Defect-related Emission Band Research Articles

Polarized photoluminescence from Sn, Fe, and unintentionally doped β-Ga2O3

In this work, we demonstrate that β-Ga2O3 shows orientation-dependent polarized photoluminescence (PL) emission and give a comprehensive insight into gallium oxide's PL spectral properties. We characterized the polarization and spectral dependencies of both the incident and emitted light for (−201) unintentionally doped (UID) as well as (−201) and (010) Sn-doped and Fe-doped crystals. We observed for UID and Sn-doped samples that the electron to self-trapped hole and native defect-related emission bands are linearly polarized with polarized emission intensities ordered as E || c (and c*) > E || a (and a*) > E || b. Furthermore, the spectral shape of emission does not change between the UID and Sn-doped samples; instead, the Sn-doping quenches the total PL spectral intensity. For Fe-doped samples, polarized red emission caused by unintentional Cr3+ doping generates emission intensities ordered E || b > E || c (and c*) > E || a (and a*). It is also observed that in some circumstances, for some doped crystals, the PL spectra can show variations not only in intensity but also in spectral shape along different polarization directions. As an example, the PL emission band for emission along c is blueshifted relative to that along a in Sn-doped β-Ga2O3.

Ultrawide bandgap willemite-type Zn2GeO4 epitaxial thin films

Willemite-type Zn2GeO4 is a promising ultrawide bandgap semiconductor material. To date, experimental results on growth and physical properties of epitaxial thin films of willemite-type Zn2GeO4 are not available. Here, we report the heteroepitaxial growth of (00.1)-oriented Zn2GeO4 thin films on c-plane sapphire substrates using pulsed laser deposition. The in-plane orientation relationships are [11.0] Zn2GeO4//[11.0] Al2O3 and [11¯.0] Zn2GeO4//[11¯.0] Al2O3. A 450 nm thick epitaxial film with a surface roughness of 2.5 nm deposited under 0.1 mbar oxygen partial pressure exhibits a full width at half maximum (FWHM) of rocking curve of (00.6) reflex of 0.35°. The direct bandgap is evaluated to be 4.9 ± 0.1 eV. The valence band maximum is determined to be 3.7 ± 0.1 eV below the Fermi level. Together with the density-functional theory band structure calculation, it is suggested that the O 2p orbital and Zn 3d orbital dominantly contribute to the valence band of Zn2GeO4. The steady-state photoluminescence (PL) spectra of the films under 266 nm excitation at room temperature exhibit a broad defect-related emission band centered at 2.62 eV with a FWHM of 0.55 eV. The origin of this native defect-related PL is suggested to correlate with Zn interstitials. This work advances the fundamental study on willemite-type Zn2GeO4 epitaxial thin films for potential device application.

Tuning defect-related optical bands by channeling implants in semiconductors

Ion implantation is an excellent method to introduce defects into semiconductors, extending their functionalities in a controllable way. Herein, we investigated an option to use crystallographically aligned implants as an additional route to control the balance between optically active defects, selecting ZnO as a test material. The optical data were correlated with the structural analysis confirming the formation of different dominating crystalline defects in samples implanted along and off [0001] direction. Specifically, we demonstrated that different proportions in the contents of the extended and point defects in the initial as-implanted states of these samples, lead to prominent variations in the defect-related luminescence upon annealing. As such, we conclude that channeling implants may have an added value in the functionalization of defects in semiconductors, e.g. to tune specific spectral contents in the defect-related emission bands.

Upconversion luminescence, scintillation properties and optical thermometry behaviors of Tm3+, Yb3+: Gd2Zr2O7 nanocrystals

The development of multifunctional nanocrystals has attracted growing interest lately in the biological and medical sciences. Herein, the dual-mode luminescence properties and optical thermometric performances of Tm3+, Yb3+: Gd2Zr2O7 nanocrystals synthesized by a facile hydrothermal method are investigated systematically. Upon 980 nm excitation, the upconversion (UC) luminescence spectra of the obtained samples consist of blue (485 nm), red (655 and 681 nm), and near-infrared (785 nm) emission bands of Tm3+ ions. Diversely, the X-ray excited luminescence (XEL) spectra contain a strong ultraviolet (UV) emission from Gd3+ ions (314 nm), a weak defect-related emission band (400–600 nm), and several tiny emissions from Tm3+ ions. To realize the optical thermometry in the biological window, the luminescence intensity ratio (LIR) between 3F2,3 → 3H6 (681 nm) and 1G4→3F4 (655 nm) transitions is employed for temperature sensing, giving high absolute and relative sensing sensitivities (Sa-max = ∼1064 × 10−4 K−1, Sr-max = ∼2.32%K−1). Therefore, the present Tm3+, Yb3+: Gd2Zr2O7 nanocrystals could be useful in optical thermometry, photodynamic therapy, and X-ray imaging.

