Dielectric Screening Effect Research Articles

Eumelanin & keratin sourced from waste: unraveling criss-cross functionalities for green electronic applications

Abstract In the framework of the Circular Economy this study provides a detailed analysis of water-based suspensions of two biopolymers derived by sustainable processes: eumelanin from insect farming and keratin from chicken feathers. The latter material was obtained via two different extraction procedures. Colloidal-like suspensions were produced in water either as a single component system or a mixture of both in selected ratios, taking advantage of their high solubility. The suspensions were examined using a comprehensive set of chemical, structural and dielectric techniques to gather information on their properties. Small-Angle x-ray Scattering results provided insights into the elemental polymer sections within the suspension, while Transmission Electron Microscopy images indicate that keratin is the component driving the shape of the aggregation structure in a colloidal environment, and, in some cases, eumelanin internalization. Furthermore, the co-presence of both polymers in water determines the aggregation dimensions and shapes. The discussion focuses on the influence of the aggregation on the dielectric proper-ties by comparing the former to the AC dynamic response returned by Broadband Dielectric Spectroscopy (BDS). Within the BDS framework various items are highlighted including dielectric relaxations, screening effects, counterion condensation and ionic charge transport. The results shown in this work let to foresee the adoption of water or biofriendly aqueous BSF-EuM:Keratin suspensions in the production of devices and sensors with low environmental impact.

Coherent Exciton Spin Relaxation Dynamics and Exciton Polaron Character in Layered Two-Dimensional Lead-Halide Perovskites.

The quantum-well-like two-dimensional lead-halide perovskites exhibit strongly confined excitons due to the quantum confinement and reduced dielectric screening effect, which feature intriguing excitonic effects. The ionic nature of the perovskite crystal and the "softness" of the lattice induce the complex lattice dynamics. There are still open questions about how the soft lattices decorate the nature of excitons in these hybrid materials. Herein, we reveal the polaronic character of excitons and coherent exciton spin relaxation dynamics in layered hybrid perovskites by using chirality-dependent impulsive vibrational spectroscopy. We identify an intrinsic exciton spin dynamics property, giving rise to a short spin relaxation lifetime in the sub-picosecond time scale. The exciton polaron formation is confirmed by the blue-shift of the phonon frequency under resonant conditions compared to that in below-resonance excitation cases. The phonon vibrational wavepackets show a cosine- and sine-like oscillation as a function of time via on- and below-resonance excitation scenarios due to the displacive and impulsive mechanisms, respectively. Our findings provide profound insights concerning the polaronic character of excitons in two-dimensional perovskites, underpinning the prospective developments in optical and optoelectronic applications.

Temperature Dependence of Optical Properties of MoS2 and WS2 Heterostructures Assessed by Spectroscopic Ellipsometry.

We report the complex dielectric function ε = ε1 + iε2 of MoS2/WS2 and WS2/MoS2 heterostructures and their constituent monolayers MoS2 and WS2 for an energy range from 1.5 to 6.0 eV and temperatures from 39 to 300 K. Comparisons between the optical properties of the heterostructures and their monolayers were conducted. Critical-point (CP) energies of the heterostructures were traced back to their origins in the monolayers. Low-temperature measurements confirmed the existence of only three excitonic CPs from 1.5 to 2.5 eV due to the overlap of trion B- of the MoS2 monolayer and exciton A0 of the WS2 monolayer. Due to the dielectric screening effect, most CPs exhibit red shifts in the heterostructures compared to their monolayer counterparts.

Dark Current Suppression in Two‐dimensional Histamine Lead Iodine Perovskite Single Crystal for X‐ray Detection and Imaging

AbstractLead halide perovskites have emerged as attractive X‐ray detector materials, owing to properties such as strong X‐ray stopping power, excellent carrier transport, and high sensitivity. Additionally, they can be easily prepared by using solution‐based synthesis approaches. However, traditional 3D (three‐dimensional) perovskites X‐ray detectors have shown limited application due to high dark currents generated under bias voltage as a result of strong ion migration. In this work, an X‐ray detector with a vertical structure device is demonstrated using 2D (two‐dimensional) histamine lead halide perovskite single crystal HAPbI4 (HPI, HA = histamine). Due to the dielectric screening effect of diamine and the vertical structure of the HPI device, the fabricated detector shows a sensitivity of 7737 µC Gyair−1 cm−2 under a bias voltage of 30 V. Furthermore, the detector shows a sensitivity of 293 µC Gyair−1 cm−2 and detection limit of 51.38 nGyair s−1 without bias voltage, wherein the dark current is almost completely suppressed. All of these properties indicate the X‐ray detection device is a promising candidate for next‐generation optoelectronic applications.

