Due to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily s... more Due to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily sheared along the basal planes. Despite a growing attention towards their use as solid lubricants, so far no head-to-head comparison has been carried out. By means of ab initio modeling of a bilayer sliding motion, we show that graphene is characterized by a shallower potential energy landscape while more similarities are attained when considering the sliding forces; we propose that the calculated interfacial ideal shear strengths afford the most accurate information on the intrinsic sliding capability of layered materials. We also investigate the effect of an applied uniaxial load: in graphene, this introduces a limited increase in the sliding barrier while in TMDs it has a substantially different impact on the possible polytypes. The polytype presenting a parallel orientation of the layers (R0) bears more similarities to graphene while that with antiparallel orientation (R180) shows deep changes in the potential energy landscape and consequently a sharper increase of its sliding barrier.
The many-body state of carriers confined in a quantum dot is controlled by the balance between th... more The many-body state of carriers confined in a quantum dot is controlled by the balance between their kinetic energy and their Coulomb correlation. In coupled quantum dots, both can be tuned by varying the inter-dot tunneling and interactions. Using a theoretical approach based on the diagonalization of the exact Hamiltonian, we show that transitions between different quantum phases can be induced through inter-dot coupling both for a system of few electrons (or holes) and for aggregates of electrons and holes. We discuss their manifestations in addition energy spectra (accessible through capacitance or transport experiments) and optical spectra.
ABSTRACT We studied the formation of graphene nanoribbons (GNRs) via the self-assembly of 10,10′-... more ABSTRACT We studied the formation of graphene nanoribbons (GNRs) via the self-assembly of 10,10′-dibromo-9,9′-bianthryl precursor molecules on gold surfaces with different synchrotron spectroscopies. Through X-ray photoemission spectroscopy core-level shifts, we followed each step of the synthetic process, and could show that the Br–C bonds of the precursors cleave at temperatures as low as 100 °C on both Au(111) and Au(110). We established that the resulting radicals bind to Au, forming Au–C and Au–Br bonds. We show that the polymerization of the precursors follows Br desorption from Au, suggesting that the presence of halogens is the limiting factor in this step. Finally, with angle-resolved ultraviolet photoemission spectroscopy and density functional theory we show that the GNR/Au interaction results in an upshift of the Shockley surface state of Au(111) by 0.14 eV, together with an increased electron effective mass.
We present a calculation of the electron optical-phonon scattering rates in GaAs/AlAs quantum wel... more We present a calculation of the electron optical-phonon scattering rates in GaAs/AlAs quantum wells, based on a very accurate microscopic description of the phonon spectra. The results show that-besides the contribution of confined modes-a very large contribution originates from interface phonons of both GaAs-like and AlAs-like character. We then compare our results for phonon displacements and potentials, as well as for scattering rates, with those obtained from several macroscopic phonon models. We are therefore able to provide indications for selecting the model which allows the most appropriate simplified description of vibrations at the wavevectors relevant to the interaction with carriers. I. Introduction The electron-optical-phonon (e-ph) interaction plays a crucial role in the transport properties of two dimensional (2D) semiconductor systems. The cooling rate of hot carriers in quantum wells (QW's) and superlattices (SL's), as well as the room temperature mobilities of modulation doping structures are determined by the strength of the electron coupling to the optical vibrations. With respect to the three dimensional bulk case, the nature of the e-ph interaction in 2D systems is drastically modified by the presence of the heteroin-terfaces between the different materials. Despite the growing interest in the field, and the availability of ultrafast spectroscopies providing direct information on the peculiarity of such processes, from the theoretical point of view a long-standing controversy is still open on the effect of the reduced dimensionality on the eph interaction'. The discussion, concerning specifically the correct description of the phonon modes which are relevant to the electronic scattering, comes in spite of the large amount of information existing on the vibrational properties of SL's2, and points out the little exchange that has occurred sofar between the researchers
The optical excitation of organic semiconductors not only generates charge-neutral electronhole p... more The optical excitation of organic semiconductors not only generates charge-neutral electronhole pairs (excitons), but also charge-separated polaron pairs with high yield. The microscopic mechanisms underlying this charge separation have been debated for many years. Here we use ultrafast two-dimensional electronic spectroscopy to study the dynamics of polaron pair formation in a prototypical polymer thin film on a sub-20-fs time scale. We observe multi-period peak oscillations persisting for up to about 1 ps as distinct signatures of vibronic quantum coherence at room temperature. The measured two-dimensional spectra show pronounced peak splittings revealing that the elementary optical excitations of this polymer are hybridized exciton-polaron-pairs, strongly coupled to a dominant underdamped vibrational mode. Coherent vibronic coupling induces ultrafast polaron pair formation, accelerates the charge separation dynamics and makes it insensitive to disorder. These findings open up new perspectives for tailoring light-to-current conversion in organic materials.
