The Raman fingerprint of rhombohedral graphite

Multi-layer graphene with rhombohedral stacking is a promising carbon phase possibly displaying correlated states like magnetism or superconductivity due to the occurrence of a flat surface band at the Fermi level. Recently, flakes of thickness up to 17 layers were tentatively attributed ABC sequences although the Raman fingerprint of rhombohedral multilayer graphene is currently unknown and the 2D resonant Raman spectrum of Bernal graphite not understood. We provide a first principles description of the 2D Raman peak in three and four layers graphene (all stackings) as well as in Bernal, rhombohedral and an alternation of Bernal and rhombohedral graphite. We give practical prescriptions to identify long range sequences of ABC multi-layer graphene. Our work is a prerequisite to experimental non-destructive identification and synthesis of rhombohedral graphite.Comment: 18 pages, 5 pages article + 13 pages supplemental materia

From Wurtzite Nanoplatelets to Zinc Blende Nanorods: Simultaneous Control of Shape and Phase in Ultrathin ZnS Nanocrystals

Ultrathin semiconductor nanocrystals (NCs) with at least one dimension below their exciton Bohr radius receive a rapidly increasing attention due to their unique physicochemical properties. These superior properties highly depend on the shape and crystal phase of semiconductor NCs. Here, we demonstrate not only the synthesis of well-defined ultrathin ZnS nanoplatelets (NPLs) with excitonic absorption and emission, but also the shape/phase transformation between wurtzite (WZ) NPLs and zinc blende (ZB) nanorods (NRs). UV–vis absorption spectra of WZ-ZnS NPLs clearly exhibit a sharp excitonic peak that is not observed in ZB-ZnS NRs. Besides, the photoluminescence characterization shows that WZ-ZnS NPLs have a narrow excitonic emission peak, while ZB-ZnS NRs exhibit a broad collective emission band consisting of four emission peaks. The appearance of excitonic features in the absorption spectra of ZnS NPLs is explained by interband electronic transitions, which is simulated in the framework of atomic effective pseudopotentials (AEP)

Disorder-perturbed Landau levels in high electron mobility epitaxial graphene

We show that the Landau levels in epitaxial graphene in presence of localized defects are significantly modified compared to those of an ideal system. We report on magneto-spectroscopy experiments performed on high quality samples. Besides typical interband magneto-optical transitions, we clearly observe additional transitions that involve perturbed states associated to short-range impurities such as vacancies. Their intensity is found to decrease with an annealing process and a partial self-healing over time is observed. Calculations of the perturbed Landau levels by using a delta-like potential show electronic states both between and at the same energies of the Laudau levels of ideal graphene. The calculated absorption spectra involving all perturbed and unperturbed states are in very good agreement with the experiments

Single‐Crystalline Colloidal Quasi‐2D Tin Telluride

Tin telluride is a narrow‐gap semiconductor with promising properties for infrared (IR) optical applications and topological insulators. A convenient colloidal synthesis of quasi‐2D SnTe nanocrystals through the hot‐injection method in a nonpolar solvent is reported. By introducing the halide alkane 1‐bromotetradecane as well as oleic acid and trioctylphosphine, the thickness of 2D SnTe nanostripes can be tuned down to 30 nm, while the lateral dimensional can reach 6 µm. The obtained SnTe nanostripes are single crystalline with a rock‐salt crystal structure. The absorption spectra demonstrate pronounced absorption features in the IR range revealing the effect of quantum confinement in such structures

Understanding and exploiting interfacial interactions between phosphonic acid functional groups and co-evaporated perovskites

Interfacial engineering has fueled recent development of p-i-n perovskite solar cells (PSCs), with self-assembled monolayer-based hole-transport layers (SAM-HTLs) enabling almost lossless contacts for solution-processed PSCs, resulting in the highest achieved power conversion efficiency (PCE) to date. Substrate interfaces are particularly crucial for the growth and quality of co-evaporated PSCs. However, adoption of SAM-HTLs for co-evaporated perovskite absorbers is complicated by the underexplored interaction of such perovskites with phosphonic acid functional groups. In this work, we highlight how exposed phosphonic acid functional groups impact the initial phase and final bulk crystal structures of co-evaporated perovskites and their resultant PCE. The explored surface interaction is mediated by hydrogen bonding with interfacial iodine, leading to increased formamidinium iodide adsorption, persistent changes in perovskite structure, and stabilization of bulk α-FAPbI3, hypothesized as being due to kinetic trapping. Our results highlight the potential of exploiting substrates to increase control of co-evaporated perovskite growth

Simulations ab-initio des spectres Raman résonants dans le graphène, les multicouches de graphène et le graphite

