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Indiana University of Pennsylvania Knowledge Repository @ IUP Theses and Dissertations 5-2015 Dilute Magnetic Semiconductor Solid Solutions of Zinc Cobalt Telluride Michael Carl Tomik Indiana University of Pennsylvania Follow this and additional works at: http://knowledge.library.iup.edu/etd Recommended Citation Tomik, Michael Carl, "Dilute Magnetic Semiconductor Solid Solutions of Zinc Cobalt Telluride" (2015). Theses and Dissertations. Paper 1282. This Thesis is brought to you for free and open access by Knowledge Repository @ IUP. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of Knowledge Repository @ IUP. For more information, please contact [email protected]. DILUTE MAGNETIC SEMICONDUCTOR SOLID SOLUTIONS OF ZINC COBALT TELLURIDE A Thesis Submitted to the School of Graduate Studies and Research in Partial Fulfillment of the Requirements for the Degree Master of Science Michael Carl Tomik Indiana University of Pennsylvania May 2015 Indiana University of Pennsylvania School of Graduate Studies and Research Department of Chemistry We hereby approve the thesis of Michael Carl Tomik Candidate for the degree of Master of Science Charles H. Lake, Ph.D. Professor of Chemistry, Advisor George R. Long, Ph.D. Professor of Chemistry Keith S. Kyler, Ph.D. Assistant Professor of Chemistry Avijita Jain, Ph.D. Assistant Professor of Chemistry ACCEPTED Randy L. Martin, Ph.D. Dean School of Graduate Studies and Research ii Title: Dilute Magnetic Semiconductor Solid Solutions of Zinc Cobalt Telluride Author: Michael Tomik Thesis Chair: Dr. Charles H. Lake Thesis Committee Members: Dr. Avijita Jain Dr. Keith S. Kyler Dr. George R. Long Dilute magnetic semiconductor (DMS) materials are a new class of materials with many interesting properties and potential applications. These materials are bulk nonmagnetic semiconductor compounds doped with magnetic cations. This project aimed to synthesize a new dilute magnetic semiconductor compound using a solid solution of zinc and cobalt telluride. Standard ceramic synthetic methods were used in an attempt to create this solid solution and its end members. Differential scanning calorimetry was used to determine a heating profile for the reaction. X-ray diffraction was used for phase identification, and paired with Rietveld refinement to do phase analysis. Scanning electron microscopy with energy dispersive spectroscopy was used to do further elemental analysis. UV-Vis-NIR spectroscopy was used to elucidate the band gaps of the synthesized materials and investigate any variance with doping. The compounds synthesized were found to contain oxide contaminations. To investigate the reason for oxide contamination the syntheses were run using various reaction methods. Initial oxide contamination was around 10 wt-% with later syntheses showing contamination around 2.5 wt-%. iii ACKNOWLEDGEMENTS I would like to thank my instructors from IUP for helping me throughout my time there. I would like to thank Dr. Lake for his insights into my project, and his help along the way. I would especially like to thank him for not giving me help when I asked for it but when I needed it pushing me to expand my boundaries as a chemist and person. I would like to thank my committee for their time and support with my project. I would like to thank Dr. Jennifer Aitken and her laboratory at Duquesne University for allowing me to use their instruments and for helping me with them when I needed it. Finally, I would like to thank my family and girlfriend for their constant support and help accomplishing my goals. iv TABLE OF CONTENTS Chapter 1 Page INTRODUCTION AND BACKGROUND ..........................................1 Microelectronics ...............................................................................1 Spintronics Theory ...........................................................................2 Interest in Spintronics............................................................3 Band Theory ....................................................................................4 Band Gaps ............................................................................5 Intrinsic and Extrinsic Semiconductors..................................6 Magnetism .......................................................................................7 What is a Solid Solution? ...............................................................10 Solid Solution Criterion........................................................10 What are DMS Materials? ..............................................................11 Previous DMS Research .....................................................13 DMS Synthesis....................................................................14 2 HYPOTHESIS................................................................................15 3 EXPERIMENTAL ...........................................................................16 Chemicals ......................................................................................16 Equipment......................................................................................17 Synthesis .......................................................................................18 Atomic Diffusion ..................................................................18 Differential Scanning Calorimetry ..................................................19 X-Ray Diffraction ............................................................................21 Method One ...................................................................................23 Method Two ........................................................................24 Rietveld Refinement ......................................................................25 Diffuse Reflectance Spectroscopy .................................................30 v Chapter Page SEM Theory ...................................................................................31 Energy Dispersive Spectroscopy ........................................32 4 RESULTS AND DISCUSSION ......................................................34 DSC Analysis .................................................................................34 Diffractogram of DSC events ..............................................35 X-Ray Diffraction ............................................................................36 Rietveld Refinement ......................................................................44 Diffuse Reflectance Spectroscopy .................................................45 Energy Dispersive Spectroscopy ...................................................48 5 SUMMARY ....................................................................................50 REFERENCES ....................................................................................................51 APPENDICES .....................................................................................................54 Appendix A -- X-Ray Diffractograms ..............................................54 Appendix B – DRS Plot ..................................................................58 Appendix C – EDS Spectra............................................................60 vi LIST OF TABLES Table Page 1 Chemicals .................................................................................................16 2 Equipment ................................................................................................17 3 Semi-quantitative elemental analysis by EDS ..........................................49 vii LIST OF FIGURES Figure Page 1 Moore’s Law ...............................................................................................1 2 3d spin-subbands .......................................................................................2 3 Giant magnetoresistance ............................................................................3 4 LCAO creating a band ................................................................................4 5 Relative band gaps .....................................................................................5 6 n- and p-type microstates ...........................................................................6 7 Magnetic moments .....................................................................................7 8 Density of states diagram ...........................................................................9 9 Color difference between Mg2SiO4 and Fe2SiO4......................................................... 