<pre>Source custom Fortran code an supporting data for "Evidence of ideal excitonic in... more <pre>Source custom Fortran code an supporting data for "Evidence of ideal excitonic insulator in bulk MoS2 under pressure", available as a preprint at <strong>arXiv:2011.02380</strong> </pre> <pre>CUSTOME FORTRAN CODE 'MoS2_continuation' (file 'MoS2_vs_P.f') MAIN AIM: To solve the self-consistent gap equation (3) of main text, within the two-band approximation. ADDITIONAL FILES: example Makefile and log files. SUPPORTING DATA: Crystal structure coordinates of phases 2H_a and 2H_c of bulk MoS_2.</pre>
We propose a theoretical approach to the general problem of phonon-coupling between localised ele... more We propose a theoretical approach to the general problem of phonon-coupling between localised electronic states in semiconductors, which takes advantage of results from first-principles calculations. Application of this scheme to the isolated antisite defect AsQ, in GaAs reveals that electron-phonon coupling can provide the mechanism for the metastable transition characteristic of EL2.
ABSTRACT We have investigated the effect of non-equilibrium phonons on carrier relaxation dynamic... more ABSTRACT We have investigated the effect of non-equilibrium phonons on carrier relaxation dynamics in a quantum wire following ultrafast photoexcitation. We show that phonon build-up produces a considerable reduction of the cooling rate of photoexcited carriers for densities of the order of 106 cm-1. In this respect, the results for quantum wires are found to be similar to the bulk case. An important consequence of the reduced dimensionality of wires is found in the non-equilibrium phonon distribution, which is populated even at very small wave vectors.
Using first-principles calculations we studied the electric field enhancement in polyacene molecu... more Using first-principles calculations we studied the electric field enhancement in polyacene molecules upon illumination. These molecules can be seen as a specific class of C-based (i.e., graphene-derived) nanostructures, recently proposed as alternative materials for plasmonics. We demonstrate that optical transitions may generate oscillating dipolar response charge, giving rise to an induced electric field near the molecule, which thus acts as a plasmon-like nanoantenna. While the field amplification in the vicinity of single acenes is rather small and decreases when the size of the system is increased, it may be selectively enhanced in the case of acene’s assemblies. This paves the way for the design of more complex C-based architectures explicitly conceived to improve the amplification factor.
ABSTRACT Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disul... more ABSTRACT Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale.
The linear and nonlinear optical properties of realistic quantum wires are studied through a theo... more The linear and nonlinear optical properties of realistic quantum wires are studied through a theoretical approach based on a set of generalized semiconductor Bloch equations. Our scheme allows a full threedimensional multisubband description of electron-hole correlation for any confinement profile, thus permitting a direct comparison with experiments for available quantum-wire structures. Our results show that electron-hole Coulomb correlation removes the one-dimensional band-edge singularities from the absorption spectra, whose shape is heavily modified with respect to the ideal free-carrier singlesubband case over the whole density range. [S0031-9007(96)00125-1]
The adsorption of the cysteine amino acid (H-SC H 2-C R H-NH 2-COOH) on the (111) surface of gold... more The adsorption of the cysteine amino acid (H-SC H 2-C R H-NH 2-COOH) on the (111) surface of gold is studied by means of periodic density functional calculations. Results for different adsorption sites and molecular configurations show that chemisorption involving S(thiolate)-Au bonds on Au(111) is favored by starting with either cysteine or cystine gas-phase molecular precursors. In the most stable adsorption configuration, the sulfur headgroup sits at the bridge site between two surface Au atoms, and the S-C bond is tilted by 57°w ith respect to the surface normal, whereas the in-plane orientation of the molecular backbone plays a secondary role. The analysis of the electronic properties shows that the hybridization of the p-like S states with the d-like Au states produces both bonding and antibonding occupied orbitals, and the process is well described by a model for the interaction of localized orbitals with narrow-band dispersive electron states. The bonding orbitals well below the Fermi level contribute to the strong chemisorption of cysteine on gold. The calculated sulfur-projected density of states allows us to locate the cysteine molecular orbitals with respect to the system Fermi level, which gives a measure of the injection barrier at the molecule/electrode junction.
