Generation Of Nanocrystals Research Articles

PbSe Quantum Dots for X-ray and γ-Ray Sensors

Nanocrystalline (NC) semiconductors exhibit exploitable properties, such as a tunable energy band gap and multiexciton generation, which arise due to strong quantum confinement. Lead selenide (PbSe), with a relatively large Bohr radius of 46 nm, possesses an extensive literature of organic synthesis routes with which one can explore the strong confinement regime in quantum dots, nanorods, and nanowires. The intrinsically high charge mobility combined with a high atomic number and density of the lead chalcogenides makes them attractive for sensing applications with highly penetrating quanta, such as X-rays and γ-rays. In sensing architectures, the exploitation of these properties for each individual nanocrystallite is hampered by the need to transport the charge carriers throughout the active volume, a motion that can be retarded by energetic surface barriers typically in the form of insulating oxides. Here, we prevent surface oxidation through the fabrication of PbSe NCs via tris(diethylamino)phosphine and a purified selenium precursor, a process that results in PbSe NCs that are chemically and optically stable for at least 1.6 years. If one endeavors to measure high-energy quanta, then the micrometer-scale stopping layers typical of optical photon sensors are insufficient and one must therefore find methods to interconnect the NCs through millimeter- to centimeter-scale thicknesses. Here, we report on the growth of millimeter-scale PbSe colloidal solids that are directly grown within the NC solution. Finally, in contrast to optical photon sensors that typically measure photocurrent, high-energy particle and photon sensors that measure the energy from each interacting quantum are typically hampered by the solid’s thermal noise and the counting statistics associated with discretizing a single quantum into a finite number of information carriers. We show that one can exploit the weaker phonon–electron coupling in nanocrystalline materials to produce room-temperature sensors of X-rays and γ-rays that have comparable resolution to state-of-the-art high-purity germanium detectors. The results thus suggest that multiexciton generation in nanocrystals can be profitably employed in sensing applications that target quanta that create hot-carrier populations that are well above the band gap.

Kinetic Model of Nanostructuring of an Amorphous Metal Film Induced by a Millisecond Thermal Pulse

A model that describes amorphous–nanocrystalline transformation induced by either thermal annealing or millisecond thermal pulse is proposed. Generation of nanocrystals in the film is a relay process in which a transformation wave moves from the heated surface. The proposed approach takes into account specific features of the pulsed thermal source: surface type, short-term effect, and high intensity. Small size, significantly nonequilibrium character of the system, thermal stress, internal thermal fluctuations, and external noise of energy source are taken into consideration.

Излучательная рекомбинация и ударная ионизация в полупроводниковых наноструктурах (О б з о р)

In this review we discuss processes of radiative recombination and impact ionization in light emitting structures based on bulk semiconductors, heterostructures with high potential barriers, nanostructures with deep quantum wells, as well as in nanocrystals with quantum dots. It is demonstrated that enhancement of quantum efficiency and optical power of luminescence in all of the structures under study is determined by the common physical mechanism. This mechanism is creation of additional electron-hole pairs during impact ionization process by hot carriers heated at high band offset on the heterointerface at current pumping or due to carrier multiplication in the process of multiexciton generation in nanocrystals at high-energy photons illumination.

NanoCrySP technology for generation of drug nanocrystals: translational aspects and business potential.

Drug nanocrystals have rapidly evolved into a mature drug delivery strategy in the last decade, with almost 16 products currently on the market. Several "top-down" technologies are available in the market for generation of nanocrystals. Despite several advantages, very few bottom-up technologies have been explored for commercial purpose. This short communication highlights a novel, bottom-up, spray drying based technology-NanoCrySP-to generate drug nanocrystals. Nanocrystals are generated in the presence of non-polymeric excipients that act as crystallization inducer for the drug. Excipients encourage crystallization of drug by plasticization, primary heterogeneous nucleation, and imparting physical barrier to crystal growth. Nanocrystals have shown significant improvement in dissolution and thereby oral bioavailability. NanoCrySP technology is protected through patents in India, the USA, and the European Union. NanoCrySP can be utilized for (i) pharmaceutical development of new chemical entities, (ii) differentiated products of existing molecules, and (iii) generic drug products. The aggregation of drug nanocrystals generated using NanoCrySP poses significant challenges in the nanocrystal-based product development. Addition of stabilizers either during spray drying or during dissolution has shown beneficial effects.

TEM Observation and Ionic Conductivity Study of Li<sub>2</sub>SiO<sub>3</sub> Thin-Film on Sapphire Substrate

Thin-film samples of the lithium ionic conductor Li2SiO3(LSO) were deposited on an A-plane sapphire substrate via the pulsed laser deposition (PLD) method, and the irreversible temperature dependence of the ionic conductivity in the thin-film samples was studied. Via transmission electron microscopy (TEM) observations of annealed LSO thin-film, it was found that the as-prepared LSO thin-film was amorphous over the temperature rangeT490 K, and that nanocrystals existed in the annealed LSO thin-film in the temperature rangeT550 K. Further more, it was clarified the irreversible temperature dependence of the ionic conductivity is due to the generation of nanocrystals.