Multi-layer graphene with rhombohedral ABC stacking is considered as a promising carbon phase possibly displaying correlated states like magnetism or high-T c superconductivity due to the occurrence of an ultraflat electronic surface band at the Fermi level. Despite Bernal graphite being the most stable form of graphite, three and four layers graphene samples with rhombohedral stacking can be synthesized. Recently, flakes of thickness up to 17 layers were tentatively attributed ABC sequences although the Raman fingerprint of rhombohedral multilayer graphene is currently unknown and the 2D two-phonon resonant Raman spectrum of Bernal graphite not completely theoretically understood. Here we provide a complete first principles description of the 2D Raman peak in three and four layer graphene for all possible stackings, as well as for bulk Bernal, rhombohedral and an alternation of Bernal and rhombohedral graphite, that can be seen as a periodic sequence of ABA and ABC trilayers. Calculations for several laser energies are performed and we give practical prescriptions are proposed to identify long range sequences of ABC multi-layer graphene flakes.Les multicouches de graphène en empilement rhomboédrique sont considérés comme une phase prometteuse du carbone. Cela est due à la particularité de cette phase de pouvoir exhiber des états à forte corrélation électronique comme le magnétisme ou la supraconductivité à haute température critique. Ce qui est due, a son tour, à l’occurrence d’un état de surface avec une dispersion d’énergie électroniques quasi-nulle à proximité du niveau de Fermi. Malgré que le graphite Bernal soit la forme la plus stable du graphite, des échantillons a trois et quatre couches de graphène en empilement rhomboédrique ont pu être synthétisés. Plus récemment, des flocons d’épaisseur dépassant les 17 couches ont été isolés et provisoirement attribués à des séquences d’empilement rhomboédrique. Cette attribution à été faite via des expériences de spectroscopie Raman sous champ magnétique, bien que l’empreinte Raman des multicouche de graphène en empilement rhomboédrique est actuellement inconnue. Même le cas simple du spectre Raman résonnant à deux phonons (le pic 2D) du graphite Bernal n’est pas totalement compris. Dans ce travail de thèse, nous fournissons une description ab-initio complète du pic Raman 2D dans les systèmes de graphène à trois et quatre couches pour tous les empilements possibles, ainsi que pour le graphite Bernal, rhomboédrique et une alternance de graphite Bernal et rhomboédrique

Ab-initio resonant Raman simulations in graphene, few layer graphene, and graphite

Les multicouches de graphène en empilement rhomboédrique sont considérés comme une phase prometteuse du carbone. Cela est due à la particularité de cette phase de pouvoir exhiber des états à forte corrélation électronique comme le magnétisme ou la supraconductivité à haute température critique. Ce qui est due, a son tour, à l’occurrence d’un état de surface avec une dispersion d’énergie électroniques quasi-nulle à proximité du niveau de Fermi. Malgré que le graphite Bernal soit la forme la plus stable du graphite, des échantillons a trois et quatre couches de graphène en empilement rhomboédrique ont pu être synthétisés. Plus récemment, des flocons d’épaisseur dépassant les 17 couches ont été isolés et provisoirement attribués à des séquences d’empilement rhomboédrique. Cette attribution à été faite via des expériences de spectroscopie Raman sous champ magnétique, bien que l’empreinte Raman des multicouche de graphène en empilement rhomboédrique est actuellement inconnue. Même le cas simple du spectre Raman résonnant à deux phonons (le pic 2D) du graphite Bernal n’est pas totalement compris. Dans ce travail de thèse, nous fournissons une description ab-initio complète du pic Raman 2D dans les systèmes de graphène à trois et quatre couches pour tous les empilements possibles, ainsi que pour le graphite Bernal, rhomboédrique et une alternance de graphite Bernal et rhomboédrique.Multi-layer graphene with rhombohedral ABC stacking is considered as a promising carbon phase possibly displaying correlated states like magnetism or high-T c superconductivity due to the occurrence of an ultraflat electronic surface band at the Fermi level. Despite Bernal graphite being the most stable form of graphite, three and four layers graphene samples with rhombohedral stacking can be synthesized. Recently, flakes of thickness up to 17 layers were tentatively attributed ABC sequences although the Raman fingerprint of rhombohedral multilayer graphene is currently unknown and the 2D two-phonon resonant Raman spectrum of Bernal graphite not completely theoretically understood. Here we provide a complete first principles description of the 2D Raman peak in three and four layer graphene for all possible stackings, as well as for bulk Bernal, rhombohedral and an alternation of Bernal and rhombohedral graphite, that can be seen as a periodic sequence of ABA and ABC trilayers. Calculations for several laser energies are performed and we give practical prescriptions are proposed to identify long range sequences of ABC multi-layer graphene flakes