Nanostructured NiS2-based flexible smart sensors for human respiration monitoring.

The growing demand for wearable healthcare devices has led to an urgent need for cost-effective, wireless and portable breath monitoring systems. However, it is essential to explore novel nanomaterials that combine state-of-the-art flexible sensors with high performance and sensing capabilities along with scalability and industrially acceptable processing. In this study, we demonstrate a highly efficient NiS2-based flexible capacitive sensor fabricated via a solution-processible route using a novel single-source precursor [Ni{S2P(OPr)2}2]. The developed sensor could precisely detect the human respiration rate and exhibit rapid responsiveness, exceptional sensitivity and selectivity at ambient temperatures, with an ultra-fast response and recovery. The device effectively differentiates the exhaled breath patterns including slow, fast, oral and nasal breath, as well as post-exercise breath rates. Moreover, the sensor shows outstanding bending stability, repeatability, reliable and robust sensing performance and is capable of contactless sensing. The sensor was further employed with a user-friendly wireless interface to facilitate smartphone-enabled real-time breath monitoring systems. This work opens up numerous avenues for cost-effective, sustainable and versatile sensors with potential applications for Internet of Things-based flexible and wearable electronics.This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.

Evaluation of Cellulose-MXene Composite Hydrogel Based Bio-Resistive Random Access Memory Material as Mimics for Biological Synapses.

We report a memory device based on organic-inorganic hybrid cellulose-Ti3C2TX MXene composite hydrogel (CMCH) as a switching layer sandwiched between Ag top and FTO bottom electrodes. The device (Ag/CMCH/FTO) was fabricated by a simple, solution-processed route and exhibits reliable and reproducible bipolar resistive switching. Multilevel switching behavior was observed at low operating voltages (±0.5 to ±1 V). Furthermore, the capacitive-coupled memristive characteristics of the device were corroborated with electrochemical impedance spectroscopy and this affirmed the filamentary conduction switching mechanism (LRS-HRS). The synaptic functions of the CMCH-based memory device were evaluated, wherein potentiation/depression properties over 8 × 103 electric pulses were observed. The device also exhibited spike time-dependent plasticity-based symmetric Hebbian learning rule of a biological synapse. This hybrid hydrogel is expected to be a potential switching material for low-cost, sustainable, and biocompatible memory storage devices and artificial synaptic applications.

Evaluation of Nanostructured NiS2 Thin Films from a Single-Source Precursor for Flexible Memristive Devices.

Herein, we report the first demonstration of a single-step, in situ growth of NiS2 nanostructures from a single-source precursor onto a flexible substrate as a versatile platform for an effective nonvolatile memristor. The low temperature, solution-processed deposition of NiS2 thin films exhibits a wide band gap range, spherical-flower-like morphology with high surface area and porosity, and negligible surface roughness. Moreover, the fabricated Au/NiS2/ITO/PET memristor device reveals reproducible bipolar resistive switching (RS) at low operational voltages under both flat and bending conditions. The flexible device shows stable RS behavior for multiple cycles with a good memory window (∼102) and data retention of up to 104 s. The switching of a device between a high-resistance state and a low-resistance state is attributed to the filamentary conduction based on sulfur ion migration and sulfur vacancies and plays a key role in the outstanding memristive performance of the device. Consequently, this work provides a simple, scalable, solution-processed route to fabricate a flexible device with potential applications in next-generation neuromorphic computing and wearable electronics.

Tin(iv) chalcogenoether complexes as single source precursors for the chemical vapour deposition of SnE2 and SnE (E = S, Se) thin films.

