Semiconductor Cathode Research Articles

Theory of photoemission from cathodes with disordered surfaces

Linear-accelerator-based applications like x-ray free electron lasers, ultrafast electron diffraction, electron beam cooling, and energy recovery linacs use photoemission-based cathodes in photoinjectors for electron sources. Most of these photocathodes are typically grown as polycrystalline materials with disordered surfaces. In order to understand the mechanism of photoemission from such cathodes and completely exploit their photoemissive properties, it is important to develop a photoemission formalism that properly describes the subtleties of these cathodes. The Dowell–Schmerge (D–S) model often used to describe the properties of such cathodes gives the correct trends for photoemission properties like the quantum efficiency (QE) and the mean transverse energy (MTE) for metals; however, it is based on several unphysical assumptions. In the present work, we use Spicer’s three-step photoemission formalism to develop a photoemission model that results in the same trends for QE and MTE as the D–S model without the need for any unphysical assumptions and is applicable to defective thin-film semiconductor cathodes along with metal cathodes. As an example, we apply our model to Cs3Sb thin films and show that their near-threshold QE and MTE performance is largely explained by the exponentially decaying defect density of states near the valence band maximum.

Bandgap Reduction and Enhanced Photoelectrochemical Water Electrolysis of Sulfur-doped CuBi2O4 Photocathode

As interest in hydrogen energy grows, eco-friendly methods of producing hydrogen are being explored. CuBi2O4 is one of the p-type semiconductor cathode materials that can be used for photoelectrochemical hydrogen production via environment-friendly water electrolysis. CuBi2O4 has a bandgap of 1.5 – 1.8 eV which allows it to photogenerate electrons and holes from the absorption of visible light. This study investigated the effect of sulfur doping on the bandgap and photoelectrochemical water reduction properties of CuBi2O4. Sulfur-doped CuBi2O4 thin films were electrochemically synthesized using a nitrate-based precursor solution with thiourea. This was followed by two-step annealing in an Ar atmosphere, which effectively prevented the oxidation of sulfur. Sulfur doping up to 0.1 at% led to the expansion of the lattice volume of the CuBi2O4. The bandgap was reduced from 1.9 eV to 1.5 eV with increasing doping concentration, which resulted in the enhancement of photoelectrochemical current density by ~240%. X-ray photoelectron spectroscopy showed that sulfur-doping reduced oxygen vacancies with increasing doping concentration, confirming that the enhanced photoelectrochemical properties resulted from the reduction in bandgap, not from any extrinsic factor such as oxygen vacancies. Further studies of sulfur-doped CuBi2O4 to improve surface coverage are expected to lead to a more promising photoelectrochemical cathode material.

Efficient H2 production in a ZnFe2O4/g-C3N4 photo-cathode single-chamber microbial electrolysis cell.

Photo-assisted single-chamber microbial electrolysis cells (MECs) incorporating semiconductor cathodes are attractively promising for exclusive hydrogen without CH4 and CO2. However, the unsustainable, high cost, and unstable metal catalysts on the cathodes along with the intricacies behind the interplay of circuital current, light illumination, and bacterial communities on both electrodes are poorly understood. Herein, photo-assisted single-chamber MECs incorporating ZnFe2O4/g-C3N4 cathodes are demonstrated to achieve efficient production of exclusive hydrogen (0.11 ± 0.01 m3/m2/day; 1.70 ± 0.04 m3/m3/day) with a solar-to-hydrogen conversion efficiency of 4.08 ± 0.17% and an energy efficiency relative to electrical input of 233 ± 5%. The ZnFe2O4/g-C3N4 structured cathodes exhibited appreciable higher photocurrents than the controls (g-C3N4: 4.3-fold; ZnFe2O4: 3.3-fold), and negligible leaking of Fe and Zn after the 4th-cycle operation. Circuital current and light illumination were proven to varying degree shape both electrodes for building up functional bacterial communities with metabolic regulation at the prolonged operation of 12 batch cycles. Energy metabolism and carbohydrate metabolism along with membrane transport, signal transduction, and cell motility based on PICRUSt functional prediction further confirmed the photo-assisted single-chamber MECs for efficient hydrogen production. This study provided a sustainable, cost-effective, and efficient approach for achieving high rates of exclusive hydrogen production and offered new insights for ingenious interplay of circuital current, light illumination, and bacterial communities for efficient hydrogen production in the photo-assisted single-chamber MECs. KEY POINTS: • ZnFe2O4/g-C3N4 cathodes of single-chamber MECs achieve efficient H2 production. • Light irradiation and circuit current shape bacterial communities on both electrodes. • Circuital current contributes to less leaking of Fe and Zn, and thus system stability.

