- Email: [email protected]
A study on conformational changes by electron charges in viologen single molecules by using STM
ARTICLE IN PRESS Ultramicroscopy 108 (2008) 1101– 1105
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
A study on conformational changes by electron charges in viologen single molecules by using STM Nam-Suk Lee a, Won-Suk Choi a, Hoon-Kyu Shin b, Dong-Jin Qian c, Young-Soo Kwon a, a b c
Department of Electrical Engineering and Nano Engineering, Dong-A University, Busan 604-714, Korea NCNT, Pohang University of Science and Technology, Pohang 790-784, Korea Institute of Advanced Materials, Fudan University, Shanghai 200433, China
a r t i c l e in fo
PACS: 07.79.Cz 33.15.e Keywords: Scanning tunneling microscopy Conformational change
abstract The topography of self-assembled viologen derivatives (VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH) molecules on an octanethiol (C8) self-assembled monolayer (SAM) modified gold surface was measured using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). We demonstrate here a novel matrix SAM appropriate for isolation of the viologen molecules. The C8 was used for a matrix SAM, in which the VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH were inserted at molecular lattice defects. The isolated single molecules of viologen derivatives inserted in the matrix SAM were observed as protrusions in STM topography using a constant current mode. We measured the topographic heights (VC8SH: 1.53 nm, VC10SH: 2.01 nm, HSC8VC8SH: 2.71 nm, and HSC10VC10SH: 3.3 nm) of the molecular protrusions using STM. Also, changes in the central axis of viologen molecules were observed as VC8SH (0.5–0.73 nm), VC10SH (0.4–0.74 nm), HSC8VC8SH (0.67–0.84 nm), and HSC10VC10SH (0.67–0.99 nm), respectively. & 2008 Elsevier B.V. All rights reserved.
1. Introduction Recent self-assembling techniques allowed us to measure electronic tunneling through a single molecule embedded in an insulating matrix using scanning tunneling microscopy (STM) [1–3]. Immersion of a preformed host self-assembled monolayer (SAM) in a dilute solution of guest molecules resulted in isolating them in the host SAM. The isolated molecules were considered to be inserted at defects of the preformed matrices, such as step edges of the Au(111) substrate surface and structural domain boundaries of the host SAM [1,4]. Isolated molecules appear as protrusions in an STM topographic image using a constant current mode. In most cases, an alkanethiol SAM was used as the host matrix. Bumm et al. [1,4] inferred that many of the like features at structural domain boundaries of the alkanethiol matrix SAM are individual molecules, because access for insertion is limited at the structural domain boundaries surrounded by the close-packed film. We used an octanethiol (C8) SAM as the host matrix and four types of viologen derivatives (N-methyl-N0 -(8-mercaptooctyl)-4, 40 -bipyridinium, VC8SH; N-methyl-N0 -(10-mercaptodecyl)-4,40 -bipyridinium, VC10SH; N-methyl-N0 -di(8-mercaptooctyl)-4,40 -bipyridinium, HSC8VC8SH; and N-methyl-N0 -di(10-mercaptodecyl)-4,
Corresponding author. Tel.: +82 51 200 6949; fax: +82 51 200 7743.
E-mail address: [email protected] (Y.-S. Kwon). 0304-3991/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2008.04.022
40 -bipyridinium, HSC10VC10SH) as the guest molecules [5]. Therefore, single guest molecules are certainly isolated in the C8 matrix SAM. These molecules are arranged to make densely packed monolayers by a sulfur atom, when their SAMs are formed on the Au(111) substrate. The synthesis procedure of viologen derivatives was reported elsewhere [5]. Fig. 1 shows molecular structures of viologen derivatives with a thiol group used in this study. The purpose of the present study is to measure the reproducibility of conformational change and confirm the topography of self-assembled viologen derivatives on the C8 matrix SAM modified gold surface at the nanometer scale using STM. It was due to the fact that the behavioral characteristics of viologen molecules were expected because of the influence of electron charges [6]. We succeeded in quantitative measurement for topography of viologen single molecules. In addition, the conformational changes of the VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH have been observed.
