Fabrication of Luminescent, Magnetic Hollow Core Nanospheres and

Oct 11, 2010 - National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110 ... Department of Physics, Jamia Millia Islamia, New Delhi 110 025...
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J. Phys. Chem. C 2010, 114, 18429–18434

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Fabrication of Luminescent, Magnetic Hollow Core Nanospheres and Nanotubes of Cr-Doped ZnO by Inclusive Coprecipitation Method1 K. Jayanthi,†,‡ Santa Chawla,*,† Amish G. Joshi,† Zahid H. Khan,‡ and R. K. Kotnala† National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110 012, India, and Department of Physics, Jamia Millia Islamia, New Delhi 110 025, India ReceiVed: July 29, 2010; ReVised Manuscript ReceiVed: September 10, 2010

Incorporation of Cr into ZnO lattice as a dopant in hollow core nanostructures has been achieved by coprecipitation in highly alkaline medium near room temperature. Energy dispersive spectroscopy and X-ray diffraction studies confirm the presence of Cr in the ZnO lattice and X-ray photoelectron spectroscopy (XPS) identified the dopant state to be Cr3+. Room temperature ferromagnetism in ZnO:Cr3+ along with structural investigations suggest an intrinsic nature of ferromagnetism in hollow core nanospheres. Unpaired spin in Cr3+ in zinc substitutional sites could create spin ordering and long-range ferromagnetic coupling. Higher Cr3+ concentration leads to the formation of hollow parallelepipeds and superparamagnetism. Photoluminescence studies reveal a sharp blue emission explicitly arising due to Cr doping. Such blue emitting magnetic nanocapsules can have immense potential as biocarriers and also in spintronics. 1. Introduction Hollow core nanoparticles of magnetic semiconductors doped with transition metals having excellent luminescence properties form a rare class of materials1-7 that have immense potential as magnetically controlled fluorescent nanocapsules in biological systems as well as for devices based on spin transport properties such as spin-LED and spintronics.8 Fabrication of shapecontrolled hollow nanoparticles with functional properties is the first step toward nanoscale building blocks for the assembly of multifunctional devices as hollow structures have lower density, higher surface area, unique optical properties for example, lasing, compared to its solid counterpart. A multifunctional semiconductor like ZnO is a very important material for morphology-controlled synthesis as undoped ZnO emits UV and visible luminescence, whereas ZnO doped with transition metals exhibits both visible luminescence and ferromagnetism (FM) at or above room temperature.9-12 Though there are some reports indicating absence of ferromagnetism due to transition metals (TMs)13,14 or magnetism arising due to precipitated secondary phase,15,16 theoretical calculations predict intrinsic ferromagnetism in transition metal-doped semiconductors. No definite agreement on the nature of the FM in dilute magnetic semiconductors (DMSs) has evolved so far. Ferromagnetism at room temperature and above has been observed for ZnO doped only with alkali metals17,18 without any TM. 3d ferromagnetism is, however, expected in ZnO doped with TM elements like Cr, Fe, Co, Ni, and Mn,19-22 of which Cr is particularly attractive. As Cr is a good emitter, Cr-based FM semiconductors have the prospect of producing ferromagnetism as well as luminescence properties at room temperature.23-25 Because of antiferromagnetic properties of Cr metal, the role of Cr precipitates in yielding spurious FM is eliminated and the only ferromagnetic oxide of Cr, CrO2 with a Tc of 386 K, is very unlikely to form under the low oxygen pressure conditions usually employed in 1

PACS numbers: 81.16.-c, 81.07.-b, 75.50.Pp, 78.55.-m. * Corresponding author Ph. 91 11 45609242, Fax: 91 11 45609310, E-mail: [email protected]. † National Physical Laboratory. ‡ Jamia Millia Islamia.

