We investigate the intercalation of epoxy resin into a Weyl semimetal, WTe_{2}, by using a scanning tunneling microscopy. We show that intercalant molecules have self-similar height variations with the silver-mean quasiperiodicity and also that they create phason and soliton defects to yield a mean coherence length of 15 sequential order. In addition, we unravel that weak quasiperiodic potentials imposed by the intercalation layer drive a WTe_{2} surface to have an enhanced conductance near the Fermi level while keeping its semimetal phase, providing a direct verification of theoretical prediction.

AbstractThe effective work‐function of metal electrode is one of the major factors to determine the threshold voltage of metal/oxide/semiconductor junction. In this work, it is demonstrated experimentally that the effective work‐function of the Aluminum (Al) electrode in Al/SiO2/n‐Si junction increases significantly by ≈1.04 eV with the graphene interlayer inserted at Al/SiO2 interface. The device‐physical analysis of solving Poisson equation analytically is provided when the flat‐band voltage is applied to the junction, supporting that the large tuning of Al effective work‐function may originate from the electric dipole layer formed by the off‐centric distribution of electron orbitals between Al and graphene layer. Our work suggests the feasibility of constructing the dual‐metal gate CMOS circuitry just by using Al electrodes with area‐specific underlying graphene interlayer.

The origin of the nonhydrogenic hole transition at similar to 23 meV in the boron-doped silicon is still under debate. Here we employ the scanning tunneling microscope to uncover that a boron-containing complex on the silicon (111) surface shows three shallow acceptor states. Among them, the first excited state has the typical energy of the nonhydrogenic hole transition. We then show that this energy gap originates from the strain-induced mixing of the valence bands, unraveling a distinct origin for the nonhydrogenic hole transition in the boron-doped silicon.

The effective work-function of metal electrode is one of the major factors to determine the threshold voltage of metal/oxide/semiconductor junction. In this work, we demonstrate experimentally that the effective work-function of Aluminum (Al) electrode in Al/SiO$_2$/n-Si junction increases significantly by $\sim$1.04 eV with the graphene interlayer inserted at Al/SiO$_2$ interface. We also provide the device-physical analysis of solving Poisson equation when the flat-band voltage is applied to the junction, supporting that the wide tuning of Al effective work-function originates from the electrical dipole layer formed by the overlap of electron orbitals between Al and graphene layer. Our work suggests the feasibility of constructing the dual-metal gate CMOS circuitry just by using Al electrodes with area-specific underlying graphene interlayer.

We generate paramagnetic centers on a heavily boron-doped Si(111) surface by using a scanning tunneling microscope and show that they mediate the spin-dependent recombination of the bound holes of the boron acceptor via direct visualization. This recombination is the intraband process and is significantly affected by the spin-orbit coupling effect. We also demonstrate that such a paramagnetic center with a boron acceptor at its neighbor site can be produced with atomic precision, which makes it a promising candidate for implementing position-controlled impurity qubits with an electrical readout mechanism in silicon.

Charge doping to Mott insulators is critical to realize high-temperature superconductivity, quantum spin liquid state, and Majorana fermion, which would contribute to quantum computation. Mott insulators also have a great potential for optoelectronic applications; however, they showed insufficient photoresponse in previous reports. To enhance the photoresponse of Mott insulators, charge doping is a promising strategy since it leads to effective modification of electronic structure near the Fermi level. Intercalation, which is the ion insertion into the van der Waals gap of layered materials, is an effective charge-doping method without defect generation. Herein, we showed significant enhancement of optoelectronic properties of a layered Mott insulator, α-RuCl3, through electron doping by organic cation intercalation. The electron-doping results in substantial electronic structure change, leading to the bandgap shrinkage from 1.2 eV to 0.7 eV. Due to localized excessive electrons in RuCl3, distinct density of states is generated in the valence band, leading to the optical absorption change rather than metallic transition even in substantial doping concentration. The stable near-infrared photodetector using electronic modulated RuCl3 showed 50 times higher photoresponsivity and 3 times faster response time compared to those of pristine RuCl3, which contributes to overcoming the disadvantage of a Mott insulator as a promising optoelectronic device and expanding the material libraries.

