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1.
Nanoscale ; 12(1): 189-194, 2020 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-31803884

RESUMO

Using state-of-the-art electron-beam lithography, Ising-type nanomagnets may be defined onto nearly any two-dimensional pattern imaginable. The ability to directly observe magnetic configurations achieved in such artificial spin systems makes them a perfect playground for the realization of artificial spin glasses. However, no experimental realization of a finite-temperature artificial spin glass has been achieved so far. Here, we aim to get a significant step closer in achieving that goal by introducing an artificial spin system with random interactions and increased effective dimension: dipolar Cayley tree. Through synchrotron-based photoemission electron microscopy, we show that an improved balance of ferro- and antiferromagnetic ordering can be achieved in this type of system. This combined with an effective dimension as high as d = 2.72 suggests that future systems generated out of these building blocks can host finite temperature spin glass phases, allowing for real-time observation of glassy dynamics.

2.
ACS Nano ; 13(12): 13910-13916, 2019 Dec 24.
Artigo em Inglês | MEDLINE | ID: mdl-31820931

RESUMO

Artificial spin ices are a class of metamaterials consisting of magnetostatically coupled nanomagnets. Their interactions give rise to emergent behavior, which has the potential to be harnessed for the creation of functional materials. Consequently, the ability to map the stray field of such systems can be decisive for gaining an understanding of their properties. Here, we use a scanning nanometer-scale superconducting quantum interference device (SQUID) to image the magnetic stray field distribution of an artificial spin ice system exhibiting structural chirality as a function of applied magnetic fields at 4.2 K. The images reveal that the magnetostatic interaction gives rise to a measurable bending of the magnetization at the edges of the nanomagnets. Micromagnetic simulations predict that, owing to the structural chirality of the system, this edge bending is asymmetric in the presence of an external field and gives rise to a preferred direction for the reversal of the magnetization. This effect is not captured by models assuming a uniform magnetization. Our technique thus provides a promising means for understanding the collective response of artificial spin ices and their interactions.

3.
J Synchrotron Radiat ; 26(Pt 3): 785-792, 2019 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-31074443

RESUMO

The successful design, installation and operation of a high spatial resolution X-ray photoelectron spectrometer at the Swiss Light Source is presented. In this instrument, a Fresnel zone plate is used to focus an X-ray beam onto the sample and an electron analyzer positioned at 45° with respect to the incoming beam direction is used to collect photoelectrons from the backside of the sample. By raster scanning the sample, transmitted current, X-ray absorption and X-ray photoemission maps can be simultaneously acquired. This work demonstrates that chemical information can be extracted with micrometre resolution; the results suggest that a spatial resolution better than 100 nm can be achieved with this approach in future. This kind of photoelectron spectromicroscope will allow in situ measurements with high spatial resolution also under ambient pressure conditions (in the millibar range). Element-specific X-ray photoemission maps can be obtained before and while exposing the sample to gas/gas mixtures to show morphological and chemical changes of the surface.

4.
Science ; 363(6434): 1435-1439, 2019 03 29.
Artigo em Inglês | MEDLINE | ID: mdl-30923219

RESUMO

Magnetically coupled nanomagnets have multiple applications in nonvolatile memories, logic gates, and sensors. The most effective couplings have been found to occur between the magnetic layers in a vertical stack. We achieved strong coupling of laterally adjacent nanomagnets using the interfacial Dzyaloshinskii-Moriya interaction. This coupling is mediated by chiral domain walls between out-of-plane and in-plane magnetic regions and dominates the behavior of nanomagnets below a critical size. We used this concept to realize lateral exchange bias, field-free current-induced switching between multistate magnetic configurations as well as synthetic antiferromagnets, skyrmions, and artificial spin ices covering a broad range of length scales and topologies. Our work provides a platform to design arrays of correlated nanomagnets and to achieve all-electric control of planar logic gates and memory devices.