vacuum deposition methods26 or in an aqueous solution in ambient conditions. However, compared with the widely studied Co- and Mn-doped ZnO system, both theoretical and experimental researches on Cr-doped ZnO are scarce. Moreover, the experimental results on the studies of Cr-doped ZnO are in conflict with each other.27-30 FM from nanopowders has been discussed controversially in the literature and also seems to be sensitive to the preparation method. In a nutshell, an unequivocal cause-effect relationship of occurrence of FM in ZnO:Cr nanocrystals do not exist. To synthesize morphology-controlled ZnO nanocrystals doped with transition metal Cr resulting in intrinsic ferromagnetism and luminescence emission simultaneously, we prepared Cr-doped ZnO by an inclusive coprecipitation route near room temperature. The morphology and properties exhibited by ZnO are greatly affected by the preparation conditions and chemical environment of the synthesis process. Formation of wurtzite ZnO structure and incorporation of substitutional defects by low-temperature synthesis is a nontrivial process and coprecipitation is one of the more successful techniques for synthesizing stable ZnO at room temperature. A particular emphasis was to prepare hollow nanostructures of undoped and Cr-doped ZnO with a different surface to volume ratio by a simple method. Formation of such hollow structures has been reported for undoped ZnO synthesized either by the template-assisted hydrothermal method or by thermal evaporation, which are complicated and instrument intensive.31-34 Fabrication of hollow core nanostructure of varied morphology in Cr-doped ZnO by a coprecipitation method at room temperature is an achievement. Hollow core nanoparicles of a multifuctional semiconductor like ZnO have great potential as novel laser material and as fluorescent nanocapsules for sensitive materials. With large open volume inside hollow magnetic nanoparticles, they can be excellent biocarriers for targeted drug delivery. 2. Experimental Procedure 2.1. Synthesis of Shape-Controlled Hollow Core ZnO and ZnO:Cr Nanostructure. Undoped ZnO was prepared by coprecipitation method using submolar aqueous stock solution of zinc

10.1021/jp107086h  2010 American Chemical Society Published on Web 10/11/2010

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acetate as a precursor material. For Cr doping, zinc acetate (Zn [(CH)3(COOH)]2 · 2H2O) stock solution and aqueous chromium sulfate (Cr2(SO4)3 · H2O) solution were mixed under continuous magnetic stirring to make a homogeneous ensemble. Cr concentration in Zn1-xCrxO was varied from x ) 0.02 to 0.1. A highly alkaline medium (pH 10) was created by adding double diluted aqueous ammonia solution and kept for one hour under vigorous stirring. The solution was covered and kept in a preheated oven at 90 °C for 24 h. The clear liquid was decanted and ZnO powder settled in the bottom was repeatedly washed with ethanol in ultrasonic bath. Cleaning was necessary to remove the surface impurities and minimize particle agglomeration. ZnO powder was dried in an oven. Any possibility of magnetic contamination through accidental or trace impurities has been meticulously avoided during sample preparation. High-purity glass ware without any metallic contamination was used for preparing the samples. The amount of trace magnetic impurities in the precursor materials was in ppm level. The same precursor (Zn [(CH)3(COOH)]2 · 2H2O) was used to synthesize undoped ZnO. 2.2. Characterization. Phase purity of the prepared samples were checked by X-ray diffraction (XRD) with Rigaku Miniflex X-ray Diffractometer using the principle of Bragg Brantano Geometry, with Cu-KR radiation (λ ) 1.54 Å). The morphology of the sample was inspected using a ZEISS EVO MA10 digital scanning electron microscope. Energy dispersive spectroscopy (EDS) was done using OXFORD INCA (ENERGY 250 EDS) system. X-ray Photoelectron Spectroscopy (XPS) was done using PerkinElmer, Model: 1257 with a non-monochromatized AlKR line at 1486.6 eV. Pass energy for general survey scan and core level spectra was kept at 143.05 and 71.55 eV, respectively. Photoluminescence (PL) spectra were recorded using PerkinElmer LS55 luminescence spectrometer with a Xe lamp source. Time resolved decay was measured using a time correlated single photon counting technique with Edinburgh Instruments FLSP920 spectrometer using a µs pulsed Xe lamp as excitation source. Magnetic measurements have been carried out with a vibrating sample magnetometer (VSM) (Lakeshore 7304) instrument using a sample holder of high-purity perspex free from any metallic impurity. Extreme care was taken to clean the holder ultrasonically to remove any magnetic material traces. The same holder was used exclusively for measurement of all ZnO samples to avoid any magnetic contamination. The same measurement procedure was followed for the empty sample holder and corresponding magnetization data of the holder were subtracted from the measured magnetic signal of the samples.

