Tomasz Wojciechowski

Close-packed monolayers of charged Janus-type nanoparticles at the air–water interface

We present a new method to obtain close-packed monolayers composed of noble metal nanoparticles (NP) possessing well-defined permanent charge of either sign. The method is based on the fact that ligands forming the protecting layer exhibit ability to rearrange at the NPs surface. We demonstrate that if the protecting layer is composed of a mixture of hydrophobic and hydrophilic charged ligands in appropriate proportion, the NP exhibits properties of a Janus-type particle with one of the hemispheres hydrophilic and the other hydrophobic. Such amphiphilic NPs self-assemble into a monolayer of well defined surface charge at the air-water interface. Due to strong stabilizing effect of the lateral electrostatic repulsions, such monolayer can be compressed to form close-packed hexagonal structure, and then easily transferred onto a solid substrate with the Langmuir-Blodgett technique.

Giant Spin Splitting in Optically Active ZnMnTe/ZnMgTe Core/Shell Nanowires

An enhancement of the Zeeman splitting as a result of the incorporation of paramagnetic Mn ions in ZnMnTe/ZnMgTe core/shell nanowires is reported. The studied structures are grown by gold-catalyst assisted molecular beam epitaxy. The near band edge emission of these structures, conspicuously absent in the case of uncoated ZnMnTe nanowires, is activated by the presence of ZnMgTe coating. Giant Zeeman splitting of this emission is studied in ensembles of nanowires with various average Mn concentrations of the order of a few percent, as well as in individual nanowires. Thus, we show convincingly that a strong spin sp-d coupling is indeed present in these structures.

Gold Micro-Flowers: One-Step Fabrication of Efficient, Highly Reproducible Surface-Enhanced Raman Spectroscopy Platform

We present a new method enabling simultaneous synthesis and deposition of gold micro-flowers (AuMFs) on solid substrates in a one-pot process that uses two reagents, auric acid and hydroxylamine hydrochloride, in aqueous reaction mixture. The AuMFs deposited onto the substrate form mechanically stable gold layer of expanded nanostructured surface. The morphology of the AuMFs depends on and can be controlled by the composition of the reaction solution as well as by the reaction time. The nanostructured metallic layers obtained with our method are employed as efficient platforms for chemical and biological sensing based on surface-enhanced Raman spectroscopy (SERS). SERS spectra recorded by such platforms for p-mercaptobenzoic acid and phage lambda exhibit enhancement factors above 106 and excellent reproducibility.

Synthesis of ZnAl2O4:(Er3+,Yb3+) spinel-type nanocrystalline upconverting luminescent marker in HeLa carcinoma cells, using a combustion aerosol method route

Efficiently upconverting, spherical ZnAl2O4 nanoparticles (NPs), doped with erbium and ytterbium, were synthesized by a combustion aerosol method (CAM) and transported to cytosol of carcinoma cell line (HeLa) for the first time. Spherical, 82–140 nm spinels were obtained at various concentrations of substrates. The nanoparticles were optimized to emit in the red luminescence range (Er3+, 661 nm, 4F9/2 → 4I15/2) when excited with near infrared light. Lower absorption and scattering by aqueous biological samples, compared to the green emission (Er3+, 550 nm, 2H11/2 → 4I15/2, 2S3/2 → 4I15/2), was responsible for the preferred upconversion. In addition, the application of the near infrared light significantly reduced the cellular autofluorescence and light scattering. X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and photoluminescence spectroscopy were employed to characterize the synthesized samples. Energy dispersive X-ray microanalysis was used to confirm the composition and distribution of the nanoparticles through the spectrum and elemental mapping. The hydrophilic, spherical NPs, coated with PVP (polyvinylpyrrolidone) in the presence of a liposomal transfection factor, lipofectamine, were endocytosed into living HeLa cells and followed as luminescent markers by confocal laser scanning microscopy. We present the optimized protocols for the NPs synthesis and delivery of the spinels to cancer cells for bioimaging.

All wurtzite (In,Ga)As-(Ga,Mn)As core-shell nanowires

Structural and magnetic properties of (In,Ga)As-(Ga,Mn)As core-shell nanowires grown by molecular beam epitaxy on GaAs(111)B substrate with gold catalyst have been investigated. (In,Ga)As core nanowires were grown at high temperature (500 °C) whereas (Ga,Mn)As shells were deposited on the {11̅00} side facets of the cores at much lower temperature (220 °C). High-resolution transmission electron microscopy images and high spectral resolution Raman scattering data show that both the cores and the shells of the nanowires have wurtzite crystalline structure. Scanning and transmission electron microscopy observations show smooth (Ga,Mn)As shells containing 5% of Mn epitaxially deposited on (In,Ga)As cores containing about 10% of In without any misfit dislocations at the core-shell interface. With the In content in the (In,Ga)As cores larger than 5% the (In,Ga)As lattice parameter is higher than that of (Ga,Mn)As and the shell is in the tensile strain state. Elaborated magnetic studies indicate the presence of ferromagnetic coupling in (Ga,Mn)As shells at the temperatures in excess of 33 K. This coupling is maintained only in separated mesoscopic volumes resulting in an overall superparamagnetic behavior which gets blocked below ∼ 17 K.

