Surface Passivation to Enhance the Interfacial Pyro-Phototronic Effect for Self-Powered Photodetection Based on Perovskite Single Crystals.

The interfacial pyro-phototronic effect (IPPE) presents a novel approach for improving the performance of self-powered photodetectors (PDs) based on metal halide perovskites (MHPs). The interfacial contact conditions within the Schottky junctions are crucial in facilitating the IPPE phenomenon. However, the fabrication of an ideal Schottky junction utilizing MHPs is a challenging endeavor. In this study, we present a surface passivation method aimed at enhancing the performance of self-powered photodetectors based on inverted planar perovskite structures in micro- and nanoscale metal-halide perovskite SCs. Our findings demonstrate that the incorporation of a lead halide salt with a benzene ring moiety for surface passivation leads to a substantial improvement in photoresponses by means of the IPPE. Conversely, the inclusion of an alkane chain in the salt impedes the IPPE. The underlying mechanism can be elucidated through an examination of the band structure, particularly the work function (WF) modulated by surface passivation. Consequently, this alteration affects the band bending and the built-in field (VBi) at the interface. This strategy presents a feasible and effective method for producing interfacial pyroelectricity in MHPs, thus facilitating its potential application in practical contexts such as energy conversion and infrared sensors.

A high-precision thermometry strategy by replacing the infrared with visible light for detection.

We have developed a high-precision thermometry strategy based on 4I9/2-4I15/2 (I800 nm) and 4S3/2-4I15/2 (I551 nm) transitions of Er3+, after replacing the measurement of the 4I11/2-4I15/2 (I1000 nm) transition with the 4S3/2-4I15/2 transition, i.e., using visible light for detection instead of infrared. Through rate equation model analysis, (I1000 nm)2 and I551 nm can be substituted for each other under certain conditions. Further, because the 4I9/2 and 4I11/2 levels of Er3+ are thermally coupled, a new idea of ratiometric thermometry is proposed based on the ratio of (I800 nm)2 and I551 nm, which has the advantages of anti-interference of excitation light source fluctuation and background-free detection. The feasibility of the idea was verified by researching the power-dependent emission spectra at different temperatures and temperature-dependent emission spectra of a CaWO4:Er3+,Yb3+ sample under 980-nm laser excitation. The maximum relative sensitivity for the new ratiometric thermometry reaches up to 7.4% K-1 and the optimal temperature uncertainty calculated is 0.03 K at 303 K. This study provides guidance for solving the problem of a weak response of an infrared detector.

Mechanical force induced luminescence ratiometric thermometry in CaZnOS:Dy3.

The 4I15/2-6H15/2 and 4F9/2-6H15/2 transitions of Dy3+ are usually used for luminescent ratiometric thermometry in the form of photoluminescence. However, here we demonstrate the possibility of using this pair of lines for luminescent ratiometric thermometry in the model of mechanoluminescence (ML) in CaZnOS:Dy3+. Upon stimulation of an external mechanical force rather than light, CaZnOS:Dy3+ emits bright yellow luminescence. The intensity ratio of 4I15/2-6H15/2 to 4F9/2-6H15/2 transitions of Dy3+ is found to increase gradually with the rise of temperature, which makes Dy3+ a qualified temperature indicator. Our work enriches the family of optical thermometry.

Mechanically induced photons from ultraviolet-C to near-infrared in Tm3+-doped MgF2.

Mechanoluminescence (ML) plays a vital role in various fields, and has gained increasing popularity over the past two decades. The widely studied materials that are capable of generating ML can be classified into two groups, self-powered and trap-controlled. Here, we demonstrate that both self-powered ML and trap-controlled ML can be achieved simultaneously in MgF2:Tm3+. Upon stimulation of external force, the 1I6→3H6 and 3H4→3H6 transitions of Tm3+ are observed, ranging from the ultraviolet-C to near-infrared. After exposure to X-rays, MgF2:Tm3+ presents a stronger ML than the uncharged sample. After cleaning up at high temperatures, the ML returns to the initial level, which is a typical characteristic of trap-controlled ML. In the end, we demonstrate the potential applications of MgF2:Tm3+ in dynamic anti-counterfeiting, and structure inspection.

Tb3+-based multi-mode optical ratiometric thermometry.