Transformation of free-standing ZnO nanorods upon Er doping

Erbium doped zinc oxide (ZnO:Er) powder was synthesized by hydrothermal growth. The morphology changed from undoped nanorods to microrods. The effect of Er doping on luminescence properties of ZnO:Er was observed. Furthermore, annealing in air at elevated temperatures up to 700 °C results in additional modification of defect states and related luminescence properties. In particular, exciton-related UV emission band and the defect-related red emission band appear in all annealed samples. Er3+infrared (IR) photoemission, the typical4I13/2→4I15/2transition, not observed in as grown samples with low Er concentration, occurs at room temperature in all ZnO:Er samples after annealing in air at elevated temperatures. Oppositely, the electron paramagnetic resonance (EPR) signal of Er3+is getting weaker upon the annealing at elevated temperatures, disappearing completely at about 500 °C. The intensity of theg = 1.96 EPR spectrum, typical for ZnO structures, is getting decreased and almost disappears after the annealing at 500 °C but increasing again after the annealing at 600 °C and 700 °C. X-ray photoelectron spectroscopy and IR Raman spectroscopy suggested that annealing in air causes diffusion of Zn from the surface to the bulk and oxidation of Er to Er3+in Er2O3and Er(OH)xforms.

Insight into Co concentrations effect on the structural, optical, and photoelectrochemical properties of ZnO rod arrays for optoelectronic applications

Using a cost-effective and time-efficient chemical bath deposition (CBD) approach, zinc oxide (ZnO) nanorod doped with various cobalt concentrations ranged from 2% to 8% have been synthesized on ITO substrates. Different methods: including X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis spectroscopy, photoluminescence spectrometry (PL), current-voltage (I–V), and photocurrent were employed to investigate the structural, morphological, optical, and photoelectrochemical. XRD results confirmed that rod arrays formed in hexagonal structure with favorite orientated in (002) plane. SEM images revealed at all doping levels that there were hexagonally shaped nanorods of ZnO, which agreed with XRD measurements. UV–visible measurements indicate an absorption band edge at 381 nm which, is mainly recognized as a characteristic direct bandgap for ZnO arrays. In doped samples; other absorption band edges observed at 562, 605, and 654 nm correspond to the d-d transformation of Co2+ ions. The calculated value of band gap energy (Eg) reduced with the increasing of the Co dopant ions. The PL spectra exposed near band edge emission and defect-related visible emission band. Photocurrent analysis showed that the fabricated arrays have an n-type conductivity. Moreover, the photocurrent enhancement with increasing doping concentration where 6% Co ions have the highest value. The results show that Co doped ZnO nanorods are talented candidates for solar cells and biosensors applications.

Excitons Bound to Defect States in Two-Dimensional (2D) MoS2

In this work, the effect of atomic defects created by gallium ion irradiation on the optical properties of single-layer molybdenum disulfide is studied by means of micro-photoluminescence measurements. The induced defects give rise to an additional emission band located at about 170 meV below the free exciton. The micro-photoluminescence intensity of this defect-related emission band is found to be proportional to the defect density. The large spectral width suggests the presence of binding sites with different binding energies available for excitons that remain optically active up to 230 K.

Effect of metal-support couplings on the photocatalytic performance of Au-decorated ZnO nanorods

We reported the effectiveness of photocatalysts based on ZnO nanorods in the pollutant degradation. UV–Vis adsorption and photoluminescence (PL) were used to verify the incorporation of the plasmonic Au NPs on the ZnO nanorods. The strong electronic interaction between Au NPs and the defect sites of ZnO can be caused by the resonant coupling near the surface of ZnO, which in turn suppresses the defect-related emission band. We significantly enhanced photocatalytic performance by the coupling ZnO nanorods to the plasmonic gold nanoparticles. Due to the surface plasmon resonance and the localized Au–ZnO Schottky barrier, the Au–ZnO nanorods demonstrated better photocatalytic efficiency (96% in 60 min) than pure ZnO nanorods (only 42% in 60 min) under visible light irradiation of the halogen lamp. The correlation between metal-support couplings and photocatalytic performance is discussed.

