Tuning the electronic properties of two-dimensional (2D) layers is a current focus of interdisciplinary research. Here, we show that the electronic properties of a surface-supported 2-dimensional (2D) layer structure can spontaneously self-texturize at nanoscale [1]. Specifically, we have characterized the 2D layer GdAu₂ forming a mismatched interface on Au(111). We demonstrate that the variable adsorption stacking configurations causes local structural relaxation processes which result in a spatially modulated layer-buckling. This is sufficient to periodically open an energy gap of ∼0.5 eV at the Fermi level at well-defined stacking configurations in an otherwise metallic layer. Additionally, this out-of-plane displacement of the Gd atoms patterns the character of the hybridized Gd-d states and shifts the center of mass of the Gd 4f multiplet proportionally to the lattice distortion. These findings demonstrate the close correlation between the electronic properties of the 2D-layer and its planarity. We demonstrate that the resulting template shows different chemical reactivity which may find important applications [2]. We will compare the properties of the 2D-layer of GdAu₂/Au(111) with the previously reported GdAg₂/Ag(111) [3].

REFERENCES:

[1]A.Correa, M.Farnesi Camellone, A.Barragan, A.Kumar,  C.Cepek,  M.Pedio,  S.Fabris, L.Vitali; Self-texturizing electronic properties of a 2-dimensional GdAu2 layer on Au(111): the role of out-of-plane atomic displacement,  Nanoscale 2017, 9, 17342

[2] Matteo Farnesi Camellone, Alexander Correa, Ana Barragán, Maddalena Pedio, Stefano Fabris, Cinzia Cepek, Lucia Vitali, Can atomic-buckling control chemical reaction?

The case of dehydrogenation of phthalocyanine molecules on a GdAu₂/Au(111). Submitted

[3] A.Correa, B.Xu, M.J.Verstraete, L.Vitali; Strain-induced effects in the electronic and spin properties of a monolayer of ferromagnetic GdAg2,  Nanoscale 2016, 8, 19148

Superconductors are commonly utilized in several applications such as energy generators and storage due to their unique capability of transferring electricity without energy losses. Miniaturization in superconductors can optimize their functionality and, furthermore, when the miniaturization reaches the nanoscale, in the range of the superconducting coherence length, novel physical phenomena can arise.

Furthermore, three-dimensional (3D) superconductors are expected to promote a change in the development of the next generation of electronic devices. Nevertheless, their fabrication is still challenging and nowadays only a few articles addressing the growth of 3D have been reported.

Here, we report for the first time the fabrication and characterization of 3D superconducting crystalline WC hollow nanowires with diameters down to 32 nm and aspect ratio above 200 (Fig. 1(a) and (b)) [1].

The method is based on using highly-focused beam of He⁺ ions to decompose a W(CO)₆ precursor molecules. The growth of the vertical WC nanowire occurs around the ion beam spot, mainly due to the interaction of secondary electrons with the adsorbed precursor molecules, whereas a cavity at the center of the nanowire is created due to the He⁺ beam milling effect on the growing material (Fig. 1(a) and (b)).  As shown by transmission electron microscopy, they display grains of large size fitting with face-centered cubic WC_1-x phase (inset of Fig. 1(c), which could present a T_c up to 10 K [2]. By studying their magnetotransport properties, we have found that nanowires exhibit 1.5 times higher superconducting critical temperatures (6.4 K) as well as 1.5 times higher upper critical magnetic fields (≈14 T) (Fig. 1(c)) when compared to nanowires grown by an Ga⁺ FIBID [3].

The fabrication of such materials with excellent properties makes this technique at the cutting edge of nanofabrication methods based on focused beams of charged particles for the development of the broad field of 3D nano-superconductivity.

Acknowledgement:
   “This project has received funding from the EU-H2020 research and innovation programme under grant agreement No 654360 NFFA-Europe.”

References

[1] Córdoba R.; Mailly D.; Ibarra A. and De Teresa J. M. Nano Lett. 2018, 18(2), 1379–1386.

[2] Kurlov A. S. and Gusev A. I. Inorg. Mater. 2006, 42, 121–7.

[3] Córdoba R. et al. Nat. Commun. 2013, 4, 1437.

Figure 1: (a) Sketch of the hollow NW growth by He⁺ FIBID. (b) SEM image of a vertical hollow NW grown by He⁺ FIBID. (c) Normalized resistance for a NW versus temperature. Left inset shows a HRTEM image of the cross-sectional view of the hollow NW indicated in b). Right inset shows an SEM image (coloured) of a NW connected by four contacts.

The nature of the oxygen species active in ethylene epoxidation is a long-standing question.

