Absorption Peak Position Research Articles

Tailor-made core/shell/shell-like Fe3O4@SiO2@PPy composites with prominent microwave absorption performance

Well-ordered core/shell/shell-like Fe3O4@SiO2@PPy microspheres with prominent electromagnetic microwave absorption performance were successfully obtained by microemulsion polymerization method. The morphology, crystal form, functional group, magnetism and microwave absorption properties were investigated. The tailored shell thickness can be tuned from 20 to 60 nm (including SiO2 layer) via changing the molar ratio of Fe3O4@SiO2 to Pyrrole. The absorption peak positions gradually move to low frequency with increment of coating thicknesses. The minimum RL reached −40.9 dB at 6 GHz with the coating thickness of 5 mm. The effective absorption bandwidth could reach 6.88 GHz from 11.12 to 18 GHz, completely covering the whole K (12–18 GHz) band. Meanwhile, it showed excellent wave absorption in 4.4–18 GHz with different coating thickness. The perfect electromagnetic wave absorption properties could be attributed to the dielectric loss from the special core/shell/shell microstructure and natural resonance from the Fe3O4 microspheres. These newly prepared Fe3O4@SiO2@PPy microspheres might be considered as prospective nominees for highly efficient microwave absorption materials with tailored nanostructures.

Plasmonic Metasurface Absorber Based on Electro-Optic Substrate for Energy Harvesting.

A highly efficient and broad light absorber capable of wide-angle absorption in the visible and near infrared range is presented and numerically investigated for energy harvesting in a simple geometry. According to the calculated results, the proposed device has a peak absorption level of about 99.95%. The actual absorption efficiency is 76.35%, which is approaching that of complex multilayer absorbers with 88 layers working in the wavelength range of 300 nm to 2000 nm. The electro-optic material has the potential of shifting the absorption peak position, compensating fabrication errors and thus reducing the fabrication technique difficulties. Also, the high electro-optic tunability can be used for filters, infrared detection, and imaging applications. More directly, the proposed absorber can be potentially deployed in solar cells and solar thermals.

Perfect Absorption in One–Dimensional Photonic Crystal with Graphene‐Dielectric Hyperbolic Metamaterials

The absorption characteristics of one‐dimensional photonic crystals (1DPC) consisting of an ordinary dielectric and graphene–dielectric hyperbolic metamaterials in terahertz (THz) are theoretically analyzed and numerically simulated using the transfer matrix method. The perfect absorption peaks can be achieved in the structure designed in our research. In the 1DPC, the dielectric layers are embedded into graphene layers and the optical axis of the graphene–dielectric hyperbolic metamaterials is normal to the graphene layers. The graphene–dielectric layers are seen as anisotropic effective medium and possess the properties of hyperbolic dispersion in THz frequency band. The locations of perfect absorptance peaks are dependent on chemical potential, the thicknesses of dielectric layers and the inclined angle. The positions of absorption peaks are blue‐shifted with increasing the chemical potential and red‐shifted by increasing the thicknesses of the dielectric layers. We testified that the orientation of the optical axis of the graphene–dielectric layers has prominent effect on the locations of the perfect absorption peaks. Moreover, multiband absorption peaks can be obtained by introducing more structure as given in our research with different parameters. The controllable perfect absorption achieved at the far infrared ray (FIR) frequencies could be applied in the design of multichannel perfect absorption filters.

N‐vinylpyrrolidone copolymers decorated with silver nanoparticles for biomedical applications

New silver nanocomposites (AgNCs) based on copolymers of N‐vinylpyrrolidone with N,N‐diallyl‐N′‐acetylhydrazine, poly (VP‐DAAH), and 3‐methacryloxytetrahydrothiophenedioxide‐1,1, poly (VP‐MTT), have been developed. The reduction of silver ions into silver nanoparticles was achieved using NaBH4 as reducing agent. UV‐spectroscopy, an atomic force microscopy and scanning electron microscopy techniques were used to characterize the formation of silver nanoparticles in copolymers. The average silver particle size ranged from 12 to 13 nm, with the corresponding UV–vis absorption peak position at 400 to 410 nm. The AgNCs have exhibited significant cytotoxic activity towards rhabdomyosarcoma and melanoma line cells. The AgNC based on poly (VP‐MTT) completely inhibited bacterial growth, including both gram‐positive and gram‐negative bacteria.

