Density Of States Distribution Research Articles

Tailoring the Density of State of n-Type Conjugated Polymers through Solvent Engineering for Organic Electrochemical Transistors.

Conjugated polymers with ethylene glycol-type side chains are commonly used as channel materials in organic electrochemical transistors (OECTs). To improve the performance of these materials, new chemical structures are often created through synthetic routines. Herein, we demonstrate that the OECT performance of these polymers can also be improved by changing their density-of-state (DOS) profile through solvent engineering. Depending on the solvent polarity, it solvates the backbone and side chains of the conjugated polymer differently, leading to differences in molecule orientation, π-stacking paracrystallinity, and film defects, such as grain boundaries and pinholes. This then results in a change in the DOS profile of the polymer. A more intense and narrow-width DOS distribution is usually observed in organic films with an "edge on" orientation and fewer film defects, while films with a "face on" orientation and apparent defects show a broadened DOS profile. The OECT devices that use the polymer film with a more intense and narrow-width DOS profile exhibit a better-normalized transconductance and figure-of-merit μC* than those with a broadened DOS profile (0.74 to 4.29 S cm-1 and 3.5 to 14.3 F cm-1 V-1 s-1). This study provides useful insights into how the DOS profile affects the mixed ionic-electronic conduction performance and presents a new avenue for improving n-type OECT materials.

Distribution of Density of States in Organic Field–Effect Transistors Based on Polymer Dielectrics

AbstractThe distribution of density of states (DOS) holds fundamental importance in determining charge transport within organic field–effect transistors (OFETs). Herein, the modulation of DOS distribution in OFET devices is demonstrated by altering the chain conformation of the polymer dielectrics. A rapid film‐formation technique, specifically the spin‐casting method, is used to fabricate the dielectric layer using poly(methyl methacrylate) (PMMA). This method allows for the retention of some memory of the chain conformations from the solution to the resulting dry film. This memory effect is employed to prepare thin PMMA films with different local chain conformations by adjusting the quality of the solvent. Good solvent forms solidified films with a reduced amount of gauche conformer in the PMMA chain, resulting in a narrow DOS distribution width. Consequently, the device exhibited enhanced charge mobility and a reduced subthreshold swing. The observed change in the width of the DOS distribution can be attributed to the alteration of the local energy state of the semiconductor, induced by the local chain conformation of PMMA dielectrics through electrostatics and steric interactions.

Impact of channel thickness on device scaling in vertical InGaZnO channel charge-trap memory transistors with ALD Al2O3 tunneling layer

This study investigates the impact of channel thickness (TCH) variation on memory performance and its physical origins in vertical channel charge trap memory (V-CTM) using InGaZnO (IGZO) channels for X–Y scaling. When the TCH decreased, the subthreshold slope (SS) increased from 0.21 to 0.30 V/dec, while the minimum program voltage decreased from 12 to 8 V, indicating a performance trade-off. The analysis of bulk trap density extracted by SS, density of states distribution, and x-ray photoelectron spectroscopy confirmed an increase in the number of trap states and hydrogen-related defects in the IGZO channel when a thin TCH is employed. As a result, it was expected that the barrier height at the active layer/TL would be lowered during program operations at relatively thin channel. However, the SS would also be degraded due to an increase in sub-gap states, such as OH- and VOH, induced by hydrogen incorporation. The V-CTM with an optimal TCH of 7 nm had a maximum memory window of 5.7 V. The program/erase states were maintained for 106 s before experiencing a 50 % charge loss, and 5-level memory states were verified to be available within a memory window of 3.2 V for 108 s with little degradation. It was noteworthy that the long-term reliability and multilevel implementation could be secured for the V-CTM with a TCH as thin as 7 nm, demonstrating the sound feasibility of its X–Y scaling. This work presents new key parameters and understanding of device integration issues for future 3D structured memory transistors, and aims to contribute to the advancement of memory technologies using oxide semiconductor channels.

