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Evaluation of the nanotube intrinsic resistance across the tip-carbon nanotube-metal substrate junction by Atomic Force Microscopy.

Dominiczak M, Otubo L, Alamarguy D, Houzé F, Volz S, Noël S, Bai J - Nanoscale Res Lett (2011)

Bottom Line: For negative tip-substrate bias, a systematic degradation in color and contrast of the electrical cartography occurs, consisting of an important and non-reversible increase of the measured resistance.This effect is attributed to the oxidation of some amorphous carbon areas scattered over the diamond layer covering the tip.For a direct polarization, the CNT and substrate surface can in turn be modified by an oxidation mechanism.

View Article: PubMed Central - HTML - PubMed

Affiliation: Lab, MSSMat, UMR CNRS 8579, Ecole Centrale Paris, Grande Voie des Vignes, Châtenay-Malabry 92290, France. frederic.houze@supelec.fr.

ABSTRACT
Using an atomic force microscope (AFM) at a controlled contact force, we report the electrical signal response of multi-walled carbon nanotubes (MWCNTs) disposed on a golden thin film. In this investigation, we highlight first the theoretical calculation of the contact resistance between two types of conductive tips (metal-coated and doped diamond-coated), individual MWCNTs and golden substrate. We also propose a circuit analysis model to schematize the «tip-CNT-substrate» junction by means of a series-parallel resistance network. We estimate the contact resistance R of each contribution of the junction such as Rtip-CNT, RCNT-substrate and Rtip-substrate by using the Sharvin resistance model. Our final objective is thus to deduce the CNT intrinsic radial resistance taking into account the calculated electrical resistance values with the global resistance measured experimentally. An unwished electrochemical phenomenon at the tip apex has also been evidenced by performing measurements at different bias voltages with diamond tips. For negative tip-substrate bias, a systematic degradation in color and contrast of the electrical cartography occurs, consisting of an important and non-reversible increase of the measured resistance. This effect is attributed to the oxidation of some amorphous carbon areas scattered over the diamond layer covering the tip. For a direct polarization, the CNT and substrate surface can in turn be modified by an oxidation mechanism.

No MeSH data available.


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Topography (left) and resistance maps (right) of raw CNTs and CNTs with AuNPs using a diamond tip for various polarizations in the range 1 to 6 V (scan size of 1 × 1 μm2).
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Figure 4: Topography (left) and resistance maps (right) of raw CNTs and CNTs with AuNPs using a diamond tip for various polarizations in the range 1 to 6 V (scan size of 1 × 1 μm2).

Mentions: In Figures 4 and 5 are represented series of AFM/Resiscope pictures of the raw CNTs and CNTs functionalized with AuNPs acquired with a diamond tip under several polarizations: from 1 up to 6 V in Figure 4 and from +1 to +3 V and -1 to -3 V in Figure 5. For the highest bias values, we can see a noticeable loss in resolution on the AFM images obtained on CNTs with AuNPs in Figure 4. The individual gold grains are not so clearly visible, but as expected the mean resistance calculated over the electrical cartography decreases as the bias is increased, except in the case of 6 V for raw CNT. The comparison of the resistance images in Figure 5 in direct and reverse polarization allows us to conclude that the current-voltage characteristic should not be symmetrical. To take into account a better approach of the conduction mechanism with the diamond tip, we adapt our resistance model (see Figure 3b) by introducing the additional contribution of a Schottky diode between the Rtip-CNT contact junction (resistance dominating in the circuit as we will see it in section of 'Sharvin's model') and RCNT, so that the diode allows the current to flow in a single direction. It was reported that the charge transport in CNTs is controlled by the Schottky barriers that forms the metal-CNT junction, the nature and geometry of this contact can strongly modify the electrical behaviour [30,31]. Let us bring up again the particularity of the results under reverse bias shown in Figure 5. The negative polarization seems to affect the tip coating. As AFM is operated in ambient air, a possible explanation could be that a local redox reaction occurs in the water meniscus at the tip apex [32], inducing an increase of the measured resistance. Such an effect could also induce on our samples a local surface modification, since the electrical contrast between the CNT and the substrate disappears between -2 and -3 V. Concerning the tip, a hypothesis could be the oxidation of small amorphous carbon domains scattered over the diamond coating of the tip for V < 0. Mahé et al. examined precisely the electrochemical reactivity effect on diamond electrodes covered with graphitic micro-domains [33]. As regards the astonishing result in direct polarization at 6 V (see Figure 4), a hypothesis could be the oxidation of amorphous carbon residues on the CNTs combined with a transfer of the oxidized material to the tip. This is corroborated by the experimental observation that the return to the initial state is difficult as if an irreversible phenomenon affected the tip surface, making it unusable. From 6 to 1 V (Figure 4) and -3 to 1 V (Figure 5), it was not possible to recover the initial resistance levels, even after several successive scans. For the following discussion only results with no oxidation suspicion will be considered.


