Half Metallic Ferromagnetism in Eu-Doped CdS Nanoparticles - The

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Half Metallic Ferromagnetism in Eu-Doped CdS Nanoparticles Rui Zhao, Pan Wang, Tianye Yang, Zhifang Li, Bingxin Xiao, and Mingzhe Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10444 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Half Metallic Ferromagnetism in Eu-Doped CdS Nanoparticles Rui Zhao†‡, Pan Wang†, Tianye Yang†, Zhifang Li†, Bingxing Xiao† and Mingzhe Zhang†* †

State Key Laboratory of Superhard Materials, Jilin University, Changchun, 130012,

People’s Republic of China. ‡

College of Computer, Jilin Normal University, Siping 136000, People's Republic of

China ∗

Corresponding author. E-mail address: [email protected]

Tel: +8643185168881; fax: +8643185168881.

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

ABSTRACT Hexagonal wurtzite structured Eu3+ doped CdS nanoparticles (CdS:Eu NPs) were synthesized via a gas-liquid phase chemical deposition method. It is found that as-prepared

CdS:Eu

NPs

exhibit

distinct

and

stronger

room-temperature

ferromagnetism (RTFM). The ferromagnetism as observed from the zero-field-cooling and field-cooling curves of CdS:Eu NPs has been further confirmed by our first-principles calculations based on the density functional theory. Because of a coupling between the 4f-related spin polarized states and the carriers, the half metallic ferromagnetic CdS:Eu NPs could be a desired material for the use in spintronics. The origin of ferromagnetism in CdS:Eu NPs is attributed primarily to the Eu atoms, and partially to the Cd vacancies as well. Keywords: CdS; Ferromagnetism; First-principles; Half metallic; doped

2


Page 3 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION To exploit the full potential of spintronics, the development of new magnetic materials, magnetic semiconductors, and half-metallic ferromagnets with Curie temperatures higher than room temperature remains a continuous priority.1,2 Toward this end, nano-scale ferromagnetic semiconductors with high Curie temperatures and high saturation magnetization (Ms) are essential items in the spintronics materials. The best choices for meeting aforementioned needs are in the classes of transition-metal

oxides,

manganese-doped

GaAs

and

GaN,

transparent

ferromagnet-doped oxides, B3 and B4 compounds, and doped semiconducting C1b alloys.2 An intuitively straightforward route is to dope the conventional semiconductors to make them magnetic.3 To date, great efforts have been devoted to the exploration of new ferromagnetic dilute magnetic semiconductor materials (DMSs). CdS is one of the wide bandgap II-VI semiconductors. There are two possible structures for CdS under normal conditions: cubic zinc-blende (B3) structures and hexagonal wurtzite (B4) structures. It can be also used as DMSs when a fraction of component ions is replaced by transition-metal /rare-earth ions impurities. More recently, there are many studies of CdS in photocatalysis, solar cells, gas sensors, biological sensors, and magneto-optical devices (magnetic field sensors, isolators, magnetic recording, and magneto-optical switches).4-10 These studies demonstrate how doping can optimize optical properties and magnetic properties. Sambandam et al. report that both the intensity and the line width of the PL emission of Mn-doped

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CdS NPs continuously but disproportionately decrease with increased addition of Mn2+. The preannealed sample of 5% Mn-doped CdS exhibits weak ferromagnetism (FM), whereas the postannealed sample shows antiferromagnetism (AFM) coupled superparamagnetism (SPM) behavior below the Neel temperature.11 Giribabu et al. find redshift of absorption edge and bandgap narrowing in Co-doped CdS NPs. Ms increases with cobalt content up to 4% (Ms=0.0088 emu/g) and then decreases (Ms=0.0027 emu/g) by RTFM hysteresis loop.12 Similar results are found by Hu et al. that the value of Ms increases initially and then drops with increasing doped concentration.13 Study shows that Gd-doped CdS nanorods exhibit ferromagnetism and the value of Ms increases upon Gd-doping.14 The electronic structure of the lanthanide ion in its most frequently occurring trivalent state consists of a common xenon core and a 4fn shell, which is progressively filled through the series. Imperfect shielding of 4fn electrons results in the electrons getting drawn inside of the 5s25p6 closed shells of the xenon core, leading to the effect known as lanthanide contraction.15 As we know, the unpaired electrons show the abnormal spin phenomenon, which can cause magnetism. Thus, lanthanide ion doped DMSs would show many novel properties. There is continuing interest in Eu-doped CdS for a variety of applications including photology,16,17 biological detection,18,19 solar cells,20 and fluorescent probes.21 Magnetism is of great concern for luminescent materials applied in magnetic resonance imaging, bioprocessing and drug delivery.22 To the best of our knowledge, however, most previous efforts in Eu-doped CdS NPs focused just on its optical properties, not on magnetic properties. It is essential to

