Study of Electric Transport in Carbon Nanofibers

Oxana V. KHARISSOVA,* Boris I. KHARISOV, Olha N. VELYCHKO, and Ubaldo ORTIZ MÉNDEZ

Universidad Autónoma de Nuevo León, Ciudad Universitaria UANL, Monterrey, 66450 Mexico. bkhariss@mail.ru

ABSTRACT

Synthesis ) of ) carbon ) nanofibers ) by ) thermal decomposition of carbon disulfide and study of their properties and electric transport as function of temperature is reported. Carbon disulfide is used as a precursor; three combinations of iron and silver were used as catalysts. When iron is used as a catalyst, an anomaly in the R(T) characteristics is observed at about 0oC due to the absorption of water molecules in carbon nanofibers. Morphology of carbon nanofibers by Atomic Force Microscopy (AFM) is studied. Keywords: carbon nanofibers, carbon nanotubes, morphology, electric transport, catalyst. *Corresponding author.

I. INTRODUCTION During more than the last decade, carbon nanotubes have been an object of a continuously increasing interest in the scientific community of materials science [1]. This interest is caused by diverse applications of carbon molecular structures in the most important scientific areas, as physics of superconductors, modern optics, biomedicine and nanotechnology. Carbon nanotubes have been considered as useful for a great number of technical applications; however, present investigations are focused on the purpose of controlled production of the most perfect nanotubes with required/programmed characteristics. In one of such investigations, carbon nanotubes were obtained [2] by thermal chemical Carbon Vapor Deposition (CVD) technique. This method is better in comparison with others in respect of purity and controlled alignment of nanotubes; present attention of researchers is paid to the development of new methods for preparation of vertically aligned carbon nanotubes using CVD. The mentioned work2 reports growth of well vertically-aligned nanotubes on silicium oxide surface, employing acetylene as a carbon precursor and cobalt as a catalyst. Before nanotube growth, the nanometric catalytic cobalt particles are formed by action of liquid fluorhydric acid, subsequent emissions of gaseous ammonia from the surface of cobalt, deposited on silicium oxide; this step is crucial to can control the size and vertical alignment of carbon nanotubes.

The growth model from the nanotube basis [3,4] supposes that rate of growth of carbon filament is inversely proportional to the size of a catalytic particle and it increases with decrease of size of a catalytic

particle due to a short length of diffusion of carbon atoms. So, when nanotube (or particle) diameter decreases, an increased growth rate results a better frequency of formation of layers between the compartments.

In the present work, we have measured resistance as a function of temperature of three types of distinct samples of catalysts, used for production of carbon nanofibers, and carried out a morphological study of the formed products.

II. EXPERIMENTAL

Three substrates were prepared containing three different catalysts on the basis of iron and silver to provide distinct characteristics of formed carbon nanofibers. The first catalyst (a) consists of a thin layer of a 120 mM solution of 243.9 mg of Fe(NO3)2·9H2O in 5 mL of ethanol, which is put on the substrate (Fig. 1a); here the formed iron functions as a catalyst. The second catalyst (b) consists of a silver film of 115 Å of thickness, obtained by decomposition of silver nitrate in a vacuum bell; a layer of solution of Fe(NO3)2·9H2O is put on this film (Fig. 1b). The third catalyst (c) consists of a silver film of 113 Å of thickness, exposed to a thermal treatment to change its morphology. After this treatment, a layer of solution of Fe(NO3)2·9H2O is put on this film (Fig. 1c).

Fig. 1. Three different types of substrates.

The next step is the growth of carbon nanotubes starting from distinct catalysts using carbon disulfide (CS2) as a precursor by CVD technique. The system used consists of a horizontal furnace containing a quartz tube (internal and external diameter of 2.2 and 2.5 cm, respectively), functioning as a reaction zone, in which the controlled flows of argon and hydrogen can be introduced directly or through a barboter with desirable liquid as a source of carbon (in our case CS2) (Fig. 2).

Fig 2. Diagram of the CVD system.

