Calibrating and controlling the quantum efficiency distribution of inhomogeneously broadened quantum rods by using a mirror ball

Biexciton Blinking in CdSe-Based Quantum Dots

Experiments on single colloidal quantum dots (QDs) have revealed temporal fluctuations in the emission efficiency of the single-exciton state. These fluctuations, often termed "blinking", are caused by opening/closing of charge-carrier traps and/or charging/discharging of the QD. In the regime of strong optical excitation, multiexciton states are formed. The emission efficiencies of multiexcitons are lower because of Auger processes, but a quantitative characterization is challenging. Here, we quantify fluctuations of the biexciton efficiency for single CdSe/CdS/ZnS core-shell QDs. We find that the biexciton efficiency "blinks" significantly. The additional electron due to charging of a QD accelerates Auger recombination by a factor of 2 compared to the neutral biexciton, while opening/closing of a charge-carrier trap leads to an increase of the nonradiative recombination rate by a factor of 4. To understand the fast rate of trap-assisted recombination, we propose a revised model for trap-assisted recombination based on reversible trapping. Finally, we discuss the implications of biexciton blinking for lasing applications.

Nanophotonic Approach to Study Excited-State Dynamics in Semiconductor Nanocrystals

In semiconductor nanocrystals, excited electrons relax through multiple radiative and nonradiative pathways. This complexity complicates characterization of their decay processes with standard time- and temperature-dependent photoluminescence studies. Here, we exploit a simple nanophotonic approach to augment such measurements and to address open questions related to nanocrystal emission. We place nanocrystals at different distances from a gold reflector to affect radiative rates through variations in the local density of optical states. We apply this approach to spherical CdSe-based nanocrystals to probe the radiative efficiency and polarization properties of the lowest dark and bright excitons by analyzing temperature-dependent emission dynamics. For CdSe-based nanoplatelets, we identify the charge-carrier trapping mechanism responsible for strongly delayed emission. Our method, when combined with careful modeling of the influence of the nanophotonic environment on the relaxation dynamics, offers a versatile strategy to disentangle the complex excited-state decay pathways present in fluorescent nanocrystals as well as other emitters.

On the choice of proper average lifetime formula for an ensemble of emitters showing non-single exponential photoluminescence decay

In this paper, we investigate non-single exponential photoluminescence decays in various disordered condensed-matter systems. For such materials, two formulas for the average lifetime of system's excited state are commonly used in the analysis of experimental data. In many cases, the choice of formula is arbitrary and lacks a clear physical justification. For this reason, our main goal is to show that the choice of correct mathematical formula should be based on the interpretation of measured photoluminescence decay curve. It is shown that depending on the investigated system, after appropriate normalization, photoluminescence decay curve can represent either a survival probability function or a probability density function of lifetime and for this reason two different formulas for the average lifetime are required. It is also shown that, depending on luminescence quantum yield, some information on the probability density function of lifetime can be lost in the process of measurement, which results in underestimated values of average lifetime. Finally, we provide an interpretation of total decay rate distributions which are frequently obtained by phenomenological modeling of non-single exponential photoluminescence decays.

Investigation of Magnetic Circular Dichroism Spectra of Semiconductor Quantum Rods and Quantum Dot-in-Rods

Anisotropic quantum nanostructures have attracted a lot of attention due to their unique properties and a range of potential applications. Magnetic circular dichroism (MCD) spectra of semiconductor CdSe/ZnS Quantum Rods and CdSe/CdS Dot-in-Rods have been studied. Positions of four electronic transitions were determined by data fitting. MCD spectra were analyzed in the A and B terms, which characterize the splitting and mixing of states. Effective values of A and B terms were determined for each transition. A relatively high value of the B term is noted, which is most likely associated with the anisotropy of quantum rods.

High Internal Emission Efficiency of Silicon Nanoparticles Emitting in the Visible Range

Light-emitting silicon nanoparticles (Si-NPs) are interesting for lighting applications due to their nontoxicity, chemical robustness, and photostability; however, they are not practically considered due to their low emission efficiencies. While large Si-NPs emitting in the red to infrared spectral region show ensemble emission quantum efficiencies up to 60%, the emission efficiencies of smaller Si-NPs, emitting in the visible spectral range, are far lower, typically below 10-20%. In this work, we test this efficiency limit by measuring for the first time the internal quantum efficiency (IQE), i.e., the higher bound of the emission quantum efficiency, considering only the emissive NPs within the ensemble, of Si-NPs emitting in the visible spectral range between 350 and 650 nm. On the basis of photoluminescence decay measurements in a Drexhage geometry, we show that Si-NPs with organic passivation (C:Si-NPs) can have high direct-bandgap-like radiative rates, which enable a high IQE over ∼50%. In this way, we demonstrate that Si-NPs can in principle be considered a competitive candidate as a phosphor in lighting applications and medical imaging also in the visible spectral range. Moreover, our findings show that the reason for the much lower ensemble emission efficiency is due to the fact that the ensemble consists of a low fraction of emissive NPs, most likely due to a low PL "blinking" duty cycle.

