Atomic Arrangement in Two-Dimensional Silica: From Crystalline to Vitreous Structures

The atomic structure of vitreous and crystalline regions of a thin silica film on Ru(0001) was investigated using noncontact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM). We were able to resolve the atomic arrangement of the Si and the O atoms in the crystalline and the vitreous structures. We discuss characteristic structural properties of the films, such as distances, orientations, and angles, and we compare our results to experiments and simulations of bulk vitreous silica networks. It was found that order in two-dimensional vitreous networks can extend up to 2 nm

Characterizing Crystalline-Vitreous Structures: From Atomically Resolved Silica to Macroscopic Bubble Rafts

A two-part experiment using bubble rafts to analyze amorphous structures is presented. In the first part, the distinctions between crystalline and vitreous structures are examined. In the second part, the interface between crystalline and amorphous regions is considered. Bubble rafts are easy to produce and provide excellent analogy to recent research results on the atomic structure of silica glass. Ring statistics are employed to characterize the 2D structures and results from student bubble raft data are compared to results from atomically resolved images of amorphous 2D silica; the bubble rafts demonstrate good qualitative agreement. In these experiments, students learn how to characterize crystalline and amorphous materials and are introduced to current research results and analysis techniques for amorphous material structures

Characterizing Crystalline-Vitreous Structures: From Atomically Resolved Silica to Macroscopic Bubble Rafts

A two-part experiment using bubble rafts to analyze amorphous structures is presented. In the first part, the distinctions between crystalline and vitreous structures are examined. In the second part, the interface between crystalline and amorphous regions is considered. Bubble rafts are easy to produce and provide excellent analogy to recent research results on the atomic structure of silica glass. Ring statistics are employed to characterize the 2D structures and results from student bubble raft data are compared to results from atomically resolved images of amorphous 2D silica; the bubble rafts demonstrate good qualitative agreement. In these experiments, students learn how to characterize crystalline and amorphous materials and are introduced to current research results and analysis techniques for amorphous material structures

Reaction of CO with Preadsorbed Oxygen on Low-Index Copper Surfaces: An Ambient Pressure X‑ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study

The reaction of CO with chemisorbed oxygen on three low-index faces of copper was studied using ambient pressure X-ray photoelectron spectroscopy (XPS) and high-pressure scanning tunneling microscopy. At room temperature, the chemisorbed oxide can be removed by reaction with gas-phase CO in the 0.01–0.20 Torr pressure range. The reaction rates were determined by measuring the XPS peak intensities of O and CO as a function of time, pressure, and temperature. On Cu(111) the rate was found to be one order of magnitude faster than that on Cu(100) and two orders of magnitude faster than that on Cu(110). The apparent activation energies for CO oxidation were measured as 0.24 eV for O/Cu(111), 0.29 eV for O/Cu(100), and 0.51 eV for O/Cu(110) in the temperature range between 298 and 473 K. These energies are correlated to the oxygen binding energies on each surface

Charge Percolation Pathways Guided by Defects in Quantum Dot Solids

Charge hopping and percolation in quantum dot (QD) solids has been widely studied, but the microscopic nature of the percolation process is not understood or determined. Here we present the first imaging of the charge percolation pathways in two-dimensional PbS QD arrays using Kelvin probe force microscopy (KPFM). We show that under dark conditions electrons percolate via in-gap states (IGS) instead of the conduction band, while holes percolate via valence band states. This novel transport behavior is explained by the electronic structure and energy level alignment of the individual QDs, which was measured by scanning tunneling spectroscopy (STS). Chemical treatments with hydrazine can remove the IGS, resulting in an intrinsic defect-free semiconductor, as revealed by STS and surface potential spectroscopy. The control over IGS can guide the design of novel electronic devices with impurity conduction, and photodiodes with controlled doping

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO

We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C₂H₄ in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C₂H₄ partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidynea strong adsorbate formed from ethylenethat does not form on the 4-fold (100)-step sites on Pt(557)