Inkjet-printed structural color patterns have attracted great attention in recent years because of their broadly promising applications. However, the patterns are usually fabricated on pretreated plastic substrates. Herein a convenient inkjet printing method was developed to fabricate large-scale computer-designed structural color patterns on photo paper without any treatment using inks containing monodisperse CdS spheres. By this strategy, not only were the single-color and multicolor structural color patterns on paper successfully obtained, but also invisible photonic anticounterfeiting was achieved without any external stimuli. The key point of this anticounterfeiting technique is printing patterns and the background with inks containing uniformed CdS spheres with different diameters but similar intrinsic colors, so that the invisible patterns can be observed clearly by simply changing the viewing angle. The invisible and visible can be realized without the change of intrinsic structure, and the patterns are all solids. The patterns will have long lifetime and good durability, which is beneficial for their practical usage.

Patterned photonic crystals (PCs) have attracted considerable attention due to their great potential in practical applications. Direct writing is an important and convenient method to fabricate patterned PCs. However, due to the limited interaction among spheres and the evaporation of ink, the obtained patterns usually suffer from poor structure strength, and non-uniform and unstable structural colors. In this work, an in situ embedding and locking strategy for fabricating tough PC patterns in one step was demonstrated. With properly dried polymer films as “paper” and dispersions of CdS spheres as “inks” to write on the “paper”, the self-assembly of CdS spheres and locking of the PC structure can be achieved simultaneously, which gives rise to tough composite patterned PCs with uniform, stable and permanent structural colors. Based on this simple method, tough PC patterns can be easily and quickly created by direct hand writing or drawing without special treatment, equipment, masks or templates. The vivid structural colors of the tough PC patterns and this simple method show great potential for practical applications.

Structural colored materials have attracted increasing attention due to their vivid color effects and non-photobleaching characteristics. However, the angle dependence of these structural colors severely restricts their practical applications, for example, in display and sensing devices. Here, a new strategy for obtaining low angle dependent structural colors is demonstrated by fabricating long-range ordered photonic crystal films. By using spheres with high refractive indices as building blocks, the angle dependence of the obtained colors has been strongly suppressed. Green, golden yellow and red structural colored films with low angle dependence were obtained by using 145 nm, 165 nm and 187 nm Cu2O spheres as building blocks, respectively. SEM images confirmed the long-range highly ordered arrays of the Cu2O photonic crystal films. Reflectance spectra and digital photographs clearly demonstrate the low angle dependence of these structural colors, which is in sharp comparison with the case of polystyrene (PS) and SiO2 photonic crystal films. Furthermore, these structural colors are vivid with high color saturation, not only under black background, but also under white background and natural light without adding any light-absorbing agents. These low angle dependent structural colors endow Cu2O photonic crystal films with great potential in practical applications. Our findings may broaden the strategies for the design and fabrication of angle independent structural colored materials.

Morphology control of upconversion nanoparticles (UCNPs) is of fundamental and technological importance to manipulate their shape/size-dependent properties for special applications. Here, we developed a method to control the morphology of the NaGdF4:Yb3+,Tm3+@NaGdF4 core–shell nanostructure by tailoring the ratio of precursors in the core and shell forming process. Through a core–shell approach, “flower-like” and “sea chestnut-like” hierarchical nanostructured UCNPs have been synthesized via a thermal decomposition method. The molar ratio of core to shell is a key factor in determining the morphology of the NaGdF4:Yb3+,Tm3+@NaGdF4 core–shell nanostructure. TEM images show that the UCNPs range from 15 nm to 100 nm with sphere, hexagonal plane, and tetrahedron morphologies and flower-like aggregates were prepared by varying the precursor amount of the core while keeping the precursor amount of the shell constant. To further understand the factors affecting the morphology of the core–shell structure, the effect of the size of the core and the heating rate on the morphology of NaGdF4:Yb3+,Tm3+@NaGdF4 were also evaluated. The results indicate that when the size of the core was about 6 nm, the atomic ratio of core/shell was 0.25, and the heating rate was 10 °C min−1, “sea chestnut-like” NaGdF4:Yb3+,Tm3+@NaGdF4 UCNP aggregates were formed. The selected area electron diffraction (SAED) pattern showed a ring-like pattern, indicating a polycrystal and multi-layer structure. This morphology control strategy may provide fundamental guidance for preparing hierarchical core–shell structured nanocrystals.

