**Scanning Probe Lithography on Organic Monolayers**

SunHyung Lee1, Takahiro Ishizaki2, Katsuya Teshima1, Nagahiro Saito3 and Osamu Takai3 *1Faculty of Engineering, Shinshu University, 2National Institute of Advanced Industrial Science and Technology (AIST), 3EcoTopia Science Institute, Nagoya University, Japan* 

#### **1. Introduction**

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Lithographic technologies for the surface modification of inorganic and organic surfaces have been developed for various devices such as sensing, data memory, single molecule electronics and biological systems. Nano and micropatterning of organic monolayers have attracted attentions for applications to biological systems in which proteins or DNA are fixed. Photolithography, microcontact printing, and electron beam lithography have usually been used as patterning techniques for organic monolayers (Hayashi et al., 2002; Hong et al., 2003; Saito et al., 2003; Hahn et al., 2004; Kidoaki & Matsuda, 1999). Although the electronbeam lithography can fabricate very small patterns, it requires an ultra-high vacuum system (Harnett et al., 2001). The resolution of photolithography is limited by the light wavelength. Moreover, these methods are based on destructive lithography, i.e., they cause damages to the organic materials.

In particular, nano-lithographic technologies have evolved in order to satisfy persistent demands for miniaturization and high-density integration of semiconductor electric circuits. Scanning probe microscopy (SPM) has been a key tool in achieving this goal. SPM can be used not only as the means that observe surface structure at sub-molecule level by a probe but also as the means that control the atomic and molecular arrangement on a substrate. As a local nano-fabrication means, the lithography technique in nanoscale range by using SPM is called to the scanning probe lithography (SPL) (Kaholek et al., 2004; Blackledge et al., 2000; Tello et al., 2002; Liu et al., 2002). In particular, a variety of lithographic techniques using SPM probe can fabricate nano-scale patterns on an organic monolayer, such as nanoshaving, nanografting, anodization SPL, dip-pen nanolithography (DPN), and electrochemical SPL. The lithography technique is used to break the material surface by using various energy sources. SPM can also be used to break the organic monolayer. For instance, nanoshaving involves mechanical scratching by physical pressure of the probe, and anodization lithography involves anodic oxidation of the substrate surface by an applied bias voltage (above 9 V) between the probe and the substrate (Jang et al., 2002; Kaholek et al., 2004; Sugimura, & Nakagiri, 1995). In the case of the anodization lithography, the oxide layer can be fabricated by anodic oxidation. As another SPL technique, the

Scanning Probe Lithography on Organic Monolayers 477

Fig. 2. Schematic illustration of dip-pen nanolithography (DPN) and electrochemical SPL. In this chapter, we introduce anodization SPL and electrochemical SPL for fabricating the nanostcuture and controlling the function group on the various organic monolayers, such as organosilane self-assembled monolayers (SAMs) and the monolayer covalently attached to silicon through Si-C bond . First, anodization SPL techniques are expected to fabricate nanostructures on surfaces of electronic device. Their technique can remove an organic monolayer and fabricate an oxide structure by applying highly bias voltage between probe and substrate. When a high positive bias voltage is applied to the probe, an oxidation reaction proceeds and an oxide structure forms on the surface. The electrochemical reactions

4*H2O* + 4e- → 2*H2* + 4OH- (1)

In these reactions, electric characteristics of coated materials on the probes are important to formation of an oxide structure. The oxide structure is easily fabricated on the substrate by anodization SPL since the silicon oxide insulator layer is removed. However, the oxide structure is difficult to control and is not attained to single nanometer level though many researchers have reported on anodization SPL. We introduced fabricating technique of a three-dimensional nanostructure (nanoline structure) of silicon oxide on the hydrogen-

(*M* : substrate) (2)

on SPM probe and substrate are shown in Eqs., respectively (Lee et al., 2009).

