Showing papers on "Scanning tunneling spectroscopy published in 2017"
TL;DR: The combined theoretical and experimental results demonstrate a concept for a quantum spin Hall wide-gap scenario, where the chemical potential resides in the global system gap, ensuring robust edge conductance.
Abstract: Quantum spin Hall materials hold the promise of revolutionary devices with dissipationless spin currents but have required cryogenic temperatures owing to small energy gaps. Here we show theoretically that a room-temperature regime with a large energy gap may be achievable within a paradigm that exploits the atomic spin-orbit coupling. The concept is based on a substrate-supported monolayer of a high–atomic number element and is experimentally realized as a bismuth honeycomb lattice on top of the insulating silicon carbide substrate SiC(0001). Using scanning tunneling spectroscopy, we detect a gap of ~0.8 electron volt and conductive edge states consistent with theory. Our combined theoretical and experimental results demonstrate a concept for a quantum spin Hall wide-gap scenario, where the chemical potential resides in the global system gap, ensuring robust edge conductance.
766 citations
TL;DR: The synthesis of an armchair graphene nanoribbon with a width of nine carbon atoms on Au(111) through surface-assisted aryl-aryl coupling and subsequent cyclodehydrogenation of a properly chosen molecular precursor is reported.
Abstract: The bottom-up approach to synthesize graphene nanoribbons strives not only to introduce a band gap into the electronic structure of graphene but also to accurately tune its value by designing both the width and edge structure of the ribbons with atomic precision. We report the synthesis of an armchair graphene nanoribbon with a width of nine carbon atoms on Au(111) through surface-assisted aryl–aryl coupling and subsequent cyclodehydrogenation of a properly chosen molecular precursor. By combining high-resolution atomic force microscopy, scanning tunneling microscopy, and Raman spectroscopy, we demonstrate that the atomic structure of the fabricated ribbons is exactly as designed. Angle-resolved photoemission spectroscopy and Fourier-transformed scanning tunneling spectroscopy reveal an electronic band gap of 1.4 eV and effective masses of ≈0.1 me for both electrons and holes, constituting a substantial improvement over previous efforts toward the development of transistor applications. We use ab initio c...
262 citations
TL;DR: Despite the ferromagnetic coupling within the chains and the strong spin-orbit coupling in the superconductor, the study of iron (Fe) chains does not find clear evidence of Majorana modes.
Abstract: Linear chains of magnetic atoms proximity coupled to an s-wave superconductor are predicted to host Majorana zero modes at the chain ends in the presence of strong spin–orbit coupling. Specifically, iron (Fe) chains on Pb(110) have been explored as a possible system to exhibit topological superconductivity and host Majorana zero modes [Nadj-Perge, S. et al., Science 2014, 346, 602−607]. Here, we study chains of the transition metal cobalt (Co) on Pb(110) and check for topological signatures. Using spin-polarized scanning tunneling spectroscopy, we resolve ferromagnetic order in the d bands of the chains. Interestingly, also the subgap Yu–Shiba–Rusinov (YSR) bands carry a spin polarization as was predicted decades ago. Superconducting tips allow us to resolve further details of the YSR bands and in particular resonances at zero energy. We map the spatial distribution of the zero-energy signal and find it delocalized along the chain. Hence, despite the ferromagnetic coupling within the chains and the strong...
153 citations
TL;DR: The energy level alignment evolution of valence and conduction bands of armchair-oriented graphene nanoribbons (aGNR) as their band gap shrinks with increasing width is reported, and valence bands are found to show Fermi level pinning as the band gap decreases below a threshold value around 1.7 eV.
Abstract: We report the energy level alignment evolution of valence and conduction bands of armchair-oriented graphene nanoribbons (aGNR) as their band gap shrinks with increasing width. We use 4,4″-dibromo-para-terphenyl as the molecular precursor on Au(111) to form extended poly-para-phenylene nanowires, which can subsequently be fused sideways to form atomically precise aGNRs of varying widths. We measure the frontier bands by means of scanning tunneling spectroscopy, corroborating that the nanoribbon’s band gap is inversely proportional to their width. Interestingly, valence bands are found to show Fermi level pinning as the band gap decreases below a threshold value around 1.7 eV. Such behavior is of critical importance to understand the properties of potential contacts in GNR-based devices. Our measurements further reveal a particularly interesting system for studying Fermi level pinning by modifying an adsorbate’s band gap while maintaining an almost unchanged interface chemistry defined by substrate and ads...
