig. 15 shows the surface potential changes corresponding to (a) "writing" and (b) "erasing" with the experimental process. In Fig. 15 (a), sixteen bright areas corresponding to the chemically converted region can be observed. These sixteen areas disappeared after "erasing," as shown in Fig. 15 (b). Although this "surface potential memory" has not yet been highly integrated, it has the potential to perform as ultraintegrated memory.

Fig. 15. Surface-potential memory: (a) "writing" state, (b) "erasing" state (c) and the experimental process.

Scanning Probe Lithography on Organic Monolayers 493

Fig. 17 (a) shows the XPS Si *2p* spectra of silicon substrate surfaces before and after the immersion in 40 % aqueous ammonium fluoride solutions (NH4F). In the spectrum of the sample surface before the immersion, the peak at 104 eV attributed to SiO2 was observed, while no appreciable peak related to the oxide was observed in the spectrum after the immersion. This indicates that the native oxide layer on the silicon substrate is completely removed after the immersion. Fig. 17 (b) show the AFM topographic image of the silicon substrate surface. The AFM image has flat terraces with the steps for silicon one-atom. The distance and the height difference between steps were evaluated to be 180 nm and 0.32±0.03 nm, respectively, as shown in Fig. 17 (b). These results reveal that the silicon surfaces are terminated with hydrogen. In order to deposit the OD monolayers, the substrate was immersed in the OD solution heated at 120 °C for 1 h. After the immersion, the water contact angle of the OD monolayers became saturated at approximately 80o at the reaction time of 1 hour. Furthermore, the film thickness of 1.2 nm corresponded approximately to the distance from Si to -CH2 end groups in the precursor, as determined by ellipsometry. Fig. 17 (c) shows the topographic image of the OD monolayer surface. The distance and the height difference between steps, as shown in Fig. 17 (c), were 120 nm and 0.28±0.03 nm, respectively. These values are well in agreement with that of hydrogen-terminated silicon surface. The OD monolayers were stably attached to the hydrogen-terminated Si surfaces, since the parallel monoatomic steps were observed on the OD molecule. This means that the OD molecule was deposited at a monolayer on the substrate. These surfaces are very stable and can be stored for several weeks without any change in the topographic properties.

Fig. 17. (a) Si2p XPS spectra of the Si(111) surface after and before etching in 40% NH4, (b)AFM image (800nm×800nm)of a H-terminated Si(111) surface, (c) AFM image (800nm×800nm) of a OD-monolayer surface [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Local Generation of Carboxyl Groups on an Organic Monolayer through Chemical conversion using Scanning Probe Anodization: Mater. Sci. Eng. C, 27, 1241-1246 (2007) Copyright@ELSEVIER (2007)]

#### **4.1 Electrochemical lithography of 1,7-octadiene monolayers covalently linked to hydrogen-terminated silicon using scanning probe microsoopy**

The organic monolayer covalently attached to silicon through Si-C bond has been expected to have better chemical resistivity compared to organosilane monolayers. The Si-C interface provides a good electronic property for molecular devices constructed on the silicon. In particular, the construction of hybrid organic- molecule/silicon devices is a promising approach for the future molecular devices. To realize such devices, it is vital to establish the fabrication technology for microstructure of the organic monolayer. The electrochemical SPL is performed through the water column condensed between the tip of SPM and the substrate surface. This water column can be used as a minute electrochemical cell. When the bias voltage is applied, a redox reaction proceeds on the substrate surface. Through reversible chemical SPL, we successfully controlled this redox reaction so that an NH2 terminated organosilane monolayer surface was converted into an NO-terminated surface. However, this organosilane monolayer suffered from electrical defects due to the presence of SiOx. A 1,7-octadiene (OD) monolayer was directly formed on a hydrogen-terminated silicon surface by radical reaction. In this section, we report the electrochemical conversion of the vinyl-terminated groups on an OD monolayer induced by a nanoprobe.

