Next Article in Journal
Reduced Graphene Oxide and Gold Nanoparticles-Modified Electrochemical Aptasensor for Highly Sensitive Detection of Doxorubicin
Previous Article in Journal
High-Performance, Degradable, Self-Healing Bio-Based Nanocomposite Coatings with Antibacterial and Antioxidant Properties
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Effects of Oxygen on Lattice Defects in Single-Crystalline Mg2Si Thermoelectrics

1
Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
2
Graduate School of Science and Technology, Nara Institute of Science and Technology, Ikoma 630-0192, Japan
3
Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
4
Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
5
Japan Synchrotron Radiation Research Institute (JASRI), Sayo 679-5198, Japan
*
Author to whom correspondence should be addressed.
Nanomaterials 2023, 13(7), 1222; https://doi.org/10.3390/nano13071222
Submission received: 31 January 2023 / Revised: 20 March 2023 / Accepted: 24 March 2023 / Published: 30 March 2023
(This article belongs to the Section Physical Chemistry at Nanoscale)

Abstract

:
Lattice defect engineering has attracted attention due to its ability to develop thermoelectric materials with low thermal conductivity. For Mg2Si single crystals (SCs), Si vacancy (VSi) defects can be introduced and consequently result in the formation of dislocation cores. These lattice defects confer Mg2Si SCs with a lower thermal conductivity compared to Mg2Si polycrystals. To reveal a mechanism for the stabilisation of VSi in the Mg2Si SCs, we investigated the effects of oxygen (O) on lattice defects by performing electronic structure calculations, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and photoelectron holography. On the basis of these calculations, we predicted that O stabilised the formation of VSi when it was located at the Si site or at an interstitial site. All experiments confirmed the presence of O inside the Mg2Si SCs. However, O was suggested to be located not at the specific site in the crystal lattice of Mg2Si but at dislocation cores. The interaction between O and the dislocation cores in the Mg2Si SC is expected to immobilise dislocation cores, leading to the stabilisation of VSi formation.

1. Introduction

Lattice defect engineering is key to developing energy-harvesting materials such as photovoltaic, dielectric, and thermoelectric (TE) materials [1,2,3]. Lattice defects such as point defects and dislocation cores should be reduced in photovoltaic and dielectric materials to achieve high energy-conversion efficiency. Although the introduction of lattice defects into TE materials also affects TE properties (the Seebeck coefficient, electrical conductivity, and thermal conductivity) [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], it can improve TE performance mainly because lattice thermal conductivity (κL) decreases due to enhanced phonon scattering by lattice defects [5,11,16,19,23,24,29,30,31]. Thus, investigating which point defect is formed and how it works in TE materials is important.
In this study, we focused on Mg2Si, which has attracted considerable attention as a potential TE material [3,8,21,32,33]. Mg2Si has an antifluorite type structure (space group: Fm 3 ¯ m), wherein the 8c(1/4 1/4 1/4) and the 4a(0 0 0) sites were occupied by Mg and Si, respectively. The most probable point of the defect in Mg2Si was theoretically predicted to be an interstitial defect in which the 4b(1/2 1/2 1/2) site was partially occupied by Mg [34,35,36]. This interstitial Mg (Mgi) was present in synthesised Mg2Si-based polycrystals (PCs) [6,7,21,37]. However, recent studies have revealed that undoped and boron (B)-doped Mg2Si single crystals (SCs) contain Si vacancy (VSi) defects [17]. In addition, VSi defects induce edge dislocation cores in these Mg2Si-based SCs [27]. As a result of the presence of VSi defects and dislocation cores, Mg2Si-based SCs exhibited lower κL than Mg2Si PCs [27]. The difference in point defects between Mg2Si-based PCs and SCs may arise from preparation temperature, which is typically 1123 K for Mg2Si-based PCs [6,7] and 1413 K for Mg2Si-based SCs [17,27]. The higher preparation temperature for Mg2Si-based SCs can induce VSi. However, Mg2Si with VSi is not expected to be stable, because VSi has higher formation energy than Mgi [34,35,36]. One possibility for the stabilisation of VSi in Mg2Si is its incorporation with oxygen (O). A theoretical study predicted that O intercalation into the 4b(1/2 1/2 1/2) site resulted in p-type Mg2Si [38]. This prediction was consistent with the findings of an experimental study on postannealed Mg2Si thin films [39], which also suggested that Mg vacancy (VMg) defects were formed through the postannealing process. The other possibility for interaction between dislocation cores and O. In a Si SC, O diffuses during preparation and precipitates at dislocation cores [40,41,42,43]. Subsequently, location of Si and O atoms were rearranged around dislocation cores, making the dislocation cores stable [41,42,43]. A similar O precipitation at the dislocation cores may occur in Mg2Si-based SCs, leading to the stabilization of VSi.
Motivated by the above studies, we investigated the effects of O on VSi and the dislocation cores in Mg2Si SCs. By using first-principles electronic structure calculations, we examined whether the incorporation of O stabilised the formation of VSi. Furthermore, we performed secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and photoelectron holography to verify the existence of O in the prepared Mg2Si SCs. In particular, on the basis of photoelectron holograms, we discussed the three-dimensional atomic arrangements of Mg, Si, and O.

