Hot Electron Emission Research Articles

Electrochemiluminescence of coumarin derivatives induced by injection of hot electrons into aqueous electrolyte solution

Hot electrons can be injected from conductor/insulator/electrolyte (C/I/E) junctions into an aqueous electrolyte solution by cathodic pulse-polarization of the electrode. Injected hot electrons induce electrogenerated chemiluminescence of various luminophores including coumarins in fully aqueous solutions. This is based on the tunnel emission of hot electrons into aqueous electrolyte solution, which can result in the generation of hydrated electrons as reducing mediators. These tunnel-emitted electrons allow also the production of highly oxidizing radicals from added precursors. This work shows that coumarin derivatives are suitable candidates as ECL labels for bioaffinity assays or other analytical applications in which detection is based on the ECL of pulse-polarized C/I/E tunnel-emission electrodes in fully aqueous solutions. The mechanisms of the ECL of coumarins are discussed and the analytical applicability of the ECL of three coumarin derivatives is studied.

Direct current-induced electrogenerated chemiluminescence of hydrated and chelated Tb(III) at aluminum cathodes

Cathodic DC polarization of oxide-covered aluminum produces electrogenerated chemiluminescence from hydrated and chelated Tb(III) ions in aqueous electrolyte solutions. At the moment of cathodic voltage onset, a strong cathodic flash is observed, which is attributed to a tunnel emission of hot electrons into the aqueous electrolyte solution and the successive chemical reactions with the luminophores. However, within a few milliseconds the insulating oxide film is damaged and finally dissolved due to (i) indiffusion of protons or alkali metal ions into the thin oxide film, (ii) subsequent hydrogen evolution at the aluminum/oxide interface and (iii) alkalization of the electrode surface induced by hydrogen evolution reaction. When the alkalization of the electrode surface has proceeded sufficiently, chemiluminescence is generated with increasing intensity. Aluminum metal, short-lived Al(II), Al(I) or atomic hydrogen and its conjugated base form, hydrated electron, can act as highly reducing species in addition to the less energetic heterogeneously transferred electrons from the aluminum electrode. Tb(III) added as a hydrated ion in the solution probably luminesces in the form of Tb(OH) 3 or Tb(OH) 4 − by direct redox reactions of the central ion whereas multidentate aromatic ligand chelated Tb(III) probably luminesces by ligand sensitized chemiluminescence mechanism in which ligand is first excited by one-electron redox reactions, which is followed by intramolecular energy transfer to the central ion which finally emits light.

Three-dimensional Child–Langmuir law for uniform hot electron emission

This paper presents a three-dimensional (3D) model of Child–Langmuir (CL) law for uniform hot electron emission in planar and cylindrical gap, including the effects of finite emission energy. It is found that the enhancement of 3D CL law (in terms of 1D CL law) can be written in a general form of JC[3D]∕JC[1D]=1+F×G, where F is the normalized mean position of 1D electron flow in classical, weakly relativistic, and quantum regime, and G is the geometrical correction factor depending on the geometrical properties of the finite emitting patches on the cathode. In particular, we present the analytical solutions for various emitting patches, such as rectangle, ellipse, square, circle, triangle, and polygon, which agree very well with 3D particle-in-cell simulation. For a cylindrical gap of finite width, it is also found that the convergent flow (cathode outside) has larger enhancement than the divergent flow (cathode inside) at a given aspect ratio of outer radius to inner radius of the gap. Smooth transition from various operating regimes is demonstrated.

Cathodic electrogenerated chemiluminescence of Ru(bpy) 32+ chelate at oxide-coated heavily doped silicon electrodes

High amplitude cathodic pulse polarization of ultra thin oxide film-coated heavily doped silicon electrodes induces tunnel emission of hot electrons into aqueous electrolyte solution, which probably results in the generation of hydrated electrons in the vicinity of the electrode surface. The method allows the detection of tris (2,2′-bipyridine) ruthenium(II) chelate at subnanomolar concentration level. This paper shows that both n- and p-type heavily doped silicon electrodes can be used, illustrates the effect of oxide film thickness upon the silicon electrode on the intensity of ECL of tris (2,2′-bipyridine) ruthenium(II) and discusses the basic features of tris (2,2′-bipyridine) ruthenium(II) chelate-specific ECL at these electrodes. Thin oxide film-coated silicon electrodes provide a lower blank emission and a higher ECL intensity of the present ruthenium chelate than oxide-covered aluminium electrodes. This suggests that thin oxide film-coated silicon is a very promising working electrode material, especially in microanalytical systems made fully or partly of silicon.

