Energy Barrier Height Research Articles

Colloid Interaction Energies for Surfaces with Steric Effects and Incompressible and/or Compressible Roughness

Colloid aggregation and retention in the presence of macromolecular coatings (e.g., adsorbed polymers, surfactants, proteins, biological exudates, and humic materials) have previously been correlated with electric double layer interactions or repulsive steric interactions, but the underlying causes are not fully resolved. An interaction energy model that accounts for double layer, van der Waals, Born, and steric interactions as well as nanoscale roughness and charge heterogeneity on both surfaces was extended, and theoretical calculations were conducted to address this gap in knowledge. Macromolecular coatings may produce steric interactions in the model, but non-uniform or incomplete surface coverage may also create compressible nanoscale roughness with a charge that is different from the underlying surface. Model results reveal that compressible nanoscale roughness reduces the energy barrier height and the magnitude of the primary minimum at separation distances exterior to the adsorbed organic layer. The depth of the primary minimum initially alters (e.g., increases or decreases) at separation distances smaller than the adsorbed organic coating because of a decrease in the compressible roughness height and an increase in the roughness fraction. However, further decreases in the separation distance create strong steric repulsion that dominates the interaction energy profile and limits the colloid approach distance. Consequently, adsorbed organic coatings on colloids can create shallow primary minimum interactions adjacent to organic coatings that can explain enhanced stability and limited amounts of aggregation and retention that have commonly been observed. The approach outlined in this manuscript provides an improved tool that can be used to design adsorbed organic coatings for specific colloid applications or interpret experimental observations.

Influence of Water on the Gas-Phase Reaction of Dimethyl Sulfide with BrO in the Marine Boundary Layer.

The effect of a single water molecule on the reaction of dimethyl sulfide (DMS) with BrO reaction has been investigated using quantum chemical calculations at the CCSD(T)/6-311++G**//BH&HLYP/aug-cc-pVTZ level of theory. Two reaction mechanisms have been considered both in the absence and the presence of water, namely, oxygen atom transfer and hydrogen abstraction, among which the oxygen atom transfer was predominant. Five reaction channels were found in the absence of water, in which the channels starting from the cis-configuration of the pre-reaction complexes were more favorable because of the low energy barrier. The inclusion of water slightly decreased the energy barrier height of most oxygen atom transfer channels, while making the hydrogen abstraction channels more complex. While the effective rate coefficients for the oxygen atom transfer paths are found to have decreased by 3–7 orders of magnitude in the presence of water relative to the water-free reactions, the negligible fraction of reactants that are effectively clustered with water does not significantly change the overall rate of the formation of dimethyl sulfoxide and Br. The present results show that the overall mechanism and rate of the DMS + BrO reaction may not be affected by humidity under atmospheric conditions.

First-passage times in protein folding: exploring the native-like states vs. overcoming the free energy barrier.

Using a model β-hairpin protein as a representative example of simple two-state folders whose kinetics are uncomplicated by the presence of on- and off-pathway intermediates, it is studied how the search for the protein's native state among native-like states affects the folding kinetics. It is revealed that the first-passage time (FPT) distributions are essentially single-exponential not only for the times to overcome the free energy barrier between the unfolded and native-like states but also for the times to find the native state among the native-like ones. The FPT distributions of this type are observed through all studied two-state-like regimes of protein folding, varying from a regime close to two-state folding to a regime close to downhill folding. If the protein explores native-like states for a time much longer than the time to overcome the free energy barrier, which is characteristic of high temperatures, the resulting FPT distribution to reach the native state remains close to exponential but the mean FPT (MFPT) is determined not by the height of the free energy barrier but by the time to explore native-like states. In particular, the mean time to overcome the free energy barrier is in reasonable agreement with the Kramers rate formula and generally far shorter than the overall MFPT to reach the native state. The observed increase of the overall MFPT, as a result of longer exploration of native-like states, may lead to an overestimate of the height of the free energy barrier between the unfolded and folded states when it is calculated from the overall MFPT.

Intercalation-deintercalation of water-in-salt electrolytes in nanoscale hydrophobic confinement.

