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Artificially nanostructured materials: semiconductor quantum dots
At the beginning of the 80s, a novel size effect induced by nanocrystals of very small size was discovered by A.L. Ekimov et al. [13]. In this paper, the exciton absorption spectrum of CuCl crystals grown in a dielectric matrix has been studied. The crystal size could be controlled from several tens of angstroms to hundreds of angstroms. With size variations, an absorption shift (of up to 0.1 eV) could be attributed to a quantum size effect [13]. In 1988, M.A. Reed et al. investigated the electronic transport through a three-dimensionally confined semiconductor quantum structure [14]. They showed that the fine structure observed in resonant tunneling through the dot corresponds to a discrete density of states of a zero-dimensional system. This zero-dimensional system was also named for the first time as « quantum dots » [14].
In the general synthesis of monodispersed nanocrystals, classic studies by La Mer et al. have shown that the production of monodisperse colloids requires a temporally discrete nucleation event followed by slower controlled growth on the existing nuclei as shown in Figure 2.1.4 [16]. It is necessary to raise the precursor concentration above the nucleation threshold via rapid addition of reagents to the reaction vessel. The result of a short nucleation burst can partially relieve the super-saturation. If the consumption of feedstock by the growing colloidal NCs is not exceeded by the rate of precursor addition to the solution, no new nuclei is formed. Because the growth of any type of NC is similar to all others, the initial size distribution is mainly determined by the time over which the nuclei are formed and begin to grow. Monodispersed NCs with diameters with standard deviations of σ ≤ 5% can be made in this way. Many types of semiconductor NCs (or QDs) have been developed during the few decades including CdSe, CdS, CdTe and PbS compounds [18, 19]. Their energy bandgaps are very different, making possibly numerous applications.
One of the most attractive characteristics of these semiconductor QDs is the size-dependent band gap which was caused by quantum confinement effect [3] which can be theoretically understood by solving the Schrödinger equation, for example, a simple case of electrons in 1-dimension [20]. Via this confinement effect, the essential bulk energy bandgap can be significantly shifted to a higher photon energy (blue-shift phenomenon) with its decreasing size [21]. As shown in Figure 2.1.6, both CdSe and PbS nanocrystals revealed a blue-shifted absorption spectra with reduced size. The same shift is also obtained on the luminescent spectrum, as shown in Figure 2.1.7.
New generation of semiconductor quantum dots:
One of the primary aims in modern materials is the design of new nanomaterials that exhibit novel properties to be utilized in various applications. With similar aims, colloidal techniques have the potential for developing elegant extensions of the synthetic NCs. These novel NCs involve the fabrication of hybrid NCs in which domains made from different materials can be assembled together in a single nano-object. These nanostructures would merge the properties of the individual materials, with new properties being likely to arise from their combination. In the synthesis of NCs made of more than one material, the formation of a large interface between the two materials is also frequently observed when the lattice constants of the two components do not differ significantly from each other [23].
Semiconductor core and core-shell quantum dots:
Generally the semiconductor quantum dots exhibit high luminescence emissions. This high efficiency of light emission is largely due to the strong overlap between the electron and hole wave functions in the confined quantum structure, whereas the exciton in bulk semiconductors is not confined in space and can rapidly dissociate, increasing the probability of nonradiative relaxation events associated with crystalline defects and charge carrier traps on the crystal surfaces [24]. However, there still exists a major source of nonradiative decay from the surface trap states, represented by unsaturated bonds on the surface [25]. Thus, it is important to have another type, namely core–shell nanocrystals, for passivating and further enhancing the luminescence from the core particles. Normally the CdSe core with a low bandgap can be covered by a wider bandgap material of ZnS as can be seen in Figure 2.1.9.
Dielectric material contains nanocrystals:
In many types of nanocrystals/dielectric nanocomposite layers, the silica-based nanocomposites are considered one of the potential systems for quantum confinement in semiconductive nanocrystallites in order to have better control of their shape and properties [33]. These embedded compositions can be considered as metal or semiconductor nanocrystals. Metal nanocrystals/dielectric nanocomposites: their optical properties have attracted numerous studies because of the unique electronic properties and surface-enhanced spectroscopic applications compared to the bulk material [34, 35]. Many studies have been reported the effect of reducing metal particle size on the optical process in these films. When such a metal–dielectric nanocomposite is excited by light, photons are coupled to the metal–dielectric interface, causing an induced charged density oscillation and showing a strong absorption maximum, called the surface plasmon resonance [36]. The resonance frequency is influenced by several factors, e.g., the size and shape of the nanoparticles and also their surrounding environment [37]. In [36], silver particles embedded in the silica nanocomposite thin films were prepared by high pressure DC sputtering technique. These increase in substrate temperature easily causing a variation in the size and shape of the nanoparticles. Films containing a distribution of particle sizes and shapes lead to an inhomogeneously broadened and asymmetrical surface plasmon resonance in the optical absorption spectra. They found that the size and shape distribution of metal nanoparticles has a profound effect on the optical absorbance spectra.
