AVS 70 Session SS+2D+AMS-WeA: Defects Nanoarchitecture and Complex Systems

Surface chemistry is critical in elucidating the functional properties of Zeolitic Imidazolate Framework-8 (ZIF-8), a versatile material with applications in gas separation, sensing, catalysis, and lithography. Comprising zinc that is tetrahedrally coordinated with 2-methylimidazolate (2mIm) linker, ZIF-8 is synthesized through various methodologies to modulate its crystalline characteristics and integration into nanocomposites including powder, film and membranes.

Despite efforts to achieve defect-free ZIF-8 structures, deviations in structure may occur due to the presence of defects such as vacancies in 2mIm ligands, zinc vacancies, and physically adsorbed 2mIm molecules within pores, substantially impacting its performance and stability. Infrared reflection absorption (IRRA) spectroscopy is employed to probe these defects, although interpretations of IR spectra vary across studies. Our investigation employs experimental IR spectroscopy alongside first-principles molecular dynamics (FPMD) simulations to scrutinize ZIF-8's vibrational spectra, with a specific focus on defect-induced signals. X-ray Photoelectron Spectroscopy (XPS) is utilized to probe the surface composition of the material up to a depth of 10 nanometers.

We have highlighted the prevalence of defects in the synthesis of ZIF-8 films/membranes, which are critical in gas separation applications. Notably, we observed the formation of SiOx as a notable defect, stemming from exposure to silicon grease in vacuum reactors. This phenomenon is pertinent to materials synthesized under vacuum conditions, as vacuum grease residues may persist over time.

Through the resolution of conflicting interpretations of IR spectra and the identification of defect signals, our objective is to gain a comprehensive understanding to enhance the quality control and design of ZIF-8-based materials for diverse applications.

1. Introduction

The basic mechanism of the resistive gas sensing has been established; it is essentially a redox reaction of the surface mediated by the negatively charged oxygen adsorbate. For example, the sensing of ethanol (EtOH) with the ionosorbed oxygen of O^2- is written as,

EtOH(ads)+O^2-(ads) → CH₃CHO+H₂O+2e-,(1)

where (ads) denotes an adsorbate. It is reasonable to assume that the gas sensing properties differ between crystallographic atomic planes. Indeed, the crystal plane dependent gas sensing response has been studied by many research groups in the last decade for EtOH sensing by ZnO, which is one of the most intensively studied target gas – sensing material combinations. The specific knowledge of the crystal plane dependence of gas sensing is useful for the development of sensing materials.

The ZnO crystal plane dependence of the gas sensing properties has typically been studied using a nanocrystal. However, the effect resulting from the contact between the particles has hindered straightforward interpretation. To overcome this problem, the ZnO crystal plane dependence of the EtOH sensing was investigated using an analytical approach in the present study, namely low-energy ion scattering spectroscopy (LEIS) combined with the pulsed jet technique.

2. Method: LEIS combined with pulsed jet

We applied a newly developed He+ LEIS combined with the pulsed jet technique to analyze the surface structure of ZnO during the EtOH sensing. In this novel technique, the free gas jet is periodically blown onto the sample surface to simulate the gas sensing surface in a vacuum as one under the realistic working condition, while the background pressure is kept low enough for the operation of He+ LEIS.

The sample was ZnO single crystals with an atomically flat mirror-polished surface. Four dominant low-index surfaces, which are Zn-terminated (0001) (c(+)), O-terminated (0001) (c(-)), (10-10) (m), and (11-20) (a), were used to evaluate the crystal orientation dependent gas sensing properties.

References

[1] N. Saito et al., Chemical Sensors 39(2023)108. (Japanese)

[2] T. Suzuki et al., Surfaces and Interfaces 35(2022)102371.

[3] T. Suzuki et al., Appl.Surf.Sci.538(2021)148102.

