Pressure-induced emission enhancement: A review

Intelligent luminescent materials that respond to external stimuli have gotten a lot of attention because of their applications in luminescence modulation, fluorescence probes, information storage, anti-counterfeiting, biological imaging, and treatment.^1,2 Mechanoluminescence (ML) is a type of light emission that is excited by external mechanical stimuli.³ In recent decades, ML materials have shown great application prospects in fields such as green lighting, mechanical display and sensing, intelligent wearability, structural health monitoring, intelligent skin, and anti-counterfeiting.^4–6 The generation of ML is closely related to the fracture and plastic deformation of materials with piezoelectric effects in the lattice, and the mechanical luminescence performance is highly dependent on the molecular stacking mode, spatial conformation, and intermolecular interactions in the solid state.^7–9 

Pressure can effectively regulate the structure and properties of materials,^10–13 aiding in the discovery of special phenomena, special mechanisms, and special materials that are difficult to achieve by other means. High pressure can control the multiple degrees of freedom coupling at different scales to achieve the regulation of functional material properties,^14,15 which is an important way and means to explore functional materials. Research under high pressure largely depends on technology, and every advancement in the latter has led to an astonishing expansion of our knowledge of material behavior under high pressure.¹⁶ In recent years, due to technological advancements in high-pressure research, particularly diamond anvil cell (DAC) technology,¹⁷ large amounts of research have been done on high-pressure luminescence. Many fascinating optical phenomena have been recorded so far under great hydrostatic pressure using DAC.^18–20 

As diamond has low birefringence and high spectral permeability, DAC is an ideal instrument for investigating the behavior of luminescent materials under high pressure.²¹ The DAC device is designed differently due to different experimental purposes but is generally composed of a gasket, diamond anvil, and support guide device²² [Fig. 1(a)]. As shown in Fig. 1(b), the working principle of DAC is to use external force to compress the gasket placed in the middle of the diamond anvil, causing high pressure to be generated in the middle sample cavity through the deformation of the gasket. Pressure calibration methods of DAC include ruby fluorescence,²³ Raman evolution of diamond sp³ vibration mode (at ∼1333 cm⁻¹), equation of state of standard materials, and so on. Various in situ characterizations based on DAC, such as infrared, x-ray diffraction (XRD),²⁴ Raman scattering, fluorescence spectroscopy, and so on, give important information about how changes in intermolecular interactions are caused by changes in pressure. Therefore, DAC can be used for in situ observation and measurement under high pressure, and it is a key piece of equipment for studying the structure and performance of substances under high pressure.

Except for the DAC technique, the luminescent properties of materials can be changed by other methods, such as mechanical grinding in a mortar pestle^25–29 and high pressure in a hydraulic press.^30–32 Mechanical grinding will generate anisotropic stress, while hydrostatic pressure generates isotropic stress, which can enrich the chemical process of luminescence.³³ However, the pressure of mechanical grinding is insufficient to disturb the molecular interactions and, thus, cannot affect the optical properties. Moreover, due to the anisotropy of the pressure in mechanical grinding, the mechanism of mechanical grinding is usually difficult to understand. For a hydraulic press, the generated pressure is much lower than that of a DAC.

Applying the DAC method to luminescent materials has opened the door for research in this field.³⁴ Usually, luminescence emission declines as the pressure goes up.^35,36 However, it has recently been found that some materials exhibit different behaviors in DAC. The luminescence intensity of these materials increases with increasing pressure. Many of the enhanced luminescence phenomena in DAC are called pressure-induced emission enhancement (PIEE).³⁷ The PIEE phenomenon opens up possibilities for new applications in fields such as optical sensors, displays, and fluorescent probes. By utilizing the enhanced emission effect of materials under high pressure, more precise and sensitive optical sensing and detection can be achieved. In this paper, we summarized the latest progress in the phenomenon of material luminescence enhancement discovered in DAC and further discussed the role of pressure in promoting the enhancement of material luminescence intensity (Table I), which provided a perspective for in-depth research on the photophysical behavior of materials and a way to explore optoelectronic materials with excellent performance.

Perovskite materials have quantum confinement properties, good stability, and controllability, enabling them to have broad application prospects in fields such as solar cells and light-emitting diodes.^38–45 However, its photoelectric performance still needs further improvement, and the internal relationship between structure and physical properties has not been well understood. The atomic spacing, crystal structure, and electronic structure of materials can be changed by means of high pressure, thus effectively changing the interaction between atoms in materials and inducing many structures and properties that have not appeared at ambient pressure.

Under 325 nm UV irradiation at atmospheric pressure, the photoluminescence (PL) spectrum of a double perovskite Cs₂Na_0.4Ag_0.6InCl₆ alloyed with Bi doping (CNAICB) revealed a bright broadband emission (BE) at 585 nm and a weak narrow emission (NE) at 435 nm.⁴⁶ Notably, in the study by Ma et al.⁴⁷ [Fig. 2(a)], with an excitation wavelength of 355 nm, the NE peak exhibited a remarkable increase in intensity, gradually evolving into an identifiable spike as pressure was raised and reaching its maximum value at 5.01 GPa. The color scale plot [Fig. 2(b)] showed that the CNAICB was continuously modulated from the starting warm white light to cool white light and finally changed to deep blue light as the pressure increased. An optical micrograph of CNAICB in DAC was shown in Fig. 2(c), and it clearly depicted the changes in the emission intensity and color. They indicated that the underlying mechanism of NE luminescence originated from the radiative recombination of self-trapped excitons populated in excited singlet states. Guo et al.⁴⁸ selected material (HA)₂(GA)Pb₂I₇ (HA = n-hexylammonium, GA = guanidinium) with unique lattice distortion as the object of study and systematically investigated the relationship between its behavior of exciton generation, defect capture and compounding, and luminescence efficiency using in situ testing techniques such as high-pressure synchrotron XRD. It was found that the PL intensity of (HA)₂(GA)Pb₂I₇ increased dramatically during compression relative to ambient conditions, reaching a 12-fold increase at 1.59 GPa, as shown in Fig. 2(d). Fluorescence micrographs at various pressures in DAC clearly showed the luminescence brightness [Fig. 2(e)].

