Athos Petrou

Athos Petrou
University at Albany, The State University of New York | UAlbany · Department of Physics

About

312
Publications
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5,954
Citations
Citations since 2016
15 Research Items
1184 Citations
2016201720182019202020212022050100150200
2016201720182019202020212022050100150200
2016201720182019202020212022050100150200
2016201720182019202020212022050100150200

Publications

Publications (312)
Article
We describe a study of the magneto-optical properties of Ag+-doped CdSe colloidal nanoplatelets (NPLs) that were grown using a novel doping technique. In this work, we used magnetic circularly polarized luminescence and magnetic circular dichroism spectroscopy to study light-induced magnetism for the first time in 2D solution-processed structures d...
Article
We used time-resolved photoluminescence spectroscopy to study the circular polarization dynamics of magnetic polarons in type-II (Zn,Mn)Te/ZnSe quantum dots in the presence of an external magnetic field. We investigated the time evolution of the peak energy of the σ+ and σ− circularly polarized photoluminescence components, and of the circular pola...
Article
We have utilized time-resolved photoluminescence (TRPL) spectroscopy to study the excitonic circular polarization (PX) from CdSe/CdMnS core/shell nanoplatelets (NPLs) with a bilayer core. This allows an extensive study of the emission dynamics as a function of magnetic field, temperature, doping concentration, and excitation wavelength. In the pres...
Article
Unlike the extensively studied, spatially direct, quantum dots (QDs) with type-I band alignment where both electrons and holes are confined in the QD, in ZnTe QDs embedded in a (Zn,Mn)Se matrix only the holes are confined in the QDs. This spatially indirect type-II system provides unexplored opportunities to control the magnetic interactions betwee...
Article
Full-text available
Lifting the valley degeneracy of monolayer transition metal dichalcogenides (TMDs) would allow versatile control of the valley degree of freedom. We report a giant valley exciton splitting of 16 meV/T for monolayer WS2, using the proximity effect from an EuS substrate, which is enhanced by nearly two orders of magnitude from that obtained by an ext...
Article
Full-text available
The effect of an external magnetic field B on magnetic materials is a subject of immense importance. The simplest and oldest manifestation of such effects is the behavior of the magnetic compass. Magnetization M plays a key role in studying the response of magnetic materials to B. In this paper, an experimental technique for the determination of M...
Preprint
Full-text available
Lifting the valley degeneracy of monolayer transition metal dichalcogenides (TMD) would allow versatile control of the valley degree of freedom. We report a giant valley exciton splitting of 18 meV/T for monolayer WS2, using the proximity effect from a ferromagnetic EuS substrate, which is enhanced by nearly two orders of magnitude from the 0.2 meV...
Article
Full-text available
Spatially indirect Type-II band alignment in magnetically-doped quantum dot (QD) structures provides unexplored opportunities to control the magnetic interaction between carrier wavefunction in the QD and magnetic impurities. Unlike the extensively studied, spatially direct, QDs with Type-I band alignment where both electrons and holes are confined...
Conference Paper
We compare the red shift of the PL peak energy with magnetic field in: a) ZnTe QDs embedded in an ZnMnSe matrix and b) ZnMnTe QDs embedded in a ZnSe matrix using two photon energies: 1. Excitation at 405 nm (3.06 eV) with photon energy above the ZnSe and ZnMnSe matrix gaps resulting in electron-hole pairs mainly in the matrices. 2. Excitation at 48...
Conference Paper
Colloidal semiconductor nanoplatelets (NPLs) are quasi 2D-nanostructures that are grown and processed inexpensively using a solution based method and thus have recently attracted considerable attention. We observe two features in the photoluminescence spectrum, suggesting two possible recombination channels. Their intensity ratio varies with temper...
Article
Exploiting the valley degree of freedom to store and manipulate information provides a novel paradigm for future electronics. A monolayer transition metal dichalcogenide (TMDC) with broken inversion symmetry possesses two degenerate yet inequivalent valleys, offering unique opportunities for valley control through helicity of light. Lifting the val...
Conference Paper
Full-text available
We studied the photoluminescence (PL) from CdSe/CdMnS/CdS core/multi-shell colloidal nanoplatelets, a versatile platform to study the interplay of optical properties and nanomagnetism. The photoluminescence (PL) exhibits σ+ polarization in the applied magnetic field. Our measurement detects the presence of even a single magnetic monolayer shell. Th...