All electronic materials contain a wide range of defects ranging in length scales from point defects to micrometer (or sub-millimeter) scale features. Some defects can be beneficial, benign, or detrimental to the functionality of the material. It’s critical to identify and locate defects, and determine their impact in the host material. Here, I will highlight the identification of defects and their impact in 2D graphene�based systems and wide bandgap semiconductors using photoemission electron microscopy (PEEM). PEEM is a nanoscale, surface-sensitive, full-field imaging technique based on the photoelectric effect. PEEM provides real space imaging of surfaces with enhanced contrast mechanism based on topographic and electronic properties, and measurement of electronic properties. In the first example, we image epitaxial graphene (EG) topography in real space and measure the electronic structure of monolayer EG regions with micrometer-scale angle resolved photoemission (μ-ARPES). We detect characteristic electronic features of graphene such as the Dirac points and the π-band, and the electronic flat band at region with different contrast [1]. Through Raman spectroscopy on the same regions that were analyzed by PEEM, and we estimated a significant amount of compressive strain (~1.2%) coinciding with the flat band region [1]. In the second example, we highlight the full-field PEEM capability by following the de�and re-intercalation of 2D Ag within EG under heating conditions. For the 2D Ag system, we find Ag clusters initially diffuse to the top EG surface and finally re-intercalate through defects with the Ag intercalation front to be 0.5 nm s-1 ± 0.2 nm s-1 [2]. The EG defects serve as intercalation “doors.” Lastly, we will show extended surface defects propagating through epitaxy GaN and β-Ga2O3 which also induce the presence of bandgap states.

[1] F. Niefind, H. G. Bell, T. Mai, A. W. Hight Walker, R. E. Elmquist, S. Pookpanratana, J. Appl. Phys. 131, 015303 (2022).

[2] F. Niefind, Q. Mao, N. Nayir, M. Kowalik, J. J. Ahn, A. Winchester, C. Dong, R. A. Maniyara, J. Robinson, A. van Duin, and S. Pookpanratana, Small 20, 2306554 (2024).

Silver iodide (AgI) is used as a cloud seeding material due to its ability to nucleate ice efficiently, which is explained by the good lattice match between AgI and hexagonal ice. The basal (0001) cleavage plane of AgI deviates from the lattice of hexagonal ice by as little as 2.5%. However, AgI consists of stacked planes of positively charged Ag⁺ alternating with negatively charged I^–. Cleaving a AgI crystal along the (0001) plane thus exposes Ag⁺ and I^– terminated surfaces. Both terminations are polar and inherently unstable.

We present atomically resolved noncontact atomic force microscopy (NC-AFM) images that show how AgI(0001) surfaces compensate for this non-zero electric dipole perpendicular to the surface. Both Ag and I terminated surfaces form reconstructions, whose structure affects their ice nucleating abilities. NC-AFM images of UHV cleaved surfaces exposed to water vapor reveal that ice forms an epitaxial layer only on the Ag terminated surface, whereas on the I termination ice forms three-dimensional clusters.

These atomic-level observations could enhance our understanding of ice formation processes in the atmosphere.

MnO_X-Na₂WO₄/SiO₂ catalyst exhibited notable C₂ selectivity/yield in the oxidative coupling of methane (OCM), a promised green chemistry reaction^{[1, 2]}. Nevertheless, the reaction mechanism of this catalyst remains a subject of contention, particularly regarding the role of Na₂WO₄ in the activation^{[3, 4]}. In this study, in situ characterizations of a TiO₂-modified MnO_X-Na₂WO₄/SiO₂ catalyst, are conducted by XRD and XPS correlating to the OCM reaction condition, focusing on the simultaneous phase transition of catalyst components within its activation temperature zone. The online MS along with XPS/XRD coupled activity study confirm that transition from Mn³⁺ to Mn²⁺ stands as a pivotal factor influencing the reactivity.In situ XRD further revealed that in this narrow temperature window there is a particular three-step Na₂WO₄ phase change, ending as molten salt, right before the substantial Mn³⁺ to Mn²⁺ transfer initiated. In addition, the rarely observed Na₂WO₄ behavior as molten salt is obtained by in situ XPS with rapid spectra collected during an on-stage heating process. A sensitive self-reduction of the tungsten upon heating to melting point is found. These comprehensive in situ catalyst characterizations, covering the extensive structure-activity relationship from solid state to partial molten salt condition, providing a interlocking pathway for the reactive oxygen species transfering at high temperature. The results provides new important insight into the complex MnO_X-Na₂WO₄/SiO₂ catalyst as a key to understand the activation mechanism of NaWMnSi catalyst in OCM.

