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While there has been significant pursuits and interest in single element metallic glass (MG), it has been extremely difficult to obtain. Despite the recent advances in preparing MGs, the massive production of stable monatomic MGs is of great challenge. Here, we show that Ta monatomic MG, presenting identical structural characteristics to those from liquid quenching, can be produced using conventional ion beam deposition on a room temperature substrate. The amorphous structure is retained even after being annealed at 573 K for 2 h, while it fully crystallizes subjected to annealing at 823 K for 5 min, corroborating the previous prediction that Ta MG is stable up to 600 K.

Nanoscale magnetism in oxides with the lateral size down to 300 nm is critical for scientific investigation and advanced technological applications such as spintronics, but often complicated to fabricate. Specifically, the emergent magnetic phenomena induced by the size effect attract tremendous attention. In this situation, fabrication of self-assembled nanoarchitectures in complex oxides and strategically modulating their properties are urgently needed. Here, we report the emerging single ferromagnetic phase state in self-assembled nanowires on the thin film surface of mixed valence manganite La0.5Sr0.5MnO3, by using low temperature magnetic force microscopy. The ferromagnetic state can be reversely switched in the presence of an external magnetic field. This work paves the way for manipulating the phase coexistence state without an external field and provides insight into the size limitation for designing next generation electronic and spintronic devices in complex oxide systems.

Taking advantage of the electron-current ability to generate, stabilize, and manipulate skyrmions prompts the application of skyrmion multilayers in room-temperature spintronic devices. In this study, the robust high-density skyrmions are electromagnetically generated from Pt/Co/Ta multilayers using Lorentz transmission electron microscopy. The skyrmion density is tunable and can be significantly enhanced. Remarkably, these generated skyrmions after optimized manipulation sustain at zero field with both the in-plane current and perpendicular magnetic field being switched off. The skyrmion generation and manipulation method demonstrated in this study opens up an alternative way to engineer skyrmion-based devices. The results also provide key data for further theoretical study to discover the nature of the interaction between the electric current and different spin configurations.

Searching strain sensitive materials for electronic skin is of crucial significance because of the restrictions of current materials such as poor electrical conductivity, large energy consumption, complex manufacturing process, and high cost. Here, we report a flexible strain sensor based on the Zr55Cu30Ni5Al10 metallic glass thin film which we name metallic glass skin. The metallic glass skin, synthesized by ion beam deposition, exhibits piezoresistance effects with a gauge factor of around 2.86, a large detectable strain range (∼1% or 180° bending angle), and good conductivity. Compared to other e-skin materials, the temperature coefficient of resistance of the metallic glass skin is extremely low (9.04 × 10−6 K−1), which is essential for the reduction in thermal drift. In addition, the metallic glass skin exhibits distinct antibacterial behavior desired for medical applications, also excellent reproducibility and repeatability (over 1000 times), nearly perfect linearity, low manufacturing cost, and neg...

Modulating energy states of metallic glasses (MGs) is significant in understanding the nature of glasses and controlling their properties. In this study, we show that high stored energy can be achieved and preserved in bulk MGs by high pressure (HP) annealing, which is a controllable method to continuously alter the energy states of MGs. Contrary to the decrease in enthalpy by conventional annealing at ambient pressure, high stored energy can occur and be enhanced by increasing both annealing temperature and pressure. By using double aberration corrected scanning transmission electron microscopy, it is revealed that the preserved high energy, which is attributed to the coupling effect of high pressure and high temperature, originates from the microstructural change that involves “negative flow units” with a higher atomic packing density compared to that of the elastic matrix of MGs. The results demonstrate that HP-annealing is an effective way to activate MGs into higher energy states, and it may assist i...

Understanding the configuration of ferroelectric domains and domain walls is critical for the fabrication and engineering of ferroelectric devices. Here, we demonstrate that scanning transmission electron microscopy (STEM) can be used to resolve a high density of 180° ferroelectric domains in Ca0.28Ba0.72Nb2O6 single crystals, previously studied by conventional transmission electron microscopy. We demonstrate the advantages of STEM for imaging in a specific set of diffraction conditions and deflection settings in conjunction with an angular dark-field detector. We specifically investigated the structure of a 180° domain wall at atomic resolution by aberration-corrected STEM under annular bright field and high angle annular dark field conditions. Our results suggest, based on observed cation displacements, that the 180° domain wall is a combination of Ising and/or Bloch-Neel types rather than pure Ising type, consistent with recent theoretical predictions.

