Patent Application: US-91288406-A

Abstract:
a method of preparing a lanthanide - doped nanoparticle sol - gel matrix film having a high signal to noise ratio is provided . the sol - gels are also provided . a method of preparing light emitting sol - gel films made with lanthanide doped nanoparticles , for the production of white light is also provided . the method comprises selecting lanthanides for the production of at least one of green , red and blue light when excited with near infrared light , preparing nanoparticles comprising the selected lanthanides , stabilizing the nanoparticles with ligands operative to stabilize the nanoparticles in an aqueous solution and selected to be substantially removed from the sol - gel matrix film during synthesis , incorporating the stabilized nanoparticles into a sol - gel matrix and heating to increase the signal to noise ratio of the luminescence by substantially removing the low molecular weight organic molecules . additionally , light emitting sol - gel films made with lanthanide doped nanoparticles are provided .

Description:
precursor nanoparticle : a nanoparticle that is used for making doped nanoparticle sol - gel films . the resulting doped nanoparticle sol - gel may or may not be comprised of the precursor nanoparticle . product nanoparticle : a doped nanoparticle sol - gel comprises product nanoparticle . the product nanoparticle may or may not comprise precursor nanoparticle . the product nanoparticle can be a core - shell nanoparticle or it may only comprise the core . temperature ranges : the temperature at which the sol - gel films are made range from approximately about 400 - 1200 c , preferably approximately about 600 - 1200 c , and more preferably approximately about 800 c . silica films with ln 3 + - doped laf 3 nanoparticles were prepared by the sol - gel method and their luminescent properties were studied as a function of temperature . significant improvements in the luminescent properties , in terms of the lifetime for the 4 i 13 / 2 level of er 3 + (˜ 10 . 9 ms ), the 4 f 3 / 2 level of nd 3 + (˜ 171 μs ) and the 5 f 3 level of ho 3 + (˜ 6 μs ) were obtained when corresponding nanoparticles were incorporated in silica films rather than the bare ions . life time values could be further improved by incorporating core - shell particles ( the doped laf 3 core is surrounded by an undoped shell of laf 3 ) in the silica matrix , as a result of further reduction of the non - radiative pathways . laf 3 : er ( 5 %) nanoparticles stabilized with citrate ligands were prepared and incorporated in silica films made by the sol - gel method . the luminescent aspects of these films were studied as a function of the annealing temperature from 400 to 800 ° c . the results were compared with that of silica films doped directly with er 3 + ions having the same er / si ratio as that of nanoparticle incorporated films . the procedure was extended to other lanthanide ions like nd 3 + and ho 3 + and also to another sol - gel matrix ( al 2 o 3 ), showing the generality of the method . laf 3 : er , laf 3 : eu , laf 3 : nd and laf 3 : ho nanoparticles , ( all doped at 5 atom % with respect to the total amount of lanthanide ions ), stabilized with citrate ligand were prepared by the co - precipitation technique in aqueous solution in presence of citrate ions . around 2 g of citric acid and 0 . 126 g naf was dissolved in 40 ml of water . the ph of the solution was adjusted to 6 by adding nh 4 oh and the solution was heated to 75 ° c . stoichiometric amounts of the nitrate salts of lanthanide ions were dissolved in 2 ml water ( for er 34 and eu 3 + ions ) or 2 ml of methanol ( for nd 3 + and ho 3 + ions ), and added drop wise . a clear solution was obtained and after two hours of reaction , the resulting solution was mixed with 150 ml of ethanol to precipitate the nanoparticles . these particles were collected by centrifugation , washed with ethanol , and dried under vacuum . formation of citrate - stabilized nanoparticles was confirmed from 1 h nmr and afm studies ( fig1 and 2 ). for the preparation of core - shell nanoparticles having a doped core covered by an undoped shell , the procedure was slightly modified . approximately 3 g of citric acid was dissolved in 35 ml of water and neutralized with nh 4 oh till the ph reaches around 6 and this solution was then heated to 75 ° c . la ( no 3 ) 3 . 6h 2 o and nd ( no 3 ) 3 . 6h 2 o or ho ( no 3 ) 3 . 5h 2 o ( 1 . 33 mmol total ) were dissolved in 3 ml of methanol and added to this followed by the dropwise addition of 3 ml water containing 0 . 266 g naf . after 10 minutes , 3 ml of a methanolic solution containing 0 . 