 *M* + 2*H2O* → *MO2* + 4*H+* + 4e-

nanografting procedure combines the fabrication of a nanopattern and the binary SAM using atomic force microscopy (AFM) (Amro et al., 2000; Xu et al., 1999; Liu et al., 2000). As the other process, a probe is scanned on the matrix SAM at a high force. The matrix SAM is removed and simultaneously replaced by another molecule in the scanning area. Fig. 1 shows the nanoshaving, nanografting, and anodization lithography techniques.

Fig. 1. Various lithographic methods using SPM probe on the organic monolayer. : (a) nanoshaving, (b) nanografting and (c) anodization lithography

In addition, dip-pen nanolithography (DPN) is a nanopatterning technique with a probe which delivers molecules to a surface via a water meniscus in the ambient atmosphere (Pena et al., 2003; Schwartz,2002; Maynor. et al., 2004). This direct-writing technique offers highresolution patterning capabilities for a number of moleculars and biomolecular materials (ink) on a variety of substrates (paper). Nanografting and DPN are recently developed as "soft" lithographic methods, which do not cause any damage to the organic monolayers. Electrochemical SPL is also one of the soft lithographic methods. This is performed through the water column condensed between the tip of the SPM and the substrate surface. This water column can be used as a minute electrochemical cell. When a bias voltage is applied, a redox reaction occurs on the substrate surface. In the case of SAM, the functional group is converted by this redox reaction and a nanopattern is formed on the SAM surface. Fig. 2 shows the DPN and electrochemical SPL techniques.

nanografting procedure combines the fabrication of a nanopattern and the binary SAM using atomic force microscopy (AFM) (Amro et al., 2000; Xu et al., 1999; Liu et al., 2000). As the other process, a probe is scanned on the matrix SAM at a high force. The matrix SAM is removed and simultaneously replaced by another molecule in the scanning area. Fig. 1

shows the nanoshaving, nanografting, and anodization lithography techniques.

Fig. 1. Various lithographic methods using SPM probe on the organic monolayer. :

In addition, dip-pen nanolithography (DPN) is a nanopatterning technique with a probe which delivers molecules to a surface via a water meniscus in the ambient atmosphere (Pena et al., 2003; Schwartz,2002; Maynor. et al., 2004). This direct-writing technique offers highresolution patterning capabilities for a number of moleculars and biomolecular materials (ink) on a variety of substrates (paper). Nanografting and DPN are recently developed as "soft" lithographic methods, which do not cause any damage to the organic monolayers. Electrochemical SPL is also one of the soft lithographic methods. This is performed through the water column condensed between the tip of the SPM and the substrate surface. This water column can be used as a minute electrochemical cell. When a bias voltage is applied, a redox reaction occurs on the substrate surface. In the case of SAM, the functional group is converted by this redox reaction and a nanopattern is formed on the SAM surface. Fig. 2

(a) nanoshaving, (b) nanografting and (c) anodization lithography

shows the DPN and electrochemical SPL techniques.

Fig. 2. Schematic illustration of dip-pen nanolithography (DPN) and electrochemical SPL.

In this chapter, we introduce anodization SPL and electrochemical SPL for fabricating the nanostcuture and controlling the function group on the various organic monolayers, such as organosilane self-assembled monolayers (SAMs) and the monolayer covalently attached to silicon through Si-C bond . First, anodization SPL techniques are expected to fabricate nanostructures on surfaces of electronic device. Their technique can remove an organic monolayer and fabricate an oxide structure by applying highly bias voltage between probe and substrate. When a high positive bias voltage is applied to the probe, an oxidation reaction proceeds and an oxide structure forms on the surface. The electrochemical reactions on SPM probe and substrate are shown in Eqs., respectively (Lee et al., 2009).

$$4H\_2O + 4e^- \rightarrow 2H\_2 + 4OH^- \tag{1}$$

$$M + 2H\_2O \rightarrow MO\_2 + 4H^\* + 4e^- \\ \text{(} M \text{: substrate)} \tag{2}$$