2. Experiments 2.1. Gold substrate preparation We prepared the gold substrate with certain surface (111) orientation by vacuum evaporation onto mica. Before the evaporation, mica was prebaked at 320 1C for 2 h in order to
ARTICLE IN PRESS 1102
N.-S. Lee et al. / Ultramicroscopy 108 (2008) 1101–1105
2.2. Preparation of the SAM
decrease the contamination of the surface. The pressure during the evaporation was kept under 6.5 107–9.0 108 Torr. The thickness of the gold thin film was 100 nm, measured using the quartz deposition monitor. The substrate was allowed to anneal at 420 1C for 4 h after the deposition and cooled down naturally [7]. From the STM image of Au(111) substrate obtained by a wide range scan (1 mm 1 mm, not shown here), the typical terrace width of Au(111) substrate was ranging from 30 to 250 nm.
Top view
The matrix SAM was fabricated using C8 as a host material, in which inserted viologen derivatives (VC8SH, VC10SH, HSC8V C8SH, and HSC10VC10SH) were used as a guest material. We prepared C8 matrix SAM by immersing Au(111) substrates in 1 mM/ml ethanol solution for 24 h and kept it in the dark during immersion to avoid photo-oxidation. After thoroughly rinsing the sample, it was immersed to a 0.1 mM/ml solution of viologen derivatives in ethanol for 30 min. The samples were rinsed with pure ethanol for a few seconds and were dried under a flow of pure N2 [8–10].
Side view
VC8SH 2.3. Scanning tunneling microscopy observation
VC10SH
A commercial ultrahigh-vacuum scanning tunneling microscopy (UHV-STM, Unisoku USM-1200) was employed in this study to avoid surface contamination after the SAM deposition. In addition, we could achieve a more stable tunneling condition in UHV than in air, especially during the observation of C8 SAM and viologen molecule. Samples were transferred into the STM vacuum chamber and observed at room temperature under the UHV condition immediately after the preparation of SAM. The samples were imaged with mechanically ground Pt/Ir tips at a pressure lower than 1.0 1011 Torr. As mentioned above, the Au(111) substrates were very flat, so the constant current mode could be employed in where the tunneling current was
HSC8VC8SH
HSC10VC10SH
Height [nm]
5 4 3 2
Height [nm]
Fig. 1. Viologen derivatives used in this study; the molecular lengths of VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH were expected to be 1.94, 2.1, 2.8, and 3.35 nm, respectively.
4 3 2 1
0
2
0
2
4
4
6
6
8
10
8 10 Distance [nm]
12
12
14
14
16
16
Fig. 2. STM images of conformational change and cross-sectional analysis of VC8SH inserted in C8 matrix SAM; (A) ON state of VC8SH, highlighted by the square. (B) The OFF state of VC8SH is low lighted by the square. Solid squares in (A) and (B) are indicated in (C) and (D) as sectional views with dotted squares, respectively.
ARTICLE IN PRESS N.-S. Lee et al. / Ultramicroscopy 108 (2008) 1101–1105
are indicated in Fig. 2C and D as a sectional view with dotted squares, respectively. In the case of other samples of viologen molecules (VC10SH, HSC8VC8SH, and HSC10VC10SH), we obtained similar characteristics. The STM images of other viologen molecules are (VC10SH, HSC8VC8SH, and HSC10VC10SH), shown in Figs. 3–5. In Figs. 4C and 5C, the small peak is estimated as a difference in contact angles with certain surfaces caused by molecular motion. It is necessary to continue further investigations. Conformational change between the ON and OFF states was reversible. A time-lapse series of images acquired over 2 h 13 min recorded the long-term behavior of the VC8SH molecules. The extracted images of this molecule are seen in the sequence shown in Fig. 6A [8]. Reversible conformational changes between the ON and OFF states occur several times in the first 27 frames, recorded at 6 min intervals. The molecule then appears to stabilize in the ON state for the remainder of the images. In the case of other samples of viologen molecules (VC10SH, HSC8VC8SH, and HSC10VC10SH), we obtained similar characteristics. The STM extracted images of other viologen molecules are (VC10SH, HSC8VC8SH, and HSC10VC10SH), shown in Fig. 6(B–D). Thus, the changed central radii were verified by overlapping each of the STM images of the measured VC8SH molecules. Therefore, the changed central radii of the VC8SH molecules were in the range of 0.5–0.73 nm. In the cases of other samples, viologen molecules were observed as VC10SH (0.4–0.74 nm), HSC8VC8SH (0.67–0.84 nm), and HSC10VC10SH (0.67–0.99 nm). They were determined as single molecules
weakly controlled. All STM images shown in this paper were measured in constant current mode with a low tunneling current as small as 0.20 nA.