5.
Sci Adv ; 5(2): eaav4489, 2019 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-30801017

RESUMO

Stable, single-nanometer thin, and free-standing two-dimensional layers with controlled molecular architectures are desired for several applications ranging from (opto-)electronic devices to nanoparticle and single-biomolecule characterization. It is, however, challenging to construct these stable single molecular layers via self-assembly, as the cohesion of those systems is ensured only by in-plane bonds. We herein demonstrate that relatively weak noncovalent bonds of limited directionality such as dipole-dipole (-CN⋅⋅⋅NC-) interactions act in a synergistic fashion to stabilize crystalline monomolecular layers of tetrafunctional calixarenes. The monolayers produced, demonstrated to be free-standing, display a well-defined atomic structure on the single-nanometer scale and are robust under a wide range of conditions including photon and electron radiation. This work opens up new avenues for the fabrication of robust, single-component, and free-standing layers via bottom-up self-assembly.

6.
Nat Nanotechnol ; 13(12): 1161-1166, 2018 12.
Artigo em Inglês | MEDLINE | ID: mdl-30275493

RESUMO

Magnetic skyrmions are knot-like quasiparticles. They are candidates for non-volatile data storage in which information is moved between fixed read and write terminals. The read-out operation of skyrmion-based spintronic devices will rely on the electrical detection of a single magnetic skyrmion within a nanostructure. Here we present Pt/Co/Ir nanodiscs that support skyrmions at room temperature. We measured the Hall resistivity and simultaneously imaged the spin texture using magnetic scanning transmission X-ray microscopy. The Hall resistivity is correlated to both the presence and size of the skyrmion. The size-dependent part matches the expected anomalous Hall signal when averaging the magnetization over the entire disc. We observed a resistivity contribution that only depends on the number and sign of skyrmion-like objects present in the disc. Each skyrmion gives rise to 22 ± 2 nΩ cm irrespective of its size. This contribution needs to be considered in all-electrical detection schemes applied to skyrmion-based devices. Not only the area of Néel skyrmions but also their number and sign contribute to their Hall resistivity.

7.
Opt Express ; 26(9): 12242-12256, 2018 Apr 30.
Artigo em Inglês | MEDLINE | ID: mdl-29716137

RESUMO

While the industrial implementation of extreme ultraviolet lithography for upcoming technology nodes is becoming ever more realistic, a number of challenges have yet to be overcome. Among them is the need for actinic mask inspection. We report on reflective-mode lensless imaging of a patterned multi-layer mask sample at extreme ultraviolet wavelength that provides a finely structured defect map of the sample under test. Here, we present the imaging results obtained using ptychography in reflection mode at 6° angle of incidence from the surface normal and 13.5 nm wavelength. Moreover, an extended version of the difference map algorithm is employed that substantially enhances the reconstruction quality by taking into account both long and short-term variations of the incident illumination.

8.
Nanotechnology ; 29(26): 265205, 2018 Jun 29.
Artigo em Inglês | MEDLINE | ID: mdl-29620015

RESUMO

Nanomagnets are a promising low-power alternative to traditional computing. However, the successful implementation of nanomagnets in logic gates has been hindered so far by a lack of reliability. Here, we present a novel design with dipolar-coupled nanomagnets arranged on a square lattice to (i) support transfer of information and (ii) perform logic operations. We introduce a thermal protocol, using thermally active nanomagnets as a means to perform computation. Within this scheme, the nanomagnets are initialized by a global magnetic field and thermally relax on raising the temperature with a resistive heater. We demonstrate error-free transfer of information in chains of up to 19 square rings and we show a high level of reliability with successful gate operations of ∼94% across more than 2000 logic gates. Finally, we present a functionally complete prototype NAND/NOR logic gate that could be implemented for advanced logic operations. Here we support our experiments with simulations of the thermally averaged output and determine the optimal gate parameters. Our approach provides a new pathway to a long standing problem concerning reliability in the use of nanomagnets for computation.

9.
Nano Lett ; 18(2): 1205-1212, 2018 02 14.
Artigo em Inglês | MEDLINE | ID: mdl-29314849

RESUMO

Large-area hexagonal boron nitride (h-BN) promises many new applications of two-dimensional materials, such as the protective packing of reactive surfaces or as membranes in liquids. However, scalable production beyond exfoliation from bulk single crystals remained a major challenge. Single-orientation monolayer h-BN nanomesh is grown on 4 in. wafer single crystalline rhodium films and transferred on arbitrary substrates such as SiO2, germanium, or transmission electron microscopy grids. The transfer process involves application of tetraoctylammonium bromide before electrochemical hydrogen delamination. The material performance is demonstrated with two applications. First, protective sealing of h-BN is shown by preserving germanium from oxidation in air at high temperatures. Second, the membrane functionality of the single h-BN layer is demonstrated in aqueous solutions. Here, we employ a growth substrate intrinsic preparation scheme to create regular 2 nm holes that serve as ion channels in liquids.

