Showing papers on "Exciton published in 2003"

Optical Cavity Effects in ZnO Nanowire Lasers and Waveguides

Abstract: Wide band gap semiconductor nanostructures with near-cylindrical geometry and large dielectric constants exhibit two-dimensional ultraviolet and visible photonic confinement (i.e., waveguiding). Combined with optical gain and suitable resonant feedback, the waveguiding behavior facilitates highly directional lasing at room temperature in controlled-growth nanowires. We have characterized the nanowire emission in detail with high-resolution optical microscopy. The waveguiding behavior of individual zinc oxide (ZnO) nanowires depends on the wavelength of the emitted light and the directional coupling of the photoluminescence (PL) to the emission dipoles of the nanowire. Polarization studies reveal two distinct regimes of PL characterized by coupling to either guided (bound) or radiation modes of the waveguide, the extent of which depends on wire dimensions. Pumping with high pulse energy engenders the transition from spontaneous to stimulated emission, and analysis of the polarization, line width, and line spacing of the laser radiation facilitates identification of the transverse and longitudinal cavity modes and their gain properties. Interpretation of the lasing spectra as a function of pump fluence, with consideration of ZnO material properties and ultrafast excitation dynamics, demonstrates a transition from exciton (fluence 1 IJ/cm 2 ) and gain saturation behavior (fluence > 3 IJ/cm 2 ) modified by the constraints of the nanoscale cylindrical cavity.

Properties of arsenic-doped p-type ZnO grown by hybrid beam deposition

Abstract: As-doped ZnO (ZnO:As) films have been characterized. ZnO:As films show p-type characteristics determined by Hall-effect and photoluminescence (PL) measurements. The hole concentration can be increased up to the mid-1017-cm−3 range. The thermal binding energy of the As acceptor (EAth-b) is 120±10 meV, as derived from temperature-dependent Hall-effect measurements. The PL spectra reveal two different acceptor levels (EAopt-b), located at 115 and 164 meV, respectively, above the maximum of the ZnO valence band, and also show the binding energy of the exciton to the As-acceptor (EAXb) is about 12 meV. The values of the ratio EAXb/(EAth-b or EAopt-b) are located in the range from 0.07 to 0.11.

Green emission to probe photoinduced charging events in ZnO-Au nanoparticles. Charge distribution and fermi-level equilibration

Abstract: Photoinduced electron accumulation in ZnO nanoparticles results in the bleaching of the exciton band as well as quenching of green emission. In the absence of an electron scavenger, photogenerated electrons are stored near the conduction band edge and promote charge recombination via a nonradiative process. By exposing the UV-irradiated ZnO suspension to an electron acceptor (O2 or thionine dye) the stored electrons are discharged and the original excitonic band and the visible emission are restored. Titration of electrons stored in ZnO nanoparticles with an electron acceptor, thionine dye, shows a linear relationship between stored electrons and the emission quenching. When gold nanoparticles are added to pre-UV-irradiated ZnO colloids, only partial recovery of the emission is seen. Pt nanoparticles on the other hand caused almost complete recovery of the quenched emission as the electrons are discharged into the solution. The charge distribution between UV-irradiated ZnO and gold nanoparticles results i...

Highly Luminescent Water-Soluble CdTe Quantum Dots

Abstract: Colloidal CdTe quantum dots prepared in TOP/DDA (trioctylphosphine/dodecylamine) are transferred into water by the use of amino−ethanethiol•HCl (AET) or mercaptopropionic acid (MPA). This results in an increase in the photoluminescence quantum efficiency and a longer exciton lifetime. For the first time, water-soluble semiconductor nanocrystals presenting simultaneously high band-edge photoluminescence quantum efficiencies (as high as 60% at room temperature), monoexponential exciton decays, and no observable defect-related emission are obtained.

Excitonic emissions observed in ZnO single crystal nanorods

Abstract: We report on the photoluminescent characteristics of ZnO single crystal nanorods grown by catalyst-free metalorganic vapor phase epitaxy. From photoluminescence (PL) spectra of the nanorods at 10 K, several PL peaks were observed at 3.376, 3.364, 3.360, and 3.359 eV. The PL peak at 3.376 eV is attributed to a free exciton peak while the other peaks are ascribed to neutral donor bound exciton peaks. The observation of the free exciton peak at 10 K indicates that ZnO nanorods prepared by the catalyst-free method are of high optical quality.

