Hot Electron Transport Research Articles

High-efficiency narrow-band plasmonic hot electron conversion from nanoscale sodium–silicon heterostructures

Plasmonic harvesting of hot electrons has stimulated intensive research activities for applications ranging from sub-bandgap photodetection to photocatalysis. Both high photoelectric conversion efficiency and tunable spectral response are pursued by manipulating resonant metal–semiconductor (M–S) nanostructures. Although noble plasmonic metals have been exclusively employed in hot electron conversion studies, exploring new materials may offer an additional degree of freedom to manipulate the hot electron generation, transport, and emission processes. In this paper, we propose to employ the low-loss alkali metal sodium as an alternate plasmonic material for developing a narrow-band resonant hot electron device. Based on a backside-illumination (BSI) configuration where plasmonic hot electrons generate locally at the M–S interface, the transport loss can be significantly suppressed. Thanks to its ultralow imaginary part of the permittivity, bringing Na into the BSI design allows for efficient shrinking of the resonant linewidth down to sub-20 nm. Another intriguing feature is that Na has more preferred electron density of state distribution for facilitating hot electron emission at the M–S junction. The optimized Na BSI device can yield a photocurrent responsivity up to 50 mA/W at a wavelength of 1400 nm as predicted by our electromagnetic simulation and theoretical model. Our study highlights that the alkali metal could be a promising alternative material for the development of high-Q resonant hot electron devices for near-infrared wavelengths.

Direct Observation of Photoinduced Charge Separation at Transition-Metal Nitride-Semiconductor Interfaces.

Optically excited hot carriers from metallic nanostructures forming metal-semiconductor heterostructures are advantageous for enhancing photoelectric conversion in the sub-band gap photon energy regime. Plasmonic gold has been widely used for hot carrier excitation, but recent works have demonstrated that plasmonic transition-metal nitrides have higher efficiencies in injecting hot electrons to adjacent n-type semiconductors and are more cost-effective. To collect direct evidence of hot carrier excitation from nanostructures, imaging of hot carriers is essential. In this work, photoexcited Kelvin probe force microscopy (KPFM) is used to image hot carriers excited in transition-metal nitride nanostructures forming heterostructures with semiconductors. Among available transition-metal nitrides, we select zirconium nitride (ZrN) for this study. Additionally, both p-type and n-type titanium dioxides (TiO2) are selected to study the transport of hot holes and hot electrons. The KPFM results indicate that for ZrN and p-type TiO2 heterostructures, hot holes are injected into the p-type TiO2 across the Schottky contact. In the case of ZrN and n-type TiO2 heterostructures, hot electrons are injected into the n-type TiO2 across the ohmic contact. Because transition-metal nitrides are known to be more effective than gold at injecting hot carriers into adjacent semiconductors, unambiguously determining the mechanisms of hot carrier transportation of transition-metal nitrides using photoexcited KPFM will facilitate additional studies on hot carrier applications with transition-metal nitrides.

Recent Advances in the Design of Plasmonic Au/TiO2 Nanostructures for Enhanced Photocatalytic Water Splitting.

Plasmonic nanostructures have played a key role in extending the activity of photocatalysts to the visible light spectrum, preventing the electron–hole combination and providing with hot electrons to the photocatalysts, a crucial step towards efficient broadband photocatalysis. One plasmonic photocatalyst, Au/TiO2, is of a particular interest because it combines chemical stability, suitable electronic structure, and photoactivity for a wide range of catalytic reactions such as water splitting. In this review, we describe key mechanisms involving plasmonics to enhance photocatalytic properties leading to efficient water splitting such as production and transport of hot electrons through advanced analytical techniques used to probe the photoactivity of plasmonics in engineered Au/TiO2 devices. This work also discusses the emerging strategies to better design plasmonic photocatalysts and understand the underlying mechanisms behind the enhanced photoactivity of plasmon-assisted catalysts.

