Publication: Organik yarıiletken filmlerin karakterizasyonu ve gaz sensörü olarak uygulanması
Abstract
Organik ince filmler, elektronikte alışılmış inorganik malzemelerle başarılamayan bir çok alanda başarı ile uygulanabilmekte ve gün geçtikçe yeni uygulama alanları bulmaktadır. Supramoleküler organik bileşikler ve polimerik ince filmler organik transistörlerden solar cell'lere, non-linear optik aygıtlardan gaz sensörlerine kadar geniş bir uygulama alanına sahiptirler. Elektronikteki önemlerinin yanı sıra bu malzemelerin iletkenliklerinin ortamdaki gaz/ gazların varlığına duyarlı olması bu malzemeleri mikroelektronik gaz sensörü yapımı konusunda bir kez daha gündeme getirmiştir. Kimyasal sensörler bir kimyasal hali giriş sinyali olarak alıp bir elektriksel veya optik çıkış sinyaline çevirirler. En basit haliyle bir kimyasal gaz sensörü kimyasal algılayıcı bir film (Coating) ve optik ya da elektriksel sinyal üreten bir transduserden ibarettir. Kimyasal gaz sensörleri kimya ve gıda sanayiinde proses kontrolünden atmosferik kirliliğe yol açan gazların dedekte edilmesine kadar farklı alanlarda uygulanabilmektedirler. Konunun bu kadar geniş olması seçici algılayıcı malzemelerin önemini ortaya koymaktadır. Ftalosyaninler gaz sensörü uygulamalarında algılayıcı malzeme olarak üzerinde en çok çalışılan arayüzey malzemeleridir. Arayüzey olarak kullanılan algılayıcı malzemelerinin gaz/ gazları algılama özelliklerinin test edilmesi için uygun bir transduser yapısı gerekir. Transduserler bu maddelerin gaz fazdaki moleküllerle etkileşmelerinden dolayı kapasite, iletkenlik, empedans, sıcaklık veya kütle değişimlerini izler. Bu çalışmada algılayıcı malzeme olarak, suda çözünebilen crosswise-disubstitute ve iki alkilsulfanil ve iki amino grupla crosswise-substitute edilmiş farklı iki grup ftalosyanin malzemelerin elektriksel karakterizasyonu yapıldıktan sonra, değişik gazlara duyarlıkları test edilmiştir. Malzemelerin değişik gaz/ gazlara duyarlığının incelenmesinde iki farklı tipte transduser kullanılmıştır. Bunlar interdijital transduser (IDT) ve Quartz Kristal Mikrobalansdır. Her iki tür transduserle gerçekleştirilen ölçümler aşağıda kısaca açıklanmıştır. Arayüzey olarak kullanılan algılayıcı malzemelerin ilgili çözücülerde çözerek elde edilen çözeltilerden fotolitografi tekniği ile cam taban malzemesi üzerinde oluşturulan IDT'ler üzerine filmleri hazırlandı. Elde edilen filmlerin oda sıcaklığında vakumda ve vakumsuz ortamda olmak üzere I-V karakteristikleri ve ac iletkenlikleri ölçüldü. Ayrıca filmlerin Br2 ve Cl2 halojenlerle kloroform, diklormetan, tetraklormetan, trikloretilen, tetrakloretilen, tetrakloretan, tolüen, hekzan, heptan ve oktan gibi kolay buharlaşabilen bu sayede gıda maddelerine ve özellikle içme sularına kolayca karışabilen organik buharlarına duyarlıkları iletkenliklerindeki değişimlerin ölçülmesi suretiyle test edilmiştir. Yedi farklı sıcaklıkta gerçekleştirilen ölçümlerde cevap süresi, duyarlık ve geri dönüşümlülük (reversibility) gibi temel sensör parametrelerinin kuvvetle sıcaklığa bağlı olduğu gözlemlenmiştir. Her iki grup ftalosyaninin de iletkenliklerinin Br2 ve Cl2 gazlarının ppb (part per billion) mertebesindeki konsantrasyonlarında dahi büyük oranda değiştiği (arttığı) gözlenirken kloroform, diklormetan, tetraklormetan, trikloretilen, tetrakloretilen, tetrakloretan, tolüen, hekzan, heptan ve oktan gibi organik buharlarının iletkenlik değişimine neden olmadığı gözlemlenmiştir. Yüzeydeki kütle değişimi sonucu rezonans frekansındaki değişimlerin ölçülmesi esasına dayanan QCR ölçümlerinde iki farklı tip QCR kullanılmıştır. Bunlardan birincisi her iki yüzeyinde de eşit ve 3.5 cm yarıçaplı metal elektrotlar bulunan 7-7 QCR ikincisi de, yüzeydeki elektrotlardan birisinin yine 3.5 cm diğerinin de 2 cm yarıçapında olduğu 4-7 QCR dır. Örneklerin kloroform, diklormetan, tetraklormetan, trikloretilen, tetrakloretilen, tetrakloretan, tolüen, hekzan, heptan ve oktan gibi organik buharlarına duyarlılıkları her iki QCR'da da hem seri hem de paralel rezonans frekansındaki değişimler ölçülmek suretiyle test edilmiştir. Oda sıcaklığında gaz konsantrasyonuna bağlı olarak gerçekleştirilen QCR ölçümlerinde ortamdaki gazın türüne bağlı olarak QCR ların duyarlıklarının değiştiği gözlemlenmiştir. İletkenlik değişimine dayalı ölçümlerle QCR ölçümlerinin karşılaştırılmasında IDT sensörlerin QCR dan daha seçici olduğu gözlenmiştir.
