LUMINESCENT PROPERTIES OF CHROMIUM AND MANGANESE IN SOL-GEL

PRODUCED ALUMINA

by

XIAOLU XI

(Under the Direction of UWE HAPPEK)

The growth of materials via the sol-gel method offers a viable alternative to traditional

high temperature melt growth processes. The sol-gel process is a wet chemical method for

producing solid inorganic materials from solutions. Sol-gel technology has evolved considerably

over the last decade and founds its way in a wide range of practical applications from catalyst

support, to thermal insulation, from solar cells to oxygen food sensors or sunscreen formulations.

The sol-gel process can be used to produce alumina, Al2O3, in a variety of forms from porous,

transparent monoliths to ceramics. In addition, the sol-gel growth technique enables the doping

of transition alumina with precise amounts of optically active impurity ions. These ions probe the

structure and dynamics of the materials through interactions with the crystal field produced by

the host ions. In this work, we report on the luminescent properties of chromium and manganese

ions doped in alumina. The goal of this dissertation is to produce Cr3+ and Mn4+ doped in

alumina using sol-gel technique and to acquire a fundamental knowledge in the area of

luminescence spectroscopy.

INDEX WORDS: sol-gel, transition alumina, rare earth ions, luminescence, chromium,

manganese

LUMINESCENT PROPERTIES OF CHROMIUM AND MANGANESE IN SOL-GEL

PRODUCED ALUMINA

by

XIAOLU XI

B.S, Tianjin Institute of Technology, China, 2001

A thesis Submitted to the Graduate Faculty of The University of Georgia in Partial Fulfillment of

the Requirements for the Degree

MASTER OF SCIENCE

ATHENS, GEORGIA

2004

© 2004

Xiaolu Xi

All Rights Reserved

LUMINESCENT PROPERTIES OF CHROMIUM AND MANGANESE IN SOL-GEL

PRODUCED ALUMINA

by

XIAOLU XI

Major Professor: Uwe Happek

Committee: Uwe Happek F. Todd Baker William M. Dennis

Electronic Version Approved: Maureen Grasso Dean of the Graduate School The University of Georgia August 2004

iv

ACKNOWLEDGEMENTS

It is a very long road that has taken me to the point where I could write this page. I never

would have completed this project without the patience and guidance of Dr. Uwe Happek. I

could not have ever asked for such a mentor, as he helped me learn to think, to write, and to find

a greater appreciation for science. Through his guidance and thirst for physics we not only

produce nice experimental results, we enjoyed the research. I also thank my other two long-

standing committee members, Dr. F. Todd Baker and Dr. William M. Dennis, for seeing me to

the finale of this project. Although the people listed above were the only ones with the teacher

status, I found many other teachers in my fellow graduate students. I would like to thank

members of my group; Long Pham, Bo Wen, Paul Schmidt, and Steve Cox. Not only did they

help make the graduate experience fun and exciting, but through many long discussions helped

to further my understanding of physics. Without the support of this large group of friends my

experience here would have been much more limited.

I also want to say thanks to my parents, who gave me my original love for academics and have

been very supportive of my graduate experience. The biggest supporter of my graduate career

has been my husband, Christopher McGee. He has always pushed me to do my best and has

never let me settle for anything less.

All of my family, friends, and colleagues have been an integral part of my education at the

University of Georgia: I thank all of you for your support and for all the good times we have had

together.

v

CONTENTS

Page

ACKNOWLEDGEMENTS........................................................................................................... iv

LIST OF FIGURES...................................................................................................................... vii

CHAPTER

1 THE SOL-GEL TECHNIQUE.......................................................................................1

1.1 INTRDUCTION................................................................................................1

1.2 THE ALKOXIDE PRODUCTION OF ALUMINA.........................................3

2 ALUMINA…………………………………………………………………………...8

2.1 HISTORICAL COMMENTS………………………………………………...8

2.2 CEMAMIC ALUMINA…………………………...………………. ………..9

2.3 NANOCRYSTALLINE ALUMINA……………………………………….10

3 POLYCRYSTALLINE ALUMINUM………………………………………….….14

3.1 BACKGOUND……………………………………………………………..14

3.2 MATERIAL STRUCTURE ISSUES………………………………………16

3.3 ENERGY LEVEL OF 3d3 ELECTRON SYSTEMS……………………....19

4 EXPERIMENT CONCERN......................................................................................27

4.1 ABSORPTION................................................................................................27

vi

4.2 EMISSION IN SOLIDS..................................................................................27

4.3 PHOTOEXCITATION……………………………………………………...29

5 CHROMIUM DOPED ALUMINA ...........................................................................30

5.1 SAMPLE PREPARATION.............................................................................30

5.2 MEASUREMENT...........................................................................................31

5.3 EMISSION SPECTRA………………………………………………………31

6 MANGANESE DOPED ALUMINA ......................................................................38

6.1 SAMPLE PREPARATION…………………………………………………38

6.2 LUMINESCENT SPECTRA……………………………………………….38

7 CONCLUSION…………………………………………………………………...42

REFERENCES..............................................................................................................................43

vii

LIST OF FIGURES

Page

Figure 1: Alumina Oxide Powder (left) and crucible (right) .........................................................11

