Exciton Absorption Research Articles

High-Figure-of-Merit Biosensing and Enhanced Excitonic Absorption in an MoS2-Integrated Dielectric Metasurface.

Among the transitional metal dichalcogenides (TMDCs), molybdenum disulfide (MoS2) is considered an outstanding candidate for biosensing applications due to its high absorptivity and amenability to ionic current measurements. Dielectric metasurfaces have also emerged as a powerful platform for novel optical biosensing due to their low optical losses and strong near-field enhancements. Once functionalized with TMDCs, dielectric metasurfaces can also provide strong photon-exciton interactions. Here, we theoretically integrated a single layer of MoS2 into a CMOS-compatible asymmetric dielectric metasurface composed of TiO2 meta-atoms with a broken in-plane inversion symmetry on an SiO2 substrate. We numerically show that the designed MoS2-integrated metasurface can function as a high-figure-of-merit (FoM=137.5 RIU-1) van der Waals-based biosensor due to the support of quasi-bound states in the continuum. Moreover, owing to the critical coupling of the magnetic dipole resonances of the metasurface and the A exciton of the single layer of MoS2, one can achieve a 55% enhanced excitonic absorption by this two-port system. Therefore, the proposed design can function as an effective biosensor and is also practical for enhanced excitonic absorption and emission applications.

Accelerated Multi‐Stage Synthesis of Indium Phosphide Quantum Dots in Modular Flow Reactors

AbstractDevelopment and scalable nanomanufacturing of high‐quality heavy metal‐free quantum dots (QDs) with high‐dimensional experimental design spaces still remain a challenge. In this work, a universal flow chemistry framework for accelerated fundamental and applied studies of heavy metal‐free QDs with multi‐stage chemistries is presented. By introducing flexible time‐ and temperature‐to‐distance transformation using modular fluidic blocks, an in‐flow synthetic route of InP QDs with the highest reported first excitonic absorption peak to valley ratio is unveiled with a reaction time one order of magnitude faster than batch reactors. The flexible time‐ and temperature‐to‐distance transformation as an enabling factor for generalization of flow reactors toward the accelerated discovery, development, and nanomanufacturing of high‐quality emerging nanomaterials for next‐generation energy, display, and chemical technologies is discussed.

Excitons and Their Fine Structure in Lead Halide Perovskite Nanocrystals from Atomistic GW/BSE Calculations

Atomistically detailed computational studies of nanocrystals, such as those derived from the promising lead-halide perovskites, are challenging due to the large number of atoms and lack of symmetries to exploit. Here, focusing on methylammonium lead iodide nanocrystals, we combine a real-space tight binding model with the GW approximation to the self-energy and obtain exciton wave functions and absorption spectra via solutions of the associated Bethe–Salpeter equation. We find that the size dependence of carrier confinement, dielectric contrast, electron–hole exchange, and exciton binding energies has a strong impact on the lowest excitation energy, which can be tuned by almost 1 eV over the diameter range of 2–6 nm. Our calculated excitation energies are about 0.2 eV higher than experimentally measured photoluminescence, and they display the same qualitative size dependence. Focusing on the fine structure of the band-edge excitons, we find that the lowest-lying exciton is spectroscopically dark and about 20–30 meV lower in energy than the higher-lying triplet of the bright states whose degeneracy is slightly broken by crystal field effects.

Hysteretic Pressure Dependence of Ca2+ Binding in LH1 Bacterial Membrane Chromoproteins

Much of the thermodynamic parameter values that support life are set by the properties of proteins. While the denaturing effects of pressure and temperature on proteins are well documented, their precise structural nature is rarely revealed. This work investigates the destabilization of multiple Ca2+ binding sites in the cyclic LH1 light-harvesting membrane chromoprotein complexes from two Ca-containing sulfur purple bacteria by hydrostatic high-pressure perturbation spectroscopy. The native (Ca-saturated) and denatured (Ca-depleted) phases of these complexes are well distinguishable by much-shifted bacteriochlorophyll a exciton absorption bands serving as innate optical probes in this study. The pressure-induced denaturation of the complexes related to the failure of the protein Ca-binding pockets and the concomitant breakage of hydrogen bonds between the pigment chromophores and protein environment were found cooperative, involving all or most of the Ca2+ binding sites, but irreversible. The strong hysteresis observed in the spectral and kinetic characteristics of phase transitions along the compression and decompression pathways implies asymmetry in the relevant free energy landscapes and activation free energy distributions. A phase transition pressure equal to about 1.9 kbar was evaluated for the complexes from Thiorhodovibrio strain 970 from the pressure dependence of biphasic kinetics observed in the minutes to 100 h time range.