Article
We used photoluminescence spectroscopy to resolve two emission features in CdSe/CdMnS/CdS and CdSe/CdS core/multi-shell nanoplatelet heterostructures. The photoluminescence from the magnetic sample has a positive circular polarization with a maximum centered at the position of the lower energy feature. The higher energy feature has a corresponding...
Article
Full-text available
InGaAs quantum well light emitting diodes (LED) with spin-injecting, epitaxial Fe contacts were fabricated using an in situ wafer transfer process where the semiconductor wafer was transferred under ultrahigh vacuum (UHV) conditions to a metals growth chamber to achieve a high quality interface between the two materials. The spin LED devices were m...
Article
Full-text available
GaAsnanostructures were grown on patternedGaAs (111) B substrates with molecular beam epitaxy. Nanopatterns were achieved by patterning a thin film of silicon dioxide (SiO2) grown on the substrate surface with a self-organized porous alumina template. Growth of patternednanostructures took place through the holes in the SiO2 film. The authors obtai...
Article
Replacing the two sublattices of carbon atoms in graphene with transition metal atoms and chalcogenide dimers results in single layers of transition metal dichalcogenides (TMDCs). TMDCs are promising new materials for light and energy harvesting, transistors, sensors and quantum information processing. One way to access the distinctive functionalit...
Article
In this work, we report the manifestations of carrier-dopant exchange interactions in colloidal Mn(2+)-doped CdSe/CdS core/multishell quantum wells. The carrier-magnetic ion exchange interaction effects are tunable through wave function engineering. In our quantum well heterostructures, manganese was incorporated by growing a Cd0.985Mn0.015S monola...
Article
Full-text available
We used continuous wave photoluminescence (cw-PL) and time resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe/(Zn,Mn)Se system the holes are confined in the non-magnetic ZnTe quantum dots (QDs), and the electro...
Article
Materials often exhibit fundamentally new phenomena in reduced dimensions that potentially lead to novel applications. This is true for single-layer, two-dimensional semiconductor crystals of transition-metal dichalcogenides, MX2 (M = Mo, W and X = S, Se). They exhibit direct bandgaps with energies in the visible region at the two non-equivalent va...
Article
Full-text available
We have studied Be-acceptors in a p-type GaInNAs epilayer using magneto-luminescence spectroscopy. The band edge photoluminescence (PL) spectra at T = 7 K contain two features: the first is associated with the free exciton while the second with the conduction band to acceptor (CB → A) transition. The intensity of the latter decreases with increasin...
Conference Paper
Full-text available
We used time resolved photoluminescence (TRPL) spectroscopy to compare the properties of magnetic polarons in two related, spatially indirect, II-VI epitaxially grown quantum dot systems. In sample A (ZnMnTe/ZnSe), the photo-excited holes are confined in the magnetic ZnMnTe quantum dots (QDs), while the electrons remain in the surrounding non-magne...
Article
We report on a highly polarized emission from InGaAs/GaAs-quantum well light-emitting diodes in which we inject spin-polarized electrons from an Fe/Schottky contact. The emission spectra consist of the e1h1 free exciton (FX) and a feature 12 meV below FX attributed to band-to-band (BB) recombination. The FX exhibits a maximum circular polarization...
Article
Photoluminescence (PL), optical pumping, and reflectance studies of nominally undoped and p-type GaInNAs epilayers are presented. The PL peak energy of the nominally undoped sample exhibits an S-shaped dependence on temperature for T < 50 K. This is attributed to recombination of bound excitons localized on traps. The energy of the PL circular-pola...
Article
Cu2O is a direct gap semiconductor with a band gap of 2.1 eV. It was considered to be a solar absorber material, while the application is hindered by its large band gap and weak stability. Here we report an electrochemical synthesis of Cu2O. By rationally control the synthetic parameters, we achieved two types of Cu2O: one of black color and the ot...
Article
Full-text available
Optical pumping experiments have been performed on as-grown and p-type MBE grown GaInNAs epilayers. The PL peak of the nominally undoped as-grown sample exhibits the characteristic S-shaped dependence of dilute nitride material for T 60 K [1]. This is associated with carrier recombination via localized states at low temperatures. The reflectance sp...
