Development of nanophosphors—A revie · 114 H. Chander/Materials Science and Engineering R 49 (2005) 113–155. Fig. 1. The PL is slightly shifted and there is a larger linewidth

Development of nanophosphors—A review

Harish ChanderLuminescent Materials and Devices Group, Electronic Materials Division, National Physical Laboratory,

Dr. K S Krishnan Road, New Delhi 110012, India

Received 19 March 2005; accepted 5 June 2005

Abstract

Nanophosphors have been extensively investigated during the last decade due to their application potential for

various high-performance displays and devices. These act as a strategic component in almost all displays. Synthesis

of nanophosphors can be accomplished in two ways namely, chemical and physical methods. Under chemical

methods, different routes such as colloidal, capping, cluster formation, sol–gel, electrochemical, etc., are being

followed. Physical methods widely used are molecular beam epitaxy, ionised cluster beam, liquid metal ion source,

consolidation, sputtering and gas aggregation of monomers. Chemical precipitation in presence of capping

agents, reaction in microemulsions, sol–gel reaction and autocombustion are commonly used techniques for

synthesis of nanophosphors. However, the particle size has to be restricted to 3–5 nm to get the real advantage

of quantum confinement. In other words, the particle size must be less than twice of Bohr radii of exciton as quantum

confinement regime is limited to that size. A brief review of different synthesis techniques employed all over the

world for the development of industrially important nanophosphors and extent of particle size reduction achieved is

discussed.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Nanophosphors; Sol–gel; Microemulsions; Nanomaterials; Phosphors; Controlled precipitation; Luminescence

The origin of nanoparticle research can be said to be in study of colloids, their synthesis and

characteristics. The quantum confinement and major changes observed in other properties [1] have

been the subject matter of intense research since last three to four decades. Nanoparticles, in general,

are supposed to have nearly half of their atoms contained in top two monolayers, which make optical

properties highly sensitive to surface morphology. Control over movement of electrons and holes and

structure of surface has been of special importance for technology development related to very low

dimension optonics and electronics. Optical properties of semiconductor nanoparticles are directly

dependent on the size. Blue shift of band gap and strong non-linear response of nanoparticles of CdS

and CdSe in glass samples were first reported [2] in the early 1980s. Enhanced quantum properties

were further confirmed with study of other semiconductor nanoparticles of ZnS, PbS, ZnSe and CdSe

[3]. Metal nanoparticles [4] were also synthesised with a view to prepare better catalysts. Conduction

band to valence band transitions also called HOMO-LUMO transitions were studied in detail using

easily available optical spectroscopy instruments.

To understand the physics and develop applications based on nanophosphors that come under the

category of doped nanocrystals (DNC) and also in field of quantum dots, huge amount of research

effort is being expanded during the recent years. The development of applications and physics of

quantum confined 2D-electron gas structures has given a boost to the effort. To utilize the phenomena

of quantum confinement further, device fabrication with nano dimensions is being done world over

with urgency. The challenge is great but will be equally rewarding.

Materials Science and Engineering R 49 (2005) 113–155

0927-796X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.mser.2005.06.001

Several nanophosphors have been synthesised mostly as powders with few exceptions in a matrix

and as films using different techniques. Synthesis techniques for nanomaterials, in general, can be

divided into two broad categories as chemical methods and physical methods. Under chemical

methods different routes, viz., colloidal, capping, cluster formation, sol–gel, electrochemical, etc., are

being followed. Physical methods mostly used are molecular beam epitaxy, ionised cluster beam,

liquid metal ion source, consolidation, sputtering and gas aggregation of monomers. Sputtering is

again achieved by either using high-energy ions or laser ablation. Again aggregation can be brought

about by one of the ways, from oven sources, laser vaporization or laser pyrolysis. In all these

techniques, the size has to be restricted to 3–5 nm, i.e., less than twice of Bohr radii of exciton as

quantum confinement regime is limited to the size.

The nanomaterials so produced are characterised by different techniques. Luminescence proper-

ties are measured by usual techniques and instruments used for studying phosphors. The measuring

set-up has to be at least one generation higher than existing ones as regards excitation and emission

range, spectral resolution, nano to picosecond time resolution and with high intensity, higher energy

excitation sources. X-ray diffraction (XRD), small angle X-ray scattering (SAXS), small angle

neutron scattering (SANS), high resolution electron microscopy (HRTEM), low frequency Raman

scattering and longitudinal-optic (LO) phonon Raman scattering, atomic force microscope (AFM),

time of flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy

(XPS) are the other characterisation techniques used for development and understanding of nanopho-

sphors.

In the present paper, a review of different synthesis techniques employed by researchers for

synthesis of nanophosphors and their development as efficient luminescent materials shall be

presented. What lies in store for nanophosphors and related devices shall be discussed. Shall we

be able to achieve nanoscale display devices, detectors and light sources? How shall we use these

devices? Are we going to stop at nanosize applications and developments or we shall continue our

march to single atom/molecule level gadgets?

Semiconductor nanoparticles also called clusters have been synthesised long back as colloidal

suspensions and studied for their electronic and optical properties since early 1980s. These have been

mostly sulfides and selenides Zn, Cd, Pb [4–11], etc. The size has been controlled by arrested

precipitation technique. The basic trick has been to synthesis and studies the nanomaterial in situ, i.e.,

in the same liquid medium avoiding the physical changes and aggregation of tiny crystallites. Thermal

coagulation and Ostwald ripening were controlled by double layer repulsion of crystallites using non-

aqueous solvents at lower temperatures for synthesis. The first report on doped nanocrystals also

abbreviated as DNCs that showed all features such as increased band gap, shift of excitation and

emission spectra of nanosized semiconductor was reported by Bhargava et al. [1] in 1994. They

synthesised manganese doped nanocrystals of zinc sulfide. The nanomaterials had external photo-

luminescence quantum efficiency of 18% and it ushered the era of nanophosphors. The synthesis

involved reaction of diethyl zinc with hydrogen sulfide in toluene. The dopant manganese is added as

ethylmanganese in tetrahydrofuran solvent to the parent solution of zinc salt before precipitation

reaction. Surfactant methacrylic acid was used to maintain separation between the particles formed.

Thus formed DNC are separated by centrifugation, washed and vacuum dried. The dried material was

further subjected to UV curing for possible polymerisation of surfactant methacrylate capping film on

the surface of Mn doped ZnS nano cluster for imparting true quantum confinement. The UV curing

effect was established by their further work on dependence of luminescence efficiency on time of

curing. The enhancement of efficiency has been explained on the basis of surface passivation of the

nanocrystals due to photopolymerization of the surfactant. The photoluminescent (PL) and photo-

luminescence excitation (PLE) spectra of the nanophosphor have been compared with bulk ZnS:Mn in

114 H. Chander / Materials Science and Engineering R 49 (2005) 113–155

Fig. 1. The PL is slightly shifted and there is a larger linewidth in the nanophosphor as compared to

bulk. It is due combination of inhomogeneous broadening and phonon assisted transitions. The large

shift in PLE spectrum is attributed to an increase in value of s–p electron band gap in the ZnS

nanocrystals as a result of quantum confinement. Further luminescent decay has been reported to

be faster by five orders of magnitude as shown in Fig. 2. The presence of an impurity within a

nanocrystal and localization of electron and hole wave function due to quantum confinement leads to

faster energy transfer to impurity in smaller particles as compared to transfer rate for band to band

transition or surface recombination. Hence, luminescence efficiency increases with decrease of

particle size.

Following the encouraging results of Bhargava et al. on Mn doped ZnS nanophosphor Khosravi

et al. [11] reported synthesis of manganese doped ZnS nanoparticles by aqueous method. They

followed process similar to the one, used by Nosaka et al. [12] for ZnS quantum dots. In this method,

aqueous solution of zinc chloride with required amount of dopant manganese chloride is mixed slowly

with mercaptoethanol under constant agitation and then reacted dropwise with sodium sulfide

solution. The reaction was carried under inert atmosphere of nitrogen gas to avoid oxidation of

freshly formed highly reactive nanoparticles. Then usual washing, centrifugation and drying are

followed to obtain the freestanding nanophosphor. They observed PLE and PL peaks at 312 and

H. Chander / Materials Science and Engineering R 49 (2005) 113–155 115

Fig. 1. Comparison of PLE and PL spectra of nanocrystalline (dashed lines) and bulk (solid lines) ZnS:Mn (from Ref. [1]).

Fig. 2. Time decay of luminescence from nanocrystalline ZnS:Mn. Inset: Time decay is separated into its exponential decaycomponents (from Ref. [1]).

600 nm, respectively. Mn concentration of 12 at wt.% was found to be optimum. About the same time,

Khosravi et al. [13] with slightly different team reported synthesis of copper doped zinc sulfide

quantum particles. Method used by Weller [14] for synthesis of CdS was employed for the purpose.

The basic idea had been the formation of polymeric chains by phosphate groups and their attachment

to metal ions. Copper doped zinc sulfide nanocrystals were synthesized by first preparing a solution of

zinc chloride, copper chloride and sodium acetate of pH 8–9. Solution of sodium hexametaphosphate

was added before precipitation reaction with sodium sulfide solution. As the sulfide solution is added,

sulfur reacts with excess metal ions forming sulfide clusters. Clusters grow by the coalescence of

smaller clusters or on metal ion seeds present in the solution. On a growing metal sulfide cluster,

phosphate chains get attached through metal ions. These chains separate the clusters due to their length

and avoid coalescence. The reaction is once again carried out in nitrogen atmosphere. Diameter of the

particles determined from XRD is reported to be �21 � 2 A. Excitation was at 300 nm which

corresponds to 4.1 eVand emission has been at �480 nm. Luminescence of undoped ZnS prepared by

same process has been compared and found to be 326 and 424 nm for excitation and emission,

respectively. Luminescence decay times were determined for both Cu doped and undoped ZnS. Two

exponential decay times of 2.9, 54.2 and 1.62, 22.12 ns were observed. Undoped sample obviously had

shorter decay times. There is no mention of curing or annealing of any type in the two methods given

by Kosaravi teams.

