Boron Nitride. can you spot the adamantane-like rings? After that little tangent on graphitic carbon nitride -back to diamondoids

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1 After that little tangent on graphitic carbon nitride -back to diamondoids Boron Nitride isoelectronic with carbon exists as various polymorphs, one is analogous to diamond and one to graphite. diamond-like polymorph is one of the hardest materials known graphite-like polymorph is a useful lubricant. Structure : cubic BN zincblende structure can you spot the adamantane-like rings? 22-1

2 Cubic boron nitride is extremely hard, although less so than diamond electrical insulator and an excellent conductor of heat. cubic boron nitride, c-bn, β-bn, or z-bn (after zincblende), widely used as an abrasive for industrial tools Preparation of cubic BN Cubic boron nitride is produced by treating hexagonal boron nitride at high pressure and temperature, much as synthetic diamond is produced from graphite. Direct conversion of hexagonal boron nitride to the cubic form occurs at pressures up to 18 GPa and temperatures between C; 22-2

3 hexagonal BN "white graphite" similar layered structure good at high temperatures 22-3

4 BN Nanostructures BN Nanotube BN Nanomesh 22-4

5 BN Nanomesh is a new nanostructured two-dimensional material. It consists of a single layer of hexagonal boron nitride on rhodium or ruthenium, forming a highly regular mesh. The distance between two pore centers is 3.2nm and the pores are 0.05 nm deep. The boron nitride nanomesh is stable under vacuum, air and some liquids, but also up to temperatures of 796 C. In addition, it traps molecules and metallic clusters BN Fullerenes 22-5

6 Properties and Synthesis boron nitride is far more resistant to oxidation than carbon and therefore suited for high-temperature applications in which carbon nanostructures would "burn." In addition, BN nanotubes are expected to be semiconducting, with predictable electronic properties that are independent of tube diameter and number of layers, unlike tubes made of carbon. Occasional four- and eightmembered rings cause boron nitride nanotubes to bend and adopt a variety of shapes (red = boron; blue = nitrogen). 22-6

7 Unlike arc-discharge methods or other techniques for making BN nanotubes in the gas phase, these are made by depositing boron and nitrogen ions on a hot, electrically biased tungsten surface. [Phys. Rev. Lett., 86, 2385 (2001)]. The single-layer nanostructures were deposited on these substrates at temperatures between ±C and a pressure of around 1023 Pa in a UHV baked system with a 1028 Pa base pressure using a negative substrate bias of V dc. Typical deposition rates were of the order of Å per second. For boron, a conventional electron-beam evaporation source was used A high-resolution electron micrograph of singlewalled BN nanostructures on tungsten substrate synthesized using ion-beam assisted deposition. Arrows 1 and 3 mark single-walled BN nanotubes (diameter approximately 0.5 nm), and arrow 2 marks a conical feature similar to a nanohorn or nanomountain. The lattice fringes in the substrate are from the(100) planes of tungsten (d spacing nm). 22-7

8 another method - uses carbon nanotubes 22-8

9 BN Nanotubes by CVD of borazine Chem. Mater., 2000, 12 (7), SEM image from a BN-nanotube deposit. The arrows identify bulbous tip features, which are present on many nanotubes. 22-9

10 During chemical vapour deposition (CVD), energy is given to a gas (the precursor gas) which contains the atom that we want to deposit. This energy dissociated the gaseous molecules by a series of chemical reactions in order to obtain a solid product (the deposit) on a surface where we want the deposition to occur (the substrate). This can be conceptualized by the following equation which describes the deposition of the gaseous species ABu : ABu A( ) + u B( ) BN from trimethoxyborane 22-10

11 Back to diamond ---diamond films CVD of methane highly dependent on gas mixture 22-11

12 nucleation growth 22-12

13 phase diagram for carbon Graphite is the thermodynamically stable form of carbon at room temperature and pressure. Diamond is only more thermodynamically stable than graphite at temperatures greater than 1300 C and pressures greater than 40 kilobar. There is only a small difference in the thermodynamic stability of the two allotropes. At a temperature of 298 K and a pressure of 1 standard atmosphere, the standard Gibbs free energy of formation of diamond is 2.9 kj mol 1 There is no easy rearrangement mechanism by which diamond can convert to graphite. The energetic activation barrier for conversion is very high and the conversion is therefore kinetically unfavourable. Hence, diamond will remain in a meta-stable state at room temperature and pressure without converting to graphite

14 future? Diamond Nanoelectrodes in Neuroscience biomedical and electrochemical applications. - implantable electrodes for in vivo sensing and neural/muscle stimulation. active areas in the nanoscale dimension. Electrochemical analysis Bioelectrochemistry Electrosynthesis transparent B-doped diamond electrode Energy storage and conversion 22-14

15 enzyme-modified nanocrystalline diamond electrode. The bilayer of two enzymes, glucose oxidase and horseradish peroxidase, is covalently immobilized to the diamond surface. The cascade of electrochemical reactions initiated by the presence of glucose is converted to an electrical current measured at the diamond electrode