AVS 69 Session SS+2D+AS+HC-WeM: Surface Science of 2D Materials

Wednesday, November 8, 2023 8:00 AM in Room D136
Wednesday Morning

Session Abstract Book
(329KB, Nov 2, 2023)
Time Period WeM Sessions | Abstract Timeline | Topic SS Sessions | Time Periods | Topics | AVS 69 Schedule

Start Invited? Item
8:00 AM SS+2D+AS+HC-WeM-1 Heterogeneous Photocatalysis: Alcohols on Bare and Metal-loaded TiO2(110) and Fe2O3(012)
Moritz Eder (TU Wien); Philip Petzoldt, Martin Tschurl (Technical University of Munich); Jiri Pavelec, Michael Schmid, Ulrike Diebold (TU Wien); Ueli Heiz (Technical University of Munich); Gareth Parkinson (TU Wien)

We investigated the (photo)chemistry of alcohols on TiO2(110) and Fe2O3(012) in ultra-high vacuum. Our studies focused on the role of the metal co-catalyst in the photocatalytic reaction by comparing the reactivity of bare and metal-loaded surfaces. We show that photocatalytic reactions are not merely a couple of redox reactions, but an interplay of thermal and photon-driven surface reactions.

Our results demonstrated that the co-catalyst plays a crucial role in the outcome of the reaction. On TiO2(110), alcohols are oxidized to the aldehyde/ketone and hydrogen surface species upon illumination. The hydrogen surface species were thermally converted to H2 by the co-catalyst, allowing for a steady-state photocatalytic conversion of alcohols and the continuous production of molecular hydrogen. Using mass spectrometry, we determined turnover frequencies and rate constants. The identification of surface mechanisms on Fe2O3 is less advanced, but there seem to be strong parallels in the photochemistry.

Our studies shed light on the fundamental processes involved in photocatalytic reactions on metal-loaded surfaces and contribute to the development of sustainable energy technologies.

8:20 AM SS+2D+AS+HC-WeM-2 Factors Governing the Reactivities of Transition Metal Carbides at Vapor/Solid and Liquid/Solid Interfaces
Samar Alhowity, Ashwin Ganesan, Mojgan Gharaee, Olatomide Omolere, Qasim Adesope, Kabirat Balogun, Precious Chukwunenye, Francis D'Souza, Thomas Cundari, Jeffry Kelber (University of North Texas)

Transition metal carbides are of broad interest for both heterogeneous and electro-catalysis. However, fundamental understanding of chemical factors governing reactivities and selectivities at the vapor/solid and liquid/solid interfaces remain sparse. Herein, in situ XPS results, electrochemical measurements, and DFT-based calculations are presented regarding the reactivities of NbC and TaC in the presence of O2 vapor, and reactivity in solution towards the reduction of N2 to NH3. NbC and TaC films were prepared by DC magnetron sputtering deposition, then exposed to O2 vapor at room temperature, and analyzed by in situ XPS without exposure to ambient. Similarly prepared samples were also analyzed by ex situ XRD. These data show that, although Nb and Ta have similar oxophilicities, (a) deposited NbC films contain significant amounts of Nb oxide phases throughout the film, whereas TaC films deposited under similar conditions do not, and (b) the exposure of NbC films to O2 at 300 K results in significant Nb oxide formation, but that TaC films remain inert towards O2 under these conditions. DFT calculations indicate that this significant reactivity difference towards O2 is due in large part to the greater Ta-C bond strength compared to Nb-C, and in part due to the relative energetic stabilities of the corresponding oxides. Electrochemical studies show that ambient-exposed NbC, with a Nb2O5 surface layer, becomes reactive towards N2 reduction to NH3 under acidic conditions, but only after etching in NaOH to remove the surface oxide layer. Additionally, chronoamperometric data indicate that this reactive NbC surface is eventually modified under electrochemical conditions and becomes relatively inert towards N2 reduction with time. Experiments involving in situ sample transfer between UHV and electrochemistry environments demonstrate that electrochemically active NbC surfaces in solution comprise Nb sub-oxide surface layers, in line with previous studies showing that effective NRR catalysts contain surface transition metal ions in intermediate oxidation states, supporting both N2 lone pair attraction and pi-backbonding to bind and activate the NN triple bond.

Acknowledgement This work was supported in part by the UNT College of Science through COS grants 1600089 and RSG-2023-002 and and in part by the NSF under grant no. DMR 2112864.