Si (111) with electrical resistivity of 10.0-20.0 Ω/cm was used as for the substrates. Fig. 16 shows schematic illustrations of the experimental process. The substrates were cleaned in piranha solution (H2SO4 : H2O2 = 3 : 1) at 100 °C for 10 min and rinsed in ultrapure water. They were then etched for 15 min by immersing in 40 % aqueous ammonium fluoride solution (NH4F). The surface was hydrogen-terminated and silicon oxide was removed by this immersion. The OD monolayer was prepared by the liquid phase method. The substrates were immersed in OD solution at 120 °C for 1h. After immersion, the samples were cleaned in toluene, acetone, ethanol and rinsed in ultrapure water.

Fig. 16. Schematic illustrations of the experimental process: (a) preparation of the ODmonolayer, (b) electrochemical scanning probe lithography and (c) chemical conversion analysis. [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7 octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning Probe Microscope, Surf. Sci., 601, 4206-4211 (2007). Copyright@ELSEVIER (2007)]

The organic monolayer covalently attached to silicon through Si-C bond has been expected to have better chemical resistivity compared to organosilane monolayers. The Si-C interface provides a good electronic property for molecular devices constructed on the silicon. In particular, the construction of hybrid organic- molecule/silicon devices is a promising approach for the future molecular devices. To realize such devices, it is vital to establish the fabrication technology for microstructure of the organic monolayer. The electrochemical SPL is performed through the water column condensed between the tip of SPM and the substrate surface. This water column can be used as a minute electrochemical cell. When the bias voltage is applied, a redox reaction proceeds on the substrate surface. Through reversible chemical SPL, we successfully controlled this redox reaction so that an NH2 terminated organosilane monolayer surface was converted into an NO-terminated surface. However, this organosilane monolayer suffered from electrical defects due to the presence of SiOx. A 1,7-octadiene (OD) monolayer was directly formed on a hydrogen-terminated silicon surface by radical reaction. In this section, we report the electrochemical conversion

**4.1 Electrochemical lithography of 1,7-octadiene monolayers covalently linked to** 

**hydrogen-terminated silicon using scanning probe microsoopy** 

of the vinyl-terminated groups on an OD monolayer induced by a nanoprobe.

were cleaned in toluene, acetone, ethanol and rinsed in ultrapure water.

Fig. 16. Schematic illustrations of the experimental process: (a) preparation of the ODmonolayer, (b) electrochemical scanning probe lithography and (c) chemical conversion analysis. [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7 octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning

Probe Microscope, Surf. Sci., 601, 4206-4211 (2007). Copyright@ELSEVIER (2007)]

Si (111) with electrical resistivity of 10.0-20.0 Ω/cm was used as for the substrates. Fig. 16 shows schematic illustrations of the experimental process. The substrates were cleaned in piranha solution (H2SO4 : H2O2 = 3 : 1) at 100 °C for 10 min and rinsed in ultrapure water. They were then etched for 15 min by immersing in 40 % aqueous ammonium fluoride solution (NH4F). The surface was hydrogen-terminated and silicon oxide was removed by this immersion. The OD monolayer was prepared by the liquid phase method. The substrates were immersed in OD solution at 120 °C for 1h. After immersion, the samples Fig. 17 (a) shows the XPS Si *2p* spectra of silicon substrate surfaces before and after the immersion in 40 % aqueous ammonium fluoride solutions (NH4F). In the spectrum of the sample surface before the immersion, the peak at 104 eV attributed to SiO2 was observed, while no appreciable peak related to the oxide was observed in the spectrum after the immersion. This indicates that the native oxide layer on the silicon substrate is completely removed after the immersion. Fig. 17 (b) show the AFM topographic image of the silicon substrate surface. The AFM image has flat terraces with the steps for silicon one-atom. The distance and the height difference between steps were evaluated to be 180 nm and 0.32±0.03 nm, respectively, as shown in Fig. 17 (b). These results reveal that the silicon surfaces are terminated with hydrogen. In order to deposit the OD monolayers, the substrate was immersed in the OD solution heated at 120 °C for 1 h. After the immersion, the water contact angle of the OD monolayers became saturated at approximately 80o at the reaction time of 1 hour. Furthermore, the film thickness of 1.2 nm corresponded approximately to the distance from Si to -CH2 end groups in the precursor, as determined by ellipsometry. Fig. 17 (c) shows the topographic image of the OD monolayer surface. The distance and the height difference between steps, as shown in Fig. 17 (c), were 120 nm and 0.28±0.03 nm, respectively. These values are well in agreement with that of hydrogen-terminated silicon surface. The OD monolayers were stably attached to the hydrogen-terminated Si surfaces, since the parallel monoatomic steps were observed on the OD molecule. This means that the OD molecule was deposited at a monolayer on the substrate. These surfaces are very stable and can be stored for several weeks without any change in the topographic properties.

