§¹i häc Quèc Gia Hµ Néi tr−êng §¹i häc C«ng NghÖ Lª Hµ Chi Nghiªn cøu §éng häc c¸c qu¸ tr×nh biÕn ®æi ®iÖn - quang - quang tö cña mµng máng vËt liÖu «xÝt kim lo¹i chuyÓn tiÕp (W, Mo) cÊu tróc nan« Ngµnh: Khoa häc vµ c«ng nghÖ nano M· sè: LuËn v¨n th¹c sÜ Ng−êi h−íng dÉn khoa häc: PGS. NguyÔn N¨ng §Þnh Hµ Néi - 2005 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com TABLE OF CONTENTS Preface. 1 Chapter 1 Overview of transition metal (W, Mo) oxides and their electrochromic properties .1 Introduction of transition metal .2 Bulk crystalline structures of tungsten oxide and molybdenum oxide [4] .3 Properties of tungsten oxide and molybdenum oxide .4 Applications for electrochromic materials. 11 Chapter 2 Photoluminescent properties of nanocomposite materials .2 Fluorescence and phosphorescence (photoluminescence) [20] .2 Physics of nanostructured materials [7] .3 Enhance photoluminescent performance of nano-composite materials .2 Preparation by electrochemical method .2 Preparation by thermal oxidation method .3 Study on morphology and structure of the films.
48 Chapter 4 Kinetics of electro-optical transformation processes of nanostructured WO3-based thin film .1 Ion intercalation/extraction studied by electrochemical techniques .2 Electro-optical properties of WO3-based electrochromic device studied in-situ by Optics Multi-canal Analyzer. 63 Chapter 5 Study on photoluminescent transformation processes of nanostructured MoO3-based nanocomposite. 68 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.2 Preparation of PVK+nc-MoO3 nanocomposite .3 Molecular bonding studied by Raman spectroscopy .3 Photoluminescent properties studied by FL 3 - 22 Spectrometer .4 I-V characteristics studied by electrochemical technique. 85 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com -1- Preface The purpose of this work is to prepare nanostructured transition metal (W, Mo) oxide - based thin films and study their kinetics of electro-optical and photoluminesent processes.
It is known that electrochromic materials have been found many interests with the respect not only to the fundamental studies, but also to the application scopes, such as solar energy management, sensors and display devices [1,3]. Among these electrochromic materials, tungsten oxide films are by far the most extensively studied. WO3 is a wide band gap semiconductor with Eg ≈ 3.2 eV, it thus transparent in the visible light range [3]. Electrochromic tungsten oxide films can be prepared by a variety of different techniques such as physical vapor [2] and chemical vapor deposition [14,29], electrochemical deposition [13,34], sol - gel [25], etc.
The electrochemical deposition is expected to be one of the most economical methods for making a large-area film as well as automatically controlling the film growth. However, these transmittances as well as the durability of the films were still limited for practical use. The aim of this work is to improve electrochromic properties of WO3 thin films deposited by electrochemical method. The morphology, electrochemical and optical properties concerning with electrochromic performance of the films are also discussed.
In addition, we tried to design a new device based on nanostructured MoO 3 thin film and poly-(N-vinyl carbazole) according to typical OLED sandwich structure. The enhanced photoluminescent performance was investigated and I-V characteristics was also studied. 1 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com -2- Chapter 1 Overview of transition metal (W, Mo) oxides and their electrochromic properties 1.1 Introduction of transition metal In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings: It commonly refers to any element in the d-block of the periodic table, including zinc and scandium. This corresponds exactly to periodic table groups 3 to 12 inclusive.
More strictly, it can refer to those elements which form at least one ion with a partially filled d shell of electrons. This is exactly the d-block with zinc and scandium excluded. The first has the attraction of apparent simplicity and is the traditional usage. However, many interesting properties of the transition elements as a group are the result of their ability to contribute valence electrons from s orbitals before d orbitals, a property which all members of the d-block except zinc and scandium share, so the more restricted definition is in many contexts the more useful.
The d orbitals are contributed after the s orbitals because once the d orbital begins to fill its electrons move closer to the nucleus, leaving the s electrons as the outermost. The 40 transition metals: The (loosely defined) transition metals are the forty chemical elements 21 to 30, 39 to 48, 71 to 80, and 103 to 112. The name transition comes from their position in the periodic table of elements. In each of the four periods in which they occur, these elements represent the successive addition of electrons to the d atomic orbitals of the atoms.
In this way, the transition metals represent the transition between group 2 elements and group 13 elements. 2 LUAN VAN CHAT LUONG download : add luanvanchat@agmail. The periodic table of the 40 transition metals 3 (III 4 (IV 5 (V 6 (VI 7 (VII 8 (VIII 9 (VIII 10 (VIII 11 (I 12 (II Group B) B) B) B) B) B) B) B) B) B) Period Sc 21 Ti 22 V 23 Cr 24 Mn 25 Fe 26 Co 27 Ni 28 Cu 29 Zn 30 4 Period Nb Y 39 Zr 40 Mo 42 Tc 43 Ru 44 Rh 45 Pd 46 Ag 47 Cd 48 5 41 Period Lu 71 Hf 72 Ta 73 W 74 Re 75 Os 76 Ir 77 Pt 78 Au 79 Hg 80 6 Period Rf Db Sg Rg Uub Lr 103 Bh 107 Hs 108 Mt 109 Ds 110 7 104 105 106 111 112 Electronic configuration: W: [Xe]6s24f145d4 Mo: [Kr]5s14d5 Variable oxidation states: The transition metals show a wide variety of oxidation states because their partially filled d orbitals can accept or donate electrons in chemical reactions. A transition element like tungsten or molybdenum has roughly linear increasing ionisation enthalpies throughout its s and d orbitals, due to the close energy difference between the 5d and 6s (W) or 4d and 5s (Mo) orbitals.
