Dependent Photoluminescence Measurements Research Articles

Top-down approach for the preparation of Au/ZnO nanostructures by glancing-angle ion irradiation: Morphological, structural and optical studies

We report the 100 keV glancing-angle Xe+ ions irradiation based top down approach for the preparation of ZnO and Au/ZnO hybrid nanostructures and its morphological, structural and optical properties. FESEM micrographs reveal Xe+ ion irradiation favours the nucleation and formation of highly aligned, dense nanostrips like structures. Crystalline structure of ZnO, Xe+ ions irradiated ZnO and Au/ZnO hybrid nanostructures were analysed by grazing incident X-ray diffraction and Raman scattering measurements. The elemental compositions and thickness of the grown nanostructures were estimated using Rutherford backscattering spectrometry analysis. The influence of Xe+ ion irradiation on the photoluminescence (PL) behaviour of ZnO and Au/ZnO hybrid nanostructures were studied in detail using temperature dependent PL measurements in the range of 80–298 K. Strong PL emission associated with free excitons (FX) and its longitudinal optical phonon replicas observed in ion irradiated Au/ZnO hybrid nanostructures clearly reveal the strong coupling of the localized surface plasmons in the Au nanoparticles to the FX in ZnO. Our experimental results reveal ion beam irradiation is one of the effective approaches to tailor the optical properties of ZnO and Au/ZnO hybrid nanostructures.

Grain boundaries are not the source of Urbach tails in Cu(In,Ga)Se2 absorbers

The presence of Urbach tails in Cu(In,Ga)Se2 (CIGSe) absorbers has been identified as a limiting factor for the performance of the CIGSe solar cells. The tail states contribute to both radiative and non-radiative recombination processes, ultimately leading to a reduction in the open-circuit voltage and, consequently, decreasing the overall efficiency of CIGSe devices. Urbach tails result from structural and thermal disorders. The Urbach tails can be characterized by the Urbach energy, which is associated with the magnitude of the tail states. Within polycrystalline CIGSe absorbers, grain boundaries can be considered as structural disorder and, therefore, can potentially contribute to the Urbach tails. In fact, it has been proposed that the band bending at grain boundaries contribute significantly to the tail states. This study focuses on examining the correlation between Urbach tails and the band bending at the grain boundaries. The Urbach energies of the CIGSe samples are extracted from photoluminescence (PL) measurements, which reveal that the introduction of Sodium (Na) into the material can lead to a reduction in the Urbach energy, and an even further decrease can be achieved through the RbF post-deposition treatment. The band bending at the grain boundaries is investigated by Kelvin probe force microscopy measurements. A thorough statistical analysis of more than 340 grain boundaries does not show any correlation between Urbach tails and grain boundaries. We measure small band bending values at the grain boundaries, in the range of the thermal energy (26 meV at room temperature). Furthermore, our intensity dependent PL measurements indicate that Urbach tails are, at least in part, a result of electrostatic potential fluctuations. This supports the model that the introduction of alkali elements mainly decreases the magnitude of electrostatic potential fluctuations, resulting in a subsequent reduction in the Urbach energy.

Mid-wave infrared photoluminescence from low-temperature-grown PbSe epitaxial films on GaAs after rapid thermal annealing

We investigate the beneficial effects of rapid thermal annealing on structure and photoluminescence of PbSe thin films on GaAs (001) grown below 150 °C, with a goal of low temperature integration for infrared optoelectronics. Thin films of PbSe deposited on GaAs by molecular beam epitaxy are epitaxial at these reduced growth temperatures, yet the films are highly defective with a mosaic grain structure with low angle and dendritic boundaries following coalescence. Remarkably, we find that rapid thermal annealing for as short as 180 s at temperatures between 300 and 425 °C in nitrogen ambient leads to extensive re-crystallization and transformation of these grain boundaries. The annealing at the same time dramatically improves the band edge luminescence at 3.7 μm from previously undetectable levels to nearly half as intense as our best conventionally grown PbSe films at 300 °C. We show using an analysis of laser pump-power dependent photoluminescence measurements that this dramatic improvement in the photoluminescence intensity is due to a reduction in the trap-assisted recombination. However, we find it much less correlated with improved structural parameters determined by x-ray diffraction rocking curves, thereby pointing to the importance of eliminating point defects over extended defects. Overall, the success of rapid thermal annealing in improving the luminescent properties of low growth temperature PbSe is a step toward the integration of PbSe infrared optoelectronics in low thermal budget, back end of line compatible fabrication processes.