Fig. 17. (a) Si2p XPS spectra of the Si(111) surface after and before etching in 40% NH4, (b)AFM image (800nm×800nm)of a H-terminated Si(111) surface, (c) AFM image (800nm×800nm) of a OD-monolayer surface [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Local Generation of Carboxyl Groups on an Organic Monolayer through Chemical conversion using Scanning Probe Anodization: Mater. Sci. Eng. C, 27, 1241-1246 (2007) Copyright@ELSEVIER (2007)]

Scanning Probe Lithography on Organic Monolayers 495

A gold-coated probe was scanned in air on OD monolayers at the bias voltage of 1 V and the scanning rate of 2 Hz. The areas scanned were a square, 20 μm on a side. Fig. 19 (a) and (b) shows the topographic and the surface potential images, respectively. The surface potential for the area scanned was 20 mV more negative than that of the area non-scanned and no change was observed in the topographic images. The value of surface potential difference between the scanned and non-scanned areas by SPL is well in agreement with that by photo oxidation, i.e., VUV light. These results indicate that the scanned OD monolayer surfaces are completely oxidized and chemically converted into COOH terminated surfaces without decomposition of the OD monolayer and siloxane networks. In order to verify that the chemical conversion with SPL was electrochemically proceeded on the areas scanned through the water column, the SPL was carried out in vacuum. The conditions were as follows: the bias voltage : 1 V, the pressure : 10-4 Pa, the scanned rate : 2 Hz, and the scanned areas : a square of 20 μm on a side. Fig. 20 shows the topographic and the surface potential images on the scanned area. In both images, no change was observed. This means that the chemical conversion on the scanned areas does not proceed under such a condition. Therefore, we can conclude that the chemical conversions with SPL are based on

Electrochemical SPL was performed on the OD monolayer at the scanning rate of 2 Hz and at bias voltages of -3 V to 3 V. Fig. 21 shows representative surface potential images, topographic images, the changes in surface potential, and the height difference against the non- lithographic regions. In topographic images, no change in height difference was observed at any of the bias voltages. This indicates that the scanning caused no change in microscopic morphology under these conditions. On the other hand, the surface potential changed remarkably. The dark and bright regions in the surface potential images correspond to low and high surface potential regions, respectively. When the bias voltage was positively applied, oxidation proceeded on the substrate. At the positive bias voltage, the scanned regions were oxidized and showed lower surface potential than the unscanned areas. The surface potential contrast was nearly constant at approximately -18 mV at bias voltages of 1 V to 2 V. However, the surface potential contrast at bias voltages of 2 V to 3 V

On the other hand, the surface potential contrast at negative bias voltages gradually increased in proportion to voltage evolution. The surface potential contrast at bias voltages of 0 V to -1 V was negative, indicating that some oxidation proceeded under these conditions. This was due to the difference in contact potential between the Au-coated probe and the Si substrate. The surface potential constant was positive at bias voltages from -1.5 V to -3 V. This change in surface potential indicates that reduction reactions occurred on the substrate surface due to the applied negative bias voltage. Considering these AFM and KPFM results, the electrochemical conversion of the vinyl-terminated groups is believed to

Using XPS, we investigated the conversion of vinyl terminated groups at each bias voltage. Fig. 22 (a) and (b) show XPS Si*2P* and C*1s* spectra for sample surfaces after probe-scanning at bias voltages of 1 V and 3 V. The C*1s* spectrum in Fig. 22 (b) shows an additional peak from -COOH groups at 288.5 eV at the bias voltage of 1 V, but not at 3 V. In addition, a silicon oxide peak at 103.4 eV can be seen in Fig. 22 (a) at the bias voltage of 3 V, but not at 1 V. The intensity of the alkyl chain peak in Fig. 22 (b) at the bias voltage of 3 V was observed to be lower than that at 1 V. These XPS results indicate that the vinyl functional groups were oxidized and converted into carboxyl groups at the bias voltage of 1 V. The surface potential

electrochemical reactions through the water column.

gradually decreased in proportion to voltage evolution.

have been governed by the applied bias voltage.

