Tuning electronic states of a CdSe/ZnS quantum dot by only one functional dye molecule

Evidence of carrier diffusion between emission states in CdSe/ZnS core-shell quantum dots: a comprehensive investigation combining fluorescence lifetime correlation spectroscopy (FLCS) and single dot photoluminescence studies

Investigation of carrier dynamics in CdSe/ZnS core-shell quantum dots (QDs) is performed using fluorescence-lifetime-correlation-spectroscopy (FLCS) and single-dot PL blinking studies. The origin of an emitted photon from a QD in an FLCS study is assigned to either an exciton state or trap state based on its excited state lifetime (τfl). Subsequently, two intrastate autocorrelation functions (ACFs) representing the exciton and trap states and one cross-correlation function (CCF) coupling these two states are constructed. Interestingly, the timescales of carrier diffusion (τR) show striking similarities across all three correlation functions, which further correlate with τR of the conventional FCS. However, ACFs notably deviate from the CCF in their μs progression patterns, with the latter showing growth, whereas the former ones display decay. This implies inter-state carrier diffusions leading to the QD blinking. Further study of single particle PL blinking on a surface-immobilized QD indicates shallow trap states near the band edge cause the blinking at low excitation power, while trion recombination becomes an additional contributing factor at higher pump power. Overall, the results highlight not only an excellent correlation between these two techniques but also the potential of our approach for achieving an accurate and comprehensive understanding of carrier dynamics in CdSe/ZnS QDs.

Ultrafast dynamics and ultrasensitive single particle spectroscopy of optically robust core/alloy shell semiconductor quantum dots

A "one-pot one-step" synthesis method of Core/Alloy Shell (CAS) quantum dots (QDs) offers the scope of large scale synthesis in a less time consuming, more economical, highly reproducible and high-throughput manner in comparison to "multi-pot multi-step" synthesis for Core/Shell (CS) QDs. Rapid initial nucleation, and smooth & uniform shell growth lead to the formation of a compositionally-gradient alloyed hetero-structure with very significantly reduced interfacial trap density in CAS QDs. Thus, interfacial strain gets reduced in a much smoother manner leading to enhanced confinement for the photo-generated charge carriers in CAS QDs. Convincing proof of alloy-shelling for a CAS QD has been provided from HRTEM images at the single particle level. The band gap could be tuned as a function of composition, temperature, reactivity difference of precursors, etc. and a high PLQY and improved photochemical stability could be achieved for a small sized CAS QD. From the ultrafast exciton dynamics in CdSe and InP CAS QDs, it has been shown that (a) the hot exciton thermalization/relaxation happens in <500 fs, (b) hot electron trapping dynamics occurs within a ∼1 ps time scale, (c) band edge exciton trapping occurs within a 10-25 ps timescale and (d) for CdSe CAS QDs the hot hole gets trapped in about 35 ps. From fast PL decay dynamics, it has been shown that the amplitude of the intermediate time constant can be correlated with the PLQY. A model has been provided to understand these ultrafast to fast exciton dynamical processes. At the ultrasensitive single particle level, unlike CS QDs, CdSe CAS QDs have been shown to exhibit (a) constancy of PL_max (i.e. no bluing) and (b) constancy of PL intensity (i.e. no bleaching) of the single CAS QDs for continuous irradiation for one hour under an air atmosphere. Thus, CAS QDs hold the promise of being a superior optical probe in comparison to CS QDs both at the ensemble and at the single particle level, leading to enhanced flexibility of the CAS QDs towards designing and developing next generation application devices.

Excited-State Charge Transfer and Extended Charge Separation within Covalently Tethered Type-II CdSe/CdTe Quantum Dot Heterostructures: Colloidal and Multilayered Systems

