Ensembles Of Nanocrystals Research Articles

Optical orientation and alignment of excitons in ensembles of inorganic perovskite nanocrystals

We demonstrate the optical orientation and alignment of excitons in a two-dimensional layer of CsPbI$_3$ perovskite nanocrystals prepared by colloidal synthesis and measure the anisotropic exchange splitting of exciton levels in the nanocrystals. From the experimental data at low temperature (2K), we obtain the average value of anisotropic splitting of bright exciton states of the order of 120{\mu}eV. Our calculations demonstrate that there is a significant contribution to the splitting due to the nanocrystal shape anisotropy for all inorganic perovskite nanocrystrals.

Size and Concentration Determination of Colloidal Nanocrystals by Small-Angle X-ray Scattering

The accurate determination of the dimensions of a nano-object is paramount to the development of nanoscience and technology. Here we provide procedures for sizing quasi-spherical colloidal nanocrystals (NCs) by means of small-angle X-ray scattering (SAXS). Using 2.5 to 10 nm PbS NCs as a model system, the protocols outline the extraction of the net NC SAXS pattern by background correction and address the calibration of scattered X-ray intensity to an absolute scale. The NC size distribution is retrieved by fitting the corrected SAXS pattern either to parametrized analytical distributions or to a distribution constructed through a Monte Carlo approach. We compare the two methods and show that they yield nearly identical estimates of the NC diameter in the case of an NC ensemble with a monodisperse and monomodal size distribution. Extending the analysis to PbSe, CdSe, and CdS NCs, we provide SAXS-calibrated sizing curves ranging from 2.5 to 7 nm, which relate the NC diameter and the NC band-gap energy as de...

Effect of Epitaxial Alignment on Electron Transport from Quasi-Two-Dimensional Iron Silicide α-FeSi2 Nanocrystals Into p-Si(001)

Self-assembled growth of α-FeSi2 nanocrystal ensembles on gold-activated and gold-free Si(001) surface by molecular beam epitaxy is reported. The microstructure and basic orientation relationship (OR) between the silicide nanocrystals and silicon substrate were analysed. The study reveals that utilisation of the gold as catalyst regulates the preferable OR of the nanocrystals with silicon and their habitus. It is shown that electron transport from α-FeSi2 phase into p-Si(001) can be tuned by the formation of (001)—or (111)—textured α-FeSi2 nanocrystals ensembles. A current-voltage characteristic of the structures with different preferable epitaxial alignment (α-FeSi2(001)/Si(100) and α-FeSi2(111)/Si(100)) shows good linearity at room temperature. However, it becomes non-linear at different temperatures for different ORs due to different Schottky barrier height governed by a particular epitaxial alignment of the α-FeSi2/p-Si interfaces.

Negligible Electronic Interaction between Photoexcited Electron–Hole Pairs and Free Electrons in Phosphorus–Boron Co-Doped Silicon Nanocrystals

Phosphorus (P) and boron (B) co-doped Si nanocrystals (NCs) have raised interest in the optoelectronic industry due to their electronic tunability, optimal carrier multiplication properties, and straightforward dispersibility in polar solvents. Yet a basic understanding of the interaction of photoexcited electron–hole (e–h) pairs with new physical features that are introduced by the co-doping process (free carriers, defect states, and surface chemistry) is missing. Here, we present the first study of the ultrafast carrier dynamics in SiO2-embedded P–B co-doped Si NC ensembles using induced absorption spectroscopy through a two-step approach. First, the induced absorption data show that the large fraction of the dopants residing on the NC surface slows down carrier relaxation dynamics within the first 20 ps relative to intrinsic (undoped) Si NCs, which we interpret as enhanced surface passivation. On longer time-scales (picosecond to nanosecond regime), we observe a speeding up of the carrier relaxation dy...

