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We propose and demonstrate a novel range of models to accurately determine the optical properties of nitrogen-free carbon quantum dots (CQDs) with ordered graphene layered structures. We confirm the results of our models against the full range of experimental results for CQDs available from an extensive review of the literature. The models can be equally applied to CQDs with varied sizes and with different oxygen contents in the basal planes of the constituent graphenic sheets. We demonstrate that the experimentally observed blue fluorescent emission of nitrogen-free CQDs can be associated with either small oxidised areas on the periphery of the graphenic sheets, or with sub-nanometre non-functionalised islands of sp 2 -hybridised carbon with high symmetry confined in the centres of oxidised graphene sheets. Larger and/or less symmetric non-functionalised regions in the centre of functionalised graphene sheet are found to be sources of green and even red fluorescent emission from nitrogen-free CQDs. We also demonstrate an approach to simplify the modelling of the discussed sp 2 -islands by substitution with equivalent strained polycyclic aromatic hydrocarbons. Additionally, we show that the bandgaps (and photoluminescence) of CQDs are not dependent on either out-of-plane corrugation of the graphene sheet or the spacing between sp 2 -islands. Advantageously, our proposed models show that there is no need to involve light-emitting polycyclic aromatic molecules (nanographenes) with arbitrary structures grafted to the particle periphery to explain the plethora of optical phenomena observed for CQDs across the full range of experimental works., (© 2023. The Author(s).)

The control of heat at the nanoscale via the excitation of localized surface plasmons in nanoparticles (NPs) irradiated with light holds great potential in several fields (cancer therapy, catalysis, desalination). To date, most thermoplasmonic applications are based on Ag and Au NPs, whose cost of raw materials inevitably limits the scalability for industrial applications requiring large amounts of photothermal NPs, as in the case of desalination plants. On the other hand, alternative nanomaterials proposed so far exhibit severe restrictions associated with the insufficient photothermal efficacy in the visible, the poor chemical stability, and the challenging scalability. Here, it is demonstrated the outstanding potential of NiSe and CoSe topological nodal-line semimetals for thermoplasmonics. The anisotropic dielectric properties of NiSe and CoSe activate additional plasmonic resonances. Specifically, NiSe and CoSe NPs support multiple localized surface plasmons in the optical range, resulting in a broadband matching with sunlight radiation spectrum. Finally, it is validated the proposed NiSe and CoSe-based thermoplasmonic platform by implementing solar-driven membrane distillation by adopting NiSe and CoSe nanofillers embedded in a polymeric membrane for seawater desalination. Remarkably, replacing Ag with NiSe and CoSe for solar membrane distillation increases the transmembrane flux by 330% and 690%, respectively. Correspondingly, costs of raw materials are also reduced by 24 and 11 times, respectively. The results pave the way for the advent of NiSe and CoSe for efficient and sustainable thermoplasmonics and related applications exploiting sunlight within the paradigm of the circular blue economy., (© 2022 The Authors. Small published by Wiley-VCH GmbH.)

The discrepancy between the bright theoretical projections for two-dimensional (2D) Janus structures and the lack of experimental realisation of these structures motivated us to study the effect of structural disorder on the stability of MoSSe, SnSSe, PtSSe, In 2 SSe and GaInSe 2 . The calculation results demonstrate that the difference between metal-sulfur and metal-selenium bonds makes Janus structures frustrated and less energetically favourable than less ordered allotropes of the same compounds. This result explains the difficulties encountered in the experimental fabrication of these materials. In the bulk, there is an additional contribution to the total energy from dipole-dipole interactions between layers with a Janus structure that can overcome the energetic cost of structural frustration in layers for compounds with sufficiently large dipole moments. However, the entropic contribution to the free energy decreases the favourability of the ordered Janus structure. The calculation results are used to make recommendations to enable the discovery and synthesis of 2D materials with Janus structures.

