Etching suppression as a means to Pt dendritic ultrathin nanosheets by seeded growth.

2D ultrathin metal nanostructures are emerging materials displaying distinct physical and chemical properties compared to their analogues of different dimensionalities. Nanosheets of fcc metals are intriguing, as their crystal structure does not favour a 2D configuration. Thanks to their increased surface-to-volume ratios and the optimal exposure of low-coordinated sites, 2D metal nanostructures can be advantageously exploited in catalysis. Synthesis approaches to ultrathin nanosheets of pure platinum are scarce compared to other noble metals and to Pt-based alloys. Here, we present the selective synthesis of Pt ultrathin nansosheets by a simple seeded-growth method. The most crucial point in our approach is the selective synthesis of Pt seeds comprising planar defects, a main driving force for the 2D growth of metals with fcc structure. Defect engineering is employed here, not in order to disintegrate, but for conserving the defect comprising seeds. This is achieved by in situ elimination of the principal etching agent, chloride, which is present in the PtCl2 precursor. As a result of etching suppression, twinned nuclei, that are selectively formed during the early stage of nucleation, survive and grow to multipods comprising planar defects. Using the twinned multipods as seeds for the subsequent 2D overgrowth of Pt from Pt(acac)2 yields ultrathin dendritic nanosheets, in which the planar defects are conserved. Using phenylacetylene hydrogenation as a model reaction of selective hydrogenation, we compared the performance of Pt nanosheets to that of a commercial Pt/C catalyst. The Pt nanosheets show better stability and much higher selectivity to styrene than the commercial Pt/C catalyst for comparable activity.

STEM-EELS investigation of c-Si/a-AlOx interface for solar cell applications.

In this article, STEM-EELS methodology is described to investigate the composition of sensitive crystalline Silicon/amorphous aluminum oxide (c-Si/a-AlOx) interface of an a AlOx/amorphous hydrogenated silicon nitride (a-AlOx/a-SiNx:H) passivation stack of a c-Si solar cell. In this stack, a-AlOx has the distinctive characteristic to provide both chemical and field effect passivation, which need further research to be more controlled in order to improve solar cell efficiency. a-AlOx is known to be unstable under the electron-beam, so we first present a detailed study on the electron-beam radiation damage to c-Si/a-AlOx interface. This interface can indeed undergo several electron-beam irradiation damage like sputtering, knock-on or radiolysis if precautions are not taken. Radiolysis damage has been found to be the dominant radiation damage. Thus, several STEM-EELS acquisition parameters like acceleration voltage, electron dose and scan orientation were taken into account and modified to limit this radiolysis damage. Once the irradiation was limited, STEM-EELS investigation was conduct using DualEELS on the Si and Al L2,3 and OK edge fines structures. The interface was found to be composed of a-SiOx and non-stoichiometric aluminum silicate with a predominance of tetrahedrally coordinated Al in its first layer.

Synthesis of hybrid colloidal nanoparticles for a generic approach to 3D electrostatic directed assembly: Application to anti-counterfeiting.

HYPOTHESIS: The capability of making 3D directed assembly of colloidal nanoparticles on surfaces, instead of 2D one, is of major interest to generate, tailor, and enhance their original functionalities. The nanoxerography technique, i.e. electrostatic trapping of nanoparticles on charged patterns, showed such 3D assembly potentialities but is presently restricted to polarizable nanoparticles with a diameter superior to 20 nm. Hence, it should be possible to exploit a generic approach based on hybrid systems using larger nanoparticles as cargos to anchor smaller ones. EXPERIMENTS: A synthesis of hybrid nanoparticles in a raspberry-like configuration was performed using 50 nm SiO2 nanoparticles and photoluminescent 3-5 nm InP@ZnS (visible emission) or PbS (infrared emission) nanoparticles. Complete topographical and photoluminescent characterizations were carried out on hybrid nanoparticle patterns assembled by nanoxerography and systematically compared to patterns obtained from single photoluminescent nanoparticles. FINDINGS: The synthesis approach is generic. Every hybrid nanoparticle system has led to 3D assemblies with improved photoluminescent signals compared to mono/bilayered assemblies. Straightforward applications for anti-counterfeiting are illustrated. The versatility of the proposed concept is expected to be applied to other nanoparticles to make the most of their magnetic, catalytic, optical etc. properties in a wide range of applications, sensors and devices.