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8:40 AM Invited SS+2D+AS+HC-WeM-3 Tunable Interfacial Electrochemistry at Moiré Material Interfaces
D. Kwabena Bediako (University of California at Berkeley)
At electrode–electrolyte interfaces, crystallographic defects are frequently implicated as active sites that mediate interfacial electron transfer (ET) by introducing high densities of localized electronic states (DOS). However, conventional defects can be challenging to deterministically synthesize and control at an atomic level, challenging the direct study of how electronic localization impacts interfacial reactivity. Azimuthal misalignment of atomically thin layers produces moiré superlattices and alters the electronic band structure, in a manner that is systematically dependent on the interlayer twist angle. Using van der Waals nanofabrication of two-dimensional heterostructures, scanning electrochemical cell microscopy measurements, and four-dimensional scanning transmission electron microscopy, we report a strong twist angle dependence of heterogeneous charge transfer kinetics at twisted bilayer and trilayer graphene electrodes with the greatest enhancement observed near the ‘magic angles’. These effects are driven by the angle-dependent engineering of moiré flat bands that dictate the electron transfer processes with the solution-phase redox couple, and the structure of the relaxed moiré superlattice. Moiré superlattices therefore serve as an unparalleled platform for systematically interrogating and exploiting the dependence of interfacial ET on local electronic structure.
9:20 AM SS+2D+AS+HC-WeM-5 Growth of Ultrathin Silica Films on Pt(111) and Rh(111): Influence of Intermixing with the Support
Matthias Krinninger, Florian Kraushofer (Technical University of Munich); Nils Refvik (University of Alberta); Friedrich Esch, Barbara A.J. Lechner (Technical University of Munich)
Silica is a widely used catalyst support material for clusters and nanoparticles. Understanding the relationship between these clusters and the support is challenging, however, because SiO2 is insulating, and in most applications not crystalline which drastically limits the use of experimental techniques to those that work on insulating samples and are not diffraction-based. Several previous studies have investigated ultrathin, quasi-2D silica films on a variety of metal supports [1], which can then be measured by scanning tunneling microscopy (STM), XPS and most other surface science methods. Previous work on Pt(111) did not result in closed films, which was attributed to lattice mismatch [2]. We show that closed films can in fact be grown on Pt(111) when silica is deposited in excess, likely due to formation of a platinum silicide layer with slightly expanded lattice constant at the interface. We also report results of film growth on Rh(111), which is a near-perfect match to the lattice constant of freestanding SiO2 films as calculated by theory. However, no high-quality films were achieved on Rh due to thermodynamic competition with a surface silicide.

References:

[1] C. Büchner, M. Heyde, Two-dimensional silica opens new perspectives, Prog. Surf. Sci., 92 (2017) 341-374.
[2] X. Yu, B. Yang, J. A. Boscoboinik, S. Shaikhutdinov, and H.-J. Freund, Appl. Phys. Lett. 100 (2012), 151608.
9:40 AM SS+2D+AS+HC-WeM-6 CO2 Adsorption on Graphitic-Like Bilayer ZnO Film Studied by NAP-XPS
Bo-Hong Liu, Shang-Hong Cheng (National Synchrotron Radiation Research Center)

CO2 activation is a fundamental process in heterogeneous catalysis. ZnO-based catalyst has been extensively used in commercial methanol synthesis from CO2 gas and the reverse water gas shift reaction. The adsorption behavior of CO2 on the catalyst surface is pivotal to the reactivity. Whereas ZnO(0001)-Zn physisorbed or weakly chemisorbed CO2,1 strong chemisorption of the molecule happens on non-polar surfaces, such as ZnO(10-10), resulting in a tridentate carbonate.2 In Operando TEM investigation during methanol synthesis shows that ZnO single atomic layer stacks distortedly around Cu nanoparticles via strong metal-support interaction. The lake of interlayer ordering between the layers suggests a weak interlayer interaction; therefore, each layer resembles a free-standing sheet.3 DFT modeling concluded that free-standing ZnO(0001) layer adopts an graphitic-like co-planner structure. The co-planner feature was verified experimentally for the bi-layer ZnO(0001) supported on Ag(111) and Au(111).4 On Au(111) substrate, TPD shows that CO2 adsorbs on the low coordinate sites at the layer edges.5 In the present study, we investigate the CO2 adsorption on bi-layer ZnO/Ag(111) film using NAP-XPS to extend the pressure condition towards reality. We found a more considerable CO2 chemisorption at elevated pressure. The presentation will also address how the surface hydroxyl group influences CO2 adsorption.

