Highly sensitive and selective SERS substrates with 3D hot spot buildings for rapid mercury ion detection.

Heavy metal ions, which are over-emitted from industrial production, pose a major threat to the ecological environment and human beings. Among the present detection technologies, achieving rapid and on-site detection of contaminants remains a challenge. Herein, capillaries with three-dimensional (3D) hot spot constructures are fabricated to achieve repaid and ultrasensitive mercury ion (Hg2+) detection in water based on surface-enhanced Raman scattering (SERS). The 4-mercapto pyridine (4-Mpy) serves as the Raman reporter with high selectivity, enabling the detection of Hg2+ by changes in adsorption configuration at the trace level. Under optimized conditions, the SERS response of 4-Mpy for Hg2+ exhibits good linearity, ranging from 1 pM to 0.1 µM in a few minutes, and the detection limit of 0.2 pM is much lower than the maximum Hg2+ concentration of 10 nM allowed in drinking water, as defined by the US Environmental Protection Agency (EPA). Simultaneously, combined with the theoretical simulation and experimental results, the above results indicate that the SERS substrates possess outstanding performances in specificity, recovery rate and stability, which may hold great potential for achieving rapid and on-site environmental pollutant detection using a portable Raman spectrometer.

Theoretical calculation of spectroscopy properties of selenium bromide cation.

In this paper, the potential energy curves of 22 Λ-S states as well as 51 Ω states were calculated using the internally contracted multiconfiguration interaction and Davidson correction method. Through the obtained transition data, the spectroscopy data of the low excitation bound state are fitted and compared with the same main group ions. The phenomenon of avoided crossing that occurs in the Ω state is analyzed, and finally it is concluded that this phenomenon mainly occurs in the energy region between 20 000 cm-1 and 40 000 cm-1. The potential laser cooling transition cycle in the Ω state is analyzed. The Franck-Condon factor, radiative lifetime and Einstein coefficient between are calculated. In this paper, we argue that direct laser cooling of SeBr+ is not feasible. The content of our study provides a theoretical basis for subsequent calculations to explore the properties of SeBr+ spectrum.

Electronic and Optical Properties of BP, InSe Monolayer and BP/InSe Heterojunction with Promising Photoelectronic Performance.

Two-dimensional (2D) materials provide a new strategy for developing photodetectors at the nanoscale. The electronic and optical properties of black phosphorus (BP), indium selenide (InSe) monolayer and BP/InSe heterojunction were investigated via first-principles calculations. The geometric characteristic shows that the BP, InSe monolayer and BP/InSe heterojunction have high structural symmetry, and the band gap values are 1.592, 2.139, and 1.136 eV, respectively. The results of band offset, band decomposed charge and electrostatic potential imply that the heterojunction structure can effectively inhibit the recombination of electron--hole pairs, which is beneficial for carrier mobility of photoelectric devices. Moreover, the optical properties, including refractive index, reflectivity, electron energy loss, extinction coefficient, absorption coefficient and photon optical conductivity, show excellent performance. These findings reveal the optimistic application potential for future photoelectric devices. The results of the present study provide new insight into challenges related to the peculiar behavior of the aforementioned materials with applications.

Theoretical Study on the Electronic Structure and Transition Properties of Low Excited States of SeCl.

For the first time, the spectroscopy and transition properties of SeCl+ are systematically reported. The potential energy curves of 22 Λ - S states and the corresponding 51 Ω states in the first and second dissociation channels of SeCl+ are calculated using the internally contracted multiconfiguration interaction and Davidson correction method. The phenomenon of avoided crossing in Ω states below 30,000 cm-1 is discussed in detail. The spectroscopy constants are obtained by fitting the potential energy curves, and also the Franck-Condon factors and radiation lifetimes of the X3Σ0+- ↔ 21Σ0++ transition are calculated. Between X3Σ0+- and 21Σ0++, the Franck-Condon factors are large, close to 1, but the radiation lifetime is large too. According to the calculation results, it is determined that direct laser cooling of SeCl+ is considered infeasible.

