Structural and optical properties characterizations
Before showing the magnetic results, we would like to present some structural and optical characterizations of Ni-doped CsPbBr3 as a representative. The samples are named as CsPb1–xNixBr3, where x is the real molar ratio of Ni relative to Pb, as determined from inductively coupled plasma mass spectrometry (Table S1, Supporting Information), and x = 0 represents pure CsPbBr3. The samples were synthesized at RT through solution processing (see Experimental section). Figs. 1(a) and 1(b) display high-resolution transmission electron microscopy (TEM) images of CsPb1–xNixBr3 with x = 0 and 0.31%, respectively. Clear lattice fringes can be observed, indicating that our samples are well crystalized. The fringe spacing determined by using fast Fourier transform patterns was found to be 0.5765 nm for both samples, which we attribute to the (100) plane. The low-resolution TEM images shown in insets reveal that both samples have typical square nanocrystal morphology, with an average size of ~ 55.2 nm. Fig. 1(c) shows X-ray diffraction (XRD) patters of CsPb1–xNixBr3 with different x values. All samples can be indexed as CsPbBr3 with cubic-phase structure. Close examination of the XRD patterns near 2θ = 21.5º reveals that the Bragg angle θ shifted slightly but systematically to higher positions as x increased, suggesting that the lattice shrank. Fig. 1(d) shows the X-ray photoelectron spectrum of the Ni 2p level in CsPb1–xNixBr3 with x = 0.31%. The peak located at around 855.42 eV is assigned to the Ni2+ 2p3/2 level. As the ion radius of Ni2+ (~0.83 Å for an octahedral site) is smaller than that of Pb2+ (~0.1 Å for an octahedral site), the lattice shrink confirms that the Ni2+ dopants were incorporated into the Pb2+ sites. Fig. 1(e) shows normalized photoluminescence spectra of CsPb1–xNixBr3 nanocrystals with different x values measured at RT. The optical gap (i.e., the energy value at the photoluminescence peak) was 2.4 eV for x = 0, which exhibits a slight red-shift with increasing x.
We adopted electron spin resonance (ESR, also known as electron paramagnetic resonance) to determine the type of defects that is predominant in our samples, as ESR is a defect-sensitive technique that is widely used to study defect physics. Fig. 1(f) shows the ESR first derivative signals as a function of the external magnetic field (Hext), obtained from CsPb1–xNixBr3 nanocrystals with x = 0 and 0.31% at RT. The two samples had the same weight (10.2 mg) for these ESR measurements. We can see that, although they are weak, clear resonance signals can be observed. Both samples exhibit the same resonance peak, same line shape, and same line width, indicating that the signals share the same origin. Using the formula g = hγ/μBHext, where h is the Planck’s constant, γ is the microwave frequency (9.85 GHz), and μB is the Bohr magnetron, the g factor was calculated to be 2.0033 for both samples, which can be assigned to the Br vacancy (VBr) donor defect. The value of the calculated g factor is very close to that of the free electron (2.0023), indicating that the donor electron is loosely bound to VBr. This means that VBr is in a shallow energy level, consistent with previous theoretical calculations[29, 30]. Given the relatively low formation energy among all possible type defect in LHPs (i.e., vacancy defect, interstitial defect, and antisite defect)[23, 29, 30], we conclude that shallow-level VBr was the predominant defect in our samples. Note that the peak intensity of the signals was almost the same for the two samples, indicating that the VBr concentration varies not much in the undoped and doped CsPbBr3.
First-principle prediction of vacancy-induced magnetism
We next elucidated the defect-induced magnetic states in CsPbBr3 through first-principle density functional theory (DFT) calculations. Based on the above ESR results, we focused only on VBr and its effects on the electronic structures of CsPbBr3. Fig. 2(a) shows the perfect 3 × 3 × 3 cubic-phase CsPbBr3 supercells (left-hand column) used in the DFT calculations. As for defective CsPbBr3 (right-hand column), a Br atom (indicated by the blue arrow) was removed from the perfect supercell to create a VBr, and the lattice was then fully relaxed to a stable state for study; see Experimental section for calculation details. Fig. 2(b) displays the slice of deformation charge density (DCD) of the Pb−Br layer from the CsPbBr3 (200) plane, without and with VBr. This permits us to study the effects of VBr on charge transfer after forming chemical bonds. The Pb−Br bonds showed ionic character, where the Br atoms gained electrons and the Pb atoms contributed electrons. The charge density distribution in perfect CsPbBr3 was highly symmetric. When VBr was introduced, it became asymmetric, particularly in the vicinity of VBr. The VBr site exhibited a charge-accumulation environment, indicating that there was strong bonding between the Pb atoms around VBr.
