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Correlation between structural, electrical, dielectric and magnetic properties of semiconducting Co doped and (Co, Li) co-doped ZnO nanoparticles for spintronics applications

A series of co-doped Zn 0.96-y Co 0.04 Li y O(0.00 ≤ y ≤ 0.10) nanoparticles has been synthesized by sol-gel technique. The quantity of each constituent calculated from Rutherford backscattering spectrometry (RBS) and electron energy loss
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  Contents lists available at ScienceDirect Physica E: Low-dimensional Systems and Nanostructures  journal homepage: Correlation between structural, electrical, dielectric and magnetic propertiesof semiconducting Co doped and (Co, Li) co-doped ZnO nanoparticles forspintronics applications Saif Ullah Awan a,b, ∗ , Zahid Mehmood b , Shahzad Hussain b,h , Saqlain A. Shah c,d , Naeem Ahmad e,f  ,Mohsin Ra fi que g,h , M. Aftab b,i , Turab Ali Abbas  j a  Department of Electrical Engineering, NUST College of Electrical and Mechanical Engineering (CEME), National University of Sciences and Technology (NUST) Islamabad, 44000, Pakistan b  Magnetism Laboratory, Department of Physics, Quaid-i-Azam University Islamabad, 44000, Pakistan c  Department of Physics, Forman Christian College (University), Lahore 54000, Pakistan d  Materials Science & Engineering, University of Washington, Seattle, WA, USA e  Spintronics Laboratory, Department of Physics, International Islamic University, Islamabad, 44000, Pakistan f   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences (CAS) Beijing, 100190, China g  State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China h  Department of Physics, COMSATS Institute of Information Technology Islamabad, 44000, Pakistan i  Department of Physics, Government Postgraduate College No.1 Abbottabad, Pakistan  j  Experimental Accelerator Physics Laboratory, National Centre for Physics Islamabad, 44000, Pakistan A B S T R A C T A series of co-doped Zn 0.96-y Co 0.04 Li y O(0.00 ≤ y ≤ 0.10) nanoparticles has been synthesized by sol-gel tech-nique. The quantity of each constituent calculated from Rutherford backscattering spectrometry (RBS) andelectron energy loss spectroscopy (EELS) has been found to be in close agreement with the nominal composi-tions. The presence of single phase hexagonal crystalline structure has been con fi rmed by re fi nement  fi ttingrietveld analysis of X-ray di ff  raction (XRD) data and high resolution transmission electron microscope (HRTEM)measurements. Elemental mapping con fi rmed the homogeneous distribution of Li in the ZnO:Co matrix usingenergy  fi ltered TEM mode. Room temperature dc-resistivity were measured 1200×10 4 ·cm for ZnO:Co while inco-doped nanoparticles the values decreased to a range 3 – 7×10 4 ·cm due to additions of hole carriers. The  p -type carriers with concentration ranging 2.9×10 18 to 9.8×10 18 /cm 3 are found using Hall measurements.From the imaginary part of dielectric measurement, we found that T c  varies in a wide range of 455 – 510K. Thesaturation magnetization at 50K lies in the range 0.18emu/g to 0.45emu/g for di ff  erent Li concentrations. Weobserved a non-monotonic dependence of hole carriers, DC resistivity, dielectric transition temperature andmagnetic moment on varying Li contents. We may argue that hole carrier concentration is responsible for sta-bilizing and promoting magnetism in co-doped (Co, Li) ZnO semiconductors for data storage and spintronicsdevices. 1. Introduction Multiferroics materials have drawn immense attention in the recentyears because of their potential highly dense data storage devices and inspintronics industry. Diluted magnetic semiconductors (DMS) are thematerial of scienti fi c interest because they posses potential for the ad-vanced generation of spintronics devices [1,2]. In this advanced gen- eration of devices, spin degree of freedom along with charge is used totransport, store and process information, providing enhanced perfor-mance and functionalities, high processing speed and storage capacityin contrast to traditional devices [3,4]. The emission of light has been demonstrated in hetero-junction light emitting diodes (LED) using  n -type ZnO with  p -layer [5 – 9] of various other materials. However, thedevelopment of highly e ffi cient, homo-junction devices [10] is chal-lenging due to the non-existence of consistent  p -type ZnO, required toproduce  p  –  n  homo-junction. Transition metals (TM) doped ZnO system 5 October 2017; Received in revised form 16 April 2018; Accepted 17 May 2018 ∗ Corresponding author. Department of Electrical Engineering, NUST College of Electrical and Mechanical Engineering (CEME), National University of Sciences and Technology (NUST)Islamabad, 44000, Pakistan.  