Donor-acceptor Pair Research Articles

Dynamic Interplay of Nonlocal Recombination Pathways in Quantum Emitters in Hexagonal Boron Nitride.

Optically active defects in wide bandgap materials play a central role in several emerging applications in quantum information and sensing as they allow for manipulating and harvesting the internal degrees of freedom of single electrons with optical means. Interactions among defect states and with the surrounding environment represent a crucial feature for sensing but can severely hamper the coherence of the quantum states and prevent an efficient integration with photonic architectures due to unpredictable spectral instability. Understanding and controlling defect interactions would mitigate the effects of spectral instabilities and enable quantum applications based on long-range interactions. Here, we investigate the photoluminescence spectral dynamics of quantum emitters in defective hexagonal boron nitride (hBN), a material whose emission spectrum notoriously displays spectral wandering and diffusion, and we identify several optical transitions with discrete energy jumps. We associate the spectral jumps with the interplay amid competing recombination pathways available to the defect states in a process like donor-acceptor-pairs (DAP). The discrete spectral jumps observed in the emission spectrum of hBN arise from interactions between the harmonic states of nitrogen π orbitals of delocalized defects, and their energies can be ascribed to a DAP-like transition sequence. Our results allow mapping of the defect geometry in an hBN lattice, setting the basis for mitigating the effects of spectral jumping in this platform and paving the way toward using the long-range interaction of defect ensembles for quantum technology.

Structural and optical properties of cubic GaN films on high-thermal conductivity substrates

A thin cubic-GaN epitaxial layer was grown by plasma-assisted molecular-beam epitaxy directly on a (100) cubic-boron-nitride nucleation layer that was previously deposited on a (100)-oriented, type IIA, single-crystal chemical-vapor deposition diamond substrate. X-ray diffraction measurements verified that (100)-plane zincblende is the dominant crystal structure of the GaN layer, with a small contribution from the wurtzite phase in some sample regions. Detailed atomic force microscopy and Raman scattering measurements confirmed the XRD findings. Low temperature photoluminescence spectra show a dominant emission line near 3.12 eV and a weaker emission line near 3.21 eV. The former is assigned to a recombination process involving electrons bound to shallow donors with holes bound to shallow acceptors (donor–acceptor pair recombination process). The latter, which becomes dominant at temperatures above 50 K, is assigned to a recombination process involving the annihilation of excitons bound to shallow impurities. Room temperature transmission measurements yielded a direct bandgap of 3.23 eV, which is close to the reported values for c-GaN. These results confirm that cubic-GaN was successfully deposited on a high thermal-conductivity diamond substrate. This opens the door to exploring the potential of cubic-GaN devices for high-power applications, given that large area diamond substrates are becoming available.

Designing Donors and Nonfullerene Acceptors for Organic Solar Cells Assisted by Machine Learning and Fragment‐Based Molecular Fingerprints

The molecular structures and properties of donor and acceptor materials for organic solar cells (OSCs) determine their photovoltaic performance; however, the complex relationship between them has hindered the development of OSC materials. To study this, we constructed the database comprising 544 donor and non‐fullerene acceptor pairs. Based on the principle of minimal rings and molecular units, each molecule in the database is cut into different fragments and defined as a new molecular fingerprint, where each bit corresponds to a fragment number in the molecule. Accordingly, the defined molecular fingerprint length is 234 and 723 bits for donors and acceptors, respectively. Random forest and extreme tree regression (ETR) are applied to predict the photovoltaic parameters, with ETR being the most effective. Through SHapley Additive exPlanations (SHAP) importance analysis, eight (10) important donor (acceptor) fragments are identified. Furthermore, by computing the cut fragment similarities with that of the important fragments obtained from SHAP analysis, fragments with similarity exceeding 0.6 are collected in order to design new molecules. By assembling the collected fragments, we designed 21 168 D‐π‐A‐π‐type donors and 1 156 400 A‐π‐D‐π‐A‐type nonfullerene acceptors, generating 24 478 675 200 donor–acceptor pairs. Based on predictions using the trained ETR model, the highest power conversion efficiency reaches 13.2%.

