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1.
Phys Rev Lett ; 124(2): 027203, 2020 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-32004033

RESUMO

Spin waves can probe the Dzyaloshinskii-Moriya interaction (DMI), which gives rise to topological spin textures, such as skyrmions. However, the DMI has not yet been reported in yttrium iron garnet (YIG) with arguably the lowest damping for spin waves. In this work, we experimentally evidence the interfacial DMI in a 7-nm-thick YIG film by measuring the nonreciprocal spin-wave propagation in terms of frequency, amplitude, and most importantly group velocities using all electrical spin-wave spectroscopy. The velocities of propagating spin waves show chirality among three vectors, i.e., the film normal direction, applied field, and spin-wave wave vector. By measuring the asymmetric group velocities, we extract a DMI constant of 16 µJ/m^{2}, which we independently confirm by Brillouin light scattering. Thickness-dependent measurements reveal that the DMI originates from the oxide interface between the YIG and garnet substrate. The interfacial DMI discovered in the ultrathin YIG films is of key importance for functional chiral magnonics as ultralow spin-wave damping can be achieved.

2.
Nat Nanotechnol ; 14(7): 691-697, 2019 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-31011219

RESUMO

Spin waves may constitute key components of low-power spintronic devices. Antiferromagnetic-type spin waves are innately high-speed, stable and dual-polarized. So far, it has remained challenging to excite and manipulate antiferromagnetic-type propagating spin waves. Here, we investigate spin waves in periodic 100-nm-wide stripe domains with alternating upward and downward magnetization in La0.67Sr0.33MnO3 thin films. In addition to ordinary low-frequency modes, a high-frequency mode around 10 GHz is observed and propagates along the stripe domains with a spin-wave dispersion different from the low-frequency mode. Based on a theoretical model that considers two oppositely oriented coupled domains, this high-frequency mode is accounted for as an effective antiferromagnetic spin-wave mode. The spin waves exhibit group velocities of 2.6 km s-1 and propagate even at zero magnetic bias field. An electric current pulse with a density of only 105 A cm-2 can controllably modify the orientation of the stripe domains, which opens up perspectives for reconfigurable magnonic devices.

3.
ACS Appl Mater Interfaces ; 11(1): 1420-1425, 2019 Jan 09.
Artigo em Inglês | MEDLINE | ID: mdl-30550263

RESUMO

Since the invention of triboelectric nanogenerators (TENGs), their output performance has been improved through various approaches such as material surface modification, device structure optimization, and so on, but rarely through the development of new friction materials. In this work, a magnetron sputtered Mg xZn1- xO film is developed as a viable friction material that rubs against polydimethylsiloxane in a TENG. The work function, measured by Kelvin probe microscopy, of the Mg xZn1- xO films can be effectively tuned by varying Mg composition, x, and exposed surface facets, which are shown to dominate the charge-transfer behavior. In addition, film thickness also plays an important role, affecting the output performance. The output voltage and total charge of a TENG with a Mg xZn1- xO film are demonstrated to be tremendously enhanced by 55 and 90 times, respectively, compared to that of a TENG with a ZnO film. Even more intriguingly, the tribo-output polarity can be reversed by adjusting the relative work function through varying the preferred growth orientation of the Mg xZn1- xO film, for a given value of Mg content.

4.
Sci Rep ; 8(1): 18034, 2018 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-30575773

RESUMO

We demonstrate significant improvement of CuO nanowire arrays as anode materials for lithium ion batteries by coating with thin NiO nanosheets conformally. The NiO nanosheets were designed two kinds of morphologies, which are porous and non-porous. By the NiO nanosheets coating, the major active CuO nanowires were protected from direct contact with the electrolyte to improve the surface chemical stability. Simultaneously, through the observation and comparison of TEM results of crystalline non-porous NiO nanosheets, before and after lithiation process, we clearly prove the effect of expected protection of CuO, and clarify the differences of phase transition, crystallinity change, ionic conduction and the mechanisms of the capacity decay further. Subsequently, the electrochemical performances exhibit lithiation and delithiation differences of the porous and non-porous NiO nanosheets, and confirm that the presence of the non-porous NiO coating can still effectively assist the diffusion of Li+ ions into the CuO nanowires, maintaining the advantage of high surface area, and improves the cycle performance of CuO nanowires, leading to enhanced battery capacity. Optimally, the best structure is validated to be non-porous NiO nanosheets, in contrary to the anticipated porous NiO nanosheets. In addition, considering the low cost and facile fabrication process can be realized further for practical applications.