E-mail addresses:, (S.U. Awan). Physica E: Low-dimensional Systems and Nanostructures 103 (2018) 110–121Available online 20 May 20181386-9477/ © 2018 Published by Elsevier B.V.    has been extensively studied as DMS over the last decade for theirunique semiconducting and magnetic properties. In particular, the Codoped ZnO(ZnO:Co) system, which is predicted to possess ferro-magnetism, has attracted increasing interest in last decade [11].However, the experimental results of ZnO:Co system have remainedhighly con fl icting with the predictions from theory. Ferromagnetismhas been reported in a number of studies in Co doped ZnO grown bydi ff  erent techniques [11 – 13]. Also there exist a number of reportswhich do not con fi rm ferromagnetism in this system [14 – 17].In previous reports, ZnO:Co DMS system either  p -type [18] or  n -type[19] carriers established long range stabilized magnetic order whileinteracting with Co ions via super-exchange or double-exchange inter-actions mechanisms [20]. However, numerous experimental studieshave indicated that homogeneous ZnO:Co exhibit paramagnetic [21] orspin-glass [22] behavior. The frequently observed ferromagneticZnO:Co system actually srcinates from the Co related micro or nano-clusters of secondary magnetic aggregations [23,24] and defect-related [25,26] mechanisms. These aggregations profoundly a ff  ect the mag-netic interaction by forming anti-ferromagnetic nanocrystals in ZnO:Cosystem [27], which leads to reduced magnetization per Co ion and re-sults in extrinsic ferromagnetism. However, for practical spintronicsapplications, DMS should present intrinsic ferromagnetism with ahomogeneous distribution of the dopants. It is therefore crucially sig-ni fi cant to manipulate the ferromagnetic coupling of the dopant ions inZnO:Co system. Though tremendous e ff  orts have been made to explorethe ZnO:Co system, however, the srcin of ferromagnetism is not clearand its properties have not been well understood yet.Theoretical studies on ZnO:Co have converged to the conclusionthat ferromagnetism requires additional carriers which can be in-troduced through doping [28 – 30]. Spaldin et al. [28] predicted that ferromagnetism would only be found in ZnO:Co in the presence of   p -type carriers. Thus the realization of   p -type ZnO:Co DMS system iscritical. In the earlier studies [30], hole doping was modeled by Znvacancies has been reported. Since Zn vacancies in ZnO are di ffi cult toachieve [31], it was suggested that co-doing of ZnO:Co co-doped withCu can be used to attain room temperature ferromagnetism. However,distinct paramagnetism and antiferromagnetism have been reported inthis system [32]. Despite of di ffi culties in fabricating and achievingsustainable  p -type ZnO, progress in  p -type Li co-doped ZnO has pro-vided some clues to realize hole doping in the ZnO:Co system. Lithium(Li) as a co-dopant to introduce  p -type ZnO:Co has been proposed andinvestigated theoretically [28 – 30,33 – 36]. During growth, Li impuritiesare likely to be incorporated both at interstitial and substitutional sites,and it is highly desired to avoid competing anti-ferromagnetic inter-actions due to Co ions. It is highly signi fi cant to note that both thesubstitutional and interstitial Li stabilize the ferromagnetism. First-principles calculations suggest that the substitutional Li (Li Zn ) acceptorsdepopulate the Co 3d minority spin-states in Li co-doped ZnO:Co [37],thus producing enhanced local magnetic moment. In co-doped ZnOsystem Sluiter et al. [35] has been calculated density of the statessuggested that defects e.g. Zn vacancy (V Zn ), play