4


Page 4 of 33

Page 5 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


elucidate the electron structures which are responsible for their magnetic properties as well as their optical properties. In this paper, we report the synthesis of high quality ferromagnetic CdS:Eu NPs, and systematically investigate the mechanism of the magnetism of CdS:Eu NPs. EXPERIMENT The synthesis of CdS:Eu NPs was carried out by gas–liquid phase chemical deposition. In the preparation stage, the reactive solutions of Cd(COOCH3)2 (purity 99.99%),

Eu(COOCH3)3

(purity

99.99%),

the

surface-active

agent

polyvinylpyrrolidone (PVP), and deionized water were mixed by a magnetic stirring apparatus. At the same time the pH was adjusted to 9.5～10 by dropwise addition of sodium hydroxide solution. After being uniformly mixed, the reactive solution was transferred to the separating funnel in a circulatory constant temperature water bath and the temperature was kept at ~ 28 ºC. There were two reactive steps in the experiment: The H2S gas was prepared by HCl reacting with Na2S according to the ratio of 1:1; then the H2S gas and the reactive solution reacted on the hemispherical crown’s polished surface in the chamber. The previous study suggests that doping needs a slow growth process for the efficient adsorption of dopants.23 Thus, the dripping speed of the reactive solution was 0.2 mol min-1 controlled by the separating funnel in the reaction process. The reactive solution dropped and spread evenly over the hemispherical crown’s polished surface, and then reacted with the H2S gas (added in excess) which carried by nitrogen. Numerous nuclei were formed and started to grow on the hemispherical glass crown

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

surface, then, the homogeneous resultants flowed into the chamber along the cambered edge of hemispherical crown. The chamber was placed in the ultrasonic bath to prevent the nanoparticles from forming aggregates. This process was repeated until reaction was completed. The precipitates were collected and washed in both deionized water and anhydrous alcohol three times, and were then dried in a nitrogen atmosphere. The pure CdS NPs were also synthesized by the same method except that the Eu(COOCH3)3 was not added. The morphology, structure, and crystal size of the nanoparticles were observed and measured by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) (TECNAI G2). The valence states of the elements were analyzed by using X-ray photoelectron spectroscopy (XPS) (ESCALAB MK II). The dopant concentrations of the samples were investigated by energy dispersive X-ray spectroscopy (EDS). The photoluminescence (PL) spectroscopy was performed via a Perkin Elmer photoluminescence. The magnetic properties were investigated with a vibrating sample magnetometer (VSM) and a Quantum Design MPMS SQUID. In order to explore magnetic mechanism, the first-principles calculations were carried out using the Vienna ab initio simulation package (VASP) based on DFT and the projector augmented plane-wave (PAW) pseudopotential. PBE approximation and generalized gradient approximation (GGA) were chosen for exchange correlation functional. The convergence tests were executed for the energy cutoff and the k-point. The convergence criterion was energy difference under 1 meV. The energy cutoff was chosen to be 380 eV. The 5×5×4 k-point meshes were used for three models. They

6


Page 6 of 33

Page 7 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were: (a) the ideal system with no defect and dopant (Cd36S36); (b) one Cd vacancy created by removing one Cd atom (Cd35S36); (c) one Cd atom selected to be substituted by one Eu atom (Cd35Eu1S36). On the basis of all unit cell, the 3×3×2 supercell includes 72 atoms. The total energy is converged to 1.0 × 10-4 eV/atom. RESULTS AND DISCUSSION The phase composition and the valence states of the elements of as-synthesized samples were analyzed by means of XRD and XPS. The dopant concentrations of the samples were measured by EDS. The EDS values show that the concentration (Atomic %) of Eu in three doped samples were 0.08 at. %, 0.19 at. %, and 0.26 at. %. The XRD pattern of pure CdS and CdS:Eu NPs are shown in Figure 1a. It can be seen from the XRD pattern that all of the diffraction peaks are indexed to the standard wurtzite structured CdS (JCPDS No.75-1545, space group: P-63mc (no. 186)). The average size of the nanoparticles is calculated to be around 8-10 nm using the Scherrer equation. It is obvious that all peak positions of the CdS:Eu NPs slightly shift to higher angles (lower d value) when compared to those of the pure CdS with increasing Eu concentration. A lattice compression phenomenon is indicated as a result of the substitution of smaller ion radius Eu3+ (0.0947 nm) for the Cd2+(0.097 nm). The lattice parameters of CdS:Eu NPs were determined as a=4.1661 Å and c=6.7775 Å (c/a=1.627). Moreover, the lack of europium clusters, europium sulfides, or europium oxides found in the XRD pattern shows that the nanoparticles are composed of pure phase.

