Diamond – an expensive allotrope of carbon
TSANG Yuk Lun and TAM Wing Tat

Introduction
Diamond is one of the allotropes of the element carbon.  The crystal structure of diamond is made of covalently bonded sp3 hybridized carbon atoms.  The tetrahedral arrangement of the carbon-carbon bonds results a very stable lattice.  The unit cell of the diamond is usually described as two intersecting f.c.c. lattice as shown in Figure 1.1.

Unique properties of diamond
The exceptional strength of the carbon-carbon bond and the covalently interlocked crystal structure accounts for the hardness and inertness of diamond.  In fact, its hardness, molar density, thermal conductivity and sound velocity are higher than that of any other known materials.  The physical properties of diamond are summarized as below:

Physical Property	Value	
Thermal expansion coefficient	0.8 ×10-6 / oC	
Thermal conductivity	20 watts / cm oC	
Molar heat capacity	6.19 J .mol oC	
Young’s modulus	105 ×1011 dynes / cm3	
Density	3.52 g / cm3	

Although pure diamond is colourless and transparent, when contaminated with other minerals, it may appear in various colours ranging from pastels to opaque black.  

Raman spectroscopy is commonly used in the characterization of diamond.  The Raman spectrum of diamond contains a characteristic peak at 1332 cm-1.

Making diamonds by Chemical Vapour Deposition (CVD)
Diamond films can be grown by hot filament chemical vapour deposition.  Methane as (CH4) and hydrogen gas (H2) are passed into the reaction chamber maintained at a very low pressure (10-4 torr).  The chamber temperature is between 1800 to 2300 oC.  Diamond films can be obtained on silicon substrates.