Intercalation-deintercalation of water-in-salt (WIS) electrolytes in nanoscale confinement is an important phenomenon relevant to energy storage and self-assembly applications. In this article, we use molecular simulations to investigate the effects of intersurface separation on the structure and free energy underlying the intercalation-deintercalation of the Li bis(trifluoromethane)sulfonimide ([Li][TFSI]) water-in-salt (WIS) electrolyte confined between nanoscale hydrophobic surfaces. We employ enhanced sampling to estimate the free energy profiles for the intercalation behaviour of WIS in confining sheets at several intersurface separations. We observe that the relative stability of the condensed and vapour phases of WIS in the confinement depends on the separation between the confining surfaces and the WIS concentration. We find that the critical separation at which the condensed and vapour phases are equally stable in confinement depends on the concentration of WIS. The relative height of the free energy barrier also strongly depends on the concentration of [Li][TFSI] inside the confined space, and we find that this concentration dependence can be attributed to changes in line tension. The process of deintercalation passes through vapour tube formation inside the confined space, and this process is initiated by vapour bubble formation. The size of the critical vapour tube required for spontaneous evaporation of WIS from the confinement is also found to depend on the intersurface separation and WIS concentration.

Nucleation rate in the two dimensional Ising model in the presence of random impurities.

Nucleation phenomena are ubiquitous in nature and the presence of impurities in every real and experimental system is unavoidable. Yet numerical studies of nucleation are nearly always conducted for entirely pure systems. We have studied the behaviour of the droplet free energy in two dimensional Ising model in the presence of randomly positioned static and dynamic impurities. We have shown that both the free energy barrier height and critical nucleus size monotonically decreases with increasing the impurity density for the static case. We have compared the nucleation rates obtained from the Classical Nucleation Theory and the Forward Flux Sampling method for different densities of the static impurities. The results show good agreement. In the case of dynamic impurities, we observe preferential occupancy of the impurities at the boundary positions of the nucleus when the temperature is low. This further boosts enhancement of the nucleation rate due to lowering of the effective interfacial free energy.

Improved rectification characteristics by engineering energy barrier height in TiOx-based RRAM

Resistive random access memory (RRAM) is a strong candidate for next-generation memory. Despite its versatility, the sneak path current severely threatens accurate read operations, preventing RRAM from being expanded to high density array. To solve this sneak path current, here we propose a method to enhance a rectification effect through barrier height engineering. We fabricated two TiOx-based devices, Al/TiOx/Al devices as a control group and Cu/TiOx/Al devices as an experimental group, and compare their rectification characteristics. As a result, a rectification ratio in Cu/TiOx/Al devices is much larger than that of Al/TiOx/Al devices, e.g., approximately 122 times for low-resistance-state and 251 times for high-resistance-state. Furthermore, we analyze the energy band diagram considering the bias polarity and electric-field-driven oxygen ion migration. It is confirmed that the rectification effect results from high energy barrier height between Cu top electrode and TiOx. In addition, it is established that the high rectification ratio of Cu/TiOx/Al devices is capable of constructing large-sized memory array with reliable read margin by simply adjusting energy barrier height

DFT Investigation of the η6 ⇌ η6-Inter-ring Haptotropic Rearrangement of the Group 8 Metals Complexes [(graphene)MCp]+ (M = Fe, Ru, Os).

Metalcyclopentadienyl complexes (MCp)+ (M = Fe, Ru, Os) bound to the large polyaromatic hydrogenated hydrocarbon (PAH) C96H24 used as a model for pristine graphene have been studied using a density functional theory (DFT) generalized gradient approximation (PBE functional) to reveal their structural features and dynamic behavior. The inter-ring haptotropic rearrangements (IRHRs) for these complexes were shown to occur via two transition states and one intermediate. The energy barriers of the η6 ⇌ η6 IRHRs of the (MCp)+ unit were found to be 30, 27, and 29 kcal/mol for M = Fe, Ru, and Os, respectively. These values are significantly lower than the values found previously for smaller PAHs. Both polar and nonpolar solvents were found not to affect significantly the energy barrier heights. Investigated transition metal complexes could be used in general as catalysts in the design of novel derivatives or materials with promising properties. Metalcyclopentadienyl complexes (MCp)+ of PAHs show catalytic properties mainly due to their structural details as well as their important characteristic of inter-ring haptotropic rearrangement. IRHRs take place usually by intramolecular mechanisms. During IRHRs, the MLn organometallic groups (OMGs) undergo shifting along the PAH plane and could coordinate additional reagents, which is important for catalysis. Large PAHs such as graphene, fullerenes, and nanotubes possess intrinsic anticancer activity, and numerous arene complexes of Ru and Os have been proven to have anticancer properties as well. We suppose that coordinating Ru or Os to very large PAHs could synergistically increase the anticancer activity of resulting complexes.

