Chapter Two: Materials Preparation and Characterization Techniques Sample Preparation and Characterization Techniques 2.1 Sample Preparation In this work, the mixed-valence manganites have been prepared in the form of polycrystalline ceramic and thin films. The two synthesis methods employed are solidstate reaction and pulsed laser deposition. Solid-state reaction method is used in an attempt to produce the polycrystalline ceramic Nd0.67Sr0.33MnO3 target. The samples are usually composed of micrometer sized grains. Pulsed laser deposition is used to produce both epitaxial and polycrystalline Nd0.67Sr0.33MnO3 thin films. Besides the above mentioned methods, wet chemical methods [68] which produce a precursor gel of intimately mixed and hydrated oxides by means of chemical co-precipitation [69, 70] technique are among other techniques that have been used to prepare the target material. However, only the solid-state reaction and pulsed laser deposition techniques will be briefly described in this chapter. 2.1.1 Solid-state reaction for the preparation of polycrystalline ceramic Nd0.67Sr0.33MnO3 target Solid-state reaction method is usually used to prepare polycrystalline ceramic because of its easy preparation technique. The standard solid-state reaction process involves mixing, grinding, compaction and firing of component oxides until 40 Chapter Two: Materials Preparation and Characterization Techniques single-phase material is achieved [71]. In this work, polycrystalline Nd0.67Sr0.33MnO3 bulk is prepared in the following five simple steps and figure – shows the flow chart for the standard solid-state reaction preparation procedure for Nd0.67Sr0.33MnO3 bulk as below: Step 1: Stoichiometric proportions of fine and analytical grade purity powders of oxides, Nd2O3, SrCO3 and MnO2 were separately weighed and prepared. Step 2: The stoichiometric portions were mixed together thoroughly in acetone using the FRITSCH centrifugal ball milling machine with zirconium oxide balls for several hours until the mixture became homogeneous. Step 3: The slurry was dried and the residue was calcined or pre-sintered in an alumina crucible at 1000 °C for 15 hrs in air to obtain better crystallization. Step 4: The calcined powder thus obtained was reground. The reground powder must be of a very fine form as the solid state reaction depends on the interdiffusion between the powders. Steps 1, 2, and were repeated several times until a single perovskite phase was obtained. Step 5: The final powder was pressed into a pellet of dimension x 10 x mm3 and sintered at 1250 °C for 24 hrs in air. Finally the desired stoichiometry is cooled down to room temperature. 41 Chapter Two: Materials Preparation and Characterization Techniques Nd2O3 + MnO2 + SrCO3 Mixture and Stirring Target Wet Grinding Sintering Pressing Drying Powder Calcination Grinding YES Single Phase Figure - Flow chart for standard preparation Nd0.67Sr0.33MnO3 target by solid-state reaction method. NO procedure of ceramic Although the wet chemical methods such as sol-gel and chemical co-precipitation techniques can be used to prepare the target material, these methods have found application only in certain specialized areas due to the higher manufacturing costs involved and the necessity of disposing large quantities of aqueous solutions. Solid-state reaction method is widely versatile and quite useful in producing pure materials which are instrumental in the study of the structure and magnetic properties of perovskite manganites. A more detailed description of this method can be found in a book by Buchanan [72] with particular reference to ferrites. 42 Chapter Two: Materials Preparation and Characterization Techniques 2.1.2 Pulsed laser deposition for the preparation of Nd0.67Sr0.33MnO3 thin film Manganese oxide thin films have been prepared by several methods such as sputtering, molecular beam epitaxy (MBE) or pulsed laser deposition (PLD) from the sintered ceramic targets. Pulsed laser deposition (PLD) is one of the physical vapor deposition (PVD) methods for thin film fabrication. The laser age began with the birth of the first laser in 1960. However, this technique went virtually unnoticed until Venkatesan and coworkers [73, 74] demonstrated the growth of newly discovered high-temperature superconductors of bulk Y-Ba-Cu material followed by annealing in air or oxygen in 1987. Since then, it had been extensively developed for cuprate superconductors and later adapted for manganites. In this project, an excimer laser KrF beam operating in the ultraviolet range with wavelength, λ = 248 nm, is employed. Oxygen gas is employed in the chamber. A simplified schematic arrangement for preparing the manganese thin films by PLD is presented in figure - 2. 