Double-peak emission of hot electrons generated by femtosecond laser interaction with solid targets

Double-peak emission of hot electrons has been observed in the interaction of a 60-fs, 125-mJ, 800-nm, p-polarized laser pulse with Al targets. One peak in the specular direction is due to the reflected laser light, which excites a plasma wave to accelerate electrons. The other peak, which is more consistent with theories of Phys. Plasmas 6, 2855 (1999)] and Phys. Rev. Lett 82, 743 (1999)], is produced by the resonance absorption at approximately 15 degrees from the target normal.

Exoelectron Emission as emission of hot electrons

The Exoelectron emission (EEE) and Thermoluminescence (TL) of a KCl thin film were investigated. The experimental results are presented and interpreted from the point of view that the EEE is the emission of hot electrons from the dielectric. This assumption for the difference of the activation energies of EEE (EE) and TL (EL), leads to the expression EE – EL= χT/Te, where T and Te are the temperatures of the lattice and of the electrons and χ is the electron affinity of the solid. This relation has been experimentally verified on a KCl thin film by measuring all five terms in the equation under the same experimental conditions. The electron affinity of the KCl film was measured from the energy distribution of the secondary electron emission. The temperatures of the exoelectrons were calculated from the tails of their energy distributions and were much higher than those of the body. The experimental results were in agreement with the expression for the difference of the activation energies. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of the hot carrier luminescence in AlxGa1?xAs/GaAs power HFET's

The processes related to the presence of hot carriers in power AlGaAs/GaAs heterostructure field effect transistors (HFET) have been studied by means of optical (electroluminescence) and electrical techniques. Electroluminescence spectra evidence the presence of three emission bands, superimposed on the maxwellian behavior typical for the hot electron emission. Two of the previous bands are related to the bandgap transitions of cold carriers in the conduction channel and in the barrier layer, while the third one is probably related to DX centers present in the Al x Ga 1-x As layers. We show that the optical measurements give more detailed information than the electrical ones; in particular the gate current seems to be a less indicative monitor of the impact ionization mechanisms than the light emitted in the conductive channel by cold carrier recombinations. No correlation between integrated light and currents is evidenced, except at low energies were an intra-band mechanism, related only to the drain current is observed. Spatial maps of the emitted light evidence that part of the cold holes produced by impact ionization move toward the source electrode and recombine with cold electrons in this region.

Double Jet Emission of Hot Electrons from a Micro-droplet Spray

Spatial distribution of hot electrons with energies above 50 keVare investigated by an ethanol micro-droplet spray irradiated bylinearly and elliptically polarized 150 fs laser pulses at an intensityof 1016W/cm2. Two symmetric hot electron jetswith respect to the laser propagation direction are observed in thepolarization plane for a linearly polarized laser field and in theplane of the long electric vector for an elliptically polarized laserfield, respectively. Particle-in-cell simulations suggest that theresonance absorption on the spherical surface of the droplets is mainlyresponsible for the generation of the double-jet emission of hotelectrons.

Z-dependence of hot electron generation in femtosecond laser interaction with solid targets

Hot electron emission is studied using 798 nm, 150 fs, p-polarized laser pulses at 1 × 1016 W cm−2, irradiating on plastic (CH), aluminium (Al), titanium (Ti) and copper (Cu) planar targets. The dependence of hot electron generation on atomic number Z is observed experimentally.

A simple estimate of resonance absorption of femtosecond laser pulses by liquid droplets

The resonance absorption of femtosecond pulses by liquid droplets is studied. The angular dependence of the laser absorption may be used to understand the symmetric double-lobe emission of hot electrons generated from liquid droplets. The total laser absorption fraction is found to be strongly related to the scale length of the microplasmas.

Metal/insulator/metal junctions for electrochemical surface science

We describe the preparation and characterization of Al-AlOx-Ag tunnel junctions and calculate the energy distribution of the tunneling hot electrons in the range 0–2.5 eV above the Fermi level of silver. Because the mean free path of the hot electrons is of the order of the thickness of the silver film of the junction, which is at the same time the electrode in contact with an electrolyte, new surface effects can be studied. Hot electrons can be injected into the nonhydrated electron band in water. Hot electrons also cause hydrogen evolution at electrode potentials more positive than the ones needed in common electrochemistry. We observed the emission of hot electrons into silver during transients of hydrogen oxidation at silver and during oxidation of overpotential hydrogen on platinum clusters deposited on the silver electrode. The tunnel current at constant tunnel voltage can be changed by faradaic reactions, but surprisingly also by nonfaradaic reactions; this is assigned to a mesoscopic quantum phenomenon.