Electrical and dielectric behaviors of Al/SiO2-surfactant/n-Si schottky structure in wide range of voltage and frequency

SiO2 surfactant insulator into Al/n-Si metal-semiconductor (MS) structure was fabricated into Al/SiO2-surfactant/n-Si metal–insulator–semiconductor (MIS) structure and its effect on the electrical properties of the final structure was investigated. The SiO2-surfactant layer is coated on the n-Si wafer by the spin coating technique. The I-V data is used to calculate the fundamental electrical parameters of this MIS structure. The density distribution of the surface states (Nss) is computed depending on the energy at forward potential. The current conduction mechanisms (CCMs) in the MIS structure are examined at the reverse and forward biases applied. To get more accurate and reliable results, the profiles of I-V and C/(G/ω)-V are measured at a wide range of bias voltage (0.25V-4V) and frequency (1kHz-1MHz), respectively. The performance of the MIS is significant due to the basic values of electrical parameters (n, I0, Rs, Rsh, Nss, ΦB0, and Rectifying Ration (RR)) and dielectric parameters (ε′, ε″, tan δ, M′, M″, Rs, and σ) compared with the MS structure. The other electrical parameters (ND, WD, Em, ΦΒ) are extracted from the slope and intercept of the reverse bias C−2-V plot as a function of frequency. Furthermore, the profile of voltage-dependent Rs and Nss was determined using different methods from I-V and C/G-V data and examined comparatively with each other. The changes in impedance properties with frequency and voltage of the MIS are discussed in detail.

Electron Scattering from Stable and Exotic Li Isotopes

The nuclear shell model was used to investigate the bulk properties of lithium isotopes (6,7,8,9,11Li), i.e., the ground state density distributions and C0 and C2 components of charge form factors. The theoretical treatment was based on supposing that the Harmonic-oscillator (HO) potential governs the core nucleons while the valence nucleon(s) move through Hulthen potential. Such assumptions were applied for both stable and exotic lithium isotopes. The HO size parameters ( and ), the core radii ( ) and the attenuation parameters ( and ) were fixed to recreate the available empirical size radii for lithium isotopes under study.

Quantum states resembling classical periodic trajectories in mesoscopic elliptic billiards.

A quantum wave function with localization on classical periodic orbits in a mesoscopic elliptic billiard has been achieved by appropriately superposing nearly degenerate eigenstates expressed as products of Mathieu functions. We analyze and discuss the rotational and librational regimes of motion in the elliptic billiard. Simplified line equationscorresponding to the classical trajectories can be extracted from the quantum state as an integral equationinvolving angular Mathieu functions. The phase factors appearing in the integrals are connected to the classical initial positions and velocity components. We analyze the probability current density, phase maps, and vortex distributions of the periodic orbit quantum states for both rotational and librational motions; furthermore, they may represent traveling and standing trajectories inside the elliptic billiard.

Phase-space path-integral representation of the quantum density of states: Monte Carlo simulation of strongly correlated soft-sphere fermions.

The Wigner formulation of quantum mechanics is used to derive a path-integral representation of the quantum density of states (DOS) of strongly correlated fermions in the canonical ensemble. A path-integral Monte Carlo approach for the simulation of DOS and other thermodynamic functions is suggested. The derived Wigner function in the phase space resembles the Maxwell-Boltzmann distribution but allows for quantum effects. We consider a three-dimensional quantum system of strongly correlated soft-sphere fermions at different densities and temperatures. The calculated properties include the DOS, momentum distribution functions, spin-resolved radial distribution functions, potentials of mean force, and related energy levels obtained from the Bohr-Sommerfeld condition. We observe sharp peaks on DOS and momentum distribution curves, which are explained by the appearance of fermionic bound states.

Field induced slow magnetic relaxation in a linear homotrinuclear manganese heterospin coordination compound with S = 7/2 ground state and intriguing spin density distribution.

We report a first example of field-induced (HDC = 2500 Oe) slow magnetization relaxation in the homotrinuclear linear heterospin manganese coordination compound with S = 7/2 ground state, based on the bidentate 3,5-di-tert-butyl-1,2-benzoquinone-1-monooxime (HL) ligand with composition {[MnL3]Mn[MnL3]}.

Application of metric space method in quantum information in quasi-periodic systems

Due to the rapid advancement of quantum information theory, some concepts such as fidelity and entanglement entropy have been introduced into the study of quantum phase transitions, which can be used not only to identify novel matter phases but also to detect the critical point and describe the critical behavior of the quantum phase transitions. From the point of view of the metric space, these physical quantities can be understood as the distance between the two functions in the metric space. In this work, we study a class of quasi-periodic system represented by the generalized Aubry-André-Harper (AAH) model, by using the distance between various wavefunctions or density distribution functions in real space. The generalized AAH model, an ideal platform to understand Anderson localization and other novel quantum phenomena, provides rich phase diagrams including extended, localized, even critical (multifractal) phases and can be realized in a variety of experimental platforms. In the standard AAH model, we find that the extended and localized phases can be identified. In addition, there exists a one-to-one correspondence between two distinct distances. We are able to precisely identify the critical point and compute the critical exponent by fitting the numerical results of different system sizes. In the off-diagonal AAH model, a complete phase diagram including extended phase, localized phase, and critical phase is obtained and the distance of critical phases is intermediate between the localized phase and extended phase. Meanwhile, we apply the metric space method to the wave packet propagation and discover that depending on the phase, the distance between wave functions or density functions exhibits varying dynamical evolution behavior, which is characterized by the exponent of the power-law relationship varying with time. Finally, the distance between the state density distribution functions is proposed, and it effectively identifies distinct matter phases and critical points. The critical phase which displays a multifractal structure, when compared with the other two phases, has the large state density distribution function distance. In a word, by defining the distances of a function under different parameters, we provide not only a physical quantity to identify familiar phase transitions but also an intuitive way to identify different matter phases of unknown systems, phase transition points, and their critical behaviors.

