Showing papers on "Photoluminescence excitation published in 2017"

Photoluminescence excitation spectroscopy of SiV− and GeV− color center in diamond

Abstract: Color centers in diamond are important quantum emitters for a broad range of applications ranging from quantum sensing to quantum optics. Understanding the internal energy level structure is of fundamental importance for future applications. We experimentally investigate the level structure of an ensemble of few negatively charged silicon-vacancy (SiV−) and germanium-vacancy (GeV−) centers in bulk diamond at room temperature by photoluminescence (PL) and excitation (PLE) spectroscopy over a broad wavelength range from 460 to and perform power-dependent saturation measurements. For SiV− our experimental results confirm the presence of a higher energy transition at . By comparison with detailed theoretical simulations of the imaginary dielectric function we interpret the transition as a dipole-allowed transition from -state to -state where the corresponding a 2u -level lies deeply inside the diamond valence band. Therefore, the transition is broadened by the diamond band. At higher excitation power of we indicate signs of a parity-conserving transition at supported by saturation measurements. For GeV− we demonstrate that the PLE spectrum is in good agreement with the mirror image of the PL spectrum of the zero-phonon line. Experimentally we do not observe a higher lying energy level up to a transition wavelength of . The observed PL spectra are identical, independent of excitation wavelength, suggesting a rapid decay to excited state and followed by optical transition to ground state. Our investigations convey important insights for future quantum optics and quantum sensing experiments based on SiV−-center and GeV−-center in diamond.

CdS QDs-Decorated Self-Doped γ-Bi2MoO6: A Sustainable and Versatile Photocatalyst toward Photoreduction of Cr(VI) and Degradation of Phenol

Abstract: In this work, CdS quantum dots (QDs)-sensitized self-doped Bi2MoO6 has been synthesized using glucose as reducing agent by hydrothermal method, followed by in situ deposition of the QDs. The synthesized catalyst has been employed to reduce toxic Cr(VI) and degrade phenol from the aqueous solution. The structural, optical, and electrochemical characterizations are performed using X-ray diffraction, UV–vis diffuse reflection, photoluminescence (PL), scanning electron microscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, and electrochemical impedance spectroscopy. The optical properties were precisely investigated by calculating the Urbach energy, PL, and photoluminescence excitation spectra. The orderly distribution of QDs is confirmed from the correlation between full width at half-maximum of PL spectra, Urbach energy, and TEM analysis. The versatile photocatalytic activity has been tested toward Cr(VI) reduction and degradation of phenol. 3% CdS QDs-sensitized self-d...

Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors with ultrabroad excitation band for white light-emitting diodes.

Abstract: A series of novel Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors have been successfully prepared by a citrate-assisted sol-gel process. Both photoluminescence excitation and emission spectra suggest that the resultant products have the strong ultrabroad absorption band ranging from 220 to 450 nm. Under the excitation of 379 nm, the characteristic emissions of Eu3+ ions corresponding to the 5D0 → 7F J transitions are observed in the doped samples. The optimal doping concentration for Eu3+ ions is found to be 12 mol% and the quenching mechanism is attributed to the dipole-dipole interaction. A theoretical calculation based on the Judd-Ofelt theory is carried out to explore the local structure environment around the Eu3+ ions. The studied samples exhibit a typical thermal quenching effect with a T0.5 value of 338 K and the activation energy is determined to be 0.427 eV. A near-ultraviolet (NUV)-based white light-emitting diode (LED) is packaged by integrating a mixture of resultant phosphors, commercial blue-emitting and green-emitting phosphors into an NUV LED chip. The fabricated LED device emits glaring white light with high color rendering index (84.6) and proper correlated color temperature (6492 K). These results demonstrate that the Eu3+-activated La2MoO6-La2WO6 compounds are a promising candidate for indoor lighting as red-emitting phosphors.