Investigation of multiband plasmonic metamaterial perfect absorbers based on graphene ribbons by the phase-coupled method

We develop an original phase-coupled method to realize multispectral metamaterial near-unity absorbers based on spatially separated graphene ribbon arrays with mid-infrared plasmonic resonances. The results both from the coupled-mode theory and finite-difference time-domain simulations reveal that in addition to the single-band absorption enabled by the bi-layer identical ribbon arrays, the outstanding dual-band perfect absorption is observed with the change in the phase between bi-layer ribbons only by varying the spacer thickness. The spectral positions of absorption peaks are tuned handily by small changes in ribbon widths and chemical potentials of graphene. Moreover, the triple-band absorber is achieved handily by the same principle and such absorbers are robust for nor-normal incident angles. The transfer matrix method is also utilized to uncover further the underlying physics of the phased-coupled-induced multispectral absorbers. Theoretical analysis are in excellent agreement with numerical calculations. The phase-coupled method thus provides new opportunities for obtaining multi-channel metamaterial perfect absorbers.

Polarization-Controlled and Flexible Single-/Penta-Band Metamaterial Absorber.

In this paper, a polarization-controlled and flexible metamaterial absorber made of a set of wires etched on ultrathin teflon dielectric substrate is proposed. The simulation results showed that the proposed absorber achieved single-band absorptivity of 99.8% at 6.64 GHz for the TM (transverse magnetic) polarization wave and penta-band absorptivity of more than 99% at 11.68 GHz, 13.58 GHz, 15.48 GHz, 17.38 GHz, and 19.28 GHz for the TE (transverse electric) polarization waves. Moreover, each absorption peak had very narrow relative bandwidth and the position of penta-band absorption peaks could be adjusted by changing the length of the corresponding wire or selecting suitable substrate material according to actual requirements, because each wire can independently respond to electromagnetic (EM) waves. Furthermore, the surface current distributions corresponding to each absorption peak were studied to demonstrate the absorption mechanism. The absorption properties of the proposed structure with different bending radii and under different incident angles of the EM waves were investigated, showing good flexibility and incident angle-insensitive properties. In addition, the simulation results were confirmed by measuring a fabricated prototype. The proposed absorber may have useful applications in polarizers, sensors, bolometers, polarization detectors, etc.

Luminescence kinetics of CdSe-based quantum dots in toluene

Luminescence kinetics of quantum dots (CdSe-CdS core-shell nanoparticles) in toluene were studied. Quantum dots were obtained through colloidal synthesis method in aqueous-organic media with the average size estimated as 2 nm and 2.9 nm from the position of the exciton absorption peak. A biexponential luminescence decays with time constants of a few and tens of picoseconds were observed via up-conversion technique

Twisted angle effects in the absorption spectra of carbon nanotube

The spectral characteristics of carbon nanotubes is essential for the development of optoelectronic components and sensors with application in many fields. The linear polarizability absorption spectra of the carbon nanotube with a twisted angle have been calculated by using the tight-binding model and sum-over-state method. First, a structural model of carbon nanotubes based on PPP(Pariser-Parr_Pople) was established, then a certain carbon atom was moved to a position where the carbon nanotubes are angled, and the torsional carbon nanotube structure model was calculated based on the change in bond length. Finally, the absorption spectrum at different twisted angles based on SOS(Sum-over-State) was calculated, and then the detailed analysis was made. It is found that the twisted angle has real impact on the absorption spectra of different types carbon nanotubes. The torsional angle leads to changes in the position and number of absorption peaks of the carbon nanotubes. Its influence on absorption spectra of different types of carbon nanotubes is different, but mainly concentrated in the low frequency region. Exactly, altering the twisted angle has an important effect on the absorption spectra of the carbon nanotube. This work offers a guidance of fabricating carbon nanotubes in the applications of photonic device in many fields.

Variation of optical spectra of water clusters with size from many-body Green's function theory.

Water clusters are an important species in the environment and atmosphere and take part in various chemical and biological reactions. How their optical properties vary with size is still an open question. Using the GW method and Bethe-Salpeter equation within the ab initio many-body Green's function theory, we study the electronic excitations in a series of water clusters (H2O)n with n = 1-48. We find that their absorption peaks blueshift with increasing cluster size due to the reducing electron-hole binding energy which arises from the enhanced electronic screening and gradually delocalized excitonic spatial distribution. The position of the first absorption peak has a close relation to the average number of hydrogen bonds per molecule. Off-diagonal matrix elements of the self-energy operator have pronounced effects on the unoccupied electronic levels and optical absorption for small clusters with n ≤ 10 when using density functional theory as the starting point for GW calculations. Although the optical absorption is predominated by delocalized excitons, highly localized excitons on a single water molecule are always present on the cluster surface in the vicinity of the absorption edge. These localized excitons may facilitate the photodissociation of water molecules. This can provide inspiration on the excited-state dynamics and photolysis in water clusters.

