Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H35: 2D Materials -Passivation, Oxidation, and FunctionalizationFocus
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Sponsoring Units: DMP Chair: Saujan Sivaram, U.S. Naval Research Laboratory Room: LACC 409B |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H35.00001: Intrinsically Patterned Two-dimensional Materials for Selective Adsorption of Molecules and Nanoclusters Invited Speaker: H.-J. Gao Two-dimensional (2D) materials have been studied extensively as monolayers, vertical or lateral heterostructures. To achieve functionalization, monolayers are often patterned using soft lithography and selectively decorated with molecules. Here, the growth of a family of 2D materials that are intrinsically patterned has been demonstrated. A monolayer of PtSe2 can be grown on a Pt substrate in the form of a triangular pattern of alternating 1T and 1H phases. Moreover, in a monolayer of CuSe grown on a Cu substrate, strain relaxation leads to periodic patterns of triangular nanopores with uniform size. Adsorption of different species at preferred pattern sites is also achieved, demonstrating that these materials can serve as templates for selective self-assembly of molecules or nanoclusters, as well as for the functionalization of the same substrate with two different species. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H35.00002: Molecular attachment onto single-layer MoS2 is facilitated by Nb doping Boyang Zheng, Youjian Tang, Vincent Crespi Nb doping in single-layer MoS2 creates an electron deficiency in the 2D layer, which may facilitate functionalization of the layer. Using first-principle calculations, we investigated the relative molecular attachment energies of –CH3, –CH2 and –CH onto single-layer MoS2 with or without Nb doping. If the number of Nb dopants in the layer matches the bond order of the ligand, the electron deficiency is compensated; this stabilizes the system by ~1 eV. Excess Nb does not contribute significantly to further stabilization but does facilitate attachment of additional ligands. The effect extends to one or two lattice constant separation between the Nb dopant(s) and the attachment site with only a modest reduction in the degree of stabilization. These results suggest that electron deficiency could be used as a general strategy for molecular attachment to 2D materials. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H35.00003: Chemically stabilized DT-MoS2 alloys for enhanced hydrogen-evolution-reaction Priyanka Manchanda, Shi-Ze Yang, Yuyang Zhang, yongji Gong, Gonglan Ye, Pulickel Ajayan, Matthew Chisholm, Wu Zhou, Sokrates Pantelides The hydrogen evolution reaction (HER) refers to the splitting of H2 molecules. Pt works best, but high cost limits its utility in large-scale applications. MoS2, owing to the low cost of its components and good stability has emerged as an alternative to Pt. However, the basal plane of the 2H phase of MoS2 is catalytically inert as only edges and voids catalyze the H production process. In this work, we use density functional theory calculations and electron microscopy to demonstrate that DT-phase MoS2 can be stabilized by alloying with 50% Re, which leads to significant improvement in HER performance. We find that DT-phase Re0.55Mo0.45S2 shows low overpotential and, therefore, exhibits enhanced catalytic activity. For DT-phase alloys with x>0.75, a high overpotential and low HER activity is observed. In order to evaluate HER activity and the mechanism responsible for low overpotential in Re-doped MoS2 monolayers, we calculated the Gibbs free energy (ΔGH) for hydrogen adsorption. We find that ΔGH for Re0.55Mo0.45S2 strongly depends on the local environment and the most active sites are S atoms surrounded by three Mo atoms with ΔGH being close to zero. Thus, the presence of Re impurities only serve to stabilize the DT phase, but are not directly involved with the catalysis process. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H35.00004: Interface-Confined Anisotropic Thermal Oxidation of 2-Dimensional MoS2 Yejin Ryu, Wontaek Kim, Seonghyun Koo, Haneul Kang, Kenji Watanabe, Takashi Taniguchi, Sunmin Ryu Despite their importance, chemical reactions confined in a low dimensional space are elusive and experimentally intractable. In this work, we report doubly anisotropic, in-plane and out-of-plane, oxidation reactions of 2-dimensional crystals, by resolving interface-confined thermal oxidation of a single and multi-layer MoS2 supported on silica substrates from their conventional surface reaction. Using optical second-harmonic generation spectroscopy of artificially-stacked multilayers, we directly proved that crystallographically-oriented triangular oxides (TOs) were formed in the bottommost layer while triangular etch pits (TEs) were generated in the topmost layer, and that both structures were terminated with zigzag edges. The formation of the Mo oxide layer at the interface demonstrates that O2 diffuses efficiently through the van der Waals (vdW) gap but not MoO3, which would otherwise sublime. The fact that TOs are several times larger than TEs indicates that oxidation is greatly enhanced when MoS2 is in direct contact with silica substrates, which suggests a catalytic effect. This study indicates that the vdW-bonded interfaces are essentially open to mass transport and can serve as a model system for investigating chemistry in low dimensional spaces. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H35.00005: Atomically thin transition metal layers: atomic layer stabilization at the oxide interface Jeongwoon Hwang, Myung Mo Sung, Kyeongjae Cho We have performed first-principles calculations to explore the possibility of synthesizing atomically thin transition metal (TM) layers. It is found that the formation energies of free-standing TM layers are significantly higher than similar sp-bonded monolayers such as silicene and borophene. It is shown that the TM layers can be stabilized by surface passivation with HS, C6H5S2, or O, and that O passivation is most effective leading to thermodynamically stable TM monolayers for most of the TM elements. For the surface oxygen passivation case, the atomic and electronic structures of stabilized TM monolayers are investigated. It is suggested that the large-area synthesis of these 2D TM layers can be extended to general TM elements not having bulk layered structure by employing metal atomic layer deposition (ALD) methods. Our prediction based on the density functional theory calculations is supported by a recent experiment, where atomically thin W film was fabricated by ALD on the oxide substrate and shown gate-controllable electronic property. In relation to the experiment, we also investigate the interface effects on the electronic property of the stabilized W monolayers. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H35.00006: Theoretical Investigation of Oxidation of Bottom-up Graphene Nanoribbons Zhongcan Xiao, Chuanxu Ma, Wenchang Lu, Alex Puretzky, Arthur Baddorf, Kunlun Hong, An-Ping Li, Jerry Bernholc Graphene nanoribbons are very promising candidates for applications in nanoelectronics. Their stability against oxidation is thus of paramount importance. It is also necessary to understand the changes in their electronic properties after oxidation. We conduct a systematic first principles investigation of various potential oxidation-induced defect groups in GNR and study their electronic properties. The results are compared with experimental observations from scanning tunneling microscopy, Raman, and x-ray photoelectron spectroscopy, and are used to determine the common types of oxidation-induced defects. We also analyze the thermodynamic stabilities of possible oxygen configurations by calculating their formation energies and compare the chemical reactivities of armchair and zigzag edges. We show that oxygen defects alter GNRs' electronic structure sufficiently to enable the use of GNRs' as high temperature oxygen sensors. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H35.00007: Oxidation of Armchair Graphene Nanoribbons Chuanxu Ma, Alexander Puretzky, Arthur Baddorf, Zhongcan Xiao, Wei-En lu, Kunlun Hong, Jerry Bernholc, An-Ping Li The stability of graphene nanoribbons (GNRs) in air is crucial for practical applications. Here we study the temperature stability of the armchair GNR with a width of seven carbon atoms (7-aGNR) after exposed to air. Combining scanning tunneling microscopy, Raman spectroscopy, x-ray photoemission spectroscopy, and first-principles theory calculations, the oxidization of the armchair GNRs is found to start from the edges at ab out 520 °C, while below 430 °C the edges are unchanged. Two different types of oxygen species are atomically identified, specifically hydroxyl (OH) and atomic oxygen bridging two carbons, both of which are common oxygen forms in oxidized graphitic lattices. The bandgap is significantly reduced from 2.6 eV to 2.3 eV and 1.9 eV in the vicinity of hydroxyl or bridge O, respectively. Our results suggest that the oxidization will greatly affect the transport properties of GNRs. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H35.00008: First-principles Characterization of the Interaction between Oxygen-Passivated Porous Graphene and Cations for Sensors and Water Filtration Jonathan Heath, Marcelo Kuroda Porous graphene has been reported as an effective membrane capable of filtering ions in water and a promising platform for gas sensor applications. Here we characterize the interactions between cations and oxygen-passivated graphene pores using the density functional theory (DFT). The coupling between porous graphene and Li, Na, K, Rb, and Cs is studied in terms of pore size and density. We find that the strongest binding occurs when the size of a pore is similar to the van der Waals radius of the ion, maximizing the electrostatic ion-pore interaction and explaining experimental observations. Analysis of the charge distribution stemming from the electron transfer to the sheet shows different behaviors for porous graphene than for pristine graphene. The latter behaves like a perfect metal, in spite of the semimetal character of graphene and its atom thickness. In contrast, charge transfer in the porous case shows oscillations that decay more rapidly than in pristine graphene. We discuss the trends found within this class of ions and their implications to the design of large-scale water desalination membranes and gas sensors. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H35.00009: Role of Pinhole Defects in Enhancing Water Transport through Graphene Oxide Laminates Vivek Saraswat, Robert Jacobberger, Joshua Ostrander, Courtney Hummell, Austin Way, Jialiang Wang, Martin Zanni, Michael Arnold Laminates made of graphene oxide nanosheets (GO) have emerged as promising materials for separation applications due to their high water permeance and salt rejection. However there is no clear consensus on the origin of ultrafast water transport through these laminates. Some studies have attributed this to the presence of a network of graphene capillaries in GO nanosheets. However, some other studies have attributed this to presence of pinhole defects in sheet interiors, which reduce the effective length that water needs to travel before progressing deeper into the laminate. To determine the primary pathway responsible for water permeance, we synthesized GO nanosheets with two orders of magnitude difference in lateral sizes and compared water permeance through laminates made by pressure-assisted deposition of these sheets. We found that water permeance through these laminates is identical despite such massive differences in lateral sizes of sheets. Furthermore we simulated water flow through these laminates using an interconnected nanochannel network model. Our simulations in tandem with the experimental data indicate that trans-sheet flow through pinhole defects in GO must be the dominant water transport pathway – as opposed to a circuitous, lateral pathway around the sheets. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H35.00010: ABSTRACT WITHDRAWN
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Tuesday, March 6, 2018 4:54PM - 5:06PM |
H35.00011: The potential for fast van der Waals computations and the study of wetting properties for layered materials Yao Zhou, Lenson Pellouchoud, Evan Reed Computation of the van der Waals (vdW) interactions plays a crucial role in the study of layered materials. The adiabatic-connection fluctuation-dissipation theorem within random phase approximation (ACFDT-RPA) has been empirically reported to be the most accurate of commonly used methods, but it is limited to small systems due to its computational complexity. Without a computationally tractable vdW correction, fictitious strains are often introduced in the study of multilayer heterostructures. We employed for the first time a defined Lifshitz model to provide the vdW potentials within 8-20% of the ACFDT-RPA calculations for a spectrum of layered materials orders of magnitude faster than the ACFDT-RPA for representative layered material structures. Using this fast Lifshitz model, we studied wetting properties of layered materials: graphite and MoS2. We find that the water contact angle of the layered materials changes with different airborne contaminants present, which theoretically demonstrates the effect of contamination on wettability of layered materials. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H35.00012: The adsorption and reaction of O2 with defects on graphene/Ru(0001) Tianbai Li, Jory Yarmoff Small molecules, such as O2 and CO, can intercalate between deposited graphene (Gr) and a substrate, and can also etch the Gr overlayer when it is heated to desorb the intercalates. The experiments performed here employ Gr grown on Ru(0001) by chemical vapor deposition into which single carbon vacancy defects are formed by Ar+ bombardment. A new adsorption site is identified on the surface of defected graphene and it is shown that these defects also affect the intercalation and etching process. The surfaces are interrogated with low energy helium ion scattering, which can monitor the adsorption and intercalation of oxygen and the etching of the graphene. The scattering angle is adjusted so that adatoms attached to the Gr surface can be distinguished from the intercalates. It is found that the oxygen deposited at room temperature at a carbon vacancy is adsorbed at a unique site and is not directly bonded to the Ru substrate. Moreover, the defects ease the intercalation of O2 and improve the etching efficiency during annealing, while the magnitude of this effect depends on the size of the defects. |
Tuesday, March 6, 2018 5:18PM - 5:30PM |
H35.00013: Nature and Evolution of Electronic Structure of Graphene Oxide: A First-Principles Perspective Leandro Seixas Rocha In this work, we investigate structural, electronic and chemical properties of graphene oxide wth varying degree of oxidation and different ratios between hydroxyls and epoxies groups. From first-principles methods based upon density functional theory, we observe how the projected density of states (PDOS) vary in these materials, and how the Fukui functions for nucleophilic and electrophilic attacks behave with the oxidation variation. The chemical reactivities of these 2D materials were studied for applications in graphene oxide functionalizations for emerging technologies in renewable energy and environment. |
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