Pressure-induced novel ZrN4 semiconductor materials with high dielectric constants: a first-principles study.

In addition to Zr3N4 and ZrN2 compounds, zirconium nitrides with a rich family of phases always exhibit metal phases. By employing an evolutionary algorithm approach and first-principles calculations, we predicted seven novel semiconductor phases for the ZrN4 system at 0-150 GPa. Through calculating phonon dispersions, we identified four dynamically stable semiconductor structures under ambient pressure, namely, α-P1̄, ß-P1̄, γ-P1̄, and ß-P1 (with bandgaps of 1.03 eV, 1.10 eV, 2.33 eV, and 1.49 eV calculated using the HSE06 hybrid density functional, respectively). The calculated work functions and dielectric functions show that the four dynamically stable semiconductor structures are all high dielectric constant (high-k) materials, among which the ß-P1̄ phase has the largest static dielectric constant (3.9 times that of SiO2). Furthermore, we explored band structures using the HSE06 functional and density of states (DOS) and the response of bandgaps to pressure using the PBE functional for the four new semiconductor configurations. The results show that the bandgap responses of the four structures exhibit significant differences when hydrostatic pressure is applied from 0 to 150 GPa.

Semiconductors with a chiral crystal structure in group IVB transition metal pernitrides.

Group IVB transition metal (TM) nitrides rarely exhibit the semiconductor phase, except for TM3N4 (TM = Ti, Zr, and Hf) compounds. In this study, using the ab initio calculations based on density functional theory, we report two chiral crystal structures, namely P3121 and P3221, of TMN2, which are dynamically stable at ambient pressure. Unlike conventional metal phases of transition metal dinitrides, the P3121 and P3221 configurations exhibit intriguing semiconductor properties (with bandgaps of 1.076 eV, 1.341 eV, and 1.838 eV for TiN2, ZrN2, and HfN2, respectively). The mechanism of metal-to-semiconductor transition from the I4/mcm to P3121 phase is deeply explored by investigating their crystal structure and electronic structures. When hydrostatic pressure is applied from 0 GPa to 200 GPa, the bandgaps of the P3121 phase of TiN2, ZrN2, and HfN2 exhibit a different response, which is related to the orbital contribution at the conduction band minimum (CBM) and valence band maximum (VBM) and the lattice constants. Furthermore, according to the calculated mechanical properties, P3121 and P3221 phases exhibit higher bulk and shear moduli than the semiconductor phases of c-Zr3N4 and c-Hf3N4 in the corresponding systems.

New multiferroic BiFeO3 with large polarization.

BiFeO3 is one of the most widely studied multiferroic materials, because of its large spontaneous polarization at room temperature, as well as ferroelasticity and antiferromagnetism. Using an ab initio evolutionary algorithm, we found two new dynamically stable BiFeO3 structures (P63 and P6322) at ambient pressure. Their energy is only 0.0662 and 0.0659 eV per atom higher than the famous R3c-BiFeO3, and they have large spontaneous polarization, i.e., 71.82 µC cm-2 and 86.06 µC cm-2, respectively. The spontaneous polarization is caused by the movement of the Bi3+ atom along the [001] direction and mainly comes from the 6s electron of Bi3+. Interestingly, there is no lone pair electron of Bi3+, which is different from R3c-BiFeO3. The new structures have the same magnetic configurations as R3c-BiFeO3 (G-type antiferromagnetism), but they are characterized by one-dimensional channels linked by a group of two via surface-sharing oxygen octahedra. Due to the similarity of the two structures, both of them have indirect bandgap structures, and the bandgaps are 2.62 eV and 2.60 eV, respectively. This work not only broadens the structural diversity of BiFeO3 but also has constructive significance for the study of spontaneous polarization of new structures of multiferroic materials.

Adjusting the structure and luminescence properties of Sr2-x Bax MgAl22 O36 :Eu2+ phosphors by Sr:Ba ratio.

