Shift Of Emission Energy Research Articles

Fabrication and characterization of VCSELs applying quantum well intermixing using spin‐on‐glass

AbstractThe applicability of the quantum well intermixing using a spin‐on‐glass (SOG) to the fabrication process of a vertical cavity surface emitting laser (VCSEL) was investigated to control the carrier confinement in the active layer. The intermixing by using the non‐doped SOG showed a large emission energy shift. The SOG coating of the mesa side‐wall of the VCSEL and following heat‐treatment are simple processes that can add to the conventional one. The fabricated VCSELs with a 30 μm‐square mesa showed lasing operation, and therefore the technique can be applicable as the VCSEL process for improving the performance of the small mesa devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum size effects in GaAs nanodisks fabricated using a combination of the bio-template technique and neutral beam etching

We successfully fabricated defect-free, distributed and sub-20-nm GaAs quantum dots (named GaAs nanodisks (NDs)) by using a novel top-down technique that combines a new bio-template (PEGylated ferritin) and defect-free neutral beam etching (NBE). Greater flexibility was achieved when engineering the quantum levels of ND structures resulted in greater flexibility than that for a conventional quantum dot structure because structures enabled independent control of thickness and diameter parameters. The ND height was controlled by adjusting the deposition thickness, while the ND diameter was controlled by adjusting the hydrogen-radical treatment conditions prior to NBE. Photoluminescence emission due to carrier recombination between the ground states of GaAs NDs was observed, which showed that the emission energy shift depended on the ND diameters. Quantum level engineering due to both diameter and thickness was verified from the good agreement between the PL emission energy and the calculated quantum confinement energy.

Tuning of the emission energy of fluorophores using solid state solvation for efficient luminescent solar concentrators

Solid state solvation is employed to reduce self-absorption, a key loss mechanism in Luminescent Solar Concentrator (LSC) devices. For an example materials system consisting of poly(methylmethacrylate), Camphoric Acid Anhydride, and the laser dye 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7,-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) a substantial shift in DCJTB emission energy of 832.2 cm−1 is observed whilst no decrease in fluorescence quantum yield occurs. When extrapolated to an LSC device a 16.1% relative improvement in efficiency is predicted with a potential for 23.1% when employing stronger dielectric additives. For a range of dicyanomethylene based fluorophores the predicted improvement in LSC efficiency is from 17.5% to 24.4%.

The effect of post‐growth thermal annealing on the emission spectra of GaAs/AlGaAs quantum dots grown by droplet epitaxy

AbstractWe fabricated GaAs/AlGaAs quantum dots by droplet epitaxy, and obtained the geometries of the dots by scanning transmission electron microscopy. Post‐growth thermal annealing is essential for the optical activation of quantum dots grown by droplet epitaxy. We measured the emission energy shifts of the dots and the underlying superlattice by post‐ growth thermal annealing, and specified the emission from dots by selectively etching the structure down to a low layer of quantum dots. We studied the influence of the degree of annealing on the optical properties of the dots from the peak shifts of the superlattice, since the superlattice has a uniform and well‐defined geometry. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Advantages of GaN based light-emitting diodes with a p-InGaN hole reservoir layer

A p-type InGaN hole reservoir layer (HRL) was designed and incorporated in GaN based light-emitting diodes (LEDs) to enhance hole injection efficiency and alleviate efficiency droop. The fabricated LEDs with p-type HRL exhibited higher light output power, smaller emission energy shift and broadening as compared to its counterpart. Based on electrical and optical characteristics analysis and numerical simulation, these improvements are mainly attributed to the alleviated band bending in the last couple of quantum well and electron blocking layer, and thus better hole injection efficiency. Meanwhile, the efficiency droop can be effectively mitigated when the p-InGaN HRL was used.