Extended star graph as a light-harvesting-complex prototype: Excitonic absorption speedup by peripheral energy defect tuning.

We study the quantum dynamics of a photoexcitation uniformly distributed at the periphery of an extended star network (with N_{B} branches of length L_{B}). More specifically, we address here the question of the energy absorption at the core of the network and how this process can be improved (or not) by the inclusion of peripheral defects with a tunable energy amplitude Δ. Our numerical simulations reveal the existence of optimal value of energy defect Δ^{*} which depends on the network architecture. Around this value, the absorption process presents a strong speedup (i.e., reduction of the absorption time) provided that L_{B}≤L_{B}^{*} with L_{B}^{*}≈12.5/ln(N_{B}). Analytical and numerical developments are then conducted to interpret this feature. We show that the origin of this speedup takes place in the hybridization of two upper-band excitonic eigenstates. This hybridization is important when L_{B}≤L_{B}^{*} and vanishes almost totally when L_{B}>L_{B}^{*}. These structural rules we draw here could represent a potential guide for the practical design of molecular nanonetwork dedicated to the realization of efficient photoexcitation absorption.

Two-Dimensional Layered Organic-Inorganic Hybrid Perovskite Thin-Film Fabrication by Langmuir-Blodgett and Intercalation Techniques.

We demonstrate the formation of a well-organized thin film of two-dimensional (2D) layered (C18H37NH3)2PbI4 hybrid perovskite by immersing octadecyl amine (ODA) Langmuir-Blodgett (LB) films in an aqueous solution of PbI2/HI. The immersed films exhibit a sharp absorption band at 486 nm (2.55 eV), which is assigned to the excitonic absorption. The film exhibits a bright green emission under ultraviolet light at room temperature. The photoluminescence spectrum has a distinct peak at 497 nm (2.49 eV) and is a mirror image of the absorption spectrum. X-ray diffraction (XRD) analyses reveal that the film has a bilayer-like structure with a d-spacing of 6.4 nm, which is equal to that of a (C18H37NH3)2PbI4 perovskite single crystal with a quantum well (QW) structure. Only intense peaks of the (0 0 l) (l = 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, and 24) reflections are observed in the out-of-plane XRD pattern, indicating that the c axis is vertically oriented with respect to the substrate surface, and the orientational order is remarkably high. Fourier transform infrared spectroscopy reveals that the ODA molecules are protonated in the PbI2/HI solution. These results suggest that the nitrogen atoms of the ODA molecules in the film are protonated in the PbI2/HI solution, and then, inorganic layers of the PbI6 octahedra are intercalated in the alkyl ammonium film to neutralize the positive charge and form a QW structure. Fluorescence microscopy observation reveals that the 2D layered (C18H37NH3)2PbI4 film has a relatively uniform surface, reflecting the well-organized layered structure of the base material (ODA LB film). Because the intercalation process can be applied to various metal cations and halogen anions, we believe that the proposed technique will aid in the development of highly efficient 2D layered organic-inorganic hybrid perovskite materials.

Synchrotron-Excited Luminescence and Converting of Defects and Quantum Dots in Modified Silica Films

