Asymptotic Correction Schemes for Semilocal Exchange-Correlation Functionals

Aiming to remedy the incorrect asymptotic behavior of conventional semilocal exchange-correlation (XC) density functionals for finite systems, we propose an asymptotic correction scheme, wherein an exchange density functional whose functional derivative has the correct (-1/r) asymptote can be directly added to any semilocal density functional. In contrast to semilocal approximations, our resulting exchange kernel in reciprocal space exhibits the desirable singularity of the type O(-1/q^2) as q -> 0, which is a necessary feature for describing the excitonic effects in non-metallic solids. By applying this scheme to a popular semilocal density functional, PBE [J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)], the predictions of the properties that are sensitive to the asymptote are significantly improved, while the predictions of the properties that are insensitive to the asymptote remain essentially the same as PBE. Relative to the popular model XC potential scheme, our scheme is significantly superior for ground-state energies and related properties. In addition, without loss of accuracy, two closely related schemes are developed for the efficient treatment of large systems.Comment: 7 pages, 2 figures, 2 tables, supplementary material not include

Epitaxial Growth of Two-dimensional Insulator Monolayer Honeycomb BeO

The emergence of two-dimensional (2D) materials launched a fascinating frontier of flatland electronics. Most crystalline atomic layer materials are based on layered van der Waals materials with weak interlayer bonding, which naturally leads to thermodynamically stable monolayers. We report the synthesis of a 2D insulator comprised of a single atomic sheet of honeycomb structure BeO (h-BeO), although its bulk counterpart has a wurtzite structure. The h-BeO is grown by molecular beam epitaxy (MBE) on Ag(111) thin films that are conveniently grown on Si(111) wafers. Using scanning tunneling microscopy and spectroscopy (STM/S), the honeycomb BeO lattice constant is determined to be 2.65 angstrom with an insulating band gap of 6 eV. Our low energy electron diffraction (LEED) measurements indicate that the h-BeO forms a continuous layer with good crystallinity at the millimeter scale. Moir\'e pattern analysis shows the BeO honeycomb structure maintains long range phase coherence in atomic registry even across Ag steps. We find that the interaction between the h-BeO layer and the Ag(111) substrate is weak by using STS and complimentary density functional theory calculations. We not only demonstrate the feasibility of growing h-BeO monolayers by MBE, but also illustrate that the large-scale growth, weak substrate interactions, and long-range crystallinity make h-BeO an attractive candidate for future technological applications. More significantly, the ability to create a stable single crystalline atomic sheet without a bulk layered counterpart is an intriguing approach to tailoring novel 2D electronic materials.Comment: 25 pages, 7 figures, submitted to ACS Nano, equal contribution by Hui Zhang and Madisen Holbroo

Quantitative determination of interlayer electronic coupling at various critical points in bilayer Mo S 2

Tailoring interlayer coupling has emerged as a powerful tool to tune the electronic structure of van der Waals (vdW) bilayers. One example is the usage of the “moiré pattern” to create controllable two-dimensional electronic superlattices through the configurational dependence of interlayer electronic couplings. This approach has led to some remarkable discoveries in twisted graphene bilayers, and transition metal dichalcogenide (TMD) homo- and hetero-bilayers. However, a largely unexplored factor is the interlayer distance, d, which can impact the interlayer coupling strength exponentially. In this letter, we quantitatively 2 determine the coupling strengths as a function of interlayer spacing at various critical points of the Brillouin zone in bilayer MoS2. The exponential dependence of the coupling parameter on the gap distance is demonstrated. Most significantly, we achieved a 280% enhancement of K-valley coupling strength with an 8% reduction of the vdW gap, pointing to a new strategy in designing a novel electronic system in vdW bilayers. gning a unique electronic system in vdW bilayers.This research was primarily supported by the NSF Materials Research Science and Engineering Centers (MRSEC) under DMR-1720595. We also acknowledge support from the Welch Foundation (F-1672 and F-1662), the US NSF (DMR-1808751) and the U.S. Air Force (FA2386-18-1-4097). C.-R.P., P.-J.C., and M.-Y.C. acknowledge the support from Academia Sinica, Taiwan. W.-H.C. acknowledges the support from the Ministry of Science and Technology of Taiwan (MOST-110-2119-M-A49-001-MBK) and the support from the Center for Emergent Functional Matter Science (CEFMS) of NYCU supported by the Ministry of Education of Taiwan. W.-T.H. acknowledges the support from the Ministry of Science and Technology of Taiwan (MOST-110-2112-M-007-011-MY3) and the Yushan Young Scholar Program from the Ministry of Education of Taiwan. C.K.S. also acknowledge the Yushan Scholar Program from the Ministry of Education of Taiwan.Center for Dynamics and Control of Material