In these reactions, electric characteristics of coated materials on the probes are important to formation of an oxide structure. The oxide structure is easily fabricated on the substrate by anodization SPL since the silicon oxide insulator layer is removed. However, the oxide structure is difficult to control and is not attained to single nanometer level though many researchers have reported on anodization SPL. We introduced fabricating technique of a three-dimensional nanostructure (nanoline structure) of silicon oxide on the hydrogen-

Scanning Probe Lithography on Organic Monolayers 479

molecule. The static water contact angle and film thickness of the monolayer was about 107° and 1.12 nm, respectively. A smooth surface coated with densely packed CH3 groups (alkyl monolayer) shows a water contact angle of approximately 110°. In addition, its thickness was considered reasonable since the chain length of 1-decane molecule was estimated to be 1.32 nm. It is indicated that the 1-decane molecules formed an organic monolayer on the silicon substrate. Fig. 3(b) shows the XPS Si2p spectrum of the Si surface coated with the 1 decane monolayer. As clearly shown in Fig. 3(b), the peak of native silicon oxide was not observed at all. These results indicate that the 1-decane molecules reacted directly to the hydrogen-terminated silicon surfaces and formed the organic monolayer there. Fig. 3(c) and 3(d) show the AFM topographic and friction images of the Si surfaces after immersing in 1 decene solution. The flat terraces with steps for silicon one-atom and the changes of high friction on steps were, respectively, observed on the topographic and friction images. In the topographic image [Fig. 3(c)], the intervals of the steps and the terraces were 180 nm and 3.2 ± 0.3 Å, respectively. In addition, the difference of friction force between the flat terraces and steps was about 10 mV. These results indicated to the surface profiles of hydrogenterminated silicon. It was therefore found that the 1-decane molecules were vertically and densely assembled, and their monolayer was formed through doubly-boned terminated

Fig. 3. (a) The chemical structure of 1-decane (b) XPS Si2p spectra (c) topographic image (d) friction image of sample surfaces after prepared 1-decane monolayer. [SH. Lee, N. Saito, O. Takai, Highly reproducible technique for three-dimensional nanostructure fabrication via

anodization scanning probe lithography, Appl. Surf. Sci., 255, 7302-7306 (2009).

Copyright@ELSEVIER (2009)]

group attached to the hydrogen-terminated silicon surface.

terminated Si substrates by anodization SPL, and the effects of coated materials of SPM probe on the sizes of oxidized structures (Lee et al., 2009).

Second, we introduce a novel soft lithography based on electrochemistry through scanning probe electrochemistry for controlling the functional group on the organic monolayers (Lee et al., 2007; Lee & Ishizaki et al., 2007; Saito et al., 2005; Sugimura et al., 2004). Scanning probe electrochemistry in which materials surfaces locally oxidized or reduced by a tip of SPM is a promising technique for constructing nanostructures consisting of organic molecules. This electrochemical SPL is performed through water column which is condensed between the tip of SPM and the substrate surface. This water column can be used as a microscopic electrochemical cell. In the case of electrochemical conversion using a nanoprobe, the electrochemical reactions which proceed at the probe-sample junction are governed by the applied bias voltage and its polarity. When the substrate is polarized positively, anodic reactions, that is, oxidation reactions, proceed on its surface. On the contrary, when the substrate is polarized negatively, cathodic reactions, that is, reductive reactions, proceed. The method is expected as a key technology for future molecular nanodevices. In addition, organosilane self-assembled monolayers (SAMs) have been applied to a resist material for SPL. Here we report the chemical conversion of an organic molecular monolayer in a reversible manner using SPM. The chemical state of the monolayer from its oxidized state to reduced state or vice versa is controlled. First, an amino-terminated SAM was chemically converted into an oxidized SAM by SPL at positivebias voltages. Moreover, this oxidized SAM was then reconverted into an amino-terminated SAM by SPL at negative bias voltages. We examine the chemical changes undergone in the scanned area from the viewpoint of surface-potential reversibility. Additionally, we introduce a electrochemical SPL to fabricate -COOH groups on an organic monolayer directly attached to silicon, which was synthesized from 1,7-octadien (OD) and hydrogenterminated silicon. The –COOH groups on the OD-monolayer were also synthesized by the properties of surface produced by SPL.

