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1.
Artigo em Inglês | MEDLINE | ID: mdl-33232102

RESUMO

We report spin-torque ferromagnetic resonance studies of the efficiency of the damping-like (ξDL) spin-orbit torque exerted on an adjacent ferromagnet film by current flowing in epitaxial (001) and (110) IrO2 thin films. IrO2 possesses Dirac nodal lines (DNLs) in the band structure that are gapped by spin-orbit coupling, which could enable a very high spin Hall conductivity, σSH. We find that the (001) films do exhibit exceptionally high ξDL ranging from 0.45 at 293 K to 0.65 at 30 K, which sets the lower bounds of σSH to be 1.9 × 105 and 3.75 × 105 Ω-1 m-1, respectively, 10 times higher and of opposite sign than the theoretical prediction. Furthermore, ξDL and σSH are substantially reduced in anisotropically strained (110) films. We suggest that this high sensitivity to anisotropic strain is because of changes in contributions to σSH near the DNLs.

2.
Sci Rep ; 10(1): 18468, 2020 10 28.
Artigo em Inglês | MEDLINE | ID: mdl-33116241

RESUMO

The development of microarray patches for vaccine application has the potential to revolutionise vaccine delivery. Microarray patches (MAP) reduce risks of needle stick injury, do not require reconstitution and have the potential to enhance immune responses using a fractional vaccine dose. To date, the majority of research has focused on vaccine delivery with little characterisation of local skin response and recovery. Here we study in detail the immediate local skin response and recovery of the skin post high density MAP application in 12 individuals receiving 3 MAPs randomly assigned to the forearm and upper arm. Responses were characterised by clinical scoring, dermatoscopy, evaporimetry and tissue viability imaging (TiVi). MAP application resulted in punctures in the epidermis, a significant transepidermal water loss (TEWL), the peak TEWL being concomitant with peak erythema responses visualised by TiVi. TEWL and TiVi responses reduced over time, with TEWL returning to baseline by 48 h and erythema fading over the course of a 7 day period. As MAPs for vaccination move into larger clinical studies more variation of individual subject phenotypic or disease propensity will be encountered which will require consideration both in regard to reliability of dose delivery and degree of inherent skin response.

3.
Nature ; 584(7822): 557-561, 2020 08.
Artigo em Inglês | MEDLINE | ID: mdl-32848225

RESUMO

Fifty years of Moore's law scaling in microelectronics have brought remarkable opportunities for the rapidly evolving field of microscopic robotics1-5. Electronic, magnetic and optical systems now offer an unprecedented combination of complexity, small size and low cost6,7, and could be readily appropriated for robots that are smaller than the resolution limit of human vision (less than a hundred micrometres)8-11. However, a major roadblock exists: there is no micrometre-scale actuator system that seamlessly integrates with semiconductor processing and responds to standard electronic control signals. Here we overcome this barrier by developing a new class of voltage-controllable electrochemical actuators that operate at low voltages (200 microvolts), low power (10 nanowatts) and are completely compatible with silicon processing. To demonstrate their potential, we develop lithographic fabrication-and-release protocols to prototype sub-hundred-micrometre walking robots. Every step in this process is performed in parallel, allowing us to produce over one million robots per four-inch wafer. These results are an important advance towards mass-manufactured, silicon-based, functional robots that are too small to be resolved by the naked eye.

4.
Nat Commun ; 11(1): 2994, 2020 06 12.
Artigo em Inglês | MEDLINE | ID: mdl-32533001

RESUMO

Both high resolution and high precision are required to quantitatively determine the atomic structure of complex nanostructured materials. However, for conventional imaging methods in scanning transmission electron microscopy (STEM), atomic resolution with picometer precision cannot usually be achieved for weakly-scattering samples or radiation-sensitive materials, such as 2D materials. Here, we demonstrate low-dose, sub-angstrom resolution imaging with picometer precision using mixed-state electron ptychography. We show that correctly accounting for the partial coherence of the electron beam is a prerequisite for high-quality structural reconstructions due to the intrinsic partial coherence of the electron beam. The mixed-state reconstruction gains importance especially when simultaneously pursuing high resolution, high precision and large field-of-view imaging. Compared with conventional atomic-resolution STEM imaging techniques, the mixed-state ptychographic approach simultaneously provides a four-times-faster acquisition, with double the information limit at the same dose, or up to a fifty-fold reduction in dose at the same resolution.