a vital role to stabi-lized ferromagnetism. Considering Li as a co-dopant, they reported thatferromagnetism is promoted by a low energy de fi cit in the neighbor-hood of two Co atoms. It is particularly signi fi cant as Sluiter et al. [35]consider 1s like state of the substitutional Li with a large radius forindirect exchange interaction.However, a number of experimental studies suggested that in Li co-doped ZnO:Co system, ferromagnetism may srcinate from boundmagnetic polaron (BMP) [38 – 40] or oxygen [36] vacancies, carriers [39] or Li defects [41]. Even so, the srcin of ferromagnetism in oxide systems remains highly controversial. Lin et al. [41] observed that inZnO:Co thin  fi lms ferromagnetism can be enhanced by co-doping of Li,which may be attributed to the indirect exchange via Li-related defects.Li et al. [38] reported ferromagnetism in (Co, Li) co-implanted ZnO fi lms and explained by considering bound magnetic polarons mediatedby defects and electrons. Magnetism in (Co, Li) co-doped ZnO  fi lms hasbeen attributed to the defects introduced by Li dopant which e ff  ectivelyenhance the ferromagnetic coupling interactions between the Co ions.  Jayakumar   et al. [36,42] showed that defects play an important role in activating ferromagnetism in (Co, Li) co-doped ZnO nanocrystals. Sur-factant treatment was also found to signi fi cantly enhance the ferro-magnetism [43]. The co-doping of Li in ZnO:Co increases magnetizationincreases by an order of magnitude due to the charge induced magneticmoment of neighboring oxygen atoms [40]. Zou et al. [39] explained that the srcin of ferromagnetism behavior in Li co-doped ZnO:Co basedon the defects and electrons that form BMP, which overlap to create aspin-split impurity band. It was found that both the substitutional, andinterstitial Li defects played important roles in enhancing the ferro-magnetic interaction between the Co 2+ ions, which were explained byBMP model [44]. It has been reported in Zn:Co co-doped Li  fi lms [45]that Li incorporation results in increased defect density and the electronconcentration and hence the ferromagnetism. Tietze et al. [46]  s ug-gested that oxygen vacancies as the intrinsic srcin for ferromagnetismin Li co-doped ZnO:Co.Despite these e ff  orts as discussed above, the magnetic interactivenature of the dopants in the (Li, Co)ZnO systems still need to be clar-i fi ed. A number of studies have revealed that when the doped Coconcentration is 5at. % or higher, there is a strong trend to form Corelated secondary phases [23,24]. This is the reason; we  fi xed the Coconcentration 4at. % of samples. This would be helpful for under-standing the important issue of how to mediate intrinsic ferro-magnetism in homogenous ZnO:Co through co-doping with Li ions. Inthis manuscript, we present microstructural measurements to in-vestigate the homogeneous crystalline hexagonal structure. We mea-sured the dc-resistivity and dielectric measurements and demonstratedthe role of hole carriers on magnetic moments. This study is focused toidentify the role of co-doped Li (i.e., hole carriers) in the presence of Coto achieve ferromagnetism in ZnO and correlate it with an increase of hole carriers. 2. Experimental details Sol – gel is an attractive technique because it has advantages of easycontrol of the dopant compositions, homogeneity, easy and low costfabrication of materials. It is possible to fabricate multi componentoxide systems such as ZnO. Li co-doped ZnO:Co [Zn 0.96-y Co 0.04 Li y O(  y  =0.00,0.02,0.04,0.06,0.08 and 0.10)] nanoparticles via sol-gelmethod as reported [47]. Brie fl y, Zinc, Cobalt and Lithium acetateswere used as starting precursors. The 0.1M solution was prepared using200ml ethylene glycol with required amount of chemicals (wt%). Thesolution was stirred for about 30min in conical  fl ask and then thesuspension was heated and optimized at 180°C for about 3h in a re- fl uxing system. After the formation of precipitation of the precursors,the mixture was cooled down to room temperature. The as-synthesizedsamples were washed twice with ethanol and distilled water and cen-trifuged at 4000rpm to get solid wet phase. This solid wet phase wasdried in an oven at 100°C overnight. To