Joung et al. Nanoscale Research Letters 2012, 7:93 http://www.nanoscalereslett.com/content/7/1/93 referenced to the residual peaks of C6D6 (δ 7.15, 1H NMR; δ 128.0, 13C{1H} NMR). EI-mass spectra were obtained from Korea Basic Science Institute. Results and discussion We designed the P precursors for InP synthesis while pre- serving the P-Si bond. The bond dissociation energy of the P-Si bond is 363.6 KJ/mol which is easier to cleave com- pared to the P-C bond (507.5 KJ/mol). We introduced var- ious side chains to enhance stability, thereby easing the control of the nucleation process during the reaction. Scheme 1 (Figure 2) shows the facile synthesis of designed phosphorus precursors such as P(SiMe3)3 (1), P(SiMe2- tert-Bu)3 (2), and P(SiMe2Ph)3 (3). Precursors 1 to 3 were synthesized using the modified procedure in the literature [13]. Specifically, the treatment of a mixture of Na/K alloy and red phosphorus with RSiMe2Cl (R = Me, tert-Bu, and Ph) in dimethoxyethane gave, after workup, a novel P (SiMe2R)3 (R = Me, 1; R = tert-Bu, 2; R = Ph, 3) as color- less compounds in 30% to 70% isolated yield as shown in Scheme 1. Compounds 1 and 3 were isolated as colorless oil; however, compound 2 was obtained as a colorless solid. Compound 1 was pyrophoric and air-sensitive, but compounds 2 and 3 were stable in air for a few hours, and they were slightly decomposed in the C6D6 solutions in capped NMR tubes at room temperature after a few days and were easy to handle. They are soluble in aromatic sol- vents and in polar organic solvents. All compounds have been fully characterized by various spectroscopic data and EI-mass analysis. The 1H NMR spectra of 1 to 3 display well-defined resonances with their expected integrations. Upon complexation to phosphorus, the proton resonances of the methyl, tert-butyl, or phenyl attached to silicon are shifted downfield relative to those of corresponding silicon precursors. Also, all protons in complexes 1 to 3 were split as doublet due to the presence of coupling between P and H. In the case of 1, the greater extent of downfield shifts for the methyl protons than for the tert-butyl pro- tons upon complexation, suggesting a strong interaction Page 4 of 8 between the P and H in methyl groups and a weak interac- tion between the P and H in tert-butyl groups. The puri- ties of 1 to 3 were checked by 31P NMR spectroscopy, which revealed only one peak near 30 ppm. We started our experiments with a synthesis of the InP NQDs using compounds 2 (Figure 3a) and 3. For a typical experiment, 0.04 mmol of In(OAc)3 was added to a mix- ture of 0.12 mmol of MA and 4 ml of ODE in a 50-ml three-necked flask. The solution was then heated to 110°C for 1.5 h in vacuum. Injection solution was prepared by 0.02 mmol of compounds 2 or 3 in 1 ml of ODE while the mixture was still degassed from the reaction vessel. Then, the solution was injected rapidly into the hot reaction solution at 250°C under N2. When we prepared InP NQDs using compound 3, we obtained a colorless solu- tion and the resulting absorption spectrum did not show any distinct peak from an excitonic transition. On the other hand, compound 2 which contains the tert-butyl group underwent the NQD formation. We further devel- oped a synthetic route of InP NQDs using P(SiMe2-tert- Bu)3. The effects of different experimental parameters on the NQDs’ growth were studied, including the reaction temperature, the ratios, and the concentrations of precur- sors. First, we studied the synthetic scheme forming InP nanocrystals with the P(SiMe3)3 precursor. The ratio range of indium to acid was varied from 1:2.5 to 1:3.5 to opti- mize the reaction condition. As suggested by other groups, MA is known to act as a ligand which allowed a balanced nucleation and growth rate desired for the growth of rela- tively monodisperse InP NQDs, resulting in uniform NQDs in the visible range [14]. Figure 1a shows that when the molar ratio of In/MA in the solution was 1:3, the reac- tion generated InP nanocrystals with a good size distribu- tion indicated by the well-distinguished absorption features. When this molar ratio was varied to 1:2.5, the reaction generated nanocrystals without any distinguish- able absorption peak, implying a broad size distribution. This result indicates that the ligand concentration window for the formation of high-quality InP nanocrystals is Figure 2 Scheme 1: Synthesis of various phosphorus precursors 1 to 3.