Session D52: Surface Studies of 2D Materials

Eu intercalation in Highly Ordered Pyrolytic Graphite (HOPG)

Recently, two-dimensional (2D) van der Waals (vdW) magnetic materials have been of significant interest. Beside the intrinsic magnetism in the 2D materials, synthesized 2D magnetic materials may provide a wider range of opportunities. To achieve this, we successfully prepared Europium (Eu) intercalated highly ordered pyrolytic graphite (HOPG) through thermal evaporation. The intercalation is evidenced by the atomic resolution scanning tunneling microscopy (STM) measurements. Observations of a shift in dI/dV spectra near the Eu intercalation site indicate the electron doping effect. Next step is to test the transport properties and seek for the magnetoresistance signal.   

First-principles study of electronic structures of two-dimensional Y2C electride surfaces

Electrides are attractive ionic compounds in which electrons confined to the narrow interior space in the material act as anions because of their exotic physical and chemical properties such as low work function and efficient electron transfer. In particular, in the case of two-dimensional electride, electrons gather at the interstitial space. In this talk, we report the electronic structure of a two-dimensional Y₂C surface using the first-principles study. We calculate the work function of Y₂C multilayers and analyze how the electronic structure of the material changes when the external electric field is applied. Finally, we discuss what happens to the electronic structure of the Y₂C surface if a Y (or C) atom is missing.   

The Moiré Superstructure of Graphene Intercalated with a Co Layer

The moiré superstructure of graphene grown on metals can drive the assembly of molecular architectures, such as metal-phthalocyanine molecules, allowing for the production of artificial molecular configurations. A detailed analysis of the Gr/Co interaction upon intercalation (including the resulting moiré pattern) is performed here by density functional theory, providing an accurate description of the template as a function of the corrugation parameters. We pay special attention to the comparison with experimental photoemission data (X-ray Photoemission Spectroscopy, XPS). We then report on empty electron states in cobalt-intercalated graphene, studied by X-ray adsorption spectroscopy and angle-resolved inverse photoemission spectroscopy. The experimental data are compared with theoretical calculations to unveil the dispersion of empty states and the hybridization of graphene π bands at the Gr/Co/Ir(111) interface.   

A non-invasive SOI gating method for probing 2D transport on pristine chemically-terminated Si surfaces

Silicon has a variety of surface terminations available to it in which surface states are passivated and where 2D electron inversion layers are possible through electrostatic gating. In the Kane lab, Si(111) surfaces are terminated with hydrogen using a simple wet-chemical treatment of high-purity, deoxygenated, aqueous NH₄F. Extremely high mobilities, in excess of 300,000 cm²/Vs [1], have been demonstrated in our previous generation devices, and further refinement of these techniques to preserve the pristine nature of these passivated Si surfaces is expected to yield even higher mobilities. To this end, we have developed a new technique to probe these pristine H-Si surfaces using a non-invasive SOI flip-chip gating assembly in which all critical device fabrication is performed on the SOI end. Additionally, our novel device architecture is fully process-compatible with existing UHV halogen-termination (Cl, Br, I) techniques providing a viable solution for 2D transport studies on these surfaces. Architecture details and 77K characterization of 4-terminal devices with newly-incorporated Au depletion gates will be discussed, as well as ongoing low temperature (4K and below) challenges.

[1] T. Kott et al, Phys. Rev. B 89, 041107(R), 2014   

Electronic Density of States of CrBr3 studied by Scanning Tunneling Microscopy and Spectroscopy (STM/S)

CrBr₃ is a van der Waals material exhibiting magnetism at the 2D limit. Despite the great attention on magnetic properties, the electronic properties of CrBr₃ are relatively unexplored. Here, the study of thin flakes of CrBr₃ exfoliated on Highly Ordered Pyrolytic Graphite (HOPG) surface was performed using STM/S. The atomic resolution topography, multi-peak dI/dV measurements on the bulk CrBr₃ as well as at the monolayer limit will be presented and discussed. Topography agrees with the crystal structures while the multi-peak dI/dV spectra agree extremely well with reported optical measurements. However, the observed energy gap extracted from the dI/dV measurements is determined to be ~0.57 eV, much smaller than the previously believed ~1.7 eV observed by absorption spectrum. DFT calculated DOS failed to reproduce two peaks near the Fermi energy. Further studies are needed to explain the discrepancy. The mono- and bi-layer flakes depict degradation at the edges due to air exposure during sample transfer. These observations provide important information towards the fundamental understanding of CrBr₃.   