6 g of la ( no 3 ) 3 . 6h 2 o was added drop - wise to the reaction mixture while stirring , for the formation of shell around the core particles . the reaction was allowed to continue for two hours and finally the nanoparticles were precipitated by the addition of excess of ethanol to the reaction mixture . formation of particles having a core - shell geometry by this procedure was confirmed from the luminescent studies of citrate stabilized laf 3 : eu — laf 3 core - shell nanoparticles , prepared by the same procedure . the details of the luminescent properties of core shell particles have been reported elsewhere ( j . w . stouwdam and f . c . j . m . van veggel , langmuir 20 , 11763 ( 2004 )). approximately 50 - 60 mg of these nanoparticles was dissolved in 1 . 5 ml water , which was then mixed with 3 ml of tetraethoxysilane ( teos ) and 7 . 8 ml of ethanol . the ph of the solution was adjusted to 2 by adding few drops of 0 . 1 n hcl and the solution was stirred for 24 hours to get a clear sol . the sol was then spin coated on a quartz substrate at 2500 rpm and heated at different temperatures under ambient environment . the films were transparent to visible light and no cracks were observed . emission spectra and decay curves from the samples were measured using a pulsed nd — yag ( nd — yag stands for nd 3 + doped yttrium aluminium garnet ) laser source attached with an optical parametric oscillator ( opo ). the pulse duration was 5 ns with a repetition frequency of 10 hz . emission spectrum of laf 3 : er nanoparticles dispersed in d 2 o obtained after exciting the sample at 488 nm , was characterized by a broad peak around 1530 nm ( full width at half maximum ( fwhm )= 69 nm ), corresponding to the 4 i 13 / 2 → 4 i 15 / 2 transition . the decay curve corresponding to the 4 i 13 / 2 level in the sample was fitted bi - exponentially with decay times 200 μs ( 82 %) and 58 μs ( 18 %) respectively ( fig3 ). fig4 shows the emission spectra and decay curves for the laf 3 : er and er 3 + incorporated silica films with er / si ratio ˜ 1 . 0 × 10 − 3 and heated in air at 400 , 600 , and 800 ° c . for 12 hours . there was significantly improved signal to noise ratio in the emission spectrum for the particles incorporated films heated at all the temperatures . furthermore , the full width at half maximum ( fwhm ) for particle - incorporated films were almost comparable for all the heat treatment temperatures . however , for silica films directly incorporated with er 3 + ions , the signal to noise ratio was poor , particularly for low temperature heat - treated films . the line width drastically decreased with increased heat treatment temperatures . the lifetime values corresponding to the 4 i 13 / 2 level of er 3 + from the 800 ° c . heated samples are shown in table 1 . ( corresponding values for the low temperature heated films are shown in table 2 ). for silica films incorporated with laf 3 : er nanoparticles , life time values were much higher at all the heat treatment temperatures compared to the directly er 3 + incorporated silica films , as can be seen from fig4 and table 1 . for nanoparticle incorporated films heated at 800 ° c ., the 1 i 13 / 2 life time was found to be 10 . 9 ms . in the case of silica film incorporated with bare er 3 + ions and heated at 800 ° c ., there was a fast decay component followed by a slow decay component . the observed fast decay component for the 800 ° c . heated film was attributed to the aggregation of er 3 + ions in the silica matrix . however , for silica films incorporated with laf 3 : er nanoparticles , no fast decay component was observed particularly for the ones heated at 600 and 800 ° c . thus , the particle - incorporated silica films offer a clear advantage in terms of the improved lifetime and absence of clustering of lanthanide ions when compared with silica films directly incorporated with the bare er 3 + ion . similar experiments were carried out for nd 3 + - and ho 3 + - incorporated samples . the citrate - stabilized nanoparticles of laf 3 : nd and laf 3 : ho were incorporated in a silica matrix by the same procedure employed for the laf 3 : er nanoparticles . fig5 shows the emission spectra and corresponding decay curves for silica films incorporated with laf 3 : nd nanoparticle and nd 3 + ion , respectively , with a nd / si ratio 0 . 9 × 10 − 3 and heated at 800 ° c . for 12 h . for silica films doped with laf 3 : nd nanoparticles , decay corresponding to 4 f 3 / 2 level was multi - exponential with a major component of ˜ 171 μs ( 72 %) and a faster component of 56 μs ( 28 %). for nd 3 + ions directly doped in silica films with the same nd / si ratio , the corresponding decay curve was characterized by a fast decay component (− 2 . 