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Recently, the ultrafast operation (∼20 ns) of a two-dimensional (2D) heterostructured nonvolatile memory (NVM) device was demonstrated, attracting considerable attention. However, there is no consensus on its physical origin. In this study, various 2D NVM device structures are compared. First, we reveal that the hole injection at the metal/MoS

Fructans such as inulin and levan accumulate in certain taxonomic groups of plants and are a reserve carbohydrate alternative to starch. Onion (Allium cepa L.) is a typical plant species that accumulates fructans, and it synthesizes inulin-type and inulin neoseries-type fructans in the bulb. Although genes for fructan biosynthesis in onion have been identified so far, no genes for fructan degradation had been found. In this study, phylogenetic analysis predicted that we isolated a putative vacuolar invertase gene (AcpVI1), but our functional analyses demonstrated that it encoded a fructan 1-exohydrolase (1-FEH) instead. Assessments of recombinant proteins and purified native protein showed that the protein had 1-FEH activity, hydrolyzing the β-(2,1)-fructosyl linkage in inulin-type fructans. Interestingly, AcpVI1 had an amino acid sequence close to those of vacuolar invertases and fructosyltransferases, unlike all other FEHs previously found in plants. We showed that AcpVI1 was localized in the vacuole, as are onion fructosyltransferases Ac1-SST and Ac6G-FFT. These results indicate that fructan-synthesizing and -degrading enzymes are both localized in the vacuole. In contrast to previously reported FEHs, our data suggest that onion 1-FEH evolved from a vacuolar invertase and not from a cell wall invertase. This demonstrates that classic phylogenetic analysis on its own is insufficient to discriminate between invertases and FEHs, highlighting the importance of functional markers in the nearby active site residues.

Terahertz (THz) time-domain emission spectroscopy was performed on layered 2H-MoSe 2 and 2H-WSe 2. The THz emission shows an initial cycle attributed to surge currents and is followed by oscillations attributed to coherent interlayer phonon modes. To obtain the frequencies of the interlayer vibrations, an analysis of the THz emission waveforms was performed, separating the two contributions to the total waveform. Results of the fitting show several vibrational modes in the range of 5.87–32.75 cm − 1 for the samples, attributed to infrared-active interlayer shear and breathing modes. This study demonstrates that THz emission spectroscopy provides a means of observing these low-frequency vibrational modes in layered materials.

The synthesis, structure and reactivity of a pyridine-stabilized silanone molybdenum complex are described.

Contact engineering of two-dimensional semiconductors is a central issue for performance improvement of micro-/nanodevices based on these materials. Unfortunately, the various methods proposed to improve the Schottky barrier height normally require the use of high temperatures, chemical dopants, or complex processes. This work demonstrates that diffused electron beam energy (DEBE) treatment can simultaneously reduce the Schottky barrier height and enable the direct writing of electrical circuitry on van der Waals semiconductors. The electron beam energy projected into the region outside the electrode diffuses into the main channel, producing selective-area n-type doping in a layered MoTe

Two-dimensional heterostructures have been extensively investigated as next-generation nonvolatile memory (NVM) devices. In the past decade, drastic performance improvements and further advanced functionalities have been demonstrated. However, this progress is not sufficiently supported by the understanding of their operations, obscuring the material and device structure design policy. Here, detailed operation mechanisms are elucidated by exploiting the floating gate (FG) voltage measurements. Systematic comparisons of MoTe2, WSe2, and MoS2 channel devices revealed that the tunneling behavior between the channel and FG is controlled by three kinds of current-limiting paths, i.e., tunneling barrier, 2D/metal contact, and p-n junction in the channel. Furthermore, the control experiment indicated that the access region in the device structure is required to achieve 2D channel/FG tunneling by preventing electrode/FG tunneling. The present understanding suggests that the ambipolar 2D-based FG-type NVM device with the access region is suitable for further realizing potentially high electrical reliability.

We investigated the synthesis of aluminum oxide (AlOx) thin films using mist chemical vapor deposition (mist-CVD) from aluminum acetylacetonate (Al(acac)3) and methanol/water (MeOH/H2O) mixture (vo...

