Tuning and Enhancing White Light Emission of II−VI BasedInorganic−Organic Hybrid Semiconductors as Single-PhasedPhosphorsXiaoming Fang,†,‡ Mojgan Roushan,† Ruibo Zhang,† Jing Peng,‡ Heping Zeng,‡ and Jing Li*,†,‡

†Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854, United States‡Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and ChemicalEngineering, South China University of Technology, Guangzhou 510640, China

*S Supporting Information

ABSTRACT: Single-phased white light emitters made of semiconductor bulkmaterials are most desirable for use in white light-emitting diodes (WLEDs)based on both photoluminescence and electroluminescence. Here wedemonstrate Cd and/or Se substituted double-layer [Zn2S2(ha)] (ha = n-hexylamine) hybrid semiconductors emit bright white light in the bulk formand their emission properties are systematically tunable. The ternaryZn2−2xCd2xS2(ha) hybrid compounds exhibit two photoluminescence (PL)emission peaks, one of which being attributed to band gap emission, and theother resulting from Cd doping and surface sites. The Cd concentrationmodulates the optical absorption edge (band gap) and the positions of the twoemission bands along with their relative intensities. The ZnS-based hybridstructures (with a nominal Cd mole fraction x = 0.25) emit bright white lightwith significantly enhanced photoluminescence quantum yield (PLQY)compared to its CdS-based hybrid analogues. For the quaternaryZn2−2xCd2xS2−2ySe2y(ha) compounds (x = 0.25 and different nominal Se mole fractions y) the synergetic effect betweendoped Cd and Se atoms leads to further tunability in the band gap and emission spectra, yielding well balanced white light of highquantum yield. Detailed analysis reveals that the PL emission properties of the ternary and quaternary hybrid semiconductorsoriginate from their unique double-layered nanostructures that combine the strong quantum confinement effect and largenumber of surface sites. The white-light emitting hybrid semiconductors represent a new type of single-phased phosphors withgreat promise for use in WLEDs.

KEYWORDS: II−VI semiconductor, inorganic−organic hybrid material, light-emitting diode (LED), white light emission

■ INTRODUCTIONIllumination shares no less than 20% of civilian electric energyconsumption, which accounts for 1.9 GT of CO2 emissions.

1

The ever-increasing energy demands and the concerns of globalwarming are pressing for the development of high-efficiencylight sources to reduce energy consumption. Solid-state lighting(SSL) in the form of light-emitting diodes (LEDs) can convertelectricity to light much more effectively than conventionallighting sources. It has been predicted that a nation-wide movetoward SSL for general illumination in the US could save 32.5quads of primary energy between 2012 and 2027.2 Therefore,high efficiency LEDs are being explored intensely, especiallywhite LEDs (WLEDs), which have been considered to be apotential light source to replace conventional lighting systemssuch as fluorescent lamps and incandescent bulbs.3−5

There are currently two major WLED systems: multichipWLEDs and one-chip WLEDs.1 In the multichip WLEDs,white light is created by combining three LED chips with colorsof red (R), green (G), and blue (B), respectively. Since eachLED requires a power source, and each source has its own

specific lighting characteristic, balancing their luminousintensity to obtain an even color mixture is a challenging taskand often results in inadequate illumination.6 In addition, RGBmultichip LEDs are most expensive. Different from themultichip WLEDs, the one-chip WLEDs consist of a LED(blue, near-ultraviolet, or ultraviolet) and phosphors, namelyphosphor-converted WLEDs (pc-WLEDs). The first commer-cialized pc-WLEDs are constructed by combining the blueInGaN chip with the yellow YAG:Ce phosphor,7 in which theblue light from the LED excites the YAG:Ce phosphor to emityellow light, which is subsequently mixed with the blue light togenerate white light. However, these WLEDs have the problemof achieving a high color rendering index of over 85 due to theirred spectral deficiency.6,8 Compared with the RGB WLEDs andblue-YAG WLEDs, the near-ultraviolet or ultraviolet LEDpumped WLEDs fabricated by UV-LED chips coated with

Received: October 19, 2011Revised: April 6, 2012Published: April 13, 2012

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white light-emitting single-phased phosphors or RGB tricolorphosphors may overcome the aforementioned shortcomingsowing to the invisible emission of the LED chip and, thus, havebeen considered an important and promising future direction ofSSL technology.9 Although NUV-LED + RGB phosphorsrepresent one of the best white light assemblies with both highluminous efficiency and high CRI, RGB phosphors obtained bymixing three phosphors with colors of red, green, and blue,respectively, suffer from complex blending of differentphosphors, lack of efficient red phosphors, and self-absorption.9,10 pc-WLEDs with single-phased white-emittingphosphors can eliminate the need of complex color mixing orconversion techniques, enabling easy fabrication with perfectcolor reproducibility, stability, and high efficiency.1 Clearly, thedevelopment of this kind of pc-WLEDs depends on thebreakthrough of the study on single-phased white-emittingphosphors.In recent years, several kinds of single-phased white-light

emitting phosphors have been developed for use in WLEDsincluding organic molecules and inorganic nanomaterials.11−15

Among them, semiconductor nanocrystals (NCs) are anintensively explored group because of their size-dependentoptical and electronic properties, cost-effective solution-basedprocessability, and high quantum yield.16 A large number ofwhite-light emitting NC systems have been synthesized,including ZnS:Pb,17 ZnSe,18 “magic-sized” CdSe,19 Mn-dopedCdS,20 Mn-doped ZnS,21 trap-rich CdS,22 onionlike CdSe/ZnS/CdSe/ZnS,22 and alloyed ZnxCd1‑xSe.

