Interband Transitions Research Articles

Inhomogeneous linear responses and transport in armchair graphene nanoribbons in the presence of elastic scattering

Within linear-response theory we derive a response function that thoroughly accounts for the influence of elastic scattering and is valid beyond the long-wavelength limit. We use the theory to evaluate the polarization function and the conductivity in metallic armchair graphene nanoribbons in the Lindhard approximation for intra-band and inter-band transitions and for a relaxation time τ that is not constant. We obtain a logarithmic behaviour in the scattering-independent polarization function not only for intra-band transitions, as is usually the case for one-dimensional systems, but also for inter-band transitions. Modifying the screening wave vector and the impurity density in the long-wavelength limit strongly influences the relaxation time. In contrast, for large wave vectors, this modification leads to a conservative value of τ . We show that the imaginary part of the impurity-dependent conductivity varies with the wave vector while its scattering-independent part exists only for a single value of the wave vector.

Evolution of plasmon-like excited states in silver nanowires and nanorods.

Silver nanowires and nanorods are useful prototypical systems to study the emergence of plasmons within a quantum mechanical context because their high aspect ratios enable plasmons to emerge in smaller systems than for roughly spherical nanoclusters. Here, we quantify the plasmon-like character of the excited states of silver nanorods and nanowires based on three nearly orthogonal criteria: (1) collectivity, (2) dipole additivity, and (3) superatomic character. Based on these three criteria, we classify the excited states as plasmon-like, collective, single-particle, interband, or as intermediate between these categories. We show that linear nanowires have a longitudinal absorption peak that has single-particle character and a transverse absorption peak that evolves from a single-particle to plasmon-like to a mix of plasmon-like and interband with increasing length. Increasing the width tends to increase the plasmon-like character of the longitudinal excited state. In contrast, increasing the nanorod width tends to decrease the length at which interband transitions start mixing significantly into the transverse plasmon-like excited states.

Disentangling Light- and Temperature-Induced Thermal Effects in Colloidal Au Nanoparticles.

We present temperature-dependent (from room temperature to 80 °C) absorption spectra of Au/SiO2 core–shell nanoparticles (NPs) (core diameter: ∼25 nm) in water in the range from 1.5 to 4.5 eV, which spans the localized surface plasmon resonance (LSPR) and the interband transitions. A decrease in absorption with temperature over the entire spectral range is observed, which is more prominent at the LSPR. These changes are well reproduced by theoretical calculations of the absorption spectra, based on the experimentally measured temperature-dependent real (ε1) and imaginary (ε2) parts of the dielectric constant of Au NPs and of the surrounding medium. In addition, we model the photoinduced response of the NPs over the entire spectral range. The experimental and theoretical results of the thermal heating and the simulations of the photoinduced heating are compared with the ultrafast photoinduced transient absorption (TA) spectra upon excitation of the LSPR. These show that while the latter is a reliable monitor of heating of the NP and its environment, the interband region mildly responds to heating but predominantly to the population evolution of charge carriers.

Nonlinear intensity dependence of photogalvanics and photoconductance induced by terahertz laser radiation in twisted bilayer graphene close to magic angle

We report on the observation of the nonlinear intensity dependence of the bulk photogalvanic current and photoconductivity in the twisted graphene with small twist angles close to the second magic angle. We show that terahertz radiation results in photoresponses, which are caused by indirect optical transitions (free carrier absorption), direct interband transitions, and optical transitions between moir\'e subbands. The relative contribution of these absorption channels depends on the Fermi-level position with respect to the multiple moir\'e subbands of the twisted graphene. The interplay of these absorption channels results in oscillations of the photoresponses with variation of the gate voltage. We show that the photoresponse saturates at high intensities. For different absorption channels it has different intensity dependencies and saturation intensities. The latter depends nonmonotonically on the Fermi-level position, which is controlled by the gate voltage.

