Dielectric Environment Research Articles

Ultrafast Exfoliation of 2D Materials by Solvent Activation and One-Step Fabrication of All-2D-Material Photodetectors by Electrohydrodynamic Printing.

Large-scale liquid exfoliation of two-dimensional materials such as molybdenum disulfide, tungsten disulfide, and graphene for the synthesis of printable inks is still inefficient due to many hours of exfoliation time needed to achieve a highly concentrated dispersion that is useful for printing. Here, we report that soaking the bulk 2D material powders in a variety of solvents (water, ethanol, isopropanol, acetone, methanol, dimethylformamide, N-methyl pyrrolidone, and hexane) briefly as short as 5 min "activates" them to be much more easily exfoliated afterward. The unsoaked powder yielded a negligible concentration of dispersed nanosheets (less than 0.01 mg/mL) even after long hours of sonication, while the powders soaked in water resulted in dispersed nanosheets of 1.21 mg/mL for MoS2 and 1.28 mg/mL for WS2 after 6 and 4 h of sonication, respectively, a more than 100 time increase. For graphene, soaking in methanol for 5 min prior to sonication for 6 h yielded an increase in the dispersed nanosheet concentration to 0.13 mg/mL, a more than 10 time increase in concentration. The enhanced exfoliation is originated not from the intercalated solvent molecules but from the slightly increased d-spacing of the bulk powders during soaking due to the different dielectric environments in the solvents, which assists in the exfoliation afterward. We further fabricated MoS2 and WS2 photodetectors with graphene as electrodes by one-step electrohydrodynamic (EHD) printing using highly concentrated inks (>2 mg/mL) obtained by ultrafast liquid exfoliation, which have light sensitivity down to 0.05 sun. We believe that this ultrafast exfoliation technique combined with the one-step device printing technique enables a big step toward the mass production of functional devices fabricated from solution-processed 2D material inks.

Chemical switching of low-loss phonon polaritons in \u03b1-MoO3 by hydrogen intercalation

Phonon polaritons (PhPs) have attracted significant interest in the nano-optics communities because of their nanoscale confinement and long lifetimes. Although PhP modification by changing the local dielectric environment has been reported, controlled manipulation of PhPs by direct modification of the polaritonic material itself has remained elusive. Here, chemical switching of PhPs in α-MoO3 is achieved by engineering the α-MoO3 crystal through hydrogen intercalation. The intercalation process is non-volatile and recoverable, allowing reversible switching of PhPs while maintaining the long lifetimes. Precise control of the intercalation parameters enables analysis of the intermediate states, in which the needle-like hydrogenated nanostructures functioning as in-plane antennas effectively reflect and launch PhPs and form well-aligned cavities. We further achieve spatially controlled switching of PhPs in selective regions, leading to in-plane heterostructures with various geometries. The intercalation strategy introduced here opens a relatively non-destructive avenue connecting infrared nanophotonics, reconfigurable flat metasurfaces and van der Waals crystals.

Review on natural ester and nanofluids as an environmental friendly alternative to transformer mineral oil

The development of reliable, environmentally safe and economic insulating oil for the transformer is an endless effort of the electrical industry. Recent research is based on natural ester fluid, the green insulating oil which exhibits excellent dielectric performance and environment friendly characteristics. Nanofluids are also emerging as potential replacement for the conventional mineral oil used in transformers. Characterisation of nanoparticle filled mineral oil and natural esters have validated their improved dielectric behaviour in comparison to the unfilled oil. Although the applications are wide, the state of the art technology requires a deeper understanding of the underlying phenomenon. Much work is expected to be done in the application of nanofluids prepared with mineral oil and natural ester, particularly its effect on the cellulose insulation. The study provides an overview of the different materials that have been used as alternatives to the conventional transformer mineral oil, with special emphasis on natural esters and natural ester nanofluids, their advantages and challenges.

Recent Advances in Plasmonic Perovskite Solar Cells.