Semiconductor nanocrystals/dielectric nanocomposites: the wide bandgap material ZnO has attracted much research attention due to its desirable physical properties. ZnO has a large exciton binding energy (around 60 meV) at room temperature [38] which can provide efficient excitonic emission at room temperature. Because ZnO emits a broad luminescence emission in the green–yellow region, this property renders it as a potential material for blue- or white-light emitting devices [39]. There are many methods for making an emitting ZnO/SiO2 nanocomposite thin film, such as the sol–gel [38], molecular capping [40] and also the sputtering method which can be employed [39]. Figure 2.1.1 clearly shows crystallized ZnO NCs embedded in a silica matrix [33].
Organic polymer containing inorganic nanocrystals:
Organic polymer/inorganic hybrid materials are promising candidates that have attracted considerable attention due to their variable physical and chemical properties for materials including nanoparticles [41]. Incorporating inorganic nanoparticles into a polymer matrix is most straightforward and common method for fabricating such hybrid materials. There have been several publications on hybrid materials with their enhanced electrical, mechanical and optical properties. To obtain tunable electrical properties with nanocomposite material, it can be achieved through hybrid polymer/metal particles. In [41], silver is incorporated into polytetrafluoroethylene (PTFE). They found that near the percolation threshold an expected sharp change in the resistivity from 107 (polymer-like material) to 10-3 for a metal-like material, when increasing the silver content in the polymer. In this case, a significant change in resistivity of several orders of magnitude occurs with just a small percentage point change in the metal amount, as clearly shown in Figure 2.2.2.
Table of contents :
Abstract
Résumé
Acknowledgement
Contents
List of figures
List of tables
1. Introduction
2. Nanomaterial evolution and nanoimprint technique
2.1 Development of nanostructured materials
2.1.1 Nanostructures in nature
2.1.2 Artificially nanostructured materials: semiconductor quantum dots
2.1.3 New generation of semiconductor quantum dots
2.2 Development of nanocomposite thin films
2.2.1 Dielectric material contains nanocrystals
2.2.2 Organic polymer containing inorganic nanocrystals
2.3 Historic development of nanoimprint lithography
2.3.1 Thermal nanoimprint lithography
2.3.2 UV-curable nanoimprint lithography
2.3.3 New material developments
3. Experimental framework and principles
3.1 Experimental framework
3.2 Spin coating method – Hybrid thin films/ QDs
3.2.1 Rotational speed and time
3.2.2 Viscosity and concentration of the solution
3.3 Measurement methods
3.3.1 Spectrophotometer- transmission and reflection measurement
3.3.2 Photoluminescence measurement
3.3.3 Spectroscopic ellipsometry
3.3.4Transmission and scanning electron microscopy
3.3.4 (a) Transmission electron microscopy
3.3.4 (b) Scanning electron microscopy
3.3.5 Atomic force microscopy
4. Hybrid organic thin films/ QDs
4.1 PMMA thin film with CdSe/ZnS QDs
4.1.1 Introduction of experimental preparation and processes
4.1.2 Electron microscopy measurement
4.2 Optical properties of PMMA thin film layers with CdSe/ZnS QDs…
4.2.1 Transmission/ reflection and absorption spectrum
4.2.2 Photoluminescence versus wavelengths measurement
4.2.3 Photoluminescence intensity versus time measurement
4.3 Frequency conversion of CdSe/ZnS QDs: Its application to solar cells
4.3.1 Using PMMA /QDs films as down-conversion frequency layer
4.3.2 QDs embedded in the active layer of organic solar cells
5. Nanostructured hybrid organic thin films/ QDs
5.1 Introduction of thermal nanoimprint organic layers
5.1.1 Heating experiment for transition temperature of organic layers
5.1.2 Periodic structure of imprinted pure organic layers
5.2 Imprinted nanostructure in hybrid organic layer with quantum dots
5.2.1 Fabrication process of imprinted hybrid layers via optimizedm heating
5.2.2 Periodic structure of imprinted hybrid layers
5.3 Optical field emission simulation for the periodic nanostructure
5.3.1 Plane wave light emission in the nanostructure
5.3.1(a) Emission light simulation in near field
5.3.1(b) Emission light simulation in far field
5.3.2 Dipole source emission light in the nanostructure
5.3.2(a) Emission light simulation in near field
5.3.2(b) Emission light simulation in far field
6. Conclusions et perspectives
Bibliography
