One of the leading directions of modern applied physics evolution is the creation of new nanomaterials and nanomaterials-based composites, as well as improving the properties of existing materials by modifying them. Among wide variety of studies, much attention is paid to the study of light-emitting properties of nanoobjects and nanocomposites, as well as to the relationship between their structural and luminescent properties. This interest is caused by the prospect to better control the characteristics of nanomaterials-based phosphors as compared with traditional phosphors. Since the significant progress has been made in recent decades in the field of light emitting technology in the visible light range, the focus of researchers' interests has shifted to the search for the phosphors in the ultraviolet and shortwave visible regions of the spectrum, which requires the use of wide-bandgap materials.
Nanomaterials based on metal oxides are often used to solve the problem light emission in the short-wave spectral region. In particular, one of the most promising are wide-band semiconductors zinc oxide (ZnO) and titanium dioxide in the crystalline phase of anatase (TiO2) with a band gap of 3.3 and 3.2 eV, respectively [1, 2]. Based on these materials, it is possible to obtain light-emitting devices with very different characteristics. Many different methods have been developed to achieve the desired light-emitting properties. In particular, the characteristics of these objects can be changed and controlled by modifying their surface, as well as incorporation into dielectric and conductive matrices. The role of these two factors in varying the properties of materials requires detailed study.
Another light-emitting material for use as short-wave phosphors, are carbon-based nanostructures. The advantage of these nanostructures as emitters is that the spectral characteristics of these objects can be controlled in a wide range by selecting the synthesis conditions. To date, many different carbon nanostructures have been synthesized, such as amorphous carbon nanoparticles, nanoclusters, graphene quantum dots, and others (such structures are referred to in the literature as "C-dots"), and the luminescent properties of these objects have been studied. However, at the beginning of this dissertation research, the question of explanation the evolution of the light-emitting properties of carbon objects remained open. Structurally close to C- dots are the nanostructures of boron nitride, BN. This wide-bandgap artificial material was first synthesized in the form of crystals, and now several modifications of nano-BN are available. The band gap of bulk boron nitride is ~ 5 eV, which makes BN a probable candidate to obtain radiation in the ultraviolet region (C-UV range). At the same time, due to the presence of defects, BN can also emit light in the near UV (A-UV range) and visible regions of the spectrum.
The aim of the dissertation is to establish the mechanisms of photoluminescence excitation and light emission processes in nanoparticles of wide-bandgap materials and composites based on them, as well as to identify the features of nanoparticle-matrix interaction.
The following scientific results were obtained during the dissertation research:
1. Photoluminescence of surface states in titanium dioxide nanoparticles as well as phenothiazine-modified nanoparticles was detected. This emission is excitation -dependent and is observed only at the excitation quanta energies exceeding Eg/ It has been shown that in TiO2 modified with phenothiazine, the excitation conditions of this emission are improved due to the filling of trap states with phenothiazine electrons.
2. The mechanism of photoluminescence excitation of ZnO nanoparticles in ZnO / ПВП nanocomposite via energy transfer from the matrix to nanoparticles is proposed. The corresponding energy scheme of transitions involved in the processes of photoluminescence excitation and light emitting of ZnO / ПВП nanocomposites is proposed.
3. The spontaneous formation of C-dots during long-term “aging” of ethanol solutions Zn(acac)2 is revealed.
4. It is shown that the photoluminescence of all types of C-dots studied (both spontaneously formed and obtained by annealing of chemically treated silica and synthesized by colloidal chemistry method in the matrix of porous SiO2) can be described by a unified scheme of C-dots radiative properties evolution.
5. In the photoluminescence spectrum of boron nitride powder synthesized by the method of carbothermal reduction from boron oxide, a long-wave wing was detected, which is due to the presence of C-dots in the material.
The practical significance of the obtained results is that a non-toxic solid-state phosphor with controlled spectral characteristics based on ZnO/polyvinylpyrrolidone nanocomposite was proposed. The dissertation also presents a model of the device based on C-dots synthesized directly in the matrix of porous silica.