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Properties of two-dimensional materials grown on metal substrates

Supercell models are proposed to investigate the properties of three types of heterostructures formed by two-dimensional (2D) materials grown on metal substrates, including (1) silicon (Si) thin films on a silver (Ag) substrate; (2) a single-layer hexagonal boron nitride (h-BN) on ruthenium (Ru) and copper (Cu) substrates; and (3) a stacked combination of lead (Pb) and Ag thin films. Coverage, orbital hybridization, and interface conditions are studied in order to tailor the electronic properties of these heterostructures. For the first system, results show that a Si coverage beyond 2.5 ML is needed for the emergence of the nearly linear energy-momentum relation. This relation is associated with the electronic states induced by the interaction between surface Si and Ag. For the second system, results demonstrate that the nitrogen (N) orbitals can hybridize with the underlying metal orbitals, and thus the regions of the h-BN monolayer with N situated on top of a metal atom will move closer to the substrate, leading to a corrugated h-BN layer. Calculated spatially-periodic modulations of the band profile and the local work function are in agreement with the experimental results. For the third system, results illustrate that the presence of the substrate alters the boundary conditions and thus can change the phase shifts of the quantum well states at the interface. The combination of Pb and Ag films creates a joint potential well that supports combined quantum well states. These findings suggest that in our studied systems, the interaction between the 2D materials and the substrates plays an important role in determining their electronic properties.Ph.D

Critical role of parallel momentum in quantum well state couplings in multi-stacked nanofilms: An angle resolved photoemission study

We use angle resolved photoemission spectroscopy to investigate the coupling of electron quantum well states (QWSs) in epitaxial thin Pb and Ag films. More specifically, we investigate the Ag/Si, Pb/Si, and Pb/Ag/Si systems. We found that the parallel momentum plays a very profound role in determining how two adjacent quantum wells are coupled electronically across the interface. We revealed that in the Pb/Ag bimetallic system, there exist two distinctly different regimes in the energy vs momentum (E vs k) space. In one regime, the electronic states in Ag and Pb are strongly coupled, resulting in a new set of QWSs for the bi-metallic system. In the other regime, the electronic states in individual metallic layers are retained in their respective regions, as if they are totally decoupled. This result is corroborated by calculations using density functional theory. We further unravel the underlying mechanism associated with the electron refraction and total internal reflection across the interface.We acknowledge the funding from the Welch Foundation (Grant No. F-1672), the U.S. Air Force (Grant No. FA2386-18- 1-4097), the NSF (Grant No. DMR-1808751), the NSF MRSEC program (Grant No. DMR-1720595), Grant No. NSF-EFMA- 1542747, and the Academia Sinica Thematic Project (Grant No. AS-TP-106-A07).Center for Dynamics and Control of Material

Tuning Band Gap and Work Function Modulations in Monolayer hBN/Cu(111) Heterostructures with Moiré Patterns

The moiré pattern formed between a two-dimensional (2D) material and the substrate has played a crucial role in tuning the electronic structure of the 2D material. Here, by using scanning tunneling microscopy and spectroscopy, we found a moiré-pattern-dependent band gap and work function modulation in hexagonal boron nitride (hBN)/Cu(111) heterostructures, whose amplitudes increase with the moiré pattern wavelength. Moreover, the work function modulation shifts agree well with the conduction band edge shifts, indicating a spatially constant electron affinity for the hBN layer. Density functional theory calculations showed that these observations in hBN/Cu(111) heterostructures mainly originated from the hybridization of the N 3pz orbital and Cu 4s orbital in different atomic configurations. Our results show that the twist-angle dependence of moiré patterns in hBN/Cu(111) heterostructures can be used to tailor the electronic properties including band gap and work function