Revealing the Band-Edge Exciton Fine Structure of Single InP Nanocrystals.

We investigate the fundamental optical properties of single zinc-blende InP/ZnSe/ZnS nanocrystals (NCs) using frequency- and time-resolved magneto-photoluminescence spectroscopy. At liquid helium temperature, highly resolved spectral fingerprints are obtained and identified as the recombination lines of the three lowest states of the band-edge exciton fine structure. The evolutions of the photoluminescence spectra and decays under magnetic fields show evidence for a ground dark exciton level 0L with zero angular momentum projection along the NC main elongation axis. It lies 300 to 600 µeV below the ±1L bright exciton doublet, which is finely split by the NC shape anisotropy. These spectroscopic findings are well reproduced with a model of exciton fine structure accounting for shape anisotropy of the InP core. Our spectral fingerprints are extremely sensitive to the NC morphologies and unveil highly uniform shapes with prolate deviations of less than 3% from perfect sphericity.

Exciton-Phonon and Trion-Phonon Couplings Revealed by Photoluminescence Spectroscopy of Single CsPbBr3 Perovskite Nanocrystals.

Lead halide perovskite nanocrystals (NCs) have outstanding photoluminescence (PL) properties and excellent potential for light-emitting diodes and single-photon sources. Here, we report the multiple-peak structures originating from excitons, trions, and biexcitons in low-temperature PL spectra of single CsPbBr3 NCs. We found fine-structure splitting in the PL peaks of bright excitons and biexcitons and also in the longitudinal-optical (LO)-phonon replicas of excitons. LO-phonon replicas of trions are clearly observed under strong photoexcitation, which do not show fine-structure splitting. From size-dependent analyses of these replicas, we clarified that both exciton-phonon and trion-phonon couplings become larger for smaller NCs and the coupling strengths of trions are larger than those of excitons in large NCs. These behaviors can be explained by the spatial distributions of the electron and hole wave functions in the NCs. Our findings provide essential information on electron-phonon couplings in perovskites and for the design of high-purity single-photon sources.

Luminescence Fine Structures in Single Lead Halide Perovskite Nanocrystals: Size Dependence of the Exciton-Phonon Coupling.

Lead halide perovskite nanocrystals (NCs) have superior photoluminescence (PL) properties, such as high PL quantum yields and wide PL wavelength tunability, for optoelectronic applications. Here, we report the PL spectra of single formamidinium lead halide perovskite FAPbX3 (X = Br, I) NCs examined by single-dot spectroscopy at low temperature. We found four PL peaks in the low-energy region below the strong exciton PL peak that originate from two longitudinal-optical (LO) phonon replicas of the exciton PL, biexcitons, and charged excitons (trions). The binding energies of the biexcitons and trions become larger as the NCs decrease in size. The LO phonon energies show no size dependence, but the Huang-Rhys factors, which reflect the strength of the exciton-phonon coupling, become larger for smaller NCs. Our findings provide important insights into the exciton properties of perovskite NCs.

Effect of A-Site Cation on Photoluminescence Spectra of Single Lead Bromide Perovskite Nanocrystals.

Lead halide perovskite (APbX3) nanocrystals exhibit photoluminescence (PL) with both wide wavelength tunability and high quantum efficiency. While the Pb-X6 octahedra mainly determines the near-band-edge optical properties and the A-site cation affects the structural stability, the role of the A-site cation in determining the optical properties is still unclear. Here, we report the PL properties of three types of lead bromide perovskite APbBr3 nanocrystals with different cations [A = HC(NH2)2+, CH3NH3+, and Cs+], as revealed by single-dot spectroscopy, and discuss the influence of the A-site cation on the PL spectrum. The nanocrystal size dependences of the PL energy and lifetime show no large variation with the species of the A-site cation. We find that the size of the A-site cation determines the coupling strength between electrons and longitudinal-optical phonons in the nanocrystal and thus affects the PL spectral shape, especially the low-energy tail.

Deciphering Charging Status, Absolute Quantum Efficiency, and Absorption Cross Section of Multicarrier States in Single Colloidal Quantum Dots.

Upon photo- or electrical-excitation, colloidal quantum dots (QDs) are often found in multicarrier states due to multiphoton absorption, photocharging, or imbalanced carrier injection of the QDs. While many of these multicarrier states are observed in single-dot spectroscopy, their properties are not well studied due to random charging/discharging, emission intensity intermittency, and uncontrolled surface defects of single QDs. Here we report in situ deciphering of the charging status, precisely assessing the absorption cross section, and determining the absolute emission quantum yield of monoexciton and biexciton states for neutral, positively charged, and negatively charged single core/shell CdSe/CdS QDs. We uncover very different photon statistics of the three charge states in single QDs and unambiguously identify their charge signs together with the information on their photoluminescence decay dynamics. We then show their distinct photoluminescence saturation behaviors and evaluate the absolute values of absorption cross sections and quantum efficiencies of monoexcitons and biexcitons. We demonstrate that the addition of an extra hole or electron in a QD not only changes its emission properties but also varies its absorption cross section.

Effect of Interfacial Alloying versus "Volume Scaling" on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots.

