Solvent Controlled Generation of Spin Active Polarons in Two-Dimensional Material under UV Light Irradiation.

Polarons belong to a class of extensively studied quasiparticles that have found applications spanning diverse fields, including charge transport, colossal magnetoresistance, thermoelectricity, (multi)ferroism, optoelectronics, and photovoltaics. It is notable, though, that their interaction with the local environment has been overlooked so far. We report an unexpected phenomenon of the solvent-induced generation of polaronic spin active states in a two-dimensional (2D) material fluorographene under UV light. Furthermore, we present compelling evidence of the solvent-specific nature of this phenomenon. The generation of spin-active states is robust in acetone, moderate in benzene, and absent in cyclohexane. Continuous wave X-band electron paramagnetic resonance (EPR) spectroscopy experiments revealed a massive increase in the EPR signal for fluorographene dispersed in acetone under UV-light irradiation, while the system did not show any significant signal under dark conditions and without the solvent. The patterns appeared due to the generation of transient magnetic photoexcited states of polaronic character, which encompassed the net 1/2 spin moment detectable by EPR. Advanced ab initio calculations disclosed that polarons are plausibly formed at radical sites in fluorographene which interact strongly with acetone molecules in their vicinity. Additionally, we present a comprehensive scenario for multiplication of polaronic spin active species, highlighting the pivotal role of the photoinduced charge transfer from the solvent to the electrophilic radical centers in fluorographene. We believe that the solvent-tunable polaron formation with the use of UV light and an easily accessible 2D nanomaterial opens up a wide range of future applications, ranging from molecular sensing to magneto-optical devices.

Multi-Sensing Platform Based on 2D Monoelement Germanane.

Covalently functionalized germanane is a novel type of fluorescent probe that can be employed in material science and analytical sensing. Here, a fluorometric sensing platform based on methyl-functionalized germanane (CH3 Ge) is developed for gas (humidity and ammonia) sensing, pH (1-9) sensing, and anti-counterfeiting. Luminescence (red-orange) is seen when a gas molecule intercalates into the interlayer space of CH3 Ge and the luminescence disappears upon deintercalation. This allows for direct detection of gas absorption via fluorometric measurements of the CH3 Ge. Structural and optical properties of CH3 Ge with intercalated gas molecules are investigated by density functional theory (DFT). To demonstrate real-time and on-the-spot testing, absorbed gas molecules are first precisely quantified by CH3 Ge using a smartphone camera with an installed color intensity processing application (APP). Further, CH3 Ge-paper-based sensor is integrated into real food packets (e.g., fish and milk) to monitor the shelf life of perishable foods. Finally, CH3 Ge-based rewritable paper is applied in water jet printing to illustrate the potential for secret communication with quick coloration and good reversibility by water evaporation.

Improving the Catalytic Performance of the Hydrogen Evolution Reaction of α-MoB2 via Rational Doping by Transition Metal Elements.

Abundant transition metal borides are emerging as promising electrochemical hydrogen evolution reaction (HER) catalysts which have a potential to substitute noble metals. Those containing graphene-like (flat) boron layers, such as α-MoB2, are particularly promising and their performance can be further enhanced via doping by the second metal. In order to understand intrinsic effect of doping and rationalize selection of dopants, we employ density functional theory (DFT) calculations to study substitutional doping of α-MoB2 by transition metals as a route towards systematic improvement of intrinsic catalytic activity towards HER. We calculated thermodynamic stability of various transition metal elements to select metals which form a stable ternary phase with α-MoB2 . We inspected surface stability of dopants and assessed catalytic activity of doped surface through hydrogen binding free energy at various hydrogen coverages. We calculated the reaction barriers and pathways for the Tafel step of HER for the most promising dopants. The results highlight iron as the best dopant, simultaneously lowering the reaction barrier of the Tafel step while having suitable thermodynamic stability within MoB2 lattice.

Graphene Field Effect Transistors: A Sensitive Platform for Detecting Sarin.