Improved electrical conductivity and thermoelectric performance of ZnO by doping with NaCl and CdO

ZnO is a promising thermoelectric (TE) material for high-temperature applications; however, its TE performance is limited by strong coupling between the electrical and thermal transport properties. The Schottky barrier at the grain boundaries, via Zn vacancies (an electron killer) and interstitial oxygen, forms the acceptor defects that restrict carrier mobility within ZnO. Increasing the carrier concentration by increasing the solubility of a trivalent/divalent/monovalent dopant in ZnO is also important. In this study, NaCl- and CdO- are co-doped with Al into ZnO samples synthesized by spark plasma sintering, and establish a strong correlation with TE properties. NaCl's alloy with ZnAlO is doped simultaneously at cationic and anionic sites, while the embedded CdO contributes to doping and surface modification. The methodology produced an ultrahigh σ ≥ 3600 S/cm at 300 K with significantly lower lattice thermal conductivity ~3.1 W/mK at high temperature. The structural, electrical, and thermal properties are analyzed to elucidate CdO's influence, and a correlation between the power factor and the energy barrier height discussed. This yields the best power factor of ~10.9 µW/cm K2 at 1173 K in the ZnAlCdO matrix, resulting because of carrier energy filtering effect, which is 1.5 times above the reported data.

Analysis and Simulation of Interface Quality and Defect Induced Variability in MgO Spin-Transfer Torque Magnetic RAMs

Device-to-device variability of CoFeB/MgO based STT-MRAMs is studied based on experiments and simulations taking into account the influence of interface quality, temperature variation and device dimensionality. Metal-induced gap states resulting from electron transfer at the ferromagnet-tunnel barrier interface significantly influence the effective energy barrier height of these devices irrespective of their diameters. Switching voltage and parallel - antiparallel resistance values vary by as much as 43% and 30% respectively for about 13% variation of the energy barrier, whereas the tunneling magnetoresistance remains typically unaffected. WRITE cycles of highly scaled STT-MRAMs are therefore more susceptible to device-to-device variations resulting from microscopic variations in the interface quality, rather than the READ cycles. Such variations are observed to be independent of temperature, as well as spatial distribution of the defects.

Performance Limits of an Alternating Current Electroluminescent Device.

The use of an alternating current (AC) voltage is a simple, versatile method of producing electroluminescence from generic emissive materials without the need for contact engineering. Recently, it was shown that AC-driven, capacitive electroluminescent devices with carbon nanotube network contacts can be used to generate and study electroluminescence from a variety of molecular materials emitting in the infrared-to-ultraviolet range. Here, performance trade-offs in these devices are studied through comprehensive device simulations and illustrative experiments, enhancing understanding of the mechanism and capability of electroluminescent devices based on alternating as opposed to direct current (DC) schemes. AC-driven electroluminescent devices can overcome several limitations of conventional DC-driven electroluminescent devices, including the requirement for proper alignment of material energy levels and the need to process emitting materials into uniform thin films. By simultaneously optimizing device geometry, driving parameters, and material characteristics, the performance of these devices can be tuned. Importantly, the turn-on voltage of AC-driven electroluminescent devices approaches the bandgap of the emitting material as the gate oxide thickness is scaled, and internally efficient electroluminescence can be achieved using low-mobility single-layer emitter films with varying thicknesses and energy barrier heights relative to the contact.

Fabrication of AlGaAs/GaAs/diamond heterojunctions for diamond-collector HBTs

Diamond is a highly attractive ultrawide bandgap semiconductor for next-generation high-power switching devices and RF devices for its superior physical and electrical properties. However, the lack of effective n-type dopants in diamond has limited the material to only unipolar p-type device applications. Heterostructure bipolar devices that use better n-type semiconductors together with p-type diamond is an approach to get high performance devices. In this work, p–n–p AlGaAs/GaAs/diamond heterojunction bipolar transistors (HBTs) are proposed and fabricated using a grafting technique. The double-heterojunction is formed by transferring an AlGaAs(p-type)/GaAs(n-type) membrane onto single-crystalline p-type doped diamond with an electron affinity of 0.32 eV. The epitaxial AlGaAs/GaAs emitter-base p–n junction shows an ideality factor of 1.09 with an Ion/Ioff of 1.53 × 107 at ± 1.5 V. The grafted GaAs/diamond n–p junction shows an ideality factor of 3.67 with an Ion/Ioff of 3.74 × 1010 at ± 5.2 V. Due to the valence-band energy barrier of 0.3 eV between the GaAs base and the diamond collector, the measured current gain for the HBT is slightly below unity. Simulations show that by reducing the electron affinity value of the p-type diamond, the base-collector energy barrier height can be correspondingly reduced, and high current gain can be expected.

