Defect Luminescence Research Articles

Optical quantum technologies with hexagonal boron nitride single photon sources

Single photon quantum emitters are important building blocks of optical quantum technologies. Hexagonal boron nitride (hBN), an atomically thin wide band gap two dimensional material, hosts robust, optically active luminescent point defects, which are known to reduce phonon lifetimes, promises as a stable single-photon source at room temperature. In this Review, we present the recent advances in hBN quantum light emission, comparisons with other 2D material based quantum sources and analyze the performance of hBN quantum emitters. We also discuss state-of-the-art stable single photon emitter’s fabrication in UV, visible and near IR regions, their activation, characterization techniques, photostability towards a wide range of operating temperatures and harsh environments, Density-functional theory predictions of possible hBN defect structures for single photon emission in UV to IR regions and applications of single photon sources in quantum communication and quantum photonic circuits with associated potential obstacles.

Experimental evidences of defect luminescence spanning red to near-infrared in strongly quantum confined sub-4 nm CuInSe2 quantum dots approaching crystallization limit

The CuInSe2 quantum dots (QDs) are “green” infrared optoelectronic materials with fruitful optically active point defects, but their roles in photodynamics remain unclear. We observe different types of stoichiometry-sensitive emission bands spanning red-near-infrared region in strongly quantum confined CuInSe2 QDs. The saturation of photoluminescence intensity versus excitation power follows law I ∝ P k with 0.54 < k <0.94 that is characteristic of defect dominated photon emission. The radiative quantum transitions have lifetimes of 15.2–32.0 μs that are far longer than usual interband transition lifetimes by three orders of magnitude. Multiple defects-involved electron transition processes are revealed.

Charge Transport in Networks of sp3-Functionalized Single-Walled Carbon Nanotubes

The controlled introduction of luminescent sp3 defects into semiconducting single-walled carbon nanotubes (SWCNTs) leads to narrowband, tunable emission in the near-infrared and increases their photoluminescence (PL) quantum yield for potential optoelectronic applications [ACS Nano 2019, 13, 9259]. While the spectroscopic properties of sp3-functionalized SWCNTs are already well-established, a detailed understanding of the impact of functionalization on their electrical performance especially in a network is still lacking. Here, we investigate charge transport in ambipolar, light-emitting field-effect transistors based on networks of pristine and sp3-functionalized (6,5) SWCNTs. While both hole and electron mobilities decrease with increasing degree of functionalization, the transistors remain fully operational, showing electroluminescence from the defect states. Charge-modulated PL spectroscopy, which exclusively probes the mobile charge carriers in SWCNT networks [ACS Nano 2020, 14, 2412], confirms that the defects are efficiently sampled by mobile carriers and consequently, sp3-functionalized SWCNTs actively participate in the charge transport within the network. Temperature-dependent transport measurements of transistors combined with ultrafast photoconductivity dynamics of individually dispersed SWCNTs measured by optical-pump THz-probe spectroscopy provide further insights into the impact of sp3-functionalization on charge transport within a single nanotube and within a network.

(Invited) Tuning the Properties of Luminescent Defects in Carbon Nanotubes for Applications

Luminescent sp3-defects (also called quantum defects or organic color centers) in single-walled carbon nanotubes (SWCNTs) with their characteristic red-shifted emission and long photoluminescence (PL) lifetimes enable higher PL quantum yields and single-photon emission at room temperature. They can be created in a controlled manner on polymer-wrapped semiconducting nanotubes via diazonium chemistry in organic solvents with a variety of functional substituents [ACS Nano 2019, 13, 9259]. Here we show, how attaching the stable and neutral perchlorotriphenylmethyl (PTM) radical to such sp3-defects impacts their emissive properties in dispersion at room temperature as well as under strong magnetic fields at cryogenic temperatures. Furthermore, the type of defect emission (E11* or E11*-) is governed by the precise binding configuration for chiral (6,5) SWCNTs. We introduce a simple synthetic protocol to obtain exclusively the more red-shifted E11*--defect emission with even longer PL lifetime and high single-photon purity. This new functionalization method relies on nucleophilic addition instead of radical-based reactions and can be transferred to other polymer-sorted nanotubes, e.g. (7,5) and (10,5) SWCNTs. The achieved tunability and control of sp3-defect emission facilitates applications in electroluminescent devices, radiative pumping of exciton-polaritons in metal-clad microcavities [ACS Photonics 2020, DOI: 10.1021/acsphotonics.0c01129] or in-vivo imaging in the 2nd biological window.

