To meet the increasing requirement, much effort has been devoted to enhance the emission intensity and tailor the emission color of rare earth phosphors. However, limited contributions have been made to the up-conversion (UC) of nanorods by complete epitaxial growth on each facet to achieve this requirement. In this study, we propose a facile epitaxial growth route to grow anisotropic hexagonal β-NaYF4:Yb3+/Ho3+@β-NaYbF4:Er3+, β-NaYF4:Yb3+/Ho3+@β-NaYF4, and β-NaYbF4:Er3+@β-NaYF4 core/shell nanorods, which were realized by adding hexagonal β-NaYF4:Yb3+/Ho3+ or β-NaYbF4:Er3+ nanorods as a core-nanostructure into a solution containing cubic α-NaYbF4:Er3+ or α-NaYF4 nanoparticles as the shell-precursor. During epitaxial growth-induced phase transformation, the precursor nanoparticles disappeared gradually in the solution and consequently corresponding β-phased shell yielded on each outer facet of each β-phased nanorod core. Eventually, the nanorod core was covered completely with a uniformly grown β-NaYbF4:Er3+ or β-NaYF4 shell. The UC emission of either β-NaYF4:Yb3+/Ho3+ or β-NaYbF4:Er3+ core can be enhanced by the outer shell due to the decrease in the number of surface defects. In addition, tailored UC emissions could be obtained by controlling the shell components and thickness, typically in the core/shell nanorods of β-NaYF4:Yb3+/Ho3+@β-NaYbF4:Er3+. The tunable colors with improved emission in these core/shell nanorods may find wider applications in multicolor labeling and anti-counterfeiting.

A series of Ca3Y(GaO)3(BO3)4:Tb3+,Eu3+ phosphors were prepared by a high-temperature solid-state reaction. Their phase structures were confirmed by powder X-ray diffraction and the element distribution was measured using transmission electron microscopy elemental mapping. The photoluminescence emission and excitation spectra and fluorescence lifetime were studied and discussed in detail. The results revealed that Eu3+ ions can be efficiently sensitized by Tb3+ ions under near-UV excitation. In addition, the energy transfer efficiency can be controlled by adjusting the ratio of Eu3+ and Tb3+ to realize colour tunable emission from green to red. For Ca3Y(GaO)3(BO3)4:0.50Tb3+,0.10Eu3+, the emission intensity at 425 K is 78.11% of that at 300 K, being available to near-UV LEDs.

A series of YGa1.5Al1.5(BO3)4:Tb3+,Eu3+ inorganic luminescence materials were successfully synthesized through a high-temperature solid-state reaction. Powder X-ray diffraction was used to confirm the crystal structure and phase purity of the obtained samples. Then, scanning electron microscopy elemental mapping was taken to characterize the distribution of the doped ions. Detailed investigations on the photoluminescence emission and excitation spectra and fluorescence lifetime revealed that trivalent europium ions can be well sensitized by trivalent terbium ions under near-ultraviolet excitation. Additionally, the energy transfer from Tb3+ to Eu3+ can be controlled to realize multi-colour emissions covering the green to red visible region. Concentration quenching does not take place in Tb3+ or Eu3+ singularly-doped YGAB due to its structure isolation. Therefore, satisfactory luminescence properties related to the structure specificity make these phosphors suitable for WLEDs.

A novel phosphate/tungstate family, K2Ln(PO4)(WO4) (Ln = Y, Gd and Lu) doped with Tb3+ and Eu3+ is synthesized via a conventional high-temperature solid-state reaction to explore new pure red phosphors with high critical concentration for white light-emitting diodes (WLEDs). The results from the Rietveld method show that the crystal structures of the hosts are composed of phosphate layers and tungstate zigzags, and the Ln3+–Ln3+-units are isolated by the [PO4]3− groups in phosphate layers. The critical concentration of Tb3+ and Eu3+ is up to 40–50% in the singly doped phosphors, which is ascribed to the interaction of the isolated Ln3+ ions being mitigated by [PO4]3− and [WO4]2− groups, such that the special structure of K2Ln(PO4)(WO4) helps the interaction of luminescence centres. The energy transfer from Tb3+ to Eu3+ in K2Ln(PO4)(WO4) is demonstrated by fluorescence decay times. By adjusting the ratio of Eu3+ and Tb3+, we can tune the emission colour of K2Ln(PO4)(WO4):Tb3+,Eu3+ from green to yellow, orange and pure red. For K2Tb0.5Eu0.5(PO4)(WO4), the internal quantum efficiency is as high as 76.45% under an excitation of 394 nm, and the emission intensity at 150 °C is 92.2% of that at 25 °C.

