Structure-Dependent Spin Polarization in Polymorphic CdS:Y

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Structure-Dependent Spin Polarization in Polymorphic CdS:Y Semiconductor Nanocrystals Pan Wang,† Bingxin Xiao,† Rui Zhao,† Yanzhang Ma,‡ and Mingzhe Zhang*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409, United States

‡

S Supporting Information *

ABSTRACT: Searching for the polymorphic semiconductor nanocrystals would provide precise and insightful structure-spin polarization correlations and meaningful guidance for designing and synthesizing high spin-polarized spintronic materials. Herein, the high spin polarization is achieved in polymorphic CdS:Y semiconductor nanocrystals. The high-pressure polymorph of rock-salt CdS:Y nanocrystals has been recovered at ambient conditions synthesized by the wurtzite CdS:Y nanocrystals as starting material under 5.2 GPa and 300 °C conditions. The rock-salt CdS:Y polymorph displays more robust room-temperature ferromagnetism than wurtzite sample, which can reach the ferromagnetic level of conventional semiconductors doped with magnetic transition-metal ions, mainly due to the significantly enhanced spin configuration and defect states. Therefore, crystal structure directly governs the spin configuration, which determines the degree of spin polarization. This work can provide experimental and theoretical methods for designing the high spin-polarized semiconductor nanocrystals, which is important for applications in semiconductor spintronics. KEYWORDS: semiconductor spintronics, polymorph transformation, spin polarization, ab initio calculations

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INTRODUCTION Spin-polarized nanosemiconductor materials as new functional materials have been the building blocks of spintronics, due to the both use of spin and charge of electrons.1,2 Seeking high spin-polarized semiconductor nanocrystals at room temperature is a great challenge in semiconductor spintronics.3−7 Spin polarization depends on spin-polarized density of states (DOS) at Fermi level produced by exchange interaction between electrons.8,9 It has been clear that there is strong interplay between the spin polarization and some factors, such as nanometer size,10 composition,11,12 and dopant5,13 in semiconductor nanocrystals. In addition, crystal structure directly governs the microscopic electronic structure, which determines the degree of spin polarization.14 Searching for the polymorphic semiconductor nanocrystals would provide precise and insightful structure-spin polarization correlations and meaningful guidance for designing and synthesizing unique spintronic materials.15,16 Moreover, it has been noted that many doped nanosemiconductor materials, such as ZnO,17−19 GaN,20,21 In2O3,22,23 and CdS,24−26 become spin-polarized at room temperature. CdS has two stable structural phases, such as fourcoordinated wurtzite and six-coordinated rock-salt phase. Rocksalt CdS of high pressure phase is difficult to be reserved under ambient conditions,27,28 and the magnetic property has not been investigated. This material has an elaborate series of structures that are fascinating in their own right, which offers a unique opportunity to study spin polarization of polymorph. Additionally, CdS nanocrystals are doped with nonmagnetic Y atom to realize intrinsic magnetism,29,30 which is beneficial to © XXXX American Chemical Society

investigate the spin polarization as a function of the crystal structure. In this work, the emphasis is on revealing the high spin polarization in polymorphic CdS:Y semiconductor nanocrystals. Y-doped wurtzite CdS nanocrystals synthesized at ambient conditions are used as starting material, and processed under high-pressure and high-temperature (HPHT) conditions then recovered to ambient conditions to get the recovered rock-salt CdS polymorph. Magnetic properties of CdS polymorphs are investigated by superconducting quantum interference device (SQUID). The spin polarization in polymorphic CdS:Y semiconductor is extensively elucidated by ab initio calculations.

