Ambipolar surface conduction in oxygen sub-stoichiometric molybdenum oxide films.

The surface electric conduction in amorphous and crystallized molybdenum oxide films was studied as a function of electronic structure by current-voltage and simultaneous spectroscopic ellipsometry measurements on structures of the kind Al/Molybdenum oxide (MoOx)/Al, at temperatures up to 400 °C and in ambient air. At room temperature, both amorphous and crystalline MoOx samples were found to be sub-stoichiometric in oxygen. The random distribution of oxygen vacancies and the imperfect atomic ordering induced the creation of an intermediate band (IB) located near the valence band and of individual electronic gap states. At temperatures below 300 °C, the conduction was found to exhibit ambipolar character in which electrons and holes participated, the former moving in the conduction band and the latter in the IB and though gap states. Above 300 °C, due to samples gradual oxidation and improvement of atomic ordering (samples crystallization), the density of states in the IB and the gap gradually decreased. The above in their turn resulted in the gradual suppression of the ambipolar character of the conduction, which at 400 °C was completely suppressed and became similar to that of ordinary n-type semiconductor. The above phenomena were found to be reversible, so as the semiconducting MoOx samples were returning to room temperature the ambipolarity of the conduction was gradually re-appearing giving rise to an unusual phenomenon of "metallic" temperature variation of electrical resistance when electrons were injected.

Investigation of mode coupling in strained and unstrained multimode step-index POFs using the Langevin equation.

The Langevin equation (LE) is used to evaluate mode coupling in multimode step-index polymer optical fiber (SI POF) that is both unstrained and strained. The numerical solution of the LE matches the numerical solution of the power flow equation (PFE). Strain-induced mode coupling is noticeably stronger in strained fiber than in unstrained fiber of the same types. Therefore, compared to similar lengths for unstrained fibers, the coupling length of the equilibrium mode distribution (EMD) is attained and the length of fiber required to produce a steady-state distribution (SSD) are both much shorter for strained fibers. We have demonstrated that the mode coupling in strained and unstrained multimode SI POFs that comes from the random perturbations (RPs) of the fiber can be successfully treated by the LE. The study's findings can be used to improve communication and sensory systems that use multimode SI POFs under different bending circumstances. Additionally, it is crucial to be able to compute the modal distribution of the SI POFs used in the optical fiber sensory system at a specific length and under various bending scenarios.

Correction: Savovic et al. Power Flow in Multimode Graded-Index Microstructured Polymer Optical Fibers. Polymers 2023, 15, 1474.

The authors wish to make three changes to their published paper [...].

Investigation of space division multiplexing in multimode step-index silica photonic crystal fibers.

The feasible distance is presented for space division multiplexed (SDM) transmission along multimode silica step-index photonic crystal fiber (SI PCF) by solving the time-independent power flow equation (TI PFE). These distances for two and three spatially multiplexed channels were determined to depend on mode coupling, fiber structural parameters, and launch beam width in order to keep crosstalk in two- and three-channel modulation to a maximum of 20% of the peak signal strength. We found that the length of the fiber at which an SDM can be realized increases with the size of the air-holes in the cladding (higher NA). When a wide launch excites more guiding modes, these lengths become shorter. Such knowledge is valuable for the use of multimode silica SI PCFs in communications.

Power Flow in Multimode Graded-Index Microstructured Polymer Optical Fibers.

We investigate mode coupling in a multimode graded-index microstructured polymer optical fiber (GI mPOF) with a solid core by solving the time-independent power flow equation (TI PFE). Using launch beams with various radial offsets, it is possible to calculate for such an optical fiber the transients of the modal power distribution, the length Lc at which an equilibrium mode distribution (EMD) is reached, and the length zs for establishing a steady-state distribution (SSD). In contrast to the conventional GI POF, the GI mPOF explored in this study achieves the EMD at a shorter length Lc. The earlier shift to the phase of slower bandwidth decrease would result from the shorter Lc. These results are helpful for the implementation of multimode GI mPOFs as a part of communications and optical fiber sensory systems.

