External Bias Voltage Research Articles

Dark electron tunneling current in metal–insulator–metal structures: modeling, fabrication, and measurement

Metal–insulator–metal (MIM) structures have been proposed for infrared photodetectors, optical rectennas, and alternative choices to conventional solar cells. Generally, if two dissimilar electrodes are separated by a sufficiently thin insulator, electrons can travel through the insulating gap region due to the quantum tunneling effect. This tunneling through the insulating gap can be driven by an electromagnetic field or an external bias voltage. In our work, we incorporate the contributions of the Poole–Frenkel (PF) effect on the energy barrier of the MIM junction and the effect of Coulomb interaction between the electron charge and its virtual image charges. Using this modified PF-Coulombic barrier, we perform quantum calculations for electron tunneling using the transfer matrix method and Wentzel–Kramers–Brillouin approximations. This, along with the density of states, the Fermi occupation probability, and the quantum confinement effect is used to derive the dark current density for the MIM diode. To test the theory, MIM Au-TiO2-Ti diodes are designed and fabricated in-house using photolithography, along with electron beam evaporation and sputter depositions. The dark current–voltage (I − V) characteristics of the fabricated MIM photodiodes are measured and show good agreement with theoretical predictions.

In-situ electrodeposition synthesis of Z-scheme rGO/g-C3N4/TNAs photoelectrodes and its degradation mechanism for oxytetracycline in dual-chamber photoelectrocatalytic system

The dual-chamber photoelectrocatalytic (PEC) system possess advantages in the degradation efficiency and processing cost of organic contaminants. In this study, TiO2 nanotube arrays modified by rGO and g-C3N4 (rGO/g-C3N4/TNAs) photoelectrodes were successfully prepared. The surface micromorphology, chemical structure, crystal structure, and basic element composition of rGO/g-C3N4/TNAs photoelectrodes were studied by SEM, FTIR, XRD, Raman, and XPS. UV–vis absorption, photoluminescence (PL) spectra, and photoelectrochemical (PECH) tests were used to explore the photoelectrochemical characteristics of rGO/g-C3N4/TNAs photoelectrodes. Under simulated sunlight illumination, the dual-chamber PEC system with external bias voltage was used to investigate the degradation of oxytetracycline (OTC) on rGO/g-C3N4/TNAs photoelectrodes. The results showed that rGO and g-C3N4 were successfully loaded on TNAs, and the separation efficiency of electrons and holes at rGO/g-C3N4/TNAs photoelectrodes was improved. The light absorption range of rGO/g-C3N4/TNAs photoelectrodes extends to the visible light region and has better light absorption performance. Compared with the photocatalytic process, when 1.2 V bias voltage was applied, the degradation efficiency of OTC in anode and cathode chambers in PEC were increased by 3.28% and 44.01% within 60 min, respectively. In addition, the anode and cathode chambers have different degradation effects on OTC. Both the external bias voltage and initial pH have significant effects in cathode chamber, but have little effect in photoanode chamber. The fluorescence excitation-emission matrix spectra and liquid chromatography-tandem mass spectrometry showed that there were different intermediates in the degradation process of OTC. This study indicated that for the dual-chamber PEC system, rGO/g-C3N4/TNAs photoelectrodes exhibited excellent photocatalytic performance and have potential application prospects in water environmental remediation.

Effect and mechanism of cyanide degradation and Cu/Zn recovery by photoelectro-catalytic oxidation

• The photoelectrocatalytic system can effectively degrade cyanide wastewater. • The bending of the TiO 2 energy band occurs to improve the catalytic performance. • The catalyst and degradation products can be separated from the electrolyte. • The catalytic mechanism of total cyanide, Cu and Zn cyanide complexes was revealed. Cyanide wastewater contains highly toxic substances such as cyanide and metals. A simple and efficient photoelectro-catalytic oxidation method was proposed. Effect and mechanism of cyanide degradation and Cu/Zn recovery by photoelectro-catalytic oxidation were investigated. The experimental results show the degradation efficiency of total cyanide and removal efficiencies of copper and zinc reached 96.90%, 99.76% and 90.56%, respectively, under the optimum conditions of 2.5 V external bias voltage and 4.0 h with air and lighting. An external electric field can significantly improve the migration and separation of photo-generated electrons ( e – ) and holes ( h + ), thereby increasing the photocatalytic efficiency. Simultaneously, the surface potential of semiconductor is lower than the body under the action of bias voltage and leads to energy band bending. XPS analysis indicates that cyanide is finally degraded into carbon oxide and nitrogen. Cu and Zn are mainly recovered as elementary substance.