Simultaneous Photoelectrocatalytic Oxidation and Nitrite‐Ammonia Conversion with Artificial Photoelectrochemistry Cells

AbstractArtificial photoelectrochemistry system harvests solar energy for solar fuel conversion by use of semiconductor anode and cathode. However, the low overall performance of this integrated system is limited by the compatibility and durability. Herein, a compact heterostructure photoanode exhibits >98% selective photoelectrocatalytic oxidation conversion and >99% Faradaic efficiency of benzyl alcohol oxidation at 0.67 V versus reversible hydrogen electrode in comparison of single‐phase photoanodes. Density functional theory calculations reveal the heterointerface is responsible for the appreciable electronic interaction and the accelerated charge transport, exploring the radical relay pathway as the selective oxidation mechanism. Especially, the integrated photoelectrochemistry cell is developed by heterostructure photoanode and protective photocathode, simultaneously enabling the efficient photoelectrocatalytic oxidation conversion and nitrite reduction reaction to ammonia synthesis under ambient conditions with a maximum Faradaic efficiency >98%. This work sheds light on rational design and the construction of artificial photoelectrochemistry cells towards solar energy conversion.

New Spin-Polarized Electron Source Based on Alkali Antimonide Photocathode.

New spin-dependent photoemission properties of alkali antimonide semiconductor cathodes are predicted based on the detected optical spin orientation effect and DFT band structure calculations. Using these results, the Na_{2}KSb/Cs_{3}Sb heterostructure is designed as a spin-polarized electron source in combination with the Al_{0.11}Ga_{0.89}As target as a spin detector with spatial resolution. In the Na_{2}KSb/Cs_{3}Sb photocathode, spin-dependent photoemission properties were established through detection of a high degree of photoluminescence polarization and high polarization of the photoemitted electrons. It was found that the multi-alkali photocathode can provide electron beams with emittance very close to the limits imposed by the electron thermal energy. The vacuum tablet-type sources of spin-polarized electrons have been proposed for accelerators, which can exclude the construction of the photocathode growth chambers for photoinjectors.

Photo-thermo-electric modeling of photon-enhanced thermionic emission with concentrated solar power

Photon-enhanced thermionic emission (PETE) is an advanced technology that combines both the photoelectric and the thermionic effects synergistically into a single device for direct electricity generation. However, its industrial-level application is still missing partly due to the lack of advanced models to analyze its operating characteristics and to understand the synergistic mechanism. Herein, we develop a numerical model of PETE by fully considering the optical, the electrical and the thermodynamic aspects with one-dimensional steady-state continuity equations of carriers in the semiconductor cathode. A hybrid PETE-Stirling cycle system is also proposed to yield an output power density of 162.65 kW/m 2 with 32.8% conversion efficiency. The PETE conversion efficiency keeps ∼20% with the optimal electron affinity increases from 0.5 to 1.14 eV as solar concentration ratio varies from 100 to 500. The cathode thickness should be optimized by considering both solar absorption and photon enhancement, where the thickness range of 0.78–1.52 μm is obtained for 50–500 suns. The interelectrode gap is also found to significantly affect the PETE performance by regulating both the space-charge effect and near-field radiation, where the range of 0.5–2 μm is recommended. This work can serve as a foundation to understand the working mechanism of PETE converters and provide guidelines for the performance evaluation. • Numerical models are developed for photon-enhanced thermionic emission converters. • Coupled effects of optical, electrical and thermodynamic fields are fully considered. • Characteristics of the converter for various scenarios are analyzed in detail. • A hybrid system is proposed with a 162.65 kW/m 2 power density and a 32.8% efficiency.