Synthesis and luminescence properties of Ba 3 Lu(PO 4 ) 3 :Sm 3+ phosphor for white light-emitting diodes

Abstract: A series of Ba3Lu(PO4)3:Sm3+ phosphors were prepared by traditional high temperature solid-state reaction methods. The site-preferred occupancy of Sm3+ in Ba3Lu(PO4)3 and the luminescence properties of Ba3Lu(PO4)3:Sm3+ were studied combined with X-ray diffraction, photoluminescence excitation (PLE) spectra, and emission (PL) spectra as well as temperature-dependent PL and decay curves. The PL intensity is improved with increasing Sm3+ content and the optimal dopant content is 0.05. The temperature-dependent PL spectra indicate that the emission intensity decreases with the temperature because of the enhancement of the non-radiative transition. The results indicate that these reddish-orange emitting phosphors could be for potential applications in w-LEDs.

Photoluminescence excitation spectroscopy of SiV$^{-}$ and GeV$^{-}$ color center in diamond

Abstract: Color centers in diamond are important quantum emitters for a broad range of applications ranging from quantum sensing to quantum optics Understanding the internal energy level structure is of fundamental importance for future applications We experimentally investigate the level structure of an ensemble of few negatively charged silicon-vacancy (SiV$^{-}$) and germanium-vacancy (GeV$^{-}$) centers in bulk diamond at room temperature by photoluminescence (PL) and excitation (PLE) spectroscopy over a broad wavelength range from 460 nm to 650 nm and perform power-dependent saturation measurements For SiV$^{-}$ our experimental results confirm the presence of a higher energy transition at ~ 231 eV By comparison with detailed theoretical simulations of the imaginary dielectric function we interpret the transition as a dipole-allowed transition from $^{2}E_{g}$-state to $^{2}A_{2u}$-state where the corresponding $a_{2u}$-level lies deeply inside the diamond valence band Therefore, the transition is broadened by the diamond band At higher excitation power of 10 mW we indicate signs of a parity-conserving transition at ~203 eV supported by saturation measurements For GeV$^{-}$ we demonstrate that the PLE spectrum is in good agreement with the mirror image of the PL spectrum of the zero-phonon line (ZPL) Experimentally we do not observe a higher lying energy level up to a transition wavelength of 460 nm The observed PL spectra are identical, independent of excitation wavelength, suggesting a rapid decay to $^{2}E_{u}$ excited state and followed by optical transition to $^{2}E_{g}$ ground state Our investigations convey important insights for future quantum optics and quantum sensing experiments based on SiV$^{-}$ center and GeV$^{-}$ center in diamond

Optical spectroscopy of excited exciton states in MoS2 monolayers in van der Waals heterostructures

Abstract: The optical properties of MoS2 monolayers are dominated by excitons, but for spectrally broad optical transitions in monolayers exfoliated directly onto SiO2 substrates detailed information on excited exciton states is inaccessible. Encapsulation in hexagonal boron nitride (hBN) allows approaching the homogenous exciton linewidth, but interferences in the van der Waals heterostructures make direct comparison between transitions in optical spectra with different oscillator strength more challenging. Here we reveal in reflectivity and in photoluminescence excitation spectroscopy the presence of excited states of the A-exciton in MoS2 monolayers encapsulated in hBN layers of calibrated thickness, allowing to extrapolate an exciton binding energy of about 220 meV. We theoretically reproduce the energy separations and oscillator strengths measured in reflectivity by combining the exciton resonances calculated for a screened two-dimensional Coulomb potential with transfer matrix calculations of the reflectivity for the van der Waals structure. Our analysis shows a very different evolution of the exciton oscillator strength with principal quantum number for the screened Coulomb potential as compared to the ideal two-dimensional hydrogen model.

Phonon-assisted oscillatory exciton dynamics in monolayer MoSe2

Abstract: In monolayer semiconductor transition metal dichalcogenides, the exciton-phonon interaction is expected to strongly affect the photocarrier dynamics. Here, we report on an unusual oscillatory enhancement of the neutral exciton photoluminescence with the excitation laser frequency in monolayer MoSe2. The frequency of oscillation matches that of the M-point longitudinal acoustic phonon, LA(M). Oscillatory behavior is also observed in the steady-state emission linewidth and in timeresolved photoluminescence excitation data, which reveals variation with excitation energy in the exciton lifetime. These results clearly expose the key role played by phonons in the exciton formation and relaxation dynamics of two-dimensional van der Waals semiconductors.

