Defect-induced Emission Research Articles

Inverted Red Quantum Dot Light-Emitting Diodes with ZnO Nanoparticles Synthesized Using Zinc Acetate Dihydrate and Potassium Hydroxide in Open and Closed Systems

We developed inverted red quantum dot light-emitting diodes (QLEDs) with ZnO nanoparticles synthesized in open and closed systems. Wurtzite-structured ZnO nanoparticles were synthesized using potassium hydroxide and zinc acetate dihydrate at various temperatures in the open and closed systems. The particle size increases with increasing synthesis temperature. The ZnO nanoparticles synthesized at 50, 60, and 70 °C in the closed system have an average particle size of 3.2, 4.0, and 5.4 nm, respectively. The particle size is larger in the open system compared to the closed system as the methanol solvent evaporates during the synthesis process. The surface defect-induced emission in ZnO nanoparticles shifts to a longer wavelength and the emission intensity decreases as the synthesis temperature increases. The inverted red QLEDs were fabricated with a synthesized ZnO nanoparticle electron transport layer. The driving voltage of the inverted QLEDs decreases as the synthesis temperature increases. The current efficiency is higher in the inverted red QLEDs with the ZnO nanoparticles synthesized in the closed system compared to the devices with the nanoparticles synthesized in the open system. The device with the ZnO nanoparticles synthesized at 60 °C in the closed system exhibits the maximum current efficiency of 5.8 cd/A.

Defect-Free, Few-Atomic-Layer Thin ZnO Nanosheets with Superior Excitonic Properties for Optoelectronic Devices.

Two-dimensional (2D) wide bandgap materials are gaining significant interest for next-generation optoelectronic devices. However, fabricating electronic-grade 2D nanosheets from non-van der Waals (n-vdW) oxide semiconductors poses a great challenge due to their stronger interlayer coupling compared with vdW crystals. This strong coupling typically introduces defects during exfoliation, impairing the optoelectronic properties. Herein, we report the liquid-phase exfoliation of few-atomic-layer thin, defect-free, free-standing ZnO nanosheets. These micron-sized, ultrathin ZnO structures exhibit three different orientations aligned along both the polar c-plane as well as the nonpolar a- and m-planes. The superior crystalline quality of the ZnO nanosheets is validated through comprehensive characterization techniques. This result is supported by density functional theory (DFT) calculations, which reveals that the formation of oxygen vacancies is energetically less favorable in 2D ZnO and that the c-plane loses its polarity upon exfoliation. Unlike bulk ZnO, which is typically dominated by defect-induced emission, the exfoliated nanosheets exhibit a strong, ambient-stable excitonic UV emission. We further demonstrate the utility of solution processing of ZnO nanosheets by their hybrid integration with organic components to produce stable light emitting diodes (LEDs) for display applications.

Lattice dynamics and electron-phonon interaction in lead-free perovskite-inspired materials with unexpected broadband NIR emission

Air-stable and Sb/Bi-based perovskite-inspired materials are attractive substitutes for their Pb-based counterparts in optoelectronic applications. The attractive low-energy broadband luminescence phenomenon has been hotly investigated. However, understanding the physical mechanism behind it is still at an early stage. Herein, high-quality Cs3SbBiI9 crystals were synthesized and accompanied with reference Cs3Sb2I9 and Cs3Bi2I9. The reformed energy band structure can be confirmed by experiment and density functional theory (DFT) calculation. Unexpected broadband near-infrared (NIR) emission has been discovered and is explained by the nonexclusive self-trapped exciton (STE) and defect-induced emission models. Temperature- dependent Raman and PL spectra confirm the diminished Stokes shift and electron-phonon coupling effect in Sb-Bi alloyed crystals driven by the local broken-symmetry. This result provides a basic physical explanation for the controversial photo-physical process in defect-type Bi/Sb alloyed perovskite-inspired family and it will inspire a feasible way to design novel Pb-free perovskites.

