ABSTRACT
Nanoparticles belong to the class of materials which is a focus of modern nanochemistry [1]. Nanoparticles may consist of organic (i.e., polymer) and inorganic (i.e., semiconductor, metal or metal oxide) materials. The development of nanochemistry in the direction of multifunctional materials gives rise to more and more sophisticated nanoparticles consisting of inorganic-inorganic, polymer-inorganic, etc., composites of various structures: core shell (also multiple shells), dumbbells, doped materials and alloys. One of the main nanochemistry approaches that allow one to obtain nanoparticles is their colloidal synthesis [2-5]. The main feature of the colloidal synthesis is the formation of nanoparticles by strictly controlled chemical precipitation in the presence of surfactants (also named ligands, capping agents, stabilizers). As it follows from the definition of a colloid, successful synthesis
should result in the formation of stable colloidal solution of nanoparticles. Besides the synthesis, post-preparative treatments of the nanoparticles (and their solutions) are very important chemical steps allowing transformation of “bare” nanoparticles into powerful, functional nano-building blocks demanded by emerging fields of nanophotonics and nanotechnology. Typical post-preparative treatments approaches include size-selective precipitation, chemical and photochemical etching, surfactant and solvent exchange, as well as mixing and forming hybrids or bulk composites with other molecules and substances.However, building blocks themselves, even the most desired and promising from the point of view of their magic functionalities, are useless if one could not properly handle them. Indeed, to fabricate nanoparticle in solution and to functionalize it are only half of the job. To make the existing particular nanoparticle useful, we have to localize it, put it in desired architecture (often with nanometre resolution!) and finally be able to address it. At present, the most successful examples of proper handling are based on so-named top-down nanotechnology approaches: the nanoparticles themselves, their superstructures and the nano-device architectures are fabricated by e-beam epitaxy, nanolithography, etc. The methods are precise but have their limitations: They are relatively expensive, yield relatively small amount of nano-objects and also applicable to a limited list of materials and substrates. Without negating the importance of top-down approaches, it is obvious that nanotechnology may only win from the development of alternative (e.g., bottom up) fabrication and assembly approaches. Indeed, the above-mentioned limitations seem not to be inherent to typical products of bottom-up synthesis, namely to colloidally synthesized nanoparticles. Indeed, 100 mL of typical colloidal solution of the type relevant to the present work contains an average of up to 1017 of the functional fluorescent semiconductor nanoparticles. The cost of such a synthesis is below €100. Consequently, the research fields devoted to efficient synthesis and handling of the colloidal nanoparticles have quickly emerged in the last few decades. The main tools allowing the manipulation of colloidal nanoparticles are assembly and self-assembly techniques.Nowadays assembly approaches are recognized as being the main working tool of bottom-up chemical nanotechnology. Colloidal semiconductor and metal nanocrystals (NCs) are used
to build up artificial molecules and solids [6-8]. The assembly of nanocrystals can be performed on surfaces of various geometries, i.e., flat, porous and spherical. Such assemblies may be very useful for thin-film technologies, doping of mesoporous materials, modification of pre-patterned substrates, creation of microshells and fabrication of microcavities [9]. Self-assembly approaches or the use of removable templates make possible the formation of nanowires [10,11], nanosheets [12] or nanoporous 3D ordered materials [13] created solely from the assembled nanoparticles. Hierarchical assembling and assembling of nanocrystals with other functional (organic or inorganic) entities opens up the possibility to achieve composites with literally unlimited functionalities. The understanding and governing of charge and energy transfer processes between the components of the composites are the key points in their efficient utilization as building blocks in novel types of LEDs, photovoltaic and photonic devices and various optical sensors [9]. In this chapter, the recent advances made in the assembling of the colloidal thiol-capped CdTe NCs for possible applications in nanophotonics and optoelectronics are discussed. 3.1 Building Blocks and Thiol-CappingAs has been already described in Chapter 2, light-emitting colloidal CdTe can successfully be synthesized in aqueous solution in the presence of short-chained thiols as stabilizing agents [14-18]. Due to quantum confinement, the photoluminescence (PL) and absorption of the NCs are dependent on their size, which in most cases can be controlled by the preparative conditions [19]. The PL of the thiol-capped NCs covers, depending on their size a very broad spectral region from green (ca. 500 nm) to the near infrared (ca. 800 nm) [17]. After synthesis, CdTe NCs can be treated to improve their size distribution, PL quantum efficiency (QE), processability and stability [16]. The PL QEs of state-of-the-art thiol-capped CdTe NCs reach 60-80% [17,20,21]. The solubility of thiol-capped CdTe NCs is not limited to aqueous solutions. NCs synthesized in water can be transferred into non-polar organic solvents, such as toluene, styrene and chloroform via a stabilizer exchange to long-chain thiols [22], by utilizing polymerizable surfactants [23] or by employing amphiphilic molecules for the
stabilization [24,25]. Being transferred to organic solutions, the NCs may be used for the fabrication of functional and processable polymer-NC composites [22,23] suitable for applications, e.g., in LEDs [26].
