Search

Transient absorption (TA) spectroscopy of semiconductor nanocrystals (NCs) is often used for excited state population analysis, but recent results suggest that TA bleach signals associated with multiexcitons in NCs do not scale linearly with exciton multiplicity. In this manuscript, we probe the factors that determine the intensities and spectral positions of exciton and biexciton components in the TA spectra of CdSe quantum dots (QDs) of five diameters. We find that, in all cases, the peak intensity of the biexciton TA spectrum is less than 1.5 times that of the single exciton TA spectrum, in stark contrast to a commonly made assumption that this ratio is 2. The relative intensities of the biexciton and exciton TA signals at each wavelength are determined by at least two factors: the TA spectral intensity and the spectral offset between the two signals. We do not observe correlations between either of these factors and the particle diameter, but we find that both are strongly impacted by replacing the native organic surface-capping ligands with a hole-trapping ligand. These results suggest that surface trapping plays an important role in determining the absolute intensities of TA features for CdSe QDs and not just their decay kinetics. Our work highlights the role of spectral offsets and the importance of surface trapping in governing absolute TA intensities. It also conclusively demonstrates that the biexciton TA spectra of CdSe QDs at the band gap energy are less than twice as intense as those of the exciton. [ABSTRACT FROM AUTHOR]

We present here a theory of Majorana excitons, photo-excited conduction electron-valence band hole pairs, interacting with Majorana Fermions in a Kitaev chain of semiconductor quantum dots embedded in a nanowire. Using analytical tools and exact diagonalization methods, we identify the presence of Majorana zero modes in the nanowire absorption spectra. [ABSTRACT FROM AUTHOR]

Fluorescence properties of quantum dots (QDs) are critically affected by their redox states, which is important for practical applications. In this study, we investigated the optical properties of MoSe2-metallic phase quantum-dots (MoSe2-mQDs) depending on the pH variation, in which the MoSe2-mQDs were dispersed in water with two sizes (Φ~3 nm and 12 nm). The larger MoSe2-mQDs exhibited a large red-shift and broadening of photoluminescence (PL) peak with a constant UV absorption spectra as varying the pH, while the smaller ones showed a small red-shift and peak broadening, but discrete absorption bands in the acidic solution. The excitation wavelength-dependent photoluminescence shows that the PL properties of smaller MoSe2-mQDs are more sensitive to the pH change compared to those of larger ones. From the time-resolved PL spectroscopy, the excitons dominantly decaying with an energy of ~3 eV in pH 2 clearly show the shift of PL peak to the lower energy (~2.6 eV) as the pH increases to 7 and 11 in the smaller MoSe2-mQDs. On the other hand, in the larger MoSe2-mQDs, the exciton decay is less sensitive to the redox states compared to those of the smaller ones. This result shows that the pH variation is more critical to the change of photophysical properties than the size effect in MoSe2-mQDs. [ABSTRACT FROM AUTHOR]

The temperature-driven plasmon-exciton coupling in thermoresponsive dextran-graft-PNIPAM/Au nanoparticle/CdTe quantum dot (D-g-PNIPAM/Au NPs/CdTe QDs) hybrid nanosystem was studied. A significant (0.84 eV) splitting of the absorption peak was observed in the absorption spectrum of the nanosystem, which reflects the fact of formation of plexcitons, occurring due to strong plasmon-exciton coupling. An increasing with time plasmonic enhancement of the photoluminescence of CdTe QDs was revealed, as a result of the penetration of quantum dots into the volume of the D-g-PNIPAM/Au NP hybrid nanosystem and bonding to it. The heating–cooling cycle of the aqueous solution of the studied nanosystem leads to a reversible quenching-recovery alteration of the QD photoluminescence. The quenching was rationalized as a result of an increased probability of nonradiative resonance energy transfer (RET) from CdTe QDs to Au NPs, which occurs due to shortening of the NP-QD distance, caused by shrinking of the macromolecule due to cooling-induced lower critical solution temperature phase transition. Increasing the NP-QD distance in the heating stage recovers the QD PL intensity. The observed effect opens up opportunities for the controlled reversible temperature-driven tuning of the photoluminescence intensity of D-g-PNIPAM/Au NP/CdTe QD nanosystem, which is highly important for its potential use in photonics and biomedical applications. [ABSTRACT FROM AUTHOR]

Low‐dimensional metal halide perovskites have been widely regarded due to their unique crystal structure and photophysical properties. Herein, blue‐emission CsPbBr3 quantum dots (QDs) or nanocrystals (NCs) with extremely small crystal size (1–2 nm) but high photoluminescence quantum yield (PLQY, 72.4%) are synthesized with a new surface ligand, single (6‐amino‐6‐deoxy) beta cyclodextrin (6A‐βCD), which also acts as the confined‐growth template for such small QDs. Detailed photoluminescence spectrum and femtosecond transient absorption spectrum study proves that the blue emission with multiple peaks originates from both band‐edge radiation and exciton recombination. Furthermore, as a result of the small size, exciton localization in the form of self‐trapped excitons (STEs) and significant quantum size confinement effect are both important reasons for the efficient and wide blue emissions of these QDs. This work provides a new strategy not only to synthesize ultrasmall perovskite QDs but also to develop blue perovskite‐QD light‐emitting devices. [ABSTRACT FROM AUTHOR]

Blue and green light emitting CsPbBr 3 and CsPb(Cl/Br) 3 perovskite quantum dots glasses (QDGs) have been fabricated in multi-component borate glass matrices by melt quenching method and following heat treatment. Absorption spectra show regular blue shift of first exciton absorption peak from CsPbBr 3 to CsPb(Cl/Br) 3 QDGs. Photoluminescence spectra show that CsPbBr 3 and CsPb(Cl/Br) 3 QDGs have adjustable emission in the wavelengths from 528 to 442 nm. The full width at half maximum (FWHM) is 15–17 nm and 21–31 nm for CsPbBr 3 and CsPb(Cl/Br) 3 QDGs respectively. Emission map and photoluminescence excitation spectra show broad band excitation characteristics of the exciton emission. Photoluminescence decay shows bi-exponential function for both CsPbBr 3 and CsPb(Cl/Br) 3 QDGs. As the ratio of Cl/Br increases from CsPbBr 3 to CsPb(Cl/Br) 3 QDGs, the short and long lifetime components roughly decrease from 9.7 to 4.0 ns and from 87.8 to 55.2 ns respectively, and the average lifetime decreases from 72.2 to 36.5 ns. Photoluminescence quantum yield changes from 82.8% to 5.7%. Temperature dependence of PL spectra in the range of 28–120 °C shows that integrated intensity obviously decreases with increasing temperature, but the emission peak wavelength and FWHM almost do not change. • Absorption spectra show blue-shift for CsPbBr 3 and CsPb(Cl/Br) 3 QDGs with different Cl/Br ratios. • Photoluminescence spectra show blue shift from 528 to 442 nm for CsPbBr 3 and CsPb(Cl/Br) 3 QDGs. • Narrow FWHM is 15–17 nm and 21–31 nm for CsPbBr 3 and CsPb(Cl/Br) 3 QDGs respectively. • With increasing Cl/Br ratio, short lifetime component and PLQY decrease with the same rule. • With increasing temperature, integrated intensity decreases, but emission peak and FWHM do not change. [ABSTRACT FROM AUTHOR]