1.4 Structure of the Thesis
2.1.2 Synthesis of Colloidal Quantum Dots
Technological as well as fundamental scientific importance of colloidal nanocrystals have led to the development of versatile and successful synthetic routes in the last three decades [Hyeon, 2003] [Kim et al., 2013]. The synthetic techniques of colloidal QDs are typically classified as ’top down’, and ’bottom up’ methods [Sytnyk, 2015] [Bera et al., 2010].
1. Top down method - Involves thinning or milling of a bulk semiconductor mechan-ically to a fine powder, which is eventually dispersed in a surfactant solution.
Syn-thesis of III-V and II-IV QDs having a particle size of approximately 30 nm has been achieved with this method [Sytnyk, 2015] [Bera et al., 2010]. Major draw-back of this method are,
• Impurities are incorporated in the QDs during milling.
• Shapes and sizes of nano-crystals are uncontrolled.
2. Bottom up method - Nanoparticles are synthesized from metal precursors where lig-ands are present to control the nanocrystal growth and prevent coagulation [Sytnyk, 2015]. Monodisperse samples synthesized this way can self assemble and form or-dered superstructures [Sytnyk, 2015] [Bera et al., 2010]. Bottom up methods can also be categorized as wet-chemical and vapour-phase methods [Bera et al., 2010].
The use of wet chemical methods have proven to be successful due to their uncomplicated nature as well as their potential for producing large batches of the product [Hyeon, 2003].
The foundation for the colloidal synthesis was laid by Victor K. LaMer and Robert H.
Dinegar in the early 40s. The kinetics of ’burst nucleation’ was described quantitatively by the classical nucleation theory while working with oil aerosols and sulfur hydrosols [Robb and Privman, 2008] [LaMer and Dinegar, 1950]. Lamer and Dinegar suggested as shown in figure 4, the driving force of a rapid nucleation is supersaturation, followed by the diffusing atomic matter absorption, known as Ostwald Ripening to the already nucleated particles that subsequently grow into bigger crystals [Robb and Privman, 2008].
Figure 4.Schematic illustration of LaMer’s model for the monodisperse colloidal particles growth [Lee et al., 2014] [Hollingsworth, 2006].
Synthesising colloidal nanocrystals using wet chemical methods require three compo-nents: precursors, organic surfactants and solvents. It is possible that surfactants may serve as solvents [Yin and Paul Alivisatos, 2005]. Typically, alkyl phosphonic acids, alkyl phosphine oxides, alkyl phosphines, some nitrogen-containing aromatics, fatty acids and amines are used as organic surfactants that are capable of solvating nanocrystals dynam-ically [Yin and Paul Alivisatos, 2005]. Various wet chemical synthesis techniques have been in use known as sol-gel process, microemulsion process and hot-solution decompo-sition method [Bera et al., 2010].
Sol-gel Process
• Metal precursors such as acetates, alkoxides or nitrates in a medium which is acidic or basic are used to prepare asol, where nanoparticles are dispersed in a solvent by brownian motion[Bera et al., 2010].
• Hydrolysis, condensation where the sol formation takes place and growth during which the gel formation occurs are the three main steps in this method [Bera et al., 2010].
• II-VI and IV-VI QDs such as PbS, ZnO and CdS have been synthesized this way [Bera et al., 2010].
• Its application is limited by wide particle size distribution as well as high number of defects [Bera et al., 2010].
Micro-emulsion Process
• Synthesis can be done at room temperature [Bera et al., 2010].
• Classified as normal microemuslsions (i.e. oil-in-water) and reverse microemul-sions (i.e. water-in-oil) [Bera et al., 2010].
• Core and core-shell QDs of group II-VI such as CdS, CdSe/ZnSe, ZnSe, ZnS/CdSe as well as QDs of group IV-VI are prepared this way [Bera et al., 2010].