Spectral (in)dependence of nonequilibrium charge carriers lifetime and density of states distribution in the vicinity of the band gap edge in F8BT polymer

Photoelectric properties of the F8BT polymer are investigated by the Constant photocurrent method (CPM) based on the photocurrent spectra Δjph(hν) and absorption spectra αCPM(hν). The negligibly weak dependence of charge carriers lifetime on the photon energy is found for hν = 2.2–2.7 eV. The peculiarities of the absorption spectra and the photoconductivity spectra are revealed to be determined mainly by the Gaussian distribution of the density of states (DOS). The density of trap states in the F8BT polymer thin films in the exponential DOS is estimated to be 103 times higher than in the Gaussian DOS.

Theoretical Study of Platinum Doped Anatase (101) Surface on Detecting Decomposition Production of Polyvinyl Chloride

The gas sensitivity of platinum doped TiO2 (101) to two kinds on polyvinyl chloride decomposition production, CO and HCl, were explored based on density functional theory. The binding energy, adsorption distance, charge transfer, and density of states distribution were simulated and discussed. The results suggest that the O oriented mode is probably the best mode for the CO gas molecules adsorption on the Pt-TiO2 and H oriented mode for the HCl adsorption system. HCl and CO molecules obtain electrons from Pt doped TiO2 surface. At room temperature, the recovery time of CO and HCl is 1.9743 × 10-10, 1.2094 × 10-10 respectively. HCl is easier to dissociate from Pt doped TiO2 surface with a little lower energy barrier compared to CO. This work provides theoretical adsorption information of Pt-TiO2 as gas sensor material for HCl and CO detection in the application of online condition monitoring and defect diagnosis in the power cables.

Understanding ovonic threshold switching of GeSe chalcogenide materials using electrical methodologies for extracting density of states

As trap-related conduction is crucial in achieving ovonic threshold switching characteristics, identifying the distribution of traps in the chalcogenide matrix is essential. Herein, the density of states (DOS) near the valence band maximum level (EV) and conduction band minimum level (EC) of GeSe layers extracted using various electrical methodologies were analyzed. When electrons in the GeSe layer participate in activation under light injection, the DOS distribution at EV can be calculated. On the other hand, capacitance measurement with respect to frequencies allows the identification of the lowest energy levels of EC containing electrons that can respond to the provided AC signals. When an As dopant is introduced into the GeSe layer, the unified DOS distribution indicates a decrease in the Se element concentration owing to the over-coordinated Ge state. Therefore, increased DOS at EC is observed, which is in good agreement with the ab initio explanation. Additional simulations were performed considering plausible scenarios in which the variation in DOS profile depends on the parameters of the transport mechanism. These results reveal the ways in which threshold switching can be controlled and facilitate the understanding of electrically achieved behavior.

Nanoadditives induced enhancement of thermal energy storage properties of molten salt: Insights from experiments and molecular dynamics simulations

Current concentrated solar power (CSP) plants use molten salts as heat storage and heat transfer medium. The thermal property enhancement of molten salts can increase the efficiency and lower the investment cost of CSP plants. In this study, the experiments and molecular dynamics (MD) simulations were employed to investigate the effect of doping Al2O3, SiO2 nanoparticles or multi-walled carbon nanotubes (MWCNTs) on the specific heat capacity (SHC) of ternary carbonate salt. The results show that the SHC of molten salt can be enhanced by adding nanoadditives. The addition of MWCNTs has the largest SHC enhancement in both solid and liquid states. Based on MD simulations, the radial distribution functions, number density distribution and vibrational density of states were further analyzed to reveal the effect of nanoadditive, base salt and interface thermal resistance between them on SHC at different temperatures. It is found that the nanoadditive agglomeration, the ion spacing of base salt, and the thermal resistance between base salt and nanoadditive is independent on temperature and cannot result in the enhanced SHC of liquid nanocomposites. Moreover, the thickness of interfacial layer changes with temperature. The thicker interfacial layer leads to the higher SHC of liquid nanocomposites. The interfacial layer also has a decisive influence on SHC of solid nanocomposites. The scanning electron microscopy images show that the layered, needle-like, and network-like interfacial layers are observed in Al2O3, SiO2 and MWCNT nanocomposites, respectively. The present findings are helpful to understand the mechanisms governing thermal energy storage, and provide guidelines for the performance optimization of molten salt based nanocomposites.

