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Carbon nanotubes and graphene

 

Thermoelectric devices from carbon nanotubes

Carbon nanotubes (CNT) are cylindrical structures made of carbon with unique mechanical and electronic properties. A CNT can be thought of as a sheet of graphene (a hexagonal lattice of carbon) rolled into a cylinder. These are large mesoscopic molecules with high aspect ratios. They could be as long as millimeters with sub-nanometer diameters. These intriguing nanostructures have sparked a lot of excitement in the last two decades.

The physical properties of carbon nanostructures are still being discovered and debated with interest. What makes it so interesting and difficult at the same time is that nanotubes have a very broad range of electronic, thermal, and structural properties that change depending on the different types of nanotubes. These properties are usually defined by the diameter, length, and chirality during their growth.

Different kinds of carbon nanotubes (CNT) exist. They can be single walled nanotubes (SWNTs) or have a multiple wall structure (cylinders inside a larger cylinder-MWNTs). They could exhibit metallic or semiconducting behavior that is determined by the growth process.

We are interested in studying electronic and thermal properties of single-walled CNTs in the mesoscopic quantum regime aimed at understanding the nature of quasi-one-dimensional nanostructures, the manifestation of quantum mechanical effects in confined geometries, and the possibilities for their applications in manufactured nano-electronic devices. We are focusing on design, fabrication and measurement of nanodevices made out of SWCNTs for studying fundamental quantum problems in solid state physics and materials and their potential applications.

We currently employ ambient CVD growth techniques in our lab to produce SWCNTs on Si substrates for further fabrication of nanodevices. By our CVD growth process, we are able to fabricate SWNTs which are 100's of microns in length. A SWNT grown using CVD process is shown in Figure 1.

Tanuj SWNT

Figure 1: Scanning electron micrograph of a Si wafer with a single SWCNT, which runs vertically from top to bottom.

Carbon nanotubes are known to have high electrical and thermal conductance, high electron mobility and can carry a huge amount of current before structural failure. These properties along with their high Seebeck coefficients make them ideal candidates for thermoelectric applications. Currently, we are studying this effect by fabricating thermoelectric Peltier coolers using SWCNTs. Such devices would be find useful in facilitating the performance of various electronics used in aircraft, spacecraft and computers.

 

Figure 2:  Images of SWNT Peltier devices.

The TEC device consists of two coupled FETs which are controlled by gate and source and drain voltages. Our TEC device in the first stage is shown in Figure 2. Our next step is to introduce special heat/electric current filter pads which would further enhance the cooling efficiency. The thermoelectric effect in our device is completely reversible and can be controlled by exploiting the van Hove and quantized state singularities which are controlled using the gate electrodes. The ultimate goal of the present project is to make powerful cooling devices made of CNT arrays and ropes.

 

Graphene Devices

Graphene is a single atomic layer of crystalline graphite. It was previously thought that two dimensional crystals, such as graphene, could not exist. Two dimensional crystals were thought to be thermodynamically unstable and would melt when isolated. However, in 2004, Andrew Geim and Konstantin Novoselov successfully isolated a single layer of graphite, and due to its stability (it is now believed that graphene is stable up to several thousand degrees) they were able to perform measurements to explore many of its unique properties. For their work with the newly found material, they were awarded the 2010 Nobel prize.

 

Since its discovery in 2004, research on graphene has exploded both due to the ease with which the material is isolated and its numerous unique and interesting physical properties. Graphene was first isolated by taking a piece high quality crystalline graphite and stripping layers off the graphite using pieces of Scotch tape. The number of layers on the tape is then thinned further by additional stripping with Scotch tape. The last piece of Scotch tape with thinned pieces of graphite on it is then pressed onto a silicon wafer and peeled off. This leaves behind many pieces of graphite and graphene with varied thicknesses. It is important to have a silicon wafer with a 300 nm oxide layer, as this allows for a strong optical contrast between graphene and the wafer, despite graphene only being one atom thick. This allows graphene to be identified quickly and without specialized equipment by using optical microscopy. The Scotch tape method, although seemingly crude, produces high quality samples of graphene without any real need for specialized equipment.

 

Despite the ease with which graphene can be isolated using the Scotch tape method, there are still major drawbacks to this method. The main problem with the Scotch tape method is that there is little to no control over the shape, size, or location of the pieces of graphene. This means to use or measure a piece of graphene, one must spend a large amount of time and resources to find, isolate, and shape the samples to one’s needs. To rectify this problem, many researches have developed ways to grow graphene in order to produce pre-engineered samples. One of the more successful methods to grow graphene has been via chemical vapor deposition (CVD). In addition to using the Scotch tape method to produce graphene, our group has adapted other groups' recipes to grow graphene by CVD as well.

 

The ease with which graphene can be produced has been a major cause of its popularity in recent research. However, graphene’s many unique physical properties have also contributed to its popularity. Physics is highly dependent on the number of available dimensions. Since graphene is a single atomic layer of crystallized carbon, electrons that travel through the material only have two free dimensions to move. This gives us a readily available two-dimensional system to test how physics changes with the number of dimensions. In addition to the 2D nature of graphene, the carbon atoms in graphene form a hexagonal (honeycomb) lattice. The overall structure of graphene (its 2D nature and its honeycomb lattice) gives rise to many interesting properties. First, even at room temperature, electron transport through graphene is ballistic, making it a material with one of the highest electron mobilities. Second, the band structure of graphene causes electrons to no longer follow simple quantum mechanics, but actual behave like relativistic quantum particles. This is despite the fact that electrons in graphene are not actually relativistic (the electrons velocity is much les than the speed of light), they only behave as such. The combination of all of these properties gives rise a very interesting system to study, not just in its isolated form, but interacting with a variety of other materials and quantum mechanical phenomenons.

 

Figure 3.  Graphene device fabricated to investigate interactions with superconductors.  Purple areas are graphene; light blue is superconductor, and gold is normal metal.

Our research group is interested is using graphene as a material in a variety of both normal metal and superconductor junctions. This is to study the interplay between the charge carriers in each of these materials and the associated Andreev physics. A graphene wore is also a strong candidate to use as a quantum dot due to its already low dimensionality. Electrons in graphene can be further confined by physically shaping the material or by using electric fields from gates. In addition to its ability to confine electrons, graphene also has a naturally low density of states. This allows single energy states to be easily isolated for use in a quantum dot. Our group is interested in researching the interactions between superconductors (as a source of entangled electrons) and graphene quantum dots.

 

 



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