Measurement of the AC conductivity in a polymer-graphene composite material∗

Gary Ciodaro Guerra† and Cristian Enrique Medina Hernandez‡

Universidad de los AndesDepartment of physics

Yenny Hernandez-Pico§

Universidad de Los AndesCarrera 1 18A-10, Bloque Ip. Bogota, Colombia

(Dated: May 24, 2013)

I. INTRODUCTION

”Graphene is a rapidly rising star on the horizon ofmaterials science and condensed-matter physics. Thisstrictly two-dimensional material exhibits exceptionallyhigh crystal and electronic quality, and, despite its shorthistory, has already revealed a cornucopia of new physicsand potential applications”[5].

This project is based on the idea that Graphene isa material with extraordinary properties, and there-fore useful for applications outside the academic field.Graphene can be obtained by several ways, such as chem-ical treatment of oxidises , tape and graphite crystals[5], and the one used here: Exfoliation of graphite in inliquid phase[7]. Graphene is a Graphite monolayer withaccepted thickness of 0.34nm [1], whose properties arecomparable with those of the pure Graphite[2], specifi-cally its thermal conductivity and stiffness. The ultimatestrength is comparable to that of the Carbon-Nanotubes.

The investigation done here deals with the applicationof polymeric-graphene composite in electronics, for doingso, the conductivity AC of the composite is to be mea-sured, and compared with other known materials such assilicon and others semiconductors.

II. EXPERIMENTAL PROCEDURE

A. Graphene dispersion

In order to obtain the monolayer graphene, a solutionwas prepared using Graphite(Branwell 99.5(uk), concen-tration of 1mg/ml), with a Sodium Cholate(Surfactant,concentration of 0.5mg/ml) mixed in millipore water(Eletronic resistivity of 18.2 Ohm). The solution was

∗ Yenny Hernandez-Pico† gj.ciodaro30@uniandes.edu.co‡ ce.medina2635§ http://fisica.uniandes.edu.co/index.php/es/personal/profesores-

de-planta/hernandez-yenny

FIG. 1. Schematic view

located in a sonic bath during 45 minutes. As a result,the exfoliation of graphite in monolayers of graphene isproduced[2], those monolayers remain in solution due thehydrophobic and hydrophilic interaction[3] with the sur-factant molecules. A prudent time has to be waited forthe decantation to happen (about 1 day), to acceleratethe process the samples where centrifuged for 20 minutesat 1000rpm, the result: a gray solution ready to be mixedwith the polymer.

B. Graphene-PVA composite

Once the Graphene solution is obtained, it is to com-bined it with Polyvinyl alcohol dispersed in millipore wa-ter at a concentration of 60mg/ml, so that, the resultantwould be 30mg/ml in the final composite. To have a nor-malized composite and to avoid compaction, the mixturewas exposed to a sonic bath for 20 minutes. To ensureit was completely diluted, the mixture was placed in thegyro magnetic plaque for 3 hours.

III. EXPERIMENTAL SETUP

the following figure show the diagram of measurement.As observed in figure 1, two electrodes of Aluminum

are required to measure the AC conductivity of the com-posite. In order to do that a mask for the Aluminum for

2

the growth was manufactured, following the plane shownin figure 2.

A ( 3 : 1 )

B ( 6 : 1 )

C ( 6 : 1 )

A

B

C

gj.ciodaro3026/02/2013

Designed by Checked by Approved byDate

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Edition Sheet

Date

1

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20

20

2

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2

3

FIG. 2. Mask

The conic geometry is used to ensure that the depo-sition of the evaporated Aluminum is uniform over thecomposite.

IV. MEASUREMENTS

To acquire the desired data, a connection box be-tween the spectrometer and the sample was manufac-tured(figure 3).

FIG. 3. Connection box

To make the bridge between the electrodes, wire bond-ing was used as first option, the connection was achievedbut once we tried to measure, nothing could be regis-tered. After failing in obtaining the data; irreversibledamage was produced in the sample as can be observedin figure 4. So a second one was needed, in this case usinga copper wire with silver paint for the connections.

FIG. 4. Damaged sample

V. RESULTS

The impedance is directly related to the conductivityas the following equations shows:

Z = Z ′ + iZ ′′ (1)

Where Z’ and Z” are the real and imaginary parts ofthe impedance respectively. The electrical modulus | σ∗ |can be written as:

| σ∗ |= l√Z ′2 + Z ′′2A

(2)

Where l is the thickness of the deposited composite,and A is its traversal area. The data was processed andthe following was obtained.

2 3 4 5 6 7 8 9 10 11

x 104

3.8

4

4.2

4.4

4.6

4.8

5

5.2

5.4

5.6

x 104

Imaginary

Rea

l

FIG. 5. Part Real Vs Imaginary of the impedance

In figure 5 two semicircles can be observed, the smallone can be attributed to the grapheme composite and the

3

second one to the contact resistance between the com-posite and the Aluminium electrodes. This behaviouris expected, since electrons, due to his wave properties,chooses the path that requires less energy.