3. Results and discussion 3.1. Conformational changes of viologen derivative molecules
Height [nm] Height [nm]
inffiffiffia nanoscale using The self-assembled C8 SAM was observed pffiffiffi p a STM. A mixed phase that includes a ð 3 3ÞR30 structure of high-density standing up-phase was observed in the C8 matrix. A stripped structure in the alkanethiol SAM has been known that can be formed in a domain which has a relatively low packing density. Moreover, a type of c (4 2) superlattice was also found in the SAM [11]. In Fig. 2A, several domain boundaries, a substrate vacancy island (in the lower right corner of the image), and one small bundle of VC8SH molecules (highlighted by the square) can be observed in the matrix SAM. But, in Fig. 2B, the small bundle of VC8SH molecules (highlighted by the square) cannot be observed in the matrix SAM. So, we refer to these as the ‘‘ON (highlighted by the square)’’ state (Fig. 2A) and the ‘‘OFF (lower lighted by the square)’’ state (Fig. 2B), which corresponds to a conformational change [8]. In this study, we defined that the height of inserted VC8SH molecule in SAM matrix is equal to or less than a few nm; it is called ‘‘ON’’ state; if the height is equal to zero or cannot be found, it is called ‘‘OFF’’ state. The solid squares in Fig. 2A and B
10 8 6 4 2
5 4 3 2
0
2
4
0
2
4
1103
6
6 Distance [nm]
8
10
8
10
12
12
Fig. 3. STM images of conformational change and cross-sectional analysis of VC10SH inserted in C8 matrix SAM; (A) ON state of VC10SH, highlighted by the square. (B) The OFF state of VC10SH is low lighted by the square. Solid squares in (A) and (B) are indicated in (C) and (D) as sectional views with dotted squares, respectively.
ARTICLE IN PRESS
Height [nm]
N.-S. Lee et al. / Ultramicroscopy 108 (2008) 1101–1105
8 6 4 2
Height [nm]
1104
6 5 4 3 2
0
2
0
2
4
4
6
6
8
10
8 10 Distance [nm]
12
12
14
14
16
16
Fig. 4. STM images of conformational change and cross-sectional analysis of HSC8VC8SH inserted in C8 matrix SAM; (A) ON state of HSC8VC8SH, highlighted by the square. (B) OFF state of HSC8VC8SH is low lighted by the square. Solid squares in (A) and (B) are indicated in (C) and (D) as sectional views with dotted squares, respectively.
because the changed central radii included one of the widths of viologen molecules. Also, two oxidation peaks and two reduction peaks were verified by cyclic voltammetry (CV). It proves that the viologen molecule represented the property of a mediator because two electrons were injected in the reduction process and two electrons were emitted in the oxidation process [11]. It is a different structure in which the VC8SH and VC10SH molecules have alkyl chains and thiol groups on one side and the HSC8VC8SH and HSC10VC10SH represent alkyl chains and thiol groups on each side. However, the characteristics of the same conformational change were verified. Thus, in the case of the VC8SH and VC10SH molecules, it is possible to estimate that the conformational change is caused by the change of tilt of the molecule brought about by the injected electron [12,13]. It needs further investigations. On the other hand, it is possible to express the conformational change of the HSC8VC8SH and HSC10VC10SH molecules as the change in polarity caused by the electron charge and to recognize the change in the width and height of the alkyl group due to the change in the polarity caused by the injected electron. It means that the electron can be injected into the viologen molecule existing in the matrix SAM when a certain negative voltage is applied to the STM probe, and the alkyl group can lie both on left and on right sides due to the loss of the polarity when the upper alkyl group is changed as a neutral stage. Also, the alkyl group can stand due to the recovery of the polarity when the electron is released. Thus, the conformation change
occurred due to the change in the width, height and tilt of the molecule based on such reasons [11].