Investigation of the thermal and light sensing properties of graphene oxide and reduced graphene oxide films obtained by spin coating method

This paper reported the thermal and light sensing properties of graphene oxide (GO) and reduced graphene oxide (RGO) films obtained by spin coating on soda lime glass. GO showed a higher thermal sensing response of 9.0 compared to RGO which had a maximum thermal sensing response of 2.2 at 623 K. The thermal conductivity of GO and RGO increased with temperature although RGO typified better thermal conductivity. For GO, a negative temperature coefficient of resistance (NTC) of 0.0032153 /K was obtained which is higher than values obtained in literatures while RGO had a NTC value of 0.00201/K. For the photo sensing, the peak photo resistance of GO corresponds to an energy band gap of 3.5 eV while the peak photo resistance of RGO corresponds to a decreased energy band gap of 2.76 eV. It was also confirmed that lower bandgap favored improved photosensing function. RGO film showed intrinsic photoconductivity as the photoconductivity increased at longer wavelengths. The photo sensing of GO and RGO were dominated by inter-band optical transitions in the UV to mid infrared region. The obtained NTC values for GO and RGO and the photoconductivity of the materials can promote further utilization in robotics and optical applications.

Plasmonic enhancement of photoacoustic strain-waves on gold gratings

In this paper, we report on the time-dependent strain-wave-induced changes in the reflection and diffraction of a gold plasmonic grating. We demonstrate efficient excitation of strain waves using enhanced absorption at and around the surface plasmon polariton resonance. In addition, we observe that the strain-wave-induced changes in the reflection and diffraction of the grating show an approximately quadratic dependence on pump fluence when probed at a wavelength of 400 nm. We tentatively attribute this non-linear behavior to strain-induced nonlinear changes of the interband transition energy. Using a model that calculates the permittivity of the gold taking into account the d to s/p interband transition, we deduce that the interband transition energy would have to change by about 0.013 eV to account for the measured changes in reflection.

Near‐Infrared Reflection Modulation Through Electrical Tuning of Hybrid Graphene Metasurfaces

AbstractGraphene, a 2D material with tunable optical properties, has recently attracted intense interest for reconfigurable metasurfaces. So far, the working wavelength of graphene‐based or hybrid graphene metasurfaces has been limited in the mid‐infrared and terahertz spectra. In this paper, by combining graphene with Au nanostructures, the authors demonstrate a near‐infrared tunable metasurface with decent modulation efficiency, weak dependence on graphene's carrier mobility, and small gate voltages, attributing to the unique interband transition of graphene. The experimental results agree well with numerical simulations. It is also shown that by properly designing the structural parameters of Au nanostructures, the hybrid graphene metasurface can be tunable in both near‐infrared and mid‐infrared regions.

MXene supported surface plasmons on telecommunications optical fibers

MXenes, an emerging class of two-dimensional materials, exhibit characteristics that promise significant potential for their use in next generation optoelectronic sensors. An interplay between interband transitions and boundary effects offer the potential to tune the plasma frequencies over a large spectral range from the near-infrared to the mid-infrared. This tuneability along with the ‘layered’ nature of the material not only offer the flexibility to produce plasmon resonances across a wide range of wavelengths, but also add a degree of freedom to the sensing mechanism by allowing the plasma frequency to be modulated. Here we show, numerically, that MXenes can support plasmons in the telecommunications frequency range and that surface plasmon resonances can be excited on a standard MXene coated side polished optical fiber. Thus, presenting the tantalising prospect of highly selective distributed optical fiber sensor networks.

Photoluminescence Evolution with Deposition Thickness of Ge Nanostructures Embedded in GaSb

Herein, low‐temperature and temperature‐dependent photoluminescence (PL) measurements are carried out on highly tensile‐strained Ge nanostructures embedded in GaSb matrix, and the effects of Ge deposition thickness are clarified. The direct‐gap transition‐related PL feature is successfully identified in the tensile‐strained Ge nanostructures. While typical PL thermal quenching is observed for the tensile‐strained Ge‐ and GaSb‐related transitions in the samples with a Ge deposition being thinner than the critical thickness, a negative thermal quenching shows up for the GaSb interband transition in the samples with Ge deposition surpassing the critical thickness at which high‐density nanoparticles form to relax the strain. A phenomenological thermal‐injection model is established of electrons from the tensile‐strained Ge layer to the GaSb matrix, the thermal quenching is accounted for, and a ladder‐like function of the strain‐relaxed Ge is clarified to favor the electron activation. The understanding of the effects of deposition thickness is helpful for the high‐performance Ge‐based light source for optoelectronic integration.

