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Exchange bias (EB) is normally created by the interfacial exchange coupling at a ferromagnetic/antiferromagnetic (FM/AFM) interface. FM/AFM interfaces have also been proved to perform enhanced spin angular momentum transfer efficiency in spin pumping (SP), compared with typical FM/nonmagnetic interfaces. Here, we report an unexpected EB and enhanced SP between a ferromagnet and semiconductor. Considerable EB has been observed in Co films grown on ZnO single crystal due to the interface antiferromagnetism of the Zn1−xCoxO (x depends on the Co solubility limit in ZnO) layer. Moreover, SP measurements demonstrate a giant spin pumping efficiency at the Co/ZnO interface with a bump (spin mixing conductance G eff ↑ ↓ = 28 nm−2) around the blocking temperature TB ∼ 75 K. The enhanced SP is further confirmed by inverse spin Hall effect measurements and the spin Hall angle θ ISHE of Zn1−xCoxO is estimated to be 0.011. The bound magnetic polarons with s–d exchange interaction between donor electrons and magnetic cation ions in Zn1−xCoxO play a key role in the formation of antiferromagnetism with giant G eff ↑ ↓ . Our work provides a new insight into spin physics at FM/semiconducting interfaces. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) allotropes of tellurium (Te), recently coined as tellurene, are currently an emerging topic of materials research due to the theoretically predicted exotic properties of Te in its ultrathin form and at the single atomic layer limit. However, a prerequisite for the production of such new and single elemental 2D materials is the development of simple and robust fabrication methods. In the present work, we report three different 2D superstructures of Te on Au(111) surfaces by following an alternative experimental deposition approach. We have investigated the superstructures using low-temperature scanning tunneling microscopy and spectroscopy, Auger electron spectroscopy (AES), and field emission AES. Three superstructures (13 × 13, 8 × 4, and √11 × √11) of 2D Te are observed in our experiments, and the formation of these superstructures is accompanied by the lifting of the characteristic 23 × √3 surface reconstruction of the Au(111) surface. Scanning tunneling spectroscopy reveals a strong dependence of the local electronic properties on the structural arrangement of the Te atoms on the Au(111) support, and we observe superstructure-dependent electronic resonances around the Fermi level and below the Au(111) conduction band. In addition to the appearance of the new electronic resonances, the emergence of band gaps with a p-type charge character has been evidenced for two out of three Te superstructures (13 × 13 and √11 × √11) on the Au(111) support. [ABSTRACT FROM AUTHOR]

Experimental evidence of topological Dirac fermion charge carriers in pure and robust α‐Sn thin films grown on InSb substrates is reported. This evidence is acquired using standard macroscopic four‐point contact resistance measurements, conducted on uncapped films with a significantly reduced bulk mobility. The electrical characteristics of the constituting components of the α‐Sn/InSb sample are analyzed and compared and a three‐band drift velocity model is proposed accordingly. A surface band, with low carrier density and high mobility, is identified as the origin of the observed Shubnikov – de Haas (SdH) oscillations. The analysis of these quantum oscillations results in a nontrivial value of the phase shift γ0, characteristic of topologically protected Dirac fermions. The momentum relaxation time τ ≈ 300 fs in pure, undoped α‐Sn thin films is estimated. [ABSTRACT FROM AUTHOR]

Arc-discharge synthesized multiwalled carbon nanotubes (AD-MWNT), or related MWNTs, exhibit a good quality compared to the more common type of MWNT synthesized by catalytic chemical vapor deposition methods. Yet experimental measurements on these are rather few and typically have not correlated data from different measurement techniques. Here, the authors report Raman spectroscopy, scanning probe microscopy, conductivity measurements, and force microscopy on single AD-MWNTs. The results demonstrate the high quality of AD-MWNTs and are compatible with the view of them as the best approximation of MWNTs as an assembly of defect-free concentric individual single-walled carbon nanotubes. The authors also demonstrate conductance measurements over a step on the surface of an AD-MWNT, which is due to an abruptly broken outer layer(s), whereby the interlayer resistance is measured. [ABSTRACT FROM AUTHOR]

Layered metal monochalcogenides have attracted significant interest in the 2D family since they show different unique properties from their bulk counterparts. The comprehensive synthesis, characterization, and optoelectrical applications of 2D‐layered tin monosulfide (SnS) grown by pulsed laser deposition are reported. Few‐layer SnS‐based field‐effect transistors (FETs) and photodetectors are fabricated on Si/SiO2 substrates. The premium 2D SnS FETs yield an on/off ratio of 3.41 × 106, a subthreshold swing of 180 mV dec−1, and a field effect mobility (µFE) of 1.48 cm2 V−1 s−1 in a 14‐monolayer SnS device. The layered SnS photodetectors show a broad photoresponse from ultraviolet to near‐infrared (365–820 nm). In addition, the SnS phototransistors present an improved detectivity of 9.78 × 1010 cm2 Hz1/2 W−1 and rapid response constants of 60 ms for grow‐time constant τg and 10 ms for decay‐time constant τd under extremely weak 365 nm illumination. This study sheds light on layer‐dependent optoelectronic properties of 2D SnS that promise to be important in next‐generation 2D optoelectronic devices. [ABSTRACT FROM AUTHOR]