Naphthalene Diimide-Embedded Donor-Acceptor Carbon Nanohoops: Photophysical, Photoconductive, and Charge Transport Properties.

Designing the architecture of donor-acceptor (D-A) pairs is an effective strategy to tailor the electronic structure of conjugated macrocycles for optoelectronic devices. Herein, we present the synthesis of three D-A nanohoops NDI-[n]CPP (n = 7, 8, 9) containing a naphthalene diimide (NDI) unit as an acceptor and [n]cycloparaphenylenes ([n]CPPs) moieties as donors. The D-A characteristics of NDI-[7-9]CPPs were substantiated through absorption and fluorescence spectroscopic studies, electrochemical investigations, and computational analysis. The device investigations demonstrated that the D-A nanohoops NDI-[7-9]CPPs can serve as the photoconductive layer and demonstrate a significant generation of photocurrent with a fast response upon exposure to light. The magnitude of photocurrent shows high dependence on the size of their rings, with an increasing tendency as the ring size decreases. The generation of photocurrent in free acceptor-based CPP has rarely been reported in the previous literature. Significantly, the C60 complexes of NDI-[7-9]CPPs exhibited a marked enhancement in photocurrent under identical conditions; in particular, the photocurrent of C60⊂NDI-[7]CPP is ca. 3.5 times greater than that of NDI-[7]CPP alone. Furthermore, the potential applications of NDI-[7-9]CPPs in electron- and hole-transport devices have also been explored, revealing the clear evidence of their bipolar behavior as an active charge transport layer.

Diacetylene-bridged covalent organic framework as crystalline graphdiyne analogue for photocatalytic hydrogen evolution.

Graphdiyne (GDY) alone as a photocatalyst is unsatisfactory because of its low crystallinity, limited regulation of the band gap, weak photogenerated charge separation, etc., and heterojunctioning with other materials is necessary to activate the photocatalytic activity of GDY. Through elaborate design, a diacetylene-rich linker (S2) was prepared and employed to construct a crystalline and structurally well-defined GDY-like covalent organic framework (COF, namely S2-TP COF) which merges the merits of both COF and GDY to boost the photocatalytic hydrogen evolution reaction (HER). By theoretical prediction on the donor-acceptor (D-A) pair, two other monoacetylene-bridged COFs (S1-TP COF and S3-TP COF) were prepared for comparison. Exhibiting enhanced separation and suppressed recombination of photogenerated excitons, Pt-photodeposited S2-TP COF showed a higher HER rate (10.16 mmol g-1 h-1) than the other two non-GDY-like COFs (3.71 and 1.13 mmol g-1 h-1). A joint experimental-theoretical study suggests that the appropriate D-A structure for photogenerated charge separation and diacetylene motif as the adsorption site are the key reasons for the boosted HER. This work opens a new avenue for the rational design of COFs as GDY mimics for photocatalytic application.

Supramolecular method enabling effective through-space charge transfer in thermally activated delayed fluorescence materials with pure orange emission

A supramolecular strategy has been developed to enhance through-space charge transfer by precisely regulating the spatial arrangement of donor–acceptor pairs, resulting in thermally activated delayed fluorescence with pure orange emission.

Microwave-assisted synthesis of highly photoluminescent core/shell CuInZnSe/ZnS quantum dots as photovoltaic absorbers.