Auger recombination is a nonradiative three-particle process wherein the electron-hole recombination energy dissipates as a kinetic energy of a third carrier. Auger decay is enhanced in quantum-dot (QD) forms of semiconductor materials compared to their bulk counterparts. Because this process is detrimental to many prospective applications of the QDs, the development of effective approaches for suppressing Auger recombination has been an important goal in the QD field. One such approach involves "smoothing" of the confinement potential, which suppresses the intraband transition involved in the dissipation of the electron-hole recombination energy. The present study evaluates the effect of increasing "smoothness" of the confinement potential on Auger decay employing a series of CdSe/CdS-based QDs wherein the core and the shell are separated by an intermediate layer of a CdSexS1-x alloy comprised of 1-5 sublayers with a radially tuned composition. As inferred from single-dot measurements, use of the five-step grading scheme allows for strong suppression of Auger decay for both biexcitons and charged excitons. Further, due to nearly identical emissivities of neutral and charged excitons, these QDs exhibit an interesting phenomenon of lifetime blinking for which random fluctuations of a photoluminescence lifetime occur for a nearly constant emission intensity.

Thick-Shell CuInS2/ZnS Quantum Dots with Suppressed "Blinking" and Narrow Single-Particle Emission Line Widths.

Quantum dots (QDs) of ternary I-III-VI2 compounds such as CuInS2 and CuInSe2 have been actively investigated as heavy-metal-free alternatives to cadmium- and lead-containing semiconductor nanomaterials. One serious limitation of these nanostructures, however, is a large photoluminescence (PL) line width (typically >300 meV), the origin of which is still not fully understood. It remains even unclear whether the observed broadening results from considerable sample heterogeneities (due, e.g., to size polydispersity) or is an unavoidable intrinsic property of individual QDs. Here, we answer this question by conducting single-particle measurements on a new type of CuInS2 (CIS) QDs with an especially thick ZnS shell. These QDs show a greatly enhanced photostability compared to core-only or thin-shell samples and, importantly, exhibit a strongly suppressed PL blinking at the single-dot level. Spectrally resolved measurements reveal that the single-dot, room-temperature PL line width is much narrower (down to ∼60 meV) than that of the ensemble samples. To explain this distinction, we invoke a model wherein PL from CIS QDs arises from radiative recombination of a delocalized band-edge electron and a localized hole residing on a Cu-related defect and also account for the effects of electron-hole Coulomb coupling. We show that random positioning of the emitting center in the QD can lead to more than 300 meV variation in the PL energy, which represents at least one of the reasons for large PL broadening of the ensemble samples. These results suggest that in addition to narrowing size dispersion, future efforts on tightening the emission spectra of these QDs might also attempt decreasing the "positional" heterogeneity of the emitting centers.

Excitation-Dependent Photoluminescence from Single-Carbon Dots.

Carbon dots (CDs) are carbon-based fluorescent nanoparticles that can exhibit excitation-dependent photoluminescence (PL) "tunable" throughout the entire visible range, interesting for optoelectronic and imaging applications. The mechanism underlying this tunable emission remains largely debated, most prominently being ascribed to dot-to-dot variations that ultimately lead to excitation-dependent ensemble properties. Here, single-dot spectroscopy is used to elucidate the origin of the excitation-dependent PL of CDs. It is demonstrated that already single CDs exhibit excitation-dependent PL spectra, similar to those of the CD ensemble. The single dots, produced by a facile one-step synthesis from chloroform and diethylamine, exhibit emission spectra with several characteristic peaks differing in emission peak position and spectral width and shape, indicating the presence of distinct emission sites on the CDs. Based on previous work, these emission sites are related to the sp2 subregions in the carbon core, as well as the functional groups on the surface. These results confirm that it is possible to integrate and engineer different types of electronic transitions at the nanoscale on a single CD, making these CDs even more versatile than organic dyes or inorganic quantum dots and opening up new routes toward light-emission engineering.

Crystallographic and Photophysical Analysis on Facet-Controlled Defect-Free Blue-Emitting Quantum Dots.

The burgeoning demand for commercializing self-luminescing quantum dot (QD) light-emitting diodes (LEDs) has stimulated extensive research into environmentally friendly and efficient QD materials. Hydrofluoric acid (HF) additive improves photoluminescence (PL) properties of blue-emitting ZnSeTe QDs, ultimately reaching a remarkable quantum yield (QY) of 97% with an ultranarrow peak width of 14 nm after sufficient HF addition. The improvement in optical properties of the QDs is accompanied by a morphology change of the particles, forming cubic-shaped defect-free ZnSeTe QDs characterized by a zinc blende (ZB) crystal structure. This treatment improves the QD-emitting properties by facilitating facet-specific growth, selectively exposing stabilized (100) facets, and reducing the lattice disorders. The facet-specific growth process gives rise to defect-free monodispersed cubic dots that exhibit remarkably narrow and homogeneous PL spectra. Meticulous time-resolved spectroscopic studies allow an understanding of the correlation between ZnSeTe QDs' particle shape and performance following HF addition. These investigations shed light on the intricacies of the growth mechanism and the factors influencing the PL efficiency of the resulting QDs. The findings significantly contribute to understanding the role of HF treatment in tailoring the optical properties of ZnSeTe QDs, thereby bringing it closer to the realization of highly efficient and bright QD-LEDs for various practical applications.

