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Room-Temperature Ferromagnetic Ultrathin α‑MoO3:Te Nanoflakes Dong Jin Lee,† Youngmin Lee,† Young H. Kwon,† Soo Ho Choi,‡ Woochul Yang,†,‡ Deuk Young Kim,†,§ and Sejoon Lee*,†,§ †
Quantum-functional Semiconductor Research Center, Dongguk University−Seoul 04623, Korea Department of Physics, Dongguk University−Seoul 04623, Korea § Department of Semiconductor Science, Dongguk University−Seoul 04623, Korea
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‡
S Supporting Information *
ABSTRACT: We materialized room-temperature ferromagnetism in ultrathin α-MoO3:Te nanoflakes. The αMoO3:Te nanoflakes, which had been grown by vaporphase epitaxy, clearly exhibited an Ag Raman band from symmetric stretching of υ(Mo−O3−Mo) in the 2D-like ultrathin α-MoO3:Te layer. Due to the intentional incorporation of smaller Te ions into bigger Mo sites, the pentacoordinated Mo5+ bonds were created inside the orthorhombic α-MoO3:Te lattice system. Since Mo5+ ions have magnetic moments from unpaired electron spins, a large number of overlapped bound magnetic polarons could be formed via ferromagnetic coupling with charged oxygen vacancies that are inevitably generated at pentacoordinated [Mo5+O5] centers. This gives rise to the increase in long-range ferromagnetic ordering and leads to roomtemperature ferromagnetism in the entire α-MoO3:Te solid-state system. The results may move a step closer to the demonstration of spin functionalities in the wide bandgap semiconductor α-MoO3:Te. KEYWORDS: orthorhombic α-MoO3:Te, ultrathin nanoflake, 2D-like layered structure, room-temperature ferromagnetism, bound magnetic polaron thermodynamically stable α-MoO3, moreover, pseudocapacitive charge-storage19 and highly sensitive photoresponse characteristics17 were achieved. To add more functionality into the nanoelectronic device schemes, one needs to introduce some kind of ferroic nature (e.g., ferromagnetic, ferroelectric, and/or piezoelectric characteristics) in the host material. In relation to ferromagnetism in α-MoO3, however, only few studies have been reported in recent years. For instance, room-temperature ferromagnetism was observed from MoO3 nanofibers,20 hierarchically branched MoO3 nanostructures,21 Co-doped MoO3 films,22 and Co- and Nicodoped MoO3 films.23 According to our best survey, no studies on room-temperature ferromagnetism in 2D and 2Dlike α-MoO3 (e.g., α-MoO3 nanosheets and α-MoO3 nanoflakes) have been conducted to date. Therefore, we have investigated the materialization of room-temperature ferromagnetic 2D-like α-MoO3:Te nanoflakes via intentional doping of Te into α-MoO3. The substitution of Te ions to Mo sites was aimed at inducing a partial lattice imperfection in α-MoO3:Te for forming the pentacoordinated Mo5+ ions
O
wing to their extraordinary structural, chemical, physical, and mechanical characteristics, two-dimensional (2D) materials (e.g., graphene and transition metal dichalcogenides) have emerged recently as highly functional semiconductors or insulators for the application of next-generation smart electronic and optoelectronic devices. For example, graphene barristors,1 graphene-based high speed devices,2,3 graphene single-electron transistors,4 graphene quantum-dot transistors,5 robust graphene flash memories,6−9 high-detectivity MoS2 phototransistors,10 and low-power h-BN memristors11 are tangible examples that can realize future nanoelectronics technology. Furthermore, spintronic devices with some distinctive functionalities have also been proposed and demonstrated on various 2D materials and their heterostructures (e.g., MoS2 spin transistors,12 LaOBiS2 spin field-effect transistors,13 graphene-WS2 spin-valve heterostructures14). In addition, such huge potential and vast interest in 2D materials and 2D oxides have triggered research on socalled 2D-like or quasi-2D materials.15 Among various 2D-like materials, α-MoO3 has become attractive because α-MoO3 is configured with an orthorhombic crystal structure in the form of 2D multilayers stacked by weak van der Waals forces along the [010] direction.16 Due to the wide band gap (∼3 eV)17 and the large work-function energy (∼6.9 eV) 18 of © XXXX American Chemical Society
Received: February 12, 2019 Accepted: July 11, 2019 Published: July 11, 2019 A
DOI: 10.1021/acsnano.9b01179 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Schematic illustration for the growth procedures of the α-MoO3:Te nanoflakes by VPE.
