Size-Dependent Optical Properties of Colloidal PbS Quantum Dots

Abstract

We quantitatively investigate the size-dependent optical properties of colloidal PbS nanocrystals or quantum dots (Qdots) by combining-the Qdot absorbance spectra with detailed elemental analysis of the Qdot suspensions. At high energies, the molar extinction coefficient epsilon increases With the Not volume d(3) and agrees with theoretical calculations using the Maxwell-Garnett effective medium theory and bulk values for the Qdot dielectric function. This demonstrates that quantum confinement has no influence on E in this spectral range, and it provides an accurate method to calculate the Qdot concentration. Around the band gap, epsilon only increases with d(1.3), and values are comparable to the epsilon of PbSe Qdots. The data are related to the oscillator strength f(if) of the band gap transition and results agree well with theoretical tight-binding calculations, predicting a linear dependence of f(if) on d. For both PbS and PbSe Qdots, the exciton lifetime tau is calculated from f(if). We find values ranging between 1 and 3 mu s, in agreement with experimental literature data from time-resolved luminescence spectroscopy. Our results provide a thorough general framework to calculate and understand the optical properties of suspended colloidal quantum dots. Most importantly, it highlights the significance of the local field factor in these systems

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Sideband pump-probe technique resolves nonlinear modulation response of PbS/CdS quantum dots on a silicon nitride waveguide

“…[1][2][3][4] This tunability, their long coherence lifetime, 5,6 and the relatively low cost and scalability of the wet-chemical production, i.e., vacuum-free deposition methods such as spincoating, make colloidal QDs an attractive choice as active medium in photonic applications. 7,8 Based on a variety of different material systems, light emitting devices and lasers have been fabricated using colloidal nanocrystals, recently even on chip.…”

“…3 An important branch of photonics is the all-optical signal amplification and modulation. Opto-electronics is currently largely based on solid-state semiconductor sources using III-V semiconductors, for which exciton lifetimes are much shorter than the data transmission rates.…”

“…An important aspect that needs consideration is the long lifetime of the exciton ground state in colloidal QDs, exceeding this time by up to three orders of magnitude. 3 A fast data manipulation mechanism therefore needs to be based on an efficient nonlinear process, 18,19 or the ground state (GS) lifetime has to be dramatically reduced. Colloidal QDs display large and size-tunable nonlinearities, [20][21][22] of which, in particular, the efficient exciton multiplication has received considerable attention due to its potential impact on QD-based solar cells.…”

“…3(c). The absorption cross section at the pump wavelength can be calculated from the intrinsic absorption coefficient 3 and is about 2 × 10 15 cm 2 for our dots at 900 nm. Using the formalism proposed in Ref.…”