Zero-dimensional Nanocrystals Research Articles

Highly reflective and fluorescent TiO2 quantum dots modified asphalt coating: Engineering characterizations and microclimatic modelling

Conventional asphalt absorbs and stores a large quantity of solar radiation, contributing to urban heat island (UHI) phenomenon. TiO2 quantum dots (QDs) are one of the important zero-dimensional nanocrystals with remarkable light reflection and fluorescence. In this study, the highly reflective and fluorescent asphalt binder modified with TiO2 QDs was proposed as an innovative UHI mitigation strategy. Optical and Superpave performance characterizations were employed on TiO2 QDs modified asphalt binder while its cooling potential was assessed by implementing microclimatic modelling. The results show that solar reflectance of asphalt binder increases from 12% to 17% when doping 5–30% TiO2 QDs compared to 3% of traditional asphalt binder. Meanwhile, fluorescent intensity of modified asphalt binder is increased by 43–289%. Compared to conventional asphalt binder, TiO2 QDs modified asphalt binder exhibits superior rutting resistance. The microclimatic modelling using ENVI-met simulation tool has revealed that asphalt coating with TiO2 QDs brings air temperature reduction by up to 2.9 °C. Moreover, economic analysis has shown that asphalt coating withTiO2 QDs could achieve desirable cost-effectiveness.

ПОВЕРХНЕВА ЛЮМІНЕСЦЕНЦІЯ НАПІВПРОВІДНИКОВИХ КВАНТОВИХ ТОЧОК А2В6 (Огляд)

Semiconductor zero-dimensional nanocrystals – quantum dots (QDs) – have been increasingly used in various fields of opto- and nanoelectronics in recent decades. This is because of the exciton nature of their luminescence, which can be controlled via the well known quantum-dimensional effect. At the same time, at small nanocrystall sizes, the influence of the surface on the optical and structural properties of nanocrystals increases significantly. The presence of broken bonds of surface atoms and point defects – vacancies and interstial atoms – can both weaken the exciton luminescence and create new effective channels of radiant luminescence. In some cases, these surface luminescence becomes dominant, leading to optical spectra broadening up to the quasi-white light. The nature of such localized states often remains unestablished due to the large number of the possible sorts of defects in both of QD and its surrounding. In contrast to exciton luminescence, which can be properly described within effective-mass approximations, the optical properties of defects relay on chemical nature of both defect itsself and its surrounding, what cannot be provided by “hydrogen-type coulomb defect” approximation. Moreover, charge state and related to this lattice relaxation must be taken into account, what requires an application of atomistic approach, such as Density functioal theory (DFT). Therefore, this review is devoted to the study of surface (defect) states and related luminescence, as well as the analysis of possible defects in nanocrystals of semiconductor compounds A2B6 (CdS, CdZnS, ZnS), responsible for luminescence processes, within ab initio approach. The review presents the results of the authors' and literature sources devoted to the study of the luminescent characteristics of ultra-small (<2 nm) QDs.

Anisotropic Transient Disordering of Colloidal, Two-Dimensional CdSe Nanoplatelets upon Optical Excitation.

Nanoplatelets (NPLs)-colloidally synthesized, spatially anisotropic, two-dimensional semiconductor quantum wells-are of intense interest owing to exceptionally narrow transition line widths, coupled with solution processability and bandgap tunability. However, given large surface areas and undercoordinated bonding at facet corners and edges, excitation under sufficient intensities may induce anisotropic structural instabilities that impact desired properties. We employ time-resolved X-ray diffraction to study the crystal structure of CdSe NPLs in response to optical excitation. Photoexcitation induces greater out-of-plane than in-plane disordering in 4 and 5 monolayer (ML) NPLs, while 3 ML NPLs display the opposite behavior. Recovery dynamics suggest that out-of-plane cooling slightly outpaces in-plane cooling in 5 ML NPLs with recrystallization occurring on indistinguishable time scales. In comparison, for zero-dimensional CdSe nanocrystals, disordering is isotropic and recovery is faster. These results favor the use of NPLs in optoelectronic applications, where they are likely to exhibit superior performance over traditional, zero-dimensional nanocrystals.