5.
Phys Rev Lett ; 120(21): 217202, 2018 May 25.
Artigo em Inglês | MEDLINE | ID: mdl-29883138

RESUMO

We observe strong interlayer magnon-magnon coupling in an on-chip nanomagnonic device at room temperature. Ferromagnetic nanowire arrays are integrated on a 20-nm-thick yttrium iron garnet (YIG) thin film strip. Large anticrossing gaps up to 1.58 GHz are observed between the ferromagnetic resonance of the nanowires and the in-plane standing spin waves of the YIG film. Control experiments and simulations reveal that both the interlayer exchange coupling and the dynamical dipolar coupling contribute to the observed anticrossings. The coupling strength is tunable by the magnetic configuration, allowing the coherent control of magnonic devices.

6.
Sci Rep ; 8(1): 7467, 2018 May 08.
Artigo em Inglês | MEDLINE | ID: mdl-29739974

RESUMO

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

7.
Nat Commun ; 9(1): 738, 2018 02 21.
Artigo em Inglês | MEDLINE | ID: mdl-29467416

RESUMO

Recent years have witnessed a rapidly growing interest in exploring the use of spin waves for information transmission and computation toward establishing a spin-wave-based technology that is not only significantly more energy efficient than the CMOS technology, but may also cause a major departure from the von-Neumann architecture by enabling memory-in-logic and logic-in-memory architectures. A major bottleneck of advancing this technology is the excitation of spin waves with short wavelengths, which is a must because the wavelength dictates device scalability. Here, we report the discovery of an approach for the excitation of nm-wavelength spin waves. The demonstration uses ferromagnetic nanowires grown on a 20-nm-thick Y3Fe5O12 film strip. The propagation of spin waves with a wavelength down to 50 nm over a distance of 60,000 nm is measured. The measurements yield a spin-wave group velocity as high as 2600 m s-1, which is faster than both domain wall and skyrmion motions.

8.
Nanoscale ; 9(37): 14039-14046, 2017 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-28894866

RESUMO

Piezoelectric nanogenerators have been investigated to generate electricity from environmental vibrations due to their energy conversion capabilities. In this study, we demonstrate an optimal geometrical design of inertial vibration direct-current piezoelectric nanogenerators based on obliquely aligned InN nanowire (NW) arrays with an optimized oblique angle of ∼58°, and driven by the inertial force of their own weight, using a mechanical shaker without any AC/DC converters. The nanogenerator device manifests potential applications not only as a unique energy harvesting device capable of scavenging energy from weak mechanical vibrations, but also as a sensitive strain sensor. The maximum output power density of the nanogenerator is estimated to be 2.9 nW cm-2, leading to an improvement of about 3-12 times that of vertically aligned ZnO NW DC nanogenerators. Integration of two nanogenerators also exhibits a linear increase in the output power, offering an enormous potential for the creation of self-powered sustainable nanosystems utilizing incessantly natural ambient energy sources.

9.
Sci Rep ; 7(1): 7135, 2017 08 02.
Artigo em Inglês | MEDLINE | ID: mdl-28769071

RESUMO

1.5-µm AlN grown by metal-organic chemical vapor deposition (MOCVD), with a single substrate temperature of 1180 °C, exhibits atomically flat surface and the XRD (102) peak width of 427 arcsec. The results are achieved with a pulsed NH3-flow condition, serving as an alternative for the commonly used temperature-varied buffer structure, which is often complicated and time-consuming. Inserting two pulsed-NH3-flow AlN layers in the epitaxial structure not only releases the lattice strain via the formation of three-dimensional nano-islands, but also smoothens the surface with prolonged lateral migration of Al adatoms. This effective growth technique substantially simplifies the manufacture of device-quality AlN.

