Alex S. Walton

Four-probe electrical transport measurements on individual metallic nanowires

This work presents nanoscale four-probe measurements on metallic nanowires using independently controlled scanning tunnelling microscope tips. This technique has allowed us to follow the change in resistance with probe separation. Gold, zinc and nickel nanowires were grown by electrodeposition within porous polycarbonate membranes. Their structure and composition were studied by transmission electron microscopy. Four-probe electrical transport measurements were taken using four independently controlled scanning tunnelling microscope tips positioned using a high resolution scanning electron microscope. Multiple I–V measurements were taken at varying tip separations, on each nanowire, and the change in resistance with separation was observed to be in good agreement with predictions based on the nanowire geometry. The resistivity values of the nanowires were found to be close to bulk values.

Structure and Electronic Properties of In Situ Synthesized Single-Layer MoS2 on a Gold Surface

When transition metal sulfides such as MoS2 are present in the single-layer form, the electronic properties change in fundamental ways, enabling them to be used, e.g., in two-dimensional semiconductor electronics, optoelectronics, and light harvesting. The change is related to a subtle modification of the band structure due to confinement in the direction perpendicular to the sheets, and there is a considerable interest in understanding how this modification can be controlled and adjusted to generate 2D-materials with functional properties. In this article we report a synthesis procedure together with scanning tunneling microscopy and X-ray photoelectron spectroscopy characterization of two-dimensional single-layer islands of MoS2 synthesized directly on a gold single crystal substrate. Thanks to a periodic modulation of the atom stacking induced by the lattice mismatch, we observe a structural buckling of the MoS2 layer resulting in a characteristic moiré pattern. X-ray photoelectron spectroscopy indicates that the system develops the characteristics of n-doped MoS2 due to electron donation. Scanning tunneling spectroscopy furthermore reflects a convolution of MoS2 and Au donor states where the MoS2 band structure appears modified at the band gap edges. This electronic effect is further modulated by the moiré periodicity and leads to small substrate-induced electronic perturbations near the conduction band minimum in the band gap of MoS2. The results may be highly relevant in the context of nanopatterned two-dimensional materials on metal surfaces, and we propose the MoS2/Au system in this article as a promising candidate to further explore the properties of supported 2D transition-metal dichalcogenides.

In Situ Detection of Active Edge Sites in Single-Layer MoS2 Catalysts.

MoS2 nanoparticles are proven catalysts for processes such as hydrodesulfurization and hydrogen evolution, but unravelling their atomic-scale structure under catalytic working conditions has remained significantly challenging. Ambient pressure X-ray Photoelectron Spectroscopy (AP-XPS) allows us to follow in situ the formation of the catalytically relevant MoS2 edge sites in their active state. The XPS fingerprint is described by independent contributions to the Mo 3d core level spectrum whose relative intensity is sensitive to the thermodynamic conditions. Density Functional Theory (DFT) is used to model the triangular MoS2 particles on Au(111) and identify the particular sulphidation state of the edge sites. A consistent picture emerges in which the core level shifts for the edge Mo atoms evolve counterintuitively toward higher binding energies when the active edges are reduced. The shift is explained by a surprising alteration in the metallic character of the edge sites, which is a distinct spectroscopic signature of the MoS2 edges under working conditions.

Nanostructured Aptamer-Functionalized Black Phosphorus Sensing Platform for Label-Free Detection of Myoglobin, a Cardiovascular Disease Biomarker

We report the electrochemical detection of the redox active cardiac biomarker myoglobin (Mb) using aptamer-functionalized black phosphorus nanostructured electrodes by measuring direct electron transfer. The as-synthesized few-layer black phosphorus nanosheets have been functionalized with poly-l-lysine (PLL) to facilitate binding with generated anti-Mb DNA aptamers on nanostructured electrodes. This aptasensor platform has a record-low detection limit (∼0.524 pg mL(-1)) and sensitivity (36 μA pg(-1) mL cm(-2)) toward Mb with a dynamic response range from 1 pg mL(-1) to 16 μg mL(-1) for Mb in serum samples. This strategy opens up avenues to bedside technologies for multiplexed diagnosis of cardiovascular diseases in complex human samples.