5.
Nano Lett ; 20(7): 4850-4856, 2020 07 08.
Artigo em Inglês | MEDLINE | ID: mdl-32525319

RESUMO

Origami design principles are scale invariant and enable direct miniaturization of origami structures provided the sheets used for folding have equal thickness to length ratios. Recently, seminal steps have been taken to fabricate microscale origami using unidirectionally actuated sheets with nanoscale thickness. Here, we extend the full power of origami-inspired fabrication to nanoscale sheets by engineering bidirectional folding with 4 nm thick atomic layer deposition (ALD) SiNx-SiO2 bilayer films. Strain differentials within these bilayers result in bending, producing microscopic radii of curvature. We lithographically pattern these bilayers and localize the bending using rigid panels to fabricate a variety of complex micro-origami devices. Upon release, these devices self-fold according to prescribed patterns. Our approach combines planar semiconductor microfabrication methods with computerized origami design, making it easy to fabricate and deploy such microstructures en masse. These devices represent an important step forward in the fabrication and assembly of deployable micromechanical systems that can interact with and manipulate micro- and nanoscale environments.

6.
Ultramicroscopy ; 215: 113019, 2020 May 26.
Artigo em Inglês | MEDLINE | ID: mdl-32521385

RESUMO

Friedel's law guarantees an inversion-symmetric diffraction pattern for thin, light materials where a kinematic approximation or a single-scattering model holds. Typically, breaking Friedel symmetry is ascribed to multiple scattering events within thick, non-centrosymmetric crystals. However, two-dimensional (2D) materials such as a single monolayer of MoS2 can also violate Friedel's law, with unexpected contrast between conjugate Bragg peaks. We show analytically that retaining higher order terms in the power series expansion of the scattered wavefunction can describe the anomalous contrast between hkl and hkl¯peaks that occurs in 2D crystals with broken in-plane inversion symmetry. These higher-order terms describe multiple scattering paths starting from the same atom in an atomically thin material. Furthermore, 2D materials containing heavy elements, such as WS2, always act as strong phase objects, violating Friedel's law no matter how high the energy of the incident electron beam. Experimentally, this understanding can enhance diffraction-based techniques to provide rapid imaging of polarity, twin domains, in-plane rotations, or other polar textures in 2D materials.

7.
Nano Lett ; 20(6): 4095-4101, 2020 Jun 10.
Artigo em Inglês | MEDLINE | ID: mdl-32396734

RESUMO

Tuning electrical conductivity of semiconducting materials through substitutional doping is crucial for fabricating functional devices. This, however, has not been fully realized in two-dimensional (2D) materials due to the difficulty of homogeneously controlling the dopant concentrations and the lack of systematic study of the net impact of substitutional dopants separate from that of the unintentional doping from the device fabrication processes. Here, we grow wafer-scale, continuous MoS2 monolayers with tunable concentrations of Nb and Re and fabricate devices using a polymer-free approach to study the direct electrical impact of substitutional dopants in MoS2 monolayers. In particular, the electrical conductivity of Nb doped MoS2 in the absence of electrostatic gating is reproducibly tuned over 7 orders of magnitude by controlling the Nb concentration. Our study further indicates that the dopant carriers do not fully ionize in the 2D limit, unlike in their three-dimensional analogues, which is explained by weaker charge screening and impurity band conduction. Moreover, we show that the dopants are stable, which enables the doped films to be processed as independent building blocks that can be used as electrodes for functional circuitry.