Yu et al. [15] prepared zinc sulfide nanoparticles with homogeneous Mn distribution and studied

optical properties along with other characteristics. Procedure followed was of reaction between

methanolic solution of zinc acetate with manganese acetate and aqueous solution of sodium sulfide.

Dispersion of precipitates was achieved with methacrylic acid. A number of methanol washings were

given to remove impurities and drying was done at 50 8C. They reported variation of crystallite sizes

from 2.3 to 2.7 nm with use of different amount of methacrylic acid. Homogeneous distribution of

Mn2+ ions in the ZnS formed was established by the workers with well-resolved ESR spectra (Fig. 3)


Fig. 3. ESR spectra of ZnS:Mn nanoparticles after drying: samples with addition of methacrylic acid: (a) 10 cm3, (b) 30 cm3

and (c) 50 cm3, (d) sample without the addition of methacrylic acid (from Ref. [15]).

of the samples with use of methacrylic acid. ESR spectra of the sample without the addition of

methacrylic acid had broad lines with unresolved structure. From the shape of hyperfine lines, these

spectra can be interpreted as the superposition of these corresponding to isolated Mn2+ ions and to

Mn2+ pairs caused by inhomogeneous distribution of manganese. TEM and XRD studies confirmed

the nanocrystalline structure of the phosphor prepared and size ranged from 2.3 to 2.7 nm. XPS studies

presented the evidence for presence of Zn, S, C, Mn and O in the phosphor. Effect of addition of

methacrylic acid on particle size, UV absorption and PL as shown in Fig. 4 has been highlighted.

Bhargava [16] elucidated in detail the effect UV curing that was mentioned in his first paper of 1994.

Fig. 5 shows increase in PLE and PL intensity with increasing time of curing. UV curing is conducive

to surface passivation which greatly improves the efficiency.

Zhou et al. [17] reported synthesis of semiconductor coated nanoparticles of CdS/PbS. Though

the team did not intend to synthesis nanophosphor but the process definitely paved the way and gave a

viable technique. The earlier workers employing similar procedure that have been referred in the

report are also contributors to the development of nanophosphors. In the technique, CdS was

precipitated from Cd salt solution with a solution containing S2� ions in presence of polyvinylpyr-


Fig. 4. PL spectra of ZnS:Mn nanoparticles (a) 2.3 nm, (b) 2.5 nm and conventional bulk ZnS:Mn (from Ref. [15]).

Fig. 5. PLE and PL variation of ZnS:Mn nanocrystals with UV curing time (from Ref. [16]).

oledone (PVP) stabilizing agent. PVP prevented coalescence of CdS colloidal particles. Subsequent

addition of Pb2+ ion aqueous solution displaced Cd and formed PbS on the surface of CdS.

Yang et al. [18] studied optical properties of manganese doped ZnS nanocrystals. Synthesis

process adopted has been of chemical precipitation in presence of a surfactant. They studied and

compared the behaviour of the nanophosphor in sol form and the dried powder obtained after

centrifugation, washing, drying and calcining at 200–700 8C for 2 h. Particle size of nanocrystals in sol

was reported to be around 5 nm and that of powder after calcining at 400 8C was �12 nm. PLE and PL

spectra of nanophosphor sol and powder were presented without and with surfactant. These were

found to be similar and there was slight shift in peaks (Fig. 6) in both cases towards shorter wavelength

for sol and powder, with and without surfactant.

Stanic et al. [19] reported in 1997 sol–gel synthesis of nanosize ZnS, a key material to a large

number of phosphors. Zinc tert-butoxide in butanol and water free toluene was taken and high purity

hydrogen sulfide gas bubbled through the solution till complete gelation occurred. Gel was allowed to

age for 24 h and dried in vacuum. Formation of ZnS was confirmed by X-ray diffraction (XRD) and IR

absorption spectra.

The exhaustive work of Kundu et al. [20] reported in 1997 on organically capped nanoclusters of

CdS, an important constituent to many phosphors, cannot be missed while discussing synthesis of

nanophosphors. They have used both non-aqueous and aqueous chemical methods. Capping agent

thiophenol has been used for non-aqueous synthesis and additives like mercaptoethanol, sodium

hexametaphosphate, ethylene glycol and ethanol have been employed for aqueous method. Cadmium

acetate was reacted with sodium sulfide in methanolic solution in presence of thiophenol. Reaction

products were centrifuged, washed and dried in air. For aqueous synthesis route, CdCl2 and Na2S

aqueous solutions of different concentrations with the additives were made to react. Thus obtained


Fig. 6. PLE (A) and PL (B) spectra of ZnS:Mn bulk and nanocrystalline powder, (a) bulk (b) without surfactant and (c) withsurfactant (from Ref. [18]).

nanoclusters of CdS were characterised by optical absorption, XRD, transmission electron microscopy

(TEM) and X-ray photoelectron spectroscopy (XPS). CdS cluster size ranged from 0.7 to 2.6 nm for

varied experimental conditions. Band gap measurement showed change from 2.98 to 4.59 eV. Cd 3d

and S 2p XPS core levels of the clusters showed shift to the higher binding energy side which is likely

to be because of increased band gap of the clusters and a corresponding shift of all occupied levels to

higher energies.

Nanocrystalline CdS was synthesised by Vogel et al. [21] using aqueous solution of CdCl2 with

mercaptoethanol under high-speed stirring. S2� ion is provided by Na2S solution with dropwise

addition. Atmosphere of nitrogen is maintained during synthesis process to avoid oxidation. Extensive

XRD analysis was performed to ascertain the crystalline nature and size distribution of the

nanocrystals. As synthesised material consisted of both �50% hexagonal wurtzite type nanoparticles

of about 2 nm and large cubic sphalerite of about 2.6 nm. The important feature of the work is Debye

function analysis simulations for model clusters (CdS)n along with experimental results. The analysis

is further supported by TEM data.

Senna et al. [22] synthesised and studied nanocrystalline ZnS:Mn phosphor from solutions

containing carboxylic acids. Methanolic solutions of zinc acetate and manganese acetate were mixed

under continuous stirring and Na2S solution in methanol was then added to the above solution. Acrylic

acid or methacrylic acid was added after precipitation while stirring was continued. The phosphor was

obtained after washing, centrifugation and drying at 50 8C for 24 h. Different characteristic were

studied with varying amounts of acrylic acid from 0 to 1.0 mol with respect to 20 mmol of ZnS. PL

intensity increased up to 0.72 mol acrylic acid Fig. 7 and then it started decreasing with increase of

acrylic acid. With acrylic acid excitation peak shifts towards shorter wavelength.

Xu et al. [23] in 1998 synthesised impurities-activated ZnS nanocrystals in microemulsion with

hydrothermal treatment. They used petroleum ether as oil phase and mixture of poly(oxyethylene)5-

nonyl phenol ether (NP-5) and poly(oxyethylene)nonyl phenol ether as surfactant phase. Two reverse

microemulsions of the system with aqueous solutions of ZnCl2 along with dopant and sodium sulfide

were prepared separately and mixed with continuous stirring. The microemulsion system was also

treated under hydrothermal conditions for surface passivation. ZnS:Cu, ZnS:Eu and ZnS:Mn


Fig. 7. PL spectra of ZnS:Mn prepared with varying amount of acrylic acid; A = 0; B = 0.14 mol; C = 0.43 mol;D = 0.72 mol; and E = 1.0 mol for 20 mmol ZnS:Mn (from Ref. [22]).

nanocrystals were prepared and particle size found to vary from 3 to 18 nm. Emission intensity

enhancement of 60 times as compared to Mn doped ZnS nanocrystals synthesized by conventional

aqueous reaction method has been reported. PL spectra of the nanophosphors synthesized is shown in

Fig. 8 Another significant observation of the workers is increase of integrated PL intensity with

increasing atomic number of dopant as shown in Fig. 9.

Bol and Meijerink [24] synthesised nanocrystalline ZnS:Mn2+ by two methods and made lifetime

measurements and conducted time-resolved spectroscopy. Synthesis involved solutions of diethyl

zinc, manganesecyclohexabutyrate and methacrylic acid in toluene. Precipitation was achieved with

saturated solution of hydrogen sulfide gas in toluene. In the second method, they used aqueous

solutions of zinc acetate, manganese acetate and Na(PO3)n and Na2S solution as precipitant. The

methods have been nearly similar to the one described earlier. They reported some contradiction to

earlier work of Bhargava [1,16] in regard to shorter decay times explained on the basis that the spin

forbidden 4T1 ! 6A1 transition of Mn2+ impurity becomes less spin forbidden due to hybridisation of

s–p states of ZnS host and the d states of Mn2+ impurity. They measured decay times at emission of 400

and 600 nm by exciting the samples at 308 nm with Excimer laser and are shown in Fig. 10. They also

recorded time-resolved emission spectra for time delay of �0 ms, 3 ms and 0.5 ms with gate width of

2 ms, 200 ms and 1 ms, respectively. Emission peaked (see Fig. 11) at 420, 420 and 590 nm (two


Fig. 8. PL spectra (room temperature) from ZnS:Mn, ZnS:Cu and ZnS:Eu nanocrystals (from Ref. [23]).

peaks), 590 nm, respectively, for the three delay times. From the data, it can be said that short decay

time is ascribed to defect related emission of ZnS and not to decay from transition of Mn2+ impurity.