Optical studies of quantum dot‐like emission from localisation centres in InGaN/GaN nanorod array LEDs

AbstractMicrophotoluminescence with one‐photon and two‐photon excitation has been employed to investigate localisation centres found in nanorod array LEDs. Quantum dot‐like features with spectral width less than 1 meV were observed originating from low dimensional carrier confinement within the nanorods. Two‐photon excitation was used to probe the individual localisation centres. The narrow peak PL energy does not shift with increasing excitation density, but rather an increase of discrete new peaks on the high energy side of the spectrum is observed. There is no temperature dependent emission energy shift observed from 4.3 K to 25 K. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells

InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0 0) and on semipolar (1 0 2), (1 1 2), (1 0 1) as well as (2 0 1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0 1) < (1 1 2) = (0 0 0 1) < (2 0 1) < (1 0 0) = (1 0 2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0 0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0 0) and the semipolar (1 0 2) and (2 0 1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1 2) and (1 0 1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

Band-gap renormalization in InP/GaxIn1−xP quantum dots

A continuous and systematic red shift of the exciton emission energy in small InP/InGaP quantum dots is observed for increasing excitation energies and intensities. Superposed on this red shift, emission energy minima appear which show a one-to-one correspondence to emission intensity maxima as a function of the excitation energy. This band-gap renormalization is attributed to the hybridization of the quantum dot excited states with the wetting layer continuum. Polarization-resolved measurements reveal that the exciton fine structure splitting can be tuned up to $10%$ in the present case by changing the excitation energy. Possible implications of the present results on the development of entangled photon pair sources are discussed.

Spectrally and time‐resolved cathodoluminescence microscopy of semipolar InGaN SQW on (11$\overline {2} $2) and (10$\overline {1} $1) pyramid facets

AbstractSemipolar InGaN/GaN single quantum wells (SQWs) grown on {11$\overline {2} $2} planes of an inverted pyramid surface and {10$\overline {1} $1} facets of single self assembled pyramids have been studied by spatially, spectrally, and time‐resolved cathodoluminescence (CL) microscopy. Mappings of local spectra and local transients provide the distribution of spectral and time‐resolved luminescence properties by peak wavelength images, time delayed CL images (TDCLIs), and initial lifetime maps. The SQW on inverse pyramids exhibit strong local differences in recombination kinetics – two orders of magnitude change in initial lifetime – correlated with a giant shift in emission energy of ∼1 eV along a facet. For single pyramids a migration process of indium adatoms from the upper facet to the edges leads to an emission of longer wavelengths at the edges and shorter wavelengths at the upper facet with respect to the base.

High wavelength tunability of InGaN quantum wells grown on semipolar GaN pyramid facets

AbstractPyramidal GaN structures were deposited by selective metal‐organic vapor phase epitaxy (MOVPE) to create semipolar facets. On top of these pyramids InGaN was deposited forming nanostructures of different dimensionality according to the varying strain regions along the pyramid.We have studied the emission properties of pyramid ensembles using low‐temperature time‐resolved photoluminescence (PL) spectroscopy. The observed PL spectra in conjunction with the revealed decay times indicate already the expectations of regions with different confinement on the pyramids. Changing the size of the pyramids and the indium supply lead to variations in the emission wavelength of the ensemble.Spatially resolved cathodoluminescence (CL) experiments were performed to analyze locally the emission properties all over the pyramidal structures. We have found that indeed the indium accumulates at the edges and the apex of the pyramids. Furthermore the emission of the quantum well (QW) sidewalls exhibit also a shift in emission energy which can be attributed again to a change of the In‐content or a change of the QW thickness.

