Film Formation of Cadmium Selenide Quantum Dots (CdSe QD) by Dip-Coating

CONTENTS

AIM OF THE WORK

CHAPTER 1
INTRODUCTION
1.1 Quantum Dot
1.2 Quantum Dots Synthesis
1.3 Dip Coating
1.4 Application
1.4.1 Tissue Regeneration
1.4.2 QD Solar Cells

CHAPTER 2
EXPERIMENTAL PART
2.1 Materials
2.2 Preparation Methods
2.2.1 Synthesis Methods for CdSe qd
2.3 Coating Procedure
2.4 Characterization Techniques
2.4.1 Fluorescence Microscopy
2.4.2 Atomic Force Microscopy (AFM)

CHAPTER 3

RESULTS AND DISCUSSION

CHAPTER 4

SUMMARY AND CONCLUSIONS

ACKNOWLEDGEMENTS

BIBLIOGRAPHY

ABSTRACT

In this work is presented the investigation on the morphology cadmium selenide quantum dots (CdSe QD) crystals, growth on glass substrates by dip coating. Two different samples have used for the film formation: one with CdSe QD and the second one with CdSe QD covered with the tripeptide L-glutathione. Both samples were synthesized using the technique of water-in-oil microemulsion, and the Dioctyl sulfosuccinate sodium salt (AOT) was used as the surfactant. The film obtained by the first sample (CdSe QD without stabilizing agent) shows the better properties: the pattern of the film is more regular and the striations are well defined with a square shape; moreover it exhibits a powerful fluorescence emission.

Furthermore, the dip-coating was carried out at different speed to verify if the film formation follows the theoretical prediction. The height of each film obtained at a different rate (using the first sample) was measured, and the results were compared with the theory using the Landau-Levich equation. It was verified a full correspondence between the experimental data and theoretical.

AIM OF THE WORK

This thesis aims to study the morphology of CdSe quantum dots crystals growth on substrates of glass by dip coating.

Mainly, CdSe nanocrystal will be prepared in inverse micelles using the surfactant AOT. The synthesis will be carried out with the technique of water-in-oil microemulsion. Two different samples will be prepared: one with only QDs in inverse micelles and the second one with QDs covered by the tripeptide L-glutathione. The purpose is to explore the difference in the film formation between the two different samples and then determine which film shows better properties.

The film will be obtained by dip coating and, to study if the film formation follows the theoretical prediction, the dip-coating will be carried out at different speed, and the results compared with the theory by the Landau-Levich equation.

AFM measures and fluorescence microscopy images will be employed to obtain information on the morphology and dimensions.

CHAPTER 1

INTRODUCTION

The attention on the nanoscale materials has been increasingly growing in recent years since new properties are acquired at this length scale, and those properties are related to their size or shape. The reasons for these unique properties are not a result of scaling factors but are related to different phenomena depending on the type of materials.

In many inorganic semiconductors, that are materials used for this work, the unique properties result from the further confinement of the electronic motion to a length scale that is comparable to, or smaller than, the length scale characterizing the electronic motion in bulk semiconducting material (called the electron Bohr radius, which is usually a few nanometers).

In recent years a lot of research groups focus them work on study and characterization of different kind of nanocrystals. Notably, a specific class of nanocrystals, named Quantum Dots, get the attention of researchers due to their unique properties.

1.1 Quantum Dot

When one of the dimensions of the semiconductor crystalline material becomes comparable with the size of the so-called exciton Bohr radius, there are the phenomena of quantum confinement, leading to the formation of discrete energy levels. The Bohr radius refers to the Bohr model that permits to calculate the radius of the electron orbit around the nucleus by the equation:

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The Coulomb force acting on the electron and the orbital angular momentum of the electron (to which have been applied the quantization condition) is considered in such model. The exciton is referred to the electrons and hole existing in a material which are subject to Coulomb interaction, and the optical nature of semiconductors can be understood by investigating the properties of the excitons. The distance between the electron and the hole within an exciton is called Bohr radius of the exciton, which typically in semiconductors is of few nanometers.

