Charge Transfer Pattern Research Articles

Triboelectric Spectroscopy for In Situ Chemical Analysis of Liquids.

Chemical analysis of ions and small organic molecules in liquid samples is crucial for applications in chemistry, biology, environmental sciences, and health monitoring. Mainstream electrochemical and chromatographic techniques often suffer from complex and lengthy sample preparation and testing procedures and require either bulky or expensive instrumentation. Here, we combine triboelectrification and charge transfer on the surface of electrical insulators to demonstrate the concept of triboelectric spectroscopy (TES) for chemical analysis. As a drop of the liquid sample slides along an insulating reclined plane, the local triboelectrification of the surface is recorded, and the charge pattern along the sample trajectory is used to build a fingerprinting of the charge transfer spectroscopy. Chemical information extracted from the charge transfer pattern enables a new nondestructive and ultrafast (<1 s) tool for chemical analysis. TES profiles are unique, and through an automated identification, it is possible to match against standard and hence detect over 30 types of common salts, acids, bases and organic molecules. The qualitative and quantitative accuracies of the TES methodology is close to 93%, and the detection limit is as low as ppb levels. Instruments for TES chemical analysis are portable and can be further miniaturized, opening a path to in situ and rapid chemical detection relying on inexpensive, portable low-tech instrumentation.

Ultra-high output triboelectric nanogenerator based on synergies of material modification and charge pumping

On the road to large-scale industrialization of triboelectric nanogenerators (TENGs), higher output is the constant quest. Here, inspired by the ubiquitous synergies in nature, an ultra-high-output TENG is proposed based on the synergistic effect of internal modification of materials along with the external integration of a charge pumping module. The carefully chosen poly (vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) film possesses strong polarization and high permittivity, which leads to excellent triboelectric properties, and can be further enhanced by doping BaTiO3 (BTO) nanoparticles. Meanwhile, the elaborately designed and optimized charge pumping module endows it with the ability to pump affluent charges. As a consequence, a TENG based on 8 µm P(VDF-HFP)/BTO 1 wt% composite film can achieve an ultra-high total transfer charge density (TTCD) of 3.5 mC/m2 under optimal charge pumping conditions. Compared to the output of the P(VDF-HFP)/BTO-based TENG (0.1 mC/m2) and the output of the P(VDF-HFP)-based TENG under charge pumping (2.6 mC/m2), the output of the P(VDF-HFP)/BTO-based TENG under charge pumping is greater than the sum of the above two devices, which indicates that the proposed synergistic effect for designing an ultra-high-output TENG is remarkable and successful. Moreover, a novel charge transfer pattern based on synergistic effect is proposed, expounding the role inherent modifications of P(VDF-HFP) and external integration of charge pumping modules play. This work presents a concept that is at the forefront of contemporary designs for ultra-high output TENGs, providing insight into the preparation of high-performance TENGs in a state-of-the-art manner.

In-situ detection technique for charge transfer behavior of direct Z-scheme BiVO4/UiO-66-NH2 composites during photocatalytic thioanisole conversion

The photocatalytic efficiency can be enhanced by constructing Z-scheme heterostructures. However, there is still a lack of comprehensive and direct evidence regarding the charge transfer pathway and mode during the photocatalytic process. A composite photocatalyst BiVO4/UiO-66-NH2 (BVO/U6N) was prepared by in-situ loading BiVO4 nanoparticles onto the surface of UiO-66-NH2 using a hydrothermal method. This catalyst effectively promotes the photocatalytic conversion of thioanisole to sulfoxide. The differences between BiVO4 and UiO-66-NH2 in band structure and Fermi energy level enable the composite to act according to the Z-scheme charge transfer pattern, which significantly enhances charge separation efficiency. In-situ X-ray photoelectron spectroscopy (in-situ XPS) combined with DFT calculation confirmed the transfer of electrons from UiO-66-NH2 to BiVO4, driven by an internal electric field (IEF) upon hybridization. This demonstrates the formation of a Z-scheme photogenerated charge transfer pathway in the BVO/U6N composite. The direct Z-scheme system significantly enhances the carrier redox, resulting in a sulfoxide yield of 95.21% for the optimized sample in methanol, which is 5 times and 4.1 times higher than that of UiO-66-NH2 and BiVO4, respectively. BVO/U6N exhibits efficient photocatalysis and selectivity towards various substrate sulfides, making it a highly promising heterogeneous photocatalyst.