Figure 2: The various mineral forms of alumina and their transformation routes.........................17

Figure 3: The unit cell of gamma-alumina....................................................................................20

Figure 4: Primitive unit cell of Mg spinel.....................................................................................21

Figure 5: Gamma-alumina sample……………………………………………………………….22

Figure 6: The structure of alpha-alumina………………………………………………………...23

Figure 7: alpha-alumina sample………………………………………………………………….24

Figure 8: Octahedral symmetry with lines connecting oxygen ions in planes perpendicular to c-

axis…………………………………………………………………………………….25

Figure 9: The energy levels of a 3d3 electronic system in an octahedral crystal field…………...26

Figure 10: Optical layout………………………………………………………………………...34

Figure 11: Experiment setup……………………………………………………………………..35

Figure 12: Emission spectra of Cr3+ doped in alpha-alumina………..…………………………..36

Figure 13: Emission spectra of Cr3+ doped in gamma-alumina at different temperatures………37

Figure 14: Emission spectra of Mn4+ doped in alpha-alumina at room temperature…….............40

Figure 15: Emission spectra of Mn4+ doped in alpha-alumina at 145K…….…………………...41

1

CHAPER 1

THE SOL-GEL TECHNIQUE

1.1 INTRODUCTION

The interest in the sol-gel processing of inorganic ceramic and glass materials began

as the mid-1900s. The sol-gel method was used in the 1950s and 1960s to synthesize a

large number of ceramic oxide compositions, involving Al, Si, Ti, which could not be

made using traditional ceramic powder methods [1]. The sol-gel process is a wet

chemical method involving the sol and an intermediate gel stage, finally converting to

solid materials. The sol-gel method has better homogeneity than mixed powder

techniques, higher purity than mineral raw materials, more flexibility in the final

materials form and the lower processing temperatures [1]. The temperature required

for the initial stages in the sol-gel method is low, usually close to room temperature.

Also, the sol-gel technique allows synthesizing different forms of compounds, and

different forms of chemicals can be produced at different temperatures. Highly porous

materials, micro-crystalline and nano-crystalline materials may be prepared in sol-gel

method [1].

Thin film or coating deposition represents the oldest commercial application of

2

sol-gel technology. Coatings for rearview mirrors and anti-reflective and architectural

applications have been in commercial production since the 1960s [2]. Today, sol-gel

thin film coatings are used for such diverse applications as protective and optical

coatings, passivation and planarization layers, sensors, high or low dielectric constant

films, inorganic membranes, electro-optic and nonlinear optical films,

electrochromics, semiconducting anti-static coatings, superconducting films,

strengthening layers and ferroelectrics.

The goal of sol-gel processing is generally to control the surfaces and interfaces of

materials during the earliest stages of production. The potential of improved

properties via growth control, higher purity and greater homogeneity has been

realized. There are three approaches to make sol-gel monoliths: 1. gelatin of a

solution of colloidal powders followed by drying of gel; 2. hydrolysis and

polycondensation of alkoxide or nitrate precursors; 3. hydrolysis and

polycondensation of alkoxide precursors followed by aging and drying under ambient

atmospheres [3]. A sol is a dispersion of colloidal particles in a liquid. A gel is an

interconnected rigid network with pores of sub micrometer dimensions and polymeric

chains whose average length is greater than a micrometer. During mixing, a

suspension of colloidal powders is formed by mechanical mixing of colloidal particles

in water at a pH that prevents precipitation. When we perform hydrolysis, a metal

alkoxide is dispersed in water to form a solution. The size of the sol particles and the

cross-linking within the particles depend on the pH value. These polymers grow to

3

colloidal dimension (0.1 to 10 micrometers), and subsequently link together to

become a three-dimensional network. The characteristics of the gel network depend

on the size of particles and extent of cross-linking. The gel is cast into a mold and left

to dry for a period of time, several hours to days to remove water and organics. The

strength of the gel increases during aging. After aging, the sample is heated to a

temperature in the range of 90˚C to 500˚C. After the drying, the final step is called

densification. Heating the gel at high temperature causes densification, and the pores

are eliminated. The temperature depends on the dimensions of the network, the

connectivity of the pores and surface area.

1.2 THE ALKOXIDE PRODUCTION OF ALUMINA

With applications in the areas of pollution control, separation technology and

microelectronics, a monolithic transparent active alumina has been developed. The

material is formed from alumina alkoxides, Al (OR)3, where R can stand for C3H7.

