Photoluminescence Wavelength Research Articles

Dissymmetric Bis(dipyrrinato)Zinc(II) Complexes: Rich Variety and Bright Red to Near-Infrared Luminescence with a Large Pseudo-Stokes Shift

Bis(dipyrrinato)metal(II) and tris(dipyrrinato) metal(III) complexes have been regarded as much less useful luminophores than their boron difluoride counterparts (4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes, BODIPYs), especially in polar solvent. We proposed previously that dissymmetry in such metal complexes (i.e., two different dipyrrinato ligands in one molecule) improves their fluorescence quantum efficiencies. In this work, we demonstrate the universality and utility of our methodology by synthesizing eight new dissymmetric bis(dipyrrinato)zinc(II) complexes and comparing them with corresponding symmetric complexes. Single-crystal X-ray diffraction analysis, 1H and 13C NMR spectroscopy, and high-resolution mass spectrometry confirm the retention of dissymmetry in both solution and solid states. The dissymmetric complexes all show greater photoluminescence (PL) quantum yields (ΦPL) than the corresponding symmetric complexes, allowing red to near-infrared emissions with large pseudo-Stokes shifts. The best performance achieves a maximum PL wavelength of 671 nm, a pseudo-Stokes shift of 5400 cm−1, and ΦPL of 0.62−0.72 in toluene (dielectric constant εs = 2.4), dichloromethane (εs = 9.1), acetone (εs = 21.4), and ethanol (εs = 24.3). The large pseudo-Stokes shift is distinctive considering BODIPYs with small Stokes shifts (∼500 cm−1), and the ΦPL values are higher than or comparable to those of BODIPYs fluorescing at similar wavelengths. Electrochemistry and density functional theory calculations illustrate that frontier orbital ordering in the dissymmetric complexes meets the condition for efficient PL proposed in our theory.

Photoluminescence and electroluminescence from Ge/strained GeSn/Ge quantum wells

Ge/strained GeSn/Ge quantum wells are grown on a 300 mm Si substrate by chemical vapor deposition. The direct bandgap emission from strained GeSn is observed in the photoluminescence spectra and is enhanced by Al2O3/SiO2 passivation due to the field effect. The electroluminescence of the direct bandgap emission of strained GeSn is also observed from the Ni/Al2O3/GeSn metal-insulator-semiconductor tunneling diodes. Electroluminescence is a good indicator of GeSn material quality, since defects in GeSn layers degrade the electroluminescence intensity significantly. At the accumulation bias, the holes in the Ni gate electrode tunnel to the strained n-type GeSn layer through the ultrathin Al2O3 and recombine radiatively with electrons. The emission wavelength of photoluminescence and electroluminescence can be tuned by the Sn content.

Control of morphology, photoluminescence, and stability of colloidal methylammonium lead bromide nanocrystals by oleylamine capping molecules

Methylammonium lead bromide (CH3NH3PbBr3) thin films and nanocrystals are useful for solar cells and LED applications. In order to improve stability in ambient environment, CH3NH3PbBr3 nanocrystals have been synthesized using oleylamine as capping molecule. It was found that by increasing the oleylamine to CH3NH3PbBr3 perovskite ratio (OPR), the photoluminescence wavelengths of CH3NH3PbBr3 nanocrystals could be varied from 505nm (green) to 450nm (blue). The change in emission wavelength is associated with a morphology change from nanoplatelets of ∼10nm width at OPR<1 to nanoparticles of ∼3nm diameter at OPR>1. It is suggested that the morphology change of nanocrystals is a result of geometric packing constraint of the sizes of oleylamine and PbBr3 octahedra. The nanocrystals with OPR=0.75 maintain photoluminescence property for more than 6months in ambient condition and can sustain temperature of 150°C for 30min.

