Tellurium (Te) films with monolayer and few-layer thickness are obtained by molecular beam epitaxy on a graphene/6H-SiC(0001) substrate and investigated by in situ scanning tunneling microscopy and spectroscopy (STM/STS). We reveal that the Te films are composed of parallel-arranged helical Te chains flat-lying on the graphene surface, exposing the (1 × 1) facet of (101̅0) of the bulk crystal. The band gap of Te films increases monotonically with decreasing thickness, reaching the near-infrared band for the monolayer Te. An explicit band bending at the edge between the monolayer Te and graphene substrate is visualized. With the thickness controlled in the atomic scale, Te films show potential applications of electronics and optoelectronics.
Exploration of efficient water oxidation catalysts (WOCs) is the primary challenge in conversion of renewable energy into fuels. Here we report a molecularly well-defined heterogeneous WOC with Aza-fused, π-conjugated, microporous polymer (Aza-CMP) coordinated single cobalt sites (Aza-CMP-Co). The single cobalt sites in Aza-CMP-Co exhibited superior activity under alkaline and near-neutral conditions. Moreover, the molecular nature of the isolated catalytic sites makes Aza-CMP-Co a reliable model for studying the heterogeneous water oxidation mechanism. By a combination of experimental and theoretical results, a pH-dependent nucleophilic attack pathway for O-O bond formation was proposed. Under alkaline conditions, the intramolecular hydroxyl nucleophilic attack (IHNA) process with which the adjacent -OH group nucleophilically attacks Co4+=O was identified as the rate-determining step. This process leads to lower activation energy and accelerated kinetics than those of the intermolecular water nucleophilic attack (WNA) pathway. This study provides significant insights into the crucial function of electrolyte pH in water oxidation catalysis and enhancement of water oxidation activity by regulation of the IHNA pathway.
The significant role of interfacial coupling on the superconductivity enhancement in FeSe films on SrTiO3 has been widely recognized. But the explicit origination of this coupling is yet to be identified. Here by surface phonon measurements using high resolution electron energy loss spectroscopy, we found electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO3 can penetrate into FeSe films and strongly interact with electrons therein. The mode-specific electronphonon coupling (EPC) constant for the ∼92 meV F-K phonon is ∼ 0.25 in the single-layer FeSe on SrTiO3. With increasing FeSe thickness, the penetrating field intensity decays exponentially, which matches well the observed exponential decay of the superconducting gap. It is unambiguously shown that the SrTiO3 F-K phonon penetrating into FeSe is essential in the interfacial superconductivity enhancement.
The recent observation of superconducting state at atomic scale has motivated the pursuit of exotic condensed phases in two-dimensional (2D) systems. Here we report on a superconducting phase in two-monolayer crystalline Ga films epitaxially grown on wide band-gap semiconductor GaN(0001). This phase exhibits a hexagonal structure and only 0.552 nm in thickness, nevertheless, brings about a superconducting transition temperature Tc as high as 5.4 K, confirmed by in situ scanning tunneling spectroscopy, and ex situ electrical magneto-transport and magnetization measurements. The anisotropy of critical magnetic field and Berezinski-Kosterlitz-Thouless-like transition are observed, typical for the 2D superconductivity. Our results demonstrate a novel platform for exploring atomic-scale 2D superconductor, with great potential for understanding of the interface superconductivity.PACS numbers: 68.37. Ef, 74.55.+v, Superconductivity has recently been observed in oneatomic-layer Pb [1][2][3][4][5] and In [6,7] films grown on Si(111) substrate, at the SrTiO 3 /LaAlO 3 interface [8], and in one-unit-cell thick FeSe films on SrTiO 3 [9,10]. This has been stimulating great attention and interest for both understanding the electron pairing in quantum confined systems and also the pursuit of emergent phases of matter in the two-dimensional (2D) systems, such as the enhancement of superconducting transition temperature T c . The recent discovery of electric field induced superconductivity at SrTiO 3 surface [11] and in 2D MoS 2 crystal [12] further demonstrates the feasibility of controlling 2D superconductivity via interface engineering. Thus far, however, the nature of interface or 2D superconductivity remains obscure. Preparing more hybird heterostructures with enhanced superconductivity is particularly required but experimentally challenging.GaN, as a wide band gap and high piezo-electric constant semiconductor [13,14], is commonly used in highspeed transistors, lasers for telecommunications, and light-emitting diodes for energy efficient displays. More significantly, it has been previously shown that GaN is often wetted with one to two atomic layers of Ga atoms [15][16][17], wherein Ga is intrinsically superconductive [18][19][20]. Therfore, Ga/GaN may possibly serve an ideal system to search for enhanced superconductivity near their interface. In this work, by in situ scanning tunneling microscopy/spectroscopy (STM/STS), ex situ electrical magneto-transport and magnetization measurements, we have unambiguously demonstrated that two-monolayer (ML) Ga films (as thin as 0.552 nm) grown on GaN form a hexagonal structure and exhibit superconductivity with a T c up to 5.4 K, which differs from any previously reported stable or crystalline Ga phases [18][19][20]. The anisotropy of critical magnetic field and BerezinskiKosterlitz-Thouless (BKT)-like transition are observed, indicative of the 2D nature of superconductivity in 2 ML Ga/GaN(0001).Our STM/STS experiments are conducted in a Unisoku ultrahigh vacuum low temperature STM ...