Figure 2. Morphological properties of the ultrathin α-MoO3:Te nanoflakes: (a) low-magnification SEM image; (b−d) high-magnification SEM images.
(Mo5+: [Kr] 4d1),24 which can increase the magnetic moment in the entire material system. In this paper, we report an enhanced ferromagnetic property in 2D-like ultrathin α-MoO3:Te nanoflakes. We prepared the ultrathin α-MoO3:Te nanoflakes by using a simple vapor-phase
epitaxy (VPE) technique, and examined their material characteristics. The morphological, structural, microstructural, optical, and magnetic properties were systematically assessed, and the origin of room-temperature ferromagnetism in 2D-like B
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ACS Nano α-MoO3:Te was carefully discussed on the basis of the experimentally observed material characteristics.
specifies that the MoO3:Te nanoflakes were crystallized with the orthorhombic structure, comprising a stack of 2D-like αMoO3 layers (see the inset of Figure 3). The slight shift of the Bragg angle is thought to originate from the incorporation of smaller Te6+ ions (0.56 Å)25,26 into bigger Mo6+ sites (0.62 Å),27,28 as discussed later. From the Bragg angles observed from XRD patterns, the lattice parameters were determined to be a = 0.3958 nm, b = 1.3756 nm, and c = 0.3759 nm, and these are consistent with the standard lattice parameters of αMoO3 (JCPDS Card No. 89-5108). Figure 4 shows the Raman scattering characteristics of the αMoO3 nanoflakes. To verify the dependence of the Raman
RESULTS AND DISCUSSION The ultrathin α-MoO3:Te nanoflakes were grown on the SiO2/ Si substrate by VPE (Figure 1). For the growth of nanostructured α-MoO3:Te, as a primary task, the Mo nanoclusters were formed at 550 °C onto the SiO2/Si substrate by thermal nucleation of the 2 nm thick Mo layer that had been coated onto the SiO2/Si substrate. Thereafter, the ultrathin α-MoO3:Te nanoflakes were grown at 550 °C in a VPE chamber through the reaction of the Mo atoms with both vaporized Te and injected O2 gas molecules. Such a growth method allows us to construct the 2D-like ultrathin αMoO3:Te nanoflakes because the microstructural shape of αMoO3:Te could become only a nanostructure due to the limitation of both the number and the location of the nucleation sites (i.e., Mo nanoclusters). Figure 2 displays the scanning electron microscopy (SEM) images of the α-MoO3:Te nanostructures grown by VPE. The α-MoO3:Te species were constructed in the form of 2D-like nanoflakes several micrometers in length (Figure 2a). As shown in high-magnification SEM images (Figure 2b,c), the nanoflakes have smooth and clean surfaces with no foreign substances or contaminants. The shape of α-MoO3:Te consists of a rectangular nanoplate (Figure 2b) and a vertical nanofan (Figure 2c), and some of them vertically cross each other (Figure 2d). The width and the height of the planar MoO3:Te nanoflakes are within a micrometer scale. From atomic force microscopy measurements, we confirmed that the thickness of the flake was less than 33 nm (see the Supporting Information). To characterize the crystal structure of the MoO3:Te nanoflakes, we carried out X-ray diffraction (XRD) measurements. As can be seen from Figure 3, the sample exhibits multiple diffraction patterns from (020), (110), (040), (130), (060), and (260) lattice planes. Although there was an extremely small shift (Δ ∼ 0.09°) of the Bragg angle, all of the observed diffraction patterns belong to the orthorhombic αMoO3 except for a strong peak from the Si substrate. This
Figure 4. Raman scattering characteristics of the α-MoO3:Te nanoflake: (a) Raman spectra at three different positions of A, B, and C, which are depicted in the (b) optical microscopy image, and mapping results of Raman scattering features at (c) 816 cm−1, (d) 284 cm−1, and (c) 157 cm−1.