152 citations
TL;DR: This work reports on a method to generate and characterize PMFs in a graphene membrane supported on nanopillars and quantifies the strain-induced PMF through the PLLs spectra observed in scanning tunneling spectroscopy.
Abstract: Graphene’s remarkable properties are inherent to its two-dimensional honeycomb lattice structure. Its low dimensionality, which makes it possible to rearrange the atoms by applying an external force, offers the intriguing prospect of mechanically controlling the electronic properties. In the presence of strain, graphene develops a pseudomagnetic field (PMF) that reconstructs the band structure into pseudo Landau levels (PLLs). However, a feasible route to realizing, characterizing and controlling PMFs is still lacking. Here we report on a method to generate and characterize PMFs in a graphene membrane supported on nanopillars. A direct measure of the local strain is achieved by using the magnifying effect of the moire pattern formed against a hexagonal boron nitride substrate under scanning tunneling microscopy. We quantify the strain-induced PMF through the PLLs spectra observed in scanning tunneling spectroscopy. This work provides a pathway to strain induced engineering and electro-mechanical graphene-...
151 citations
TL;DR: An experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) by molecular-beam epitaxy using density functional theory calculations reveals rectangular surface cells with the cell size consistent with the theoretically predicted β-tellurene.
Abstract: Monolayer tellurium (Te) or tellurene has been suggested by a recent theory as a new two-dimensional (2D) system with great electronic and optoelectronic promises. Here we present an experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) by molecular-beam epitaxy. Scanning tunneling microscopy of ultrathin layers of Te reveals rectangular surface cells with the cell size consistent with the theoretically predicted β-tellurene, whereas for thicker films, the cell size is more consistent with that of the [100] surface of the bulk Te crystal. Scanning tunneling spectroscopy measurements show that the films are semiconductors with the energy band gaps decreasing with increasing film thickness, and the gap narrowing occurs predominantly at the valence-band maximum (VBM). The latter is understood by strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory calculations.
140 citations
TL;DR: In this article, the role of dimensionality on the electronic properties of a van der Waals GaSe/graphene heterostructure was investigated using scanning tunneling spectroscopy.
Abstract: Two-dimensional few-layered materials exhibit unique and exotic electronic properties when compared to their bulk counterparts. Here, the authors report the role of dimensionality on the electronic properties of a van der Waals GaSe/graphene heterostructure. The strong thickness dependence of the GaSe band gap is demonstrated experimentally using scanning tunneling spectroscopy. The GaSe band gap decreases as the number of layers increases due to quantum confinement effects. The particular GaSe `Mexican hat' band structure is further explored by angle-resolved photoemission spectroscopy and by theoretical calculations. Interestingly, an electron transfer from graphene to GaSe has also been detected. It results in $n$-type doped thin GaSe films, opposite to the $p$-type character GaSe bulk.
110 citations
TL;DR: In this paper, the growth of Pt on Ge(110) using scanning tunneling microscopy and spectroscopy was investigated using annealing at 1100 K and the outermost layer of these nanocrystals exhibits a honeycomb structure.
Abstract: We have investigated the growth of Pt on Ge(110) using scanning tunneling microscopy and spectroscopy The deposition of several monolayers of Pt on Ge(110) followed by annealing at 1100 K results in the formation of three-dimensional metallic Pt-Ge nanocrystals The outermost layer of these crystals exhibits a honeycomb structure The honeycomb structure is composed of two hexagonal sub-lattices that are displaced vertically by 02 A with respect to each other The nearest-neighbor distance of the atoms in the honeycomb lattice is 25${\pm}$01 A, ie very close to the predicted nearest-neighbor distance in germanene (24 A) Scanning tunneling spectroscopy reveals that the atomic layer underneath the honeycomb layer is more metallic than the honeycomb layer itself These observations are in line with a model recently proposed for metal di-(silicides/)germanides: a hexagonal crystal with metal layers separated by semiconductor layers with a honeycomb lattice Based on our observations we propose that the outermost layer of the Ge2Pt nanocrystal is a germanene layer
110 citations
TL;DR: In this paper, the authors focus on the new physical properties of some of the thinnest superconducting films that can be fabricated and studied in situ nowadays with state-of-the-art methods.
