- Email: [email protected]
polymer hybrid nanowires: Control of the metal growth morphology using functional monomers
Electrochemistry Communications 11 (2009) 550–553
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Templating Ag on DNA/polymer hybrid nanowires: Control of the metal growth morphology using functional monomers Said A. Farha Al-Said a, Reda Hassanien a, Jennifer Hannant a, Miguel A. Galindo a, Stela Pruneanu b, Andrew R. Pike a, Andrew Houlton a, Benjamin R. Horrocks a,* a b
School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK National Institute for R&D of Isotopic and Molecular Technologies, 65-103 Donath Str., P.O. Box 700 RO-400293, Cluj-Napoca, Romania
a r t i c l e
i n f o
Article history: Received 27 November 2008 Received in revised form 8 December 2008 Accepted 9 December 2008 Available online 24 December 2008 Keywords: Nanowire Conductive polymer DNA Template Silver Nanoparticle
a b s t r a c t Ag nanowires and nanoparticles have been formed on hybrid k-DNA/conducting polymer templates. The strong, but non-covalent, interaction of the conducting polymer with the double helix allowed us to incorporate chemical functionalities (alkynyl) into the DNA/conducting polymer strands by synthesis of functional monomers. Oxidative polymerisation of alkynyl-thienylpyrrole in the presence of k-DNA produced conductive nanowires bearing alkyne groups; we show, using a combination of AFM, cAFM and EFM phase measurements that the alkyne functionality strong influences the subsequent templating reaction of Tollens’ reagent to produce uniform conductive nanowires comprised of many connected Ag clusters. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction DNA has been used as a template on which to deposit metals and conducting polymers in the form of nanowires [1]. The advantages of this technique are the simplicity of the chemical procedures and the robust nature of DNA, which is available in precisely defined lengths. However, DNA-templated metal nanowires are often very rough, even dendritic in appearance, whereas DNA-templated polymers are smooth, but less conductive [2,3]. In this work, we show that metal-binding functionality (alkynyl) can be introduced into DNA-templated polymer nanowires by chemical modification of the monomer and used to improve the morphology of Ag deposited on this hybrid template. 2. Experimental 2.1. Materials General reagents of AnalaR grade or equivalent were purchased from Sigma–Aldrich. 2-(Thiophen-2-yl)-1H-pyrrole (TP) was prepared as in Ref. [4] and 1-(pent-4-yn-1-yl)-2-(thiophen2-yl)-1H-pyrrole (alkynyl-TP) was prepared by alkylation of TP with 5-chloropent-1-yne using sodium hydride in DMF. k-DNA * Corresponding author. Tel.: +44 (0) 191 222 5619; fax: +44 (0) 191 222 6929. E-mail address: [email protected] (B.R. Horrocks). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.12.031
(500 ng lL 1) was purchased from New England Biolabs (cat no. N30011S, Hitchin, Herts. SG4 0TY United Kingdom). Tollens’ reagent was prepared fresh before use. Ag2O was precipitated from a mixture of 200 lL aqueous AgNO3 (0.5% wt/vol) and 5 lL NaOH (5% wt/vol) and the precipitate was dissolved in 600 lL NH4OH (conc. NH4OH diluted 2% in H2O). 2.2. Methods 2.2.1. Templated electroless deposition of Ag k-DNA solution (20 lL), or equivalent amount of polymer-templated DNA, was mixed with 20 lL Tollens’ reagent and heated at 50 °C for 10 min. 2.2.2. Polymer (TP or alkynyl-TP) templating on DNA k-DNA solution (20 lL) was mixed with monomer (5 lL, 3 mM), aqueous MgCl2 (5 lL, 0.5 mM) and FeCl3 (5 lL, 1 mM). 2.2.3. Probe microscopy DNA nanostructures were deposited on 1 cm2 Si chips with a 200 nm-thick oxide layer for probe microscopy; the oxide was silanised in Me3SiCl vapor for 10 min as in Ref. [5]. Nanowires were aligned by molecular combing. Tapping mode AFM images were acquired in air using a Dimension Nanoscope V system (Veeco Inc.) with NanoProbe tips (Veeco Inc.). The cantilever was 200–250 lm long, with 1–5 N m 1 spring constant and 252 kHz resonant
S.A. Farha Al-Said et al. / Electrochemistry Communications 11 (2009) 550–553
551
Scheme 1. DNA-templated synthesis of poly(alkynyl-TP) nanowires and deposition of Ag nanocrystals.
