Dielectric Confinement Effect Research Articles

Interfacial and Defective Construction from Diverse CuxSy Quantum Dots toward Broadband Carbon-Based Microwave Absorber.

In this study, highly monodisperse copper sulfide (CuxSy) quantum dots (QDs) have been successfully obtained using a ligand-chemistry strategy, and then a variety of S-deficient CuxSy/nitrogen-doped carbon (NC) heterointerfaces are constructed by compositional fine-tuning (Cu9S5 → Cu1.96S → Cu). First-principles calculations show that the S-deficient domains of CuxSy QDs and N-doped domains of carbon synergistically enhance the electron transfer from CuxSy to NC. In addition, the finite element simulations demonstrate that the diverse CuxSy QDs exhibit their intrinsic size and dielectric confinement effects to precisely manipulate the electric field distortion and improve the relaxation polarization. Consequently, CuxSy@NC achieves excellent impedance matching and a strong loss mode dominated by dielectric polarization. Among them, CuxSy@NC-650 has a maximum effective absorption bandwidth of 7.7 GHz at 2.5 mm, while CuxSy@NC-700 features a minimum reflection loss of -66.7 dB at 13.7 GHz, respectively. Furthermore, the simulations of radar cross-sections have confirmed that the CuxSy@NC series is promising in the field of radar stealth.

Nonlinear Absorption in 2D Ruddlesden-Popper Perovskites: Pathways to Ultrafast Optical Applications.

Owing to their distinctive photophysical properties resulting from their larger exciton binding energy and the influence of dielectric and quantum confinement effects, considerable research interest has been directed toward two-dimensional Ruddlesden-Popper halide perovskites (2D RP-HPs). Particularly, 2D RP-HPs exhibit exceptional multiphoton absorption (MPA) effects that reveal versatile applications. In this work, two-photon absorption (2PA) and three-photon absorption (3PA) in 2D RP-HPs, specifically (PEA)2PbBr4 and (BA)2PbBr4 platelets, have been demonstrated through their photoluminescence spectra under multiphoton excitation, revealing a power law relationship with excitation energy. On the other hand, polarization-dependent MPA measurements are also conducted to obtain their anisotropy properties. The excellent 2PA and 3PA effects of 2D RP-HPs have found applications in ultrafast optics to construct fringe-resolved autocorrelation traces, enabling the retrieval of pulse duration not only passively mode-locked Ti:sapphire at 800 nm but also Yb-doped fiber lasers at 1030 nm.

Fluorinated Organic Cations Derived Chiral 2D Perovskite Enabling Enhanced Spin-Dependent Oxygen Evolution Reaction.

Chirality-induced spin selectivity observed in chiral 2D organic-inorganic hybrid perovskite holds promise to achieve spin-dependent electrochemistry. However, conventional chiral 2D perovskites suffer from low conductivity and hygroscopicity, limiting electrochemical performance and operational stability. Here, a cutting-edge material design is introduced to develop a stable and efficient chiral perovskite-based spin polarizer by employing fluorinated chiral cation. The fluorination approach effectively promotes the charge carrier transport along the out-of-plane direction by mitigating the dielectric confinement effect within the multi-quantum well-structured 2D perovskite. Integrating the fluorinated cation incorporated spin polarizer with BiVO4 photoanode considerably boosts the photocurrent density while reducing overpotential through a spin-dependent oxygen evolution reaction. Furthermore, the hydrophobic nature of fluorine in spin polarizer endows operational stability to the photoanode, extending the durability by 280% as compared to the device with non-fluorinated spin polarizer.

Reducing Dielectric Confinement Effect Enhances Carrier Separation in Two-dimensional Hybrid Perovskite Photocatalysts.

