Evaluation of the nanotube intrinsic resistance across the tip-carbon nanotube-metal substrate junction by Atomic Force Microscopy
© Dominiczak et al; licensee Springer. 2011
Received: 6 January 2011
Accepted: 14 April 2011
Published: 14 April 2011
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 R tip-CNT, R CNT-substrate and R tip-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.
Since the official publication of the carbon nanotubes (CNTs) images during the period of 1950 to 1990 , these allotropes have become very promising candidates for various applications because of their outstanding electrical, mechanical and thermal characteristics. They have competed for a high-level development in many fields such as nanoelectronic devices and nanoelectromechanical technologies: for example field-effect transistors (FETs), nano electro mechanical systems (NEMS), nano random access memories (NRAMs), nanoelectronic logic circuits and also nanomotors based on semiconducting CNTs [2–6]. A single-walled carbon nanotube (SWCNT) may behave either as a conductor or as a semiconductor. Electrical properties of nanotube are highly dependant on their atomic structure ; for example the conductivity of SWCNTs depends on their chirality in the honeycomb lattice structure of graphene and their diameter  as well as the electrical contact nature. CNTs have gained a renewed interest in the past few years, owing to their high conductance and high electron mobility [9, 10]. The strength of the sp2 (C-C) covalent hybridization bonds brings carbon nanotubes noteworthy mechanical properties too [11–13]. Multi-walled carbon nanotubes (MWCNTs) consist of several concentric SWCNTs held together by Van der Waals interactions. The spacing between two consecutive graphene sheets is about of 3.4 Å and the intershell conduction is governed by the electron hopping mechanism, which depends on the overlap of the carbon π-orbitals between neighboring layers. MWCNTs present an anisotropic metallic behaviour  because of the stacking of the graphite sheets. Multi-walled carbon nanotubes have the advantage to be easier to connect and give contact resistances lower than SWCNTs ones. Indeed, the contact resistance between a SWCNT and a metal contact cannot be lower than a few kΩ [15–18]. In the literature, researches based on the electrical contact resistance on MWCNTs have been previously published . Lan et al. studied the electrical contact between an individual MWCNT and a deposited metallic film. The contact resistance is modelled as a sequence of resistors that tie the CNT along its entire length. An uncovered length of the CNT bridges the gap between the two separate Ti/Au electrodes on which is applied a bias voltage.
In this paper, investigations are focused by another approach than , on the study of the radial contact resistance between a conductive tip and a single MWCNT, then between this CNT and a metal substrate. By means of conductive probe atomic force microscopy (CP-AFM), we characterize at room temperature CNTs by electrical imaging in order to measure their local resistance. The key requirements allowing to deduce the CNT intrinsic radial resistance are discussed by proposing a resistance model for the «tip-CNT-substrate» junction. The contact resistance R of each contribution as R tip-CNT, R CNT-substrate and R tip-substrate can be calculated by combining the Hertz's mechanical formula of contact area and the Sharvin's ballistic resistance model [20, 21]. The functionalization of CNTs with gold nanoparticles (AuNPs) is also investigated as a possible mean to improve their electrical conductivity. Finally, the contiguous question of local modification of the CNT and substrate surface is raised after operating at various bias voltages with diamond tips.
Methods and materials
Elaboration and purification of the MWCNTs
Carbon nanotubes have been elaborated by chemical vapour deposition (CVD) in a tubular furnace through a reactor (quartz tube) under a mixture of argon, hydrogen and acetylene gas. This production method can fabricate MWCNTs in large quantity. Observations in transmission electron microscopy (TEM) showed an entanglement of synthesized MWCNTs, which grow from catalysts in different geometrical configurations as straight or helical shapes. The catalytic activity realized with a mixture of ferrocene and xylene (as carbon source) was obtained by heating up to 750°C for 10 min. By thermal oxidation, the amorphous carbon structure was eliminated at 300°C during 1 h 30 min in air for purification. CNTs were then mixed with a nitric acid treatment for removing the metallic catalyst impurities [22, 23]. By acid treatment, it has been observed at optical microscope with Surf substrate («Nanolane» manufacturer, France) that the CNTs were best-purified. The as-prepared solution was uniformly dispersed by sonication during 2 min to separate the aggregations and then filtered. These CNTs were then ultrasonically diluted with DMF (N,N-dimethylformamide) solvent for 4 min, before AuNPs grafting for some of them (see further the first section of Results and discussion).
Au surface preparation
The golden substrates used for the study were 5 × 5 mm2 coupons obtained from a Si wafer covered with a 10-nm Cr adhesion layer and an Au layer of about 200 nm by physical vapour deposition (PVD). Gold has been considered as a reference material surface to investigate the electrical transport properties of the MWCNTs. The dispersion solution containing CNTs was then deposited onto these substrates.