Europium ion (Eu2+ )-doped phosphors exhibit adjustable photoluminescence due to the sensitivity of their luminescence to the local environment. It is of great significance to adjust the luminescence of Eu2+ by changing their local environment in the host. In this work, we investigated the effect of strontium/barium (Sr:Ba) ratio on the structure and luminescence properties of Sr2-x Bax MgAl22 O36 :Eu2+ phosphors. Our investigation indicates that with the decrease of Sr:Ba ratio, the matrix lattice gradually expands and the peak wavelength for the luminescence of Eu2+ presents an obvious blue shift. The occupancy of Eu2+ was analyzed and the reason for the blue shift was explained. Thermal stability for the luminescence of Eu2+ can also be adjusted by changing the Sr:Ba ratio. This work has a positive effect on the regulation of the emission of phosphors and the improvement of thermal stability, which will promote the application of Sr2-x Bax MgAl22 O36 :Eu2+ phosphors in the field of white light emitting diodes.

Efficient and Thermally Stable Broad-Band Near-Infrared Emission in a KAlP2O7:Cr3+ Phosphor for Nondestructive Examination.

Broad-band near-infrared (NIR) phosphors are essential to assembling portable NIR light sources for applications in spectroscopy technology. However, developing inexpensive, efficient, and thermally stable broad-band NIR phosphors remains a significant challenge. In this work, a phosphate, KAlP2O7, with a wide band gap and suitable electronic environment for Cr3+ equivalent substitution was selected as the host material. The synthesized KAlP2O7:Cr3+ material exhibits a broad-band emission covering 650-1100 nm with a peak centered at 790 nm and a full width at half-maximum (fwhm) of 120 nm under 450 nm excitation. The internal quantum efficiency (IQE) was determined to be 78.9%, and the emission intensity at 423 K still maintains 77% of that at room temperature, implying the high efficiency and excellent thermal stability of this material. Finally, a NIR phosphor-converted light-emitting diode (pc-LED) device was fabricated by using the as-prepared material combined with a 450 nm blue LED chip, which presents a high NIR output power of 32.1 mW and excellent photoelectric conversion efficiency of 11.4% under a drive current of 100 mA. Thus, this work not only provides an inexpensive broad-band NIR material with high performance for applications in NIR pc-LEDs but also highlights some strategies to explore this class of materials.

Accurately Controlling the Occupation of Eu2+ in Cs(K, Na)3(Li3SiO4)4 to Achieve Narrow-Band Green Emission for Wide Color Gamut Displays.

The development of narrow-band phosphors for wide color gamut displays in multimodal phosphors through selective site occupancy engineering is an important challenge. In this work, by replacing Na ions with K ions in the cyan-green double-band emitting phosphor CsK0.9Na2(Li3SiO4)4: 10%Eu2+, the occupation of Eu2+ in Cs(K, Na)3(Li3SiO4)4 was accurately controlled from occupying three sites of Cs, K1, and Na to occupying only one site of K2/Na. The obtained phosphor CsK1.9Na(Li3SiO4)4: 10%Eu2+ exhibits a single narrow-band green emission at 531 nm (the full width at half-maximum of 46 nm) with excellent thermal stability of luminescence from 80 to 523 K (96.3% @423 K of the intensity of integrated emission at room temperature and 94.9% @300 K of the intensity of integrated emission at 80 K). The white light-emitting diode (wLED) that was fabricated by combining a blue LED chip with this narrow-band green phosphor and red phosphor K2SiF6: Mn4+ presents a satisfactory wide color gamut of 128.1% of the National Television Standards Committee, which demonstrates the important application value of the phosphor in the wide color gamut displays field. This work provides an effective design strategy for exploring narrow-band phosphors through selective site occupancy engineering, which will facilitate the exploration of relevant narrow-band emitters in the future.