We used N,N'-dicyclohexylcarbodiimide (DCC) coupling chemistry to synthesize (1) heterostructures of CdSe and CdTe quantum dots (QDs) in colloidal dispersions and (2) heterostructures of CdSe and CdTe QDs, as well as CdS and CdSe QDs, immobilized on metal oxide thin films. The DCC-mediated formation of amide bonds between terminal carboxylic acid and amine groups of ligands on different QDs drove the formation of heterostructures. This cross-linking mechanism selectively yields heterostructures and prohibits the undesired formation of homostructures consisting of just one type of QD. Products of adsorption, ligand-exchange, and covalent-coupling reactions were characterized by transmission electron microscopy and ATR-FTIR, ¹H NMR, electronic absorption, steady-state emission, and time-resolved emission spectroscopy. Ground-state absorption spectra of constituent QDs were unperturbed upon incorporation into heterostructures, enabling control over electronic properties. Heterostructures of CdSe and CdTe QDs exhibit type-II interfacial energetic offsets that promote charge separation following excitation of either QD. Indeed, photoexcited CdTe QDs transferred electrons to CdSe, and photoexcited CdSe QDs transferred holes to CdTe, on time scales of 10-100 ns, as evidenced by dynamic quenching of band-edge and trap-state emission. Mixed dispersions of noninteracting QDs did not undergo excited-state charge transfer. Constructing heterostructures on TiO₂ thin films introduced an additional charge-transfer pathway, electron transfer from QDs to TiO₂, which occurred on subnanosecond time scales and enabled extended spatial separation of photogenerated electrons and holes. Our results reveal that carbodiimide coupling chemistry can be used to tether colloidal QDs selectively and covalently to each other, yielding dispersed or immobilized heterostructures with programmable compositions and energetic offsets that can undergo efficient excited-state interfacial electron transfer.

Hybrid Complexes of Photosensitizers with Luminescent Nanoparticles: Design of the Structure

Increasing the efficiency of the photodynamic action of the dyes used in photodynamic therapy is crucial in the field of modern biomedicine. There are two main approaches used to increase the efficiency of photosensitizers. The first one is targeted delivery to the object of photodynamic action, while the second one is increasing the absorption capacity of the molecule. Both approaches can be implemented by producing dye-nanoparticle conjugates. In this review, we focus on the features of the latter approach, when nanoparticles act as a light-harvesting agent and nonradiatively transfer the electronic excitation energy to a photosensitizer molecule. We will consider the hybrid photosensitizer-quantum dot complexes with energy transfer occurring according to the inductive-resonance mechanism as an example. The principle consisting in optimizing the design of hybrid complexes is proposed after an analysis of the published data; the parameters affecting the efficiency of energy transfer and the generation of reactive oxygen species in such systems are described.

Observation of ordered organic capping ligands on semiconducting quantum dots via powder X-ray diffraction

Powder X-ray diffraction is one of the key techniques used to characterize the inorganic structure of colloidal nanocrystals. The comparatively low scattering factor of nuclei of the organic capping ligands and their propensity to be disordered has led investigators to typically consider them effectively invisible to this technique. In this report, we demonstrate that a commonly observed powder X-ray diffraction peak around [Formula: see text] observed in many small, colloidal quantum dots can be assigned to well-ordered aliphatic ligands bound to and capping the nanocrystals. This conclusion differs from a variety of explanations ascribed by previous sources, the majority of which propose an excess of organic material. Additionally, we demonstrate that the observed ligand peak is a sensitive probe of ligand shell ordering. Changes as a function of ligand length, geometry, and temperature can all be readily observed by X-ray diffraction and manipulated to achieve desired outcomes for the final colloidal system.

Self-Assembly of Semiconductor Quantum Dots using Organic Templates

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are regarded as brightly photoluminescent nanomaterials possessing outstanding photophysical properties, such as high photodurability and tunable absorption and emission wavelengths. Therefore, QDs have great potential for a wide range of applications, such as in photoluminescent materials, biosensors and photovoltaic devices. Since the development of synthetic methods for accessing high-quality QDs with uniform morphology and size, various types of QDs have been designed and synthesized, and their photophysical properties dispersed in solutions and at the single QD level have been reported in detail. In contrast to dispersed QDs, the photophysical properties of assembled QDs have not been revealed, although the structures of the self-assemblies are closely related to the device performance of the solid-state QDs. Therefore, creating and controlling the self-assembly of QDs into well-defined nanostructures is crucial but remains challenging. In this Minireview, we discuss the notable examples of assembled QDs such as dimers, trimers and extended QD assemblies achieved using organic templates. This Minireview should facilitate future advancements in materials science related to the assembled QDs.

