Room-temperature Emission Research Articles

Sol-gel synthesis and nanostructured semiconductor analysis of undoped and Cd-doped ZnO thin films

The semiconductor thin films of undoped and cadmium (Cd) doped zinc oxide (ZnO) via different concentrations (Cd: 2, 5 and 10 wt.%) were prepared by sol-gel route technique using a dip-coating method onto glass and silicon substrates. The Zinc acetate dehydrate (ZnAc), the Cadmium acetate dehydrate (CdAc), the 2-Methoxyethanol and the Ethanolamine were used as a starting material, doping, solvent and stabilizer, respectively. All films were annealed for 120 min at 500 °C. The samples were characterized by different analysis techniques to understand their structural, morphological and optical properties and its components such as the thin films surface are a nanometric crystallite size with a ganglia-like structure as observed by Environmental Scanning Electron Microscopy. The films were characterized by X-ray diffraction, it was crystallized in a hexagonal crystal structure with a highly c-axis preferred (002) orientation, and the crystallites size decrease from 30 to 23 nm with the increasing of cadmium doping. The Rutherford Backscattering Spectrometry and the Auger Electron Spectroscopy spectra given an idea in the thickness of the film by simulation and relation to the etching time respectively, where did the two agree that the film of ZnO doped 10 wt.% Cd is the thickest. All films exhibit an optical transmittance above 75 to 85% nm and a sharp absorption onset about 375 nm corresponding to the fundamental absorption edge 3.08 to 3.21 eV. The room-temperature luminescence emission spectra of these films were excited under the same conditions with 250 nm Xenon lamp excitation, showed an ultraviolet and visible emissions.

Multiphoton Upconversion Emission from Diamond Single Crystals.

Room-temperature upconversion emission up to eight-photon absorption is demonstrated from diamond single crystals under femtosecond laser excitation for the first time. The low concentration of defects and impurities is attributed to the support of free excitons emission at 235 nm. Nonlinear optical properties are also investigated by using an open-aperture Z-scan technique. The corresponding three-, five-, and eight-photon absorption coefficients of the diamonds are found to be 1.8 × 10-2 cm3/GW2, 5 × 10-9 cm7/GW4, and 1.6 × 10-19 cm13/GW7, respectively. Considering its high hardness and high thermal conductivity, diamonds are a versatile nonlinear optical material suitable for high-power deep ultraviolet applications under multiphoton excitation.

Transient Spectroscopy of Glass-Embedded Perovskite Quantum Dots: Novel Structures in an Old Wrapping

Abstract Semiconductor doped glasses had been used by the research and engineering communities as color filters or saturable absorbers well before it was realized that their optical properties were defined by tiny specs of semiconductor matter known presently as quantum dots (QDs). Nowadays, the preferred type of QD samples are colloidal particles typically fabricated via organometallic chemical routines that allow for exquisite control of QD morphology, composition and surface properties. However, there is still a number of applications that would benefit from the availability of high-quality glass-based QD samples. These prospective applications include fiber optics, optically pumped lasers and amplifiers and luminescent solar concentrators (LSCs). In addition to being perfect optical materials, glass matrices could help enhance stability of QDs by isolating them from the environment and improving heat exchange with the outside medium. Here we conduct optical studies of a new type of all-inorganic CsPbBr3 perovskite QDs fabricated directly in glasses by high-temperature precipitation. These samples are virtually scattering free and exhibit excellent waveguiding properties which makes them well suited for applications in, for example, fiber optics and LSCs. However, the presently existing problem is their fairly low room-temperature emission quantum yields of only ca. 1%–2%. Here we investigate the reasons underlying the limited emissivity of these samples by conducting transient photoluminescence (PL) and absorption measurements across a range of temperatures from 20 to 300K. We observe that the low-temperature PL quantum yield of these samples can be as high as ~25%. However, it quickly drops (in a nearly linear fashion) with increasing temperature. Interestingly, contrary to traditional thermal quenching models, experimental observations cannot be explained in terms of a thermally activated nonradiative rate but rather suggest the existence of two distinct QD sub-ensembles of “emissive” and completely “nonemissive” particles. The temperature-induced variation in the PL efficiency is likely due to a structural transformation of the QD surfaces or interior leading to formation of extremely fast trapping sites or nonemissive phases resulting in conversion of emissive QDs into nonemissive. Thus, future efforts on improving emissivity of glass-based perovskite QD samples might focus on approaches for extending the range of stability of the low-temperature highly emissive structure/phase of the QDs up to room temperature.