10 10 Na2ZnSiO4 crystal structure1 ....................................................................11 11 Substitution in a DMS material .................................................................11 12 Mn-doped DMS Curie temperatures .........................................................13 13 Pellet formed using a pellet press.............................................................18 14 Graphite crucible inside fused silica tube .................................................19 15 NETZSCH Pegasus F3 Differential Scanning Calorimeter .......................20 16 Rigaku Miniflex II X-Ray Diffractometer ....................................................21 17 Bragg equation .........................................................................................22 18 Preliminary heating profile ........................................................................24 19 Fused silica tube apparatus ......................................................................24 20 Apparatus and visual for fire-sealing fused silica tube ..............................25 21 Pre-refinement liveplot ..............................................................................26 22 Liveplot making use of difference plot ......................................................28 viii Figure Page 23 Cary 5000 UV-Vis-NIR Spectrometer .......................................................30 24 SEM image of CoTe .................................................................................31 25 SEM image showing area for EDS ...........................................................32 26 Sample EDS spectra ................................................................................32 27 DSC plot of 10% (Zn,Co)Te ......................................................................34 28 Diffractogram based on DSC heating profile ............................................35 29 Modified heating .......................................................................................36 30 CoTe run under flowing argon ..................................................................37 31 CoTe after changes to tube furnace fittings and seals..............................38 32 First reaction with graphite crucible in fused silica tube ............................39 33 Diffractogram without monochromator......................................................40 34 Diffractogram with monochromator...........................................................41 35 Diffractogram of compound with extended vacuum time ..........................42 36 Synthesis of (Zn,Co)Te2 ............................................................................................................... 43 37 GSAS/EXPGUI Uiso refinement problem ...................................................44 38 DRS plot of ZnTe ......................................................................................46 39 DRS plot of 5% (Zn,Co)Te ........................................................................46 40 EDS spectra of CoTe................................................................................48 ix x CHAPTER ONE INTRODUCTION AND BACKGROUND Microelectronics The advent of the age of information came about with the invention of the computer and the internet whose ability to reproduce, disseminate and store information has allowed for this revolution.1 These two technologies rely on both semiconductors and ferromagnetic materials to process and store information, respectively. Semiconductors are a substance with electrical conductivity between that of an insulator and conductor, and are used in the central processing unit (CPU) and random access memory (RAM). This is where binary information is encoded and transmitted as charge on a capacitor.1 The CPU and RAM were developed from the integrated circuit, in which large numbers of transistors were able to be manufactured on a single chip.2 Figure 1. Moore's Law. 1 Technological progress in miniaturization of the electronic components has increased the number of transistor’s built into a chip according to Moore’s Law. Moore’s Law states that the number of transistors on a single chip will double every 18 months.3 As this trend continues, eventually the size required for further miniaturization will be on the atomic scale where quantum effects take over.3 In addition, semiconductor memory faces another, more basic problem. Information contained in RAM is lost on powering down as current RAM relies on charge. Read-only memory (ROM) on the other hand is non-volatile, but it is not in use today. This type of memory persists even after power is removed from the system because the data is stored in the physical layout of the memory. Spintronics Theory Typical electronics (computers) use charge in order to process information. Spintronics, an area of future development in electronics, utilizes the intrinsic spin of an Figure 2. 3d spin-subbands separated by exchange energy. electron or atomic nucleus.(1, 4) This is done through the manipulation and transport of spin-polarized carriers.1 One way to create spin-polarized current is to pass it through a 2 ferromagnetic material. The d-bands of a ferromagnetic material’s spin-up and spindown sub bands are unequal and shifted by an amount of energy called the exchange energy or Uex.1 The difference in the spin-up and spin-down subband’s energy can be seen in figure 2. This is the cause of the spontaneous magnetization in a permanent magnetic material, the presence of more spin-up or spin-down electrons.1 It is also true that the spin-up and spin-down sub bands have different conductance’s, so that when a current is passed through a ferromagnetic material it will choose the path of least resistance and thus create an excess of either spin-up or down electrons.1 Interest in Spintronics Significant research committed to the development of new materials and applications for spintronics. Current iterations of spintronics materials are in use in magnetic hard drives in the form of giant magnetoresistant (GMR) read heads. Giant http://www.research.ibm.com/research/gmr.html Figure 3. Giant magnetoresistance. magnetoresistance was discovered in the late 1980s Peter Gruenberg and Albert Fert simultaneously.5 GMR read heads consist of a non-magnetic layer sandwiched by two ferromagnetic layers, where one is pinned in its orientation and the other is free. The free layer is allowed to alter its alignment based upon the alignment of magnetic fields that is passes over. As electricity is passed through the read head the relative alignments of the two ferromagnetic layers varies the resistance. These read heads are 3 extremely sensitive because of the enormous difference in conductance when these ferromagnetic layers are aligned parallel versus antiparallel, allowing very minute magnetic fields to be registered. Spintronics in general use magnetic fields, as opposed to electric fields, to control the spin of the charge carriers. Further attempts at using spintronic materials in computers are aimed at introducing magnetic properties into conventional electronic devices. Advantages of spintronics materials are; very low power consumption, faster read/write times, the possibility of instant-on computers, and infinite storage using spin orientation.1 Band Theory Electronic band structure can be visualized by using linear combination of atomic orbital (LCAO) theory. The atomic orbitals of two atoms can overlap if the energy and symmetry are compatible results in two molecular orbitals, one bonding and one Figure 4. LCAO creating a band. http://images.flatworldknowledge.com/averillfwk/averillfwk-fig12_021.jpg antibonding.6 If four atomic orbitals overlap it will lead to two antibonding and two bonding molecular orbitals.6 This progression can be followed until Avogadro’s number of atoms is combined to form NA/2 bonding molecular orbitals and NA/2 antibonding 4 orbitals. The addition of more molecular orbitals to the system reduces the energy difference between adjacent levels. At this point, the energy difference between adjacent energy levels is small enough to be negligible, and thus creates a continuum called a band shown in figure 4.7 The valence band and the conduction band represent the continuum of bonding and antibonding molecular orbitals respectively. The energy difference between the top of the valence band and the bottom of the conduction band is referred to as the band gap. Band Gaps The different classes of electronic materials