The OD monolayers were selectively oxidized in air through the photomask by vacuum ultraviolet (VUV) light irradiation for 10 min. The areas irradiated were converted into -COOH groups due to photochemical oxidation, thus dividing the small surface into distinct -CH2 and -COOH end groups regions. Fig. 18 (a) shows the XPS C*1s* spectra of OD monolayer surfaces before and after the irradiation. Its peak of 289.6 eV is assigned to the carboxyl group. Fig. 18 (b) shows the surface potential image (KFM) of the OD monolayer. The dark and bright regions correspond to the CPD images of low and high surface potential, respectively. In this figure, the surface potential for the irradiated OD monolayer surfaces was 20 mV lower than that of the unirradiated surfaces. The change of the surface potential indicates that -CH2 end groups on the OD monolayer were chemically converted into -COOH end groups. The end groups of the OD monolayers were confirmed by the selective adsorption of amino-modified fluorescence spheres in a pH 4 solution. The -COOH and -NH2 groups in the pH 4 solution were converted into -COO- and -NH3 + ion groups, so that the selective adsorption of fluorescence spheres on to the substrate proceeded due to their attractive interaction to the surface. Under this pH condition, the regions of –CH2 end groups on the surface were not negatively charged and the amino-modified polystyrene fluorescence spheres did not adsorb onto it. Fig. 18 (c) shows an image acquired by dark field microscopy of the micropatterned CH2 / COOH sample after immersion. The lighter areas between the dark rectangular regions correspond to the COOH terminated regions. This dark-field image indicates that aminomodified polystyrene fluorescence spheres selectively adsorbed on the -COOH end group regions since scattered light due to surface roughness can be observed. From these results, we determined that an -CH2 end group had been successfully converted into the COOH terminated surface through chemical lithography, i.e., photolithography.

Fig. 18. (a) C1s XPS spectra for VUV irradiation of 0 and 10min, (b) KFM image (150μm×150μm) of OD-monolayer irradiated for 10min, (c) optical microscope image of the irradiated OD-monolayer which adsorbed amino terminated particles.[SH. Lee, T. Ishizaki, N. Saito, O. Takai, Local Generation of Carboxyl Groups on an Organic Monolayer through Chemical conversion using Scanning Probe Anodization: Mater. Sci. Eng. C, 27, 1241-1246 (2007). Copyright@ELSEVIER (2007)]

The OD monolayers were selectively oxidized in air through the photomask by vacuum ultraviolet (VUV) light irradiation for 10 min. The areas irradiated were converted into -COOH groups due to photochemical oxidation, thus dividing the small surface into distinct -CH2 and -COOH end groups regions. Fig. 18 (a) shows the XPS C*1s* spectra of OD monolayer surfaces before and after the irradiation. Its peak of 289.6 eV is assigned to the carboxyl group. Fig. 18 (b) shows the surface potential image (KFM) of the OD monolayer. The dark and bright regions correspond to the CPD images of low and high surface potential, respectively. In this figure, the surface potential for the irradiated OD monolayer surfaces was 20 mV lower than that of the unirradiated surfaces. The change of the surface potential indicates that -CH2 end groups on the OD monolayer were chemically converted into -COOH end groups. The end groups of the OD monolayers were confirmed by the selective adsorption of amino-modified fluorescence spheres in a pH 4 solution. The -COOH and -NH2 groups in the pH 4 solution

fluorescence spheres on to the substrate proceeded due to their attractive interaction to the surface. Under this pH condition, the regions of –CH2 end groups on the surface were not negatively charged and the amino-modified polystyrene fluorescence spheres did not adsorb onto it. Fig. 18 (c) shows an image acquired by dark field microscopy of the micropatterned CH2 / COOH sample after immersion. The lighter areas between the dark rectangular regions correspond to the COOH terminated regions. This dark-field image indicates that aminomodified polystyrene fluorescence spheres selectively adsorbed on the -COOH end group regions since scattered light due to surface roughness can be observed. From these results, we determined that an -CH2 end group had been successfully converted into the COOH

terminated surface through chemical lithography, i.e., photolithography.