Doping, incorporating appropriate and functional atoms or ions into host lattices of colloid nanoparticles, is an important approach to modifying the shapes and sizes of nanoparticles and endowing them with new and useful properties. Herein, we demonstrate a “core–shell” synthetic strategy to control the shape of core–shell structured NaGdF4 upconversion nanoparticles (UCNPs) by introducing an impurity dopant in the core region. We introduced Cr3+ ions in the core region of NaGdF4 to synthesize “Jasminum sambac flower” shaped non-classic core–shell NaGdF4:Cr3+@NaGdF4:Yb3+,Tm3+ UCNPs. First of all, the effect of doping concentration on the shape of NaGdF4 core was investigated. Then, the core–shell strategy was applied to obtain a shape-controlled hierarchical nanostructure, and the effects of Cr3+ concentration on the phases, shapes and sizes of the products were investigated in detail. TEM images indicate that NaGdF4:Cr3+@NaGdF4:Yb3+,Tm3+ nanocrystals with Cr3+ concentration of 40–50% show highly uniform “Jasminum sambac flower” shaped structures. TEM images and XRD patterns of NaGdF4:Cr3+@NaGdF4:Yb3+,Tm3+ at different reaction times indicate that, initially, the flower shaped products are cubic phase and the size of the flowers is about 50 nm; with an increase of the reaction time, the cubic phase nanocrystals discompose to small ones and then transform to a bigger hexagonal phase “Jasminum sambac flower”. This “core/shell” doping strategy presents a general route to achieving controlled morphology in lanthanide-doped nanocrystal systems.

Photonic crystals (PCs) have long been considered effective for tuning upconversion luminescence due to their photonic band gap (PBG) and the redistribution of density of optical states (DOS). Although the emission intensity can be changed obviously by the PC effect, rarely an obvious lifetime change consistent with theory is observed due to the low refractive index of PS or SiO2 spheres in the commonly used PCs. Herein, CdS/NaYF4:Yb3+,Er3+ composite PCs with a high refractive index contrast are fabricated in one step with upconversion nanoparticles filled inside CdS PCs. When the upconversion emission peak lies at the edge of the PBGs of the composite PCs, a dramatic decrease in lifetime by 28% and 41% is observed for the green and red emissions, respectively. At the same time, obvious emission intensity enhancements are also observed. In contrast, PS PCs with a low refractive index contrast show a slight effect on the lifetime of upconversion luminescence with their emission peak at the edge of the PBGs. Our results agree well with theory and prove that a sufficiently large refractive index contrast is necessary for PCs to dramatically tune the luminescence lifetime and intensity simultaneously.

Monodisperse semiconductor colloidal spheres with a high refractive index hold great potential for building photonic crystals with a strong band gap, but the difficulty in separating the nucleation and growth processes makes it challenging to prepare highly uniform semiconductor colloidal spheres. Herein, real monodisperse Cu2O spheres were prepared via a hot-injection & heating-up two-step method using diethylene glycol as a milder reducing agent. The diameter of the as prepared Cu2O spheres can be tuned from 90 nm to 190 nm precisely. The SEM images reveal that the obtained Cu2O spheres have a narrow size distribution, which permits their self-assembly to form photonic crystals. The effects of precursor concentration and heating rates on the size and morphology of the Cu2O spheres were investigated in detail. The results indicate that the key points of the method include the burst nucleation to form seeds at a high temperature followed by rapid cooling to prevent agglomeration, and appropriate precursor concentration as well as a moderate growth rate during the further growth process. Importantly, photonic crystal films exhibiting a brilliant structural color were fabricated with the obtained monodisperse Cu2O spheres as building blocks, proving the possibility of making photonic crystals with a strong band gap. The developed method was also successfully applied to prepare monodisperse CdS spheres with diameters in the range from 110 nm to 210 nm.