The molecular Sn(iv) complexes, [SnCl4{nBuS(CH2)3SnBu}] (2), [SnCl4(nBu2S)2] (3) and [SnCl4(nBu2Se)2] (4) have been prepared in good yield from reaction of SnCl4 with the appropriate chalcogenoether ligand in anhydrous hexane and, together with the known [SnCl4{nBuSe(CH2)3SenBu}] (1), employed as single source precursors for the low pressure chemical vapour deposition of the corresponding tin dichalcogenide thin films. At elevated temperatures the bidentate ligand precursors, (1) and (2), also form the tin monochalcogenides, SnSe and SnS, respectively. In contrast, (3) gave a mixture of phases, SnS2, Sn2S3 and SnS and (4) gave SnSe2 only. The morphologies, elemental compositions and crystal structures of the resulting films have been determined by scanning electron microscopy, energy dispersive X-ray spectroscopy, grazing incidence X-ray diffraction and Raman spectroscopy. Van der Pauw measurements on the SnS2, SnS and SnSe2 films confirm their resistivities to be 2.9(9), 266(3) and 4.4(3) Ω cm, respectively.

Nanoscale arrays of antimony telluride single crystals by selective chemical vapor deposition.

Arrays of individual single nanocrystals of Sb2Te3 have been formed using selective chemical vapor deposition (CVD) from a single source precursor. Crystals are self-assembled reproducibly in confined spaces of 100 nm diameter with pitch down to 500 nm. The distribution of crystallite sizes across the arrays is very narrow (standard deviation of 15%) and is affected by both the hole diameter and the array pitch. The preferred growth of the crystals in the <1 1 0> orientation along the diagonal of the square holes strongly indicates that the diffusion of adatoms results in a near thermodynamic equilibrium growth mechanism of the nuclei. A clear relationship between electrical resistivity and selectivity is established across a range of metal selenides and tellurides, showing that conductive materials result in more selective growth and suggesting that electron donation is of critical importance for selective deposition.

Extending the chemistry of carbones: P-N bond cleavage via an S(N)2'-like mechanism.

The reactivity of nucleophilic carbodiphosphorane (C(PPh3)2, 1) and carbodicarbene (C(C(NMe)2C6H4)2, 2) towards various dichlorophosphines has been explored. In most cases the expected carbone-for-chloride ligand exchange was observed. However, the use of MeN(PCl2)2 resulted in a unique P-N bond cleavage that, according to computational studies, occurred via an SN2'-like mechanism.

A dicationic iminophosphane.

A novel dicationic system containing a PN fragment has been synthesized and structurally characterized. According to the solid-state analysis and theoretical investigation, the dicationic iminophosphane resonance form is the most appropriate description for the dication. However, the contribution from the phosphorus mononitride resonance form is not negligible.

Niobium(V) and tantalum(V) halide chalcogenoether complexes--towards single source CVD precursors for ME2 thin films.

A series of pentavalent niobium and tantalum halide complexes with thio-, seleno- and telluro-ether ligands, [MCl5(E(n)Bu2)] (M = Nb, Ta; E = S, Se, Te), [TaX5(TeMe2)] (X = Cl, Br, F) and the dinuclear [(MCl5)2{o-C6H4(CH2SEt)2}] (M = Nb, Ta), has been prepared and characterised by IR, (1)H, (13)C{(1)H}, (77)Se, (93)Nb and (125)Te NMR spectroscopy, as appropriate, and microanalyses. Confirmation of the tantalum(V)-telluroether coordination follows from the crystal structure of [TaCl5(TeMe2)], which represents the highest oxidation state transition metal complex with telluroether coordination structurally authenticated. The Ta(V) monotelluroether complexes are much more stable than the Nb(V) analogues. In the presence of TaCl5 the ditelluroether, CH2(CH2Te(t)Bu)2, is decomposed; one of the products is the dealkylated [(t)BuTe(CH2)3Te][TaCl6], whose structure was determined crystallographically. Crystal structures of [(MCl5)2{o-C6H4(CH2SEt)2}] (M = Nb, Ta) show ligand-bridged species. The complexes bearing ß-hydrogen atoms on the terminal alkyl substituents have also been investigated as single source reagents for the deposition of ME2 thin films via low pressure chemical vapour deposition. While the tantalum complexes proved to be unsuitable, the [NbCl5(S(n)Bu2)] and [NbCl5(Se(n)Bu2)] deposit NbS2 and NbSe2 as hexagonal platelets onto SiO2 substrates at 750 °C and 650 °C, respectively. Grazing incidence and in-plane X-ray diffraction confirm both materials adopt the 3R-polytype (R3mh), and the sulfide shows preferred orientation with the crystallites aligned predominantly with the c axis perpendicular to the substrate. Scanning electron microscopy and Raman spectra are consistent with the X-ray data.

