Publications in 2019

1. Miguel Urbaneja Torres, Kristjan Ottar Klausen, Anna Sitek, Sigurdur I Erlingsson, Vidar Gudmundsson and Andrei Manolescu.
Anisotropic electromagnetic field emitted by core-shell semiconductor nanowires driven by an alternating current.
arXiv e-prints, pages arXiv:1912.10284 (December 2019).
Abstract We consider tubular nanowires with polygonal cross-section, whose geometry has important implications for the electron localization and separates the lowest energy groups into two sets of corner and side-localized states. The presence of an external magnetic field transversal to the nanowires imposes an additional localization mechanism, the electrons being pushed to the sides relative to the direction of the field. This has important implications on the current density, as it creates current loops induced by the Lorentz force. We calculate numerically the electromagnetic field radiated by hexagonal, square, and triangular nanowires. We demonstrate that, because of the aforementioned localization properties, the radiated field has an anisotropic behavior that can reproduce the internal geometry of the nanowire. Consequently, core-shell nanowires made of semiconductor materials, driven by an alternating current, may act as an anisotropic emitter nanoantenna, at least in the neighborhood of the nanowire.
arXiv BibTeX

@article{2019arXiv191210284U ,
abstract = "We consider tubular nanowires with polygonal cross-section, whose geometry has important implications for the electron localization and separates the lowest energy groups into two sets of corner and side-localized states. The presence of an external magnetic field transversal to the nanowires imposes an additional localization mechanism, the electrons being pushed to the sides relative to the direction of the field. This has important implications on the current density, as it creates current loops induced by the Lorentz force. We calculate numerically the electromagnetic field radiated by hexagonal, square, and triangular nanowires. We demonstrate that, because of the aforementioned localization properties, the radiated field has an anisotropic behavior that can reproduce the internal geometry of the nanowire. Consequently, core-shell nanowires made of semiconductor materials, driven by an alternating current, may act as an anisotropic emitter nanoantenna, at least in the neighborhood of the nanowire.",
adsnote = "Provided by the SAO/NASA Astrophysics Data System",
archiveprefix = "arXiv",
arxiv = "https://arxiv.org/abs/1912.10284",
author = "{Urbaneja Torres}, Miguel and {Klausen}, Kristjan Ottar and {Sitek}, Anna and {Erlingsson}, Sigurdur I. and {Gudmundsson}, Vidar and {Manolescu}, Andrei",
eid = "arXiv:1912.10284",
eprint = "1912.10284",
journal = "arXiv e-prints",
keywords = "Condensed Matter - Mesoscale and Nanoscale Physics",
month = 12,
pages = "arXiv:1912.10284",
primaryclass = "cond-mat.mes-hall",
title = "{Anisotropic electromagnetic field emitted by core-shell semiconductor nanowires driven by an alternating current}",
year = 2019
}

2. M U Torres, A Sitek and A Manolescu.
Transverse polarization light scattering in tubular semiconductor nanowires.
In 2019 21st International Conference on Transparent Optical Networks (ICTON) (). (2019), 1-4.
BibTeX

@inproceedings{8840569 ,
author = "M. U. {Torres} and A. {Sitek} and A. {Manolescu}",
booktitle = "2019 21st International Conference on Transparent Optical Networks (ICTON)",
number = "",
pages = "1-4",
title = "Transverse polarization light scattering in tubular semiconductor nanowires",
volume = "",
year = 2019
}

3. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Oscillations in electron transport caused by multiple resonances in a quantum dot-QED system in the steady-state regime.
arXiv preprint arXiv:1903.03655 (2019).
Abstract We model the electron transport current as the photon energy is swept through several resonances of a multi-level quantum dot, embedded in a short quantum wire, coupled to photon cavity. We use a Markovian quantum master equation appropriate for the long-time evolution and include the electron-electron and both the para- and the diamagnetic electron-photon interactions via diagonalization in a truncated many-body Fock space. Tuning the photon energy, several anti-crossings caused by Rabi-splitting in the energy spectrum of the quantum dot system are found. The strength of the Rabi-splittings and the photon-exchange between the resonant states depend on the polarization of the cavity photon field. We observe oscillations of the charge in the system and several resonant transport current peaks for the photon energies corresponding to the resonances in the steady-state regime.
arXiv BibTeX

@article{abdullah2019oscillations ,
abstract = "We model the electron transport current as the photon energy is swept through several resonances of a multi-level quantum dot, embedded in a short quantum wire, coupled to photon cavity. We use a Markovian quantum master equation appropriate for the long-time evolution and include the electron-electron and both the para- and the diamagnetic electron-photon interactions via diagonalization in a truncated many-body Fock space. Tuning the photon energy, several anti-crossings caused by Rabi-splitting in the energy spectrum of the quantum dot system are found. The strength of the Rabi-splittings and the photon-exchange between the resonant states depend on the polarization of the cavity photon field. We observe oscillations of the charge in the system and several resonant transport current peaks for the photon energies corresponding to the resonances in the steady-state regime.",
arxiv = "https://arxiv.org/abs/1903.03655",
author = "Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "arXiv preprint arXiv:1903.03655",
title = "Oscillations in electron transport caused by multiple resonances in a quantum dot-QED system in the steady-state regime",
year = 2019
}

4. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
The photocurrent generated by photon replica states of an off-resonantly coupled dot-cavity system.
Scientific reports 9, 1–10 (2019).
Abstract Transport properties of a quantum dot coupled to a photon cavity are investigated using a quantum master equation in the steady-state regime. In the off-resonance regime, when the photon energy is smaller than the energy spacing between the lowest electron states of the quantum dot, we calculate the current that is generated by photon replica states as the electronic system is pumped with photons. Tuning the electron-photon coupling strength, the photocurrent can be enhanced by the influences of the photon polarization, and the cavity-photon coupling strength of the environment. We show that the current generated through the photon replicas is very sensitive to the photon polarization, but it is not strongly dependent on the average number of photons in the environment.
DOI BibTeX

@article{abdullah2019photocurrent ,
abstract = "Transport properties of a quantum dot coupled to a photon cavity are investigated using a quantum master equation in the steady-state regime. In the off-resonance regime, when the photon energy is smaller than the energy spacing between the lowest electron states of the quantum dot, we calculate the current that is generated by photon replica states as the electronic system is pumped with photons. Tuning the electron-photon coupling strength, the photocurrent can be enhanced by the influences of the photon polarization, and the cavity-photon coupling strength of the environment. We show that the current generated through the photon replicas is very sensitive to the photon polarization, but it is not strongly dependent on the average number of photons in the environment.",
author = "Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
doi = "10.1038/s41598-019-51320-8",
journal = "Scientific reports",
number = 1,
pages = "1--10",
publisher = "Nature Publishing Group",
title = "The photocurrent generated by photon replica states of an off-resonantly coupled dot-cavity system",
volume = 9,
year = 2019
}

