Electron Transit Time Research Articles

Do electrons diffusing in a nanostructured semiconductor have time to attain local equilibrium?

The electron diffusion in the nanostructured photoanode of a dye-sensitized solar cell is affected by both the nanoparticle film morphology and the electron traps in the nanoparticles. The effect of the traps is to reduce the diffusion coefficient from its trap free value, which is deleterious to the cell performance. However, there is a time for the effect of the traps to be felt, which can be identified with a local or quasi equilibration time. We address this issue here and show how the volume of the nanoparticles, the equilibrium chemical potential and the temperature affect this time and can make the local equilibration time comparable to the electron transit time across the film.

InGaP/GaAsSb/InGaAsSb/InP Double Heterojunction Bipolar Transistors With Record f t of 813 GHz

We fabricated InGaP/GaAsSb/InGaAsSb/InP double heterojunction bipolar transistors (DHBTs) with an aggressive lateral and vertical scaling technology to improve the current gain cutoff frequency ( ${f}_{\text {t}}$ ) further. A 13-nm-thick GaAsSb/InGaAsSb base and a 40-nm-thick InP collector are used to reduce electron transit time. In addition, the width of the base electrode on each side of the emitter is reduced to about 0.05 $\mu \text{m}$ to suppress increases in parasitic collector capacitance. A fabricated DHBT with the emitter size of 0.24 $\mu \text{m}\,\,\times7.8\,\,\mu \text{m}$ exhibits maximum differential current gain of ~95 and collector-emitter breakdown voltage of 2.6 V. At a collector current density of 18 mA/ $\mu \text{m}^{{2}}$ , the DHBT exhibits ${f}_{\text {t}}$ of 813 GHz, which is the highest among all types of transistors measured at a room temperature.

Realtime kinetics of the light driven steps of photosynthetic water oxidation in living organisms by “stroboscopic” fluorometry

We develop a rapid “stroboscopic” fluorescence induction method, using the fast repetition rate fluorometry (FRRF) technique, to measure changes in the quantum yield of light emission from chlorophyll in oxygenic photosynthesis arising from competition with primary photochemical charge separation (P680* ➔ P680+QA−). This method determines the transit times of electrons that pass through PSII during the successive steps in the catalytic cycle of water oxidation/O2 formation (S states) and plastoquinone reduction in any oxygenic phototroph (in vivo or in vitro). We report the first measurements from intact living cells, illustrated by a eukaryotic alga (Nannochloropsis oceanica). We demonstrate that S state transition times depend strongly on the redox state of the PSII acceptor side, at both QB and the plastoquinone pool which serve as the major locus of regulation of PSII electron flux. We provide evidence for a kinetic intermediate S3′ state (lifetime 220 μs) following formation of S3 and prior to the release of O2. We compare the FRRF-detected kinetics to other previous spectroscopic methods (optical absorbance, EPR, and XES) that are applicable only to in vitro samples.

Terahertz Ultralow-Voltage Gyrotron With Upstream Output

A medium-power broadband-tuning terahertz (THz) gyrotron operating at low voltages is especially attractive for applications in biomedicine. This article tends to elaborate on the challenges related to startup oscillation and ohmic loss suppression of an extremely low-voltage THz gyrotron. High interaction efficiency is achievable by applying long transit time of electrons to compensate the unfavorable decline of the beam-wave coupling strength for low-energy electron beams. A cavity structure which favors backward-wave interaction is proposed to extend the frequency tuning range. The theoretical study predicts that the optimal cavity contributes to the robust frequency tunability at low voltages. The results indicate that a continuous tuning range of 10.7 GHz around the 330-GHz band with output power higher than 1 W can be expected by utilizing a 0.3 kV, 0.5 A helical electron beam.

Increasing the Efficiency of Dye-Sensitized Solar Cells by Adding Nickel Oxide Nanoparticles to Titanium Dioxide Working Electrodes

In this study, nickel oxide (NiO) nanoparticles were added to a titanium dioxide (TiO2) nanoparticle paste to fabricate a dye-sensitized solar cell (DSSC) working electrode by using a screen-printing method. The effects of the NiO proportion in the TiO2 paste on the TiO2 working electrode, DSSC devices, and electron transport characteristics were comprehensively investigated. The results showed that adding NiO nanoparticles to the TiO2 working electrode both inhibited electron transport (a negative effect) and prevented electron recombination with the electrolyte (a positive effect). The electron transit time was extended following an increase in the amount of NiO nanoparticles added, confirming that NiO inhibited electron transport. Furthermore, the energy level difference between TiO2 and NiO generated a potential barrier that prevented the recombination of the electrons in the TiO2 conduction band with the I3- ions in the electrolyte. When the TiO2–NiO ratio was 99:1, the positive effects outweighed the negative effects. Therefore, this ratio was the optimal TiO2–NiO ratio in the electrode for electron transport. The DSSCs with a TiO2–NiO (99:1) working electrode exhibited an optimal power conversion efficiency of 8.39%, which was higher than the DSSCs with a TiO2 working electrode.