During the process alkoxides are hydrolyzed and the resultant hydrate is peptized to a

clear sol. When hydrolyzation and peptization are performed under certain conditions,

a clear sol is produced which may be gelled and pyrolyzed to produce nanoparticulate

transparent alumina [3]. According to the Yoldas paper [4], peptization requires that a

critical amount of certain acids be introduced into the slurry, and the slurry must be

kept above 80˚c for some time� Aluminum hydroxide sols can be created from a

number of different aluminum compounds. The reaction forms the gel by releasing

4

the respective alkane:

Al(OR)3 + 2H2O � ALOOH + 3ROH

Their hydrolysis is easier to control. The lower alkoxides are associated, even in

solution and in the vapor phase. The degree of association depends on the bulkiness of

the alkoxy group, eg, the freshly distilled isopropylate is trimetric [5]:

Since the aluminum hydroxides’s solubility in solutions of intermediate pH is very

low, supersaturating and rapid precipitation is caused by a little change in the pH of

the solution. The acid must also strong enough to produce the charge effect with

respect to aluminum concentration. In other words, the amount of acid in relation to

aluminum must not be too large to prevent the formation of a continuous aluminum

bonding through oxygen or hydroxides.

1.2.1 HYDROLYSIS AND CONDENSATION

5

The technique used in my work, which is based on Yolda’s method [4], starts with the

hydrolysis of an aluminum alkoxide, Al(OR)3. The hydrolysis must be done at a

temperature above 80oC to prevent the formation of the stable precipitate Al(OH)3,

aluminum hydroxide [4]. Hydrolysis and polycondensation occur at the same time.

Al(OR)3 + 2H2O � ALOOH + 3ROH

Stirring the solution for about 30 mins, 0.73 moles of HNO3 are added per mole of

alkoxide. The variables of major importance are temperature, nature of the solvent,

and type of alkoxide precursor. The melting point of alumina oxide is 2020˚C. But by

using sol-gel method, the metal oxide bonds can form at temperatures of 90˚C to

100˚C. The condensation is irreversible and bonds cannot be hydrolyzed after they are

formed, the condensation process may form cross-links, finally forming the 3-D

network. After the formation of the polymer network and the removal of excess water,

a specific concentration is reached, and the forming of the gel starts.

1.2.2 GELATION

The gelling process is easy to observe qualitatively and easy defined in abstract terms,

but difficult to measure analytically. As the sol particles grow and collide,

condensation occurs and macro particles form. The sol becomes a gel when it can

support stress [3]. The sol changes from a viscous fluid to an elastic gel. This change

6

is gradual as more and more particles become interconnected. The initial gel has very

high viscosity and low elasticity. Then the doped sol is heated to the boiling point to

remove excess water and the alcohol via heating and drying. After removal of 80

percent of the volume, the viscosity undergoes an abrupt increase, thus obtaining the

doped alumina gel.

1.2.3 AGEING

The structure and properties of a gel continue to change. The gel is kept in a mold for

a period of time, hours to days - a process called aging. There are four processes that

can occur. These are labeled polycondensation, synerisis, coarsering, and phase

transformation [3]. Ageing effects are often considered as a significant disadvantage

in the use of sol-gel materials in technological applications. However, ageing usually

improves the properties of the material. The ageing process can be controlled varying

the pH, temperature, pressure, ageing liquid medium and initial precursor mixture

composition.

1.2.4 DRYING

During heat treatment, the remaining water and organic materials from the solution

are removed in the form of gases. As the residual gases are removed, the colloidal

7

particles of the gel cement together into a continuous phase. There are four main

stages in the drying of a gelled sample [2]:

1. The constant rate period. When the water has evaporated, a gel will shrink first.

This phase can only occur in gels which are flexible and can be adjusted to the

reduced volume.

2. The critical point. As the gel dries and shrinks, its more compact structure and

associated cross-linking lead to increased stiffness. At this point the liquid begins to

recede into the porous structure of the gel.

3. First falling-rate period. Liquid transport occurs by flow through the surface films

that cover partially empty pores. The liquid flows to the surface where evaporation

takes place.

4. second falling-rate period. At this stage, the pores have emptied and surface films

along the pores can not be sustained. The remaining liquid can escape by evaporation

from within the pores and diffusion of vapor to the surface.

1.2.5 HEAT TREATMENT

The remaining water and alcohol are removed from the solution during heat treatment.

The treatment temperatures are 750˚C and 1200˚C for gamma and alpha alumina. The

materials are held at the temperatures for one hour, which allows enough time for

complete conversion.