2. Calculation and Experimental Methods

We used the full-potential linearised augmented plane wave (FLAPW) method implemented in the Wien2k code [44] and the Korringa-Kohn-Rostoker (KKR) method under the coherent potential approximation implemented in the AkaiKKR code [45] to reduce the cost for the electronic structure calculation. Although the GW approximation, using the Green’s function G and the screened Coulomb potential W, has been reported to be the most accurate method for the calculation of Mg2Si, Mg2Ge, and Mg2Sn [46], the FLAPW and/or KKR methods are known to be sufficient for determining the conduction type and comparing the total energy, E, of different crystal structure models [8,10,12,18,19,20,22,25,28]. The local exchange-correlation potential in generalised gradient approximation was used for calculation through the FLAPW method. Twelve crystal structure models with a 2 × 2 × 2 cubic supercell were used, as shown in Figures S1–S3. Model 1 is ‘Mg64Si32′, which contains 64 Mg atoms at the 8c site and 32 Si atoms at the 4a site without any point defects. Model 2a is ‘Mg64Si32+Mgi’, which contains one additional Mgi. Models 2b and 2c contain one Mgi and an O atom at the 4b site (Oi) with different distances between Mgi and Oi, namely, ‘Mg64Si32+Mgi+Oi’. Model 3a is designated as ‘Mg63Si32′, i.e., one VMg exists in a supercell. Models 3b and 3c also contain one VMg and one Oi with different distances between VMg and Oi and are expressed as ‘Mg63Si32+Oi’. One VSi exists in Model 4a, and one additional Oi exists in Models 4b and 4c; these models are represented as ‘Mg64Si31′ and ‘Mg64Si31+Oi’, respectively. Models 3d and 4d are denoted as ‘Mg63Si32+OMg’ and ‘Mg64Si31+OSi’, respectively, where an O atom can be substituted for an Mg/Si atom. We investigated the stability of the point defects in each crystal structure model on the basis of the calculation of total energy E. For the calculation, the E values of Mg, Si, and/or O atoms were added to equalise the number of atoms.
E = E Mg 64 a Si 32 b + c Mg i + d O i + e O Mg + f O Si + 1 + a c E Mg + b · E Si + 1 d e f E O ,
where af is a constant (0 or 1). The numbers of k-points in the Brillouin zone for each model, Mg, and Si, were 108, 2028, and 3430, respectively. We also calculated the electronic density of states (DOS) to examine the conduction type. In the KKR method, the generalised gradient approximation and Perdew–Burke–Ernzerhof functional were used. The angular momentum cut-off was 2. The imaginary part added to the Fermi energy was set to 0.0001 Ry to calculate the DOS of Mg2Si, Mg2Si0.97, Mg2Si0.97+0.03OSi, Mg2Si0.99, and Mg2Si0.99+0.01OSi. The number of k-points was 2168.
The preparation procedure of the Mg2Si SC that contained VSi and dislocation cores is described elsewhere [17]. The depth profiles of O, carbon (C), hydrogen (H), Si, and MgSi in the prepared Mg2Si SCs were determined through SIMS (IONTOF, TOF-SIMS5-100) in negative polarity mode. The XPS spectra of the Mg2Si SC were acquired in a vacuum (1.2 × 10−5 Pa) by using Al Kα radiation as a light source (ThermoFisher Scientific, Theta Probe). A cleavage surface was obtained just before the sample was introduced into the XPS chamber. Surface etching was performed in the chamber by using Ar ion milling. Photoelectron holography, a type of atomic resolution holography that can directly reveal a three-dimensional local structure around a target atom [47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63], was performed by using soft X-ray as a light source at the beamline 25SU [64] of the synchrotron radiation facility Super Photon ring-8 GeV (SPring-8), Japan. A wide-angle display-type retarding field analyser was used for the measurement [65]. A cleaved surface was obtained after the introduction of the sample into the vacuum chamber. The vacuum pressure during the measurement was 6.5 × 10−8 Pa. From photoelectron holography, an atomic arrangement of O on W(110) [62] and a defect structure including O at the interface between Al2O3 and diamond [63] was revealed. Thus, the position of O in the Mg2Si SC can be determined if a structure around O has a long-range order.