The screening of Coulomb interaction controls many-body physics in carbon nanotubes, as it tunes ... more The screening of Coulomb interaction controls many-body physics in carbon nanotubes, as it tunes the range and strength of the force that acts on charge carriers and binds electron-hole pairs into excitons. In doped tubes, the effective Coulomb interaction drives the competition between Luttinger liquid and Wigner crystal, whereas in undoped narrow-gap tubes it dictates the Mott or excitonic nature of the correlated insulator observed at low temperature. Here, by computing the dielectric function of selected narrow-and zero-gap tubes from first principles, we show that the standard effective-mass model of screening systematically underestimates the interaction strength at long wavelength, hence missing the binding of low-energy excitons. The reason is that the model critically lacks the full three-dimensional topology of the tube, being adapted from graphene theory. As ab inito calculations are limited to small tubes, we develop a two-band model dielectric function based on the plane-wave expansion of Bloch states and the exact truncated Coulomb cutoff technique. We demonstrate that our-computationally cheap-approach provides the correct screening for narrow-gap tubes of any size and chirality. A striking result is that the screened interaction remains long-ranged even in gapless tubes, as an effect of the microscopic local fields generated by the electrons moving on the curved tube surface. As an application, we show that the effective electron-electron force that is felt at distances relevant to quantum transport experiments is super Coulombic.
Due to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily s... more Due to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily sheared along the basal planes. Despite a growing attention towards their use as solid lubricants, so far no head-to-head comparison has been carried out. By means of ab initio modeling of a bilayer sliding motion, we show that graphene is characterized by a shallower potential energy landscape while more similarities are attained when considering the sliding forces; we propose that the calculated interfacial ideal shear strengths afford the most accurate information on the intrinsic sliding capability of layered materials. We also investigate the effect of an applied uniaxial load: in graphene, this introduces a limited increase in the sliding barrier while in TMDs it has a substantially different impact on the possible polytypes. The polytype presenting a parallel orientation of the layers (R0) bears more similarities to graphene while that with antiparallel orientation (R180) shows deep changes in the potential energy landscape and consequently a sharper increase of its sliding barrier.
The many-body state of carriers confined in a quantum dot is controlled by the balance between th... more The many-body state of carriers confined in a quantum dot is controlled by the balance between their kinetic energy and their Coulomb correlation. In coupled quantum dots, both can be tuned by varying the inter-dot tunneling and interactions. Using a theoretical approach based on the diagonalization of the exact Hamiltonian, we show that transitions between different quantum phases can be induced through inter-dot coupling both for a system of few electrons (or holes) and for aggregates of electrons and holes. We discuss their manifestations in addition energy spectra (accessible through capacitance or transport experiments) and optical spectra.
ABSTRACT We studied the formation of graphene nanoribbons (GNRs) via the self-assembly of 10,10′-... more ABSTRACT We studied the formation of graphene nanoribbons (GNRs) via the self-assembly of 10,10′-dibromo-9,9′-bianthryl precursor molecules on gold surfaces with different synchrotron spectroscopies. Through X-ray photoemission spectroscopy core-level shifts, we followed each step of the synthetic process, and could show that the Br–C bonds of the precursors cleave at temperatures as low as 100 °C on both Au(111) and Au(110). We established that the resulting radicals bind to Au, forming Au–C and Au–Br bonds. We show that the polymerization of the precursors follows Br desorption from Au, suggesting that the presence of halogens is the limiting factor in this step. Finally, with angle-resolved ultraviolet photoemission spectroscopy and density functional theory we show that the GNR/Au interaction results in an upshift of the Shockley surface state of Au(111) by 0.14 eV, together with an increased electron effective mass.