The electron motion (charge carrier) confined within the nanostructure can be described, in the first approximation, similar to the movement of a particle in the box through the resolution of the Schroedinger equation to it associated:

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This equation is a time-independent bidimensional equation where is the Planck´s constant , m is the mass of the particle, is the wave function, x and y the dimension of “box” and E the total energy of the particle.

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Figure 1. Schematic description of the confinement of a particle in a bidimensional box.¹

For a nanocrystal, this approach allows to obtain a discretization of energy levels in the vicinity of the bands, that is a distinctive feature of the nanostructures compared to the continuous energy levels associated with the corresponding bulk solid. This phenomenon is commonly referred to as Quantum Confinement ^{1 2} that describes how the electronic properties, i.e. the organization of energy levels into which electrons can climb or fall, and optical properties change when the material size is less than 10 nm. The changing in such properties results from electrons and holes being squeezed into a dimension that approaches a critical quantum measurement, called the exciton Bohr radius. The hole is the positive charge species left over when an electron vacates its position in a crystal.

The density of the quantum states can be derived as a function depending on the dimensionality of the potential well (1, 2 or 3-D).

There are three different kinds of nanostructures by the dimensionality, i.e. the number of dimensions in the order of nanometers in which the charge carriers are confined (Figure 2):

- 2D: one of the three dimensions is nanometer; therefore the electrons are free to move only in two dimensions (quantum well);
- 1D: two of the three dimensions are nanometer; consequently the electrons are free to move only in one dimension (quantum wire);
- 0D: all three dimensions are nanometer, therefore the electrons are confined within a defined space, and they can assume only quantized values of energy (quantum dots).

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Figure 2. Quantum confinement in three dimensions.³

In the zero-dimensional system (quantum dot), where the quantum confinement exists in three dimensions, the distribution of states N(E) is a function very similar to that of an atomic system. The smaller number of allowed quantum states in a quantum dot compared to a quantum well or a quantum wire allows the more excellent tunability of the states accessible to electrons, and consequently the electromagnetic properties.

The quantum dots are the smallest nanostructures, and their dimensions vary between 2 nm and 10 nm in diameter, which corresponds to several atoms between 500 and 15,000. In these nanocrystals, the quantum confinement has a strong effect because their radius is comparable with the excitonic diameter, where the electron cannot move freely. It is precisely these reduced dimensions that make the quantum dots a class of semiconductor unique in their optical properties and electrical conductivity.

In the nanocrystals, the energetic state of the charge carriers is quantized due to their small size, which is of the same order of magnitude as the De Broglie wavelength of an electron and a hole at room temperature. In spherical nanocrystals, which are the quantum dots, electrons and holes are confined in all three dimensions, so the movement of charge carriers is entirely governed by quantum mechanics resulting in discrete electronic levels, whose density is a function inversely proportional to the square of the size of nanocrystal. Accordingly, through the dimensions of the quantum dot, it is possible to tune the energy associated to the recombination of one electron in the conduction band with one hole in the valence band and thus gain the emission of photons at the desired wavelength.

Moreover, the spherical shape of the nanostructures allows to have a single plasmonic band and therefore a unique characteristic band of absorbance. The plasmonic band, or better the phenomena of surface plasmon absorption, is another consequence of nanocrystal size. The semiconductor nanoparticles are smaller than the wavelength in UV-Vis (200-800 nm). The electric field associated to the electromagnetic radiation lead a polarization in the free electron of semiconductor (electron in conduction band) which move from their equilibrium position with longer distances than the positively charged nuclei (that are much heavier). This move generates a Coulomb force attraction which tends to return the electrons in the equilibrium position. Such force creates an oscillation on the charge density of the nanoparticle that is an oscillating electric dipole with a characteristic period T. The absorption of EMR and the generation of this oscillation is described by Mie theory.⁴

The peculiarity of these nanostructures is their capacity to absorb the wavelengths along the entire spectrum of UV-Vis, emitting then at specific λ, as shown in Figure 3. Cadmium selenide quantum dots (CdSe QDs) present a further peculiar characteristic that makes them particularly interesting. These QDs possess a “big” dimension of the Bohr radius of the exciton (~5 nm) compared to other materials, allowing to tune the band-gap energy with the size in a wide range.