The lifetimes of the excited electrons (photoluminescence lifetime) of quantum dots (QDs) have an important effect on the electron transfer efficiency between QDs and other substances and thus determine their application. Here, a new strategy to prolong the photoluminescence (PL) lifetime of quantum dots (QDs) by using π-conjugated ligands together with thioglycolic acid (TGA) as the QD's ligand shell is reported. 4-Mercaptobenzoic acid (4-MBA), 4-methylbenzeneththiol (4-MBT) or 2-mercaptobenzothiazole (2-MBTH) was selected as π-conjugated ligand together with thioglycolic acid (TGA) to synthesize aqueous CdTe quantum dots (QDs). The decay curves of TGA–CdTe, 4-MBA–TGA–CdTe, 4-MBT–TGA–CdTe and 2-MBTH–TGA–CdTe (λem = 550 nm) were recorded and their average PL lifetimes were calculated. The HOMO and LUMO energy levels of 4-MBA, 4-MBT and 2-MBTH were calculated with Gaussian 09W. By comparing lifetimes of QDs capping by different ligands relating to the HOMO and LUMO energy levels of ligands, we presume that in the QDs/π-conjugated ligand hybrid, an electron delocalized field is formed by mixing the LUMO energy levels of π-conjugated ligand with conductive band-edge energy (Ecb) of CdTe QDs, which will supply the excited electron with a more stable environment. As a result, the PL lifetime of CdTe QDs is prolonged greatly (from 49 ns to 80 ns) when a π-conjugated ligand with appropriate LUMO energy level was used as ligand shell.

In this work, the ligand dynamic effect was utilized to synthesize hexagonal NaYF4 (β-NaYF4) crystals with controllable morphology by a hydrothermal method. A series of fatty acids was chosen as organic ligands to investigate the ligand dynamic effect on the crystal morphology. Confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray power diffraction (XRD), the dynamic nature of ligands has strong influences on the crystal morphology, crystal growth and phase transition processes. Moreover, possible crystal growth processes with different chain length ligands have been proposed.

Due to the strong tissue absorption of short-wave-length light below 600 nm, in vivo imaging based on UCNPs as luminescent probes is still limited. The NIR spectral range (700–1100 nm) and the red region (600–700 nm) are referred to as the “optical window” of the biological tissues because the light scattering, absorbance and autofluorescence of tissue are minimum in this range. Here we synthesized NaYF4:Yb3+,Tm3+/CdS nanoheterostructures. In the nanoheterostructures, the red (700 nm) and NIR (802 nm) emissions of NaYF4:Yb,Tm were enhanced by 7.3 times, while the blue emission peak at 476 nm nearly disappeared and no peak of CdS appeared. Transmission electron microscopy (TEM) images show that the morphology of the binary nanoparticles is predominantly peanut-like and the X-ray diffraction (XRD) patterns of the NaYF4:Yb3+,Tm3+/CdS nanoheterostructure were assignable to both wurtzite phase CdS and hexagonal phase NaYF4:Yb3+,Tm3+. In comparison, introducing Cd2+ ions or S2− ions or preparation of physical mixture of NaYF4:Yb3+,Tm3+ and CdS did not enhance the red (700 nm) and NIR (802 nm) emissions of NaYF4:Yb, Tm. These results indicated that the interactions in the NaYF4:Yb3+,Tm3+/CdS nanoheterostructure played a key role for enhancing the NIR emission of NaYF4:Yb3+,Tm3+ nanocrystals. QDs act as a sub energy level to mediate energy transfer of Tm3+. To evaluate the effect of other QDs on the upconverted fluorescence spectrum of NaYF4:Yb3+,Tm3+, NaYF4:Yb3+,Tm3+/CdSe nanoheterostructures were also prepared by a similar method. The upconverted fluorescence spectra indicated that by conjugation with CdSe, the NIR emission at 802 nm of NaYF4:Yb3+,Tm3+ was also enhanced greatly.

