Electronic Materials

Heteroepitaxial growth of Ge films on Si is necessary for the progress of integrated Si photonics technology. In this work, an in-house assembled plasma enhanced chemical vapor deposition reactor was used to grow high quality epitaxial Ge films on Si (100) substrates. Low economic and thermal budget were accomplished by the avoidance of ultra-high vacuum conditions or high temperature substrate pre-deposition bake for the process. Films were deposited with and without plasma assistance using germane (GeH₄) precursor in a single step at process temperatures of 350–385 °C and chamber pressures of 1–10 Torr at various precursor flow rates. Film growth was realized at high ambient chamber pressures (>10⁻⁶ Torr) by utilizing a rigorous ex situ substrate cleaning process, closely controlling substrate loading times, chamber pumping and the dead-time prior to the initiation of film growth. Plasma allowed for higher film deposition rates at lower processing temperatures. An epitaxial growth was confirmed by X-Ray diffraction studies, while crystalline quality of the films was verified by X-ray rocking curve, Raman spectroscopy, transmission electron microscopy and infra-red spectroscopy. Full article

In this paper, we investigate a commercial nematic liquid crystal (LC) mixture namely E7 dispersed with small concentrations of multi-walled carbon nanotubes (MWCNTs). The dielectric and electro-optical characterizations have been carried out in the homogeneously and vertically aligned LC cells. The electro-optical response of LC molecules has been enhanced by 60% after the addition of MWCNTs, which is attributed to the reduced rotational viscosity in the composites. MWCNTs act like barricades for ionic impurities by reducing them up to ∼34.3% within the dispersion limit of 0.05 wt%. The nematic–isotropic phase transition temperature (T${}_{NI}$) of the E7 LC has also been shifted towards the higher temperature, resulting in a more ordered nematic phase. The enhanced birefringence and orientational order parameter in the LC-MWCNTs are attributed to $\pi$-$\pi$ electron stacking between the LC molecules and the MWCNTs. The outlined merits of the LC-MWCNTs composites evince their suitability for ultrafast nematic-based electro-optical devices. Full article

Polyacrylonitrile (PAN) fibers with unique properties are becoming increasingly important as precursors for the fabrication of carbon fibers. Here, we suggest the preparation of porous PAN composite fibers to increase the homogeneity and thermal stability of the fibers. Based on the thermodynamics of polymer solutions, the ternary phase diagram of the PAN/H₂O/Dimethylformamide (DMF) system has been modeled to introduce porosity in the fibers. Adding a conscious amount of water (4.1 wt.%) as a non-solvent to the PAN solution containing 1 wt.% of graphene oxide (GO), followed by wet spinning, has led to the preparation of porous composite fibers with high thermal stability and unique physicochemical properties. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) results elucidate that PAN/GO/H₂O porous composite fibers have a higher thermal decomposition temperature, increased residual weight, reduced heat release rate, and higher crystallinity in comparison with the pristine PAN fibers, being a promising precursor for the development of high-performance carbon fibers. The results show a promising application window of the synthesized PAN fibers in electronic and electrochemical devices. Full article

In this work, we demonstrate P3HT (poly 3-hexylthiophene) organic rectifier diode both in rigid and flexible substrate with a rectification ratio up to 10⁶. This performance has been achieved through tuning the work function of gold with a self-assembled monolayer of 2,3,4,5,6-pentafluorobenzenethiol (PFBT). The diode fabricated on flexible paper substrate shows a very good electrical stability under bending tests and the frequency response is estimated at more than 20 MHz which is sufficient for radio frequency identification (RFID) applications. It is also shown that the low operating voltage of this diode can be a real advantage for use in a rectenna for energy harvesting systems. Simulations of the diode structure show that it can be used at GSM and Wi-Fi frequencies if the diode capacitance is reduced to a few pF and its series resistance to a few hundred ohms. Under these conditions, the DC voltages generated by the rectenna can reach a value up to 1 V. Full article

Solid solutions in the Ba(Pb_1−xSr_x)O_3−z system were prepared by aliovalent substitution of Pb⁴⁺ by Sr²⁺ ions to investigate the effect of cation stoichiometry on thermal and electrical properties as x was varied between 0 and 0.4, in the temperature range 300–523 K. The starting compound, barium plumbate (BaPbO₃), has a perovskite structure and is known to exhibit metallic conductivity due to an overlap of the O2p nonbonding and the Pb–O spσ antibonding band, which is partially filled by the available electrons. The large difference in the ionic radii between the Pb⁴⁺ and Sr²⁺ ions introduces significant strain into the (Pb/Sr)O₆ octahedra of the perovskite structure. Additionally, charged defects are created on account of the different oxidation states of the Pb⁴⁺ and Sr²⁺ ions. Evidence of a metal to insulator transition (MIT) of the Mott–Hubbard type has been observed at a critical concentration of Sr²⁺ ions. Full article