Abstract: The purpose of this review is to focus from an experimental point-of-view on the new physical properties of some of the thinnest superconducting films that can be fabricated and studied in situ nowadays with state-of-the-art methods. An important characteristic of the films we address is that the underlying electronic system forms a two-dimensional electron gas (2DEG). Up to now there are only few of these systems. Such true 2D superconductors can be divided into two classes: surface-confined or interface-confined films. Because the second types of films are burried below the surface, they are not accessible to purely surface-sensitive techniques like angular-resolved photoemission spectroscopy (ARPES) or scanning tunneling spectroscopy (STS). As a consequence the bandstructure characteristics of the 2DEG cannot be probed nor the local superconducting properties. On the other hand, in situ prepared surface-confined films are nowadays accessible not only to ARPES and STS but also to electrical transport measurements. As a consequence surface-confined systems represent at present the best archetypes on which can be summarized the new properties emerging in ultimately thin superconducting films hosting a 2DEG, probed by both macroscopic and microscopic measurement techniques. The model system we will widely refer to consists of a single atomic plane of a conventional superconductor, like for example lead (Pb), grown on top of a semiconducting substrate, like Si(111). In the introductory part 1 we first introduce the topic and give historical insights into this field. Then in the section 2, we introduce useful concepts worked out in studies of so-called 'granular' and 'homogeneous' superconducting thin films that will be necessary to understand the role of non-magnetic disorder on 2DEG superconductors. In this section, we also briefly review the superconducting properties of crystalline Pb/Si(111) ultrathin films grown under ultrahigh vacuum (UHV) conditions in order to illustrate their specific properties related to quantum-size effects. In the next section 3 we review the growth methods and structural properties of the presented 2DEG surface-confined superconductors. In section 4, we review the electronic structure and Fermi surface properties as measured by macroscopic ARPES and confront them to ab initio DFT calculations based on the characterized atomic structures of the monolayers. The following section 5 reviews the macroscopic properties inferred from in situ electrical transport measurements methods, including attempts to study the Berezinsky–Kosterlitz–Thouless 2D regime. In the last section 6, we summarize the emerging local spectroscopic properties measured by STS. These latter demonstrate variations of the local superconducting properties at a scale much shorter than the superconducting coherence length due to a combined effect of non-magnetic disorder and two-dimensionality. Further peculiar local spectroscopic effects are presented giving evidence for the presence of a mixed singlet-triplet superconducting order parameter induced by the presence of a strong Rashba spin–orbit coupling term at the surface. These local signatures will be discussed along with ARPES and transport measurements in parallel high magnetic field on closely related systems. Finally, we present in anisotropic Pb and In monolayers the peculiar role played by atomic steps on vortex properties, leading to the observation by STS of mixed Abrikosov–Josephson vortices in agreement with in situ macroscopic transport measurements. From the overview of all recent experimental and theoretical results it appears that these surface 2D superconductors, such as one monolayer of Pb on Si(111), are ideal templates to engineer and realize topological superconductivity.
107 citations
TL;DR: To prevent extensive Te loss at higher temperatures, where Mo6Te6 nanowire formation and substantial desorption-induced etching effects will take place simultaneously, surface and edge passivation with a monolayer graphene coverage on MoTe2 is tested and is greatly enhanced up to 500 °C without apparent structural defects.
