Defect Emission Research Articles

Direct Synthesis and Characterization of Hydrophilic Cu-Deficient Copper Indium Sulfide Quantum Dots.

Copper indium sulfide (CIS) nanocrystals constitute a promising alternative to cadmium- and lead-containing nanoparticles. We report a synthetic method that yields hydrophilic, core-only CIS quantum dots, exhibiting size-dependent, copper-deficient composition and optical properties that are suitable for direct coupling to biomolecules and nonradiative energy transfer applications. To assist such applications, we complemented previous studies covering the femtosecond-picosecond time scale with the investigation of slower radiative and nonradiative processes on the nanosecond time scale, using both time-resolved emission and transient absorption. As expected for core particles, relaxation occurs mainly nonradiatively, resulting in low, size-dependent photoluminescence quantum yield. The nonradiative relaxation from the first excited band is wavelength-dependent with lifetimes between 25 and 150 ns, reflecting the size distribution of the particles. Approximately constant lifetimes of around 65 ns were observed for nonradiative relaxation from the defect states at lower energies. The photoluminescence exhibited a large Stokes shift. The band gap emission decays on the order of 10 ns, while the defect emission is further red-shifted, and the lifetimes are on the order of 100 ns. Both sets of radiative lifetimes are wavelength-dependent, increasing toward longer wavelengths. Despite the low radiative quantum yield, the aqueous solubility and long lifetimes of the defect states are compatible with the proposed role of CIS quantum dots as excitation energy donors to biological molecules.

Decisive role of preparation technique on the structural, electrical and magnetic properties of vanadium doped ZnO nanoparticles

1% vanadium doped ZnO powders have been successfully synthesized via two different preparation techniques viz., glycine assisted combustion method and mechanical attrition method to understand their impact on the structure-property relation. X-ray Diffraction study validates the Wurtzite phase formation of ZnO with hexagonal structure. X-ray diffraction peak discloses the marked role of size and strain component. Evidently, the shift of lattice strain with the preparation techniques from compressive to tensile nature show the importance of structure property relations in the ZnO compounds. SEM micrographs depict the formation of densely populated nanograins with networking like structures which is characteristics of combustion synthesized product whereas as top down approach exhibits the presence of coarse grains amidst the broken sea of fine particles. Selected area electron diffraction (SAED) images display ring and bright circular spot pattern with changes in method of preparation, which further clarifies the distinct nature of investigated ZnO:V particles. Vibrational spectral study gives clear evidence on the structural disorder as deduced from the changes associated with longitudinal optical mode phonon scattering. Interestingly, the UV spectral study divulges the increment in optical band gap as well as a strong visible light absorption (about 30 %) for milled ZnO:V over their chemically prepared counterparts. Photoluminescence spectra of milled V doped ZnO upheld the structural disorder taken place with the suppression of Near Band Edge (NBE) transition and the shifting of broad asymmetric defect emission band towards the lower wavelength regime. Impedance study enlightens the crucial role of synthesis method with 5 times enhanced grain boundary resistance along with suppression of grain contribution in the milled samples. Magnetic study highlights the remarkable distinction of preparation route which alters the ferromagnetic ordering in the combustion synthesized samples to conventional diamagnetic signature of the host ZnO in the milled samples.

Modulating ultrafast carrier dynamics behavior via vacancy engineering of ReSe2 with Se vacancy for efficient electrochemical activity

