Electronic and optical properties of a novel two-dimensional semiconductor material TlPt2S3: a first-principles study.

Two-dimensional (2D) materials have attracted widespread attention due to their unique physical and chemical properties. Here, by using density functional theory calculations, we suggest a novel 2D TlPt2S3 material whose layered bulk counterpart was synthesized in 1973. Theoretical calculation results indicate that the exfoliating energy of monolayer and bilayer TlPt2S3 is 34.96 meV Å-2 and 36.03 meV Å-2. We systematically studied the electronic and optical properties of monolayer and bilayer TlPt2S3, and revealed that they are indirect band gap semiconductors with band gaps of 2.26 eV and 2.10 eV, respectively. Monolayer and bilayer TlPt2S3 exhibit superior carrier mobility (901.63 cm2 V-1 s-1 and 13635.04 cm2 V-1 s-1 for electron mobility of the monolayer and bilayer, respectively) and photocatalytic performance (as high as 1 × 105 light absorption coefficient in the visible light region). Interestingly, we find that monolayer TlPt2S3 has significant hydrogen evolution performance, while in the bilayer, the electron band distribution shows complete oxygen evolution ability, which indicates that the proposed monolayer and bilayer TlPt2S3 are potential novel 2D materials suitable for photocatalytic water splitting driven by visible light.

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.

"Roller coaster"-like thermal evolution of the Er3+ ion's red photoluminescence in CaWO4:Yb3+/Er3+ phosphors.

The abnormal "roller coaster"-like thermal evolution of the Er3+ ion's red photoluminescence (corresponding to the F9/24-I415/2 transition) in CaWO4:Yb3+/Er3+ phosphors is observed. This red emission suffers from a strong thermal quenching in the 293-573 K temperature range, followed by a sharp increase on further increasing the temperature. The mechanism behind this phenomenon is confirmed to be from the dynamic temperature-dependent multiple mechanisms imposed on the F9/24 state. At relatively low temperatures, the two-photon upconversion mechanism plays a leading role while, with the increasing of temperature, the one-photon channel, ascribed to the thermal population from the lower I9/24 state, gradually takes a dominant place.

Highly sensitive optical ratiometric thermometry by exciting Eu3+/Tb3+'s unusual absorption lines.

A guideline for designing sensitive thermometry is proposed. It reveals that using two emission bands that possess the opposite change tendencies against temperature makes it easier to achieve a larger relative sensitivity. Based on the guidelines, a highly sensitive strategy for optical thermal detection that depends on the Tb3+-to-Eu3+ emission ratio is designed by exciting Eu3+/Tb3+'s unusual absorption lines. This can be easily driven by a commonly used and cheap 405 nm laser diode. Moreover, its maximum relative sensitivity reaches up to 2.02% K-1 at 610 K, one of the largest sensitivities reported so far.

Relative sensitivity variation law in the field of fluorescence intensity ratio thermometry.

We study the variation law of relative sensitivity in the field of fluorescence intensity ratio thermometry. It is theoretically demonstrated that there must be only one maximum value of relative sensitivity in the case in which there is a positive offset in fitting function. Moreover, the method to obtain this maximum is proposed. Experimental results, taking the D15/D50 levels of Eu3+ as examples, are in excellent accordance with the conclusion. The mechanism behind is then investigated, and other populating processes imposed on the D15 level, which exert negative outcome on thermal sensitivity, are found to play a key role in determination of this unique variation law.

Modified calculation method of relative sensitivity for fluorescence intensity ratio thermometry.

The calculation method of relative sensitivity (Sr) for fluorescence intensity ratio (FIR) thermometry is discussed, taking the F33-H63 and H43-H63 transitions of Tm3+ as examples. The value of Sr is calculated using its original definition, and is found to largely deviate from the result obtained using the conventional method that is widely used at present. This deviation is found to stem from the neglect of an offset. A modified expression of Sr is proposed, which shows the true performance of FIR technology and makes it possible to precisely compare the Sr values obtained using various methods.