Structural and optical properties of micro-diamonds with SiV− color centers

Isolated, micro-metre sized diamonds are grown by micro-wave plasma chemical vapour deposition technique on Si(001) substrates. Each diamond is uniquely identified by markers milled in the Si substrate by Ga+ focused ion beam. The morphology and micro-grain structure analysis, indicates that the diamonds are icosahedral or bi-crystals. Icosahedral diamonds have higher (up to = 2.3 GPa), and wider distribution ( = 4.47 GPa) of hydrostatic stress built up at the micro-crystal grain boundaries, compared to the other crystals. The number and spectral shape of SiV− colour centres incorporated in the micro-diamonds (MDs) is analysed, and estimated by means of temperature dependent photoluminescence measurements, and Monte Carlo simulations. The Monte Carlo simulations indicates that the number of SiV− colour centres is a few thousand per MD.

Cellulose‐Assisted Formation of 2D Hybrid Halide Perovskite Nanocrystals with Enhanced Stability for Light‐Emitting Devices

AbstractA common approach to enhance the stability of metal halide perovskites (MHPs) implemented in optical and optoelectronic devices is to incorporate polymer additives into the perovskite layer. A β‐(1,4) cellulose oligosaccharide (COS) synthesized by mechanocatalytic depolymerization of cellulose has been incorporated to films of the BA2MA4Pb5I16 (BA = n‐butylammonium and MA = methylammonium) two‐dimensional (2D) Ruddlesden‐Popper (RP) hybrid perovskite. Scanning Electron Microscopy (SEM) images displayed a three‐fold reduction of the 2D RP perovskite grains (≈30–40 nm), close to the quantum confinement scale. The analysis of the dark J–V curves of single carrier devices by using the space charge limited current (SCLC) method resulted in a rise of the defect concentration. A notable 14‐fold increase in the photoluminescence (PL) signal at high COS content is detected. Moreover, the analysis of the temperature dependence PL measurements (80–300 K) resulted in a larger exciton binding energy, Eb = 180 to 370 meV, at high COS content. Light emitting diodes of the 2D RP perovskites (PeLEDs) are fabricated w/o the COS compound. The stability test performed under operation (5 V) displays 20 times higher operational lifetimes at high COS content while the luminance is also increased in the devices with the COS compound.

Formation of light-emitting defects in silicon by swift heavy ion irradiation and subsequent annealing

High-resistivity floating-zone silicon (FZ-Si) and n-type (4.5 Ohm·cm) Czochralski silicon (CZ-Si) wafers were irradiated at room temperature with 167 MeV Xe ions to fluences ranging from 5·1010 to 5·1011 cm−2. Temperature dependent photoluminescence (PL) measurements were performed to characterize the as-implanted and annealed silicon wafers. It was found that irradiation with swift heavy ions (SHI) leads to the formation of light-emitting defects in both types of silicon wafers. Low-temperature annealing (400 °C) leads to a significant increase of the photoluminescence intensity. In addition, the photoluminescence decreases for increasing measurement temperatures and eventually quenching is observed for temperatures larger than 80 K. The type of silicon wafer (FZ or CZ) does not significantly influence the defect-related light-emitting properties. This implies that low fluence SHI irradiation is a promising method for creation of light-emitting defects in silicon.

Deciphering the Role of Hole Transport Layer HOMO Level on the Open Circuit Voltage of Perovskite Solar Cells

AbstractWith the rapid development of perovskite solar cells, reducing losses in open‐circuit voltage (Voc) is a key issue in efforts to further improve device performance. Here it is focused on investigating the correlation between the highest occupied molecular orbital (HOMO) of device hole transport layers (HTLs) and device Voc. To achieve this, structurally similar HTL materials with comparable optical band gaps and doping levels, but distinctly different HOMO levels are employed. Using light‐intensity dependent Voc and photoluminescence measurements significant differences in the behavior of devices employing the two HTLs are highlighted. Light‐induced increase of quasi‐Fermi level splitting (ΔEF) in the perovskite layer results in interfacial quasi‐Fermi level bending required to align with the HOMO level of the HTL, resulting in the Voc measured at the contacts being smaller than the ΔEF in the perovskite. It is concluded that minimizing the energetic offset between HTLs and the perovskite active layer is of great importance to reduce non‐radiative recombination losses in perovskite solar cells with high Voc values that approach the radiative limit.

