Amorphous Hosts Research Articles

A link between the size and composition of comets

ABSTRACT All cometary nuclei that formed in the early Solar System incorporated radionuclides and therefore were subject to internal radiogenic heating. Previous work predicts that if comets have a pebble-pile structure internal temperature build-up is enhanced due to very low thermal conductivity, leading to internal differentiation. An internal thermal gradient causes widespread sublimation and migration of either ice condensates, or gases released from amorphous ice hosts during their crystallization. Overall, the models predict that the degree of differentiation and re-distribution of volatile species to a shallower near-surface layer depends primarily on nucleus size. Hence, we hypothesize that cometary activity should reveal a correlation between the abundance of volatile species and the size of the nucleus. To explore this hypothesis, we have conducted a thorough literature search for measurements of the composition and size of cometary nuclei, compiling these into a unified data base. We report a statistically significant correlation between the measured abundance of CO/H2O and the size of cometary nuclei. We further recover the measured slope of abundance as a function of size, using a theoretical model based on our previous thermophysical models, invoking re-entrapment of outward migrating high volatility gases in the near-surface pristine amorphous ice layers. This model replicates the observed trend and supports the theory of internal differentiation of cometary nuclei by early radiogenic heating. We make our data base available for future studies, and we advocate for collection of more measurements to allow more precise and statistically significant analyses to be conducted in the future.

Dynamic Ultra‐long Room Temperature Phosphorescence Enabled by Amorphous Molecular “Triplet Exciton Pump” for Encryption with Temporospatial Resolution

AbstractOrganic ultra‐long room‐temperature phosphorescence (RTP) materials in the amorphous state have attracted widespread attention due to their simple preparation and flexibility to adopt various forms in sensors, bioimaging, and encryption applications. However, the amorphous molecular host for the host–guest RTP systems is highly demanded but limited. Here, a universal molecular host (DPOBP−Br) has been designed by integration of an amorphous moiety of diphenylphosphine oxide (DPO) and an intersystem crossing (ISC) group of 4‐bromo‐benzophenone (BP−Br). Various commercial fluorescence dyes were doped into the tight and transparent DPOBP−Br film, respectively, resulting in amorphous host–guest systems with ultra‐long RTP colors from green to red. It was found that DPOBP−Br acted as a universal “triplet exciton pump” for promoting the generation of triplet excitons in the guest, through energy transfer processes and external heavy‐atom effect based on DPOBP−Br. Interestingly, dynamic RTP was achieved by controlling residual oxygen concentration in the amorphous matrix by UV irradiation. Therefore, multi‐dimensional anti‐counterfeiting coatings were realized even on curved surfaces, simultaneously exhibiting spatial and 2D‐time dependence. This work provides a strategy to design new amorphous molecular hosts for RTP systems and demonstrates the advanced information encryption with tempo‐spatial resolution based on the dynamic ultra‐long RTP of an amorphous system.

Dynamic Ultra-long Room Temperature Phosphorescence Enabled by Amorphous Molecular "Triplet Exciton Pump" for Encryption with Temporospatial Resolution.

Organic ultra-long room-temperature phosphorescence (RTP) materials in the amorphous state have attracted widespread attention due to their simple preparation and flexibility to adopt various forms in sensors, bioimaging, and encryption applications. However, the amorphous molecular host for the host-guest RTP systems is highly demanded but limited. Here, a universal molecular host (DPOBP-Br) has been designed by integration of an amorphous moiety of diphenylphosphine oxide (DPO) and an intersystem crossing (ISC) group of 4-bromo-benzophenone (BP-Br). Various commercial fluorescence dyes were doped into the tight and transparent DPOBP-Br film, respectively, resulting in amorphous host-guest systems with ultra-long RTP colors from green to red. It was found that DPOBP-Br acted as a universal "triplet exciton pump" for promoting the generation of triplet excitons in the guest, through energy transfer processes and external heavy-atom effect based on DPOBP-Br. Interestingly, dynamic RTP was achieved by controlling residual oxygen concentration in the amorphous matrix by UV irradiation. Therefore, multi-dimensional anti-counterfeiting coatings were realized even on curved surfaces, simultaneously exhibiting spatial and 2D-time dependence. This work provides a strategy to design new amorphous molecular hosts for RTP systems and demonstrates the advanced information encryption with tempo-spatial resolution based on the dynamic ultra-long RTP of an amorphous system.