0 μs , 48 %) as can be seen from the inset of fig5 ( bottom right ), and a slow decay component ( 130 μs , 52 %). the fast component is attributed to the clusters of nd 3 + ions formed in the silica matrix . 35 a comparison of the life time values shown in table 1 and the decay curves shown in fig5 clearly reveals that there is an improvement of the luminescent properties , in terms of improved life time and absence of lanthanide ion clustering , when the nanoparticles are incorporated in the silica films rather than the bare ions . for silica films incorporated with laf 3 : ho nanoparticles with a ho / si ratio around 1 . 5 × 10 − 3 and heated at 800 ° c ., luminescence was observed both in the visible and near - infrared region . the emission spectrum in the nir region along with the decay curve corresponding to the 5 f 3 level of ho 3 + from this sample are shown in fig6 ( left ). the lifetime value of 5 f 3 level was found to be 6 μs ( 75 %) and 12 μs ( 25 %), with no faster decay component , indicating the absence of ho 3 + clustering in the sample . in contrast to this , when ho 3 + ions are directly doped in silica films with the same ho / si ratio , no emission was observed in the visible and near - infrared region . laf 3 : eu nanoparticle stabilized with citrate ions were prepared , incorporated in silica matrix and subjected to heat treatments at different temperatures . fig6 ( right ) show the emission spectra of the laf 3 : eu incorporated silica film heated at 800 ° c . in air . the intensity of the 5 d 0 → 7 f 2 emission peak (˜ 615 nm ) for this sample was found to be significantly larger than that of the 5 d 0 → 7 f 1 emission peak ( 591 nm ). which is characteristic of eu 3 + surrounded by oxygen ions . as both 5 d 0 and 7 f 0 levels are non - degenerate , the transition between the levels can be used as a probe to understand the environment around the eu 3 + ions in the lattice . the high resolution emission spectrum corresponding to the 5 d 0 → 7 f 0 transition for laf 3 : eu nanoparticles incorporated silica films ( shown as inset of fig6 c ) clearly shows an asymmetric peak which could be deconvoluted into two gaussians centered around 576 . 9 and 578 . 2 nm , respectively , indicating that more than one type of eu 3 + is present in the films . for laf 3 : eu nanoparticles relatively sharper and a more symmetric peak around 578 nm was observed corresponding to the 5 d 0 - 7 f 0 transition ( inset of fig6 d ). comparing the spectra in fig6 ( c and d ), it is clear that eu 3 + is existing in more than one crystallographic phase in laf 3 : eu incorporated silica films . x - ray diffraction studies carried out on a sample of silica film incorporated with 25 wt % of laf 3 : eu nanoparticles and heated at 800 ° c ., revealed the presence of a non - stoichiometric lanthanum silicate phase , ( la 9 . 31 si 6 . 24 o 26 ), along with the laf 3 phase as can be seen from fig7 , roughly in a 1 : 1 ratio . the eu 3 + thus occurs in two different phases , which confirmed the luminescence data . it is likely that the surface of the laf 3 : eu nanoparticles reacted with the silanol groups of the matrix to form the eu 3 + - doped lanthanum silicate surrounding a core of unreacted laf 3 : eu . one skilled in the art would conclude that the same occurs for all laf 3 : ln ( ln — er , nd , and ho ) doped sio 2 films in this study . the life time values of the lanthanide ion containing silica films can be further improved by incorporating the core - shelf nanoparticles having a doped core covered by an undoped shell . ( core - shell nanoparticles doped with er 3 + ions in the core were found to be less soluble in water and hence good quality sol - gel films could not be obtained ). the life time values observed for 4 f 3 / 2 level of nd 3 + and 5 f 3 level of ho 3 + in laf 3 : nd — laf 3 and laf 3 : ho — laf 3 core - shell nanoparticle incorporated films are shown in table 1 . there was an improvement in the life time of the core - shell particles incorporated films compared to the core particle incorporated films . in order to further substantiate the generality of the method , the above experiments were repeated by taking al 2 o 3 as the sol - gel matrix . al 2 o 3 sols were prepared based on the procedure similar to that of ishizaka et al . 36 hydrous aluminum hydroxide was precipitated by adding aqueous 6m nh 3 solution to a 0 . 2 m al ( no 3 ) 3 . 9h 2 o solution drop wise under stirring . the precipitated hydroxide was aged for 12 h without stirring , then centrifuged and washed with water . this was then mixed with glacial acetic acid and heated at 80 ° c . for 8 h . the viscous sol obtained thus was mixed with around 3 . 