Reference no: EM133774030

Graphene and electrical conductivity

OBJECTIVE

To study the electrical conductivity of different materials and to make and study graphene flakes on a silicon wafer.

INTRODUCTION

Graphene:
Carbon exists in a number of allotropic forms, coal, graphite, diamond, Buckminsterfullerene, carbon nanotubes, and graphene. Coal is formed from organic matter that has decayed and been compressed under great pressure. It is an amorphous form of carbon that has no definite crystal structure.

Graphite consists of layers of carbon atoms in a hexagonal arrangement. There are only weak bonds between layers, allowing the layers to slide over one another, Figure 1. Graphite is used in the "lead" of lead pencils, as a dry lubricant, and in electroplating of substances. Diamond has a framework structure where each carbon atom is bonded to 4 other carbons in a three- dimensional structure. Diamond is the hardest naturally occurring mineral found in nature. Used for drills, cutting wheels, and polishing of many substances, as well as for jewellery.

Buckminsterfullerene, consisting of 60 carbon atoms in a spherical shape similar to a soccer ball, is formed by electrically evaporating graphite in an atmosphere of He gas. Note that the structure consists of both 5 and 6 member carbon rings. Because the shape is similar to that of a geodesic dome invented by R. Buckminster Fuller, it was named Buckminsterfullerene or "Bucky Ball". Fullerenes have been prepared with as few as 20 carbon atoms and more than 80 atoms. Applications include superconductors, along with compounds with interesting electrical, magnetic, and optical properties.

Carbon nanotubes, discovered in 1952, and again noted in subsequent years, were overlooked until about 1991. Single wall carbon nanotubes are essentially a single sheet of graphite rolled into a seamless tube approximately 1 nm in diameter. They have very high strength and high electrical and heat conductivity. Multi-wall carbon nanotubes can also be synthesized.

Graphene is a single planar sheet of carbon atoms that are densely packed in a honeycomb crystal lattice, Figure 2. Graphene is the basic structural element for all other graphitic materials including graphite and carbon nanotubes. It was first made in 2004 when physicists from the University of Manchester and the Institute for Microelectronics Technology, Chernogolovka, Russia, found a way to isolate graphene by peeling it off from graphite with Scotch tape and optically identified it by transferring it to a silicon dioxide layer on Si wafer. This is partly what you will redo in this lab.

Graphene's high electrical conductivity and high optical transparency make it a candidate for transparent conducting electrodes, required for such applications as touchscreens, liquid crystal displays, organic photovoltaic cells, and Organic light-emitting diodes.

Electrical conductivity:
One of the most basic and important laws of electric circuits is Ohm's law. Ohm's law states that the voltage (U) across a conductor is directly proportional to the current (I) flowing through it (U = RI), provided all physical conditions and temperatures remain constant. The constant of proportionality, R, is called resistance of the conductor and is measured in Ohms (W). Low resistance indicates high electrical conductivity in a material and high resistance implies low conductivity. Very low electrical conductivity as in for example insulators and undoped semiconducting materials are very difficult to measure accurately and will not be discussed in this lab.
To determine the conductivity of a thin film the four-probe technique are normally used. The four-point probe set up (Figure 3) consists of four equally spaced metal tips with small radius. Each tip is supported by springs on the end to minimize sample damage during measurements. The four metal tips are part of a mechanical stage which are moved up and down during measurements. A constant current is supplied through the outer two probes, a voltmeter measures the voltage across the inner two probes to determine the sample resistivity. Typical probe spacing of ~ 2 mm is used. These inner probes draw no current because of the high input impedance voltmeter in the circuit, thus, unwanted voltage drops (RI drop) caused by contact resistance between probes and the sample is eliminated from the potential measurements. These contact resistances are very sensitive to pressure and to surface condition (such as oxidation of either surface) and must be avoided.