8.
Ultramicroscopy ; 214: 112994, 2020 May 04.
Artigo em Inglês | MEDLINE | ID: mdl-32413681

RESUMO

Scanning nanobeam electron diffraction (NBED) with fast pixelated detectors is a valuable technique for rapid, spatially resolved mapping of lattice structure over a wide range of length scales. However, intensity variations caused by dynamical diffraction and sample mistilts can hinder the measurement of diffracted disk centers as necessary for quantification. Robust data processing techniques are needed to provide accurate and precise measurements for complex samples and non-ideal conditions. Here we present an approach to address these challenges using a transform, called the exit wave power cepstrum (EWPC), inspired by cepstral analysis in audio signal processing. The EWPC transforms NBED patterns into real-space patterns with sharp peaks corresponding to inter-atomic spacings. We describe a simple analytical model for interpretation of these patterns that cleanly decouples lattice information from the intensity variations in NBED patterns caused by tilt and thickness. By tracking the inter-atomic spacing peaks in EWPC patterns, strain mapping is demonstrated for two practical applications: mapping of ferroelectric domains in epitaxially strained PbTiO3 films and mapping of strain profiles in arbitrarily oriented core-shell Pt-Co nanoparticle fuel-cell catalysts. The EWPC transform enables lattice structure measurement at sub-pm precision and sub-nm resolution that is robust to small sample mistilts and random orientations.

9.
Phys Rev Lett ; 124(5): 057201, 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-32083901

RESUMO

Magnets with chiral crystal structures and helical spin structures have recently attracted much attention as potential spin-electronics materials, but their relatively low magnetic-ordering temperatures are a disadvantage. While cobalt has long been recognized as an element that promotes high-temperature magnetic ordering, most Co-rich alloys are achiral and exhibit collinear rather than helimagnetic order. Crystallographically, the B20-ordered compound CoSi is an exception due to its chiral structure, but it does not exhibit any kind of magnetic order. Here, we use nonequilibrium processing to produce B20-ordered Co_{1+x}Si_{1-x} with a maximum Co solubility of x=0.043. Above a critical excess-Co content (x_{c}=0.028), the alloys are magnetically ordered, and for x=0.043, a critical temperature T_{c}=328 K is obtained, the highest among all B20-type magnets. The crystal structure of the alloy supports spin spirals caused by Dzyaloshinskii-Moriya interactions, and from magnetic measurements we estimate that the spirals have a periodicity of about 17 nm. Our density-functional calculations explain the combination of high magnetic-ordering temperature and short periodicity in terms of a quantum phase transition where excess-cobalt spins are coupled through the host matrix.

10.
Nat Nanotechnol ; 2020 Feb 10.
Artigo em Inglês | MEDLINE | ID: mdl-32042164

RESUMO

Although conventional homoepitaxy forms high-quality epitaxial layers1-5, the limited set of material systems for commercially available wafers restricts the range of materials that can be grown homoepitaxially. At the same time, conventional heteroepitaxy of lattice-mismatched systems produces dislocations above a critical strain energy to release the accumulated strain energy as the film thickness increases. The formation of dislocations, which severely degrade electronic/photonic device performances6-8, is fundamentally unavoidable in highly lattice-mismatched epitaxy9-11. Here, we introduce a unique mechanism of relaxing misfit strain in heteroepitaxial films that can enable effective lattice engineering. We have observed that heteroepitaxy on graphene-coated substrates allows for spontaneous relaxation of misfit strain owing to the slippery graphene surface while achieving single-crystalline films by reading the atomic potential from the substrate. This spontaneous relaxation technique could transform the monolithic integration of largely lattice-mismatched systems by covering a wide range of the misfit spectrum to enhance and broaden the functionality of semiconductor devices for advanced electronics and photonics.

