CHAPTER - 3 CHARACTERIZATION OF SINGLE CRYSTALS USING DIFFERENT TECHNIQUES. 3.1 Instrumentations used for characterization

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1 CHAPTER - 3 CHARACTERIZATION OF SINGLE CRYSTALS USING DIFFERENT TECHNIQUES 3.1 Instrumentations used for characterization Crystals grown using any method of crystal growth have to be characterized for a better understanding of its microscopic nature, followed by its application. Thus a variety of techniques are to be followed according to the need and applications. The following techniques are used for the characterization of single crystals in the present study. (i) Crystal structure by single crystal X-ray diffraction technique: This technique is used to measure lattice parameters and for finding out the crystal structure of the material under process. (ii) FTIR and FT Raman spectral techniques: These techniques are used to identify and assign the vibrational band of frequencies present in the compounds and complexes under study. (iii) Powder SHG measurements by Kurtz method: The nonlinear optical activity of the material can be tested using these measurements. (iv) UV-Visible-NIR spectral analysis: This technique is used to find out the UV cut-off and transparency of the material in the UV-visible- NIR region from the absorption data, a basic criterion for nonlinear optical activity. (v) Thermo gravimetric analysis and differential thermal analysis: The chemical stability and thermal hardness of samples under study can be tested using this technique. (vi) Elemental analysis: Chemical composition of the crystals can be found by using Inductively Coupled Plasma (ICP) / Optical Emission Spectrometer(OES). 29

2 3.1.1 Single crystal X-Ray diffraction analysis Data collection can be carried out using a variety of single crystal diffractometers with minimal differences in their design. The present study makes use of a single crystal diffractometer, details of which are given below. (i) ENRAF NONIUS CAD-4 Single crystal diffractometer with CCD (area detector) Data are collected at room temperature on ENRAF NONIUS CAD-4 Single crystal diffractometer with CCD (area detector). The absorption correction was done using the method inserted in SHELXL-NT V5.1 [36]. The structure was solved by direct methods using the software SHELXL-NT V5.1. (ii) Auto indexing (cell parameter determination and symmetry) In the beginning of the procedure for data collection the auto indexing along with the determination of consequent cell parameters is performed using an algorithm operating on difference vectors from the input reflections. A reflections array with a maximum of 999 reflections is used. However auto indexing and determination of crystal orientation requires only a small fraction of this number. Usually 25 reflections are the minimum with reflections as the optimal. At the end of the data collection least squares calculation is used to have the best cell parameters. In this case, all the reflections with the intensities >10sigma are used. But these programs do not report their θ values, as these reflections are normally found to have a very high number. (iii) X-ray crystal structure solution and refinement (Siemens, Philips,CAD4) The structure is solved using direct methods with SIR2004 program and further refined by full matrix least squares with SHELX97 [37]. All the 30

3 non hydrogen atoms are refined anisotropically while the hydrogen atoms, found in a difference Fourier map, are refined with isotropic thermal parameters. (iv) Crystal decay In the time decay correction the output reflection file (or merged file if more than 1 run was processed) is read and the observation with the same hkl, direction cosines, swing angle, and X,Y position are used to compute the time decay. The hkls swing and position must be identical. Symmetry equivalents and the same hkl at different psi are not used for this purpose. The decay of each reflection (Fsquare versus time in hours) as fit to a least squares line is stored, as well as its sin (θ/λ). After all multiple observations have been processed this way, decay rate versus [sin (θ/λ)] square is fit by linear least squares, and the slope and intercepts are used to correct the intensities of all reflections. So it is clear that the frames are not measured at intervals of time to check the eventual decay, and the eventual decay is always calculated and corrected at the end of the data collection based on the same observation measured in different frames [38] FTIR instrumentation This instrument is most useful for identifying chemical substances, both organic and inorganic [39]. It can also be used for the quantitative analysis of the compounds of an unknown mixture. It can be applied to the analysis of solids, liquids, and gases. Sample preparation is the most time-consuming, error-prone and labour-intensive step of an IR analysis. The easy availability of computers and high energy sources such as lasers has made a remarkable impact on the instrumentation of FTIR spectrometer. Figure 3.1 shows the block diagram of a Fourier transform infrared spectrometer. It consists of a movable mirror, a stationary mirror and a beam splitter. 31

4 Source Beam splitter Stationary mirror Movable mirror Sample position Fig. 3.1 Block diagram of FTIR spectrometer Radiation from an infrared source is passed on to the mirror through a beam splitter, which partly transmits and partly reflects the incident light. The transmitted and reflected beams fall on the stationary and movable mirrors respectively. These beams, which are sent in two directions at right angles to each other, are brought together after reflection to interfere with each other. Generally the beam splitters used in the mid-ir region are thin films of germanium or silicon deposited on cesium iodide or bromide, sodium chloride, or potassium bromide. The modern double beam infrared spectrometer consists of four parts (40). i. radiation source ii. sampling area iii. monochromator and iv. detector. Infrared radiation is produced by electrically heated sources such as Nernst filament, globar sources and nichrome coil. Mercury arc lamp and 32

