High Local Field Research Articles

Correlation of optical sensing with extinction coefficient and local field enhancement in gold nanosphere dimer

In this paper we systematically study the optical extinction, local field enhancement, and resonance peak shift of basic single/double gold nanosphere system. We find that in the double gold nanosphere system, the incident light can excite the coupled resonance modes when the two gold nanospheres are approaching to each other, leading the local field to be enhanced greatly. Interestingly, limited by the scant volume of local field, the extinction coefficient of the double gold nanosphere system of 2 nm gap with a high local field enhancement factor is greatly reduced, so that its optical sensing sensitivity and extinction coefficient are smaller than the 5 nm gap system’s. Studies show that the optical sensing sensitivity of the double gold nanosphere system is not directly determined by the local field enhancement amplitude, but has a similar change behavior to the extinction coefficient of the system. These results can offer us a useful route and hint for designing the gold nanoparticle systems used in the surface Raman scattering enhancement and high performance optical sensing.

Static and ultrafast optical response of two metal nanoparticles glued with a semiconductor quantum dot

A hot-spot of high local field can be created between two closely placed metal nanoparticles and by irradiating them with an appropriate wavelength and polarization of incident light. The strength of the field at the gap between the particles is expected to get enhanced by several orders of magnitude higher than that of the applied field. Placing a semiconductor quantum dot or an analyte molecule at the hot-spot is an essential step towards harnessing the enhanced field for various applications. This article shows that it is possible to position a CdTe quantum dot (QD) between two larger silver nanospheres in a colloidal solution. The extinction spectra measured during growth suggests that the final hybrid nanostructure have two touching Ag nanoparticles (NPs) and a CdTe QD in between them close to the point of contact. Using ultrafast transient measurements, it has been shown that the presence of CdTe QD strongly influences the dynamics when the probe excites the gap plasmon. The method demonstrated here to place the semiconductor QD in between the two Ag NPs is an important step in the area of colloidal self-assembly and for the applications of hot-spot in plasmonic sensing, optoelectronics, energy-harvesting, nanolithography and optical nano-antennas.

Gold cauldrons as efficient candidates for plasmonic tweezers

In this report, gold cauldrons are proposed and proved as efficient candidates for plasmonic tweezers. Gold cauldrons benefit from high field localization in the vicinity of their apertures, leading to particle trapping by a reasonably low power source. The plasmonic trapping capability of a single gold cauldron and a cauldrons cluster are studied by investigating the plasmon-induced variations of the optical trap stiffness in a conventional optical tweezers configuration. This study shows that the localized plasmonic fields and the consequent plasmonic forces lead to enhanced trap stiffness in the vicinity of the cauldrons. This observation is pronounced for the cauldrons cluster, due to the additive plasmonic fields of the neighboring cauldrons. Strong direct plasmonic tweezing by the gold cauldrons cluster is also investigated and confirmed by our simulations and experimental results. In addition to the presented plasmonic trapping behavior, gold cauldrons benefit from a low cost and simple fabrication process with acceptable controllability over the structural average dimensions and plasmonic behavior, making them attractive for emerging lab-on-a-chip optophoresis applications.

Characterization of Bilayer Organic Solar Cells with Carbon Nanotubes Using Impedance Analysis.

Four organic solar cell (OSC) devices with the bilayer heterojunction architecture were investigated, where carbon nanotubes (CNTs) were doped within the acceptor layer. The power conversion efficiency (PCE) of the CNT-incorporated device with a concentration of 0.004 wt% is approximately 20% point higher than that of the reference one. As the concentration of CNTs became higher, the PCE of the devices deteriorated; this could be caused by the percolative connection of CNTs within the layer. The voltage dependence on the effective lifetime of the charge carriers, determined by Cole-Cole curves of the impedance analysis, was different for the reference and CNT-incorporating devices-the lifetime of the CNT-incorporated ones was shorter, possibly owing to the high local electric field near the CNTs. Controlling the concentration of CNTs below the critical concentration of percolation is a key factor in achieving high photovoltaic performance.

