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

The production of an antireflective filter from titanium dioxide and zinc oxide

Sol-jel döndürerek kaplama yöntemiyle hazırlanmış TiO2 (titanyum dioksit) ve ZnO (çinko oksit) ince filmleri kullanılarak görünür bölgenin belli bir dalgaboyu aralığı için çok katmanlı yansıtmayıcı filtre elde edilmiştir. Filtre tasarımı ZnO/TiO2/taşıyıcı/TiO2/ZnO olarak gerçekleştirilmiş ve taşıyıcı olarak Corning 2947 camı kullanılmıştır. Filtredeki filmlerin optik özellikleri belirlenerek tek-katman film sonuçlarıyla kıyaslanmışlardır. Başka bir karşılaştırma, tek-katman filmler ile filtrenin yüzey pürüzlülükleri arasında yapılmıştır. Tek-katman ZnO filmi ve filtre içindeki ZnO filminin optik band aralığı değeri neredeyse aynı iken, TiO2 filminin optik band aralığı filtrede tek-katman TiO2 filmine göre oldukça yüksek çıkmıştır. Filtre içinde hem ZnO hem de TiO2 filmlerinin kalınlıkları tek-katman filmlere göre daha ince bulunmuştur. Ayrıca ZnO'te TiO2'e göre daha fazla incelme gözlemlenmiştir. Kırma indisi değerleri 550 nm dalgaboyunda TiO2 için tek-katmanda ve filtrede neredeyse aynı bulunmuştur, aynı durum ZnO için de geçerlidir. Tek-katman TiO2 ve ZnO filmlerinin yüzeyleri oldukça pürüzsüzken filtrenin yüzeyinin daha pürüzlü olduğu ortaya çıkarılmıştır; yani ZnO filmi cam yerine TiO2 üstüne kaplandığında yüzeyi daha pürüzlü olmaktadır. Elde edilen filtrede, yansıtmanın minimumu 650 nm dalgaboyunda %0.808 değeri ile meydana gelmekte, ayrıca yansıtma, 635-665 nm dalgaboyu aralığı için %1'den, 585-720 nm için %4'den az çıkmaktadır. Sonuçta, sol-jel yöntemi kullanılarak elde edilen ZnO filminin, TiO2 filmiyle  beraber  yansıtmayıcı filtrelerde kullanılabileceği gösterilmiştir. Anahtar Kelimeler: ZnO, TiO2, sol-jel, döndürerek kaplama, yansıtmayıcı filtre.Antireflective filters are mostly used to increase the efficiency of solar cells. They vary from each other in their method of coating, the filter design, the materials that are used, and where their minimum occurs. In the present work, we coated TiO2 (titanium dioxide) and ZnO (zinc oxide) thin films through the sol-gel spin coating method to obtain an antireflective filter. First, single films were coated, producing a substrate/film system. The transmittance and reflectance of this system were obtained in the UV-visible region. The thicknesses, refractive indices and extinction coefficients of the films were determined. By choosing the appropriate film thicknesses and refractive indices needed for an antireflective filter in the desired wavelength region, the single films were combined to build filters with a ZnO/TiO2/substrate/TiO2/ZnO structure. Then the optical properties of the individual films in the multilayer stack were extracted and were compared to the single film results. The surface roughness of the single films was also compared to the filter's surface rouhgness. The experimental procedure involves sol preparation, coating of the films, and characterization methods. The sol-gel coating method was chosen for a variety of reasons: it is cheap to fabricate, there is minimal loss of material  during coating, it is suitable for mass production, homogenous films can be obtained, and by changing the amount of the chemical components and coating parameters, one can get a desired film thickness. For the TiO2 sol, ethyl alcohol, titanium (IV) butoxide and acetic acid were used. 2-propanol, Zinc acetate dehydrate, diethanolamine and distilled water were used to prepare the ZnO sol. Both the single layer films and the filter were coated on Corning 2947 as a substrate. The films in the filter were coated at different rotation speeds and number of layers. All the sub layers were preheated at the same temperature with the same duration time, which was 1 min at 250oC. Finally the stack firing method was applied to the filter for 15 min at 450oC. As for characterization methods, the transmittance and reflectance of the single films and the filter were obtained through a UV-visible spectrophotometer. The thicknesses, refractive indices and extinction coefficients were determined by fitting to a modified Tauc-Lorentz model for the ZnO films and a Tauc-Lorentz model for the TiO2 films. The surface roughness of the single films and the filter was determined by atomic force microscopy. The characterization of the films and filter lead to the following results. The optical bandgap value of the TiO2 film was found to be 3.37 eV in the single layer and 3.53 eV in the stack. ZnO had a bandgap value of 3.25 eV in the single layer which shifted to 3.29 eV in the stack. Thus, the bandgap value of ZnO stays almost the same while the one of TiO2 increases by a substantial amount. The thickness value of TiO2 in the single layer was 168 nm and dropped to 157 nm in the stack. The ZnO film had a thickness of 111 nm for the single layer but  81 nm for the one in the stack. As a percentage of the single layer thickness, the reduction in size was larger for ZnO than TiO2. The refractive index of TiO2 at =550 nm in the single layer was 2.04 and 2.02 in the stack. The ZnO layer had a refractive index of 1.40 in the single layer and 1.38 in the stack. This shows that for both of the films, the indices in the stack and single layers stay almost the same, which allows the single layer properties to be useful in predicting the behavior of the stack. The root mean square roughness values for the single layer TiO2 and ZnO films are 0.5 and 5.2 nm, respectively. When the ZnO film is fabricated in the filter, its roughness value increased to 9.2 nm, which means that if the ZnO film is coated onto TiO2 instead of glass, its surface becomes slightly rougher. The filter had a minimum at =650 nm with 0.808% reflection. The reflection for the wavelength range  635-665 nm was less than  1%  and for 585-720 nm it was less than 4%. To summarize, it is shown here that sol-gel-made ZnO films can be used in an antireflective filter incorporating a TiO2 layer. Keywords: ZnO, TiO2, sol-gel, spin coating, antireflective fitler

