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Theses and Dissertations
5-2015
Dilute Magnetic Semiconductor Solid Solutions of
Zinc Cobalt Telluride
Michael Carl Tomik
Indiana University of Pennsylvania
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Tomik, Michael Carl, "Dilute Magnetic Semiconductor Solid Solutions of Zinc Cobalt Telluride" (2015). Theses and Dissertations.
Paper 1282.
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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. Some steps were taken in order to eliminate the contamination such as:
installation of a dry box, purchase of new chemicals, and the utilization of inert
atmosphere techniques. It was found that the intended products could indeed be
synthesized; however the oxide contamination was present in every product. The steps
that were carried out were able to reduce the quantities of the oxide contamination, but
were not able to eliminate it. The cause for the oxide contamination is not known at this
point. Further research will focus on eliminating the oxide contamination as well as
determining the cause of the oxide contamination.
51
REFERENCES
1. Johnson, Mark Spintronics. J. Phys. Chem. B, 2005, 109, 14278-14291.
2. Central Processing Unit. http://en.wikipedia.org/wiki/Central_processing_unit. (accessed April 2014)
3. Moore’s Law. http://en.wikipedia.org/wiki/Moore%27s_Law. (accessed April 2014)
4. Sankar Das Sarma, Spintronics http://www.physics.umd.edu/cmtc/earlier_papers/AmSci.pdf.
(accessed March 2014)
5. The Giant Magnetoresistive Head: A Giant Leap for IBM Research.
http://www.research.ibm.com/research/gmr.html. (accessed March 2014)
6. Atkins, Peter.; de Paula, Julio.; Friedman, Ron. Quanta Matter and Change, 2nd Edition, New York,
W.H. Freeman, 2008.
7. West, Anthony R. Basic Solid State Chemistry, second edition, New York, Wiley and Sons Inc., 1999.
8. Robey, Richard F.; Dix, William M. Magnetism and Chemical Constitution. JChemEd., 1937, 14, 414.
9. Callister Jr., William D. Materials Science and Engineering, and Introduction, 7th Edition, New York,
John Wiley and Sons Inc., 2007.
10. Michael W. Davidson, Lenz’s Law. http://micro.magnet.fsu.edu/electromag/java/lenzlaw/. (accessed
summer 2013)
11. Klein, Cornelis.; Hurlbut, Cornelius S. Manual of Mineral Science, 22nd Edition, New York, John Wiley
& Sons, Inc., 2001
12. Forsterite-fayalite series. http://www.britannica.com/EBchecked/topic/214039/forsterite-fayaliteseries. (accessed May 2014),
13. Dana, James D.; Hurlbut Jr., Cornelius S. Manual of mineralogy seventeenth edition, New York, John
Wiley & Sons Inc., 1963.
14. Yao, Tao et al. High-Temperature Ferromagnetism of Hybrid Nanostructure Ag-Zn0.92Co0.08O Dilute
Magnetic Semiconductor. Journal of Physical Chemistry C, 2009, 113, 3581–3585.
15. Rajaram, Rekha. Study of Magnetism in Dilute Magnetic Semiconductors Based on III-V Nitrides,
2007.
16. Bhattacharyya, Sayan et al. A One-step, Template-free Synthesis, Characterization, Optical and
Magnetic Properties of Zn1-xMnxTe Nanosheets. ChemMat., 2009, 21, 326–335.
17. Dietl, T.; Ohno, H.; Ferromagnetism in III-V and II-VI semiconductor structures. Physica E., 2001, 9,
185-193.
52
18. Barakat, Samira, et. al. High-pressure investigations of the itinerant ferromagnet CoS2. Physica B.,
2005, 359–361, 1216–1218.
19. Greenwood, N. N.; and Earnshaw, A. Chemistry of the Elements, Burlington, Elsevier Ltd., 1997.
20. Ohta, Satoru;, Kaneko, Takejiro.; Yoshida , Hajime. Anisotropic thermal expansion and effect of
pressure on magnetic transition temperatures in chromium chalcogenides Cr3X 4 with X = Se and Te.
Journal of Magnetism and Magnetic Materials., 1996, 163, 117-124.
21. What is diffuse reflectance spectroscopy?.
http://www.nuance.northwestern.edu/KeckII/Instruments/FT-IR/keck-ii%20pages2.html. (accessed on
August 25, 2013)
22. Bloss, Donald F. Crystallography and Crystal Chemistry, Holt, Rinehart, and Winston Inc., 1994.
23. Andara, Angel Jose´; Heasman, David M.; Fernandez-Gonzalez, Ängeles; Prieto, Manuel.
Characterization and Crystallization of Ba(SO4,SeO4) Solid Solution, Crystal Growth and Design, 2005, 5,
1371-1378.
24. F. Albert Cotton, Geoffrey Wilkinson, Carlos A. Murillo, and Manfred Bochmann, Advanced Inorganic
Chemistry, New York, John Wiley and Sons Inc., 1999.
25. Virginia Lea Millera, Wei-li Lee, Gavin Lawes, Nai-PhuanOng, Robert J. Cava, Synthesis and properties
of the Co7Se8_xSxand Ni7Se8_xSxsolid solutions, Journal of Solid State Chemistry, 2005, 178, 1508–1512.
26. G. R. Waitkins, A. E. Bearse, and R. Shutt, Industrial Utilization of Selenium and Tellurium, Industrial
and Engineering Chemistry, 1942, 34, 899-910.
27. Pavle V. Radovanovic,Carl J. Barrelet,SilvijaGradecak,Fang Qian,and Charles M. Lieber, General
Synthesis of Manganese-Doped II−VI and III−V Semiconductor Nanowires, Nano Letters, 2005, 5, 14071411.
28. Xiao-Hong Xu, Xiu-Fang Qin, Feng-Xian Jiang, Xiao-Li Li, Ya Chen, G.A. Gehring, The dopant
concentration and annealing temperature dependence of ferromagnetism in Co-doped ZnO thin films,
Applied Surface Science, 2008, 254, 4956–4960.
29. Gavrichev, K.S., Sharpataya, G.A., Guskov, V.N., Greenberg, J.H., Feltgen, T., Fiederle, M., Benz, K. W.,
High-temperature heat capacity and thermodynamic functions of zinc telluride, Thermochimica Acta,
2002, 381, 133-138.
30. Schmitt, Andrew L., Higgins, Jeremy M., Song Jin, Chemical Synthesis and Magnetotransport of
Magnetic Semiconducting Fe1-xCoxSi Alloy Nanowires, Nano Letters, 2008, 8, 810-815.
31. A. Ioachim, M.I. Toacsan, M.G. Banciu, L. Nedelcu, H. Alexandru, C. Berbecaru, D. Ghetu, G. Stoica
BNT ceramics synthesis and characterization, Materials Science and Engineering B, 2004, 109, 183-187.
53
32. G. Korotcenkov, B.K. Cho b, M. Nazarov, Do Young Noh, E.V. Kolesnikova, Cathodoluminescence
studies of un-doped and (Cu, Fe, and Co)-doped tin dioxide films deposited by spray pyrolysis, Current
Applied Physics, 2010, 10, 1123–1131.
33. Lisa C. Roof, and Joseph W. Kolis, New Developments in the Coordination Chemistry of Inorganic
Selenide and Telluride Ligands, Chemistry Review, 1993, 93, 1037-1080.
34. Charles V. Rice, Guinevere A. Giffin, Quantum Dots in a Polymer Composite: A Convenient Particle-ina-Box Laboratory Experiment, Journal of Chemical Education, 2008, 85, 842-844.
35. 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