Perovskite solar cells (PSCs) have emerged recently as promising candidates for next generation photovoltaics and have reached power conversion efficiencies of 25.2%. Among the various methods to advance solar cell technologies, the implementation of nanoparticles with plasmonic effects is an alternative way for photon and charge carrier management. Surface plasmons at the interfaces or surfaces of sophisticated metal nanostructures are able to interact with electromagnetic radiation. The properties of surface plasmons can be tuned specifically by controlling the shape, size, and dielectric environment of the metal nanostructures. Thus, incorporating metallic nanostructures in solar cells is reported as a possible strategy to explore the enhancement of energy conversion efficiency mainly in semi‐transparent solar cells. One particularly interesting option is PSCs with plasmonic structures enable thinner photovoltaic absorber layers without compromising their thickness while maintaining a high light harvest. In this Review, the effects of plasmonic nanostructures in electron transport material, perovskite absorbers, the hole transport material, as well as enhancement of effective refractive index of the medium and the resulting solar cell performance are presented. Aside from providing general considerations and a review of plasmonic nanostructures, the current efforts to introduce these plasmonic structures into semi‐transparent solar cells are outlined.

Concerted Photoluminescence of Electrochemically Self-Assembled CuSCN / Stilbazolium Dye Hybrid Thin Films

Whether they are inorganic or organic, the key words for creation of new functional materials should be (1) solution, (2) low temperature, and (3) rare-element-free for sustainable development. As a leading example, we explore electrochemical self-assembly (ESA) of inorganic / organic hybrid thin films. In addition to the interests to study evolution of unique hybrid structures, concerted new functionalities are anticipated due to intimate interaction between the inorganic and organic constituents. Herein, we report ESA of CuSCN / 4-(N,N-dimethylamino)-4’-(N’-methyl)stilbazolium (DAS) hybrid thin films and its concerted photoluminescence (PL) behavior [1]. Hybrid thin films of crystalline CuSCN/DAS in three distinctively different nanostructures were obtained by ESA from a single pot containing all the chemical ingredients. Adsorption of DAS during the electrochemical precipitation of CuSCN result in significant change of film morphology, crystal structure and its orientation, as DAS “occluded” into crystal grains of β-CuSCN, DAS phase separated nano-“haircomb” shape β-CuSCN, and DAS phase-separated in nano-“scale” shape α-CuSCN, by simply increasing DAST concentration in the bath [2]. Their optical properties for UV-vis-NIR absorption, photoluminescence (PL), and PL excitation (PLE) spectra were examined between 77 and 298 K, in comparison with solution and solid powder of DAS tosylate (DAST). While all the fluorescent dyes we previously found to hybridize with CuSCN underwent quenching due to hole injection from dye excited state to the valence band of CuSCN, DAS presented the first and only example to exhibit PL when combined with CuSCN [3]. DAST exhibited a strong exciton-phonon coupling to weaken, broaden, and red shift PL at room temperature, so that it inversely is strongly enhanced, sharpened, and blue-shifted at 77 K. However, the PL of the same dye in the hybrid thin film shows a slight red shift and not much intensified. Dielectric environment as well as ordered alignment of DAS greatly stabilize exciton against thermalization loss by suppressing twisted intramolecular charge transfer (TICT) and exciton-phonon coupling. The smallest temperature dependence, thus to be interpreted as the strongest exciton stabilization, was found for the “haircomb” β-CuSCN/DAS hybrid. The PLE spectra of the “scale” α-CuSCN/DAS hybrid show a prominent sharp peak at 380 nm and another double-peak between 400 and 450 nm by cooling temperature. The former corresponds to the band-edge absorption of CuSCN, while the latter being similar to the LMCT absorption of [Cu(SCN)]+ complex. It has been confirmed that those peaks in PLE spectra indicate the contribution of CuSCN thin film without DAS for the generation of PL. These contributions of the light absorption by CuSCN to the PL from DAS clearly indicate energy transfer from CuSCN to DAS as a concerted PL mechanism. Concerted PL by energy transfer from CuSCN to DAS has also been found to occur efficiently, especially for the “scale” α-CuSCN/DAS with the largest contact area due to interpenetrating hybrid nanostructure in the smallest domain size. Those hybrid thin film having the concerted PL mechanism (Fig. 1) can possibly make these materials useful as LEDs.[1] K. Uda et al., ACS Omega, 2019, 4 (2), 4056-4062.[2] Y. Tsuda et al., Chem., 2017, 148, 845-854.[3] K. Uda et al., ECS Trans., 2019, 88, 323-333. Figure 1