Anomalous correlation between quantum efficiency and transverse momentum spread in semiconductor cathode photoemission

The next generation of accelerator based scientific facilities, such as the megahertz repetition rate free electron laser and electron-ion collider, relies on photoelectron sources of high peak and high average brightness. Therefore, photocathodes with both low transverse momentum spread (TMS) and high quantum efficiency (QE) are required. The conventional positive correlation between QE and TMS leads to a tradeoff between peak and average brightness. In this study, a negative correlation between QE and TMS is experimentally observed for the first time on a semiconductor photocathode (${\mathrm{Cs}}_{2}\mathrm{Te}$). Two models are proposed to explain such an anomalous correlation. One model considers the junction between the semiconductor cathode and the metal substrate in the case of slightly excessive cesium deposition, and the other model considers different Cs-Te composites. Our studies inspire a new way to break the conventional correlation between QE and TMS, and open a new perspective to advance the beam brightness frontiers.

Direct measurement of photocathode time response in a high-brightness photoinjector

Electron photoinjectors provide high-brightness electron beams to numerous research applications in physics, chemistry, material, and life sciences. Semiconductor photocathodes are widely used here, as they enable the production of low-emittance beams with variable charge at high repetition rates. One of the key figures of merit of photocathodes is the minimum achievable bunch length. In semiconductor cathodes, this is dominated by scattering effects and varying penetration depths of the extracting photons, which leads to a characteristic electron emission function. We present a method to determine this cathode time response with resolution on the tens of femtoseconds level, breaking the resolution barrier encountered in previous studies. The method is demonstrated with cesium-telluride (Cs2Te) and gold cathodes, revealing response times of (184 ± 41) fs up to (253 ± 58) fs for the semiconductor and an upper limit of (93 ± 17) fs for the metal. Monte Carlo simulations of Cs2Te emission benchmarked to these results give detailed information about the cathode material.

The applications of semiconductor materials in air batteries

Air batteries are promising energy storage technologies that have gained continuous attraction due to their high energy densities. At present, investigations on anodes of air batteries are usually focused on various metals such as Li, Zn, Al. In contrast, the semiconductor anodes like Si and Ge are less investigated. Si-air battery possesses a high theoretical energy density and Ge-air battery has a high actual power density and ideal safety. Besides anodes, air cathodes where oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are also the key components in air batteries. To further promote the discharging performance and facilitate energy conversion/storage, semiconductor materials have been introduced in electrochemical cells like Li-O2 and Zn-air batteries. This review briefly summarizes semiconductor materials utilized in various air batteries, including the progress of Si-air and Ge-air batteries and recent advances in semiconductor cathodes catalysts. Finally, the remaining challenges and further perspective are discussed.

Computational screening of photocathodes based on layered MXene coated Cs<sub>3</sub>Sb heterostructures