43 Chapter Two: Materials Preparation and Characterization Techniques Figure - A simplified diagram of the pulsed laser deposition (PLD) system The KrF excimer laser is focused through a UV-transmitting window onto a rotating ceramic Nd0.67Sr0.33MnO3 target, prepared by the method as described in section 2.1.1, by focusing lens. The energy per pulse or fluence density used is about J/cm2. When the laser beam hits the target, plasma which takes the form of a plume, expanding several centimeters long normal to the target towards the substrates is created. The plasma plume contains both ground and excited state neutral atoms and ions, as well as electrons. These undergo collisions in the high-density plasma region near the target, resulting in a highly directional flow in front of the target surface and deposition on the substrate surface. Thus a layer of thin film is built up. The critical variables for thin film deposition are the gas pressure in the chamber (in this case, the oxygen pressure), the substrate temperature and thin film annealing time/gas pressure. Compounds such as La1-xSrxMnO3 [1], La0.67Ba0.33MnO3 44 Chapter Two: Materials Preparation and Characterization Techniques [75], La1-xCaxMnO3 [76, 77], Nd1-xSrxMnO3 with < x < [78] and Pr0.7Sr0.3MnO3 [79] have been deposited on different substrates such as LaAlO3 (LAO), MgO, SrTiO3 (STO) and Si. Optimal conditions for the growth of high quality manganites thin films as recorded in the literature are temperature ranging from 600 – 800 °C and an oxygen pressure of – × 10-1 mbar, followed by proper annealing under oxygen pressure. With careful selection of the type of substrate, epitaxial and polycrystalline thin films can be fabricated. For example, we can fabricate (001)-oriented epitaxial manganite thin film on a (001)-oriented × 10 mm2 SrTiO3 substrate. Polycrystalline manganite thin film can be fabricated on a Si substrate. In this work, high quality epitaxial Nd0.67Sr0.33MnO3 monolayered and bilayered La0.67Sr0.33MnO3/Nd0.67Sr0.33MnO3 thin films were deposited on LAO and STO substrates. 2.2 Sample Characterization Techniques This section aims to review a number of techniques used to analyze the structural, magnetic and electrical transport properties of a material. Among the wide variety of techniques that are available to measure and analyze these quantities, we illustrate the most common methods by examples involving the samples we have fabricated in this project. X-ray diffraction (XRD) is the conventional bulk sensitive technique used for structural characterization. Other techniques such as scanning electron microscopy (SEM), atomic force microscope (AFM), and magnetic force microscope (MFM) are employed for surface morphologies characterizations. To determine the magnetic properties of these half-metallic ferromagnetic manganese oxides, vibrating sample magnetometer (VSM) is employed. The four-point probe technique is used to measure 45 Chapter Two: Materials Preparation and Characterization Techniques the electrical transport property of the material. X-ray photoelectron microscope (XPS) is used to analyze the surface concentration and the chemical environment of each element in the outer surface of ∼100 Å of the samples. 2.2.1 X-ray Diffraction X-rays were discovered by W. C. Roentgen in 1895 [80]. X-ray diffraction (XRD) is a non-destructive technique widely applied for identification of compounds by their diffraction pattern. Among the various kinds of micro- and nano-crystalline materials for characterization are inorganics, organics, drugs, minerals, catalysts, ceramics or metals. The physical state of the materials can be loose powders, thin films or bulk materials. In this work, the samples investigated are ceramic Nd0.67Sr0.33MnO3, Fedoped Nd0.67Sr0.33MnO3, La0.67Sr0.33MnO3 bulk and thin films. Collimating slit Sample Holder Scattering Detector slit Receiving θ slit X-ray Tube Tower Figure - Photograph of the Phillips PW1710 X-ray Diffractometer with the important parts labeled. 46 Chapter Two: Materials Preparation and Characterization Techniques Figure - above shows the Phillips PW1710 X-ray Diffractometer used in our laboratory. Striking a pure anode of a particular metal with high-energy electrons in a sealed vacuum tube (x-ray tube as indicated in the picture) generates X-rays usable for x-ray diffraction. Here we employed the Copper (Cu) Kα (1.54056 Å) radiation source. A coherent beam of monochromatic CuKα from the x-ray tube is directed at the sample. Interaction of X-rays with sample creates secondary diffracted beams of X-rays which are related to the interplanar spacings in the crystalline materials according to Bragg's Law [81]: nλ = 2dsinθ where n is an integer, d is