are differentiated by the band gap. In a conductor the valence band and conduction band overlap therefore conductors have no band gap. An insulator has a very large band gap, typically over 4 eV. A semiconductor is intermediate between a conductor and insulator and has a band gap Figure 5. Relative band gaps. 5 between 0-4 eV. This can be observed graphically in figure 5. At absolute zero semiconductors behave as insulators, but as temperature rises, electrons are promoted from the valence band to the conduction band, resulting in conductivity. This happens because the Fermi (Ef) level exists just above the valence band, which means that the valence band is completely filled with the conductance band completely empty. The Fermi level is the theoretical level that states the highest occupied energy level. Intrinsic and Extrinsic Semiconductors Semiconductors can be further divided into subcategories which are intrinsic and extrinsic. Extrinsic semiconductors have their electronic properties entirely controlled by dopant atoms present in the bulk structure. Extrinsic semiconductors have atoms doped into them that are either electron rich or deficient. When an impurity atom has an extra electron it is a donor and makes an n-type semiconductor. This donor electron exists in a microstate that lies just below the conduction band and only requires a small amount Figure 6. (left) p-type microstates (right) n-type microstates. of energy to be promoted. If the impurity atom has fewer electrons than the host, it makes a p-type semiconductor. In p-type semiconductors the electron occupies a microstate just above the valence band and allows an electron from the valence band to be promoted easily. When an electron is promoted from the valence band, it leaves 6 behind a positively charged hole that allows conduction. These microstates are shown in Figure 6. In this scope of this project, intrinsic semiconductors are the object of interest because the compound zinc telluride is a wide band intrinsic semiconductor. Intrinsic semiconductors are semiconductors by themselves without dopant atoms. An example of this would be silicon, which is the basis for computer processors as was mentioned before. The conductivity properties of an intrinsic semiconductor can be altered by either crystalline defects or by excitation of electrons from the valence band to conduction band. When an electron is promoted from the valence band to the conduction band, a semiconductor is able to conduct. In the conduction band, conduction is occurring through the electron moving towards an anode. However, when an electron is promoted this also leaves behind a positively charged hole that the electron inhabited. This type of conduction can be seen as moving in the opposite direction where the hole will move towards a cathode. Magnetism Magnetism is a property that is possessed by materials that are both naturally occurring and synthetic. In 1845 Michael Faraday discovered that all materials possess some form of magnetism.8 Magnetism in materials can be divided into multiple Figure 7. Magnetic moments in (a) absence of a magnetic field and (b) presence of a magnetic field. 7 categories: dia-, para-, antiferro-, ferro- and ferri-magnetism. A diamagnetic compound has no unpaired electrons so that all magnetic moments are cancelled resulting in zero net magnetic moment for the bulk compound. Diamagnetic compounds repel magnets or magnetic fields due to Lenz's Law.9 Lenz’s Law states that as a magnet is moved towards a loop of wire, the magnet will induce a current in the wire. The direction of current flow will generate a magnetic field that opposes the incoming magnet.10 In a compound this means that the electron that is aligned parallel to the incoming field slows its spin down, while the electron opposing the field speeds up. In effect, this reduces the magnetic moment of the electron with a parallel moment, and increases the magnetic moment of the electron opposing the incoming field. The strength of the diamagnetic forces is very weak compared to other magnetic forces and is negligible when those other forces exist. A paramagnetic compound has any number of unpaired electrons.9 Paramagnetic materials contain randomly oriented magnetic moments that cancel each other out in a bulk material resulting in zero net magnetic moment. However, in the presence of a magnetic field, the magnetic moments align themselves parallel to magnetic field lines figure 7.9 This causes paramagnetic materials to become attracted to external magnetic fields. For paramagnets thermal vibrational energy exceeds electron-electron coupling at high temperature resulting in random orientation of the unpaired spins. As the temperature decreases, an alignment of the electron spins can occur in one of two ways. If the spins align parallel the material is a ferromagnet and has dropped below the Curie temperature, and if the spins align antiparallel the material is an antiferromagnet and has dropped below the Néel temperature. The electrons in an antiferromagnet align antiparallel because there is a 8 slight overlap of their orbitals and the Pauli Exclusion Principle states that no two identical particles may occupy the same space.8 Ferromagnetism is caused by an effect called exchange coupling, which locks the moments of neighboring electrons in a parallel fashion.9 This occurs because the electrons move slightly farther apart than they would be normally, and this reduction in the electrostatic repulsion of the electrons is more energetically favorable than pairing the electrons. When looking at Figure 8. Density of states (DOS) diagram. ferromagnetism, it is also helpful to consider the density of states (DOS). At the Fermi level there is a large number of degenerate states in the d-band that allow for the parallel alignment of the electrons, which can be seen in figure 8. Ferromagnetism is distinguished from paramagnetism by the exhibition of magnetic forces greater that the applied field, as well as the force being sustained after the applied field is removed.8 Above the Curie temperature this spin-spin coupling will no longer be retained, the spin orientation randomizes and it becomes a paramagnet.9 In an antiferromagnetic material, the exchange coupling causes the magnetic dipoles to align in an antiparallel fashion.9 This results in the bulk-material having zero net magnetic moment. The antiferromagnetic coupling occurs at a Néel temperature above which the thermal vibrations overcome the exchange coupling and it becomes a paramagnet. A subset of 9 antiferromagnetic materials is ferrimagnets. These materials have moments of different strengths which align antiparallel so there is a net magnetic moment. What is a Solid Solution? Compositional variation in extended structures can occur in several ways: Figure 9. Color difference between end members of the solid solution series (left) Mg2SiO4 and (right) Fe2SiO4. substitutionally, interstitially, or by omission.11 This variation is allowed because in extended structures, there are no discrete molecules. In a compound with the generic formula A+X-, the cation/anion may be partially substituted by a different cation/anion such as in the replacement of magnesium in Mg2SiO4 by iron.11, 12 The difference between these two members of the series can be seen by the striking color difference between, as seen in figure 9. If this substitution is completely randomized so as to form a homogenous bulk structure, then it is called a solid solution. If the substitution is not entirely random, phase separation will occur and rather than a solid solution we will see two separate compounds separated by phase boundaries. When a solid solution is achieved, a new formula can be used to denote this varied phase: (Mg,Fe)2SiO4 for the above example. Solid Solution Criterion 10 The relative sizes of the ions in question must be compatible to form a solid solution. If the difference in the ionic radii is less that 15% complete substitution is possible.13 When the difference in ionic radii is below 30 percent limited substitution is Figure 11. Dilute magnetic semiconductor solid solution example. Figure 10. Crystal structure of Na2ZnSiO4. possible which can cause distortions in the crystal lattice.13 A difference in radii of more than 30 percent effectively eliminates the possibility of substitution. Charge balance is the second criteria. Chemical compounds must maintain electrical neutrality. For example, Fe2+ cations can replace the Mg2+ cations in the fosterite-fayalite