Fang et al.⁴⁹ used the pressure effect to regulate the accumulation of organic layers in the (BA)₄AgBiBr₈ [BA = CH₃(CH₂)₃NH₃⁺] crystal structure and the degree of distortion of the connection between the two octahedra, thus inducing a structural phase transition and producing the phenomenon of pressure-induced self-trapped excitons luminescence. The PL spectrum of (BA)₄AgBiBr₈ under high pressure was shown in Fig. 2(f), and the optical micrographs revealed the trend of PL brightness with pressure [Fig. 2(g)]. Under environmental conditions, the sample did not emit light. As the pressure increased, the unexpected emission of intrinsic non-emitting (BA)₄AgBiBr₈ occurred at 2.5 GPa. As the pressure increased to 8.2 GPa, the luminescence intensity reached its maximum value, accompanied by a blue shift in the PL peak position display. Above 8.2 GPa, the PL intensity decreased, with a red shift displayed at the PL peak position, until the pressure reached 25 GPa and the PL almost completely disappeared. Wang et al.⁵⁰ studied the changes in the structures and optical properties of Sb³⁺-doped Cs₂InBr₅·H₂O under high pressure. They found that the PL intensity of Sb³⁺-doped Cs₂InBr₅·H₂O gradually increased with increasing pressure and reached its maximum at 0.6 GPa. The structural analysis results showed that the enhanced luminescence intensity of Sb³⁺-doped Cs₂InBr₅·H₂O could be attributed to the small energy difference between the ³P₁ and self-trapped exciton states of Sb³⁺, as well as the reduced nonradiative energy loss.

Jing et al.⁵¹ studied the unique optical and electrical properties of Eu³⁺-doped CsPbCl₃ quantum dots under high pressure. It was discovered that the PL peak of CsPbCl₃ underwent a continuous red shift as the pressure increased, while the PL intensity decreased gradually and disappeared at ∼1.4 GPa, which was attributed to the significant changes in the crystal structures. The PL intensity of Eu³⁺ ions tended to increase up to 10.1 GPa, was enhanced by a factor of about three, and still maintained a higher fluorescence intensity at 22 GPa. They speculated that this was due to increased energy transfer from chalcogenide to Eu³⁺ ions. Fang et al.⁵² did tests with a (2meptH₂)PbCl₄ crystal under high pressure to change their optical properties and crystal structures. They found that the sample maintained a rare warm white light emission throughout the pressurization process, and the PL rose sharply at 2.1 GPa and continued to rise until 9.9 GPa, as shown in Fig. 3(a). The corresponding micrographs [Fig. 3(b)] clearly illustrate the variations in PL luminance and color during the compression process. Further research demonstrated that as the pressure was increased from 2.1 to 9.9 GPa, the stiffness of the lattice shrinkage rose, and the pressure had a big effect on the nonradiative loss of self-trapped excitons, which made more PL.

Zhang et al.⁵³ investigated the structural, electrical, and optical pressure response of Cs₃Bi₂I₉ using a high-pressure technique. They conducted photoluminescence (PL) experiments at high pressure to examine the effect of lattice compression on emission. The PL micrographs [Fig. 3(c)] illustrate the PL intensity transition at different pressures. As shown in Fig. 3(d), PL intensity increased sharply at mild pressure and then gradually decreased until PL disappeared totally at 9.3 GPa. The increase in PL intensity was linear up to 0.9 GPa, after which it increased exponentially by a factor of 10 and then slowed down with increasing pressure. The emission spectrum maintained a continuous, slow redshift throughout the compression process. They indicated that the sharp PL enhancement of Cs₃Bi₂I₉ under moderate pressure was due to the increase in exciton binding energy during lattice compression. Samanta et al.⁵⁴ detailedly studied the material Cs₃Bi₂Br₉ halide perovskite under high pressure. The bulk Cs₃Bi₂Br₉ did not exhibit any detectable PL under ambient conditions. However, as pressure increased, luminescence appeared at ∼1.4 GPa. The additional pressure increase resulted in an abrupt increase in PL intensity, which reached a maximum of ∼2.9 GPa. The theoretical results showed that the large distortion and secondary elongation of BiBr₆ octahedra were the reasons for the enhancement of PL intensity. They concluded that structural distortion was an important factor affecting the luminescence of lead-free halide perovskite.

