National Institute Of Materials Physics - Romania

Nanoscale Condensed Matter Laboratory

Group of Surface and Interface Science

A. Group of Surface and Interface Science

B. Group leader: Cristian Mihail Teodorescu, PhD, Habil., Senior Scientist I

C. Personnel: 31

3 Senior Scientist I, 4 Senior Scientists II, 8 Senior Scientists III, 1 Development Engineer III, 2 Researchers, 9 Research Assistants, 4 Technicians.

D. Main research topics: surface science, ferroelectrics, catalysis, magnetic materials, epitaxy, fundamental aspects of ferroic systems

E. Experimental setups:

1. Experimental surface science setup, comprized by: (i) molecular beam epitaxy, with in situ analysis by low energy electron diffraction (LEED), reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES), sample preparation by ion sputtering, annealing, plasma source, Knudsen cells, electron bombardment evaporators, residual gas analysis; (ii) an installation for scanning tunneling microscopy and spectroscopy (STM–STS), at variable temperature; (iii) an installation for angle- and spin-resolved photoelectron spectroscopy, allowing X-ray photoelectron spectroscopy (XPS), photoelectron diffraction (PED, XPD), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) also with spin resolution. Actually, this cluster is delocalized on the SuperESCA beamlie at the Elettra synchrotron radiation facility in Trieste, Italy and is available for external users, feed with synchrotron radiation with continuous spectrum with energy between 90–1200 eV, produced by an undulator, with linear polarization. The beam is concentrated on a spot of approximately 10 x 100 μm2 and the flux is in the range of 1013 photons/s with a resolving power E/ΔE in the range of 10,000. This allows one to record high resolution ultrafast photoelectron spectra and to follow-up in situ various surface processes. The Romanian team has attributed yearly 23 days of beamtime, while the access to use the installled laboratory sources (X-ray guns, UV lamp, STM, MBE) is unlimited. Outside the beamtime allocated by Elettra, photoemission can still be performed by using monochromated Al Kα (1486.7 eV)/ Ag La (2984.3 eV) radiation and high power UV lamp (300 W) with He I (21.2 eV) and He II (40.8 eV) radiation. Manufacturer: Specs, Germany.

Instalatie_1
Instalatie_2

2. Experimental cluster comprising (i) a molecular beam epitaxy (with in situ LEED, RHEED and AES analyses), sample preparation (sputtering, annealing, Knudsen cells, e-beam evaporators), residual gas analysis, gas cabinet; (ii) a chamber for scanning tunneling microscopy and spectroscopy (STM – STS); (iii) chamber for photoelectron spectroscopy (XPS, UPS). Available excitation sources: monochromated Al Kα radiation (1486.7 eV), dual Al Kα (1486.7 eV) / Mg Kα (1253.6 eV) anode, UV lamp with He I (21.2 eV) radiation. Manufacturer: Specs, Germany.

3. Automated installation for XPS with spatial resolution (2 mm) coupled to a reaction cell for online studies of surface reactions at high temperatures and pressures (1000 °C, 4 bar), with gas cabinet with 4 ways. Excitation sources: monochromated Al Kα (1486.7 eV), dual Al Kα (1486.7 eV) / Mg Kα (1253.6 eV) anode. Manufacturer: Kratos Analytical, U. K.

4. Installation for low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM). Available techniques: (i) dark or bright field LEEM with 4 nm lateral resolution; (ii) PEEM using excitation with Hg lamp or with UV lamp with He I and He II radiation, lateral resolution 15 nm; (iii) mirror electron microscopy; (iv) micro-LEED (with mm lateral resolution); k-space mapping with sub-micrometer lateral resolution (vi) Possibility to follow in real time LEEM, PEEM, MEM, LEED during thermal treatment, ion bombardment, thin film growth. Manufacturer: Specs, Germany.

