This study concerns atomistic and coarse-grained Molecular Dynamics simulations of pristine hexab... more This study concerns atomistic and coarse-grained Molecular Dynamics simulations of pristine hexabenzocoronene (HBC) molecular crystals. HBC is a symmetric graphene flake of nanometric size that falls in the category of polyaromatic hydrocarbons, finding numerous applications in the field of organic electronics. The HBC molecule is simulated in its crystalline phase initially by means of an all-atom representation, where the molecules self-organize into well aligned molecular stacks, which in turn create a perfect monoclinic molecular crystal. The atomistic model reproduces fairly well the structural experimental properties and thus can be used as a reliable starting point for the development of a coarse-grained model following a bottom-up approach. The coarse-grained model is developed by applying Iterative Boltzmann Inversion, a systematic coarse-graining method which reproduces a set of target atomistic radial distribution functions and intramolecular distributions at the coarser level of description. This model allows the simulation of HBC crystals over longer time and length scales. The crystalline phase is analyzed in terms of the Saupe tensor and thermomechanical properties are probed at the atomistic level.
This study concerns atomistic and coarse-grained molecular dynamics simulations of two disk-shape... more This study concerns atomistic and coarse-grained molecular dynamics simulations of two disk-shaped molecules in the crystalline phase. The coarse-grained models were developed by applying the Iterative Boltzmann Inversion method which is a systematic coarse-graining method. In particular, a set of radial distribution functions and intramolecular distributions are reproduced at the coarse-grained level. Before applying coarse-graining, a reliable atomistic model was developed to reproduce the main experimental properties of these molecules. The crystalline phases are analyzed in terms of the Saupe ordering tensor.
The structural and dynamical properties of molecular wires comprised of nanographene molecules wi... more The structural and dynamical properties of molecular wires comprised of nanographene molecules with linear aliphatic chains grafted around their periphery are studied via empirical force field-based Molecular Dynamics simulations. A detailed analysis of the internal structure of the wires is carried out, focusing on the stacking patterns of the molecules and the way side chains protrude from each molecular pillar and occupy the surrounding space. Coupled with dynamical studies that quantify the molecular mobility, this analysis helps draw a unified picture of the temporal behavior of the molecular wires.
Discotic polyaromatic molecules, similar to nanometric graphene flakes, constitute an interesting... more Discotic polyaromatic molecules, similar to nanometric graphene flakes, constitute an interesting class of materials for organic electronic applications. Grafting flexible side chains around the periphery of such molecules enhances their processability and gives rise to diverse behaviours, such as the manifestation of liquid-crystalline character and anisotropic mechanical response. In this work, we examine by means of molecular dynamics simulations the properties of molecular crystals comprised of alkyl-substituted hexa-peri-hexabenzocoronene mesogens. Pristine and mono-substituted systems by hydrogen or iodine atoms are modelled, with variable side chain length. A general structural and mechanical robustness to peripheral substitution is reported, with the mesogens forming tightly packed molecular wires even at elevated temperature and pressure. In their discotic ordering, the molecules present relatively low translational mobility, a beneficial phenomenon for charge transport. A thermotropic dependence of the mechanical response is identified, with the systems behaving differently in their room-temperature crystalline phase and in their liquid-crystalline phase at elevated temperatures. The melting process is also examined, elucidating an initial negative expansion along a high symmetry direction and the existence of a metastable state, before falling into the final liquid-crystalline state.
In this work, we examine the vibrational and mechanical properties of clamped-clamped rectangular... more In this work, we examine the vibrational and mechanical properties of clamped-clamped rectangular SixGe1−x and Si/SixGe1−x nanowires (NWs) using molecular dynamics simulations. A virtual atomic force microscope nanotip is used to drive the vibration. The frequency response, the beat vibration phenomenon and the calculation of mechanical properties such as quality factor Q and Young's modulus E are thoroughly analyzed. The influence of added mass and hydrogen passivation on the NW resonator performance is also demonstrated. The decreasing frequency trend with increasing Ge concentration was different for binary alloys and alloy superlattices, while it remained unaffected by the superlattice period. The beat vibration phenomenon driven by a single excitation was observed at elevated temperatures for all studied configurations. The resonance frequency decreases linearly with increasing temperature whereas the Q-factor follows a power law decrease. The Young's modulus E obtained through stress–strain computational experiments is found to be overestimated compared to the classical beam theory. Frequency decreases linearly as more atoms are added to the resonator. For low temperatures, the quality factor of composite Si/Ge resonators increased from 104 to 105 at T = 23 K, after performing hydrogen passivation on the NW free surfaces. The frequency shift due to the change in stoichiometry in a Si NW, alongside with representative snapshots of the vibrational behavior.In this work, we examine the vibrational and mechanical properties of clamped-clamped rectangular SixGe1−x and Si/SixGe1−x nanowires (NWs) using molecular dynamics simulations. A virtual atomic force microscope nanotip is used to drive the vibration. The frequency response, the beat vibration phenomenon and the calculation of mechanical properties such as quality factor Q and Young's modulus E are thoroughly analyzed. The influence of added mass and hydrogen passivation on the NW resonator performance is also demonstrated. The decreasing frequency trend with increasing Ge concentration was different for binary alloys and alloy superlattices, while it remained unaffected by the superlattice period. The beat vibration phenomenon driven by a single excitation was observed at elevated temperatures for all studied configurations. The resonance frequency decreases linearly with increasing temperature whereas the Q-factor follows a power law decrease. The Young's modulus E obtained through stress–strain computational experiments is found to be overestimated compared to the classical beam theory. Frequency decreases linearly as more atoms are added to the resonator. For low temperatures, the quality factor of composite Si/Ge resonators increased from 104 to 105 at T = 23 K, after performing hydrogen passivation on the NW free surfaces. The frequency shift due to the change in stoichiometry in a Si NW, alongside with representative snapshots of the vibrational behavior.