The effect of annealing temperature on optical and structural properties of ZnO thin films

Çinko oksit (ZnO) yasak band aralığı yaklaşık 3.3 eV olan ve elektromanyetik spektrumun geniş bir aralığında yüksek geçirgenliğe sahip yarıiletken bir malzemedir. Çinko oksit ucuzluğu, sağlığa zararlı olmaması ve diğer şeffaf iletken malzemelere alternatif olma potansiyelinden dolayı son yıllarda yaygın olarak çalışılmaktadır. Bu çalışmada sol-jel döndürerek kaplama metodu ile Corning 2947 taşıyıcılar üzerine çinko oksit ince filmler hazırlanmıştır. Hazırlanan filmlere 100, 250, 350 ve 550oC’de ısıl işlem uygulamıştır. Farklı ısıl işlem sıcaklıklarının çinko oksit ince filmlerin optik ve yapısal özellikleri üzerine etkisi araştırılmıştır. Optik parametreler NKD spektrofotometresinden elde edilmiştir. Filmlerin kristal yapısı ve yüzey morfolojisi sırasıyla X-Işını Kırınımı (X-Ray Diffraction, XRD) ve Atomik Kuvvet Mikroskobu (Atomic Force Microscopy, AFM) ile analiz edilmiştir. Farklı ısıl işlem uygulanan filmlerin tamamının görünür bölgede oldukça yüksek geçirgenliğe sahip oldukları görülmüştür. 300–400 nm dalgaboyu aralığında ise ısıl işlem sıcaklığının etkisi daha belirgin ortaya çıkmıştır. XRD sonuçlarına göre 100, 250 ve 350oC ısıl işlem sıcaklığına tabi tutulan filmlerin amorf yapıda oldukları; 550oC’de ısıl işlem uygulanan filmin ise kristal yapıda olduğu görülmüştür. AFM resimlerinden elde edilen sonuçlara göre ısıl işlem sıcaklığının artmasıyla filmlerin yüzey morfolojilerinin de önemli ölçüde değiştiği görülmüştür. Hazırlanan ZnO filmlerin tamamı uygulanan bütün ısıl işlem sıcaklıklarında 7.5 nm ve altındaki Rms değerleriyle oldukça pürüzsüz yüzeylere sahiptir. En düşük pürüzlülüğe sahip film 350oC’de elde edilmiştir. Anahtar Kelimeler: Çinko oksit, sol-jel, optik özellikler, yapısal özellikler.Zinc oxide (ZnO) is an n-type semiconducting material with a ~3.3 eV band gap and high transparency over a wide range of the electromagnetic spectrum. Zinc oxide has been studied widely due to its inexpensiveness, non-toxicity and since it is a potential alternative to other transparent conducting materials. ZnO thin films are important due to their potential for applications such as gas sensors, solar cells, optoelectronic devices, transparent conducting electrodes and optical waveguides. ZnO thin films have been prepared by various methods such as chemical vapor deposition, pulsed laser deposition, sputtering, r.f. magnetron sputtering, spray pyrolysis and the sol-gel process. Among them, the sol-gel method is preferred since it has the advantages of easy control of the film composition and easy fabrication of large-area films with low cost. There are many factors that affect the optical and microstructural properties of sol-gel-made films.   