Optically active defects in modified silicon oxide films on a silicon substrate have been studied by low-temperature photoluminescence (PL) spectroscopy using excitation by synchrotron radiation in the vacuum ultraviolet region. Films of dry thermal silicon oxide obtained by treatment of stoichiometric SiO2 in hydrogen plasma, films of wet thermal silicon oxide, and films with low dielectric constant (so called "low-k" dielectrics) have been studied. Investigations of various type SiO2 films detect the PL centers, which can be conditionally divided into two groups. The first group includes intrinsic point defects, the different variants of oxygen-deficient centers (ODCs). The second group contains centers like the spatially confined excitons in silicon quantum dots (SiQDs). It is shown that SiQDs differ in spectral characteristics, are formed in different ways, due to the transformation and clustering of point defects such as E'-centers, ODC(I) and ODC(II). In this case, the size and spectral properties of quantum dots depend on the mechanism of their formation. Schemes for the conversion of these defects are proposed.It was found that for wet films and nonstoichiometric SiOx films treated in hydrogen plasma, "metastable" centers ODC(I) and ODC(II) arise only under the action of synchrotron radiation in the region of interband or exciton absorption and then they decay because of radiative relaxation. The absence of stable ODC centers in these films is due to their clustering and the formation of SiQDs. In "low-k" mesoporous films, on the contrary, silicon quantum dots SiQDs are not formed, but ODC(I) and ODC(II) centers are clearly manifested. A scheme of electronic transitions upon indirect excitation of silicon quantum dots through exciton states of the SiO2 matrix is considered. The presented results demonstrate the possibility of controlling the optical properties of silicon thin-film structures by creating in them both oxygen-deficient point defects and quantum dots of various sizes.

High anisotropy in titanium trisulfide monolayer: Ultrahigh carrier mobilities and large excitonic absorption

Two-dimensional titanium trisulfide (TiS3) attracts much attention due to its high carrier mobilities and anisotropic physical properties. In this study, based on the ab-initio method, the vibrational, carrier transport and excitonic properties for TiS3 monolayer are investigated in detail. The results show that highly anisotropic structure of monolayer TiS3 leads to its anisotropic physical properties. The hole mobilities are larger than electron mobilities, which can be further enhanced by strain engineering. A strong absorption peak at 0.59 eV for light polarization along b direction is observed within the quasi-particle bandgap, which is 15 times larger than that at 0.98 eV for light polarization along a direction. The absorption peaks are contributed dominantly by bright excitons whose binding energies are 0.43 eV and 0.82 eV along a and b directions, both of which are delocalized Wannier-Mott types. The ultrahigh and flexibly strain-engineering transport properties and the significant excitonic effects of anisotropic TiS3 monolayer make possible applications in flexible electronic and optoelectronic devices.

Biosynthesis and Characterization of ZnO Nanoparticles by using Leaf Extractionof Allium Calocephalum Wendelbow Plant

Zinc Oxide (ZnO) nanoparticles (NPs) were biosynthesized by using a leaf extract of Allium Calocephalum Wendelbow (ACW) plant. The impact of various zinc salts on the characteristic properties of synthesized ZnO NPs and which salt is more suitable for the synthesis of ZnO NPs were investigated. The used different zinc salts were Zinc Nitrate hexahydrate (ZNH), Zinc Acetate (ZA), and Zinc Chloride (ZC). The properties of synthesized NPs were studied using a variety of characterization techniques. The FTIR analysis and UV-Visible spectroscopy of ACW plant leaf extract proved to suggest this extract is a better choice for the green synthesis of ZnO NPs. The UV–Visible spectra of leaf extract showed two distinct absorption peaks in the region of 262 nm and 350 nm at ambient temperature. The FE-SEM analysis revealed a spherical form of ZnO NPs having an (average) mean size in the range of (21.61-63.12) nm. XRD results revealed the formation of a hexagonal wurtzite structure. The crystal size of produced ZnO NPs along the (002) diffraction peak was in the range of (16.91 to 28.19) nm for different Zinc salt. The EDX analysis shows that the produced ZnO NPs are very pure. The FTIR results displayed there is no obvious peak in the monitoring range, suggesting that the ZnO NPs created via using ACW Leaf Extract. The UV-Vis. results of ZnO NPs showed that the sharpness in this region, exciton absorption, and UV absorption edge were found (381-397) nm, which is corresponding to the Eg of the ZnO NPs, and the investigated Eg of ZnO NPs was in the range of (3.12-3.25) eV. All obtained results of the fabricated ZnO NPs, the ZnO NPs synthesized from zinc salt which is the zinc Nitrate hexahydrate showed very high-quality and improved compared to ZnO NPs synthesized from other zinc salts.