Vacancy engineering is powerful for manipulating material properties. Selenium vacancies (VSe) exert significantly impact on the physicochemical properties of rhenium diselenide (ReSe2), triggering applications in catalysis and photoelectric devices. However, the effects on electronic structure, carrier lifetime, and electrochemical activity are inadequately explored and poorly elucidated. Herein, hydrothermal-controlled growth of ReSe2 with VSe is synthesized by different strategies. The ethanol-assisted hydrothermal method can adjust VSe concentration of ReSe2 on carbon paper by time parameters. Mediated excitonic effects were characterized in the transient absorption spectroscopy and photoluminescence of VSe-regulated ReSe2. The defect level produces unique defect emission peak, and the VSe-mediated exciton dynamics problem was discussed. The transient absorption result shows that high VSe concentration can optimize carrier lifetime, carrier separation efficiency, and electron utilization. The electrocatalytic hydrogen evolution reaction of ReSe2 catalyst revealed the electrochemical activity regulation by VSe concentration, which provides ideas for enhancing catalyst performance. Density functional theory result confirms the role of selenium vacancies in the regulation of the electronic structure of ReSe2. This work proffers an optimal solution strategy for hydrothermal preparation of ReSe2, and demonstrates the feasibility of regulating electrochemical activity by vacancy engineering. The excellent performance is beneficial for water cracking and physical devices.

Zinc Dopant-Induced Modulation of Electronic Structure and Defect Emissions in Monoclinic Gallium Oxide

Zinc Dopant-Induced Modulation of Electronic Structure and Defect Emissions in Monoclinic Gallium Oxide

Exceptionally strong coupling of defect emission in hexagonal boron nitride to stacking sequences

Van der Waals structures present a unique opportunity for tailoring material interfaces and integrating photonic functionalities. By precisely manipulating the twist angle and stacking sequences, it is possible to elegantly tune and functionalize the electronic and optical properties of layered van der Waals structures. Among these materials, two-dimensional hexagonal boron nitride (hBN) stands out for its remarkable optical properties and wide band gap, making it a promising host for solid state single photon emitters at room temperature. Previous investigations have demonstrated the observation of bright single photon emission in hBN across a wide range of wavelengths. In this study, we unveil an application of van der Waals technology in modulating their spectral shapes and brightness by carefully controlling the stacking sequences and polytypes. Our theoretical analysis reveals remarkably large variations in the Huang-Rhys factors–an indicator of the interaction between a defect and its surrounding lattice–reaching up to a factor of 3.3 for the same defect in different stacking sequences. We provide insights into the underlying mechanism behind these variations, shedding light on the design principles necessary to achieve rational and precise control of defect emission. This work paves the way for enhancing defect identification and facilitating the engineering of highly efficient single photon sources and qubits using van der Waals materials.

Optical, electrical and thermal investigations on brucinium di-hydrogen citrate tri-hydrate single crystal: An optimistic tool for microelectronics, OPO and OLED applications

Highly capable nonlinear optical brucinium di-hydrogen citrate tri-hydrate (BDHCTH) single crystal under organic material category has been grown-up successfully by slow evaporation method. Optical, electrical, and thermal investigations were made on title crystal to discover the suitability of fabrication of an optical as well as electrical device for the first time. Crystal structure analysis confirms that, the BDHCTH crystal belongs to triclinic crystal system with non-centro symmetric space group P1. The SLDT value of BDHCTH crystal was found to be 4.22 GW/cm2. Second harmonic generation efficiency of the BDHCTH compound is 1.08 times superior to the reference sample KDP. An attribution of single and sharp peak with lesser full width half maximum value confirms the presence of very fewer defects in the grown crystal. An electric response of the title compound shows the normal dielectric behavior. The piezoelectric charge co-efficient (d33) was found to be 4.5 p C/N. Photo luminescence study confirms that the synthesized sample possess violet, blue and red emission behaviors. Presences of various functional groups in the title sample were identified by FT-IR study. Thermal stability of the compound BDHCTH was found to be 135 °C. Kinetic parameters such as activation energy, Arrhenius factor, standard entropy of activation, standard enthalpy of activation, standard Gibb's energy of activation (ΔG*) and induction time of the grown sample were found to be 161.6396 k J mol−1, 7.4854 s−1, -230.8278 J K mol−1, 154.854 k J mol−1, 249.0314 k J mol−1 and 52 days respectively. At the end of an optical, electrical and thermal analysis, we concluded that the grown crystal is an optimistic tool for microelectronics, OPO and OLED applications due to existence of good SHG value, lesser defect and red emission behavior.

