Free Exciton Emission Research Articles

High-performance UV photodetectors and temperature-dependent photoluminescence of individual ZnO hexagonal-prism microwire

ZnO hexagonal-prism microwires (HPMs) with the average width of about 50 m have been fabricated by a floating zone method. Their structural, temperature-dependent photoluminescence (PL) and UV photoresponse based on an individual ZnO HPMs were systematically investigated. For the temperature-dependent PL properties, different transitions including free exciton emission, bound exciton emission and free-to-bound transition were clearly observed at 83 K. The individual ZnO HPM-based UV photodetector showed a response cut-off wavelength of 390 nm and an ultraviolet/visible ratio of about two orders of magnitude with an applied bias of 5 V. 2014 Springer-Verlag Berlin Heidelberg

Selective area growth of GaN nanostructures: A key to produce high quality (11–20) a-plane pseudo-substrates

Selective area growth of GaN nanostructures was performed on (11-20) a-plane GaN/sapphire templates. The dominant lateral growth rate along the in-plane c-direction produces the coalescence of the individual nanostructures into a continuous film. Photoluminescence measurements show the appearance of donor-bound and free exciton emissions in individual nanostructures, pointing towards an improvement of the material quality as compared to the original template. Upon nanostructures coalescence a decrease of the full width half maximum value, down to 2 meV, is observed. These results reveal the high quality of the coalesced film, opening the way to fabricate high quality, non-polar GaN pseudo-substrates.

In-situ optical transmission electron microscope study of exciton phonon replicas in ZnO nanowires by cathodoluminescence

The cathodoluminescence spectrum of single zinc oxide (ZnO) nanowires is measured by in-situ optical Transmission Electron Microscope. The coupling between exciton and longitudinal optical phonon is studied. The band edge emission varies for different excitation spots. This effect is attributed to the exciton propagation along the c axis of the nanowire. Contrary to free exciton emission, the phonon replicas are well confined in ZnO nanowire. They travel along the c axis and emit at the end surface. Bending strain increases the relative intensity of second order phonon replicas when excitons travel along the c-axis.

Analysis of optoelectronic properties of TiO2 nanowiers/Si heterojunction arrays

The optoelectronic properties of n-TiO2NW/p-Si heterojunction fabricated by depositing TiO2 nanowires on a p-Si substrate are studied. Under excitation at a wavelength of 370 nm, the TiO2 nanowires produce a light emission at 435 nm due to the emission of free excitons. The I—V characteristics are measured to investigate the heterojunction effects under the dark environment and ultraviolet (UV) illumination. n-TiO2NW/p-Si has a p—n junction formed in the n-TiO2/p-Si heterojunction. TiO2NW/Si photodiode produces a photocurrent larger than dark current under UV illumination. It is observed that UV photons are absorbed in TiO2 and the heterojunction shows a 0.034-A/W responsivity at 4-V reverse bias.

Synthesis of Functional ZnO Nanowall Networks Using Simple Solution Etching

We report the synthesis of high quality single crystalline ZnO nanowall networks (NNs) using ammonia solution etching of a ZnO nanowire-nanowall heterojunction (NNH) structure. Synchrotron X-ray diffraction revealed that the full-width-at-half-maximum of the ZnO (0002) peak in the ZnO NN sample was much narrower than that of the ZnO NNH sample. Temperature-dependent photoluminescence (PL) measurements revealed more intense and narrower bound exciton peak emission in the NN sample compared to that in the NNH sample. Moreover, the clear observation of free exciton emission in the PL spectrum of the NN sample, even at temperatures as low as 30 K, suggested incorporation of a small number of impurities into the NN sample. In addition, I-V characteristics confirm the higher conductivity of the ZnO NNs as compared to those of the NNH structures, indicating that the NN sample had a superior crystalline property than NNH sample.

