AVS 69 Session SS-ThP: Surface Science Poster Session

Pulse deposition, a novel sample deposition technique for scanning tunneling microscopy (STM), sprays molecules onto a surface through a process that forces non-equilibrium, high-energy structures. Electrospray ionization mass spectrometry (ESI-MS) is a “soft” ionization method; molecules of interest do not fully fragment, but instead can be observed whole, as can dimers, trimers, and other supramolecular structures, which is often seen as a limitation. However, this clustering of molecules allows for a “screening” of sorts for pulse-deposited STM experiments, which are significantly more time-intensive. ESI-MS and pulse deposition have similar sample delivery methods, and thus can be compared to each other and used to study molecules in tandem. For example, melamine and cyanuric acid together form highly toxic complexes in the body, but the precise crystallization and intermolecular forces that drive the creation of the toxic clusters is unknown. Using the ESI-MS as a precursor to investigation with pulse deposited STM imaging, we observed several stable noncovalent clusters of melamine and cyanuric acid, ranging from mixed dimers to mixed nonamers (four melamines and five cyanuric acids). In this presentation, I will discuss the results of our ESI-MS experiments on melamine and cyanuric acid as well as comment on STM experiments of different solution ratios and how ESI-MS and STM results can be jointly interpreted.

Magnetite (Fe₃O₄) is an attractive catalyst for the water-gas shift reaction (WGSR), owing to its low cost and minimal environmental impact. However, the reaction path remains unclear despite extensive experimental and theoretical studies. This is partly due to the difficulty in conducting experiments and the complexity of analyzing data on co-adsorbed systems, which has resulted in a lack of clarity in the overall picture.

To clarify the mechanism of the WGSR on magnetite, we performed scanning tunneling microscopy (STM) to observe CO exposed, H₂O adsorbed, and H₂O-CO co-adsorbed Fe₃O₄(111) surfaces. Our results confirmed that CO molecules do not adsorb on the bare surface at room temperature, agreeing with the previous study based on temperature programmed desorption[1]. Two types of adsorbate species appeared on the surface after the exposure to H₂O, which we attribute to dissociated species, OH, and molecular water. We also found a larger adsorbate on the H₂O-CO co-adsorbed surface, although it is unclear if this species is an intermediate of the WGSR due to the complexity of the STM image. We are currently developing image analysis methods to identify and classify the adsorbed species. Our study provides a new insight into the atomistic mechanism of the WGSR on Fe₃O₄(111).

[1]C. Lemire et al., Surf. Sci.572, 103(2004).

Graphene, a two-dimensional (2D) carbon crystal with sp² hybridization, has attracted much attention in recent years due to its novel chemical, electrical, and mechanical properties. Even materials that do not form cluster superlattices upon room temperature deposition may be grown into such by low-temperature deposition. A graphene monolayer was prepared on an Ir (111) single crystal, ethylene (C₂H₄) is pyrolytically cleavaged on the surface. The resulting superstructure is examined using scanning tunneling microscopy (STM) and was identified as a well-aligned, incommensurate pattern known as moiré. This moiré pattern arises from overlapping the graphene lattice and the Ir (111) lattice, resulting in alternating bright and dark regions.

The moiré patterns in graphene act as nucleation sites for the growth of plasmonic metal nanoclusters. When Ag/Au metal is deposited on graphene, it nucleates at these sites to form nanoclusters on the surface. Through STM, we have examined the nucleation and growth of these nanoclusters, studying their shape, organization, and structural evolution. Additionally, we have also investigated the stability of these nanoclusters at different temperatures.

Zirconium diboride ZrB₂ is an extremely hard material with a high melting point of 3246 °C; given these properties, ZrB₂ can be used for various applications, such as high-resistant coatings for body armors and tanks. In addition, it has also been explored as a diffusion barrier in microelectronics. Industrially, highly conformal thin films of ZrB₂ are grown via chemical vapor deposition (CVD), using zirconium borohydride Zr(BH₄)₄ as a precursor. While surface chemistry plays a central role in the CVD of ZrB₂ from the Zr(BH₄)₄ precursor, the surface mechanism is yet to be explored. In this study, we investigated the surface mechanism of Zr(BH₄)₄ decomposition on a ZrB₂(0001) surface with reflection absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The RAIRS spectra obtained on exposing the ZrB₂(0001) surface at 90K to Zr(BH₄)₄(g)closely matched that of the pure compound, indicating adsorption of Zr(BH₄)₄ without decomposition. However, new surface intermediates were formed upon heating to 280 K, as shown by the retention of the vB-Ht stretch (2569 cm^-1) and δH-B-H bend (1228 & 1057 cm^-1) in the RAIRS spectra. These surface intermediates were tentatively identified as either BH₄ or BH₃ and were found stable up to 330 K. Temperature-programmed desorption studies revealed the desorption of B₂H₆ and H₂ at around 470 K.