Li et al.⁵⁵ achieved continuous and significant enhancement of self-trapped exciton emission under high pressure by controllably distorting tetrahedral units in CsCu₂I₃ with tetrahedral units. They claimed that the visible structural deformation between and within the tetrahedra was to blame for the considerable increase in high-pressure phase emission. Cao et al.⁵⁶ studied the high-pressure behaviors of Mn²⁺-Doped Cesium Lead Chloride Perovskite Nanocrystals (CsPb_xMn_1−xCl₃ NCs). The optical micrographs of CsPb_xMn_1−xCl₃ NCs were depicted in Fig. 4(a), clearly illustrating the trend of PL luminosity after compression. The color of PL shifted from a beautiful orange to a brighter red as the pressure increased. They discovered that the highest PL intensity at high pressure was much higher than the starting PL intensity under environmental circumstances. The calculation of density functional theory indicated that the PIEE phenomenon of CsPb_xMn_1−xCl₃ NCs occurred during the phase transition process, which was attributed to the increased energy release of Mn from ⁴T₁–⁶A₁. Shi et al.⁵⁷ achieved unexpected emissions through high-pressure treatment of one-dimensional organic tin bromide perovskite C₄N₂H₁₄SnBr₄. They found that the PL intensity continued to increase with increasing pressure until reaching a maximum value of ∼8.01 GPa [Fig. 4(b)]. The first-principles calculations results showed that the enhanced transition dipole moment and increased binding energy of self-trapped excitons after structural phase transition were the main reasons for significant pressure-induced emission enhancement.

Fu et al.⁵⁸ accomplished PIEE in (C₆H₅CH₂CH₂NH₃)₂PbCl₄ [(PEA)₂PbCl₄] nanocrystals (NCs). Under a mild pressure of 0.4 GPa, a considerable 5-fold pressure-induced emission was achieved, which was strongly connected with improved radiative recombination of self-trapped excitons. Following the complete release of pressure, the 1.6 time emission of dense (PEA)₂PbCl₄ NCs persisted, and the color of the light changed from “warm” (4403 K) to “cold” (14 295 K) white. Wang et al.⁵⁹ achieved a 90% photoluminescence quantum yield in C₄N₂H₁₄PbBr₄ through pressure suppression of nonradiative loss. During compression, the PL intensity first declined as the pressure was raised from ambient pressure to 1.5 GPa [Fig. 4(c)]. As the pressure increased from 1.5 to 2.8 GPa, the PL intensity increased sharply. The integrated PL intensity grew more than four times from its original value of 2.8 GPa. After further compression, the intensity of PL decreased and underwent a blue shift, resulting from a pressure-induced phase transition. The PL photos under different pressures [Fig. 4(d)] demonstrated the changes in emission brightness.

Under pressure, there will be a certain degree of pressure-induced photoluminescence enhancement. However, it is still difficult to stabilize its enhanced characteristics under environmental pressure or lower pressures.^53,60 Shen et al.⁶¹ demonstrated pressure-induced PL enhancement and significant broadband white light emission, covering the entire visible spectrum in BA₂PbBr₄. As the pressure was released to 1.8 GPa, these optical characteristics stabilized. Fang et al.⁶² modified perovskite by means of high pressure to emit high-quality “cold” or “warm” white light and attained luminescence up to 35.1 GPa in C₄N₄H₁₄PbCl₄. During compression, the emission chromaticity of the initial broadband white emission C₄N₄H₁₄PbCl₄ could be continuously and broadly adjusted, and the C₄N₄H₁₄PbCl₄ crystal exhibited periodic pressure-induced emission before 35.1 GPa. Zhang et al.⁶³ maintained a narrow bandgap and enhanced PL in Mn-doped and undoped Cs₂NaBiCl₆ double perovskites. After pressure treatment, Mn-doped and undoped Cs₂NaBiCl₆ exhibited a majority of the reserved bandgap narrowing relative to its initial state of 12.2% through compression–decompression cycles, as well as persistent stability under environmental conditions. In moderate compression, especially at a pressure below 0.5 GPa, it was found that Mn²⁺ ions had significantly enhanced pressure-induced emission, which was due to the increase in energy transfer efficacy from Bi³⁺ to Mn²⁺ ions as a result of electron wave function superposition.

Ma et al.⁶⁴ systematically investigated the structure–property relationship between pressure-modulated self-trapped excitons (STE) emission and one-dimensional halide perovskite C₄N₂H₁₄PbBr₄ nanocrystals (NCs). As shown in Fig. 5(a), in situ high-pressure PL measurements of C₄N₂H₁₄PbBr₄ NCs indicated that STE emissions endured a clear pressure-sensitive evolution with increasing pressure. When the pressure exceeded 1.75 GPa, STE emissions substantially increased, ultimately reaching their maximum at 6.18 GPa. They also derived the pressure-dependent integral intensity of STE emission, which showed significant anomalies near 1.75 GPa [Fig. 5(b)]. The cumulative emissions significantly increased between 1.75 and 6.18 GPa under pressure. Figure 5(c) also clearly demonstrated the trend of C₄N₂H₁₄PbBr₄ NC emission brightness from white to blue–white colors. Kong et al.⁶⁵ investigated the deuterated perovskite CD₃ND₃PbI₃ using in situ high-pressure photoluminescence, infrared spectroscopy based on optics and synchrotron, and neutron/x-ray diffraction. Compared to their non-deuterated CH₃NH₃PbI₃ counterparts, they discovered that under high pressure, lattice distortion was substantially reduced, followed by a significant increase in PL intensity, a delayed bandgap blue shift, and delayed metallization.