Among them are the chemical composition and concentration of the solution, the coating parameters, the thickness of the film, the preheating temperature and time between each layer deposition, and the final annealing temperature. In this study, transparent ZnO thin films were prepared on Corning 2947 substrates by the sol-gel spin coating method. ZnO films were annealed at 100, 250, 350 and 550oC for 1 hour. The effects of annealing temperature on microstructure and optical properties were investigated. The coating solution was prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)2 .2H2O, 98%, Aldrich) in 2-propanol (99.5%, Aldrich) and stirring by a magnetic stirrer at 60oC for 10 min. Diethanolamine (DEA, 99%, Aldrich) was added drop by drop to the solution during stirring. Then, distilled water was slowly added to the solution and stirred 10 min more. Finally, a clear and homogeneous solution was obtained. (DEA:ZnAc=1:1 [mol ratio] and H2O:ZnAc=2:1 [mol ratio]). An amount of ZnO solution was dropped onto Corning 2947 glass substrates on the spin coater, which were rotated at 3000 rpm for 30 s. The films were dried at 100oC for 2 min to evaporate the solvent and remove organic residuals. The coating procedure was repeated six times to reach the desired thickness. Finally, the films were heat treated at 100, 250, 350 and 550oC for 1 hour. All coatings were made at room temperature (~21oC) and ~55% relative humidity. Optical transmittance and reflectance of the ZnO films were measured in the spectral range of 300?1000 nm using an NKD 7000 (Aquila, UK) spectrophotometer. Measurements were made with a beam incidence angle of 30o using s-polarized light. AFM images of the films were obtained using a SPM-9500J3 (Shimadzu, Japan) scanning probe microscope in contact mode. The crystalline structures of the films were analyzed by an X-ray diffractometer (Philips PW3710, CuKα radiation). All the ZnO thin films were highly transparent with a transmittance of ~85% in the visible range. The ZnO thin films annealed at 100, 250, 350 and 550oC had transmittance values of 88.4%, 86%, 87.1%, 86.7% and reflectance values of 10.5%, 13.8%, 12.7%, 13.2% at the wavelength of 550 nm, respectively. The effect of the annealing temperature was significant between the wavelength range of 300?400 nm. The peaks in this range for films annealed at 350 and 550oC are a property of semiconducting materials due to excitonic absorption.  ZnO has a high excitonic binding energy (60 meV) which causes exciton-related emission in the ultraviolet region. The XRD diffraction peaks belonging to the (100), (002), (101), (102) (110), (103) and (112) planes were seen only in ZnO films annealed at 550oC for 1 hour. The film annealed at 550oC was crystalline with hexagonal wurtzite structure. However, there were no peaks for the other films annealed at 100, 250 and 350oC for 1 hour. ZnO thin films had smooth surfaces with RMS values of 7.5, 1.2, 0.5 and 3.3 nm for annealing temperatures of 100, 250, 350 and 550oC, respectively. This study showed that annealing temperature has a considerable effect on the optical and structural properties of ZnO thin films. Keywords: Zinc oxide, sol-gel, optical properties, structural properties