8

CHAPTER 2

ALUMINA

2.1 HISTORICAL COMMENTS

Aluminum oxide is low-cost high-performance ceramic. Because of its high hardness,

it can be used as cutting tools and abrasives. It can also be used as electrical insulation

because of its high electrical resistivity. In addition, the material is chemically inert,

and has high melting points[6]. By using the sol-gel method, a number of different

compounds can be produced at different temperatures. γ-alumina is one of many

polytypes of Al2O3 that is used extensively as a catalytic support material because of

its high porosity and large surface area. However, at temperatures in the range

1000-1200ºC, γ-alumina transforms rapidly into the stable α-alumina form

(corundum), and the pores get closed, thus suppressing the catalytic activity of the

system. The form transformation can be slowed down by doping γ-alumina with one

of many elements such as Mn and Cr [7].

The incorporation of various transition metal cations can cause alumina coloration.

Alumina doped with Cr3+ is red, while alumina doped with Mn4+ is pink. Though the

science of form transformations is highly developed, the atomic-scale mechanisms

9

and the role of dopants in specific systems remain wide open [8]. In contrast to the

transition alumina, nanocrystalline α-Al2O3 traditionally has been difficult to

synthesize. Over the past year, two new routes to nanocrystalline α-Al2O3 have been

identified. An exciting aspect of these processes is that they use precursors that are

both synthesized and transformed to α-Al2O3 at certain pressure. Hence, they

circumvent the principle difficulty associated with synthesis from α-AlOOH and may

well offer additional advantages. A major obstacle to support and adsorbent

applications still exists, however, because nanocrystalline α-Al2O3 is prone to rapid

sintering and surface-area loss between 500°C and 1000°C, temperatures where

γ-Al2O3 and the other transition aluminas resist coarsening. Prior studies have shown

that dopants and impurities can have significant effects on the coarsening of transition

alumina and their conversion to α-Al2O3, as well as sintering and microstructure

development in bulk α-Al2O3. Although the mechanisms underlying such effects are

often subject to debate, dopants have the potential to impact both the thermodynamic

(e.g., interfacial energies) and kinetic factors (e.g., diffusivities and boundary

mobilities) that are pertinent to the respective processes.

2.2 CERAMIC ALUMINA

Alumina Ceramics can be use as fire retarders, grinding powders, catalysts and

catalyst supports [9,10,11,12]. In 1979, the alumina-catalyzed dehydration of ethanol

was discovered by Dutch chemists. Since then, the applications of alumina in catalytic

10

has been increased. The increasing emphasis on the development of structural

ceramics for high-temperature causes a higher need for understanding creep fracture

in the materials.

2.3 NANOCRYSTALLINE ALUMINA

Nanosized materials are those particles - organic, inorganic or a combination – that

are on a nanometer scale (in the order of one-billionth of a meter, or 10-9 meter).

These particles can be amorphous, semi-crystalline (certain rocks composed partly of

crystalline, partly of amorphous matter), or crystalline. In general, they are less than

100nm in size. We distinguish two main classes of nanosized material: nanophase and

nanocomposite. Nanophase materials provide closer control of product properties.

Nanophase ceramics are stronger and more ductile than conventional ceramics. They

can also be sintered at low temperatures, increasing the range of possible substrates

and lowering processing costs. Nanophase materials are generally monolithic pure

materials, such as titanium oxide. In nanocomposite ceramics, a small ceramic particle

will be entrapped within another particle [8]. An example of a nanocomposite is

nanometer-sized silicon carbide inside of alumina. Nanocomposite ceramics can

exhibit good wear resistance, chemical inertness, corrosion resistance and thermal

insulating properties. The first alumina form produced in the sol-gel process is called

γ � alumina� γ � alumina is highly porous material composed of nanoscale crystallites

11

Fig 1. Aluminum oxide powder (left) and crucible (right).

12

[13]. Nanosized or nanostructured materials exhibit many interesting properties and

are increasingly being used for new and innovative applications. These properties are

caused by the small scale over which the material possesses translational symmetry.

With the often pronounced difference between optical properties, the development of

new emissive materials depends on our understanding of the basic processes affecting

the structure and dynamics of these new materials. The synthesis of nanophase

materials, including nanoceramics, has lead to the potential to become another

materials revolution. Nanophase material properties can be manipulated to provide a

designer with the specific material needed to build a product that is strong and can

withstand wide temperature variations, while having special optical, electrical or

magnetic properties. The ability to manufacture a nanocrystalline ceramic material at

lower temperatures is a great advantage that could result in economical production of

flawless, high-precision ceramics using techniques similar to those in the

powder-metal industry. Nanostructured materials are being developed and used for

diverse applications that exploit their magnetic, optical, electronic, catalytic and other

properties. The unique properties of nanocrystalline ceramics have opened a wide

range of applications, including durable ceramic parts for automotive engines, cutting

tools, ultrafine filters, abrasive, flexible superconducting wire and fiber-optic

connector components [14]. Nanocrystalline materials have large surface-to-volume

ratios, comparing to bulk materials, whose surface layers comprise a small fraction of

the material.

