Quantum-Confined and Enhanced Optical Absorption of Colloidal PbS Quantum Dots at Wavelengths with Expected Bulk Behavior

Abstract

Nowadays it is well-accepted to attribute bulk-like optical absorption properties to colloidal PbS quantum dots (QDs) at wavelengths above 400 nm. This assumption permits to describe PbS QD light absorption by using bulk optical constants and to determine QD concentration in colloidal solutions from simple spectrophotometric measurements. Here we demonstrate that PbS QDs experience the quantum confinement regime across the entire near UV−vis−NIR spectral range, therefore also between 350 and 400 nm already proposed to be sufficiently far above the band gap to suppress quantum confinement. This effect is particularly relevant for small PbS QDs (with diameter of ≤4 nm) leading to absorption coefficients that largely differ from bulk values (up to ∼40% less). As a result of the broadband quantum confinement and of the high surface-to-volume ratio peculiar of nanocrystals, suitable surface chemical modification of PbS QDs is exploited to achieve a marked, size-dependent enhancement of the absorption coefficients compared to bulk values (up to ∼250%). We provide empirical relations to determine the absorption coefficients at 400 nm of as-synthesized and ligand-exchanged PbS QDs, accounting for the broadband quantum confinement and suggesting a heuristic approach to qualitatively predict the ligand effects on the optical absorption properties of PbS QDs. Our findings go beyond formalisms derived from Maxwell Garnett effective medium theory to describe QD optical properties and permit to spectrophotometrically calculate the concentration of PbS QD solutions avoiding underestimation due to deviations from the bulk. In perspective, we envisage the use of extended π-conjugated ligands bearing electronically active substituents to enhance light-harvesting in QD solids and suggest the inadequacy of the representation of ligands at the QD surface as mere electric dipoles.

Semiconductor quantum dots reveal dipolar coupling from exciton to ligand vibration

“…In this context, most of the studies focus on the influence of ligands on the optoelectronic properties of NCs [30][31][32][33][34][35][36][37] . In particular, somespecific surface modifications with organic ligands proved to enhance light absorption and fluorescence emission [38][39][40] , which is a clear advantage for photovoltaic, photodetection and photoemission applications. It has also been shown that ligands condition the quantum confinement and the phonon behaviour of NCs, thus affecting exciton dynamics and transport phenomena [41][42][43] .…”

Synthesis of ultra-narrow PbTe nanorods with extremely strong quantum confinement

“…In addition, Murphy et al reported the large anisotropy inherent to the bandstructure of PbTe (a B ∼ 12 nm, a B⊥ ∼ 152 nm), which is most pronounced among the lead chalcogenide systems, compared to 66 nm for PbSe and 23.5 nm for PbS [29]. Therefore, our synthesized PbTe NRs with different lengths may present quantum-confinement effects in three dimensions and reveal how their anisotropic properties affect the optoelectronic properties of PbTe NCs [41]. In the near infrared region, the first excitonic absorption peak of PbTe NRs red-shifts from 1182 to 1713 nm as the average diameter of corresponding NRs increases from 2.8 to 5.4 nm (Fig.…”

The size controlled synthesis of Cu2S/P25 hetero junction solar-energy-materials and their applications in photocatalytic degradation of dyes

“…Quantum connement of carriers and enhanced surface effects at nanodimensions are well known to result in large modications in the structural, optical, and electronic properties of nanoparticles. 38,39 Balamurugan B. has reported the sizedependent changes in Cu 2 O nanoparticles, result in conductivity type inversion on reduction of nanoparticle size. [40][41][42] So it is necessarily to know how and what is the extent the quantum connement of Cu 2 S effect on the visible light catalytic performance.…”