Self-organizing nanodot structures on InP surfaces evolving under low-energy ion irradiation: analysis of morphology and composition

Surfaces of InP were bombarded by 1.9 keV Ar(+) ions under normal incidence. The total accumulated ion fluence Φ the samples were exposed to was varied from 1 × 10(17) cm(−2) to 3 × 10(18) cm(−2), and ion fluxes f of (0.4 − 2) × 10(14) cm(−2) s(−1) were used. The surface morphology resulting from these ion irradiations was examined by atomic force microscopy (AFM). Generally, nanodot structures are formed on the surface; their dimensions (diameter, height and separation), however, were found to depend critically on the specific bombardment conditions. As a function of ion fluence, the mean radius r, height h, and spacing l of the dots can be fitted by power-law dependences: r ∝ Φ(0.40), h ∝ Φ(0.48), and l ∝ Φ(0.19). In terms of ion flux, there appears to exist a distinct threshold: below f ~ (1.3 ± 0.2) × 10(14) cm(−2) s(−1), no ordering of the dots exists and their size is comparatively small; above that value of f, the height and radius of the dots becomes substantially larger (h ~ 40 nm and r ~ 50 nm). This finding possibly indicates that surface diffusion processes could be important. In order to determine possible local compositional changes in these nanostructures induced by ion impact, selected samples were prepared for atom probe tomography (APT). The results indicate that APT can provide analytical information on the composition of individual InP nanodots. By means of 3D APT data, the surface region of such nanodots evolving under ion bombardment could be examined with atomic spatial resolution. At the InP surface, the values of the In/P concentration ratio are distinctly higher over a distance of approximately 1 nm and amount to 1.3 to 1.7

Science learning environment—outside-class experience: design, evaluation and challenge

Traditional university science (physics) teaching is based on the lecture-laboratory/tutorial delivery scheme. Also, this type of training is usually in the form of puzzle-solving strategy on a particular class of problems which comprises successful aspects of a topic/discipline (textbook-science). In addition to that the segmentation of teaching where the knowledge transferred has been broken up into separate courses (topics) increases tendency to omit as much as possible of the material that does not fit exactly the course objectives. As a result, some important in teaching and practice of science topics falls between and are not presented at all. This is especially true for issues related to contemporary, cutting-edge science. Science and science-oriented students enrolled in two introductory physics courses presented by the Discipline of Physics at the University of Newcastle were/are exposed to a number of diverse topics (fundamental physics) with little time for expanding and overlapping their course-based knowledge with more and more multidisciplinary science environment. In order to encourage students to test and expand their knowledge a two-A4-page format bulletin was designed and widely distributed among about 450 students in these two level-one introductory physics courses (Figure 1). Qualitative evidence regarding student’s perceived value of the bulletin shows that the proposed design was as a successful and valuable learning experience for a vast majority of the first year students

On the Notion of Conceptual Learning in Undergraduate Physics

Methods for improving the quality of student learning continues to be central to research and practice in teaching and learning in tertiary education. Extensive studies have shown that student learning approaches influence their learning conception and outcomes. In particular, it has been shown that learning as simple reproduction of knowledge leads to surface learning approaches, while learning as transformation of knowledge leads to deep learning approaches [1,2]. An important factor in the way students learn appears to be the way knowledge is structured when presented. In this paper we illustrate how concept maps of the hierarchical organisation of cognitive structure may benefit the preparation, presentation and assessment of course material. The results of a comparison between the concepts presented with those actually assessed in a one semester first year university, introductory physics course show a large imbalance. Concept maps may be effectively used to improve the planning and structure of all components of course presentation in order to more realistically match assessment tasks. Concept maps also help develop relationships and perspectives related to the physics ideas by the students. We illustrate how the maps were used to organize the discussed concepts, and how the new concepts could be incorporated into the map so that the new knowledge becomes an integral part of the existing structure

The role of biotic factors during plant establishment in novel communities assessed with an agent-based simulation model