Tuning Single Quantum Dot Emission with a Micromirror

The photoluminescence of single quantum dots fluctuates between bright (on) and dark (off) states, also termed fluorescence intermittency or blinking. This blinking limits the performance of quantum dot-based devices such as light-emitting diodes and solar cells. However, the origins of the blinking remain unresolved. Here, we use a movable gold micromirror to determine both the quantum yield of the bright state and the orientation of the excited state dipole of single quantum dots. We observe that the quantum yield of the bright state is close to unity for these single QDs. Furthermore, we also study the effect of a micromirror on blinking, and then evaluate excitation efficiency, biexciton quantum yield, and detection efficiency. The mirror does not modify the off-time statistics, but it does change the density of optical states available to the quantum dot and hence the on times. The duration of the on times can be lengthened due to an increase in the radiative recombination rate.

Determining the internal quantum efficiency of shallow-implanted nitrogen-vacancy defects in bulk diamond

It is generally accepted that nitrogen-vacancy (NV) defects in bulk diamond are bright sources of luminescence. However, the exact value of their internal quantum efficiency (IQE) has not been measured so far. Here we use an implementation of Drexhage's scheme to quantify the IQE of shallow-implanted NV defects in a single-crystal bulk diamond. Using a spherical metallic mirror with a large radius of curvature compared to the optical spot size, we perform calibrated modifications of the local density of states around NV defects and observe the change of their total decay rate, which is further used for IQE quantification. We also show that at the excitation wavelength of 532 nm, photo-induced relaxation cannot be neglected even at moderate excitation powers well below the saturation level. For NV defects shallow implanted 4.5 ± 1 and 8 ± 2 nm below the diamond surface, we determine the quantum efficiency to be 0.70 ± 0.07 and 0.82 ± 0.08, respectively.

Drexhage's Experiment for Sound

Drexhage's seminal observation that spontaneous emission rates of fluorophores vary with distance from a mirror uncovered the fundamental notion that a source's environment determines radiative linewidths and shifts. Further, this observation established a powerful tool to determine fluorescence quantum yields. We present the direct analogue for sound. We demonstrate that a Chinese gong at a hard wall experiences radiative corrections to linewidth and line shift, and extract its intrinsic radiation efficiency. Beyond acoustics, our experiment opens new ideas to extend the Drexhage experiment to metamaterials, nanoantennas, and multipolar transitions.

Non-blinking single-photon emitters in silica

Samples for single-emitter spectroscopy are usually prepared by spin-coating a dilute solution of emitters on a microscope cover slip of silicate based glass (such as quartz). Here, we show that both borosilicate glass and quartz contain intrinsic defect colour centres that fluoresce when excited at 532 nm. In a microscope image the defect emission is indistinguishable from spin-coated emitters. The emission spectrum is characterised by multiple peaks with the main peak between 2.05 and 2.20 eV, most likely due to coupling to a silica vibration with an energy that varies between 160 and 180 meV. The defects are single-photon emitters, do not blink, and have photoluminescence lifetimes of a few nanoseconds. Photoluminescence from such defects may previously have been misinterpreted as originating from single nanocrystal quantum dots.

The local density of optical states of a metasurface

While metamaterials are often desirable for near-field functions, such as perfect lensing, or cloaking, they are often quantified by their response to plane waves from the far field. Here, we present a theoretical analysis of the local density of states near lattices of discrete magnetic scatterers, i.e., the response to near field excitation by a point source. Based on a pointdipole theory using Ewald summation and an array scanning method, we can swiftly and semi-analytically evaluate the local density of states (LDOS) for magnetoelectric point sources in front of an infinite two-dimensional (2D) lattice composed of arbitrary magnetoelectric dipole scatterers. The method takes into account radiation damping as well as all retarded electrodynamic interactions in a self-consistent manner. We show that a lattice of magnetic scatterers evidences characteristic Drexhage oscillations. However, the oscillations are phase shifted relative to the electrically scattering lattice consistent with the difference expected for reflection off homogeneous magnetic respectively electric mirrors. Furthermore, we identify in which source-surface separation regimes the metasurface may be treated as a homogeneous interface, and in which homogenization fails. A strong frequency and in-plane position dependence of the LDOS close to the lattice reveals coupling to guided modes supported by the lattice.