Substrate-dependent degradation of thin TMDC layers in ambient conditions

In this work, substrate-dependent degradation of thin layers of transition metal dichalcogenides (TMDC) is demonstrated. The energy barrier heights of the MoSe2/Au and MoSe2/Ni heterojunctions were studied using Kelvin probe force microscopy. The effect of the barrier height on the photodegradation process is shown. The results show that the Ni-covered substrate promotes the photodegradation process in the few-layer MoSe2 flake. While the Au-covered substrate depresses it. The results of this work show the importance of choosing a substrate for the stability of devices based on thin MoSe2 layers and might be useful for creating devices based on thin TMDC layers.

Molecular rotors with designed polar rotating groups possess mechanics-controllable wide-range rotational speed

Molecular rotors with controllable functions are promising for molecular machines and electronic devices. Especially, fast rotation in molecular rotor enables switchable molecular conformations and charge transport states for electronic applications. However, the key to molecular rotor-based electronic devices comes down to a trade-off between fast rotational speed and thermal stability. Fast rotation in molecular rotor requires a small energy barrier height, which disables its controllability under thermal excitation at room temperature. To overcome this trade-off dilemma, we design molecular rotors with co-axial polar rotating groups to achieve wide-range mechanically controllable rotational speed. The interplay between polar rotating groups and directional mechanical load enables a “stop-go” system with a wide-range rotational energy barrier. We show through density functional calculations that directional mechanical load can modulate the rotational speed of designed molecular rotors. At a temperature of 300 K, these molecular rotors operate at low rotational speed in native state and accelerates tremendously (up to 1019) under mechanical load.

Gallium Doped Zinc Oxide Work Function Extraction Using Transparent Gate MOSFET Devices

Recent commercialization has peaked interest in transparent conducting oxides being implemented in transparent applications. Gallium doped zinc oxide (GZO) films exhibit excellent transmission characteristics in the visible spectrum while maintaining high electrical conductivity. In majority applications the work function of the material used has an impact on the device performance as it affects the energy barrier height at the hetero-junction interface. Hence, the work function identification is of critical importance.In this article, we discuss work function extraction of GZO from a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) device for the first time. Four level mask sets were used to fabricate two MOSFET devices following exact same conditions. Source and drain contacts were made using aluminum metal for both MOSFETs. One of the MOSFET was fabricated with aluminum as gate contact and the other MOSFET with transparent conducting GZO as its gate contact. GZO used in this research were RF sputtered. From the threshold voltage equation of both the fabricated MOSFETs, work function of GZO was extracted. The electrical and optical transmission studies were also performed on the sputtered GZO thin films and are reported in this study.

Ion conductor barrier height for the switching time improvement of all-thin-film inorganic complementary electrochromic devices

In this study, an all-thin-film inorganic complementary electrochromic device (ECD) with the lithium doping zirconium dioxide (ZrO2:Li) ion conductor layer has demonstrated for the lower cost ECD applications. Even though the ZrO2:Li has demonstrated a lower lithium (Li) ion conductivity of 1.62 × 10−8 S/cm and ion mobility of 8.67 × 10−12 cm2/Vs, the switching characteristic of the ECDs has been improved. According to the refractive index measurement, the ion conductor ZrO2:Li with the higher refractive index value is also indicated a higher energy bandgap material. As a result, the possible mechanism is due to the electron energy barrier height 1.2 eV between the interface of electrochromic layer tungsten oxide (WO3) and ion conductor layer ZrO2:Li. Besides, the induction of an electric field between the WO3 and high bandgap ZrO2:Li material interface also accelerates the Li ion transported through the interface. Furthermore, the ZrO2:Li with the lower ion conductivity and mobility is due to the dense material structure, which is verified by the cross-sectional field emission scanning electron micrograph profile. Additionally, the dense material structure has also provided better surface morphology for thin-film device structures with the layer-by-layer deposition process.

Effect of Thermodynamic Factors on Colloid Stability

Colloid is more and more widely used in daily life. No matter medical application, or food additive in the food industry, or electrolyte in the lead battery, all are deeply connected with it. Among them, the stability of colloid is an important direction for people to explore. According to DLVO theory, the stability of colloid is determined by the height of energy barrier. In this paper, the effect of temperature on colloid aggregation or sedimentation will be discussed by calculating and observing the settling process of colloid. This paper will also put forward the optimum temperature position to keep the highest energy barrier height and the way to estimate the value of energy barrier by reaction, which gives new possibilities for controlling colloid stability.