Under reaction conditions, two broad types of oxygen species are present at the O/Ag surface during epoxidation.^1-2 These species can be distinguished by XPS. One type (nucleophilic oxygen) has an O 1s binding energy (BE) of 528-528.5 eV and the other type (electrophilic oxygen) has a BE of 530-531 eV.^2-3 The latter has been shown to participate in epoxidation.⁴ Yet, while the atomic structure of nucleophilic oxygen is known,^5-6 the atomic structure of the active species is debated.^2,7-9 This electrophilic species was thought to be weakly bound oxygen and, following early assignments,^10-11 has been extensively interpreted as being unreconstructed adsorbed or dissolved atomic O.^2-3 However, it was recently demonstrated by means of DFT calculations that the spectroscopic features of unreconstructed atomic oxygen do not agree with those measured for the electrophilic species with a BE of 530-531 eV.⁷  Here, we use both in-situ and UHV X-Ray Photoelectron Spectroscopy (XPS) to study the interaction of oxygen with a silver surface. We show experimental evidence that unreconstructed adsorbed atomic oxygen has a BE ≤ 528 eV, showing a core-level shift (CLS) to lower BE with respect to the O-reconstructions, as previously predicted by DFT.^7-8 Thus, contrary to the frequent assignment, adsorbed atomic oxygen cannot account for the electrophilic oxygen species with an O 1s binding energy (BE) of 530-531 eV. Moreover, we show that adsorbed atomic O is present at very low O-coverages during in-situ XPS measurements and that it can be obtained at slightly higher coverages in UHV at low temperature. These findings suggest that a high coverage of unreconstructed atomic O is not likely to be present at the silver surface under ethylene epoxidation conditions, at odds with the high concentration of electrophilic oxygen thought to participate in epoxidation. This has important implications on the understanding of the role of the different oxygen species in ethylene epoxidation on silver catalysts.

[1] V.I. Bukhtiyarov et al., J Catal, 238, 260 (2006).
[2] T.C.R. Rocha et al., J Catal, 312, 12 (2014).
[3] T.C.R. Rocha et al., PCCP, 14, 4554 (2012).
[4] V.I. Bukhtiyarov et al., Surf Sci 320, L47 (1994).
[5] T.E. Jones et al., J Phys. Chem. C 120, 28630 (2016).
[6] C.T. Campbell et al., Surf Sci 139, 396 (1984).
[7] T.E. Jones et al., PCCP, 17, 9288 (2015).
[8] T.E. Jones et al., Acs Catal 5, 5846 (2015).
[9] T.E. Jones et al. ACS Catal. 8, 3844 (2018).
[10] R.W. Joyner et al., Chem. Phys. Lett. 60, 459 (1979).
[11] C.T. Au et al.,. J Chem Soc Farad T 1, 79, 1779 ( 1983).

Herein we report the easy incorporation of brightly phosphorescent cationic iridium(III) tetrazole complexes into a silica based ceramic matrix via an easily scalable colloidal process. For this purpose, two cationic Ir(III) emitters bearing 5-aryl tetrazole ligands (R-CN4) were selected: blue [F2IrPTZ-Me]+ and red [IrQTZ-Me]+. The cationic complexes were readily adsorbed to negatively charged silica nanoparticles and trapped in the sol–gel matrix. The sol-to-solid phase transfer was performed by using an innovative spray-freeze-drying technique, leading to the formation of phosphorescent solid micro-granules. The structural and optical characterisation of the Ir(III) complexes together with SiO2 nanoparticles, nanosols (Ir@SiO2) and powders (Ir@SiO2 powders), revealed how the presence of the Ir(III)-based complexes did not alter the morphology of the colloidal silica or granulated phases. Moreover, the ceramic matrix did not interfere with the optical properties of the embedded complexes. The distribution of Ir(III) complexes in the spray-freeze-dried powders was qualitatively evaluated by fluorescence microscopy, revealing how the luminescent particles were homogeneously dispersed all over the silica matrix. Interestingly, release of complex [F2IrPTZ-Me]+ from the corresponding Ir@SiO2 powder in aqueous and organic solutions is almost negligible, therefore suggesting that a strong interaction occurs between the host–silica matrix and the Ir(III) guest complex. In conclusion, we designed a multi-scale process, and obtained luminescent nano-structured powders that preserved their optical properties from the molecular scale to the microscale, providing a self-marking ceramic platform that was stable and can be easily processed at dry or wet dispersed state. Nevertheless as further application of these luminescent nanosol systems, we produced Ir@SiO2 ceramic coated cotton fabrics by dip-pad-dry-cure technique. Exploiting self-marking properties to trace silica nanoparticles functionalization itself on the substrate, we evaluated once more time a negligible release of Ir@SiO2 from fabric if exposed to human skin, showing preliminary a reduce human impact.
These results prove as the integration of such materials in a lot of nano/biotechnology relevant products (LED colour converters; photonics, optoelectronics; photo-induced catalysis; industrial anti-counterfeiting; bio-imaging/targeting/sensing), where long-term stability with high luminescence efficiency is required, will strongly expand the traditional ceramic market.