Precise determination on the upconversion processes of the ultraviolet upconversion fluorescence of Ho3+-doped Y2O3 ceramic by excitation of a 532-nm continuous-wave laser.

Aiming at developing a Ho3+-based continuous-wave ultraviolet laser, the method of determining the weight of the excited-state absorption (ESA) process occupied in the populating of the D33 level is studied. Using the rate equation model, the weight is found to be related to the weighting factors of the double-exponential decay components of the D33 level. Using time-resolved spectroscopy, the weight of the ESA process for Y2O3 ceramic doped with 0.1 mol. % Ho3+ is experimentally determined to be 83%. Furthermore, the fluorescence dynamic properties of the Ho3+-doped Y2O3 ceramic with different doping concentrations were analyzed, and the weight was found to decrease with the doping concentration but by no less than 70%.

Influence of phase transitions on green fluorescence intensity ratio in Er3+ doped K0.5Na0.5NbO3 ceramic.

The fluorescence intensity ratio (FIR) method is a non-contact temperature (T) measurement technique based on thermally coupled levels of rare earth ions in a doped host. Green fluorescence originating from 2H11/2 and 4S3/2 states of Er3+ doped K0.5Na0.5NbO3 (KNN) ceramic are studied in the temperature range of 300 K to 720 K. The fluorescence intensities change dramatically around phase transition points where the crystal symmetry changes, inducing deviation of the FIR from Boltzmann's law. The temperature determined by the FIR method deviates from thermocouple measurements by 7 K at the orthorhombic to tetragonal phase transition (TO-T) point and 13 K at the Curie point (TC). This finding gives guidance for developing fluorescent T sensors with ferroelectrics and may also provide a fluorescent method to detect phase transitions in ferroelectric materials.

Fluorescence intensity ratio method for temperature sensing.

A thermometry method based on the ratio between the valley intensity formed by fluorescence peak overlap and the peak fluorescence intensity has been developed. Excited by a 405 nm laser, the valley to peak ratio (VPR) of the emissions originating from 5D0 to 7F2 Stark sublevels in Eu3+-doped CaWO4 shows a monotonic change with temperature. Spectrum analysis indicates that this monotonic increase is caused by the homogeneous broadening of the spectral lines as the temperature increases. The relative sensitivity S(r) is in the magnitude of 10(-4) K(-1) in the experimental temperature range of 303-573 K.

Luminescence and photosensitivity of gadolinium labeled hematoporphyrin monomethyl ether.

Photodynamic therapy for deep-lying lesions needs an appropriate imaging modality, precise evaluation of tissue oxygen and an effective photosensitizer. Gadolinium based metalloporphyrins Gd(III)-HMME is proposed in this study as a potential multifunctional theranostic agent, as photosensitizer, ratiometric oxygen sensor and MRI contrast agent. The time resolved spectroscopy revealed the luminescence peak of Gd(III)-HMME at 710 and 779 nm with a lifetime of 64 µs in oxygen-free methanol to be phosphorescent. This phosphorescence is strongly dependent on dissolved oxygen concentration. Its intensity in oxygen saturated methanol solution is 21% of that in deoxygenated solution. The singlet oxygen quantum yields ΦΔ of HMME and Gd(III)-HMME in air saturated methanol solution were determined to be 0.79 and 0.40 respectively using comparative spectra method. These phenomena indicate that the oxygen sensibility and production of singlet oxygen of Gd(III)-HMME can fulfill the requirement of PDT treatment.

Large-scale sensitivity adjustment for Gd-HMME room temperature phosphorescence oxygen sensing.

The oxygen sensing enhancement based on room temperature phosphorescence (RTP) of Gd-HMME adjusted by imidazole was studied. The phosphorescence intensity IP0 and the Stern-Volmer equations under different imidazole concentration were obtained, and the physical mechanism of imidazole regulating the oxygen quenching constant KSV was analyzed. It was found that the KSV value increased by ∼46 folds in the range of 12.4(1)-576.1(5) kPa-1, and the large-scale variation of KSV is conducive to the realization of high precision oxygen concentration measurement in a wide range. In addition, the standard deviation σ of continuous measurement results was given, and the limit of detection (LOD) was determined to be 6.6 ppm.