Jayanthi et al.

Figure 1. Color online X-ray diffraction patterns of ZnO:Cr nanopowders. The inset illustrates the changes in the intensity and 2θ values of ZnO:Cr with different Cr concentrations.

TABLE 1: Change in Lattice Parameters in ZnO with Cr Doping lattice parameters (Å) sample details

a (or) b

c

undoped ZnO ZnO:Cr (2%) ZnO:Cr (10%)

3.2666 3.2546 3.2505

5.2304 5.2126 5.2098

variation of lattice parameters a and c are shown in Table 1. Noticeable peak shift and decreased a and c parameters due to Cr doping support substitutional incorporation of Cr in to the ZnO lattice. The decrease in lattice parameters is due to the smaller ionic radii Cr3+ (0.61 Å) substituting larger host ion Zn2+ (0.74 Å) at its site in ZnO:Cr. Such changes due to Cr in ZnO lattice have been observed in films.35 Therefore, the doping limit for Cr in ZnO is below 10%. EDS analysis of the 2% Cr-doped ZnO reconfirms the incorporation of Cr in the ZnO lattice as shown in Figure 2. The analysis also shows that the samples are deficient in Zn (Zn0.44O0.54:Cr0.008). A precursor solution with high pH implies more alkalinity and an oxygen-rich fluidic environment for growth. The incorporation of dopant Cr in ZnO and presence of zinc vacancy in the synthesized ZnO: Cr show a similar trend as that in our earlier reported work on Mn-doped ZnO.36 The chemical route for formation of ZnO particles in highly alkaline environment can be expressed as follows: Zn(CH3COO)2 + 2NH4OH f Zn(OH)2 + 2NH4CH3COO

(1)

3. Results and Discussion The XRD patterns of undoped and Cr-doped ZnO nanopowders are shown in Figure 1. Well-defined peaks of wurtzite hexagonal structure of ZnO were observed. Formation of well crystalline structure of pure ZnO from aqueous solution near room temperature is a very tedious task and needs a careful balance of pH and lowtemperature reaction parameters. All peak positions of undoped as well as Cr-doped samples correspond to the standard diffraction patterns of wurtzite hexagonal structure [JCPDS Card No: 36-1451]. Undoped ZnO shows diffraction peaks mainly along the [100], [002], [101] direction with an average particle size of about 44 nm. Peak intensities of 2% Cr-doped ZnO nanopowders decreased compared to undoped ZnO without the presence of any secondary phase. However, with a further increase of Cr concentration to 10%, [002], [101] peak intensities decreased but peak intensity of [100] increased compared to 2% Cr-doped ZnO with formation of Cr2O5 as a secondary phase [JCPDS Card No: 36-1329] (Figure 1). The shift of corresponding diffraction peaks are shown in the inset. The

Figure 2. EDS analysis of 2% Cr-doped ZnO nanopowders.

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Zn(OH)2 + 4NH3 f Zn2+ + 2OH- + 4NH3 f Zn(NH3)2+ 4 + 2OH

f ZnO(s) + 4NH3(g) + H2O(aq)

(2) (3)