Photoluminescence Investigations of Dy3+-Doped Silicate Xerogels and SiO2-LaF3 Nano-Glass-Ceramic Materials.

In this work, the series of Dy3+-doped silicate xerogels were synthesized by sol-gel technique and further processed at 350 °C into SiO2-LaF3:Dy3+ nano-glass-ceramic materials. The X-ray diffraction (XRD) measurements, along with the thermal analysis, indicated that heat-treatment triggered the decomposition of La(TFA)3 inside amorphous sol-gel hosts, resulting in the formation of hexagonal LaF3 phase with average crystal size at about ~10 nm. Based on the photoluminescence results, it was proven that the intensities of blue (4F9/2 → 6H15/2), yellow (4F9/2 → 6H13/2), and red (4F9/2 → 6H11/2) emissions, as well as the calculated yellow-to-blue (Y/B) ratios, are dependent on the nature of fabricated materials, and from fixed La3+:Dy3+ molar ratios. For xerogels, the emission was gradually increased, and the τ(4F9/2) lifetimes were elongated to 42.7 ± 0.3 μs (La3+:Dy3+ = 0.82:0.18), however, for the sample with the lowest La3+:Dy3+ molar ratio (0.70:0.30), the concentration quenching was observed. For SiO2-LaF3:Dy3+ nano-glass-ceramics, the concentration quenching effect was more visible than for xerogels and started from the sample with the highest La3+:Dy3+ molar ratio (0.988:0.012), thus the τ(4F9/2) lifetimes became shorter from 1731.5 ± 5.7 up to 119.8 ± 0.4 μs. The optical results suggest, along with an interpretation of XRD data, that Dy3+ ions were partially entered inside LaF3 phase, resulting in the shortening of Dy3+-Dy3+ inter-ionic distances.

Influence of crystalline mediums KCl and KDP on structural and optical properties of embedded CdSe quantum dots

In order to pass comfortably to the practicalities of nanomaterials it is necessary to use, in general, a support like: films, amorphous hosts or crystalline hosts. So it is important to know the effect of hosts on structural and optical properties of nanomaterials, in this context, this investigation suggests to survey the behavior of CdSe quantum dots (QDs) dispersed in two different crystalline hosts the potassium chlorade KCl and the potassium Di-hydrogen phosphate KH2PO4 (KDP) single crystals, where they are prepared using tow different technics: the Czochralski (Cz) method and the lowering temperature from aqueous solution technic, respectively. The obtained results from X-ray diffraction (XRD) demonstrate that CdSe QDs have different preferred orientations inside these two hosts. This behavior will be discussed in detail. Nevertheless, the UV–vis and Phtoluminiscance (PL) spectroscopy prove that the optical properties of CdSe QDs have great similarity in these two hosts.

EPR characterization of erbium in glasses and glass ceramics

Electron paramagnetic resonance (EPR) is a well-established spectroscopic technique for electronic structure characterization of rare-earth ion impurities in crystalline and amorphous hosts. EPR spectra of erbium-doped glass matrices and nanocomposites can provide information about local structure variations induced by changes in chemical composition or crystallization processes. Characterization possibilities of Er3+ ions in glasses and glass ceramics including direct EPR measurements, indirect investigations via secondary paramagnetic probes, and optically detected magnetic resonance techniques are considered in this article.

Organic Light-Emitting Diode Employing Metal-Free Organic Phosphor.