Tin(II) fluoride vs. tin(II) chloride--a comparison of their coordination chemistry with neutral ligands.

Reaction of SnF2 in MeOH with the appropriate neutral N- or O-donor ligands produces [SnF(2,2'-bipy)]2SnF6, [SnF(1,10-phen)]2SnF4 and [SnF2(L)] L = Me3PO, dmso or pyNO). The X-ray structures of [SnF(2,2'-bipy)]2SnF6, [SnF(1,10-phen)]2SnF4 and [SnF2(dmso)], reveal trigonal pyramidal Sn(II) cores with longer fluorine bridges completing distorted 5- or 6-coordination. Attempts to prepare SnF2 adducts with various phosphine or diphosphine ligands in MeCN failed, whilst in CH2Cl2 solution complex reactions involving the solvent occurred. The NHC, 1,3-(2,6-di-isopropylphenyl)imidazol-2-ylidene (IDiPP) and SnF2 produced the imidazolium salt, [IDiPPH]SnF3, the crystal structure of which revealed the first example of a discrete trifluorostannate(II) ion. In contrast, diphosphine complexes of tin(II) chloride formed readily, including [SnCl2{Me2P(CH2)2PMe2}], [SnCl2{o-C6H4(PMe2)2}], [SnCl2{o-C6H4(PPh2)2}] and [(SnCl2)2(µ-Ph2P(CH2)2PPh2)], which were characterised by X-ray crystallography. The structures of [SnCl2{Me2P(CH2)2PMe2}] and [SnCl2{o-C6H4(PMe2)2}] reveal chloride-bridged dimers, but [SnCl2{o-C6H4(PPh2)2}], although also dimeric, has very asymmetric diphosphine coordination best described as κ(1). The structures of [(SnCl2)2(µ-Ph2P(CH2)2PPh2)] and of [SnCl{o-C6H4(AsMe2)2}]SnCl3 reveal trigonal pyramidal cores, but with longer Sn···Cl bridges affording polymeric structures. The synthesis of [SnCl2(R3EO)2] (R = Ph, E = P or As; and R = Me, E = P) are also reported, along with the structure of [SnCl2(Me3PO)2], which contains distorted tetragonal pyramidal Sn(II) coordination. X-ray structures are also reported for [(PMe3)2CH2][SnCl3]2 and [Ph2P(H)(CH2)2P(H)Ph2][SnCl3]2, obtained as by-products from the attempts to synthesise phosphine complexes, as well as [(o-C6H4(PMe2)2CH2]I2. All complexes were characterised by microanalysis, IR and multinuclear NMR spectroscopy ((1)H, (19)F{(1)H}, (31)P{(1)H } and, where solubility allowed, (119)Sn). Comparisons are drawn with corresponding Sn(IV) and Ge(II) complexes.

Area Selective Growth of Titanium Diselenide Thin Films into Micropatterned Substrates by Low-Pressure Chemical Vapor Deposition.

