Excited Hot Electrons Research Articles

Formation of hot-electron ensembles quasiequilibrated in momentum space by ultrafast momentum scattering of highly excited hot electrons photoinjected into the Γ valley of GaAs

We study ultrafast scattering dynamics of hot electrons photoinjected with high excess energies in the \ensuremath{\Gamma} valley of the conduction band of GaAs, using time- and angle-resolved photoemission spectroscopy and ab initio calculations. At ultrafast rates of the order of 10 fs, the packets in the \ensuremath{\Gamma} valley are transformed into hot-electron ensembles (HEEs) quasiequilibrated in momentum space but not in energy space. The energy relaxation of the HEEs takes place as a whole on a longer time scale with rates dependent only on the excess energy, irrespective of the momenta of hot electrons. Both momentum scattering and energy relaxation are ruled by the electron-phonon interaction.

Surface Plasmon Catalytic Aerobic Oxidation of Aromatic Amines in Metal/Molecule/Metal Junctions

The surface plasmon catalytic selective aerobic oxidation of aromatic amines to aromatic azo compounds in metal/molecule/metal junctions was explored by density functional theory. The overall reaction could be divided into the initial plasmon-induced oxygen activation and the subsequent photothermal-driven dehydrogenation process. The activation of oxygen on silver and gold surfaces is proposed through a surface plasmon-mediated hot electron injection mechanism at solid/gas interface. Resonance absorption of incident light by metal nanostructures generates energetic electron–hole pairs. Time-dependent density functional theory calculations illustrate that the excited hot electrons created on metal surfaces can transfer to the antibonding 2π* orbital of adsorbed oxygen, which facilitates the dissociation of O2 on metal surfaces. Silver shows better catalytic performance for the oxygen activation due to its stronger surface plasmon resonance absorption intensity and higher hot electron energy level. Aromati...

Anomalous ultrafast dynamics of hot plasmonic electrons in nanostructures with hot spots.

The interaction of light and matter in metallic nanosystems is mediated by the collective oscillation of surface electrons, called plasmons. After excitation, plasmons are absorbed by the metal electrons through inter- and intraband transitions, creating a highly non-thermal distribution of electrons. The electron population then decays through electron-electron interactions, creating a hot electron distribution within a few hundred femtoseconds, followed by a further relaxation via electron-phonon scattering on the timescale of a few picoseconds. In the spectral domain, hot plasmonic electrons induce changes to the plasmonic resonance of the nanostructure by modifying the dielectric constant of the metal. Here, we report on the observation of anomalously strong changes to the ultrafast temporal and spectral responses of these excited hot plasmonic electrons in hybrid metal/oxide nanostructures as a result of varying the geometry and composition of the nanostructure and the excitation wavelength. In particular, we show a large ultrafast, pulsewidth-limited contribution to the excited electron decay signal in hybrid nanostructures containing hot spots. The intensity of this contribution correlates with the efficiency of the generation of highly excited surface electrons. Using theoretical models, we attribute this effect to the generation of hot plasmonic electrons from hot spots. We then develop general principles to enhance the generation of energetic electrons through specifically designed plasmonic nanostructures that could be used in applications where hot electron generation is beneficial, such as in solar photocatalysis, photodetectors and nonlinear devices.

Ultrafast Dynamics of Photoexcited Gold Nanostructures on Mica Substrates Probed by Transient Absorption Spectroscopy and Time-resolved X-ray Diffraction

Abstract Gold nanostructures deposited on mica were excited with 400-nm sub-ps light pulses. Sub-ps transient absorption spectroscopy showed that excited hot electrons in the gold nanostructures decayed with a time constant of 13 ps through electron–phonon coupling in the nanostructures. Sub-ps time-resolved X-ray diffraction revealed that the (0010) mica lattice constant in a 1.7 µm thick surface layer expanded with a time constant of 6 ps.

Light-concentrating plasmonic Au superstructures with significantly visible-light-enhanced catalytic performance.

Noble metals are well-known for their surface plasmon resonance effect that enables strong light absorption typically in the visible regions for gold and silver. However, unlike semiconductors, noble metals are commonly considered incapable of catalyzing reactions via photogenerated electron-hole pairs due to their continuous energy band structures. So far, photonically activated catalytic system based on pure noble metal nanostructures has seldom been reported. Here, we report the development of three different novel plasmonic Au superstructures comprised of Au nanoparticles, multiple-twinned nanoparticles and nanoworms assembling on the surfaces of SiO2 nanospheres respectively via a well-designed synthetic strategy. It is found that these novel Au superstructures show enhanced broadband visible-light absorption due to the plasmon resonance coupling within the superstructures, and thus can effectively focus the energy of photon fluxes to generate much more excited hot electrons and holes for promoting catalytic reactions. Accordingly, these Au superstructures exhibit significantly visible-light-enhanced catalytic efficiency (up to ∼264% enhancement) for the commercial reaction of p-nitrophenol reduction.