Fig. 1. Tapping mode AFM images of nanowires on SiO2/Si. (a) Ag/DNA, height; (b) Ag/poly(alkynyl-TP)/DNA, height; (c) Ag/poly(TP)/DNA, height; (d) Ag/poly(alkynyl-TP)/ DNA, phase; (e) Ag/poly(TP)/DNA, larger scale 7 7 lm, height image; and (f) Ag/poly(TP)/DNA, phase (g) and (h) show expanded views of (a) and (d), respectively. All scale bars are 1 lm.
frequency. Conductive AFM measurements (cAFM) were performed using MESP probes, n-doped Si cantilevers coated with a Co/Cr layer (Veeco). For EFM phase imaging (‘‘scanned conductance microscopy”), [6,7] the lift height was 60 nm, the tip resonant frequency
was about 74 kHz, spring constant = 2.8 N m 1 and the quality factor = 260. The tip was grounded and a dc bias was applied to the Si chip (Si(1 0 0), p++, doping density 1021 cm 3, 200 nm-thick oxide).
552
S.A. Farha Al-Said et al. / Electrochemistry Communications 11 (2009) 550–553
3. Results and discussion We compare three types of Ag nanostructure formed on DNA templates and show that the morphology of the Ag deposit can be controlled by the incorporation of Ag-binding groups in the template (Scheme 1). FTIR spectra of poly(alkynyl-TP) films prepared by FeCl3 oxidation show a complex fingerprint region typical of conjugated polymers and the two major bands expected for the alkynyl group: a carbon-carbon triple bond stretch at 2120 cm 1 and a C–H stretch due to the sp hybridised C–H bonds at 3302 cm 1. The TP units in our polymer probably do not couple strictly head-to-tail, but with a random orientation [8,9]. However, this does not affect our measurements and the TP monomer is convenient for incorporation of alkynyl functionality by N-alkylation. Poly(alkynyl-TP)/DNA denotes polymer templated on DNA and Ag/DNA or Ag/poly(alkynyl-TP)/DNA denote the structure formed after electroless deposition of Ag on the template. Fig. 1 shows AFM images (height and phase) of these nanostructures. Fig. 1a was prepared by the reaction of Tollen’s reagent with k-DNA under ambient light conditions and with mild heating (50 °C, 10 min). The growth of Ag nanocrystals along the DNA molecule is clearly observed and two-terminal conductance measurements (data not shown) demonstrate that the nanocrystals are in electrical contact. This beads-on-a-string morphology is quite typical of DNA-templated metals [1]. We have previously shown that conductive polymers produce much more regular and smooth nanowires when templated on DNA by oxidative chemical polymerisation of the monomer in a solution of k-DNA [3]. Fig. 1b and c are typical AFM images for Ag/poly(alkynyl-TP)/DNA and Ag/poly(TP)/DNA; these nanowires are quite thick (20 nm) and rather smooth. The wires lie straight in these images because they have been ‘‘combed” across the surface by the flow in a dragged water droplet. Fig. 1b–d show another phenomenon which has been noted previously: the compensation of the charge on the double helix by the polymer causes nanowires to wrap around each other to form a nanorope [5]. It is also possible that the Ag nanowire in Fig. 1a comprises more than one DNA molecule. The conductive polymer-based nanowires are smooth and regular in structure, but their conductance is much less than for true metals [3]. Nevertheless, chemical functionality can easily be incorporated into the polymer nanowire using a functional monomer. We chose to modify the TP monomer by attaching a propyl chain with a pendant alkyne group to the N-atom of the pyrrole unit (Scheme 1). DNA-templated polymerisation of this monomer produces nanowires and nanoropes indistinguishable (by AFM) from those formed by the unmodified TP. However, when the TP nanowires are treated with Tollens’ reagent, the growth of Ag deposits on the nanowires is strongly influenced by alkynyl group (c.f. Fig. 1c,f and b,d). On TP/DNA, the Ag forms a relatively small number of nanocrystals and, compared to those on the alkynyl polymer, they are quite large. This is probably due to the absence of a strong Ag+ binding site on poly(TP), which results in sporadic nucleation of only a few Ag clusters. On poly(alkynyl-TP), the nucleation of Ag is facile because of the well-known interaction of alkynes and Ag(I): initially Ag+ forms a sigma bond to the sp carbon after loss of the alkynyl proton, but polymerisation of such complexes via p-interactions also occurs [10]. In fact, many Ag(I) complexes with high Ag: alkyne stoichiometric ratios have been reported [11]. On the poly(alkynyl-TP)/DNA nanowire, many small nanocrystals are formed and they are so close together and uniformly distributed over the template that they are only distinguished in the phase image (Fig. 1d). In order to determine which of the nanostructures are conductive, we used two-point i–V measurements in conductive AFM (cAFM) and a variant of electric force microscopy (EFM) called scanned conductance microscopy [6,7]. Fig. 2 shows a current im-
Fig. 2. A cAFM current image of an Ag/poly(alkynylTP)/DNA nanowire. The tip/ sample bias potential was 10 V, the scale bar is 1 lm and the grayscale corresponds to 100 nA.