(Invited) Tailoring of Single-Walled Carbon Nanotube Luminescence as Photoswitchable Near-Infrared Emitters

Single-molecule localization microscopy (SMLM) has set a new paradigm in the field of optical imaging by delivering super-resolution images i.e. with resolution much better than the diffraction limit. We demonstrated over the last years that such approaches can be designed to understand basic excitonic processes in carbon nanotubes [1-2]. In the field of bioimaging, SMLM is currently limited to the visible or far-red range, missing the near-infrared region where biological tissues are however the most transparent. One reason for this is that single-molecule emitter optical properties still need to be tailored in the near-infrared, including photoswitching capabilities which is a basic ingredient to achieve single-molecule SMLM. To fill this gap, optical imaging of single-wall carbon nanotubes can first be greatly facilitated in brain tissues by covalent functionalization of luminescent defects, reducing the required excitation light intensity by one order of magnitude [3]. In addition, we have introduced a novel type of hybrid nanomaterials consisting of single-wall carbon nanotubes covalently functionalized with photo-switching molecules that are used to control the intrinsic luminescence of the single nanotubes in the near-infrared (beyond 1 µm) [4]. Through the control of photoswitching, we demonstrate super-localization imaging of nanotubes unresolved by diffraction limited microscopy opening the route toward SMLM in the near-infrared for biological applications. Photocontrol of individual near-infrared emitters will also be highly desirable for elementary optical molecular switches or information storage elements since most communication data transfer protocols are established in this spectral range.

Charge Transport in and Electroluminescence from sp3-Functionalized Carbon Nanotube Networks.

The controlled covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent sp3 defects leads to additional narrow and tunable photoluminescence features in the near-infrared and even enables single-photon emission at room temperature, thus strongly expanding their application potential. However, the successful integration of sp3-functionalized SWCNTs in optoelectronic devices with efficient defect state electroluminescence not only requires control over their emission properties but also a detailed understanding of the impact of functionalization on their electrical performance, especially in dense networks. Here, we demonstrate ambipolar, light-emitting field-effect transistors based on networks of pristine and functionalized polymer-sorted (6,5) SWCNTs. We investigate the influence of sp3 defects on charge transport by employing electroluminescence and (charge-modulated) photoluminescence spectroscopy combined with temperature-dependent current–voltage measurements. We find that sp3-functionalized SWCNTs actively participate in charge transport within the network as mobile carriers efficiently sample the sp3 defects, which act as shallow trap states. While both hole and electron mobilities decrease with increasing degree of functionalization, the transistors remain fully operational, showing electroluminescence from the defect states that can be tuned by the defect density.

Origin of defect luminescence in ultraviolet emitting AlGaN diode structures

Light emitting diode structures emitting in the ultraviolet spectral range are investigated. The samples exhibit defect luminescence bands. Synchrotron-based photoluminescence excitation spectroscopy of the complicated multi-layer stacks is employed to assign the origin of the observed defect luminescence to certain layers. In the case of quantum well structures emitting at 320 and 290 nm, the n-type contact AlGaN:Si layer is found to be the origin of defect luminescence bands between 2.65 and 2.8 eV. For 230 nm emitters without such n-type contact layer, the origin of a defect double structure at 2.8 and 3.6 eV can be assigned to the quantum wells.

Ultraviolet-Cathodoluminescent 330 nm light source from a 2-inch wide CNT electron-beam emission under DC electric field

We report a 2-inch wide-area AlGaN-based ultraviolet (UV) – cathodoluminescence (CL) light source emission using electron beam (EB) pumped source under DC electric field from an AlGaN/GaN multi-quantum-well grown on a sapphire substrate. The EB-pumping is achieved by wide-area carbon nanotubes (CNT) based field emitters and is arranged via a metal mesh, thereby acting as a gate to pump the electron flow. We have carried out UV–CL measurements with a turn-on field emission in the anode voltage ranging between 5 kV and 9 kV at anode current up to 1 mA. The best results are obtained at the low consumption energy of 7 W (anode current 1 mA; anode voltage 7 kV). The 330 nm UV–CL emission shows an output power of ~225 mW, with an as-calculated power efficiency of ~3.6%. The CL measurements show (5–8) % defect luminescence in the visible region.