Two-dimensional conductive surface oxide on CrN(001) and Cr1-xAlxN(001) films

CrN is a Mott-Hubbard insulator but forms a conductive surface oxide with a sheet conductance G_s = 5.9×10^-5 [Ω/sq]^-1 when exposed to oxygen. This is demonstrated by in situ transport measurements on epitaxial CrN(001) layers during a continuously increasing pressure dp/dt = 0.05 Pa/s of a 90% Ar - 10% O₂ mixture, suggesting the formation of a thin n-type doped layer through substitutional replacement of N surface atoms with O. Low-temperature transport measurements indicate incomplete carrier delocalization in the 2D conductive oxide. Alloying with AlN to form cubic Cr_1-xAl_xN(001) films results in similar conductive surface oxides for x < 0.62, an increase in the room temperature resistivity from ρ = 0.070 to 26 Ω-cm for x = 0-0.46, and an increase in the optical band gap from 0.8 to 4.0 eV for x = 0-0.85. Direct oxygen incorporation during reactive deposition in a Ar-N₂-O₂ mixture yields epitaxial CrN_1-xO_x(001) films with a sheet resistance that decreases by over two orders-of-magnitude from 4.4×10³ to 7.5 Ω/sq with an increasing O₂ partial pressure P_O2 = 0-0.060 mTorr, and an insulator-to-metal transition at P_O2 = 0.015 mTorr. The overall results show that oxygen can be used to control transport in CrN, with potential applications in 2D electronics and thermoelectrics.   

Topological charge pumping in twisted bilayer graphene

We show that a sliding motion between the two layers of a \moire superlattice induces an electric current and realizes a two-dimensional version of the topological Thouless pump when the Fermi energy lies in one of the minigaps. Interestingly, a chiral charge pump, namely, a transverse current induced by the sliding motion, is possible in twisted homobilayers. This result is confirmed by a concrete calculation of the adiabatic current in twisted bilayer graphene. Our work reveals an interesting link between mechanical motion and electricity unique to \moire superlattices, and may find applications in nanogenerators and nanomotors.   

Edge states and no wetting layer in ultrathin Bi(110) films grown on graphene/6H-SiC(0001)

Ultrathin Bi(110) films have attracted significant attraction recently as a potential candidate for two-dimensional topological insulators. Grown on different substrates, ultrathin Bi(110) films exhibit totally different topological properties. Clarifying charge doping from substrates and wetting layer existing or not will profoundly deepen our understanding of possible topological properties in ultrathin Bi(110) films, which are still under fierce debate. In this talk, we unambiguously demonstrate no wetting layer existing in epitaxial ultrathin Bi(110) films grown on graphene/6H-SiC(001) using low-temperature scanning tunneling microscopy/spectroscopy (STM/S). Obvious moiré pattern and (6 × 6) reconstruction originating from SiC(001) are clearly observed on the three-monolayer Bi(110) islands, confirming sharp and clean interface between graphene and Bi(110). Gapless edge states are also detected on island edges and domain boundaries, showing a signature of topological nontrivial electronic states in ultrathin Bi(110) films.   

Superconductivity in monolayer graphene driven by Ca and K doping

Superconductivity (SC) induced in graphene has been attracting much attention recently [1, 2]. An example is Ca-intercalated bilayer graphene on SiC [1]. Our structural analysis has unveiled that Ca is intercalated in graphene-buffer interlayer [3], not in graphene-graphene interlayer as analogy of graphite intercalation. Because the position of Ca atoms is on the buffer layer, it can be expected to intercalate Ca atoms into monolayer graphene on SiC. In this study, we have fabricated three samples: Ca-intercalated monolayer graphene grown on SiC(0001), and K or Ca+K adsorbed one on it to change the carrier concentration. Transport measurements have been performed in situ in ultrahigh vacuum. The result shows SC transition at (T_c =) 5.5 K, 3.2 K and 1.6 K, respectively. This is the first report of SC in monolayer graphene which has Dirac electrons. The relation between the T_c and the carrier density is different from that of the conventional SC. In the presentation, we will show the details of the experiments and discuss the mechanism of SC. This work was supported by JSPS KAKENHI Grant Number 18K18732, 18K19014, and JSPS fellows.   