Real time, rapid, and accurate detection of chemical warfare agents (CWA) is an ongoing security challenge. Typical detection methods for CWA are adapted from traditional chemistry techniques such as chromatography and mass spectrometry, which lack portability. Here, we address this challenge by evaluating graphene field effect transistors (GFETs) as a sensing platform for sarin gas using both experiment and theory. Experimentally, we measure the sensing response of GFETs when exposed to dimethyl methylphosphonate (DMMP), a less toxic compound used as simulant due to its chemical similarities to sarin. We find low detection limits of 800 ppb, the highest sensitivity reported up to date for this type of sensing platform. In addition to changes in resistance, we implement an in-operando monitor of the GFETs characteristics during and after exposure to the analyte, which gives insights into the graphene-DMMP interactions. Moreover, using theoretical calculations, we show that DMMP and sarin interact similarly with graphene, implying that GFETs should be highly sensitive to detecting sarin. GFETs offer a versatile platform for the development of compact and miniaturized devices that can provide real-time detection of dangerous chemicals in the local environment.

Silver Covalently Bound to Cyanographene Overcomes Bacterial Resistance to Silver Nanoparticles and Antibiotics.

The ability of bacteria to develop resistance to antibiotics is threatening one of the pillars of modern medicine. It was recently understood that bacteria can develop resistance even to silver nanoparticles by starting to produce flagellin, a protein which induces their aggregation and deactivation. This study shows that silver covalently bound to cyanographene (GCN/Ag) kills silver-nanoparticle-resistant bacteria at concentrations 30 times lower than silver nanoparticles, a challenge which has been so far unmet. Tested also against multidrug resistant strains, the antibacterial activity of GCN/Ag is systematically found as potent as that of free ionic silver or 10 nm colloidal silver nanoparticles. Owing to the strong and multiple dative bonds between the nitrile groups of cyanographene and silver, as theory and experiments confirm, there is marginal silver ion leaching, even after six months of storage, and thus very high cytocompatibility to human cells. Molecular dynamics simulations suggest strong interaction of GCN/Ag with the bacterial membrane, and as corroborated by experiments, the antibacterial activity does not rely on the release of silver nanoparticles or ions. Endowed with these properties, GCN/Ag shows that rigid supports selectively and densely functionalized with potent silver-binding ligands, such as cyanographene, may open new avenues against microbial resistance.

Two-Dimensional Functionalized Germananes as Photoelectrocatalysts.

Succeeding graphene, monoelemental two-dimensional (2D) materials such as germanene and silicene, coined as "Xenes", have attracted vast scientific and technological interests. Adding covalently bonded hydrogen on both sides of germanene leads to germanane (i.e., hydrogen-terminated germanene, GeH). Further, the covalent functionalization of germanane allows the tuning of its physical and chemical properties. Diverse variants of germananes have been synthesized, but current research is primarily focused on their fundamental properties. As a case in point, their applications as photo- and electrocatalysts in the field of modern energy conversion have not been explored. Here, we prepare 2D germanene-based materials, specifically germanane and germananes functionalized by various alkyl chains with different terminal groups-germanane with methyl, propyl, hydroxypropyl, and 2-(methoxycarbonyl)ethyl-and investigate their structural, morphological, optical, electronic, and electrochemical properties. The bond geometries of the functionalized structures, their formation energies, and band gap values are investigated by density functional theory calculations. The functionalized germananes are tested as photoelectrocatalysts in the hydrogen evolution reaction (HER) and photo-oxidation of water. The performance of the germananes is influenced by the functionalized groups, where the germanane with -CH2CH2CH2OH termination records the lowest HER overpotentials and with -H termination reaches the highest photocurrent densities for water oxidation over the entire visible spectral region. These positive findings serve as an overview of organic functionalization of 2D germananes that can be expanded to other "Xanes" for targeted tuning of the optical and electronic properties for photo- and electrochemical energy conversion applications.

