Diamond nitrogen impurities

The color of diamond due to nitrogen impurities has been described in Section 9.6.3 It has been found that nitrogen impurities that are located next to a carbon vacancy in diamond thin films endow the solid with quite new properties, somewhat similar to the properties of a solid containing FLi centers compared with ordinary F centers. The diamond structure is built up of carbon atoms each surrounded by four... 

For a diamond lattice containing a nitrogen impurity or a vacancy a large but finite cluster model (of up to 70 atoms) was employed by Watkins and Messmer 162> to calculate the distortional modes due to electron-vibrational coupling. From their computed data it appears that the distortion takes place along several normal modes of the cluster. This may be also the consequence of the anharmonic nature of the model. 

Samples of diamond sometimes also exhibit considerable fluorescence. Especially type I diamonds bearing impurities of nitrogen have a pronounced spectrum with two maxima already known from UV/Vis absorption. (A = 415 nm most likely from transitions without participation of foreign atoms or vacancies, but at defects generated from the breaking of C-C-a-bonds and A = 503 nm from transitions including foreign atoms on lattice positions.)... 

Carbon in the structural form of diamond is the only element used industrially as a hard material. Each year about ten tons of natural diamond and about twenty tons of synthetic diamond (produced via high temperature high pressure synthesis) are marketed as hard materials. While pure diamond is transparent, a yellow tint results from the replacement of some carbon atoms by nitrogen, and a blue, yellow, or even green tint through substitution of carbon by boron atoms. Polycrystalline diamond with impurities, used as an abrasive, is often black. 

Nitrogen. Nitrogen impurity in diamond is detected and characterized by IR absorption and paramagnetic resonance. The majority of nitrogen atoms within the diamond structure are arranged in pairs as shown in Fig. 

Almost all natural diamonds are impure and contain substitutional nitrogen. Clusters of N atoms are present in Type la diamonds (the most common defect structure), while Type lb diamonds contain well-separated N atoms, (a) What is meant by substitutional nitrogen (b) In what type of environment will an N atom reside in natural diamond (c) Sketch a diagram to illustrate the band strucmre of diamond, assuming no defects. Explain why diamond is an insulator, (d) The donor-level associated with the nitrogen lies about 1.7 eV below the conduction band. How does this donor level arise Why do natural diamonds not behave as semiconductors ... 

H Hanzawa, N Umemura, Y Nisida, H Kanda, M Okada, M Kobayashi. Disorder effecet of nitrogen impurities, irradiation-induced defects, and " C isotope composition on the Raman spectra in synthetic lb diamond. Phys Rev B 54 3793-3799, 1996. 

Type I. These have IR bands at 1400-1100 cm of variable intensity. They are due to nitrogen impurity. About 99% of all diamonds are of this type. 

Processes involving defect energy levels are responsible for coloration of diamonds containing races of nitrogen or boron impurities. Diamond has a band gap of about 8.65 x 10-19 J (5.4 eV), which is too large to absorb visible light and... 

Figure 9.27 A (N-V) center in diamond, consisting of a carbon atom vacancy and a neighboring nitrogen atom impurity.

The diamond is found in natural deposits in many parts of the world. Also, it can be synthesized from graphite or other carbonaceous materials. Graphite can be converted to diamond under high temperatures (about 1,400°C) and very high pressure (in the range 4,000-5,000 atm) in the presence of a metal catalyst such as iron or nickel. Presence of trace impurities can impart different coloration to diamonds. For example, introducing trace boron or nitrogen causes blue or yellow coloration. 

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