Preparation and comparison of standard and mesoporous WO3 films

Bu çalışmada, standart ve gözenekli WO3 filmler, başlangıç malzemesi olarak tungsten hekzaklorit (WCl6) ve ethanol kullanılarak sol-gel yöntemlerinden olan döndürme tekniğiyle hazırlanmıştır. Gözenekli tungsten oksit filmler, tungsten oksit solune dört farklı tip polimer eklenerek kaplanmıştır. Söz konusu polimerler: (BASFTM Pluronic p123) (p1), poly(ethylene glycol) (p2), polyethylene glycol-ran-propylene glycol (p3) ve polystyrene-co-allyl-alchl (p4)’dır. Bu polimerler kullanılarak gözenekli WO3 filmler hazırlanmış ve filmlerin optik, yapısal ve elektrokromik özellikleri standart (s1) WO3 filmler ile kıyaslanmıştır. Filmlerden UV aydınlatma yöntemi ile polimer kalıpların uzaklaştırılması başarılmıştır. Aynı zamanda UV aydınlatma işlemi uygulanmış tungsten oksit filmlerin renklenme etkinliği değerlerinde düzelme gözlenmiştir. Anahtar Kelimeler: Elektrokromism, gözenekli WO3 filmler, iyon giriş-çıkış kinetiği, sol-gel, uv aydınlatma.In this study, standard and mesoporous sol-gel WO3 thin films were prepared by a spin coating technique from an ethanolic solution of tungsten hexachloride (WCl6). Different types of polymers were employed as a template to generate the mesoporous structure. These polymers: (BASFTM Pluronic p123) (p1), poly(ethylene glycol) (p2), polyethylene glycol-ran-propylene glycol (p3) and polystyrene-co-allyl-alchl (p4). To avoid the varying degrees of crystallinity as a result of thermal treatment, a UV illumination method has been employed to remove the polymer surfactant. This room temperature approach uses ozone generated during UV illumination in air to oxidize the organic compounds.  The electrochromic and optical properties of the mesoporous films are described and compared to standard sol-gel WO3 films. Results are also presented on the samples prepared by thermal treatment. We demonstrate that the UV illumination treatment is a superior method to remove templates which enables us to more effectively investigate the effect of mesoporosity on the electrokinetics of ion insertion into WO3 films. These mesoporous materials exhibit superior high rate ion-insertion performance when used as electrochromic layers, which is attributed to the high surface area of mesoporous WO3. The films were characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), ellipsometre and cyclic voltammetry. Keywords: Electrochromism, insertion kinetics, mesoporous WO3 films, sol-gel, uv illumination