Study on the spectroscopy and transition properties of TeCl.

For the first time, the spectroscopy data of TeCl+ ion and the transition data between low excited states are systematically calculated. The potential energy curves of 22 Λ-S states and 51 Ω states are calculated by the internally contracted multiconfiguration interaction and Davidson correction method. By solving the one-dimensional radial Schrödinger equation, the spectroscopy data of Λ-S states and Ω states are obtained. The phenomenon of avoided crossing in Ω state is analyzed in detail, which is mainly concentrated in the region of 20000 cm-1 to 35000 cm-1. The Franck-Condon factors, Einstein coefficients and spontaneous radiative lifetimes of [Formula: see text] transitions are calculated. According to the calculation results, it is preliminarily judged that the direct laser cooling of TeCl+ ion is not feasible.

Dielectric-loading approach for extra electric field enhancement and spatially transferring plasmonic hot-spots.

Plasmonic nanoantennas have been widely explored for boosting up light-matter interactions due to their ability of providing strongly confined and highly enhanced electric near fields, so called 'hot-spots'. Here, we propose a dielectric-loading approach for hot-spots engineering by coating the conventional plasmonic nanoantennas with a conformal high refractive index dielectric film and forming dielectric-loaded plasmonic nanoantennas. Compared to the conventional plasmonic nanoantennas, the corresponding dielectric-loaded ones that resonate at the same frequency are able to provide an extra enhancement in the local electric fields and meanwhile spatially transfer the hot spots to the dielectric surfaces. These findings have important implications for the design of optical nanoantennas with general applications in surface enhanced linear and nonlinear spectroscopies. As a demonstration application, we show that the maximum achievable fluorescence intensity in the dielectric-loaded plasmonic nanoantennas could be significantly larger than that in the conventional plasmonic nanoantennas.

Theoretical study of the low-lying electronic states, including the spin-orbit interactions, of the sulfur monochloride cation.

High-level ab initio computations have been performed on the experimentally unknown species SCl+. The low-lying Λ-S electronic states correlated to the first and the second dissociation channels as well as their corresponding Ω states have been investigated by the icMRCI+Q methodology employing basis sets up to quintuple-ζ quality. Information about potential energy curves, electron configurations, spectroscopic constants, dipole moments and transition properties are derived and discussed. The results for SCl+ represent an improvement over our previous theoretical descriptions for the ground state. In addition, several low-lying excited states that have not been accessed experimentally and theoretically are also been well characterized in this work. The accuracy of our predictions for SCl+ are verified by comparisons of spectroscopic constants and vibrational levels between our accompany SCl computations and those reported in literatures for the neutral species. The feasibility of performing laser cooling of SCl+ has also been discussed and the photoelectron spectrum of SCl+(X3Σ-) + e â† SCl(X2Π) is simulated.

Graphene ultraviolet ultrahigh-Q perfect absorption for nanoscale optical sensing.

We propose an ultraviolet perfect ultranarrow band absorber by coating a dielectric grating on the monolayer graphene-dielectric-metal structure. The absorber presents an ultranarrow Fano lineshape with quality (Q) factor of 70 and a nearly perfect absorption of over 99.9% in the ultraviolet region, which is ascribed to the near field coupling of the optical dissipation of graphene and guide mode resonance of the dielectric grating. Structure parameters to the influence of the performance are investigated. The structure exhibits the high optical sensitivity (S = 150 nm/RIU, S* = 48/RIU) and figure of merit (FOM = 50, FOM* = 25374) and can also be used to detect the nanoscale analyte layer of sub-nanometer thickness, suggesting great potential applications in ultra-compact efficient biosensors for a much more sensitive detection of small refractive index changes.

Ab initio investigation on the low-lying electronic states of magnesium antimonide.