Two-dimensional organic-inorganic hybrid perovskites ( OIHPs) with alternating structure of the organic and inorganic layers have a natural quantum well structure. The difference of dielectric constants between organic and inorganic layers in this structure results in the enhancement of dielectric confinement effect, which exhibits a large exciton binding energy and hinders the separation of electron-hole pairs. Herein, a strategy to reduce the dielectric confinement effect by narrowing the dielectric difference between organic amine molecule and [PbBr6]4- octahedron is put forward. The Ethanolamine (EOA) contains hydroxyl groups, resulting in the positive and negative charge centers of O and H non-overlapping,which generated a larger polarity and dielectric constant. The reduced dielectric constant produces a smaller exciton binding energy (71.03 meV) of (C2H7NO)2PbBr4 ((EOA)2PbBr4) than (C8H11N)2PbBr4 ((PEA)2PbBr4 (156.07 meV), and promotes the dissociation of electrons and holes. The increasing of lifetime of photogenerated carrier in (EOA)2PbBr4 are proved by femtosecond transient absorption spectra. DFTcalculations have also indicated that the small energy shift of the total density of states (DOS) between the C/H/N and the Pb/Br in (EOA)2PbBr4 favors the separation of electrons and holes. In addition, this work demonstrates the application of (PEA)2PbBr4 and (EOA)2PbBr4 in the field of photocatalytic CO2 reduction.

Broken symmetries in quasi-2D charged systems via negative dielectric confinement

We report spontaneous symmetry breaking (SSB) phenomena in symmetrically charged binary particle systems under planar nanoconfinement with negative dielectric constants. The SSB is triggered solely via the dielectric confinement effect, without any external fields. The mechanism of SSB is found to be caused by the strong polarization field enhanced by nanoconfinement, giving rise to charge/field oscillations in the transverse directions. Interestingly, dielectric contrast can even determine the degree of SSB in transverse and longitudinal dimensions, forming charge-separated interfacial liquids and clusters on square lattices. Furthermore, we analytically show that the formed lattice constant is determined by the dielectric mismatch and the length scale of confinement, which is validated via molecular dynamics simulations. The novel broken symmetry mechanism may provide new insights into the study of quasi-2D systems and the design of future nanodevices.

Regulation of the quantum barrier and carrier transport toward high-efficiency quasi-2D Dion-Jacobson tin perovskite solar cells

Quasi-2D Dion-Jacobson (DJ) tin halide perovskite has attracted much attention due to its elimination of Van der Waals gap and enhanced environmental stability. However, the bulky organic spacers usually form a natural quantum well structure, which brings a large quantum barrier and poor film quality, further limiting the carrier transport and device performance. Here, we designed three organic spacers with different chain lengths (ethylenediamine (EDA), 1,3-propanediamine (PDA), and 1,4-butanediamine (BDA)) to investigate the quantum barrier dependence. Theoretical and experimental characterizations indicate that EDA with short chain can reduce the lattice distortion and dielectric confinement effect, which is beneficial to the effective dissociation of excitons and the inhibition of trap-free non-radiative relaxation. In addition, EDA cation shows strong interaction with the inorganic octahedron, realizing large aggregates in precursor solution and high-quality films with improved structural stability. Furthermore, femtosecond transient absorption proves that EDA cations can also weaken the formation of small n-phases with large quantum barrier to achieve effective carrier transport between different n-phases. Finally, the quasi-2D DJ (EDA)FA9Sn10I31 solar cells achieves a 7.07% power conversion efficiency with good environment stability. Therefore, this work sheds light on the regulation of the quantum barrier and carrier transport through the chain length of organic spacer for quasi-2D DJ lead-free perovskites.

On the specific behavior of the work function and surface potential of an asymmetric metal-dielectric nanosandwich

We examine thin film on a dielectric substrate (vacuum/Al/SiO2) in the stabilized jellium model and the Kohn–Sham method. We investigate surface and size effects on the effective potential and the electron work function, and analyze the spatial distributions of electrons and potentials. It is found that a dielectric environment generally leads to a decrease in the work function. The effect of dielectric confinement for the electron work function of the asymmetric metal-dielectric nanosandwiches is reduced only by the surface area weighted average value of the dielectric constants. This conclusion follows from the application of the Gauss theorem for a conducting sphere with an inhomogeneous dielectric coating. The flow of electrons from the dielectric face to the vacuum one due to the contact potential difference manifests itself in the appearance of an additional dipole between the left and right face within the spatial distributions of ions. This leads to the fact that in a vacuum the electrostatic and effective potentials change sign twice, as a result of which a potential barrier appears above the vacuum level. We introduced the position of an electron conduction band in the dielectric as the input parameter in the self-consistency procedure for one of the sandwich approximations. As it turned out, the barrier height depends only on the used local or non-local approximation of the exchange-correlation energy. The nontrivial origin and behavior of the calculated effective potential on the vacuum side of the film, as well as the reasons for it, are discussed.