Chemical Ordering in Bimetallic FeCo Nanoparticles: From a Direct Chemical Synthesis to Application As Efficient High-Frequency Magnetic Material.

Single-crystalline FeCo nanoparticles with tunable size and shape were prepared by co-decomposing two metal-amide precursors under mild conditions. The nature of the ligands introduced in this organometallic synthesis drastically affects the reactivity of the precursors and, thus, the chemical distribution within the nanoparticles. The presence of the B2 short-range order was evidenced in FeCo nanoparticles prepared in the presence of HDAHCl ligands, combining 57Fe Mössbauer, zero-field 59Co ferromagnetic nuclear resonance (FNR), and X-ray diffraction studies. This is the first time that the B2 structure is directly formed during synthesis without the need of any annealing step. The as-prepared nanoparticles exhibit magnetic properties comparable with the ones for the bulk ( Ms = 226 Am2·kg-1). Composite magnetic materials prepared from these FeCo nanoparticles led to a successful proof-of-concept of the integration on inductor-based filters (27% enhancement of the inductance value at 100 MHz).

Shape selection through epitaxy of supported platinum nanocrystals.

Supported nanocrystals of original shapes are highly desirable for the development of optimized catalysts; however, conventional methods for the preparation of supported catalysts do not allow shape control. In this work, we have synthesized concave platinum nanocubes exposing {110} crystallographic facets at 20 °C. In the presence of a crystallographically oriented Pt(111) support in the reaction medium, the concave nanocubes grow epitaxially on the support, producing macroscopic nanostructured surfaces. Higher reaction temperature produces a mixture of different nanostructures in solution; however, only the nanostructures growing along the 111 direction are obtained on the Pt(111) support. Therefore, the oriented surface acts as a template for a selective immobilization of specific nanostructures out of a mixture, which can be regarded as an "epitaxial resolution" of an inhomogeneous mixture of nanocrystals. Thus, a judicious choice of the support crystallographic orientation may allow the isolation of original nanostructures that cannot be obtained in a pure form.

Noble Metal Nanocluster Formation in Epitaxial Perovskite Thin Films.

We studied the synthesis of nanocomposite materials consisting of noble metal clusters embedded in an oxide semiconductor matrix. The embedded nanostructures form in a simple self-organized single-step growth process. The primary interest is in developing materials for photo-electrochemical energy conversion where spatially inhomogeneous band structures can enhance photogenerated charge separation and carrier extraction from a semiconductor. We show that spontaneous segregation of metallic Ir occurs during the initial growth of an Ir:SrTiO3 thin film. Cross-sectional transmission electron microscopy suggests that the nanoscale Ir clusters are epitaxial with the host lattice, and their presence is not detectable by surface morphology measurements.

Performance Enhancement via Incorporation of ZnO Nanolayers in Energetic Al/CuO Multilayers.

Al/CuO energetic structure are attractive materials due to their high thermal output and propensity to produce gas. They are widely used to bond components or as next generation of MEMS igniters. In such systems, the reaction process is largely dominated by the outward migration of oxygen atoms from the CuO matrix toward the aluminum layers, and many recent studies have already demonstrated that the interfacial nanolayer between the two reactive layers plays a major role in the material properties. Here we demonstrate that the ALD deposition of a thin ZnO layer on the CuO prior to Al deposition (by sputtering) leads to a substantial increase in the efficiency of the overall reaction. The CuO/ZnO/Al foils generate 98% of their theoretical enthalpy within a single reaction at 900 °C, whereas conventional ZnO-free CuO/Al foils produce only 78% of their theoretical enthalpy, distributed over two distinct reaction steps at 550 °C and 850 °C. Combining high-resolution transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry, we characterized the successive formation of a thin zinc aluminate (ZnAl2O4) and zinc oxide interfacial layers, which act as an effective barrier layer against oxygen diffusion at low temperature.