Pressure-sensitive blackbody point radiation induced by infrared diode laser irradiation.

Ultrabroadband radiation from Yb(2)O(3) at ambient and low air pressures was investigated under the excitation of a 980 nm diode laser. The radiation was confirmed to be blackbody radiation, and it is sensitive to environmental air pressure in the way that the integrated radiation intensity decreases linearly with increasing air pressure. An ideal gas model may be employed to interpret the linear dependence. The pressure-sensitive radiation characteristic provides a potential method for noncontact measurement of air pressure with high accuracy.

Computational Screening of High Activity and Selectivity TM/g-C3N4 Single-Atom Catalysts for Electrocatalytic Reduction of Nitrates to Ammonia.

Electrocatalytic reduction of nitrates (NO3RR) selectively generating ammonia (NH3) opens up a new idea for treating nitrates in wastewater, which not only reduces nitrates but also obtains the valuable product ammonia. By first-principles calculations, we explore the activity and selectivity for NO3RR to NH3 of TM/g-C3N4 single-atom catalysts. Six TM/g-C3N4 catalysts (TM = Ti, Os, Ru, Cr, Mn, and Pt) are selected by a four-step screening method. Ru/g-C3N4 is the most promising of these six TM/g-C3N4 catalysts because of its lowest energy barrier and extraordinary selectivity. The origin of the NO3RR activity of Ru/g-C3N4 is explained from the viewpoint of NO3- adsorption. In addition, the hydrogen evolution reaction has also been implied to be uncompetitive for the poor adsorption on H atoms. This work provides a screening mechanism for finding new catalysts for NO3RR to NH3, promotes the development of NO3RR, and provides a stimulating impetus for further experimental exploration.

An effective oxygen content detection in phosphorescence of PtOEP-C6/Poly (St-co-TFEMA).

The phosphorescence of PtOEP-C6/Poly (St-co-TFEMA) has been investigated to achieve an accurate oxygen content, which is always puzzled as its extreme temperature sensitivity. The relations of oxygen content and phosphorescence intensity ratio can be perfectly fitted by the Stern-Volmer equation at different temperatures, meanwhile the monotonic quenching constant KSV is obtained, which enables us to develop a method of temperature correction to realize the intrinsic oxygen content. Then a clear fundamental picture of the temperature quenching mechanism of PtOEP is drawn by the time-resolved spectroscopy, the temperature sensitivity of phosphorescence arises from the enhanced quenching effect of oxygen by temperature. Our results provide an effective method to gain accurate oxygen content by simple optical measurement.

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.

The effect of imidazole on the enhancement of gadolinium-porphyrin phosphorescence at room temperature.

A mechanism for the enhanced room-temperature phosphorescence (RTP) of gadolinium-coordinated hematoporphyrin monomethyl ether (Gd-HMME) in the presence of imidazole and free gadolinium ions (Gd3+) is revealed. Imidazole, the molten solvent used in the synthesis of Gd-HMME, was effective in enhancing the Gd-HMME RTP. In the presence of imidazole, further enhancement of the Gd-HMME RTP was observed upon adding Gd3+. Overall, a 40-fold enhancement of Gd-HMME RTP intensity was achieved by adding both imidazole and Gd3+. In addition, there was an increase in the RTP lifetime. Through spectroscopic analysis, we deduced that a protective medium is formed by the imidazole and the degree of this protection is further increased by Gd3+. The protective medium enhances the Gd-HMME RTP by partially inhibiting energy transfer from the lowest triplet state of Gd-HMME to oxygen. This was demonstrated by the presence of lower levels of singlet oxygen in the Gd-HMME solution after the addition of imidazole. These results indicate that imidazole could have potential application as an RTP enhancer or triplet state protector.