Fig. 18. (a) C1s XPS spectra for VUV irradiation of 0 and 10min, (b) KFM image

(150μm×150μm) of OD-monolayer irradiated for 10min, (c) optical microscope image of the irradiated OD-monolayer which adsorbed amino terminated particles.[SH. Lee, T. Ishizaki, N. Saito, O. Takai, Local Generation of Carboxyl Groups on an Organic Monolayer through Chemical conversion using Scanning Probe Anodization: Mater. Sci. Eng. C, 27, 1241-1246

+ ion groups, so that the selective adsorption of

were converted into -COO- and -NH3

(2007). Copyright@ELSEVIER (2007)]

A gold-coated probe was scanned in air on OD monolayers at the bias voltage of 1 V and the scanning rate of 2 Hz. The areas scanned were a square, 20 μm on a side. Fig. 19 (a) and (b) shows the topographic and the surface potential images, respectively. The surface potential for the area scanned was 20 mV more negative than that of the area non-scanned and no change was observed in the topographic images. The value of surface potential difference between the scanned and non-scanned areas by SPL is well in agreement with that by photo oxidation, i.e., VUV light. These results indicate that the scanned OD monolayer surfaces are completely oxidized and chemically converted into COOH terminated surfaces without decomposition of the OD monolayer and siloxane networks. In order to verify that the chemical conversion with SPL was electrochemically proceeded on the areas scanned through the water column, the SPL was carried out in vacuum. The conditions were as follows: the bias voltage : 1 V, the pressure : 10-4 Pa, the scanned rate : 2 Hz, and the scanned areas : a square of 20 μm on a side. Fig. 20 shows the topographic and the surface potential images on the scanned area. In both images, no change was observed. This means that the chemical conversion on the scanned areas does not proceed under such a condition. Therefore, we can conclude that the chemical conversions with SPL are based on electrochemical reactions through the water column.

Electrochemical SPL was performed on the OD monolayer at the scanning rate of 2 Hz and at bias voltages of -3 V to 3 V. Fig. 21 shows representative surface potential images, topographic images, the changes in surface potential, and the height difference against the non- lithographic regions. In topographic images, no change in height difference was observed at any of the bias voltages. This indicates that the scanning caused no change in microscopic morphology under these conditions. On the other hand, the surface potential changed remarkably. The dark and bright regions in the surface potential images correspond to low and high surface potential regions, respectively. When the bias voltage was positively applied, oxidation proceeded on the substrate. At the positive bias voltage, the scanned regions were oxidized and showed lower surface potential than the unscanned areas. The surface potential contrast was nearly constant at approximately -18 mV at bias voltages of 1 V to 2 V. However, the surface potential contrast at bias voltages of 2 V to 3 V gradually decreased in proportion to voltage evolution.

On the other hand, the surface potential contrast at negative bias voltages gradually increased in proportion to voltage evolution. The surface potential contrast at bias voltages of 0 V to -1 V was negative, indicating that some oxidation proceeded under these conditions. This was due to the difference in contact potential between the Au-coated probe and the Si substrate. The surface potential constant was positive at bias voltages from -1.5 V to -3 V. This change in surface potential indicates that reduction reactions occurred on the substrate surface due to the applied negative bias voltage. Considering these AFM and KPFM results, the electrochemical conversion of the vinyl-terminated groups is believed to have been governed by the applied bias voltage.

Using XPS, we investigated the conversion of vinyl terminated groups at each bias voltage. Fig. 22 (a) and (b) show XPS Si*2P* and C*1s* spectra for sample surfaces after probe-scanning at bias voltages of 1 V and 3 V. The C*1s* spectrum in Fig. 22 (b) shows an additional peak from -COOH groups at 288.5 eV at the bias voltage of 1 V, but not at 3 V. In addition, a silicon oxide peak at 103.4 eV can be seen in Fig. 22 (a) at the bias voltage of 3 V, but not at 1 V. The intensity of the alkyl chain peak in Fig. 22 (b) at the bias voltage of 3 V was observed to be lower than that at 1 V. These XPS results indicate that the vinyl functional groups were oxidized and converted into carboxyl groups at the bias voltage of 1 V. The surface potential