The alkali-based semiconductor cathodes, such as Cs<sub>3</sub>Sb that possesses high quantum efficiency, low electron emittance and short spectral response time, can be considered as ideal next-generation electron sources. However, the alkali-based emitters are found to be sensitive to the oxygen gases, which causes a series of problems such as structural instability, short lifetime, and reduced electron emitting efficiency. It is known that the employing of the ultra-thin layered two-dimensional (2D) materials to protect Cs<sub>3</sub>Sb basement can promote the development of novel cathodes with excellent performances. However, there is a lack of efficient 2D materials to maintain low work-function (<i>W</i> ) and high quantum efficiency. Recently, the MXene materials which contain layered transitional metal carbides, nitrides and carbonitrides, have attracted great attention particularly in the fields of catalysis and energy. Notably, their flexible types of dangling bonds can lead to tunable structural and electronic properties of MXene-based materials. Here in this work, the MXene-Cs<sub>3</sub>Sb heterostructures are modeled by using home-made script and systematically investigated by using first-principle calculations based on density functional theory. Further, the effects of transitional metal element (M), M/C ratio, stacking configuration and types of dangling bonds on the calculated <i>W</i> of heterostructures are studied. The result indicates that the type of dangling bond shows a more pronounced effect, and the MXene-Cs<sub>3</sub>Sb heterostructures with —OCH<sub>3</sub>/—OH possess lower W than other dangling bonds. The charge density difference and band alignment analysis are further used to illustrate the underlying reason for the change of <i>W</i>. And it is found that interlayer charge redistribution can result in different surface dipole directions, and thus emitting electrons with varying barriers. After computational screening based on the change of W, the M<sub>2</sub>C(OH)<sub>2</sub> (M = V, Ti, Cr) and M<sub>2</sub>C(OCH<sub>3</sub>)<sub>2</sub> (M = Ti, Cr, Nb) can be potentially considered as ideal coating materials, and especially for V<sub>2</sub>C(OH)<sub>2</sub>-Cs<sub>3</sub>Sb (<i>W</i> = 1.602 eV) and Ti<sub>2</sub>C(OCH<sub>3</sub>)<sub>2</sub>-Cs<sub>3</sub>Sb (<i>W</i> = 1.877 eV) with significantly reduced <i>W</i>. Finally, we believe that this work can not only give an in-depth insight into the electronic and optical properties of Cs<sub>3</sub>Sb-MXene heterostructures, but also provide the useful criteria for the computational screening of superior cathodes. Meanwhile, we further urgently expect the cooperative efforts from an experimental perspective to demonstrate the superior performances of those screened MXene-Cs<sub>3</sub>Sb photocathodes for practical applications.

MULTI-POINTED FIELD-EMISSION CATHODE AS A GENERATOR OF HIGHFREQUENCY OSCILLATIONS

Semiconductor field-emission cathodes have gained considerable popularity in modern radio electronics and electronic optics due to the high-power generation of the electron beam in the external electric field at temperatures close to the room ones. However, their wide application is restricted by the high dependence of the electron emission current on the value of the applied field and geometrical parameters of the cathode. This study aimed to examine the effect of resonance processes on amplifying the field emission of the multi-pointed semiconductor cathode. Modeling the behavior of resonant tunneling of electrons from semiconductors to vacuum was simulated by solving the one-dimensional Schrodinger’s equation, and the amplification due to resonant processes was estimated. The modeling results showed that as the electric field increases, the resonance conditions shift towards low energy levels. With the increase in the width of the barrier for the electron inside the solid body, the resonance conditions shift towards higher energies. It has been established that in onedimensional semiconductors with electrons of low conductivity width, the resonant energy coincides with the Fermi level. These cathode properties are optimal for amplifying the emission current and reducing failures of vacuum electronic devices based on semiconductive field cathodes. The proposed technique can be used to study the regularities of emission amplification due to resonant processes in multipoint semiconductor cathodes with multilayered structure and with metal tips.

Effect of the electrolyte chemical nature on the formation and characteristics of TiO2 nanotubes synthesized by anodic oxidation using a Ti cathode

