Showing papers by "Torsten Fritz published in 2020"

Giant persistent photoconductivity in monolayer MoS2 field-effect transistors

Abstract: Monolayer transition metal dichalcogenides (TMD) have numerous potential applications in ultrathin electronics and photonics. The exposure of TMD based devices to light generates photo-carriers resulting in an enhanced conductivity, which can be effectively used, e.g., in photodetectors. If the photo-enhanced conductivity persists after removal of the irradiation, the effect is known as persistent photoconductivity (PPC). Here we show that ultraviolet light (wavelength = 365 nm) exposure induces an extremely long-living giant PPC (GPPC) in monolayer MoS2 (ML-MoS2) field-effect transistors (FET) with a time constant of ~30 days. Furthermore, this effect leads to a large enhancement of the conductivity up to a factor of 107. In contrast to previous studies in which the origin of the PPC was attributed to extrinsic reasons such as trapped charges in the substrate or adsorbates, we unambiguously show that the GPPC arises mainly from the intrinsic properties of ML-MoS2 such as lattice defects that induce a large amount of localized states in the forbidden gap. This finding is supported by a detailed experimental and theoretical study of the electric transport in TMD based FETs as well as by characterization of ML-MoS2 with scanning tunneling spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. The obtained results provide a basis towards the defect-based engineering of the electronic and optical properties of TMDs for device applications.

Effect of an electric field during the deposition of silicon dioxide thin films by plasma enhanced atomic layer deposition : an experimental and computational study

Abstract: The growth, chemical, structural, mechanical, and optical properties of oxide thin films deposited by plasma enhanced atomic layer deposition (PEALD) are strongly influenced by the average-bias voltage applied during the reaction step of surface functional groups with oxygen plasma species. Here, this effect is investigated thoroughly for SiO2 deposited in two different PEALD tools at average-bias voltages up to -300 V. Already at a very low average-bias voltage (< -10 V), the SiO2 films have significantly lower water content than films grown without biasing together with the formation of denser films having a higher refractive index and nearly stoichiometric composition. Substrate biasing during PEALD also enables control of mechanical stress. The experimental findings are supported by density functional theory and atomistic simulations. They demonstrate that the application of an electric field during the plasma step results in an increased energy transfer between energetic ions and the surface, directly influencing relevant surface reactions. Applying an electric field during the PEALD process leads to SiO2 thin films with significantly improved properties comparable to films grown by ion beam sputtering.

Hybridization vs decoupling: influence of an h-BN interlayer on the physical properties of a lander-type molecule on Ni(111).

Abstract: 2D materials such as hexagonal boron nitride (h-BN) are widely used to decouple organic molecules from metal substrates. Nevertheless, there are also indications in the literature for a significant hybridization, which results in a perturbation of the intrinsic molecular properties. In this work we study the electronic and optical properties as well as the lateral structure of tetraphenyldibenzoperiflanthene (DBP) on Ni(111) with and without an atomically thin h-BN interlayer to investigate its possible decoupling effect. To this end, we use in situ differential reflectance spectroscopy as an established method to distinguish between hybridized and decoupled molecules. By inserting an h-BN interlayer we fabricate a buried interface and show that the DBP molecules are well decoupled from the Ni(111) surface. Furthermore, a highly ordered DBP monolayer is obtained on h-BN/Ni(111) by depositing the molecules at a substrate temperature of 170 °C. The structural results are obtained by quantitative low-energy electron diffraction and low-temperature scanning tunneling microscopy. Finally, the investigation of the valence band structure by ultraviolet photoelectron spectroscopy shows that the low work function of h-BN/Ni(111) further decreases after the DBP deposition. For this reason, the h-BN-passivated Ni(111) surface may serve as potential n-type contact for future molecular electronic devices.

Role of Initial and Final States in Molecular Spectroscopies:Example of Tetraphenyldibenzoperiflanthene (DBP) on Graphite

Abstract: Electronic devices based on organic semiconductors are a fast-growing field of technology. A detailed understanding of the interplay between optical and electronic properties of organic molecular t...

An efficient method for indexing grazing‐incidence X‐ray diffraction data of epitaxially grown thin films

Abstract: Crystal structure identification of thin organic films entails a number of technical and methodological challenges. In particular, if molecular crystals are epitaxially grown on single-crystalline substrates a complex scenario of multiple preferred orientations of the adsorbate, several symmetry-related in-plane alignments and the occurrence of unknown polymorphs is frequently observed. In theory, the parameters of the reduced unit cell and its orientation can simply be obtained from the matrix of three linearly independent reciprocal-space vectors. However, if the sample exhibits unit cells in various orientations and/or with different lattice parameters, it is necessary to assign all experimentally obtained reflections to their associated individual origin. In the present work, an effective algorithm is described to accomplish this task in order to determine the unit-cell parameters of complex systems comprising different orientations and polymorphs. This method is applied to a polycrystalline thin film of the conjugated organic material 6,13-pentacene­quinone (PQ) epitaxially grown on an Ag(111) surface. All reciprocal vectors can be allocated to unit cells of the same lattice constants but grown in various orientations [sixfold rotational symmetry for the contact planes (102) and (102)]. The as-determined unit cell is identical to that reported in a previous study determined for a fibre-textured PQ film. Preliminary results further indicate that the algorithm is especially effective in analysing epitaxially grown crystallites not only for various orientations, but also if different polymorphs are present in the film.