The substrates with catalysts (a), (b) and (c) were collocated into the centre of a furnace, where the temperature of the internal part of quartz tube was calibrated earlier. At the beginning of the process, pure argon (126 sccm) was directly introduced into the tube in furnace for some minutes to displace air; then temperature was elevated to a desirable point (900oC for approximately 40 min.). Reaching this temperature, argon flow was turned off and pure hydrogen (450 sccm) was directly introduced for 5 min. to replace argon. Then a hydrogen flow (88 sccm) passed through barboter containing 20 mL of carbon disulfide (CS2). At the end of the process, a hydrogen flow was retained by the barboter and was substituted by pure argon (126 sccm, during 15 min.) to replace the remaining mixture of hydrogen and CS2 to avoid any deposit of carbonous material non-desirable during cooling of the furnace. To measure electric properties, samples in the form of thin film were prepared [5], where a cleaned glass substrate was collocated on the fused quartz substrate, in which carbon nanofibers are present, and it was pressed (Fig. 3a). Thus, a thin film on the basis of carbon nanofibers on glass substrate is obtained. For any type of sample, two silver electrodes of approximately 500 Å of thickness, prepared from silver nitrate, were collocated and separated by a distance of approximately 1 mm between them; they serve to put two electric terminals (Fig. 3b). Starting from three forms of catalysts (a), (b) and (c), three types of samples with carbon nanofibers were prepared (Film-A, Film-B and Film-C, respectively).

Fig. 3. (a) Elaboration process of samples of the type of

thin film; (b) evaporated silver electrodes.

Determination of electric properties of the obtained samples was carried out in a vacuum box, equipped with a heater; it can be cooled by liquid nitrogen. Electric measurements were made at a constant voltage, varying temperature from 123 to 427 K.

III. RESULTS AND DISCUSSION

) The substrates with catalysts (a), (b) and (c) were analyzed by AFM to see their morphology (Fig. 4) and measure particle size. It is found that, in case of the substrate (a), conglomerates with diameters of 100-400 nm are formed (Fig. 5a).

Fig. 4. AFM image at scanning of 10 µm showing the

substrates with the catalysts (a), (b) and (c), respectively.

For the substrate with the catalyst (b), conglomerates with diameter of 300-400 nm are formed by lesser-size particles with medium width of 90 nm (Fig. 5b), meanwhile, for the catalyst (c), conglomerates with diameter of 200-400 nm are formed by particles with medium width of 70 nm (Fig. 5c).

Fig. 5. AFM image at scanning of 1 µm showing the conglomerates formed by the catalysts (a), (b) and (c)

with their respective dimensional measurements. In order to verify presence of nanofibers, different thin films were analysed by AFM, which revealed homogeneously dispersed nanofibers in the substrate (Fig. 6).

Fig. 6. AFM images: (a) scanning of 18 µm, showing formed nanofibers (Film-A); (b) scanning of 10 µm,

showing formed nanofibers (Film-B).

Measurements of diameters for obtained nanofibers were made in the samples Film-A and Film-B; a medium diameter is 413 nm and 251 nm, respectively (Fig.7). These dimensions are in agreement with medium ranges of size of catalysts in the form of conglomerates, from which the nanofibers grew; growth process from nanotube corresponds to the model described by R. T. K. Baker [3,4].

Fig. 7. AFM image at scanning of (a) 5 µm and (b) 1 µm, showing formed nanofibers in Film-A and Film-B,

respectively, and their dimensional measurements. Fig. 8 (image AFM 3D) shows that the nanofibers are closed in the Film-A (Fig. 8a); Fig. 8b (Film-B) reveals a sectional nanofiber structure of bamboo type.

Fig. 8. AFM 3D image with (a) dimensions of 677 × 677 × 108 nm, showing nanofibers in Film-A, and (b) dimensions of 277 × 277 × 202 nm, part of a nanofiber

in Film-B, showing sectional structure.

A non-ohmic behaviour was obtained in case of the sample Film-A graphic I(V), Fig. 9a, so it was chosen to take only the voltage values from -1 V to 1 V, the region where an ohmic behaviour is present. Fig. 9b shows the graphic of R(T), where a hysteresis in the process of increasing and decreasing of temperature is observed. An anomaly in the curve R(T) at about 273 k is seen: a jump in the electrical resistance in the cooling process and a more complicated structure in the heating process. This phenomenon could be attributed to the presence of water molecules in carbon nanofibers. Introducing humidity to the vacuum box, this anomaly is revealed more as in cooling as heating processes (Fig. 9c).

-6 -4 -2 0 2 4 6

-4.0x10-8

-3.0x10-8

-2.0x10-8

-1.0x10-8

0.0

1.0x10-8

2.0x10-8

3.0x10-8

I [A

]

V [V]

(a)Film-A

150 200 250 300 350 400 450

1x107

2x107

3x107

4x107

5x107

6x107

7x107

8x107

9x107

1x108

R [Ω

]

T [K]

(b)Film-A

150 200 250 300 350 400

0.0

2.0x108

4.0x108

6.0x108

8.0x108

1.0x109

(c)Film-A

R [

Ω ]

T [K]

Fig. 9. (a) Graphic of electric current against voltage for Film-A, where a ohmic behaviour can be associated between the values of -1 V y 1 V. (b) Resistance against temperature, where the upper line indicates the phase of increasing temperature and the lower line indicates the decreasing phase. (c) Resistance against temperature with humidity inside the vacuum box; irregularity in the resistance is observed.