Adsorption of CO molecules on Si(111)-(7×7) is investigated by using scanning tunneling microscopy (STM) and density-functional theory calculations. The most reactive site on the Si(111)-(7×7) surface is the corner adatom in the faulted half unit (FHU), followed by the center adatom in the FHU. The initial sticking probability of CO molecules on Si(111)-(7×7) at room temperature (RT) is estimated to be ∼4×1010 molecules/(cm2·Langmuir), which is comparable with that on Si(001)-(2×1). From the experiments and theoretical calculations, the adsorption of CO molecules are found to occur on Si adatoms either in the upright on-top configuration or in the back-bond inserted configuration, while the adsorption on the rest and the corner hole atoms (which are theoretically probable) are not observable using STM due to their low-lying geometries. Though the sticking probability is very low, every surface dangling bond on the surface can bind strongly with the C atom of a CO molecule even at temperatures higher than RT.

Kekulé distortion in graphene is a subject of extensive theoretical studies due to its non-trivial material properties. Yet, experimental observation of its formation mechanism and electronic structures is still elusive. Here, we used scanning tunneling microscopy to visualize two different phases of the Kekulé distortion in graphene along with experimental evidence that local strain is responsible for the formation of such distortions. In addition, we directly measured the electronic structures of the two phases of the Kekulé distortion in graphene revealing that one opens an energy gap whereas the other maintains a linear density profile. These are consistent with the calculated band structures of the two phases of the Kekulé distortion, respectively, providing a direct verification of the theoretical predictions.

The (001) surface of heavily boron (B) doped silicon is investigated by employing the scanning tunneling microscopy measurements and the density functional theory calculations. Two different defect structures are found in the surface layers, both of which evolve the characteristic spectral features near the valence band maximum. One of the two incorporates a B atom in the second layer with the Si dimers intact and is scattered randomly in the layer. The other one incorporates a B atom in the fourth layer with a dimer vacancy produced directly above it and tends to get together with nearby dimer vacancies to grow the extended or line defects along the perpendicular direction to the dimer rows. Such defect formation is energetically favored to enhance the B populations in the second and fourth layers by similar to 120 and similar to 80 times, respectively, when compared to that in the bulk layer.

A charged vacancy on the Si surface, which generates deep levels within the band gap to accommodate multiple charge states, becomes increasingly important with the device scaling. Yet its characteristics have not been addressed as comprehensively as the bulk vacancy in Si. Here we generate the adatom vacancies on a heavily B doped Si(111) surface and measure their structural and electronic properties by using scanning tunneling microscopy. We find that they are Jahn-Teller distorted in the ground state but undergo the symmetry-restoring transition when gated by the external bias. We also determine the stabilization energy of the Jahn-Teller transition as well as the hopping energy between the dangling bonds of the adatom vacancy quantitatively.

We perform first-principles calculation to investigate the possible magnetism on the Si(111)-\(\sqrt 3 \times \sqrt 3 \) surface, which is stabilized for highly boron-doped samples. When the silicon adatom on top of a boron atom is removed to form a defect structure, three silicon dangling bonds are exposed, generating half-filled doubly degenerate energy levels in the band gap, which stabilizes a local magnetic moment of 2 μB. When many such defect structures are adjacent to one another, they are found to align antiferromagnetically. However, we demonstrate that the ferromagnetism can be stabilized by adjusting the number of electrons in the defects, suggesting a possibility towards spintronic applications for this unique silicon surface structure.

