Nitrogen-Doped Graphene and Its Iron-Based Composite As Efficient Electrocatalysts for Oxygen Reduction Reaction

The high cost of platinum-based electrocatalysts for the oxygen reduction reaction (ORR) has hindered the practical application of fuel cells. Thanks to its unique chemical and structural properties, nitrogen-doped graphene (NG) is among the most promising metal-free catalysts for replacing platinum. In this work, we have developed a cost-effective synthesis of NG by using cyanamide as a nitrogen source and graphene oxide as a precursor, which led to high and controllable nitrogen contents (4.0% to 12.0%) after pyrolysis. NG thermally treated at 900 °C shows a stable methanol crossover effect, high current density (6.67 mA cm^–2), and durability (∼87% after 10 000 cycles) when catalyzing ORR in alkaline solution. Further, iron (Fe) nanoparticles could be incorporated into NG with the aid of Fe(III) chloride in the synthetic process. This allows one to examine the influence of non-noble metals on the electrocatalytic performance. Remarkably, we found that NG supported with 5 wt % Fe nanoparticles displayed an excellent methanol crossover effect and high current density (8.20 mA cm^–2) in an alkaline solution. Moreover, Fe-incorporated NG showed almost four-electron transfer processes and superior stability in both alkaline (∼94%) and acidic (∼85%) solutions, which outperformed the platinum and NG-based catalysts

Chemisorbed Monolayers of Corannulene Penta-Thioethers on Gold

Penta­(<i>tert</i>-butylthio)­corannulene and penta­(4-dimethylaminophenylthio)­corannulene form highly stable monolayers on gold surfaces, as indicated by X-ray photoelectron spectroscopy (XPS). Formation of these homogeneous monolayers involves multivalent coordination of the five sulfur atoms to gold with the peripheral alkyl or aryl substituents pointing away from the surface. No dissociation of C–S bonds upon binding could be observed at room temperature. Yet, the XPS experiments reveal strong chemical bonding between the thioether groups and gold. Temperature-dependent XPS study shows that the thermal stability of the monolayers is higher than the typical stability of self-assembled monolayers (SAMs) of thiolates on gold

Cooperative Self-Assembly of Discoid Dimers: Hierarchical Formation of Nanostructures with a pH Switch

Derivatives of the self-complementary 2-guanidiniocarbonyl pyrrole 5-carboxylate zwitterion (<b>1</b>) (previously reported by us to dimerize to <b>1</b>•<b>1</b> with an aggregation constant of ca. >10¹⁰ M^–l in DMSO) aggregate in a diverse manner depending on, e.g., variation of concentration or its protonation state. The mode of aggregation was analyzed by spectroscopic (NMR, UV) and microscopic (AFM, SEM, HIM, and TEM) methods. Aggregation of dimers of these zwitterions to higher supramolecular structures was achieved by introduction of <i>sec</i>-amide substituents at the 3-position, i.e., at the rearward periphery of the parent binding motif. A butyl amide substituent as in <b>2b</b> enables the discoid dimers to further aggregate into one-dimensional (rod-like) stacks. Quantitative UV dilution studies showed that this aggregation is strongly cooperative following a nucleation elongation mechanism. The amide hydrogen seems to be essential for this rod-like aggregation, as neither <b>1</b> nor a corresponding <i>tert</i>-amide congener <b>2a</b> form comparable structures. Therefore, a hydrogen bond-assisted π–π-interaction of the dimeric zwitterions is suggested to promote this aggregation mode, which is further affected by the nature of the amide substituent (e.g., steric demand), enabling the formation of bundles of strands or even two-dimensional sheets. By exploiting the zwitterionic nature of the aggregating discoid dimers, a reversible pH switch was realized: dimerization of all compounds is suppressed by protonation of the carboxylate moiety, converting the zwitterions into typical cationic amphiphiles. Accordingly, typical nanostructures like vesicles, tubes, and flat sheets are formed reversibly under acidic conditions, which reassemble into the original rod-like aggregates upon readjustment to neutral pH

Atomic-scale characterization of contact interfaces between thermally self-assembled Au islands and few-layer MoS2 surfaces on SiO2

The interaction between metallic nanoparticles and transition metal chalcogenides (TMDs) can realize new functionalities in thriving technologies such as optoelectronics and nanoengineering. Here we have investigated the self-assembly of triangular-shaped crystalline Au nanoislands on MoS2 flakes mechanically exfoliated or grown by chemical vapor deposition (CVD). The density and size of the islands are determined by substrate temperature, deposition flux, and subsurface morphology. The thickness of the MoS2 layers is measured by Raman spectroscopy, which also enables the evaluation of the strain and doping distributions induced by the Au islands. Top and cross-sectional images of the Au-MoS2 interface are obtained by scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Sub-nanometer resolution of the Au, Mo and S layers reveals that the MoS2 flakes follow the corrugation of the SiO2 substrate, with flattening and wrinkling effects induced by the growth of the Au islands on top