5 mg of er ( no 3 ) 3 . 5h 2 o or around 30 mg laf 3 : er nanoparticles stabilized with citrate ligand and stirred for 24 . the sot was then transferred to a petri dish and dried under ambient conditions followed by heating at 800 ° c . for 8 hours . similar to sio 2 matrix , significant improvement in the life time values were observed when laf 3 : er nanoparticles were incorporated in the films compared to the bare er 3 + incorporated films ( fig8 ). in conclusion , a general method , from readily available and cheap starting materials , that combines the advantages of both nanoparticles and the sol - gel method , has been demonstrated for making silica and alumina films containing highly luminescent lanthanide ions . the improved luminescent properties of nanoparticle incorporated films have been attributed to the effective isolation of lanthanide ions from the high phonon energy matrix , residual oh groups , and absence of lanthanide ion clustering . white light was generated from a silica or zirconia thin film made with yb 0 . 75 la 0 . 2 eu 0 . 05 f 3 , la 0 . 45 yb 0 . 5 er 0 . 05 f 3 , and la 0 . 75 yb 0 . 2 tm 0 . 05 f 3 nanoparticles by exciting with a single source near infrared light ( 980 nm cw diode laser ). eu 3 + and tm 3 + ions are responsible for red and blue emission respectively . er 3 + ion is responsible for green as well as red emission . the commission internationale de l &# 39 ; eclairage ( cie ) coordinates of the resulting light were easily adjusted by controlling the concentration of lanthanide ions in the nanoparticles 27 - 31 as well as the concentration of nanoparticles ( ln 3 + doped ) in the sol - gel thin layer . more specifically , there is spatial isolation of the three pairs of precursor ln 3 + ions ( i . e . tm 3 + / yb 3 + , er 3 + / yb 3 + , and eu 3 + / yb 3 + ) that generate blue , green plus red , and red emission , respectively . silica thin film made with la 0 . 75 yb 0 . 2 ho 0 . 05 f 3 nanoparticles produced bright green light by exciting with near infrared light ( 980 nm cw diode laser ) which can be also used in the generation of white light . in the case of zro 2 as the sol - gel matrix we see la 2 zr 2 o 7 as phase , which is a low - phonon matrix . this has the advantage of leading to less quenching than would occur in high - phonon matrices . la 0 . 45 yb 0 . 5 er 0 . 05 f 3 , la 0 . 75 yb 0 . 2 tm 0 . 05 f 3 , la 0 . 75 yb 0 . 2 ho 0 . 05 f 3 , and yb 0 . 75 la 0 . 2 eu 0 . 50 f 3 nanoparticles , stabilized with citrate ligand were prepared by the co - precipitation technique in aqueous solution in presence of citrate ions . around 2 g of citric acid and 0 . 126 g naf was dissolved in 40 ml of water . the ph of the solution was adjusted to 6 by adding nh 4 oh and the solution was heated to 75 ° c . stoichiometric amounts of the nitrate salts of lanthanide ions were dissolved in 2 ml of methanol and added drop wise . a clear solution was obtained and after two hours of reaction , the resulting solution was mixed with 70 ml of ethanol to precipitate the nanoparticles . these particles were collected by centrifugation at 3000 rpm , washed with ethanol , and dried under vacuum . after drying the particles can easily be dispersed in water . formation of citrate stabilized nanoparticles was confirmed from 1 h nmr and afm studies ( van veggel c . s . chem . mater . 2005 , 17 , 4736 ). 50 mg of ln 3 + doped laf 3 nanoparticles were dissolved in 2 ml water , which was then mixed with 3 ml of tetraethoxyorthosilane ( teos ) and 7 . 8 ml of ethanol . the ph of the solution was adjusted to 2 by adding a few drops of 0 . 1 n hcl and the solution was stirred for 24 hours to get a clear sol . the sol was then spin coated on a quartz substrate at 2500 rpm and heated to 400 ° c . from 25 ° c . in 1 . 40 hr , staying at 400 ° c . for 30 min and then heated to 800 ° c . in 2 hr and staying at 800 ° c . for 12 hr under ambient environment . 1 mg of la 0 . 45 yb 0 . 5 er 0 . 05 f 3 , 100 mg of la 0 . 75 yb 0 . 2 tm 0 . 05 f 3 , and 150 mg of yb 0 . 75 la 0 . 2 eu 0 . 05 f 3 in 4 ml water were used for the material which gave white light emission . up - conversion emission spectra from the samples were measured using a 980 nm cw semiconductor laser source . fig9 shows the up - conversion emission spectra of silica films made with la 0 . 45 yb 0 . 5 er 0 . 05 f 3 , la 0 . 75 yb 0 . 2 tm 0 . 05 f 3 , and yb 0 . 75 la 0 . 2 eu 0 . 05 f 3 nanoparticles . emission peaks at red , green and blue region can be seen . the calculated colour coordinates are 0 . 