Amorphous transparent conducting oxides (a-TCOs) have seen substantial interest in recent years due to the significant benefits that they can bring to transparent electronic devices. One such material of promise is amorphous ZnxSn1-xOy (a-ZTO). a-ZTO possesses many attractive properties for a TCO such as high transparency in the visible range, tunable charge carrier concentration, electron mobility, and only being composed of common and abundant elements. In this work, we employ a combination of UV-vis spectrophotometry, X-ray photoemission spectroscopy, and in situ scanning tunneling spectroscopy to investigate a 0.33 eV blue shift in the optical bandgap of a-ZTO, which we conclude to be due to quantum confinement effects.

High-quality epitaxial p-type V2O3 thin films have been synthesized by spray pyrolysis. The films exhibited excellent electrical performance, with measurable mobility and high carrier concentration. The conductivity of the films varied between 115 and 1079 Scm−1 while the optical transparency of the films ranged from 32 to 65% in the visible region. The observed limitations in thinner films’ mobility were attributed to the nanosized granular structure and the presence of two preferred growth orientations. The 60 nm thick V2O3 film demonstrated a highly competitive transparency-conductivity figure of merit compared to the state-of-the-art.

Transparent conductive oxides (TCOs) are important materials for a wide range of optoelectronic devices. Amorphous zinc tin oxide (a-ZTO) is a TCO and one of the best nontoxic, low-cost replacements for more expensive amorphous indium-gallium-zinc oxide. Here, we employ spray pyrolysis (SP), an inexpensive and versatile chemical vapor deposition-based technique, to synthesize a-ZTO with an as-deposited conductivity of ≈300 S/cm-the highest value hitherto among the reported solution-processed films. Compositional analysis via X-ray photoelectron spectroscopy reveals a nonstoichiometric transfer of Zn and Sn from the dissolved precursors into the film, with the best electrical properties achieved at a film composition of xfilm = 0.38 ± 0.04 ((ZnO)x(SnO2)1-x (0 < x < 1)). The morphology of these films is compared to films synthesized by physical vapor deposition (PVD), and a strong correlation between morphology and electrical properties is revealed. The granular nature of the SP-grown films, which seems like a drawback at first glance, brings about the prospect of using a-ZTO in ink-jet-printed films from a nanoparticle suspension for the room-temperature deposition. Brief post-anneal cycles in N2 gas improve the conductivity of the films by means of grain boundary (GB) passivation.

The dewetting resistance of Au 50 nm films fabricated atop Ti/Ta/W/Cr/Al adhesion layers (0.5-5 nm) was investigated. Results show sub-nanometer Ta has superior stability under thermal stress, while W & Ti show best plasmonic response.

Improving the thermal stability of Au thin films is critical if thermo-plasmonic applications such as heat-assisted magnetic recording are to become commercially viable. In this work, Al capping layers are deposited on M/Au films, where adhesion metal M = Ti or Ta, and are investigated for their utility in stabilizing Au thin films against dewetting. Contrary to previous investigations, it was found that thinner capping layers (0.5 nm) result in a greater stability than that conferred by thicker (1-5 nm) layers. Deposition of 0.5 nm of Al, which subsequently oxidizes when exposed to air, resulted in up to 10x stability enhancement. Similarly, capping with 0.5 nm Al resulted in less damping of the plasmonic response than with thicker capping layers. Finally, based on this work and previous work on metallic adhesion layers, optimally stable systems for thermo-plasmonic applications are proposed.

Structural colour generation by nanoscale plasmonic structures is of major interest for non-bleaching colour printing, anti-counterfeit measures and decoration applications. We explore the physics of a two-metal plasmonic nanostructure consisting of metallic nanodiscs separated from a metallic back-reflector by a uniform thin polymer film and investigate the potential for vibrant structural colour in reflection. We demonstrate that light trapping within the nanostructures is the primary mechanism for colour generation. The use of planar back-reflector and polymer layers allows for less complex fabrication requirements and robust structures, but most significantly allows for the easy incorporation of two different metals for the back-reflector and the nanodiscs. The simplicity of the structure is also suitable for scalability. Combinations of gold, silver, aluminium and copper are considered, with wide colour gamuts observed as a function of the polymer layer thickness. The structural colours are also shown to be insensitive to the viewing angle. Structures of copper nanodiscs with an aluminium back-reflector produce the widest colour gamut.