The acoustic phonon transport through an asymmetric y-branch semiconductor quantum structure is investigated by use of the scattering-matrix method. It is found that the quantum structure exhibits mode-splitting behavior, resonant transmission, and the noninteger quantized thermal conductance at very low temperatures. The thermal transport behavior sensitively depends on the geometric configuration of the structure. Our work suggests that such a y-branch structure with specific structural parameters could be used as a splitter of phonon modes.

Resistance switching phenomena were studied by varying temperature in SrTiO3 single crystal. The resistance hysteresis loops appear at a certain temperature ranging from 340 K to 520 K. With the assistance of 375 nm ultraviolet continuous laser, the sample resistance is greatly reduced, leading to a stable effect than that in dark. These resistance switching phenomena only exist in samples with enough oxygen vacancies, which is confirmed by spherical aberration-corrected scanning transmission electron microscopy measurements, demonstrating an important role played by oxygen vacancies. At temperatures above 340 K, positively charged oxygen vacancies become mobile triggered by external electric field, and the resistance switching effect emerges. Our theoretical results based on drift-diffusion model reveal that the built-in field caused by oxygen vacancies can be altered under external electric field. Therefore, two resistance states are produced under the cooperative effect of built-in field and external f...

We propose and demonstrate a spin-filter diode based on semimagnetic semiconductor ZnSe/Zn1−xMnxSe/Zn1−yMnySe/ZnSe heterostructures. It is found that the degree of spin polarization greatly depends on the electric field direction and its magnitude. There is significant difference of the spin polarization between under forward bias and under reverse bias. It is also found that the spin polarization will reverse under relatively small magnetic field, which is originated from resonant enhancement effect for spin-up electrons tunneling through effective steplike potential of the corresponding structure.

By use of first-principles calculations, we investigate the effect of introducing substitutional boron or nitrogen atom in the tip on field-emission properties of capped single-wall carbon nanotubes. We find that the substitution in the tip can significantly enhance the localized density of states at the Fermi level, which is important for field emission. The substitution of nitrogen introduces unsaturated dangling bond states at the tip, yielding a reduction of the work function of the tip. Our results suggest that the capped tube with substitutional nitrogen or boron atom in the tip could have much larger field-emission current than the tube without substitution.

We find that the conventional method to calculate the soft-mode frequency of the pseudospin model is incorrect, and present a valid modified method. It is demonstrated that the conventional method does not show the soft-mode frequency going to zero at a critical temperature in the presence of random internal fields, while the frequency calculated by our modified method goes to zero in random fields at a critical temperature. For the field-induced phase transition relaxor ferroelectrics, the soft-mode frequency decreases to zero at the first-order phase transition temperature though the symmetry has been destroyed at high temperatures under an external field.

In this letter, by use of the first-principles calculations, field emission properties of the body are investigated for a series of dimension-variational single-walled carbon nanotubes (SWNTs). We find that in the course of increasing tube diameter, at low emission currents, field emission properties worsen, chemical stability enhances and binding energy increases, which indicates that optimized field emitter could be achieved; while at high applied voltages, the highest emission current density may appear for the SWNT with some peculiar dimension. We predict that the body of the SWNT could behave as a peculiar field emitter with high threshold voltage and high current density.

The low-energy theory for finite long carbon nanotube is derived and numerically examined. It shows that the electronic structure is dominated by the quantum confining, which determines the profile of wave functions as well as the eigen energies; while the details of the wave functions are resolved by the structure of the nanotubes. This behavior is attributed to the peculiar electronic structure of the nanotubes. Because of the slow variation of the profile of electron wave functions, the measured conductance is NOT independent of the position to measure it, which is evident in the multiprobe experiment.

The room-temperature ferroelectricity of SrTiO3 is promising for oxide electronic devices controlled by multiple fields. An effective way to control the ferroelectricity is highly demanded. Here, we show that the off-centered antisite-like defects in SrTiO3 films epitaxially grown on Si (001) play the determinative role in the emergence of room-temperature ferroelectricity. The density of these defects changes with the film cation concentration sensitively, resulting in a varied coercive field of the ferroelectric behavior. Consequently, the room-temperature ferroelectricity of SrTiO3 films can be effectively modulated by tuning the temperature of metal sources during the molecular beam epitaxy growth. Such an easy and reliable modulation of the ferroelectricity enables the flexible engineering of multifunctional oxide electronic devices.

