Ultrafast Carrier Dynamics in Semiconductor Nanowires

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

“…By tuning the probe photon energy through the MoS 2 band gap (both indirect and direct), our UOM measurements show that conduction and valence band states are rapidly populated on a sub-picosecond (ps) time scale in a MoS 2 monolayer after photoexcitation at 3.1 eV, consistent with previous work [14][15][16][17][18] . Pump fluence-dependent measurements reveal that subsequent carrier relaxation in our samples is primarily due to surface-related defects and trap states, not the Auger processes observed in previous measurements on MoS 2 and other semiconductor nanosystems 12,[18][19][20][21][22][23] . We also observed an increase in the carrier relaxation time with an increase in the number of MoS 2 layers, likely due to the well known layer-dependent changes in the electronic structure 16,24 .…”

“…3(b), which shows that τ increases by a factor of two and the maximum value of Δ T/T increases by more than a factor of three over the measured fluence range, saturating at the highest fluences (likely due to state filling near the band edge). This dependence of τ on F is the opposite of what one would expect for interband Auger recombination, a three-carrier non-radiative process that often dominates carrier relaxation at high fluence in semiconductor nanostructures [19][20][21][22][23]27 . This is also inconsistent with Auger-induced capture into defect states, as described in refs.…”

“…1(a)). In our system, a 100 kHz regeneratively amplified laser producing 50 femtosecond (fs), 10 μ J pulses at 800 nm is used to pump a visible optical parametric amplifier (OPA) 21 . The signal wavelength from the OPA was tuned to generate linearly polarized probe pulses (with polarization perpendicular to the pump pulses) from 550 nm (2.24 eV) to 700 nm (1.77 eV), covering the bandgap (both indirect and direct) of MoS 2 , as well as the well-known A and B exciton transitions 16,26 .…”

“…The pump and probe spot diameters were 63 μ m and 30 μ m, respectively, which fully covered a single MoS 2 flake without overlapping other flakes. The normalized photoinduced change in transmission (Δ T/T), which is directly proportional to the photoexcited carrier population at a given point in time and space 15,21,27 , is measured at the detector. Real time monitoring of both focused spots and the sample position using a CCD camera with a 50X objective and accompanying beam expander allows us to selectively measure ultrafast carrier dynamics in individual nanomaterials, removing the detrimental influence of inhomogeneous broadening 12,13 .…”

(Invited) III-Nitride Nanowires: Emerging Materials for Lighting and Energy Applications

“…Transmission electron microscopy showed the nanowires are singlecrystalline, free of threading dislocations, and have triangular cross-sections. The high degree of vertical alignment is most likely the result of the crystallographic match between the [11][12][13][14][15][16][17][18][19][20] oriented nanowires and the r-plane sapphire surface. Uniform, dense, aligned growth could be achieved over wafer-scale areas by controlling the Ni thickness and growth conditions precisely, as shown in Figure 1.…”