Electron Tunneling Effect Research Articles

Quantum mechanical effects in plasmonic sub-nanometer cavities formed by a tip and substrate structure

Enhancing local field intensity through light field compression is one of the core issues in surface plasmon-enhanced spectroscopy. The theoretical framework for the nanostructure composed of a tip and a substrate has predominantly relied on classical electromagnetic models, ignoring the electron tunneling effect. In this paper, we investigate the plasmonic near-field characteristics in the sub-nanometer cavity formed by the tip and the substrate using a quantum-corrected model. Additionally, we analyze the local electric field and Raman enhancement when hexagonal boron nitride (h-BN) monolayer is used as a decoupling layer for the nanocavity. The results indicate that classical electromagnetic theory fails to accurately describe the plasmonic electric field in smaller sub-nanometer gaps. When the gap is reduced to 0.32 nm, the quantum-corrected model shows that the local electric field in the sub-nanometer cavity is significantly reduced due to the tunneling current, aligning more closely with experimental results. Moreover, adding a high-barrier h-BN layer effectively prevents the occurrence of tunneling current, allowing for a strong local electric field even when the gap is less than 0.32 nm. The calculated maximum Raman enhancement reaches up to 15 orders of magnitude. Our research results provide a deep understanding of quantum mechanical effects in tip-enhanced spectroscopy systems, enabling the potential applications based on quantum plasmons in nanocavity.

Reductive Degradation of Florfenicol by Electrogenerated Hydrated Electrons via the Electron Tunneling Effect

Reductive Degradation of Florfenicol by Electrogenerated Hydrated Electrons via the Electron Tunneling Effect

Theoretical study of tip-enhanced photoluminescence of single molecule in nanocavity formed by apex protrusion and substrate

Tip-enhanced photoluminescence techniques, characterized by ultra-high detection sensitivity and spatial resolution, have exhibited unprecedented capabilities in exploring optoelectronic phenomena and unraveling intrinsic physical mechanism at the single-molecule level. To suppress the quenching of molecular luminescence and achieve high-resolution optical imaging, a rational design of the tip-substrate configuration is required. In this paper, we systematically investigate the tip-enhanced photoluminescence characteristics of single molecules located within the plasmonic nanocavity formed by the tip protrusion and the substrate from a theoretical perspective. The impacts of electron tunneling and electron-hole pair creation on localized electric field and quantum yield are discussed using the quantum correction model and nonlocal dielectric theory, respectively. The crucial roles of atomic-scale protrusion at the tip apex and the decoupling layer on the substrate in enhancing vertical and horizontal electric fields as well as quantum yields have been emphasized. The results indicate that in smaller sub-nanometer cavities, electron tunneling and nonlocal dielectric effects greatly weaken the electric field intensity and fluorescence quantum yield. Meanwhile, due to the sharp lightning rod effect, the presence of atomic-level protrusion at the tip apex can provide a strong and highly localized plasmonic field. We find that the added NaCl layer not only blocks charge transfer between molecules and the substrate but also effectively prevents fluorescence quenching. The maximum fluorescence and Raman enhancement factors near the tip apex approach up to 5 and 12 orders of magnitude, respectively, achieving sub-nanometer spatial resolution. Finally, compared to vertical electric field and dipole, sharp protrusion can excite a stronger horizontal electric field and couple with horizontal dipoles to yield a higher quantum yield, resulting in a three orders of magnitude higher in the fluorescence enhancement of horizontal dipole compared to the tip without protrusion. Our theoretical research not only confirms the experimental results of spectral enhancement with the proximity of the tip to the molecule in tip-enhanced photoluminescence, but also provides effective guidance for a deeper understanding of the mechanism of tip-enhanced fluorescence and Raman spectroscopy, as well as optimizing the experimental system.