23 In particular, the“magic-sized” CdSe NCs (average diameter less than 2 nm)have become a topic of intensive interest.24−30 Unliketraditional NCs (diameters larger than 2 nm) that exhibitnear-monochromatic band-edge photoluminescence, the magic-sized CdSe NCs emit a broad white light that covers the entirevisible spectrum as a consequence of very high surface-to-volume ratio which leads to a significantly large number ofmidgap surface sites.24,25,28 Most recently, WLEDs based onthe electroluminescence of the magic-sized CdSe NCs that haveexcellent color characteristics and high color rendering indexesare reported.31 However, these WLEDs suffer from very lowluminous efficiency (∼1.0 lm/W).16 The weak correlationsamong the quantum dots make it difficult to achieve highconductivity and mobility required for a LED. Semiconductorbulk materials that have good transport properties and canconvert electricity directly to white light are most desirable.The inorganic−organic hybrid semiconductors built on II−

VI nanolayers are a unique family of semiconductor bulkmaterials32,33 and show great promise to be used as a singlephase white-light-emitting source in a LED configuration.34

These highly crystalline materials are composed of two-dimensional (2D) layers of II−VI semiconductor motifs(inorganic component) that are bonded by amine molecules(organic component) to form perfectly ordered crystal lattices.They not only possess a number of enhanced semiconductorproperties with respect to their parent II−VI binary compoundsbut also exhibit very strong structure-induced quantumconfinement effect (QCE),35,36 to a higher extent than thoseof the smallest colloidal QDs reported to date. Moresignificantly, they represent the first examples of single-phasewhite-light phosphors in bulk form of semiconductor materials.These materials have the following desirable features comparingwith the CdSe NCs: First of all, their crystal structures are well-defined and characterized. Second, the infinite 2D II−VI layersmay provide efficient conduction pathways for the carriers

(electrons and holes), thus, attaining high transport properties.Third, the crystal structures and optical properties can besystematically tuned. Finally these materials can be solutionprocessed at low cost and large scale and in bulk form withease. In an earlier report,34 we showed, as a concept-provingcase study, that a select group of these materials, namelyCd2S2(L) (L = n-proylamine, n-butylamine, n-hexylamine),emit over the entire visible region. However, their quantumefficiencies are very low (∼4−5%). In this work, we focus on aseries of Zn2S2(L) based and Cd and/or Se substituted phases.We have carried out a systematic study to investigate andoptimize their light emitting properties. We demonstrate thatwhite-light emitting ternary Zn2−2xCd2xS2(L) and quarternaryZn2−2xCd2xS2−2ySe2y(L) hybrid systems have significantlyimproved quantum yield and show strong potential for use asa new type of single-phase white light phosphors.

■ EXPERIMENTAL SECTIONMaterials and Synthesis. ZnCl2 (98%), Zn(NO3)2·6H2O (98%),

CdCl2(99%), Se (99.5%), S (99%), n-propylamine (pa, 99%), n-butylamine (ba, 99%), and n-hexylamine (ha, 99%), dimethyl sulfoxide(DMSO, 99.99%), trans-stilbene, and hexane (99.99%) were used asreceived without further purification. Hybrid compounds wereprepared by solvothermal reactions in acid digestion bombs (23mL). The solid reactants were weighed and transferred to the bombs,followed by injection of solvents. The bombs were then sealed andheated in ovens at elevated temperatures. After the reactions werecomplete, the bombs were cooled down naturally to roomtemperature. The final products were washed with 30% and 80%ethanol followed by drying at 50 °C in a vacuum oven. Specifically,Zn2S2(ha) was synthesized by reacting 0.272 g of ZnCl2 (2 mmol) and0.032 g of S (1 mmol) in 3 mL of ha at 120 °C for 2 days. Cdsubstituted Zn2−2xCd2xS2(ha), Se substituted Zn2S2−2ySe2y(ha), and Cdand Se cosubstituted Zn2−2xCd2xS2−2ySe2y(ha) samples were synthe-sized by reactions at various mole fractions of CdCl2 (x = 0−0.25)and/or Se (y = 0−0.25) under the same conditions as for Zn2S2(ha).For comparison purposes, selected samples were heated at 450 °Cunder N2 flow for 30 min to generate the corresponding inorganiccounterparts.

Characterization. Powder X-ray diffraction (PXRD) patterns ofsamples were performed on a Rigaku D/M-2200T automateddiffraction system (Ultima+) using Cu Kα radiation (λ = 1.5406 Å)operated at 40 kV and 40 mA with a step size of 0.02°. Optical diffusereflectance spectra were measured at room temperature on a Perkin-Elmer Lambda 850 UV/vis spectrometer. Room temperaturephotoluminescence (PL) emission spectra were recorded at roomtemperature using a Horiba/Jobin-Yvon Fluorolog-3TM doublegrating−double grating fluorescence spectrophotometer at anexcitation wavelength of 360 nm. The measurement conditions wereidentical in all cases, and therefore relative intensities can be compared.In order to limit the intensities of PL emission spectra for all samplesto be less than 2 × 106 CPS, the two slits were fixed at 0.8 nm.Furthermore, the photoluminescence quantum yields (PLQYs) of thesamples were measured by the relative (comparative) method and theabsolute method (see the Supporting Information), respectively, andthe results obtained from the two methods were found to be in a goodagreement.