4. Conclusions We observed the single viologen molecule in the C8 matrix SAM by using STM. Also, we confirmed the conformational change of the viologen molecule is due to change of the width and height of molecule. We demonstrated here a novel matrix SAM appropriate for isolation of the viologen molecules. The C8 was used as a matrix SAM, in which the VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH were inserted at molecular lattice defects. The isolated single molecules of viologen derivatives inserted into the matrix SAM were observed as protrusions in STM topography using a constant current mode. We measured the heights of the VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH to be 1.53, 2.01, 2.71, and 3.3 nm, respectively. In addition, changes in the central axis of viologen molecules were observed as VC8SH (0.5–0.73 nm), VC10SH (0.4–0.74 nm), HSC8VC8SH (0.67–0.84 nm), and HSC10VC10SH (0.67–0.99 nm). Thus, it is possible to express the conformational change of the viologen derivatives molecules as the change of tilt and the movement of molecules which recognize the change in the width and height of the alkyl group due to the change in the polarity caused by the electron charge.
ARTICLE IN PRESS N.-S. Lee et al. / Ultramicroscopy 108 (2008) 1101–1105
1105
19 nm
Height [nm]
8 6 4 2 0
Height [nm]
19 nm
5 4 3 2
0
2
4
6
0
2
4
6
8
8
10
10 Distance [nm]
12
12
14
14
16
16
18
18
Fig. 5. STM images of conformational change and cross-sectional analysis of HSC10VC10SH inserted in C8 matrix SAM; (A) ON state of HSC10VC10SH, highlighted by the square. (B) OFF state of HSC10VC10SH is low lighted by the square. Solid squares in (A) and (B) are indicated in (C) and (D) as sectional views with dotted squares, respectively.
VC8SH 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Acknowledgments This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. R01-2006-000-11120-0). References
19
20
21
22
23
24
25
26
27
VC10SH
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18 HSC8VC8SH
1
2
3
4
5
6
7
8
9 HSC10VC10SH
1
2
3
4
5
6
7
8
9
10
Fig. 6. Characteristics of conformational change of viologen derivatives. Reversible conformational change between the ON and OFF states occurs several times in the first 27, 18, 9, and 10 frames, recorded at 6 min intervals for (A) VC8SH, (B) VC10SH, (C) HSC8VC8SH, and (D) HSC10VC10SH, respectively.
[1] L.A. Bumm, J.J. Arnold, M.T. Cygan, T.D. Dunbar, T.P. Burgin, L. Jones II, D.L. Allara, J.M. Tour, P.S. Weiss, Science 271 (1996) 1705. [2] S. Wakamatsu, S. Fujii, U. Akiba, M. Fujihira, Ultramicroscopy 97 (2003) 19. [3] J. Gaudioso, W. Ho, J. Am. Chem. Soc. 123 (2001) 10095. [4] M.T. Cygan, T.D. Dunbar, J.J. Arnold, L.A. Bumm, N.F. Shedlock, T.P. Burgin, L. Jones II, D.L. Allara, J.M. Tour, P.S. Weiss, J. Am. Chem. Soc. 120 (1998) 2721. [5] N. Nakamura, H.X. Huang, D.J. Qian, J. Miyake, Langmuir 18 (2002) 5804. [6] L. Michaelis, E.S. Hill, J. Am. Chem. Soc. 55 (1933) 1481. [7] J. Noh, T. Murase, K. Nakajima, H. Lee, M. Hara, J. Phys. Chem. B 104 (2000) 7411. [8] Z.J. Donhauser, B.A. Mantooth, K.F. Kelly, L.A. Bumm, J.D. Monnell, J.J. Stapleton, D.W. Price Jr., A.M. Rawlett, D.L. Allara, J.M. Tour, P.S. Weiss, Science 292 (2001) 2303. [9] K. Kobayashi, T. Horiuchi, H. Yamada, K. Matsushige, Thin Solid Films 331 (1998) 210. [10] N.-S. Lee, J.-S. Chang, Y.-S. Kwon, J. Electron. Eng. Technol. 1 (2006) 366. [11] N.-S. Lee, W.-S. Choi, H.-K. Shin, D.-J. Qian, Y.-S. Kwon, Jpn. J. Appl. Phys. 47 (2008) 1173. [12] P.A. Lewis, C.E. Inman, F. Maya, J.M. Tour, J.E. Hutchison, P.S. Weiss, J. Am. Chem. Soc. 127 (2005) 17421. [13] T. Ishida, E. Koyama, H. Tokuhisa, A. Belaissaoui, Y. Nagawa, M. Nakano, W. Mizutani, M. Kanesato, Jpn. J. Appl. Phys. 43 (2004) 4561.