5 tungsten filament lamp also serve as sources. A tunable carbon dioxide laser also serves as an infrared source [41, 42]. An important property of the laser source is the radiant power available in each line, which is several orders of magnitude greater than that of blackbody sources. Thus, laser sources are useful for quantitative determination of an important species. In recent years tunable dye lasers are emerging as precise sources. The sample area of a precision spectrophotometer accommodates a wide variety of sampling accessories such as gas cells and micro cells. Reference and sample beams enter the sampling area and pass through the reference cell and sample cell respectively. Opaque shutters mounted on the source housing permit blocking of either beam independently. A monochromator is used to separate polychromatic radiations with a suitable monochromatic form. A monochromator has three important funcitons [43]. (i) It disperses the radiation into its wavelength components. (ii) It restricts the radiation falling on the detector into a narrow wave number range. (iii) It maintains the energy incident on the detector to an approximately constant level when no sample is present throughout the wavenumber range of the instrument. A detector with a high sensitivity and quick response time is used to measure the radiant energy by means of its heating effect [43]. Two common types of detectors generally used are thermocouple and bolometer. There are other faster response time detectors such as Golay neumatic detector, pyroelectric detector and photo conducting cells, which are superior to the conventional detectors. The radiant energy received by the detector is converted into a measurable electrical signal and is amplified by amplifiers. A recorder or a 33

6 plotter, which records the transmittance of the sample, as a function of the wavenumber, registers the amplified signal. The spectrum is recorded with frequency in wavenumber units (cm -1 ) on the abscissa against percent transmittance (%T) on the ordinate. Intensities are usually labeled as strong, very strong, medium strong, medium, weak medium, weak etc. The spectrophotometer is calibrated using a substance (polystyrene) whose exact infrared absorption band positions are known. The spectra obtained with spectrophotometers having double monochromator have higher resolution. Optical diagram of an FTIR instrument is as shown in Figure 3.2. HeNe Laser Glass window Desiccant box Shield Source Coil Interferometer flat mirror Beam Spiltter Interferometer scan mirrors IR detector Fixed toroidal windows IR beam Laser fringe detector Optical stop Interferometer Flat mirror KB Windows Sample area Purge cover Adjustable toroidal window Fig. 3.2 Optical diagram of FTIR instrument 34

7 3.1.3 FT Raman instrumentation Fourier transform spectrometer provides maximum Raman collection efficiency and uses the light most efficiently in converting to a spectrum. The FT Raman spectrometer has a near IR laser, a Michelson-Interferometer and Fourier transform processors [44]. The near IR laser is used for the measurement of the spectra while the interferometer and Fourier transform processor are used for the collection and analysis of the scattered light. Wavelength separation in Raman spectroscopy is based on either interference using a Michelson interferometer or diffraction (dispersive) using a grating monochromator (Figure 3.3). Concave mirrors גּ גּ 2 1 Entrance Slit Reflecting grating A Exit Slit B Focal plane Fig. 3.3 Grating monochromator The spectra are exclusively obtained with and Nd: YAG laser of wavelength 1064 nm. A tunable filter comprising of an acousto-optic device disposed along the optical path and positioned so as to intersect the out coming beam from the laser to perfect its monochromaticity. So, the selected laser radiation is highly monochromated and then focused on to the sample. Light reflected and scattered from the sample is directed to a Raman spectrometer for 35

8 analysis. The spectrometer has a back scattering arrangement which removes the Rayleigh scattered light and allow only the Raman scattering to pass. The noise associated with the intense Rayleigh scattering is distributed over the entire spectrum in the Fourier transformation step and seriously degrades the desired Raman spectrum [43]. So a laser line rejection filter is used to suppress the strong Rayleigh scattering to obtain good quality FT Raman spectra. The latter then passes through a Michelson interferometer identical to that used in FT infrared spectroscopy. The output of the interferogram is collected and detected on a near infrared detector. The detector signal is digitized and Fourier transformed to generate a spectrogram as obtained in the infrared analogue. The spectrum is recorded as intensity of scattering versus frequency shift using software. A Fourier transform spectrometer, which can be utilized to record, the Raman spectrum is as shown in Figure 3.4. Sample Laser Beam Positioner Cool phototube Double monohromator Amplifier Discriminator Pulse Shaper Scaler RC Circuit Impedance Matcher Recorder Fig. 3.4 Block diagram of Raman spectrometer Monochromator and detector play a vital role in the recording of a Raman spectrum and are dealt with. Raman scattering process requires high 36