13

Corundum contains ten ions per unit cell. The large unit cell makes the structure of

sapphire and forces calculations of the surface structures relatively complicate. We

like to note that the chemical complexity of metal oxides is very important. Because

of that, the electronic structure of the metal oxides encompasses systems ranging from

large bandgap insulators to superconductors.

14

CHAPTER 3

POLYCRYSTALLINE ALUMINA

3.1 BACKGROUND

Alumina is the most cost efficient, versatile and widely used material in the family of

engineering ceramics. The raw materials from which this high performance technical

grade ceramic is made are readily available and reasonably priced, resulting in a good

cost return for fabricated alumina parts. With an excellent combination of properties

and an attractive price, it is no surprise that fine grain technical grade alumina has a

very wide range of applications. Aluminum oxide, commonly referred to as alumina,

possesses strong ionic interatomic bonding giving rise to its desirable material

characteristics. It can exist in seven crystalline phases which all revert to the most

stable hexagonal alpha phase at elevated temperatures.

Alpha phase alumina is the strongest and hardest of the oxide ceramics. Its hardness,

excellent dielectric properties, refractoriness and good thermal properties make it the

material of choice for a wide range of applications. The type or structure of each

alumina and its temperature range of existence are determined by the structure of the

starting hydroxide; they are different for gibbsite, bayerite, nordstrandite, boehmite or

15

diaspore. Extensive literature exists on the dehydroxilation of the crystalline

hydroxides, especially on gibbsite, because it is the phase formed in the industrial Bay

Process. The seven aluminas are called “Transition Aluminas” and are labeled by

Greek letters to identify them: gamma; delta; theta; kappa; chi; eta and rho. The

transition aluminas are widely used in industry as adsorbents, catalysts or catalyst

carriers, coatings and soft abrasives because they have fine particle, high surface area

and the catalytic activities of their surfaces [7]. The transition alumina (the different

phase of alumina due to different temperature) refer to the group of partially

dehydrated alumina hydroxides. Fig 2 shows the various mineral formations of

alumina and their transformation routes to the final form of corundum. The

temperatures depend on the purity and crystallinity of starting materials and the

thermal treatment. The temperature ranges of stability for the transition aluminas are

approximate. All the transition aluminas are reproducible and stable at room

temperature, but the transformation is irreversible upon cooling. Thus they are called

transition forms instead of different phases. These properties of the transition

aluminas have attracted a lot of attention to their structures. Most research has focused

on dehydroxylation and the transformation mechanism, porosity and specific surface

area, surface structure and chemical reactivity, and the defect crystal structure. The

numerous applications and material have made the production and distribution of

transition aluminas more and more popular.

As mentioned above, we distinguish seven different transition aluminas: Gamma,

16

delta, theta, kappa, chi, eta and rho. 1. Gamma-alumina has a defect spinel structure

2. Delta-alumina are pseudomorphs from rhombs of gamma-alumina and from

boehmite. 3. Eta-alumina has small round particles, probably from bayerite crystal.

4. Kappa-alumina is only formed from heating chi-alumina and that it transforms

above 1000˚C into alpha-alumina. 5. Rho-alumina is unstable and must be kept

permanently under high vacuum. 6. Theta-alumina, are agglomerates of round

particles. 7. Chi-alumina can only be produced by gibbsite crystals and form

hexagonal plates. Apha-alumina is not a transition alumina, because it is the final

form of the alumina. Beta-alumina is not a transition alumina, it has the composition

Na20.11Al2O3.

3.2 MATERIAL STRUCTURE ISSUES

3.2.1 GAMMA ALUMINA

Gamma-alumina is an enormously important material in catalysis. It is used as a

catalyst in hydrocarbon conversion (petroleum refining), and as a support for

automotive and industrial catalysts [15] � Gamma-alumina behaves as a “ reactive

Sponge” in that it can store and release water in a reactive way. This chemical activity

offers a basis for understanding long-standing puzzles in the behavior of aluminas in

catalytic systems [15]. Even through gamma alumina is so widely used, the

experimental data available about its structure has given rise to a long history of