Plasmon-coupled sub-bandgap photoluminescence enhancement in ultra-wide bandgap CuO through hot-hole transfer

Ultra-wide bandgap oxide semiconductors are garnering widespread interest for numerous applications, including high-power electronics, deep-UV opto-electronics, quantum devices, and applications in hostile environments. CuO has recently been shown to be a good contender for ultra-wide bandgap oxide materials. CuO's ultra-wide bandgap property is still in its infancy as far as scientific understanding is concerned. Herein, sub-bandgap emission properties of ultra-wide bandgap CuO thin films, deposited using a facile and low-thermal budget solution-process have been systematically investigated using temperature-dependent photoluminescence measurements. Interestingly, near-infrared (NIR) emission has been observed in ultra-wide bandgap CuO with sub-bandgap (488 nm) excitation suggesting involvement of defect and tail states in emission properties. Further, with the incorporation of Au nanoparticles, four-fold enhancement in the NIR emission was observed. Temperature dependent photoluminescence measurements have suggested enhanced Purcell effect in Au and CuO. The enhanced NIR emission can be assigned concertedly to metal-enhanced fluorescence, plasmon-coupling with photoluminescence and hot-carrier transfer from the metal. Time-resolved photoluminescence measurements have shown clear change in the emission dynamics with Au coating (decay time changed from 7.6 ns to 3 ns). Further, systematic DFT studies have been performed to clearly understand the effect of defects on the emission and absorption properties of ultra-wide bandgap CuO. Finally, the effect of Au on other optical and electrical features has been studied using electron paramagnetic resonance measurements and current-voltage characteristics. This study will expand the understanding of the optical and electrical properties of ultra-wide bandgap CuO for better performing optoelectronic devices.

Interfacial Grown Zinc Oxide Origami Structure for Broadband Visible Light Driven Photocatalysts

AbstractA novel route to synthesize large area ZnO/Zn(OH)2 origami thin films at air/liquid interface is reported. ZnO materials derived from this route show the largest blue‐shifted band‐to‐band emission at 3.85 eV (3.37 eV in bulk ZnO) so far, resulting from the quantum confinement in the ultrathin ZnO/Zn(OH)2 nanosheets. Interband defect levels (IDLs) related broadband photoluminescence is observed from UV to red region by excitation wavelength dependent photoluminescence measurements. The ZnO origami structures show photocatalytic methylene blue degradation (MB‐degradation) activity even under sub‐bandgap visible light illumination from 405 nm to green region at 505 nm, measured by low power monochromatic light emitting diode light sources. From the consistent light absorption, luminescence, and MB‐degradation activity, it can be concluded that the broadband visible light MB‐degradation activity is attributed to photogenerated carriers via IDLs. The 2D ZnO‐nanosheets in the origami structure better solve the dilemma of the IDLs in bulk materials, i.e., they are beneficial for light absorption in visible range but detrimental for enhanced recombination loss of carriers, and they can be quickly extracted to the surface without substantial recombination. This finding makes the ZnO origami a promising candidate for efficient photocatalyst that harvesting the sunlight and for indoor applications.

Trapping the carrier in the spin-locked MoS2 atomic valley by absorption of chiral L-cysteine

This work demonstrates enhanced valley contrasting spin-momentum locked chiral states at the van der Waals interface of chiral L-cysteine and single layer (SL) MoS2 placed on a Si/SiO2 substrate at ambient conditions. Helicity dependent photoluminescence and resonance Raman measurements highlight spin-locked transitions for the chiral L-cysteine modified SL-MoS2 at ambient conditions. Selective adsorption of chiral L-cysteine dimer/cysteine stabilizes the in-plane effective magnetic field due to the Si/SiO2 substrate and blocks the intervalley spin relaxation. The observed polarization efficiency can be useful for improving the functionality of valley-based light emitting diodes and encoding information in logical devices.

Sb3+ -Er3+ -Codoped Cs2 NaInCl6 for Emitting Blue and Short-Wave Infrared Radiation.

