The determination of band gap using Tauc method

U radu je predstavljena metoda određivanja zabranjene zone poluvodiča korištenjem UV-Vis difuzne refleksijske spektroskopije. Detaljno je opisana sama tehnika difuzne refleksijske spektroskopije, postupak transformacije difuzijskog refleksijskog spektra u Kubelka-Munk funkciju kao i konstrukcija Taucova grafičkog prikaza, odnosno određivanje zabranjene zone. Uzorci TiO2, ZnO te ZrO2 prvo su podvrgnuti rendgenskoj difrakcijskoj analizi kako bi se dobio uvid u njihov fazni sastav, odnosno čistoću. Utvrđeno je da su tri uzorka monofazna te se sastoje od anatasa, rutila i cinkita dok je četvrti uzorak smjesa monoklinskog i kubičnog ZrO2. Monofazni uzorci dalje su analizirani metodom UV-Vis difuzne refleksijske spektroskopije. Dobiveni spektri su transformirani u Kubelka-Munk funkciju te su konstruirani grafički prikazi ovisnosti Kubelka-Munk funkcije o energiji. Potom su konstruirani Taucovi grafički prikazi i iz njih su određene zabranjene zone anatasa, rutila i cinkita. Dobivene su zabranjene zone od 3,22 eV za anatas, 3,17 eV za rutil i 3,28 eV za cinkit. Dobive vrijednosti za energiju zabranjene zone uspoređene su s literaturnim vrijednostima.The method for the determination of semiconductor band gap using UV-Vis diffuse reflectance spectroscopy has been presented. Diffuse reflectance spectroscopy technique, the transformation procedure of diffuse reflectance spectra to Kubelka-Munk function and construction of Tauc's plot, i.e. the determination of band gap, have been described in details. The samples of TiO2, ZnO and ZrO2 have been first subjected to X-ray diffraction analysis in order to gain insight in their phase composition, i.e. phase purity. It has been determined that three samples are monophasic and composed of anatase, rutile and zincite while fourth sample is mixture of monoclinic and cubic zirconia. Monophasic samples were further analyzed using UV-Vis diffuse reflectance spectroscopy. Obtained spectra were transformed to Kubelka-Munk function and the dependences of Kubelka-Munk functions on energy have been graphically displayed. Subsequently Tauc's plots have been constructed and using them band-gaps of anatase, rutile and zincite have been determined. Band gaps of od 3.22 eV, 3.17 eV and 3.28 have been obtained for anatase, rutile and zincite, respectively. The obtained band gaps were compared with literature data

The determination of band gap using Tauc method

U radu je predstavljena metoda određivanja zabranjene zone poluvodiča korištenjem UV-Vis difuzne refleksijske spektroskopije. Detaljno je opisana sama tehnika difuzne refleksijske spektroskopije, postupak transformacije difuzijskog refleksijskog spektra u Kubelka-Munk funkciju kao i konstrukcija Taucova grafičkog prikaza, odnosno određivanje zabranjene zone. Uzorci TiO2, ZnO te ZrO2 prvo su podvrgnuti rendgenskoj difrakcijskoj analizi kako bi se dobio uvid u njihov fazni sastav, odnosno čistoću. Utvrđeno je da su tri uzorka monofazna te se sastoje od anatasa, rutila i cinkita dok je četvrti uzorak smjesa monoklinskog i kubičnog ZrO2. Monofazni uzorci dalje su analizirani metodom UV-Vis difuzne refleksijske spektroskopije. Dobiveni spektri su transformirani u Kubelka-Munk funkciju te su konstruirani grafički prikazi ovisnosti Kubelka-Munk funkcije o energiji. Potom su konstruirani Taucovi grafički prikazi i iz njih su određene zabranjene zone anatasa, rutila i cinkita. Dobivene su zabranjene zone od 3,22 eV za anatas, 3,17 eV za rutil i 3,28 eV za cinkit. Dobive vrijednosti za energiju zabranjene zone uspoređene su s literaturnim vrijednostima.The method for the determination of semiconductor band gap using UV-Vis diffuse reflectance spectroscopy has been presented. Diffuse reflectance spectroscopy technique, the transformation procedure of diffuse reflectance spectra to Kubelka-Munk function and construction of Tauc's plot, i.e. the determination of band gap, have been described in details. The samples of TiO2, ZnO and ZrO2 have been first subjected to X-ray diffraction analysis in order to gain insight in their phase composition, i.e. phase purity. It has been determined that three samples are monophasic and composed of anatase, rutile and zincite while fourth sample is mixture of monoclinic and cubic zirconia. Monophasic samples were further analyzed using UV-Vis diffuse reflectance spectroscopy. Obtained spectra were transformed to Kubelka-Munk function and the dependences of Kubelka-Munk functions on energy have been graphically displayed. Subsequently Tauc's plots have been constructed and using them band-gaps of anatase, rutile and zincite have been determined. Band gaps of od 3.22 eV, 3.17 eV and 3.28 have been obtained for anatase, rutile and zincite, respectively. The obtained band gaps were compared with literature data