P-Doped α-Ca2SiO4 and (Si, P)-Codoped CaO with Special Defect Microstructures and Luminescence due to an Enhanced Solid Solution by a Pulsed Laser Ablation Condensation Route

The α-Ca2SiO4 and rocksalt-type (R) CaO condensates/particulates with an enhanced solid solution of P and Si + P ions, respectively, were produced via pulsed laser ablation of a silicocarnotite (Ca5(PO4)2SiO4) target under a relatively high peak power density of 1.8 × 109 Watt/cm2 in high vacuum (8 × 10–5 torr). The condensates were dispersed in an amorphous phase of Ca–Si–P–O linkage as spheres up to submicrons in size or coalesced in particulates by the {100}R, {111}R, and {1̅21̅0}α facets to follow a preferred crystallographic relationship of (1̅1̅1)R//(1̅21̅0)α; [101]R//[101̅1]α, with a fair three-dimensional match of the lattice planes. The 1-D commensurate superstructures n × (200)R and n × (1̅21̅0)a, where n = 1.5 and 3, were commonly noted for the condensates in the particulates. The P-doped α-Ca2SiO4 and (Si, P)-codoped CaO showed characteristic luminescence at 407 and 537 nm, respectively, under the combined influences of internal compressive stress as well as substitutional and/or interstitial defects with charge/volume-compensating cation vacancies for potential optoelectronic and photocatalytic applications.

Controlled preparation of undoped Zn2GeO4 microcrystal and the luminescent properties resulted from the inner defects

Zn2GeO4 microcrystals were prepared by hydrothermal method. Two synthesis routes were tried and the products were studied by analyzing the morphology and the crystalline phases. According to the experimental phenomenon and the XRD spectra of the products obtained at different stages in the preparation process, the possible chemical reactions in the synthesis process were analyzed. Homogeneous dispersion system is needed for preparing well dispersed nanometer or micron Zn2GeO4. PH value of the reaction system and the synthesis route affect the particle size and the morphology of the samples deeply. Heat treatment changes the optical properties of the samples and too high temperature leads to the formation of new phase. With the increase of annealing temperature, the emission spectrum first becomes wider and then narrower. This is because the defect kind and the defect intensity vary with sintering temperature. As a result, the CIE Chromaticity coordinates correspond to the emission spectra move from white area to green area. This change is the inevitable result of defect luminescence mechanism.

Synthetic control over the binding configuration of luminescent sp3-defects in single-walled carbon nanotubes

The controlled functionalization of single-walled carbon nanotubes with luminescent sp3-defects has created the potential to employ them as quantum-light sources in the near-infrared. For that, it is crucial to control their spectral diversity. The emission wavelength is determined by the binding configuration of the defects rather than the molecular structure of the attached groups. However, current functionalization methods produce a variety of binding configurations and thus emission wavelengths. We introduce a simple reaction protocol for the creation of only one type of luminescent defect in polymer-sorted (6,5) nanotubes, which is more red-shifted and exhibits longer photoluminescence lifetimes than the commonly obtained binding configurations. We demonstrate single-photon emission at room temperature and expand this functionalization to other polymer-wrapped nanotubes with emission further in the near-infrared. As the selectivity of the reaction with various aniline derivatives depends on the presence of an organic base we propose nucleophilic addition as the reaction mechanism.

Role of each step in the combined treatment of reactive ion etching and dynamic chemical etching for improving the laser-induced damage resistance of fused silica.

We investigate the role of each step in the combined treatment of reactive ion etching (RIE) and dynamic chemical etching (DCE) for improving the laser-induced damage resistance of fused silica optics. We employ various surface analytical methods to identify the possible damage precursors on fused silica surfaces treated with different processes (RIE, DCE, and their combination). The results show that RIE-induced defects, including F contamination, broken Si-O bonds, luminescence defects (i.e., NBOHCs and ODCs), and material densification, are potential factors that limit the improvement of laser-induced damage resistance of the optics. Although being capable of eliminating the above factors, the DCE treatment can achieve rough optical surface with masses of exposed scratches and pits which might serve as reservoirs of the deposits such as inorganic salts, thus limiting the further improvement in damage resistance of fused silica. The study guides us to a deep understanding of the laser-induced damage process in achieving fused silica optics with enhanced resistance to laser-induced damage by the combined treatment of RIE and DCE.

