Transformation of Colloidal Quantum Dot: From Intraband Transition to Localized Surface Plasmon Resonance

Abstract

An increase in the carrier density of semiconductor nanocrystals can gradually change the origin of the optical property from the excitonic transition to the localized surface plasmon resonances. Here, we present the evolution of the electronic transition of self-doped Ag2Se colloidal quantum dots, from the intraband transition to the localized surface plasmon resonances along with a splitting of the intraband transition (1Pe–1Se). The minimum fwhm of the split intraband transition is only 23.7 meV, which is exceptionally narrow compared to that of metal oxide nanocrystals showing LSPRs, inferring that the electron–electron scattering is significantly suppressed due to the smaller carrier density. The splitting of the intraband transition mainly results from the asymmetrical crystal structure of the tetragonal Ag2Se CQDs and becomes distinct when the nanocrystal changes its crystal structure from the cubic to tetragonal structure. Maximizing the discrete energy levels in the quantum dot along with mixing with plasmonic characters may provide opportunities to fully harness merits of both the quantum confinement effect and localized surface plasmon resonance characters.

Colloidal Alloyed Quantum Dots with Enhanced Photoluminescence Quantum Yield in the NIR-II Window

“…Taking Ag 2 Se as an example, it has a narrow direct band gap of 0.15 eV for bulk materials and shows great PL properties in the NIR-II window, being a promising alternative as whole NIR-II emitting nanoprobes. However, in comparison with visible-emission QDs, many of which can be up to near-unity PLQY, the NIR-II QDs often exhibit poor fluorescence intensity. − In the case of Ag 2 Se, the high mobility of the Ag ion gives rise to abundant cation vacancies and crystal defects, leading to a low absolute PLQY below 1%. One of the alternative strategies to improve the optical properties of QDs is the epitaxial growth of shell semiconductor materials to construct a core–shell structure, especially with a wider band gap compared to that of the core QDs (type I core–shell QDs) to gain vast enhancement in the fluorescence intensity and stability of QDs.…”

Pb‐Doped Ag2Se Quantum Dots with Enhanced Photoluminescence in the NIR‐II Window

“…[ 4,12 ] In particular, recent developed NIR‐II Ag 2 X (X = S, Se, Te) QDs have been reported for their excellent optical properties and biocompatibility. [ 13–18 ] Among them, Ag 2 Se is a typical direct semiconductor with narrow bandgap of 0.15 eV, [ 19 ] promising it as an ideal nanoprobe with emission in whole NIR‐II window according to the size‐dependent emission of QDs. [ 20 ] Nevertheless, it suffers from the abundant cation vacancies and crystal defects caused by high Ag + mobility, resulting low PLQY.…”

Beyond the Bandgap Photoluminescence of Colloidal Semiconductor Nanocrystals

“…Temperature. The intraband PL peak does not significantly shift in frequency with temperature variation in the case of the Ag 2 Se or HgSe/CdSe QDs, 29,31 whereas reduction in the emission bandwidth has been observed by decreasing the temperature. Also, the integrated PL intensities of the various HgSe/CdX (X = S, Se, Te) core/shell structures do not differ on a large scale across the temperatures.…”

“…Furthermore, with an increase of the carrier density, the electronic transition of selfdoped CQDs transforms from the excitonic transition to the localized surface plasmon resonances (LSPR) along with peak splitting. 31 Interestingly, an exceptionally narrow fwhm of 23.7 meV for the split intraband absorption peak has been observed in Ag 2 Se CQDs, which is unprecedented considering the bandwidth of LSPR for the metal oxide nanocrystals. 32 The degree of intraband transition splitting gradually increases with an increase of the size of the nanocrystal and continues to redshift, which results from the quantum confinement effect mixed with the LSPR character.…”