Hot solution decomposition method
Hot solution decomposition method also known as the hot injection method is a well established method that revolutionized the research in the field of nanotechnology and it has become the widely used method for preparing QDs [Bera et al., 2010] [Yin et al., 2016]. A precursor solution usually kept at room temperature is rapidly injected into a warm surfactant solution in this method. The high temperature facilitates chemical transformation of precursors into monomers [Yin et al., 2016]. This subsequently leads to the formation of monodisperse QDs (standard deviation of 5% about the average particle size) and the presence of surfactant molecules affects the nanocrystal growth [Lim et al., 2012] [Yin et al., 2016] [Bera et al., 2010].
The two steps of nanocrystal formation are,
• Nucleation- Precursor decomposition at high temperature leads to instantaneous monomer supersaturation that occurs at the critical point of nucleation within a small time span followed by a burst of nanocrystal nucleation [Lim et al., 2012] [Yin et al., 2016]. The temperature is decreased immediately after hot injection and nucleation is seized as a result, which prevents the consumption of all the precursor that could drive fast aggregation and ripening process [Murray et al., 1993].
• Growth- Additional monomers present in the solution drives the nuclei growth to bigger crystals at the reduced temperature [Yin et al., 2016] [Murray et al., 1993].
Figure 5. Illustration of the experimental apparatus used during the synthesis of CQDs by hot-injection method [Kim et al., 2013] [Hollingsworth, 2006]
This hot injection method generates nanocrystals based on the condition that monomers are able to anneal and rearrange during growth [Yin et al., 2016]. It is crucial to know the cohesive energy per atom that coordinates with the solid melting temperature for deter-mining the optimal conditions for nanocrystal growth [Yin et al., 2016]. Thus, deciding the ideal temperature that allows monomer rearrangement and annealing within a growing nanocrystal is crucial [Yin et al., 2016].
Identification of ideal surfactants as well as precursors is also pivotal. Choosing or-ganic surfactant is based on their tendency to adhere to a growing nanocrystal [Yin et al., 2016]. The metal coordinating and solvophilic groups inhibit the growth and aggregation of nanocrystals due to their electron donating nature that allow coordination to electron-deficient metal atoms at the nanocrystal surface. Nanocrystal solubility varies as the sec-ond end of the surfactant extends to the solvent leaving nanocrystals with a hydrophobic
surface [Yin et al., 2016]. Accordingly, the presence of organic surfactants also deter-mines the size, spatial arrangement as well as shape of some of the inorganic solids dur-ing the colloidal nanocrystal growth [Yin et al., 2016]. Precursors such as organometal-lic compounds are chosen for their propensity to react or disintegrate rapidly, yielding monomers at the nanocrystal growth temperature that drives nanocrystal nucleation and growth [Yin et al., 2016].
This approach was first experimented by Bawendi and co-workers in 1993 for synthesiz-ing nanocrystals of cadmium chalcogenides (CdS, CdSe, CdTe) [Bera et al., 2010] [Mur-ray et al., 1993] [Hyeon, 2003]. Cadmium precursor has been chosen as dimethyl cad-mium, ME2Cd, while chalcogen precursors were chosen to be tri-n-octyl-phosphine tel-luride (TOPTe) and tri-n-octyl-phosphine selenide (TOPSe) [Murray et al., 1993] [Kim et al., 2013]. Coordinating solvent is a mixture of trioctylphosphine oxide (TOPO) and tri-octylphosphine (TOP), which is added into a three-neck round bottomed flask followed by degassing. The precursor solution is made with ME2Cd and TOPSe. This is then injected at a temperature of 300oC into a hot TOPO placed in the flask. CdSe nuclei are instantly formed followed by QDs growth throughOstwald Ripening [Murray et al., 1993] [Bera et al., 2010] [Mahajan et al., 2013]. As a coordinating solvent, TOPO stabilizes the dispersion of QDs, enhances surface passivation as well as slows the growth of QDs by providing an adsorption barrier [Bera et al., 2010]. Precursor reactivity as well as reaction variables such as temperature, concentration, time and coordinating solvents/surfactants strongly affect the nucleation and growth kinetics [Lim et al., 2012] [Bera et al., 2010].
This facilitates the synthesis of crystalline, monodisperse and highly luminescent QDs across a broad range of well controlled and narrowly distributed sizes [Kim et al., 2013].