Article
We have extended our previous investigation of time evolution of PL from (Zn,Mn)Te/ZnSe quantum dots in a magnetic field B [1]. PL studies at T=5 K in these type-II dots reveal formation of magnetic polarons (MP). We find their formation time τMP to be 0.5 ns, which varies little with B. The circular polarization P of the emission shows a surprisin...
Article
Electromagnetic Fields and Waves (EFW) courses for undergraduate students are usually taught in traditional lecture style. The instructors that teach EFW courses using this conventional method encounter several problems including these: 1. There is disconnect between the mathematics used to describe electromagnetic phenomena and the physics, even w...
Article
In this paper the authors review their work on spin-LEDs they have carried out. The general operating principles as well as the measuring techniques of spin-LEDs are discussed. Then particular examples of work on spin-LEDs are given using paramagnetic ZnMnSe and ferromagnetic Fe contacts are presented. Finally injection of spin polarized electrons...
Article
Full-text available
We report studies of two key factors involved in the use of MnAs as a spin injector, namely, Mn diffusion and spin injection properties when it is grown on GaAs-based semiconductor heterostructures. Depth profile of Mn concentration in a sample consisting of a MnAs film grown on GaAs is investigated with time-of-flight secondary ion mass spectrosco...
Article
Full-text available
Carrier-dopant exchange interactions in Mn-doped PbS colloidal quantum dots were studied by circularly polarized magneto-photoluminescence. Mn substitutional doping leads to paramagnetic behavior down to 5 K. While undoped quantum dots show negative circular polarization, Mn doping changes its sign to positive. A circular polarization value of 40%...
Article
Full-text available
The intensity of photoluminescence emission in InGaAs/GaAs quantum wells (QWs) exhibits local maxima as a function of magnetic field applied perpendicular to the QW plane at low temperatures. These maxima are attributed to the optical Aharonov-Bohm effect associated with spatially indirect excitons that are formed in the vicinity of indium-rich InG...
Article
We report magneto-photoluminescence studies on manganese-doped lead sulfide (Pb1-xMnxS, x = 0 to 5%) quantum dot system synthesized by solution-phase chemical method. The size (3 to 15nm), temperature (7K to 100K), magnetic field (0 to 7T), and laser power dependence (1mW to 20mW) of photoluminescence were systematically investigated, with a focus...
Article
We have carried out a magneto-PL study of a single layer of (Zn,Mn)Te/ZnSe quantum dots. The CW PL from a previously studied 5 layer sample exhibits high circular polarization but very small Zeeman splitting. Time-resolved PL exhibited a temperature independent temporal red shift, associated with the magnetic polaron formation. These data have been...
Article
We have carried out Hanle spin relaxation time measurements in InGaAs quantum well structures as well as in Fe-based spin-LEDs that incorporate InGaAs QWs. In the InGaAs QW structures the spin lifetime TS at T = 5 K is equal to 2 ns while in spin LEDs TS is only 0.17 ns. For the undoped QWs, TS increases from 2ns at 5K to 4.5ns at 15K and then decr...
Article
The photoluminescence (PL) from Al0.08Ga0.92As/Al.25Ga0.75As quantum wells (QW) was studied as function of magnetic field applied along the normal to the QW planes. The PL intensity exhibits two maxima at 2.3 and 4.9 tesla. The time-resolved PL from the same sample has a decay time which is one order of magnitude longer than the PL from a GaAs/AlGa...
Article
Full-text available
Diluted magnetic semiconductor quantum dots are interesting model systems for the investigation of carrier dopant exchange interactions. In this work, we report the carrier spin polarization studies in narrow band gap Mn-doped PbS quantum dots, a much less studied system compared to their II-VI counterparts. The PbMnS quantum dots were synthesized...
Article
We have grown single crystal zincblende epilayers of the diluted magnetic semiconductors Zn1−xFexSe and Zn1−xCoxSe on GaAs(001) by molecular beam epitaxy. We summarize the growth and the structural, magnetic and optical properties of these materials, and contrast their properties with those of the more well-studied Mn-based family of diluted magnet...
Article
We present recent experimental results that show strong optical AB effects in the magneto‐photoluminescence of columnar II‐VI quantum dots (QDs). The AB effects manifest themselves as oscillations in the photoluminescence energy and intensity with magnetic field. It will be shown that this coherent behavior is remarkably robust, persisting at 180...
