7 μm. In agreement with the extremely small diameter obtained, an intense room temperature PL coming from quantum-confined Si nanostructures occurs under a 488-nm excitation, as shown in Figure 5a; the PL spectrum consists of a broadband centered at about 670 nm which strongly resembles that one previously observed and reported for pure Si NWs [2, 12]. A similar PL spectrum, although less intense, was observed in shorter NWs. No Ge-related PL signals are detected in the IR region at room temperature. Figure 5 PL spectra of Si/Ge NWs. (a) Room temperature

spectrum in the visible region. (b) Spectrum in the IR region obtained at 11 K. Both spectra were obtained with a photon flux of 3.1 × 1020 cm−2 · s−1. Relevant variations of the PL spectrum are found by decreasing the temperature down to 11 K. Indeed, Decitabine ic50 Nutlin-3 in vivo the intensity of the Si-related signal strongly decreases by decreasing temperature, as previously reported in the case of pure Si NWs [12]. On the other hand, a PL signal appears in the IR region at about 1,240 nm (red squares), as shown in Figure 5b. The peak position is in agreement with literature data concerning light emission from Ge nanostructures [19–21]. It is noteworthy that the emission is enhanced by about a factor of 5 with respect to that one coming from the unetched MQW, shown in the same figure as blue squares, which suggests that stronger quantum confinement effects are operating in the NWs (where

Ge regions can be considered as nanodots) with respect to the MQW. To this end, we also underline that NWs cover only about the 50% of the sample surface, so that the actual enhancement factor of the PL intensity for Si/Ge NWs accounts for at least an order of magnitude. Although ultrathin Si/Ge NWs were already successfully synthesized [6, 14], to our knowledge, the above-reported data constitute the first evidence of simultaneous light emission

from both Si and Ge nanostructures in Si/Ge NWs. Since the properties of the Si-related PL signal observed in Si/Ge NWs tightly resemble those found in pure Si NWs [2, 12], in the rest of the work, we mainly focused our attention on the Ge-related emission. In particular, we studied in detail the IR PL emission as a function of the temperature, as reported in Figure 6a. We observed that by decreasing the temperature, click here the PL intensity monotonically increases, due to a reduced efficiency of non-radiative phenomena. Furthermore, it can be noticed that the PL emission exhibits a blueshift toward shorter wavelengths by decreasing temperature, in agreement with the well-known dependence of the Ge bandgap on temperature. Figure 6 PL properties of Si/Ge NWs as a function of temperature. (a) PL spectra in the IR region of Si/Ge NWs from 11 K to room temperature. (b) PL time-decay curves measured at 1,220 nm and at temperatures in the 11- to 80-K range. All measurements were performed with a photon flux of 3.1 × 1020 cm−2 · s−1.