Rengin Peköz

Hyperbranched Unsaturated Polyphosphates as a Protective Matrix for Long-Term Photon Upconversion in Air

The energy stored in the triplet states of organic molecules, capable of energy transfer via an emissive process (phosphorescence) or a nonemissive process (triplet-triplet transfer), is actively dissipated in the presence of molecular oxygen. The reason is that photoexcited singlet oxygen is highly reactive, so the photoactive molecules in the system are quickly oxidized. Oxidation leads to further loss of efficiency and various undesirable side effects. In this work we have developed a structurally diverse library of hyperbranched unsaturated poly(phosphoester)s that allow efficient scavenging of singlet oxygen, but do not react with molecular oxygen in the ground state, i.e., triplet state. The triplet-triplet annihilation photon upconversion was chosen as a highly oxygen-sensitive process as proof for a long-term protection against singlet oxygen quenching, with comparable efficiencies of the photon upconversion under ambient conditions as in an oxygen-free environment in several unsaturated polyphosphates. The experimental results are further correlated to NMR spectroscopy and theoretical calculations evidencing the importance of the phosphate center. These results open a technological window toward efficient solar cells but also for sustainable solar upconversion devices, harvesting a broad-band sunlight excitation spectrum.

Ab initio characterization of graphene nanoribbons and their polymer precursors

Bottom-up fabrication of graphene nanoribbons (GNRs) from halogen-terminated aromatic precursors is a promising method for achieving atomically precise nanoribbons at competitive yields. GNR fabrication proceeds via the polymerization of the precursors and successive dehydrogenation. By first principles density functional theory calculations, we perform a systematic characterization of the polymeric precursors and the corresponding graphene nanoribbons in terms of structural and electronic properties, and we compute the Raman and infrared spectra. The band structure properties are examined by considering the bonding features and the partial charge densities of the structures. The physical origin of the infrared and Raman peaks is investigated in terms of the morphology and vibrational properties of the precursors and products. We show that light spectroscopy provides a unique fingerprint for each type of GNR, which may be used to monitor the quality of the final products and the kinetics of the synthesis process.

Effect of van der Waals interactions on the chemisorption and physisorption of phenol and phenoxy on metal surfaces

The adsorption of phenol and phenoxy on the (111) surface of Au and Pt has been investigated by density functional theory calculations with the conventional PBE functional and three different non-local van der Waals (vdW) exchange and correlation functionals. It is found that both phenol and phenoxy on Au(111) are physisorbed. In contrast, phenol on Pt(111) presents an adsorption energy profile with a stable chemisorption state and a weakly metastable physisorbed precursor. While the use of vdW functionals is essential to determine the correct binding energy of both chemisorption and physisorption states, the relative stability and existence of an energy barrier between them depend on the semi-local approximations in the functionals. The first dissociation mechanism of phenol, yielding phenoxy and atomic hydrogen, has been also investigated, and the reaction and activation energies of the resulting phenoxy on the flat surfaces of Au and Pt were discussed.

Amorphous Phase Change Materials: Structure, Stability and Relation with Their Crystalline Phase

Phase Change Materials should be stable enough in their amorphous phase to achieve a durable data retention, however they should also be bad glass formers to be able to recrystallise at high speed. To understand these contradicting properties, we construct models of amorphous Ge–Sb–Te systems using Ab Initio Molecular Dynamics and analyse the structures in relation with the relevant crystalline state. We show that structural patterns that are precursors of the crystalline phase exist in the amorphous state and we identify the signature of the various types of local atomic orders in the X-ray absorption spectra that we compute using Density Functional Theory. We first analyse the mechanical properties of the amorphous phase in the framework of the Maxwell rigidity theory, showing that all efficient Phase Change Materials deviate from the perfect glass and are mechanically stressed-rigid. Additionally, we show that the stability of Phase Change Materials is related to the density of low frequency vibrational modes (Boson peak). We describe how an adequate doping can result in an increased stability of the amorphous phase while keeping intact the phase change ability of the material.