Water-dispersible core/shell CuInZnSe/ZnS (CIZSe/ZnS) quantum dots (QDs) were efficiently synthesized under microwave irradiation using N-acetylcysteine (NAC) and sodium citrate as capping agents. The photoluminescence (PL) emission of CIZSe/ZnS QDs can be tuned from 593 to 733 nm with varying the Zn : Cu molar ratio in the CIZSe core. CIZSe/ZnS QDs prepared with a Zn : Cu ratio of 0.5 exhibit the highest PL quantum yield (54%) and the longest PL lifetime (515 ns) originating from the recombination of donor-acceptor pairs. The potential of CIZSe/ZnS QDs as photoabsorbers in QD-sensitized solar cells was also evaluated. An adequate type-II band alignment is observed between TiO2 and CIZSe/ZnS QDs, indicating that photogenerated electrons in CIZSe/ZnS QDs could efficiently be injected into TiO2.

Manipulating Oxygen Vacancies in γ-Ga2O3 Nanocrystals: Correlation between Defect Location, Charge State, and Photophysical Properties.

Recent advancements in colloidal synthesis have enabled precise control of extrinsic dopants in semiconductor nanocrystals (NCs), enriching our understanding of dopant-exciton interactions and opening new avenues for controlling NC properties. However, the manipulation of intrinsic defects in colloidal NCs remains challenging. Here, we demonstrate regulation of oxygen vacancy concentration and location in γ-Ga2O3 NCs, significantly altering their photoluminescent properties. Spectroscopic analysis and density functional theory calculations reveal that bulk oxygen vacancies are mostly neutral and lead to the formation of a deep donor band that contributes to the UV emission. Conversely, surface-proximate oxygen vacancies, influenced by the band bending effect, exhibit a tendency toward double ionization, giving rise to the characteristic donor-acceptor pair emission. This work highlights the correlation between the oxygen vacancy location and charge states, leading to diverse defective states and distinct photophysical processes. Precise defect manipulation offers new insights into structure-property relationships and the design of functional nanomaterials.

Ligand-coupled conformational changes in a cyclic nucleotide-gated ion channel revealed by time-resolved transition metal ion FRET

Cyclic nucleotide-binding domain (CNBD) ion channels play crucial roles in cellular-signaling and excitability and are regulated by the direct binding of cyclic adenosine- or guanosine-monophosphate (cAMP, cGMP). However, the precise allosteric mechanism governing channel activation upon ligand binding, particularly the energetic changes within domains, remains poorly understood. The prokaryotic CNBD channel SthK offers a valuable model for investigating this allosteric mechanism. In this study, we investigated the conformational dynamics and energetics of the SthK C-terminal region using a combination of steady-state and time-resolved transition metal ion Förster resonance energy transfer (tmFRET) experiments. We engineered donor-acceptor pairs at specific sites within a SthK C-terminal fragment by incorporating a fluorescent noncanonical amino acid donor and metal ion acceptors. Measuring tmFRET with fluorescence lifetimes, we determined intramolecular distance distributions in the absence and presence of cAMP or cGMP. The probability distributions between conformational states without and with ligand were used to calculate the changes in free energy (ΔG) and differences in free energy change (ΔΔG) in the context of a simple four-state model. Our findings reveal that cAMP binding produces large structural changes, with a very favorable ΔΔG. In contrast to cAMP, cGMP behaved as a partial agonist and only weakly promoted the active state. Furthermore, we assessed the impact of protein oligomerization and ionic strength on the structure and energetics of the conformational states. This study demonstrates the effectiveness of time-resolved tmFRET in determining the conformational states and the ligand-dependent energetics of the SthK C-terminal region.

Ligand-coupled conformational changes in a cyclic nucleotide-gated ion channel revealed by time-resolved transition metal ion FRET.