series without other variations. If the charges of the dopant ions differ from the host ions, additional substitutions are needed to maintain electrical neutrality, this may include vacancies. What are DMS Materials? A dilute magnetic semiconductor (DMS) material is a substitutional solid solution of a semiconductor host material doped with magnetic transition metal cations. A quaternary 11 semiconductor material that has been investigated is Na2ZnSiO4, with Na2CoSiO4 being an isostructural antiferromagnetic compound. This is a hometype of the wurtzite (ZnS) structure with all ions tetrahedrally coordinated. Na2ZnSiO4 is a semiconducting compound with a band gap of ~1.7 eV. The crystal structure of this compound can be seen in figure 10. Upon one percent doping of Co2+ ions into the divalent sites of Na2ZnSiO4 the band gap remains at 1.7 eV, which shows that doping magnetic cations into a semiconductor host does not distort the electronic properties. Na2ZnSiO4 is a diamagnetic material that upon doping 1% cobalt into the divalent sites shows paramagnetic character. For application towards technologies such as “spintronics,” DMS materials have attracted a lot of attention.14 Dilute magnetic semiconductor materials randomly incorporate magnetic ions into diamagnetic host structures, as shown in figure 11. The amount of cationic sites that are substituted by doped magnetic ions is generally less than 10%.15 A formula such as Na2Zn1-xCoxSiO4 denotes a solid solution where x represents the degree of substitution. Remember, to be a solid solution the dopant ions must be randomly distributed throughout the host structure or a separation of two distinct crystalline phases will occur. These magnetic solid solutions show promise because they tend to exhibit relatively large magnetic moments with doping as low as one percent.16 A current area of investigation in DMS materials is that of "tuning" the transition temperatures ultimately creating high temperature DMS materials. This is currently not very well understood. Previous DMS Research 12 While DMS materials have been theorized to have many advantages, their development has progressed slowly. Some of these problems include phase separation, very low Curie/Neel temperatures, and the inherent difficulty in making pure solid state compounds. As a result, novel materials are needed if the use of dilute magnetic semiconductors in spintronics. Most binary compounds have been predicted to have very low curie temperatures, well below the boiling point of liquid nitrogen (77 K), making them of limited utility to modern technology.17 However, it has been discovered that one binary system does have a Curie temperature above liquid nitrogen temperatures: CoS2. Cobalt disulfide has been found to possess a curie temperature at 122 K.18 This mineral contains cobalt (II) in a structure with disulfide as the counter ion. The pyrite crystal structure has the cubic space group Pa3. Other binary iterations of CoS exist; however they are part of a continuum that is CoxSy. 19 CoS2 is one of the few stoichiometries that exist normally, with another being Co3S4.19 In this case it can be assumed that CoS is a sulfur deficient example of this continuum. Figure 12. Theoretical Curie temperatures of Mn-doped DMS materials. 13 DMS Synthesis Synthesizing a compound with a curie temperature above liquid nitrogen can be problematic, though guides exist. Promising evidence for the use of tellurium in this experiment is found in the paper by Satoru Ohta et al. “Anisotropic thermal expansion and effect of pressure on magnetic transition temperatures in chromium chalcogenides Cr3X4 with X = Se and Te.” In this research it was found that when looking at the chromium chalcogenide compound mentioned was found to transition from paramagnetism at different temperatures with selenium and tellurium. The Néel temperature for the chromium selenide compound was found to be 173 K; The Curie temperature for the telluride compound was found to be around 330 K.20 Another reason for using tellurium is the prediction made based on the Zener model for ferromagnetism. This model shows that room temperature ferromagnetism should be easier to achieve when the bulk semiconductor compound is a wide-band semiconductor. 14 CHAPTER TWO HYPOTHESIS The proposed project will investigate two solid-solution series and determine their feasibility as DMS materials. A worthwhile starting point is the determination of magnetic and electronic properties of CoTe and CoTe2. These cobalt tellurides are analogs of CoS and CoS2, the latter having a Curie temperature of 121 K which is well above the boiling point of liquid nitrogen and thus has the potential for industrial applications.18 Following this, the synthesis and characterization of the solid-solution series, (Zn,Co)Te and (Zn,Co)Te2 will be investigated. These solid-solutions will be created through doping of cobalt ions into the diamagnetic zinc telluride host structure. 15 Initial studies will focus on the (Zn,Co)Te. This synthesis must be conducted under anaerobic conditions to avoid forming unwanted products such as TeO2. The (Zn,Co)Te system will be more extensively studied as it is a simpler system than (Zn,Co)Te2. The structural properties of these materials will be examined to ensure that completely random solid-solutions have formed. The electronic and magnetic properties of the compounds will be investigated. The information elucidated from this research is of potential interest to the spintronics community. This new area of study utilizes the spin on an electron as well as charge and could allow for near infinite storage, instant boot times and decreased energy usage. CHAPTER THREE EXPERIMENTAL Chemicals Table 1. Chemicals. Company Reagent Specs Lot # Alfa Aesar Zinc powder 99.9% ~100 A10Y002 mesh Alfa Aesar Tellurium 99.5% ~200 16 G14W014 Strem powder mesh Cobalt powder 99.80% 22386800 Chemicals Equipment Table 2. Equipment. Equipment Specs Company Alumina Crucible 20 mL capacity Sigma Aldritch Pellet Press - - Agate Mortar and I.D. 50 mm Sigma Aldritch Pestle O.D. 65 mm Analytical Balance Model A-250 17 Denver Instrument ~0.0001g Co. Fused Silica Tube - - Graphite Crucible - - Synthesis 18 Ceramic synthetic methods are used for synthesis in the solid state. Reactions in the solid-state require the atoms/ions in question to interact, therefore in a synthesis the atoms need to diffuse within a solid in order to interact.9 The first step in a solid state synthesis is to determine the heating profile using differential scanning calorimetry. After Figure 13. Pellet formed using a pellet press. the reaction temperatures have been discovered, stoichiometric amounts of the reactants will be massed and ground in an agate mortar and pestle to an average particle size of 45 microns. Grinding the reactants increases the surface area of the particles and effectively the concentration of the reactants.9 After the reactants are ground they are pressed into pellets to increase surface contact and minimize void space present.9 The pellet is placed into a crucible and both are then put into a high temperature furnace to react. Atomic Diffusion Solid-state chemical reactions occur at the boundaries of particles where phases are in contact. This process is proportional to temperature since as temperature increases, the rate of diffusion increases.9 As the reaction proceeds the compound of interest forms at the particle boundaries creating a barrier to diffusion. This decreases 19 the rate of reaction, for the reactants have to diffuse across larger distances. In order to address this problem, the reactant mixture is reground and repelletized. If an atom is to diffuse through a solid two conditions need to be met: there must be a neighboring site Figure 14. Fused silica tube containing reaction vessel held in place using an alumina boat crucible. that is empty, and the atom has to have enough energy to break its bonds and then cause some lattice distortion.9 The energy that is utilized is the vibrational energy that is inherent to all systems, and an increase in temperature increases the number of vibrational modes available to an atom.2 Diffusion can occur in two different ways, through vacancies or through interstitial sites.2 Vacancy diffusion will be looked at for this reaction since the atoms that are being reacted are too large to diffuse through interstitial sites. While the atoms involved play a role in the rate of diffusion through a solid, temperature has the largest influence. An increase in temperature