Exciton binding energies in organic semiconductors

Abstract: The exciton binding energy is one of the key parameters that govern the physics of many opto-electronic organic devices. It is shown that the previously reported values for the exciton binding energies in many organic semiconductors, which differ by more than an order of magnitude, can be consistently rationalized within the framework of the charging energy of the molecular units, with a simple dependence of the exciton binding energy on the length of these units. The implications of this result are discussed.

Temperature dependence of the free-exciton transition energy in zinc oxide by photoluminescence excitation spectroscopy

Abstract: Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies are used to track the temperature dependence of the A exciton energy (EXA) in undoped bulk ZnO crystals grown by the seeded-chemical-vapor-transport method. For T>150 K, the edge emission becomes broad as the A exciton recombination and its longitudinal-optical (LO) phonon replica become superimposed. We use PLE to determine the temperature dependence of EXA by monitoring the broad green emission commonly observed in as-grown ZnO crystals, and thus have established the energy difference between the EXA and PL emission peak energies. The PL emission at 3.26 eV at room temperature is shown to be offset by about 50 meV to lower energy than the actual EXA transition. The temperature dependence of the energy difference between the EXA and PL peaks is compared with predictions based on the lineshape function for the EXA– LO recombination. At 300 K, the PL is predominantly composed of EXA– LO recombination. Further, the temperature depe...

Ultrafast terahertz probes of transient conducting and insulating phases in an electron–hole gas

Abstract: Many-body systems in nature exhibit complexity and self-organization arising from seemingly simple laws For example, the long-range Coulomb interaction between electrical charges has a simple form, yet is responsible for a plethora of bound states in matter, ranging from the hydrogen atom to complex biochemical structures Semiconductors form an ideal laboratory for studying many-body interactions of electronic quasiparticles among themselves and with lattice vibrations and light1,2,3,4 Oppositely charged electron and hole quasiparticles can coexist in an ionized but correlated plasma, or form bound hydrogen-like pairs called excitons5,6 The pathways between such states, however, remain elusive in near-visible optical experiments that detect a subset of excitons with vanishing centre-of-mass momenta In contrast, transitions between internal exciton levels, which occur in the far-infrared at terahertz (1012 s-1) frequencies7,8,9, are independent of this restriction, suggesting10 their use as a probe of electron–hole pair dynamics Here we employ an ultrafast terahertz probe to investigate directly the dynamical interplay of optically-generated excitons and unbound electron–hole pairs in GaAs quantum wells Our observations reveal an unexpected quasi-instantaneous excitonic enhancement, the formation of insulating excitons on a 100-ps timescale, and the conditions under which excitonic populations prevail

Multiple temperature regimes of radiative decay in CdSe nanocrystal quantum dots: Intrinsic limits to the dark-exciton lifetime

Abstract: We investigate the strongly temperature-dependent radiative lifetime of electron–hole excitations in colloidal CdSe nanocrystal quantum dots over nearly three orders of magnitude in temperature (300 K to 380 mK). These studies reveal an intrinsic, radiative upper limit of ∼1 μs for the storage of excitons below 2 K. At higher temperatures, exciton lifetimes are consistent with thermal activation from the dark-exciton ground state, but with two different activation thresholds.

Photoenhancement of Luminescence in Colloidal CdSe Quantum Dot Solutions

Abstract: Enhancement of the photoluminescence (PL) of colloidal CdSe and (core)shell (CdSe)ZnS quantum dots has been observed when the dots are illuminated above the band-gap energy. The effect occurs in dots suspended in a variety of organic or aqueous environments. During periods of constant illumination, the exciton PL quantum yield was found to reach a value of up to 60 times that of the solution of as-prepared quantum dots and, if illumination continued, subsequently declined slowly because of photooxidation. When returned to the dark, the PL reverted to near its original value. The rate and magnitude of photoenhancement are found to depend on the illumination wavelength, the presence of a ZnS shell, the solvent environment, and the concentration of surfactant molecules. Time-resolved measurements of the fluorescence decay reveal multiexponential kinetics and an average lifetime that lengthens during the illumination period and shortens when quantum dots are returned to darkness. It is postulated that the stabilization of surface trap states, lengthening their average lifetime, could occur by a light-activated rearrangement of surfactant molecules, thus increasing the probability of thermalization back to the lowest emitting exciton state and enhancing the quantum dot PL.