Owing to some special superiority, luminescence ratiometric thermometry, mainly including dual excitations single emission and single excitation dual emissions, has gained popularity over the past few years. However, developing novel ratiometric thermometry that can work in multi-mode is still a challenge. Here we report a temperature measurement method based on the photoinduced luminescence of Tb3+ in the low-cost and easy to prepare calcium tungstate. Both the conventional luminescence intensity ratio (LIR) and recently developed single-band ratiometric (SBR) strategies have been achieved in our materials. On the one hand, upon excitation of the charge transfer state, the emissions from the excited 5D4 and 5D3 states present different responses to temperature. A thermometry depending on the LIR between these two emissions has thus been developed, with adjustable relative sensitivity that is sensitive to the excitation wavelength. On the other hand, both the emissions from the excited 5D4 and 5D3 states respond dissimilarly to the temperature variation. A SBR thermometer has thus been constructed with two excitation modes, reaching the maximum relative sensitivity of 1.83% K-1 at 573 K. The present work is expected to inspire other researchers to exploit more multi-mode optical ratiometric thermometries.

MgF2:Mn2+: novel material with mechanically-induced luminescence.

Mechanoluminescent (ML) materials can directly convert external mechanical stimulation into light without the need for excitation from other forms of energy, such as light or electricity. This alluring characteristic makes ML materials potentially applicable in a wide range of areas, including dynamic imaging of force, advanced displays, information code, storage, and anti-counterfeiting encryption. However, current reproducible ML materials are restricted to sulfide- and oxide-based materials. In addition, most of the reported ML materials require pre-irradiation with ultraviolet (UV) lamps or other light sources, which seriously hinders their practical applications. Here, we report a novel ML material, MgF2:Mn2+, which emits bright red light under an external dynamic force without the need for pre-charging with UV light. The luminescence properties were systematically studied, and the piezophotonic application was demonstrated. More interestingly, unlike the well-known zinc sulfide ML complexes reported previously, a highly transparent ML film was successfully fabricated by incorporating MgF2:Mn2+ into polydimethylsiloxane (PDMS) matrices. This film is expected to find applications in advanced flexible optoelectronics such as integrated piezophotonics, artificial skin, athletic analytics in sports science, among others.

Persistent visible luminescence of SrF2:Pr3+ for ratiometric thermometry.

Luminescence-based thermometry, especially the ratiometric temperature sensing technology, has attracted considerable attention recently due to its characteristics such as non-contact operating mode and strong capacity of resisting disturbance. Differing from the conventional strategy that usually needs continuous excitation, here an optical thermometry, which we have named the persistent luminescence intensity ratio (PLIR) thermometry, is proposed. The PLIR thermometry relies on the optical material SrF2:Pr3+ that could emit luminescence for several hours and even longer after being charged by X-ray. It has been demonstrated that the PLIR is sensitive to the variation of temperature and complies with the Boltzmann distribution. More importantly, the reliability of the proposed PLIR thermometry is verified. Our work may inspire others to develop more persistent luminescence thermometry.

Visible-to-ultraviolet-C upconverted photon for multifunction via Ca2SiO4:Pr3.

The ultraviolet C (UVC) photon plays a key role in a broad spectrum of fields. With the implementation of the Minamata Convention, searching for a new way to achieve UVC light is highly desired. Here we develop a material of Ca2SiO4:Pr3+ that can emit UVC light upon excitation of a 450-nm laser or even a very cheap 450-nm LED, a fact confirmed by using a solar blind camera to capture UVC emission from Ca2SiO4:Pr3+. In addition, smart anti-counterfeiting and inactivation of Bacillus subtilis applications using Ca2SiO4:Pr3+ are also confirmed.

Boltzmann-distribution-dominated persistent luminescence ratiometric thermometry in NaYF4:Pr3.

A novel, to the best of our knowledge, optical temperature measurement method is proposed, i.e., persistent luminescence intensity ratio (PLIR) thermometry. The PLIR thermometry relies on the micro-sized NaYF4:Pr3+ material that can emit persistent luminescence (PersL) uninterruptedly after being charged by x ray irradiation. The 3P1→3H5 and 3P0→3H5 PersL transitions, locating separately at ∼ 522 and 538 nm, have been confirmed to follow the Boltzmann distribution. The emitting intensity ratio of this pair of PersL lines is thus found to be a good indicator of the variation of temperature. Our work is expected to enrich the optical temperature sensing family.

Mechanism of the trivalent lanthanides' persistent luminescence in wide bandgap materials.

The trivalent lanthanides have been broadly utilized as emitting centers in persistent luminescence (PersL) materials due to their wide emitting spectral range, which thus attract considerable attention over decades. However, the origin of the trivalent lanthanides' PersL is still an open question, hindering the development of excellent PersL phosphors and their broad applications. Here, the PersL of 12 kinds of the trivalent lanthanides with the exception of La3+, Lu3+, and Pm3+ is reported, and a mechanism of the PersL of the trivalent lanthanides in wide bandgap hosts is proposed. According to the mechanism, the excitons in wide bandgap materials transfer their recombination energy to the trivalent lanthanides that bind the excitons, followed by the generation of PersL. During the PersL process, the trivalent lanthanides as isoelectronic traps bind excitons, and the binding ability is not only related to the inherent arrangement of the 4f electrons of the trivalent lanthanides, but also to the extrinsic ligand field including anion coordination and cation substitution. Our work is believed to be a guidance for designing high-performance PersL phosphors.