Analyzing Acceptor-like State Distribution of Solution-Processed Indium-Zinc-Oxide Semiconductor Depending on the In Concentration.

Understanding the density of state (DOS) distribution in solution-processed indium-zinc-oxide (IZO) thin-film transistors (TFTs) is crucial for addressing electrical instability. This paper presents quantitative calculations of the acceptor-like state distribution of solution-processed IZO TFTs using thermal energy analysis. To extract the acceptor-like state distribution, the electrical characteristics of IZO TFTs with various In molarity ratios were analyzed with respect to temperature. An Arrhenius plot was used to determine electrical parameters such as the activation energy, flat band energy, and flat band voltage. Two calculation methods, the simplified charge approximation and the Meyer-Neldel (MN) rule-based carrier-surface potential field-effect analysis, were proposed to estimate the acceptor-like state distribution. The simplified charge approximation established the modeling of acceptor-like states using the charge-voltage relationship. The MN rule-based field-effect analysis validated the DOS distribution through the carrier-surface potential relationship. In addition, this study introduces practical and effective approaches for determining the DOS distribution of solution-processed IZO semiconductors based on the In molarity ratio. The profiles of the acceptor-like state distribution provide insights into the electrical behavior depending on the doping concentration of the solution-processed IZO semiconductors.

Electronic structures and Aharonov–Bohm effect in higher-order topological Dirac Semimetal nanoribbons with strong confinements

Electronic structures and magnetotransport properties of topological Dirac semimetal (TDSM) nanoribbons are studied by adopting the tight-binding lattice model and the Landauer–Büttiker formula based on the non-equilibrium Green’s function. For concreteness, the TDSM material Cd3As2 grown along the experimentally accessible [110] crystallographic direction is taken as an example. We found that the electronic structures of the TDSM nanoribbon depend on both the strength and direction of the magnetic field (MF). The transversal local charge density (LCD) distribution of the electronic states in the TDSM nanoribbon is moved gradually from the center toward the hinge of each surface as a [010] direction MF strength is increased, forming the two-sided hinge states. However, one-sided surface states are generated in the TDSM nanoribbon when a [001] direction MF is applied. As a result, one-sided hinge states can be achieved once a tilted MF is placed to the TDSM nanoribbon. The underlying physical mechanism of the desired one-sided hinge states is attributed to both the orbital and Zeeman effects of the MF, which is given by analytical analyses. In addition, typical Aharonov–Bohm interference patterns are observed in the charge conductance of the two-terminal TDSM nanoribbon with a tilted MF. This conductance behaviour originates from the unique interfering loop shaped by the one-sided hinge states. These findings may not only further our understanding on the external-field-induced higher-order (HO) topological phases but also provide an alternative method to probe the HO boundary states.

Analysis of trap distribution and NBTI degradation in Al2O3/SiO2 dielectric stack

In this work, p-substrate MOS capacitors with plasma-enhanced atomic-layer-deposited Al2O3/ thermally-grown SiO2 dielectric stack (with a target equivalent oxide thickness of 4.2 nm) are investigated by means of the conductance method in a temperature range from 25 °C to 150 °C. The trap state density and energy level distribution in the silicon bandgap are extracted over the bandgap. Furthermore, negative bias temperature instability is analyzed by two-point capacitance–voltage characterization to evaluate the electrical stability and dielectric reliability of the Al2O3/SiO2 stack. The contributions from interface and oxide trap generations are detailed. Our results indicate that plasma-enhanced atomic layer deposited Al2O3 on thermal SiO2 stack features low interface trap density, and comparable dielectric reliability to conventional SiO2, SiOxNy, and high-k dielectric stacks.