Xu and Ji [25] gave an interesting new route for preparation of nanoparticles of ZnS. The new

route is via synthesis of Zn nanoparticles by inert-gas evaporation with induction heating. These

particles were made to react with sodium sulfide aqueous solution under ultrasonic radiation at 50 8C.

XRD analysis showed highly crystalline phase of b-ZnS and TEM confirmed spherical particles with

narrow size distribution averaging 40 nm. However, presence of some unreacted zinc was observed by

XRD.

Pingbo et al. [26] prepared nanocrystalline ZnS:Mn using surfactant DBS—dodecyl benzene

sulfonic acid sodium salt. Chemical precipitation method has been used employing aqueous solutions

of Zn(NO3)2, MnCl2 and Na2S. Different amount of Na2S was taken to create Zn2+ vacancies. They

studied PL properties as shown in Fig. 12 varying Mn concentration and also compared the


Fig. 9. Integrated PL intensity of doped ZnS nanocrystals vs. atomic number of dopants (from Ref. [23]).

Fig. 10. Decay curves of emissions of nanocrystalline ZnS:Mn at (a) l = 400 nm, t � 50 and 200 ns, (b) l = 600 nm, t � 40and 250 ns and (c) l = 600 nm, t � 0.4 and 1.9 ms. Excitation at 308 nm; measurement at 300 K (from Ref. [24]).


Fig. 11. Time-resolved emission spectra of nanocrystalline ZnS:Mn: (a) time delay � 0 ms, gate width = 2 ms, (b) timedelay � 3 ms, gate width = 200 ms, (c) time delay � 0.5 ms, gate width = 1 ms, (d) time-averaged PL spectra with excitationat 266 nm (from Ref. [24]).

Fig. 12. Orange band luminescence of normal nanocrystalline ZnS:Mn (A) and surface modified samples (B) vs. Mn2+

concentration (A1 and B1, 0.005 Mn; A2 and B2, 0.01 Mn; A3 and B3, 0.015 Mn) (from Ref. [26]).

luminescence with and without surface-modification. In this work, fluorescence lifetime have also

been analysed for different emission wavelengths and observed that nanosecond decay is due to zinc

vacancies and millisecond decay is attributed to Mn. Photo aging was also studied for these samples.

Fig. 13 shows XPS measurements, which indicate effect of surface-modification on the surface

structure.

Wang and Hong [27] reported in 2000, a new preparation procedure for nanosized zinc sulfide

particles by solid-state method at low temperature. Zinc acetate and thioacetamide were milled

separately, mixed and further milled for through and uniform dispersion of the components in the

mass. The mixture was heated in an oven at different temperatures up to 300 8C for 4 h and Fig. 14

show XRD obtained. Formation of nanocrystalline zinc sulfide with size of 3.2 nm at 100 8C is

confirmed. TEM and PL studies also establish nanocrystalline nature of the sample prepared.

Park et al. [28] in 2000 made YAG–Y3Al5O12:Tb phosphor in nanocrstallized form by a sol–gel

process. Y2O3, Al(NO3)3�9H2O and Tb4O7 were taken in stoichiometric amount and dissolved in

dilute nitric acid. Twice the moles of citric acid were added to the solution. The sol was allowed to cure

at 60 8C for several days and a yellow gel was obtained. The gel was further dried at 126 8C and fired

800–1300 8C. Particle size of the phosphor samples was between 25 and 45 nm. XRD pattern of

samples prepared at different temperatures shows progress of phase formation. Samples are amor-

phous in nature up to 800 8C firing temperature and PLE spectrum is substantially different from the

samples fired at higher temperatures. PLE and PL spectra of samples are given in Fig. 15. From decay

time and comparative PLE studies workers stake the claim that the phosphor is good for plasma

display panel (PDP) use.

Kezuka et al. [29] in 2000 reported fabrication of composite films of nanophosphor ZnS:Mn and

polymer and studied the properties. The nanophosphor was prepared by co-precipitation method in

methanol. Polyacrylic acid (PAA) was added to the colloidal suspension of the nanophosphor and the

composite film of ZnS:Mn–PAA was obtained on a glass slide mounted in the suspension by

evaporation of methanol and water using a rotary evaporator. Composite film with poly methyl


Fig. 13. XPS of normal nanocrystalline ZnS:Mn (A) and surface modified samples (B) (from Ref. [26]).

methacrylate (PMMA) was made by first removing methanol–water from the phosphor, re-dispersing

in tetrahydrofuran and mixing poly methyl methacrylate. The film was made on slide by dip coating

and drying in vacuum after concentrating composite suspension using evaporation for desired

viscosity. The weight ratio of nanophosphor to polymer was kept 1:1. Polyacrylic acid solution

was coated on poly methyl methacrylate composite film by spinning to form a bilayer composite film.

PL and PLE spectra of ZnS:Mn–PMMA and ZnS:Mn–PAA composite are compared in Fig. 16. Also


Fig. 14. XRD patterns of ZnS nanoparticles prepared at 80, 100, 150, 200 and 300 8C, respectively (from Ref. [27]).

Fig. 15. PLE and PL spectra of samples prepared at 800 and 1200 8C. PLE recorded at 544 nm emission and for PLexcitation was by 255 and 277 nm for 800 and 1200 8C samples, respectively (from Ref. [28]).

compared are PL spectra of PMMA composite film and PMMA/PAA bilayer film. Intensity obtained

for bilayer is more than twice of PMMA composite layer and it is eight times for ZnS:Mn/PAA

compared to ZnS:Mn/PMMA. It is concluded that exchange interaction is more effective for energy

transfer from PAA to Mn2+ than multipolar interaction.

Haase et al. [30] in 2000 synthesised colloidal lanthanide doped nanophosphors of YVO4 and

LaPO4 in high boiling coordinating solvents or by hydrothermal means. Aqueous solutions of the

lanthanide nitrates in required proportion were precipitated with NaOH solution and the obtained

composition was reacted with sodium vandate/phosphate. The pH of the resulting mass was adjusted

to 12.5 and heated to 200 8C for 1–2 h in a Teflon-lined autoclave with stirring. Solids were separated

by centrifugation and washed with dilute nitric acid or 1-hydroxyethane-1,1-diphosphonic acid.

Centrifuged again to get solids and remixed with water to form colloidal solution by peptization. The

colloids thus prepared were subjected to centrifugation at high g. In other scheme, doped LaPO4:Eu

was prepared by reacting chlorides of La and Eu with phosphoric acid in tributylphosphate at 200 8C.

Particles of size from 4 to 90 nm have been obtained depending on pH and other reaction parameters.

The materials have been characterised by TEM, XRD, absorption and luminescence spectroscopy. By

these studies, it is observed that dopant ions enter the same lattice sites as in corresponding bulk

material. Hydrothermally prepared YVO4 nanoparticles exhibit similar tetragonal zircon structure as

that of bulk material. In case of LaPO4, monazite structure is observed and there is energy transfer

between the host and dopant ion. In LaPO4:Ce,Tb energy transfer between cerium and terbium ions is

evident (Fig. 17).

Yu et al. [31] in 2001 synthesised nanostructured g-Al2O3 by sol–gel process. Aluminium

isopropoxide was prepared by the reaction of aluminium foil with anhydrous isopropyl alcohol in

presence of iodine as catalyst. Aluminium isopropoxide was purified by distillation and it was

dissolved in aliphatic hydrocarbon so as make 0.1–0.2 M solution. It was then hydrolysed to get g-

Al2O3. The powder so obtained was calcined at 800 C in air for 3 h. Nanocrystalline g-Al2O3 powder

of 50 nm size was obtained as confirmed by XRD analysis. PLE was recorded at emission wavelength

of 422 and 405 nm is shown in Fig. 18. Four peaks at 238, 255, 278.5 and 348.5 are observed for

emission at 422 nm, whereas at 405 nm emission, three excitation peaks located at 235.5, 255 and

270.5 nm have been noticed. PL emission spectrum with excitation of 238 nm has been recorded and is

shown in Fig. 19. Spectrum has a broad band with a maximum at �422 nm with two shoulders at

404.5, 447 and 484.5 nm.

Nanda and Sarma [32] in 2001 synthesised ZnS nanocrystallites of average size 1.8, 2.5 and

3.5 nm and compared the properties. The method consists of taking a fixed quantity of zinc acetate,


Fig. 16. PL and PLE spectra of ZnS:Mn–PMMA (dotted line) and ZnS:Mn–PAA (solid line) composite films. Inset:Magnified PLE of ZnS:Mn–PMMA (from Ref. [29]).

sodium sulfide and 1-thioglycerol in dimethyl foramide and the reaction continued for 8–10 h under

reflux in argon atmosphere. The phosphor powder is obtained after condensing the solution and then

adding acetone. Size depended on ratio of sulfide to thioglycerol. Nano ZnS with size 3.5 nm was

obtained using thiourea instead of using sodium sulfide. Extensive studies on XPS for Zn and S core

levels have been reported in the work. These studies indicate that Zn atoms have a similar chemical

surrounding for both bulk and nanocrystallites. But several different species of sulfur coexist for the

nanocrystalline samples. It is observed that a core–shell structure of the nanocrystallites as shown in

Fig. 20 with a relative increase in the surface component there is decrease in the size.

Konrad et al. [33] in 2001 synthesised nanocrystalline cubic yttria and compared its features with

bulk phosphor. Route followed was chemical vapour synthesis. Samples showed slight agglomeration,


Fig. 17. Absorption spectrum-broken lines and PLE spectrum—solid line of colloidal solution of YVO4:Eu (top). PLspectrum of nanocrystalline LaPO4:Ce—dashed line and LaPO4:Ce,Tb—solid line. PLE at 542 nm emission of LaPO4:Ce,Tb as solid line in UV region (from Ref. [30]).