Factors that determine and limit the resistivity of high-quality individual ZnO nanowires

Knowing and controlling the resistivity of an individual nanowire (NW) is crucial for the production of new sensors and devices. For ZnO NWs this is poorly understood; a 10(8) variation in resistivity has previously been reported, making the production of reproducible devices almost impossible. Here, we provide accurate resistivity measurements of individual NWs, using a four-probe scanning tunnelling microscope (STM), revealing a dependence on the NW dimensions. To correctly interpret this behaviour, an atomic level transmission electron microscopy technique was employed to study the structural properties of the NWs in relation to three growth techniques: hydrothermal, catalytic and non-catalytic vapour phase. All NWs were found to be defect free and structurally equivalent; those grown with a metallic catalyst were free from Au contamination. The resistivity measurements showed a distinct increase with decreasing NW diameter, independent of growth technique. The increasing resistivity at small NW diameters was attributed to the dominance of surface states removing electrons from the bulk. However, a fundamental variance in resistivity (10(2)) was observed and attributed to changes in occupied surface state density, an effect which is not seen with other NW materials such as Si. This is examined by a model to predict the effect of surface state occupancy on the measured resistivity and is confirmed with measurements after passivating the ZnO surface. Our results provide an understanding of the primary influence of the reactive nature of the surface and its dramatic effect on the electrical properties of ZnO NWs.

Edge reactivity and water-assisted dissociation on cobalt oxide nanoislands

Transition metal oxides show great promise as Earth-abundant catalysts for the oxygen evolution reaction in electrochemical water splitting. However, progress in the development of highly active oxide nanostructures is hampered by a lack of knowledge of the location and nature of the active sites. Here we show, through atom-resolved scanning tunnelling microscopy, X-ray spectroscopy and computational modelling, how hydroxyls form from water dissociation at under coordinated cobalt edge sites of cobalt oxide nanoislands. Surprisingly, we find that an additional water molecule acts to promote all the elementary steps of the dissociation process and subsequent hydrogen migration, revealing the important assisting role of a water molecule in its own dissociation process on a metal oxide. Inspired by the experimental findings, we theoretically model the oxygen evolution reaction activity of cobalt oxide nanoislands and show that the nanoparticle metal edges also display favourable adsorption energetics for water oxidation under electrochemical conditions.

Vanadium( iii ) phenoxyimine complexes for ethylene or ε-caprolactone polymerization : mononuclear versus binuclear pre-catalysts

The mononuclear {[C6H4NCH(ArO)]2VCl(THF)} (Ar = 2,4-t-Bu2C6H2 (1), Ar = C6H4 (2)), {O[C6H4NCH(ArO)]2}VCl(THF) (Ar = 2,4-t-Bu2C6H2 (3), Ar = C6H4 (4)) and the binuclear vanadium(III) complexes {[C6H4NCH(ArO)]VCl2(THF)2}2(μ-CH2CH2) (Ar = 2,4-t-Bu2C6H2 (5), Ar = C6H4 (6)), have been synthesized and fully characterized. The compounds [C6H5NCH(ArO)]VCl2(THF)2 (Ar = 2,4-t-Bu2C6H2 (7), Ar = C6H4 (8)), [2,4,6-Me3–C6H2NCH(ArO)]VCl2 (Ar = 2,4-t-Bu2C6H2 (9), Ar = C6H4 (10)) and [2,6-i-Pr2-C6H3NCH(ArO)]VCl2(THF)2 (Ar = 2,4-t-Bu2C6H2 (11), Ar = C6H4 (12)), {μ-CH2CH2[NCH(C6H4O)]2VCl(THF)} (14) and {C6H4[NCH(C6H4O)]2VCl(THF)} (15) were synthesized for comparative polymerization studies. The dizwitterionic compound [2,6-i-Pr2-C6H3N+(H)CH(C6H4O)]2VCl2O (13) was also isolated, and presumably formed via a fortuitous hydrolysis reaction. The complexes 2, 5 and 13 have been structurally characterized; the molecular structure of the parent ligand (L5) in 5 is also reported. All complexes have been screened for ethylene as well as e-caprolactone polymerization, and results are compared against those for known related mono- and bi-nuclear counterparts to evaluate for possible cooperative effects. The compounds 10 and 12 have been supported on modified SiO2, analysed by XPS and subjected to homo-polymerization (ethylene) and co-polymerization (1-hexene and ethylene) studies.