Effect of epitaxial alignment on electron transport from quasi-two-dimensional iron silicide alpha-FeSi-=SUB=-2-=/SUB=- nanocrystals into p-Si(001)

AbstractSelf-assembled growth of α-FeSi_2 nanocrystal ensembles on gold-activated and gold-free Si(001) surface by molecular beam epitaxy is reported. The microstructure and basic orientation relationship (OR) between the silicide nanocrystals and silicon substrate were analysed. The study reveals that utilisation of the gold as catalyst regulates the preferable OR of the nanocrystals with silicon and their habitus. It is shown that electron transport from α-FeSi2 phase into p-Si(001) can be tuned by the formation of (001)—or (111)—textured α-FeSi2 nanocrystals ensembles. A current-voltage characteristic of the structures with different preferable epitaxial alignment (α-FeSi_2(001)/Si(100) and α-FeSi_2(111)/Si(100)) shows good linearity at room temperature. However, it becomes non-linear at different temperatures for different ORs due to different Schottky barrier height governed by a particular epitaxial alignment of the α-FeSi_2/ p -Si interfaces.

Спиновая физика экситонов в коллоидных нанокристаллах

AbstractWe present a review of spin-dependent properties of excitons in semiconductor colloidal nanocrystals. The photoluminescences (PL) properties of neutral and charged excitons (trions) are compared. The mechanisms and the polarization of radiative recombination of a “dark” (spin-forbidden) exciton that determines the low-temperature PL of colloidal nanocrystals are discussed in detail. The radiative recombination of a dark exciton becomes possible as a result of simultaneous flips of the surface spin and electron spin in a dark exciton that leads to admixture of bright exciton states. This recombination mechanism is effective in the case of a disordered state of the spin system and is suppressed if the polaron ferromagnetic state forms. The conditions and various mechanisms of formation of the spin polaron state and possibilities of its experimental detection are discussed. The experimental and theoretical studies of magnetic field-induced circular polarization of PL in ensembles of colloidal nanocrystals are reviewed.

Determination of quantum yields of semiconductor nanocrystals at the single emitter level via fluorescence correlation spectroscopy.

Comparing the photoluminescence (PL) properties of ensembles of nanocrystals like semiconductor quantum dots (QDs) with single particle studies is of increasing interest for many applications of these materials as reporters in bioimaging studies performed under very dilute conditions or even at the single particle level. Particularly relevant is here the PL quantum yield (ΦF), which determines the signal size together with the reporter's molar extinction coefficient and is a direct measure for nanocrystal quality, especially for the inorganic surface passivation shell and its tightness, which can be correlated also with nanocrystal stability and the possible release of heavy metal ions. Exemplarily for red and green emitting CdTe nanocrystals, we present a method for the determination of ΦF of nanoparticle dispersions at ultralow concentration compared to cuvette measurements using fluorescence correlation spectroscopy (FCS), a single molecule method, and compared to molecular dyes with closely matching spectral properties and known ΦF. Our results underline the potential of this approach, provided that material-inherent limitations like ligand- and QD-specific aggregation affecting particle diffusion and QD drawbacks such as their complex and power-dependent blinking behavior are properly considered as shown here.

Dephasing and Quantum Beating of Excitons in Methylammonium Lead Iodide Perovskite Nanoplatelets

Perovskite nanocrystals have emerged as an interesting material for light-emitting and other optoelectronic applications. Excitons are known to play an important role in determining the optical properties of these nanocrystals and their energetic levels as well as quantization properties have been extensively explored. Despite this, there are still many aspects of perovskites that are still not well-known, for example, the homogeneous and inhomogeneous line widths of the energetic transitions, quantities that cannot be directly extracted by linear absorption optical spectroscopy on nanocrystal ensembles. Here, we present temperature-dependent absorption and four-wave mixing (FWM) experiments on thick methylammonium lead iodide (MAPI) perovskite nanoplatelets exhibiting bulk-like absorption and emission spectra. Dephasing times T2 of excitons are determined to lie in the range of several hundreds of femtoseconds at low temperatures. This value enables us to distinguish between the homogeneous and inhomogen...