[1] Lunsford JH, Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century. Catalysis today 63 (2000) 165-174;

[2] Lunsford JH The Catalytic Oxidative Coupling of Methane. Angewandte Chemie International Edition in English 34 (1995) 970-980;

[3] Si J, Zhao G, Sun W, Liu J, Guan C, Yang Y, Shi XR, Lu Y Oxidative Coupling of Methane: Examining the Inactivity of the MnOx‐Na2WO4/SiO2 Catalyst at Low Temperature. Angewandte Chemie International Edition 61 (2022) e202117201

[4] Fang X, Li S, Gu J, Yang D Preparation and characterization of W-Mn catalyst for oxidative coupling of methane. J Mol Catal 6 (1992) 255-261

Plastic particles in the ocean have become a contaminant of emerging concern due to their damage to humans and marine animals. Of all plastic production, which is increasing still, it has been shown that more than 99% of plastic waste which ends up in the oceans cannot be accounted for. The belief is that all of this plastic degrades to a nano-sized scale which is extremely hard to detect. In order to understand the fate of nanoplastics in aquatic environments, we must have a better understanding of the degradation mechanisms at an atomic and chemical level. In this project, we have developed procedures to evaporate powdered materials and deposit them onto a prepared surface. The deposition of nanoplastics is confirmed by mass spectrometry, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. Using atomic force microscopy combined with scanning tunneling microscopy, the deposited plastics are investigated.

We also investigate plastics samples after several degradation mechanisms, such as oxygenation, hydrogenation, UV exposure, and thermal annealing. After degradation, the plastics samples are studied in ultra-high vacuum with atomic force microscopy combined with scanning tunneling microscopy and with X-ray photoelectron spectroscopy.

Nickel-chromium based superalloys combine good mechanical strength with excellent resistance to corrosion over a wide range of conditions. Passivity stems primarily from a thin layer of chromium oxides and hydroxides and increasing Cr content above a threshold of 11-15wt% results in a protective passive layer. Adding minor alloying elements like Mo has an outsized impact on corrosion resistance, but there is debate in literature over the mechanism of this action. To investigate the interplay of Cr concentration and Mo alloying, the early-stage oxidation (0-65 Langmuir of O₂ ) of Ni22Cr, Ni5Cr, and Ni22Cr6Mo were studied on a clean metal surface and moderate temperatures (450-500˚C). The oxidization was studied in-situ using x-ray photoelectron microscopy (XPEEM), which yields x-ray absorption hyperspectral images at 0, 5, 20, and 65 L of exposure, and a timeseries observing the oxide evolution from 0 to 65L at a single photon energy representative of Cr₂O₃. XPEEM valence band spectra, and conventional XPS study of the same samples complement the experiment. To analyze the ~10⁷ spectra produced by hyperspectral imaging, several data dimensionality reduction techniques including principal component analysis and non-negative matrix analysis were used to gain insight into the oxide evolution, the species present, and their spatial distribution. The amounts of each species present in the hyperspectral images were quantified using cosine similarity. Oxidation of Ni22Cr produces islands of Cr₂O₃ along with a surface oxide, while on Ni5Cr larger, sparser islands are observed. For both binary alloys the oxide grows in a layer-plus-island morphology. The oxide island chemistry of Ni5Cr appears to include some NiO as well as Cr₂O₃. Ni22Cr6Mo, on the other hand, does not nucleate oxide islands visible with XPEEM but instead forms a continuous oxide layer whose thickness increases over time. This is commensurate with a layer-by-layer growth mode which is a significant advantage for protective function. This observation has implications for Mo’s protective mechanism in the passive film, suggesting that Mo may be protecting from localized breakdown by altering the morphology of the oxide to produce a more uniformly protective oxide layer.