from 500 to 900°C can increase the diffusion coefficient of the self-diffusion of iron by around six orders of magnitude.9 Differential Scanning Calorimetry The first step in a solid-state synthesis is to determine the reaction heating profile through the use of a differential scanning calorimeter (DSC). In a DSC experiment, the temperature of an alumina reference crucible is compared to that of an equivalent 20 crucible containing a reaction mixture. As the reaction undergoes a thermal event, the voltage of the sample crucible is varied to hold it constant to that of the reference crucible. In an endothermic event, the sample requires more energy to continue heating at a constant rate and an upward peak is seen as voltage to the sample is increased. In an exothermic event, the sample is generating the energy necessary to continue heating at a constant rate and a downward peak is seen as the voltage to the sample is Figure 15. NETZSCH Pegasus F3 Differential Scanning Calorimeter. decreased. This release of energy typically means a reaction has taken place for the reaction products have adopted a lower energy state. The DSC helps to generate heating profiles for solid state reactions and coupled with x-ray diffraction, can provide insight into the solid state reaction mechanism. To gain this insight, diffractograms before and after each thermal event are studied. X-Ray Diffraction 21 X-rays provide structural information because they possess wavelengths with the same order of magnitude as the spacing between planes of atoms in crystalline material. The crystals act as a diffraction grating to x-rays. The generated interference patterns contain all the information needed to solve and refine crystal structures. X-rays are generated by bombarding a metal target with a high energy (~35 kV) electron beam. Figure 16. Rigaku Miniflex II X-Ray Diffractometer. The target material determines the properties of the x-rays produced. All experiments in this work use x-rays generated from a copper target with a λkα = 1.5418 Å. The generated x-rays are directed towards the crystal where the electronic component of the x-rays interacts with the electrons of the structure. Secondary wavelets scattered by the vibrating electrons generate interference patterns which contain the structural information. While x-ray diffraction only identifies the location of electrons, the majority of these electrons are congregated around atomic nuclei allowing for the identification of 22 the atomic sites. If planes of electron density are geometrically related so the Bragg, shown below, equation is solved, constructive/destructive interference may occur. = 2 ( ) When constructive interference occurs, a peak is generated on the diffractogram. To solve the Bragg equation wave 2 must travel an extra distance 2dsinθ shown in bold in figure 17. Each crystal structure generates a unique diffraction pattern based upon the Wave 1 Wave 2 Figure 17. Visual representation of the Bragg equation. unit cell parameters and the fractional coordinates of the atoms contained inside the unit cell. This makes this technique extremely valuable for phase analysis of crystalline materials. The ICDD PDF 4 and PDF 2 databases contain vast libraries of known diffraction patterns. Experimental diffraction patterns are compared to these databases for phase identification of products. Products that are isostructural with crystal structures collected in the databases can be used as starting models for Rietveld refinement avoiding the need for difficult structure elucidation from powder diffraction data. All Rietveld refinements will be attempted with the GSAS/EXPGUI program package. This 23 program package is freely available from Argonne National Laboratory and is currently the most used refinement package for powder diffraction. Method One All reactions must be under anaerobic conditions to prevent oxygen from oxidizing the tellurium reactant. Initially stoichiometric amounts of starting reagents were measured. The reactants were then ground for 20 minutes in an agate mortar and pestle which can reduce particle sizes down to 45 microns and increase the randomness of the reaction mixture. The reaction mixture was then pelletized to increase the contact area of the crystallites. The pellet was then transferred to an alumina crucible and placed in a tube furnace. Before heating, the tube furnace was sealed at both ends and purged with argon for one hour. As heating occurred, argon flowed continuously through the tube and out a mineral oil bubbler to prevent oxygen from contaminating the reaction. The argon gas flow was increased upon cooling to prevent atmospheric gases from entering the reaction chamber. After the hour long purge, the reaction was run using the heating profile developed from the DSC measurements. The reaction of interest was identified as the exothermic event that occurred at 420 °C. The heating profile temperature was increased to 700 °C to promote diffusion of the ions across crystallite grain boundaries. 24 After heating for 30 hours, the reaction was allowed to cool and the reactants were removed from the tube furnace. Characterization was accomplished utilizing a Rigaku Figure 18. Preliminary heating profile. Figure 19. Fused silica tube containing graphite crucible sealed with vacuum tube and pinch clamp. Miniflex 2 bench-top x-ray diffractometer. Phase analysis of the products was conducted with MDI Jade software, and the ICDD PDF 2 and PDF 4 databases. If phase analysis revealed the reaction was incomplete, the products were reground and repelletized and the heating was repeated. This was repeated until no further change was identified in the products. Method Two A second method for the synthesis of anaerobic materials was investigated. Stoichiometric amounts of reactants were ground and mixed in an agate mortar and pestle in an argon filled dry box. The reaction mixture was then added to a graphite crucible which was placed inside a fused silica tube. This fused silica tube was connected to a piece of vacuum tubing that was pinch-clamped in the dry box. 25 The fused silica tube was then taken out of the dry box, and attached to a vacuum pump figure 20. The fused silica tube was then evacuated for 30-40 minutes and firesealed with a propane-oxygen torch as shown in Figure 20. Figure 20. (left) Fused silica tube attached to vacuum pump and (above) fire-sealing fused silica tube. Once the graphite crucible was sealed inside the evacuated fused silica tube, it was placed inside the tube furnace and run with the same heating profile as the argon purged attempt. Rietveld Refinement Rietveld refinement, using the GSAS/EXPGUI program package, was used to refine the structure models. Rietveld refinement is a whole pattern fitting technique that 26 uses non-linear least squares analysis to minimize the difference between the experimental diffraction pattern and a theoretical diffraction pattern mathematically calculated from the proposed model. Structural parameters based upon the proposed model are altered to improve the fit of the calculated data to that of the experimental diffraction pattern. Starting models were taken from PDF2/PDF4 crystallographic databases. Rietveld refinement of powder diffraction data is more complicated than Figure 21. Pre-refinement liveplot. similar techniques used for single crystal analysis. In addition to all the parameters needed to fit single crystal data, the Rietveld analysis also needs to fit diffraction peak shape, background and various instrument and crystallite effects not important in single crystal diffraction. This can be very difficult for it is the result of many instrumental and crystal parameters which may be correlated. The initial structure model is represented in figure 21. The “X’s” represent the histogram of the experimental diffraction data. X-ray powder diffraction data were collected with a step size of 0.01 °, and a step time of five 27 seconds. The step width was chosen so that at least 7-13 steps were across each Bragg peak. The purple tick marks on the plot represent the possible peak locations as determined by the initial structure model. A red line represents the calculated data determined from the proposed model. A green line represents the calculated background. In this figure the calculated data are only a small fraction of the experimental data. This is due to an incorrect scaling of the experimental data and is easily corrected. The blue line represents the difference plot between the experimental and calculated data. In an ideal situation this blue line should be flat, if the model exactly represents the actual crystal structure. Close evaluation of this difference plot reveals hints as to how the proposed model should be modified. Figure 22 shows the effects of refining the scale factor. Close observation of the three peaks reveals that the first peak has incorrect peak width and therefor the peak profile parameters should be refined. The second and third peaks represent intensity issues which reveal a poor crystal structure model. Continuing modification of the structural and profile parameters will result in an improved crystal structure model that better fits the experimental data collected. 