Rhenium Doping of Layered Transition-Metal Diselenides Triggers Enhancement of Photoelectrochemical Activity.

The ever decreasing sources of fossil fuels have launched extensive research of alternative materials that might play a key role in their replacement. Therefore, the scientific community continuously investigates the possibilities of maximizing the working capacity of such materials in order to fulfill energy challenges in the near future. In this context, doping of the semiconducting materials is a versatile strategy to trigger their physicochemical properties as well their electrochemical performance. Herein, the impact of rhenium doping toward photoelectrochemical activity of MoSe2 and WSe2 was studied. Our results indicate that rhenium as a dopant contributes to better overall electrochemical performance, that is, an easier electron transfer of these materials compared to pristine compounds. Additionally, the photoelectrochemical measurements revealed that the doping with rhenium generated an enhancement of the photocurrent response of MoSe2 as well as WSe2 under UV light illumination.

1D Coordination π-d Conjugated Polymers with Distinct Structures Defined by the Choice of the Transition Metal: Towards a New Class of Antiaromatic Macrocycles.

Recently π-d conjugated coordination polymers have received a lot of attention owing to their unique material properties, although synthesis of long and defect-free polymers remains challenging. Herein we introduce a novel on-surface synthesis of coordination polymers with quinoidal ligands under ultra-high vacuum conditions, which enables formation of flexible coordination polymers with lengths up to hundreds of nanometers. Moreover, this procedure allows the incorporation of different transition-metal atoms with four- or two-fold coordination. Remarkably, the two-fold coordination mode revealed the formation of wires constituted by (electronically) independent 12-membered antiaromatic macrocycles linked together through two C-C single bonds.

Atomic-Scale Edge Morphology, Stability, and Oxidation of Single-Layer 2H-TaS2.

Tantalum disulphide belongs to the group of transition metal dichalcogenides (TMDs) and has attracted attention for its unique structural, electronic, and catalytic properties. Herein, we report the edge properties of single-layer 2H-TaS2 studied by using density functional theory calculations, because the knowledge of the edge morphology, stability, and surface energy is essential for the determination of nanoparticle shapes and understanding the nature of catalytically active sites. We calculate the grand canonical potential of TaS2 clusters having various edge morphologies to evaluate the edge energies of the Ta-edge and S-edge terminated surfaces. Under S-rich conditions, the most likely shape of TaS2 is a deformed hexagon dominated by the Ta-edge covered by S monomers, while the triangular shape is preferred under S-poor conditions. Exposed edges of the single-layer TaS2 are susceptible to oxidation in air because both oxygen adsorption and substitution at the edge are strongly exothermic, -0.96 and -2.20 eV for single O atom, respectively. The XPS calculation shows that specific initial steps of oxidative process (adsorption, vacancy creation, substitution) are unlikely to be distinguished in the XPS spectra due to small shift of respective binding energies, but initial edge oxidation of TaS2 should be observable by an asymmetry of the Ta 4f doublet towards higher binding energies.

Tailoring Electronic and Magnetic Properties of Graphene by Phosphorus Doping.

The electronic and magnetic properties of graphene can be modulated by doping it with other elements, especially those with a different number of valence electrons. In this article, we first provide a three-dimensional reconstruction of the atomic structure of a phosphorus substitution in graphene using aberration-corrected scanning transmission electron microscopy. Turning then to theoretical calculations based on the density functional theory (DFT), we show that doping phosphorus in various bonding configurations can induce magnetism in graphene. Our simulations reveal that the electronic and magnetic properties of P-doped (Gr-P) and/or phosphono-functionalized graphene (Gr-PO3H2) can be controlled by both the phosphorus concentration and configurations, ultimately leading to ferromagnetic (FM) and/or antiferromagnetic (AFM) features with the transition temperature up to room temperature. We also calculate core-level binding energies of variously bonded P to facilitate X-ray photoelectron spectroscopy-based identification of its chemical form present in P-doped graphene-based structures. These results may enable the design of graphene-based organic magnets with tailored properties for future magnetic or spintronic applications.