High entropy alloys (HEAs) are a widely studied family of materials that typically contain five or more elements. There are many combinations of elements that can create HEAs, and material properties can be tuned simply by changing elemental compositions. These properties of HEAs result in numerous applications, such as catalysis, energy storage, and aerospace engineering refractory materials. Severe-lattice-distortion is identified as one of the four core effects impacting the physical properties in HEAs. In this work, we demonstrate atomic resolution images of the surface of a CrMnFeCoNi HEA (Cantor alloy) using scanning tunneling microscopy (STM). This data allows us to determine lattice local distortions unambiguously. Additionally, we report our findings on the types of local defects on the surface, such as grain boundaries, phase changes, and amorphization, and how these defects can impact local distortions of the crystals nearby.

The atomic-scale investigation of the “bilayer” reconstruction of Sn on heavily doped n-type Si(111) was performed with scanning tunneling microscopy. When cooled to 55K, the bilayer reconstruction undergoes both a structural and electronic transition. Structurally, the dimer units of the bilayer shift to form a herringbone pattern with a rhombohedral ordering. The electronic structure transitions into a very uniform square lattice with similar dimensions to the structural dimers. There are distinct differences between the structural and electronic spacing that resolves itself when counting multiple periods. This ordering is like behavior observed in materials that exhibit charge density waves. Furthermore, STS point spectra show a transition from a small gap to a large insulating gap at low temperature that is consistent with a transition to a Mott insulating ground state. The charge ordering coupled with the relaxation to a Mott insulating phase upon cooling the Sn bilayer presents unique physics when the dimensionality is reduced from the bulk.

Supramolecular nanoarchitectures have been widely explored to precisely design low-dimensional materials at atomic and molecular levels [1]. Such control is mainly based on bottom-up fabrication methods, e.g. on-surface synthesis, by combining molecular building blocks and atoms to engineer novel nanomaterials [2]. Particularly, the self-assembled monolayers (SAMs) of organic molecules present the potential for applications in nanoelectronics due to their reversible non-covalent interactions [3]. Such intermolecular interactions allow the fabrication of almost defect-free supramolecular nanostructures. Typically, the geometry of these nanoarchitectures can be controlled by the insertion of metal and non-metal atoms in the reaction process. To date, the adatom-mediated SAM fabrication concentrates on the use of metal adatoms, especially d metals (Cu, Co, Au, and Fe) [4]. Herein, by combining scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations, we report the 2D self-assembled of 1,3,5-tris[4-(pyridin-4-yl)-[1,1’-biphenyl])benzene (TPyPPB) molecules on Ag(111) in the presence of Cl adatoms. The adsorption of the TPyPPB molecules on the clean Ag(111) surface forms an almost defect-free porous SAM stabilized by hydrogen bonds, so-called triangular packing. Such packing can be explored as a host-guest material for atom/molecular confinement. However, in the presence of Cl adatoms, the molecular arrangement changes dramatically. The molecular assembly changes its geometry, forming a non-porous SAM stabilized by H···Cl···H bonds. Such halogen-mediated SAM presents the advantage that the adatom used to stabilize the nanostructure has less influence on the electronic density compared to the typical metal adatoms.

Keywords: On-surface synthesis, STM, Self-assembly monolayer, Nanoporous networks.

Acknowledgments: This work was financially supported by FAPESP (2021/04411-1), FAPESP (2022/12929-3), CNPq, and CAPES (627946/2021-00).

References

1. Fan, Q. et al. Accounts of Chemical Research 48, 2484–2494 (2015).

2. Pawlak, R. et al. Journal of the American Chemical Society 142, 12568–12573 (2020)

3. Casalini, S. et al. Chemical Society Reviews 46, 40–71 (2017)

4. Shi, Z. et al. Journal of the American Chemical Society 133, 6150–6153 (2011)