Absorption of femtosecond laser pulses in metallic and dielectric slabs

Laser absorption, and hot electron emission and its energy spectrum are studied in the interaction between P-polarized femtosecond laser pulses at 8×1015 W/cm2 and solid-density plasmas. Comparison is made between th e different b ehaviours of metallic and dielectric slabs irradiated by femtosecond laser pulse s. It appears that there are less laser absorption, fewer hot electrons and high er hot electron temperature from dielectric slabs compared with metallic ones. T hese are concluded to be due to the lower conductivity and consequently lower ch arge separation potential of the dielectric slabs.

Effects of hot electron emission on a low-conductivity tetracyanoethylene polymer layer including studies of the corrugation of the film surface

The effect of strong field electron emission (FEE) on a tetracyanoethylene (TCNE) polymer layer was studied by Field Ion Microscopy (FIM) using TCNE and Ne as the imaging gases. The TCNE polymer was formed on each tungsten tip by radical polymerisation before FEE. The FIM images show field emission spots all over the surface of the tip. The FEM images show a random distribution of several field emission areas at the onset of FEE. After sometime at a current of about 1 μA, there is a transition to higher currents at the same voltage, in which the electron emission pattern changes to have only one emitting area. After this transition, two different types of FIM images were observed, depending on the imaging gas that was used. Neon FIM images at low tip voltages show spots in the areas where the electron emission current was greatest, and at much higher voltages these images show emission from other areas with lower surface corrugation. However, the FIM images with TCNE as the imaging gas do not show any differences between the areas with and without electron emission. The FIM images remain as before FEE, which can be explained by the formation of a new polymer by the reaction of the surface layer with the imaging gas. It is assumed that chemically reactive fragments at the polymer/vacuum interface, which are needed for the polymerisation reaction, are formed by pyrolysis and sputtering processes during FEE.

Fabrication of a novel micro time-of-flight mass spectrometer

This paper presents the fabrication and the tests of a novel micro-mass spectrometer fabricated by micro-machining and emission test of hot electron for ionization. A micro-mass spectrometer consists of a micro-ion source and a micro-ion separator. The micro-ion source consists of a hot filament and grid electrodes. Electrons emitted from a hot filament are to ionize some sample molecules. The ions are accelerated to an ion detector by an electric field of the ion separator. Mass can be analyzed by using the time-of-fight depending on the mass-to-charge ratio. The current of hot electron emission from the hot filament is measured for various input voltages and the ion separation is tested for acetone.

Electrochemiluminoimmunoassay of hTSH at disposable oxide-coated n-silicon electrodes

Cathodic pulse polarisation of thin insulating film-coated electrodes enables the generation of electrochemiluminescence (ECL) by tunnel emission of hot electrons from the Fermi level of the conductor material of the conductor–insulator–aqueous electrolyte solution junction to the solutes at the vicinity of the electrode surface and probably also to the conduction band of water. The latter process can generate hydrated electrons as strongly reducing slightly longer-lived cathodic intermediate s, which are known to be able to induce chemiluminescence (CL) of various types of luminophores having very different photophysical and chemical properties. The generation of the above-mentioned cathodic primary species provides good possibilities to use many types of luminophores as label molecules in sensitive immuno and DNA-probing assays. This paper introduces an electrochemiluminoimmunoassay (ECLIA) for human thyroid stimulating hormone (hTSH) at oxide-coated n-silicon electrodes and demonstrates the suitability of silicon electrodes covered with thermally grown silicon dioxide film as disposable working electrodes (WEs) in sensitive time-resolved ECL (tr-ECL) measurements in aqueous solution. The label chelate can be detected almost down to picomolar level and the calibration curve of the chelate covers more than five orders of magnitude of chelate concentration. Also the calibration curve of the immunometric hTSH assay was found to be linear over a wide range of hTSH concentration, the detection limit of the hormone being below 1 mU l −1 (4 pmol l −1).