Upconverting/magnetic: Gd2O3:(Er3+,Yb3+,Zn2+) nanoparticles for biological applications: effect of Zn2+ doping

Upconverting Gd2O3 nanoparticles (NPs) doped 1% Er3+ and 18% Yb3+ permits one to perform optical imaging. Because of the presence of Gd3+ they are useful in MRI. The main challenge is to enhance the NPs upconversion efficiency. As a result of co-doping the NPs with Zn2+ ions, achieved using microwave-induced solution combustion synthesis, we obtained optimal upconversion quantum yields (UQYs). The breakdown of the local crystal field symmetry around the rare earth ions, maximal in the presence of 5% of zinc, may be responsible for the highest observed UQY. The upconversion of IR light results in emission of visible red light mainly at 660 nm and at 550 nm. Optimized red photoluminescence of the samples observed in an organic environment was examined as a function of the laser power density to explain the mechanism of the upconversion emission. Paramagnetic properties of the NPs were determined by superconducting quantum interference device measurements. The non-functionalized nanoparticles incubated with HeLa cells were endocytosed and imaged by confocal laser scanning microscopy. We investigated their localization inside HeLa cells for various incubation times and NPs concentrations. PrestoBlue toxicity assay was performed to test the NPs bio-efficacy.

Structural and optical properties of self-catalytic GaAs:Mn nanowires grown by molecular beam epitaxy on silicon substrates

Mn-doped GaAs nanowires were grown in the self-catalytic growth mode on the oxidized Si(100) surface by molecular beam epitaxy and characterized by scanning and transmission electron microscopy, Raman scattering, photoluminescence, cathodoluminescence, and electron transport measurements. The transmission electron microscopy studies evidenced the substantial accumulation of Mn inside the catalyzing Ga droplets on the top of the nanowires. Optical and transport measurements revealed that the limit of the Mn content for self-catalysed growth of GaAs nanowires corresponds to the doping level, i.e., it is much lower than the Mn/Ga flux ratio (about 3%) used during the MBE growth. The resistivity measurements of individual nanowires confirmed that they are conductive, in accordance with the photoluminescence measurements which showed the presence of Mn(2+) acceptors located at Ga sites of the GaAs host lattice of the nanowires. An anomalous temperature dependence of the photoluminescence related to excitons was demonstrated for Mn-doped GaAs nanowires.

Autonomous Self‐Assembly of Ionic Nanoparticles into Hexagonally Close‐Packed Lattices at a Planar Oil–Water Interface

Lets get charged! Positively charged nanoparticles (NPs) spontaneously self-assemble into hexagonally close-packed lattices at a planar CH(2)Cl(2)-water interface. The self-assembly process is fully autonomous and occurs without any external manipulation.

Spin Splitting Anisotropy in Single Diluted Magnetic Nanowire Heterostructures

We study the impact of the nanowire shape anisotropy on the spin splitting of excitonic photoluminescence. The experiments are performed on individual ZnMnTe/ZnMgTe core/shell nanowires as well as on ZnTe/ZnMgTe core/shell nanowires containing optically active magnetic CdMnTe insertions. When the magnetic field is oriented parallel to the nanowire axis, the spin splitting is several times larger than for the perpendicular field. We interpret this pronounced anisotropy as an effect of mixing of valence band states arising from the strain present in the core/shell geometry. This interpretation is further supported by theoretical calculations which allow to reproduce experimental results.

Morphological changes of gold nanoparticles due to adsorption onto silicon substrate and oxygen plasma treatment

Deposition of gold nanoparticles (AuNPs) on a solid substrate followed by oxygen plasma treatment is a commonly used protocol for surface functionalization. Surprisingly, the effect of the deposition process and oxygen plasma on the morphology of the AuNPs is usually overlooked in research. Here, we investigated morphological changes caused by (i) adsorption of small ligand-capped AuNPs (∼5 nm in diameter) onto a silicon substrate and (ii) subsequent oxygen plasma treatment. AuNPs coated with positively and negatively charged as well as uncharged ligands have been investigated. It is found that upon the adsorption the AuNPs undergo plastic deformations and their shapes can be approximated by spherical caps. The degree of the deformation depends strongly on the AuNP coating. During the plasma treatment the AuNPs behave like droplets of a non-wetting liquid, exhibiting the ability to move and merge. We argue that the AuNP coarsening is dominated by the diffusion coalescence mechanism and show that time evolution of the surface AuNP density follows the Smoluchowski coagulation equation. The diffusivity of the AuNP scales with its mass as D(m) ∼ m−α with α = 2.6.