Electrical behavior of multi-walled carbon nanotube network embedded in amorphous silicon nitride
- Ionel Stavarache1,
- Ana-Maria Lepadatu1,
- Valentin Serban Teodorescu1,
- Magdalena Lidia Ciurea1Email author,
- Vladimir Iancu^2,
- Mircea Dragoman3,
- George Konstantinidis4 and
- Raluca Buiculescu5
© Stavarache et al; licensee Springer. 2011
Received: 2 August 2010
Accepted: 17 January 2011
Published: 17 January 2011
The electrical behavior of multi-walled carbon nanotube network embedded in amorphous silicon nitride is studied by measuring the voltage and temperature dependences of the current. The microstructure of the network is investigated by cross-sectional transmission electron microscopy. The multi-walled carbon nanotube network has an uniform spatial extension in the silicon nitride matrix. The current-voltage and resistance-temperature characteristics are both linear, proving the metallic behavior of the network. The I-V curves present oscillations that are further analyzed by computing the conductance-voltage characteristics. The conductance presents minima and maxima that appear at the same voltage for both bias polarities, at both 20 and 298 K, and that are not periodic. These oscillations are interpreted as due to percolation processes. The voltage percolation thresholds are identified with the conductance minima.
The carbon nanotubes (CNTs), either single-walled (SWCNTs) or multi-walled (MWCNTs), have a quasi-1D behavior that results from their nanometric diameters and micrometric lengths [1–6]. While the SWCNT structures correspond to the rolling up of one graphene sheet, the MWCNTs consist of several concentric sheets.
The electrical behavior of SWCNTs is determined by their chirality, either metallic or semiconductor . The longitudinal conductance of a metallic one is quantified, namely, G = nG 0, with G 0 = 2e 2/h = 77.47 μS and n a natural number. The behavior of MWCNTs is metallic if, at least, one sheet has a metallic chirality. A theoretical analysis on the conductance of infinitely long, defect-free MWCNTs shows that the tunneling current between states on different walls is vanishingly small , which leads to the quantization of the conductance. In the frame of this model, the authors showed that in a finite nanotube, the interwall conductance is negligible compared to the intrawall ballistic conductance. Abrikosov et al.  calculated the electron spectrum of a metallic MWCNT with an arbitrary number of concentric sheets. They calculated the entropy and density of states for an MWCNT and analyzed the tunneling between the nanotube and a metal electrode. The authors proved that measuring the tunneling conductivity at low temperatures, the one-electron density of states can be directly determined. They also give the necessary restrictions on temperature.
Kuroda and Leburton  modeled the linear behavior of the R-T characteristics measured at low field in SWCNTs, by taking into account the mean free paths determined by the interactions of electrons with acoustic and optical phonons. Their results are in good agreement with the data from Refs [11, 12]. This model is generalized for MWCNTs in Ref. .
Li et al.  measured in individual vertical MWCNTs with large diameters very large currents at low bias voltage and they determined a very high conductance, G = 490G 0, much higher than the value of 2G 0, predicted in the literature for perfect metallic SWCNTs. They explained this behavior by a multichannel quasiballistic transport of electrons in the inner walls. In Ref. , Collins et al., studying the limits of high energy transport in MWCNTs, showed that the nanotubes fail via a series of sharp and equal current steps, in contrast to metal wires that fail continuously and in accelerating mode.
The percolation phenomena in films with MWCNTs are extensively investigated in the literature, related to film composition and thickness, temperature, nanotubes concentration and shape, and so on. The electrical conductivity of oxidized MWCNT-epoxy composites was investigated in Ref. . The MWCNTs were oxidized under both mild and strong conditions. Strong oxidation conditions produce partially damaged nanotubes. Consequently, their conductivity decreases and the percolation threshold increases. On the contrary, the MWCNTs oxidized under mild conditions present a high conductivity, independent of oxidation conditions. The study of the conductivity as a function of film thickness and nanotube volume fraction  shows that reducing the film thickness to a value comparable with the MWCNT length, the percolation threshold significantly diminishes. The authors explain this considering that different conductive paths appear with different probabilities in a film of MWCNT embedded in polyethylene.
The MWCNT-PMMA [poly(methyl methacrylate)] composites also exhibit percolation phenomena. The dc conductivity increases with increasing the MWCNTs concentration or mass [18–21], a typical percolation behavior. A percolation threshold of 0.4 wt% was reported in Ref. . Using other polymers as a matrix, e.g., polydimethylsiloxane and styrene acrylic emulsion-based polymer, percolation thresholds of 1.5 wt%  and 0.23 wt% were found for MWCNTs . The electrical behavior of the composite formed by an MWCNT network embedded in PMMA is explained by a combination of Sheng's fluctuation induced tunneling and 1D variable range hopping models . Percolation in a 2D MWCNT network  is strongly influenced by the MWCNT sizes and shape.
In the present letter we report on the electrical behavior of an MWCNT network embedded in amorphous silicon nitride matrix. The sample preparation and microstructure investigations are presented. The voltage and temperature dependences of the current were measured and the current-voltage, conductance-voltage, and resistance-temperature characteristics are discussed. The observed conductance minima are interpreted as voltage percolation thresholds, analogous to those previously observed on nanostructures formed by nanocrystalline silicon dots embedded in amorphous silicon dioxide matrix, and also in nanocrystalline porous silicon .
Cross-sectional transmission electron microscopy (XTEM) investigations were made on a Jeol TEM 200CX instrument. The XTEM specimen was prepared by a conventional method using mechanical polishing and ion thinning in a Gatan PIPS device. Electrical measurements were performed in a Janis CCS-450 cryostat at room temperature (298 K) and low temperature (20 K), using a Keithley 6517A electrometer.
Results and discussions
We can suppose that such a CNT network keeps the same morphology during the deposition of the SiN matrix. The final XTEM specimen consists only in a slice of about 50 nm thick from the MWCNT network present in the SiN matrix. Consequently, in the XTEM specimen, the presence of MWCNTs will be rarely observed, in the very thin part of the specimen. However, the repetitive observations of the same XTEM specimen after a series of sequential small duration of ion milling allow us to observe different areas with MWCNT network embedded in the SiN matrix.
Conductance oscillations are previously presented in articles where they are attributed to Coulomb blockade effect [27, 28], most of these results being observed in SWCNTs. The oscillations found by Ahlskog et al.  practically disappear when the sample temperature is increased from 4.6 to 20 K. On the other hand, the oscillations observed by LeRoy et al.  measured at 4.5 K are periodically depending on the voltage.
The structure formed by the MWCNT network embedded in SiN was XTEM investigated. The TEM investigations, performed on nanotubes deposited directly on the carbon grid, reveal a uniform spatial extension of MWCNT network. In our opinion, this structure is preserved when MWCNT network is embedded in SiN.
The Cr/Al/MWCNT-SiN/Cr/Al samples present a metallic behavior, which is proved by the linear character of both the I-V and R-T characteristics.
The oscillations of the I-V and G-V curves are interpreted as due to percolation processes, as they are symmetric in bias polarization, are not periodic and are temperature independent. The voltage percolation thresholds of 20 and 30 mV on both bias polarities and both temperatures (20 and 298 K) are given by the conductance minima.
multi-walled carbon nanotubes
single-walled carbon nanotubes
cross-sectional transmission electron microscopy.
The Romanian contribution to this work was supported by the Romanian National Authority for Scientific Research through the CNMP Contract 10-009/2007, the Ideas Program Contract 471/2009 (ID 918/2008), and the Core Program Contract PN09-45.
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