Study of Single and Multipass f-rGO Inkjet-Printed Structures with Various Concentrations: Electrical and Thermal Evaluation.

Reduced graphene oxide (rGO) is a derivative of graphene, which has been widely used as the conductive pigment of many water-based inks and is recognized as one of the most promising graphene-based materials for large-scale and low-cost production processes. In this work, we evaluate a custom functionalised reduced graphene oxide ink (f-rGO) via inkjet-printing technology. Test line structures were designed and fabricated by the inkjet printing process using the f-rGO ink on a pretreated polyimide substrate. For the electrical characterisation of these devices, two-point (2P) and four-point (4P) probe measurements were implemented. The results showed a major effect of the number of printed passes on the resulting resistance for all ink concentrations in both 2P and 4P cases. Interesting results can be extracted by comparing the obtained multipass resistance values that results to similar effective concentration with less passes. These measurements can provide the ground to grasp the variation in resistance values due to the different ink concentrations, and printing passes and can provide a useful guide in achieving specific resistance values with adequate precision. Accompanying topography measurements have been conducted with white-light interferometry. Furthermore, thermal characterisation was carried out to evaluate the operation of the devices as temperature sensors and heaters. It has been found that ink concentration and printing passes directly influence the performance of both the temperature sensors and heaters.

Magnetic Resonance Studies of Hybrid Nanocomposites Containing Nanocrystalline TiO2 and Graphene-Related Materials.

Nanocomposites based on nanocrystalline titania modified with graphene-related materials (reduced and oxidized form of graphene) showed the existence of magnetic agglomerates. All parameters of magnetic resonance spectra strongly depended on the materials' modification processes. The reduction of graphene oxide significantly increased the number of magnetic moments, which caused crucial changes in the reorientation and relaxation processes. At room temperature, a wide resonance line dominated for all nanocomposites studied and in some cases, a narrow resonance line derived from the conduction electrons. Some nanocomposites (samples of titania modified with graphene oxide, prepared with the addition of water or butan-1-ol) showed a single domain magnetic (ferromagnetic) arrangement, and others (samples of titania modified with reduced graphene oxide) exhibited magnetic anisotropy. In addition, the spectra of EPR from free radicals were observed for all samples at the temperature of 4 K. The magnetic resonance imaging methods enable the capturing of even a small number of localized magnetic moments, which significantly affects the physicochemical properties of the materials.

Carbon Nanodots as Electron Transport Materials in Organic Light Emitting Diodes and Solar Cells.

Charge injection and transport interlayers play a crucial role in many classes of optoelectronics, including organic and perovskite ones. Here, we demonstrate the beneficial role of carbon nanodots, both pristine and nitrogen-functionalized, as electron transport materials in organic light emitting diodes (OLEDs) and organic solar cells (OSCs). Pristine (referred to as C-dots) and nitrogen-functionalized (referred to as NC-dots) carbon dots are systematically studied regarding their properties by using cyclic voltammetry, Fourier-transform infrared (FTIR) and UV-Vis absorption spectroscopy in order to reveal their energetic alignment and possible interaction with the organic semiconductor's emissive layer. Atomic force microscopy unravels the ultra-thin nature of the interlayers. They are next applied as interlayers between an Al metal cathode and a conventional green-yellow copolymer-in particular, (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1',3}-thiadiazole)], F8BT)-used as an emissive layer in fluorescent OLEDs. Electrical measurements indicate that both the C-dot- and NC-dot-based OLED devices present significant improvements in their current and luminescent characteristics, mainly due to a decrease in electron injection barrier. Both C-dots and NC-dots are also used as cathode interfacial layers in OSCs with an inverted architecture. An increase of nearly 10% in power conversion efficiency (PCE) for the devices using the C-dots and NC-dots compared to the reference one is achieved. The application of low-cost solution-processed materials in OLEDs and OSCs may contribute to their wide implementation in large-area applications.