Photoluminescence Excitation Spectroscopy Research Articles

In-Situ Tailoring of Vertically Coupled InAs p-i-p Quantum-Dot Infrared Photodetectors: Toward Homogeneous Dot Size Distribution and Minimization of In–Ga Intermixing

The authors report a detailed analysis of an epitaxial growth technique for indium arsenide (InAs) Quantum-dot infrared photodetectors circumvent the detrimental effects arising from the progressively increasing dot-size in vertically coupled heterostructures. Constant overgrowth percentage of the vertically coupled dot-layers has been achieved with the implementation of the growth strategy, which has been validated by cross-sectional transmission electron microscopy (X-TEM) images of the samples. The optical characteristics of these samples have been analyzed through photoluminescence spectroscopy and photoluminescence excitation spectroscopy (PL and PLE) measurements which show longer wavelength response and reduced full width at half-maxima (fwhm) upon implementation of the growth strategy. X-TEM and in-plane and out-of-plane high resolution X-ray diffraction (HR-XRD) measurements suggest morphological improvement upon implementation of the growth strategy, with a reduction in the indium desorption and lowering of defects and dislocation densities. Excellent correlation has been found between the different experimental results and also their theoretical simulations. The fabricated single-pixel photodetectors at low temperature (T = 14 K) show a broad response extending up to the MWIR region (∼4.5 μm) for one of the samples. Also, a strong spectral response in the SWIR region is obtained even at room temperature (T = 300 K). The highest responsivity (Rp) and specific detectivity (D*) values obtained are 166.17 A/W and 8.39 × 1010 cm Hz1/2 W–1 at a bias of 5 V and 300 K temperature.

Concentration-Dependent Fluorescence and Judd–Ofelt Analysis of Trivalent-Praseodymium-Doped Alkali Fluoroborate Glass

The effects of different concentrations of Pr3+ ions on the optical properties of (70−x)B2O3 + 10BaO + 10K2O + 10ZnF2 + xPr6O11 (x = 0.02 mol.%, 0.03 mol.%, 0.05 mol.%, 0.1 mol.%, and 0.5 mol.%) glass prepared by the melt quenching technique have been studied. The structural and luminescence behavior of the prepared glasses were studied by x-ray diffraction (XRD) analysis, Fourier-transform infrared (FTIR) spectroscopy, optical absorption spectroscopy, and photoluminescence excitation and emission spectroscopy. The XRD spectrum confirmed the amorphous nature while the FTIR spectrum revealed the presence of different structural groups in the prepared glass samples. The optical absorption spectrum was used to determine important parameters such as the nephelauxetic ratio, bonding parameter, optical bandgap energy, and Urbach energy. The PL spectra of the prepared glasses showed four prominent peaks at wavelengths of 484 nm, 601 nm, 693 nm, and 725 nm, related to the 3P0 → 3H4, 1D2 → 3H4, 3P0 → 3F3, and 3P0 → 3F4 transitions, respectively. Furthermore, the Judd–Ofelt (JO) intensity parameters Ωλ (λ = 2, 4, 6) and other important spectroscopic parameters such as the transition probability (A), radiative lifetime (τR), and branching ratio (βR) were calculated to evaluate the radiative properties of Pr3+levels from the optical absorption spectrum. In the host matrix, the JO parameters followed the trend Ω2 > Ω4 > Ω6. The emission spectra suggested that the prepared glass samples are excellent orange–red light-emitting materials under excitation at 442 nm (3H4 → 3P2). Quenching of the fluorescence intensity with increase of the Pr3+ concentration was also observed. The emission spectrum was used to determine the Commission Internationale de I’Eclairage (CIE) 1931 color coordinates. The results suggested the suitability of the prepared materials for various orange–red light-emitting applications, mainly in display devices, lasers, and optical amplifiers.