Germanane (GeH) and silicane (SiH) are the fully hydrogenated forms of germanene and silicene, th... more Germanane (GeH) and silicane (SiH) are the fully hydrogenated forms of germanene and silicene, the Ge- and Si-analogues of graphene. Here we use density-functional theory calculations to probe the properties of GeH and SiH sheets and their dependence on applied uni-axial compression. We find that GeH polymorphs with distinct hydrogen arrangements have markedly different energy band gaps. We also show that, when compressed, GeH forms superstructures with parts in low- and wide-gap geometries, enabling the creation of alternating polymorph nano-ribbons. An alternative to superstructure formation is the adoption by GeH of a corrugated form with extreme bending. Silicane shows similar behavior under compression, with either high corrugation, or successive parts with different geometries. Finally, we demonstrate that interaction with a substrate can influence the relative stability of GeH overlayer polymorphs. Overall, the results reveal ways to enhance the functionalities of these two-dimensional materials through the formation of superstructures with sizeable quantum well effects or outstanding mechanical response.
Journal of physics. Condensed matter : an Institute of Physics journal, 2013
Hydrogenated graphene, also known as graphane, is a wide band gap semiconductor in its various po... more Hydrogenated graphene, also known as graphane, is a wide band gap semiconductor in its various possible conformations. Here we show with first-principles calculations that application of uniaxial pressure can give rise to stripes of the so-called washboard conformation between segments of the most stable chair geometry. Compressed washboard-chair superlattices follow distinct zigzag patterns with interchanging chair and washboard parts. Hybrid systems of this type could be employed in various applications, such as in the templated growth of nanosystems, or as parts of nanomechanical systems.
This study concerns atomistic and coarse-grained Molecular Dynamics simulations of pristine hexab... more This study concerns atomistic and coarse-grained Molecular Dynamics simulations of pristine hexabenzocoronene (HBC) molecular crystals. HBC is a symmetric graphene flake of nanometric size that falls in the category of polyaromatic hydrocarbons, finding numerous applications in the field of organic electronics. The HBC molecule is simulated in its crystalline phase initially by means of an all-atom representation, where the molecules self-organize into well aligned molecular stacks, which in turn create a perfect monoclinic molecular crystal. The atomistic model reproduces fairly well the structural experimental properties and thus can be used as a reliable starting point for the development of a coarse-grained model following a bottom-up approach. The coarse-grained model is developed by applying Iterative Boltzmann Inversion, a systematic coarse-graining method which reproduces a set of target atomistic radial distribution functions and intramolecular distributions at the coarser level of description. This model allows the simulation of HBC crystals over longer time and length scales. The crystalline phase is analyzed in terms of the Saupe tensor and thermomechanical properties are probed at the atomistic level.
This study concerns atomistic and coarse-grained molecular dynamics simulations of two disk-shape... more This study concerns atomistic and coarse-grained molecular dynamics simulations of two disk-shaped molecules in the crystalline phase. The coarse-grained models were developed by applying the Iterative Boltzmann Inversion method which is a systematic coarse-graining method. In particular, a set of radial distribution functions and intramolecular distributions are reproduced at the coarse-grained level. Before applying coarse-graining, a reliable atomistic model was developed to reproduce the main experimental properties of these molecules. The crystalline phases are analyzed in terms of the Saupe ordering tensor.
The structural and dynamical properties of molecular wires comprised of nanographene molecules wi... more The structural and dynamical properties of molecular wires comprised of nanographene molecules with linear aliphatic chains grafted around their periphery are studied via empirical force field-based Molecular Dynamics simulations. A detailed analysis of the internal structure of the wires is carried out, focusing on the stacking patterns of the molecules and the way side chains protrude from each molecular pillar and occupy the surrounding space. Coupled with dynamical studies that quantify the molecular mobility, this analysis helps draw a unified picture of the temporal behavior of the molecular wires.