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with the ultraflat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of Δn ∼ 2 × 1012 cm-2 μN-1, resulting in the significant intensity of trions in photoluminescence spectra of MoSe2 at low temperature. We found that a hBN/MoSe2/hBN prepared with the "graphene-capping-assisted AFM nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual line width of the exciton peak is only 2.2 meV, which is comparable to the homogeneous limit. Furthermore, in this high-quality sample, we found that the formation of biexciton occurs even at extremely low excitation power (Φph ∼ 2.3 × 1019 cm-2 s-1) due to the enhanced collisions between excitons.

Van der Waals heterostructures are the ideal material platform for tunnel field effect transistors (TFETs) because a band-to-band tunneling (BTBT) dominant current is feasible at room temperature (RT) due to ideal, dangling bond free heterointerfaces. However, achieving subthreshold swing (SS) values lower than 60 mVdec-1 of the Boltzmann limit is still challenging. In this work, we systematically studied the band alignment and heterointerface quality in n-MoS2 channel heterostructure TFETs. By selecting a p+-MoS2 source with a sufficiently high doping level, stable gate modulation to a type III band alignment was achieved regardless of the number of MoS2 channel layers. For the gate stack formation, it was found that the deposition of Al2O3 as the top gate introduces defect states for the generation current under reverse bias, while the integration of an h-BN top gate provides a defect-free, clean interface, resulting in the BTBT dominant current even at RT. All 2D heterostructure TFETs produced by combining the type III n-MoS2/p+-MoS2 heterostructure with the h-BN top gate insulator resulted in low SS values at RT.

Asparagus (Asparagus officinalis L.) accumulates inulin and inulin neoseries-type fructans in root, which are synthesized by three fructosyltransferases-sucrose:sucrose 1-fructosyltransferase (1-SST, EC 2.4.1.99), fructan:fructan 1-fructosyltransferase (1-FFT, EC 2.4.1.100), and fructan:fructan 6G-fructosyltransferase (6G-FFT, EC 2.4.1.243). Fructans in roots are considered as energy sources for emerging of spears, and it has been demonstrated that a gradual decrease in root fructan content occurs during the spear harvesting season (budding and shooting up period). However, the roles of certain three fructosyltransferases during the harvest season have not yet been elucidated. Here, we investigated the variation in enzymatic activities and gene expression levels of three fructosyltransferases and examined sugar contents in roots before and during the spear harvest period. Two cDNAs, aoft2 and aoft3, were isolated from the cDNA library of roots. The respective recombinant proteins (rAoFT2 and rAoFT3), produced by Pichia pastoris, were characterized: rAoFT2 showed 1-FFT activity (producing nystose from 1-kestose), whereas rAoFT3 showed 1-SST activity (producing 1-kestose from sucrose). These reaction profiles of recombinant proteins were similar to those of native enzymes purified previously. These results indicate that aoft2 and aoft3 encoding 1-FFT and 1-SST are involved in fructan synthesis in roots. A gradual downregulation of fructosyltransferase genes and activity of respective enzymes was observed in roots during the harvest period, which also coincided with the decrease in fructooligosaccharides and increase in fructose due to fructan exohydrolase activity. These findings suggest that downregulation of fructosyltransferases genes during harvest time may contribute to efficient degradation of fructan required for the emergence of spears.

Flexible manipulation of the carrier transport behaviors in two-dimensional materials determines their values of practical application in logic circuits. Here, we demonstrated the carrier-type mani...

The crystal structure of a material creates a periodic potential that electrons move through giving rise to the electronic band structure of the material. When two-dimensional materials are stacked, the twist angle between the layers becomes an additional degree freedom for the resulting heterostructure. As this angle changes, the electronic band structure is modified leading to the possibility of flat bands with localized states and enhanced electronic correlations. In transition metal dichalcogenides, flat bands have been theoretically predicted to occur for long moir\'e wavelengths over a range of twist angles around 0 and 60 degrees giving much wider versatility than magic angle twisted bilayer graphene. Here we show the existence of a flat band in the electronic structure of 3{\deg} and 57.5{\deg} twisted bilayer WSe2 samples using scanning tunneling spectroscopy. Direct spatial mapping of wavefunctions at the flat band energy have shown that the flat bands are localized differently for 3{\deg} and 57.5{\deg}, in excellent agreement with first-principle density functional theory calculations., Comment: 15 pages, 4 figures. Added data for 57 degree twist angle sample

