Bulk Mobility Research Articles

The effect of positional disorder and the Beer-Lambert law in organic photovoltaics : A kinetic Monte Carlo simulation analysis.

It is urgent to address climate change by radically changing our energy sources. Organic photovoltaics (OPVs) are a competitive clean energy emerging technology and will undoubtedly have a market niche in a world that needs to take advantage of every possible type of renewable energy. Recent studies have brought relevant improvements on internal efficiency, focusing on two properties at the interface: energetic disorder and bending. However, how positional disorder affects internal efficiency is still an open question. Here, we show that positional disorder is desired at the interface, but only up to a threshold value of 0.2 nm for poly p-phenylene vinylene. Using a kinetic Monte Carlo simulator, we realized that not enough excitons were reaching the interface, and introduced the Beer-Lambert law of attenuance to correct it. Furthermore, we realized that the same disorder that facilitates charge separation at the interface diminishes exciton and charge mobility in bulk, so we propose here a new morphology for the active layer of OPVs. Our suggestion implicates in better overall performance, improving not just the internal but the overall cell efficiency.

Understanding synaptic characteristics of nonvolatile analog redox transistor based on mobile ion-modulated-electrolyte thickness model for neuromorphic applications

Linear and symmetric updates of the channel current of the redox transistor are involved in bulk mobile ion motion. In this study, we introduce the concept of a variable effective electrolyte thickness (teff) precisely tuned by gate voltage-driven ions into the drain current equation of a conventional transistor. In order to understand the switching characteristics of a nonvolatile analog redox transistor that serves as an artificial synapse for neuromorphic systems, we developed a physics-based model in MATLAB. The simulated synaptic update curves obtained using identical gate pulses were in good agreement with the fabricated Cu-ion-actuated CuOx/HfOx/WOx redox transistor. We then analyzed the impact of geometrical and material-related parameters on the synaptic behavior, taking into account the ion speed and the degree of allowable electric field through the electrolyte. In addition, we performed Monte Carlo simulation to create a non-uniformly changed teff circumstance. With this, we reproduced the fluctuated update of the channel current every gate pulse, which is occasionally observed experimentally when mobile ions are easily moved randomly. Our simulation results revealed that the redox transistor immune to the unevenly changed teff can be achieved by lowering the ion velocity.

Easily Accessible Topologically Protected Charge Carriers in Pure and Robust α‐Sn Films

Experimental evidence of topological Dirac fermion charge carriers in pure and robust α‐Sn thin films grown on InSb substrates is reported. This evidence is acquired using standard macroscopic four‐point contact resistance measurements, conducted on uncapped films with a significantly reduced bulk mobility. The electrical characteristics of the constituting components of the α‐Sn/InSb sample are analyzed and compared and a three‐band drift velocity model is proposed accordingly. A surface band, with low carrier density and high mobility, is identified as the origin of the observed Shubnikov – de Haas (SdH) oscillations. The analysis of these quantum oscillations results in a nontrivial value of the phase shift γ0, characteristic of topologically protected Dirac fermions. The momentum relaxation time τ ≈ 300 fs in pure, undoped α‐Sn thin films is estimated.

A study on MOCVD growth window for high quality N-polar GaN for vertical device applications

Lateral N-polar GaN devices have outperformed Ga-polar GaN counterparts in mm-wave applications. A vertical N-polar GaN device structure with higher breakdown voltage is expected to further improve the output power density. This paper is aimed at understanding the growth conditions for such a vertical N-polar GaN device stack. A vertical device stack requires high-quality n-GaN layer with low unintentional dopant concentration and high electron mobility for better ON resistance–breakdown voltage performance. A systematic study by varying reactor temperature, pressure and NH3 flow rate is performed in the metal organic chemical vapor deposition growth of N-polar GaN for vertical device structure. Structural quality of the sample is increased with increase growth temperature and higher NH3 flow rate, yet an increase in surface roughness is observed for highest temperature used. The N-polar GaN grown at the optimized conditions exhibited a x-ray diffraction rocking curve full width half maximum of symmetric (0002) and skew-symmetric (20 ) scans of 358″ and 569″ respectively with a low oxygen and carbon concentration of 6 × 1016 cm−3 and 5 × 1016 cm−3, respectively. The optimized sample exhibited a low screw dislocation density of 2.5 × 108 cm−2 on sapphire substrate. A gradual increase in the bulk mobility of electrons with decrease in screw dislocation density in N-polar GaN is also reported.