In-plane compressively strained α-Sn films have been theoretically predicted and experimentally proven to possess non-trivial electronic states of a 3D topological Dirac semimetal. The robustness of these states typically strongly depends on purity, homogeneity, and stability of the grown material itself. By developing a reliable fabrication process, we were able to grow pure strained α-Sn films on InSb(100), without heating the substrate during growth nor using any dopants. The α-Sn films were grown by molecular beam epitaxy, followed by experimental verification of the achieved chemical purity and structural properties of the film's surface. Local insight into the surface morphology was provided by scanning tunneling microscopy. We detected the existence of compressive strain using Mössbauer spectroscopy, and we observed a remarkable robustness of the grown samples against ambient conditions. The topological character of the samples was confirmed by angle-resolved photoemission spectroscopy, revealing the Dirac cone of the topological surface state. Scanning tunneling spectroscopy, moreover, allowed us to obtain an improved insight into the electronic structure of the 3D topological Dirac semimetal α-Sn above the Fermi level. [ABSTRACT FROM AUTHOR]

The electric field emission behavior of vertically aligned few-layer graphene was studied in a parallel plate–type setup. Few-layer graphene was synthesized in the absence of any metallic catalyst by microwave plasma enhanced chemical vapor deposition with gas mixtures of methane and hydrogen. The deposit consists of nanostructures that are several micrometers wide, highly crystalline stacks of four to six atomic layers of graphene, aligned vertically to the substrate surface in a high density network. The few-layer graphene is found to be a good field emitter, characterized by turn-on fields as low as 1 V/μm and field amplification factors up to several thousands. We observe a clear dependence of the few-layer graphene field emission behavior on the synthesis parameters: Hydrogen is identified as an efficient etchant to improve field emission, and samples grown on titanium show lower turn-on field values and higher amplification factors when compared to samples grown on silicon. [ABSTRACT FROM AUTHOR]

The training effect in polycrystalline exchange bias Co/CoO bilayers has been investigated with low-temperature magnetic force microscopy (MFM). After field cooling the bilayer to 13 K, no MFM contrast was detected but a clear MFM contrast related to the domain structure appears after the first magnetization reversal of the ferromagnetic Co layer. Once formed, the ferromagnetic domains survive even at very large fields and cannot be erased by the application of a magnetic field along the cooling field direction. On the other hand, it is possible to partially remove the magnetic domains by performing a hysteresis loop with a magnetic field perpendicular to the cooling field direction. [ABSTRACT FROM AUTHOR]

The supramolecular organization of a new polychlorotriphenyl (PTM) radical bearing three long alkyl chains is analyzed by scanning tunneling microscopy (STM) at the liquid-solid interface. The radical has self-assembled hierarchically on graphite forming head-to-head dimers that is organized in rows following a spin-containing two-leg molecular ladder topology, in which the alkyl chains have determined the space between the radical rows and have acted as diamagnetic barriers.

The simultaneous formation of single domain (3×3) and multi domain (√7×√7)R(±19.1°) germanene phases on Al(111) surface in the sub-monolayer range was studied using scanning tunneling microscopy (STM) and density functional theory (DFT) based simulations. Experimental results revealed that both germanene phases nucleate and grow independently from each other and regardless of Al substrate temperature within significantly expanded range Ts = 27–200 °C. Our results unambiguously showed that STM images with hexagonal contrast yield correct-resolved structure for both germanene phases, while honeycomb contrast is a result of an artificial tip-induced STM resolution. First-principles calculations suggested atomic models with strongly buckled germanene (2×2)/Al(111)(3×3) and (√3×√3)R30°/Al(111)(√7×√7)R(±19.1°) with one of eight and one of six Ge atoms protruding upward respectively, that consistently describe the experimentally observed STM images both for single and multi domain surface phases. According to the DFT based simulations both germanene (2×2) and (√3×√3)R30° superstructures have a stretched lattice strain with respect to the ideal free-standing germanene by 6.2% and 13.9%, respectively. Hence, numerous small domains separated by domain boundaries in the (√3×√3)R307Al(111)(√7×√7)R(±19.1°) germanene phase tend to reduce the surface energy and prevent the formation of extended single domains, in contrast to the (2×2)/Al(111)(3×3) phase. However, our experimental results showed that the nucleation and growth of germanene on Al(111) surface yield strong modifications of Al surface even at room temperature (RT), which may be contributed to the formation of Al-Ge alloy due to Ge surface solid-states reactivity that was ignored in recent studies. It is already evident from our present findings that the role of Al atoms in the formation of (3×3) and (√7×√7)R(±19.1°) germanene phases is worthy to be carefully studied in the future, which could be an important knowledge for large-quantity fabrication of germanene on aluminum. [ABSTRACT FROM AUTHOR]

Membranes with high solvent permeance and excellent molecular sieving characteristics are highly desirable for energy-efficient organic liquid separation. However, the "trade-off" relationship between permeability and solute selectivity remains a challenge for thin film composite (TFC) polymeric membranes. Herein, we report a facile and scalable fabrication technique for a high performance and solvent-resistant TFC membrane based on a nanofibrous hydrogel network. Aramid nanofibers (ANF) with a diameter of ∼12.3 nm were used to construct a three-dimensional interconnected hydrogel structure and to obtain a homogeneous diffusion of the aqueous solution. A polyamide (PA) nanofilm with a thickness down to 62 nm was successfully fabricated on the nanofibrous hydrogel support via controlled interfacial polymerization (IP) and was found to have an unprecedented solvent permeation for mild polar solvents (methanol: 54.0 L m−2 h−1 bar−1) and aggressive organic solvents (acetone: 70.0 L m−2 h−1 bar−1, dimethyl formamide: 34.0 L m−2 h−1 bar−1). These permeabilities are over two orders of magnitude higher than those of commercial membranes, with a similar selectivity. The ease of preparation stimulates industrial scale-up and opens up future possibilities for applications in molecular separation, solvent recovery, water treatment and water desalination. [ABSTRACT FROM AUTHOR]

We probe the electron–phonon coupling for in situ engineered porphyrin-based magnetic molecular layers supported on weakly reactive surfaces. Using high-resolution scanning tunneling microscopy and spectroscopy at 4.5 K we show that the electronic and magnetic properties of the engineered molecules are the result of interplay between many-body spin–flip excitations and electron–phonon interactions. [ABSTRACT FROM AUTHOR]

We demonstrate the effective confinement of surface electrons in the pores of molecular networks formed by dehydrobenzo[12]annulene (DBA) molecules with butoxy groups (DBA-OC4) on Au(111). Investigation of the network formation starting from single molecules reveals a considerable interaction of the molecules with the substrate, which is at the origin of the observed confinement. [ABSTRACT FROM AUTHOR]

The monolayer adsorption of sulfur on Ge(100) surfaces from aqueous (NH4)2S solution is an approach to saturate, i.e., to passivate broken surface bonds and hence to reduce the electrical and chemical activity of this semiconductor surface. Despite its importance in view of developing Ge-based microelectronic devices, we still lack a fundamental understanding of how this treatment modifies the electrical and chemical properties of the Ge surface. In this study, the electronic properties and ambient stability of sulfurized p-type Ge surfaces are investigated using a variety of complementary spectroscopic techniques. Based on these results we evaluate the degree of electrical and chemical passivation that can be achieved by sulfur adsorption from (NH4)2S solution. We find that sulfur atoms chemically bind to Ge surface atoms within the first few seconds after immersion in solution. Saturation is achieved after approximately 30 s at a maximum sulfur coverage below half of a monolayer. The Ge–S bonds have a partial ionic character, causing depletion of the majority charge carriers near the surface. The band gap measured at the surface exhibits a lower density of surface states compared to the clean Ge surface, indicating that the S/Ge surface is electrically passivated. The Ge–S bonds are preserved upon moderate exposure to ambient conditions (ca. 2 hours), but a small fraction of the sulfur is oxidized. The steady increase of the oxygen coverage with increasing exposure time suggests a growth of Ge oxides, indicating limited resistance of the sulfurized Ge surfaces to oxidizing conditions. [ABSTRACT FROM AUTHOR]

We present the first measurements of both the morphology and electronic structure of preformed Co nanoclusters in the size range of a few up to several hundreds of atoms by means of low-temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Co clusters are produced in the gas phase and deposited under controlled ultra-high vacuum conditions onto clean Au(111) with low density, well below complete coverage of the Au(111) substrate. We find that smaller clusters, which typically contain less than 20 atoms, exhibit a significant surface mobility with subsequent aggregation, whereas larger clusters turn out to be immobile. From a systematic analysis of the cluster height distribution, we infer that the approximately spherical clusters experience only a restricted flattening and have a multilayered structure, which sometimes is observed to exhibit hexagonal facets, pointing to a truncated octahedral shape. Furthermore, detailed STS measurements on individual Co clusters reveal the presence of various size- and shape-dependent maxima with large energy spacings of the order of 100 meV, as well as an occupied state around -200 meV that originates from the Co 3d band. Our findings provide direct evidence for the existence of strong electron confinement effects in the Co clusters stemming from delocalized Co valence electrons. Our results provide an original contribution to the understanding of cluster physics and in particular of the specific electronic structure. [ABSTRACT FROM AUTHOR]

We report on detailed low temperature scanning tunneling spectroscopy measurements performed on nanoscale Co islands on Au(111) films. At low coverages, Co islands self-organize in arrays of mono- and bilayer nanoscale structures that often have an hexagonal shape. The process of self-organization is induced by the Au(111) 'herringbone' reconstruction. By means of mapping of the local density of states with lock-in detection, electron standing wave patterns are resolved on top of the atomically flat Co islands. The surface state electrons are observed to be strongly confined laterally inside the Co nanosized islands, with their wavefunctions reflecting the symmetry of the islands. To complement the experimental work, particle-in-a-box calculations were performed. The calculations are based on a newly developed variational method that can be applied to '2D boxes' of arbitrary polygonal shape. The experimental patterns are found to fit nicely to the calculated wavefunctions for a box having a symmetry corresponding to the experimental island symmetry. The small size of the Co islands under study (down to 7.7 nm2) is observed to induce a strong discretization of the energy levels, with very large energy separations between the eigenstates up to several 100 meV. The observed standing wave patterns are identified either as individual eigenstates or as a 'mixture' of two or more energetically close-lying eigenstates of the cobalt island. Additionally, the Co surface state appears not to be limited to mono- and bilayer islands, but this state remains observable for multilayered islands up to five monolayers of Co. [ABSTRACT FROM AUTHOR]

We report the direct observation by low temperature scanning tunneling microscopy and scanning tunneling spectroscopy of the d-orbital of a Mn p-type impurity appearing on a cleaved InAs(110) surface. STM images reveal remarkable cross-like protrusions and round depressions around isolated impurity atom on the surface at opposite polarity of tunneling bias voltage. By means of diagram technique for nonequilibrium processes we show that the crucial interplay between nonequilibrium charging effects and many-particle interaction leading to Coulomb singularities provides a consistent description of the experimental results. [ABSTRACT FROM AUTHOR]

The intrinsic domain wall resistance (DWR) of 180° Néel walls in a polycrystalline Fe film is determined by creating a periodic domain pattern, obtained by locally inducing exchange bias. After field cooling, the coercivity is spatially modulated, resulting in periodic 180° domain walls. To determine the intrinsic DWR, a rotating magnetic field is used to reversibly create and annihilate the domain walls. After correcting for the anisotropic magnetoresistance, the extracted DWR is positive. [ABSTRACT FROM AUTHOR]

We studied surface morphology and magnetic properties of Fe/MgO(001) films deposited at an angle varying between 0° and 60° with respect to the surface normal and with azimuth along the Fe[010] or the Fe[110] direction. Due to shadowing, elongated grains appear on the film surface for deposition at sufficiently large angle. X-ray reflectivity reveals that, depending on the azimuthal direction, films become either rougher or smoother for oblique deposition. For deposition along Fe[010] the pronounced uniaxial magnetic anisotropy (UMA) results in the occurrence of “reversed” two-step and of three-step hysteresis loops. For deposition along Fe[110] the growth-induced UMA is much weaker, causing a small rotation of the easy axes. [ABSTRACT FROM AUTHOR]

Grazing-incidence Ar+ ion sputtering has been used to produce nanoscale ripples on the surface of the Fe/Mg(001) system. This way, a uniaxial anisotropy with both controllable strength and orientation can be superimposed on top of the cubic anisotropy, resulting in Fe/MgO(001) films with unusual anisotropy symmetry. By combining longitudinal and transverse Kerr-effect measurements, different switching processes are revealed. Depending on the orientation of the external magnetic field, one-jump, two-jump, and “reverse” two-jump magnetization reversals can be observed. A simple model, which takes into account the relevant anisotropy energies, is developed to explain the experimentally observed switching fields and to evaluate the domain wall pinning energies of the sputtered sample. [ABSTRACT FROM AUTHOR]

2D Optoelectronics: Layer-Dependent Optoelectronic Properties of 2D van der Waals SnS Grown by Pulsed Laser Deposition (Adv. In article number 1901020, Xinhua Pan, Yu-Jia Zeng, Chris Van Haesendonck, and co-workers successfully grow 2D layered tin monosulfide (SnS) by pulsed laser deposition. They demonstrate how the properties of SnS depend on the number of monolayers. [Extracted from the article]