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a) XRD patterns of CdS:Eu NPs with dopant content 0.00 at. %, 0.08 at. %, 0.19 at. % and 0.26 at. %; (b) S 2p3/2 and S 2p1/2 XPS spectrum; (c) Cd 3d5/2 and Cd 3d3/2 XPS spectrum; (d) Eu 3d5/2 and Eu 3d3/2 XPS spectrum. To further investigate the chemical composition and the bonding state, we performed XPS measurements on the 0.19 at. % CdS:Eu sample. The S 2p, Cd 3d, and Eu 3d XPS spectrums are shown in Figure 1b, Figure 1c, and Figure 1d, respectively. According to the result, the S 2p level was resolved into a single spin-orbit splitting of Gaussian component. The spin-orbit splitting separation was allowed to vary during fitting. The binding energy values of S 2p3/2 and S 2p1/2 are 161.15 eV and 162.28 eV with separation of 1.13 eV, respectively. The Cd 3d spectrum has two peaks due to spin-orbit splitting, resulting in to 3d5/2 and 3d3/2 peaks. The binding energy value of Cd 3d5/2 is 404.8 eV and Cd 3d3/2 is 411.5 eV with separation 6.7 eV. The characteristic binding energies of XPS of S and Cd agree with those reported literature.24 Figure 1d presents the Eu 3d XPS spectra. The two characteristic peaks at 1134.08eV and 1164.79 eV are attributable to the core levels of 8


Page 8 of 33

Page 9 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Eu 3d3/2 and Eu 3d5/2, respectively, which indicates that the Eu ions are trivalent.25 Thus, it is strongly evidenced that we have successfully incorporated the lanthanide Eu3+ ions and dispersed them in the CdS main structure. Microstructures of CdS and various CdS:Eu NPs are characterized by HRTEM and selected-area electron diffraction (SAED) in Figure 2a - 2h, respectively. The results reveal that the nanocrystals are faceted, crystalline, and have an average size of 8∼10 nm, which is consistent with that estimated from XRD pattern broadening by using the Scherrer equation. The SAED pattern indicates that the diffraction rings correspond to the XRD pattern without any impurities, revealing only a single phase

Figure 2. HRTEM images and SAED pattern of Eu-doped CdS NPs (i.e. 0.00 at. %, 0.08 at. %, 0.19 at. % and 0.26 at. % Eu)

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(hexagonal wurtzite) in CdS:Eu NPs. To investigate the effect of doped ions, a typical PL emission spectrum is shown for as-prepared pure CdS and CdS:Eu NPs with different doping concentration in Figure 3. For pure CdS NPs, broad emission results in the region of 450-650 nm peaking at 533 nm when excited with 325 nm radiation in Figure 3a. This single broad emission is interpreted to be a cadmium-vacancy-related defect states (VCd).11 Such vacancies present either as homometallic Cd2+-Cd2+ vacant pairs close to each other, or as isolated single Cd2+ vacancies. It can be observed that the intensities of the VCd emission with the various kinds doped concentrations are lower than pure CdS NPs. It is due to the reduction in the number of Cd vacancy sites by the occupation of Eu3+. This type of quenching has been reported in many literatures.11, 26 It is noteworthy that the peak positions of VCd emission have slight change with varying the doping concentration of Eu3+ ions from 0.08 at. % to 0.26 at. %. The peak position of VCd shows a red shift during the initial addition of Eu3+ in contrast with pure CdS Nps which can be attributed to the reduction in the number of isolated VCd. But the blue shift of VCd peak position with the increasing of the doped concentration is attributed to the reduction in the number of VCd intersite interactions specially when they are close to each other which in turn is actually responsible for the formation of exchange coupled dimers when occupied by Eu3+ ions.11 The Eu3+ ions possessing multiple sites in nanoparticles exhibit different luminescent properties for each single site. The 5D0-7F2 transition of Eu3+ ion is an electric-dipole (ED) nature and very sensitive to its site symmetry. However, 5D0-7F1

10


Page 10 of 33

Page 11 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transition is a magnetic-dipole (MD) nature and insensitive to site symmetry. In Figure 3b - 3d, The characteristic peaks of Eu3+ ion at 591 nm and 615nm correspond to the MD (5D0-7F1) and the ED (5D0-7F2), respectively. 15

Figure 3. PL spectrums for (a) un-doped (0%); (b) 0.08 at. %; (c) 0.19 at. % and (d) 0.26 at. % CdS:Eu NPs using excitation at 325 nm. As we know the f-f transitions arise from forced electric dipole which are parity forbidden, but it become partially allowed when the ion is situated at a low symmetry site. As a result the radiative emission rate increases.27 The 5D0-7F2 luminescence intensities of the three CdS:Eu samples increase with the rising of the concentration of Eu3+ ions, which indicates that Eu3+ ions locate at a low-symmetry site without inversion center.28 The PL measurement proves the existence of Eu3+ in the CdS hosts. Figure 4a presents a study of the magnetic property of CdS:Eu NPs in different doped concentrations with a magnetic field from -6000 to 6000 Oe using VSM. The 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

characteristics of the magnetization hysteresis loops demonstrate that CdS:Eu NPs have RTFM. The saturation magnetizations are determined to be 0.0175 emu/g, 0.0322 emu/g, and 0.0088 emu/g for samples 0.08 at. % Eu, 0.19 at. % Eu, and 0.26 at. % Eu, respectively. The corresponding coercivities are revealed at 101.9 Oe, 59.05 Oe, and 59.76 Oe in Figure 4b. Remarkably, as the doped concentration is increased,