Chapter
States of charged magnetoexcitons in quasi-two-dimensional systems are investigated. The effect of excess electrons on internal transitions of negatively charged excitons X− in GaAs quantum wells is studied experimentally by optically detected resonance (ODR) spectroscopy and theoretically. An experimentally observed blue shift with excess electron...
Article
An efficient electron spin-relaxation mechanism has been observed in InAs quantum dots (QDs) that manifests itself as a sharp drop in the circular polarization of the light emitted by Fe spin-light emitting diodes, which incorporate a single layer of InGaAs QDs, for a narrow range of magnetic fields around 5 T. The underlying mechanism occurs when...
Article
Recent studies of ferromagnetic MnAs have revealed a wide range of properties desirable for spintronic applications. In this work, ferromagnetic MnAs contacts have been used to inject spin polarized electrons into AlGaAs(n)/GaAs(i)/AlGaAs(p) light emitting diodes. The band-edge electroluminescence emitted from these devices has a saturation circula...
Article
We have observed magnetic field oscillations in the photoluminescence (PL) intensity from InGaAs quantum wells (QWs) with indium compositions of 5% and 15% with laser excitation close to the bandgap for temperatures < 20 K. For all samples the intensity maxima occur at 2.2 and 4.5 tesla when the magnetic field is applied perpendicular to the QW pla...
Article
We present a magneto-optical study of magnetic polarons in Mn-doped II-VI colloidal quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. The long lifetime of the excitons allows the observation of the complete formation process of the mag...
Article
We have recorded time-resolved emission spectra from a series of MBE grown (Zn,Mn)Te/ZnSe quantum dots (QDs) at 7 K in the 0 - 4 tesla magnetic field range. The photoluminescence (PL) spectra were analyzed into their σ+ and σ- circularly-polarized components. The holes in this type-II system are confined in the (Zn,Mn)Te QDs, while the electrons re...
Article
Cadmium Sulfide (CdS) thin films had been fabricated by a laser deposition technique. Our previous results indicated that the hexagonal structured films could be formed at a substrate temperature of 400 C or higher. Reducing the temperature to 200 C or lower, cubic dominant films had been fabricated. To further understand the effects of other depos...
Article
The magnetic and electronic properties of some I-VI semiconductors with 3d transition metals other than Mn are presented. For example, the nonzero orbital moment in Fe2+ leads to more complex electronic energy levels than for Mnt ions. In (CdFe)Se, inelastic light scattering experiments demonstrate that the ground state is nonmagnetic (Van Vleck io...
Article
Hydrogenated amorphous silicon thick films deposited by dc glow discharge on molybdenum substrates were annealed by a pulsed Nd:glass laser. Mass spectrometry showed hydrogen remaining in all the laser annealed films. The amount of hydrogen remaining decreased with decreasing scan rate. The hydrogen evolved upon heating at 365 °C and mainly at 658...
Article
We present the results of an electrical injection study of spin polarized electrons from ferromagnetic Fe contacts into electronic shells of self-assembled InAs quantum dots (QDs) incorporated in GaAs/AlGaAs spin LED structures. The circular polarization of the emitted light was measured as function of current and magnetic field. The polarization o...
Article
We present a magnetoreflectivity study of the excitons in MBE grown Zn1−xFexSe crystals for x=0.017, 0.027 and 0.043. The excitonic spinsplitting was studied as function of magnetic field in both the Faraday and Voigt geometries. The exchange integrals were determined from the conduction and valence band spin-splittings. The exciton spin-splitting...
Article
A detailed kinetic model of diamond-like film growth from methane diluted in hydrogen using low-pressure, filament-assisted chemical vapor deposition (FACVD) has been developed. The model includes both gas-phase and surface reactions. The surface kinetics include adsorption of CH3· and H·, abstraction reactions by gas-phase radicals, desorption, an...
Article
Full-text available
Studies of ferromagnetic MnAs in recent years have revealed a wide range of properties desirable for spintronic applications. Previously studied MnAs spin-light-emitting-diodes exhibited a low value of spin injection into the device active region. In this work, we have investigated injection of spin polarized electrons from MnAs into AlGaAs(n)/GaAs...