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
A study on conformational changes by electron charges in viologen single molecules by using STM Nam-Suk Lee a, Won-Suk Choi a, Hoon-Kyu Shin b, Dong-Jin Qian c, Young-Soo Kwon a, a b c
Department of Electrical Engineering and Nano Engineering, Dong-A University, Busan 604-714, Korea NCNT, Pohang University of Science and Technology, Pohang 790-784, Korea Institute of Advanced Materials, Fudan University, Shanghai 200433, China
a r t i c l e in fo
PACS: 07.79.Cz 33.15.e Keywords: Scanning tunneling microscopy Conformational change
abstract The topography of self-assembled viologen derivatives (VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH) molecules on an octanethiol (C8) self-assembled monolayer (SAM) modified gold surface was measured using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). We demonstrate here a novel matrix SAM appropriate for isolation of the viologen molecules. The C8 was used for a matrix SAM, in which the VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH were inserted at molecular lattice defects. The isolated single molecules of viologen derivatives inserted in the matrix SAM were observed as protrusions in STM topography using a constant current mode. We measured the topographic heights (VC8SH: 1.53 nm, VC10SH: 2.01 nm, HSC8VC8SH: 2.71 nm, and HSC10VC10SH: 3.3 nm) of the molecular protrusions using STM. Also, changes in the central axis of viologen molecules were observed as VC8SH (0.5–0.73 nm), VC10SH (0.4–0.74 nm), HSC8VC8SH (0.67–0.84 nm), and HSC10VC10SH (0.67–0.99 nm), respectively. & 2008 Elsevier B.V. All rights reserved.
1. Introduction Recent self-assembling techniques allowed us to measure electronic tunneling through a single molecule embedded in an insulating matrix using scanning tunneling microscopy (STM) [1–3]. Immersion of a preformed host self-assembled monolayer (SAM) in a dilute solution of guest molecules resulted in isolating them in the host SAM. The isolated molecules were considered to be inserted at defects of the preformed matrices, such as step edges of the Au(111) substrate surface and structural domain boundaries of the host SAM [1,4]. Isolated molecules appear as protrusions in an STM topographic image using a constant current mode. In most cases, an alkanethiol SAM was used as the host matrix. Bumm et al. [1,4] inferred that many of the like features at structural domain boundaries of the alkanethiol matrix SAM are individual molecules, because access for insertion is limited at the structural domain boundaries surrounded by the close-packed film. We used an octanethiol (C8) SAM as the host matrix and four types of viologen derivatives (N-methyl-N0 -(8-mercaptooctyl)-4, 40 -bipyridinium, VC8SH; N-methyl-N0 -(10-mercaptodecyl)-4,40 -bipyridinium, VC10SH; N-methyl-N0 -di(8-mercaptooctyl)-4,40 -bipyridinium, HSC8VC8SH; and N-methyl-N0 -di(10-mercaptodecyl)-4,
Corresponding author. Tel.: +82 51 200 6949; fax: +82 51 200 7743.
E-mail address: [email protected] (Y.-S. Kwon). 0304-3991/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2008.04.022
40 -bipyridinium, HSC10VC10SH) as the guest molecules [5]. Therefore, single guest molecules are certainly isolated in the C8 matrix SAM. These molecules are arranged to make densely packed monolayers by a sulfur atom, when their SAMs are formed on the Au(111) substrate. The synthesis procedure of viologen derivatives was reported elsewhere [5]. Fig. 1 shows molecular structures of viologen derivatives with a thiol group used in this study. The purpose of the present study is to measure the reproducibility of conformational change and confirm the topography of self-assembled viologen derivatives on the C8 matrix SAM modified gold surface at the nanometer scale using STM. It was due to the fact that the behavioral characteristics of viologen molecules were expected because of the influence of electron charges [6]. We succeeded in quantitative measurement for topography of viologen single molecules. In addition, the conformational changes of the VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH have been observed.