Abstract: Understanding the structural stability of transition-metal dichalcogenides is necessary to avoid surface/interface degradation. In this work, the structural stability of 2H-MoTe2 with thermal treatments up to 500 °C is studied using scanning tunneling microscopy and scanning transmission electron microscopy. On the exfoliated sample surface at room temperature, atomic subsurface donors originating from excess Te atoms are observed and presented as nanometer-sized, electronically-induced protrusions superimposed with the hexagonal lattice structure of MoTe2. Under a thermal treatment as low as 200 °C, the surface decomposition-induced cluster defects and Te vacancies are readily detected and increase in extent with the increasing temperature. Driven by Te vacancies and thermal energy, intense 60° inversion domain boundaries form resulting in a “wagon wheel” morphology after 400 °C annealing for 15 min. Scanning tunneling spectroscopy identified the electronic states at the domain boundaries and the domain ...
105 citations
TL;DR: In this article, the authors introduced a centrifugally-cast thin-film formation technique to 2D transition metal dichalcogenides (TMDs) formed through a liquid-based exfoliation method.
TL;DR: It is found that many half-Heusler compounds exhibit multiple triple points along four independent C_{3} axes, through which the doubly degenerate conduction bands and the nondegenerate valence band cross each other linearly nearby the Fermi energy.
Abstract: We predict the existence of triple point fermions in the band structure of several half-Heusler topological insulators by ab initio calculations and the Kane model We find that many half-Heusler compounds exhibit multiple triple points along four independent C_{3} axes, through which the doubly degenerate conduction bands and the nondegenerate valence band cross each other linearly nearby the Fermi energy When projected from the bulk to the (111) surface, most of these triple points are located far away from the surface Γ[over ¯] point, as distinct from previously reported triple point fermion candidates These isolated triple points give rise to Fermi arcs on the surface, that can be readily detected by photoemission spectroscopy or scanning tunneling spectroscopy
TL;DR: The thermally induced dehydrogenation uncovered the isomerization of intermediate dihydrononacene species, which allowed for their in-depth structural and electronic characterization.
Abstract: The on-surface synthesis of nonacene has been accomplished by dehydrogenation of an air-stable partially saturated precursor, which could be aromatized by using a combined scanning tunneling and atomic force microscope as well as by on-surface annealing. This transformation allowed the in-detail analysis of the electronic properties of nonacene molecules physisorbed on Au(111) by scanning tunneling spectroscopy measurements. The spatial mapping of molecular orbitals was corroborated by density functional theory calculations. Furthermore, the thermally induced dehydrogenation uncovered the isomerization of intermediate dihydrononacene species, which allowed for their in-depth structural and electronic characterization.
TL;DR: Density of states measurements indicate that the domain walls host an extra population of fermions near the Fermi level which may play a role in the emergence of superconductivity in this system.
Abstract: We report a low-temperature scanning tunneling microscopy study of the charge density wave (CDW) order in 1T-TiSe_{2} and Cu_{0.08}TiSe_{2}. In pristine 1T-TiSe_{2} we observe a long-range coherent commensurate CDW (CCDW) order. In contrast, Cu_{0.08}TiSe_{2} displays an incommensurate CDW (ICDW) phase with localized CCDW domains separated by domain walls. Density of states measurements indicate that the domain walls host an extra population of fermions near the Fermi level which may play a role in the emergence of superconductivity in this system. Fourier transform scanning tunneling spectroscopy studies suggest that the dominant mechanism for CDW formation in the ICDW phase may be electron-phonon coupling.
TL;DR: A novel approach to access the frontier states of armchair graphene nanoribbons (AGNRs) through surface alloying suppresses the strong contribution of the Au(111) surface state and allows for an unambiguous determination of the frontier electronic states of both wide and narrow band gap AGNRs.
Abstract: The electronic properties of graphene nanoribbons grown on metal substrates are significantly masked by the ones of the supporting metal surface. Here, we introduce a novel approach to access the frontier states of armchair graphene nanoribbons (AGNRs). The in situ intercalation of Si at the AGNR/Au(111) interface through surface alloying suppresses the strong contribution of the Au(111) surface state and allows for an unambiguous determination of the frontier electronic states of both wide and narrow band gap AGNRs. First-principles calculations provide insight into substrate induced screening effects, which result in a width-dependent band gap reduction for substrate-supported AGNRs. The strategy reported here provides a unique opportunity to elucidate the electronic properties of various kinds of graphene nanomaterials supported on metal substrates.