Implantable bioelectrodes have the potential to advance neural sensing and muscle stimulation, mainly in patients with peripheral nerve injuries. They function as the transducer at the interface between the damaged nerve and the muscle which is controlled by that nerve. This work reports the fabrication and characterization of novel, low-cost, flexible bioelectrodes based on polyaniline (PANI) and supported with silicone polymer. The fabricated electrodes were evaluated for their electrical and mechanical characteristics. PANI was used as the main transducer component in this fabrication. The characterization methods included electrical conductivity, capacitive behavior, long-term electrical impedance, and mechanical evaluation. The results of the fabricated PANI-silicone-based samples displayed a bulk impedance of 0.6 kΩ with an impedance of 1.6 kΩ at the frequency of 1 kHz. Furthermore, the bioelectrodes showed a charge storage capacity range from 0.0730 to 4.3124 C/cm². The samples were stable when subjected to cyclic voltammetry tests. The bioelectrodes revealed very flexible mechanical properties as observed from the value of Young’s modulus (in the order of MPa) which was less than that of skin. Hence, the PANI-based bioelectrodes reported herein showed promising electrochemical characteristics with high flexibility. Full article

In the field of printed electronics, there is a pressing need for printable resistors, particularly ones where the resistance can be varied without changing the size of the resistor. This work presents ink synthesis and printing results for variable resistance, inkjet-printed patterns of a novel and sustainable carbon nanomaterial—multilayer graphene nanoshells. Dispersed multilayer graphene nanospheres are sterically stabilized by a surfactant (Triton X100), and no post-process is required to achieve the resistive functionality. A surface tension-based adsorption analysis technique is used to determine the optimal surfactant dosage, and a geometric model explains the conformation of adsorbed surfactant molecules. The energetic interparticle potentials between approaching particles are modeled to assess and compare the stability of sterically and electrostatically stabilized multilayer graphene nanoshells. The multilayer graphene nanoshell inks presented here show a promising new pathway toward sustainable and practical printed resistors that achieve variable resistances within a constant areal footprint without post-processing. Full article

Using a combination of first principles and semiempirical calculation, we explore the structural, electronic, and optical properties of a wide range of perovskite ($AB{X}_3$) nanoparticle of different size and composition. The variations of the $B{X}_3$ backbone structure considered include all possible combinations of the cations $B=Pb,Sn$ and the anions $X=Cl,Br,I$, while the interstitial cation A is either methylamonium (MA), or formamidinium (FA), or guanidine amine (GA), or dimethylamine (DEA). Our results indicate that the orientational disorder of the A moieties may affect the structural and electronic properties of the NPs while the optical properties exhibit a clear dependence on the NPs’ size and the types of B cations and X anions, but they are quite insensitive to the type of A cation. Full article

In this study, we undertake a Bayesian optimization of the Hubbard U parameters of wurtzite GaN and InN. The optimized Us are then tested within the Hubbard-corrected local density approximation (LDA+U) approach against standard density functional theory, as well as a hybrid functional (HSE06). We present the electronic band structures of wurtzite GaN, InN, and (1:1) InGaN superlattice. In addition, we demonstrate the outstanding performance of the new parametrization, when computing the internal electric-fields in a series of [InN]${}_1$–[GaN]${}_n$ superlattices (n = 2–5) stacked up along the c-axis. Full article

A multi-level cell (MLC) operation as a 1–3 bit/cell of the FeFET emerging memory is reported by utilizing optimized Si doped hafnium oxide (HSO) and hafnium zirconium oxide (HZO) based on ferroelectric laminates. An alumina interlayer was used to achieve the thickness independent of the HSO and HZO-based stack with optimal ferroelectric properties. Various split thicknesses of the HSO and HZO were explored with lamination to increase the FeFET maximum memory window (MW) for a practical MLC operation. A higher MW occurred as the ferroelectric stack thickness increased with lamination. The maximum MW (3.5 V) was obtained for the HZO-based laminate; the FeFETs demonstrated a switching speed (300 ns), 10 years MLC retention, and 10⁴ MLC endurance. The transition from instant switching to increased MLC levels was realized by ferroelectric lamination. This indicated an increased film granularity and a reduced variability through the interruption of ferroelectric columnar grains. The 2–3 bit/cell MLC levels and maximum MW were studied in terms of the size-dependent variability to indicate the impact of the ferroelectric area scaling. The impact of an alumina interlayer on the ferroelectric phase is outlined for HSO in comparison to the HZO material. For the same ferroelectric stack thickness with lamination, a lower maximum MW, and a pronounced wakeup effect was observed in HSO laminate compared to the HZO laminate. Both wakeup effect and charge trapping were studied in the context of an MLC operation. The merits of ferroelectric stack lamination are considered for an optimal FeFET-based synaptic device operation. The impact of the pulsing scheme was studied to modulate the FeFET current to mimic the synaptic weight update in long-term synaptic potentiation/depression. Full article