(Invited) Momentum-Resolved Dielectric Response of Free Standing Black Phosphorus Down to the Monolayer

Atomic layers of Black Phosphorus (BP) have been recently isolated, ten years after graphene. BP stands out in the 2D-materials panorama by its unique semiconducting properties: direct bandgap which can be tuned by the layer number in a wide range of wavelengths from visible (monolayer) to midinfrared (bulk) [1]. This tunability combined the anisotropy of the structure therefore offers promising perspectives in various fields such as electronics and photonics. However, the fast photooxidation in ambient condition, coupled to a high sensitivity to quantum confinement and dielectric environment in ultrathin BP, make very difficult the investigations on its intrinsic optical properties [2]. Further, as screening effects may strongly affect electronic and spectroscopic properties of 2D materials, it is highly desirable to investigate intrinsic properties of free-standing layers as well the ones of the bulk material which remain poorly known.To start with, we have investigated photoluminescence (PL) and absorption properties of high quality BP single crystals at 2K [FTIR]. The PL intensity appears comparable to high quality InAs crystals confirming the suitability of BP for infrared applications. Two peaks are evidenced at 0.275 eV and 0.26 eV. The highest energy peak is particularly narrow (FWHM = 3.8meV) and is tentatively attributed to exciton recombination, thanks to the temperature dependence of PL spectra from 2K to 300K. Further, we have measured the gap related energy shift of the PL spectra as a function of the BP thickness in a series of mechanically exfoliated samples with thicknesses down to 10 nm, complementary to the previous measures done on thinner samples [3].As far as the thinnest layers are concerned, which cannot manipulated in air, we have shown that Angular resolved Electron energy loss spectroscopy implemented in Transmission Electron Microscopy (Ar-EELS-TEM) offers a unique way to investigate dielectric response of free-standing layers related to valence band and plasmon excitations with the advantage to get access to their q dispersion and their symmetry properties [4]. By combining this technique with suitable ab initio calculations, we have studied the dielectric response of free-standing BP layers as a function of the number of layers. We found optical bandgap values of 1.9 eV, 1.4 eV and 1.1 eV for the mono- bi- and trilayer respectively. Moreover, by combining our results with a simple variational model, we correlate the exciton energy with the dielectric screening. We hence demonstrate that the variations of the electronic gap are sizeably larger than the variations of the binding energy. Finally, we probe and analyze the volume and surface plasmons dispersion as a function of momentum for the 1-3 BP layers and bulk and highlight a deviation and linearization of the parabolic dispersion with strong anisotropic fingerprints [5].[1] G. Zhang et al., Nat. Com., 8, 14071, (2017)[2] A. Favron, E. Gaufres et al Nature Mat. 14 (2015) 826.[3] C. Chen et al., NanoLett., (2019)[4] F. Fossard et al, Phys. Rev. B 96, 115304 (2017)[5] E. Gaufres et al, Nanoletters (2019); DOI: 10.1021/acs.nanolett.9b03928

Plasmon modes characterization in hybrid noble/ ferromagnetic nanodisks using an effective medium approximation

Magneto-plasmonic systems composed by noble/ferromagnetic metals structures can be used to control light polarization. The extraordinary optical properties arising from combining strong local enhancements of electromagnetic fields in surface plasmon excitations with the magneto-optical activity inherent to ferromagnetic materials, can be controlled by external magnetic fields that have demonstrated the possibility to control and amplify the magneto-optical properties via plasmonic excitations. In this work, the anisotropic optical and ellipsometric sensitivity to dielectric environment of multi-layered hybrid gold/cobalt magnetoplasmonic nanodisks are studied in the framework of the effective-medium approximation, where the components of the dielectric tensor of the system are modelled using Lorentz-like oscillators. Furthermore, full electromagnetic simulations were performed using the standard Finite Element Method, which allow us to characterize the modes and explore the role of the dielectric environment of the nanodisks in the optical properties of the nanostructures via the variation of a coating silicon dioxide (SiO2) layer thickness. Our model and numerical results show a very good agreement with spectral ellipsometry measurements, where two orthogonal plasmonic modes one in-plane and one out-of-plane- are well characterized. These results lead us to conclude that the effective-medium approximation is good enough to accurately describe the plasmonic behavior of multi-layered confined nanostructures.