Carrier interactions in 2D nanostructures are of central importance not only in condensed-matter physics but also for a wide range of optoelectronic and photonic applications. Here, new insights into the behavior of photoinduced carriers in layered platinum diselenide (PtSe2 ) through ultrafast time-resolved pump-probe and nonlinear optical measurements are presented. The measurements reveal the temporal evolution of carrier relaxation, chemical potential and bandgap renormalization in PtSe2 . These results imply that few-layer PtSe2 has a semiconductor-like carrier relaxation instead of a metal-like one. The relaxation follows a triple-exponential decay process and exhibits thickness-dependent relaxation times. This occurs along with a band-filling effect, which can be controlled based on the number of layers and may be applied in saturable absorption for generating ultrafast laser pulses. The findings may provide means to study many-body physics in 2D materials as well as potentially leading to applications in the field of optoelectronics and ultrafast photonics.

The use of a metallic adhesion layer is known to increase the thermo-mechanical stability of Au thin films against solid-state dewetting, but in turn results in damping of the plasmonic response, reducing their utility in applications such as heat-assisted magnetic recording (HAMR). In this work, 50 nm Au films with Ti adhesion layers ranging in thickness from 0 to 5 nm were fabricated, and their thermal stability, electrical resistivity, and plasmonic response were measured. Subnanometer adhesion layers are demonstrated to significantly increase the stability of the thin films against dewetting at elevated temperatures (>200 °C), compared to more commonly used adhesion layer thicknesses that are in the range of 2-5 nm. For adhesion layers thicker than 1 nm, the diffusion of excess Ti through Au grain boundaries and subsequent oxidation was determined to result in degradation of the film. This mechanism was confirmed using transmission electron microscopy and X-ray photoelectron spectroscopy on annealed 0.5 and 5 nm adhesion layer samples. The superiority of subnanometer adhesion layers was further demonstrated through measurements of the surface-plasmon polariton resonance; those with thinner adhesion layers possessed both a stronger and spectrally sharper resonance. These results have relevance beyond HAMR to all Ti/Au systems operating at elevated temperatures.

We investigate nanolamination as a means of increasing the refractive index of materials, with a focus on [amorphous InGaZnO (a-IGZO)/TiO2](i) nanolaminates as composite transparent conducting oxides with a tunable index. We demonstrate that by periodic layering with TiO2 the refractive index of a-IGZO can be increased by up to Delta n approximate to 0.39 while the carrier mobility and conductivity retain 70% of their original values. This is shown to be a result of the alignment of the conduction bands of a-IGZO and TiO2. We also use optical modeling to verify the integral refractive index behavior of the nanolaminates and outline its range of applicability.

Ultrathin (>= 0.5 nm) adhesion and capping layers were used to inhibit dewetting in 50 nm Au thin films. A 1500x stability enhancement was found over standard 2 nm adhesion Ti layers used in HAMR.

Relaxor ferroelectrics exhibit a range of interesting material behavior, including high electromechanical response, polarization rotations, as well as temperature and electric field-driven phase transitions. The origin of this unusual functional behavior remains elusive due to limited knowledge on polarization dynamics at the nanoscale. Piezoresponse force microscopy and associated switching spectroscopy provide access to local electromechanical properties on the micro- and nanoscale, which can help to address some of these gaps in our knowledge. However, these techniques are inherently prone to artefacts caused by signal contributions emanating from electrostatic interactions between tip and sample. Understanding functional behavior of complex, disordered systems like relaxor materials with unknown electromechanical properties therefore requires a technique that allows distinguishing between electromechanical and electrostatic response. Here, contact Kelvin probe force microscopy (cKPFM) is used to gain insight into the evolution of local electromechanical and capacitive properties of a representative relaxor material lead lanthanum zirconate across the phase transition from a ferroelectric to relaxor state. The obtained multidimensional data set was processed using an unsupervised machine learning algorithm to detect variations in functional response across the probed area and temperature range. Further analysis showed the formation of two separate cKPFM response bands below 50 °C, providing evidence for polarization switching. At higher temperatures only one band is observed, indicating an electrostatic origin of the measured response. In addition, the junction potential difference, which was extracted from the cKPFM data, becomes independent of the temperature in the relaxor state. The combination of this multidimensional voltage spectroscopy technique and machine learning allows to identify the origin of the measured functional response and to decouple ferroelectric from electrostatic phenomena necessary to understand the functional behavior of complex, disordered systems like relaxor materials.

The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to the magnetization of a magnetically ordered metallic sample; in metallic ferrimagnets with chemically distinct sublattices, such as rare-earth transition-metal alloys, it depends on the difference between the sublattice contributions. Here we show that in a highly spin polarized, fully compensated ferrimagnet, where the sublattices are chemically similar, a signal is observed even when the net moment is negligible. We analyze the spectral ellipsometry and MOKE of Mn2Rux Ga and show that this behavior is due to a highly spin-polarized conduction band dominated by one of the two manganese sublattices (4c) which creates helicity-dependent reflectivity dominated by a broad Drude tail. Our findings open prospects for studying spin dynamics in the infrared.