10.
ACS Appl Mater Interfaces ; 9(14): 12859-12864, 2017 Apr 12.
Artigo em Inglês | MEDLINE | ID: mdl-28318226

RESUMO

Ultrathin three-dimensional topological insulator films are promising for use in field effect devices. (Bi1-xSbx)2Se3 ultrathin films were fabricated on SrTiO3 substrate, where large resistance changes of ∼25 000% could be achieved using the back gate voltage. We suggest that the large ON/OFF ratio was caused by the combined effect of Sb-doping and the reduction of film thickness down to the ultrathin regime. The crossover of different quantum transport under an electric field may form the basis for topological insulators (TI)-based spin transistors with large ON/OFF ratios in the future.

11.
Nanoscale ; 8(1): 634-40, 2016 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-26646378

RESUMO

Surface free energy, as an intrinsic property, is essential in determining the morphology of materials, but it is extremely difficult to determine experimentally. We report on the derivation of the SE of different facets of ZnO and GaN experimentally from the holes developed using electron beam drilling with transmission electron microscopy. Inverse Wullf's construction is employed to obtain polar maps of the SE of different facets to study different nanomaterials (ZnO and GaN) in different morphologies (nanorod, nanobelt and thin film) to prove its versatility and capability. The results show that the SE of ZnO{10-13} is derived to be 0.99 J m(-2), and the SE of ZnO{10-10} is found to be less than {0002} and {11-20}. A GaN thin film also exhibits a similar trend in the SE of different facets as ZnO and the SE of GaN{10-13} is determined to be 1.36 J m(-2).

12.
Adv Mater ; 27(40): 6289-95, 2015 Oct 28.
Artigo em Inglês | MEDLINE | ID: mdl-26349632

RESUMO

Alx Ga1-x N thin-film-based piezotronic strain sensors with ultrahigh strain sensitivity are fabricated through alloying of AlN with GaN. The strain sensitivity of the ternary compound Alx Ga1-x N is higher than those of the individual binary compounds GaN and AlN. Such a high performance can be attributed to the piezoelectric constant enhancement via intercalation of Al atoms into the GaN matrix, the effect of residual strain, and a suppressed screening effect.

13.
Adv Mater ; 27(33): 4845-50, 2015 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-26178685

RESUMO

Semipolar {101¯1} InGaN quantum wells are grown on (001) Si substrates with an Al-free buffer and wafer-scale uniformity. The novel structure is achieved by a bottom-up nano-heteroepitaxy employing self-organized ZnO nanorods as the strain-relieving layer. This ZnO nanostructure unlocks the problems encountered by the conventional AlN-based buffer, which grows slowly and contaminates the growth chamber.


Assuntos
Gálio/química , Nanotecnologia/métodos , Nanotubos/química , Silício/química , Equipamentos e Provisões Elétricas , Nanotecnologia/instrumentação
14.
Sci Rep ; 5: 12533, 2015 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-26211998

RESUMO

The island nucleation in the context of heterogeneous thin film growth is often complicated by the growth kinetics involved in the subsequent thermodynamics. We show how the evolution of sputtered Zn island nucleation on Si(111) by magnetron sputtering in a large area can be completely understood as a model system by combining reflective second harmonic generation (RSHG), a 2D pole figure with synchrotron X-ray diffraction. Zn dots are then oxidized on the surfaces when exposed to the atmosphere as Zn/ZnO dots. Derived from the RSHG patterns of Zn dots at different growth times, the Zn dots grow following a unique transition from kinetic to thermodynamic control. Under kinetic-favoring growth, tiny Zn dots prefer arranging themselves with a tilted c-axis to the Si(111) substrate toward any of the sixfold in-plane Si<110> directions. Upon growth, the Zn dots subsequently evolve themselves to a metastable state with a smaller tilting angle toward selective <110> directions. As the Zn dots grow over a critical size, they become most thermodynamically stable with the c-axis vertical to the Si(111) substrate. For a system with large lattice mismatch, small volume dots take kinetic pathways with insignificant deviations in energy barriers.