3. Results and Discussion

By using the FLAPW method, we investigated whether point defects in each supercell were stable or not. Figures S1–S3 show that the E of Models 2a (Mg64Si32+Mgi), 3a (Mg63Si32), and 4a (Mg64Si31) were higher than that of Model 1 (Mg64Si32). This result indicates that the formation energy of Mgi, VMg, and VSi was positive (+0.197 eV/cell, +0.266 eV/cell, and +0.293 eV/cell, respectively). Consistent with the results of previous studies [34,35,36], VSi showed the highest E, i.e., it had the highest formation energy, among the point defects. However, Models 2b, 2c, 3b, 3c, 3d, 4b, 4c, and 4d, which all contained O, exhibited a lower E than Model 1, indicating that the introduction of O could stabilise the formation of Mgi, VMg, and VSi. Models 2c (Mg64Si32+Mgi+Oi), 3b (Mg63Si32+Oi), and 4d (Mg64Si31+OSi) had the lowest E in the cases of Mgi, VMg, and VSi, respectively. The E values of Models 1, 2a, 3a, 4a, 2c, 3b, and 4d are summarised in Figure 1. Although Model 4a had the highest E, the introduction of O into the Si site (Model 4d) resulted in the lowest E value. The formation energy of Mgi+Oi (Model 2c), VMg+Oi (Model 3b), and OSi (Model 4d) were −0.272 eV/cell, −0.193 eV/cell, and −0.583 eV/cell, respectively. In addition, the E values of Models 4b and 4c, which contained VSi and Oi (see Figure S3), respectively, were lower or equal to those of Models 2c and 3b, which contained Mgi/VMg and Oi, respectively. The formation energy of VSi+Oi (Models 4b and 4c) was −0.211 eV/cell and −0.296 eV/cell, respectively. In other words, the incorporation of O stabilised the formation of VSi rather than Mgi and VMg, regardless of whether O was located at the Si site or at the interstitial site. (The formation energy of each defect is summarised in Table S1).
Next, we examined the conduction type of Models 1, 4a, and 4d by calculating their DOS through the FLAPW method. By introducing one VSi defect into Mg64Si32, p-type conductivity changed to n-type one (see Figure 2a,b). The band gap increased with the introduction of the VSi defect from 0.11 eV to 0.19 eV. Similar to the introduction of one Mgi into Mg64Si32 and Mg216Si108 [21], the substitution of one O for the Si site caused an in-gap state just at the bottom of the conduction band (Figure 2c). Given that the Fermi level (EF) was located inside the conduction band, Mg64Si31+OSi was found to have n-type conductivity. These calculation results were reproduced using the KKR method, as shown in Figure 2d–f. Note that the fraction of VSi or OSi in the crystal structure models used for the calculation of Figure 2b,c,e,f was approximately 3%. By contrast, the VSi fraction in the prepared Mg2Si SCs was reported to be approximately 1%. Thus, we calculated the DOS of Mg2Si0.99 by using the KKR method (Figure 2g). The n-type conductivity was confirmed, which was consistent with a previous calculation and used another KKR code [67]. The increase in the band gap was also found in the KKR calculation. The band gap of Mg2Si was 0.10 eV, whereas that of Mg2Si0.99 and Mg2Si0.97 was 0.15 eV and 0.20 eV, respectively. The band gap of Mg2Si and Mg2Si0.97 was in good agreement with the FLAPW calculation. For Mg2Si0.99+0.01OSi (Figure 2h), the in-gap state became smaller than that shown in Figure 2c,f, but EF continued to exist inside the conduction band. From the above calculations, we found that the presence of VSi or OSi led to n-type conductivity of Mg2Si. This result was consistent with the experiments showing that the prepared Mg2Si SCs had a negative Seebeck coefficient, i.e., they had n-type conductivity [17]. Figure 2i presents the lattice constant at the minimum E, which was used for the calculation of the results given in Figure 2a,h. The introduction of VSi or OSi reduced the lattice constant. Experimentally, the smaller lattice constant resulted in an increase in the VSi fraction in Mg2Si SCs [17]. A similar tendency was reported for the prepared Mg2Sn SCs, wherein the VMg fraction increased with the decrease in the lattice constant [16,27]. As expected from the above calculations, O can be Oi or OSi to stabilise VSi if it exists in the crystal lattice of Mg2Si.
We acquired the depth profiles of O, C, H, Si, and MgSi by using SIMS, as shown in Figure 3, to investigate whether O was present or absent in the Mg2Si SC. O, C, and H were mainly detected below 200 nm, indicating that the surface of the Mg2Si SC was contaminated with O, C, and H. As the depth increased to 200 nm, the H intensity decreased to a noise level (<10 counts), and Si and MgSi were clearly detected instead. The C intensity also decreased and reached the noise level at 1600 nm. These results suggested that surface contamination with C and H could be eliminated by etching or cleaving the Mg2Si SC. On the other hand, O intensity decreased but remained constant above 1200 nm. Thus, O was expected to exist in the Mg2Si SC after etching or cleaving.
The presence of O inside the Mg2Si SC was also confirmed by acquiring the XPS spectra before and after etching, as shown in Figure 4a. C and O peaks, in addition to Mg and Si peaks, were observed before etching. This finding indicated that the surface of the Mg2Si SC was contaminated and had oxidised. In fact, the surface of Mg2Si SC is known to oxidise when it is placed in the atmosphere [68]. After etching, the C peak disappeared, but the O peaks remained. Additional detailed information is provided in Figure 4b. Before etching, two peaks were observed in the C 1s XPS spectrum (upper-left figure). These peaks, which were assigned to C-O (290 eV [69]) and C (285 eV), were diminished after etching (lower-left figure). A broad peak consisting of C-O (532 eV), Si-O (532 eV [70]), and Mg-O (530 eV) components were observed in the O 1s XPS spectrum before etching (upper-middle figure). The surface oxidation and/or the presence of O inside the Mg2Si SC were ascribed to the Si-O and Mg-O components. By etching the surface, the broad O 1s peak changed into double peaks (lower-middle figure) due to a decrease in C-related contamination. In other words, the C-O component at 532 eV disappeared, making the Si-O and Mg-O components more evident after etching. The remaining Si-O and Mg-O components indicated that O existed inside the Mg2Si SC. The removal of surface oxidation was confirmed by comparing the Mg 2p XPS spectra before and after etching (upper-right and lower-right figures, respectively). A broad peak was observed in the spectra. This peak could be separated into two components at 50 eV and 49 eV. Such components were also found in an Sb-doped Mg2Si0.4Sn0.6 PC and were assigned to the Mg of Mg2Si (50 eV) and Mgi (49 eV) components [71]. However, this assignment can be disputed, given the absence of Mgi in the Mg2Si SC [17]. Studies [72,73,74] have reported that the Mg-O and Mg peaks of Mg2Si components appeared in the Mg 2p XPS spectra at lower and higher binding energies, respectively. In consideration of these studies, as well as the O 1s XPS spectra in this study, we concluded that the components observed at 50 eV and 49 eV were Mg-O and Mg in Mg2Si components, respectively. Etching the surface changed the intensity ratio of these components. After etching, the Mg-O component decreased, whereas the Mg of the Mg2Si component increased. This result indicates that although the oxidised surface had been removed, O remained in the Mg2Si SC.