Fig. 18. PLE spectra for the nanometer-sized g-Al2O3 powder recorded at: (a) lem = 422 nm and lem = 405 nm (from Ref.[31]).

cubic crystal structure and 10 nm size. Size of crystallite was found to increase to 20 and 50 nm,

respectively, upon firing at 900 and 1100 8C for 5 h in air. PLE, PL and absorption spectra of

nanocrystalline and bulk yttria have been measured at 300 and 80 K and compared. There is

broadening of absorption edge and blueshift of PL spectra depending on particle size. It is found

by the contributors that the energy emitted due to recombination of a bound exciton does not depend

on particle size. In their opinion, the changes of spectrum are not due to quantum/phonon confinement.

An explanation based on quantum mechanical configurational coordinate diagram has been given and

shown in Fig. 21.

Konshi et al. [34] in 2001 prepared hybridised ZnS:Mn nanocrystals with polymerised acrylic

acid. Methanolic solution of zinc acetate and manganese acetate was put in water + methanol (1:1)

solution of Na2S. The mass was stirred for 20 min and then required amount of acrylic acid (AA) was

added. Stirring continued for another 15 min. Centrifugation and drying was done at 50 8C. Thereafter

powder was aged at 80 8C for different time duration and studied. For polyacrylic acid (PAA)

hybridisation, 35 wt.% aqueous solution was mixed with methanol and added to ZnS:Mn suspension

made following earlier procedure. Intense mixing for 5 min and drying was done at 60 8C in steps in

vacuum at pressure of 100, 60 and 20 Torr. Aging of the samples with AA at 80 8C enhances the PL

intensities at 580 nm shown in Fig. 22 due to d–d transition of Mn2+ ions and at 440 nm due to

carboxyl groups of polymerised acrylic acid. Carboxyl groups form –S–O–C( O)– with sulfur and

thereby C O group remains unchanged. Whereas when PAA is used, C O group is lost due to

interaction with metallic ions to produce –C–O–Me–. The hypothesis is corroborated by infrared

absorption spectra of various samples shown in Fig. 23 and XPS spectra.


Fig. 19. PL spectrum for nanometer-sized g-Al2O3 powder at lex = 238 nm (from Ref. [31]).

Fig. 20. Schematic model of a ZnS nanocrystallite based on the photoemission core level analysis. R0 is the radius of core, R1

is radius including surface layer and R2 is radius including the capping layer (from Ref. [32]).


Fig. 21. Qualitative configurational coordinate diagram for coarse grinded and nanocrystalline yttria at 300 K. The particlesize dependence of the exciton excitation energy is mainly due to an increasing of the slope of excitation parabola withreducing particle size (from Ref. [33]).

Fig. 22. The change in PL spectra of acrylic acid-modified ZnS:Mn nanocrystals aged at 80 8C for 0 h (a), 6 h (b), 12 h (c)and 24 h (d) (from Ref. [34]).

Dijken et al. [35] in 2001 prepared colloidal solutions of nanocrystalline ZnO particles and

studied quantum efficiency with particle size. NaOH solution is added slowly to zinc acetate solution.

Both the solutions are in 2-propanol and pre-cooled to 0 8C. Colloidal suspension of ZnO particle of

0.7 nm radius is generated. Particle size grows with time due to aging. Growth up to 3 nm was recorded

and analysed. Quantum efficiency of the colloidal suspension of the sample is determined by

comparing the emission spectrum of the sample with a reference solution of coumarone 153 in 2-

propanol. Quantum efficiency of the visible emission has been found to decrease as particle size

increases. As shown in Fig. 24, efficiency of 20% for 0.7 nm particle decreases to 12% non-linearly for

1.0 nm particle. The workers have found no evidence of quantum size effect that can be attributed to

decrease of quantum efficiency.

Yang et al. [36] in 2001 synthesised ZnS nanocrystals co-activated with Cu and rare-earth metals

like Ce, Y, Nd, Er, Tb. Doped zinc sulfide nanoparticles were made by precipitation from homo-

geneous solution. Zinc acetate solution with dopants in form of chloride salt and solution of

thioacetamide were separately heated to 80 8C and mixed, pH of the solution raised to 2.0 and

reaction time of 30 min was given. Reaction was arrested by cooling the solution to less than 10 8C.

Centrifugation, washing with water and isopropyl alcohol and then drying at 80 8C for 10 h gave the

desired ZnS nanocrystals of 2–3 nm. XRD patterns of samples prepared with different impurities were

recorded and it was found that there is no difference in these due to doping of nano ZnS samples either

with Cu2+ up to 1 mol% or with rare-earth impurities or Cu2+ together with rare-earths. PL spectra for

all combinations was measured and analysed. Spectra with rare-earth impurities showed practically no


Fig. 23. Infra red absorption spectra of acrylic acid monomer (a), acrylic acid-modified ZnS:Mn nanocrystals aged at 80 8Cfor 0 h (b), 12 h (c), 24 h (d) and the sample modified with polyacrylic acid (from Ref. [34]).

change in emission peak from that of pure ZnS. However, intensity enhancement by factor of 5–6 was

observed. In case of double doping with Cu and rare-earth peak PL emission was around 540–550 nm

shown in Fig. 25 with 15 times increase in intensity for Tb3+, Cu2+ sample compared to samples of

pure ZnS.

Chen et al. [37] in 2001 reported detailed synthesis and studies on nanosized ZnS:Mn with size

variation and in cavities of ultrastable zeolite—Y. The nanosized ZnS:Mn was prepared by slow

simultaneous addition of aqueous solutions of Na2S and zinc nitrate with manganese nitrate to a

container with DI water. The mass was kept stirred, nitrogen atmosphere maintained and heated to

80 8C for 24 h. Centrifugation and drying in vacuum at room temperature was done to get the

nanoparticles. ZnS:Mn nanophosphors of size �4.5 and 3.5 nm were also prepared in methacrylic

acid/ethanol and methacrylic acid/citric acid ethanol solution, respectively. ZnS:Mn nanoparticles

were placed in zeolite matrix by mixing 100 mg of the phosphor of �10 nm size and 2 g of zeolite

powder and pressing to form pellets which were heated at 900 8C in vacuum (10�5 Torr) for 2 days.

XRD measurements and TEM micrographs confirmed nanocrystalline nature of the samples prepared.

Cryo EFTEM was performed on samples to confirm actual doping of samples. ESR spectra were


Fig. 24. Room temperature luminescence quantum efficiencies (QE) vs. particle radius for nanocrystalline ZnO particles(from Ref. [35]).

Fig. 25. PL spectra of ZnS nanocrystallites co-doped with Cu2+ and rare-earth ions (from Ref. [36]).

recorded for the samples in order to determine the valence and distribution of Mn2+ ions in

nanoparticles. PL spectra of the samples (Fig. 26) gave emission peaks at 591, 588, 581 and 570

for the particles of 10, 4.5, 3.5 nm and the clusters in zeolites. It was concluded that the main factor for

the shift is the phonon coupling, whose strength is size dependent and is determined by both the size

confinement and surface-modification. The crystal field strength dependent on particle size is not

contributing substantially to the emission shift of Mn2+.

Igarashi et al. [38] in 2001 synthesised ZnS:Mn nanocrystals coated with polyacrylic acid

following chemical precipitation route in methanolic media and compared with ZnS:Mn phosphor

prepared in normal way (bulk sample) of ZnS and Mn diffusion at high temperature. Mn2+

coordination states inside and near the surface of the phosphor were studied by electron paramagnetic

resonance (EPR). The EPR spectra measured at 35 GHz is shown in Fig. 27. Signals I and II are for two

types of Mn2+ sites located inside and on the surface of particles, respectively. The symmetry of both

sites of nanocrystalline sample is lower than that of the bulk sample due to larger lattice distortion and

larger zero field splitting constant. This is responsible for the increase of forbidden d–d transition of

Mn2+ for nanocrystalline sample. The values of DHpp for signals I and II of nanocrystalline sample do

not change with temperature. This behaviour is attributed to the strong interaction between ZnS and

Mn2+ and between polyacrylic acid and Mn2+. This explains PL intensity increase due to effective

energy transfer between Mn2+ and ZnS as well as between PAA and Mn2+.

Kulkarni et al. [39] in 2001 reported investigations on chemically capped CdS, ZnS and ZnCdS

nanoparticles. The samples were synthesised by wet chemical route using organic capping molecules

for passivation. Interesting feature of the work is high-resolution photoelectron spectroscopy XPS

study to give detailed information about the surface of nanoparticles. Fig. 28 shows the proposed

model with position of different elements such as Cd, Zn and S. Other studies only reinforce the

already established observations.

Lee et al. [40] in 2002 prepared ZnS nanocluster thin films by solution growth technique.

Solutions of zinc acetate, aqueous ammonia, triethanolamine were mixed at room temperature.


Fig. 26. PLE and PL spectra of ZnS:Mn2+/USYand ZnS:Mn nanoparticles. Inset shows the change of luminescence intensitywith size (from Ref. [37]).

Thoroughly cleaned glass slides were vertically held in the solution and the solution heated on water

bath till temperature of 75, 80, 85, 88 8C for different samples. To this hot solution, thiourea solution

was added and system kept heated for 5–120 min interval. Thin bright silver, tightly adherent ZnS

layer was grown on the substrate and growth was studied for various parameters such as time of

deposition, concentration of reactants, temperature of reaction and properties like XRD, optical

absorption, SEM and PL. Film formed were of cubic crystal structure. Optimum growth conditions

were: (i) growth temperature = 75 8C; (ii) Zn salt:thiourea = 1 M:2 M; aqueous ammonia = 14 M,

band gap = 3.69–3.91, PL peaks were at 402–412 and intensity increase with time of deposition

(Fig. 29). The results indicate strong quantum confinement.