Temperature and doping concentration dependence of the energy band gap in β-Ga_2O_3 thin films grown on sapphire

Abstract: This paper presents the effects of temperature and n-type doping concentration on the energy band gap of β-Ga2O3 thin films grown on c-plane sapphire substrates by low pressure chemical vapor deposition (LPCVD). The β-Ga2O3 thin films were grown using high purity gallium (Ga) and oxygen (O2) as precursors, and Si as the n-type dopant. The transmission electron microscopy (TEM) diffraction pattern showed that the thin films are single crystals that have a monoclinic crystal structure. The dependence of the energy band gap on temperature and n-type doping concentration have been experimentally determined from photoluminescence excitation (PLE) and absorbance spectra. The PLE spectra were measured in the temperature range of 77-298 K. The results indicate that both temperature and carrier concentration play important roles in determining the energy band gap of β-Ga2O3 thin films. The optical gap increased with the electron concentration for ne ≤ 2.52x1018 cm−3, which is due to the dominant Burstein-Moss (BM) shift. The sudden decrease in the energy gap at a doping concentration of 6.23x1018 – 3.05x1019 cm−3 is consistent with the theoretical prediction of Mott criterion for Ga2O3 semiconductor-metal transition. The energy band gap shrinks with an increasing temperature from 77 to 298 K.

Substrate screening effects on the quasiparticle band gap and defect charge transition levels in MoS$_2$

Abstract: Monolayer MoS$_2$ has emerged as an interesting material for nanoelectronic and optoelectronic devices. The effect of substrate screening and defects on the electronic structure of MoS$_2$ are important considerations in the design of such devices. Here, we present ab initio density functional theory (DFT) and GW calculations to study the effect of substrate screening on the quasiparticle band gap and defect charge transition levels (CTLs) in monolayer MoS$_2$. We find a giant renormalization to the free-standing quasiparticle band gap by 350 meV and 530 meV in the presence of graphene and graphite as substrates, respectively. Our results are corroborated by recent experimental measurements on these systems using scanning tunneling spectroscopy and photoluminescence excitation spectroscopy. Sulfur vacancies are the most abundant native defects found in MoS$_2$. We study the CTLs of these vacancies in MoS$_2$ using the DFT+GW formalism. We find (+1/0) and (0/-1) CTLs appear in the pristine band gap of MoS$_2$. Substrate screening results in renormalization of the (0/-1) level, with respect to the valence band maximum (VBM), by the same amount as the gap. This results in the pinning of the (0/-1) level about $\sim$500 meV below the conduction band minimum for the free-standing case as well as in the presence of substrates. The (+1/0) level, on the other hand, lies less than 100 meV above the VBM for all the cases.

A one-pot synthesis of water soluble highly fluorescent silica nanoparticles.

Abstract: We report a one-pot synthesis of water dispersible fluorescent silica nanoparticles (NPs) functionalized with terminal amine groups, starting from silicon tetrabromide (SiBr4) and aminopropyltriethoxy silane (APTES). The NPs range from 1 to 2 nm in diameter, and exhibit an intense blue emission with a quantum yield (QY) of around 34% in water. They were characterized using XRD, XPS, TEM and FTIR spectroscopy for structural analysis. A tentative mechanism explaining the origin of the NPs emission in the blue region is presented based on the distinctive features of their low temperature photoluminescence (PL), photoluminescence excitation (PLE) spectrum and time correlated single photon counting lifetime decay profiles. The outstanding PL QY and photostability of the NPs, together with their water dispersibility and biocompatibility, constitute a unique set of properties among existing silica NPs and enable the application of the NPs in various fields.

