National Institute Of Materials Physics - Romania

The effects of a turnstile operation on the current-induced vibron dynamics in nanoelectromechanical systems (NEMS) are analyzed in the framework of the generalized master equation. In our simulations each turnstile cycle allows the pumping of up to two interacting electrons across a biased mesoscopic subsystem which is electrostatically coupled to the vibrational mode of a nanoresonator. The time-dependent mean vibron number is very sensitive to the turnstile driving, rapidly increasing/decreasing along the charging/discharging sequences. This sequence of heating and cooling cycles experienced by the nanoresonator is due to specific vibron-assisted sequential tunneling processes along a turnstile period. At the end of each charging/discharging cycle the nanoresonator is described by a linear combination of vibron-dressed states s(v). associated to an electronic configuration nu. If the turnstile operation leads to complete electronic depletion the nanoresonator returns to its equilibrium position, i.e., its displacement vanishes. It turns out that a suitable bias applied on the NEMS leads to a slow but complete cooling at the end of the turnstile cycle. Our calculations show that the quantum turnstile regime switches the dynamics of the NEMS between vibron-dressed subspaces with different electronic occupation numbers. We predict that the turnstile control of the electron-vibron interaction induces measurable changes on the input and output transient currents.

Among the prerequisites for the progress of single-molecule-based electronic devices are a better understanding of the electronic properties at the individual molecular level and the development of methods to tune the charge transport through molecular junctions. Scanning tunneling microscopy (STM) is an ideal tool not only for the characterization, but also for the manipulation of single atoms and molecules on surfaces. The conductance through a single molecule can be measured by contacting the molecule with atomic precision and forming a molecular bridge between the metallic STM tip electrode and the metallic surface electrode. The parameters affecting the conductance are mainly related to their electronic structure and to the coupling to the metallic electrodes. Here, the experimental and theoretical analyses are focused on single tetracenothiophene molecules and demonstrate that an in situ-induced direct desulfurization reaction of the thiophene moiety strongly improves the molecular anchoring by forming covalent bonds between molecular carbon and copper surface atoms. This bond formation leads to an increase of the conductance by about 50 % compared to the initial state.

The effect of UV light on the cationic photopolymerization of the SU8 negative photoresist is shown by photoluminescence (PL) studies. Our results demonstrate that the cationic photopolymerization reaction of the SU8 photoresist takes place predominantly under the influence of the UVA light. Using UVA light, the influence of carbon nanotubes [of the types single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), multiwalled carbon nanotubes (MWNTs), and SWNTs functionalized with carboxyl groups (SWNTs-COOH)] on the cationic photopolymerization process of the SU8 photoresist is shown by PL studies. The cationic photopolymerization of the SU8 photoresist is monitored by the variations of the two emission bands with maxima at similar to 400-429 nm and 523-556 nm. The increase in the relative intensity of the PL band at similar to 523-556 nm is dependent on (i) the carbon nanotube concentration in the SU8 photoresist matrix; (ii) the type of carbon nanotubes, i.e., SWNTs, DWNTs, and MWNTs; and (iii) the nonfunctionalized and functionalized state of SWNTs. The results reported in this work demonstrate that PL can be used as a complementary method to Raman scattering and IR spectroscopy in the investigation of the cationic photopolymerization reaction of the SU8 negative photoresist. A decrease in the wrapping angle of carbon nanotubes with the SU8 photoresist is highlighted by anisotropic PL studies.

The underlying mechanisms of the metal-insulator transition in correlated oxides are a rich source of interesting physics and a topic of long-standing investigation. Here, the authors use angle-resolved photoelectron spectroscopy to investigate changes in charge carrier properties and electron-phonon interactions as a function of Ce-doping across the metal-insulator transition in CaMnO3. Many transition metal oxides (TMOs) are Mott insulators due to strong Coulomb repulsion between electrons, and exhibit metal-insulator transitions (MITs) whose mechanisms are not always fully understood. Unlike most TMOs, minute doping in CaMnO3 induces a metallic state without any structural transformations. This material is thus an ideal platform to explore band formation through the MIT. Here, we use angle-resolved photoemission spectroscopy to visualize how electrons delocalize and couple to phonons in CaMnO3. We show the development of a Fermi surface where mobile electrons coexist with heavier carriers, strongly coupled polarons. The latter originate from a boost of the electron-phonon interaction (EPI). This finding brings to light the role that the EPI can play in MITs even caused by purely electronic mechanisms. Our discovery of the EPI-induced dichotomy of the charge carriers explains the transport response of Ce-doped CaMnO3 and suggests strategies to engineer quantum matter from TMOs.

In view of the HL-LHC upgrade, radiation-tolerant silicon sensors containing low-resistivity p-type implants or substrates, like LGAD or HV-CMOS devices, are being developed in the framework of ATLAS, CMS, RD50 and other sensor R&D projects. These devices are facing a particular problem - the apparent deactivation of the doping due to the irradiation, the so-called acceptor removal effect. In the present work proton and neutron fluence-dependent radiation damage effects, including the change in leakage current and effective doping concentration, space charge sign-inversion, but also introduction and annealing of point- and cluster-like defects have been studied in Si pad diodes fabricated from p-type EPI material of different resistivity (10-1000 Omega cm). Standard electrical characterisations (IV, CV), TCT (Transient Current Technique) and TSC (Thermally Stimulated Current) techniques were applied. A correlation between effective doping concentration obtained from CV measurements and defect concentration N-t extracted from TSC measurements for both - neutron and proton - irradiations was observed pointing towards the microscopic origin of the acceptor removal. A detailed analysis of the dominant TSC peaks - E(30), BiOi and three main deep acceptor levels H(116), H(140) and H(152) - responsible for the changes in the effective space charge is performed. The origin and annealing behaviour of E(30) and H(40) and other cluster-related defects are discussed as well.