Self-Organized Al2Cu Nanocrystals at the Interface of Aluminum-Based Reactive Nanolaminates to Lower Reaction Onset Temperature.

Nanoenergetic materials are beginning to play an important role in part because they are being considered as energetic components for materials, chemical, and biochemical communities (e.g., microthermal sources, microactuators, in situ welding and soldering, local enhancement of chemical reactions, nanosterilization, and controlled cell apoptosis) and because their fabrication/synthesis raises fundamental challenges that are pushing the engineering and scientific frontiers. One such challenge is the development of processes to control and enhance the reactivity of materials such as energetics of nanolaminates, and the understanding of associated mechanisms. We present here a new method to substantially decrease the reaction onset temperature and in consequence the reactivity of nanolaminates based on the incorporation of a Cu nanolayer at the interfaces of Al/CuO nanolaminates. We further demonstrate that control of its thickness allows accurate tuning of both the thermal transport and energetic properties of the system. Using high resolution transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry to analyze the physical, chemical and thermal characteristics of the resulting Al/CuO + interfacial Cu nanolaminates, we find that the incorporation of 5 nm Cu at both Al/CuO and CuO/Al interfaces lowers the onset temperature from 550 to 475 °C because of the lower-temperature formation of Al-Cu intermetallic phases and alloying. Cu intermixing is different in the CuO/Cu/Al and Al/Cu/CuO interfaces and independent of total Cu thickness: Cu readily penetrates into Al grains upon annealing to 300 °C, leading to Al/Cu phase transformations, while Al does not penetrate into Cu. Importantly, θ-Al2Cu nanocrystals are created below 63% wt Cu/Al, and coexist with the Al solid solution phase. These well-defined θ-Al2Cu nanocrystals seem to act as embedded Al+CuO energetic reaction triggers that lower the onset temperature. We show that ∼10 nm thick Cu at Al/CuO interfaces constitutes the optimum amount to increase both reactivity and overall heat of reaction by a factor of ∼20%. Above this amount, there is a rapid decrease of the heat of reaction.

Interface Magnetoelectric Coupling in Co/Pb(Zr,Ti)O3.

Magnetoelectric coupling at multiferroic interfaces is a promising route toward the nonvolatile electric-field control of magnetization. Here, we use optical measurements to study the static and dynamic variations of the interface magnetization induced by an electric field in Co/PbZr0.2Ti0.8O3 (Co/PZT) bilayers at room temperature. The measurements allow us to identify different coupling mechanisms. We further investigate the local electronic and magnetic structure of the interface by means of transmission electron microscopy, soft X-ray magnetic circular dichroism, and density functional theory to corroborate the coupling mechanism. The measurements demonstrate a mixed linear and quadratic optical response to the electric field, which results from a magneto-electro-optical effect. We propose a decomposition method of the optical signal to discriminate between different components involved in the electric field-induced polarization rotation of the reflected light. This allows us to extract a signal that we can ascribe to interface magnetoelectric coupling. The associated surface magnetization exhibits a clear hysteretic variation of odd symmetry with respect to the electric field and nonzero remanence. The interface coupling is remarkably stable over a wide frequency range (1-50 kHz), and the application of a bias magnetic field is not necessary for the coupling to occur. These results show the potential of exploiting interface coupling with the prospect of optimizing the performance of magnetoelectric memory devices in terms of stability, as well as fast and dissipationless operation.

Size-Specific Spin Configurations in Single Iron Nanomagnet: From Flower to Exotic Vortices.

