# Simple Synthesis Method and Characterizations of Aggregation-Free Cysteamine Capped PbS Quantum Dot
## 1 Introduction  
Over the past decade, quantum dots have attracted great attention in various research fields and industries because of their distinct electrical and optical properties from bulk materials. Three-dimensional quantum confinements are caused by particle size below Bohr's exciton radius effects on the energy band structure, which is deeply related to the electrical and optical properties of quantum dots [1; 2]. Unlike in the bulk semiconductors, the dimension-dependent density of states has the shape of delta function in the quantum dots. This yields the absorption and emission spectra of quantum dots, with narrow full width half maximum (FWHM) centered at particular wavelength. The quantum dots have engineering advantages where the absorption and emission spectra can be adjusted by changing the synthesis conditions. These properties of quantum dots provide high wavelength-selectivity, accuracy, and sensitivity [3; 4; 5; 6]. Therefore, quantum dots are considered one of the most promising materials in various applications, including optoelectronics [7; 8], biosensors [9; 10; 11; 12; 13; 14], energy harvesters [15; 16; 17], and quantum dot-based lasers [18; 19].  
In general, the core quantum dot consists of semiconductors, especially the compounds of II-VI or IV-VI materials. The PbS quantum dots, one of IV-VI quantum dots, have been studied in various research and industry fields because it absorbs and emits infrared (IR) radiation in the wavelength range of near and shortwave IR [20; 21; 22], which corresponds to the absorption energy range between 0.71 to 1.28 eV [23]. The applications of PbS quantum dots with IR have been considered as one of the major subjects in research and industries, including optoelectronics and sensors [24; 25; 26].  
While the core of a quantum dot determines the electrical and optical properties of the quantum dot, the ligand of the quantum dot generally characterizes chemical interactions with other materials including solvents, surfaces, and particles [27]. The development of ligands has made it possible to carry diverse chemical properties to quantum dots, making them more attractive to sensor applications [28; 29]. In ligand groups comprising trioctylphosphine oxide (TOPO), trioctylphosphine (TOP), amines, and carboxyl acids, thiols are mainly used to synthesize gold-adherent quantum dots [30; 31]. Since gold is one of the most used materials in biosensors [32; 33; 34; 35], PbS quantum dots passivated with thiol-based ligands can have many advantages over other ligands for quantum dots in biosensor applications. Cysteamine, one of the thiols, is distinguished from other thiols because it is soluble in water, the most common and safe solvent. Thus, the various types of cysteamine capped quantum dots have been studied in the fields [36; 37; 38]. However, the synthesis of cysteamine capped PbS quantum dot has been considered difficult, due to the aggregation caused by hydrogen bonds between cysteamine molecules [39].  
In this study, we describe a simple synthesis method of aggregation-free cysteamine capped PbS quantum dots. We employed sodium docusate as a stabilizing agent to maintain lead ions to be monomers, and 2-mercaptoethanolethanesulfonate as an additional ligand to prevent the aggregation. The whole process was conducted in one pot and did not require any extra complex procedures. The size and lattice structure of the quantum dots were characterized by the transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS) confirmed that cysteamine and 2-mercaptoethanolethanesulfonate were well-adhered to the surface of the quantum dot. The absorption characteristics of the quantum dot analyzed by the spectrophotometer shows high absorbance at the wavelength of 1450 nm. The synthesized quantum dots maintain dispersion stability longer than one month.