Strain-Balanced InGaAsP/GaInP Multiple Quantum Well Solar Cells With a Tunable Bandgap (1.65–1.82 eV)

Currently available materials for III–V multijunction solar cells lattice matched to GaAs covering the spectral range from 1.65 to 1.82 eV are composed of either immiscible quaternary alloys or contain aluminum. We report the fabrication of a novel aluminum-free In $_x$ Ga $_{1-x}$ As $_{1-z}$ P $_z$ /Ga $_{1-y}$ In $_y$ P ( x > y ) strain-balanced multiple quantum-well (SBMQW) p-i-n solar cell structure lattice matched to GaAs, grown by metal–organic chemical vapor deposition. SBMQWs consist of alternating layers of In $_x$ Ga $_{1-x}$ As $_{1-z}$ P $_z$ wells and Ga $_{1-y}$ In $_y$ P barriers ( x > y ) under compressive and tensile strain, respectively. When compared with standard GaInP devices, SBMQW structures exhibit longer photoluminescence wavelength (680–780 nm) emission and enhanced light absorption with improved short-circuit current density. In this study, the SBMQW emission and absorption wavelength is controlled by adjusting the layer thickness of InGaAsP wells, while the arsenic and indium compositions are fixed. We show that carriers generated in QWs are extracted via thermionic emission. The proposed SBMQWs allow more flexibility in the design of current multijunction solar cells and future cells with more than four junctions. InGaAsP/GaInP SBMQWs may also be used in applications other than solar cells, such as light-emitting diodes (LEDs) and lasers, with the advantages of tuning the emission and absorption processes.

Emergence of new red-shifted carbon nanotube photoluminescence based on proximal doped-site design.

Single-walled carbon nanotubes (SWNTs) show unique photoluminescence (PL) in the near-infrared (NIR) region. Here we propose a concept based on the proximal modification in local covalent functionalization of SWNTs. Quantum mechanical simulations reveal that the SWNT band gap changes specifically based on the proximal doped-site design. Thus, we synthesize newly-designed bisdiazonium molecules and conduct local fucntionalisation of SWNTs. Consequently, new red-shifted PL (E112*) from the bisdiazonium-modified SWNTs with (6, 5) chirality is recognized around 1250 nm with over ~270 nm Stokes shift from the PL of the pristine SWNTs and the PL wavelengths are shifted depending on the methylene spacer lengths of the modifiers. The present study revealed that SWNT PL modulation is enable by close-proximity-local covalent modification, which is highly important for fundamental understanding of intrinsic SWNT PL properties as well as exciton engineering–based applications including photonic devices and (bio)imaging/sensing.

Measuring photoluminescence spectra of self-assembly array nanowire of colloidal CdSe quantum dots using scanning near-field optics microscopy

A novel periodic array CdSe nanowire is prepared on a substrate of the porous titanium dioxide by using a self-assembly method of the colloidal CdSe quantum dots (QDs). The experimental results show that the colloidal CdSe QDs have renewedly assembled on its space scale and direction in process of losing background solvent and form the periodic array nanowire. The main peak wavelength of Photoluminescence (PL) spectra, which is measured by using a 100-nm aperture laser beam spot on a scanning near-field optics microscopy, has shifted 60 nm with compared to the colloidal CdSe QDs. Furthermore, we have measured smaller ordered nanometer structure in thin QDs area as well, a 343-nm periodic nanowire in thick QDs area and the colloidal QDs in edge of well-ordered nanowire.

Dissymmetric Bis(dipyrrinato)zinc(II) Complexes: Rich Variety and Bright Red to Near-Infrared Luminescence with a Large Pseudo-Stokes Shift.

Bis(dipyrrinato)metal(II) and tris(dipyrrinato)metal(III) complexes have been regarded as much less useful luminophores than their boron difluoride counterparts (4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes, BODIPYs), especially in polar solvent. We proposed previously that dissymmetry in such metal complexes (i.e., two different dipyrrinato ligands in one molecule) improves their fluorescence quantum efficiencies. In this work, we demonstrate the universality and utility of our methodology by synthesizing eight new dissymmetric bis(dipyrrinato)zinc(II) complexes and comparing them with corresponding symmetric complexes. Single-crystal X-ray diffraction analysis, (1)H and (13)C NMR spectroscopy, and high-resolution mass spectrometry confirm the retention of dissymmetry in both solution and solid states. The dissymmetric complexes all show greater photoluminescence (PL) quantum yields (ϕPL) than the corresponding symmetric complexes, allowing red to near-infrared emissions with large pseudo-Stokes shifts. The best performance achieves a maximum PL wavelength of 671 nm, a pseudo-Stokes shift of 5400 cm(-1), and ϕPL of 0.62-0.72 in toluene (dielectric constant εs = 2.4), dichloromethane (εs = 9.1), acetone (εs = 21.4), and ethanol (εs = 24.3). The large pseudo-Stokes shift is distinctive considering BODIPYs with small Stokes shifts (∼500 cm(-1)), and the ϕPL values are higher than or comparable to those of BODIPYs fluorescing at similar wavelengths. Electrochemistry and density functional theory calculations illustrate that frontier orbital ordering in the dissymmetric complexes meets the condition for efficient PL proposed in our theory.