Time-resolved electrochemiluminescence of platinum(II) coproporphyrin

Cathodic pulse polarisation of oxide-covered aluminium electrodes can generate electrochemiluminescence (ECL) from metalloporphyrins. This is based on the tunnel emission of hot electrons into aqueous electrolyte solution, which probably results in the generation of hydrated electrons as reducing mediators. These tunnel emitted electrons allow the production of highly reactive radicals, such as sulfate radicals from peroxodisulfate ions, which can induce strong redox luminescence from various organic chemiluminophores including metalloporphyrins. The work presented here illustrates the generation of ECL from platinum(II) coproporphyrin (PtCP) and its bovine serum albumin (BSA) conjugate. This allows the detection of these molecules below nanomolar concentrations and over several orders of magnitude of concentration. The relatively long luminescence lifetime of PtCP allows discrimination from the background ECL signal using time resolved measurements, leading to higher sensitivity and the detection of PtCP-BSA indicates the potential use of metalloporphyrins as labels in ECL-based bioassays such as immunoassays and DNA-binding assays.

Effects of Polarization on Super-hot Electron Generation inFemtosecond Laser-Plasma Interaction

The effects of laser polarization on super-hot electron (>100 keV) generationhave been studied in the interaction of femtosecond laser light (800 nm,150 fs, 6×1015W·cm-2) with a pre-formed plasma from aslab Cu target. For p-polarized laser pulses, high-energy γ-rays ofthe energy ~400 keV were detected. The electron temperatures deducedfrom the γ-ray spectra were 66 and 52 keV, respectively, in normal andreflective directions of the solid target, and hot electrons were emittedout of the plasma mainly in the normal direction. In contrast, there werenearly no γ-rays >100 keV found for s-polarized laser pulses. The hotelectron temperature was 26 keV and the emission of hot electrons wasparallel to the laser field. The superposition of resonant field withelectrostatic field excited by escaping electrons may contribute to thehigh-energy γ-ray or super-hot electron (>100 keV) generation.

Experimental study of a subpicosecond pulse laser interacting with metallic and dielectric targets.

We have studied laser absorption, hot electron emission, and the energy spectrum of hot electrons produced during the interaction of a 150 fs, 5 mJ, 800 nm p-polarized laser pulse at 8x10(15) W/cm(2) with metallic and dielectric target materials. Because dielectric targets are much less conductive, the charge separation potential in dielectric targets is higher than that of metallic targets. This leads to a smaller laser absorption, fewer emitted electrons, and a lower hot electron temperature in dielectric than in metallic targets.

Low-frequency excess noise induced by hot-carrier injection in polysilicon thin-film transistors

Low-frequency noise has been investigated in self-aligned (SA) and gate overlapped lightly doped drain (GOLDD) polysilicon thin-film transistors (TFTs). We found that, at low Vds, both device structures are characterised by 1/f noise, originating from carrier number fluctuations. However, when the SA devices were operated in the kink regime, an excess noise was observed, in contrast to GOLDD TFTs. This has been explained by attributing the excess noise to supplemental oxide charge fluctuations induced by hot-carrier injection into the gate oxide. By using two-dimensional numerical simulations, we show that the hot-electron emission current in SA polysilicon TFTs is much higher than in the GOLDD structure.

Conservation of Lateral Momentum in Heterostructure Integrated Thermionic Coolers

ABSTRACTThin film thermionic coolers use selective emission of hot electrons over a heterostructure barrier layer from emitter to collector resulting in evaporative cooling. In this paper a detailed theory of electron transport perpendicular to the multilayer superlattice structures is presented. Using Fermi-Dirac statistics, density-of-states for a finite quantum well and the quantum mechanical reflection coefficient, the currentvoltage characteristics and the cooling power density are calculated. The resulting equations are valid in a wide range of temperatures and electric fields. It is shown that conservation of lateral momentum plays an important role in the device characteristics. If the lateral momentum of the hot electrons is conserved in the thermionic emission process, only carriers with sufficiently large kinetic energy perpendicular to the barrier can pass over it and cool the emitter junction. However, if there is no conservation of lateral momentum, the number of electrons participating in thermionic emission will dramatically increase. The theoretical calculations are compared with the experimental dark current characteristics of quantum well infrared photodetectors and good agreement over a wide temperature range is obtained. Calculations for InGaAs/InGaAsP superlattice structures show that the effective thermoelectric power factor (electrical conductivity times the square of the effective Seebeck coefficient) can be improved comparing to that of bulk material. We will also discuss methods by which the conservation of lateral momentum in thermionic emission process can be altered such as by creating a controlled roughness at the interface of the superlattice barriers. The improvement in the effective power factor through thermionic emission can be combined with the other methods to reduce the phonon thermal conductivity in superlattices and thus obtain higher thermoelectric figure-of-merit ZT.