Delaminated Graphene at Silicon Carbide Facets: Atomic Scale Imaging and Spectroscopy

Atomic-resolution structural and spectroscopic characterization techniques (scanning transmission electron microscopy and electron energy loss spectroscopy) are combined with nanoscale electrical measurements (conductive atomic force microscopy) to study at the atomic scale the properties of graphene grown epitaxially through the controlled graphitization of a hexagonal SiC(0001) substrate by high temperature annealing. This growth technique is known to result in a pronounced electron-doping (∼10¹³ cm^–2) of graphene, which is thought to originate from an interface carbon buffer layer strongly bound to the substrate. The scanning transmission electron microscopy analysis, carried out at an energy below the knock-on threshold for carbon to ensure no damage is imparted to the film by the electron beam, demonstrates that the buffer layer present on the planar SiC(0001) face delaminates from it on the (112̅<i>n</i>) facets of SiC surface steps. In addition, electron energy loss spectroscopy reveals that the delaminated layer has a similar electronic configuration to purely sp²-hybridized graphene. These observations are used to explain the local increase of the graphene sheet resistance measured around the surface steps by conductive atomic force microscopy, which we suggest is due to significantly lower substrate-induced doping and a resonant scattering mechanism at the step regions. A first-principles-calibrated theoretical model is proposed to explain the structural instability of the buffer layer on the SiC facets and the resulting delamination

Revealing a Discontinuity in the Degradation Behavior of CH₃NH₃PbI₃ during Thermal Operation

The advance of innovative photovoltaics based on hybrid perovskites is currently forced to face their stability and durability through the rationalization of the phenomena occurring into the lattice under conditions which mimic the material operation. In this framework, we study the structural modifications of MAPbI₃ layers by in situ structural and optical analyses upon recursive thermal cycles from 30 to 80 °C in different annealing environments. We reveal an acceleration of the material modification, above what expected, as the threshold of the tetragonal to cubic transition (∼50 °C) is surpassed. This produces discontinuities in the degradation rate, bandgap value, and dielectric behavior of the MAPbI₃ layer. The phenomenon is put in relationship with the order–disorder lattice modifications described by Car–Parrinello molecular dynamics calculations and reveals that the action of species from humid air becomes largely more effective above 50 °C for reasons related to the increased accessibility/reactivity of the lattice which, in turn, impacts on defects generation

Texture of MAPbI₃ Layers Assisted by Chloride on Flat TiO₂ Substrates

In this paper, we use X-ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy to investigate the structural and morphological properties of methylammonium lead iodide (MAPbI₃) thin films deposited on flat TiO₂ substrates, as obtained with and without the use of chlorine precursors. We demonstrate that the presence of Cl precursors assists the growth of oriented MAPbI₃ domains along a specific growth direction. Such features are attributed to the proximity of the Cl species to the interface with the TiO₂ substrate

From PbI₂ to MAPbI₃ through Layered Intermediates

In the present work we describe the process of formation of methylammonium lead iodide perovskite (MAPbI₃) by reaction between [001] oriented PbI₂ thin films and MAI. The reaction leads to a rapid change in the optical properties with the formation of a brown phase having a large band in the visible range and a contribution in the near-UV range (375 nm). X-ray diffraction analyses indicate that the MAI + PbI₂ as-obtained material mainly consists of a highly oriented matrix of MAI that intermixes with the Pb–I precursor. A chemical exfoliation of the initial PbI₂ structure with the massive intercalation of the methylammonium iodide ions suitably describes the optical and structural findings. Short thermal treating (100 °C) of this as-obtained sample results in the formation of a second oriented intermediate phase, attributed to the presence of a low-dimensional perovskite structure. Such low-dimensional metastable phase is no longer visible by increasing the annealing time or the annealing temperature (150 °C), with the 3D MAPbI₃ tetragonal phase left in the XRD pattern

In-Situ Degradation Pathway Analyses on Hybrid Perovskites with Mixed Cations and Anions

Multication and multianion hybrid perovskites are among the most attractive materials currently under investigation for tandem photovoltaic applications because of the possibility they offer to finely tune the band gap, thus allowing the coupling with other semiconductors. If mixing different ions in the perovskite compositions can, on one hand, give the possibility to create different useful materials with different properties, on the other hand it could be detrimental to the hybrid perovskite stability. The presence of different ions leads to the formation of different polytypes that could open new pathways for the degradation of the prepared films. In this work, we have investigated the role of different polytypes on the degradation and stability of mixed hybrid perovskites by in situ X-ray diffraction analyses. We found that the use of even a small amount of methylammonium cations opens a new route for the degradation of the perovskite film, triggering a more advanced degradation beyond the loss of the methylammonium cations. By in-situ analyses during focused thermal treatments, we unveil an interplay between the different polytypes triggered by temperature. In particular, we show how the 4H polytype of the formamidinium lead iodide (FAPbI3) is recovered with an annealing at 100 °C in N2 that restores the 3C polytype. We propose the use of 1,3:2,4-bis-O-(4-methylbenzylidene) d-sorbitol (MDBS) additive as a remedy to increase the stability of the perovskite film even in the presence of the methylammonium cation by making the perovskite films more compact, thus reducing the surface/volume ratio

Ambipolar MoS₂ Transistors by Nanoscale Tailoring of Schottky Barrier Using Oxygen Plasma Functionalization

One of the main challenges to exploit molybdenum disulfide (MoS₂) potentialities for the next-generation complementary metal oxide semiconductor (CMOS) technology is the realization of p-type or ambipolar field-effect transistors (FETs). Hole transport in MoS₂ FETs is typically hampered by the high Schottky barrier height (SBH) for holes at source/drain contacts, due to the Fermi level pinning close to the conduction band. In this work, we show that the SBH of multilayer MoS₂ surface can be tailored at nanoscale using soft O₂ plasma treatments. The morphological, chemical, and electrical modifications of MoS₂ surface under different plasma conditions were investigated by several microscopic and spectroscopic characterization techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), conductive AFM (CAFM), aberration-corrected scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Nanoscale current–voltage mapping by CAFM showed that the SBH maps can be conveniently tuned starting from a narrow SBH distribution (from 0.2 to 0.3 eV) in the case of pristine MoS₂ to a broader distribution (from 0.2 to 0.8 eV) after 600 s O₂ plasma treatment, which allows both electron and hole injection. This lateral inhomogeneity in the electrical properties was associated with variations of the incorporated oxygen concentration in the MoS₂ multilayer surface, as shown by STEM/EELS analyses and confirmed by ab initio density functional theory (DFT) calculations. Back-gated multilayer MoS₂ FETs, fabricated by self-aligned deposition of source/drain contacts in the O₂ plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio <i>I</i>_on/<i>I</i>_off ≈ 10³ and field-effect mobilities of 11.5 and 7.2 cm² V^–1 s^–1 for electrons and holes, respectively. The electrical behavior of these novel ambipolar devices is discussed in terms of the peculiar current injection mechanisms in the O₂ plasma functionalized MoS₂ surface