Miao and Zhang Nanoscale Research Letters 2012, 7:321 http://www.nanoscalereslett.com/content/7/1/321 Page 4 of 6 Figure 4 Room-temperature PL spectra of InP nanowires grown for (a) 2, (b) 5, and (c) 7 min. Trace (d) is for the nanowires grown for 10 min based on the smaller catalyst fabricated. to the bulk InP, there is blueshift which decreases as the growing time accumulates. It is found that the amount of the blueshift of PL is dependent on the time of the nanowire growth, which in turn is dependent on the dimensions of the nanowire. The blueshift has a relation with the dimension of the nanowires. The diameter and the length of the nano- wires are both increasing as the growing stage is pro- moted. The quantity is obtained by the SEM observation and listed in Table 1 with the corresponding blueshift. The blueshift of PL is from 66 to 18 meV, and the nano- wire diameter is from 40 to 93 nm. The blueshift of PL varies with the diameter of the nanowires. It is to prove that the blueshift of PL is directly related with the diam- eter of the nanowires. For comparison, the nanowires with smaller diameter and larger length are fabricated. It is realized by reducing the time for catalyst fabrication. Trace (d) is for these nanowires. Taking traces (a) and (d) for study, their diameters are close while their lengths are quite different. It can be seen that their blue- shifts have a little difference. This means that the PL blueshift of the nanowires has direct relations with the diameter, rather than the length. In the nanowire Table 1 PL blueshift of the nanowires with different diameters and lengths Nanowires for trace a b c d Diameter (nm) 40 67 93 45 Length (nm) 800 1,750 2,467 2,500 Blueshift (meV) 66 47 18 63 growth, the diameter increases as the growing time accumulates and the PL blueshift of the nanowires decreases. Raman spectra for nanowires at different growing stages were collected at room temperature and shown in Figure 5. Traces (a) to (d) separately correspond to the nanowires grown for 2, 5, 7, and 15 min, respectively. The peaks correspond to the transverse optical phonon (TO), and the longitudinal optical phonon (LO) is vis- ible. As the growing time accumulates, the LO phonon gets close to the bulk InP. It means that the stress in the nanowires is being released. The full width at half max- imum of the TO phonon and the LO phonon gets smal- ler from 2 to 5 min of growing time and gets larger after 5 min. It indicates that the quality of the crystal in InP nanowires gets better at the beginning of the nanowire growth and falls down as the growing time increases to a certain level. At the original growing stage, crystal mis- match exists between the InP nanowires and the Si sub- strate and the crystal quality is low. As the growing time accumulates, stress is released and the quality rises. Along with the growth, the diameter of the nanowires increases and the disfigurement on the sidewall accumu- lates; as a result, the crystal quality falls down. Conclusions The catalyst-free MOCVD growth of the InP nanowires on Si (100) substrates is demonstrated as a dynamic process. The density of the nanowires is increasing as time accumulates. The catalyst fabricated in the nano- wire growing process is caused by two reasons. One is the temperature region for indium droplet fabrication and InP nanowire growth matched together. The other