5. D V Anghel, G A Nemnes, Ioana Pintilie and A Manolescu.
Modelling J\textendashV hysteresis in perovskite solar cells induced by voltage poling.
Physica Scripta 94, 125809 (October 2019).
Abstract We present an extension of the dynamic electrical model, which enable us to explain some important features of the perovskite solar cells (PSC), like the shape of the hysteresis and the appearance of the ’bump’ in the so called reverse scan, without requiring any additional assumptions. We give analytical expressions in terms of the Lambert’s function W for the open circuit voltage, the stationary current, and the instantaneous current, which can be written also in terms of elementary functions for the most part of the ranges of the physical parameters. The initial polarization of the cell, modeled as the charging of a capacitor with voltage dependent capacitance, is consistently determined in the model, from the initial stationary conditions. This is inline with a previously observed sharp increase of the PSC capacitance beyond the open-circuit voltage. Besides the known features, we obtain characteristics that were not yet analyzed experimentally, like the change of the bump from the reverse scan branch of the J–V characteristic to the forward scan, with the increase of the poling voltage (or the increase of the PSC capacitance).
URL, DOI BibTeX

@article{Anghel_2019 ,
abstract = "We present an extension of the dynamic electrical model, which enable us to explain some important features of the perovskite solar cells (PSC), like the shape of the hysteresis and the appearance of the ’bump’ in the so called reverse scan, without requiring any additional assumptions. We give analytical expressions in terms of the Lambert’s function W for the open circuit voltage, the stationary current, and the instantaneous current, which can be written also in terms of elementary functions for the most part of the ranges of the physical parameters. The initial polarization of the cell, modeled as the charging of a capacitor with voltage dependent capacitance, is consistently determined in the model, from the initial stationary conditions. This is inline with a previously observed sharp increase of the PSC capacitance beyond the open-circuit voltage. Besides the known features, we obtain characteristics that were not yet analyzed experimentally, like the change of the bump from the reverse scan branch of the J–V characteristic to the forward scan, with the increase of the poling voltage (or the increase of the PSC capacitance).",
author = "D V Anghel and G A Nemnes and Ioana Pintilie and A Manolescu",
doi = "10.1088/1402-4896/ab347d",
journal = "Physica Scripta",
month = "oct",
number = 12,
pages = 125809,
publisher = "{IOP} Publishing",
title = "Modelling J{\textendash}V hysteresis in perovskite solar cells induced by voltage poling",
url = "https://doi.org/10.1088%2F1402-4896%2Fab347d",
volume = 94,
year = 2019
}

6. George Alexandru Nemnes, Cristina Besleaga, Andrei Gabriel Tomulescu, Lucia Nicoleta Leonat, Viorica Stancu, Mihaela Florea, Andrei Manolescu and Ioana Pintilie.
The hysteresis-free behavior of perovskite solar cells from the perspective of the measurement conditions.
J. Mater. Chem. C 7, 5267-5274 (2019).
Abstract We investigate how far the hysteresis-free behavior of perovskite solar cells can be reproduced using particular pre-conditioning and measurement conditions. As there are currently more and more reports of perovskite solar cells without J–V hysteresis it is crucial to distinguish between genuine performance improvements and measurement artifacts. We focus on two of the parameters that influence the dynamic J–V scans, namely the bias scan rate and the bias poling voltage, and point out measurement conditions for achieving a hysteresis-free behavior. In this context we discuss the suitability of defining a hysteresis index (HI) for the characterization of dynamic J–V scans. Using HI, aging effects are also investigated, establishing a potential connection between the sample degradation and the variation of the maximal hysteresis on one hand, and the relaxation time scale of the slow process on the other hand. Pre-poling induced recombination effects are identified. In addition, our analysis based on sample pre-biasing reveals potential indications regarding two types of slow processes, with two different relaxation time scales, which provides further insight regarding ionic migration.
URL, DOI BibTeX

@article{C8TC05999C ,
abstract = "We investigate how far the hysteresis-free behavior of perovskite solar cells can be reproduced using particular pre-conditioning and measurement conditions. As there are currently more and more reports of perovskite solar cells without J–V hysteresis it is crucial to distinguish between genuine performance improvements and measurement artifacts. We focus on two of the parameters that influence the dynamic J–V scans{,} namely the bias scan rate and the bias poling voltage{,} and point out measurement conditions for achieving a hysteresis-free behavior. In this context we discuss the suitability of defining a hysteresis index (HI) for the characterization of dynamic J–V scans. Using HI{,} aging effects are also investigated{,} establishing a potential connection between the sample degradation and the variation of the maximal hysteresis on one hand{,} and the relaxation time scale of the slow process on the other hand. Pre-poling induced recombination effects are identified. In addition{,} our analysis based on sample pre-biasing reveals potential indications regarding two types of slow processes{,} with two different relaxation time scales{,} which provides further insight regarding ionic migration.",
author = "Nemnes, George Alexandru and Besleaga, Cristina and Tomulescu, Andrei Gabriel and Leonat, Lucia Nicoleta and Stancu, Viorica and Florea, Mihaela and Manolescu, Andrei and Pintilie, Ioana",
doi = "10.1039/C8TC05999C",
issue = 18,
journal = "J. Mater. Chem. C",
pages = "5267-5274",
publisher = "The Royal Society of Chemistry",
title = "The hysteresis-free behavior of perovskite solar cells from the perspective of the measurement conditions",
url = "http://dx.doi.org/10.1039/C8TC05999C",
volume = 7,
year = 2019
}

7. Vidar Gudmundsson, Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Valeriu Moldoveanu.
Cavity-Photon-Induced High-Order Transitions between Ground States of Quantum Dots.
Annalen der Physik 531, 1900306 (2019).
Abstract Abstract It is shown that quantum electromagnetic transitions to high orders are essential to describe the time-dependent path of a nanoscale electron system in a Coulomb blockade regime when coupled to external leads and placed in a 3D rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads, a slow high-order transition between the ground states of the two quantum dots is identified. The Fourier power spectrum for photon–photon correlations in the steady state shows a Fano type of resonance for the frequency of the slow transition. Full account is taken of the geometry of the multilevel electronic system, and the electron–electron Coulomb interactions together with the para- and diamagnetic electron–photon interactions are treated with step-wise exact numerical diagonalization and truncation of appropriate many-body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.
URL, DOI BibTeX

@article{doi:10.1002/andp.201900306 ,
abstract = "Abstract It is shown that quantum electromagnetic transitions to high orders are essential to describe the time-dependent path of a nanoscale electron system in a Coulomb blockade regime when coupled to external leads and placed in a 3D rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads, a slow high-order transition between the ground states of the two quantum dots is identified. The Fourier power spectrum for photon–photon correlations in the steady state shows a Fano type of resonance for the frequency of the slow transition. Full account is taken of the geometry of the multilevel electronic system, and the electron–electron Coulomb interactions together with the para- and diamagnetic electron–photon interactions are treated with step-wise exact numerical diagonalization and truncation of appropriate many-body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.",
author = "Gudmundsson, Vidar and Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Moldoveanu, Valeriu",
doi = "10.1002/andp.201900306",
eprint = "https://onlinelibrary.wiley.com/doi/pdf/10.1002/andp.201900306",
journal = "Annalen der Physik",
number = 11,
pages = 1900306,
title = "Cavity-Photon-Induced High-Order Transitions between Ground States of Quantum Dots",
url = "https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.201900306",
volume = 531,
year = 2019
}

