Search Results (195)

Bilayer transition metal dichalcogenides (TMDs) are promising materials for next-generation field-effect transistors (FETs) due to their atomically thin structure and favorable transport properties. In this study, we employ density functional theory (DFT) to compute the electronic band structures and phonon dispersions of bilayer WS${}_2$, WSe${}_2$, and MoS${}_2$, and the electron-phonon scattering rates using the EPW (electron-phonon Wannier) method. Carrier transport is then investigated within a semiclassical full-band Monte Carlo framework, explicitly including intrinsic electron-phonon scattering, dielectric screening, scattering with hybrid plasmon–phonon interface excitations (IPPs), and scattering with ionized impurities. Freestanding bilayers exhibit the highest mobilities, with hole mobilities reaching 2300 cm${}^2$/V·s in WS${}_2$ and 1300 cm${}^2$/V·s in WSe${}_2$. Using hBN as the top gate dielectric preserves or slightly enhances mobility, whereas HfO${}_2$ significantly reduces transport due to stronger IPP and remote phonon scattering. Device-level simulations of double-gate FETs indicate that series resistance strongly limits performance, with optimized WSe${}_2$ pFETs achieving ON currents of 820 A/m, and a 10% enhancement when hBN replaces HfO${}_2$. These results show the direct impact of first-principles electronic structure and scattering physics on device-level transport, underscoring the importance of material properties and the dielectric environment in bilayer TMDs. Full article

We report calculations for electron–radon scattering using a complex relativistic optical potential method. The energy range of this study is 0–1000 eV, with results for the elastic (total, momentum-transfer and viscosity-transfer) cross section, summed discrete electronic-state integral excitation cross sections and electron-impact ionization cross sections presented. Here, we obtain our cross sections from a single theoretical relativistic calculation. Since radon is a heavy element, a relativistic treatment is very desirable. The electron transport coefficients are subsequently calculated for reduced electric fields ranging from 0.01 to 1000 Td, using a multi-term solution of Boltzmann’s equation. Full article

Given the pivotal role of coumarins as tunable nonlinear optical (NLO) materials for advanced photonics, this study aims to decipher the regulatory mechanisms governing their excited-state dynamics and nonlinear absorption. In this study, two amino-coumarin dyes (102 and 153) differing in electron-withdrawing groups are synthesized to probe the impact of intramolecular charge transfer (ICT) on transient dynamics and nonlinear absorption. Frontier orbital and natural transition orbital analyses reveal subtle alterations in the ICT characteristics of amino-coumarin molecules. These minor modifications induce a significant red shift in the stimulated emission band within transient absorption spectroscopy, ultimately triggering a transition from reverse saturable absorption (RSA) to saturable absorption (SA) at 515 nm. Our findings demonstrate that, with straightforward molecular modifications, coumarins emerge as promising dual-function materials for saturable absorption and optical limiting. Full article

Gallium Nitride (GaN) is a wide-bandgap semiconductor that has revolutionized optoelectronic applications, enabling blue/white light-emitting devices and high-power electronics. Point defects in GaN strongly influence its optical and electronic properties, producing both beneficial and detrimental effects. This review provides a comprehensive update on the current understanding of point defects in GaN and their impact on photoluminescence (PL). Since our earlier review (Reshchikov and Morkoç, J. Appl. Phys. 2005, 97, 061301), substantial progress has been made in this field. PL bands associated with major intrinsic and extrinsic defects in GaN are now much better understood, and several defects in undoped GaN (arising from unintentional impurities or specific growth conditions) have been identified. Notably, the long-debated origin of the yellow luminescence band in GaN has been resolved, and the roles of Ga and N vacancies in the optical properties of GaN have been revised. Zero-phonon lines have been discovered for several defects. Key parameters, such as electron- and hole-capture coefficients, phonon energies, electron–phonon coupling strength, thermodynamic charge transition levels, and the presence of excited states, have been determined or refined. Despite these advances, several puzzles associated with PL remain unsolved, highlighting areas for future investigation. Full article

