12 Colloidal synthesis of Quantum Dots (QDs)

S.S. islam

epgp books

 

 

 

Introduction

 

QDs have been studied for approximately two decades with views toward the aforementioned applications as well as for the pure science aspects concerned with the physical, electronic, and non-linear optical properties that lie between the molecular and the bulk. While quantum dots have dimensions that are on the order of nanometers, they still contain hundreds to thousands of atoms. Most of the QDs that have been studied to date are dispersed in a host matrix such as glass or a crystalline counterpart. The newest research is often being carried out using colloidal quantum dots that may be placed on a substrate in a self-assembly process to produce films, patterns, and wires.

 

A capping structure is used in colloidal quantum dots to charge-passivate the surface and defects. Capping structures act as an interface between the QDs and their surroundings. With appropriate capping, QDs can be used in sensors or integrated with biological systems. For these reasons, an investigation into how to make cadmium selenide (CdSe) quantum dots with varied capping structures, particularly, how to make the robust ZnS capped CdSe variety for biological marker or polymer microsphere tagging applications, is of interest. The use of trioctylphosphine oxide (TOPO), stearic acid (SA), dodecylamine (DA), phenyl sulfone (PS), aminophenyl sulfone (APS) and other organic molecules as the capping structures of CdSe QDs need to be investigated during initial production of the nanoparticles. In the larger scientific scope, these capping structures could facilitate integration of CdSe QDs into biological or chemical sensing systems, polymeric systems, LEDs, photovoltaics and many other functional applications. Additionally, the inorganic capping structure will be used as the primary ligands for high temperature, airless, organo-metallic based colloidal synthesis of CdSe quantum dots.

 

The advantages of a colloidal system over a matrix/QD system are many and varied. Three-dimensional interaction of quantum dots with their surroundings is desirable as it allows for applications such as tagging of DNA or aqueous monitoring for biological and chemical changes in a given environment. The colloidal system allows greater available surface area and environmental contact than the matrix/QD or substrate/QD system. The colloidal system also provides the ability to change the solvent surrounding the quantum dots, deposit the dots through precipitation onto a substrate, or diffuse them into a polymer. Such flexibility makes bulk manufacturing of quantum dots desirable. Quantum dots are being produced at the laboratory scale out of a variety of materials in accordance with the properties desired.

 

1. The Colloidal Quantum Dot System

 

Colloidal processing is liquid phase condensation wherein chemical reactions supersaturate the precursors and cause nucleation as well as growth of nanoparticles. Post-synthesis treatments are very crucial for applications of these QDs because the as-synthesized QDs have poor quantum yields for luminescence; additionally, it is presently not possible to synthesise nanoparticles with efficient luminescence by other techniques. We will now discuss the most frequently used physical methods for post-synthesis treatments of QDs.

 

1.1 Micellar synthesis of colloidal quantum dots

 

Water-in-oil, a reverse micro-emulsion technique, to prepare colloidal quantum dots was first proposed in the last two decades of the twentieth century. The technique involves a chemical reaction which can be controlled by inter-micellar exchange of reactants, resulting in nucleation and growth of the nanoparticles. This process has been employed to produce a variety of nanoparticles from different substances, such as metals, metallic oxides, silver halides, and semiconductors. Earlier, nanoparticle growth via reverse microemulsions was assumed to be limited by reverse micelle shells. As reverse micelle are monodisperse, the nanoparticles, thus produced, should also be monodisperse. Based on this logic, the size of the nanoparticles could be altered since the size of reverse micelles is controllable by variations in the concentrations of water and surfactants.

 

1.2. High-temperature colloidal synthesis

 

Murray, Norris and Bawendi were the first to report colloid preparation at elevated temperatures. They had produced colloidal quantum dots of CdS, CdSe and CdTe having adjustable luminescence, via a one step process, called, pyrolysis of organometallic precursors at 300 °C. The process involved injecting cold precursors (a mixture of TOP-Se and dimethylcadmium in tri-n-octylphosphine) in hot (300 °C) tri-n-octylphosphine oxide. This resulted in abrupt nucleation and a subsequent decrease in temperature of mixture (~180 °C). After this point nucleation stopped. Then, temperature of the reaction mixture was slowly increased to 230-260 °C. The temperature was maintained at this value during the nanoparticle growth. Nanoparticle synthesis at such high temperatures caused annealing of the produced particles, thereby producing defect free crystals. Thus prepared colloidal quantum dots exhibit strong band-to-band fluorescence. However, luminescence from deep traps could not be seen.

 

The colloidal quantum dots (CQDs) produced by this method are of high-quality, i.e.,

 

a.     Temporal separation of nucleation and growth processes

b.    The produced nanoparticles are annealed.

c.     Elevated temperature synthesis produces defect-free QDs having high quantum yields of fluorescence as well as narrow size distributions in comparison to liquid-phase processes.

 

1.3 High-temperature colloidal synthesis of AIIIBV semiconductor quantum dots

 

CdSe Qds have impressive properties, however, their use is limited owing to their high toxicity (Cd being a carcinogen). Therefore, alternative methods have been developed to coat CdSe QDs in order to reduce the toxic effects. Examples include silicon dioxide or amphiphilic polymer, the coating of these shells decreases the amount of cadmium (Cd2+) ions escaping from the nanoparticle cores. Nonetheless, these compounds are still toxic and remain unsafe for use. Less toxic and cadmium free alternatives (such as AIIIBV semiconductors) have garnered huge attention these days. Examples are GaAs, InP, InAs and InSb. Indium phosphide matches CdSe in terms of physicochemical behaviour and is the potential candidate to replace CdSe.