age of an Ag/poly(alkynylTP)/DNA nanowire on an Si chip acquired by cAFM. One contact to the nanowire is a drop of In/Ga eutectic placed on the chip and the other is the metallised AFM tip. When tip contacts the oxide no current is observed (black background in Fig. 2.). However, when the tip touches the nanowire a current flows (ca. 10 7 A @10 V in Fig. 2). The current pathway to the remote In/Ga contact exceeds the scan range and therefore cAFM is unsuited to a precise determination of conductivity, but it does convincingly demonstrate the nanowire is conductive. As we have previously noted, [5] nanowires are easily disturbed during contact mode imaging and therefore the current image does not have the resolution of the tapping mode images of Fig. 1. To extract further information on the conduction pathway, we employed scanned conductance microscopy [6,7]. This is a twopass technique in which a dc bias is applied between a conductive tip and the underlying Si substrate. In the first pass, the surface topography is imaged as in tapping mode; in the second pass, the tip is lifted a set height above the surface. The phase of the tip oscillation in the second pass reflects changes in the tip/substrate capacitance (C) because the electrical energy stored is equivalent to a change in the cantilever spring constant of ½C’’V2 where the primes indicate differentiation with respect to height. Previous workers have shown that extended objects which are merely polarisable, rather than conductive, produce positive phase shifts with respect to background, but that conductive objects can produce negative phase shifts [6,7]. This is because the nanowire/substrate capacitance is much larger when the whole of the object is polarised. Fig. 3 shows scanned conductance (phase) images of several nanowires. Ag/DNA nanowires give large, bright spots corresponding to Ag clusters which are presumably not in good electrical contact with the rest of the wire (dark shadow). Of the other structures, only the Ag/poly(alkynyl-TP)/DNA nanowire shows similarly large negative phase shifts. Those wires (Fig. 3e) are also much more uniform than the pure Ag/DNA wire and this confirms that most of the Ag clusters of Fig. 1d are in good electrical contact. We observed the expected parabolic dependence of phase shift on bias for voltages between 7 and +7 V; this confirms the phase is due to the ‘‘scanned conductance effect” rather than
S.A. Farha Al-Said et al. / Electrochemistry Communications 11 (2009) 550–553
553
Fig. 3. EFM phase images (‘‘scanned conductance microscopy”, Refs. [6,7]) of various nanostructures. (a) poly(TP)/DNA; (b) poly(alkynyl-TP)/DNA; (c) Ag/DNA; (d) Ag/ poly(TP)/DNA and (e) Ag/poly(alkynyl-TP)/DNA. The lift height was 60 nm and the grayscale corresponds to 3°. The scan sizes (lm) are (a) 6.3, (b) 4.1, (c) 3.3, (d) 5.4 and (e) 4.0.
trapped charge which produces phase shifts that depend linearly on bias voltage [12]. In the absence of deposited Ag, the polymers show smaller phase shifts and we observe the negative–positive– negative phase profile previously observed for conductive polymer fibres [13]. 4. Conclusions We have shown that alkynyl functional groups can be incorporated in conductive polymer nanowires in a convenient manner by chemical modification of the monomer. Alkynyl groups are known to bind Ag+ and were chosen to encourage nucleation and growth of Ag on the polymer/DNA template; this was demonstrated using tapping mode AFM to image the morphology of the nanowires and conductive AFM and EFM phase imaging to demonstrate their electrical conductivity. Acknowledgements One NorthEast, EPSRC, the government of Saudia Arabia and the EU are thanked for funding.