Eu3+-based dual-excitation single-emission luminescent ratiometric thermometry.

Recently, single-band ratiometric (SBR) thermometry becomes a hot-spot in the research field of optical thermometry. Here we propose a new SBR thermometry by combining the temperature-induced red shift of charge transfer state (CTS) of W-O and Eu-O with the ground state absorption (GSA) and excited state absorption (ESA) of Eu3+. The emitting intensity of the 5D0-7F2 transition of Eu3+ is monitored under CTS, GSA and ESA excitations at different temperatures. It is found that the SBR thermometry, depending on the combination of [GSA + CTS] of Eu3+ doped calcium tungstate, has the highest relative sensitivity of 1.25% K-1 at 573 K, higher than conventional luminescent ratiometric thermometry such as the 2H11/2 and 4S3/2 thermally coupled states of Er3+.

Ultra-sensitive luminescence ratiometric thermometry from 2E→4A2 transitions of AlTaO4:Cr3.

In recent years, non-contact ratiometric luminescence thermometry has continued to gain popularity among researchers, owing to its compelling features, such as high accuracy, fast response, and convenience. The development of novel optical thermometry with ultrahigh relative sensitivity (Sr) and temperature resolution has become a frontier topic. In this work, we present a novel, to the best of our knowldege, luminescence intensity ratio (LIR) thermometry method that relies on AlTaO4:Cr3+ materials, based on the fact that they possess both anti-Stokes phonon sideband emission and R-line emission at the 2E→4A2 transitions and have been confirmed to follow the Boltzmann distribution. In the temperature range 40-250 K, the emission band of the anti-Stokes phonon sideband shows an upward trend, while the bands of the R-lines show the opposite downward trend. Relying on this fascinating feature, the newly proposed LIR thermometry achieves a maximum relative sensitivity of 8.45%K-1 and a temperature resolution of 0.038 K. Our work is expected to provide guiding insights for optimizing the sensitivity of Cr3+-based LIR thermometers and provide some novel entry points for designing excellent and reliable optical thermometers.

Ratiometric optical thermometry based on upconversion luminescence with different multi-photon processes in CaWO4:Tm3+/Yb3+ phosphor.

Ratiometric optical thermometry based on upconversion (UC) luminescence with different multi-photon processes in CaWO4:Tm3+,Yb3+ phosphor was developed. A new fluorescence intensity ratio (FIR) thermometry, utilizing the ratio of the cube of 3F2,3 emission to the square of 1G4 emission of Tm3+ and retaining the feature of anti-interference of excitation light source fluctuations, is proposed. Under the hypotheses of the UC terms being neglected in the rate equations and the ratio of the cube of 3H4 emission to the square of 1G4 emission of Tm3+ being a constant in a relatively narrow temperature range, the new FIR thermometry is valid. The correctness of all hypotheses was confirmed by testing and analyzing the power-dependent emission spectra at different temperatures and the temperature-dependent emission spectra of CaWO4:Tm3+,Yb3+ phosphor. The results prove that the new ratiometric thermometry based on UC luminescence with different multi-photon processes is feasible through optical signal processing, and maximum relative sensitivity of the thermometry is 6.61% K-1 at 303 K. This study provides guidance in selecting UC luminescence with different multi-photon processes to construct ratiometric optical thermometers with anti-interference of excitation light source fluctuation.

Mechanoluminescence ratiometric thermometry via MgF2:Tb3.

Mechanoluminescent materials have attracted considerable attention over the past two decades, owing to the ability to convert external mechanical stimuli into useful photons. Here we present a new, to the best of our knowledge, type of mechanoluminescent material, i.e., MgF2:Tb3+. In addition to the demonstration of traditional applications, such as stress sensing, we show the possibility of ratiometric thermometry using this mechanoluminescent material. Under stimulation of an external force, rather than the conventional photoexcitation, the luminescence ratio of 5D3→7F6 to 5D4→7F5 emission lines of Tb3+ is confirmed to be a good indicator of temperature. Our work not only expands the family of mechanoluminescent materials, but also provides a new and energy-saving route for temperature sensing.

High performance FIR thermometry on the basis of the redshift of CTB by dual-wavelength alternative excitation in Eu3+:YVO4.