Halide effects on formation and physicochemical properties of mercury(II) complexes containing Y-type tridentate N-donor

Self-assembly of HgX2 (X− = Cl−, Br−, and I−) with Y-type 2,6-bis[(2-isonicotinoyloxy-5-methylphenyl)methyl]-1-isonicotinoyloxy-4-methylbenzene (L) yields 2D consisting of alternate prismatic P- and M-helical-linked-layers, 1D consisting of P- and M-helices, and simple 2D sheet in a unique Y-type mode, respectively. The L/Hg(II) ratio of each product (3/3 for Cl−; 2/3 for Br−; 1/3 for I−) is dependent on the nature of the halide anions. The coordinating environments around of Hg(II) ion approximate to a square pyramid for Cl−, a square planar and a distorted tetrahedral geometry for Br−, and distorted tetrahedral arrangement for I−, respectively. Photoluminescence wavelengths are strongly depending on the halide anions, and coordination ability to L is in the order of X− = Cl− > Br− > I−. Such physicochemical properties were explained by electronic and steric natures of halide anions.

Facile Synthesis of Cadmium-Free Zn-In-S:Ag/ZnS Nanocrystals for Bio-Imaging.

High quality cadmium-free Zn-In-S:Ag doped-nanocrystals (d-NCs) were synthesized via a simple one-step noninjection route using silver nitrate, indium acetate, zinc acetate, oleylamine, S powder and 1-dodecanethiol as starting materials in an organic phase. The size and optical properties can be effectively tailored by controlling the reaction time, reaction temperature, Ag+ dopant concentration, and the molar ratio of In to Zn. The photoluminescence wavelength of as-prepared Zn-In-S:Ag NCs covered a broad visible range from 458 nm to 603 nm. After being passivated by protective ZnS shell, the photoluminescence quantum yield (PLQY) of Zn-In-S:Ag+ /ZnS was greatly improved to 43.5%. More importantly, the initial high PLQY of the obtained core/shell d-NCs in organic media can be preserved when being transferred into the aqueous media via ligand exchange. Finally, high quality Zn-In-S:Ag+ /ZnS d-NCs in aqueous phase were applied as bio-imaging agents for identifying living KB cells.

Towards industrialisation of GaN‐on‐Si based high brightness blue LEDs

AbstractThe manufacturability of blue LED structures grown on 6‐inch Si (111) substrates is reported. The totally epi‐structure thickness is only 3.75 μm, which allows faster epitaxy process throughput and lower manufacturing costs. Well controlled strain engineering leads to a room temperature wafer bow of 0 ± 5 μm and a highly uniform photoluminescence wavelength standard deviation of 1.1 nm. XRD FWHM (002) and (102) are 380 and 390 arcsec, respectively. For a blue 1×1 mm2 vertical thin film die with silicone dome lens, the optical light output power of 563 mW at an operating voltage of 3.05 V is achieved at 350 mA. Excellent leakage current (IR) yield is achieved with over 95% of dies exhibiting IR &lt; 0.1 μA at ‐5 V. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of GaAs spacer layer thickness on optical properties of multi-stacked InAs/GaAs quantum dots

Effect of GaAs spacer layer thickness (dGaAs) on multi-stacked InAs/GaAs quantum dots is investigated by photoluminescence (PL) and excitation wavelength (λexc) dependent pump–probe reflection spectroscopy. Dominance of light hole exciton transition in the PL spectra is observed at smaller dGaAs (<15nm). Double maxima (ΔR/R)max1 and (ΔR/R)max2 appear in the differential reflection spectra (DRS) at intermediate λexc beyond which positive to negative reversal of the DRS is observed due to dominating effect of inter band absorption in InAs wetting layer. The λexc at which double maxima occur, and the positive to negative reversal starts is found to be dependent on dGaAs.