28 Figure 22. Zoomed-in liveplot showing the use of the difference plot. Some of the more important parameters that need to be modeled are: background, phase fractions, sample shift, sample transparency, Gaussian peak shape, Lorentzian peak shape, peak asymmetry, fractional coordinates, unit cell parameters, and atomic displacement parameters. Peak shapes demonstrate a combination of Gaussian and Lorentzian character and are generally modeled as a pseudo-Voigt function. How the full width at half maximum (FWHM) for each Bragg diffraction peak changes with diffraction angle are modeled with the Caligoti terms GU, GV, GW. As 2θ increases the peak widths become wider. These Caligoti terms model this phenomenon. GU and GW must be positive and GV must be negative otherwise the model makes little chemical sense. The Lorentzian character of the peaks is generally related to crystallite size and strain. Other non-structural parameters that need to be refined in Rietveld analysis is sample transparency, sample displacement, anisotropic strain broadening, and possibly preferred orientation. Once the peak profiles are modeled adequately, actual structural 29 parameters such as fractional coordinates of the atoms, atomic displacement parameters, and fractional occupancies can be investigated. The correctness of the model will be evaluated using residual indices (R-factors), goodness of fit (chi2) and chemical intuition. Rietveld refinement considers every point in a data set valid for computing least squares, with the following equation representing the quantity being minimized: ( − ) With wi being the weighting factor of the point, yoi being the observed intensity of the ith point, and yci being the calculated intensity of the point in the model. The weighting factor deals quantifies how reliable a given data point is using the following equation: = 1 With σ being the standard deviation and the σ2 term as the variance. During a refinement, the parameters are varied to generate new sets of yci to more accurately model the experimental data. There are several factors which allow for the monitoring of the refinement quality with the most useful being the weighted R-factor: = (| | − | |) In this equation Fo is the measured structure factor, Fc is the calculated structure factor, k is a scaling factor an w is the weighting factor. A typically used quantity for 30 determining the goodness of fit of the calculated model is the quantity χ2. This quantity is calculated as shown in the following equation and should approach unity. = ( ( !) "#! ) This equation shows the comparison of Rwp versus Rexp which are the weighted profile R-factor and expected R-factor respectively. The most important aspect of the refinements “correctness” is does the final structure make chemical sense. It happens quite often that the statistics of the refinement look very favorable with a crystal structure model that violates good chemical sense. This idea of good chemical sense is the most important aspect of crystal structure analysis. Peak shape modeling is unique to Rietveld analysis. Diffuse Reflectance Spectroscopy Diffuse reflectance spectroscopy (DRS) is the technique that will be used in order to study the electronic properties of the solid-solution series. DRS uses UV-Vis-NIR Figure 23. Cary 5000 UV-Vis-NIR Spectrometer. radiation to examine electronic transitions in semiconductor materials. Of particular interest are those transitions in the visible region which corresponds to energies in the 31 semiconductor band gap range. For measurements, light is directed at the sample where one of two things can happen; the light can be reflected and “lost,” or the light can be transmitted to the next particle in line where the same two things can occur.21 This can occur many times over increasing the pathlength the radiation follows.21 The scattered radiation is collected by a spherical mirror which focuses the light onto a detector.21 The generated IR light is partially absorbed by the sample and gives knowledge about the band gap of the sample.21 The data gathered from DRS is given as absorbance vs wavelength, and is shown below. This absorbance data can then be translated into a more meaningful format where the band gap can be seen. SEM Theory A scanning electron microscope does not use light like a traditional microscope, but rather uses a focused electron beam. This beam is generated through the use of an electron gun, which produces electrons by thermionic emission. After the electron beam Figure 24. SEM image of synthesized CoTe. is generated, it is passed through electromagnets in order to focus the beam upon the sample. The sample is scanned in a raster pattern, which is a line by line scan of the 32 sample. An SEM can give various types of information about the sample such as topography and crystal morphology as shown in figure 24. Energy Dispersive Spectroscopy Energy dispersive spectroscopy is a semi-quantitative measurement of the composition Figure 25. SEM image of CoTe with the yellow square showing the area to be examined by EDS. cps/eV 230 7 6 5 4 3 C O Te Co Te Co 2 1 0 2 4 6 8 keV Figure 26. Sample EDS spectra. 33 10 12 14 of a sample. EDS uses secondary electrons to investigate the sample. These secondary electrons are accelerated at the sample at around 15 keV, which is about 3-5 times higher than when only magnifying the sample. An area of the sample is chosen as shown in figure 25.When the electrons strike the sample they eject core electrons, causing outer electrons to drop into the energy vacancies. This relaxation emits energy in the form of x-rays, in much the same way as x-rays are generated for x-ray diffraction. Depending on which shell the outer electrons are in when they relax, various amounts of energy are released which are specific to each element. The x-rays are detected and used to generate a spectrum as shown in figure 26. 34 CHAPTER FOUR RESULTS AND DISCUSSION DSC analysis 3 1 4 2 Figure 27. DSC plot of (Zn,Co)Te with 10% cobalt doping. The DSC is used in order to determine the temperature at which thermal events occur. In the plot in figure 27, there are potentially four events shown. The first three exothermic events shown, numbers one, two and three, are difficult to investigate. There are several possibilities for these peaks; they could be three separate exothermic events, a single event that has been split into “separate” peaks, or a mix of these two scenarios. The second option of the three peaks really being a single peak would make sense with the melting points of zinc and tellurium being 419.5 and 449.5 °C respectively. The breaks between the exothermic peaks occur at temperatures that are a little below the melting points of both zinc and tellurium, which makes sense with freezing/melting point depression of a mixture. If the three peaks really are a single 35 peak it could be due to the melting zinc and tellurium. Further investigation of these theories is impractical for the scope of this project because the peaks are too close together to test separately. The “peak” towards the end of the DSC run on this plot is more obscure. During this run, the DSC’s thermocouple failed. This last peak could simply be an artifact of this failure, or it could be another exothermic event. To date, the DSC has not been fixed so that rerunning this experiment isn’t possible. Diffractogram based on DSC events Figure 28. Diffractogram of 10% (Zn,Co)Te using heating profile based on DSC. The diffractogram in figure 28 is an experiment run on the compound formed using the reaction temperatures that the DSC showed, ten percent doping of cobalt into zinc telluride. This diffractogram shows the reaction products of a reaction run 700°C. By using this reaction temperature the intended compound, zinc telluride, is indeed 36 being synthesized. However, it can also be seen that an unwanted product is also being produced, the zinc oxide or zincite. X-Ray Diffraction The first diffractogram is a look at initial attempts at the synthesis cobalt telluride. For this experiment, the reactants were stored, weighed, and pressed into a pellet in the in the glove box in an attempt to avoid oxidation. Once the reactants were pressed into a pellet, they were transferred into and sealed in the tube furnace as quickly as possible to minimize time spent in an oxygenated environment. The tube was purged with argon gas for an hour to try and eliminate the presence of any oxygen. Once the reaction chamber had been purged, the furnace was turned on and put through the heating program outlined in figure 29, with argon flowing through the reaction chamber: Degrees C 800 700 600 500 400 Degrees C 300 200 100 0 0 10 20 30 40 Figure 29. Modified heating profile used for later experiments. 