[Luminescence investigation of Na(z)Ca(1-x-2y-z)Bi(y)MoO4 : Eu(x+y)3+, red phosphors].

A series of red phosphors with the composition Na(z)Ca(1-x-2y-z), Bi(y) MoO4 : Eu(x+y)3+ (y, z = 0, x = 0.24, 0.26, 0.30, 0.34, 0.38; x = 0.30, y = 0.01, 0.02, 0.03, 0.03, 0.05, 0.06, 0.07; x = 0.30, y = 0.04, z = 0.38) were prepared via traditional solid-state method. The crystal structures of the obtained phosphors were identified by X-ray powder diffraction (XRD) method. The photoluminescence properties of the samples were characterized by fluorescence spectrophotometer. The results indicated that the concentration of Eu3+ single doped Ca(1-x) MoO4 : Eu3+ with the maximum luminescence intensity was found to be 0.30 (namely, Ca0.70 MoO4 : Eu(0.30)3+); the photoluminescence properties with different ratio of Bi3+/Eu3+ codoped Ca0.70-2y Bi(y) MoO4 : Eu(0.30+y)3+, were also investigated, and the results showed that the charge band (CTB) reached the maximum value when the y value was equal to 0.03; for the characteristic excitation peaks of Eu3+, however, the intensity of the excitation spectral line locating at 393 nm was stronger than that at 464 nm when y < 0.03, while the intensity at 464 nm was greater than that at 393 nm when y > or = 0.03; the intensity of excitation peaks locating at 393 and 464 nm respectively both reached the maximum intensity when the y value was 0.04. The relative intensity of the excitation and emission of the above phosphor was enhanced greatly when Na2CO3 acting as charge compensation was added. The above results showed that the relative intensity between 393 and 464 nm could be changed by adjusting the ratio of Bi3+ /Eu3+ codoping concentrations.

The design of dual-switch fluorescence intensity ratio thermometry with high sensitivity and thermochromism based on a combination strategy of intervalence charge transfer and up-conversion fluorescence thermal enhancement.

Currently, the temperature sensing performances of inorganic photoluminescence materials based on fluorescence intensity ratio technology have become a research hotspot in the optical thermometry field due to their non-contact sensing, fast response and high stability. However, several problems have obstructed the development of optical temperature sensing materials, including low sensitivity and narrow temperature measurement ranges. In view of the above dilemma, a new optical thermometer La2Mo3O12:Yb3+,Pr3+ designed based on the combination strategy of intervalence charge transfer and up-conversion fluorescence thermal enhancement was developed. Under excitation at 450 nm, the thermometer can work in a range from 298 to 648 K and the relative sensitivity reaches as high as 2.000% K-1 at 648 K. Under excitation at 980 nm, the thermometer can sense temperature with a wide range from 298 to 748 K and the relative sensitivity reaches as high as 4.325% K-1 at 598 K. A dual-switch optical temperature sensing material with high-sensitivity and a wide temperature measurement range has been successfully developed. Our research design strategies will give inspiration to the research on multi-switch temperature sensing materials with high sensitivity and a wide temperature measurement range.

Trivalent Chromium Ions Doped Fluorides with Both Broad Emission Bandwidth and Excellent Luminescence Thermal Stability.

Recently, trivalent chromium ion doped phosphors have exhibited significant application potential in broadband near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs). However, developing an NIR phosphor with both broad emission bandwidth and excellent luminescence thermal stability is still a great challenge. Here, we demonstrate an NIR phosphor, ScF3:Cr3+, which can fulfill both conditions simultaneously. The prepared phosphors show broadband emission in the range of 700 to 1100 nm, with a full width at half-maximum (FWHM) of 140 nm peaking at 853 nm. These phosphors also demonstrate an excellent luminescence thermal stability (the emission intensity of ScF3:Cr3+ keeps 85.5% at 150 °C compared with the value at room temperature). An NIR pc-LED based on blue LED chips was fabricated and tested. The results show that the NIR pc-LED can yield strong broadband NIR emission. This work not only provides a promising phosphor for the application of NIR pc-LEDs but also has important guiding significance for effect of synthesis conditions on the luminescence properties of Cr3+-doped fluorides.