Decoupling excitons behavior of two-dimensional Ruddlesden-Popper PEA2PbI4 nanosheets

Two-dimensional (2D) Ruddlesden-Popper hybrid organic-inorganic perovskites are the most promising candidates for highly efficient optoelectronic devices due to their remarkable exciton characteristics, adjustable band structure and long-term environmental stability. However, excitons emission behavior of 2D Ruddlesden-Popper perovskites is still unclear. Here we theoretically-experimentally decoupled the excitons behavior in 2D Ruddlesden-Popper PEA2PbI4 nanosheets. Theoretically, the Elliott theory was used to separate exciton and band edge absorptions of PEA2PbI4 nanosheets and determine an exciton binding energy of about 274 meV. Experimentally, spectral-dependent photoluminescence lifetime and excitation powder density-dependent photoluminescence spectra revealed that a distinct photoluminescence tail emission should be derived from localized excitons with lower emission energy than free excitons. Further, the luminescent component of the free excitons and localized excitons with their corresponding densities of states was theoretically decoupled by line-shape analysis of PEA2PbI4 nanosheets photoluminescence spectra, which is consistent with the experimental results demonstrating the existence of localized excitons. Hence, our work provides understanding on the exciton behavior and luminescence mechanism of 2D Ruddlesden-Popper perovskites, which would benefit the prediction of high-performance excitonic devices based on 2D Ruddlesden-Popper perovskites.

Fabrication of ZnO nanorods decorated Au nanoparticles by hydrothermal and magnetron sputter techniques

In this study, ZnO nanorods decorated with gold (Au) nanoparticles with the desired size parameters were fabricated using a simple, low-cost, and highly efficient method. The ZnO nanorod structure was fabricated using a hydrothermal method on a ZnO seed layer with hydrothermal solution concentrations varying from 20 mM to 90 mM. Au nanoparticles were coated on the ZnO nanorod structure by magnetron sputtering with a sputtering time from the 40s to 70 s. The characteristics of the fabricated samples were investigated through SEM images and optical absorption spectroscopy. The results show that the fabricated ZnO nanorods are relatively uniform, with a cylindrical shape and hexagonal cross-section when the solution concentration is less than 70 mM. Au nanoparticles were attached to the surface of the ZnO nanorods with average sizes of 30-50 nm. The optical absorption spectroscopy results showed that the ZnO nanorods’ absorption edge appeared at a wavelength of approximately 395 nm. In addition, the exciton absorption peak of Au nanoparticles was between 550 nm and 600 nm and there was a shift towards shorter wavelengths as the size of the Au nanoparticle decreased. This result opens up potential applications of this material such as increasing photocatalytic efficiency and its, use in photonic devices, etc.

Osmotic-Enhanced-Photoelectrochemical Hydrogen Production Based on Nanofluidics

Osmotic-Enhanced-Photoelectrochemical Hydrogen Production Based on Nanofluidics

Hot-carrier tunable abnormal nonlinear absorption conversion in quasi-2D perovskite

Controlling the high-power laser transmittance is built on the diverse manipulation of multiple nonlinear absorption (NLA) processes in the nonlinear optical (NLO) materials. According to standard saturable absorption (SA) and reverse saturable absorption (RSA) model adapted for traditional semiconductor materials, the coexistence of SA and RSA will result in SA induced transparency at low laser intensity, yet switch to RSA with pump fluence increasing. Here, we observed, in contrast, an unusual RSA to SA conversion in quasi-two-dimensional (2D) perovskite film with a low threshold around 2.6 GW cm−2. With ultrafast transient absorption (TA) spectra measurement, such abnormal NLA is attributed to the competition between excitonic absorption enhancement and non-thermalized carrier induced bleaching. TA singularity from non-thermalized “Fermi Sea” is observed in quasi-2D perovskite film, indicating an ultrafast carrier thermalization within 100 fs. Moreover, the comparative study between the 2D and 3D perovskites uncovers the crucial role of hot-carrier effect to tune the NLA response. The ultrafast carrier cooling of quasi-2D perovskite is pointed out as an important factor to realize such abnormal NLA conversion process. These results provide fresh insights into the NLA mechanisms in low-dimensional perovskites, which may pave a promising way to diversify the NLO material applications.