Sol-gel Synthesis and Characterization of Lithium and Cerium Codoped Perovskite

Perovskites are an important group of ceramic materials with a structural formula ABO3 and wide array of potential applications in electronics, superconductors, catalysis, etc. CaTiO3, by which the whole group was named for, is particularly significant due to its use in catalysis, but its photocatalytic activity is limited by a large band gap value (~3.5 eV). A possible solution is the substitution of A and B cations with foreign cations which causes the alteration of properties, including photocatalytic efficiency. The aim of this work was the sol-gel synthesis of lithium and cerium codoped CaTiO3, characterization of the prepared gel and ceramics obtained by its thermal treatment. Samples of codoped perovskite, Ca1-xLixCexTiO3, where x = 0, 0.01, 0.02, 0.03 and 0.04, were prepared and characterized using powder X–ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), differential thermal and thermo- gravimetric analysis (DTA-TGA), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Photocatalytic activity was evaluated through the study of methylene blue photocatalytic degradation. XRD analysis showed that the prepared samples consisted of calcium nitrate and titanium chelate. In accordance with the established thermal evolution path, all samples were thermally treated at 500 °C for 2 hours. Beside perovskite, Ca2Ti2O6 appeared as a secondary phase in all thermally treated samples. SEM analysis of thermally treated samples showed the presence of agglomerates of irregular morphology and the decrease of primary particles size with the increase of dopants concentration. The sample with x=0.04 showed an increased photocatalytic activity

Deposition of Thin Alumina Films Containing 3D Ordered Network of Nanopores on Porous Substrates

Self-supporting thin films containing nanopores are very promising materials for use for multiple applications, especially in nanofiltration. Here, we present a method for the production of nanomembranes containing a 3D ordered network of nanopores in an alumina matrix, with a diameter of about 1 nm and a body centered tetragonal structure of the network nodes. The material is produced by the magnetron sputtering deposition of a 3D ordered network of Ge nanowires in an alumina matrix, followed by a specific annealing process resulting in the evaporation of Ge. We demonstrate that the films can be easily grown on commercially available alumina substrates containing larger pores with diameters between 20 and 400 nm. We have determined the minimal film thickness needed to entirely cover the larger pores. We believe that these films have the potential for applications in the fields of filtration, separation and sensin

Measurement and Control: Determination of the Semiconductors Band Gap by UV-Vis Diffuse Reflectance Spectroscopy