Improved dielectric breakdown strength and energy storage properties in Er2O3 modified Sr0.35Bi0.35K0.25TiO3

The development of lead-free dielectric ceramics with excellent energy storage properties has received extensive research attention. Herein, Er2O3 modified Sr0.35Bi0.35K0.25TiO3 (SBKT) composite ceramic is investigated. As 2 wt.% Er2O3 is added, a certain amount of insulating second phase with fewer regions of high local electric field (numerically simulated by COMSOL) is formed, giving rise to the increase in the dielectric breakdown strength (BDS) due to the increased electrical resistivity and band gap, the decreased average grain size and the decreased leakage current density. This optimal composition exhibits many excellent energy storage properties, such as high energy storage density of 2.59 J/cm3, high recoverable energy storage density (Wrec) of 2.17 J/cm3 with high efficiency (η) of 83.9% at 245 kV/cm, superior stability of temperature (<1% variation of Wrec and η in −25 ~ 150 °C), frequency (~1.6% variation of Wrec with ~3.6% variation of η in 10 ~ 500 Hz) and cycling (almost no change of Wrec and η within 105 cycles), and high discharge energy density of 1.51 J/cm3 with high power density of 39.6 MW/cm3 at 200 kV/cm. In this work, we provide a novel strategy and understanding to design the advanced pulse energy storage ceramics.

Ag modified Cu hollow fiber for gas-phase CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; electrocatalytic conversion to oxygenates

<p indent="0mm">The conversion of CO₂ to oxygenates driven by renewable electricity is of great importance, since not only the carbon emission would be reduced by utilizing CO₂ as feedstock, but also the renewable energy could be stored as high value-added chemicals. The development of gas-phase electrocatalytic system is significant for the practical application of CO₂ electroreduction, which could avoid the obstacles of liquid-phase CO₂ electroreduction system, such as the insufficient CO₂ solubility and separation dilemma of soluble products. In this work, Ag modified Cu hollow fibers via galvanic replacement are utilized as the electrode for gas-phase CO₂ electroreduction to produce multi-carbon oxygenates. Hollow fiber with its large surface area, adjustable pore structure, good chemical stability and conductivity shows unique advantage in electrocatalysis application. The morphology, surface and bulk structure characterizations of Ag-Cu hollow fiber electrodes indicate that the modification via galvanic replacement forms Ag nanodendrites in different scales on the surface of Cu hollow fiber electrodes. The relationship between the surface structures and electrocatalytic performances of the as-prepared Ag-Cu electrodes is also unveiled. Compared with Cu hollow fiber electrode, not only the current densities but also the faradaic efficiencies of C₃ products (propanal and acetone) for Ag-Cu hollow fiber electrodes are greatly improved. 0.1wt.% Ag-Cu hollow fiber electrode shows an elevated current density of −0.19, −0.35 and <sc>−0.72 mA cm⁻²</sc> at the potential of −1.4, −1.6 and <sc>−1.8 V</sc> vs. counter electrode respectively, compared with those of −0.11, −0.16 and <sc>−0.33 mA cm⁻²</sc> for Cu hollow fiber. And 0.1wt.% Ag-Cu hollow fiber electrode also gives promoted faradaic efficiencies of not only C₂ product ethanol (with a yield of <sc>56 nmol cm⁻² h⁻¹),</sc> but also C₃ product acetone. Although the further increase of Ag loading narrows the current density difference comparing with Cu hollow fiber, the faradaic efficiencies and yields of C₃ products propanal and acetone are greatly improved for 0.3wt.% and 1wt.% Ag-Cu hollow fiber electrodes. 1wt.% Ag-Cu hollow fiber electrode provides faradaic efficiencies of 19% for propanal and 7% for acetone with a current density of <sc>0.13 mA cm⁻²</sc> at the potential of −1.4 V vs. counter electrode. And its propanal yield reaches <sc>57 nmol cm⁻² h⁻¹,</sc> even higher than the ethanol yield of other electrocatalysts in reported gas-phase CO₂ electroreduction works, indicating the better activity of Ag-Cu hollow fiber electrode on multi-carbon product formation. The modification of Ag dendrites on hollow fiber surface not only promotes the activity of gas-phase CO₂ electroreduction, but also facilitates the formation of multi-carbon oxygenates. The fractal dendrite structure of Ag forms abundant conductive networks on the surface of Cu hollow fiber electrode, which facilitates the electron transport and mass transfer processes, leading to superior kinetics and elevated current densities for gas-phase CO₂ electroreduction. Also, the high curvature surface of Ag dendrites induces high local electric field and high local concentration of CO₂ as well as active intermediates, which would be beneficial for the C-C coupling and the formation of multi-carbon products. The present study provides an inspiration and scientific instruction for designing and constructing gas-phase CO₂ electrocatalytic systems to produce multi-carbon oxygenates with high activity and high selectivity.