Bandgap engineering of CdTe/CdSe rod-shaped core/shell and CdTeSe ellipsoidal alloy quantum dots with tunable and intense emission

In this work, core CdTe, core-shell CdTe/CdSe and alloy CdTeSe QDs are synthesized by aqueous solvent. TEM visually shows the rod-shaped core-shell structure with a transverse to longitudinal ratio of 6.5 and the ellipsoidal alloy structure with different crystal planes competing for growth. The XRD and Raman spectra of the three QDs illustrate the characteristics of the internal lattice structure. With the increase of reaction time, CdTe emission maximum (EM) shows a red shift of 84 nm. Stokes shift changes little, and full width half maximum (FWHM) increases slightly. Compared with the CdTe, the red shift of CdTe/CdSe EM is slowly first and then faster, with a total shift of 125 nm. Stokes shift increases obviously. The initial FWHM is larger than CdTe and the widening speed is gradually accelerated. The initial EM of CdTeSe is red shifted compared with CdTe, and the total red shift of EM is 72 nm. Stokes shift increases obviously. The initial FWHM is significantly wider than CdTe. FWHM first decreases and then increases with reaction time. CdTe shows obvious band edge exciton absorbance of photoluminescence excitation (PLE) spectrum between 356 nm and 400 nm. CdTe/CdSe shows double band edge exciton absorption at 354–373 nm and 401–436 nm due to wave function separated of core-shell structure. The PLE spectrum of CdTeSe shows an obvious upward trend at 306–373 nm due to high energy level nonradiation. The average lifetime of CdTe/CdSe increases by 53 ns compared with CdTe due to the spatially separated electrons and holes. The defect state of CdTeSe increases the proportion of fast delay, which is 2 ns shorter than the average lifetime of CdTe. The transition mechanism of core-shell indirect band gap and alloy direct band gap is explained according to the valence bands of each element shown by XPS and the simulation of energy band density of states. The thickness shell and the doping elements can be adjusted to provide rich variation in the optical properties. This provides a more definite direction for the applications of cadmium chalcogenide in the fields of photoelectric devices and biology. • Type I and type II transformation through Rod-shaped core/shell and ellipsoidal alloy QDs. • Tunable emission achieved by adjusting morphology, lattice structure and valence bond connection. • Precise and rich regulation of band gap engineering. • Providing a more definite direction for the applications of cadmium chalcogenide QDs.

Effects of external perturbations on light absorption by light/heavy hole excitons in a semi-parabolic quantum well

The effects of electric, magnetic, and intense terahertz laser fields on optoelectronic properties are investigated in a GaAs/AlxGa1-xAs semi-parabolic quantum well. The binding energy, exciton light absorption coefficients, interband transition energies, and dipole elements in the matrix are studied within the structure of the parabolic band and effective-mass approximations. The transition energy difference between the ground-state electron energy level and the ground-state heavy hole (light hole) energy level and matrix elements are carried out as functions of external perturbations with a constant well size. The results exhibit that the exciton binding energies of heavy and light holes reduce as long as the laser dressing parameter is larger. The optical absorption of light hole exciton in the system is stronger than the heavy hole exciton. And the excitonic energies, transition energies, and absorption coefficients are much more sensitive and controlled by changing the external perturbations.

The suppressing of excitonic effects in Cu-chalcogenides for solar cell applications

Excitonic effect can usually significantly enhance the optical absorption coefficient in wide-gap insulators. Cu-chalcogenides are widely used as thin-film solar cell absorbers due to their high light utilization efficiency. Theoretical investigations on the excitonic absorption in Cu-chalcogenides are rare because of the complicated two-particle nature of an exciton. Here, we theoretically evaluate the continuum-excitonic effect in CuGaS2 and CuInS2 by solving the Bethe-Salpeter equation. Despite the large bandgap of 2.4 eV for CuGaS2, the excitonic effect induces moderate absorption enhancement when compared with the single-particle scenario. Moreover, the experimental results are better reproduced using the latter approach, indicating that the intrinsic excitonic effect is nearly suppressed in the practical experiments. The moderate intrinsic excitonic effect and its further suppressing in the experiments are explained by strong electronic screening effects in these materials. Justification of the simple single-particle approach would be convenient for future theoretical studies, which otherwise require more complex methods with more expensive computational efforts.

Plasmon-Induced Enhanced Light Emission and Ultrafast Carrier Dynamics in a Tunable Molybdenum Disulfide-Gallium Nitride Heterostructure.