15.
Dalton Trans ; 44(17): 7982-90, 2015 May 07.
Artigo em Inglês | MEDLINE | ID: mdl-25826776

RESUMO

We have synthesized novel metal organic hybrid mixed compounds of bis (acetylacetonato κ-O, O') [zinc(ii)/copper(ii)]. Taking C10H14O4Zn0.7Cu0.3 (Z0.7C0.3AA) as an example, the crystals are composed of Z0.7C0.3AA units and uncoordinated water molecules. Single-crystal X-ray diffraction results show that the complex Z0.7C0.3AA crystallizes in the monoclinic system, space group P21/n. The unit cell dimensions are a = 10.329(4) Å, b = 4.6947(18) Å, and c = 11.369(4) Å; the angles are α = 90°, ß = 91.881(6)°, and γ = 90°, the volume is 551.0(4) Å(3), and Z = 2. In this process, the M(ii) ions of Zn and Cu mix and occupy the centers of symmetrical structural units, which are coordinated to two ligands. The measured bond lengths and angles of O-M-O vary with the ratio of metal species over the entire series of the complexes synthesized. The chemistry of the as-synthesized compounds has been characterized using infrared spectroscopy, mass spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analysis, and the morphology of the products has been characterized using scanning electron microscopy. The thermal decomposition of the Z0.7C0.3AA composites measured by thermogravimetric analysis suggests that these complexes are volatile. The thermal characteristics of these complexes make them attractive precursors for metal organic chemical vapor deposition.

16.
Small ; 10(22): 4718-25, 2014 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-25044675

RESUMO

This study investigates the role of carrier concentration in semiconducting piezoelectric single-nanowire nanogenerators (SNWNGs) and piezotronic devices. Unintentionally doped and Si-doped GaN nanowire arrays with various carrier concentrations, ranging from 10(17) (unintentionally doped) to 10(19) cm(-3) (heavily doped), are synthesized. For SNWNGs, the output current of individual nanowires starts from a negligible level and rises to the maximum of ≈50 nA at a doping concentration of 5.63 × 10(18) cm(-3) and then falls off with further increase in carrier concentration, due to the competition between the reduction of inner resistance and the screening effect on piezoelectric potential. For piezotronic applications, the force sensitivity based on the change of the Schottky barrier height works best for unintentionally doped nanowires, reaching 26.20 ± 1.82 meV nN(-1) and then decreasing with carrier concentration. Although both types of devices share the same Schottky diode, they involve different characteristics in that the slope of the current-voltage characteristics governs SNWNG devices, while the turn-on voltage determines piezotronic devices. It is demonstrated that free carriers in piezotronic materials can influence the slope and turn-on voltage of the diode characteristics concurrently when subjected to strain. This work offers a design guideline for the optimum doping concentration in semiconductors for obtaining the best performance in piezotronic devices and SNWNGs.

18.
Adv Mater ; 26(16): 2572-9, 2014 Apr 23.
Artigo em Inglês | MEDLINE | ID: mdl-24470383

RESUMO

In purely bent ZnO microwires, the excitons can be effectively driven and concentrated by the elastic strain-gradient towards the tensile outer side of the purely bent wire. Experimental and theoretical approaches are combined to investigate the dynamics of excitons in an inhomogeneous strain field with a uniform elastic strain-gradient. Cathodoluminescence spectroscopy analysis on purely bent ZnO microwires verifies that excitons can be effectively driven and concentrated along the elastic strain-gradient.

20.
Opt Express ; 20(3): 2015-24, 2012 Jan 30.
Artigo em Inglês | MEDLINE | ID: mdl-22330442

RESUMO

A combined method of modified oblique-angle deposition and hydrothermal growth was adopted to grow an optically anisotropic nanomaterial based on single crystalline ZnO nanowire arrays (NWAs) with highly oblique angles (75°-85°), exhibiting giant in-plane birefringence and optical polarization degree in emission. The in-plane birefringence of oblique-aligned ZnO NWAs is almost one order of magnitude higher than that of natural quartz. The strong optical anisotropy in emission due to the optical confinement was observed. The oblique-aligned NWAs not only allow important technological applications in passive photonic components but also benefit the development of the optoelectronic devices in polarized light sensing and emission.


Assuntos
Nanopartículas Metálicas/química , Nanopartículas Metálicas/ultraestrutura , Ressonância de Plasmônio de Superfície/métodos , Óxido de Zinco/química , Anisotropia , Birrefringência , Luz , Teste de Materiais , Espalhamento de Radiação , Propriedades de Superfície
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