Four-point probe-based instruments use a long-established technique to measure the average resistance of a thin layer or sheet by passing current through the outside two points of the probe and measuring the voltage across the inside two points. If the spacing between the probe points is constant, and the conducting film thickness is less than 40% of the spacing, and the edges of the film are more than 4 times the spacing distance from the measurement point, the average conductivity of the film or the sheet conductivity (s) is given by:

σ = 1/4.53.U/I.t = 1/4.53.R.t

Where t is the thickness of the film/sheet in cm. The unit of conductivity is normally reported as S/cm (1/ Ω cm) instead of the SI units S/m.

EXPERIMENTAL PROCEDURE

Graphene flakes on a silicon wafer:

1. Take a piece of scotch tape about 10 cm long. Fold over about 1 cm of each end of the tape to make "handles" for holding the tape.

2. Place the tape onto the graphite sample from First Graphene 1 cm from one of the "handles". Smooth the tape with your finger or plastic forceps. Slowly, lift the tape from the graphite surface.

3. Fold the tape in half, sticky sides together, over the graphite area and slowly peel it apart. Fold the tape in half over the graphite area again and peel it apart again. Do this several more times.

4. Place the tape with the last graphite layer onto a clean silicon wafer disk with a thick oxide layer. Take care as the silicon wafers are fragile and break easily. Gently rub the tape with your plastic forceps for about 5 minutes to transfer the graphite/grapheme to the silicon wafer. Do not press hard!

5. Using your forceps to hold the silicon wafer in place, SLOWLY peel off the tape. This may take about one minute. Do not pull hard as it will cause the silicon wafer to break!

6. To observe the graphite/grapheme, use the forceps to place the silicon wafer on a microscope stage. Illuminate the wafer from above. Focus using the 10x objective. The graphite/graphene material may look like red or pink spots on the silicon surface. Once you have located the graphite/graphene debris, rotate the 40x objective into place. Focus using the fine focusing knob only. If your microscope is equipped with a 50x, 60x, or 75x objective, you can try to use them to view the graphite/graphene.

Electrical conductivity measurements:
Using a lead pencil, draw a dark rectangle, approximately 3 cm x 1 cm on a white paper. Make sure it is uniformly black.

Repeated step 1 on a separate white paper but use a piece of graphite from First Graphene (99% graphite) to "paint" a dark rectangle of the same size.

Test the dark rectangles for conductivity by pressing the two electrodes of the small multimeter (setting to Ohm) against the dark rectangles with 2 mm between the two electrodes and record the resistance, compare with the unpainted paper. What is the resistance between the electrodes for the pure paper, coated using a lead pencil and coated with pure graphite? What happens if you increase the distance between the two electrodes when you measure the resistance? Rub your finger over the dark rectangle and record your observations.

Measure the resistance using the small multimeter (setting on Ohm) on a piece of coal and a clean graphite piece (from First Graphene). Record the resistance.

Measure the resistance using the small multimeter (setting to Ohm) on an Indium Tin Oxide (ITO) coated glass slide on both sides and a piece of titanium foil. Have approximately 2 mm between the two electrodes. Record the resistance.

Repeat the measurements under 3 and 5 using the four-probe equipment. Do not disconnect the wires and be careful with the four-probe needles! Turn on the source meter. Put the sample under the four-needles and record the values (voltage and current) on the source meter without lowering the needles onto the sample. Carefully lower the needles so that they make contact with the sample. Record the voltage and the current values. Lift the needles from the sample and put your next sample in position. Repeat the procedure for all the flat samples (used in 3 and 5 (both sides for the ITO coated glass)). Calculate the electrical conductivity in S/cm for the samples assuming that the thickness of the painted graphite layers on the papers are 10 micrometers. The thickness of the ITO is 140 nm, and the thickness of the titanium foil is 200 micrometers.

ANALYSIS OF RESULTS

QUESTION 1. What can you see using the microscope when looking at the silicon wafer?

QUESTION 2. Is there a difference in resistance of the led pencil painted paper and unpainted area? Explain the results

QUESTION 3. What is the conductivity of the ITO layer measured with the four-probe technique and the two probe technique? Calculate the conductivity value for the two probe technique by using the measured resistivity value for ITO instead of the (U/I) in the formula for calculating the conductivity. If there is a difference in conductivity, explain the difference.