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Iman S . Roqan

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Arizona State University

Alberto Pimpinelli

Université Clermont Auvergne

University of Galway

UPM - Universiti Putra Malaysia

Suseendran Jayachandran

IMEC

University of Dr moulay taher

Kalasalingam University

Richard Gauthier

Santa Rosa Junior College

University of Regensburg

University of Bern

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Papers by Iman S . Roqan

Structural, magnetic and electronic properties of two dimensional NdN: an ab initio study

RSC Advances, 2019

The peculiar magnetic properties of rare earth nitrides (RENs) make them suitable for a wide rang... more The peculiar magnetic properties of rare earth nitrides (RENs) make them suitable for a wide range of applications. Here, we report on a density functional theory (DFT) study of an interesting member of the family, two-dimensional (2D) NdN film, using the generalized gradient approximation (GGA), including the Hubbard (U) parameter. We consider different film thicknesses, taking into account the effects of N vacancies (V N) and dopants (C and O). Formation energy values show that, even though N vacancy is the predominant defect, C and O dopants are also probable impurities in these films. Individual Nd and N magnetic moments oscillate in the presence of V N and dopants owing to the induced lattice distortions. The density of states calculations show that the 2D NdN film has a semi-metallic nature, while the f orbitals are separated into fully filled and empty bands. A magnetic anisotropy energy of $50 meV is obtained, and the easy axis aligns along the film orientation as the film thickness increases, revealing that such films are ideal candidates for spintronic applications.

Rare earth doping of III-nitride alloys by ion implantation

Physica Status Solidi A-applications and Materials Science, 2008

Abstract The implantation damage and rare earth (RE) luminescence in the wide band gap ternaries ... more

Correction: The origin of room temperature ferromagnetism mediated by Co–V<sub>Zn</sub> complexes in the ZnO grain boundary

RSC Advances, 2016

The authors regret that the equation for the exchange coupling parameter shown in the nal paragr... more The authors regret that the equation for the exchange coupling parameter shown in the nal paragraph of the Results and discussion is incorrect in the original article. The correct version of the equation, in which the positions of the ferromagnetic (FM) and anti-ferromagnetic (AFM) congurations are reversed, is: DE ¼ E AFM À E FM. The subsequent discussion on DE is unaffected by this typographical error.

Photonics: Enhanced Performance of MoS<sub>2</sub> Photodetectors by Inserting an ALD-Processed TiO<sub>2</sub> Interlayer (Small 5/2018)

Small, Feb 1, 2018

Europium doping of zincblende GaN by ion implantation

Journal of Applied Physics, Jun 1, 2009

Eu was implanted into high quality cubic ͑zincblende͒ GaN ͑ZB-GaN͒ layers grown by molecular beam... more Eu was implanted into high quality cubic ͑zincblende͒ GaN ͑ZB-GaN͒ layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction ͑XRD͒ and Rutherford backscattering/channeling spectrometry. A low concentration ͑Ͻ10%͒ of wurtzite phase inclusions was observed by XRD analysis in as-grown samples with their ͑0001͒ planes aligned with the ͕111͖ planes of the cubic lattice. Implantation of Eu causes an expansion of the lattice parameter in the implanted region similar to that observed for the c-lattice parameter of wurtzite GaN ͑W-GaN͒. For ZB-GaN:Eu, a large fraction of Eu ions is found on a high symmetry interstitial site aligned with the ͗110͘ direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal annealing, but an increase in the wurtzite phase fraction was observed at the same time. Cathodoluminescence, photoluminescence ͑PL͒, and PL excitation spectroscopy revealed several emission lines which can be attributed to distinct Eu-related optical centers in ZB-GaN and W-GaN inclusions.