Synthesis and optical properties of Mn-doped CaWO4 nanoparticles

Pure and Mn-doped calcium tungstate (Mn:CaWO4) nanoparticles (NPs) were synthesized by hydrothermal method which lead to the formation of high quality and narrow sized particles. The concentration of Mn in CaWO4 NPs was varied from 1% to 4% by weight. A distinct peak of Mn (d–d) transition was observed which is actually spin-forbidden but may be allowed due to crystal field effects. Electron spin resonance measurements revealed inclusion of Mn2+ at the substitutional site.. Room temperature photoluminescence emission (PL) and excitation (PLE) spectroscopies were carried out to understand the interactions of sp-d levels. PL spectra revealed the presence of both band-edge (∼450 nm) and Mn2+ related orange luminescence (∼530 nm). Time dependent photoluminescence (TDPL) studies were also carried out where the life time value of 4.3 ms obtained was in accordance with measurement for Mn in other systems. Thus TDPL further strengths the presence of Mn in 2+ state in CaWO4 NPs.

Interplay between emission wavelength and s-p splitting in MOCVD-grown InGaAs/GaAs quantum dots emitting above 1.3 μ m

The electronic structure of strain-engineered single InGaAs/GaAs quantum dots emitting in the telecommunication O band is probed experimentally by photoluminescence excitation spectroscopy. The observed resonances can be attributed to p-shell states of individual quantum dots. The determined energy difference between the s-shell and the p-shell shows an inverse dependence on the emission energy. The experimental data are compared with the results of confined state calculations, where the impact of the size and composition in the investigated structures is simulated within the 8-band k·p model. On this basis, the experimental observation is attributed mainly to changes in the indium content within individual quantum dots, indicating a way of engineering and selecting a desired quantum dot whose electronic structure is the most suitable for a given nanophotonic application.

Modified vertical Bridgman method: Time and cost effective tool for preparation of Cs2HfCl6 single crystals

Abstract Time and cost effective methods are highly desirable in research and development of new scintillators. Modern techniques like micro-pulling down (μ-PD) are suitable for material screening but are unfit for the growth of some crystals. These crystals must be grown by different methods that are usually very time demanding. Modification of halide μ-PD apparatus by custom made elements allowed us to grow cesium hafnium chloride (Cs2HfCl6) by vertical Bridgman method (VB) with significantly reduced growth time. Structural and optical properties of samples prepared from as-grown crystals were studied and compared to crystals grown by standard VB method. The X-ray diffraction confirmed the formation of cesium hafnium chloride single-phase and natural cleavage of the crystals along the (1 1 1) crystallographic plane. Photoluminescence emission, excitation, absorption, and radioluminescence spectroscopy revealed that the optical quality of the crystals grown by modified VB method was comparable to the quality of the crystals grown by standard VB method. Therefore we can use samples prepared by modified VB to estimate and optimize the performance of Cs2HfCl6 based crystals in scintillation detectors. This setup allows the time and cost-effective material screening and it is a powerful tool for the development of new halide based single crystal scintillators.

Spectroscopic investigations of negatively charged tin-vacancy centres in diamond

The recently discovered negatively charged tin-vacancy centre in diamond is a promising candidate for applications in quantum information processing (QIP). We here present a detailed spectroscopic study encompassing single photon emission and polarisation properties, the temperature dependence of emission spectra as well as a detailed analysis of the phonon sideband and Debye–Waller factor. Using photoluminescence excitation spectroscopy we probe an energetically higher lying excited state and prove fully lifetime limited linewidths of single emitters at cryogenic temperatures. For these emitters we also investigate the stability of the charge state under resonant excitation. These results provide a detailed insight into the spectroscopic properties of the SnV− centre and lay the foundation for further studies regarding its suitability in QIP.