Supramoleculer organic compaunds and polymeric thin films are often used as prototype materials for both, basic research and practical applications in the field of moleculer electronic or non-linear optical devices, organic transistors, solar cells and gas sensors. Together with the importance in the field of electronic these materials are known to exibit large conductivity changes on exposure to gas and their applications as sensitive films for semiconducting gas sensors are currently under investigations. Chemical sensors are a device which is capable of converting a chemical quantity into an electrical or optical signal. A gas sensor consists of a chemically sensitive layer (coating) and a transducer unit which produces an output signal. Chemical gas sensors can be used in the field from the chemistry and food industry in process control to the medical sciences. The phthalocyanines are a class of organic semiconductors, whose electrical properties have recently attracted considerable attention owing to their potentiall applications in gas sensor systems. There has been increasing interest in recent years in the use of metal phthalocyanine films for gas sensing. Specifically, they can be used as thin film semiconducting gas sensors for the detection of halogens, such as chlorine, bromine, nitrogen dioxide as well as organic vapours. The interaction between the gas molecules and the coatings result in detectable changes in the physical properties of the coating which include conductivity, mass density, and optical properties. Such changes can be used to implement any kind of gas sensors using a transducer. In order to develop new gas sensors and to gain more insight into the interaction between phthalocyanine and organic vapours of interest, a series of new soluble phthalocyanines, such as,-2,3,16,17-di(n- dodecylsulfanyl)-diaminophthalocyanine were used. To investigate the gas sensing properties of these materials two kind of transducer were used; one is the interdigital transducer to detect conductivity changes and other is Quartz Crystal Microbalance to detect mass changes. The sensor device for the dc and ac conductivity change measurements consist of a split finger interdigital electrode structure were prepared on glass substrate photolithographically. The sensitive coating were prepared by dipping using diluted polymer solutions in volatile organic solvents. For the electrical characterization current-voltage (I-V) characteristics and ac conductivity measurement were carried out under vacuum and without vacuum at room temperature. We also measured dc and ac conductivity changes when the films exposed to analytes molecules. The analytes investigated are volatile organic compaund such as, trichloromethane, tetrachloromethane, trichloroethene, tetrachloroethene, tetrachloroethane, toluene, nitrobenzene, n-hexane, n- heptane, n- octane and halogens, such as chlorine and bromine . The measurments were carried out in the 5-75 0C temperature range. İn these measurements we observed that, while the very low concentration (ppb) of halogens cause a very large conductivity changes the volatile organic compaunds don't contribute to the conductivity. We also observed that the sensors parameters such as, sensitivity, selectivity, response and recovery time depend strongly on temperature. In QCR measurements a conventional 11 MHz. 7-7 QCR and 4-7 QCR were used. İn the 4-7 QCR, the upper electrode in contact with the coating has a diameter of 4 mm and the lower electrode is maintained at 7 mm. The sensors were tested in a computer controlled flow setup at room temperature. Frequency changes of the QCR sensors were measured using oscillator circuits. Both the series-resonant (fs) and parallel-resonant (fp) frequencies are measured to illustrate the possible changes in the electrical properties of the various polymer coatings. These measurement and calculations showed that the selectivity of the IDT sensors better than QCR sensors.