Analytical Modelling and Performance Characterization of Single Electron Transistor

One important component of the present nanotechnology research field is single electron transistor (SET) which has unrealized quality to operate at breakneck speeds with ultralow power consumption. The single electron transistor is a novel nanoscale switching device that can regulate the motion of a single electron in addition to maintaining its scalability down to the atomic level. In this context, scalability refers to the ability of electronic device to function better as their dimensions get smaller. The basic device properties that this single electron transistor (SET) operates on "quantized current-voltage (I-V) characteristic," "single electron tunneling effect," and "Coulomb blockade," are also covered. This study presents some of the key SET-related topics, such as modelling and simulation methods. Simulation of Nanostructures Method (SIMON) is circuit simulator for tunneling device with just a single electron device based on Monte Carlo approach. It enables simulation of any kind of circuits with voltage source, junctions in tunnels and capacitors in quasi stationary mode and transient mode. The most significant feature of SET that is applied in the research work is Coulomb blockade oscillation, so at the conclusion of this paper, a number of simulations utilizing the SIMON simulator are shown. The simulation results show that for gate capacitance CG = 2af, C∑ = 3af, IBIAS = 2nA, tunnel barrier resistance RT > 4.1KΩ and operating temperature T=21K the model reflects real SET characteristic with maximum voltage gain.

A multiscale modeling framework for predicting strain‐dependent electrical conductivity of carbon nanotube‐incorporated nanocomposites considering the electron tunneling effect

AbstractThe present work proposes a multiscale modeling framework for predicting the strain‐dependent electrical conductivity of carbon nanotube (CNT)‐incorporated nanocomposites considering the electron tunneling effect. A micromechanical model taking into account the waviness of the CNTs is utilized and molecular dynamics simulations estimating changes in distance between CNTs in the polymer matrix are conducted in an attempt to incorporate the electron tunneling effect. A series of numerical parametric studies are carried out to examine the influence of model parameters (e.g., CNT length, number of CNT segments, and intrinsic interfacial resistivity) on the strain‐dependent electrical conductivity of the nanocomposites. In addition, CNT/polydimethylsiloxane samples are fabricated and their strain‐dependent electrical conductivity is experimentally evaluated. Finally, to verify the predictive capability of the proposed modeling framework, the present predictions are compared with experimental results.Highlights A multiscale modeling framework of CNT‐incorporated nanocomposites is proposed. A micromechanics model and molecular dynamics simulations are used. The study includes a numerical parametric investigation. The present predictions are compared with those obtained experimentally.

Insights into the efficient water treatment over N-doped carbon nanosheets with layered minerals as template: The role of interfacial electron tunneling and transfer

Peroxymonosulfate (PMS) oxidation reactions have been extensively studied recently. Due to the high material cost and low catalytic capability, PMS oxidation technology cannot be effectively applied in an industrial water treatment process. In this work, we developed a modification strategy based on enhancing the neglected electron tunneling effect to optimize the intrinsic electron transport process of the catalyst. The 2D nitrogen-doped carbon-based nanosheets with small interlayer spacing were prepared by self-polymerization of dopamine hydrochloride inserted into the natural layered bentonite template. Systematic characterizations confirmed that the smaller layer spacing in the 2D nitride-doped carbon-based nanosheets reduces the depletion layer width. The weak electronic shielding effect derived by the small layer spacing on the material subsurface enhanced the bulk electron tunneling effect. More bulk electrons could be migrated to the catalyst surface to activate PMS molecules. The PMS activation system showed ultrafast oxidation capability to degrade organic pollutants and strong ability to resist interference from environmental matrixes due to the optimized electron transfer process. Furthermore, the developed membrane reactor exhibited strong catalytic stability during the continuous degradation of P-Chlorophenol (CP).

