Room Temperature Excitonic Absorption Research Articles

Strong room temperature exciton absorption of TiO2 nanorods prepared by Ti and Cu ion implantation

High purity SiO2 substrates were implanted by 45 keV Ti ions or sequentially implanted by 45 keV Ti and Cu ions at different fluences. After annealing at 900 °C, rutile TiO2 nanorods were synthesized in the samples sequentially implanted with Ti and Cu ions. Besides, strong room temperature exciton absorption was observed near the band edge of TiO2 in these samples. The position and intensity of the TiO2 exciton absorption peak can be regulated by changing the fluence of Ti and Cu ions. Finally, through analyzing the valence and distribution of elements of the samples, we put forward a possible mechanism about the formation of the surface polarized dipoles layer on the surface of TiO2 nanorods to explain the exciton-enhanced phenomenon. The experimental results provide an effective way to enhance the exciton absorption of TiO2, which could be used to fabricate more efficient optoelectronic devices.

Influence of π -conjugated cations and halogen substitution on the optoelectronic and excitonic properties of layered hybrid perovskites

Low-cost chemical engineering of two-dimensional layered hybrid halide perovskite structures allows for the design of hybrid semiconductor quantum wells with tailored room-temperature excitonic optical absorption, emission, and charge carrier transport properties. Here density functional theory and the Bethe-Salpeter equation are used to predict the electronic structure and optical response of layered perovskites with two representative single-ring conjugated organic spacers, ammonium-propyl-imidazole (API) and 2-phenethylammonium (PEA). The inorganic perovskite quantum well properties are further tuned by analyzing the effect of halogen (X = I, Br, Cl) substitution. We found that visible light absorption occurs primarily within the perovskite layer and that UV light absorption induces partial electron-hole separation between layers. In addition, a strong exciton binding energy and influence on absorption spectrum is found by solving the Bethe-Salpeter equation. Our results suggest that further engineering is necessary beyond the single-ring limit, by introducing more conjugated rings and/or heavier nuclei into the organic spacer. This is a promising future direction to achieve photoinduced charge separation and more generally hybrid heterostructures with attractive optoelectronic properties.

Saccharide capped CdSe quantum dots grown via electron beam irradiation

CdSe quantum dots were synthesized in aqueous solutions using equimolar cadmium sulfate and sodium selenosulfate in aqueous solutions through 7 MeV electron beam irradiation in a linear electron accelerator, LINAC in the presence of different saccharides, such as fructose, glucose, sucrose and starch. The as grown CdSe quantum dots in colloidal solution were fairly stable under ambient conditions and exhibit room temperature excitonic absorption and broad fluorescence. Among the saccharides used in this study, starch was found to have a better capping property, whereas glucose was found to reduce their cytotoxicity effect to a larger extent as compared to other saccharides. The mechanism of radiolytic synthesis of these quantum dots was investigated by pulse radiolysis studies.

Strong room-temperature ultraviolet to red excitons from inorganic organic-layered perovskites, (R-NH3)2MX4 (M=Pb2+, Sn2+, Hg2+; X=I−, Br−)

Many varieties of layered inorganic-organic (IO) perovskite of type (R-NH 3 ) 2 MX 4 (where R: organic moiety, M: divalent metal, and X: halogen) were successfully fabricated and characterized. X-ray diffraction data suggest that these inorganic and organic structures are alternatively stacked up along c -axis, where inorganic mono layers are of extended corner-shared MX 6 octahedra and organic spacers are the bi-layers of organic entities. These layered perovskites show unusual room-temperature exciton absorption and photoluminescence due to the quantum and dielectric confinement-induced enhancement in the exciton binding energies. A wide spectral range of optical exciton tunability (350 to 600 nm) was observed experimentally from systematic compositional variation in (i) divalent metal ions (M=Pb 2+ , Sn 2+ , Hg 2+ ), (ii) halides (X=I − and Br − ), and (iii) organic moieties (R). Specific photoluminescence features are due to the structure of the extended MX 2− 4 network and the eventual electronic band structure. The compositionally dependent photoluminescence of these IO hybrids could be useful in various photonic and optoelectronic devices.