WLED Fabrication. The two white light emitting hybrids emi conduc to r s , Zn 2− 2 xCd2 xS 2 (ha ) (x = 0 . 25) andZn2−2xCd2xS2−2ySe2y(ha) (x = 0.20, y = 0.15), were chosen for use asphosphors in WLED devices. First, 50 mg of the samples wasdispersed into 10 mL of DMSO and treated by an ultrasonic processor(Model VCX-750, Sonics & Materials, Inc.) at 30 W for 30 s at roomtemperature, respectively. Second, the resultant suspensions werecentrifuged to remove some DMSO followed by carefully grinding toobtain a “gel-like” pastes. Finally, a commercial 360 nm UV-LED(commercially available from Le Group Fox, Inc.) was dipped into the

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pastes, respectively, followed by rolling to make the pastes coated ontoits surface uniformly.

■ RESULTS AND DISCUSSIONStructure and Optical Emission Properties of BinaryZn2S2(L). As previously reported,36 all double-layer[M2Q2(L)] (where M = Zn, Cd; Q = S, Se) hybridsemiconductors with an even number of carbon atoms intheir organic amine molecules are isostructural and thus havesimilar crystal structures. The crystal structure of Zn2S2(ha)simulated based on that of Zn2Se2(ba)

36 is illustrated in Figure1a, and a view of the double layer [Zn2S2] is displayed in Figure

1b. Zn2S2(ha) is composed of double-layer [Zn2S2] slabs, andthe slabs are sandwiched by coordinated n-hexylamine. All zincatoms have tetrahedral coordination. There are two crystallo-graphic independent Zn atoms, one of which bonds to four Satoms to form a distorted tetrahedron, and the othercoordinates to three S atoms and one N atom of n-hexylaminemolecule. The double layer [Zn2S2] slab can be regarded as a“slice” cut from the (110) crystal plane of the hexagonalstructure of ZnS (or wurtzite structure). The thickness of thedouble-layer [Zn2S2] slab has been estimated to be far less thanthe exciton Bohr radius of ZnS (2.4 nm37), which results in verystrong confinement of electrons/holes in the c direction andthe formation of 2D exciton analogous to semiconductorsuperlattices.38

The optical absorption spectra obtained from diffusereflectance and the room temperature PL emission spectra ofZn2S2(ha) and the reference ZnS are shown in Figure 2a and2b, respectively. Compared to 3.5 eV of ZnS, the absorptionedge of Zn2S2(ha) is estimated to be 4.0 eV, indicating a blueshift of 0.5 eV (Figure 2a). The blue-shift is due to the QCEinduced in the layered nanostructure of Zn2S2(ha).

32 At anexcitation energy of 360 nm, Zn2S2(ha) and ZnS exhibit asimilar emission profile, with one broad peak centered at 430and 420 nm, respectively (Figure 2b). It has been reported thatthe nanosized ZnS particles prepared inside MCM-41 hostswith and without ethylenediamine (en) as a functional groupshow one emission peak centered at ∼430 and ∼450 nm,respectively, which is attributed to the defects related to sulfurvacancies.39 The inorganic/organic ZnS/NaSCH2COONananocomposite synthesized under hydrothermal conditionsgives a blue emission peak at ∼425 nm, which is ascribed to thesulfur vacancies.40 Pure ZnS nanoparticles often exhibit only a

luminescent peak in the range of 450−420 nm.41 Analogously,the PL emission band at 430 nm for Zn2S2(ha) may also beattributed to the presence of the sulfur vacancies in the doublelayer [Zn2S2] slabs. Note Zn2S2(ha) exhibits much strongeremission intensity over ZnS. Such a luminescence-enhance-ment effect has been studied in the ZnS-en-MCM-41mesoporous nanosized composites39 and Zn1‑xMnxSe(L)1/2and Cd1‑xMnxSe(L)1/2 (L = diamines) 2D-dilute magneticsemiconductors.42,43 These studies indicate that diamines (asLewis bases for Zn2+ and Mn 2+ ions) can act as electron donorsto reduce the nonradioactive decay, thus enhancing dramati-cally the PL intensity.42,43 For the present work, n-hexylaminemolecule not only is a Lewis base for Zn2+ ions but also directlycoordinates with Zn atoms to form the Zn−N bonds in theZn2S2(ha) crystal lattice. We infer that n-hexylamine can alsoact as an electron donor to reduce the nonradiative decaythrough the Zn−N bonds. In addition, unlike the referenceZnS, the organic and inorganic interface in the two-dimensionalZn2S2(ha) hybrid structure can induce strong interactionsbetween the ZnS slabs and the organic amines and more sulfurvacancies.39 Consequently, a considerable enhancement of thePL intensity for Zn2S2(ha) over ZnS has been observed. Athigher excitation energies (e.g., 280 nm), band gap emission isalso observed with a very low intensity (Figure SI-1, SupportingInformation).The emission intensity of [Zn2S2](L) is enhanced monotoni-

cally as the length of L increases from pa and ba to ha (FigureSI-2, Supporting Information). These monoamines act asspacers between, and induce confinement within, the [Zn2S2]layers. The confined luminescence in the 2D [Zn2S2] layers is

Figure 1. (a) Side view of the double-layer 2D-[Zn2S2(ha)] basedcrystal structure. (b) The double layer of ZnS in 2D-[Zn2S2(ha)].(The light blue balls are Zn; red balls, S; blue balls, N; gray balls, C.Hydrogen atoms are omitted for clarity.)

