AVS2012 Session NS-ThM: Nanoscale Imaging and Microscopy

With the advent of scanning probe microscopy techniques that involve a tip and a sample in relative motion in the contact or non-contact regime, the microscopic aspects of friction have become a major branch of research called nanotribology. A significant number of recent studies in this field have concentrated on the distinction between electronic and phononic contributions to friction. For the present study, we have used the combination of spin-polarized scanning tunneling microscopy [1] and single-atom manipulation in order to move individual magnetic atoms over a magnetic template [2]. By monitoring the spin-resolved manipulation traces and comparing them with results of Monte-Carlo simulations, we have been able to reveal the characteristic friction force variations resulting from the occurrence of spin friction on the atomic scale [3].

[1] R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009) .

[2] D. Serrate, P. Ferriani, Y. Yoshida, S.-W. Hla, M. Menzel, K. von Bergmann, S. Heinze,

A. Kubetzka, and R. Wiesendanger, Nature Nanotechnology 5, 350 (2010).

[3] B. Wolter, Y. Yoshida, A. Kubetzka, S.-W. Hla, K. von Bergmann, and R. Wiesendanger,

Phys. Rev. Lett. (2012), in press.

Geologic sequestration of CO2 has become an emerging enterprise for reduction of greenhouse gas emissions. Because CO2 will be injected and stored in host rock at depths >800m, lithostatic pressure will cause the CO2 to remain in supercritical fluid state. Knowledge of mineral-fluid chemical transformation rates at geologically relevant temperatures and pressures is expected to be an important aspect of predicting reservoir stability. Many mechanisms of mineral transformation reactions where scCO2 is the dominant phase and water availability is low have so far remained unstudied. We have developed an atomic force microscope capable of observing in-situ mineral transformations under supercritical conditions (i.e., >72.8 atm and >304K) in real time.

Observations of the disappearance of a 1.5nm layer on the surface calcite is evident in anhydrous scCO2 are consistent with the dehydration of a hydrated calcium carbonate layer and is consistent with measurements from piezoelectric force microscopy. We have also followed the formation of a water film on the surface of geologically more relevant forsterite, which is deemed to be essential in the transformation of this silicate mineral into a carbonate, and have related film thickness to water content in the scCO2.

Vitreous silica is the basis of traditional glasses. Furthermore, it is the prototype oxide network former. Hence, it has been extensively investigated by diffraction methods, like X-ray and neutron scattering. Therefore, it is a great surprise how little is known about its atomic structure. It is impossible to directly extract ring statistics or local ring environments from diffraction. Theoretical structural models have been correlated to the diffraction data with reasonable agreement. Nevertheless, such agreement can never be unambiguously elucidate the structure of an amorphous material.

Modern imaging techniques render the investigation of two dimensional (2D) glass systems possible. Recently, we reported on the atomic structure of a thin metal-supported vitreous silica film on Ru(0001) using scanning tunneling microscopy (STM) [1]. For the first time, it was possible to verify Zachariasen's continuous random network [2] in real space. The existence of a 2D silica glass on a graphene support has been shown by scanning transmission electron microscopy [3] suggesting that further 2D glass systems may be prepared. The investigation of 2D glass models provides the unique possibility to study unexplored properties of amorphous materials.

Herein we report on the first non-contact atomic force microscopy (nc-AFM) images of a 2D silica glass grown on Ru(0001). We will present a thorough statistical analysis and a comparison to diffraction data of 3D silica glass as well as to theoretical models, hereby showing that a 2D thin film of vitreous silica can act as a model system for the amorphous 3D network.

[1] L. Lichtenstein, C. Büchner, B. Yang, S. Shaikhutdinov, M. Heyde, M. Sierka, R. W łodarczyk, J. Sauer, and H.-J. Freund, " The Atomic Structure of a Metal-Supported Vitreous Thin Silica Film", Angew. Chem. Int. Ed. 51, 404 (2012).

[2] W. H. Zachariasen, " The Atomic Arrangement in Glass", J. Am. Chem. Soc. 54, 3841 (1932).

[3] P. Y. Huang, S. Kurasch, A. Srivastava, V. Skakalova, J. Kotakoski, A. V. Krasheninnikov, R. Hovden, Q. Mao, J. C. Meyer, J. Smet, D. A. Muller, and U. Kaiser, "Direct Imaging of a Two-Dimensional Silica Glass on Graphene", Nano Lett. 12, 1081 (2012).