37 50 It can be seen in the diffractogram, figure 30, that there is some cobalt oxide present in the product formed. The presence of the cobalt oxide points to the tube Figure 30. Diffractogram of CoTe run in tube furnace under flowing argon. furnace having a leak that is allowing oxygen in, even when under positive pressure from constantly passing argon through the tube. While in this product the cobalt oxide is still a minority phase, the oxide accounts for about 13.75 wt.%. Curiously, while this sample has a 13.75 wt.% cobalt oxide contamination there is something missing. Having oxygen reacting with some of the cobalt begs the question, what is happening to the tellurium that cannot be reacting with this cobalt? So far this cannot be definitively answered as the tellurium is not showing up in the diffractograms. A possibility is that the tellurium, since the reaction temperature is above its melting point, is melting and the vapor pressure is high enough that some is being lost to the flowing argon. 38 In figure 31 is the diffractogram from a reaction run under the same conditions after some changes were made to the tube furnace and its fittings, in an attempt to plug any leaks Figure 31. CoTe reacted with flowing argon after changes to tube furnace seals and fittings. that may have been present. The tubing connecting the argon cylinder was using a connector that was covered in parafilm and sealed, in addition to vacuum grease being spread on the tube and o-rings of the inlet/outlet flanges. At first glance, the cobalt oxide phase present appears reduced when compared to the previous reaction’s product. This was confirmed through rietveld refinement, which showed that the cobalt oxide phase present in this product accounted for ~3 wt.% of the total. This number is slightly skewed because the refinement was done on a diffractogram that was taken before the monochromator was added. Again, it can be seen in this product that there is cobalt oxide present and no excess tellurium. 39 Figure 32. First attempt at CoTe synthesis using graphite crucible in fused silica tube. Further attempts at eliminating the oxide contamination followed a new direction. Instead of leaving the reactants open to even the argon atmosphere, they were sealed inside an evacuated fused silica tube. The procedure for this follows the one outlined above in the experimental section. The first attempt of this the fused silica tube was allowed to vacuum for only the 1-2 minutes stated. At this point the fused silica tube was put into a muffle furnace and allowed to run through the aforementioned heating program. The initial diffractogram of this product, run through the miniflex, showed no cobalt oxide. However, this was simply due 40 to the amount of noise present before the monochromator was installed in the miniflex. Figure 33. Diffractogram pre-monochromator. When the experiment was rerun with the monochromator, the cobalt oxide phase was present in a contaminated products, a couple modifications were tried. The difference between pre- and post-monochromator experiments can be seen in figures 32 and 33. The monochromator reduced the noise present in the experiments, although this comes at a cost. The use of a monochromator reduces the intensity of the incident radiation by about 40-50%. In the cases of experiments run for this project, it turned a ~9 hour experiment into one that took more than 24 hours to finish. 41 Figure 34. Diffractogram post-monochromator. In one of the final attempts to stem the oxide contamination the previous procedure was slightly modified. In this attempt, the reactants were kept under a vacuum by the vacuum pump for around 30 minutes instead of the one-two minutes of previous trials. The diffractogram for this reaction, shown in figure 34, and subsequent refinement showed that there was still an oxide contamination. The fraction of contamination however had gone down even further to around 4.6 wt.%. With the difference in oxide percentages, there remains the question of where the contamination is coming from. Having a lower percentage with a longer vacuum time gives us some insight into some possibilities. There is possibly a small amount of oxygen in the glove 42 Figure 35. Diffractogram of compound run after extended vacuum time. box, so that when the fused silica tube is sealed inside some oxygen is present, if the O2 sensor is not working correctly. There could also be a problem with the vacuum pump and some of air is backfilling the fused silica tube when it is running. A third, and troubling, possibility is that the fused silica tube itself is leeching oxygen into the reaction vessel; This last possibility means a new reaction route would be necessary other than using a fused silica tube or a tube furnace. While the samples that have been discussed thus far have only been for the cobalt telluride end member, an attempt at various solid solutions has been made. When investigating solid solutions, it is important to cover a range of doping percentages in order to discover whether it is possible as well as the extent to which doping can occur. In this project four different amounts of cobalt doping were tried: one percent, five percent, ten percent and twenty percent. The diffractograms from these 43 products are included in Appendix A. These attempts had more pronounced problems than the reactions where just the end members were synthesized. There were more phases present, as the ten percent cobalt zinc telluride had zinc telluride, zinc oxide, cobalt oxide, cobalt ditelluride, and other peaks that couldn’t be assigned. Further runs in the furnace could have possibly allowed for the cobalt to substitute into the zinc telluride phase; however this direction was not pursued because of the amounts of oxide impurities present. No refinement was attempted on these products as the purity was poor, and peak overlap became a problem with this many phases present. Figure 36. Synthesis of (Zn,Co)Te2. In a parallel to the experiments previously discussed, an attempt was made to synthesize the ditelluride analogue (Zn,Co)Te2. The steps of this synthesis were taken 44 from the synthesis of the CoTe/ZnTe/(Zn,Co)Te syntheses. This synthesis was run using the heating profile seen in figure 28. It was run in a graphite crucible sealed inside an evacuated fused silica tube as the later experiments were run. The diffractogram can be seen in figure 35 and shows similar characteristics as the analogous experiments. It can be seen that oxide contamination is present as well as the zinc telluride that was intended. The cobalt cannot be seen in this diffractogram because the percent doping in this was not significant. Rietveld Refinement The first refinement of a cobalt telluride resulted in a chi2 of 1.932. This would Figure 37. GSAS/EXPGUI showing a problem with Uiso refinement. usually be a very good fit; however, there was a lot of noise in the experimental data. 45 During the refinement of one of the model’s parameters gave cause for further investigation. This was the atomic displacement parameter, Uiso. When this parameter was refined with oxygen in the cobalt oxide phase, the Uiso term refined to a negative, which can be seen in figure 36; a negative Uiso is the program is trying to fit the electron density present around the atom into a singularity. To look at another possibility, this same term was refined with a tellurium ion present instead of an oxygen in the supposedly cobalt oxide phase. When it was refined using these conditions, a much more appropriate number was reached. At this point it seemed possible that the oxide