#### **2. Anodization scanning probe lithography on the organic monolayer on the hydrogen-terminated Si substrate**

In this chapter, we introduced fabricating technique of a three-dimensional nanostructure of organic monolayer on the hydrogen-terminated Si substrates by anodization SPL. First, we fabricated the organic monolayer on the hiydrogen-termicated Si(111) wafers with an electrical resistance of 10.0-20.0 Ω-cm. Si substrates were sonicated in acetone and ethanol for 10 min, and then, cleaned by an ultraviolet (UV) light/ozone cleaning method. The substrates were exposed to vacuum UV (VUV) light (172 nm) from an excimer lamp for 30 min under atmospheric pressure and room temperature. Subsequently, 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). As a result, the native silicon oxide layer was removed from the substrates, and hydrogen-terminated surfaces were formed. 1-decane monolayer was prepared by a liquid phase method. The hydrogen-terminated substrates were immersed in the solution of 1-decane molecules at 150 °C for 3 h . After the immersion, the organic monolayer coated substrates were cleaned in toluene, acetone, and ethanol, and rinsed in ultrapure water.

Firstly, the 1-decane monolayer was fabricated on the hydrogen-terminated Si substrates through a liquid phase method at 150 °C. Fig. 3(a) shows the chemical structure of 1-decane

terminated Si substrates by anodization SPL, and the effects of coated materials of SPM

Second, we introduce a novel soft lithography based on electrochemistry through scanning probe electrochemistry for controlling the functional group on the organic monolayers (Lee et al., 2007; Lee & Ishizaki et al., 2007; Saito et al., 2005; Sugimura et al., 2004). Scanning probe electrochemistry in which materials surfaces locally oxidized or reduced by a tip of SPM is a promising technique for constructing nanostructures consisting of organic molecules. This electrochemical SPL is performed through water column which is condensed between the tip of SPM and the substrate surface. This water column can be used as a microscopic electrochemical cell. In the case of electrochemical conversion using a nanoprobe, the electrochemical reactions which proceed at the probe-sample junction are governed by the applied bias voltage and its polarity. When the substrate is polarized positively, anodic reactions, that is, oxidation reactions, proceed on its surface. On the contrary, when the substrate is polarized negatively, cathodic reactions, that is, reductive reactions, proceed. The method is expected as a key technology for future molecular nanodevices. In addition, organosilane self-assembled monolayers (SAMs) have been applied to a resist material for SPL. Here we report the chemical conversion of an organic molecular monolayer in a reversible manner using SPM. The chemical state of the monolayer from its oxidized state to reduced state or vice versa is controlled. First, an amino-terminated SAM was chemically converted into an oxidized SAM by SPL at positivebias voltages. Moreover, this oxidized SAM was then reconverted into an amino-terminated SAM by SPL at negative bias voltages. We examine the chemical changes undergone in the scanned area from the viewpoint of surface-potential reversibility. Additionally, we introduce a electrochemical SPL to fabricate -COOH groups on an organic monolayer directly attached to silicon, which was synthesized from 1,7-octadien (OD) and hydrogenterminated silicon. The –COOH groups on the OD-monolayer were also synthesized by the