Plasmonic photocatalysis and SERS sensing using ellipsometrically modeled Ag nanoisland substrates

Silver nanoislands are key platforms for plasmonic photocatalysis, SERS sensing and optical metamaterials due to their localized surface plasmon resonances. The low intrinsic loss in Ag enables high local electromagnetic field enhancements. Solution-based fabrication techniques, while cheap, result in highly non-reproducible plasmonic substrates with wide sample-to-sample variability in geometry, optical resonances and Q-factors. Herein, we present a non-lithographic method of forming silver nanoislands based on sputter deposition of Ag films followed by elevated temperature annealing to induce spontaneous dewetting. The resulting plasmonic substrates show reproducible, well-defined LSPR resonances with high ensemble Q-factors whose optical properties could be modeled using spectroscopic ellipsometry to yield n and k values across the visible range. UV–Vis-NIR, and XRD analyses define the optical and crystallographic characteristics of the Ag nanoisland samples. FESEM was utilized to discern the geometry and architecture of the Ag nanoisland as well as their uniformity and monodispersity. Our vacuum deposited Ag nanoislands demonstrated excellent photocatalytic activity for the transformation of 4-nitrobenzenethiol (4-NBT) and 4-aminothiophenol (PATP) into p,p’-dimercaptoazobenzene (DMAB).

Silver SERS Adenine Sensors with a Very Low Detection Limit.

The detection of adenine molecules at very low concentrations is important for biological and medical research and applications. This paper reports a silver-based surface-enhanced Raman scattering (SERS) sensor with a very low detection limit for adenine molecules. Clusters of closely packed silver nanoparticles on surfaces of discrete ball-like copper bumps partially covered with graphene are deposited by immersion in silver nitrate. These clusters of silver nanoparticles exhibit abundant nanogaps between nanoparticles, where plasmonic coupling induces very high local electromagnetic fields. Silver nanoparticles growing perpendicularly on ball-like copper bumps exhibit surfaces of large curvature, where electromagnetic field enhancement is high. Between discrete ball-like copper bumps, the local electromagnetic field is low. Silver is not deposited on the low-field surface area. Adenine molecules interact with silver by both electrostatic and functional groups and exhibit low surface diffusivity on silver surface. Adenine molecules are less likely to adsorb on low-field sensor surface without silver. Therefore, adenine molecules have a high probability of adsorbing on silver surface of high local electric fields and contribute to the measured Raman scattering signal strength. We demonstrated SERS sensors made of clusters of silver nanoparticles deposited on discrete ball-like copper bumps with very a low detection limit for detecting adenine water solution of a concentration as low as 10−11 M.

Ultrabright Fluorescence Readout of an Inkjet-Printed Immunoassay Using Plasmonic Nanogap Cavities.