TL;DR: In this article, the formation of Cu 2 FeSnS 4 (CFTS) thin-films through successive ionic layer adsorption and reaction (SILAR) method is presented.
TL;DR: Using spin-resolved scanning tunneling spectroscopy with simulations, it is demonstrated that the pair of peaks associated with the YSR states of an individual Fe atom coupled to an oxygen-reconstructed Ta surface gets spin polarized in an external magnetic field.
Abstract: A magnetic atom in a superconducting host induces so-called Yu-Shiba-Rusinov (YSR) bound states inside the superconducting energy gap. By combining spin-resolved scanning tunneling spectroscopy wit ...
TL;DR: Two reproducible new phases of 2D boron sheets have been found on Ag(1 1’�1 and one of them shares the identical atomic structure of the previously reported S1 phase but has a different rotational relationship with the substrate, and thus exhibits very different features in scanning tunneling microscopy (STM) images.
Abstract: Two reproducible new phases of 2D boron sheets have been found on Ag(1 1 1). One of them shares the identical atomic structure of the previously reported S1 phase (β 12 sheet) but has a different rotational relationship with the substrate, and thus exhibits very different features in scanning tunneling microscopy (STM) images. The other new phase has a hexagonal symmetry and is proposed to be the long-expected α-sheet. Both of these two boron sheets are confirmed to be metallic by scanning tunneling spectroscopy.
TL;DR: In this paper, the experimental progress in the unusual Landau quantized behaviors of Dirac fermions in monolayer and multilayer graphene by using scanning tunneling microscopy (STM) and scan tunneling spectroscopy(STS) was reviewed.
Abstract: When electrons are confined in a two-dimensional (2D) system, typical quantum–mechanical phenomena such as Landau quantization can be detected. Graphene systems, including the single atomic layer and few-layer stacked crystals, are ideal 2D materials for studying a variety of quantum–mechanical problems. In this article, we review the experimental progress in the unusual Landau quantized behaviors of Dirac fermions in monolayer and multilayer graphene by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Through STS measurement of the strong magnetic fields, distinct Landau-level spectra and rich level-splitting phenomena are observed in different graphene layers. These unique properties provide an effective method for identifying the number of layers, as well as the stacking orders, and investigating the fundamentally physical phenomena of graphene. Moreover, in the presence of a strain and charged defects, the Landau quantization of graphene can be significantly modified, leading to unusual spectroscopic and electronic properties.
TL;DR: It is found that the monolayer MoS2 prepared by chemical vapor deposition is nearly defect-free after annealing under ultrahigh vacuum conditions at ≈400 K, as evidenced by scanning tunneling microscopy observations.
Abstract: The existence of defects in 2D semiconductors has been predicted to generate unique physical properties and markedly influence their electronic and optoelectronic properties. In this work, it is found that the monolayer MoS2 prepared by chemical vapor deposition is nearly defect-free after annealing under ultrahigh vacuum conditions at ≈400 K, as evidenced by scanning tunneling microscopy observations. However, after thermal annealing process at ≈900 K, the existence of dominant single sulfur vacancies and relatively rare vacancy chains (2S, 3S, and 4S) is convinced in monolayer MoS2 as-grown on Au foils. Of particular significance is the revelation that the versatile vacancies can modulate the band structure of the monolayer MoS2 , leading to a decrease of the bandgap and an obvious n-doping effect. These results are confirmed by scanning tunneling spectroscopy data as well as first-principles theoretical simulations of the related morphologies and the electronic properties of the various defect types. Briefly, this work should pave a novel route for defect engineering and hence the electronic property modulation of three-atom-thin 2D layered semiconductors.
TL;DR: In this paper, on-surface synthesis of 7-armchair graphene nanoribbons (7-AGNRs) substituted with nitrile (CN) functional groups is reported.