Strong and sharp photocurrent peak at longitudinal optical (LO) phonon frequency (8.87 THz) is found in GaAs/(Al,Ga)As terahertz quantum-well photodetectors (QWPs). Two mesa-structure terahertz QWPs with and without one-dimensional metal grating are fabricated to investigate the behavior of such photoresponse peak. The experimental and simulation results indicate that the photocurrent peak originates from a two-step process. First, at the LO phonon frequency, a large number of non-equilibrium LO phonons are excited by the incident electromagnetic field, and the electromagnetic energy is localized and enhanced in the thin multi-quantum-well layer. Second, through the Frohlich interaction, the localized electrons are excited to continuum states by absorbing the non-equilibrium LO phonons, which leads to the strong photoresponse peak. This finding is useful for exploring strong light-matter interaction and realizing high sensitive terahertz photodetectors.

The design, fabrication, and characterization of terahertz (THz) quantum well photodetectors with one-dimensional reflection-grating coupler are presented. It is found that the reflection gratings could effectively couple the THz waves normally incident to the device. Compared with the 45-degree facet sample, the peak responsivity of this grating-coupled detector is enhanced by over 20%. The effects of the gratings on the photocurrent spectra are also analyzed.

A method to prepare silicon nitride nanoscale rods using carbon nanotube as a template has been presented in this letter. The products of the reaction of carbon nanotubes with a mixture of Si and SiO2 powder in nitrogen atmosphere are β-Si3N4, α-Si3N4, and Si2N2O nanorods. The sizes of the nanorods are 4–40 nm in diameter and up to several microns in length. The formation mechanism of the nanorods has also been discussed.

The dielectric constant e*(ω,T)=e′(ω,T)+ie″(ω,T) of relaxor ferroelectrics is simulated by the Monte Carlo method. It is proved that the polar microregions switch among different polarization directions as a whole under the electrical field, giving rise to the high dielectric constant. The relaxor characteristics are caused by the randomly distributed interactions between the polar microregions. Factors which influence the dielectric response process are discussed.

We report that thin films of a prototype topological insulator, Bi$_{2}$Se$_{3}$, can be epitaxially grown onto the (0001) surface of BaFe$_{12}$O$_{19}$(BaM), a magnetic insulator with high Curie temperature and large perpendicular anisotropy. In the Bi$_2$Se$_3$ thin films grown on non-magnetic substrates, classic weak antilocalization (WAL) is manifested as cusp-shaped positive magnetoresistance (MR) in perpendicular magnetic fields and parabola-shaped positive MR in parallel fields, whereas in Bi$_{2}$Se$_{3}$/BaM heterostructures the low field MR is parabola-shaped, which is positive in perpendicular fields and negative in parallel fields. The magnetic field and temperature dependence of the MR is explained as a consequence of the suppression of WAL due to strong magnetic interactions at the Bi$_{2}$Se$_{3}$/BaM interface., 5 pages, 5 figures, supplemental material available upon request or visit the APL website

The glass-forming ability (GFA) of an alloy, closely related to its ability to resist crystallization, is a crucial issue in condensed matter physics. So far, the studies on GFA are mostly statistical and empirical guides. Benefiting from the ultrahigh thermal stability of ultrathin metallic glassy film and high resolution spherical aberration-corrected transmission electron microscope, the crystallization of atomically thin ZrCu and its microalloyed ZrCuAl glasses with markedly different GFA was investigated at the atomic scale. We find the Zr diffusivity estimated from the density of nuclei is dramatically decreased by adding of Al, which is the major reason for the much better GFA of the ZrCuAl metallic glass.

We report the fabrication and study of thermal stability of atomically thin ZrCu-based metallic glass films. The ultrathin films exhibit striking dynamic properties, ultrahigh thermal stability, and unique crystallization behavior with discrete crystalline nanoparticles sizes. The mechanisms for the remarkable high stability and crystallization behaviors are attributed to the dewetting process of the ultrathin film. We demonstrated a promising avenue for understanding some fundamental issues such as glassy structure, crystallization, deformation, and glass formation through atomic resolution imaging of the two dimensional like metallic glasses.

Although topological defects, such as domain walls (DWs) or vortices, are naturally protected by topological invariance, yet, we discover an exception that the six-state topology of the vortex with Z2 × Z3 symmetry is broken by a partial edge dislocation (PED) in hexagonal Y0.67Lu0.33MnO3, where the topologies of the four-state vortex or closed DWs emerges. Using aberration-corrected scanning transmission electron microscopy, we found that the PED plays an important role in changing the phase of translation domain. The PED at the vortex core leads to the formation of the four-state vortex, while the ones at closed DWs connect different types of DWs, both corresponding to continuous phase changes. These results indicate that PEDs can change the topology of translation-related domain vortices and more vortices with even domains can be expected.

