Preface to the Special Issue on 2D-Materials-Related Physical Properties and Optoelectronic Devices

Recent advances in two-dimensional (2D) materials following the successful fabrication of graphene in 2004 by Novoselov and Geim is expected to grow into the new silicon, offering a lifeline for Moore’s law. With the rapid development of the synthesis methods, more and more 2D materials, such as transition metal dichalcogenides (TMDs, MX2), black phosphorus (BP) and InSe with a finite gap are reported to be more promising for achieving this dream since they often offer alternative solutions to compensate for the gapless graphene’s weaknesses.

Electronic structure of molecular beam epitaxy grown 1T’-MoTe2 film and strain effect

Atomically thin transition metal dichalcogenide films with distorted trigonal(1T') phase have been predicted to be candidates for realizing quantum spin Hall effect. Growth of 1T' film and experimental investigation of its electronic structure are critical. Here we report the electronic structure of 1T'-MoTe2 films grown by molecular beam epitaxy(MBE).Growth of the 1T'-MoTe2 film depends critically on the substrate temperature, and successful growth of the film is indicated by streaky stripes in the reflection high energy electron diffraction(RHEED) and sharp diffraction spots in the low energy electron diffraction(LEED). Angle-resolved photoemission spectroscopy(ARPES) measurements reveal a metallic behavior in the as-grown film with an overlap between the conduction and valence bands. First principles calculation suggests that a suitable tensile strain along the a-axis direction is needed to induce a gap to make it an insulator. Our work not only reports the electronic structure of MBE grown 1T'-MoTe2 films, but also provides insights for strain engineering to make it possible for quantum spin Hall effect.

The proximity between hydroxyl and single atom determines the catalytic reactivity of Rh1/CeO_(2) single-atom catalysts

The local structure of the metal single-atom site is closely related to the catalytic activity of metal single-atom catalysts(SACs).However,constructing SACs with homogeneous metal active sites is a challenge due to the surface heterogeneity of the conventional support.Herein,we prepared two Rh1/CeO_(2)SACs(0.5Rh1/r-CeO_(2)and 0.5Rh1/c-CeO_(2),respectively)using two shaped CeO_(2)(rod and cube)exposing different facets,i.e.,CeO_(2)(111)and CeO_(2)(100).In CO oxidation reaction,the T100 of 0.5Rh1/r-CeO_(2)SACs is 120°C,while the T100 of 0.5Rh1/c-CeO_(2)SACs is as high as 200°C.Via in-situ CO diffuse reflectance infrared Fourier transform spectroscopy(CO-DRIFTS),we found that the proximity between OH group and Rh single atom on the plane surface plays an important role in the catalytic activity of Rh1/CeO_(2)SAC system in CO oxidation.The Rh single atom trapped at the CeO_(2)(111)crystal surface forms the Rh1(OH)adjacent species,which is not found on the CeO_(2)(100)crystal surface at room temperature.Furthermore,during CO oxidation,the OH group far from Rh single atom on the 0.5Rh1/c-CeO_(2)disappears and forms Rh1(OH)adjacent species when the temperature is above 150°C.The formation of Rh1(OH)adjacentCO intermediate in the reaction is pivotal for the excellent catalytic activity,which explains the difference in the catalytic activity of Rh single atoms on two different CeO_(2)planes.The formed Rh1(OH)adjacent-O-Ce structure exhibits good stability in the reducing atmosphere,maintaining the Rh atomic dispersion after CO oxidation even when pre-reduced at high temperature of 500°C.Density functional theory(DFT)calculations validate the unique activity and reaction path of the intermediate Rh1(OH)adjacentCO species formed.This work demonstrates that the proximity between metal single atom and hydroxyl can determine the formation of active intermediates to affect the catalytic performances in catalysis.

