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The quantum interference patterns induced by impurities in graphene can form the (√3 × √3)R30° superlattice with intervalley scattering. This superlattice can lead to the folded Dirac cone at the center of Brillouin zone by coupling two non-equivalent valleys. Using angle-resolved photoemission spectroscopy (ARPES), we report the observation of suppression of the folded Dirac cone in mono- and bilayer graphene upon potassium doping. The intervalley coupling with chiral symmetry broken can persist upon a light potassium-doped level but be ruined at the heavily doped level. Meanwhile, the folded Dirac cone can be suppressed by the renormalization of the Dirac band with potassium doping. Our results demonstrate that the suppression of the intervalley scattering pattern by potassium doping could pave the way toward the realization of novel chiraltronic devices in superlattice graphene.

The Mn-Bi-Te class of compounds are recently discovered topological insulators with broken time-reversal-symmetry, which host unique quantum anomalous Hall and axion insulator states. Their key characteristics are believed to be sufficiently understood by models in a single-particle picture. Here, we apply scanning tunneling microscopy to study the electronic properties of MnBi2Te4 and MnBi4Te7. Unexpectedly, our quasiparticle interference (QPI) results demonstrate that rotational symmetry of the crystal breaks, i.e. a nematic-like pattern arises, in certain energy range but persists in others. Moreover, our data in the presence of an external magnetic field rule out the possibility of the material magnetism as an origin of the C2 symmetric QPI pattern. This study reveals that the interaction in the Mn-Bi-Te class of topological materials may play an essential role in their electronic states, and thus opens a new path for investigating the interplay between wavefunction topology and symmetry breaking phases.

For the inverted organic solar cells (OSCs), the interface contacts between the ZnO electron transporting layer and the organic active layer play an important role in the device performance and stability. Since the solution-processed ZnO surface always contains some base or zinc salt contaminants, we explored how the surface pH conditions influence the performance and stability of the nonfullerene acceptor (NFA) cells. A tight relationship between the surface pH condition and the device performance and stability was established. Specifically, device performance and stability were improved by treating the ZnO films with acid solutions but worsened after base treatment. The large number of hydroxyl groups on the surface of the solution-processed ZnO films was proved to be the main reason for the surface pH condition-related performance, which caused oxygen-deficient defects and unfavorable vertical phase separation in the blend films, hindered the photogenerated charge transfer and collection, and consequently resulted in low short-circuit current density and power conversion efficiency (PCE). The surface -OH groups also boosted the photocatalytic activity and led to fast degradation of the nonfullerene acceptor. Removal of the surface -OH groups can alleviate such problems. Different acid solutions, ZrAcac, 2-phenylethylmercaptan (PET), and glutamic acid (GC), were used to treat the ZnO films, and PET treatment was the most effective treatment for performance improvement. An efficiency of 16.46% was achieved for the PM6:Y6 cells and the long-term stability under continuous illumination conditions was significantly improved with a T80 lifetime of over 4000 h (4410 h), showing the excellent long-term stability of this heterojunction solar cell. Our understanding of the surface pH condition-related device performance and stability would guide the development of a feasible method for solving the interface problems in OSCs. We also provide a practical strategy to modify ZnO with acid solutions for high-performance and stable NFA OSCs.

Although perovskite solar cells have achieved great development in the device performance, the device stability is still a crucial issue hinder the further development. The ion migration is a main reason for the poor stability of the perovskite solar cells. Here, we use a cross-linking material, polydimethylsiloxane (PDMS) to dope the perovskite layer to improve the operational stability. We systematically studied the effect of curing agent content in PDMS on the device performance and stability. The characterization of perovskite film indicates that PDMS primarily doped in the perovskite layer and as well as enriched at the surface of perovskite film. Doping of PDMS promoted the crystallization of perovskite phase, leading to increase of short circuit density (JSC) and fill factor (FF) of the solar cells. In addition, PDMS doping improved the operational stability of device due to the suppressing the migration of iodide ions during aging. Moreover, the higher content of curing agents used in PDMS, the better stability of the devices. This work indicates PDMS doping the perovskite layer with a suitable content of curing agent can produce a better operational stability of the perovskite solar cells.

