Pressure-induced Luminescence Research Articles

Pressure-induced emission enhancement: A review

The luminescent properties of some materials undergo significant changes under compression. High pressure generated by a diamond anvil cell (DAC) is often used as an external stimulus to explore the relationship between the structures and luminescent properties of materials, provide traceable color and structural changes, and quantify the environment in which the materials are located. Under high pressure, the luminous intensity or color of materials changes, which has important potential applications in fields such as safety detection, information storage, optoelectronic devices, and mechanical sensing. Recently, many phenomena of pressure-induced luminescence enhancement have been discovered in DAC, commonly referred to as pressure-induced emission enhancement. In this review, recent pressure-induced emission enhancement phenomena have been collected, and the role of pressure in promoting the luminescent enhancement of materials in DAC was revealed and discussed, which helps to design some materials with specific emission characteristics and provides a perspective for in-depth research on the photophysical behavior of materials.

Pressure-induced luminescence evolution of 3,3′-diamino-4,4′-azofurazan: Role of restricting chemical bond vibration and conformational modification

The luminescence and electronic structure of 3,3′-Diamino-4,4′-azofurazan (DAAzF) were studied under high pressure conditions through experimental and calculation approaches. The transition of π* → π was primarily responsible for DAAzF's broad light emission. Upon applying pressure to DAAzF, high-pressure-stiffened hydrogen-bond interactions enable the restriction of the stretching vibration of NH2 group. The reduced energy loss through nonradiative rotational relaxation and molecular motions lead to a ∼20 times luminescent enhancement of DAAzF from 1 atm to 8.9 GPa. With the further strengthening of interlayer hydrogen bond interactions at higher pressure, the deviation of hydrogen atoms in amino groups from the molecular plane lessens the radiation transition efficiency. In addition, the bending of the C–C–N=N bond further leads to molecular conformation changes at approximately 20.7 GPa, which induces an abrupt redshift and moderate quenching of the luminescence. Furthermore, the band gap of DAAzF is significantly influenced by pressure. As the color undergoes a transition from yellow to red, and becomes darker as the pressure increases, the absorption edge shifted towards red. At 3.4, 9, and 21 GPa, three conformational variations were identified in conjunction with electronic structural alterations.

Flexible 2D graphene-coupled ZnS:Mn2+ mechanodetectors for heart rate monitoring

Materials with pressure-induced luminescence is a type of important energy-storage materials that can release the stored energy in form of light upon an external mechanical stimulus, which can be used for light sources, visualization of stress distribution and mapping of dynamic pressure of the materials. However, using such type of materials as the wearable applications to monitor health problems receives little attention in the previous works. Herein, the mechanoluminescent (ML) materials, i.e., ZnS:Mn2+, with an broad emission band peaking at 586 nm upon excitation at 320 nm, are prepared by using an auto-clave method by addition of Na2S to a solution containing Zinc Oleate and Mn(NO3)2. Since the pure ZnS:Mn2+ can release the ML under external press, here we have coupled it with the 2D graphene to form 2D graphene-coupled ZnS:Mn2+ mechanodetector in order to make the ZnS:Mn2+ having the supercapacity property and keep the heart rate monitoring when there is no external press. The crystal structure, photoluminescence, ML and electrochemical properties of ZnS:Mn2+ with and without 2D graphene are characterized by several techniques, including X-ray diffraction, transmission electron microscopy (TEM), photoluminescence (PL) and pressure-induced ML spectra, galvanostatic charge-discharge and cyclic voltammetry. Our findings reveal that 2D graphene-coupled ZnS:Mn2+ possesses pressure-controlled luminescence. By dispersing highly conductive polyoxometalate (POM) into graphene-coupled ZnS:Mn2+ and then detecting the relative ML intensity and pressure value, we find the ML intensity and pressure value feature a linear relationship within a certain range and can basically come back its original state after several cycles. Due to the introduction of the conductive 2D graphene and POM, the graphene-coupled ZnS:Mn2+ is not only sensitive to the external mechanical stimulus but also can possess the electric conduction due to the supercapacity property. The heart rate monitoring results of the device show great potential for the wearable applications to monitor human health problem.

Influence of pressure on the structure and luminescence properties of AMP-protected gold nanoparticles as revealed by fluorescence spectra and 2D correlation analysis

In the present study we have applied in situ fluorescence spectroscopy, in combining with two-dimensional correlation spectroscopy (2DCOS), to investigate the dynamic fluorescence changes of adenosine 5′-monophosphate capped gold nanoclusters (AuNCs@AMP) versus pressure in solid. Several weak and overlapped spectral components, developed with pressure, are differentiated and assigned in detailed by 2DCOS analysis. In addition, the sequential spectral variations are also revealed, which take place as follows. The ligands-surface Au(I) (denoting as 450, 480 nm), surface Au(I)⋯Au(I) (as 515, 520 and 565 nm), and even the inner metal core Au–Au interactions (as 605 nm), take place at pressure range from low to high. Moreover, the pressure-induced luminescence performance is also compared between AuNCs@AMP being prepared at 60 and 80 °C, respectively. It implies that it is more stable for AuNCs prepared at 60 °C as which has better pressure tolerance. Finally, it is confirmed that the orientation of AMP at the gold surface plays a crucial role in the tuning of luminance performance, especially the emission intensity to compression.