Lifetime effects and satellites in the photoelectron spectrum of tungsten metal

Tungsten is an important and versatile transition metal and has a firm place at the heart of many technologies. A popular experimental technique for the characterisation of tungsten and tungsten-based compounds is X-ray photoelectron spectroscopy (XPS), which enables the assessment of chemical states and electronic structure through the collection of core level and valence band spectra. However, in the case of metallic tungsten, open questions remain regarding the origin, nature, and position of satellite features that are prominent in the photoelectron spectrum. These satellites are a fingerprint of the electronic structure of the material and have not been thoroughly investigated, at times leading to their misinterpretation. The present work combines high-resolution soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) with reflection electron energy loss spectroscopy (REELS) and a multi-tiered ab-initio theoretical approach, including density functional theory (DFT) and many-body perturbation theory (G0W0 and GW+C), to disentangle the complex set of experimentally observed satellite features attributed to the generation of plasmons and interband transitions. This combined experiment-theory strategy is able to uncover previously undocumented satellite features, improving our understanding of their direct relationship to tungsten's electronic structure. Furthermore, it lays the groundwork for future studies into tungsten based mixed-metal systems and holds promise for the re-assessment of the photoelectron spectra of other transition and post-transition metals, where similar questions regarding satellite features remain.

Ultrafast optical control over spin and momentum in solids

Employing state-of-the-art first principles calculations, we show that hybrid laser pulses combining linearly and circularly polarized light allow for the excitation of free carriers of chosen spin at the arbitrary k-vector with the Brillouin zone of WSe2. The linear terahertz linear component of the pulse induces adiabatic intra-band evolution, serving to link an arbitrary point k0 to one of the high symmetry K points over a single pulse cycle. The circularly polarized component induces interband transitions at one of the two inequivalent K points that, due to the linear pulse, results at full cycle in laser excited charges at the desired k-vector k0 and not at the high symmetry K point. We, thus, theoretically predict a generalized form of the spin-valley locking in dichalcogenides, opening the way to the preparation of arbitrary momenta and spin specified excited states at ultrafast time scales.

Uniaxial hybridization in the quasi-one-dimensional Kondo lattice CeCo2Ga8

We report the electron dynamics of the recently discovered quasi-one-dimensional Kondo lattice compound ${\mathrm{CeCo}}_{2}{\mathrm{Ga}}_{8}$ by optical spectra measurements using polarized light. For an electric field along the $c$ axis, the high-temperature Drude-like peak is found to split into two distinct components below the coherence temperature ${T}^{*}$, including a narrower Drude-like peak at low frequencies characterizing the emergent heavy electron state with a large mass enhancement, and a midinfrared Lorentz-like peak originating from interband transitions across the hybridization gap. The splitting is, however, absent for an electric field along the perpendicular directions, suggesting that the hybridization only occurs along the $c$ axis, although the high-temperature optical spectra from the background conduction electrons are isotropic. Our observations are in agreement with the resistivity data that show a broad maximum at around 20 K only along the $c$ axis but which keep increasing with a lowering temperature below 50 K along the $ab$ plane. Our work confirms the quasi-one-dimensional nature of the heavy electron physics in ${\mathrm{CeCo}}_{2}{\mathrm{Ga}}_{8}$ and establishes a direct correspondence between the bulk transport properties and microscopic electronic structures. It also pushes the previous observation of anisotropic coherence temperature in many heavy fermion compounds to an extreme situation and reveals the importance of intersite magnetic correlations beyond the local Kondo picture.

3D Dirac Semimetal Supported Tunable TE Modes

AbstractThe tunable propagation properties of transverse electric (TE) plasmonic modes have been investigated in the mid‐IR spectral region by using air‐Dirac semimetal (DSM)‐Si asymmetric structures, including the effects of Fermi levels, temperatures, and scattering times. The results manifest that on the condition that the Fermi level of DSM layer is not very large (2|μc|<ℏω) and temperature is relatively high (about 100 K), the proposed DSM asymmetric structure supports obvious TE modes, which is quite different from the graphene symmetric structure limited to low liquid helium temperature. The transition frequency increases with Fermi level, the peak position of the real part and the saturation region of the imaginary part of effective index indicates an obvious blue shift. As temperature decreases, the contribution of interband transition becomes stronger, the 3D DSM indicates better dielectric properties and sustains stable TE mode. If scattering time is shorter than 0.1 ps, its effects on the propagation properties are very strong, resulting in a redshift of the real part of effective index. The results are very helpful to understand the tunable properties of DSM TE structures and aid the design of novel plasmonic devices, such as polarizers, modulators, and filters.