Nonmagnetic Spoof Plasmonic Isolator Based on Parametric Amplification

AbstractDriven by the miniaturization of integrated electronics, research on spoof plasmonic circuits has recently aroused widespread interest. On the other hand, nonreciprocal devices, such as isolators and circulators, are key components of integrated electronic systems. However, bulky magnets required to realize isolation and circulation prevent the application of traditional nonreciprocal technologies to integrated systems. Here, parametric amplification is explored to achieve magnetic‐free plasmonic isolation, and an ultrathin reconfigurable spoof plasmonic isolator is realized experimentally. In this isolation system, the forward signal amplified by a spoof plasmonic parametric amplifier is coupled to a second linear plasmonic waveguide via a spoof localized surface plasmon resonator, whereas the transmission from the inverse direction is prohibited, giving rise to a measured isolation ratio of up to 20 dB. By tuning the nonlinear phase‐matching condition through external bias voltage, multifrequency isolation of spoof surface plasmon polariton (SSPP) signals is also realized experimentally. This work demonstrates the possibility of producing miniaturized and low‐cost nonreciprocal SSPP devices, holding great promise for applications in nonmagnetic information processing and radar detection.

Tunable dual plasmon-induced transparency and slow-light analysis based on monolayer patterned graphene metamaterial

We propose a novel terahertz metamaterial structure based on patterned monolayer graphene. This structure produces an evident dual plasmon-induced transparency (PIT) phenomenon due to destructive interference between bright and dark modes. Since the Fermi level of graphene can be adjusted by an external bias voltage, the PIT phenomenon can be tuned by adjusting the voltage. Then the coupled-mode theory (CMT) is introduced to explore the internal mechanism of the PIT. After that, we investigate the variation of absorption rate at different graphene carrier mobilities, and it shows that the absorption rate of this structure can reach 50%, which is a guideline for the realization of graphene terahertz absorption devices. In addition, through the study of the slow-light performance for this structure, it is found that its group index is as high as 928, which provides a specific theoretical basis for the study of graphene slow-light devices.

High-power terahertz pulse generation from bias-free nanoantennas on graded composition InGaAs structures.

We present a bias-free photoconductive emitter that uses an array of nanoantennas on an InGaAs layer with a linearly graded Indium composition. The graded InGaAs structure creates a built-in electric field that extends through the entire photoconductive active region, enabling the efficient drift of the photo-generated electrons to the nanoantennas. The nanoantenna geometry is chosen so that surface plasmon waves are excited in response to a 1550 nm optical pump to maximize photo-generated carrier concentration near the nanoantennas, where the built-in electric field strength is maximized. With the combination of the plasmonic enhancement and built-in electric field, high-power terahertz pulses are generated without using any external bias voltage. We demonstrate the generation of terahertz pulses with 860 µW average power at an average optical pump power of 900 mW, exhibiting the highest radiation power compared to previously demonstrated telecommunication-compatible terahertz pulse emitters.

Preparation of Nanostructured HgI2 Nanotubes/Si Photodetector by Laser Ablation in Liquid

