Joung et al. Nanoscale Research Letters 2012, 7:93 http://www.nanoscalereslett.com/content/7/1/93 Page 5 of 8 Figure 3 One-pot synthesis of core-shells and their optical properties. (a) One-pot synthesis of InP with P(SiMe2-tert-Bu)3 followed by ZnS shell overcoating. (b) Size-tunable emission from InP/ZnS NQDs synthesized using a new P precursor (red-green). narrow. Figure 1b shows the UV-Vis absorption spectra of NQDs prepared in a temperature range of 250°C to 280°C, using otherwise the parameters of the fixed molar ratio of In/MA to 3. It is evident that with increasing reaction temperature, the NQDs’ excitonic peak is being red shifted, which implied the formation of bigger InP QDs. Figure 1c,d shows the UV-Vis absorption spectra of InP NQDs prepared using a new P precursor, P(SiMe2-tert- Bu)3, in a temperature range of 250°C to 280°C. It also shows that with increasing reaction temperature, the NQDs’ excitonic peak is being red shifted. While phosphorus precursor 3 did not undergo InP for- mation (see Table 1), precursor 2 gave highly monodis- perse crystalline InP NQDs when we used the same reaction protocol optimized for the P(SiMe3)3 precursor. By varying reaction temperature, we obtained an InP 1s absorption spanning from 560 nm to 640 nm. When com- pared with 1, reactions using precursor 2 resulted in a lar- ger size as shown in Figure 4. At the same reaction temperature, InP synthesized using 2 showed 20 to 40 nm more of red-shifted first excitonic transition (Figure 4b). Unlike II-VI NQDs, growth of InP nanocrystal is sug- gested as dominated by interparticle ripening. Increasing growth temperature does not expect to significantly broaden the excitonic feature. Rather, the myristic acid in the solution interferes with the ripening, thereby inducing broadening in excitonic transition [15]. We suggest that Table 1 Synthesis of InP NQDs using various P precursors. P Precursors P(SiMe3)3 (1) P(SiMe2-tert-Bu)3 (2) P(SiMe2Ph)3 (3) FWHM, full width at half maximum. introducing the bulky tert-butyl group creates less nuclei at the same injection temperature and allows further growth using unreacted P precursors in the solution. With the previously used precursor 1, we only obtained the dots with 1s absorption maxed at 580 nm in our reaction con- dition. The new P precursor with tertiary butyl group enabled a further shift to red up to 640 nm without com- promising the size uniformity. InP synthesized using new P precursors has poor band edge photoluminescence [PL] (Figure 5a) possibly due to surface traps, dangling bonds, and staking faults in the crys- tal as commonly seen in III-V NQDs compared with II-VI NQDs. Strategies to enhance the PL include chemically modifying the particle surface or epitaxial growth in a shell with a large bandgap material such as ZnS. In situ synthesis of InP/ZnS was reported with limited control in their sizes [7]. Here, we used a single-molecular ZnS precursor, diethyldithiocarbamate, for shell formation (Figure 3a). Slow addition of the air-stable ZnS precursor (diethyldithio- carbamate) at elevated temperature allowed shell formation on the surface of each particle, thereby enhancing PL prop- erties (Figure 5b). The PL QY of the as-prepared InP NQDs was 1% (standard: Rhodamine 6G). The complete surface passivation procedure strongly enhanced the PL QY of the InP NQDs up to 18% to 28% (Figure 5b). Figure 5c shows a typical transmission electron microscope [TEM] image of an InP NC with mean diameters of InP (nm) 495 to 601 560 to 640 No InP formation FWHM (nm) 49 to 60 50 to 62 - Reaction temperature (°C) 230 to 300 210 to 300 -