The C1s shakeup spectra of Buckminsterfullerene, acenaphthylene, and naphthalene, studied by high resolution x‐ray photoelectron spectroscopy and quantum mechanical calculations

Electronic structure of graphene films prepared from water dispersions and their energy level alignments with organic semiconductors

The unique physical and chemical properties of graphene offer significant potential in a wide range of applications, particularly as a flexible electrode in electronic devices. Solution-processable graphene, especially graphene dispersion in water (GDW), has emerged as a promising candidate for cost-effective, environmentally friendly, and large-scale production. However, the energy level alignment between GDW and semiconductors, which is critical for designing efficient device architectures, remains insufficiently understood. In this study, we investigated the interfacial electronic structures of GDW electrodes with organic semiconductors (C60 and rubrene) using in situ x-ray/ultraviolet photoelectron spectroscopy. We also explored the effect of ultraviolet-ozone (UV-O3) treatment on the charge injection barriers. After 5 min of UV-O3 treatment, the work function of GDW increased by ∼0.4 eV due to surface oxidation, shifting the electron and hole injection barriers for C60 from 0.59 and 1.66 eV to 0.84 and 1.41 eV, respectively, and shifting those for rubrene from 1.82 and 0.74 eV to 2.14 and 0.42 eV, respectively. Weak interactions of both organic semiconductors with the GDW were observed, in contrast to metal electrodes. These results provide valuable insights into the design of future high-efficiency devices using GDW.

Competing pathways to aromaticity governed by amine dehydrogenation and metal-organic complexation in on-surface synthesis

We investigated the reactivity of a gem-dichlorovinyl-carbazole precursor in the on-surface synthesis approach. Our findings reveal that, on the Au(111) surface, the thermally-induced dehalogenation reaction led to the formation of cumulene dimers. Contrastingly, the more reactive Cu(111) surface promoted the formation of a polyheterocyclic compound exhibiting extended aromaticity. The latter was found to be related to the dehydrogenation of the amine groups, which did not occur on Au(111), thus promoting the different reactivity observed. At higher annealing temperature, selective C-H activation led to the formation of well-defined organometallic chains. In addition, we found that the amine complexation with metal adatom on Cu(111) was an inhibiting factor for the dimerization reaction, a challenge that could be overcome through proper control of the deposition conditions.

Exceptional ultrafast nonlinear optical response of functionalized silicon nanosheets

The present work reports on the ultrafast saturable absorption (SA), optical limiting (OL), and the nonlinear refractive response of hydride-terminated silicon nanosheets (SiNS-H) differently functionalized with styrene and tert-butyl methacrylate (tBuMA), namely, SiNS-styrene and SiNS-tBuMA, using 50 fs, 400 nm and 70 fs, 800 nm laser pulses. SiNS-styrene and SiNS-tBuMA exhibit dramatically enhanced nonlinear optical (NLO) responses compared to SiNS-H, with their absorptive nonlinearity strongly dependent on the laser excitation wavelength. More specifically, the studied functionalized SiNSs reveal strong SA behavior under 400 nm laser excitation, with NLO absorption coefficients, saturable intensities, and modulation depths comparable to various two-dimensional (2D) materials, known to exhibit strong SA, such as graphene, black phosphorous (BP), some transition metal dichalcogenides (TMDs), and some MXenes. On the other hand, under 800 nm laser excitation, SiNS-styrene and SiNS-tBuMA show highly efficient OL performance with OL onset values of about 0.0045 and 0.0065 J cm-2, respectively, which are significantly lower than those of other 2D nanostructures. In addition, it is shown that both SiNS samples have great potential in already existing Si-based optoelectronic devices for optical-switching applications since they exhibit very strong NLO refraction comparable to that of bulk Si. The results of the present work demonstrate that the chemical functionalization of SiNSs provides a highly efficient strategy for the preparation of 2D Si-based nanostructures with enhanced NLO response in view of several optoelectronic and photonic applications, such as OL, SA, and all-optical switching.