In this study, we present a high-performance HgI2 nanotubes/Si heterojunction photodetector by a simple one-step pulsed laser ablation in ethanol at a laser fluence of 12.7 J/cm2. The structural and optical properties of HgI2 nanotubes (NTs) are studied. The X-ray diffraction (XRD) measurement shows the presence of the mixture of tetragonal and orthorhombic phases of HgI2. The optical properties results show that the optical energy gap of HgI2 NTs was 2.98 eV measured at room temperature. The photoluminescence (PL) spectrum of HgI2 displays a single emission centered at 535 nm. Scanning electron microscope (SEM) image show the formation of nanotubes and spherical nanoparticles with an average particle size of 23 nm. The transmission electron microscope (TEM) image of HgI2 confirms the formation of nanotubes morphology with an average diameter of 80 nm and an average length of 1.6 μm. The current–voltage characteristics of HgI2 NTs/p-Si heterojunction were measured at dark and illumination. The HgI2 nanotubes/Si photodetector is sensitive to the spectral region ranging from visible to near infrared region. This photodetector presents a responsivity of up-to 1.09 A/W at 450 nm under an external bias voltage of 5 V. The specific detectivity (D*) and external quantum efficiency (EQE) of the photodetector are 3.6 × 1012 Jones and 3 × 102% at 400 nm, respectively, without using post annealing. The fabricated photodetector has good linearity characteristics with a large linear dynamic region, and the saturation in photocurrent is observed at 180 mW/cm2. The figures of merit of the fabricated photodetector are compared with those of some heterojunction-based silicon photodetectors.

Strain modulation on the spin transport properties of PTB junctions with MoC2 electrodes.

Based on MoC2 nanoribbons and poly-(terphenylene-butadiynylene) (PTB) molecules, we designed MoC2-PTB molecular spintronic devices and investigated their spin-dependent electron transport properties by using spin-polarized density functional theory and the non-equilibrium Green's function method. As a typical MXene material, it is found that the magnetic contribution of MoC2 nanoribbons mainly comes from the delocalized 3d electron of edge Mo atoms. Owing to the obvious spin-splitting near the Fermi level of the MoC2 nanoribbon electrode, the spin states can be effectively injected into the central scattering region under an external bias voltage. In addition, we also studied the effects of z-axis strain on the spin transport properties of the PTB molecular device, where the strain was controlled within the range of -9% to 9%. Under a compressed strain, spin current increases obviously, and the spin-filtering efficiency (SFE) decreases slightly. Nevertheless, under a tensile strain, we found that the SFE increases but spin current decreases. Moreover, z-axis strain can induce a negative differential resistance (NDR) effect at a high bias point. This work would expand the potential applications of new two-dimensional (2D) materials in the field of molecular spintronic devices.

Design of a Novel THz Modulator for B5G Communication

Wireless data traffic has expanded at a rate that reminds us of Moore’s prediction for integrated circuits in recent years, necessitating ongoing attempts to supply wireless systems with ever-larger data rates in the near future, despite the under-deployment of 5G networks. Terahertz (THz) communication has been considered a viable response to communication blackout due to the rapid development of THz technology and sensors. THz communication has a high frequency, which allows for better penetration. It is a fast expanding and evolving industry, driven by an increase in wireless traffic volume and data transfer speeds. A THz modulator based on a hybrid metasurface was devised and built in this work. The device’s modulation capabilities were modelled and proved experimentally. The electrolyte is an ion-gel medium implanted between graphene and metasurface, and the active material is graphene. On the metasurface, the interaction between the THz wave and graphene is improved. Additionally, an external bias voltage was employed to actively regulate the THz waves by tuning the electrical conductivity of graphene. The results show that with a minimal bias voltage, the device can achieve a modulation depth of up to 73% at the resonant frequency. Furthermore, during the modulation process, the resonance frequency remains almost constant. As a result, the proposed gadget offers a unique tool for substantial THz amplitude modulation at low voltages.

High power density and bias-free reverse electrowetting energy harvesting using surface area enhanced porous electrodes

Reverse electrowetting-on-dielectric (REWOD) is a novel energy harvesting technology with a significant advantage over other energy harvesting technologies due to its effective performance at a low-frequency range and not requiring resonance frequency of solid structures. However, REWOD energy harvesting based on planar electrodes has a limited surface area and therefore a lower power output. In this work, we present a novel approach for enhancing power output from a REWOD energy harvester by significantly increasing the total available surface area using perforated silicon wafer electrodes. Without applying any external bias voltage, maximum current and voltage densities per unit planar area were measured to be 3.77 μA/cm 2 and 1.05 V/cm 2 , respectively, for a 38 μm pore-size electrode at 5 Hz modulation frequency. RMS power density output was 4.8 μW/cm 2 , which is ∼23 times higher than that from our prior work on planar electrodes showing the significance of porous electrodes in REWOD energy harvesting. A simple capacitive theoretical model validating experimental results was developed and justified. The novelty of this work lies in the combination of a bias-free approach to REWOD energy harvesting coupled with significant enhancement in electrode surface area per planar area to increase the output power density. • Low frequency (1–5 Hz) motion energy harvesting using high surface area electrodes. • Highest reported REWOD power density (4.8 μW/cm 2 ) without bias voltage. • Experimentally supported capacitive theoretical modeling. • Demonstration of increasing power density with shrinking pore size electrodes. • Power density increased by ∼23 times compared to planar electrodes.