Li et al.⁶⁶ proposed a method for enhancing the luminescence intensity of neodymium-doped CsPbBr₃ perovskite nanocrystals by doping Nd³⁺ ions and introducing pressure. They found that the photoluminescence intensity of CsPbBr₃: Nd nanocrystals increased abnormally from 0.19 to 0.34 GPa, as shown in Fig. 6(a). Pressure contributed to the recombination of self-trapped excitons and inhibited the recombination of Nd energy levels, as determined by time-resolved photoluminescence measurements. Wang et al.⁶⁷ prepared two-dimensional perovskites [(CH₃)₂CH(CH₂)₂NH₃]₂PbI₄ [(PNA)₂PbI₄] and [CH₃(CH₂)₄NH₃]₂PbI₄ [(PA)₂PbI₄] with isomer-dependent organic molecules as cationic spacers and studied their structures and optical behaviors as measured by their PL spectra at various pressures. In comparison to (PA)₂PbI₄, the PL intensity of (PNA)₂PbI₄ at 1.3 GPa increased nearly 15-fold, and it exhibited an ultra-broad visible light spectral range of 300 nm at 7.48 GPa. Li et al.⁶⁸ studied the high-pressure influence on the structure, electronic, and optical properties of CsCu₂I₃ using DAC. Figure 6(b) depicts the fluorescence images of CsCu₂I₃ under various pressures. Under environmental conditions, the sample would emit a very weak yellow light. The color of the emission shifted from yellow to green as the pressure rose, and fluorescence increased at pressures ranging from 0 to 3.34 GPa. The first-principles calculations results indicated that PL emission might be related to intrinsic defects such as V_I in CsCu₂I₃. Fang et al.⁶⁹ explored the effects of pressure on the crystal structures and optical properties of [(HO) (CH₂)₂NH₃]₂PbI₄ (ETA₂PbI₄). Figure 6(c) depicts the pressure-dependent PL spectrum of ETA₂PbI₄ under increased pressure. In addition to a more symmetrical photoluminescence (PL) peak, they found that the emission increased fourfold at 1.5 GPa, which was attributed to the pressure suppression of nonradiative recombination and the increase in exciton binding energy.

High-brightness yellow light emission was observed in zero-dimensional lead-free double-perovskite Rb₂TeCl₆ microcrystals under high pressure, as reported by Zhao et al.⁷⁰ During the initial compression stage, the PL intensity gradually increased with pressure. It was noteworthy that at ∼1.0 GPa, the integrated yellow PL intensity experienced a significant enhancement of more than seven times compared to the initial intensity. The PL peak intensity decreased and nearly disappeared at around 8.4 GPa due to a possible isostructural phase transition. They attributed this PIE to variations in nonradiative recombination caused by variations in electron–phonon coupling strength and the contraction of halide octahedra influenced by pressure processing.

Metal halides have attracted great attention due to their unique electronic structure and excellent luminescent properties.^71–74 The broadband emission generated by their self-trapped excitons is expected to be applied to single component white light LEDs. In order to achieve practical applications of such materials, it is necessary to have a deeper understanding of their structural and physical properties to provide guidance for further optimizing their luminescent properties.

Li et al.⁷⁵ innovatively combined high-pressure and low-temperature characterizations in their high-pressure research on zero-dimensional metal halides 0D Cs₂InBr₅·H₂O and obtained a rare phenomenon of inverse excitation-dependent dual-band emission. They pointed out that the contraction of the non-uniformly coordinated InBr₅O octahedron could effectively enhance the intensity of STE luminescence, increase the luminescence energy, and gradually transform the luminescence of the system from orange–red to yellow. Zhao et al.⁷⁶ achieved green emission enhancement [PP14]₂[MnBr₄] (N-butyl-N-methylpiperidinium [PP14]⁺) materials in DAC. When the pressure was completely released from 15.2 GPa to ambient pressure, [PP14]₂[MnBr₄] showed a quantum yield of up to 90.8%, and the luminous intensity was nearly three times higher. They concluded that the PIEE phenomenon was caused by increasing the Br⋯H interaction to limit the degree of freedom of motion of [MnBr₄]²⁻. A systematic high-pressure study of a 0D organic-inorganic metal halide perovskites (OMHPs) [(C₆H₁₁NH₃)₄BiBr₆]Br·CH₃CN (Cy₄BiBr₇) was conducted by Sun et al.⁷⁷ Initially, Cy₄BiBr₇ did not display any PL response to external pressure, but it exhibited bright blue emission with increasing pressure. The PL intensity of Cy₄BiBr₇ increased noticeably with a further increase in pressure, eventually reaching a maximum of 4.9 GPa. The PL intensity decreased gradually with increased pressure until it reached its minimum at 13.0 GPa. The observed PIE might be due to the enhanced exciton binding energy related to the distortion of the [BiBr₆]³⁻ octahedron under pressure. Geng et al.⁷⁸ used in situ high-pressure PL spectra to study the emission characteristics of Cs₃Bi₂Cl₉ NCs. No emission signal was found, according to spectroscopy analysis at ambient pressure. When the pressure increased to 0.8 GPa, the Cs₃Bi₂Cl₉ NCs displayed an anomalous broad emission, indicating an intriguing emission ranging from “0” to “1” under high pressure. The increased emission was a result of both the weakened electron–phonon coupling and the relaxed halide octahedral distortion benefiting from the vacancy-ordered structure. Above 9.5 GPa, the PL nearly vanished due to a combination of structural collapse and deviatoric stress. When the pressure was fully released to ambient conditions, pressure-induced PL reversibility was observed.

Shi et al.⁷⁹ achieved significant enhancement of self-trapped exciton emission related to energy exchange between multiple self-trapped states in Cs₃Cu₂I₅ Nanocrystals (NCs) using DAC. As shown in Fig. 7(a), STE emissions underwent a clear pressure-sensitive evolution with increasing pressure. When the pressure increased to 1.0 GPa, the original emission wavelength of 440 nm decreased progressively, and a slight blue shift occurred. In addition, the PL emission curve with a low-energy wavelength of ∼550 nm increased slowly. The sensitive changes in peak position led to the phenomenon of pressure-induced color changes, manifested as the color changes of Cs₃Cu₂I₅ NCs [Fig. 7(b)].