The development of on-demand individual deep impurities in silicon is motivated by their employment as a physical substrate for qubits [1], nanoscale transistors [2], single photon emitters [3] and to fabricate Hubbard-like quantum systems [4].
Single-atom silicon devices based on conventional doping elements such as phosphorous [5], arsenic [6] and boron [7] can operate only at cryogenic temperature due to shallow weakly localized ground state impurity levels.
Differently, the implantation of Ge dopants in silicon and the subsequent annealing is expected to generate stable Ge vacancy complexes [8] that are promising candidates to achieve single-atom quantum effects at room temperature.
These hybrid impurities combine indeed the properties of the silicon vacancy, which carries deep states in the bandgap, with the accurate spatial controllability of the defect obtainable through state of the art single-ion implantation of Ge atom.
Two NFFA-EUROPE Theory projects (ID 188 and ID 517) were assigned to UMIL on request of the experimental User Prof. T. Tanii (Waseda University) which developed the ion implantation technique and performed the electrical characterization of GeV-silicon channels, in collaboration with E. Prati (IFN-CNR).
By means of ab initio Density Functional Theory calculation with screened-exchange hybrid functional, that solves the “gap and delocalization problem” of standard DFT, we characterize structural and electronic properties of different GeV_n defects.
The calculated excitation energies relative to the addition of electrons into the defect, are in very good agreement with the available experiments, demonstrating the occurrence of a very deep state in the gap. Accordingly, we show that the electrons are more localized than in conventional dopants, decaying in a radius of about 0.5 nanometers from the defect
[9].
Being characterized by very large on-site repulsion U and small electronic hopping t (U/t~250) this system is expected to be a platform to study the Mott transition and magnetic correlation.
By mapping the ab initio DFT results in an extended Hubbard formalism we are able to shed light on the transport mechanisms through an array of GeV complexes in silicon showing that both temperature activated and resonanttransport related features contribute to the conductance, in agreement with the experimental findings [10].

[1]. Pla, J. J. et al. “A single-atom electron spin qubit in silicon.”, Nature 489, 541 (2012).
[2]. Shinada, T. et al. “Opportunity of single atom control for quantum processing in silicon and diamond.” in Silicon, Nanoelectronics Workshop (SNW), 1–2 (IEEE, 2014).
[3]. Aharonovich, I., Englund, D. & Toth, M. “Solid-state single-photon emitters.” Nat. Photonics 10, 631 (2016).
[4]. Fratino, L., Semon, P., Charlebois, M., Sordi, G. & Tremblay, A.-M. “Signatures of the Mott transition in the antiferromagnetic state of the two-dimensional Hubbard model.” Phys. Rev. B 95, 235109 (2017).
[5]. Tan, K. Y. et al. “Transport spectroscopy of single phosphorus donors in a silicon nanoscale transistor.” Nano Lett. 10, 11–15 (2009).
[6]. Hori, M., Shinada, T., Guagliardo, F., Ferrari, G. & Prati, E. “Quantum transport property in FETs with deterministically implanted single-arsenic ions using single-ion implantation.” In Silicon Nanoelectronics Workshop (SNW), 1–2 (IEEE, 2012).
[7]. Khalafalla, M., Ono, Y., Nishiguchi, K. & Fujiwara, A. “Identification of single and coupled acceptors in silicon nano-fieldeffect transistors.” Appl. Phys. Lett. 91, 263513 (2007).
[8] Suprun-Belevich, Y. & Palmetshofer, L. “Deep defect levels and mechanical strain in Ge+ implanted Si.” Nucl. Instr. Methods Phys. Res. B 96, 245–248 (1995).
[9] S. Achilli, N. Manini, G. Onida, T. Tanii, T. Shinada, E. Prati, “GeVn complexes for silicon-based room-temperature single-atom nanoelectronics”, Sci. Rep. 8, 18054 (2018).
[10] S. Achilli, N. Manini, N. H. Le, T. Tanii, T. Shinada, E. Prati, G. Onida, “Quantum transport in GeV:silicon channels”, in preparation.