Evaluation of the nanotube intrinsic resistance across the tip-carbon nanotube-metal substrate junction by Atomic Force Microscopy.

Dominiczak M, Otubo L, Alamarguy D, Houzé F, Volz S, Noël S, Bai J - Nanoscale Res Lett (2011)

Topography (left) and resistance maps (right) of raw CNTs and CNTs with AuNPs using a diamond tip for various polarizations in the range 1 to 6 V (scan size of 1 × 1 μm2).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211423&req=5

Figure 4: Topography (left) and resistance maps (right) of raw CNTs and CNTs with AuNPs using a diamond tip for various polarizations in the range 1 to 6 V (scan size of 1 × 1 μm2).
Mentions: In Figures 4 and 5 are represented series of AFM/Resiscope pictures of the raw CNTs and CNTs functionalized with AuNPs acquired with a diamond tip under several polarizations: from 1 up to 6 V in Figure 4 and from +1 to +3 V and -1 to -3 V in Figure 5. For the highest bias values, we can see a noticeable loss in resolution on the AFM images obtained on CNTs with AuNPs in Figure 4. The individual gold grains are not so clearly visible, but as expected the mean resistance calculated over the electrical cartography decreases as the bias is increased, except in the case of 6 V for raw CNT. The comparison of the resistance images in Figure 5 in direct and reverse polarization allows us to conclude that the current-voltage characteristic should not be symmetrical. To take into account a better approach of the conduction mechanism with the diamond tip, we adapt our resistance model (see Figure 3b) by introducing the additional contribution of a Schottky diode between the Rtip-CNT contact junction (resistance dominating in the circuit as we will see it in section of 'Sharvin's model') and RCNT, so that the diode allows the current to flow in a single direction. It was reported that the charge transport in CNTs is controlled by the Schottky barriers that forms the metal-CNT junction, the nature and geometry of this contact can strongly modify the electrical behaviour [30,31]. Let us bring up again the particularity of the results under reverse bias shown in Figure 5. The negative polarization seems to affect the tip coating. As AFM is operated in ambient air, a possible explanation could be that a local redox reaction occurs in the water meniscus at the tip apex [32], inducing an increase of the measured resistance. Such an effect could also induce on our samples a local surface modification, since the electrical contrast between the CNT and the substrate disappears between -2 and -3 V. Concerning the tip, a hypothesis could be the oxidation of small amorphous carbon domains scattered over the diamond coating of the tip for V < 0. Mahé et al. examined precisely the electrochemical reactivity effect on diamond electrodes covered with graphitic micro-domains [33]. As regards the astonishing result in direct polarization at 6 V (see Figure 4), a hypothesis could be the oxidation of amorphous carbon residues on the CNTs combined with a transfer of the oxidized material to the tip. This is corroborated by the experimental observation that the return to the initial state is difficult as if an irreversible phenomenon affected the tip surface, making it unusable. From 6 to 1 V (Figure 4) and -3 to 1 V (Figure 5), it was not possible to recover the initial resistance levels, even after several successive scans. For the following discussion only results with no oxidation suspicion will be considered.

Bottom Line: For negative tip-substrate bias, a systematic degradation in color and contrast of the electrical cartography occurs, consisting of an important and non-reversible increase of the measured resistance.This effect is attributed to the oxidation of some amorphous carbon areas scattered over the diamond layer covering the tip.For a direct polarization, the CNT and substrate surface can in turn be modified by an oxidation mechanism.

View Article: PubMed Central - HTML - PubMed

Affiliation: Lab, MSSMat, UMR CNRS 8579, Ecole Centrale Paris, Grande Voie des Vignes, Châtenay-Malabry 92290, France. frederic.houze@supelec.fr.

ABSTRACT
Using an atomic force microscope (AFM) at a controlled contact force, we report the electrical signal response of multi-walled carbon nanotubes (MWCNTs) disposed on a golden thin film. In this investigation, we highlight first the theoretical calculation of the contact resistance between two types of conductive tips (metal-coated and doped diamond-coated), individual MWCNTs and golden substrate. We also propose a circuit analysis model to schematize the «tip-CNT-substrate» junction by means of a series-parallel resistance network. We estimate the contact resistance R of each contribution of the junction such as Rtip-CNT, RCNT-substrate and Rtip-substrate by using the Sharvin resistance model. Our final objective is thus to deduce the CNT intrinsic radial resistance taking into account the calculated electrical resistance values with the global resistance measured experimentally. An unwished electrochemical phenomenon at the tip apex has also been evidenced by performing measurements at different bias voltages with diamond tips. For negative tip-substrate bias, a systematic degradation in color and contrast of the electrical cartography occurs, consisting of an important and non-reversible increase of the measured resistance. This effect is attributed to the oxidation of some amorphous carbon areas scattered over the diamond layer covering the tip. For a direct polarization, the CNT and substrate surface can in turn be modified by an oxidation mechanism.

No MeSH data available.


Related in: MedlinePlus