Zhao et al.⁸⁰ achieved significantly enhanced emission in (C₅H₇N₂)₂ZnBr₄ [(4AMP)₂ZnBr₄] in DAC at room temperature and retained the high-pressure metastable phase with high luminescence efficiency at atmospheric pressure. As shown in Fig. 7(c), within the pressure range of 5.07 GPa, the fluorescence intensity continued to increase, emitting a very bright sky-blue light. Further pressure decreased the fluorescence intensity. Unusually, after pressure relief, the retained fluorescence intensity exceeded ten times the initial intensity. The quenching emission intensity significantly increased to ∼10 times the emission intensity of 1 atm, as confirmed by the changes in PL images in the sample chamber [Fig. 7(d)].

The above research indicated that pressure could effectively regulate the self-limiting exciton behavior of metal halides and improve their luminescence efficiency. High-pressure regulation combined with in situ characterization of the physical properties of structures could deepen the understanding of the structure–activity relationship of materials and provide a way to explore optoelectronic materials with excellent performance.

Carbon dots (CDs), as a type of fluorescent carbon nanomaterials, have high fluorescence quantum yield, photostability, and excellent biocompatibility, which have broad application prospects in fields such as photocatalysis, sensing, optoelectronic devices, biological imaging, photodynamic therapy, etc.⁸¹ In recent years, significant progress has been made in modifying and exploring the optical properties of CDs using high pressure. The discovery of the luminescence characteristics of stimulus-response under high pressure will undoubtedly provide guidelines for the further development of CDs.

Jiang et al.⁸² observed pressure-induced aggregation-induced emission enhancement and reversible thermochromic behavior in F, N-doped carbon dots. They found that the PL intensity of F, N-doped carbon dots progressively increased as the pressure increased to 0.65 GPa, as shown in Figs. 8(a) and 8(b). The distances between adjacent particles would be shortened due to an increase in external pressures, thereby increasing the van der Waals force and hydrogen bonds between each other, leading to the aggregation of F, N-doped carbon dots and exhibiting aggregation-induced emission enhancement behavior. Wang et al.⁸³ synthesized red light carbon dots (R-CDs) from 1,3,5-benzene-trithiol as a raw material and achieved pressure-triggered aggregation-induced emission enhancement. They found that R-CDs exhibited significant emission enhancement at an initial pressure of 0.05 GPa, and the fluorescence intensity within the pressure range of 0.05–0.5 GPa was better than that at 1 atm. The compressed R-CDs exhibited strong intramolecular charge transfer from their natural transition orbit, which led to enhanced emission under high pressure.

Lu et al.⁸⁴ reported yellow emissive CDs (Y-CDs) with significant piezochromic luminescence under high pressure. They found that applying pressure ranging from 0 to 22.84 GPa caused the color of Y-CDs to change from yellow to blue–green, and the fluorescence of Y-CDs increased with increasing pressure, as shown in Fig. 8(c). First-principles calculations indicated that the sp² domains of Y-CDs transformed into sp³-hybridized domains under high pressure, exhibiting significantly different fluorescence emissions during the sp² to sp³ phase transition. Lou et al.⁸⁵ used sodium hydroxide as the matrix material to prepare sodium hydroxide confined carbon dots samples. They found that the photoluminescence of carbon dots confined within sodium hydroxide could be enhanced with increasing pressure, and their fluorescence was enhanced by 1.6 times compared to the original CDs. After the pressure was relieved, the enhanced luminescence under high pressure could be retained at ambient pressure. Experimental and theoretical calculations had shown that the vibration of functional groups on the surface of carbon dots was limited under spatial confinement, reducing nonradiative transitions and exhibiting enhanced pressure-induced photoluminescence.

Organic luminescent materials have advantages such as high photoelectric efficiency, fast response, and molecular flexibility. They have been widely used in organic electroluminescent devices, chemical sensing, biological probes, and other fields.^86,87 The luminescent properties of some organic luminescent materials undergo significant changes under high pressure, such as luminescence discoloration or enhancement, which have many potential applications in anti-counterfeiting, force sensors, data recording and storage, and luminescent devices.

Yang et al.⁸⁸ discovered that the room-temperature phosphorescence (RTP) emission intensity of selenanthrene crystals increased progressively under high pressure, accompanied by a slight red shift from green to chartreuse [Fig. 9(a)]. They indicated that the PIEE of RTP was due to the increase in the triplet state radiation transition rate, according to experimental and theoretical investigations. Fu et al.⁸⁹ examined the evolution of fluorescence in crystals of salicylic acid under high pressure and observed an increase in emission during compression. After 4 GPa, the excited-state intramolecular proton transfer process of salicylic acid was inhibited, and the enhancement of intermolecular hydrogen bonds reduced nonradiative decay, resulting in increased emission. Gu et al.⁹⁰ discovered that PIEE and multicolor behavior could be detected in 1,2,3,4-tetraphenyl-1,3-cyclopentadiene crystals. Figure 9(b) depicts the multicolor process in which the emission color transitions from blue to yellow progressively. Within the pressure range of 1.0–10.3 GPa, they discovered a significant increase in emissions. They demonstrated that PIEE was caused by the restriction of intramolecular rotation by the C–H⋯C hydrogen bond.

