Exciton Magnetic Polaron Research Articles

Photon–carrier–spin coupling in a one-dimensional Ni(II)-doped ZnTe nanostructure

Transition metal (TM) ion doping in II–VI semiconductors can produce exciton magnetic polarons (EMPs) and localized EMPs containing longitudinal optical (LO) phonon coupling, which will be discussed in this paper. TM ion doping in II–VI semiconductors for a dilute magnetic semiconductor show emission via magnetic polarons (MPs) together with hot carrier effects that need to be understood via its optical properties. The high excitation power that is responsible for hot carrier effects suppresses the charge trapping effect in low exciton binding energy (8.12 meV) semiconductors, even at room temperature (RT). The large polaron radius exhibits strong interaction between the carrier and MP, resulting in anharmonicity effects, in which the side-band energy overtone to LO phonons. The photon-like polaritons exhibit polarized spin interactions with LO phonons that show strong spin–phonon polaritons at RT. The temperature-dependent photoluminescence spectra of Ni-doped ZnTe show free excitons (FX) and FXs interacting with 2LO phonon–spin interactions, corresponding to 3T1(3F) → 1T1(1G) and EMP peaks with ferromagnetically coupled Ni ions at 3T1(3F) → 1E(1G). In addition, other d–d transitions of single Ni ions (600–900 nm) appear at the low-energy side. RT energy shifts of 14–38 meV are observed due to localized states with density-of-states tails extending far into the bandgap-related spin-induced localization at the valence band. These results show spin–spin magnetic coupling and spin–phonon interactions at RT that open up a more realistic new horizon of optically controlled dilute magnetic semiconductor applications.

Enhanced room-temperature magnetic field effect on spin-polarized excitons of cesium lead bromine-magnetite nanocomposites

The generation and manipulation of spin-polarized electrons are the basis of spintronic applications. Spin injection from a ferromagnetic surface or heterojunction to a semiconductor adjacent layer is often used to create spin-polarized carriers; however, for halide perovskites, due to the mismatch of lattice parameters with most ferromagnets at room temperature, this spin injection is challenging. In this study, we synthesize all-inorganic perovskite CsPbBr3 surrounded by Fe3O4 magnetic nanoparticles. It is found that this nanocomposite shows both magnetization and magnetic field-induced circularly polarized photoluminescence at room temperature. Specifically, with the attachment of Fe3O4 nanoparticles closely on the surface of CsPbBr3 nanocrystals, both the degree of circular polarization and g-factor enhanced by 3.5 times compared with that of pure CsPbBr3. The phenomenon should be due to the formation of exciton magnetic polaron through the coupling of the magnetic aligned nanoparticles with the excitonic state of the host semiconductor in the external magnetic field. The effective spin injection provides a method of controlling the excitonic spin polarization of all-inorganic halide perovskites for spintronic applications.

Nickel(II)-Doped Lead-Free Halide Crystals Exhibiting Highly Efficient Tunable Blue-Emitting out of Antiferromagnetic Ni-Ni Coupling.

Metal halide crystals are widely used in optoelectronic fields due to their excellent optical properties. The hunt for a lead-free halide semiconductor with superior optical performance is a particularly fascinating topic in order to avoid the toxicity of lead. Here, we incorporate Ni2+ into a series of halide nanocrystals (NCs) through solution-phase synthesis. By modifying the A-site and varying the halide compositions, we successfully achieved significant tunability of the blue emission of the Ni2+-doped AX (A = K+, Rb+, NH2CH = NH2+ (FA), CH3NH3+ (MA); X = Br, I) NCs, ranging from 375 to 490 nm, due to the antiferromagnetic polaron (AMP), which is in contrast with the excitonic magnetic polarons (EMP) from those with ferromagnetic (FM) coupling between transition metal ions in similar compounds. This work shows that Ni2+-doped halide crystals could become a typical example providing AMP excitation as the optional emission centers for use in light emitting devices.