Coverage-Dependent Modulation of Charge Density at the Interface between Ag(001) and Ruthenium Phthalocyanine

When an organic film is deposited on a metal surface, charge layers are formed at the interface. These are an important feature of the interface electronic structure and play a crucial role as separation layers between electrodes and active layers in organic devices. Here, we report on a study of the interface between diruthenium phthalocyanine, (RuPc)2, and the Ag(001) surface. The molecules form two different commensurable arrangements on the substrate, a low density one for a coverage well below the first monolayer and a high density one up to the completion of the monolayer. The focus of this study is on the interface states evolution with the molecular density on the metal surface and the charge distribution in the thin interfacial layer between molecules and substrate. From this investigation, conducted by low energy electron diffraction, scanning tunneling microscopy/spectroscopy, photoemission spectroscopy, and density functional theory, we have found that, even if individual molecules are characterized by a quite similar surface-to-molecule charge transfer pattern, the two molecular arrangements present different valence band structures and, more interestingly, different modulations of the interface charge. These charge modulations are governed by interfacial states energetically resonant with the molecular states, localized at the position of the molecules as well as by a reaction of the electronic cloud of the metal surface to the molecular adsorption due to a Pauli pushback effect. This complex, spatial charge modulation makes the (RuPc)2/Ag(001) an interesting case of interaction intermediate between physisorption and chemisorption.

Structural, spectroscopic, FMOs, and non-linear optical properties exploration of three thiacaix(4)arenes derivatives

Computational exploration has been carried out on three p-tertbutylthiacalix[4]arene derivatives, namely cone (a), partial-cone (b), and 1,3-alternate (c). These three compounds are recently synthesized in high yields (80, 95, and 85%, respectively) and characterized by our group, which showed high uptake efficacy of alkali, heavy, and transition metal ions via liquid–liquid extraction. Significantly, these compounds exhibited high uptake capacity for Cu(II) (74.9 %) and Pb(II) (80.0%) upon using cone (a), and partial-cone (b), respectively. In this study, in order to optimize how the thiacalix[4]arene (as a nucleophilic) interacts with the metal ion (as an electrophile), DFT calculations were undertaken. Theoretical IR and UV–Vis spectral features of the titled compounds were computed and compared with their experimental values. Chemical reactivity was ranked based on selected electronic chemical descriptors. Moreover, the active sites of the three compounds were determined by molecular electrostatic potential (MEP) maps. Furthermore, non-linear optical (NLO) properties were investigated and compared with urea as reference material. The results of NLO characteristics showed that the subject compounds are excellent candidates as NLO materials. The enhancement of NLO properties can be qualitatively explained by means of high charge transfer pattern.

Electronic bandstructure modulation of MoX2/ZnO(X:S,Se) heterostructure by applying external electric field

The electronic properties of 2-Dimensional Van der Waals heterostructure of MX2(M:Mo, X:S, Se) and graphene-like ZnO by applying an external electric field(EF) in the range -0.50V/Å to 1.0V/Å is studied using first-principles calculations. The effect of the transverse electric field is measured in terms of modulation of the electronic energy bandgap and change in the band alignment properties. The bandstructure of MoS2/ZnO heterostructure(HS) shows an indirect bandgap of 1.61eV with a type-II band alignment and a large built-in electric field of 7.42eV with a valence band offset of 1.22eV across the interface. The bandstructure of MoSe2/ZnO shows a direct bandgap of 1.81eV with type-I alignment and a built-in electric field of 3.64eV with a band offset of 0.31eV. The charge density is localized on MoS2 and ZnO in VBM and CBM respectively in MoS2/ZnO HS, whereas in MoSe2/ZnO, both VBM and CBM are localized on MoSe2. With a perpendicular electric field applied across the HS, re-alignment of bandstructure and modulation of bandgap in both the HSs occurs. The energy bandgap increases linearly with the applied electric field for MoS2/ZnO(1.1–2.2eV) and remains almost constant(1.81eV) in the range -0.50V/Å to 0.50V/Å followed by a small decrease with an increase in electric field (1.60 for EF=±1.0V/Å). A cross-over in the bandgap type from indirect(type-II)→direct(type-I) in MoS2-ZnO and direct(type-I) to indirect(type-II) in MoSe2/ZnO has been observed at a critical value of electric field EF=0.75V/Å. The cross-over in band structure is consistent with the charge transfer pattern observed on the application of electric field. Tuning the electronic bandgap and changing the band-alignment with an external electric field opens a way to design futuristic electronic and optical devices.

