Aryl-viologen pentapeptide self-assembled conductive nanofibers.

A pentapeptide sequence was functionalized with an asymmetric arylated methyl-viologen (AVI3D2) and through controllable β-sheet self-assembly, conductive nanofibers were formed. Using a combination of spectroscopic techniques and conductive atomic force microscopy, we investigated the molecular conformation of the resultant AVI3D2 fibers and how their conductivity is affected by β-sheet self-assembly. These conductive nanofibers have potential for future exploration as molecular wires in optoelectronic applications.ERC (‘ASPiRe’, 240629) Marie Curie (FP7 ‘SASSYPOL’ ITN, 607602

Atomic assessment of paramagnetic defects in 2-dimensional semiconductor layers: MoS2

For decades, the semiconductor industry has pursued the extensive miniaturization of the transistor. Conventional Si-based devices are however reaching their scaling limits since undesired 'physics' effects arise when Si-based transistors are ultra-scaled. A possible solution to overcome these electrostatic control issues and lithography challenges is to incorporate novel 2D materials in future nanoelectronic devices. In this regard, TMDs, and in particular MoS2, emerged as promising candidates to create novel low-power devices with enhanced functionalities. Crucially, two paramount issues need to be solved before MoS2 can be widely applied as a 2D channel material in future devices: A reliable synthesis method and a robust doping procedure need to be developed. In this respect, ESR serves as a most useful non-destructive technique to assess the quality and doping properties of 2D materials by characterizing intrinsic and extrinsic point defects. An extensive investigation of defects in synthetic and geological MoS2 has therefore been performed in this work by ESR, a technique that achieves exceptional sensitivity and possesses eminent discriminative and quantitative capabilities. This work presents a detailed multi-frequency ESR analysis of a newly observed impurity in a CVD-grown bulk 2H MoS2 crystal. The previously unreported signal of axial symmetry exhibits a g anisotropy typical of acceptors and is, after careful consideration, identified as originating from N acceptor dopants, which are found to be substituting for S sites in bulk MoS2 with a density of ~ 2E17 cm-3, thus predominantly accounting for the p-type sample doping. The thermal stability, spatial distribution, and activation energy (~ 45 meV) of the N acceptor is also studied in detail. Ultimately, substantial N contamination is revealed to be an inherent trait of the specific CVD method applied for the synthesis of the studied MoS2 material. The thermal stability, activation energy (~ 0.7 meV), and temperature dependence of the ESR spectral characteristics of the As acceptor dopant (As substituting for S) in geological bulk 2H MoS2 is also assessed in this work. In general, As is confirmed as a promising candidate for stable covalently bonded p-type doping of MoS2. Additionally, these findings indicate that the As acceptor emerges as a shallower and more robust dopant than the N acceptor. Next, this work deals with an ESR investigation of point defects present in transferred synthetic few-layer MoS2. The ESR investigation is closely combined with an in-depth analysis by an assortment of other experimental techniques, including AFM, RBS, XPS, and TEM, to ultimately result in the assignment of the ESR signal to a defect of intrinsic nature, most likely a Mo vacancy related defect located at MoS2 grain edges or boundaries. The oxidation of the 2D material at grain edges and boundaries combined with the applied water-based transfer procedure is demonstrated to play a crucial role in the generation of the newly observed defect, thus exposing a weakness in the process method. The final part of this thesis presents a comparative multi-frequency ESR analysis of various geological MoS2 crystals which reveals numerous kinds of bulk and surface contamination related defects. Different dopant regimes (n,p, and mixed) are uncovered, with As, N, and Re being traced as effective dominant impurity dopants. These observations emphasize the necessity of rigorous surface cleaning and even removing surface layers to obtain a pristine MoS2 parent crystal suitable for the exfoliation of high-quality flakes intended for fundamental analysis or state-of-the-art applications.status: publishe

Nitrogen acceptor in 2H-polytyp synthetic MoS2 assessed by multifrequency electron spin resonance