11.
Nat Mater ; 19(2): 176-181, 2020 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-31873229

RESUMO

Epitaxial strain can unlock enhanced properties in oxide materials, but restricts substrate choice and maximum film thickness, above which lattice relaxation and property degradation occur. Here we employ a chemical alternative to epitaxial strain by providing targeted chemical pressure, distinct from random doping, to induce a ferroelectric instability with the strategic introduction of barium into today's best millimetre-wave tuneable dielectric, the epitaxially strained 50-nm-thick n = 6 (SrTiO3)nSrO Ruddlesden-Popper dielectric grown on (110) DyScO3. The defect mitigating nature of (SrTiO3)nSrO results in unprecedented low loss at frequencies up to 125 GHz. No barium-containing Ruddlesden-Popper titanates are known, but the resulting atomically engineered superlattice material, (SrTiO3)n-m(BaTiO3)mSrO, enables low-loss, tuneable dielectric properties to be achieved with lower epitaxial strain and a 200% improvement in the figure of merit at commercially relevant millimetre-wave frequencies. As tuneable dielectrics are key constituents of emerging millimetre-wave high-frequency devices in telecommunications, our findings could lead to higher performance adaptive and reconfigurable electronics at these frequencies.

12.
Vaccines (Basel) ; 7(4)2019 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-31756967

RESUMO

Dengue virus is the most important arbovirus impacting global human health, with an estimated 390 million infections annually, and over half the world's population at risk of infection. While significant efforts have been made to develop effective vaccines to mitigate this threat, the task has proven extremely challenging, with new approaches continually being sought. The majority of protective, neutralizing antibodies induced during infection are targeted by the envelope (E) protein, making it an ideal candidate for a subunit vaccine approach. Using truncated, recombinant, secreted E proteins (sE) of all 4 dengue virus serotypes, we have assessed their immunogenicity and protective efficacy in mice, with or without Quil-A as an adjuvant, and delivered via micropatch array (MPA) to the skin in comparison with more traditional routes of immunization. The micropatch contains an ultra-high density array (21,000/cm2) of 110 µm microprojections. Mice received 3 doses of 1 µg (nanopatch, intradermal, subcutaneous, or intra muscular injection) or 10 µg (intradermal, subcutaneous, or intra muscular injection) of tetravalent sE spaced 4 weeks apart. When adjuvanted with Quil-A, tetravalent sE vaccination delivered via MPA resulted in earlier induction of virus-neutralizing IgG antibodies for all four serotypes when compared with all of the other vaccination routes. Using the infectious dengue virus AG129 mouse infectious dengue model, these neutralizing antibodies protected all mice from lethal dengue virus type 2 D220 challenge, with protected animals showing no signs of disease or circulating virus. If these results can be translated to humans, MPA-delivered sE represents a promising approach to dengue virus vaccination.

13.
Nano Lett ; 19(12): 9154-9159, 2019 12 11.
Artigo em Inglês | MEDLINE | ID: mdl-31738851

RESUMO

The local structure and geometry of catalytic interfaces can influence the selectivity of chemical reactions. Selectivity is often critical for the practical realization of reactions such as the electroreduction of carbon dioxide (CO2). Previously developed strategies to manipulate the structure and geometry of catalysts for electroreduction of CO2 involve complex processes or fail to efficiently alter the selectivity. Here, using a prestrained polymer, we uniaxially and biaxially compress a 60 nm gold film to form a nano-folded electrocatalyst for CO2 reduction. We observe two kinds of folds and can tune the ratio of loose to tight folds by varying the extent of prestrain in the polymer. We characterize the nano-folded catalysts using X-ray diffraction, scanning, and transmission electron microscopy. We observe grain reorientation and coarsening in the nano-folded gold catalysts. We measure an enhancement of Faradaic efficiency for carbon monoxide formation with the biaxially compressed nano-folded catalyst by a factor of about nine as compared to the flat catalyst (up to 87.4%). We rationalize this observation by noting that an increase of the local pH in the tight folds of the catalyst outweighs the effects of alterations in grain characteristics. Together, our studies demonstrate that nano-folded geometries can significantly alter grain characteristics, mass transport, and catalytic performance.