Understanding the Hydration Process of Salts: The Relation between Surface Mobility and Metastability

In this work, the relation between mobility and metastability is investigated. This is done under pure water vapor vacuum conditions. First, the properties of the metastable zone under pure water vapor conditions are investigated and compared to those under atmospheric conditions. Second, mobility is investigated using electrochemical impedance spectroscopy, whereby we differentiate between bulk and surface mobility. Using surface mobility, we showed that mobility is a prerequisite for the nucleation of K2CO3. Sufficient mobility must be introduced for the diffusion of ions and water, whereby an increase of almost two orders of magnitude (800 times) is observed on a hydrated sample. It has been shown that the metastable zone is both a consequence of the energy barrier for nucleation and the low ion mobility at low vapor pressures.

Reliable evaluation method for interface state density and effective channel mobility in lateral 4H-SiC MOSFETs

The electrical performance of silicon carbide metal-oxide-semiconductor field effect transistors (4H-SiC MOSFETs) are strongly related to the presence of interface states at the silicon dioxide (SiO2)/4H-SiC interface, whose density is much higher than in the Si/SiO2 system. In particular, the charged interface states determine a degradation of the carrier mobility in the channel of the MOSFET with respect to the bulk mobility. A reliable and consistent method for the evaluation of the interface state density (D it) and the effective channel mobility (μ ch) in these devices is presented in this work. The two quantities are simultaneously extracted by a combined fit of the current–voltage (I–V) and capacitance–voltage (C–V) electrical characteristics collected on a single device. The simultaneous fit of the I–V and C–V characteristics, which can be easily measured on the same device, noticeably improve the reliability of D it and μ ch estimation. The results obtained at different temperatures indicate an increase of the μ ch with T, in agreement with a dominance of the Coulomb scattering effect as the degradation mechanism of the carrier channel mobility induced by the presence of charged interface states.

Fluorescent Probe of Aminopolymer Mobility in Bulk and in Nanoconfined Direct Air CO2 Capture Supports

Poly(ethylenimine) (PEI) is widely recognized as an efficient carbon capture medium. When loaded onto mesoporous oxide supports, the polymer becomes particularly attractive for direct air capture (DAC) applications given the high surface area of the composites, the low volatility of the polymer, and the excellent cyclability of the system. As polymer segmental mobility is coupled with CO2 uptake and diffusion, understanding how that mobility is influenced by nanoconfinement will ultimately be critical to the development of more efficient DAC systems. Here, we discuss our development of a fluorescent probe molecule based on tetrakis(4-hydroxyphenyl)ethylene. As the fluorescence intensity of this molecule and the shape of the emission spectra are strongly dependent on the viscosity of the supporting medium, doping PEI-composites with this fluorescent probe can provide sensitive indication of polymer glass transition and/or melting temperatures across a wide range of temperatures (−100 to +100 °C). Herein, we demonstrate how this molecule can be used as a ratiometric probe to study bulk PEI dynamics and confinement effects in mesoporous silica as influenced by pore functionality, polymer fill fraction, and polymer architecture.