Size Dependence of Trion and Biexciton Binding Energies in Lead Halide Perovskite Nanocrystals.

Lead halide perovskite nanocrystals (NCs) have attracted much attention as light-source materials for light-emitting diodes, lasers, and quantum light emitters. The luminescence properties of perovskite NCs and the performance of NC-based light-source devices depend on trion and biexciton dynamics. Here, we examined the size dependence of trion and biexciton binding energies by conducting low-temperature single-dot spectroscopy on three different perovskite NCs: CsPbBr3, CsPbI3, and FAPbBr3. While the photoluminescence spectral widths of the all-inorganic CsPbBr3 and CsPbI3 NCs were narrow, compared with those of the organic-inorganic hybrid FAPbBr3 NCs, the binding energies of trions and biexcitons of all three samples showed similar size dependences, independent of the A-site cation and halogen. The effective-mass approximation calculations implied the importance of dynamical dielectric screening on the formation of trions and biexcitons.

Band-Edge Energy Levels of Dynamic Excitons in Cube-Shaped CdSe/CdS Core/Shell Nanocrystals.

An electron-hole pair in a cube-shaped CdSe/CdS core/shell nanocrystal exists in the form of dynamic excitons across the strongly and weakly confined regimes under ambient temperatures. Photochemical doping is applied to distinguish the band-edge electron and hole levels, confirming an effective mass model with universal constants. Reduction of the optical bandgap upon epitaxy of the CdS shells is caused by lowering the band-edge electron level and barely affecting the band-edge hole level. Similar shifts of the electron levels, yet retaining the hole levels, can switch the order in energy of the three lowest-energy transitions. Thermal distribution of 1-4 electrons among the two thermally accessible electron levels follows number-counting statistics, instead of Fermi-Dirac distribution.

Shape-Tuned Multiphoton-Emitting InP Nanotetrapods.

As the properties of a semiconductor material depend on the fate of the excitons, manipulating exciton behavior is the primary objective of nanomaterials. Although nanocrystals exhibit unusual excitonic characteristics owing to strong spatial confinement, studying the interactions between excitons in a single nanoparticle remains challenging due to the rapidly vanishing multiexciton species. Here, a platform for exciton tailoring using a straightforward strategy of shape-tuning of single-crystalline nanocrystals is presented. Spectroscopic and theoretical studies reveal a systematic transition of exciton confinement orientation from 3D to 2D, which is solely tuned by the geometric shape of material. Such a precise shape-effect triggers a multiphoton emission in single nanotetrapods with arms longer than the exciton Bohr radius of material. In consequence, the unique interplay between the multiple quantum states allows a geometric modulation of the quantum-confined Stark effect and nanocrystal memory effect in single nanotetrapods. These results provide a useful metric in designing nanomaterials for future photonic applications.

Unraveling the Emission Pathways in Copper Indium Sulfide Quantum Dots.

Semiconductor copper indium sulfide quantum dots are emerging as promising alternatives to cadmium- and lead-based chalcogenides in solar cells, luminescent solar concentrators, and deep-tissue bioimaging due to their inherently lower toxicity and outstanding photoluminescence properties. However, the nature of their emission pathways remains a subject of debate. Using low-temperature single quantum dot spectroscopy on core-shell copper indium sulfide nanocrystals, we observe two subpopulations of particles with distinct spectral features. The first class shows sharp resolution-limited emission lines that are attributed to zero-phonon recombination lines of a long-lived band-edge exciton. Such emission results from the perfect passivation of the copper indium sulfide core by the zinc sulfide shell and points to an inversion in the band-edge hole levels. The second class exhibits ultrabroad spectra regardless of the temperature, which is a signature of the extrinsic self-trapping of the hole assisted by defects in imperfectly passivated quantum dots.

Single-Nanocrystal Photoluminescence Spectroscopy Studies of Plasmon-Multiexciton Interactions at Low Temperature.

Using thick-shell or "giant" CdSe/CdS nanocrystal quantum dots (g-NQDs), characterized by strongly suppressed Auger recombination, we studied the influence of plasmonic interactions on multiexciton emission. Specifically, we assessed the separate effects of plasmonic absorption and plasmonic emission enhancement by a systematic analysis of the pump fluence dependence of low-temperature photoluminescence (low-T PL) derived from individual CdSe/CdS g-NQDs deposited on nanoroughened silver films. Our study reveals that (1) the multiexciton (MX) emissions in g-NQD coupled to silver films were enhanced not only through the creation of more excitons via enhancement of absorption but also through the direct modification of the competition between the radiative and nonradiative recombination processes of MXs; (2) strong enhancement in absorption is not necessary for strong multiexciton emission; and (3) the emission of MXs can become stronger with the increase of multiexciton order. We also exploited the strong enhancement of MX emission to perform second-order photon correlation and cross-correlation experiments using very low pump fluences and observed a strong photon bunching that decays with increasing pump fluence.