Cyclic nucleotide-binding domain (CNBD) ion channels play crucial roles in cellular-signaling and excitability and are regulated by the direct binding of cyclic adenosine- or guanosine-monophosphate (cAMP, cGMP). However, the precise allosteric mechanism governing channel activation upon ligand binding, particularly the energetic changes within domains, remains poorly understood. The prokaryotic CNBD channel SthK offers a valuable model for investigating this allosteric mechanism. In this study, we investigated the conformational dynamics and energetics of the SthK C-terminal region using a combination of steady-state and time-resolved transition metal ion Förster resonance energy transfer (tmFRET) experiments. We engineered donor-acceptor pairs at specific sites within a SthK C-terminal fragment by incorporating a fluorescent noncanonical amino acid donor and metal ion acceptors. Measuring tmFRET with fluorescence lifetimes, we determined intramolecular distance distributions in the absence and presence of cAMP or cGMP. The probability distributions between conformational states without and with ligand were used to calculate the changes in free energy (ΔG) and differences in free energy change (ΔΔG) in the context of a simple four-state model. Our findings reveal that cAMP binding produces large structural changes, with a very favorable ΔΔG. In contrast to cAMP, cGMP behaved as a partial agonist and only weakly promoted the active state. Furthermore, we assessed the impact of protein oligomerization and ionic strength on the structure and energetics of the conformational states. This study demonstrates the effectiveness of time-resolved tmFRET in determining the conformational states and the ligand-dependent energetics of the SthK C-terminal region.

Harnessing Free Sulfate Groups in Glycosylation Reactions.

General strategies for synthesizing sulfated oligosaccharides employ the protection of sulfate groups for glycosylation or post-sulfation after the synthesis of oligosaccharides. However, the chemical behavior of free sulfate groups in glycosylation reactions has not been thoroughly studied. We examined several glycosyl donors with free sulfate groups, but neither glycosyl imidates nor thioglycosides achieved products. Conversely, activating 6-sulfated GalNAc donors with either a 2-NTroc group or 2-O-acetyl group using an ortho-hexynylbenzoate group at the anomeric position yielded β-glycosides in good yield. These results indicate that glycosylations with a free sulfate group can be performed and that the neighboring group participation of 2-NHTroc and 2-O-acetyl groups works even with an unprotected sulfate group at the 6-position. Ab initio calculations supported the formation of acyloxonium-cation via 2-O-acyl participation. Additionally, glycosylation reactions under various counter cations of the sulfate group, such as Na+, Li+, K+, Ba2+, and pyridinium were examined. Results showed that sodium and lithium salt donors yielded the products in good yield. These results were also supported by ab initio calculations. Practical glycosylation reactions between disaccharide donor-acceptor pairs with free sulfate groups successfully yielded the sulfated tetrasaccharides. This study also discusses how a sodium salt acts as a protecting group during glycosylation.

Vinylene-Linked Covalent Organic Frameworks for Visible-Light-Promoted Selective Oxidation of Styrene in Water.

Vinylene-linked COFs, as an emerging class of crystalline porous polymers, have been regarded as ideal heterogenous photocatalysts due to their ordered structure, tailored pore size, outstanding stability and fully π-conjugated structure. Unfortunately, their photocatalytic performances are usually impeded by high exciton binding energy and unsatisfactory exciton dissociation efficiency. Herein, the authorsbroke through this dilemma by arrangement of complementary donor-acceptor (D-A) pairs within the COF skeleton to improve charge transfer/separation. Two vinylene-linked COFs (TMT-BT-COF and TMT-TT-COF) are synthesized by Aldol condensation using highly photoactive thienothiophene and benzothiazole groups as donor and electron-deficient triazine units as acceptor. Photochemical/electrochemical studies as well as DFT calculation suggest that these D-A type vinylene-linked COFs endow high charge transfer efficiency and low charge recombination. As a result, both of them demonstrate remarkably catalytic activity in the oxidation of styrene to benzaldehyde with molecular oxygen, with an exceptionally high conversion rate (≥92%) and selectivity (≥90%). Intriguingly, in the presence of NaHCO3, the above COFs could photocatalyze epoxidation styrene in water, and the styrene oxide selectivity reached 53%. This work elucidates the prominent capability of vinylene-linked COFs in the photocatalytic transformation of organic compounds in aqueous media, which may pave a new avenue for their future development.