Infrared Activities of Adsorbed Species on Metal Surfaces: The Puzzle of Adsorbed Methyl (CH3).

Reflection-absorption infrared spectroscopy (RAIRS) is widely used to identify molecular adsorbates on metals during surface chemical reactions, but the interpretation of RAIRS data can be difficult with experiment alone. Here, we reveal from first-principles calculations the origin of the contrasting RAIRS spectra of methyl adsorbed on Pt(111) and Ni(111). We find that the dynamic dipole associated with the symmetric C-H stretch vibration of CH3 along surface normal is significant on Pt(111) but negligibly small on Ni(111), explaining the strong IR activity in the former and the absence of any RAIRS peaks in the latter. This difference is correlated to different charge transfer patterns between metals and the adsorbate, which are determined by the different preferred adsorption sites of methyl on the two surfaces. This work highlights the need of electronic structure calculations in interpreting RAIRS spectra of adsorbates on metal surfaces.

Second-order NLO properties and two-state switching effects of transition metal redox complexes of iron and cobalt: A DFT study

Designing switchable materials with large contrasts of nonlinear optical properties has been the focus of research in recent decades because of their widespread applications. Redox-active metal complexes due to charge transfer excitation are suitable to produce switchable nonlinear optical (NLO) material. In this regard, we present here the redox switchable NLO response of active metal complexes of iron and cobalt. The geometric, electronic, molecular absorption, nonlinear optical properties, and switch “ON/OFF” style of these metal complexes are studied at the CAM-B3LYP/6-31 + G(d) level of theory. NLO responses of these redox metal complexes are described in terms of change in the charge transfer (CT) patterns by time dependent density functional theory (TD-DFT). The highest βo value of 301534 × 10−30 esu is noticed in [Fe-ethynyl-ZnP]1+ complex, because of obvious charge transfer transition from metal to ligand i.e meatal-ligand charge transfer (MLCT) in redox metal complex. In each redox metal isomeric pair, the greater hyperpolarizability value of individual isomer is quite consistent with its smaller energy gap (H-Lgap) between highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), low crucial excitation energy, and bathochromic shift of λmax. The remarkable βo contrasts of these isomeric redox complexes illustrate that they can be appropriate for effective redox-triggered NLO switches. Thus, the results reveal that these redox pair complexes show two-state switching “ON/OFF” effect.

Structures, energies, and bonding in the microsolvation of Na+

A stochastic search of the potential energy surfaces for the interctions between the sodium cation and up to six explicit water molecules was performed under Møller–Plesset second–order perturbation theory (MP2) with extended basis sets. At these low molecularties, the sodium cation is more likely to occupy the interior of the cluster, energetically favoring a maximum coordination number of four, even when five or six water molecules are available. An energy decomposition analysis reveals a major contribution from electrostatic interactions to cluster stabilization, with non-negligigle induction contributions accounting for ≈20% of the total interaction. Topological analysis of the electron distribution unambiguously describe H2O⋯Na+ ionic interactions. Hydrogen bond networks seem to arrange in order to maximize the stability of Na+ via a clear charge transfer pattern, where electron density is withdrawn from the O–H bonds towards Na+. Because of this reorganization of the electron density, and because of the perturbative effect of the formal charge, all H2O⋯H2O hydrogen bonds are also highly ionic in nature with some degree of covalency.

