Modulation Of Photoluminescence Intensity Research Articles

Curvature-dependent strain-induced photoluminescence modulations in flexible Er-doped Pb0.98La0.02Zr0.95Ti0.05O3 antiferroelectric thin films prepared by the sol-gel method

Tunable photoluminescence (PL) has wide applications in optical waveguides and communication. Since the PL intensity is strongly dependent on the crystal field symmetry, curvature-dependent strain-induced PL modulation is expected in flexible antiferroelectric (AFE) thin film materials. Here, flexible Er-doped Pb0.98La0.02Zr0.95Ti0.05O3 (PLZT) AFE thin films were prepared via the sol-gel method. A giant and reversible PL intensity modulation (482%) was achieved during the bending process. Based on density functional theory (DFT) calculations and Raman spectra measurements, the relationship amongst the curvature-dependent strains, crystal structure, and PL modulation was discussed. The induced strain leads to lattice distortion and even a phase transition, both of which contribute to the reduction in crystal symmetry and thus significantly enhance the PL intensity.

Modulation of photoluminescence intensity by surface defects of MAPbBr3 crystals

MAPbBr3 single crystals have attracted much attention because of their high quantum efficiency and photoluminescence (PL) peaks. It has been researched that the crystal has two PL peaks and the additional PL peak comes from the “PbBr2” defect in the crystal. This can be well applied to dual-wavelength lasers. However, the change rule of the PL two peaks intensity is not clear. Herein. Crystals with two PL peaks are obtained by inverse temperature crystallization. We research the crystal structures of MABr and PbBr2 in different proportions, and find that these crystals have varying degrees of defects. Moreover, we confirm that these defects are Schottky defects based on experimental and simulated XRD. The optical properties of the samples are studied, which suggest that the PL intensity is related to the ratio of MABr to PbBr2. The energy level distribution of the crystal is calculated by energy level simulation, which further explains that the 536 nm is generated by “PbBr2” and the 567 nm is generated by MAPbBr3. All the results illustrate that the intensity of two peaks can be adjusted by the ratio of MABr to PbBr2. The maximum intensity is at 536 nm for MABr: PbBr2= 2 and 567 nm for MABr: PbBr2= 0.5. We explain this change rule of two peaks intensities from the energy level point of view and obtained MAPbBr3 crystals with high PL intensity corresponding to different wavelengths. This research provides a theoretical basis for the change in PL bimodal intensity, and an experimental reference for perovskite dual-wavelength laser.

Effect of Photonic Band Gap on Photoluminescence in a Dye-Doped Blue Phase Liquid Crystal.

Tunability of fluorescence intensity is an essential parameter for enhancing the versatility of devices like emissive displays and solar cells. Soft photonic crystals, with their tunable photonic band gap (PBG), are highly sought-after systems for such purposes. Here, we report modulation of photoluminescence (PL) intensity in a fluorescent dye-doped blue phase liquid crystal, a 3D soft photonic crystal. On cooling, from the isotropic fluid phase, the PL intensity gets enhanced due to the overlapping of the emission wavelength of the dye with the photonic band edge. However, the PL intensity decreases on the application of an electric field, despite both thermal and electric fields having a similar effect (red shift) on the PBG. The contrasting behavior of PL intensity, also observed in composites obtained by varying the dye and the chiral dopant (handedness), is discussed in terms of scattering pathways for the emitted photons. The time-resolved PL studies show a reduction in the lifetime of the excited species upon cooling, validating the thermal dependence of PL intensity modulation due to Purcell effect. The facile modulation of PL intensity in the dye-doped blue phase system makes it appealing from the point of view of high-performance photonic applications.

All-Optical Reversible Manipulation of Exciton and Trion Emissions in Monolayer WS2.

Monolayer transition metal dichalcogenides (TMDs) are direct gap semiconductors with promising applications in diverse optoelectronic devices. To improve devices’ performance, recent investigations have been systematically focused on the tuning of their optical properties. However, an all-optical approach with the reversible feature is still a challenge. Here we demonstrate the tunability of the photoluminescence (PL) properties of monolayer WS2 via laser irradiation. The broad-range and continuous modulation of PL intensity, as well as the conversion between neutral and charged excitons have been readily and reversibly achieved by only switching the two laser power densities. We attribute the reversible manipulation to the laser-assisted adsorption and desorption of gas molecules, which will deplete or release free electrons from the surface of WS2 and thus modify its PL properties. This all-optical manipulation, with advantages of reversibility, quantitative control, and high spatial resolution, suggests promising applications of TMDs monolayers in optoelectronic and nanophotonic applications, such as erasable optical data storage, micropatterning, and display.