Anisotropic structural and optical properties of semi-polar (11-22) GaN grown on m-plane sapphire using double AlN buffer layers.

We report the anisotropic structural and optical properties of semi-polar (11–22) GaN grown on m-plane sapphire using a three-step growth method which consisted of a low temperature AlN buffer layer, followed by a high temperature AlN buffer layer and GaN growth. By introducing double AlN buffer layers, we substantially improve the crystal and optical qualities of semi-polar (11–22) GaN, and significantly reduce the density of stacking faults and dislocations. The high resolution x-ray diffraction measurement revealed that the in-plane anisotropic structural characteristics of GaN layer are azimuthal dependent. Transmission electron microscopy analysis showed that the majority of dislocations in the GaN epitaxial layer grown on m-sapphire are the mixed-type and the orientation of GaN layer was rotated 58.4° against the substrate. The room temperature photoluminescence (PL) spectra showed the PL intensity and wavelength have polarization dependence along parallel and perpendicular to the [1–100] axis (polarization degrees ~ 0.63). The realization of a high polarization semi-polar GaN would be useful to achieve III-nitride based lighting emission device for displays and backlighting.

Control of wavelength and decay time of photoluminescence for InAs quantum dots grown on InP(311)B using the digital embedding method

A lattice-matched InGaAs and InAlAs superlattice (SL) was used to embed InAs quantum dots (QDs) on an InP(311)B substrate; this is called the digital embedding method (DEM). We controlled the emission wavelength of the InAs QDs by changing the period of the SL or the ratio of the thickness of InGaAs to that of InAlAs. In addition, we investigated the time-resolved photoluminescence (PL) of the InAs QDs using the DEM. The decay time of the PL from the QDs was estimated by fitting a single exponential curve and was 380 ps in the DEM sample and 700 ps in the reference sample with a conventional InGaAlAs barrier. The decay time was clearly altered by changing the embedding structure. Therefore, carrier dynamics could be controlled using the DEM.

Photoassisted photoluminescence fine-tuning of gold nanodots through free radical-mediated ligand-assembly.

In this study, we have developed a simple photoassisted ligand assembly to fine-tune the photoluminescence (PL) of (11-mercaptoundecyl)-N,N,N-trimethylammonium bromide-capped gold nanodots (11-MUTAB-Au NDs). The 11-MUTAB-Au NDs (size: ca. 1.8 nm), obtained from the reaction of gold nanoparticles (ca. 3 nm) and 11-MUTAB, exhibited weak, near-infrared (NIR) PL at 700 nm with a quantum yield (QY) of 0.37% upon excitation at 365 nm. The PL QY of the Au NDs increased to 11.43% after reaction with 11-mercaptoundecanoic acid (11-MUA) for 30 min under ultraviolet (UV) light, which was accompanied by a PL wavelength shift to the green region (∼520 nm). UV-light irradiation accelerates 11-MUA assembly on the 11-MUTABAu NDs (11-MUA/11-MUTAB-Au NDs) through a radical-mediated reaction. Furthermore, the PL wavelength of the 11-MUA/11-MUTAB-Au NDs can be switched to 640 nm via cysteamine under UV-light irradiation. We propose that the PL of the Au NDs with NIR and visible emissions was originally from the surface thiol-Au complexes and the Au core, respectively. These dramatically different optical properties of the Au NDs were due to variation in the surface ligands, as well as the densities and surface oxidant states of the surface Au atoms/ions. These effects can be controlled by assembling surface thiol ligands and accelerated by UV irradiation.