An ab initio modelling for pristine and defect-induced emissions from NaCe(WO4)2: a potential material for solid-state lighting application

An ab initio modelling for pristine and defect-induced emissions from NaCe(WO4)2: a potential material for solid-state lighting application

(Invited) Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects

Individual single-walled carbon nanotubes with covalent sidewall defects have emerged as a new class of photon sources whose photoluminescence spectra can be tailored by the carbon nanotube chirality and the attached functional group/molecule. Here we present electroluminescence spectroscopy data from single-tube devices based on (7,5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes. We observe electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes. The defect-induced emissions are assigned to excitonic and trionic recombination processes by correlating electroluminescence excitation maps with electrical transport and photoluminescence data. At cryogenic conditions additional, gate-dependent emission lines appear which are assigned to phonon-assisted hot-exciton electroluminescence from quasi-levels. Similar results were obtained with functionalized (6,5) nanotubes. We also compare functionalized (7,5) electroluminescence data with photoluminescence of pristine and functionalized (7,5) nanotubes redox-doped using gold(III) chloride solution. This work shows that electroluminescence excitation is selective toward neutral defect-state configurations with the lowest transition energy, which in combination with gate-control over neutral versus charged defect-state emission leads to high spectral purity.Reference: Min-Ken Li et al., ACS Nano 2022 (doi: 10.1021/acsnano.2c03083) Figure 1

Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects.

Individual single-walled carbon nanotubes with covalent sidewall defects have emerged as a class of photon sources whose photoluminescence spectra can be tailored by the carbon nanotube chirality and the attached functional group/molecule. Here we present electroluminescence spectroscopy data from single-tube devices based on (7, 5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes. We observe electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes. The defect-induced emissions are assigned to excitonic and trionic recombination processes by correlating electroluminescence excitation maps with electrical transport and photoluminescence data. At cryogenic conditions, additional gate-dependent emission lines appear, which are assigned to phonon-assisted hot-exciton electroluminescence from quasi-levels. Similar results were obtained with functionalized (6, 5) nanotubes. We also compare functionalized (7, 5) electroluminescence data with photoluminescence of pristine and functionalized (7, 5) nanotubes redox-doped using gold(III) chloride solution. This work shows that electroluminescence excitation is selective toward neutral defect-state configurations with the lowest transition energy, which in combination with gate-control over neutral versus charged defect-state emission leads to high spectral purity.

Low lead migration 0D Cs4PbBr6 nanocrystal glass with super stability as a new member of the luminous family

In this study, by adjusting the cesium carbonate concentration and tuning the heat treatment temperature, zero dimensional (0D) Cs4PbBr6 perovskite nanocrystals (NCs) with different sizes were encapsulated into glass. The resultant Cs4PbBr6 NC glass exhibited strong green emission with a narrow full width at half maximum (FWHM, ~ 20 nm) and superior water resistance, which is a novel addition to the literature. In addition, Cs4PbBr6 NC glass exhibited a unique quantum behavior that could render it applicable in a novel type of optoelectronic device. Furthermore, a white-light-emitting diode (WLED) with color coordinates of (0.325, 0.331) and good long-term stability was successfully fabricated. The prepared Cs4PbBr6 NC glass also achieved up-conversion luminescence. Compared with CsPbX3 materials, the Cs4PbBr6 NC glass displayed unique optoelectronic properties that were attributed to the defect-induced emission, thereby advancing the study of these materials.

A defect-induced emission material with turn-on mechanoresponsive luminescence serving as a data storage

Most of the mechanoresponsive luminescent (MRL) materials function based on phase transition (crystalline-to-amorphous or crystalline-to-crystalline) or solid-state chemical reaction of the organic solid emitters. However, it is difficult to enable a turn-on type mechanoresponsive luminescence using such strategy. In this work, the defect-induced emission (DIE) of (E)-(4-((2-hydroxy-4-methoxybenzylidene)amino)phenyl)(phenyl) methanone (HMPM) was disclosed, which endows the resulting material with reversible turn-on MRL. The crystalline powder of HMPM is non-emissive, but the emission intensity can be greatly enhanced by external mechanical stimuli, showing high contrast. In the semiquantitative experiment, pressure up to 1.78 N would generate crystal defects, and further induces DIE. Crystallographic data demonstrate that the dark-state stems from the closely-connected plane-style aggregation of neighboring molecules, which allows the intermolecular orbital-overlapping to the disadvantage of radiative transition. Mechanical stimuli would destroy the plane-style aggregation to block intermolecular orbital overlap and the possible energy transfer, resulting in emission-enhancement. Taking advantage of the DIE nature, an information storage film is fabricated, capable of data-writing via mechanical force and data-erasing by fumigation treatment, showing rewritability as well as high signal contrast.

Vacancy-Induced Visible Light-Driven Fluorescence in Toxic Ion-Free Resorbable Magnetic Calcium Phosphates for Cell Imaging Applications.