Surface versus Bulk Currents and Ionic Space-Charge Effects in CsPbBr3 Single Crystals

CsPbBr3 single crystals have potential for application in ionizing-radiation detection devices due to their optimal optoelectronic properties. Yet, their mixed ionic–electronic conductivity produces instability and hysteretic artifacts hindering the long-term device operation. Herein, we report an electrical characterization of CsPbBr3 single crystals operating up to the time scale of hours. Our fast time-of-flight measurements reveal bulk mobilities of 13–26 cm2 V–1 s–1 with a negative voltage bias dependency. By means of a guard ring (GR) configuration, we separate bulk and surface mobilities showing significant qualitative and quantitative transport differences. Our experiments of current transients and impedance spectroscopy indicate the formation of several regimes of space-charge-limited current (SCLC) associated with mechanisms similar to the Poole–Frenkel ionized-trap-assisted transport. We show that the ionic-SCLC seems to be an operational mode in this lead halide perovskite, despite the fact that experiments can be designed where the contribution of mobile ions to transport is negligible.

Effect of Diode Field on Time-Resolved Photoluminescence of CdTe-Based Solar Cells

Time-resolved photoluminescence (TRPL) measurements with externally applied voltage bias were performed on a variety of thin-film CdTe-based solar cells. These measurements revealed a substantial variation in luminescence decays with the application of external voltage bias in many of the samples. Experimental results were replicated with a MATLAB-based modeling tool. Decay variations are attributed to the changing strength of internal p-n junction electric fields as bias is modulated; this exercise, therefore, illustrates whether or not field effects influence a particular TRPL measurement and how. It is found via modeling that the field influence on TRPL decays is dependent on the cell's bulk minority carrier lifetime, mobility, and carrier concentration, as well as the laser excitation power during a measurement. While nearly insignificant in poorer quality legacy cells, the field effects are substantial in state-of-the-art CdTe cells with high lifetime, mobility, and doping, complicating determination of bulk lifetime. It is found that forward bias is effective in eliminating junction fields. It is also shown that fields can be effectively suppressed by increasing excitation power, but doing so is imprecise and can also introduce additional errors unrelated to fields. It is therefore concluded that lifetime is most accurately measured in completed cells under forward bias at low excitation intensity.

Defect-rich BN-supported Cu with superior dispersion for ethanol conversion to aldehyde and hydrogen

Copper-based heterogeneous catalysts commonly exhibit uncontrolled growth of copper species under reaction conditions because of the low Hüttig temperature (surface mobility of atoms) and Tamman temperature (bulk mobility) for copper at just 134 and 405 °C, respectively. Herein, we report the use of defect-enriched hexagonal boron nitride nanosheets (BNSs) as a support to anchor the Cu species, which resulted in superior dispersion of the Cu species. The obtained Cu/BNS catalyst was highly stable for ethanol dehydrogenation, with a high selectivity of 98% for producing acetaldehyde and an exceptionally high acetaldehyde productivity of 7.33 gAcH gcat–1 h–1 under a weight hourly space velocity of 9.6 gEtOH gcat–1 h–1. The overall performance of our designed catalyst far exceeded that of most reported heterogeneous catalysts in terms of the stability of the Cu species and the yield of acetaldehyde in this reaction. The hydroxyl groups at the defect edges of BNS were responsible for the stabilization of the copper species, and the metal-support interaction was reinforced through charge transfer, as evidenced by coupling atomic resolution images with probe molecule infrared spectroscopy and X-ray photoelectron spectroscopy. A designed in situ diffuse reflectance infrared Fourier transform spectroscopy study of ethanol/acetaldehyde adsorption further revealed that Cu/BNS favored ethanol adsorption while suppressing acetaldehyde adsorption and further side reactions. This study demonstrates a new method for designing highly dispersed Cu-based catalysts with high durability.

Body doping dependence of field-effect mobility in both n- and p-channel 4H-SiC metal-oxide-semiconductor field-effect transistors with nitrided gate oxides

Both n- and p-channel SiC MOSFETs, the gate oxides of which were annealed in NO, with various body doping concentrations were fabricated. Despite the large difference in bulk mobility between electrons (1020 cm2 V−1 s−1) and holes (95 cm2 V−1 s−1), the maximum field-effect mobility in heavily-doped (∼5 × 1017 cm−3) MOSFETs was 10.3 cm2 V−1 s−1 for the n-channel and 7.5 cm2 V−1 s−1 for the p-channel devices. The measurements using body bias revealed that the field-effect mobility in both n- and p-channel SiC MOSFETs is dominated by the effective normal field rather than the body doping.