Oleic acid together with sodium hydroxide (NaOH) was widely used to control the size, phase and morphology of NaYF4 UCNCs. However, reports are scarce about the modulation of the luminescence intensity and color output of NaYF4 UCNCs by using different amounts of NaOH. Here, we synthesized NaYF4:Yb3+, Er3+ nanocrystals by a hydro/solvent thermal method using oleic acid as ligand with different amounts of NaOH and systematically investigated their upconverted fluorescence spectra. The results indicated that the emission intensity of NaYF4:Yb3+,Er3+, especially the green emission at 540 nm was dramatically increased as NaOH decreased. The enhancement factors of 540 nm emissions were 48, 42 and 32 for samples prepared with 0 g, 0.2 g and 0.4 g NaOH (the corresponding pH of the reaction solution were 5.9, 6.9 and 7.4), respectively, compared with that of the product prepared with the usually used 0.7 g NaOH (the corresponding pH was 8.3). Importantly, the ratio of green to red (RGR) increased from 0.4 to 5.9 when the pH decreased from 8.3 to 5.9. That is to say, the color output was tuned from orange yellow to nearly pure green. This resulted from the high energy vibration of OH−, which led to nonradiative relaxation from green emission levels (2H11/2 and 4S3/2) to the red emission level (4F9/2) and nonradiative relaxation from the red emission level (4F9/2) to the no emission level (4I11/2) of Er3+.

•The emission of a given UCNP was greatly enhanced by using short chain length ligands.•The RGR of a given UCNP can be modulated by tuning chain length of ligands.•Ligand with longer chain length caused greater nonradiative decay of UCNP.Here, hydro/solvent thermal method was applied to synthesize lanthanide doped NaYF4 upconversion nanoparticles (UCNPs). In order to overcome the quenching effect of (CH2)n due to their high energy stretching vibration, a series of new chelating ligands such as, butanoic acid, hexanoic acid, decanoic acid, and tetradecanoic acid were selected for hydrothermal synthesis of NaYF4:Yb, Er nanocrystals. For comparison, commonly used oleic acid was also used as ligand for hydrothermal synthesis of UCNPs. The effect of the carbon chain length of chelating ligands on the NaYF4 nanocrystals’ emission intensity was systematically investigated. It was found that relative short length of the carbon chain in chelating ligands resulted in enhancement of luminescent intensity and modulation of the ratios of green to red (RGR) of NaYF4:Yb, Er nanocrystals. In contrast to the product prepared with oleic acid as ligand, the enhancement factors of 540 nm emissions were 25, 21, 6.5, and 3 for samples prepared with butanoic acid, hexanoic acid, decanoic acid, and tetradecanoic acid as ligands, respectively. The FT-IR spectra of NaYF4:Yb, Er UCNPs prepared with different ligands indicated that ligands with relatively short chain length indeed have weak IR absorption in 2800–2950 cm−1, which resulted in weak quenching effect of Er3+. We also synthesized NaYF4:Yb, Tm UCNPs with these ligands. The results showed that chain length of ligands has similar effect on emission intensity of NaYF4:Yb, Tm. It is expected that this method can also be applied to enhance the luminescence intensity of other luminescent materials.