The twelve Λ-S electronic states of the first four dissociation limits of the MgSb molecule have been examined at the icMRCI+Q level employing basis sets of quintuple-ζ quality. The potential energy curves, vibrational levels and spectroscopic constants of the species have been investigated. The permanent dipole moments of the interested states are derived, and the transition dipole moments, Einstein emission coefficients, radiation lifetimes and Franck-Condon factors between selected states are also determined. Four Λ-S states of the first two dissociation limits split into seven Ω states under the effect of spin-orbit coupling. Characterizations of the MgSb low-lying Ω states are performed for the first time. In addition, the results and relevant data provided in this work on MgSb are compared with the antimony-IIA group and magnesium-VA group diatomic species. It is anticipated that this work will shed some light on further investigations of MgSb and other antimony-IIA group systems.

The study of laser cooling of TeH- anion in theoretical approach.

The probabilities of laser cooling of TeH- anion via a spin-forbidden transition and a three-electronic-level transition are proposed. The potential energy curves of the X1Σ+, a3∏, A1∏, and b3Σ+ electronic states of tellurium monohydride anion (TeH-) are calculated using multi-reference configuration interaction method. Davidson corrections, core-valence correlations and spin-orbit coupling effects are also considered. The AWCV5Z-PP pseudopotential basis set of Te atom is used. Spectroscopic parameters of the Λ-S and Ω states are obtained by solving radial Schrodinger equation. These results are reported at the first time. Permanent dipole moments of the Ω states and transition dipole moments of the a21↔X0+ and A1↔X0+ transitions are also calculated. Highly diagonally distributed Franck-Condon factors of the a21↔X0+ and A1↔X0+ transitions are obtained, the value of f00 is 0.9970 and 0.9980, respectively. Spontaneous radiative lifetimes of the a21 and A1 excited states are predicted. i.e. τ(a21) = 200.3 ns and τ(A1) = 84.3 ns. Only the main pump laser is required to driving a21↔X0+ and A1↔X0+ transitions. The laser wavelengths both are in the visible region. Doppler temperatures and recoil temperatures of laser cooling TeH- anion are also predicted.

Characterization of the low-lying electronic states of tin monohydride cation including the spin-orbit coupling effect: A theoretical perspective.

High-level ab initio computations have been performed on SnH+. The potential energy curves and spectroscopic constants of the low-lying Λ-S electronic states, as well as their associated Ω states, are derived at the icMRCI + Q level employing basis sets of quintuple-ζ quality. The transition dipole moments, Einstein coefficients, radiative lifetimes and Franck-Condon factors of three spin-forbidden transition bands ( [Formula: see text] , [Formula: see text] and [Formula: see text] ) are determined. Comparisons between our predictions and available experimental results indicate reasonable agreement. The spin-orbit coupling effect has been proved to affect these low-lying electronic states significantly.

Theoretical investigation on the low-lying electronic states of beryllium antimonide.

The Λ-S electronic states with respect to the lowest four dissociation limits of BeSb are investigated theoretically on the icMRCI + Q level employing basis set of quintuple-ζ quality. The geometrical parameters, potential energy curves, vibrational energy levels, spectroscopic constants for the twelve Λ - S states are obtained, analyzed and compared with those of the Beryllium-VA group diatomic family species where data are available. The permanent dipole moments, transition dipole moments, Einstein emission coefficients, radiative lifetimes and Franck-Condon factors for interested Λ - S states are also derived. Further assessments of the spin-orbit coupling effect are performed for states associated with the first two dissociation asymptotes of BeSb. Four Λ - S states split into seven Ω states, and some of the PECs are distorted significantly through the spin-orbit coupling effect, which is similar to its isovalent diatomics BeAs. In consideration of potential risks of manipulating beryllium-containing species directly, the information associated with molecular structures, spectroscopic parameters as well as transition properties that provide in this paper is anticipated to serve as guidelines for further researches of BeSb.

An ab initio investigation on the low-lying electronic states of NaMg.