Photocatalysis using complexes of d⁰ metals with ligand-to-metal charge-transfer (LMCT) excited states is an active area of research. Because titanium is the second most abundant transition metal in the earth’s crust, d⁰ complexes of Ti^IV are an appropriate target for this research. Recently, our group has demonstrated that the arylethynyltitanocene Cp₂Ti(C₂Ph)₂CuBr is not emissive in room-temperature fluid solution, whereas the corresponding Cp* complex, Cp*₂Ti(C₂Ph)₂CuBr, is emissive. The Cp* ligand is hypothesized to provide steric constraint that inhibits excited-state structural rearrangement. However, modifying the structure also changes the orbital character of the excited state. To investigate the impact of the excited-state orbital character on the photophysics, herein we characterize complexes similar to Cp*₂Ti(C₂Ph)₂CuBr—but one with a more electron-rich arylethynyl ligand, ethynyldimethylaniline (C₂DMA), and one with a more electron-poor arylethynyl ligand, ethynyl-α,α,α-trifluorotoluene. We have also prepared complexes with the C₂DMA ligand but with different Cp ligands that adjust the steric bulk and constraint around the Ti, by replacing the Cp* ligands with either indenyl ligands or an ansa-cyclopentadienyl ligand where the two Cp ligands are bridged by a dimethylsilylene. All four target complexes have been characterized crystallographically and structure activity relationships are highlighted. Full article

Nanodiamonds in water can generate solvated electrons under ultraviolet (UV) excitation, but UV light constitutes only a small portion of solar energy. To harvest solar energy in the visible range, we investigate band gap reduction via surface amino functionalization and examine its impact on photo-excited charge transfer to water. Adamantane, the smallest nanodiamond, is used as a model due to its electron emission properties. Liquid water is first represented using water dimers and then complete solvation shell structures surrounding the adamantane. By systematically analyzing different functionalized adamantane structures, we find that nitrogen serves as the primary electron donor to nearby water molecules. Furthermore, the negative electron affinity of adamantane, which determines its emission capability, is preserved with half of the amino group functionalization on the surface. Our findings motivate further experimental verification using nanodiamonds with amino-functionalized surfaces. Full article

Non-thermal plasma technology is used in the food sector due to its many advantages such as low operating costs, fast and efficient processing at low temperatures, minimal environmental impact, and preservation of sensory and nutritional properties. In this article, the plasma was generated using a high-voltage electrical discharge (HVED) with argon at a voltage of 35 kV and a frequency of 60 Hz. Plasma monitoring and diagnostics were performed using optical emission spectroscopy (OES) to optimise the process parameters and for quality control. OES was used as a non-invasive sensor to collect useful information about the properties of the plasma and to identify excited species. The values obtained for electron temperature and electron density (up to $2.3 \mathrm{eV}$ and up to ${10}^{23} {\mathrm{m}}^{-3}$) confirmed that the generated plasma is a non-thermal plasma. Therefore, the use of OES is recommended in the daily control of food processing, as this is necessary to confirm that the processes are non-thermal and suitable for the food sector. Full article

Split-shaft wind turbines decouple the turbine’s shaft from the generator’s shaft, enabling several modifications in the drivetrain. One of the significant achievements of a split-shaft drivetrain is the reduction in size of the excitation circuit. The grid-side converter is eliminated, and the rotor-side converter can safely reduce its size to a fraction of a full-size excitation. Therefore, this low-power-rated converter operates at low voltage and handles regular operations well. However, fault conditions may expose weaknesses in the converter and push it to its limits. This paper investigates the effects of the reduced-size rotor-side converter on the voltage ride-through capabilities required from all wind turbines. Four different protection circuits, including the active crowbar, active crowbar along a resistor–inductor circuit (C-RL), series dynamic resistor (SDR), and new-bridge fault current limiter (NBFCL), are employed, and their effects are investigated and compared. Wind turbine controllers are also utilized to reduce the impact of faults on the power electronic converters. One effective method is to store excess energy in the generator’s rotor. The proposed low-voltage ride-through strategies are simulated in MATLAB Simulink (2022b) to validate the results and demonstrate their effectiveness and functionality. Full article

