Miao and Zhang Nanoscale Research Letters 2012, 7:321 http://www.nanoscalereslett.com/content/7/1/321 Methods InP nanowires were grown by low-pressure MOCVD at the pressure of 10 kPa, with trimethylindium (TMIn) and phosphine (PH3) as precursor materials, transported in H2 as carrier gas. For In droplet fabri- cation, TMIn was introduced to the reactor at a mole flux of 4.78 × 10−6 mol/min for 30 s at 330 °C. Then, PH3 was introduced into the reactor immediately to start the nanowire growth. The nanowire was grown at 330 °C. For the precursors, the mole flux of TMIn is 4.78 × 10−6 mol/min and that of the PH3 is 1.4 × 10−4 mol/ min; the V/III ratio for nanowire growth is 30. After the growth was completed, TMIn was turned off and PH3 was continuously inputted to protect the substrate until the temperature drops to 250 °C. The time for nanowire growth changed from 30 s, 1 min, 2 min, 3 min, 5 min, 7 min, till 15 min. The InP nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction, PL, and Raman. For PL, the 532-nm wavelength of a laser and a liquid N2-cooled Ge detector were used. The 488- nm wavelength of a laser is used as excitation source for Raman diffraction. Results and discussion Top-view images of indium catalyst droplets and InP nanowires at different growing stages are presented in Figure 1. It clearly shows that the density of the nano- wires increases as the growing time gets longer. This phenomenon will not appear in the traditional Au- catalyzed nanowire MOCVD growth, for the density of the gold droplets is certain as soon as they were fabri- cated in the pretreatment. Thus, the density of the nano- wires grown on Au droplets is certain. The catalyst-free InP nanowire growth is divided into two steps: catalyst fabrication and nanowire growth. Nanowires grow on Page 2 of 6 the base of the indium droplets fabricated in the front step. The density of the nanowires increases as the time accumulates, which means that besides the nanowires grown on the catalyst, the new nanowires are fabricated in the nanowire growing process. In order to verify whether new nanowires could grow in the nanowire growing process, we cancel indium droplet fabrication and the Si substrate with no indium droplets is put in the circumstance of nanowire growth for 10 min. The corresponding SEM images of the sam- ple are given in Figure 2. There are some nanowires and abundant pellets on the substrate. Figure 2b shows the nanowires on the substrate. There is a catalyst nanopar- ticle at the top, meaning that these nanowires are grown on the base of the catalyst. The two images indicate that, in the atmosphere of TMIn and PH3, indium droplets can still be fabricated and provide seeds for new nano- wire growth. There are two possible reasons for that. One is the physical condition especially the temperature. The cata- lyst indium droplets are fabricated by the thermal de- composition of TMIn. For catalyst-free InP nanowire growth, it has been tested in our experiments before that the temperature is in the range of 330 °C to 370 °C [17,18]. In this range, indium droplets can be fabricated [19]. This provides physical possibility for indium drop- let fabrication during the nanowire growing step. The other is the TMIn on the surface of the substrate which provides material for the fabrication of new in- dium droplets. In the model of the nanowire growing process on the sidewall, the precursor material on the surface of the substrate can be absorbed by the nano- wires in the way of adatom diffusion and every single nanowire can only absorb a limited area around itself [20]. Figure 1a shows that the catalyst droplet, which Figure 1 SEM images of In droplets and grown nanowires. (a) In droplets. Nanowires grown for (b) 30 s, (c) 1 min, (d) 2 min, (e) 3 min, (f) 5 min, (g) 7 min, and (h) 15 min. The scale bar is 1 μm and applies for all images.