Optical switching of defect charge states in 4H-SiC.

Abstract: We demonstrate optically induced switching between bright and dark charged divacancy defects in 4H-SiC. Photoluminescence excitation and time-resolved photoluminescence measurements reveal the excitation conditions for such charge conversion. For an energy below 1.3 eV (above 950 nm), the PL is suppressed by more than two orders of magnitude. The PL is recovered in the presence of a higher energy repump laser with a time-averaged intensity less than 0.1% that of the excitation field. Under a repump of 2.33 eV (532 nm), the PL increases rapidly, with a time constant 30 μs. By contrast, when the repump is switched off, the PL decreases first within 100–200 μs, followed by a much slower decay of a few seconds. We attribute these effect to the conversion between two different charge states. Under an excitation at energy levels below 1.3 eV, VSiVC 0 are converted into a dark charge state. A repump laser with an energy above 1.3 eV can excite this charged state and recover the bright neutral state. This optically induced charge switching can lead to charge-state fluctuations but can be exploited for long-term data storage or nuclear-spin-based quantum memory.

Anomalous Above-Gap Photoexcitations and Optical Signatures of Localized Charge Puddles in Monolayer Molybdenum Disulfide

Abstract: Broadband optoelectronics such as artificial light harvesting technologies necessitate efficient and, ideally, tunable coupling of excited states over a wide range of energies. In monolayer MoS2, a prototypical two-dimensional layered semiconductor, the excited state manifold spans the visible electromagnetic spectrum and is comprised of an interconnected network of excitonic and free-carrier excitations. Here, photoluminescence excitation spectroscopy is used to reveal the energetic and spatial dependence of broadband excited state coupling to the ground-state luminescent excitons of monolayer MoS2. Photoexcitation of the direct band gap excitons is found to strengthen with increasing energy, demonstrating that interexcitonic coupling across the Brillouin zone is more efficient than previously reported, and thus bolstering the import and appeal of these materials for broadband optoelectronic applications. Narrow excitation resonances that are superimposed on the broadband photoexcitation spectrum are ide...

Efficient Diffusive Transport of Hot and Cold Excitons in Colloidal Type II CdSe/CdTe Core/Crown Nanoplatelet Heterostructures

Abstract: Cadmium chalcogenide colloidal quantum wells or nanoplatelets (NPLs), a class of new materials with atomically precise thickness and quantum confinement energy, have shown great potential in optoelectronic applications. Short exciton lifetimes in two-dimensional (2D) NPLs can be improved by the formation of type II heterostructures, whose properties depend critically on the mechanism of exciton transport. Herein, we report a study of room-temperature exciton in-plane transport mechanisms in type-II CdSe/CdTe core/crown (CC) colloidal NPL heterostructures with the same CdSe core and different CdTe crown sizes. Photoluminescence excitation measurements show unity quantum efficiency for transporting excitons created at the crown to the CdSe/CdTe interface (to form lower-energy charge-transfer excitons). At near band edge excitation, the crown-to-core transport time increases with crown size (from 2.7 to 5.6 ps), and this size-dependent transport can be modeled well by 2D diffusion of thermalized excitons in ...

Adjusting the band structure and defects of ZnO quantum dots via tin doping

Abstract: The structures and band structures of Sn doped ZnO were investigated by density functional theory (DFT). Sn-doped ZnO quantum dots were also synthesized via an ultrasonic sol–gel method. The charge of Sn ions was also taken into consideration and Sn2+ and Sn4+ were employed. UV-Vis absorption spectra and photoluminescence excitation spectra were used to elucidate the band gap of Sn-doped ZnO QDs. Photoluminescence spectra were employed to study the change in the defects of Sn-doped ZnO QDs. Sn doping reduced the symmetry of the ZnO cell and stretched the cell. Furthermore, it also could change ZnO from a direct gap semiconductor to an indirect gap semiconductor. The experimental results and DFT calculations matched quite well. Both the DFT calculations and experimental results showed that with an increasing Sn content, the band gap of Sn-doped ZnO first decreased and then increased, whereas the band gap of the ZnO base only increased. Sn doping also changed the optical defects of the quantum dots from defects to OZn and Oi defects.