37 and 0 . 32 . 35 these values fall within the white region of 1931 commission internationale de l &# 39 ; eclairage ( cie ) diagram . 37 this white light was bright and can been seen by the naked eye even at a laser pump power of only 200 mw . there is no virtually change in the colour coordinates of the white light with a change in the excitation power . the weak emissions at region 590 and 612 nm from eu 3 + ions help keep the colour coordinates from moving slightly towards green region ( 0 . 3 , 0 . 41 ). in order to show that using three different ln 3 + / yb 3 + pairs in a silica thin film does not lead to a thin film capable of emitting white light , a thin film was prepared with the same concentrations of la 3 + er 3 + , tm 3 + , and eu 3 + ions with yb 3 + ions by direct incorporation and subjected to the same heat treatment . the results show only green and red emission from er 3 + ions and no blue and red emission from tm 3 + ions and eu 3 + ions respectively ( fig1 ). the emission band around 470 nm is assigned to the 1 g 4 to 3 h 6 transition of tm 3 + ions . an emission band of tm 3 + ions at 790 nm ( 3 h 4 to 3 h 6 transition ) was also observed . er 3 + gave emission peaks around 515 , 540 nm and 645 nm which are assigned to the 2 h 11 / 2 to 4 i1 15 / 2 , 4 s 3 / 2 to 4 i1 15 / 2 , 4 f 9 / 2 to 4 i1 15 / 2 transitions , respectively . the intensity ratio of red to green emission from er 3 + ions can be tuned by changing the concentration of yb 3 + ions in the gd 2 o 3 nanoparticle ( guo , h . ; dong , n . ; yin , m . ; zhang , w . ; lou , l . ; xia , s . j . phys . chem . 2004 , 108 , 19205 ). we also found similar green to red ratio dependence by changing the yb 3 + concentration in the nanoparticle . the emission bands around 590 nm and 612 nm are assigned to the 5 d 0 to 7 f 1 and 5 d 0 to 7 f 2 transitions of eu 3 + ions , respectively . fig1 shows the up - conversion spectra of la 0 . 75 yb 0 . 2 ho 0 . 05 f 3 incorporated in silica film under 980 nm cw laser excitation . ho 3 + ions gave two emission bands at approximately 540 nm and 640 nm , which are assigned to the 5 s 2 to 5 i 8 and 5 f 5 to 5 i 8 transitions , respectively . the green luminescence intensity is very high when compared with the red emission and can easily be seen with the naked eye at laser pump power of only 200 mw . this can be also used as green and red light source of the white light combination . lifetime of 5 s 2 level is 250 μs which indirectly shows that the up - conversion process is efficient . to our knowledge , no such studies have been reported regarding up - converted green and red emission from ho 3 + ions in sol - gel derived oxide nanoparticles by exciting yb 3 + ions . ln 3 + ( ho 3 + , tm 3 + , eu 3 + ) ions with the yb 3 + ions individually incorporated in silica thin film and subjected to same heat treatment didn &# 39 ; t show any up - conversion . this clearly demonstrates the advantage of nanoparticles used in silica thin film rather than direct doping with lanthanide ions . the up - conversion luminescence can be improved by using laf 3 : ln 3 + core - shell nanoparticles ( the doped laf 3 core is surrounded by an undoped shell of laf 3 ) in the silica matrix and then used as precursor nanoparticles in the thin film formation . possible mechanisms for the up - conversion processes are , photoavalanche ( pa ), excited state absorption ( esa ), energy transfer ( et ). a schematic diagram showing the energy level of ho 3 + , tm 3 + , er 3 + , eu 3 + and yb 3 + as well as possible up - conversion mechanisms for the blue , green , and red emissions under 980 nm excitation are shown in fig1 and 13 . fig1 shows the dependence of the up - conversion emission intensity on the excitation power in different samples a ) la 0 . 75 yb 0 . 2 ho 0 . 05 f 3 , b ) la 0 . 75 yb 0 . 2 tm 0 . 05 f 3 c ) la 0 . 45 yb 0 . 5 er 0 . 05 f 3 nanoparticles individually incorporated in silica film . blue emission from tm 3 + ions is three photon process . green and red emission from er 3 + and ho 3 + ions are two photon processes . power dependence graphs ( fig1 ) show a slight decrease in the slope when the laser power is increased . this can be attributed to a ground state depletion caused by the population build - up of the yb 3 + excited level and in turn in the saturation of corresponding levels in the lanthanide ions . when the zro 2 films were made with la 0 . 45 yb 0 . 5 er 0 . 05 f 3 , la 0 . 75 yb 0 . 2 tm 0 . 05 f 3 , and yb 0 . 75 la 0 . 2 eu 0 . 05 f 3 nanoparticles white light was observed with the coordinates of 0 . 37 , 0 . 