Fluorescence-based microarrays are promising diagnostic tools due to their high throughput, small sample volume requirements, and multiplexing capabilities. However, their low fluorescence output has limited their implementation for in vitro diagnostics applications in point-of-care (POC) settings. Here, by integration of a sandwich immunoassay microarray within a plasmonic nanogap cavity, we demonstrate strongly enhanced fluorescence which is critical for readout by inexpensive POC detectors. The immunoassay consists of inkjet-printed antibodies on a polymer brush which is grown on a gold film. Colloidally synthesized silver nanocubes are placed on top and interact with the underlying gold film creating high local electromagnetic field enhancements. By varying the thickness of the brush from 5 to 20 nm, up to a 151-fold increase in fluorescence and 14-fold improvement in the limit-of-detection is observed for the cardiac biomarker B-type natriuretic peptide (BNP) compared to the unenhanced assay, paving the way for a new generation of POC clinical diagnostics.

Controllable Vacancy Redistributions in Embedded Thin Films Under Concentrated Electric Fields

The redistribution and accumulation of oxygen vacancies under electric field underpins both the functionality and degradation of electrical and electrochemical devices comprising oxide layers. Interfacial accumulations of oxygen vacancies form defect dipoles, significantly reduce the breakdown strength, increase the breakdown path area, yet facilitates charge transfer reactions. Confinement of both oxygen vacancy redistributions and their mobility remains critical to preserving function in oxide systems.Here we study one of the most important, yet random transport processes: the redistribution of oxygen vacancies in binary oxides under high local electric fields. The ability to identify or predefine the shape, size, and location of vacancy distributions is critical to controlling interfacial properties of functional oxides. We achieved superior uniformity in HfO2 films by embedding highly-ordered metal nanoisland (NI) arrays to act as electric field concentrators. Embedded films exhibit significant reduction in operating voltages while displaying enhanced uniformity of resistance states. This behavior is attributed to the concentration of electric fields along Pt and Ti NIs and their interactions with the surrounding oxide film matrix environment, which induce separate and distinct filamentary formation mechanisms that affect the stability. A comparison of the density and distribution of the oxygen vacancies responsible driving filament dynamics is made via combined electrostatic force microscopy (EFM) and conductive atomic force microscopy (c-AFM) studies. Finally, the morphological evolution of conducting vacancy filaments is enabled by three-dimensional (3-D) c-AFM nanotomography and cross-sectional scanning transmission electron microscopy – energy dispersive spectroscopy (STEM-EDS) to provide direct correlations of nanoisland-oxide interactions with overall switching performance. The application of these exciting results to other vacancy-mediated electronic and electrochemical processes will also be discussed.

Desorption of Positive Metal Ions from the Surface of a Massive Anode on a Zeolite Plate

The transfer of positive metal ions from the surface of a massive anode on a zeolite plate in an electric field that is anomalously low for this process has been considered. An explanation given to this phenomenon relies on the fact that an electric field produces a negative charge in the conductive pores of zeolite near the anode. This charge localizes on the walls of pores and generates an anomalously high local electric field on the surface of the anode. This field provides ion transfer from the anode to the zeolite surface. The presence of a threshold external field for this phenomenon has been explained.

Monolithic Metal Dimer-on-Film Structure: New Plasmonic Properties Introduced by the Underlying Metal.

Dimers, two closely spaced metallic nanostructures, are one of the primary nanoscale geometries in plasmonics, supporting high local field enhancements in their interparticle junction under excitation of their hybridized "bonding" plasmon. However, when a dimer is fabricated on a metallic substrate, its characteristics are changed profoundly. Here we examine the properties of a Au dimer on a Au substrate. This structure supports a bright "bonding" dimer plasmon, screened by the metal, and a lower energy magnetic charge transfer plasmon. Changing the dielectric environment of the dimer-on-film structure reveals a broad family of higher-order hybrid plasmons in the visible region of the spectrum. Both of the localized surface plasmons resonances (LSPR) of the individual dimer-on-film structures as well as their collective surface lattice resonances (SLR) show a highly sensitive refractive index sensing response. Implementation of such all-metal magnetic-resonant nanostructures offers a promising route to achieve higher-performance LSPR- and SLR-based plasmonic sensors.