the interplanar spacing in the crystalline phase, θ is the incidence angle and λ is the wavelength of the incident x-ray. The diffracted beams are then collected by the detector through the receiving slit. The diffraction peaks are measured along a 2θ diffractometer circle whereby the x-ray tube is fixed and the specimen moves at half the angular rate of the detector to maintain the θ-2θ geometry. The angle of the diffraction is related to the interplanar spacing, d and the intensity of the diffraction peak is related to the strength of those diffractions in the specimen. Figure shows the X-ray diffraction pattern for Nd0.67Sr0.33MnO3 target by solid-state reaction method recorded in terms of 2θ using a computer running PC-APD analytical software. 47 Chapter Two: Materials Preparation and Characterization Techniques 400 (310) 350 300 250 50 (213) (422) (312) (311) 100 (201) 150 (420) 200 (210) Intensity (counts) Nd0.67Sr0.33MnO3 target Tetragonal Structure 10 30 2Θ (degree) 50 70 Figure - The X-ray diffraction pattern for Nd0.67Sr0.33MnO3 target by solid-state reaction method Based on figure - 4, by using fitting program, Winfit, adopted freely from Stefan Krumm [82], the unit cell parameters and microstructural parameter (grain size) can be obtained. The polycrystalline ceramic Nd0.67Sr0.33MnO3 target can be indexed according to a tetragonal structure with lattice parameters, a = 8.635 Å and c = 3.848 Å. One can also refer to Powder diffraction file [83] available in the market for determining the structural phase of the sample. In this day of automated data collection and analysis, the preparation of sample is usually the most critical factor influencing the quality and reliability of the collected analytical data. The three parameters of special interest in a diffraction pattern are (1) the position of the diffraction peaks, (2) the peak intensities, and (3) the intensity distribution as a function of diffraction angle. How accurate these experimental results represent the sample in terms of these parameters determine whether the results are useful for phase 48 Chapter Two: Materials Preparation and Characterization Techniques identification, or more detailed analyses like crystallite size and distribution, stress and strain or quantitative determination of different phases in a multi-phase sample. Therefore, to be able to see all the diffraction peaks, the powdered sample must present a large number of crystallites in a random orientation to the incident beam. For thin film analysis, it is important to mount the film on a zero-background plate to ensure that the surface of the film is parallel to the reference plane in order to fulfill Bragg’s diffraction condition. 2.2.2 Vibrating Sample Magnetometer (VSM) Vibrating sample magnetometer (VSM) has become widely accepted as a standard technique for magnetic measurements. This technique was first highlighted by Foner [84] in 1959. It is a relative measurement giving the total magnetic moment of a sample. A VSM is an example of an induction technique whereby the AC field produced by an oscillating sample moment located in the center of two counter-wound coils spaced a small distance apart is measured. The motion of the magnetic sample creates a changing flux in the coils which induces a voltage across the pick up coils as shown in figure - 5. 49 Chapter Two: Materials Preparation and Characterization Techniques Transducer Driver Transducer Audio Amplifier Lock-in Amplifier XY Recorder Pick up coils Magnet Magnet Power Supply Figure - A simplified diagram of the VSM system. The pair of pick up coils is wound in opposition because by doing so, the induced voltage from one coil due to the vibrating sample is added to that from the other coil. The voltages from the changing background field are equal in magnitude but opposite in direction and will cancel each other out. However, it is important to calibrate the voltage against a sample of known moment using calibration materials such as nickel, palladium or iron. The signals from the pick up coils are sent to two variable gain amplifiers through a balanced secondary microphone transformer and then amplified by a lock-in amplifier. Lastly the signals are processed and data collected by a computer. 