Vertically Coupled Hybrid InAs Sub-Monolayer on InAs Stranski–Krastanov Quantum Dot Heterostructure: Toward Next Generation Broadband IR Detection

In the present article, we are introducing a novel heterogeneously coupled InAs Submonolayer (SML) on Stranski-Krastanov (SK) quantum dot (QD) heterostructure with reduced strain accumulation. Theoretical comparison manifests the superiority of this hybrid SML-SK QDs over bilayer SK QDs. Photoluminescence and photoluminescence excitation spectroscopy are employed to characterize the electronic interaction and carrier tunneling in between this hybrid quantum dot assembly. Growth rate is optimized at 0.1 ML/sec. To tune the coupling, the barrier layer thickness is varied from 5 nm to 10 nm. Even up to the highest barrier thickness, no signature of SML peak is obtained in the PL response. But, PLE depicts the presence of SML peak, which is partially overlapped with the SK 2nd excited peak. The sample with 7.5 nm GaAs barrier shows perfect resonance between SML ground state and SK 2nd excited state. Based on experimental and analytical results, this SML on SK (SML-SK) configuration is compared with SK on SML (SK-SML) configuration; where SML-SK exhibits better inter dot electronic interaction than its reciprocal configuration. For ex situ tailoring of this heterogeneous electronic coupling, rapid thermal annealing has been done. It shows tunability of inter dot interaction with improved crystalline quality. Finally, we have shown a SML-SK quantum dot infrared photodetector with a broad spectral response (from SWIR to near LWIR).

Giant Valley-Zeeman Splitting from Spin-Singlet and Spin-Triplet Interlayer Excitons in WSe2/MoSe2 Heterostructure.

Transition metal dichalcogenides (TMDCs) heterostructure with a type II alignment hosts unique interlayer excitons with the possibility of spin-triplet and spin-singlet states. However, the associated spectroscopy signatures remain elusive, strongly hindering the understanding of the Moiré potential modulation of the interlayer exciton. In this work, we unambiguously identify the spin-singlet and spin-triplet interlayer excitons in the WSe2/MoSe2 heterobilayer with a 60° twist angle through the gate- and magnetic field-dependent photoluminescence spectroscopy. Both the singlet and triplet interlayer excitons show giant valley-Zeeman splitting between the K and K' valleys, a result of the large Landé g-factor of the singlet interlayer exciton and triplet interlayer exciton, which are experimentally determined to be ∼10.7 and ∼15.2, respectively, which is in good agreement with theoretical expectation. The photoluminescence (PL) from the singlet and triplet interlayer excitons show opposite helicities, determined by the atomic registry. Helicity-resolved photoluminescence excitation (PLE) spectroscopy study shows that both singlet and triplet interlayer excitons are highly valley-polarized at the resonant excitation with the valley polarization of the singlet interlayer exciton approaching unity at ∼20 K. The highly valley-polarized singlet and triplet interlayer excitons with giant valley-Zeeman splitting inspire future applications in spintronics and valleytronics.

Investigation of enhanced far-red emitting phosphor GdAlO3:Mn4+ by impurity doping for indoor plant growth LEDs

Phytochrome is indispensable for plant growth, which can absorb light useful for sprout, blossom, fruits. Generally, Phytochrome PR absorbs red light and Phytochrome PFR absorbs far-red light. The emission of Mn4+ doped GdAlO3 is situated in far-red light region. Herein, GdAlO3:Mn4+, Mg2+(GAL:Mn4+, Mg2+) phosphors were successfully synthesized via sol-gel method and the crystalline structure and luminescence properties were investigated systematically. Impressively, the emission intensity of GAL: Mn4+ phosphors was raised by Mg2+ and Ge4+ doping, and the mechanism of improved luminescence was also discussed. Depending on photoluminescence excitation and photoluminescence spectroscopy, the crystal field strength of the GdAlO3 host was calculated. In addition, excellent color purity and good thermal stability were found in Mg2+ co-doped GdAlO3:Mn4+ phosphors. Furthermore, the luminescence spectrum of GAL: Mn4+, Mg2+ phosphor was agreed well with the absorption of Phytochrome PFR, which convincingly confirmed that Mg2+ co-doped GdAlO3:Mn4+ phosphors show promise in indoor plant growth LEDs.

