Evaluating strain and doping of Janus MoSSe from phonon mode shifts supported by ab initio DFT calculations.

With the study of Janus monolayer transition metal dichalcogenides, in which one of the two chalcogen layers is replaced by another type of chalcogen atom, research on two-dimensional materials is advancing into new areas. Yet only little is known about this new kind of material class, mainly due to the difficult synthesis. In this work, we synthesize MoSSe monolayers from exfoliated samples and compare their Raman signatures with density functional theory calculations of phonon modes that depend in a nontrivial way on doping and strain. With this as a tool, we can infer limits for the possible combinations of strain and doping levels. This reference data can be applied to all MoSSe Janus samples in order to quickly estimate their strain and doping, providing a reliable tool for future work. In order to narrow down the results for our samples further, we analyze the temperature-dependent photoluminescence spectra and time-correlated single-photon counting measurements. The lifetime of Janus MoSSe monolayers exhibits two decay processes with an average total lifetime of 1.57 ns. Moreover, we find a strong trion contribution to the photoluminescence spectra at low temperature which we attribute to excess charge carriers, corroborating our ab initio calculations.

Structural Insights into Hysteretic Spin-Crossover in a Set of Iron(II)-2,6-bis(1H-Pyrazol-1-yl)Pyridine) Complexes.

Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT1/2 ) spin-state switching are desirable for molecule-based switching and memory applications. In this study, we report on structural facets governing hysteretic SCO in a set of iron(II)-2,6-bis(1H-pyrazol-1-yl)pyridine) (bpp) complexes - [Fe(bpp-COOEt)2 ](X)2 ⋅CH3 NO2 (X=ClO4 , 1; X=BF4 , 2). Stable spin-state switching - T1/2 =288 K; ΔT1/2 =62 K - is observed for 1, whereas 2 undergoes above-room-temperature lattice-solvent content-dependent SCO - T1/2 =331 K; ΔT1/2 =43 K. Variable-temperature single-crystal X-ray diffraction studies of the complexes revealed pronounced molecular reorganizations - from the Jahn-Teller-distorted HS state to the less distorted LS state - and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2. Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule-based switching and memory elements.

Large-Area, Two-Dimensional MoS2 Exfoliated on Gold: Direct Experimental Access to the Metal-Semiconductor Interface.

Two-dimensional semiconductors such as MoS2 are promising for future electrical devices. The interface to metals is a crucial and critical aspect for these devices because undesirably high resistances due to Fermi level pinning are present, resulting in unwanted energy losses. To date, experimental information on such junctions has been obtained mainly indirectly by evaluating transistor characteristics. The fact that the metal-semiconductor interface is typically embedded, further complicates the investigation of the underlying physical mechanisms at the interface. Here, we present a method to provide access to a realistic metal-semiconductor interface by large-area exfoliation of single-layer MoS2 on clean polycrystalline gold surfaces. This approach allows us to measure the relative charge neutrality level at the MoS2-gold interface and its spatial variation almost directly using Kelvin probe force microscopy even under ambient conditions. By bringing together hitherto unconnected findings about the MoS2-gold interface, we can explain the anomalous Raman signature of MoS2 in contact to metals [ACS Nano. 7, 2013, 11350] which has been the subject of intense recent discussions. In detail, we identify the unusual Raman mode as the A1g mode with a reduced Raman shift (397 cm-1) due to the weakening of the Mo-S bond. Combined with our X-ray photoelectron spectroscopy data and the measured charge neutrality level, this is in good agreement with a previously predicted mechanism for Fermi level pinning at the MoS2-gold interface [Nano Lett. 14, 2014, 1714]. As a consequence, the strength of the MoS2-gold contact can be determined from the intensity ratio between the reduced A1greduced mode and the unperturbed A1g mode.

Towards field-effect controlled graphene-enhanced Raman spectroscopy of cobalt octaethylporphyrin molecules.

During the last decade graphene-enhanced Raman spectroscopy has proven to be a powerful tool to detect and analyze minute amounts of molecules adsorbed on graphene. By using a graphene-based field-effect device the unique opportunity arises to gain a deeper insight into the coupling of molecules and graphene as graphene's Fermi level can be controlled by the transistor`s gate voltage. However, the fabrication of such a device comes with great challenges because of contaminations stemming from processing the device inevitably prevent direct adsorption of the molecules onto graphene rendering it unsuitable for field-effect controlled graphene-enhanced Raman spectroscopy measurements/experiments. In this work, we solve this problem by establishing two different fabrication procedures for such devices, both of which are in addition compatible with large area and scalable production requirements. As a first solution, selective argon cluster irradiation is shown to be an efficient way to remove resist residues after processing. We provide evidence that after the irradiation the enhancement of the molecular Raman signal can indeed be measured, demonstrating that this procedure cleans graphene's surface sufficiently enough for direct molecular adsorption. As a second solution, we have developed a novel stacking method to encapsulate the molecules in between two graphene layers to protect the underlying graphene and molecular layer from the harsh conditions during the photolithography process. This method combines the advantages of dry stacking, which leads to a perfectly clean interface, and wet stacking processes, which can easily be scaled up for large area processing. Both approaches yield working graphene transistors with strong molecular Raman signals stemming from cobalt octaehtylporphyrin, a promising and prototypical candidate for spintronic applications, and are therefore suitable for graphene based molecular sensing applications.