  1. Wang, J.;Hokkanen, B.; Burghaus, U., Adsorption of CO2 on pristine Zn–ZnO (0 0 0 1) and defected Zn–ZnO (0 0 0 1): A thermal desorption spectroscopy study. Surf. Sci. 2005,577 (2-3), 158-166.
  2. Schott, V.;Oberhofer, H.; Birkner, A.;Xu, M.;Wang, Y.;Muhler, M.;Reuter, K.; Wöll, C., Chemical activity of thin oxide layers: strong interactions with the support yield a new thinā€film phase of ZnO. Angewandte Chemie International Edition 2013,52 (45), 11925-11929.
  3. Lunkenbein, T.;Schumann, J.;Behrens, M.;Schlögl, R.; Willinger, M. G., Formation of a ZnO overlayer in industrial Cu/ZnO/Al2O3 catalysts induced by strong metal–support interactions. Angewandte Chemie 2015,127 (15), 4627-4631.
  4. Tusche, C.;Meyerheim, H.; Kirschner, J., Observation of depolarized ZnO (0001) monolayers: formation of unreconstructed planar sheets. Phys. Rev. Lett. 2007,99 (2), 026102.
  5. Deng, X.;Sorescu, D. C.; Lee, J., Enhanced adsorption of CO 2 at steps of ultrathin ZnO: the importance of Zn–O geometry and coordination. Phys. Chem. Chem. Phys. 2017,19 (7), 5296-5303.
10:00 AM BREAK - Complimentary Coffee in Exhibit Hall
11:00 AM SS+2D+AS+HC-WeM-10 Investigation of Nitride Spintronic and Kagome-Structured Intermetallic Topological Materials Using Molecular Beam Epitaxy and Scanning Tunneling Microscopy
Arthur R. Smith (Ohio University Physics and Astronomy Department)
Owing to the overwhelming interest in topological [1] and spintronic materials [2], it is imperative to investigate these down to the atomic scale for their possible use in advanced devices. Many promising properties discovered among nitride materials, such as chemical stability and wide band gaps [3], may be combined with the equally promising aspects of topological materials, such as the topological Hall and Nernst effects [4]. Very recent work illustrates that spin-polarized scanning tunneling microscopy is a powerful tool for exploring topological band-structured Kagome antiferromagnets [5]. In our current work, we investigate both nitride material systems grown using molecular beam epitaxy as well as the growth of topological systems such as Kagome antiferromagnetic materials. Ongoing work in our group encompasses the investigation of Mn_3Sn, FeSn, CrSn, Mn_3Ga, and as a spintronic and topological nitride, Mn_3GaN. These materials are grown in combined UHV MBE and scanning tunneling microscopy chamber systems in which the grown samples are first fabricated using MBE and after that investigated for their structural, electronic, and magnetic properties including using STM and tunneling spectroscopy. Our goal is also to investigate these materials using spin-polarized STM as a function of temperature and applied magnetic field. Our current results show that these materials can be fabricated effectively using molecular beam epitaxy and investigated using various in-situ techniques such as reflection high energy electron diffraction and STM. Results from multiple on-going investigations will be presented with a birds-eye view of the progress. Especially to be presented will be STM and STS results in these Kagome systems grown using MBE.

This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317.

[1] P. Liu et al., "Topological nanomaterials," Nat. Rev. Mater. 4, 479 (2019).

[2] A. Hirohata et al., "Review on spintronics: Principles and device applications," Journal of Magnetism and Magnetic Materials 509, 166711 (2020).

[3] M. Xu et al., "A review of ultrawide bandgap materials: properties, synthesis and devices," Oxford Open Materials Science 2(1), itac004 (2022).

[4] S. Roychowdhury et al., "Giant Topological Hall Effect in the Noncollinear Phase of Two-Dimensional Antiferromagnetic Topological Insulator MnBi4Te7," Chemistry of Materials33, 8343 (2021).

[5] H. Li et al., "Spin-polarized imaging of the antiferromagnetic structure and field-tunable bound states in kagome magnet FeSn," Scientific Reports12, 14525 (2022).