Figure 2. (a) Room temperature optical absorption spectra of 2D-[Zn2S2(ha)] and reference ZnS, obtained from diffuse reflectance data.(b) Room temperature PL emission spectra of 2D-[Zn2S2(ha)] andreference ZnS (λex = 360 nm). The intensity of the hybrid structure isdoubled with respect to that of ZnS.

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similar to that in quantum wells or superlattices.32 Longeramine molecules give rise to larger [Zn2S2] interlayer distance,which further weakens the interlayer interactions. Conse-quently, the confinement effect of the PL process becomesmore prominent, which contributes to the luminescenceenhancement.42,43

Structure and Optical Emission Properties of TernaryZn2−2xCd2xS2(ha). Plotted in Figure 3 are the PXRD patterns

of the Cd substituted Zn2−2xCd2xS2(ha) hybrid compoundswith the Cd mole fractions (x) of 0.10, 0.15, 0.20, and 0.25,respectively, together with those of Zn2S2(ha) (x = 0) andCd2S2(ha) (x = 1). Since Zn2S2(ha) and Cd2S2(ha) areisostrucutural phases, they are expected to have similar PXRDpatterns, with small difference in their diffraction peaks.Zn2S2(ha) shows three diffraction peaks located at 3.4°,7.24°, and 28.74°(2θ), respectively, while the correspondingpeaks for Cd2S2(ha) are found at 3.6°, 26.68°, and 29.04°,respectively. As shown in Figure 3, the PXRD patterns of theternary Zn2−2xCd2xS2(ha) compounds are almost identical tothat of Zn2S2(ha), having their first diffraction peaks at ca.3.5°(2θ). This confirms that Cd substituted Zn2−2xCd2xS2(ha)compounds have the same layered nanostructure as that of[Zn2S2](ha). No other peaks appear in their PXRD patterns,indicating that samples are in high purity. In addition, theintensities of the first diffraction peaks for the Zn2−2xCd2xS2(ha)hybrid compounds gradually decrease with the increase in xfrom 0, 0.1, 0.15, 0.2, and 0.25 to 1, and Cd2S2(ha) (x = 1) hasthe lowest intensity, which suggests a decline in crystallinitywith increasing x.The band gaps of the ternary Zn2−2xCd2xS2(ha) are very

sensitive to the Cd content. Even a small amount of x leads to alarge red shift in their optical absorption edge (Figure 4).Specifically, the band gap of Zn2−2xCd2xS2(ha) is estimated tobe 3.15, 3.05, 3.0, and 2.95 eV for x = 0.1, 0.15, 0.2, and 0.25,respectively (see Table 1). Note at x = 0.25, the band gap ofZn2−2xCd2xS2(ha) is nearly the same as that of the binaryCd2S2(ha) (x = 1). Thus, substitution of Zn by a nominalamount of Cd can effectively modulate the band gap of aternary phase. Considering the fact that Cd2S2(ha) issynthesized at 50 °C,36 a much lower temperature than thatof Zn2S2(ha) (120 °C), substitution of Zn by Cd in the doublelayer [Zn2S2] slabs can be readily achieved. Moreover, similar toZn2S2(ha) and Cd2S2(ha), ternary Zn2−2xCd2xS2(ha) phaseswith various x values exhibit sharp and single absorption edgesbetween those of Zn2S2(ha) and Cd2S2(ha) (Figure 4),

confirming that there is no phase separation throughout theentire composition range (x = 0 to 1).44

Note that the absorption edges of Zn2−2xCd2xS2(ha) andCd2S2(ha) are around 3 eV, far away from 4.0 eV of Zn2S2(ha)(Figure 4). The room temperature PL excitation (PLE) spectraof the ternary Zn2−2xCd2xS2(ha) hybrid compounds (x = 0.15,0.20, and 0.25) and Cd2S2(ha) (x = 1) were measured with theemission wavelength fixed at 420 nm, while that of Zn2S2(ha)(x = 0) was measured with the emission wavelength fixed at325 nm. The obtained PLE spectra (Figure SI-3) clearlyindicate that the suitable excitation wavelength range for theternary Zn2−2xCd2xS2(ha) hybrid compounds and binaryCd2S2(ha) are quite different from that for Zn2S2(ha).Consequently, the room temperature PL emission spectra ofthe ternary Zn2−2xCd2xS2(ha) hybrid compounds (x = 0.15,0.20, and 0.25) and Cd2S2(ha) (x = 1) were measured at theexcitation wavelength of 360 nm, while that of Zn2S2(ha) wasmeasured at the excitation of 280 nm. As shown in Figure 5, theCd2S2(ha) phase emits white light and shows one broad andlow-intensity emission peak (maximum at ∼530 nm) thatcovers the entire visible spectrum. As previously stated,34 the2D double-layered cadmium based Cd2S2(L) (L = monoamine)hybrid compounds have a large number of surfaces sites withineach crystal due to the nature of its layered structure, and theboard white light emission for Cd2S2(L) is attributedpredominantly to the recombination of the charge carrierswithin surface states. Different from previously reportedCd2S2(L) where band gap emission was recognizable,34 bandgap emission is not recognizable visible in the Cd2S2(ha),largely due to its very low overall intensity. All ternaryZn2−2xCd2xS2(ha) compounds show two PL emission peakswith significantly enhanced emission intensities compared toCd2S2(ha), which are centered at 408 and 500 nm for x = 0.15,410 and 510 nm for x = 0.20, and 420 and 530 nm for x = 0.25,respectively (see Table 1). The first (high energy) peak in allcases is attributed to the band gap emission, and the intensitydecreases as x value increases. A red shift is observed as afunction of increasing x. The first emission peak of theZn2−2xCd2xS2(ha) compounds are at 408, 410, and 420 nm forx = 0.15, 0.20, and 0.25, respectively. This lowering in emissionenergy is a result of band gap decrease as the content of Cdincreases, which is also confirmed from their optical absorptionedges shown in Figure 4. Moreover, we also observe a red shiftin the second emission peaks of the Zn2−2xCd2xS2(ha) ternarycompounds, from 500, 510 to 530 nm as x increases from 0.15,0.20 to 0.25, respectively. The second emission peak of the