10.
Nat Mater ; 16(11): 1106-1111, 2017 11.
Artigo em Inglês | MEDLINE | ID: mdl-29058727

RESUMO

Modern nanofabrication techniques have opened the possibility to create novel functional materials, whose properties transcend those of their constituent elements. In particular, tuning the magnetostatic interactions in geometrically frustrated arrangements of nanoelements called artificial spin ice can lead to specific collective behaviour, including emergent magnetic monopoles, charge screening and transport, as well as magnonic response. Here, we demonstrate a spin-ice-based active material in which energy is converted into unidirectional dynamics. Using X-ray photoemission electron microscopy we show that the collective rotation of the average magnetization proceeds in a unique sense during thermal relaxation. Our simulations demonstrate that this emergent chiral behaviour is driven by the topology of the magnetostatic field at the edges of the nanomagnet array, resulting in an asymmetric energy landscape. In addition, a bias field can be used to modify the sense of rotation of the average magnetization. This opens the possibility of implementing a magnetic Brownian ratchet, which may find applications in novel nanoscale devices, such as magnetic nanomotors, actuators, sensors or memory cells.

11.
Nat Commun ; 8(1): 700, 2017 09 26.
Artigo em Inglês | MEDLINE | ID: mdl-28951540

RESUMO

Oxidation of bromide in aqueous environments initiates the formation of molecular halogen compounds, which is important for the global tropospheric ozone budget. In the aqueous bulk, oxidation of bromide by ozone involves a [Br•OOO-] complex as intermediate. Here we report liquid jet X-ray photoelectron spectroscopy measurements that provide direct experimental evidence for the ozonide and establish its propensity for the solution-vapour interface. Theoretical calculations support these findings, showing that water stabilizes the ozonide and lowers the energy of the transition state at neutral pH. Kinetic experiments confirm the dominance of the heterogeneous oxidation route established by this precursor at low, atmospherically relevant ozone concentrations. Taken together, our results provide a strong case of different reaction kinetics and mechanisms of reactions occurring at the aqueous phase-vapour interface compared with the bulk aqueous phase.Heterogeneous oxidation of bromide in atmospheric aqueous environments has long been suspected to be accelerated at the interface between aqueous solution and air. Here, the authors provide spectroscopic, kinetic and theoretical evidence for a rate limiting, surface active ozonide formed at the interface.

12.
Sci Rep ; 7(1): 7253, 2017 08 03.
Artigo em Inglês | MEDLINE | ID: mdl-28775262

RESUMO

The advent of x-ray free electron lasers has extended the unique capabilities of resonant x-ray spectroscopy techniques to ultrafast time scales. Here, we report on a novel experimental method that allows retrieving with a single x-ray pulse the time evolution of an ultrafast process, not only at a few discrete time delays, but continuously over an extended time window. We used a single x-ray pulse to resolve the laser-induced ultrafast demagnetisation dynamics in a thin cobalt film over a time window of about 1.6 ps with an excellent signal to noise ratio. From one representative single shot measurement we extract a spin relaxation time of (130 ± 30) fs with an average value, based on 193 single shot events of (113 ± 20) fs. These results are limited by the achieved experimental time resolution of 120 fs, and both values are in excellent agreement with previous results and theoretical modelling. More generally, this new experimental approach to ultrafast x-ray spectroscopy paves the way to the study of non-repetitive processes that cannot be investigated using traditional repetitive pump-probe schemes.