The aim of this work is to show the effect of the electrolyte chemical nature on the formation of self-organized TiO2 nanotubes (TNT) arrays synthesized via anodic oxidation, when using a Ti cathode. The synthesis was performed in situ in a potentiostatic cell provided with an anode and a cathode (both Ti substrates pretreated) under constant hydrodynamic conditions and different anodizing times, temperatures and electrical potentials. Ti electrodes were immersed in 300 mL of each of an inorganic and organic electrolyte. Then, a thermal treatment varying the temperature and the heating rate in each case was applied to convert the amorphous TiO2 to crystalline TiO2. TNT were characterized by field-emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and atomic force microscopy (AFM). FESEM showed the formation of nanotubular structures perpendicular to Ti substrate with average inner diameters of 84 and 57 nm for TNT synthesized in the electrolytes with different nature tested (inorganic and organic), and labeled hereafter as TNT-I and TNT-O, respectively. EDS spectra from different zones of the substrate confirmed the presence of Ti (~ 34.2%) and O (~ 66%) on the surface of the TNT. The anatase (~ 86%) and rutile (~ 14%) crystalline phases were detected via XRD in both cases. AFM provided information about the topographic profile of TNT and the roughness of the substrates. Thus, the use of a semiconductor cathode allowed the successful synthesis of the TNT and the electrolyte chemical nature was found to influence its morphology, dimensions and formation mechanism.

Development of Technological Principles for Creating a System of Microfocus X-Ray Tubes Based on Silicon Field Emission Nanocathodes

The technological prospects for the creation of a system of microfocus X-ray tubes with the use of silicon field emission of nanocathodes have been discussed. A numerical analysis of the field-emission current from a nanoscale semiconductor cathode regulated by voltage on a grid electrode has been carried out on the basis of which a scheme for controlling the elements of the matrix of field-emission cathode assemblies has been proposed. The current–voltage characteristics of silicon field emission nanocathodes have been measured. They are in good agreement with the theoretical estimates of the field-emission current. A full technological cycle of the development of elements of microfocus X-ray tubes (a set of field-emission cathode assemblies and a set of anode assemblies) has been performed. The results can be used to create systems of microfocus X-ray tubes for nanolithographic equipment of a new generation.

Improvement of technology for the production of a semiconductor cathode of oxide-semiconductor capacitors

Improvement of technology for the production of a semiconductor cathode of oxide-semiconductor capacitors

Разработка технологических принципов создания системы микрофокусных рентгеновских трубок на основе кремниевых автоэмиссионных нанокатодов

Abstract The technological prospects for the creation of a system of microfocus X-ray tubes with the use of silicon field emission of nanocathodes have been discussed. A numerical analysis of the field-emission current from a nanoscale semiconductor cathode regulated by voltage on a grid electrode has been carried out on the basis of which a scheme for controlling the elements of the matrix of field-emission cathode assemblies has been proposed. The current–voltage characteristics of silicon field emission nanocathodes have been measured. They are in good agreement with the theoretical estimates of the field-emission current. A full technological cycle of the development of elements of microfocus X-ray tubes (a set of field-emission cathode assemblies and a set of anode assemblies) has been performed. The results can be used to create systems of microfocus X-ray tubes for nanolithographic equipment of a new generation.

Observed nonlinearities in a DC semiconductor-gas discharge system

Nonlinear behaviour of a direct current (DC) driven semiconductor-gas discharge plasma is investigated experimentally. The setup consists of two planar electrodes separated by a gap of 1 mm. Plasma glow is generated between a semiconductor cathode and a transparent anode using nitrogen gas at partial atmospheric pressure. Nonlinear behavior of the discharge is investigated by varying the applied DC voltage while monitoring the plasma current (I), voltage (V), and the optical emission, (i.e., amplified voltage measurements obtained using a photodiode and an amplifier). The discharge behaves oscillatory in time. By increasing the applied voltage, the system evolves into higher frequency oscillations with sub-harmonics. In fact, period doublings are observed in these systems, as shown in earlier publications by our research group. In this study, we investigate the mode coupling nature of these oscillations using a Fourier transform based spectral analysis.

