Diamond has several applications: it is used for decoration, to manufacture high quality stereo equipment in addition to being included in cutting instruments. It is therefore important to determine the quality of the diamond. This can be done a simple, nondestructive method that is known as Raman spectroscopy. This method is used in the characterization of the morphology of the molecules of carbon materials. The bands that are detected on the Raman spectrum are dependent on the frequency of the vibrations which is attributed to the bonds within the molecule. This technique has several applications in diamond science: it is used in the detection of doping, measurement of temperature by non-contact among other uses.
1.0 Introduction
At some point in history diamond was only regarded as decoration by the wealthy and famous. They flaunted it on their wrists, fingers, necks and ears. Wars were paid for by this precious metal and one’s status in society was gauged by the diamonds that they had. However with time, diamond which is a tetrahedral arrangement of carbon found its way beyond decoration. It is used is cutting instruments, thermal management systems and in high end stereo systems. With the increase of the demand for diamonds, there are several methods that scientists have come up with in the recent years to grow “synthetic diamond”.
One of the methods is known as chemical vapor deposition which relies on high substrate temperatures (about 9000 C). It entails the deposition of diamond like substances on a material such as glass or plastic. Once the film has been deposited, it is important to find out the quality of the diamond that has been made. This can be achieved through several methods such as X-Ray diffraction and transmission electron microscopy. However both of these methods are time consuming and involve destruction of the sample. Based on these challenges, there is need to come up with an efficient method.
Raman Spectroscopy offers a way of determining the quality of the diamond. It is fast, does not involve the destruction of the sample and allows the characterization of ambiguous forms of carbon deposits. It requires little preparation of the specimen and can produce resolved maps of different forms of carbon in a specimen. The technique utilizes a laser beam that is focused through a microscope onto the sample surface. The scattered light is directed to a spectrometer which later disperses the light onto a charge coupled detector. Based on the use, the choice of the laser wavelength can be varied. Other choices include: near infra-red, visible light and ultra violet light. This paper shall therefore analyze the use of Raman spectroscopy for diamonds.
2.0 Relationship between the spectrum of graphite and amorphous carbon and that of diamond.
Raman spectroscopy is particularly useful when it comes to the characterization of the morphology of the molecules in carbon materials. This is because of its sensitivity to the symmetric covalent bonds which exhibit little or no dipole moment. Each band within the Raman spectrum corresponds to a specific vibrational frequency of a bond found within the molecule. The frequency of the vibration determines the position of the Raman and is dependent on the orientation of the bonds in addition to the weight exerted by the atoms on either side of the bond. For instance, the Raman spectrum that is produced by diamond is different from the one produced by silicon or germanium in spite the fact that they share the same configuration of the tetrahedral structure. The heavier atoms of silicon and germanium slow down the frequency of vibration hence they result in lower Raman spectra.
In order to understand the interpretation of the Raman spectrum of diamond, it is imperative to understand that of both graphite and amorphous carbon. It has been shown that single crystal graphite produces a single peak at 1575cm-1 while other forms of graphite produce a second peak at 1355 cm-1. The intensity of the second peak increases in relation to that of the first peak depending on: the increase in amount of disorganized carbon and the decrease in the crystal size of the graphite.
The first peak is called the G-peak while the second peak is known as the D-peak. The D-peak is caused by the breakdown of the solid state Raman selection rules which hinder it from appearing from the spectrum of the perfect crystal. Amorphous carbon is made up of unstructured mixtures of sp2 (as in graphite) and sp3 (as in diamond).the ratio of sp2/sp3 is dependent on the deposition conditions. When studying a substance comprising of two components such as amorphous carbon, it is important to understand the relative polarizability of each component. The П bonds formed by the sp2 hybrids exhibit a greater tendency to polarize as compared to σ bonds formed by sp3 hybrids hence have a larger Raman cross section.
The Raman spectrum of amorphous carbon is similar to that of graphite as with both G and D peaks with greater peak widths based on the ordering of the sample. The ratio of the D to the G peak provides information on the amount of disorder in the sample.
Figure 2: First order spectrum of graphite
3.0 Raman spectrum of diamond
3.1 Microcrystalline diamond
Microcrystalline diamond is ideal for microelectronic mechanical devices because it has a smooth surface, has a homogenous film and possesses a high Young Modulus. The Raman spectrum of microcrystalline diamond can be inferred from different contributions of 1332cm-1of the diamond phonon and the D and G peaks of amorphous carbon. The relative intensity of the diamond to non-diamond signals is dependent on the wavelength of the excitation. It is not clearly understood whether this is as a result of the enhanced resonance of the sp3 component or of the decreased enhancement of the sp2 component.
Experiments show that there is an increase in the D: G ratio with the decrease in the excitation wavelength. It is therefore important that the excitation wavelength be chosen carefully based on the type of experiment being conducted. If analysis of a micro crystalline sample is done using a near infrared signal, the diamond line would not be visible but those from the amorphous carbon would be strongly detected.
Excitation with ultra violet rays would however produce a different result. It would therefore result in a small diamond signal hence proving that the sample is indeed micro crystalline diamond and not amorphous carbon. However in order to differentiate between two diamond samples, it would be better to use near infrared radiation as compared to using U.V since the emissions from the sp2 would be more enhanced.
The relative intensity of the D peak as compared to that of the G peak is used as a crude measure of the phase purity of chemical vaporized diamond. This comes in handy when relating the conditions under which the deposition was done to the quality of the sample.