References [1] E. Braun, K. Keren, Adv. Phys. 53 (2004) 441. [2] Y. Ma, J. Zhang, G. Zhang, H. He, J. Am. Chem. Soc. 126 (2004) 7097. [3] L.Q. Dong, T. Hollis, S. Fishwick, B.A. Connolly, N.G. Wright, B.R. Horrocks, A. Houlton, Chem. Eur. J. 13 (2007) 822. [4] R.E. Niziurski-Mann, Adv. Mater. 5 (1993) 547. [5] S. Pruneanu, S.A. Farha Al-Said, L.Q. Dong, T.A. Hollis, M.A. Galindo, N.G. Wright, A. Houlton, B.R. Horrocks, Adv. Func. Mat. 18 (2008) 2444. [6] C. Staii, A.T. Johnson Jr., N.J. Pinto, Nano Lett. 4 (2004) 859. [7] M. Bockrath, N. Markovic, A. Shepard, M. Tinkham, L. Gurevich, L.P. Kouwenhoven, M.W. Wu, L.L. Sohn, Nano Lett. 2 (2002) 187. [8] R.E. Niziurski-Mann, C. Scordilis-Kelley, T.-L. Liu, M.P. Cava, R.T. Carlin, J. Am. Chem. Soc. 115 (1993) 887. [9] A. Pelter, M. Rowlands, I.H. Jenkins, Tetrahedron Lett. 28 (1987) 5213. [10] S.Q. Zang, T.C.W. Mak, Inorg. Chem. 47 (2008) 7094. [11] G.C. Guo, G.D. Zhou, T.C.W. Mak, J. Am. Chem. Soc. 121 (1999) 3136. [12] T.S. Jespersen, J. Nygard, Nano Lett. 5 (2005) 1838. [13] Y. Zhou, M. Freitag, J. Hone, C. Staii, A.T. Johnson, N.J. Pinto, A.G. MacDiarmid, Appl. Phys. Lett. 83 (2003) 3800.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Templating Ag on DNA/polymer hybrid nanowires: Control of the metal growth morphology using functional monomers Said A. Farha Al-Said a, Reda Hassanien a, Jennifer Hannant a, Miguel A. Galindo a, Stela Pruneanu b, Andrew R. Pike a, Andrew Houlton a, Benjamin R. Horrocks a,* a b
School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK National Institute for R&D of Isotopic and Molecular Technologies, 65-103 Donath Str., P.O. Box 700 RO-400293, Cluj-Napoca, Romania
a r t i c l e
i n f o
Article history: Received 27 November 2008 Received in revised form 8 December 2008 Accepted 9 December 2008 Available online 24 December 2008 Keywords: Nanowire Conductive polymer DNA Template Silver Nanoparticle
a b s t r a c t Ag nanowires and nanoparticles have been formed on hybrid k-DNA/conducting polymer templates. The strong, but non-covalent, interaction of the conducting polymer with the double helix allowed us to incorporate chemical functionalities (alkynyl) into the DNA/conducting polymer strands by synthesis of functional monomers. Oxidative polymerisation of alkynyl-thienylpyrrole in the presence of k-DNA produced conductive nanowires bearing alkyne groups; we show, using a combination of AFM, cAFM and EFM phase measurements that the alkyne functionality strong influences the subsequent templating reaction of Tollens’ reagent to produce uniform conductive nanowires comprised of many connected Ag clusters. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction DNA has been used as a template on which to deposit metals and conducting polymers in the form of nanowires [1]. The advantages of this technique are the simplicity of the chemical procedures and the robust nature of DNA, which is available in precisely defined lengths. However, DNA-templated metal nanowires are often very rough, even dendritic in appearance, whereas DNA-templated polymers are smooth, but less conductive [2,3]. In this work, we show that metal-binding functionality (alkynyl) can be introduced into DNA-templated polymer nanowires by chemical modification of the monomer and used to improve the morphology of Ag deposited on this hybrid template. 