Manipulation of the electrical and memory properties of MoS2 field-effect transistors by highly charged ion irradiation.

Field-effect transistors based on molybdenum disulfide (MoS2) exhibit a hysteresis in their transfer characteristics, which can be utilized to realize 2D memory devices. This hysteresis has been attributed to charge trapping due to adsorbates, or defects either in the MoS2 lattice or in the underlying substrate. We fabricated MoS2 field-effect transistors on SiO2/Si substrates, irradiated these devices with Xe30+ ions at a kinetic energy of 180 keV to deliberately introduce defects and studied the resulting changes of their electrical and hysteretic properties. We find clear influences of the irradiation: while the charge carrier mobility decreases linearly with increasing ion fluence (up to only 20% of its initial value) the conductivity actually increases again after an initial drop of around two orders of magnitude. We also find a significantly reduced n-doping (≈1012 cm-2) and a well-developed hysteresis after the irradiation. The hysteresis height increases with increasing ion fluence and enables us to characterize the irradiated MoS2 field-effect transistor as a memory device with remarkably longer relaxation times (≈ minutes) compared to previous works.

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.

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.