Mechanistic Insight into the Limiting Factors of Graphene-Based Environmental Sensors.

Graphene has demonstrated great promise for technological use, yet control over material growth and understanding of how material imperfections affect the performance of devices are challenges that hamper the development of applications. In this work, we reveal new insight into the connections between the performance of the graphene devices as environmental sensors and the microscopic details of the interactions at the sensing surface. We monitor changes in the resistance of the chemical-vapor deposition grown graphene devices as exposed to different concentrations of ethanol. We perform thermal surface treatments after the devices are fabricated, use scanning probe microscopy to visualize their effects down to nanometer scale and correlate them with the measured performance of the device as an ethanol sensor. Our observations are compared to theoretical calculations of charge transfers between molecules and the graphene surface. We find that, although often overlooked, the surface cleanliness after device fabrication is responsible for the device performance and reliability. These results further our understanding of the mechanisms of sensing in graphene-based environmental sensors and pave the way to optimizing such devices, especially for their miniaturization, as with decreasing size of the active zone the potential role of contaminants will rise.

Positive and Negative Effects of Dopants toward Electrocatalytic Activity of MoS2 and WS2: Experiments and Theory.

Two-dimensional transition-metal dichalcogenides (TMDs) are lately in the scope within the scientific community owing to their exploitation as affordable catalysts for next-generation energy devices. Undoubtedly, only precise tailoring and control over the catalytic properties can ensure high efficiency and successful implementation of such devices in day-to-day practical utilization. However, contrary to theoretical predictions, systematic experimental work dealing with the doped materials and their impact to electrocatalysis are relatively underrated despite the considerable effect that it could bring into this field. Herein, we investigate the effect of four different dopants (i.e., Ti, V, Mn, and Fe) incorporated to both layered MoS2 and WS2 as solid-state solution toward their electrocatalytic performance through their evaluation as catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Our results pointed out that doping by Mn and Fe can enhance the electrocatalytic performance toward ORR, whereas doping by Ti and V revealed poor electrocatalytic effects (inhibition) compared to both undoped MoS2 and WS2. Surprisingly, none of the dopants contributed to the improvement of either MoS2 or WS2 toward HER activity. Therefore, in addition to the experimental data, density functional theory calculations were performed to further investigate the role of the dopants in the performance of MoS2 toward HER. According to these calculations, all dopants preferably occupied the edges of the crystal structure and thus could affect the electrocatalytic properties of the initial material. However, the observed ΔG values for hydrogen adsorption revealed that MoS2 is the best catalyst with a subsequent trend for doped materials following the less negative binding energies V < Ti < Mn < Fe, which was in good agreement with experimentally obtained overpotentials of the respective samples. This study thus elucidates the reasons for negative effects of doping in TMDs. This study brings an insight that not all dopants are beneficial and not all reactions are affected in the same way by dopants in TMDs.

Acetonitrile-assisted exfoliation of layered grey and black arsenic: contrasting properties.

In recent years, two-dimensional monoelemental nanostructures beyond graphene have received great attention due to their outstanding properties. Out of these elements, only arsenic is known to form different allotropes with a layered structure in the bulk form. Orthorhombic arsenic, also termed "black arsenic", is a metastable form of arsenic with a structure analogous to that of black phosphorus and rhombohedral arsenic is known as "grey arsenic". Here, we compare the exfoliation of black and grey arsenic in acetonitrile in high yield forming stable colloidal solutions of exfoliated materials. Together with the exfoliation procedure, detailed structural and chemical analyses are provided and potential applications in gas sensing and photothermal absorption are demonstrated for potential future arsenic-based devices.

Microwave Energy Drives "On-Off-On" Spin-Switch Behavior in Nitrogen-Doped Graphene.