Unusual magnetic properties are found in ultrathin La0.9Sr0.1MnO3 films by systematically investigating the films with the thicknesses varying from 200 to 6 unit cells. Post annealing in oxygen can significantly enhance the Curie temperature and saturation magnetization by complementing oxygen vacancies. We observe that oxygen vacancies around the surfaces are much more than those close to the interfaces using an aberration-corrected scanning transmission electron microscopy for both the as-grown and post-annealed ultrathin films. The Curie temperature up to 325 K, much higher than that of the bulk, is found in the annealed films with the thickness of 50 unit cells.

The atomic and electronic structures of transition metal-metalloid based Zr60Co(Ni,Cu)28Al12 bulk metallic glasses (MGs) are probed by 27Al nuclear magnetic resonance (NMR) spectroscopy. The magnitude and evolution of 27Al isotropic shifts indicate that the Fermi level moves toward a minimum upon transition metal 3d sub-band filling and provides experimental evidence for pd hybridization bonding mechanism, which is further supported by the observation of Co L23 edges, and the prepeaks near Al L23 and K edges in electron energy loss spectroscopy. This investigation provides insight into the bonding mechanisms in MGs, which are relevant for understanding of their mechanical behaviors.

We study the evolution of surface reconstructions on SrTiO3 (110) determined by cation concentration ratio and find it detectable by high energy electron diffraction (RHEED) even at high temperature up to 800 °C. The evaporation rate of Sr and Ti sources can be calibrated precisely by monitoring RHEED patterns in situ and in real time during the extended homogeneous oxide molecule beam epitaxy growth of SrTiO3 (110) film. High quality film is obtained with deviation of cation stoichiometry less than 0.5%.

High-quality wurtzite MgZnO film was deposited on Si(111) substrate via a delicate interface engineering using BeO, by which solar-blind ultraviolet photodetectors were fabricated on the n-MgZnO(0001)/p-Si(111) heterojunction. A thin Be layer was deposited on clean Si surface with subsequent in situ oxidation processes, which provides an excellent template for high-Mg-content MgZnO growth. The interface controlling significantly improves the device performance, as the photodetector demonstrates a sharp cutoff wavelength at 280 nm, consistent with the optical band gap of the epilayer. Our experimental results promise potential applications of this technique in integration of solar-blind ultraviolet optoelectronic device with Si microelectronic technologies.

We demonstrate a three-dimensionally macroporous Si 9R polytype that exhibits a different electronic structure than bulk diamond-structured Si. Unlike the latter one which has an indirect-bandgap transition close to the zone boundary, the conduction band minimum in this material, as revealed by valence electron energy-loss spectroscopy in a monochromated transmission electron microscope, significantly shifts toward the Γ point within a range from 0.6 to 5.6 nm−1, indicating substantially less momentum transfer required to fulfill the bandgap transition.

From first-principles calculations, we find that in a (110) Si film with surface etching along the [001] direction, the holes can be confined underneath the patterned layer. This effect arises from the interplay between the anisotropic carriers and the patterning-induced quantum confinement. An anisotropy coefficient K=mz/my, which is the ratio between the out-of-plane effective mass and the in-plane effective mass of the charge carriers in the film, is introduced to explain the orientation dependence. We propose that a modulation-dopinglike effect can be achieved in the (110) nanopatterned Si film by selective doping in the top patterned layer.

Using spin-polarized density functional theory calculations, we demonstrate that hybrid BN–C nanotubes (BN-CNTs) have diverse electronic and magnetic properties depending on their percentage of carbon and BN components. Typically, a BN-CNT is converted from a nonmagnetic semiconductor to a spin-polarized metal and then to a nonmagnetic semiconductor by increasing the ratio of BN component. The intrinsic half-metallicity could be achieved when the percentage of carbon component in the tube is within a certain ratio, and is insensitive to the tube curvature. Our findings suggest that BN-CNTs may offer unique opportunities for developing nanoscale spintronic materials.

Local investigations using high-angle annular-dark-field imaging combined with electron low-energy-loss spectroscopy were performed to closely characterize the GaN/Al0.5Ga0.5N quantum dots heterostructure. It is found that the Al0.5Ga0.5N barrier tends to exhibit phase separation. Gallium-rich arms arise from the top rims of the truncated quantum dots while the space between these arms is filled with aluminum-rich AlGaN. This phase separation, due to morphological and strain nonuniformities of the GaN front surface, provokes an optical-property modulation in the neighborhood of the quantum dots which, from a practical point of view, could affect the electronic barrier homogeneity.