© 2018 Published by the AVS. Electron spin resonance (ESR) study on 2H-polytype synthetic MoS2 revealed the N acceptor dopants as being characterized by a spectrum of axial symmetry [g∥ = 2.032(2); g⊥ = 2.270(2)], typical for a hole-type center in MoS2. The N impurities substitute for S sites, with a density of ∼2.3 × 1017 cm−3, which accounts for the overall p-type doping. With respect to measurements for the applied magnetic field directed along the c-axis, the signal consists of a 14N primary hyperfine triplet of splitting constant A∥ = 14.7 ± 0.2 G superimposed on a correlated Gaussian single central line of peak-to-peak width ΔBpp = 15.3 ± 0.5 G, the latter making up only ∼26% of the total signal intensity. The current work extends on these results through extensive monitoring of the temperature (T) dependence of salient ESR parameters and studying the impact of thermal treatment. ESR signal saturation studies indicate a N acceptor spin-lattice relaxation time T1 (4.2 K) ≈ 3 × 10−4 s, notably different from the much smaller As acceptor’s T1 in geological MoS2. Concerning the thermal stability of the dopant, the N acceptor is found to be drastically passivated when exposed to H2 at ∼500 °C. Yet, subsequent reactivation attempts in vacuum at temperatures up to 740 °C appear unsuccessful, urging great caution with conventional forming gas treatments at T ≳ 500 °C. Combination of careful K- and Q-band ESR monitoring of the T-dependent signal intensity resulted in the consolidation of the N dopant as a shallow acceptor of activation energy Ea = 45 ± 7 meV. The consolidated results establish N as a promising candidate for stable covalently bonded p-type doping of MoS2 layers intended for application in novel nanoelectronic devices.status: publishe

Degradation of sulfamethoxazole by heat-activated persulfate oxidation: elucidation of the degradation mechanism and influence of process parameters

In this article, heat-activated persulfate oxidation was investigated as a promising technique for the removal of sulfamethoxazole from an aqueous environment. It was found that the degradation efficiency of sulfamethoxazole increases with increasing persulfate concentration due to the increased •SO4- production. As suggested by the Arrhenius equation, the sulfamethoxazole degradation rate constant increased with increasing temperature. An activation energy of 103 kJ/mol was determined. Furthermore, the initial pH of the reaction mixture had a large influence on the degradation of sulfamethoxazole. At higher initial pH values, the degradation of sulfamethoxazole increased. The main cause for this increase is a difference in sulfamethoxazole distribution: at higher pH, the deprotonated form of sulfamethoxazole is present and found to be more susceptible to degradation. A second reason was found to be the formation of •OH at higher initial pH values, although this contribution was smaller. To elucidate the degradation process, six intermediates were identified, and the difference in formation of these compounds at different initial pH values was revealed. Through ECOSAR modeling, some degradation products were found to be of main interest when monitoring the toxicity of the degradation mixture.status: Published onlin

Aryl-viologen pentapeptide self-assembled conductive nanofibers

A pentapeptide sequence was functionalized with an asymmetric arylated methyl-viologen (AVI3D2) and through controllable β-sheet self-assembly, conductive nanofibers were formed. Using a combination of spectroscopic techniques and conductive atomic force microscopy, we investigated the molecular conformation of the resultant AVI3D2 fibers and how their conductivity is affected by β-sheet self-assembly. These conductive nanofibers have potential for future exploration as molecular wires in optoelectronic applications.status: publishe

Structural Properties of Al-O Monolayers in SiO2 on Silicon and the Maximization of Their Negative Fixed Charge Density

Al2O3 on Si is known to form an ultrathin interfacial SiO2 during deposition and subsequent annealing, which creates a negative fixed charge ( Qfix) that enables field-effect passivation and low surface recombination velocities in Si solar cells. Various concepts were suggested to explain the origin of this negative Qfix. In this study, we investigate Al-O monolayers (MLs) from atomic layer deposition (ALD) sandwiched between deliberately grown/deposited SiO2 films. We show that the Al atoms have an ultralow diffusion coefficient (∼4 × 10-18 cm2/s at 1000 °C), are deposited at a constant rate of ∼5 × 1014 Al atoms/(cm2 cycle) from the first ALD cycle, and are tetrahedral O-coordinated because the adjacent SiO2 imprints its tetrahedral near-order and bond length into the Al-O MLs. By variation in the tunnel-SiO2 thickness and the number of Al-O MLs, we demonstrate that the tetrahedral coordination alone is not sufficient for the formation of Qfix but that a SiO2/Al2O3 interface within a tunneling distance from the substrate must be present. The Al-induced acceptor states at these interfaces have energy levels slightly below the Si valence band edge and require charging by electrons from either the Si substrate or from Si/SiO2 dangling bonds to create a negative Qfix. Hence, tunneling imposes limitations for the SiO2 and Al2O3 layer thicknesses. In addition, Coulomb repulsion between the charged acceptor states results in an optimum number of Al-O MLs, i.e., separation of both interfaces. We achieve maximum negative Qfix of ∼5 × 1012 cm-2 (comparable to thick ALD-Al2O3 on Si) with ∼1.7 nm tunnel-SiO2 and just seven ALD-Al2O3 cycles (∼8 Å) after optimized annealing at 850 °C for 30 s. The findings are discussed in the context of a passivating, hole-selective tunnel contact for high-efficiency Si solar cells.status: publishe