Figure 3. PXRD patterns of the Cd substituted Zn2−2xCd2xS2(ha)hybrid compounds with the nominal Cd mole fractions (x) of 0.10,0.15, 0.20, and 0.25, respectively, together with those of Zn2S2(ha) (x= 0) and Cd2S2(ha) (x = 1).

Figure 4. Optical absorption spectra of the Cd substitutedZn2−2xCd2xS2(ha) hybrid compounds with the nominal Cd molefractions (x) of 0.10, 0.15, 0.20, and 0.25, respectively, together withthose of Zn2S2(ha) (x = 0) and Cd2S2(ha) (x = 1).

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Zn2−2xCd2xS2(ha) hybrid compound with x of 0.25 is very closeto that of Cd2S2(ha), following the same trend as in theiroptical absorption edges. Analogous to the previouslyinvestigated Cd2S2(L) system,34 we infer that the secondemission band of the ternary Zn2−2xCd2xS2(ha) hybrids resultsfrom the substituted Cd atoms in the double-layer[Zn2−2xCd2xS2] slabs and the surfaces sites related to theirlayered structure. The increase of the Cd content results in thered-shift. Moreover, the PL emission spectrum of Zn2S2(ha)measured at the excitation wavelength of 280 nm also exhibitstwo emission peaks, one of which is located at 325 nmcorresponding to band gap emission, and the other peak mayoriginate from the presence of the sulfur vacancies in thedouble layer [Zn2S2] slabs, as described above.It has been reported that the band-edge emission with

narrow spectral width dominates the PL emission feature of thehigh-quality alloyed ZnxCd1‑xS nanocrystals (quantum dots)synthesized at high temperatures,44,45 which is associated withthe QCE of the charge carriers in all three dimensions due totheir particle radius close to their corresponding Bohr radii.44 Ithas been observed that the original deep-trap emission rangingfrom 500 to 600 nm is gradually eliminated with the increase inthe high-temperature annealing time owing to the removal ofthe crystallite defects within the nanocrystals.44 On the otherhand, the CdxZn1‑xS nanoparticles prepared at room temper-ature exhibit only one broad emission peak centered at 480,505, 540, 543, and 550 nm for Cd0.1Zn0.9S, Cd0.22Zn0.78S,Cd0.72Zn0.28S, Cd0.80Zn0.20S, and Cd0.95Zn0.05S, respectively,

46

which accordingly coresponds to the deep-trap emission relatedto the crystallite defects within the nanoparticles. Obviously,the ternary Zn2−2xCd2xS2(ha) hybrid compounds combine thetwo features, one of which (1st peak) is similar to the alloyedZnxCd1‑xS QDs synthesized at high temperatures,44,45 and theother (2nd peak) is just like the nanoparticles synthesized atroom temperature. Such a phenomenon is most likelyoriginated from their unique layered nanostructure combingwith the strong QCE and large surface sites. Since the Cdsubstituted Zn2−2xCd2xS2(ha) compounds have the samelayered nanostructure as that of [Zn2S2](ha), the same QCE

exists in Zn2−2xCd2xS2(ha), which leads to the first emissionpeaks that are attributed to band gap emission. Since[Zn2S2](ha) does not exhibit any emission peak between 500and 550 nm, it can be inferred that the second emission peakranging from 500 to 530 nm in the Zn2−2xCd2xS2(ha) ternarycompounds is associated with the doped Cd atoms in thedouble-layer [Zn2−2xCd2xS2] slabs, along with some defectsrelated to the surfaces sites, similar to the CdxZn1‑xSnanoparticles prepared at room temperature.46 Note that thewell-defined Cd ion related broad emission is either attributedto direct band-to-band recombination in CdS-based phases ordue to recombination of Cd2+ ions as an impurity center.47 ThePXRD analysis clear shows that there is no phase separationand that Cd ions are indeed doped onto the double-layer[Zn2S2] slabs. Consequently, the second emission peak inZn2−2xCd2xS2(ha) should be attributed to the recombination ofCd2+ ions as an impurity center. The increase of the Cd contentresults in the red-shift, very much like the red shift in the broademission peak of the CdxZn1‑xS nanoparticles prepared at roomtemperature.46 Significantly different from the magic-size CdSeNCs,19 the surface sites of the hybrid nanostructuredcompounds are produced by the nature of their double-layeredstructure, not by chance.22