Unsaturated Ligands Seed an Order to Disorder Transition in Mixed Ligand Shells of CdSe/CdS Quantum Dots

A phase transition within the ligand shell of core/shell quantum dots is studied in the prototypical system of colloidal CdSe/CdS quantum dots with a ligand shell composed of bound oleate (OA) and octadecylphosphonate (ODPA). The ligand shell composition is tuned using a ligand exchange procedure and quantified through proton NMR spectroscopy. Temperature-dependent photoluminescence spectroscopy reveals a signature of a phase transition within the organic ligand shell. Surprisingly, the ligand order to disorder phase transition triggers an abrupt increase in the photoluminescence quantum yield (PLQY) and full-width at half-maximum (FWHM) with increasing temperature. The temperature and width of the phase transition show a clear dependence on ligand shell composition, such that QDs with higher ODPA fractions have sharper phase transitions that occur at higher temperatures. In order to gain a molecular understanding of the changes in ligand ordering, Fourier transform infrared and vibrational sum frequency generation spectroscopies are performed. These measurements confirm that an order/disorder transition in the ligand shell tracks with the photoluminescence changes that accompany the ligand phase transition. The phase transition is simulated through a lattice model that suggests that the ligand shell is well-mixed and does not have completely segregated domains of OA and ODPA. Furthermore, we show that the unsaturated chains of OA seed disorder within the ligand shell.

Photoinduced Reversible Modulation of Fluorescence of CdSe/ZnS Quantum Dots in Solutions with Diarylethenes

Steady-state absorption and fluorescence spectra, fluorescence decay kinetics of CdSe/ZnS quantum dots (QD) with photochromic diarylethenes (DAE) in toluene have been studied. Two kinds of QDs emitting at 525 and 600 nm were investigated and DAE were selected to ensure good overlap of their photoinduced absorption band with QDs emission spectra. It has been found that photochromic molecules form complexes with QD which results in partial fluorescence quenching. A reversible modulation of QDs emission intensity which correlates with magnitude of transient photoinduced absorption band of the diarylethenes during photochromic transformations has been demonstrated.

Two-Component Analysis of Photoluminescence Bands for Semiconductor Quantum Dots in Solutions

We present the quantitative analysis of the photoluminescence (PL) obtained for semiconductor TOPO-capped CdSe/ZnS QDs in solutions at 77–293[Formula: see text]K. The PL bands are approximated more accurately when assuming the superposition of at least two Gaussian components differing considerably in the linewidth (FWHM) and having different nature.

Temperature Dependence of Photoluminescence for Spin-Coated Semiconductor Quantum Dots and Quantum Dot-Dye Nanoassemblies on Quartz Substrate

The attachment of pyridyl substituted porphyrin molecule to the surface of CdSe/ZnS quantum dots in solutions is realized in the competition with capping ligand TOPO molecules resulting in the specific change of photoluminescence for the quantum dots across the temperature range of 77–290[Formula: see text]K. We have shown that fixation of alone quantum dots or quantum dot-porphyrin nanoassemblies on quartz substrate changes significantly temperature dependence of photoluminescence. In contrast to the samples in a glass-forming solution no phase transition of the TOPO capping layer was observed upon removal of the capping layer.

Quantum efficiency of the photo-induced electronic transfer in dye-TiO2 complexes

We present a method based on a time-dependent self-consistent density functional tight-binding (TD-DFTB) approach, able to predict the quantum efficiency of the photoinjection process in a dye-TiO₂ complex from a fully atomistic picture. We studied the process of charge transfer of three systems with different dyes: catechol (CAT), alizarin (ALZ) and FSD101. Each system was excited with lasers of different energies in the range of 300-2500 nm, studying the efficiency of the induced charge transfer process at the incident energies. We show that the perturbation can produce either hole transfer or electron transfer from the dye to the nanoparticle, therefore affecting the efficiency of the charge transfer in the solar cell when illuminated by broadband radiation.