Blue cathodoluminescence from thulium implanted AlxGa1−xN and InxAl1−xN

Superlattices and Microstructures, Oct 1, 2006

Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted AlxGa1− xN with differ... more

Lattice site location of optical centers in GaN:Eu light emitting diode material grown by organometallic vapor phase epitaxy

Applied Physics Letters, Sep 13, 2010

Analysis on the energetics, magnetism and electronic properties in a 45° ZnO grain boundary doped with Gd

RSC Advances, 2018

The structural stability and magnetic properties of a grain boundary (GB) formed by aligning two ... more The structural stability and magnetic properties of a grain boundary (GB) formed by aligning two ZnO single crystals oriented at an angle of 45 is investigated by density functional theory, using generalized gradient approximation (GGA) and taking the U parameter into consideration for the 4f impurity states. We found that the GB is stable with no dangling bonds and inter-granular structures. The stability of defects such as Gd substituted to the Zn site (Gd Zn), Zn vacancy (V Zn) and O vacancy (V O) as well as defect complexes

Highly Stable and Ultrafast Hydrogen Gas Sensor Based on 15 nm Nanogaps Switching in a Palladium-Gold Nanoribbons Array

Advanced Materials Interfaces, Dec 27, 2018

The photoluminescence/excitation (PL/E) spectroscopy of Eu-implanted GaN

Optical Materials, May 1, 2011

Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently... more Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, Eu Ga , is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N 2. According to structural determinations, such samples exhibit an essentially complete removal of lattice damage caused by the implantation process. A second centre, Eu1, probably comprising Eu Ga in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the 5 D 0 to 7 F 2 spectral region near 620 nm. We can readily distinguish Eu1 and Eu2 by their excitation spectra, in particular through their different sensitivities to above-gap and below-gap excitation. The present study extends recent work on photoluminescence/excitation (PL/E) spectroscopy of Eu1 and Eu2 to arrive at an understanding of these mechanisms in terms of residual optically active defect concentrations. We also report further on the 'host-independent' excitation mechanism that is active in the case of a prominent minority centre. The relevance of this work to the operation of the red GaN:Eu light-emitting diode is discussed.

Enhanced Performance of MoS<sub>2</sub> Photodetectors by Inserting an ALD-Processed TiO<sub>2</sub> Interlayer

Small, Dec 4, 2017

2D molybdenum disulfide (MoS2 ) possesses excellent optoelectronic properties that make it a prom... more 2D molybdenum disulfide (MoS2 ) possesses excellent optoelectronic properties that make it a promising candidate for use in high-performance photodetectors. Yet, to meet the growing demand for practical and reliable MoS2 photodetectors, the critical issue of defect introduction to the interface between the exfoliated MoS2 and the electrode metal during fabrication must be addressed, because defects deteriorate the device performance. To achieve this objective, the use of an atomic layer-deposited TiO2 interlayer (between exfoliated MoS2 and electrode) is reported in this work, for the first time, to enhance the performance of MoS2 photodetectors. The TiO2 interlayer is inserted through 20 atomic layer deposition cycles before depositing the electrode metal on MoS2 /SiO2 substrate, leading to significantly enhanced photoresponsivity and response speed. These results pave the way for practical applications and provide a novel direction for optimizing the interlayer material.

Defect induced <i>d</i><sup>0</sup> ferromagnetism in a ZnO grain boundary

Journal of Chemical Physics, Dec 8, 2015

Several experimental studies have referred to the grain boundary (GB) defect as the origin of the... more Several experimental studies have referred to the grain boundary (GB) defect as the origin of the ferromagnetism in ZnO. However, the mechanism of this hypothesis has never been confirmed. Present study investigates the atomic structure and the effect of point defects in a ZnO GB using the GGA+U approximation. The relaxed GB possesses large periodicity and channels with 8 and 10 numbered atoms having 4 and 3 fold coordination. Unlike O vacancy (V O), Zn vacancy (V Zn) is preferentially aggregated at the GB, relative to the bulk-like region, indicating the possibility of obtaining p-type conductivity in polycrystalline ZnO. Although no magnetization is obtained from point defect-free GB, V Zn induces spin polarization as large as 0.68 µ B /atom to the O sites at the GB. Ferromagnetic exchange energy > 150 eV is obtained by increasing the concentration of V Zn and by the injection of holes into the system. Electronic structure analysis indicates that the spin polarization without external dopants originates from the O 2p orbitals, a common feature of d 0 semiconductors.

