Giovanni Cantele1∗, Domenico Ninno1,2, Robero Nunzio D’Amico1
1CNR-SPIN, Dipartimento di Fisica
Complesso Universitario Monte Sant’Angelo Via Cintia, 80126 Napoli, Italy
2Universit`a degli Studi di Napoli “Federico II”, Dipartimento di Fisica
Complesso Universitario Monte Sant’Angelo Via Cintia, 80126 Napoli, Italy
Abstract. In this report, the study of electronic processes and barrier heights at semicon- ductor-semiconductor and semiconductor-metal interfaces, of interest for applications in (nano)electronics, energy conversion and optoelectronic devices, is presented. Two dif- ferent systems are chosen, namely, the SrTiO3–TiO2interface and graphene nanoribbons
covalently attached to a metallic substrate. Fundamental properties, following the for- mation of the interface, are: charge transfer and interface dipole, band offset, role of defects.
It is shown that the local chemistry and the interface stoichiometry play a fundamental role in determining the interface electronic properties. As such, a deep understanding at microscopic level is needed, as provided by accurate first principle approaches.
1
Introduction
The most recent advances in thin-film synthesis techniques and two-dimensional ma- terials fabrication has opened new perspectives in the interface physics and its use in (nano)electronics, optoelectronic and energy conversion devices. In particular, the abil- ity of engineering atomically precise complex interfaces, of controlling their stoichiometry and of combining the properties of individual materials into various heterostructures paves the way to new functional devices. Among the many, special mention deserve complex oxide interfaces [1]: a wide range of crystalline structures exhibit, already in the bulk, an incredible variety of physical phenomena, such as piezo-, pyro-, and ferroelectricity, superconductivity, and so on. It is expected that an even more wide range of properties and applications can be observed in atomically sharp oxide heterostructures. Beyond ox- ide electronics, organic electronic applications are also foreseen, thanks to interfaces with organic layer in devices, such as light-emitting diodes and organic photovoltaic cells [2]. Even more exciting is the physics of two-dimensional materials [3, 4], that has emerged since the discovery of graphene. Graphene nanoribbons (GNR), one dimensional graphene
Figure 1: The SrTiO3–TiO2 interface [5].
stripes, have been shown as promising materials for nanoelectronics: at variance with graphene, that exhibits a zero energy gap, quantum confinement in GNR channels guar- antees band energy gaps as large as ∼ 1 eV for ∼ 1 nm wide stripes. The presence of this gap is fundamental for the realization of a switching device. However, metallic contacts are required in practical applications and the understanding of the metal contact–GNR interaction is of utmost importance for predicting and engineering the device properties. In this report fundamental properties, arising at complex interfaces, are addressed, choos- ing as model systems the SrTiO3–TiO2 oxide-based interface and a GNR–based system, where the GNR is covalently immobilized on the metallic substrate by means of an organic layer.
2
Software and computational resources
All calculations have been performed in the framework of Density Functional Theory, as implemented in the open source Quantum Espresso package [6]. This package uses atomic pseudopotentials to mimic the ion cores and plane waves to expand the electronic wave functions and charge density. The periodic supercell approach is used to study low dimensional systems, such as surfaces/interfaces/two-dimensional materials, nanowires, zero-dimensional nanostructures and molecules. As such, vacuum space is needed in these cases, to avoid spurious interactions between the periodic replicas. The number of basis set functions needed to provide converged properties changes with the atomic pseudopoten- tial, number of atoms in the supercell and supercell size. Therefore, huge computational resources are usually needed for the study of interfaces, mostly in the presence of defects.
Figure 2: A graphene nanoribbon covalently immobilized on a metallic gold substrate [5].
The results shown in the present report have been obtained with the use of high per- formance computing facilities, in particular the CRESCO3 cluster located at the Portici ENEA center. Typical jobs require 64 to 256 computing units and several tens Gb of disk storage (for electronic wave functions and charge density, needed both during the optimization tasks and for post processing purposes). Parallelization is implemented in the used package through the MPI environment.
3
Results
3.1 The SrTiO3–TiO2 interface
One of the most important properties that characterize an interface is the band offset, that is, the relative position of the energy levels on the two sides of the interface. In particular, the valence band offset, VBO (the conduction band offset, CBO) can be defined as the difference between the positions of the top valence bands (bottom conduction bands) of the two materials.
X-ray photoelectron spectroscopy has revealed a VBO for the SrTiO3–TiO2interface rang- ing from -0.06 eV to +0.16 eV, even though theoretical predictions (∼ 0.5 eV) largely overestimate such experimental outcomes [7, 8].
The theoretical simulations of this interface, schematically shown in Fig. 1, show that the barrier heights at the interface are lowered by the presence of oxygen vacancies in the near interface region, indicating a possible explanation of the near zero band offsets that have been experimentally observed. Profound modification of the interface electronic structure are induced in the presence of a defected (non stoichiometric) interface, giving indication of a possible tool to tailor the interface properties [9].
3.2 GNRs covalently immobilized on a metallic substrate
Recent experiments have made available the possibility of immobilizing graphene and related nanostructure onto either metallic or semiconducting substrates, using an organic layer (thiol-functionalized per-fuorophenyl azide, PFPA) as a coupling agent [10, 11]. A schematic view of the hybrid organic-inorganic heterostructure is shown in Fig. 2. Large-scale ab initio calculations have allowed to address two fundamental issues, that is, how and to which extent the presence of both the substrate and the covalent functional- ization of the GNR modifies the device electronic properties [12, 13]. In particular, it has been shown that: i) the presence of a significant GNR band energy gap is not altered by the presence of the metal–PFPA and PFPA–GNR interfaces, ii) a substantial electrical decoupling between the metal and GNR electronic states shows up, iii) no doping effect (charge transfer from the metal to the GNR) takes place. The results suggest a route for the the engineering and nanopatterning of GNR-based nanodevices.
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