Spectral and Dynamic Properties of Excitons and Biexcitons in Type-II Semiconductor Nanocrystals

Using time-resolved photoluminescence spectroscopy, we analyze single and multiexciton emission energies and decay dynamics of CdS/ZnSe core/shell nanocrystals (NCs). The NCs exhibit a characteristic type-II band alignment that leads to spatially separated charges; this effect is at the origin of long radiative exciton lifetimes, repulsive biexciton interaction energies, and reduced Auger recombination efficiencies. We determine these properties as a function of ZnSe shell thickness and find that the exciton emission energies and the decay times depend little on this parameter. In contrast, the spectral and dynamic properties of biexcitons vary strongly with the shell thickness. The considerable shell dependence of the biexciton decay lets us conclude that the associated Auger process involves the excitation of holes localized in the ZnSe shell. In NCs with thick shells, the large blue shift of the biexciton emission energy is mainly caused by Coulomb repulsion between electrons localized in the CdS core. The different sensitivity of exciton and multiexciton characteristics on the ZnSe shell thickness provides a unique opportunity to tune them independently.

Indium Nitride Nanometric-Objects on c-Sapphire Grown by Plasma-Assisted Molecular Beam Epitaxy

The indium nitride (InN)-based nanometric-objects were grown directly on a c-sapphire substrate by using plasma-assisted molecular beam epitaxy (PAMBE) at different substrate temperatures. High resolution X-ray diffraction (HRXRD) reveals the InN (0002) reflection and full width at half maximum (FWHM) found to be decreased with increasing the growth temperature. The size, height and density of the grown nanometric-objects studied by scanning electron microscopy (SEM) has remarkable differences, evidencing the decisive role of substrate temperature. Photoluminescence (PL) studies revealed that the emission energy is shifted towards the higher side from the bulk value, i.e., a blue shift in the PL spectra was observed. The temperature dependence of the PL peak position shows an ``S-shaped'' emission energy shift, which can be attributed to the localization of carriers in the nanometric-objects.

Low-Temperature Growth and Characterization of Single Crystalline ZnO Nanorod Arrays Using a Catalyst-Free Inductively Coupled Plasma-Metal Organic Chemical Vapor Deposition

Vertically aligned ZnO nanorod arrays have been synthesized on c-plane sapphires at a low temperature of 400 degrees C using catalyst-free inductively coupled plasma (ICP) metal organic chemical vapor deposition (MOCVD) technique by varying the ICP powers. Diameters of the ZnO nanorods changed from 200 nm to 400 nm as the ICP power increased from 200 to 400 Watt. TEM and XRD investigations indicated that the ZnO nanorod arrays grown at ICP powers above 200 Watt had a homogeneous in-plane alignment and single crystalline nature. PL study at room temperature (RT) and 6 K confirmed that the ZnO nanorod arrays in the present study are of high optical quality as well as good crystalline quality, showing only exciton-related emission peaks without any trace of defect-related deep level emissions in visible range. The blueshift of exciton emission peak in RTPL spectra was also found as rod diameter decreased and it is deduced that this shift in emission energy may be due to the surface resonance effect resulted from the increased surface-to-volume ratio, based on the observation and behavior of the surface exciton (SX) emission in the high-resolution 6 K PL spectra.

Radiative recombination dynamics in tetrapod-shaped CdTe nanocrystals: Evidence for a photoinduced screening of the internal electric field

We study the radiative recombination processes in CdTe tetrapod nanocrystals at 10K. Two intrinsic emission bands, namely the ground state (GS) and the excited state (EX), decay with three time constants, due to a power dependent Auger-like recombination process (tens of picoseconds), to the intrinsic emission of the two states (hundreds of picoseconds) and to emission from defect states (a few nanoseconds). The existence of an internal electric field originating from the e-h separation induced by the peculiar symmetry of the GS is demonstrated by a dynamical shift of the GS emission energy that is correlated to the EX population.