Design of nanophotonic, hot-electron solar-blind ultraviolet detectors with a metal-oxide-semiconductor structure

Solar-blind ultraviolet (UV) detection refers to photon detection specifically in the wavelength range of 200 nm–320 nm. Without background noises from solar radiation, it has broad applications from homeland security to environmental monitoring. The most commonly used solid state devices for this application are wide band gap (WBG) semiconductor photodetectors (Eg > 3.5 eV). However, WBG semiconductors are difficult to grow and integrate with Si readout integrated circuits (ROICs). In this paper, we design a nanophotonic metal-oxide-semiconductor structure on Si for solar-blind UV detectors. Instead of using semiconductors as the active absorber, we use Sn nano-grating structures to absorb UV photons and generate hot electrons for internal photoemission across the Sn/SiO2 interfacial barrier, thereby generating photocurrent between the metal and the n-type Si region upon UV excitation. Moreover, the transported hot electron has an excess kinetic energy >3 eV, large enough to induce impact ionization and generate another free electron in the conduction band of n-Si. This process doubles the quantum efficiency. On the other hand, the large metal/oxide interfacial energy barrier (>3.5 eV) also enables solar-blind UV detection by blocking the less energetic electrons excited by visible photons. With optimized design, ∼75% UV absorption and hot electron excitation can be achieved within the mean free path of ∼20 nm from the metal/oxide interface. This feature greatly enhances hot electron transport across the interfacial barrier to generate photocurrent. The simple geometry of the Sn nano-gratings and the MOS structure make it easy to fabricate and integrate with Si ROICs compared to existing solar-blind UV detection schemes. The presented device structure also breaks through the conventional notion that photon absorption by metal is always a loss in solid-state photodetectors, and it can potentially be extended to other active metal photonic devices.

Exploring size and state dynamics in CdSe quantum dots using two-dimensional electronic spectroscopy

Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques.

Extracting the temperature of hot carriers in time- and angle-resolved photoemission

The interaction of light with a material's electronic system creates an out-of-equilibrium (non-thermal) distribution of optically excited electrons. Non-equilibrium dynamics relaxes this distribution on an ultrafast timescale to a hot Fermi-Dirac distribution with a well-defined temperature. The advent of time- and angle-resolved photoemission spectroscopy (TR-ARPES) experiments has made it possible to track the decay of the temperature of the excited hot electrons in selected states in the Brillouin zone, and to reveal their cooling in unprecedented detail in a variety of emerging materials. It is, however, not a straightforward task to determine the temperature with high accuracy. This is mainly attributable to an a priori unknown position of the Fermi level and the fact that the shape of the Fermi edge can be severely perturbed when the state in question is crossing the Fermi energy. Here, we introduce a method that circumvents these difficulties and accurately extracts both the temperature and the position of the Fermi level for a hot carrier distribution by tracking the occupation statistics of the carriers measured in a TR-ARPES experiment.

Probing of molecular adsorbates on Au surfaces with large-amplitude temperature jumps

Methods are described to probe vibrational transitions of molecules adsorbed on Au films subjected to calibrated ultrafast large-amplitude temperature jumps (T-jumps). The probe technique, vibrational sum-frequency generation (SFG), can monitor vibrations localized on specific parts of adsorbate molecules in the form of self-assembled monolayers (SAMs). Substrates had a thin Cr adhesion layer and an Au film that could withstand millions of T-jumps without laser damage of film or adsorbate. The substrate flash-heating process was characterized using ultrafast reflectance measurements. Reflectance transients induced by both 800 nm or 400 nm femtosecond pulses had overshoot-decay-plateau structures. The overshoots and decays represented optically generated hot electrons, and the plateaus gave the equilibrium temperature increase ΔT, which was in the 30–175 K range. The combination of SFG adsorbate and Au surface reflectance measurements was used to assess the effects of adsorbate vibrational heating by both hot electrons and the hot Au lattice. Two types of SAMs were investigated, nitrobenzenethiolate (NBT), where SFG probed nitro groups located 4 carbon atoms from the surface, and octadecylthiolate (ODT), where SFG probed terminal methyl groups 17 carbon atoms from the surface. With ΔT = 175 K, the NBT nitro transition νs(NO2) showed time-dependent intensity loss, redshifting, and broadening. These three kinds of transients also had overshoot-decay-plateau structures, which resulted from the interplay of hot electron excitation of higher-frequency vibrations including the probed vibration, and Au lattice heating of lower-energy vibrations and the conformational modes that cause reversible disordering of the SAM structure. The relative importance of these effects was different for the overshoot and plateau regions, and for the intensity, redshifting, and broadening effects. With ODT, T-jumps caused the terminal methyl groups to become disordered, and the disordering process was nonexponential in time. From the ratio of symmetric to antisymmetric CH-stretching intensities, the ensemble-averaged methyl tilt angle could be determined. With smaller T-jumps, the methyl groups gradually increased their tilt by a small amount during ∼200 ps, while with larger T-jumps where ΔT = 175 K, the methyl groups abruptly reoriented toward the surface normal and then tilted gradually away from the normal in the next 20 ps.