Selective Generation of Luminescent Defects in Hexagonal Boron Nitride

AbstractSingle photon emitters from atomic defects in crystals like hexagonal boron nitride (hBN) are vital for quantum technologies. Although various techniques are devised to obtain defects emission in hBN, simultaneous control over defects position, type, and emission spectrum has not been achieved yet. Here, ion implantation with 12C, 20Ne, and 69Ga are used to create a composite defects population with emission ≈820 nm. The correlation of Raman and photoluminescence (PL) spectroscopy helps to identify the defects’ type. After selecting Ga as the ion species yielding the maximum emitter brightness, a strategy based on thermal annealing is developed to modify the composition of the induced defects. This results in an emitter ensemble with selected spectral properties, even when starting from different implantation conditions. Specifically, thermal annealing induces a defect transmutation from one type to another, shifting the emission wavelength from 820 to 625 nm. Moreover, sample patterning is combined with focused ion beam implantation and subsequent annealing in an efficient method to deterministically set the defects position as well as the PL spectral composition. These results offer a practical avenue to achieve in situ positioning and tuning of ensembles of emitters in hBN, promising for quantum information and sensing applications.

Ratiometric fluorescent sensing of pyrophosphate with sp³-functionalized single-walled carbon nanotubes

Inorganic pyrophosphate is a key molecule in many biological processes from DNA synthesis to cell metabolism. Here we introduce sp3-functionalized (6,5) single-walled carbon nanotubes (SWNTs) with red-shifted defect emission as near-infrared luminescent probes for the optical detection and quantification of inorganic pyrophosphate. The sensing scheme is based on the immobilization of Cu2+ ions on the SWNT surface promoted by coordination to covalently attached aryl alkyne groups and a triazole complex. The presence of Cu2+ ions on the SWNT surface causes fluorescence quenching via photoinduced electron transfer, which is reversed by copper-complexing analytes such as pyrophosphate. The differences in the fluorescence response of sp3-defect to pristine nanotube emission enables reproducible ratiometric measurements in a wide concentration window. Biocompatible, phospholipid-polyethylene glycol-coated SWNTs with such sp3 defects are employed for the detection of pyrophosphate in cell lysate and for monitoring the progress of DNA synthesis in a polymerase chain reaction. This robust ratiometric and near-infrared luminescent probe for pyrophosphate may serve as a starting point for the rational design of nanotube-based biosensors.

Investigating the effects of varying sulfur concentration on ZnSxSe1-x (0 ≤ x ≤ 1.0) thin films prepared by photo-assisted chemical bath method

Zinc sulfur-selenide (ZnSxSe1-x) (0 ≤ x ≤ 1.0) thin films were grown on glass substrates using photo-assisted chemical bath deposition technique and the deposited samples were annealed in an open-air furnace at 250 ℃ for 3 h. X-ray diffraction analysis revealed a phase transition from hexagonal ZnSe to cubic ZnS as x changes from 0 to 1.0. Raman scattering showed two longitudinal optical modes due to ZnSe structure. The effects of S and Se ions variation is observed by the change in surface morphology from nanoflakes to spherical particles as x increased. The presence of the expected elementary composition was confirmed by the energy dispersive x-ray spectroscopy. The bandgap of the films was found to increase from 2.68 to 3.50 eV with a corresponding decrease in crystallite size (11.9 – 7.1 nm) as x varied from 0 to 1.0. Photoluminescence excitation and emission showed defect emission peaks ascribed to intrinsic and extrinsic deep level defects such as Zn2+ or S2- ions vacancies and interstitials. The emission colours were tuned from red to yellowish-green with increased x values as confirmed from Commission Internationale de L’Eclairage analysis. The tunability of the ZnSxSe1-x structural, optical and morphological properties makes it a good candidate for optoelectronic applications.

