Temperature Dependent Resistance and Magnetoresistance of Single

Chapter 8

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Temperature Dependent Resistance and Magnetoresistance of Single Wall Carbon Nanotubes Mounted on Silica Fiber Surfaces Q. Lu,1 V. Samuilov,1,2,3 V. Ksenevich,2 T. Dauzhenka,2,3 and R. S. Helburn*,4 1Department

of Physics, St. John’s University, Jamaica, NY 11439 of Physics, Belarus State University, 220030 Minsk, Belarus 3CNRS, LNCMI, 31400 Toulouse France & Universite de Toulouse, UPS, INSA, LNCMI, 31077 Toulouse, France 4Department of Chemistry & Physics, St. Francis College, Brooklyn Heights, NY 11201 *[email protected] 2Department

Single wall carbon nanotubes (SWCNTs) possess multiple analytically useful properties, intrinsic and extrinsic, as well as some that are of potential value. This chapter focuses on the magnetotransport properties of SWCNT coated 0.11mm diameter quartz silica fibers. The fibers were prepared by functionalizing the silica surface with a hydrocarbon layer. SWCNTS (oxidized or un-oxidized) were adsorbed onto the functionalized surface and the entire unit was annealed. Resistance and magnetoresistance in the 1.8–250 K and 1.8 – 8 K ranges, respectively, were examined and modeled to show the existence of two charge transport mechanisms, variable range hopping (VRH) and fluctuation induced tunneling.

Introduction Carbon nanotubes (CNTS) are rapidly finding their place in analytical chemistry, especially in those methodologies that involve an interface or an interfacial layer. CNTS are a class of graphitic materials that can be organized into two categories, single- and multi- walled carbon nanotubes, i.e. SWCNTs and MWCNTs respectively. A SWCNT is a seamlessly rolled up graphene sheet © 2011 American Chemical Society In Interfaces and Interphases in Analytical Chemistry; Helburn, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


(Figures 1a-c) (1–3). Likewise, a MWCNT can be envisioned as a rolled up stack of graphene sheets such that the cross section of the MWCNT appears as a set of concentric circles. The surfaces of SWCNTs are hydrophobic due to their π bonding configuration and π -π interactions among the aromatic rings with the result being that individual SWCNTs tend to aggregate into bundles (Figure 1e) (2). This aspect of SWCNTs can make them difficult to work with as they do not disperse well in many solvents (4, 5). Accordingly, both SWCNTS and MWCNTs have strong hydrophobic binding capability (6, 7). The abundance of fundamental studies of SWCNTs suggests that they are the most well characterized of the two CNT forms.

SWCNTS: Interfacial Properties and Analytical Applications The properties of CNTs that make them attractive in the design of sensing probes and chemical separations are those of: 1. a conductor, 2. semiconductor, 3. electrocatalytic surface and 4. an adsorption medium with large surface-to-volume ratio. Mechanical strength and heat tolerance also are valuable attributes (6–9). Note, that the extent to which SWCNTs are conducting vs. semiconducting relates to the spatial orientation of individual hexagons in the rolled up graphene sheet (Figure 1a), more specifically the chiral angle (1, 10, 11), as well as the diameter of the roll (Figure 1c) (1). As an example in analytical sensing, the unadulterated SWCNT can be an effective transducer in a gas sensing device (11, 12). Here, SWCNTs behave as an adsorption medium where the SWCNT-analyte surface interaction simultaneously results in a change in the electric properties of the material (12), e.g. conductance (S), resistance (Ω), current (i). The electric signal is further propagated via SWCNTs’ ability to make contact with other conducting materials (13–16), thus providing seamless information transfer between the adjacent small molecule and its corresponding concentration based signal. Greater selectivity and a wider range of analyte interactions on the front end of the sensor are realized when CNT surfaces are chemically modified via covalent processes, or they are inserted into a layered composite. The surfaces of SWCNTs may be directly functionalized, however, harsh and high concentration conditions often are needed to produce polar functional groups on the graphitic surface, moieties that can be further modified through covalent and/or non-covalent processes. Some examples of surface formed reactive groups are -F and -COOH (17, 18). More recently, reactive radicals and an alkyne functionalized SWCNT for use in Click reactions have been achieved (19, 20). To date, combinations of covalent and non specific interactions have been used to tether a variety of species including biomolecules (e.g. DNA fragments) and fluorescent probes (21). Some issues to contend with in this respect are that covalent modification can introduce defects that disrupt the CNT sp2 structure resulting in deterioration of a CNT’s intrinsic electronic and mechanical properties (22), a phenomenon that is not encountered when SWCNTs are merely added into a layered material. Composites that include CNTs as part of a layered material have been deposited on gold and glassy carbon electrodes (10, 23). In this latter context, the material’s electrical conductivity depends on factors such as the intrinsic conductivity of the 186 In Interfaces and Interphases in Analytical Chemistry; Helburn, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

SWCNT (or MWCNT), the volume of SWCNT within the composite as well the nature of a contacting polymer or ceramic (13).

CNTS and Magnetoresistance


Principles of Magnetoresistance A relatively new property of CNTs that holds promise towards their use in fundamental aspects of sensing and interfacial signal transfer is magnetoresistance (MR). This electric property refers to the change of resistance in a material when an external magnetic field is applied. It is also a measure of the resistance shift in a material as defined by the following relation.