Tailoring light-matter interaction in WS2 –gold nanoparticles hybrid systems

Recently, considerable attention has been paid to tune the emission using hybrid systems composed of layered transition-metal dichalcogenides and metal nanoparticles (NPs) since metal NPs have the ability to enhance and localize the incident electromagnetic field. Furthermore, these hybrid systems show great interest from the standpoint of fundamental science as it constitutes an atomic scale prototype of charge-transfer complexes. Here, we realized ${\mathrm{WS}}_{2}$--gold (Au) NPs hybrids by chemically growing Au NPs at the edges of the mechanically exfoliated bilayer ${\mathrm{WS}}_{2}$. The Au NPs significantly increase the light-matter interaction which has been studied through Raman and photoluminescence (PL) spectroscopy. A substantial enhancement of the PL intensity in the ${\mathrm{WS}}_{2}$--Au composite concerning the pristine ${\mathrm{WS}}_{2}$ has been observed, and it increases as the number and size of the Au NPs on ${\mathrm{WS}}_{2}$ is increased. Geometry-dependent modification of plasmon resonance energy of Au NP alters the coupling strength between the emission pathways of ${\mathrm{WS}}_{2}$ and the plasmon which is manifested by a change in relative intensity between trion $({X}^{\ensuremath{-}})$ and exciton $(X)$ emissions. We probe the mechanism of the PL intensity modulation through polarization-dependent measurements and simulation. We have demonstrated that, in ${\mathrm{WS}}_{2}$, the internal quantum efficiency increases and activation energy decreases due to coupling with Au NPs. Compared to pristine ${\mathrm{WS}}_{2}$, a faster change in optical band gap with temperature in ${\mathrm{WS}}_{2}$--Au may be due to enhancing electron-phonon interaction and lattice expansion in the latter. Our paper indicates the possibility to develop high performance transition-metal dichalcogenide-based photonic devices.

Thermally driven reversible photoluminescence modulation in WS2/VO2 heterostructure

We demonstrate reversible modulation of photoluminescence in chemical vapor deposition (CVD) grown WS2 monolayer transferred on VO2 thin films that undergo insulator-metal transition. Thermally driven phase transition in VO2 thin film involving structural change as well as thermal expansion mismatch at interfaces induce strain and able to reversibly tune photoluminescence (PL) of atomic thin WS2 layer. PL intensity is increased with enhancement factor of 1.83 which corresponds to ~250% enhancement when heated from 30 °C to 70 °C. With further increase of temperature, PL intensity is found to be decreasing due to thermal quenching. While interference effect arising from metallic VO2/WS2 interface may also contribute for luminescence enhancement in metallic state, the observed increase in photoluminescence intensity during heating within the insulating state reveals the role of thermal strain. Systematic in-situ Raman and PL measurements revealed role of phase transition in controlling the PL intensity of excitons in WS2 monolayer. Single crystalline VO2 microplates also have been used to confirm the PL intensity modulation across phase transition. The observed direct correlation between photoluminescence and phase change interaction of VO2 with atomic thin layer of WS2 provide novel platform to tune the optical properties for diverse smart photonic applications.

Room Temperature Electrical Control of Single Photon Sources at 4H-SiC Surface

Single photon source (SPS) providing nonclassical light states on demand is one of the key technologies for the application of quantum communication and optical quantum computer. In this paper, room temperature electrical control of single photon emission from defects at 4H-SiC surface is presented. Planar-type 4H-SiC p+nn+ diodes are fabricated and defects that act as SPS are formed on the surface of n-type epi-layer by field oxidation process. The photon emission properties of SPSs are investigated using a home-built confocal microscopy. Results show that the electroluminescence (EL) intensity of SPS can be controlled by minority carrier injection of forward bias voltages, while the photoluminescence (PL) intensity of SPS can be controlled by reverse bias voltages. No significant variations due to applied bias voltages are observed in the EL and PL spectra, indicating the defect structure and charge state are unchanged. The PL intensity modulation by switching a reverse bias voltage is also demonstrated.