Dynamically tunable dendritic graphene-based absorber with thermal stability at infrared regions

The infrared polarization-insensitive absorber, which is composed of dendritic metal, graphene layer, silicon dioxides layer, gallium arsenide substrate, and metal plate, is investigated theoretically and numerically. The tunability can be realized by loading a graphene layer into the structure. The position of absorption peak can be tuned by manipulating the graphene’s Fermi energy. Compared with the previously reported graphene-based absorbers, the system has the advantage of temperature-independent high absorption. The results indicate that the proposed absorber can be used in the applications of the refractive index sensor with a sensitivity of 587.8 nm/refractive index unit and temperature-insensitive infrared absorber.

Experimental and theoretical study of fingerprint spectra of 2-(4-fluorophenyl)benzimidazole and 2-(4-chlorophenyl)benzimidazole in terahertz range

Due to wide variety of biological and pharmacological activities of benzimidazole derivatives, the terahertz fingerprint spectra of 2-(4-fluorophenyl)benzimidazole and 2-(4-chlorophenyl)benzimidazole are researched by employing terahertz time-domain spectroscopy and density functional theory systematically. Although the substituent at the para position on the benzene moiety are the same family elements (F, Cl) in the periodic table, both experimental and theoretical results show that there are substantial differences in their fingerprint spectra in the range of 0.2–2.5 THz, such as the amount, amplitude and frequency position of absorption peaks. The validity of these results was confirmed by isolated-molecule and solid-state calculations based on density functional theory. The possible reasons of these differences are different intramolecular hyperconjugative interactions, different elongation of the corresponding bond lengths, different HOMO-LUMO energy gaps, as well as different atomic motions within in the unit cell owing to the electron-withdrawing effects of different halogen atoms at the para position on the benzene moiety. These results indicate the importance of this spectral range as a conformational fingerprint region where even minor changes in the molecular configuration lead to major differences in its THz absorption.

Theory of a Carbon-Nanotube Polarization Switch

Recently, it was suggested that the polarization dependence of light absorption to a single-walled carbon nanotube is altered by carrier doping. We specify theoretically the doping level at which the polarization anisotropy is reversed by plasmon excitation. The plasmon energy is mainly determined by the diameter of a nanotube, because pseudospin makes the energy independent of the details of the band structure. We find that the effect of doping on the Coulomb interaction appears through the screened exchange energy, which can be observed as changes in the absorption peak positions. Our results strongly suggest the possibility that oriented nanotubes function as a polarization switch.

Terahertz absorption in graphite nanoplatelets/polylactic acid composites

The electromagnetic properties of composite materials based on poly(lactic) acid (PLA) filled with graphite nanoplatelets (GNP) were investigated in the microwave (26–37 GHz) and terahertz (0.2–1 THz) frequency ranges. The maximum of the imaginary part of the dielectric permittivity was observed close to 0.6 THz for composites with 1.5 and 3 wt.% of GNP. The experimental data of complex dielectric permittivity of GNP/PLA composites was modelled using the Maxwell-Garnett theory. The effects of fine dispersion, agglomeration, and percolation in GNP-based composites on its electromagnetic constitutive parameters, presence, and position of THz absorption peak are discussed on the basis of the modeling results and experimental data. The unique combination of conductive and geometrical parameters of GNP embedded into the PLA matrix below the percolation threshold allow us to obtain the THz-absorptive material, which may be effectively used as a 3D-printing filament.

Size and edge dependence of two-photon absorption in rectangular graphene quantum dots.

The size and edge-dependence of two-photon absorption (TPA) for rectangular graphene quantum dots (GQDs) is investigated theoretically in the framework of Dirac equation under hard wall boundary conditions. The TPA cross section associated with interband transitions around K point is derived and the transition selection rules are obtained. Results reveal that when the size of zigzag-edge M = 3M0 ± 1 (M0 is an integer), the GQD exhibits a semiconductor while for M = 3M0 it is metallic. For semiconducting rectangular GQDs, TPA is tuned by the sizes of both edges in GQDs and the armchair-edge dimension contributes more to TPA. While for metallic rectangular GQDs, zigzag-edge dimension affects TPA little and the position of absorption peak and the magnitude of the TPA coefficient are determined by the size of armchair-edge and the resonant enhancement occurs.