Graphene-based terahertz bias-driven negative-conductivity metasurface.

A graphene-based terahertz negative-conductivity metasurface based on two types of unit cell structures is investigated under the control of an external bias voltage. Electrical characterization is conducted and verification is performed using a finite-difference time-domain (FDTD) and an optical-pump terahertz (THz)-probe system in terms of simulation and transient response analysis. Owing to the metal-like properties of graphene, a strong interaction between the metasurface and monolayer graphene yields a short-circuit effect, which considerably weakens the intensity of the resonance mode under passive conditions. Under active conditions, graphene, as an active load, actively induces a negative-conductivity effect, which enhances the THz transmission and recovers the resonance intensity gradually because of the weakening of the short-circuit effect. The resulting resonance frequency shows a blue shift. This study provides a reference value for combining graphene exhibiting the terahertz bias-driven negative-conductivity effect with metasurfaces and its corresponding applications in the future.

Deep-ultraviolet electroluminescence and photocurrent generation in graphene/hBN/graphene heterostructures

Hexagonal boron nitride (hBN) is a van der Waals semiconductor with a wide bandgap of ~ 5.96 eV. Despite the indirect bandgap characteristics of hBN, charge carriers excited by high energy electrons or photons efficiently emit luminescence at deep-ultraviolet (DUV) frequencies via strong electron-phonon interaction, suggesting potential DUV light emitting device applications. However, electroluminescence from hBN has not been demonstrated at DUV frequencies so far. In this study, we report DUV electroluminescence and photocurrent generation in graphene/hBN/graphene heterostructures at room temperature. Tunneling carrier injection from graphene electrodes into the band edges of hBN enables prominent electroluminescence at DUV frequencies. On the other hand, under DUV laser illumination and external bias voltage, graphene electrodes efficiently collect photo-excited carriers in hBN, which generates high photocurrent. Laser excitation micro-spectroscopy shows that the radiative recombination and photocarrier excitation processes in the heterostructures mainly originate from the pristine structure and the stacking faults in hBN. Our work provides a pathway toward efficient DUV light emitting and detection devices based on hBN.

Inter-grain Li+ conduction in Sc and Y doped LATP compounds

Low value of grain boundary conductivity in Li-NASICON materials is an important limitation that needs to be overcome. Inter-grain conduction of Li + ions in Li1·3Al0.3-xScxTi1.7(PO4)3 (LASTP) and Li1·3Al0.3-xYxTi1.7(PO4)3 (LAYTP) (where x = 0.01 and 0.07) samples is investigated. The samples, prepared by solid state reaction method, were characterised by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. Electrochemical Impedance Spectroscopy (EIS) was performed in the frequency range of 20 MHz to 1 Hz, and temperature range of 20 °C–100 °C. The grain boundary impedance and capacitance was found to vary with an external dc bias voltage. Higher grain boundary conductivity was found for sample containing higher yttrium concentration. Effect of space charge on grain boundary conductivity is evaluated in light of two competing models.

Electrically tunable SERS based on plasmonic gold nanorod-graphene/ion-gel hybrid structure with a low voltage

Actively controlling the performance of surface-enhanced Raman scattering (SERS) holds great promise in enabling novel applications of this technique. As a popular two-dimensional material, graphene shows intriguing tunable properties and thus has become a promising material in many potential active device applications. Here we proposed and experimentally demonstrated low-voltage electrically tunable SERS by using a plasmonic gold nanorod-graphene/ion-gel hybrid structure. The SERS signals of molecules adsorbed on such a structure were found to be continuously tunable through an external small bias voltage. As such a tuning process is highly related with the charge transfer between the adsorbed molecules and graphene, the proposed design is also helpful in understanding the charge transfer-based enhancement mechanism of SERS.