Abstract: We report the on-surface synthesis of 7-armchair graphene nanoribbons (7-AGNRs) substituted with nitrile (CN) functional groups The CN groups are attached to the GNR backbone by modifying the 7-AGNR precursor Whereas many of these groups survive the on-surface synthesis, the reaction process causes the cleavage of some CN from the ribbon backbone and the on-surface cycloisomerization of few nitriles onto pyridine rings Scanning tunneling spectroscopy and density functional theory reveal that CN groups behave as very efficient n-dopants, significantly downshifting the bands of the ribbon and introducing deep impurity levels associated with the nitrogen electron lone pairs
TL;DR: High quality single crystal ZrSiS as a theoretically predicted Dirac semimetal has been grown successfully using a vapor phase transport method and the transport and thermodynamic properties suggest a Fermi liquid behavior with two Fermani pockets at low temperatures.
Abstract: High quality single crystal ZrSiS as a theoretically predicted Dirac semimetal has been grown successfully using a vapor phase transport method. The single crystals of tetragonal structure are easy to cleave into perfect square-shaped pieces due to the van der Waals bonding between the sulfur atoms of the quintuple layers. Physical property measurement results including resistivity, Hall coefficient (RH), and specific heat are reported. The transport and thermodynamic properties suggest a Fermi liquid behavior with two Fermi pockets at low temperatures. At T = 3 K and magnetic field of Hǁc up to 9 Tesla, large magneto-resistance up to 8500% and 7200% for Iǁ(100) and Iǁ(110) were found. Shubnikov de Haas (SdH) oscillations were identified from the resistivity data, revealing the existence of two Fermi pockets at the Fermi level via the fast Fourier transform (FFT) analysis. The Hall coefficient (RH) showed hole-dominated carriers with a high mobility of 3.05 × 104 cm2 V-1 s-1 at 3 K. ZrSiS has been confirmed to be a Dirac semimetal by the Dirac cone mapping near the X-point via angle resolved photoemission spectroscopy (ARPES) with a Dirac nodal line near the Fermi level identified using scanning tunneling spectroscopy (STS).
TL;DR: More complex bipartite and spin-chains were discussed in this article, which can lead to the formation of new quantum states which emerge by interatomic entanglement, and correlated many-particle states can emerge due to the interaction with itinerant electrons of the electrodes.
24 Jul 2017
TL;DR: In this paper, the electronic properties of two-dimensional transition metal dichalcogenides (TMDs) were investigated by using scanning tunneling microscopy and spectroscopy (STM/STS) in ultra-high vacuum.
Abstract: We have investigated the electronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs), namely trilayer WSe2 and monolayer MoSe2, deposited on epitaxial graphene on silicon carbide, by using scanning tunneling microscopy and spectroscopy (STM/STS) in ultra-high vacuum. Depending on the number of graphene layers below the TMD flakes, we identified variations in the electronic dI/dV(V) spectra measured by the STM tip: the most salient feature is a rigid shift of the TMD spectra (i.e. of the different band onset positions) towards occupied states by about 120 mV when passing from bilayer to monolayer underlying graphene. Since both graphene phases are metallic and present a work function difference in the same energy range, our measurements point towards the absence of Fermi-level pinning for such van der Waals 2D TMD/Metal heterojunctions, following the prediction of the Schottky-Mott model.
TL;DR: A new approach for clean deposition of solution-synthesized atomically precise chevron GNRs onto H:Si(100) under ultrahigh vacuum is developed and the results of STM measurements were shown to be in good agreement with first-principles computational modeling.
Abstract: There has been tremendous progress in designing and synthesizing graphene nanoribbons (GNRs). The ability to control the width, edge structure, and dopant level with atomic precision has created a large class of accessible electronic landscapes for use in logic applications. One of the major limitations preventing the realization of GNR devices is the difficulty of transferring GNRs onto nonmetallic substrates. In this work, we developed a new approach for clean deposition of solution-synthesized atomically precise chevron GNRs onto H:Si(100) under ultrahigh vacuum. A clean transfer allowed ultrahigh-vacuum scanning tunneling microscopy (STM) to provide high-resolution imaging and spectroscopy and reveal details of the electronic structure of chevron nanoribbons that have not been previously reported. We also demonstrate STM nanomanipulation of GNRs, characterization of multilayer GNR cross-junctions, and STM nanolithography for local depassivation of H:Si(100), which allowed us to probe GNR–Si interactio...