The effect of elevated temperatures on excitonic emission and degradation processes of WS2 monolayers.

Controlled heating experiments in an inert environment have been performed on WS2 monolayers, in order to clarify the conflicting reports on the high-temperature photoluminescent response of 2D TMDs. We find that in contrast to some previous results on both WS2 and MoS2, the photoluminescent intensity shows a consistent reduction above room temperature. This is accompanied by an almost linear redshift of the peak maximum, and a nearly linear increase in the peak width, which is attributed to an enhanced interaction with optical phonons. Moreover, by fitting the photoluminescence integral intensity with an Arrhenius type dependence, we demonstrate that the center of the WS2 monolayer flake starts to undergo irreversible degradation at a temperature of 573 K in an inert environment. Regions close to flake edges in contrast, with a more intense room temperature PL response, remain stable. The macroscopic PL signal is largely recovered in these regions following subsequent cooling to room temperature.

Apparent differences between single layer molybdenum disulphide fabricated via chemical vapour deposition and exfoliation.

Innovative applications based on two-dimensional solids require cost-effective fabrication processes resulting in large areas of high quality materials. Chemical vapour deposition is among the most promising methods to fulfill these requirements. However, for 2D materials prepared in this way it is generally assumed that they are of inferior quality in comparison to the exfoliated 2D materials commonly used in basic research. In this work we challenge this assumption and aim to quantify the differences in quality for the prototypical transition metal dichalcogenide MoS2. To this end single layers of MoS2 prepared by different techniques (exfoliation, grown by different chemical vapour deposition methods, transfer techniques and as vertical heterostructure with graphene) are studied by Raman and photoluminescence spectroscopy, complemented by atomic force microscopy. We demonstrate that as-prepared MoS2, directly grown on SiO2, differs from exfoliated MoS2 in terms of higher photoluminescence, lower electron concentration and increased strain. As soon as a water film is intercalated (e.g. by transfer) underneath the grown MoS2, in particular the (opto)electronic properties become practically identical to those of exfoliated MoS2. A comparison of the two most common precursors shows that the growth with MoO3 causes greater strain and/or defect density deviations than growth with ammonium heptamolybdate. As part of a heterostructure directly grown MoS2 interacts much stronger with the substrate and in this case an intercalated water film does not lead to the complete decoupling, which is typical for exfoliation or transfer. Our work shows that the supposedly poorer quality of grown 2D transition metal dichalcogenides is indeed a misconception.

Electron Irradiation of Metal Contacts in Monolayer MoS2 Field-Effect Transistors.

Metal contacts play a fundamental role in nanoscale devices. In this work, Schottky metal contacts in monolayer molybdenum disulfide (MoS2) field-effect transistors are investigated under electron beam irradiation. It is shown that the exposure of Ti/Au source/drain electrodes to an electron beam reduces the contact resistance and improves the transistor performance. The electron beam conditioning of contacts is permanent, while the irradiation of the channel can produce transient effects. It is demonstrated that irradiation lowers the Schottky barrier at the contacts because of thermally induced atom diffusion and interfacial reactions. The simulation of electron paths in the device reveals that most of the beam energy is absorbed in the metal contacts. The study demonstrates that electron beam irradiation can be effectively used for contact improvement through local annealing.

Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions.

Porous single-layer molybdenum disulfide (MoS2) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS2. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the critical role of energy deposition from the ion to the material through electronic excitation in the defect creation process and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Analysis of the irradiated samples with atomic resolution scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradiation with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm.

Highly active single-layer MoS2 catalysts synthesized by swift heavy ion irradiation.

Two-dimensional molybdenum-disulfide (MoS2) catalysts can achieve high catalytic activity for the hydrogen evolution reaction upon appropriate modification of their surface. The intrinsic inertness of the compound's basal planes can be overcome by either increasing the number of catalytically active edge sites or by enhancing the activity of the basal planes via a controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface using swift heavy ion irradiation. The creation of nanometer-scale structures by an ion beam, in combination with the partial sulfur depletion of the basal planes, leads to a large increase of the number of low-coordinated Mo atoms, which can form bonds with adsorbing species. This results in a decreased onset potential for hydrogen evolution, as well as in a significant enhancement of the electrochemical current density by over 160% as compared to an identical but non-irradiated MoS2 surface.

High affinity peptide inhibitors of the hepatitis C virus NS3-4A protease refractory to common resistant mutants.

Hepatitis C virus (HCV) NS3-4A protease is essential for viral replication. All current small molecular weight drugs against NS3-4A are substrate peptidomimetics that have a similar binding and resistance profile. We developed inhibitory peptides (IPs) capping the active site and binding via a novel "tyrosine" finger at an alternative NS3-4A site that is of particular interest for further HCV drug development. The peptides are not cleaved due to a combination of geometrical constraints and impairment of the oxyanion hole function. Selection and optimization through combinatorial phagemid display, protein crystallography, and further modifications resulted in a 32-amino acid peptide with a K(i) of 0.53 nm. Inhibition of viral replication in cell culture was demonstrated by fusion to a cell-penetrating peptide. Negligible susceptibility to known (A156V and R155K) resistance mutations of the NS3-4A protease was observed. This work shows for the first time that antiviral peptides can target an intracellular site and reveals a novel druggable site on the HCV protease.