**2. Anodization scanning probe lithography on the organic monolayer on the** 

In this chapter, we introduced fabricating technique of a three-dimensional nanostructure of organic monolayer on the hydrogen-terminated Si substrates by anodization SPL. First, we fabricated the organic monolayer on the hiydrogen-termicated Si(111) wafers with an electrical resistance of 10.0-20.0 Ω-cm. Si substrates were sonicated in acetone and ethanol for 10 min, and then, cleaned by an ultraviolet (UV) light/ozone cleaning method. The substrates were exposed to vacuum UV (VUV) light (172 nm) from an excimer lamp for 30 min under atmospheric pressure and room temperature. Subsequently, 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). As a result, the native silicon oxide layer was removed from the substrates, and hydrogen-terminated surfaces were formed. 1-decane monolayer was prepared by a liquid phase method. The hydrogen-terminated substrates were immersed in the solution of 1-decane molecules at 150 °C for 3 h . After the immersion, the organic monolayer coated substrates were cleaned in toluene, acetone, and ethanol, and rinsed in ultrapure water. Firstly, the 1-decane monolayer was fabricated on the hydrogen-terminated Si substrates through a liquid phase method at 150 °C. Fig. 3(a) shows the chemical structure of 1-decane

probe on the sizes of oxidized structures (Lee et al., 2009).

properties of surface produced by SPL.

**hydrogen-terminated Si substrate** 

molecule. The static water contact angle and film thickness of the monolayer was about 107° and 1.12 nm, respectively. A smooth surface coated with densely packed CH3 groups (alkyl monolayer) shows a water contact angle of approximately 110°. In addition, its thickness was considered reasonable since the chain length of 1-decane molecule was estimated to be 1.32 nm. It is indicated that the 1-decane molecules formed an organic monolayer on the silicon substrate. Fig. 3(b) shows the XPS Si2p spectrum of the Si surface coated with the 1 decane monolayer. As clearly shown in Fig. 3(b), the peak of native silicon oxide was not observed at all. These results indicate that the 1-decane molecules reacted directly to the hydrogen-terminated silicon surfaces and formed the organic monolayer there. Fig. 3(c) and 3(d) show the AFM topographic and friction images of the Si surfaces after immersing in 1 decene solution. The flat terraces with steps for silicon one-atom and the changes of high friction on steps were, respectively, observed on the topographic and friction images. In the topographic image [Fig. 3(c)], the intervals of the steps and the terraces were 180 nm and 3.2 ± 0.3 Å, respectively. In addition, the difference of friction force between the flat terraces and steps was about 10 mV. These results indicated to the surface profiles of hydrogenterminated silicon. It was therefore found that the 1-decane molecules were vertically and densely assembled, and their monolayer was formed through doubly-boned terminated group attached to the hydrogen-terminated silicon surface.

Fig. 3. (a) The chemical structure of 1-decane (b) XPS Si2p spectra (c) topographic image (d) friction image of sample surfaces after prepared 1-decane monolayer. [SH. Lee, N. Saito, O. Takai, Highly reproducible technique for three-dimensional nanostructure fabrication via anodization scanning probe lithography, Appl. Surf. Sci., 255, 7302-7306 (2009). Copyright@ELSEVIER (2009)]

Scanning Probe Lithography on Organic Monolayers 481

line structures drastically decreased from 375 to 125 nm. For Si probe (uncoated Si probe), the line width gradually decreased with increasing scanning rate, reaching about 100 nm at 5 m/s. The width variations were Au-coated Si probe > Si probe (uncoated) > diamondcoated Si probe. These variations of the line width could be explained the band-gap energy of the coated materials. The band-gap energies of Au, Si and diamond are respectively about 0, 1.2 and 5.47 eV. That is to say, their conductive property is thought to be greatly dependent on the line width. These results indicate that high reproducibility of oxide nanoline structures is attainable by means of anodization SPL using the diamond-coated

Fig. 5. The change of the line width with increasing scanning rate [SH. Lee, N. Saito, O. Takai, Highly reproducible technique for three-dimensional nanostructure fabrication via