To study whether or not the charge distribution asymmetry could induce magnetic states, calculations on spin-resolved density of states (DOSs) were carried out; the results are shown in Fig. 2(c). As expected, the total DOSs of the perfect CsPbBr3 showed high spin-degeneracy, that is, the distribution of the spin-up and spin-down electrons was completely symmetrical, indicating the nonmagnetic nature of perfect CsPbBr3. The total DOSs of the perfect CsPbBr3 were discrete and sharp, revealing that the electronic states were rather localized. No states were present inside the bandgap of perfect CsPbBr3. Regarding defective CsPbBr3, clear spin splitting can be seen from its total DOSs (i.e., the spin degeneracy has lifted). The magnitude of the spin splitting near the valence band maximum was ~ 38 meV. The net magnetic moment of the defective supercell was calculated to be 6μB. Moreover, the defective CsPbBr3 exhibited extended DOSs, indicating that the electronic states in defective CsPbBr3 were much more delocalized than those in perfect CsPbBr3. Particularly, some impurity states were present inside the bandgap of defective CsPbBr3. Analysis of the partial DOSs of the defective CsPbBr3 revealed that: (1) the conduction band consisted of Pb 6p orbitals (predominant) and Br 4s and 4p orbitals; (2) the valence bands were formed by Br 4p orbitals (predominant) and Pb 6s and 6p orbitals; (3) Pb 6s and 6p orbitals exhibited strong hybridization with Br 4s and 4p orbitals; and (4) the impurity states were mainly composed of Pb 6p orbitals, whereas the Br 4p orbital also contributed a small part of the impurity states, due to its hybridization with the Pb 6p orbital. The exchange interaction between vacancies was studied by calculating the total energy of a 3 × 3 × 3 CsPbBr3 supercell containing two VBr and comparing the energy for the ferromagnetic (EFM) and antiferromagnetic (EAFM) states. It was found that EFM was lower than EAFM, with an energy of 3.73 meV, suggesting that the bivacancy system had a ferromagnetic ground state.
Experimental demonstration of vacancy-induced ferromagnetism
In experiment, we measured the magnetic properties of a number of LHPs including pure CsPbCl3, CsPbBr3, CsPbI3, and CH3NH3PbBr3, using a vibrating sample magnetometer (see Experimental section). Fig. 3(a) displays the magnetization versus Hex curves of RT-synthesized CsPbBr3 nanocrystals at measuring temperatures of 4, 100, 200, 300, and 400 K. The linear diamagnetic backgrounds have already been subtracted. All curves show a clear ferromagnetic behavior with S-shape signals. That is, the magnetization increased with Hex before then becoming saturated at a certain Hex value. The curves show little or no hysteresis (inset of Fig. 3(a)), and the saturation magnetization (Ms) does not change much with temperature (Fig. 3(b)), features of d0 ferromagnetism[38, 39]. The ferromagnetic behaviors observed at low temperatures persisted as the temperature rose to 400 K, suggesting that the Curie temperature of the CsPbBr3 nanocrystals is above 400 K.
Moreover, as shown in Fig. 3(c), the ferromagnetism of the CsPbBr3 nanocrystal could be tuned by treating the nanocrystal surfaces with oleylammonium bromide (OAmBr): the Ms decreased from 0.99 memu/g before the treatment to 0.57 memu/g after the treatment. We also studied the magnetic properties of CsPbBr3 quantum dots synthesized by hot injection at 170 ºC and CsPbBr3 single crystals (see Experimental section for synthesis details). We found that both samples only showed diamagnetic background signals at 300 K (Fig. S1, Supporting Information), which indicates that they were nonmagnetic at 300 K. Surface treatment with OAmBr can passivate the VBr and thus decrease VBr concentration near the surfaces. Hot-injection synthesized and single-crystal CsPbBr3 also have reduced VBr, as indicated by our ESR measurements (Fig. S2, Supporting Information). Together with the first-principle calculation results, the magnetic results of the surface-passivation and high-quality CsPbBr3 samples confirmed the VBr origin of the ferromagnetism observed in the RT-synthesized CsPbBr3 nanocrystals.