Demonstration of a graphene-base heterojunction transistor with saturated output current

A novel transistor with a graphene base embedded between two n-type silicon emitter and collector layers (graphene-base heterojunction transistor) is fabricated and characterized electrically. The base voltage controlled current of the device flows vertically from the emitter via graphene to the collector. Due to the extremely short transit time for electrons passing the ultimately thin graphene base, the device has a large potential for high-frequency RF applications. The transistor exhibits saturated output currents and a clear modulation of the collector current by means of the graphene base voltage. The vertical transfer current from the emitter via the graphene base to the collector is much lower than expected from device simulations. A comparison of the graphene-base transistor and a reference silicon n-p-n bipolar transistor is performed with respect to the main DC transistor characteristics. A common-emitter gain of larger than one has been achieved for the reference device while the graphene-base transistor so far exhibits a much lower gain.

Experimental Observation of Chaotic Generation at 1.5% Spectral Width in a Gyrotron under Large Supercriticality Conditions

A complex chaotic dynamics in a gyrotron was experimentally observed under conditions of high supercriticality corresponding to 1000–1500-fold excess of the beam current parameter over the starting value. At electron beam currents typical of gyrotron test setups, this excess has been achieved by a transition to excitation of the lowest mode of a cylindrical waveguide. Chaotic generation regimes were observed in a 33 GHz frequency range with a spectral width of 500 MHz and record narrow relative width below 1.5% These results imply considerable progress in comparison to previous experiments with gyrotron type devices and are in good agreement with theoretical analysis based on a system of averaged evolution equations derived with allowance for finiteness of the transit time of electrons via the interaction space.

Electrochemical Capacitance and Transit Time in Quantum Hall Conductors

In a two dimensional electron gas, low energy transport in presence of a magnetic field occurs in chiral 1D channels located on the edge of the sample. In the AC description of quantum transport, the emittance determines the amplitude of the imaginary part of the admittance, whose sign and physical meaning are determined by the sample topology: a Hall bar is inductive while a Corbino ring is capacitive. In this article, the perfect capacitive character of Corbino samples in the quantum Hall effect regime is shown. A vanishing conductance and an electrochemical capacitance which depends on the density of states of 1D channels are measured. Our samples have no gate, neither on the side nor on the top, and the inner capacitances are measured. The transit time of electrons across the device is obtained and the drift velocity of carriers is deduced.

Экспериментальное наблюдение хаотической генерации с шириной спектра 1.5% в гиротроне в условиях большой надкритичности

The complex dynamics of the gyrotron is experimentally studied for operating current which is 1000–1500 times higher than the gyrotron starting current. Whereas in the experiment the electron beam current was only 0.2 A, a very high normalized current parameter is obtained through the operation at the lowest mode of a cylindrical waveguide. At the frequencies of about 33 GHz, the chaotic generation was observed with a spectrum bandwidth of up to 500 MHz that correspond to a relative bandwidth of 1.5 %. This value is significantly higher than the bandwidths of chaotic signals obtained in previous experiments with gyrotron devices. The experimental results are in good agreement with the results of the theoretical analysis, which are based on the averaged equations taking into account the finiteness of the electron transit time through the interaction region.

Quantifying electron transit in donor-bridge-acceptor systems using probabilistic confidence

A probabilistic approach to characterizing transit times for quantum particles is generalized to a system of more than two spatial regions and applied to the transport of charge in donor-bridge-acceptor systems. The approach is based on applying conditional probability analysis to a discrete representation of the time-dependent probability density as generated by numerical solution of the time-dependent Schrödinger equation for an initially localized electron. To carry out this analysis, it is first necessary to cast the conditional probability analysis approach in matrix form. The results afford a quantification of the electron transit time and may provide a tool to gain insight into the mechanism of charge transport.