Multifunctional nanosized particles are very beneficial in the field of biomedicine. Bioactive and highly biocompatible calcium phosphate (CaP) nanoparticles (∼50 nm) exhibiting both superparamagnetic and fluorescence properties were synthesized by incorporating dual ions (Fe3+ and Sr2+) in HAp (hydroxyapatite) [Ca10(PO4)6(OH)2]. Insertion of Fe3+ creates oxygen vacancies at the PO43- site, thereby destabilizing the structure. Thus, in order to maintain the structural stability, Sr2+ has been incorporated. This incorporation of Sr2+ leads to an intense emission at 550 nm. HAp nanoparticles when subjected to thermal treatment (800 °C) transform to β-TCP, exhibiting emission at 710 nm due to the emergence of an intermediate band. Moreover, these nanoparticles exhibit fluorescence in visible light when compared to the other UV and IR fluorescence excitation sources which could damage the tissues. The synthesis involving the combination of ultrasound and microwave techniques resulted in the distribution of Fe3+ in the interstitial sites of CaP, which is responsible for the excellent fluorescent properties. Moreover, thermally treated CaP becomes superparamagnetic, without affecting the desired optical properties. The bioactive, biocompatible, magnetic, and fluorescent properties of this resorbable CaP which is free from toxic heavy metals (Eu, Gd, etc.) could help in overcoming the long-term cytotoxicity. This could also be useful in tracking the location of the nanoparticles during drug delivery and magnetic hyperthermia. The bioactive fluorescent CaP nanoparticle helps in monitoring the bone growth and in addition, it could be employed in cell imaging applications. The in vitro MCF-7 imaging using the nanoparticles after 24 h of uptake at 465 nm evidences the bioimaging capability of the prepared nanoparticles. The reproducibility of the defect level is essential for the defect-induced emission properties. The synthesis of nontoxic fluorescent CaP is highly reproducible with the present synthesis method. Hence, it could be safely employed in various biomedical applications.

Controllable synthesis of Y doped AlN microtubes, prism-arrayed microstructures and microflowers via an improved DC arc discharge plasma method

The yttrium doped AlN (AlN:Y) microtubes, prism-arrayed microstructures and microflowers were fabricated through the direct reaction of Al:Y alloy and N2 without extra catalyst. AlN microstructures can be controlled by varying deposition temperature or changing deposition area. The structure and morphology of AlN:Y microstructures were characterized by X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy. The results indicate that Y3+ ions incorporate inside the AlN nanostructures. The photoluminescence spectra of the AlN:Y microstructures show strong defect-induced emissions in the visible range of 400–750 nm. Room-temperature ferromagnetism is observed in AlN:Y microstructures which is caused by high concentration of Al vacancies. The AlN:Y microstructures with interesting geometries possess excellent optical and magnetic properties, suggesting potential applications in optoelectronic and spintronic microdevices.

Exploring Defect-Induced Emission in ZnAl2O4: An Exceptional Color-Tunable Phosphor Material with Diverse Lifetimes.

Activator-free zinc aluminate (ZA) nanophosphor was synthesized through a sol-gel combustion route, which can be used both as a blue-emitting phosphor material and a white-emitting phosphor material, depending on the annealing temperature during synthesis. The material also has the potential to be used in optical thermometry. These fascinating color-tunable emission characteristics can be linked with the various defect centers present inside the matrix and their changes upon thermal annealing. Various defect centers, such as anionic vacancy, cationic vacancy, antisite defect, etc., create different electronic states inside the band gap, which are responsible for the multicolor emission. The color components are isolated from the complex emission spectra using time-resolved emission spectroscopy (TRES) study. Interestingly, the lifetime values of the various defect centers were found to change significantly from milliseconds to microseconds upon thermal annealing, which makes the phosphors more diverse (i.e., either long-persistent blue-emitting phosphors or short-persistent white-emitting phosphors). Fourier transform infrared (FTIR) and diffuse reflectance spectroscopy (DRS) confirmed the presence of antisite defect centers such as AlZn+ or ZnAl- in the matrix. X-ray absorption fine structure (EXAFS) study showed that the spinel structure was more disordered in nature for low-temperature-annealed compounds. Electron paramagnetic resonance (EPR) and positron annihilation lifetime spectroscopy (PALS) studies were also carried out in order to characterize various anionic and cationic vacancies and their clusters present in the compounds. Antisite defect centers such as AlZn+ or ZnAl-, which act as an electron or hole trap, were found to be responsible for the diverse lifetime behavior. To gain insight about the electronic states inside the band gap, density functional theory (DFT)-based calculations were performed for both pure and various vacancy-introduced spinel structures. Finally, based on the theoretical and experimental results, for the first time, a detailed investigation of various defect-induced emission behavior in ZA is presented, which also explains the mechanism of color tunability and dynamic lifetimes.