Heterojunctions are widely used as an essential building block in advanced semiconductor devices because of their multiple functionalities. The electrical and optical properties of heterojunctions are strongly governed by their electronic band alignment.

High-resolution X-ray photoemission spectroscopy (HR-XPS) is proven to be a powerful way of measuring the valence band offsets in semiconductor heterojunctions. This study aims to determine the band alignment to different semiconductor heterojunctions by direct X-ray photoelectron spectroscopy (XPS) measurements. Type-I and Type-II band alignment were obtained.

Design of heterojunction based electronic/photonic devices requires an accurate determination of the band offset. This parameter is crucial to tailor heterojunction based devices as per the operating requirement.

Probing the effect of the local chemical environment of surface nanostructures is a challenge because the spatial resolution of conventional spectroscopic techniques is limited by the diffraction limit of light. Coupling light with plasmonic nano-objects creates highly localized surface plasmons (LSPs), which allows us to break this limit. Tip-enhanced Raman spectroscopy (TERS) is one such surface spectroscopic technique that uses the apex of the tip of a scanning tunneling microscope made from a plasmonic metal as a nano object to couple light to the near field. The Raman modes of the nanostructure underneath this tip are greatly enhanced which allows us to obtain chemical information with Angstrom scale spatial resolution. Thus, TERS can probe intermolecular interactions, molecule-substrate interactions, organic-2D material heterostructures, and the reactivity of 2-D materials to reveal how the local chemical environment affects the chemical and physical interactions of a molecule or a nanostructure on a surface. Apart from probing the local environment, the highly localized nature of LSPs can be used to drive energy-intensive and unselective thermally activated chemical reactions using light with a lower energy input and a site-selective manner. The controllability of LSPRs was demonstrated by dissociating a single bond inside a molecule in the presence of multiple equivalent bonds. The insights obtained into the local environment enable precise control of self-assemblies, on-surface reactions, and LSPRs and expand the ability to synthesize nanostructures with tailored electronic, optical, and magnetic properties required for next-generation nanodevices.

Many important processes such as energy conversion, electrochemical, corrosion, and biological processes take place at solid-gas and solid-liquid interfaces [1-3]. The molecular beam epitaxy (MBE) method is the one of powerful techniques for creating new nanomaterials and it is the key to improving the performance of novel battery generation or renewable energy sources such as solar, wind, or hydropower energy conversion devices. We would like to promote an original, modular ultra-high vacuum system for the fabrication of heterostructed nanomaterials by the molecular beam epitaxy (MBE) method. Its basic element is a vacuum installation, which consists of a sample loading chamber, a substrate preparation (cleaning) chamber, and a proper MBE chamber for the deposition of selected nanomaterials. All vacuum chambers are connected by appropriate vacuum locks and magnetic sample transfers between the above-mentioned chambers, which enables the implementation of all technological and research works without contact of deposited nanomaterials with the atmospheric environment. Vacuum conditions in all of the above-mentioned vacuum chambers are created using independent systems of various types of vacuum pumps. For the control of the substrate cleaning process, the deposition of nano-layers of selected electronic materials, and their initial characterization, the above-mentioned vacuum installation is equipped with electronic control systems and measurement data acquisition systems. Correct operation of the designed and the completed installation has been verified on the example of the deposition of Mg nano-layers on the Si substrate. The conducted technological works and preliminary research works as well as the obtained results confirmed that the designed and assembled modular vacuum system can be very useful for the deposition of nanolayers of selected electronic materials using the MBE method, on the one hand on terms of their potential research applications, and on the other - in terms of their potential industrial applications, incl. for the production of photovoltaic renewable energy sources.

References:

[1]S. Choudhury et al. C 2021, 7, 28.

[2]A. Asyuda et al. Phys. Chem. Chem. Phys., 2020,22, 10957-10967.

[3]H. Aldahhak et al. . J. Phys. Chem. C 2020, 124, 11, 6090–6102.