features on the flake stack numbers, we collected Raman signals (Figure 4a) from three different points (A, B, and C) marked in Figure 4b. At the SiO2/Si substrate region (i.e., position A), no significant Raman scattering occurs except for the strong peak from Si. However, at the B and C positions, three dominant Raman scattering features arise at Ag/B1g ∼157 cm−1, B2g ∼ 284 cm−1, and Ag ∼ 816 cm−1. The Ag/B1g band arises from the translational rigid-chain mode of δ(O2Mo2)n,29 the B2g band originates from the wagging mode of δ(O3−Mo− O3),29−32 and the Ag band comes from the υ(Mo−O3−Mo) stretching mode.29−32 According to the works of Yan et al.31 and Ou et al.,32 the presence of the Ag band obviously depicts that α-MoO3 has a 2D-like layered morphology. At position B (i.e., single-flake region), the intensity ratio of Ag/(Ag/B1g + B2g) is greater than that at the position C (i.e., double-flake region). From Raman mapping images, additionally, one can
Figure 3. XRD pattern of the α-MoO3:Te nanoflakes. The inset shows the schematic configuration of the layered lattice structure of the orthorhombic α-MoO3:Te. C
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Figure 5. (a) Bright-field TEM image and the SAED pattern (inset) of the single α-MoO3:Te nanoflake, (b) high-resolution TEM image, (c) high-magnification TEM image and the fast Fourier transform pattern, and (d) average lattice spacings of a (top) and c (bottom) observed from TEM images.
find that the Ag band clearly appears at the entire area (Figure 4c), while both the Bg2 band (Figure 4d) and the Ag/B1g band (Figure 4e) dominate at the double-flake region. Namely, Raman scattering of Ag only occurs at the single nanoflake region. Since the strong Ag peak represents a predominance of υ(Mo−O3−Mo) symmetric stretching along the a-axis of the ultrathin α-MoO3 layer,30 we can conjecture that our αMoO3:Te nanoflakes were composed of the ultrathin αMoO3:Te stacked-layers that are spread along the direction normal to the a-axis. After confirming the 2D-like morphological nature, we examined the microstructural properties of α-MoO3:Te by using a cleaved single nanoflake. As shown in the bright-field transmission electron microscopy (TEM) image and its corresponding selective-area electron-diffraction (SAED) pattern (Figure 5a), the α-MoO3:Te nanoflake is a single crystal grown along the direction normal to [00l]. The absence of stacking faults and microtwins in high-resolution TEM confirms the high crystallinity of the α-MoO3 nanoflake (Figures 5b,c). In addition, the regularly spaced spots in the fast Fourier transform pattern further elucidate the uniform orthorhombic arrangement of the lattice species (see the inset of Figure 5c). From the high-resolution TEM image, the lattice spacings of a and c were estimated to be 0.39 and 0.38 nm on average (Figures 5d), and these unit cell parameters coincide
with those from XRD results as well as standard lattice parameters of α-MoO3 (JCPDS Card No. 89-5108). Such a result identifies that the ultrathin α-MoO3:Te nanoflake definitely belongs to the orthorhombic 2D-like layered structure having a- and c-axis planes. To examine further insight into the bonding states of the lattice components in 2D-like α-MoO3:Te, we performed Xray photoelectron spectroscopy (XPS) measurements. For the XPS spectra of the Mo 3d core levels (Figure 6a), the sample clearly exhibits two typical emission peaks from Mo 3d5/2 and 3d3/2. As represented by best-fitted curves, both Mo 3d5/2 and 3d3/2 peaks can be deconvoluted by two different valence states of 5+ (i.e., 3d5/2 at ∼231.8 eV and 3d5/2 at ∼234.9 eV)33 and 6+ (i.e., 3d5/2 at ∼232.5 eV and 3d5/2 at ∼235.8 eV).33 This indicates that both Mo5+ and Mo6+ ions coexist in the αMoO3:Te nanoflake. The presence of Mo5+can be explained by the incorporation of Te into α-MoO3. The sample clearly displays the XPS peaks from the Te 3d core levels (Figure 6b), and the concentration of the incorporated Te species was confirmed to be ∼1.0 atom %. As shown in Figure 6b, a large portion of Te6+ is bonded with O (i.e., Te−O bond: 3d5/2 at ∼576.4 eV and 3d5/2 at ∼586.8 eV),34 whereas the portion of the Te−Mo bond (i.e., Te−Mo bond: 3d5/2 at ∼573.1 eV and 3d5/2 at ∼583.8 eV)34 is relatively small. The appearance of the O 1s peak at ∼530.5 eV further corroborates the presence of D
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Figure 7. PL spectra at 20−300 K of the α-MoO3:Te nanoflakes. Black dots, red lines, and green lines are experimental data, bestfitted results, and fitting curves, respectively. The inset represents the optical emission processes in α-MoO3 and pentacoordinated Mo5+ ions. CB and VB denote the conduction band and the valence band of α-MoO3, respectively.