Time- and Frequency-Resolved Photoelectrochemical Measurements on Dye Sensitized Solar Cells Employing (102) and (100) Dominated ZnO Nanocrystals

We have succeeded in microwave-assisted hydrothermal synthesis of ZnO nanocrystals in distinctively different structures by use of structure directing agents (SDAs). Use of triethanolamine resulted in hexagonal bi-pyramidal (HBP) shape with 84% with predominant exposoure of (102) facet, whereas benzene tetracarboxylic acid yielded nanosheet (NS) with 76% (100) facet. They were compared as photoelectrodes of dye sensitized solar cells (DSSCs) employing D149 and eosinY as sensitizers. Dye coverage at the highest density was found for HPB for both dyes, whereas NS the lowest. Denser packing resulted in higher current, not because of the difference of absolute light harvesting efficiency, but improved efficiency of electron injection. On the other hand, highest voltage was always observed for NS. The values of MZ appeared always between those of HBP and NS, in fact, resulting in the highest efficiency among the three due to its best-balanced performance. These observations left us some questions. Why does the (102) facet densely covered with dyes become leaky to result in low voltage, while loosely covered (100) was able to block the back reaction to achieve high Voc? In this paper, we have carried out intensity modulated photocurrent/voltage spectroscopies (IMPS/IMVS) of the DSSCs employing these different ZnO nanocrystals to elucidate their transport and back reaction kinetics. Pastes made of different ZnO nanoparticles were doctor bladed on FTO glass substrate using Scotch® tape to control their thickness to 10 mm. After drying, the geometric area of the films were regulated to be 0.25 cm2 by scratching off the redundant part. Then the films were dipped into 60°C warm water to promote necking for 10 min. and dried in an oven at 100°C for 30 min. The films were immersed into 0.5 mM DN216 (same chromophore as D149 but with improved adsorption stability due to extra anchor, developed by Chemicrea) in tert-butanol/acetonitrile (1/1 in volume) solution for sensitization. Sandwich cells were made by attaching platinized FTO glass counter electrodes in a face-to-face geometry with a thermoplastic film. The cells were sealed with a cover glass after injecting electrolyte of 0.5 M propyl-methyl-imidazolium iodide and 0.05 M iodine in acetonitrile. Monochromatic LED (625 nm) were used as light sources for IMPS and IMVS. The background light intensity was varied between 1 and 250 W m-2, and the amplitude of the light modulation was set to 10% of the back-ground light intensity, where frequency range from 0.3 Hz to 100 kHz was set for IMPS and IMVS. The electron transit time and lifetime were calculated from the frequency of the minimum in the imaginary component from IMPS (short circuit condition) and IMVS (open circuit condition), respectively. The fitting values were plotted against light intensity in log scale, as shown in Fig. 1 where open marks correspond to transit time and solid ones to lifetime. From IMPS results, longer transit times were revealed for HBP and NS compared to MZ, which means it takes longer for the photoinjected electrons to reach the back contact. This factor is influenced by the film thickness and dye loading. As the film thickness was constant at 10 µm, the difference may rather come from the total amount of dye loading. As described earlier the dye loading was largest on MZ because of the larger surface area. Also, the transit time depends on the trap concentrations in the films. Due to the larger particle size in HBP and NS films, the particles in the film may not be as well connected as in the films of small MZ particles, which will create more traps to limit electron transport. In the IMVS results, NS shows longer electron lifetime than MZ and HBP. A slower back reaction of photogenerated electrons to tri-iodide is thereby indicated under open circuit conditions. Hence, a stronger passivation against this recombination reaction in the NS film than in MZ and HBP is observed, which gives one reason why NS cells always exhibit higher Voc than the others. In order to quantify this passivation behavior, electrochemical impedance spectroscopy (EIS) was also carried out to clarify the factors such as recombination resistance, chemical capacitance, etc and will be discussed in this study. Figure 1

Comparison of performance and optoelectronic processes in ZnO and TiO2 nanorod array-based hybrid solar cells

