Mikael Kepenekian

Energetics and stability of dangling-bond silicon wires on H passivated Si(100)

We evaluate the electronic, geometric and energetic properties of quasi 1D wires formed by dangling-bonds on Si(100)-H(2 × 1). The calculations are performed with density functional theory (DFT). Infinite wires are found to be insulating and Peierls distorted, however finite wires develop localized electronic states that can be of great use for atomic scale devices. The ground state solution of finite wires does not correspond to a geometrical distortion but rather to an antiferromagnetic ordering. For the stability of wires, the presence of abundant H atoms in nearby Si atoms can be a problem. We have evaluated the energy barriers for intradimer and intrarow diffusion, finding all of them about 1 eV or larger, even in the case where a H impurity is already sitting on the wire. These results are encouraging for using dangling-bond wires in future devices.

Electron transport through dangling-bond silicon wires on H-passivated Si(100)

We compute the electron transmission through different types of dangling-bond wire on Si(100)-H (2 × 1). Recent progress in the construction of atomic-size interconnects (Weber et al 2012 Science 335 64) shows the possibility to achieve atomic-size circuits via atomic-size wires using silicon surfaces. Hence, electron transport through quasi-1D Si-based structures is a compelling reality. Prior to these achievements, wires formed by controlled desorption of passivating H atoms off the monohydride Si(100) surface have been shown to be subject to 1D correlations and instabilities (Hitosugi et al 1999 Phys. Rev. Lett. 82 4034). The present calculations are based on density functional theory and evaluate the electron transmission though the minimum-energy 1D structures that can be formed when creating dangling-bonds on Si(100)-(2 × 1)-H. The purpose of this study is twofold: (i) to assess the transport properties of these atomic-size wires in the presence of 1D instabilities; (ii) to provide a fingerprint for experimental identification of the instability through the transport characteristics of the wires. To these aims, we evaluate the electron transport through the wires in the absence of instabilities, in the presence of distortions (Jahn-Teller instabilities) and in the presence of magnetic instabilities (ferro- and antiferro-ordering). We find that instabilities substantially reduce the transport capabilities of dangling-bond wires leading to transmissions that vary so differently with electron energy that an unambiguous identification of the wire type should be accessible in transport experiments.

Scaling law for excitons in 2D perovskite quantum wells

Ruddlesden–Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A’n-1MnX3n+1, where optoelectronic properties can be tuned by varying the perovskite layer thickness (n-value), and have recently emerged as efficient semiconductors with technologically relevant stability. However, fundamental questions concerning the nature of optical resonances (excitons or free carriers) and the exciton reduced mass, and their scaling with quantum well thickness, which are critical for designing efficient optoelectronic devices, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modeling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with both exciton reduced masses and binding energies decreasing, respectively, from 0.221 m0 to 0.186 m0 and from 470 meV to 125 meV with increasing thickness from n equals 1 to 5. Based on this study we propose a general scaling law to determine the binding energy of excitons in perovskite quantum wells of any layer thickness.Hybrid 2D layered perovskites are solution-processed quantum wells whose optoelectronic properties are tunable by varying the thickness of the inorganic slab. Here Blancon et al. work out a general behavior for dependence of the excitonic properties in layered 2D perovskites.

First-Principles Simulations of Electronic Transport in Dangling-Bond Wires

It has recently become possible to calculate at an ab initio level electronic transport in atomic and molecular systems connected to semi-infinite electrodes under an applied bias. In this chapter, we show how electronic structure calculations based on the density functional theory (DFT), followed by the use of nonequilibrium Green’s functions (NEGF), allow one to simulate the electronic transport in various systems. This method is apply to the problem of electronic transport in dangling-bond wires built on the Si(100) surface.