8. Maximilian M Sonner, Anna Sitek, Lisa Janker, Daniel Rudolph, Daniel Ruhstorfer, Markus Döblinger, Andrei Manolescu, Gerhard Abstreiter, Jonathan J Finley, Achim Wixforth, Gregor Koblmüller and Hubert J Krenner.
Breakdown of Corner States and Carrier Localization by Monolayer Fluctuations in Radial Nanowire Quantum Wells.
Nano Letters 19, 3336-3343 (2019).
Abstract We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickest and that even minute deviations of the perfect hexagonal shape strongly perturb the commonly assumed 6-fold symmetric ground state.
URL, DOI BibTeX

@article{doi:10.1021/acs.nanolett.9b01028 ,
abstract = "We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickest and that even minute deviations of the perfect hexagonal shape strongly perturb the commonly assumed 6-fold symmetric ground state.",
author = "Sonner, Maximilian M. and Sitek, Anna and Janker, Lisa and Rudolph, Daniel and Ruhstorfer, Daniel and Döblinger, Markus and Manolescu, Andrei and Abstreiter, Gerhard and Finley, Jonathan J. and Wixforth, Achim and Koblmüller, Gregor and Krenner, Hubert J.",
doi = "10.1021/acs.nanolett.9b01028",
eprint = "https://doi.org/10.1021/acs.nanolett.9b01028",
journal = "Nano Letters",
note = "PMID: 31013103",
number = 5,
pages = "3336-3343",
title = "Breakdown of Corner States and Carrier Localization by Monolayer Fluctuations in Radial Nanowire Quantum Wells",
url = "https://doi.org/10.1021/acs.nanolett.9b01028",
volume = 19,
year = 2019
}

9. Valeriu Moldoveanu, Andrei Manolescu and Vidar Gudmundsson.
Generalized Master Equation Approach to Time-Dependent Many-Body Transport.
Entropy 21 (2019).
Abstract We recall theoretical studies on transient transport through interacting mesoscopic systems. It is shown that a generalized master equation (GME) written and solved in terms of many-body states provides the suitable formal framework to capture both the effects of the Coulomb interaction and electron–photon coupling due to a surrounding single-mode cavity. We outline the derivation of this equation within the Nakajima–Zwanzig formalism and point out technical problems related to its numerical implementation for more realistic systems which can neither be described by non-interacting two-level models nor by a steady-state Markov–Lindblad equation. We first solve the GME for a lattice model and discuss the dynamics of many-body states in a two-dimensional nanowire, the dynamical onset of the current-current correlations in electrostatically coupled parallel quantum dots and transient thermoelectric properties. Secondly, we rely on a continuous model to get the Rabi oscillations of the photocurrent through a double-dot etched in a nanowire and embedded in a quantum cavity. A many-body Markovian version of the GME for cavity-coupled systems is also presented.
URL, DOI BibTeX

@article{e21080731 ,
abstract = "We recall theoretical studies on transient transport through interacting mesoscopic systems. It is shown that a generalized master equation (GME) written and solved in terms of many-body states provides the suitable formal framework to capture both the effects of the Coulomb interaction and electron–photon coupling due to a surrounding single-mode cavity. We outline the derivation of this equation within the Nakajima–Zwanzig formalism and point out technical problems related to its numerical implementation for more realistic systems which can neither be described by non-interacting two-level models nor by a steady-state Markov–Lindblad equation. We first solve the GME for a lattice model and discuss the dynamics of many-body states in a two-dimensional nanowire, the dynamical onset of the current-current correlations in electrostatically coupled parallel quantum dots and transient thermoelectric properties. Secondly, we rely on a continuous model to get the Rabi oscillations of the photocurrent through a double-dot etched in a nanowire and embedded in a quantum cavity. A many-body Markovian version of the GME for cavity-coupled systems is also presented.",
article-number = 731,
author = "Moldoveanu, Valeriu and Manolescu, Andrei and Gudmundsson, Vidar",
doi = "10.3390/e21080731",
issn = "1099-4300",
journal = "Entropy",
number = 8,
title = "Generalized Master Equation Approach to Time-Dependent Many-Body Transport",
url = "https://www.mdpi.com/1099-4300/21/8/731",
volume = 21,
year = 2019
}

10. Vidar Gudmundsson, Hallmann Gestsson, Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Valeriu Moldoveanu.
Coexisting spin and Rabi oscillations at intermediate time regimes in electron transport through a photon cavity.
Beilstein Journal of Nanotechnology 10, 606-616 (2019).
DOI BibTeX

@article{Gudmundsson2019 ,
author = "Vidar Gudmundsson and Hallmann Gestsson and Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Valeriu Moldoveanu",
doi = "10.3762/bjnano.10.61",
issn = "2190-4286",
journal = "Beilstein Journal of Nanotechnology",
pages = "606-616",
title = "Coexisting spin and Rabi oscillations at intermediate time regimes in electron transport through a photon cavity",
volume = 10,
year = 2019
}

11. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Thermoelectric Inversion in a Resonant Quantum Dot-Cavity System in the Steady-State Regime.
Nanomaterials 9 (2019).
Abstract We theoretically investigate thermoelectric effects in a quantum dot system under the influence of a linearly polarized photon field confined to a 3D cavity. A temperature gradient is applied to the system via two electron reservoirs that are connected to each end of the quantum dot system. The thermoelectric current in the steady state is explored using a quantum master equation. In the presence of the quantized photons, extra channels, the photon replica states, are formed generating a photon-induced thermoelectric current. We observe that the photon replica states contribute to the transport irrespective of the direction of the thermal gradient. In the off-resonance regime, when the energy difference between the lowest states of the quantum dot system is smaller than the photon energy, the thermoelectric current is almost blocked and a plateau is seen in the thermoelectric current for strong electron–photon coupling strength. In the resonant regime, an inversion of thermoelectric current emerges due to the Rabi-splitting. Therefore, the photon field can change both the magnitude and the sign of the thermoelectric current induced by the temperature gradient in the absence of a voltage bias between the leads.
URL, DOI BibTeX