Bio-based activated carbons with very high specific surface area of >3.000 m² g⁻¹ (based on CO₂ adsorption isotherms) and a high proportion of micropores (87% of total SSA) are produced by corncobs via pyrolysis and chemical activation with KOH. The activated carbon is further doped with different proportions of the highly pseudocapacitive transition metal oxide RuO₂ to obtain enhanced electrochemical properties and tune the materials for the application in electrochemical double-layer capacitors (EDLC) (supercapacitors). The activated carbon and composites are extensively studied regarding their physico-chemical and electrochemical properties. The results show that the composite containing 40 wt.% RuO₂ has an electric conductivity of 408 S m⁻¹ and a specific capacitance of 360 Fg⁻¹. SEM-EDX, XPS, and XRD analysis confirm the homogenous distribution of partly crystalline RuO₂ particles on the carbon surface, which leads to a biobased composite material with enhanced electrochemical properties. Full article

Tungsten oxide is a widely used electrochromic material with important applications in variable-transmittance smart windows as well as in other optoelectronic devices. Here we report on electrochemical impedance spectroscopy applied to hydrous electrochromic tungsten oxide films in a wide range of applied potentials. The films were able to reversibly bleach and color upon electrochemical cycling. Interestingly, the bleaching potential was found to be significantly higher than in conventional non-hydrous tungsten oxide films. Impedance spectra at low potentials showed good agreement with anomalous diffusion models for ion transport in the films. At high potentials, where little ion intercalation takes place, it seems that parasitic side reactions influence the spectra. The potential dependence of the chemical capacitance, as well as the ion diffusion coefficient, were analyzed. The chemical capacitance is discussed in terms of the electron density of states in the films and evidence was found for a band tail extending below the conduction band edge. Full article

Currently, a large number of neurostimulators are commercially available for the treatment of drug-resistant diseases and as an alternative to pharmaceuticals. According to the current state of the art, such highly engineered electroceuticals require bulky battery units and necessitate the use of leads and extensions to connect the implantable electronic device to the stimulation electrodes. The battery life and the use of wired electrodes constrain the long-term use of such implantable systems. Furthermore, for therapeutic success and patient safety, it is of utmost importance to keep the stimulation current within a safe range. In this paper, we propose an implantable system design that consists of a low number of passive electronic components and does not require a battery. The stimulation parameters and power are transmitted inductively using an extracorporeal wearable transmitter at frequencies below 1 MHz. A simple circuit design approach is presented to achieve a closed-loop control of the stimulation current by exploiting the nonlinear properties of ferroelectric materials in ceramic capacitors. Twenty circuit topologies of series- and/or parallel-connected ceramic capacitors are investigated by measurement and are modeled in Mathcad. An approximately linear increase in the stimulation current, a stabilization of the stimulation current and an unstable state of the system were observed. In contrast to previous results, specific plateau ranges of the stimulation current can be set by the investigated circuit topologies. For further investigations, the consistency of the proposed model needs to be improved for higher induced voltage ranges. Full article

We report an optimized chemical vapor transport method to grow single crystals of (Mn_1−xNi_x)₂P₂S₆ where x = 0, 0.3, 0.5, 0.7, and 1. Single crystals up to 4 mm × 3 mm × 200 μm were obtained by this method. As-grown crystals are characterized by means of scanning electron microscopy and powder X-ray diffraction measurements. The structural characterization shows that all crystals crystallize in monoclinic symmetry with the space group C2/m (No. 12). We have further investigated the magnetic properties of this series of single crystals. The magnetic measurements of the all as-grown single crystals show long-range antiferromagnetic order along all principal crystallographic axes. Overall, the Néel temperature T_N is non-monotonous; with increasing Ni²⁺ doping, the temperature of the antiferromagnetic phase transition first decreases from 80 K for pristine Mn₂P₂S₆ (x = 0) up to x = 0.5 and then increases again to 155 K for pure Ni₂P₂S₆ (x = 1). The magnetic anisotropy switches from out-of-plane to in-plane as a function of composition in (Mn_1−xNi_x)₂P₂S₆ series. Transport studies under hydrostatic pressure on the parent compound Mn₂P₂S₆ evidence an insulator-metal transition at an applied critical pressure of ~22 GPa. Full article