We report enhanced interlayer tunneling with reduced linewidth at zero interlayer bias in a twist-controlled double monolayer graphene heterostructure in the quantum Hall regime, when the top ($\nu_{\mathrm{T}}$) and bottom ($\nu_{\mathrm{B}}$) layer filling factors are near $\nu_{\mathrm{T}}=\pm1/2, \pm3/2$ and $\nu_{\mathrm{B}}=\pm1/2, \pm3/2$, and the total filling factor $\nu = \pm1$ or $\pm3$. The zero-bias interlayer conductance peaks are stable against variations of layer filling factor, and signal the emergence of interlayer phase coherence. Our results highlight twist control as a key attribute in revealing interlayer coherence using tunneling., Comment: 5 pages, 4 figures, includes supplementary material

The doping technique is vital for applying two-dimensional (2D) materials such as transition metal dichalcogenide (TMDC)-based field effect transistors (FETs), which can control the channel polarity and improve the performance. However, as conventional doping techniques for silicon-based FET are not suitable for 2D materials, the realization of heavy doping of TMDC materials is challenging, especially for n-type heavy doping. This study reports a simple, regioselective, controllable, and chemically stable heavy doping method for 2H-MoTe2 crystal via high-density laser irradiation. The polarity of the doping can be controlled by changing the irradiation environment. For the MoTe2-nFET, good performance with enhanced contact properties was obtained using the contact doping method via laser irradiation in a vacuum environment.

Selective fragmentation of α,β-unsaturated esters into CC, CO, and OR fragments was investigated with the assistance of a gallane(pyridyl)iron complex Cp*(OC)Fe{(η1-HGaMes2)(η1-2-C5H4N)} (1, Cp*: η5-C5Me5, Mes: 2,4,6-Me3C6H2). The reaction of 1 with methyl acrylate in C7D8 afforded vinyliron complex Cp*(OC)2Fe(CHCH2) (2) and 4-membered Ga2O2 cyclic gallane [Mes2GaOMe]2 (3). The C–O and C–C bonds in t-butyl acrylate were cleaved by the reaction with complex 1, producing complex 2 and pyridine-coordinated gallane Mes2(t-BuO)Ga(pyridine) (4). The selective bond cleavage reactions proceeded by the reactions of 1 with methyl methacrylate and methyl 2-butenoate to afford Cp*(OC)2Fe(CR1CHR2) (R1 = Me, R2 = H (5), R1 = H, R2 = Me (6)) and 3. In contrast, compound 3 was formed in 11% NMR yield by the reaction of 1 with methyl 3-butenoate (β,γ-unsaturated ester), and complex mixtures of unidentified products were obtained by the reaction of 1 with methyl 4-pentenoate (γ,δ-unsaturated ester) and methyl propionate (saturated ester). The treatment of a cyclic ester, α-methylene-γ-butyrolactone, with 1 afforded a 4-membered Ga2O2 cyclic gallane dimer with Cp*(OC)2FeC(CH2)CH2CH2 substituents on the oxygen atoms, namely [Cp*(OC)2FeC(CH2)CH2CH2OGaMes2]2 (7). In addition, the bond cleavage reaction occurred by the reaction of 1 with allyl methyl ether to afford the (η3-allyl)iron complex Cp*(OC)Fe(η3-CH2CHCH2) (8) and 3. These results indicate that the existence of the –CC–C–OR fragment, regardless of the presence or absence of the CO group between the CC and OR fragments, is essential for the bond cleavage reaction.

We report the fabrication of self-assembled surface-passivated [CH(NH2)2]0.8Cs0.2PbI3 thin films using a one-step method with a fluorinated polymer (FP) as an additive without post-treatment. The F...

The oxygen (O2)-dependent resistance change of multilayered molybdenum ditelluride (MoTe2) channels was characterized. A variation of the channel resistance could reproducibly determine relative O2...