Faster photoswitching and amplified NBE emission from mg doped sol-gel derived ZnO thin films

ABSTRACT Photoconductivity and photoluminescence of highly transparent predominantly c-axis oriented hexagonal wurtzite nanocrystalline Mg doped zinc oxide thin films are reported here. These films prepared by sol-gel spin coating are useful for switching and UV detector applications. The increase in dopant concentration tunes the optical band-gap from 3.15 to 3.41 eV. Amplified NBE emission with low defect emission is obtained in appropriately doped sample along with high suitability for switching applications since the photocurrent sharply falls to a very low value as the illumination is put off and the rise and decay pattern is reproducible for several consecutive cycles. The films have crystallite size less than 16 nm and rod-like structures of diameter 50 to 100 nm are seen to grow out of spherical granular background. Doping reduces the size of grains and rods as well as the surface roughness.

Leveraging plasmonic hot electrons to quench defect emission in metal-semiconductor nanostructured hybrids.

Modeling light-matter interactions in hybrid plasmonic materials is vital to their widening relevance from optoelectronics to photocatalysis. Here, we explore photoluminescence (PL) from ZnO nanorods (ZNRs) embedded with gold nanoparticles (Au NPs). A progressive increase in Au NP concentration introduces significant structural disorder and defects in ZNRs, which paradoxically quenches defect related visible PL while intensifying the near band edge (NBE) emission. Under UV excitation, the simulated semi-classical model realizes PL from ZnO with sub-bandgap defect states, eliciting visible emissions that are absorbed by Au NPs to generate a non-equilibrium hot carrier distribution. The photo-stimulated hot carriers, transferred to ZnO, substantially modify its steady-state luminescence, reducing NBE emission lifetime and altering the abundance of ionized defect states, finally reducing visible emission. The simulations show that the change in the interfacial band bending at the Au-ZnO interface under optical illumination facilitates charge transfer between the components. This work provides a general foundation to observe and model the hot carrier dynamics and strong light-matter interactions in hybrid plasmonic systems.

Effect of annealing temperature on the structure and optical properties of ZnO thin films

The effect of annealing temperature on the microstructure, defects and optical properties of ZnO thin films are investigated using sol–gel based spin coating method for a range of annealing temperatures from 200 ∘C to 500 ∘C. The correlation among the microstructure, defects, impurity content and the optical band gap of films of thickness about 10–12 nm is elucidated. The particle size increases and the optical band gap reduces with the annealing temperature. At 200 ∘C, amorphous films were formed with particle size less than 10 nm with an optical band gap of about 3.41 eV. As the temperature increases the grain size increases and the defect, impurity content as well as the optical band gap reduces. This could be due to the reduction in the lattice strain. For an average grain size of about 35 nm and above, the band gap asymptotically approaches the theoretical value of ZnO (3.37 eV). The photoluminescence (PL) spectra show a systematic red-shift in the excitonic levels corresponding to the variation in the optical band-gap. The defect emission from Zn-vacancies is observed in the PL spectra and are further supported by the positron annihilation measurements.

Identifying Luminescent Boron Vacancies in h-BN Generated Using Controlled He+ Ion Irradiation.

The defect emission from h-BN at 1.55 eV is interesting as it enables optical readout of spins. It is necessary to identify the nature of the relevant point defects for its controlled introduction. However, it is challenging to engineer point defects in h-BN without changing the local atomic structure. Here, we controllably introduce boron vacancies in h-BN using an ultrahigh spatial resolution and low-energy He+ ion beam. By optimizing the He+ ion irradiation conditions, we control the quantity and location of defects spatially and along the depth of h-BN to achieve a robust photoluminescence emission at 1.55 eV from 10 K to room temperature. We show that as-generated defects activate an additional Raman mode at 1295 cm-1. Electron energy loss spectroscopy confirms introduction of boron vacancies without modification of the local h-BN crystal structure. Our results provide a deterministic strategy to create scalable boron vacancy emitters in h-BN for quantum photonics.