Fig. 27. EPR spectra, measured at the Q-band (microwave frequency: 35 GHz) and room temperature, for NC–ZnS:Mnnanocrystals and SMP–ZnS:Mn submicro particles usual method (from Ref. [38]).

Fig. 28. A model for ZnCdS nanoparticles based on photoemission results illustrating various geometrical positions of Zn,Cd and S atoms (from Ref. [39]).

Qiao et al. [41] in 2002 reported a route to synthesis Wurtzite ZnS and CdS nanorods. Zinc

terephthalate 4,40-bipyricine or cadmium terephthalate 4,40-bipyricine and thiourea were added in to a

Teflon-lined stainless steel autoclave which had been filled with ethanol up to 90% of its capacity.

Temperature of autoclave was maintained at 140 8C for 10 h and then cooled to room temperature

naturally. White ZnS powder and yellow CdS powder was obtained by centrifugation, washing and

drying. The morphology determined by TEM gives the average diameters of width/length as 50/200

and 20/75 nm for ZnS and CdS, respectively. XRD and XPS spectra proved that the as-prepared

products were pure ZnS and CdS.

Ishizaka et al. [42] in 2002 prepared Tb3+ and Eu3+ doped alumina films by gel technique and

studied their luminescence properties. Hydrous aluminium oxide was prepared through the reaction of

solutions of aluminium chloride and ammonium hydroxide. The precipitates were aged and then

washed and peptised to clear sol using acetic acid under reflux at 80 8C. For doping, the viscous sol

was mixed with the chloride solution of the rare-earth. The sol was put into a Petri dish and gelation

carried out by dehydration at room temperature in a glove box. The films obtained were heated in air at

200–800 8C for 2 h and studied. PL and decay curve was recorded. Quantitative treatment was applied

to lifetime studies considering cross-relaxation and phonon relaxation by OH groups. Cross-relaxation

hardly occurred in doped film and relaxation by OH groups was dominant on the lifetime. Tb3+ and

Eu3+ co-doped alumina films showed sensitised Eu3+ luminescence by energy transfer from Tb3+

Fig. 30 shows PL spectra of Tb–Eu co-doped alumina films at a fixed 1 mol% Eu and various Tb

concentration.

Lu and Jagannathan [43] in 2002 synthesised Y3Al5O12:Ce3+ (YAG) nanophosphor by sol–gel

pyrolysis. Stoichiometric amounts of aqueous solutions of yttrium, aluminium, cerium and barium

were taken and reacted with equivalent quantity of urea in presence of polyvinyl alcohol at 150 8C for

2 h under intense homogenisation. Gelling was achieved at 250 8C for another 2 h. Heat treatment at

800–1100 8C for 2–6 h yielded the required phosphor. Nanosize of the phosphor was confirmed by

XRD as a small shift towards higher 2u values is observed with respect to the standard pattern. In this

study, samples with compositions Sample A—(Y0.95Ce0.05)Al5O12, Sample B—(Y0.85Ba0.10Ce0.05)-

Al5O12, Bulk sample C—(Y0.95Ce0.05)Al5O12, fired at 1450 8C for 6 h. Fig. 31 shows PL spectra of the


Fig. 29. PL intensity vs. deposition time. Bath conditions zinc acetate dihydrate 1.5 M, thiourea 3 M Ammonia 14 M andgrowth temperature 88 8C (from Ref. [40]).

samples at 460 nm excitation. For reflectance spectra, a blue shift of about 720 cm-1 has been observed

in case of (Y0.95Ce0.05)Al5O12 nanoparticles with respect to submicron powders.

Yang et al. [44] in 2002 reported lowering of synthesis temperature of doped SrAl2O4 solid-state

reaction if the precursor was made in nanophase by sol–gel method. The precursor was made by

dropwise reacting mixed aqueous solution of strontium acetate, aluminium chloride and acetates of

dopants Cu, Dy and ammonia solution. Slow gelling and then drying gave xerogel which was heat

treated at 1200 8C for 2 h. PLE and PL studies were performed of the samples. The excitation and

emission spectra of the CuDy:SrAl2O4 sample are almost the same as those of the Dy:SrAl2O4 sample.

As shown in Fig. 32, PL intensity of sample with double dopant, i.e., Cu and Dy is much greater than

the sample with single dopant Dy. Excitation spectra of different samples and emission spectra of Cu

doped sample establish that there is sensitised transfer from Cu2+ ions to Dy3+ ions in the lattice.

Qu et al. [45] in 2002 prepared ZnS:Eu3+ nanocrystals in water methanol solution. Binary

solution of water/methanol with 30% methanol by volume of zinc chloride and sodium sulfide were


Fig. 30. PL spectra of Tb3+–Eu3+ co-doped alumina films at 1 mol% Eu concentration and varying concentrations of Tb(from Ref. [42]).

Fig. 31. PL spectra with excitation at 460 nm of Ce3+ (5 mol%) in YAG samples prepared under different heating conditions:(i) Sample A 1000 8C/4 h; (ii) Sample B 1000 8C/4 h; (iii) 800 8C/4 h and 1100 8C/6 h; (iv) bulk 1450 8C/6 h (from Ref.[43]).

mixed dropwise under vigorous stirring. a-Methacrylic acid was used as stabilizer and capping agent.

Dopant Eu was also added to ZnCl2 solution. Refluxing at 85 8C gave stable nanophosphor.

Centrifugation and drying at 50 8C was carried out to get dried product. XRD and HRTEM established

nano character of the prepared phosphor. Eu concentration was varied between 0 and 5 mol%. Four

samples a, b, c and d with 0, 1, 2 and 5% Eu, respectively, were studied for PL and the spectra for the

same is shown in Fig. 33. Partially overlapping twin peaks is observed.

Ebenstein et al. [46] in 2002 reported fluorescence quantum yield of CdSe/ZnS core/shell

nanophosphors synthesised by them. The particles were made by following the route of high

temperature pyrolysis of organometallic precursors in coordinating solvents. Alternatively, some

samples were prepared with trioctylphosphine oxide (TOPO) and hexadecyl amine capping. Corre-

lated atomic force and fluorescence microscopy were used in this work to study single particle versus

ensemble fluorescence quantum yields (QY) of semiconductor nanocrystals by measuring a simulta-


Fig. 32. PL spectrum of SrAl2O4:Cu,Dy sample and SrAl2O4:Dy sample (from Ref. [44]).

neous map of the topography and the single particle fluorescence. A significant portion of dark

particles is detected. Comparison with the ensemble solution QY shows that samples with higher QY

have a larger fraction of bright particles accompanied by an increased single particle QY. Saturated

emission from single nanocrystals could not be detected because of particle darkening under high

power excitation.

Nandakumar et al. [47] in 2002 synthesised CdS quantum dots in polymer matrix Nafion

following ion exchange reaction. Nafion 117@ sheets are first cleaned by boiling these in70% HNO3

for 30 min and it gives the films free from inorganic impurities. Then thorough washings by D I water

are given. Films are then soaked in cadmium acetate solution and then dried under vacuum for 1 h.

Films are treated with ammonia gas for half an hour, which render passive the surface of quantum dots.

Films dried again in vacuum and then treated with H2S gas. Films contain CdS quantum dots as

confirmed by XRD with sizes ranging from 1.4 to 6 nm. PL spectra of different samples with excitation

at 350 nm are shown in Fig. 34. There are two emission bands in all the samples. The one at higher

energy is more pronounced and closer to the band edge. It shows blueshift with size. Second band is

seen at lower energies and intensity is low.

Cao et al. [48] reported in 2002 preparation of core–shell ZnS:Mn/ZnS nanoparticles and studied

enhancement of luminescence. A microemulsion of aqueous solution of Zn and Mn acetates with

0.04 M AOT heptane solution was prepared. H2S gas was introduced to form ZnS:Mn and then

nitrogen gas passed through the solution to drive off any excess H2S. Then calculated amount of Zn2+

ion solution was introduced into the microemulsion and H2S was passed again to give coat of ZnS on

ZnS:Mn. PL spectra for two type of samples one with coating of ZnS and other uncoated has been


Fig. 33. PL spectra of ZnS and ZnS:Eu3+ nanocrystals with different Eu concentrations: (a) 0, (b) 1%, (c) 2%, (d) 5% (fromRef. [45]).

compared. Coated sample has nearly eight times more intensity for Mn transition whereas the part of

emission due to host lattice is comparable. But when emission spectra is compared after exposing both

samples to exposed to 305 nm radiations for 1 h intensity of uncoated samples increase for both peaks

and there is not much change in coated sample. PL intensity in case of uncoated sample increases with

time of exposure to reach a saturation whereas in case of coated sample it decreases by �15% shown

by Fig. 35 and both seem to stabilize at nearly equal intensity.

Pan et al. [49] in 2003 reported synthesis of Pr3+ doped CaTiO3 nanophosphor and red

luminescence from polymer precursor. Mixed solution Ca and Pr nitrate was added dropwise into

solution of n-titanium butoxide in citric acid and ethylene glycol kept under constant stirring. Then the

solution was heated at 200 8C for 5 h. After removal of solvent by continued heating, dark brown

glassy resin like mass was obtained. Dark solid lumps are formed upon further heating at 400 8C which

ground to powder form and the powder is heated in air at 600 8C or above for 5+ h to get the

nanophosphor. Particle size as calculated by Scherrer’s equation has been around 10 nm. Thermo-

gravimetric (TG) analysis done on precursor material and is shown in Fig. 36 indicates that at 600 8Cnetwork of organic molecules is completely broken and most of the volatiles are removed. XRD


Fig. 34. PL spectra (normalised) of CdS quantum dots at 350 nm excitation. The spectra are shifted along the Y-axis forclarity (from Ref. [47]).