The Increase of Radiative Lifetime of Free Excitons in Selectively Si-doped GaAs/AlxGa1-xAs Heterostructures

The time resolved photoluminescence spectra of selectively Si-doped GaAs/AlxGa1-xAs heterostructures have been investigated over a wide temperature range from 3.6 K to 300 K in order to identify possible mechanisms behind the observed increase in radiative lifetime of free excitons. Possible mechanisms of carrier recombination are discussed with emphasis on the unique traits of excitonic photoluminescence. The intensive lines found in the spectra of the heterostructures are associated with the formation and enhancement of free exciton emission in the flat band region of an active i-GaAs layer. We have established that the free exciton radiative lifetime in the heterostructures increases about two times, up to 1.44 ns in comparison with lifetime 0.6 ns of i-GaAs layer without a heterostructure for first sample and up to 0.92 ns from 0.4 ns for second sample at 3.6 K temperature. DOI: http://dx.doi.org/10.5755/j01.ms.20.2.6329

Optical properties of strain-free AlN nanowires grown by molecular beam epitaxy on Si substrates

The optical properties of catalyst-free AlN nanowires grown on Si substrates by molecular beam epitaxy were investigated. Such nanowires are nearly free of strain, with strong free exciton emission measured at room temperature. The photoluminescence intensity is significantly enhanced, compared to previously reported AlN epilayer. Moreover, the presence of phonon replicas with an energy separation of ∼100 meV was identified to be associated with the surface-optical phonon rather than the commonly reported longitudinal-optical phonon, which is further supported by the micro-Raman scattering experiments.

Luminescence properties of tetrapod ZnO nanostructures

The tetrapod ZnO nanostructures are synthesized on the Si (100) substrates using the chemical vapor deposition (CVD) method at 1000 °C. Each nanostructure has four arms which are about 3–10 μm in length and 0.2–1.5 μm in diameter. Further analyses on structure demonstrate that the tetrapod ZnO nanostructures have single crystalline wurtzite hexagonal structure preferentially oriented in c-axis. The photoluminescence (PL) measurements of the tetrapod ZnO nanostructures revealed a UV peak at 382 nm corresponding to the free exciton emission, and a green peak at 523 nm arising from deep level emission. For comparative analysis, cathodoluminescence (CL) spectra obtained from different regions of an individual tetrapod are investigated. Moreover, a possible growth mechanism of the tetrapod ZnO nanostructures is also discussed based on the experimental results.

Temperature dependent photoluminescence of nanocrystalline γ-CuCl hybrid films

Organic–inorganic hybrid films combine the basic properties of organic and inorganic materials and offer special advantages that enhance optical, thermal and mechanical properties. We have studied the temperature dependent photoluminescence (PL) of nanocrystalline γ-CuCl hybrid films from 15K to room temperature in order to investigate the electronic transitions of the hybrid films. The I1 impurity bound exciton peak is the most intense emission peak at 15K but the peak intensity decreases rapidly with increasing temperature due to the low binding energy of this exciton bound to an impurity center and above 80K all three excitonic and biexcitonic peaks, except the Z3 free exciton emission peak disappeared. The biexciton emission peak intensity follows a quadratic dependency on power in the excitation power range <10 kWcm−2. The integrated Z3 excitonic PL intensity is almost independent of the temperature below 80K, while above 100K the PL emission intensity decreases rapidly. Thermal quenching of the Z3 free exciton PL emission in hybrid films has been observed. The full width at half maximum (FWHM) of the free exciton peak was investigated as a function of temperature and was explained by a theoretical model which considers the scattering of excitons with acoustic phonons and longitudinal optical phonons. The FWHM of the Z3 free exciton emission peak increases with increasing temperature, and a value of ~76meV was deduced for the FWHM at room temperature, which is comparatively better than ZnO (106meV) and GaN (100meV) nanostructures. The Z3 free exciton energy of the hybrid films exhibits a blue shift of 3meV at 15K compared to the bulk CuCl samples which may be due to a dead layer effect near the CuCl nanocrystal surface. The exciton energy also presents a blue shift of ~41meV with increasing temperature from 15K to room temperature. The results obtained for the γ-CuCl hybrid films are comparable to those of vacuum evaporated and sputtered CuCl films reported in the literature.

Synthesis and optical properties of pencil-like and shuttle-like ZnO microrods

The pencil-like and shuttle-like ZnO microrods have been fabricated on Si (100) substrates by chemical vapor deposition. Structure characterization results show that the microrods are perfect single crystals with the wurtzite structure along the [0001] growth direction and have diameters ranging from 100 nm to 2 μm and lengths up to 10 μm. Room-temperature photoluminescence measurements of ZnO microstructures exhibit an intensive ultraviolet peak at 390 nm and a broad peak centered at about 526 nm, which can be attributed to the free exciton emission and the deep level emission, respectively. Cathodoluminescence measurements show the same ultraviolet and green emissions as seen in the photoluminescence results. A possible growth mechanism of ZnO microrods is finally proposed.