5. Setup for laboratory extended X-ray absorption fine structure (EXAFS). Excitation: Mo Kα1 (17479.34 eV), W La1 (8397.6 eV), power 3 kW (40 kV, 75 mA); monochromators Ge(220), Ge(400), Ge(840); detectors: proportional or scintillators, measurements in transmission or fluorescence, simulation and analysis software. Manufacturer: Rigaku, Japan.

6. Chemistry laboratory, with basic instrumentation: glassware, balances, ultrasonic shafts, vortex, water distillation, pressure reactors, heating and stirring systems, photocatalytic reactions, vacuum stove (Memmert), rotvapor (Heidolph), etc. Dedicated to catalyst preparation and performance evaluation for oxidation, hydrogenation, coupling reactions etc. The laboratory is assisted by: (a) Analytical methods for chemical compound identification (gas chromatography coupled to mass spectroscopy GC-MS QP2010 Ultra, manufactured by Shimadzu, Japan). This equipment uses two injectors, two detectors (MS and BID), pressure valve for real time analysis, auto-sampler, detects gaseous compounds or volatile liquids. (b) Analysis system for materials characterization, using cumulative characterization techniques, such as temperature programmed desorption, reduction or oxidation (TPD, TPR, TPO), pulsed chemisorption, nitrogen physisorption (Brunauer-Emmett-Teller BET) for pore size analysis. (c) Raman spectroscopy (AvaRaman 532) and UV-Vis-NIR spectro-photometry (AvaSpec-ULS 2048 L-RS-USB2) used for reaction follow-up, product identification, detection and characterization of nanoparticles in suspension. Irradiation sources: 532 nm laser for Raman spectroscopy and Xe source with emission 200 – 1100 nm, for the UV-Vis-NIR spectro-photometer. The latter is equipped with an integrating sphere with 80 mm internal diameter and flux cell analysis with transverse Swagelok feedthroughs and two collimating lenses UV-Vis-NIR; maximum temperature 80 °C, maximum pressure 10 bar.

F. Services offered:

1. Photoelectron spectroscopy-based techniques: X-ray photoelectron spectroscopy (XPS) and diffraction (XPD), ultraviolet photoelectron spectroscopy (UPS), angle-resolved UPS (ARUPS), spin-resolved ARUPS.

2. Auger electron spectroscopy (AES) and diffraction (AED).

3. Low energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED) characterization of surfaces.

4. Scanning tunneling microscopy (STM) and spectroscopy (STS) at variable temperature.

5. Sample depth profiling by ion sputtering assisted by XPS or AES.

6. Surface cleaning and synthesis of epitaxial thin films by molecular beam epitaxy (MBE).

7. Thermally-programmed desorption of molecules from surfaces by residual gas analysis (RGA).

8. Low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM), micro-LEED and micro-ARUPS.

8. Extended X-ray absorption fine structure (EXAFS).

9. Positron annihilation-induced Auger electron spectroscopy (PAES).

G. Output in 2020: 43 papers in ISI-ranked journals, 1 patent application, 2 granted patents.

Output in 2019: 53 papers in ISI-ranked journals, 6 patent applications

H. Highlights in 2020

1. Degenerated semiconductor catalysts with internal junctions

Water photocatalytic reduction with or without sacrificial agents was demonstrated by UV irradiation by using mixed oxide photocatalysts of Ni, Zn and Ti synthesized by a cheap method (co-precipitation). The material containing Ni:Zn in ratio 9:1 exhibits a photocatalytic activity of  about 17 mmol per gram per hour, being 1000 times more effective than the Evonik TiO2 standard. Band alignment examination by X-ray photoelectron spectroscopy yields the fact that titania becomes a degenerated semiconductor, which accumulates electrons under UV irradiation and transfers these electrons towards Ni nanoparticles, while holes are driven towards ZnO, thus enhancing the reduction or oxidation of these components, respectively. Reference: F. Neațu, L. E. Abramiuc, M. M. Trandafir, R. F. Negrea, M. Florea, C. M. Teodorescu, and Ș. Neațu, ChemCatChem 12, 4642–4651 (2020).