Targeted alleviation of ischemic stroke reperfusion via atorvastatin-ferritin Gd-layered double hydroxide

In acute ischemic stroke therapy,potent neuroprotective agents are needed that prevent neural injuries caused by reactive oxygen species(ROS)during ischemic reperfusion.Herein,a novel 2D neuroprotective agent(AFGd-LDH)is reported,comprising Gd-containing layered double hydroxide nanosheets(Gd-LDH,as a drug nanocarrier/MRI contrast agent),atorvastatin(ATO,as a neuroprotective drug)and the ferritin heavy subunit(FTH,as a blood brain barrier transport agent).Experiments revealed AFGd-LDH to possess outstanding antioxidant activity,neuroprotective properties,blood‒brain barrier transit properties,and biocompatibility.In vitro studies demonstrated the ROS scavenging efficiency of AFGd‒LDH to be~90%,surpassing CeO_(2)(50%,a ROS scavenger)and edaravone(52%,a clinical neuroprotective drug).Ischemia‒reperfusion model studies in mice showed AFGd‒LDH could dramatically decrease apoptosis induced by reperfusion,reducing the infarct area by 67%and lowering the neurological deficit score from 3.2 to 0.9.AFGd-LDH also offered outstanding MRI performance,thus enabling simultaneous imaging and ischemia reperfusion therapy.

Rota-Baxter TD Algebra and Quinquedendriform Algebra

A dendriform algebra defined by Loday has two binary operations that give a two-part splitting of the associativity in the sense that their sum is associative. Sim- ilar dendriform type algebras with three-part and four-part splitting of the associativity were later obtained. These structures can also be derived from actions of suitable linear operators, such as a Rota-Baxter operator or TD operator, on an associative algebra. Mo- tivated by finding a five-part splitting of the associativity, we consider the Rota-Baxter TD （RBTD） operator, an operator combining the Rota-Baxter operator and TD oper- ator, and coming from a recent study of Rota＇s problem concerning linear operators on associative algebras. Free RBTD algebras on rooted forests are constructed. We then introduce the concept of a quinquedendriform algebra and show that its defining relations are characterized by the action of an RBTD operator, similar to the cases of dendriform and tridendriform algebras.

Crossover from 2D metal to 3D Dirac semimetal in metallic PtTe2 films with local Rashba effect

PtTe2 and PtSe2 with trigonal structure have attracted extensive research interests since the discovery of type-II Dirac fermions in the bulk crystals. The evolution of the electronic structure from bulk 3D topological semimetal to 2D atomic thin films is an important scientific question. While a transition from 3D type-II Dirac semimetal in the bulk to 2D semiconductor in monolayer(ML) film has been reported for PtSe2, so far the evolution of electronic structure of atomically thin PtTe2 films still remains unexplored.Here we report a systematic angle-resolved photoemission spectroscopy(ARPES) study of the electronic structure of high quality PtTe2 films grown by molecular beam epitaxy with thickness from 2 ML to 6 ML.ARPES measurements show that PtTe2 films still remain metallic even down to 2 ML thickness, which is in sharp contrast to the semiconducting property of few layer PtSe2 films. Moreover, a transition from 2D metal to 3D type-II Dirac semimetal occurs at film thickness of 4–6 ML. In addition, Spin-ARPES measurements reveal helical spin textures induced by local Rashba effect in the bulk PtTe2 crystal, suggesting that similar hidden spin is also expected in few monolayer PtTe2 films. Our work reveals the transition from2D metal to 3D topological semimetal and provides new opportunities for investigating metallic 2D films with local Rashba effect.

Pressure-induced Lifshitz transition in the type Ⅱ Dirac semimetal PtTe_2

Numerous exotic properties have been discovered in Dirac Semimetals(DSMs) and Weyl Semimetals(WSMs). In a given DSM/WSM, the Dirac/Weyl nodes usually coexist with other bulk states, making their respective contribution elusive. In this work, we distinguish the role of bulk states from the tilted Dirac nodes on the transport properties in DSMs. Specifically, we applied pressure to a type-II DSM material, PtTe2, and studied its pressure modified electronic and lattice structure systematically by using in situ transport measurements and X-ray diffraction(XRD). A pressure-induced transition at about 20 GPa is revealed in the transport properties, while the layered lattice structure is robust against pressure as illustrated in XRD measurement results.Density functional theory(DFT) calculations suggest that this is originated from the Lifshitz transition in the bulk states. Our findings provide evidence to identify the bulk states' influence on transport from the topologically-protected DSM states in the DSM material.