Cs2 NaInCl6 double perovskite is stable, environmentally benign and easy to prepare. But it has a wide band gap (5.1 eV), and therefore, does not show optical and optoelectronic properties in the visible and short-wave infrared (SWIR) region. Here we introduce such functionalities in Cs2 NaInCl6 by codoping Sb3+ (s-electron doping) and Er3+ (f-electron doping) ions. Sb3+ doping introduces optically allowed 5s2 5s1 5p1 electronic absorption at the sub-band gap level, which then emits blue photoluminescence with ≈93 % quantum yield. But f-f electronic absorption of Er3+ is parity forbidden. Codoping Sb3+ -Er3+ , leads to transfer of excitation energy from Sb3+ to Er3+ , yielding SWIR emission at 1540 nm. Temperature (6 to 300 K) dependent photoluminescence measurements elucidate the excitation and emission mechanism. A phosphor converted light emitting diode (pc-LED) fabricated by using the codoped sample emits stable blue and SWIR radiation over prolonged (84 hours) operation at 5.1 V.

Fluorenone and triphenylamine based donor–acceptor–donor (D–A–D) for solution-processed organic light-emitting diodes

In this work, we describe the design and synthesis of a novel, organic emissive small donor–acceptor–donor molecule, 3,6-bis(4-(bis(4-methoxyphenyl)amino)phenyl)-9H-fluoren-9-one, and its incorporation into functional organic light-emitting diode (OLED) devices. The molecule was developed through a three-step procedure with high yields using low-cost and commercially available materials. Photophysical analysis shows emission from both locally excited (LE) and intramolecular charge transfer (ICT) fluorescent states, leading to broad emission spectra with a high solution photoluminescence quantum yield of 73%. Time resolved and temperature dependent photoluminescence measurements were utilized to obtain insights about emission mechanism from LE and ICT states. The presence of broad emission was further confirmed using density functional theory calculations. After optimal design and selection of the film thickness and host matrix, an OLED using a solution-processed host: dopant emissive layer with 10 wt% dopant exhibited a maximum current efficiency, power efficiency and external quantum efficiency of 2.6 cd A−1, 2.5 lm W−1 and 1.5%, respectively.

Sb3+–Er3+‐Codoped Cs2NaInCl6 for Emitting Blue and Short‐Wave Infrared Radiation

AbstractCs2NaInCl6 double perovskite is stable, environmentally benign and easy to prepare. But it has a wide band gap (5.1 eV), and therefore, does not show optical and optoelectronic properties in the visible and short‐wave infrared (SWIR) region. Here we introduce such functionalities in Cs2NaInCl6 by codoping Sb3+ (s‐electron doping) and Er3+ (f‐electron doping) ions. Sb3+ doping introduces optically allowed 5s2 5s15p1 electronic absorption at the sub‐band gap level, which then emits blue photoluminescence with ≈93 % quantum yield. But f‐f electronic absorption of Er3+ is parity forbidden. Codoping Sb3+–Er3+, leads to transfer of excitation energy from Sb3+ to Er3+, yielding SWIR emission at 1540 nm. Temperature (6 to 300 K) dependent photoluminescence measurements elucidate the excitation and emission mechanism. A phosphor converted light emitting diode (pc‐LED) fabricated by using the codoped sample emits stable blue and SWIR radiation over prolonged (84 hours) operation at 5.1 V.

Plasmon enhanced optical properties of Ag nanowire decorated conjugated polymer

The effect of Ag nanowires (NWs) on the optical properties of optoelectronic polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) have been systemically investigated. The decoration of Ag NWs with an average diameter of 160 nm leads to improved absorption and photoluminescence (PL) ability due to the coexistence of localized surface plasmons and propagating surface plasmons. Results of steady-state spectra exhibit 180% enhancement of absorption and 190% enhancement at ∼630 nm of PL. Temperature and excitation intensity dependent PL measurements demonstrate that Ag NWs can effectively suppress non-radiative recombination and enhance radiative recombination. Time-resolved PL measurement reveals the interaction between plasmons and interchain species plays a dominant role in the enhanced PL. These results will provide an effective way for further improving the optical properties of optoelectronic devices based on conjugated polymers.

Bright excitonic multiplexing mediated by dark exciton transition in two-dimensional TMDCs at room temperature.