n–i–p Type perovskite solar cells generally require air oxidation of the Spiro-OMeTAD layer to achieve high power conversion efficiency (PCE). However, the detailed oxidation mechanism is still not fully understood. In this paper, oxidation of Spiro-OMeTAD was demonstrated via a non-contact electrochemical route using UV-Vis absorption, laser beam induced current (LBIC) imaging and secondary ion mass spectrometry (SIMS) profiling of the Spiro-OMeTAD films. At the cathode, oxygen is reduced to form OH− with the help of H2O, while the anodic reaction is the oxidation of Spiro-OMeTAD to form Spiro-OMeTAD+. Diffusion of Li+ towards the surface of the Ag electrode completes the electrochemical cycle and increases the conductivity of the hole-transporting layer. SIMS analyses of the completed devices demonstrate that the oxidation of Spiro-OMeTAD also leads to migration of Li+ through the perovskite layer into SnO2, which supposedly leads to an increase of the built-in voltage. We verify these results by incorporation of the experimentally measured Li+ concentration into a numerical drift-diffusion simulation, to replicate solar cell J–V-curves. This work provides a new insight into the oxidation of Spiro-OMeTAD in perovskite solar cells, and demonstrates that Li+ migration is involved in the oxidation of Spiro-OMeTAD.

Flat bands (FBs), presenting a strongly interacting quantum system have drawn increasing interest recently, as partly reflected by its discovery in twisted bilayer graphene in association with superconductivity. However, experimental realization of topological FBs has been hampered by lacking FB materials, and especially remained elusive in the ideal platform of monolayer materials where they arise from destructive quantum interference as predicted in 2D lattice models. Here, we report experimental synthesis of mesoscale ordered self-assembled monolayer of 2D hydrogen-bond (H-bond) organic frameworks (HOFs) of 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) on Au(111) surface and the observation of related topological FB. Its formation, atomic and electronic structures have been characterized with a suite of experimental and theoretical techniques in excellent agreement. High-resolution scanning tunneling microscopy/spectroscopy (STM/STS) shows mesoscale, highly-ordered and uniform THPB-HOF domains, while angle-resolved photoemission spectroscopy (ARPES) highlights a FB over the whole Brillouin zone (BZ). Density-functional-theory (DFT) calculations and analyses reveal that the observed topological FB arises from a hidden electronic breathing-Kagome lattice without atomically breathing bonds. Our findings demonstrate that self-assembly of HOFs provides a viable approach for synthesis of 2D organic topological materials, paving the way to explore many-body quantum states of topological FBs.

Microscopic imaging of molecules is important for determination of molecular structures by providing real space snapshots. The scanning tunneling microscope (STM) is able to offer local perturbatio...

The growth of graphene on Ni magnetic films is of great significance for graphene spintronics, whereas the existence of grain boundaries and twin crystal structures in Ni films is an obstacle for obtaining the large-scale and uniform graphene. In this paper, an epitaxial wafer-scale single-crystalline Ni(111) film with the flat and clean surface was successfully prepared on the commercial α-Al2O3(0001) substrate by a two-step method, which was demonstrated with several characterization methods. According to the abnormal grain growth mechanism, the clean and uniform sapphire surface plays a key role for the single crystallization of Ni films as it induces a weak interface energy difference between two atomic stacking structures (ABC and ACB), thus stimulating the evolution of Ni films from (111) out-of-plane textures to single crystals. Furthermore, an ultra-flat and wrinkle-free graphene monolayer was synthesized on the prepared Ni film, which further verified its high quality and effectiveness.

A charge density wave (CDW) is a collective quantum phenomenon in metals and features a wavelike modulation of the conduction electron density. A microscopic understanding and experimental control of this many-body electronic state in atomically thin materials remain hot topics in materials physics. By means of material engineering, we realized a dimensionality and Zr intercalation induced semiconductor-metal phase transition in 1

Ni-rich layered oxides (LiNixMnyCozO2, x ≥ 0.6, x + y + z = 1) are promising positive electrode materials for high energy density lithium-ion batteries thanks to their high specific capacity. Howev...