Liu et al.⁹¹ designed a compound, 9-[3-(1,2,2-triphenylvinyl) phenyl]-anthracene (mTPE-AN), by combining tetraphenylethylene (TPE) and discrete π–π anthracene (AN) into one molecule. They found that mTPE-AN exhibited a rare piezoceramic luminescence behavior when pressure was applied to the crystal [Fig. 9(c)]. As depicted in Fig. 9(d), blue shift emission and enhancement occurred when the pressure exceeded 1.23 GPa and reached its maximal value at 4.28 GPa. Pressure-dependent structural simulation showed that the synergistic effects between aggregation-induced emission of TPE units and energy-transfer suppression from TPE to an AN excimer were the main cause of the PIEE phenomenon.

Gu et al.⁹² analyzed the high-pressure optical properties of trans-stilbene crystals using in situ high-pressure PL spectroscopy. They found that trans-stilbene exhibited two distinct PIEE behaviors in separate pressure regions, as shown by the PL results. The experimental structural characterizations revealed that the first emission enhancement was attributable to a decrease in nonradiative transitions, and the second emission enhancement was ascribed to the enhanced C–H⋯C interactions brought about by the inversion of the aromatic ring.

Yuan et al.⁹³ conducted in-depth research on tetraphenylethene (TPE) crystals under high pressure. As pressure increased, the PL of TPE changed from dark blue to sky blue and finally to spring green. As shown in Fig. 10(a), the emission intensity increased dramatically from 1.5 to 5.3 GPa, and the emission intensity of 5.3 GPa was about three times that of 1.5 GPa. The reason for the increase in PL intensity was that the enhancement of intermolecular interactions could more effectively suppress the energy loss of intramolecular motion. Li et al.⁹⁴ found that triphenylethylene (TriPE) exhibited significant PIEE and compression-induced color change behavior in the pressure range of 0.0–0.8 GPa, as shown in Fig. 10(b). In situ high-pressure infrared spectroscopy and angular dispersion XRD analysis indicated that the related C–H⋯π and C–H⋯C hydrogen bonds were thought to be responsible for the increased emission of TriPE. These hydrogen bonds could inhibit or limit the movement of aromatic parts, which would reduce the energy loss caused by intramolecular motion. Further research also found that the decrease in crystallinity of the crystal led to a decrease in emission as the pressure exceeded 0.8 GPa.

Zhang et al.⁹⁵ conducted piezochromic measurement on the single crystal of 4-(2-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)-1H-phenanthro[9,10-d]imidazole-1-yl)benzonitrile with metastable triangular cone conformation. As the pressure increased from 2.50 to 4.55 GPa, the emission intensity of the crystal rose while the peak continuously shifted red. Their research indicated that the enhancement of PL was due to the special rehybridization of nitrogen atoms under pressure. Gu et al.⁹⁶ found that carbazole crystals exhibited significant PIEE from 0.0 to 1.0 GPa, as shown in Fig. 10(c). Their theoretical and experimental results indicated that the PIEE phenomenon was caused by the diminution of nonradiative vibration processes.

Under pressure, Wu et al.⁹⁷ found that the emission intensity of C₁₈H₆N(TPA) increased several times [Fig. 10(d)] and underwent a weak red shift. Research showed that TPA did not occur during the phase transition during the high-pressure loading process but rather formed a metastable structure under high pressure. Even though the spacing between molecules decreased, no p–p accumulation was observed, resulting in a weak red shift. Simultaneously, as a result of limitations in the vibration and rotation of the benzene ring, nonradiative transitions became weaker, resulting in increased fluorescence.

Yang et al.⁹⁸ utilized a DAC to expose the crystalline state of a flex-activated mechanophore at high pressure and observed its fluorescence properties. Below 2.0 GPa, a greatly increased blue fluorescence was observed, corresponding to crystalline anthracene emission. The fluorescence maximum values were redshifted from 478 to 506 nm, and the fluorescence intensity steadily decreased above 2.0 GPa. The emission wavelength reverted to 477 nm upon pressure released to ambient conditions, and it displayed strong blue light with almost 2.5-fold the intensity of its slightly emissive initial state, signifying an irreversible mechanofluorescence transition.

In addition to the above-mentioned perovskite materials, metal halide materials, carbon dots, and organic luminescent materials, pressure-induced emission enhancement has also been found in many other materials,⁹⁹ such as metal–organic frameworks, compound semiconductor materials, transparent conductive oxides, organic inorganic hybrid materials, etc.

Wang et al.¹⁰⁰ achieved high photoluminescence performance in green-emitting Tb(BTC) (H₂O)₆. When the samples were put under pressure, the quantum yield of photoluminescence went from 50.6 to 90.4%. They discovered that the strengthened hydrogen bonds kept the carboxyl group and benzene ring locked in a conjugated shape, allowing the promoted system crossing to efficiently drive ligand-to-metal energy transfer. Wisniewski et al.¹⁰¹ conducted a room-temperature and high-pressure luminescence study on Eu-doped GaN polycrystalline phosphors. They pointed out that the enhancement of Eu³⁺ ion emission intensity was caused by the stronger localization of bound excitons on the rare earth ions in a structured isoelectronic trap caused by applied pressure. Hong et al.¹⁰² found that pressure could help simultaneously enhance the emission of red and green light in Er: GdVO₄. They found that the main modes of red and green light emission got four times brighter at 11.7 GPa, while the intensity of PL as a whole got twice as bright. The red and green emission spectra are shown in Figs. 11(a) and 11(b). Synchrotron XRD showed that the reason for the increase was the structural phase transition. Zhao et al.¹⁰³ found abnormal pressure-induced photoluminescence enhancement and phase decomposition in pyrochlore La₂Sn₂O₇. At room temperature and 2 GPa, they found pressure-induced visible narrow band PL in La₂Sn₂O₇, and the visible PL spectral intensity remained stable between 6.6 and 16.6 GPa with changes in pressure, resulting in a flat maximum PL. They pointed out that the occurrence of this abnormal PL behavior was related to the lattice symmetry distortion caused by phase decomposition.

