4.3.1
Infrared Spectrometer
FTIR spectroscopy is used to study the one, two and three phonon region of the diamond spectra. Intrinsic diamond does not absorb in the one phonon region (below 1332 cm−1) but defects do. It is particularly useful for looking at different forms of nitrogen and single substitutional boron.
IR spectra were obtained with a PerkinElmer FTIR System Spectra GX spec- trometer and were taken at room temperature. The spectrometer consists of an interferometer, a schematic of which is shown in figure4.11. An interferometer is
(a) Oscilloscope image of U1 spectrometer output triggering AOM. The U1 pulse, shown in blue, is 1.5 ms long.
(b) Oscilloscope image of end of trigger to laser fully on. ‘a’ indicates the U1 pulse switching on and ‘b’ indicates the laser being on. The time between the start of the U1 pulse and laser fully on is ∼882 ns.
Figure 4.10: Oscilloscope measurement of AOM switching efficiency. Blue is the trigger from the pulse spectrometer which triggers the AOM and yellow is the laser response.
Figure 4.11: Schematic of an interferometer. Moving mirror scans with a fre- quency of∼10 kHz.
composed of a broadband light source. This light is split into two beams using a beam splitter; a half-silvered mirror which is coated in aluminium and allows half the light to be transmitted and half to be reflected. These two beams of light are then reflected by a stationary mirror and a moving mirror respectively. The two beams are then recombined. The result is then shone onto the detector. This gives an interferogram which can be Fourier transformed to produce the source spectrum. The same is done with a sample in place. The two Fourier transformed spectra can be compared and the absorption of the sample will be given by the difference.
The two sources used were a tungsten halogen lamp in conjunction with a quartz beam splitter for the near-IR range and a temperature stabilised wire coil at 1350 K in conjunction with a potassium bromide (KBr) beam splitter for the mid-IR region.
The IR spectra obtained were fitted with the ‘SpectrumManipulator’ program for Matlab, detailed in Dale’s thesis [135]. This program compares the absorption spectrum with spectra of samples with known concentration in order to calculate concentrations of the species within.
4.3.2
UV/visible Spectrometer
The UV/visible absorption spectrometer used was a PerkinElmer Lambda 1050 UV/vis/NIR spectrometer which has a range of 175 nm to 3300 nm. When tak- ing measurements below room temperature this was used in conjunction with an Oxford Instruments optical cryostat and an Oxford Instruments ITC5035 temper-
Figure 4.12: Schematic of light path within UV/visible spectrometer.
ature controller. The two sources used were a deuterium halogen lamp for the UV region and a tungsten halogen lamp for the visible and NIR region.
Unlike the FTIR spectrometer discussed previously, a UV/visible spectrometer measures the absorption of each wavelength in turn. A schematic of the spec- trometer is shown in figure 4.12. It consists of a broadband light source which is spread into a rainbow using a diffraction grating. A single wavelength can then be selected by using slits. The size of the slits used and the number of lines on the grating determine the resolution of the scan. Once the wavelength is selected, the beam of light is split in two by a chopper. One of the resulting beams goes through the sample and the other passes though the reference. The spectrum is created by measuring the absorption by the sample at each wavelength and comparing this to the reference beam.
The absorption was measured. The intensity of light is measured differently de- pending on the range. A photomultiplier tube (PMT) is used in the range from 170 to 800 nm. An indium gallium arsenide (InGaAs) detector is used in the range from 800 to 1500 nm. A zinc selenide (ZnSe) detector is used in the range from 1500 to 3500 nm.
4.3.3
Raman/Photoluminescence Spectrometer
Both Raman and photoluminescence (PL) measurements were performed with a Renishaw inVia Raman Microscope. Cooling to liquid nitrogen temperatures was performed with a Linkam THMS600 optical cryostat. A selection of different excitation wavelengths were used as detailed in table 4.2.
A Raman/photoluminescence spectrometer works by shining a laser on to a sample and interpreting the scattered light. A schematic of this is shown in figure 4.13. The laser is focused down to a sharp point using a lens and a pinhole. It is then reflected up to a dichroic mirror which reflects the laser but not the light at different frequencies. The dichroic reflects the laser light onto a second mirror
where it then is focused by an objective lens onto the sample. The inelastically and elastically scattered light is then collected by the objective and is reflected to the dichroic. At this point much of the light intensity is lost because it is not scattered towards the lens. The dichroic only transmits light which is not at the wavelength of the laser, and the laser light that was inelastically scattered can pass through. An edge filter is also used at this point to remove any remaining laser light. The luminescence spectra of the material is separated out into its different wavelength components using a diffraction grating. A lens allows one wavelength to be measured at a time by a charge-coupled device (CCD). The wavelength versus the photon count can be plotted. In the case of a Raman experiment this can give information about the strain and isotopic content of the sample and in the case of a PL experiment this can give information about the defects present. The Raman line is a signal produced by Stokes scattering. It is equivalent to the laser wavelength minus the characteristic phonon of energy of the material being studied. It can be used to normalise spectra. The signal is produced by the inelastically scattered light, however, this forms a minority of the light collected from the sample, approximately one photon in 100000. The photoluminescence (PL) signal also consists of significantly less light that the reflected laser light. It is imperative to remove as much of the laser light as possible in order to detect the smaller Raman or PL signal.
4.3.4
Fluorescence and Phosphorescence Imaging
Fluorescence and phosphorescence imaging was performed by a DiamondViewTM.
The instrument images the fluorescence and phosphorescence of diamond samples after ultraviolet illumination by a xenon flashlamp [140].
Figure 4.13: Schematic of light path within Raman spectrometer.
4.4
Cathodoluminescence
A cathodoluminescence experiment involves studying visible light emitted from a sample which has been bombarded with electrons. For more details about the theory of this refer to section 3.8.
The sample surface is bombarded with electrons using a high voltage cathode gun (at 4 to 30 keV) and the resulting emitted photons are studied with an visible light microscope. The sample is kept in a vacuum.
A cathodoluminescence microscope was used to qualitatively study the samples discussed in Chapter 6. The CL images were taken with a cold cathode, CITL 8200 Mk 3A mounted on a Nikon Optiphot petrological microscope at the Grant Institute at the University of Edinburgh.