How to electrically control magnetic properties of a magnetic material is promising towards spintronic applications, where the investigation of carrier doping effects on antiferromagnetic (AFM) materials remains challenging due to their zero net magnetization. In this work, we found electron doping dependent variation of magnetic orders of a two-dimensional (2D) AFM insulator NiPS3, where doping concentration is tuned by intercalating various organic cations into the van der Waals gaps of NiPS3 without introduction of defects and impurity phases. The doped NiPS3 shows an AFM-ferrimagnetic (FIM) transition at doping level of 0.2-0.5 electrons/cell and a FIM-AFM transition at doping level of >= 0.6 electrons/cell. We propose that the found phenomenon is due to competition between Stoner exchange dominated inter-chain ferromagnetic order and super-exchange dominated inter-chain AFM order at different doping level. Our studies provide a viable way to exploit correlation between electronic structures and magnetic properties of 2D magnetic materials for realization of magnetoelectric effect., 18 pages, 7 figures

Interfacial electron-phonon coupling in ultrathin films has attracted much interest recently. Here, by combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we report quantized electronic states and strong interfacial electron-phonon coupling in ultrathin Yb films on graphite. We observed clear kinks in the energy-momentum dispersion of quantum well states, and the kink positions agree well with the energies of optical phonons of graphite. The extracted coupling strength $\ensuremath{\lambda}$ is largest for the thinnest film with a preferred (``magic'') thickness of four monolayers and exhibits a strong band dependence, which can be qualitatively accounted for by a simple model. The interfacial electron-phonon coupling also gives rise to characteristic steplike structures in the $dI/dV$ spectra, implying dominant coupling with the phonons with zero in-plane momentum. A Lifshitz transition occurs at higher coverage, where quantum well states derived mainly from $5d$ electrons dominate near the Fermi level and possess large effective mass (up to $\ensuremath{\sim}19\phantom{\rule{0.28em}{0ex}}{m}_{e}$). Our results highlight the potentially important role of interfacial electron-phonon interaction for ultrathin films and provide spectroscopic insight to understand this cross-interface fermion-boson interaction.

The scattering process induced by impurities in graphene plays a key role in transport properties. Especially, the disorder impurities can drive the ordered state with a hexagonal superlattice on graphene by electron-mediated interaction at a transition temperature. Using angle-resolved photoemission spectroscopy (ARPES), we reveal that the epitaxial monolayer and bilayer graphene with various impurities display global elastic intervalley scattering and quantum interference below the critical temperature (34 K), which leads to a set of new folded Dirac cones at the Brillouin-zone center by mixing two inequivalent Dirac cones. The Dirac electrons generated from intervalley scattering without chirality can be due to the breaking of the sublattice symmetry. In addition, the temperature-dependent ARPES measurements indicate the thermal damping of quantum interference patterns from Dirac electron scattering on impurities. Our results demonstrate that the electron scattering and interference induced by impurities can completely modulate the Dirac bands of graphene.

As a special order of electronic correlation induced by spatial modulation, the charge density wave (CDW) phenomena in condensed matters attract enormous research interests. Here, using scanning-tunneling microscopy in various temperatures, we observe a new (2×1) CDW phase besides the (√7×√3) CDW phase in epitaxial monolayer 1T-VSe2 film. Combining the variable-temperature angle-resolved photoemission spectroscopic (ARPES) measurements, we discover an anisotropic CDW gap and a two-step transition associated with the different CDW phases, which were observed below 135 K for the (√7×√3) CDW phase and between 135 K to 330 K for the (2×1) CDW phase respectively. The (√7×√3) CDW phase results a full gap, while the (2×1) CDW phase shows highly momentum dependence and results a partial gap structure at the Fermi surface. This two-step transition with anisotropic gap opening and the resulted evolution in ARPES spectra are corroborated by our theoretical calculation based on a phenomenological form for the self-energy containing a two-gap structure. Our findings provide significant information and deep understanding on the CDW phases in monolayer 1T-VSe2 film as a 2D material.