Figure 2: Raman spectrum of natural diamond showing active mode at 1332cm-1.The Raman signal intensity is taken in arbitrary units (a.u)
3.2 Nano crystalline diamond
As the size of the diamond decreases from micrometers to nanometers, the analysis of the Raman Spectrum becomes slightly more complex. When using a large crystal of diamond as a sample where the size of the lattice is infinite, only a given set of phonons remains active hence the spectra that are produced are relatively simple to interpret.
In nano-crystals, the selection rules are more active hence giving rise to many vibration modes. This therefore gives rise to several spectra that make it difficult for interpretation to be carried out. The peaks appear to be shifting and are asymmetrical. In addition to that, there are new spectra that are created as a result of the disorder that is created during the vibration. The shifting of the peaks and asymmetry has been observed at 1332 cm -1 mode of shock synthesized nano crystallized diamond but in diamond films, a shift in the peaks is attributed to the compressive or the tensile stress within the film.
Experiments involving nano crystallized diamond particles have shown that in addition to the G and D peaks that appear, there is also appearance of a broad signal at the 1150cm-1. Initially it was speculated that the appearance of this signal was as a result of the disturbances caused by vibration. However it is now speculated that the peak arises from sp2 hybridized structure such as poly-acetylene molecules.
3.3 Diamondoid hydrocarbons
If the size of the diamond crystals is further reduced then the resulting molecule is more closely related to a hydrocarbon than diamond. The resulting molecules have a structure that can be super imposed on the diamond lattice and are known as diamondoids. They are found in petroleum and exist in such minute concentrations that it is often difficult to isolate them.
The Raman spectra of the diamondoids contain several lines just as exhibited by hydrocarbons such as cyclohexane. Through the use of computational chemistry, it is possible to calculate the vibrational frequency of these peaks and therefore assign the peaks.
4.0 Applications of Raman spectrum in diamond science
4.1 Non-contact measurement of temperature
It is often possible to measure temperature by non-contact methods. Some of the applications that require measurement of temperature in situ during chemical vaporized diamond growth include: accurate measurement of diamond temperature during in an ultra-high vacuum chamber and the measurement of the temperature in a diamond anvil pressure cell. Optical pyrometry is often used although the accuracy of the technique is pegged on the emissivity of diamonds that is in turn dependent on the defects and impurities.
Raman spectroscopy provides an exact non-contact solution. This can be done through two methods. The first makes use of the Stokes and the anti-Stokes peaks as a ratio given as a frequency of the laser and the phonons respectively. If detailed and accurate measurements are provided, this method can provide the temperature of the given sample. The other method involves the deduction of the temperature from the change of the position of the Raman line. This method offers a great alternative because the change can be detected on a curve fitting 0.3cm-1 translating to a precision of temperature within _ or + 10K.
The temperature is calculated using the formula below:
IAS/IS= (ωI + ω P /ωI - ω p)4 γЂω/ kT
Where:
AS- anti stokes
S- Stokes
ω I – laser frequency
ω P - phonon frequency
T is less than 273 K
4.2 Probing defects and annealing
Experiments have been done in which the defects on diamond were introduced at high temperatures using He- ion radiation. The damages diamond films are then studied under Raman spectrum in order to detect the difference in the spectra obtained. It has been observed that under such conditions, new peaks appear at 1490cm-1 and 1630cm-1. It is speculated that the appearance of these peaks is as a result of the vacancy in the diamond films and the interstitial defects. At higher levels of damages, broad spectrums have been known to appear at 1400 and 1200 cm-1 respectively which are consistent with the peaks of phonon density of diamond. This happens because the ion beam creates amorphous zones within the diamond in which the selection rules are broken hence allowing for the observation of the phonon density of diamond.
The identification of the peaks at 1490 and 1630 cm-1 respectively mean that Raman spectroscopy can be used to study the effects of annealing diamond point defects. The presence of such spectrum can be used to detect the presence of residual defects that have been introduced by ion implantation. As a result it is possible to detect the defects that are as a result of doping and those that are as a result of damages to the diamond. The unique features of the spectrum could also be used to study the recovery of the diamond lattice after the implantation of damage or an impurity. The spectrum also comes in handy in the interpretation of the spectra for nano crystallized diamond.
4.3 Doping
This is an important aspect when dealing with diamond hence researchers have devoted time to find out how Raman spectroscopy can be used in the assessment of the level of doping. The signatures are often subtle owing to the interaction between the vibrational frequency of the diamond lattice and the electronic continuum states that are as a result of the presence of dopants.
The Raman spectrum is most sensitive to the concentration of the dopant although it is also sensitive to the interaction between the diamond lattice and the electronic continuum. The detection is further complicated by the fact that the morphology, growth rate and size of the crystallite are all sensitive to the amount of dopant in the mixture. In spite of the complications, studies have shown that Raman spectrum can be used in the detection of boron doped diamond.
4.4 Measuring phase purity and crystalline perfection of chemical vapor derived diamond (CVD)
Given that the Raman spectrum of CVD diamond can be attributed to the diamond and non-diamond components, the ratio can be used to determine the phase purity of CVD diamond films. For instance the observation of the Raman spectrum at 1332cm-1 without another peak at 1500-1550 cm-1 is taken as an indication of the presence of pure diamond with literally no non diamond components present. This is especially if the measurement is carried out using infra-red excitation which enhances the sensitivity of the Raman spectrum to the sp2 component.
In conclusion, Raman spectroscopy has several applications in diamond science. Studies should be carried out in order to enhance understanding of the merits and demerits of the technique.
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