A novel red-emitting phosphor Ca8MgLu(PO4)7:Eu3+ was synthesized by a high-temperature solid-state reaction method. Its crystal structure, photoluminescence emission and excitation spectra, and decay time were investigated in detail. X-ray diffraction (XRD) results indicate that Ca8MgLu(PO4)7 crystallizes in single-phase component with a whitlockite-like structure and the space group R3c of β-Ca3(PO4)2. The emission spectrum shows a dominant peak at 612 nm due to the dipole 5D0→7F2 transition of Eu3+, and the luminescence intensity keeps increasing with increasing the content of Eu3+ to 100%. The excitation spectrum is coupled well with the emission of near ultraviolet (NUV) LED (380–410 nm). The CIE coordinates of Ca8MgLu(PO4)7:Eu3+ phosphor is (0.654, 0.346), being close to the standard value of National Television Standard Committee (NTSC) for red phosphor, (0.670, 0.330). The internal quantum efficiency of the phosphor is 69% under the excitation of 394 nm. The results show that Ca8MgLu(PO4)7:Eu3+ is a very appropriate red-emitting phosphor with a high ratio of red and orange for NUV-based white LEDs.

Two series of single-composition Ca8MgLu(PO4)7:Tb3+ and Ca8MgTb(PO4)7:Eu3+ phosphors were prepared by a high-temperature solid-state reaction technique, and their phase structures were characterized by powder X-ray diffraction (XRD). The excitation and emission spectra, and fluorescence decays were measured and discussed in detail. The results reveal that Tb3+ can efficiently transfer excitation energy to Eu3+ via its 4f states and therefore sensitizes Eu3+ emission under NUV excitation. By adjusting the ratio of Eu3+ and Tb3+, we can tune the emission color of Ca8MgTb(PO4)7:Eu3+ from green to yellow, orange and pure red. For Ca8MgTb0.1(PO4)7:0.9Eu3+, the emission intensity at 150 °C is 87.44% of that at 25 °C, which makes it be a potential pure red phosphor for NUV LEDs.

The application of white LEDs is hindered by the low efficiency of commercial red phosphors. Here, a novel narrow-line red phosphor is produced by a terbium chain in the form of Ce3+–(Tb3+)n–Eu3+ in the Na2Y2B2O7 host and is characterized with X-ray diffraction, photoluminescence (PL), PL excitation (PLE), and fluorescence lifetime, and the energy transfer (ET) processes between rare-earth ions in the host are discussed. The formation of terbium chain with a quite low content of Tb3+ in Na2Y2B2O7 is realized by the ET processes of Ce3+–Tb3+ and Tb3+–Eu3+, and a new concept of saturation distance is put forward as an explanation for the first time. An energy level diagram is proposed to explain the ET processes in the phosphor of Na2Y2B2O7:Ce3+,Tb3+,Eu3+. The emitting colour of the phosphor can be tuned from blue to green or yellow and finally to the orange–red region with increasing the content of Tb3+. The quantum efficiency of the phosphor with an optimized ratio of rare-earth ions, Na2Y2B2O7:0.5% Ce3+,60% Tb3+,0.5% Eu3+, is up to 77% under the excitation of 365 nm, which indicates that the as-synthesized phosphor is applicable to near-UV white LEDs.

Nanostructured tetragonal LaVO4 with sheaf-like and prickly spherical morphologies has been hydrothermally synthesized with the assistance of ethylenediaminetetraacetic acid (EDTA). In the hydrothermal process, EDTA not only acts as a chelating reagent to facilitate the formation of t-LaVO4, but also acts as a surface capping agent to adhere to the newly created surface and to promote the crystal splitting. t-LaVO4 nanostructures with different morphologies can be achieved by adjusting the molar ratio of EDTA/La3+, the concentration of La3+, and the total volume of the reaction mixture, which result in the change of the crystal growth rate. Nanostructured Eu3+-doped t-LaVO4 was also synthesized and showed intense red emission under near UV-light excitation.

Terbium chain in the form of S → (Tb3+)n → A (S = Ce3+ or Eu2+, A = Eu3+), as a promising energy transfer (ET) approach, has been proposed to enhance Eu3+ emission for solid-state lighting. However, the viewpoint of ET from S to A via the terbium chain (Tb3+–Tb3+–Tb3+–...) is very doubtful. Here, hosts of Ba3Ln(PO4)3, LnPO4, LnBO3, and Na2Ln2B2O7 doped with Ce3+ → (Tb3+)n → Eu3+ or (Tb3+)n → Eu3+ are synthesized to prove the universality of S → (Tb3+)n → A in inorganic hosts and to study the unsolved issues. Saturation distance of Tb3+–Eu3+, estimated with the empirical data of different hosts, is proposed to be a criterion for determining whether a spectral chromaticity coordinate keeps constant. A branch model is put forward to replace the chain model to explain the role of (Tb3+)n in ET from Ce3+ to Eu3+ and the necessity of high content of Tb3+; the term “terbium bridge” is used to replace “terbium chain”, and the value of n is determined to be two or three. The intensity quenching of Eu3+ emission is attributed to the surface defects ascribed to the smaller particles and larger specific surface area rather than the concentration quenching of Tb3+. Based on the saturation distance and the mechanism of luminescence quenching, the necessary concentration of Tb3+ for (Tb3+)n can be estimated as long as the cell parameters are already known and the luminescent efficiency of Eu3+ can be further improved by optimizing the synthesis method to decrease the quantity of surface defects.