Influence of resonant plasmonic nanoparticles on optically accessing the valley degree of freedom in 2D semiconductors

The valley degree of freedom is one of the most intriguing properties of atomically thin transition metal dichalcogenides. Together with the possibility to address this degree of freedom by valley-contrasting optical selection rules, it has the potential to enable a completely new class of future electronic and optoelectronic devices. Resonant optical nanostructures emerge as promising tools for controlling the valley degree of freedom at the nanoscale. However, a critical understanding gap remains in how nanostructures and their nearfields affect the polarization properties of valley-selective chiral emission hindering further developments in this field. In order to address this issue, our study delves into the experimental investigation of a hybrid model system where valley-specific chiral emission from monolayer molybdenum disulfide is interacting with a resonant plasmonic nanosphere. Contrary to the intuition suggesting that a centrosymmetric nanoresonator preserves the degree of circular polarization in the farfield, our cryogenic photoluminescence microscopy reveals almost complete depolarization. We rigorously study the nature of this phenomenon numerically considering the monolayer-nanoparticle interaction at different levels including excitation and emission. We find that the farfield degree of polarization strongly reduces in the hybrid system when including excitons emitting from outside of the system's symmetry point, which in combination with depolarisation at the excitation level causes the observed effect. Our results highlight the importance of considering spatially distributed chiral emitters for precise predictions of polarization responses in these hybrid systems. This finding advances our fundamental knowledge of the light-valley interactions at the nanoscale but also unveils a serious impediment of the practical fabrication of resonant valleytronic nanostructures

Few-cycle laser pulse characterization on-target using high-harmonic generation from nano-scale solids

We demonstrate high-harmonic generation for the time-domain observation of the electric field (HHG-TOE) and use it to measure the waveform of ultrashort mid-infrared (MIR) laser pulses interacting with ZnO thin-films or WS$_2$ monolayers. The working principle relies on perturbing HHG in solids with a weak replica of the pump pulse. We measure the duration of few-cycle pulses at 3100\,nm, in reasonable agreement with the results of established pulse characterization techniques. Our method provides a straightforward approach to accurately characterize femtosecond laser pulses used for HHG experiments right at the point of interaction

Exciton Dynamics in MoS₂‑Pentacene and WSe₂‑Pentacene Heterojunctions

We measured the exciton dynamics in van der Waals heterojunctions of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSs). TMDCs and OSs are semiconducting materials with rich and highly diverse optical and electronic properties. Their heterostructures, exhibiting van der Waals bonding at their interfaces, can be utilized in the field of optoelectronics and photovoltaics. Two types of heterojunctions, MoS2-pentacene and WSe2-pentacene, were prepared by layer transfer of 20 nm pentacene thin films as well as MoS2 and WSe2 monolayer crystals onto Au surfaces. The samples were studied by means of transient absorption spectroscopy in the reflectance mode. We found that A-exciton decay by hole transfer from MoS2 to pentacene occurs with a characteristic time of 21 ± 3 ps. This is slow compared to previously reported hole transfer times of 6.7 ps in MoS2-pentacene junctions formed by vapor deposition of pentacene molecules onto MoS2 on SiO2. The B-exciton decay in WSe2 shows faster hole transfer rates for WSe2-pentacene heterojunctions, with a characteristic time of 7 ± 1 ps. The A-exciton in WSe2 also decays faster due to the presence of a pentacene overlayer; however, fitting the decay traces did not allow for the unambiguous assignment of the associated decay time. Our work provides important insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions

A Universal Scheme to Convert Aromatic Molecular Monolayers into Functional Carbon Nanomembranes

Free-standing nanomembranes with molecular or atomic thickness are currently explored for separation technologies, electronics, and sensing. Their engineering with well-defined structural and functional properties is a challenge for materials research. Here we present a broadly applicable scheme to create mechanically stable carbon nanomembranes (CNMs) with a thickness of ∼0.5 to ∼3 nm. Monolayers of polyaromatic molecules (oligophenyls, hexaphenylbenzene, and polycyclic aromatic hydrocarbons) were assembled and exposed to electrons that cross-link them into CNMs; subsequent pyrolysis converts the CNMs into graphene sheets. In this transformation the thickness, porosity, and surface functionality of the nanomembranes are determined by the monolayers, and structural and functional features are passed on from the molecules through their monolayers to the CNMs and finally on to the graphene. Our procedure is scalable to large areas and allows the engineering of ultrathin nanomembranes by controlling the composition and structure of precursor molecules and their monolayers

2D materials production and generation of functional inks: general discussion

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