The established application of graphene in organic/inorganic spin-valve spintronic assemblies is as a spin-transport channel for spin-polarized electrons injected from ferromagnetic substrates. To generate and control spin injection without such substrates, the graphene backbone must be imprinted with spin-polarized states and itinerant-like spins. Computations suggest that such states should emerge in graphene derivatives incorporating pyridinic nitrogen. The synthesis and electronic properties of nitrogen-doped graphene (N content: 9.8%), featuring both localized spin centers and spin-containing sites with itinerant electron properties, are reported. This material exhibits spin-switch behavior (on-off-on) controlled by microwave irradiation at X-band frequency. This phenomenon may enable the creation of novel types of switches, filters, and spintronic devices using sp2 -only 2D systems.

Chalcogenide vacancies drive the electrocatalytic performance of rhenium dichalcogenides.

The hydrogen evolution reaction (HER) is one of the most promising ways to produce clean energy. However, its wide-spread use is hindered by the price of the state-of-the-art catalysts based on precious metals. Transition metal dichalcogenide (TMD) nanomaterials are a cheap alternative, but a relatively large portion of them remains unexplored in terms of the HER. Here we report the HER performance of rhenium disulfide and diselenide in an acidic environment. We used sodium naphthalenide as an exfoliation agent. In comparison with other TMDs, the degree of exfoliation was relatively low. On the other hand, rhenium disulfide and diselenide both exhibit high chemical stability towards oxidation even after the exfoliation. The reported HER performance of the exfoliated rhenium dichalcogenides was strongly dependent on electrochemical treatment and to further enhance the performance, we used a series of electrochemical pretreatments at potentials as high as -2 V in sulfuric acid. While the morphology of the samples remained unchanged, the surface was found out to be chalcogen deficient, pointing out the formation of chalcogen vacancies. Consequentially, the HER performance was substantially enhanced. These results were corroborated by theoretical calculations showing improved bonding of hydrogen by chalcogenide vacancies at the surface. Our results show that a very simple electrochemical procedure can be used to improve the electrocatalytic performance of rhenium dichalcogenides and, possibly, also other TMDs.

Surface termination of MgB2 unveiled by a combination of adsorption experiments and theoretical calculations.

Superconductivity in polycrystalline and thin-film MgB2 is strongly affected by the termination of its surface, but a reliable determination of the surface termination is still a challenging task of surface chemistry. Here, the surface properties of superconducting MgB2 were investigated using a combination of inverse gas chromatography and van der Waals corrected density functional theory calculations. The dispersive surface energy was measured as a function of the surface coverage and its value (58 mJ m-2 to 48 mJ m-2) was verified by high-level non-local EXX + RPA calculations, which predicted that the dispersive contribution to the cleavage energy was 56 mJ m-2. The isosteric adsorption enthalpies of cyclohexane, dioxane, acetone and acetonitrile molecules were measured on an MgB2 sample and compared to the DFT calculated enthalpies for the Mg-terminated MgB2, B-terminated MgB2 and MgO(001) surfaces. The close agreement between theory and experiment for the Mg-terminated surface suggested that the magnesium termination is the dominant surface phase of MgB2. Thus, combining inverse gas chromatography experiments with theoretical calculations may provide information about the surface termination.

Role of the puckered anisotropic surface in the surface and adsorption properties of black phosphorus.

Nanomaterials have a high surface-to-mass ratio and their surface properties significantly affect their features and application potential. Phosphorene, a single layer of black phosphorus (BP), was the first homoatomic two-dimensional material to be prepared after the discovery of graphene. The structure of phosphorene resembles the honeycomb arrangement of graphene, but its layers are buckled and highly anisotropic. We studied how this difference affects the surface properties of BP, namely the free surface energy and adsorption affinity of various organic molecules. Using inverse gas chromatography, we measured the total surface free energy of BP powder to be 90 mJ m-2 and showed that it was dominantly determined by dispersion forces, but, unlike on graphene, with a notable contribution from specific acid-base interactions. We further measured adsorption enthalpies of volatile organic compounds on BP and rationalized them using density functional theory calculations. Polar molecules showed an increased affinity due to a significant contribution of dipole-dipole interactions to the molecule-surface bonding, because the buckled surface of BP causes higher diffusion barriers than those on graphene, hinders molecular in-plane motion and supports mutual orientation of molecular dipoles over longer distances, in contrast to graphene.