A simple and economical one-pot method to synthesize high-quality water soluble CdTe QDs is reported. An 83% quantum yield (QY) was reached using TGA as the ligand in aqueous solution without protection of nitrogen gas when using K2TeO3 as a stable Te source and NaBH4 as reducing agent. The size of the CdTe NCs could be tuned by the duration of reflux and easily monitored by absorption and PL spectra. At the optimum conditions (pH = 10.5, the concentration of Cd2+ was 1.0 mmol l−1 and the ratio of Cd2+:Te2−:TGA:NaBH4 was 1:0.2:1:10), the maximal QY reached 83%, which was much higher than that of samples prepared by the traditional refluxing aqueous solution method. The X-ray diffraction (XRD) patterns indicated that CdS was formed in the preparation process of CdTe NCs. This CdS shell could effectively passivate the surface trap states, and enhance the PL QY and stability of the CdTe QDs. The temporal evolution of the absorption spectrum of thioglycolic acid indicated that TGA was decomposed and released S2− during the preparation of CdTe QDs. The S2− reacted with Cd2+ to form CdS, this is consistent with the XRD results. 3-Mercaptopropionic acid (MPA) was also used as a ligand in this system and the results indicate that high quality multi colour CdTe NCs could also be obtained using MPA as the ligand. By this simple method, there is no need of N2 protection, buffering solution, special ligands, large quantity of N2H4 or particular treatment (such as microwave, or ultrasonication).

Here we elucidate the “cooperative effect” of carboxylic acid/amine on the size, shape, and multicolor output of fluoride upconversion nanoparticles (UCNPs) utilizing the amidation reaction. During the synthesis of NaYF4 UCNPs using oleic acid (OA) and octadecylamine (OM) as ligands, the evolution of the reaction solution was monitored by FT-IR and proton nuclear magnetic resonance (1H-NMR) spectroscopy. It is shown that N-octadecyloleamide (OOA) was formed and tightly bonded to the surface of the fluoride UCNPs. FT-IR and 1H-NMR analyses point towards a two-step OOA formation mechanism where OA initially reacted with OM to form an acid–base complex (C17H33CO2− +NH3C18H37) in reaction mixture, which further transformed into OOA before the nucleation of fluoride UCNPs at elevated temperature. Importantly, we find that the interaction of OOA with the surfaces of the fluoride UCNPs was stronger than that of OM, and that OOA could strongly quench the green-emitting levels of excited Er3+ ions. A series of control experiments and analyses demonstrate that OOA was not only responsible for the control of the size and shape of NaYF4: 20% Yb, 2% Er UCNPs, but also for the multicolor tuning of them. Those observations reveal that the “cooperative effect” between carboxylic acid and amine results from the amidation reaction and its product. This paper provides us with some new deep insights into the “cooperative effect” of carboxylic acid and amine during the synthesis of inorganic nanoparticles under thermolysis. Furthermore, other lanthanide-ion doped fluoride UCNPs with controlled size, shape and multicolor output can also be obtained in the same way.

In this paper, we report a general and facile approach for subtly tuning the multicolor output of lanthanide-ion doped NaYF4 upconversion nanoparticles (UCNPs) within a given composition under single wavelength excitation of 980 nm. By adjusting the molar ratio of octadecylamine (OM) and oleamide in reaction solution under a thermolysis procedure, the color output of NaYF4:20%Yb,2%Er UCNPs can be fine tuned. X-Ray diffraction (XRD), transmission electron microscopy (TEM), upconversion photoluminescence spectroscopy, FT-IR and 1H-NMR were employed to characterize the obtained samples. 1H-NMR analysis and a series of control experiments have revealed that the tunability of multicolor output mainly resulted from different ratios of two coordinating ligands bonded onto the particle surface. We also synthesized NaYF4:20%Yb,2%Er UCNPs with multicolor output in three pairs of mixtures, namely OM and oleic acid (OA), OM and N-octadecyloleamide (OOA), as well as OA and OOA, based on the above method. Moreover, this approach was further extended to the preparation of multicolored NaYF4:20%Yb,2%Ho and NaYF4:20%Yb,0.5%Tm UCNPs. It is expected these multicolored UCNPs have great potential for applications in biology, displays and other optical technologies.

