Carbon Application Note 01

Carbon 01 Impact of Raman Spectroscopy On Technologically Important Forms of Elemental Carbon Pure carbon can occur in many forms reflected While diamond is the hardest substance in by the nearest neighbour bonds, and by mid- nature, it is actually a metastable form that range and long-range ordering of the solid. precipitates from high pressure, high Different forms of carbon are being exploited in temperature melts. Diamond is a unique newly engineered applications. For example, material because of its physical properties, diamond can be doped and used as the which include: substrates for integrated circuits with properties superior to silicon, and hard carbon . high strength films are being used to coat computer discs. optical transparency Raman spectra have been shown to be . electrical insulation (when pure) uniquely diagnostic of the many forms of . semi-conductivity (when doped) carbon, even when the material is present in . high thermal conductivity small quantities or thin films. The following . corrosion resistance. discussion should provide an aid to the use of Raman spectroscopy for characterising Because of these properties there has been carbons in the design, monitoring, and control interest in utilising diamond in engineered of manufacturing processes. The carbon atom materials. Diamond fibres (and graphite fibres, has four valence orbitals, which can hybridise as well) are used in metal or polymer matrix to form sp2 and sp3 orbital combinations. The composites for reinforcement. The high sp2 orbital forms planar structures with 3-fold thermal conductivity and the robustness of symmetry at the carbon atom, and is found in diamond makes it an ideal candidate for heat aromatic molecules and in crystalline graphite. sink substrates which are used for the The sp3 orbital forms the basis for 3- dissipation of heat generated by high power dimensional, tetrahedrally bonded carbon as in micro or opto-electronics devices such as laser methane (CH4) and in diamond. The most diodes. Diamond doped with nitrogen, boron, well-known forms of elemental carbon found in or phosphorous becomes the semi-conducting nature are graphite and diamond. In recent material of choice for electronic devices such years exotic sp2 forms known as Fullerenes, as transistors. Due to its high reflectivity in the and including the carbon nanotubes, have also UV range, diamond could be used in the future drawn interest. Raman spectra of some of the for realisation of UV optics (gratings, mirrors). common forms of carbon are shown in Figure 1. The salient features of these spectra will be However, as long as the only method to grow discussed below. diamond requires high temperatures and pressures, this is not an economically feasible Diamond material for most applications. Diamond (D) is formed by an infinite extension of sp3 carbon-carbon bonds. Over the last 10 years many workers have demonstrated the ability to grow diamond films in CVD (chemical vapour deposition) reactors under conditions close to ambient. However, since diamond is not a thermodynamically stable phase, much sp2-bonded carbon often accompanies the diamond deposition. The sensitivity of Raman spectroscopy to even very small amounts of sp2 carbon makes it the technique of choice to study these films. Figure 1. 1/4 Carbon 01 Aside from the ability of Raman spectroscopy electrical conductivity and a 0eV bandgap. Van to demonstrate the presence of diamond in der Waals bonding between the planes is weak CVD films, there is also information in the which is why the planes can lip, and the Raman band of the diamond itself. The Raman material can be used as a solid lubricant or as line appears at 1332 cm-1 in single crystal an additive to an oily lubricant (for valves, diamond. Under compressive and tensile mechanical seals, sliders...) stress in the film, the diamond line will shift respectively to lower and higher frequency. In Figure 1 shows the graphite first order phonon these films the line can additionally broaden at about 1580 cm-1 which is fairly sharp, due to heterogeneity of the sample. Workers although not as sharp as that of diamond. This using Raman spectroscopy to characterise line has been documented to move with stress these films can develop a quality index that and temperature, and because of the potential measures the following characteristics: for heating by laser absorption, stress- and temperature- induced effects can be difficult to . Diamond frequency separate. Diamond linewidth . Sp2 carbon background When the long-range order of the graphite lattice is disrupted, a second line appears at Figure 2 shows a typical Raman spectrum of a about 1360 cm-1. In a grinding study Tuinstra diamond film containing substantial amounts of and Koenig (J. Chem. Phys. 53, 1126, 1970) non-diamond carbon. The spectrum has been showed that the intensity of the peak at 1360 band-fit to quantify the contributions of the cm-1 scaled with the size of the crystal, as various components. It is also useful to note derived from XRD (x-ray