Manipulation of plasmon-induced hot electron transport in Pd/MoO3-x@ZIF-8: Boosting the activity of Pd-catalyzed nitroaromatic hydrogenation under visible-light irradiation

Controlling the charge carrier transfer pathway in a plasmonic catalyst is the key to developing efficient catalyst. Herein, a visible-light responsive plasmonic catalyst is prepared by depositing Pd nanoparticles on ZIF-8 (zeolitic imidazolate framework) coated plasmonic MoO3-x composites (Pd/MoO3-x@ZIF-8). The as-prepared catalyst allows effective plasmon-induced electron transfer from plasmonic MoO3-x to Pd active site by taking advantages of the Schottky junction formed between Pd metal nanoparticles and MoO3-x@ZIF-8 composite as well as heterojunction formed between ZIF-8 and MoO3-x, effectively retarding the recombination of the plasmonic induced hot electron-hole pairs in MoO3-x. As a result, the Pd/MoO3-x@ZIF-8 presents exceptionally higher catalytic activities for the hydrogenation of nitroaromatics under visible-light irradiation than that under dark conditions, which are far superior to those of Pd/MoO3-x and Pd/ZIF-8. The enhanced activity can be attributed to the cooperative promoting effects between Pd nanoparticles and plasmonic MoO3-x@ZIF-8 on facilitating the plasmon-induce electron transfer, and the possible reaction mechanism is proposed. This study expands the scope of reductive organic chemical transformation catalyzed by plasmonic catalysts and sheds light on design of efficient plasmonic catalysts by steering an electron flow to promote the photogenerated electron transfer from plasmonic semiconductor to catalytic active site.

Transient magnetic gratings on the nanometer scale.

Laser-driven non-local electron dynamics in ultrathin magnetic samples on a sub-10 nm length scale is a key process in ultrafast magnetism. However, the experimental access has been challenging due to the nanoscopic and femtosecond nature of such transport processes. Here, we present a scattering-based experiment relying on a laser-induced electro- and magneto-optical grating in a Co/Pd ferromagnetic multilayer as a new technique to investigate non-local magnetization dynamics on nanometer length and femtosecond timescales. We induce a spatially modulated excitation pattern using tailored Al near-field masks with varying periodicities on a nanometer length scale and measure the first four diffraction orders in an x-ray scattering experiment with magnetic circular dichroism contrast at the free-electron laser facility FERMI, Trieste. The design of the periodic excitation mask leads to a strongly enhanced and characteristic transient scattering response allowing for sub-wavelength in-plane sensitivity for magnetic structures. In conjunction with scattering simulations, the experiment allows us to infer that a potential ultrafast lateral expansion of the initially excited regions of the magnetic film mediated by hot-electron transport and spin transport remains confined to below three nanometers.

Absence of Nonlocal Manipulation of Oxygen Atoms Inserted below the Si(111)-7×7 Surface.

The injection of electrons from the scanning tunneling microscope tip can be used to perform nanoscale chemistry and study hot electron transport through surfaces. While nonlocal manipulation has been demonstrated primarily for aromatic adsorbates, here we confirm that oxygen atoms bonded to the Si(111) surface can also be nonlocally manipulated, and we fit the measured manipulation data to a single channel decay model. Unlike aromatic adsorption systems, oxygen atoms also insert below the surface of silicon. Although the inserted oxygen can be manipulated when the tip is directly over the relevant silicon adatom, it is not possible to induce nonlocal manipulation of inserted oxygen atoms at the same bias. We attribute this to the electrons injected at +4 eV initially relaxing to couple to the highest available surface state at +3.4 eV before laterally transporting through the surface. With a manipulation threshold of 3.8 eV for oxygen inserted into silicon, once carriers have undergone lateral transport, they do not possess enough energy to manipulate and remove oxygen atoms inserted beneath the surface of silicon. This result confirms that nonlocal nanoscale chemistry using the scanning tunneling microscope tip is dependent not only on the energy required for atomic manipulation, but also on the energy of the available surface states to carry the electrons to the manipulation site.