Highly Active, Thermally Stable, Ethylene-Polymerisation Pre-Catalysts Based on Niobium/TantalumImine Systems

The reactions of MCl5 or MOCl3 with imidazole-based pro-ligand L(1)H, 3,5-tBu2-2-OH-C6H2-(4,5-Ph2-1H-)imidazole, or oxazole-based ligand L(2)H, 3,5-tBu2-2-OH-C6H2 (1H-phenanthro[9,10-d])oxazole, following work-up, afforded octahedral complexes [MX(L(1,2))], where MX=NbCl4 (L(1), 1a; L(2), 2a), [NbOCl2(NCMe)] (L(1), 1b; L(2), 2b), TaCl4 (L(1), 1c; L(2), 2c), or [TaOCl2(NCMe)] (L(1), 1d). The treatment of α-diimine ligand L(3), (2,6-iPr2C6H3N=CH)2, with [MCl4(thf)2] (M=Nb, Ta) afforded [MCl4(L(3))] (M=Nb, 3a; Ta, 3b). The reaction of [MCl3(dme)] (dme=1,2-dimethoxyethane; M=Nb, Ta) with bis(imino)pyridine ligand L(4), 2,6-[2,6-iPr2C6H3N=(Me)C]2C5H3N, afforded known complexes of the type [MCl3(L(4))] (M=Nb, 4a; Ta, 4b), whereas the reaction of 2-acetyl-6-iminopyridine ligand L(5), 2-[2,6-iPr2C6H3N=(Me)C]-6-Ac-C5H3N, with the niobium precursor afforded the coupled product [({2-Ac-6-(2,6-iPr2C6H3N=(Me)C)C5H3N}NbOCl2)2] (5). The reaction of MCl5 with Schiff-base pro-ligands L(6)H-L(10)H, 3,5-(R(1))2-2-OH-C6H2CH=N(2-OR(2)-C6H4), (L(6)H: R(1)=tBu, R(2)=Ph; L(7)H: R(1)=tBu, R(2)=Me; L(8)H: R(1)=Cl, R(2)=Ph; L(9)H: R(1)=Cl, R(2)=Me; L(10)H: R(1)=Cl, R(2)=CF3) afforded [MCl4(L(6-10))] complexes (M=Nb, 6a-10a; M=Ta, 6b-9b). In the case of compound 8b, the corresponding zwitterion was also synthesised, namely [Ta(-)Cl5(L(8)H)(+)]·MeCN (8c). Unexpectedly, the reaction of L(7)H with TaCl5 at reflux in toluene led to the removal of the methyl group and the formation of trichloride 7c [TaCl3(L(7-Me))]; conducting the reaction at room temperature led to the formation of the expected methoxy compound (7b). Upon activation with methylaluminoxane (MAO), these complexes displayed poor activities for the homogeneous polymerisation of ethylene. However, the use of chloroalkylaluminium reagents, such as dimethylaluminium chloride (DMAC) and methylaluminium dichloride (MADC), as co-catalysts in the presence of the reactivator ethyl trichloroacetate (ETA) generated thermally stable catalysts with, in the case of niobium, catalytic activities that were two orders of magnitude higher than those previously observed. The effects of steric hindrance and electronic configuration on the polymerisation activity of these tantalum and niobium pre-catalysts were investigated. Spectroscopic studies ((1)H NMR, (13)C NMR and (1)H-(1)H and (1)H-(13)C correlations) on the reactions of compounds 4a/4b with either MAO(50) or AlMe3/[CPh3](+)[B(C6F5)4](-) were consistent with the formation of a diamagnetic cation of the form [L(4)AlMe2](+) (MAO(50) is the product of the vacuum distillation of commercial MAO at +50 °C and contains only 1 mol% of Al in the form of free AlMe3). In the presence of MAO, this cationic aluminium complex was not capable of initiating the ROMP (ring opening metathesis polymerisation) of norbornene, whereas the 4a/4b systems with MAO(50) were active. A parallel pressure reactor (PPR)-based homogeneous polymerisation screening by using pre-catalysts 1b, 1c, 2a, 3a and 6a, in combination with MAO, revealed only moderate-to-good activities for the homo-polymerisation of ethylene and the co-polymerisation of ethylene/1-hexene. The molecular structures are reported for complexes 1a-1c, 2b, 5, 6a, 6b, 7a, 8a and 8c.