Preparation, characterization, and photoluminescence property of core-shell (BiO)2CO3@Bi2S3 nanowires

Bi2S3-containing nanocomposites have attracted considerable attention due to their outstanding photocatalytic activity. However, little study was focused on the preparation and PL property of core-shell (BiO)2CO3@Bi2S3 nanowires. Herein we report the in situ preparation, characterization and PL property of (BiO)2CO3@Bi2S3 core-shell nanowires. (BiO)2CO3@Bi2S3 core-shell nanowires with different sulfur content were synthesized in situ by irradiating (BiO)2CO3 nanowires in xanthate solution. These nanowires showed strong photoluminescence (PL) emission from violet to blue light range. The XRD patterns, EDS mapping images, HRTEM images, and XPS results demonstrated the formation of Bi2S3 shell on the surface of (BiO)2CO3 core. SEM and TEM images indicated that the core-shell nanowires were 20–40 nm in diameter and up to tens of micrometers in length. The sulfur content in core-shell nanowires increased from 0.31% to 1.13% with increasing reacted times. EDS mapping showed that S element was uniformly distributed on the surface of nanowires. The PL peaks at 395, 418, 441 and 467 nm of (BiO)2CO3@Bi2S3 core-shell nanowires with low S concentration were contributed to the intrinsic emission of (BiO)2CO3 core, Bi-containing intermediate-induced emission, BiO cluster-induced emission, and surface defect-induced emission of (BiO)2CO3 core, respectively. A new shoulder peak at 427 nm appeared in the PL spectrum of (BiO)2CO3@Bi2S3 core-shell nanowires with 1.13% S, which could be contributed the electronic coupling effect between Bi2S3 shell and (BiO)2CO3 core. We anticipate that the core-shell nanowires will find use in photochemical and photo-electronic fields.

A turn-on type mechanochromic fluorescent material based on defect-induced emission: implication for pressure sensing and mechanical printing

Defect-induced emission enables a turn-on mechanochromic fluorescence for pressure sensing and mechanical printing.

Static fault localization of subtle metallization defects using near infrared photon emission microscopy

In this paper, two electroluminescence phenomena, which enabled the static electrical fault localization of subtle back-end-of-line metallization defects using near-infrared photon emission microscopy in the logic circuitry and the memory array, are described. In the logic circuitry, through the study of the defect-induced hot carrier emissions from the combinational logic gates, distinctive differences in emission characteristic between open and short defects are identified. Using this defect induced emission characterization approach, together with layout trace and analysis, the type of defect can be predicted. The defect physical location, which yielded no detectable hotspot signal, can also be narrowed down along the long failure net. This allows for the selection of the most appropriate physical failure analysis approach for defect viewing and thus achieving significant reduction in failure analysis cycle time. In the memory array, the weak emission from partially turned-on pass gate transistor is leveraged to localize marginal opens and shorts on the wordline node of the pass-gate transistor. These approaches are applied with great success in the foundry environment to localize yield limiting defects that resulted in SCAN and memory build-in self-test failure, without memory bitmap, diagnostic support or measurable IDD leakage, on advanced technology nodes devices. A discussion on the factors that influence the success rate of this approach is also provided.

Analysis of defect luminescence in Ga-doped ZnO nanoparticles.

We applied cathodoluminescence (CL) spectroscopy to evaluate the defect-induced luminescence within ZnO and Ga-doped ZnO (GZO) nanoparticles. The observed emissions from defect sites present in the GZO lattice exhibited a strong dependence on both dopant content and synthesis methods. The strong and broad defect-induced emissions and inhomogeneous population of intrinsic defects in nano-sized ZnO particles could effectively be suppressed by Ga doping, although large dopant amounts caused the generation of negatively-charged defects, VZn and Oi, with a subsequent increase of the luminescence. Upon deconvolution of the retrieved CL spectra into individual sub-bands, the physical origin of all the sub-bands could be clarified, and related to sample composition and synthesis protocol. This study lays the foundation of quantitative CL evaluation of defects to assess the quality of GZO optoelectronic devices.

Subpicosecond Exciton Dynamics and Biexcitonic Feature in Colloidal CuInS2 Nanocrystals: Role of In-Cu Antisite Defects.