Topography-induced variations of localized surface plasmon resonance in tip-enhanced Raman configuration

We report on topography-induced changes of the localized surface plasmon resonance (LSPR) enhanced luminescence of gold tip on SrTiO3 nanostructures with apertureless scanning near-field optical microscopy (aSNOM) in tip-enhanced Raman spectroscopy (TERS) configuration. Our experimental and simulated results indicate that the averaged refractive index of the dielectric environment of the tip apex containing both air and SrTiO3 in variable volume ratios, is dependent on the topography of the sample. This reveals that the local topography has to be taken into consideration as an additional contribution to the position of the LSPR.

Collective excitations in gapped graphene-GaAs double-layer structures

The aim of this work is to determine the collective excitations and decay rate of plasma oscillations in a gapped graphene-GaAs double-layer structure within random phase approximation without temperature effects. Firstly, we utilize long wavelength approximation expansions of polarization functions and Coulomb bare interactions to observe the analytical expressions for plasmon frequencies. These analytical formulae demonstrate that the band gap has significant contributions to collective excitations. Secondly, our numerical calculations illustrate that in-phase optical plamon branch continues in single-particle excitation area while acoustic out-of-phase one merges into the boundary of this area then disappears. Besides, taking into account the band gap decreases remarkably optical plasmon frequency meanwhile carrier density affects on acoustic branch, dissimilar to that in other double-layer structures. Finally, the interlayer separation and the inhomogeneous dielectric environment affect on both plasmon modes, similar to those obtained in previous works.

Tunable Fano Resonances in an Ultra-Small Gap

A Fano resonance is experimentally observed in a single silver nanocube separated from a supporting silver film by a thin aluminum oxide film. The resonance spectrum is modulated by changing the size of the silver nanocube and its distance from the silver film. The system is fabricated by a bottom-up process with an accurately controlled nanogap at the sub-6-nm scale. The simulation result shows that the destructive interference between the dipole mode and the quadrupole mode in this “nanocube on mirror” (NCoM) structure is responsible for the resonance. The spectra red-shifted as the size of the silver nanocube increased and its distance from the silver film decreased. In addition, a refractive index sensitivity of the spectrum of 140 meV/RIU (refractive index unit), with a 2.4 figure of merit, is obtained by changing the dielectric environment around the silver nanocube. This work will enable the development of high-performance tunable optical nanodevices based on NCoM structures.

Polar vibrational and dielectric properties of monolayer transition metal dichalcogenides from macroscopic equations

Long wavelength polar vibrations in monolayer (ML) transition metal dichalcogenides (TMDs) are systematically studied for in-plane and out-of-plane motions, using two pairs of macroscopic equations deduced from a microscopic dipole lattice model accounting for local field effects (LFEs) and electronic polarization (EP). Longitudinal and transverse optical modes and out-of-plane modes are derived, and the analytical expressions describe previous first-principles calculations very well. Owing to the LFEs, the in-plane dielectric susceptibilities of ML TMDs are one order of magnitude greater than the out-of-plane susceptibilities. Furthermore, the effects of the dielectric environment on the polar vibrations are studied. Both EP and LFEs should be accounted for obtaining an accurate evaluation of dielectric susceptibility and key lattice-dynamical properties such as Born charge and phonon dispersion. A two-dimensional (2D) Lyddane–Sachs–Teller relation and a frequency–susceptibility relation are derived for in-plane and out-of-plane motions, relating the 2D dielectric functions or susceptibilities to the polar phonon frequencies. The results are also compared in detail with those of ML hexagonal boron nitride.

Effects of Kinetic Dielectric Decrement on Ion Diffusion and Capacitance in Electrochemical Systems.