Tuning exciton dynamics by the dielectric confinement effect in quasi-two-dimensional perovskites

The dielectric confinement effect plays an essential role in optoelectronic devices. Existing studies on the relationship between the dielectric confinement and the photoelectric properties are inadequate. Herein, three organic spacers with different dielectric constants are employed to tune the exciton dynamics of quasi-two-dimensional (quasi-2D) Ruddlesden–Popper perovskite films. Femtosecond transient absorption spectroscopy reveals that the small dielectric constant ligand enables a weak dynamic disorder and a large modulation depth of the coherent phonons, resulting in a more complete energy transfer and the inhibition of a trap-mediated nonradiative recombination. Additionally, the increase in the bulk-ligand dielectric constant reduces the corresponding exciton binding energy and then suppresses the Auger recombination, which is beneficial for high-luminance light-emitting diodes. This work emphasizes the importance of dielectric confinement for regulating the exciton dynamics of layered perovskites.

Effect of Dielectric Confinement on Energetics of Quantum Metal Films: Analysis of Calculation Results

Effect of Dielectric Confinement on Energetics of Quantum Metal Films: Analysis of Calculation Results

High dielectric constant ethanolamine based two-dimensional perovskite thin films and their application in perovskite PSCs

In recent years, the extensive exploration of two-dimensional (2D) perovskites in the perovskite solar cell field has been driven by their remarkable humidity stability and tunable optoelectrical properties. However, the device performance in solar cells is hampered by the constraints posed by the quantum and dielectric confinement effects. In this study, ethanolamine (EA+) with a high dielectric constant was introduced as a spacer cation to fabricate 2D perovskite (EA)2(MA)4Pb5I16 films. The investigation focused on evaluating the influence of various additives on the crystallinity, surface morphology, light absorption capacity, and carrier lifetime of EA+-based perovskite thin films. By incorporating MACl as an additive, we achieved smooth and compact perovskite thin films of superior quality, leading to perovskite solar cells (PSCs) with a remarkable photovoltaic efficiency of 10.24%. Furthermore, the unencapsulated (EA)2(MA)4Pb5I16-based device maintained 83% of its original efficiency after 840 h of storage in ambient conditions, outperforming the 2D perovskite counterpart that employed phenethylamine (PEA+) as the spacer cation with a dielectric constant of 3.314. These results convincingly demonstrate that the strategic incorporation of high dielectric constant organic cations into 2D PSCs offers a promising avenue for elevating their efficiency and humidity stability.

Bioinspired construction of size adjustable hollow polydopamine microspheres for ultra-low dielectric polyimide composites

Polydopamine (PDA) has been extensively investigated due to its excellent adhesion ability and versatile chemical potential for secondary reactions. However, the potential application of hollow polydopamine microspheres (H-PDA) in the field of low-dielectric electronic packaging remains an unexplored area. Here, H-PDA microspheres with adjustable size were synthesized by a template-assisted approach, and the influence of size and content of H-PDA on the comprehensive performance of PI/H-PDA composites was explored in detail. The morphology observation revealed that the presence of H-PDA could be homogeneously dispersed in the PI matrix, and the PI/H-PDA composite containing 2 wt% H-PDA-800 achieved the lowest dielectric constant of 1.96 with a dielectric loss of 0.0123 at 1 MHz. It was attributed to the synergistic effects of nano-sized air bubbles from H-PDA and the dielectric confinement effect. Meanwhile, PI/H-PDA composites maintained high thermal stability (T5> 517 °C). In addition, the incorporation of H-PDA had a strengthening and toughening effect on PI composites, which can be explained by the sliding theory of macromolecular chains. This work offers a feasible strategy to obtain ultra-low dielectric constant of porous PI composites while maintaining good mechanical properties and thermal stability.