9 rejection of light especially when low frequency modes are measured. The most important property of any monochromator system used for Raman spectroscopy is its spectral purity [45]. This can be achieved using two or more monochromators in series in the path of scattered radiation. The present day monochromators used holographic grating and the most of the modern Raman spectrographs are dispersive in nature. Detectors are in general photo multipliers and the photon incident on the photocathode caused the emission of electrons. Nowadays detectors are charge coupled device (CCD) detectors. Charge Coupled Device Detector (camera) is used to detect the Raman spectrum. A CCD detector is a two dimensional array of very low noise silicon detectors. These detectors are like the chips used in modern digital cameras, but are of much higher specification and sensitivity. High performance CCD detectors should be cooled for optimum performance. A regular CCD is a front illuminated CCD. There are wide varieties of detectors depending on the type of laser system under use. They are (i) Open electrode CCD (ii) Back illuminated CCD (iii) Deep depletion CCD and (iv) Electron Multiplying CCD The back of CCD has no electrodes on it and the surface is clear of obstruction and hence sensitive. This is a back illuminated CCD and it cannot be used for near IR laser. A deep depletion CCD has a deep photoactive region, allowing it to measure photons that even pass through the detector. Signal levels (electrons) are multiplied or otherwise amplified to detect even less sensitive photons in electron multiplying CCD. The advantages of Raman spectroscopy over IR are 37

10 (i) It may be used for a wide variety of the samples (ii) No medium such as KBr or solvent is needed. (iii) Solid can be packed into a capillary tube as a powder. It may be packed in transparent glass or plastic containers. (iv) A large proportion of the vibrational IR spectrum of aqueous samples is being masked by the intense water signals. With Raman spectroscopic techniques aqueous samples can be performed with ease as Raman signals from the water molecule are relatively weak(46). The Raman spectra can be recorded for solids such as polycrystalline material or single crystal. A few milligrams of the solid samples are required. The crystal can be mounted in a goniometer on a glass or silica fibre. The spectra can be measured for different orientations of the crystal. When polarized incident radiation falls on a signal crystal, the output of the Raman spectrum varies depending on the direction or orientation of the crystal Instrumentation for NLO activity The Kurtz and Perry powder SHG method [47] is used (Figure 3.5) to measure the NLO efficiency of the materials under scrutiny. In this method, powdered sample is densely packed in a capillary tube and irradiated with high intense infrared beam produced by a Q-switched Nd-YAG laser of wavelength 1064 nm with a pulse width of 8 ns and a repetition rate of 10 Hz. Potassium dihydrogen orthophosphate (KDP) crushed into samples of identical size is used as reference material. The output of laser beam having the bright green emission of wavelength 532 nm confirms the second harmonic generation output. 38

11 HV Supply Q SWITCHED LASER 1055 EMI 9254 PHOTO MULTIPLIER PRE AMP ADYV A 102E RCA 925 PHOT TEXTRONIX 555 (2W) TRIGGE Ω (W) Ω CH1 CH2 (W) Fig. 3.5 Powder SHG instrument Second order NLO materials are used in optical switching (modulation), frequency conversion (SHG, wave mixing), and electro-optic applications, especially in EO modulators. All of these applications rely on the manifestation of the molecular hyperpolarizability of the materials. The variety of applications has led to the invention new nonlinear optical materials [48] UV Visible NIR spectral analysis To perform multi various tasks, there are hundred of spectrometers available in the UV Visible - NIR range from simple, inexpensive to complex and costly. Commercial instruments (UV Visible - NIR range) designed, to perform complicated tasks, which connot be done with simpler ones has been discussed in the cited references [49, 50]. 39

12 A schematic diagram of a simple typical spectrometer is shown in the Figure 3.6. The functioning of this instrument is relatively straightforward. A beam of light from a visible and / or UV light source (colored red) is separated into its component wavelengths by a prism or diffraction grating. Each monochromatic (single wavelength) beam in turn is split into two equal intensity beams by a half-mirrored device. One beam, the sample beam (colored magenta), passes through a small transparent container (cuvette) containing a solution of the compound being studied in a transparent solvent. The other beam, the reference (colored blue), passes through an identical cuvette containing only the solvent. The intensities of these light beams are then measured by electronic detectors and compared. The intensity of the reference beam, which should have suffered little or no light absorption, is defined as I 0. The intensity of the sample beam is defined as I. Over a short period of time, the spectrometer automatically scans all the component wavelengths in the manner described. The ultraviolet (UV) region scanned is normally from 100 to 400 nm, the visible portion is from 400 to 700 nm and the NIR portion is from nm. In recording the solid UV spectrum for the sample, the instrument used for taking liquid samples can also be used simply by changing the sample holder. Here the powder sample is powdered well and then a drop of liquid paraffin is added and made into a paste. This paste is smeared on a Whattman filter paper and the spectra are recorded. A blank filter paper which is cut to the same size as the sample paper is kept on the reference side. Otherwise it is left as such and the baseline correction is done with AIR as reference. 40