Discotic polyaromatic molecules, similar to nanometric graphene flakes, constitute an interesting... more Discotic polyaromatic molecules, similar to nanometric graphene flakes, constitute an interesting class of materials for organic electronic applications. Grafting flexible side chains around the periphery of such molecules enhances their processability and gives rise to diverse behaviours, such as the manifestation of liquid-crystalline character and anisotropic mechanical response. In this work, we examine by means of molecular dynamics simulations the properties of molecular crystals comprised of alkyl-substituted hexa-peri-hexabenzocoronene mesogens. Pristine and mono-substituted systems by hydrogen or iodine atoms are modelled, with variable side chain length. A general structural and mechanical robustness to peripheral substitution is reported, with the mesogens forming tightly packed molecular wires even at elevated temperature and pressure. In their discotic ordering, the molecules present relatively low translational mobility, a beneficial phenomenon for charge transport. A thermotropic dependence of the mechanical response is identified, with the systems behaving differently in their room-temperature crystalline phase and in their liquid-crystalline phase at elevated temperatures. The melting process is also examined, elucidating an initial negative expansion along a high symmetry direction and the existence of a metastable state, before falling into the final liquid-crystalline state.
In this work, we examine the vibrational and mechanical properties of clamped-clamped rectangular... more In this work, we examine the vibrational and mechanical properties of clamped-clamped rectangular SixGe1−x and Si/SixGe1−x nanowires (NWs) using molecular dynamics simulations. A virtual atomic force microscope nanotip is used to drive the vibration. The frequency response, the beat vibration phenomenon and the calculation of mechanical properties such as quality factor Q and Young's modulus E are thoroughly analyzed. The influence of added mass and hydrogen passivation on the NW resonator performance is also demonstrated. The decreasing frequency trend with increasing Ge concentration was different for binary alloys and alloy superlattices, while it remained unaffected by the superlattice period. The beat vibration phenomenon driven by a single excitation was observed at elevated temperatures for all studied configurations. The resonance frequency decreases linearly with increasing temperature whereas the Q-factor follows a power law decrease. The Young's modulus E obtained through stress–strain computational experiments is found to be overestimated compared to the classical beam theory. Frequency decreases linearly as more atoms are added to the resonator. For low temperatures, the quality factor of composite Si/Ge resonators increased from 104 to 105 at T = 23 K, after performing hydrogen passivation on the NW free surfaces. The frequency shift due to the change in stoichiometry in a Si NW, alongside with representative snapshots of the vibrational behavior.In this work, we examine the vibrational and mechanical properties of clamped-clamped rectangular SixGe1−x and Si/SixGe1−x nanowires (NWs) using molecular dynamics simulations. A virtual atomic force microscope nanotip is used to drive the vibration. The frequency response, the beat vibration phenomenon and the calculation of mechanical properties such as quality factor Q and Young's modulus E are thoroughly analyzed. The influence of added mass and hydrogen passivation on the NW resonator performance is also demonstrated. The decreasing frequency trend with increasing Ge concentration was different for binary alloys and alloy superlattices, while it remained unaffected by the superlattice period. The beat vibration phenomenon driven by a single excitation was observed at elevated temperatures for all studied configurations. The resonance frequency decreases linearly with increasing temperature whereas the Q-factor follows a power law decrease. The Young's modulus E obtained through stress–strain computational experiments is found to be overestimated compared to the classical beam theory. Frequency decreases linearly as more atoms are added to the resonator. For low temperatures, the quality factor of composite Si/Ge resonators increased from 104 to 105 at T = 23 K, after performing hydrogen passivation on the NW free surfaces. The frequency shift due to the change in stoichiometry in a Si NW, alongside with representative snapshots of the vibrational behavior.
Germanane (GeH) and silicane (SiH) are the fully hydrogenated forms of germanene and silicene, th... more Germanane (GeH) and silicane (SiH) are the fully hydrogenated forms of germanene and silicene, the Ge- and Si-analogues of graphene. Here we use density-functional theory calculations to probe the properties of GeH and SiH sheets and their dependence on applied uni-axial compression. We find that GeH polymorphs with distinct hydrogen arrangements have markedly different energy band gaps. We also show that, when compressed, GeH forms superstructures with parts in low- and wide-gap geometries, enabling the creation of alternating polymorph nano-ribbons. An alternative to superstructure formation is the adoption by GeH of a corrugated form with extreme bending. Silicane shows similar behavior under compression, with either high corrugation, or successive parts with different geometries. Finally, we demonstrate that interaction with a substrate can influence the relative stability of GeH overlayer polymorphs. Overall, the results reveal ways to enhance the functionalities of these two-dimensional materials through the formation of superstructures with sizeable quantum well effects or outstanding mechanical response.
Journal of physics. Condensed matter : an Institute of Physics journal, 2013
Hydrogenated graphene, also known as graphane, is a wide band gap semiconductor in its various po... more Hydrogenated graphene, also known as graphane, is a wide band gap semiconductor in its various possible conformations. Here we show with first-principles calculations that application of uniaxial pressure can give rise to stripes of the so-called washboard conformation between segments of the most stable chair geometry. Compressed washboard-chair superlattices follow distinct zigzag patterns with interchanging chair and washboard parts. Hybrid systems of this type could be employed in various applications, such as in the templated growth of nanosystems, or as parts of nanomechanical systems.
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Papers by Orestis Ziogos