14.
Proc Natl Acad Sci U S A ; 116(49): 24425-24432, 2019 12 03.
Artigo em Inglês | MEDLINE | ID: mdl-31727846

RESUMO

Designing high-performance nonprecious electrocatalysts to replace Pt for the oxygen reduction reaction (ORR) has been a key challenge for advancing fuel cell technologies. Here, we report a systematic study of 15 different AB2O4/C spinel nanoparticles with well-controlled octahedral morphology. The 3 most active ORR electrocatalysts were MnCo2O4/C, CoMn2O4/C, and CoFe2O4/C. CoMn2O4/C exhibited a half-wave potential of 0.89 V in 1 M KOH, equal to the benchmark activity of Pt/C, which was ascribed to charge transfer between Co and Mn, as evidenced by X-ray absorption spectroscopy. Scanning transmission electron microscopy (STEM) provided atomic-scale, spatially resolved images, and high-energy-resolution electron-loss near-edge structure (ELNES) enabled fingerprinting the local chemical environment around the active sites. The most active MnCo2O4/C was shown to have a unique Co-Mn core-shell structure. ELNES spectra indicate that the Co in the core is predominantly Co2.7+ while in the shell, it is mainly Co2+ Broader Mn ELNES spectra indicate less-ordered nearest oxygen neighbors. Co in the shell occupies mainly tetrahedral sites, which are likely candidates as the active sites for the ORR. Such microscopic-level investigation probes the heterogeneous electronic structure at the single-nanoparticle level, and may provide a more rational basis for the design of electrocatalysts for alkaline fuel cells.

15.
Science ; 366(6471): 1379-1384, 2019 12 13.
Artigo em Inglês | MEDLINE | ID: mdl-31699884

RESUMO

The large-scale synthesis of high-quality thin films with extensive tunability derived from molecular building blocks will advance the development of artificial solids with designed functionalities. We report the synthesis of two-dimensional (2D) porphyrin polymer films with wafer-scale homogeneity in the ultimate limit of monolayer thickness by growing films at a sharp pentane/water interface, which allows the fabrication of their hybrid superlattices. Laminar assembly polymerization of porphyrin monomers could form monolayers of metal-organic frameworks with Cu2+ linkers or covalent organic frameworks with terephthalaldehyde linkers. Both the lattice structures and optical properties of these 2D films were directly controlled by the molecular monomers and polymerization chemistries. The 2D polymers were used to fabricate arrays of hybrid superlattices with molybdenum disulfide that could be used in electrical capacitors.

16.
Nano Lett ; 19(11): 7901-7907, 2019 11 13.
Artigo em Inglês | MEDLINE | ID: mdl-31596599

RESUMO

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using "phonon currents". With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the room-temperature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10-9 K m2 W-1), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO3. This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics.

17.
Nano Lett ; 19(11): 8287-8293, 2019 11 13.
Artigo em Inglês | MEDLINE | ID: mdl-31661615

RESUMO

Quantum computing based on superconducting qubits requires the understanding and control of the materials, device architecture, and operation. However, the materials for the central circuit element, the Josephson junction, have mostly been focused on using the AlOx tunnel barrier. Here, we demonstrate Josephson junctions and superconducting qubits employing two-dimensional materials as the tunnel barrier. We batch-fabricate and design the critical Josephson current of these devices via layer-by-layer stacking N layers of MoS2 on the large scale. Based on such junctions, MoS2 transmon qubits are engineered and characterized in a bulk superconducting microwave resonator for the first time. Our work allows Josephson junctions to access the diverse material properties of two-dimensional materials that include a wide range of electrical and magnetic properties, which can be used to study the effects of different material properties in superconducting qubits and to engineer novel quantum circuit elements in the future.