50 Chapter Two: Materials Preparation and Characterization Techniques Mangetization, M (emu/g) Nd0.67Sr0.33MnO3 La0.67Sr0.33MnO3 50 150 250 350 Temperature, T (K) 450 Figure - The temperature dependent magnetization, M(T) for Nd0.67Sr0.33MnO3 and La0.67Sr0.33MnO3 at a fixed magnetic field of 200 Oe from 77 to 470 K. In this work, we employed a computer controlled MagLab VSM system from the Oxford Instruments. Common features of this MagLab VSM system include automated systems for changing the temperature and magnetic field applied to the sample. The automated MagLab VSM system operates in a temperature from boiling point of liquid helium (5 K) to 300 K and a maximum magnetic field up to Tesla. In this work, both the isothermal magnetic hysteresis loop, M(H) and the temperature dependent magnetization, M(T) at a fixed field were evaluated. The covered range of temperature was from 77 to 300 K and the fixed magnetic field adopted for the M(T) curves ranged from 80 to 2000 Oe. It is important to note that the measured moment of the sample is the sum of the moments of the sample itself, the substrate (for example thin film), the sample holder and any medium which is moving inside the sample region. Therefore, in order to 51 Chapter Two: Materials Preparation and Characterization Techniques determine the characteristics of the sample, it is necessary to minimize or subtract the contributions from the outer components. There may be little option in choosing the substrate as this has an important bearing on the sample preparation itself. Its contribution to the total signal is best determined by a separate measurement of the substrate material itself. In addition, the sample holder can be constructed from a diamagnetic material and shaped so as to be as symmetrical as possible about the centre of the coil system in order to eliminate the contribution from it. 2.2.3 Scanning Electron Microscope (SEM) The scanning electron microscope (SEM) is used to study the surface, or near surface structure of bulk specimens. Unlike optical microscope which uses light beam, SEM employs a beam of electrons directed at the specimen. An electron gun, usually of the tungsten filament thermionic emission type, produces electrons, and accelerates them to an energy of about - 40 keV. Two or three condenser lenses then demagnify the electron beam until, as it hit the specimen, it may have a diameter of only – 10 nm. The fine beam of electrons is scanned across the specimen by the scan coils, while a detector counts the number of low energy secondary electrons, or other radiation, given off from each point on the surface. At the same time, the spot of a cathode ray tube (CRT) is scanned across the screen, while the brightness of the spot is modulated by the amplified current from the detector. The electron beam and CRT spot are both scanned in a rectangular set of straight lines known as a raster. The mechanism by which the image is magnified involves no lenses at all. It can easily be obtained by making the raster scanned by the electron beam on the specimen smaller than the raster displayed on the 52 Chapter Two: Materials Preparation and Characterization Techniques CRT. Figure - shows the surface morphology of Nd0.67Sr0.33MnO3 target using backscattered electron image (SEI). Average Grain Size Accelerating Voltage Magnification Figure - SEM for Nd0.67Sr0.33MnO3 target with tetragonal structure prepared by solid-state reaction method. The SEM study in our work was mainly carried out by means of a JEOLJSM-S35 SEM Model equipped with energy dispersive x-ray spectrometer (EDX) for compositional mapping. All SEMs normally have the facilities for detecting secondary (10 – 50 eV) and backscattered electrons for imaging while X-rays is used primarily for chemical analysis. For secondary and backscattered electrons imaging, accelerating voltage of about – 10 kV is usually employed whereas for compositional imaging, a higher accelerating voltage from 10 – 20 kV is used. There are some demerits in increasing the accelerating voltage such as higher possibility of charge-up, higher possibility of specimen damage and remarkable edge effect. At higher accelerating 53 Chapter Two: Materials Preparation and Characterization Techniques voltages, the beam penetration and diffusion area become larger, resulting in unnecessary signals being generated from within the specimen. These signals reduce the image contrast and veil the fine surface structures. Thus, it is especially desirable to use low accelerating voltage for observation of low-concentration substances. In order to correctly select the optimum observation conditions for various specimen, one can refer to “A Guide to Scanning Microscope Observation” [85] issued by JEOL DATUM. 