(Invited) Aryl-Functionalized Single-Walled Carbon Nanotubes Embedded into Metallo-Dielectric Antennas

Creating exciton localization via covalent sp3 functionalization of single-walled carbon nanotubes (SWCNTs) is a promising route to enhance the optical quantum yield and can furthermore lead to single-photon emission at room temperatures [1]. Of recent interest are aryl-functionalized SWCNTs that are protected by polyfluorene, for which we have recently shown that a variety of chemically and energetically distinct defect state exist that have their origin in topological variation in the chemical binding configuration of the monovalent aryl groups [2]. Here we extend our study by embedding aryl-functionalized SWCNTs into metallo-dielectric antennas that provide broadband coupling with near-unity light collection efficiencies by trapping air gaps on chip that form cavity modes. Scalable implementation is realized by employing polymer layer dry-transfer techniques that avoid solvent incompatibility issues, as well as a planar design that avoids solid-immersion lenses, resulting in a narrow output cone of ±15° that enables a priori fiber-to-chip butt coupling. In these devices the exciton emission from the aryl-functionalized sides (E11*) remains spectrally sharp with linewidth values near the resolution limit of about 35 meV. Interestingly, we find that the pump-induced dephasing for E11* exciton (0D exciton) is significantly reduced by more than 300% as compared to the E11 exciton (1D exciton), thereby preserving exciton coherence even into the high pump power regime, as needed for practical devices in quantum photonics. We will discuss how these findings provide a foundation to build unified descriptions on emergence of novel optical behavior from the interplay of covalent dopants, dispersants, and excitons in SWCNTs. [1] He, X. et al. Tunable room-temperature single-photon emission at telecom wavelengths from sp3 defects in carbon nanotubes. Nature Photonics 11, 577–582 (2017). [2] He, X. et al. Low-Temperature Single Carbon Nanotube Spectroscopy of sp3 Quantum Defects, ACS Nano, Articles ASAP (2017); DOI:10.1021/acsnano.7b03022

Constraining Photoluminescent Defect States in Chirality-Sorted Covalently Doped Single-Walled Carbon Nanotubes

Recent developments in carbon nanotube surface chemistry are leading to the emergence of unexpected new optical behaviors. Specifically, introduction of covalently-bound defect sites into the nanotube structure generates a new photoluminescent (PL) emitting state that is significantly red-shifted from the normal excitonic emitting state that results from optical excitation.1 These new states have been shown to significantly increase emission quantum yields and introduce new functionality including room-temperature single photon emission, near-IR photon upconversion, and multifunctionality generated by localization of excitons at these defect sites.2,3,4 The new optical behaviors present a broad range of unexplored photophysics yet to be fully understood. Of particular interest is the use of emerging functionalization chemistries to introduce sp3 defects that have highly tunable properties. An important challenge arising from the functionalization chemistry is that seemingly similar and structurally simple functionalization can give rise to a broad diversity in observed PL spectral features.1,3,5 Low-temperature spectroscopy along with quantum chemistry calculations demonstrated that these multiple emissive defect states are likely due to the occurrence of multiple possible, chemically distinct, binding configurations on the nanotube surface.6 Following these results, we hypothesize that the diverse emissive states are also influenced by the interplay of nanotube chirality, structure of the dopant, and surface structures on the tube. In this regard, we aim to control the number of emissive states through control of these factors. We have studied a number of different chiral structures for aryldiazonium functionalization, ranging from so-called near-armchair to near-zigzag structures of varying diameter. We have demonstrated that zigzag carbon nanotubes tend to constrain photoluminescent defect states more efficiently than other chiralities. Density functional theory (DFT) calculations suggest that symmetry of the zigzag tube is the primary factor in creating a less diversified emission spectrum. Our findings on controlling the defect peak diversity could enhance the predictable functionality of these doped carbon nanotubes.

Room-temperature single-photon emitters in titanium dioxide optical defects.

Fluorescence properties of crystallographic point defects within different morphologies of titanium dioxide were investigated. For the first time, room-temperature single-photon emission in titanium dioxide optical defects was discovered in thin films and commercial nanoparticles. Three-level defects were identified because the g(2) correlation data featured prominent shoulders around the antibunching dip. Stable and blinking photodynamics were observed for the single-photon emitters. These results reveal a new room-temperature single-photon source within a wide bandgap semiconductor.