Nanosculpting of Atomically Thin 2D Materials for Site‐Specific Photoluminescence Modulation

AbstractNanomanufacturing techniques to generate patterned structures of mechanically exfoliated two‐dimensional (2D) materials by lithography, gas phase etching, and electron beam/ion beam irradiation have gained significant interest. Such control over patterning in monolayers of WS2 and MoS2 will aid the development of novel, miniaturized devices for nanoelectronic and nanophotonic applications. In this study a controlled site‐specific nanosculpting of WS2 and MoS2 monolayers grown on SiO2/Si substrate by atmospheric pressure chemical vapor deposition (CVD) is reported. Nanoindentation by sharp diamond Berkovich tip with ≈50 nm radius is used to make periodic arrays of indents to spatially modulate photoluminescence (PL) intensity up to more than an order of changes. To demonstrate systematic strain variation, relatively shallow indentations are also carried out with conical tip of ≈5 µm radius. Raman and PL measurements carried out near the indented region indicate a systematic variation in strain with respect to load and location. Further, nanosculpting to design nonperiodic structures is achieved by performing nanoindentation in close proximity to each other with detailed Raman and PL mapping. The developed approach of indentation‐based site‐specific nanosculpting of 2D materials and modulation of PL intensity provides direct pathways to design a variety of patterned structures and nanoscale devices for photodetector applications.

Spatial Control of Photoluminescence at Room Temperature by Ferroelectric Domains in Monolayer WS2/PZT Hybrid Structures.

Single-monolayer transition metal dichalcogenides exhibit exceptionally strong photoluminescence (PL), dominated by a combination of distinct neutral and charged exciton contributions. We show here that the surface charge associated with ferroelectric domains patterned into a lead zirconium titanate film with an atomic force microscope laterally controls the spatial distribution of neutral and charged exciton populations in an adjacent WS2 monolayer. This is manifested by the intensity and spectral composition of the PL measured in air at room temperature from the areas of WS2 over a ferroelectric domain with a polarization dipole pointed either out of the surface plane or into the surface plane. This approach enables spatial modulation of PL intensity and trion/neutral exciton populations and fabrication of lateral quantum dot arrays in any geometry, with potential applications in nonvolatile optically addressable memory or optical quantum computation.

Photoluminescence intensity modulation by charge carrier injection in silicon nanocrystals at room temperature

Hysteretic intensity modulation of the photoluminescence (PL) of silicon nanocrystals (Si-NCs) embedded in silicon oxide (SiO2) is observed in metal-oxide-semiconductor (MOS) structures at room temperature during gate voltage sweeps of ±3 V. This PL intensity modulation is correlated with simultaneous current density measurements in the different operating regimes of these MOS devices. In particular, PL intensity enhancement is shown to result from electron injection into the oxide. The different mobilities of the charge carriers in SiO2 and the competing effects of charge trapping in the Si-NC and the surrounding oxide defects are used to explain the observed PL modulation.

Effect of electric field on the photoluminescence intensity of single CdSe nanocrystals

An investigation of the effect of an applied electric field on the photoluminescence (PL) intensity of single CdSe nanocrystals has revealed a measurable field induced PL modulation for a large fraction of the nanocrystals studied. The field induced intensity modulation characteristics (i.e. modulation sign and depth) were observed to vary from particle to particle, and even for different time periods for the same particle in many cases. Simultaneous intensity and frequency resolved PL measurement show that the PL intensity modulation is in fact due to an electric field effect on the PL quantum yield. The results are consistent with a model in which the energies of surface charge trapping sites are modulated by the applied electric field, causing in turn a modulation of the rates of exciton quenching by these sites. The complex observed field effects can be explained by the superposition of the applied and internal electric fields due to deeply trapped charges on the surface of the nanoparticle.

Field-Induced Photoluminescence Modulation of MEH−PPV under Near-Field Optical Excitation

Electric field-induced modulation of the near-field photoluminescence of thin films of the conjugated polymer MEH−PPV was measured. A voltage bias applied between the near-field probe and the substrate results in a highly spatially confined electric field. The near-field photoluminescence intensity decreases when the sample bias is positive with respect to the probe while the intensity increases when the sample bias is negative. The modulation of photoluminescence intensity provides a sensitive measure of changes in the local carrier density induced by the applied electric field. Experiments performed with a blocking layer indicate that carriers are generated by photoexcitation in the bulk of the polymer film. On the basis of the modulation results, we estimate the concentration of holes under optical and electrical excitation to be 1017 cm-3. The temporal response of the fluorescence upon the application of voltage is described in terms of the carrier mobility and electric field.