Effect of excavation stress condition on hydraulic fracture behaviour

The fracture behaviour of single- and multi-stage hydraulic fracturing (HF) under varying excavation stress conditions was studied using a flow-coupled discrete element method (DEM). Conventional HF theories and analytical solutions for excavation stress redistribution are used to verify the numerical model. The numerical simulation results indicated that for the single-stage HF, both the in situ stresses and injection location have major influences on the HF propagation path, caused by the redistributions of the local maximum and minimum principal stresses under excavation conditions. Fractures generally propagate perpendicular to the local minimum principal stress (σmin) and also propagate preferentially towards the area with lower σmin. Hydraulic fractures propagation is dominated by tensile failure, and the generation of fracture branches is disfavored because of the high local compressive stress field induced by the HF process itself. As for multi-stage HF, different excavation stress conditions may cause different fracture development performances because of alterations of the preferred fracture orientation, blunting of propagating fractures, or creating a more complex fracture geometry in the affected zone. Therefore, when using high-pressure HF for preconditioning the rock mass to assist mechanical excavation in hard rock mining, performance can be enhanced by understanding the impact of stress changes and careful selection of appropriate HF methods depending on the in situ stress.

Optical Trapping, Sizing, and Probing Acoustic Modes of a Small Virus

Prior opto-mechanical techniques to measure vibrational frequencies of viruses work on large ensembles of particles, whereas, in this work, individually trapped viral particles were studied. Double nanohole (DNH) apertures in a gold film were used to achieve optical trapping of one of the smallest virus particles yet reported, PhiX174, which has a diameter of 25 nm. When a laser was focused onto these DNH apertures, it created high local fields due to plasmonic enhancement, which allowed stable trapping of small particles for prolonged periods at low powers. Two techniques were performed to characterize the virus particles. The particles were sized via an established autocorrelation analysis technique, and the acoustic modes were probed using the extraordinary acoustic Raman (EAR) method. The size of the trapped particle was determined to be 25 ± 3.8 nm, which is in good agreement with the established diameter of PhiX174. A peak in the EAR signal was observed at 32 GHz, which fits well with the predicted value from elastic theory.

Plasmon mediated decomposition of brominated nucleobases on silver nanoparticles \u2013 A surface enhanced Raman scattering (SERS) study

The localized surface plasmon resonances (LSPRs) of silver nanoparticles (AgNPs) give rise to the generation of so called hot electrons and a high local electric field enhancement, which enable an application of AgNPs in different fields ranging from catalysis to sensing. Hot electrons generated upon the decay of LSPRs are transferred to molecules adsorbed on the surface of the NPs and trigger chemical reactions via dissociative electron attachment (DEA). Herein, we report on the hot electron induced decomposition of the brominated nucleobases – 8-bromoadenine, 8-bromoguanine, 5-bromocytosine and 5-bromouracil on laser illuminated AgNP surfaces. Surface enhanced Raman scattering (SERS) spectra of all canonical nucleobases and their brominated analogues have been recorded at different laser illumination times, and for the very first time we present SERS measurements of 8-bromoguanine and 5-bromocytosine. Reaction products have been identified by their vibrational fingerprint revealing the cleavage of the carbon bromide bond in all cases even under mild illumination conditions. These results indicate that the well-known reactions from DEA experiments in the gas phase (i) are also taking place on nanoparticle surfaces under ambient conditions, (ii) can be monitored by SERS, and (iii) are also of importance in analytical SERS applications involving electrophilic molecules, as the bands originating from reaction products need to be identified.Graphical abstract

Long-range Graphene Surface Plasmon Polariton induced Tunable Optical Bistability in Terahertz Range

