Air-Stable PbSe/PbS and PbSe/PbSe_xS_1-x Core−Shell Nanocrystal Quantum Dots and Their Applications

Abstract

The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.

PbSe/CdSe Thin-Shell Colloidal Quantum Dots

“…Other unique properties of IV-VI nanostructures are small effective masses [7] of electrons and holes, large dielectric constants [8], and a relatively large effective exciton Bohr radius [4]. The above properties make IV-VI CQDs indispensable in such emerging applications as CQD gain devices [9,10], biological markers [11][12][13], photovoltaic cells (PVCs) [14][15][16][17][18][19][20], Q-switches [21,22], and thermoelectric devices [23][24][25]. For example, due to their broad-band absorption profile, light harvesting in IV-VI CQD-based PVCs is extremely large, resulting in power conversion efficiency of up to 8% [14][15][16][17][18][19][20].…”

Nanoheterostructure Cation Exchange: Anionic Framework Conservation

“…The exciton absorption of the PbSe seed is red-shifted (by 0.1 and 0.25 eV for larger and smaller seed, respectively) from the expected value for the corresponding bare PbSe, 25 consistent with embedment in a PbS matrix. 21,26,27 The rods also show seed sizedependent NIR emission.…”

Charge Transport in Cu2S Nanocrystals Arrays: Effects of Crystallite Size and Ligand Length

“…Different approaches are practiced for overcoming the insulating nature of the ligands. The long organic capping molecules are commonly replaced by shorter organic or inorganic ligands [10][11][12], or may be decomposed via device thermal annealing [13][14][15]. These processes increase the device conductance by decreasing the distance between neighboring NCs and therefore reducing the inter-particle tunneling barrier.…”