The Role of Symmetry Breaking on the Optical Transitions in Lead-Salt Quantum Dots

Abstract

Lead (II) oxide powder (PbO, 99.999%), Bis (trimethylsilyl) sulfide (TMS), Oleic acid (OA, technical grade 90%), 1-Octadecene (ODE, technical grade 90%), and anhydrous Toluene were purchased from Sigma-Aldrich. Distilled Acetone was purchased from Caledon Lab. Bistrimethylsilylselenide (TMSe>97%) was purchased from Gelest. ODE was degassed by pre-pumping at 80oC for 16 hours. QDs synthesis:The QDs were synthesized and purified using standard air-free techniques in two-neck flask equipped with magnetic stirrer, thermocouple, and heating mantle.Lead oleate stock solution (SS) used for PbS and PbSe synthesis was prepared by pumping the mixture of 4.0 mmol of PbO (0.9 g), 9.5mmol of OA (2.67 g) and 18.8 mmol of ODE (4.73 g) at 100oC for 16 hours. The sulfur or selenium precursors were prepared in a Nitrogen-filled Glove box. A mixture of 4.5 mL of SS and 15 mL of ODE was pre-pumped at 100 0 C for 30 min for each synthesis. The sulfur or selenium precursors were quickly injected at a certain temperature. Than the heater then was turned off without removing the heating mantel and the reaction flask was let to cool down slowly.

Ultrafast Supercontinuum Spectroscopy of Carrier Multiplication and Biexcitonic Effects in Excited States of PbS Quantum Dots

“…The narrow size distribution of QD samples allows the clear identification of the set of optical absorption peaks. Our measurements evidence the first and second absorption peaks in spectral position reported in literature [30,31]. Some samples also show the presence of a weak absorption peak in-between.…”

“…Following the energy-level calculation presented in literature and based on the four-band k·p envelope model [35] with account of band anisotropy [36] or wave function anisotropy [30] we attribute the second peak to the one-photon allowed 1P h -1P e transition [37]. The minima inbetween is associated to the one-photon absorption (1PA) forbidden 1S h,e -1P h,e transitions [33].…”

“…Some samples also show the presence of a weak absorption peak in-between. While the first exciton peak is traditionally associated to the 1S h -1S e transition, the identification of the second peak in linear absorption in PbS QDs has been the subject of debate recently [30,[32][33][34].…”

“…In particular the isotropic model cannot explain the 1PA transition among the S and P levels because of the wavefunction parity, without inclusion of the energy band anisotropy. Recent work [30] pointed that symmetry breaking in the wave function can explain the 1PA peaks at the 1S h,e -1P h,e transitions as well as two-photon absorption peaks at the 1S h -1S e and 1P h -1P e transitions.…”

Two-Photon Absorption Spectrum of a Single Crystal Cyanine-like Dye

“…6−8 The search for large 2PA materials spans from bulk solid-state materials (e.g., semiconductors 9 ), to engineered nanostructured materials (e.g., semiconductor quantum dots 10 ), and to organic molecules. 11,12 Organic molecules possess the advantage of greater degrees of freedom in custom material design and fabrication.…”