Influence of Mo doping on the luminescence properties and defect states in ZnO nanorods. Comparison with ZnO:Mo thin films

ZnO:Mo powders of nano- and microrods were fabricated by the hydrothermal growth method. ZnO:Mo thin films deposited on a fused silica glass substrate were synthesized for comparison as well. X-ray diffraction and fluorescence (XRD, XRF), scanning electron microscopy (SEM), electron paramagnetic resonance (EPR), photo- and radioluminescence (PL, RL) were applied to characterize these materials. The samples were also annealed in air at elevated temperatures to study the defects and luminescence modification. In particular, EPR spectra in the powder and thin film samples were composed of the signals originating from shallow donors before any treatment. The Mo5+ signal appeared after the annealing at 350 °C in the powder spectra. It existed prior to the treatment in the thin film samples. New EPR signal appears after X-ray irradiation in the powder samples. Red luminescence occurs in all powder samples. The intensity of this band demonstrated different dependences on the annealing temperatures in the samples with different Mo content. The exciton-related band at 380 nm never observed in the powder samples before the annealing, appears after the annealing at 350 °C. The strongest it was in the ZnO:Mo powder with low Mo content.

Tunable blue and green emissions of sol–gel synthesized transparent nano-willemite codoped with nano-pyroxmangite and dysprosium

Sol–gel method was utilized for synthesis of transparent bulk-samples of undoped, Mn-doped and Dy-Mn-codoped zinc silicate (Zn2SiO4). The products were characterized in two forms which are the xerogels and after heat treatment at 750 °C for 3 h. Formation of nano-crystalline willemite in the three compositions was confirmed by X-ray diffraction (XRD). In case of 2 mol% Mn-doping, XRD revealed formation of nano-pyroxmangite, i.e. manganese silicate (MnSiO3). Microstructure of the nanocrystalline products was characterized by high resolution transmission electron microscopy (HRTEM). XRD and HRTEM indicated that the size of formed nano-crystals varied from 15 to 40 nm. Photoluminescence (PL) spectroscopy revealed tunable blue and bright green emissions from the pure nano-willemite upon ultraviolet excitation at 254, 300, 350 and 365 nm. CIE chromaticity diagrams provided definitely the overall color of light emitted from each spectrum. These emissions are attributed to optically active luminescent defects of various types. Mn and Dy play no crucial role in the luminescence behavior, but only slightly alter the nano-willemite structure causing different luminescent defects that, in turn, produces the difference in light tunes. The obtained results lead to the assumption that 1 mol% of Dy2O3 was entirely masked within the nanowillemite lattice which is evidenced by absolute absence of any peak belonging to Dy3+ ions in the ultraviolet–visible–near infrared optical absorption spectra. The present willemites can find potential application as blue and green phosphors in luminescent optical and optoelectronic devices.

Interaction of Luminescent Defects in Carbon Nanotubes with Covalently Attached Stable Organic Radicals.

The functionalization of single-walled carbon nanotubes (SWCNTs) with luminescent sp3 defects has greatly improved their performance in applications such as quantum light sources and bioimaging. Here, we report the covalent functionalization of purified semiconducting SWCNTs with stable organic radicals (perchlorotriphenylmethyl, PTM) carrying a net spin. This model system allows us to use the near-infrared photoluminescence arising from the defect-localized exciton as a highly sensitive probe for the short-range interaction between the PTM radical and the SWCNT. Our results point toward an increased triplet exciton population due to radical-enhanced intersystem crossing, which could provide access to the elusive triplet manifold in SWCNTs. Furthermore, this simple synthetic route to spin-labeled defects could enable magnetic resonance studies complementary to in vivo fluorescence imaging with functionalized SWCNTs and facilitate the scalable fabrication of spintronic devices with magnetically switchable charge transport.

Creation of luminescent defects in crystals by coherent pairs of femtosecond laser pulses

The processes of creation of luminescence centers by laser radiation in transparent anisotropic crystals are investigated using MgF2 as an example. To monitor the concentration of the created centers, a luminescent method was developed, which ensured a direct proportionality between the luminescence intensity and the concentration of centers. By the joint action of coherent pairs of femtosecond laser pulses, the longitudinal positioning of the action of light on the medium was performed during the creation of spatially modulated structures. Laser filaments resulting from self-focusing contained both ordinary and extraordinary components. A simple method is substantiated for studying the efficiency of the formation of quantum systems depending on the polarization state of the acting radiation in one experiment. It is shown that the experiments implemented an intermediate mechanism of multiphoton-tunneling ionization of a crystal during laser defects formation.