Pd/ n-SiC nanofilm sensor for molecular hydrogen detection in oxygen atmosphere

We demonstrate a concept of molecular hydrogen sensor operable continuously in oxygen-rich atmosphere, using a Pd/ n-SiC heterojunction structure with nanometer thickness metallization. The sensing mechanism is based on the excitation of hot electrons in the Pd nanofilm traveling ballistically over the Schottky barrier and toward the semiconductor layer to constitute a detectable electric current. If molecular oxygen is also present in the host ambient, the hydrogen sensitivity of the device increases by an order of the magnitude and the generated current acquires a stationary character. Magnitude of the stationary current is linearly proportional to the mixture pressure and to the rate of hydrogen-to-water oxidation process occurring on the Pd nanofilm surface. Hydrogen sensors of the present type can have very fast response and recovery times of only several seconds.

Compact deposition system for device-based ultrathin crystalline film growth

The study of hot electron excitation at surfaces requires the deposition of ultrathin metal films. To probe the role of particle bombardment in such film excitations, homogeneous, atomically ordered, and relatively defect free thin films must be deposited in the same ultrahigh vacuum system where they will be studied. With these constraints in mind, the authors designed a compact deposition chamber that allows for in situ growth and analysis of metal layers, which are only a few monolayers. This deposition chamber is attached to the commercial variable temperature scanning tunneling microscope (STM) and has an internal volume of 500cm3. The target substrates for deposition are compatible with the STM design and are held in place in a specially designed clamping slot that enables low temperature growth. They used the custom built chamber to deposit Ag top layers with thicknesses between 8 and 15nm on Si(100). Electronic and morphological characteristics of the prototype Ag∕n-Si(100) devices are presented.

Kinetic electronic excitation of solids by fast-particle bombardment

The kinetic excitation of hot electrons and conduction-band vacancies following the impact of an energetic particle onto a solid surface was studied using metal-insulator-metal tunnel junctions. The top metal layer polycrystalline silver was bombarded by charged and neutral Ar projectiles of kinetic energies between 1 and 15 keV. Hot charge carriers generated within the collision cascade initiated by the projectile impact were detected as a tunneling current across the oxide barrier into the underlying substrate metal electrode. The tunneling yield is shown to depend monotonously on the kinetic impact energy with no notable contribution of potential emission. The dependence, however, is different for singly charged and neutral projectiles. Applying a bias voltage between the two metal electrodes, information about the energy spectrum of the excited carriers is obtained. The experimental data are interpreted in terms of a simple two-temperature tunneling model, yielding a kinetically induced transient electron “temperature” on the order of 10 4 K.

Coherent Quantum Control of Excitons at Ultracold and High Density inCu2Owith Phase Manipulated Pulses

By phase manipulation of a short laser pulse, it is possible to selectively generate ultracold excitons in a two-photon process while quenching the multiphoton excitation of hot electrons and holes. We show how this technique allows us to suppress the heating of n=1 orthoexcitons in Cu(2)O at high density. Using a laser pulse having an energy of a few microJ and duration of 100 fs, we are thus able to produce a cold exciton gas up to a density of 10(15) cm(-3).