Atomic force microscope
For all the experiments reported below, we used a D.I. Nanoscope IIIa Multimode AFM equipment associated with a LGEP home-made system called 'Resiscope'  dedicated to the local electrical resistance measurement. The as-prepared substrates are then fixed with silver paint on the steel sample holder placed on the AFM piezoelectric actuator. The surface morphology of the CNTs was imaged at room temperature (300 K), in the standard contact mode. We used two types of commercial conductive probes: (i) N-doped silicon probes coated with a P-doped diamond layer and (ii) Pt/Ir coated Si probes, both of them with a nominal k spring constant in the range 1 to 5 N/m («Veeco Probes» manufacturer, USA). The average curvature radius (r t) of the diamond tip is of about 150 nm and the Pr/Ir tip one of 20 nm. Topography and resistance cartographies were simultaneously recorded, applying a DC bias between the substrate and the tip. The Resiscope range covers ten decades from 102 to 1012 Ω. For a given zone, successive scans at different bias voltages were performed in order to determine the sensitivity to this parameter.
Results and discussion
Contact resistance measurement methods
Comparison between Pt/Ir and diamond tips
Average of the whole <Log(R)> values of several rectangles selected along the CNT length on raw CNT and CNT functionalized with AuNPs (direct bias +1 V).
R Total (Ω)
2 × 105
5 × 105
1 × 104
R Total (Ω)
2 × 104
3.2 × 104
2 × 104
On the other hand, R Total must be measured with a Pt/Ir tip because R Pt/Ir-tip is very low compared to R diamond-tip. Moreover, we have pointed out the resistance measured with a Pt/Ir tip is higher (one decade) on the CNT than on the substrate. Accordingly, there is no resistance filtering with a Pt/Ir tip, but it is not true with a diamond tip (see Table 1). R diamond-coated tip and CNT is approximately similar to R diamond-coated tip and substrate. We do not distinguish them plainly because the intrinsic diamond tip resistance brings a very important and non-negligible contribution across the «tip-CNT-substrate» junction. Hence, we consider that measurements with a diamond tip are not as reliable as the ones with a Pt/Ir tip. We also noticed that the resistance value is nearly one decade higher on the CNT when measured with a Pt/Ir tip than when obtained with a diamond one. A larger contact area, at a given force, for the diamond tip apex  could explain why R diamond-CNT is lower than R Pt/Ir-CNT.
«Tip-CNT-substrate» junction analysis model
The bias voltage V applied between the tip and the substrate, supplies the two junctions in series tip-CNT and CNT-substrate. The CNT-substrate interface is supposed to be formed by a number of elementary contacts at the top of roughness hills and therefore simulated by a parallel resistance network. The current trajectory across the MWCNT is anisotropic (Figure 3a).
DC voltage effects
In this paragraph, a rough model is proposed in order to estimate each contact resistance contribution in Equation 1. These contributions are related to the constriction of the current lines at the tip-CNT, CNT-substrate and tip-substrate interfaces, therefore two combined models are required: a mechanical model giving the contact area, and an electrical model physically adapted to this size allowing to calculate the resulting resistance.
Mechanical and electrical parameters of the various materials used for the junction «tip-CNT-substrate», with E i (Young's moduli), ν i (Poisson's ratio's), r t (curvature radius), ρ (resistivity) and l (electron mean free path).
ρ (Ω m)
2.35 × 10-82
4 × 10-5
2.35 × 10-82
Calculation of the different contact pressure and radii for the tip-CNT, CNT-substrate and tip-substrate junction.
a (1 N/m)(nm)
a (5 N/m)(nm)
Po (1 N/m)(MPa)
Po (5 N/m)(MPa)
Sharvin's model and calculations of the junction contributions
Results of the contact resistance calculations for each interface (see Equation 1).
Conducting probe atomic force microscopy in ambient air was used to investigate the local electrical resistance of MWCNTs disposed on thin gold films. The whole setup can be considered as a «tip-CNT-substrate» junction. By imaging individual CNTs, we were able to deduce their intrinsic radial resistance from the global one measured experimentally and the electrical contact ones calculated across the junction via a series-parallel resistance network model. Using a conductive Pt/Ir tip, we found a high resistance value of about 105 Ω for a cantilever load-force of about 16 to 80 nN with our AFM setup. For an application in electronic devices, this suggests the need to reduce the contact resistance by applying a more important load and to optimize the CNTs functionalization. Through this study, parasitic phenomena were also evidenced with diamond tips for negative bias voltages as well as some high positive ones, causing an irreversible increase of the measured electrical resistance. This observation was attributed to the redox reactions at the tip and/or sample surface leading to a local surface modification of the CNTs and substrate.
atomic force microscope
conductive probe atomic force microscopy
chemical vapour deposition
multi-walled carbon nanotubes
nano electro mechanical systems
nano random access memories
physical vapour deposition
single-walled carbon nanotube
transmission electron microscopy.
We would like to thank J. Sobotka (SPMS, ECP) for the elaboration of the gold substrates, and O. Schneegans (LGEP-SUPELEC) for enlightening discussions. This project was financially supported by the Carnot C3S "Centrale SUPELEC Sciences des Systèmes" Institute.
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