Hydrothermally synthesized Cd-doped ZnO nanostructures with efficient sunlight-driven photocatalytic and antibacterial activity

The doping in the metal oxide is key factors that can affect the photocatalytic and antibacterial activity. In present work, Cd-doped ZnO nanostructures have been synthesized by using a facile hydrothermal method. The sunlight induced photocatalytic activity of Cd-doped ZnO nanostructures was evaluated towards the degradation of methylene blue dye. The antibacterial activity of ZnO nanoparticles against Staphylococcus aureus was investigated. The XRD patterns exhibit a wurtzite hexagonal crystal structure with crystallite size in the range of 18–23 nm. Field emission scanning electron microscopy revealed the plate-like morphology of undoped ZnO and it transforms into small nanorods and aggregated irregularly shaped nanoparticles with the incorporation of Cd into ZnO. Energy dispersive spectroscopy analysis confirms the presence of cadmium in Cd-doped ZnO sample. The UV–Visible absorption spectra showed a redshift with an increase in Cd dopant concentration in ZnO. The PL spectra show near band edge emission and defect-related visible emission band. The Cd-doped ZnO exhibits excellent photocatalytic degradation of methylene blue dye compared to undoped ZnO within 30 min. The Cd-doped ZnO (4 wt%) shows maximum photocatalytic degradation efficiency of 84% under sunlight irradiation.

Composition-gradient ZnO/Zn(S,O) heterostructure nanorod arrays and their cathodoluminescence

Well-aligned ZnO/Zn(S,O) heterostructure nanorods have been synthesized through a well-designed chemical vapor deposition synthesis process. The as-synthesized ZnO/Zn(S,O) heterostructures exhibit a single crystal characteristic and a transitive interface with obvious composition gradient. X-ray Energy dispersive spectrometer (EDS) analysis demonstrates that ZnS-phase is rich in the nanorod bottom during the initial nucleation process and the S element content has a drastic decrease along the heterostructure nanorod from the bottom to the top, while O element is spatially detected in the whole rod. Room-temperature cathodoluminescence measurements verify two UV emissions at 330 nm and 385 nm for respective Zn(S,O) and ZnO phases with a strong defect-related emission band in visible range. Finally, the growth mechanism and optical emissions of ZnO/Zn(S,O) heterostructure nanorods are discussed.

Franck Condon shift assessment in 2D MoS2

Optical spectroscopy (OS) techniques are often coupled with first-principles density functional theoretical (DFT) calculations for determining the precise influence of defects on the electronic and structural properties of two-dimensional (2D) transition metal dichalcogenides. Such calculations are carried out presuming there is little or no effect of vibrational transitions on the observed electronic spectrum. However, if the effect of change in vibrational energy (Franck Condon (FC) shift) associated with such a transition is large, it could possibly lead to a different origin for the observed peak. One such instance is the attribution of the 0.75 eV cathodoluminescence peak by Fabbri et al (2016 Nat. Commun. 7 13044) to an optical transition from an S vacancy level in the band gap, under the assumption that the FC shift is negligible. Here, by first principles constrained DFT calculations using hybrid HSE06 functional we show that this combined prediction of OS and DFT calculations is valid for 2D MoS2 since the FC shift associated with electronic transitions from a sulfur vacancy is indeed small ~28 meV. Based on our calculations we conclude that it is reasonable to make a direct connection between DFT calculations and optical spectroscopy techniques in this material, hence, establishing a one to one relation between defect related emission bands and electronic transitions from the defect levels.

Carrier recombination in sonochemically synthesized ZnO powders

Abstract ZnO powders with particle size in the nm to μm range have been fabricated by sonochemical method, utilizing zinc acetate and sodium hydroxide as starting materials. Carrier recombination processes in the powders have been investigated using the photoluminescence, FT-IR and surface photovoltage techniques. It has been shown that the photoluminescence spectra exhibit a number of defect-related emission bands which are typically observed in ZnO lattice and which depend on the sonication time. It has been found that the increase of the stirring time results in a faster decay of the photovoltage transients for times shorter than approximately 5 ms. From the obtained data it has been concluded that the sonication modifies the complicated trapping dynamics from volume to surface defects, whereas the fabrication method itself offers a remarkably convenient means of modifying the relative content of the surface-to-volume defect ratio in powder grains and altering the dynamics of photoexcited carriers.