We report for the very first time the crystal structure and self-assembly of a new aggregation-induced emission enhancement (AIEE) dye 4-(5-methoxythiazolo[4,5- b ]pyridin-2-yl)- N , N -dimethylaniline (TPA) and its application in sensing dichromate ions. TPA reveals cyan blue emission under UV and visible light. The self-assembly properties of TPA were studied extensively by scanning electron microscopy (SEM) which revealed the formation of beautiful flower-like morphologies. These structures revealed both green and red fluorescence under FITC and rhodamine filters respectively when observed through fluorescence microscopy connoting the panchromatic emission properties of TPA from blue to red. The interactions which cause self-assembled structure formation in TPA were also validated theoretically using density functional theory (DFT) calculations. Crystal and molecular structure analysis of TPA was carried out via single-crystal X-ray diffraction to visualize the intermolecular interactions occurring in the solid-state and to study the structure-photophysical property relationship in the aggregated state. The photophysical properties of TPA were also studied extensively by UV-visible and fluorescence spectroscopy and its quantum yield and fluorescence lifetime were calculated by time-correlated single-photon counting (TCSPC). Interestingly, TPA could efficiently sense dichromate (Cr 2 O 7 2- ) ions in an acidic medium and an interesting morphological transition from a fluorescent flower to non-fluorescent disassembled structures could also be observed. The limit of detection of TPA for Cr 2 O 7 2- ions was found to be as low as 5.5 nM, suggesting its exceptional sensitivity. More importantly, TPA could selectively sense Cr 2 O 7 2- ions in real water samples even in the presence of other metal ions routinely present in polluted water, hence making it practically useful for water quality monitoring.

Surface terminations of two-dimensional materials should have a strong influence on the nonlinear optical (NLO) properties, but the relationship between surface terminations and NLO properties has not yet been reported. In this work, switching the NLO properties of MXenes (Ti 3 C 2 T x ) via "surface terminations modulation" is explored. The surface terminations of Ti 3 C 2 T x are modulated by electrochemical treatment, resulting in different states ( viz ., Ti 3 C 2 T x (pristine), Ti 3 C 2 T x (═O rich), and Ti 3 C 2 T x (-OH rich)). The sign and magnitude of the effective NLO absorption coefficient (β eff ) change with the surface terminations. Ti 3 C 2 T x (═O rich) shows a relatively large saturable absorption (SA) with laser excitation at 515 nm (β eff = -1020 ± 136.2 cm GW -1 ), while reverse saturable absorption (RSA) is found in Ti 3 C 2 T x (pristine) and Ti 3 C 2 T x (-OH rich). The RSA of Ti 3 C 2 T x (pristine) and Ti 3 C 2 T x (-OH rich) is attributed to excited-state absorption, while the SA of Ti 3 C 2 T x (═O rich) is associated with Pauli blocking. With laser excitation at 800 nm, the β eff of Ti 3 C 2 T x (-OH rich) is 113 ± 3.2 cm GW -1 , 1.68 times that of Ti 3 C 2 T x (pristine); the RSA is caused by photon-induced absorption. Our results reveal a correlation between surface terminations and NLO properties, highlighting the potential of MXenes in photoelectronics.

Due to their peculiar quasiparticle excitations, topological metals have high potential for applications in the fields of spintronics, catalysis, and superconductivity. Here, by combining spin- and angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and density functional theory, we discover surface-termination-dependent topological electronic states in the recently discovered mitrofanovite Pt 3 Te 4 . Mitrofanovite crystal is formed by alternating, van der Waals bound layers of Pt 2 Te 2 and PtTe 2 . Our results demonstrate that mitrofanovite is a topological metal with termination-dependent (i) electronic band structure and (ii) spin texture. Despite their distinct electronic character, both surface terminations are characterized by electronic states exhibiting strong spin polarization with a node at the Γ point and sign reversal across the Γ point, indicating their topological nature and the possibility of realizing two distinct electronic configurations (both of them with topological features) on the surface of the same material.

The knowledge concerning the influence of defects on the nonlinear optical response of materials remains scarce so far. In this work, we have successfully introduced defects into SnS 2 nanosheets by plasma treatment and shown that a defect generation is an effective approach to significantly improve the reverse saturable absorption of SnS 2 . The SnS 2 nanosheets treated with Ar plasma for 40 s exhibit a nonlinear absorption coefficient (β 0 ) as large as (2.9 ± 0.12) × 10 4 cm GW -1 , which is nearly 9 times that of the untreated sample. The influence of Ar-plasma-treatment time, defect type, and defect number on the nonlinear absorption of SnS 2 nanosheets are also studied. Structure and spectroscopy characterization confirms the introduction of S and Sn vacancies with Ar-plasma etching. Surface photovoltage spectroscopy and density functional theory calculation indicate that S vacancies can induce in-gap states in the band gap. These in-gap states act as intermediate states for the successive absorption of photons during femtosecond laser excitation (namely, excited-state absorption). In contrast, Sn defects cannot lead to in-gap states and have a limited contribution to nonlinear absorption. Our result would provide a promising way to improve optical nonlinearities.