13.
Nature ; 541(7635): 68-71, 2017 01 04.
Artigo em Inglês | MEDLINE | ID: mdl-28054605

RESUMO

Hydrogen spillover is the surface migration of activated hydrogen atoms from a metal catalyst particle, on which they are generated, onto the catalyst support. The phenomenon has been much studied and its occurrence on reducible supports such as titanium oxide is established, yet questions remain about whether hydrogen spillover can take place on nonreducible supports such as aluminium oxide. Here we use the enhanced precision of top-down nanofabrication to prepare controlled and precisely tunable model systems that allow us to quantify the efficiency and spatial extent of hydrogen spillover on both reducible and nonreducible supports. We place multiple pairs of iron oxide and platinum nanoparticles on titanium oxide and aluminium oxide supports, varying the distance between the pairs from zero to 45 nanometres with a precision of one nanometre. We then observe the extent of the reduction of the iron oxide particles by hydrogen atoms generated on the platinum using single-particle in situ X-ray absorption spectromicroscopy applied simultaneously to all particle pairs. The data, in conjunction with density functional theory calculations, reveal fast hydrogen spillover on titanium oxide that reduces remote iron oxide nanoparticles via coupled proton-electron transfer. In contrast, spillover on aluminium oxide is mediated by three-coordinated aluminium centres that also interact with water and that give rise to hydrogen mobility competing with hydrogen desorption; this results in hydrogen spillover about ten orders of magnitude slower than on titanium oxide and restricted to very short distances from the platinum particle. We anticipate that these observations will improve our understanding of hydrogen storage and catalytic reactions involving hydrogen, and that our approach to creating and probing model catalyst systems will provide opportunities for studying the origin of synergistic effects in supported catalysts that combine multiple functionalities.

14.
Phys Chem Chem Phys ; 18(42): 29506-29515, 2016 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-27747349

RESUMO

Despite the ubiquitous nature of aqueous solutions across the chemical, biological and environmental sciences our experimental understanding of their electronic structure is rudimentary-qualitative at best. One of the most basic and seemingly straightforward properties of aqueous solutions-ionization energies-are (qualitatively) tabulated at the water-air interface for a mere handful of solutes, and the manner in which these results are obtained assume the aqueous solutions behave like a gas in the photoelectron experiment (where the vacuum levels of the aqueous solution and of the photoelectron analyzer are equilibrated). Here we report the experimental measure of a sizeable offset (ca. 0.6 eV) between the vacuum levels of an aqueous solution (0.05 M NaCl) and that of our photoelectron analyzer, indicating a breakdown of the gas-like vacuum level alignment assumption for the aqueous solution. By quantifying the vacuum level offset as a function of solution chemical composition our measurements enable, for the first time, quantitative determination of ionization energies in liquid solutions. These results reveal that the ionization energy of liquid water is not independent of the chemical composition of the solution as is usually inferred in the literature, a finding that has important ramifications as measured ionization energies are frequently used to validate theoretical models that posses the ability to provide microscopic insight not directly available by experiment. Finally, we derive the work function, or the electrochemical potential of the aqueous solution and show that it too varies with the chemical composition of the solution.

15.
Sci Rep ; 6: 18818, 2016 Jan 06.
Artigo em Inglês | MEDLINE | ID: mdl-26732372

RESUMO

Understanding the chemistry of nanoparticles is crucial in many applications. Their synthesis in a controlled manner and their characterization at the single particle level is essential to gain deeper insight into chemical mechanisms. In this work, single nanoparticle spectro-microscopy with top-down nanofabrication is demonstrated to study individual iron nanoparticles of nine different lateral dimensions from 80 nm down to 6 nm. The particles are probed simultaneously, under same conditions, during in-situ redox reaction using X-ray photoemission electron microscopy elucidating the size effect during the early stage of oxidation, yielding time-dependent evolution of iron oxides and the mechanism for the inter-conversion of oxides in nanoparticles. Fabrication of well-defined system followed by visualization and investigation of singled-out particles eliminates the ambiguities emerging from dispersed nanoparticles and reveals a significant increase in the initial rate of oxidation with decreasing size, but the reactivity per active site basis and the intrinsic chemical properties in the particles remain the same in the scale of interest. This advance of nanopatterning together with spatially-resolved single nanoparticle X-ray absorption spectroscopy will guide future discourse in understanding the impact of confinement of metal nanoparticles and pave way to solve fundamental questions in material science, chemical physics, magnetism, nanomedicine and nanocatalysis.

16.
J Synchrotron Radiat ; 22(6): 1528-30, 2015 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-26524318

RESUMO

A 30 µm pinhole is introduced in the intermediate focus of the SIM beamline at the Swiss Light Source to improve the spot size at the second downstream focus, which is used here for liquid jet X-ray photoelectron spectroscopy experiments. The 30 µm pinhole reduces the beam dimensions from 250 (v) × 100 (h) µm to 75 × 45 µm for a vertical exit slit of 100 µm. The smaller X-ray spot results in a substantial decrease in the gas-phase contribution of the spectra from 40% down to 20% and will help to simplify the interpretation and peak assignments of future experiments.