Background Establishment success of non-native species is not only influenced by environmental conditions, but also by interactions with local competitors and enemies. The magnitude of these biotic interactions is mediated by species traits that reflect competitive strength or defence mechanisms. Our aim was to investigate the importance of species traits for successful establishment of non-native species in a native community exhibiting biotic resistance in the form of competition and herbivory. Methods We developed a trait-based, individual-based simulation model tracking the survival of non-native plants in a native community. In the model, non-native plants are characterized by high or low values of competition and defence traits. Model scenarios included variation of initial number of non-natives, intensity of competitive interaction, density of herbivores and density as well as mixture of the native community. Results Traits related to competition had a much greater impact on survival of non-native species than traits related to defence. Survival rates of strong competitors never fell below 50% while survival of weak competitors averaged at about 10%. Weak competitors were also much more susceptible to competitive pressures such as community density, composition and competition intensity. Strong competitors responded negatively to changes in competition intensity, but hardly to composition or density of the native community. High initial numbers of non-native individuals decreased survival rate of strong competitors, but increased the survival rate of weak competitors. Survival under herbivore attack was only slightly higher for plants with high defensive ability than for those with low defensive ability. Surprisingly, though, herbivory increased survival of species classified as weak competitors. Discussion High survival rates of strong non-native competitors relate to a higher probability of successful establishment than for weak competitors. However, the reduced survival of strong competitors at high initial numbers indicates a self-thinning effect, probably mediated by a strongly competitive milieu. For weak competitors, our model emphasizes positive effects of high propagule pressure known from field studies. General effects of herbivory or defence abilities on survival were not supported by our model. However, the positive effect of herbivory on survival of weak competitors indicated side effects of herbivory, such as weakening resident competitors. This might play an important role for establishment of non-natives in a new community

Mixing knowledge, attitude and experience–teamwork approach in the 1st year undergraduate physics laboratory

We report on a new laboratory program implemented in the first year, lower-level service physics course at the University of Newcastle in 2005. It is shown that the new laboratory environment has resulted in very high overall students’ satisfaction for the new program and positively influenced the way they look at their laboratory-based learning experience. In particular it is shown that their study approaches have been influenced in a positive manner, with students acknowledging that the new laboratory structure has developed their teamwork skills and helped them to better understand the lecture material and the underlying physical principles

Structure, Stability, and (Non)Reactivity of the Low-Index Surfaces of Crystalline B2O3−I

Diboron trioxide (B2O 3) assumes critical importance as an effective oxidation inhibitor in prominent chemical applications. For instance, it has been extensively used in electrolysis and ceramic/glass technology. Results are presented of accurate quantum mechanical calculations using the PW1PW hybrid HF/DFT functional of four low- index surfaces of the low-pressure phase of B2O : (101), (100), (011), and (001). Bond lengths, bond angles, and net Mulliken charges of the surface atoms are analyzed in detail. Total and projected density of states as well as surface energies are discussed. The occurrence of tetrahedral BO 4 units on the lowest energy structures of two of these surfaces has been demonstrated for the first time. The corresponding surface orientations incur larger energies in reference to the two orientations featuring only BO3 units. All of the four investigated lowest energy structures have no dangling bonds, which reasonably relates to the experimentally observed low reactivity of this compound. Findings in this paper pave the way for potential interest in the perspective of future studies on the surfaces of amorphous B2O3, as well as on the hydroxylation of both crystalline and amorphous B2O3

STM and DFT study on formation and characterization of Ba-incorporated phases on a Ge(001) surface

We characterize the incorporation of Ba adatoms into the Ge(001) surface, resulting in the formation of one-dimensional structures with an internal 2×3 periodicity, after the deposition of Ba atoms at 970 K or at room temperature followed by a 770 K anneal. Scanning tunneling microscopy (STM) data were compared with theoretically simulated STM images generated by density functional theory electronic structure calculations. Excellent agreement between experiment and simulation was found when using an adopted structural model that assumes partial removal of the surface Ge dimers in the [1–10] surface direction and subsequent addition of a single Ba atom to the substrate second layer. Structural assignments for a number of defects observed within regions of the 2×3 reconstruction were also obtained