The growth of lithium (Li) dendrites and the huge volume change are the critical issues for the practical applications of Li‐metal anodes. In this work, a spatial control strategy is proposed to address the above challenges using lotus‐root‐like Ni–Co hollow prisms@carbon fibers (NCH@CFs) as the host. The homogeneously distributed bimetallic Ni–Co particles on the N‐doped carbon fibers serve as nucleation sites to effectively reduce the overpotential for Li nucleation. Furthermore, the 3D conductive network can alter the electric field. More importantly, the hierarchical lotus‐root‐like hollow fibers provide sufficient void space to withstand the volume expansion during Li deposition. These structural features guide the uniform Li nucleation and non‐dendritic growth. As a result, the NCH@CFs host enables a very stable Li metal anode with a low voltage hysteresis during repeated Li plating/stripping for 1200 h at a current density of 1 mA cm−2.
Caffeoylquinic acids (CQAs) are main constituents in many herbal medicines with various biological and pharmacological effects. However, CQAs will degrade or isomerize when affected by temperature, pH, light, etc. In this study, high-performance liquid chromatography with photodiode array detection (HPLC-PDA) and high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) was utilized to study the stability and degradation of CQAs (three mono-acyl CQAs and four di-acyl CQAs) under various ordinary storage conditions (involving different temperatures, solvents, and light irradiation). The results indicated that the stability of CQAs was mainly affected by temperature and light irradiation, while solvents did not affect it in any obvious way under the conditions studied. Mono-acyl CQAs were generally much more stable than di-acyl CQAs under the same conditions. Meanwhile, the chemical structures of 30 degradation products were also characterized by HPLC-MS n , inferring that isomerization, methylation, and hydrolysis were three major degradation pathways. The result provides a meaningful clue for the storage conditions of CQAs standard substances and samples.
Molecular beam epitaxy is used to grow TiSe2 ultrathin films on graphitized SiC(0001) substrate. TiSe2 films proceed via a nearly layer-by-layer growth mode and exhibit two dominant types of defects, identified as Se vacancy and interstitial, respectively. By means of scanning tunneling microscopy, we demonstrate that the well-established charge density waves can survive in single unit-cell (one triple layer) regime, and find a gradual reduction in their correlation length as the density of surface defects in TiSe2 ultrathin films increases. Our findings offer important insights into the nature of charge density wave in TiSe2, and also pave a material foundation for potential applications based on the collective electronic states. 68.37.Ef Transition metal dichalcogenides (TMDCs) typically crystallize into layered structures via weak van der Waals attraction between adjacent layers and exhibit a variety of technologically fascinating physical properties. Like graphene, a body of distinctively promising phenomena emerges when the TMDC bulk crystals are thinned down to mono-or few-layers, which have recently attracted considerable interests in condensed matter physics and materials science. 1 These phenomena include, for example, realization of the two-dimensional (2D) semiconductor with a direct band gap in the visible range, 2,3 broken parity symmetry, 4,5 pronounced spinorbital coupling/splitting, 6,7 and extremely large exciton binding energy. 8 The intriguing physical properties in TMDC monolayers can be employed to develop applications in optoelectronics, valleytronics, spintronics and energy storages. 1-8 Some of layered TMDCs are found to exhibit generic instabilities towards the symmetryreducing charge density wave (CDW) and superconductivity, and therefore provide unprecedented opportunities to investigate their interplays. Parallels between TMDCs and cuprates, both of which share similar ground states, have indeed been recently claimed. 9 Titanium diselenide (TiSe 2 ), a semimetal in nature with hexagonally packed TiSe 6 octahedra (1T ), 10-12 represents a widely studied and interesting TMDC. It undergoes a second-order phase transition to nonchiral CDW with a commensurate 2 × 2 × 2 superstructure at the CDW transition temperature T CDW ∼ 200 K, 13 and then to chiral CDW at a slightly lower temperature. 14 Yet, in spite of more than three decades of intensive experimental and theoretical endeavors, the driving force for the CDW transition remains unsettled. Upon intercalation with copper 15 or applying pressure, 16 the CDW ordering melts and superconductivity develops with a critical transition temperature of several Kelvin, indicating com-petition between CDW and superconductivity in TiSe 2 . Recently, self-induced topologically nontrivial and chiral superconducting phases have been predicted in pressurized TiSe 2 17 and TiSe 2 monolayer, 18 respectively, which might harbor the long-pursuing Majorana fermions. 19 According to Raman spectroscopy study, T CDW can be enhanced as the thickness of mechan...
Chirality is a particularly important concept in nature and exists at all length scales, ranging from the molecular level to the supramolecular level. Over the last two decades, various design strategies have been developed to construct chiral materials based on perylene diimides (PDIs) and to mimic the chiral assembly process in biological systems, but applications of these chiral aggregates are still at an early stage. This Minireview summarizes recent progress in the synthesis and properties of chiral PDIs. The chirality in PDI‐based materials can be generated by three different approaches: from the twisted planes of PDIs, the chiral substituents of PDIs, and the co‐assembly of achiral PDIs and chiral guests. A comprehensive understanding of the applications of chiral PDIs as well as potential future developments is also provided.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.