Gamma–Radiation-Assisted Synthesis of Luminescent ZnO/Ag Heterostructure Core–Shell Nanocomposites

There is increasing interest in tuning the physical properties of semiconductor nanostructures using metal nanoparticles. In this work, ZnO nanosphere covered with Ag nanoparticles were synthesized using gamma–radiation-assisted method. The amount of deposited Ag nanoparticles is controlled by changing irradiation dose in the range of 30–100 kGy in order to tune the semiconductor–metal interaction. The successful deposition of Ag on the ZnO nanoparticles is examined by analyzing the morphology, microstructure, optical, and magnetic properties of ZnO/Ag nanoparticles through field emission scanning electron (FESEM), microscopy X-ray diffraction spectra, UV-visible absorption, photoluminescence measurement, and vibrating sample magnetometer. FESEM and elemental mapping results confirmed that Ag nanoparticles have been concentrated at the surface of spherical ZnO particles. Moreover, formation of pure metallic Ag nanoparticles has been confirmed by XRD analysis. UV-visible absorption spectra of obtained ZnO/Ag showed two combined peaks, a weak peak at the shoulder around 360 nm corresponds to ZnO and a sharp absorption at 420 nm refers to spherical Ag nanoparticles. Obtained results from photoluminescence revealed that the near-band-edge emission and defect-related visible emission bands of ZnO could be enhanced dramatically at the same time by deposition of Ag nanoparticles, which was ascribed to localized surface plasmon–exciton coupling and surface plasmon scattering. Controlling the semiconductor and metal coupling effect is interesting because of its application in highly efficient optoelectronic devices and biosensor.

Effect of annealing temperature and ambient on the structural and optical properties of Eu3+-doped ZnS nanocrystals

This work explores the influence of post-synthesis annealing temperature and ambient on the structural and optical properties of Eu3+ ion doped in ZnS nanocrystals prepared via chemical precipitation route. Annealing was perform in nitrogen, sulfurizing and reducing ambient at three different temperatures (200 °C, 300 °C, and 400 °C) each. All the nanocrystals show predominantly the cubic phase of ZnS crystal structure without oxidation. Slight variations in lattice constant due to the substitution of Eu3+ at the Zn2+ site was observed especially in samples treated in reducing ambient. Energy transfer between ZnS and Eu3+ was observed. Photoluminescence emission intensity of doped nanocrystals treated at 200 °C was dominated by the dopant ion emission lines, while the defect bands dominated after annealing at 300 °C in reducing and sulfurizing the ambient. The strength of the emission intensities were in the increasing order of treatment ambient; sulfurizing > nitrogen > reducing at 200 °C and 300 °C and the reverse order at 400 °C. The combination of the defect related emission bands and Eu3+ emission lines gave white light spectrum which was confirmed using the CIE chromaticity coordinate. White light emission was obtained only when the samples were annealed at 300 °C in the three ambient. Excitation wavelength for white light emission is 395 nm, while the same samples produced red emission when excited at 465 nm. Both wavelengths are ideal for application with nUV and blue InGaN-based LEDs.

Study of the defects in La3Ta0.5Ga5.5O14 single crystals

Defects that are formed during crystal growth pose a serious obstacle for potential application of La3Ga5.5Ta0.5O14 (LGT) as a laser or piezoelectric crystal. We have performed the study of the defects origin in LGT crystals grown in different atmospheres using optical, EPR and time-resolved luminescence characterization methods. The absorption bands detected in the transparency region at 290, 360 and 490nm (T=300K) demonstrate different dependence on crystal annealing in vacuum and air. EPR analysis demonstrated that the defects responsible for these bands are non-paramagnetic. X-ray irradiation results in hole trapping by oxygen ions thus forming O− centers perturbed by neighboring defects. New arguments in favor of the existence of TaGa(2) antisite defects in LGT are presented. The absorption bands at 255 and 290nm are related to oxygen vacancies localized at the different sites of crystal lattice. The absorption band at 360nm is related to cation vacancies (VGa3−orVLa3−). Time-resolved luminescence spectroscopy allowed to identify excitation processes responsible for the defect-related emission bands.