The application of white LEDs is hindered by the low efficiency of commercial red phosphors. Here, a novel narrow-line red phosphor is produced by a terbium chain in the form of Ce3+–(Tb3+)n–Eu3+ in the Na2Y2B2O7 host and is characterized with X-ray diffraction, photoluminescence (PL), PL excitation (PLE), and fluorescence lifetime, and the energy transfer (ET) processes between rare-earth ions in the host are discussed. The formation of terbium chain with a quite low content of Tb3+ in Na2Y2B2O7 is realized by the ET processes of Ce3+–Tb3+ and Tb3+–Eu3+, and a new concept of saturation distance is put forward as an explanation for the first time. An energy level diagram is proposed to explain the ET processes in the phosphor of Na2Y2B2O7:Ce3+,Tb3+,Eu3+. The emitting colour of the phosphor can be tuned from blue to green or yellow and finally to the orange–red region with increasing the content of Tb3+. The quantum efficiency of the phosphor with an optimized ratio of rare-earth ions, Na2Y2B2O7:0.5% Ce3+,60% Tb3+,0.5% Eu3+, is up to 77% under the excitation of 365 nm, which indicates that the as-synthesized phosphor is applicable to near-UV white LEDs.

A series of YGa1.5Al1.5(BO3)4:Tb3+,Eu3+ inorganic luminescence materials were successfully synthesized through a high-temperature solid-state reaction. Powder X-ray diffraction was used to confirm the crystal structure and phase purity of the obtained samples. Then, scanning electron microscopy elemental mapping was taken to characterize the distribution of the doped ions. Detailed investigations on the photoluminescence emission and excitation spectra and fluorescence lifetime revealed that trivalent europium ions can be well sensitized by trivalent terbium ions under near-ultraviolet excitation. Additionally, the energy transfer from Tb3+ to Eu3+ can be controlled to realize multi-colour emissions covering the green to red visible region. Concentration quenching does not take place in Tb3+ or Eu3+ singularly-doped YGAB due to its structure isolation. Therefore, satisfactory luminescence properties related to the structure specificity make these phosphors suitable for WLEDs.

A series of Ca3Y(GaO)3(BO3)4:Tb3+,Eu3+ phosphors were prepared by a high-temperature solid-state reaction. Their phase structures were confirmed by powder X-ray diffraction and the element distribution was measured using transmission electron microscopy elemental mapping. The photoluminescence emission and excitation spectra and fluorescence lifetime were studied and discussed in detail. The results revealed that Eu3+ ions can be efficiently sensitized by Tb3+ ions under near-UV excitation. In addition, the energy transfer efficiency can be controlled by adjusting the ratio of Eu3+ and Tb3+ to realize colour tunable emission from green to red. For Ca3Y(GaO)3(BO3)4:0.50Tb3+,0.10Eu3+, the emission intensity at 425 K is 78.11% of that at 300 K, being available to near-UV LEDs.

A novel phosphate/tungstate family, K2Ln(PO4)(WO4) (Ln = Y, Gd and Lu) doped with Tb3+ and Eu3+ is synthesized via a conventional high-temperature solid-state reaction to explore new pure red phosphors with high critical concentration for white light-emitting diodes (WLEDs). The results from the Rietveld method show that the crystal structures of the hosts are composed of phosphate layers and tungstate zigzags, and the Ln3+–Ln3+-units are isolated by the [PO4]3− groups in phosphate layers. The critical concentration of Tb3+ and Eu3+ is up to 40–50% in the singly doped phosphors, which is ascribed to the interaction of the isolated Ln3+ ions being mitigated by [PO4]3− and [WO4]2− groups, such that the special structure of K2Ln(PO4)(WO4) helps the interaction of luminescence centres. The energy transfer from Tb3+ to Eu3+ in K2Ln(PO4)(WO4) is demonstrated by fluorescence decay times. By adjusting the ratio of Eu3+ and Tb3+, we can tune the emission colour of K2Ln(PO4)(WO4):Tb3+,Eu3+ from green to yellow, orange and pure red. For K2Tb0.5Eu0.5(PO4)(WO4), the internal quantum efficiency is as high as 76.45% under an excitation of 394 nm, and the emission intensity at 150 °C is 92.2% of that at 25 °C.