0 0.5 1 1.5 2 2.5 3 3.5 4

x 107

0

2

4

6

8

10

12

14x 10

4

Frecuency (Hz)

Impe

danc

e (O

hm)

FIG. 6. Impedance Vs Frequency

Plotting equation 2 in logarithmic scale, we found:

FIG. 7. Grafica log

In figure 7, for low frequencies (lower than 104hz), thespectrometer failed acquiring the data, specifically, itpresented discontinuities in frequency. The reasons areunknown and more investigation is needed for getting asatisfactory explanation.

In figure 7 and estimation of the l was done using [4].So that l = 4µm. It can be observed, that there is aregion where the conductivity has a constant behaviour,that is, independently from the frequency, therefore im-plying the equality of the current AC and DC. Averagingover the constant range, we found the experimental con-ductivity.

σ = 1.24 ∗ 10−4S

m(3)

VI. CONCLUSIONS

The idea of this project was to measure the AC con-ductivity σ∗ of our solution Polyvinyl-graphene and com-pare it with data of literature to test the effectiveness ofthe sample. The measure of conductivity is of the or-der of 10−4 compared with [3] has, approximately thesame order of magnitude of carbon fibbers, which showsthat our solution is effective, and that the addition ofpolyvinyl has little affects on the properties of graphene.Compared to other types of materials best known, wecan see that the conductivity of the solution is part ofsemiconductors region which makes it ideal for electronicapplications. A comparison in orders of magnitude canbe done with silicon, one material that is widely used inthe microelectronics industry and has a conductivity of10−5 just one order of magnitude lower than our com-posite.

VII. APPENDIX

1. Aluminium evaporation

A. Purpose

For this article we saw the need to create an exper-imental protocol for the evaporation on the aluminumon silicon mask. Although the process is carried under avery efficient vacuum chamber they spare us error factorsas air flows, temperature changes or human errors, theprocess should be take under strict process so the resultwould be satisfactory.

The idea of the process is to create six points of alu-minum of 30 nm of thickness over the Graphene solutionover an aluminum sheet over the silicon. At this pointwe located two rows of SiO2 substrates (three each), cre-ating a render sequences that allowed us to have severalexperimental data for each samples.

B. materials.

• 5 grams of aluminum

• Graphene solution

• Silicon wafer

• Evaporation mask

• Tungsten boat

• Vacuum chamber

1. Vacuum chamber

the chamber we used, is a basic prototype that allowsus to have pressures less than 10−3 Tor, a gap more than

4

enough in our project, and could achieves a temperatureabove the 5000 degrees Celsius through a potential dif-ferences on the tungsten boat. The chamber also has athickness monitor that shows the deposition rate as theevaporation occurs. The tungsten boat goes on the baseof the chamber and over it the aluminum piece. It takes acurrent of 120 to 125 A to get to the point of evaporationof aluminum while the mask and the silicon are placeddirectly over the steam.

2. The mask

The mask is an aluminum plate whit six holes andan opening where fits the silicon substrate. The shapeof the holes is essential to obtain the circular contactswe want, its must be cylinders slightly conical for steameasily stagnate without causing disturbance. Figure (2)shows the model used on the experiment.

3. Graphene solution

To add the layer of composite on silicon, we first try toput drops of solution while the layer spinning in a cen-

trifuge, but it was a failure because the composite layerhad excessive bubbles, so we try to put drops of Graphenebetween two layers of silicon at 60 degrees celsius (solid-ification point of the solution)

C. Procedure

1. create the graphene layer over the silicon

2. Depressurise the vacuum chamber

3. Add the tungsten plate whit the aluminum, closethe chamber and start the vacuum process.

4. Once the pressure kept constant at 10−1 Tor turnon the diffraction bomb until the pressure stabilizesin a range of 10−3Tor.

5. Slowly increase the current up to 120-125 A. it isimportant to slow because aluminium is quite tem-peramental and if the process is does not evaporateevenly and also is possible its start to get out thetungsten plate.

6. Wait to the mass counter reaches the 30 mn.

[1] Stankovich, S., Dikin, D. A., Dommett, G. H., Kohlhaas,K. M., Zimney, E. J., Stach, E. A., & Ruoff, R. S. (2006).Graphene-based composite materials. Nature, 442(7100),282-286.

[2] Eda, G., & Chhowalla, M. (2009). Graphene-based com-posite thin films for electronics. Nano letters, 9(2), 814-818.

[3] Hernandez, Y., Nicolosi, V., Lotya, M., Blighe, F. M., Sun,Z., De, S., & Coleman, J. N. (2008). High-yield productionof graphene by liquid-phase exfoliation of graphite. NatureNanotechnology, 3(9), 563-568.

[4] Hernandez, Y. Electrical Conductivity of Polymer-CarbonNanotube Composites.Transfer Report. School of PhysicsTrinity College Dublin (2006).

[5] A. K. Geim1 & K. S. Novoselov1,The rise of graphene ,Nature Materials 6, 183 - 191 (2007)

[6] Jun Zhu1, Graphene production: New solutions to a newproblem, Nature Nanotechnology 3, 528 - 529 (2008).

[7] Zhao, Weifeng and Fang, Ming and Wu, Furong and Wu,Hang and Wang, Liwei and Chen, Guohua, Preparation ofgraphene by exfoliation of graphite using wet ball milling,J. Mater. Chem.(2010)