achieve the required singlephase hexagonal (wurtzite) structure, these samples were annealed atan optimum temperature at 973K in a forming gas (5%H 2  + 95%Ar)for 6 h.Rutherford backscattering spectrometry (RBS) was used to  fi nd thecomposition of the samples. A collimated beam (2.085MeV He 1+ )produced by an accelerator (5UDH Pelletron Tandem) was used forRBS. The back scattered angle ( ∼ 170°) with solid state barrier detector( ∼ 20keV energy resolution). X-ray powder di ff  raction data were col-lected in the range 20º-80° with an integration time of 2s per 0.02° stepusing a PANAlytical X-ray di ff  ractometer with Cu Ka radiation( λ =1.5409 Å ). Structural re fi nements have been performed with theTOPAS program (version 4.1) by employing the rietveld re fi nementtechnique. Transmission electron microscopy (TEM) of (FEI, Titan G260 – 300 CT) was used to analyze the samples by operating the micro-scope at 300kV. We dispersed our sample in di ff  erent solvents e.g.  S.U. Awan et al.  Physica E: Low-dimensional Systems and Nanostructures 103 (2018) 110–121 111  ethanol, 2-propanol, and distilled water and observed that more sta-bilized solution was prepare in distilled water for TEM sample pre-paration. For obtaining higher homogeneous mixture, we used soni-cator to separate the particles at maximum scale for 40min. Wecarefully analyzed that there was no agglomeration and nanoparticleswere fully dissolved in solvent. Then we add a single stable drop of nanoparticles solution (used a pipette of 5 micro liters) onto the sampleholder (carbon coated copper grid of 400 mesh) and dried in a vacuumoven. Both selected area electron di ff  ractions (SAEDs) and TEM Image-mode micrographs were recorded on a 4k×4k CCD camera (US4000,Gatan, Inc., Pleasanton, CA). Elemental analysis was carried out byusing Electron Energy-Loss Spectroscopy (EELS) Tridiem TEM energy- fi lter (Gatan Inc.). Commercial van der Pauw Hall system (HMS-5500)was used for measuring the conductivity type at room temperature. DCresistivity and dielectric properties were studied by two probe con fi g-urations via WAYNE KERR WK-4275 LCR meter with varying thetemperature from 300K to 650K. Powder polycrystalline samples werecompacted into circular pellets by an uni-axial press under identicalconditions (e.g., 6-ton pressure) to retain the similar density over theentire composition range, thereby minimizing the possible variation inthe inter-grain resistivity contribution. Thin layer of silver paint wasused for making parallel plate capacitor geometry to study the electricalproperties. Vibrating sample magnetometer (VSM) of Versa-Lab com-mercial system was used for investigating the magnetic properties. 3. Results and discussion 3.1. Rutherford backscattering spectrometry  Rutherford backscattering spectrometry (RBS) identi fi es di ff  erentelements because of the energy loss by the incident beam (He 1+ ions inour case) due to its interaction with di ff  erent elements while passingthrough a specimen (0.5mm thick pellets). The simulated (red) usingthe RUMP program [48]and experimental (black) curves for y=0.00and 0.10 samples can be seen in Fig. 1(a,b). These spectral lines con- fi rmed the presence of Zn, O and Co. The position of Li ions in thespectra cannot be detected by RBS due to the limitations of instrumentfor smaller ions. Under such conditions, an alternate approach has beenproposed by Valentini et al. [49] for the calculation of Li concentrationwhich is calculated with respect to the concentrations of O, Co and Znobtained from RBS spectra. The calculated Li concentrations are listedin Table 1 with a close agreement with the nominal compositionswithin±0.05 error. 