Designing multifunctional single-molecule devices by mononuclear or binuclear manganese phthalocyanines

Molecular-scale magnetic devices have attracted great attention of research in recent years. In this work, by using the non-equilibrium Green’s function (NEGF) method in combination with density functional theory (DFT), we investigated the spin transport properties of manganese phthalocyanine based spintronic devices constructed by a mononuclear manganese phthalocyanine (MnPc) or binuclear manganese phthalocyanine (Mn2Pc2) sandwiched between two zigzag-edge graphene nanoribbon (zGNR) electrodes. The calculation results show that both MnPc and Mn2Pc2 devices are good spin filters manifesting excellent spin filtering effect, which is originated from the distinct difference in the energy alignments of the spin-up and spin-down molecular electronic states with respect to the Fermi energy (EF) of zGNR electrodes. More interestingly, the spin polarization of the tunneling electrons through the devices is intimately opposite to that of Mn atom(s). Specifically, for a spin-up polarization of Mn in the devices, almost only the spin-down electrons are allowed to go through the device, and vice versa. In addition, spin filtering efficiency (SFE) of the devices is evidently enhanced in Mn2Pc2 devices in comparison with that of the MnPc devices, of which the value of SFE can reach almost 100%. This enhancement is attributed to the fact that the spin electronic states of Mn2Pc2 devices are closer to EF and there is a weaker localization in their spatial distributions induced by the external bias voltage. Furthermore, both MnPc and Mn2Pc2 devices can act as NOT logic gates by taking the spin polarization characteristics of Mn atom(s) and current of devices as input and output signals, respectively. This work demonstrates that mononuclear MnPc and binuclear Mn2Pc2 molecules have potential applications in designing multifunctional spintronic single-molecule devices.

Bias-driven terahertz negative conductivity and transmission enhancement

Bias-driven terahertz (THz) negative conductivity and transmission enhancement based on structures composed of monolayer graphene, MXene (Ti 3 C 2 T x ) and poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate):dimethyl sulfoxide (PEDOT:PSS:DMSO) caused by photo-excited or bias-driven are investigated. The THz signals that pass through the monolayer graphene/quartz, MXene/quartz, thinner and thicker PEDOT:PSS:DMSO/quartz structures are enhanced effectively under photo excitation or application of an external bias voltage in a THz time-domain spectroscopy (THz-TDS) system. This enhancement of THz transmission indicates that negative conductivity occurs, which is consistent with the results based on the monolayer graphene, thinner PEDOT:PSS:DMSO samples detected using an optical pump-THz probe (OPTP) system. The conductivity reduction mechanism is explained using an established Drude model. After photo excitation, this results in the generation of photo-induced carriers and an increase in the average scattering rate of the carriers. And as a consequence of the energy injection into the carrier system of the sample after applying a bias, the carrier temperature increases rapidly; this results in the generation of carriers and an increase in the average scattering rate of the carriers. However, the increase in the average scattering rate of the carriers is much higher than the increase in the average carrier density, which leads to the negative conductivity and the increased THz transmission. • THz signals that pass through the structures are enhanced under photo excitation or application of an external bias voltage. • THz transmission enhancement indicates that negative conductivity occurs, which is consistent with the results from optical pump-THz probe system. • This results in the generation of hot carriers and an increase in the average scattering rate of the carriers.

Electrically tunable absorber based on a graphene integrated lithium niobate resonant metasurface.

Perfect absorbers are of great importance in various applications such as photodetectors, optical sensors and optical modulators. Recently, perfect absorption metasurface based on monolayer graphene has attracted lots of research interest. In this paper, a graphene-lithium niobate (LN) perfect absorption metasurface is constructed, where graphene works as a thin absorptive layer as well as a conductive electrode. The proposed device achieves 99.99% absorption at 798.42 nm and 1.14 nm redshift of the absorption peak is realized at 300 V(from -150 V to 150 V) external bias voltage through the electro-optical effect of LN, which enables the proposed device work as a electrically tunable absorber in the visible and near infrared range. The switching ratio of reflected light R/R0 could reach -44.08 dB with an applied voltage tuning from -150 V to 0 V at 798.42 nm. Our work demonstrates the potential of LN integrated high-Q resonant metasurface in realizing electro-optic tunable nanophotonic devices in the visible and near infrared band. It will promote the research of graphene integrated optoelectronic devices as well as LN based tunable nanophotonic devices which have widespread applications such as modulators and optical phase arrays.