The reported efficiencies of pristine ZnO nanorod array (NRA)-based polymer-inorganic hybrid solar cells (HSCs) are normally lower than those of their pristine TiO2 NRA-based counterparts. This difference typically results from the lower short-circuit current density (Jsc) of the ZnO NRA device. This paper presents a comparative study of pristine ZnO and TiO2 NRA-based HSCs. We investigate the morphological structure (length, diameter, number density, area of nanorod laterals), photovoltaic performance (current density-voltage J–V, external quantum efficiency EQE), and optoelectronic processes related to electron transfer (electron mobility μe, electron diffusion length LD, electron lifetime τe and electron transit time τt related electron collecting efficiency ηcc, electron injection ηinj, surface potential SP, photoluminescence PL, bound charge pairs BCP) in HSCs, with ZnO and TiO2 NRA as electron acceptor. Our comparative investigations reveal that the factors relating to the interface area, μe, LD, and ηcc are not the key factors responsible for the difference in the value of Jsc in ZnO and TiO2 NRA-based HSCs with the same device structure. In fact, the crucial step for a lower Jsc in ZnO NRA-based HSCs than in TiO2 NRA-based HSCs is attributed to the less efficient transfer of photo-generated electrons at the charge separation interface in ZnO NRA-based HSCs. Dynamic characterizations indicate that the transfer of interfacial photo-generated electrons in TiO2 NRA-based HSCs is more efficient than ZnO NRA-based HSCs, and is confirmed by Kelvin probe force microscopy (KPFM) and PL studies. The reason for the better interface charge transfer property in MEH-PPV/TiO2 NRA than that of in MEH-PPV/ZnO NRA is further investigated by Marcus model, we find that more trapped BCP states are generated in the ZnO NRA based HSCs, which resulting in lower interfacial electron injection efficiency from polymer to ZnO NRA.

Multipactor susceptibility chart of coaxial transmission lines with stationary statistical modeling

Multipactor breakdown is a detrimental electromagnetic phenomenon caused by resonant secondary electron emissions synchronizing with field oscillation, which frequently takes place in powerful microwave devices and accelerating structures. Regarded as the principal failure mode of space microwave systems, multipactor may cause the performance to degenerate or even hardware operation to deteriorate catastrophically, thus multipactor becomes a major limitation in promoting the further development of space communication technology. Meanwhile, higher power capacity and volume integration accordingly lead to continuously growing multipactor hazard. In order to prevent multipactor from occurring, the accurate predictive technique to determine multipactor susceptibility has become a key issue for the mechanical design and performance optimization of microwave devices in the ground stage. Compared with the existing approaches to investigating the multipactor, statistical theories are able to conduct multipactor threshold calculation and mechanism analysis, with the stochastic nature of secondary emission fully considered from the probabilistic perspective. Currently, stationary statistical theory of multipactor has been developed for efficient multipactor threshold analysis of the parallel-plate geometry. However, it has not been further extended to the coaxial geometry which is commonly involved in radio frequency (RF) systems. For this reason, the stationary statistical modeling of the coaxial multipactor with all influencing factors considered is detailed in this paper. Due to the field nonuniformity and the secondary emission randomness, analytic equation of electron trajectories in the coaxial geometry is approximately derived by using the perturbation approach. Based on the implicit correlation between electron emission velocity and transit time, the joint probability density function is constructed for the calculation of the probability density distribution of electron transit time. Afterwards, a system of integral equations for depicting electron multiplication process in the coaxial geometry is formulated and solved with a novel and general iteration method. Finally, this stationary statistical theory is applied to the full multipactor susceptibility chart of coaxial transmission lines with typical coating materials in space engineering, such as silver, copper, alumina and alodine. A comparison shows that the calculation results are in reasonable agreement with the experimental measurements provided by the Europe Space Agent. What is more, there exists significant difference between multipactor susceptibility curves of the parallel-plate geometry and the coaxial geometry. This research is of great significance for optimizing the mechanism design and material selection of multipactor-free microwave devices.

Gyrotron generation of broadband chaotic radiation under overlapping of high- and low-frequency resonances

Significant (up to 10%) broadening of the band of noise-like radiation in gyrotrons is possible due to specific tuning of gyrofrequency relative to the critical frequency of the working mode when the high- and low-frequency cyclotron resonances are substantially separated at the given translation and transverse velocities of electrons. The generation bands at the resonances are overlapped under operation above the threshold. The analysis with allowance for finiteness of the electron transit time in the interaction space and, hence, different slopes of the dispersion characteristics of electron beam and wave is critically important for correct investigation of the generation regimes in the framework of the average approach. The results are verified using direct 3D particle-in-cell simulation of the chaotic generation regimes of the gyrotron generation in the 8-mm-wavelength range.