@article{nano9050741 ,
abstract = "We theoretically investigate thermoelectric effects in a quantum dot system under the influence of a linearly polarized photon field confined to a 3D cavity. A temperature gradient is applied to the system via two electron reservoirs that are connected to each end of the quantum dot system. The thermoelectric current in the steady state is explored using a quantum master equation. In the presence of the quantized photons, extra channels, the photon replica states, are formed generating a photon-induced thermoelectric current. We observe that the photon replica states contribute to the transport irrespective of the direction of the thermal gradient. In the off-resonance regime, when the energy difference between the lowest states of the quantum dot system is smaller than the photon energy, the thermoelectric current is almost blocked and a plateau is seen in the thermoelectric current for strong electron–photon coupling strength. In the resonant regime, an inversion of thermoelectric current emerges due to the Rabi-splitting. Therefore, the photon field can change both the magnitude and the sign of the thermoelectric current induced by the temperature gradient in the absence of a voltage bias between the leads.",
article-number = 741,
author = "Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
doi = "10.3390/nano9050741",
issn = "2079-4991",
journal = "Nanomaterials",
number = 5,
pubmedid = 31091757,
title = "Thermoelectric Inversion in a Resonant Quantum Dot-Cavity System in the Steady-State Regime",
url = "https://www.mdpi.com/2079-4991/9/5/741",
volume = 9,
year = 2019
}

12. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Manifestation of the Purcell Effect in Current Transport through a Dot–Cavity–QED System.
Nanomaterials 9 (2019).
Abstract We study the transport properties of a wire-dot system coupled to a cavity and a photon reservoir. The system is considered to be microstructured from a two-dimensional electron gas in a GaAs heterostructure. The 3D photon cavity is active in the far-infrared or the terahertz regime. Tuning the photon energy, Rabi-resonant states emerge and in turn resonant current peaks are observed. We demonstrate the effects of the cavity–photon reservoir coupling, the mean photon number in the reservoir, the electron–photon coupling and the photon polarization on the intraband transitions occurring between the Rabi-resonant states, and on the corresponding resonant current peaks. The Rabi-splitting can be controlled by the photon polarization and the electron–photon coupling strength. In the selected range of the parameters, the electron–photon coupling and the cavity-environment coupling strengths, we observe the results of the Purcell effect enhancing the current peaks through the cavity by increasing the cavity–reservoir coupling, while they decrease with increasing electron–photon coupling. In addition, the resonant current peaks are also sensitive to the mean number of photons in the reservoir.
URL, DOI BibTeX

@article{nano9071023 ,
abstract = "We study the transport properties of a wire-dot system coupled to a cavity and a photon reservoir. The system is considered to be microstructured from a two-dimensional electron gas in a GaAs heterostructure. The 3D photon cavity is active in the far-infrared or the terahertz regime. Tuning the photon energy, Rabi-resonant states emerge and in turn resonant current peaks are observed. We demonstrate the effects of the cavity–photon reservoir coupling, the mean photon number in the reservoir, the electron–photon coupling and the photon polarization on the intraband transitions occurring between the Rabi-resonant states, and on the corresponding resonant current peaks. The Rabi-splitting can be controlled by the photon polarization and the electron–photon coupling strength. In the selected range of the parameters, the electron–photon coupling and the cavity-environment coupling strengths, we observe the results of the Purcell effect enhancing the current peaks through the cavity by increasing the cavity–reservoir coupling, while they decrease with increasing electron–photon coupling. In addition, the resonant current peaks are also sensitive to the mean number of photons in the reservoir.",
article-number = 1023,
author = "Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
doi = "10.3390/nano9071023",
issn = "2079-4991",
journal = "Nanomaterials",
number = 7,
pubmedid = 31319544,
title = "Manifestation of the Purcell Effect in Current Transport through a Dot–Cavity–QED System",
url = "https://www.mdpi.com/2079-4991/9/7/1023",
volume = 9,
year = 2019
}

13. G A Nemnes, T L Mitran, A Manolescu and Daniela Dragoman.
Electric and thermoelectric properties of graphene bilayers with extrinsic impurities under applied electric field.
Physica B: Condensed Matter 561, 9 - 15 (2019).
Abstract In contrast to monolayer graphene, in bilayer graphene (BLG) one can induce a tunable bandgap by applying an external electric field, which makes it suitable for field effect applications. Extrinsic doping of BLGs enriches the electronic properties of the graphene-based family, as their behavior can be switched from an intrinsic small-gap semiconductor to a degenerate semiconductor. In the framework of density functional theory (DFT) calculations, we investigate the electronic and thermoelectric properties of BLGs doped with extrinsic impurities from groups III (B, Al, Ga), IV (Si, Ge) and V (N, P, As), in the context of applied external electric fields. Doping one monolayer of the BLG with p- or n-type dopants results in a degenerate semiconductor, where the Fermi energy depends on the type of the impurity, but also on the magnitude and orientation of the electric field, which modifies the effective doping concentration. Doping one layer with isoelectronic species like Si and Ge opens a gap, which may be closed upon applying an electric field, in contrast to the pristine BLG. Furthermore, dual doping by III-V elements, in a way that the BLG system is formed by one n-type and one p-type graphene monolayer, leads to intrinsic semiconductor properties with relatively large energy gaps. Si-Si and Ge-Ge substitutions render a metallic like behavior at zero field similar to the standard BLG, however with an asymmetric density of states in the vicinity of the Fermi energy. We analyze the suitability of the highly doped BLG materials for thermoelectric applications, exploiting the large asymmetries of the density of states. In addition, a sign change in the Seebeck coefficient is observed by tuning the electric field as a signature of narrow bands near the Fermi level.
URL, DOI BibTeX

@article{NEMNES20199 ,
abstract = "In contrast to monolayer graphene, in bilayer graphene (BLG) one can induce a tunable bandgap by applying an external electric field, which makes it suitable for field effect applications. Extrinsic doping of BLGs enriches the electronic properties of the graphene-based family, as their behavior can be switched from an intrinsic small-gap semiconductor to a degenerate semiconductor. In the framework of density functional theory (DFT) calculations, we investigate the electronic and thermoelectric properties of BLGs doped with extrinsic impurities from groups III (B, Al, Ga), IV (Si, Ge) and V (N, P, As), in the context of applied external electric fields. Doping one monolayer of the BLG with p- or n-type dopants results in a degenerate semiconductor, where the Fermi energy depends on the type of the impurity, but also on the magnitude and orientation of the electric field, which modifies the effective doping concentration. Doping one layer with isoelectronic species like Si and Ge opens a gap, which may be closed upon applying an electric field, in contrast to the pristine BLG. Furthermore, dual doping by III-V elements, in a way that the BLG system is formed by one n-type and one p-type graphene monolayer, leads to intrinsic semiconductor properties with relatively large energy gaps. Si-Si and Ge-Ge substitutions render a metallic like behavior at zero field similar to the standard BLG, however with an asymmetric density of states in the vicinity of the Fermi energy. We analyze the suitability of the highly doped BLG materials for thermoelectric applications, exploiting the large asymmetries of the density of states. In addition, a sign change in the Seebeck coefficient is observed by tuning the electric field as a signature of narrow bands near the Fermi level.",
author = "G.A. Nemnes and T.L. Mitran and A. Manolescu and Daniela Dragoman",
doi = "https://doi.org/10.1016/j.physb.2019.02.044",
issn = "0921-4526",
journal = "Physica B: Condensed Matter",
keywords = "Bilayer graphene, Extrinsic doping, Field effect, Seebeck coefficient",
pages = "9 - 15",
title = "Electric and thermoelectric properties of graphene bilayers with extrinsic impurities under applied electric field",
url = "http://www.sciencedirect.com/science/article/pii/S0921452619301334",
volume = 561,
year = 2019
}