In the reaction process, the formation of the amino complex Zn(NH3)42+ is dependent critically upon the alkalinity (pH) of the solution.37,38 By increasing pH of the solution with the addition of more NH4OH, more amino complex is formed. These Zn(NH3)42+ complexes can act as nucleation centers and dehydration of these Zn(NH3)42+ complex at a slightly elevated temperature leads to formation of ZnO with hollow core as the gaseous component of the nucleating center evaporates during decomposition. The morphology of synthesized ZnO and ZnO:Cr is exemplified in the SEM micrographs (Figure 3). There is a distinguishable change in the morphology due to dopant Cr compared to undoped ZnO. Nano structure varies from hollow hexagonal nanotubes (along [001] direction) for undoped ZnO to hollow nanospheres for 2% Cr-doped ZnO to hollow nano-parallelepipeds for 10% Cr-doped ZnO. For undoped ZnO, uniform hexagonal nanotubes have length around 450-750 nm, width 200 nm, and wall thickness about 20-40 nm (part a of Figure 3). As the direction of anisotropy for wurtzite ZnO is along c axis (along [001] direction) hexagonal crystal growth occurs along c axis. A hollow nanospherical shape has been observed with 2% Cr-doped ZnO with a size in the range of 50-300 nm (part b of Figure 3). Long rectangular sticklike nanoparallelopipeds of micrometer length formed in 10% Cr-doped ZnO with a tube width of about 500 nm and a wall thickness of about 150 nm (part c of Figure 3). 10% Cr-doped ZnO also showed formation of some fine nanoclusters of size ∼50 nm. The most striking feature is the hollow core structure of the tubes, spheres, and sticks as illustrated in the insets of Figure 3. Hollow ZnO structures have been mostly reported for hydrothermal growth with gas bubbles as a soft template for hollow growth.31 In the present synthesis, the amino complex Zn(NH3)42+, which forms in abundance for high alkaline environment makes the soft gas template. One interesting feature to note is that the direction of growth for undoped ZnO is along the [001] direction resulting in hexagon sided nanorods, with 2% Cr doping the shape is near spherical and with increase an in Cr concentration (10%) nano-parallelepipeds preferentially grow along the [100] direction. Such a change in the orientation of growth in ZnO with dopant concentration has been reported for Al-doped ZnO thin films.39 Because of the tetrahedral coordination of Zn atoms with four O atoms, Zn d-electrons hybridize with the O p-electrons and the bonding is highly ionic as the electronegativity value of Zn (1.65) differs significantly from that of O (3.44). Two interconnecting sublattices of Zn2+ and O2- forming the crystal structure give rise to polar symmetry along the [001] direction. The polar nature of the lattice plays a major role in crystal growth, etching, and defect generation. Wurtzite ZnO has four common face terminations, which are the polar Znterminated (001) and O-terminated (001) faces (c axis oriented), and the nonpolar (110) and (100) (a axis oriented) faces. The chemical and physical properties of the polar faces are different. The polar surfaces and the (100) surface are found to be stable, whereas the (110) face has less stability. Hence, growth in ZnO can take place either in the [001] or the [100] direction. X-ray photoelectron spectroscopy (XPS) was used to ascertain the presence of Cr and its oxidation state in ZnO. The XPS survey spectrum of the 2% Cr-doped ZnO nanopowders (part a

Figure 3. SEM images of (a) undoped (b) 2% (c) 10% Cr-doped ZnO nanopowders. The insets in (a), (b), and (c) show the hollow structure, notice the change from hexagonal rod to spherical structure to rectangular rods.

of Figure 4) is recorded after Ar ion sputtering for 2 min (to remove any absorbed surface contaminants). The survey spectrum exhibits sharp peaks corresponding to Zn 2p3/2 (1022.4 eV), Zn2p1/2(1045.4 eV), C 1s (284.5 eV), and O 1s (531.0 eV) levels. The core level binding energy observed for the Cr 2p3/2 primary peak at 577.5 ( 0.1 eV (part b of Figure 4) is clearly different from 574.02 eV of Cr metal and 576.0 eV of Cr2+.40-42 These peaks can be indexed to Cr3+ ions as they match well with reported binding energy (577.2 ( 0.2 eV) of Cr3+ states.40-43 The fact that peaks corresponding to Cr3+ could be

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Jayanthi et al. doping, Cr would be precipitated on the surface where it has 3-fold coordination44 resulting in an increase in Cr-O bond length pushing the Cr atoms further apart. The change in morphology due to Cr doping can also be explained on the basis of the ionic fraction of the bond in Crdoped ZnO. Takeshi et al45 used the concept of electronegativity (EN) (the ratio of negative ions to electron densities of element) to estimate the ionic fraction of the bond. In ZnO, though the tetrahedral coordination is an indicator of sp3 covalent bonding, the Zn-O bond also has strong ionic character and thus ZnO lies on the borderline between being classed as a covalent and ionic compound with an ionicity of 0.616 on the Phillips ionicity scale. Degree of ionic bonding can be estimated from the following formula:

I.F ) exp(-0.25 × ∆E2)

Figure 4. Color online (a) XPS survey spectrum of ZnO:Cr (2%) nanopowders. (b) XPS spectra of Cr 2p core level showing binding energy corresponding to Cr3+ state.

