Polarized Photoluminescence Measurements Research Articles

Isoemissive Photoluminescence from a Quaternary System of Valley-Polarized, Defect-Bound Excitons and Trions in Two-Dimensional Transition Metal Dichalcogenides

Interest in the use of local band extrema, known as valleys, as a possible information carrier has in recent years been advanced by the isolation of 2D transition metal dichalcogenides (TMDCs). In some monolayer TMDCs, the structural inversion asymmetry leads to spin-valley locked states that present potential advantages for information storage and manipulation beyond existing charge- and spin-based semiconductor technologies. However, understanding of the role of defects on exciton recombination and valley scattering remains lacking and limits progress toward applications. Here, we report gate voltage-dependent polarized photoluminescence measurements on CVD-grown monolayer WS2 and MoS2 field-effect transistors and observe a quaternary system of valley polarized excitons and trions. Intriguingly, we find the spectra to exhibit isoemissive points, which were previously seen only as evidence of a binary system. We propose a rate model to explain the existence of isoemission in a quaternary system and find that the presence of isoemissive points imply a kinetic relationship between recombination and valley scattering. This relationship sheds light on the inverse relation seen between the polarization degree and photoluminescence intensity. Further consideration of the phenomenon also led to the identification of a possible new mechanism for charge carrier detrapping using the energy released from trion recombination.

1D Mixed-Stack Cocrystals Based on Perylene Diimide toward Ambipolar Charge Transport.

There is very limited repertoire of organic ambipolar semiconductors to date. Electron donor-acceptor alternative stacking is a unique and important binary motif for 1D mixed-stack cocrystals, opening up possibilities for the development of organic ambipolar semiconductors. Herein, four 1D mixed-stack cocrystals using N,N'-bis(perfluorobutyl)-1,7-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDICNF) as the acceptor and anthracene, pyrene, perylene, and meso-diphenyl tetrathia[22]annulene[2,1,2,1] (DPTTA) as the donors are achieved in a stoichiometric ratio (D:A=1:1) through solution or vapor processed methods. Their packing structures, energy levels, charge transfer interactions, coassembling behaviors, and molecular orientations are systematically investigated by single-crystal X-ray analysis, absorption spectra, fluorescence quenching, Job's curve plot, and polarized photoluminescence measurements with the help of theoretical calculations. The donor-acceptor alternative stacking direction coincides with the long axis for all the four cocrystals. The field-effect transistors based on Pyrene-PDICNF show the electron mobility up to 0.19cm2 V-1 s-1 , which is the highest value among perylene diimide-based cocrystals. Moreover, DPTTA-PDICNF cocrystals possess well-balanced electron and hole mobility with 1.7×10-2 and 2.0×10-2 cm2 V-1 s-1 respectively due to both hole and electron strong superexchange interactions, shedding light on the design of 1D mixed-stack cocrystals with excellent ambipolar transport behaviors.

Enhanced directional emission of monolayer tungsten disulfide (WS2) with robust linear polarization via one-dimensional photonic crystal (PhC) slab

Abstract Objectives Monolayer transition metal dichalcogenides (TMDCs) have been regarded as promising candidates for the future light-emitting devices. To date, though the modulation of emission intensity and directionality in monolayer TMDCs has received considerable scholarly attention, there has been no systematic investigation on the underlying critical polarization. The intensity, directionality and robust polarization are highly favorable and pivotal for the future on-chip optoelectronic emission devices based on TMDCs. Methods We explore the emission features of the monolayer TMDCs in the photonic crystal (PhC) platform at room temperature. A monolayer tungsten disulfide (WS2) is specifically integrated with a tailored PhC structure. Angle-resolved photoluminescence (PL), time-resolved PL and polarized PL measurements are carried out to study the enhanced emission and polarization properties. Results The photoluminescence (PL) of WS2 is greatly enhanced by over 300-fold, resulting from a ∼fivefold enhancement (from 1.5 to 7.2%) of the PL efficiency with accelerated spontaneous emission rates. Additionally, the overall polarized emission is obtained with the degree of linear polarization (DLP) up to 60%, which is independent of the excitation polarization. Moreover, two branched directional emissions with horizontal polarization are also achieved at a divergency angle of only 3.5°, accompanied by a surprising near-100% DLP at ±8° directions. Conclusions This comprehensive study sets out to assess the feasibility of the high-performance light emission device based on the monolayer TMDCs and PhC structures.

