Measurements at appropriate spatial and temporal scales are essential for understanding and monitoring spatially heterogeneous environments with complex and highly variable emission sources, such as ...in urban areas. However, the costs and complexity of conventional air quality measurement methods means that measurement networks are generally extremely sparse. In this paper we show that miniature, low-cost electrochemical gas sensors, traditionally used for sensing at parts-per-million (ppm) mixing ratios can, when suitably configured and operated, be used for parts-per-billion (ppb) level studies for gases relevant to urban air quality. Sensor nodes, in this case consisting of multiple individual electrochemical sensors, can be low-cost and highly portable, thus allowing the deployment of scalable high-density air quality sensor networks at fine spatial and temporal scales, and in both static and mobile configurations.
In this paper we provide evidence for the performance of electrochemical sensors at the parts-per-billion level, and then outline results obtained from deployments of networks of sensor nodes in both an autonomous, high-density, static network in the wider Cambridge (UK) area, and as mobile networks for quantification of personal exposure. Examples are presented of measurements obtained with both highly portable devices held by pedestrians and cyclists, and static devices attached to street furniture. The widely varying mixing ratios reported by this study confirm that the urban environment cannot be fully characterised using sparse, static networks, and that measurement networks with higher resolution (both spatially and temporally) are required to quantify air quality at the scales which are present in the urban environment. We conclude that the instruments described here, and the low-cost/high-density measurement philosophy which underpins it, have the potential to provide a far more complete assessment of the high-granularity air quality structure generally observed in the urban environment, and could ultimately be used for quantification of human exposure as well as for monitoring and legislative purposes.
► Suitably configured electrochemical sensors can be used for air quality studies. ► Evidence of performance of electrochemical sensors at parts-per-billion levels. ► Sensors are sensitive, low noise, highly linear and generally highly selective. ► Measurement density (space and time) unachievable using current methods. ► Show low-cost air quality sensor networks are now feasible for widespread use.
The formation of a metal–organic framework (MOF) with nodes that have single‐molecule magnet (SMM) behaviour has been achieved by using mononuclear lanthanoid analogues, also known as single‐ion ...magnets (SIMs), which enormously simplifies the challenging issue of making SMM‐MOFs. Here we present a rational design of a family of MOFs, Ln(bipyNO)4(TfO)3⋅x solvent (Ln=Tb (1); Dy (2); Ho (3); Er (4); TfO=triflate), in which the lanthanoid centres have an square‐antiprismatic coordination environment suitable for SIM behaviour. Magnetic measurements confirm the existence of slow magnetic relaxation typical of SMMs, which has been rationalised by means of a radial effective charge model. In addition, we have explored the incorporation of bulky polyoxometalates (POMs) into the cavities of the SIM‐MOF by anion exchange, finding that they do not interfere with the slow magnetic relaxation. This demonstrates the robustness of the frameworks and opens the possibility of incorporating non‐innocent anions.
SIMs to be in (3D) order: The formation of a single‐ion magnet/metal–organic framework (SIM‐MOF) is presented, which enormously simplifies the challenging issue of making SMM‐MOFs. The incorporation of bulky polyoxometalates (POMs) into the cavities does not interfere with the slow magnetic relaxation, demonstrating the robustness of the frameworks and opening the possibility of incorporating non‐innocent anions.
Tetrathiafulvalene‐lanthanide (TTF‐Ln) metal–organic frameworks (MOFs) are an interesting class of multifunctional materials in which porosity can be combined with electronic properties such as ...electrical conductivity, redox activity, luminescence and magnetism. Herein a new family of isostructural TTF‐Ln MOFs is reported, denoted as MUV‐5(Ln) (Ln=Gd, Tb, Dy, Ho, Er), exhibiting semiconducting properties as a consequence of the short intermolecular S⋅⋅⋅S contacts established along the chain direction between partially oxidised TTF moieties. In addition, this family shows photoluminescence properties and single‐molecule magnetic behaviour, finding near‐infrared (NIR) photoluminescence in the Yb/Er derivative and slow relaxation of the magnetisation in the Dy and Er derivatives. As such properties are dependent on the electronic structure of the lanthanide ion, the immense structural, electronic and functional versatility of this class of materials is emphasised.
Hot MOFs: Herein a new family of isostructural TTF‐Ln MOFs (Ln=Gd, Tb, Dy, Ho, Er), named MUV‐5, are presented, exhibiting electrical conductivity, NIR luminescence and magnetic properties.
We report two new single‐ion magnets (SIMs) of a family of oxydiacetate lanthanide complexes with D3 symmetry to test the predictive capabilities of complete active space ab initio methods (CASSCF ...and CASPT2) and the semiempirical radial effective charge (REC) model. Comparison of the theoretical predictions of the energy levels, wave functions and magnetic properties with detailed spectroscopic and magnetic characterisation is used to critically discuss the limitations of these theoretical approaches. The need for spectroscopic information for a reliable description of the properties of lanthanide SIMs is emphasised.
Benchmarking predictive capabilities: Ab initio complete active space (CASSCF and CASPT2) and semiempirical radial effective charge (REC) theoretical methods are tested on a family of lanthanoid oxydiacetate single‐ion magnets (see figure). Comparison of their predictions concerning energy levels, wave functions and magnetic properties with detailed spectroscopic and magnetic characterisation is used to critically discuss their performances.