Luminescence Mechanisms of Quaternary Zn-Ag-In-S Nanocrystals: ZnS:Ag, In or AgInS2:Zn?

Highly emissive Zn-Ag-In-S nanocrystals have attracted attention as derivatives of I-III-VI2-type nanocrystals without the use of toxic elements. The wide tunability of their luminescence wavelengths is attributed to the controllable bandgap of the solid solution between ZnS and AgInS2. However, enhancement of the photoluminescence quantum yield (PL-QY) depending on the chemical composition has not been elucidated. Here, the luminescence mechanisms of Zn-Ag-In-S nanocrystals were studied from the perspective of ZnS doped with Ag and In, although previous research has proposed a hypothesis that Zn is a radiative recombination centre in the AgInS2 host. The Zn-Ag-In-S nanocrystals were synthesized by systematically varying the Zn, Ag, and In contents. The nanocrystals exhibit a structure in which a part of the Zn in the cubic ZnS is substituted with Ag and In. Luminescence was ascribed to a donor-acceptor pair (DAP) recombination between electrons trapped in In donors and holes trapped in Ag acceptors. The composition-dependent enhancement of PL-QYs was attributed to an increase in donor and acceptor concentrations. The DAP characteristics were maintained over a wide range of Ag and In contents because of the localized character of the band edge states dominated by Ag and In orbitals, as suggested formerly by simulation.

Design of Clickable Aromatic Donor-Acceptor Pairs for Easy Installation onto Polymers.

New intrinsically unsymmetric aromatic donors (D) and acceptors (A) were designed to simplify the incorporation of a single reactive group. Naphthalene diimide (NDI) was desymmetrized by replacing one of the imide units with two nitro groups to yield 3,6-dinitronaphthalene monoimide (NMI(NO2)2); likewise, 3,6-dimethoxycarbazole (CBZ(OMe)2) was designed as a 3-ring aromatic donor. Different derivatives of naphthalene monoimide (NMI) and carbazole (CBZ) were prepared, and charge-transfer complex formation between them was studied using NMR and UV-visible titrations; the estimated association constants (Ka) were compared with the common pair, NDI, and 1,5-dialkoxy naphthalene (DAN). To rationalize the variation of Ka values, the electrostatic potential maps and the energies of highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) of different derivatives were computed by using density functional theory. Some of the new D-A pairs outperform the DAN-NDI pair; as expected, the best pair was NMI(NO2)2 and CBZ(OMe)2. The Ka for this pair was 50 M-1, which is slightly higher than that of NDI-DAN but with the advantage that a single reactive handle, like azide, can be easily incorporated and used to click them onto supramolecular motifs or polymers. To illustrate this feature, a low Tg copolymer of n-butyl acrylate and propargyl acrylate (10 mol %) was clicked with the azido-derivatives of the new donors and acceptors; the influence of the charge-transfer interaction on the mechanical properties of blends of these copolymers carrying D and A units was examined using nanoindentation studies. Additionally, the propargyl acrylate copolymers were also coclicked with both D and A units, and their properties were examined as a function of the density of interacting sites. In summary, this study illustrates an alternate design strategy for aromatic donors and acceptors that are easier to synthesize, scale-up, and derivatize; importantly, they can be readily installed onto different motifs for the efficient design of supramolecular materials and polymers.

Charge Transfer Donor-Acceptor Guests Confined within Metal-Organic Framework Crystals