Establishment of the Mathematical Model for PMI Estimation Using FTIR Spectroscopy and Data Mining Method

To analyse the relationship between Fourier transform infrared （FTIR） spectrum of rat's spleen tissue and postmortem interval （PMI） for PMI estimation using FTIR spectroscopy combined with data mining method. Rats were sacrificed by cervical dislocation, and the cadavers were placed at 20 ℃. The FTIR spectrum data of rats' spleen tissues were taken and measured at different time points. After pretreatment, the data was analysed by data mining method. The absorption peak intensity of rat's spleen tissue spectrum changed with the PMI, while the absorption peak position was unchanged. The results of principal component analysis （PCA） showed that the cumulative contribution rate of the first three principal components was 96%. There was an obvious clustering tendency for the spectrum sample at each time point. The methods of partial least squares discriminant analysis （PLS-DA） and support vector machine classification （SVMC） effectively divided the spectrum samples with different PMI into four categories （0-24 h, 48-72 h, 96-120 h and 144-168 h）. The determination coefficient （R²） of the PMI estimation model established by PLS regression analysis was 0.96, and the root mean square error of calibration （RMSEC） and root mean square error of cross validation （RMSECV） were 9.90 h and 11.39 h respectively. In prediction set, the R² was 0.97, and the root mean square error of prediction （RMSEP） was 10.49 h. The FTIR spectrum of the rat's spleen tissue can be effectively analyzed qualitatively and quantitatively by the combination of FTIR spectroscopy and data mining method, and the classification and PLS regression models can be established for PMI estimation.

The coverage control and electric desorption kinetics of polypyridyl ruthenium dye molecule fixed on the interface of nanoporous TiO2

This paper conducted researches on electric desorption kinetics, desorption mechanism and quantitative desorption of the adsorbed polypyridyl ruthenium based dye on the surface of nanoporous TiO2 film under negative potential. Change external negative potential to obtain the relationship between dye desorption amount of sensitized-TiO2 film and potential. After the negative potential exceeds about −0.5 V, amount of dye left on the surface of TiO2 film decreases rapidly. According to the pseudo-first-order kinetic equation, dye desorption rate kd under potential of −0.6 V is 6.92 × 10−3 s−1. Research the relationship between the amount of desorbed dye and the potential duration, ionic concentration in supporting electrolyte solution as well as film thickness. It implies that the amount of adsorbed dye can be accurately controlled. Based on the measurement and analysis of the adsorption property of desorbed dye through UV–Visible absorption spectroscopy, there is no change of number of absorption peaks of the desorbed dye or no occurrence of new peak, while the absorption peak position of the desorbed dye shows a blue shift. Cyclic voltammetry is used to research the TiO2/dye/electrolyte interface and TiO2/electrolyte interface, and analyze the influence of dye desorption on the cyclic voltammetric response. Two possible dye desorption mechanisms are provided: one is the fracture of ester bond between dye and TiO2 under negative potential; another is the electrostatic repulsion desorption mechanism. Work in this paper is of theoretical guiding significance for compounding new dyes, defining assembly mechanism of dye, enhancing the stability of solar cell, and improving solar cell efficiency.

Research on the differences between 2-(2-Chlorophenyl)benzimidazole and 2-(4-Chlorophenyl)benzimidazole based on terahertz time domain spectroscopy

Due to wide variety of biological and pharmacological activities of benzimidazole derivatives, the differences between 2-(2-Chlorophenyl)benzimidazole and 2-(4-Chlorophenyl) benzimidazole were researched by employing terahertz time-domain spectroscopy and density functional theory systematically. Although the only difference between their molecular configurations is the arrangement of chlorine atom on chlorophenyl ring, there are distinctive differences in their fingerprint spectra in the range of 0.2–2.5THz, such as amount, amplitude, and frequency position of absorption peaks. The validity of these results was confirmed by the theoretical results simulated by using density functional theory. The possible reasons of these differences originate from the different van der Waals forces and the different dihedral angles of the molecules within crystal cell. These results indicate the importance of this spectral range as a conformational fingerprint region where even minor changes in the molecular configuration lead to major differences in its THz absorption.