Phase-transition field-effect transistors (FETs) are a class of steep-slope devices that show abrupt on/off switching owing to the metal-insulator transition (MIT) induced in the contacting materials. An important avenue to develop phase-transition FETs is to understand the charge injection mechanism at the junction of the contacting MIT materials and semiconductor channels. Here, toward the realization of high-performance phase-transition FETs, we investigate the contact properties of heterojunctions between semiconducting transition-metal dichalcogenides (TMDCs) and vanadium dioxide (VO2) that undergoes a MIT at a critical temperature (Tc) of approximately 340 K. We fabricated transistors based on molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) in contact with the VO2 source/drain electrodes. The VO2-contacted MoS2 transistor exhibited n-type transport both below and above Tc. Across the MIT, the on-current was observed to increase only by a factor of 5, in contrast to the order-of-magnitude change in the resistance of the VO2 electrodes, suggesting the existence of high contact resistance. The Arrhenius analyses of the gate-dependent drain current confirmed the formation of the interfacial barrier at the VO2/MoS2 contacts, irrespective of the phase state of VO2. The VO2-contacted WSe2 transistor showed ambipolar transport, indicating that the Fermi level lies near the mid gap of WSe2. These observations provide insights into the contact properties of phase-transition FETs based on VO2 and TMDCs and suggest the need for contact engineering for high-performance operations.

We report the fabrication of highly crystalline, large-grained organic-inorganic perovskite [CH(NH2)2]0.8Cs0.2PbI3 thin films on TiO2 layers by a one-step method using 1-Cyclohexyl-2-pyrrolidone and thiosemicarbazide as additives without using an antisolvent. The perovskite thin films have no pinhole, grain size of micrometer order, and high crystallinity, with a crystallite size greater than 200 nm. An n-i-p structured [CH(NH2)2]0.8Cs0.2PbI3 thin film solar cell exhibits power conversion efficiencies of 17.8% and 13.8% during reverse and forward scanning, respectively.

Onion (Allium cepa L.) produces a fructose-based oligosaccharide (fructan) in the bulb as the major reserve carbohydrate. We previously found varietal differences in fructan quantity in the harvested bulbs; ‘Kita-momiji 2000’ accumulated a high level of fructans with a high degree of polymerization, while ‘Pole Star’ had low amounts and accumulated mainly monosaccharides and sucrose. In this study, we examined fructan accumulation and enzymatic activities involved in the fructan metabolic pathway in more detail in both aerial and subterranean tissues that were harvested throughout onion development. Analyses of fructan levels in the two cultivars revealed that levels in fructan accumulation were almost the same in aerial and basal leaves before bulb formation, but fructan levels differed significantly in the late stage after bulb formation; varietal differences in fructan content in the harvested bulb were apparent only during the 2 or 3 weeks before harvest. At an early developmental stage, fructan levels in ‘Pole Star’ were equivalent to those in ‘Kita-momiji 2000’, indicating that ‘Pole Star’ can synthesize highly polymerized fructans. In analyses of enzymatic activities, ‘Pole Star’ had high activity for synthesizing fructan after bulb formation compared to ‘Kita-momiji 2000’ and also greater potential for degrading sucrose and fructan before bulb formation, leading to less sucrose accumulation both in aerial and basal leaves. These results suggest that high activities for fructan degradation enzymes in ‘Pole Star’ cause less fructan accumulation in the bulb than in ‘Kita-Momiji 2000’ because this cultivar cannot sustain high levels of sucrose, the starting substrate for fructan synthesis.

The cationic silanethione-coordinated transition-metal complexes [Cp*(OC)3W{S═SiR2(DMAP)}]TFPB (R = Me (6a), Ph (6b), Cp* = η5-C5Me5, DMAP = 4-(dimethylamino)pyridine, TFPB– = B{3,5-(CF3)2C6H3}4–) were isolated by H– abstraction from silylsulfanyl complexes Cp*(OC)3WSSiR2H (5) with Ph3CTFPB, followed by the addition of DMAP. The reaction of 6b with 1 equiv of MeOH afforded methoxysilylsulfanyl complex Cp*(OC)3W(SSiPh2OMe) (7) and [H·DMAP]TFPB (8).

Molybdenum disulfide (MoS

This chapter covers metal complexes with ligating Group 13 element atoms. Since these elements have ns2p1 electron configurations, they are considered to be Lewis acidic and electron-deficient, with a vacant p orbital. As a result, their complexes show a range of characteristics not found for carbon, such as the existence of three-center-two electron bonds, π-interactions different to those in M–Si compounds, the formation of Lewis acid–base adducts and high reactivity. The first sections are dedicated to examples of boryl and borylene complex synthesis and reactions due to their uniqueness and significant differences to heavier boron congeners. The later sections deal with the remaining Group 13 elements. Bonding in the different structural types is explained in detail, since this is central to understanding the chemistry. A number of approaches to synthesis are covered, with examples of each.