Photoluminescence properties of silicon nanocrystals grown by nanosecond laser ablation of solid-state targets in an inert gas atmosphere

It was found that the photoluminescence intensity decay kinetics of nanocrystalline silicon layers formed by nanosecond laser ablation of crystalline silicon targets in a helium atmosphere exhibit a power-law behavior with an exponent from 0.9 to 1.5, depending on the temperature and luminescence photon energy in the range of 1.4–1.8 eV, which indicates photoexcited carrier recombination controlled by dissipative tunneling processes in silicon nanocrystal ensembles in a suboxide matrix.

Thermally stimulated exciton emission in Si nanocrystals

Increasing temperature is known to quench the excitonic emission of bulk silicon, which is due to thermally induced dissociation of excitons. Here, we demonstrate that the effect of temperature on the excitonic emission is reversed for quantum-confined silicon nanocrystals. Using laser-induced heating of silicon nanocrystals embedded in SiO2, we achieved a more than threefold (>300%) increase in the radiative (photon) emission rate. We theoretically modeled the observed enhancement in terms of the thermally stimulated effect, taking into account the massive phonon production under intense illumination. These results elucidate one more important advantage of silicon nanostructures, illustrating that their optical properties can be influenced by temperature. They also provide an important insight into the mechanisms of energy conversion and dissipation in ensembles of silicon nanocrystals in solid matrices. In practice, the radiative rate enhancement under strong continuous wave optical pumping is relevant for the possible application of silicon nanocrystals for spectral conversion layers in concentrator photovoltaics.

Ensemble and Single Particle Fluorescence Characterization of Dye-Labeled Cellulose Nanocrystals.

Cellulose nanocrystals (CNCs) have been covalently labeled with both fluorescein and rhodamine and studied by a combination of UV-vis absorption spectroscopy and ensemble and single molecule fluorescence spectroscopy. For all samples, the fluorescence anisotropy and lifetimes were consistent with effects expected for covalently bound dye molecules. Low dye loading levels (∼0.1 dye/particle) were estimated for the fluorescein-labeled CNC which coupled with the strong pH dependence make this a less suitable fluorophore for most applications. Rhodamine-labeled CNCs were prepared from both sulfated and carboxylated CNCs and had loading levels that varied from 0.25 to ∼15 dye molecules/CNC. For the sulfated samples, the absorption due to (nonfluorescent) dimeric dye increased with dye loading; in contrast, the carboxylated sample, which had the highest rhodamine content, had a low dimer yield. Single particle fluorescence studies for two of the rhodamine-labeled CNCs demonstrated that individual particles are readily detected by their stepwise blinking/bleaching behavior and by polarization effects. Overall, the results indicate the importance of understanding the effects of loading on dye photophysics to select an optimal dye concentration to maximize sensitivity while minimizing the effect of the dye on the CNC behavior. The results also demonstrate that CNCs with relatively low dye loadings (e.g., ∼1 dye/particle) are readily detectable by fluorescence and should be adequate for use in fluorescence-based biological assays or to probe the distribution of CNCs in composite materials.

Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films.

The optical properties of the newly developed near-infrared emitting formamidinium lead triiodide (FAPbI3 ) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady-state and time-resolved photoluminescence. The excitonic emission is dominant in NC ensemble because of the localization of electron-hole pairs. A promisingly high quantum yield above 70%, and a large absorption cross-section (5.2 × 10-13 cm-2 ) are measured. At high pump fluence, biexcitonic recombination is observed, featuring a slow recombination lifetime of 0.4 ns. In polycrystalline thin films, the quantum efficiency is limited by nonradiative trap-assisted recombination that turns to bimolecular at high pump fluences. From the temperature-dependent photoluminescence (PL) spectra, a phase transition is clearly observed in both NC ensemble and polycrystalline thin film. It is interesting to note that NC ensemble shows PL temperature antiquenching, in contrast to the strong PL quenching displayed by polycrystalline thin films. This difference is explained in terms of thermal activation of trapped carriers at the nanocrystal's surface, as opposed to the exciton thermal dissociation and trap-mediated recombination, which occur in thin films at higher temperatures.