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RESULTS AND DISCUSSION The phase transformation from wurtzite to rock-salt is demonstrated in the X-ray diffraction (XRD) patterns (Figure 1) and transmission electron microscopy (TEM) images (Figure S2) measured at ambient conditions of the starting material and HPHT products which are fabricated at different temperatures and a fixed pressure of 5.2 GPa then recovered to the ambient conditions. The downmost XRD curve (Figure 1) of Y-doped CdS nanocrystals (starting material) crystallizes in a pure wurtzite CdS structure (space group P63mc) with the lattice parameters, a = 4.111 Å and c = 6.726 Å, and the particle size is about 7−9 nm, which is consistent with TEM images Received: December 22, 2015 Accepted: February 23, 2016

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DOI: 10.1021/acsami.5b12542 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

material and rock-salt CdS:Y nanocrystals based on energydispersive X-ray spectroscopy (EDS) analysis (Figures S4 and S5 and Tables S1 and S2). To investigate the magnetic properties of 0.2 atom % wurtzite (starting material) and rock-salt CdS:Y nanocrystals, we carried out SQUID measurements. All zero-field-cooled (ZFC) and field-cooled (FC) curves (Figure 2) of both of the above samples show irreversible as a function of temperature ranging from 2 to 380 K, indicating that the ferromagnetic transition temperature is higher than 380 K.33 For the wurtzite starting material, ZFC curve (Figure 2a) reaches a minimum at 8 K, which corresponds to the quantum tunneling of magnetization from magnetic long-range order state to quantum superparamagnetic state.34 Above 8 K, there is a cusp at 72 K attributed to the blocking states of small particles in ZFC curve (Figure 2a).35 Then ZFC curve (Figure 2a) reaches a maximum at 274 K, defined as blocking temperature TB, ascribed to the magnetic blocking mechanism caused by the competition between thermal energy and magnetic anisotropic energy,36,37 and does not coincide with FC until 380 K, which reveals that the thermal energy cannot disturb the magnetic ordered state.38 For the rock-salt sample, the TB point of the recovered rock-salt CdS sample is 66 K deduced from the maximum of ZFC curve (Figure 2c). Finally the ZFC and FC curves (Figure 2c) just coincide with each other at 380 K. Therefore, both the blocking and overlap temperatures of the rock-salt sample are lower (Figure 2c) than those of the wurtzite sample (Figure 2a), suggesting that the ferromagnetic transition temperature of the rock-salt sample is probably lower.37,39 All M−H curves (Figure 2b, d) of the wurtzite and rock-salt samples measured at 10 and 380 K show the neglectable magnetic hysteresis, reaffirming that the both samples maintain the room-temperature ferromagnetic properties until 380 K.40 The saturation magnetizations (MS) at 10 and 380 K are 0.031 and 0.023 emu/g for the wurtzite sample,

Figure 1. XRD patterns of CdS:Y polymorphs. XRD patterns of Ydoped CdS nanocrystals synthesized by gas-spray phase chemical reaction as starting material (at ambient conditions), 3 HPHT samples fabricated by starting material under a constant pressure of 5.2 GPa and 100−300 °C, measured at ambient conditions.

(Figure S2a, b). When the synthesis temperature increased to 300 °C, all XRD diffraction peaks of recovered sample (Figure 1) match that of pure rock-salt CdS structure with the lattice parameter a = 5.402 Å (space group Fm3m), further verified by TEM images (Figure S2c, d). Compared with the wurtzite starting material, the slight shift to higher angles of diffraction peaks (see Figure 1) of the recovered rock-salt sample are observed, resulting from the shrink of lattice induced by pressure.31,32 Despite the introduction of high temperature in synthesis condition, the broad diffraction peaks (Figure 1) of recovered rock-salt sample are still observed, and the particle sizes are calculated about 8−10 nm. In addition, according to X-ray photoelectron spectroscopy (XPS) (Figure S3) of wurtzite starting material, Y is in nonmagnetic Y3+ state into host CdS semiconductor. Moreover, the Y contents are basically constant and about 0.2 atom % in wurtzite starting

Figure 2. SQUID experiments on 0.2 at % CdS:Y polymorphs. (a, c) M−T curves obtained in the temperature range 2−380 K under FC and ZFC measurement models at applied magnetic field of 500 Oe, and (b, d) M−H curvesmeasured at 10 and 380 K and field range −60−60 kOe, correspond to the wurtzite starting material and recovered rock-salt sample, respectively. B