Effect of water content on features of W/O emulsion during water flooding in heavy oil reservoir: Bulk properties and mobility control characteristics

The positive role of water-in-oil (W/O) emulsion on recovering heavy oil during water flooding has started arousing attention. Whereas, few studies have focused on exploring its mobility control characteristics, which was greatly affected by water content(ϕ). Thus, this paper targeted the effect of water content on its bulk properties and mobility control performance. Results showed that the increase of ϕ weakened the emulsion stability and enlarged the droplet size. The viscosity of W/O emulsion was 1.23–16.57 times higher than that of original oil except a few cases (ϕ ≥ 50%, γ˙≥600s−1), largely elevating the displacing-displaced phase mobility ratio. Besides, for emulsions (ϕ ≥ 40%), viscosities exhibited the strong shear rate dependence effect, and the values of flow consistency index ranged between 0.71803 and 0.83766. More obvious shear-thinning behaviors were observed in W/O emulsions with higher ϕ, which made W/O emulsion exhibit a certain self-adjustment ability to plug the layer. Compared with water-only flooding, the presence of W/O emulsion significantly increased the pressure drop, suggesting the excellent performance in permeability reduction. Due to the blocking of large-size droplets, the pressure drops in the subsequent water injection stage were significantly increased for emulsions (ϕ ≥ 70%), prolonging the duration of mobility control. Meanwhile, the presence of W/O emulsion improved fluid distribution and stabilized the flood front. Emulsions forming in higher ϕ presented stronger ability to reduce permeability and control profile. This study is helpful to clarify the key role of W/O emulsion.

Molecular mobility in bulk and in near-surface nano-layers of ultra-high-molecular-weight polyethylene

A comparative study of molecular dynamics in near-surface nanolayers and in a bulk of polyethylene is presented. Molecular mobility in near-surface nanolayers of polyethylene reactor powders and melt-crystallized films of various molecular weights prepared in different crystallization conditions were studied with the help of thermoluminescence technique using high frequency glow dicharge Ar plasma for surface activation. Molecular mobility in a bulk of the same samples was investigated by the method of radio thermoluminescence, in which a deeply penetrating γ-quanta from 60Co radiation was used for activation of the bulk. A marked difference in a temperature position of the peaks on the glow curves of plasma-induced thermoluminescence and those of radiothermoluminescence correspoding to γ- and ß-transitions was found to depend on crystallisation conditions and molecular weight. Quasi-independent segmental mobility (ß-relaxation) in the near-surface nano-layers of UHMWPE reactor powder particles synthesized at low temperatures appeared to be unexpectedly unfreezed at the temperature higher than that in the bulk. It was supposed that this happend because of crystallization under the confinement conditions during low-temperature polymerization leads to formation of a specific surface structure.

Volume and composition of semi-adsorbed stationary phases in hydrophilic interaction liquid chromatography. Comparison of water adsorption in common stationary phases and eluents

Pycnometric and homologous series retention methods are used to determine the volume and mean composition of the water-rich layers partially adsorbed on the surface of several hydrophilic interaction liquid chromatography (HILIC) column fillings with acetonitrile-water and methanol-water as eluents. The findings obtained in this work confirm earlier studies using direct methods for measuring the stationary phase water content performed by Jandera's and Irgum's research groups. Water is preferentially adsorbed on the surface of the HILIC bonded phase in hydroorganic eluents containing more than 40% acetonitrile or 70% methanol, and a gradient of several water-rich transition layers between the polar bonded phase and the poorly polar bulk mobile phase is formed. These layers of reduced mobility act as HILIC stationary phases, retaining polar solutes. The volume of these layers and concentration of adsorbed water is much larger for acetonitrile-water than for methanol-water mobile phases.In hydroorganic eluents with less than 20-30% acetonitrile or 40% methanol the amount of preferentially adsorbed water is very small, and the observed retention behavior is close to the one in reversed-phase liquid chromatography (RPLC). In eluents with intermediate acetonitrile-water or methanol-water compositions a mixed HILIC-RPLC behavior is presented.Comparison of several HILIC columns shows that the highest water enrichment in the HILIC retention region for acetonitrile-water mobile phases is observed for zwitterionic and aminopropyl bonded phases, followed in minor grade for diol and polyvinyl alcohol functionalizations. Pentafluorophenyl bonded phase, usually considered a HILIC column, does not show significant water adsorption, nor HILIC retention.