Reactivity of fluorographene is triggered by point defects: beyond the perfect 2D world.

Preparation of graphene derivatives using fluorographene (FG) as a precursor has become a key strategy for the large-scale synthesis of new 2-D materials (e.g. graphene acid, cyanographene, allyl-graphene) with tailored physicochemical properties. However, to gain full control over the derivatization process, it is essential to understand the reaction mechanisms and accompanying processes that affect the composition and structure of the final products. Despite the strength of C-F bonds and high chemical stability of perfluorinated hydrocarbons, FG is surprisingly susceptible to reactions under ambient conditions. There is clear evidence that nucleophilic substitution on FG is accompanied by spontaneous defluorination, and solvent-induced defluorination can occur even in the absence of any nucleophilic agent. Here, we show that distributed radical centers (fluorine vacancies) on the FG surface need to be taken into account in order to rationalize the defluorination mechanism. Depending on the environment, these radical centers can react as electron acceptors, electrophilic sites and/or cause homolytic bond cleavages. We also propose a new radical mechanism of FG defluorination in the presence of N,N'-dimethylformamide (DMF) solvent. Spin-trap experiments as well as 19F NMR measurements unambiguously confirmed formation of N,N'-dimethylformyl radicals and also showed that N,N'-dimethylcarbamoyl fluoride plays a key role in the proposed mechanism. These findings imply that point defects in 2D materials should be considered as key factor determining their chemical properties and reactivity.

TaS3 Nanofibers: Layered Trichalcogenide for High-Performance Electronic and Sensing Devices.

Layered materials, like transition metal dichalcogenides, exhibit broad spectra with outstanding properties with huge application potential, whereas another group of related materials, layered transition metal trichalcogenides, remains unexplored. Here, we show the broad application potential of this interesting structural type of layered tantalum trisulfide prepared in a form of nanofibers. This material shows tailorable attractive electronic properties dependent on the tensile strain applied to it. Structure of this so-called orthorhombic phase of TaS3 grown in a form of long nanofibers has been solved and refined. Taking advantage of these capabilities, we demonstrate a highly specific impedimetric NO gas sensor based on TaS3 nanofibers as well as construction of photodetectors with excellent responsivity and field-effect transistors. Various flexible substrates were used for the construction of a NO gas sensor. Such a device exhibits a low limit of detection of 0.48 ppb, well under the allowed value set by environmental agencies for NOx (50 ppb). Moreover, this NO gas sensor also showed excellent selectivity in the presence of common interferences formed during fuel combustion. TaS3 nanofibers produced in large scale exhibited excellent broad application potential for various types of devices covering nanoelectronic, optoelectronic, and gas-sensing applications.

Surface properties of MoS2 probed by inverse gas chromatography and their impact on electrocatalytic properties.

Layered transition metal dichalcogenides (TMDs) are at the forefront of materials research. One of the most important applications of these materials is their electrocatalytic activity towards hydrogen evolution, and these materials are suggested to replace scarce platinum. Whilst there are significant efforts towards this goal, there are various reports of electrocatalysis of MoS2 (which is the most commonly tested TMD) with large variations of the reported electrocatalytic effect of the material, with overpotential varying by several hundreds of millivolts. Here, we analyzed surface properties of various bulk as well as single layer MoS2 samples using inverse gas chromatography. All samples displayed significant variations in surface energies and their heterogeneities. The surface energy ranged from 50 to 120 mJ m-2 depending on the sample and surface coverage. We correlated the surface properties and previously reported structural features of MoS2 with their electrochemical activities. We concluded that the observed differences in electrochemistry are caused by the surface properties. This is an important finding with an enormous impact on the whole field of electrocatalysis of layered materials.