Multiple exciton generation in nano-crystals revisited: Consistent calculation of the yield based on pump-probe spectroscopy

Multiple exciton generation (MEG) is a process in which more than one exciton is generated upon the absorption of a high energy photon, typically higher than two times the band gap, in semiconductor nanocrystals. It can be observed experimentally using time resolved spectroscopy such as the transient absorption measurements. Quantification of the MEG yield is usually done by assuming that the bi-exciton signal is twice the signal from a single exciton. Herein we show that this assumption is not always justified and may lead to significant errors in the estimated MEG yields. We develop a methodology to determine proper scaling factors to the signals from the transient absorption experiments. Using the methodology we find modest MEG yields in lead chalcogenide nanocrystals including the nanorods.

Charge trapping processes at memory window formation in single- and double nanocrystal layered NVMs

Graphical abstractDisplay Omitted HighlightsBoth antipolar and unipolar programming possible in nanocrystal memory samples.The surface potential fluctuations caused by electron trapping on nanocrystals.Double layered memory samples has better window retention than single layered. The paper is devoted to detailed study of charge trapping processes occurred during memory window formation in nanocrystal memory (NCM) devices. The objects are silicon MOS capacitors with one and two nanocystal layers inside of silicon dioxide. The memory window formation methods are based on the programming pulses application combined with high speed capacitance measurements. The differences in memory devices operation under antipolar and unipolar biasing were compared and discussed. It was shown that unipolar recharging which is specific process of positive charge generation in nanocrystals leads to more reliable and long-retain operation of non-volatile memory devices in comparison with standard bipolar recharging.

Theory of multiexciton generation in semiconductor nanocrystals

We develop a generalized framework based on a Green’s function formalism to calculate the efficiency of multiexciton generation in nanocrystal quantum dots. The direct/indirect absorption and coherent/incoherent impact ionization mechanisms, often used to describe multiexciton generation in nanocrystals, are reviewed and rederived from the unified theory as certain approximations. In addition, two new limits are described systematically – the weak Coulomb coupling limit and the semi-wide band limit. We show that the description of multiexciton generation in nanocrystals can be described as incoherent process and we discuss the scaling of multiexciton generation with respect to the photon energy and nanocrystal size. Illustrations are given for three prototype systems: CdSe, InAs and silicon quantum dots.

Multiple Exciton Generation in InAs Nanocrystals

Limitations of the efficiency of the existing solar cells motivate continuous search for new technological solutions that might lead to more efficient photovoltaic conversion. Among systems that are investigated for possible use in solar energy conversion devices are semiconductor quantum dots. A few ways of employing these nanometer-scale objects in the photoconversion process have been proposed [1, 2], one of which is multiple exciton generation in nanocrystals. Such an effect, consisting in generation of two or more electron–hole pairs by a single absorbed photon, has been observed in a few recent experiments performed on nanocrystals formed of various narrow-gap semiconductors [3–5]. It is a matter of discussion whether the excitons are created coherently during the absorption process or incoherently, due to carrier kinetics involving some kind of dissipative processes at a later time. In any case, the process results from the Coulomb coupling between electron and trion states. As a result, an intraband transition (relaxation) of the electron may be accompanied by the creation of another electron–hole pair, as shown in Fig. 1a. This Auger-type process is referred to as impact ionization. Let us note that the hole, which is initially created in the same confined state as the electron, does not take part in the impact ionization process. In this paper we formulate and study a simple theoretical model of coherent multiple exciton generation by a single high-energy photon absorbed in an InAs nanocrystal. We calculate the interband Coulomb matrix element between an excited electron state and a ground state trion. We determine the degree of mixing

Microstructure and nanoscale composition analysis of the mechanical alloying of Fe xCu 100 − x ( X = 16, 60)

The microstructures of Fe 16Cu 84 and Fe 60Cu 40 (atomic percent) during mechanical alloying (MA) were studied by high resolution electron microscopy (HREM). Nanoscale composition distribution in Fe 16Cu 84 was determined using a HF 2000 FEG TEM. In the Fe 16Cu 84 specimen, a number of deformation twins were observed. In the Fe 60Cu 40 specimen, shear band and generation of nanocrystals in the shear band were observed, which is shown to be a typical mechanism for grain size reduction during MA. In both specimens, the b.c.c. grains tend to be very small (<5 nm) before alloying, which is shown to be a prerequisite condition for the dissolution of Fe in Cu and is also direct evidence to support the thermodynamical model proposed by Yavari et al. Nanoscale composition analysis in Fe 16Cu 84 specimen shows that the average Fe contents in both the interior of grains and the grain boundaries (GBs) are close to the designed composition, thus proving that a supersaturated solid solution has really formed. However, the Fe contents in both cases are rather inhomogeneous, indicating that the mixing of Fe and Cu during MA is inhomogeneous. The process of MA is suggested to be divided into two stages: at the early stage, the grain sizes reduce quickly to a steady value due to the mobility of dislocations; further deformation can be fully accommodated by GBs. As a result, very fast volume diffusion and GB diffusion are achieved. NC-structure and greatly enhanced diffusion coefficients allow the formation of supersaturated solid solutions in immiscible systems with positive enthalpy of mixing.