observed only after Ar ion sputtering lends credence to the fact that Cr3+ ions are embedded within ZnO lattice and not as surface states. The distinct two states of Cr (2p3/2) and Cr (2p1/2) observed at 577.5 eV and 587.4 eV are due to spin-orbit splitting. This result clearly shows that Cr has been incorporated as Cr3+ in ZnO lattice. Charge compensation in ZnO doped with Cr3+ would require that two Cr3+ ions are substituted for three Zn2+ ions. For trivalent state of Cr3+ dopant, overall charge neutrality in the lattice could be maintained either by creating one Zn2+ vacancy for incorporation of each two Cr3+ ions or introducing one oxygen interstitial (Oi2-) defect in the following manner: 3+ 3Zn2+ ) 2CrZn + VZn2+

(4)

3+ or, 3Zn2+ ) 2CrZn + Zn2+ + Oi2-

(5)

As substitutional Cr3+ has a smaller ionic radius (0.61 Å) than Zn2+ (0.74 Å), Cr-O bond contraction would result in clustering of Cr3+ ions around oxygen atoms. Because EDS results indicate Zn vacancy and surplus oxygen, both defect species may be present for charge compensation. Zinc vacancies have very low formation energy and are more favorable in an oxygen-rich atmosphere. Photoluminescence results as discussed later strengthen the presence of zinc vacancies and Zn vacancy centers are the active point defect in the synthesized ZnO:Cr3+. As the substitution demands the presence of vacancy in a neighboring position for charge compensation, the strain in the ZnO lattice will be more for Cr3+ substitution. Contraction of Cr-O bond length coupled with the lattice strain would result in a spherical structure of 2% Cr-doped ZnO. With excess Cr

(6)

where, I.F is the fraction of ionic bonding and ∆E is the difference in the electronegativities of the elements. The electronegativity of Zn is 1.65 on the Pauling scale, whereas for Cr it is 1.66 on the same scale, that is the ionic fraction is low (∼1), giving rise to longitudinal structure with increased Cr concentration. As formation of elongated structure is indicative of high growth rate along a particular crystallographic direction by addition of oriented nuclei, this is believed to be facilitated by higher electronegativity in Cr-rich precursor solution. Because of the partial ionic character of bonding, electrostatic as well as van der Waals interactions would facilitate oriented linear growth in a direction that leads to minimum surface energy. Magnetization versus magnetic field (M-H) loops measured at room temperature for undoped and Cr-doped ZnO are shown in Figure 5. Undoped ZnO shows diamagnetic behavior (part a of Figure 5). A typical ferromagnetic narrow hysteresis loop is observed for 2% Cr-doped ZnO with maximum magnetization of 8 memu/g, whereas Cr- (10 at %) doped ZnO exhibited super paramagnetic behavior (part b of Figure 5). Such superparamagnetic behavior has been reported for Li-doped ZnO nanocrystals17 and ZnO:Ni thin films.46 For investigating the origin of ferromagnetism in Cr-doped ZnO, the possibility of spurious ferromagnetism due to magnetic impurities or Cr related secondary magnetic phases have to be considered first. The same precursor material [Zn(CH)3(COOH)]2 · 2H2O] with very high purity (99.5%) was used to prepare the samples. Iron (Fe) is the only trace magnetic impurity present in zinc acetate in proportion of 0.0005% and also in the dopant precursor. As undoped ZnO exhibits diamagnetic behavior and same concentration and amount of precursor material [Zn(CH)3(COOH)]2 · 2H2O] has been used for synthesis of all ZnO:Cr samples, trace iron as source of DMS in ZnO:Cr can be immediately ruled out. If trace iron in the dopant precursor could be the cause for ferromagnetism, then magnetization would have increased with an increase in dopant concentration. However, experimental results indicate that 10% Cr-doped ZnO exhibits superparamagnetic behavior. The next possibility is Cr related secondary precipitated magnetic phases. As we discussed in XPS analysis, dopant Cr3+ ion in ZnO lattice is clearly different from the observed secondary phase of Cr2O5 as well as the only ferromagnetic phase CrO2 (Tc ) ∼386 K)35 among all possible Cr oxide phases. This means that there is no precipitated ferromagnetic impurity CrO2 phase formed in the samples. The available theories on DMS of TM-doped ZnO have two basic approaches. In the mean field theory, a single phase random alloy of Zn1-xTMxO, with TM atoms in substitutional lattice

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Figure 6. Color online PL excitation and emission spectrum of Crdoped ZnO nanopowders. The inset shows the luminescence decay profile at 425 nm emission for 2% Cr-doped ZnO.