Yin et al.¹⁰⁴ conducted theoretical simulations on the luminescence characteristics of FCO-CzS under pressure. When the pressure reached 20 GPa, they discovered that FCO-CzS exhibited anomalous emission enhancement and a photoinduced blue shift. They noted that this anomalous pressure response in PL must be associated with the pressure-induced modification of the geometry and electronic structure of FCO-CzS. Zhang et al.¹⁰⁵ investigated the effects of up to 65 GPa of pressure at room temperature on the photoelectric properties and structure of (CH₃NH₃)₃Bi₂I₉ crystal. Figures 11(c) and 11(d) depict the PL spectrum and PL intensity of the (CH₃NH₃)₃Bi₂I. As the pressure increased, they observed a gradual redshift in the PL band, accompanied by ∼14-fold PL enhancement. Figure 11(e) clearly shows the significant changes from transparent red to dark black in the single crystal sample. They found that the crystal structure of (CH₃NH₃)₃Bi₂I₉ exhibited irreversible quantum confinement effects, which could potentially increase exciton binding energy under moderate pressure, leading to PIEE. Hu et al.¹⁰⁶ conducted systematic high-pressure research on few layered g-C₃N₄ (FL-CN). They found that applying high pressure could achieve significant enhancements in photoluminescence and FL-CN tunable emission colors. Further research has shown that the increase in pressure-induced interlayer interactions could affect the charge separation of photo-induced electrons and holes, leading to PL enhancement. Zou et al.¹⁰⁷ described a tetracoordinate boron complex with polymorphism (Y-phase and O-phase)-dependent luminescence. They discovered that Y-phase crystals exhibited uncommon piezoceramic luminescence characteristics, such as pressure-induced blue shift and enhanced emission. According to their findings, this phenomenon was brought about by the interactions between the changeable charge transfer behavior and the restriction of intramolecular motion under high pressure.

Xiao et al.¹⁰⁸ showed that by compressing CdSe nanocrystals (NCs), the amount of light they gave off could be raised by more than ten times. Figure 12(a) depicts the PL spectra of variously shaped CdSe NCs. They found that the brightest emission could be made at the pressure where CdSe NCs of different sizes went through a phase change. First-principles calculations indicated that the increase in Hirshfeld charges transferred from oleic acid (OA) molecules to CdSe NCs under high pressure, accompanied by the disappearance of surface-related trap states, was the reason for PIEE in CdSe NCs. Ma et al.¹⁰⁹ discovered pressure-induced light emission enhancement in GaAs nanowires by measuring in situ high-pressure photoluminescence with nitrogen as the pressure transport medium. They discovered that the PL intensity increased dramatically with increasing pressure from 0.0 to 2.2 GPa, and the transition from direct to indirect bandgap caused an abrupt decrease in photoluminescence intensity above 2.2 GPa.

Zheng et al.¹¹⁰ analyzed the luminescence characteristics of Ca₂Gd_7.76Ce_0.24Si₆O₂₆ phosphor as a function of pressure. Figure 12(b) depicts the normalized emission spectra of Ca₂Gd_7.76Ce_0.24Si₆O₂₆ phosphors measured under various pressure values and UV illumination excitation. As the pressure increased, an aberrant increase in emission intensity and a significant redshift in the emission band were observed. Lv et al.¹¹¹ found that zinc blend cadmium selenide nanocrystals (Zb-CdSe NCs) had interesting pressure-induced emission enhancement and maturation processes by using in situ high-pressure optical studies. Under high pressure, they discovered that the contact between the stearic acid layer and the Zb-CdSe nucleus was reinforced, resulting in an almost sevenfold increase in the intensity of Zb-CdSe NCs emissions. In addition, the large separation between the photoluminescence and absorption spectra at 4 GPa could be explained by the fact that the Zb-CdSe NCs grew larger as the sample aged under high pressure.

At an excitation wavelength of 980 nm, Zhang et al.¹¹² found that the upconversion luminescence (UCL) intensity of LiErF₄:0.5%Tm³⁺@LiYF₄ (Er:Tm@Y) showed a gradual increase with increasing pressure. When ∼6 GPa of pressure was applied, the intensity of UCL increased by 2.6 times. Density functional theory calculations indicated that the symmetry distortion of the LiErF₄ crystal reached its maximum when pressurized to 6 GPa, while an Er-4f state emerged, significantly reducing the bandgap from 8.3 to 5.7 eV. They pointed out that local symmetry distortion and different energy transfer modes under different excitations were the reasons for the UCL enhancement. Through further experimental comparative analysis and theoretical simulation calculations, they pointed out that the strength enhancement of UCL was due to local symmetry distortion caused by doping and different energy transfer modes under different excitations. Shen et al.¹¹³ observed pressure-induced PL enhancement in In₂O₃ nanowires (NWs). The PL spectrum of In₂O₃ NW was shown in Fig. 12(c), showing the maximum PL at a pressure close to 16.31 GPa, which corresponds to a 4.4-fold increase in the initial PL intensity at 1.91 GPa. Density functional theory calculations indicated that the orbital overlap between the oxygen-induced defect energy levels and the conduction band minimum might be the reason for the pressure-induced photoluminescence enhancement in nanowires. Zhao et al.¹¹⁴ used in situ PL and XRD measurements to explore the PL and phase transition of CeF₃:Tb³⁺ nanoparticles under high pressure. They found that the photoluminescence of CeF₃:Tb³⁺ nanoparticles showed an enhancement from 18.3 to 33.4 GPa under high pressure. They pointed out that this was related to the changes in distances between the emission centers, which weakened the negative quenching effect and improved transmission efficiency. Li et al.¹¹⁵ conducted systematic studies of face-centered cubic and bitetrahedral nanoclusters under high pressure. Figure 12(d) illustrates that the PL of these nanoclusters increased by up to two orders of magnitude above 7 GPa. They noted that the pressure-induced increase in PL was due to the enhancement of the near-band-edge transition strength, suppression of the nonradiative vibrations, and hindrance of the excited-state structural distortions.