Laser-assisted photoemission from surfaces

We investigate the laser-assisted photoelectric effect from a solid surface. By illuminating a $\mathrm{Pt}(111)$ sample simultaneously with ultrashort 1.6 and $42\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ pulses, we observe sidebands in the extreme ultraviolet photoemission spectrum, and accurately extract their amplitudes over a wide range of laser intensities. Our results agree with a simple model, in which soft x-ray photoemission is accompanied by the interaction of the photoemitted electron with the laser field. This strong effect can definitively be distinguished from other laser surface interaction phenomena, such as hot electron excitation, above-threshold photoemission, and space-charge acceleration. Thus, laser-assisted photoemission from surfaces promises to extend pulse duration measurements to higher photon energies, as well as opening up measurements of femtosecond-to-attosecond electron dynamics in solid and surface-adsorbate systems.

Reactivation of damaged rare earth luminescence centers in ion-implanted metal–oxide–silicon light emitting devices

Charge trapping and quenching of electroluminescence (EL) in SiO2 layers implanted by Ge and rare earth (RE) ions during hot electron injection were investigated. In case of the SiO2:Ge layer the EL quenching is caused by the transformation of the luminescent defects (\(\equiv\)Ge–Si\(\equiv\) or \(\equiv\)Ge–Ge\(\equiv\)) to optically inactive centers during hot electron excitation, whereas the EL from rare earth centers is quenched due to the electron trapping by RE-centers or their surroundings, but not due to their optical deactivation. Therefore, the flash lamp post-injection annealing releasing trapped electrons reactivates RE centers and increases the operating time of metal–oxide–silicon light emitting devices (MOSLEDs).

Electroluminescence and its excitation mechanism of SiO x films deposited by electron-beam evaporation

Blue electroluminescence from SiO x films deposited by electron beam evaporation was observed. This blue emission blueshifted from 450 to 410 nm with increasing applied voltage. The dependences of blue emission on applied voltage, frequency and conduction current were studied. Our experimental data support that blue emission from SiO x films is the result of both recombination of charge carriers injected from opposite electrodes and impact excitation of hot electrons, the recombination of carriers injected is dominant in low and medium electric fields but hot electron impact excitation is dominant under high electric fields.

Hot carrier type exchange in inorganic electroluminescent thin films

The authors have observed the hot carrier type (holes or electrons) exchange in rare-earth-ion-activated strontium thiogallate (SrGa2S4) thin films by measuring the transient electroluminescent wave forms of the devices having a single insulating thin film. Measured wave forms revealed that the green electroluminescence of europium activated SrGa2S4 thin film occurs due to hot hole excitation. In contrast, the blue electroluminescence of cerium activated SrGa2S4 thin film occurs due to hot electron excitation. Hence, the hot carrier type is exchanged by the different rare-earth-ion doping.

Study on impurity desorption induced by femtosecond pulse laser based on a stochastic process model

With the advantages on non-equilibrium heating and desorption induced by electronic transition, the femtosecond pulse laser introduces a new way for solving the problem of impurity pollution adsorbed on a solid thin film in micro-electro-mechanical systems (MEMS). A model based on stochastic processes was established for stimulated desorption induced by the femtosecond pulse laser to interpret the interaction of the opti-cally excited hot electrons with the adsorbed molecules in a metal substrate. Numerical simulation results reveal a time-dependent desorption probability of adsorbed molecules and indicate that how key parameters of femtosecond pulse laser, such as incident laser energy flux, pulse duration, and wavelength of pulse, have a great effect on the desorption probability.

Specifics of substrate-mediated photo-induced chemical processes on supported nm-sized metal particles

Substrate-mediated mechanism of photochemical reactions on metals includes direct and/or plasmon-related excitation of electron–hole pairs in the bulk by photons and subsequent chemical transformation due to interaction of excited (hot) electrons (or holes) with the adsorbate. In nm-sized metal particles, the excited electrons are confined. We show in detail that this factor is beneficial for photochemistry under low-intensity steady-state conditions (linear regime) and also during intensive sub-ps laser pulses (non-linear regime). In both cases, the confinement may increase the contribution of secondary hot electrons to the photochemical conversion. In addition, in the non-linear regime the decrease of electron temperature after heating may be much slower than that on the surfaces of bulk samples and accordingly photochemical reactions may be more probable.

Ultrafast Electron Dynamics in Metals

During the last decade, significant progress has been achieved in the rapidly growing field of the dynamics of hot carriers in metals. Here we present an overview of the recent achievements in the theoretical understanding of electron dynamics in metals, and focus on the theoretical description of the inelastic lifetime of excited hot electrons. We outline theoretical formulations of the hot-electron lifetime that originates in the inelastic scattering of the excited quasiparticle with occupied states below the Fermi level of the solid. First-principles many-body calculations are reviewed. Related work and future directions are also addressed.