Transition metal ions are therefore commonly found in very high states. The oxidation states found in compounds of W and Mo are changed from 2 to 6. Properties with respect to the stability of oxidation states: Higher oxidation state ions become less stable across the period. Ions in higher oxidation states tend to make good oxidizing agents, whereas elements in low oxidation states become reducing agents.
The 2+ ions across the period start as strong reducing agents, and become more stable. 3 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com -4- The 3+ ions start stable and become more oxidizing across the period.2 Bulk crystalline structures of tungsten oxide and molybdenum oxide [4] 1.1 Crystal structures of tungsten oxide and molybdenum oxide For Mo oxide, just as for W oxide, the basic structural element is an octahedron with a metal atom at the center and oxygen atoms at the corners, Figure 1. Deviations from the ideal cubic perovskite-like structure correspond to antiferroelectric displacements of W atoms and to mutual rotations of oxygen octahedra. The magnitude of the distortion depends on the temperature, which is in agreement with the behavior of most perovskites, and pure WO3 single crystals go through structural transformations according to the sequence tetragonal → orthorhombic → monoclinic → triclinic → monoclinic as the temperature is lowered from 900 to -189oC.
Tungsten oxide has a tendency to form substoichiometric phases containing edge-sharing octahedra. (W, Mo) atoms Oxygen atoms Figure 1. Schematic illustrating a corner-sharing arrangement of octahedra in a W oxide or Mo oxide crystal. The crystal structure has been studied by high-resolution electron microscopy, and extended defects characterized by crystallographic shear planes, pentagonal 4 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com -5- bipyramidal columns, and hexagonal tunnels have been identified.2 demonstrates arrangements of WO6 octahedra surround large defects with hexagonal and pentagonal cross-sections.
Interpretation of high-resolution transmission electron micrographs for two crystals of WO3-z with different stoichiometry. Hexagonal WO3 phases are of particular relevance to electrochromism, as will be mentioned later. Hexagonal phases are characterized by a one-dimensional tunnel structure extending through the material. An even more open pyrochlore structure of WO3, with a three-dimensional tunnel structure, was discovered recently.
It contains some W and O vacancies as well as H3O+ for charge neutrality.2 Crystal structures of (W, Mo) bronzes and ion intercalated (W, Mo) oxide Tungsten bronzes can be represented as MxWO3 with M being an atom from the first column in the Periodic Table. Their crystal structure depends on the type and density of the species added to the WO3 host. MxWO3 bronzes with 0 ≤ x ≤ 1 and M = Li, Na, K, Rb, and Cs (with ionic radii 0. The phase domains are approximate only.
Cubic (perovskite) phases are found within a 5 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com -6- range that is displaced towards increased x value for increased ionic radii. Such a structure does not exist in pure WO3 but it is possible to extrapolate a lattice parameter for a hypothetical material. Tetragonal phases are found at low to intermediate x values for LixWO3 and NaxWO3 and at intermediate x values for KxWO3. Hexagonal phases occur for small incorporation of large ions: KxWO3, CsxWO3, InxWO3 and LixWO3.
In case of HxWO3, the hydrogens are thought to be statistically attached to the oxygens as hydroxyl groups, so the material may be adequately represented as WO3-x(OH)x. There are reports about an orthorhombic phase at x = 0.1, tetragonal phases for x = 0.33, and a cubic phase for x = 0. Modifications of the crystalline structure during Li+ intercalation/extraction are of particular concern for electrochromic devices. Tungsten trioxide crystalline structure with ion M+ (H+, Li+, Na+) intercalation have perovskite-like atomic configurations based on corner-sharing WO6 ,MoO6 octahedra.
It seen from Figure 1.3 that ion M+ intercalation makes the sample transform according to monoclinic → tetragonal → cubic with intermediate mixed phases. The WO6 octahedra are shown as well as the sites available for ion intercalation. From an 6 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com -7- inspection of the structures, it is reasonable to expect that only small ions (H+, Li+, Na+) can be accommodated in the cubic configuration. The open crystal structures of Mo oxide and its hydrates make these materials excellent as intercalation hosts for H+, Li+, and other ions.3MoO3 can serve as a host for cyclic Li intercalation/deintercalation.
It is possible to prepare LixMoO2 and NaxMoO2 with x up to ~ 1. The materials can serve as intercalation hosts and are of interest in battery technology.3 Properties of tungsten oxide and molybdenum oxide Molybdenum oxide films show pronounced electrochromism and have many properties in similar with tungsten oxide. The discussion below covers the optical properties, the electrical properties and electrochromism of these oxide films in common.1 Optical properties [4, 14] WO3 crystals have an average refractive index for white light of 2. Color changes appear in WO3-z when z is increased, as investigated by Glemser and Sauer.
Intercalation of alkali ions, so that tungsten oxide bronzes are created, also leads to the development of colors. The colors are indicative of a strongly wavelength dependent reflectance. Diffuse spectral reflectance of NaxWO3 in the luminous and near-infrared spectral range was reported by Brown and Banks with a reflectance maximum at ~ 0.2 and a reflectance minimum at ~ 0. In the latter samples, there is high reflectance beyond a certain wavelength that shifts towards smaller values as the Na content is increased.5, the reflectance lies primarily in the infrared range, and the moderately high reflectance of blue light appears in the visible range.5, there is high reflectance in the long- wavelength part of the luminous spectrum, and consequently the visible appearance is reddish or yellowish.
7 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com -8- In addition, Faughnan et al.