Electronic, magnetic and optical properties of Co(II) doped and (Al, Co) co-doped CdS nanowires: An ab initio study

In this work, we have carried out first-principles calculations for understanding the optoelectronic and magnetic properties of Co(II)-doped and (Al, Co) co-doped CdS nanowires. In the free carrier Co(II)-doped CdS NW, the spin-down Co(II)-ta state is completely empty, leading to a super-exchange process. The exchange coupling of the electron carrier (provided by Al co-doping) with the spin-down Co(II)-ta state promote a ferromagnetic ordering via double exchange interactions with a Curie temperature above room temperature. From the optical analysis, we found that Co(II) doping causes a blue-shift of the fundamental energy gap of CdS nanowire. The observed d-d spin-allowed transition peak of the Co(II) ion at 1.97 eV in the visible region is consistent with the experiments. The n-type Al co-doping not only generates an exciton-magnetic polaron (EMP) peak next to the fundamental bandgap but also exhibits the red-shift of the fundamental energy gap and Co(II)'s d-level-to-d-level transition bands. The correlation between spin-spin interaction and optical absorption shows that the d-d spin-allowed transition and optical energy gap both exhibit a blue-shift (red-shift) in the AFM (FM), supporting the experimental observation. Due to the paramagnetic behavior of the Co ions in the far configuration, there is no shift in the d-d transition peak or optical bandgap in the AFM and FM.

Excitonic magnetic polarons in II-VI diluted magnetic semiconductor nanostructures

Exciton is the elementary excitation in semiconductors with energy close to Eg, bound by electron and hole. Spin is an intrinsic form of angular momentum carried by elementary particles, especially for electron, transition metal(TM) ion and photon. Here we discuss the excitonic magnetic polaron-----coupling of exciton or electron with coupled spins of doped Transition metal(TM) ions in the micrometer sized diluted magnetic semiconductor(DMS) nanostructures, and their formation, dynamic spectral responses, collective behaviors have been discussed in this review, because they have important influences on their optical properties, reflected by the profound spin-related emission behaviors and the collective magnetic excitonic polaron condensation processes in such micrometer sized DMS space, which may have valuable applications in the quantum modulation and lighting technology.

The magnetic polaron modulated luminescence bands of organic-inorganic hybrid ferroelectric anti-perovskite (C3H9N)3Cd2Cl7 doped with Mn2+

Recently, the excellent optical properties of organic-inorganic hybrid metal halides have attracted much attention in the optoelectronic field. However, their complicated preparation processes seriously influence their properties and applications. In this work, we developed a series of organic-inorganic hybrid metal halides (C3H9N)3Cd2Cl7:x%Mn2+ with an antiperovskite structure with ferroelectrics in an early report, giving tunable emissions contemporarily with different manganese (Mn)2+ concentrations via a simple mechanochemical method. Meanwhile, their single crystals were also grown by a slow thermal evaporation method. The as-grown products with Mn dopants exhibited diluted magnetic semiconductor behavior and varied emission profiles by different excitation wavelengths, which could be modified by the heat treatment. All the emission bands come from the different magnetic polarons with enhanced electron-phonon coupling or self-trapped exciton formation. Ferromagnetic coupling Mn–Mn pairs or clusters in the doped lattice favor the magnetic polaron and red emission at room temperature and even give much stronger emission above room temperature. The excitonic magnetic polaron and local excitonic magnetic polaron were detected at about 309 nm and 398 nm, respectively, with Mn doping. Without Mn2+ dopant, the weak emission band at about 398 nm can also be detected from an intrinsically bound exciton or confined exciton from the amine incorporated metal chlorides. This Mn-doped anti-perovskite Cd halides may find applications in the solid display and lighting, as well as the magneto-optical devices.

Magnetic Effect of Dopants on Bright and Dark Excitons in Strongly Confined Mn-Doped CsPbI3 Quantum Dots

We investigated the magnetic effect of Mn2+ ions on an exciton of Mn-doped CsPbI3 quantum dots (QDs), where we looked for the signatures of an exciton magnetic polaron known to produce a large effective magnetic field in Mn-doped CdSe QDs. In contrast to Mn-doped CdSe QDs that can produce ∼100 T of magnetic field upon photoexcitation, manifested as a large change in the energy and relaxation dynamics of a bright exciton, Mn-doped CsPbI3 QDs exhibited little influence of a magnetic dopant on the behavior of a bright exciton. However, a μs-lived dark exciton in CsPbI3 QDs showed 40% faster decay in the presence of Mn2+, equivalent to the effect of ∼3 T of an external magnetic field. While further study is necessary to fully understand the origin of the large difference in the magneto-optic property of an exciton in two systems, we consider that the difference in antiferromagnetic coupling of the dopants is an important contributing factor.