Probing Homogeneous Line Broadening in CdSe Nanocrystals Using Multidimensional Electronic Spectroscopy.

The finite spectral line width of an ensemble of CdSe nanocrystals arises from size and shape inhomogeneity and the single-nanocrystal spectrum itself. This line width directly limits the performance of nanocrystal-based devices, yet most optical measurements cannot resolve the underlying contributions. We use two-dimensional electronic spectroscopy (2D ES) to measure the line width of the band-edge exciton of CdSe nanocrystals as a function of radii and surface chemistry. We find that the homogeneous width decreases for increasing nanocrystal radius and that surface chemistry plays a critical role in controlling this line width. To explore the hypothesis that unpassivated trap states serve to broaden the homogeneous line width and to explain its size-dependence, we use 3D ES to identify the spectral signatures of exciton-phonon coupling to optical and acoustic phonons. We find enhanced coupling to optical phonon modes for nanocrystals that lack electron-passivating ligands, suggesting that localized surface charges enhance exciton-phonon coupling via the Fröhlich interaction. Lastly, the data reveal that spectral diffusion contributes negligibly to the homogeneous line width on subnanosecond time scales.

Light-induced nonthermal population of optical phonons in nanocrystals

Raman spectroscopy is widely used to study bulk and nanomaterials, where information is frequently obtained from spectral line positions and intensities. In this study, we monitored the Raman spectrum of ensembles of semiconductor nanocrystals (NCs) as a function of optical excitation intensity (optical excitation experiments). We observe that in NCs the red-shift of the Raman peak position with increasing light power density is much steeper than that recorded for the corresponding bulk material. The increase in optical excitation intensity results also in an increasingly higher temperature of the NCs as obtained with Raman thermometry through the commonly used Stokes/anti-Stokes intensity ratio. More significantly, the obtained dependence of the Raman peak position on temperature in optical excitation experiments is markedly different from that observed when the same NCs are excited only thermally (thermal excitation experiments). This difference is not observed for the control bulk material. The inefficient diffusion of photogenerated charges in nanoparticulate systems, due to their inherently low electrical conductivity, results in a higher steady-state density of photoexcited charges and, consequently, also in a stronger excitation of optical phonons that cannot decay quickly enough into acoustic phonons. This results in a nonthermal population of optical phonons and thus the Raman spectrum deviates from that expected for the temperature of the system. Our study has major consequences to the general application of Raman spectroscopy to nanomaterials.

Exciton-to-Dopant Energy Transfer in Mn-Doped Cesium Lead Halide Perovskite Nanocrystals.

We report the one-pot synthesis of colloidal Mn-doped cesium lead halide (CsPbX3) perovskite nanocrystals and efficient intraparticle energy transfer between the exciton and dopant ions resulting in intense sensitized Mn luminescence. Mn-doped CsPbCl3 and CsPb(Cl/Br)3 nanocrystals maintained the same lattice structure and crystallinity as their undoped counterparts with nearly identical lattice parameters at ∼0.2% doping concentrations and no signature of phase separation. The strong sensitized luminescence from d-d transition of Mn2+ ions upon band-edge excitation of the CsPbX3 host is indicative of sufficiently strong exchange coupling between the charge carriers of the host and dopant d electrons mediating the energy transfer, essential for obtaining unique properties of magnetically doped quantum dots. Highly homogeneous spectral characteristics of Mn luminescence from an ensemble of Mn-doped CsPbX3 nanocrystals and well-defined electron paramagnetic resonance spectra of Mn2+ in host CsPbX3 nanocrystal lattices suggest relatively uniform doping sites, likely from substitutional doping at Pb2+. These observations indicate that CsPbX3 nanocrystals, possessing many superior optical and electronic characteristics, can be utilized as a new platform for magnetically doped quantum dots expanding the range of optical, electronic, and magnetic functionality.