3.2. Structural characterizations The crystal structure of the entire series of co-doped samples wasinvestigated by X-Ray di ff  raction (XRD). Fig. 2 shows the Rietveld re- fi nement fi tting results of Zn 0.96-y Co 0.04 Li y O (  y  =0.00, 0.02, 0.04, 0.06,0.08, 0.10) samples. The crystal structure and lattice parameters ex-tracted via Rietveld  fi tting for all compositions are summarized inTable 2. We remained very careful to analyze the crystalline structure of mixed metal oxides like e.g. LiCoZnO 2 . Our XRD patterns and Rietveldre fi nement data con fi rmed the single phase mixed metal oxide notcomplexes. We observed single phase crystalline structure and absenceof any type of secondary type metallic or impurity phases. Transmissionelectron microscope (TEM) measurements were performed for furtheranalysis. Fig. 3(a) depicts the image of Zn 0.94 Co 0.04 Li 0.02 O as acquiredfrom High resolution transmission electron microscope (HRTEM),which shows the homogeneous crystalline structure without the pre-sence of any amorphous phase. Fig. 3(b) shows HRTEM micrograph forZn 0.90 Co 0.04 Li 0.06 O sample. The  d -spacing was extracted from the se-lected area electron di ff  raction (SAED) patterns (seen insetFig. 3(a – b)).The deviation in  d -values probes the role of Li dopantwhether it occupies substitutional or interstitial position in hexagonalstructure of ZnO. These  d -values measured from XRD and TEM are ingood agreement and corresponds to hexagonal wurtzite (SG  P  6 3 mc , a =0.32nm,  c =0.52nm) crystalline structure without any metallic orsecondary phase. We observed that  d -spacing of the sample with 6% Li Fig. 1.  Rutherford backscattering spectrometry (RBS) spectra recorded suing2.085MeV He 1+ beam from a sample (a) Zn 0.96 Co 0.04 O and (b)Zn 0.86 Co 0.04 Li 0.10 O. The black data points represent the srcinal data collectedduring experiments, whereas the red solid traces correspond to their best  fi ttedsimulation. (For interpretation of the references to color in this  fi gure legend,the reader is referred to the Web version of this article.) Table 1 Zn, Co, O and Li concentrations in nanoparticles samples from RBS/C analysis. Sample Comp.(Nominal)Measured Compositions (±0.05)Zn (Conc.)(%)O (Conc.)(%)Li (Conc.) a (%)Co (Conc.)(%)Zn 0.096 Co 0.04 O 95.97 98.92 0.00 3.98Zn 0.94 Co 0.04 Li 0.02 O 93.98 98.93 1.98 3.96Zn 0.92 Co 0.04 Li 0.04 O 91.95 98.94 3.91 3.97Zn 0.90 Co 0.04 Li 0.06 O 89.94 98.96 5.93 3.94Zn 0.88 Co 0.04 Li 0.08 O 87.92 98.92 7.96 3.95Zn 0.86 Co 0.04 Li 0.10 O 85.90 98.95 9.95 3.96 a The Li concentration is calculated with respect to ZnO:Co quantity obtainedby RBS.  S.U. Awan et al.  Physica E: Low-dimensional Systems and Nanostructures 103 (2018) 110–121 112  was smaller than that of 2%, which can be attributed to the compressivestrain in the unit cell because of the smaller size of Li dopant. As Lisubstitutes Zn, lattice parameter decreases. On the other hand, if Lioccupies interstitial sites, lattice parameters are expected to increase[50,51]. As EDS spectra cannot detected Li due to the limitations of Bewindow. The presence of Li was investigated by electron energy loss(EELS) spectroscopy and presented in Fig. 4(a) for Zn 0.90 Co 0.04 Li 0.06 O Fig. 2.  Rietveld re fi nement  fi tting results of the X-ray powder di ff  raction patterns of Zn 0.96-y Co 0.04 Li y O (  y  =0.00,0.02,0.04,0.06,0.08, 0.10) nanoparticles samplesmeasured at 300K showing the observed pattern (diamonds in red color), the best  fi t Rietveld pro fi les (black solid line), re fl ection markers (vertical bars), anddi ff  erence plot at the bottom (blue solid line) [Orignal XRD data has been published S.U. Awan et al. J. Phys. Condens. Matter 25, 156005 (2013) Copy rights IoP].(For interpretation of the references to color in this  fi gure legend, the reader is referred to the Web version of this article.)  S.U. Awan et al.  Physica E: Low-dimensional Systems and Nanostructures 103 (2018) 110–121 113  sample. The O-K, Co-L 23  and Zn-L 23  edges can be seen in full scan range(400 – 1400eV) of EELS spectrum for Zn 0.90 Co 0.04 Li 0.06 O nanoparticles,while Fig. 4(b) depicts EELS spectra for a range of 45 – 250eV forZn 0.90 Co 0.04 Li 0.06 O sample. Zn-M 23 edge and Li-K edge are more clear inthe scan range (50 – 180eV). No other element has been detected inEELS spectra. We performed an energy  fi ltered transmission electronmicroscopy (EF-TEM) measurement for Li mapping. Using EF-TEMmode low resolution image of Zn 0.90 Co 0.04 Li 0.06 O sample was measuredas shown in Fig. 5(a). Li mapping was attained from Fig. 5(a) and mi- crograph shown in Fig. 5(b). We have presented dark  fi eld TEM imagesof few selected sample in Fig. 5(a). This TEM data re fl ects