The intensity of the second emission peak of theZn2−2xCd2xS2(ha) hybrid is considerably higher than that ofthe broad emission of Cd2S2(ha) at ∼530 nm. One of thepossible reasons for the very low emission intensity ofCd2S2(ha) is its low crystallinity (Figure 3). Materials withhigher crystallinity will more likely possess less nonradiativerecombination centers, leading to an increase in their PLemission intensity.49 Note the overall emission intensity forZn2−2xCd2xS2(ha) gradually decreases as the Cd contentincreases. Such a decrease in the PL emission intensity is alsoobserved for the CdxZn1‑xS nanoparticles prepared at roomtemperature.46 We attribute the lowering in the PL emission tothe lattice mismatch induced by doping Cd atoms into the[Zn2S2] slabs. The defects originated from the lattice mismatchcan act as the nonradiative recombination centers, resulting inthe decrease in PL emission.48 Consequently, Zn2−2xCd2xS2(ha)is superior in PL emission intensity over Zn2S2(ha) (x = 0) andCd2S2(ha) (x = 1), and its emission can be well modulated andbalanced by controlling the x values. The CIE coordinates ofvarious Zn2−2xCd2xS2(ha) can be tuned by changing thecomposition (Figure SI-4, Supporting Information). The CIEcoordinates for the Zn2−2xCd2xS2(ha) hybrid compound with x= 0.25 are calculated to be (0.30, 0.38), which are well withinthe white-light region. With a PLQY measured to be as high as9.9%, this compound demonstrates a considerable increase(doubling) in its emission efficiency compared to theCd2S2(ba)-based white light emitters (4−5%).34Structure and Optical Emission Properties of Quaternary

Zn2−2xCd2xS2−2ySe2y(ha). At a fixed Cd mole fraction x = 0.20,Cd and Se cosubstituted quaternary Zn2−2xCd2xS2−2ySe2y(ha)hybrid compounds with the nominal Se mole fractions (y) of0.05, 0.10, and 0.15, respectively, are synthesized at 120 °C (2days) . The PXRD pat t e rns o f the qua te rna ry

Table 1. Optical Absorption Edges and PL Emission Peaks of the Zn2‑2xCd2xS2(ha) Hybrid Compounds (λex = 360 nm)

nominal Cd mole fraction (x) 0 [Zn2S2(ha)] 0.15 0.2 0.25 1 [Cd2S2(ha)]

estimated band gap (eV) 4.0 3.05 3.0 2.95 2.9PL emission first peak (nm) 430 408 410 420 none

second peak (nm) none 500 510 530 530

Figure 5. Room temperature PL emission spectra (λex = 360 nm) ofCd2S2(ha) (x = 1) and the Cd substituted Zn2−2xCd2xS2(ha) hybridcompounds with the nominal Cd mole fractions (x) of 0.15, 0.20, and0.25, respectively, together with that (λex = 280 nm) of Zn2S2(ha) (x =0).

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Zn2−2xCd2xS2−2ySe2y(ha) (x = 0.20, y = 0.05, 0.10, and 0.15,respectively) compounds are nearly identical to that ofZn1.6Cd0.4S2(ha) (x = 0.20) (Figure SI-5, SupportingIn format ion) , wh ich c lea r l y ind ica te s tha t theZn2−2xCd2xS2−2ySe2y(ha) compounds are single-phased andpossess the same layered nanostructure as the ternaryZn2−2xCd2xS2(ha). Figure 6 displays the optical absorption

spectra of the Zn2−2xCd2xS2−2ySe2y(ha) (x = 0.20, y = 0, 0.05,0.10, and 0.15, respectively) compounds. Again, theirabsorption edge gradually shifts to lower energy with anincreasing y value. The estimated band gaps ofZn1.6Cd0.4S2−2ySe2y(ha) (x = 0.20) are 2.85, 2.80, and 2.75 eVfor y = 0.05, 0.10, and 0.15, respectively (see Table 2).

Compared with the band gap of the ternary Zn2−2xCd2xS2(ha)(x = 0.20) reference structure, 3.0 eV, the extent of red shifthere is relatively small. Interestingly, the absorption intensityincreases monotonically as a function of y to a much largerextent. It is also interesting to note that Se substitution makes anegligible effect on the band gap and emission properties of thebinary Zn2S2(ha) structures prepared at the same conditions(120 °C for 2 days, see Figures SI-6 and SI-7, SupportingInformation). This observation is quite different from thealloyed ZnSxSe1‑x nanowires that exhibit the tunable band gapemissions through composition (x) modulation.50 Since 2D-[Zn2Se2(ha)] was synthesized at 140 °C,36 a higher temper-ature than 120 °C used for the synthesis of Zn2S2(ha), one mayspeculate that substitution of S by Se in the double-layer[Zn2S2] slabs to form ZnS2−2ySe2y(ha) at 120 °C may not bethe optimum temperature. However, since 2D-[Cd2Se2(ha)]was indeed synthesized at the same temperature of 120 °C asthat for preparing Zn2S2(ha),

