AVS2015 Session SS+AS+NS-WeM: Metals, Alloys & Oxides: Reactivity and Catalysis

Self-assembly of functional molecular building blocks on well-defined surfaces is a promising approach for the bottom-up fabrication of two-dimensional nanostructures with outstanding properties. In this respect, porphyrins are particularly attractive because of their distinct chemical and physical properties.^1,2

In this presentation, we investigate and compare the adsorption behaviour of 2H-5,10,15,20-Tetrakis-(3,5-di-tert-butyl)-phenylporphyrin (2HTTBPP) on different substrates, i.e., Cu(111),^3-5 Cu(110) and Cu(110)-(2X1)O, under ultra-high vacuum conditions by scanning tunneling microscopy (STM). At room temperature, supramolecular arrangements of 2HTTBPP are observed on Cu(111) and Cu(110)-(2X1)O, while on Cu(110) individual 2HTTBPP molecules are observed in a disorderd layer. Interestingly, the intramolecular conformations of the molecules are quite different on the investigated substrates, as determined by STM.⁴ The corresponding findings are interpreted by accounting for specific molecule-molecule and molecule-substrate interactions. In addition, the so-called self-metalation of 2HTTBPP with Cu atoms will be reported and discussed.^4,5

References:

1. Auwärter, et al., Nat. Chem., 2015, 7, 105.

2. Gottfried, Surf. Sci. Rep., 2015, doi:10.1016/j.surfrep.2015.04.001.

3. Ditze et al., J. Am. Chem. Soc., 2014, 136, 1609.

4. Stark et al., Chem. Commun., 2014, 50, 10225.

5. Marbach et al., Chem. Commun., 2014, 50, 9034.

Bismuth (Bi) takes the rhombohedral (A7) crystalline structure in bulk. However, it also takes the black phosphorous (BP) structure in ultrathin films. Theoretically, a few bilayer thick Bi(110) films with the BP structure were predicted to be a nontrivial two-dimensional topological insulator by removing the buckling at the surfaces [1]. In the meantime, ultrathin Bi(110) films with the A7 structure was suggested to realize the Dirac electron system at the surfaces [2]. From these viewpoints, the growth, structure, and electronic states of the Bi(110) ultra-thin films are of great interest. In this study, we investigated the details of the atomic arrangements and electronic states at the Bi(110) islands on the Si(111)√3 × √3-B substrate experimentally using scanning tunneling microscope (STM) [3] and angle-resolved photoelectron spectroscopy (ARPES) in a synchrotron radiation facility.

In the study, we found that atomically flat, long, narrow Bi(110) islands grew along specific orientations on the Si(111) )√3 × √3-B substrate. The orientations belonged to one of the two sets of three-fold rotational axes, which differed by 26° each other. The preference of the specific orientations were reasonably attributed to the commensuration of the diagonal of the rectangular Bi(110) lattice to the √3 × √3 substrate unit cell. The islands grew as to make their edges parallel to the short side of the Bi(110) rectangular unit cell. The combination of the Bi domains of different orientations caused various types of boundaries on the wide terraces of the Bi(110) islands. In particular, the domains along ±87° from the {1-10} direction were found to be connected perfectly on the atomic scale at the straight boundary by inserting a local switching of the bond direction to the zigzag chains of the in-plane bonds. On the Si(111) )√3 × √3-B substrate, both the odd layer thick A7 and even layer thick BP Bi(110) islands appeared. The dispersive surface bands and their Fermi surface mapping with characteristic electron and hole pockets were observed in ARPES spectra. Details will be reported in the presentation.

[1] Y. Lu, W. Xu, M. Zeng, G. Yao, L. Shen, M. Yang, Z. Luo, F. Pan, K. Wu, T. Das, P. He, J. Jiang, J. Martin, Y. P. Feng, H. Lin, X. Wang, Nano Lett. 15, 80 (2015).

[2] G. Bian, X. Wang, T. Miller, T. -C. Chiang, P. J. Kowalczyk, O. Mahapatra, S. A. Brown, Phys. Rev. B 90, 195409 (2014).

[3] I. Kokubo, Y. Yoshiike, K. Nakatsuji, H. Hirayama, Phys. Rev. B 91, 075429 (2015).