31 ( fig1 ). a zro 2 thin film prepared with the same concentrations of la 3 + , er 3 + , tm 3 + , and eu 3 + ions with yb 3 + ions by direct incorporation only showed green and red emission from er 3 + ions and no blue and red emission from tm 3 + ions and eu 3 + ions , respectively , which substantiates the importance of the role of three different nanoparticles . our recent report showed the presence of a non - stoichiometric lanthanum silicate phase ( la 9 . 31 si 6 . 24 o 26 ) along with the expected laf 3 phase from x - ray diffraction ( xrd ) studies carried out on a silica thin film sample ( van veggel c . s . chem . mater . 2005 , 17 , 4736 ). xrd studies carried out on zro 2 thin films made with nanoparticles showed the presence of lanthanum zirconate ( la 2 zr 2 o 7 ), but little or no laf 3 ( fig1 ). we conclude that the nanoparticles have reacted with oh groups present in zro 2 sol - gel to form ln 3 + doped lanthanum zirconate . in spite of the formation of lanthanum zirconate , the three pairs of ln 3 + ions are spatially isolated in the sol - gel layer made with nanoparticles . accordingly , the pairs of ln 3 + ions are still effectively in a nanoparticle that has very low phonon energy ( perhaps in the range of ˜ 300 cm − 1 ). bright white light was generated from sio 2 and zro 2 sol - gel thin film made with four different combinations of lanthanide - doped nanoparticles . a 33 - fold increase in the white light emission intensity was observed with the commission internationale de l &# 39 ; eclairage ( cie ) co - ordinates of 0 . 39 , 0 . 31 from silica thin film made with combination 2 ( yb / tm and yb / er ) nanoparticles when compared to our previous thin film of example 2 . we have estimated the efficiency of production of the resulting white light as 25 % based on the efficiency of energy transfer and quantum yield of the ln 3 f emissions . similarly , silica thin film made with combination 1 ( yb / tm , yb / ho , and yb / er ), combination 3 ( yb / tm , yb / tb , and yb / er ), and combination 4 ( yb / tm , yb / tb , and yb / eu ) nanoparticles also produced white light with higher efficiency when compared to our previous thin films of example 2 ( 26 , 11 , 2 times , respectively ). zro 2 thin films made with these new combinations of nanoparticles also showed similar increases in the efficiency of white light . the combinations of ln 3 + - doped laf 3 nanoparticles , stabilized by citrate ligands , incorporated in sol - gel thin films used to achieve the white light are given in table 3 . the films were transparent to visible light and no cracks were observed under an optical microscope . film formation characteristics have been reported by us . 38 fig1 shows the digital image of bright white light emission from silica thin film made with nanoparticles of combination 1 under 980 nm cw laser excitation . bright white light can be seen very clearly from the thin film even at a laser pump power of only 300 mw . fig1 a shows the up - conversion emission spectra of silica thin film made with nanoparticles of combination 1 . emission peaks at red , green and blue region can clearly be seen in fig1 a . the calculated cie colour coordinates of the combination 1 are 0 . 39 and 0 . 31 . these values fall within the white region of 1931 commission internationale de l &# 39 ; eclairage ( cie ) diagram . a 26 - fold increase in the efficiency of generation of white light was observed when compared to our earlier thin film of example 1 . in this new combination red emission at 640 nm from ho 3 + ions compensates for the absence of red emission from eu 3 + ions and keeps the colour co - ordinates in the white light region . in addition , the energy transfer from yb 3 + ions to ho 3 + ions is very efficient when compared to co - operative up - conversion of yb 3 + ions to eu 3 + ions which makes our new combination overall more efficient . like our previous thin film , there is virtually no change in the cie colour coordinates of the white light with a change in the excitation power . table 4 amount of nanoparticles used for making thin films combination amount of nanoparticles used 1 la 0 . 75 yb 0 . 20 tm 0 . 05 f 3 ( 150 mg ), la 0 . 75 yb 0 . 20 ho 0 . 05 f 3 ( 0 . 5 mg ) and la 0 . 45 yb 0 . 50 er 0 . 05 f 3 ( 0 . 5 mg ) 2 la 0 . 75 yb 0 . 20 tm 0 . 05 f 3 ( 150 mg ) and yb 0 . 75 la 0 . 20 er 0 . 05 ( 1 mg ) 3 la 0 . 75 yb 0 . 20 tm 0 . 05 f 3 ( 100 mg ) la 0 . 45 yb 0 . 5 er 0 . 05 f 3 ( 0 . 5 mg ) and yb 0 . 75 la 0 . 20 tb 0 . 05 ( 100 mg ) 4 la 0 . 75 yb 0 . 20 tm 0 . 05 f 3 ( 100 mg ) yb 0 . 75 la 0 . 20 tb 0 . 05 ( 80 mg ) and yb 0 . 75 la 0 . 20 eu 0 . 