Inverse Sauter effect through a smooth potential step in binary waveguide arrays

We have systematically investigated the quantum relativistic inverse Sauter effect which is the prohibition of the inverse Klein tunneling effect of electrons through a smooth inverse potential step if its declining transition width is large enough. We can simulate this inverse Sauter effect by launching a Dirac soliton in binary waveguide arrays where the smooth transition region of the inverse potential step can be easily realized if we modify the effective refractive indices of the waveguides so that the smooth declining transition region of the inverse potential step can be described by a linear, exponential, or sinusoidal function. As expected, if the transition obeys the linear and sinusoidal laws, then the inverse Klein tunneling is practically suppressed if the transition width is comparable to or larger than the Compton length. Meanwhile, in the case of the exponential transition law analyzed in this work, the inverse Klein tunneling still can take place even if the transition width is significantly larger than the Compton length.

Tunneling induced transparency and slow light in an asymmetric double quantum dot molecule—Metal nanoparticle hybrid

We investigate the optical properties appearing in a nanostructure that is composed of an asymmetric double semiconductor quantum dot (SQD) molecule and a metal nanoparticle (MNP). The profile of the total linear absorption spectrum is proportional to the SQD contribution, while the MNP contribution is important. The profile of the doublet of resonances detected on the total linear absorption spectrum creates a transparency window. The doublet is asymmetric for small SQD-MNP distances and has a narrow peak and a wide peak. The width of the transparency window is increased, either with the enhancement of the rate at which the electron tunneling effect takes place within the double SQD molecule or with the decrease of the distance that separates the SQD molecule from the center of the MNP. The steep slope detected on the linear dispersion spectrum for frequencies laying within the transparency window owes its presence to the tunneling induced transparency and leads to slow light production. The corresponding value of the slow down factor is maximized for low values of the electron tunneling rate as well as for low center-to-center distances between the components of the hybrid nanostructure.

Deep trap assisting elastico-mechanoluminescence in diphase non-piezoelectrics induced by tunneling effect

The sustainable conversion of mechanical energy into light (elastico-mechanoluminescence, EML) opens up possibilities for energy-saving, which is of pivotal significance in addressing the energy crisis. The concepts of piezophotonics and the piezoelectric field's dependence on the probability of charge carriers detrapping have been thoroughly developed in explaining EML. Nevertheless, in contrast to the EML triggered by the piezoelectricity model, strong elastico-mechanoluminescence phenomena have also been frequently discovered in non-piezoelectric materials. Is the working principle different? This paper provides physical insight into the reconfigurable EML phenomena of intrinsic non-piezoelectric systems. It emphasizes the exploration of the mechanism through comprehensive analysis of trap information, de-trapping processes, and the lifetime of charge carriers in traps. We demonstrate the assistance of deep trap to enhance the red EML mode in a diphase centrosymmetric luminescent host through the electron tunneling effect. This advancement supports the progress of non-piezoelectric EML dielectrics and offers an appealing alternative approach in this field.

Tunneling induced swapping of orbital angular momentum in a quantum dot molecule

In this paper, we have examined the effectiveness exchange of optical vorticity via three-wave mixing (TWM) technique in a four-level quantum dot (QD) molecule by means of the electron tunneling effect. Our analytical analysis demonstrates that the TWM procedure can result in the production of a new weak signal beam that may be absorbed or amplified within the QD molecule. We have taken into account the electron tunneling as well as the relative phase of the applied lights to assess the absorption and dispersion characteristics of the newly generated light. We have discovered that the slow light propagation and signal amplification can be achieved. Our results show that the exchange of the orbital angular momentum of light can transfer from coupling optical vortex light to the new generated light in high efficiency.

Transition from optical bistability to multistability in a tunneling quantum dot via structure hybrid light

In this paper, we proposed a new model based on the electron tunneling effect in a four-level quantum dot molecule (QDM) for studying the optical bistability (OB) and optical multistability (OM). The QDM interacts with a probe and two coupling and Laguerre–Gaussian (LG) fields. We found that by adjusting the electron tunneling effect and the parametric controlling of LG light, the transition from OB to OM or vice versa is possible. Moreover, due to the simultaneous interaction between coupling and LG lights with the same optical transition adjusting the threshold of OB and OM by orbital angular momentum (OAM) of the LG light becomes achievable. Our results show that by adjusting the simultaneous effect of electron tunneling and OAM state of the vortex light, the favorable OB and OM patterns with adjustable intensity thresholds are achievable. Our proposed model may have potential application in quantum information science based on quantum dot (QD) devices.

