Intersubband Relaxation in CdSe Colloidal Quantum Wells

Abstract

The dynamics of intersubband relaxation are critical to quantum well technologies such as quantum cascade lasers and quantum well infrared photodetectors. Here, intersubband relaxation in CdSe colloidal quantum wells, or nanoplatelets, is studied via pump–push–probe transient spectroscopy. An initial interband pump pulse is followed by a secondary infrared push excitation, resonant with intersubband absorption, which promotes electrons from the first conduction band of the quantum well to the second conduction band. A probe pulse monitors subsequent electron cooling to the band edge of the quantum well. Using this technique, intersubband relaxation is studied as a function of critical variables such as colloidal quantum well size and thickness, surface ligand chemistry, temperature, and excitation pulse intensity. Larger quantum well sizes, judicious selection of surface ligand chemistry (e.g., thiolates), low temperatures, and elevated push pulse fluences slow intersubband relaxation. However, compared to resonant intraband relaxation in colloidal quantum dots (up to hundreds of picoseconds), intersubband relaxation in colloidal quantum wells is rapid (<1 ps) under all examined conditions. These experiments indicate that rapid relaxation is driven by both LO phonon and surface scattering. The short time scale of relaxation observed in these materials may hinder intersubband technologies such as mid-infrared detectors, although such rapid relaxation may prove valuable in optical switching.

2D II–VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

“…371 Three pulse pump−push−probe experiments, in which a secondary "push" pulse is resonant with photoinduced intersubband transitions, revealed a slowing of electron cooling at 5 K (350 fs) compared to 300 K (700 fs) for one sample, as well as ligand-dependent cooling that was faster for phosphonates (350 fs) than thiolates (800 fs). 329 Intraband relaxation in NPLs is slowed at higher excitation intensities. For example, measurements of optical pumping followed by a photoelectron probe from Sippel et al shown in Figure 28 indicate that the kinetic energy of liberated electrons falls from an effective temperature of 3000 K to ∼650 K in 1 ps, but this is followed by a much slower fall to ambient temperature.…”

“…112 In contrast to CdSe QD, where intraband relaxation times are reported to be in the tens of picoseconds, 328 NPL intersubband relaxation is subpicosecond. 329 3.4. Linear and Circular Polarization of Optical Properties.…”

“…In addition to thickness and temperature, the energy of the observed intersubband transitions depends, like interband transitions, on the ligand chemistry of the NPL . In contrast to CdSe QD, where intraband relaxation times are reported to be in the tens of picoseconds, NPL intersubband relaxation is subpicosecond …”

Transient reshaping of intraband transitions by hot electrons

“…51 An additional possibility for the faster cooling of thinner CQWs is that surface phonon modes (not just LO phonons) play an important role in cooling, which was also suggested previously. 55,56 The line-shape of the intersubband transition also reveals fluence-dependence of the dynamics. The cooling times observed here under low fluence excitation are much faster than those reported at higher fluence.…”

Quantum Nature of THz Conductivity: Excitons, Charges, and Trions in 2D Semiconductor Nanoplatelets and Implications for THz Imaging and Solar Hydrogen Generation

“…As the effective mass also shifts the energetic levels, resonances of heavier particles densify in the low-frequency regime. A rather small dephasing rate of Γ = 1.5 ps –1 at room temperature for all charged particles is used, and the sharp absorption characteristics remain visible. In contrast, the excitonic mobility is orders of magnitude smaller due to its polarization nature.…”