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Register a new userProbing the Local Dielectric Function by Near Field Optical Microscopy Operating in the Visible Spectral Range
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The optoelectronic properties of nanoscale systems such as carbon nanotubes (CNTs), graphene nanoribbons and transition metal dichalcogenides (TMDCs) are determined by their dielectric function. This complex, frequency dependent function is affected by excitonic resonances, charge transfer effects, doping, sample stress and strain, and surface roughness. Knowledge of the dielectric function grants access to a materials transmissive and absorptive characteristics. Here we introduce the dual scanning near field optical microscope (dual s-SNOM) for imaging local dielectric variations and extracting dielectric function values using a mathematical inversion method. To demonstrate our approach, we studied a monolayer of WS$_2$ on bulk Au and identified two areas with differing levels of charge transfer. Our measurements are corroborated by atomic force microscopy (AFM), Kelvin force probe microscopy (KPFM), photoluminescence (PL) intensity mapping, and tip enhanced photoluminescence (TEPL). We extracted local dielectric variations from s-SNOM images and confirmed the reliability of the obtained values with spectroscopic imaging ellipsometry (SIE) measurements.
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We present an imaging modality that enables detection of magnetic moments and their resulting stray magnetic fields. We use wide-field magnetic imaging that employs a diamond-based magnetometer and has combined magneto-optic detection (e.g. magneto-optic Kerr effect) capabilities. We employ such an instrument to image magnetic (stripe) domains in multilayered ferromagnetic structures.
Imaging dynamical processes at interfaces and on the nanoscale is of great importance throughout science and technology. While light-optical imaging techniques often cannot provide the necessary spatial resolution, electron-optical techniques damage the specimen and cause dose-induced artefacts. Here, Optical Near-field Electron Microscopy (ONEM) is proposed, an imaging technique that combines non-invasive probing with light, with a high spatial resolution read-out via electron optics. Close to the specimen, the optical near-fields are converted into a spatially varying electron flux using a planar photocathode. The electron flux is imaged using low energy electron microscopy, enabling label-free nanometric resolution without the need to scan a probe across the sample. The specimen is never exposed to damaging electrons.
The integration of two-dimensional transition metal dichalcogenide crystals (TMDCs) into a dielectric environment is critical for optoelectronic and photonic device applications. Here, we investigate the effects of direct deposition of different dielectric materials (Al$_2$O$_3$, SiO$_2$, SiN$_x$) onto atomically thin (monolayer) TMDC WS$_2$ on its optical response. Atomic layer deposition (ALD), electron beam evaporation (EBE), plasma enhanced chemical vapour deposition (PECVD), and magnetron sputtering methods of material deposition are investigated. The photoluminescence (PL) measurements reveal quenching of the excitonic emission after all deposition processes. The reduction in neutral exciton PL is linked to the increased level of charge doping and associated rise of the trion emission, and/or the localized (bound) exciton emission. Furthermore, Raman spectroscopy allows us to clearly correlate the observed changes of excitonic emission with the increased levels of lattice disorder and defects. Overall, the EBE process results in the lowest level of doping and defect densities and preserves the spectral weight of the exciton emission in the PL, as well as the exciton oscillator strength. Encapsulation with ALD appears to cause chemical changes, which makes it distinct from all other techniques. Sputtering is revealed as the most aggressive deposition method for WS$_2$, fully quenching its optical response. Our results demonstrate and quantify the effects of direct deposition of dielectric materials onto monolayer WS$_2$, which can provide a valuable guidance for the efforts to integrate monolayer TMDCs into functional optoelectronic devices.
We present numerical simulations of scattering-type Scanning Near-Field Optical Microscopy (s-SNOM) of 1D plasmonic graphene junctions. A comprehensive analysis of simulated s-SNOM spectra is performed for three types of junctions. We find conditions when the conventional interpretation of the plasmon reflection coefficients from s-SNOM measurements does not apply. Our results are applicable to other conducting 2D materials and provide a comprehensive understanding of the s-SNOM techniques for probing local transport properties of 2D materials.
Bias stress degradation in conjugated polymer field-effect transistors is a fundamental problem in these disordered materials and can be traced back to interactions of the material with environmental species,1,2,3 as well as fabrication-induced defects.4,5 However, the effect of the end groups of the polymer gate dielectric and the associated dipole-induced disorder on bias stress stability has not been studied so far in high-performing n-type materials, such as N2200.6,7 In this work, the performance metrics of N2200 transistors are examined with respect to dielectrics with different end groups (Cytop-M and Cytop-S8). We hypothesize that the polar end groups would lead to increased dipole-induced disorder, and worse performance.1,9,10 The long-time annealing scheme at lower temperatures used in the paper is assumed to lead to better crystallization by allowing the crystalline domains to reorganize in the presence of the solvent.11 It is hypothesized that the higher crystallinity could narrow down the range at which energy carriers are induced and thus decrease the gate dependence of the mobility. The results show that the dielectric end groups do not influence the bias stress stability of N2200 transistors. However, long annealing times result in a dramatic improvement in bias stress stability, with the most stable devices having a mobility that is only weakly dependent on or independent of gate voltage.
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