In this work, we have used Quantum Dots of semiconductors, in the specific cadmium selenide quantum dots (CdSe QDs) which represent a nanometer inclusion of one semiconductor within another.

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Figure 3. Colours corresponding to different emission of the nanocrystals: A) the nanocrystals absorb and emit light at different wavelengths therefore with different colours, according to their size, B) different solutions of nanocrystals illuminated with UV lamp at the same wavelength.⁵

This peculiarity in the regulation of band-gap energy is reflected in the emission when these nanocrystals absorb a photon. The energy associated with the luminescence is strongly determined by the energy of band-gap that can be easily seen in the emission spectra of CdSe nanocrystals recorded at different growth times but at the same wavelength of excitation. Observing the spectra in Figure 4 can be seen clearly as with the advance of the growth process of the nanocrystals, the emission bands shift to longer wavelengths.

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Figure 4. Absorption and fluorescence spectra of CdSe nanocrystals that show the variation of the energy band-gap and of the fluorescence depending on the size.⁶

1.2 Quantum Dots Synthesis

In the literature are described several methods for the preparation of nanocrystals. In this work has been used as a solution of colloidal quantum dots. In general, the synthesis of colloidal solution of nanocrystals starts from precursor compounds dissolved in solution, as often happens in traditional chemical processes.

For the quantum dots, the synthesis is based on a system with three components: precursor, organic surfactants and solvents.

The solution is heated at a sufficiently high temperature to facilitate the solubilization of the precursors and promoting the transformation in the monomers. Once the precursors have reached a reasonably high level of concentration, the nanocrystals begin to be formed through a process of nucleation. Once created, these nuclei grow thermodynamically stable.

In summary, the process of nanoparticles formation can be divided into two stages: nucleation and growth, the first endergonic and the second exergonic.

In general, terms, when Quantum dots are synthesized (for example the CdSe one) the process, includes a chemical reaction between a metal ion, such as cadmium, and a species that is capable of donating an ion such as selenium. These reactions create crystals of cadmium selenide.

At this point, it is necessary to avoid that the small crystals bind each other while they grow to the desired size.

To “isolate” the particles from each other to prevent aggregation, the reaction is carried out in the presence of organic molecules, called stabilizers which coat the surface of each crystal of cadmium selenide while it is growing. These stabilizers interact closely with the cluster surface in a way that the growth of nanocrystals is kinetically interrupted by the chemical and physical capping of these agents on the crystal surface. Stabilizers commonly used include reverse micelles, polymers, glass, sol-gel, film di Langmuir-Blodgett.

Regarding the samples used for this work, the microemulsion method has been used for the synthesis, because, in contrast to the most common processes of synthesis, this process allows to avoid the use of extreme conditions. The microemulsion is a simple and efficient method consisting of a homogenous dispersion, optically transparent and isotropic, of two immiscible fluids formed by nanoscale domains of a liquid within the other liquid, mediated by a particular class of molecules know by the name of surfactants. The microemulsion can be described as a reduced-scale version of emulsion, in which form aggregates of a pseudo spherical structure consisting of microdroplets of water in oil, with diameters that reach to the maximum 100 nm.

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Figure 5. Different positioning of the surfactant in direct and inverse micelles.

Depending on whether the dispersing phase is water or an organic solvent, the microemulsion is classified as o/w (oil-in-water), or w/o (water-in-oil), as shown in the schematic representation below (Figure 5).