Theoretical investigations for NaMg have been performed on the icMRCI + Q level employing basis set of quintuple-ζ quality with corrections of core-valence correlation and scalar relativistic effect. The geometrical parameters, potential energy curves, vibrational energy levels, spectroscopic constants for the eight Λ-S states, with respect to the lowest four dissociation limits, are investigated. Through the spin-orbit coupling effect, these states split into fourteen Ω states. The permanent dipole moments, transition dipole moments, Einstein emission coefficients, radiative lifetimes and Franck-Condon factors for all Ω states are studied. The feasibility of performing laser cooling of NaMg has also been discussed. Our predictive results are anticipated to serve as guidelines for further researches on NaMg.

Laser cooling of the OH- molecular anion in a theoretical investigation.

The schemes for laser cooling of the OH- anion are proposed using an ab initio method. Scalar relativistic corrections are considered using the Douglas-Kroll Hamilton. Spin-orbit coupling (SOC) effects are taken into account at the MRCI+Q level. SOC effects play important roles in the transition properties of the OH- anion. Transition strengths for the transition of the OH- anion cannot be ignored. Large vibrational branching ratios for the and transitions are determined. Short spontaneous radiative lifetimes for the a3Π1 and A1Π1 states are also predicted for rapid laser cooling. The vibrational branching loss ratio to the intervening states a3Π0 and a3Π1 for the transition is small enough to enable the building of a laser cooling project. The three required laser wavelengths for the and transitions are all in the visible region. The results imply the probability of laser cooling of the OH- anion via both a spin-forbidden transition and a three-electronic-level transition.

Theoretical investigation on spin-forbidden cooling transitions of gallium hydride.

Herein, the spin-forbidden cooling of a gallium hydride molecule is investigated using ab initio quantum chemistry. The cooling transition and the corresponding potential energy curves including , a3Π0-, a3Π0+, a3Π1, a3Π2, A1Π1, , 13Σ, , , and 23Σ states are simulated based on the multi-reference configuration interaction approach plus Davidson corrections method. By solving the nuclear Schrödinger equation, we calculate the spectroscopic constants of these states, which are in good agreement with the available experimental values. Based on the transition data, there seems to be a theoretical puzzle: highly diagonally distributed Franck-Condon factor f00 for transitions , , and for the gallium hydride molecule but the intervening state A1Π1 for transition is prohibitive to laser cooling. In addition, the transition does not have a suitable rate of optical cycling owing to a large radiative lifetime for state. Our theoretical simulation indicates the solution to the puzzle: the transition has a high emission rate, and there is a suitable radiative lifetime for a3Π1 state, which can ensure rapid and efficient laser cooling of gallium hydride. The proposed laser drives transition by using three wavelengths (main pump laser λ00; two repumping lasers λ10 and λ21). These results demonstrate the possibility of laser-cooling the gallium hydride molecule, and a sub-microkelvin cool temperature can be reached for this molecule.

Comparative studies on the quality factors of whispering gallery modes and hybrid plasmon photon modes.

We theoretically and experimentally investigate the multipolar hybrid plasmon-photon modes supported by a dielectric-metal core-shell resonator consisting of a dielectric core wrapped by a thin silver shell and the whispering-gallery modes in its pure dielectric counterpart (the dielectric sphere with the same size). We theoretically demonstrate that in a certain wavelength range the achievable maximum Q-factors of hybrid modes could be either larger or smaller than that of whispering-gallery modes, depending on the size of the resonator. By means of the coupling of the dye molecules to the hybrid and whispering-gallery modes, the reshaped fluorescence spectra are measured for resonators containing two different sized dye-doped dielectric spheres, which allow us to compare the Q-factors of hybrid and whispering-gallery modes, providing direct experimental support to the theoretical predictions. Our results provide guidance for appropriately choosing plasmonic core-shell (hybrid modes) or dielectric resonators (whispering-gallery modes) in applications such as ultrasensitive bio-sensors, low-threshold lasing, slow-light and nonlinear optical devices.

Ultra-broadband Tunable Resonant Light Trapping in a Two-dimensional Randomly Microstructured Plasmonic-photonic Absorber.