Formation of oriented luminescent organic thin films on modified polymer substrate

Nanostructured organic thin films with polarized luminescence are obtained by means of the slow vapor deposition of fuorescent liquid crystalline dipolar molecules on the surface of the rubbed polyamide layer. Based on the polarized photoluminescence measurements, the polarization degree of thin film based on blue-emitting organic liquid crystalline molecules 4-(n-pentyl)-3-chloro-4′′′-trifluoromethoxy-[1,1′, 4′, 1′′, 4′′, 1′′′-quaterphenyl] (M-BLUE) was found to be 29%, and for red-emitting 4-(dicyanomethylene)-2-(4-amylcyclohexyl)-6[4-(dimethylaminostyryl)]-4H-pyran molecule (M-RED) it is as high as 78%. It can be connected with the higher dipole momentum of M-RED molecule (15) than M-BLUE (3.45), and can be ascribed to its higher interaction with polyamide matrix.

Epitaxial Single-Layer MoS2 on GaN with Enhanced Valley Helicity.

Engineering the substrate of 2D transition metal dichalcogenides can couple the quasiparticle interaction between the 2D material and substrate, providing an additional route to realize conceptual quantum phenomena and novel device functionalities, such as realization of a 12-time increased valley spitting in single-layer WSe2 through the interfacial magnetic exchange field from a ferromagnetic EuS substrate, and band-to-band tunnel field-effect transistors with a subthreshold swing below 60 mV dec-1 at room temperature based on bilayer n-MoS2 and heavily doped p-germanium, etc. Here, it is demonstrated that epitaxially grown single-layer MoS2 on a lattice-matched GaN substrate, possessing a type-I band alignment, exhibits strong substrate-induced interactions. The phonons in GaN quickly dissipate the energy of photogenerated carriers through electron-phonon interaction, resulting in a short exciton lifetime in the MoS2 /GaN heterostructure. This interaction enables an enhanced valley helicity at room temperature (0.33 ± 0.05) observed in both steady-state and time-resolved circularly polarized photoluminescence measurements. The findings highlight the importance of substrate engineering for modulating the intrinsic valley carriers in ultrathin 2D materials and potentially open new paths for valleytronics and valley-optoelectronic device applications.

In-Plane Anisotropic Molecular Orientation of Pentafluorene and Its Application to Linearly Polarized Electroluminescence.

By preparing parallelly aligned 1.9-μm-high SiO2 microfluidic channels on an indium tin oxide substrate surface, the solution flow direction during spin-coating was controlled to be parallel to the grating. Using this technique, a pentafluorene-4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) binary solution in chloroform was spin-coated to embed a 40-50 nm-thick 10 wt %-pentafluorene:CBP thin film in the channels. In-plane polarized photoluminescence measurements revealed that the pentafluorene molecules tended to orient along the grating, demonstrating that one-dimensional fluid flow can control the in-plane molecular orientation. Furthermore, the dependences of the photoluminescence anisotropy on the spin speed and substrate material suggest that the velocity of the solution flow and/or its gradient in the vertical direction greatly affects the resulting orientation. This indicates that the mechanism behind the molecular orientation is related to stress such as the shear force. The effect of the solution flow on the molecular orientation was demonstrated even in organic light-emitting diodes (OLEDs). Linearly polarized electroluminescence was obtained by applying the in-plane orientation to OLEDs, and it was found that the dichroic ratio of the electroluminescence orthogonal (x) and parallel (y) to the grating is x/y = 0.75.