Mechanistic insight into deep holes from interband transitions in Palladium nanoparticle photocatalysts

SummaryUtilizing hot electrons generated from localized surface plasmon resonance is of widespread interest in the photocatalysis of metallic nanoparticles. However, hot holes, especially generated from interband transitions, have not been fully explored for photocatalysis yet. In this study, a photocatalyzed Suzuki-Miyaura reaction using mesoporous Pd nanoparticle photocatalyst served as a model to study the role of hot holes. Quantum yields of the photocatalysts increase under shorter wavelength excitations and correlate to “deeper” energy of the holes from the Fermi level. This work suggests that deeper holes in the d-band catalyze the oxidative addition of aryl halide R-X onto Pd0 at the nanoparticles' surface to form R-PdII-X complex, thus accelerating the rate-determining step of the catalytic cycle. The hot electrons do not play a decisive role. In the future, catalytic mechanisms induced by deep holes should deserve as much attention as the well-known hot electron transfer mechanism.

Theoretical studies on optical absorption in twisted bilayer graphene under vertical electric field

The interlayer twist angle is an important parameter that can tune the physical properties of graphene in a wide wavelength range. In this paper, we employ an effective continuum model to calculate the band structure of twisted bilayer graphene with different twist angles in the presence and absence of vertical electric field. Based on the transition rate of the electron-photon interaction, we calculate and simulate the optical absorption spectra caused by the interband and intraband transitions around the van Hove singularities. The calculation results show that the optical absorption caused by the interband transitions occurs in the wavelength range from visible light to near-infrared while it appears in far-infrared for intraband transitions. The optical absorption coefficient of the intra-band transitions is almost two orders of magnitude larger than that of inter-band transitions. In the absence of an external electric field, as the twist angle increases, the absorption peak of the inter band transition moves from the infrared light band to the visible light band, but the resonant peak position of its intra-band transition does not change. At the same time, the absorption coefficient values corresponding to the above two transitions will increase. When an electric field is applied perpendicular to the twisted bilayer graphene, the symmetry of the initial band structure of bilayer graphene is destroyed, which results in the splitting of the absorption peaks associated the with interband transitions, and the distance between the two splitting peaks increases with the electric field intensity increasing; while the position and amplitude of the absorption peak associated with the intraband transition are completely unaffected by the applied electric field. The theoretical calculation results in this paper can provide the theoretical guidance for further applying twisted graphene to optoelectronic devices such as tunable dual-band filters.

Chiral biosensing at both interband transition and plasmonic extinction regions using twisted-stacked nanowire arrays.

Chiral metal nanostructures that exhibit strong chiroptical properties and enhanced light-matter interactions have recently attracted great interest due to their potential applications including chiral sensing and asymmetric synthesis. Most studies in this field focused on chiral sensing using circular dichroism (CD) responses at the plasmonic extinction region. In comparison, little is known about their CD responses at interband transition regions and their utility in chiral biosensing. Herein, we constructed a series of twisted-stacked silver nanowire arrays (TNAs) featuring CD signals at both the interband transition and plasmonic extinction regions and that are independently controllable. These TNAs are highly sensitive towards protein secondary structures. Proteins containing more β-sheets are more sensitive toward strong chiral plasmonic fields, whereas proteins rich in α-helices tend to generate larger CD shifts at the interband transition region of TNAs. The mutually independent optical activities at the interband transition and plasmonic extinction regions complement each other, providing more sensitivity and reliability in chiral biosensing.