Figs. 3(d)–3(f) present the magnetic properties of RT-synthesized tetragonal-phase CsPbCl3, orthorhombic-phase CsPbI3, and cubic-phase CH3NH3PbBr3 (structure and optical characterizations are shown in Fig. S3, Supporting Information), respectively. They all exhibited clear d0 ferromagnetism at 300 K, and surface passivation suppressed it. Moreover, similar to the case of CsPbBr3, no ferromagnetism was observed in the hot-injection synthetized CsPbCl3, CsPbI3, and CH3NH3PbBr3 at a measuring temperature of 300 K (Fig. S4, Supporting Information). Accordingly, we conclude that vacancy-induced d0 ferromagnetism should be universal in LHP materials.
Ferromagnetism enhancement with 3d ion doping
For practical device applications, the ferromagnetism should be as strong as possible, to stabilize the spins against external thermal fluctuations. As it has a defect-origin nature, d0 ferromagnetism can in principle be enhanced by increasing the defect concentration. However, having too many vacancies in LHPs is potentially hazardous to their structural stability, due to the vacancy-mediated ionic migration effect, a tough issue that remains to be solved. Recently, doping LHPs with TM ions has been shown to improve both the optical properties and structural stability of LHPs. Therefore, we attempted to dope the RT-synthesized CsPbBr3 nanocrystals with 3d TM ions, and studied whether this could enhance the d0 ferromagnetism.
Fig. 4(a) presents the magnetization versus Hex curves of RT-synthesized CsPbBr3 nanocrystals doped with Mn, Fe, Co, Ni, Cu, and Zn, measured at 300 K (The XRD study confirmed the successful incorporation of these 3d dopants into the Pb site; see Fig. S5, Supporting Information). Even a tiny fraction (< 1%) of 3d dopants led to a significant modulation of the ferromagnetism of the CsPbBr3 nanocrystals. Compared with pure CsPbBr3, as shown in Fig. 4(b), doping with 0.54% Fe, 0.87% Co, and 0.31% Ni enhanced the Ms by a factor of three, two, and four, respectively, while doping with 0.35% Cu and 0.86% Zn impaired the Ms. The remarkable variation of the Ms indicates that the exchange coupling is sensitive to 3d ions doped, which have variable electron configuration in the 3d orbitals. As a representative, Fig. 4(c) shows the Ni2+ dopant concentration dependence of magnetic properties in CsPb1–xNixBr3 (see Fig. 1 for structural characterizations). The Ms roughly increased as increased from 0 to 0.46% (inset of Fig. 4(c)). Due to solubility limitation, we were unable to investigate the effects of higher x on the Ms. Fig. 4(d) displays the magnetization curves of CsPb1–xNixBr3 with x = 0.31% at measuring temperatures of 4, 100, 200, 300, and 400 K. In contrast to pure CsPbBr3, where the Ms did not vary significantly with temperature (Figs. 3(a) and 3(b)), the Ms of Ni-doped CsPbBr3 exhibited strong temperature dependence behavior: it decreased by approximately 42% as the temperature increased from 4 to 400 K. Nevertheless, all of the magnetization curves presented in Figs. 4(c) and 4(d) showed little or no hysteresis, indicating that the ferromagnetism of Ni-doped CsPbBr3 should also originate from VBr – the same as for pure CsPbBr3. The dramatic temperature dependence of the Ms reflects that there was strong coupling between the VBr and Ni2+ dopants. Only paramagnetism was found in the hot-injection-synthesized Ni-doped CsPbBr3 quantum dots (Fig. S6, Supporting Information), further confirming the VBr origin of the ferromagnetism.