Diamond Stars

When Ms. Jane Taylor composed the famous lyric "diamond in the sky" about 200 years ago, she could not have imagined that her figurative poetry would foretell the monumentous scientific discovery made in the 20th century. As it turns out, real diamonds are not only in the sky, but it may even be the most common solid in the universe. In addition, as will be introduced in Chapters 2 and 3, diamond is also a very unique and an essential material for a myriad of different applications. Thus, the most abundant material is also the most versatile. Diamond is truly the king of all jewels.

Carbon is the fourth most abundant element in the universe, after hydrogen, helium, and oxygen. However, these more abundant elements are gases, so carbon becomes the most abundant solid in the universe. The majority of carbon is locked up deep inside of stars where the pressure is exceedingly high. Since diamond is the stable form of carbon under pressure, it is natural to assume that diamond is commonplace inside a large star.

Stars are large nuclear reactors that generate heat by fusing nuclei together. The first step of nuclear fusion is to merge hydrogen atoms in order to form helium atoms. For an ordinary star, this process may take a long time to complete. For example, the Sun

Diamond Nanotechnology: Synthesis and Applications by James C Sung & Jianping Lin

Copyright © 2009 by Pan Stanford Publishing Pte Ltd

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has been shining for 5 billion years and it will continue the process for another 5 billion years. During the sustaining period of hydrogen fusion, stars maintain its stability by balancing two opposing forces, an inward pulling gravity against the outward moving photons. However, when a star with a size comparable to our Sun exhausts its nuclear fuel, it will shrink to become a white dwarf. Inside such white dwarfs lie colossal mines of diamond and the more massive the dwarf, the more plentiful its core of diamonds. A typical white dwarf is about a million times more massive than that of Earth; hence, chances are that most white dwarfs contain a mine of diamond much larger than the entire Earth.

In 1969, Saslaw and Gaustad proposed that diamond might exist in space. In 1987, Levis and others identified nanodiamonds 3-5 nm in size in meteorites. Such diamonds belongs to a rare clase known as lonsdaleite (hexagonal diamond). The most common polymorph of diamond from both natural and synthetic origins is the cubic diamond. Hexagonal diamond is often formed by shock wave transformation (e.g. the collision of two meteorites) of the common form of graphite — the hexagonal graphite. In contrast, cubic diamond must take time to form under sustained conditions of high pressure and high temperature, such as inside a large star.

Diamonds have been found in the sky based on the spectra of interstellar light and the measured absorption spectra of nanodi-amond. For example, the surface of a nanodiamond is often terminated by the absorption of hydrogen. These carbon-hydrogen (C-H) bonds may vibrate to emit or absorb electromagnetic radiation with a characteristic wavelength. Matching extraterrestrial light with lab measurements of diamond spectra provides clues to the existence of diamond in the sky (Figs. 1.1 and 1.2).

Nanodiamond may also form inside a giant star that undergoes an explosion to form a supernova. However, more commonly, nanodiamond particles may be formed when carbon-containing molecules collide with one another in interstellar dusts. Most of these nanodiamond particles contain hydrogen terminated bonds on its surface. The C-H bonds vibrate at a characteristic frequency that can shed light on the size and structure of the interior nanodiamond.

Starlight may also be compared with the frequency of diamond lattice resonance. There are only a handful of stars that lie within 20 light years from the Earth. In 1998, astronomer Steve Kawaler

Waveniimber (cm I

Figure 1.1. The similarity between an emission spectrum from an interstellar object and the absorption spectrum from stretching C-H bonds on the surface of nanodiamond (top diagram). The bottom diagram demonstrates that the absorption spectrum is dependent on the size of the nanodiamond. Source: Data provided by Huan-Cheng Chang.

.1050 2950 2K50 2750 2650 Frequency (cm 1 )

Figure 1.1. The similarity between an emission spectrum from an interstellar object and the absorption spectrum from stretching C-H bonds on the surface of nanodiamond (top diagram). The bottom diagram demonstrates that the absorption spectrum is dependent on the size of the nanodiamond. Source: Data provided by Huan-Cheng Chang.

discovered that one of them was a diamond star (BPM37093) that emitted the characteristic resonance frequency. This star is located 17 light years from the Earth. It belongs to the Constellation Centauras which is visible only from the southern hemisphere. This diamond star is 12,800 km in diameter and is almost the same size as Earth. However, it is a white dwarf and is the ruminant of a

Figure 1.2. Idealized nanodiamond structures that may be formed by collisions of carbon containing molecules in space as inferred from their emission spectra. Source: Data provided by Huan-Cheng Chang.

sun-like star after it exhausted its fuel for nuclear fusion. Such a cinder of dead star is very dense, and a teaspoonful of material may weigh over one ton.

The surface of this diamond star reaches about 12,000°C twice that of our Sun. It emits a greenish blue light that reveals the characteristic absorption spectrum of diamond. It would appear that the diamond it contains might also be blue in color. As most diamonds found on Earth are either colorless or yellowish, blue diamonds are extremely scarce. The blue color is caused by the incorporation of minute amount of boron atoms that absorb the yellow color. This could be formed in conjunction of carbon during nuclear synthesis. Boron-doped diamond is an electron-receiving (P-type) semiconductor due to the presence of electron deficient holes. Boron-doped diamond may out perform boron doped silicon or other P-type semiconductors in hole mobility and in electrical conductivity.

The famous Hope diamond is the largest blue diamond known. It was originally embedded in the forehead of Rama-Sita in an Indian temple. The French adventurer J. J. Tavernier adventured to India many times, and at the age of 80, he made the last trip to India where he stole the blue diamond from Rama-Sita. Upon his return to France, he sold this 112 carat diamond to King Louis XIV. This blue diamond was eventually passed on to Louis XVI who was beheaded during the French Revolution. In 1792, the blue diamond was robbed away from the palace by a mob. When it reappeared in 1839, it was re-cut into a much smaller 44.5-carat gem. This blue diamond was said to carry a curse of Rama-Sita that led to tragedy for many of its owners, including the death of Louis XVI.

Eventually, the blue diamond fell into the hands of the English banker Henry Hope and hence it bears his name since. In 1958, the American gem collector Harry Winston presented it to the Smithsonian Museum in Washington, D.C. The Hope diamond has become the most visited attraction for tourists who visit the museum (Fig. 1.3). Although the Hope diamond may be the most glamorous gem ever existed, it could easily be dwarfed by a chip from the blue diamond star of BMP37093.

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