14. GA Nemnes, TL Mitran and A Manolescu.
Gap prediction in hybrid graphene-hexagonal boron nitride nanoflakes using artificial neural networks.
Journal of Nanomaterials 2019 (2019).
Abstract The electronic properties of graphene nanoflakes (GNFs) with embedded hexagonal boron nitride (hBN) domains are investigated by combined ab initio density functional theory calculations and machine-learning techniques. The energy gaps of the quasi-0D graphene-based systems, defined as the differences between LUMO and HOMO energies, depend not only on the sizes of the hBN domains relative to the size of the pristine graphene nanoflake but also on the position of the hBN domain. The range of the energy gaps for different configurations increases as the hBN domains get larger. We develop two artificial neural network (ANN) models able to reproduce the gap energies with high accuracies and investigate the tunability of the energy gap, by considering a set of GNFs with embedded rectangular hBN domains. In one ANN model, the input is in one-to-one correspondence with the atoms in the GNF, while in the second model the inputs account for basic structures in the GNF, allowing potential use in upscaled systems. We perform a statistical analysis over different configurations of ANNs to optimize the network structure. The trained ANNs provide a correlation between the atomic system configuration and the magnitude of the energy gaps, which may be regarded as an efficient tool for optimizing the design of nanostructured graphene-based materials for specific electronic properties.
DOI BibTeX

@article{nemnes2019gap ,
abstract = "The electronic properties of graphene nanoflakes (GNFs) with embedded hexagonal boron nitride (hBN) domains are investigated by combined ab initio density functional theory calculations and machine-learning techniques. The energy gaps of the quasi-0D graphene-based systems, defined as the differences between LUMO and HOMO energies, depend not only on the sizes of the hBN domains relative to the size of the pristine graphene nanoflake but also on the position of the hBN domain. The range of the energy gaps for different configurations increases as the hBN domains get larger. We develop two artificial neural network (ANN) models able to reproduce the gap energies with high accuracies and investigate the tunability of the energy gap, by considering a set of GNFs with embedded rectangular hBN domains. In one ANN model, the input is in one-to-one correspondence with the atoms in the GNF, while in the second model the inputs account for basic structures in the GNF, allowing potential use in upscaled systems. We perform a statistical analysis over different configurations of ANNs to optimize the network structure. The trained ANNs provide a correlation between the atomic system configuration and the magnitude of the energy gaps, which may be regarded as an efficient tool for optimizing the design of nanostructured graphene-based materials for specific electronic properties.",
author = "Nemnes, GA and Mitran, TL and Manolescu, A",
doi = "10.1155/2019/6960787",
journal = "Journal of Nanomaterials",
publisher = "Hindawi",
title = "Gap prediction in hybrid graphene-hexagonal boron nitride nanoflakes using artificial neural networks",
volume = 2019,
year = 2019
}

15. M Nita, M Tolea, A Manolescu and D C Marinescu.
HUND RULE OR MINIMAL SPIN IN CIRCULAR MOLECULES AND BIPARTITE LATTICES.
New Frontiers in Chemistry 28, 20-21 (2019).
Abstract Hund rule's relevance has long exceeded the boundaries of atomic physics where it was first formulated, over the years being investigated in many other systems such as quantum dots 1], artificial molecules created by quantum dots 2], metal clusters 3], bipartite lattices 4, 5], trilayer graphene 6], etc. In this paper we discuss the spin properties of circular molecules with an arbitrary number of atoms whose one-particle spectrum, in general, shows a ladder of double degenerate electronic states. We show 5] that an anti-Hund rule always defines the ground state in a circular molecule with 4N atoms at half-filling. In all other cases (i.e. number of atoms not multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction. In a celebrated paper, E. Lieb 7] gives exact results regarding the net spin of a socalled bipartite latice (i.e. which can be divided in "A" and "B" sub-groups of sites, hopping existing only between sites from different sub-groups) at half filling, in the presence of on-site Hubbard interaction. We study 8] the validity of the Lieb rule for a generalized Hamiltonian (including long-range interactions) using perturbatives plus Configuration-Interaction approaches.
URL BibTeX

@article{Nita2019hund ,
abstract = {Hund rule's relevance has long exceeded the boundaries of atomic physics where it was first formulated, over the years being investigated in many other systems such as quantum dots 1], artificial molecules created by quantum dots 2], metal clusters 3], bipartite lattices 4, 5], trilayer graphene 6], etc. In this paper we discuss the spin properties of circular molecules with an arbitrary number of atoms whose one-particle spectrum, in general, shows a ladder of double degenerate electronic states. We show 5] that an anti-Hund rule always defines the ground state in a circular molecule with 4N atoms at half-filling. In all other cases (i.e. number of atoms not multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction. In a celebrated paper, E. Lieb 7] gives exact results regarding the net spin of a socalled bipartite latice (i.e. which can be divided in "A" and "B" sub-groups of sites, hopping existing only between sites from different sub-groups) at half filling, in the presence of on-site Hubbard interaction. We study 8] the validity of the Lieb rule for a generalized Hamiltonian (including long-range interactions) using perturbatives plus Configuration-Interaction approaches.},
author = "Nita,M. and Tolea,M. and Manolescu,A. and Marinescu,D. C.",
isbn = 23932171,
journal = "New Frontiers in Chemistry",
keywords = "Chemistry; Circularity; Atomic physics; Graphene; Quantum dots; Ground state; Onsite; Electron states; Atomic properties; Metal clusters",
language = "English",
note = "Copyright - Copyright West University of Timisoara, Faculty of Chemistry 2019; Last updated - 2020-04-02",
number = 1,
pages = "20-21",
title = "HUND RULE OR MINIMAL SPIN IN CIRCULAR MOLECULES AND BIPARTITE LATTICES",
url = "https://search.proquest.com/docview/2385400650?accountid=28419",
volume = 28,
year = 2019
}

16. Valeriu Moldoveanu, Ion Viorel Dinu, Andrei Manolescu and Vidar Gudmundsson.
Backaction effects in cavity-coupled quantum conductors.
Phys. Rev. B 100, 125416 (September 2019).
Abstract We study the electronic transport through a pair of distant nanosystems (Sa and Sb) embedded in a single-mode cavity. Each system is connected to source and drain particle reservoirs and the electron-photon coupling is described by the Tavis-Cummings model. The generalized master equation approach provides the reduced density operator of the double system in the dressed-states basis. It is shown that the photon-mediated coupling between the two subsystems leaves a signature on their transient and steady-state currents. In particular, a suitable bias applied on subsystem Sb induces a photon-assisted current in the other subsystem Sa which is otherwise in the Coulomb blockade. We also predict that a transient current passing through one subsystem triggers a charge transfer between the optically active levels of the second subsystem even if the latter is not connected to the leads. As a result of backaction, the transient current through the open system develops Rabi oscillations (ROs) whose period depends on the initial state of the closed system.
URL, DOI BibTeX