Resonant tunneling of carriers in silicon nanocrystals

The rates of resonant and nearly resonant tunnel transitions have been calculated within the envelope function approximation for electrons and holes in silicon nanocrystals embedded in a silicon dioxide matrix. It is shown that, if the nanocrystals are close enough, the rates of resonant tunneling reach the values of the order of 1012–1014 s−1, which considerably exceed the rates of radiative recombination and other basic non-radiative processes, such as the Auger recombination and capture on surface defects. The transition rate is found to be very sensitive to inter-crystallite distance, crystallite size, and effective mass of the carriers in the oxide matrix. Electron tunneling turns out to be faster than the hole one, especially, at greater distances between the nanocrystals. Thus, the tunnel migration in a dense ensemble of nanocrystals is mainly electronic.

Potentiometric Titrations for Measuring the Capacitance of Colloidal Photodoped ZnO Nanocrystals.

Colloidal semiconductor nanocrystals offer a unique opportunity to bridge molecular and bulk semiconductor redox phenomena. Here, potentiometric titration is demonstrated as a method for quantifying the Fermi levels and charging potentials of free-standing colloidal n-type ZnO nanocrystals possessing between 0 and 20 conduction-band electrons per nanocrystal, corresponding to carrier densities between 0 and 1.2 × 10(20) cm(-3). Potentiometric titration of colloidal semiconductor nanocrystals has not been described previously, and little precedent exists for analogous potentiometric titration of any soluble reductants involving so many electrons. Linear changes in Fermi level vs charge-carrier density are observed for each ensemble of nanocrystals, with slopes that depend on the nanocrystal size. Analysis indicates that the ensemble nanocrystal capacitance is governed by classical surface electrical double layers, showing no evidence of quantum contributions. Systematic shifts in the Fermi level are also observed with specific changes in the identity of the charge-compensating countercation. As a simple and contactless alternative to more common thin-film-based voltammetric techniques, potentiometric titration offers a powerful new approach for quantifying the redox properties of colloidal semiconductor nanocrystals.

Single-exciton optical gain in semiconductor nanocrystals: Positive role of electron-phonon coupling

The possibility of obtaining optical gain in an ensemble of semiconductor nanocrystals without involvement of multiexcitons is very attractive for low-threshold laser applications. Here, we reexamine theoretically the conditions required to reach this single-exciton gain regime in nanocrystals. We show that the electron-phonon interaction can play a very positive role, in addition to the exciton-exciton interaction. In presence of both interactions, the optical gain regime can be reached even when the population of nanocrystals containing single excitons is below 10%. For these reasons, we suggest that ultrasmall nanocrystals, or nanocrystals with deep defects at their surface, could be promising materials for light amplification.

Effect of dielectric confinement on optical properties of colloidal nanostructures

We review the effects caused by a large difference in the dielectric constants of a semiconductor and its surrounding in colloidal semiconductor nanostructures (NSs) with various shapes, e.g., nanocrystals, nanorods, and nanoplatelets. The difference increases the electron–hole interaction and consequently the exciton binding energy and its oscillator transition strength. On the other hand, this difference reduces the electric field of a photon penetrating the NS (the phenomenon is called the local field effect) and reduces the photon coupling to an exciton. We show that the polarization properties of the individual colloidal NSs as well as of their randomly oriented ensemble are determined both by the anisotropy of the local field effect and by the symmetry of the exciton states participating in optical transitions. The calculations explain the temperature and time dependences of the degree of linear polarization measured in an ensemble of CdSe nanocrystals.

Quantum theory of electroabsorption in semiconductor nanocrystals.

We develop a simple quantum-mechanical theory of interband absorption by semiconductor nanocrystals exposed to a dc electric field. The theory is based on the model of noninteracting electrons and holes in an infinitely deep quantum well and describes all the major features of electroabsorption, including the Stark effect, the Franz-Keldysh effect, and the field-induced spectral broadening. It is applicable to nanocrystals of different shapes and dimensions (quantum dots, nanorods, and nanoplatelets), and will prove useful in modeling and design of electrooptical devices based on ensembles of semiconductor nanocrystals.