Diminishing the Induced Strain and Oxygen Incorporation on Aluminium Nitride Films Deposited Using Pulsed Atomic-Layer Epitaxy Techniques at Standard Pressure MOCVD

A pulsed atomic-layer epitaxy growth technique has been introduced to substantially diminish the induced strain and oxygen incorporation on aluminium nitride films grown at standard pressure by metal organic chemical vapour deposition. The qualities of the as-deposited aluminium nitride films were studied by varying the aluminium nitride nucleation layer growth temperature at 700°C, 800°C, 900°C, 1000°C and 1100°C, respectively. The compressive strain inside the as-deposited aluminium nitride films, induced by the hetero-epitaxial growth on sapphire, was investigated through Raman spectroscopy by focusing on the evolution of E2 (high) peak frequency, where almost stress-free aluminium nitride films were attained at nucleation layer growth temperature of 1100°C. Then, the correlation between luminescence defect and level of foreign impurities respective to the varied nucleation layer growth temperatures were also systematically analysed through photoluminescence spectroscopy and x-ray photoelectron spectroscopy, respectively.

Gallium Concentration Optimisation of Gallium Doped Zinc Oxide for Improvement of Optical Properties

Abstract The near-band luminescence of doped ZnO is promising for advanced scintillators; however, the dopant type and concentration effects require a detailed study. Undoped and Ga-doped ZnO nanopowders were prepared by a microwave-assisted solvothermal method and the gallium concentration effect on luminescence properties was studied. The near-band luminescence peak position dependence on gallium concentration was observed. Near-band luminescence intensity versus defect luminescence intensity ratio was explored for different gallium concentrations and the optimal value was determined. Samples were prepared with dopant concentrations between 0.2 and 1.5 at%, XRD analysis confirmed that samples contained only zinc oxide hexagonal wurtzite phase. The results of the research showed that ZnO:Ga containing 0.9 at.% gallium was promising for scintillators.

Prominent luminescence of silicon-vacancy defects created in bulk silicon carbide p\u2013n junction diodes

We investigate fluorescent defect centers in 4H silicon carbide p–n junction diodes fabricated via aluminum-ion implantation into an n-type bulk substrate without the use of an epitaxial growth process. At room temperature, electron-irradiated p–n junction diodes exhibit electroluminescence originating from silicon-vacancy defects. For a diode exposed to an electron dose of 1 times 10^{18},{{mathrm{cm}}}^{-2} at 800,{{mathrm{keV}}}, the electroluminescence intensity of these defects is most prominent within a wavelength range of 400–1100,{{mathrm{nm}}}. The commonly observed {{mathrm{D}}}_1 emission was sufficiently suppressed in the electroluminescence spectra of all the fabricated diodes, while it was detected in the photoluminescence measurements. The photoluminescence spectra also displayed emission lines from silicon-vacancy defects.

Особенности фотолюминесценции в кристаллах ниобата лития, легированных цинком в широком диапазоне концентраций

The concentration changes in the photoluminescence spectra of LiNbO3 : Zn crystals (0.004 ÷ 6.5 mol.% ZnO) were studied. It was found that with the increase of zinc concentration from 0.004 to 1.42 mol.% ZnO, the intensity decrease of luminescence bands caused by VLI, NbNb, and NbNb−NbLi defects was observed. As the crystal composition approached the second concentration threshold (≈ 7.0 mol.% ZnO), the luminescent halo shifted by ≈ 0.41 eV to the high-energy region of the spectrum and the intensity of the luminescence centers increased at 2.66 and 2.26 eV. It was caused by the appearance of ZnLi point defects. It was shown that in the LiNbO3 : Zn(4.69 mol.% ZnO) crystal obtained by homogeneous doping technology, there is a greater number of luminescence centers of different origin than in congruent and zinc-doped crystals obtained by direct melt doping technology. In the LiNbO3 : Zn crystal (4.52 mol.% ZnO), the luminescence of the main defects (VLi, NbNb, ZnLi) was quenched by increasing the fraction of nonradiative transitions relative to other LiNbO3 : Zn crystals in the concentration range [ZnO] = 4.46 ÷ 6.50 mol.%.