Dielectric screening modulating charge carrier recombination in high-performance of CsPbI2Br carbon-based perovskite solar cells

As a type of ionic crystal, CsPbI2Br perovskite inevitably generates a significant number of defects during the rapid crystallization process that occurs from deposition to annealing. The non-radiative recombination of charge carriers caused by these defects severely limits the performance of perovskite solar cells (PSCs). Here, 2,2,2-trifluoroethylamine hydrochloride (F3EACl) with permanent dipole moment is devised to modify the perovskite interface. Highly electronegative trifluoro groups can accelerate the dielectric response of perovskite films, intensify the dielectric screening effect, and significantly suppress non-radiative recombination of charge carriers. Meanwhile, the –NH3+ group in F3EACl effectively passivates the defects at the perovskite surface, further alleviating the defect-induced carrier recombination loss. Consequently, the CsPbI2Br carbon-based PSCs treated with F3EACl achieved an impressive power conversion efficiency (PCE) of 14.69 % at a higher open-circuit voltage (1.28 V), along with excellent environmental stability. This work demonstrates a unique tactic to achieve efficient and stable PSCs by modulating charge carrier recombination utilizing the synergistic effects of dielectric screening and defect passivation.

Impurity affected transport properties of quantum well heterostructures with electronic and high-κ dielectric quantum screening

The effect of electronic and high-κ dielectric quantum screening (ES and DS) on the impurity-limited electron transport properties of quantum well/high-κ dielectric barrier type heterostructures with two-dimensional electron gas (2DEG) is studied theoretically. Characteristic of these heterosystems the 2D compound forms of screened impurity potential are employed for the first time. In the framework of 2D Debye-Hückel potential form an electron momentum relaxation time (τ) expression depending on the impurity 2D screening radius is obtained analytically. A numerical analysis of τ is carried out for the realistic HfO/InSb/HfO2QW heterostructure taking account both the finite mismatch of the energy bands at the heterointerface and the energy band non-parabolicity of InSb. The contributions of screened potential compound 2D forms to τ are established depending on QW width. A significant suppression of the scattering rate τ−1 (by an order of magnitude) is received with accounting of ES + DS combined effect.

Effects of dynamical dielectric screening on the excitonic spectrum of monolayer semiconductors

Effects of dynamical dielectric screening on the excitonic spectrum of monolayer semiconductors

Thickness-modulated electronic band structures and exciton behavior of non-van-der-waals 2D Bi2O2Se films

Bi2O2Se, as a non-van-der-waals (non-vdW) interlayer coupling two-dimensional material, its electronic band structures and exciton behavior still merit further in-depth studies, especially in the consideration of thickness tuning degrees of freedom. Here, we reveal the broadband excitonic performance and critical point properties of Bi2O2Se films with different thicknesses (3.54–16.33 nm). Five critical points are determined and assigned to specific optical transitions. Moreover, we observe that electronic bandstructures in the Γ valley are stable with variable thicknesses, while the deeper electron levels received more pronounced thickness modulation. And we first report four excitons in wide spectral range (1.24–6.20 eV) through absorption coefficient fitting. Significantly, due to the strong localization of electrons and holes induced by the intralayer built-in electric field, the exciton binding energy (Eb) value of 3.54 nm-Bi2O2Se (234 meV at Γ position) is apparently larger than typical multi-layer TMDCs, which offering unprecedented opportunities in applications in optoelectronic devices. Furthermore, Eb of two excitons at low energy region (Eb1, Eb2) red-shift with increasing thickness, while Eb of higher-energy excitons (Eb3, Eb4) blue-shift, owing to the dispersion characteristic of the effect dielectric screening effects. Our results are vital to wide-ranging applications of Bi2O2Se from fundamental science to optoelectronic devices.

