Showing papers by "An-Ping Li published in 2020"
TL;DR: Large-area, environmentally stable, single-crystal 2D gallium, indium and tin that are stabilized at the interface of epitaxial graphene and silicon carbide that demonstrate compelling superconducting properties are demonstrated.
Abstract: Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, single-crystal 2D gallium, indium and tin that are stabilized at the interface of epitaxial graphene and silicon carbide. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to the graphene overlayer; that is, they are 'half van der Waals' metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals offer compelling opportunities for superconducting devices, topological phenomena and advanced optoelectronic properties. For example, the reported 2D Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly free-electron Fermi surface that closely approaches the Dirac points of the graphene overlayer.
102 citations
TL;DR: An on-surface synthesis approach to forming atomically precise GNRs directly on semiconducting metal oxide surfaces and the formation of planar armchair GNRs terminated by well-defined zigzag ends is confirmed by scanning tunneling microscopy and spectroscopy, which reveal weak interaction between GNRs and the rutile titanium dioxide substrate.
Abstract: Atomically precise graphene nanoribbons (GNRs) attract great interest because of their highly tunable electronic, optical, and transport properties However, on-surface synthesis of GNRs is typically based on metal surface-assisted chemical reactions, where metallic substrates strongly screen their designer electronic properties and limit further applications Here, we present an on-surface synthesis approach to forming atomically precise GNRs directly on semiconducting metal oxide surfaces The thermally triggered multistep transformations preprogrammed in our precursors' design rely on highly selective and sequential activations of carbon-bromine (C-Br) and carbon-fluorine (C-F) bonds and cyclodehydrogenation The formation of planar armchair GNRs terminated by well-defined zigzag ends is confirmed by scanning tunneling microscopy and spectroscopy, which also reveal weak interaction between GNRs and the rutile titanium dioxide substrate
90 citations
TL;DR: In this article, the surface states of magnetic topological insulators with Sb substitution were obtained by shifting the Fermi level into the bulk band gap, showing a band gap of 50 meV at the Dirac point.
Abstract: The interplay between magnetism and nontrivial topology in magnetic topological insulators (MTIs) is expected to give rise to exotic topological quantum phenomena like the quantum anomalous Hall effect and the topological axion states. A key to assessing these novel properties is to realize gapped topological surface sates. $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ possesses nontrivial band topology with an intrinsic antiferromagnetic state. However, the highly electron-doped nature of the $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ crystals obstructs the exhibition of the surface band gap. Here, we tailor the material through Sb substitution to reveal the gapped surface states in $\mathrm{Mn}{\mathrm{Bi}}_{2\text{\ensuremath{-}}x}{\mathrm{Sb}}_{x}{\mathrm{Te}}_{4}$. By shifting the Fermi level into the bulk band gap, we access the surface states and show a band gap of 50 meV at the Dirac point from quasiparticle interference measured by scanning tunneling microscopy (STM). Surface-dominant conduction is confirmed through transport spectroscopy measured by multiprobe STM below the N\'eel temperature. The surface band gap is robust against the out-of-plane magnetic field despite the promotion of field-induced ferromagnetism. The realization of bulk-insulating MTIs with the large exchange gap offers a promising platform for exploring emergent topological phenomena.
32 citations
TL;DR: It is shown for the first time that size-selective interfacial polymerization after high-density nanopore formation in graphene not only seals larger defects and macroscopic tears, but also successfully preserves the smaller sub-nanometer pores.
Abstract: Atomically thin graphene with a high-density of precise subnanometer pores represents the ideal membrane for ionic and molecular separations However, a single large-nanopore can severely compromise membrane performance and differential etching between pre-existing defects/grain boundaries in graphene and pristine regions presents fundamental limitations Here, we show for the first time that size-selective interfacial polymerization after high-density nanopore formation in graphene not only seals larger defects (>05 nm) and macroscopic tears but also successfully preserves the smaller subnanometer pores Low-temperature growth followed by mild UV/ozone oxidation allows for facile and scalable formation of high-density (4-55 × 1012 cm-2) useful subnanometer pores in the graphene lattice We demonstrate scalable synthesis of fully functional centimeter-scale nanoporous atomically thin membranes (NATMs) with water (∼028 nm) permeance ∼23× higher than commercially available membranes and excellent rejection to salt ions (∼066 nm, >97% rejection) as well as small organic molecules (∼07-15 nm, ∼100% rejection) under forward osmosis
29 citations
TL;DR: This result proves that by gating the probe, scanning probe microscopy can be used as an easy tool for characterizing sublayer defects in a nondestructive way.