退火对Ni/TiO 2 纳米复合材料吸波性能的影响 *

采用机械化学法制备了Ni/TiO2纳米复合材料,制备态样品由六角形结构的金红石TiO2和面心立方结构的Ni组成。退火使样品晶格缺陷和内应力减少,Ni晶粒尺寸和饱和磁化强度明显增大。退火后,TiO2的本征发光峰、自由激子及束缚激子引起的发光峰都发生红移,其原因是能带结构的畸变。制备态样品在15 GHz产生强烈的非线性介电共振,并呈现明显的Cole-Cole半圆。当吸收厚度为8 mm时,在14-16 GHz范围内的反射损耗值(RL)都超过了-10 dB,且在15 GHz处出现最优RL值-32 dB。退火使制备态样品在2.8和5.2 GHz处的自然共振明显加强,2-6 GHz范围的吸波能显著提高。

Synthesis and cathodoluminescence characteristics of caltrop-like ZnO nanostructures

Novel caltrop-like ZnO nanostructures have been synthesized on Si (100) substrates using a simple carbon thermal method at 1050°C. Structure characterizations indicate that the as-prepared nanostructures have a single crystalline wurtzite structure preferentially oriented in the [0001] direction. Cathodoluminescence measurements of ZnO nanostructures exhibit an intensive ultraviolet peak at 384nm and four broad green peaks (at 492, 504, 524 and 524nm, respectively), which can be attributed to the free exciton emission and the deep level emission, respectively. The prominent optical properties of the caltrop-like ZnO nanostructures make them promising candidates for fabricating optoelectronic nanodevices. A possible growth mechanism is also discussed.

Anionic polymerization in Co and Fe doped ZnO: Nanorods, magnetism and photoactivity

The growth mechanism of Zn1−xCoxO (ZC) and Zn1−xFexO (ZF) nanorods, and resulting magnetic and optical properties have been studied. The ZC and ZF nanorods were prepared by sol–gel synthesis route. X-ray diffraction results in polycrystalline phase with wurtzite structure of ZC and ZF nanorods. The transmission electron microscopy images show the formation of nanorods. The growth mechanism of nanorods is explained on the basis of agglomeration of Zn2+ with OH− ions which is react with poly vinyl alcohol involve anionic polymerization of oriented growth. Magnetic measurement of ZC and ZF nanorods exhibit superferromagnetic behavior and the large value of saturation magnetization observed at room temperature. The magnetization below room temperature measurement confirms the origin of observed magnetism. Raman and photoluminescence spectra show good photoactivity. The observed Raman active modes show wurtzite structure belongs to C6v symmetry group. Photoluminescence measurements of ZC and ZF nanorods exhibit ultraviolet peaks at 413.90 nm (∼3 eV) due to free exciton emission and at 546.31 nm (∼2.27 eV) due to transition from deep donor states which arises from oxygen vacancy.

Anomalous anticrossing of neutral exciton states in GaAs/AlGaAs quantum dots

We study the effects of heavy hole--light hole (HH-LH) mixing on fine-structure and polarization properties of neutral excitons (${\text{X}}^{0}$) confined in single GaAs/AlGaAs quantum dots (QDs) under the application of anisotropic biaxial stress. In the large HH-LH mixing regime, these properties are substantially different from the usually observed properties in the case of small or no mixing. By varying the applied stress, the mixing in the initially strain-free QDs changes from $\ensuremath{\sim}$0 to $\ensuremath{\sim}$70$%$ and an anomalous anticrossing of the ${\text{X}}^{0}$ bright states is observed. The latter is attributed to stress-induced rotation of the in-plane principal axis of the QD confinement potential. We show that the analysis of free-excitonic emission of bulk GaAs surrounding the QDs not only allows estimation of the stress and mixing in the QDs, but also provides the quantum-confinement-induced HH-LH splitting of the as-grown QDs.

Laser-Excited Excitonic Luminescence of Nanocrystalline TiO2 Powder

Titanium dioxide (TiO2) nanocrystalline powders were prepared by the thermal hydrolysis method in the form of pure anatase or rutile and were investigated by X-ray diffraction, X-ray fluorescence, FT-Raman spectroscopy, optical absorption, and photoluminescence (PL) methods. PL spectra were studied under the intense UV laser excitation at 337.1 nm (3.68 eV) at room temperature. Some interesting features in the PL spectra including the well-resolved peaks of excitonic and band-band transitions in TiO2 were observed for the first time. It is shown that PL bands with peaks at 2.71–2.81 eV and its phonon replicas in anatase and rutile TiO2 arise from the excitonic e− − ℎ+ recombination via oxygen vacancies. The excitonic peak at 2.91 eV is attributed to the recombination of self-trapped excitons in anatase or free excitons in rutile TiO2. The PL peaks within 3.0–3.3 eV in anatase TiO2 are ascribed to indirect allowed transitions due to the band-band e− − ℎ+ recombination. The peaks at 3.03 and 3.26 eV are attributed to the free exciton emission near the fundamental band edge of rutile and anatase TiO2, respectively.