The electrical, crystallographic, and magnetic properties of ultrathin magnetite (Fe3O4) have been studied in detail, by employing superlattice structures of Fe3O4/MgFe2O4 and Fe3O4/MgO on a variety of substrates. By careful analysis of their properties, the influence of substrate stoichiometry, Fe3O4 thin film thickness, antiphase boundaries on the magnetic properties can be separated. In particular, the controversial enhanced magnetic moment in ultrathin films (<5 nm) was confirmed to be related to the substrate stoichiometry, specifically the migration of oxygen vacancies into the Fe3O4 thin films. The multilayer concept can be employed with many other such systems and offers methods of tuning the properties of thin magnetic oxides.

The delafossite structured CuCrO2 system is well known as p-type transparent conducting oxide. We have synthesized a Cu deficient form at low process temperature maintaining its good conductive properties.

Propagating surface plasmon polaritons (SPPs) at metal–dielectric interfaces allow extreme light confinement enabling important technologies such as label-free real-time sensing and nanoscale optical wave guiding. In this work, we experimentally and theoretically map the surface plasmon dispersion of thin films of sequentially deposited silver and gold based on the Kretschmann–Raether configuration using spectroscopic ellipsometry. The results show efficient excitation of SPP modes related to the symmetric and antisymmetric modes of an insulator–metal–insulator waveguide. Since the symmetric modes have a larger electric field inside the bimetallic layer than the antisymmetric modes, we show that the dispersion for both modes can be tuned independently by controlling the thickness ratio and deposition order of metals in the film. This behavior is fundamentally different than modes in a single film or a thin film alloy. Results are in agreement with the analytical eigenmode dispersion of these modes and with a full transfer matrix model. Our results provide a complete understanding of surface plasmon modes in thin bilayer or multilayer metallic films and their ability to influence the propagation of surface plasmon waves.

We demonstrate an alternative approach to tuning the refractive index of materials. Current methodologies for tuning the refractive index of a material often result in undesirable changes to the structural or optoelectronic properties. By artificially layering a transparent conducting oxide with a lower refractive index material the overall film retains a desirable conductivity and mobility while acting optically as an effective medium with a modified refractive index. Calculations indicate that, with our refractive index change of 0.2, a significant reduction of reflective losses could be obtained by the utilisation of these structures in optoelectronic devices. Beyond this, periodic superlattice structures present a solution to decouple physical properties where the underlying electronic interaction is governed by different length scales.

Off-normal, polarization dependent second-harmonic generation (SHG) measurements were performed ex situ on plasmonic nanostructures grown by self-assembly on nanopatterned templates. These exploratory studies of Ag nanoparticle (NP) arrays show that the sensitivity of SHG to the local fields, which are modified by the NP size, shape and distribution, makes it a promising fixed wavelength characterization technique that avoids the complexity of spectroscopic SHG. The off-normal geometry provides access to the out-of-plane SH response, which is typically an order-of-magnitude larger than the in-surface-plane response measured using normal incidence, for example in SHG microscopy. By choosing the plane of incidence orthogonal to the NP array direction, it was shown that the p-polarized SH response, as a function of input polarization, is very sensitive to NP morphology, with a change of 20% in the aspect ratio of the NPs producing a variation of a factor of 30 in the easily measureable ratio of the p-polarized SH field strength for s- and p-polarized input. The results show that such a fixed geometry could be used for the in situ characterization of anisotropic nanostructure morphology during growth by self-assembly, which could be particularly useful in situations where rotating the sample may be neither desirable nor easily accomplished.

Reflectance anisotropy spectroscopy (RAS) has been used to measure the optical anisotropies of bulk and thin-film Fe3O4(110) surfaces. The spectra indicate that small shifts in energy of the optical transitions, associated with anisotropic strain or electric field gradients caused by the (110) surface termination or a native oxide layer, are responsible for the strong signal observed. The RAS response was then measured as a function of temperature. A distinct change in the RAS line-shape amplitude was observed in the spectral range from 0.8 to 1.6 eV for temperatures below the Verwey transition of the crystal. Finally, thin-film magnetite was grown by molecular beam epitaxy on MgO(110) substrates. Changes in the RAS spectra were found for different film thickness, suggesting that RAS can be used to monitor the growth of magnetite (110) films in situ. The thickness dependence of the RAS is discussed in terms of various models for the origin of the RAS signal.