05 ( 150 mg ) silica and zirconia thin films were made with 6 ml of teos and 4 ml of zirconium propoxide , respectively . the emission band around 470 nm is assigned to the 1 g 4 to 3 h 6 transition of tm 3 + ions . both er 3 + and ho 3 + ions are responsible for green and red emission . er 3 + ions gave emission peaks around 515 , 540 , and 665 nm , which are assigned to the 2 h 11 / 2 to 4 i 15 / 2 , 4 s 3 / 2 to 4 i 15 / 2 , 4 f 9 / 2 to 4 i 15 / 2 transitions , respectively . ho 3 + ions gave two emission bands at approximately 540 nm and 640 nm , which are assigned to the 5s 2 to 5 i 8 and 5 f 5 to 5 i 8 transitions , respectively . a control silica thin film was made with the sane concentration of la 3 + , er 3 + , ho 3 + , tm 3 + , and yb 3 + ions by direct incorporation only showed green and red emission from er 3 + ions , consistent with earlier observations . silica thin films with ho 3 + / yb 3 + or tm 3 + / yb 3 + ions didn &# 39 ; t show any up - conversion which also clearly demonstrates that different nanoparticles are necessary to produce white light . fig1 b shows the up - conversion emission spectrum of silica thin film made with nanoparticles of combination 2 . a 33 - fold increase in the efficiency of white light generation was observed . the calculated cie colour coordinates are 0 . 30 and 0 . 34 . the efficiency generation of white light from combination 2 has been improved here in a different way than in combination 1 by increasing the red to green emission ratio from er 3 + ions . the increase in the red to green emission ratio has been achieved by increasing the concentration of yb 3 + in the nanoparticle . zhang and co - workers 39 and capobianco and co - workers 40 have also investigated this dependence of red to green ratio on yb 3 + concentration in matrices like gd 2 o 3 and y 2 o 3 , respectively . an 11 - fold and 2 - fold increase in the efficiency of generation of white light has been achieved from combination 3 ( fig1 ) and combination 4 ( fig2 ), respectively . the emission peaks at 542 , 586 and 623 nm are assigned to 5 d 4 to 7 f 5 , 7 f 4 and 7 f 3 transitions of tb 3 + ion , respectively . the co - operative up - conversion of yb — tb ions is more efficient than yb — eu ions because energy transfer can happen relatively easily from the virtual state (− 490 nm ) of two excited yb 3 ions to the 5 d 4 level ( 490 nm ) of tb 3 + ions , where these two levels are resonant in energy . in the case of yb — eu up - conversion process some energy has to be dumped into the matrix during the energy transfer from the yb 3 + ions to the 5 d 1 level ( 520 nm ) of eu ions . gudel and co - workers 41 have reported co - operative up - converted emission mechanism in cs 3 tb 2 br 9 : yb 3 + single crystal . preliminary results into the mechanism of the up - conversion process suggests that it is occurring via energy transfer ( et ) rather than an excited state absorption ( esa ). up - conversion from tm 3 + and eu 3 + ions are due to energy transfer processes , because both ions have no ground or excited state absorption that matches the 980 nm photon . green and red emission from er 3 + ions are predominantly due to energy transfer processes and may be due to a photoavalanche ( pa ) process and little contribution is from er 3 + excited state absorption , as can be seen from a silica thin film made with la 0 . 45 yb 0 . 5 er 0 . 05 f 3 nanoparticles that showed intense luminescence when compared to a silica thin film made with la 0 . 95 er 0 . 05 f 3 nanoparticles . the energy level of ho 3 + , tm 3 + , er 3 + , tb 3 + , eu 3 + and yb 3 + as well as possible up - conversion mechanisms for the blue , green , and red emissions under 980 nm excitation are given in the fig2 . we can calculate the efficiency of energy transfer ( η τ ) from yb 3 + to ln 3 + ions from η τ = 1 −( τ da / τ d ), where , τ da is lifetime of donor in the presence of acceptor and τ d is the lifetime of donor in the absence of acceptor . the effective lifetime of the 2 f 5 / 2 level of yb 3 + ions in the silica thin film incorporated with la 0 . 45 yb 0 . 50 y 0 . 05 nanoparticles is 1 . 1 ms ( fig2 a ). lifetime of yb 3 + when it is co - doped with er 3 + ( fig2 b ), ho 3 + and tm 3 + are 428 μs , 475 μs , and 600 μs , respectively . thus , the efficiency of energy transfer to er 3 , ho 3 + and tm 3 + were determined to be 0 . 6 , 0 . 6 and 0 . 5 , respectively . the effective lifetime 1 g 4 level of tm 3 + ions and 5 s 2 level of ho 3 + ions in the sample was found to be 300 μs ( τ r = 837 μs 42 and 378 μs , ( τ r = 489 μs 42 , respectively . the effective lifetime of 4 s 3 / 2 level and 4 f 9 / 2 level of er 3 + ions was found to be 525 μs ( τ r = 778 μs | 17 | ) and 418 μs , ( τ r = 1 . 