The synthesis and luminescence properties of a novel red-emitting phosphor: Eu 3+ -doped Ca 9 La(PO 4 ) 7

Abstract: A series of novel red-emitting phosphors Ca9La1–x (PO4)7: xEu3+ were synthesized by high-temperature solid state reactions. The photoluminescence excitation and photoluminescence spectra of these phosphors were investigated in detail. O2−-Eu3+ charge transfer band peaking at about 261 nm is dominant in the PLE spectra of Eu3+-doped Ca9La(PO4)7, indicating that the phosphors are suitable for tricolor fluorescent lamps. The phosphors also show a good absorption in near ultraviolet (around 395 nm) and blue (around 465 nm) spectral region, which indicates that it can be pumped with NUV and blue chips for white light-emitting diodes. The transition of 5D0 → 7F2 of Eu3+ in this lattice can emit bright red light. Ca9La(PO4)7 could accommodate a large amount of Eu3+ with an optimal concentration of 60 mol%. The dipole–dipole interaction between Eu3+ is the dominant mechanism for concentration quenching of Eu3+. The calculated color coordinates lie in red region (x = 0.64, y = 0.36), which is close to Y2O3: 0.05Eu3+ (x = 0.65, y = 0.34). The integral emission intensity of Ca9La0.4(PO4)7: 0.6Eu3+ is 1.9 times stronger than that of widely used commercial red phosphor Y2O3: 0.05Eu3+. All these results indicate that Eu3+-doped Ca9La(PO4)7 is a promising red-emitting phosphor which can be used in tricolor fluorescent lamps and white light-emitting diodes .

Exciton–phonon interactions in the Cs3Bi2I9 crystal structure revealed by Raman spectroscopic studies

Abstract: The enhancement of the Raman scattering in Cs3Bi2I9 is evaluated by the ratio IT/I300 K between the relative intensities of the Raman line peaked at 146 cm−1, when the spectra are recorded in the temperature range of 88–300 K, as a signature of exciton–phonon interactions In the resonant and nonresonant conditions, excitation wavelengths 476, 561, and 660 nm, respectively, are used in order to overlap with great accuracy the bands disclosed by diffuse reflection, photoconductivity (PC), photoluminescence (PL), and photoluminescence excitation (PLE) spectra Based on the experimental analyses, the strength of exciton–phonon interaction is dependent on the defects in the crystal and the type–range interaction of the excitations in an independent Bi2I93− cluster The noticeable PL band, attributed to excitons trapped on different stacking faults, manifests some defects in crystal that diminish the movement of excitons This effect significantly decreases the overlaps of excitons with the phonons, resulting in a reduced exciton–phonon coupling

Optical switching of defect charge states in 4H-SiC

Abstract: We demonstrate optically induced switching between bright and dark charged divacancy defects in 4H-SiC. Photoluminescence excitation and time-resolved photoluminescence measurements reveal the excitation conditions for such charge conversion. For an energy below ~1.3 eV (above ~950 nm), the PL is suppressed by more than two orders of magnitude. The PL is recovered in the presence of a higher energy repump laser with a time-averaged intensity less than 0.1% that of the excitation field. Under a repump of 2.33 eV (532 nm), the PL increases rapidly, with a time constant ~30 $\mu$s. By contrast, when the repump is switched off, the PL decreases first within ~100-200 $\mu$s, followed by a much slower decay of a few seconds. We attribute these effects to the conversion between two different charge states. Under an excitation at energy levels below 1.3 eV, V$_{Si}$V$_C$$^0$ are converted into a dark charge state. A repump laser with an energy above 1.3 eV can excite this charged state and recover the bright neutral state. This optically induced charge switching can lead to charge-state fluctuations but can be exploited for long-term data storage or nuclear-spin-based quantum memory.