For the inverted organic solar cells (OSCs), the interface contacts between the ZnO electron transporting layer and the organic active layer play an important role in the device performance and stability. Since the solution-processed ZnO surface always contains some base or zinc salt contaminants, we explored how the surface pH conditions influence the performance and stability of the nonfullerene acceptor (NFA) cells. A tight relationship between the surface pH condition and the device performance and stability was established. Specifically, device performance and stability were improved by treating the ZnO films with acid solutions but worsened after base treatment. The large number of hydroxyl groups on the surface of the solution-processed ZnO films was proved to be the main reason for the surface pH condition-related performance, which caused oxygen-deficient defects and unfavorable vertical phase separation in the blend films, hindered the photogenerated charge transfer and collection, and consequently resulted in low short-circuit current density and power conversion efficiency (PCE). The surface -OH groups also boosted the photocatalytic activity and led to fast degradation of the nonfullerene acceptor. Removal of the surface -OH groups can alleviate such problems. Different acid solutions, ZrAcac, 2-phenylethylmercaptan (PET), and glutamic acid (GC), were used to treat the ZnO films, and PET treatment was the most effective treatment for performance improvement. An efficiency of 16.46% was achieved for the PM6:Y6 cells and the long-term stability under continuous illumination conditions was significantly improved with a

Charge density wave (CDW) is a collective quantum phenomenon in metals and features a wave-like modulation of the conduction electron density. A microscopic understanding and experimental control of this many-body electronic state in atomically thin materials remain hot topics in materials physics. By means of material engineering, we realized a dimensionality and Zr intercalation induced semiconductor-metal phase transition in 1T-ZrX$_2$ (X = Se, Te) ultra-thin films, accompanied by a commensurate 2 $\times$ 2 CDW order. Furthermore, we observed a CDW energy gap up to 22 meV around the Fermi level. Fourier-transformed scanning tunneling microscopy and angle-resolved photoemission spectroscopy reveal that 1T-ZrX$_2$ films exhibit the simplest Fermi surface among the known CDW materials in TMDCs, consisting only of Zr 4d-derived elliptical electron conduction band at the corners of the Brillouin zone., 29 pages, 4 figures, 4 Supplementary notes

Selective C(sp3) −H activation is of fundamental importance in processing alkane feedstocks to produce high-value-added chemical products. By virtue of an on-surface synthesis strategy, we report selective cascade dehydrogenation of n-alkane molecules under surface constraints, which yields monodispersed all-trans conjugated polyenes with unprecedented length controllability. We are also able to demonstrate the generality of this concept for alkyl-substituted molecules with programmable lengths and diverse functionalities, and more importantly its promising potential in molecular wiring., Direct transformation of n-alkanes and alkyl-substituted derivatives has been achieved under surface constraints to yield conjugated polyenes with programmed length and functionalities.

Recently, intriguing physical properties have been unraveled in anisotropic layered semiconductors, in which the in-plane electronic band structure anisotropy often originates from the low crystallographic symmetry and thus a thickness-independent character emerges. Here, we apply high-resolution angle-resolved photoemission spectroscopy to directly image the in-plane anisotropic energy bands in monoclinic gallium telluride (GaTe). Our first-principles calculations reveal the in-plane anisotropic energy band structure of GaTe measured experimentally is dominated by a strong bulk-surface interaction rather than geometric factors, surface effect and quantum confinement effect. Furthermore, accompanied by the thickness of GaTe increasing from mono- to few-layers, the strong interlayer coupling of GaTe induces direct-indirect-direct band gap transitions and the in-plane anisotropy of hole effective mass is reversed. Our results shed light on the physical origins of in-plane anisotropy of electronic structure in GaTe, paving the way for the design and device applications of nanoelectronics and optoelectronics based on anisotropic layered semiconductors., Comment: To be published in Communications Physics

The phenomenon of oxygen incorporation induced superconductivity in iron telluride (Fe1+yTe, with antiferromagnetic (AFM) orders) is intriguing and quite different from the case of FeSe. Until now, the microscopic origin of the induced superconductivity and the role of oxygen are far from clear. Here, by combining in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and x-ray photoemission spectroscopy (XPS) on oxygenated FeTe, we found physically adsorbed O2 molecules crystallized into c(2/3x2) structure as an oxygen overlayer at low temperature, which was vital for superconductivity. The O2 overlayer were not epitaxial on the FeTe lattice, which implied weak O2-FeTe interaction but strong molecular interactions. Energy shift observed in the STS and XPS measurements indicated hole doping effect from the O2 overlayer to the FeTe layer, leading to a superconducting gap of 4.5 meV opened across the Fermi level. Our direct microscopic probe clarified the role of oxygen on FeTe and emphasized the importance of charge transfer effect to induce superconductivity in iron-chalcogenide thin films., Comment: 15 pages,4 figures

