PbS Sub-micrometer Structures with Anisotropic Shape: Ribbons, Wires, Octapods, and Hollowed Cubes

Star-shaped colloidal PbS nanocrystals: structural evolution and growth mechanism

Branched nanostructures have attracted considerable interest due to their large surface-to-volume ratio with benefits in photocatalysis and photovoltaic applications. Here we discuss the tailoring of branched structures with a shape of a star based on PbS semiconductor. It exposes the reaction mechanism and the controlling factors that template their morphology. For this purpose, we varied the primary lead precursors, types of surfactant, lead-to-surfactant molar ratio, temperature and duration of the reaction. Furthermore, intermediate products in a growth reaction were thoroughly examined using X-ray diffraction, transmission electron microscopy, Raman scattering, optical absorbance and Fourier transform infrared spectroscopy. The results designated a primary formation of truncated octahedral seeds with terminating {100} and {111} facets, followed by the selective fast growth of pods along the 〈100〉 directions toward the development of a star-like shape. The examined intermediates possess a cubic rock salt structure. The observations indicated that small surfactant molecules (e.g. acetate) evolve the branching process, while long-chain surfactants (e.g. oleate) stabilize the long pods as well as mitigate the aggregation process. This study conveys fundamental knowledge for the design of other branched structures, that are attractive for practical use in catalysis, electrochemistry and light-harvesting.

Tailoring of branched structures in the shape of stars based on PbS semiconductor, revealing the reaction mechanism and controlling factors that dictate their morphology and associated optical properties.

The non-stationary case of the Maxwell-Garnett theory: growth of nanomaterials (2D gold flakes) in solution

The solution-based growth mechanism is a common process for nanomaterials. The Maxwell-Garnett theory (for light-matter interactions) describes the solution growth in an effective medium, homogenized by a mean electromagnetic field, which applies when materials are in a stationary phase. However, the charge transitions (inter- and intra-transitions) during the growth of nanomaterials lead to a non-stationary phase and are associated with time-dependent permittivity constant transitions (for nanomaterials). Therefore, time-independence in the standard Maxwell-Garnett theory is lost, resulting in time dependence, ε _i(t). This becomes important when the optical spectrum of a solution needs to be deconvoluted at different reaction times since each peak represents a specific charge/energy transfer with a specific permittivity constant. Based on this, we developed a time-resolved deconvolution approach, f(t) ∝ ε _i(t), which led us to identify the transitions (inter- and intra-transitions) with their dominated growth regimes. Two gold ion peaks were precisely measured (322 nm and 367 nm) for the inter-transition, and three different polyaniline oxidation states (PAOS) for the intra-transition, including A (372 nm), B (680 nm), and C (530 nm). In the initial reaction time regime (0-90 min), the permittivity constant of gold was found to be highly dependent on time, i.e. f _E ∝ ε _i(t), since charge transfer takes place from the PAOS to gold ions (i.e. inter-transition leads to a reduction reaction). In the second time regime (90-180 min), the permittivity constant of gold changes as the material deforms from 3D to 2D (f _S ∝ ε _3D-2D), i.e. intra-transition (combined with thermal reduction). Our approach provides a new framework for the time-dependent modelling of (an)isotropic solutions of other nanomaterials and their syntheses.

Fabrication and characterization of hierarchical multipod silver citrate complex microcrystals with excellent SERS properties

Novel hierarchical multipods silver citrate complexes (SCC) microcrystals have been successfully synthesized in aqueous solution for the first time. The multipods SCC microcrystals showed excellent performance and long-term-stability as SERS substrate.

PbSe-Based Colloidal Core/Shell Heterostructures for Optoelectronic Applications

Lead-based (IV–VI) colloidal quantum dots (QDs) are of widespread scientific and technological interest owing to their size-tunable band-gap energy in the near-infrared optical region. This article reviews the synthesis of PbSe-based heterostructures and their structural and optical investigations at various temperatures. The review focuses on the structures consisting of a PbSe core coated with a PbSe_xS_1–x (0 ≤ x ≤ 1) or CdSe shell. The former-type shells were epitaxially grown on the PbSe core, while the latter-type shells were synthesized using partial cation-exchange. The influence of the QD composition and the ambient conditions, i.e., exposure to oxygen, on the QD optical properties, such as radiative lifetime, Stokes shift, and other temperature-dependent characteristics, was investigated. The study revealed unique properties of core/shell heterostructures of various compositions, which offer the opportunity of fine-tuning the QD electronic structure by changing their architecture. A theoretical model of the QD electronic band structure was developed and correlated with the results of the optical studies. The review also outlines the challenges related to potential applications of colloidal PbSe-based heterostructures.

Tuning of electronic properties in IV-VI colloidal nanostructures by alloy composition and architecture

Colloidal lead chalcogenide (IV-VI) quantum dots and rods are of widespread scientific and technological interest, owing to their size tunable energy band gap at the near-infrared optical regime. This article reviews the development and investigation of IV-VI derivatives, consisting of a core (dot or rod) coated with an epitaxial shell, when either the core or the shell (or both) has an alloy composition, so the entire structure has the chemical formula PbSexS1-x/PbSeyS1-y (0 ≤ x(y) ≤ 1). The article describes synthesis procedures and an examination of the structures' chemical and temperature stability. The investigation of the optical properties revealed information about the quantum yield, radiative lifetime, emission's Stokes shift and electron-phonon interaction, on the variation of composition, core-to-shell division, temperature and environment. The study reflected the unique properties of core-shell heterostructures, offering fine electronic tuning (at a fixed size) by changing their architecture. The optical observations are supported by the electronic band structure theoretical model. The challenges related to potential applications of the colloidal lead chalcogenide quantum dots and rods are also briefly addressed in the article.

Solvothermal synthesis, growth mechanism, and photoluminescence property of sub-micrometer PbS anisotropic structures

The sub-micrometer PbS with anisotropic microstructures including fishbone-like dendrites, multipods, cubes, corallines, and hopper cubes were successfully prepared by the solvothermal process. Different morphologies can be obtained by adjusting the reaction temperatures or using the nontoxic controlled reagents which can tune the relative growth rate in the <100> direction and the <111> direction of PbS nuclei. Based on the viewpoint of crystallography about face-centered cubic crystal structure, the possible formation mechanism for sub-micrometer anisotropic structures has been discussed. The difference between the enhanced growth rates on the {100} and {111} planes induced the change of ratio between the growth rates in the <100> and <111> directions, which resulted in the formation of the different PbS anisotropic microstructures. The PbS anisotropic structures exhibited the different visible emission with a peak in the red regions mainly attributed to the variation of shape, size, and the trap state of as-obtained PbS.

Octapod-shaped colloidal nanocrystals of cadmium chalcogenides via "one-pot" cation exchange and seeded growth

The growth behavior of cadmium chalcogenides (CdE = CdS, CdSe, and CdTe) on sphalerite Cu(2-x)Se nanocrystals (size range 10-15 nm) is studied. Due to the capability of Cu(2-x)Se to undergo a fast and quantitative cation exchange reaction in the presence of excessive Cd(2+) ions, no Cu(2-x)Se/CdE heterostructures are obtained and instead branched CdSe/CdE nanocrystals are built which consist of a sphalerite CdSe core and wurtzite CdE arms. While CdTe growth yields multiarmed structures with overall tetrahedral symmetry, CdS and CdSe arm growth leads to octapod-shaped nanocrystals. These results differ significantly from literature findings about the growth of CdE on sphalerite CdSe particles, which until now had always yielded tetrapod-shaped nanocrystals.