Disclosing crystal nucleation mechanism in lithium disilicate glass through molecular dynamics simulations and free-energy calculations

Unraveling detailed mechanism of crystal nucleation from amorphous materials is challenging for both experimental and theoretical approaches. In this study, we have examined two methods to understand the initial stage of crystal precipitation from lithium disilicate glasses using molecular dynamics simulations. One of the methods is a modified exploring method to find structurally similar crystalline clusters in the glass models, enabling us to find three different embryos, such as Li2Si2O5 (LS2), Li2SiO3 (LS) and Li3PO4 (LP), in the 33Li2O·66SiO2·1P2O5 glass (LS2P1), in which P2O5 is added as a nucleating agent. Interestingly, LS2 and LP crystals were found inside the LS2P1 glass while LS crystal appeared on the glass surface, which agrees with experimental observations. The other method is free energy calculation using a subnano-scale spherical crystal embedded in the glass model. This method, which we called Free-Energy Seeding Method (FESM), allows us to evaluate free energy change as a function of crystal radius and to identify critical size of the crystal precipitation. The free energy profiles for LS and LS2 crystal nuclei in the LS2 glass models possess maximum energy at a critical radius as expected by classical nucleation theory. Furthermore, the critical radius and the energy barrier height agree well with recent experimental investigation, proving the applicability of this method to design glass–ceramics by atomistic modeling.

Improved dielectric properties of indium and tantalum co-doped CaCu3Ti4O12 ceramic prepared by spark plasma sintering

The current-voltage characteristics and dielectric properties of indium and/or tantalum doped CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (CCTO) ceramics prepared by spark plasma sintering are investigated in this work. The X-ray diffraction patterns reveal the formation of pure cubic perovskite CCTO with no impurity phase. The grain size is suppressed by doping In and Ta, and the densification of the CCTO ceramics is achieved, showing a relative density of about 97.5%. The approach of 0.5 mol% In and Ta co-doping results in enhanced energy density and electrical properties. The energy density is increased by more than 30 % and the breakdown field E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> is improved to 9.67 kV/cm. Similarly, the lowest dielectric loss of 0.024 at nearly 1 kHz can be obtained in the co-doped sample, which can be attributed to the enhancement of the conductance activation energy and Schottky barrier height.

Enhancement of thermoelectric performance of PbTe by embedding NaCl

Different contents of NaCl (2%, 4%, 6%, and 8 wt%) was incorporated in polycrystalline PbTe samples and consolidated via spark-plasma-sintering (SPS). NaCl acted as a dopant material during SPS. The effect of doping through NaCl increased the carrier concentration without deteriorating mobility. The incorporation of NaCl helped the formation of a porous structure which facilitated high rate of phonon scattering and decreased the thermal conductivity. Experimental observation suggests that the origin of electrical property is due to the extra carriers given by Na, and that Fermi level is well optimized by the compensation effect acquired by Cl doping. The energy barrier heights (EB) for charge carriers in PbTe/NaCl samples is lower than the EB of reference PbTe. High Seebeck coefficient of 300 µV/K at 773 K combined with the lower thermal conductivity of 0.55 W/m-K, yielded an improved figure-of-merit (zT) value as high as 1.2 at 773 K in a 6wt%NaCl:PbTe. In the PbTe/NaCl series, PbTe exhibited the reduced lattice thermal conductivity, enhanced charge carrier concentration as well as mobility, and high zT values. As a result, the incorporation of NaCl acted as a dopant well as aided in pore formation, which can be a potential pathway to optimize thermoelectric performances.

Direct detection and stochastic analysis on thermally activated domain-wall depinning events in micropatterned Nd-Fe-B hot-deformed magnets

Although the magnetization reversal process in the permanent magnets has long been studied, it has not been fully revealed. The recent progress of the computational science realizes the atomistic calculations for the thermally activated magnetization reversal process in permanent magnets. In contrast, the experimental study on the magnetization reversal has remain to be the macroscopic evaluation. In this study, Nd-Fe-B hot-deformed magnets are micropatterned, and a staircase-like magnetization curve corresponding to the elemental domain-wall depinning event at each grain boundary is successfully observed. Each elemental domain-wall depinning event fluctuates with respect to thermal activation, and the stochastic analysis based on the Néel-Arrhenius model gives the energy barrier parameters H0 and E0 of each depinning event, which are the intrinsic domain-wall depinning field and energy barrier height, respectively. Three types of Nd-Fe-B hot-deformed magnets with different coercivities are adopted for this stochastic analysis. As a result, E0 exhibits very little dependence on H0, and its slope becomes steeper for the lower-coercivity magnet. The stochastic micromagnetics simulation based on the Landau-Lifshitz-Gilbert equation for the two-grain model with various inter-grain exchange coupling reproduces the experimentally observed relationship between E0 and H0. Moreover, the behavior for the lower-coercivity magnet can be explained by assuming the presence of a low magnetic anisotropy layer on the grain surface.