the low re-solution image with full presentation of all elements e.g (Zn, Co, Li andO species. The bright areas in Fig. 5(b) only re fl ecting the distributionof Li i.e. these bright spots are showing presence of Li in ZnO:Cosample. This micrograph con fi rms the homogeneous and uniform dis-tribution of Li into the ZnO:Co crystal without any precipitates orclusters. These results showed the absence of any metallic or externalimpurity. We make sure properly and analyzed that the thickness of prepared sample is homogeneous otherwise the Co and Li signals withtheir proper distribution will be not obtained. As, we obtained thesignals of Li and Co of well strength so this is the actual result of how weprepare well homogeneous sample for TEM measurements to obtainedhomogeneous distribution. Our data of EELS and EF-TEM re fl ects thehomogeneous distribution of Li into the matrix of ZnO:Co sample. Aftercon fi rmation of structure and morphology, we investigate the electricaland magnetic properties of semiconducting co-doped ZnO samples. 3.3. Hall measurements Hall E ff  ect measurements were carried out on the square(10×10mm 2 ) type pelletized samples of thickness 1mm for a series of Zn 0.96-y Co 0.04 Li y O nanoparticles with di ff  erent compositions(0.00 ≤ y ≤ 0.10). We found that undoped ZnO and Co doped ZnOsamples exhibited  n -type semiconductor conductivity. Theoretical andexperimental reports have demonstrated that shallow donor levels inZnO systems (doped) may give rise to  n -type, whereas acceptor levelsinduce  p -type semiconductor conductivity. The measured values of   n -type charge carriers for ZnO and ZnO:Co are 4.4×10 15 /cc and6.7×10 15 /cc respectively in agreement with the literature [33,52]. We observed that co-doped compositions showed a non-monotonic trend of   p -type conductivity in the range 2.9×10 18 to 9.8×10 18 /cc versus Licompositions. For a series of Li co-doped samples, type of carriers, re-sistivity, mobility and carrier concentrations were measured and pre-sented in Table 3. In Li co-doped ZnO:Co system, many types of defects,e.g., interstitial Li and oxygen, substitutional Li and Co, and oxygen andzinc vacancies, can supply free charge carriers (e.g. electron and hole)[28]. In the Hall measurement shows a net amount of charge carrierswith contribution from above mentioned sources for each compositionas presented in Table 3. These results clearly showed that the compo-sitions having the highest hole concentrations correspond to the lowestvalue of the resistivity (e.g. for y=0.06), whereas, y=0.02, 0.08 and0.10 samples carried lower carrier concentrations and higher resistivityvalues. This may be due to the fact that the prominent role of holecarriers might have been compensated by  n -type carriers in these co-doped systems. Interestingly, we found that for y=0.04 and y=0.06compositions, considerable decrease in resistivity, and  p -type charge Table 2 Structural parameters co-doped ZnO:Co [Zn 0.96-y Co 0.04 Li y O (  y  =0.00,0.02,0.04,0.06,0.08 and 0.10)] nanoparticles samples obtained from XRD data using Rietveldre fi nement  fi tting method. Sample ID a (Å) b (Å) c (Å) Volume (Å 3 ) Density gm/cm 3 R p  R wp  χ  2 ZnO 3.250808(±0.000092)3.250808(±0.000092)5.207338(±0.000240)47.657(±0.003)5.671 0.1252 0.2112 1.119y=0 3.251096(±0.000084)3.251096(±0.000084)5.207348(±0.000215)47.666(±0.002)5.670 0.1342 0.2208 1.23y=2 3.251353(±0.000074)3.251353(±0.000074)5.207448(±0.000189)47.674(±0.002)5.669 0.1335 0.2210 1.272y=4 3.250150(±0.000062)3.250150(±0.000062)5.206621(±0.000170)47.631(±0.002)5.674 0.1239 0.2101 1.24y=6 3.250027(±0.000062)3.250027(±0.000062)5.206460(±0.000171)47.626(±0.002)5.675 0.1222 0.2096 1.234y=8 3.250485(±0.000089)3.250485(±0.000089)5.206511(±0.000273)47.640(±0.002)5.673 0.1446 0.2254 1.252y=10 3.250134(±0.000050)3.250134(±0.000050)5.206152(±0.000128)47.627(±0.001)5.675 0.1393 0.2183 1.205 Fig. 3.  High resolution transmission electron microscopy (HR-TEM) images of (a) Zn 0.94 Co 0.04 Li 0.02 O nanoparticles and (b) Zn 0.90 Co 0.04 Li 0.06 O nanoparticles.  S.U. Awan et al.  Physica E: Low-dimensional Systems and Nanostructures 103 (2018) 110–121 114
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