Hybrid solar cells with β- and γ- gallium oxide nanoparticles

Herein, hybrid composite layers made of nanocrystalline β-Ga2O3 or γ-Ga2O3 within poly(3,4-ethylenedioxythiophene):-polystyrene (PEDOT:PSS) have been studied in order to be incorporated into hybrid solar cells. Structural, morphological, optical and electrical characterization has been carried out with X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), PL excitation (PLE), cathodoluminescence (CL) and Raman spectroscopy. Bandgaps of 4.7 and 4.9 eV for γ- and β- phases, respectively, are obtained from PLE. Beside broad luminescence bands on 2.7–2.9 eV for the two phases, a band on 4.7 eV is obtained only for γ-Ga2O3, coincident with the observed PLE band. Minority carrier lifetimes are higher when γ-Ga2O3 nanoparticles are used for the hybrid system.

Confirmation of the compensation of unintentional donors in AlGaN/GaN HEMT structures by Mg-doping during initial growth of GaN buffer layer

Effect of partial Mg doping on the compensation of unintentional donors at epilayer/template interface and in the GaN channel layer of AlGaN/GaN High Electron Mobility Transistor (HEMT) structures is investigated. Photoluminescence excitation spectroscopy and surface photovoltage spectroscopy measurements reveal the signature of high density of unintentional shallow donors and deep level defects throughout the GaN buffer layer along with a strong dominance at the GaN/Fe-GaN template interface. Mg doping during the initial growth of GaN buffer layer on Fe-GaN template is found to compensate unintentional donors which therefore reduces the density of deep defects. Hall measurements also confirm systematic improvement in the electronic transport properties of AlGaN/GaN HEMT for an optimized Mg doping. Further, Mg-doping driven enhancement of 2-dimensional electron gas confinement at AlGaN/GaN interface is also observed in the spectroscopy measurements. It is shown that the electrical and optical properties can be improved by an optimized Mg-doping of the initial part of GaN buffer layer in AlGaN/GaN HEMT structures.

Remarkable influence of alkaline earth ions on the enhancement of fluorescence from Eu3+ ion doped in sodium ortho-phosphate phosphors

Abstract A series of different alkaline earth based sodium-ortho phosphate (NaMPO4, where M = Mg, Ca, Sr and Ba) phosphors doped with 1 mol% of Europium (Eu3+) were prepared by solution combustion method. The phase evolution controlled morphology and fluorescence properties were systematically examined by various techniques such as X-Ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and photoluminescence excitation (PLE) and photoluminescence (PL) spectroscopy. Our materials crystallized into monoclinic and orthorhombic phases that were found to be consistent with those listed in the standard JCPDS cards. The XRD results confirmed that the samples contained a mixture of phases in the crystals. The PLE spectra of the NaMPO4 phosphors consisted of a broad band with a maximum at 278 nm, while the Eu3+ doped phosphors contained a similar broad band of lower intensity and characteristic Eu3+excitation bands. These phosphors could be efficiently excited by near ultraviolet at 393 nm and were found to exhibit excellent red emission at 593 nm corresponding to the 5D0→7F4 transition of the Eu3+ ion. The calculated Commission Internationale de l’Eclairge (CIE) chromaticity color coordinates were calculated. The color of the emitting light could be tuned from orange-red to red through incorporation of different alkaline metal ions. All the results suggested that the examined sodium-ortho phosphates could potentially be used as sources of red light in tricolor white light emitting diodes.

Optical coherence of implanted silicon vacancy centers in thin diamond membranes.

We report the fabrication and optical characterization of thin diamond membranes implanted with negatively charged silicon vacancy (SiV-) centers. The variations in the membrane thickness enable the experimental study of optical coherence of SiV- centers as the membrane thickness is varied from 100 nm to 1100 nm. Photoluminescence excitation spectroscopy at low temperature shows that most of the SiV- centers in these membranes feature an optical linewidth ranging between 200 and 300 MHz. Furthermore, there is no discernable dependence of the optical linewidth on the membrane thickness for membranes as thin as 100 nm, indicating the feasibility of incorporating SiV- centers in a varity of diamond nanostructures and still maintaining the excellent optical coherence of these color centers.