Figure 4. (a) Magnetization hysteresis loop of CdS:Eu nanoparticles at room temperature for 0.08 at%, 0.19 at% and 0.26 at % Eu; (b) enlarged view in the field between -200 and 200 Oe at 300 K; (c) Magnetization plots of 0.26 at. % CdS:Eu nanoparticles as a function of the applied fields (-60 and 60 kOe) at 10 K, 150 K, 300K and 380 K. 12


Page 12 of 33

Page 13 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the value of the Ms increased initially and then decreased, as seen in some other systems, thus suggesting new operative mechanisms of ferromagnetic behaviour.12, 13, 29

The preceding report suggests that the introduction of parent ion vacancies leads to

magnetic moment reduction, albeit marginally. However, with some Co impurity fraction in the octahedral interstitial site inside the wurtzite CdS:Co cage, the magnetic moment drops drastically.29 Thus, as the concentration of Eu grows, the decreases of the parent ion vacancies and substitutions of the cations result in the increase of Ms. But, after the substitutions balance, an increasing fraction of substitutions is pushed to the interstitial site which causes the decreases of Ms. The Figure 4c displays the variation of magnetization loops for 0.26 at. % CdS:Eu NPs at 10K, 150K, 300K and 380K in a field between -60 and 60 kOe. The magnetization decreases from 0.029 emu/g to 0.0084 emu/g at 60 kOe from 10K to 380K. It is clear that the two curves at 300K and 380K tend to be in a state of superposition. This indicates that the saturation magnetization is nearly constant above room temperature. There are relevant reports suggesting that negligible coercivity and remanence in the loops indicate SPM, which is a characteristic of small ferromagnetic particles.30, 31 In order to accurate confirm the magnetic phase transition, the temperature dependence of magnetization curves were measured. Figure 5 shows the zero-field-cooling (ZFC) and field-cooling (FC) curves of CdS:Eu NPs with dopant contents of 0.08 at. % , 0.19 at. %, and 0.26 at. % measured at an applied magnetic field of 500Oe up to room temperature. As is shown in Figure 5, the distinct divergence between corresponding ZFC and FC curves of all samples indicate

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

occurrence of ferromagnetism in the whole temperature range. The similar behavior of ZFC-FC has also been observed in the case of (Mn, Zn) Co-doped CdS nanowires.10 The behaviour with steep rise observed at low temperature was explained in terms of polaron-percolation-theory.32 The feature around 61 K in ZFC-FC curves of 0.19 at. % sample may appear because of contamination from molecular oxygen.33 To further demonstrate the origin of magnetism of CdS:Eu NPs, we investigate the magnetic interactions in three supercell models of wurtzite CdS by performing the first-principles spin-polarization density of states (DOS) calculations. Three models have been built. Figure 6a is corresponding to the ideal system of pure CdS (Cd36S36) with the optimized lattice constants (a=4.127 Å, c=6.717 Å, c/a=1.628 ), which agree well with the experimental values. Figure 6b and 6c are corresponding to Cd35S36 and Cd35Eu1S36, respectively. Figure 7 shows the spin-polarized DOS of three models. For Cd36S36, the computational result indicates that the total magnetic moment of the system is zero,

Figure

5.

Magnetization

versus temperature for field cooled (FC) and

zero-field-cooled (ZFC) measurements with H=500 Oe. (a) 0.08 at. % CdS:Eu; (b)

14


Page 14 of 33

Page 15 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.19 at. % CdS:Eu; (c) 0.26 at. % CdS:Eu. The insets are respectively corresponding to the high-magnification profiles of the three curves between 260 K and 300 K.

Figure 6. Three calculation models: (a) the ideal system with no defects or dopants (Cd36S36); (b) one Cd vacancy created by removing one Cd atom (Cd35S36), and the black circle is the site of the removable Cd atom; (c) one Cd atom replaced by one Eu atom (Cd35Eu1S36).