Two-dimensional honeycomb crystals have inspired intense research interest for their novel properties and great potential in electronics and optoelectronics. Here, through molecular beam epitaxy on SrTiO3(001), we report successful epitaxial growth of metal-rich chalcogenide Fe2Te, a honeycomb-structured film that has no direct bulk analogue, under Te-limited growth conditions. The structural morphology and electronic properties of Fe2Te are explored with scanning tunneling microscopy and angle resolved photoemission spectroscopy, which reveal electronic bands cross the Fermi level and nearly flat bands. Moreover, we find a weak interfacial interaction between Fe2Te and the underlying substrates, paving a newly developed alternative avenue for honeycomb-based electronic devices., Comment: 12 pages,5 figures

The nature of the parent compound is intimately connected to the doping-induced emergence of superconductivity and the pairing mechanism in high-temperature superconductors. In high-temperature cuprate superconductors, the parent phase is believed to be a Mott insulator and superconductivity is realized by doping the Mott insulator. In iron-based superconductors, on the other hand, the parent compounds discovered so far are all bad metals which are either antiferromagnetic or nematic. Here we report direct spectroscopic evidence that the parent phase of the single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_{3}$ (FeSe/STO) films is insulating. It represents a case in the iron-based superconductors in which the parent compound is an insulator. We also find that the single-layer FeSe/STO films exhibit a distinct doping evolution to superconductivity that is similar to doping a Mott insulator in cuprate superconductors. These observations provide key insights in understanding the record high- ${T}_{C}$ superconductivity in single-layer FeSe/STO films.

Recently, the observations of spontaneous out-of-plane electric polarization and ferroelectric switching in WTe2 related devices with semimetal property a have generated considerable attentions. The polar structure of WTe2 was proposed to be vital, however there is still no consensus about the physical origin, which deserves detailed surface analysis. Here, by using angle-dependent X-ray photoelectron spectroscopy (ADXPS), we observed an obvious downward band bending in WTe2 after slightly surface oxidation, which can't be observed on the freshly cleaved WTe2. And it will disappear after fully oxidation, demonstrating strong correlation between band bending and surface oxidation. Our study suggests surface oxidation and resulted band bending in WTe2 should be taken into account in the WTe2-related devices, such as ferroelectric.

The stability of the self-assembled RS–Au–SR (R = CH2CH3)/Au(111) interface at room temperature has been investigated using scanning tunneling microscopy (STM) in conjunction with density functiona...

The accuracy and error propagation for determining the low specific contact resistance of Ohmic contacts on III-V wide band-gap semiconductors based on the circular transmission line model have been analyzed and the validity of this method is discussed in detail. The accuracy is more susceptible to the factors including data fitting method, electrical measurement technique and contact area correction. By using the equations of the original circular transmission line model to extract the fitting parameters, the calculation accuracy is much improved and the inapplicability of the linear least-square fitting is prevented. To further improve the accuracy, a four-probe current-voltage measurement technique was adopted to reduce the parasitic series resistances and the uncertainty bound, especially for the Ohmic contact with low sheet resistance of the semiconductor. Moreover, we have studied the size effect of contact pads of patterns and demonstrated that contact area correction is necessary for the semiconductor with high sheet resistance. A comprehensive error analysis is also performed to fully understand all the impact factors on this advanced method of specific contact resistance measurement, which is benefit for device performance evaluation and failure analysis.

Surface band bending and composition of Ga-polar n-GaN with different surface treatments were characterized by using angular dependent X-ray photoelectron spectroscopy. Upward surface band bending of varying degree was observed distinctly upon to the treatment methods. Besides the nitrogen vacancies, we found that surface states of oxygen-containing absorbates (O-H component) also contribute to the surface band bending, which lead the Fermi level pined at a level further closer to the conduction band edge on n-GaN surface. The n-GaN surface with lower surface band bending exhibits better linear electrical properties for Ti/GaN Ohmic contacts. Moreover, the density of positively charged surface states could be derived from the values of surface band bending.