Ultralow Detection Limit and Anisotropic X‐Ray Detection Behaviors Based on A Novel Oxide KCsMoP2O9 Single Crystal

AbstractX‐ray detection has drawn great attention in industrial examination, medical imaging, scientific research, and beyond. Herein, the anisotropic X‐ray detection performances are investigated based on a novel oxide crystal KCsMoP2O9 (KCMP). The large dielectric screening effect and 1D infinite chain structure consisting of MoO6 octahedron and two PO4 tetrahedrons are conducive to efficient carrier transport along the X‐direction, leading to a higher µτ (4.77 × 10−3 cm2 V−1) of X direction than that of Z direction (1.05 × 10−4 cm2 V−1). Additionally, the KCMP‐based X‐ray detector along the X direction shows weaker ion migration than that of the Z direction due to the large migration barrier of O2− (8.0 eV) and high ion active energy (784.9 meV) along the X direction. Owing to the high resistivity (2.29 × 1013 Ω cm), high µτ, high ion activation energy, suppressed ion migration, and ultralow dark current, the X‐ray detection performance of the KCMP detector along the X direction is superior to that of the Z direction. Specifically, the high sensitivity (131.2 µC Gy−1 cm−2) and ultralow detection limit (8.9 nGys−1) are achieved in the KCMP detector along the X direction. This work clarifies anisotropic engineering to enhance the X‐ray detection performance of oxide crystals and proposes a novel X‐ray detection crystal with an ultralow detection limit.

Enhancing the Performance of Perovskite Light-Emitting Diodes via Synergistic Effect of Defect Passivation and Dielectric Screening

HighlightsSynergistic passivation mechanism of the dual functional compound of potassium bromide for metal halide perovskite films, that is through chemical bonding and physical screening towards the various types of defects.Metal-ion additives can simultaneously passivate the defects of halide vacancies with bromine anions and dangling bond defects with metal cations.Achieve the maximum external quantum efficiency of ~21% and maximum luminance of ~60,000 cd m−2.

Revealing the Role of Polyacrylonitrile for Highly Efficient and Stable Perovskite Solar Cells at Extremely Low Temperatures

AbstractPerovskite solar cells (PSCs) are considered to be a promising candidate for near‐space applications due to their high specific power, excellent radiation resistance, and compatibility with flexible substrates. Nevertheless, the low‐temperature cycles impose considerable challenges to their long‐term stability. Herein, the extremely low‐temperature photovoltaic process of PSCs is investigated and the effect mechanisms of PAN (Polyacrylonitrile) as an additive are revealed. It is found that the additive PAN regulates perovskite lattice stress and stabilizes perovskite lattice structure and thus retards perovskite phase transition at low temperatures. Moreover, the PAN improves perovskite dielectric properties and ion migration activation energy and effectively passivates perovskite defects at low temperatures. Therefore, the carrier transport/extraction/collection abilities at low temperatures are enhanced effectively by alleviating exciton effect and increasing dielectric screening effect. Benefiting from these positive effects of PAN, the PAN‐modified device achieves a maximum efficiency of 24.34% at 150K and maintains 72% of its initial value after 120 thermal cycles (290‐130K). Also, the PAN‐modified flexible devices exhibit excellent bending stability and thermal cycle stability. This study provides a deep insight into understanding the extremely low‐temperature photovoltaic behavior of PSCs and demonstrates the potentiality of PAN additive strategy for achieving efficient and stable PSCs at low temperatures.

Charged Exciton Generation by Curvature-Induced Band Gap Fluctuations in Structurally Disordered Two-Dimensional Semiconductors.

Curvature is a general factor for various two-dimensional (2D) materials due to their flexibility, which is not yet fully unveiled to control their physical properties. In particular, the effect of structural disorder with random curvature formation on excitons in 2D semiconductors is not fully understood. Here, the correlation between structural disorder and exciton formation in monolayer MoS2 on SiO2 was investigated by using photoluminescence (PL) and Raman spectroscopy. We found that the curvature-induced charge localization along with band gap fluctuations aid the formation of the localized charged excitons (such as trions). In the substrate-supported region, the trion population is enhanced by a localized charge due to the microscopic random bending strain, while the trion is suppressed in the suspended region which exhibits negligible bending strain, anomalously even though the dielectric screening effect is lower than that of the supported region. The redistribution of each exciton by the bending strain leads to a huge variation (∼100-fold) in PL intensity between the supported and suspended regions, which cannot be fully comprehended by external potential disorders such as a random distribution of charged impurities. The peak position of PL in MoS2/SiO2 is inversely proportional to the Raman peak position of E12g, indicating that the bending strain is correlated with PL. The supported regions exhibit an indirect portion that was not shown in the suspended regions or atomically flat substrates. The understanding of the structural disorder effect on excitons provides a fundamental path for optoelectronics and strain engineering of 2D semiconductors.