13 Light Source UV Diffraction Grating Mirror 1 Skit 2 Slit 1 Light Source Vis Mirror 4 Filter Reference Beam Reference Cuvette Detector 2 Lens 1 I 0 Half Mirror Mirror 3 Mirror 2 Example Beam Sample Cuvette Lens 2 Detector 1 I Fig. 3.6 Block diagram of simple UV spectrometer If the sample compound does not absorb light of a given wavelength, I = I 0. However, if the sample compound absorbs light then I is less than I 0, and this difference may be plotted on a graph versus wavelength. Absorption may be presented as transmittance (T = I/I 0 ) or absorbance (A= log I 0 /I). If no absorption has occurred, T = 1.0 and A = 0. Most spectrometers display absorbance on the vertical axis, and the commonly observed range is from 0 (1005 transmittance) to 2 (15 transmittance). The wavelength of maximum absorbance is a characteristic value, designated as λ max Thermal analysis Thermal analysis describes a set of techniques, which are widely used in both academic research and industry [51, 52]. These techniques are generally straightforward to use and are able to characterize a wide range of materials and their properties. 41

14 (i) Thermo gravimetric analysis The TG curve is a characteristic of a particular compound, because of physiochemical events, which occur under particular conditions over the temperature concerned. The technique is simple in concept and provides valuable qualitative and quantitative information concerning the existence and type of hydrate and solvate present. The TGA method [53] simply measures the weight change of a sample as a function of temperature or time (isothermal dehydration) generally called thermogram or a thermal decomposition curve [54]. Such analysis relies on a high degree of precision in three measurements: weight, temperature, and temperature change. As many weight loss curves look similar, the weight loss curve may require transformation before results may be interpreted. A derivative weight loss curve can be used to tell the point at which weight loss is most apparent. Again, interpretation is limited without further modifications and deconvolution of the overlapping peaks may be required. The analyzer as shown in Figure 3.7 usually consists of a high-precision balance with a pan loaded with the sample. The sample is placed in a small electrically heated oven with a thermocouple to measure the temperature accurately. The atmosphere may be purged with an inert gas to prevent oxidation or other undesired reactions. A change in sample mass causes a deflection of the beam, which interposes a light shutter between a lamp and one of two photo diodes. The resulting imbalance in the photo diode current is amplified and fed into coil E, which is situated between the poles of a permanent magnet F. the magnetic field generated by the current in the coil restores the beam to its original position. The amplified photodiode current is monitored and transformed into mass or mass loss information by the data acquisition system. Analysis is carried out by raising the temperature gradually and plotting weight against temperature. After the data is obtained, curve smoothing and other operations may be done such as to find the exact points of inflection. 42

15 Fig. 3.7 Thermogravimetric analyzer (ii) Differential thermal analysis A technique, which shares much in common with differential scanning calorimetry, is differential thermal analysis or DTA. In this technique [53] it is the heat flow to the sample and reference that remains the same rather than the temperature. When the sample and reference are heated identically phase changes and other thermal processes cause a difference in temperature between the sample and the reference (Figure 3.8) S R ΔT Single Heater Fig. 3.8 Apparatus for differential thermal analysis 43

16 A few milligrams of the sample (S) and an inert reference substance(r) are contained in small aluminum dishes that are located above sample and reference thermocouples in an electrically heated furnace [55]. The reference material is an inert substance such as alumina, silicon carbide, or glass beads. The output potential E, from the sample thermocouple passes into a microcomputer where it is made to control the current input to the furnace in such a way that the sample temperature increases linearly and at a predetermined rate. The sample thermocouple signal is also converted to temperature T s and is then recorded as the abscissa of the differential thermogram. The output across the sample and reference thermocouple ΔE is amplified and converted to a temperature difference ΔT, which serves as the ordinate of the thermogram. Generally an inert gas, such as nitrogen or a reactive gas such as oxygen or air is permitted to circulate through the sample and reference chambers Elemental analysis It is carried out by using Inductively Coupled Plasma (ICP) Optical Emission Spectrometer (OES) PERKIN ELMER 2100 DV. Using this technique, chemical composition of a compound mainly heavy metals and their proportion by weight in the grown crystals can be obtained. 44

25 Instruments for Optical Spectrometry

25 Instruments for Optical Spectrometry 25A INSTRUMENT COMPONENTS (1) source of radiant energy (2) wavelength selector (3) sample container (4) detector (5) signal processor and readout (a) (b) (c) Fig.