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ACS Applied Materials & Interfaces and 0.210 and 0.107 emu/g for the rock-salt sample, respectively, manifesting the more robust ferromagnetism of rock-salt CdS:Y nanocrystals. The room-temperature MS of the rock-salt CdS nanocrystals doped with nonmagnetic ion Y3+ can reach the ferromagnetic level of conventional semiconductors doped with magnetic transition-metal ions,41−43 which may result from the HPHT synthesis method and this high-pressure polymorph of CdS nanocrystals. To shed light on the origin of the ferromagnetism and get deeper insight into the structure-dependent spin polarization, first-principles calculations are performed based on the densityfunctional theory. Because of the excessive H2S in experimental synthesis process, we consider the S-rich growth condition. The main defects existence in CdS:Y nanocrystals are four cases for wurtzite (see structural models in Figure S6a2−a5) and rocksalt (see structural models in Figure S6b2−b5), one Y dopant defect (YCd) (Cd31YS32), one Cd vacancy defect (VCd) (Cd31S32), one S antisite defect (SCd) (Cd31S33), and one S interstitial defect (Si), separately. The comparative analysis of spin-polarized DOS for five wurtzite and rock-salt CdS systems (Figures 3 and 4) provide

Figure 4. Spin-polarized projected DOS of three 4 × 4 × 1 wurtzite and 2 × 2 × 2 rock-salt CdS systems, for (a, b) one Cd vacancy created by removing one Cd atom (VCd) (Cd31S32), (c, d) one S atom in the Cd-substitutional position defect (SCd) (Cd31S33), (e, f) one S atom in the interstitial position inside the monoclinic cage (Si) (Cd32S33), separately. The black vertical dotted lines represent Fermi level (EF = 0). The asterisks and frames display the split positions of spinpolarized DOS.

between the S 3p and Cd 4d orbitals, but also results in a spinpolarized conduction band for rock-salt structure (Figure 4b), which is primarily derived from the significant coupling between S 3p and Cd 5s orbitals, suggesting the much larger degree of spin polarization of rock-salt CdS structure with VCd defect. The calculated total magnetic moments of wurtzite and rock-salt structures with VCd defects are 0.030 and 0.379 μB. The SCd defects in both structures only result in the spin splits of the conduction bands (Figure 4c, d), which mainly come from the couplings between the S 3p and Cd 4p orbitals in the conduction bands. The calculated total magnetic moments for wurtzite and rock-salt CdS structure with SCd defects are 0.003 and 0.014 μB, respectively. The Si defect only results in the spin polarization of rock-salt structure (Figure 4f) with a 0.014 μB total magnetic moment, which mainly comes from the couplings between the S 3p and Cd 4p orbitals in the conduction bands. As revealed in the above spin-polarized DOS and magnetic moments, the magnetic property of rock-salt CdS structure is significantly stronger than wurtzite structure, which is consistent with the changing trend of the saturation magnetizations of the wurtzite and rock-salt CdS:Y nanocrystals (Figure 2b ,d), and the VCd defect is the mainly responsible for the ferromagnetic properties of the wurtzite and rock-salt CdS:Y nanocrystals. Above theoretical analyses demonstate that the dopant Y atom contributes no magnetic moment to the wurtzite and rock-salt CdS systems. As compared with the pure wurtzite CdS nanocrystals briefly reported eleswhere,44 a strong increase in saturation magnetization of Y doped CdS

Figure 3. Spin-polarized total DOS (density of states) of CdS:Y polymorphs. Spin-polarized total DOS of two 4 × 4 × 1 wurtzite and 2 × 2 × 2 rock-salt CdS systems, for (a1, b1) the ideal systems (Cd32S32), (a2, b2) one Cd atom replaced with one Y dopant atom (YCd) (Cd31YS32), separately. The Fermi level (EF = 0) is indicated by the black vertical dotted lines.

