Chemical method of synthesis of semiconductor nanoparticles, Langmuir-Blodgett film methods,

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Chemical method of synthesis of semiconductor nanoparticles, Langmuir-Blodgett film methods, micro-emulsion and sol gel method

Chemical method of Synthesis of semi conductor nano particles

semiconductor are of great importance in the quickly growing field of hybrid organic/inorganic electronics since they can serve as active components of photovoltaic cells, light emitting diodes, photodetectors and other devices

Chemical methods

The advantage of chemical synthesis is its versatility in designing and synthesizing new materials that can be refined into the final product.

Synthesis of semiconductor nanoparticles in colloidal solution

The semiconductors nanoparticles could be prepared by chemical synthesis in homogeneous solution, in different surfactant assemblies like micelles, vesicles, and Langmuir –Blodgett films in polymers, glasses zeolites and β-cyclodextrin.

     The easiest and most common method for the preparation of semiconductors nanoparticles is the synthesis from the starting reagents in solution by arresting the reaction at a definite moment of time. This is the so-called method of arrested precipitation.

Nanoparticles of metal sulfides are usually synthesized by a reaction of  a water soluble metal salt and H₂S (or Na₂S in the presence of an appropriate stabilizer such as sodium metaphosphate. For example, the CdS nanoparticles can be synthesized by mixing Cd(ClO4)₂ and Na₂S solutions:

Cd(ClO₄)2 + Na₂S = CdS +2NaClO₄                                        (1)

The growth of the CdS nanoparticles in the course of reaction is arrested by an abrupt increase in pH of the solution.

Colloidal particles of metal oxides can be obtained by hydrolysis of the corresponding salts. For example, the TiO₂ nanoparticles are readily formed in the hydrolysis of titanium tetrachroride.

TiCl₄ + 2H₂O = TiO₂ + 4HCl                                                    (2)

Formation of TiO2 nanoparticles via reaction 2

           Unfortunately, most of the colloidal solutions of nanoparticles; have low stability towards coagulation and possess a large size dispersion. Coagulation can be prevented by passivation of the surface of nanoparticles by hydroxyl ions, amines, or ammonia. Yet another procedure for the stabilization of colloidal solutions of nanoparticles is the coating of their surface with polyphosphates or thiols. As a result, one can obtain a stable colloidal solution of nanoparticles, isolations the nanoparticles as a powder, and then prepare a colloidal solution again by dispersing the powder in a solvent.

Usually the method of arrested precipitation results in a non uniform size distribution of nanoparticles. It is possible to decrease the width of this distribution by monitoring the synthetic procedures and using high-pressure liquid chromatography and capillary eletrophoresis. In the latter case, the separation of nanoparticles is achieved due to the different charge/size ratios for nanoparticles of different sizes.

Small monodisperse semiconductor cluster (like e.g.Cd₄S₄) can be obtained by performance the synthesis inside zeolite cages. Larger semiconductor nanoparticles of fixed size could be synthesized by introducing additional molecules to a small initial cluster stabilized by organic ligands in a colloidal solution.

Dispersion of macroscopic particles in solutions

It is possible to obtain semiconductor nanoparticles by sonication of colloidal solutions of large particles. Nanoparticles of layered semiconductors are also formed upon mere dissolution of large particles in an appropriate solvent, which was observed for MoS₂ and WS₂. Layered MoS₂ – type semiconductors are characterized by a weak van der Waals interaction between separate S – Mo – S layers. In the course of dissolution, the solvent molecules penetrate between the layers of the semiconductor and destroy large particles In the case of MoS₂, the process of destruction can be proceed until the formation of a two-layer particle. No further splitting of the semiconductor crystal occurs, since the formation of single-layer particles is accompanied by a considerable increase in the free energy of the system.

          Nanocrystals of layered PbI₂ –type semiconductors have a disk-like shape and discrete "magic" sizes of disks. For these semiconductors, a stable nanoparticle of a minimum size is assumed to be the smallest crystallite conserving the hexagonal symmetry of the macroscopic crystal. Such a crystallite is composed of two seven –atom iodine layers and two lead layers. Large stable nanoparticles are obtained from this seed by the layer-by-layer addition of extra iodide caps symmetrically around the perimeter. An analogous structure is also assumed for MoS₂ nanoparticles.