An idea towards the low threshold optical bistability and tunability of bistable thresholds at THz frequency under realistic situation is analyzed through a proposed multilayered configuration. The proposed configuration comprises a coupling prism of silicon, and monolayer graphene sheets spatially separated through 35 nm thick polymer which is sandwiched between an air gap of thickness 4.6 μm and a kerr polymer (GaAs) at 300 K temperature and at an incident light frequency of 2 THz. Utilizing the large nonlinear effect of the configuration obtained by the nonlinearity of the kerr polymer and the high local field effect due to long-range graphene surface plasmon resonance, the switching-down and switching-up threshold values for optical bistability have been markedly reduced upto 1.21×105 V/m and 1.24×105 V/m respectively thereby rendering a minimum threshold intensity of 7.2 kW/cm2 than the articles reported till date. Tunability of the switching-down and switching-up threshold values have also been realized from 1.21×105 V/m to 5.18×105 V/m and 1.24×105 V/m to 1.424×106 V/m respectively through electrical or chemical modification of the charge carrier density and hence fermi energy of the graphene from 0.23 eV to 0.41 eV and from 1.21×105 V/m to 1.56×105 V/m and 1.24×105 V/m to 2.12×105 V/m respectively for the variation in incident angle from 70° to 88°. The effect of damping rate of graphene on optical bistability behavior has also been observed. Therefore the proposed optical bistable configuration finds potential applications in the field of all-optical switching, optical transistor, optical logic, nano-illumination and optical memory with low input threshold at THz frequency and can also be used to realize tunable optical bistable device for future THz optical communication technology.

A microfabricated, 3D-sharpened silicon shuttle for insertion of flexible electrode arrays through dura mater into brain

Objective. Electrode arrays for chronic implantation in the brain are a critical technology in both neuroscience and medicine. Recently, flexible, thin-film polymer electrode arrays have shown promise in facilitating stable, single-unit recordings spanning months in rats. While array flexibility enhances integration with neural tissue, it also requires removal of the dura mater, the tough membrane surrounding the brain, and temporary bracing to penetrate the brain parenchyma. Durotomy increases brain swelling, vascular damage, and surgical time. Insertion using a bracing shuttle results in additional vascular damage and brain compression, which increase with device diameter; while a higher-diameter shuttle will have a higher critical load and more likely penetrate dura, it will damage more brain parenchyma and vasculature. One way to penetrate the intact dura and limit tissue compression without increasing shuttle diameter is to reduce the force required for insertion by sharpening the shuttle tip. Approach. We describe a novel design and fabrication process to create silicon insertion shuttles that are sharp in three dimensions and can penetrate rat dura, for faster, easier, and less damaging implantation of polymer arrays. Sharpened profiles are obtained by reflowing patterned photoresist, then transferring its sloped profile to silicon with dry etches. Main results. We demonstrate that sharpened shuttles can reliably implant polymer probes through dura to yield high quality single unit and local field potential recordings for at least 95 days. On insertion directly through dura, tissue compression is minimal. Significance. This is the first demonstration of a rat dural-penetrating array for chronic recording. This device obviates the need for a durotomy, reducing surgical time and risk of damage to the blood-brain barrier. This is an improvement to state-of-the-art flexible polymer electrode arrays that facilitates their implantation, particularly in multi-site recording experiments. This sharpening process can also be integrated into silicon electrode array fabrication.