Disorder and phase diagrams of higher-order topological insulators

In this work, we study the disorder effects on the bulk-boundary correspondence of two-dimensional higher-order topological insulators (HOTIs). We concentrate on two cases: (i) bulk-corner correspondence, (ii) edge-corner correspondence. For the bulk-corner correspondence case, we demonstrate the existence of the mobility gaps and clarify the related topological invariant that characterizes the mobility gap. Furthermore, we find that, while the system preserves the bulk-corner correspondence in the presence of disorder, the corner states are protected by the mobility gap instead of the bulk gap. For the edge-corner correspondence case, we show that the bulk mobility gap and edge band gaps of HOTIs are no longer closed simultaneously. Therefore, a rich phase diagram is obtained, including various disorder-induced phase transition processes. Notably, a disorder-induced transition from the non-trivial to trivial phase is realized, distinguishing the HOTIs from the other topological states. Our results deepen the understanding of bulk-boundary correspondence and enrich the topological phase transitions of disordered HOTIs.

A New Definition of the Stationary Phase Volume in Mixed-Mode Chromatographic Columns in Hydrophilic Liquid Chromatography.

Polar columns used in the HILIC (Hydrophilic Interaction Liquid Chromatography) systems take up water from the mixed aqueous–organic mobile phases in excess of the water concentration in the bulk mobile phase. The adsorbed water forms a diffuse layer, which becomes a part of the HILIC stationary phase and plays dominant role in the retention of polar compounds. It is difficult to fix the exact boundary between the diffuse stationary and the bulk mobile phase, hence determining the column hold-up volume is subject to errors. Adopting a convention that presumes that the volume of the adsorbed water can be understood as the column stationary phase volume enables unambiguous determination of the volumes of the stationary and of the mobile phases in the column, which is necessary for obtaining thermodynamically correct chromatographic data in HILIC systems. The volume of the aqueous stationary phase, Vex, can be determined experimentally by frontal analysis combined with Karl Fischer titration method, yielding isotherms of water adsorbed on polar columns, which allow direct prediction of the effects of the composition of aqueous–organic mobile phase on the retention in HILIC systems, and more accurate determination of phase volumes in columns and consistent retention data for any mobile phase composition. The n phase volume ratios of 18 columns calculated according to the new phase convention strongly depend on the type of the polar column. Zwitterionic and TSK gel amide and amine columns show especially strong water adsorption.

ZnO a multifunctional material: Physical properties, spectroscopic ellipsometry and surface examination

The investigation reports on the transparent and conducting pure and metal doped zinc oxide (ZnO) thin layer. The layers are ultrasonic sprayed onto microscope glass substrates at 350 °C. The effect of Sn doping level (from 0% to 3%) is discussed in one hand and metal (Al, Cu and Sn) doping effect on the properties in other hand. Spectroscopic ellipsometry, optical, structural and electrical analyses of the as-grown films have been achieved. The (002)-oriented hexagonal wurtzite crystal structure is revealed by X-ray pattern having a grain size of 4.5–15 nm which confirms the nanosized aspect. The UV–VIS–IR spectrophotometer measurements reveal that the as-grown films are highly transparent in the visible and IR ranges (T~90%). A slight influence of dopants on the band gap energy which ranges between 2.24 and 2.29 eV is emphasized. The electrical parameters such as bulk density, resistivity and mobility are measured by Hall measurement system (HMS). The films thickness is estimated by spectroscopic ellipsometry values and found to be of 178–330 nm. The electrical properties confirm an enhancement of bulk density and conductivity within the range − 1.06 × 1012–5.09 × 1013 (cm−3) and 3.2 × 10−5–1.05 × 10−3(1/Ω cm) due to dopants incorporation as predicted. Inversion to p-type is demonstrated in Sn-doped ZnO sample. The nanostructures are observed in 2D scanned AFM views, size are of micro/nanometer and roughness is of 3.39–15 nm.

