Tuning the Inter-Nanoplatelet Distance and Coupling Strength by Thermally Induced Ligand Decomposition.

CdSe nanoplatelets (NPLs) are promising 2D semiconductors for optoelectronic applications, in which efficient charge transport properties are desirable. It is reported that thermal annealing constitutes an effective strategy to control the optical absorption and electrical properties of CdSe NPLs by tuning the inter-NPL distance. Combining optical absorption, transmission electron microscopy, and thermogravimetric analysis, it is revealed that the thermal decomposition of ligands (e.g., cadmium myristate) governs the inter-NPL distance and thus the inter-NPL electronic coupling strength. Employing ultrafast terahertz spectroscopy, it is shown that this enhanced electronic coupling increases both the free carrier generation efficiency and the short-range mobility in NPL solids. The results show a straightforward method of controlling the interfacial electronic coupling strength for developing functional optoelectronic devices through thermal treatments.

Solution-Synthesized Extended Graphene Nanoribbons Deposited by High-Vacuum Electrospray Deposition.

Solution-synthesized graphene nanoribbons (GNRs) facilitate various interesting structures and functionalities, like nonplanarity and thermolabile functional groups, that are not or not easily accessible by on-surface synthesis. Here, we show the successful high-vacuum electrospray deposition (HVESD) of well-elongated solution-synthesized GNRs on surfaces maintained in ultrahigh vacuum. We compare three distinct GNRs, a twisted nonplanar fjord-edged GNR, a methoxy-functionalized "cove"-type (or also called gulf) GNR, and a longer "cove"-type GNR both equipped with alkyl chains on Au(111). Nc-AFM measurements at room temperature with submolecular imaging combined with Raman spectroscopy allow us to characterize individual GNRs and confirm their chemical integrity. The fjord-GNR and methoxy-GNR are additionally deposited on nonmetallic HOPG and SiO2, and fjord-GNR is deposited on a KBr(001) surface, facilitating the study of GNRs on substrates, as of now not accessible by on-surface synthesis.

Facilitating the acidic oxygen reduction of Fe-N-C catalysts by fluorine-doping.

As the alternatives to expensive Pt-based materials for the oxygen reduction reaction (ORR), iron/nitrogen co-doped carbon catalysts (FeNC) with dense FeNx active sites are promising candidates to promote the commercialization of proton exchange membrane fuel cells. Herein, we report a synthetic approach using perfluorotetradecanoic acid (PFTA)-modified metal-organic frameworks as precursors for the synthesis of fluorine-doped FeNC (F-FeNC) with improved ORR performance. The utilization of PFTA surfactants causes profound changes of the catalyst structure including F-doping into graphitic carbon, increased micropore surface area and Brunauer-Emmett-Teller (BET) surface area (up to 1085 m2 g-1), as well as dense FeNx sites. The F-FeNC catalyst exhibits an improved ORR activity with a high E1/2 of 0.83 V (VS. RHE) compared to the pristine FeNC material (E1/2 = 0.80 V). A fast decay occurs in the first 10 000 potential cycles for the F-FeNC catalyst, but high durability is still maintained up to another 50 000 cycles. Density functional theory calculations reveal that the strongly withdrawing fluorine atoms doped on the graphitic carbon can optimize the electronic structure of the FeNx active center and decrease the adsorption energy of ORR intermediates.

Chemical vapour deposition of graphene on copper-nickel alloys: the simulation of a thermodynamic and kinetic approach.

Chemical vapour deposition (CVD) of graphene on transition metals is generally believed to be the fabrication route best suited for the production of high-quality large-area graphene sheets. The mechanism of CVD graphene growth is governed by interactions in both the gas phase and at the surface. Here we present a simulation of the CVD graphene growth mechanism which includes thermodynamics, gas phase kinetics and the surface reaction in a sequential manner. The thermodynamic simulation shows that the deposition driving force is the greatest for high carbon to hydrogen ratios and reaches a maximum at around 850 °C. No graphene growth is observed below this temperature. The surface kinetic model also shows that below this temperature, the carbon surface concentration is less than the solubility limit, thus no film can grow. The effect of the reaction chamber geometry on the product concentrations was clear from the gas phase decomposition reactions. The gas residence times studied here (around 0.07 s) show that the optimum gas phase composition is far from that expected at thermodynamic equilibrium. The surface kinetics of CH4 reactions on Ni, Cu and Cu-Ni surfaces shows good agreement with the experimental results for different growth pressures (0.1 to 0.7 mbar), temperatures (600 to 1200 °C) and different Ni thicknesses (25-500 µm). Also, the model works well when substrates with various C solubilities are used. The thermodynamic and kinetic models described here can be used for the design of improved reactors to optimise the production of graphene with differing qualities, either single or multi-layer and sizes. More importantly, the transfer to a continuous process with a moving substrate should also be possible using the model if it is extended from 2D to 3D.