Controlled anisotropic growth of layered perovskite nanocrystals for enhanced optoelectronic properties

We synthesized cubic-phase layered perovskite nanocrystals (PNCs) of different shapes, namely thin nanoplatelets, thick nanoplatelets, and nanocubes, to investigate their photoresponse characteristics in a phototransistor device. The PNCs were prepared using a one-pot hot-injection method, and their shape evolved from nanoplatelets to nanocubes (with zero-dimensional nanocrystals as a control) by controlling the reaction temperature and amount of cesium precursor. In addition, the PNCs were deposited into a film with a thickness of a few tens of nanometers by shear force using a solvent washing process combined with the spin-coating technique. The surface ligands and contaminating organic molecules were effectively removed from the deposited PNCs, and consequently, the drain current in the PNC-based phototransistors increased by 37–349% under illumination compared with the controls (purified or annealed). Of the four PNC shapes, the nanocubes showed the longest radiative lifetime and highest photosensitivity, with a photocurrent of up to 1.74 µA cm−2 under a 3.0 V bias. This work provides in-depth insight into the dependence of the intrinsic photophysical properties of PNCs on their morphology. Moreover, the morphology of the PNCs determined how they assembled into channels to bridge the transistor electrodes, which offers new opportunities for the development of high-performance, long-lifetime phototransistors and photovoltaics.

Size-dependent optical properties of MoS2 nanoparticles and their photo-catalytic applications

While two-dimensional (2D) layered MoS2 nanosheets have been extensively studied owing to their fascinating optoelectronic properties, less attention has been paid to the corresponding zero-dimensional nano-crystals. In this contribution, we report the efficacy of MoS2 nanocrystals for their size tunable properties for optical and photocatalytic applications. We have synthesized differently sized (10–70 nm) crystalline, hexagonal 2H-MoS2 nanoparticles (NPs) dispersed in DMF solvent using a simple exfoliation technique. Synthesized NPs are found to exhibit size-dependent optical properties and excitation-dependent fluorescence characteristics in the visible region, which are otherwise not observed in bulk or 2D MoS2 layers. Size tunable bandgap and broad absorbance and emission spectrum covering the visible range could be exploited in the fabrication of various opto-electronic devices. Charge carrier emission dynamics of differently sized MoS2 NPs are investigated using time correlated single photon counting (TCSPC) spectroscopic technique. We found two time components, one in the order of several hundreds of ps, which arises due to the radiative recombination of charge carriers, while the other one is of the order of a few ns, which emanates from the defect states of MoS2 NPs. The average time constants are found to decrease with increase in particle size. A noticeable photocatalytic activity of the synthesized MoS2 NPs under visible light illumination for the degradation of brilliant green dye is also demonstrated for the first time and the effect of size variation of NPs in the dye degradation process is reported.

Near–infrared electrochemiluminesence biosensor for high sensitive detection of porcine reproductive and respiratory syndrome virus based on cyclodextrin-grafted porous Au/PtAu nanotube

Self-supported one-dimensional nanocatalysts with better electrical transport and less vulnerable to dissolution and aggregation than zero-dimensional nanocrystals have aroused particular attention in mimic-enzyme catalysis. Herein, the porous Au/PtAu bimetallic heterojunction nanotube (PtAu BNT) as a remarkable catalyst was firstly introduced into the near–infrared electrochemiluminesence (NIR ECL) immunoassay for enhancing detection sensitivity of porcine reproductive and respiratory syndrome virus (PRRSV). Meanwhile, the unique and strong host-guest recognition between β-cyclodextrin and adamantine was chose as the linker to assemble the PtAu BNTs catalysts onto PRRSV antibody, which clearly surpassed the unstable electrostatic assembly between cationic surfactant and biosamples. The formed supramolecular “bridge” could extend the space to accommodate more PtAu BNTs catalysts and reduce the blocking effect. The fabricated sandwich immunosensor exhibited excellent analytical performance for PRRSV with a better linear detection range from 1:103 to 1:106 (dilution ratio) and a sensitive detection limit (10.8pg/mL). Good stability, acceptable reproducibility and accuracy of the immunosensor suggested its potential applications in clinical diagnostics.

Multiband tight-binding theory of disordered A x B1- x C semiconductor quantum dots

Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a broad range of applications, as their spectrum and thus their excitation gap can be tailored by variation of their size. Additionally, nanocrystals of the type ABC can be realized by alloying of two pure compound semiconductor materials AC and BC, which allows for a continuous tuning of their absorption and emission spectrum with the concentration x. We use the single-particle energies and wave functions calculated from a multiband sp^3 empirical tight-binding model in combination with the configuration interaction scheme to calculate the optical properties of CdZnSe nanocrystals with a spherical shape. In contrast to common mean-field approaches like the virtual crystal approximation (VCA), we treat the disorder on a microscopic level by taking into account a finite number of realizations for each size and concentration. We then compare the results for the optical properties with recent experimental data and calculate the optical bowing coefficient for further sizes.