InGaAs/InP quantum wires grown on silicon with adjustable emission wavelength at telecom bands

We report the growth of vertically stacked InGaAs/InP quantum wires on (001) Si substrates with adjustable room-temperature emission at telecom bands. Based on a self-limiting growth mode in selective area metal–organic chemical vapor deposition, crescent-shaped InGaAs quantum wires with variable dimensions are embedded within InP nano-ridges. With extensive transmission electron microscopy studies, the growth transition and morphology change from quantum wires to ridge quantum wells (QWs) have been revealed. As a result, we are able to decouple the quantum wires from ridge QWs and manipulate their dimensions by scaling the growth time. With minimized lateral dimension and their unique positioning, the InGaAs/InP quantum wires are more immune to dislocations and more efficient in radiative processes, as evidenced by their excellent optical quality at telecom-bands. These promising results thus highlight the potential of combining low-dimensional quantum wire structures with the aspect ratio trapping process for integrating III–V nano-light emitters on mainstream (001) Si substrates.

Single-Photon Emitters in Boron Nitride Nanococoons.

Quantum emitters in two-dimensional hexagonal boron nitride (hBN) are attractive for a variety of quantum and photonic technologies because they combine ultra-bright, room-temperature single-photon emission with an atomically thin crystal. However, the emitter's prominence is hindered by large, strain-induced wavelength shifts. We report the discovery of a visible-wavelength, single-photon emitter (SPE) in a zero-dimensional boron nitride allotrope (the boron nitride nanococoon, BNNC) that retains the excellent optical characteristics of few-layer hBN while possessing an emission line variation that is lower by a factor of 5 than the hBN emitter. We determined the emission source to be the nanometer-size BNNC through the cross-correlation of optical confocal microscopy with high-resolution scanning and transmission electron microscopy. Altogether, this discovery enlivens color centers in BN materials and, because of the BN nanococoon's size, opens new and exciting opportunities in nanophotonics, quantum information, biological imaging, and nanoscale sensing.

New insights into the complex photoluminescence behaviour of titanium white pigments

This work reports the analysis of the time-resolved photoluminescence behaviour on the nanosecond and microsecond time scale of fourteen historical and contemporary titanium white pigments. The pigments were produced with different production methods and post-production treatments, giving rise to a remarkable variability of titanium dioxide powders and, in some cases, to the formation of a complex surface of the crystal agglomerates. The pigments have been further characterized by Raman spectroscopy, scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy and inductively coupled plasma atomic emission spectrometry. Our study provides a clear view of the main features of the photoluminescence (PL) emission of anatase- and rutile-based pigments. For both the polymorphs of titanium dioxide the room-temperature photoluminescence emission is complex and involves different relaxation paths, related to shallow levels close to the conduction bands and mid-gap trap states. The PL behaviour appears to be little affected by post-production treatments such as organic and inorganic coatings. Instead, the presence of niobium impurities in the TiO2 crystal lattice, as residues of the sulphate synthesis process, induce a remarkable quenching of the visible emission of anatase-based pigments. We confirm that rutile-based and anatase-based pigments are significantly different in terms of photoluminescence behaviour. This clear distinction is a valuable point for non-invasive pigment identification by in-situ photoluminescence spectroscopy. In particular, while many organic binding media emit in the visible region, the near-infrared emission of rutile is specific and can likely be used to identify the pigment in more complex materials as paints. This research paves the way to future studies of the photo-physical properties of titanium white pigments, which is imperative to understand the risk of degradation induced by the well-known photocatalytic activity of this widely used 20th century pigment.

Growth of AlGaN-based multiple quantum wells on SiC substrates

Al0.2Ga0.8N/Al0.45Ga0.55N multiple quantum wells (MQWs) were grown on SiC substrates by metal–organic chemical vapor deposition. We studied the influence of well width and barrier width on the structural and optical properties of AlGaN MQWs in details. The MQWs structures prepared in this work all exhibited good periodicity and abrupt interfaces. When the barrier width was fixed, the thinner well was benefit to increase the integrated photoluminescence (PL) intensity of the MQWs and obtain a shorter ultraviolet wavelength. And the thicker barrier was also contributed to improve the optical properties while the well width remained the same. Compared with the effect of the well width, the emission wavelength of MQWs was less dependent on the barrier width. A 318-nm room-temperature PL emission was achieved when the well width and barrier width were 2.5 and 11 nm, respectively.

Transfer printing of semiconductor nanowire lasers

The authors review their work on the accurate positioning of semiconductor nanowire (NW) lasers by means of nanoscale Transfer Printing (nano-TP). Using this hybrid nanofabrication technique, indium phosphide NWs are successfully integrated at selected locations onto heterogeneous surfaces with high positioning accuracy. Moreover, they show that NW lasers can also be organised to form bespoke spatial patterns, including (one-dimensional) 1D or 2D arrays, or complex configurations with a defined number of NWs and controlled separation between them. Besides, their nano-TP technique also permits the integration of NWs with different dimensions in a single system. Notably, the nano-TP fabrication protocols do not affect the optical or structural properties of the NWs and they retain their room-temperature lasing emission after their positioning onto all investigated receiving surfaces. This developed nano-TP technique offers therefore new exciting prospects for the fabrication of hybrid bespoke nanophotonic systems using NW lasers as building blocks.