2.2.4 Atomic Force Microscopy (AFM) Atomic force microscopy (AFM) belongs to a family of microscopy technique called the Scanning Probe Microscopy (SPM). It was invented by G. Binning, Calvin F. Quate and Christopher Gerber as a collaboration effort between IBM Zurich and Stanford University in 1986 [86]. AFM differs from traditional optical microscopy in that it does not rely on lenses, light beams or electron beams to generate an optical image of the specimen. It employs a sharp mechanical probe to scan over the specimen surface. At the same time, a flexible cantilever undergoes deflection or oscillation changes, depending on how the probe interacts with the surface topography. The probe used is made from Silicon. Here, both cantilever and tip are an integrated assembly etched out of a single Silicon crystal. The force constant of the cantilever is small so that forces exerted between the tip and the sample would not become so great as to displace atoms in the specimen or damage the cantilever tip. Using the feedback of the cantilever deflection/oscillation, the atomic force microscope is able to deduce and construct a precise representation of the specimen's surface topography. This operation is precise. It 54 Chapter Two: Materials Preparation and Characterization Techniques allows scanning to be performed under ambient conditions and serves as a useful analytical tool for resolving surface features right down to the atomic scale (0.1 Å). It is a suitable method for surface imaging as the latent damage done to the specimen is minimal. The probe tip does not contact the specimen surface perpetually, but only at the bottom of each oscillation. Hence, any possible damage to the surface arising from a stronger contacting force is limited. It is important to note that any excessively large spring constant will risk the softer specimen being ruined badly. Dimension AFM Head Rotatable Stage Figure - Photograph of the Digital Dimension 3000 as used in our laboratory. In our work, a Digital Dimension 3000 as shown in figure – is employed to the characterization of Nd0.67Sr0.33MnO3 thin films. It has altogether variant modes of operation. They are contact, tapping and non-contact mode. We have 55 Chapter Two: Materials Preparation and Characterization Techniques adopted the tapping mode in our experiments. In tapping mode AFM, the probe surveys the topography of a sample surface by essentially "tapping" the probe tip lightly on it, contacting the surface at the bottom of each oscillation. This tapping motion is driven by the oscillation of the cantilever, which vibrates at or near its resonance frequency with amplitude typically ranging from 20 - 100 nm. Figure - shows the AFM morphology for Nd0.67Sr0.33MnO3 film deposited on LaAlO3 substrate by PLD. Besides surface imaging, some other quantitative information regarding the surface roughness, sample thickness, average grain size and grain size distribution can also be obtained. Figure - AFM morphology for Nd0.67Sr0.33MnO3 film on LAO by PLD. 56 Chapter Two: Materials Preparation and Characterization Techniques 2.2.5 Four – Point Probe Technique The four-point probe technique is used to measure the electrical transport property of the sample. The system requires the introduction of a test current into the specimen and the voltage of the sample is measured [87]. The four probe tips are arranged in a linear array as shown in figure - 10. It is customary to have the outer two probes carry current and the inner probes measure the resultant voltage of the sample. The current, I is supplied through a current source while the potential difference, V across the two electrodes is collected by a nanovoltmeter. D1 D2 D3 Figure – 10 Conventional electrode configuration of a linear four-point probe array where D1 = D2 = D3 is the probe spacing. Since the input impedance of the nanovoltmeter is very high (typically > 10 GOhm), thus the current running through the inner nanovoltmeter probe is negligible as compared to the current applied to the sample across the two outer electrodes. Probe force, probe travel, tip radius and probe material must be selected with consideration for the resistivity, hardness, and thickness of the layer to be measured accurately so that the 57 Chapter Two: Materials Preparation and Characterization Techniques contribution from the probe contact and probe material resistance will have negligible affects on the measured four-point probe resistance, R = V/I. In practice, the accuracy and repeatability of the measurements suffer from the geometrical effects caused by slight variations in probe spacing and geometry of the specimen. Indeed, the probe spacing varies slightly each time the probe array contacts the specimen. This is due to the geometry dependence of the current distribution in the sample. Fortunately, there exists a self-compensation technique for eliminating geometric sources of error. Ideally, for a rectangular dimension specimen, the four probes should be parallel to each other and the contact area between the probe tip and specimen should not be too large. For a circular specimen, the four probes should be parallel to each other and the radius of the circular specimen. Sample holder Dewar filled with Liquid Electromagnet of H = ±4 kOe Figure - 11 Picture of the experimental setup for the four-point probe method for Nd0.67Sr0.33MnO3 target measurement. 