A one-step method assisted by hydrothermal treatment was approached to obtain nanocrystalline 1 and 10 mol. % In doped 2 mol.% Pd-SnO2 powders using a non-ionic surfactant Brij52 and Polyethylene glycol 6000 (PEG) as templates. Depending on In content, the samples were labeled as Pd1InSn and Pd10InSn. The obtained materials consist of nanosized crystallites packed into micrometric grains with a high porosity, as revealed by the morphological and structural investigations (SEM, TEM). A dependence of the grain size with respect to the In content has been revealed i.e. the sample Pd1InSn was showing an average grain size of around 10 nm, whilst for the sample Pd10InSn the average grain size was found to be around 5 nm. The XPS investigations highlighted the differences occurred in the surface chemistry in terms of surface hydroxylation as well as the chemical states of Pd. The sensing properties towards different CO concentrations have been examined under infield background conditions, at low operating temperature of 50 degrees C. The sensing mechanism model for CO was discussed in detail according to the possible interplay between oxygen and water related species based on the experimentally results acquired through simultaneous electrical resistance and work function measurements.

Curcumin (CR) is a natural antioxidant compound extracted from Curcuma longa (turmeric). Until now, researches related to the incorporation of CR into layered double hydroxides (LDHs) were focused only on hybrid structures based on a MgxAl-LDH matrix. Our studies were extended towards the incorporation of CR in another type of LDH-matrix (Zn3Al-LDH) which could have an even more prolific effect on the antioxidant activity due to the presence of Zn. Four CR-modified Zn3Al-LDH solids were synthesized, e.g., PZn3Al-CR(Aq), PZn3Al-CR(Et), RZn3Al-CR(Aq) and RZn3Al-CR(Et) (molar ratio CR/Al = 1/10, where P and R stand for the preparation method (P = precipitation, R = reconstruction), while (Aq) and (Et) indicate the type of CR solution, aqueous or ethanolic, respectively). The samples were characterized by XRD, Attenuated Total Reflectance Fourier Transformed IR (ATR-FTIR) and diffuse reflectance (DR)-UV-Vis techniques and the CR-release was investigated in buffer solutions at different pH values (1, 2, 5, 7 and 8). XRD results indicated a layered structure for PZn3Al-CR(Aq), PZn3Al-CR(Et), RZn3Al-CR(Aq) impurified with ZnO, while RZn3Al-CR(Et) contained ZnO nano-particles as the main crystalline phase. For all samples, CR-release revealed a decreasing tendency towards the pH increase, and higher values were obtained for RZn3Al-CR(Et) and PZn3Al-CR(Et) (e.g., 45% and 25%, respectively at pH 1).

Herein we report on novel multiferroic core shell nanostructures of cobalt ferrite (CoFe2O4) bismuth, sodium titanate doped with barium titanate (BNT-BT0.08), prepared by a two step wet chemical procedure, using the sol gel technique. The fraction of CoFe2O4 was varied from 1:0.5 to 1:1.5 = BNT-BT0.08/CoFe2O4 (molar ratio). X-ray diffraction confirmed the presence of both the spinel CoFe2O4 and the perovskite Bi0.5Na0.5TiO3 phases. Scanning electron microscopy analysis indicated that the diameter of the core shell nanoparticles was between 15 and 40 nm. Transmission electron microscopy data showed two phase composite nanostructures consisting of a BNT-BT0.08 core surrounded by a CoFe2O4 shell with an average thickness of 4-7 nm. Cole-Cole plots reveal the presence of grains and grain boundary effects in the BNT-BT0.08/CoFe2O4 composite. Moreover, the values of the dc conductivity were found to increase with the amount of CoFe2O4 semiconductive phase. Both X-ray photoelectron spectroscopy (XPS) and Mossbauer measurements have shown no change in the valence of the Fe3+, Co2+, Bi3+ and Ti4+ cations. This study provides a detailed insight into the magnetoelectric coupling of the multiferroic BNT-BT0.08/CoFe2O4 core shell composite potentially suitable for magnetoelectric applications.

The structural investigations of SrFe11.9In0.1O19 compound synthesized by solid-state method have been carried by neutron diffraction method in a wide temperature range. The appearace of spontaneous polarization that coexists with ferrimagnetic ordering has been found out at room temperature in strontium hexaferrite partially substituted with In ions. In order to explaine the presence of ferroelectric properties in SrFe11.9In0.1O19 compound, its crystal structure has been refined within the framework of both centrosymmetric P6(3)/mmc (No. 194) and non-centrosymmetric P6(3)mc (No. 186) space groups. The analysis of refinement results allows to understand microscopic mechanism of appearance ferroelectric properties in strontium hexaferrites.

In this work, the effect of aliovalent substitution of Pb2+ by Eu3+ on structural, morphological and optical properties of CH3NH3PbI3 (MAPbI(3)) was studied, aiming to improve the properties of perovskite films used in solar cells application. The surface morphology, the microstructure and the optical properties of the obtained films containing different Europium (Eu) concentrations were characterized by atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, UV-vis spectroscopy and photoluminescence spectroscopy.