2. Experiments 2.1. Gold substrate preparation We prepared the gold substrate with certain surface (111) orientation by vacuum evaporation onto mica. Before the evaporation, mica was prebaked at 320 1C for 2 h in order to
ARTICLE IN PRESS 1102
N.-S. Lee et al. / Ultramicroscopy 108 (2008) 1101–1105
2.2. Preparation of the SAM
decrease the contamination of the surface. The pressure during the evaporation was kept under 6.5 107–9.0 108 Torr. The thickness of the gold thin film was 100 nm, measured using the quartz deposition monitor. The substrate was allowed to anneal at 420 1C for 4 h after the deposition and cooled down naturally [7]. From the STM image of Au(111) substrate obtained by a wide range scan (1 mm 1 mm, not shown here), the typical terrace width of Au(111) substrate was ranging from 30 to 250 nm.
Top view
The matrix SAM was fabricated using C8 as a host material, in which inserted viologen derivatives (VC8SH, VC10SH, HSC8V C8SH, and HSC10VC10SH) were used as a guest material. We prepared C8 matrix SAM by immersing Au(111) substrates in 1 mM/ml ethanol solution for 24 h and kept it in the dark during immersion to avoid photo-oxidation. After thoroughly rinsing the sample, it was immersed to a 0.1 mM/ml solution of viologen derivatives in ethanol for 30 min. The samples were rinsed with pure ethanol for a few seconds and were dried under a flow of pure N2 [8–10].
Side view
VC8SH 2.3. Scanning tunneling microscopy observation
VC10SH
A commercial ultrahigh-vacuum scanning tunneling microscopy (UHV-STM, Unisoku USM-1200) was employed in this study to avoid surface contamination after the SAM deposition. In addition, we could achieve a more stable tunneling condition in UHV than in air, especially during the observation of C8 SAM and viologen molecule. Samples were transferred into the STM vacuum chamber and observed at room temperature under the UHV condition immediately after the preparation of SAM. The samples were imaged with mechanically ground Pt/Ir tips at a pressure lower than 1.0 1011 Torr. As mentioned above, the Au(111) substrates were very flat, so the constant current mode could be employed in where the tunneling current was
HSC8VC8SH
HSC10VC10SH
Height [nm]
5 4 3 2
Height [nm]
Fig. 1. Viologen derivatives used in this study; the molecular lengths of VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH were expected to be 1.94, 2.1, 2.8, and 3.35 nm, respectively.
4 3 2 1
0
2
0
2
4
4
6
6
8
10
8 10 Distance [nm]
12
12
14
14
16
16
Fig. 2. STM images of conformational change and cross-sectional analysis of VC8SH inserted in C8 matrix SAM; (A) ON state of VC8SH, highlighted by the square. (B) The OFF state of VC8SH is low lighted by the square. Solid squares in (A) and (B) are indicated in (C) and (D) as sectional views with dotted squares, respectively.
ARTICLE IN PRESS N.-S. Lee et al. / Ultramicroscopy 108 (2008) 1101–1105
are indicated in Fig. 2C and D as a sectional view with dotted squares, respectively. In the case of other samples of viologen molecules (VC10SH, HSC8VC8SH, and HSC10VC10SH), we obtained similar characteristics. The STM images of other viologen molecules are (VC10SH, HSC8VC8SH, and HSC10VC10SH), shown in Figs. 3–5. In Figs. 4C and 5C, the small peak is estimated as a difference in contact angles with certain surfaces caused by molecular motion. It is necessary to continue further investigations. Conformational change between the ON and OFF states was reversible. A time-lapse series of images acquired over 2 h 13 min recorded the long-term behavior of the VC8SH molecules. The extracted images of this molecule are seen in the sequence shown in Fig. 6A [8]. Reversible conformational changes between the ON and OFF states occur several times in the first 27 frames, recorded at 6 min intervals. The molecule then appears to stabilize in the ON state for the remainder of the images. In the case of other samples of viologen molecules (VC10SH, HSC8VC8SH, and HSC10VC10SH), we obtained similar characteristics. The STM extracted images of other viologen molecules are (VC10SH, HSC8VC8SH, and HSC10VC10SH), shown in Fig. 6(B–D). Thus, the changed central radii were verified by overlapping each of the STM images of the measured VC8SH molecules. Therefore, the changed central radii of the VC8SH molecules were in the range of 0.5–0.73 nm. In the cases of other samples, viologen molecules were observed as VC10SH (0.4–0.74 nm), HSC8VC8SH (0.67–0.84 nm), and HSC10VC10SH (0.67–0.99 nm). They were determined as single molecules
weakly controlled. All STM images shown in this paper were measured in constant current mode with a low tunneling current as small as 0.20 nA.
