ESSAY ON TWO DIFFERENT NANOSTRUCTURED ALLOTOPES OF CARBON ENEE416 GROUP ACTIVITY 7 Jeremy Feldman Daniel Gerzhoy Keywords Chemical Vapor Deposition CVD Cathodic Arc Physical Vapor Deposition Arc PVD INTRODUCTION Carbon nanostructures come in at least three to four allotropic forms fullerenes graphene and graphite carbon nanotubes and diamond Diamond has been researched mostly for its use in tribological and thermal coatings while graphene a mono layer of graphite has gained popularity for use in functionalized coatings and electronics The key to understanding the various structural thermal and electric properties of carbon nanostructures is the carbon bond which can be single double or triple bond order Graphene is held together by sp2 bonds and diamond is held together by the stronger sp3 bonds Diamond is one of the most well known allotropes of carbon and is both one of the hardest known natural minerals it has the highest heat conductivity 1 Diamond also has a wide bandgap and high optical dispersion 1 Diamonds are one of the most popular gemstones for jewelry due to their high optical dispersion luster and mechanical properties Diamond is created naturally in high temperature high pressure environments such as the earth s mantle taking billions of years to form from carbon deposits hundreds of kilometers below the earth s surface Synthetic Diamond Manufacture CVD is most often used in research and the electronics industry while Arc PVD is mostly used for tool coatings 3 Diamond is a good insulator though some natural blue diamonds and doped diamond act as a semiconductor 4 Diamond has remarkable material properties and has been highly valued Figure 1 Diamond on the left and graphene on the right are two allotropes of carbon in both mechanical and the electronics industry as much if not more so than the jewelry industry Graphene carbon is rapidly becoming a major field of study in the field of microelectronics It is a single layer of hexagonal carbon crystal It has some amazing properties such as its high electrical and thermal conductivity lack of band gap ambipolarity low light absorption see through and high tensile strength These unique properties are crucial to the application of Graphene in high quality electronics such as transistors and sensors However one of the biggest problems with Graphene is the difficulty with which it is manufactured and the characterization of the manufactured product Many of the properties of Graphene come from the perfection of the crystalline lattice Graphene with flaws often has different properties than perfect Graphene 5 DIAMOND STRUCTURE PROPERTIES Figure 2 Different view angles of a diamond face centered cubic lattice unit cell Diamond is a transparent crystal made up of a repeating pattern of 8 carbon atoms The pattern makes up one unit cell of the diamonds face centered cubic bravis lattice seen in figure 2 There are two tetrahedrally bonded atoms in each primitive cell 6 The atomic packing factor of the diamond cubic structure is about 0 34 which is the fraction of volume in a crystal structure that is occupied by atoms An important characteristic of materials with diamond blende structures used in fabrication is that they present open hexagonal ion channels which enable more efficient ion implantation if done from any of the Figure 3 Diamond cubic 110 directions seen crystal viewed from a in figure 3 These 110 direction Diamond structures display octahedral cleavage with fewer bonds and exposing points of structural weakness This characteristic can guide chemical etching to produce pyramidal structures as mesas points or etch pits and can be useful in MEMS fabrication 7 8 In addition to the aforementioned properties diamond is also valued for Scratch resistance Low coefficient of friction for nanostructured diamond like carbon films especially for polished NCD films 9 Resists abrasive and adhesive wear 9 Provides lubricity both in and out of vacuum 9 Has good chemical resistance to corrosion 12 Is a good insulator if no impurities 10 Can be doped and used as a semiconductor P and N 10 High thermal conductivity Bio compatible Chemically very stable 11 Superconductive at 11 4 K heavily boron doped 12 13 Highest thermal conductivity of any known solid at room temperature 14 15 DEPOSITION OF DIAMOND Two deposition methods for depositing and growing crystalline diamond discussed in this paper are CVD and Arc PVD Arc PVD is a physical vapor deposition done with a high temperature electrical arc and is used mostly for coating cutting or grinding tools for industry CVD is currently the only way to grow good quality single crystal electronics grade diamond The process for diamond growth using CVD consists of applying a seed layer to the substrate the new diamond needs something to grow on then under low vacuum of around 20 torr and heated to around 800 C Running a carbon based gas such as CH4 and H2 about 1 100 CH4 H2 and creating a plasma to break apart the molecules into chemically active radicals in plasma A simplified visualization of the concept can be seen in figure 4 3 GRAPHENE PROPERTIES STRUCTURE Many people use a material related to Graphene each day Graphite Graphite has the same hexagonal structure as Graphene but consists of layers of these hexagonal sheets Bulk Graphene is sometimes referred to when speaking about a thin but not single layer of graphite A perfect single sheet of Graphene has some amazing properties It has very high thermal and electrical conductivity whichespecially when coupled with its transparencymakes it an attractive option for electrodes on solar cells panels and other electrical devices This conductivity can alter the properties of other materials when made into a composite with Graphene as well Another important property of Graphene is its lack of a band gap 5 When semiconductor materials are voltage biased in one direction or another electrons enter the conduction band positive bias or holes enter the valence band negative bias The holes and electrons in these two situations then become the dominant mobile carriers 18 essentially Graphene can be switched from ptype to n type simply by changing the bias voltage Graphene is thus deemed ambipolar giving rise to flexible circuits that can create junctions on demand for various logic purposes For example it is possible to take a sheet of Graphene place a layer of silicon oxide between it and an electrode and place electrodes on either side of the Graphene