The nature of non-FRET photoluminescence quenching in nanoassemblies from semiconductor quantum dots and dye molecules

Nanoassemblies formed via self-assembly based on colloidal CdSe quantum dots (QDs) and porphyrin (H2P) dye molecules show Fluorescence Resonant Energy Transfer (FRET) and non-FRET quenching of QD photoluminescence (PL). We present a procedure to unravel and quantify these two relaxation pathways via dynamic and static PL quenching experiments. Accordingly, FRET amounts at maximum to 10% of the total quenching efficiency. Since the degree of ligand coverage is inhomogeneously distributed across the QD ensemble PL quantum yields vary broadly. The attachment of H2P molecules occurs preferentially to those QDs with low ligand coverage. Along with that, nanoassembly formation deviates strongly from Poisson statistics. Like FRET, non-FRET depends on the QD size. We assign non-FRET quenching to the formation of specific new Cd2+ trap states following depletion of several ligands by the spacious dye molecules. While FRET follows quantitatively the Förster model, non-FRET appears on time scales of 1-3 ns in new and enhanced non-radiative near-band-edge QD PL decay channels caused by a trapping of the electrons in long-lived intra-gap states which then manifests itself in a subsequent weak PL emission. We assign the related intra-band emission to a recombination of deep-trap electrons and shallow-trap holes.

Evaluation of Neuroimaging Findings of Central Nervous System Complications in Heart Transplant Recipients

OBJECTIVES

In this study, we presented neuroradiologic findings and diagnoses of neurologic complications in a series of heart transplant recipients.

MATERIALS AND METHODS

A retrospective review was conducted at Başkent University Hospital. We searched the hospital and radiology databases and identified 109 heart transplant recipients. Thirty-one of these recipients had neuroradiologic evaluations secondary to presentation of neurologic symptoms after heart transplant, with 18 patients evaluated with computed tomography and 22 patients evaluated with magnetic resonance imaging (overlap of imaging-defined groups occurred in 9 recipients). Computed tomography and magnetic resonance imaging studies were retrieved from the Picture Archiving and Communication System, with each type of imaging retrospectively evaluated on consensus by 2 radiologists.

RESULTS

Radiopathologic findings related to symptoms were detected in 12 of the 31 study patients. The most common abnormality was posterior reversible leukoencephalopathy syndrome (5 patients, 4.6%). The other abnormalities were ischemic stroke (3 patients, 2.8%), hemorrhagic stroke (1 patient, 0.9%), intracranial abscess (2 patients, 1.8%), and intracranial dissemination of sinusoidal fungal infection and related hemorrhagic infarct (1 patient, 0.9%). The other 19 heart transplant recipients who underwent computed tomography and/or magnetic resonance imaging for neurologic complaints showed no neuroradiologic findings related to neurologic symptoms.

CONCLUSIONS

Posterior reversible leukoencephalopathy syndrome and ischemic stroke were the most common neurologic complications in our heart transplant recipients. The other complications were hemorrhagic stroke, intracranial abscess, and intracranial dissemination of sinusoidal fungal infection. Neurologic complications are common in heart transplant recipients and should be identified promptly for early treatment. For the recognition of these complications, computed tomography should be performed for initial evaluation to rule out edema or hemorrhage. However, in the presence of serious neurologic symptoms that cannot be explained by computed tomography, magnetic resonance imaging should be indicated.

Application of semiconductor quantum dots in bioimaging and biosensing

In this review we present new concepts and recent progress in the application of semiconductor quantum dots (QD) as labels in two important areas of biology, bioimaging and biosensing.

Super-Resolution Optical Fluctuation Bio-Imaging with Dual-Color Carbon Nanodots

Success in super-resolution imaging relies on a proper choice of fluorescent probes. Here, we suggest novel easily produced and biocompatible nanoparticles-carbon nanodots-for super-resolution optical fluctuation bioimaging (SOFI). The particles revealed an intrinsic dual-color fluorescence, which corresponds to two subpopulations of particles of different electric charges. The neutral nanoparticles localize to cellular nuclei suggesting their potential use as an inexpensive, easily produced nucleus-specific label. The single particle study revealed that the carbon nanodots possess a unique hybrid combination of fluorescence properties exhibiting characteristics of both dye molecules and semiconductor nanocrystals. The results suggest that charge trapping and redistribution on the surface of the particles triggers their transitions between emissive and dark states. These findings open up new possibilities for the utilization of carbon nanodots in the various super-resolution microscopy methods based on stochastic optical switching.