Surface Self-Assembly of Functionalized Molecules on Ag(111): More Than Just Chemical Intuition

Abstract: The fabrication of nanomaterials involves self-ordering processes of functional molecules on inorganic surfaces. To obtain specific molecular arrangements, a common strategy is to equip molecules with functional groups. However, focusing on the functional groups alone does not provide a comprehensive picture. Especially at interfaces, processes that govern self-ordering are complex and involve various physical and chemical effects, often leading to structures that defy chemical intuition, as we showcase here on the example of a homologous series of quinones on Ag(111). From chemical intuition one could expect that such quinones, which all bear the same functionalization, form similar motifs. In salient contrast, our joint theoretical and experimental study shows that profoundly different structures are formed. Using a machine-learning-based structure search algorithm, we find that this is due to a shift of the balance of three antagonizing driving forces: adsorbate-substrate interactions governing adsorption sites, adsorbate-adsorbate interactions favoring close packing, and steric hindrance inhibiting certain otherwise energetically beneficial molecular arrangements. The theoretical structures show excellent agreement with our experimental characterizations of the organic/inorganic interfaces, both for the unit cell sizes and the orientations of the molecules within. With a detailed examination of all driving forces, we are further able to devise a design principle for self-assembly of functionalized molecules. The non-intuitive interplay of similarly strong interaction mechanisms will continue to be a challenging aspect for the design of functional interfaces. Our agreement between theory and experiment combined with the new physical insights indicates that these methods have now reached the necessary accuracy to do so.

Morphology of Thin Films of Aromatic Ellagic Acid and Its Hydrogen Bonding Interactions

Abstract: Ellagic acid (EA), an antioxidant from fruits or other plants, has recently evoked interest in the field of organic electronics because of its weak electron donor properties. In this work, the prep...

High Potassium Concentrations Nested in Epitaxial Monolayers of a Flexible Lander-Type Molecule on Ag(111)

Abstract: The structural, electronic, and optical properties of organic adlayers can be tuned to a large extent by incorporating metal atoms. Naturally, the tunability of those properties is limited by the thermodynamic stability of the intercalated phases obtained and the segregation tendency, which often prevents the nesting of high metal atom concentrations in homogeneous epitaxial compound films. Here, we employ scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) to investigate monolayers of the polycyclic aromatic hydrocarbon tetraphenyldibenzoperiflanthene (DBP, C64H36) epitaxially grown on Ag(111) and intercalated with potassium. This lander-type molecule contains four phenyl substituents that are nearly perpendicular to the aromatic backbone, and its flexibility enables rather complex adlayer structures. We succeeded in preparing highly ordered (mixed) monolayers with up to six potassium atoms per DBP. For increasing K concentrations we find that DBP changes its shape from a consi...

Explaining the non-intuitive surface self-assembly of acenequinones on Ag(111): More than just LEGO

Abstract: The rapid development of nanotechnology requires optimizing materials and material combinations at a molecular level. A common tool is to exploit chemical intuition and equip molecules with functional groups to obtain specific molecular arrangements. However, whether functionalization yields the expected results depends sensitively on the parent molecules. Our experimental investigations show that despite chemical similarity, profoundly different structures occur for the prototypical system of acenequinones on Ag(111). To understand this behavior from first principles, we perform in-depth investigations with machine-learning-assisted quantum mechanical modeling. This allows us to understand the monolayer motifs and gain insight into the interaction mechanisms at play. We attribute the different structures to a non-intuitive molecule-size-dependent tradeoff between adsorbate-substrate interactions governing molecular orientations, adsorbate-adsorbate interactions favoring close-packing, and steric hindrance partly inhibiting otherwise beneficial structures. This demonstrates that for the prediction of surface structures an in-depth understanding about the specific interactions, beyond chemical intuition, is vital.

High Potassium Concentrations Nested in Epitaxial Monolayers of a Flexible Lander-Type Molecule on Ag(111)

Abstract: The structural, electronic, and optical properties of organic adlayers can be tuned to a large extent by incorporating metal atoms. Naturally, the tunability of those properties is limited by the thermodynamic stability of the intercalated phases obtained and the segregation tendency, which often prevents the nesting of high metal atom concentrations in homogeneous epitaxial compound films. Here, we employ scanning tunneling microscopy and low-energy electron diffraction to investigate monolayers of the polycyclic aromatic hydrocarbon tetraphenyldibenzoperiflanthene (DBP, C₆₄H₃₆) epitaxially grown on Ag(111) and intercalated with potassium. This lander-type molecule contains four phenyl substituents that are nearly perpendicular to the aromatic backbone, and its flexibility enables rather complex adlayer structures. We succeeded in preparing highly ordered (mixed) monolayers with up to six potassium atoms per DBP. For increasing K concentrations, we find that DBP changes its shape from a considerably bent geometry (pristine DBP and K₂DBP phases) to a molecule with a planar backbone (K₆DBP phase), which is known to occur in free DBP molecules. By means of density functional theory (DFT) calculations, it is elucidated that the added K atoms adsorb underneath the molecule and thereby weaken the direct bonding channels between DBP and Ag while adding new bonding channels via the K atoms. This is accompanied by a gradually increasing charge transfer into the lowest unoccupied molecular orbital of DBP. The combination of structural data, results of different spectroscopy methods, and state-of-the-art DFT calculations leads to a comprehensive view on this rather complex host molecule–guest atom–substrate system.