Luminescence Thermochromism of Gold(I) Phosphane-Iodide Complexes: A Rule or an Exception?

A series of gold(I) iodide complexes 1-11 have been prepared from di-, tri-, and tetraphosphane ligands. Crystallographic studies reveal that the di- (1-7) and tetrametallic (11) compounds feature linearly coordinated gold(I) ions with short aurophilic contacts. Their luminescence behavior is determined by the combined influence of the phosphane properties, metal-metal interaction, and intermolecular lattice-defined interactions. The proposed variable contribution of 3 (X+M)-centered (X=halogen; M=metal) and 3 XLCT (halogen to ligand charge transfer) electronic transitions into the lowest lying excited state, which is influenced by supramolecular packing, is presumably responsible for the alteration of room-temperature emission color from green (λ=545 nm, for 11) to near-IR (λ=698 nm, for 2). Dinuclear compounds 6 and 7 exhibit distinct luminescence thermochromism with a blueshift up to 5750 cm-1 upon cooling. Such dramatic change of emission energy is assigned to the presence of two coupled triplet excited states of 3 ππ* and 3 (X+M)C/3 XLCT nature, the presence of which depends on both molecular structure and the crystal lattice arrangement.

Strong room-temperature emission from defect states in CVD-grown WSe2 nanosheets

Monolayer transition metal dichalcogenides (TMDCs), as direct bandgap semiconductors, show promise for applications in ultra-thin flexible optoelectronic devices. However, the optical properties and device performance are greatly affected by defects, such as vacancies, present in these materials. Vacancies exist unavoidably in mechanically exfoliated or grown by chemical vapor deposition (CVD) monolayer TMDCs; therefore, their influence on the electric and optical properties of host materials has been widely studied. Here, we report a new defect state located at 1.54 eV, which is 70 meV lower than the neutral exciton energy in as-prepared MoS2 monolayers grown by CVD. This defect state is clearly observed in photoluminescence (PL) and Raman spectra at ambient conditions. PL mapping, Raman mapping, and atomic force microscopy analysis indicate a solid-vapor reaction growth mechanism of the defect state formation. During a certain growth stage, nuclei with the composition of WOxSey do not fully react with the Se vapor, leading to the defect formation. This type of defects permits radiative recombination of bound neutral excitons, which can make the PL intensity as strong as the intrinsic excitation. Our findings reveal a new way to tailor the optical properties of two-dimensional TMDCs without any additional processes performed after growth.

Influence of n-type doping in germanium crystalline films grown on silicon substrates on room-temperature photoluminescence and optical absorption

We investigate the influence of n-type doping in germanium crystalline films grown on silicon substrates on room-temperature light emission and absorption. The photoluminescence and optical absorption of germanium films on silicon with various carrier concentrations were measured at room temperature. The photoluminescence intensity increased with increasing carrier concentration under low-power excitation, but it quenched at high carrier concentration under high-power excitation. This carrier concentration quenching works against the heavy n-type doping proposed for achieving germanium lasing action. For the undoped germanium film, the absorption at energies near the band edge was enhanced by excitonic effects. There was no enhancement in the absorption spectra of the doped germanium films. The direct band-gap narrowing observed in these absorption measurements was estimated to be about 14 meV at a carrier concentration of 4 × 1019 cm−3. This value was considerably smaller than the value (40 meV) reported in a study employing photoluminescence measurements.

One-step synthesis of cyclic compounds towards easy room-temperature phosphorescence and deep blue thermally activated delayed fluorescence.

With the advantages of facile one-pot synthesis and THF-irrigating purification, we first developed novel cyclic compounds with deep blue thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) dual emission even in the as-spun films. Careful crystal analysis, combined with theoretical calculations, reveal that abundant inter- and intra-molecular interactions, whatsoever the molecular packing, are obligated to unique RTP properties. These results open up a new study of compounds with multiple acceptors and donors, and represent an important step in the further development of deep blue TADF and easy RTP emission.