Finally, the effect of applied bias voltage on nanoline width of silicon oxide was investigated by use of various SPM probes. Fig. 6(a)-(c) show the topographic images of SiOx nanoline structures fabricated by diamond-coated Si, Si (uncoated) and Au-coated Si probes, respectively. The applied bias voltage and scanning rate were, respectively, fixed at 7 V and 1 m/s. The obtained widths were 15, 60 and 100 nm in Fig. 6(a)-(c), respectively. The line width fabricated at the bias voltage of 7 V decreased compared to that at 9 V when Au-coated Si and uncoated Si probes were used. For the diamond-coated probe, the nanoline structure maintained the width of 15nm, that is, the applied bias voltages had no effect on the nanoline width. These results are also attributed to the band-gap energy of the coated materials. In particular, three-dimensional oxide nanostructures fabricated by the diamond-coated probe showed highly reproducibility even though various scanning rates and applied bias voltages are used in aondization SPL. Therefore, this technique is expectable to be applied to fabricate a wide variety of nanodevices, that have three-dimensional structures, in various industrial fields.

anodization scanning probe lithography, Appl. Surf. Sci., 255, 7302-7306 (2009).

Copyright@ELSEVIER (2009)]

probe or a probe which has relatively low conductivity.

Fig. 4. The topographic images of SiOx line structure fabricated on the 1-decane monolayer and profiles for each topographic image by using various tip: (a) diamond coated Si tip, (b) Si tip and (c) Au coated Si tip. [SH. Lee, N. Saito, O. Takai, Highly reproducible technique for three-dimensional nanostructure fabrication via anodization scanning probe lithography, Appl. Surf. Sci., 255, 7302-7306 (2009). Copyright@ELSEVIER (2009)]

Next, we investigated that the effect of scanning rate on nanoline width of silicon oxide fabricated by anodization SPL. An anodization SPL was carried out on the 1-decane monolayer in air (humidity in ranging from 30 to 40 %) at the applied bias voltage of 9 V. In the anodization SPL, 1-decane monolayer was removed and the SiOx nanoline structures were formed by scanning the probes. Fig. 4(a)-4(c) show the topographic images and profiles of SiOx nanoline structure fabricated by the anodization SPL. In these experiments, various probes were used to fabricate oxide nanostructures [Fig. 4(a); diamond-coated Si probe, Fig. 4(b); Si probe (i.e., uncoated Si probe), Fig. 4(c); Au-coated Si probe]. Additionally, we investigated the effect of coated materials on the formation of the oxide nanostructure. The ranging of scanning rates for SPL is 0.1 to 5 m/s. In these topographic images, the nanoline structure and flat terraces with steps were obviously observed under all scanning conditions. However, the widths of nanoline structures were changed with the scanning rates. Fig. 5 shows the variation in the width of nanoline structures with scanning rates and surface compositions of probes. When the diamond coated probe is used for anodization SPL, the nanoline widths were found to remain constant under all scanning conditions, and the scanning rates had no effect on the line width. The width was approximately 15 nm, which is one of the finest nanostructures in the field of SPL technique. The highly reproducible structure was fabricated by anodization SPL using the diamondcoated Si probe, and this technique is thought to be able to apply various industrial fields. On the other hand, when the Si probe (uncoated-Si) and Au-coated Si probe were used for anodization SPL, the line widths were markedly changed with the scanning rates. In the case of Au-coated Si probe, as the scanning rate increased from 0.1 to 5 m/s, the width of

Fig. 4. The topographic images of SiOx line structure fabricated on the 1-decane monolayer and profiles for each topographic image by using various tip: (a) diamond coated Si tip, (b) Si tip and (c) Au coated Si tip. [SH. Lee, N. Saito, O. Takai, Highly reproducible technique