36 this temperature should besuitable for the substitution of Zn and S atoms by Cd and Se,respectively, to form quaternary Zn2−2xCd2xS2−2ySe2y com-

pounds, as a result of synergetic effect on Cd and Se codoping.Optical absorption spectra (Figure 6) are suggestive of theformation of single phased Zn2−2xCd2xS2−2ySe2y(ha), rather thana mixture of Zn2S2(ha), Cd2S2(ha), and Cd2Se2(ha).Figure 7 shows the PL emission spectra of the quaternary

Zn1.6Cd0.4S2−2ySe2y(ha) (x = 0.20, y = 0, 0.05, 0.10, and 0.15,

respectively) compounds, together with that of the referenceCd2S2(ha). Similar to the ternary Zn1.6Cd0.4S2(ha) (x = 0.20)systems that show characteristic two emission peaks centered at410 and 510 nm, respectively (Figure 7, x = 0.2, y = 0), thequaternary Zn1.6Cd0.4S2−2ySe2y(ha) compounds also exhibit twoPL emission peaks centered at 436 and 515 nm for y = 0.05,439, and 520 nm for y = 0.10, and 445 and 525 nm for y = 0.15,respectively (Table 2), although the relative intensity of the firstpeak drops significantly as the y value increases. In addition, ared shift is observed in the first peak as a function of increasingy, from 410 nm to 436, 439, and 445 nm for y = 0 to 0.05, 0.10,and 0.15, respectively. Such a red shift is attributed to thelowering of the band gaps with higher y (see Figure 6). Unlikethe obvious red shift in their first emission bands, no significantshift is found in their second emission peaks, in accordancewith the fact that their second emission bands are attributed tothe substituted Cd atoms, rather than Se atoms. The ymodulation also leads to a variation in the CIE coordinates ofthe quaternary Zn2−2xCd2xS2−2ySe2y(ha) compounds (Figure SI-8, Supporting Information). For Zn1.6Cd0.4S1.7Se0.3(ha) (x =0.20, y = 0.15) the CIE values are (0.28, 0.33), well within thewhite-light region and closer to the pure white CIE (0.31, 0.31)than the ternary phase Zn1.6Cd0.4S2(ha). The PL emissioni n t e n s i t i e s o f t h e Cd a nd S e c o s u b s t i t u t e dZn2−2xCd2xS2−2ySe2y(ha) compounds decrease with the increasein y from 0 to 0.15 (Figure 7), which can be also attributed tothe lattice mismatch induced by doping Cd and Se atoms intothe [Zn2S2] slabs. The PLQY of Zn1.6Cd0.4S1.7Se0.3(ha) (x =0.20, y = 0.15) is calculated to be 6.8%, 70% increase of thebinary Cd2S2(ba)-based system.34

In addition, the quaternary Zn1.5Cd0.5S1.7Se0.3(ha) (x = 0.25,y = 0.15) hybrid compound was also synthesized at 120 °C(reaction time: 2 days). Compared with 2.95 eV of theabsorption edge for the ternary Zn1.5Cd0.5S2(ha) compound,the absorption edge for Zn1.5Cd0.5S1.7Se0.3(ha) is estimated tobe 2.75 eV, indicating a small red shift of ca. 0.2 eV (Figure SI-9, Supporting Information). Figure 8 displays the PL emissionspectra of Zn1.5Cd0.5S2(ha) and Zn1.5Cd0.5S1.7Se0.3(ha), togetherwith that of Cd2S2(ha) for comparison. Similar toZn1.5Cd0.5S2(ha) that exhibits two emission peaks centered at420 and 530 nm, Zn1.5Cd0.5S1.7Se0.3(ha) shows two emission

Figure 6. Optical absorption spectra of the Zn1.6Cd0.4S2−2ySe2y(ha) (y= 0, 0.05, 0.10, and 0.15) compounds.

Table 2. Optical Absorption Edges and PL Emission Peaksof the Zn2‑2xCd2xS2‑2ySe2y(ha) Hybrid Compounds (λex = 360nm)

nominal Cd mole fraction(x) 0.2 0.25

nominal Se mole fraction(y) 0 0.05 0.1 0.15 0 0.15

estimated band gap (eV) 3.0 2.85 2.8 2.75 2.95 2.75PLemission

first peak(nm)

410 436 439 445 420 450

second peak(nm)

510 520 530 540 530 530

Figure 7. Room temperature PL emission spectra of theZn1.6Cd0.4S2−2ySe2y(ha) (y = 0, 0.05, 0.10, and 0.15) compounds,along with that of Cd2S2(ha) (λex = 360 nm).