17

Fig 2. The various mineral forms of alumina and their transformation routes to the

final form of corundum

18

disagreement among scientists[16]. Gamma alumina (Fig 5) forms with a defect

spinel structure: in the normal spinel (MgAl2O4), 32 oxygen anions and 24 cations

form the unit cell. In gamma alumina, Al atoms occupy both the tetrahedral and

octahedral cation positions among the cubic close-packed oxygens. To satisfy the

Al2O3 stoichiometry of aluminum oxide, in gamma alumina 2 2/3 of the 24 cation

sites need to be vacant (Fig. 3). That is the first point of view, obtained through X-ray

diffraction. The second point of view comes from chemical analysis. Gamma alumina

actually contains hydrogen (H) in its composition. These reports cast doubt on the

validity of the widely accepted defect spinel structure [16]. In a paper by Karl

Sohlberg[16], calculations provide an energy profile for dehydration of gamma

alumina and show that HAl5O8 is a perfect hydrogen spinel. It is easily described in

terms of the closely related Mg spinel. The cubic cell of Mg spinel has 24 cations and

32 anions, (Mg8Al16O32) but the primitive unit cell is a rhombohedron with the

contents Mg2Al4O8 (Fig 4).

3.2.2 ALPHA ALUMINA

After water is removed from the gamma alumina structure and the order of the cation

lattice increases, around 1250˚C, alpha alumina (Fig. 7) is formed, which has the

corundum structure (Fig. 6). Alpha alumina is important because of its great

absorptive power and its catalytic properties, and it is also used in the manufacture of

synthetic sapphire. Corundum is comprised of its constituent elements (aluminum and

19

oxygen) arranged in the trigonal system of crystal symmetry. The ionic radius of O2-

is 1.35 Ao and two thirds of the interstices between the oxygen layers are filled with

Al3+, with radius 0.54 Ao. Each Al3+ ion is octahedrally coordinated, with a single site

symmetry of C3v (Fig 8) [5].

3.3 ENERGY LEVEL OF 3d3 ELECTRON SYSTEMS

Both Cr3+ and Mn4+, the ions used in this study, have the electronic 3d3 configuration

(Fig. 9). The ground state of the electronic configuration is a 4F state. Under the

influence of an octahedral field the ground state splits to three energy sublevels

(4A2,4T1,

4T2). Mn4+ (3d3) is isoelectronic with Cr3+, and thus the spectra of the two

ions are similar. In both of the samples, the 2E level is assumed to be the lowest

excited. The 4A2 -> 4T2 absorption transition is a broad band. A strong zero-phonon

line is observed together with a weak vibronic sideband which appears on the high

energy side of the sharp line in absorption, and on the low energy side in emission

[17]. In Cr3+ doped alumina, the electrons promoted into the 4T2 state rapidly relax

back to the 2E state non-radiatively, and then decay radiatively from 2E state and the

ground state.

20

Fig 3 the unit cell of gamma alumina. Large open circles are oxygen ions. The small

open circles are octahedrally coordinated alumina. The full circles are tetrahedrally

coordinated alumina [20]

21

.

Fig 4 Primitive unit cell for Mg spinel (Mg2Al4O8). The Mg atoms (green) are

tetrahedrally coordinated by O (����� ), and the Al (purple)atoms are octahedrally

coordinated by O [12].

22

Fig 5 gamma-alumina sample

23

Fig. 6: the structure of alpha alumina, corundum. The open circles are the oxygen

anions. The full circles are the octahedral positions of the aluminum cations [5].

24

Fig. 7: Alpha-alumina sample.

25

Fig. 8: Octahedral symmetry with lines connecting oxygen ions in planes

perpendicular to the ( 111 ) or c-axis [21].

26

Fig. 9: The energy levels of a 3d3 electronic system in an octahedral crystal field [14].

27

CHAPTER 4

EXPERIMENTAL CONCERNS

4.1 ABSORPTION

The Cr3+ ions in sapphire crystals give ruby its red color. The absorption spectra of

impurity ions are responsible for the coloration of many materials. The total transition

probability per unit time is given by Fermi’s Golden Rule [18]:

Γ=2π/h|�f � e.r � i � |2ρδ(Ef-Ei-hν),

where the delta function is included to emphasize energy conservation. In the

identification of impurity states, absorption spectroscopy is very useful because we

can map out the energy levels of the ion. The result of illumination a low temperature

sample will be the absorption of photons that have energy resonant with a ground

state to excited state transition. Thus, absorption spectra give the energy levels of the

excited state levels.

4.2 EMISSION IN SOLIDS

Atoms emit light by spontaneous emission when electrons in excited states relax to a

lower level by radiation. In solids the radiative emission process is called

28

luminescence [17]. In our experiment, each photon of green light (532 nm) absorbed

by the alpha or gamma Al2O3 raises the energy of a Cr ion to an excited state. This

excited state rapidly decays to the metastable 2E state with a lifetime τ. Before the

ions are excited, virtually all the Cr ions are in the ground state, because the large

energy difference between the ground state and first excited state is much larger than

kT. After turning on the laser, the number of excited ions increases. The radiative

emission rate for radiative transitions between two levels is:

(d N/dt)radiative = -AN

A is Einstein A coefficient, N is population [18].