Concept of Lattice Mismatch and Emergence of Surface States in Two-dimensional Hybrid Perovskite Quantum Wells

Surface states are ubiquitous to semiconductors and significantly impact the physical properties and, consequently, the performance of optoelectronic devices. Moreover, surface effects are strongly amplified in lower dimensional systems such as quantum wells and nanostructures. Layered halide perovskites (LHPs) are two-dimensional solution-processed natural quantum wells where optoelectronic properties can be tuned by varying the perovskite layer thickness n, i.e., the number of octahedra spanning the layer. They are efficient semiconductors with technologically relevant stability. Here, a generic elastic model and electronic structure modeling are applied to LHPs heterostructures with various layer thickness. We show that the relaxation of the interface strain is triggered by perovskite layers above a critical thickness. This leads to the release of the mechanical energy arising from the lattice mismatch, which nucleates the surface reorganization and may potentially induce the formation of previously observed lower energy edge states. These states, which are absent in three-dimensional perovskites are anticipated to play a crucial role in the design of LHPs for optoelectronic systems.

Two-Dimensional Halide Perovskites Incorporating Straight Chain Symmetric Diammonium Ions, (NH3CmH2mNH3)(CH3NH3)n−1PbnI3n+1 (m = 4–9; n = 1–4)

Low-dimensional halide perovskites have recently attracted intense interest as alternatives to the three-dimensional (3D) perovskites because of their greater tunability and higher environmental stability. Herein, we present the new homologous 2D series (NH3C mH2 mNH3)(CH3NH3) n-1Pb nI3 n+1 ( m = 4-9; n = 1-4), where m represents the carbon-chain number and n equals layer-thickness number. Multilayer ( n > 1) 2D perovskites incorporating diammonium cations were successfully synthesized by the solid-state grinding method for m = 4 and 6 and by the solution method for m = 7-9. Structural characterization by single-crystal X-ray diffraction for the m = 8 and m = 9 series ( n = 1-4) reveals that these compounds adopt the Cc space group for even n members and Pc for odd n members. The optical bandgaps are 2.15 eV for two-layer ( n = 2), 2.01 eV for three-layer ( n = 3), and 1.90 eV for four-layer ( n = 4). The materials exhibit excellent solution processability, and casting thin-films of the n = 3 members was successfully accomplished. The films show a clear tendency for the higher- m members to have preferred orientation on the glass substrate, with m = 8 exhibiting almost perfect vertical layer orientation and m = 9 displaying both vertical and parallel layer orientation, as confirmed by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The vertical layer orientation for the (NH3C8H16NH3)(CH3NH3)2Pb3I10 member results in the best thermal, light, and air stability within this series, thus showing excellent potential for solar cell applications.

Theoretical studies of Rashba and Dresselhaus effects in hybrid organic-inorganic perovskites for optoelectronic applications

In this paper, we propose a description of the Rashba-Dresselhaus effect in Hybrid Organic Perovskite (HOP). We show how the loss of the inversion symmetry leads to the loss of the spin degeneracy. An example of structure where both Rashba and Dresselhaus operate is illustrated with the formamidinium tin iodide CH(NH2)2SnI3. The control of this effect is as well addressed by two examples. A first example concerns the control with the temperature and is demonstrated for the 2D HOP Bz2PbCl4 (Bz = benzylammonium). Then the control with an external field is established for the 3D HOP CH3NH3PbBr3.

Dielectric properties of hybrid perovskites and drift-diffusion modeling of perovskite cells

A method based on DFT is used to obtained dielectric profiles. The high frequency Ɛ∞(z) and the static Ɛs(z) dielectric profiles are compared for 3D, 2D-3D and 2D Hybrid Organic Perovskites (HOP). A dielectric confinement is observed for the 2D materials between the high dielectric constant of the inorganic part and the low dielectric constant of the organic part. The effect of the ionic contribution on the dielectric constant is also shown. The quantum and dielectric confinements of 3D HOP nanoplatelets are then reported. Finally, a numerical simulation based on the SILVACO code of a HOP based solar cell is proposed for various permittivity of MAPbI3.

Simulations of Constant Current STM Images of Open-Shell Systems

In this chapter we review the main methods for simulating STM images, mainly the Tersoff–Hamman approximation based on the Bardeen approach and full-fledge Landauer calculations based on nonequilibrium Green’s functions (NEGF). However, these methods are built on the electronic structure of the system as computed by density functional theory (DFT). This theory has important limitation for open-shell systems. As a matter of fact, an open-shell molecule on a metallic substrate can lead to correlations among the electrons of the metal. This gives rise to the Kondo effect. Hence, we briefly present a method to implement STM image simulations including the Kondo effect.