Optical properties of ZnO rods formed by metalorganic chemical vapor deposition

Abstract: High-quality ZnO rods were formed directly on sapphire (0001) substrates by metalorganic chemical vapor deposition. The rods exhibited free exciton and very sharp bound exciton emissions at low temperatures. By increasing the excitation intensity, biexciton emission was observed. Temperature dependence of the emission spectra suggested that the emission peak at ∼3.315 eV, which had been attributed to neutral acceptor-bound exciton emission, is due to donor-acceptor pairs. The acceptor binding energy was determined to be about 107 meV, which agrees well with that estimated from a hydrogen-atom-like acceptor model.

Band structure and fundamental optical transitions in wurtzite AlN

Abstract: With a recently developed unique deep ultraviolet picoseconds time-resolved photoluminescence (PL) spectroscopy system and improved growth technique, we are able to determine the detailed band structure near the Γ point of wurtzite (WZ) AlN with a direct band gap of 6.12 eV. Combined with first-principles band structure calculations we show that the fundamental optical properties of AlN differ drastically from that of GaN and other WZ semiconductors. The discrepancy in energy band gap values of AlN obtained previously by different methods is explained in terms of the optical selection rules in AlN and is confirmed by measurement of the polarization dependence of the excitonic PL spectra.

Optical properties of single-crystalline ZnO nanowires on m-sapphire

Abstract: ZnO nanowires have been synthesized using a catalyst-assisted heteroepitaxial carbothermal reduction approach on a m-sapphire substrate. Intricate and uniform arrays have been obtained with each nanowire forming an angle ∼30° with the substrate normal. Photoluminescence studies at room temperature for wavelengths between 335 and 620 nm reveal a strong single exciton peak at ∼380 nm (3.26 eV) with accompanying deep-level blueshifted emission peaks at ∼486, 490, and 510 nm. UV resonant Raman spectroscopy has been used to characterize the nanowires at room temperature with multiphonon scattering exhibiting phonon quantum confinement.

Theory of optical processes in semiconductors : bulk and microstructures

Abstract: 1 INTRODUCTION 2 CLASSICAL THEORY OF OPTICAL PROCESSES 3 PHOTONS 4 ELECTRON BAND STRUCTURE AND ITS MODIFICATIONS 5 INTERBAND AND IMPURITY ABSORPTIONS 6 EXCITONIC ABSORPTION 7 ABSORPTION AND REFRACTION IN AN ELECTRIC FIELD 8 INTERBAND MAGNETO-OPTICAL EFFECTS 9 FREE CARRIER PROCESSES 10 RECOMBINATION PROCESSES 11 INTRODUCTION TO TWO-DIMENSIONAL SYSTEMS 12 OPTICAL PROCESSES IN QUANTUM WELLS 13 EXCITONS AND IMPURITIES IN QUANTUM WELLS 14 OPTICAL PROCESSES IN QUANTUM WIRES AND DOTS 15 SUPERLATTICES 16 STRAINED LAYERS 17 EFFECTS OF ELECTRIC FIELD ON LOW DIMENSIONAL SYSTEMS

Why is exciton dissociation so efficient at the interface between a conjugated polymer and an electron acceptor

Abstract: Although doping of a conjugated polymer by electron acceptors strongly facilitates exciton dissociation into geminate pairs of carriers, the yield of free carrier photogeneration can be high only at high doping levels, that is, in polymer/acceptor blends. We suggest a model that explains how excitons can efficiently dissociate into free carriers at an intrinsic polymer/acceptor interface despite the Coulomb interaction between the charges within precursor geminate pairs.

Temperature dependent exciton photoluminescence of bulk ZnO

Abstract: Temperature dependent (4.2–300 K) photoluminescence (PL) of bulk (0001)-oriented ZnO in the range of free- and bound-exciton emission is presented. Emission from several bound excitons and the free A exciton were observed from the low temperature (20 K) PL spectrum. The temperature dependence of the free-exciton peak position was fit using the Manoogian-Woolley equation and the coefficients obtained show reasonable agreement both with first-principle theoretical calculations and empirical values of the coefficients for other II–VI semiconductors. The strongest bound-exciton line with a width (full width at half maximum) of about 1 meV exhibited a thermal activation energy of approximately 14 meV, consistent with the exciton-defect binding energy. It was not observed at temperatures above 150 K. Additional analysis of this particular bound-exciton peak suggests it dissociates into a free exciton and a neutral-donor-like defect-pair complex with increasing temperature.