Scanning Probe Lithography on Organic Monolayers 497

Fig. 20. (a) Topographic image and (b) surface potential image after the scanning in vacuum.[SH. Lee, T. Ishizaki, N. Saito, O. Takai, Local Generation of Carboxyl Groups on an Organic Monolayer through Chemical conversion using Scanning Probe Anodization:

Mater. Sci. Eng. C, 27, 1241-1246 (2007). Copyright@ELSEVIER (2007)]

of the carboxyl surface was lower than that of the vinyl-terminated surface because the carboxyl groups had more negative dipole moment. This agrees with the KFM results. In addition, OD molecules on the sample surface were decomposed and silicon oxide was formed at the bias voltage of 3 V. However, the fact that there was no change of AFM morphology at the bias voltage of 3 V was probably due to the formation of "depthless" silicon oxide, that is, the partial decomposition of OD molecules, and to the effect of absorbed water on the sample surface.

Fig. 19. (a) Topographic images and (b) surface potential image after the scanning at the rate of 2Hz. [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Local Generation of Carboxyl Groups on an Organic Monolayer through Chemical conversion using Scanning Probe Anodization: Mater. Sci. Eng. C, 27, 1241-1246 (2007). Copyright@ELSEVIER (2007)]

of the carboxyl surface was lower than that of the vinyl-terminated surface because the carboxyl groups had more negative dipole moment. This agrees with the KFM results. In addition, OD molecules on the sample surface were decomposed and silicon oxide was formed at the bias voltage of 3 V. However, the fact that there was no change of AFM morphology at the bias voltage of 3 V was probably due to the formation of "depthless" silicon oxide, that is, the partial decomposition of OD molecules, and to the effect of

Fig. 19. (a) Topographic images and (b) surface potential image after the scanning at the rate of 2Hz. [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Local Generation of Carboxyl Groups on an Organic Monolayer through Chemical conversion using Scanning Probe Anodization:

Mater. Sci. Eng. C, 27, 1241-1246 (2007). Copyright@ELSEVIER (2007)]

absorbed water on the sample surface.

Fig. 20. (a) Topographic image and (b) surface potential image after the scanning in vacuum.[SH. Lee, T. Ishizaki, N. Saito, O. Takai, Local Generation of Carboxyl Groups on an Organic Monolayer through Chemical conversion using Scanning Probe Anodization: Mater. Sci. Eng. C, 27, 1241-1246 (2007). Copyright@ELSEVIER (2007)]

Scanning Probe Lithography on Organic Monolayers 499

Fig. 22. (a) XPS Si*2P* spectra and (b) XPS C*1s* spectra of sample surfaces after probe-scanning

monolayer surface and a sample surface after probe-scanning at the bias voltage of -3V. [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7-octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning Probe

Fig. 22 (c) and (d) show XPS Si*2P* and C*1s* spectra of an unscanned OD-monolayer surface and a sample surface scanned at the bias voltage of -3 V. In the C*1s* XPS spectrum in Fig. 22 (d), the intensity of the alkyl chain peak for the surface scanned at the bias voltage of -3 V was the same as that for the OD monolayer. The silicon oxide peaks in the Si*2p* spectra were not observed in all cases [Fig. 22 (a) and (c)]. These XPS results indicate that OD molecules were not decomposed at negative bias voltage. We believe that the vinyl functional groups were reduced and converted into cyclobutane rings. No peak for these cyclobutane rings was observed in the C*1s* spectra since such a peak is generally weak. However, in view of the AFM and KFM results, we consider that cyclobutane rings form in the same manner as

Fig. 23 shows a schematic illustration of the mechanism of electrochemical SPL on the OD monolayer. In this Section, two factors were considered: the alkyl radical reaction from frictional heat due to the probe scanning, and the redox reaction on the sample surface

at bias voltages of 1V and 3V. (c) XPS Si*2P* spectra and (d) XPS C*1s* spectra of an OD-

Microscope, Surf. Sci., 601, 4206-4211 (2007). Copyright@ELSEVIER (2007)]

they are known to in photochemical and thermal reactions.