3D simulation of hexagonal current pattern formation in a dc-driven gas discharge gap with a semiconductor cathode

The formation of hexagonal current pattern in a dc-driven thin planar gas discharge gap with a semiconductor cathode is studied on the basis of a 3D drift-diffusion discharge model. The model includes continuity equations for electron and ion densities, the electrostatic equation for electric field and heat conduction equation for gas temperature. Solving these equations by the time-relaxation method, we investigate the genesis of ordered current pattern and its subsequent rearrangement into the hexagonal pattern, starting from initial homogeneous discharge state without any initial perturbations. For the case of the dc discharge with a semiconductor cathode and cryogenic conditions, the 3D calculation has been performed for the first time. The resulting current pattern is in agreement with experiment. The results of the simulation confirm that the discharge instability and the patterns formation are due to an ionization-overheating (thermal) mechanism, as was proposed before in [Raizer Yu P and Mokrov M S 2010 J. Phys. D: Appl. Phys. 43 225204] on the basis of approximate theory.

A photo-assisted microbial electrolysis cell for the exclusive biohydrogen production using a MoS2-coated p-type copper oxide

A microbial electrolysis cell (MEC) has been regarded as an emerging new technology for the biohydrogen production from various organic substances, even from wastewater. One major problem is, however, that methane dominates produced gases in a long-term operation. Here we report that a photo-assisted MEC (PAMEC) is an efficient way to produce hydrogen with a p-type semiconductor cathode. When Cu2O coated with MoS2 as cocatalyst (MoS2/Cu2O) is employed, only hydrogen with essentially no methane and carbon dioxide was produced from acetate at −0.8 V bias under visible light illumination at a rate of 2.72 m3H2 m−3d−1. No appreciable performance degradation is observed over 50 days of operation. At lower bias voltage, methane and carbon dioxide begins to be produced. Energy efficiency based on input electricity and hydrogen yield are 225% and 3.4, respectively. This excellent feature of PAMEC is attributed to p-type semiconductor characteristics of Cu2O and proton reduction activity of MoS2. Electrons generated from acetate oxidation at the anode are photoexcited to the conduction band at the Cu2O acquiring enough reduction potential to reduce protons. The concept of PAMEC can be extended to wastewater treatment for the hydrogen production.

Near-field enhanced thermionic energy conversion for renewable energy recycling

This article proposes a new energy harvesting concept that greatly enhances thermionic power generation with high efficiency by exploiting the near-field enhancement of thermal radiation. The proposed near-field enhanced thermionic energy conversion (NETEC) system is uniquely configured with a low-bandgap semiconductor cathode separated from a thermal emitter with a subwavelength gap distance, such that a significant amount of electrons can be photoexcited by near-field thermal radiation to contribute to the enhancement of thermionic current density. We theoretically demonstrate that the NETEC system can generate electric power at a significantly lower temperature than the standard thermionic generator, and the energy conversion efficiency can exceed 40%. The obtained results reveal that near-field photoexcitation can enhance the thermionic power output by more than 10 times, making this hybrid system attractive for renewable energy recycling.

The Features of GaAs and GaP Semiconductor Cathodes in an Infrared Converter System

The aim of this study is to examine the electrical and optical comparative analysis of semi-insulating GaAs and GaP photoconductive electrodes in an infrared converter system with a resistivity of >107 Ω cm for the same interelectrode distance d and gas pressure p experimentally and theoretically, when the discharge cell has been filled by argon. To provide the stability of the semiconductor electrode in Ar media, the experiments were carried out in Townsend and glow discharge regimes for various parameter sets of pressure, interelectrode gap and discharge voltage. When the discharge exceeds a critical voltage value, some N-shape CVCs, which stem from both semiconductors and Ar gas, have been observed. To compare the features of the GaAs and GaP cathodes, the COMSOL multiphysics programme is used under the Ar media. The mean electron energy, thermal velocity, surface charge density, space charge and initial electron densities, and electron mobilities have been calculated for both semiconductor materials. It has been found that the electron mobility μe, electron thermal velocity, surface charge density σ and mean electron energy of GaAs is higher than those of GaP; hence, GaAs has better opto-electronic features compared to GaP. In addition, the experiments on the optical explorations prove that GaAs exhibit better optical response in the infrared region. The explored transport characteristics of the semiconductor electrodes are of importance, and they have to be taken into account when studying plasma cells.