Photoluminescence from c-axis oriented ZnO films synthesized by sol-gel with diethanolamine as chelating agent

ZnO films were synthesized on SiO2/Si substrates through the sol-gel technique using diethanolamine as chelating agent and annealed in Ar+O2 atmospheres with different O2 flow-rates in the 10–100sccm range. Samples were studied by scanning electron microscopy and X-ray diffraction, evidencing a nanostructured morphology with a preferential orientation along the (0 0 2) direction (c-axis orientation), which is uncommon when diethanolamine is used as the chelating agent. The room temperature photoluminescence spectra show strong UV emissions at around 375 and 384nm from near band-edge transitions and phonon replica, and a broad defect-related band extending from the visible to near infrared (∼500–800nm). The analysis of the defect-related emission band and its various components as a function of the O2 flow-rate is discussed in terms of contributions from specific luminescent point defect centers established during annealing.

Very slow decay of a defect related emission band at 2.4 eV in AlN: Signatures of the Si related shallow DX state

We report on a defect related luminescence band at 2.4 eV in aluminum nitride bulk crystals, for which we find strong indications to be related to silicon DX centers. Time resolved photoluminescence spectroscopy using a sub-bandgap excitation reveals two different recombination processes with very long decay times of 13 ms and 153 ms at low temperature. Based on the results of temperature and excitation dependent photoluminescence experiments, the process with the shorter lifetime is assigned to a donor-acceptor pair transition involving the shallow silicon donor state, which can be emptied with a thermal dissociation energy of 65 meV. The slower process with a thermal quenching energy of 15 meV is assigned to the slightly deeper Si DX state known from electron paramagnetic resonance experiments, which is transferred back to the shallow donor state.

Light emission enhancement based on surface plasmon resonance in zinc oxide/silver film on digital video disk template

ABSTRACTThe photoluminescence signals of zinc oxide/silver films based on digital video disk template were carried out compared with that of the planar mode. The spectra that on silicon showed a strong bandgap emission around 378 nm and a defect related emission band centered at ∼470 nm. After adding digital video disk template, it resulted in an increased signal of both the ultraviolet and the visible region, but the ultraviolet emission enhancement was notable. That the enhancement of band edge emission resulted from surface plasmon resonance coupling between silver and zinc oxide was discussed in detail.

Dependence of pyrolytic temperature on the growth of high quality MnO2 nanofibers

Growth of highly (101) oriented MnO2 nanofibers were demonstrated by spray pyrolysis technique for different temperatures. The influence of deposition temperature on the orientation, microstructure, morphology and optical properties of MnO2 films were discussed. Based on X-ray diffraction results, the growth resulted in a well-defined crystal structure with preferred (101) orientation for MnO2 films prepared at 300°C. The band gap decreased with increase in particle size, due to the effect of linear expansion with increase in temperature. The improved crystallinity of the MnO2 films revealed from increase in intensity of photoluminescence emission, with a substantially suppressed defect related emission band, established the development possibility of the nanofibers for biosensor devices. Scanning electron microscopy images show the uniform and well-aligned growth of MnO2 nanofibers at 300°C, which provides higher capture efficiency favorable for bio sensing application.

Synthesis, structural and optical properties of alloyed Ti(1−x)SnxO2 nanoparticles

Strain minimized Ti(1−x)SnxO2 (x=0.0, 0.25, 0.50, 0.75 and 1.0) nanoparticles were successfully synthesized by an efficient and cost effective solvothermal microwave irradiation (SMI) method. The average crystallite size value determined by XRD profile broadening and high resolution transmission electron microscopy is within the range of 4–9nm (±0.64nm). The morphologies and composition of synthesized nanocrystals were characterized by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX), respectively. The direct band gap values determined by UV–Vis. technique is within the range 3.358–3.777eV (±0.027eV). The photoluminescence (PL) spectra exhibit two prominent bands, which can be ascribed to the band-edge and defect-related emission bands. The optical emission is tuned from 460nm (2.699eV) to 521nm (2.381eV) with the increase of Sn composition in Ti(1−x)SnxO2 nanoparticles.