3. Results and discussion 3.1. Conformational changes of viologen derivative molecules
Height [nm] Height [nm]
inffiffiffia nanoscale using The self-assembled C8 SAM was observed pffiffiffi p a STM. A mixed phase that includes a ð 3 3ÞR30 structure of high-density standing up-phase was observed in the C8 matrix. A stripped structure in the alkanethiol SAM has been known that can be formed in a domain which has a relatively low packing density. Moreover, a type of c (4 2) superlattice was also found in the SAM [11]. In Fig. 2A, several domain boundaries, a substrate vacancy island (in the lower right corner of the image), and one small bundle of VC8SH molecules (highlighted by the square) can be observed in the matrix SAM. But, in Fig. 2B, the small bundle of VC8SH molecules (highlighted by the square) cannot be observed in the matrix SAM. So, we refer to these as the ‘‘ON (highlighted by the square)’’ state (Fig. 2A) and the ‘‘OFF (lower lighted by the square)’’ state (Fig. 2B), which corresponds to a conformational change [8]. In this study, we defined that the height of inserted VC8SH molecule in SAM matrix is equal to or less than a few nm; it is called ‘‘ON’’ state; if the height is equal to zero or cannot be found, it is called ‘‘OFF’’ state. The solid squares in Fig. 2A and B
10 8 6 4 2
5 4 3 2
0
2
4
0
2
4
1103
6
6 Distance [nm]
8
10
8
10
12
12
Fig. 3. STM images of conformational change and cross-sectional analysis of VC10SH inserted in C8 matrix SAM; (A) ON state of VC10SH, highlighted by the square. (B) The OFF state of VC10SH is low lighted by the square. Solid squares in (A) and (B) are indicated in (C) and (D) as sectional views with dotted squares, respectively.
ARTICLE IN PRESS
Height [nm]
N.-S. Lee et al. / Ultramicroscopy 108 (2008) 1101–1105
8 6 4 2
Height [nm]
1104
6 5 4 3 2
0
2
0
2
4
4
6
6
8
10
8 10 Distance [nm]
12
12
14
14
16
16
Fig. 4. STM images of conformational change and cross-sectional analysis of HSC8VC8SH inserted in C8 matrix SAM; (A) ON state of HSC8VC8SH, highlighted by the square. (B) OFF state of HSC8VC8SH is low lighted by the square. Solid squares in (A) and (B) are indicated in (C) and (D) as sectional views with dotted squares, respectively.
because the changed central radii included one of the widths of viologen molecules. Also, two oxidation peaks and two reduction peaks were verified by cyclic voltammetry (CV). It proves that the viologen molecule represented the property of a mediator because two electrons were injected in the reduction process and two electrons were emitted in the oxidation process [11]. It is a different structure in which the VC8SH and VC10SH molecules have alkyl chains and thiol groups on one side and the HSC8VC8SH and HSC10VC10SH represent alkyl chains and thiol groups on each side. However, the characteristics of the same conformational change were verified. Thus, in the case of the VC8SH and VC10SH molecules, it is possible to estimate that the conformational change is caused by the change of tilt of the molecule brought about by the injected electron [12,13]. It needs further investigations. On the other hand, it is possible to express the conformational change of the HSC8VC8SH and HSC10VC10SH molecules as the change in polarity caused by the electron charge and to recognize the change in the width and height of the alkyl group due to the change in the polarity caused by the injected electron. It means that the electron can be injected into the viologen molecule existing in the matrix SAM when a certain negative voltage is applied to the STM probe, and the alkyl group can lie both on left and on right sides due to the loss of the polarity when the upper alkyl group is changed as a neutral stage. Also, the alkyl group can stand due to the recovery of the polarity when the electron is released. Thus, the conformation change
occurred due to the change in the width, height and tilt of the molecule based on such reasons [11].
