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Very large lattice strain and strain-induced polarization are achieved in KNbO _3 using epitaxial growth of a thin KNbO _3 film onto a (001)-oriented SrTiO _3 single-crystal substrate. We demonstrate experimentally that epitaxy produces dramatic changes of interband transitions in the film compared to those of a reference KNbO _3 crystal: the energies of transitions change, some transitions are substantially suppressed and new ones appear in the film. A comparison of the experimental observations with theoretical calculations points to yet unexplored phenomena. Our results indicate that optical refraction and electro-optical coefficients of ferroelectric films can be controlled by epitaxial growth, which is of importance for emerging photonic and optoelectronic applications.

Comparative studies of ZnO crystalline and amorphous thin films and nanocrystalline powders are reported. The UV-visible optical spectra were analyzed with special attention paid to the direct optical bandgap. Atmospheric radio-frequency barrier torch discharge and pulsed hollow cathode sputtering techniques for the film fabrication were used. For the crystalline films, similar values of the direct optical bandgap were found independent of the growth method used. The analysis of the amorphous films and powders revealed a pronounced Urbach-like exponential absorption tail approaching the bandedge. For the powders, the bandgap energies were larger than those for the crystalline and amorphous films. A decrease in the powder particle size in the powders leads to an increased direct optical bandgap. [ABSTRACT FROM AUTHOR]

Optical spectra of CoxC60 mixture films (x  <  10) were studied in order to obtain insights into electronic interactions in an organometallic system that is attractive for spin-transfer electronics. The optical absorption (OA) spectra were described in terms of the Lorentz oscillators model, allowing the quantification of the spectral variations driven by Co concentration x. The variations are found to be most pronounced for 0  <  x  <  2–4, suggesting the effects of the electronic interactions in the Co2C60 fulleride formed in the mixture. The Co–C60 electronic interactions evidenced by the OA edge shift and by the OA peak detected at approximately 1 eV give rise to the Jahn–Teller (JT) splitting of the t1u electronic levels in C60 occupied by the electrons transferred from Co. The JT effect was confirmed by observation of the downshifted T1u(4) infrared-active mode modified by electron–phonon coupling at the t1u levels. [ABSTRACT FROM AUTHOR]

Localisation phenomena in highly disordered metals close to the extreme conditions determined by the Mott-Ioffe-Regel (MIR) limit when the electron mean free path is approximately equal to the interatomic distance is a challenging problem. Here, to shed light on these localisation phenomena, we studied the dc transport and optical conductivity properties of nanoscaled multilayered films composed of disordered metallic Ta and magnetic FeNi nanoisland layers, where ferromagnetic FeNi nanoislands have giant magnetic moments of 10[Formula: see text]-10[Formula: see text] Bohr magnetons ([Formula: see text]). In these multilayered structures, FeNi nanoisland giant magnetic moments are interacting due to the indirect exchange forces acting via the Ta electron subsystem. We discovered that the localisation phenomena in the disordered Ta layer lead to a decrease in the Drude contribution of free charge carriers and the appearance of the low-energy electronic excitations in the 1-2 eV spectral range characteristic of electronic correlations, which may accompany the formation of electronic inhomogeneities. From the consistent results of the dc transport and optical studies we found that with an increase in the FeNi layer thickness across the percolation threshold evolution from the superferromagnetic to ferromagnetic behaviour within the FeNi layer leads to the delocalisation of Ta electrons from the associated localised electronic states. On the contrary, we discovered that when the FeNi layer is discontinuous and represented by randomly distributed superparamagnetic FeNi nanoislands, the Ta layer normalized dc conductivity falls down below the MIR limit by about 60%. The discovered effect leading to the dc conductivity fall below the MIR limit can be associated with non-ergodicity and purely quantum (many-body) localisation phenomena, which need to be challenged further.

Controllable access to the hybrid plasmonic nanostructures built of small metal nanoparticles and organic spacer offers a tempting set of electronic excitations, which proper handling promises valuable applications and bright fundamental prospect. Here, we report on remarkable plasmonic properties of the Au x C 60 hybrid nanostructures formed through self-assembling the depositing mixture of metal and fullerene. Using optical absorption spectra, we demonstrate establishing of quantum plasmon (QP) excitations upon the controllable increase of spatial density and size of the Au clusters formed in the films. Detection of two plasmonic modes evidences the QP hybridization enabling by nm-scaled proximity of the neighboured Au clusters. Variation of the QP mode parameters with gradual decrease of the inter-cluster spacing ΔL to the sub-nanometre scale driven by the Au concentration in the film x allowed us to evidence the quantum tunnelling regime in the QP hybridization launching at ΔL ≈ 0.9 nm. The later result designates an important role of the C 60 molecules, separating the Au clusters, in design of plasmonic and transport properties of the hybrid films. The obtained results represent the self-assembled Au x C 60 nanocomposites as the promising plasmonic materials with potential for application in nanoplasmonics, nanoelectronics, and nanomedicine.

Allying epitaxial strain and synthesis conditions allows for the introduction of specific point defects in perovskite oxide films. In ferroelectric films, such defects lead to essential polar and electronic properties, which can enable advanced applications. Here, to elucidate the nature of the defects, optical constants are investigated in the spectral range of 0.7-8.8 eV in epitaxial ferroelectric BaTiO 3 films, which are synthesized under different conditions. It is demonstrated that oxygen-vacancy-related defects are responsible for a peculiar transition below the bandgap at ∼2.7-2.9 eV and significant blueshifts of ∼0.3-0.4 eV of the gap and the main interband transitions. These observations suggest that the defects are dipolar complexes comprising titanium cations and oxygen vacancies (Ti 3+ -V O ).