further insight into the mechanism of ferromagnetism as a function of crystal structure. Due to symmetric total spinpolarized DOS for the cases of ideal systems (Figure 3a1, b1) and YCd (Figure 3a2, b2) of both structures, there are no resultant spin polarization and magnetic moments. We thus conclude that dopant Y defect has no influence on the obseveration of the ferromagnetism in wurtzite and rock-salt 0.2 atom % CdS:Y nanocrystals, whereas in the cases of VCd and SCd defects, asymmetric DOS for spin-up and spin-down channels (highlighted by the dotted circles) of both structures are evident in the Figure 4. The VCd defect not only leads to a significant spin polarization of valence band in both structures (Figure 4a, b) which mainly stems from the strong coupling C


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ACS Applied Materials & Interfaces nanocrystals is observed herein. To figure out this conflict, the defect formation energies (more details in part 6 of the Supporting Information) of wurtzite Cd31S32 and Cd30YS32 systems are calculated as 0.625 and −3.685 eV, suggesting that the dopant Y atom can strongly decrease the defect formation energy to produce more Cd vacancies and improve the magnetic property of the wurtzite CdS system, separately. Additionally, the defect energies of the rock-salt Cd31S32 and Cd30YS32 systems are −1.185 and −4.158 eV, which are lower than those of the corresponding wurtzite CdS systems, implying that the dopant Y atom in the rock-salt CdS system is more in favor of the formation of the Cd vacancy defect. Moreover, according to EDS analysis of wurtzite and rock-salt CdS:Y samples (Figures S4 and S5 and Tables S1 and S2), the HPHT synthesis method can also increase the defects including Cd vacancy defects.45,46 Therefore, both of the rock-salt CdS structure and this HPHT sysnthesis method are beneficial to the formation of Cd vacancy defects to improve the ferromagnetic property of the recovered rock-salt CdS:Y sample. VCd defect is the key factor in influencing the magnetic property of the both wurtzite and rock-salt CdS structures and will be mainly considered in the research of the intrinsic mechanism of structure-dependent spin polarization. The spin density (Δρ = ρ↑ − ρ↓) distributions are shown in Figure 5a, c

1↓ 3↑ 1↓ 44 combine into high-spin configurations via (a1↑ 1 )(a1 )(t2 )(t2 ) ↑ 3↑ 2↑ and (a1)(t2p)(eg ), separately. Thus, the spin configuration of rock-salt CdS system with Cd vacancy is much higher than that of the corresponding wurtzite system, manifesting that the Cd vacancy in rock-salt structure is probably in favor of the stronger magnetic property. Other relevant works also reported the ferromagnetism in undoped CdS nanocrystals. The undoped zinc-blende CdS nanoparticles show room-temperature ferromagnetism with MS of ∼4 × 10−3 emu/g, which origins from the defects at surface.47 The pure wurtzite CdS nanorods display roomtemperature ferromagnetism with MS of ∼2.63 × 10−3 emu/g and ferromagnetic transition temperature of 305 K, which is as a result of the Cd vacancy.44 Compared with the abovereported pure CdS nanocrystals, the wurtzite CdS nanoparticles doped with nonmagnetic Y3+ ions in this work have significantly stronger room-temperature ferromagnetism with MS of 0.023 emu/g at 380 K. To the best of our knowledge, there are no reports on the magnetic property of rock-salt CdS:Y nanoparticles, and herein MS at 380 K is 0.107 emu/g. The rock-salt CdS:Y polymorph displays more robust room-temperature ferromagnetism than wurtzite sample, mainly because of the significantly enhanced spin configuration and defect states. In summary, the high spin polarization is achieved in polymorphic CdS:Y semiconductor nanocrystals. The highpressure polymorph of rock-salt CdS:Y nanocrystals has been recovered at ambient conditions synthesized by wurtzite CdS:Y nanocrystals as starting material under 5.2 GPa and 300 °C conditions. The rock-salt CdS:Y polymorph displays more robust room-temperature ferromagnetism than wurtzite sample, which can reach the ferromagnetic level of conventional semiconductors doped with magnetic transition-metal ions, mainly due to the significantly enhanced spin configuration and defect states. Therefore, crystal structure directly governs the spin configuration which determines the degree of spin polarization. This work can provide experimental and theoretical methods for designing the high spin-polarized semiconductor nanocrystals, which is important for applications in semiconductor spintronics.