18.
Science ; 365(6460): 1454-1457, 2019 09 27.
Artigo em Inglês | MEDLINE | ID: mdl-31604274

RESUMO

A high-conductivity two-dimensional (2D) hole gas, analogous to the ubiquitous 2D electron gas, is desirable in nitride semiconductors for wide-bandgap p-channel transistors. We report the observation of a polarization-induced high-density 2D hole gas in epitaxially grown gallium nitride on aluminium nitride and show that such hole gases can form without acceptor dopants. The measured high 2D hole gas densities of about 5 × 1013 per square centimeters remain unchanged down to cryogenic temperatures and allow some of the lowest p-type sheet resistances among all wide-bandgap semiconductors. The observed results provide a probe for studying the valence band structure and transport properties of wide-bandgap nitride interfaces.

19.
ACS Appl Mater Interfaces ; 11(44): 41178-41187, 2019 Nov 06.
Artigo em Inglês | MEDLINE | ID: mdl-31600433

RESUMO

Lithium nickel manganese cobalt oxide (NMC) materials, with low cost and high energy density, are considered to be among the most promising cathode materials for Li-ion batteries (LIBs). However, several issues have hindered their further deployment, particularly for high-powered applications, including limited rate capability, capacity loss during cycling (especially at high temperatures and high voltages), and difficulty in reproducibly preparing the desired particle morphology. In this work, we have developed a robust LiNi0.33Mn0.33Co0.33O2 cathode material (NMC-111) capable of high-rate performance for LIBs. Our high power NMC-111 (HP-NMC) cathode materials showed significantly enhanced electrochemical performance, relative to a commercial NMC-111 (c-NMC), with discharge capacities of 138 and 131 mAh/g at high current rates of 20 and 30 C, respectively. The material also exhibited enhanced cycling stability under both room temperature and at 50 °C. We ascribe the high performance of our material to a unique crystalline microstructure observed by electron microscopy characterization, which showed preferential orientation of the Li-diffusing channels radially outward. This HP-NMC material achieved one of the highest performance metrics among NMC materials reported to date, especially for high-powered electric vehicles.

20.
Nano Lett ; 19(10): 6845-6852, 2019 10 09.
Artigo em Inglês | MEDLINE | ID: mdl-31478675

RESUMO

Two-dimensional (2D) layered semiconductors, with their ultimate atomic thickness, have shown promise to scale down transistors for modern integrated circuitry. However, the electrical contacts that connect these materials with external bulky metals are usually unsatisfactory, which limits the transistor performance. Recently, contacting 2D semiconductors using coplanar 2D conductors has shown promise in reducing the problematic high contact resistance. However, many of these methods are not ideal for scaled production. Here, we report on the large-scale, spatially controlled chemical assembly of the integrated 2H-MoTe2 field-effect transistors (FETs) with coplanar metallic 1T'-MoTe2 contacts via phase engineered approaches. We demonstrate that the heterophase FETs exhibit ohmic contact behavior with low contact resistance, resulting from the coplanar seamless contact between 2H and 1T'-MoTe2 confirmed by transmission electron microscopy characterizations. The average mobility of the heterophase FETs was measured to be as high as 23 cm2 V-1 s-1 (comparable with those of exfoliated single crystals), due to the large 2H-MoTe2 single-crystalline domain size (486 ± 187 µm). By developing a patterned growth method, we realize the 1T'-MoTe2 gated heterophase FET array whose components of the channel, gate, and contacts are all 2D materials. Finally, we transfer the heterophase device array onto a flexible substrate and demonstrate the near-infrared photoresponse with high photoresponsivity (∼1.02 A/W). Our study provides a basis for the large-scale application of phase-engineered coplanar MoTe2 semiconductor-metal structure in advanced electronics and optoelectronics.

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