Identifying Clusters and/or Small-Size Quantum Dots in Colloidal CdSe Ensembles with Optical Spectroscopy.

It is well-known that optical absorption and photoluminescence (PL) provide information that is sensitive to the size and size distribution of colloidal binary semiconductor quantum dots (QDs). To explore the nature of reaction products, clusters, and/or small-size QDs, we show that it is important to perform as well photoluminescence excitation (PLE) spectroscopy. For two non-hot-injection reactions of cadmium oleate (Cd(OA)2) and selenium (Se) in 1-octadecene (ODE), we show that sequentially extracted products displayed a similar apparent red shift in both absorption and PL with a full width at half-maximum (fwhm) of ∼30 nm. We demonstrate that one reaction (with the presence of diphenyl phosphine (HPPh2)) produced multiple types of clusters (with slightly different optical properties) in one ensemble, while the other reaction (without HPPh2) yielded primarily small-size QDs. Our findings provide evidence for the probable existence of clusters within small-size CdSe QD products, the existence of which complicates the size determination of small-size CdSe QDs.

Effect of PO43−/VO43− proportion on structure and photoluminescence properties of Gd(PyV1−y)O4: X at.% Tm3+ phosphors

A series of Gd(PyV1−y)O4: x at.% Tm3+ (x = 1, 2, 3, 4 and 5; y = 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1) phosphors were prepared by chemical co-precipitation method. The phosphors were characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, photoluminescence excitation and emission spectroscopy. XRPD result suggests that when the concentration ratio of VO43− to PO43− exceeds 3∕7, the crystal structure of phosphors changes from monoclinic structure to tetragonal structure. Tm3+ ion has a typical blue emission peak around 477 nm, which is caused by the transition of 1D4→3H6 energy level. By mixing PO43− and VO43− groups, the photoluminescence intensity and color purity of the phosphors are improved, and the optimal PO43− to VO43− concentration ratio is identified to be 1∕9. Different concentrations of Tm3+ were then used as activating ions of Gd(P0.1V0.9)O4 matrix, and the quenching concentration is found to be 2 at.%, with the quenching mechanism being revealed to be dipole-dipole interaction.

Effect of lithium addition on Te4+ emission in TeO2-Li2O glasses

Lithium tellurite glasses with xTeO2 + (100 − x)Li2O nominal composition, where x = 95, 90, 85, 80, 75 and 70 mol%, have been prepared to investigate the dependence of the Te4+ emission as a function of the lithium modifier. Optical and structural properties of the glasses were explored by using UV–Vis absorption, photoluminescence and photoluminescence excitation spectroscopies, photoluminescence lifetime and Raman spectroscopy. Besides, the influence of Li2O in the glass structure was verified by density, glass transition temperature, thermal stability, glass-forming tendency, optical band edge and the number of non-bridging oxygen. The results support there is a good correlation between the glass structure and the Te4+ center concentration: the Li2O content contributes to increase the Te4+ in the glass structure.

Identification of spin, valley and moiré quasi-angular momentum of interlayer excitons

Moiré superlattices provide a powerful way to engineer the properties of electrons and excitons in two-dimensional van der Waals heterostructures1–8. The moiré effect can be especially strong for interlayer excitons, where electrons and holes reside in different layers and can be addressed separately. In particular, it was recently proposed that the moiré superlattice potential not only localizes interlayer exciton states at different superlattice positions, but also hosts an emerging moiré quasi-angular momentum (QAM) that periodically switches the optical selection rules for interlayer excitons at different moiré sites9,10. Here, we report the observation of multiple interlayer exciton states coexisting in a WSe2/WS2 moiré superlattice and unambiguously determine their spin, valley and moiré QAM through novel resonant optical pump–probe spectroscopy and photoluminescence excitation spectroscopy. We demonstrate that interlayer excitons localized at different moiré sites can exhibit opposite optical selection rules due to the spatially varying moiré QAM. Our observation reveals new opportunities to engineer interlayer exciton states and valley physics with moiré superlattices for optoelectronic and valleytronic applications. Stacked 2D materials can host excitons with distinct valley selection rules due to the spatial variation of the moiré pattern. The authors demonstrate this via optical spectroscopy, opening a route for control of optoelectronic devices.