The PIEE in most materials is attributed to the regulation of crystal and electronic structures by pressure. Pressure usually increases the exciton binding energy, changes the intrinsic defect concentration, and controls the radiative recombination process or nonradiative energy loss by means of structural phase transition and/or lattice distortion, thus enhancing the PL. For perovskite materials, the PIEE phenomenon is caused by improved activation energy for detrapping. For metal halide materials, the PIEE is affected by increasing exciton binding energy, reducing carrier scattering, and suppressing nonradiative decay. For carbon dots, the PIEE phenomenon is related to the sp² to sp³ transition and intramolecular charge transfer, leading to enhanced hydrogen bonding interaction and reduced nonradiative recombination. For organic luminescent materials, the pressure-induced weakening of nonradiative transitions, interactions between chemical bonds and molecules, and energy-transfer suppression can enhance the luminescence.

In this review, we summarize the recently discovered phenomenon and potential mechanism of pressure-induced material luminescence enhancement in DAC. Among the materials described in this review, perovskite materials have quantum confinement properties, good stability, and controllability, making them have broad application prospects in fields such as solar cells and light-emitting diodes. Metal halide materials exhibit significant quantum confinement effects and highly localized charge density characteristics, resulting in high exciton binding energy and stable exciton emission. The broadband emission generated by its self-trapped excitons is expected to be applied to single-component white LEDs. Carbon dots have excellent properties such as easy preparation, tunable optical and surface properties, and good biocompatibility and are widely used in the fields of biosensors, biological imaging, drug delivery, and photocatalysis. However, fluorescence quenching hinders the application of carbon dots in solid-state luminescence. The PIEE phenomenon under high pressure provides an approach to solving the problem of the self-quenching of carbon dots. Organic luminescent materials have the advantages of diverse varieties, good adjustability, rich colors, and flexible molecular design and are widely used in anti-counterfeiting, force sensors, and light-emitting devices. The enhancement of luminescence in most materials is attributed to the regulation of crystal and electronic structures by high pressure. Most of the mechanisms of pressure-induced luminescence and enhancement have been identified as the mechanisms of restriction of intramolecular rotation, the effect of aggregation-induced emission, and energy-transfer suppression. This work reveals the role of pressure in promoting the luminescent enhancement of materials in DAC, which helps to design materials with specific emission characteristics and provides a perspective for in-depth research on the photophysical behavior of materials.

Although the luminescence of most materials in DAC is enhanced under high pressure, when the pressure is completely released, the luminescence intensity of the sample returns to its initial state or even becomes weaker. Therefore, the future research direction will be devoted to designing materials that can achieve high luminous quantum yields through pressure treatment engineering. Furthermore, materials with the PIEE phenomenon also show great application potential in pressure sensing and detection in extreme environments. PIEE-based devices with high sensitivity and reliability remain a technical challenge, which will provide an opportunity to perform real-time monitoring, fault diagnosis, and safety assessment in harsh environments. Recently, the rotational diamond anvil cell (RDAC) has been widely used to study the behavior of materials under high pressure and shear. The introduction of plastic shear under high pressure has led to many exciting phenomena, including reducing the phase transition pressure, discovering hidden phases and phase transition pathways, maintaining the high-pressure phase after pressure release, and so on. The impact of both pressure and shear on the optical characteristics and luminescence mechanisms of materials will be studied in the future, and more phenomena and mechanisms are expected to be discovered.

This work was supported by the National Key R&D Program of China (Grant No. 2023YFA1406200), the National Natural Science Foundation of China (Grant Nos. 11604133, 12174146, and 11874174), the Science and Technology Plan of Youth Innovation Team for Universities of Shandong Province (Grant No. 2019KJJ019), the Introduction and Cultivation Plan of Youth Innovation Talents for Universities of Shandong Province, the Natural Science Foundation of Shandong Province (Grant Nos. ZR2021QA087, ZR2021QA092, and ZR2017QA013), the Guangyue Young Scholar Innovation Team of Liaocheng University (Grant No. LUGYTD2023-01), and the Special Construction Project Fund for Shandong Province Taishan Scholars.

The authors have no conflicts to disclose.

Boyu Zou, Yingxue Han, and Zhihao Yang contributed equally to this work.

Boyu Zou: Data curation (equal); Investigation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Yingxue Han: Data curation (equal); Investigation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Zhihao Yang: Data curation (equal); Investigation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Qinglin Wang: Conceptualization (equal); Supervision (lead); Writing – review & editing (equal). Guangyu Wang: Conceptualization (supporting); Data curation (supporting); Investigation (supporting); Supervision (supporting). Guozhao Zhang: Conceptualization (equal); Supervision (equal); Writing – review & editing (equal). Yinwei Li: Conceptualization (supporting); Data curation (supporting); Investigation (supporting); Supervision (supporting). Cailong Liu: Conceptualization (equal); Supervision (equal); Writing – review & editing (equal).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.