Harnessing Conformational Disorder of Organic Cations for Efficient Blue Quasi‐2D Perovskite LEDs

AbstractDespite quasi‐2D perovskite offering great control over the optoelectronic properties, disordered organic cations are often perceived as detrimental to device performance, primarily affecting charge carrier mobility. However, it is proposed that such disordered organic cations‐enabled excellent excitonic properties can be beneficial for fabricating high‐efficiency perovskite light‐emitting diodes (PeLEDs) facilitated by reduced dielectric screening effect. Here, by incorporating acetamidinium bromide additives, the conformational disorder of organic cation is precisely manipulated and meticulously probed using sum frequency generation vibrational spectroscopy. Finally, a distinctive inverse relationship is elucidated between the degree of conformational order, characterized as relative structure ordering , and key performance metrics such as photoluminescence quantum yield and external quantum efficiency (EQE). By optimizing the configurational disorder, sky‐blue (485 nm) PeLEDs achieve a noteworthy EQE of 14.42% and exhibit significantly prolonged operational stability in open‐air conditions. This finding underscores the potential advantages of disordered organic cations in enhancing exciton properties and radiative recombination efficiency.

Modulation of Auger recombination via facet engineering in CsPbBr3 perovskite nanocrystals

We demonstrate the modulation of Auger recombination via facet engineering in CsPbBr3 perovskite nanocrystals, where the static dielectric screening effect is revealed to prevail over the dynamic polaronic screening effect.

Boosting Photovoltaic Output by Dual External Manipulations on 1D Ferroelectric Nanorods Solar Cell: Electric Field and Giant Dielectric Constant Variation

Unlike common photovoltaic (PV) cells whose performance has been anchored, the merits for the cells made of ferroelectric crystals, due to the non‐centrosymmetric structure, provide possibility to further enhance the efficiency by dual external manipulations through increasing the internal electric field strength and the dielectric screening effect, based on anomalous photovoltaic (APV) arisen by the external poling and giant dielectric constant variation through temperature‐controlled ferro‐paraelectric phase transition. In this study, a synergistic effect achieving a 67% improvement in power conversion efficiency (PCE) of the n‐i‐p configuration solar cell by ferroelectric polarization and high dielectricity is observed, taking c‐axis‐aligned single‐crystalline ferroelectric SbSI nanorods as the active layer. The opposite poling effects are revealed leading to asymmetric characteristics in the n‐i‐p configuration. The enhancements of Jsc induced by inherent giant variation imply strong dielectric screening facilities to lower carrier cross section captured by charged defects, uniformly suppressing various recombination mechanisms, understood by the Langevin model. Therefore, solar cells employing n‐i‐p structured ferroelectric thin films as absorbing layers hold promising potential for achieving defect‐tolerance high performance through rational design, external polarization, and dielectric property control.

Elucidating the microscopic origin of observed anti-correlation between the protein-protein and protein-water interaction energies

Water as a solvent plays an important role in dictating the structure and dynamics of the large biomolecules like proteins. It has been proposed that the protein dynamics and energetics are closely coupled to the water molecules in the hydration shell. In particular, it has been shown earlier that the fluctuations in intra-protein (protein-protein) interaction energy is strongly anti-correlated with the protein-water interaction energy (solvation energy). Moreover, the dynamical signatures of water gets reflected into the protein dynamics in terms of a dominant power law behavior. In this work, we have carried out systematic molecular dynamics (MD) simulations of three different protein systems to understand the key physical factors that contribute to such anti-correlations. First, we compare the dynamics in globular folded proteins like Lysozyme with intrinsically disordered peptides (IDPs) like Amyloid beta to show that enhanced conformational fluctuations lead to significantly stronger anti-correlation. Trp-cage miniprotein shows a similar behavior distributed over two sub-ensembles of folded and unfolded states. We also investigate what type of interactions and molecular processes are responsible for the observed anti-correlation. In particular, we observe that the extent of correlation decreases significantly in the order of charged, polar and non-polar amino acid residues. Charged residues that form salt bridge interactions show a dominant anti-correlation. Surprisingly, a few non-polar (hydrophobic and buried) residues demonstrate significant anti-correlation as well. We attribute this to the structural fluctuations that lead to an equilibrium between buried and solvent exposed states of these residues, where the protein-protein and protein-water interactions compensate each other. Our studies indicate that the molecular origin of the observed anti-correlation can be vary depending on the type of the interaction. The overall coupling/correlation is dominated by the electrostatic interactions and charged residues due to a general dielectric screening effect, whereas a few buried hydrophobic residues may also show such coupling due to structural dynamics between buried and exposed states.