Fig. 35. Variation of luminescence intensity at 580 nm with exposure time under 305 nm irradiation for (a) uncoatedZnS:Mn2+ nanoparticle and (b) ZnS:Mn2+ nanoparticle coated with ZnS (from Ref. [48]).

establishes formation of CaTiO3:Pr nanophosphor. Higher temperature and longer time firing increase

particle size of the phosphor and thereby increase PL intensity of the material. Figure shows PLE and

PL spectra of the phosphor. The emission at 612 nm is in agreement with the phosphor prepared using

alternate technique of solid-state reaction.

Synthesis and photoluminescent properties ZnS nanocrystals doped with Cu and halogen was

reported by Manzoor et al. [50] in 2003. Usual colloidal precipitation route with organic surfactant

PVP was employed for synthesis. Aqueous solutions of zinc sulfate, Cu(II) acetate and sodium sulfide

were the main starting chemicals. Sodium sulfite was employed to reduce Cu(II) to Cu(I). Cen-

trifugation and washing with DI water and drying were done to obtain the nanophosphor ZnS:Cu. The

particle size estimate by XRD and TEM was reported around 2 nm. PLE and PL spectra have been

recorded for different samples prepared by the method. Samples that have been studied are undoped

ZnS, halogen doped ZnS, ZnS doped with varying amount of Cu and fixed amount of F, ZnS doped

with fixed amount of Cu (0.01%) and varying amounts of F (0.5–20%) and ZnS:1%Cu,10%F with

different amount of sulfide used. For undoped ZnS, emission is reported to be peaked at 434 nm with a

shoulder at 464 nm (Fig. 37) with excitation peak at 319 nm. The emission called self-activated is

attributed to sulfur vacancies contrary to Zn vacancy related activated luminescence in bulk ZnS. The

authors have studied undoped ZnS and some other ZnS:Cu,F samples with variation in stoichiometry,

i.e., X defined as ratio of sulfur ions to zinc ions in starting composition has been varied from 0.5 to 1.5.

Interestingly, there is no change in spectra except for change of PL intensity, maximum being for

X = 0.5 to minimum for X = 1.5. It seems their lattice is varying from either from a mixed lattice of

ZnO and ZnS to a polysulfide of Zn or it is ZnS with sulfur/zinc vacancies as claimed. In latter case,

one has to assume that excess of soluble salts are going to drain as washings. In case of halogen doped

ZnS, different halogen doping with concentration variation from 0.5 to 20% has been tried. There is

almost no change in emission spectrum as compared to undoped ZnS samples. This shows that halogen

doping do not significantly change the sulfur vacancy related self-activated luminescence. Thereby, it

is observed that the sulfur vacancy related luminescence is quite efficient. The sample of

ZnS:Cu0.01%,xF have PL spectrum with two peaks at 434 and 472 nm. Peak at 472 nm becomes

more pronounced as F content is increased and is due to excitation at 375 nm. In case of samples with

fixed concentration of F at 10% and varying Cu from 0.05 to 1.5%, the excitation band at 375 nm

increases to 411 and emission peak shifts from 472 to 498 nm. The authors report further increase in

wavelength of emission peaks with increase in ratio of S2� to Zn2+ ions in starting composition. The

authors have also tried to explain the various emission peaks on the basis of observed excitation spectra


Fig. 36. PLE (a) at lem = 612 nm and PL (c) spectra at lex = 340 nm of Pr3+ doped CaTiO3, (b) shows enlarged PLE in therange for clarity (from Ref. [49]).

and possible creation of different states within band gap due to formation of nanophase. However, the

paper reports that emission from 434 to 514 nm can be achieved in ZnS:Cu nanophosphor by slight

variation in preparation parameters.

Yang and Holloway [51] in 2003 synthesised CdS:Mn/ZnS core/shell quantum dots and found

substantially enhanced PL to CdS:Mn quantum dots prepared by other techniques say organic capping.

Basic technique was based on reverse micelle method. Mn doped CdS core nanocrystals were formed

by mixing (Cd2+ + Mn2+) and S2� containing micellar solutions rapidly for 10–15 min. Then Zn2+

containing micellar solution was added at a slow rate into the core forming micelle. A surplus of sulfur

ions was maintained for proper growth of ZnS shell. PLE and PL spectra of n-dodecanethiol capped

CdS:Mn and ZnS capped CdS:Mn have been compared and shown in Fig. 38. Both spectra are peaked

for ZnS capped CdS:Mn. Thus it has been concluded that surface passivation in case of ZnS capping is

more effective than n-dodecanethiol capping.

Wang et al. [52] in 2003 synthesised cubic nanocrystalline Y2O3:Tb phosphor by combustion

technique and studies luminescence enhancement under various conditions. Aqueous solutions of

Y(NO3)3, Tb nitrate and glycine were taken in appropriate amounts and concentrated by heating to a

thick paste. The paste was subject to further heating with rising temperature till the mass got ignited

spontaneously. Fine powdered phosphor was obtained whose particle size depended upon proportion

of glycine used. The particle size estimated by XRD was in the range of 35–70 nm. Interesting feature

of the study is increase in emission intensity in the nanophosphor formed with time of irradiation of

samples with 250 nm radiation as also shown in Fig. 39. The authors have shown with the help of ESR

studies that the irradiation passivates Y–O surface dangling bonds.

Heo et al. [53] in 2003 reported preparation of nanorods of ZnMgO employing catalysis-driven

molecular beam epitaxy and measured optical properties. In this, Mg doped ZnO nanorods were

deposited on Ag coated silicon substrate. An ozone/oxygen mixture was used as the oxidizing source.

The nanorods were 15–40 nm in diameter and about 1 mm in length is formed and it has been

confirmed by TEM and SEM. PL spectra of samples grown with substrate temperatures of 400 and


Fig. 37. PLE at lem = 434 nm and PL spectra at lex = 319 nm of undoped ZnS nanoparticles. Inset: PL spectra of undopedZnS showing reduction in blue emission intensities with stoichiometric variations of X = [S2�/Zn2+] (from Ref. [50]).

500 8C has been recorded and shown in Fig. 40. The emission has been attributed to the radiative

recombination of photogenerated holes with electrons occupying the oxygen vacancy.

Natter and Hempelmann [54] in 2003 developed process for tailor-made nanomaterials using

electrochemical methods. The crystallite size of the nanoparticles can be controlled by variation of

physical and chemical parameters. Pulsed electrodeposition and DC-plating procedures have been

employed for preparation of catalyst films. Large quantities of nanostructured metal oxides were

reportedly prepared with electrodeposition under oxidizing conditions that is based on the reduction of

metal ions generated from the anodic dissolution of a sacrificial anode with subsequent oxidation

of the formed metal crystals. Using the technique, many metal oxides and mixed oxides such as


Fig. 38. PL (a) lex = 400 nm and PLE (b) spectra of unpassivated, n-dodecanethiol capped lem = 570 nm and ZnS cappedCdS:Mn nanocrystals lem = 585 nm (from Ref. [51]).

ZnO, Mn3O4, CuO, In2O3, In2O3/SnO2 and others have been prepared at a rate of 1 kg per day.

Preparation of nanophosphors in bulk amounts should be possible with the process after some

modifications.

Anh et al. [55] in 2003 reported preparation, optical properties and application potential of two

nanophosphors, Y2O3 and SiO2–TiO2/ZrO2 doped with various rare-earths. Y2O3 phosphor was

prepared by combustion method from carbonates not from nitrates that are normally employed. The

other phosphor thin film was prepared via hydrolysis and condensation of tetraethoxysilane,

Ti(OC3H7)4 and Zr(OC3H7)4. Si substrate and spin coating was employed for film making. Particle

size ranged from 4.4 to 72 nm. PL spectrum of yttrium oxide samples doped with different proportions

of Eu/Tb is shown in Fig. 41 and ratio of 8:2 had best luminescence yield.

Water-soluble CdS nanoparticles were synthesised by Chory et al. [56] in 2003 and studied for

structure and spectroscopic properties. Aqueous solution of glutathione and cadmium chloride was

taken and tetramethylammoniumhydroxide and ethanol were added. The composition was kept stirred

and hexamethyldisilathiane was added rapidly. Slightly yellow glutathione coated CdS colloidal

particles resulted. After 1 h, adding tetrahydrofuran precipitated the particles. The particles of CdS are

re-dispersible in water. The size of the particles is about 2.3 nm, with CdS core of 1.3 nm and an

organic shell of 0.5 nm thickness which is thermally stable up to 200 8C. Absorption and PL spectra of

the stabilized CdS particles which were redispersed was recorded and is shown in Fig. 42. Excitation

was by argon ion laser (361 and 351 nm) of varying power. PL peak shift towards shorter wavelength is


Fig. 39. PL spectra before and after irradiation at 250 nm for 40 min at lex = 275 nm of Y2O3:Tb having 35 nm size. Inset:Dependence of PL intensity at 542 nm on irradiation time (from Ref. [52]).

Fig. 40. PL spectra for ZnO rods grown at 400 and 500 8C (from Ref. [53]).

observed with increase in excitation energy power that is explained on the basis of extrinsic states

created because of in complete surface passivation.

