Younes Makoudi

Faceting, composition and crystal phase evolution in III-V antimonide nanowire heterostructures revealed by combining microscopy techniques

III-V antimonide nanowires are among the most interesting semiconductors for transport physics, nanoelectronics and long-wavelength optoelectronic devices due to their optimal material properties. In order to investigate their complex crystal structure evolution, faceting and composition, we report a combined scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM) study of gold-nucleated ternary InAs/InAs(1-x)Sb(x) nanowire heterostructures grown by molecular beam epitaxy. SEM showed the general morphology and faceting, TEM revealed the internal crystal structure and ternary compositions, while STM was successfully applied to characterize the oxide-free nanowire sidewalls, in terms of nanofaceting morphology, atomic structure and surface composition. The complementary use of these techniques allows for correlation of the morphological and structural properties of the nanowires with the amount of Sb incorporated during growth. The addition of even a minute amount of Sb to InAs changes the crystal structure from perfect wurtzite to perfect zinc blende, via intermediate stacking fault and pseudo-periodic twinning regimes. Moreover, the addition of Sb during the axial growth of InAs/InAs(1-x)Sb(x) heterostructure nanowires causes a significant conformal lateral overgrowth on both segments, leading to the spontaneous formation of a core-shell structure, with an Sb-rich shell.

Complete Supramolecular Self-Assembled Adlayer on a Silicon Surface at Room Temperature

The engineering of a complete adlayer of organic nanolines by supramolecular self-assembly has been achieved for the first time on a silicon-based surface at room temperature and has been studied by scanning tunneling microscopy. This complete adlayer has been successfully obtained thanks to the combination of a specific Si(111)-B square root 3x square root 3R30 degrees semiconductive surface and of strong hydrogen bonds between a pair of dipolar molecules.

Persistent enhancement of the carrier density in electron irradiated InAs nanowires

We report a significant and persistent enhancement of the conductivity in free-standing non-intentionally doped InAs nanowires upon irradiation in ultra-high vacuum. Combining four-point probe transport measurements performed on nanowires with different surface chemistries, field effect based measurements and numerical simulations of the electron density, the change in the conductivity is found to be caused by an increase in the surface free carrier concentration. Although an electron beam of a few keV, typically used for the inspection and the processing of materials, propagates through the entire nanowire cross-section, we demonstrate that the electrical properties of the nanowire are predominantly affected by radiation-induced defects occurring at the nanowire surface and not in the bulk.

Room‐Temperature Electronic Template Effect of the SmSi(111)‐8×2 Interface for Self‐Alignment of Organic Molecules

This work describes an innovative concept for the development of organized molecular systems based on the template effect of the pre-structured semi-conductive SmSi(111) interface. This substrate is selected because Sm deposition in the submonolayer range leads to a 8x2-reconstruction, which is a well-defined one-dimensional semi-metallic structure. Adsorption of aromatic molecules [1,4-di-(9-ethynyltriptycenyl)-benzene] on SmSi(111)- 8x2 and Si(111)-7x7 interfaces is investigated by scanning tunneling microscopy (STM) at room temperature. Density functional theory (DFT) and semi-empirical (ASED+) calculations define the nature of the molecular adsorption sites of the target molecule on SmSi as well as their self-alignment on this interface. Experimental data and theoretical results are in good agreement.

Large-Scale Patterning of Zwitterionic Molecules on a Si(111)-7 7 Surface

The formation of a large scale pattern on Si(111)-7 × 7 reconstruction is still a challenge. We report herein a new solution to achieve this type of nanostructuration by using of zwitterionic molecules. The formation of a large-scale pattern is successfully obtained due to the perfect match between the molecular geometry and the surface topology and to electrostatic interactions between molecules and surface. The adsorption is described by high-resolution scanning tunneling microscopy (STM) images and supported by density functional theory and STM calculations.

STM and DFT Investigations of Isolated Porphyrin on a Silicon‐Based Semiconductor at Room Temperature

Metalloporphyrins represent a class of flexible molecules with a nearly square planar core conformation and a two dimensional conjugated p-electron delocalization. Due to their interesting physicochemical properties, metalloporphyrins adsorbed on a surface can be used in many technological applications such as molecular electronics, light-harvesting arrays for solar energy generation, catalysts, sensors, etc. The fine determination of the conformation and arrangement of adsorbed molecules on a surface are key points, since they are strongly related to the physical and chemical properties of the final organic–inorganic interfaces. They are changed by the subtle balance of internal deformation and substrate–molecule interactions, leading to a conformational adaptation of the molecule on the substrate lattice. These features are even more relevant in semiconductors than in metals because the moleculesemiconductor interactions are usually greater than molecule– metal interactions. Although scanning tunneling microscopy (STM) is a remarkable tool to investigate individual adsorbed molecules on semiconductors, experimental STM images of metalloporphyrins were achieved only on metals and only an unique very recent article investigates theoretically the adsorption of a metalloporphyrin on a Si(111)-H surface. Herein, we report the first experimental investigation at room temperature of the adsorption of Cu-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl) porphyrin (Cu-TBPP) as a model of metalloporphyrin on a passivated silicon based surface (Si(111)-B) using STM and by theoretical calculations in order to fully understand the conformational adaptation of the Cu-TBPP on a Si(111)-B surface.

Extended monolayer of cyano-ended oligo(para-phenylenes) at the air/HOPG interface investigated by high-resolution AFM.

The formation of functional networks on surfaces is one of the main challenges in the field of nanotechnologies. In this paper, we shall propose a very simple process which can be used to achieve the formation of extended monolayer of functional oligo(para-phenylenes) molecules at the air/graphite interface. By developing a convergent strategy, we successfully achieved the synthesis of oligo(para-phenylenes) molecules with a tuneable length. The photophysical properties of these new oligomers were characterized by UV-vis absorption and fluorescence spectroscopy. Deposition of these molecules by a simple spin-coating process on a highly oriented pyrolytic graphite (HOPG) surface leads to the formation of extended monolayered 2D networks. These networks were characterized by atomic force microscopy experiments under ambient conditions with submolecular resolution thus providing the adsorption model of these molecules on an HOPG surface.

Single Molecular Machines on Semiconductor Surfaces

The development of nanomachines is a key challenge for the future electronics, energy conversion, biology, and medicine. Semiconductor surfaces have been one of the basic elements of many technologies for 40 years. However, despite their promising interest, semiconductor-based nanomachines are almost unstudied. In this work, a panel of single nanomachines-based semiconductor surfaces is described. The role of molecule–surface interaction for the development of nanomachines is highlighted.

Double Decoration of a Si(111)-7 x 7 Surface at Room Temperature by Chemisorption of an Organic Salt

This work describes the chemisorption of an organic salt, which is constituted by an iodide anion and a pyridinium-based cation, on the Si(111)-7 × 7 surface at room temperature. The adsorption of both iodide anion and organic cation has been justified by the study of the STM images polarity dependency. The regioselectivity of the chemisorptive process is explained by the electrostatic interactions between substrate and ionic species.