Performance evaluation of Ce3+ doped flexible PVDF fibers for efficient optical pressure sensors

This work proposes a highly novel centrifugally spun lanthanide doped nonwoven fibrous mat as a non-contact optical pressure sensor with a wide linear dynamic range and good pressure sensitivity. The sensor properties are based on the spectral shift, broadening and intensity enhancement of Ce3+ ion in Ce doped PVDF fiber upto significantly high pressure. Two different systems: Ce(NO3)3·6H2O and (NH4)4Ce(SO4)4·2H2O doped PVDF flexible fibers (CeN-PF and CeS-PF) were produced using the Forcespinning® technique. Both CeN-PF and CeS-PF fibers displayed violet-blue emission under UV irradiation due to a 5d-4f transition of Ce3+ ions. Our emission results show that both CeN-PF and CeS-PF spectral characteristics are influenced by high pressures, inducing significant spectral ref shift in 5d-4f. The pressure-induced monotonous changes in bandwidth and emission intensity enhancement along with red shift suggesting the potential application of these fibers for pressure sensing applications. The CeN-PF fiber exhibits a high-pressure sensitivity (dλ/dP ≈ 0.28 nm/GPa) under a comprehensive linear dynamic range (0–64 GPa) with no pressure-induced luminescence quenching. The changes in CeS-PF is less pronounced with a lower pressure sensitivity of 0.10 nm/GPa compared to CeN-PF due to large crystal field splitting energy of nitrate ion compared to sulphate ion. This work presents a highly efficient, cost effective, scalable lanthanide doped flexible fibrous based system with negligible high pressure quenching and a wider linear dynamic range for optical pressure sensing applications.

Pressure-induced luminescence quenching in KY(WO4)2:Pr3+

The quenching of the red Pr3+ (1D2 → 3H4) luminescence in a single crystal of KY(WO4)2 doped with Pr3+ is investigated at room temperature under high hydrostatic pressure. The quenching is ascribed to a pressure-induced downshift of the Pr3+ → W6+ metal-to-metal charge transfer (or impurity trapped exciton) state. The concomitant decrease of the 1D2 → 3H4 emission lifetime is reproduced using a phenomenological model. The fitting allows the determination of the pressure-induced shrinkage of the Pr3+(Y3+)-W6+ distance in the crystal. The value is consistent with the quantity previously determined in CaWO4 by means of X-ray diffraction.

Pressure-induced luminescence of cerium-doped gadolinium gallium garnet crystal

Studies of the spectroscopic properties of Ce${}^{3+}$ dopant in bulk Gd${}_{3}$Ga${}_{5}$O${}_{12}$:Ce crystal under pressure are presented. In spite of strong intershell $4f\ensuremath{\rightarrow}5d$ absorption bands at ambient pressure, the cerium luminescence in Gd${}_{3}$Ga${}_{5}$O${}_{12}$ is entirely quenched even at low temperature. It has been shown that applying pressure allows for recovery of the $5d\ensuremath{\rightarrow}4f$ radiative transitions. Further increase of pressure improves the emission efficiency. This effect is analyzed in terms of two possible phenomena: (i) by pressure-induced electronic crossover of the excited 5$d$ energy level of the Ce${}^{3+}$ with the conduction band bottom of the host crystal, and (ii) by decrease of electron-lattice coupling with increasing pressure, resulting in reduction of the Stokes shift and nonradiative transitions between the low vibrational levels of the 5$d$ state and high vibrational levels of the ground 4$f$ state. The results of high-pressure absorption and luminescence measurements point out that the ambient-pressure luminescence quenching is caused by the donor-like charge transfer processes due to the resonant location of the Ce${}^{3+}$ 5$d$ electronic levels with respect to the host conduction band. In such a situation, the ionization of Ce${}^{3+}$ to Ce${}^{4+}$ occurs, accompanied by large lattice relaxation, which enables the nonradiative recombination to the Ce 4$f$ state. The pressure-induced approach of the conduction-band bottom of the host crystal by the excited 5$d$ energy level of the Ce${}^{3+}$ results in mixing between conduction band states and the 5$d$ state, and the broadband luminescence from this mixed state is observed at pressures from 30 up to about 60 kbar. Then, for pressures exceeding 60 kbar, this luminescence is replaced by the classic ${\mathrm{Ce}}^{3+}\phantom{\rule{0.16em}{0ex}}5d\ensuremath{\rightarrow}4f$ transitions. Using a simple anticrossing model and configuration coordinate model in the appropriate pressure range, quantitative description of the system has been carried out, and the pressure-induced decrease of electron-lattice coupling has been shown.