2. “2D nanoreactors” between graphene and the metal substrate

Graphene deposited on Pt(001) with the hex reconstruction is the weakest interacting graphene deposited on a metal single crystal known to date. In these conditions, small molecules such as CO may be trapped between the graphene and metal substrate, as is demonstrated in our work by photoelectron spectroscopy. Upon CO dosing of graphene/Pt(001), the Pt 4f signal decreases, but the C 1s signal is almost constant, which is a sign that CO is intermixing below the graphene layer. Therefore, these molecules may be confined in these 2D structures, being ready for further reactions, such as simple oxidation of Fischer-Tropsch reactions. Reference: N. G. Apostol, I. C. Bucur, G. A. Lungu, C. A. Tache, and C. M. Teodorescu, Catal. Today, accepted (2020). DOI: 10.1016/j.cattod.2020.02.006.

3. Microscopic model of ferroelectricity

Charge accumulation at the surfaces or interfaces of a ferroelectric thin film with perpendicular polarization was seen as a pre-requisite to compensate the depolarization field produced by the fixed, terminating, charges of the dipoles inside the material. In this work, starting from the assumption that for a large enough system, the dipolar interaction is weak, one considers the stabilization energy of an elementary electric dipole inside the material with the charge accumulated near the outer surfaces and interfaces. As soon as the system is able to provide these charge accumulated at surfaces and interfaces (located in metal outer electrodes or contaminants, provided by chemical reconstruction, self-doping, photogenerated carriers), the ferroelectric state becomes more stable in energy. This allows one to write down the microscopic interaction energy for a dipole, then to apply the Curie-Weiss mean field theory to derive the thermo-ferroelectric properties of the system, i. e. the hysteresis cycles and their dependence on temperature, all starting with only two parameters: the maximum value of the elementary dipole and a “dielectric constant” which in fact represents the absolute value of the negative slope unifying the point (– coercive field, + saturation polarization) with (coercive field, – saturation polarization), at zero temperature. The analytical formulas derived for the dependence of the applied field on the polarization and temperature allows one to derive values for the free energy of the ferroelectric system, beyond the phenomenological Landau-Ginsburg-Devonshire approach. Reference: C. M. Teodorescu, Phys. Chem. Chem. Phys., DOI: 10.1039/D0CP05617K (2020).

Highlights in 2019

1. Epitaxial growth of Ag(111) on Si(111) 7 x 7.

This growth was performed up to thick layers (30 nm) in view of the cheap synthesis of single crystal Ag(111) substrates for growth of 2D systems. In the figure below (a, b) represent XPS spectra confirming layer-by-layer growth, (c, d) are LEED images pointing long range order and (e) is a STM image of the 30 nm Ag(111) film exhibiting good surface morphology. Ref. A.E. Bocîrnea, R.M. Costescu, N.G. Apostol, C.M. Teodorescu, Appl. Surf. Sci. 473, 433–441 (2019).

2. Origin and sense of resistance hysteresis in graphene-like layers grown on ferroelectric surfaces

sp2 graphene-like layers were synthesized on atomically clean lead zirco-titanate (001) in a dedicated setup for in situ surface analysis together with electrical measurements for different substrate polarizations (I, below). Characterization was performed by high resolution XPS (II) allowing to deride carbon coverage and electronic structure of graphene-like layers, near-edge absorption fine structure (NEXAFS, III) allowing one to derive the percentage of in-plane sp2 bonds, STM (IV) and resistance measurements vs. voltages applied on the ferroelectric (V). Layers with thicknesses of 1 monolayer (1 ML) or below exhibit ‘anti-hysteretic’ behavior, while starting with 2 ML the ‘normal’ hysteresis is obtained. The explanation proposed takes into account combined transport through graphene-like layers and the ferroelectric material near surface, when accumulated charge carriers are available due to some orientations of the ferroelectric polarization. Ref. N.G. Apostol, D. Lizzit, G.A. Lungu, P. Lacovig, C.F. Chirilă, L. Pintilie, S. Lizzit, C.M. Teodorescu, RSC Adv. 10, 1522–1534 (2020).



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