Gresback et al. Nanoscale Research Letters 2011, 6:68 http://www.nanoscalereslett.com/content/6/1/68 Transmission electron microscopy (TEM) was per- formed by dropping a small amount of the InP colloid onto a thin carbon-coated TEM grid. A FEI Tecnai T12 (FEI Company, Hillsboro, OR, USA) operated at 120 kV was used. X-ray diffraction (XRD) was performed on a Bruker-AXS Microdiffractometer (Bruker Scientific Instruments, Billerica, MA, USA) with a 2.2-kW sealed Cu X-ray source. Raman spectroscopy was performed using a Witec Alpha300R (WITec GmbH, Ulm, Germany) confocal Raman microscope. Samples for XRD and Raman spectroscopy were prepared by casting drops of concentrated solutions onto glass substrates. Photoluminescence (PL) measurements were preformed on a Photon Technology International QuantaMaster 40 UV-Vis (Photon Technology International, Inc. Birming- ham, NJ, USA) spectrofluorometer. UV-Vis absorption measurements were performed using a HR2000 spectro- meter (Ocean Optics, Inc. Dunedin, FL, USA) with a combination of deuterium and tungsten halogen lamps (DH-2000-BALL). Results and discussion Bare InP NCs were found to oxidize rapidly making characterization of dry powder difficult. Hence, organic ligands are used to inhibit oxidation, afford disper- sibility, and to allow for subsequent material charac- terization. Figure 2 shows a TEM image of InP NCs synthesized with a 10 ms residence time and passivated with myristic acid. The (111) lattice fringes are clearly visible in several NCs. The inset in Figure 2 shows a Page 3 of 6 selected area diffraction (SAD) pattern with the rings of the (111), (220), and (311) InP planes being clearly visi- ble. The average NC size determined by counting more than 50 particles was 4.3 nm with a standard deviation of 0.8 nm. The size distribution is broad compared with liquid-phase synthesis techniques, where a standard deviation of less than 10% of the average size is com- mon [14]; however, with a standard deviation of approximately 20% of the mean size, the size distribu- tion is rather narrow for gas-phase synthesized NCs [15]. Raman spectroscopy and XRD were used to investi- gate further the NC microstructure and size. Figure 3 shows an XRD spectrum of NCs synthesized with a resi- dence time of 6 ms. The (111), (220), and (311) peaks are clearly visible with a small signal from the (200) peak. This confirms the presence of InP with a zinc- blend structure. Debye-Scherrer analysis of XRD peak broadening gives a NC size of 3.4 nm, while TEM results found a slightly larger mean size of 3.8 nm. The Raman spectra in Figure 4 shows InP NCs synthesized with residence times of 4, 6, and 10 ms and passivated with myristic acid. The peak of the longitudinal optical mode decreases while the transverse optical mode increases, with decreasing residence time and NC size. This was also observed by Guzelian et al. [16] and was thought to be because of a loss of symmetry as NC size decreases. Figure 2 TEM image of InP NCs synthesized with a 10-ms residence time. The average size is 4.3 nm. Insets: SAD showing (111), (220), and (311) planes and HR image. Figure 3 XRD spectrum of InP NCs synthesized with a 6-ms residence time. The size given by the Debye-Scherrer equation is 3.6 nm.