A simple and economical one-pot method to synthesize high-quality water soluble CdTe QDs is reported. An 83% quantum yield (QY) was reached using TGA as the ligand in aqueous solution without protection of nitrogen gas when using K2TeO3 as a stable Te source and NaBH4 as reducing agent. The size of the CdTe NCs could be tuned by the duration of reflux and easily monitored by absorption and PL spectra. At the optimum conditions (pH = 10.5, the concentration of Cd2+ was 1.0 mmol l−1 and the ratio of Cd2+:Te2−:TGA:NaBH4 was 1:0.2:1:10), the maximal QY reached 83%, which was much higher than that of samples prepared by the traditional refluxing aqueous solution method. The X-ray diffraction (XRD) patterns indicated that CdS was formed in the preparation process of CdTe NCs. This CdS shell could effectively passivate the surface trap states, and enhance the PL QY and stability of the CdTe QDs. The temporal evolution of the absorption spectrum of thioglycolic acid indicated that TGA was decomposed and released S2− during the preparation of CdTe QDs. The S2− reacted with Cd2+ to form CdS, this is consistent with the XRD results. 3-Mercaptopropionic acid (MPA) was also used as a ligand in this system and the results indicate that high quality multi colour CdTe NCs could also be obtained using MPA as the ligand. By this simple method, there is no need of N2 protection, buffering solution, special ligands, large quantity of N2H4 or particular treatment (such as microwave, or ultrasonication).

The lifetimes of the excited electrons (photoluminescence lifetime) of quantum dots (QDs) have an important effect on the electron transfer efficiency between QDs and other substances and thus determine their application. Here, a new strategy to prolong the photoluminescence (PL) lifetime of quantum dots (QDs) by using π-conjugated ligands together with thioglycolic acid (TGA) as the QD's ligand shell is reported. 4-Mercaptobenzoic acid (4-MBA), 4-methylbenzeneththiol (4-MBT) or 2-mercaptobenzothiazole (2-MBTH) was selected as π-conjugated ligand together with thioglycolic acid (TGA) to synthesize aqueous CdTe quantum dots (QDs). The decay curves of TGA–CdTe, 4-MBA–TGA–CdTe, 4-MBT–TGA–CdTe and 2-MBTH–TGA–CdTe (λem = 550 nm) were recorded and their average PL lifetimes were calculated. The HOMO and LUMO energy levels of 4-MBA, 4-MBT and 2-MBTH were calculated with Gaussian 09W. By comparing lifetimes of QDs capping by different ligands relating to the HOMO and LUMO energy levels of ligands, we presume that in the QDs/π-conjugated ligand hybrid, an electron delocalized field is formed by mixing the LUMO energy levels of π-conjugated ligand with conductive band-edge energy (Ecb) of CdTe QDs, which will supply the excited electron with a more stable environment. As a result, the PL lifetime of CdTe QDs is prolonged greatly (from 49 ns to 80 ns) when a π-conjugated ligand with appropriate LUMO energy level was used as ligand shell.