The band alignment of Mg(OH)₂-based heterostructures is investigated based on first-principles calculation. (111)-MgO/(0001)-Mg(OH)₂ and (0001)-wurtzite ZnO/(0001)-Mg(OH)₂ heterostructures are considered. The O 2s level energy is obtained for each O atom in the heterostructure supercell, and the band edge energies are evaluated following the procedure of the core-level spectroscopy. The calculation is based on the generalized gradient approximation with the on-site Coulomb interaction parameter U considered for Zn. For MgO/Mg(OH)₂, the band alignment is of type II, and the valence band edge of MgO is higher by 1.6 eV than that of Mg(OH)₂. For ZnO/Mg(OH)₂, the band alignment is of type I, and the valence band edge of ZnO is higher by 0.5 eV than that of Mg(OH)₂. Assuming the transitivity rule, it is expected that Mg(OH)₂ can be used for certain types of heterostructure solar cells and dye-sensitized solar cells to improve the performance. Full article

The article demonstrates an original, non-destructive technique that could be used to in situ monitor charge transport in organic light-emitting diodes. Impedance spectroscopy was successfully applied to determine an OLED’s charge carrier mobility and average charge density in the hole- and electron-transport layer in a range of applied voltages. The fabricated devices were composed of two commercially available materials: NPB (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) and TPBi (2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) as hole- and electron-transport layers, respectively. By varying the thicknesses of the hole-transport layer (HTL) and the electron-transport layer (ETL), correlations between layer thickness and both charge carrier mobility and charge density were observed. A possibility of using the revealed dependencies to predict diode current–voltage characteristics in a wide range of applied voltage has been demonstrated. The technique based on a detailed analysis of charge carrier mobilities and densities is useful for choosing the appropriate transport layer thicknesses based on an investigation of a reference set of samples. An important feature of the work is its impact on the development of fundamental research methods that involve AC frequency response analysis by providing essential methodology on data processing. Full article

In this article, we summarize the synthetic approaches developed in our research groups during the last decade to efficiently tune the optical, electrochemical and morphological characteristics of oligothiophene–naphthalimide assemblies. Different variables were tuned in these organic semiconductors, such as the planarity and the length of their π-conjugated backbones, the topology and energy levels of the frontier molecular orbitals (HOMO and LUMO) and their molecular dipole moments. The tuning of these properties can be connected with the microstructure properties observed by atomic force microscopy (AFM) and X-ray diffraction (XRD) in thin films as well as with the performances in organic field-effect transistors (OFETs). The possibility of incorporating these donor-acceptor assemblies into macromolecular structures is also addressed, and some innovative applications for these macromolecular systems, such as the degradation of organic pollutants in aqueous media, are also presented. Full article

Organic optoelectronic device behaviour is heavily dependent on interfacial effects due to the device architecture and thickness. Interfaces between the inorganic electrodes and the active organic layers play a defining role in the all of the electronic and stability processes that occur in organic light emitting diodes (OLEDs) and organic solar cells (OPVs). Amongst the many interlayers introduced at these interfaces to improve charge carrier movement and stability, LiF has proven to be the most successful and it is almost ubiquitous in all organic semiconductor devices. Implemented at both top and bottom contact interfaces, doped into the charge transporting layers, and used as encapsulants, LiF has played major roles in device performance and lifetime. This review highlights the use of LiF at both top and bottom contacts in organic optoelectronics, discusses the various mechanisms proposed for the utility of LiF at each interface, and explores its impact on device lifetimes. From examples relating to charge carrier flow, interfacial electronic level modification, and interfacial stability, a comprehensive picture of the role of LiF in organic devices can be formed. This review begins with a brief overview of the role of the interface in OLEDs and OPVs, and the general properties of LiF. Then, it discusses the implementation of LiF at the top contact electrode interface, followed by the bottom substrate contact electrode, examining both performance and degradation effects in both cases. Full article

Silk fibroin is a well-known biopolymer that is used in several applications in which interactions with biological tissue are required. Fibroin is extremely versatile and can be shaped to form several constructs that are useful in tissue engineering applications. Confocal imaging is usually performed to test cell behavior on a construct, and, in this context, the fibroin intrinsic fluorescence is regarded as a problem. In addition, the intrinsic fluorescence is not intense enough to provide useful morphological images. In fact, to study the construct’s morphology, other techniques are used (i.e., SEM and Micro-CT). In this work, we propose a method based on fluorescence energy transfer (FRET) to suppress the fibroin intrinsic fluorescence and move it to a higher wavelength that is accessible to confocal microscopy for direct imaging. This was done by creating two FRET couples by dispersing two fluorophores (2,5-diphenyloxazole (PPO) and Lumogen F Violet 570 (LV)) into the fibroin matrix and optimizing their percentages to suppress the fibroin intrinsic fluorescence. With the optimized composition, we produced an electrospun mat, and the dimensions of the fibers were accurately determined by confocal microscopy. Full article