DBR Laser with over 20nm Wavelength Tuning Range

We report a widely tunable distributed Bragg reflector (DBR) laser, in which InGaAsP with 1.4- $\mu \text{m}$ photoluminescence wavelength is butt-jointed as the DBR section material. A titanium (Ti) thin-film heater is integrated in the DBR section of the device. With the help of the tuning effect of the heater, an over 20-nm wavelength tuning range is obtained. The DBR laser is a promising light source for future wavelength division multiplexing passive optical networks.

Quantum dots protected from oxidative attack using alumina shells synthesized by atomic layer deposition.

Applications of luminescent quantum dots require the materials to be stable under a wide range of temperatures, photon fluxes and chemical environments. In this work, we demonstrate that Al2O3 shells synthesized by atomic layer deposition on films of CdTe quantum dots are effective to prevent chemical degradation for up to 17 hours under continuous illumination at 90 °C in ambient air. Control samples with no Al2O3 coating experienced extensive oxidation and severe quenching of the photoluminescence intensity under these conditions.

A Study of Structural and Photoluminescence for Al-Doped CdO Thin Films

Al-doped CdO thin films were prepared by radio frequency magnetron sputtering at different deposition time and substrate temperature. X-ray diffraction showed that the changes in the intensities of the (200), (220), and (311) planes followed a similar trend with increase in deposition time. The surface of the thin film was examined by scanning electron microscopy. Grain sizes of Al-doped CdO thin films increased significantly with increasing deposition time. The film thicknesses were 0.09, 0.12, 0.20, and 0.225 μm for the deposition times of 1, 2, 3, and 4 h, respectively. The photoluminescence spectra of the Al-doped CdO thin films were measured at room temperature. The photoluminescence wavelength changed in the sequence, green, blue, green, and blue, with increasing deposition time, which indicates that blue light emitting films can be fabricated by adjusting the processing parameters.

Direct observation of strain in InAs quantum dots and cap layer during molecular beam epitaxial growth using in situ X-ray diffraction

Direct measurements on the growth of InAs quantum dots (QDs) and various cap layers during molecular beam epitaxy are performed by in situ X-ray diffraction (XRD). The evolution of strain induced both in the QDs and cap layers during capping is discussed based on the XRD intensity transients obtained at various lattice constants. Transients with different features are observed from those obtained during InGaAs and GaAs capping. The difference observed is attributed to In-Ga intermixing between the QDs and the cap layer under limited supply of In. Photoluminescence (PL) wavelength can be tuned by controlling the intermixing, which affects both the strain induced in the QDs and the barrier heights. The PL wavelength also varies with the cap layer thickness. A large redshift occurs by reducing the cap thickness. The in situ XRD observation reveals that this is a result of reduced strain. We demonstrate how such information about strain can be applied for designing and preparing novel device structures.

Material research on the InGaAs-emitting-layer VECSEL grown on GaAs substrate

Wafers of InGaAs-emitting-layer vertical external cavity surface emitting semiconductor laser (VECSEL) gain chip and separate active region were grown on semi-insulator GaAs substrates by low pressure metal–organic vapor phase epitaxy (MOVPE). Photoluminescence (PL) wavelength of the active region could be adjusted linearly about 1nm for increasing 1sccm H2 flow rate through TMIn under AsH3 flow rates of 150sccm. The complicated surface-emitted PL signal of the VECSEL gain chip was strongly modulated by interferences within the multilayer and was interpreted by the aberrance of the quantum wells emission with a profile filtered by a micro-cavity resonance in the longitudinal confinement factor. Material tests of the VECSEL wafer showed the reflectivity of the DBR mirrors was in good agreement with the active region photoluminescence, and the wafer was obtained with high crystal quality.

Effect of the Barrier Thickness on the Optical Properties of InGaN/GaN/Al2O3 (0001) LED Heterostructures

The results of numerical and experimental study of the electric field strength, photoluminescence wavelength, and internal quantum efficiency of InGaN/GaN (0001) blue LED heterostructures consisting of InGaN multiple quantum wells and GaN barrier layers with the thicknesses of 3, 10, and 15 nm are presented. It is shown that a decrease in the thicknesses of the GaN barrier layers results in a blue shift of the wavelength of LED structures and in an increase of internal quantum efficiency of the structure at high excitation power density.