Ground State Structure of Helium and Phosphorus Isotopes using the Radial Wave Functions of Harmonic-Oscillator and Hulthen Potentials

The ground state density distributions and electron scattering Coulomb form factors of Helium (4,6,8He) and Phosphorate (27,31P) isotopes are investigated in the framework of nuclear shell model. For stable (4He) and (31P) nuclei, the core and valence parts are studied through Harmonic-oscillator (HO) and Hulthen potentials. Correspondingly, for exotic (6,8He) and (27P) nuclei, the HO potential is applied to the core parts only, while the Hulthen potential is applied to valence parts. The parameters for HO and Hulthen are chosen to reproduce the available experimental size radii for all nuclei under study. Finally, the CO component of electron scattering charge form factors are also investigated. Unfortunately, there is no analytic solution to the Hulthen potential except for the states which are harnessed to the current calculation.

Theoretical Modeling of a Temperature-Dependent Threshold-Voltage Shift in Self-Aligned Coplanar IZTO Thin-Film Transistors

Here, we investigate the effect of increasing temperature on threshold voltage (VTH) stability in self-aligned coplanar amorphous indium-zinc-tin oxide (a-IZTO) thin-film transistors (TFTs). An analytical model of the temperature dependency of VTH stability reveals the importance of device geometry and subgap density-of-state (DOS) reductions in highly stable a-IZTO TFTs. The validity of the analytical model is confirmed using experiments and technology computer-aided design simulations to predict quantitative relationships under various shifts in temperatures, subgap DOS, and VTH. The role of hydrogen impurity in performance and VTH stability is also examined. The incorporation of hydrogen in a-IZTO channels improves TFT performance and thermal stability (ΔVTH/ΔT = 3.18 mV/K) due to hydrogen’s role as a passivation center. However, excessive incorporation of hydrogen increases subgap DOS distribution, causing a slight deterioration in thermal stability (ΔVTH/ΔT = 3.31 mV/K). This suggests that hydrogen can be converted from a shallow donor to an acceptor-like deep trap state. Our findings inform future designs of stable oxide semiconductor TFTs in terms of thermal stability in emerging organic light-emitting diode, augmented and virtual reality, memory, logic, and monolithic three-dimensional applications.

Electrical Stability Modeling Based on Surface Potential for a-InGaZnO TFTs under Positive-Bias Stress and Light Illumination.

In this work, an electrical stability model based on surface potential is presented for amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) under positive-gate-bias stress (PBS) and light stress. In this model, the sub-gap density of states (DOSs) are depicted by exponential band tails and Gaussian deep states within the band gap of a-IGZO. Meanwhile, the surface potential solution is developed with the stretched exponential distribution relationship between the created defects and PBS time, and the Boltzmann distribution relationship between the generated traps and incident photon energy, respectively. The proposed model is verified using both the calculation results and experimental data of a-IGZO TFTs with various distribution of DOSs, and a consistent and accurate expression of the evolution of transfer curves is achieved under PBS and light illumination.

Terpyridine ruthenium–triarylamine asymmetrical mixed-valence systems: Syntheses, (Spectro) electrochemistry and theoretical calculations

A series of asymmetrical terpyridine ruthenium-TAA redox-active organometallic complexes was prepared and characterized. The bonding and electronic properties of these complexes were investigated by spectro-electrochemistry and theoretical calculations. Electrochemical results revealed that four complexes exhibited two successive single-electron redox processes, in which the first-step electrochemical behavior of tpyRu-ethynyl-TAA complexes occurred at the triarylamine moiety, the first-step based-metal oxidation of Cl-tpyRu-TAA complex was decided, and their half-wave potential difference ∆E decreased gradually. The near-infrared (NIR) spectroscopic results of mixed-valence states obtained by spectro-electrochemistry showed that they all have NIR multiple absorption characteristics. The maximum absorption wavelength of singly-oxidized tpyRu-ethynyl-TAA molecules redshifted with bridge conjugate extension, whereas Cl-tpyRu-TAA mixed-valence state only displayed weak absorption at high-energy region, as later confirmed by TDDFT calculation. The electron coupling parameters Hab obtained from the inter-valence charge transfer analysis of mixed-valence states by the Marcus–Hush theory decreased gradually with the extension of the bridge chain, consistent with the electrochemical results. In addition, DFT calculation reflected that the HOMO contribution in bridge chain of neutral molecules increased with the conjugate bridge extension, and the spin density distribution of mixed-valence states was mainly localized on the aryl bridge and NAr2 and featured the less metal character and hence less charge delocalization. The above results concluded that the degree of electronic coupling and the order of redox processes are greatly affected by conjugated bridge and the coordination model between terpyridine and ruthenium.