2. Experimental 2.1. Materials General reagents of AnalaR grade or equivalent were purchased from Sigma–Aldrich. 2-(Thiophen-2-yl)-1H-pyrrole (TP) was prepared as in Ref. [4] and 1-(pent-4-yn-1-yl)-2-(thiophen2-yl)-1H-pyrrole (alkynyl-TP) was prepared by alkylation of TP with 5-chloropent-1-yne using sodium hydride in DMF. k-DNA * Corresponding author. Tel.: +44 (0) 191 222 5619; fax: +44 (0) 191 222 6929. E-mail address: [email protected] (B.R. Horrocks). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.12.031
(500 ng lL 1) was purchased from New England Biolabs (cat no. N30011S, Hitchin, Herts. SG4 0TY United Kingdom). Tollens’ reagent was prepared fresh before use. Ag2O was precipitated from a mixture of 200 lL aqueous AgNO3 (0.5% wt/vol) and 5 lL NaOH (5% wt/vol) and the precipitate was dissolved in 600 lL NH4OH (conc. NH4OH diluted 2% in H2O). 2.2. Methods 2.2.1. Templated electroless deposition of Ag k-DNA solution (20 lL), or equivalent amount of polymer-templated DNA, was mixed with 20 lL Tollens’ reagent and heated at 50 °C for 10 min. 2.2.2. Polymer (TP or alkynyl-TP) templating on DNA k-DNA solution (20 lL) was mixed with monomer (5 lL, 3 mM), aqueous MgCl2 (5 lL, 0.5 mM) and FeCl3 (5 lL, 1 mM). 2.2.3. Probe microscopy DNA nanostructures were deposited on 1 cm2 Si chips with a 200 nm-thick oxide layer for probe microscopy; the oxide was silanised in Me3SiCl vapor for 10 min as in Ref. [5]. Nanowires were aligned by molecular combing. Tapping mode AFM images were acquired in air using a Dimension Nanoscope V system (Veeco Inc.) with NanoProbe tips (Veeco Inc.). The cantilever was 200–250 lm long, with 1–5 N m 1 spring constant and 252 kHz resonant
S.A. Farha Al-Said et al. / Electrochemistry Communications 11 (2009) 550–553
551
Scheme 1. DNA-templated synthesis of poly(alkynyl-TP) nanowires and deposition of Ag nanocrystals.
Fig. 1. Tapping mode AFM images of nanowires on SiO2/Si. (a) Ag/DNA, height; (b) Ag/poly(alkynyl-TP)/DNA, height; (c) Ag/poly(TP)/DNA, height; (d) Ag/poly(alkynyl-TP)/ DNA, phase; (e) Ag/poly(TP)/DNA, larger scale 7 7 lm, height image; and (f) Ag/poly(TP)/DNA, phase (g) and (h) show expanded views of (a) and (d), respectively. All scale bars are 1 lm.
frequency. Conductive AFM measurements (cAFM) were performed using MESP probes, n-doped Si cantilevers coated with a Co/Cr layer (Veeco). For EFM phase imaging (‘‘scanned conductance microscopy”), [6,7] the lift height was 60 nm, the tip resonant frequency
was about 74 kHz, spring constant = 2.8 N m 1 and the quality factor = 260. The tip was grounded and a dc bias was applied to the Si chip (Si(1 0 0), p++, doping density 1021 cm 3, 200 nm-thick oxide).