temperature range, the sample reveals a prominent PL feature at the ultraviolet−visible wavelength region. At 300 K, as represented by Gaussian fitting curves, the PL spectrum could be deconvoluted by five peaks at P1 ∼ 394 nm, P0 ∼ 430 nm, P2 ∼ 467 nm, P2′ ∼ 517 nm, and P3 ∼ 592 nm. As the temperature decreases to 20 K, only P0 shows a blue shift, while other peaks exhibit almost no temperature dependence of their peak positions. In semiconducting materials, the excitonic emission energy depends on the temperature because of both the exciton−phonon interaction and the thermal expansion of the emission lines.36,37 However, the emission energies of the d−d intraband transitions from metallic dopant ions are independent of the temperature.38 Therefore, the strong blue emission at P0 (∼2.88 eV) can be assigned as resulting from near-band-edge (NBE) emission,39,40 and four other emission peaks can be allocated by the lattice imperfection.39,40 As mentioned above, the incorporation of Te into α-MoO3 leads to the partial distortion of the octahedron centers, and it results in the formation of pentacoordinated Mo5+ ions. According to Łabanowska’s quantum-chemical calculation41 and Dieterle et al.’s optical characterization,42 the intraband transitions can take place at the pentacoordinated [Mo5+O5] centers. Based on their theoretical and experimental results, we can therefore ascribe P1 (∼3.15 eV), P2 (∼2.66 eV), and P2′ (∼2.40 eV) to the intraband of d1xz-d1yz, d2yz-d2xz, and d2yz−d2xy transitions from pentacoordinated [Mo5+O5] centers (see also the inset of
Figure 6. XPS spectra of (a) Mo 3d, (b) Te 3d, and (c) O 1s core levels for the α-MoO3:Te nanoflakes.