Synthesis and Thin Films of Thermally Robust Quartet (S = 3/2) Ground State Triradical

High-spin (S = 3/2) organic triradicals may offer enhanced properties with respect to several emerging technologies, but those synthesized to date typically exhibit small doublet quartet energy gaps and/or possess limited thermal stability and processability. We report a quartet ground state triradical 3, synthesized by a Pd(0)-catalyzed radical-radical cross-coupling reaction, which possesses two doublet-quartet energy gaps, ΔE_DQ ≈ 0.2-0.3 kcal mol^-1 and ΔE_DQ2 ≈ 1.2-1.8 kcal mol^-1. The triradical has a 70+% population of the quartet ground state at room temperature and good thermal stability with onset of decomposition at >160 °C under an inert atmosphere. Magnetic properties of 3 are characterized by SQUID magnetometry in polystyrene glass and by quantitative EPR spectroscopy. Triradical 3 is evaporated under ultrahigh vacuum to form thin films of intact triradicals on silicon substrate, as confirmed by high-resolution X-ray photoelectron spectroscopy. AFM and SEM images of the ∼1 nm thick films indicate that the triradical molecules form islands on the substrate. The films are stable under ultrahigh vacuum for at least 17 h but show onset of decomposition after 4 h at ambient conditions. The drop-cast films are less prone to degradation in air and have a longer lifetime.

Stability of radical-functionalized gold surfaces by self-assembly and on-surface chemistry

We have investigated the radical functionalization of gold surfaces with a derivative of the perchlorotriphenylmethyl (PTM) radical using two methods: by chemisorption from the radical solution and by on-surface chemical derivation from a precursor. We have investigated the obtained self-assembled monolayers by photon-energy dependent X-ray photoelectron spectroscopy. Our results show that the molecules were successfully anchored on the surfaces. We have used a robust method that can be applied to a variety of materials to assess the stability of the functionalized interface. The monolayers are characterized by air and X-ray beam stability unprecedented for films of organic radicals. Over very long X-ray beam exposure we observed a dynamic nature of the radical-Au complex. The results clearly indicate that (mono)layers of PTM radical derivatives have the necessary stability to withstand device applications.

Hypergolics in Carbon Nanomaterials Synthesis: New Paradigms and Perspectives

Recently we have highlighted the importance of hypergolic reactions in carbon materials synthesis. In an effort to expand this topic with additional new paradigms, herein we present novel preparations of carbon nanomaterials, such-like carbon nanosheets and fullerols (hydroxylated fullerenes), through spontaneous ignition of coffee-sodium peroxide (Na₂O₂) and C₆₀-Na₂O₂ hypergolic mixtures, respectively. In these cases, coffee and fullerenes played the role of the combustible fuel, whereas sodium peroxide the role of the strong oxidizer (e.g., source of highly concentrated H₂O₂). The involved reactions are both thermodynamically and kinetically favoured, thus allowing rapid product formation at ambient conditions. In addition, we provide tips on how to exploit the released energy of such highly exothermic reactions in the generation of useful work.

Interfacing a Potential Purely Organic Molecular Quantum Bit with a Real-Life Surface

By using a multidisciplinary and multitechnique approach, we have addressed the issue of attaching a molecular quantum bit to a real surface. First, we demonstrate that an organic derivative of the pyrene-Blatter radical is a potential molecular quantum bit. Our study of the interface of the pyrene-Blatter radical with a copper-based surface reveals that the spin of the interface layer is not canceled by the interaction with the surface and that the Blatter radical is resistant in presence of molecular water. Although the measured pyrene-Blatter derivative quantum coherence time is not the highest value known, this molecule is known as a "super stable" radical. Conversely, other potential qubits show poor thin film stability upon air exposure. Therefore, we discuss strategies to make molecular systems candidates as qubits competitive, bridging the gap between potential and real applications.

Monolayer of Hydrazine Facilitates the Direct Covalent Attachment of C60 Fullerene to a Silicon Surface

The development of oxygen-free organic-inorganic interfaces has led to new schemes for the functionalization of silicon surfaces with nitrogen-based chemical groups. However, building layers of large structures directly on this functionalized surface has remained elusive. This work confirms the path to form a stable interface between silicon and buckminsterfullerene C₆₀ based on covalent chemical bonds. The starting point for this modification is the hydrazine-reacted Si(111) surface with the diamine functionality, which is further reacted directly with the C₆₀ molecules. The chemistry of this process is confirmed spectroscopically and microscopically and can be used to form organic-inorganic interfaces separated by a single layer of nitrogen.