Laser Dressed Magnetic Polaron In Semimagnetic Core/Shell Nanostructure

The combined effects of magnetic and intense laser fields on the properties of the ground state exciton in semimagnetic Cd0.7Mn0.3Te/CdTe Core/Shell Nanostructure (CSN) are analyzed using the variational method in the effective mass approximation. The magnetic field of 5 to 20 T, a laser with the frequency of 10 THz and its intensity ranging from 0.44 to 7.11 μW nm−2 are considered as external perturbing fields. The evolution of Exciton Magnetic Polaron (EMP) in response to the external fields is explored via Magnetization (M) and Zeeman energy shift (∆EZE). The variation in binding energy (BE) of an exciton, M and ∆EZE are examined for different shell widths, magnetic and laser fields. The laser-induced diamagnetism due to the dressed energy state of carriers reduces the polarization of EMP. Similarly, the external magnetic field reduces the exchange interaction between carriers and weakens EMP. The combined effect of perturbing fields on an EMP will give an insight on controlling the performance of magneto-optic devices.

Optical manifestation of magnetic polarons bound to excitons and resident holes in a (Cd,Mn)Te quantum well

Photoluminescence and multiple ${\mathrm{Mn}}^{2+}$ spin-flip Raman scattering (SFRS) are studied in the (Cd,Mn)Te/(Cd,Mg)Te quantum well with excess resident hole concentration under resonance continuous-wave photoexcitation in the localized states of excitons and trions at $T=1.5$ K. A magnetic polaron shift of both exciton and trion photoluminescence is observed to be increasing with an increase in the magnetic field up to 6 T applied perpendicularly to the sample growth axis (Voigt geometry). Exciton magnetic polaron shift in magnetic field is attributed to the adiabatic energy transfer from the exchange reservoir to the Zeeman reservoir during the precession of the ${\mathrm{Mn}}^{2+}$ ions in the summary field of external magnetic and exchange hole fields. A mechanism of the magnetic polaron shift of trion photoluminescence is suggested. It is related to direct photoexcitation of the triplet trion state, which is stable in the conditions which are provided by the resident hole magnetic polarons. In these conditions, the ${\mathrm{Mn}}^{2+}$ magnetic moment experiences additional magnetization in the double exchange field provided by two holes in the triplet trion state. It is observed that the maximum efficiency of multiple ${\mathrm{Mn}}^{2+}$ spin-flip Raman scattering falls into the energy range corresponding to the states of localized excitons where the exciton magnetic polaron shift is observed. From this we conclude that the magnetic polaron state could be an intermediate state of the multiple ${\mathrm{Mn}}^{2+}$ SFRS process.

Magnetic polaron-related optical properties in Ni(II)-doped CdS nanobelts: Implication for spin nanophotonic devices

Emissions by magnetic polarons and spin-coupled d–d transitions in diluted magnetic semiconductors (DMSs) have become a popular research field due to their unusual optical behaviors. In this work, high-quality NiI2(II)-doped CdS nanobelts are synthesized via chemical vapor deposition (CVD), and then characterized by scanning electron microscopy (SEM), x-ray diffraction, x-ray photoelectron spectroscopy (XPS), and Raman scattering. At low temperatures, the photoluminescence (PL) spectra of the Ni-doped nanobelts demonstrate three peaks near the band edge: the free exciton (FX) peak, the exciton magnetic polaron (EMP) peak out of ferromagnetically coupled spins coupled with FXs, and a small higher-energy peak from the interaction of antiferromagnetic coupled Ni pairs and FXs, called antiferromagnetic magnetic polarons (AMPs). With a higher Ni doping concentration, in addition to the d–d transitions of single Ni ions at 620 nm and 760 nm, two other PL peaks appear at 530 nm and 685 nm, attributed to another EMP emission and the d–d transitions of the antiferromagnetic coupled Ni2+–Ni2+ pair, respectively. Furthermore, single-mode lasing at the first EMP is excited by a femtosecond laser pulse, proving a coherent bosonic lasing of the EMP condensate out of complicated states. These results show that the coupled spins play an important role in forming magnetic polaron and implementing related optical responses.