TL;DR: By systematically exploring the interfacial electronic structures of individual perovskite grains, enhanced charge separation and reduced back recombination were observed when an optimal design of interfacial PbI2 passivation layers consisting of a thickness less than 20 nm at perovkite crystal grains was applied.
Abstract: The presence of the PbI2 passivation layers at perovskite crystal grains has been found to considerably affect the charge carrier transport behaviors and device performance of perovskite solar cells. This work demonstrates the application of a novel light-modulated scanning tunneling microscopy (LM-STM) technique to reveal the interfacial electronic structures at the heterointerfaces between CH3NH3PbI3 perovskite crystals and PbI2 passivation layers of individual perovskite grains under light illumination. Most importantly, this technique enabled the first observation of spatially resolved mapping images of photoinduced interfacial band bending of valence bands and conduction bands and the photogenerated electron and hole carriers at the heterointerfaces of perovskite crystal grains. By systematically exploring the interfacial electronic structures of individual perovskite grains, enhanced charge separation and reduced back recombination were observed when an optimal design of interfacial PbI2 passivation...
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TL;DR: Scanning tunneling spectroscopy and density functional theory reveal that CN groups behave as very efficient n-dopants, significantly downshifting the bands of the ribbon and introducing deep impurity levels associated with the nitrogen electron lone pairs.
Abstract: We report on the on-surface synthesis of 7 armchair graphene nanoribbons (7-AGNRs) substituted with nitrile (CN) functional groups. The CN groups are attached to the GNR backbone by modifying the 7-AGNR precursor. While many of these groups survive the on-surface synthesis, the reaction process causes the cleavage of some CN from the ribbon backbone and the on-surface cycloisomerization of few nitriles onto pyridine rings. Scanning Tunneling Spectroscopy and Density Functional Theory reveal that CN groups behave as very efficient n-dopants, significantly downshifting the bands of the ribbon, and introducing deep impurity levels associated to the nitrogen electron lone pairs.
TL;DR: An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states and the experimental data indicates that the interlayer current stems primarily from energy and in-plane momentum conserving 2D-2D tunneling, with minimal contributions from inelastic or non-momentum-conserving tunneling.
Abstract: We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe2. We observe large interlayer current densities of 2 and 2.5 μA/μm2 and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates that the interlayer current stems primarily from energy and in-plane momentum conserving 2D–2D tunneling, with minimal contributions from inelastic or non-momentum-conserving tunneling. We demonstrate narrow tunneling resonances with intrinsic half-widths of 4 and 6 meV at 1.5 and 300 K, respectively.
TL;DR: In this article, a review summarizes recent progress in understanding the roles of defects and boundaries in electronic, magnetic, thermoelectric, and transport properties of 2D layered materials.
TL;DR: High-quality, single-layer MoSe2 islands on the Au(111) surfaces in ultrahigh vacuum by molecular beam epitaxy have hexagonal or triangular shapes with two kinds of well-defined edges, showing notable differences in positive sample bias for the two types of edges.
Abstract: Recently, single-layer transition-metal dichalcogenides have drawn significant attention due to their remarkable physical properties in the monolayer as well as at the edges. Here, we constructed high-quality, single-layer MoSe2 islands on the Au(111) surfaces in ultrahigh vacuum by molecular beam epitaxy. All of the islands have hexagonal or triangular shapes with two kinds of well-defined edges. Scanning tunneling spectroscopy (STS) curves show notable differences in positive sample bias for the two types of edges. Density functional theory calculations for several edge configurations of MoSe2 confirm that the STS differences are attributed to the coupling between the pz orbital of Se atoms and the dxz orbital of Mo atoms, and the two types of observed edge terminations are the bare Se edge and selenium-saturated Mo edge.