Substrate-tuned dielectric screening effect on optical properties of monolayer MoSe2

Grasping the intricate mechanisms for the influences of dielectric screening from diverse substrates on the optical properties of ultra-thin two-dimensional materials is crucial for related device design. This study investigates the substrate impacts on the optical properties of monolayer MoSe2 by combining the spectroscopic ellipsometry (SE) and first-principle calculations. Featured absorption peaks of monolayer MoSe2 on substrates of quartz, silicon, and sapphire are meticulously analyzed via critical point (CP) analysis on the dielectric functions determined by SE. Results indicate that variation in substrates induces notable shifts in energy positions of the absorption peaks due to different dielectric screening effects. The study further uncovers electron transfer from silicon substrate to monolayer MoSe2, leading to alterations in the band structure. By integrating CP analysis with first-principle calculations, we gain a comprehensive understanding of the optical transitions corresponding to CPs in the monolayer MoSe2 on various substrates. This establishes that the energy shifts of CPs regulated by substrate-induced dielectric screening are highly correlated with the positions of their corresponding optical transitions in momentum space. These findings underscore the pivotal role of substrate selection in customizing the optical properties of two-dimensional materials, providing valuable insights for the design and optimization of MoSe2-based photonic devices.

Effect of environmental screening and strain on optoelectronic properties of two-dimensional quantum defects

Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control of SPEs requires in-depth understanding of their optoelectronic properties. However, how the surrounding environment of host materials, including the number of layers, substrates, and strain, influences SPEs has not been fully understood. In this work, we study the dielectric screening effect due to the number of layers and substrates, and the strain effect on the optical properties of carbon dimer and nitrogen vacancy defects in hBN from first-principles many-body perturbation theory. We report that environmental screening causes a lowering of the quasiparticle gap and exciton binding energy, leading to nearly constant optical excitation energy and exciton radiative lifetime. We explain the results with an analytical model starting from the Bethe–Salpeter equation Hamiltonian with Wannier basis. We also show that optical properties of quantum defects are largely tunable by strain with highly anisotropic response, in good agreement with experimental measurements. Our work clarifies the effect of environmental screening and strain on optoelectronic properties of quantum defects in two-dimensional insulators, facilitating future applications of SPEs and spin qubits in low-dimensional systems.

Giant excitonic effects in vacancy-ordered double perovskites

Using first-principles GW plus Bethe-Salpeter equation calculations, we identify anomalously strong excitonic effects in several vacancy-ordered double perovskites Cs2MX6 (M = Ti, Zr; X = I, Br). Giant exciton binding energies about 1 eV are found in these moderate-gap, inorganic bulk semiconductors, pushing the limit of our understanding of electron-hole (e-h) interaction and exciton formation in solids. Not only are the exciton binding energies extremely large compared with any other moderate-gap bulk semiconductors, but they are also larger than typical 2D semiconductors with comparable quasiparticle gaps. Our calculated lowest bright exciton energy agrees well with the experimental optical band gap. The low-energy excitons closely resemble the Frenkel excitons in molecular crystals, as they are highly localized in a single [MX6]2- octahedron and extended in the reciprocal space. The weak dielectric screening effects and the nearly flat frontier electronic bands, which are derived from the weakly bonded [MX6]2- units, together explain the significant excitonic effects. Spin-orbit coupling effects play a crucial role in red-shifting the lowest bright exciton by mixing up spin-singlet and spin-triplet excitons, while exciton-phonon coupling effects have minor impacts on the strong exciton binding energies.

Dielectric screening at TMD:hBN interfaces: Monolayer-to-bulk transition, local-field effect, and spatial dependence

The dielectric effects of a substrate have been shown to be important for modulating the electronic properties of an adsorbate, especially in van der Waals heterostructures. Here, using the first-principles dielectric embedding $GW$ approach within the framework of many-body perturbation theory, we perform a case study on the dielectric screening effects of hexagonal boron nitride (hBN) on various transition-metal dichalcogenides (TMDs). We consider three systems, monolayer ${\mathrm{MoS}}_{2}$, bilayer ${\mathrm{MoS}}_{2}$, and a mixed ${\mathrm{WS}}_{2}/{\mathrm{MoS}}_{2}$ bilayer adsorbed on hBN, and examine three aspects of the substrate dielectric screening: (i) the thickness dependence and the monolayer-to-bulk transition, for which we consider the effects of one-, two-, three-, and four-layer hBN; (ii) the local-field effect, for which we numerically assess a common approximation of neglecting the in-plane local-field components of the substrate polarizability; and (iii) the spatial dependence, for which we consider the mixed ${\mathrm{WS}}_{2}/{\mathrm{MoS}}_{2}$ bilayer adsorbed on hBN with either side facing the substrate. Our results provide quantitative insight into how the substrate screening effects can be leveraged for band structure engineering.