Polarized Raman and photoluminescence studies of a sub-micron sized hexagonal AlGaN crystallite for structural and optical properties

The polarized Raman spectroscopy is capable of giving confirmation regarding the crystalline phase as well as the crystallographic orientation of the sample. In this context, apart from crystallographic x-ray and electron diffraction tools, polarized Raman spectroscopy and corresponding spectral imaging can be a promising crystallographic tool for determining both crystalline phase and orientation. Sub-micron sized hexagonal AlGaN crystallites are grown by a simple atmospheric pressure chemical vapor deposition technique using the self catalytic vapor-solid process under N-rich condition. The crystallites are used for the polarized Raman spectra in different crystalline orientations along with spectral imaging studies. The results obtained from the polarized Raman spectral studies shows single crystalline nature of sub-micron sized hexagonal AlGaN crystallites. Optical properties of the crystallites for different crystalline orientations are also studied using polarized photoluminescence measurements. The influence of internal crystal field to the photoluminescence spectra is proposed to explain the distinctive observation of splitting of emission intensity reported, for the first time, in case of c-plane oriented single crystalline AlGaN crystallite as compared to that of m-plane oriented crystallite.

Optically Induced Nuclear Spin Polarization in the Quantum Hall Regime: The Effect of Electron Spin Polarization through Exciton and Trion Excitations.

We study nuclear spin polarization in the quantum Hall regime through the optically pumped electron spin polarization in the lowest Landau level. The nuclear spin polarization is measured as a nuclear magnetic field B(N) by means of the sensitive resistive detection. We find the dependence of B(N) on the filling factor nonmonotonic. The comprehensive measurements of B(N) with the help of the circularly polarized photoluminescence measurements indicate the participation of the photoexcited complexes, i.e., the exciton and trion (charged exciton), in nuclear spin polarization. On the basis of a novel estimation method of the equilibrium electron spin polarization, we analyze the experimental data and conclude that the filling factor dependence of B(N) is understood by the effect of electron spin polarization through excitons and trions.

Effects of strain on the valence band structure and exciton-polariton energies in ZnO

The uniaxial stress dependence of the band structure and the exciton-polariton transitions in wurtzite ZnO is thoroughly studied using modern first-principles calculations based on the HSE+G0W0 approach, k p modeling using the deformation potential framework, and polarized photoluminescence measurements. The ordering of the valence bands [A(G7), B(G9), C(G7)] is found to be robust even for high uniaxial and biaxial strains. Theoretical results for the uniaxial pressure coefficients and splitting rates of the A, B, and C valence bands and their optical transitions are obtained including the effects of the spin-orbit interaction. The excitonic deformation potentials are derived and the stress rates for hydrostatic pressure are determined based on the results for uniaxial and biaxial stress. In addition, the theory for the stress dependence of the exchange interaction and longitudinal-transversal splitting of the exciton-polaritons is developed using the basic exciton functions of the quasi-cubic approximation and taking the interaction between all exciton states into account. It is shown that the consideration of these effects is crucial for an accurate description of the stress dependence of the optical spectra in ZnO. The theoretical results are compared to polarized photoluminescence measurements of different ZnO substrates as function of uniaxial pressure and experimental values reported in the literature demonstrating an excellent agreement with the computed pressure coefficients.

One-dimensional miniband formation in closely stacked InAs/GaAs quantum dots

We have studied the electronic states of closely stacked InAs/GaAs quantum dots (QDs) with a 4.0-nm spacer layer using linearly polarized photoluminescence (PL) and time-resolved PL measurements. An increase in the stacking-layer number (SLN) leads to an increase in the linear polarization anisotropy in the (001) plane; the [$\ensuremath{-}$110]-polarization component becomes dominant. These SLN-dependent polarization characteristics result from the valence-band mixing induced by the vertically coupled electronic states. The PL spectrum of the stacked QDs shows clear blueshifts with an increase in the excitation power because of the band filling. In addition, the radiative recombination lifetime has been found to obey the ${T}^{1/2}$ dependence, which directly confirms the one-dimensional translational motion of excitons in the closely stacked QDs.