phase present in the product was not an oxide at all, but rather a cobalt telluride with an atypical structure. While not all of the products investigated showed this characteristic, having it happen in multiple samples gives cause for further investigation. Diffuse Reflectance Spectroscopy To investigate the electronic properties, mainly the band gap, of the synthesized compounds a UV-Vis-NIR spectrometer was used. The raw data collected with this instrument provides a plot of wavelength vs. absorbance. From here, a conversion was done using the equation: $% &'%( )* = % +1 − ,100/0 46 % 2 ∗ ,100/ 3.5 3 2.5 Absorbance (α α/s) 2 1.5 1 0.5 0 0 1 2 3 eV 4 5 6 Figure 38. DRS plot of ZnTe. Where %R is the percent reflectance, turning transmittance into absorbance. While the compounds that were investigated were not pure, they were run to get an idea of what the band gap may be. Figure 38 is the absorbance plot of zinc telluride, revealing a band gap of ~2.1 eV. Figure 39 is the absorbance plot of zinc telluride with 5% cobalt 3.5 3 2.5 Absorbance (α α/s) 2 1.5 1 0.5 0 0 1 2 eV 3 4 Figure 39. DRS plot of 5% (Zn,Co)Te. 47 5 6 doping, also revealing a band gap of ~2.1 eV. This shows that the doping of magnetic cations into semiconductor host structures has little effect on the band gap of the substance. The absorbance spectra for each of the compounds were not ideal, as the compounds had very low absorption even though the compounds were relatively dark. While this seemed to be an issue, the compounds gave workable spectra when converted using the template. When determining the band gap of a compound from the absorption spectra, following the tangent to the maximum curvature of the slope, as shown in figures 37 and 38, down to the x-axis. In the diffuse reflectance spectra of zinc telluride, the eV versus absorbance diagram can be seen above. The spectra shown tells us that the band gap for this compound is about 2.1 eV. This corresponds well with the band gap for zinc telluride, which is 2.25 eV. The difference between the experimental and the actual band gap for this compound is most likely explained by the zinc oxide impurity present in the product. The next spectra shown is the zinc telluride with five percent cobalt doping. In both of these spectra, the band gap is found to be just shy of the 2.2 eV mark. This shows us that at small doping percentages, the dopant has little effect on the band gap of the host semiconductor. Additional band gap measurements are in Appendix B. 48 Energy Dispersive Spectroscopy cps/eV 230 7 6 5 4 3 C O Te Co Te Co 2 1 0 2 4 6 8 10 12 14 keV Figure 40. EDS spectra of CoTe. During Rietveld refinements on some of the products synthesized, an anomaly was discovered; the refinements were saying that it was a very real possibility that there was no oxygen in the product, but rather another telluride phase. To get definitive data on this topic elemental analysis or something similar was needed. To investigate the chemical composition of the product materials, a Scanning Electron Microscope (SEM) with the Energy Dispersive Spectroscopy (EDS) attachment was used. The compound used for this investigation was the cobalt telluride made with the thirty minute vacuum time, which seemingly had the least cobalt oxide present thus far. Several representative sections of the product were scanned to form a more complete picture of the sample. These analyses showed that in all locations cobalt, tellurium, oxygen and carbon were found which can be seen. The data are collected in Table 3. While this just shows the presence of oxygen in the compound, the semi-quantitative determination of 49 the relative amounts of each of these elements can also be seen. The data shows that Spectrum: El AN 230 Series unn. C norm. C Atom. C Error (1 Sigma) [wt.%] [wt.%] [at.%] [wt.%] ----------------------------------------------------C 6 K-series 0.58 0.51 3.72 0.14 O 8 K-series 0.43 0.37 2.05 0.11 Co 27 K-series 37.05 32.58 48.49 1.14 Te 52 L-series 75.67 66.54 45.74 2.25 ----------------------------------------------------Total: 113.73 100.00 100.00 Table 3. Semi-quantitative elemental analysis by EDS. the atoms are present in amounts which are consistent with the amount of cobalt telluride and cobalt oxide in the synthesized products. The data of the quantitative analysis can be seen in Table 3. When the reactants were measured and added into the graphite crucible, the cobalt and tellurium were added in one to one stoichiometric amount. As we can see in the data, there is seemingly less tellurium than cobalt. This is consistent with there being cobalt oxide seen in the diffractogram, without any excess tellurium present. Also in this data, there is carbon present though not detected in any x-ray experiments. This can be explained by the sample being adhered to carbon tape in order to examine it. Additional spectra and data tables can be found in Appendix C. 50 CHAPTER FIVE SUMMARY The goal of this project was to synthesize dilute magnetic semiconductor solid solutions of zinc cobalt telluride. Initial syntheses attempted to produce pure end members of zinc and cobalt telluride. Problems were encountered when the phase analysis of the products showed the presence of oxide contamination in significant quantities. 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Darrell Henry, Cathodoluminescence Theory. http://serc.carleton.edu/research_education/geochemsheets/CLTheory.html. (accessed summer 2013) 36. Jian-Tao Han, Yun-Hui Huang, Wei Huang, Solvothermal synthesis and magnetic properties of pyrite Co1-xFexS2 with various morphologies, Materials Letters, 2005, 60, 1805-1808. 54 Appendix A: X-Ray Diffractograms 1. Figure 11: Diffractogram of Zinc Telluride 2. Figure 12: Driffractogram of Zinc Telluride 55 3. Figure 13: Diffractogram of Zinc Telluride 4. Figure 14: Diffractogram of 1% Cobalt Zinc Telluride 56 5. Figure 15: Diffractogram of 1% Cobalt Zinc Telluride 6. Figure 16: Diffractogram of 5% Cobalt Zinc Telluride 57 7. Figure 17: Diffractogram of 10% Cobalt Zinc Telluride 8. Figure 18: Diffractogram of 20% Cobalt Zinc Telluride 58 Appendix B: DRS Plots 1. 3.5 Absorbance (α α/s) 3 2.5 2 1.5 1 0.5 0 0 1 2 Figure 19: DRS plot of 1% Cobalt Zinc Telluride 3 eV 4 5 6 3 eV 4 5 6 2. 3.5 3 Absorbance (α α/s) 2.5 2 1.5 1 0.5 0 0 1 2 Figure 20: DRS plot of 5% Cobalt Zinc Telluride 59 3. 3.5 3 Absorbance (α α/s) 2.5 2 1.5 1 0.5 0 0 1 2 Figure 21: DRS plot of 10% Cobalt Zinc Telluride 3 eV 4 5 6 3 eV 4 5 6 4. 3.5 3 Absorbance (α α/s) 2.5 2 1.5 1 0.5 0 0 1 2 Figure 22: DRS plot of 20% Cobalt Zinc Telluride 60 Appendix C: EDS Spectra 1. cps/eV 228 7 6 5 4 3 C O Te Co Te Co 2 1 0 2 4 6 8 10 keV Spectrum: El AN 228 Series unn. C norm. C Atom. C Error (1 Sigma) [wt.%] [wt.%] [at.%] [wt.%] ----------------------------------------------------C 6 K-series 0.57 0.59 4.49 0.15 O 8 K-series 0.00 0.00 0.00 0.00 Co 27 K-series 28.37 29.26 45.32 0.89 Te 52 L-series 68.02 70.15 50.19 2.03 ----------------------------------------------------Total: 96.96 100.00 100.00 Figure 23: EDS Spectra and analysis ofCoTe 61 12 14 2. cps/eV 3.5 231 3.0 2.5 2.0 1.5 Te Co C O Te Co 1.0 0.5 0.0 2 4 6 8 10 keV Spectrum: El AN 231 Series unn. C norm. C Atom. C Error (1 Sigma) [wt.%] [wt.%] [at.%] [wt.%] ----------------------------------------------------C 6 K-series 0.72 0.71 4.12 0.16 O 8 K-series 1.44 1.41 6.16 0.23 Co 27 K-series 58.31 57.12 67.49 1.77 Te 52 L-series 41.60 40.76 22.24 1.25 ----------------------------------------------------Total: 102.08 100.00 100.00 Figure 24: EDS Spectra of CoTe 62 12 14 3. cps/eV 232 3.0 2.5 2.0 1.5 Te Co C O Te Co 1.0 0.5 0.0 2 4 6 8 10 keV Spectrum: El AN 232 Series unn. C norm. C Atom. C Error (1 Sigma) [wt.%] [wt.%] [at.%] [wt.%] ----------------------------------------------------C 6 K-series 0.53 0.45 3.27 0.15 O 8 K-series 0.52 0.44 2.40 0.13 Co 27 K-series 40.29 34.37 50.44 1.24 Te 52 L-series 75.89 64.73 43.88 2.26 ----------------------------------------------------Total: 117.24 100.00 100.00 Figure 25: EDS Spectra and analysis of CoTe 63 12 14 4. cps/eV 233 3.0 2.5 2.0 1.5 Te Co C O Te Co 1.0 0.5 0.0 2 4 6 8 10 keV Spectrum: El AN 233 Series unn. C norm. C Atom. C Error (1 Sigma) [wt.%] [wt.%] [at.%] [wt.%] ----------------------------------------------------C 6 K-series 0.41 0.39 2.75 0.11 O 8 K-series 0.52 0.48 2.59 0.11 Co 27 K-series 38.23 35.88 52.18 1.17 Te 52 L-series 67.39 63.25 42.48 2.00 ----------------------------------------------------Total: 106.55 100.00 100.00 Figure 26: EDS Spectra and analysis of CoTe 64 12 14