Anomalous nonlinear optical effect and enhanced emission by magnetic excitons in CVD grown cobalt-doped ZnSe nanoribbon

The magnetic excitons in diluted magnetic semiconductor (DMS) have varied formats due to the inhomogeneous phases out of doping concentration and/or structural relaxations or defects. Here the high quality cobalt-doped zinc blende ZnSe nanoribbons (NRs) were synthesized, showing bright and color-variable emissions from blue, yellow to a little mixed white colors. Their power and temperature dependent micro-photoluminescence (PL) spectra have been obtained in which two emission bands, one magnetic exciton band near the band-edge and a Co2+ high-level d–d transition emission band at 550 nm out of their ferromagnetic (FM) coupled aggregates in ZnSe lattice, both bands could also be reflected by a nonlinear optical absorption enhancement. The easy formed stacking fault defects in a chemical vapor deposition (CVD) grown ZnSe zincblende NR took part in the above optical processes out of magnetic polaronic excitons (PXs). The femtosecond (fs) laser pulse pumping on single ZnSe:Co NR produces obvious lasing behavior but with profile of a complicated magnetic exciton interactions with indication of a crossover from collective exciton magnetic polarons (EMP) to bound magnetic polaron (BMP) scattering in Co doped ZnSe NR. These findings indicate the complication of the magnetic coupling natures in varied DMS structures, whose optical properties have been found to be highly nonlinear, due to the involvement of the spin–spin, spin–exciton and spin–phonon interactions, verified by the theoretic calculation in Yang X-T et al (2019 Interstitial Zn-modulated ferromagnetism in Co-doped ZnSe Mater. Res. Express 6 106121).

Substitutional site effects of Cr(II) ions on optical and magnetic properties of 1D CdS semiconductor nanoneedles for optoelectronic and spintronic applications

Spin controlled diluted magnetic semiconductors (DMSs) could bring revolution in the processing speed as well as in enhancing the storage capacity. To achieve its industrial application, several impurities have been doped from the 3d/4f into semiconductors, however, Cr has not yet been doped into CdS 1D nanostructures. Here, in the present study, we report on the Cr-doped CdS 1D nanoneedles synthesized by CVD and studied its optical and magnetic properties. The successful incorporation of Cr-ions into CdS confirmed by EDX and XRD. Along with the increase in Cr-ions concentration, first PL-peak shifted from 514.1 to 558.3 nm associated with the formation of excitonic magnetic polarons (EMPs) and the second peak centered at 993.8 nm may correspond to the aggregation of Cr-ions. For the first time, we tuned CdS emission up to ~ 482 nm covering visible spectral region up to near-infrared (NIR) region. With increasing the Cr-ions concentration up to 6.2%, saturation magnetization (Ms) was significantly enhanced from 0.614 × 10−3 Am2/kg to 0.987 × 10−3 Am2/kg. These findings will promote their future applications in the optoelectronic and spintronic devices.

Activation of infrared emission in (iodine, nickel) Co-Doped CdS nanobelts for solar cells and optoelectronic applications

Achieving tunable emission in semiconductors over a wide-spectral range has always remained a challenge for its potential applications. For this purpose, transition metals (TMs) have been widely doped into II-VI semiconductors, however, least efforts have been made on adding non-metals. Here, we co-doped non-metal and TM, i.e. iodine + nickel (I + Ni) into CdS 1D nanostructures and explored optical properties for the first time. FESEM shows that among CdS: (F + Ni, Cl + Ni, Br + Ni, I + Ni), I + Ni-doped CdS nanobelts (NBs) were thinner and have high crystallinity confirmed by XRD and Raman spectroscopy. Successful incorporation of iodine into CdS NBs was confirmed by EDX. Low temperature (80 K) photoluminescence (PL) of I + Ni doped CdS shows multiple emission centred at 525, 583, 758, and 916 nm, respectively. The first emission corresponds to exciton magnetic polaron (EMP) and the next two emissions correspond to d-d transition (3A2g → 1Eg) and (3T1P → 3T1F) of Ni ions, respectively. The last emission (916 nm) originated only in the I + Ni co-doped CdS NB which may correspond to the iodine incorporated into belts. According to the best knowledge, iodine related emission was detected for the first time. This work will promote their future applications in solar cells and optoelectronic devices.