Article
We have studied the circular polarization of the electroluminescence emitted from Fe spin-LEDs which incorporate a single layer of InAs QDs at the center of the device as function of magnetic field applied along the growth axis. The circular polarization below 50 K shows a dramatic drop around 5 tesla, indicating the presence of an efficient spin r...
Article
We report the magnetic and magneto-optical properties of Mn2+ doped CdSe nanoparticles synthesized by hot colloidal solution method. Magnetic hysteresis measurements on a particle ensemble show that they are paramagnetic at room temperature, and become ferromagnetic below about 50 K. The coercivity reaches to about 0.4 Tesla at 6K. The carrier spin...
Article
We present the results of a time-resolved photoluminescence (TRPL) study of type-II (Zn,Mn)Te/ZnSe quantum dots. The sample consists of 5 layers of (Zn,Mn)Te QDs separated by ZnSe spacers. We observe magnetic ordering in these QDs through measurements of the peak energy (E) of the PL vs. time. The large red shift with time (˜ 40 meV at low temperat...
Article
Full-text available
We present evidence of magnetic ordering in type-II (Zn, Mn) Te quantum dots. This ordering is attributed to the formation of bound magnetic polarons caused by the exchange interaction between the strongly localized holes and Mn within the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to ~ 200 K, with...
Article
We have fabricated spin light emitting diodes (LEDs) with Fe as the spin injector and 100å In0.1Ga0.9As/GaAs QWs as the detector. The emission efficiency from the InGaAs QW is extremely high, with a narrow linewidth of 4meV at 5K. The free exciton exhibits 25% optical polarization due to the injection of spin polarized carriers from the reverse-bia...
Article
The circular polarization P of light emitted by Fe/InAs QDs spin LEDs has been studied as function of magnetic field B and temperature T. P shows a pronounced decrease around Bo = 5 T, in the form of a resonance with a full width of 0.75 T with the following characteristics: (i) The resonance strength is quite sensitive to the bias voltage V. At lo...
Article
We have carried out optical pumping, Hanle and longitudinal Hanle studies of InGaAs:Al/GaAs single quantum wells. The circular polarization at zero magnetic field has a maximum around 50 K indicating that at low temperatures the recombination is associated with a bound electron. The measured spin lifetimes at low temperatures are an order of magnit...
Article
We have studied as function of magnetic field the electroluminescence spectra from an n-i-p LED that incorporates three GaAs quantum wells in the intrinsic region. This device has excess n-type doping and as a result, the quantum wells are populated by a two-dimensional electron gas. The broad zero field emission band evolves into a series of discr...
Article
Temperature dependant magneto-photoluminescence studies of type-II diluted magnetic semiconductor (ZnMn)Te/ZnSe quantum dots (QDs) will be presented. As expected, the exchange interaction between the Mn spins and charge carriers results in a strong optical polarization of the luminescence at low temperature in a magnetic field. In addition, however...
Article
The photoluminescence of Mn2+ doped CdSe nanoparticles synthesized by solution phase method has been studied as a function of nanocrystal size and the circular polarization of the light emitted from Cd1-xMnxSe nanoparticles has been investigated as a function of applied magnetic field in the 7-100 K temperature range. Spin polarized photoluminescen...
Article
We compare the electroluminescence spectra (spectral composition and polarization characteristics) of two types of Fe-based AlGaAs/GaAs n-i-p spin light emitting diodes (spin LEDs). In type A spin LEDs the GaAs quantum well (QW) does not contain any confined carriers, while in type B LEDs the GaAs QW is occupied by confined electrons generated by e...
Article
GaAs-based spin LEDs were used to study spin injection through a Schottky barrier from a ferromagnetic contact formed by an in-situ technique. Optical measurements were performed in both oblique Hanle and Faraday geometry. In addition to direct recombination at the Gamma-point in the quantum well, emission peaks were seen due to phonon-assisted rec...
Article
Full-text available
We report sequential spin population of individual shell states of self-assembled InAs quantum dots controlled by a spin-polarized current from an Fe contact, and determine the s-p and p-d intershell exchange energies. We resolve excitonic features in the electroluminescence (EL) spectra associated with individual quantum levels. In contrast with s...
Article
We report results for the photoluminescence emission intensity from ZnSe quantum-dot dispersions in liquids as a function of particle concentration. The observed emission intensity initially increases with dilution, exhibits a maximum, and then decreases. The optimal particle concentration yielding the maximum emission intensity increases as the pa...