A tetravalent uranium compound with a radical azobenzene ligand, namely, {(SiMe2NPh)3‐tacn}UIV(η2‐N2Ph2.) (2), was obtained by one‐electron reduction of azobenzene by the trivalent uranium compound ...UIII{(SiMe2NPh)3‐tacn} (1). Compound 2 was characterized by single‐crystal X‐ray diffraction and 1H NMR, IR, and UV/Vis/NIR spectroscopy. The magnetic properties of 2 and precursor 1 were studied by static magnetization and ac susceptibility measurements, which for the former revealed single‐molecule magnet behaviour for the first time in a mononuclear UIV compound, whereas trivalent uranium compound 1 does not exhibit slow relaxation of the magnetization at low temperatures. A first approximation to the magnetic behaviour of these compounds was attempted by combining an effective electrostatic model with a phenomenological approach using the full single‐ion Hamiltonian.
Uranium magnet: The mononuclear UIV complex with an azobenzene radical ligand {(SiMe2NPh)3‐tacn}UIV(η2‐N2Ph2) exhibits slow relaxation of magnetization with an energy barrier of 17.6 K. This unprecedented behaviour among UIV complexes, due to the interaction of the metal ion with the paramagnetic ligand, was investigated by means of an effective electrostatic model and provides a new strategy to design single‐ion magnets with non‐Kramers ions (see figure).
Here we develop a general approach to calculating the energy spectrum and the wave functions of the low-lying magnetic levels of a lanthanoid ion submitted to the crystal field created by the ...surrounding ligands. This model allows us to propose general criteria for the rational design of new mononuclear lanthanoid complexes behaving as single-molecule magnets (SMMs) or acting as robust spin qubits. Three typical environments exhibited by these metal complexes are considered, namely, (a) square antiprism, (b) triangular dodecahedron, and (c) trigonal prism. The developed model is used to explain the properties of some representative examples showing these geometries. Key questions in this area, such as the chemical tailoring of the superparamagnetic energy barrier, tunneling gap, or spin relaxation time, are discussed. Finally, in order to take into account delocalization and/or covalent effects of the ligands, this point-charge model is complemented with ab initio calculations, which provide accurate information on the charge distribution around the metal, allowing for an explanation of the SMM behavior displayed by some sandwich-type organometallic compounds.
Early actinide ions have large spin‐orbit couplings and crystal field interactions, leading to large anisotropies. The success in using actinides as single‐molecule magnets has so far been modest, ...underlining the need for rational strategies. Indeed, the electronic structure of actinide single‐molecule magnets and its relation to their magnetic properties remains largely unexplored. A uranium(III) single‐molecule magnet, UIII{SiMe2NPh}3‐tacn)(OPPh3) (tacn=1,4,7‐triazacyclononane), has been investigated by means of a combination of magnetic, spectroscopic and theoretical methods to elucidate the origin of its static and dynamic magnetic properties.
That's the one! A comprehensive study involving several methods was performed to determine the electronic structure of a uranium(III) single‐ion magnet. The methods included high‐frequency electron paramagnetic resonance (HFEPR), far‐infrared (FIR) and magnetic circular dichroism (MCD) spectroscopy, complemented by theoretical ab initio calculations and crystal field (CF) modelling.
Molecular nanomagnets based on mononuclear metal complexes, also known as single-ion magnets (SIMs), are crossing challenging boundaries in molecular magnetism. From an experimental point of view, ...this class of magnetic molecules has expanded from lanthanoid complexes to both d-transition metal and actinoid complexes. From a theoretical point of view, more and more improved models have been developed, and we are now able not only to calculate the electronic structure of these systems on the basis of their molecular structures but also to unveil the role of vibrations in the magnetic relaxation processes, at least for lanthanoid and d-transition metal SIMs. This knowledge has allowed us to optimize the behavior of dysprosocenium-based SIMs until reaching magnetic hysteresis above liquid-nitrogen temperature. In this contribution, we offer a brief perspective of the progress of theoretical modeling in this field. We start by reviewing the developed methodologies to investigate the electronic structures of these systems and then move on focus to the open problem of understanding and optimizing the vibrationally induced spin relaxation, especially in uranium-based molecular nanomagnets. Finally, we discuss the differences in the design strategies for 4f and 5f SIMs, including an analysis of the metallocenium family.
A dysprosium based single-ion magnet is synthesized and characterized by the angular dependence of the single-crystal magnetic susceptibility.
and effective electrostatic analyses are performed using ...the molecular structures determined from single crystal X-ray diffraction at 20 K, 100 K and 300 K. Contrary to the common assumption, the results reveal that the structural thermal effects that may affect the energy level scheme and magnetic anisotropy below 100 K are negligible.
Due to their anisotropy, layered materials are excellent candidates for studying the interplay between the in-plane and out-of-plane entanglement in strongly correlated systems. A relevant example is ...provided by 1T-TaS2, which exhibits a multifaceted electronic and magnetic scenario due to the existence of several charge density wave (CDW) configurations. It includes quantum hidden phases, superconductivity and exotic quantum spin liquid (QSL) states, which are highly dependent on the out-of-plane stacking of the CDW. In this system, the interlayer stacking of the CDW is crucial for interpreting the underlying electronic and magnetic phase diagram. Here, atomically thin-layers of 1T-TaS2 are integrated in vertical van der Waals heterostructures based on few-layers graphene contacts and their electrical transport properties are measured. Different activation energies in the conductance and a gap at the Fermi level are clearly observed. Our experimental findings are supported by fully self-consistent DFT+U calculations, which evidence the presence of an energy gap in the few-layer limit, not necessarily coming from the formation of out-of-plane spin-paired bilayers at low temperatures, as previously proposed for the bulk. These results highlight dimensionality as a key effect for understanding quantum materials as 1T-TaS2, enabling the possible experimental realization of low-dimensional QSLs.
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