Growth of Silicene on Iodine/Silver (111) studied by low temperature scanning tunneling microscopy

Silicene is the 2D form of silicon. It grows readily on silver (111) as single layers or even bi-layers showing several surface reconstructions in scanning tunneling microscopy images. In an attempt to produce more insulated silicene sheets we grew ~ 0.5 ML of silicon on an almost fully iodine terminated terminated Ag(111) single crystal. For the most part silicene seems to be forming underneath the iodine layer and appears as iodine terminated silicene. In several areas clean silicene sheets were observed. Topographic and spectroscopic data will be discussed.   

Emerging electronic states in van der Walls hetero-bilayers

Bilayer heterostructures have received tremendous attentions in recent years. It is usually believed that electronic structures of stacked van der Waals (vdW) bilayers are simply superposition of their monolayer electronic structures. Here, we show that significant interlayer electronic hybridization occurs even for the most weak-interlayer coupled MoS₂/WSe₂ bilayer among MoS₂/WSe₂ , MoSe₂/PtSe₂ and PtS₂/PtSe₂ bilayers. Several interface states originating from the interlayer S-Se or Se-Se hybridization are explicitly identified by our density functional theory calculations of up to 4000 atoms and confirmed by electrostatic force microscopy (EFM) imaging. In addition, our results also explained the found emerging states within the bandgap of the MoS₂/WSe₂ bilayer.
  

Selective adsorption of Dy on intercalated epitaxial graphene

Manipulation of metal deposition and growth morphology on graphene is essential for promising applications of graphene, e.g., magnetic nanoislands for computer memory and stable nanoparticles for catalytic activity. In this study, we present an effective method to control the adsorption and nucleation property of metal ions on graphene via metal intercalation. We experimentally demonstrate that Dy adatoms show highly selective adsorption behavior on Dy-intercalated epitaxial graphene on SiC(0001) by scanning tunnelling microscopy. This adsorption selectivity is theoretically explained by comparing adsorption energies on graphene regions with different intercalation configurations using first-principles methods based on density functional theory. Our results show that the metal intercalation can be an effective way for the functional manipulation of graphene and enlarge the scope of graphene application.   

Theory of Epitaxial Growth of Borophene on Layered Electride: Thermodynamic Stability and Kinetic Pathway

Based on first-principles calculations, we propose that the layered electride can serve as an effective substrate, in place of metal substrate, to grow borophene. We first confirm the thermodynamic stability of B@Sr₂N heterostructures by energetics analysis. Then, kinetically, we identify the atomistic pathways for a preferred 2D growth mode with a mixed triangle-hexagon configuration over 3D growth on the surface of Sr₂N, indicating the feasibility of epitaxial growth of borophene on layered electrides. As a weak metal, the significantly reduced density of states from the electride at the Fermi level helps to retain the most intrinsic electronic and transport properties of borophene, a significant advantage over metal substrate. We envision that layered electrides provide an attractive family of substrates for epitaxial growth of a range of 2D materials that can otherwise only be grown on undesirable metal substrates.   

Electronic structures of air-exposed few-layer black phosphorus by optical spectroscopy

The electronic structure of few-layer black phosphorus(BP) sensitively depends on the sample thickness, strain and doping. Here, we show that it’s also vulnerable to air-exposure. The oxidation of BP caused by air-exposure gives several optical signatures, including the broadening of resonance peaks and increased Stokes shift between infrared absorption and photoluminescence peaks. More importantly, air exposure causes blue shifts of all resonance peaks in infrared absorption and photoluminescence spectra, with more prominent effects for thinner samples and higher order subband transitions. Our study may provides a convenient and exotic way for band-structure engineering of few-layer black phosphorus through controllable air-exposure or defect creation.