The development of alternative energy is crucial to realizing the goals of the Paris Agreement. Hydrogen is a key energy carrier, and ammonia is considered its practical storage medium due to its high H₂ content and efficient storage and transportation. However, efficient NH₃ decomposition methods are needed to recover stored hydrogen. Plasma-assisted decomposition offers a potential solution, but high energy consumption, mainly due to inefficient power supply systems, remains a challenge. This study examines the impact of varying the driving frequency of a gliding discharge plasma system on ammonia decomposition, comparing low-frequency 50 Hz and high-frequency 20 kHz power supplies. Results show that high-frequency plasma enhances electron density and energy distribution, increasing the amount of vibrationally excited nitrogen molecules. This improves catalyst activation, leading to higher ammonia conversion and hydrogen production. Compared to the thyristor-powered system, the high-frequency system increased ammonia decomposition productivity by 30% and reduced energy consumption by 36% using a coprecipitated catalyst. These findings emphasize the importance of a plasma generator optimizing plasma-assisted ammonia decomposition and improving efficiency in hydrogen production. Full article

Monolayer (ML) molybdenum disulfide (MoS₂) is a typical valleytronic material which has important applications in, for example, polarization optics and information technology. In this study, we examine the effect of continuous wave (CW) laser pumping on the basic optoelectronic properties of ML MoS₂ placed on a sapphire substrate, where the pump photon energy is larger than the bandgap of ML MoS₂. The pump laser source is provided by a compact semiconductor laser with a 445 nm wavelength. Through the measurement of THz time-domain spectroscopy, we obtain the complex optical conductivity for ML MoS₂, which are found to be fitted exceptionally well with the Drude–Smith formula. Therefore, we expect that the reduction in conductivity in ML MoS₂ is mainly due to the effect of electronic backscattering or localization in the presence of the substrate. Meanwhile, one can optically determine the key electronic parameters of ML MoS₂, such as the electron density n_e, the intra-band electronic relaxation time τ, and the photon-induced electronic localization factor c. The dependence of these parameters upon CW laser pump intensity is examined here at room temperature. We find that 445 nm CW laser pumping results in the larger n_e, shorter τ, and stronger c in ML MoS₂ indicating that laser excitation has a significant impact on the optoelectronic properties of ML MoS₂. The origin of the effects obtained is analyzed on the basis of solid-state optics. This study provides a unique and tractable technique for investigating photo-excited carriers in ML MoS₂. Full article

A potential field of study for improving the efficiency of next-generation photovoltaic devices hot carriers in perovskite solar cells is investigated in this review paper. Considering their relevance to hot carrier dynamics, the paper thoroughly studies metal halide perovskites’ essential characteristics and topologies. We review important aspects like carrier excitation, exciton binding energy, phonon coupling, carrier excitation, thermalization, and hot hole and hot electron dynamics. We investigate, in particular, the significance of relaxation mechanisms, including thermalization and the Auger heating effect. Moreover, the bottleneck effect and defect management are discussed with an eye on their impact on device performance and carrier behaviour. A review of experimental methods for their use in investigating hot carrier dynamics, primarily transient photovoltage measurements, is included. Utilizing this thorough investigation, we hope to provide an insightful analysis of the difficulties and techniques for reducing the effect of hot carriers in perovskite solar cells and optimizing their performance. Full article

This is a numerical investigation of optical and electronic characteristics of GaAs spherical quantum dots based on single and double quartic potentials and presenting a hydrogenic impurity at their center. The radial Schrödinger equation was solved using the finite difference method (FDM) to obtain the energy levels and the wavefunctions. These physical quantities were then used to compute the dipole matrix elements, the total optical absorption coefficient (TOAC), and the binding energies. The impact of the structural parameters in the confining potentials on the red and blue shifts of the TOAC is discussed in the presence and absence of hydrogenic impurity. Our results indicate that the structural parameter $k$ in both potentials plays a crucial role in tuning the TOAC. In the case of single quartic potential, increasing $k$ produces a blue shift; however, its augmentation in the case of double quartic potential displays a blue shift at first, and then a red shift. Furthermore, the augmentation of the parameter $k$ can control the binding energies of the two lowest states, ($1s$) and ($1p$). In fact, enlarging this parameter reduces the binding energies and converges them to constant values. In general, the modification of the potential’s parameters, which can engender two shapes of confining potentials (single quartic and double quartic), enables the experimenters to control the desired energy levels and consequently to adjust and select the suitable TOAC between the two lowest energy states (ground ($1s$) and first excited ($1p$)). Full article