 

InP (indium phosphide) is a direct bandgap semiconductor and has a bandgap of 1.27 eV, for comparison, CdSe has a bandgap of 1.74 eV. InP has tetrahedral structure of wurtzite or sphalerite type. Since, contributions of ionic bonding is ~15%, covalent bonding predominates the crystal structure. The predominance of covalent bonding increases the exciton Bohr radius to about 10.6 nm, which is much larger than that of CdSe (5.2 nm). Thus, size effects are more pronounced in InP in comparison to CdSe. Furthermore, strong covalent banding enhances the photo-stability of InP Qds. Thus, these Qds can be used as a substitute to CdSe QDs in applications such as in biomedicine.

 

Even though InP QDs offer better properties than CdSe QDs, InP QDs have not been synthesised similar to CdSe. The elevated temperature synthesis under conditions, solvents, precursors, etc. similar to CdSe, have not produced InP satisfactorily. The reaction rates of nucleation and nanoparticle growth are very slow and production of nanoparticles of specific dimensions requires long time durations (sometimes hundreds of hours). Additionally, it is very difficult to get mono-disperse QDs. This is because, owning to the strong covalent bonds, it becomes difficult to separate the nucleation and growth of nanoparticles. This results in a broad size distribution of the synthesised nanoparticles, which in turn results in broad luminescence bands, namely full width at half maximum (FWHM). FWHM is greater than 60nm for InP, while it is 30nm in CdSe.

 

Consequently, several synthesis routes have been developed to prepare InP QDs. The use of octadecene, as solvent, and fatty acids, such as palmitic acid as indium-coordinating ligand, has caused considerable reduction in the synthesis time duration. The use of these reagents have also helped in producing nanoparticles having narrow size distributions (~4.7% variance) in few hours. Also, precursors such as indium carboxylates (i.e., lauratre, myristate, stearate) promote the synthesis of InP QDs. The choice of precursors has great influence on the size of the synthesised nanoparticles. The reactivity of indium carboxylates (and synthesis rate) can be enhanced by using fatty amines as activating agents. Thus different sized InP QDs can be produced having tunable spectrum (390-720 nm) within one hour.

 

Phosphorous source is also important for synthesising InP QDs. Mostly used precursors such as tris(trimethylsilyl)phosphine cannot be used to grow bigger nanoparticles since it is highly reactive due to which it is consumed in a very short time after introducing into the reaction chamber. In this case, the nucleation as well as growth of the particles occur almost simultaneously, thereby precluding any possibility of achieving mono disperse phase. In addition to this, this precursor is very expensive, hazardous, and spontaneously inflammable material. Therefore, any large scale production of InP QDs requires development of either alternate precursors or efficient synthesis techniques.

 

To this end, alternate sources of phosphorous have been explored. These include yellow phosphorus, gaseous phosphine (PH3). PH3 is produced by the reaction of calcium phosphide (Ca3P2) or zinc phosphide (Zn3P2) with HCl. Tris(dimethylamino)phosphine is another less expensive precursor for phosphorus which is relatively safer to use. Tris(dimethylamino)phosphine was used for the first time by Matsumoto, Maenosono and Yamaguchi and Song to produce InP QDs at elevated temperatures and having narrow size distributions. Luminescence spectrum for these dots was tunable between 500 and 600 nm by simply adjusting the synthesis time only.

 

Thus, reactants and reaction conditions can be adjusted to produce InP QDs having narrow size distribution in relatively shorter time periods. Additionally, the size of the particles can be controlled to vary in a broad range. Nonetheless, the InP QDs produced till now, have shown very poor quantum yields of luminescence (around 1%).

 

This has been attributed to the presence of covalent bonds which form several deep surface traps (or unsaturated bonds and packing defects) in the indium phosphide QDs. The activation energies to overcome these traps are more than that in AIIBVI semiconductors. By proper selection of the semiconductors having wide bandgaps to modify the QDs’ surfaces, it is possible to increase the quantum yield of photoluminescence. For instance, HF or NH4F treated (chemically annealed) colloidal quantum dots demonstrated intense band to band luminescence. In this case, the quantum yield of luminescence was found to be increased by more than 30%. InP colloidal quantum dots when dispersed in tetrahydrofuran, exhibited stable and bright luminescence for over two weeks. This is attributed to the occupation of phosphorus vacancies by fluoride anions during chemical annealing. This resulted in the removal of oxygen from the oxide layer.

 

In another technique, chemical and photochemical annealing processes can be combined. Thus, a further UV illumination for about 8 hours almost doubled the quantum yield. Additionally, covering the InP surface by a wide bandgap semiconductor is an effective method to enhance the quantum yield of the InP QDs.

 

The growth of ZnS shells on InP cores increased the quantum yield of luminescence from <1% to >40%. This core-shell structure also improved the air stability of the QDs. There have been numerous efforts to prepare different core-shell architectures by used various semiconductor shells (e.g., CdSe, CdS, ZnSe, ZnSe, ZnS, or ZnCdSe2). Out of which ZnS based QDs have demonstrated the best performance. The semiconductor shells can either be produced after the synthesis process or during the synthesis process of the InPQDs.

 

Chemical annealing followed by growth of wide bandgap semiconductor shell is an attractive mean. But chemical annealing of InP core may deteriorate the structure of ZnS shells. This may happen because HF blocks the surface of InP colloidal quantum dots. ZnS shell over InP enhanced the quantum yield of luminescence. Additionally, the formation of InP QDs in presence of zinc ions reduced the surface defects and facilitates the synthesis of ZnS shells. Further, zinc ions also increase the stability of the QDs’ surface.

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