Study on kinetics of electro-optical and photoluminescent processes in nanostructured transition metal (W, Mo) oxide-based thin films

Đại học Quốc Gia Hà Nội trờng Đại học Công Nghệ Lê Hà Chi Nghiên cứu Động học trình biến đổi điện - quang - quang tử màng mỏng vật liệu ôxít kim loại chuyển tiếp (W, Mo) cấu trúc nanô Ngành: Khoa học công nghệ nano MÃ số: Luận văn thạc sĩ Ngời hớng dẫn khoa học: PGS TS Nguyễn Năng Định Hà Nội - 2005 TABLE OF CONTENTS Preface Chapter Overview of transition metal (W, Mo) oxides and their electrochromic properties 1.1 Introduction of transition metal 1.2 Bulk crystalline structures of tungsten oxide and molybdenum oxide [4] 1.3 Properties of tungsten oxide and molybdenum oxide 1.4 Applications for electrochromic materials 11 Chapter Photoluminescent properties of nanocomposite materials 20 2.2 Fluorescence and phosphorescence (photoluminescence) [20] 20 2.2 Physics of nanostructured materials [7] 23 2.3 Enhance photoluminescent performance of nano-composite materials 33 Chapter Experiments 39 3.2 Preparation by electrochemical method 39 3.2 Preparation by thermal oxidation method 45 3.3 Study on morphology and structure of the films 48 Chapter Kinetics of electro-optical transformation processes of nanostructured WO3-based thin film 55 4.1 Ion intercalation/extraction studied by electrochemical techniques 55 4.2 Electro-optical properties of WO3-based electrochromic device studied in-situ by Optics Multi-canal Analyzer 63 Chapter Study on photoluminescent transformation processes of nanostructured MoO3-based nanocomposite 68 5.2 Preparation of PVK+nc-MoO3 nanocomposite 68 5.3 Molecular bonding studied by Raman spectroscopy 69 5.3 Photoluminescent properties studied by FL - 22 Spectrometer 75 5.4 I-V characteristics studied by electrochemical technique 79 Conclusion 83 References 85 -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 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 -2- Chapter 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 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 elements and group 13 elements -3Table 1.1 The periodic table of the 40 transition metals Group (III (IV B) B) (V B) (VI B) (VII (VIII (VIII 10 (VIII 11 (I B) B) B) B) B) 12 (II B) Period Sc 21 Ti 22 V 23 Cr 24 Mn 25 Fe 26 Co 27 Ni 28 Cu 29 Zn 30 Period Y 39 Zr 40 Nb 41 Mo 42 Tc 43 Ru 44 Rh 45 Pd 46 Ag 47 Cd 48 Os 76 Ir 77 Pt 78 Au 79 Hg 80 Period Lu 71 Hf 72 Ta 73 W 74 Re 75 Rf Period Lr 103 104 Bh 107 Hs 108 Db 105 Sg 106 Mt 109 Ds 110 Rg 111 Uub 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 to 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 -4 The 3+ ions start stable and become more oxidizing across the period 1.2 Bulk crystalline structures of tungsten oxide and molybdenum oxide [4] 1.2.1 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 1.1 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 1.1 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 -5bipyramidal columns, and hexagonal tunnels have been identified Figure 1.2 demonstrates arrangements of WO6 octahedra surround large defects with hexagonal and pentagonal cross-sections Figure 1.2 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 1.2.2 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 ≤ x ≤ and M = Li, Na, K, Rb, and Cs (with ionic radii 0.060, 0.095, 0.133, 0.148 and 0.169 nm, respectively) The phase domains are approximate only Cubic (perovskite) phases are found within a -6range 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 = 0.23 and x = 0.33, and a cubic phase for x = 0.5 Modifications of the crystalline structure during Li+ intercalation/extraction are of particular concern for electrochromic devices - M - W, Mo - O Figure 1.3 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 -7inspection 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 K0.3MoO3 can serve as a host for cyclic Li intercalation/deintercalation It is possible to prepare LixMoO2 and NaxMoO2 with x up to ~ The materials can serve as intercalation hosts and are of interest in battery technology 1.3 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 1.3.1 Optical properties [4, 14] WO3 crystals have an average refractive index for white light of 2.5 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 ~ 0.5 μm for x < 0.2 and a reflectance minimum at ~ 0.5 to ~ 0.7 μm for x ≥ 0.2 In the latter samples, there is high reflectance beyond a certain wavelength that shifts towards smaller values as the Na content is increased At 0.2 < x < 0.5, the reflectance lies primarily in the infrared range, and the moderately high reflectance of blue light appears in the visible range At 0.2 < x < 0.5, there is high reflectance in the longwavelength part of the luminous spectrum, and consequently the visible appearance is reddish or yellowish - 74 Typical strong Raman scatterers are moieties with distributed electron clouds, such as carbon-carbon double bonds The pi-electron cloud of the double bond is easily distorted in an external electric field Bending or stretching the bond changes the distribution of electron density substantially, and causes a large change in induced dipole moment Chemists generally prefer a quantum-mechanical approach to Raman scattering theory, which relates scattering frequencies and intensities to vibrational and electronic energy states of the molecule The standard perturbation theory treatment assumes that the frequency of the incident light is low compared to the frequency of the first electronic excited state The small changes in the ground state wave function are described in terms of the sum of all possible excited vibronic states of the molecule Polarization Effects: Raman scatter is partially polarized, even for molecules in a gas or liquid, where the individual molecules are randomly oriented The effect is most easily seen with an exciting source which is plane polarized In isotropic media polarization arises because the induced electric dipole has components which vary spatially with respect to the coordinates of the molecule Raman scatter from totally symmetric vibrations will be strongly polarized parallel to the plane of polarization of the incident light The scattered intensity from non-totally symmetric vibrations is 3/4 as strong in the plane perpendicular to the plane of polarization of the incident light as in the plane parallel to it The situation is more complicated in a crystalline material In that case the orientation the crystal is fixed in the optical system The polarization components depend on the orientation of the crystal axes with respect to the plane of polarization of the input light, as well as on the relative polarization of the input and the observing polarizer 5.2.2 Experimental data and analysis 74 - 75 - Intensity (a.u)  MoO3 and PNM composite studied by Raman spectroscopy PNM 725 435 529 819 292 MoO3 200 996 664 336 470 400 600 800 1000 1200 -1 Wavenumber (cm ) Figure 5.4 Raman spectroscopy of MoO3 and PNM composite The Raman Effect is used in materials analysis The frequency of light scattered from a molecule may be changed based on the structural characteristics of the molecular bonds A monochromatic light source (laser) is required for illumination, and a spectrogram of the scattered light then shows the deviations caused by state changes in the molecule Here, one can see Raman spectroscopy of MoO3 and PNM composite (Figure 5.4) measured by LABRAM - 1B Raman scattering spectrometer (Dilor JobinYvon Spex, France) in Raman spectroscopy laboratory, IMS, VAST Figure 5.4 represents that PNM spectroscopy covered both characteristic peaks of MoO3 (292, 336, 470, 664, 819, 996 cm-1) and the others of PVK (435, 529, 725 cm-1) Therefore, we can estimate that PVK was absorbed as very thin layer onto MoO3 to form a PNM composite 5.3 Photoluminescent properties studied by FL - 22 Spectrometer 75 - 76 5.3.1 Description of the measurement Photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called "photo-excitation." One way this excess energy can be dissipated by the sample is through the emission of light, or luminescence In the case of photo-excitation, this luminescence is called "photoluminescence." The intensity and spectral content of this photoluminescence is a direct measure of various important material properties Specifically, photo-excitation causes electrons within the material to move into permissible excited states When these electrons return to their equilibrium states, the excess energy is released and may include the emission of light (a radiative process) or may not (a nonradiative process) The energy of the emitted light or photoluminescence is related to the difference in energy levels between the two electron states involved in the transition that is, between the excited state and the equilibrium state The quantity of the emitted light is related to the relative contribution of the radiative process Figure 5.5 Typical experimental set-up for PL measurements 5.3.2 Experimental data and analysis Poly(N-vinylcarbazole) (PVK) is not only a photoconductive polymer [15,16], but also a photoluminescent one By adding the PVK as a hole transport layer (HTL) sandwiched between an anode and emitter layers of the device one can expect the 76 - 77 equalization of injection rates of hole and electron, consequently to obtain a higher electroluminescent efficiency of the OLED Moreover, PVK itself is an photoluminescent (PL) material, the PL emission from PVK film extends from 350 to 600 nm with a maximum at 404 nm and a shorter wavelength shoulder at 385 nm [15] However, using single PVK as an emitter material is not very efficient because the PL intensity is low and the onset electric field is rather high Recently [9,19], the use of the composite of polymer (like MEH-PPV) and nanocrystalline inorganic material (like Ge, Si) have proved the advantage in the optical and electrical properties of the composite structure in comparison with those of the homogeneous polymers 4000000 3500000 PNM Intensity (a.u) 3000000 2500000 2000000 1500000 1000000 PVK 500000 350 400 450 500 550 600 650 700 Wavelength (nm) Figure 5.6 Photoluminescent spectrometry of PVK and PNM composite Photoluminescent properties of PVK and PNM composit were investigated by FL 22 Spectrometer in Faculty of Physics, HUS, VNUH The intensity and spectral content of this photoluminescence is a direct measure of various important material properties As seen from figure 5.6, PVK has an absorption maximum at about 404 nm 77 - 78 consisting with the absorption range of glass However, studying the photoluminescent (PL) property of the films enables us to be aware of useful information about the polymer and the composite An interesting result of this study is represented in Figure 5.6 where there is observed a considerably large difference in the PL intensity between homogenous PVK and the PNM composite, i.e., the presence of nanostructured MoO3 shows the significant enhancement of photoluminescence of PNM approximately 12 fold higher intensity than homogenous PVK To explain the difference we consider the PNM composite as a system consisting of oxide-particles/polymer transition boundaries The scheme of the energy bands of the system is shown in Figure 5.7, where PVK is the emissive polymer Figure 5.7 Schematic drawing of a MoO3 nanoparticle / PVK junction with nanocomposite in OLED before (a) and after (b) laser excitation As discussed above, the return to equilibrium, also known as "recombination", can involve both radiative and nonradiative processes The amount of photoluminescence and its dependence on the level of photo-excitation and temperature are directly related to the dominant recombination process Analysis of photoluminescence helps to 78 - 79 understand the underlying physics of the recombination mechanism Research interest in nanoparticles has been stimulated by the realization that their optical properties may be different from those of bulk materials Nanoparticles may act to harvest energy from absorption of photons and subsequently transfer this energy to a luminescent centre, i.e they may act as sensitizers for radiative relaxation processes Thus the improvement of PL efficiency may be explained with this model as follows The efficiency of the photoluminescence intensity could be determined by recombination of the holes and electrons injected Since MoO3 is a wide-band gap semiconductor of n-type, while the MoO3 nanoparticles are excited by the energy of nitrogen laser beam, electrons from the valence bands can jump to the conducting ones, this results in the decrease of the Shorttky barrier height of the oxide particles/polymer transition boundaries On the one hand some of these electrons diffuse to the LUMO level of the polymer, on the other hand the holes at the valence bands of MoO3 may oxidize the electrons from the HOMO level of polymer, forming holes at this level This results in generation of an additional amount of excitons Moreover, TiO2 nanoparticles surface enables create a larger charge space, consequently to increase the probability of the hole-electron recombination and to raise the PL intensity 5.4 I-V characteristics studied by electrochemical technique 5.4.1 Description of the measurement The role of the nanocomposite emitting material in the electroluminescent devices can be seen by comparing of I-V characteristics of the devices made from the standard homogenous PVK and PNM composite To measure I-V characteristics, we designed a 4-layers device based on composite nc-MoO3/ PVK where the Mo was used as anode A very thin aluminum layer was thermally evaporated onto composite PNM layer to be used as cathode So that, we have a sandwich device to study I-V characteristics by using electrochemical technique, see Figure 5.8 79 - 80 - Al _ PVK nc-MoO3 + Mo Figure 5.8 Schematic draw of a 4- layers device (+) Mo/nc-MoO3/PVK/Al (-) This 4-layer device has a typical structure of an Organic Light-Emitting Diode (OLED) Organic electroluminescence (EL) is the electrically driven emission of light from non-crystalline organic materials, which was first observed and extensively studied in the 1960s [19] In 1987, a team in Kodak introduced a double layer OLED, which combined modern thin film deposition techniques with suitable materials and structure to give moderately low bias voltages and attractive luminance efficiency Since then, there have been increasing interests and research activities in this new field, and enormous progress has been made in the improvements of color gamut, luminance efficience and device reliability The growing interest is largely motivated by the promise of the use of this technology in flat panel displays Here, we made an effort to fabricate a new device based on nanostructured MoO3 thin film and poly-(N-vinyl carbazole) according to typical OLED sandwich structure with simple technology An OLED has an organic EL medium consisting of extremely thin layers (< 0.2 µm in combined thickness) sandwiched by two electrodes 80 - 81 - Figure 5.9 Schematic of Recombination Processes in a Single-Layer OLED 5.4.2 Experimental data and analysis 0.025 PNM Current (A) 0.020 0.015 0.010 PVK 0.005 0.000 10 Voltage (V) Figure 5.10 I-V characteristics of PVK and composite PNM I-V characteristics of two devices with structure like ITO/PVK/Al and Mo/ncMoO3/PVK/Al were studied on the Auto-lab PGS-30 system in Joint laboratory of thin film technology, IMS, VAST As we can see from the figure 5.10, the I-V characteristics studied by electrochemical technique demonstrate that composite PNM 81 - 82 has diode characteristics better than PVK Composite PNM show the much smaller threshold voltage (~ 2V) in comparison with PVK (~ 5V) In addition, the effect of reverse current of composite PNM is less than PVK Consideration of the reason why the composite PNM has good I-V characteristics is also the related question of whether or not the Mo/nc-MoO3 film has good contact We not know of any detailed theoretical discussion of the cohesive forces between an oxide layer and a metal substrate, or any experimental measurement of the surface energy of the interface However, all oxides are at least partly polar and the charges on the metal and oxygen ions must be strongly attracted to the substrate metal Thus strong cohesive forces between metal and oxide must exist 82 - 83 - Conclusion New modern technologies require new materials During the past decade, the movement towards nanodimensions in many areas of technology raises a huge interest in nanostructurized materials In this thesis, we reported the main results of the synthesis of nanocrystalline materials by electrodeposition method and thermal oxidation method as well as structure-property relationships for nanostructured transition metal (W, Mo) oxide - based thin films WO3/ITO thin films were successfully deposited by electrochemical method The films exhibit nanoscale structure with an average size of grains of 80-100 nm It was clearly demonstrated that under action of an electrical field the Li+ ions have inserted (extracted) into (out of) the films, respectively resulting in coloration and bleaching of these films The WO3 based electrochromic device cells possess good electrochromic performance with a response time less than 10 s, a high efficiency and reversibility These obtained parameters are considerably enhanced in comparison with previous ones With these properties, WO3 thin films can be suggested for many potential applications, such as electrochromic smart windows, sun glasses, automotive glasses, motorcycle helmets, etc Nanostructured MoO3 thin film with an average diameter size of 15 nm and length of 40 nm was fabricated by thermal oxidation method Then we spincoated PVK on this thin film that has been prepare to investigate the enhanced photoluminescent performance and I-V characteristics We designed a new device based on nanostructured MoO3 thin film and poly(N-vinyl carbazole) according to typical OLED sandwich structure 83 - 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