New Aspects of Carrier Multiplication in Semiconductor Nanocrystals

One consequence of strong spatial confinement of electronic wave functions in semiconductor nanocrystals (NCs) is a significant enhancement in carrier-carrier Coulomb interactions. This effect leads to a number of novel physical phenomena including ultrafast decay of multiple electron-hole pairs (multiexcitons) by Auger recombination and high-efficiency generation of mutiexcitons by single photons via carrier multiplication (CM). Significant recent interest in multiexciton phenomena in NCs has been stimulated by studies of NC lasing, as well as potential applications of CM in solar-energy conversion. The focus of this Account is on CM. In this process, the kinetic energy of a "hot" electron (or a "hot" hole) does not dissipate as heat but is, instead, transferred via the Coulomb interaction to the valence-band electron, exciting it across the energy gap. Because of restrictions imposed by energy and translational-momentum conservation, as well as rapid energy loss due to phonon emission, CM is inefficient in bulk semiconductors, particularly at energies relevant to solar energy conversion. On the other hand, the CM efficiency can potentially be enhanced in zero-dimensional NCs because of factors such as a wide separation between discrete electronic states, which inhibits phonon emission ("phonon bottleneck"), enhanced Coulomb interactions, and relaxation in translational-momentum conservation. Here, we investigate CM in PbSe NCs by applying time-resolved photoluminescence and transient absorption. Both techniques show clear signatures of CM with efficiencies that are in good agreement with each other. NCs of the same energy gap show moderate batch-to-batch variations (within approximately 30%) in apparent multiexciton yields and larger variations (more than a factor of 3) due to differences in sample conditions (stirred vs static solutions). These results indicate that NC surface properties may affect the CM process. They also point toward potential interference from extraneous effects such as NC photoionization that can distort the results of CM studies. CM yields measured under conditions when extraneous effects are suppressed via intense sample stirring and the use of extremely low pump levels (0.02-0.03 photons absorbed per NC per pulse) reveal that both the electron-hole creation energy and the CM threshold are reduced compared with those in bulk solids. These results indicate a confinement-induced enhancement in the CM process in NC materials. Further optimization of CM performance should be possible by utilizing more complex (for example, shaped-controlled or heterostructured) NCs that allow for facile manipulation of carrier-carrier interactions, as well as single and multiexciton energies and dynamics.

Carrier multiplication in semiconductor nanocrystals via intraband optical transitions involving virtual biexciton states

We propose and analyze a physical mechanism for photogeneration of multiexcitons by single photons (carrier multiplication) in semiconductor nanocrystals, which involves intraband optical transitions within the manifold of biexciton states. In this mechanism, a virtual biexciton is generated from nanocrystal vacuum by the Coulomb interaction between two valence-band electrons, which results in their transfer to the conduction band. The virtual biexciton is then converted into a real, energy-conserving biexciton by photon absorption on an intraband optical transition. The proposed mechanism is inactive in bulk semiconductors as momentum conservation suppresses intraband transitions. However, it becomes highly efficient in the case of zero-dimensional nanocrystals, where quantum confinement results in relaxation of momentum conservation, which is accompanied by the development of strong intraband absorption. Our calculations show that the efficiency of the carrier multiplication channel mediated by intraband optical transitions can be comparable to or even greater than that for impact-ionization-like processes mediated by interband transitions.

Synthesis of inorganic nanomaterials

Synthesis forms a vital aspect of the science of nanomaterials. In this context, chemical methods have proved to be more effective and versatile than physical methods and have therefore, been employed widely to synthesize a variety of nanomaterials, including zero-dimensional nanocrystals, one-dimensional nanowires and nanotubes as well as two-dimensional nanofilms and nanowalls. Chemical synthesis of inorganic nanomaterials has been pursued vigorously in the last few years and in this article we provide a perspective on the present status of the subject. The article includes a discussion of nanocrystals and nanowires of metals, oxides, chalcogenides and pnictides. In addition, inorganic nanotubes and nanowalls have been reviewed. Some aspects of core-shell particles, oriented attachment and the use of liquid-liquid interfaces are also presented.

Visible luminescence from silicon quantum dots and wells

We discuss the mechanism of efficient photoluminescence (PL) from Si quantum dots and wells. Luminescence properties of SiO 2-capped Si nanocrystals are different from those of H-terminated Si nanocrystals, but are similar to those of two-dimensional Si quantum wells sandwiched by SiO 2 layers. The size dependence of PL properties and resonantly excited PL spectra of SiO 2-capped Si nanocrystals indicate that excitons are localized near the nanocrystal surface and the strong coupling of localized excitons to surface oxide vibrations causes broad PL spectra in the visible spectral region. The localization of excitons near the interface between crystalline Si (c-Si) and surface oxide layer is attributed to efficient luminescence in nanoscale c-Si/SiO 2 systems such as zero-dimensional nanocrystals and two-dimensional quantum wells