Exciton migration in β -phase poly(9,9-dioctylfluorene)

Abstract: We have studied the dynamics of optically generated excitations in spin-coated glassy films of poly(9,9-dioctylfluorene) (PFO) and in $\ensuremath{\beta}$-phase PFO films using picosecond time resolved photoluminescence (PL) spectroscopy, performed both at room temperature (RT) and at 5 K. We also present measurements of the PL emission of PFO and $\ensuremath{\beta}$-phase PFO at RT and 5 K following continuous wave (cw) excitation. We show that the cw emission from $\ensuremath{\beta}$-phase PFO at 5 K is very highly resolved, permitting us to make an assignment of the different vibrational modes of the molecule that couple to the ${S}_{1}\ensuremath{\rightarrow}{S}_{0}$ transition. Via time-dependent spectroscopy measurements performed at 5 K, we are able to follow exciton diffusion and relaxation through an energetically broadened density of states to polymer chains having a longer conjugation length and lower energy gap. By comparing the relative emission intensity of the different vibronic transitions as a function of time, we are able to directly demonstrate that the lower energy emissive states are associated with longer conjugation length polymeric chains that have enhanced rigidity. At room temperature, we find that these relaxation processes occur faster than the resolution of our detector due to thermally assisted energy migration.

High efficiency transparent organic light emitting devices

Abstract: A highly transparent non-metallic cathode is disclosed that comprises a metal-doped organic electron injection layer that is in direct contact with a transparent non-metallic electron injecting cathode layer, such as indium tin oxide (ITO), wherein the metal-doped organic electron injection layer also functions as an exciton blocking or hole blocking layer. The metal-doped organic electron injection layer is created by diffusing an ultra-thin layer of about 5-10 Å of a highly electropositive metal such as Li throughout the layer. A representative embodiment of the highly transparent non-metallic cathode comprises a layer of ITO, a layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), which acts as an electron injection, exciton blocking, and hole blocking layer, and an ultra-thin layer of lithium, which degenerately dopes the layer of BCP, improving the electron injecting properties of the BCP layer. This cathode is demonstrated for use in an OLED having a transparency of about 90% or higher combined with a device external quantum efficiency of about 1% or higher.

Substituent-control exciton in J-aggregates of protonated water-insoluble porphyrins.

Abstract: A series of protonated porphyrin J-aggregates of various water-insoluble tetraphenylporphyrin derivatives was prepared by aggregation at the liquid-liquid or gas-liquid interface. Using atomic force microscopy, we observed microcrystalline porphyrin J-aggregates. The J-aggregates have two strong exciton bands corresponding to the B (Soret)- and Q-bands of the protonated porphyrin. Interestingly, the excitation energy of the lower exciton (denoted by S1) markedly depends on the meso-substituents, whereas that of the higher exciton (denoted by S2) does not depend on them. These results indicate that the nature of the exciton coupling of the S1 transition dipole moment can be systematically changed by the substituents.

Size Effects in ZnO: The Cluster to Quantum Dot Transition

Abstract: The use of tetraalkylammonium hydroxides to prepare ZnO colloids with diameters ranging from 1 to 6 nm is described. The position of the first excitonic transition has been measured by UV-vis spectrometry and correlated with the particle size, which has been measured using high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and ultracentrifugation (UC). The exciton transition is first visible at 265–270 nm corresponding to particle diameters around 1 nm; the exciton absorption band then becomes sharper and narrower, while the band red-shifts only slowly. Based on the sizing data from HRTEM, XRD, and UC, it is concluded that the quantum size effect at sizes less than the Bohr radius is significantly less than predicted from the Kayanuma equation. Based on the blue-shift in the trap emission as a function of nanocrystal size, the effective masses of the electron and hole (me, mh) remain constant in particles down to 1 nm in diameter, with a relative value given by me/(me+mh)=0.55 ± 0.04.

Electromagnetically Induced Transparency in Semiconductors via Biexciton Coherence

Abstract: We report an experimental demonstration and theoretical analysis of electromagnetically induced transparency in a GaAs quantum well, in which the absorption of an exciton resonance is reduced by more than twentyfold. The destructive quantum interference in this scheme is set up by a control pulse that couples to a resonance of biexcitons. These studies illustrate that many-particle interactions, which are inherent in semiconductors and are often detrimental to quantum coherences, can also be harnessed to manipulate these coherences.