The study of the excited-state properties of diamond is crucial for understanding its electronic structure and surface physicochemical properties, providing theoretical support for its applications in optoelectronic devices, quantum technologies, and catalysis. This research employs Density Functional Theory (DFT) with the fixed electron occupation method to simulate the electron excitation. Using the Generalized Gradient Approximation (GGA) within DFT, we systematically investigated the excited-state characteristics of diamond by simulating the transfer of a fraction of electrons from the Highest Occupied Crystal Orbital (HOCO) to the Lowest Unoccupied Crystal Orbital (LUCO). Theoretical calculations indicate that with increasing electron excitation levels, the diamond crystal structure transitions from cubic to tetragonal, accompanied by a gradual decrease in the bandgap. Mechanical property analysis reveals that both Young’s modulus and shear modulus decrease with increasing excitation rate, while the bulk modulus remains nearly constant. These findings indicate a significant impact of electronic excitation on the mechanical stability of diamond. Phonon dispersion curves exhibit reduced degeneracy in high-frequency optical branches and a marked decrease in crystal symmetry upon excitation. This study not only advances the understanding of diamond’s excited-state properties but also offers valuable theoretical insights into its structural evolution and performance tuning under such extreme conditions. Full article

This study presents a comprehensive analysis of the modifications in electronic structure and magnetism resulting from electronic excitation in pulsed laser-deposited Ba_0.7Sr_0.3Fe_xTi_(1−x)O₃ thin films, specifically for compositions with x = 0, 0.1, and 0.2. To investigate the effects of electronic energy loss (S_e) within the lattice, we performed 120 MeV Ag ion irradiation at varying fluences (1 × 10¹² ions/cm² and 5 × 10¹² ions/cm²) and compared the results with those of the pristine sample. The S_e induces lattice damage by generating ion tracks along its trajectory, which subsequently leads to a reduction in peak intensity observed in X-ray diffraction patterns. Atomic force microscopy micrographs indicate that irradiation resulted in a decrease in average grain height, accompanied by a more homogeneous grain distribution. X-ray photoelectron spectroscopy reveals a significant increase in oxygen vacancy (V_O) concentration as ion fluence increases. Ferromagnetism exhibits progressive deterioration with rising irradiation fluence. Due to the high S_e and multiple ion impact processes, cation interstitial defects are highly likely, which may overshadow the influence of V_O in inducing ferromagnetism, thereby contributing to an overall decline in magnetic properties. Furthermore, the elevated S_e potentially disrupts bound magnetic polarons, leading to a degradation of long-range ferromagnetism. Collectively, this investigation elucidates the electronic excitation-induced modulation of ferromagnetism, employing Fe impurity incorporation and irradiation techniques for precise defect engineering. Full article

Photodynamic therapy (PDT) is a minimally invasive technique—used for the local eradication of neoplastic cells—that exploits the interaction of light, oxygen, and a photo-responsive drug called photosensitizer (PS) for the local generation of lethal ROS. Push-pull chromophores, that bear electron donor (D) and acceptor (A) groups linked through a π-electron bridge, are characterized by a non-homogeneous charge distribution in their excited state, with charge transfer from one extremity of the chain to the other one (Internal Charge Transfer—ICT). This phenomenon has a direct impact on the photophysical features of the push-pull compounds, as the bathochromic shift of the emission maxima and intersystem crossing (ISC) of the excited state are directly connected with the production of reactive oxygen species (ROS). In continuing our research regarding the synthesis and use of oligophenylene ethynylenes (OPEs) in PDT, two new push-pull glycosyl OPE-NOF and OPE-ONF—featuring electron-donor N,N-dimethylamino (N) and dimetoxyaryl (O) and acceptor tetrafluoroaryl (F) moieties on the OPE chain—have been efficiently prepared. The interchanged position of the D groups onto the conjugated skeleton was aimed to tune and optimize the push-pull effect, while the introduction of glucoside terminations was directed to give biocompatibility and bioaffinity to the chromophores. OPE-NOF, OPE-ONF, and the synthetic intermediates were fully characterized, and their photophysical properties were investigated by using UV-Vis absorption and emission spectroscopy. OPE-NOF showed a strong charge-transfer character and high PDT effect on HeLa cancer cells when irradiated with non-harmful blue light, causing massive cancer cell death. Full article