Figure 5. Color online M vs H curve of (a) undoped ZnO and ZnO: Cr (2%) and (b) 10% Cr-doped ZnO nanopowders.

sites is presumed where interaction between local moments of TM atoms mediated by free carriers gives rise to ferromagnetism. In the second approach, the precipitated magnetic phase of the dopant magnetic atoms within the semiconductor has been thought to be the reason for DMS. An understanding of the origin of ferromagnetism, therefore, needs a thorough analysis of synthesis conditions, observed structural and magnetic properties to establish an unequivocal cause-effect relationship. On the basis of the analysis of experimental results, the second mechanism can be ruled out in the present case. Therefore, results clearly show that observed ferromagnetism is intrinsic in origin and due to ferromagnetic coupling of Cr3+ ions, which is in substitutional sites of Zn. Change in Cr concentration in ZnO leads to a change in the nature of magnetic coupling resulting in a change of its magnetic state. A first principle study of magnetic behavior of Cr-doped ZnO47,48 showed that ferromagnetic coupling between Cr atoms is favored as energy gained from double exchange interaction (d-d) dominates that of superexchange antiferromagnetic interaction. Further, it was shown that a low concentration of Cr and nanodimensions favor stable ferromagnetic state with a higher Tc compared to bulk material. As the present ZnO:Cr nanoparticles exhibit a unique hollow core nanostructure, shape dependence of the observed magnetic behavior needs special mention. Hollow core nanosphere shell particles of 2% Cr-doped ZnO allow magnetic domain wall growth hence exhibiting ferromagnetism. Addition of further Cr (10%) atoms into ZnO transforms the shape into hollow nano-parallelopipeds (part c of Figure 3). It is well-known that the shape of magnetic particles is unequivocally one of the key factors governing their

magnetic behavior. For hollow nano-parallelopipeds, eight edges of its six faces are highly stressed due to bending at right angles. Local stress field developed in and around edges of different faces of parallelopiped gives rise to highly imhomogenous microstrains within a solid phase. Precipitipated Cr2O5 phase worsens the local strain situation. Such local strains do not allow a magnetic domain wall to exist in the region as microstrain points act as a pinning center for the domain wall. Hence, parallelopiped faces become free of the domain wall resulting into an ideal region for single domain development to manifest superparamagnetism. Photoluminescence (PL) excitation and emission spectra of undoped and Cr-doped ZnO hollow nanostructures are shown in Figure 6. The excitation peak of undoped and Cr-doped samples is centered at 325 nm. PL emission spectra of undoped ZnO show a broad peak around 394-415 nm and can be attributed to band edge emission and low intensity peaks in the visible region (440, 476, and 521 nm) due to intrinsic defect levels in ZnO. The disappearance of UV emission band in ZnO: Cr nanoparticles are due to Cr incorporation into ZnO lattice and Cr is a strong quencher of the band edge emission of ZnO.49,50 Cr-doped ZnO shows a strong sharp emission peak at 420 nm and low intensity peaks at 440, 455, 484, and 528 nm. The peaks at 420 nm and 455 nm are present only in Cr-doped ZnO, whereas other peaks are also present in undoped ZnO with some spectral shift arising possibly due to a change in local charge environment because of the presence of the dopant ions and the redistribution of point defects in the lattice and on the surface. The distinct sharp blue emission peak at 420 nm in Cr-doped ZnO arises from recombination of a bound exciton between a shallow Cr3+ donor level and valence band hole. The small emission peak at 455 nm arises due to recombination between a Cr3+ donor level and a VZn2+ acceptor center. From the EDS analysis, we confirm that samples are deficient in Zn. Other PL emission peaks occur due to different donor-acceptor pair transitions. Time resolved luminescence decay at 420 nm emission (insert of part a of Figure 6) shows that the photophysical process of luminescence recombination occurs in the microsecond range and supports bound exciton pair recombination. The decay curve could be fitted into a biexponential equation with estimated decay times of 0.64 µs (38% contribution) and 7.29 µs (62% contribution). 4. Conclusions Stand alone hollow core nanoparticles of undoped and Cr3+doped ZnO have been successfully prepared by the coprecipi-