High Curvature PEDOT:PSS Transport Layer Toward Enhanced Perovskite Light-Emitting Diodes.

The rapidly developed metal halide perovskite light-emitting diodes (PeLEDs) are considered as a promising candidate for next-generation display and illumination, but the unbalanced charge transport is still a hard-treat case to restrict its efficiency and operational stability. Here, a high curvature PEDOT:PSS transport layer is demonstrated via the self-assembly island-like structures by the incorporation of alkali metal salts. Benefiting from the dielectric confinement effect of the high curvature surface, the modified CsPbBr3 -based PeLEDs present a 2.1 times peak external quantum efficiency (EQE) from 6.75% to 14.23% and a 3.3 times half lifetime (T50 ) from 3.96 to 13.01h. Besides, the PeLEDs show high luminance up to 44834cdm-2 . Evidently, this work may provide a deep insight into the structure-activity relationship between the micro-structures at the PEDOT:PSS/perovskite interface and the performance of PeLEDs, and crack the codes for ameliorating the performance of PeLEDs via interfacial micro-structured regulation.

Phase Distribution Manipulation through Molecular Interaction Enables Efficient Quasi-2D Perovskite Light-Emitting Diodes

Quasi-two-dimensional (quasi-2D) metal halide perovskites exhibit excellent electroluminescence (EL) performance promising for display and lighting applications, which benefits from the quantum and/or dielectric confinement effects as well as the unique energy-funneling process. However, the inefficient energy transfer caused by the intricate phase distribution with different thick nanoplatelets (n-values), as well as the existence of numerous defects, could deteriorate the EL performance of the resulting perovskite light-emitting diodes (PeLEDs). Here, a multifunctional small molecule, bistrifluoromethanesulfonimide lithium (LiTFSI), was introduced to address the above issues. The strongly electronegative fluorine atoms in LiTFSI form hydrogen bonding interactions with the ammonium heads of organic spacers, which suppress the formation of perovskite ultrathin nanoplatelet phases with small n-values, thus smoothing the energy transfer process. Meanwhile, the Lewis base sulfur oxide groups are capable of effectively passivating the uncoordinated lead ion defects and then reducing nonradiative recombination loss. Eventually, a green emission quasi-2D PeLED with an external quantum efficiency of 21.0% was achieved. This work provides a facile method to boost the performance of quasi-2D PeLEDs.

Ag NPs and MoS2 QDs double modified graphene/GaAs near-infrared photodetector

The light absorption enhancement ability of quantum dots (QDs) and the localized surface plasmon resonance (LSPR) effect of metal nanoparticles (NPs) can effectively improve the performance of photodetectors. In this paper, an Ag NPs and MoS2 QDs double modified graphene/GaAs near-infrared photodetector is investigated. The performance of the detector is improved by adopting the LSPR effect of Ag NPs and the dielectric confinement effect of MoS2 QDs. After subsequent testing and analysis, the photodetector responsivity is 21.1 mA W−1 and the detectivity is 8.4 × 1012 cm Hz1/2 W−1 at 808 nm wavelength; the responsivity and detectivity at 1064 nm wavelength can be up to 18.4 mA W−1 and 3.39 × 1012 cm Hz1/2 W−1, respectively. At the same time, under a 4 kHz pulsed illumination, the rise time and fall time are 15.87 μs and 89.95 μs respectively. Compared with previous devices, the new device has lower dark current and higher detectivity while having good responsivity and response time.