Fig. 21. (a) Representative topographic images, (b) representative surface potential images, (c) change of surface potential against non-lithographic regions; and (d) height difference against non-lithographic regions. Electrochemical SPL was performed at bias voltages of -3V to 3V. [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7 octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning Probe Microscope, Surf. Sci., 601, 4206-4211 (2007). Copyright@ELSEVIER (2007)]

Fig. 21. (a) Representative topographic images, (b) representative surface potential images, (c) change of surface potential against non-lithographic regions; and (d) height difference against non-lithographic regions. Electrochemical SPL was performed at bias voltages of -3V to 3V. [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7 octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning

Probe Microscope, Surf. Sci., 601, 4206-4211 (2007). Copyright@ELSEVIER (2007)]

Fig. 22. (a) XPS Si*2P* spectra and (b) XPS C*1s* spectra of sample surfaces after probe-scanning at bias voltages of 1V and 3V. (c) XPS Si*2P* spectra and (d) XPS C*1s* spectra of an ODmonolayer surface and a sample surface after probe-scanning at the bias voltage of -3V. [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7-octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning Probe Microscope, Surf. Sci., 601, 4206-4211 (2007). Copyright@ELSEVIER (2007)]

Fig. 22 (c) and (d) show XPS Si*2P* and C*1s* spectra of an unscanned OD-monolayer surface and a sample surface scanned at the bias voltage of -3 V. In the C*1s* XPS spectrum in Fig. 22 (d), the intensity of the alkyl chain peak for the surface scanned at the bias voltage of -3 V was the same as that for the OD monolayer. The silicon oxide peaks in the Si*2p* spectra were not observed in all cases [Fig. 22 (a) and (c)]. These XPS results indicate that OD molecules were not decomposed at negative bias voltage. We believe that the vinyl functional groups were reduced and converted into cyclobutane rings. No peak for these cyclobutane rings was observed in the C*1s* spectra since such a peak is generally weak. However, in view of the AFM and KFM results, we consider that cyclobutane rings form in the same manner as they are known to in photochemical and thermal reactions.

Fig. 23 shows a schematic illustration of the mechanism of electrochemical SPL on the OD monolayer. In this Section, two factors were considered: the alkyl radical reaction from frictional heat due to the probe scanning, and the redox reaction on the sample surface

Scanning Probe Lithography on Organic Monolayers 501

terminated groups of the OD monolayer were converted into COOH terminated groups by

Fig. 24. (a) Selective adsorption of amino-modified fluorescent spheres on a surface scanned at the bias voltage of 1V; (b) dark field image of a patterned surface after immersion in a pH 4 solution containing amino-modified fluorescent spheres [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7-octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning Probe Microscope, Surf. Sci., 601, 4206-4211

In this chapter, we introduced the chemical conversion of functional groups on the organic monolayer by electrochemical SPL. The three-dimensional nanostructures of silicon oxide were successfully fabricated by decomposing the 1-decane monolayer and subsequent oxidizing the hydrogen-terminated Si surfaces via anodization SPL. The size and reproducibility of oxide nanoline structures were greatly dependent on the sorts of probes for anodization SPL. In the case of Au-coated Si and uncoated Si probes, the obtained nanoline structures were changed with the scanning rates and the applied bias voltages. On the other hand, the nanotexture fabrication using the diamond-coated probe showed one of the finest structures (15 nm nanoline) and highly reproducibility even though any fabrication conditions such as scanning rate and applied bias voltage are used in the anodization SPL.The amino surface on SAM was oxidized and converted into a nitroso surface at bias voltages of 0.5 to 3 V. The functional groups on APhS SAM were reversibly converted by controlling the applied bias voltage. It was also demonstrated that the surface-potential memory was based on surface potential reversibility. In addition, the vinyl-terminated groups of the OD monolayer were site-selectively oxidized and chemically converted into carboxyl groups at bias voltages of 1 to 2 V. OD molecules on the sample surface were decomposed and silicon oxide was formed at bias voltages greater than 3 V. On the other hand, CH2-terminal groups were converted into

scanning at the applied bias voltage of 1 V.

(2007). Copyright@ELSEVIER (2007)].