Metal-free organic phosphorescent materials are promising alternatives to the organometallic counterparts predominantly adopted in organic light-emitting diodes due to their low cost, chemical stability, and large molecular design window. However, only a few reports on OLED devices incorporating metal-free organic phosphors have been presented due to the lack of understanding on material properties, device physics, and device fabrication processes. Here, we report a tailor-designed novel fluorene-based organic phosphor with efficient spin-orbit coupling activated by bromine, aromatic carbonyl, and spiro-annulated phenyl moieties. Photoluminescence quantum yield of 24.0% was achieved when doped in optically inert amorphous polymer hosts. Effects of OLED host materials on the phosphor were investigated in terms of color purity, suppression of exciplex emission, and restraint of molecular motion. Bright green phosphorescence emission (1430 cd/m2 at 100 mA/cm2) was realized with 2.5% maximum external quantum efficiency at 1 mA/cm2.

Spectroscopic Properties of Eu3+ Ions in Sol–Gel Materials Containing Calcium Fluoride Nanocrystals

In this study, a series of Eu3+‐doped silicate xerogels with modified initial compositions is prepared and heat treated at 350 °C to fabricate glass‐ceramic materials containing CaF2 nanocrystals. The impact of an N,N‐dimethylformamide (DMF) additive and a catalyst agent (CH3COOH, HNO3) on the sol–gel transformation from liquid sols through silicate gels and xerogels into glass‐ceramics is investigated using infrared spectroscopy. To verify the formation of CaF2 nanocrystals, X‐ray diffraction (XRD) measurements are performed. The optical behavior of Eu3+ ions in the prepared sol–gel materials is evaluated based on photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra as well as luminescence decay analysis. The distribution of Eu3+ ions between the amorphous silicate hosts and CaF2 nanocrystals in the prepared glass‐ceramic samples is found due to the biexponential character of registered decay curves. In addition, R/O‐ratio (R/O is defined as ratio I(‐)/I(‐) analysis is conducted to characterize the efficiency of Eu3+ ion migration into precipitated CaF2 nanocrystals. The lack of DMF as well as HNO3 as a catalyst in the initial composition significantly promotes the incorporation of Eu3+ ions into CaF2 nanocrystals, resulting in long‐lived luminescence with an estimated lifetime τ() = 12.84 ms.

Use of effective fragment potentials for simulation of excited states in an inhomogeneous environment

AbstractSinglet and triplet spectra of phosphorescent organic light‐emitting diode dopants such as bis(2‐methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) in inhomogeneous amorphous hosts are simulated by time‐dependent density functional theory (TDDFT) using molecular dynamics and effective fragment potentials (EFPs). The EFPs of the host molecules N,N′‐di(1‐naphthyl)‐N,N′‐diphenyl‐(1,1′‐biphenyl)‐4,4′‐diamine and 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl are constructed from small fragments. The procedure for breaking large molecules into fragments and constructing an EFP is presented. It is demonstrated that polarizable inhomogeneous environment affects the position, intensity, and width of the spectral bands and, therefore, should be taken into account in accurate simulations of spectral bands.

Lead fluoride β-PbF 2 nanocrystals containing Eu 3+ and Tb 3+ ions embedded in sol-gel materials: Thermal, structural and optical investigations

