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

Two-step flux synthesis of ultrapure transition metal dichalcogenides

Here, we describe synthesis of TMD crystals using a two-step flux growth method that eliminates a major potential source of contamination. Detailed characterization of TMDs grown by this two-step method reveals charged and isovalent defects with densities an order of magnitude lower than in TMDs grown by a single-step flux technique. Initial temperature-dependent electrical transport measurements of monolayer WSe2 yield room-temperature hole mobility above 840 cm2/Vs and low-temperature disorder-limited mobility above 44,000 cm2/Vs. Electrical transport measurements of graphene-WSe2 heterostructures fabricated from the two-step flux grown WSe2 also show superior performance: higher graphene mobility, lower charged impurity density, and well-resolved integer quantum Hall states

Engineering two-dimensional materials : discovery, defects, and environment

The discovery of graphene and its unprecedented properties inspired an extraordinary increase in research progress, launching an era of two-dimensional (2D) electronic materials. These stable crystalline atomic layers enable the design of ultrathin 2D devices by combining different 2D materials as the foundational components. In order to control the properties of these devices, materials with a variety of electronic properties must be available. In this dissertation, we explore three distinct paths to achieve this goal: expanding the library of 2D materials, post synthesis defect engineering, and proximity engineering of the electrostatic environment. First, we report the MBE synthesis and STM/S characterization of a new 2D insulator, honeycomb structure BeO. In addition to determining the atomic structure and density of states, we used moiré pattern analysis to demonstrate the high crystallinity of the BeO and determined the work function modulation across the moiré pattern. We illustrate that the scalable growth, weak substrate interactions, and long-range crystallinity make honeycomb BeO an attractive candidate for future technological applications. The next focus of this work was defect engineering of monolayer WS₂ by UHV annealing. A high concentration of S vacancies was generated by UHV annealing of the WS2, leading to S vacancy defect-defect coupling. Using STM/S we determined that the interaction of nearby S vacancies leads to an increase of deep in-gap states for different divacancy geometries. This indicates that vacancy engineering can be a useful tool to controllably manipulate 2D material electronic properties. Finally, we demonstrate the creation of a nanoscale planar p-n junction within a single monolayer of MoSe₂ by modulating the electronic properties of the underlying substrate. By intercalating Se at the interface of the hBN/Ru substrate, the hBN becomes decoupled from the Ru, changing its conductivity and work function. We find that this change in the electronic landscape tunes the band gap of the overlying MoSe₂, by screening and shifting the MoSe₂ work function. Thus, this dissertation shines a light on the vast opportunities 2D materials provide for exploration of novel approaches to materials engineering, and demonstrates a tool set for manipulating the electronic properties of these fascinating materials.Physic

2010-06-15

2010-06-1

Variation in Setal Micromechanics and Performance of Two Gecko Species

Biomechanical models of the gecko adhesive system typically focus on setal mechanics from a single gecko species, Gekko gecko. In this study, we compared the predictions from three mathematical models to experimental observations considering an additional gecko species Phelsuma grandis, to quantify interspecific variation in setal micromechanics. We also considered the accuracy of our three focal models: the frictional adhesion model, work of detachment model, and the effective modulus model. Lastly, we report a novel approach to quantity the angle of toe detachment using the Weibull distribution. Our results suggested the coupling of frictional and adhesive forces in isolated setal arrays first observed in G. gecko is also present in P. grandis although P. grandis displayed a higher toe detachment angle, suggesting they produce more adhesion relative to friction than G. gecko. We also found the angle of toe detachment accurately predicts a species’ maximum performance limit when fit to a Weibull distribution. When considering the energy stored during setal attachment, we observed less work to remove P. grandis arrays when compared to G. gecko, suggesting P. grandis arrays may store less energy during attachment, a conclusion supported by our model estimates of stored elastic energy. Our predictions of the effective elastic modulus model suggested P. grandis arrays to have a lower modulus, Eeff, but our experimental assays did not show differences in moduli between the species. The considered mathematical models successfully estimated most of our experimentally measured performance values, validating our three focal models as template models of gecko adhesion (see Full and Koditschek 1999), and suggesting common setal mechanics for our focal species and possibly for all fibular adhesives. Future anchored models, built upon the above templates, may more accurately predict performance by incorporating additional parameters, such as variation in setal length and diameter. Variation in adhesive performance may affect gecko locomotion and as a result, future ecological observations will help to determine how 31 species with different performance capabilities use their habitat

Data from: Variation in setal micromechanics and performance of two gecko

Biomechanical models of the gecko adhesive system typically focus on setal mechanics from a single gecko species, Gekko gecko. In this study, we compared the predictions from three mathematical models with experimental observations considering an additional gecko species Phelsuma grandis, to quantify interspecific variation in setal micromechanics. We also considered the accuracy of our three focal models: the frictional adhesion model, work of detachment model, and the effective modulus model. Lastly, we report a novel approach to quantify the angle of toe detachment using the Weibull distribution. Our results suggested the coupling of frictional and adhesive forces in isolated setal arrays, first observed in G. gecko is also present in P. grandis although P. grandis displayed a higher toe detachment angle, suggesting they produce more adhesion relative to friction than G. gecko. We also found the angle of toe detachment accurately predicts a species’ maximum performance limit when fit to a Weibull distribution. When considering the energy stored during setal attachment, we observed less work to remove P. grandis arrays when compared with G. gecko, suggesting P. grandis arrays may store less energy during attachment, a conclusion supported by our model estimates of stored elastic energy. Our predictions of the effective elastic modulus model suggested P. grandis arrays to have a lower modulus, E eff, but our experimental assays did not show differences in moduli between the species. The considered mathematical models successfully estimated most of our experimentally measured performance values, validating our three focal models as template models of gecko adhesion (see Full and Koditschek in J Exp Biol 202(23):3325–3332, 1999), and suggesting common setal mechanics for our focal species and possibly for all fibular adhesives. Future anchored models, built upon the above templates, may more accurately predict performance by incorporating additional parameters, such as variation in setal length and diameter. Variation in adhesive performance may affect gecko locomotion and as a result, future ecological observations will help to determine how species with different performance capabilities use their habitat

2010-04-30

2010-04-3

2010-06-14

2010-06-1

2010-05-05

2010-05-0