Three-dimensional (3D) topological insulators are known for their strong spin-orbit coupling (SOC) and the existence of spin-textured surface states that might be potentially exploited for ..."topological spintronics." Here, we use spin pumping and the inverse spin Hall effect to demonstrate successful spin injection at room temperature from a metallic ferromagnet (CoFeB) into the prototypical 3D topological insulator Bi2Se3. The spin pumping process, driven by the magnetization dynamics of the metallic ferromagnet, introduces a spin current into the topological insulator layer, resulting in a broadening of the ferromagnetic resonance (FMR) line width. Theoretical modeling of spin pumping through the surface of Bi2Se3, as well as of the measured angular dependence of spin-charge conversion signal, suggests that pumped spin current is first greatly enhanced by the surface SOC and then converted into a dc-voltage signal primarily by the inverse spin Hall effect due to SOC of the bulk of Bi2Se3. We find that the FMR line width broadens significantly (more than a factor of 5) and we deduce a spin Hall angle as large as 0.43 in the Bi2Se3 layer.
A structural study of a hierarchical zeolite X, which is similar to the one first synthesized by Inayat et al.,12 was performed using transmission electron microscopy imaging and diffraction. ...Evidence is provided, by comparison to simulations, that this material is an intergrowth of FAU and EMT and a conceptual model is presented for the growth of the FAU material with a small fraction of EMT in an atypical morphology of assembled sheets with well‐defined intersection angles.
Atypical morphology: Hierarchical faujasite is identified as an intergrowth of FAU and EMT. It is proposed that, under the appropriate growth conditions of faceting and branching, this intergrowth with a very small fraction of EMT, not detectable by XRD, leads to the repetitively branched morphology of faujasite.
Thin zeolite films are attractive for a wide range of applications, including molecular sieve membranes, catalytic membrane reactors, permeation barriers, and low-dielectric-constant materials. ...Synthesis of thin zeolite films using high-aspect-ratio zeolite nanosheets is desirable because of the packing and processing advantages of the nanosheets over isotropic zeolite nanoparticles. Attempts to obtain a dispersed suspension of zeolite nanosheets via exfoliation of their lamellar precursors have been hampered because of their structure deterioration and morphological damage (fragmentation, curling, and aggregation). We demonstrated the synthesis and structure determination of highly crystalline nanosheets of zeolite frameworks MWW and MFI. The purity and morphological integrity of these nanosheets allow them to pack well on porous supports, facilitating the fabrication of molecular sieve membranes.
Single‐unit‐cell Sn‐MFI, with the detectable Sn uniformly distributed and exclusively located at framework sites, is reported for the first time. The direct, single‐step, synthesis is based on ...repetitive branching caused by rotational intergrowths of single‐unit‐cell lamellae. The self‐pillared, meso‐ and microporous zeolite is an active and selective catalyst for sugar isomerization. High yields for the conversion of glucose into fructose and lactose to lactulose are demonstrated.
The adventures of thin tin: Sn is introduced in the framework of single‐unit‐cell (2 nm thick) zeolite MFI lamellae, which are intergrown in a self‐pillared arrangement to obtain a hierarchical micro/mesoporous Lewis acid catalyst. Its conversion and selectivity are better than those of other micro‐ and mesoporous Sn‐containing catalysts for isomerization of mono‐ and disacharides.
A detailed description of the electronic properties, chemical state, and structure of uniform single and few‐layered graphene oxide (GO) thin films at different stages of reduction is reported. The ...residual oxygen content and structure of GO are monitored and these chemical and structural characteristics are correlated to electronic properties of the thin films at various stages of reduction. It is found that the electrical characteristics of reduced GO do not approach those of intrinsic graphene obtained by mechanical cleaving because the material remains significantly oxidized. The residual oxygen forms sp3 bonds with carbon atoms in the basal plane such that the carbon sp2 bonding fraction in fully reduced GO is ∼0.80. The minority sp3 bonds disrupt the transport of carriers delocalized in the sp2 network, limiting the mobility, and conductivity of reduced GO thin films. Extrapolation of electrical conductivity data as a function of oxygen content reveals that complete removal of oxygen should lead to properties that are comparable to graphene.
A detailed description of electronic properties, chemical state and structure of uniform single and few‐layered graphene oxide (GO) thin films at different stages of reduction is reported. The residual oxygen content and carbon sp2 bonding is monitored and correlated to electronic properties at various stages of reduction. This analysis reveals that removal of oxygen to achieve sp2 carbon fraction of >0.95 in GO should lead to properties that are comparable to graphene.
•Different milling times affected the structural defects in graphene and its distribution in the n-type Bi2Te2.7Se0.3 matrix.•Long milling time induced higher levels of structural defects in graphene ...and reduced the composite’s electrical conductivity•Ten minutes of mechanical milling was the optimum time to enhance the thermoelectric properties of the composite.•The improved ZT is attributed to the low level of structural defects and reduced agglomeration of graphene in the composite.
This study examines the effects of the structural integrity of graphene on the thermoelectric properties of n-type bismuth telluride alloy. Graphene/Bi2Te2.7Se0.3 composites were prepared via mechanical alloying and spark plasma sintering techniques. Different graphene concentrations (0.05 and 0.5 wt%) and addition times (20 hrs, 10 mins, and 1 min) were considered. The thermoelectric properties were measured, and the results showed that the milling time affects graphene structure as well as its agglomeration. It is revealed that the optimum time to add the two-dimensional filler is during the last phase of mechanical milling as it will preserve graphene’s structure and boost the electrical conductivity. It is also shown that as the milling time of graphene increases, the Seebeck coefficient improves. Even though an increase in the thermal conductivity is expected due to the high electrical conductivity, a clear reduction in the lattice thermal conductivity part was obtained due to the increased scattering at the new interfaces. The figure-of-merit for the optimum sample with 0.05 wt% graphene added in the last 10 mins of milling had an improvement of 19% at room temperature reaching a value of 0.5, and 25% at 160 °C achieving a final value of 0.81.
Abstract
The rapid discovery of two-dimensional (2D) van der Waals (vdW) quantum materials has led to heterostructures that integrate diverse quantum functionalities such as topological phases, ...magnetism, and superconductivity. In this context, the epitaxial synthesis of vdW heterostructures with well-controlled interfaces is an attractive route towards wafer-scale platforms for systematically exploring fundamental properties and fashioning proof-of-concept devices. Here, we use molecular beam epitaxy to synthesize a vdW heterostructure that interfaces two material systems of contemporary interest: a 2D ferromagnet (1T-CrTe
2
) and a topological semimetal (ZrTe
2
). We find that one unit-cell (u.c.) thick 1T-CrTe
2
grown epitaxially on ZrTe
2
is a 2D ferromagnet with a clear anomalous Hall effect. In thicker samples (12 u.c. thick CrTe
2
), the anomalous Hall effect has characteristics that may arise from real-space Berry curvature. Finally, in ultrathin CrTe
2
(3 u.c. thickness), we demonstrate current-driven magnetization switching in a full vdW topological semimetal/2D ferromagnet heterostructure device.
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