In Equation 1, ΔR is the change of the resistance caused by the applied magnetic field and R is the resistance in the absence of the magnetic field (24). This effect was first observed in ferromagnetic thin sheets by Lord Kelvin (25). However, only subtle 0.033% increases in electrical resistance were reported in that work. The effect is also prevalent in non-magnetic metals and semiconductors. However, no more than 5% resistance shift is usually observed in pure materials (26, 27).

Figure 1. Graphene sheet showing lattice vectors along which the sheet can be rolled (a-b), a zig-zag SWCNT formed by rolling along the 8,0 vector (c); an atomic resolution scanning tunneling microscope (STM) image of a CNT (d), aggregation of SWCNTs via weak interactions and hydrophobic character to form a bundle (e). Adapted and reproduced from References (1–3). (see color insert) 187 In Interfaces and Interphases in Analytical Chemistry; Helburn, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


When a magnetic field is applied perpendicular to an electrical current occuring in a material, the electrons are deflected by the Lorentz force which builds up on one side of the sample surface parallel to the electrical current until a counterbalanced Hall electric field is established (28). Under this equilibrium condition, all electrons should follow the same straight paths as if the magnetic field were nonexistent. However, the velocity at which the electrons drift is not always the same as that demanded by their statistical distribution. For those electrons whose drift velocity deviates from the equilibrium condition, their paths are twisted by the magnetic field to form cyclotrons (29). This impeded travel of the electrons gives rise to MR (29). MR varies for different mechanisms of scattering and valence band structure. The magnetic force causes electrons to move along a circular or helical orbit. The ratio of the magnetic field to resistance depends on how many times electrons go around the cyclotron orbit between successive collisions. This value reflects the ratio of the electronic mean path to the orbit radius, which affects MR as found by Kohler. Kohler also pointed out that the MR depends on the relative orientations of current, magnetic field and the crystalline axes. Thus, MR has long been used as an experimental tool to deduce the Fermi surface and the crystalline structure of solid materials (30). It was not until the discovery of giant magnetoresistance (GMR), with an effect as much as 50%, in thin metallic sandwiches of Fe-Cr-Fe (31, 32) that this century-old phenomenon began to find uses beyond the arena of theoretical physics. As an example, GMR is making its way into the realm of biosensing and biomolecular detection. In this application, a magnetic tag is used in place of a fluorescent label, e.g. as a protein biomarker on a microarray substrate. Analyte detection occurs at a selective probe continuous with the surface of a GMR sensor (33, 34). GMR effects are usually observed in layered thin films of two or more ferromagnetic materials separated by very thin non-ferromagnetic spacers. The ferromagnetic alignment realized through thin film layering enables GMR materials to induce a far more pronounced shift in MR than regular (non-magnetic) materials do. The much weaker anisotropic magnetoresistive effect (AMR) is at work in regular materials, and thus only an insignificant magnetoresistive effect is observed (35). It is of interest to note that GMR effects up to ≥ 90% have been measured in systems where CNTs comprise the non-ferromagnetic layer and for other Fe-CNT composites (36). The MR to be described in this study, which was measured on SWCNTs coated on fused silica fibers, is attributed to an AMR effect. CNTS and MR: Previous Studies Magnetotransport properties of SWCNTs have been previously reported but in many cases without a clarified explanation (i.e. a mechanistic interpretation) (37, 38). One set of measurements performed on SWCNT bundles and thin films showed negative MR (39, 40) attributed to the weak localization arising from the extrinsic network effect. A more recent study performed on sorted metallic (i.e. conducting) SWCNTs revealed positive MR owing to the Aharonov-Bohm (AB) effect, which is thought to be an intrinsic effect of the magnetic field on SWCNTs 188 In Interfaces and Interphases in Analytical Chemistry; Helburn, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

(41). In this work we have measured and interpreted the temperature dependent resistance and MR for a series of SWCNT coated 0.11 mm diameter quartz fibers.


Preparation of SWCNT Coated Silica Fibers The quartz silica fibers were modified with SWCNTs via the following general steps: (1) preparation of the silica surface, (2) creation of a bonded hydrophobic layer on the silica surface via silanization, (3) non-covalent adsorption of SWCNTs (oxidized vs. unmodified) to the hydrocarbon modified surface and (4) annealing at 250-300°C. The silanization reactions that were used are shown below in Figure 2. Figure 3 illustrates a graphic representation of SWCNTs adsorbed onto the covalently attached hydrocarbon layer. Once the hydrocarbon layer was covalently attached, the SWCNTs were added in by physical adsorption. The fiber was then annealed.

Experimental Section Preparation of Coated Fibers Reagents and Materials Octadecyl trimethoxysilane (90%), phenyl trimethoxysilane (97%), phenethyl trimethoxysilane (98%), methyl trimethoxysilane (98%), and SWCNTs produced by carbon vapor deposition (CVD) method of avg. diam x length of 1.1 nm x 0.5100 um (Lot # 12526AE) were purchased from Sigma Aldrich; the lot consisted of >50% (volume percent) SWCNT and ~40% other nanotubes, amorphous carbon < 5%; ash content reported as Co, Mg, Mo and silicates < 2%. Super purified (SP) research grade SWCNTs manufactured by high pressure CO processing method (HiPco®) were obtained from Carbon Nanotechnologies Inc. CNI–Unidym (Lot # SP0334); lot consisted of < 3% TGA (thermal gravimetric analysis) residuals. Purified (P) grade HiPco® SWCNTs with

Temperature Dependent Resistance and Magnetoresistance of Single

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