With the size reduction of structures in current electronic devices, differences in the electronic properties caused, for example, by the structural nonuniformity of each element have an ever-increasing effect on macroscopic functions. The study of nonequilibrium quantum dynamics in materials with small structures is of great importance not only from the fundamental viewpoint but also as a basis for the further development of functional devices. Real-space imaging of the transient carrier transport and transitions in nanostructures is desired to obtaine a deeper understanding of current semiconductor physics. Probing the effect of local electronic structures of nano-clusters on energy transfer is important for the analysis of chemical reactions in catalytic activities and also for the development of organic solar devices. To advance such studies, a method that enables the probing of local carrier dynamics with high spatial and temporal resolution is necessary.

Recently, femtosecond time-resolved scanning tunneling microscopy (STM), which enables ultrafast phenomena on a target material to be probed with the spatial resolution of STM, has been realized by the combination of STM with ultrashort-pulse laser technologies [1-4]. In time-resolved STM, the tunnel gap of STM is illuminated by a sequence of paired laser pulses, and the change in tunneling current ΔI is measured as a function of the delay time between the paired pulses (t_d). A high temporal resolution in the femtosecond range, which is limited only by the optical pulse width, is obtained simultaneously with the atomic spatial resolution of STM. This microscopy technique is applicable to systems in which the response of the tunneling current has a nonlinear dependence on the optical excitation intensity. I n the case of semiconductors, for example, the magnitude of ΔI obtained for a certain delay time t_d reflects the density of photogenerated minority carriers at t_d after the first pulse excitation, and the carrier decay processes can be observed by analyzing the delay-time dependence of ΔI. Using polarized light, spin dynamics can be probed, and the detection of signals such as phonons is also possible.

In this talk, I would like to introduce this new microscopy technique with some new results.

References

[1] Y. Terada, S. Yoshida, O. Takeuchi and H. Shigekawa, J. of Physics: Condensed Matter 22, 264008 (2010). [2] Y. Terada, S. Yoshida, O. Takeuchi and H. Shigekawa, Nature Photonics, 4, 12, 869 (2010). [3] Y. Terada, S. Yoshida, O. Takeuchi and H. Shigekawa, Advances in Optic.Tech., 2011, 510186 (2011). [4] S. Yoshida, Y. Terada, R. Oshima, O. Takeuchi and H. Shigekawa, Nanoscale, 2012, 4, 757 (2012).

We have used scanning tunneling microscopy (STM) to characterize the growth of iridium onto Ge(111). Iridium was deposited onto the Ge(111) c(2x8) surface at different coverages less than 1ML, and the samples were annealed to temperatures between 550K and 750K. A new form of growth was observed, consisting of pathways connecting larger iridium islands. As the annealing temperature increased, the iridium growth first formed unusual shapes with finger-like protrusions. Next, these shapes broke apart into smaller islands, which ultimately formed into larger islands at higher temperatures. High resolution images have been obtained, which allow insight into the atomic arrangements. We propose a model relating the activation energy of specific binding sites to the formation of various observed structures on the surface.

Funding from NSF CHE-0719504 and NSF PHY-1004848 (REU)

In this work, we present the nanoscale characterization of AlInN semiconductors and AlInN/GaN high electron mobility transistor (HEMT) structures using a combination of transmission electron microscopy (TEM) and laser assisted atom probe tomography (APT), and correlate these with the structures’ electronic and optical properties, as well as the effects of irradiation. APT is an emerging technique based on the field ion emission from a needle-shaped region of interest and is capable of yielding 3D chemical mapping with atomic sensitivity and sub-nanometer spatial resolution. We report here a study of the field evaporation mechanisms from wide bandgap AlInN and GaN using a visible ps laser during APT experiments and correlate them with APT experiments (e.g. laser pulse energy, …) in order to establish approaches for reliable chemical analysis at the nanoscale of AlInN compound alloys. We proceed to investigate the fundamental material characteristics of interest that can be extracted from a combined APT and TEM analysis, including indium segregation phenomena in AlInN, interdiffusion near the AlInN/GaN channel interfaces and interface roughness, as well as the effects of irradiation on the channel properties.