Mutual Energy Transfer in a Binary Colloidal Quantum Well Complex.

Förster resonance energy transfer (FRET) is a fundamental process that is key to optical biosensing, photosynthetic light harvesting, and down-converted light emission. However, in total, conventional FRET in a donor-acceptor pair is essentially unidirectional, which impedes practical application of FRET-based technologies. Here, we propose a mutual FRET scheme that is uniquely bidirectional in a binary colloidal quantum well (CQW) complex enabled by utilizing the d orbital electrons in a dopant-host CQW system. Steady-state emission intensity, time-resolved, and photoluminescence excitation spectroscopies have demonstrated that two distinct CQWs play the role of donor and acceptor simultaneously in this complex consisting of 3 monolayer (ML) copper-doped CQWs and 4 ML undoped CQWs. Band-edge excitons in 3 ML CQWs effectively transfer the excitation to excitons in 4 ML CQWs, whose energy is also harvested backward by the dopants in 3 ML CQWs. This binary CQW complex, which offers a unique mutual energy-transfer mechanism, may unlock revolutionary FRET-based technologies.

Lanthanide-doped lanthanum hafnate nanoparticles as multicolor phosphors for warm white lighting and scintillators

Designing luminescent materials especially nanomaterials with multifunctional applications is highly challenging and demanding. In this work, we explored pyrochlore La2Hf2O7 nanoparticles (NPs) singly and triply codoped with Eu3+, Tb3+ and Dy3+. Under both ultraviolet and X-ray irradiations, the La2Hf2O7 NPs singly doped with Eu3+, Tb3+ and Dy3+ displayed red, green and yellowish-blue emission, respectively. The concentration quenching study revealed a non-radiative energy transfer in Eu3+ doped La2Hf2O7 NPs, which takes place via dipole-quadrupole mechanism. On the other hand, a dipole-dipole interaction prevails in Tb3+ and Dy3+ doped La2Hf2O7 NPs. Lifetime spectroscopy reveals the stabilization of Eu3+ and Dy3+ ions at La3+ site at low doping concentration whereas a fraction of them migrates to Hf4+ site at high doping concentration. For the La2Hf2O7:Tb3+ NPs, Tb3+ ions are localized at Hf4+ site at all doping concentrations. Furthermore, when triply codoped with Eu3+, Tb3+ and Dy3+ ions, the La2Hf2O7 NPs display beautiful warm white light as a new strategy for color tunability through doping percentage. To sum, our complete spectrum of studies on the structure, UV excited photoluminescence, concentration quenching, and local site spectroscopy of the La2Hf2O7:Ln3+ NPs suggests that they are potential candidates as single-component multicolor-emitting phosphors for lighting and scintillating applications.

An optical thermometry based on abnormal negative thermal quenching of the charge transfer band edge

In this work the temperature-dependent photoluminescence excitation and photoluminescence spectroscopies of Eu3+-doped LuVO4 phosphors have been performed in the 298–448 K range. It is found that the charge transfer band (CTB) of [VO4]3- unites undergoes a red-shift, mainly owing to thermal population of the vibronic sublevels of the ground electronic energy level, as the temperature increases. Due to this, the edge of CTB in PLE exhibits an abnormal negative thermal quenching behavior compared to those of the maximum CTB transition and the intra-configuration f-f transitions of Eu3+ ions. Accordingly, a novel thermometry strategy for realizing ratiometric optical thermometer is proposed by considering different excitation paths taking advantage of their diverse thermal behaviors. For Eu3+-doped LuVO4 compound, our experimental results demonstrate its great potential for high performance optical thermometry, promising with a remarkable relative sensitivity and excellent repeatability in the 298–448 K range.