@article{PhysRevB.100.125416 ,
abstract = "We study the electronic transport through a pair of distant nanosystems (Sa and Sb) embedded in a single-mode cavity. Each system is connected to source and drain particle reservoirs and the electron-photon coupling is described by the Tavis-Cummings model. The generalized master equation approach provides the reduced density operator of the double system in the dressed-states basis. It is shown that the photon-mediated coupling between the two subsystems leaves a signature on their transient and steady-state currents. In particular, a suitable bias applied on subsystem Sb induces a photon-assisted current in the other subsystem Sa which is otherwise in the Coulomb blockade. We also predict that a transient current passing through one subsystem triggers a charge transfer between the optically active levels of the second subsystem even if the latter is not connected to the leads. As a result of backaction, the transient current through the open system develops Rabi oscillations (ROs) whose period depends on the initial state of the closed system.",
author = "Moldoveanu, Valeriu and Dinu, Ion Viorel and Manolescu, Andrei and Gudmundsson, Vidar",
doi = "10.1103/PhysRevB.100.125416",
issue = 12,
journal = "Phys. Rev. B",
month = "Sep",
numpages = 9,
pages = 125416,
publisher = "American Physical Society",
title = "Backaction effects in cavity-coupled quantum conductors",
volume = 100,
year = 2019
}

17. Daniel J Carney, Halldor G Svavarsson, Hafez Hemmati, Alexander Fannin, Jae W Yoon and Robert Magnusson.
Refractometric Sensing with Periodic Nano-Indented Arrays: Effect of Structural Dimensions.
Sensors 19 (2019).
Abstract Fabrication and sensor application of a simple plasmonic structure is described in this paper. The sensor element consists of nano-patterned gold film brought about from two-dimensional periodic photoresist templates created by holographic laser interference lithography. Reflectance spectroscopy revealed that the sensor exhibits significant refractive index sensitivity. A linear relationship between shifts in plasmonic resonances and changes in the refractive index were demonstrated. The sensor has a bulk sensitivity (SB) of 880 nm/refractive index unit and work under normal incidence conditions. This sensitivity exceeded that of many common types of plasmonic sensors with more intricate structures. A modeled spectral response was used to study the effect of its geometrical dimensions on plasmonic behavior. A qualitative agreement between the experimental spectra and modeled ones was obtained.
URL, DOI BibTeX

@article{s19040897 ,
abstract = "Fabrication and sensor application of a simple plasmonic structure is described in this paper. The sensor element consists of nano-patterned gold film brought about from two-dimensional periodic photoresist templates created by holographic laser interference lithography. Reflectance spectroscopy revealed that the sensor exhibits significant refractive index sensitivity. A linear relationship between shifts in plasmonic resonances and changes in the refractive index were demonstrated. The sensor has a bulk sensitivity (SB) of 880 nm/refractive index unit and work under normal incidence conditions. This sensitivity exceeded that of many common types of plasmonic sensors with more intricate structures. A modeled spectral response was used to study the effect of its geometrical dimensions on plasmonic behavior. A qualitative agreement between the experimental spectra and modeled ones was obtained.",
article-number = 897,
author = "Carney, Daniel J. and Svavarsson, Halldor G. and Hemmati, Hafez and Fannin, Alexander and Yoon, Jae W. and Magnusson, Robert",
doi = "10.3390/s19040897",
issn = "1424-8220",
journal = "Sensors",
number = 4,
title = "Refractometric Sensing with Periodic Nano-Indented Arrays: Effect of Structural Dimensions",
url = "https://www.mdpi.com/1424-8220/19/4/897",
volume = 19,
year = 2019
}

18. Anna Sitek, Miguel Urbaneja Torres and Andrei Manolescu.
Corner and side localization of electrons in irregular hexagonal semiconductor shells.
Nanotechnology 30, 454001 (August 2019).
Abstract We discuss the low energy electronic states in hexagonal rings. These states correspond to the transverse modes in core–shell nanowires built of III–V semiconductors which typically have a hexagonal cross section. In the case of symmetric structures the 12 lowest states (including the spin) are localized in the corners, while the next following 12 states are localized mostly on the sides. Depending on the material parameters, in particular the effective mass, the ring diameter and width, the corner and side states may be separated by a considerable energy gap, ranging from few to tens of meV. In a realistic fabrication process geometric asymmetries are unavoidable, and therefore the particles are not symmetrically distributed between all corner and side areas. Possibly, even small deformations may shift the localization of the ground state to one of the sides. The transverse states or the transitions between them may be important in transport or optical experiments. Still, up to date, there are only very few experimental investigations of the localization-dependent properties of core–shell nanowires.
URL, DOI BibTeX

@article{Sitek_2019 ,
abstract = "We discuss the low energy electronic states in hexagonal rings. These states correspond to the transverse modes in core–shell nanowires built of III–V semiconductors which typically have a hexagonal cross section. In the case of symmetric structures the 12 lowest states (including the spin) are localized in the corners, while the next following 12 states are localized mostly on the sides. Depending on the material parameters, in particular the effective mass, the ring diameter and width, the corner and side states may be separated by a considerable energy gap, ranging from few to tens of meV. In a realistic fabrication process geometric asymmetries are unavoidable, and therefore the particles are not symmetrically distributed between all corner and side areas. Possibly, even small deformations may shift the localization of the ground state to one of the sides. The transverse states or the transitions between them may be important in transport or optical experiments. Still, up to date, there are only very few experimental investigations of the localization-dependent properties of core–shell nanowires.",
author = "Anna Sitek and Miguel Urbaneja Torres and Andrei Manolescu",
doi = "10.1088/1361-6528/ab37a1",
journal = "Nanotechnology",
month = "aug",
number = 45,
pages = 454001,
publisher = "{IOP} Publishing",
title = "Corner and side localization of electrons in irregular hexagonal semiconductor shells",
url = "https://doi.org/10.1088%2F1361-6528%2Fab37a1",
volume = 30,
year = 2019
}