Due to the strong tissue absorption of short-wave-length light below 600 nm, in vivo imaging based on UCNPs as luminescent probes is still limited. The NIR spectral range (700–1100 nm) and the red region (600–700 nm) are referred to as the “optical window” of the biological tissues because the light scattering, absorbance and autofluorescence of tissue are minimum in this range. Here we synthesized NaYF4:Yb3+,Tm3+/CdS nanoheterostructures. In the nanoheterostructures, the red (700 nm) and NIR (802 nm) emissions of NaYF4:Yb,Tm were enhanced by 7.3 times, while the blue emission peak at 476 nm nearly disappeared and no peak of CdS appeared. Transmission electron microscopy (TEM) images show that the morphology of the binary nanoparticles is predominantly peanut-like and the X-ray diffraction (XRD) patterns of the NaYF4:Yb3+,Tm3+/CdS nanoheterostructure were assignable to both wurtzite phase CdS and hexagonal phase NaYF4:Yb3+,Tm3+. In comparison, introducing Cd2+ ions or S2− ions or preparation of physical mixture of NaYF4:Yb3+,Tm3+ and CdS did not enhance the red (700 nm) and NIR (802 nm) emissions of NaYF4:Yb, Tm. These results indicated that the interactions in the NaYF4:Yb3+,Tm3+/CdS nanoheterostructure played a key role for enhancing the NIR emission of NaYF4:Yb3+,Tm3+ nanocrystals. QDs act as a sub energy level to mediate energy transfer of Tm3+. To evaluate the effect of other QDs on the upconverted fluorescence spectrum of NaYF4:Yb3+,Tm3+, NaYF4:Yb3+,Tm3+/CdSe nanoheterostructures were also prepared by a similar method. The upconverted fluorescence spectra indicated that by conjugation with CdSe, the NIR emission at 802 nm of NaYF4:Yb3+,Tm3+ was also enhanced greatly.

Here we elucidate the “cooperative effect” of carboxylic acid/amine on the size, shape, and multicolor output of fluoride upconversion nanoparticles (UCNPs) utilizing the amidation reaction. During the synthesis of NaYF4 UCNPs using oleic acid (OA) and octadecylamine (OM) as ligands, the evolution of the reaction solution was monitored by FT-IR and proton nuclear magnetic resonance (1H-NMR) spectroscopy. It is shown that N-octadecyloleamide (OOA) was formed and tightly bonded to the surface of the fluoride UCNPs. FT-IR and 1H-NMR analyses point towards a two-step OOA formation mechanism where OA initially reacted with OM to form an acid–base complex (C17H33CO2− +NH3C18H37) in reaction mixture, which further transformed into OOA before the nucleation of fluoride UCNPs at elevated temperature. Importantly, we find that the interaction of OOA with the surfaces of the fluoride UCNPs was stronger than that of OM, and that OOA could strongly quench the green-emitting levels of excited Er3+ ions. A series of control experiments and analyses demonstrate that OOA was not only responsible for the control of the size and shape of NaYF4: 20% Yb, 2% Er UCNPs, but also for the multicolor tuning of them. Those observations reveal that the “cooperative effect” between carboxylic acid and amine results from the amidation reaction and its product. This paper provides us with some new deep insights into the “cooperative effect” of carboxylic acid and amine during the synthesis of inorganic nanoparticles under thermolysis. Furthermore, other lanthanide-ion doped fluoride UCNPs with controlled size, shape and multicolor output can also be obtained in the same way.

In this paper, we report a general and facile approach for subtly tuning the multicolor output of lanthanide-ion doped NaYF4 upconversion nanoparticles (UCNPs) within a given composition under single wavelength excitation of 980 nm. By adjusting the molar ratio of octadecylamine (OM) and oleamide in reaction solution under a thermolysis procedure, the color output of NaYF4:20%Yb,2%Er UCNPs can be fine tuned. X-Ray diffraction (XRD), transmission electron microscopy (TEM), upconversion photoluminescence spectroscopy, FT-IR and 1H-NMR were employed to characterize the obtained samples. 1H-NMR analysis and a series of control experiments have revealed that the tunability of multicolor output mainly resulted from different ratios of two coordinating ligands bonded onto the particle surface. We also synthesized NaYF4:20%Yb,2%Er UCNPs with multicolor output in three pairs of mixtures, namely OM and oleic acid (OA), OM and N-octadecyloleamide (OOA), as well as OA and OOA, based on the above method. Moreover, this approach was further extended to the preparation of multicolored NaYF4:20%Yb,2%Ho and NaYF4:20%Yb,0.5%Tm UCNPs. It is expected these multicolored UCNPs have great potential for applications in biology, displays and other optical technologies.