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A pinboard by
Arthur Shapiro

My name is Arthur Shapiro, a PhD student at the faculty of chemistry at the Technion-Israel institute of Technology under the supervision of Prof. Efrat Lifshitz.

When I started my Bachelor's degree I encountered many obstacles and didn't imagine that one day I will start a PhD, what today seems to me so natural.

Besides science sport is an integral part of my life.

I deal with nano particles and I like to create such a tiny objects with unique features due to their dimensions.

As a chemist I have a great influence and control on the final product. I am amazed when I am taking my particles to electronic microscope and see what I have synthesized with ability to see single atoms.

PINBOARD SUMMARY

Solution for the air-stability problem of IV-VI CQDs and the strain in the interface are addressed

Lead chalcogenide (group IV-VI semiconductors) CQDs have raised scientific and technological interest due to their optical tunability in the IR spectral regime [so-called near infrared (NIR), 0.7-1.5 µm and shortwave infrared (SWIR), 1.5-3 µm ]. Therefore, these CQDs can be potentially employed in numerous applications, such as solar cells, field-effect transistor (FETs), SWIR applications and up-conversion (UC) devices. Although, lead chalcogenides have many benefits, they are extremely sensitive in ambient conditions. The limited chemical stability of lead chalcogenides prevents a study of their properties under ambient conditions and technological implementation. Therefore, the passivation of QDs has become an important issue. To tackle the problem, several strategies of post-synthesis treatment of CQDs can be applied, such as halogen passivation, which provides atomic surface passivation, or by using heterostructures such as core/shell. Aside to benefits of the core/shell heterostructures, unintended strain at an interface between the core and the shell can be generated due to the constituents' different crystallographic parameters. This strain results in the formation of dislocations or vacancy defect sites, as a strain relief process. These defects can become a site for non-radiative recombination in CQDs, resulting in the reduction of fluorescent efficiency. For example, a PbS shell exhibits a small deviation (~3%) of lattice parameter (6.12 Å and 5.94 Å for bulk PbSe and PbS, respectively) and the same rock-salt crystal structure with respect to a PbSe core, indicating a good candidate as a shell. However, our recent study has demonstrated that PbSe/PbS CQDs experience the strain force despite close crystalline matching. Previous works have shown that an annealing process can relax the strain by introducing an alloying layer at the interface between core and shell. By post-treatment at optimal annealing temperature, the increase of photoluminescence (PL) quantum yield (QY) and lifetime was observed. Another advantage of alloying is providing tuneability of electronic structure by smoothing the core-to-shell boundary potential, enabling the influence of energy band offsets and band structure by a strain relief.

9 ITEMS PINNED

Air-stable PbSe/PbS and PbSe/PbSexS1-x core-shell nanocrystal quantum dots and their applications.

Abstract: The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.

Pub.: 15 Dec '06, Pinned: 31 Aug '17

Composition-tunable optical properties of colloidal IV-VI quantum dots, composed of core/shell heterostructures with alloy components.

Abstract: Colloidal quantum dots (CQDs) attract worldwide scientific and technological attention due to the ability to engineer their optical properties by the variation of their size. However, several important applications, such as biological tagging and photovoltaic cells, impose a limit on their size yet demand tunability and thermal stability of the optical band edge. This work introduces a new class of heterostructures, composed of PbSe or PbSe(y)S(1-y) cores, coated by PbS or PbSe(x)S(1-x) shells, with different core-radius/shell-width division, with a radial gradient composition (with 0 < y < 1, 0 < x < 1), which offer a control of the band edge properties by varying the CQDs' composition. Continuous-wave and transient photoluminescence measurements over a wide temperature range (1.4-300 K) revealed a distinct behavior of the heterostructures with respect to that of pure PbSe cores: (i) increase of the emission quantum yield; (ii) red-shift of the absorption edge but a decrease of the emission Stokes shift; (iii) alleviation of a dark exciton recombination, viz., a reduction of an exchange interaction; (iv) tuning of the radiative lifetime with shell width and composition; (v) reduction of the band edge temperature coefficient, dE/dT, viz., induction of thermal stability. The k·p envelope function calculation, considering abrupt or smooth alloying continuation of the potential at the core-shell interface, revealed a delocalization of the hole wave function over the entire volume of the CQDs, as a partial explanation for the marked tunability, nonetheless preserving a desired size.

Pub.: 16 Oct '10, Pinned: 29 Aug '17

Interface control of electronic and optical properties in IV-VI and II-VI core/shell colloidal quantum dots: a review.

Abstract: Semiconductor colloidal quantum dots (CQDs) have attracted vast scientific and technological interest throughout the past three decades, due to the unique tuneability of their optoelectronic properties by variation of size and composition. However, the nanoscale size brings about a large surface-to-bulk volume ratio, where exterior surfaces have a pronounced influence on the chemical stability and on the physical properties of the semiconductor. Therefore, numerous approaches have been developed to gain efficient surface passivation, including a coverage by organic or inorganic molecular surfactants as well as the formation of core/shell heterostructures (a semiconductor core epitaxially covered by another semiconductor shell). This review focuses on special designs of core/shell heterostructures from the IV-VI and II-VI semiconductor compounds, and on synthetic approaches and characterization of the optical properties. Experimental observations revealed the formation of core/shell structures with type-I or quasi-type-II band alignment between the core and shell constituents. Theoretical calculations of the electronic band structures, which were also confirmed by experimental work, exposed surplus electronic tuning (beyond the radial diameter) with adaptation of the composition and control of the interface properties. The studies also considered strain effects that are created between two different semiconductors. It was disclosed experimentally and theoretically that the strain can be released via the formation of alloys at the core-shell interface. Overall, the core/shell and core/alloyed-shell heterostructures showed enhancement in luminescence quantum efficiency with respect to that of pure cores, extended lifetime, uniformity in size and in many cases good chemical sustainability under ambient conditions.

Pub.: 21 Dec '16, Pinned: 29 Aug '17