Probing the energy levels in hole-doped molecular semiconductors

Understanding the nature of polarons – the fundamental charge carriers in molecular semiconductors – is indispensable for rational material design that targets superior (opto-) electronic device functionality. The traditionally conceived picture of the corresponding energy levels invokes singly occupied molecular states within the energy gap of the semiconductor. Here, by employing a combined theoretical and multi-technique experimental approach, we show that this picture needs to be revised. Upon introducing an excess electron or hole into the material, the respective frontier molecular level is split by strong on-site Coulomb repulsion into an upper unoccupied and a lower occupied sub-level, only one of which is located within the semiconductor gap. By including also inter-site Coulomb interaction between molecular ions and circumjacent neutral molecules, we provide a complete picture for the electronic structure of molecular semiconductors in the presence of excess charges. With this understanding, a critical re-examination of previous results is called for, and future investigations of the properties and dynamics of polarons in weakly interacting molecular systems are put on sound footing.Peer Reviewe

Transduction and encoding sensory information by skin mechanoreceptors

International audienc

Membrane cholesterol depletion as a trigger of Nav1.9 channel‐mediated inflammatory pain

International audienc

The Importance of Ligand Selection on the Formation of Bimetallic Phosphide Catalysts Derived from Metal-Organic Frameworks

Coordination polymers (CPs) and metal-organic frameworks (MOFs) have emerged as versatile precursors for transition-metal phosphides catalysts. However, the controlled synthesis of MOF-derived bimetallic phosphides remains a challenge, as mixtures of various phosphide phases are often formed. Here, it is shown that controlling the formation of pure CoMoP and CoMoP2 requires a careful choice of the ligands used to construct the MOF precursors, based on the chemical properties of the metals. In particular, the nature and number of the coordination moieties of the ligand play a key role. CoMoP and CoMoP2 particles coated with N-doped carbon were derived from phosphonate-based MOFs and compared as hydrogen evolution reaction (HER) electrocatalysts in acidic medium. CoMoP2 is more active and shows a turnover frequency (TOF) of 0.9 s-1 compared to 0.4 s-1 for CoMoP. The higher intrinsic activity of the CoMoP2 catalytic sites correlates with the differences in the electronic structure of the materials, with a larger charge transfer from the molybdenum to the phosphorous found for CoMoP2

Electronic structure of CoPc adsorbed on Ag(100): Evidence for molecule-substrate interaction mediated by Co 3d orbitals

International audienceThe electronic structure of cobalt-phthalocyanine (CoPc) molecules adsorbed on Ag(100) is investigated by photoemission spectroscopy. The results are compared to first-principles electronic structure calculations, based on many-body perturbation theory in the GW approximation. The photoemission data, obtained from both multilayer and monolayer films of CoPc, show that charge transfer occurs between the first molecular layer and the metal surface. Varying the photon energy, to tune the photoionization cross sections, reveals that the charge-transfer-related interface states mainly involve the Co 3d atomic orbitals of the Co central atom. GW calculations for the neutral CoPc molecule and its anion compare well with the experimental observations for a multilayer and a monolayer CoPc film, respectively. They confirm the major role played by the Co atom in the charge-transfer process and elucidate the complex energy rearrangement of the molecular electronic levels upon metal adsorptio

Activating Ru in the pyramidal sites of Ru2P-type structures with earth-abundant transition-metals for achieving extremely high HER activity while minimizing noble metal content

