arXiv subjects

Milan Kivala

Publications and source records attributed to Milan Kivala.

6 recordsLinked to original sources

arXiv2026

Sulfur-rich Spirofluorene-Bridged N Heterotriangulene Redox-Active Polymers

Sulfur-rich spirofluorene-bridged N-heterotriangulene (FTN) polymers featuring covalently linked oligosulfide units and a terthiophene-based analogue were synthesized via nucleophilic aromatic substitution and Stille cross-coupling polymerization. The resulting materials are amorphous, insoluble solids with high thermal stability and sulfur contents up to 25 wt%. Structural and compositional analyses by combustion analysis, ToF-SIMS, FT-IR, XPS, and solid-state NMR confirm the efficient incorporation of short oligosulfide to disulfide linkages and well-defined terthiophene units in the respective polymers. Electrochemical characterization in lithium half-cells reveals a reversible, high-voltage oxidation of the FTN unit at 3.8-4.0 V (vs. Li/Li+), accompanied by low-voltage sulfur- or terthiophene-based redox processes between 1.5-2.5 V (vs. Li/Li+). Sulfur incorporation markedly increases the theoretical and initial discharge capacities (up to 129 mA h g-1), while the sulfide conversion processes exhibit rapid fading and poor reversibility due to sulfide dissolution. In contrast, the terthiophene-linked polymer shows only transient low-voltage activity while maintaining high Coulombic efficiencies (ca. 99.7%) governed by the persistent FTN backbone redox event. Our results highlight how different redox-active linkers influence the electrochemical behavior of FTN-based polymers and provide insights into the design of functional organic cathode materials featuring multi-redox processes.

cond-mat.mtrl-sci↗

arXiv2025

Zero-energy band observation in an interfacial chalcogen-organic network

Structurally-defined molecule-based lattices such as covalent organic or metal-organic networks on substrates, have emerged as highly tunable, modular platforms for two-dimensional band structure engineering. The ability to grow molecule-based lattices on diverse platforms, such as metal dichalcogenides, would further enable band structure tuning and alignment to the Fermi level, which is crucial for the exploration and design of quantum matter. In this work, we study the emergence of a zero-energy band in a triarylamine-based network on semiconducting 1T-TiSe2 at low temperatures, by means of scanning probe microscopy and photoemission spectroscopy, together with density-functional theory. Hybridization between the position-selective nitrogens and selenium p-states results in CN-Se interfacial coordination motifs, leading to a hybrid molecule-semiconductor band at the Fermi level. Our findings introduce chalcogen-organic networks and showcase an approach for the engineering of organic-inorganic quantum matter.

physics.chem-ph↗

arXiv2024

Moir\'e lattice of twisted bilayer graphene as template for non-covalent functionalization

We present a novel approach to achieve spatial variations in the degree of non-covalent functionalization of twisted bilayer graphene (tBLG). The tBLG with twist angles varying between ~ 5{\deg} and 7{\deg} was non-covalently functionalized with 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN) molecules. Our results show a correlation between the degree of functionalization and the twist angle of tBLG. This correlation was determined through Raman spectroscopy, where areas with larger twist angles exhibited a lower HATCN peak intensity compared to areas with smaller twist angles. We suggest that the HATCN adsorption follows the moir\'e pattern of tBLG by avoiding AA-stacked areas and attach predominantly to areas with a local AB-stacking order of tBLG, forming an overall ABA-stacking configuration. This is supported by density functional theory (DFT) calculations. Our work highlights the role of the moir\'e lattice in controlling the non-covalent functionalization of tBLG. Our approach can be generalized for designing nanoscale patterns on two-dimensional (2D) materials using moir\'e structures as a template.

cond-mat.mtrl-sci↗

arXiv2024

Photochemistry upon charge separation in triphenylamine derivatives from fs to $\mathrmμ$s

Quantum chemical methods and time-resolved laser spectroscopy are employed to elucidate ultrafast charge separation processes in triphenylamine (TPA) derivatives upon photoexcitation. When changing the ambient solvent from generic ones to those capable of accepting electrons, such as chloroform, a vastly extended and multifaceted photochemistry is observed. Following the initial excitation, two concurrent charge transfer processes are identified. Firstly, when the TPA derivative and solvent molecules are correctly positioned, an electron transfer to the solvent molecule with immediate charge separation takes place. Consequently, this process gives rise to the formation of the corresponding radical cation of the TPA derivative. This highly reactive species can subsequently combine with other TPA derivative molecules to yield dimeric species. Secondly, when the molecular positioning upon photoexcitation is not optimal, relaxation back to the $\mathrm{S_1}$ state occurs. From this state, an electron transfer process leads to the formation of a charge transfer complex. In this complex, the negatively charged solvent molecule remains closely associated with the positively charged TPA derivative. Within 30 picoseconds, the charges within this complex recombine, yielding a triplet state. This transition to the triplet state is driven by a lower reaction barrier for charge separation compared to the formation of the singlet state.

physics.chem-ph↗

arXiv2024

Electronic Properties of Interfaces between N-Heterotriangulene Donors and Strong Tetracyanoquinodimethane Acceptors

N-heterotriangulenes (N-HTAs) represent a class of functional molecules with high potential for optoelectronic materials, for example as electron donating compounds in donor/acceptor (D/A) systems. The capability of two different N-HTAs, N-HTA 550 and N-HTA 557, the latter containing an additional 7-membered ring, to act as electron donors at interfaces with strong tetracyanoquinodimethane (TCNQ and F4TCNQ) acceptors is studied using high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. For TCNQ/N-HTA bilayer systems adsorbed on Au(111) Low-energy (< 2.5 eV) electronic transitions which are attributed to charge transfer (CT) states for all four D/A combinations are identified. Based on substantial quantum chemical calculations a generation of ground state CT complexes is excluded. Instead, CT in the excited state, in which an electron-stimulated CT from the N-HTAs to TCNQs is the underlying process, is proposed. The energies of the CT states are determined by the values of the ionization potential and electron affinity of the involved donor and acceptor.

cond-mat.mtrl-sci↗

arXiv2024

Determination of Energetic Positions of Electronic States and the Exciton Dynamics in a pi-Expanded N-Heterotriangulene Derivative Adsorbed on Au(111)

Bridged triarylamines, so-called N-heterotriangulenes (N-HTAs) are promising organic semiconductors for applications in optoelectronic devices. Thereby the electronic structure at organic/metal interfaces and within thin films as well as the electronically excited states dynamics after optical excitation is essential for the performance of organic-molecule-based devices. Here, we investigated the energy level alignment and the excited state dynamics of a N-HTA derivative adsorbed on Au(111) by means of energy- and time-resolved two-photon photoemission spectroscopy. We quantitatively determined the energetic positions of several occupied and unoccupied molecular (transport levels) and excitonic states (optical gap) in detail. A transport gap of 3.20 eV and an optical gap of 2.58 eV is determined, resulting in an exciton binding energy of 0.62 eV. With the first time-resolved investigation on a N-HTA compound we gained insights into the exciton dynamics and resolved processes on the femtosecond to picosecond timescale.

cond-mat.mtrl-sci↗