2D-semiconductors with strong light-matter interaction are attractive materials for integrated and tunable optical devices. Here, we demonstrate room-temperature wavelength multiplexing of the two-primary bright excitonic channels (Ab-, Bb-) in monolayer transition metal dichalcogenides (TMDs) arising from a dark exciton mediated transition. We present how tuning dark excitons via an out-of-plane electric field cedes the system equilibrium from one excitonic channel to the other, encoding the field polarization into wavelength information. In addition, we demonstrate how such exciton multiplexing is dictated by thermal-scattering by performing temperature dependent photoluminescence measurements. Finally, we demonstrate experimentally and theoretically how excitonic mixing can explain preferable decay through dark states in MoX2 in comparison with WX2 monolayers. Such field polarization-based manipulation of excitonic transitions can pave the way for novel photonic device architectures.

Investigation of deep electronic levels in n‐type and p‐type 3C‐SiC using photoluminescence

Among the various SiC polytypes, cubic 3C‐SiC is much more difficult to grow in high crystalline quality than the commercially introduced hexagonal 6H‐SiC and 4H‐SiC counterparts. Besides some benefits of 3C‐SiC for transistor applications related to a greater electron mobility and a lower metal‐oxide‐semiconductor interface trap density compared to 4H‐SiC, new potential optoelectronic applications have been introduced very recently. Boron doped 3C‐SiC may act as an ideal candidate for an intermediate band (IB) solar cell material. Aluminum doped p‐type 3C‐SiC could lead to the development of efficient optoelectrochemical water splitting cells. Finally, 3C‐SiC with its various intrinsic point defects has been considered as a suitable candidate for future spintronic‐applications. All these applications will critically depend on further understanding defect behaviour on atomic level. In our study we investigated free standing n‐type and p‐type 3C‐SiC material grown in our lab. Temperature dependent photoluminescence measurements revealed the presence of carbon vacancy related VC and VC-CSi defect transitions in the p‐type materials but not in the n‐type materials. This observation present in as grown 3C-SiC is believed to have significant impact on the optoelectronic applications. Copyright © 2017 VBRI Press.

Study of color centers in radiation-modified diamonds

We report on the optical properties of He-related color centers created by He-ion implantation and subsequent thermal annealing in natural diamonds, including the temperature (300–700 K) and excitation power (1–1800 kW/cm2)-dependent photoluminescence (PL) measurements. The prospects for the use of He-implanted diamonds for temperature sensing are discussed. The effect of fast neutron irradiation on the optical properties of Si-V color centers in CVD diamonds were also examined.

Photoluminescence study of solution-deposited Cu2BaSnS4 thin films

To experimentally identify the character of radiative transitions in trigonal Cu2BaSnS4, we conduct temperature and excitation intensity dependent photoluminescence (PL) measurements in the temperature range of 15–300 K. The low-temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon-assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor–acceptor-pair recombination and the weak broad emission at 1.6 eV to the free-to-bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonons.

Optical Properties from a Single GaAs Quantum Dot Structure.

We observed exciton and biexciton states in a single GaAs quantum dot at 4 K using micro pho-toluminescence system and investigated power dependent photoluminescence measurements to identify both exciton and biexciton states. The biexciton and exciton states showed quadratic (a~2.2) and linear (a~0.95) increasing power factor, respectively. The large energy difference (~0.2 meV) from exciton states for the perpendicular polarization was observed.

The effect of thermal annealing on the optical properties of Mg-doped zincblende GaN epilayers

The effects of thermal annealing on the optical properties of Mg-doped cubic zincblende GaN epilayers grown by metalorganic chemical vapor deposition on 3C-SiC/Si (001) substrates are investigated. The photoluminescence spectra show near band edge features and a blue luminescence band that depend on Mg concentration, temperature, and excitation power density. Annealing the sample in a N2 atmosphere causes the intensity of the blue band to increase by a factor of 5. Power dependent photoluminescence measurements show a reduction in the laser excitation density required for saturation of the blue band after annealing, indicating an increase in the recombination lifetime. Time decay measurements confirm this increase, which is attributed to a reduction in the concentration of non-radiative defects after annealing. The results presented here are compared to those reported previously for Mg-doped hexagonal wurtzite GaN.