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bands centered at ca. 450 and 530 nm, respectively. Theirsecond emission peaks are located at the same wavelength of530 nm, since both have the same nominal Cd mole fractions of0.25. Their first emission peaks shift from 420 nm for y = 0.0 to450 nm for y = 0.15, corresponding to the red shift in the bandgap (from 2.95 to 2.75 eV). Clearly, Se doping leads to a muchmore balanced white light emission spectrum for the quaternaryphase Zn1.5Cd0.5S1.7Se0.3(ha). Consequently, the CIE coordi-nates move from (0.30, 0.38) for Zn1.5Cd0.5S2(ha) to (0.28,0.32) for Zn1.5Cd0.5S1.7Se0.3(ha), with the latter closer to thepure white color (0.31, 0.31) (Figure SI-10, SupportingInformation). These results further demonstrate that thesynergetic effect between doped Cd and Se atoms tunes theoptical edges and PL emission spectra of the quaternaryZn2−2xCd2xS2−2ySe2y(ha) compounds, giving rise to a morebalanced white light.Dependence of PL Emission on Layered Nanostructure. In

order to elucidate the relationship between the layerednanostructure and the PL emission properties of the doubled-layered II−VI hybrid compounds, the ternary Zn2−2xCd2xS2(ha)(x = 0.25) and quaternary Zn2−2xCd2xS2−2ySe2y(ha) (x = 0.20, y= 0.15) hybrid compounds that emit white light were calcinedat 450 °C for 30 min to remove the organic ligand n-hexylamine (ha), and thus their inorganic counterparts wereobtained. Figure 9 shows comparison on the PXRD patterns(a) and the PL emission spectra (b) of Zn1.5Cd0.5S2(ha) andZn1.5Cd0.5S1.7Se0.3(ha) before and after calcination, respectively.After calcination at 450 °C for 30 min, their first diffractionpeaks located at around 3.5°(2θ) disappear (Figure 9a), clearlyindicating that their layered nanostructure is destroyed due tothe evaporation of n-hexylamine during the calcination.Accordingly, the PL emission spectra of the inorganiccounterparts are quite different from those of the as-madehybrid structures (Figure 9b). Both show only one very broademission band (∼450 nm), and the intensity is reduceddrastically. Note the similarity of these bands to those of theZnS-based materials mentioned in Section 3.1, which originatefrom the presence of the sulfur vacancies in the ZnS-basedmaterials. On the contrary, the two PL emission bands of thet e r n a r y Z n 2 − 2 xC d 2 x S 2 ( h a ) a n d q u a t e r n a r yZn2−2xCd2xS2−2ySe2y(ha) hybrid compounds originate fromtheir unique double-layered nanostructure.Zn2−2xCd2xS2(ha) Based White LED Assemblies. Figure 10

illustrates photos of the white LEDs fabricated using the ternaryZn1.5Cd0.5S2(ha) and quaternary Zn1.5Cd0.5S1.7Se0.3(ha) sam-ples. The reference UV LED (360 nm) emits blue light (Figure10a). Upon coating its surface with a thin layer of

Zn1.5Cd0.5S2(ha) (Figure 10b), the LED change the emissioncolor from blue to white (Figure 10c). Coating a layer ofZn1.5Cd0.5S1.7Se0.3(ha) (Figure 10d) gives white light emissionshown in Figure 10e.

■ SUMMARYCd and/or Se-substituted Zn2S2(L)-based hybrid semiconduc-tors have been synthesized and structurally characterized. TheZn2−2xCd2xS2−2ySe2y(ha) hybrid compounds not only are singlephased but also have the same layered nanostructure as that ofZn2S2(ha). The ternary Zn2−2xCd2xS2(ha) and quaternaryZn2−2xCd2xS2−2ySe2y(ha) hybrid compounds exhibit two PLemission peaks, with one of which attributed to the band gapemission, and the other, originated from the doped Cd atomsand the surface sites related to their layered nanostructure. The

Figure 8. Room temperature PL emission spectra of theZn1.5Cd0.5S2−2ySe2y(ha) compounds (y = 0 and 0.15), along withthat of Cd2S2(ha) (λex = 360 nm).

Figure 9. PXRD patterns (a) and PL emission spectra (b) (λex = 360nm) of A and B before and after the calcinations at 450 °C for 30 min,respectively. A: Zn1.5Cd0.5S2(ha); B: Zn1.6Cd0.4S1.7Se0.3(ha).

Figure 10. White-light assemblies built on the 2D-[Zn2−2xCd2xS2(ha)]and 2D-[Zn2−2xCd2xS2−2ySe2y(ha)] phosphors. (a) A 5 mm referenceUV LED (360 nm) illuminates blue light (commercially available fromLe Group Fox, Inc.); (b) the same LED coated with a thin layer ofZn1.5Cd0.5S2(ha); (c) the same LED in (b) illuminating; (d) the sameLED coated with a thin layer of Zn1.6Cd0.4S1.7Se0.3(ha); (e) the sameLED in (d) illuminating.

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amount of Cd and Se and synergetic effect between the twoelements play a key role in affecting the optical absorption edgeand the location of the two emission bands along with theirrelative intensities, as well as the quantum efficiency and colorquality of the emitted white light. The ZnS-based ternary andquaternary hybrid semiconductors exhibit significantly en-hanced PLQY compared to the white-light emitting CdS-based hybrid compounds. Upon removal of organic amines, theemission intensity of the inorganic counterpart is greatlyreduced and is shifted from white light to a broad blue-greenregion (∼450 nm). Clearly, it is the unique layerednanostructure and the QCE and the surface sites associatedwith these nanostructures that play a critical role in theemission properties of the hybrid semiconductors. The white-light emitting ternary Zn2‑xCd2xS2(ha) and quaternaryZn2−2xCd2xS2−2ySe2y(ha) compounds are promising for use asa new type of phosphors in WLEDs.

■ ASSOCIATED CONTENT*S Supporting InformationText and Figures SI-1−SI-10. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: jingli@rutgers.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by the National Science Foundation(DMR-0706069) and the National Natural Science Foundationof China (Project Number 60976053).

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