We have fabricated a resistive memory device made of the ultrathin (1.6–3.6 nm) layer of silicon oxide, for which the electrical hysteresis is hardly induced via the conventional electroforming process or field-driven structural modification because non-dissipative quantum tunneling across it hampers this process. We show that such thin silicon oxide comes to exhibit the hysteretic behavior successfully when a large amount of boron atoms are incorporated into it: a typical ratio of high-to-low resistance is as high as >300 and maintained up to the set/reset cycles of >50000, and the projected retention time is longer than a couple of months. We also propose an electronic model for its hysteretic mechanism where the charge trapping into the boron-induced defects in silicon oxide and the alteration of its energy band near the interfacial region play the active roles.

Low-temperature scanning tunneling microscopy measurements and first-principles calculations are employed to characterize edge structures observed for graphene nanoislands grown on the Co(0001) surface. Images of these nanostructures reveal straight well-ordered edges with zigzag orientation, which are characterized by a distinct peak at low bias in tunneling spectra. Density functional theory based calculations are used to discriminate between candidate edge structures. Several zigzag-oriented edge structures have lower formation energy than armchair-oriented edges. Of these, the lowest formation energy configurations are a zigzag and a Klein edge structure, each with the final carbon atom over the hollow site in the Co(0001) surface. In the absence of hydrogen, the interaction with the Co(0001) substrate plays a key role in stabilizing these edge structures and determines their local conformation and electronic properties. The calculated electronic properties for the low-energy edge structures are consistent with the measured scanning tunneling images.

Dielectric screening is essential in determining semiconductor properties. Its assessment on the surface, however, is beyond the capability of conventional techniques due to their lack of surface sensitivity. Here we present the surface-sensitive measurement of the dielectric screening by using scanning tunneling microscopy and spectroscopy. Both single-atom and single-electron manipulations on the B $\ensuremath{\delta}$-doped Si(111) surface unravel that the dielectric screening on this surface is much in excess of what the classical image-charge model predicts, which we ascribe to the strained bonds and the ionic character of the surface layers. Also, as an exemplary application of the measured screening parameters, we demonstrate determining the ionization state of a surface defect from the defect-induced band bending.

An enormous amount of research activities has been devoted to developing new types of non-volatile memory devices as the potential replacements of current flash memory devices. Theoretical device modeling was performed to demonstrate that a huge change of tunnel resistance in an Edge Metal-Insulator-Metal (EMIM) junction of metal crossbar structure can be induced by the modulation of electric fringe field, associated with the polarization reversal of an underlying ferroelectric layer. It is demonstrated that single three-terminal EMIM/Ferroelectric structure could form an active memory cell without any additional selection devices. This new structure can open up a way of fabricating all-thin-film-based, high-density, high-speed, and low-power non-volatile memory devices that are stackable to realize 3D memory architecture.

We show that each surface atom of heavily boron-doped, (111)-oriented silicon with a √3 × √3 reconstruction has electrically switchable two charge states due to the strong electron-lattice coupling at this surface. The structural and electronic properties of the two charge states as well as their energetics are uncovered by employing scanning tunneling microscopy measurements and density functional theory calculations, which reveals that one of the two is a two-electron bound state or surface bipolaron. We also execute the single-atom bit operations on individual surface atoms by controlling their charge states while demonstrating implementation of the atomic scale memory at a silicon surface with an unprecedented recording density.

Exchange bias was discovered in surface-oxidized cobalt particles six decade ago, yet its microscopic mechanism is still being debated because of the lack of atomic-scale measurements at the interfacial region between the CoO and the Co layers. Here, we report our atom-resolved measurements by using scanning tunneling microscopy/spectroscopy of an ultrathin CoO layer grown on a Co(0001) single crystal. We have found that the layer undergoes an order-to-disorder transition upon cooling from a crystalline structure at room temperature to a disordered, glassy state at low temperatures. Accordingly, its electronic state changes from a metallic character in the crystalline phase to an Efros-Shklovskii Coulomb gap state in the disordered one. These findings provide direct evidence that exchange bias in the CoO/Co heterostructure is mediated by the atomic-scale disorder in or the spin-glass-like phase of the CoO layer.