552
S.A. Farha Al-Said et al. / Electrochemistry Communications 11 (2009) 550–553
3. Results and discussion We compare three types of Ag nanostructure formed on DNA templates and show that the morphology of the Ag deposit can be controlled by the incorporation of Ag-binding groups in the template (Scheme 1). FTIR spectra of poly(alkynyl-TP) films prepared by FeCl3 oxidation show a complex fingerprint region typical of conjugated polymers and the two major bands expected for the alkynyl group: a carbon-carbon triple bond stretch at 2120 cm 1 and a C–H stretch due to the sp hybridised C–H bonds at 3302 cm 1. The TP units in our polymer probably do not couple strictly head-to-tail, but with a random orientation [8,9]. However, this does not affect our measurements and the TP monomer is convenient for incorporation of alkynyl functionality by N-alkylation. Poly(alkynyl-TP)/DNA denotes polymer templated on DNA and Ag/DNA or Ag/poly(alkynyl-TP)/DNA denote the structure formed after electroless deposition of Ag on the template. Fig. 1 shows AFM images (height and phase) of these nanostructures. Fig. 1a was prepared by the reaction of Tollen’s reagent with k-DNA under ambient light conditions and with mild heating (50 °C, 10 min). The growth of Ag nanocrystals along the DNA molecule is clearly observed and two-terminal conductance measurements (data not shown) demonstrate that the nanocrystals are in electrical contact. This beads-on-a-string morphology is quite typical of DNA-templated metals [1]. We have previously shown that conductive polymers produce much more regular and smooth nanowires when templated on DNA by oxidative chemical polymerisation of the monomer in a solution of k-DNA [3]. Fig. 1b and c are typical AFM images for Ag/poly(alkynyl-TP)/DNA and Ag/poly(TP)/DNA; these nanowires are quite thick (20 nm) and rather smooth. The wires lie straight in these images because they have been ‘‘combed” across the surface by the flow in a dragged water droplet. Fig. 1b–d show another phenomenon which has been noted previously: the compensation of the charge on the double helix by the polymer causes nanowires to wrap around each other to form a nanorope [5]. It is also possible that the Ag nanowire in Fig. 1a comprises more than one DNA molecule. The conductive polymer-based nanowires are smooth and regular in structure, but their conductance is much less than for true metals [3]. Nevertheless, chemical functionality can easily be incorporated into the polymer nanowire using a functional monomer. We chose to modify the TP monomer by attaching a propyl chain with a pendant alkyne group to the N-atom of the pyrrole unit (Scheme 1). DNA-templated polymerisation of this monomer produces nanowires and nanoropes indistinguishable (by AFM) from those formed by the unmodified TP. However, when the TP nanowires are treated with Tollens’ reagent, the growth of Ag deposits on the nanowires is strongly influenced by alkynyl group (c.f. Fig. 1c,f and b,d). On TP/DNA, the Ag forms a relatively small number of nanocrystals and, compared to those on the alkynyl polymer, they are quite large. This is probably due to the absence of a strong Ag+ binding site on poly(TP), which results in sporadic nucleation of only a few Ag clusters. On poly(alkynyl-TP), the nucleation of Ag is facile because of the well-known interaction of alkynes and Ag(I): initially Ag+ forms a sigma bond to the sp carbon after loss of the alkynyl proton, but polymerisation of such complexes via p-interactions also occurs [10]. In fact, many Ag(I) complexes with high Ag: alkyne stoichiometric ratios have been reported [11]. On the poly(alkynyl-TP)/DNA nanowire, many small nanocrystals are formed and they are so close together and uniformly distributed over the template that they are only distinguished in the phase image (Fig. 1d). In order to determine which of the nanostructures are conductive, we used two-point i–V measurements in conductive AFM (cAFM) and a variant of electric force microscopy (EFM) called scanned conductance microscopy [6,7]. Fig. 2 shows a current im-
Fig. 2. A cAFM current image of an Ag/poly(alkynylTP)/DNA nanowire. The tip/ sample bias potential was 10 V, the scale bar is 1 lm and the grayscale corresponds to 100 nA.