Experimental and atomistic theoretical study of degree of polarization from multilayer InAs/GaAs quantum dot stacks

Recent experimental measurements, without any theoretical guidance, showed that isotropic polarization response can be achieved by increasing the number of quantum-dot (QD) layers in a QD stack. Here we analyze the polarization response of multilayer QD stacks containing up to nine QD layers by linearly polarized photoluminescence (PL) measurements and by carrying out a systematic set of multimillion atom simulations. The atomistic modeling and simulations allow us to include correct symmetry properties in the calculations of the optical spectra, a factor critical to explain the experimental evidence. The values of the degree of polarization (DOP) calculated from our model follows the trends of the experimental data. We also present detailed physical insight by examining strain profiles, band edges diagrams, and wave function plots. Multidirectional PL measurements and calculations of the DOP reveal a unique property of InAs QD stacks that the TE response is anisotropic in the plane of the stacks. Therefore, a single value of the DOP is not sufficient to fully characterize the polarization response. We explain this anisotropy of the TE modes by orientation of hole-wave functions along the [$\overline{1}10$] direction. Our results provide a new insight that isotropic polarization response measured in the experimental PL spectra is due to two factors: (i) TM${}_{001}$-mode contributions increase due to enhanced intermixing of HH and LH bands, and (ii) TE${}_{110}$-mode contributions reduce significantly due to hole-wave function alignment along the [$\overline{1}10$] direction. We also present optical spectra for various geometry configurations of QD stacks to provide a guide to experimentalists for the design of multilayer QD stacks for optical devices. Our results predict that the QD stacks with identical layers will exhibit lower values of the DOP than the stacks with nonidentical layers.

Optical anisotropy of InAs quantum dots

The optical anisotropy of InAs quantum dots (QDs) synthesized in the regime of either continuous or submonolayer deposition on a singular GaAs(100) surface have been studied using polarized photoluminescence measurements. It is established that an isolated array of QDs formed in a continuous deposition regime possesses a weak (<1–2%) optical anisotropy, whereas the vertical matching (coupling) of such QDs via less than 15-nm-thick spacer layers leads to an 8% linear polarization of PL along the [0\( \bar 1 \)1] crystallographic direction. QDs formed in the regime of submonolayer deposition exhibit a strong (17–20%) anisotropy of emission from the ground and excited states of QDs in the same crystallographic direction.

Spin injection and circular polarized electroluminescence from InAs-based spin-light emitting diode structures

We have investigated circularly polarized electroluminescence (EL) from hybrid II-Mn-VI/III–V light emitting diodes (LED’s) at low temperatures in magnetic fields upto 10 T. Both magnetic (the Brillouin paramagnet Cd1−xMnxSe) and nonmagnetic (CdSe) injectors were studied. Electrons, spin unpolarized (n-CdSe) or spin-polarized (n-CdMnSe), were injected into wide InAs quantum wells, where they recombined with unpolarized holes injected from p-type InAs/AlAsSb layers. Detailed measurements and modeling of the circular polarization of the resulting midinfrared EL were carried out to explore and quantify the additional complexities of this materials system compared with the extensively studied GaAs-based spin-LED structures. We show that optical and spin polarization in narrow gap semiconductors such as InAs are not simply related to each other. To analyze the complex relationship, we have developed and used a detailed rate equation model, which incorporates the band-structure of electrons and holes in a magnetic field, a finite ratio of recombination and spin-flip times, and the spin polarization of the CdMnSe spin-aligner as a function of injection current. The latter was determined in situ by circular polarized photoluminescence measurements on the injector material. Experimentally, the circular polarization degrees of magnetic and nonmagnetic structures are observed to be very similar, when the magnetic samples have low effective Mn incorporation. This results from a combination of the consequently low spin polarization of the aligner and comparable spin and recombination life times in InAs.

Photoluminescence from single isoelectronic traps in nitrogen delta-doped GaAs grown on GaAs(1 1 1)A

We have studied photoluminescence (PL) observed from single isoelectronic traps formed by nitrogen pairs in nitrogen δ-doped GaAs layers grown on GaAs(1 1 1)A substrates. The PL was composed of a single peak with a narrow linewidth of ∼80 μeV. Polarized PL measurements confirmed that the emission from single isoelectronic traps in nitrogen δ-doped GaAs(1 1 1) is unpolarized irrespective of nitrogen pair arrangements. These results can be explained by in-plane isotropy of the samples, which is consistent with the symmetrical property of GaAs(1 1 1), and demonstrate that utilizing (1 1 1) substrate is an effective means for obtaining unpolarized single photons, which are desirable for the application to quantum cryptography.