The 2-D double layer structures of indium–benzenemulticarboxylate coordination polymers with green fluorescence

Two coordination polymers, [In(btc)(2,2′-bpy)(H 2O)] n · nH 2O ( 1) and [In 2(btec)(2,2′-bpy) 2Cl 2] n ( 2) (H 3btc = 1,3,5-benzenetricarboxylic acid, H 4btec = 1,2,4,5-benzenetetracarboxylic acid, 2,2′-bpy = 2,2′-bipyridine), have been prepared under hydrothermal conditions. Single-crystal X-ray diffraction analyses reveal that compounds 1 and 2 consist of two-dimensional double layer structures with (3, 3) grids and (4, 2) grids, respectively. The grids along two different directions in 1 are of the opposite chirality, while the interactions of double-layer sheets in 2 create a three-dimensional supramolecular network with one-dimensional tunnels. The additional green luminescences for both 1 and 2 in the solid state at low temperature imply the red shift of emission energy in the compounds, which may derive from LMCT. Their X-ray powder diffractions and thermogravimetric analyses are also discussed.

Theoretical studies on square-planar pincer platinum(II) complex and its SO 2 adducts: SO 2 detector and potential luminescent probe

We explored the electronic structures and spectroscopic properties of [Pt(Me 4NCN)]Cl ( 1) (NCN − = [2,6-(CH 2NMe 2) 2-C 6H 3] −) and its SO 2 adducts, [Pt(Me 4NCN)(SO 2)]Cl, which show three penta-coordinated modes ( trans- η 1-pyramidal ( 2), cis- η 1-pyramidal ( 3) and η 1-coplanar ( 4)), using the theoretical methods. For 2, a comparative study of ab initio and density functional theory (DFT) shows that the DFT method cannot describe molecular structures appropriately, especially the Pt–S bonding. Therefore, the structures of 1– 4 in the ground state were optimized by the ab initio MP2 method and those of the first two complexes in the lowest-lying triplet excited state were studied by the unrestricted MP2 method. The present studies reveal that trans- η 1-pyramidal SO 2 coordination mode is the most stable in the three conformations of [Pt(Me 4NCN)(SO 2)]Cl because of its lower total energy as well as the most overlapping between the HOMO and LUMO of SO 2 and the filled Pt d orbital, and least repulsion between the lone pair orbitals of SO 2 and the Lewis base (PtNCN). The spectroscopic calculations at the TD-DFT associated with the Polarizable Continuum Model (PCM) level indicate that the adsorption of SO 2 changes the nature of low-lying absorptions of 1, agreeing well with the color change observed in experiments. We predicted the phosphorescent emissions in the CH 2Cl 2 solution at 778 and 552 nm for 1 and 2, respectively. The coordination of SO 2 into Pt atom results in a significant blue shift in emission energy.

Synthesis and characterization of semiconductor polymers having different phenylene–vinylene conjugation lengths

We have synthesized phenylene–vinylene (PV) polymers containing segments with different conjugation lengths interspaced by random distributed aliphatic segments. Infrared (IR) and ultraviolet–visible (UV–vis) spectroscopies, hydrogen nuclear magnetic resonance (1H NMR) spectrometry and differential scanning calorimetry (DSC) were used to characterize the prepared copolymers’ structures. Polymers molecular weights were determined by gel permeation chromatography (GPC). The effect of polymer structure and composition on emission properties was studied by fluorescence (PL) spectroscopy under different irradiation wavelength. The emission energy shift due to segments with longer conjugation lengths was minor owed to the low polymerization degree achieved.

Location of quantum dots identified by microscopic photoluminescence changes during nanoprobe indentation with a horizontal scan

The location of quantum dots (QDs) embedded in a GaAs matrix is successfully identified by microscopic photoluminescence (PL) measurement during nanoprobe indentation with a horizontal scan at low temperature $(10\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. By introducing a high-sensitive load cell and a focused ion beam-fabricated flat-apex nanoprobe, the indentation force and PL of QDs are measured simultaneously at each point in the direction of the nanoprobe horizontal scanning with a constant indentation force. The experimental results show that the emission energy from QDs has a strong dependence on the location of QDs relative to the nanoprobe-indented location. Consequently, the emission energy shifts with the nanoprobe scan, generating an emission trace with a constant indentation force. Based on the energy shift of the ground state of the QD simulated by the combination of three-dimensional finite element strain calculation and strain-dependent $\mathbf{k}\mathbf{∙}\mathbf{p}$ Hamiltonian, an estimation method is developed, which enables us to identify the location of the QD from its emission trace in the nanoprobe indentation experiment. Results indicate that all the emission traces clearly observed are emitted from the QDs around the edge of the nanoprobe-indented area. Further discussion reveals that the evolution of shear strain generates hole accumulation around the contact edge of the indented nanoprobe. Consequently, the high density of holes enhances the PL of the QDs in that region.