Recently, techniques involving random patterns have made it possible to control the light trapping of microstructures over broad spectral and angular ranges, which provides a powerful approach for photon management in energy efficiency technologies. Here, we demonstrate a simple method to create a wideband near-unity light absorber by introducing a dense and random pattern of metal-capped monodispersed dielectric microspheres onto an opaque metal film; the absorber works due to the excitation of multiple optical and plasmonic resonant modes. To further expand the absorption bandwidth, two different-sized metal-capped dielectric microspheres were integrated into a densely packed monolayer on a metal back-reflector. This proposed ultra-broadband plasmonic-photonic super absorber demonstrates desirable optical trapping in dielectric region and slight dispersion over a large incident angle range. Without any effort to strictly control the spatial arrangement of the resonant elements, our absorber, which is based on a simple self-assembly process, has the critical merits of high reproducibility and scalability and represents a viable strategy for efficient energy technologies.

Strong tunable absorption enhancement in graphene using dielectric-metal core-shell resonators.

We theoretically investigate light absorption by a graphene monolayer that is coated on the outside of dielectric-metal core-shell resonators (DMCSRs). We demonstrate that light absorption of graphene can be greatly enhanced in such multi-layered core-shell architectures as a result of the excitation of the hybridized bonding plasmon resonance supported by the DMCSRs. We also demonstrate that the absorption enhancement in graphene can be easily tuned over a wide range from the visible to the near-infrared, and particularly the enhancement factor can be optimally maximized at any selective wavelength, by simultaneously varying the dielectric core size and the metal shell thickness. Our results suggest that the graphene-wrapped DMCSRs with strong and highly wavelength-tunable absorption enhancement in graphene could be attractive candidates for applications in graphene-based photodetectors and image sensors.

Vibrational branching ratios and radiative lifetimes in the laser cooling of AlBr.

The feasibility of laser cooling of the AlBr molecule is investigated using ab initio quantum chemistry. Potential energy curves, permanent dipole moments, and transition dipole moments for the ground state X1Σ+ and the first two excited states (a3Π and A1Π) are calculated using the multi-reference configuration interaction plus Davidson corrections (MRCI+Q) method with the ACVQZ basis set; the spin-orbit coupling effects are also taken into account in electronic structure calculations at the MRCI level. Based on the acquired potential energy curves and transition dipole moments, highly diagonally distributed Franck-Condon factors (f00 = 0.9540, f11 = 0.8172) and vibrational branching ratios (R00 = 0.9708, R11 = 0.8420) for the transition are determined. Radiative lifetime calculations of the A1Π1 (ν' = 0-4) state are found to be short (9.16-11.48 ns) enough for rapid laser cooling. The proposed main cycling laser drives the transition at the wavelength λ00 = 279.19 nm. The vibrational branching loss ratios of the A1Π1 (ν') state to the intervening states a3Π0+ and a3Π1 are small (<5.2 × 10-6) enough to be negligible. The present theoretical results indicate that the AlBr molecule is a promising candidate for laser cooling.

Strong coupling between few molecular excitons and Fano-like cavity plasmon in two-layered dielectric-metal core-shell resonators.

We theoretically investigate the coupling between molecular excitons and dipolar Fano-like cavity plasmon resonance in two-layered core-shell resonators consisting of a dielectric core with high refractive index and a thin metal outer shell gapped by a low refractive index thin dielectric layer containing molecules. We demonstrate that associated with the excitation of the dipolar Fano-like cavity plasmon, the electric fields can be highly localized within the dielectric gap shell, leading to very small mode volumes. By using the three-oscillator temporal coupled model to describe the proposed plasmon-exciton system, we are able to demonstrate that the coupling between molecular excitons and cavity plasmon resonance can reach the strong coupling regime. Furthermore, we also demonstrate that reducing the thickness or the refractive index of the dielectric gap shell layer can result in further compression of the mode volumes, and consequently decrease the minimum number of the coupled excitons that are required to fulfill the criteria for strong coupling.