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Figure 5. (a, c) Spin density distributions around VCd and (b, d) structural configurations of 4 × 4 × 1 wurtzite and 2 × 2 × 2 rock-salt CdS systems, separately.

EXPERIMENTAL SECTION

Sample Preparation. The synthesis process of CdS:Y nanocrystals included gas-spray phase chemical reaction and HPHT experimental synthesis (more details in parts 1 and 2 of the Supporting Information). In the first step, the reactive solution which was the mixture of a certain proportion of Cd2+ and Y3+ was first atomized, then reacted with excessive H2S gas (see experimental setup in Figure S1). In the second step, the sample synthesized by the first step was pressed into a cylinder, and subsequently processed in a HPHT apparatus under certain temperature and pressure. Structural Characterization and Magnetic Measurements. The phase impurity, structure, and crystal size were determined by Xray diffraction (XRD) recorded by the X-ray power diffractometer (Shimadzu, XRD-6000) with Cu Kα radiation (λ = 1.5406 Å). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected-area electron diffraction (SAED) images performed on JEOL JEM-2200FS at 200 kV were used for analyzing the sizes and microstructures of samples. The valence state of dopant Y was characterized by X-ray photoelectron spectroscopy (XPS) (ESCALAB MK II). The compositions of samples were investigated by energy-dispersive X-ray spectroscopy (EDS) attached to scanning electron microscope (SEM) (Magellan-400). Magnetic properties were investigated by using superconducting quantum interference device (SQUID) (Quantum Design) and vibrating sample magnetometer (Lakeshore Model 7410 VSM).

to further analyze the spin polarization around VCd defects in wurtzite and rock-salt structures. The spin density distribution of VCd defect in the wurtzite structure (Figure 5a) is strongly localized on the four nearest neighboring S atoms, and six nearest neighboring S atoms in rock-salt structure (Figure 5c), respectively. The spin-density distribution of rock-salt CdS structure with VCd defect is much larger than that of the wurtzite structure, which is consisitent with the above calculated magnetic moments and spin-polarized DOS. To underatand the much stronger spin polarization for the rocksalt CdS structure with VCd defects, the comparative insight into the spin configurations of the CdS polymorphs is investigated. As illustated in structural configurations of two structures (Figure 5b, d), in wurtzite CdS structure, each Cd cation is surrounded by a tetrahedron (Td symmetry) of four S anions with four sp 3 orbitals, and an octahedron (O h symmetry) of six S anions with six sp3d2 orbitals in rock-salt CdS structure, respectively. When a Cd vacancy is created, two holes are introduced. According to the molecular orbital theory, the four sp3 and six sp3d2 orbitals of neighbors of Cd vacancy D


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ACS Applied Materials & Interfaces First-Principles Calculations. Vienna ab initio simulation package (VASP) software was applied to theoretical calculation (more details in part 5 and 6 of the Supporting Information) based on density functional theory (DFT) and projector augmented wave (PAW) pseudopotential. The exchange and correlated functions were implemented by the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) gradient-corrected functions. 380 eV is used as the cutoff energy, and the convergence thresholds are set at values of 1 × 10−3 eV for energy and 0.05 eV Å−1 for force, respectively. For the ideal CdS systems, the ionic positions, cell volumes, and cell shapes are all allowed to be relaxed (ISIF = 3, IBRION = 2). In case of defects in CdS systems, only the ionic positions are allowed to be relaxed (ISIF = 2, IBRION = 2). For the calculation of the total energy we use the tetrahedron method with Blöchl corrections (ISMEAR = −5). The k-point meshes used for the first Brillouin zone integration were generated by Monkhorst−Pack scheme.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12542. Experimental and theoretical details, additional figures including TEM, XPS and EDS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Science Foundation of China, 11174103 and 11474124. We also acknowledge the High Performance Computing Center of Jilin University for calculation resources.

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