We present a calculation of the electron optical-phonon scattering rates in GaAs/AlAs quantum wel... more We present a calculation of the electron optical-phonon scattering rates in GaAs/AlAs quantum wells, based on a very accurate microscopic description of the phonon spectra. The results show that-besides the contribution of confined modes-a very large contribution originates from interface phonons of both GaAs-like and AlAs-like character. We then compare our results for phonon displacements and potentials, as well as for scattering rates, with those obtained from several macroscopic phonon models. We are therefore able to provide indications for selecting the model which allows the most appropriate simplified description of vibrations at the wavevectors relevant to the interaction with carriers. I. Introduction The electron-optical-phonon (e-ph) interaction plays a crucial role in the transport properties of two dimensional (2D) semiconductor systems. The cooling rate of hot carriers in quantum wells (QW's) and superlattices (SL's), as well as the room temperature mobilities of modulation doping structures are determined by the strength of the electron coupling to the optical vibrations. With respect to the three dimensional bulk case, the nature of the e-ph interaction in 2D systems is drastically modified by the presence of the heteroin-terfaces between the different materials. Despite the growing interest in the field, and the availability of ultrafast spectroscopies providing direct information on the peculiarity of such processes, from the theoretical point of view a long-standing controversy is still open on the effect of the reduced dimensionality on the eph interaction'. The discussion, concerning specifically the correct description of the phonon modes which are relevant to the electronic scattering, comes in spite of the large amount of information existing on the vibrational properties of SL's2, and points out the little exchange that has occurred sofar between the researchers
The optical excitation of organic semiconductors not only generates charge-neutral electronhole p... more The optical excitation of organic semiconductors not only generates charge-neutral electronhole pairs (excitons), but also charge-separated polaron pairs with high yield. The microscopic mechanisms underlying this charge separation have been debated for many years. Here we use ultrafast two-dimensional electronic spectroscopy to study the dynamics of polaron pair formation in a prototypical polymer thin film on a sub-20-fs time scale. We observe multi-period peak oscillations persisting for up to about 1 ps as distinct signatures of vibronic quantum coherence at room temperature. The measured two-dimensional spectra show pronounced peak splittings revealing that the elementary optical excitations of this polymer are hybridized exciton-polaron-pairs, strongly coupled to a dominant underdamped vibrational mode. Coherent vibronic coupling induces ultrafast polaron pair formation, accelerates the charge separation dynamics and makes it insensitive to disorder. These findings open up new perspectives for tailoring light-to-current conversion in organic materials.
<pre>Source custom Fortran code an supporting data for "Evidence of ideal excitonic in... more <pre>Source custom Fortran code an supporting data for "Evidence of ideal excitonic insulator in bulk MoS2 under pressure", available as a preprint at <strong>arXiv:2011.02380</strong> </pre> <pre>CUSTOME FORTRAN CODE 'MoS2_continuation' (file 'MoS2_vs_P.f') MAIN AIM: To solve the self-consistent gap equation (3) of main text, within the two-band approximation. ADDITIONAL FILES: example Makefile and log files. SUPPORTING DATA: Crystal structure coordinates of phases 2H_a and 2H_c of bulk MoS_2.</pre>
We propose a theoretical approach to the general problem of phonon-coupling between localised ele... more We propose a theoretical approach to the general problem of phonon-coupling between localised electronic states in semiconductors, which takes advantage of results from first-principles calculations. Application of this scheme to the isolated antisite defect AsQ, in GaAs reveals that electron-phonon coupling can provide the mechanism for the metastable transition characteristic of EL2.
ABSTRACT We have investigated the effect of non-equilibrium phonons on carrier relaxation dynamic... more ABSTRACT We have investigated the effect of non-equilibrium phonons on carrier relaxation dynamics in a quantum wire following ultrafast photoexcitation. We show that phonon build-up produces a considerable reduction of the cooling rate of photoexcited carriers for densities of the order of 106 cm-1. In this respect, the results for quantum wires are found to be similar to the bulk case. An important consequence of the reduced dimensionality of wires is found in the non-equilibrium phonon distribution, which is populated even at very small wave vectors.