Electrically injected 13-μm quantum-dot photonic-crystal surface-emitting lasers

We have fabricated electrically injected InAs/InGaAs/GaAs quantum-dot (QD) photonic-crystal (PC) surface-emitting lasers (SELs) and successfully demonstrated room-temperature lasing emissions at 1.3-μm wavelength for the first time. The PCSEL device fabrication was greatly simplified by deposition of transparent conducting layer of indium-tin-oxide over “PC slab-on-substrate” structure. The threshold current density per QD layer was as low as 50 A/cm2/layer; however, the optical output was limited to 2 mW. The band-edge lasing mode was identified and near-circular beam with narrow divergence angle less than 2° was achieved.

Investigating new activators for small-bandgap LaX3 (X = Br, I) scintillators

Luminescence and scintillation properties of Bi3+, Sb3+, and Eu2+-doped LaI3 and LaBr3 were explored. Out of the three dopants investigated, Eu2+ was the most promising new activator for small-bandgap LaX3 (X = Br, I) and was further studied in the mixed-halide LaBr3−xIx. Crystals were grown from the melt using the vertical Bridgman method. LaBr3:Eu2+ 0.5% (mol) had the most favorable scintillation properties with a light output of 43,000 ph/MeV and 6% energy resolution at 662 keV. Performance of LaBr3−xIx:Eu2+ worsened for most samples as iodide concentration was increased. Room-temperature scintillation of LaI3:Eu2+ 0.1% and 0.5% was observed and is the first case of room-temperature emission reported for doped LaI3.

Room-Temperature Mid-Infrared Emission from Faceted InAsSb Multi Quantum Wells Embedded in InAs Nanowires.

There is considerable interest in the development of InAsSb-based nanowires for infrared photonics due to their high tunability across the infrared spectral range, high mobility, and integration with silicon electronics. However, optical emission is currently limited to low temperatures due to strong nonradiative Auger and surface recombination. Here, we present a new structure based on conical type II InAsSb/InAs multiquantum wells within InAs nanowires which exhibit bright mid-infrared photoluminescence up to room temperature. The nanowires are grown by catalyst-free selective area epitaxy on silicon. This unique geometry confines the electron-hole recombination to within the quantum wells which alleviates the problems associated with recombination via surface states, while the quantum confinement of carriers increases the radiative recombination rate and suppresses Auger recombination. This demonstration will pave the way for the development of new integrated quantum light sources operating in the technologically important mid-infrared spectral range.

Intensities and spectral features of the – potential laser transition of Er3+ centers in CaF2–CeF3 disordered crystal**Project supported by Shanghai Engineering Research Center for Sapphire Crystals, China (Grant No. 14DZ2252500), the Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics Chinese Academy of Sciences (Grant No. 2008DP17301), the Fundamental Research Funds for the Central Universities of China, the National

A CaF2–CeF3 disordered crystal containing 1.06% of Er3+ ions was grown by the temperature gradient technique. Optical absorption and emission spectra recorded at room temperature and at 10 K, luminescence decay curve recorded at room temperature, and extended x-ray-absorption fine structure spectra were analyzed with an intention to assess the laser potential related to the transition of Er3+. In addition, the thermal diffusivity of the crystal was measured at room temperature. The analysis of room-temperature spectra revealed that the emission is long-lived with a radiative lifetime value of 5.5 ms, peak emission cross section of , and large spectral width pointing at the tunability of the emission wavelength in the region stretching from approximately 1480 nm to 1630 nm. The energies of the crystal field components for the ground and excited multiplets determined from low-temperature absorption and emission spectra made it possible to predict successfully the spectral position and shape of the room-temperature emission band. Based on the correlation of the optical spectra and dynamics of the luminescence decay, it was concluded that in contrast to Yb3+ ions in heavily doped CaF2 erbium ions in the CaF2–CeF3 crystal reside in numerous sites with dissimilar relaxation rates.

Purely excitonic lasing in ZnO microcrystals: Temperature-induced transition between exciton-exciton and exciton-electron scattering

Since the seminal observation of room-temperature laser emission from ZnO thin films and nanowires, numerous attempts have been carried out for detailed understanding of the lasing mechanism in ZnO. In spite of the extensive efforts performed over the last decades, the origin of optical gain at room temperature is still a matter of considerable discussion,. We show that ZnO microcrystals with a size of a few micrometers exhibit purely excitonic lasing at room temperature without showing any symptoms of electron-hole plasma emission. We then present the distinct experimental evidence that the room-temperature excitonic lasing is achieved not by exciton-exciton scattering, as has been generally believed, but by exciton-electron scattering. As the temperature is lowered below ~150 K, the lasing mechanism is shifted from the exciton-electron scattering to the exciton-exciton scattering. We also argue that the ease of carrier diffusion plays a significant role in showing room-temperature excitonic lasing.