Manipulating single excess electrons in monolayer transition metal dihalide

Polarons are entities of excess electrons dressed with local response of lattices, whose atomic-scale characterization is essential for understanding the many body physics arising from the electron-lattice entanglement, yet difficult to achieve. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we show the visualization and manipulation of single polarons in monolayer CoCl2, that are grown on HOPG substrate via molecular beam epitaxy. Two types of polarons are identified, both inducing upward local band bending, but exhibiting distinct appearances, lattice occupations and polaronic states. First principles calculations unveil origin of polarons that are stabilized by cooperative electron-electron and electron-phonon interactions. Both types of polarons can be created, moved, erased, and moreover interconverted individually by the STM tip, as driven by tip electric field and inelastic electron tunneling effect. This finding identifies the rich category of polarons in CoCl2 and their feasibility of precise control unprecedently, which can be generalized to other transition metal halides.

A High-Randomness and High-Stability Electronic Quantum Random Number Generator without Post Processing

Random numbers are one of the key foundations of cryptography. This work implements a discrete quantum random number generator (QRNG) based on the tunneling effect of electrons in an avalanche photo diode. Without any post-processing and conditioning, this QRNG can output raw sequences at a rate of 100 Mbps. Remarkably, the statistical min-entropy of the 8,000,000 bits sequence reaches 0.9944 bits/bit, and the min-entropy validated by NIST SP 800-90B reaches 0.9872 bits/bit. This metric is currently the highest value we have investigated for QRNG raw sequences. Moreover, this QRNG can continuously and stably output raw sequences with high randomness over extended periods. The system produced a continuous output of 1,174 Gbits raw sequence for a duration of 11,744 s, with every 8 Mbits forming a unit to obtain a statistical min-entropy distribution with an average value of 0.9892 bits/bit. The statistical min-entropy of all data (1,174 Gbits) achieves the value of 0.9951 bits/bit. This QRNG can produce high-quality raw sequences with good randomness and stability. It has the potential to meet the high demand in cryptography for random numbers with high quality.

Effect of N2 flux on the field emission properties of synthesized ZnO/ZrN core-shell nanostructures

In this paper, the highly oriented ZnO/ZrN nanoneedle emitters with excellent field emission performance were fabricated by sputtering ZrN particles with different N2 flux to modify ZnO nanoneedle arrays. The effects of different N2 flow on the microscopic morphology, structure and field emission properties of ZnO nanoneedles were investigated. The results shows that the ZnO/ZrN nanoneedles (ZnO/ZrN-1.6) emitter prepared when the flow of N2 gas is 1.6 SCCM exhibits the best field emission performance, with the lowest turn-on electric field Eto at 0.81 V/μm, and a field emission current density of 25 mA/cm2 which was 25 times of pure ZnO. In addition to the improvement of band structure, the improvement of field emission performance can also be attributed to the tunneling effect of electrons through heterostructure. This well-aligned ZnO/ZrN core-shell cathode with excellent emission characteristics grown on carbon cloth substrate has the potential applications in future flexible light source devices.

Demonstrating the electron blocking effect of AlGaN/GaN superlattice cladding layers in GaN-based laser diodes