N-rich single crystalline SiOxNy nanowires with three-dimensional (3D) branches have been synthesized and characterized. In contrast to the reported SiOxNy nanowires, our products exhibit a predominant N concentration and a single crystalline phase throughout the 3D branched structure. These properties are highly favorable for applications on which high dielectric constant and high refractive index are required. Valence electron energy-loss spectroscopy utilizing monochromated electrons reveals that the band gap of the materials is 4.4±0.2 eV. Energy-loss near edge structures of Si L2,3, N K, and O K edges display a strong orbital hybridization.

Our first-principles calculations indicate the possibility of preparing spin-polarized scanning tunneling microscopy (SP-STM) probes from Fe-doped capped carbon nanotubes (CNTs). The structural stability, magnetic moment, and electronic property of hybrid systems are found to depend on the Fe adsorption site, which is attributed to the hybridization between Fe 3d and C 2p orbitals. The CNTs with Fe atoms adsorbed at the tip-top are demonstrated to be promising candidates for the SP-STM probe, with a high spin polarization leading to a completely spin-polarized current at lower voltages. In contrast, the CNTs encapsulating Fe atom are basically nonmagnetic, and thus useless for the SP-STM probe application in nature.

Using first-principle electronic structure calculations, we show a metal- semiconductor transition of a metallic graphene nanoribbon with zigzag edges induced by substitutional doping of Nitrogen or Boron atoms at the edges. A field effect transistor consisting of a metal-semiconductor-metal junction can then be constructed by selective doping of the ribbon edges. The current-voltage characteristics of such a prototype device is determined by the first-principle quantum transport calculations., 4 pages, 3 figures. Accepted in APL

First-principles calculations of crystalline silicon nanotubes SiNTs show that nonuniformity in wall thickness can cause sizable variation in the band gap as well as notable shift in the optical absorption spectrum. A unique quantum confinement behavior is observed: the electronic wave functions of the valence band maximum and conduction band minimum are due mainly to atoms located in the thicker side of the tube wall. This is advantageous to spatially separate the doping impurities from the conducting channel in doped SiNTs. Practically, the performance of the SiNT-based transistors may be substantially improved by selective p/n doping in the thinner side of the tube wall in the spirit of modulation doping. © 2007 American Institute of Physics. DOI: 10.1063/1.2778756

Density functional calculations are performed to investigate the functionalization of carbon nanotubes with the vacancy and carboxyl group that always appear in pairs after chemical oxidation (such as oxidative acid treatment). It is found that the geometrical structures and electronic properties of the nanotubes are changed significantly, with the formation of a strong covalent bond between the carboxyl and the defected tube. The electrical conductivity of the system could be considerably enhanced, which sensitively depends upon the axial density of vacancy-carboxyl pairs. A promising approach, controllable oxidative acid treatment, is designed to make carbon nanotube-based chemical and nanoelectronic devices.

First-principles calculations are performed to study the mechanical properties, electronic structure, and uniaxial-stress effects of β-SiC nanowires (NWs). It is found that the band gap of SiC NWs becomes larger as their diameter decreases because of the quantum confinement effect, but increases (decreases) slightly with increasing tensile (compressive) stress up to about 12GPa. The calculated Young’s modulus and tensile strength of SiC NWs are about 620 and 52GPa, respectively, in accordance with the experimental data. The characteristics of their mechanical and electronic properties suggest that β-SiC NWs may be used in electronic composites as reinforcement nanomaterials or in nanoscale electronic/photoelectric devices under harsh environments.

Ab initio calculations reveal that ultrashort-period epitaxial KNbO3∕(KTaO3)m superlattice exhibits typical ferroelectricity when m=1 or 2; while it becomes paraelectric when m⩾3. We observe considerable and tantamount dipole moment in each cell, which can be attributed to the continuity of dipole moment at the interface and the short period of the heterostructure. The polarization behavior of KNbO3∕KTaO3 superlattices sensitively depends upon the ratio of two constituents. Our results directly support the experimental finding of the antiferroelectric behavior in the 1×1 KNbO3∕KTaO3 superlattice.

Using a first-principles-based approach, we investigate the strain–temperature phase diagram of Pb(Zr0.5Ti0.5)O3 ultrathin film without surface charge screening. We find that the compressive strain dramatically affects the sequence of phase transitions, and observe a rich variety of ferroelectric phases, including a monoclinic phase and a stripe domain forbidden in the bulk crystal. The vortex stripe structure leads to an unusual characteristic of the phase diagram.