In this chapter, we delineate the present state-of-the-art of solution-processed PEDOT:PSS/n-Si heterojunction solar cells in detail. Here, we discuss the emergence, principle of operation, fabrication process, carrier transport properties, and evolution of the efficiency of the PEDOT:PSS/n-Si heterojunction solar cells. We also discuss with the challenges of the solar cells and propose few design guidelines to further improve the efficiency of the solar cells in future. The SCAPS-1D simulation reveals that the use of n+ CdS or In3Se4 BSF layer which can be deposited by simple solution process enhances the efficiency of the PEDOT:PSS/n-Si heterojunction solar cells to 30.94–35.05% with a higher VOC of 0.89 V. The short-circuit current of the solar cells can be further increased by the use of proper ARC layer on the top of the PEDOT:PSS/n-Si heterojunction solar cells.

Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with the ultraflat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of Δ

Flexible manipulation of the carrier transport behaviors in two-dimensional materials determines their values of practical application in logic circuits. Here, we demonstrated the carrier-type manipulation in field-effect transistors (FETs) containing α-phase molybdenum ditelluride (MoTe

In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in $\text{MoSe}_{\text{2}}$/$\text{WSe}_{\text{2}}$ twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1$^\circ$ to 3.5$^\circ$. Using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moir\'e potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of "twistronics".

Vanadium dioxide (VO2) shows promise as a building block of switching and sensing devices because it undergoes an abrupt metal-insulator transition (MIT) near room temperature, where the electrical resistivity changes by orders of magnitude. A challenge for versatile applications of VO2 is to control the MIT by gating in the field-effect device geometry. Here, we demonstrate a gate-tunable abrupt switching device based on a VO2 microwire that is monolithically integrated with a two-dimensional (2D) tungsten diselenide (WSe2) semiconductor by van der Waals stacking. We fabricated the WSe2 transistor using the VO2 wire as the drain contact, titanium as the source contact, and hexagonal boron nitride as the gate dielectric. The WSe2 transistor was observed to show ambipolar transport, with higher conductivity in the electron branch. The electron current increases continuously with gate voltage below the critical temperature of the MIT of VO2. Near the critical temperature, the current shows an abrupt and discontinuous jump at a given gate voltage, indicating that the MIT in the contacting VO2 is thermally induced by gate-mediated self-heating. Our results have paved the way for the development of VO2-based gate-tunable devices by the van der Waals stacking of 2D semiconductors, with great potential for electronic and photonic applications.

We investigated the synthesis of amorphous aluminum titanium oxide Al1− xTi xO y thin films from a Al(acac)3 and Ti(acac)4 mixture using CH3OH/H2O as a solvent through mist chemical vapor deposition (mist-CVD) for application as a high dielectric material. The Ti composition ratio x in the Al1− xTi xO y thin films depends on the Al(acac)3 and Ti(acac)4 mixing ratios and CH3OH/H2O volume ratio. A bandgap energy of Al1− xTi xO y films was decreased from 6.38 to 4.25 eV and the surface roughness also decreased when the Ti composition ratio was increased from 0 to 0.54. The capacitance–voltage plot revealed that the dielectric constant of Al1− xTi xO y thin films increased from 6.23 to 25.12. Consequently, Al1− xTi xO y thin films with a bandgap energy of 5.12 eV and a dielectric constant of 13.8 were obtained by adjusting the ratio x of 0.26. This Al0.74Ti0.26O y layer was applied as a gate dielectric layer for metal-oxide-semiconductor field-effect transistors (MOSFETs) using a mechanically exfoliated two-dimensional (2D) transition metal dichalcogenide (TMDC), MoSe2, and As-doped WSe2 flakes as a channel layer. The MoSe2-based MOSFETs with source/drain gold electrodes exhibit n-channel behavior with a field-effect mobility of 85 cm2/(V s), a threshold voltage of 0.92 V. On the other hand, an on/off ratio of ∼106. As-doped WSe2-based MOSFETs with source/drain platinum electrodes also showed an ambipolar behavior, which was applied for use in logic applications. These findings suggest that Al0.74Ti0.26O y by mist-CVD is promising as a high- k material for TMDC-based MOSFETs.