Reducing dielectric confinement effect in ionic covalent organic nanosheets to promote the visible-light-driven hydrogen evolution

Ultra-thin two-dimensional (2D) organic semiconductors are promising candidates for photocatalysts because of the short charge diffusion pathway and favorable exposure of active sites plus the versatile architecture. Nonetheless, the inherent dielectric confinement of 2D materials will induce a strong exciton effect hampering the charge separation. Herein, we demonstrated an effective way to reduce the dielectric confinement effect of 2D ionic covalent organic nanosheets (iCONs) by tailoring the functional group via molecular engineering. Three ultra-thin CONs with different functional groups and the same ionic moieties were synthesized through Schiff base condensation between ionic amino monomer triaminoguanidinium chloride (TG) and aldehyde linkers. The integration of the hydroxyl group was found to significantly increase the dielectric constant by enhancing the polarizability of ionic moieties, and thus reduced the dielectric confinement and the corresponding exciton binding energy (Eb). The champion hydroxyl-functional iCON exhibited promoted exciton dissociation and in turn a high photocatalytic hydrogen production rate under visible-light irradiation. This work provided insights into the rationalization of the dielectric confinement effect of low-dimensional photocatalysts.

Photogenerated charge transfer in Dion-Jacobson type layered perovskite based on naphthalene diimide.

Incorporating organic semiconducting spacer cations into layered lead halide perovskite structures provides a powerful approach to mitigate the typical strong dielectric and quantum confinement effects by inducing charge-transfer between the organic and inorganic layers. Herein we report the synthesis and characterization of thin films of novel DJ-phase organic-inorganic layered perovskite semiconductors using a naphthalene diimide (NDI) based divalent spacer cation, which is shown to accept photogenerated electrons from the inorganic layer. With alkyl chain lengths of 6 carbons, an NDI-based thin film exhibited electron mobility (based on space charge-limited current for quasi-layered 〈n〉 = 5 material) was found to be as high as 0.03 cm2 V-1 s-1 with no observable trap-filling region suggesting trap passivation by the NDI spacer cation.

Ultrafast Excitonic Response in Two-Dimensional Hybrid Perovskites Driven by Intense Midinfrared Pulses.

Two-dimensional organic-inorganic hybrid perovskites (2DHPs) are natural quantum-well-like materials, in which strong quantum and dielectric confinement effects due to the organic spacers give rise to tightly bound excitons with large binding energy. To examine the mutual interactions between the organic spacer cations and the inorganic charge-residing octahedral framework in 2DHPs, here we perform femtosecond pump-probe spectroscopy by direct vibrational pumping of the organic spacers, followed by a visible-to-ultraviolet probe covering their excitonic resonances. Measurements on prototypical lead-bromide based 2DHP compounds, (BA)_{2}PbBr_{4} and (BA)_{2}(FA)Pb_{2}Br_{7} (BA^{+}=butylammonium; FA^{+}=formamidinium), reveal two distinct regimes of the temporal response. The first regime is dominated by a pump-induced transient expansion of the organic spacer layers that reduces the exciton oscillator strength, whereas the second regime arises from pump-induced lattice heating effects primarily associated with a spectral shift of the exciton energy. In addition, vibrational excitation enhances the biexciton emission, which we attribute to a stronger intralayer exciton confinement as well as vibrationally induced exciton detrapping from defect states. Our study provides fundamental insights regarding the impact of organic spacers on excitons in 2DHPs, as well as the excited-state dynamics and vibrational energy dissipation in these structurally diverse materials.

Dielectric effects, crystal field, and shape anisotropy tuning of the exciton fine structure of halide perovskite nanocrystals