Tin diselenide (SnSe 2 ) is a van der Waals semiconductor, which spontaneously forms a subnanometric SnO 2 skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO 2 -SnSe 2 heterojunction in both oxidative and humid environments. Explicitly, we find that the work function of the pristine SnSe 2 surface increases by 0.23 and 0.40 eV upon exposure to O 2 and air, respectively, with a charge transfer reaching 0.56 e - /SnO 2 between the underlying SnSe 2 and the SnO 2 skin. Remarkably, both pristine SnSe 2 and defective SnSe 2 display chemical inertness toward water, in contrast to other metal chalcogenides. Conversely, the SnO 2 -SnSe 2 interface formed upon surface oxidation is highly reactive toward water, with subsequent implications for SnSe 2 -based devices working in ambient humidity, including chemical sensors. Our findings also imply that recent reports on humidity sensing with SnSe 2 should be reinterpreted, considering the pivotal role of the oxide skin in the interaction with water molecules.

Room-temperature ferromagnetism in monolayer vanadium diselenide (VSe2) on graphite is the object of a controversial debate. Herein, we unveil the contribution from extrinsic factors to the magnetic properties of monolayer VSe2 by means of density functional theory. Specifically, we demonstrate that either intrinsic defects or the adsorption of molecules enhances ferromagnetic interactions. The expansion of the VSe2 lattice increases the magnetic moment on vanadium ions, whereas both compression and out-of-plane distortion withdraw magnetic moments. The exchange interactions between vanadium ions and magnetic defects (vacancies and impurities) in the surface and subsurface layers of the substrate are able to turn the unstable two-dimensional (2D) ferromagnetism into stable three-dimensional (3D) ferromagnetism. Definitely, the combination of effects related to chemisorption, substrate-induced distortion and magnetic defects of the substrate could enhance or suppress ferromagnetism in monolayer VSe2.

By means of experiments and theory, the gas-sensing properties of tin diselenide (SnSe 2 ) were elucidated. We discover that, while the stoichiometric single crystal is chemically inert even in air, the nonstoichiometric sample assumes a subnanometric SnO 2 surface oxide layer once exposed to ambient atmosphere. The presence of Se vacancies induces the formation of a metastable SeO 2 -like layer, which is finally transformed into a SnO 2 skin. Remarkably, the self-assembled SnO 2 /SnSe 2-x heterostructure is particularly efficient in gas sensing, whereas the stoichiometric SnSe 2 sample does not show sensing properties. Congruently with the theoretical model, direct sensing tests carried out on SnO 2 /SnSe 2- x at an operational temperature of 150 °C provided sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) [ppm] -1 for NO 2 and H 2 , respectively, in dry air. The corresponding calculated limits of detection are (0.36 ± 0.01) and (3.6 ± 0.1) ppm for NO 2 and H 2 , respectively. No detectable changes in gas-sensing performances are observed in a time period extended above six months. Our results pave the way for a novel generation of ambient-stable gas sensor based on self-assembled heterostructures formed taking advantage on the natural interaction of substoichiometric van der Waals semiconductors with air.

