# Simple Synthesis Method and Characterizations of Aggregation-Free Cysteamine Capped PbS Quantum Dot
## 3 Results and Discussion  
The colloid of resulting cysteamine capped PbS quantum dot has been well-dissolved in double distilled water over a month. As it contained sodium docusate as a stabilizing agent, lead ion from lead chloride developed in the form of monomers. If the lead precursor is not dissolved as monomers, nuclear growth occurs before nuclei are created, which makes it difficult to control the size of the quantum dots. When the lead precursor is injected to water in which docusate is dissolved, the docusate molecules bind around the lead monomer, and the color of solution turns white. Subsequently, when the lead monomers react with the sulfur precursor, nuclei of PbS quantum dot start to form, and the size of the quantum dot gradually increases according to Ostwald ripening. The size of the quantum dots is usually proportional to the reaction time and temperature. After nuclear growth, cysteamine aqueous solution is added to induce ligand exchange on the surface of the quantum dot. However, since cysteamine has an amine group, there is a possibility that quantum dots may aggregate over time by forming hydrogen bonds in an aqueous solution, and it is necessary to additionally attach another ligand to prevent the aggregation. Since 2-mercaptoethanolhanesulfonate is anionized in an aqueous solution, quantum dots can maintain dispersion stability even if they have cysteamine as a ligand, due to the electrical repulsion generated therefrom. The synthesized aggregation-free PbS quantum dots,  
Figure 1: A brief description of synthesis of aggregation-free PbS quantum dots. At the first step, lead chloride aqueous solution was added into double distilled water containing docusate sodium as a stabilizing agent. A sodium sulfide aqueous solution was injected to the solution to cause formation of PbS quantum dot. For ligand exchange, cysteamine aqueous solution was dropped into the colloid of PbS quantum dot, and subsequently 2-mercaptoethanolhanesulfonate aqueous solution was added into the solution for preventing the aggregation of cysteamine. as described above, it was confirmed that they did not aggregate and maintained dispersion stability even after one month. Although, some white and yellow precipitates were formed on the bottom of the solution. We attribute the white and yellow precipitates to lead compounds, including lead sulphate, lead carbonate, and lead oxide, caused by lead atoms not involved in quantum dot formation. Figure 2 shows the images of quantum dots as synthesized and after one month from synthesis. Figure 3a shows the TEM images of the synthesized quantum dot. The TEM images shows that the particles with particular diameter are evenly distributed on the grid. Figure 3b shows a TEM image of quantum dots synthesized without adding 2-mercaptoethanoltonate and severe aggregation of the quantum dots. Figure 3: Transmission electron microscopy (TEM) images of cysteamine capped PbS quantum dots. (**a**) Image of cysteamine capped PbS quantum dot with 2-mercaptoethanoltonate as additional ligand (**b**) Image of cysteamine capped PbS quantum dot without 2-mercaptoethanoltonate. Unlike (**a**,**b**) shows the severe aggregation of PbS quantum dots caused from hydrogen bonds of cysteamine molecules. Figure 2: Image of cysteamine capped PbS quantum dot dispersed in water. (left) Image of quantum dot as synthesized, (right) Image of quantum dot one month after synthesis. In Figure 3b, all particles are connected and aggregated with the hydrogen bond from amine functional groups of cysteamine. However, in Figure 3a, the separated particles are laid over the layers. From above, attaching 2-mercaptoethanol methanesulfonate as an additional ligand was an efficient method to prevent the aggregation of cysteamine. Figure 4 shows the TEM image of the synthesized quantum dots in a higher magnitude than in Figure 3. The particles in the image have three types of lattice structures which are PbS [111], PbS [200], and PbS [220]. The lattice constant of PbS is known as 5.963 A.