Supramoleculer organic compaunds and polymeric thin films are often used as prototype materials for both, basic research and practical applications in the field of moleculer electronic or non-linear optical devices, organic transistors, solar cells and gas sensors. Together with the importance in the field of electronic these materials are known to exibit large conductivity changes on exposure to gas and their applications as sensitive films for semiconducting gas sensors are currently under investigations. Chemical sensors are a device which is capable of converting a chemical quantity into an electrical or optical signal. A gas sensor consists of a chemically sensitive layer (coating) and a transducer unit which produces an output signal. Chemical gas sensors can be used in the field from the chemistry and food industry in process control to the medical sciences. The phthalocyanines are a class of organic semiconductors, whose electrical properties have recently attracted considerable attention owing to their potentiall applications in gas sensor systems. There has been increasing interest in recent years in the use of metal phthalocyanine films for gas sensing. Specifically, they can be used as thin film semiconducting gas sensors for the detection of halogens, such as chlorine, bromine, nitrogen dioxide as well as organic vapours. The interaction between the gas molecules and the coatings result in detectable changes in the physical properties of the coating which include conductivity, mass density, and optical properties. Such changes can be used to implement any kind of gas sensors using a transducer. In order to develop new gas sensors and to gain more insight into the interaction between phthalocyanine and organic vapours of interest, a series of new soluble phthalocyanines, such as,-2,3,16,17-di(n- dodecylsulfanyl)-diaminophthalocyanine were used. To investigate the gas sensing properties of these materials two kind of transducer were used; one is the interdigital transducer to detect conductivity changes and other is Quartz Crystal Microbalance to detect mass changes. The sensor device for the dc and ac conductivity change measurements consist of a split finger interdigital electrode structure were prepared on glass substrate photolithographically. The sensitive coating were prepared by dipping using diluted polymer solutions in volatile organic solvents. For the electrical characterization current-voltage (I-V) characteristics and ac conductivity measurement were carried out under vacuum and without vacuum at room temperature. We also measured dc and ac conductivity changes when the films exposed to analytes molecules. The analytes investigated are volatile organic compaund such as, trichloromethane, tetrachloromethane, trichloroethene, tetrachloroethene, tetrachloroethane, toluene, nitrobenzene, n-hexane, n- heptane, n- octane and halogens, such as chlorine and bromine . The measurments were carried out in the 5-75 0C temperature range. İn these measurements we observed that, while the very low concentration (ppb) of halogens cause a very large conductivity changes the volatile organic compaunds don't contribute to the conductivity. We also observed that the sensors parameters such as, sensitivity, selectivity, response and recovery time depend strongly on temperature. In QCR measurements a conventional 11 MHz. 7-7 QCR and 4-7 QCR were used. İn the 4-7 QCR, the upper electrode in contact with the coating has a diameter of 4 mm and the lower electrode is maintained at 7 mm. The sensors were tested in a computer controlled flow setup at room temperature. Frequency changes of the QCR sensors were measured using oscillator circuits. Both the series-resonant (fs) and parallel-resonant (fp) frequencies are measured to illustrate the possible changes in the electrical properties of the various polymer coatings. These measurement and calculations showed that the selectivity of the IDT sensors better than QCR sensors.