Abstract Nanocrystalline β-PbF2 phase singly-doped with Eu3+ and Tb3+ ions have been successfully synthesized using sol-gel technique and subsequent heat-treatment of xerogels at 350 °C. Thermal behavior and structural properties of obtained materials were studied using thermogravimetric analysis (TG), differential scanning calorimetry (DSC) as well as FT-IR and Raman techniques. XRD results confirmed formation of β-PbF2 nanocrystals embedded in silica amorphous hosts after annealing at 350 °C. Moreover, the photoluminescence properties have been investigated based on excitation and emission spectra as well as decay analysis from the 5D0 (Eu3+) and the 5D4 (Tb3+) excited states. The sharp intraconfigurational 4f6−4f6 and 4f8−4f8 emission transitions assigned as the 5D0→7FJ (J=0–4) of Eu3+ and the 5D4→7FJ (J=6-3) of Tb3+ bands, respectively, were registered. The most prominent bands in studied xerogels and glass-ceramic materials are related to the following electronic transitions: 5D0→7F1 (orange) and 5D0→7F2 (red) (Eu3+) as well as 5D4→7F5 (green) and 5D4→7F6 (blue) (Tb3+). Thus, the R/O (Eu3+) and G/B (Tb3+) luminescence intensity ratios were calculated and analyzed. Luminescence decay kinetic clearly indicated a presence of two different surroundings around Eu3+ and Tb3+ dopants in β-PbF2-based glass-ceramic samples. In such singly-doped with Eu3+ and Tb3+ materials, the longer luminescence lifetimes (Eu3+: τ1(5D0)=0.90 ms, τ2(5D0)=5.15 ms; Tb3+: τ1(5D4)=0.48 ms, τ2(5D4)=4.01 ms) of an appropriate excited states were achieved in comparison to xerogel hosts (Eu3+: τ(5D0)=0.38 ms; Tb3+: τ(5D4)=0.49 ms). The obtained results indicate the incorporation of Eu3+ and Tb3+ ions into nanocrystalline phase during ceramization process.

Absorption Spectroscopy Analysis of Calcium-Phosphate Glasses Highly Doped with Monovalent Copper.

CaO-P2 O5 glasses with high concentrations of monovalent copper ions were prepared by a simple melt-quench method through CuO and SnO co-doping. Spectroscopic characterization was carried out by optical absorption with the aim of analyzing the effects of Cu(+) ions on the optical band-gap energies, which were estimated on the basis of indirect-allowed transitions. The copper(I) content is estimated in the CuO/SnO-containing glasses after the assessment of the concentration dependence of Cu(2+) absorption in the visible region for CuO singly doped glasses. An exponential dependence of the change in optical band gaps (relative to the host) with Cu(+) concentration is inferred up to about 10 mol %. However, the entire range is divided into two distinct linear regions that are characterized by different rates of change with respect to concentration: 1) below 5 mol %, where the linear dependence presents a relatively high magnitude of the slope; and 2) from 5-10 mol %, where a lower magnitude of the slope is manifested. With increasing concentration, the mean Cu(+) -Cu(+) interionic distance decreases, thereby decreasing the sensitivity of monovalent copper for light absorption. The decrease in optical band-gap energies is ultimately shown to follow a linear dependence with the interionic distance, suggesting the potential of the approach to gauge the concentration of monovalent copper straightforwardly in amorphous hosts.

Efficient and long-lived Zeeman-sublevel atomic population storage in an erbium-doped glass fiber

Long-lived population storage in optically pumped levels of rare-earth ions doped into solids, referred to as persistent spectral hole burning, is of significant fundamental and technological interest. However, the demonstration of deep and persistent holes in rare-earth ion doped amorphous hosts, e.g. glasses, has remained an open challenge since many decades -- a fact that motivates our work towards a better understanding of the interaction between impurities and vibrational modes in glasses. Here we report the first observation and detailed characterization of such holes in an erbium-doped silica glass fiber cooled to below 1 K. We demonstrate population storage in electronic Zeeman-sublevels of the erbium ground state with lifetimes up to 30 seconds and 80\% spin polarization. In addition to its fundamental aspect, our investigation reveals a potential technological application of rare-earth ion doped amorphous materials, including at telecommunication wavelength.

Enhanced Terbium Emission Due to Plasmonic Silver Nanoparticles in Bismuth Silicate

A simple way to select a suitable host material, when doped with any rare‐earth (RE) ion and incorporated with silver nanoparticles (NPs), to cause overlap between an excitation band of the RE ions and the localized surface plasmon resonance (LSPR) of the metallic NPs to study possible plasmonic enhancement is presented using Mie theory calculations. Unlike in previous studies, plasmonic enhancement was studied in a crystalline host instead of amorphous hosts. Bismuth silicate was synthesized using the sol–gel method and successfully doped with only terbium or silver, or co‐doped with terbium and silver. The formation of silver NPs was investigated using diffuse reflectance spectroscopy, transmission electron microscopy, and Auger electron spectroscopy (AES). Luminescence properties of the terbium‐doped bismuth silicate containing silver NPs were explored in detail and an enhancement of the emission from terbium ions at 545 nm when excited at 485 nm of about 2.5 times is attributed to amplification of the electric field associated with the LSPR of the silver NPs.