Figure 7. Total and partial DOS of three models. (a) the ideal system (Cd36S36), (b) one Cd vacancy created by removing one Cd atom (Cd35S36), (c) one Cd atom replaced by one Eu atom (Cd35Eu1S36). The vertical line denotes the position of the

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fermi energy, which has been chosen to be 0.0 (eV). Positive values correspond to the spin up and negative values to the spin down. which is evidenced by the symmetrical DOS in Figure 7a. For Cd35S36 and Cd35Eu1S36, all of their total spin-polarized DOS are split corresponding to Figure 7b and 7c, respectively. Hence, the observed ferromagnetism in CdS:Eu NPs arises not only from the Cd vacancies, but also from the Eu atoms. In order to provide a qualitative explanation of the orbital origins of the different total DOS. We also display the partial densities of Cd35S36 and Cd35Eu1S36. According to Figure 7b, it is clear that the Fermi level crosses the minority spin states while it crosses the majority spin states, which demonstrates that the CdS with Cd vacancy are just magnetic semiconductors. It is noticed that both the S-3p states and Cd-4d states are at the Fermi level, indicating a strong p-d hybridization. The total DOS of Figure 7c shows a typical half metallic characteristic. The Eu 4f majority spin states cross the Fermi level, whereas the minority spin states have a large band gap at the Fermi level and are partially occupied for both compounds. According to XPS, the chemical valence of Eu atom in CdS:Eu is trivalent, while the average chemical valence of Cd atom in CdS:Eu is bivalent. Therefore, one Cd atom is substituted by one Eu atom corresponds to one extra electron in CdS:Eu. This extra electron occupies the bottom of the CdS conduction band, which presents electric neutrality. As a result, this neutrally charged Eu impurity in CdS has its total spin associated to both the 4f-related states and the delocalized spin-polarized carrier in the conduction band. To analyze the spin-polarization induced by the Eu atom and the Cd vacancy, we 16


Page 16 of 33

Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


illustrate the spin-density distribution and discuss the origin of the magnetic moment. As shown in Figure 8a, the most spin-densities are localized on the four nearest neighboring S atoms surrounding the vacancy site with a fraction distributing on the next nearest neighboring S atoms. The distribution is highly anisotropic. Obviously, the magnetic moment of Cd35S36 (MTotal = 0.998 µB) was mainly contributed by the S atoms (MS = 0.916 µB). In Figure 8b, one can see that the spin-up density is restricted mainly on the Eu atom and the next nearest neighboring S atoms. The four nearest neighboring S atoms of Eu atom exhibit the spin-down density, indicating that the conduction band carriers having mostly S-p orbitals interact antiferromagnetically with Eu (MS = -0.05 µB ). The total magnetic moment of Cd35Eu1S36 (MTotal = 6.729 µB) is mainly from the Eu atom in CdS:Eu (MEu = 6.760 µB), which is consistent with the results of DOS. The total magnetic moment of Cd35Eu1S36 amounts to 6.729 µB in contrast to 0.998 µB of Cd35S36. It demonstrates that the ferromagnetism of the samples mainly results from the substitution of Eu atoms, which make the major contribution.

Figure 8. The spatial distribution of the spin density for (a) Cd35S36 and (b) Cd35Eu1S36 in the FM state. The yellow isosurface corresponds to the spin-up density 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and the blue isosurface is spin-down density. The black circle is the site of the removable Cd atom. CONCLUSIONS In summary, in this paper we report the magnetic properties of the Eu3+-doped CdS NPs. The stronger room-temperature ferromagnetism is observed in CdS:Eu NPs with a well-crystallized hexagonal wurtzite structure. There are no traces of any secondary phases. The highest Ms is 0.0322 emu/g for 0.19 at. % Cd:Eu NPs. The ZFC-FC curves show the ferromagnetism of CdS:Eu NPs. The characteristic peaks of Eu3+ (5D0–7F1 and 5D0–7F2) are proved by the PL measurement, which demonstrates the existence of Eu3+ in the CdS hosts. Half metallic ferromagnetism characteristic is presented in DOS by first-principles calculation. The Eu substitution is the primary origin of magnetism relative to the Cd vacancy. The Eu 4f orbitals are the most contributor of the magnetic moment. There is the strong coupling between the negative carrier and the 4f-related states, which indicates its potential application in spintronics. ACKMOWLEDGMENTS This work was funded by the National Science Foundation of China, no. 11174103 and 11474124, and Specialized Research Fund for the Doctoral Program of Higher Education of China, no. 20130061110012, and Program for the development of Science and Technology of Jilin province (No. 20140101206JC-07). Our calculated work was supported by High Performance Computing Center of Jilin University, China. REFERENCES 18