Diffusion of ionic components in electrolytes not only eliminates the gradients of ionic concentrations but also alters the local dielectric environment, and the coupling effect between kinetic dielectric decrement and ionic concentration gradient on the diffusion dynamics is not well understood. Herein, taking the charging process in electrical double layer systems as a case study, we conduct a multiscale investigation of ion diffusions in aqueous electrolytes by combining the dynamic density functional theory and an ion-concentration-dependent dielectric constant model. By properly considering the time evolutions of local dielectric constant coupled with ion density, we report an interesting phenomenon on the suppression of surface charge density that is not captured by conventional models. In addition, we show that the usage of aqueous electrolyte with small dielectric decrement coefficients promotes the capacitance, in quantitative agreement with experimental measurements.

Interlayer excitons in van der Waals heterostructures: Binding energy, Stark shift, and field-induced dissociation

Photoexcited intralayer excitons in van der Waals heterostructures (vdWHs) with type-II band alignment have been observed to tunnel into interlayer excitons on ultrafast timescales. Such interlayer excitons have sufficiently long lifetimes that inducing dissociation with external in-plane electric fields becomes an attractive option of improving efficiency of photocurrent devices. In the present paper, we calculate interlayer exciton binding energies, Stark shifts, and dissociation rates for six different transition metal dichalcogenide (TMD) vdWHs using a numerical procedure based on exterior complex scaling (ECS). We utilize an analytical bilayer Keldysh potential describing the interaction between the electron-hole pair, and validate its accuracy by comparing to the full multilayer Poisson equation. Based on this model, we obtain an analytical weak-field expression for the exciton dissociation rate. The heterostructures analysed are MoS2/MoSe2, MoS2/WS2, MoS2/WSe2, MoSe2/WSe2, WS2/MoSe2, and WS2/WSe2 in various dielectric environments. For weak electric fields, we find that WS2/WSe2 supports the fastest dissociation rates among the six structures. We, furthermore, observe that exciton dissociation rates in vdWHs are significantly larger than in their monolayer counterparts.

The Effect of the Dielectric Environment on Electron Transfer Reactions at the Interfaces of Molecular Sensitized Semiconductors in Electrolytes

Electron transfer theories predict that rates of charge transfer vary with the dielectric properties of the environment where the reaction occurs. An appropriate description of this relation for mo...

Ultrathin Plasmonic Tungsten Oxide Quantum Wells with Controllable Free Carrier Densities.

Localized surface plasmon resonances (LSPR) of nanostructures can be tuned by controlling their morphology, local dielectric environment, and free carrier concentration. We report the colloidal synthesis of an ∼3 tungsten-oxygen (W-O) layer thick (∼1 nm), two-dimensional (2D) WO3-x nanoplatelets (NPLs) (x ≈ 0.55-1.03), which display tunable near-infrared LSPR properties and additionally high free electron density (Ne) that arises predominantly from the large shape factor of 2D NPLs. Importantly, the W to O composition ratios inferred from their LSPR measurements show much higher percentage of oxygen vacancies than those determined by X-ray diffraction analysis, suggesting that the aspect ratio of ultrathin WO3-x NPLs is the key to producing an unprecedentedly large Ne, although synthesis temperature is also an independent factor. We find that NPL formation is kinetically controlled, whereas thermodynamic parameter manipulation leads to Ne values as high as 4.13 × 1022 cm-3, which is close to that of plasmonic noble metals, and thus our oxide-based nanostructures can be considered as quasi-metallic. The unique structural properties of 2D nanomaterials along with the high Ne of WO3-x NPLs provide an attractive alternative to plasmonic noble metal nanostructures for various plasmon-driven energy conversions and design of photochromic nanodevices.