The water-in-oil microemulsion, (the one used in this work), are formed by reverse micelles: spheroidal aggregates formed by surfactant in an apolar medium. In this kind of microemulsion, the polar heads of surfactants molecules are oriented towards the interior of aggregate forming a pole core which can solubilize the water (water pool). At the same time, the apolar alkyl tails are exposed to the organic solvent. Because of the limited size of the micellar aggregates, the microemulsion is thermodynamically stable systems. This stability is due to the low value of the interfacial tension between water and oil microdomains.

Two factors have to been taken into consideration for the preparation of a microemulsion:

1) The critical micelle concentration (CMC), i.e. the minimum amount of a surfactant to the above which molecules of surfactants no longer able to disposed at the interface between the two immiscible liquids and forming the micelles;
2) The radius of equilibrium Re, whose value depends on the ratio between the overall concentration of surfactant and the total concentration of dispersed phase: in the case of reverse micelles this relationship is denoted by w.

In microemulsion, it is possible to obtain monodisperse particle with nanoscopic size, whether they are constituted by semiconductors, metals or oxides, using this system as real microreactors. In general, the monodisperse particles prepared in microemulsion have the characteristic of possessing tiny average size, a narrow distribution and high stability. The reagents are dissolved in the water pool of the reverse micelles. They can react through the micelle communication during the dynamic processes of a collision, which leads to nucleation and growth of QDs within the micelles.

Ideally, it is realized a separation between the nucleation process and the growth process, which allows obtaining a tiny particle. Initially, in the water pools, the process of nucleation starts. Given the small size of the microreactor, the formation of the nucleus is restricted to the availability of the monomer and, once the monomer is absorbed, it cannot take part in any further nucleation process; hence the growth can proceed only for the exchange of monomers.

This synthesis technique offers several advantages:

- particles sufficiently small and monodisperse can be obtained;
- the particle size can be modulated by varying the experimental parameters of the microemulsion.

Moreover, it is an easy technique to apply because it does not require particularly harsh conditions of pressure and temperature, and that allows us to realize an organic or inorganic capping of the nanocrystalline semiconductors.

The nanoparticles dispersion used in this work was carried out by mixing two identical solutions of the two reagents in the microemulsion. The chemical reaction between the reactants involves the reagents exchange within the micelles through the coalescence between two microdroplets.

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Figure 6. Dioctyl sulfosuccinate sodium salt (AOT) chemical structure.⁷

For the microemulsion used in this work, Dioctyl sulfosuccinate sodium salt (AOT) was chosen as the anionic surfactant, the chemical structure is shown below (Figure 6)

The choice is motivated by the surfactant ability to form the microemulsion in an extensive range of reagents concentrations and consisting of well-defined micelles.

Unfortunately, the microemulsion is not a stable system in time, and the compartmentalization of the reaction is not definitive because, over time, the micelles tend to fuse, and the process of growth does not cease. For this reason, it was added a stabilizing agent to prevent the growth and to stop the process when the nanocrystals arise the desired size.

The stabilizer agent used for this purpose it was the tripeptide L-glutathione (GSH). The choice of this molecule was based on studies carried out in previous work.

The advantage of using this synthesis process is the possibility to obtain Quantum Dots with even size distribution and without defect on the surface that show an intense fluorescence emission. It is possible to verify these QDs properties observing the absorbance and emission spectra of these nanocrystals (Figure 7).

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Figure 7. Absorption and Emission spectra of CdSe Quantum Dots in microemulsion recorded at different time from synthesis.⁸

In the absorbance spectra, the band-gap (corresponding to the shoulder of absorption bands) changes moving to higher , but the bands are well narrow even after 11 days from synthesis. The fluorescence spectra show that the maximum of fluorescence emission moves to longer wavelengths (accordingly with the behaviour shown on absorbance spectra) and the intensity of emission decrease with passing of time. Still, the bands become more well defined than the initial ones. These results confirm that the structural and emission properties of nanocrystals create with this process of synthesis are comparable with nanocrystal obtained by different synthetic techniques, but also proves that this method does not stop the growth of QDs. For this reason, the process was adjusted, adding L-glutathione to the solution to stop the growth process.