Page 18 of 33

Page 19 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(1) Dietl, T. A Ten-Year Perspective on Dilute Magnetic Semiconductors and Oxides. Nat. Mater. 2010, 9, 965-974. (2) Felser, C.; Fecher, G. H.; Balke, B. Spintronics: A Challenge for Materials Science and Solid-State Chemistry. Angew. Chem. Int. Ed. 2007, 46, 668-699. (3) Ohno, H. Toward Functional Spintronics. Science 2001, 291, 840-841. (4) Kim, D. S.; Cho, Y. J.; Park, J.; Yoon, J.; Jo, Y.; Jung, M. (Mn, Zn) Co-Doped CdS Nanowires. J. Phys. Chem. C 2007, 111, 10861-10868. (5) Cao, M. H.; Wang, P. F.; Ao, Y. H.; Wang,C.; Hou, J.; Qian, J. Investigation on Graphene and Pt Co-Modified CdS Nanowires with Enhanced Photocatalytic Hydrogen Evolution Activity under Visible Light Irradiation. Dalton Trans. 2015, 44, 16372-16382. (6) Nascimento, C. C.; Andrade, G. R. S.; Neves, E. C.; Barbosa, C. D. A. E. S.; Costa, L. P.; Barreto, L. S.; Gimenez, I. F. Nanocomposites of CdS Nanocrystals with Montmorillonite Functionalized with Thiourea Derivatives and Their Use in Photocatalysis. J. Phys. Chem. C 2012, 116, 21992−22000. (7) Sun, W. T.; Yu, Y.; Pan, H. Y.; Gao, X. F.; Chen, Q.; Peng, L. M. CdS Quantum Dots Sensitized TiO2 Nanotube-Array Photoelectrodes. J. Am. Chem. Soc. 2008, 130, 1124-1125. (8) Zhang, Q. X.; Guo, X. Z.; Huang, X. M.; Huang, S. Q.; Li, D. M.; Luo, Y. H.; Shen, Q.; Toyoda, T.; Meng, Q. B. Highly Efficient CdS/CdSe-Sensitized Solar Cells Controlled by the Structural Properties of Compact Porous TiO2 Photoelectrodesw. Phys. Chem. Chem. Phys. 2011, 13, 4659-4667.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(9) Zhai, J. L.; Wang, L. L.; Wang, D. J.; Li, H. Y.; Zhang, Y.; He, D. Q.; Xie, T. F. Enhancement of Gas Sensing Properties of CdS Nanowire/ZnO Nanosphere Composite Materials at Room Temperature by Visible-Light Activation. ACS Appl. Mater. Inter. 2011, 3, 2253-2258. (10) Chronopoulos, D. D.; Karousis, N.; Zhao, S.; Wang, Q.; Shinohara, H.; Tagmatarchis, N. Photocatalytic Application of Nanosized CdS Immobilized onto Functionalized MWCNTs. Dalton Trans. 2014, 43, 7429-7434. (11) Sambandam, B.; Rajendran, N.; Kanagaraj, M.; Arumugam, S.; Manoharan, P. T. Switching on Antiferromagnetic Coupled Superparamagnetism by Annealing Ferromagnetic Mn/CdS Nanoparticles. J. Phys. Chem. C 2011, 115, 11413-11419. (12) Giribabu, G.; Murali, G.; Reddy, D. A.; Liu, C. L.; Vijayalakshmi, R. P. Structural, Optical and Magnetic Properties of Co Doped CdS Nanoparticles. J. Alloy. Compd. 2013, 581, 363-368. (13) Hu, T. T.; Zhang, M. Z.; Wang, S. D.; Shi, Q. J.; Cui, G. L.; Sun, S. S. CdS:Co Diluted Magnetic Semiconductor Nanocrystals: Synthesis and Ferromagnetism Study. CrystEngComm 2011, 13, 5646-5649. (14) Kaur, K.; Lotey, G. S.; Verma, N. K. Structural, Magnetic, Dielectric and Magnetodielectric Properties of Gd-Doped CdS Nanorods. Mat. Sci. Semicom. Proc. 2014, 19, 6-10. (15) Goldys, E. M.; Drozdowicz-Tomsia, K.; Sun, J. J.; Dosev, D.; Kennedy, I. M.; Yatsunenko, S.; Godlewsk, M. Optical Characterization of Eu-Doped and Undoped Gd2O3 Nanoparticles Synthesized by the Hydrogen Flame Pyrolysis Method. J. Am.

20


Page 20 of 33

Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chem. Soc. 2006, 128, 14498-14505. (16) Yang, G. L.; Xu, G. Y.; Chen, B. K. General Synthesis and White Light Emission of Diluted Magnetic Semiconductor Nanowires Using Single-Source Precursors. Chem. Mater. 2013, 25, 3260-3266. (17) Deng, L.; Shan, Y.; Xu, J. J.; Chen, H. Y. Electrochemiluminescence Behaviors of Eu3+-Doped CdS Nanocrystals Film in Aqueous Solution. Nanoscale 2012, 4, 831-836. (18) Valera, E.; García-Febrero, R.; Pividori, I.; Sánchez-Baeza, F.; Marco, M. P. Coulombimetric Immunosensor for Paraquat Based on Electrochemical Nanoprobes. Sens. Actuator. B 2014, 194, 353-360. (19) Zhou, H.; Zhang, Y. Y.; Liu, J.; Xu, J. J.; Chen, H. Y. Electrochemiluminescence Resonance Energy Transfer Between CdS:Eu Nancrystals and Au Nanorods for Sensitive DNA Detection. J. Phys. Chem. C 2012, 116, 17773-17780. (20) Sun, H. C.; Pan, L. K.; Zhu, G.; Piao, X. L.; Zhang, L.; Sun, Z. Long Afterglow Sr4Al14O25:Eu,Dy Phosphors as both Scattering and Down Converting Layer for CdS Quantum Dot-Sensitized Solar Cells. Dalton Trans. 2014, 43, 14936-14941. (21) Zhang, K. X.; Yu, Y. X.; Sun, S. Q. Influence of Eu Doping on the Microstructure and Photoluminescence of CdS Nanocrystals. Appl. Surf. Sci. 2012, 258, 7658-7663. (22) Han, L. L.; Wang, Y. H.; Guo, L. N.; Zhao, L.; Tao, Y. Multifunctional ScF3:Ln3+ (Ln = Tb, Eu, Yb, Er, Tm and Ho) Nano/Microcrystals: Hydrothermal/Solvothermal Synthesis, Electronic Structure, Magnetism and Tunable Luminescence Properties. Nanoscale 2014, 6, 5907-5917.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