4. Conclusions We observed the single viologen molecule in the C8 matrix SAM by using STM. Also, we confirmed the conformational change of the viologen molecule is due to change of the width and height of molecule. We demonstrated here a novel matrix SAM appropriate for isolation of the viologen molecules. The C8 was used as a matrix SAM, in which the VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH were inserted at molecular lattice defects. The isolated single molecules of viologen derivatives inserted into the matrix SAM were observed as protrusions in STM topography using a constant current mode. We measured the heights of the VC8SH, VC10SH, HSC8VC8SH, and HSC10VC10SH to be 1.53, 2.01, 2.71, and 3.3 nm, respectively. In addition, changes in the central axis of viologen molecules were observed as VC8SH (0.5–0.73 nm), VC10SH (0.4–0.74 nm), HSC8VC8SH (0.67–0.84 nm), and HSC10VC10SH (0.67–0.99 nm). Thus, it is possible to express the conformational change of the viologen derivatives molecules as the change of tilt and the movement of molecules which recognize the change in the width and height of the alkyl group due to the change in the polarity caused by the electron charge.
ARTICLE IN PRESS N.-S. Lee et al. / Ultramicroscopy 108 (2008) 1101–1105
1105
19 nm
Height [nm]
8 6 4 2 0
Height [nm]
19 nm
5 4 3 2
0
2
4
6
0
2
4
6
8
8
10
10 Distance [nm]
12
12
14
14
16
16
18
18
Fig. 5. STM images of conformational change and cross-sectional analysis of HSC10VC10SH inserted in C8 matrix SAM; (A) ON state of HSC10VC10SH, highlighted by the square. (B) OFF state of HSC10VC10SH is low lighted by the square. Solid squares in (A) and (B) are indicated in (C) and (D) as sectional views with dotted squares, respectively.
VC8SH 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Acknowledgments This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. R01-2006-000-11120-0). References
19
20
21
22
23
24
25
26
27
VC10SH
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18 HSC8VC8SH
1
2
3
4
5
6
7
8
9 HSC10VC10SH
1
2
3
4
5
6
7
8
9
10
Fig. 6. Characteristics of conformational change of viologen derivatives. Reversible conformational change between the ON and OFF states occurs several times in the first 27, 18, 9, and 10 frames, recorded at 6 min intervals for (A) VC8SH, (B) VC10SH, (C) HSC8VC8SH, and (D) HSC10VC10SH, respectively.
[1] L.A. Bumm, J.J. Arnold, M.T. Cygan, T.D. Dunbar, T.P. Burgin, L. Jones II, D.L. Allara, J.M. Tour, P.S. Weiss, Science 271 (1996) 1705. [2] S. Wakamatsu, S. Fujii, U. Akiba, M. Fujihira, Ultramicroscopy 97 (2003) 19. [3] J. Gaudioso, W. Ho, J. Am. Chem. Soc. 123 (2001) 10095. [4] M.T. Cygan, T.D. Dunbar, J.J. Arnold, L.A. Bumm, N.F. Shedlock, T.P. Burgin, L. Jones II, D.L. Allara, J.M. Tour, P.S. Weiss, J. Am. Chem. Soc. 120 (1998) 2721. [5] N. Nakamura, H.X. Huang, D.J. Qian, J. Miyake, Langmuir 18 (2002) 5804. [6] L. Michaelis, E.S. Hill, J. Am. Chem. Soc. 55 (1933) 1481. [7] J. Noh, T. Murase, K. Nakajima, H. Lee, M. Hara, J. Phys. Chem. B 104 (2000) 7411. [8] Z.J. Donhauser, B.A. Mantooth, K.F. Kelly, L.A. Bumm, J.D. Monnell, J.J. Stapleton, D.W. Price Jr., A.M. Rawlett, D.L. Allara, J.M. Tour, P.S. Weiss, Science 292 (2001) 2303. [9] K. Kobayashi, T. Horiuchi, H. Yamada, K. Matsushige, Thin Solid Films 331 (1998) 210. [10] N.-S. Lee, J.-S. Chang, Y.-S. Kwon, J. Electron. Eng. Technol. 1 (2006) 366. [11] N.-S. Lee, W.-S. Choi, H.-K. Shin, D.-J. Qian, Y.-S. Kwon, Jpn. J. Appl. Phys. 47 (2008) 1173. [12] P.A. Lewis, C.E. Inman, F. Maya, J.M. Tour, J.E. Hutchison, P.S. Weiss, J. Am. Chem. Soc. 127 (2005) 17421. [13] T. Ishida, E. Koyama, H. Tokuhisa, A. Belaissaoui, Y. Nagawa, M. Nakano, W. Mizutani, M. Kanesato, Jpn. J. Appl. Phys. 43 (2004) 4561.