Semiconductor systems, nanostructures

In recent years, nanotechnology has opened a new window on the physical-chemical properties of semiconductor materials nanostructuration. The development of nanostructured materials (nanowires, nanotubes, nanoparticles etc.) enables improvements in the response and recovery times of the sensor signal, due to the reduction of gas diffusion effects in the bulk of the material. Additionally, the sensitivity and the sensor response are also improved by the changes in the conduction mechanisms at the nanoscale and, finally, the reduction of the dimensions of the device allows the reduction of the power consumption by the system. 

In this chapter we concentrate on nano-semiconductor systems, where it has become possible during the last 15 years to handle and control single excitons. More specifically we focus on excitons and exciton-polaritons in semiconductor QDs. This chapter presents first a theoretical background of the solid-state theory, the exciton and the coupling between the exdton and the photon (excitonic polariton). Optical properties of exciton polaritons in nanostructures and their photonic engineering are then discussed. 

The development of high quality semiconductor systems where quantum eon-finement occurs, manifested in the creation of discrete, atom-like , electronie states rather than continuous bulk states, has boosted the interest of the PV research community in the implementation of solar cell geometries based on nanostructures. Particularly, colloidal quantum dots have emerged as potential candidates for developing inexpensive and highly efficiency concepts, including tandem solar cells,hot carrier solar cells and cells based on carrier multiplication. " ... 

Solar cells based on hot carrier extraction and CM rely on precise control of hot carrier relaxation were expected to be realized in nanostructured semiconductors e.g. QDs) because of enhanced carrier arrier interactions and discretized energy levels. As will be shown below, TRTS is capable of probing charge carrier dynamics at early times after photoexdtation, including intraband relaxation and CM in bulk materials and quantum dots. As such, TRTS represents a powerful technique for evaluating novel semiconductor systems that may be used in the design of more efficient solar cells. 

Answer. There has been little effective interplay between experimental results obtained on single nanostructures grown as quantum-wells and studied by optical-pumping methods and those obtained on bulk nanoscale semiconductors by more conventional NMR approaches. However, this situation may change, since the former studies can provide information about the effects of, e.g., charge carriers or strain or compositional interfaces upon NMR parameters such as chemical and Knight shifts and EFGs in reasonably well-defined systems. 

We use the same approach to classify the different nanostructures for Titania. The term one-dimensional (ID) nanostructures indicate nanocrystals in which elongation only in one direction is above this threshold (about 10 nm). This class of ID nanostructures comprises different types of nano-ordered materials, such as nanorods, -wires, -coils, -fibers, -pillars (or -columns) and -tubes. We prefer to use the term quasi one-dimensional nanostructures, because often the dimensions are larger than the indicated threshold, although elongation along one main axis still exists. When the diameter of the nanorod, nanowire or nanotube becomes smaller, there is often a significant change in the properties with respect to crystalline solids or even two-dimensional systems. A bismuth nanowire is an excellent example, which transforms into a semiconductor, as the wire diameter becomes smaller.145... 

K. Vinodgopal, I. Bedja, P.V. Kamat, Nanostructured semiconductor films for photocatalysis. Photoelectrochemical behavior of Sn02/Ti02 composite systems and its role in photocatalytic degradation of a textile azo dye, Chem. Mater. 8 (1996) 2180-2187. 

ZnS-CdS (bandgap = 2.3-2.4 eV) composite semiconductor photoelectrodes show a broad spectral response and n-type behavior, with saturation of the anodic photocurrent upon increasing anodic potential making the system suitable for use as a photoelectrochemical cell photoanode [72], Nanostructured ZnS-CdS thin film electrodes show that anodic photocurrent saturation can be attained with the application of a small, 0.1 V, bias [73], while hydrogen evolution is observed at the Pt cathode. The performance of the ZnS-CdS photoanodes appear strongly dependent upon the method of film preparation [72,73], with Zn rich films demonstrating superior photocurrent generation, and stability, in comparison to Cd rich films. 

Unfortunately, the author has not come so far across any publication on concerning inorganic semiconductor surfaces (2D) or linear ID systems. The problem of correct measurement of local densities or distances between PCs in nanostructured low-dimensional systems is even more complicated. Indeed, using modem EPR technique, one can measure 1/T2 values up to 5-6 nm [124]. But it is the very size of colloidal and aggregated nanoparticles Is it possible to use the pure 2D model in this case, or is it necessary to take into consideration an input of 3D interaction Our group is working on this problem now, trying to understand where is a border between 3D and 2D cases in terms of quantitative analysis of dipole-dipole interaction. 

We have studied photocatalytic properties of nanostructured metal-semiconductor composites, made from mesoporous samples of Ti02 and a number of metals (Cu2+, Ni2+, Co2+, Cd2+, Fe2+, Ag+, Zn2+, Pb2+) in hydrogen evolution from water-ethanol solutions. Correlations between the quantum yields of the photoreaction and various parameters of the reacting system (such as the metal nature and concentration, photocatalyst quantity, light intensity, temperature) have been found and discussed. It has been shown, that maximal quantum yield of hydrogen production (y = 0.44) could be achieved in case of Ti02/Cu composite. 

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