Spin-related optical behaviors of dilute magnetic semiconductor ZnSe:Ni(II) nanobelts

There are varied spin states in dilute magnetic semiconductors, and carriers are not the only elementary excitations that carry the spin. This article reports a study of spin interactions in excitons of ZnSe:NiI(II) nanostructures. High-quality ZnSe:NiI(II) nanobelts (NBs) prepared by chemical vapor deposition show a zinc blende structure by x-ray diffraction and Raman spectroscopy. The temperature-dependent photoluminescence spectra of doped NBs show independent free exciton (FX) and exciton magnetic polaron (EMP) peaks at room temperature with ferromagnetically coupled Ni ions. A single-mode lasing profile was obtained with femtosecond laser excitation due to condensation of EMPs over a threshold. The luminescence lifetimes at different pump powers indicated different relaxation profiles, confirming the formation of coherent EMP aggregates. At a slightly higher dopant concentration, a weak peak at the high-energy side of the FX peak showed up separately at low temperature; this should be the magnetic polaron emission band from the antiferromagnetically coupled Ni(II) pair binding with a FX (antiferromagnetic magnetic polaron). These results illustrate the typical spectroscopic characteristics of spin–spin magnetic coupling, exciton–spin or phonon interactions in dilute magnetic semiconductor nanostructures, showing that their different coupled spin types could work as exciton binders for their collective excitons, with possible use in spin nanophotonic devices and quantum modulations.

Spin-polarized exciton formation in Co-doped GaN nanowires

Diluted magnetic semiconductors often show a spin-dependent interaction and induced ferromagnetism with high Curie temperature which is a desirable feature to be utilized for magneto-electronic devices. However, the origin of ferromagnetism and optical emission dynamics has not yet been well-understood due to their complicated microstructural and compositional characters. Herein, un-doped and Co-doped GaN nanowires have been synthesized by chemical vapor deposition and the effect of Co dopant on structural, magnetic and optical properties has been investigated. Electron-phonon couple caused by Co dopant, plays a dominant role for enhancement and shift of LO- phonon mode toward higher wavenumber. The incorporation of Co (II) ion in GaN lattice is attributed to the presence of room temperature ferromagnetism which influences their optical properties through exciton-spin interactions. It happens when the Co atom prefers to substitute on Ga sites and induces coupling with neighboring Co atoms to align ferromagnetically, leading to the formation of independent exciton magnetic polarons, as a new elementary excitation that is one of the origins of the bosonic lasing. These findings can pave new way in the future solid-state optoelectronic quantum information technology.

Directed Exciton Magnetic Polaron Formation in a Single Colloidal Mn2+:CdSe/CdS Quantum Dot.

One of the most prominent signatures of transition-metal doping in colloidal nanocrystals is the formation of charge carrier-induced magnetization of the dopant spin sublattice, called exciton magnetic polaron (EMP). Understanding the direction of EMP formation, however, is still a major obstacle. Here, we present a series of temperature-dependent photoluminescence studies on single colloidal Mn2+:CdSe/CdS core/shell quantum dots (QDs) performed in a vector magnetic field providing a unique insight into the interaction between individual excitons and numerous magnetic impurities. The energy of the QD emission and its full width at half-maximum are controlled by the interplay of EMP formation and statistical magnetic fluctuations, in excellent agreement with theory. Most important, we give the first direct demonstration that anisotropy effects-hypothesized for more than a decade-dominate the direction of EMP formation. Our findings reveal a pathway for directing the orientation of optically induced magnetization in colloidal nanocrystals.

Bosonic Lasing of Collective Exciton Magnetic Polarons in CuCl2-Doped CdS Nanoribbons: Implications for Quantum Light Sources

The diluted magnetic semiconductor (DMS) is an important system for realizing room-temperature ferromagnetism. In the DMS, the exchange interactions between the spins of the carrier or exciton and ...