Effect of matrix material on the morphology and optical properties of InP-based InAsSb nanostructures

This paper presents a study on the effect of matrix material on the morphology and optical properties of self-assembled InP-based InAsSb nanostructures. Due to the differences in surface roughness of the growth front, In0.53Ga0.47As matrix layer induces the formation of short quantum dashes (QDashes) and elongated quantum dots, while InP and In0.52Al0.48As matrix layers promote the formation of long QDashes and quantum wires, respectively. The shape anisotropy of InAsSb nanostructures on In0.53Ga0.47As, InP, and In0.52Al0.48As layers is further investigated with polarized photoluminescence measurements. The InAsSb nanostructures show a luminescence polarization degree of 8.5%, 14.3%, and 29% for In0.53Ga0.47As, InP, and In0.52Al0.48As matrixes, which corresponds well with the shape anisotropy observed with atomic force microscope. Furthermore, InAsSb/In0.53Ga0.47As nanostructures also show the longest, thermally stable emission wavelength, which serves as a promising material system for fabricating midinfrared emitters.

Gallium-nitride nanorods serve as subwavelength optical media

GaN-based LEDs and laser diodes have become the devices of choice for optoelectronic applications operating in the shortwavelength range. One of the remaining challenges for improving GaN-based LEDs is to reduce their optical losses, which are caused by the high refractive index of GaN (∼ 2.5).1 To overcome this difficulty, the low optical reflection from low-effectiverefractive-index (low-n) nanorod arrays could be exploited. To date, there have been few reports about light-extraction enhancement using transparent, conductive indium-tin oxide2 and zinc oxide3 nanorod arrays deposited on top of GaN LEDs. But GaN-nanorod arrays have been shown to have the same physical and thermal properties as the bulk GaN crystal, making them suitable and durable for high-power, GaN-based optoelectronic applications. By analyzing the reflectivity interference fringes, we have quantitatively determined that GaN-nanorod arrays behave as low-n transparent media in the entire visible spectra region.4 Moreover, the polarized properties of single GaN nanorods have been demonstrated and studied in detail.5 By measuring linearly polarized photoluminescence (PL) from individual, isolated GaN nanorods with diameters in the subwavelength regime (30–90nm), we are able to present clear experimental evidence for the size dependence of polarization anisotropy. This can resolve the long-standing issue related to the giant luminescence-polarization anisotropy observed from various semiconductor nanorods and nanowires. In our investigations, high-density, vertically aligned (along the wurtzite c-axis) GaN nanorod arrays were grown using a catalyst-free, self-organized method based on plasma-assisted molecular-beam epitaxy (PA-MBE).6 Judging from results of field-emission, scanning-electron microscopy (FE-SEM), both the average rod diameter and nearest-neighbor separation are in the subwavelength regime of visible light. There are two major advantages of using PA-MBE-grown GaN-nanorod arrays. First, the thickness and filling ratio (volume fraction) of the GaNnanorod array can be tuned by controlling the growth parameters. And, the interfaces formed by PA-MBE are very abrupt, allowing for interferometer-type measurements. We observed strong modulations for all nanorod arrays in the reflection spectra measured in the transparentspectral region. This can be attributed to the effects of Fabry-Perot microcavities formed within the nanorod arrays by optically flat air/nanorod and nanorod/substrate interfaces. Figure 1 shows a representative normal-incident reflectivity spectrum in the near-ultraviolet and visible regions of a GaN-nanorod array. Flat and featureless reflectivity spectra can be observed in the opaque region. Strongly modulated reflectivity spectra are measured in the transparent region. Using an analysis technique based on the spectral positions of the reflectivity fringes, we determined the effective refractive index of the GaN-nanorod array (ne f f ): see Figure 1. The important finding in this analysis is that the effective refractive indices of nanorod arrays in the transparent region can be approximated to the entire dispersion relationship of bulk GaN reflective index by multiplying with a constant of ∼0.65. This value also agrees with that obtained for the opaque region using a filling ratio of ∼0.5. Several semiconductor nanowires have giant emission anisotropy, which is highly desirable in a variety of applications such as polarized LEDs and flat-panel displays. J. Wang and colleagues proposed that the phenomenon is caused by the large dielectric contrast between the nanowire and the surrounding environment, as opposed to quantum-confinement effects.7 H. E. Ruda and A. Shik calculated the detailed effects of dielectric confinement for semiconductor nanowires.8 One important conclusion of their study is the prediction of strong size-dependence of emission anisotropy in the subwavelength regime. Therefore, it is critical to study individual, isolated nanorods with a size distribution within that regime. For our work, the GaN nanorod samples for the polarized PL measurements were prepared with a nanomanipulator system installed