High circular dichroism and robust performance in planar plasmonic metamaterial made of nano-comma-shaped resonators

Circularly dichroic metasurfaces are highly sought for a plethora of applications; while many alternative options are present in the literature, it is hard to select an approach that combines high circular dichroism (CD) with stable performances and easy reproducibility. In this work, we have designed and experimentally investigated a planar plasmonic metamaterial based on a comma-shaped geometry, which features such characteristics. We have focused the complexity of realization in the design process, which combines intrinsic chirality and high field localization, while fabrication was executed by using a standard single-step lift-off procedure. We have produced two classes of samples, closely related in shape but differing slightly in designing method and results, both reaching high levels of CD. Experiments on the first reveal the sensibility of the metasurface to geometrical variations due to fabrication non-idealities, as often happens in metamaterials based on surface plasmons and resonances; on the other hand, with the second, we demonstrate that it is possible to guarantee stable peak values on specific wavelength ranges, even when dealing with relevant fabrication tolerances. Moreover, numerical analyses suggest the possibility to reach values converging toward unity. The ease of implementation by using standard fabrication procedures and the robustness of the performance even with fabrication imprecisions make our proposed metamaterial eligible for adoption for further research in high-precision spectroscopy and implementation at industrial scale.

CVD grown nitrogen doped graphene is an exceptional visible-light driven photocatalyst for surface catalytic reactions

The photocatalytic potential of large area CVD grown nitrogen doped graphene (NGr) has been explored though the chemical transformation of 4-nitrobenzene thiol into p,p′-dimercaptoazobenzene. Decoration of NGr with Ag nanocubes with rounded edges to form NGr/Ag nanohybrids resulted in a slight increase in the work-function and a decrease in the n-type character of NGr due to ground state transfer of negative charge from NGr to Ag. The Ag nanocubes exhibited a localized surface plasmon resonance (LSPR) at ~425 nm. When the NGr/Ag nanohybrids were illuminated with visible light of wavelength close to the LSPR peak, Kelvin probe force microscopy (KPFM) indicated a dramatic change in surface potential of −225 mV and Raman spectra detected electron accumulation in NGr, which are attributed to a high local field enhancement-mediated hot electron injection into NGr and the formation of long-lived charge separated states. Pristine nitrogen doped graphene and its coupled system with plasmonic Ag nanoparticles showed superior photocatalytic performance compared to bare plasmonic Ag catalyst. While standalone Ag NPs were unable to complete the transformation of 4-NBT into DMAB even at a laser power of 10 mW, NGr/Ag nanohybrids completed this transformation at a laser power of 1 mW, pointing to the high photoreduction strength of NGr/Ag. Density functional theory (DFT) based computational modeling was used to examine the electronic structure of graphene doped with graphitic, pyridinic and pyrrolic nitrogen dopant atoms. DFT results indicated an enhanced chemical reactivity of NGr due to stronger localization of charge at the dopant sites and a pronounced difference in the projected density of states (PDOS) for carbon atoms in proximity to, and distant from, the nitrogen dopant sites.

Gold‐ and Silver‐Coated Barium Titanate Nanocomposites as Probes for Two‐Photon Multimodal Microspectroscopy

AbstractImproved multiphoton‐excited imaging and microspectroscopy require nanoprobes that can give different nonlinear optical signals. Here, composite nanostructures with a barium titanate core and a plasmonic moiety at their surface are synthesized and characterized. It is found that the core provides a high second‐order nonlinear susceptibility for sensitive second harmonic generation (SHG) imaging in living cells. As a second function in the two‐photon regime, the plasmonic part yields high local fields for resonant and nonresonant surface enhanced hyper Raman scattering (SEHRS). SEHRS complements the one‐photon surface enhanced Raman scattering (SERS) spectra that are also enhanced by the plasmonic shells. Barium titanate silver core–shell (Ag@BaTiO3) composites are specifically suited for SEHRS and SHG excited at 1064 nm, while gold at barium titanate (Au@BaTiO3) nanoparticles can be useful in a combination of SHG and SERS at lower wavelengths, here at 785 nm and 850 nm. The theoretical models show that the optical properties of the BaTiO3 dielectric core depend on probing frequency, shape, size, and plasmonic properties of the surrounding gold nanoparticles so that they can be optimized for a particular type of experiment. These versatile, tunable probes give new opportunities for combined multiphoton probing of morphological structure and chemical properties of biosystems.