Organic donor-acceptor (D-A) co-crystals have emerged as a promising class of semiconductors for photovoltaic applications.1,2 These co-crystals, which are packed through π-stacking interactions, can form charge transfer (CT) excitons within donor-acceptor pairs. These materials can exhibit unique and rare properties such as ambipolar charge transport, room-temperature ferroelectricity, and non-linear optical properties. Confinement of such D-A pairs within crystalline porous host materials such as metal-organic materials (MOFs)3 has rarely been studied. This approach can improve energy and charge transfer interactions by constraining their interaction and minimizing nonradiative energy transfer. Additionally, the formation of D-A pairs as dimers or a polymeric phase without altering the CT interactions can be controlled; band gap tunability is also possible. We hypothesize that confining D-A pairs within a porous network will also enhance the exciton lifetime compared to its dimer state in a crystal or in solution. In 2014, we reported an example of successful confinement of dihexyl-sexithiophene (acceptor) and [6,6]-phenyl-C61-butyric acid methyl ester (donor) molecules within the pores of MOF-177.4 In the present work, we focused on different sets of D-A pairs to provide direct structural insights, such as the location within the pore, host-guest interactions, and guest-guest orientation. For this purpose, we dissolved naphthalene-TCNQ or pyrene-TCNQ in dichloromethane (DCM) at a 1:1 ratio and soaked MOF-177 crystals in the solution for 3 days. UV-Vis spectroscopy reveals the difference in the absorption bands of the isolated molecules vs. D-A pairs in MOF pores. To confirm the presence of guest species inside the MOF and to determine the donor vs. acceptor ratio we obtained nuclear magnetic resonance (NMR) spectra. Interestingly, our preliminary results confirm that both naphthalene-TCNQ and pyrene-TCNQ pairs were accommodated inside the MOF but in different stoichiometric ratios. We also collected single crystal X-ray diffraction of the MOF crystals after being soaked after infiltration with D-A pairs. Structural refinement is underway to determine their position and interactions. Our approach provides new mechanistic insights and also produces new benchmark materials with long exciton lifetimes, which can be utilized to advance current photonic technologies. References Abou Taka, J. E. Reynolds Iii, N. C. Cole-Filipiak, M. Shivanna, C. J. Yu, P. L. Feng, et al. Phys. Chem. Chem. Phys. 2023, 25, 27065.Sun, Y. Wang, F. Yang, X. Zhang and W. Hu. Adv. Mater. 2019, 31, 1902328.Shivanna, Q.-Y. Yang, A. Bajpai, E. Patyk-Kazmierczak and M. J. Zaworotko, Nat. Commun. 2018, 9, 3080.K. Leong, M. E. Foster, B. M. Wong, E. D. Spoerke, D. Van Gough, J. C. Deaton, et al. J. Mat. Chem. A 2014, 2, 3389.

Delayed luminescence and thermoluminescence in laboratory-grown diamonds

The “blue-green” phosphorescence/thermoluminescence is most commonly observed in diamonds following excitation at or above the indirect band gap and has been explained by a substitutional nitrogen-boron donor-acceptor pair recombination model. “Orange” and “red” phosphorescence have also been frequently observed in laboratory-grown near-colorless high-pressure high-temperature diamonds following optical excitation and their luminescence mechanisms are shown to be different from that of the blue-green phosphorescence. The physics of the orange and red luminescence and phosphorescence bands including the optical-excitation dependency (UV-NIR), temperature dependency (20K–573K), and related charge transfer process are investigated by a combination of self-built time-resolved imaging/spectroscopic techniques. In this paper, an alternative model for long-lived phosphorescence based on charge trapping is proposed to explain the orange phosphorescence/thermoluminescence band. Additionally, the red phosphorescence band is attributed to point defect, which possibly has a three-level phosphorescence system. Published by the American Physical Society 2024

ChemFET Anion Sensor Based on MOF Nanoparticles.

Nanoparticles of metal-organic frameworks (nanoMOFs) possess the unusual combination of both internal and external surfaces. While internal surfaces have been the focus of fundamental and applications-based MOF studies, the chemistry of the external surfaces remains scarcely understood. Herein we report that specific ion interactions with nanoparticles of Cu(1,2,3-triazolate)2 (Cu(TA)2) resemble the Hofmeister behavior of proteins and the supramolecular chemistry of synthetic macromolecules. Inspired by these anion-selective interactions, we tested the performance of Cu(TA)2 nanoparticles as chemical field effect transistor (ChemFET) anion sensors. Rather than size-based selectivity, the detection limits of the devices exhibit a Hofmeister trend, with the greatest sensitivity towards anions perchlorate, iodide, and nitrate. These results highlight the importance of the pore-based supramolecular interactions, rather than localized donor-acceptor pairs, in designing MOF-based technologies.