Electron leakage currents seriously limit the power conversion efficiencies (PCEs) of gallium nitride (GaN)-based laser diodes (LDs). To minimize the leakage currents, electron blocking layers are generally applied in the p-type region. However, few works have discussed the electron blocking effect of a p-cladding layer, which is found to be critical in suppressing the leakage currents of an LD. In this work, we compare the blocking performance of uniform AlGaN p-cladding layers and AlGaN/GaN superlattice (SL) p-cladding layers with the same average Al component respectively. Both light-emitting diodes (LEDs) and LDs with the same epitaxy structures are characterized by light–current (L–I) and current–voltage (I–V) measurements. The latest analytical model of leakage currents is applied to fit the L–I curves of LEDs, where smaller leakage coefficients are observed in the SL structures compared with the uniform-layer structures. Eighty LDs with varying ridge widths are studied by comparing the threshold current densities, slope efficiencies, and PCEs. The SL-based p-cladding layer shows statistically significant advantages over a uniform AlGaN layer. The blocking effects of both scattering- and bound-state electrons in SLs are investigated theoretically. Repetitive reflection and thermal relaxation are responsible for the blocking effect of scattering-state electrons. Simulation results indicate that the tunneling effect of bound-state electrons through a miniband mechanism is insignificant at a large injection level due to a negative differential conductivity by the Esaki–Tsu effect. We demonstrate a better electron blocking performance of p-cladding layers based on SLs than uniform AlGaN layers in GaN-based LDs.

Evolution Between Exciton and Exciplex Emission in Planar Heterojunction OLEDs with Different Hole-Injection Characteristics

Planar heterojunction-based organic light-emitting diodes (PHJ OLEDs) usually have simultaneous exciton and exciplex emissions and understanding their evolution processes is crucial for designing high-performance white OLEDs, but relevant evolution mechanisms are still unclear. Here, unreported competitions between exciton and exciplex emissions and an undiscovered energy-transfer channel from triplet exciton to triplet exciplex states in PHJ OLEDs with different hole-injection abilities are investigated by separately measuring their current-dependent electroluminescence (EL) spectra and magneto-EL (MEL) traces at different temperatures. Interestingly, the ratio of emission intensity between exciton and exciplex states ($R={I}_{\mathrm{exciton}}/{I}_{\mathrm{exciplex}}$) from the device with a good hole-injection ability rises with increasing bias current at each temperature, whereas that from the device with a poor hole-injection ability hardly changes. In addition, R from the devices with good and poor hole-injection abilities show nonmonotonic variation and monotonic reduction with decreasing operational temperature at each bias current, respectively. These various current- and temperature-dependent competitions between exciton and exciplex emissions are attributed to distinct current- and temperature-dependent electron-tunneling effects occurring at organic heterojunction interfaces of devices with different hole-injection abilities. More intriguingly, using MEL as a fingerprint detection tool, we find there is a Dexter energy-transfer (DET) channel from the triplet exciton to triplet exciplex (${T}_{1}\ensuremath{\rightarrow}{\mathrm{EX}}_{3}$) state and the DET can enhance the reverse intersystem crossing (RISC) process from triplet to singlet exciplexes (${\mathrm{EX}}_{3}\ensuremath{\rightarrow}{\mathrm{EX}}_{1}$), which cannot be found using the conventional probing tool, EL spectra. Because the numbers of ${T}_{1}$ excitons in these devices have different current and temperature dependencies, various current- and temperature-dependent DET and RISC processes happen, which cause abundant MEL behaviors. Obviously, this work deepens the physical understanding of the competition and DET processes between exciton and exciplex states in PHJ OLEDs.

A Novel Low On−State Resistance Si/4H−SiC Heterojunction VDMOS with Electron Tunneling Layer Based on a Discussion of the Hetero−Transfer Mechanism

In this study, we propose a novel silicon (Si)/silicon carbide (4H−SiC) heterojunction vertical double−diffused MOSFET with an electron tunneling layer (ETL) (HT−VDMOS), which improves the specific on−state resistance (RON), and examine the hetero−transfer mechanism by simulation. In this structure, the high channel mobility and high breakdown voltage (BV) are obtained simultaneously with the Si channel and the SiC drift region. The heavy doping ETL on the 4H−SiC side of the heterointerface leads to a low heterointerface resistance (RH), while the RH in H−VDMOS is extremely high due to the high heterointerface barrier. The higher carrier concentration of the 4H−SiC surface can significantly reduce the width of the heterointerface barrier, which is demonstrated by the comparison of the conductor energy bands of the proposed HT−VDMOS and the general Si/SiC heterojunction VDMOS (H−VDMOS), and the electron tunneling effect is significantly enhanced, leading to a higher tunneling current. As a result, a significantly improved trade−off between RON and BV is achieved. With similar BV values (approximately 1525V), the RON of the HT−VDMOS is 88% and 65.75% lower than that of H−VDMOS and the conventional SiC VDMOS, respectively.