(23) Bose, R.; Manna, G.; Pradhan, N. Surface Doping for Hindrance of Crystal Growth and Structural Transformation in Semiconductor Nanocrystals. J. Phys. Chem. C 2013, 117, 20991-20997. (24) Hota, G.; Idage, S. B.; Khilar, K. C. Characterization of Nano-sized CdS-Ag2S Core-Shell Nanoparticles Using XPS Technique. Colloid. Surf. A 2007, 293, 5-12. (25) Yang, L. L.; Wang, Z.; Zhang, Z. Q.; Sun, Y. F.; Gao, M.; Yang, J. H.; Yan, Y. S. Surface Effects on the Optical and Photocatalytic Properties of Graphene-like Zno:Eu3+ Nanosheets. J. Appl. Phys. 2013, 113, 033514(1)-033514(8). (26) Sambandam, B.; Muthu, S. E.; Arumugamb, S.; Manoharan, P. T. Coexistence of Antiferromagnetism and Ferromagnetism in Mn2+/CdS Nanocrystals and Their Photophysical Properties. RSC Adv. 2013, 3, 5184-5195. (27) Chowdhury, P. S.; Patra, A. Role of Dopant Concentration and Surface Coating on Photophysical Properties of CdS: Eu3+ Nanocrystals. Phys. Chem. Chem. Phys. 2006, 8, 1329–1334. (28) Liu, Y. H.; Luo, W. Q.; Li, R. F.; Liu, G. K.; Antonio, M. R.; Chen, X. Y. Optical Spectroscopy of Eu3+ Doped ZnO Nanocrystals. J. Phys. Chem. C 2008, 112, 686-694. (29) Bogle, K. A.; Ghosh, S.; Dhole, S. D.; Bhoraskar, V. N.; Fu, L. F.; Chi, M. F.; Browning, N. D.; Kundaliya, D.; Das, G. P.; Ogale, S. B. Co:CdS Diluted Magnetic Semiconductor

Nanoparticles:

Radiation

Synthesis,

Dopant-Defect

Complex

Formation, and Unexpected Magnetism. Chem. Mater. 2008, 20, 440-446. (30) Yang, T. Y.; Yang, H.; Wang, P.; Zhao, R.; Xiao, C. H.; Wang, S. M.; Xiao, B. X.;

22


Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Li, Z. F.; Zhang, M. Z. Magnetic Phase Transition of Ag2S:Eu Diluted Magnetic Semiconductor Nanoparticles. RSC Adv., 2014, 4, 33645-33650. (31) Yao, B. B.; Wang, P.; Wang, S. M.; Zhang, M. Z. Ce Doping Influence on the Magnetic Phase Transition in In2S3:Ce Nanoparticles. CrystEngComm 2014, 16, 2584-2588. (32) Kaminski, A.; Sarma, S. D. Polaron Percolation in Diluted Magnetic Semiconductors. Phys. Rev. Lett. 2002, 88, 247202(1)-247202(4). (33) Inamdar, D. Y.; Lad, A. D.; Pathak, A. K.; Dubenko, I.; Ali, N.; Mahamuni, S. Ferromagnetism in ZnO Nanocrystals: Doping and Surface Chemistry. J. Phys. Chem. C 2010, 114, 1451-1459. TOC.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure captions Figure 1. (a) XRD patterns of CdS:Eu NPs with dopant content 0.00 at. %, 0.08 at. %, 0.19 at. % and 0.26 at. %; (b) S 2p3/2 and S 2p1/2 XPS spectrum; (c) Cd 3d5/2 and Cd 3d3/2 XPS spectrum; (d) Eu 3d5/2 and Eu 3d3/2 XPS spectrum. Figure 2. HRTEM images and SAED pattern of Eu-doped CdS NPs (i.e. 0.00 at. %, 0.08 at. %, 0.19 at. % and 0.26 at. % Eu) Figure 3. PL spectrums for (a) un-doped (0%); (b) 0.08 at. %; (c) 0.19 at. % and (d) 0.26 at. % CdS:Eu NPs using excitation at 325 nm. Figure 4. (a) Magnetization hysteresis loop of CdS:Eu nanoparticles at room temperature for 0.08 at%, 0.19 at% and 0.26 at % Eu; (b) enlarged view in the field between -200 and 200 Oe at 300 K; (c) Magnetization plots of 0.26 at. % CdS:Eu nanoparticles as a function of the applied fields (-60 and 60 kOe) at 10 K, 150 K, 300K and 380 K. Figure

5.