We further proved that O was present in Mg2Si SC through the use of photoelectron holography. Figure 5a shows the Mg 2p spectrum (dots), which could be deconvoluted into two components (blue and red curves). The spin-orbit splitting for each component was set at 0.6 eV. In consideration of the XPS results shown previously, we assigned the components at higher and lower binding energies to Mg-O and Mg in Mg2Si, respectively. The difference in the energy position between the two components was 1 eV and was consistent with that in the Mg 2p XPS spectra. The insets in Figure 5a are the holograms derived from each component (blue-black colour scale). In particular, the hologram for the component at a lower binding energy well coincided with the hologram simulated using the regular atomic arrangement of the Mg and Si atoms of Mg2Si around an emitter Mg atom (yellow-black colour scale). Thus, the assignment of the components in the Mg 2p spectrum was found to be valid. The Si 2p spectrum (dots) is shown in Figure 5b. The spin-orbit splitting for each component was set at 0.6 eV. Similar to a previous result [68], a component of Si for Mg2Si at the lowest binding energy and Si-O components at a higher binding energy were present in the Si 2p spectrum (coloured curves). The Si-O components were Si+, Si2+, Si3+, and Si4+ at 0.5 eV, 0.9 eV, 1.3 eV, and 4.5 eV, respectively. The energy positions of these components were evaluated relative to that of the Si component and were found to correspond reasonably to values in the literature [75,76,77]. An inset in Figure 5b is an experimental hologram derived from the Si component (left) and a simulated hologram constructed on the basis of the regular atomic arrangement of the Mg and Si atoms of Mg2Si around an emitter Si atom (right). Given that the experimental hologram coincided with the simulated one, the assignment of the components was confirmed to be valid for the Si 2p spectrum. Although the cleavage surface was prepared in a vacuum, the Mg-O and Si-O components were found in Mg 2p and Si 2p spectra, respectively, verifying that O was present inside the Mg2Si SC. The remaining O in an Ar gas and/or the aluminum crucible was used for the preparation of the Mg2Si SC, which could be a source of O.
Here, we discuss the position of O in the Mg2Si SC. A hologram derived from the Mg-O component is shown in the inset of Figure 5a, which is rather unclear compared with those derived from the Mg and Si components. We reconstructed a simulated hologram of Model 4c (Mg64Si31+Oi) and Model 4d (Mg64Si31+OSi), as shown in Figure 6, to examine the position. The characteristic features in the simulated holograms were not identified in the experimental hologram. Thus, we could not conclude that O existed at a specific site of the crystal lattice of Mg2Si. Instead, O was highly likely to be present at the dislocation cores. O was likely diffused and segregated to the dislocation cores during crystal growth. O segregation resulted in the immobilisation of the dislocation cores through the reconstruction of Mg, Si, and O atom locations around the dislocation cores. The reconstruction process reduced the total energy, which, in turn, stabilised VSi. Note that reconstructed atomic arrangements lacked a long-range order; therefore, the hologram derived from the Mg-O component was featureless. In the future, other experiments, such as x-ray absorption spectroscopy and positron annihilation spectroscopy, will be performed to determine the position of O precisely.
In summary, using the FLAPW and KKR methods, we theoretically predicted that the presence of O stabilised the formation of Mgi, VMg, and VSi in Mg2Si SCs. The formation energy of VSi+Oi (Models 4b and 4c) and OSi (Model 4d) was lower or equal to that of the other lattice defects, indicating that Mg2Si SCs with VSi were stabilised through the incorporation of O. The OSi defect showed the lowest formation energy of the −0.583 eV/cell. The calculated DOS indicated that the electrical conduction of Mg2Si changed from p-type to n-type by introducing VSi. The n-type conductivity was maintained for the additional introduction of O into Mg2Si with VSi. The presence of C and O in the prepared Mg2Si SC was revealed by SIMS and XPS. C existed in the vicinity of the surface of the Mg2Si SC, whereas O was present not only at the surface but also inside the Mg2Si SC. The presence of O was also confirmed by photoelectron holography measurements on the cleavage surface of the Mg2Si SC. In the Mg 2p spectrum, Mg and Mg-O components were observed, which was consistent with the XPS measurements. In the Si 2p spectrum, Si and Si-O components were present. An experimental hologram derived from the Mg and Si components was reproduced by a simulated hologram constructed on the basis of the regular atomic arrangement of the Mg and Si atoms of Mg2Si around an Mg and Si atom, respectively. On the other hand, an experimental hologram derived from the Mg-O component was featureless and did not coincide with a simulated hologram of Mg2Si with VSi or OSi. This result indicated that O could be located at dislocation cores, not at the interstitial site or at the Si site in the Mg2Si SC. The Mg2Si SC with VSi may have become stable due to the interaction between O and the dislocation cores. The effect of O as well as the higher preparation temperature of the Mg2Si SC, are the reasons for the formation of VSi in the Mg2Si SC.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano13071222/s1, Figure S1: Volume dependence of the total energy for the four crystal structure models relative to the minimum energy of Model 1. Crystal structures are drawn by using VESTA [66]; Figure S2: Volume dependence of the total energy for the five crystal structure models relative to the minimum energy of Model 1; Figure S3: Volume dependence of the total energy for the five crystal structure models relative to the minimum energy of Model 1; Table S1: Formation energy of Mgi, VMg, VSi, and their complex defects in combination with O.

