Nanomaterial evolution and nanoimprint technique

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Development of nanostructured materials

For nanostructured materials, especially nanocrystals, we now provide a brief overview of the main studies regarding them carried out until now. Thus the section is organized from nanostructures existing in nature, artificial nanostructured materials and the new generation. The main focus in our study is on nanocrystal materials, because of early developments which have lead to a lot of useful applications. Furthermore the new generation nanocrystals of a core-shell type are also considered and compared with single core nanocrystals.
When changing over from bulk to nanocrystal semiconductors both the electronic and optical properties are affected. In Figure 2.1.1, the density of states is varied from a bulk to a 0D (dot) semiconductor with a change from continuous to discrete energy states induced by confinement effects [3]. The 1D and 0D structures are analogous to quantum wires and quantum dots [2, 3].
Figure 2.1.1: The “Density of States” (DOS) function evolves from a bulk (3D solid) to a 2D, 1D and 0D system with varying degrees of confinement. [2, 3].

Nanostructures in nature:

Nanotechnology has been quickly developing during these last few decades. However, we are already actually surrounded by many amazing nano-materials in nature. The most typical one is the “lotus” noted for its water-repellent and self-cleaning surfaces. Figures 2.1.2 (a) and (b) who a lotus leaf surface covered by micrometer-sized papillae decorated with nanometer branch-like protrusions [10]. The roughness of papillae reduces the area of contact between the surface and a liquid drop. This ideal structure has inspired scientists to “imitate” it for fabricating artificial self-cleaning surfaces as in Figure 2.1.2 (d) and (e) [10].
Figure 2.1.2: (a) SEM image of the surface of a lotus leaf and (b) a higher magnification with hierarchical structures clearly resolved. (c) A water drop on the surface of the lotus leaf attains a nearly spherical shape. (d) SEM image of the artificial laser- structured silanized silicon surface and (e) a higher magnification showing the dual length-scale roughness. (f) A water drop on the structured surface. [10] It is however not only plants like the lotus, but also many insects and animals have their own nanostructure, for example beetles, sea mouse and butterflies [11]. Due to genetic evolution, their structures which are built up in the nano-sized range are naturally created by their molecular precursors. Another example is our own body, comprised of a systematic composition of many nanostructured materials such as the structure of DNA, a nanostructure existing within the nucleus of a cell which stores information on evolution.
As shown in Figure 2.1.3 (a) and (b), the natural structure of butterfly wings is quite interesting. Most of the butterflies of the Papilio species present a common bulk and surface structure. The upper membrane is constituted of a multilayered air/chitin film [12]. An important property of the super hydrophobic surface of butterfly wings is found with a contact angle of about  [12] which is close to that of the lotus. In addition, the natural structure of160 butterfly wings can be replicated as can be seen in Figure 2.1.3 (c) and (d).

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].

General synthesis and processing of monodispersed nanocrystals [15]:

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.
Figure 2.1.5 shows high resolution TEM observations of the size and shape of CdSe NCs. This image indicates a nearly spherical shape with a size of around 3.9 nm that was clearly produced. [15, 17].
Figure 2.1.4: (A) Depiction of the stages of nucleation and growth for the preparation of monodispersed NCs in the framework of the La Mer model. As NCs grow with time, a size series of NCs may be isolated by periodically removing aliquots from the reaction vessel. (B) Representation of the simple synthetic apparatus employed in the preparation of monodisperse NC samples. [15, 16].
Figure 2.1.5: Collection of high resolution TEM images for typical NC materials such as (A) <100>-oriented CdSe (scale bar =15 Å), (B) <001>-oriented CdSe (scale bar =15 Å). [15, 17 ].

General energy bandgap blue shift with size:

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.
Figure 2.1.6: (a) Size tuning of CdSe nanocrystal absorption spectra spanning the visible spectral region. (b) Size tuning of PbS nanocrystal absorption spectra spanning the near-infrared (NIR) spectral region. (c) Transmission electron microscope (TEM) images of different PbS nanocrystal samples with average sizes ranging from 2.3 to 5.5 nm. [18, 19].
A red-shift can be observed between the absorption and emission of QDs. Some possible explanations are illustrated in Figure 2.1.8 [22]. This figure reveals different possibilities linked to the electronic energy transition due to the splitting of the discrete energy levels caused by quantum confinement in the semiconductor nanoparticles [3].
Figure 2.1.7: Colloidal suspensions of CdSe quantum dots of increasing size from left (approximately 1.8 nm diameter) to right (approximately 4.0 nm in diameter). Bottom: Samples viewed in ambient light vary in color from green–yellow to orange–red. Top: The same samples viewed under long-wave ultraviolet illumination vary in color from blue to yellow. [22]
Figure 2.1.8: Relationship between absorption and emission energies for discrete molecules (left) and for semiconductor nanoparticles (right). When a photon is absorbed thereby exciting an electron from the ground state (1) to an excited state (2) the bond order decreases and the atoms relax to a longer internuclear distance (3) before the emission of a photon (4) and relaxation to the ground state (1). In semicond uctor nanoparticles the process is the same (steps 1–4) but there are more possibilities for the energy of the electron under excitation from a photon to match an electronic energy transition. [22].

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.
Figure 2.1.9: The band gaps and their relative alignment for selected semiconductor materials with the band offsets. The numbers in the bars indicate the width of the bulk band gap in electron volt. [26].
Figure 2.1.10: (a) Fluorescence spectra of one type of CdSe nanocrystals overcoated with shells of ZnS of different thicknesses. The CdSe cores have a diameter of 4.2 10% nm and the shells have a thickness of (a) 0, (b) 0.65, (c) 1.3, (d) 2.6, and (e) 5.3 monolayers of ZnS. In addition to the variations of the quantum yield (see inset), one can also observe a red shift and a broadening of the spectra with an increasing shell thickness. (b) Radial probability density of the electron and the hole for the lowest energy states for CdSe cores and CdSe/ZnS core–shells. Due to the lower step in the potential at the s urface of the CdSe section in the case core–shell nanocrystals, the probability density in the ZnS section is increased when compared to the respective volume in the case of the bare CdSe cores. This leads to an efficient increase of the confinement volume and thus to a slight red shift of the optical properties. [27, 28].
In Figure 2.1.10, the area under the wave function of both the electron and the hole in the matrix surrounding the nanocrystal is decreased in the case of the core–shell nanocrystals, and thus the probability of finding excitons in this area is also diminished. Therefore, the probability of an interaction between the excitons and the host matrix is reduced as well [28].

Type I and type II semiconductor core-shell quantum dots:

The core-shell Type 1 QDs are described. The band edges of the core material are localized in the band gap of the shell material. CdSe/ZnS is a typical case for Type I. Moreover, in 2003, Bawendi and co-workers proposed a new type of core-shell semiconductor heterostructure for which the conduction and valence bands of the core and shell material are staggered, resulting in the segregation of electrons and holes between the core and shell materials [29].

Table of contents :

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 optimized 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

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