Regularities of behavior of temperature dependences of resistivity of crystals of solid solutions (Bi-=SUB=-2-x-=/SUB=-Sb-=SUB=-x-=/SUB=-)Te-=SUB=-3-=/SUB=- (0<x<2)

It is shown that the change in the resistivity of crystals of solid solutions (Bi2-xSbx)Te3 (0<x<2) p-type in the temperature range preceding the onset of intrinsic conductivity is due not only to a change in the static relaxation time, the behavior of which in the temperature range from 80 to 300 K is determined mainly by carrier scattering on the vibrations of the crystal lattice, but also by changing the concentration of light holes. The latter is a consequence of the transition of charge carriers from the subzone of heavy holes to the subzone of light holes, as a result of which the concentration of light holes, which make the main contribution to electrical conductivity, decreases with increasing temperature. As a consequence, the plasmon energy proportional to the concentration of charge carriers also decreases with increasing temperature, and the energy of this transition, comparable to the plasmon energy, increases. In this regard, crystals (Bi2-xSbx)Te3 have a specific feature due to the convergence of the plasmon energy and the interband transition, which creates conditions for increasing the intensity of the electron-plasmon interaction. Keywords: thermoelectric materials, bismuth and antimony tellurides, resistivity, interband transitions, plasma of free charge carriers.

Закономерности поведения температурных зависимостей удельного сопротивления кристаллов твердых растворов (Bi-=SUB=-2-x-=/SUB=-Sb-=SUB=-x-=/SUB=-)Te-=SUB=-3-=/SUB=- (0<x<2)

It is shown that the change in the resistivity of crystals of solid solutions (Bi2-xSbx)Te3 (0<x<2) p-type in the temperature range preceding the onset of intrinsic conductivity is due not only to a change in the static relaxation time, the behavior of which in the temperature range from 80 to 300 K is determined mainly by the scattering of carriers on oscillations crystal lattice, but also by changing the concentration of light holes. The latter is a consequence of the transition of charge carriers from the subzone of heavy holes to the subzone of light holes, as a result of which the concentration of light holes, which make the main contribution to electrical conductivity, decreases with increasing temperature. As a consequence, the plasmon energy proportional to the concentration of charge carriers also decreases with increasing temperature, and the energy of this transition, comparable to the plasmon energy, increases. In this regard, crystals (Bi2-xSbx)Te3 have a specific feature due to the convergence of the plasmon energy and the interband transition, which creates conditions for increasing the intensity of the electron-plasmon interaction. Keywords: thermoelectric materials, bismuth and antimony tellurides, resistivity, interband transitions, plasma of free charge carriers.

Ultrafast electronic dynamics and vibrational dynamics of SWCNT under alkylation and annealing

Using ultrashort visible pulse laser, we have performed transient absorption spectroscopy to study the electronic relaxation and vibrational dynamics for three SWCNT samples of non-substituted SWCNT, alkylated SWCNT, and annealed alkylated SWCNT. Estimated lifetimes of ∼100 fs and ∼1 ps can be assigned to intraband and interband relaxation processes, respectively. Compared with the non-substituted sample, the intraband transition rate was found to be 7% smaller for the alkylated sample and nearly equal to the annealed alkylated sample within 2%. It implies that the intraband relaxation rate is not affected by alkylation itself but inhibited by excess chemical modification. Conversely, the interband relaxation rate shows no difference between the three samples. Short-time Fourier transform analysis of the measured traces of pump–probe delay dependent difference absorbance visualizes the change of molecular vibration frequency reflecting the energy transfer from (6,5)-SWCNT to (6,4)-SWCNT and the molecular structure change during the interband transition.

Features in the behavior of temperature dependences of magnetic susceptibility of crystals of Bi-=SUB=-2-=/SUB=-Te-=SUB=-3-=/SUB=--Sb-=SUB=-2-=/SUB=-Te-=SUB=-3-=/SUB=- solid solutions containing from 25 to 70 percent antimony telluride

Temperature dependences of the magnetic susceptibility of p-type crystals of Bi2Te3-Sb2Te3 solid solutions containing from 25 to 70 mol percent antimony telluride were examined in the range from 2 to 400 K. A correlation was found between the features observed in the behavior of temperature dependences of the magnetic susceptibility of crystals containing 60 and 70 mol percent Sb2Te3 and the ratio of the plasmon energy to the energy of the gap between the chemical potential level and the heavy hole subband. An assumption is formulated that in the case of convergence of these energies, the increasing electron--plasmon interaction affects the electron system state and, consequently, the magnitude of magnetic susceptibility. Keywords: bismuth and antimony tellurides, magnetic susceptibility, plasmon, interband transitions, electron--plasmon interaction.