Synthesis, structural, thermal and optical studies of inorganic-organic hybrid semiconductors, R-PbI4

Wide varieties of naturally self-assembled two-dimensional inorganic-organic (IO) hybrid semiconductors, (4-ClC6H4NH3)2PbI4, (C6H9C2H4NH3)2PbI4, (CnH2n+1NH3)2PbI4 (where n = 12, 16, 18), (CnH2n-1NH3)2PbI4 (where n = 3, 4, 5), (C6H5C2H4NH3)2PbI4, NH3(CH2)12NH3PbI4, and (C4H3SC2H4NH3)2PbI4, were fabricated by intercalating structurally diverse organic guest moieties into lead iodide perovskite structure. The crystal packing of all these fabricated IO-hybrids comprises of well-ordered organic and inorganic layers, stacked-up alternately along c-axis. Almost all these hybrids are thermally stable upto 200 °C and show strong room-temperature exciton absorption and photoluminescence features. These strongly confined optical excitons are highly influenced by structural deformation of PbI matrix due to the conformation of organic moiety. A systematic correlation of optical exciton behavior of IO-hybrids with the organic/inorganic layer thicknesses, intercalating organic moieties, and various structural disorders were discussed. This systematic study clearly suggests that the PbI layer crumpling is directly responsible for the tunability of optical exciton energy.

Synthesis, structure and optical studies of inorganic–organic hybrid semiconductor, NH 3(CH 2) 12NH 3PbI 4

Synthesis, crystal structure and optical properties of two-dimensional layered inorganic–organic hybrid NH 3(CH 2) 12NH 3PbI 4 are presented. Intercalation strategy has also been used to fabricate single crystal and thin films of the same. As thin film, they are well oriented and stacked along the [l 0 0] direction with strong room-temperature excitonic absorption and emission characteristics. Exciton features were correlated with the relative inorganic network crumpling and conformation of alkylammonium chains. The structural features are compared with those of similar hybrid, (H(CH 2) 12NH 3) 2PbI 4.

Characterization of ZnO and ZnO:Al thin films deposited by the sol–gel dip-coating technique

Nanocrystalline zinc oxide films have been obtained by the sol–gel process. The films were deposited from precursor solutions by dip-coating on quartz substrates, and subsequently transformed into nanocrystalline pure or aluminium-doped ZnO films after a thermal treatment. The film microstructure and composition characterization was studied by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The optical properties were studied by transmittance spectroscopy. The water adsorption energy was measured by temperature programmed desorption (TPD) in the range 90–700 K. The optical transmittance in the UV region gives bandgap energy values of 3.27 eV for undoped samples, and higher than 3.30 eV for the Al-doped ones. The increase in bandgap energy in Al-doped samples may be explained by band-filling effects. The band edge absorption coefficient increases monotonically for the Al-doped samples but has a shoulder for the undoped ones, which may be assigned to room-temperature excitonic absorption.

Electroabsorption in lattice-matched InGaAlAs-InAlAs quantum wells at 1.3 μm

Electroabsorption properties of (In/sub 0.53/Ga/sub 0.47/As)/sub 0.7/ (In/sub 0.52/Al/sub 0.48/As)/sub 0.3/-In/sub 0.52/Al/sub 0.48/As quantum wells were investigated experimentally and analytically in order to form a semi-empirical model for 1.3 /spl mu/m optical modulator applications. The observed exciton energy shifts and changes in electron-hole wave function overlap integrals are in agreement with calculation for the quantum confined Stark effect. Empirically, we found that the room-temperature exciton absorption peak can be described by a Gaussian peak, and that the residual absorption should be characterized by an exponential tail. In order to provide realistic linewidth broadening parameters, empirical expressions are summarized here for this material. >

Characterization of high quality GaInP/GaAs superlattices grown on GaAs and Si substrates by gas source molecular beam epitaxy

We report an analysis of the heteroepitaxial interfaces in high quality GaInP–GaAs superlattices grown simultaneously on GaAs and Si substrates by gas source molecular beam epitaxy. These two superlattices have been studied using high resolution x-ray diffraction measurements. Sharp superlattice satellites, with very little broadening, are observed within a 6° range for the sample on GaAs. Photoluminescence peaks with full widths at half-maximums of 5 and 7 meV are obtained at 4 K for samples with 58 Å wells on GaAs and Si, respectively. Room temperature exciton absorption is observed in the photovoltage measurements for a superlattice grown on Si substrate. The thicknesses determined by x-ray analysis are consistent with those obtained by a Kronig–Penny model fitting of the photovoltage spectroscopy.

Momentum-space solution of exciton excited states and heavy-hole-light-hole mixing in quantum wells.

A full valence-band mixing model for excitons in quantum wells is presented. The exciton equation is solved in momentum space using the modified Gaussian quadrature method, taking into account the angular dependence of the excitons. The theory accounts for the mixing of the ground state of the light-hole exciton and the excited states of the heavy-hole exciton at low temperatures reported previously by Vi\~na et al. [Phys. Rev. Lett. 58, 832 (1987)]. Our calculations reproduce correctly the resonant field, the minimum energy split at the resonant field, and the fine structure of the competing 2s and 2p states resolved in the experimental data. For the s-states-dominated spectra, the full model can be simplified to include only s-states coupling. We show that the numerical results based on the simplified model match very well the polarization-dependent room-temperature exciton absorption spectra of Miller et al. [IEEE J. Quantum Electron. QE-22, 1816 (1986)].