Long-lived Triplet Excitons Allowed by Intermolecular Hydrogen Bonding in Amorphous Matrix

A purely organic guest material with a long triplet excited state lifetime (τ > 1 s) was doped into two different π-conjugated amorphous hosts with similar glass transition temperatures (Tg). The phosphorescence quenching behavior of the guest material was investigated under vacuum conditions. When doped into a host that did not exhibit intermolecular hydrogen bonding, the τ of the guest decreased significantly at temperatures below the Tg of the host. Conversely, when doped into a host that did exhibit intermolecular hydrogen bonding, the τ remained almost unaffected at temperatures below the Tg of the host. The diffusion coefficient of oxygen, which was largely removed from the host materials under vacuum, was similar for both host materials. These results indicated that weak thermal diffusional motion of a host material intrinsically quenches long-lived triplet excitons of purely organic guest materials.

(Invited) Rare Earth Luminescence in Nanostructured Amorphous Silicon Alloys

Rare Earth (RE) doped amorphous silicon alloys can be prepared by reactive RF sputtering from a Si target partially covered with metallic or oxide RE platelets using appropriate reactive atmospheres. We have studied Er3+, Nd3+, Eu3+ and Tb3+ in a-Si:H, a-SiOx:H and a-SiNx:H. Annealing optimizes the photoluminescence and can induce the formation of Si nanocrystals. The RE act as nucleation centers. The RE ions present intense photoluminescence at room temperature. EXAFS measurements reveal a highly non-centrosymetric lattice site for Er in a-SiOx:H. This partially breaks the selection rule that forbids intra-4f transitions. The RE present two luminescence lifetimes, the fast component determined by the host and the slow component associated to the local symmetry ofthe ions. The amorphous and nanostructured hosts increase the cross section for RE excitation by a few orders of magnitude, making the materials practical for applications as phosphors and active amplifiers.

Highly efficient p-dopants in amorphous hosts

We study the influence of the molecular energy levels on doped organic layers, using four different combinations of two amorphous hosts (MeO-TPD and BF-DPB) and two efficient p-dopants (F6-TCNNQ and C60F36). Conductivity and Seebeck studies are performed in situ, varying the doping concentration over more than two orders of magnitude. Whereas trends of doping are in agreement with the hosts’ energy levels, trends deviate from the expectation based on the dopants’ energy levels. A lower limit for the mobility can be derived from conductivity data, which for samples of F6-TCNNQ increases with doping, even exceeding the measured OFET-mobility of intrinsic MeO-TPD.

The composition, structure and optical properties of weakly magnetic Co-containing amorphous Si and Ge films

This study investigated the composition, structure and optical properties of amorphous SiCo and GeCo films. The samples were prepared by radio frequency sputtering. Films were deposited with Co atomic concentrations in the range of 1.7–10.3 at.%. After deposition, the films were submitted to thermal treatments up to 900 °C and investigated by energy dispersive X-ray spectrometry, X-ray diffraction, Raman scattering and optical transmission spectroscopy. Additionally, magnetic force microscopy measurements were performed at room temperature. For comparison purposes, Co-free samples were also prepared, annealed and characterized following a similar procedure. The experimental results indicated the following: (1) the Co atoms were effectively and homogeneously incorporated into the amorphous hosts; (2) the as-deposited films (either pure or containing Co) were essentially amorphous; (3) annealing the films at high temperatures induced crystallization; (4) after crystallization, non-magnetic CoSi2 (silicide) and CoGe2 (germanide) phases were identified in the Co-containing Si and Ge films, respectively; (5) the optical properties of the films were significantly affected by the insertion of Co and by the annealing temperature; and (6) the samples exhibited a reduced magnetic signal at room temperature. These experimental observations were systematically studied, which are presented and discussed in this report.