TiO2 photocatalysts

Son yıllarda, titanyum dioksit (TiO2) üzerinde, fotokatalitik aktivite özelliğinden dolayı yoğun olarak çalışılmaktadır. TiO2, UV ışığı ile uyarıldığı zaman fotoaktif özellik gösteren ve organik grupları parçalayabilen yarıiletken bir malzemedir. TiO2, ışığa maruz bırakıldığında, suyun arıtılmasında, kendi kendini temizleyebilen, buğulanmayan yüzeylerin elde edilmesinde, fotokimyasal olarak kanser tedavisi uygulamalarında, havanın arındırılmasında kullanılabilir. TiO2 filmler, kimyasal buhar biriktirme, sıçratma, elektron demeti ile buharlaştırma, iyon ışını destekli biriktirme ve sol-jel gibi yöntemlerle değişik yüzeyler üzerine kaplanabilirler. TiO2, anataz, rutil ve brukit olmak üzere üç farklı kristal yapıya sahiptir. Birçok uygulamada TiO2’in anataz formu en iyi fotoaktivite özelliği göstermektedir. TiO2’in, solar spektrumun çok az bir bölümünü oluşturan UV ışığı ile aktive edilebiliyor olması bu malzemenin pratik uygulamalardaki kullanımını sınırlandırmaktadır. Bundan dolayı, pratik uygulamalar için, TiO2’in fotoaktivitesinin geliştirilmesi gerekmektedir. Fotoaktiviteyi arttırmanın bir yolu, TiO2’in geçiş metalleri veya soy metallerle katkılandırma işlemi yaparak soğurma (absorption) bandının UV bölgesinden, görünür bölgeye kaydırılmasıdır. Literatürde, titanyum oksit filmlere gümüş, tungsten ve molibden katkılandırılmasına yönelik çalışmalar yapılmış ve üç katkılandırmanın da, titanyum oksit filmlerin fonksiyonalitesine farklı mekanizmalar üzerinden ciddi katkılar yapacak nitelikte olduğu belirtilmiştir. Bu çalışmada, TiO2’in fotokatalitik aktivite mekanizması tartışılmış ve TiO2’in kullanım alanları anti-bakteriyel özelliklerine odaklanarak özetlenmiştir. Buna ek olarak, gümüş, tungsten ve molibden katkısının TiO2’in, anti-bakteriyel aktivitesine olan etkileri tartışılmıştır.  Anahtar Kelimeler: TiO2, sol-jel, anti-bakteriyel, fotokatalitik aktivite.Recently, titanium dioxide (TiO2) has been studied extensively due to its high photocatalytic activity for handling of several types of environmental problems. Major areas of activity in TiO2 photocatalysis are; water purification, photochemical cancer treatment, air purification, self-sterilizing, fog-proof and self-cleaning surfaces. Photocatalysis can be defined as ?acceleration of a photoreaction by the presence of a photocatalyst?. Photocatalytic reactions necessitate a photocatalyst that absorbs the phonons and drives the redox reactions. TiO2 is a semiconductor and it can be chemically activated by UV light. TiO2 has three different crystal structures which are anatase, brookite and rutile. TiO2 in the anatase form is the most efficient of photocatalysts for many applications. The band gap energy of anatase TiO2 is 3.2 eV and it can be only activated by UV light. Although UV light is present in the solar spectrum it is only a very limited part. For practical applications the photocatalytic activity of TiO2 needs further improvement. Doping TiO2 with transition metals or noble metals is an effective way to improve photocatalytic activity. When TiO2 is exposed to UV light, electron-hole pairs are created. The photogenerated holes in the valence band, which has strong oxidizing power, diffuse to the surface and react with adsorbed water in order to produce hydroxyl radicals (?OH). These hydroxyl radicals participate in oxidizing organic molecules. On the other hand, electrons in the conduction band react with molecular oxygen in the air to produce the superoxide radical anion (O2-?), which also participates in further oxidation processes. The photocatalytic efficiency of TiO2 strongly depends on surface area and electron-hole recombination rate. The surface area of the photocatalyst increases with a smaller particle size and the active surface sites increase. For improving photocatalytic efficiency, electron-hole recombination rate should be reduced. An effective way to seperate electron-hole pairs is to introduce foreign materials into TiO2 matrix. As mentioned above, TiO2 can be used in different application areas. One of the remarkable property of TiO2 is its self-cleaning effect. The surfaces of glasses, ceramic tiles can be contaminated by organic particles such as smoke residue, oil and dirt. TiO2 thin films can be applied to these surfaces in order to decompose those organic species. Another excellent property of TiO2 photocatalysts is their anti-bacterial effect. TiO2 can decompose bacteria and virus when it is exposed to UV light. TiO2 has advantages over conventional self-sterilizing surfaces. For instance, in the case of E.coli, TiO2 decompose both the living cells as well as the endotoxin released from these cells during their death. TiO2 photocatalysts can also be used for cancer treatment. TiO2 particles which are injected to the tumor  clearly inhibit the tumor growth. In literature, there are several studies which are related to the doping effect of silver. Studies which are performed on the effect of silver dopant are focused on the change of optical and electronical properties of TiO2. Moreover, since silver itself is known as strong anti-bacterial agent it is used as dopant for improving anti-bacterial properties of TiO2. Doping silver can give rise to the separation of electron-hole pairs and can accelerate the formations of oxidative species. In addition to this, silver can reduce particle size which is needed for increasing surface area of TiO2. In order to obtain anti-bacterial effect in the dark, energy storage photocatalyst can be produced by doping TiO2 with tungsten. TiO2-WO3 photocatalyst can be photo-charged by irradiating their surfaces with UV light. Photo-charged tungsten doped TiO2 films are able to show anti-bacterial effect when the light is turned off. Molybdenum also is an energy storage material and it can be used as an alternative to those of tungsten. In this study, the mechanism of photocatalytic activity is discussed and the application fields of TiO2 photocatalysts were summarized by focussing on the bacterial activity of TiO2. Moreover, the effect of silver, tungsten and molybdenum dopants on the bacterial activity of TiO2 were discussed.  Keywords: TiO2, sol-gel, photocatalyst, anti-bacterial, photocatalytic activity

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