Long-range surface plasmons on duplex metal nanolayers

Planar structures with two metal nanolayers supporting long-range surface plasmons (LRSPs) are described. We introduce a design where a one-dimensional photonic crystal (1D PC) is used as a support for a planar dielectric film sandwiched between two metal nanolayers. This quasisymmetric design is compared with a symmetric where two metal nanolayers deposited on opposite sides of the planar dielectric film have the same semi-infinite dielectric environment from both sides. Such multilayer structures are useful when electrical power is supplied to the planar film through LRSP-supporting metal nanolayers on opposite sides of the film. This planar film may be, for example, an electro-optical crystal or an (organic) electroluminescent film. A method for designing such LRSP-supporting duplex metal structures based on an impedance approach is presented. Calculations of the optimal thickness for the metal-bounded dielectric film, which provides minima of optical field inside the metal nanolayers, are given for both symmetric and quasisymmetric cases.

Impact of dielectric environment on the linear and nonlinear optical properties for CdS/ZnS cylindrical core/shell quantum dots

By the mean of the effective mass approximation (E.M.A) and compact density matrix method, the energy eigenvalues, linear and nonlinear dielectric function have been theoretically investigated for CdS/ZnS cylindrical core/shell quantum dot embedded in PVA, PMMA and SiO2 matrices. Simulation results showed that the presence of the dielectric mismatch in the QD-matrix system can lead to a significant enhancement of the nonlinear dielectric function. According to our calculations, peaks of the real and imaginary part of the dielectric function can experience a redshift or a blue shift with the appropriate dielectric environment.

Charge Transfer through Redox Molecular Junctions in Nonequilibrated Solvents.

Molecular conduction operating in dielectric solvent environments is often described using kinetic rates based on the Marcus theory of electron transfer at a molecule-metal electrode interface. However, the successive nature of charge transfer in such a system implies that the solvent does not necessarily reach equilibrium in such processes. Here we generalize the theory to account for solvent nonequilibrium and consider a molecular junction consisting of an electronic donor-acceptor system coupled to two metallic electrodes and placed in a polarizable solvent. We determine the nonequilbrium distribution of the solvent by solving diffusion equations in the strong- and weak-friction limits and calculate the charge current and its fluctuating behavior. In extreme limits, the absence of the solvent or fast solvent relaxation, the charge-transfer statistics is Poissonian, while it becomes correlated by the dynamic solvent between these limits. A Kramers-like turnover of the nonequilibrium current as a function of the solvent damping is found. Finally, we propose a way to tune the solvent-induced damping using geometrical control of the solvent dielectric response in nanostructured solvent channels.

Layer dependence of defect charge transition levels in two-dimensional materials

Point defects in two-dimensional (2D) materials hold great promise for optoelectronic and quantum technologies. Their properties depend sensitively on the dielectric environment and number of 2D layers, but this has remained a challenge to include in first-principles calculations on account of the high computational cost. Recent first-principles techniques facilitate efficient prediction of substrate effects on defect charge transition levels in 2D materials, replacing the substrate by a continuum dielectric model. Here, we show that analogous continuum treatment of defect-free layers in multilayer 2D materials accurately predicts defect energies compared to explicit multilayer calculations, but at a small fraction of the computational cost. Applications of this technique to one- to five-layer and bulk hexagonal boron nitride reveal that defect ionization energies systematically decrease with an increasing number of layers and for defects in inner layers due to increased dielectric screening. Our results highlight the dominant role of electrostatic screening in the effect of the environment and the feasibility of tuning defect levels in 2D materials using material thickness and defect location within the material.

Hole Transfer Promoted by a Viscosity Additive in an All-Polymer Photovoltaic Blend.

Viscosity modifiers are widely applied to improve the mechanical compliance of organic optoelectronic devices. However, the effect of the viscosity additives on the charge dynamics remains poorly understood. Here, we report the observation of markedly different effects of a high-viscosity polymeric additive on the electron- and hole-transfer dynamics in all-polymer organic photovoltaic blends. By using ultrafast transient absorption spectroscopy, we determine that hole transfer from charge-transfer excitations in the acceptor is markedly promoted while the electron transfer from local excitations in the donor remains nearly unchanged upon introduction of viscosity additives into the blends. We argue that the modification of dielectric screening is the mechanism underlying the effect of the additive on the charge dynamics. This finding suggests a new strategy for designing high-performance flexible organic photovoltaic devices by manipulating the dielectric environment.