“Comprehensive study about new hybrid materials based on TiO2 and BiVO4 modified by alizarin compounds”
(Dr.Eng. Justyna Mech)
The main goal of the project entitled “Comprehensive study about new hybrid materials based on TiO2 and BiVO4 modified by alizarin compounds” was to observe and apply the photocurrent switching effect in information processing. Photocurrent can be described as movement of charge carriers induced by absorption of light in material. For semiconductors like TiO2 and BiVO4 the main charge carriers are electrons. Photocurrent switching is just a change of direction of photocurrent propagation between electrodes: cathode and anode. To ensure charge carrier movement the electrical contact between electrodes must be ensured by conducting medium. The most commonly used electrolytes are aqueous solutions (ionic substances dissociated in water). For electronic applications more practical are solid state materials which does not contain any liquid phase. Desired electrolyte should contain following features: good adhesion with porous structure of electrode, high electrical conductivity and solid state structure. In such a case, very useful are ionic liquids, which are good electrical conductors and at the same time have high enough viscosity. In photoswitchable devices such electrolyte plays also a role of binder (fig.1).
Coming back to photocurrent switching effect the indisputable advantage is simple and fast change of direction of current propagation by e.g. alternating pulsed lighting of the system. Those are relevant elements of nowadays electronic and optoelectronic. So far, in this fields silicon transistors are applied. The demand for more and more powerful and fast electronic devices from one side and less energy consuming from the otherresults in progressive miniaturization. These days silicon transistors are produced in 14 nm technology but this is not the dimension of whole transistor, rather dimension of the smallest element. In fact the transistor scale is much bigger. What is more fabrication of this sophisticated structures encounter technological obstacles. Further lowering of the silicon transistor dimension is very difficult, moreover high cost of production force to seek alternating solutions. One of them is the construction of logical gates using photodiodes based on the hybrid materials exhibiting photocurrent switching effect. Logical operations are realized as in the case of silicon transistors but at the same time logical gates consist of fewer logic elements, and what is more, the single element (photodiode) can be scaled to single semiconductor grain. In presented project AND gate is realized by only one photodiode, and XOR gate consists of two connected in series.
Other aspect of the research concerning TiO2/BiVO4 semiconductors modified by alizarin and alizarin red S is gaining knowledge about processes occurring at organic/inorganic interface. Hybrid materials like inorganic semiconductor and organic molecule are nowadays commercially used in catalysis, energy generation (dye sensitized solar cells), automotive industry (protecting coatings), ionics (flexible batteries), food industry (food packaging). Specially interesting process occurring in this type of materials is interfacial electron transfer. Thus this phenomenon is the object of intense research in electronics, optoelectronics, detection, energetics, automotive or biomedicine. Undertake research on the application of unusual photoelectrochemical phenomena for constructing information processing devices (logical gates, sensors) as well as searching for new materials meeting the highest requirements can be crucial for widely understood electronics and for medical and industry analytics.
“Photoelectrochemistry of bismuth oxyiodide and antimony sulfoiodide – towards novel semiconducting materials for optoelectronic applications”
(Dr. Eng. Przemysław Kwolek, Nr UMO-2012/05/N/ST5/00327)
The aim of the project was the synthesis and comprehensive characterization of two V-VI-VII semiconductors: bismuth oxyiodide (BiOI) and antimony sulphoiodide (SbSI). They were obtained via different routes (i.e. hydrothermal, microwave assisted and sonochemical) and characterized using various spectroscopic and electrochemical techniques in order to evaluate their electronic structures. Obtained results indicated that the band structures of both materials are similar, including relatively narrow band gap enabling absorption of visible light, the same conductivity type (BiOI and SbSI are both the n-type semiconductors) as well as similar conduction and valence band edge potentials. Thus, spectroscopic and electrochemical properties of these materials are similar. Applied synthetic techniques enabled obtaining materials characterized with relatively high surface-to-volume ratio facilitating electron (or hole) transfer at the semiconductor-electrolyte interface. Thereafter, photoelectrochemical investigation was performed. Upon immersion of a semiconductor into electrolyte, strong electric field at the semiconductor–electrolyte interface arises. Since it facilitates electron-hole pair separation, it is utilized for efficient photocurrent generation (upon illumination of the solid-liquid junction) for example in photoassisted water splitting. It may also be applied in unconventional information processing. Namely, one may control the direction of the photocurrent flow by changing the electric field strength at the interface. It may be realized via appropriate polarization of the semiconducting electrode, sometimes it is also possible to change the photocurrent polarity via changing the wavelength of incident light. Such a phenomenon is called the photoelectrochemical photocurrent switching effect and is sometimes observed for n-type semiconductors. Both BiOI and SbSI exhibit the switching effect which depends on the presence of efficient electron donor and acceptor in the electrolyte (e.g. iodide, triiodide species or molecular oxygen) as well as the potential barrier at the semiconductor-electrolyte interface. The second main goal of the project was the construction of simple logic devices on the basis of obtained materials, working in the solid state. Sandwich-type structures i.e. conductive, transparent substrate / semiconductor / electrolyte / conductive substrate were constructed. In order to obtain the switching effect in these simple devices, gelatin-based electrolyte with iodide species was applied. Constructed sandwich-like devices, upon illumination behave as the AND gate. Light is the information source whereas appropriate polarization of the working electrode (i.e. semiconductor deposited onto a conductive, transparent substrate) controls the work of the device. These are the inputs, photocurrent generated at the semiconductor-electrolyte interface is the output. Anodic photocurrent is observed exclusively when the device is illuminated within appropriate wavelength range and properly polarized. The same construction may also behave as the switch (1:2 demultiplexer), since the input (light) is directed into one of the two outputs (anodic or cathodic photocurrent) using appropriate polarization. Using such a simple structures, also OR and XOR gates were constructed. In the case of the former, two devices were connected in series i.e. in such a way that concomitant illumination of both devices yields sum of the photocurrent generated at each device. Illumination of only one component yields low intensity photocurrent at the output. Opposite polarization of two devices gives the functionality of XOR gate. In such a case photocurrent is observed only when exclusively one component is illuminated. Concomitant illumination of two devices gives null signal at the output. Thus, potential application of SbSI and BiOI in construction of logic gates was proved.
SbSI and BiOI increases the number of materials exhibiting photocurrent switching effect which are potential candidates for logic gates and switches working in nano- and molecular-scale. So far, there is not so much publications concerning switching effect observed for unmodified semiconductors. Understanding of the mechanism of this phenomenon is essential from the point of view of their application in information processing. Construction of unconventional logic devices based on the photoelectrochemical photocurrent switching effect is a prospective alternative for classical, silicon based technology. The necessity of looking for such an unusual ways of information processing stems on one hand from booming demand for fast and powerful computers, on the other hand from severe technological constrains of silicon-based technology. It is worth to note that materials such as BiOI or SbSI may be also promising alternative for surface-modified semiconductors where degradation of a dye may be a serious drawback. In addition, synthesis of BiOI and SbSI is simple and non-toxic. It enables potential fast and cheap fabrication of logic devices. V-VI-VII semiconductors (especially BiOI) are also promising photocatalysts. Understanding of the electronic structure of applied material is essential in designing and realizing photocatalytic processes. Usually these materials are applied for oxidation of organic pollutants from wastewater using hole from the valence band. However, the switching effect (which practically means possibility of cathodic photocurrent generation using n- type semiconductor) may be applied in photoreduction of heavy metals from industrial wastewater (e.g. from non-ferrous metals plants or spent electroplating baths). (Dr. Eng. Przemysław Kwolek)