A warm white emission of Bi3+-Eu3+ and Bi3+-Sm3+ codoping Lu2Ge2O7 phosphors by energy transfer of Bi3+-sensitized Eu3+/Sm3.

In the last few years, multicolor-tunable phosphors, especially single-composition white-light-emitting phosphors, have attracted increasing attention and interest for UV-converted white LEDs. In this paper, Lu2Ge2O7:Bi3+ phosphor presents a doublet emission ranging from UV to visible spectrum with a high QE of 76%. To obtain a warm white emission, we tried to codope Eu3+ or Sm3+ into Lu2Ge2O7 with Bi3+ ions. Multicolor-tunable emission colors of Bi3+-Eu3+ and Bi3+-Sm3+ codoped Lu2Ge2O7 samples are adjusted from cold white light, warm white light, light pink, pink, to red by changing the Eu3+ or Sm3+ doping concentration. Energy transfer process occurring from Bi3+ to Eu3+ or Sm3+ is discussed in detail and the corresponding ηETE of Bi3+-Eu3+ and Bi3+-Sm3+ can reach as high as 42% and 35%, respectively. This paper not only provides a novel UV-converted multicolor-tunable phosphor, but also discovery novel single-composition white-light-emitting phosphors for UV-converted white LEDs.

Highly bright multicolor-tunable KSrLu(PO4)2:Ce3+, Tb3+, Mn2+ phosphors via efficient energy transfer.

Research hotspots about Ce3+-Tb3+ and Ce3+-Mn2+ codoped multicolor-tunable materials, has grown rapidly and is still growing. However, few feasible strategies can be employed to achieve the multicolor-tunable luminescence under the premise of maintaining high QE. In this paper, KSrLu(PO4)2:0.25Ce presents a high luminescence efficiency with approximately 78% quantum efficiency. Furthermore, Ce3+-Tb3+ and Ce3+-Mn2+ codoped optimal samples give a green and red luminescence, which corresponds to a high 61% and 55% quantum efficiency due to a high energy transfer efficiency of Ce3+→Tb3+ and Ce3+→Mn2+. Besides, we achieved the precise adjustment of blue, green, orange and pink emissions of KSrLu(PO4)2:Ce3+, Tb3+, Mn2+ samples. Meanwhile, KSrLu(PO4)2:Ce3+, Tb3+ and KSrLu(PO4)2:Ce3+, Mn2+ phosphors present a weak thermal quenching: their integrated emission intensity at 500K still keep more than 80% of their initial intensity at room temperature. Therefore, the results might provide a novel multicolor-tunable phosphor for backlight display application.

Tunable Polarity Behavior and High-Performance Photosensitive Characteristics in Schottky-Barrier Field-Effect Transistors Based on Multilayer WS2.

Schottky-barrier field-effect transistors (SBFETs) based on multilayer WS2 with Au as drain/source contacts are fabricated in this paper. Interestingly, the novel polarity behavior of the WS2 SBFETs can be modulated by drain bias, ranging from p-type to ambipolar and finally to n-type conductivity, due to the transition of band structures and Schottky-barrier heights under different drain and gate biases. The electron mobility and the on/off ratio of electron current can reach as high as 23.4 cm2/(V s) and 8.5 × 107, respectively. Moreover, the WS2 SBFET possesses high-performance photosensitive characteristics with response time of 40 ms, photoresponsivity of 12.4 A/W, external quantum efficiency of 2420%, and photodetectivity as high as 9.28 × 1011 cm Hz1/2/W. In conclusion, the excellent performance of the WS2 SBFETs may pave the way for next-generation electronic and photoelectronic devices.