CdSe/ZnS quantum dots as sensors for the local refractive index

We explore the potential of CdSe/ZnS colloidal quantum dots (QDs) as probes for their immediate dielectric environment, based on the influence of the local refractive index on the fluorescence dynamics of these nanoemitters. We first compare ensembles of quantum dots in homogeneous solutions with single quantum dots dispersed on various dielectric substrates, which allows us to test the viability of a conceptual framework based on a hard-sphere region-of-influence and the Bruggeman effective-medium approach. We find that all our measurements can be integrated into a coherent description, provided that the conceptualized point-dipole emitter is positioned at a distance from the substrate that corresponds to the geometry of the QD. Three theoretical models for the evolution of the fluorescence decay rate as a function of the local refractive index are compared, showing that the classical Lorentz approach (virtual cavity) is the most appropriate for describing the data. Finally, we use the observed sensitivity of the QDs to their environment to estimate the detection limit, expressed as the minimum number of traceable streptavidin molecules, of a potential QD-nanosensor based on fluorescence lifetime.

Microcantilever based distance control between a probe and a surface

We demonstrate a method to control the distance between a custom probe and a sample on a μm to nm scale. The method relies on the closed-loop feedback on the angular deflection of an in-contact AFM microcantilever. High performance in stability and accuracy is achieved in this method by taking advantage of the small mechanical feedback path between surface and probe. We describe how internal error sources that find their origin in the microcantilever and feedback can be minimized to achieve an accurate and precise control up to 3 nm. In particular, we investigated how hysteresis effects in the feedback caused by friction forces between tip and substrate can be minimized. By applying a short calibration procedure, distance control from contact to several micrometers probe-sample distance can be obtained with an absolute nanometer-scale accuracy. The method presented is compatible with any probe that can be fixed on a microcantilever chip and can be easily built into existing AFM systems.

Lanthanide-doped CaS and SrS luminescent nanocrystals: a single-source precursor approach for doping

The incorporation of dopants with optical or magnetic functionalities into colloidal nanocrystals (NCs) has been a longstanding challenge for nanomaterial research. A deeper understanding of the doping kinetics will aid a better control of the doping process. In particular, alkaline-earth sulfides are an important class of host materials for a range of luminescent dopants, including transition-metal and lanthanide ions. Their nanocrystalline analogues have many potential applications. However, the lack of synthetic methodologies hampers their development. Here we introduce a single-source precursor approach that successfully leads to Ce(3+)- and Eu(2+)-doped CaS and SrS luminescent NCs with diameters of ∼10 nm and with luminescent properties similar to those of the bulk analogues. The characteristic absorption and luminescence of Ce(3+) and Eu(2+) depend on the local coordination and are applied to probe dopant ion internalization. We demonstrate that controlling the reactivity of the precursors is crucial for achieving effective doping. By designing the chemical structure of the dopant precursor to vary the reactivity relative to that of the host precursor, the doping efficiency can be controlled. In addition, we have applied a growth doping strategy to further improve internalization of the dopants. Finally, we demonstrate nucleation doping as an alternative method to achieve lanthanide NC doping for dopant and host precursors with strongly different reactivities. The single-source precursor approaches proposed here allow for a flexible design of synthesis strategies and have the potential to be widely applicable to the doping of colloidal chalcogenide NCs with transition-metal and lanthanide dopant ions.

Reduced Auger recombination in single CdSe/CdS nanorods by one-dimensional electron delocalization

Progress to reduce nonradiative Auger decay in colloidal nanocrystals has recently been made by growing thick shells. However, the physics of Auger suppression is not yet fully understood. Here, we examine the dynamics and spectral characteristics of single CdSe-dot-in-CdS-rod nanocrystals. These exhibit blinking due to charging/discharging, as well as trap-related blinking. We show that one-dimensional electron delocalization into the rod-shaped shell can be as effective as a thick spherical shell at reducing Auger recombination of the negative trion state.