Author Contributions

Calculation, K.H. (Kei Hayashi); investigation, K.H. (Kei Hayashi), S.K., Y.H., N.A., Z.H., W.S., K.T. and K.H. (Koichi Hayashi), and T.M.; writing—original draft preparation, K.H. (Kei Hayashi); writing—review and editing, K.H. (Kei Hayashi), T.M., and Y.M.; supervision, K.H. (Kei Hayashi) and Y.M.; funding acquisition, K.H. (Kei Hayashi), K.H. (Koichi Hayashi), and T.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partly supported by the Grant-in-Aid for Scientific Research (B) (Grant Nos. 17H03398, 20H01841, 22H02161) and the Grants-in-Aid for Transformative Research Area (A) ’Hyper-Ordered Structures Science’ (Grant Nos. 20H05878, 20H05881, 20H5882, 20H05884) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and was partly based on collaborative research between Sumitomo Metal Mining Co., Ltd. and Tohoku University, which is part of the Vision Co-creation Partnership. The synchrotron radiation experiments were performed with the approval of Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2021A1018).

Data Availability Statement

Not applicable.

Acknowledgments

Kei Hayashi is grateful to R. Shishido from the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University for the SIMS measurement. The authors are also grateful to T. Muro from JASRI for the photoelectron holography measurements.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Park, J.-S.; Kim, S.; Xie, Z.; Walsh, A. Point defect engineering in thin-film solar cells. Nat. Rev. Mater. 2018, 3, 194–210. [Google Scholar] [CrossRef]
  2. Balaraman, A.A.; Dutta, S. Inorganic dielectric materials for energy storage applications: A review. J. Phys. D Appl. Phys. 2022, 55, 183002. [Google Scholar] [CrossRef]
  3. Shi, X.-L.; Zou, J.; Chen, Z.-G. Advanced Thermoelectric Design: From Materials and Structures to Devices. Chem. Rev. 2020, 120, 7399–7515. [Google Scholar] [CrossRef] [PubMed]
  4. Liu, W.-S.; Zhang, B.-P.; Li, J.-F.; Zhao, L.-D. Effects of Sb compensation on microstructure, thermoelectric properties and point defect of CoSb3 compound. J. Phys. D Appl. Phys. 2007, 40, 6784–6790. [Google Scholar] [CrossRef]
  5. Jiang, G.; He, J.; Zhu, T.; Fu, C.; Liu, X.; Hu, L.; Zhao, X. High Performance Mg2(Si,Sn) Solid Solutions: A Point Defect Chemistry Approach to Enhancing Thermoelectric Properties. Adv. Funct. Mater. 2014, 24, 3776–3781. [Google Scholar] [CrossRef]
  6. Kubouchi, M.; Hayashi, K.; Miyazaki, Y. Quantitative analysis of interstitial Mg in Mg2Si studied by single crystal X-ray diffraction. J. Alloys Compd. 2014, 617, 389–392. [Google Scholar] [CrossRef]
  7. Kubouchi, M.; Ogawa, Y.; Hayashi, K.; Takamatsu, T.; Miyazaki, Y. Effect of Interstitial Mg in Mg2+xSi on Electrical Conductivity and Seebeck Coefficient. J. Electron. Mater. 2016, 45, 1589–1593. [Google Scholar] [CrossRef]
  8. Kubouchi, M.; Hayashi, K.; Miyazaki, Y. Electronic structure and thermoelectric properties of boron doped Mg2Si. Scr. Mater. 2016, 123, 59–63. [Google Scholar] [CrossRef]
  9. Tamaki, M.; Sato, H.K.; Kanno, T. Isotropic Conduction Network and Defect Chemistry in Mg3+δSb2-Based Layered Zintl Compounds with High Thermoelectric Performance. Adv. Mater. 2016, 28, 10182–10187. [Google Scholar] [CrossRef]
  10. Hayashi, K.; Eguchi, M.; Miyazaki, Y. Structural and Thermoelectric Properties of Ternary Full-Heusler Alloys. J. Electron. Mater. 2017, 42, 2710–2716. [Google Scholar] [CrossRef]
  11. Wu, D.; Wu, L.; He, D.; Zhao, L.-D.; Li, W.; Wu, M.; Jin, M.; Xu, J.; Jiang, J.; Huang, L.; et al. Direct observation of vast off-stoichiometric defects in single crystalline SnSe. Nano Energy 2017, 35, 321–330. [Google Scholar] [CrossRef] [Green Version]
  12. Saito, W.; Hayashi, K.; Nagai, H.; Miyazaki, Y. Preparation and thermoelectric properties of mixed valence compound Sn2S3. JPN J. Appl. Phys. 2017, 56, 061201. [Google Scholar] [CrossRef]
  13. Hayashibara, Y.; Hayashi, K.; Ando, I.; Kubouchi, M.; Ogawa, Y.; Saito, W.; Miyazaki, Y. Fabrication and Thermoelectric Properties of Al/Mg2Si Composite Materials. Mater. Trans. 2018, 59, 1041–1045. [Google Scholar] [CrossRef] [Green Version]
  14. Li, H.; Hayashi, K.; Miyazaki, Y. Design and fabrication of full-Heusler compound with positive Seebeck coefficient as a potential thermoelectric material. Scr. Mater. 2018, 150, 130–133. [Google Scholar] [CrossRef]
  15. Zhang, Q.; Gu, B.; Wu, Y.; Zhu, T.; Fang, T.; Yang, Y.; Liu, J.; Ye, B.; Zhao, X. Evolution of the Intrinsic Point Defects in Bismuth Telluride-Based Thermoelectric Materials. ACS Appl. Mater. Interfaces 2019, 11, 41424–41431. [Google Scholar] [CrossRef]
  16. Saito, W.; Hayashi, K.; Dong, J.; Li, J.-F.; Miyazaki, Y. Control of the Thermoelectric Properties of Mg2Sn Single Crystals via Point-Defect Engineering. Sci. Rep. 2020, 10, 2020. [Google Scholar] [CrossRef] [Green Version]
  17. Hayashi, K.; Saito, W.; Sugimoto, K.; Ohoyama, K.; Hayashi, K.; Happo, N.; Harada, M.; Oikawa, K.; Inamura, Y.; Miyazaki, Y. Preparation, thermoelectric properties, and crystal structure of boron-doped Mg2Si single crystals. AIP Adv. 2020, 10, 035115, Erratum in AIP Adv. 2021, 11, 029903. [Google Scholar] [CrossRef] [Green Version]
  18. Li, H.; Hayashi, K.; Dong, J.; Li, J.-F.; Miyazaki, Y. Distinct impact of order degree on thermoelectric power factor of p-type full-Heusler Mn2VAl compounds. Mater. Res. Express 2020, 7, 055503. [Google Scholar] [CrossRef]
  19. Huang, Y.; Hayashi, K.; Miyazaki, Y. Electron Conduction Mechanism of Deficient Half-Heusler VFeSb Compound Revealed by Crystal and Electronic Structure Analyses. Chem. Mater. 2020, 32, 5173–5181. [Google Scholar] [CrossRef]