Electronic structure investigation of biphenylene films

Photoelectron Spectroscopy (PS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy have been used to investigate the occupied and empty density of states of biphenylene films of different thicknesses, deposited onto a Cu(111) crystal. The obtained results have been compared to previous gas phase spectra and single molecule Density Functional Theory (DFT) calculations to get insights into the possible modification of the molecular electronic structure in the film induced by the adsorption on a surface. Furthermore, NEXAFS measurements allowed characterizing the variation of the molecular arrangement with the film thickness and helped to clarify the substrate-molecule interaction.

Mechanical ball-milling preparation of fullerene/cobalt core/shell nanocomposites with high electrochemical hydrogen storage ability

The design and synthesis of new hydrogen storage nanomaterials with high capacity at low cost is extremely desirable but remains challenging for today's development of hydrogen economy. Because of the special honeycomb structures and excellent physical and chemical characters, fullerenes have been extensively considered as ideal materials for hydrogen storage materials. To take the most advantage of its distinctive symmetrical carbon cage structure, we have uniformly coated C60's surface with metal cobalt in nanoscale to form a core/shell structure through a simple ball-milling process in this work. The X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectra, high-solution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectrometry (EDX) elemental mappings, and X-ray photoelectron spectroscopy (XPS) measurements have been conducted to evaluate the size and the composition of the composites. In addition, the blue shift of C60 pentagonal pinch mode demonstrates the formation of Co-C chemical bond, and which enhances the stability of the as-obtained nanocomposites. And their electrochemical experimental results demonstrate that the as-obtained C60/Co composites have excellent electrochemical hydrogen storage cycle reversibility and considerably high hydrogen storage capacities of 907 mAh/g (3.32 wt % hydrogen) under room temperature and ambient pressure, which is very close to the theoretical hydrogen storage capacities of individual metal Co (3.33 wt % hydrogen). Furthermore, their hydrogen storage processes and the mechanism have also been investigated, in which the quasi-reversible C60/Co↔C60/Co-Hx reaction is the dominant cycle process.

Electronic relaxation effects in condensed polyacenes: A high-resolution photoemission study

We present a high-resolution photoelectron spectroscopy investigation of condensed films of benzene, naphthalene, anthracene, tetracene, and pentacene. High spectroscopic resolution and a systematic variation of the molecular size allow a detailed analysis of the fine structures. The line shapes of the C 1s main lines are analyzed with respect to the different contributions of inhomogeneous broadening, vibronic coupling, and chemical shifts. The shake-up satellite spectra reveal trends, which give insight into the charge redistribution within the molecule upon photoexcitation. In particular, the shake-up between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) increases in intensity and moves closer toward the C 1s main line if the size of the aromatic system is increased. An explanation is given on the basis of the delocalization of the aromatic system and its capability in screening the photogenerated core hole. A comparison of the HOMO-LUMO shake-up position to the optical band gap gives additional insight into the reorganization of the electronic system upon photoexcitation.

Shake-up and shake-off excitations with associated electron losses in X-ray studies of proteins

Photoionization of an atom by X-rays usually removes an inner shell electron from the atom, leaving behind a perturbed "hollow ion" whose relaxation may take different routes. In light elements, emission of an Auger electron is common. However, the energy and the total number of electrons released from the atom may be modulated by shake-up and shake-off effects. When the inner shell electron leaves, the outer shell electrons may find themselves in a state that is not an eigen-state of the atom in its surroundings. The resulting collective excitation is called shake-up. If this process also involves the release of low energy electrons from the outer shell, then the process is called shake-off. It is not clear how significant shake-up and shake-off contributions are to the overall ionization of biological materials like proteins. In particular, the interaction between the outgoing electron and the remaining system depends on the chemical environment of the atom, which can be studied by quantum chemical methods. Here we present calculations on model compounds to represent the most common chemical environments in proteins. The results show that the shake-up and shake-off processes affect approximately 20% of all emissions from nitrogen, 30% from carbon, 40% from oxygen, and 23% from sulfur. Triple and higher ionizations are rare for carbon, nitrogen, and oxygen, but are frequent for sulfur. The findings are relevant to the design of biological experiments at emerging X-ray free-electron lasers.