Constructing a compact heterojunction structure of Ag2CO3/Ag2O in-situ intermediate phase transformation decorated on ZnO with superior photocatalytic degradation of ibuprofen

In this study, the ZnO/Ag2CO3/Ag2O photocatalyst had been successfully fabricated via the facile two-step synthesis method, which included the heterojunction of Ag2CO3 on ZnO, followed by the in-situ phase transformation method of Ag2CO3/Ag2O. The phase transformation from Ag2CO3 to Ag2O over ZnO surfaces was observed in different phase structures and the O atom possessed by Ag2O had diffused towards the Ag2CO3 surface, signifying the presence of Ag2CO3/Ag2O heterostructure over the ZnO. The result indicates that the heterojunction of Ag2CO3/Ag2O over ZnO surfaces exhibited a 99.3% photodegradation of Ibuprofen (IBF) solution under visible-light irradiation, with 10-folds higher reaction rate compared to the pristine ZnO. Compact interfacial heterojunction in ZnO–Ag2CO3-Ag2O sample had promoted effective interfacial charge transfers due to synergistic effect of Ag2CO3/Ag2O heterostructure. This situation leads an extended light absorption due to light sensitive Ag2CO3 and Ag2O which can all lead to an improved photocatalytic activity of the ZnO/Ag2CO3/Ag2O. Experimental and characterization results have shown that a plausible mechanism of highlighting the surface of redox reaction and charge transfer pattern on ZnO/Ag2CO3/Ag2O had been attained. Hence, heterojunction with interface structure could be a promising photocatalyst for photocatalytic applications.

Improvement in visible-light-induced photocatalytic activity of Ag–CeO2 Schottky-type contact heterostructures

This study has widely described the fabrication and performance of a series of Ag–CeO2 Schottky plasmonic photocatalysts under visible light irradiation. A conventional wetness incipient impregnation technique has been utilized to load Ag metal on the surface of CeO2 nanoparticles synthesized by the hydrothermal method. The as-synthesized photocatalysts have been identified using combined characterizations of XRD, FTIR, FESEM, EDS, TEM and UV–Vis DRS. The photocatalytic activity of as-synthesized samples has been investigated in terms of degradation efficiency of methylene blue (MB) as a function of involved operating factors including irradiation photon energy, irradiation time, pH, initial MB concentration, photocatalyst dose and amount of loaded Ag. All of the Ag–CeO2 samples have exhibited higher photocatalytic efficiencies than pure CeO2 with the order of 8%-Ag–CeO2 > 5%-Ag–CeO2 > 2%-Ag–CeO2 > pure CeO2. The enhanced photocatalytic activity has been ascribed to the improved light absorption toward visible irradiation photons due to the surface plasmon resonance, supported by the DRS results. A Schottky scheme photocatalytic mechanism that highlights charge transfer pattern has been developed for photodegradation rate. Photocatalytic process kinetics has been found to obey the pseudo-first-order expression according to the Langmuir–Hinshelwood model.

In situ self-assembly synthesis of sandwich-like TiO2/reduced graphene oxide/LaFeO3 Z-scheme ternary heterostructure towards enhanced photocatalytic hydrogen production

In the present work, an indirect Z-schematic heterojunction system of TiO2 and LaFeO3 (LFO) with a solid state electron mediator (RGO) is proposed and applied for photocatalytic hydrogen (H2) evolution. Through a facile 2-step synthesis strategy including adsorption of LFO on GO sheets and growth of TiO2 between the LFO-GO frameworks, a Ti-O-C bonds strongly linked sandwich-like TiO2/RGO/LFO composite is obtained. The structure, morphology, optical properties and catalytic mechanism of the Z-scheme heterojunction are thoroughly investigated. The obtained visible light responded TiO2/RGO/LFO composite displays significantly elevated photocatalytic performance for H2 evolution with excellent stability, and its H2 production rate (0.893 mmol h−1 g-1) is approximately 3.2, 14.4 and 11.4 times superior to the direct Z-scheme TiO2/LFO composite, pure TiO2 and LFO, respectively. The sandwich-like structure with strong interaction between three components and short diffusion distance for impactful separation of e--h+ pairs bring about a vast improvement in H2 evolution performance of TiO2/RGO/LFO composite. In addition, Ti-O-C bonds could also enhance electronic conductivity and structure stability as a bridge of photogenerated carriers. Based on the investigation results, possible mechanisms for synthesis and photocatalytic charge transfer pattern for TiO2/RGO/LFO are developed.