Single-molecule probing of incommensurate biphenyl

Our data on the distribution of purely electronic linewidths of single molecules of terrylene in incommensurate biphenyl crystals are compared with the data obtained by other groups for different low-temperature organic solid hosts and with results of numerical simulations. The first two moments of the distributions measured within a narrow temperature interval have been used to calculate a single dimensionless parameter characterizing each of the respective hosts—the variation coefficient. It appears that different amorphous hosts have similar values of this coefficient, but the value obtained for the incommensurate crystal of biphenyl is significantly different. One can conclude that the remarkable single-molecule line broadening in biphenyl at 1.8K cannot be solely explained by the interaction with two-level systems, which is considered to cause the broadening in amorphous hosts.

Design of star-shaped molecular architectures based on carbazole and phosphine oxide moieties: towards amorphous bipolar hosts with high triplet energy for efficient blue electrophosphorescent devices

With a carbazole moiety as the electron donor and a phosphine-oxide moiety as the electron acceptor, two novel star-shaped bipolar hosts, 4,4′,4″-tri(N-carbazolyl)triphenylphosphine oxide (TCTP) and 3,6-bis(diphenylphosphoryl)-9-(4′-(diphenylphosphoryl)phenyl)carbazole (TPCz), have been designed and synthesized. Their topology structure differences are that the phosphine-oxide moiety is located in the molecular centre and the periphery for TCTP and TPCz, respectively. The star-shaped architecture imparts them with high decomposition temperatures (Td: 497 °C for TCTP and 506 °C for TPCz) and results in the formation of a stable amorphous glassy state (Tg: 163 °C for TCTP and 143 °C for TPCz), while the phosphine oxide linkage ensures the disrupted conjugation and the high triplet energy (>3.0 eV). In addition, both TCTP and TPCz possess a bipolar transporting capability. However, TCTP mostly transports holes and TPCz primarily conducts electrons. On the basis of appropriate device configurations, high performance blue electrophosphorescent devices with comparable efficiency (35.0–36.4 cd A−1, 15.9–16.7%) have been realized using TCTP and TPCz as the host for the blue phosphor, respectively. Compared with the unipolar host, 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA, 15.9 cd A−1, 7.8%), the efficiency is improved by more than two-fold. As far as the obtained state-of-the-art performance is concerned, we think that these novel materials should provide an avenue for the design of amorphous bipolar hosts with high triplet energy used for blue PhOLEDs on a star-shaped scaffold.

Blue Light Emitting Ir(III) Compounds for OLEDs - New Insights into Ancillary Ligand Effects on the Emitting Triplet State

The sky-blue emitting phosphorescent compound Ir(4,6-dFppy)(2)(acac) (FIracac) doped into different matrices is studied under ambient conditions and at cryogenic temperatures on the basis of broadband and high-resolution emission spectra. The emitting triplet state is found to be largely of metal-to-ligand charge transfer (MLCT) character. It is observed that different polycrystalline and amorphous hosts distinctly affect the properties of the triplet. Moreover, a comparison of FIracac with the related Ir(4,6-dFppy)(2)(pic) (FIrpic), differing only by the ancillary ligand, reveals obvious changes of properties of the emitting state. These observations are explained by different effects of acac and pic on the Ir(III) d-orbitals. In particular, the occupied frontier orbitals, strongly involving the t(2g)-manifold, and their splitting patterns are modified differently. This influences spin-orbit coupling (SOC) of the emitting triplet state to higher-lying (1,3)MLCT states. As a consequence, zero-field splittings, radiative decay rates, and phosphorescence quantum yields are changed. The important effects of SOC are discussed qualitatively and are related to the emission properties of the individual triplet substates, as determined from highly resolved spectra. The results allow us to gain a better understanding of the impact of SOC on the emission properties with the aim to develop more efficient triplet emitters for OLEDs.