diffraction). Material that Yarbrough and Roy (MRS 1988 with shorter range order will be denoted Symposium, Diamond and Diamond-like microcrystalline graphite (pG). A typical Material Synthesis, ed. Johnson, et.al, pp.33- spectrum is shown in Figure 1.The relative 38) have noted that under CVD conditions that peak intensities of these two lines at 1580 and are marginal for the deposition of diamond, a 1360 cm-1, denoted respectively as G and D new phase is detected. XRD (x-ray diffraction) lines in the literature (graphite and defect), can in this article indicates nanocrystalline vary enormously. diamond (nD), but the Raman spectrum is completely different from anything that has In addition, the D band will shift with excitation been seen in other carbons. These materials wavelength, appearing at lower frequencies show new diffuse bands at ca. 1140 and 1450 with longer wavelength excitation lasers (Wang, cm-1. Alsmeyer, and McCreery, Raman Spectroscopy of Carbon Materials: Structural Basis of Observed Spectra, Chemistry of Materials 2, 557- 563 (1990). Since the relative intensity is determined by the degree of long range order of the lattice, it is not surprising that the linewidths of these lines also vary. Consequently, it has in recent years, been deemed more appropriate to calculate the ratio of the integrated intensities of the lines, using bandfitting algorithms to separate the components (LabSpec and DiskSpec softwares, Jobin Yvon). Figure 2. The Raman spectrum of many carbon materials, including some films and graphitized Graphite fibres. exhibits such extensive broadening and overlap between the two bands that only band- Graphite (G) is formed from stacks of infinitely fitting would permit appropriate use of the data. extended planes of fused aromatic rings. The π electrons in the planes provide for high 2/4 Carbon 01 Carbons showing this behaviour will be Engineered Carbons denoted disordered carbon (DC). Spectra of two samples are shown in Figure 1. Raman spectra can be used to quantify the degree of structural order as well as orientation In addition, it has been shown (Rouzaud, et.al., in technologically important carbons. Spectra Thin Solid Films, 105, 75-96, 1983) that some of carbon fibres manufactured from pitch and carbon films require a second defect band at PAN (polyacrylonitrile) showed not only a about 1500 cm-1 to flatten the residual of the difference in long range order but polarisation- band-fitted Raman spectra. These authors dependent intensities related to the orientation attribute this band to the presence of interstitial of the graphitic planes in the fibers (Adar and defects between the graphitic layers or Noether, in Microbeam Analysis-1 983, 269- between structural units in the layers. In 273). contrast the D band at 1360 cm-1 is attributed to in-plane defects between basic structural Hard carbon films are being used universally units. in the manufacture of hard discs in the computer industry and on read-write heads. When band-broadening and overlap in carbon Engineering the parameters of the deposition films is so extensive that separate bands are equipment is controlling the physical and not discernible before the band-fit, the material tribological properties of the films. Raman is called "diamond-like carbon" (DLC). This spectra are being used in this industry as a material was originally termed "diamond-like" rapid confirmation of the properties of the film because of its relative transparency and because the spectra have been correlated with electrical insulation, properties that result from the tribological properties of interest (J. Ager, the limited size range of the graphitic structural IEEE Trans. Magn, 1992). units and their π systems. DLC films can be used as passivation layers in magnetic storage Such hard films can also be used to improve devices. the wear properties of cutting tools and surgical/scalpel blades, as well as to passivate A material called "Glassy Carbon" (GC) has medical implants. Indeed, their chemical the intensity ratio of the D and G bands inertness makes them bio-compatible. reversed (D band higher), with the bands fairly sharp. Ironically this material is composed of Figures 3a and b exhibit spectra of carbon highly crystalline regions, but with very small films that require a fit with 2 and 3 bands dimensions (private communication, Wm. respectively. The film whose spectrum is White, Penn State U.). A spectrum of glassy shown in Figure 3b has some interstitial carbon is shown in Figure 1. defects as discussed above. It is worthwhile to make another comment on the dependence of these spectra on excitation Buckeyballs or Fullerenes, Nanotubes wavelength. Because the D and G bands are related to different sizes of graphitic structural After the theoretical proposal that 60- and 70- units, enhancement of the two Raman bands atom clusters of carbon might exist in nature, that depends on the proximity of the laser fullerenes were isolated from carbon soot wavelength to electronic transitions, is (Kroto; et al., Nature 318, 162, 1985).

Load more