11:20 AM SS+2D+AS+HC-WeM-11 Molecular Beam Epitaxial Growth and Investigations of FeSn on LaAlO3
Tyler Erickson, Sneha Upadhyay, Hannah Hall, David C. Ingram, Savas Kaya, Arthur R. Smith (Ohio University)
Kagome antiferromagnetic and ferromagnetic materials provide an interesting avenue for research through the investigation of frustrated magnetism, band topology and electronic correlations [1-4]. FeSn is a layer-wise antiferromagnetic Kagome structured material with characteristic dispersion-less flat bands and Dirac cones at the Brillouin zone boundaries. Li et al. have presented exciting spin-polarized scanning tunneling microscopy results revealing surface electronic and magnetic properties of in-situ cleaved bulk FeSn [1]. Zhang et al. reported strain engineering of FeSn on SrTiO3 (111) with precise control of the stanene layers [2]. Kawakami et al. reported Fe3Sn2 growth on Pt buffer layers on top of Al2O3 and studied various topological phenomena of this topological Kagome material [3,4]. Bhattarai et. al. studied the magnetotransport properties of FeSn grown on silicon substrates [5]. Here, we study the growth of FeSn directly on LaAlO3 and report the successful growth of high-quality crystalline thin-films of FeSn. Reflection high-energy electron diffraction and x-ray diffraction are used to discover the in-plane and out-of-plane lattice constants, while atomic force microscopy and Rutherford backscattering provide topographical and stoichiometric characterization. Preliminary results indicate in-plane and out-of-plane lattice constants of 5.290 Å and 4.56 Å compared to the expected results of 5.297 Å and 4.481 Å, respectively. Besides discussing the thin film FeSn growth results, we also plan to present scanning tunneling microscopy results on the MBE-grown surfaces.
This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317.
[1] H. Li et al., Scientific Reports, 12 14525 (2022)
[2] H. Zhang et al., Nano Lett. 23, 239 – 2404 (2023)
[3] I. Lyalin et al., Nano Lett. 21, 6975 – 6982 (2021)
[4] S. Cheng et al., APL Mater. 10, 061112 (2022)
[5] N. Bhattarai et al., Phys. Status Solidi A, 220: 2200677 (2023)
11:40 AM SS+2D+AS+HC-WeM-12 AVS Graduate Research Awardee Talk: Molecular Beam Epitaxial Growth, Structural Properties, and Surface Studies of a-Plane-Oriented Mn3Sn on C-Plane Al2O3
Sneha Upadhyay, Tyler Erickson (Ohio University); Juan Carlos Moreno Hernandez (Universidad Autonoma de Puebla, Mexico); Hannah Hall, Kai Sun (Ohio University); Gregorio Hernandez Cocoletzi (Universidad Autonoma de Puebla, Mexico); Noboru Takeuchi (Universidad Nacional Autonoma de Mexico, Mexico); Arthur Smith (Ohio University)

Recently, Chen et al. reported the observation of tunneling magnetoresistance in an all-antiferromagnetic tunnel junction consisting of Mn3Sn/MgO/Mn3Sn.1 Furthermore, Bangar et al. demonstrated a technique for engineering the spin Hall conductivity of Mn3Sn films by changing the Mn: Sn composition.2 These works show the potential of studying this Kagome antiferromagnetic material and the importance of being able to grow smooth films. This work uses molecular beam epitaxy to investigate the growth of Mn3Sn (1120) on Al2O3 (0001). The growth is monitored in-situ using reflection high energy electron diffraction and measured ex-situ using X-ray diffraction, Rutherford backscattering, and atomic force microscopy.In our previous work, we carried out a single-step growth at 450°C, which resulted in a crystalline but discontiguous a-plane-oriented (~43% 1120) Mn3Sn film with a mix of other orientations including 0002.3 Leading from this work, changes were made to the growth recipe, which involved carrying out a two-step growth procedure at room temperature, resulting in a contiguous, epitaxial Mn3Sn film with up to ~82% 1120-orientation. We are also exploring the effect of varying the Mn: Sn flux ratio and the film thicknesses (in the range of 5 – 200 nm) on the film crystallinity and orientation. We observe that varying the Mn: Sn flux ratio leads to a change in the RHEED patterns from pointy to streaky, and the XRD shows that the 1120 peak can be varied between~ 82 % to ~38 % of all the peaks’ total intensity. We also plan to present the first results on ultra-high vacuum scanning tunneling microscopy imaging of the (11­20) Mn3Sn surface.

Acknowledgments:

The authors acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317. The authors would like to thank Dr. Eric Stinaff and his students for back-coating the sapphire (0001) substrates.

1 X. Chen et al., "Octupole–driven magnetoresistance in an antiferromagnetic tunnel junction.” Nature 613, 490 (2023).

2 H. Bangar et al., “Large Spin Hall Conductivity in Epitaxial thin films of Kagome Antiferromagnet Mn3Sn at room temperature”, Adv.Quantum Technol. 6, 2200115 (2023).

3 S. Upadhyay et al., “Molecular beam epitaxy and crystal structure of majority a-plane oriented and substrate strained Mn3Sn thin films grown directly on sapphire (0001)”, Journal of Vacuum Science and Technology A, to be published (2023).

Session Abstract Book
(329KB, Nov 2, 2023)
Time Period WeM Sessions | Abstract Timeline | Topic SS Sessions | Time Periods | Topics | AVS 69 Schedule