Correction: Analysis on the energetics, magnetism and electronic properties in a 45° ZnO grain boundary doped with Gd

RSC Advances, 2019

The authors regret that an incorrect grant number was shown in the Acknowledgements section of th... more

Identifying the influence of the intrinsic defects in Gd-doped ZnO thin-films

Journal of Applied Physics, Feb 8, 2016

Gd-doped ZnO thin films were prepared using pulsed laser deposition at different oxygen pressures... more Gd-doped ZnO thin films were prepared using pulsed laser deposition at different oxygen pressures and varied Gd concentrations. The effects of oxygen deficiency-related defects on the Gd incorporation, optical and structural properties, were explored by studying the impact of oxygen pressure during deposition and post-growth thermal annealing in vacuum. Rutherford Backscattering Spectrometry revealed that the Gd concentration increases with increasing oxygen pressure for samples grown with the same Gd-doped ZnO target. Unexpectedly, the c-lattice parameter of the samples tends to decrease with increasing Gd concentration, suggesting that Gd-defect complexes play an important role in the structural properties. Using low-temperature photoluminescence (PL), Raman measurements and density functional theory calculations, we identified oxygen vacancies as the dominant intrinsic point defects. PL spectra show a defect band related to oxygen vacancies for samples grown at oxygen deficiency. V

Nature of low dimensional structural modulations and relative phase stability in MoS2/WS2-ReS2 transition metal dichalcogenide alloys

arXiv (Cornell University), Nov 2, 2016

We report on the various types of Peierls like two dimensional structural modulations and relativ... more We report on the various types of Peierls like two dimensional structural modulations and relative phase stability of 2H and 1T poly-types in MoS 2-ReS 2 and WS 2-ReS 2 alloy system. Theoretical calculation predicts a polytype phase transition cross over at ~50 at.% of Mo and W in ReS 2 in both monolayer and bulk form, respectively. Experimentally, two different types of structural modulations at 50% and a modulation corresponding to trimerization at 75% alloy composition is observed for MoS 2-ReS 2 and only one type of modulation is observed at 50% WS 2-ReS 2 alloy system. The 50% alloy system is found to be a suitable monolithic candidate for metal semiconductor transition with minute external perturbation. ReS 2 is known to be in 2D Peierls distorted 1T d structure and forms a chain like superstructure. Incorporation of Mo and W atoms in the ReS 2 lattice modifies the metal-metal hybridization between the cations and influences the structural modulation and electronic property of the system. The results offer yet another effective way to tune the electronic 2 structure and poly-type phases of this class of materials other than intercalation, strain, and vertical stacking arrangement. I.

The magnetic ordering in high magnetoresistance Mn-doped ZnO thin films

AIP Advances, Mar 1, 2016

We studied the nature of magnetic ordering in Mn-doped ZnO thin films that exhibited ferromagneti... more We studied the nature of magnetic ordering in Mn-doped ZnO thin films that exhibited ferromagnetism at 300 K and superparamagnetism at 5 K. We directly interrelated the magnetisation and magnetoresistance by invoking the polaron percolation theory and variable range of hopping conduction below the metal-to-insulator transition. By obtaining a qualitative agreement between these two models, we attribute the ferromagnetism to the s-d exchange-induced spin splitting that was indicated by large positive magnetoresistance (∼40 %). Low temperature superparamagnetism was attributed to the localization of carriers and non-interacting polaron clusters. This analysis can assist in understanding the presence or absence of ferromagnetism in doped/un-doped ZnO.