Exciton-phonon interactions observed in blue emission band in Te-delta-doped ZnSe

Photoluminescence (PL) in the blue wavelength region originating from a no-phonon (NP) transition at 2.734 eV and longitudinal optical (LO) phonon sidebands of Te isoelectronic centers (ICs) were clearly resolved after thermal annealing by δ-doping of Te in ZnSe layers. Broad luminescence (conventionally called the S1 band) had been previously observed in this region (i.e., around 2.65 eV). The PL intensities of the NP line and the phonon replicas followed a Poisson distribution with a mean phonon number (Huang–Rhys factor) of S∼1.1. This indicates that the broadening of the 2.65 eV emission band is due to the superposition of the NP lines and their LO phonon replicas originating from the Te ICs with slight variations in their transition energies. The origin of the luminescence is discussed in relation to linearly polarized PL measurements.

Optical properties of InGaAs/GaAs quantum chains

We have investigated the steady-state and transient optical properties of InGaAs/GaAs quantum chains and found that the photoluminescence (PL) decay time exhibits a strong photon energy dependence. It increases with the decrease of the emission energy. It is also found that the PL decay time increases with the excitation power. When the excitation power is large enough the PL decay time tends to be saturated. All these experimental results show that there is a strong carrier coupling along the chain direction in the quantum dot chain structure. The polarization PL measurements further confirm the carrier transfer process along the chain direction.

Optical Anisotropic Properties of m-Plane GaN Film Grown by Metalorganic Chemical Vapor Deposition

Spontaneous and piezoelectric polarization could lower the efficiency of GaN-based light-emitting diodes. In order to eliminate or reduce this undesirable effect, m-plane GaN film was prepared by metalorganic chemical vapor deposition (MOCVD) on LiAlO 2(100) substrate with a GaN buffer layer. Since the c axis of the m-plane GaN lays in the grown plane, the breakage of in-plane symmetry gave rise to an optical anisotropy, which was revealed by a difference in refractive indexes of E-field parallel and perpendicular to the c axis of the m-plane GaN measured by polarized reflection measurement. In addition, in-plane strain anisotropy due to the lattice mismatch between the GaN film and the LiAlO 2 substrate changed the Electronic Band Structure (EBS). The change of EBS also led to an in-plane optical anisotropy. Polarized absorption and Photoluminescence (PL) measurements showed a split in energy of 32 meV for optical absorption edge and 37 meV for PL peak energy respectively.

Magneto-photoluminescence spectroscopy of ZnMnTe/ZnMgTe spin superlattice

In this work, we studied the magneto-optical properties of ZnMnTe/ZnMgTe spin superlattices by using photoluminescence (PL) spectroscopy at low temperature. The circularly polarized PL measurements in an increasing magnetic field were performed. Detailed characteristics of the level crossing behavior for the spin-polarized excitonic states due to the giant Zeeman splitting were observed. The results show that an increase of the magnetic field transfers the spin superlattices from direct band gap system to indirect band gap system, and spatially separates the electrons and the holes by changing type-I band alignment to type-II band alignment. These features are confirmed by the behavior of the PL intensity in magnetic field, and agree well with the theoretical expectation basing on the sp–d exchange interaction.