Gold diggers: Altered reconstruction of the gold surface by physisorbed aromatic oligomers

We present electronic structure theory calculations and scanning tunneling microscopy experiments for the adsorption of $\ensuremath{\alpha}$-sexithiophene on the 100 surface of gold. Our density-functional theory calculations show that $\ensuremath{\alpha}$-sexithiophene prefers to adjust an energetically unfavorable adsorption site by modifying the gold surface over seeking out more favorable adsorption sites. Molecular adsorption results in a complex charge transfer pattern, with more charge transfer in more stable sites. Our results challenge the current paradigm that weakly interacting (e.g., physisorbed) molecules perceive metal surfaces as rigid templates with preordained adsorption sites.

Fabrication of highly efficient and stable indirect Z-scheme assembly of AgBr/TiO2 via graphene as a solid-state electron mediator for visible light induced enhanced photocatalytic H2 production

In this study, indirect Z-schematic assembly of AgBr and TiO2 via graphene as a solid-state electron mediator was developed and investigated for photocatalytic H2 production. The AgBr/rGO/TiO2 was fabricated using facile two-step synthesis method which includes the growth of AgBr and deposition of TiO2 on graphene oxide sheets followed by partial reduction through reflux method. The photocatalysts were characterized using TEM, XRD, XPS, FTIR, nitrogen (N2) adsorption and desorption, Raman, PL and UV–Vis spectroscopy to understand morphology, structure, chemical and optical properties. Alterations in band structures, elevation of conduction band positions and reduction in band gap energies of rGO-modified AgBr/TiO2 photocatalysts were evaluated. The performance of AgBr/rGO/TiO2 exhibited 2025 ppm of H2 production, which was 7 fold higher than AgBr/TiO2 (289 ppm), 2.3 fold higher than rGO/TiO2 (885 ppm) and 13.4 fold higher than TiO2 (151 ppm). Enhanced photocatalytic activity of Z-schematic composites can be attributed to strong interfacial bonding (AgBr—rGO–TiO2), efficient transfer of electrons due to synergistic effect of AgBr and rGO as well as extended light absorption due to highly light sensitive AgBr. In addition, yield of H2 production was decreased above 5% AgBr loading and dosage of 0.10 g of photocatalyst due to incompatibility of ratio and shielding effect of particles. Moreover, with increase of temperature and concentration of hole scavenger, yield of H2 production was gradually increased which demonstrated the contribution of H2 from photoreforming of hole scavenger. Based on the experimental and characterizations results, a possible mechanism that highlighted the surface redox reactions and charge transfer pattern on AgBr/rGO/TiO2 was developed. Thus, indirect Z-scheme assembly of AgBr/rGO/TiO2 could be a promising photocatalyst for solar energy assisted H2 production.

Improving the NLO response of bis-cyclometalated iridium(Ⅲ) complexes by modifying ligands: A DFT study

Density functional theory (DFT) have been carried out to investigate the nine recently reported iridium (Ⅲ) complexes which substituted various cyclometalated ligands (CˆN) and aryl isocyanide ancillary ligands (CNAr). The aim of this paper is to study the substituents effect on geometrical structures, electronic and second-order nonlinear optical (NLO) properties, UV-vis absorption spectra. It is found that the first hyperpolarizabilities can be easily modulated by the introduction of different electron-withdrawing/donating ligands on CˆN and CNAr. Complexes with 2-benzothienylpyidine (btp) CˆN ligands or the substituents on CNAr are electron-withdrawing groups -NO2 possess larger first hyperpolarizabilities values compared to remaining complexes. Significantly, the changes of the first hyperpolarizability can be qualitatively explained by means of the charge transfer pattern. It is anticipative that the theoretical investigation of these iridium complexes will be helpful for designing high-performance and versatile NLO materials.

Facet-dependent interfacial charge separation and transfer in plasmonic photocatalysts

Surface plasmon resonance (SPR) induced plasmonic photocatalysis provides a brand new way for more efficient light absorption and utilization to achieve better solar light conversion. Although the SPR effect in metal-semiconductor photocatalysis has been widely investigated, the SPR-driven interfacial charge separation and transfer patterns between the two counterparts have not yet been fully revealed. The plasmonic metal-semiconductor photocatalytic systems require to be rationally designed, especially for the facet-aspect of the semiconductor, which can dominantly endow the contacting interface with diverse charge transfer patterns. Taken Bi metal deposited at the typical (001) and (010) facets of BiOBr nanosheets as a case study, we demonstrate that the surface charge alternation on the (010) facet highly favors the interfacial charge separation and transfer, by providing a new route of [Bi2O2]2+ → plasmonic metal → Br− for interfacial carriers transfer. The charge alternation on discrepant semiconductor facets in essence makes the plasmonic photocatalytic system be different in charge transportation pattern. These new findings are further validated in extensive composite systems composed of alternative plasmonic metal (Ag and Au) and semiconductors (BiOCl and BiOI). The perspective here can open numerous possibilities for the rational design of more efficient plasmonic photocatalysts.