Strain-relaxed, high-speed In 0.2Ga 0.8As MQW p-i-n photodetectors grown by MBE

We have fabricated p-i-n photodetectors with 10, 15 and 20 well In 0.2Ga 0.8As/GaAs multiple quantum wells (MQWs) in the intrinsic region, whose bandwidths exceed 23 GHz. Cathodoluminescence (CL) images are characterized by rectangular grids, indicating strain relaxation through the formation of misfit dislocations. The MQW photoluminescence (PL) is strongly affected by the junction electric field, but PL from the p ++ GaAs cap shows that increasing the number of QWs enhances the non-radiative recombination associated with the misfit dislocation network. Infrared absorption measurements reveal sharp, room temperature excitonic absorption at 980 nm, which shifts to 1000 nm with a 6 V reverse bias. The peak excitonic absorption intensity and linewidth improve as the number of wells increases. Room temperature dark current measurements on 250 μm x 250 μm p-i-n photodetectors yield leakage currents as low as 2.5 nA for a reverse bias of 4.4 V. These results demonstrate the usefulness the usefulness of strain-relaxed InGaAs/GaAs MQW photodetectors for high speed applications.

Growth, optical, and optoelectronic properties of CdZnTe/ZnTe multiple quantum wells

Abstract The use of molecular beam epitaxy has allowed growth of CdZnTe/ZnTe quantum wells with optical properties that match those of well-established infrared III-V quantum well systems. The sharp room temperature exciton absorption peaks that these II to VI quantum wells exhibit have been used in image processing devices and electroabsorption modulators operating at visible wavelengths. The large exciton-binding energy, very large saturation intensity, and strong exciton-LO phonon coupling exert a major influence on both the device characteristics and the basic physical properties of these films. The progress in these quantum wells has been achieved despite complications in the growth that result from the large lattice mismatches and the proximity, for many CdZnTe compositions, to a miscibility gap.

Room temperature excitonic absorption in CdZnTe/ZnTe quantum wells: Contributions to exciton linewidth

We explore the inhomogeneous and homogeneous contributions to the exciton linewidth that have allowed the recent observation of room temperature excitonic absorption in II–VI semiconductor quantum wells. Our measurements of the absorption spectrum in a range of CdxZn1−xTe/ZnTe quantum well heterostructures indicate that temperature-dependent contributions to the exciton linewidth are influenced by alloy composition.

Room-temperature excitonic saturation in CdZnTe/ZnTe quantum wells

We present the first measurements of room-temperature excitonic absorption saturation in a II-VI semiconductor quantum well. Strong room-temperature excitonic absorption in CdZnTe/ZnTe quantum wells is found to saturate at an incident optical intensity that is considerably higher than that for III-V quantum wells. We show that this phenomenon can be interpreted in terms of the smaller excitonic Bohr radius characteristic of wide-gap II-VI compounds.

Mode locking of semiconductor diode lasers using saturable excitonic nonlinearities

Multiple-quantum-well (MQW) structures of GaAs and GaAlAs have been used for passive mode locking of commercial GaAs semiconductor diode lasers. We present an extended discussion of this application of the sensitive room-temperature excitonic absorption saturation in MQW material. We review the criteria for passive mode locking and discuss two methods—carrier diffusion and proton bombardment—for reducing the aborption recovery time without destroying the excitonic nonlinearity. A simple probabilistic theory is derived for the effect of bombardment on the excitonic effects that is in order-of-magnitude agreement with experiment. We have performed experiments using MQW material to mode lock a GaAs laser. A continuous train of pulses as narrow as 1.6 psec has been obtained with a pulse-repetition rate of 2 GHz.

Room temperature excitonic nonlinear absorption and refraction in GaAs/AlGaAs multiple quantum well structures

We present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime. Nonlinear absorption and degenerate four-wave mixing in the vicinity of the room temperature exciton resonances are observed and analyzed. Spectra of the real and imaginary parts of the nonlinear cross section as a function of wavelength are obtained, and these are in excellent agreement with experimental data. A simple model for excitonic absorption saturation is proposed; it accounts qualitatively for the very low saturation intensities of room temperature excitons in MQWS.

Luminescence phenomena in CsI:Na

Absorption, emission and excitation spectra of CsI : Na were measured on bulk single crystals and thin films at room temperature (RT) and liquid nitrogen temperature (LNT) Excitonic absorption peaks as well as absorption in the long-wavelength tail of the fundamental absorption which is not present in pure CsI were observed. In addition to emission at 4250 Å, excitonic emission at 3400 Å, normally found at LNT, has also been observed at RT, This room temperature UV emission is closely related to the heat treatment process that the specimen received.