Controlling the Electrical Transport Properties of Nanocontacts to Nanowires

The ability to control the properties of electrical contacts to nanostructures is essential to realize operational nanodevices. Here, we show that the electrical behavior of the nanocontacts between free-standing ZnO nanowires and the catalytic Au particle used for their growth can switch from Schottky to Ohmic depending on the size of the Au particles in relation to the cross-sectional width of the ZnO nanowires. We observe a distinct Schottky to Ohmic transition in transport behavior at an Au to nanowire diameter ratio of 0.6. The current-voltage electrical measurements performed with a multiprobe instrument are explained using 3-D self-consistent electrostatic and transport simulations revealing that tunneling at the contact edge is the dominant carrier transport mechanism for these nanoscale contacts. The results are applicable to other nanowire materials such as Si, GaAs, and InAs when the effects of surface charge and contact size are considered.

An investigation of the growth of bismuth whiskers and nanowires during physical vapour deposition

Bismuth thin films of thickness in the region of 500 nm have been prepared by planar magnetron sputtering onto glass, silicon and GaAs substrates. Electron microscopy of these films reveals that bismuth whiskers grow spontaneously when the substrate is heated to temperatures between 110 and 140 °C during deposition and the optimum temperature for such growth is largely independent of substrate. Depositing films under similar conditions using thermal evaporation does not, however, produce the whisker growth. X-ray diffraction has been employed to investigate film texture with temperature and it has been shown that the film crystallites are predominantly [1 1 0] and [1 1 1] oriented. The [1 1 0] orientation of the crystallites dominates at deposition temperatures above 110 °C for sputter deposition and the [1 1 1] at lower temperatures. The optimum temperature for whisker growth coincides with the temperature for the change between predominant orientations. While sputter deposition appears to favour films with crystallite orientation of [1 1 0], thermal evaporation favours [1 1 1] and has a higher change-over temperature. The whiskers that grow from the film emerge at off-normal angles between 43.3° and 69.2° with a mean of 54 ± 3°. The projected length of whiskers on a 500 nm film on a GaAs substrate shows a wide distribution to a maximum of more than 100 µm. The mean projected length for this sample was 16 ± 1 µm and the diameter is around 0.5 µm. Measurements of the electrical properties of the whiskers at room temperature reveals ohmic behaviour with an estimated resistivity of 2.2 ± 0.2 µΩ m. Detailed examination of scanning electron micrographs, eliminates all growth mechanisms except tip growth by a non-catalysed vapour–solid/vapour–liquid–solid method. By depositing thinner films it is shown that this spontaneous growth of whiskers offers a route to fabricate high quality bismuth nanowires of lengths exceeding 10 µm.