Explicit extraction of absorption peak positions, widths and heights using higher order derivatives of total shape spectra by nonparametric processing of time signals as complex damped multi-exponentials

Sole lineshape estimation of non-parametrically computed higher-order derivatives of spectral envelopes in different modes (complex, real, imaginary, magnitude) is investigated. The processed time signals are sums of complex attenuated exponentials (harmonics). The peak parameters of the derivative spectra are directly connected to those of the customary (non-derivative) absorption lineshapes. Crucially, this permits the reconstruction of the latter from the former parameters (the latter are sought since they are unknown, whereas the former are extractable from the derivative envelopes). The cross-checking relationships of the lineshapes for the magnitude modes with the real and imaginary parts of the complex-valued envelopes (total shape spectra) are established. The explicit procedure and the necessary analytical expressions are presented for reconstruction of the exact locations, widths and heights of all the retrieved physical resonances (spectral peaks). These facets are illuminated in the derivative fast Padé transform (dFPT) using its non-parametric version, i.e. without solving the quantification problem (no polynomial rooting, no tackling of eigen-value problems, etc.). Two kinds of illustrations for derivative spectra are reported. One deals with the general Breit-Wigner resonance formula and its first three derivatives. The other is concerned with the dFPT in clinical diagnostics of relevance to breast cancer detection by magnetic resonance spectroscopy. A systematic parallel between these two examples is drawn to highlight, in a stepwise manner, the role of paramount importance played by derivative lineshapes, especially for disentangling overlapping resonances that invariably plague all quantitative analyses of spectra.

Possibility to Use Hydrothermally Synthesized CuFeS2 Nanocomposite as an Acceptor in Hybrid Solar Cell

Here we have approached the plausible use of CuFeS2 nanocomposite as an acceptor in organic–inorganic hybrid solar cell. To produce CuFeS2 nanocomposite, hydrothermal strategy was employed. The room-temperature XRD pattern approves the synthesized material as CuFeS2 with no phase impurity (JCPDS Card no: 37-0471). The elemental composition of the material was analyzed from the TEM-EDX data. The obtained selected area electron diffraction (SAED) planes harmonized with the XRD pattern of the synthesized product. Optical band gap (4.14 eV) of the composite from UV–Vis analysis depicts that the synthesized material is belonging to wide band gap semiconductor family. The HOMO (− 6.97 eV) and LUMO (− 2.93 eV) positions from electrochemical study reveal that there is a possibility of electron transfer from MEH-PPV to CuFeS2. The optical absorption and photoluminescence spectra of MEH-PPV:CuFeS2 (donor:acceptor) composite were recorded sequentially by varying weight ratios. The monotonic blue shifting of the absorption peak position indicated the interaction between donor and acceptor materials. The possibility of electron transfer from donor (MEH-PPV) to acceptor (CuFeS2) was approved with photoluminescence analysis. Subsequently, we have fabricated a hybrid solar cell by incorporating CuFeS2 nanocomposite with MEH-PPV in open atmosphere and obtained 0.3% power conversion efficiency.

Excitation spectra of retinal by multiconfiguration pair-density functional theory.

Retinal is the chromophore in proteins responsible for vision. The absorption maximum of retinal is sensitive to mutations of the protein. However, it is not easy to predict the absorption spectrum of retinal accurately, and questions remain even after intensive investigation. Retinal poses a challenge for Kohn-Sham density functional theory (KS-DFT) because of the charge transfer character in its excitations, and it poses a challenge for wave function theory because the large size of the molecule makes multiconfigurational perturbation theory methods expensive. In this study, we demonstrate that multiconfiguration pair-density functional theory (MC-PDFT) provides an efficient way to predict the vertical excitation energies of 11-Z retinal, and it reproduces the experimentally determined absorption band widths and peak positions better than complete active space second-order perturbation theory (CASPT2). The consistency between complete active space self-consistent field (CASSCF) and KS-DFT dipole moments is demonstrated to be a useful criterion in selecting the active space. We also found that the nature of the terminal groups and the conformations of retinal play a significant role in the absorption spectrum. By considering a thermal distribution of conformations, we predict an absorption spectrum of retinal that is consistent with the experimental gas-phase spectrum. The location of the absorption peak and the spectral broadening based on MC-PDFT calculations agree better with experiments than those of CASPT2.