Solenoid transport of beams with current-dependent initial conditions

Intense charged particle beams will generally be formed with an initial correlation between their longitudinal properties, including longitudinal variations in current, and their transverse properties, including their radius and divergence. This is due to the competition between the transverse focusing fields in the beam source and the time-varying space charge forces in the beam. In DC electron guns where the current modulation is slow compared to the electron transit time, the nature of these correlations was previously shown to depend on the gun geometry, exhibiting a linear dependence of the beam radius and divergence on the beam current at the gun exit. Here, we extend the previous work to study the transport of beams with such correlation in uniform and periodic solenoid channels. For each transport channel configuration studied, the transverse envelope equation is used to calculate the envelope of 101 beam slices differing in their slice currents, as well as initial radius and divergence (due to their dependence on slice current). For each channel configuration, these calculations are performed 546 times, with each of these iterations considering a different degree of correlation between the radius and divergence, and the slice current. It is found that some degree of correlation between the initial radius and slice current actually aids in beam transport, and the required strength of correlation can be estimated with simple models. Increasing the degree of correlation between the initial divergence and slice current is generally counterproductive, and the degree of sensitivity to such correlations depends on the design of the transport channel.

Low-temperature thermal transport and thermopower of monolayer transition metal dichalcogenide semiconductors

We study the low temperature thermal conductivity of single-layer transition metal dichalcogenides (TMDCs). In the low temperature regime where heat is carried primarily through transport of electrons, thermal conductivity is linked to electrical conductivity through the Wiedemann–Franz law (WFL). Using a k.p Hamiltonian that describes the and valley edges, we compute the zero-frequency electric (Drude) conductivity using the Kubo formula to obtain a numerical estimate for the thermal conductivity. The impurity scattering determined transit time of electrons which enters the Drude expression is evaluated within the self-consistent Born approximation. The analytic expressions derived show that low temperature thermal conductivity (1) is determined by the band gap at the valley edges in monolayer TMDCs and (2) in presence of disorder which can give rise to the variable range hopping regime, there is a distinct reduction. Additionally, we compute the Mott thermopower and demonstrate that under a high frequency light beam, a valley-resolved thermopower can be obtained. A closing summary reviews the implications of results followed by a brief discussion on applicability of the WFL and its breakdown in context of the presented calculations.

Multimode time-dependent gyrotron equations for different time scales

Development of gyrotrons requires careful understanding of different regimes of gyrotron oscillations. For a long time, the gyrotron theory was developed assuming that the transit time of electrons through the interaction space is much shorter than the cavity fill time. Correspondingly, it was assumed that during this transit time, the amplitude of microwave oscillations remains constant. However, there are situations when this assumption is not fulfilled, or is marginally fulfilled. In such cases, a different mathematical formalism has to be used. The present paper generalizes the new formalism to the multi mode case. The particular example considered indicates that in some cases the results obtained by means of the old and the new formalism differ significantly.

Remote electrical arc suppression by laser filamentation: erratum.

A correction in the transit time of electrons between the filaments and the electrodes leads us to reattribute the remote unloading to ions rather than to electrons. The experimental results reported in [Opt. Express23, 286407 (2015)] about remote electrical unloading and discharge suppression, as well as the analogy with the analogy with a supercorona, remain valid.

Influence of surface modification with D205 dye on charge dynamics of hybrid ZnO nanorods/polymer solar cells

ABSTRACTInfluence of surface modification with D205 dye on charge dynamics of hybrid solar cells based on ZnO nanorods and poly(3-hexylthiophene) or P3HT was investigated. Electrochemical impedance spectroscopy analysis reveals that ZnO nanorods/D205/P3HT device shows longer effective lifetime (less charge recombination) in comparison with ZnO nanorods/P3HT one. Moreover, transit time of electrons travelling to the electrode for the D205-modified device is shorter than that of the unmodified one. These results indicate that the adsorption of D205 dye promotes efficient charge collection, leading to the improvement in fill factor. The surface modification efficiently enhances charge dynamics of hybrid ZnO/P3HT devices.

Self-consistent non-stationary theory of the gyrotron

For a long time, the gyrotron theory was developed assuming that the transit time of electrons through the interaction space is much shorter than the cavity fill time. Correspondingly, it was assumed that during this transit time, the amplitude of microwave oscillations remains constant. A recent interest to such additional effects as the after-cavity interaction between electrons and the outgoing wave in the output waveguide had stimulated some studies of the beam-wave interaction processes over much longer distances than a regular part of the waveguide which serves as a cavity in gyrotrons. Correspondingly, it turned out that the gyrotron theory free from the assumption about constant amplitude of microwave oscillations during the electron transit time should be developed. The present paper contains some results obtained in the framework of such theory. The main attention is paid to modification of the boundary between the regions of oscillations with constant amplitude and automodulation in the plane of normalized parameters characterizing the external magnetic field and the beam current. It is shown that the theory free from the assumption about the frozen wave amplitude during the electron transit time predicts some widening of the region of automodulation.