Transition group metals are used as catalysts in various oxidation reactions. A common example is silver which found industrial usage in the epoxidation of ethylene to the ethylene oxide. To optimize such processes it is required to understand the dynamics and microscopic details.
In this talk, the velocity and angular distribution of NO₂ and recombinatively-desorbing oxygen from Ag(111) will be presented. Experimentally, we combined velocity-map imaging (VMI) and temperature-programmed desorption (TPD). NO₂ decomposes into NO and O after adsorption on silver. At 510 K, a clean p(4 × 4)-O reconstruction with a maximum oxygen coverage of θ_O = 0.375 ML forms on Ag(111).¹ The reaction probability S of NO₂ decomposition was studied at this temperature as a function oxygen coverage θ_O. S is coverage-independent up to θ_O = 0.3 ML which is a clear indication for a precursor-mediated mechanism.
In TPD spectra, the recombinative desorption of O atoms occurs as a defined O₂ desorption feature around 600 K. In contrast to NO₂ whose desorption appears thermal, these oxygen molecules exhibit a clearly hyper-thermal velocity distribution; ⟨v⟩ is >200 m ⋅ s⁻¹ above a flux-weighted, thermal velocity distribution. Compared to Rh(111),² we observe a significantly narrower cos⁸(θ) angular distribution for the flux density of O₂ and the velocity distribution differs stronger from a thermal one.
Finally, first results for the epoxidation of styrene to styrene oxide will be shown. We observe that the epoxide forms only at high oxygen coverage.

[1] A. Michaelides, K. Reuter, and M. Scheffler, J. Vac. Sci. Technol. A23, 1487 (2005).
[2] A.C. Dorst, F. Güthoff, D. Schauermann, A.M. Wodtke, D.R. Killelea, and T. Schäfer, Phys. Chem. Chem. Phys. 24, 26421 (2022).

With the use of electrospray ionization mass spectroscopy (ESI-MS) we are able to observe clusters of cyanuric acid and melamine in solution. The clusters consist of homogenous and heterogenous mixtures with varying ratios of molecules. Several of these clusters are observed forming in high concentrations. Using density functional theory (DFT), calculations are run to model possible cluster structures. Both homogenous and heterogenous clusters arrange in a lattice structure. When there are clusters with disproportionality more cyanuric acid or melamine the molecular structure has variations leading to the molecule not lying flat. Larger clusters show increased stability when compared to smaller clusters.

The supramolecular assembly on metal surfaces is governed by a subtle balance between intermolecular interaction and molecule-substrate interaction. Even on chemically similar metals, such as Cu, Ag, and Au, the deposition of the same molecule under the same condition may result in different types of assembled structures. We have observed such a case with a molecule called diarylethene, famous for its photochromism and expected to work as single molecule switching device.

After depositing molecular powder of the open-form isomer on metal surfaces at room temperature, scanning tunneling microscopy (STM) was performed in ultra-high vacuum at liquid helium temperature. Our STM images revealed the presence of both the open-form and closed-form isomers on all three metals. This is due to a stability reversal on metals compared to the gas phase, arising from charge transfer between molecules and metallic substrates[1]. Isolated adsorption was predominant on Cu(111) even after annealing. Co-deposition of NaCl was necessary to form a homogeneous monolayer, which was achieved via ion-dipole interaction[2]. On Au(111), closed-form isomers formed small clusters, such as trimers and tetramers, while the open-from remained isolated. In contrast, larger clusters made of both the open- and closed-from isomers were found on Ag(111), including chain-type clusters and three-fold symmetric chiral clusters. Mild annealing transformed all the open-form isomers to the closed-form isomers, and only three-fold symmetric clusters made of nine or more closed-form isomers were observed. The clusters found on Ag(111) have three-dimensional structures, suggesting local and weak intermolecular interaction. Theoretical analysis is underway to clarify the exact structures and interactions involved.

References

[1] T. K. Shimizu, et al., Chem. Commun. 49, 8710-8712 (2013).

[2] T. K. Shimizu, et al., Angew. Chem. Int. Ed. 53, 13729-13733 (2014).

High entropy materials, including high entropy alloys (HEAs), high entropy Van der Waals materials (HEX), and high entropy oxides (HEOs), have drawn the attention of scientists and engineers for their various functionalities and properties. While a wide variety of properties are being studied in these materials, a microscopic understanding is still missing. In this work, the spatial resolving power of scanning tunneling microscopy (STM) is combined with the elemental resolving X-ray absorption spectroscopy (XAS) to achieve this goal. With the unique X-ray assisted tunneling effect, the elemental distributions on the surface of a HEA at the sub-nm scale were revealed by a synchrotron X-ray scanning tunneling microscope (SX-STM). The elemental distribution at sub-nm scale was revealed by maximizing the correlation coefficient between the collected XAS mappings and the atomic scale elemental modeling. The results shown here demonstrate that SX-STM is a promising tool to reveal elements at the sub-nm scale, even for high entropy materials.