Fig. 10a shows a totally ohmic behaviour in the graphic I(V) for the sample Film-B. A behaviour of semiconductor type [7] in function of temperature without any hysteresis is observed in the graphic of R(T).

-0.006 -0.004 -0.002 0.000 0.002 0.004 0.006-4.0x10-8

-3.0x10-8

-2.0x10-8

-1.0x10-8

0.0

1.0x10-8

2.0x10-8

3.0x10-8

(a)Film-B

I [A

]

V [V]

100 150 200 250 300 350 400 450

0.0

5.0x106

1.0x107

1.5x107

2.0x107

2.5x107

R [Ω

]

T [K]

(b)Film-B

Fig. 10. Characteristics of the electric transport in Film-B. (a) An ohmic behaviour in the graphic of current against voltage, (b) Graphic of resistance against temperature, where the line indicates increasing of temperature and (o) decreasing phase (coinciding below with the main line), where the behaviour of a semiconductor is observed.

Carbon nanofibers, considered as unidimensional conductors, present behaviour of liquids of Luttinger [5,7], a model, in which the conductance is given in the function of temperature as G ~ ATα and dI/dV ~ Vα, and as G = 1/R. It is possible to put a function R(T) according to the model of liquids of Luttinger in the form:

R(T) = R0T-α

with an experimental value of α of 3.0113 and 2.90835 for increasing and decreasing in temperature, respectively. Fig. 11 shows an adjustment with values of constants R0.

100 150 200 250 300 350 400 450

0.0

5.0x106

1.0x107

1.5x107

2.0x107

Modelo: R=R0T -α

R0= 3.258E13 ± 2.7095E12

α = 3.0113R [

Ω ]

T [K]

(a)Film-B (decremento en T)

100 150 200 250 300 350 400 450

0.0

5.0x106

1.0x107

1.5x107

2.0x107

2.5x107

(b)Film-B (incremento en T)

Modelo: R=R0T -α

R0= 1.9901E13 ± 0.79999E12

α = 2.90835

R [

Ω ]

T [K]

Fig. 11. Adjusted values to the model of liquid of Luttinger for the sample Film-B: (a) phase of decreasing temperature, (b) phase of increasing temperature.

For the sample Film-C, new electrodes should be evaporated, since, with one separation of 1 mm, the sample was extremely resistive and it was not possible to detect any current. So, the electrodes, separated by a distance of ~74 µm were made, improving enormously the contacts with the sample and increasing its conductance. An ohmic behaviour is observed in Fig. 12 graphic of I(V).

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-4.0x10-7

-2.0x10-7

0.0

2.0x10-7

4.0x10-7

I [A

]

V [V]

Film-C

Fig. 12. Graphic of current against voltaje showing an ohmic behaviour for the sample Film-C.

Fig. 13 graphic of R(T) shows that the sample acts as a switch for the phase of decreasing of temperature, probably, due to the attraction and repulsion forces, existing between nanofibers and permitting a better or worse contact between the electrodes and the sample.

150 200 250 300 350 4007.5x105

8.0x105

8.5x105

9.0x105

9.5x105

1.0x106

1.1x106

R[Ω

]

T [K]

Film-C

Fig. 13. Graphic of resistance against temperature for the processes of increasing (lower line) and decreasing

(upper line) of temperature for the sample Film-C.

IV. CONCLUSIONS

It is confirmed that the growth process of carbon nanofibers coincides with Baker’s3 model, forming those having a similar diameter which have conglomerates, formed with use of catalysts of each type of samples. Resistance as a function of temperature was measured for three types of distinct samples. In case of Film-A, an anomaly in resistance is observed, close to 273.15 K, caused by presence of water molecules in the nanofibers. In case of Film-B, it is possible to adjust behaviour of the sample to the model of liquids of Luttinger, adding a resistance term in order to take into account interconnections between nanofibers. In case of Film-C, switch-type behaviour is observed which is attributed to the attraction forces between nanofibers, allowing a better or worse contact between the electrodes and sample.

V. ACKNOWLEDGEMENTS

The authors are very grateful to the Mexican Academy of Sciences for financial support during XVI Summer of Research, IIM-UNAM for laboratory space, Mr. Carlos Flores for his help in obtaining AFM images, and Dr. Doroteo Mendoza López for useful suggestions and consultation, and to Professor Miguel Jose Yacaman (University of Texas at San Antonio, USA) for critical revision of this manuscript.

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