Ultrafast carrier dynamics, band-gap renormalization, and optical properties of ZnSe nanowires

In this paper, we present a comprehensive study of the carrier dynamics and optical properties of ZnSe nanowires (NWs). The transparency of the sample, obtained by the growth of the ZnSe NWs on glass, allowed us to perform transmittance, reflectance, photoluminescence (PL), time-resolved PL, and pump-probe transient absorption spectroscopy on as-grown samples. All measurements were performed at room temperature. Strong light trapping at the band-gap energy has been observed in reflectivity measurements. Fast transient absorption bleaching due to band filling and band-gap renormalization has been observed. The band-gap renormalization has a rise time constant of about 170 fs and a decay time of about 4 ps. Fast transient absorption bleaching is also observed at energies below the band gap, suggesting that intrinsic processes prevail over extrinsic photoinduced transitions in our high-quality NWs. The PL reveals the presence at room temperature of excitonic emission that shows a decay time of 0.5 ns. All of these features indicate that our ZnSe NWs have quality comparable to epitaxial films and can be used for optical devices and nonlinear optics.

Figure 1

(a) Transmittance (black curve) and reflectance (red dashed curve) of an as-grown ZnSe NW sample. (b) Plot of ${\left(A\left(h\upsilon \right)·h\upsilon \right)}^2$, as extracted from the transmission and reflectivity curve shown in (a). The red dotted line is the linear fit of the near band-gap absorption curve. The intercept value to zero absorption gives the band-gap value (see text). The substrate is transparent in the investigated energy range (not shown).

Figure 2

(a) Two-dimensional color map of the absorbance difference $\mathrm{\Delta }A$ as a function of probe energy ($x$ axis) and of the delay time between pump and probe ($y$ axis). Notice the logarithmic scale of the $y$ axis. (b) Spectral dependence of $\mathrm{\Delta }A$ for different delays, up to 0.6 ps. In this region, the absolute value of $\mathrm{\Delta }A$ increases for increasing delays. (c) Same as (b) but for delay times from 0.6 to 880 ps. In this time region, the absolute value of $\mathrm{\Delta }A$ decreases as the time delay increases. All data shown in the figure were taken at a fluence of $60\mu \mathrm{J}/\mathrm{c}{\mathrm{m}}^2$.

Figure 3

(a) Time dependence of $\mathrm{\Delta }A$ at different probe energies. (b) Detail relative to the time dependence of $\mathrm{\Delta }A$ for $E=2.627\mathrm{eV}$ (energy of the narrow dip, see Fig. 2). The time scale is limited to 2 ps. (c) Spectral dependence of the decay time of $\mathrm{\Delta }A$.

Figure 4

Energy position of the narrow dip observed in the $\mathrm{\Delta }A$ measurements (see Fig. 2) as a function of the pump-probe delay.

Figure 5

(a) Time dependence of $\mathrm{\Delta }A$ at the probe wavelength of 550 nm for two values of the pump fluence. At high fluence, a sizeable positive signal is observed at $t>5\mathrm{ps}$. (b) Time dependence of $\mathrm{\Delta }A$ for different probe energies for $150\mu \mathrm{J}/\mathrm{c}{\mathrm{m}}^2$ of pump power. Notice the logarithmic scale of the time axis.

Figure 6

(a) The PL spectra at two different excitation intensities. (b) Comparison between absorption and luminescence spectra.

Figure 7

(a) The NBE emission. (b) Time dependence of the PL at the peak energy (2.68 eV).