  Sol-gel synthesis technique

Sol-gel processing is a wet chemical synthesis approach that can be used to generate nanoparticles by gelation, precipitation, and hydrothermal treatment. Size distribution of semiconductors, metal, metal oxide nanoparticles can be manipulated by either dopant introduction or heat treatment. Better size and stability control of quantum-confined semiconductor nanoparticles can be achieved through the use of inverted micelles, polymer matrix architecture based on block copolymers  or polymer blends, porous glasses and ex-situ particle capping techniques.

Chemical vapor deposition (CVD)

Nanostructured materials are also prepared by chemical vapor deposition (CVD) or chemical vapor condensation (CVC). In these processes, a chemical precursor is converted to the gas phase and it then undergoes decomposition at either low or atmospheric pressure in a carrier gas and collected on a cold substrate, from where they are scraped and collected.  The CVC method may be used to produce a variety of powders and fibers of metals, compounds, or composites. The CVD method has been employed to synthesize several ceramic metals, intermetallics, and composite materials. For example, nanophase Si-N-C-containing ceramic particles were obtained by the thermal decomposition of liquid silazane precursors having the general formula [CH₃SiHNH]_x, x = 3 or 4 , with 80% of the cyclic being x = 4. It is believed that in the pyrolysis reaction the -SiH-NH- groups were responsible for the extensive cross linking and the nucleophilic displacements on the neighboring Si atoms, resulting in a three-dimensional network.

Laser chemical vapor deposition (LCVD) technique

In this technique, photo-induced processes are used to initiate the chemical reaction. Three different types of activation are usually considered during LCVD. If the thermalization of the Laser energy is faster than the chemical reaction, pyrolytic and/or photothermal activation is responsible for the activation. In photolytical (non-thermal) processes, the first chemical reaction step is faster than the thermalization of the excitation energy. In addition, combinations of the different types of activation are often encountered.

In pyrolytic LCVD (thermally activated process), the focused laser beam (usually at perpendicular incidence of the substrate) is used as a source of heat to induce the chemical reaction leading to CVD. The main advantage are that a pyrolytic process depends only slightly on the Laser wavelength (i.e., many different sources can be used), and that high rates of deposition can be reached. In addition localized and small deposits can be easily achieved (sub-micron patterning).

In photolytic LCVD is based on selective excitation of precursor molecules and laser beam is usually aligned parallel to the substrate as shown in Figure (4). Since the majority of the desired transitions (resulting in efficient decomposition) of the precursor molecules correspond to UV radiation, the number of available, powerful laser sources is limited. Commonly, excimer lasers are used to initiate photolytic LCVD. A combination of pyrolytic and photolytic LCVD is usually referred to as photophysical LCVD (or hybrid-LCVD), and this type of activation make it possible to make the best of the advantages and disadvantages of pyrolytic and photolytic LCVD. The setup used is usually a twin beam (UV + longer wavelength) or a single –beam (at intermediate wavelength) to activate a combined pyrolytic/photolytic process.  

 * A Langmuir–Blodgett film contains one or more monolayers of an organic material, deposited from the surface of a liquid onto a solid by immersing (or emersing) the solid substrate into (or from) the liquid. A monolayer is adsorbed homogeneously with each immersion or emersion step, thus films with very accurate thickness can be formed. This thickness is accurate because the thickness of each monolayer is known and can therefore be added to find the total thickness of a Langmuir-Blodgett Film. The monolayers are assembled vertically and are usually composed of amphiphilic molecules with a hydrophilic head and a hydrophobic tail (example: fatty acids). Langmuir–Blodgett films are named after Irving Langmuir and Katharine B. Blodgett, who invented this technique while working in Research and Development for General Electric Co. An alternative technique of creating single monolayers on surfaces is that of self-assembled monolayers.

Langmuir-Blodgett Films should not be confused with Langmuir films, which tends to describe an organic monolayer submersed in an aqueous solution