Extremely reduced dielectric confinement in two-dimensional hybrid perovskites with large polar organics

Communications physics, Nov 15, 2018

Two dimensional inorganic-organic hybrid perovskites (2D perovskites) suffer from not only quantu... more Two dimensional inorganic-organic hybrid perovskites (2D perovskites) suffer from not only quantum confinement, but also dielectric confinement, hindering their application perspective in devices involving the conversion of an optical input into current. In this report, we theoretically predict that an extremely low exciton binding energy can be achieved in 2D perovskites by using high dielectric-constant organic components. We demonstrate that in (HOCH 2 CH 2 NH 3) 2 PbI 4 , whose organic material has a high dielectric constant of 37, the dielectric confinement is largely reduced, and the exciton binding energy is 20-times smaller than that in conventional 2D perovskites. As a result, the photo-induced excitons can be thermally dissociated efficiently at room temperature, as clearly indicated from femtosecond transient absorption measurements. In addition, the mobility is largely improved due to the strong screening effect on charge impurities. Such low dielectric-confined 2D perovskites show excellent carrier extraction efficiency, and outstanding humidity resistance compared to conventional 2D perovskites.

Evidence of Carrier Localization in AlGaN/GaN‐Based UV Multiple Quantum Wells with Opposite Polarity Domains Provided by Nanoscale Imaging

physica status solidi (RRL) – Rapid Research Letters

AlGaN‐based multiple quantum wells (MQWs) incorporating opposite polarity domains are grown by me... more AlGaN‐based multiple quantum wells (MQWs) incorporating opposite polarity domains are grown by metal–organic chemical vapor deposition (MOCVD). A direct demonstration of the carrier localization effect is provided by a combination analysis of space‐resolved luminescence peak position and Ga/Al composition distribution. Furthermore, through Raman spectroscopy, it is found that compressive strain plays a key role in improving the optical properties of UV‐MQWs despite the inferior crystalline quality in the N‐polar domains. This suggests that incorporating sub‐micrometer‐scale polarity domains in the MQWs is promising for the development of efficient UV emitters.

Gallium nitride on gallium oxide substrate for integrated nonlinear optics

2017 Photonics North (PN), 2017

A solid–liquid two-phase precipitation method for the growth of fullerene (C60) nanowires

CrystEngComm, 2021

A schematic diagram of the preparation of C60 nanowires by the solid–liquid two-phase precipitati... more

Structural, magnetic and electronic properties of two dimensional NdN: an ab initio study

RSC Advances, 2019

The peculiar magnetic properties of rare earth nitrides (RENs) make them suitable for a wide rang... more The peculiar magnetic properties of rare earth nitrides (RENs) make them suitable for a wide range of applications. Here, we report on a density functional theory (DFT) study of an interesting member of the family, two-dimensional (2D) NdN film, using the generalized gradient approximation (GGA), including the Hubbard (U) parameter. We consider different film thicknesses, taking into account the effects of N vacancies (V N) and dopants (C and O). Formation energy values show that, even though N vacancy is the predominant defect, C and O dopants are also probable impurities in these films. Individual Nd and N magnetic moments oscillate in the presence of V N and dopants owing to the induced lattice distortions. The density of states calculations show that the 2D NdN film has a semi-metallic nature, while the f orbitals are separated into fully filled and empty bands. A magnetic anisotropy energy of $50 meV is obtained, and the easy axis aligns along the film orientation as the film thickness increases, revealing that such films are ideal candidates for spintronic applications.

Rare earth doping of III-nitride alloys by ion implantation

Physica Status Solidi A-applications and Materials Science, 2008

Abstract The implantation damage and rare earth (RE) luminescence in the wide band gap ternaries ... more

Correction: The origin of room temperature ferromagnetism mediated by Co–V<sub>Zn</sub> complexes in the ZnO grain boundary

RSC Advances, 2016

The authors regret that the equation for the exchange coupling parameter shown in the nal paragr... more The authors regret that the equation for the exchange coupling parameter shown in the nal paragraph of the Results and discussion is incorrect in the original article. The correct version of the equation, in which the positions of the ferromagnetic (FM) and anti-ferromagnetic (AFM) congurations are reversed, is: DE ¼ E AFM À E FM. The subsequent discussion on DE is unaffected by this typographical error.