Conductive Path Mechanism for Bipolar Resistive Switching Characteristics in Lead‐Free Perovskite CsSnBr3‐Based Nonvolatile Memories

Halide perovskites are intensely studied due to their potential excellent properties in electronic devices. However, the toxicity of lead remains as an obstacle on their way toward large‐scale applications. Herein, the lead‐free all‐inorganic perovskite CsSnBr3 film is used as the switching layer to construct nonvolatile resistive switching (RS) memories and the underlying switching mechanism is systematically explicated. Both memories with Ag/CsSnBr3/Pt and Pt/CsSnBr3/Pt structures exhibit reproducible bipolar RS characteristics, including the large on/off ratios and low reset currents. The RS characteristics are attributed to the formation/rupture of nano‐cone‐shaped conductive paths under external voltage bias. In addition, compared with Pt/CsSnBr3/Pt device, the Ag/CsSnBr3/Pt device manifests lower operation set/reset voltages, revealing the formation of higher density or stronger conductive paths. This work might pave the way for the lead‐free all‐inorganic perovskite‐based RS memory devices in high performance emerging commercial applications.

Oxides for Rectenna Technology.

The quest to harvest untapped renewable infrared energy sources has led to significant research effort in design, fabrication and optimization of a self-biased rectenna that can operate without external bias voltage. At the heart of its design is the engineering of a high-frequency rectifier that can convert terahertz and infrared alternating current (AC) signals to usable direct current (DC). The Metal Insulator Metal (MIM) diode has been considered as one of the ideal candidates for the rectenna system. Its unparalleled ability to have a high response time is due to the fast, femtosecond tunneling process that governs current transport. This paper presents an overview of single, double and triple insulator MIM diodes that have been fabricated so far, in particular focusing on reviewing key figures of merit, such as zero-bias responsivity (β0), zero-bias dynamic resistance (R0) and asymmetry. The two major oxide contenders for MInM diodes have been NiO and Al2O3, in combination with HfO2, Ta2O5, Nb2O5, ZnO and TiO2. The latter oxide has also been used in combination with Co3O4 and TiOx. The most advanced rectennas based on MI2M diodes have shown that optimal (β0 and R0) can be achieved by carefully tailoring fabrication processes to control oxide stoichiometry and thicknesses to sub-nanometer accuracy.

Dynamically tunable broadband absorber with a single ultra-thin layer of graphene in the terahertz regime

We have proposed a broadband near perfect terahertz absorber based on a single ultra-thin layer of graphene with incident polarization angle and oblique incidence angle insensitivity. The absorber consists of a patterned graphene metasurface, a polyethylene cycloolefin copolymer (TOPAS) dielectric layer, and a gold film. The structure of the graphene metasurface includes a cross-shaped graphene patch with four corners cut off at the center of the unit cell, and four arc-shaped graphene patches at the four corners of the unit cell. Numerical simulations show that the proposed absorber can achieve absorbance higher than 97% in the wideband frequency range of 1.45–2.99 THz under normal incident conditions with an absolute bandwidth reaches to 1.54 THz. For oblique incidence, the absorber remains the same absorbance over a wide frequency range, which infers that the proposed broadband absorber is incident angle insensitive. Moreover, the tunable property of graphene was also studied, which indicated that absorbance can be tuned from 11% to nearly 100% by changing the chemical potential of the graphene via external bias voltage. Furthermore, we introduced the impedance matching theory and interference theory to verify the absorption rate of the absorber. The calculated results are nearly the same as the simulated results. Due to its simple structure and flexible tunability, the proposed metamaterial absorber has potential application prospects in modulators, sensors, and other multispectral devices.