Luminescence properties of dislocations in α-Ga2O3

Abstract Dislocations in epitaxial lateral overgrown α-Ga2O3 are investigated using hyperspectral cathodoluminescence spectroscopy. The dislocations are associated with a reduction of self-trapped hole-related luminescence (ca. 3.6 eV line) which can be ascribed to their actions as non-radiative recombination sites for free electrons, to a reduction in free electron density due to Fermi level pinning or to electron trapping at donor states. An increase in the intensity of the ca. 2.8 eV and 3.2 eV lines are observed at the dislocations, suggesting an increase in donor–acceptor pair transitions and providing strong evidence that point defects segregate at dislocations.

Improving broadband absorption of carbon dot conjugated melanin via pre-doping strategy as interfacial photothermal evaporator

Photothermal seawater desalination harnesses solar energy to induce heat and expediates seawater evaporation and purification. The desalination efficacy heavily relies on the light absorption capability and microstructure of the solar evaporator. Polydopamine (PDA) represents a typical synthetic melanin with broad spectral absorption characteristics. However, its absorption capacity within the near-infrared (NIR) region is limited, leading to inadequate photothermal conversion efficiency. To overcome this challenge, we adopt a straightforward approach involving the pre-doping of N,S-doped carbon quantum dots (N,S-CQDs) with dopamine through covalent interaction to fabricate CQDs-loaded mesoporous polydopamine (MPDA-8). This strategy effectively reduces the band gap of PDA by fostering additional donor–acceptor pairs and π-stacking structures, thereby broadening the absorption capacity across the UV–Vis-NIR spectrum through nonradiative transitions. The MPDA-8 coated Balsa wood and cellulose evaporator achieved high evaporation rates of 1.82 kg m−2 h−1 and 2.10 kg m−2 h−1, respectively, overcoming the limit of 2D interfacial evaporator. The small pores and fiber channels within the wood, coupled with the hydrophilicity of MPDA-8, facilitates efficient water transportation and reduces evaporation enthalpy. Outdoor evaporation experiments conducted under sunlight corroborated the practical viability of this material. This study introduces a new perspective for advancing synthetic melanins in photothermal applications through the nanoparticle pre-doping strategy.

Single ZnO Nanowire for Electrical and Optical NO2 Gas Sensing: Origin of Reversible and Irreversible Gas Effects Investigated by Photoluminescence Spectroscopy.

In this work, the gas sensing properties of a single ZnO nanowire (NW) are investigated, simultaneously in terms of photoluminescence (PL) and photocurrent (PC) response to NO2 gas, with the purpose of giving new insights on the gas sensing mechanism of a single 1D ZnO nanostructure. A single ZnO NW sensing device was fabricated, characterized, and compared with a sample made of bundles of ZnO NWs. UV near-band-edge PL emission spectroscopy was carried out at room temperature and by lowering the temperature down to 77 K, which allows detection of resolved PL peaks related to different excitonic transition regions. Surface effects were observed in PL maps, considering different nano and microstructures. Electrical and optical measurements were acquired at the same time during the NO2 gas exposure, allowing for the comparison of PL and PC response times and signal recovery. During NO2 gas desorption, irreversible behavior in the surface-related and donor-acceptor pair (DAP) regions is interpreted as the effect of an initial transient when electronic transfer from the gas molecules to the bulk occurs through the ZnO NW surface which acts as a channel. To the best of our knowledge, this is the first work which investigates the simultaneous PL optical and PC electrical response signals of a single ZnO NW to gas exposure.