age of an Ag/poly(alkynylTP)/DNA nanowire on an Si chip acquired by cAFM. One contact to the nanowire is a drop of In/Ga eutectic placed on the chip and the other is the metallised AFM tip. When tip contacts the oxide no current is observed (black background in Fig. 2.). However, when the tip touches the nanowire a current flows (ca. 10 7 A @10 V in Fig. 2). The current pathway to the remote In/Ga contact exceeds the scan range and therefore cAFM is unsuited to a precise determination of conductivity, but it does convincingly demonstrate the nanowire is conductive. As we have previously noted, [5] nanowires are easily disturbed during contact mode imaging and therefore the current image does not have the resolution of the tapping mode images of Fig. 1. To extract further information on the conduction pathway, we employed scanned conductance microscopy [6,7]. This is a twopass technique in which a dc bias is applied between a conductive tip and the underlying Si substrate. In the first pass, the surface topography is imaged as in tapping mode; in the second pass, the tip is lifted a set height above the surface. The phase of the tip oscillation in the second pass reflects changes in the tip/substrate capacitance (C) because the electrical energy stored is equivalent to a change in the cantilever spring constant of ½C’’V2 where the primes indicate differentiation with respect to height. Previous workers have shown that extended objects which are merely polarisable, rather than conductive, produce positive phase shifts with respect to background, but that conductive objects can produce negative phase shifts [6,7]. This is because the nanowire/substrate capacitance is much larger when the whole of the object is polarised. Fig. 3 shows scanned conductance (phase) images of several nanowires. Ag/DNA nanowires give large, bright spots corresponding to Ag clusters which are presumably not in good electrical contact with the rest of the wire (dark shadow). Of the other structures, only the Ag/poly(alkynyl-TP)/DNA nanowire shows similarly large negative phase shifts. Those wires (Fig. 3e) are also much more uniform than the pure Ag/DNA wire and this confirms that most of the Ag clusters of Fig. 1d are in good electrical contact. We observed the expected parabolic dependence of phase shift on bias for voltages between 7 and +7 V; this confirms the phase is due to the ‘‘scanned conductance effect” rather than
S.A. Farha Al-Said et al. / Electrochemistry Communications 11 (2009) 550–553
553
Fig. 3. EFM phase images (‘‘scanned conductance microscopy”, Refs. [6,7]) of various nanostructures. (a) poly(TP)/DNA; (b) poly(alkynyl-TP)/DNA; (c) Ag/DNA; (d) Ag/ poly(TP)/DNA and (e) Ag/poly(alkynyl-TP)/DNA. The lift height was 60 nm and the grayscale corresponds to 3°. The scan sizes (lm) are (a) 6.3, (b) 4.1, (c) 3.3, (d) 5.4 and (e) 4.0.
trapped charge which produces phase shifts that depend linearly on bias voltage [12]. In the absence of deposited Ag, the polymers show smaller phase shifts and we observe the negative–positive– negative phase profile previously observed for conductive polymer fibres [13]. 4. Conclusions We have shown that alkynyl functional groups can be incorporated in conductive polymer nanowires in a convenient manner by chemical modification of the monomer. Alkynyl groups are known to bind Ag+ and were chosen to encourage nucleation and growth of Ag on the polymer/DNA template; this was demonstrated using tapping mode AFM to image the morphology of the nanowires and conductive AFM and EFM phase imaging to demonstrate their electrical conductivity. Acknowledgements One NorthEast, EPSRC, the government of Saudia Arabia and the EU are thanked for funding.
References [1] E. Braun, K. Keren, Adv. Phys. 53 (2004) 441. [2] Y. Ma, J. Zhang, G. Zhang, H. He, J. Am. Chem. Soc. 126 (2004) 7097. [3] L.Q. Dong, T. Hollis, S. Fishwick, B.A. Connolly, N.G. Wright, B.R. Horrocks, A. Houlton, Chem. Eur. J. 13 (2007) 822. [4] R.E. Niziurski-Mann, Adv. Mater. 5 (1993) 547. [5] S. Pruneanu, S.A. Farha Al-Said, L.Q. Dong, T.A. Hollis, M.A. Galindo, N.G. Wright, A. Houlton, B.R. Horrocks, Adv. Func. Mat. 18 (2008) 2444. [6] C. Staii, A.T. Johnson Jr., N.J. Pinto, Nano Lett. 4 (2004) 859. [7] M. Bockrath, N. Markovic, A. Shepard, M. Tinkham, L. Gurevich, L.P. Kouwenhoven, M.W. Wu, L.L. Sohn, Nano Lett. 2 (2002) 187. [8] R.E. Niziurski-Mann, C. Scordilis-Kelley, T.-L. Liu, M.P. Cava, R.T. Carlin, J. Am. Chem. Soc. 115 (1993) 887. [9] A. Pelter, M. Rowlands, I.H. Jenkins, Tetrahedron Lett. 28 (1987) 5213. [10] S.Q. Zang, T.C.W. Mak, Inorg. Chem. 47 (2008) 7094. [11] G.C. Guo, G.D. Zhou, T.C.W. Mak, J. Am. Chem. Soc. 121 (1999) 3136. [12] T.S. Jespersen, J. Nygard, Nano Lett. 5 (2005) 1838. [13] Y. Zhou, M. Freitag, J. Hone, C. Staii, A.T. Johnson, N.J. Pinto, A.G. MacDiarmid, Appl. Phys. Lett. 83 (2003) 3800.