Electronic Structures and Spectroscopic Properties of [Pt(CNMe)2(CN)2]n (n = 1–4): A Theoretical Exploration of Promising Phosphorescent Materials

AbstractThe structures of [Pt(CNMe)2(CN)2]n (n = 1–4) in the ground states (S0) and lowest‐energy triplet excited states (T1) were calculated by using the second‐order Møller–Plesset perturbation (MP2) and density functional theory (DFT) methods, respectively. The MP2 results show that the formation of the dimer causes a significant red shift in emission energy, and the frequency calculations reveal that a weak metal–metal interaction exists in the S0 state, which is greatly enhanced in the 3[dσ*pσ] excited state. The aggregation of [Pt(CNMe)2(CN)2]n (n = 1–4) was explored by using the slate‐type VWN functional in the DFT method. The 3Bu→1Ag transition in the dimer at 509 nm corresponds to the experimental higher‐energy emission at 530 nm in CH3CN solution, while the 3A′→1A′ transitions in the trimer and tetramer at 557 and 650 nm, respectively, are responsible for the low‐energy emission at 584 nm observed experimentally. The analyses of the Wiberg bond indices for the Pt–Pt bond indicate that the dimer may be the most stable form in solution and that the oligomer species (n = 3 and 4) can be treated as a special dimer in which the excess z electron ligand is bonded to the Pt atoms of the central dimer.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)

Excited State Intramolecular Proton Transfer in 2-(2‘-Arylsulfonamidophenyl)benzimidazole Derivatives: Insights into the Origin of Donor Substituent-Induced Emission Energy Shifts

Donor-substituted 2-(2'-arylsulfonamidophenyl)benzimidazoles undergo efficient excited-state intramolecular proton transfer (ESIPT) upon photoexcitation. The tautomer emission energy depends strongly on the substituent attachment position on the fluorophore pi-system. While substitution with a donor group in the para-position relative to the sulfonamide moiety yields an emission energy that is red-shifted relative to the unsubstituted fluorophore, fluorescence of the meta-substituted derivative appears blue-shifted. To elucidate the origin of the surprisingly divergent emission shifts, we performed detailed photophysical and quantum chemical studies with a series of methoxy- and pyrrole-substituted derivatives. The nature and contribution of solvent-solute interactions on the emission properties were analyzed on the basis of solvatochromic shift data using Onsager's reaction field model, Reichardt's empirical solvent polarity scale ET(30), as well as Kamlet-Abboud-Taft's empirical solvent index. The studies revealed that all ESIPT tautomers emit from a moderately polarized excited-state whose dipole moment is not strongly influenced by the donor-attachment position. Furthermore, the negative solvatochromic shift behavior was most pronounced in protic solvents presumably due to specific hydrogen-bonding interactions. The extrapolated gas-phase emission energies correlated qualitatively well with the trends in Stokes shifts, suggesting that solute-solvent interactions do not play a significant role in explaining the divergent emission energy shifts. Detailed quantum chemical calculations not only confirmed the moderately polarized nature of the ESIPT tautomers but also provided a rational for the observed emission shifts based on the differential change in the HOMO and LUMO energies. The results gained from this study should provide guidelines for tuning the emission properties of this class of ESIPT fluorophores with potential applications in analytical chemistry, biochemistry, or materials science.