Tuning phase and photoluminescent properties of ZrO2:Eu3+ coatings formed by plasma electrolytic oxidation and Judd-Ofelt analysis of composite materials

Eu3+ doped Zirconia coatings were synthesized by the electrochemical plasma electrolytic oxidation method in only 8 min from the pure zirconium substrate. The phase constitution from pure monoclinic to pure tetragonal and exactly in between was achieved by using different concentrations of Na3PO4 and NaAlO2 electrolytes. The complex emission spectra composed of Eu2+, Eu3+, and ZrO2 defect emission greatly depend on the excitation wavelength and phase constitution. Eu3+ photoluminescence properties depend on the phase, which is reflected by the different Stark splitting and different intensities of transitions. Thus, both phase and photoluminescent properties of ZrO2 coatings are fine-tuneable. Judd-Ofelt analysis was performed from the emission and excitation spectra, showing that all 3 intensity parameters depend linearly on the phase constitution, being largest in the pure monoclinic phase. The mixed phase has Judd-Ofelt parameters between those in pure phases. Eu3+ has 2.45 times more preference to get incorporated into the tetragonal than in the monoclinic phase in the mixed phase samples. The general equation for Judd-Ofelt parameters in a compound with mixed constituents and probability of incorporation is introduced, allowing also for a prediction of the spectrum shape based on the Judd-Ofelt parameters of pure-phase compounds. Python software code for estimating the percent of incorporation by multiple linear regression model is also provided.

Hydrothermally grown molybdenum doped ZnO nanorod arrays. The concept of novel ultrafast nanoscintillator

Molybdenum doped ZnO was hydrothermally grown as the arrays of nanorods deposited onto the fused silica glass substrate. The molybdenum doping level varied from 1 to 30 %. The influence of Mo on the electronic and crystalline structure as well as luminescence and defects in the ZnO nanorods is under study of the bundle of experimental techniques complemented with density functional theory calculations. The tendency of Mo to create energy states within the bandgap of ZnO and their influence on the energy levels of native defects as well as excitons were proven by the synergy of experiment and theory. The improvement of the timing characteristics of the exciton- and zinc vacancy-related emission bands upon Mo doping (1–10 %) was observed. This paves the way for the defect engineering strategy in the search of effective and ultrafast scintillator with the improved light yield as well as compared to other materials. The new concept is based on the combination of the exciton and the defect emission. It is expected to have the potential of application in the detection of gamma rays implemented in time-of-flight positron emission tomography (TOFPET). The 20 and 30 % Mo doping levels resulted in the zinc molybdates creation strongly outnumbering ZnO nanorods.

Efficient 2D to 0D Energy Transfer in HgTe Nanoplatelet-Quantum Dot Heterostructures through High-Speed Exciton Diffusion.

Large area absorbers with localized defect emission are of interest for energy concentration via the antenna effect. Transfer between 2D and 0D quantum-confined structures is advantageous as it affords maximal lateral area antennas with continuously tunable emission. We report the quantum efficiency of energy transfer in in situ grown HgTe nanoplatelet (NPL)/quantum dot (QD) heterostructures to be near unity (>85%), while energy transfer in separately synthesized and well separated solutions of HgTe NPLs to QDs only reaches 47 ± 11% at considerably higher QD concentrations. Using Kinetic Monte Carlo simulations, we estimate an exciton diffusion constant of 1-10 cm2/s in HgTe NPLs, the same magnitude as that of 2D semiconductors. We also simulate in-solution energy transfer between NPLs and QDs, recovering an R-4 dependence consistent with 2D-0D near-field energy transfer even in randomly distributed NPL/QD mixtures. This highlights the advantage of NPLs 2D morphology and the efficiency of NPL/QD heterostructures and mixtures for energy harvesting.