Gu et al. [57] in 2003 prepared SnO2:Dy nanophosphor and reported its luminescence char-

acteristics. Sol–gel route was adopted. Aqueous solutions of tin chloride and dysprosium nitrate were

mixed and to this ammonia solution was added dropwise. Opal gel obtained was washed with distilled

water and dried at 80 8C. The powder was heat treated at 400, 500 and 600 8C for 2 h. Particle size of

the nanophosphor as estimated from XRD and Debye–Scherrer equation was around 2.8 nm and was

about 3 nm as observed by TEM. Absorption, PLE and PL spectra of pure SnO2 and SnO2 doped with

varying amounts of Dy were recorded and are shown in Fig. 43. Dy is exhibiting prominent emission at

575 nm due to 4F9/2 ! 6H13/2 transition. Optimum concentration of Dy was found to be 1.5%.

Tissue and Yuan [58] in 2003 reported Y2O3:Eu3+ nanophosphor via gas phase condensation.

Dependence of structure on annealing conditions and corresponding changes in luminescence spectra

were also studied. CO2 laser under vacuum of 10 and 400 Torr of nitrogen for two different samples

heated sintered pellet of Y2O3:Eu3+. Condensed samples were collected on stainless steel cone placed

3.5 cm above heated spot. Sample prepared at 10 and 400 Torr had particle size of 5 and 13 nm,

respectively. Annealing at 800 8C for long duration �24 h increased size by a factor of 1.5–2. The

crystal structure changed from mixed phase to single phase cubic in case of 5 nm particles prepared at

10 Torr and in case of 13 nm 400 Torr sample annealing did not significantly change the monoclinic

crystal structure. This was established by excitation spectra as indicated by Fig. 44.


Fig. 41. PL spectra of Y2O3:Tb,Eu with mole ratio of Eu/Tb: 7/3, 8/2, 9/1 at lex = 337.1 nm (from Ref. [55]).

Fig. 42. Absorption and PL spectra of glutathione-stabilized CdS nanoparticles at room temperature. Excitation by argon ionlaser of 364 and 351 nm with 0.30 mW power (from Ref. [56]).

Synthesis and PL of nano-Y2O3:Eu3+ phosphor has been reported by He et al. [59] in 2003. Wet

chemical method has been followed. Chloride solutions of yttrium and Eu were mixed in proper

proportion with butanol which acted as surfactant. To this, 0.3 M solution of sodium carbonate was

added slowly with constant stirring. Samples were made at different pH, washed and dried at 60 8C.

The desired nanophosphor was obtained after firing at 800 8C for 1 h in air. Size of the samples was

around 50 nm. Particle size of phosphor obtained was compared in case of samples prepared with

surfactant and without surfactant and size with surfactant was smaller. Particle size of prepared

samples increased with pH of the reaction composition. PL intensity of sample was maximum with

4% Eu.


Fig. 43. Absorption (a), PLE (b) and PL spectra for pure SnO2 sample and PL spectrum of Dy3+ doped SnO2 sample (d) withDy concentration = 1.5% (from Ref. [57]).

Fig. 44. Broad band PLE spectra of as-prepared and annealed 0.1% Eu3+:Y2O3 nanoparticles. 5D0 ! 7F2 luminescence wasmonitored at 612 nm with a bandpass of 6 nm to include all phases (from Ref. [58]).

Zhang and Li [60] prepared ZnO nanoparticles by precipitation transformation method. Solution

of zinc sulfate was reacted with sodium carbonate solution with vigorous stirring. To this, NaOH

solution was added dropwise. The reaction was performed at temperatures from 25 to 70 8C. Washing,

filtration and drying followed to obtain powder ZnO nanoparticles. Zn5(CO3)2(OH)6 was formed as

intermediate and phase transformation in presence of NaOH solution led to formation of ZnO.

Lan et al. [61] in 2003 reported synthesis of ZnS nanorods by annealing precursor ZnS

nanoparticles in NaCl flux. ZnCl2 and Na2S were ground and then mixed with C16H37O(CH2-

CH2O)10H and ground for 30 min and washed with distilled water in ultrasonic bath and then with

ethyl alcohol and dried at 60 8C. The obtained nano ZnS precursor was mixed with NaCl five time by

weight and heated at 800 8C for 2 h in air. The sample was washed and dried and was found to be

nanorod ZnS with 40 nm diameter and length in many micrometers.

Lee et al. [62] studied in 2004 effect of synthesis temperature on particle size/shape of ZnS:Cu

nanocrystals. For synthesis, aqueous solutions of thiourea, urea and triethanolamine were mixed using

magnetic stirrer. To this, Zn and Cu acetate solution was added dropwise. Various synthesis

temperatures from 70 to 95 8C were used. Particle size increased with increase in synthesis

temperature. There is no change in crystal structure with increase of synthesis temperature and it

remains cubic. Most suitable temperature of synthesis was found to be 85 8C. PL spectra measured at

different synthesis temperature is shown in Fig. 45 and from XPS studies it was concluded that Cu is

transformed to CuO at synthesis temperatures >90 8C.

Karar et al. [63] in 2004 prepared nanocrystalline (Zn/Mn)S using polyvinylpyroledone capping

employing chemical method. Requisite amounts of Zn and Mn acetate solutions were taken in iso-

propanol medium and polyvinylpyroledone was added to control particle size. Sodium sulfide solution

was used to provide sulfur ion for the reaction. The reactants were stirred for 6 h and then reactants

allowed to settle. Subsequently precipitates were centrifuged, washed and dried. Mn was varied from 0

to 40%. Particle size of the samples made was �2 nm as determined from XRD analysis. TEM image

of a typical sample (Fig. 46) shows a narrow particle size distribution with peak around 4 nm. PL

spectra of the samples with 0, 10, 20, 30 and 40% Mn was recorded and is shown in Fig. 47 with

deconvoluted peaks. Mn concentration of 20% was found to be optimum from PL intensity data.

Deconvoluted peaks were at 463, 603, 640 and 692 nm and were attributed to higher Mn content and

Mn–S related interaction.

Morita et al. [64] in 2004 prepared metal ion doped silica glasses by sol–gel technique and studied

the luminescence properties of these nanophosphors. Tetraethooxysilane (TEOS), diethoxydimetyl-

silane, metal nitrate (1–0.5 mol%) were dissolved in a solution of water and ethyl alcohol and the

solution was stored at room temperature. Xerogel was obtained after 2–3 months as wet solid blocks,

which, after annealing at 600 8C for 1 day, turned into transparent, and isotropic sol–gel glasses

containing dopants and their PL and time-resolved spectra were analysed. PL spectra recorded by N2

laser excitation for a typical sample SiO2:Cr at 10 K is shown in Fig. 48. In other sample of ZnS:Cu,Al

in sol–gel matrix, though the PL is similar to bulk ZnS:Cu,Al phosphor but luminescence decay profile

shown in Fig. 49 gives lifetime of 355 ps at 10 K attributed to nanophase formation within the sol–gel

matrix.

Nanosized spherical particles of doped yttrium oxysulfide were synthesised by Pires et al. [65] in

2004 and studied for up-conversion luminescence. Nitrate solution of constituent elements in required

proportion was taken; citric acid, ethylene glycol and urea was added; heated with stirring until resin

carbonisation resulting in basic carbonate precursor. Next sulfurization process was carried out with

sulfur vapour and argon gas at 750 8C for 4 h. It yielded well crystalline Y2O2S:RE nanophosphor in

size range of 15–20 nm. Nanophosphors Y2O2S:4%Yb,0.1%Er and Y2O2S:4%Yb,0.1%Tm were

made and studied for up-conversion PL results are shown in Fig. 50. With excitation at 280 nm, Er



Fig. 45. PL spectra and PL intensity (a and b) of ZnS:Cu solution grown samples plotted as a function of synthesistemperature (from Ref. [62]).

Fig. 46. A representative TEM picture (magnification: 120,000�) showing particle size distribution in ZnS:Mn nanopho-sphor (about 4 nm) (from Ref. [63]).


Fig. 47. Room Temperature PL spectra of (a) ZnS: 0% Mn, (b) ZnS: 10% Mn, (c) ZnS: 20% Mn, (d) ZnS: 30% Mn and (e)ZnS: 40% Mn (from Ref. [63]).

Fig. 48. PL spectrum of SiO2:Cr sol–gel glass at 10 K in the near infrared region. The sample was prepared at 1300 K underoxygen gas flow (from Ref. [64]).

doped samples emit in green and red due to 2H11/2, 4S3/2 ! 4I15/2 and 4F9/2 ! 4I15/2, respectively. For

Tm doped samples strong blue and red emission is obtained due to 1H4 ! 3H6 and 1G4 ! 3F4. The

results have been compared with samples made using carbonate salts as starting material and the

present method has shown better luminescence characteristics.

Yan and Zhou [66] in 2004 prepared YxGd2�xO3:Eu3+ nanophosphor via sol–gel technique using

different set of chemicals. Salicylic acid was dissolved in 95% ethanol and pH adjusted to 7 with

ammonia solution. Then nitrate solutions of rare-earths in required ratio were added, mixed

thoroughly. Now solutions of urea and PVA were added and heated to 60 8C and pH adjusted to

alkaline side. Solutions were then heated to 100 8C for drying. The precursor thus obtained was heated


Fig. 49. PL spectra of ZnS:Cu,Al as bulk powder (I) and nanocrystals in xerogel (II) at room temperature under N2 laserexcitation (from Ref. [64]).

Fig. 50. PL spectra under 980 nm excitation (Yb3+ 2F7/2 ! 2F5/2 transition) and room temperature of (a) Y2O2S:Yb(4%),Er(0.1%) and (b) Y2O2S:Yb(4%), Tm(0.1) (from Ref. [65]).

to 800 8C to get the nanophosphor of 20–70 nm particle sizes. The samples made by the technique

were compared with samples with same procedure except using salicylic acid. Composition with

x = 0.4 gave the best luminescence intensity and as shown in Fig. 51 both types of samples had

identical PL spectrum and comparable intensity.