A novel route to visualize the hot electron transfer process in Ag@SiO2-TiO2 for solar-hydrogen conversion

The wrapping of plasmonic metal particles with silica shell is considered to be one of the effective means to make full use of hot electrons for solar-hydrogen conversion. Ag@SiO2-TiO2 structures with different shell thicknesses (2, 3, 5, 7 and 11 nm) as hot-electron devices were fabricated, and their local electric field was simulated by the COMSOL software. A novel method was proposed to visually characterize the migration of hot electrons, which was represented by multiphysics coupling. The simulated current flow indicates the transfer direction of hot electrons, and the variation of electron-hole concentration implies the gain and loss of electrons. The results demonstrate that the applied thinner silica shell not only decreases the barrier to the hot electron transport out of shell, but also intensify local electric field. When the silica shell thickness is 2 nm, the amount of hot electrons migrated out of shell is high, such that the photogenerated electrons in TiO2 have more chances for jumping to higher energy states, which results in the enhanced separation of electrons and holes. The photocurrent and photovoltage of Ag@SiO2 (2 nm)-TiO2 were respectively 1.68 and 1.54 times higher than those of Ag-TiO2 denoting the highest carriers’ migration and utilization efficiency.

Domain wall dynamics due to femtosecond laser-induced superdiffusive spin transport

Manipulation of magnetic domain walls via a helicity-independent laser pulse has recently been experimentally demonstrated and various physical mechanisms leading to domain wall dynamics have been discussed. Spin-dependent superdiffusive transport of hot electrons has been identified as one of the possible ways how to affect a magnetic domain wall. Here, we develop a model based on superdiffusive spin-dependent transport to study the laser-induced transport of hot electrons through a smooth magnetic domain wall. We show that the spin transfer between neighboring domains can enhance ultrafast demagnetization in the domain wall. More importantly, our calculations reveal that when the laser pulse is properly focused on to the vicinity of the domain wall, it can excite sufficiently strong spin currents to generate a spin-transfer torque that can rapidly move the magnetic domain wall by several nanometers in several hundreds of femtoseconds, leading to a huge nonequilibrium domain wall velocity.

Symmetric bow-tie diode for terahertz detection based on transverse hot-carrier transport

Room temperature terahertz detection based on the transverse transport of hot electrons in symmetric bow-tie diode was demonstrated and studied in this work. More than an order of magnitude improvement was achieved in detector responsivity and noise equivalent power compared to the previously reported bow-tie diodes of the asymmetric design. High detection efficiency is based on the electric field enhancement in the active part of the symmetric diode, as well as on reducing the device resistance and improved impedance matching between the detector and integrated bow-tie antenna. Hydrodynamic modeling was verified by the ensemble Monte Carlo simulations. Ways to further improve the performance of the symmetric hot-carrier detector are discussed.

Estimation of the charged defect density from hot-electron transport studies in epitaxial ZnO

High-field electron transport measurements by applying short (few ns) voltage pulses on nominally undoped n-type Zn-polar ZnO epilayers are reported and interpreted in terms of the Boltzmann kinetic equation. The transient measurements do not demonstrate a significant change in the electron density up to 320 kV/cm electric field. This result together with the experimental data on the current allows one to estimate the electron drift velocity from the measured current: the highest value of ~2.9 × 107 cm/s is obtained at the pre-breakdown field of 320 kV/cm for the ZnO layer with the electron density of 1.5 × 1017 cm–3. The densities of double-charged oxygen vacancies (~1.6 × 1017 cm–3) and other charged centres (~1.7 × 1017 cm–3) are assumed for the best fit of the simulated and measured hot-electron effect. A correlation with the epilayer growth conditions is demonstrated: the higher Zn cell temperature favours the formation of a higher density of the oxygen vacancies (1.9 × 1017 cm–3 at 347°C).

Study of Effects of Hot Phonon on Hot Electron Transport

We study the influence of hot phonons on hot electron transport within the drain region is investigated. The ensemble Monte Carlo (MC) method self-consistently coupled with Poisson’s equation is used. This MC simulation is three-dimensional in k-space and two-dimensional in real space. The two-dimensional model is possible if physical quantities have no significant variation along the third direction. The phonon Boltzmann transport equation (BTE) is solved and the mean heat generation rate is calculated.