  20. Yoshioka, S.; Hayashi, K.; Yokoyama, A.; Saito, W.; Li, H.; Takamatsu, T.; Miyazaki, Y. Crystal structure, electronic structure and thermoelectric properties of β- and γ-Zn4Sb3 thermoelectrics: A (3+1)-dimensional superspace group approach. J. Mater. Chem. C 2020, 8, 9205–9212. [Google Scholar] [CrossRef]
  21. Hayashi, K.; Miyazaki, Y.; Saito, W.; Kubouchi, M.; Ogawa, Y.; Suzuki, S.; Hayashibara, Y.; Ando, I. Thermoelectric properties of Mg2Si and its derivatives: Effects of lattice defects and secondary phases. In Thermoelectric Materials; Kurosaki, K., Takagiwa, Y., Shi, X., Eds.; De Gruyter: Berlin, Germany, 2020; pp. 99–116. [Google Scholar]
  22. Hayashi, K.; Li, H.; Eguchi, M.; Nagashima, Y.; Miyazaki, Y. Magnetic Full-Heusler Compounds for Thermoelectric Applications. In Magnetic Materials and Magnetic Levitation; Sahu, D.R., Ed.; IntechOpen: Rijeka, Croatia, 2020; Chapter 4. [Google Scholar]
  23. Saito, W.; Hayashi, K.; Huang, Z.; Dong, J.; Li, J.-F.; Miyazaki, Y. Enhancing the Thermoelectric Performance of Mg2Sn Single Crystals via Point Defect Engineering and Sb Doping. ACS Appl. Mater. Interfaces 2020, 12, 57888–57897. [Google Scholar] [CrossRef] [PubMed]
  24. Kanno, T.; Tamaki, H.; Yoshiya, M.; Uchiyama, H.; Maki, S.; Takata, M.; Miyazaki, Y. High-Density Frenkel Defects as Origin of N-Type Thermoelectric Performance and Low Thermal Conductivity in Mg3Sb2-Based Materials. Adv. Funct. Mater. 2021, 31, 2008469. [Google Scholar] [CrossRef]
  25. Li, H.; Hayashi, K.; Nagashima, Y.; Yoshioka, S.; Dong, J.; Li, J.-F.; Miyazaki, Y. Effects of Disorder on the Electronic Structure and Thermoelectric Properties of an Inverse Full-Heusler Mn2CoAl Alloy. Chem. Mater. 2021, 33, 2543–2547. [Google Scholar] [CrossRef]
  26. Yoshioka, S.; Hayashi, K.; Yokoyama, A.; Saito, W.; Miyazaki, Y. Crystal structure, microstructure, and electronic transport properties of β-Zn4Sb3 thermoelectrics: Effects of Zn intercalation and deintercalation. Mater. Today Energy 2021, 21, 100723. [Google Scholar] [CrossRef]
  27. Saito, W.; Hayashi, K.; Huang, Z.; Sugimoto, K.; Ohoyama, K.; Happo, N.; Harada, M.; Oikawa, K.; Inamura, Y.; Hayashi, K.; et al. Chemical-Pressure-Induced Point Defects Enable Low Thermal Conductivity for Mg2Sn and Mg2Si Single Crystals. ACS Appl. Energy Mater. 2021, 4, 5123–5131. [Google Scholar] [CrossRef]
  28. Huang, Y.; Hayashi, K.; Miyazaki, Y. Outstanding thermoelectric performance of n-type half-Heusler V(Fe1-xCox)Sb compounds at room-temperature. Acta Mater. 2021, 215, 117022. [Google Scholar] [CrossRef]
  29. Huang, Z.; Hayashi, K.; Saito, W.; Miyazaki, Y. Realizing p-type Mg2Sn Thermoelectrics via Ga-Doping and Point Defect Engineering. ACS Appl. Energy Mater. 2021, 4, 13044–13050. [Google Scholar] [CrossRef]
  30. Hayashi, K. Enhancement of thermoelectric performance of Mg2Sn single crystals via lattice-defect engineering. JSAP Rev. 2022, 2022, 220403. [Google Scholar]
  31. Huang, Z.; Hayashi, K.; Saito, W.; Pei, J.; Li, J.-F.; Miyazaki, Y. Enhanced Thermoelectric Performance of p-type Mg2Sn Single Crystals via Multi-scale Defect Engineering. J. Mater. Chem. A 2023, 11, 2652–2660. [Google Scholar] [CrossRef]
  32. Ning, H.; Mastrorillo, G.D.; Grasso, S.; Du, B.; Mori, T.; Hu, C.; Xu, Y.; Simpson, K.; Maizza, G.; Reece, M.J. Enhanced thermoelectric performance of porous magnesium tin silicide prepared using pressure-less spark plasma sintering. J. Mater. Chem. A 2015, 3, 17426–17432. [Google Scholar] [CrossRef] [Green Version]
  33. Gao, P.; Davis, J.D.; Poltavets, V.V.; Hogan, T.P. The p-type Mg2LixSi0.4Sn0.6 thermoelectric materials synthesized by a B2O3 encapsulation method using Li2CO3 as the doping agent. J. Mater. Chem. C 2016, 4, 929–934. [Google Scholar] [CrossRef]
  34. Kato, A.; Yagi, T.; Fukusako, N. First-principles studies of intrinsic point defects in magnesium silicide. J. Phys. Condens. Matter 2009, 21, 205801. [Google Scholar] [CrossRef]
  35. Jund, P.; Viennois, R.; Colinet, C.; Hug, G.; Fèvre, M.; Tédenac, J.-C. Lattice stability and formation energies of intrinsic defects in Mg2Si and Mg2Ge via first principles simulations. J. Phys. Condens. Matter 2013, 25, 035403. [Google Scholar] [CrossRef] [Green Version]
  36. Liu, X.; Xi, L.; Qiu, W.; Yang, J.; Zhu, T.; Zhao, X.; Zhang, W. Significant roles of intrinsic point defects in Mg2X (X = Si, Ge, Sn) thermoelectric materials. Adv. Electron. Mater. 2016, 2, 1500284. [Google Scholar] [CrossRef]
  37. Farahi, N.; Prabhudev, S.; Botton, G.A.; Zhao, J.; Tse, J.S.; Liu, Z.; Salvador, J.R.; Kleinke, H. Local structure and thermoelectric properties of Mg2Si0.977-xGexBi0.023 (0.1 ≤ x ≤ 0.4). J. Alloys Compd. 2015, 644, 249–255. [Google Scholar] [CrossRef] [Green Version]
  38. Imai, Y.; Sohma, M.; Suemasu, T. Effect of oxygen incorporation in the Mg2Si lattice on its conductivity type—A possible reason of the p-type conductivity of postannealed Mg2Si thin film. J. Alloys Compd. 2016, 676, 91–95. [Google Scholar] [CrossRef]
  39. Kurokawa, M.; Shimizu, T.; Uehara, M.; Katagiri, A.; Akiyama, K.; Matsushima, M.; Uchida, H.; Kimura, Y.; Funakubo, H. Control of p- and n-type Conduction in Thermoelectric Non-doped Mg2Si Thin Films Prepared by Sputtering Method. MRS Adv. 2018, 3, 1355–1359. [Google Scholar] [CrossRef]
  40. Möller, H.J. The movement of dissociated dislocations in the diamond-cubic structure. Acta Metall. 1978, 26, 963–973. [Google Scholar] [CrossRef]
  41. Umerski, A.; Jones, R. The interaction of oxygen with dislocation cores in silicon. Philos. Mag. A 1993, 67, 905–915. [Google Scholar] [CrossRef]
  42. Yonenaga, I.; Sumino, K. Influence of oxygen precipitation along dislocations on the strength of silicon crystals. J. Appl. Phys. 1996, 80, 734–738. [Google Scholar] [CrossRef]