Multinuclear “Staircase” Oligomers Based on the (Et2C2B4H4)Fe(η6-C6H6) Sandwich Unit: Quantitative Tailorable and Redox Switchable Nonlinear Optics

To provide a new approach for enhancing second-order nonlinear optical (NLO) response, a series of multinuclear “staircase” oligomers composed of Fe(Et2C2B4H4) ferracarborane clusters and benzene rings have been systematically designed and investigated. The calculations of geometric and electronic structures, electronic absorption spectra, first hyperpolarizabilities, and first hyperpolarizability densities were carried out by density functional theory and time-dependent density functional theory. It is found that the NLO properties have obvious multiple relationships with the numbers of the sandwich unit. Moreover, all of the possibilities for one-, two-, and three-electron oxide complexes have also been considered. The results first revealed some remarkable changes upon charge transfer pattern which is from the unexpected “interrupted” to desired “continuous” forms in the two-electron oxidation reactions, accompanied by significant differences in the relevant second-order NLO properties. The considerabl...

Second-order nonlinear optical responses of carboranyl-substituted indole/indoline derivatives: impact of different substituents

Carborane has been the subject of great interest over the last decades due to its high structural, chemical, biological stability and diverse applications. In the present work, carboranyl-substituted indole/indoline compounds and their functionalized derivatives have been systematically investigated by density functional theory (DFT) method with the view of assessing their electronic structures and first hyperpolarizabilities. Significantly, the first hyperpolarizabilities can be obviously enhanced by the introduction of a strong electron-withdrawing group for closed-ring forms, while the strong electron-donating group is beneficial for large first hyperpolarizabilities for open-ring forms. It indicates that the NLO properties of these compounds can be enhanced by controlling their relative substituent groups. Furthermore, the time-dependent DFT calculation illustrates that the enhancement of the first hyperpolarizabilities are found due to the obvious charge transfer (CT) transition, and closed-ring forms have a significant difference on the CT patterns versus open-ring ones. Investigation of the structure–property relationship and substituent effects at the molecular level can benefit for further exploration of carboranyl-substituted indole/indoline derivatives with versatile and fascinating NLO properties.

Effects of particle size on the triboelectrification phenomenon in pharmaceutical excipients: Experiments and multi-scale modeling

Particle sizes play a major role to mediate charge transfer, both between identical and different material surfaces. The study probes into the probable mechanism that actuates opposite polarities between two different size fractions of the same material by analyzing the charge transfer patterns of two different sizes of microcrystalline cellulose (MCC). Quantum scale calculations confirmed alteration of charge transfer capacities due to variation of moisture content predicted by multiple surface and bulk analytical techniques. Discrete Element Method (DEM) based multi-scale computational models pertinent to predict charge transfer capacities were further implemented, and the results were in accordance to the experimental charge profiles.

Exploring Second-Order Nonlinear Optical Properties and Switching Ability of a Series of Dithienylethene-Containing, Cyclometalated Platinum Complexes: A Theoretical Investigation.

The second-order nonlinear optical (NLO) properties of a series of dithienylethene- (DTE-) containing Pt(II) complexes have been investigated by density functional theory calculations. The first hyperpolarizabilities β of studied systems can be greatly enhanced by simple ligand substitutions. Because of the nature of DTE units, the β values also can be varied by the use of lights in the studied systems. The highest β difference between photoisomers can over 1000 × 10(-30) esu, with the contrast around five times. Thus, the studied systems can act as effective photoswitchable second-order NLO materials. The time-dependent density functional theory calculations revealed that the charge transfer patterns of studied systems have special characters compared to other reported DTE-containing NLO switched chromogens, the DTE units mainly act as electron-donors in studied systems, and the variation of β can be viewed as alternation of donor abilities of DTE units; thus, our work also proposed a new mechanism for designing photoswitched NLO multifunctional materials.