O−Te bonds (Figure 6c).35 Since a lot of positively ionized Te ions (i.e., Te6+) are bonded with O2− ions, the number of unoccupied oxygen sites would be increased inside the αMoO3:Te lattices. Furthermore, the substitution of smaller Te6+ ions to bigger Mo6+ sites might also shrink the lattice spacings, as confirmed from XRD (i.e., a slight shift of the Bragg angle). This will eventually result in a partial distortion of octahedron centers inside the α-MoO3 lattice system. Therefore, the metastable bonds could be formed in the crystal lattice because of the substantial movement of the unpaired Mo ions toward the energetically stable sites. Since the formation enthalpy of the oxygen vacancy (VO) is low under the oxygen deficient condition and the 5+ valence state of Mo is preferable for the formation of VO,19 the large numbers of VO point defects and their corresponding Mo5+ ions (i.e., pentacoordinated Mo5+ ions) could be created in orthorhombic α-MoO3:Te. Figure 7 displays the photoluminescence (PL) spectra at 20−300 K of the α-MoO3:Te nanoflakes. In a whole E
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5.8 × 10−2 emu/g, the remanent magnetization (Mr) of 9.2 × 10−3 emu/g, and the coercive magnetic field (Hc) of 93.6 Oe. Surprisingly, the sample maintains its ferromagnetic hysteresis loop even at 300 K with Ms of 4.2 × 10−2 emu/g, Mr of 7.1 × 10−3 emu/g, and Hc of 72.7 Oe. As can be confirmed from Figure 8b, the magnetization vs temperature (M−T) curves further substantiate room-temperature ferromagnetism in the α-MoO3:Te nanoflakes. From the field-cooled (FC) M−T curve, one can observe that the ferromagnetic-to-paramagnetic transition starts occurring at temperature above 350 K (Figure 8b). In addition, the zero field-cooled (ZFC) M−T curve reveals no antiferromagnetic features at a whole measurement temperature range. According to previous reports on the diluted magnetic semiconductors,43−46 furthermore, there exists obvious ferromagnetism at a temperature range up to the Curie temperature (TC) if ΔM (= MFC − MZFC) is not zero. Thus, the above results accentuate that the sample has room-temperature ferromagnetism persisting up to TC > 350 K. To compare ferromagnetic properties of our α-MoO3:Te with other state-of-the art ferromagnetic α-MoO3 samples, we summarized key ferromagnetic parameters of Ms, Mr, Hc, and TC (Table 1). Ms, Mr, and Hc at 300 K of the α-MoO3:Te nanoflakes are 4.2 × 10−2 emu/g, 7.1 × 10−3 emu/g, and 72.7 Oe, respectively, and these are greater than those of magnetic ion doped MoO3 thin films and are comparable with undoped MoO3 nanostructures. We ascribe the enhanced roomtemperature ferromagnetic properties of α-MoO3:Te to the formation of metastable Mo5+ bonds and their corresponding VO native point defects through intentional doping of Te into α-MoO3. Since the Mo5+ ion has an unpaired electron spin,24 Mo5+ ions will provide magnetic moments for the α-MoO3:Te solid-state system. In addition, VO defects are inevitably formed at the pentacoordinated [Mo5+O5] centers.41,42 Then the charged VO defects may help form the bound magnetic polarons (BMPs)47 via coupling with the localized Mo5+ ions. Furthermore, since the random distribution of both Mo5+ and VO would increase the number of overlapped BMPs, the longrange ferromagnetic order could be enhanced in the entire αMoO3:Te lattice system (see also the inset of Figure 8b).
Figure 7).42 In addition, P3 (∼2.09 eV) can also be attributed to the intervalence charge-transfer transition. The presence of the pentacoordinated [Mo5+O5] centers may affect the magnetic properties of the α-MoO3:Te nanoflakes because the Mo5+ ions provide the magnetic moments due to their unpaired electron spins (i.e., Mo5+: [Kr] 4d1).24 We, therefore, evaluated the magnetization characteristics of the α-MoO3:Te nanoflakes through superconducting quantum interference device (SQUID) measurements. Figure 8a shows the magnetization vs magnetic field (M−H) curves of
Figure 8. (a) M−H curves of the α-MoO3:Te nanoflakes at 5 and 300 K. The inset of (a) displays an enlarged view of M−H curves. (b) M−T curves of the α-MoO3:Te nanoflakes measured under ZFC and FC (H = 3000 Oe) modes. The inset of (b) schematically illustrates the overlapped BMPs in α-MoO3:Te.