  43. Ide, T.; Harada, H.; Miyamura, Y.; Imai, M.; Nakano, S.; Kakimoto, K. Relationship between Dislocation Density and Oxygen Concentration in Silicon Crystals during Directional Solidification. Crystals 2018, 8, 244. [Google Scholar] [CrossRef] [Green Version]
  44. Blaha, P.; Schwarz, K.; Madsen, G.; Kvasnicka, D.; Luitz, J. WIEN2k, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties; Schwarz, K., Ed.; Techn. Universität Wien: Wien, Austria, 2001. [Google Scholar]
  45. AkaiKKR Machikaneyama, Ab-Initio Electronic-Structure Calculation Code. Available online: http://kkr.issp.u-tokyo.ac.jp/ (accessed on 6 October 2022).
  46. Shi, G.; Kioupakis, E. Relativistic quasiparticle band structures of Mg2Si, Mg2Ge, and Mg2Sn: Consistent parameterization and prediction of Seebeck coefficients. J. Appl. Phys. 2018, 123, 085114. [Google Scholar] [CrossRef]
  47. Szöke, A. X-ray and electron holography using a local reference beam. AIP Conf. Proc. 1986, 147, 361–367. [Google Scholar]
  48. Matsushita, T.; Yoshigoe, A.; Agui, A. Electron holography: A maximum entropy reconstruction scheme. Europhys. Lett. 2005, 71, 597–603. [Google Scholar] [CrossRef]
  49. Matsushita, T.; Guo, F.Z.; Matsui, F.; Kato, Y.; Daimon, H. Three-dimensional atomic-arrangement reconstruction from an Auger-electron hologram. Phys. Rev. B 2007, 75, 085419. [Google Scholar] [CrossRef]
  50. Matsushita, T.; Guo, F.Z.; Suzuki, M.; Matsui, F.; Daimon, H.; Hayashi, K. Reconstruction algorithm for atomic-resolution holography using translational symmetry. Phys. Rev. B 2008, 78, 144111. [Google Scholar] [CrossRef]
  51. Matsushita, T.; Matsui, F.; Daimon, H.; Hayashi, K. Photoelectron holography with improved image reconstruction. J. Electron Spectrosc. Relat. Phenom. 2010, 178–179, 195–200. [Google Scholar] [CrossRef]
  52. Matsui, H.; Matsui, F.; Maejima, N.; Matsushita, T.; Daimon, H. Stacking registry determination of graphene grown on the SiC(0001) by photoelectron holography. Surf. Sci. 2015, 635, 1–4. [Google Scholar] [CrossRef]
  53. Matsui, F.; Eguchi, R.; Nishiyama, S.; Izumi, M.; Uesugi, E.; Goto, H.; Matsushita, T.; Sugita, K.; Daimon, H.; Hamamoto, Y.; et al. Photoelectron Holographic Atomic Arrangement Imaging of Cleaved Bimetal-intercalated Graphite Superconductor Surface. Sci. Rep. 2016, 6, 36258. [Google Scholar] [CrossRef] [Green Version]
  54. Fukami, S.; Taguchi, M.; Adachi, Y.; Sakaguchi, I.; Watanabe, K.; Kinoshita, T.; Muro, T.; Matsushita, T.; Matsui, F.; Daimon, H.; et al. Correlation Between High Gas Sensitivity and Dopant Structure in W-doped ZnO. Phys. Rev. Appl. 2017, 7, 064029. [Google Scholar] [CrossRef]
  55. Tsutsui, K.; Matsushita, T.; Natori, K.; Muro, T.; Morikawa, Y.; Hoshii, T.; Kakushima, K.; Wakabayashi, H.; Hayashi, K.; Matsui, F.; et al. Individual Atomic Imaging of Multiple Dopant Sites in As-Doped Si Using Spectro-Photoelectron Holography. Nano Lett. 2017, 17, 7533–7538. [Google Scholar] [CrossRef]
  56. Matsushita, T. Algorithm for Atomic Resolution Holography Using Modified L1-Regularized Linear Regression and Steepest Descent Method. Phys. Status Solidi B 2018, 255, 1800091. [Google Scholar] [CrossRef]
  57. Yokoya, T.; Terashima, K.; Takeda, A.; Fukura, T.; Fujiwara, H.; Muro, T.; Kinoshita, T.; Kato, H.; Yamasaki, S.; Oguchi, T.; et al. Asymmetric Phosphorus Incorporation in Homoepitaxial P-Doped (111) Diamond Revealed by Photoelectron Holography. Nano Lett. 2019, 19, 5915–5919. [Google Scholar] [CrossRef]
  58. Yamamoto, Y.; Ang, A.K.R.; Kimura, K.; Matsushita, T.; Hirose, Y.; Oka, D.; Hayashi, K. Anion arrangement analysis of oxynitride perovskite thin film with inverse photoelectron holography. J. Electron Spectr. Relat. Phenom. 2021, 246, 147018. [Google Scholar] [CrossRef]
  59. Tang, J.; Takeuchi, S.; Tanaka, M.; Tomita, H.; Hashimoto, Y.; Nagata, T.; Chen, J.; Ohkochi, T.; Kotani, Y.; Matsushita, T.; et al. Direct Observation of Atomic Structures and Chemical States of Active and Inactive Dopant Sites in Mg-Doped GaN. ACS Appl. Electron. Mater. 2022, 4, 4719–4723. [Google Scholar] [CrossRef]
  60. Uenuma, M.; Kuwaharada, S.; Tomita, H.; Tanaka, M.; Sun, Z.; Hashimoto, Y.; Fujii, M.N.; Matsushita, T.; Uraoka, Y. Atomic structure analysis of gallium oxide at the Al2O3/GaN interface using photoelectron holography. Appl. Phys. Express 2022, 15, 085501. [Google Scholar] [CrossRef]
  61. Li, Y.; Sun, Z.; Kataoka, N.; Setoguchi, T.; Hashimoto, Y.; Takeuchi, S.; Koga, S.; Muro, T.; Demura, S.; Noguchi, K.; et al. Incorporation Site and Valence State of Sn Atoms in Sn-Substituted La(O,F)BiS2 Superconductor. J. Phys. Soc. Jpn. 2022, 91, 054602. [Google Scholar] [CrossRef]
  62. Takeuchi, S.; Hashimoto, Y.; Daimon, H.; Matsushita, T. High-precision atomic image reconstruction from photoelectron hologram of O on W(110) by SPEA-L1. J. Electron Spectr. Relat. Phenom. 2022, 256, 147177. [Google Scholar] [CrossRef]
  63. Fujii, M.N.; Tanaka, M.; Tsuno, T.; Hashimoto, Y.; Tomita, H.; Takeuchi, S.; Koga, S.; Sun, Z.; Enriquez, J.I.; Morikawa, Y.; et al. Atomic Imaging of Interface Defects in an Insulating Film on Diamond. Nano Lett. 2023, 23, 1189–1194. [Google Scholar] [CrossRef]
  64. Senba, Y.; Ohashi, H.; Kotani, Y.; Nakamura, T.; Muro, T.; Ohkochi, T.; Tsuji, N.; Kishimoto, H.; Miura, T.; Tanaka, M.; et al. Upgrade of beamline BL25SU for soft x-ray imaging and spectroscopy of solid using nano- and micro-focused beams at SPring-8. AIP Conf. Proc. 2016, 1741, 030044. [Google Scholar]
  65. Muro, T.; Ohkochi, T.; Kato, Y.; Izumi, Y.; Fukami, S.; Fujiwara, H.; Matsushita, T. Wide-angle display-type retarding field analyzer with high energy and angular resolutions. Rev. Sci. Instrum. 2017, 88, 123106. [Google Scholar] [CrossRef] [PubMed]
  66. Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272–1276. [Google Scholar] [CrossRef]