Influence of post-deposition annealing on the transport properties of sputtered Bi2Se3 thin films

The fabrication of chalcogenide-based topological insulator (TI) thin films with low defects and vacancies is an important research problem. In the past decade, Bi2Se3 is one of the most investigated TI materials. The physical properties of Bi2Se3 are dictated by selenium vacancies irrespective of the growth method. Controlling the number of vacancies is very challenging, but it can be influenced by substrate and growth temperature. In this work, we demonstrate that post-deposition annealing temperature is also effective in controlling the carrier concentration of Bi2Se3 thin films. Ultra-thin Bi2Se3 films were fabricated on quartz substrates using magnetron sputtering at room temperature, and their resistivity, bulk carrier concentration, and bulk mobility are measured as a function of the post-deposition annealing temperature under vacuum conditions ranging from 180–350 °C. We find that the carrier concentration can vary by an order of magnitude, and we obtained values as low as ~1 × 1019 cm−3 between 200-250 °C which compares very well with some of the best literature reports on films made by molecular beam epitaxy. Room temperature bulk mobility values in excess of 100 cm2/Vs were recorded for the optimally annealed samples. Energy dispersive x-ray spectroscopy measurements showed that selenium-rich Bi2Se3 films have the highest carrier concentration values. X-ray diffraction measurements showed that the best crystallographic properties are obtained for stoichiometric Bi2Se3. Our results indicate that higher mobility values may be possible if better crystal structure and lower resistivity are obtained in selenium-rich films.

Dislocation Assisted Phase Separation: A Phase Field Study

Defects play a key role in deciding the mechanisms and kinetics of phase transformations. In this paper, we show how dislocations influence phase separation in alloys with miscibility gap. Specifically, depending on the ratio of pipe mobility to bulk mobility, it is seen that even in a system with nominal compositions outside the spinodal limit, spinodal phase separation is possible. Surprisingly, phase separation through both nucleation and growth, and spinodal decomposition, is seen concurrently (for the case of intersecting dislocations). Finally, the prominent role played by dislocations in influencing the morphology of precipitates is explored. We show that these results agree qualitatively with recent experimental results in iron based systems obtained using Atom Probe Tomography (APT).

Charge trapping analysis in sputtered BixSe1-x based accumulation-mode FETs. II. Gate capacitance characteristics

In this study, we extend the analyses done on sputtered BixSe1-x based accumulation mode FETs. Previously, we studied the basic electrical and leakage properties of these FET devices. We extend our analyses to obtain key parameters of the BixSe1-x (x = 0.44) film at various gate voltages. We start by extracting the sheet carrier density and bulk mobility for different gate voltages, using the Drude model with the previously obtained semi-empirical relationship between carrier concentration and bulk mobility for BixSe1-x. The change in sheet carrier density is a result of accumulation or depletion or majority carriers from the BixSe1-x/SiO2 interface, which show hysteretic behavior. This allows us to calculate the surface sheet carrier density and the quasi-static capacitance at various gate voltages. We use a simple capacitive model to separate the capacitance originating from the gate and the film bulk. The capacitance from the film bulk is due to the surface charge thickness and is directly dependent on the Debye length. From the change of capacitance, with respect to gate voltage, we were able to identify the characteristics of the conduction band edge and the bulk band gap/Dirac cone.