Photonics: Enhanced Performance of MoS<sub>2</sub> Photodetectors by Inserting an ALD-Processed TiO<sub>2</sub> Interlayer (Small 5/2018)

Small, Feb 1, 2018

Europium doping of zincblende GaN by ion implantation

Journal of Applied Physics, Jun 1, 2009

Eu was implanted into high quality cubic ͑zincblende͒ GaN ͑ZB-GaN͒ layers grown by molecular beam... more Eu was implanted into high quality cubic ͑zincblende͒ GaN ͑ZB-GaN͒ layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction ͑XRD͒ and Rutherford backscattering/channeling spectrometry. A low concentration ͑Ͻ10%͒ of wurtzite phase inclusions was observed by XRD analysis in as-grown samples with their ͑0001͒ planes aligned with the ͕111͖ planes of the cubic lattice. Implantation of Eu causes an expansion of the lattice parameter in the implanted region similar to that observed for the c-lattice parameter of wurtzite GaN ͑W-GaN͒. For ZB-GaN:Eu, a large fraction of Eu ions is found on a high symmetry interstitial site aligned with the ͗110͘ direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal annealing, but an increase in the wurtzite phase fraction was observed at the same time. Cathodoluminescence, photoluminescence ͑PL͒, and PL excitation spectroscopy revealed several emission lines which can be attributed to distinct Eu-related optical centers in ZB-GaN and W-GaN inclusions.

Blue cathodoluminescence from thulium implanted AlxGa1−xN and InxAl1−xN

Superlattices and Microstructures, Oct 1, 2006

Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted AlxGa1− xN with differ... more

Lattice site location of optical centers in GaN:Eu light emitting diode material grown by organometallic vapor phase epitaxy

Applied Physics Letters, Sep 13, 2010

Analysis on the energetics, magnetism and electronic properties in a 45° ZnO grain boundary doped with Gd

RSC Advances, 2018

The structural stability and magnetic properties of a grain boundary (GB) formed by aligning two ... more The structural stability and magnetic properties of a grain boundary (GB) formed by aligning two ZnO single crystals oriented at an angle of 45 is investigated by density functional theory, using generalized gradient approximation (GGA) and taking the U parameter into consideration for the 4f impurity states. We found that the GB is stable with no dangling bonds and inter-granular structures. The stability of defects such as Gd substituted to the Zn site (Gd Zn), Zn vacancy (V Zn) and O vacancy (V O) as well as defect complexes

Highly Stable and Ultrafast Hydrogen Gas Sensor Based on 15 nm Nanogaps Switching in a Palladium-Gold Nanoribbons Array

Advanced Materials Interfaces, Dec 27, 2018

The photoluminescence/excitation (PL/E) spectroscopy of Eu-implanted GaN

Optical Materials, May 1, 2011

Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently... more Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, Eu Ga , is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N 2. According to structural determinations, such samples exhibit an essentially complete removal of lattice damage caused by the implantation process. A second centre, Eu1, probably comprising Eu Ga in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the 5 D 0 to 7 F 2 spectral region near 620 nm. We can readily distinguish Eu1 and Eu2 by their excitation spectra, in particular through their different sensitivities to above-gap and below-gap excitation. The present study extends recent work on photoluminescence/excitation (PL/E) spectroscopy of Eu1 and Eu2 to arrive at an understanding of these mechanisms in terms of residual optically active defect concentrations. We also report further on the 'host-independent' excitation mechanism that is active in the case of a prominent minority centre. The relevance of this work to the operation of the red GaN:Eu light-emitting diode is discussed.