Ratio problem in single carbon nanotube fluorescence spectroscopy

Abstract: The electronic band gaps measured in fluorescence spectroscopy on individual single wall carbon nanotubes isolated within micelles show significant deviations from the predictions of one electron band theory. We resolve this problem by developing a theory of the electron-hole interaction in the photoexcited states. The one-dimensional character and tubular structure introduce a novel relaxation pathway for carriers photoexcited above the fundamental band edge. Analytic expression for the energies and line shapes of higher subband excitons are derived, and a comparison with experiment is used to extract the value of the screened electron-hole interaction.

Variational approach to excitons in carbon nanotubes

Abstract: Excitons in quasi-one-dimensional semiconductors may lower the optical transition energies by a substantial amount. To quantify the effect in carbon nanotubes, we apply a simple variational approach. In excitonic units, apower law with exponent ∼-0.6 is obtained for the binding energy dependence on nanotube radius. When converted to ordinary units, the ratio of binding energy to tight-binding band gap yields a roughly constant value of nearly 40%. This substantial ratio implies that exciton effects are of prime importance for the optical properties of carbon nanotubes.

Intensity dependence and transient dynamics of donor–acceptor pair recombination in ZnO thin films grown on (001) silicon

Abstract: We report room-temperature time-integrated and time-resolved photoluminescence (PL) measurements on a nominally undoped wurtzite ZnO thin film grown on (001) silicon. A linear and sublinear excitation intensity Iex dependence of the PL intensity were observed for the 379.48-nm exciton line and the weak broad green band (∼510 nm), respectively. The green luminescence was found to decay as hyperbolic t−1, and its peak energy was observed to increase nearly logarithmically with increased Iex. These results are in an excellent agreement with the tunnel-assisted donor–deep-acceptor pair (DAP) model so that its large blueshifts of about 25 meV per decade increase in Iex can be accounted for by the screening of the fluctuating impurity potential. Also, the 30-ps fast decay of the exciton emission was attributed to the rapid trapping of carriers at luminescent impurities, while the short lifetime of τ1/e=200 ps for the green luminescence may be due to an alternative trapping by deeper centers in the ZnO. Finally, singly ionized oxygen and zinc vacancies have been tentatively invoked to act as donor–deep-acceptor candidates for the DAP luminescence, respectively.

Trion, biexciton, and exciton dynamics in single self-assembled CdSe quantum dots

Abstract: We present an analysis of time- and polarization-resolved data taken in microphotoluminescence experiments on individual CdSe/ZnSe quantum dots grown by molecular beam epitaxy. The identification of individual dots was performed by a spectral jitter correlation technique and by their polarization properties and density dependences. Decay times are given for exciton, trion, and biexciton states and evidence is shown for a spin-relaxation-limited energy relaxation of the trion. For the bright-exciton state the temperature dependence of the decay time is studied and a repopulation from the dark-exciton state is observed. Trion binding energies of 15–22 meV and biexciton binding energies of 19–26 meV are found.

Dielectric functions (1 to 5 eV) of wurtzite MgXZn1 -XO (x≤0.29) thin films

Abstract: The optical dielectric functions for polarization perpendicular and parallel to the c-axis (optical axis) of pulsed-laser-deposition grown wurtzite MgxZn1−xO (0⩽x⩽0.29) thin films have been determined at room temperature using ellipsometry for photon energies from 1 to 5 eV. The dielectric functions reveal strong excitonic contributions for all Mg concentrations x. The band gap energies (E0A=3.369 eV for ZnO to 4.101 eV for x=0.29) show a remarkable blueshift. The exciton binding energy (61 meV for ZnO) decreases to approximately 50 meV for x≈0.17 and increases to approximately 58 meV for x=0.29. In contrast to ZnO, the MgxZn1−xO alloys are found uniaxial negative below the band gap energy, opposite to previously reported results.

Degenerate p-type conductivity in wide-gap LaCuOS1−xSex (x=0–1) epitaxial films

Abstract: Epitaxial films of LaCuOS1−xSex (x=0–1) solid solution were grown on MgO (001) substrates and their electrical and optical properties were examined. Sharp emission due to room-temperature exciton with binding energy of ∼50 meV is observed for all x values. Hall mobility becomes large with an increase in the Se content and it reaches 8.0 cm2V−1s−1 in LaCuOSe, a comparable value to that of p-type GaN:Mg. Doping of Mg2+ ions at La3+ sites enhances a hole concentration up to 2.2×1020 cm−3, while maintaining the Hall mobility as large as 4.0 cm2V−1s−1. Consequently, a degenerate p-type electrical conduction with a conductivity of 140 S cm−1 was achieved.