19. Muhammad Taha Sultan, Adrian Valentin Maraloiu, Ionel Stavarache, Tómas Jón Gudmundsson, Andrei Manolescu, Valentin Serban Teodorescu, Magdalena Lidia Ciurea and Halldór Gudfinnur Svavarsson.
Fabrication and characterization of Si1−xGex nanocrystals in as-grown and annealed structures: a comparative study.
Beilstein Journal of Nanotechnology 10, 1873-1882 (2019).
Abstract Multilayer structures comprising of SiO2/SiGe/SiO2 and containing SiGe nanoparticles were obtained by depositing SiO2 layers using reactive direct current magnetron sputtering (dcMS), whereas, Si and Ge were co-sputtered using dcMS and high-power impulse magnetron sputtering (HiPIMS). The as-grown structures subsequently underwent rapid thermal annealing (550–900 °C for 1 min) in N2 ambient atmosphere. The structures were investigated using X-ray diffraction, high-resolution transmission electron microscopy together with spectral photocurrent measurements, to explore structural changes and corresponding properties. It is observed that the employment of HiPIMS facilitates the formation of SiGe nanoparticles (2.1 ± 0.8 nm) in the as-grown structure, and that presence of such nanoparticles acts as a seed for heterogeneous nucleation, which upon annealing results in the periodically arranged columnar self-assembly of SiGe core–shell nanocrystals. An increase in photocurrent intensity by more than an order of magnitude was achieved by annealing. Furthermore, a detailed discussion is provided on strain development within the structures, the consequential interface characteristics and its effect on the photocurrent spectra.
DOI BibTeX

@article{Sultan2019 ,
abstract = "Multilayer structures comprising of SiO2/SiGe/SiO2 and containing SiGe nanoparticles were obtained by depositing SiO2 layers using reactive direct current magnetron sputtering (dcMS), whereas, Si and Ge were co-sputtered using dcMS and high-power impulse magnetron sputtering (HiPIMS). The as-grown structures subsequently underwent rapid thermal annealing (550–900 °C for 1 min) in N2 ambient atmosphere. The structures were investigated using X-ray diffraction, high-resolution transmission electron microscopy together with spectral photocurrent measurements, to explore structural changes and corresponding properties. It is observed that the employment of HiPIMS facilitates the formation of SiGe nanoparticles (2.1 ± 0.8 nm) in the as-grown structure, and that presence of such nanoparticles acts as a seed for heterogeneous nucleation, which upon annealing results in the periodically arranged columnar self-assembly of SiGe core–shell nanocrystals. An increase in photocurrent intensity by more than an order of magnitude was achieved by annealing. Furthermore, a detailed discussion is provided on strain development within the structures, the consequential interface characteristics and its effect on the photocurrent spectra.",
author = "Muhammad Taha Sultan and Adrian Valentin Maraloiu and Ionel Stavarache and Jón Tómas Gudmundsson and Andrei Manolescu and Valentin Serban Teodorescu and Magdalena Lidia Ciurea and Halldór Gudfinnur Svavarsson",
doi = "10.3762/bjnano.10.182",
issn = "2190-4286",
journal = "Beilstein Journal of Nanotechnology",
pages = "1873-1882",
title = "Fabrication and characterization of Si1−xGex nanocrystals in as-grown and annealed structures: a comparative study",
volume = 10,
year = 2019
}

20. M T Sultan, J T Gudmundsson, A Manolescu, T Stoica, M L Ciurea and H G Svavarsson.
Enhanced photoconductivity of embedded SiGe nanoparticles by hydrogenation.
Applied Surface Science 479, 403 - 409 (2019).
Abstract We investigate the effect of room-temperature hydrogen-plasma treatment on the photoconductivity of SiGe nanoparticles sandwiched within SiO2 layers. An increase in photocurrent intensity of more than an order magnitude is observed after the hydrogen plasma treatment. The enhancement is attributed to neutralization of dangling bonds at the nanoparticles and to passivation of nonradiative defects in the oxide matrix and at SiGe/matrix interfaces. We find that increasing the partial pressure of hydrogen to pressures where H3+ and H2+ were the dominant ions results in increased photocurrent.
URL, DOI BibTeX

@article{SULTAN2019403 ,
abstract = "We investigate the effect of room-temperature hydrogen-plasma treatment on the photoconductivity of SiGe nanoparticles sandwiched within SiO2 layers. An increase in photocurrent intensity of more than an order magnitude is observed after the hydrogen plasma treatment. The enhancement is attributed to neutralization of dangling bonds at the nanoparticles and to passivation of nonradiative defects in the oxide matrix and at SiGe/matrix interfaces. We find that increasing the partial pressure of hydrogen to pressures where H3+ and H2+ were the dominant ions results in increased photocurrent.",
author = "M.T. Sultan and J.T. Gudmundsson and A. Manolescu and T. Stoica and M.L. Ciurea and H.G. Svavarsson",
doi = "https://doi.org/10.1016/j.apsusc.2019.02.096",
issn = "0169-4332",
journal = "Applied Surface Science",
keywords = "SiGe, SiO, Nanoparticles, Photocurrent, Hydrogen plasma, Passivation",
pages = "403 - 409",
title = "Enhanced photoconductivity of embedded SiGe nanoparticles by hydrogenation",
url = "http://www.sciencedirect.com/science/article/pii/S0169433219304398",
volume = 479,
year = 2019
}

21. Muhammad Taha Sultan, Andrei Manolescu, Jon Tomas Gudmundsson, Kristinn Torfason, George [Alexandru Nemnes], Ionel Stavarache, Constantin Logofatu, Valentin Serban Teodorescu, Magdalena Lidia Ciurea and Halldor Gudfinnur Svavarsson.
Enhanced photoconductivity of SiGe nanocrystals in SiO2 driven by mild annealing.
Applied Surface Science 469, 870 - 878 (2019).
Abstract Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization of photosensitive Si1-xGex NCs sandwiched between SiO2 matrix. A radio-frequency magnetron sputtering was applied to obtain a multilayer-structures (MLs) by depositing SiO2/SiGe/SiO2 films on Si (0 0 1) substrate. The Si1-xGex NCs were formed by a post-deposition annealing at 100–700 °C for 1–5 min. The effect of annealing temperature and time on MLs morphology and NCs size and density was studied using grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and measurements of spectral distribution of photocurrent. It is demonstrated how the photoconductive properties of the MLs can be enhanced and tailored by controlling the NCs formation conditions and the presence of stress field in MLs and defects acting as traps and recombination centers. All these features can be adjusted/controlled by altering the annealing conditions (temperature and time). The MLs photosensitivity was increased of more than an order of magnitude by the annealing process. A mechanism, where a competition between crystallization process (NCs formation and evolution i.e. size and shapes) and stress field appearance determines the peak position in the photocurrent spectra, was identified.
URL, DOI BibTeX