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tation method near room temperature. The morphology varied from hollow hexagonal nanotubes for undoped ZnO to a hollow core nanosphere for ZnO:Cr (2%) and hollow nano-parallelepipeds for 10% Cr-doped ZnO. Experimental evidence such as change in lattice parameters and presence of Cr in XRD, EDS, and XPS confirmed the incorporation of Cr in to ZnO lattice as Cr3+. Room temperature ferromagnetism and visible luminescence emission from ZnO: Cr3+ hollow core nanoparticles formed by a simple aqueous growth technique make it an attractive material for a wide variety of applications mainly spin LEDs and nanocapsules for biological applications. Acknowledgment. K. Jayanthi acknowledges the Council of Scientific and Industrial Research, Govt. of India, for Research Fellowship. The authors are thankful to Ms. Jyoti shah for magnetic measurements, and Mr. K. N. Sood and Mr. Jay Tawale for SEM measurements. References and Notes (1) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science. 2000, 287, 1019. (2) Dietl, T.; Ohno, H. Physica E 2001, 9, 185. (3) Fukumura, T.; Jin, Z.; Ohtomo, A.; Koinuma, H.; Kawasaki, M. Appl. Phys. Lett. 1999, 75, 3366. (4) Fukumura, T.; Jin, Z.; Kawasaki, M.; Shono, T.; Hasegawa, T.; Koshihara, S.; Koinuma, H. Appl. Phys. Lett. 2001, 78, 958. (5) Jin, Z.; Fukumura, T.; Kawasaki, M.; Ando, K.; Saito, H.; Yoo, Y. Z.; Murakami, M.; Matsumoto, Y.; Hasegawa, T.; Koinuma, H. Appl. Phys. Lett. 2001, 78, 3824. (6) Ando, K.; Saito, H.; Jin, Z.; Fukumura, T.; Kawasaki, M.; Matsumoto, Y.; Koinuma, H. J. Appl. Phys. 2001, 89, 7284. (7) Sonoda, S.; Shimizu, S.; Sasaki, T.; Yamamoto, Y.; Hori, H. J. Cryst. Growth. 2002, 237-239, 1358–1362. (8) Ozgur, U.; Ozgur, U.; Alivov, Ya. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S.-J.; Morkoc, H. J. Appl. Phys. 2005, 98, 041301. (9) Ozgur, U.; Teke, A.; Lui, C.; Cho, S. J.; Morkoc, H. Appl. Phys. Lett. 2004, 84, 3223. (10) Chakraborti, D.; Narayan, J.; Prater, J. T. Appl. Phys. Lett. 2007, 90, 062504. (11) Chu, D.; Zeng, Y. P.; Jiang, D. Solid State Commun. 2007, 143, 308. (12) Roberts, B. K.; Pakhomov, A. B.; Krishnan, K. M. J. Appl. Phys. 2008, 103, 07D133. (13) Jin, Z. W.; Fukumura, T.; Kawasaki, M.; Ando, K.; Saito, H.; Sekiguchi, T.; Yao, Y. Z.; Murakami, M.; Matsumoto, Y.; Hasegawa, T.; Koinuma, H. Appl. Phys. Lett. 2001, 78, 3824. (14) Li, W.; Kang, Q. Q.; Lin, Z.; Chu, W. S.; Chen, D. L.; Wu, Z. Y.; Yan, Y.; Chen, D. G.; Huang, F. Appl. Phys. Lett. 2006, 89, 112507. (15) Park, J. H.; Kim, M. G.; Jang, H. M.; Ryu, S.; Kim, Y. M. Appl. Phys. Lett. 2004, 84, 1338. (16) Deka, S.; Pasricha, R.; Joy, P. A. Phys. ReV. B 2006, 74, 033201. (17) Chawla, S.; Jayanthi, K.; Kotnala, R. K. Phys. ReV. B 2009, 79, 125204. (18) Chawla, S.; Jayanthi, K.; Kotnala, R. K. J. Appl. Phys. 2009, 106, 113923.

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