Compared with the forbidden 4f transition of rare earth ions, the strong absorption of the charge transfer band (CTB) enabled fluorescence thermometry to have high luminescence efficiency. Based on the temperature induced redshift of CTB, a high performance fluorescence intensity ratio (FIR) thermometry performed by dual-wavelength alternative excitation was studied. By way of the rising and falling edges of CTB in Eu3+ doped YVO4, monochrome sensitivity as a function of excitation wavelength was studied in the range of 303-783 K. The excitation wavelength with the highest positive monochrome sensitivity was determined, as well as that with the negative one. The optimum FIR temperature sensing strategy is proposed, and the theoretical highest relative sensitivity (Sr) is calculated to be 1.86% K-1, with the lowest uncertainty (ΔT) of 0.1 K at 783 K.

Luminescence properties, energy transfer and thermal stability of white emitting phosphor Sr3(PO4)2:Ce3+/Tb3+/Mn2+ for white LEDs.

A series of Sr3(PO4)2:Ce3+/Mn2+/Tb3+ phosphors were synthesized by a high temperature solid phase method. After introducing Ce3+ as sensitizer in Sr3(PO4)2:Ce3+/Mn2+, the efficient energy transfer from Ce3+ to Mn2+ was observed and analyzed in detail, and Sr3(PO4)2:Ce3+/Mn2+ was demonstrated to be color tunable, changing from blue to orange red. In addition, Tb3+ ion, which mainly emits green light, was further added into the Sr3(PO4)2:Ce3+/Mn2+. Due to the addition of this green emission, the white emitting phosphors with good quality were obtained. At the same time, the energy transfer mechanisms among Ce3+, Tb3+ and Mn2+ ions were also analyzed in detail. The results show that Sr3(PO4)2:Ce3+/Mn2+/Tb3+ is a promising candidate for white light emitting diodes.

Stably and highly sensitive FIR thermometry over a wide temperature range of 303-753†K based on the GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles.

Fluorescence intensity ratio (FIR) temperature sensors provide an effective method to control or study fine variations in physical and biological research because of their high sensitivity, accuracy, and spatial resolution. However, it is difficult to maintain high sensitivity over a wide temperature range using FIR temperature sensors because of the limits of the Boltzmann distribution law. In this study, sensitivity amplification for a wide temperature range in FIR thermometry based on GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles is achieved. The mechanism of the non-monotonic temperature dependence of the relative sensitivity (Sr) is studied. The results demonstrate that the Sr stably keeps ∼2.4% per K over a wide temperature range of 303-753 K, thus providing a basis for the extensive application of FIR temperature sensors.

On the Er3+ NIR photoluminescence at 800†nm.

Here, we study the Er3+ NIR 4I9/2-4I15/2 photoluminescence peaking at 800 nm. It can be detected with a good signal-to-noise for the prepared CaWO4:Yb3+,Er3+ phosphors upon excitation at 980 nm. When directly exciting the Er3+ green and red emitting states over the 333-773 K temperature range, the 800 nm photoluminescence for the samples is undetectable. It shows that the non-radiative relaxation from the upper excited states to the 4I9/2 emitting state is extremely inefficient. Moreover, the 800 nm photoluminescence decay curve is measured at high temperatures. It is found that the 800 nm emission always has a similar lifetime with the Er3+ 4I11/2-4I15/2 transition. This reminds us that the Er3+ 4I9/2 state is mainly populated by the adjacent lower 4I11/2 state by a thermally coupled way.

Eu3+-based luminescence ratiometric thermometry.

Recently, luminescence ratiometric thermometry has gained ever-increasing attention due to its merits of rapid response, non-invasiveness, high spatial resolution, and so forth. For research fields relying on temperature measurements, achieving a higher relative sensitivity of this measurement is still an important task. In this work, we developed a strategy for achieving a more sensitive temperature measurement, one merely depending on the photoluminescence of Eu3+. We showed that using the 5D1-7F1 transition and the hypersensitive 5D0-7F2 transition of Eu3+ can boost the relative sensitivity compared with the method relying on the 5D1-7F1 and 5D0-7F1 transitions of Eu3+. The difference between these two strategies was studied and was explained by the hypersensitive 5D0-7F2 transition more steeply decreasing than the 5D0-7F1 transition with a rise in temperature. Our work is expected to help researchers design sensitive optical thermometers via proper use of this hypersensitive transition.

Origin of the giant thermal enhancement of the Er3+ ion's 4I9/2-4I15/2 photoluminescence.

The Er3+ ion's 4I9/2-4I15/2 emission spectrum, which was rarely reported before, was successfully observed in the as-prepared scheelite-structured CaWO4:Yb3+,Er3+ phosphors, upon excitation at 980 nm. This photoluminescence, peaking at ca. 800 nm, was found to undergo a monotonous and giant enhancement with increasing the temperature from 333 to 813 K. Its dependence on pump power revealed that this emission spectrum was from one-photon pumping mechanism. Together with the analysis on luminescence intensity ratio thermometry, it was confirmed that the giant enhancement of the 800 nm emission spectrum was most likely to come from the adjacent lower 4I11/2 state via a thermally coupled way.