Asparagus (Asparagus officinalis L.) accumulates inulin- and inulin neoseries-type fructans. Fructose released by the hydrolysis of fructans is an energy source for emerging asparagus spears. Plant fructans are hydrolyzed by fructan exohydrolases (FEHs), whose presence in asparagus has not yet been fully characterized. Here, we describe for the first time the purification and characterization of an FEH from asparagus, and the functional analysis of its gene. The purified enzyme was predicted to exist as a dimer (approximately 130 kDa) consisting of two polypeptides with a molecular mass of approximately 68 kDa. N-terminal sequences of the purified enzyme were matched with the amino acid sequences of aoeh4a and aoeh4b cDNAs isolated from asparagus (cv. Gijnlim and Taihouwase). Native enzymes obtained from asparagus roots and recombinant enzymes produced by Pichia pastoris showed fructan 1-exohydrolase (1-FEH) activity via the hydrolysis of inulin-type fructan. Unlike other 1-FEHs, these enzymes showed minimal hydrolysis of 1-kestose but efficiently hydrolyzed neokestose. Therefore, the enzyme was termed 6G&1-FEH. Gene expression studies in asparagus roots showed that aoeh4 increased during root storage at 2 °C and spear harvesting. These findings suggest that 6G&1-FEH may be involved in fructan hydrolysis in asparagus roots to provide an energy source for emerging asparagus spears.

We fabricated organic-inorganic perovskite [CH(NH2)2]0.8Cs0.2PbI3 thin films on compact TiO2 layers using a one-step method with 1-cyclohexyl-2-pyrrolidone as an additive without using an antisolve...

Realization of unconventional physical properties in two-dimensional (2D) transition-metal dichalcogenides (TMDs) is currently one of the key challenges in condensed-matter systems. However, the electronic properties of 2D TMDs remain largely unexplored compared to those of their bulk counterparts. Here, we report the fabrication of a high-quality monolayer NbSe2 film with a trigonal prismatic structure by molecular beam epitaxy, and the study of its electronic properties by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and electrical transport measurements, together with first-principles band-structure calculations. In addition to a charge density wave (CDW) with 3 × 3 periodicity and superconductivity below 1.5 K, we observed sizable (~0.1 eV) band splitting along the Γ-K cut in the Brillouin zone due to inversion symmetry breaking in the monolayer crystal. This splitting is highly anisotropic in k space, leading to a spin-split van-Hove singularity in the band structure. The present results suggest the importance of spin–orbit coupling and symmetry breaking for unconventional superconductivity and CDW properties in monolayer TMDs. The interplay between symmetry breaking, superconductivity and charge density wave in monolayer NbSe2 is unveiled by spectroscopic techniques. A team led by Takafumi Sato at Tohoku University used molecular beam epitaxy to fabricate atomically thin NbSe2 films on bilayer graphene, and investigated their electronic properties by a combination of electrical transport measurements, scanning tunneling microscopy, and angle-resolved photoemission spectroscopy. A charge density wave transition with a strong periodic charge modulation was detected below 1.5 K, accompanied by the emergence of superconductivity. Owing to the inversion symmetry breaking occurring in monolayer NbSe2, a band splitting developed along the Γ-Κ direction of the Brillouin zone. Such band splitting was found to be related to the robustness of the observed superconductivity, as well as the formation of the charge density wave.

We have fabricated bismuth (Bi) ultrathin films on a charge-density-wave (CDW) compound 1T-TaS2 and elucidated electronic states by angle-resolved photoemission spectroscopy and first-principles band-structure calculations. We found that the Bi film on 1T-TaS2 undergoes a structural transition from (111) to (110) upon reducing the film thickness, accompanied by a drastic change in the energy band structure. We also revealed that while two-bilayer-thick Bi(110) film on Si(111) is characterized by a dispersive band touching the Fermi level (EF), the energy band of the same film on 1T-TaS2 exhibits holelike dispersion with a finite energy gap at EF. We discuss the origin of such intriguing differences in terms of the CDW proximity effect.