Magnetization

versus temperature for field cooled (FC) and

zero-field-cooled (ZFC) measurements with H=500 Oe. (a) 0.08 at. % CdS:Eu; (b) 0.19 at. % CdS:Eu; (c) 0.26 at. % CdS:Eu. The insets are respectively corresponding to the high-magnification profiles of the three curves between 260 K and 300 K. Figure 6. Three calculation models: (a) the ideal system with no defects or dopants (Cd36S36); (b) one Cd vacancy created by removing one Cd atom (Cd35S36), and the black circle is the site of the removable Cd atom; (c) one Cd atom replaced by one Eu atom (Cd35Eu1S36). Figure 7. Total and partial DOS of three models. (a) the ideal system (Cd36S36), (b) one Cd vacancy created by removing one Cd atom (Cd35S36), (c) one Cd atom replaced by one Eu atom (Cd35Eu1S36). The vertical line denotes the position of the Fermi energy, which has been chosen to be 0.0 (eV). Positive values correspond to the spin up and negative values to the spin down.

24


Page 24 of 33

Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. The spatial distribution of the spin density for (a) Cd35S36 and (b) Cd35Eu1S36 in the FM state. The yellow isosurface corresponds to the spin-up density and the blue isosurface is spin-down density. The black circle is the site of the removable Cd atom.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a) XRD patterns of CdS:Eu NPs with dopant content 0.00 at. %, 0.08 at. %, 0.19 at. % and 0.26 at. %; (b) S 2p3/2 and S 2p1/2 XPS spectrum; (c) Cd 3d5/2 and Cd 3d3/2 XPS spectrum; (d) Eu 3d5/2 and Eu 3d3/2 XPS spectrum. 82x91mm (600 x 600 DPI)


Page 26 of 33

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. HRTEM images and SAED pattern of Eu-doped CdS NPs (i.e. 0.00 at. %, 0.08 at. %, 0.19 at. % and 0.26 at. % Eu) 137x250mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. PL spectrums for (a) un-doped (0%); (b) 0.08 at. %; (c) 0.19 at. % and (d) 0.26 at. % CdS:Eu NPs using excitation at 325 nm. 107x153mm (600 x 600 DPI)


Page 28 of 33

Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Magnetization hysteresis loop of CdS:Eu nanoparticles at room temperature for 0.08 at%, 0.19 at% and 0.26 at % Eu; (b) enlarged view in the field between -200 and 200 Oe at 300 K; (c) Magnetization plots of 0.26 at. % CdS:Eu nanoparticles as a function of the applied fields (-60 and 60 kOe) at 10 K, 150 K, 300K and 380 K. 227x446mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Magnetization versus temperature for field cooled (FC) and zero-field-cooled (ZFC) measurements with H=500 Oe. (a) 0.08 at. % CdS:Eu (b) 0.19 at. % CdS:Eu (c) 0.26 at. % CdS:Eu. The insets are respectively corresponding to the high-magnification profiles of the three curves between 260 K and 300 K. 53x38mm (600 x 600 DPI)


Page 30 of 33

Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Three calculation models: (a) the ideal system with no defects or dopants (Cd36S36); (b) one Cd vacancy created by removing one Cd atom (Cd35S36), and the black circle is the site of the removable Cd atom; (c) one Cd atom replaced by one Eu atom (Cd35Eu1S36). 34x14mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Total and partial DOS of three models. (a) the ideal system (Cd36S36), (b) one Cd vacancy created by removing one Cd atom (Cd35S36), (c) one Cd atom replaced by one Eu atom (Cd35Eu1S36). The vertical line denotes the position of the Fermi energy, which has been chosen to be 0.0 (eV). Positive values correspond to the spin up and negative values to the spin down. 167x349mm (600 x 600 DPI)


Page 32 of 33

Page 33 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. The spatial distribution of the spin density for (a) Cd35S36 and (b) Cd35Eu1S36 in the FM state. The yellow isosurface corresponds to the spin-up density and the blue isosurface is spin-down density. The black circle is the site of the removable Cd atom. 43x23mm (600 x 600 DPI)


Half Metallic Ferromagnetism in Eu-Doped CdS Nanoparticles - The

Recommend Documents