We present a theoretical study of the excitonic band edge states applied to the more commonly studied inorganic perovskite nanocrystals: ${\mathrm{CsPbBr}}_{3}$. We highlight the key role played by the electron-hole exchange interaction, including long-range and short range terms, in the positioning and splitting of dark and bright exciton sublevels, and discuss the influence of dielectric confinement, crystal field and shape anisotropy effects on the excitonic fine structure. The electron-hole exchange interaction splits the four excitonic states into three bright excitonic states at higher energy and a singlet dark state at lower energy. We find that, whatever the crystal phase, the bright-dark splitting is weakly sensitive to shape anisotropy. However, a strong enhancement, of about 50%, is obtained at the maximum of the dielectric contrast. For two crystalline symmetries, we particularly analyze how the bright exciton triplet energies vary, taking into account the states polarizations and considering the directions along which shape elongation/contraction applies. The main effect of the dielectric contrast is to increase significantly the exciton energies, for all the configurations. For a cubic crystal---${\mathrm{O}}_{h}$ symmetry---the bright exciton triplet degeneracy can be partially or fully lifted with anisotropic shape along one or two directions, respectively. For a tetragonal crystal---${\mathrm{D}}_{4h}$ symmetry---the partial triplet degeneracy can be completely lifted by a single distortion from the cubic shape along a direction perpendicular to the tetragonal axis but a partial degeneracy is maintained for small distortions along the tetragonal axis. The amplitude of bright exciton splittings are basically driven by the shape anisotropy and is almost insensitive to the dielectric contrast. On the basis of this model, we discuss recent results on ${\mathrm{CsPbBr}}_{3}$ nanocrystals photoluminescence.

A Novel Multiple‐Ring Aromatic Spacer Based 2D Ruddlesden–Popper CsPbI3Solar Cell with Record Efficiency Beyond 16%

AbstractTwo‐dimensional (2D) Ruddlesden–Popper (RP) CsPbI3perovskite possesses superior phase stability by introducing steric hindrance. However, due to the quantum and dielectric confinement effect, 2D structures usually exhibit large exciton binding energy, and the charge tunneling barrier across the organic interlayer is difficult to eliminate, resulting in poor charge transport and performance. Here, a multiple‐ring aromatic ammonium, 1‐naphthylamine (1‐NA) spacer is developed for 2D RP CsPbI3perovskite solar cell (PSC). Theoretical simulations and experimental characterizations demonstrate that the 2D RP CsPbI3perovskite using 1‐NA spacer with extended π‐conjugation lengths reduces the exciton binding energy and facilitates the efficient separation of excitons. In addition, its cations have a significant contribution to the conduction band, which can reduce the bandgap, promote electronic coupling between organic and inorganic layers, and improve interlayer charge transport. Importantly, the strong π–π conjugation of 1‐NA spacer can enhance intermolecular interactions and hydrogen bonding, and prepare high‐quality films with preferred vertical orientation, resulting in lower defect density, and directional charge transport. As a result, the (1‐NA)2(Cs)3Pb4I13PSC exhibits a record 16.62% performance with enhanced stability. This work provides an efficient approach to improve charge transport and device performance by developing multiple‐ring aromatic spacers.

Space-confined fabrication of hydrophobic magnetic carbon nanofibers for lightweight and enhanced microwave absorption

Advanced microwave absorbers (MAs) containing magnetic particles and carbon nanofibers (CNFs) have raised widespread attention in the rapid popularization of electronic communication equipment. However, it is quite challenging to control the growth of magnetic particles in CNFs to improve electromagnetic waves (EMWs) loss and attenuation. Herein, a novel space-confined strategy was adopted to develop Co/CoO/SiO2/CNFs, in which Co/CoO nanoparticles (NPs) with single domain size and dispersed distribution could achieve enhanced magnetic loss and dielectric loss. The introduction of nano-SiO2 in the system could enhance the dielectric confinement effect of the nanofibers (NFs), while an interfacial confinement network could also be constructed to realize the refinement of Co/CoO NPs, enhance the electromagnetic (EM) loss, and ultimately ameliorate the microwave absorption performance of as-obtained NFs. When the additive amount of nano-SiO2 in the spinning solution was 1.0 mg, the as-prepared Co/CoO/SiO2/CNFs reached the optimal reflection loss (RL) of −52.9 dB at 12.64 GHz with a thickness of 2.5 mm, and the effective bandwidth (EBW) reached 5.67 GHz (10.46–16.13 GHz). Furthermore, the excellent hydrophobic properties of the CNFs endow them with potential self-cleaning function. The confinement strategy proposed in this work lays a foundation for the design of low-filling ratio and high-performance CNFs-based microwave absorbers, and presents a general approach for the synthesis of one-dimensional composite materials with controllable structures.