Two-Dimensional Crystal Grain Size Tuning in WS2 Atomic Layer Deposition: An Insight in the Nucleation Mechanism

© 2018 American Chemical Society. When two-dimensional (2D) group-VI transition metal dichalcogenides such as tungsten disulfide (WS 2 ) are grown by atomic layer deposition (ALD) for atomic growth control at low deposition temperatures (≤450 °C), they often suffer from a nanocrystalline grain structure limiting the carrier mobility. The crystallinity and monolayer thickness control during ALD of 2D materials is determined by the nucleation mechanism, which is currently not well understood. Here, we propose a qualitative model for the WS 2 nucleation behavior on dielectric surfaces during plasma-enhanced (PE-) ALD using tungsten hexafluoride (WF 6 ), dihydrogen (H 2 ) plasma and dihydrogen sulfide (H 2 S) based on analyses of the morphology of the WS 2 crystals. The WS 2 crystal grain size increases from ∼20 to 200 nm by lowering the nucleation density. This is achieved by lowering the precursor adsorption rate on the starting surface using an inherently less reactive starting surface, by decreasing the H 2 plasma reactivity, and by enhancing the mobility of the adsorbed species at higher deposition temperature. Since silicon dioxide (SiO 2 ) is less reactive than aluminum oxide (Al 2 O 3 ), and diffusion and crystal ripening is enhanced at higher deposition temperature, WS 2 nucleates in an anisotropic island-like growth mode with preferential lateral growth from the WS 2 crystal edges. This work emphasizes that increasing the crystal grain size while controlling the basal plane orientation is possible during ALD at low deposition temperatures, based on insight in the nucleation behavior, which is key to advance the field of ALD of 2D materials. Moreover, this work demonstrates the conformal deposition on three-dimensional (3D) structures, with WS 2 retaining the basal plane orientation along topographic structures.status: publishe

Two-Dimensional Crystal Grain Size Tuning in WS2 Atomic Layer Deposition: An Insight in the Nucleation Mechanism

© 2018 American Chemical Society. When two-dimensional (2D) group-VI transition metal dichalcogenides such as tungsten disulfide (WS 2 ) are grown by atomic layer deposition (ALD) for atomic growth control at low deposition temperatures (≤450 °C), they often suffer from a nanocrystalline grain structure limiting the carrier mobility. The crystallinity and monolayer thickness control during ALD of 2D materials is determined by the nucleation mechanism, which is currently not well understood. Here, we propose a qualitative model for the WS 2 nucleation behavior on dielectric surfaces during plasma-enhanced (PE-) ALD using tungsten hexafluoride (WF 6 ), dihydrogen (H 2 ) plasma and dihydrogen sulfide (H 2 S) based on analyses of the morphology of the WS 2 crystals. The WS 2 crystal grain size increases from ∼20 to 200 nm by lowering the nucleation density. This is achieved by lowering the precursor adsorption rate on the starting surface using an inherently less reactive starting surface, by decreasing the H 2 plasma reactivity, and by enhancing the mobility of the adsorbed species at higher deposition temperature. Since silicon dioxide (SiO 2 ) is less reactive than aluminum oxide (Al 2 O 3 ), and diffusion and crystal ripening is enhanced at higher deposition temperature, WS 2 nucleates in an anisotropic island-like growth mode with preferential lateral growth from the WS 2 crystal edges. This work emphasizes that increasing the crystal grain size while controlling the basal plane orientation is possible during ALD at low deposition temperatures, based on insight in the nucleation behavior, which is key to advance the field of ALD of 2D materials. Moreover, this work demonstrates the conformal deposition on three-dimensional (3D) structures, with WS 2 retaining the basal plane orientation along topographic structures.status: publishe