58 Chapter Two: Materials Preparation and Characterization Techniques Figure – 11 shows the picture of the four-point probe system employed in our laboratory. The sample holder is inserted in a Dewar filled with liquid nitrogen for temperature and field dependent of resistance, R(T) and R(H) measurements. The Dewar is placed in the center of a pair of electromagnet with maximum magnetic field of ± kOe in magnitude. The magnetic field is read by a magnetometer with a Hall probe attached to one site of the electromagnet and the data collected by a multimeter, sent to and recorded by a computer program. 2.2.6 X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS), which is also known as electron spectroscopy for chemical analysis (ESCA), exploits the photoelectric effect to obtain information about the chemical composition and structure of the specimen’s surface. When a photon source (e.g. X-rays) is directed at the specimen, the photons interact with the electrons present in the sample material. If the photon has sufficient energy, it will cause an electron to be emitted from its orbital. The simple theoretical relationship that describes this process is KE = hν – BE where KE is the kinetic energy of the emitted photoelectron, hν is the energy of the photon, and BE is the binding energy for the emitted photoelectron. KE is measured in the ESCA experiment, hν is known, and BE can be calculated, yielding the energy with which the electron was held in its atomic or molecular environment. For photoemission from solids, the work function term must be added to this equation. A unique work 59 Chapter Two: Materials Preparation and Characterization Techniques function is established for each ESCA instrument. This term expresses the additional energy required after the ionization process to get the emitted electron away from the surface and into the surrounding gas or vacuum space. Thus, the measured kinetic energy of the electron will be indicative of the element from which it came and the chemical environment of that element. For ESCA analysis, a sample is placed in an ultrahigh vacuum environment, typically less than 10-8 Torr. The sample is then exposed to a low-energy, monochromatic X-ray source which causes the emission of photoelectrons from atomic shells of the elements present on the surface. In our work, Mg Kα radiation with hν = 1253.6 eV is employed for sample analysis. These electrons possess an energy characteristic of the element and molecular orbital from which they are emitted. The electrons are detected and counted according to the energy they possess. By counting the number of electrons detected at each energy value (KE < hν), a spectrum of peaks corresponding to the elements on the surface is generated. Figure – 12 shows the wide scan XPS spectrum of Nd0.67Sr0.33MnO3 target. 60 Chapter Two: Materials Preparation and Characterization Techniques La 3d Signal (cps) Mn LMV O 1s Mn 2p Mn 3s 1000 800 600 400 BE (eV) 200 Figure – 12 The wide scan XPS spectrum of Nd0.67Sr0.33MnO3 target. The area under these peaks is a measure of the relative amounts of each element present, while the shape and position of the peaks reflect the chemical environment of each element. ESCA is a surface sensitive technique as only those electrons that leave the surface without energy loss will contribute to the peak signifying that element. Those electrons originating from far below the surface (>100 Å) suffer energy loss through collisions and are unable to make it out of the surface, or they escape the surface with considerable energy loss. 61 Chapter Two: Materials Preparation and Characterization Techniques 2.3 Conclusion Table – below illustrates the similarities and differences between the five characterization techniques used and how each of these techniques may be chosen to suit the investigation at hand. Table – The differences and similarities between the five characterization techniques with the different capabilities. ITEM XRD SEM AFM Surface XPS Surface Purpose Illuminating Beam Medium Structural Surface (< 0.1 Å) (< 100 Å) X-ray Beam Air Collimating, Scattering lens Electron Beam Vacuum Nil Air Electron lens SEI, BEI and others Nil Lens Obtaining Images User Friendliness Image Interpretation Straightforward Specimen Any shape XRD Very easy Easy to use Fairly straightforward Small and flat, conducting AFM Easy to use Fairly straightforward Small and flat Mg Kα Vacuum Analyzer lens Wide and small scan RT Electrical Transport Nil Air/Vacuum Nil R-T; R-H Fairly easy Direct Complicated Thin and round Direct Small; flat 62 [...]