Using first-principles calculations we studied the electric field enhancement in polyacene molecu... more Using first-principles calculations we studied the electric field enhancement in polyacene molecules upon illumination. These molecules can be seen as a specific class of C-based (i.e., graphene-derived) nanostructures, recently proposed as alternative materials for plasmonics. We demonstrate that optical transitions may generate oscillating dipolar response charge, giving rise to an induced electric field near the molecule, which thus acts as a plasmon-like nanoantenna. While the field amplification in the vicinity of single acenes is rather small and decreases when the size of the system is increased, it may be selectively enhanced in the case of acene’s assemblies. This paves the way for the design of more complex C-based architectures explicitly conceived to improve the amplification factor.
ABSTRACT Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disul... more ABSTRACT Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale.
The linear and nonlinear optical properties of realistic quantum wires are studied through a theo... more The linear and nonlinear optical properties of realistic quantum wires are studied through a theoretical approach based on a set of generalized semiconductor Bloch equations. Our scheme allows a full threedimensional multisubband description of electron-hole correlation for any confinement profile, thus permitting a direct comparison with experiments for available quantum-wire structures. Our results show that electron-hole Coulomb correlation removes the one-dimensional band-edge singularities from the absorption spectra, whose shape is heavily modified with respect to the ideal free-carrier singlesubband case over the whole density range. [S0031-9007(96)00125-1]
The adsorption of the cysteine amino acid (H-SC H 2-C R H-NH 2-COOH) on the (111) surface of gold... more The adsorption of the cysteine amino acid (H-SC H 2-C R H-NH 2-COOH) on the (111) surface of gold is studied by means of periodic density functional calculations. Results for different adsorption sites and molecular configurations show that chemisorption involving S(thiolate)-Au bonds on Au(111) is favored by starting with either cysteine or cystine gas-phase molecular precursors. In the most stable adsorption configuration, the sulfur headgroup sits at the bridge site between two surface Au atoms, and the S-C bond is tilted by 57°w ith respect to the surface normal, whereas the in-plane orientation of the molecular backbone plays a secondary role. The analysis of the electronic properties shows that the hybridization of the p-like S states with the d-like Au states produces both bonding and antibonding occupied orbitals, and the process is well described by a model for the interaction of localized orbitals with narrow-band dispersive electron states. The bonding orbitals well below the Fermi level contribute to the strong chemisorption of cysteine on gold. The calculated sulfur-projected density of states allows us to locate the cysteine molecular orbitals with respect to the system Fermi level, which gives a measure of the injection barrier at the molecule/electrode junction.
The screening of Coulomb interaction controls many-body physics in carbon nanotubes, as it tunes ... more The screening of Coulomb interaction controls many-body physics in carbon nanotubes, as it tunes the range and strength of the force that acts on charge carriers and binds electron-hole pairs into excitons. In doped tubes, the effective Coulomb interaction drives the competition between Luttinger liquid and Wigner crystal, whereas in undoped narrow-gap tubes it dictates the Mott or excitonic nature of the correlated insulator observed at low temperature. Here, by computing the dielectric function of selected narrow-and zero-gap tubes from first principles, we show that the standard effective-mass model of screening systematically underestimates the interaction strength at long wavelength, hence missing the binding of low-energy excitons. The reason is that the model critically lacks the full three-dimensional topology of the tube, being adapted from graphene theory. As ab inito calculations are limited to small tubes, we develop a two-band model dielectric function based on the plane-wave expansion of Bloch states and the exact truncated Coulomb cutoff technique. We demonstrate that our-computationally cheap-approach provides the correct screening for narrow-gap tubes of any size and chirality. A striking result is that the screened interaction remains long-ranged even in gapless tubes, as an effect of the microscopic local fields generated by the electrons moving on the curved tube surface. As an application, we show that the effective electron-electron force that is felt at distances relevant to quantum transport experiments is super Coulombic.
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Papers by Elisa MOLINARI