The different spin configurations in the vicinity of the single-domain/vortex transition are reported in isolated magnetic nanoparticles. By combining chemical synthesis, electron holography in a dedicated transmission electron microscope and micromagnetic simulations, we establish the "magnetic configurations vs size" phase diagram of Fe single-crystalline nanocubes. Room temperature high resolution magnetic maps reveal the transition between single-domain and vortex states for Fe nanocubes from 25 to 27 nm, respectively. An intermediate spin configuration consisting of an ⟨111⟩ vortex is for the first time evidenced.

Oriented Metallic Nano-Objects on Crystalline Surfaces by Solution Epitaxial Growth.

Chemical methods offer the possibility to synthesize a large panel of nanostructures of various materials with promising properties. One of the main limitations to a mass market development of nanostructure based devices is the integration at a moderate cost of nano-objects into smart architectures. Here we develop a general approach by adapting the seed-mediated solution phase synthesis of nanocrystals in order to directly grow them on crystalline thin films. Using a Co precursor, single-crystalline Co nanowires are directly grown on metallic films and present different spatial orientations depending on the crystalline symmetry of the film used as a 2D seed for Co nucleation. Using films exposing 6-fold symmetry surfaces such as Pt(111), Au(111), and Co(0001), the Co heterogeneous nucleation and epitaxial growth leads to vertical nanowires self-organized in dense and large scale arrays. On the other hand, using films presenting 4-fold symmetry surfaces such as Pt(001) and Cu(001), the Co growth leads to slanted wires in discrete directions. The generality of the concept is demonstrated with the use of a Fe precursor which results in Fe nanostructures on metallic films with different growth orientations which depend on the 6-fold/4-fold symmetry of the film. This approach of solution epitaxial growth combines the advantages of chemistry in solution in producing shape-controlled and monodisperse metallic nanocrystals, and of seeded growth on an ad hoc metallic film that efficiently controls orientation through epitaxy. It opens attractive opportunities for the integration of nanocrystals in planar devices.

Enhancing the Reactivity of Al/CuO Nanolaminates by Cu Incorporation at the Interfaces.

In situ deposition of a thin (∼5 nm) layer of copper between Al and CuO layers is shown to increase the overall nanolaminate material reactivity. A combination of transmission electron microscopy imaging, in situ infrared spectroscopy, low energy ion scattering measurements, and first-principles calculations reveals that copper spontaneously diffuses into aluminum layers (substantially less in CuO layers). The formation of an interfacial Al:Cu alloy with melting temperature lower than pure Al metal is responsible for the enhanced reactivity, opening a route to controlling the stochiometry of the aluminum layer and increasing the reactivity of the nanoenergetic multilayer systems in general.

Solution epitaxial growth of cobalt nanowires on crystalline substrates for data storage densities beyond 1 Tbit/in2.

The implementation of nano-objects in numerous emerging applications often demands their integration in macroscopic devices. Here we present the bottom-up epitaxial solution growth of high-density arrays of vertical 5 nm diameter single-crystalline metallic cobalt nanowires on wafer-scale crystalline metal surfaces. The nanowires form regular hexagonal arrays on unpatterned metallic films. These hybrid heterostructures present an important perpendicular magnetic anisotropy and pave the way to a high density magnetic recording device, with capacities above 10 Terabits/in(2). This method bypasses the need of assembling and orientating free colloidal nanocrystals on surfaces. Its generalization to other materials opens new perspectives toward many applications.

Stabilizing vortices in interacting nano-objects: a chemical approach.

We report a chemical method to prepare metallic Fe porous nanocubes. The presence of pores embedded inside the cubes was attested by electron tomography. Thanks to electronic holography and micromagnetic simulations, we show that the presence of these defects stabilizes the vortices in assembly of interacting cubes. These results open new perspectives toward magnetic vortex stabilization at relatively low cost for various applications (microelectronics, magnetic recording, or biological applications).