The effect of localized plasmon on the photoemission and absorption in hybrid molybdenum disulfide-Gallium nitride (MoS2-GaN) heterostructure has been studied. Localized plasmon induced by platinum nanoparticles was resonantly coupled to the bandedge states of GaN to enhance the UV emission from the hybrid semiconductor system. The presence of the platinum nanoparticles also increases the effective absorption and the transient gain of the excitonic absorption in MoS2. Localized plasmons were also resonantly coupled to the defect states of GaN and the exciton states using gold nanoparticles. The transfer of hot carriers from Au plasmons to the conduction band of MoS2 and the trapping of excited carriers in MoS2 within GaN defects results in transient plasmon-induced transparency at ~1.28 ps. Selective optical excitation of the specific resonances in the presence of the localized plasmons can be used to tune the absorption or emission properties of this layered 2D-3D semiconductor material system.

Spheroidal Cesium Lead Chloride-Bromide Quantum Dots and a Fast Determination of Their Size and Halide Content.

Lead halide perovskite (LHP) quantum dots (QDs), with their bright and narrow emission, are promising candidates for LEDs, lasers, and quantum light sources. However, current methods to synthesize monodisperse CsPb(Cl:Br)3 and CsPbCl3 QDs exhibiting multiple sharp absorption resonances are not as well developed compared to CsPbBr3. Furthermore, both quantum confinement and the halide ratio in CsPb(Cl:Br)3 QDs strongly influence the bandgap, making it impossible to optically determine their size. In this work, monodisperse spheroidal CsPb(Cl:Br)3 QDs are synthesized in the 4-10 nm range, at any Cl:Br ratio, with up to five excitonic absorption transitions. Furthermore, in situ spectroscopy was used to cross-correlate the size and composition of these QDs directly to the energy of the first two excitonic absorption transitions. This work therefore provides not only a method for monodisperse CsPb(Cl:Br)3 QDs but also a protocol to determine their size, concentration, and halide ratio, circumventing conventional expensive and time-consuming techniques.

Giant quantum anharmonic effects on the stability, vibrational and optical properties of cyclo[4n+2]carbon

Cyclo[ 4 n + 2 ]carbons are sp-bonded carbon rings in which Hückel rule predicts a fully symmetric structure that is, however, in competition with the second order Jahn–Teller (Peierls) distortion. This picture, however, neglects the crucial role played by nuclear quantum effects. Here we investigate the magnitude of nuclear quantum effects on the stability, vibrational and optical properties of cyclo[ 4 n + 2 ]carbons ( n = 1 , 2 , 3 , 4 ) in vacuum. The quantum structural minimization reduces the energy separation between the different isomers and determines that the most stable C 14 isomer is the cumulenic one, setting the transition from the polyyenic to the cumulenic form at n = 3 (at odd with the classical structural optimization setting the transition at n = 2 ). Moreover, the quantum anharmonic effects generate very large frequency shifts, linewidth broadenings and satellites in the phonon spectral weight hindering any possible interpretation based on the non-interacting harmonic spectrum. The optical absorbance is completely reshaped by quantum anharmonic vibrations with redshifts ranging from 0.4 to 1.0 eV in the first excitonic absorption with respect to the static ionic picture. Finally, we also determine the crystal structure of C 18 on a NaCl bilayer to be a buckled polyynic phase in agreement with recent experimental findings. Our work outlines the crucial role of nuclear quantum effects in the understanding of carbon molecular systems.

Influence of external fields on the exciton binding energy and interband absorption in a double inverse parabolic quantum well

Ground-state binding energy and optical absorption coefficient of exciton in a symmetric double inverse parabolic quantum well system in the presence of external electric and magnetic fields are investigated theoretically. Numerical calculations have been carried out with the variational technique considering the single-band effective mass approximations. Results reveal that the excitonic binding energy and interband optical absorption depend on significantly on the geometrical and structural parameters defining carriers’ double inverse parabolic confinement potential, the aluminum concentration at the well center, the electric field, and the magnetic field. The exciton binding energy is a decreasing (increasing) function of the electric field (magnetic field). The increase in aluminum concentration creates a blueshift at the resonance peak positions. As a result, the tunability of the excitonic effects is expected to be of importance in developing stable and high-efficiency nanoscale excitonic optoelectronic devices . • Analyses of double inverse parabolic quantum well. • Exciton binding energy. • Interband transitions. • Electric and magnetic field effects.