Correlations between reverse bias leakage current, cathodoluminescence intensity and carbon vacancy observed in 4H-SiC junction barrier Schottky diode

Reverse bias currents of ten commercial junction barrier Schottky diodes were measured, and the dies were studied by scanning electron microscope (SEM) and cathodoluminescence (CL) after the de-capsulation of the diodes. Defect emissions (DEs) of 2.62 eV were observed in all the CL spectra. By comparing the SEM images, the integral CL intensity spatial mappings and the reverse bias leakage currents, correlations between the leakage current, the integral CL intensity and the Al-implantation process were established. The data of reverse bias leakage current against the reverse bias voltage taken at room temperature followed the Poole Frenkel emission from the Z 1/Z 2 carbon vacancy states to the conduction band. The DE at 2.62 eV is associated with the electronic transition from Z 1/Z 2 to the valence band. The current observation also opens up the feasibility of screening off SiC diodes with large leakage current during production by inspecting the CL intensity before the device fabrication is complete.

Defect Emission and Its Dipole Orientation in Layered Ternary Znln2 S4 Semiconductor.

Defect engineering is promising to tailor the physical properties of 2D semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors are not clarified. Here, the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2 S4 is thoroughly studied via versatile spectroscopic tools in combination with theoretical calculations. It is demonstrated that the Zn-In antistructural defects induce the formation of a series of donor and acceptor energy levels and sulfur vacancies induce donor energy levels, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair in Znln2 S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef ). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2 S4 is directly revealed by back focal plane imagining, where it presents obviously in-plane dipole orientation within a dozen-layer thickness of Znln2 S4 . These unique features of defects in Znln2 S4 including extrinsic absorption, rich recombination paths, gate tunability, and in-plane dipole orientation are definitely a benefit to the advanced orientation-functional optoelectronic applications.

Energy transfer dynamics between carbon quantum dots and molybdenum disulfide revealed by transient absorption spectroscopy

Zero-dimensional environmentally friendly carbon quantum dots (CQDs) combined with two-dimensional materials have a wide range of applications in optoelectronic devices. We combined steady-state and transient absorption spectroscopies to study the energy transfer dynamics between CQDs and molybdenum disulfide (MoS2). Transient absorption plots showed photoinduced absorption and stimulated emission features, which involved the intrinsic and defect states of CQDs. Adding MoS2 to CQDs solution, the lowest unoccupied molecular orbital of CQDs transferred energy to MoS2, which quenched the intrinsic emission at 390 nm. With addition of MoS2, CQD-MoS2 composites quenched defect emission at 490 nm and upward absorption, which originated from another energy transfer from the defect state. Two energy transfer paths between CQDs and MoS2 were efficiently manipulated by changing the concentration of MoS2, which laid a foundation for improving device performance.

Modulating the Properties of SnO2 Thin Film by Post-Deposition UV-Ozone Treatment

AbstractTin(IV) oxide (SnO2) is a metal oxide renowned for its excellent optoelectronic properties. With the use of simple post-processing methods, the characteristics of SnO2 may be easily modified. In the current work, SnO2 thin films were prepared using the spray pyrolysis technique and were subjected to post-UV-ozone (UVO) treatment for different durations. Characterization techniques including x-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive spectroscopy, UV–visible spectroscopy, and photoluminescence spectroscopy were employed to assess the effects of UVO treatment. It was found that UVO treatment had no significant impact on the film's structural characteristics. However, after exposure to UVO, the bandgap was seen to decrease from 3.04 eV to 2.84 eV. Also, photoluminescence investigations revealed that UVO treatment increased the defects in the films with a decrease in the ratio between band-to-band emission and defect emissions. The results indicate that UVO treatment is an effective strategy for tuning the optical properties of SnO2 thin films by precisely managing the bandgap.