Charge carrier dynamics of multinary quantum dots like CuInS2 (CIS) nanocrystals (NCs) is not clearly understood, especially in ultrafast time scales. Herein we have synthesized colloidal CIS NCs that show defect-induced emission between donor (antisite) and acceptor (internal/surface) states as indicated from steady-state and time-resolved photoluminescence (PL) measurements. Subpicosecond transient absorption (TA) spectra of CIS NCs reveal a gradient of electronic states that exists above the conduction band edge. The electron cooling rate has been determined to be ∼0.1-0.15 eV/ps. The cascade of electron cooling dynamics was monitored after following the TA kinetics at different electronic states. Interestingly, the kinetics at the antisite state unveil a biexcitonic feature, which has been enlightened through a probe-induced biexciton mechanism. With progressively higher fluence (⟨N⟩), the biexciton binding energy increases, and the electron cooling to the antisite state considerably slows down. Extra energy released during Auger recombination of bi/multiexcitons are used to re-excite the electron to a further high energy level, resulting in longer electron cooling time to the antisite states.

MgAl2O4 spinel: Synthesis, carbon incorporation and defect-induced luminescence

The present work explores the synthesis of carbon-doped MgAl2O4 and investigates the effect of doping on the photophysical properties of MgAl2O4. Pure MgAl2O4 spinel was synthesized by gel combustion followed by annealing at 1100°C. The carbon doping was performed by two methods. The first method involved heating the sample with electron beam (from electron gun) in graphite crucible (A) and second method involved heating the sample up to 2100°C in graphite furnace (B). The photoluminescence spectroscopy exhibited defect-induced emissions with enhanced intensity in the case of sample B. A significant blue shift in the emission band was also observed in the case of sample B. The photoluminescence decay studies indicated that multiple trapping and detrapping events are experienced before the radiative recombination process, which eventually occurs. Average lifetime was observed to be 4.83μs which is typical of defect-related emission. The results were complimented by electron paramagnetic resonance (EPR) technique. The CIE co-ordinates for sample B were found to be x=0.231 and y=0.227 which establish it as a blue-emitter.

Defect-sensitive crystals based on diaminomaleonitrile-functionalized Schiff base with aggregation-enhanced emission

In this work, we report the synthesis and photophysical studies of a new luminogen, A3MN, a diaminomaleonitrile-functionalized Schiff base. A3MN is aggregation-enhanced emission (AEE)-active: the emission of A3MN is enhanced with the aggregate formation. A3MN also possesses twisted intramolecular charge transfer (TICT) properties, showing noticeable solvatofluorochromism. Interestingly, the crystals of A3MN are nonemissive; the defect areas of the crystal, however, are highly emissive, as confirmed by spectroscopic methods and confocal microscopy. By taking advantage of this defect sensitive feature, a “turn-on” type of mechanofluorochromic material is developed, the emission of which is significantly enhanced under pressure or shear force. The detection limit reaches 0.1 Newton owing to its “turn-on” nature. Such defect-induced emission also renders A3MN sensitive to various kinds of mechanical actions, including hitting, friction, sculpture, and ultrasonic vibration.

Visible-light photoresponse in a hollow microtube–nanowire structure made of carbon-doped ZnO

A hollow microtube–nanowire structure of carbon-doped ZnO was fabricated via using carbon fibers as the sacrificed substrates. The hollow microtube–nanowire architecture was characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman, photoluminescence and electrical measurements. The results indicate that carbon is self-doped into ZnO primarily substituting oxygen in the growth and annealing processes. The room temperature photoluminescence spectrum of the carbon-doped ZnO shows a strong defect-induced emission in the visible range of 400–800 nm. The optoelectronic properties in the range of the visible light are demonstrated by operating an organic/inorganic p–n heterojunction and a metal–semiconductor–metal (M–S–M) structure, which indicates that carbon doping in ZnO extends its photoelectric specifics in the visible light region.

Role of oxygen vacancies in visible emission and transport properties of indium oxide nanowires

We report on the effect of oxygen vacancies on the defect-related emission and the electronic properties of In2O3 nanowires. The nanowires were synthesized by vapor phase transport and had diameters ranging from 80–100 nm and lengths over 10–20 μm, with a growth direction of [0 0 1]. The as-grown nanowires connected in an FET type of configuration show n-type conductivity, which is ascribed to the presence of intrinsic defects like oxygen vacancies in the nanowire. The resistivity, transconductance, field effect mobility and carrier concentration of the In2O3 nanowires were determined to be 1.82 × 10−2 Ω cm, 11.2 nS, 119 cm2 V−1 s−1 and 4.89 × 1017 cm−3, respectively. The presence of oxygen vacancies was also confirmed by photoluminescence measurements, which show a strong UV emission peak at 3.18 eV and defect peaks in the visible region at 2.85 eV, 2.66 eV and 2.5 eV. We present a technique of post-growth annealing in O2 environment and passivation with (NH4)2S to reduce the defect-induced emission.