CONCLUSIONS The room-temperature ferromagnetic α-MoO3:Te nanoflakes were grown by VPE. The α-MoO3:Te nanoflakes revealed a strong Raman peak from the Ag band, which represents a symmetric stretching mode of υ(Mo−O3−Mo) along the a-
the α-MoO3:Te nanoflakes. At 5 K, a ferromagnetic hysteresis loop clearly appears with the saturation magnetization (Ms) of
Table 1. Magnetic Properties of Various Undoped and Transition Metal-Doped MoO3 material
type
TC (K)
MoO3 MoO3 MoO3 MoO3:Co(1%) MoO3:Co(2%) MoO3:Co(3%) MoO3:Co(4%) MoO3 MoO3:Ni (1%)Co (2%) MoO3:Ni(2%)Co(1%) MoO3:Ni(2%)Co(2%) MoO3:Te
nanobranch nanofiber thin film thin film thin film thin film thin film thin film thin film thin film thin film nanoflake
>300 >300 >300 >300 >300 >300 >300 >300 >300 >300 >300 >350
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
10−2 10−2 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−2
1.5 × 10−2
56 144 56.38 70.28 64.8 99.81 109.34 26.41 51.15 47.88 22.0 72.7
2.78 1.46 2.96 2.85 3.25 1.64 1.79 5.43 4.86 4.45 1.69 4.2 F
× × × × × × × × × × × ×
8.10 7.56 5.49 3.61 5.91 9.82 8.06 7.06 0.68 7.1
× × × × × × × × × ×
10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−3
ref 21 20 22
23
this work
DOI: 10.1021/acsnano.9b01179 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano axis of 2D-like ultrathin α-MoO3:Te. Due to the intentional incorporation of smaller Te6+ ions into bigger Mo6+ sites, the pentacoordinated Mo5+ ions and their corresponding VO defects were effectively formed in α-MoO3:Te. Since the randomly distributed magnetic Mo5+ ions (i.e., Mo5+: [Kr] 4d1) create the overlapped BMPs via coupling with the charged VO defects, the degree of long-range ferromagnetic ordering could be enhanced in the α-MoO3:Te solid state system. This enables us to demonstrate enhanced room-temperature ferromagnetism in ultrathin α-MoO3:Te nanoflakes.
Research Programs (Grant Nos.: 2016R1D1A1B03935948, 2016R1A6A1A03012877, 2017R1A2B4004281, and 2019R1I1A1A01063238) funded by Korean Governments.
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METHODS The α-MoO3:Te nanoflakes were grown by VPE. First, the Mo-coated SiO2/Si substrate was prepared by depositing a 2 nm thick Mo (99.999%) layer onto the SiO2/Si substrate through an electron-beam evaporation method. Then the Mo/SiO2/Si substrate was mounted on the main reaction zone in a quartz tube of the VPE system. We note here that 0.5 mg of Te grains (99.999%) was placed at the extra impurity zone in the same quartz tube. The distance between the former zone and the latter zone was 7 cm. After the quartz tube was purged for 5 min by blowing pure Ar gas (99.99%), we increased the reaction zone temperature up to 550 °C so as to nucleate the Mo nanoclusters on the SiO2 surface. Thereafter, we started the Te vaporization by increasing the impurity zone temperature up to 550 °C. Then we subsequently added both the reaction gas (O2: 10 sccm) and the carrier gas (Ar: 110 sccm) in the quartz tube to perform the VPE growth of α-MoO3:Te. We kept such a growth condition for 60 min, during which time the Mo nanoclusters are oxidized and the Te vapors are incorporated into the solid solution. Since the nucleation sites (i.e., Mo nanoclusters) are locally limited onto the SiO2 surface, the shape of α-MoO3:Te could become a nanostructure (i.e., growth of 2D-like ultrathin α-MoO3:Te nanoflakes). As soon as the growth process was completed, the temperature of the quartz chamber was cooled to 27 °C in Ar atmosphere. The morphology and the crystallographic structure of α-MoO3:Te were examined by SEM and XRD measurements, respectively. Raman scattering characteristics were measured under the back-scattering geometry by using a green laser excitation source (λ = 514 nm). TEM and in situ SAED measurements were performed to monitor the microstructural properties of α-MoO3:Te. The chemical bonding states of the material components were investigated by XPS. The optical and the magnetic properties were characterized by PL spectroscopy using an excitation source of the 325 nm He−Cd laser and SQUID magnetometry, respectively.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b01179. Surface topography of the ultrathin α-MoO3:Te nanoflake (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Woochul Yang: 0000-0003-3726-2269 Sejoon Lee: 0000-0002-4548-7436 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) through the Basic Science G
DOI: 10.1021/acsnano.9b01179 ACS Nano XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsnano.9b01179 ACS Nano XXXX, XXX, XXX−XXX