Using scanning tunneling microscopy (STM), we have characterized the surface of clean Ge(111) dosed with 0.66 to 2.0 monolayers (ML) of Ir and then annealed to temperatures between 550 K and 800 K. We observed a broad range of surface formations, including Ir adatom clusters and various stages of island formation. Islands with winding, wormy shapes formed around 580K. As the annealing temperature increased above 650K, round islands formed. In addition, a new type of growth is observed in which the Ir gathers along the antiphase domain boundaries between competing surface domains of the Ge surface reconstruction; this gives the appearance of the Ir forming pathways interconnecting different Ir islands. The low energy electron diffraction (LEED) pattern for this surface shows domains with (√3 x √3)R30° reconstruction and becomes sharper as the temperature is increased. In the STM images, the Ge top layer reconstruction, the Ir adatom clusters, the pathways, and the Ir round islands all have √3-spacing between features. X-ray photoemission spectroscopy (XPS) was used to determine that the IR coverage was ~ 2.0 monolayers when low energy electron microscopy (LEEM) images showed completion of 1 overlayer of Ir grown on Ge(111) at 600C. We present a model consistent with our XPS and LEEM data that suggests that each Ir adatom cluster observed in STM images corresponds to three Ir adatoms. To model the surface-adsorption processes for the Ir/Ge(111) system, we used simple Monte Carlo simulations with pair-wise surface potentials and random walks of atoms to imitate surface diffusion. Particular parameter choices yielded growth along pathways between domain boundaries of the substrate, in agreement with the experimental data.

Although steps have been made to decrease toxic gas emissions globally, these emissions persistently cause detrimental health effects worldwide. Current household gas sensors are limited in their abilities to detect sensitively and selectively at or below relevant toxicity levels for many gases. Tin(IV) oxide (SnO₂) nanomaterials are well-equipped to address some of these limitations as a result of dual valency (Sn²⁺ and Sn⁴⁺) and high surface area, thus creating diverse surface chemistry. These properties are advantageous for gas sensing devices because SnO₂ functions as a sensor via gas-surface interactions, facilitated by adsorbed oxygen species. By measuring changes in resistance upon gas exposure, sensitivity and selectivity are observed. To increase sensitivity through maximizing gas surface interactions, chemical vapor deposition-grown SnO₂ nanowires and commercial nanoparticles were treated with an Ar/O₂ and H₂O_(v) plasma resulting in increased oxygen adsorption. Surface and bulk characterization throughout the plasma treatment process demonstrate an increase in adsorbed oxygen content over a 30 - 150 W applied power range regardless of plasma precursor, in addition to showing that tin reduction occurs upon H₂O_(v) plasma treatment. Gas sensing performance was initially explored by exposing SnO₂ sensors to air at temperatures of 25-300° C to determine base resistance of the materials in an ambient atmosphere. The data show changes in resistance that are dependent upon nanomaterial architecture, plasma treatment conditions, and sensor temperature. Base resistance changes for specific plasma and sensor conditions will be discussed as well as sensor responses and selectivity upon exposure to toxic gases including benzene and carbon monoxide. By combining materials characterization with gas sensor responses, we can optimize sensor sensitivity and selectivity by tuning plasma modification conditions with aims for targeted gas sensing applications.

Ultrathin layers of ZnO grown on the Au(111) substrate have been characterized using low-temperature scanning tunneling microscopy (STM). Under reactive deposition condition, the ZnO layers have been found to grow by forming islands. Detailed analysis of electronic structure have been conducted using scanning tunneling spectroscopy (STS) and the density functional theory (DFT) calculations. The band gap of ultrathin layers of ZnO is found to be larger compared to the bulk ZnO wurtzite structure. The density functional theory calculations provides understanding of the increased band gaps of thin ZnO layers. The lattice constants of ultrathin ZnO layers on Au(111) are also found to be larger than that of the bulk lattice constant, which could be explained by the formation of graphitic-like ZnO layers.

The adsorption of water onto metal surfaces yields a host of intricate adsorbate structures at

coverages less than a single layer. Complex adsorption structures arise due to the delicate balance

of hydrogen bonding between water molecules as well as attractive forces between water molecules

and the metal surface. Therefore, the complexity of the system provides an excellent opportunity to

refine models of water-water and water-surface interactions. Water molecules are largely confined

to a single plane and, as a result, the geometry of aligned water molecules gives rise to frustrated

hydrogen bonding. At low coverages of water on metal surfaces, these frustrated intermolecular

interactions result in the formation of structures other than the classic hexagons of bulk water. We

will report the results of a combined theory-experiment study of water adsorption on planar Pt(111)

and stepped Pt(553). Experimentally, temperature programmed desorption (TPD) and ultra-high

vacuum scanning tunneling microscopy (UHV-STM) were used to quantify water coverage and to

image the resultant surface structures. On Pt(111), 5, 6, and 7-membered rings were found to form

across the Pt surface, in agreement with previously reported experimental results and electronic

structure calculations. On Pt(553), however, tetragonal structures that have not been previously

observed were found to form across monatomic steps. These observations confirm DFT

calculations for submonolayer water coverage on Pt(553) and provide fine details as to how water-

water and water-surface interactions are balanced on active metal surfaces.