Enhanced Performance of MoS<sub>2</sub> Photodetectors by Inserting an ALD-Processed TiO<sub>2</sub> Interlayer

Small, Dec 4, 2017

2D molybdenum disulfide (MoS2 ) possesses excellent optoelectronic properties that make it a prom... more 2D molybdenum disulfide (MoS2 ) possesses excellent optoelectronic properties that make it a promising candidate for use in high-performance photodetectors. Yet, to meet the growing demand for practical and reliable MoS2 photodetectors, the critical issue of defect introduction to the interface between the exfoliated MoS2 and the electrode metal during fabrication must be addressed, because defects deteriorate the device performance. To achieve this objective, the use of an atomic layer-deposited TiO2 interlayer (between exfoliated MoS2 and electrode) is reported in this work, for the first time, to enhance the performance of MoS2 photodetectors. The TiO2 interlayer is inserted through 20 atomic layer deposition cycles before depositing the electrode metal on MoS2 /SiO2 substrate, leading to significantly enhanced photoresponsivity and response speed. These results pave the way for practical applications and provide a novel direction for optimizing the interlayer material.

Defect induced <i>d</i><sup>0</sup> ferromagnetism in a ZnO grain boundary

Journal of Chemical Physics, Dec 8, 2015

Several experimental studies have referred to the grain boundary (GB) defect as the origin of the... more Several experimental studies have referred to the grain boundary (GB) defect as the origin of the ferromagnetism in ZnO. However, the mechanism of this hypothesis has never been confirmed. Present study investigates the atomic structure and the effect of point defects in a ZnO GB using the GGA+U approximation. The relaxed GB possesses large periodicity and channels with 8 and 10 numbered atoms having 4 and 3 fold coordination. Unlike O vacancy (V O), Zn vacancy (V Zn) is preferentially aggregated at the GB, relative to the bulk-like region, indicating the possibility of obtaining p-type conductivity in polycrystalline ZnO. Although no magnetization is obtained from point defect-free GB, V Zn induces spin polarization as large as 0.68 µ B /atom to the O sites at the GB. Ferromagnetic exchange energy > 150 eV is obtained by increasing the concentration of V Zn and by the injection of holes into the system. Electronic structure analysis indicates that the spin polarization without external dopants originates from the O 2p orbitals, a common feature of d 0 semiconductors.

Correction: Analysis on the energetics, magnetism and electronic properties in a 45° ZnO grain boundary doped with Gd

RSC Advances, 2019

The authors regret that an incorrect grant number was shown in the Acknowledgements section of th... more

Identifying the influence of the intrinsic defects in Gd-doped ZnO thin-films

Journal of Applied Physics, Feb 8, 2016

Gd-doped ZnO thin films were prepared using pulsed laser deposition at different oxygen pressures... more Gd-doped ZnO thin films were prepared using pulsed laser deposition at different oxygen pressures and varied Gd concentrations. The effects of oxygen deficiency-related defects on the Gd incorporation, optical and structural properties, were explored by studying the impact of oxygen pressure during deposition and post-growth thermal annealing in vacuum. Rutherford Backscattering Spectrometry revealed that the Gd concentration increases with increasing oxygen pressure for samples grown with the same Gd-doped ZnO target. Unexpectedly, the c-lattice parameter of the samples tends to decrease with increasing Gd concentration, suggesting that Gd-defect complexes play an important role in the structural properties. Using low-temperature photoluminescence (PL), Raman measurements and density functional theory calculations, we identified oxygen vacancies as the dominant intrinsic point defects. PL spectra show a defect band related to oxygen vacancies for samples grown at oxygen deficiency. V

Nature of low dimensional structural modulations and relative phase stability in MoS2/WS2-ReS2 transition metal dichalcogenide alloys