4 ms 43 ) respectively . the effective lifetime of 5 d 0 level of eu 3 + and 5 d 4 level of tb 3 + ions was found to be 2 . 8 ms ( τ r = 6 . 7 ms 44 ) and 3 . 8 ms , ( τ r = 4 . 9 ms 45 ), respectively . the effective lifetimes and radiative lifetimes ( τ r ) of above lanthanide ions suggest that the estimated quantum yield ( qy = τ eff / τ r ) of resulting white light is on the order of 50 %. thus , the efficiency of the resulting white light is on the order of 25 %. the power dependence of the up - conversion emission intensity of silica film individually made with la 0 . 75 yb 0 . 2 ho 0 . 05 f 3 nanoparticles was measured ( fig2 ), showing that the green and red emission from ho 3 + ions are two - photon processes . the power dependence graphs show a slight decrease in the slope when the laser power is increased . this can be attributed to a ground state depletion caused by the population build - up of the yb 3 + excited level and in turn to the saturation of corresponding levels in the lanthanide ions . the green and red emission from er 3 + ions are two - photon processes and the blue emission from tm 3 + ions is a three - photon process . in order to substantiate further the generality of the method , above experiments were repeated by taking zro 2 as the sol - gel matrix . similar to sio 2 matrix , white light was observed with the zro 2 films made with combination 1 ( fig2 ), combination 2 ( fig2 ), and combination 3 ( fig2 ) nanoparticles . zro 2 thin films made with these combinations of nanoparticles also showed similar increase in the efficiency of white light like silica thin film . the calculated cie colour co - ordinates for combination 1 and combination 2 are 0 . 37 , 0 . 40 and 0 . 34 , 0 . 29 respectively . the calculated cie colour co - ordinates for combination 3 are 0 . 34 and 0 . 37 . a zro 2 thin film prepared with the same concentrations of la 3 + , er 3 + , tm 31 , and ho 3 + ions with yb 3 + ions by direct incorporation only showed green and red emission from er 3 + ions and no emission from tm 3 + ions and ho 3 + ions was observed . similarly , zro 2 thin film prepared with the same concentrations of la 3 + , tb 3 + , tm 3 + , and er 3 + ions with yb 3 + ions showed green and red emission which is from er 3 + ions . this again substantiates the importance of the role of three different nanoparticles to produce white light . in conclusion , up to a 33 - fold increase in the efficiency of the conversion of 980 nm light into white light has been achieved in sol - gel derived thin films by a judicious choice of upconverting ln 3 + - doped nanoparticles that were co - doped with yb 3 + . experimental : for nanoparticles amounts see table 4 . effective lifetimes were calculated using origin 7 software based on the following equation , all the calculations were done based on duplicate measurements and the values have estimated errors of 5 %. the foregoing is a description of embodiments of the invention . as would be known to one skilled in the art , variations would be contemplated that would not alter the scope of the invention . for example , this method can be extended to other luminescent ln 3 ions , i . e . ce , pr , sm , gd , tb , dy , tm , or yb , other nanoparticles , and to other matrices for example , but not limited to tio 2 , zro 2 , hfo 2 , ta 2 o 5 , nb 2 o 5 , geo 2 , y 2 o 3 , and gd 2 o 3 . further , other carboxylates can be employed , provided that they are substantially removed during heating of the sol - gel , as can some neutral molecules . white light can easily be generated by incorporating ln 3 + doped nanoparticles in sol - gel thin films other than sio 2 and zro 2 for example , but not to be limiting , y 2 o 3 , gd 2 o 3 tio 2 , al 2 o 3 , geo 2 , hfo 2 , nb 2 o 5 , ta 2 o 5 either individually or in combination . additionally , other core - shell nanoparticles could be used comprising lanthanides suitable for the production of core - shell nanoparticles . in general , other nanoparticles , such as oxides , could be used as well . additionally , the foregoing methods and products can be used to produce individual colors of light . for example , but not limited to , green and some red via nanoparticles as shown in fig1 a , blue via nanoparticles as shown in fig1 b , and red and some green via nanoparticles as shown in fig1 c . yb 3 + / eu 3 + / er 3 + / tm 3 + / la 3 + in sio 2 or zro 2 produces green and red from er 3 + . the fact that there is no light generate from eu 3 + and tm 3 + could be a result of energy transfer to er 3 + and / or quenching of the excited eu 3 + and tm 3 + , leading to non - 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