U radu je dan pregled pojmova i jednadžbi vezanih uz određivanje zabranjene zone metodom difuzne refleksijske spektroskopije i uporabu Taucova grafičkog prikaza. Kako bi se demonstrirao postupak i vrednovala sama metoda, primjenom metode određena je zabranjena zona komercijalnih uzoraka anatasa, rutila, cinkita i hematita. Na temelju eksperimentalno dobivenih vrijednosti širine zabranjene zone komentirana je točnost metode te osjetljivost pri razlikovanju poluvodiča s direktnim i indirektnim prijelazima. Pokazano je da Taucova metoda nije besprijekorna niti u pogledu točnosti niti razlikovanja indirektnih i direktnih elektronskih prijelaza u poluvodičkim materijalima, ali je vrlo praktičan način određivanja širine zabranjene zone poluvodiča budući da ne zahtijeva pretjerano skupu instrumentaciju a obrada eksperimentalnih podataka relativno je jednostavna. Ovo djelo je dano na korištenje pod licencom Creative Commons Imenovanje 4.0 međunarodna.For the application of semiconductors, an important factor is the band gap, i.e., the minimum energy required for the transfer of electrons from the valence to the conduction band. One of the possible methods for band gap determination is diffuse reflectance spectroscopy and Tauc plot. In this paper, an overview of the terms and equations related to the said method is given, as well as its utilization in the determination of band gaps of commercial samples of various metal oxides. Thus, the procedure is demonstrated and evaluated through the determination of indirect and direct band gap values of anatase (TiO2), rutile (TiO2), zincite (ZnO), and hematite (Fe2O3). All samples were beforehand analysed and identified by X-ray powder diffraction on Shimadzu XRD 6000 diffractometer with CuKα radiation working in a step scan mode with steps of 0.02° and counting time of 0.6 s. It was determined that all samples are well crystallized with relatively large crystallite sizes. UV-Vis spectra of the samples, as well as barite, which was used as a reference, were obtained on the UV-Vis spectrometer with an integrating sphere in total reflectance mode. The UV-Vis DRS spectra were transformed to Kubelka-Munk function, after which Tauc plot was used for the determination of the indirect and direct band gap values of all samples. The obtained values for anatase were 3.20 eV for indirect transition and 3.41 eV for direct transition, and for rutile 3.00 eV for indirect transition and 3.11 eV for direct transition. The zincite sample showed an indirect band gap of 3.19 eV and direct band gap of 3.25 eV, while the obtained indirect band gap value for hematite was 1.96 eV and direct band gap value 2.15 eV. As may be seen, the method is not particularly useful when distinguishing direct from indirect semiconductors, since, for all samples, the curves in Tauc plot for both indirect and direct electron transitions possess a linear dependence region from which the band gap value is estimated. However, the obtained band gap values for all the studied semiconductors are in relatively good concordance with literature references. The method is perhaps most useful in monitoring the variation of band gap depending on the dopant content. Namely, the studied metal oxides are used in photocatalysis where the addition of dopants is expected to reduce the band gap to visible light area, and thus improve the photocatalytic activity of the semiconductor. It can be concluded that the Tauc method is not perfect in terms of accuracy and differentiation between indirect and direct electron transitions in semiconductors. Nevertheless, it is a very practical way of band gap assessment for semiconducting materials, because it requires no excessively expensive instrumentation, and the processing of experimental data is rather simple. This work is licensed under a Creative Commons Attribution 4.0 International License

Ge/Al and Ge/Si3N4/Al Core/Shell Quantum Dot Lattices in Alumina: Boosting the Spectral Response by Tensile Strain

We investigated the production conditions and optoelectrical properties of thin film material consisting of regularly ordered core/shell Ge/Al and Ge/Si3N4/Al quantum dots (QDs) in an alumina matrix. The materials were produced by self– assembled growth achieved by means of multilayer magnetron sputtering deposition. We demonstrated the successful fabrication of well-ordered 3D lattices of Ge/Al and Ge/Si3N4/Al core/shell quantum dots with a body-centred tetragonal arrangement within the Al2O3 matrix. The addition of shells to the Ge core enables a strong tuning of the optical and electrical properties of the material. An Al shell induces a bandgap shift toward smaller energies, and, in addition, it prevents Ge oxidation. The addition of a thin Si3N4 shell induces huge changes in the material spectral response, i.e., in the number of extracted excitons produced by a single photon. It increases both the absolute value and the width of the spectral response. For the best sample, we achieved an enhancement of over 250% of the produced number of excitons in the measured energy range. The observed changes are, as it seems, the consequence of the large tensile strain in Ge QDs which is induced by the Si3N4 shell addition and which is measured to be about 3% for the most strained QDs. The tensile strain causes activation of the direct bandgap of germanium, which has a very strong effect on the spectral response of the material

Multiple exciton generation in 3D ordered networks of Ge quantum wires in alumina matrix

Thin films containing 3D-ordered semiconductor quantum wires offer a great tool to improve the properties of photosensitive devices. In the present work, we investigate the photogenerated current in thin films consisting of an interconnected 3D-ordered network of Ge quantum wires in an alumina matrix. The films are prepared using nitrogen-assisted magnetron sputtering co-deposition of Ge and Al2O3. We demonstrate a strong photocurrent generation in the films, much stronger than in similar films containing Ge quantum dots. The enhanced photocurrent generation is the consequence of the multiple exciton generation and the films’ specific structure that allows for efficient carrier transport. Thin film with the largest nitrogen content showed enhanced performance compared to other thin films with 1.6 excitons created after absorption of a single photon at an energy nearly equal to the double bandgap value. The bandgap value depends on the geometrical properties of the quantum wires, and it is close to the maximum of the solar irradiance in this case. In addition, we show that the multiple exciton generation is the most pronounced at the photon energy values equal to multiple values of the thin film bandgap