TeYu Chien

Self-assembled nanorings in Si-capped Ge quantum dots on (001)Si

Nanorings with an average height and diameter of 1.2 and 65 nm, respectively, were observed to form in Si-capped Ge quantum dots grown at 600 °C by ultrahigh-vacuum chemical vapor deposition. The nanorings were captured with the rapid cooling of the samples with appropriate amount of Si capping. Based on the results of transmission electron microscopy and Raman spectroscopy, the formation of nanorings is attributed to alloying and strain relief in the Si/Ge/(001)Si system. The self-assembly of nanorings provides a useful scheme to form ultrasmall ring-like structure and facilitates the characterization of the physical properties of unconventional quantum structures.

Visualizing short-range charge transfer at the interfaces between ferromagnetic and superconducting oxides

The interplay between antagonistic superconductivity and ferromagnetism has been a interesting playground to explore the interaction between competing ground states. Although this effect in systems of conventional superconductors is better understood, the framework of the proximity effect at complex-oxide-based superconductor/ferromagnet interfaces is not so clear. The main difficulty originates from the lack of experimental tools capable of probing the interfaces directly with high spatial resolution. Here we harness cross-sectional scanning tunnelling microscopy and spectroscopy together with atomic-resolution electron microscopy to understand the buried interfaces between cuprate and manganite layers. The results show that the fundamental length scale of the electronic evolution between YBa2Cu3O(7-δ) (YBCO) and La2/3Ca1/3MnO3 (LCMO) is confined to the subnanometre range. Our findings provide a complete and direct microscopic picture of the electronic transition across the YBCO/LCMO interfaces, which is an important step towards understanding the competition between ferromagnetism and superconductivity in complex-oxide heterostructures.

Combining scanning tunneling microscopy and synchrotron radiation for high-resolution imaging and spectroscopy with chemical, electronic, and magnetic contrast

The combination of high-brilliance synchrotron radiation with scanning tunneling microscopy opens the path to high-resolution imaging with chemical, electronic, and magnetic contrast. Here, the design and experimental results of an in-situ synchrotron enhanced x-ray scanning tunneling microscope (SXSTM) system are presented. The system is designed to allow monochromatic synchrotron radiation to enter the chamber, illuminating the sample with x-ray radiation, while an insulator-coated tip (metallic tip apex open for tunneling, electron collection) is scanned over the surface. A unique feature of the SXSTM is the STM mount assembly, designed with a two free-flex pivot, providing an angular degree of freedom for the alignment of the tip and sample with respect to the incoming x-ray beam. The system designed successfully demonstrates the ability to resolve atomic-scale corrugations. In addition, experiments with synchrotron x-ray radiation validate the SXSTM system as an accurate analysis technique for the study of local magnetic and chemical properties on sample surfaces. The SXSTM systems capabilities have the potential to broaden and deepen the general understanding of surface phenomena by adding elemental contrast to the high-resolution of STM.

Templating Sub-10 nm Atomic Layer Deposited Oxide Nanostructures on Graphene via One-Dimensional Organic Self-Assembled Monolayers

Molecular-scale control over the integration of disparate materials on graphene is a critical step in the development of graphene-based electronics and sensors. Here, we report that self-assembled monolayers of 10,12-pentacosadiynoic acid (PCDA) on epitaxial graphene can be used to template the reaction and directed growth of atomic layer deposited (ALD) oxide nanostructures with sub-10 nm lateral resolution. PCDA spontaneously assembles into well-ordered domains consisting of one-dimensional molecular chains that coat the entire graphene surface in a manner consistent with the symmetry of the underlying graphene lattice. Subsequently, zinc oxide and alumina ALD precursors are shown to preferentially react with the functional moieties of PCDA, resulting in templated oxide nanostructures. The retention of the original one-dimensional molecular ordering following ALD is dependent on the chemical reaction pathway and the stability of the monolayer, which can be enhanced via ultraviolet-induced molecular cross-linking.

Nanometer-scale striped surface terminations on fractured SrTiO{sub 3} surfaces.

Using cross-sectional scanning tunneling microscopy on in situ fractured SrTiO(3), one of the most commonly used substrates for the growth of complex oxide thin films and superlattices, atomically smooth terraces have been observed on (001) surfaces. Furthermore, it was discovered that fracturing this material at room temperature results in the formation of stripe patterned domains having characteristic widths ( approximately 10 to approximately 20 nm) of alternating surface terminations that extend over a long range. Spatial characterization utilizing spectroscopy techniques revealed a strong contrast in the electronic structure of the two domains. Combining these results with topographic data, we are able to assign both TiO(2) and SrO terminations to their respective domains. The results of this experiment reveal that fracturing this material leads to reproducibly flat surfaces that can be characterized at the atomic-scale and suggest that this technique can be utilized for the study of technologically relevant complex oxide interfaces.

X-ray nanotomography of SiO2-coated Pt90Ir10 tips with sub-micron conducting apex

Hard x-ray nanotomography provides an important three-dimensional view of insulator-coated “smart tips” that can be utilized for modern emerging scanning probe techniques. Tips, entirely coated by an insulating SiO2 film except at the very tip apex, are fabricated by means of electron beam physical vapor deposition, focused ion beam milling and ion beam-stimulated oxide growth. Although x-ray tomography studies confirm the structural integrity of the oxide film, transport measurements suggest the presence of defect-induced states in the SiO2 film. The development of insulator-coated tips can facilitate nanoscale analysis with electronic, chemical, and magnetic contrast by synchrotron-based scanning probe microscopy.

Visualizing nanoscale electronic band alignment at the La 2 / 3 Ca 1 / 3 MnO 3 / Nb : SrTiO 3 interface

Cross-sectional scanning tunnelling microscopy and spectroscopy (XSTM/S) were used to map out the band alignment across the complex oxide interface of La

Controllable local modification of fractured Nb-doped SrTiO3 surfaces

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Self-Assembled Two-Dimensional Heteromolecular Nanoporous Molecular Arrays on Epitaxial Graphene

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Spin-dependent synchrotron x-ray excitations studied by scanning tunneling microscopy

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