An alternative string theory: The current‐versus‐potential behavior of metal atom strings (Ni, Co, Cr) is dependent on the strength of the d‐orbital coupling along the metal atom chain. Penta‐ and heptachromium strings each exhibit two sets of primary I–V curves, which depend on whether the CrCr bonds alternate and are localized, or are delocalized (see picture).

Two new linear pentanickel complexes [Ni5(bna)4(Cl)2][PF6]2 (1) and [Ni5(bna)4(Cl)2][PF6]4 (2; bna=binaphthyridylamide), were synthesized and structurally characterized. A derivative of 1, [Ni5(bna)4(NCS)2][NCS]2 (3), was also isolated for the purpose of the conductance experiments carried out in comparison with [Ni5(tpda)4(NCS)2] (4; tpda=tripyridyldiamide). The metal framework of complex 2 is a standard [Ni5]10+ core, isoelectronic with that of [Ni5(tpda)4Cl2] (5). Also as in 5, complex 2 has an antiferromagnetic ground state (J=-15.86 cm(-1)) resulting from a coupling between the terminal nickel atoms, both in high-spin sate (S=1). Complex 1 displays the first characterized linear nickel framework in which the usual sequence of NiII atoms has been reduced by two electrons. Each dinickel unit attached to the naphthyridyl moieties is assumed to undergo a one-electron reduction, whereas the central nickel formally remains NiII. DFT calculations suggest that the metal framework of the mixed-valence complex 1 should be described as intermediate between a localized picture corresponding to NiII-NiI-NiII-NiI-NiII and a fully delocalized model represented as (Ni2)3+-NiII-(Ni2)3+. Assuming the latter model, the ground state of 1 results from an antiferromagnetic coupling (J=-34.03 cm(-1)) between the two (Ni2)3+ fragments, considered each as a single magnetic centre (S=3/2). An intervalence charge-transfer band is observed in the NIR spectrum of 1 at 1186 nm, suggesting, in accordance with DFT calculations, that 1 should be assigned to Robin-Day class II of mixed-valent complexes. Scanning tunnelling microscopy (STM) methodology was used to assess the conductance of single molecules of 3 and 4. Compound 3 was found approximately 40% more conductive than 4, a result that could be assigned to the electron mobility induced by mixed-valency in the naphthyridyl fragments.

The realization of molecular electronics requires comprehension of single-molecule I-V characteristics. Aside from the electron-transport properties of the molecular framework, the molecule-electrode binding contributes significantly to the contact resistance, R n)0 , and thus to the values of single-molecule resistance. Isothiocyanate (-NCS), a versatile ligand for organometallics, can bind to a metal substrate to complete a metal-moleculemetal configuration for external measurements. Isothiocyanate has the advantage of being a π-conjugated moiety that presumably exhibits a relatively smaller impedance than the commonly used methylene thiol headgroup (-CH 2 SH) in many molecular wires. For example, this study shows that the single-molecule conductance of n-butanediisothiocyanate is an order of magnitude better than that of n-octanedithiol even though they both contain 10 atoms counted from sulfur to sulfur. For a homologous series of molecules, R n)0 can be extrapolated from the intercept of the resistance obtained by the repeated formation of molecular junctions using scanning tunneling microscopy. To isolate the contact effect of the -NCS-Au electrode from other factors, alkanediisothiocyanates were studied because the large HOMO-LUMO gap of alkyl chains is not sensitive to the number of methylene units. The results show two sets of R n)0 values, with the smaller set being 128 kΩ, about 1/12 the other value. A detailed examination of the results suggests that the preferential adsorption site for isothiocyanate on gold is the atop site rather than the 3-fold-hollow sites of thiol on gold.

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Monolayer transition-metal dichalcogenides (TMDCs) in the 2H-phase are promising semiconductors for opto-valleytronic and opto-spintronic applications because of their strong spin-valley coupling. Here, we report detailed studies of opto-valleytronic properties of heterogeneous domains in CVD-grown monolayer WS2 single crystals. By illuminating WS2 with off-resonance circularly polarized light and measuring the resulting spatially resolved circularly polarized emission (P circ), we find significantly large circular polarization (P circ up to 60% and 45% for α- and β-domains, respectively) already at 300 K, which increases to nearly 90% in the α-domains at 80 K. Studies of spatially resolved photoluminescence (PL) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Kelvin-probe force microscopy, and conductive atomic force microscopy reveal direct correlation among the PL intensity, defect densities, and chemical potential, with the α-domains showing lower defect densities and a smaller work function by 0.13 eV than the β-domains. This work function difference indicates the occurrence of type-two band alignments between the α- and β-domains. We adapt a classical model to explain how electronically active defects may serve as nonradiative recombination centers and find good agreement between experiments and the model. Scanning tunneling microscopic/spectroscopic (STM/STS) studies provide further evidence for tungsten vacancies (WVs) being the primary defects responsible for the suppressed PL and circular polarization in WS2. These results therefore suggest a pathway to control the opto-valleytronic properties of TMDCs by means of defect engineering.

a b s t r a c tWe report a single-step growth process of graphene nanostripes (GNSPs) by adding certain substituted aromatics (e.g., 1,2-dichlorobenzene) as precursors during the plasma enhanced chemical vapor deposition (PECVD). Without any active heating and by using low plasma power ( 60 W), we are able to grow GNSPs vertically with high yields up to (13 ± 4) g/m 2 in 20 min. These GNSPs exhibit high aspect ratios (from 10:1 to >~130:1) and typical widths from tens to hundreds of nanometers on various transitionmetal substrates. The morphology, electronic properties and yields of the GNSPs can be controlled by the growth parameters (e.g., the species of seeding molecules, compositions and flow rates of the gases introduced into the plasma, plasma power, and the growth time). Studies of the Raman spectra, scanning electron microscopy images, ultraviolet photoelectron spectroscopy, transmission electron microscopy images, energy-dispersive x-ray spectroscopy and electrical conductivity of these GNSPs as functions of the growth parameters confirm high-quality GNSPs with electrical mobility~10 4 cm 2 /V-s. These results together with residual gas analyzer spectra and optical emission spectroscopy taken during PECVD growth suggest the important roles of both substituted aromatics and hydrogen plasma in the rapid vertical growth of GNSPs with large aspect ratios.

Etching studies involving citric acid/hydrogen peroxide false(C6H8O7:H2O2false) at volume ratios from 0.5:1 to 50:1 were found to provide good selective etching of various III‐V semiconductor materials grown on normalGaAs and normalInP substrates using molecular beam epitaxy. Both selective and uniform (nonselective) etching regions were found between these material systems by choosing different concentration volume ratios of citric acid/hydrogen peroxide false(χC6H8O7:1H2O2false) . Etchant selectivities, defined as a ratio of the etch rates, for the normalGaAs‐normalbased materials were measured to be as high as 116 for normalGaAs/As0.3Ga0.7AS and 120 for In0.2Ga0.8normalAs/Al0.3Ga0.7normalAs . In addition, the normalInP system had selectivities of approximately 60 and 100 for In0.53Ga0.47normalAs/In0.52Al0.48normalAs and In0.52Al0.48normalAs/normalInP , with the highest selectivity of 473 found for In0.53Ga0.47normalAs/normalInP . The citric acid/hydrogen peroxide system can be used as a stop etch for normalInP‐normalbased devices, as normalInP is virtually unaffected by this etchant. Finally, citric acid/hydrogen peroxide can be used to preferentially etch these materials through a photoresist mask, since it does not erode photoresist at any volume ratio.

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