Pressure-dependent fluorescence studies of Sm3+-doped fluorophosphate glass

Pressure-induced luminescence spectra and decay curves for the 4G5/2 level of Sm3+ ions in fluorophosphate glass (PKFBASm 40:54 P2O5+14 K2O+13 BaO+6 KF+9 Al 2 O 3+4 Sm 2O 3) have been measured as a function of pressure up to 36.3 GPa at room temperature. In this pressure range, a red shift of barycenter and Stark level splittings are observed for the 4G5/2→6HJ (J=9/2, 7/2 and 5/2) multiplet transitions. The decay curves of the 4G5/2 level show non-exponential behavior in the entire pressure range, which has been explained by energy transfer due to the dipole–dipole interaction. The effect of releasing pressure on the luminescence spectra and decay curves has also been studied. The results obtained for this glass have been compared with Sm3+-doped fluorophosphate, phosphate, lead phosphate, lithium borate and tellurite glasses.

High-Pressure Studies of 1,3,5,7-Cyclooctatetraene: Experiment and Theory

High-pressure studies of 1,3,5,7-cyclooctatetraene have been performed by using Raman spectroscopy up to 16 GPa and compared with complementary density functional calculations. Angular-dispersive X-ray diffraction studies were also performed in the solid state at elevated pressure. The lattice constants of solid 1,3,5,7-cyclooctatetraene obtained from the X-ray diffraction pattern taken at 3.8 GPa and room temperature are in good agreement with theoretical results. At least two phase transitions were observed during pressure increase followed by the loss of long-range crystallographic order, which was also associated with a strong pressure-induced luminescence that allowed estimation of band gap alterations with pressure.

Pressure-induced luminescence from broadband to narrow-line emission in Cr3+-doped LiCaAlF6 at room temperature

This work investigates pressure-induced excited-state-crossover phenomena in Cr3+-doped LiCaAlF6 at room temperature. The aim is to find the structural requirements yielding the change in the Cr3+ photoluminescence (PL) behaviour from a single broadband emission, which is located at 1.62 eV (765 nm) at ambient conditions, to a ruby-like narrow-line emission at 1.87 eV (662 nm). For this purpose, emission and excitation spectroscopy as well as lifetime measurements as a function of pressure were performed in the 0–35 GPa range. Interestingly, we have been able to transform the broadband Cr3+ PL at ambient pressure into a ruby-like emission at 28 GPa. This behaviour, together with the variations of the 2E and 4T2 excited-state energies, and the PL lifetime with pressure, can be explained on the basis of the electron–ion coupling associated with the 4T2 and 2E states.

Pressure-induced luminescence quenching of terbium-doped oxide sulfides

The photoluminescence of Tb[sup 3+]- and Eu[sup 3+]-doped rare earth oxide sulfides has been studied as a function of pressure. The luminescence intensity and lifetime of the [sup 5]D[sub 3] level of Tb[sup 3+] is quenched by pressure as a result of the pressure-induced red shift of the interconfigurational 4f[sup 7]5d band, in agreement with the energy gap law of radiationless transitions. 33 refs., 7 figs.

Structural transformations in N-isopropylcarbazole crystal monitored by pressure-induced luminescence

The phase diagram for solid/3-solid transitions in a N-isopropylcarbazole single crystal is determined by monitoring pressure-induced luminescence (PIL) pulses appearing at transition points. The structural transitions at room temperature which occur under hydrostatic pressure 0.6 and 0.46 GPa are suggested to be equivalent to previously observed normal pressure transitions at ≈ 137 and ≈ 200 K. The results have been verified by calorimetric studies. The transition parameters and, in particular, transition enthalpies, are found to be strongly dependent on crystal history (thermal and pressure treatment before measurements). The effect is assigned to local strain relaxations. The pressure-induced phase transition at 0.6 GPa has been found to appear as a discontinuity in the pressure dependence of the maximum position of the photoluminescence spectrum and luminescence efficiency of the crystal.

Structural transitions in N-isopropylcarbazole single crystal monitored by pressure-induced luminescence

Abstract Phase diagram for solid-solid transitions in N-isopropyl-carbazole single crystal is determined by monitroing pressure-induced-luminescence (PIL) pulses appearing at transition points. A comparison of the data obtained from PIL and from microcalorimetric measurements shows remarkable difference in the transition temperature at ambient pressure and enormous discrepancy in the transition enthalpy. These discrepances are discussed in terms of local strains influence on parameters of phase transitions.

Pressure-induced luminescence of N-isopropylcarbazole single crystal

The spontaneous emission of light from single crystals of N-isopropylcarbazole under changing hydrostatic pressure (pressure-induced luminescence) has been observed for the first time. The luminescence consists of bursts of light around characteristic pressures at which pressure-induced structural transitions are believed to take place. This new phenomenon is interpreted in terms of the electrical discharge in the ambient gas involving high electric fields produced by spontaneous polarization at fresh microcrystal faces emerging during crystal cracking on structural transformations.