Carrier and spin dynamics of high-density exciton magnetic polarons in Cd0.8Mn0.2Te

We investigated the carrier and spin dynamics of high-density exciton magnetic polarons (HD-EMPs) in Cd0.8Mn0.2Te based on the measurement of their time-resolved photoluminescence (PL) spectra and polarization states, and the utilization of photo-induced Faraday rotation techniques. The PL from the HD-EMPs were collected in a forward scattering configuration, and was observed as a pulsed emission of a few picoseconds duration, exhibiting a blue-shift with time evolution. The blue shift originated from the refractive-index dispersion of the sample. By excluding the influence of the refractive-index dispersion on the time profile, it was revealed that the ultra-short pulsed emission with a time width smaller than 1 ps was initially radiated with a time delay of ~2.4 ps after photoexcitation. From the results of time evolution of the polarization states, it is concluded that the exciton–Mn spin interactions occurs immediately after the excitation, which causes the Mn ion spins to align to follow the spin states of photoexcited excitons. The alignment of the Mn ion spins through the formation of the HD-EMPs was significantly faster than that of the localized EMP. On the other hand, the time evolution of the photo-induced Faraday rotation showed two decay components attributed to spin relaxations of the excitons and Mn ions within the HD-EMP. The observation of the Faraday rotation signal due to the Mn ion spins further confirms that these spins were aligned by the photo-excited spin-aligned excitons. Our findings suggest a novel mechanism for the effective optical control of spins in a semimagnetic semiconductor, which is associated with a multi-exciton system and its localized state.

The contribution of Cr(III)-doping on the modulation of magnetic and luminescence properties of GaN nanowires

An external incorporation of magnetic ions into the parent compounds is often used to tune their semiconducting behavior. For example, doping of a small amount of transition metal ion in a semiconductor can lead to the formation of dilute magnetic semiconductors (DMSs) that can exhibit some novel properties related to spin interactions. Here, we report on Cr(III)-doped GaN nanowires (NWs), prepared by a thermal evaporation method. X-ray diffraction analysis indicates the single crystalline nature of as-synthesized NWs. Each NW is composed of hexagonal-facet with sharp conical shaped tip investigated by a scanning electron microscope (SEM). The appearance of 2LO mode in micro-Raman spectra indicates a strong electron-phonon coupling with the A 1 (LO) mode induced by Cr(III) ion. The occurrence of ferromagnetic domains as observed in GaN plays an important role in spin ordering which comes from mixed spin-spin, exciton-spin, and spin-carrier interaction. These findings provide a new path to realize new spin-polarized photonic devices . • Well-shaped Cr-doped GaN nanowires grown by CVD. • A large redshift effect of A 1 (LO) as compared with bulk GaN. • Ferromagnetic coupling between transition metal ions and GaN lattice. • Formation of exciton magnetic polaron (EMP) due to high exciton binding energy. • Strong interactions resulting in exciton-spin or phonon interactions.

Single-channel dual tunable emission in the visible and near-infrared region using aggregations of Mn(II) ions in an individual Mn-doped CdS nanosheet

In this study, high-quality single crystalline Mn-doped CdS nanosheets were synthesized by the chemical vapor deposition method and their photoluminescence (PL) properties were determined. Energy dispersive X-ray analysis, the non-uniform peak shifts in the Raman spectra, and micro-PL mapping demonstrated the presence of Mn ions in the nanosheets. The temperature-dependent (78–400 K) micro-PL spectra showed that the multi-peak emissions ranged from 455.302 to 749.901 nm. The band-edge emission exhibited a pronounced red shift (∼8 nm) as the temperature increased from 78 to 400 K due to the formation of exciton magnetic polarons (EMPs). The orange emission (597.621 nm) was due to the 4T1(4G) → 6A1(6S) d–d transition between the excited state and ground state of Mn2+. The shorter lifetimes compared with the forbidden d-d transition and deep traps due to structural relaxation confirmed that the red (670.832 nm) and near-infrared (749.901 nm) emissions were associated with localized EMPs. The PL emission centered at 471.780 nm appeared at 80 K due to free-exciton C (FXC). Emissions centered at 455.302 and 463.501 nm were detected for the first time in our samples. Interestingly, both of these emissions appeared on the higher energy side of FXC and unlike EMP, they did not follow the Varshni function. Therefore, we assigned these emissions to bound magnetic polarons. These novel emissions in dilute magnetic semiconductors could be beneficial for further applications in photovoltaics, full-color displays, and nanoscale devices.