After time t of weak excitation, the number of excited ions is given by

N (t) = No [1- exp (- t /τ)],

Where No is the number of ions which are excited after the crystal has been weakly

illuminated for a time t >>τ. Neglecting stimulated emission, the excitation process

comes to equilibrium when the rate of excitation equals the rate of decay from the

metastable state.

The emission spectra cannot only provide information on the energy levels, but

enission can also be used to obtain the temperature of the sample, in cases where two

or more closely spaced (in terms of energy) emission lines are available. We assume

the impurity is in thermal equilibrium with its surroundings, so the ratio of ions in any

two energy levels is:

N2/N1= e-∆E/KT

29

We find in our experiments that the temperature of the cold finger is much lower that

the temperature of the sample.

4.3 PHOTOEXCITATION

Since absorption measurements depend on the change in transmitted light as a

function of frequency, they have a large background and small absorption coefficients

can be difficult to detect. The absorption signal is given by the product of coefficient

absorption and sample thickness. The combination of absorption data and

photoexcitation measurements aids in assigning the excited state levels. In

photoexcitation, a particular emission from the excited state manifold is monitored,

while the excitation is tuned through the absorption region. When the excitation

comes into resonance with an excited state level, a peak is seen in the emission.

Photoexcitation measurements are more specific than absorption in that they show

only absorption features that lead to emission at a specific energy, and are

site-selective. Photoexcitation has the benefit that it is more sensitive than regular

absorption measurements, due to being a zero background technique [19].

30

CHAPTER 5

CHROMIUM DOPED ALUMINA

5.1 SAMPLE PREPARATION

In order to synthesize chromium doped alumina, we followed the sol-gel method

described by Yoldas [4]. The following recipe is used:

1. Measure 16.67 ml deionized water, and heat it to 90˚C.

2. Add Chromium(III) nitrate, 98.5% (Assay), Cr(NO3)3� 9H2O.

3. Slowly add 2.12 g Al(OC3H7)3 while stirring. The Cr(NO3)3 solution be 0.1%

molar of the Al(OC3H7)3. Thus 0.0000103mol (0.000413g) Cr(NO3)3� 9H2O

should be added. Since the purity of Chromium(III) nitrate is 98.5%, add

0.000413/0.985=0.0004196 g.

4. Stir for 30 minutes.

5. Add 0.73 ml 1M HNO3.

6. Let the sample stay in room temperature for a night, then move it into furnace

preheated to 95˚C.

31

7. Heat the sample to a temperature between 760˚C and 1100˚C, which results

γ-alumina.

8. To obtain α-alumina, heat to 1200˚C.

5.2 MEASUREMENTS

Alpha-alumina doped with Cr3+ is so called Ruby. Visible radiation resonant with the

4T2 state or 4T1 state can be used to optically excite the Cr3+ ion. The life times of

these levels are short, so that the excited atoms quickly relax by making a transition to

the long-lived metastable 2E state. The energy that is lost in this process is

nonradiative and goes into heating the crystal by generating phonon or vibrational

excitations of the crystal atoms. This metastable 2E energy level has a lifetime of a 3

milliseconds and emits the so-called “R-line” at a wavelength of 694.3 nm.

5.3 EMISSION SPECTRA

Three different laser sources were used to excite the alpha alumina and gamma

alumina at different temperature. The three laser sources were 632.8 nm He-Ne laser,

a 532 nm green frequency doubled Nd:YAG laser and an argon-ion laser. The

32

argon-ion laser is similar in some respects to the He-Ne laser. Fig 9 shows the

experiment setup. The laser first is attenuated by a neutral density filter wheel, which

decreases the intensity of the laser beam, and then is reflected off two mirrors, which

are used to steer the laser beam onto the sample.. The samples (alpha alumina or

gamma alumina) were attached to the copper cold finger with thin tungsten wire. The

sample emission is focused by lens onto the entrance slit of a monochromator. The

focus length of the lens is 5cm. Thus, the distance between the lens and sample and

the distance between lens

and detector are both equal to 10 cm. The 0.275m focal length monochromator

(SpectraPro 275) is equipped with an SBIG CCD camera. The SpectraPro is an f/3.8

monochromator, triple indexable gratings, microprocessor control and computer

compatibility. The remote scan controller can change gratings, and is used for

33

scanning the instrument. Fig. 7 shows inside the SpectraPro. The SpectraPro has no

heat generation motors or electronics inside the optical chamber, therefore any

potential thermal distortion problems are eliminated.

Fig. 12 shows the emission spectra of the Cr doped alpha alumina. It contains two

sharp lines at 694.3nm and 692.9nm (R1 and R2 lines). They correspond to the 2E->

4A2 in Cr3+ ions in the octahedral crystal field. The absorption spectrum is determined

by the two strong transitions 4A2->4T2,

4T1. In ruby, these strong absorptions occur in

the green and blue regions. Incidentally, the absorption band gives ruby its red color,

not the 693 nm emission.