  67. Zwolenski, P.; Tobola, J.; Kaprzyk, S. KKR-CPA study of electronic structure and relative stability of Mg2X (X = Si, Ge, Sn) thermoelectrics containing point defects. J. Alloys Compd. 2015, 627, 85–90. [Google Scholar] [CrossRef]
  68. Esaka, F.; Nojima, T.; Udono, H.; Magara, M.; Yamamoto, H. Non-destructive depth analysis of the surface oxide layer on Mg2Si with XPS and XAS. Surf. Interface Anal. 2016, 48, 432–435. [Google Scholar] [CrossRef]
  69. Shchukarev, A.V.; Korolkov, D.V. XPS study of group IA carbonates. Cent. Eur. J. Chem. 2004, 2, 347–362. [Google Scholar] [CrossRef] [Green Version]
  70. Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers—The Scienta ESCA300 Database; Wiley Interscience: New York, NY, USA, 1992. [Google Scholar]
  71. Du, Z.; Zhu, T.; Chen, Y.; He, J.; Gao, H.; Jiang, G.; Tritt, T.M.; Zhao, X. Roles of interstitial Mg in improving thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6 solid solutions. J. Mater. Chem. 2012, 22, 6838–6844. [Google Scholar] [CrossRef]
  72. Sekino, K.; Midonoya, M.; Udono, H.; Yamada, Y. Preparation of Schottky contacts on n-type Mg2Si single crystalline substrate. Phys. Proc. 2011, 11, 171–173. [Google Scholar] [CrossRef] [Green Version]
  73. Liao, Y.; Xie, J.; Lv, B.; Xiao, Q.; Xie, Q. X-ray photoelectron spectroscopy characterization of band offsets of MgO/Mg2Si and SiO2/Mg2Si heterojunctions. Surf. Interface Anal. 2021, 53, 852–859. [Google Scholar] [CrossRef]
  74. Liao, Y.; Xie, J.; Lv, B.; Xiao, Q.; Xie, Q. The Degradation Mechanism of Mg2Si during Exploitation at High Temperature. Phys. Status Solidi 2021, 258, 2100425. [Google Scholar] [CrossRef]
  75. Alfonsetti, R.; De Simone, G.; Lozzi, L.; Passacantando, M.; Picozzi, P.; Santucci, S. SiOx Surface Stoichiometry by XPS: A Comparison of Various Methods. Surf. Interface Anal. 1994, 22, 89–92. [Google Scholar] [CrossRef]
  76. Raider, S.I.; Fritsch, R. Silicon Monoxide Thin Films. J. Electrochem. Soc. 1976, 123, 1754–1757. [Google Scholar] [CrossRef]
  77. Nguyen, T.P.; Lefrant, S. XPS study of SiO thin films and SiO-metal interfaces. J. Phys. Condens. Matter 1989, 1, 5197–5204. [Google Scholar] [CrossRef]
Figure 1. Volume dependence of the total energy of seven crystal structure models relative to the minimum energy of Model 1. Crystal structures were drawn using VESTA [66].
Figure 1. Volume dependence of the total energy of seven crystal structure models relative to the minimum energy of Model 1. Crystal structures were drawn using VESTA [66].
Nanomaterials 13 01222 g001
Figure 2. Electronic density of states (DOS) for (a) Model 1, (b) Model 4a, and (c) Model 4d shown in Figure 1. Electronic DOS of the 1 × 1 × 1 cubic cell of (d) Mg2Si, (e) Mg2Si0.97, (f) Mg2Si0.97+0.03OSi, (g) Mg2Si0.99, and (h) Mg2Si0.99+0.01OSi. (i) Lattice constant used for the calculation of (ah), which provided the minimum total energy.
Figure 2. Electronic density of states (DOS) for (a) Model 1, (b) Model 4a, and (c) Model 4d shown in Figure 1. Electronic DOS of the 1 × 1 × 1 cubic cell of (d) Mg2Si, (e) Mg2Si0.97, (f) Mg2Si0.97+0.03OSi, (g) Mg2Si0.99, and (h) Mg2Si0.99+0.01OSi. (i) Lattice constant used for the calculation of (ah), which provided the minimum total energy.
Nanomaterials 13 01222 g002
Figure 3. Depth profiles of O, C, H, Si, and MgSi of the Mg2Si SC.
Figure 3. Depth profiles of O, C, H, Si, and MgSi of the Mg2Si SC.
Nanomaterials 13 01222 g003
Figure 4. (a) X-ray photoelectron spectroscopy survey spectra and (b) core level spectra of the Mg2Si SC before and after etching (upper and lower, respectively).
Figure 4. (a) X-ray photoelectron spectroscopy survey spectra and (b) core level spectra of the Mg2Si SC before and after etching (upper and lower, respectively).
Nanomaterials 13 01222 g004
Figure 5. (a) Mg 2p and (b) Si 2p spectra taken in photoelectron holography measurements of the Mg2Si SC (dots). Each spectrum is deconvoluted into several components (coloured curves). The sum of all components is drawn as a black curve. Insets are holograms derived from measured Mg, Mg-O, and Si components (blue-black colour scale) and simulated holograms (yellow-black colour scale).
Figure 5. (a) Mg 2p and (b) Si 2p spectra taken in photoelectron holography measurements of the Mg2Si SC (dots). Each spectrum is deconvoluted into several components (coloured curves). The sum of all components is drawn as a black curve. Insets are holograms derived from measured Mg, Mg-O, and Si components (blue-black colour scale) and simulated holograms (yellow-black colour scale).
Nanomaterials 13 01222 g005
Figure 6. Simulated holograms of Model 4c (Mg64Si31+ Oi) and Model 4d (Mg64Si31+OSi).
Figure 6. Simulated holograms of Model 4c (Mg64Si31+ Oi) and Model 4d (Mg64Si31+OSi).
Nanomaterials 13 01222 g006
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Hayashi, K.; Kawamura, S.; Hashimoto, Y.; Akao, N.; Huang, Z.; Saito, W.; Tasaki, K.; Hayashi, K.; Matsushita, T.; Miyazaki, Y. Effects of Oxygen on Lattice Defects in Single-Crystalline Mg2Si Thermoelectrics. Nanomaterials 2023, 13, 1222. https://doi.org/10.3390/nano13071222

AMA Style

Hayashi K, Kawamura S, Hashimoto Y, Akao N, Huang Z, Saito W, Tasaki K, Hayashi K, Matsushita T, Miyazaki Y. Effects of Oxygen on Lattice Defects in Single-Crystalline Mg2Si Thermoelectrics. Nanomaterials. 2023; 13(7):1222. https://doi.org/10.3390/nano13071222

Chicago/Turabian Style

Hayashi, Kei, Sota Kawamura, Yusuke Hashimoto, Noboru Akao, Zhicheng Huang, Wataru Saito, Kaichi Tasaki, Koichi Hayashi, Tomohiro Matsushita, and Yuzuru Miyazaki. 2023. "Effects of Oxygen on Lattice Defects in Single-Crystalline Mg2Si Thermoelectrics" Nanomaterials 13, no. 7: 1222. https://doi.org/10.3390/nano13071222

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop