Preparation and Characterization of Self-Assembled Thin Film of MPS-Capped ZnS Quantum Dots for Optical Applications

For this study, we prepared colloidal ZnS quantum dots using 3-mercaptopropyltrimethoxysilane (MPS) as the capping agent. Colloidal ZnS quantum dots were directly deposited on glass substrates by a spin coating process. Therefore, self-assembled films made of ZnS quantum dots in a SiO2 network were obtained using only one production step. The films were heat-treated at 100°, 125°, 150°, 175° and 200°C in an N2 atmosphere. The results showed that the dimension of quantum dots changed from 2.8 nm to 3.2 nm by heat treatment. The refractive index, extinction coefficient, thickness, and dielectric coefficient values of the films were calculated. The present study showed that size and the refractive indices of films can be controlled by the heat treatment. Therefore, such films can be a good candidate in optical filter applications

Properties of tantalum mixed vanadium oxide thin film

Birçok geçiş metal elementleri oksitlerinin ince filmleri elektrokromik özellik göstermektedir. Bugüne kadar üzerinde çok çalışılan elektrokromik malzemelerden bazıları tungsten, vanadyum, molibden, tantalum, titanyum vb. geçiş metal oksitlerdir. Uygulanan bir dış potansiyel altında optik geçirgenliklerini değiştiren ve uygulanan potansiyelin işareti değiştirildiğinde tekrar önceki geçirgenlik değerine dönen cihazlara elektrokromik cihazlar denir. Renklenme mekanizmasının esaslarını anlamak için vanadyum oksit filmler en ideal malzemelerden biridir, çünkü vanadyum oksit filmler; termokromizm, fotokromizm ve elektrokromizm gibi çeşitli tiplerde renklenme özellikleri gösterirler. Vanadyum dioksit bilinen en eski termokromik malzemedir. Vanadyum pentoksit ise hem katodik hem de anodik olarak renklenebilen elektrokromik malzemedir. Vanadyum pentoksit gelişmiş elektrokimyasal özelliğinden dolayı üzerinde çok çalışılmış bir malzemedir. Elektrokromik cihazlarda, termokromik cihazlarda, güneş pillerinin pencerelerinde, yüksek kapasiteli lityum pillerinin elektrotlarında, elektronik ve optik anahtarlama cihazlarında kullanılmaktadır. Literatürde Vanadyum oksit ince filmlerin hazırlanmasında birçok yönteme rastlanmaktadır. Tozutma yöntemi, vakum buharlaştırma, elektrokimyasal yöntemler ve sol jel yöntemi kaplama yöntemlerine bazı örneklerdir. Bu çalışmada Vanadium (V) Oxytriisopropoxide ve Tantalum(V) Ethoxide başlangıç maddeleri ile sol-jel daldırma ve döndürme yöntemi kullanılarak ince filmler hazırlanmıştır. Hazırlanan vanadyum ve tantalum katkılı vanadyum oksit filmler sarı renkte olup elektrolit içerisinde elektrik potansiyeli uygulandığında tersinir olarak önce saydamlaşır, sonra açık kahverengi renge dönüşür. Tantalum katkısı, filmlerin optik ve yapısal özelliklerini değiştirmiş, elektrokromik özelliklerini iyileştirmiştir. Anahtar Kelimeler: Vanadyum, tantalum, sol-jel, ince film, elektrokromik, elektrokromiz.The enormous development of the electronic age after the discovery of solid state bipolar junction transistor, thin film technology made possible to improve the capabilities, reduce the size and cut down the price of many electronic devices. Besides of the capabilities to produce high efficient electronic device in small volumes, they benefit also from low power consumption. Nowadays Resistors, capacitors, inductors, as well as semiconductor such as diode, transistor and integrated circuit in electronic application, reflecting, anti-reflecting, light polarizing, optical band pass filters in optical industry and anti corrosive and anti abrasion coating in mechanical application take use of thin film technology. The use of thin films is extended to lithium battery electrodes, heat, temperature, pressure, humidity and gas sensors. Chromogenic devices comprices also of chromegenic thin film coatings. The influence of ambient condition such as light, temperature, pressure on the color of the material is refered as chromegenic. Chromegenic material returns to its initial state when the ambient conditon returns to its initial condition. Chemocromism, gaschromism, photochromism, thermochromism and electrochromic are some sort of chromogenic effects. The most desirable application of electrochromic (EC) device are smart windows. Electrochromic smart window colors upon applied potential. By reversing the potential it becomes again belached. Smart windows in comparison with other building coating application, can reduce the energy consumption by means of heating and illuminating conditioning for indoor of large buildings. Due to unresolved problems their use is still not widely considered. Heat sensitive coating can also be used to minimize the heating or cooling costs. Some automobile rear  view mirrors and automatically belaching sensor driven devices uses electrochromic coatings. Electrochromic reflecting mirror system can also be continously adjusted to reflect the desired amount of light. It is not always possible to use photochromic device since they change their optical properties due to the incoming light. Electrochromic device colors or bleaches when electric potential is applied to its terminals. By using an external electronic control unit the coloration or bleaching on-demand or dimming can be achived easily at a desirable speed within a maximum and minimum transmittance value. Many works have been carried on most of the electrochromic materials but many of them are interested in vanadium oxide and tungsten oxide. Vanadium is the most studied powerful element which has an oxidation state V5+ up to V2+. Vanadium oxide films possess metal-semiconductor phase transition, photochromic, electrochromic, and thermochromic properties. Vanadium is the rare material which exhibits the three chromogenic properties. The anodic and cathodic coloration of vanadium oxide and the oxidation states resulting in coloration of more then one color makes vanadium oxide preferable to use them in EC application. They can be used as EC active layer and also as complimentary layer in EC devices. This work states the study on vanadium oxide and tantalum oxide mixed vanadium oxide thin films. Material mixture and coating conditions for fabricating high performance EC device has been investigated. Vanadium and tantalum mixed vanadium oxide thin films have been prepared by sol-gel dip and spin coating process from vanadium (V) oxytriisopropoxide and tantalum (V) ethoxide precursors. Heat treatment at 100°C and 300°C has been applied for 2h. Optical, electrochemical and surface analyses have been investigated. Results show that the surface structure and electrochromic properties can be easily adjusted with the ingredient tantalum percentage. Increasing tantalum improves the optical transmittance of the coatings. Charge capacity of the EC film also has been increased with increasing tantalum content in the studied range. The calculated highest diffusion coefficient is 4.11 10-12 cm2/s  which correspond to 10% tantalum mixed vanadium oxide thin films. Increasing heat treatment temperature causes porosity that reveals the improvement of EC performance as a result of easier ion injection. Keywords: Vanadium, tantalum, sol-gel, thin film, electrochromic, electrochromism

Optical, structural and electrochromic properties of titanium dioxide mixed niobium pentoxide films

Elektrokromik camlar, uygulanan bir gerilim ile optik özelliklerini değiştirebildikleri için son yıllarda yoğun ilgi konusu olmuşlardır. Bu camlara uygulanan gerilimin ters yönde çevrilmesi ile camların optik özelliklerinin tekrar eski durumuna geri dönmesi, elektrokromik camları teknolojik açıdan çok önemli bir konuma getirmektedir. Elektrokromik camlar; arabalarda (tavan camlarında, yan camlarda, ön camda ve dikiz aynalarında), binalardaki pencerelerde ve ekran uygulamalarında kullanılmaktadırlar. Bu camlar gerek mimari açıdan görsel bir güzellik sağlamakta, gerekse ısı ve ışık kontrolü yaptıkları için enerji tasarrufu sağlamaktadırlar. Niobyum pentoksit, elektrokromik açıdan oldukça verimli bir malzemedir. Birçok elektrokromik malzeme kristal halde iyi derecede elektrokromik özellik göstermezken niobyum pentoksit bu özelliği göstermektedir. Ayrıca, niobyum pentoksit amorf halde iken kahverengi, kristal halde iken mavi renklenme gösterir. Bu özelliği ile diğer elektrokromik malzemelerden ayrılır. Niobyum pentoksit, kendisine katkılanan malzemeye göre de farklı renkte elektrokromik renklenme gösterebilmektedir. Bu çalışmada saf niobyum pentoksitin yanısıra hacimce %20, %40, %60 ve %80 titanyum dioksit katkılı niobyum pentoksit filmler de hazırlanmıştır. Elde edilen filmlerin optik, yapısal ve elektrokromik özellikleri incelenmiştir. Bu filmlerin bazıları ısıl işleme tabi tutulmuş ve ısıl işlemin filmlere etkisi incelenmiştir. Titanyum dioksit katkı miktarının artması ile filmlerin yük tutabilme kapasitesinin arttığı gözlenmiştir. Anahtar Kelimeler: Niobyum pentoksit, sol-jel, daldırarak kaplama, elektrokromizm.Electrochromism is the phenomenon displayed by some chemical species of reversibly changing color when a voltage is applied. As the color change is persistent and energy need only be applied to effect a change, electrochromic materials are used to control the amount of light and heat allowed to pass through windows, and has also been applied in the automobile industry to automatically tint rear-view mirrors in various lighting conditions. Electrochromic windows (or "smart windows") are windows that can be darkened or lightened electronically. A small voltage applied to the windows will cause them to darken; reversing the voltage causes them to lighten. This capability allows for the automatic control of the amount of light and heat that passes through the windows, thereby presenting an opportunity for the windows to be used as energy-saving devices. Electrochromic properties of niobium pentoxide were firstly reported in 1980. After the first study of electrochromic properties of sol-gel deposited niobium pentoxide in 1991 it became an extensively studied electrochromic material. Niobium pentoxide coated electrochromic films show both bronze and pale blue coloration if coated films are amorphous or crystalline, respectively. Niobium chloride (NbCl5) and niobium ethoxide (Nb(OC2H5)5) are the most used precursors to obtain niobium pentoxide sol. Sol-gel dip coating and spin coating methods are the most used methods to prepare niobium pentoxide sol. Not only sol-gel method but also reactive DC magnetron sputtering, indirect reactive sputtering, thermal oxidation, pulsed laser and chemical vapor deposition methods were frequently used for obtaining niobium pentoxide thin films. There are only a few studies concerning doped niobium pentoxide thin films. Some of these studies can be summarized as Sn, Zr, Ti, Mo, Li and titanium dioxide (TiO2). Although there are many studies concerning electrochemical properties of niobium pentoxide only a few of studies were interested in optical properties. Structural studies on niobium pentoxide showed that it is amorphous up to 450°C and starts crystallization above this temperature. At least 12 crystal structures of niobium pentoxide have been identified and the most often phases have been labeled as TT, T, M, B and H. Low temperature (~500°C), medium temperature (~800°C) and high temperature (~1000°C) forms of niobium pentoxide are called TT (or T), M (or B) and H phases, respectively (Ko and Weissman, 1990). Phase transformation in titanium dioxide has been widely studied for optical and electronic applications because they have a high refractive index, a high dielectric constant, high photocatalytic activity, and good physical and chemical stability (Oh et. al., 2003). The high refractive index and low absorption coefficient of titanium dioxide make it suitable for optical coating in silicon solar cell and optical thin film device. Titanium dioxide films have also attracted attention for use in the fabricating capacitors of microelectronic devices due to their high dielectric constants. There are also some studies concerning electrochromic properties of titanium dioxide. In this study we investigated and compared the structural, optical and electrochemical properties of titanium dioxide mixed niobium pentoxide films deposited by sol-gel dip coating method at 134 mm/min dipping rate. The films have high transmittance values of between 0.74 and 0.88. A steady increase was observed in the thickness of the films with increasing titanium dioxide concentration for both as deposited and heat treated films. Refractive indices of the films at 550 nm wavelength lie between 1.80 and 1.91. All as deposited films have band gap values about 3.35 ± 0.02 eV whereas those of heat treated films are about 3.18 ± 0.02 eV. Height profile analysis of three dimensional atomic force microscope pictures shows that surface smoothness of the films decreases with increasing titanium dioxide concentration. It was found that charge density values of the films increase with increasing titanium dioxide doping concentration for as deposited films. Charge density values of as deposited films are 5.5, 7.7, 8.8 and 13.3 mC/cm2 for 0%, 5%, 10% and 15% titanium dioxide mixed niobium pentoxide films, respectively. On the other hand, charge density values of heat treated films did not show important variation with increasing titanium dioxide concentration. Keywords: Niobium pentoxide, sol-gel, dip coating, electrochromism

Growth kinetics of MPS-capped CdS quantum dots in self-assembled thin films

For this study, we prepared colloidal CdS quantum dots using 3-mercaptopropyltrimethoxysilane as capping agent. Colloidal CdS quantum dots were directly deposited on glass substrates by a spin-coating process. Coated substrates were heat-treated between 225A degrees C and 325A degrees C for various heat treatment time intervals to investigate the growth kinetics of the quantum dots. Results showed that sizes of the CdS quantum dots grew approximately from 2.9 to 4.6 nm, and the E (1s1s) energy values shifted approximately from 3.3 to 2.7 eV. Results showed that the average size of quantum dots increase by thermal treatment due to Ostwald ripening. The thermal process used to grow the size of quantum dots was examined according to the Lifshitz-Slyozov-Wagner theory. The activation energy of CdS quantum dots in thin films was calculated at approximately 44 kJ/mol