Theoretical study of Scattering of Electron Transport

The effect of the scattering-direction of hot electrons in the drain of ballistic \(n^{+}\)-\(i\)-\(n^{+}\) diodes is studied by a semi-classical Monte Carlo method. At low doping concentrations, the ionized impurity scattering has a weak influence on hot electron transport, although it is an elastic scattering. At sufficiently high doping concentrations on the other hand, the ionized impurity scattering enhances the backward flow of hot electrons, and severely degrades the peak of mean carrier-velocity in the channel and also the steady-state current. We argue that the scattering direction of hot electrons is the main reason behind these results.

Hanbury Brown and Twiss exchange correlations in a graphene box

Quadratic detection in linear mesoscopic transport systems produces cross terms that can be viewed as interference signals reflecting statistical properties of charge carriers. In electronic systems these cross term interferences arise from exchange effects due to Pauli principle. Here we demonstrate fermionic Hanbury Brown and Twiss (HBT) exchange phenomena due to indistinguishability of charge carriers in a diffusive graphene system. These exchange effects are verified using current-current cross correlations in combination with regular shot noise (autocorrelation) experiments at microwave frequencies. Our results can be modeled using semiclassical analysis for a square-shaped metallic diffusive conductor, including contributions from contact transparency. The experimentally determined HBT exchange factor values lie between the calculated ones for coherent and hot electron transport.

Expansion of Solar Coronal Hot Electrons in an Inhomogeneous Magnetic Field: 1D PIC Simulation

Abstract The expansion of hot electrons in flaring magnetic loops is crucial to understanding the dynamics of solar flares. In this paper we investigate, for the first time, the transport of hot electrons in a magnetic mirror field based on a 1D particle-in-cell simulation model. The hot electrons with small pitch angles transport into the cold plasma, which leads to the generation of Langmuir waves in the cold plasma and ion acoustic waves in the hot plasma. The large pitch angle electrons can be confined by the magnetic mirror, resulting in the different evolution timescale between electron parallel and perpendicular temperatures. This will cause the formation of electron temperature anisotropy, which then generates the whistler waves near the interface between hot electrons and cold electrons. The whistler waves can scatter the large pitch angle electrons to smaller value through the cyclotron resonance, leading to electrons escaping from the hot region. These results indicate that the whistler waves may play an important role in the transport of electrons in flaring magnetic loops. The findings from this study provide some new insights to understand the electron dynamics of solar flares.

Ballistic hot-electron transport in a quantum Hall edge channel defined by a double gate

Ballistic transport of hot electrons in a quantum Hall edge channel is attractive for studying the electronic analog of quantum optics, where the edge potential profile is an important parameter that governs the charge velocity and scattering by longitudinal-optical (LO) phonons. Here, we use a parallel double gate to control the electric field of the edge potential and investigate the ballistic length of the channel by using hot-electron spectroscopy. The ballistic length is significantly enhanced by reducing the LO phonon scattering rate in the tailored potential.

Effect of Co and Fe stoichiometry on terahertz emission from Ta/(CoxFe1−x)80B20/MgO thin films

Laser pulse induced terahertz (THz) emission from Ta/${({\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x})}_{80}{\mathrm{B}}_{20}$/MgO thin films, with varying compositions $x$ and annealing temperatures, is investigated. With increasing annealing temperature, the THz emission intensity exhibits significant dependence on $x$, with maxima at $x$ of 0.1--0.3. The $x$ dependence of the THz emission originated from the composition dependence of the spin currents induced in ${\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x}$ formed by the crystallization of amorphous CoFeB after the annealing. The origin of the laser-induced spin current is qualitatively discussed in terms of a ballistic transport of hot electrons and a spin-dependent Seebeck effect with different compositions.

Elongated Lifetime and Enhanced Flux of Hot Electrons on a Perovskite Plasmonic Nanodiode.