Abstract: The atomic and electronic structures of pristine PdSe2 as well as various intrinsic vacancy defects in PdSe2 are studied comprehensively by combining scanning tunneling microscopy, spectroscopy, an...
27 citations
TL;DR: N narrow-band light emission from covalent heterostructures fused to the edges of graphene nanoribbons (GNRs) is reported by controllable on-surface reactions from molecular precursors by highlighting a route to programmable and deterministic creation of quantum light emitters.
Abstract: Solid-state narrow-band light emitters are on-demand for quantum optoelectronics. Current approaches based on defect engineering in low-dimensional materials usually introduce a broad range of emission centers. Here, we report narrow-band light emission from covalent heterostructures fused to the edges of graphene nanoribbons (GNRs) by controllable on-surface reactions from molecular precursors. Two types of heterojunction (HJ) states are realized by sequentially synthesizing GNRs and graphene nanodots (GNDs) and then coupling them together. HJs between armchair GNDs and armchair edges of the GNR are coherent and give rise to narrow-band photoluminescence. In contrast, HJs between the armchair GNDs and the zigzag ends of GNRs are defective and give rise to nonradiative states near the Fermi level. At low temperatures, sharp photoluminescence emissions with peak energy range from 2.03 to 2.08 eV and line widths of 2-5 meV are observed. The radiative HJ states are uniform, and the optical transition energy is controlled by the band gaps of GNRs and GNDs. As these HJs can be synthesized in a large quantity with atomic precision, this finding highlights a route to programmable and deterministic creation of quantum light emitters.
23 citations
TL;DR: Density functional theory calculations produce a semiconducting density of states (LDOS) of isolated PNRs and find that the band gap narrows and disappears quickly once considering coupling between PNR stacking layers or interaction with the PdSe2 substrate.
Abstract: We report atomically precise pentagonal PdSe2 nanoribbons (PNRs) fabricated on a pristine PdSe2 substrate with a hybrid method of top-down and bottom-up processes. The PNRs form a uniform array of dimer structure with a width of 2.4 nm and length of more than 200 nm. In situ four-probe scanning tunneling microscopy (STM) reveals metallic behavior of PNRs with ballistic transport for at least 20 nm in length. Density functional theory calculations produce a semiconducting density of states of isolated PNRs and find that the band gap narrows and disappears quickly once considering coupling between PNR stacking layers or interaction with the PdSe2 substrate. The coupling of PNRs is further corroborated by Raman spectroscopy and field-effect transistor measurements. The facile method of fabricating atomically precise PNRs offers an air-stable functional material for dimensional control.
22 citations
TL;DR: At cryogenic temperatures, the bias voltage from an STM tip can propel a large organic molecule, dibromoterfluorene, long distances—tens of nanometers along straight tracks on the flat silver surface (see the Perspective by Esch and Lechner).
Abstract: Spatial control over molecular movement is typically limited because motion at the atomic scale follows stochastic processes. We used scanning tunneling microscopy to bring single molecules into a stable orientation of high translational mobility where they moved along precisely defined tracks. Single dibromoterfluorene molecules moved over large distances of 150 nanometers with extremely high spatial precision of 0.1 angstrom across a silver (111) surface. The electrostatic nature of the effect enabled the selective application of repulsive and attractive forces to send or receive single molecules. The high control allows us to precisely move an individual and specific molecular entity between two separate probes, opening avenues for velocity measurements and thus energy dissipation studies of single molecules in real time during diffusion and collision.
16 citations
TL;DR: In this paper, the interlayer magnetic ordering of Fe$3-x}$GeTe$_2$ is investigated by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations and STM simulations.
Abstract: Fe$_{3-x}$GeTe$_2$ is a layered van der Waals magnetic material with a relatively high ordering temperature and large anisotropy. While most studies have concluded the interlayer ordering to be ferromagnetic, there have also been reports of interlayer antiferromagnetism in Fe$_{3-x}$GeTe$_2$. Here, we investigate the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations and STM simulations. We conclude that the layers of Fe$_{3-x}$GeTe$_2$ are coupled ferromagnetically and that in order to capture the magnetic and electronic properties of Fe$_{3-x}$GeTe$_2$ within density functional theory, Hubbard U corrections need to be taken into account.
15 citations
Journal Article•
TL;DR: In this paper, the energy profiles of various reaction pathways using density functional theory and the nudged elastic band method were investigated for cyclodehydrogenation of a 7-atom-wide armchair GNR made from 10,10′-dibromo-9,9′-bianthryl (DBBA) precursors on metal substrates through dehalogenation/polymerization followed by cyclode hydrogenation.
