M. Arciniegas

In situ microscopy of the self-assembly of branched nanocrystals in solution

Solution-phase self-assembly of nanocrystals into mesoscale structures is a promising strategy for constructing functional materials from nanoscale components. Liquid environments are key to self-assembly since they allow suspended nanocrystals to diffuse and interact freely, but they also complicate experiments. Real-time observations with single-particle resolution could have transformative impact on our understanding of nanocrystal self-assembly. Here we use real-time in situ imaging by liquid-cell electron microscopy to elucidate the nucleation and growth mechanism and properties of linear chains of octapod-shaped nanocrystals in their native solution environment. Statistical mechanics modelling based on these observations and using the measured chain-length distribution clarifies the relative importance of dipolar and entropic forces in the assembly process and gives direct access to the interparticle interaction. Our results suggest that monomer-resolved in situ imaging combined with modelling can provide unprecedented quantitative insight into the microscopic processes and interactions that govern nanocrystal self-assembly in solution.

Nanocrystal Film Patterning by Inhibiting Cation Exchange via Electron-Beam or X‑ray Lithography

In this Letter we report patterning of colloidal nanocrystal films that combines direct e-beam (electron beam) writing with cation exchange. The e-beam irradiation causes cross-linking of the ligand molecules present at the nanocrystal surface, and the cross-linked molecules act as a mask for further processing. Consequently, in the following step of cation exchange, which is performed by directly dipping the substrate in a solution containing the new cations, the regions that have not been exposed to the electron beam are chemically transformed, while the exposed ones remain unchanged. This selective protection allows the design of patterns that are formed by chemically different nanocrystals, yet in a homogeneous nanocrystal film. Spatially resolved compositional analysis by energy-dispersive X-ray spectroscopy (EDS) corroborates that the selective exchange occurs only in the nonirradiated regions. We demonstrate the utility of this lithography approach by fabricating conductive wires and luminescent patterns in CdSe/CdS nanocrystal films by converting nonirradiated regions to Cu2-xSe/Cu2-xS. Furthermore, we show that X-ray irradiation too can lead to protection from cation exchange.

Quantum parameters for guiding the design of Ti-alloys with shape memory and/or low elastic modulus

Electronic structure calculations based on density functional theory have been applied to clusters of titanium with different alloying elements in order to obtain quantum parameters that give some information on the interaction between the mother and alloying atoms. Average values of these parameters weighted with the molar fractions have been calculated for 146 titanium alloys that exhibit shape memory and/or low elastic modulus. These values have been mapped in order to identify zones that group the alloys with either property. This information was used as a guide for designing seven new alloys with desired properties. These have been microstructurally and mechanically characterized; the results confirm the usefulness of the method.

N-Methylformamide as a Source of Methylammonium Ions in the Synthesis of Lead Halide Perovskite Nanocrystals and Bulk Crystals

We report chemical routes for the synthesis of both nanocrystals and bulk crystals of methylammonium (MA) lead halide perovskites employing N-methylformamide (NMF) as a source of MA ions. Colloidal nanocrystals were prepared by a transamidation reaction between NMF and an alkyl amine (oleylamine). The nanocrystals showed photoluminescence quantum yields reaching 74% for MAPbBr3 and 60% for MAPbI3. Bulk crystals were grown at room temperature, with no need for an antisolvent, by the acid hydrolysis of NMF. Important advantages of using NMF instead of MA salts are that the syntheses involve fewer steps and less toxic and less expensive chemicals.

In vitro response of preosteoblastic MG63 cells on Ni-free Ti shape memory substrates.

Ni-free Ti alloys are a potential strategy to overcome the risk of Ni-adverse reactions and rigidity mismatch for implant materials. Here, we report the biological behavior induced for two promising candidate alloys--Ti19.1Nb8.8Zr (M(S) temperature of 46°C and elastic modulus of 74 GPa) and Ti41.2Nb6.1Zr (elastic modulus of 67 GPa)--on cultured MG63 cells, as well as their physical and chemical properties. Contact angle results revealed the hydrophobic character of the former alloy (59.02° ± 2.35°) attributed to the presence of the martensitic phase, while the latter one presented a hydrophilic response (67.77° ± 2.78°). Results showed also that the cell adhesion response (after 4 and 8 h of incubation) in both substrates was not statistically different to that obtained in the cp Ti as control material. These surfaces induced well spread cell morphology with cytoplasmic extension like filopodia of up to 100 μm even at short culture times and presented an uninterrupted proliferation after longer incubation times (9 days). A decrement in the proliferation rate was appreciated from the Ti19.1Nb8.8Zr surface at that time, which was attributed to an earlier activation of the cell differentiation stage, as confirmed by the twofold increment of alkaline phosphatase activity. The results also evidenced that the presence of a 2 nm thick layer of amorphous Nb₂O₅, which was detected on both alloys, has a significant effect on cell behavior favoring the cell adhesion and morphology response of the new alloys studied.

New Ni-free superelastic alloy for orthodontic applications

A potential new Ni-free Ti alloy for biomedical applications was assessed in order to investigate the superelastic behavior, corrosion resistance and the biocompatibility. The alloy studied was Ti19.1Nb8.8Zr. The chemical composition was determined by X-ray microanalysis, the thermoelastic martensitic transformation was characterized by high sensitivity calorimeter. The critical stresses were determined by electromechanical testing machine and the corrosion behavior was analyzed by potentiostatic equipment in artificial saliva immersion at 37°C. The results were compared with six different NiTi orthodontic archwire brands. The biocompatibility was studied by means of cultures of MG63 cells. Ni-free Ti alloy exhibits thermoelastic martensitic transformation with Ms=45°C. The phase present at 37°C was austenite which under stress can induce martensite. The stress-strain curves show a superelastic effect with physiological critical stress (low and continuous) and a minimal lost of the recovery around 150 mechanical cycles. The corrosion resistance improves the values obtained by different NiTi alloys avoiding the problem of the Ni adverse reactions caused by Ni ion release. Cell culture results showed that adhered cell number in new substrate was comparable to that obtained in a commercially pure Ti grade II or beta-titanium alloy evaluated in the same conditions. Consequently, the new alloy presents an excellent in-vitro response.

Ptychographic Imaging of Branched Colloidal Nanocrystals Embedded in Free-Standing Thick Polystyrene Films

Research on composite materials is facing, among others, the challenging task of incorporating nanocrystals, and their superstructures, in polymer matrices. Electron microscopy can typically image nanometre-scale structures embedded in thin polymer films, but not in films that are micron size thick. Here, X-ray Ptychography was used to visualize, with a resolution of a few tens of nanometers, how CdSe/CdS octapod-shaped nanocrystals self-assemble in polystyrene films of 24 ± 4 μm, providing a unique means for non-destructive investigation of nanoparticles distribution and organization in thick polymer films.