arXiv (Cornell University), Nov 2, 2016

We report on the various types of Peierls like two dimensional structural modulations and relativ... more We report on the various types of Peierls like two dimensional structural modulations and relative phase stability of 2H and 1T poly-types in MoS 2-ReS 2 and WS 2-ReS 2 alloy system. Theoretical calculation predicts a polytype phase transition cross over at ~50 at.% of Mo and W in ReS 2 in both monolayer and bulk form, respectively. Experimentally, two different types of structural modulations at 50% and a modulation corresponding to trimerization at 75% alloy composition is observed for MoS 2-ReS 2 and only one type of modulation is observed at 50% WS 2-ReS 2 alloy system. The 50% alloy system is found to be a suitable monolithic candidate for metal semiconductor transition with minute external perturbation. ReS 2 is known to be in 2D Peierls distorted 1T d structure and forms a chain like superstructure. Incorporation of Mo and W atoms in the ReS 2 lattice modifies the metal-metal hybridization between the cations and influences the structural modulation and electronic property of the system. The results offer yet another effective way to tune the electronic 2 structure and poly-type phases of this class of materials other than intercalation, strain, and vertical stacking arrangement. I.

The magnetic ordering in high magnetoresistance Mn-doped ZnO thin films

AIP Advances, Mar 1, 2016

We studied the nature of magnetic ordering in Mn-doped ZnO thin films that exhibited ferromagneti... more We studied the nature of magnetic ordering in Mn-doped ZnO thin films that exhibited ferromagnetism at 300 K and superparamagnetism at 5 K. We directly interrelated the magnetisation and magnetoresistance by invoking the polaron percolation theory and variable range of hopping conduction below the metal-to-insulator transition. By obtaining a qualitative agreement between these two models, we attribute the ferromagnetism to the s-d exchange-induced spin splitting that was indicated by large positive magnetoresistance (∼40 %). Low temperature superparamagnetism was attributed to the localization of carriers and non-interacting polaron clusters. This analysis can assist in understanding the presence or absence of ferromagnetism in doped/un-doped ZnO.

Extremely reduced dielectric confinement in two-dimensional hybrid perovskites with large polar organics

Communications physics, Nov 15, 2018

Two dimensional inorganic-organic hybrid perovskites (2D perovskites) suffer from not only quantu... more Two dimensional inorganic-organic hybrid perovskites (2D perovskites) suffer from not only quantum confinement, but also dielectric confinement, hindering their application perspective in devices involving the conversion of an optical input into current. In this report, we theoretically predict that an extremely low exciton binding energy can be achieved in 2D perovskites by using high dielectric-constant organic components. We demonstrate that in (HOCH 2 CH 2 NH 3) 2 PbI 4 , whose organic material has a high dielectric constant of 37, the dielectric confinement is largely reduced, and the exciton binding energy is 20-times smaller than that in conventional 2D perovskites. As a result, the photo-induced excitons can be thermally dissociated efficiently at room temperature, as clearly indicated from femtosecond transient absorption measurements. In addition, the mobility is largely improved due to the strong screening effect on charge impurities. Such low dielectric-confined 2D perovskites show excellent carrier extraction efficiency, and outstanding humidity resistance compared to conventional 2D perovskites.

Evidence of Carrier Localization in AlGaN/GaN‐Based UV Multiple Quantum Wells with Opposite Polarity Domains Provided by Nanoscale Imaging

physica status solidi (RRL) – Rapid Research Letters

AlGaN‐based multiple quantum wells (MQWs) incorporating opposite polarity domains are grown by me... more AlGaN‐based multiple quantum wells (MQWs) incorporating opposite polarity domains are grown by metal–organic chemical vapor deposition (MOCVD). A direct demonstration of the carrier localization effect is provided by a combination analysis of space‐resolved luminescence peak position and Ga/Al composition distribution. Furthermore, through Raman spectroscopy, it is found that compressive strain plays a key role in improving the optical properties of UV‐MQWs despite the inferior crystalline quality in the N‐polar domains. This suggests that incorporating sub‐micrometer‐scale polarity domains in the MQWs is promising for the development of efficient UV emitters.

Gallium nitride on gallium oxide substrate for integrated nonlinear optics

2017 Photonics North (PN), 2017

A solid–liquid two-phase precipitation method for the growth of fullerene (C60) nanowires

CrystEngComm, 2021

A schematic diagram of the preparation of C60 nanowires by the solid–liquid two-phase precipitati... more