Fig 13 shows the emission spectra of gamma alumina. The disorder in the spinel

γ-Al2O3 lattice results in strong inhomogeneous broadening of lines in the optical

spectra. Via the upper Cr3+ states, the R-line is observed which is due to 2E->4A2

transitions in Cr3+ in a strong octahedral field. The asymmetric long-wavelength wing

of this line is due to vibronic sidebands of the zero-phonon R-transition.

34

Fig. 10: Optical Layout

Laser Beam

Density Filter

Wheel

Sample

Focus Lens

Filter

Detector

Mirror

Mirror

35

Fig 11: Experimental setup.

36

Fig. 12: Emission spectra of Cr3+ doped in alpha-alumina.

680 690 700 7100.0

0.2

0.4

0.6

0.8

1.0

680 690 700 710

0.0

0.2

0.4

0.6

0.8

1.0

arb

inte

rsity

wavelength(nm)

37

Fig. 13: Emission spectra of the gamma alumina at different temperatures.

500 600 700 800 900 1000 11000.0

0.2

0.4

0.6

0.8

1.0

500 600 700 800 900 1000 1100

0.0

0.2

0.4

0.6

0.8

1.0

wavelength(nm)

arb

inte

nsity

T285K T75K

38

CHAPTER 6

MANGANESE DOPED ALUMINA

6.1 SAMPLE PREPARATION

In order to synthesize manganese doped alumina, we followed the sol-gel method

used to obtain Cr doped alumina (see chapter 5), but replaced CrNO3 with MnNO3.

The obtained samples have a slightly different color, while Cr doped alpha alumina is

red and Mn doped alpha alumina is pink.

6.2 LUMINESCENCE SPECTRA

The Mn2+ doped in alumina oxide was excited with a 470nm LED. The emission is

again collected and imaged onto a fast f/3.8 monochromator fitted with a 600

groove/mm grating. The signal is detected and collected by a liquid nitrogen cooled

CCD. Mn4+ (3d3) is isoelectronic with Cr3+. That is why their spectra are similar. The

only difference is a shift of 20 nm in the spectral position of the Mn4+ R-lines relative

to those of Cr3+. Mn4+ contains two R-lines (2E-4A2) of Mn4+ at 672.7 and 676.3nm.

Weaker lines due to Mn4+ were observed in fluorescence at both higher and lower

39

frequencies. Some of these may be due to sites with different charge compensation,

some of pairs of ions, and some to vibrational side-bands. The widths of the R1 and R2

lines were about 2nm. During the experiment, the Oxford ITC601 temperature

controller showed 80K. But according to the N2/N1= e-∆E/kT, from the obtained from

the data, N2/N1=0.2, ∆E=hν2- hν1, where k is Boltzmann’s constant. From this, we

calculated a temperature of T=145K, much higher than the temperature of the cold

finger.

40

Fig. 14: Emission spectrum of Mn4+ in alpha-alumina at room temperature.

660 670 680 690 700 710 7200.0

0.2

0.4

0.6

0.8

1.0

660 670 680 690 700 710 720

0.0

0.2

0.4

0.6

0.8

1.0

arb

inte

nsity

wavelength(nm)

room tempexcitation:470nm LED

41

Fig. 15: Emission spectrum of Mn4+ in alumina at 145K.

660 670 680 690 700 710 7200.0

0.2

0.4

0.6

0.8

1.0

660 670 680 690 700 710 720

0.0

0.2

0.4

0.6

0.8

1.0

arb

inte

rsity

wavelength(nm)

80K, excitation:470nm LED

42

CHAPTER 7

CONCLUSION

The goal of this thesis is to produce Cr3+ and Mn4+ doped in alumina using sol-gel

technique and to learn the basic optical spectroscopy of these samples. The sol-gel

technique offers a low-temperature method for combining materials that are either

totally inorganic in nature or both inorganic and organic. The process, which is based

on the hydrolysis and condensation reaction of organometallic compounds in

alcoholic solutions, offers many advantages for the fabrication of coatings, including

excellent control of the stoichiometry of precursor solutions, ease of compositional

modifications, customizable microstructure, ease of introducing various functional

groups or encapsulating sensing elements, relatively low annealing temperatures, the

possibility of coating deposition on large-area substrates, and simple and inexpensive

equipment. Sol-gel technology has evolved considerably and founds its way in a wide

range of practical applications from catalyst support, to thermal insulation, from solar

cells to oxygen food sensors or sunscreen formulations. The increasing success of

sol-gel technology at solving technological challenges as well as the variety of its

field of applications will continue to attract condensed matter researchers to this field.

43

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