Abstract: Graphene nanoribbons (GNRs) can be synthesized from molecular precursors with atomic precision. A prominent case is the 7-atom-wide armchair GNR made from 10,10′-dibromo-9,9′-bianthryl (DBBA) precursors on metal substrates through dehalogenation/polymerization followed by cyclodehydrogenation. We investigate the key aspects of the cyclodehydrogenation process by evaluating the energy profiles of various reaction pathways using density functional theory and the nudged elastic band method. The metal substrate plays a critical catalytic role by providing stronger adsorption for products and facilitating H desorption. For polyanthrylene on an extra layer of GNR on Au, the underlying GNR insulates it from the Au substrate and increases the reaction barriers, rendering the polyanthrylene “quasi-freestanding”. However, positive charge injection can induce localized cyclodehydrogenation. We find that this is due to the stabilization of an intermediate state through an arenium ion mechanism and favorable orbital symmetries. These results provide mechanistic insight into the effects of the metal substrate and charge injection on cyclodehydrogenation during GNR synthesis and offer guidance for the design and growth of new graphitic structures.
6 citations
TL;DR: S spatially resolved measurement of the spin-dependent thermovoltage in a tunneling junction formed by ferromagnetic Co nano-islands and a Ni tip is reported usingspin-dependent scanning tunneling thermvoltage microscopy (SP-STVthM) to resolve the nanoscale thermoelectric powers with respect to spin polarization, nano- island size, stacking order of Co layers on a Cu substrate, as well as local sample heterogeneities.
Abstract: The Seebeck effect explains the generation of electric voltage as a result of a temperature gradient. Its efficiency, defined as the ratio of the generated electric voltage to the temperature diffe...
TL;DR: In this paper, the authors investigated the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations, and STM simulations.
Abstract: ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{GeTe}}_{2}$ is a layered van der Waals magnetic material with a relatively high ordering temperature and large anisotropy. While most studies have concluded the interlayer ordering to be ferromagnetic, there have also been reports of interlayer antiferromagnetism in ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{GeTe}}_{2}$. Here, we investigate the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations, and STM simulations. We conclude that the layers of ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{GeTe}}_{2}$ are coupled ferromagnetically and that in order to capture the magnetic and electronic properties of ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{GeTe}}_{2}$ within density functional theory, Hubbard U corrections need to be taken into account.
TL;DR: In this article, the structural and electronic properties of a tetragonal CuMnAs thin film studied using scanning tunneling microscopy (STM) and density functional theory (DFT).
Abstract: Epitaxial thin films of CuMnAs have recently attracted attention due to their potential to host relativistic antiferromagnetic spintronics and exotic topological physics. Here, we report on the structural and electronic properties of a tetragonal CuMnAs thin film studied using scanning tunneling microscopy (STM) and density functional theory (DFT). STM reveals a surface terminated by As atoms, with the expected semi-metallic behavior. An unexpected zigzag step edge surface reconstruction is observed with emerging electronic states below the Fermi energy. DFT calculations indicate that the step edge reconstruction can be attributed to an As deficiency that results in changes in the density of states of the remaining As atoms at the step edge. This understanding of the surface structure and step edges on the CuMnAs thin film will enable in-depth studies of its topological properties and magnetism.
Posted Content•
TL;DR: In this paper, using monolayer MoS$_2$ as a model system, atomistic imaging and spectroscopy reveal that metal substitution into sulfur vacancy results in a nonvolatile change in resistance.
Abstract: Non-volatile resistive switching, also known as memristor effect in two terminal devices, has emerged as one of the most important components in the ongoing development of high-density information storage, brain-inspired computing, and reconfigurable systems. Recently, the unexpected discovery of memristor effect in atomic monolayers of transitional metal dichalcogenide sandwich structures has added a new dimension of interest owing to the prospects of size scaling and the associated benefits. However, the origin of the switching mechanism in atomic sheets remains uncertain. Here, using monolayer MoS$_2$ as a model system, atomistic imaging and spectroscopy reveal that metal substitution into sulfur vacancy results in a non-volatile change in resistance. The experimental observations are corroborated by computational studies of defect structures and electronic states. These remarkable findings provide an atomistic understanding on the non-volatile switching mechanism and open a new direction in precision defect engineering, down to a single defect, for achieving optimum performance metrics including memory density, switching energy, speed, and reliability using atomic nanomaterials.