17.
J Phys Chem A ; 119(19): 4600-8, 2015 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-25530167

RESUMO

A more detailed understanding of the heterogeneous chemistry of halogenated species in the marine boundary layer is required. Here, we studied the reaction of ozone (O3) with NaBr solutions in the presence and absence of citric acid (C6H8O7) under ambient conditions. Citric acid is used as a proxy for oxidized organic material present at the ocean surface or in sea spray aerosol. On neat NaBr solutions, the observed kinetics is consistent with bulk reaction-limited uptake, and a second-order rate constant for the reaction of O3 + Br(-) is 57 ± 10 M(-1) s(-1). On mixed NaBr-citric acid aqueous solutions, the uptake kinetics was faster than that predicted by bulk reaction-limited uptake and also faster than expected based on an acid-catalyzed mechanism. X-ray photoelectron spectroscopy (XPS) on a liquid microjet of the same solutions at 1.0 × 10(-3)-1.0 × 10(-4) mbar was used to obtain quantitative insight into the interfacial composition relative to that of the bulk solutions. It revealed that the bromide anion becomes depleted by 30 ± 10% while the sodium cation gets enhanced by 40 ± 20% at the aqueous solution-air interface of a 0.12 M NaBr solution mixed with 2.5 M citric acid in the bulk, attributed to the role of citric acid as a weak surfactant. Therefore, the enhanced reactivity of bromide solutions observed in the presence of citric acid is not necessarily attributable to a surface reaction but could also result from an increased solubility of ozone at higher citric acid concentrations. Whether the acid-catalyzed chemistry may have a larger effect on the surface than in the bulk to offset the effect of bromide depletion also remains open.


Assuntos
Ar , Brometos/química , Ozônio/química , Compostos de Sódio/química , Água/química , Ácido Cítrico/química , Cinética , Oceanos e Mares , Espectroscopia Fotoeletrônica , Soluções , Propriedades de Superfície , Tensoativos/química
18.
J Am Chem Soc ; 136(26): 9355-63, 2014 Jul 02.
Artigo em Inglês | MEDLINE | ID: mdl-24960576

RESUMO

The formation of on-surface coordination polymers is controlled by the interplay of chemical reactivity and structure of the building blocks, as well as by the orientating role of the substrate registry. Beyond the predetermined patterns of structural assembly, the chemical reactivity of the reactants involved may provide alternative pathways in their aggregation. Organic molecules, which are transformed in a surface reaction, may be subsequently trapped via coordination of homo- or heterometal adatoms, which may also play a role in the molecular transformation. The amino-functionalized perylene derivative, 4,9-diaminoperylene quinone-3,10-diimine (DPDI), undergoes specific levels of dehydrogenation (-1 H2 or -3 H2) depending on the nature of the present adatoms (Fe, Co, Ni or Cu). In this way, the molecule is converted to an endo- or an exoligand, possessing a concave or convex arrangement of ligating atoms, which is decisive for the formation of either 1D or 2D coordination polymers.

19.
Phys Rev Lett ; 112(10): 107201, 2014 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-24679323

RESUMO

X-ray photoemission electron microscopy combined with x-ray magnetic circular dichroism is used to study the magnetic properties of individual iron nanoparticles with sizes ranging from 20 down to 8 nm. While the magnetocrystalline anisotropy of bulk iron suggests superparamagnetic behavior in this size range, ferromagnetically blocked particles are also found at all sizes. Spontaneous transitions from the blocked state to the superparamagnetic state are observed in single particles and suggest that the enhanced magnetic energy barriers in the ferromagnetic particles are due to metastable, structurally excited states with unexpected life times.

20.
Chem Commun (Camb) ; 50(32): 4242-4, 2014 Apr 25.
Artigo em Inglês | MEDLINE | ID: mdl-24633394

RESUMO

Depth resolved X-ray photoelectron spectroscopy (XPS) combined with a 25 µm liquid jet is used to quantify the spatial distribution of 3 nm SnO2 nanoparticles (NPs) from the air-water interface (AWI) into the suspension bulk. Results are consistent with those of a layer several nm thick at the AWI that is completely devoid of NPs.

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