Ferroelectric films may lose polarization as their thicknesses decrease to a few nanometers because of the depolarizing field that opposes the polarization therein. The depolarizing field is minimized when electrons or ions in the electrodes or the surface/interface layers screen the polarization charge or when peculiar domain configuration is formed. Here, we demonstrate ferroelectric phase transitions using thermooptical studies in ∼5-nm-thick epitaxial Pb 0.5 Sr 0.5 TiO 3 films grown on different insulating substrates. By comparing theoretical modeling and experimental observations, we show that ferroelectricity is stabilized through redistribution of charge carriers (electrons or holes) inside ultrathin films. The related high-density of screening carriers is confined within a few-nanometers-thick layer in the vicinity of the insulator, thus resembling a two-dimensional carrier gas.

Driving by interplay between plasmonic and magnetic effects in organic composite semiconductors is a challenging task with a huge potential for practical applications. Here, we present evidence of a quantum plasmon excited in the self-assembled Co x C 60 nanocomposite films with x > 15 (interval of the Co cluster coalescence) and analyse it using the optical absorption (OA) spectra. In the case of Co x C 60 film with x = 16 (LF sample), the quantum plasmon generated by the Co/CoO clusters is found as the 1.5 eV-centred OA peak. This finding is supported by the establishment of four specific C 60 -related OA lines detected at the photon energies E p  > 2.5 eV. Increase of the Co content up to x = 29 (HF sample) leads to pronounced enhancement of OA intensity in the energy range of E p  > 2.5 eV and to plasmonic peak downshift of 0.2 eV with respect to the peak position in the LF spectrum. Four pairs of the OA peaks evaluated in the HF spectrum at E p  > 2.5 eV reflect splitting of the C 60 -related lines, suggesting great change in the microscopic conditions with increasing x. Analysis of the film nanostructure and the plasmon-induced conditions allows us to propose a Rashba-like spin splitting effect that suggests valuable sources for spin polarization.

Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO 3 , KNbO 3 (KNO), and NaNbO 3 (NNO) with a thickness of 10-30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO 3 (001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices.

Optical index of refraction n is studied by spectroscopic ellipsometry in epitaxial nanofilms of NaNbO 3 with thickness ∼10 nm grown on different single-crystal substrates. The index n in the transparency spectral range ( n  ≈ 2.1 - 2.2) exhibits a strong sensitivity to atmospheric-pressure gas ambience. The index n in air exceeds that in an oxygen ambience by δn  ≈ 0.05 - 0.2. The thermo-optical behaviour n ( T ) indicates ferroelectric state in the nanofilms. The ambience-sensitive optical refraction is discussed in terms of fundamental connection between refraction and ferroelectric polarization in perovskites, screening of depolarizing field on surfaces of the nanofilms, and thermodynamically stable surface reconstructions of NaNbO 3 .

We report the creation of a functional nanostructure on a Si crystal surface by 200 keV C60(++) cluster ion bombardment (CIB). We found that the modified layer produced by CIB includes two sublayers with different nanostructures. The top 24-nm-thick sublayer is an agglomeration of 5-nm-sized amorphous Si nanodots (a-Si NDs). The deeper 10-nm-thick sublayer is a transient layer of disordered Si as an interface between the a-Si top sublayer and the bulk Si(100). The top a-Si sublayer and the nc-Si transient layer are formed by the local heating effect and shock wave effect, respectively, induced by the cluster ion impacts. The photoluminescence (PL) spectra of the CIB-modified Si samples revealed an emission line centered at a photon energy of 1.92 eV. The absorption spectra of the modified samples exhibit enhanced light absorption at this photon energy. The parameters of the PL line require ascribing the emission origin to the quantum-confinement-induced optical transitions in the a-Si nanodots. The core-shell structure of a-Si NDs is confirmed by detection of an additional PL line centered at 2.5 eV. Analysis of the Rutherford backscattering (RBS) and the PL spectra implies the existence of -Si--O- bonds in the nanodot outer shells, which are responsible for the additional PL line. The obtained results demonstrate the valuable potential of CIB for the controllable fabrication of Si surface nanostructures, which is attractive for optoelectronics and nanoelectronics. The obtained results elucidate the evolution of structure modification occurring in silicon due to the injection of energetic C60 cluster ions with an energy of hundreds of keV.

Ultrathin (12-15 nm) SrTiO(3) films are grown by pulsed laser deposition on various single-crystal substrates. The crystal structure, orientation, and strain state of the films are studied by x-ray diffraction. The room-temperature optical properties of the films are experimentally determined using ellipsometric spectroscopy in the 1-6 eV spectral range. Epitaxial films with biaxial in-plane strain are obtained on LaAlO(3), DyScO(3), and KTaO(3) substrates, with the critical thickness for pseudomorphic growth being less than 10 nm. Abrupt strain relaxation has been detected. The optical properties of the films with different microstructure are compared with each other and with those of single-crystal SrTiO(3). Based on the comparison, surface effects are suggested to be dominant in the visible range, while the interband transitions are smeared and suppressed due to small film thickness and the presence of biaxial strain. The energies of gaps can increase due to strain-induced polarization. The absorption edge is affected by all the factors mentioned (surface, thickness, strain, and polarization).