Metal-organic frameworks (MOFs) are recognized as advanced sorbents for the effective removal and recovery of various hazardous pollutants in liquid and gaseous environments. In this research, the potential applicability of two Zr-based MOFs (UiO-66 (U6) and its amine counterpart UiO-66-NH 2 (U6N)) was investigated relative to activated carbon (AC, tested as a reference adsorbent) for the purification of industrial organic solvents (e.g., methanol) from six different carbonyl impurities (CCs (C 1 to C 5 ): formaldehyde (FA, CH 2 O), acetaldehyde (AA, CH 3 CHO), propionaldehyde (PA, C 3 H 6 O), butyraldehyde (BA, C 4 H 8 O), isovaleraldehyde (IA, C 5 H 10 O), and valeraldehyde (VA, C 5 H 10 O)). In the sorptive removal of these CCs (both individually and in binary mixtures with FA), U6N showed higher efficacy in capturing all of the target CCs than U6 and AC. The adsorption selectivity of U6N toward single CC compounds was in the order of PA (165.1 mg g -1 ) > BA (158.9 mg g -1 ) > IA (154 mg g -1 ) > AA (136 mg g -1 ) > VA (131.5 mg g -1 ) > FA (120 mg g -1 ). In all binary mixtures, U6N selectively captured FA over the heavier CCs (C 2 -C 5 ) by 1.5-3.3 times due to the steric hindrance of the C 2 -C 5 aliphatic tails in the pore diffusion mechanism. The preferential adsorption of FA onto U6N can also be accounted for by the contribution of chemical bonding (Schiff base interaction) between the -NH 2 groups in U6N and the C═O functionalities (aldehyde molecules) and physisorption, as confirmed by density functional theory (DFT) calculations. Theoretical DFT simulations also revealed that the competition between aldehyde molecules for Brønsted acidic sites (μ 3 -OH of Zr-clusters) created minor distortions in the U6/U6N frameworks.

First principles calculations of the magnetic and electronic properties of VSe2 describing the transition between two structural phases (H,T) were performed. The results of the calculations evidence a rather low energy barrier (0.60 eV for the monolayer) for the transition between the phases. The energy required for the deviation of a Se atom or whole layer of selenium atoms by a small angle of up to 10° from their initial positions is also rather low, 0.32 and 0.19 eV/Se, respectively. The changes in the band structure of VSe2 caused by these motions of Se atoms should be taken into account for analysis of the experimental data. Simulations of the strain effects suggest that the experimentally observed T phase of the VSe2 monolayer is the ground state due to substrate-induced strain. Calculations of the difference in the total energies of the ferromagnetic and antiferromagnetic configurations evidence that the ferromagnetic configuration is the ground state of the system for all stable and intermediate atomic structures. Calculated phonon dispersions suggest a visible influence of the magnetic configurations on the vibrational properties.

Lead-free, water-resistant photovoltaic absorbers are of significant interest for use in environment-friendly and water-stable thin film solar cells. However, there are no reports on the water-resistance characteristics of such photoactive materials. Here, we demonstrate that silver bismuth sulfide (AgBiS2) nanocrystal solids exhibit inherent water resistance and can be employed as effective photovoltaic absorbers in all-solid-state thin film solar cells that show outstanding air and moisture stabilities under ambient conditions. The results of X-ray photon spectroscopy (XPS) and X-ray diffraction (XRD) analyses show that there is no change in the chemical composition and crystal structure of the AgBiS2 nanocrystal solids after a water treatment. Based on these results, AgBiS2 nanocrystal solar cells are fabricated. These devices also do not show any drop in performance after a water treatment, confirming that the AgBiS2 nanocrystal solids are indeed highly water-resistant. In contrast, lead sulfide (PbS) colloidal quantum dot (CQD) solar cells show significant decrease in performance after a similar water treatment. Using XPS analysis and density functional theory (DFT) calculations, we confirm that the iodine removal and the surface hydroxylation of the water-treated PbS CQD solids are the primary reasons for the observed decrease in the device performance of the CQD solar cells.

Among the various two-dimensional semiconductors, indium selenide has recently triggered the interest of scientific community, due to its band gap matching the visible region of the electromagnetic spectrum, with subsequent potential applications in optoelectronics and especially in photodetection. In this feature article, we discuss the main issues in the synthesis, the ambient stability and the application capabilities of this novel class of two-dimensional semiconductors, by evidencing open challenges and pitfalls. In particular, we evidence how the growth of single crystals with reduced amount of Se vacancies is crucial in the road map for the exploitation of indium selenide in technology through ambient-stable nanodevices with outstanding values of both mobility of charge carriers and ON/OFF ratio. The surface chemical reactivity of the InSe surface, as well as applications in the fields of broadband photodetection, flexible electronics and solar energy conversion are also discussed., Competing Interests: The authors declare no conflict of interest.