@article{SULTAN2019870 ,
abstract = "Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization of photosensitive Si1-xGex NCs sandwiched between SiO2 matrix. A radio-frequency magnetron sputtering was applied to obtain a multilayer-structures (MLs) by depositing SiO2/SiGe/SiO2 films on Si (0 0 1) substrate. The Si1-xGex NCs were formed by a post-deposition annealing at 100–700 °C for 1–5 min. The effect of annealing temperature and time on MLs morphology and NCs size and density was studied using grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and measurements of spectral distribution of photocurrent. It is demonstrated how the photoconductive properties of the MLs can be enhanced and tailored by controlling the NCs formation conditions and the presence of stress field in MLs and defects acting as traps and recombination centers. All these features can be adjusted/controlled by altering the annealing conditions (temperature and time). The MLs photosensitivity was increased of more than an order of magnitude by the annealing process. A mechanism, where a competition between crystallization process (NCs formation and evolution i.e. size and shapes) and stress field appearance determines the peak position in the photocurrent spectra, was identified.",
author = "Muhammad Taha Sultan and Andrei Manolescu and Jon Tomas Gudmundsson and Kristinn Torfason and George [Alexandru Nemnes] and Ionel Stavarache and Constantin Logofatu and Valentin Serban Teodorescu and Magdalena Lidia Ciurea and Halldor Gudfinnur Svavarsson",
doi = "https://doi.org/10.1016/j.apsusc.2018.11.061",
issn = "0169-4332",
journal = "Applied Surface Science",
keywords = "SiO, SiGe nanocrystals, Strain, Radio frequency magnetron sputtering, Thermal annealing, Photoconductivity",
pages = "870 - 878",
title = "Enhanced photoconductivity of SiGe nanocrystals in SiO2 driven by mild annealing",
url = "http://www.sciencedirect.com/science/article/pii/S0169433218331295",
volume = 469,
year = 2019
}

22. M T Sultan, J T Gudmundsson, A Manolescu, V S Teodorescu, M L Ciurea and H G Svavarsson.
Efficacy of annealing and fabrication parameters on photo-response of SiGe in TiO2 matrix.
Nanotechnology 30, 365604 (June 2019).
Abstract SiGe nanoparticles dispersed in a dielectric matrix exhibit properties different from those of bulk and have shown great potential in devices for application in advanced optoelectronics. Annealing is a common fabrication step used to increase crystallinity and to form nanoparticles in such a system. A frequent downside of such annealing treatment is the formation of insulating SiO2 layer at the matrix/SiGe interface, degrading the optical properties of the structure. An annealing process that could bypass this downside would therefore be of great interest. In this work, a short-time furnace annealing of a SiGe/TiO2 system is applied to obtain SiGe nanoparticles without formation of the undesired SiO2 layer between the dielectric matrix (TiO2) and SiGe. The structures were prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A wide range spectral response with a response-threshold up to ∼1300 nm was obtained, accompanied with an increase in photo-response of more than two-orders of magnitude. Scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy and grazing incidence x-ray diffraction were used to analyze the morphological changes in respective structures. Photoconductive properties were studied by measuring photocurrent spectra using applied dc-voltages at various temperatures.
URL, DOI BibTeX

@article{Sultan_2019 ,
abstract = "SiGe nanoparticles dispersed in a dielectric matrix exhibit properties different from those of bulk and have shown great potential in devices for application in advanced optoelectronics. Annealing is a common fabrication step used to increase crystallinity and to form nanoparticles in such a system. A frequent downside of such annealing treatment is the formation of insulating SiO2 layer at the matrix/SiGe interface, degrading the optical properties of the structure. An annealing process that could bypass this downside would therefore be of great interest. In this work, a short-time furnace annealing of a SiGe/TiO2 system is applied to obtain SiGe nanoparticles without formation of the undesired SiO2 layer between the dielectric matrix (TiO2) and SiGe. The structures were prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A wide range spectral response with a response-threshold up to ∼1300 nm was obtained, accompanied with an increase in photo-response of more than two-orders of magnitude. Scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy and grazing incidence x-ray diffraction were used to analyze the morphological changes in respective structures. Photoconductive properties were studied by measuring photocurrent spectra using applied dc-voltages at various temperatures.",
author = "M T Sultan and J T Gudmundsson and A Manolescu and V S Teodorescu and M L Ciurea and H G Svavarsson",
doi = "10.1088/1361-6528/ab260e",
journal = "Nanotechnology",
month = "jun",
number = 36,
pages = 365604,
publisher = "{IOP} Publishing",
title = "Efficacy of annealing and fabrication parameters on photo-response of {SiGe} in {TiO}2 matrix",
url = "https://doi.org/10.1088%2F1361-6528%2Fab260e",
volume = 30,
year = 2019
}

23. Miguel Urbaneja Torres, Anna Sitek and Andrei Manolescu.
Anisotropic light scattering by prismatic semiconductor nanowires.
Opt. Express 27, 25502–25514 (September 2019).
Abstract Anisotropic transverse light scattering by prismatic nanowires is a natural outcome of their geometry. In this work, we perform numerical calculations of the light scattering characteristics for nanowires in the optical and near-infrared range and explore the possibility of tuning the directivity by changing the angle of light incidence. The scattering cross section and the directivity of the scattered light when it is incident perpendicular to a facet or to an edge of the prism are investigated both with transverse electric and with transverse magnetic polarizations. The phenomenology includes Mie resonances and guided modes yielding together rich and complex spectra. We consider nanowires with hexagonal, square and triangular cross sections. The modes that are most sensitive to the incidence angle are the hexapole for the hexagonal case and the quadrupole for the square case. Higher order modes are also sensitive, but mostly for the square geometry. Our results indicate the possibility of a flexible in-situ tunability of the directivity simply by rotating the nanowire profile relatively to the direction of the incident light which could offer potential advantages in applications such as switching or sensing.
URL, DOI BibTeX

@article{UrbanejaTorres:19 ,
abstract = "Anisotropic transverse light scattering by prismatic nanowires is a natural outcome of their geometry. In this work, we perform numerical calculations of the light scattering characteristics for nanowires in the optical and near-infrared range and explore the possibility of tuning the directivity by changing the angle of light incidence. The scattering cross section and the directivity of the scattered light when it is incident perpendicular to a facet or to an edge of the prism are investigated both with transverse electric and with transverse magnetic polarizations. The phenomenology includes Mie resonances and guided modes yielding together rich and complex spectra. We consider nanowires with hexagonal, square and triangular cross sections. The modes that are most sensitive to the incidence angle are the hexapole for the hexagonal case and the quadrupole for the square case. Higher order modes are also sensitive, but mostly for the square geometry. Our results indicate the possibility of a flexible in-situ tunability of the directivity simply by rotating the nanowire profile relatively to the direction of the incident light which could offer potential advantages in applications such as switching or sensing.",
author = "Miguel Urbaneja Torres and Anna Sitek and Andrei Manolescu",
doi = "10.1364/OE.27.025502",
journal = "Opt. Express",
keywords = "Forward scattering; Guided mode resonance; Light matter interactions; Light scattering; Mie resonances; Resonant modes",
month = "Sep",
number = 18,
pages = "25502--25514",
publisher = "OSA",
title = "Anisotropic light scattering by prismatic semiconductor nanowires",
url = "http://www.opticsexpress.org/abstract.cfm?URI=oe-27-18-25502",
volume = 27,
year = 2019
}