Topological semimetals, in which conduction and valence bands cross each other at either discrete points or along a closed loop with symmetry protected in the momentum space, exhibited great potential in applications of optical devices as well as heterogeneous catalysts or antiferromagnetic spintronics, especially when the crossing points/lines matches Fermi level (E F). It is intriguing to find the ‘ideal’ topological semimetal material, in which has a band structure with Dirac band-crossing located at E F without intersected by other extraneous bands. Here, by using angle resolved photoemission spectroscopy, we investigate the band structure of the so-called ‘square-net’ topological material ZrGeS. The Brillouin zone (BZ) mapping shows the Fermi surface of ZrGeS is composed by a diamond-shaped nodal line loop at the center of BZ and small electron-like Fermi pockets around X point. The Dirac nodal line band-crossing located right at E F, and shows clearly the linear Dirac band dispersions within a large energy range >1.5 eV below E F, without intersected with other bands. The obtained Fermi velocities and effective masses along Γ–X, Γ–M and M–X high symmetry directions were 4.5–5.9 eV Å and 0–0.50 m e, revealing an anisotropic electronic property. Our results suggest that ZrGeS, as a promising topological nodal line semimetal, could provide a promising platform to investigate the Dirac-fermions related physics and the applications of topological devising.

We investigate electrical manipulation of magnetism in Ta/Pd/Co/HfO2 films probed by anomalous Hall Effect (AHE). The coercivity of the one monolayer cobalt and the transition temperature for the disappearance of usual AHE curves could be modified by a low voltage below ±2 V via the usage of ionic liquid. The unexpected emergence of inverted Hall loops above the transition temperature but far below the Curie temperature performed on the magnetometer could be explained by the competition between skew scattering and intrinsic/side-jump contributions of AHE. Our finding provides a different avenue for exploring the origin of AHE by electrical manipulation.

We present a systematic study of surface band bending in Ga-polar n-GaN with different Si doping concentrations by angular dependent X-ray photoelectron spectroscopy (ADXPS). The binding energies of Ga 3d and N 1 s core levels in n-GaN films increase with increasing the emission angle, i. e., the probing depth, suggesting an upward surface band bending. By fitting the Ga 3d core level spectra at different emission angles and considering the integrated effect of electrostatic potential, the core level energy at the topmost surface layer is well corrected, therefore, the surface band bending is precisely evaluated. For moderately doped GaN, the electrostatic potential can be reflected by the simply linear potential approximation. However, for highly doped GaN samples, in which the photoelectron depth is comparable to the width of the space charge region, quadratic depletion approximation was used for the electrostatic potential to better understand the surface band bending effect. Our work improves the knowledge of surface band bending determination by ADXPS and also paves the way for studying the band bending effect in the interface of GaN based heterostructures.

Efficient electrical switching of antiferromagnets (AFMs) is key to their use in high-density, ultrafast, nonvolatile spintronic memory. Mn${}_{2}$Au, an AFM with opposite spin sublattices, is a unique metallic material, in that fieldlike spin torque can switch its AFM moments. However, switching induced by antidamping torque remains to be verified in metallic AFMs. Here the authors demonstrate current-induced switching of AFM moment in both a (103)-oriented Mn${}_{2}$Au single layer and a Mn${}_{2}$Au/Pt heterojunction by fieldlike torque and antidamping torque respectively. The simultaneous realization of both torque types in metallic Mn2Au makes it a promising candidate for AFM spintronics.

Current-driven domain-wall motion has attracted intense attention, due to its rich physics and promising application in ``racetrack'' nonvolatile memory. However, the interplay between antiferromagnetic/ferromagnetic (AFM/FM) phase boundaries and current has remained elusive. To investigate, the authors use pulses of current to induce the formation of anisotropic FM domains in an AFM matrix. The phase boundaries can be reversibly oriented by the applied (writing) current, giving rise to the orthogonal alignment of phase boundaries and current, which could pave the way for multistage digital memory based on AFM/FM boundaries.