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Figure 8. Absorption and Emission spectra of CdSe Quantum Dots in microemulsion stabilized with L-glutathione recorded at a different time from synthesis.⁸

The absorbance and emission spectra of QDs stabilized with L-glutathione are shown in Figure 8.

In the absorbance spectra, with time, the band-gap of absorption bands does not move to higher wavelengths, and the bands remain well defined. Also in the fluorescence spectra, the maximum of emission does not move to longer wavelengths, moreover, with time the emission bands become more narrow and the intensity higher than the initial one.

Finally, it is essential to describe the crystal structure of CdSe Quantum Dot.

This kind of nanocrystals typically has two different types of crystal structure:

- Wurtzite (hexagonal)⁹

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- Sphalerite (cubic)⁹

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When they are synthesized in the microemulsion, due to the shape of micelles, they have a nearly spherical shape, as shown by AFM images (Figure 9A) and TEM images (Figure 9B).

In summary, colloidal semiconductors nanocrystals Quantum Dots have been extensively studied to take advantage of their unique properties such as zero-dimensional discrete electronic energy feature, tunable and bright emission, high photostability, and solution processability.

They are promising for many potential applications, including light-emitting diodes (LEDs), lasers, biological fluorescent probes and solar cells.

For these reasons, we chose to study these structures and use them for forming films on the substrate of glass via dip-coating technique (to exploit such properties in films).

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Figure 9. Images of CdSe quantum dots synthesized using microemulsion technique: A) AFM image; B) TEM images (scale bar 100 nm).⁸

1.3 Dip Coating

The CdSe QD solution previously described was used to prepare films on glass substrates by dip-coating. The formation of thin films usually involves the use of sol-gel techniques.

The term sol-gel is referred to a particular class of solutions, where:

- Sol is a stable dispersion of colloidal particles or polymers in a solvent.

The particles may be amorphous or crystalline. The colloidal particles are not dissolved but do not agglomerate or sediment. Term colloid means particles with tiny dimensions and an average diameter between 1 to 100 nm that is not affected by gravitational forces but tend to move in the liquid phase by Brownian type motion.

- A gel consists of a three-dimensional continuous network, which encloses a liquid phase. In a colloidal gel, the network is built from the agglomeration of colloidal particles. In a polymer gel, the particles have a polymeric sub-structures made by aggregates of sub-colloidal particles. Generally, the Sol particles may interact by Van der Waals forces or hydrogen bonds. A gel may also be formed from linking polymer chains. In most gel system used for materials synthesis, the interactions are covalent, and the gel process is irreversible. The gelation process may be reversible if other interactions are involved.

The idea behind sol-gel synthesis is to dissolve the compound in a liquid to bring it back as a solid in a controlled manner.

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Figure 10. Schematic description of Sol-Gel process.¹⁰

The sol-gel synthesis allows preparing the material with a lot of different shapes, such as porous structures, thin fibers, dense powders and thin films (Figure 10).

The sol-gel processing is technologically important because, before gelation, the fluid sol or solution is ideal for preparing thin films by such standard processes as dipping, spinning or spraying. Compared to the conventional thin films processes (CVD, evaporation or sputtering) sol-gel film formation requires considerably less equipment and is potentially less expensive. However, the most crucial advantage of sol-gel processing is the ability to control the microstructure of the deposited film precisely.

Sol-gel dip-coating consists of the withdrawal of a substrate from a fluid sol: gravitational draining and solvent evaporation, accompanied by further condensation reactions, resulting in the deposition of a solid film.

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