A fundamental understanding of hot electron transport is critical for developing efficient hot-carrier-based solar cells. There have been significant efforts to enhance hot electron flux, and it has been found that a key factor affecting the hot electron flux is the lifetime of the hot electrons. Here, we report a combined study of hot electron flux and the lifetime of hot carriers using a perovskite-modified plasmonic nanodiode. We found that perovskite deposition on a plasmonic nanodiode can considerably improve hot electron generation induced by photon absorption. The perovskite plasmonic nanodiode consists of MAPbI3 layers covering a plasmonic-Au/TiO2 Schottky junction that is composed of randomly connected Au nanoislands deposited on a TiO2 layer. The measured incident photon-to-electron conversion efficiency and the short-circuit photocurrent show a significantly improved solar-to-electrical conversion performance of this nanodiode. Such an improvement is ascribed to the improved hot electron flux in MAPbI3 caused by effective light absorption from near-field enhancement of plasmonic Au and the efficient capture of hot electrons from Au nanoislands via the formation of a three-dimensional Schottky interface. The relation between the lifetime and flux of hot electrons was confirmed by femtosecond transient absorption spectroscopy that showed considerably longer hot electron lifetimes in MAPbI3 combined with the plasmonic Au structure. These findings can provide a fundamental understanding of hot electron generation and transport in perovskite, which can provide helpful guidance to designing efficient hot carrier photovoltaics.

Formation of Surface Silver Nano-network Structures through Hot Electron Regulated Diffusion-limited Aggregation

A surface Ag nano-network pattern is formed by first depositing Ag nanoparticles (NPs) on a conductive template, which has a certain defect structure, and then illuminating the Ag NPs with ultraviolet (UV) light in a moist environment. Such an Ag nano-network pattern consists of multiple connected Brownian trees (BTs), which are produced through the diffusion-limited aggregation (DLA) process. In the DLA process, diffuse Ag+ ions, which are generated by UV light illumination and dissolved by a thin adsorbed water layer on the surfaces of the Ag NPs and used GaN template, settle to form a BT through the combination with excited hot electrons migrating into the template from the Ag NPs. The lateral transport of hot electrons in the template is regulated by the distributions of threading dislocation and point defect cluster in the template, which eventually become the centers of BTs. The structure of a surface Ag nano-network can potentially serve as a transparent conductor.

Ultrafast demagnetization in buried Co80Dy20 as fingerprint of hot-electron transport

Abstract The generation of ultrashort hot-electron pulses in metallic heterostructures offers attractive perspectives for the ultrafast spin manipulation on the picosecond time scale. In such approach, the hot-electron pulses are produced by exciting a non-magnetic capping layer with femtosecond infrared laser pulses. These hot-electron pulses propagate towards a buried magnetic layer to trigger ultrafast demagnetization. Lately, it was shown that the demagnetization onset and characteristic demagnetization times are both affected by the transport regime (ballistic or diffusive) of the photo-excited hot-electrons. In this work, we show that the hot-electron pulses produced by photo-exciting a Al(3)/Ta(3)/Cu(60) capping layer undergo a temporal stretching and delays when they go across a [Co(0.1)/Ni(0.6)]x5 multilayer. These information were extracted from a study of hot-electron induced demagnetization in CoDy alloys by means of Time-Resolved X-Ray Magnetic Circular Dichroism.

Spectroscopic study on hot-electron transport in a quantum Hall edge channel

Hot electron transport in a quantum Hall edge channel of an AlGaAs/GaAs heterostructure is studied by investigating the energy distribution function in the channel. Ballistic hot-electron transport, its optical-phonon replicas, weak electron-electron scattering, and electron-hole excitation in the Fermi sea are clearly identified in the energy spectra. The optical-phonon scattering is analyzed to evaluate the edge potential profile. We find that the electron-electron scattering is significantly suppressed with increasing the hot-electron's energy well above the Fermi energy. This can be understood with suppressed Coulomb potential with longer distance for higher energy. The results suggest that the relaxation can be suppressed further by softening the edge potential. This is essential for studying non-interacting chiral transport over a long distance.