Positive temperature variation of the bandgap energy in the single-crystalline chalcopyrite semiconductor AgInS2

Optical absorption spectra have been measured on the single-crystalline chalcopyrite semiconductor AgInS2 using polarized light at T = 10–300 K. The bandgap energy Eg of AgInS2 shows unusual temperature dependence at low temperatures. The resultant temperature coefficient ∂Eg/∂T is found to be positive at T &amp;lt; 130 K and negative above 130 K. This result has been successfully explained by considering the effects of thermal expansion and electron–phonon interaction. The free-exciton emission of photoluminescence spectra also indicates positive temperature dependence of the peak energies at low temperatures. The exciton binding energy of AgInS2 is determined to be 26.4 meV.

ZnO Multi-Branched Microflowers: Fabrication and Temperature-Dependent Photoluminescence Properties

ZnO multi-branched microflowers were synthesized on Si substrate using a simple vapor transport process without any catalysts. X-ray diffractometer (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM) were used to characterize the morphologies and structures. The XRD results showed that the as-grown ZnO microflowers possessed hexagonal wurtzite structure with preferred c-axis orientation. The FESEM results indicated that the microflowers had multi-branches, and each branch showed the nanonail morphology with the length of about 1.5 mu m. Their photoluminescence properties were found to be strongly temperature-dependent. The different transitions including free exciton emission, bound exciton emission and the free-to-bound (FB) transition were clearly observed at 86 K. The bound exciton emission and FB emission became weak with the increase of the testing temperature, while the free exciton (FE) emission got relatively strong. The 1-LO phonon replica of FB was rather strong in the multi-branched ZnO microflowers.

ZnS grain size effects on near-resonant Raman scattering: optical non-destructive grain size estimation

Near-resonant Raman scattering measurements of zinc sulfide nanoparticles and thin films have been made and correlated to grain and particle size, respectively, using a 325 nm wavelength excitation source. The area ratios between the first, second, and third order peaks of ZnS identified as the T2(LO) mode decrease with increasing ZnS grain size. This is an effect attributed to changes in the bandgap energy from quantum confinement due to the varying grain size between the films/particles, as noted by a shift in the room temperature photoluminescence emission corresponding to the free exciton emission energy. While Raman scattering spectroscopy is typically limited to identification of phases and their crystalline properties, it is possible to attain more than such straightforward information by calibrating the spectral features to variations between sets of samples. These results open the possibility of making a quantitative grain size estimation in ZnS thin films and nanostructures, as well as in other material systems where ZnS may be expected as a secondary phase, such as Cu2ZnSnS4. Additionally, more commonly used excitation wavelengths for Raman scattering, such as 514 and 532 nm, are shown to be of limited use in characterizing ZnS thin films due to the extremely low Raman scattering efficiency of ZnS in films with sub-micron thicknesses.

Tailored defect-induced sharp excitonic emission from microcrystalline CuI and its ab initio validation

Iodine concentration modulated free excitonic emission of CuI thin films developed by wet chemical reaction at room temperature.

Excitonic recombination in epitaxial lateral overgrown AlN on sapphire

Excitonic emission in heteroepitaxially grown aluminum nitride (AlN) with reduced defect density due to the epitaxial lateral overgrowth (ELO) of patterned AlN/sapphire templates has been investigated by photoluminescence spectroscopy and compared to AlN/sapphire and homoepitaxially grown AlN. The ELO sample exhibits small linewidths of the free exciton and two different bound exciton emission bands. The free exciton emission energy is shifted by 58.5 meV with respect to unstrained homoepitaxially grown AlN attributed to compressive strain. A donor bound exciton D0X with an exciton localization energy of 13.0–13.5 meV is dominating in the photoluminescence spectra of ELO AlN/sapphire. This D0X does not show strong phonon replica and is dominant at elevated temperatures in ELO AlN/sapphire. The optical quality of heteroepitaxial AlN is significantly improved using the ELO technique and therefore suitable for high efficiency ultraviolet light emitters.