... then amplified by a lock -in amplifier Lastly the signals are processed and data collected by a computer 50 Chapter Two: Materials Preparation and Characterization Techniques Mangetization, M (emu/g) 7 Nd0. 67Sr0. 33MnO3 6 5 4 La0 .67Sr0. 33MnO3 3 2 1 0 50 150 25 0 350 Temperature, T (K) 450 Figure 2 - 6 The temperature dependent magnetization, M(T) for Nd0. 67Sr0. 33MnO3 and La0 .67Sr0. 33MnO3 at a fixed magnetic. .. Nd0. 67Sr0. 33MnO3 thin films It has altogether 3 variant modes of operation They are contact, tapping and non-contact mode We have 55 Chapter Two: Materials Preparation and Characterization Techniques adopted the tapping mode in our experiments In tapping mode AFM, the probe surveys the topography of a sample surface by essentially "tapping" the probe tip lightly on it, contacting the surface at the bottom of each... – 20 kV is used There are some demerits in increasing the accelerating voltage such as higher possibility of charge-up, higher possibility of specimen damage and remarkable edge effect At higher accelerating 53 Chapter Two: Materials Preparation and Characterization Techniques voltages, the beam penetration and diffusion area become larger, resulting in unnecessary signals being generated from within... Surface XPS Surface Purpose Illuminating Beam Medium Structural Surface (< 0.1 Å) (< 100 Å) X-ray Beam Air Collimating, Scattering lens Electron Beam Vacuum Nil Air Electron lens SEI, BEI and others Nil Lens Obtaining Images User Friendliness Image Interpretation Straightforward Specimen Any shape XRD Very easy Easy to use Fairly straightforward Small and flat, conducting AFM Easy to use Fairly straightforward... voltages from the changing background field are equal in magnitude but opposite in direction and will cancel each other out However, it is important to calibrate the voltage against a sample of known moment using calibration materials such as nickel, palladium or iron The signals from the pick up coils are sent to two variable gain amplifiers through a balanced secondary microphone transformer and. .. important bearing on the sample preparation itself Its contribution to the total signal is best determined by a separate measurement of the substrate material itself In addition, the sample holder can be constructed from a diamagnetic material and shaped so as to be as symmetrical as possible about the centre of the coil system in order to eliminate the contribution from it 2. 2.3 Scanning Electron Microscope... probe material must be selected with consideration for the resistivity, hardness, and thickness of the layer to be measured accurately so that the 57 Chapter Two: Materials Preparation and Characterization Techniques contribution from the probe contact and probe material resistance will have negligible affects on the measured four-point probe resistance, R = V/I In practice, the accuracy and repeatability... Two: Materials Preparation and Characterization Techniques 2. 3 Conclusion Table 2 – 1 below illustrates the similarities and differences between the five characterization techniques used and how each of these techniques may be chosen to suit the investigation at hand Table 2 – 1 The differences and similarities between the five characterization techniques with the different capabilities ITEM XRD SEM AFM... dispersive x-ray spectrometer (EDX) for compositional mapping All SEMs normally have the facilities for detecting secondary (10 – 50 eV) and backscattered electrons for imaging while X-rays is used primarily for chemical analysis For secondary and backscattered electrons imaging, accelerating voltage of about 5 – 10 kV is usually employed whereas for compositional imaging, a higher accelerating voltage from... modulated by the amplified current from the detector The electron beam and CRT spot are both scanned in a rectangular set of straight lines known as a raster The mechanism by which the image is magnified involves no lenses at all It can easily be obtained by making the raster scanned by the electron beam on the specimen smaller than the raster displayed on the 52 Chapter Two: Materials Preparation and . Chapter Two: Materials Preparation and Characterization Techniques 2 Sample Preparation and Characterization Techniques 2. 1 Sample Preparation In this work, the mixed-valence manganites. They are contact, tapping and non-contact mode. We have 55 Chapter Two: Materials Preparation and Characterization Techniques adopted the tapping mode in our experiments. In tapping mode AFM,. related to the interplanar spacings in the crystalline materials according to Bragg's Law [81]: nλ = 2dsinθ where n is an integer, d is the interplanar spacing in the crystalline phase,