Harish et al. [67] reported in 2004 synthesis of nanophosphor crystals of long persisting

SrAl2O4:Eu2+, Dy3+ by modified combustion technique. In this technique, mixture of nitrates of

strontium, aluminium and the rare-earths with water, added boric acid and urea and the container

placed in a partially closed ceramic/quartz tube and heated from 400 to 600 8C. Combustion reaction

started with bright yellow flame and continued only for few seconds. Tube was taken out of furnace.

Low-density mass of the nanophosphor less than 50 nm in size was obtained. The samples obtained by

combustion method were compared with bulk samples made by solid-state reaction technique. SEM

micrographs, luminescent spectra and decay profile of both the samples are shown in Figs. 52–54,

respectively. Higher excitation energy and lower decay times are attributed to quantum confinement of

dopants because of nanostructure formed in the samples.

In 2004, Manzoor et al. [68] prepared two types ZnS based nanophosphors with pyridine and

polyvinyl pyroledone as organic capping agents and elaborated on role of capping agents in energy

transfer. Method involving wet chemical colloidal precipitation was employed. Nanophosphors of

ZnS:Cu, ZnS:Cu,Al, ZnS:Mn were prepared. Interesting feature of the study has been PLE spectra of

samples capped with pyridine, denoted as P–ZnS, capped with different amounts PVP denoted as

PVP–ZnS and bulk ZnS. PLE spectra (Fig. 55) becomes broader with increase in PVP concentration

and on deconvolution as many as five band are resolved. Hence, PL spectra are accordingly effected.

PVP itself shows emission (Fig. 56) around 430 nm, thereby leading to possibility of energy transfer.

Haranath et al. [69] in 2004 from author’s group reported controlled growth of ZnS nanophosphor

in porous silica matrix in an effort to replace organic capping agents with inorganic ones which are

more stable and robust. Synthesis involved preparation of silica alcogel from tetraethylorthosilicate

with ethanol and water as diluents and hydrochloric acid as catalyst. Aqueous solutions of zinc and

manganese were added in colloidal solution of silica before gelling followed by addition of Na2S

solution. Subsequently the solution was cast into glass vials for gelation and then dried at 50 8C.

Samples with varying ratios of ZnS and SiO2 were prepared and their annealing at different

temperatures was studied. XRD analysis indicated crystallite sizes ranging from 5 to 7 nm and

the patterns as shown in Fig. 57 indicate superimposition of pattern nano ZnS on the pattern of silica.

PLE and PL spectra of sample were recorded after annealing at different temperatures and are shown

in Fig. 58. Crystal size is increased and band gap have decreased as annealing temperature is raised.


Fig. 51. PL intensities of Y0.4Gd1.6O3:Eu3+; (A) RE(NO3)3–PVA and (B) [RE(sal)3]n–PVA precursors (from Ref. [66]).

It is found that ZnS goes to wurtzite phase after annealing at 900 8C, which has been reported for the

first time.

Another effort for inorganic capping agents, Karar et al. [70] in 2004 again from author’s group,

synthesised and reported studies on ZnS:Mn nanophosphor capped with ZnO. The nanophosphor was

prepared by reacting acetates of zinc and manganese with sodium sulfide. Thereafter, in the same

reaction medium zinc acetate along with ammonia solution were reacted leading to formation of zinc

hydroxide, i.e., ZnO on drying on zinc sulfide. Capping was confirmed by TEM micrography Fig. 59

and XPS with sputtering Fig. 60. PL spectra of samples with different level of ZnO coating have been

shown in Fig. 61. Spectra have been found to be more narrow, i.e., sharper peak as compared to PVP

capped sample with same concentration of Mn.

Huang and Yan [71] reported in 2004 synthesis of Gd2SiO5:Tb nanophosphor using polymer

precursors. They selected rare-earth coordination polymers of salicylic acid as the precursors of

corresponding rare-earth oxides for their infinite chain-like polymeric structure similar to organic

polymer template. Polyacrylamide was used as dispersing medium and the hybrid polymeric precursor

were assembled with other functional components such as tetraethylorthosilicate, urea for fuel. Thus,

polybasic hybrid polymeric precursors were assembled and nanometer gadolinium silicate phosphor

was obtained by thermolysis at 850 8C for 4 h. Particles of the phosphor synthesised were in the range

of 40–100 nm in size. PL spectrum of the nanophosphor prepared was recorded and is shown in Fig. 62


Fig. 52. SEM micrographs of phosphors prepared by (a) solid-state reaction (SRA) and (b) combustion process (SRAC)(from Ref. [67]).


Fig. 53. PLE (dotted line) and PL (solid line) spectra of (a) SRA and SRAC phosphors (from Ref. [67]).

Fig. 54. Decay pattern of SRA and SRAC phosphors (from Ref. [67]).

Fig. 55. PLE spectra corresponding to dopant related emissions from ZnS phosphors (a) Bulk ZnS, (b) Pyridine capped-ZnSnanoparticles, (c–h) PVP capped ZnS nanoparticles with increase in PVP concentration. (i) Deconvoluted PLE of 1% PVP–ZnS showing presence of multiple excitation bands in PVP capped nanoparticles. Schematic: Illustration of PVP capped ZnSnanoparticles (from Ref. [68]).

and is found to have characteristic green emission of terbium. This indicates that nanophosphor of

Gd2SiO5:Tb is successfully synthesised using sol–gel technique.

Summarising, it is observed that field of nanophosphor synthesis is very dynamic. Many

processes such as chemical precipitation with and without capping agents, sol–gel, sol–gel with

heating, microemulsion, solid-state heating, chemical vapour synthesis, hydrothermal synthesis,

chemical synthesis within matrix, molecular beam epitaxy, electrochemical route, autocombustion,

chemical precipitation from homogeneous solution have been developed for synthesis of nanopho-

sphors. Among these chemical precipitation with capping of organic polymers has been studied most.

Perhaps it is because of simpler experimental set-up required. Commercial viability of the technique

may be low due to costly capping agents and processing difficulties of finished nanophosphors for

display devices. The processes of sol–gel, sol–gel with heating, microemulsion, chemical vapour

synthesis, molecular beam epitaxy, autocombustion are not likely to be economically and envir-

onmentally friendly as these are chemical and energy intensive. Chemical precipitation with capping

using inorganic compounds, chemical precipitation from homogeneous solution, synthesis within a

matrix, electrolysis based process, hydrothermal synthesis are the techniques that are likely to be


Fig. 56. (a) PLE spectrum of PVP solution at 430 nm emission. (b) PL spectrum of PVP solution. (c) PL spectrum of PVP–ZnS:Mn under 235 and 310 nm excitations. Inset: increase in integrated PLE intensity with (i) PVP concentration and (ii) Mnconcentration (from Ref. [68]).

Fig. 57. XRD patterns of the pure silica (X) and nanophosphors with ZnS/SiO2 molar ratios of 3.11 � 10�4 (Y) and1.5 � 10�1 (Z) (from Ref. [69]).


Fig. 58. Temperature dependence of the PL of a ZnS:Mn nanophosphor with a ZnS/SiO2 molar ratios of 3.11 � 10�4

annealed at in the range of 400–900 8C. Dotted lines and solid lines represent PLE and PL spectra, respectively (from Ref.[69]).

Fig. 59. (a) Representative XRD pattern for ZnS:Mn/ZnO sample (inset shows the fitted peaks for ZnS and ZnO; (b)Representative TEM micrograph showing the particle with capped film (magnification: 89,000�) (from Ref. [70]).


Fig. 60. Representative XPS spectra of ZnS:Mn/ZnO (for 50% ZnO capping) showing relative change in counts of theoxygen/sulfur ratio with sputtering time. Inset shows a schematic model of ZnO capped ZnS:Mn samples and effect ofsputtering (from Ref. [70]).

Fig. 61. PL spectra of ZnS:Mn/ZnO samples. Inset shows he corresponding optical absorption pattern (from Ref. [70]).

Fig. 62. PL spectrum of nanometer Gd2SiO5:Tb3+ by [Gd(sal)n]–PAN precursors (from Ref. [71]).

technologically acceptable. Nanophosphors prepared using the processes include a wide variety. The

list, which by no means can be complete, has ZnS:Mn, ZnS:Cu, ZnS:Cu,Al, YAG, YVO4, LaPO4,

Al2O3, Y2O3:Eu, ZnO, ZnS:CuRE, SrAl2O4, CdS, ZnS:Mn/ZnS and CdS:Mn/ZnS in core/shell form,

ZnMgO, SnO2:Dy, (Zn + Mn)S, ZnO capped ZnS:Mn, ZnS:Mn in SiO2 matrix and Gd2SiO5:Tb.

ZnS:Mn has received maximum attention of researchers. Early success of ZnS:Mn nanophosphor

prepared by Bhargava et al. with increased efficiency and spectrum shift has been the inspiration for

the large quantum of work in the field. With science and art of synthesising nanophosphors having

been mastered, attention is gradually shifting to fabrication of display devices. A number of reports

have appeared on nanophosphors based photoluminescent as well as electroluminescent (EL) displays

with improved features [72–78] of higher resolution, lower dose rates and low voltage EL operation

(�10 V). The work on the devices is still at laboratory scale requiring efforts to streamline reliability

and reproducibility aspects but multicoloured displays based on low voltage EL may replace plasma

display panels and field emission display devices in near future.

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Documents

Development of nanophosphors—A revie · 114 H. Chander/Materials Science and Engineering R 49 (2005) 113–155. Fig. 1. The PL is slightly shifted and there is a larger linewidth