The neutral, distorted octahedral complex [TiCl4(Se n Bu2)2] (1), prepared from the reaction of TiCl4 with the neutral Se n Bu2 in a 1:2 ratio and characterized by IR and multinuclear (1H, 13C{1H}, 77Se{1H}) NMR spectroscopy and microanalysis, serves as an efficient single-source precursor for low-pressure chemical vapor deposition (LPCVD) of titanium diselenide, TiSe2, films onto SiO2 and TiN substrates. X-ray diffraction patterns on the deposited films are consistent with single-phase, hexagonal 1T-TiSe2 (P3̅m1), with evidence of some preferred orientation of the crystallites in thicker films. The composition and structural morphology was confirmed by scanning electron microscopy (SEM), energy dispersive X-ray, and Raman spectroscopy. SEM imaging shows hexagonal plate crystallites growing perpendicular to the substrate, but these tend to align parallel to the surface when the quantity of reagent is reduced. The resistivity of the crystalline TiSe2 films is 3.36 ± 0.05 × 10-3 Ω·cm with a carrier density of 1 × 1022 cm-3. Very highly selective film growth from the reagent was observed onto photolithographically patterned substrates, with film growth strongly preferred onto the conducting TiN surfaces of SiO2/TiN patterned substrates. TiSe2 is selectively deposited within the smallest 2 µm diameter TiN holes of the patterned TiN/SiO2 substrates. The variation in crystallite size with different diameter holes is determined by microfocus X-ray diffraction and SEM, revealing that the dimensions increase with the hole size, but that the thickness of the crystals stops increasing above ∼20 µm hole size, whereas their lengths/widths continue to increase.

Synthesis, characterisation and structures of thio-, seleno- and telluro-ether complexes of indium(III) halides.

The indium(III) halo-bridged octahedral dimers [InX(2)(L-L)(mu-X)(2)InX(2)(L-L)] (X = Cl: L-L = MeS(CH(2))(2)SMe, MeSe(CH(2))(2)SeMe, (n)BuSe(CH(2))(2)Se(n)Bu), the ionic trans-[InX(2)(L-L)(2)][InX(4)] (X = Cl: L-L = (i)PrS(CH(2))(2)S(i)Pr; X = Br: L-L = MeS(CH(2))(2)SMe, (i)PrS(CH(2))(2)S(i)Pr, MeSe(CH(2))(2)SeMe), cis-[InCl(2)(thiamacrocycle)][InCl(4)] (thiamacrocycle = [12]aneS(4) or [14]aneS(4)) and the neutral, octahedral [InCl(3)([9]aneS(3))] and [InCl(3){MeC(CH(2)SMe)(3)}] were obtained in good yield by the reaction of 1:1 molar ratios of InX(3) with the ligand in anhydrous CH(2)Cl(2) solution. The distorted tetrahedral [InX(3)(Me(2)Se)] (X = Cl, Br or I) and [InX(3)(Me(2)Te)] (X = Br or I) were obtained from 1:3 and 1:2 molar ratios respectively of InX(3) and Me(2)E (E = Se or Te) also in CH(2)Cl(2). The ligand-bridged, distorted tetrahedral dimers [(InCl(3))(2){micro(2)-o-C(6)H(4)(CH(2)SMe)(2)}] and [(InCl(3))(2){micro(2)-MeTe(CH(2))(3)TeMe}] are formed even from a 1:1 In:ligand ratio. Key structure types were confirmed from crystal structures of [InCl(2){RSe(CH(2))(2)SeR}(micro-Cl)(2)InCl(2){RSe(CH(2))(2)SeR(2)}] (R = Me or (n)Bu), trans-[InX(2){(i)PrS(CH(2))(2)S(i)Pr}(2)][InX(4)] (X = Cl or Br), trans-[InBr(2){MeSe(CH(2))(2)SeMe}(2)][InBr(4)], cis-[InCl(2)([14]aneS(4))][InCl(4)] and [InBr(3)(Me(2)Se)]. The bulk complexes have been characterised by IR and Raman spectroscopy and microanalyses, while (1)H, (77)Se{(1)H} and (125)Te{(1)H} NMR spectroscopy show that the compounds are extremely labile in solution and undergo rapid dynamic exchange equilibria. Comparisons are drawn between these structurally rather diverse In(III) chalcogenoether complexes and the corresponding Ga(III) species (all of which are neutral and involve distorted tetrahedral coordination). The reaction of TlCl(3) with Me(2)E (E = Se or Te) shows that chlorination of Me(2)E rather than adduct formation occurs, while no reaction occurred between TlCl(3) and Me(2)S, consistent with Tl(III) being a very poor Lewis acid.