Next, we investigated that the effect of scanning rate on nanoline width of silicon oxide fabricated by anodization SPL. An anodization SPL was carried out on the 1-decane monolayer in air (humidity in ranging from 30 to 40 %) at the applied bias voltage of 9 V. In the anodization SPL, 1-decane monolayer was removed and the SiOx nanoline structures were formed by scanning the probes. Fig. 4(a)-4(c) show the topographic images and profiles of SiOx nanoline structure fabricated by the anodization SPL. In these experiments, various probes were used to fabricate oxide nanostructures [Fig. 4(a); diamond-coated Si probe, Fig. 4(b); Si probe (i.e., uncoated Si probe), Fig. 4(c); Au-coated Si probe]. Additionally, we investigated the effect of coated materials on the formation of the oxide nanostructure. The ranging of scanning rates for SPL is 0.1 to 5 m/s. In these topographic images, the nanoline structure and flat terraces with steps were obviously observed under all scanning conditions. However, the widths of nanoline structures were changed with the scanning rates. Fig. 5 shows the variation in the width of nanoline structures with scanning rates and surface compositions of probes. When the diamond coated probe is used for anodization SPL, the nanoline widths were found to remain constant under all scanning conditions, and the scanning rates had no effect on the line width. The width was approximately 15 nm, which is one of the finest nanostructures in the field of SPL technique. The highly reproducible structure was fabricated by anodization SPL using the diamondcoated Si probe, and this technique is thought to be able to apply various industrial fields. On the other hand, when the Si probe (uncoated-Si) and Au-coated Si probe were used for anodization SPL, the line widths were markedly changed with the scanning rates. In the case of Au-coated Si probe, as the scanning rate increased from 0.1 to 5 m/s, the width of

for three-dimensional nanostructure fabrication via anodization scanning probe lithography, Appl. Surf. Sci., 255, 7302-7306 (2009). Copyright@ELSEVIER (2009)] line structures drastically decreased from 375 to 125 nm. For Si probe (uncoated Si probe), the line width gradually decreased with increasing scanning rate, reaching about 100 nm at 5 m/s. The width variations were Au-coated Si probe > Si probe (uncoated) > diamondcoated Si probe. These variations of the line width could be explained the band-gap energy of the coated materials. The band-gap energies of Au, Si and diamond are respectively about 0, 1.2 and 5.47 eV. That is to say, their conductive property is thought to be greatly dependent on the line width. These results indicate that high reproducibility of oxide nanoline structures is attainable by means of anodization SPL using the diamond-coated probe or a probe which has relatively low conductivity.

Fig. 5. The change of the line width with increasing scanning rate [SH. Lee, N. Saito, O. Takai, Highly reproducible technique for three-dimensional nanostructure fabrication via anodization scanning probe lithography, Appl. Surf. Sci., 255, 7302-7306 (2009). Copyright@ELSEVIER (2009)]

Finally, the effect of applied bias voltage on nanoline width of silicon oxide was investigated by use of various SPM probes. Fig. 6(a)-(c) show the topographic images of SiOx nanoline structures fabricated by diamond-coated Si, Si (uncoated) and Au-coated Si probes, respectively. The applied bias voltage and scanning rate were, respectively, fixed at 7 V and 1 m/s. The obtained widths were 15, 60 and 100 nm in Fig. 6(a)-(c), respectively. The line width fabricated at the bias voltage of 7 V decreased compared to that at 9 V when Au-coated Si and uncoated Si probes were used. For the diamond-coated probe, the nanoline structure maintained the width of 15nm, that is, the applied bias voltages had no effect on the nanoline width. These results are also attributed to the band-gap energy of the coated materials. In particular, three-dimensional oxide nanostructures fabricated by the diamond-coated probe showed highly reproducibility even though various scanning rates and applied bias voltages are used in aondization SPL. Therefore, this technique is expectable to be applied to fabricate a wide variety of nanodevices, that have three-dimensional structures, in various industrial fields.

Scanning Probe Lithography on Organic Monolayers 483

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

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.

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

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

**4. Nano-probe electrochemistry on amino-terminated self-assembled** 

substrate resulting in the fabrication of an amino-terminated SAM.

**monolayers toward nano memory** 

Fig. 6. Topographic images of line structure fabricated by (a) diamond coated Si tip, (b) Si tip and (c) Au coated Si tip, respectively. The applied bias voltage and scanning rate are 7 V and 1m/s, respectively. [SH. Lee, N. Saito, O. Takai, Highly reproducible technique for threedimensional nanostructure fabrication via anodization scanning probe lithography, Appl. Surf. Sci., 255, 7302-7306 (2009). Copyright@ELSEVIER (2009)]