Synthesis of TiO2/Al2O3 Double-Layer Inverse Opal by Thermal and Plasma-Assisted Atomic Layer Deposition for Photocatalytic Applications.

In comparison to conventional nano-infiltration approaches, the atomic layer deposition (ALD) technology exhibits greater potential in the fabrication of inverse opals (IOs) for photocatalysts. In this study, TiO2 IO and ultra-thin films of Al2O3 on IO were successfully deposited using thermal or plasma-assisted ALD and vertical layer deposition from a polystyrene (PS) opal template. SEM/EDX, XRD, Raman, TG/DTG/DTA-MS, PL spectroscopy, and UV Vis spectroscopy were used for the characterization of the nanocomposites. The results showed that the highly ordered opal crystal microstructure had a face-centered cubic (FCC) orientation. The proposed annealing temperature efficiently removed the template, leaving the anatase phase IO, which provided a small contraction in the spheres. In comparison to TiO2/Al2O3 plasma ALD, TiO2/Al2O3 thermal ALD has a better interfacial charge interaction of photoexcited electron-hole pairs in the valence band hole to restrain recombination, resulting in a broad spectrum with a peak in the green region. This was demonstrated by PL. Strong absorption bands were also found in the UV regions, including increased absorption due to slow photons and a narrow optical band gap in the visible region. The results from the photocatalytic activity of the samples show decolorization rates of 35.4%, 24.7%, and 14.8%, for TiO2, TiO2/Al2O3 thermal, and TiO2/Al2O3 plasma IO ALD samples, respectively. Our results showed that ultra-thin amorphous ALD-grown Al2O3 layers have considerable photocatalytic activity. The Al2O3 thin film grown by thermal ALD has a more ordered structure compared to the one prepared by plasma ALD, which explains its higher photocatalytic activity. The declined photocatalytic activity of the combined layers was observed due to the reduced electron tunneling effect resulting from the thinness of Al2O3.

Extending Plasmonic Enhancement Limit with Blocked Electron Tunneling by Monolayer Hexagonal Boron Nitride.

Fabricating ultrasmall nanogaps for significant electromagnetic enhancement is a long-standing goal of surface-enhanced Raman scattering (SERS) research. However, such electromagnetic enhancement is limited by quantum plasmonics as the gap size decreases below the quantum tunneling regime. Here, hexagonal boron nitride (h-BN) is sandwiched as a gap spacer in a nanoparticle-on-mirror (NPoM) structure, effectively blocking electron tunneling. Layer-dependent scattering spectra and theoretical modeling confirm that the electron tunneling effect is screened by monolayer h-BN in a nanocavity. The layer-dependent SERS enhancement factor of h-BN in the NPoM system monotonically increases as the number of layers decreases, which agrees with the prediction by the classical electromagnetic model but not the quantum-corrected model. The ultimate plasmonic enhancement limits are extended in the classical framework in a single-atom-layer gap. These results provide deep insights into the quantum mechanical effects in plasmonic systems, enabling the potential novel applications based on quantum plasmonic.

Hot carrier-based near-field thermophotovoltaics with energy selective contacts

A model of the thermophotovoltaic device combining a near-field thermal emitter and a hot-carrier solar cell is established. The fluctuating electromagnetic near-field theory for the radiative thermal transport and Landauer's formula for the carrier extraction are introduced. Expressions for the efficiency and the power output of the device are derived. How the voltage and the extraction energy of the energy selective contacts affect the performance of the device is revealed. The results show that the efficiency of the proposed device can be greatly enhanced by exploiting the radiation between the emitter and the cell and extracting carriers through electron tunneling effects.