**5. Conclusion** 

caused by polarization due to the applied bias voltage. First, alkyl radicals were formed by frictional heat. Next, the redox reaction occurred on the sample surface in the radical atmosphere. With positive bias voltages, the oxidation reaction easily occurred on the sample surface due to polarization in adsorbed water. Thus, the conversions on the sample surface were governed by the oxidation reaction. The vinyl-terminated groups of the OD monolayer were converted into carboxyl groups at positive bias voltage. However, the reduction reaction on the sample surface rarely occurred at negative bias voltages because the dissolved oxygen was preferentially reduced in adsorbed water. Thus, in this case, the surface reaction was governed by alkyl radicals. The formation of cyclobutane rings was considered to have occurred due to alkyl radical combinations at the negative bias voltage.

Fig. 23. Schematic illustration of the redox reaction induced by electrochemical SPL [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7-octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning Probe Microscope, Surf. Sci., 601, 4206-4211 (2007). Copyright@ELSEVIER (2007)].

In support of the SPM and XPS results, the oxidized groups of the OD monolayer were confirmed by the selective adsorption of amino-modified fluorescent spheres. The -COOH and -NH2 groups in the pH 4 solution were converted into –COOand -NH3 + ion groups. Thus, the selective adsorption of fluorescent spheres onto the COOH regions proceeded due to attractive electrostatic interaction. In the pH 4 solution, regions with vinyl terminated groups were not negatively charged, and the amino-modified polystyrene fluorescent spheres were repulsed. Fig. 24 (a) shows the mechanism of this selective adsorption of the amino-modified fluorescence spheres. Fig. 24 (b) shows a dark field image of samples scanned at the bias voltage of 1 V after immersion in the solution of amino-modified fluorescent spheres. The bright areas correspond to the areas scanned at the bias voltage of 1 V which site-selectively adsorbed the fluorescent spheres. This confirms that vinyl

caused by polarization due to the applied bias voltage. First, alkyl radicals were formed by frictional heat. Next, the redox reaction occurred on the sample surface in the radical atmosphere. With positive bias voltages, the oxidation reaction easily occurred on the sample surface due to polarization in adsorbed water. Thus, the conversions on the sample surface were governed by the oxidation reaction. The vinyl-terminated groups of the OD monolayer were converted into carboxyl groups at positive bias voltage. However, the reduction reaction on the sample surface rarely occurred at negative bias voltages because the dissolved oxygen was preferentially reduced in adsorbed water. Thus, in this case, the surface reaction was governed by alkyl radicals. The formation of cyclobutane rings was considered to have occurred due to alkyl radical combinations at the negative bias voltage.

Fig. 23. Schematic illustration of the redox reaction induced by electrochemical SPL [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7-octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning Probe

In support of the SPM and XPS results, the oxidized groups of the OD monolayer were confirmed by the selective adsorption of amino-modified fluorescent spheres. The -COOH

Thus, the selective adsorption of fluorescent spheres onto the COOH regions proceeded due to attractive electrostatic interaction. In the pH 4 solution, regions with vinyl terminated groups were not negatively charged, and the amino-modified polystyrene fluorescent spheres were repulsed. Fig. 24 (a) shows the mechanism of this selective adsorption of the amino-modified fluorescence spheres. Fig. 24 (b) shows a dark field image of samples scanned at the bias voltage of 1 V after immersion in the solution of amino-modified fluorescent spheres. The bright areas correspond to the areas scanned at the bias voltage of 1 V which site-selectively adsorbed the fluorescent spheres. This confirms that vinyl

and -NH3+ ion groups.

Microscope, Surf. Sci., 601, 4206-4211 (2007). Copyright@ELSEVIER (2007)].

and -NH2 groups in the pH 4 solution were converted into –COO-

terminated groups of the OD monolayer were converted into COOH terminated groups by scanning at the applied bias voltage of 1 V.

Fig. 24. (a) Selective adsorption of amino-modified fluorescent spheres on a surface scanned at the bias voltage of 1V; (b) dark field image of a patterned surface after immersion in a pH 4 solution containing amino-modified fluorescent spheres [SH. Lee, T. Ishizaki, N. Saito, O. Takai, Electrochemical Soft Lithography of an 1,7-octadiene Monolayer Covalently Linked to Hydrogen-Terminated Silicon using Scanning Probe Microscope, Surf. Sci., 601, 4206-4211 (2007). Copyright@ELSEVIER (2007)].