Preparation and structures of tellurium(IV) halide complexes with thioether coordination.

The first chalcogenoether complexes of Te(iv) chloride and bromide are prepared by reaction of the thioether with a suspension of TeX(4) in anhydrous CH(2)Cl(2), and the products characterised by IR, Raman, (1)H and (125)Te{(1)H} NMR spectroscopy and microanalysis. The structures of the distorted octahedral chelate complexes [TeX(4){RS(CH(2))(2)SR}] (X = Cl or Br; R = Me or (i)Pr) and the centrosymmetric halo-bridged dimers [{X(3)(Me(2)S)Te}(2)(micro-X)(2)] (X = Cl or Br) involving Te(iv) are reported and the structures interpreted in terms of a three-centre-four-electron bonding model, with weak, secondary Te-S interactions. The structure of a unique Te(ii) thioether complex, [TeCl{(i)PrS(CH(2))(2)S(i)Pr}][Te(2)Cl(9)] obtained as a decomposition product from a sample of [TeCl(4){(i)PrS(CH(2))(2)S(i)Pr}], in which a Te(ii) thioether cation and Te(iv) chloride anion are weakly associated via micro(2)- and micro(3)-bridging Cl ligands, is also described. In this case distortions in the coordination environment at the Te(ii) ion are attributed to the effects of the Te-based lone pairs.

Synthesis, characterisation and structures of thio-, seleno- and telluro-ether complexes of gallium(III).

The reactions of GaX3 (X = Cl, Br or I) with SMe2, SeMe2 and TeMe2 (L) in non-coordinating solvents produces only the pseudo-tetrahedral [GaX3L], which have been characterised by IR, Raman and multinuclear NMR (1H, 71Ga, 77Se or 125Te) spectroscopy, and by the crystal structure of [GaCl3(SeMe2)]. The 71Ga NMR resonances show small low frequency shifts for fixed halides as the neutral donors change from S --> Se --> Te. Bidentate ligands including MeS(CH2)2SMe, PhS(CH2)2SPh, MeSe(CH2)2SeMe, nBuSe(CH2)2Se(n)Bu and MeTe(CH2)3TeMe (L-L) also produce complexes with 4-coordinate gallium centres, [(GaX3)2(mu-L-L)], confirmed by the crystal structures of [(GaI3)2(mu-MeS(CH2)2SMe)], [(GaCl3)2(mu-PhS(CH2)2SPh)] and [(GaCl3)2(mu-nBuSe(CH2)2Se(n)Bu)]. The structural data are consistent with the weaker Lewis acidity of the gallium as the halide co-ligands become heavier. Multinuclear NMR studies suggest that in chlorocarbon solutions partial dissociation of the ligands occur, which increases with the halide co-ligand Cl < Br < I. The o-xylyl dithioether, o-C6H4(CH2SMe)2, despite being pre-organised for chelation, also forms [(GaCl3)2(mu-L-L)]. The corresponding diselenoether complex decomposes in solution with C-Se bond cleavage to form the selenonium salt [o-C6H4CH2Se(Me)CH2][GaCl4], which was structurally characterised. The ditelluroether o-C6H4(CH2TeMe)2 undergoes rapid C-Te bond fission and rearrangement upon reaction with GaCl3, and the telluronium species [o-C6H4CH2Te(Me)CH2]+ and [MeTe(CH2(o-C6H4)CH2TeMe)2]+ have been identified by ES+ mass spectrometry from their characteristic isotope patterns.