Rational design of efficient pH-universal hydrogen evolution reaction (HER) catalysts to enable large-scale hydrogen production via electrochemical water splitting is of great significance, yet a challenging task. Herein, Ru atoms in the Ru2P structure were replaced with M=Co, Ni, or Mo to produce M2-xRuxP nanocrystals. The metals show strong site preference, with Co and Ni occupying the tetrahedral sites and Ru the square pyramidal sites of the CoRuP and NiRuP Ru2P-type structures. The presence of Co or Ni in the tetrahedral sites leads to charge redistribution for Ru and, according to density functional theory (DFT) calculations, to a significant increase in the Ru d-band centers. As a result, the intrinsic activity of CoRuP and 1 NiRuP increases considerably compared to Ru2P in both acidic and alkaline media. The effect is not observed for MoRuP, in which Mo prefers to occupy the pyramidal sites. In particular, CoRuP shows state-of-the-art activity, outperforming Ru2P with Pt-like activity in 0.5 M H2SO4 (η10=12.3 mV; η100=52 mV; turnover frequency (TOF)=4.7 s-1). It remains extraordinarily active in alkaline conditions (η10=12.9 mV; η100=43.5 mV) with a TOF of 4.5 s-1, which is 4x higher than that of Ru2P and 10x that of Pt/C. Further increase of the Co content does not lead to drastic loss of activity, especially in alkaline medium, where for example the TOF of Co1.9Ru0.1P remains comparable to that of Ru2P and higher than Pt/C, highlighting the viability of the adopted approach to prepare cost efficient catalysts

Operando Diffuse Reflectance UV-VIS Spectroelectrochemistry for Investigating Oxygen Evolution Electrocatalysts

This is a complete self-standing study on operando diffuse reflectance UV-vis spectroelectrochemistry for investigating oxygen evolution electrocatalysts.</div

On the Role of Heterojunctions of Core-Shell Heterostructures in Gas Sensing

Heterostructures made from semiconducting metal oxides (SMOX) are fundamental for the development of high-performance gas sensors. Yet, despite the recognition of their importance in real applications, the understanding of the transduction mechanism either related to the heterojunction, or simply to the core and shell materials is still lacking. A better understanding of the sensing response of heterostructured nanomaterials requires the engineering of heterojunctions with well-defined core and shell layers. Here, we introduce a series of prototypes nSMOX-CNT, pSMOX-CNT, and pSMOX-nSMOX-CNT and nSMOX-pSMOX-CNT hierarchical core-shell heterostructures (CSHS) permitting us to directly relate the sensing response to the SMOX shell, or to the p-n heterojunction. The carbon nanotubes are here used as highly conductive substrates permitting to operate the devices at relatively low temperature and are not involved in the sensing response. NiO and SnO2 are selected as representative p- and n-type SMOX, respectively, and the response of a set of samples is studied toward hydrogen considered as model analyte. The n,pSMOX-CNT CSHS exhibit response related to the n,pSMOX-shell layer. On the other hand, the pSMOX-nSMOX-CNT and nSMOX-pSMOX-CNT CSHS show sensing responses, which in certain cases are governed by the heterojunctions between nSMOX and pSMOX and strongly depends on the thickness of the SMOX layers. Due to the fundamental nature of this study, these findings are important for the development of next generation gas sensing devices

Type-I Energy Level Alignment at the PTCDA—Monolayer MoS2 Interface Promotes Resonance Energy Transfer and Luminescence Enhancement

Van der Waals heterostructures consisting of 2D semiconductors and conjugated molecules are of increasing interest because of the prospect of a synergistic enhancement of (opto)electronic properties. In particular, perylenetetracarboxylic dianhydride (PTCDA) on monolayer (ML)-MoS2 has been identified as promising candidate and a staggered type-II energy level alignment and excited state interfacial charge transfer have been proposed. In contrast, it is here found with inverse and direct angle resolved photoelectron spectroscopy that PTCDA/ML-MoS2 supported by insulating sapphire exhibits a straddling type-I level alignment, with PTCDA having the wider energy gap. Photoluminescence (PL) and sub-picosecond transient absorption measurements reveal that resonance energy transfer, i.e., electron–hole pair (exciton) transfer, from PTCDA to ML-MoS2 occurs on a sub-picosecond time scale. This gives rise to an enhanced PL yield from ML-MoS2 in the heterostructure and an according overall modulation of the photoresponse. These results underpin the importance of a precise knowledge of the interfacial electronic structure in order to understand excited state dynamics and to devise reliable design strategies for optimized optoelectronic functionality in van der Waals heterostructures.Peer Reviewe