Growth of large scale PtTe,PtTe_(2) and PtSe_(2) films on a wide range of substrates

1T phase of transition metal dichalcogenides(TMDCs)formed by group 10 transition metals(e.g.Pt,Pd)have attracted increasing interests due to their novel properties and potential device applications.Synthesis of large scale thin films with controlled phase is critical especially considering that these materials have relatively strong interlayer interaction and are difficult to exfoliate.Here we report the growth of centimeter-scale PtTe,1T-PtTe2 and 1T-PtSe2 films via direct deposition of Pt metals followed by tellurization or selenization.We find that by controlling the Te flux,a hitherto-unexplored PtTe phase can also be obtained,which can be further tuned into PtTe2 by high temperature annealing under Te flux.These films with different thickness can be grown on a wide range of substrates,including NaCl which can be further dissolved to obtain free-standing PtTe2 or PtSe2 films.Moreover,a systematic thickness dependent resistivity and Hall conductivity measurements show that distinguished from the semiconducting PtSe2 with hole carriers,PtTe2 and PtTe films are metallic.Our work opens new opportunities for investigating the physical properties and potential applications of group 10 TMDC films and the new monochalcogenide PtTe film.

Multimodal bioimaging based on gold nanorod and carbon dot nanohybrids as a novel tool for atherosclerosis detection

Advanced biocompatible and robust platforms equipped with diverse properties are highly required in biomedical imaging applications for the early detection of atherosclerotic vascular disease and cancers. Designing nanohybrids composed of noble metals and fluorescent materials is a new way to perform multimodal imaging to overcome the limitations of single-modality counterparts. Herein, we propose the novel design of a multimodal contrast agent; namely, an enhanced nanohybrid comprising gold nanorods （GNRs） and carbon dots （CDs） with silica （SiO2） as a bridge. The nanohybrid （GNR@SiO2@CD） construction is based on covalent bonding between SiO2 and the silane-functionalized CDs, which links the GNRs with the CDs to form typical core-shell units. The novel structure not only retains and even highly improves the optical properties of the GNRs and CDs, but also possesses superior imaging performance in both diffusion reflection （DR） and fluorescence lifetime imaging microscopy （FLIM） measurements compared with bare GNRs or fluorescence dyes and CDs. The superior bioimaging properties of the GNR@SiOa@CD nanohybrids were successfully exploited for in vitro DR and FLIM measurements of macrophages within tissue-like phantoms, paving the way toward a theranostic contrast agent for atherosclerosis and cancer.

Function toggle of tumor microenvironment responsive nanoagent for highly efficient free radical stress enhanced chemodynamic therapy

In contrast to reactive oxygen species(ROS),the generation of oxygen-irrelevant free radicals is oxygen-and H2O2-independent in cell,which can offer novel opportunities to maximum the chemodynamic therapy(CDT)efficacy.Herein,an H2O2-independent“functional reversion”strategy based on tumor microenvironment(TME)-toggled C-free radical generation for CDT is developed by confining astaxanthin(ATX)on the NiFe-layered double hydroxide(LDH)nanosheets(denoted as ATX/LDH).The unique ATX/LDH can demonstrate outstanding TME-responsive C-free radical generation performance by proton coupled electron transfer(PCET),owing to the specific ATX activation by unsaturated Fe sites on the LDH nanosheets formed under TME.Significantly,the Brönsted base sites of LDH hydroxide layers can promote the generation of neutral ATX C-free radicals by capturing the protons generated in the ATX activation process.Conversely,ATX/LDH maintain antioxidant performance to prevent normal tissue cancerization due to the synergy of LDH nanosheets and antioxidative ATX.In addition,C-free radical can compromise the antioxidant defense in cells to the maximum extent,compared with ROS.The free radicals burst under TME can significantly elevate free radical stress and induce cancer cell apoptosis.This strategy can realize TME-toggled C free radical generation and perform free radical stress enhanced CDT.